Leaching of cations during displacement by acid solutions through columns of cecil soil

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Title:
Leaching of cations during displacement by acid solutions through columns of cecil soil
Physical Description:
xvi, 221 leaves : ill. ; 28 cm.
Language:
English
Creator:
Liu, Ko-Hui, 1951-
Publisher:
s.n.
Publication Date:

Subjects

Subjects / Keywords:
Soils -- Leaching   ( lcsh )
Soils -- Effect of acid rain on   ( lcsh )
Ion exchange   ( lcsh )
Soil Science thesis Ph. D
Dissertations, Academic -- Soil Science -- UF
Genre:
bibliography   ( marcgt )
non-fiction   ( marcgt )

Notes

Thesis:
Thesis (Ph. D.)--University of Florida, 1987.
Bibliography:
Bibliography: leaves 215-220.
Statement of Responsibility:
by Ko-Hui Liu.
General Note:
Typescript.
General Note:
Vita.

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Source Institution:
University of Florida
Rights Management:
All applicable rights reserved by the source institution and holding location.
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oclc - 17625202
notis - AEW2000
sobekcm - AA00004842_00001
System ID:
AA00004842:00001

Full Text

190


Concentrations of Al3+ in the solution phase were relatively

high for both treatments. If one examined only the

shallowest depth (at the end of the column receiving acid

input solution), cation concentrations in the exchange phase

were in the order Ca2+ > Al > Mg2+ > K > Na for pH

treatment 4.9 and in the order Al3+ > Ca2+> Mg2+ > K > Na+

for pH treatment 3.9. Higher Al3+ concentrations but lower

concentrations of the basic cations Ca2+, Mg 2+, K and Na

were found at the .input end of the soil column for the pH

3.9 case.

Charge Balance Using All Major Cations for Treated and
Mixed Soil Columns

The charge balance of all major cations for pretreated

and mixed topsoil and subsoil are given in Tables 4-22

through 4-27. The theoretical charge balance of major

cations in each soil column can be described by the

following relationship

Total H+ (mmole(+)) added + Exchangeable cations (mmole(+))

(includes exchangeable H )

initially present on soil

exchange sites

= Total H+ (mmole(+)) and cations (mmole(+))

exported from the soil column

+ Exchangeable cations (mmole(+)) (includes H+ ions)

remaining in the soil column. (4-3]

Details for the calculation procedure are the same as

stated in chapter three. In the following text the








column experiments were 0.370 and 0.458 M3 M3 for topsoil

and subsoil, respectively. A range of volumetric water-

content values of 2% deviation from the experimental data,

such as 0.360 and 0.380 M3 M-3 for topsoil and 0.468 and

0.448 M3 M3 for subsoil, respectively, was tested to

determine the sensitivity of 8 as an input parameter to the

model. Results show that the effect of variations in 8 on

prediction of Mg2+ concentration was small for topsoil and

had essentially no effect for subsoil. The effect of 8 is

shown by the term 0/8 (the soil-to-solution ratio) in the

cation-retardation factor (R). The smaller values of 8 give

the higher ratios of a0/ and the most retarded cation

movement.

Bulk density. Computed and observed depth

distributions of Mg2+ concentration in the solution phase

are given in Figs. 2-9 and 2-10, where a sensitivity

analysis was performed for bulk density (a). Experimental

bulk density values obtained from the column experiment were

1.64 and 1.42 Mg M-3 for topsoil and subsoil, respectively.

Values with 2% deviation from the experimental data gave

values of 1.67 and 1.61 Mg M-3 and 1.45 and 1.39 Mg M-3 for

use in the sensitivity analysis, respectively. The effect

of a occurs through the ratio 0/8 in R. The sensitivity

analysis revealed that even slight changes in values for a

could substantially influence simulated cation

concentrations.





126


soils are amphoteric, exhibiting both acidic and basic

properties and undergoing exchange with cations as well as

anions. Also, if neutral salt solution is added to a soil,

cations from the salt replace part of the exchangeable H+

and Al3+ present on the soil exchange phase and thereby

increase the base saturation of the soil in proportion to

the magnitude of the exchange acidity. The resulting

acidity in turn decreases the pH of the soil solution. With

continuous addition of salt solution the initial acidity in

the solution will gradually be leached, resulting in a

corresponding rise of soil pH (Wiklander, 1975). Thus, the

higher the normality of the input neutral salt solution, the

more complete the saturation of exchange sites with a

specific cation species.

Mechanisms of H+ Replacement of Exchangeable cations on
Soil Exchange Sites

When an acid soil is transformed from (H+Al)-soil to

one saturated with a specific cation species (examples: K ,
2+ 2+
Mg or Ca ), the acid groups in the soil are successively

neutralized and the base saturation of the soil is

increased. During subsequent removal of basic cations by

leaching with an acid solution, the replacement efficiency

of H+ for basic cations on the soil can be expressed by the

ratio 8M/8H (Wiklander and Andersson, 1972), where 8M is the

number of equivalents of cations removed and 6H is the

number of equivalents of H+ added. A single-species

cation-saturated soil always gives a high degree of base





207


separated by a centrifuge method. In the solution and

exchange phases, distributions of cation species and CEC

varied with depth in the columns. Al3+ comprised about 25

and 50% of the total exchangeable cations in subsoil and

topsoil columns, respectively. The large aluminum contents

of the Cecil soil are possibly due to dissolution of

interlayered vermiculite (M+ (Mg,Fe)3 (S1.A1)4 010 (OH)2),

and of hydrous Al oxides.

The Mg2+ exchange isotherm curve for the binary Mg ->

Ca2+ reaction was concave in shape and indicated that

exchange sites in both the topsoil and subsoil preferred
2+ 2+
Ca2+ over Mg2+. Average magnitudes of selectivity

coefficients for the topsoil and subsoil were 0.225 and

0.798, respectively. Hydrodynamic dispersion coefficients

obtained from experimental Cl breakthrough curves were 1.85

x 10-4 and 3.03 x 10-4 m2 h-1, respectively, for subsoil and

topsoil columns.

A computer model was developed for predicting the

transport and exchange of two cation species in Cecil soil

columns. The model is based upon a Galerkin finite-element

numerical solution of the convective-dispersive partial

differential equation. The governing equation describes

one-dimensional transport and binary exchange of cations

under steady water flow through soil. The exchange term in

the transport equation is treated as an explicit function of

total solution concentration, cation exchange capacity,

valence and cation concentrations in the solution phase.





103


Table 3-4 Concentrations of cations in solution
and exchange phases for topsoil after
leaching with pH 4.9 solution


Depth
(cm)


1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0


Ca


K Na Al Sum


Solution phase
mmole (+) L
0.030 0.021 0.125 0.130 0.172 0.478
0.032 0.037 0.104 0.113 0.942 1.228
0.032 0.066 0.122 0.113 1.800 2.133
0.023 0.058 0.128 0.130 1.300 1.638
0.040 0.075 0.158 0.139 1.260 1.672
0.027 0.058 0.164 0.135 1.200 1.584
0.020 0.074 0.180 0.152 1.570 1.996
0.025 0.090 0.196 0.141 2.130 2.583
0.020 0.025 0.156 0.146 0.297 0.644
0.042 0.054 0.179 0.163 0.724 1.162


Depth Ca Mg K Na Al Sum
(cm) Exchange phas-
mmole (+) Kg soil
1.0 0.714 0.074 0.151 0.178 7.895 9.012
2.0 0.883 0.123 0.123 0.074 7.917 9.120
3.0 0.883 0.173 0.133 0.061 7.984 9.234
4.0 0.883 0.181 0.133 0.087 7.583 8.867
5.0 0.863 0.181 0.136 0.078 8.117 9.375
6.0 0.843 0.214 0.166 0.087 7.828 9.138
7.0 0.973 0.123 0.237 0.054 6.638 8.026
8.0 0.963 0.206 0.225 0.109 7.394 8.897
9.0 0.913 0.197 0.171 0.056 5.715 7.053
10.0 0.893 0.197 0.200 0.165 6.427 7.882












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*-O 0

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0 e M









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sites (Helfferich, 1962; Cho, 1985). The value of CT is

assumed to be constant for a given soil material. For

numerical simulations in this work the magnitude of K
s was

assumed to be constant (Valocchi et al., 1981) during the

ion exchange transport process.

Transport with Ion Exchange

For the case of binary cation exchange and steady water

flow, the convective-dispersive mass-transport equation

[2-1] can be explicitly written as

2
aC ac ac 1
+ = D 2 v [2-15]
at 8 at ax ax

and
2
3C2 C 2 C 2 3C2
+ -D v [2-16]
at e at 3x2 ax

Equations [2-14] and [2-15] are coupled through additional

equations for the time rate of change of adsorbed-phase

terms (the second terms on the left-hand sides of equations

[2-15] and [2-16]) to specifically describe the cation-

exchange process. If one assumes the soil cation exchange

capacity CT to be time invariant at any given location in

the soil, it can be expressed as the local sum of the

concentrations of cations in the exchange phase as


C = C1 + C2, 2
T 1 2


[2-17]





129


1.10 cm h-1 ( 3%) Darcy velocity to each column. A check

for concentrations of cations in the column effluent was

made periodically. A conventional way to estimate if the

soil exchange sites are saturated with a specific cation is

by monitoring concentrations of the specific cation in the

effluent with time. If the cation concentration in the

effluent becomes equal to that of the input solution then

the soil exchange sites are commonly considered to be

saturated with that cation species. Such a technique,

however, is not without difficulties, particularly if the

exchange sites initially contained highly-preferred species

of cations. In this investigation concentrations of a

specific cation in the effluent reached 99% of the input

solution concentration after about 3-4.5 pore volumes of

elution, but very small concentrations of other ion species

remained even after leaching of the column for a month or

more. After the flow was terminated, the columns were

leached with 1.5 liters of 95% ethanol to remove all soluble

salts. Soil was then removed from each column, air-dried,

ground, passed through a 2-mm sieve and stored in large

plastic bottles for future use. Three columns of subsoil

were prepared similarly to the aforementioned procedure for

topsoil columns. Therefore, Ca-saturated, Mg-saturated,

K-saturated topsoil and subsoil materials were obtained.

Pretreated mixed-topsoil and mixed-subsoil were obtained by

carefully mixing equal weight ratios (1:1:1 for Ca:Mg:K) of

Ca-saturated, Mg-saturated, and K-saturated soil materials.







of catchments in response to changes in acid deposition was

proposed by Cosby et al. (1985b). Due to the lack of

long-term records of catchment-water quality, however,

results obtained from their model need further verification.

Any criteria used for assessing soil sensitivity to

cation leaching by acid rain should include cation-exchange

capacity (CEC), base saturation and mobilized Al3+, pH and

carbonate content (McFee, 1980). Critical properties, such

as CEC and cation-selectivity coefficients, are especially

needed in the investigation of effects of acid deposition

upon soils.

The first objective of this study was to determine the

influence of input solution pH upon leaching of basic

cations during continuous displacement by aqueous HC1

solutions through hand-packed columns of Cecil topsoil and

subsoil. The second objective was to determine the

influence of input solution pH upon distributions of cation

concentrations in soil columns after leaching experiments,

and the third objective was to use cation concentrations in

the solution and exchange phases to calculate binary

selectivity coefficients for ion-pairs.

Basic Theory

Surfaces of Soil Particles

Constant electrostatic surface charge for a clay

mineral is derived from isomorphous substitution of Si4+ by

Al3+ or of Mg2+ for Al3+ within the crystalline structure.

Mica and related 2:1 lattice-type minerals, smectites,




179


concentrations tended to decrease. Increased concentrations

of cations in the exchange phase during acid leaching are

explained by the argument given in the section on treated

subsoil, since K Na and Ca occur naturally between the

lattices or interlayers of interlayer-hydroxy vermiculite

(Dixon and Weed, 1977; Bohn et al., 1985). Due to the

dissolution or weathering of such minerals and of

carbonates, the release of cations from nonexchangeable

forms tends to offset leaching losses of cations due to acid

deposition. Calcium was not decreased greatly due to

leaching for either of the acid treatments, since Ca was

more preferred on soilexchange sites.

Tables 4-14 and 4-15 provide concentrations of cations

in solution and exchange phases for K-topsoil columns which

received input HC1 solutions with pH 3.9 and 4.9,

respectively. The concentrations of cations in the solution

phase were in the order Na+ > Mg2+ = K+ > Ca2+ 3+> Al and

for the exchange phase the concentrations were in order Al3+
2+ + 2+
> K > Ca > Na > Mg2. If one compared the

concentrations of cations in exchanger phase for Tables 4-14

and 4-15 with values in Table 4-4 for original K-topsoil,

concentrations of exchangeable K+ were significantly less.

Concentrations of other cations such as Ca2+, Mg 2+, Na and

Al tended to increase for both treatments, however.

Reasons similar to those mentioned earlier were used to

explain the increased concentrations of specific cations in

the exchange phase. The column that received pH 4.9 HC1







2-21 Simulation results pnd experimental data for
distributions of Mg concentrations in the solution
phase for the subsoil column after miscible
displacement with 3.6 pore volumes................. 70

2-22 Simulation results Pnd experimental data for
distributions of Mg concentrations in the exchange
phase for the subsoil column after miscible
displacement with 3.6 pore volumes................. 71

2-23 Effluent pH values for the topsoil and subsoil
columns ......................................... 72

3-1 Breakthrough curves for pH in the effluent from
Cecil topsoil columns which had received two input
HC1 solutions with different values of pH........... 88

3-2 Breakthrough curves for cation concentrations in the
effluent from Cecil topsoil columns which had
received pH 3.9 input HC1 solution................. 89

3-3 The effect of input solution pH upon the break-
through curves of K from Cecil topsoil columns.... 91

3-4 The effect of input solution pH upon the break-
th5rugh curves of summed concentrations of Ca ,
Mg K and Na in effluent from topsoil columns. 92

3-5 The effect of input slution pH upon the break-
through curves of Al from Cecil topsoil
columns ................. ............... 93

3-6 Breakthrough curves for pH in the effluent from
Cecil subsoil columns which received two input HC1
solutions with different values of pH.............. 94

3-7 Breakthrough curves for cations in the effluent
from Cecil subsoil columns which received pH 4.9
input HC1 solution................................. 96

3-8 The effect of input solution pH upon the break-
through curves of Ca from Cecil subsoil columns 97

3-9 The effect of input solution pH upon the break-
through curves of summed concentrations of Ca ,
Mg K and Na in effluent from subsoil columns. 98

3-10 The effect of input Polution pH upon the break-
through curves of Al from Cecil subsoil columns 99

4-1 Breakthrough curves for pH in the effluent from
Ca-topsoil columns which received input HC1


xii







Table 3-2 Initial concentrations of exchangeable cations,
pH, and CEC for Cecil topsoil and subsoil

Parameter Topsoil Subsoil

pH (1:1 water:soil) 4.46 4.86
pH2(1:1 KC1: soil) 3.85 3.95
Ca (NH OAc) 1.60 (11%) 8.80 (41%)
2 (mm6le(+) Kg" soil)
Mg (NH OAc) 2.50 (18%) 2.20 (10%)
(mm le(+) Kg~1 soil)
K (NH OAc) 1 0.70 ( 5%) 0.80 ( 4%)
Na+O~~c) -1 (8 20 1%
(mmAle(+) Kg1 soil)
Na (NH OAc) 2.60 (18%) 2.80 (13%)
-1
3+ (nimmle(+) Kg1 soil)
Al (KC1) -1 6.70 (48%) 7.00 (32%)
(nmqole(+) Kg soil)
CEC (NH OAc+KCl) 1 14.10 21.70
-1
(mmole(+) Kg soil)

* Numbers enclosed in parentheses represent fractions of
each ion in the exchange phase.


From Table 3-2, dominant basic cations in the topsoil
2+ 2+
in decreasing order of abundance are shown to be Mg2+, Ca2

and Na but were Ca2+, Na and Mg2+ in the subsoil. Base

saturation was 52% and 68% for topsoil and subsoil,

respectively. Soil pH values determined by KCl were less

than those determined by distilled water (Table 3-2),

indicating that the variable-charge surfaces of the soil

particles were predominantly negatively charged. Also, K+

exchanged with H+ on the soil surfaces, resulting in the

solution becoming even more acid.




178


Table 4-13 Concentrations of cations in solution
and exchange phases for Ca-topsoil
after leaching with pH 4.9 HC1 solution


Depth
(cm)


1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
11.0
12.0
13.0
14.0
15.0
16.0
17.0
18.0
19.0
20.0


Ca Mg K Na Al Sum
Solution phase
mmole(+) L
1.330 0.204 0.565 0.268 0.006 2.373
0.913 0.103 0.453 1.090 0.012 2.571
0.606 0.086 0.413 0.891 0.016 2.012
0.718 0.174 0.632 1.020 0.016 2.560
0.731 0.197 0.577 0.939 0.014 2.459
0.521 0.089 0.445 0.646 0.018 1.719
0.687 0.215 0.465 1.100 0.016 2.483
0.599 0.148 0.256 0.630 0.019 1.652
0.695 0.109 0.200 0.522 0.009 1.535
0.692 0.207 0.387 0.770 0.018 2.074
0.674 0.070 0.224 0.500 0.012 1.480
0.958 0.074 0.216 0.600 0.012 1.860
0.852 0.194 0.401 0.852 0.011 2.310
0.888 0.140 0.380 0.826 0.012 2.246
0.946 0.156 0.299 0.750 0.014 2.166
0.727 0.046 0.176 0.496 0.023 1.469
0.862 0.099 0.259 0.665 0.022 1.907
1.080 0.155 0.186 0.565 0.076 2.062
1.230 0.270 0.265 0.609 0.020 2.394
1.010 0.165 0.164 0.435 0.039 1.813


Depth Ca


K Na


Sum


(cm) Exchange phas
mmole (+) Kg soil
1.0 6.610 0.082 0.249 0.130 6.249 13.320
3.0 10.500 0.082 0.288 0.174 5.393 16.437
6.0 9.480 0.062 0.160 0.272 5.526 15.500
8.0 10.100 0.062 0.205 0.217 5.582 16.166
10.0 10.900 0.062 0.275 0.283 5.582 17.102
12.0 11.100 0.021 0.230 0.261 5.315 16.927
14.0 10.500 0.021 0.179 0.283 5.404 16.387
16.0 11.500 0.000 0.217 0.304 5.010 17.031
18.0 13.000 0.103 0.294 0.326 4.759 18.482
20.0 11.400 0.103 0.480 0.370 6.416 18.769







and { S }I = [ -Cr ,0,.....,0, Cn+

Consequently, one has

{ C" } = [ A ] [ B i { C .} + [ A ] { S } [2-35]

Alternatively, in component form

n+1 ,
Ci (xt) = Eia jC (t) 6Pi C1 (t) + 60i,n+lCn+l (t) .
j=-1
where [ a ] = [A1 [B] (2-36]

and [ P, ] = [A]"1

Substituting equation (2-35) into equation (2-29), the cubic

polynomial approximating function g(z,t) for C(x,t) in the

ith element, in polynomial of degree three in z, is given by

the following formula:

n+1
g.(z,t)= E a (z)C (t) + b (z)C (t) + b C (t)
j=1 i,1 1 i,n+ n+1

where [2-37]

z -3z2+2zh2 z -zh2
a. .(z)=-a. + a -
ij' i'j 6h. i+1,j 6h.
i 6hi

z-h z
Sij + i+l,j [2-38]
hi h.


z3-3z2zh+2zh. 2 z3-zhi 2
bi, (z) = 1 i+1_ h
1i i
and
a -z3z2h.+2zh z -3zh 2
i,n+ in+l hi + i+n+1 hi

in which 6 is the Kronecker's delta. Therefore the cubic





149


0




C:
W a









.Q


0
4- 4I












o
V) p















4.o
(o a 4 -i
OE 0
X tr






Ur
0
:3 +






0 0 r -




O sX c
Cl 00,
-
(Ur
nr

i **-*
_i -- | i -- -- i -- -- |


(E/ (+) a I OWW)


uoTq-euqueouoo


II I





139


for the remainder (approximately 25 pore volumes) of each

run. Concurrently, high concentrations of basic and acidic

cations appeared early in the effluent and then greatly

decreased with increasing volumes of effluent. Finally,

fairly stable values for concentrations of basic cations in

the effluent were maintained. Using input HC1 solutions

with two different pH values resulted in similar effects

upon column effluent. Figs. 4-1 and 4-2 present the

effluent pH from Ca-topsoil and Ca-subsoil, respectively.

In general, columns that received pH 4.9 solution resulted

in higher pH values for the effluent than when pH 3.9

solution was applied.

Cation Concentrations in Effluent from Treated Soil Columns

Concentrations of specific.basic cations in effluent

from pretreated topsoil columns were observed to be very

high for each of the first-collected samples (about

0.14-0.17 pore volume). For example, K-topsoil, Ca-topsoil

and Mg-topsoil columns that received pH 3.9 HC1 solution had

initial concentrations of K Ca2+ and Mg2+ of 71.63,

164.87, and 249.28 mmole(+) L 1, respectively. For columns

receiving pH 4.9 HC1 solution, initial concentrations of

70.13, 101.60, and 204.03 mmole(+) L~1 were observed for K ,

Ca2+, and Mg 2+, respectively. High initial concentrations

of specific cations in effluent from correspondingly

pretreated subsoil columns were also observed. For

K-subsoil, Ca-subsoil, and Mg-subsoil columns that received

pH 3.9 HC1 solution, concentrations of K Ca2+ and Mg2+







using these two additional sets of CEC values was to

determine if exclusion of exchangeable Al from the CEC, or

if exclusion of all of the existing cations species except

Ca2+ and Mg2+, would improve simulated results. First,

modified cation-exchange capacity values were obtained by

summing the exchange phase concentrations of all cations

species except Al3+ over all depths as given in Tables 2-3

and 2-4, and then taking means for topsoil and subsoil,

respectively. Magnitudes for subsoil and topsoil were 12.73

and 4.97 mmole(+) Kg1 soil, respectively. Using these

modified cation exchange capacity values, calculated results
2+
for Mg concentration distributions in solution and

adsorbed phases are given in Figs. 2-15 and 2-16 for subsoil

and in Figs. 2-17 and 2-18 for topsoil, respectively.

Agreement between simulated and experimental data was better

for the subsoil than for the topsoil. Simulations for Mg2+

concentrations in the topsoil showed an overestimation of

Mg2+ concentrations in both the solution and exchange

phases.

The second method for obtaining a better estimation of

cation exchange capacity values was obtained by summing the

exchange-phase concentrations of Ca2+ and Mg2+ for all

depths (Tables 2-3 and 2-4) and then taking the mean for

topsoil and subsoil, respectively. Magnitudes of CEC for

subsoil and topsoil were 12.1 and 4.7 mmole(+) Kg-1 soil,

respectively. Based upon these second modified CEC values,

predicted results for distributions of Mg2+ concentrations













LIST OF TABLES


Table Page

2-1 Soil parameters used in column experiments ....... 35

2-2 Properties of Cecil topsoil and subsoil .......... 35

2-3 Concentrations of cations in solution and exchange
phases for topsoil after miscible displacement
with 4.5 pore volumes ............................ 44

2-4 Concentrations of cations in solution and exchange
phases for subsoil after miscible displacement
with 3.6 pore volumes ............................ 45

3-1 Physical and chemical parameters for Cecil topsoil
and subsoil columns ............................. 85

3-2 Initial concentrations of exchangeable cations, pH,
and CEC for Cecil topsoil and subsoil ............ 86

3-3 Concentrations of cations in solution and exchange
phases for topsoil after leaching with pH 3.9
solution ..........................................102

3-4 Concentrations of cations in solution and exchange
phases for topsoil after leaching with pH 4.9
solution ..........................................103

3-5 Concentrations of cations in solution and exchange
phases for subsoil after leaching with pH 3.9 HC1
solution ........................................... 104

3-6 Concentrations of cations in solution and exchange
phases for subsoil after leaching with pH 4.9 HCl
solution ...................... ....................105

3-7 Topsoil selectivity coefficients as determined
after leaching with pH 3.9 HCl solution ...........108

3-8 Topsoil selectivity coefficients as determined
after leaching with pH 4.9 HCl solution ..........109

3-9 Subsoil selectivity coefficients as determined
after leaching with pH 3.9 HCl solution ..........110

3-10 Subsoil selectivity coefficients as determined


vii













CHAPTER III
CATION LEACHING DURING CONTINUOUS DISPLACEMENT
BY AQUEOUS HYDROCHLORIC ACID SOLUTION
THROUGH COLUMNS OF CECIL SOIL

Introduction

Acid rain is commonly considered to be a serious

environmental problem for industrialized nations. In

particular, soil scientists are concerned that acid
2+
atmospheric inputs could accelerate cation (examples: Ca2+

Mg 2+, K etc.) leaching from the soil profile. Nutrient

deficiency accelerated by leaching of cations and

mobilization of toxic Al+3 in the soil solution may lead to

eventual decline in productivity of certain soils. Since

agricultural soils routinely receive limestone and

fertilizer as a crop-management aid, whereas forest soils do

not, scientists are generally more concerned about effects

of acid precipitation upon forest soils than upon

agricultural soils. Acid rain in the forms of H2SO4 or HNO3
2+
may affect the status of forest nutrients such as Ca and

Mg 2+, either positively or negatively. This phenomenon has

been extensively investigated by Abrahamsen (1980). In

cases where nutrient cations are abundant and S and N

concentrations are at plant-deficiency levels in the soil,

moderate inputs of acid rain may actually cause increased

forest growth. At the other extreme, however, where soil







method of Whittig (1965). The soil was treated with

hypochlorous acid to remove carbonates and organic matter,

followed by use of the dithionite method to remove Fe

oxides. The clay fraction was separated by alternatively

adding pH-10 water followed by centrifuging for different

combinations of speed and time. The clay (< 2 mu) fraction

was prepared on a ceramic tile such that one was Mg-glycerol

saturated and air-dried, and the other was K-saturated and

air-dried. The samples were then X-rayed at room

temperature (250 C). Samples were next heated at 110 0C and

300 0C by sucessive treatment and X-rayed after each

treatment. As the last step, K-saturated sample was heated

at 550 oC before being X-rayed.

Column Preparation and Displacement Procedure

The column consisted of a stack of 1-cm diameter

lathe-cut plexiglass cylinders, with dimensions of 0.10 m

length and 0.0375 m inside diameter to give a total internal

volume of 1.105 x 10 m The rings were held together by

wrapping water-proof, acid-resistant electrical tape along

the circumference in order to give a water-impervious

column. Soil was held in the column by placing fine nylon

mesh and Whatman number 42 filter paper over a thin plastic

disc with small holes distributed on the surface in each of

the inflow and outflow endplates. A check for water leaks

through the column wall was made prior to packing soil into

each column. Each column was then placed in the vertical

position and was sequentially packed by slowly adding







0


/0 r *
4O *
01 -H -

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l r.4 u


sO 0/

BOE 0 4/


00 4 r.o
IE U


0* 0
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oE O O Q

XnUllR /O u C


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) r-4 '
I1 / 0/ C 44 U2
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S. i -rlI 0



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I
/ 0)
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uo!T(ej+u)uoo


(T:/ (+) e ouju




187


Table 4-19 Concentrations of cations in solution and
exchange phases for mixed-cation subsoil
after leachingwith pH 4.9 HC1 solution

Depth Ca Mg K Na Al Sum
(cm) Solution phase
mmole (+) L


1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
11.0
12.0
13.0
14.0
15.0
16.0
17.0
18.0
19.0
20.0


0.062
0.047
0.027
0.035
0.057
0.032
0.030
0.025
0.092
0.025
0.027
0.027
0.020
0.037
0.027
0.077
0.007
0.020
0.075
0.018


0.041
0.033
0.013
0.008
0.016
0.008
0.016
0.008
0.021
0.008
0.008
0.004
0.008
0.012
0.012
0.037
0.008
0.008
0.025
0.008


0.414
0.267
0.354
0.247
0.421
0.656
0.344
0.342
0.384
0.265
0.307
0.288
0.434
0.404
0.377
0.794
0.287
0.411
0.625
0.375


0.120
0.122
0.086
0.107
0.183
0.087
0.124
0.126
0.217
0.122
0.130
0.098
0.135
0.167
0.133
0.128
0.104
0.200
0.170
0.091


0.006
0.011
0.018
0.011
0.017
0.000
0.022
0.011
0.011
0.006
0.006
0.006
0.028
0.017
0.011
0.022
0.028
0.028
0.020
0.028


0.643
0.480
0.498
0.408
0.695
0.784
0.537
0.512
0.725
0.426
0.478
0.423
0.625
0.637
0.561
1.059
0.435
0.667
0.915
0.520


Depth Ca


K Na


Sum


(cm) Exchange phas -
mmole (+) Kg soil
1.0 3.590 3.620 1.430 0.313 2.635 11.588
3.0 4.540 2.960 3.480 0.196 1.190 12.366
5.0 4.490 2.630 4.550 0.200 0.956 12.826
7.0 4.690 2.300 4.940 0.196 1.012 13.138
9.0 4.040 2.570 5.350 0.200 0.878 13.038
11.0 3.940 2.500 5.550 0.183 0.756 12.929
14.0 4.040 2.390 5.830 0.130 0.812 13.202
16.0 4.190 2.330 6.060 0.191 0.701 13.472
18.0 4.090 2.220 5.940 0.235 0.689 13.174
20.0 4.240 2.100 6.640 0.243 0.767 13.990





137


Table 4-5 Concentrations of exchangeable cations for
pretreated and mixed topsoil and subsoil

Soil Concentrations of exchangeable cations
(mmole(+) kg soil)

K Na Mg2+ Ca Al3+

K-topsoi 17.20 0.30 0.08 0.60 5.80
Mg-topsoil 0.20 0.30 11.70 0.90 0.50
Ca-topsoil 0.10 0.00 0.02 11.70 2.20
K-subsoil 50.20 0.40 0.10 1.40 0.70
Mg-subsoil 0.30 0.20 96.80 0.30 1.40
Ca-subsoil 0.20 0.00 0.10 54.40 0.20
Mixed-topsoil 6.10 0.60 6.50 5.10 3.00
Mixed-subsoil 16.10 0.50 31.50 26.30 0.80

CEC- Dominant cation %
-1
mmgle(+) kg soil for this specific
Al included CEC with Al
included

K-topsoil 23.68 (K) 72
Mg-topsoil 13.60 (Mg) 86
Ca-topsoil 14.02 (Ca) 84
K-subsoil 52.80 (K) 95
Mg-subsoil 99.00 (Mg) 98
Ca-subsoil 54.90 (Ca) 95
Mixed-topsoil 21.30 *
Mixed-subsoil 75.20 *

CEC Dominant cation %
-1
mmWge(+) kg soil for this specific
Al not included CEC with Al not
included


K-topsoil
Mg-topsoil
Ca-topsoil
K-subsoil
Mg-subsoil
Ca-subsoil
Mixed-topsoil
Mixed-subsoil


18.18
13.10
11.82
52.11
97.60
54.70
18.30
74.40


(K)
(Mg)
(Ca)
(K)
(Mg)
(Ca)
*
*


95
89
99
96
99
99.1







then resulting in mixing of displacing and displaced

solutions in the porous media. In some investigations (Lai

and Jurinak, 1971; Reiniger and Bolt, 1972; Persaud and

Wierenga, 1982), empirical equations have been fitted to

measured adsorption data in order to obtain adsorption

isotherms for incorporation into the transport equation.

Ion chromatography theory is based upon differences of ionic

migration rates for different ions in a packed bed. In

modeling, the movement of these ionic species is based upon

the principle of conservation of mass. In multicomponent

chromatography theory (Bolt 1967; Helfferich and Klein,

1970), where the local equilibrium condition is also

assumed, the exchange relationship between any two given ion

species in the system can be described by a Vanselow

selectivity coefficient. This coefficient is valid when

ionic strength and ion-pair corrections are made for cation

activities in the solution phase (Babcock and Schulz 1963;

van Beek and Bolt, 1973). Therefore, quantitative

relationships between the equilibrium constants for ion

exchange, concentrations, and valences of each pair of ions

can be obtained. Using multicomponent chromatography, Rubin

and James (1973) proposed a general mathematical form for

transport and exchange for each of several ion species by

using a generalized Vanselow selectivity function. The

model was later used successfully to describe the processes
2+
of exchange and transport for three major cations (Ca2+

Mg 2+, Na ) in an aquifer system (Valocchi et al., 1981). In







superscript notation for C was only used for formulation

purposes in the finite-element method.

A Galerkin Finite-Element Numerical Method

Finite-Element Method

Finite-element methods typically incorporate an

approximating integral equation to replace the original

governing partial differential equation (Pinder and Gray,

1977). Variational and weighted-residual techniques are two

methods most commonly used to obtain the approximate

integral equation. The Galerkin finite-element weighted-

residual method has been widely used for the mass-transport

equation (Price et al., 1968; Pinder, 1973) and is the

method used here. In the finite-element method, the domain

of interest is discretized into a number of subdomains

called elements. In this study, a line-segment element was

used and a cubic-spline function was chosen as the

interpolation function. For the Galerkin approximation,

consider a linear differential operator of the form

L (u) = 0 on domain D [2-24]

To solve u, a trial function u(x,t) is assumed which is

composed of a linear combination of approximation functions

containing time-dependent, undetermined coefficients G (t)

and specified-shape functions N (x) that satisfy the given

boundary condition of the problem. The trial function can

be expressed as

u(x,t) = u(x,t) = E G.(t) N.(x) [2-24]
j=1







Soil samples were air-dried, passed through a 2-mm sieve,

and stored in covered plastic buckets.

Values of pH for the soil samples were determined in

soil-water suspensions (1:1 soil:water) using a glass

electrode. Organic matter content was determined by the

Walkley-Black method (Allison, 1965). Particle-size

distribution was estimated by the pipette method (Day,

1965). Exchangeable cations were determined with neutral 1

M NH4OAc, by placing 5 g of 2-mm air-dried soil in duplicate

50-ml centrifuge tubes, and adding 25 ml of 1 M NH OAc to

each. All tubes were then stoppered and shaken for 30 min,

with the tubes then being placed in a centrifuge and spun at

2000 rpm for 10 min. Number 42 Whatman filter paper was

used to collect the supernatant into a 50-ml volumetric

flask. The same procedure was repeated, with the volume

then brought up to 50 ml with 1 M NH4OAc (Thomas, 1982).

Exchangeable Al was determined by 1 M KC1 extraction

of 10-g, air-dried, soil samples in duplicate 50-ml

volumetric flasks, with 25 ml of KC1. The soil and KC1 were

mixed, allowed to stand for 30 minutes, and then transferred

to Buchner funnels fitted with number 42 Whatman filter

paper mounted on 250-ml vacuum flasks. An additional 125 ml

volume of KC1 solution was added in 25-ml increments to give

a final volume of 150 ml (McClean, 1965; Thomas, 1982).

X-Ray Diffraction

X-ray diffraction analysis of the soil clay was

performed for topsoil and subsoil, respectively, by the




74


soil-to-solution ratio a/9. The exact magnitude of R could

not be obtained, since R involved unknown concentration

variables. The cation exchange capacity is the most

critical input parameter to the simulation. This is a

result of the cation retardation function R = (1+(o/e)F),

where the function F in equation [2-20] is a function of C .

Therefore, cation movement undergoes greater retardation in

soil with a larger value of CEC (and thus a larger value of

R) than in soil with a small CEC.




131


Ca-saturated topsoil, Mg-saturated subsoil, K-saturated

subsoil and mixed subsoil, and mixed topsoil for a total of

16 soil columns. Miscible-displacement experiments were

performed for one of the pretreated and mixed soil columns

using pH 3.9 displacing HC1 solution and for another column

using pH 4.9 HC1 displacing solution. Darcy velocities

ranged from 1.09 x 102 to 1.16 x 10-2 ( 2%) m h1. A

four-channel peristaltic pump as described in an earlier

chapter was used to deliver the applied solution into the

bottom of each soil column. Effluent from the top of the

column was collected by the fraction collector described

previously and effluent samples were stored in a

refrigerator for later analysis. After termination of acid

application to a given column, the pore volume was obtained

from the mass difference between wetted and air-dried soil

columns. Corrections were made to cumulative effluent

volume for amounts of solution held inside each of the

endplates. Initially the soil was air-dry but water

saturation soon was approached. Eventually the soil columns

reached steady-state water flow, which was maintained

thereafter. Experimental measurements of dispersion

coefficient for each column were not conducted, but were

assumed using the magnitude of the dispersion coefficients

for the topsoil and subsoil as given in chapter two. Since

all of the soil columns have the same length, the deviation

of bulk density was within 3 and 1% for topsoil and

subsoil, respectively. Moreover, the deviation for the







exchange phases for K-topsoil after leaching
with pH 3.9 HC1 solution.........................180

4-15 Concentrations of cations in solution and
exchange phases for K-topsoil after leaching
with pH 4.9 HC1 solution..........................181

4-16 Concentrations of cations in solution and
exchange phases for Mg-topsoil after leaching
with pH 3.9 HC1 solution..........................183

4-17 Concentrations of cations in solution and
exchange phases for Mg-topsoil after leaching
with pH 4.9 HC1 solution..........................184

4-18 Concentrations of cations in solution and
exchange phases for mixed-cation subsoil after
leaching with pH 3.9 HC1 solution.................186

4-19 Concentrations of cations in solution and
exchange phases for mixed-cation subsoil after
leaching with pH 4.9 HC1 solution................. 187

4-20 Concentrations of cations in solution and
exchange phases for mixed-cation topsoil after
leaching with pH 3.9 HC1 solution..................188

4-21 Concentrations of cations in solution and
exchange phases for mixed-cation topsoil after
leaching with pH 4.9 HC1 solution..................189

4-22 Charge balance of cations for columns of
Ca-topsoil..................................191

4-23 Charge balance of cations for columns of
K-topsoil...........................................192

4-24 Charge balance of cations for columns of
Mg-topsoil.........................................193

4-25 Charge balance of cations for columns of
Ca-subsoil ............ ...............................194

4-26 Charge balance of cations for columns of
K-subsoil............. ......... ........... .. ...... 195

4-27 Charge balance of cations for columns of
Mg-subsoil.........................................196

4-28 Charge balance of cations for columns of
mixed-cation subsoil................. .......... 201

4-29 Charge balance of cations for columns of
mixed-cation topsoil............................... 202

ix





147


O I
O
d
o o


m I



T T




0 m)




+ I

oou
4 'Q)
a O c? M










)* 3 e( .







0 0 U
OK > 4-I




S( 0 *1
L 4






o (o



O/+0 0 U0(1
CCo
J3 g ^




203


proportional to the H+ concentration of the applied solution

for the mixed soil, but the application of pH 3.9 acid

solution did increase the quantities of cations leached

compared to the case where pH 4.9 solution was applied.

Base saturation of cations was apparently decreased by acid

application to the soil columns.

Conclusions

The clay content of treated subsoil was 2.7-fold

greater than for treated topsoil, but organic matter content

was 1.6-fold greater for the topsoil than for the subsoil.

Exchange sites of pretreated Cecil subsoil gave

equivalent fractions of 95 and 99 % saturation with respect

to the major cation of saturation (K +, Ca +, or Mg 2+) in the

cases of where exchangeable Al was considered or not in

the calculation of cation exchange capacity. For pretreated

Cecil topsoil, however, equivalent fractions totalled 85 and

95% saturation with respect to the major cation of

saturation (K Ca2+, or Mg2+). Leaching of Cecil soil with

unbuffered salt solution resulted in an increase in the

cation exchange capacity of the soil.

All of the first few samples of effluent from soil

columns that received HC1 solutions were observed to have

low pH but high concentrations of cations. This observation

could be explained by the replacement efficiency of H + for

the cation-saturated sites as well as by a salt effect.

Soil chemically pretreated with a given cation was observed

to be more sensitive to the pH effect of the input solution








that study, cation-exchange reactions were assumed to be

controlled by local equilibrium, the soil column was assumed

to be water saturated, water flow was assumed to be

one-dimensional and steady, and exchange selectivity

coefficients for each pair of ion species were assumed to be

constant. Although the model successfully described the

movement of several major cations, the selectivity

coefficients for binary pairs of ions are not constant in

soil but are known to vary as the solution composition

changes (Sposito, 1981; Mansell et al., 1986).

The convection-dispersion transport model for cations

in porous media is governed by a system of nonlinear partial

differential equations which can be solved by employing

different numerical techniques with the aid of a digital

computer. Lai and Jurinak (1971) used a finite-difference

method and a single-component exchange isotherm to model

cation adsorption in soil. They found, for the Ca2+ -> Mg2+

binary reaction (i.e. exchange sites initially occupied with

Mg2+ and displacement by Ca 2+), the use of a nonlinear

exchange function demonstrated good agreement between

computed and experimental results. Rubin and James (1973)

later formulated the governing transport equation for

multi-ion species in one-dimensional space by applying the

Galerkin finite-element method with linear-basis shape

functions over the space domain. The resulting system of

nonlinear ordinary differential equations was solved using

Crank-Nicholson method and predictor-corrector




107


coefficients are presented in Tables 3-7, 3-8, 3-9 and 3-10

for each pair of cations at specific column depths for both

topsoil and subsoil after displacement with the two HC1

solutions. The average cation exchange capacity for each

column was obtained by summing up the 'sums' in the exchange

phase that appeared for each depth in Tables 3-3, 3-4, 3-5

and 3-6, respectively, and then averaging them.

Corresponding total solution-phase concentrations for each

column were likewise obtained by summing up the 'sums' in

the solution phase for each depth. Also, in the calculation

of selectivity coefficients, the higher valenced species for

any two cations was assumed to pseudo-saturate the soil

exchange sites. For example, for the calculated selectivity

coefficient _K->Al in Table 3-5 for topsoil, the exchange

reaction for these two specific cation species can be

described by the equation Al-(ads) + 3 K+ -> 3 K-(ads) +

Al with the cation concentrations at specific depths

being taken from the table. In the calculation of

selectivity coefficients, if two cation species had the same

valence, the cation with the greater atomic weight was

assumed to pseudo-saturate the soil exchange sites. For

example, for the calculated selectivity coefficients KMg->ca

in Table 3-5 for topsoil, the exchange reaction for these

two specific cation species can be described by the equation

Ca-(ads) + Mg2+ -> Mg-(ads) + Ca2+ and cation

concentrations in solution and exchange phases for specific

depth can be taken from the table. Solution activity








Concentrations and pH of Column Effluent

Figs. 3-1 and 3-2 show breakthrough curves (BTC) for pH

and cation concentrations in the effluent from the Cecil

topsoil column which received pH 3.9 HC1 solution. For the

first few effluent samples the pH reading was near pH 4.0

and the concentrations of all reported cations were high.

With increasing numbers of pore volumes of effluent, the pH

abruptly increased and concentrations of cations sharply

declined. From about 10 pore volumes to the end of each run

(30 to 37 pore volumes), concentrations of cations and pH

values for the effluent were essentially stable and the

effluent pH remained higher than that for the input

solution. These quasi-stable pH readings for the effluent

were also considerably higher than corresponding values for

stirred soil-water suspensions (Table 3-2). A similar

effect was observed for effluent collected from the topsoil

column that received pH 4.9 HC1 input solution (data not

shown). Effluent pH was lower with the pH 3.9 input

solution than when pH 4.9 input solution was applied. Fig.

3-2 shows concentrations of Ca2+, Mg 2+, K and Na in

effluent from the topsoil column that received pH 3.9

solution. For the first two collected effluent samples
2+ 2+
(0.14 and 0.28 pore volumes), Mg and Ca were the

dominant species. Thereafter, K+ was the dominant species

in the effluent until about 16 pore volumes. A similar

trend was observed for column effluent of input pH 4.9.

Concentrations of K+ in effluent from topsoil columns







the whole system was of equilibrium. The next day, with the

column still in the vertical position, the tape was removed

carefully. After removing the outflow endplate, a piece of

parafilm was placed over the end of the column and a fine

steel thread was forced between consecutive rings in order

to slice the column into sections. Resulting sections of

soil were carefully removed and placed on a piece of

parafilm. Each whole section of soil was then packed into

a prenumbered small centrifuge tube which had a predrilled

small hole on the closed end. A Whatman number 42 filter

paper was place on top of the hole, inside the tube. Each

small centrifuge tube was then placed into a corresponding

large-size centrifuge tube along with a glass bead to

separate the extracted solution from soil in the small

centrifuge tube. Paired tubes were carefully placed in the

centrifuge and spun at 4000 rpm for 30 min. Soil samples

were then removed from the small centrifuge tubes and placed

into a weighing boat, and wet soil weights were recorded.

Soil in each weighing boat was air-dried and the soil weight

recorded. Exchange-phase concentrations of Ca 2+, Mg2+, K

and Na+ were obtained using a neutral 1 M NH OAc extraction

method (Thomas, 1982). Correction was made for entrapped

equilibrium solution remaining in the exchange phase after

centrifuging, by taking differences in equivalents per L for

the extractant and the residual solution. Unbuffered 1 M

KC1 (Thomas, 1982) was used in order to obtain exchangeable

Al concentrations. Concentrations of Ca 2+, Mg 2+, K+ and




175


Table 4-11 Concentrations of cations in solution
and exchange phases for Mg-subsoil after
leaching with pH 4.9 HC1 solution


0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0


Depth
(cm)


Ca Mg


K Na


Al Sum


Exchange phas I-
----- mmole (+) Kg soil
0.823 9.050 0.192 0.359 3.514 13.938
0.749 9.870 0.211 .0.370 2.113 13.313
0.786 9.670 0.230 0.348 1.968 13.002
0.811 9.460 0.217 0.435 1.868 12.791
0.674 9.670 0.243 0.337 1.935 12.859
0.699 9.050 0.211 0.283 1.334 11.577
0.724 8.840 0.230 0.326 1.301 11.421
1.620 9.260 0.294 0.326 1.190 12.690
0.773 9.460 0.301 0.391 1.801 12.726
0.711 9.870 0.249 0.348 2.168 13.346


Ca Mg K Na Al Sum
Solution phase
mmole(+) L
.419 0.090 0.184 1.000 0.022 1.716
.294 0.086 0.139 0.935 0.022 1.477
.259 0.086 0.134 0.804 0.041 1.325
.511 0.119 0.243 1.110 0.017 2.000
.205 0.090 0.105 0.676 0.203 1.280
.302 0.074 0.134 0.817 0.001 1.328
.264 0.078 0.114 0.809 0.029 1.294
.419 0.090 0.152 0.965 0.002 1.629
.432 0.103 0.136 1.360 0.016 2.047
.319 0.103 0.164 1.110 0.130 1.826
.479 0.109 0.526 1.140 0.003 2.257
.356 0.079 0.330 0.939 0.003 1.707
.050 0.099 0.243 1.290 0.002 1.684
.389 0.082 0.243 1.190 0.004 1.909
.269 0.084 0.235 0.916 0.048 1.552
.557 0.099 0.152 1.270 0.003 2.081
.434 0.165 0.148 1.710 0.001 2.458
.452 0.107 0.171 1.280 0.010 2.020
.467 0.128 0.321 1.570 0.017 2.503
.392 0.107 0.129 1.200 0.014 1.843


1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
11.0
12.0
13.0
14.0
15.0
16.0
17.0
18.0
19.0
20.0


Depth
(cm)

2.0
4.0
5.0
7.0
9.0
12.0
14.0
16.0
18.0
20.0







Clemson, South Carolina. The exact location of the site was

reported earlier by Dr. V. L. Quisenberry (Cassel, 1985)

from the Agronomy Department of Clemson University.

Reported texture of the Cecil soil profile varied from site

to site, with the clay content of the Ap horizon ranging

from 6 to 38 percent depending upon the amount of subsoil

mixing which had occurred subsequent to soil erosion. Clay

content in the B horizon ranged from 42 to 72 percent

(Cassel, 1985). In situ values of unsaturated hydraulic

conductivity reported (Cassel, 1985) for the 0-30 cm depth

ranged from 1.70 x 10 to 4.51 x 102 cm h 1, whereas those

for the 30-60 cm depth ranged from 4.32 x 10-4 to 4.6 x 10-1

cm h 1, respectively. In situ values of soil water content

for the 0-30 cm depth ranged from 0.275 to 0.495 cm3 cm3

whereas values ranged from 0.409 to 0.560 cm3 cm3 for the

30-60 cm depth (Cassel, 1985). Typic Hapludult soils are

freely drained with great or moderate depth to hard rock,

have an ochric epipedon that is not both thick and sandy,

and have a loamy or clayey particle-size class in an

argillic horizon.

Cecil topsoil and subsoil bulk samples for this

research were obtained from the 0-30 and 30-60 cm depths,

respectively, of the profile. The soil was air-dried,

passed through a 2-mm sieve, and stored in covered plastic

buckets.

Values of pH for the soil samples were determined in a

soil:water suspension (1:1 soil:water) using a glass







evaluated by using Gaussian-Legendre quadrature formula

(Krylov, 1962)

J1 m(M) F( (M) [2-531
-1F(p) df = t A F(mE)


where A (M) and %(M) are the set of weighting coefficients

and roots associated with M-th degree Legendre polynomials.

The quadrature formula is exact if F(p) is a polynomial of

degree less than or equal to 2M-1. In the present work, m=4

was used. In applying equation [2-53] to the equations

[2-48] through [2-52] one must first transform the interval

[O0, h ] onto [-1, 11. This can be accomplished by the

transformation


W h (1 + (p) [2-541
2

The system of equations [2-46] is large and 'stiff'. This

is typical for the system of ordinary differential equations

arising from the application of the method of lines to

partial differential equations (Hindmarsh, 1981). In

solving the system of ordinary differential equations, if

the magnitude of eigenvalues cover a wide range, an

undesirably small time-step size is required. A problem of

this type is called 'stiff'. A computer model for

simulating convective-dispersive ion exchange/transport

consists of 28 elements with this proposed method, giving

rise to 29 ordinary differential equations to be solved

simultaneously. Due to the presence of stiffness,

conventional numerical integration of the equations [2-46]




166


SI


I I


0


0


41


I


0 t


O

3


0ofP
.I 0I .O


a I


a A a


a a 1 I 1 9 1 8 -


(t/ (+) ar oWuuW)


uo ejq-uueouoo


I


--


'n
0 (
u

4 -
0



CO
ow









0-4
I 4al



0o
4- a 0)





O xB
u 0 -H-I
o -o
>| m 0



Uo w


o.
I >. 4

44 ri-
0 0 0
E "


I)
J> u4



L 4J >






rd
*1
$4l








M
and Mg [2-5]

M
where RM represents the sum of MA and MB (mole per kg)

of the soil. The relationship between activities and mole

fractions for ion species in the exchange phase is given by

Sposito (1981) as


[A] = A MA ; [2-6]

[B] = u M ;
where iLA and iBq are activity coefficients for cations A and

B on the exchange phase, and MA and MB are mole fractions

for cations A and B on the exchange phase, respectively.

The relationship between activities and concentrations

for the solution phase is given by Sposito (1981) as

[A] = rA MA ; [2-7]

[B] = rB MB ;

where rA and rB are ion activity coefficients for cations A

and B in the solution phase and MA and MB are molar

concentrations for cations A and B in the solution phase.

Substitution of equations [2-6] and [2-7] into equation

[2-8] yields



K (u MB*) (rA MA) 2
K = [B2-8]




195


Table 4-26 Charge balance
K-subsoil


K-subsoil

H + Ca2+

Initial cations 0.430
Total input 0.271
Final # 0.050
solution phase
Final # 0.372
exchange phase
Total 0.009 0.151
output in effluent

K-subsoil pH

H Ca 2

Initial cations 0.424
Total input 0.026
Final # 0.044
solution phase
Final # 0.393
exchange phase
Total 0.008 0.082
output in effluent

#: undetermined


of cations for columns of


pH 3.9 (mmole(+))

Mg2+ K

0.031 15.421

0.008 0.051

0.021 3.721

0.030 11.140


4.9 (mmole(+))

Mg2+ K

0.030 15.199

0.006 0.042

0.026 4.208

0.019 6.872


Na

0.123

0.207

0.208

0.030


Na

0.121

0.176

0.214

0.021


A13+

0.215

0.002

0.450

0.015


Al1

0.212

0.001

0.303

0.010


--







as acid solution infiltrates soil is central to evaluating

acid-rain effects upon forest soils. Predictions based upon

established soil physical-chemical methods would be the most

reliable method of evaluating long-term effects of acid rain

on soil (Reuss, 1983).

Objectives

The ion-exchange process is one of the most important

soil chemical processes which influences cation leaching

during infiltration of acid rain into soil. Thus, a

computer model solely based on binary ion-exchange

equilibrium and saturated steady water flow was developed

for predicting the movement of cation species in soil. The

response of cation leaching to input acid solution at two

different pH values was determined using columns of Cecil

soil (Typic Hapludult).

There are three objectives in this study. The first

objective was to evaluate a numerical model by simulating

binary cation exchange and transport during miscible

displacement of electrolyte solutions through columns of

Cecil soil. The second objective was to experimentally

determine effects upon cation leaching of applying

artificial acid-rain solutions (hydrochloric acid) to

columns of Cecil soil The third objective was to

determine the leaching effect of applied artificial acid-

rain solutions (hydrochloric acid) to columns of Cecil soil

pretreated to saturate exchange sites with specific cation




158




++ 0
cu cI + '-
(U Cm+ (0
I, o
II I -Y z

0.

c 4-)W
C 0
4- ^u




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+- 4-
*u1


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+ wO
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+ a 0
+ E w
+ a1 a
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+
+ >
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+ 0 0M
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+
+ 0) I-I

+ [ W 4
+ 0
+ +-
++ O
++ -
+ 0 I

0 t ., .P4 .
0 (0 ( 0
Tl

(T/(+) StOWW) U0T eJ4U93UO0











































































I0 (U 0) (0 ) 0
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a



X -U
SQ)
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S (1)




0 0
_^ *o >












c(aoa


0
CQ)


: 3





P4-1
0Ot,


*H HC









CM,





ci)


uoT ejuueouoo


T TOs BA/ (+) --t ouIu







in the solution phase). The selectivity coefficients for
2+ 2+
Mg -> Ca exchange are considerably less than unity at

all values of solution concentration, implying that exchange

sites in Cecil topsoil and subsoil preferred Ca2+ over Mg2+.

The observed effect was greater for subsoil than for

topsoil, however. After displacing 4.5 pore volumes of

effluent through the topsoil, Mg2+ occupied 30% to 60% of

the cation exchange capacity throughout the column. After

displacing 3.6 pore volume of effluent through the subsoil,

however, Mg2+ occupied from 15% to 80% of the cation

exchange capacity. The average of the total solution cation

concentrations in both the topsoil and subsoil columns was

about 3% less than the CT value (Tables 2-3 and 2-4). This

3% may represent the H+ concentration in the solution phase.

The average of the total cation content on the exchange

sites of topsoil and subsoil columns was about 37% less than

the corresponding ET value (Tables 2-3 and 2-4). The

displacing cation Ca2+, although initially saturating the

soil exchange sites, due to the unknown masking effects of

interlayer-hydroxy vermiculite (M (Mg,Fe)3(Si,Al)4010(OH)2)

could become a non-exchange cation.

Exchange selectivity Coefficients (Ks) for Soil Columns

From the soil-column experiments, concentrations of

cations in the solution and exchange phases are given in

Tables 2-3 and 2-4 for Cecil topsoil and subsoil,

respectively. The selectivity coefficients (KMg-Ca) were

obtained by using equation [2-14]. The cation exchange







in solution and exchange phases are given in Figs. 2-19 and

2-20 for topsoil and in Figs. 2-21 and 2-22 for subsoil,

respectively. Simulated results for the subsoil were

greatly improved as compared to the case where Al was not

included among the exchangeable cations. Simulated results

for topsoil better approximated the observed data, but an

overestimation for concentrations in both the solution and

exchange phases was observed. Careful examination of

constituent cations located on exchange sites of the topsoil

revealed that Al was the dominant cation species,

comprising about 50% of the total exchangeable cations.

Cation concentrations in the exchange phase for the topsoil

decreased in the order Al3+ > Mg2+ > Ca2+ > Na > K+. The
2+
topsoil column was obviously no longer dominated by Mg
2+ 3+
For the subsoil column, however, the order was Mg > Al >

Ca2+ > K+ Na This phenomenon can be attributed to the

topsoil containing more interlayer-hydroxy vermiculites than

the subsoil. Displacing salt solution through either

topsoil or subsoil resulted in the exchange of Ca2+ or Mg2+

with Al 3+. Additional acidity (Fig. 2-23) would be caused

by the hydrolysis of Al 3+, which would tend to induce

further mineral dissolution. The subsequent further

production of Al3+ would then compete with divalent Ca2+ and

Mg2+ for soil exchange sites. The result would be a higher

saturation of exchange sites with Al .


















LEACHING OF CATIONS DURING DISPLACEMENT BY ACID
SOLUTIONS THROUGH COLUMNS OF CECIL SOIL






BY






KO-HUI LIU


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN
PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA


1987





















































































(t/ (+) 8TOWUUJ)


C

-4J
1t






U
0H




4-(00
40 U






m>





- Io (0
ul











Q) *



S-H
OrO
0 -4 .
> X




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ri H
0 -0
















0)
+ a

cO Q)

4 0-

00









-H
4 r-
ro





Er 4
410






*d'g


0a r-I i
*Hi : R
k *din
+>U -P
tor E -
*H *H
*o gm

Q)
^ n
10 (
+>3
CIH


uoT;ej;uemuoo




101


solution as compared to columns receiving pH 4.9 HC1

solution. This can be explained by increased solubility and

mobility of complex aluminum as acidity increased.

Concentrations of Cations in Solution and Exchange Phases

Distributions of Ca2+, Mg 2+, K Na and Al in

solution and exchange phases for columns of Cecil topsoil

after leaching with pH 3.9 and 4.9 HCl solutions are

presented in Tables 3-3 and 3-4, respectively. When an acid

solution is displaced through a soil column, cations with

higher affinities for exchange sites would be expected to

exchange strongly or tend to displace exchangeable cations

with lesser affinities from exchange sites (Helfferich,

1962; Mansell, 1983). For the case where equivalent

fractions of cations initially in the exchange phase are

approximately equal for all ion species, concentrations in

the solution phase of K+ and Na+ should be higher than

those of Ca2+, and Mg2+ and Al3+. For the Cecil soil the

situation was more complicated since the initial composition

of the exchange phase was relatively high in trivalent Al3+

as well as divalent Ca2+ and Mg2+ species. For the exchange

phase, concentrations of Ca2+, Mg 2+, Al3+ would be dominant

over monovalent species. Results in Tables 3-3 and 3-4

indicate that, in the corresponding solution phase, cation

concentrations were in the order K = Na Al > Ca 2+>
Mg2+
Mg

Tables 3-5 and 3-6 present distributions of cation

concentrations in solution and exchange phases for columns













REFERENCES


Abrahamsen, G. 1980. Acid precipitation, plant nutrients and
forest growths. p.58-63. In Ecology Impact of Acid
Precipitation. D. Drablos and A. Tollan (eds.). The
Norwegian Interdisciplinary Research Programme: Acid
Precipitation-Effects on Forest and Fish, Oslo-As, Norway.

Ahlberg, J.H., E.N. Nilson, and J.L. Walsh. 1967. Theory of
Splines and Their Application. Academic Press., New York.

Allison, L.E. 1965. Walkley-Black method. p.1372-1376.
In Methods of Soil Analysis. Part 2. C.A. Black (ed.).
American Society of Agronomy, Madison, WI.

Babcock, K.L., and R.K. Schulz. 1963. Effect of anion on the
sodium-calcium exchange in soil. Soil Sci. Soc. Am. Proc.
27:630-632.

Bohn. H.L., B.L. McNeal, and G.A. O'Connor. 1985. Soil
Chemistry, 2nd edition. Wiley-Interscience. New York.

Bolt, G.H. 1967. Cation exchange equations in soil science:
A review. Neth. J. Agric. Sci. 15:81-103.

Brenner, H. 1962. The diffusion model of longitudinal mixing
in beds of finite length: Numerical values. Chem. Eng.
Sci. 17:229-243.

Cassel, D.K. 1985. Physical characteristics of soils of the
southern region: Summary of in-situ unsaturated hydraulic
conductivity. Southern Cooperative Series, Bulletin 303,
Regional Research Project S-124. North Carolina State
University, Raleigh.

Chatterjee, B., and C.E. Marshall. 1950. Studies in the
ionization of magnesium, calcium and barium clay. J. Phys.
Colloid. Chem. 54:671-681.

Cho, C.M. 1985. Ionic transport in soil with ion-exchange
reaction. Soil Sci. Soc. Am. J. 49:1379-1386.

Cosby, B.J., R.F. Wright, G.M. Hornberger, and J. N.
Galloway. 1985a. Modeling the effects of acid deposition:
Assessment of a lumped-parameter model of soil water and
streamwater chemistry. Water Resour. Res. 21:51-63.


215







polynomial g(z,t), approximating C(x,t) in the ith element,

is a linear function of all the nodal values of C(x,t) and

the slopes at extreme nodes. The resulting trial function

(x,t) of classical cubic spline approximation for the

function C(x,t) at a specific length of soil column

(profile) is given by



C(x,t) = Ei g.(x,t) where 6 1 for x < x < xi+l.
j=1 0 otherwise.

[2-39]
It is obvious that cubic spline function C(x,t) agrees with

C(x,t) at nodes and is twice continuously differentiable in

the interval of interest.

Method of Weighted Residuals

Substituting the boundary conditions of equation [2-23]

into equation [2-37], the approximation function g(z,t)

becomes:


n+l v
g (z,t) = E a (z) C(t) + b, (C1 -Cf) [2-40]
j=1 1, D

Therefore, the unknown coefficients to be determined are C1,

C2, C3, ..... C n+l Substituting equation [2-40] into

equation [2-39] yields

C(x,t) = n"lN (Z) C (t) + B(z) Cf [2-41]
j=l D

where

n v n
N = E 68 a. ,(z) + --- E 6 b. ((z)
1 i=l 1 ,1 D i=l1 1







sites have adequate N and S concentrations but are deficient

in nutrient cations, acid rain in sufficient amounts may

decrease productivity (Johnson et al., 1982).

In all cases, anions associated with acid inputs or

present in the soil must be mobile in the soil solution if

basic cations are to be transported through the soil

profile. That is, cations cannot leach from soil without an

equivalent concentration of accompanying anions in solution.

Immobilization of anions can effectively prevent cation

leaching; therefore, the entire process of soil leaching is

strongly dependent on the input solution or on the internal

production of mobile anions (Johnson and Cole, 1980).

However, anion-leaching theory has been questioned by Krug

and Frink (1983). They point out that some of the factors

commonly considered to make soils susceptible to

acidification by acid rain are the same as those which

acidify soil through natural processes. Reuss (1983) used

cation-exchange equilibrium to predict the leaching of Ca2+

and Al3+ following the application of H2SO4 to soil. Cosby

et al. (1985a) presented an equilibrium model which included

equilibrium soil-solution cation exchange, inorganic

aluminum reactions and dissolved inorganic reactions. The

model demonstrated the interaction of soil chemical

processes as a key to controlling stream water chemistry. A

mathematical model which used quantitative descriptions of

soil chemical processes to estimate long-term chemical

changes that occur in the soil, soil water and surface water







Conclusions

The principal minerals in the Cecil topsoil and subsoil

are kaolinite, interlayer-hydroxy vermiculite and quartz.

Gibbsite was also found in the subsoil. This highly

weathered acid soil also contains amorphous materials in the

form of Al and Fe oxides. The clay content in the subsoil

is almost three-fold greater than for the topsoil, and

organic matter in the topsoil is 1.6 times greater than for

the subsoil.
2+ 2+
Exchange isotherms for Mg -> Ca reaction in Cecil

soil were concave in shape for both soils, indicating that
2+ 2+
Ca2+ was adsorbed preferentially to Mg on the soil

exchange sites. This result was as expected.

Excellent agreement between numerical and analytical

solutions demonstrated that the computer program for

convective-dispersive transport of two ion species

functioned as designed for the cases of both non-reactive

and reactive solute species.

Results from sensitivity analysis showed that the

model-simulation results were essentially insensitive to

small changes in the dispersion coefficient and selectivity

coefficient. The dispersion coefficient was apparently

independent from the cation retardation factor R, although

the selectivity coefficient is actually embedded in the

retardation factor. The volumetric water content and soil

bulk density both showed slight effects upon model-predicted

results, which can be explained by the inclusion in R by the







coefficients (D) of 1.85*10-4 and 3.03*104 m 2/h were

obtained from experimental Cl breakthrough curves for

subsoil and topsoil columns, respectively. Values of the

dispersion coefficient that had 2% deviation from

experimental dispersion coefficients were used in model

simulations for both topsoil and subsoil. Experimental and

simulated cation-concentration distributions for D values of

3.09*10-4, 3.06*10-4 and 3.03*10-4 M2 h- 1 for the topsoil

column are presented in Fig. 2-5. Experimental and

simulated cation-concentration distributions for D values of

1.90*10-4, 1.85*10-4, and 1.80*10-4 M2 h-1 for the subsoil

column are presented in Fig. 2-6. For both topsoil and

subsoil columns, the effect of 2% variation in D upon

predicted profiles of Mg2+ in the solution phase was

extremely small. This analysis revealed that the model is

relatively insensitive to small changes in values of D.

In these simulations, the effect of the dispersion

coefficient upon distributions of Mg2+ concentration in the

soil was apparently slight due to the fact that the

dispersion coefficient is independent from the cation-

retardation factor R.

Volumetric water content. Predicted and observed

distributions of Mg2+ concentration in the solution phase

with column depth are presented in Figs. 2-7 (topsoil) and

2-8 (subsoil), where a sensitivity analysis was performed

with respect to the volumetric water content (9). Average

volumetric water contents obtained experimentally from





108


Table 3-7 Topsoil selectivity coefficients as determined
after leaching with pH 3.9 HC1 solution


Mg-->Ca
0.29335
0.18077
0.75109E-01
0.12798
0.69720E-01
0.30525E-01
0.42872E-01
0.26389E-01
0.41085E-01
0.18000


K-->Ca
0.18174E-01
0.70553E-02
0.97137E-02
0.63549E-02
0.63531E-02
0.31815E-02
0.26237E-02
0.20953E-02
0.20387E-02
0.21101E-02


Na-->Ca
0.40481E-01
0.12418E-01
0.14524E-01
0.82523E-02
0.10183E-02
0.40369E-02
0.10465E-01
0.83075E-02
0.12087E-01
0.12170E-01


Na-->K
1.4925
1.3267
1.2228
1.1396
0.4004
1.1264
1.9971
1.9912
2.4349
2.4015


K-->Mg
0.61120E-01
0.39029E-01
0.12933
0.49655E-01
0.91123E-01
0.10423
0.61199E-01
0.79398E-01
0.49622E-01
0.11723E-01


Average selectivity values

0.10718 0.59700E-02 0.12376E-01 1.5533 0.67642E-01


Na-->Mg Al-->Ca Mg-->Al K-->Al Na-->Al

1 0.13614 4761.6 0.55214E-05 0.35506E-04 0.11803E-03
2 0.68694E-01 37.536 0.15738E-03 0.96728E-04 0.22587E-03
3 0.19337 23.566 0.17981E-04 0.19721E-03 0.36056E-03
4 0.64482E-01 10.694 0.19602E-03 0.15492E-03 0.22925E-03
5 0.14606E-01 1.355 0.25008E-03 0.43499E-03 0.27915E-04
6 0.13225 0.148 0.19225E-03 0.46656E-03 0.66683E-03
7 0.24409 0.044 0.17958E-02 0.64156E-03 0.51104E-02
8 0.31480 0.15444E-02 0.11899E-01 0.24405E-02 0.19267E-01
9 0.29419 0.52535E-02 0.13200E-01 0.12700E-02 0.18333E-01
10 0.06761 0.72402E-01 0.80550E-01 0.36022E-03 0.49893E-02

Average selectivity values

0.15302 483.50 0.10826E-01 0.60982E-03 0.49328E-02










C
C 0


0P

r 0
0O0
(0


I I

O I


o3/0)


0 0 0
uoT qgu-ueouoo


4 tr



-_ -
4Q)



00

141

W *r-
O( a
Q- a
0 rU





ul

E O*
CLO
















00
r-4 U








0 0 w
O r
*H ,c

10)





El
In 0o0
Ua "


*A; ^
OLd^
0co

















0 0eet








Table 2-4 Concentrations of cations in solution and
exchange phases for subsoil after miscible
displacement with 3.6 pore volumes


Depth Ca
(cm) -

1 0.1
2 0.1
3 0.1
4 0.1
5 0.2.
6 0.2
7 0.2:
8 0.2
9 0.4
10 0.6'
11 0.8
12 1.6'
13 2.1:
14 3.41
15 3.7:
16 4.4:
17 5.1
18 5.31
19 5.5.
20 6.51


88
48
48
44
12
44
24
99
35
67
53
00
10
50
30
20
40
60
40
60


Mg K Na Al Sum
Solution phase
mmole(+) L


9.420
9.510
9.250
9.220
8.950
8.890
8.490
8.630
8.760
8.300
8.110
7.340
7.180
5.930
5.730
4.530
3.790
3.420
3.620
2.070


0.143
0.190
0.176
0.135
0.205
0.203
0.131
0.137
0.174
0.258
0.240
0.184
0.209
0.276
0.143
0.171
0.258
0.194
0.269
0.371


0.470
0.518
0.511
0.483
0.435
0.424
0.477
0.497
0.351
0.407
0.370
0.306
0.282
0.337
0.157
0.157
0.209
0.170
0.235
0.365


0.064 10.285
0.060 10.426
0.056 10.141
0.055 10.037
0.052 9.854
0.051 9.812
0.049 9.371
0.048 9.611
0.044 9.764
0.042 9.674
0.040 9.613
0.039 9.469
0.019 9.800
0.024 10.017
0.024 9.784
0.021 9.299
0.022 9.419
0.020 9.164
0.020 9.684
0.016 9.382


Average 9.731
Na Al Sum


Ca


Exchange phase
mmole(+) Kg
0.609 13.700 0.177 0.235 *
0.509 13.000 0.184 0.226 1.993 15.91:
0.489 12.100 0.197 0.243 *
0.549 11.600 0.258 0.243 4.663 17.31;
0.499 11.300 0.230 0.226 *
0.589 11.200 0.235 0.226 4.974 17.22L
0.599 10.900 0.238 0.235 *
0.793 11.200 0.287 0.243 4.396 16.915
0.913 10.000 0.317 0.243 *
1.300 10.500 0.333 0.235 4.072 16.44(
1.200 9.960 0.243 0.226 *
2.590 9.710 0.266 0.226 4.264 17.05(
3.540 7.570 0.261 0.252 *
4.340 6.670 0.304 0.243 4.266 15.824
6.090 5.760 0.279 0.235 *
4.740 7.160 0.289 0.261 4.511 16.963
8.280 4.530 0.312 0.217 *
8.930 3.870 0.327 0.226 4.534 17.881
9.430 3.130 0.404 0.243 *
.0.300 2.470 0.476 0.235 5.190 18.673


3




I

3

)


* undetermined Average 17.020


Deptt
(cm)


1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20 1


* undetermined


Average 17.020





191


Table 4-22 Charge balance
Ca-topsoil

Ca-topsoil

H Ca2+

Initial cations 4.163
Total input 0.284
Final # 0.075
solution phase
Final # 3.682
exchange phase
Total 0.004 4.060
output in effluent

Ca-topsoil

H CaC2+

Initial cations 4.240
Total input 0.029
Final # 0.070
solution phase
Final # 3.810
exchange phase.
Total 0.003 2.120
output in effluent


of cations for columns of


pH 3.9

Mg2+

0.007

0.060

0.017

0.016


(mmole(+))

K

0.036

0.017

0.103

0.024


pH 4.9 (mmole(+))

Mg2+ K

0.007 0.036

0.012 0.029

0.022 0.093

0.042 0.019


Na

0.000

0.169

0.116

0.100


Na

0.000

0.059

0.095

0.023


#: undetermined


A13+

0.783

0.004

0.518

0.116


Al3+

0.800

0.002

2.000

0.193







are chosen to be the interpolation (shape) functions of the

assumed solution, such as

N = Nj [2-45]

Computational Method

After rearranging, equation [2-44] becomes

aC (t)
[Q] { = ([R] [U]) {C (t)} + ({V}-{S})Cf ,
at
[2-46]
where:

a n+1
[Q] = (1 + F) Nm(z) j.l N.(z)dx
0 o j-l j

n+l 2N.(z)
[RI = D f N(z) 3 dx ,
Sx

L n+1 aN.(z)
[U] = v f N (z)jl ax 3 dx ,



{V} = D N(z) dx [2-47]
0 ax


L aB1(z)
and {S} = v f N (z) ax dx

In equation [2-46], undetermined coefficients C (x,t)

do not appear explicitly but are embedded in the function F.

This nonlinear term is linearlized by employing the previous
*
time-step solution of C. (x,t) into the F term for

manipulation. For example, if we consider m = 2, 3,...,

n+1, the matrix coefficients at the left-hand side of

equation [2-37] can be written as








a I I II 1i1


+ +


O U


0x


I I


0:

0
0
0

0
0*
Ox


O0
O*



Ox





Ox
0


Oc


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217


laminar boundary-layer flow. p.497-504. In Numerical
Methods in Laminar and Turbulent Flow. C. Taylor and B.A.
Schrefler (eds.). Pineridge Press, Swansea, UK.

Hutchinson, T.C., and M. Havas. (eds.). 1980. Effects of
Acid Precipitation on Terrestrial Ecosystems. Plenum, New
York.

Jennings, A.A., D.J. Kirkner, and T.L. Theis. 1982.
Multicomponent equilibrium chemistry in groundwater
quality models. Water Resour. Res. 18:1089-1096.

Johnson, D.W., and D.W. Cole. 1980. Anion mobility in soil:
Relevance to nutrient transport from forest ecology.
Environ. Internat. 3:79-80.

Johnson, D.W., J. Turner, and J.M. Kelly. 1982. The effects
of acid rain on forest nutrient status. Water Resour. Res.
18:449-461.

Keng, J.C.W., and G. Uehera. 1974. Chemistry, mineralogy and
taxonomy of oxisols and ultisols. Soil Crop Sci. Soc. Fl.
Proc. 33:119-126.

Kissel, D.E., E.P. Gentzsch, and G.W. Thomas. 1971. Hydrogen
of nonexchangeable acidity in soils during soil
extractions of exchangeable acidity. Soil Sci.
111:293-297.

Krishnamoorthy, C., and R. Overstreet. 1950. Behavior of
hydrogen in ion-exchange reactions. Soil Science
69:87-93.

Krug. E.C., and C.R. Frink. 1983. Acid rain on acid soil: A
new perspective. Science 221:520-525.

Krylov, V.I. 1962. Approximation calculation of integrals.
Macmillen, New York.

Lai, S.H., and J.J. Jurinak. 1971. Numerical approximation
of cation exchange in miscible displacement through soil
columns. Soil Sci. Soc. Am. Proc. 35:849-899.

Lapidus, L., and N.R. Amundson. 1952. Mathematics of
adsorption in beds: VI. The effect of longitudinal
diffusion in ion exchange and chromatographic columns. J.
Phys. Chem. 56:984-988.

Mansell, R.S. 1983. Ion exchange coupled with
convective-dispersive transport of cations during acid
rain infiltration in soil: A review. p.256-264. In Acid
Deposition: Causes and Effects. A.E.S. Green and W.H.
Smith (eds.). Government Institute, Inc., Rockville, MD.







z(z-h )(z+h ) i z
+ C C+1 (t) + --- C (t)
6h. h
1 1h [2-30]

The requirement of slope continuity at nodes is further

imposed as

9'i(h i) = g'i+1(O) ,
g'1(O) = C'1(t) (2-31]

and gn (h ) = C n+(t) .



The following system of n+1 equations is thus

obtained

h1 h_ "_ C1 C 2
Co + C + Cl
3 6 2 h h


h. ,, (h +hj+1 h.+l C
^ + c c -
6 3 j+1 6 j+2

C. 1 1 1
( + ) Cj + ---- C.
h hi h j+1 h j+2
j j j+1 j+1

h n h n Cn Cn+1
C + C n= h + C n+
n n+1 n+1
6 3 h h
n n [2-32]

The system of equations given as equation (2-31) can be

written in matrix form simply as

[A] {C" } = [B] {C i + {S} [2-33]

where [A] and [B] are nonsingular, symmetric, tridiagonal

(n+1)-by-(n+1) constant matrices, and

{ C T =[ C C2 ...... C ]n+

{ C.i T = C1, C2, .....* Cn+ ]i [2-34]







capacity was approximated as being the average sum of the

concentrations of Ca2+, Mg2+, K+, Na+, and Al3+ in the

exchange phase. Since 5 species were involved, the

equivalent fractions of Ca2+ and Mg2+ did not sum to unity.

Thus, values obtained for Ks were approximated. Values for

KMg-Ca varied with depth since solution and exchange phases
of cation concentrations varied and ranged from 0 to 1. In

order to simplify model simulation, KMg-Ca was assumed

constant, and the input value for KMg-Ca was obtained by

taking the mean of KMg-Ca obtained for all depths for the

Cecil topsoil and subsoil columns, respectively. The

magnitude for KMg-Ca was 0.225 for Cecil topsoil, and 0.798

for Cecil subsoil, indicating that Ca2+ was preferred over

Mg2+ by both topsoil and subsoil but that Mg2+ was preferred

more by subsoil exchange sites than by topsoil exchange

sites.

Model Sensitivity Analysis

A sensitivity analysis for dispersion coefficient (D),

volumetric water content (8), bulk density (a), selectivity

coefficient (KMg-Ca), and cation exchange capacity (CT) was

performed for the numerical model with respect to the

experimental data from the soil columns.

Dispersion coefficient. Figures 2-5 and 2-6 present

predicted and observed distributions for Mg2+ concentrations

in the soil solution phase with column depth where a

sensitivity analysis was performed for the dispersion

coefficient (D) for both topsoil and subsoil. Dispersion









since retardation R is a function of both CT and CT.

Therefore, using large values of CEC in the simulation would

tend to retard cation movement.

Discrepancies observed between predicted and observed

distributions of Mg2+ concentration using CEC values of 10.6

and 17.0 mmole(+) Kg soil for topsoil and subsoil columns,

respectively, were attributed to several factors. First,

certain of the model assumptions were not strictly met.

Careful examination of the soil system reveals that a

contradiction occurs between some of the assumptions and

reality. The cation exchange capacity was not constant but

actually varied from point to point along the soil column as

shown in Tables 2-3 and 2-4. At any given point, the extent

that a specific ion could occupy soil exchange sites would

also be affected by the presence of other ion species in the

system. Exchangeable Al comprised about 25% and 50% of

total exchangeable cations for Cecil subsoil and topsoil,

respectively, after leaching with MgCl2. This probably

means that Ca occupied only 50-75% of the exchange sites

initially, which is considerably less than 100% as assumed

for the model. Experimental conditions involved Ca2+, Mg2+

Al K Na and H whereas the model assumptions only
2+ 2+
allowed for Ca and Mg2. Model simulations performed here

did not sufficiently account for this discrepancy.

Two other sets of cation exchange capacity values were

calculated for Cecil topsoil and subsoil. The reason for







Table 2-3 Concentrations of cations in solution and
exchange phases for topsoil after miscible
displacement with 4.5 pore volume

Depth Ca Mg K Na Al Sum
(cm) Solution pEase
mmole(+) L


0.224 9.050
0.188 9.150
0.184 8.990
0.211 8.670
0.319 8.560
0.259 8.590
0.323 8.780
0.419 8.160
0.511 11.900
0.387 8.560
0.547 8.330
0.459 8.230
0.487 8.230
0.523 7.900
0.607 8.030
0.675 7.670
0.705 7.960
0.799 8.070
0.898 7.340
0.978 7.410


0.231
0.174
0.143
0.127
0.311
0.127
0.341
0.272
0.409
0.262
0.346
0.272
0.270
0.256
0.256
0.260
0.274
0.327
0.149
0.258


0.348 0.014 9.867
0.459 0.016 9.987
0.414 0.016 9.747
0.470 0.016 9.494
0.599 0.013 9.802
0.459 0.016 9.451
0.517 0.013 9.974
0.334 0.012 9.197
0.487 0.013 13.320
0.431 0.012 9.652
0.894 0.013 10.130
0.424 0.012 9.397
0.327 0.012 9.326
0.410 0.013 9.102
0.435 0.013 9.341
0.348 0.013 8.966
0.485 0.019 9.443
0.434 0.017 9.647
0.334 0.017 8.738
0.431 0.016 9.093


Depth Ca


K Na


Average 9.684
Al Sum


(cm) Exchange phase
------- mmole(+) kg soil


0.609
0.614
0.544
0.564
0.584
0.659
0.589
0.664
0.709
0.828
0.813
0.973
0.913
0.938
1.060
1.070
1.100
1.140
1.270
1.500


6.580
4.940
4.860
4.120
3.950
3.700
3.460
3.460
3.790
3.620
3.540
3.790
3.540
3.290
3.790
3.370
3.210
3.290
3.290
2.960


0.125
0.146
0.113
0.107
0.133
0.113
0.105
0.125
0.110
0.128
0.194
0.141
0.164
0.153
0.166
0.189
0.248
0.177
0.215
0.205


0.265
0.257
0.261
0.265
0.270
0.261
0.270
0.257
0.257
0.261
0.287
0.278
0.270
0.287
0.261
0.278
0.270
0.283
0.278
0.287


*
4.280 10.237
*
5.592 10.648
*
5.792 10.525
*
5.859 10.365
*
5.826 10.663
*
5.992 11.174
*
5.603 10.271
*
5.692 10.599
*
5.647 10.537
*
6.014 10.966


* undetermined Average 10.598


* undetermined


Average 10.598










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Table 3-10


Subsoil selectivity coefficients as determined
after leaching with pH 4.9 HC1 solution


Mg->Ca K-->Ca Na-->Ca Na->K K-->Mg

1 1.0571 0.10071E-01 0.18402E-03 0.13517 0.95277E-02
2 0.83329 0.87928E-02 0.90796E-04 0.10162 0.10552E-01
3 0.68700 0.17035E-01 0.56657E-03 0.18237 0.24796E-01
4 0.70282 0.69391E-02 0.26874E-04 0.06223 0.98732E-02
5 0.67556 0.13804E-01 0.87994E-04 0.07984 0.20434E-01
6 0.59744 0.22818E-01 0.13039E-03 0.07559 0.38194E-01
7 0.74461 0.43823E-01 0.21775E-03 0.07049 0.58853E-01
8 0.64961 0.26035E-01 0.45399E-03 0.13205 0.40077E-01
9 0.69344 0.41166E-01 0.43954E-03 0.10333 0.59365E-01
10 0.65642 0.17224E-01 0.25094E-03 0.12070 0.26240E-01

Average selectivity values

0.72973 0.20771E-01 0.24489E-03 0.10634 0.29791E-01


Na-->Mg Al->Ca Mg-->Al K-->Al Na-->Al
1 0.17408E-03 540.47 0.21853E-02 0.43475E-04 0.10737E-06
2 0.10896E-03 18.528 0.31230E-01 0.19155E-03 0.20100E-06
3 0.82470E-03 13.935 0.23269E-01 0.59562E-03 0.36127E-05
4 0.38238E-04 5.8117 0.59736E-01 0.23978E-03 0.57790E-07
5 0.13025E-03 5.1617 0.59731E-01 0.71390E-03 0.36332E-06
6 0.21824E-03 3.6995 0.57643E-01 0.17921E-02 0.77406E-06
7 0.29243E-03 26.964 0.15311E-01 0.17667E-02 0.61877E-06
8 0.69886E-03 6.7631 0.40534E-01 0.16153E-02 0.37196E-05
9 0.63386E-03 27.173 0.12271E-01 0.16023E-02 0.17678E-05
10 0.38228E-03 54.483 0.51913E-02 0.30626E-03 0.53854E-06

Average selectivity values

0.35019E-03 70.299 0.30710E-01 0.88669E-03 0.11761E-05







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112


coefficients were assumed to be unity for all cations specie

in the calculations.

After leaching, exchangeable Al3+ comprised 75-95% of

the total exchangeable cations in topsoil columns for the pH

3.9 HC1 treatment and 80-86% for the pH 4.9 treatment.

These values greatly exceed the 48% of Al3+ initially

present on the soil exchange phase. The acid leaching

processes obviously resulted in more exchangeable Al as a

consequence of acid dissolution of soil clay minerals.

Exchangeable Al3+ comprised about 40-60% of total

exchangeable cations in the subsoil after leaching with both

treatments of HC1 solution, where as Al3+ initially

accounted for only 32% of total exchangeable cations. Hence

acid leaching also resulted in forming more exchangeable

Al3+ in the subsoil column. However, the magnitudes were

not as large as observed for the topsoil. These

observations suggest that, of all the cation species, Al3+

was the most preferred ion for exchange sites of both

topsoil and subsoil, and also dominant in terms of

exchangeable and nonexchangeable forms for this acid Cecil

soil. Calculated selectivity coefficients for Al3+ -> Ca2+

revealed a significant preference of exchange sites for Al3+

over Ca2+ in the subsoil but the opposite in the topsoil.

Overall, the approximate values of selectivity coefficients

for the same ion pair varied from depth to depth, and were

highly dependent upon the local concentrations of cations in





168


Table 4-6 Concentrations of cations in solution
and exchange phases for K-subsoil
after leaching with pH 3.9 HC1 solution

Depth Ca Mg K Na Al Sum
(cm) Solution phase
mnnole (+) L
1.0 0.354 0.058 0.215 1.130 0.014 1.771
2.0 0.287 0.054 0.413 1.230 0.009 1.992
3.0 0.619 0.107 0.640 2.420 0.014 3.801
4.0 0.372 0.062 0.454 1.480 0.007 2.374
5.0 0.344 0.049 0.370 1.310 0.046 2.119
6.0 0.489 0.099 0.570 3.700 0.008 4.865
7.0 0.564 0.115 0.532 2.680 0.009 3.900
8.0 0.531 0.099 0.350 1.930 0.021 2.931
9.0 0.473 0.064 0.390 1.530 0.008 2.465
10.0 0.459 0.066 0.384 1.220 0.110 2.239
11.0 0.297 0.078 0.480 1.720 0.024 2.600
12.0 0.437 0.084 0.502 2.210 0.009 3.242
13.0 0.312 0.058 0.434 1.200 0.029 2.033
14.0 0.374 0.064 0.454 2.180 0.011 3.083
15.0 0.803 0.140 0.734 2.960 0.009 4.646
16.0 0.713 0.104 0.962 2.680 0.009 4.468
17.0 0.342 0.058 0.315 1.340 0.014 2.069
18.0 0.332 0.069 0.465 1.580 0.028 2.474
19.0 0.743 0.118 0.622 2.990 0.019 4.492
20.0 0.813 0.111 0.458 2.320 0.006 3.708

Depth Ca Mg K Na Al Sum
(cm) Exchange phas
mmole (+) Kg soil
1.0 0.661 0.041 1.820 0.652 9.885 13.059
3.0 1.070 0.041 13.400 0.685 0.512 15.708
5.0 1.100 0.123 14.100 0.674 0.701 16.698
7.0 1.520 0.103 14.800 0.739 0.467 17.629
9.0 1.250 0.041 15.100 0.652 0.789 17.832
11.0 1.380 0.082 13.000 0.674 0.556 15.692
13.0 1.330 0.062 14.300 0.652 0.441 16.785
16.0 1.470 0.062 14.100 0.685 0.311 16.628
18.0 1.170 0.062 14.400 0.652 0.523 16.807
20.0 1.160 0.062 14.100 0.696 0.478 16.496




198


initial quantities for Mg-topsoil. For all of the

pretreated topsoil, quantities of all cations leached were

not directly proportional to the H+ concentration of the

applied solution, but larger quantities of major cations

were leached from columns receiving pH 3.9 HC1 than for pH

4.9 HC1. For example, for Ca-topsoil that received pH 3.9

HC1, 4.06 mole(+) of Ca2+ were leached but only 2.12

nmmole(+) were leached when pH 4.9 HC1 was applied. Mineral

dissolution was implied from concentrations of cations in

the leachate and on exchange sites. For example for

Ca-topsoil, K+ and Na+ were initially 0.036 and 0.0 mmole(+)

in the exchange phase but became 0.103 and 0.116 mmole(+)

after leaching with pH 3.9 HC1. Similar results were

observed for all treated topsoil columns of all cations

species examined. The increase of exchangeable-cation

concentration was most dramatic for Al3+. Output from the

treated topsoil was observed to be greater than input, and

the resulting positive-charge-balance errors can be

attributed to the dissolution or decomposition of

interlayer-hydroxy vermiculite, as well as to other mineral-

weathering processes in this soil.

Treated subsoil. Tables 4-25 through 4-27 present the

charge balance for Ca-subsoil, K-subsoil and Mg-subsoil,

respectively. Inputs were 18.336 and 18.089 mmole(+) and

outputs were 19.473 and 18.418 mmole(+) for Ca-subsoil that

received pH 3.9 and 4.9 HC1 solutions, respectively.

Ca-subsoil gave charge-balance errors of + 6 and + 2 % for













































































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209


subsoil. Samples of Cecil topsoil and subsoil were treated

with electrolyte solutions in order to saturate exchange

sites with a single (K Ca2+ or Mg 2+) cation (treated

soil). Mixed soils were obtained by mixing equal masses of

K -, Ca -, and Mg saturated soils. Packed soil columns

used in these experiments were 10-cm long for nontreated

soil, and 20-cm long for treated and mixed soils. Dilute

aqueous HC1 solutions of pH 3.9 or 4.9 were applied at 1.0

cm h-1 Darcy flux to separate air-dry soil columns, until

approximately 30 pore volumes of effluent had been

collected. Initially, liquid flow into the soil was

transient as the wetting front moved through the unsaturated

soil, and effluent outflow did not occur until the wetting

front had reached the end of the column. Steady flow

occurred after a short transition period.

For all columns, regardless of applied-solution pH,

initial samples of effluent were characterized by low pH and

high concentrations of basic (Ca2+, Mg2+, K Na ) and

acidic (Al 3+) cation species. These observations were

attributed primarily to miscible displacement of soil

solution originally present within small pores, and

secondarily to the combined effects of removal of

exchangeable cations by ion exchange with H+ ions, and

release of cations by mineral hydrolysis, as the acid

wetting front penetrated the initially air-dry soil column.

With increasing volume of column effluent, cation

concentrations dramatically decreased and pH increased up to




199


pH 3.9 and 4.9 HCl solutions, respectively. Inputs were

16.491 and 16.012 mmole(+) and outputs were 16.465 and

12.425 mmole(+) for K-subsoil receiving pH 3.9 and 4.9 HC1

solutions, respectively. K-subsoil thus gave + 4 and + 22 %

charge-balance errors for treatments of pH 3.9 and 4.9,

respectively. Inputs were 30.917 and 30.701 mmole(+), and

outputs were 27.020 and 26.931 nmmole(+) for Mg-subsoil that

received pH 3.9 and 4.9 HC1 solutions, respectively.

Mg-subsoil thus gave 13 and 12% charge-balance errors

for treatments of pH 3.9 and 4.9, respectively. For all of

the pretreated subsoil, quantities of cations leached were

not directly proportional to H+ concentration of the applied

HCl solution for the major saturating cation. For example

for K-subsoil that received pH 3.9 solution, 11.140 mmole(+)

of K+ was leached, but only 6.872 mmole(+) was leached when

pH 4.9 acid was applied. The leaching of other cations was

also less than proportional to the applied solution pH. The

result of mineral dissolution was evident from the

concentrations of cations in the leachate and an exchange

phases, but was not as much as for the topsoil. For

Mg-subsoil, for example, concentrations of Na+ and Al

initially on the exchange sites were 0.006 and 0.436

mmole(+) but, after leaching with pH 3.9 HCl solution, these

concentrations became 0.041 and 0.725 mmole(+),

respectively. For all of the treated Cecil subsoil columns

the outputs were much less than were inputs, and negative

charge-balance errors occurred. This can be explained by








2-10 Experimental and simulated distributions of Mg2+
concentrations for three values of bulk density for
the subsoil column after miscible displacement with
3.6 pore volumes.................................. 54

2-11 Experimental distributions of Mg2+ concentrations for
the topsoil column after miscible displacement with
4.5 pore volumes, along with simulation results for
three values of the selectivity coefficient ....... 56

2-12 Experimental distributions of Mg2+ concentrations for
the subsoil column after miscible displacement with
3.6 pore volumes, along with simulated results for
three values of the selectivity coefficient ....... 57

2-13 Experimental distributions of Mg2+ concentrations for
the topsoil column after miscible displacement with
4.5 pore volumes, along with simulation results for
two values of the cation exchange capacity ........ 59

2-14 Experimental distributions of Mg2+ concentrations for
the subsoil column after miscible displacement with
3.6 pore volumes, along with simulation results for
two values of the cation exchange capacity ........ 60

2-15 Calculated and experimental distributions of Mg2+
concentrations in the solution phase for the subsoil
column after miscible displacement with 3.6
pore volumes ....................................... 63

2-16 Calculated and experimental distributions of Mg2+
concentrations in the exchange phase for the subsoil
column after miscible displacement with 3.6
pore volumes..........................................64

2-17 Calculated and experimental distributions of Mg2+
concentrations in the solution phase for the topsoil
column after miscible displacement with 4.5
pore volumes .................................... 65

2-18 Calculated and experimental distributions of Mg2+
concentrations in the exchange phase for the
topsoil column after miscible displacement with
4.5 pore volumes................. .................. 66

2-19 Calculated and experimental distributions of Mg2+
concentrations in the solution phase for the topsoil
column after miscible displacement with 4.5
pore volumes....................................... 68

2-20 Calculated and experimental distributions of Mg2+
concentrations in the exchange phase for the topsoil
column after miscible displacement with 4.5
pore volumes ...................................... 69











































































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110


Table 3-9 Subsoil selectivity coefficients as determined
after leaching with pH 3.9 HC1 solution


K-->Ca
0.10565E-01
0.18616E-02
0.12097E-02
0.17322E-02
0.23342E-02
0.33944E-02
0.26102E-02
0.35146E-02
0.13292E-02
0.23357E-02


Na-->Ca
0.63078E-02
0.11321E-02
0.32949E-03
0.38949E-03
0.32048E-03
0.21863E-03
0.23781E-03
0.26898E-03
0.32188E-03
0.34229E-03


Na-->K
0.77267
0.77982
0.52189
0.47419
0.37054
0.25379
0.30184
0.27664
0.49210
0.38281


K- >Mg
0.23385E-01
0.43760E-02
0.18491E-02
0.26239E-02
0.38122E-02
0.54578E-02
0.42387E-02
0.53978E-02
0.23759E-02
0.43464E-02


Average selectivity values

0.57896 0.30887E-02 0.98689E-03 0.46263 0.57862E-02


Na-->Mg Al->Ca Mg-->Al K-->Al Na-->Al
1 0.13961E-01 26816. 0.34395E-05 0.66319E-05 0.30593E-05
2 0.26611E-02 72.997 0.10547E-02 0.94013E-05 0.44582E-05
3 0.50364E-03 773.49 0.36202E-03 0.15129E-05 0.21505E-06
4 0.59001E-03 304.80 0.94389E-03 0.41293E-05 0.44030E-06
5 0.52340E-03 27.830 0.82487E-02 0.21377E-04 0.10875E-05
6 0.35153E-03 15.629 0.15392E-01 0.50024E-04 0.81771E-06
7 0.38618E-03 121.03 0.19294E-02 0.12122E-04 0.33335E-06
8 0.41310E-03 46.725 0.59079E-02 0.30482E-04 0.64535E-06
9 0.57535E-03 35.173 0.49785E-02 0.81711E-05 0.97375E-06
10 0.63695E-03 51.489 0.30140E-02 0.15732E-04 0.88254E-06

Average selectivity values

0.20602E-02 2826.5 0.41835E-02 0.15958E-04 0.12913E-05


Mg-->Ca
0.45181
0.42541
0.65423
0.66015
0.61230
0.62193
0.61580
0.65112
0.55946
0.53739





116


and cations exported + cations still remaining in the

column) were 2.733 mmole(+) and 2.086 mmole(+),

respectively, for pH 3.9 input solution and topsoil (Table

3-11). Overall, charge balance error was 7.5% for the pH

3.9 treatment, indicating a loss of 0.647 mmole(+) of charge

during application of the HC1. The corresponding total

charge balances were 2.087 mmole(+) and 1.869 mmole(+) for

input and output solutions, respectively, for pH 4.9 input

solution and topsoil. Overall charge balance error was -

10.5% for treatment pH 4.9, also indicating a loss in charge

during application of HC1 to the soil.

From Table 3-12 for the subsoil column the total charge

balances of major cations for the initial column and after

leaching were 3.446 and 2.702 mmole(+) for the pH 3.9

treatment, respectively. The corresponding charge balance

error was 28% indicating a sizeable loss in charge. For

pH 4.9 treatment, values of 3.285 and 2.197 mmole(+) were

observed for charge balance of major cations, respectively

for the initial column and after leaching. The

corresponding charge balance error was 33%, also

indicating a substantial loss in charge.

For all of the columns, outputs of cation charge were

less than inputs of charge for topsoil and subsoil,

indicating a net loss of charge. Sources of charge-balance

error can be attributed to the difficulty in precisely

determining H in the solution and exchange phases, to the

fact that Al analyses were not specific for Al3+ forms, and





200


the red-orange color of the treated Cecil subsoil, which is

due to substantial amounts of Fe oxides. pH-Dependent sites

for these oxides tend to develop a negatively charged

surface at moderate to high pH (pH > 5.0). The stable pH

readings for the samples of effluent and of soil solution

were near 6.5. Therefore, some of the exchangeable cations

were adsorbed for charge-balance purpose, resulting in a

decrease in the quantity of exchangeable cations or cations

exported.

Mixed soil. Tables 4-28 and 4-29 present the charge

balance for mixed subsoil and topsoil, respectively.

Inputs were 8.161 and 7.841 mmole(+) and outputs were 11.576

and 12.656 mmole(+) for mixed topsoil that received pH 3.9

and 4.9 HCl solution, respectively. Charge-balance errors

for mixed topsoil were + 42 and + 61% for treatments pH 3.9

and 4.9, respectively. The large charge-balance error was

explained by mineral dissolution. Inputs were 24.825 and

24.582 mmole(+) and outputs were 26.889 and 30.360 mmnole(+)

for mixed subsoil that received pH 3.9 and 4.9 acid,

respectively. Charge-balance errors for mixed subsoil were

8% and 23% for treatments of pH 3.9 and 4.9, respectively.

Mixed soils obtained by mixing the three pretreated soils in

a 1: 1: 1 weight ratio resulted in Mg2+ domination of

exchange sites. Concentrations of cations in the leachate

were also dominated by Mg2. Thus leaching losses of Mg

were greater than for all other cations. Total

concentrations of each cation in the effluent were not
















































































S S
(0 1)


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iuen~liEl


10


0

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Na in the column effluent, solution and exchange phases of

the soil were analyzed using an atomic absorption

spectrometer. Total Al3+ was determined by optical emission

spectroscopy (inductively coupled plasma argon ICAP).

Results and Discussion

Soil parameters used in the leaching experiment are

presented in Table 3-1. Clay minerals contained in the

Cecil topsoil and subsoil were kaolinite,

interlayered-hydroxy vermiculite, quartz, as well as Al and

Fe hydrous oxides. In addition, gibbsite was found in the

subsoil. Other soil properties, such as particle size

distribution and texture are presented in Table 2-2.

Concentrations of exchangeable cations in Cecil topsoil and

subsoil are presented in Table 3-2.


Table 3-1 Physical and chemical parameters for Cecil topsoil
and subsoil columns

Parameter Topsoil Topsoil Subsoil Subsoil

pH of input 3.9 4.9 3.9 4.9
solution
Bulk density3 1.57 1.58 1.37 1.37
(mg cm )
Volumetric 0.40 0.40 0.49 0.48
water content
(cm cm )
Pore velocity 2 0.027 0.027 0.022 0.023
(cm h x 10x )
Column length2 0.10 0.10 0.10 0.10
(cm x 10 )
Pore volume (L) 0.044 0.045 0.054 0.053
Input total H 125 12.5 125 12.5
concentration -
(mmole(+) L x 10 )
Total number 37.8 36.9 30 29
of pore volumes
collected












































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Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy

LEACHING OF CATIONS DURING DISPLACEMENT BY ACID
SOLUTIONS THROUGH COLUMNS OF CECIL SOIL

By

KO-HUI LIU

August, 1987

Chairman: R.S. Mansell
Major Department: Soil Science


Cation transport under conditions of steady liquid flow

was investigated, using columns filled with water-saturated

Cecil (Typic Hapludult) topsoil and subsoil. Initially the

soils were saturated with Ca 2+, using CaCl2 solution, which

was then miscibly displaced by MgCl2 solution.

A Galerkin finite-element method with cubic-spline

shape functions was used to numerically solve the equation

for one-dimensional transport and binary exchange of cations

in the Cecil soil columns. The numerical model was verified

by assuming that the solute was non-reactive solute or that

paired cations had no preference for soil exchange sites.

Cation exchange capacity (CEC) was shown to be the most

sensitive input parameter when the model was used to

simulate cation transport in the soil columns. Increase of

CEC increased retardation of cation movement. Relatively




120


basic cations in the exchange phase were decreased and

significant hydrolysis of Al3+ in the soil occurred during

leaching with HC1. This effect was more pronounced for

topsoil that received pH 3.9 solution. For the subsoil,

only the end of the column that received acid input solution

showed significant losses of cations and dissolution of

Al Generally, exchange-phase concentrations of basic

cations as determined with NH4OAc extraction before and

after leaching with HC1 solutions were decreased for both

soils. The percent of base saturation before HC1

application was 52%, for the topsoil. After HC1

application, base saturation wass 0.1% and 1.5% for pH 3.9

and 4.9 treatments, respectively. For the subsoil, base

saturation was initially 68%. After acid application, base

saturation was 5.4% and 4.4% for the pH 3.9 and 4.9

treatments, respectively.

The distribution of cations in the exchange phase

followed the order Al3+ > Ca2+ > Mg2+ > K = Na for both

soils. Al was the preferred ion on the exchange sites, as

expected due to its large valence and large equivalent

fraction initially on the exchanger. In the solution phase,

the distribution of cation concentrations followed the order

K Na = Al3+ > Ca2+ > Mg2+ for topsoil and Na+ > K+ = Ca2+

>Mg2+ > Al3+ for subsoil. The exchange selectivity

coefficients for each pair of cations as estimated from

distributions of ion concentrations in solution and on

exchange phases at specific column depths varied with depth







a C (t) n+l hi n
Qm,j t 21hi am i (z) (1 + T+W)2
( CT + W)


n+1 C.(t)
E a. dz -
i=l 1, at


[2-481


where n = CT K C ,


and


n+1 n+1
W = CT + (K 1) ( E 6 E a., C.
i=l i=1


n+1
+ E 6 b (-Cf q/D))
i=1 i,1 f
i=1

For the right-hand side of equation [2-46], the matrix

coefficients are given as:

n+1 n+1 h a (zia
Rm'j = D ( 1 i 1 h am i(z)ai.j (z) dz )) [2-49]


n+1 n+1 0 m
Umj = ( 2 Oi a () a.j (z) dz )) [2-501


n+1 n+1 h "
Vm,1 = q Cf (j2 i 1 f ami(z) bi,1 (z) dz )), [2-511


and

n+1 n+1 h q
Sm,1 j-2 ( Oi a i() bi, (z) (- D Cf) dz)
(2-52]

In the process of numerical integration, the definite

integral is given by equations [2-48] through [2-52]. Since

these equations are very complex, an explicit expression can

not be obtained. However, this integral can be effectively










































































In o
n Ye


C/ (+) B ouWW)


UCT 429~u9ou0


ii +
m


44
0



In
u


0



4O



(as
0)




0


o :






0



0I-I


t)
S ra




0)







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154


The cause of this is uncertain. Figs. 4-12, 4-13, and 4-14

present Al3+ concentrations in effluent from columns of

K-topsoil, Ca-topsoil and Mg-topsoil, respectively. The

effect of input solution pH upon Al3+ concentration in

column effluent was such that, the more acidic the input

solution, the more Al3+ was leached. This effect was not in

direct proportion to the H+ ion concentration, however.

Cation Concentrations in Effluent from Chemically Pretreated
Mixed Soil Columns

Figs. 4-15 and 4-16 present concentrations for cations

eluted from columns of mixed topsoil and mixed subsoil that

received pH 3.9 HC1 solution, respectively. Very high

concentrations of each cation were observed initially in

effluent. For example, for mixed topsoil that received pH

3.9 HC1 solution, initial concentrations of Ca2+, Mg2+, K+

and Na+ were 58.8, 114.36, 24.0 and 0.63 mmole(+) L1,

respectively. For effluent from mixed topsoil columns that

received pH 4.9 HC1 solution, initial concentrations of

Ca2+ Mg2+, K+ and Na4 were 54.8, 115.18, 25.84 and 0.56
-1
mmole(+) L respectively. For mixed subsoil that received

pH 3.9 HC1, initial concentrations of Ca2+, Mg 2+, K and Na+

in column effluent were 788.42, 756.89, 112.30 and 1.2

mmole(+) L1 respectively; and for mixed subsoil that

received pH 4.9 HC1 solution, initial concentrations of

Ca2+, Mg2+, K Na' were 459.08, 699.03, 125.61 and 1.36
-1
mmole(+) L respectively. These observations can be

explained once more by the salt effect and by the replacing













CHAPTER I
INTRODUCTION

Description of Problem

During recent decades, the environmental impact of acid

precipitation has become a major concern in many

industrialized countries. Concerns about possible harmful

effects on soil, vegetation and surface-water supplies have

resulted in intensive research in this area. The most

serious effects of acid deposition upon forest soils are

commonly thought to be the potentials for accelerating

cation leaching, for increasing soil acidity, and hence for

decreasing forest productivity. Acidification of surface

waters resulting in the release of toxic aluminum ions

(Al 3+) from sediments in lakes and streams is harmful to

aquatic ecology (Hutchinson and Havas, 1980).

During rain events, rainwater which does not run off

the surface or undergo evapotranspiration infiltrates into

the soil profile. As the soil water moves by mass flow,

hydrodynamic dispersion mixes the native soil solution with

the incoming rainwater. Ion species moving with the soil

solution may undergo ion-exchange with counter ions

initially present on the soil exchange phase. In addition

to ion exchange phenomena, the presence of H+ in the

rainwater results in secondary reactions such as acid




63

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144


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9*


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165


exchange sites, but was not as effective in enhancing

competition of H+ ions with divalent cations such as Mg2+

and Ca2+

Concentrations of Al3+ in Effluent from Mixed Soil Columns

Concentrations of Al in effluent from mixed

topsoil and subsoil columns that received HC1 solutions. are

presented in Figs. 4-21 and 4-22, respectively. Initial
3+ -1
concentrations of Al3+ were approximately 2.80 mmole(+) L 1

for effluent from mixed topsoil that received pH 3.9 HC1

solution, and 0.47 mmole(+) L-1 from subsoil columns that

received pH 4.9 HC1 solution. In general, concentrations of

eluted Al in the effluent from columns of mixed subsoil

were quite low after the first few samples of effluent, and

no differences in these values were observed for applied HC1

solutions with different pH. For mixed topsoil columns,

concentrations of Al3+ decreased gradually and the pH of

input HC1 solution did effect the concentration of Al3+ in

the effluent. Higher concentrations of Al3+ occurred

in initial effluent from columns receiving pH 3.9 solution

than at pH 4.9. Total quantities of Al in the effluent

from columns receiving pH 3.9 solution were not reasonably

greater than for pH 4.9, however. The reason for this

observation was uncertain.

Concentrations of Cations in Solution and Exchange Phases
for Treated and Mixed Soil Columns after Application of 25
Pore Volumes of HC1 Solution

Treated Subsoil. Concentrations of cations in solution

and exchange phases are presented in Tables 4-6 and 4-7 for




197


terminology of "input" and "output" refers to the left- and

right- hand sides of equation [4-3], respectively.

Treated Topsoil. Tables 4-22 through 4-24 present the

charge balance of Ca-topsoil, K-topsoil and Mg-topsoil,

respectively. Inputs were 5.272 and 5.112 mmole(+) and

outputs were 8.893 and 8.590 mmole(+) for Ca-topsoil that

received treatment pH 3.9 and 4.9 HC1 solutions,

respectively. Therefore, very large charge-balance errors,

of + 69 and + 68%, were observed for Ca-topsoil for

treatments of pH 3.9 and 4.9, respectively. From Table 4-22

the sum of concentrations of Ca2+ in the exchange phase and

effluent exceeded the initial quantities of Ca2+ in the

Ca-topsoil, indicating that soil minerals underwent

dissolution during leaching with the HC1 solutions. Inputs

were 9.353 and 8.718 mmole(+) and outputs were 12.031 and

10.275 mmole(+) for K-topsoil that received pH 3.9 and 4.9

HC1 solutions, respectively. Charge-balance errors for

K-topsoil were + 29 and + 15% for the pH 3.9 and 4.9

treatments, respectively. Inputs were 5.206 and 4.954

mmole(+), and outputs were 15.734 and 8.183 mmole(+) for

Mg-topsoil that received pH 3.9 and 4.9 HC1 solutions,

respectively. Charge-balance errors were + 202 and + 8% for

treatments of pH 3.9 and 4.9 respectively. The extremely

large error observed for the pH 3.9 treatment with

Mg-topsoil can possibly be explained by the effect of
2+
mineral dissolution, since concentrations of Mg in the

exchange phase and effluent were 3-fold greater than the




201


Table 4-28 Charge balance of cations for columns of
mixed-cation subsoil


Mixed-cation subsoil

H Ca2+C


pH 3.9

Mg2+


Initial cations 8.544 10.233
Total input 0.267
Final # 0.004 0.002
solution phase
Final # 1.473 0.793
exchange phase
Total 0.008 9.900 8.970
output in effluent

Mixed-cation subsoil pH 4.9

H Ca2+ Mg2+

Initial cations 8.544 10.233
Total input 0.024
Final # 0.003 0.002
solution phase
Final # 1.360 0.832
exchange phase
Total 0.007 8.183 8.000
output in effluent

#: undetermined


(mmole(+))

K+

5.360

0.040

1.352

3.720


(mmole(+))

K

5.360

0.039

1.617

4.640


Na+

0.162

0.017

0.067

0.028


Na

0.162

0.013

0.068

0.036


Al3+

0.260

0.002

0.500

0.017


Al3+
Al3

0.260

0.002

0.338

0.020


*







The two different CEC values for the Cecil soil

resulted from the two extraction methods used for

exchangeable acidity. Exchangeable acidity as determined by

BaC12-TEA extraction has more practical utility for

calcareous than for acid soils (Thomas and Hargrove, 1984),

and KCl extraction is reflects the immediate need for lime

(Thomas, 1982). Thomas (1982) suggests that the best method

for determining exchangeable acidity for acid soils is to

use a neutral, unbuffered salt to remove the acidity at the

pH of the salt solution-soil mixture.

Clay content of the Cecil subsoil was 2.7 times greater

than that for the topsoil, but organic matter content of the

topsoil was only 1.6 times greater than that for the

subsoil. The ratio of subsoil to topsoil CEC values was

somewhat less (1.3 to 1.6) than the 2.7 ratio for clay

contents. The higher clay content in the subsoil did,

however, result in a higher CEC and volumetric water content

for the subsoil. The texture of the topsoil was sandy loam

as compared to sandy clay loam for the subsoil. The type of

clay minerals in the topsoil and subsoil indicates that both

soils were highly weathered and had electrostatic charge

sites which were predominately pH-dependent charge surfaces.

Verification of the Numerical Model

Predictive accuracy of the computer program for the

numerical solution to the convective-dispersive ion-

transport equation was evaluated for a simple problem of

transport of a conservative solute (i.e. where Ks = 0) for




177


Table 4-12 Concentrations of cations in solution
and exchange phases for Ca-topsoil after
leaching with pH 3.9 HC1 solution


Depth Ca Mg K
(cm) Solution
-- nmmole(+)


1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
11.0
12.0
13.0
14.0
15.0
16.0
17.0
18.0
19.0
20.0


0.799
1.260
0.948
0.741
0.633
0.723
0.804
0.656
0.838
1.320
0.912
1.320
1.190
0.858
0.705
0.907
0.880
0.979
1.010
0.859


0.726
0.792
1.120
1.330
0.437
0.318
0.207
1.220
0.592
1.000
0.918
0.067
0.481
1.670
0.489
1.550
0.141
0.896
0.444
0.222


0.184
0.490
0.127
0.122
0.134
0.166
0.364
0.136
0.350
0.364
0.200
0.219
0.127
0.274
0.076
0.246
0.087
0.186
0.269
0.158


Na Al Sum
phase
L


1.210
1.780
0.939
0.998
1.430
1.390
2.410
1.350
2.320
2.110
1.130
1.410
1.370
3.990
0.743
1.860
10.200
1.530
1.260
1.680


0.002
0.017
0.060
0.022
0.066
0.217
0.009
0.009
0.007
0.004
0.012
0.038
0.012
0.407
0.054
0.046
0.018
0.008
0.007
0.006


2.921
4.339
3.194
3.213
2.700
2.814
3.794
3.371
4.107
4.798
3.172
3.053
3.180
7.199
2.067
4.609
11.326
3.599
2.990
2.925


K Na


(cm) Exchange phase
------ mmole (+) Kg
1.0 3.620 0.041 0.192 0.337
3.0 10.500 0.041 0.294 0.304
5.0 10.400 0.021 0.237 0.283
7.0 9.360 0.021 0.211 0.261
10.0 11.700 0.041 0.301 0.337
11.0 12.700 0.041 0.269 0.359
14.0 10.600 0.041 0.269 0.283
16.0 11.700 0.062 0.326 0.370
18.0 12.000 0.082 0.499 0.413
20.0 10.900 0.082 0.288 0.304


Al Sum

soil
5.182 9.372
1.511 12.650
1.156 12.097
1.023 10.876
1.012 13.391
1.201 14.570
0.867 12.060
0.845 13.303
0.978 13.972
0.789 12.363


Depth Ca





114


each have pH-dependent characteristics. Poorly-crystalline

clay minerals that are coated with Al compounds act as sinks

for H+ by transforming the soil exchange phase from H to

Al3+- saturated (Thomas and Hargrove, 1984). Hydrolysis of

Al in the soil then causes the formation of H during

extraction. This so-called 'exchangeable' H+ is a result of

hydrolysis and really does not exist as such on the exchange

sites of soil clay minerals or organic matter (Kissel et

al., 1971). Therefore, the ion-exchange behavior of H+ in

acid soil is not easily described in a meaningful way

(Thomas and Hargrove, 1984).

Charge Balance of Major Cations for Topsoil and Subsoil

The charge balance of major cations in columns of

topsoil and subsoil which received two HC1 solutions with

different pH values are given in Tables 3-11 and 3-12,

respectively, with units of mmole(+). The overall charge

balance of major cations in each soil column can be

described by the following relationship:

Total H added as HC1 + All cations initially present on
soil exchange sites
(1) (2)

= Total H+ exported or leached in the column effluent

(3)

+ Total cation exported or leached in the column effluent

(4)

+ All cations still remaining in the soil columns in the
exchange and solution phases after leaching with the
HC1 solutions.
(5) (3-5]





141


o E D cuI ED
O P m It gO
* S
N(D n in

3uanT^3 ^o Hd


0


0




*


mU
m>










4-40

0 0a
10 r-*











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>4 >0
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0
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W U
4> I
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S0 j




41 0
FZ 11 4
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3 r
43 &
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AO







value between 7 and 20 pore volumes of effluent. Between 20

and 28 pore volumes the pH gradually decreased once more

because the acid-buffering capacity of the soil had by then

been exceeded. As expected, effluent from treatment pH 3.9

had slightly lower pH compared to effluent from treatment pH

4.9. Effluent concentrations of Ca2+, Mg K and Na

from the subsoil column which received pH 4.9 HC1 are

presented in Fig. 3-7. Magnitudes of cation concentrations

in the effluent from columns treated with either pH 3.9 or

4.9 solutions were in the order Ca2+ > Mg2+ > Na ,

which is approximately the same order as quantities of the

various ion species initially on the exchange phase of the

subsoil (Table 3-2).

The pH effect of input HC1 upon concentrations of Ca2+

in the effluent is shown in Fig. 3-8. More Ca2+ was leached

from the subsoil column receiving treatment pH 3.9 than for

input pH 4.9. BTC for summed concentrations of Ca 2+, Mg2+

K and Na+ in the effluent from subsoil columns which

received pH 3.9 and 4.9 HC1 solutions are presented in Fig.

3-9. The subsoil column (pH 3.9) which received tenfold

higher H+ concentrations, resulted in leaching of

approximately a two-fold greater quantity of basic cations

in the effluent. Fig. 3-10 indicates that the concentration

of Al3+ in the effluent from columns of subsoil receiving

the two different pH treatments. Higher concentrations of

Al were observed in the effluent for treatment pH 3.9.






























To My Parents





117


Table 3-11 Charge balance of cations for columns of topsoil

Topsoil pH 3.9 (mmole(+))

H Ca2+ Mg K Na Al3+

Initial cations 0.278 0.043 0.121 0.451 1.162
Total input 0.202
Final # 13820 1.970 7.920 8.040 2.870
solution phase (*10 )
Final # 0.150 0.012 0.027 0.044 1.527
exchange phase
Total 0.07 0.058 0.064 0.069 0.025 0.110
output in effluent

Topsoil pH 4.9 (mmole(+))

H Ca2+ Mg K Na Al3

Initial cations 0.279 0.044 0.122 0.454 1.170
Total input 0.018
Final # 1.]00 2.460 6.730 6.070 0.051
solution phase (*10 )
Final # 0.154 0.029 0.029 0.017 1.275
exchange phase
Total 0.068 0.040 0.043 0.064 0.023 0.017
output in effluent

# : undetermined





160


efficiency of H+ for basic cations described in the

aforementioned paragraph. After the first collected sample

of effluent, concentrations of cations were observed to

decrease drastically in the second or third samples, and

then afterwards concentrations gradually decreased with

time. The initially larger concentrations for mixed subsoil

than for mixed topsoil are attributed to the fact that the

CEC of the pretreated mixed subsoil was three-fold greater

than for mixed topsoil. For mixed topsoil and subsoil,

concentrations of K+ were the greatest, concentrations of

Mg and Ca were less than for K and concentrations of
2+ 2+
Mg and Ca were difficult to distinguish from each other.

The same concentration behavior was also found when pH 4.9

HC1 was applied to columns of mixed topsoil and subsoil,

respectively. This occurred because divalent Ca2+ and Mg2+

cations are more preferred than monovalent K+ and Na+

cations for the Cecil soil exchange sites.

The effect of pH of the input HC1 solutions upon

elution of K and Ca2+ from mixed topsoil columns is shown

in Figs 4-17 and 4-18, and the effect on Mg2+ and K+ from

mixed subsoil is shown in Figs 4-19 and 4-20. The pH of

input HC1 solution had a greater effect upon concentrations
+ 2+ 2+
of K in column effluent than on Ca or Mg2. This

observation implies that a lower pH (higher H+

concentration) of input solution enhanced competition of H+

ions with monovalent cations such as K+ and Na+ for soil












+1)
*He











OOI
+1
cN,
c)p
0U.

C-'-4




04
4+ E






4-1 H

SW(0
S0
* ,C

4.)
+1 0
COO


+HO
a)

crk
-1 0
4 44C ()
C1) E
X 01 r-

(!.>


00
V'*I- 04
H0+

0 1

r-i 0)) 4


try
1-1






-4


Uorq.Jquaouoo


(T/ (+) at ouI i)








functions possess a number of desirable optimal properties

(Ahlberg et al., 1967).

For a selected soil column of length L, suppose that

the space interval 0 < X < L is properly discretized into n

interconnected elements with n+l nodal points at 0 = x1 <.x2

< x3 <...< x n+ = L, where values of C(x.,t) and its

derivatives with respect to x at nodal point j are defined

as

C (x.,t) = C (t) ,

C.'(t) = I [2-28]
ax xi
ax

and C. (t)= ax2 x


For the ith element, one defines the element size h. as

h. = xi+ x.i and the local coordinate z as z = x xi with

0 < z < h One further expresses the cubic polynomial

function approximating C(x,t) in the ith element as g(z,t).

If g(z,t) is made to satisfy the conditions

g.(0,t) = Ci(t) ,

gi(h,t) = Ci+l(t) 2-29]

g"i(0,t) = C" (t) ,
and g"i+1(h,t) = C"i+ (t)

g(z,t) is given by

z(z-h )(z-2hi) i z-h.
gi(z,t) = C (t) C (t)
6h. h 1
i i







becomes inefficient and a method that is more suitable for

stiff equations should be employed. The LSODE solver

developed by A. C. Hindmarsh at the Lawrence Livermore

National Laboratory was used in this study as a part of the

main subroutine throughout the program. The LSODE

subroutine (Hindmarsh, 1980, 1983) is capable of solving

systems of ordinary differential equations of the form

dy
= f(t,y) [2-55]
dt

and y(t,O) = f0

where y, f, and f0 are vectors of length N.

In the computation, the spatial discretization was made

to give a small element-size in the immediate vicinity of

the inlet boundary, and gradually coarser elements were used

for increasing distances from the inlet. Element sizes were

obtained by trial and error in order to provide solutions

without oscillations. Mass-balance error was not allowed to

exceed 2% in a given simulation. A small initial time step

(5 1010 sec) was used for generating stable solutions,

after which larger time steps (300 sec) were used. All

computations were performed with the IFAS computer network

on a VAX 11/750 computer at the University of Florida.

Double-precision arithmetic was used in all computations.

Material and Methods

Soil

The Cecil (Typic Hapludult) soil used in this study was

obtained from a forest site located at Clemson University in





134


Table 4-1 Concentrations of exchangeable cations for
nontreated Cecil topsoil and subsoil


Exchangeable cations
(mmole(+) Kg soil)
(mmole(+) Kg soil)


CEC
(mmole(+) Kg soil)
(mmole(+) Kg soil)


Ca2+ Mg2+ K +


Na


Al3+


Topsoil 1.60 0.30 0.70 2.60 6.70 11.90
Subsoil 8.80 2.20 0.80 2.80 7.00 21.60



Table 4-2 Soil parameters for chemically pretreated topsoil
columns after leaching with HC1 solutions

Parameter Ca-topsoil Ca-topsoil Mg-topsoil

Input solution pH 3 3.9 4.9 3.9
Bulk density (MG M ) 1.61 1.64 1.64
Volumetric water 0.37 0.38 0.37
content (M M )
Dispersion -3.03*10 3.03*10 3.03*10
coefficient (M1 h )
-1
Pore velocity (M h ) 0.030 0.028 0.030
Pore volume (L) 0.082 0.083 0.082
Column length (M) 0.20 0.20 0.20
Input concentration i0.125 0.0125 0.125
of H (mmole(+) L )
Total amount of H 0 284 0.029 0.277
applied (mmole(+) L )
Total no. of pore 26 28 26
volumes collected

Parameter Mg-topsoil K-topsoil K-topsoil

Input solution pH 3 4.9 3.9 4.9
-3
Bulk density (MG M ) 1.61 1.65 1.64
Volumetric water 3 0.38 0.37 0.37
content (M M ) -
-4 -4 -4
Dispersion 2 13.03*10 3.03*10 3.03*10
coefficient (M1 h )
Pore velocity (M h ) 0.029 0.034 0.028
Pore volume (L) 0.083 0.081 0.083
Column length (M) 0.20 0.20 0.20
Input concentration i0.0125 0.125 0.0125
of H (mmole{+) L )
Total amount of H 0.025 0.348 0.029
applied mmole(+) L
Total no. of pore 27 34 27
volumes collected


Soil





205


treated soil tended to accelerate the leaching loss of

nutrient cations (Ca2+, Mg 2+) and induced the transformation

of nonexchangeable Al to exchangeable A13+

Acid deposition tended to adversely effect Cecil soil

by decreasing its fertility status. Therefore, under forest

conditions, fertilizer and lime application might be needed

in the future to replace Ca2+ and Mg2+ leached from Cecil

soil if prolonged acid precipitation occurs.





155


o O
0 0



(T/ (-n) BtO uW)


o o


uoTqe;UjUDUoo


1'-




I-




4-
0


0



>


U)
E

03

0



(U
L
0
in -


0i
a)







0-I 0
E)











0 -
Oe


1 4
a44
o-
pra,
r (



4-14

r-P

0 3



> 0
4-4

m En

1 -H
*H


-I

S-H




B-H


CNe
i-t





4

species. The format for this dissertation follows the

sequence of stated objectives.










K *


I


ID

0

(PaqJoS)


* I


I


' I


' I


* I


0 0

UDT43B2Jd


I


0

*ATnb3


0


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4-4
(0


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ot
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113


solution and on the exchange phase as well as upon cation

valence and pH of the applied HC1 solution.

If one assumed the input solution concentration for H+

to be equal to the total solution concentration for all

cation species in the soil column and the cation-exchange

capacity of the soil column to be invariant with depth and

time then, by taking differences between input solution

concentrations and the sum of solution-phase concentrations

in Tables 3-3, 3-4, 3-5 and 3-6, the solution-phase

concentrations of H+ can be obtained for each treatment and

depth of topsoil and subsoil. In the same manner,

subtracting the sum of exchangeable cations at each depth in

Tables 3-3, 3-4, 3-5 and 3-6 from the appropriate choice of

CEC obtained from Table 3-2, gives the exchange-phase

concentrations of H+ for each depth of topsoil and subsoil

in the different treatments. Therefore, the selectivity

coefficients between basic cations and H+ can be calculated.

Calculations were not performed initially for H versus

basic cations, for the reason discussed earlier by

Krishnamoorthy and Overstreet (1950) that the exchangeable

hydrogen in clays and soil colloids is not completely

disassociated. Thus, conventional ion exchange formulations

are not valid for calculating these selectivity

coefficients. Obtaining such selectivity coefficients for

H + is especially difficult for Cecil topsoil and subsoil,

which contain substantial amounts of Fe and Al hydrous

oxides as well as of interlayer-hydroxy vermiculite, which













CHAPTER IV
CATION LEACHING DURING CONTINUOUS DISPLACEMENT
BY HYDROCHLORIC ACID SOLUTION THROUGH COLUMNS OF
CHEMICALLY-PRETREATED CECIL SOIL

Introduction

Leaching of cations and movement of organic and

inorganic anions in response to acid rain deposition in

forest soils has been investigated by Mollitor and Raynal

(1982) and Ulrich et al. (1980). Cation exchange is

recognized as one of the important soil processes which

control detrimental effects of acid rain upon the nutrient

status of forest soils (Wiklander, 1975; Reuss, 1983; Cosby

et al., 1985a, 1985b). Experimental results have shown that

neutral salts added to soil-water suspensions or to dilute

solutions percolating through soil columns tend to decrease

acidification of the soil (Wiklander, 1975).

Very few published papers report investigations of the

leaching of soil cations during the displacement of aqueous

acid solutions through columns of soil where the exchange

sites have initially been saturated with a single specific
+ 2+ 2+
cation species such as K Ca or Mg Such an

investigation was performed during the research reported in

this chapter. The advantage of using chemically-pretreated

soil is that the soil system involves fewer species of

cations and thus the data are more easily interpreted.


123








receiving pH 3.9 and 4.9 treatments (Fig. 3-3) were similar.
2+
Breakthrough curves for summed concentrations of the Ca2+

Mg2+, K and Na species of basic cations in the effluent

from topsoil columns which received applications of pH 3.9

and 4.9 HC1 solutions are presented in Fig. 3-4. Generally,

the more acidic the input solution the higher the

concentrations of cations observed in the leachate.

Breakthrough curves for concentrations of Al3+ in the

effluent are reported in Fig. 3-5 for treatments pH 3.9 and

4.9, respectively. Initial Al concentrations in the

effluent were as high as 0.25 mmole(+) L- and a second

peak in the Al3+ concentration of about 0.11 mmole(+) L

occurred after about 15 pore volumes elution for input pH

3.9. Corresponding values were 0.06 mmole(+) L-1 and 24

pore volumes for input pH 4.9. This elution pattern

indicates that Al is much preferred on soil exchange
2+ 2+ + + 3+
sites over Ca 2+, Mg 2+, K and Na which causes the Al BTC

curves to be retarded. The more acidic the input solution,

the more Al3+ which should come into the solution phase.

Therefore the Al3+ BTC for input pH 3.9 should be less

retarded than for input pH 4.9, and should also have a

higher peak concentration.

Effluent pH from Cecil subsoil columns which received

pH 3.9 and 4.9 input solutions, respectively, are presented

in Fig. 3-6. Effluent pH initially was 4.2 but abruptly

increased to 6.0 after about 2 pore volumes of elution.

Thereafter, the effluent pH decreased to a relatively stable




214


effects of acid deposition are likely to go unnoticed.

However, for certain forest soils, periodical fertilizer

application may be needed in the future to minimize adverse

effects of acid deposition upon soil fertility.




119


to the fact that not all ion species (i.e. NH 4) were

included in the analyses. Essentially all of the column

effluent and soil-solution phase had a pH of less than 5.6

to 6.0, as shown in Fig. 3-1 and 3-5. Another reason could

be due to experimental error to soil chemical reactions that

act as sinks for applied H +, and to chemical dissolution of

Al3+ from nonexchangeable form in oxides and clay minerals.

The overall charge balance of major cations in each

soil column was described in equation [3-5]. A sixth term

would include sinks for H and a seventh term would include

a source for Al3+ by acid dissolution of soil oxides and

clay minerals. Although these two terms were not measured

in this investigation, they are important to the overall

balance. The reported charge-balance values are thus in

error, but do give apparent charge-balance values.

Conclusions

Cecil soil is known to be a highly weathered acid soil.

The principal soil minerals observed in Cecil topsoil and

subsoil were kaolinite, interlayer-hydroxy vermiculite, and

quartz. Gibbsite was also observed in the subsoil. A "salt

effect" caused the first few effluent samples to be

uncommonly low in pH and high in concentrations of several

cation species. Application of HC1 solutions with two

different pH values to the Cecil soil resulted in larger

quantities of cations being leached by pH 3.9 solution

relative than of pH 4.9. The reverse effect was observed

for quantities of Al3+ exported, however. Concentrations of













LIST OF FIGURES


Figure Page

2-1 Numerical and analytical solutions obtained using
topsoil parameters for the case where the binary
exchange selectivity coefficient equals zero ...... 39

2-2 Numerical and analytical solutions obtained
using topsoil parameters for the case where
the exchange selectivity coefficient equals unity.. 40

2-3 Mg2+ exchange isotherm curve obtained from the
topsoil column after miscible displacement with
4.5 pore volumes.................................... 41

2-4 Mg2+ exchange isotherm curve obtained from the
subsoil column after miscible displacement with 3.6
pore volumes. ........... ....................... 42

2-5 Experimental distributions of Mg2+ concentrations
in the solution phase of the topsoil column after
miscible displacement with 4.5 pore volumes, along
with calculated results obtained using three values
for the dispersion coefficient.................... 47

2-6 Experimental distributions of Mg2+ concentrations
in the solution phase of the subsoil column after
miscible displacement with 3.6 pore volumes, along
with calculated results obtained using three values
for the dispersion coefficient..................... 48

2-7 Experimental distributions of Mg2+ concentrations
in the solution phase of the topsoil column after
miscible displacement with 4.5 pore volumes, along
with calculated results obtained using three values
for volumetric water content....................... 50

2-8 Experimental distributions of Mg2+ concentrations
in the solution phase of the subsoil column after
miscible displacement with 3.6 pore volumes, along
with calculated results obtained using three values
for the volumetric water content....................51

2-9 Experimental and simulated distributions of Mg2+
concentrations for three values of bulk density for
the topsoil column after miscible displacement with
4.5 pore volumes............................... 53




180


Table 4-14 Concentrations of cations in solution
and exchange phases for K-topsoil after
leaching with pH 3.9 HC1 solution


C

C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
0
0
0
0
1
0
0
0
0
0
0
0
0
0
0
0
0
0


Ca


K Na


Sum


Depth
(cm)


Na Al


Sum


Exchange phae e
mmole(+) Kg soil
0.499 0.082 0.761 0.543 7.583 9.468
0.923 0.123 1.400 0.652 6.794 9.892
0.986 0.123 1.540 0.554 5.660 8.863
0.948 0.123 2.220 0.565 5.838 9.694
1.010 0.144 2.850 0.446 5.860 10.310
0.848 0.123 2.790 0.435 6.271 10.467
1.040 0.165 3.270 0.630 5.982 11.087
1.050 0.123 3.180 0.478 5.504 10.335
0.823 0.123 3.000 0.663 6.038 10.647
1.060 0.185 3.470 0.707 5.104 10.526


Solution phase
mmole (+) L
).296 0.622 0.090 0.587 0.004 1.599
).844 0.874 0.583 1.370 0.004 3.675
).449 0.852 0.205 0.783 0.006 2.295
.130 1.220 0.246 0.593 0.002 2.191
.817 1.110 0.592 1.240 0.006 3.765
.692 0.666 0.638 1.130 0.006 3.132
.190 8.400 1.600 4.370 0.048 15.608
.110 1.660 0.652 1.470 1.140 5.032
.221 1.250 0.725 0.061 0.681 2.938
.298 1.110 0.905 1.160 0.006 3.479
.850 1.500 1.450 2.540 0.003 6.343
.435 0.406 1.030 1.670 0.003 3.544
.259 0.118 1.060 1.610 0.004 3.051
.220 1.160 1.360 0.765 0.033 3.538
.229 1.760 1.110 1.660 0.007 4.766
.269 0.526 0.999 0.900 0.016 2.710
.184 1.180 1.200 1.350 0.036 3.950
.153 1.250 1.080 1.510 0.014 4.008
.404 0.940 0.960 2.780 0.011 5.095
.856 2.410 1.970 2.400 0.150 7.786


1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
11.0
12.0
13.0
14.0
15.0
16.0
17.0
18.0
19.0
20.0


Depth
(cm)

1.0
4.0
6.0
8.0
10.0
12.0
14.0
16.0
18.0
20.0








a aC1 C1 3C1
( 1 + F ) = D v [2-22]
e at ax ax

The term R= 1+(o/8)F is referred to as the retardation

function for transport of ion 1 through the soil. For

binary nonpreferential (Ks = 1) homovalent exchange

F = CT/ CT and R = 1 + (o/9) (CT/CT), so that, for constant

cation exchange capacity, retardation of cation transport

tends to increase as CT decreases towards small values such

as those that normally occur in rain water. Also, for this

case, cation retardation can be expected to be greatest in

soils with highest values for CT and with highest

soil-to-solution ratios (0/l). Appropriate initial and

boundary conditions for equation [2-22] are


t = 0 x > 0 C = Ci [2-23a]


t O x = 0 v C = D + v C and
ax

aC I[2-23b]
t 2 0 x = L 0 =
ax

Partial differential equations given as [2-22] and

[2-23] constitute a mathematical description of cation

transport and exchange in a soil column of finite length L

during steady water flow, they were solved in this research

by use of a Galerkin finite-element method. For simplicity,

concentration C1 for ion species 1 is denoted by C

throughout the remainder of the dissertation. Subscript or














CHAPTER V
SUMMARY AND CONCLUSIONS

Summary

Cecil (Typic Hapludult) soil was used in investigations

of cation movement during steady displacement of electrolyte

solution and of cation leaching during application of acid

solutions under constant-flux conditions. Cecil soil was

characterized as a highly weathered soil. The principal

clay minerals in topsoil (0-30 cm depth) and subsoil (30-60

cm depth) were interstratified or interlayered vermiculite,

kaolinite, and quartz. Gibbsite was found only in the

subsoil. Topsoil and subsoil textures were classified as

sandy loam and sandy clay loam, respectively. Organic

matter (O.M) contents for topsoil and subsoil were 1.60 and

1.04%, respectively.

Studies of cation transport were conducted using

columns filled with water-saturated Cecil topsoil or

subsoil. Using a steady liquid flux of 1 cm h-1, soil

columns were initially saturated with Ca2+, using 0.005 M

CaCl2, and then miscibly displaced by 0.005 M MgCl2

solution. After displacing 4.5 and 3.6 pore volumes of

MgC12 solution for topsoil and subsoil columns,

respectively, the flow was terminated. Soil columns were

equilibrated overnight, before dissection into sections the

next day. Cations in the solution and exchange phases were


206




185


for mixed subsoil columns that received input HC1 solutions

of pH 3.9 and 4.9, respectively. Concentrations of cations

throughout the column for the solution phase were generally

in the order K+ > Na+ > Ca2+ > Mg2+ > Al3+ but, for the

exchange phase the order was K > Ca > Mg > Al > Na.

If one only examines the shallowest depth for the column

receiving the pH 3.9 input acid solution, the exchange-phase

concentrations were in the order Al > Ca2+ > Mg2+ > K >

Na+ and for pH treatment 4.9 in the order Ca2+ > Mg2+ > Al3+

> K > Na. Concentrations of the basic cations Ca2+, Mg2+

K and Na were much less than the Al3+ concentration for

the pH 3.9 case. The pH 4.9 treatment had a reverse effect.

A comparison of concentrations on the exchange phase in

Tables 4-18 and 4-19 with values in Table 4-4 for the

original exchangeable cations in mixed subsoil showed that

concentrations of the basic cation Mg2+, Ca2+, K and Na
3+
were decreased but those Al remained fairly stable. The

base-cation saturation was remarkedly decreased by leaching.

Mixed topsoil. Tables 4-20 and 4-21 present

concentrations of cations in the solution and exchange

phases for mixed topsoil columns after receiving pH 3.9 and

4.9 input HC1 solutions, respectively. Concentrations of

cations through the soil columns for the solution phase were

generally in the order Al3+ > Ca2+ > Mg2+ > K > Na for pH

treatment 3.9 and in the order Al3+ > Mg2+ > K+ > Ca2+ > Na+

for pH treatment 4.9. The exchange phase was in the order

Ca2+ > Al3+ > Mg2+ > K > Na for both treatments.




142


were 60.89, 94.86 and 186.75 mmole(+) L respectively.

For columns that received pH 4.9 HC1 solution,

concentrations of 33.51, 145.62 and 139.72 mmole(+) were

observed for K Ca2+ and Mg 2+, respectively. Beginning

with the third collected sample (about 0.5 pore volume) of

effluent, cation concentrations underwent a drastic decrease

to below 10 mmole(+) L for all of the pretreated topsoil

and subsoil columns. Concentrations of other cations for

each soil column were detectable but usually so small as to

be negligible.

High concentrations of cations that appeared in the

first few collected effluent samples from each pretreated

soil column can be explained by a "salt effect" and by the

replacement efficiency of H+ for exchangeable cations

(Wiklander, 1975; Reuss and Johnson, 1986). All air-dry

pretreated topsoil and subsoil contained 0.6 and 1% water

content by volume, respectively. Since the soil columns

were initially air-dry, as the acid solution infiltrated

into each soil column H+ in the moving wetting front tended

to replace cations from variably-charged sites effectively.

This resulted in high concentrations of cations in the

wetting front. Thus, due to the high ratio of 6M/6H for the

chemically pretreated soil columns, the replacing efficiency

of H+ for basic cations (Wiklander and Andersson, 1972) was

enhanced. Thereafter, these high concentrations of cations

induced cation exchange with exchangeable (Al + H ),

resulting in low pH for the soil solution. This phenomenon





146


I I


uoTqeJ.uaouoo3


i -J
C











0












E

rH
O-
4-







C"-


a)
L
0


'r




0)


>



OH




o ri
0
-P








I
0)










4-ri
0
CO








I 0
r-
ia



oa



0r-4
4o =


S-Pc
S0
O *H





Lnt

t4
0




ki+
m 3
(U


(T/ +) TouWu







0
ID








00
o o
r-I
Cri
o OQ
o o

0
r-I
6K 0
E oL





0 04




OO
0 0L0





o 1 It




o oo



0- 0 0 0 4


Hd 4uanTj.^








Exchange Phases for Treated and Mixed
Soil Columns after Application of 25 Pore
Volumes of HC1 Solution ................ 165
Charge Balance Using All Major Cations
for Treated and Mixed Soil Columns ...... 190
Conclusions ................................. 203

V SUMMARY AND CONCLUSIONS ...................... 206
Summary .................................... 206
Conclusions .................................. 212

REFERENCES ............................................ 215

BIOGRAPHICAL SKETCH .................................. 221








soil pH, particle size distribution, clay content, and

cation exchange capacity, are presented in Table 2-2.





Table 2-1 Soil parameters used in the column experiments

Parameter Topsoil Subsoil


-1
Darcy velocity (M H ) -1
Dispersion coeff. (1 H )
Bulk density (Mg M )
Volumetric water content
(M- M )
Bulk volume of soil (L)
Column length (M)
Total concentration
(MMOLE(+) L)
Porosity
Pore volume (L)
Total number of pore
volumes collected
Cation exchange-capacity
(MMOLE(+) Kg soil)
Selectivity coeff.(KM^ C)


-2
1.11 x 10
-4
3.03 x 10
1.64
0.37
-1
2.21 x 101
-2
20.05 x 10 2
0.01

0.38
-3
81.97 x 10
4.50

10.6

0.225


-2
1.08 x 10-2
-4
1.85 x 10
1.42
0.46


-1
2.21 x 10-1
-2
20.05 x 10
0.01

0.46
-3
101.47 x 10
3.62

17.0

0.798


Table 2-2 Properties of Cecil topsoil and subsoil

Topsoil Subsoil


pH(1: 1)
Sand(%)
Silt(%)
Clay(%)
O.M. (%)
CEC from NHOA4c+KC1
(mmole(+) Kg soil)
CEC from
NH OAc+BaCl -TEA
molele() K& soil)
Soil texture
Clay minerals Int
Ve
Ka
Qu
Amorphous Al


4.46
76.90
14.00
9.10
1.60
17.00

54.00


Sandy loam
erlayered-hydroxy
rmiculite,
olinite,
artz.
,Fe hydrous oxides


4.80
61.60
13.70
24.70
1.04
27.00

70.00


Sandy clay loam
Interlayered-hydroxy
Vermiculite,
Kaolinite, Gibbsite,
Quartz.
Al,Fe hydrous oxides







Selectivity coefficient. Figs. 2-11 and 2-12 present

simulated and observed distributions for Mg2+ in the

solution phase with depth for topsoil and subsoil columns,

respectively. The approximate selectivity coefficients

(KMg-Ca) obtained from column experiments were 0.225 and
0.798 for topsoil and subsoil, respectively. A deviation of

2% from these experimental values were used in simulations

to determine sensitivity to this input parameter. These

values were 0.230 and 0.220 for topsoil and 0.782 and 0.814

for subsoil, respectively. The'effect of KMg-Ca upon the

simulation of Mg2+ concentration distributions in the

solution phase was minor for both topsoil and subsoil.

Although the selectivity coefficient occurs within the

cation retardation factor R, sensitivity analysis showed

that slight changes in KMg-Ca had little effect upon the

Mg 2+-concentration profile distribution.

Cation exchange capacity. Sensitivity of the model to

change in cation exchange capacity (CT or CEC) was examined

by using two values of CEC in separate simulations. Values

of the CEC of 70 and 54 mmole(+) Kg-1 soil were obtained

experimentally from the buffered 1 M NH40OAc plus BaC12-TEA
-l
method, whereas values of 27 and 17 mmole(+) Kg were

obtained from the buffered 1 M NH OAc plus unbuffered 1 M

KC1 method, for original subsoil and topsoil, respectively.
-1
CEC values of 17.0 and 10.6 mmole(+) Kg soil were the mean

CEC values obtained from all dissected soil sections (Tables

2-3 and 2-4) in columns of subsoil and topsoil,




159


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(D 0)+ IO


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+B
+
+
+
+
+
+
+1
+

+
+
+
+


+
+

+I


+
+
+
+

+
+ -
+ +
+ +
+ +
++
+



O+

.V a O..
I *V vi .,


uoTaeuauaouoo


0


*

r(0

0
P0
44O


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4- a04


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(JO






a)
r i




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> w
co














44
l. *a
~ >




0 0

+i 0
,' c'
\ 0 <


.


- -- --


(T/ (+) GTOWWU)














CHAPTER II
MODEL SIMULATION OF BINARY CATION EXCHANGE
AND TRANSPORT IN COLUMNS OF CECIL SOIL

Introduction

The exchange of cations between exchange and solution

phases in a soil system is an important phenomenon which

greatly influences nutrient movement in soil, leaching of

cations by acid-rain infiltration, reclamation of

salt-affected soil, contaminant migration, and other

processes. Ion transport in packed exchanger beds and in

soil columns has been investigated for many decades and

several approaches have been used to model the ion-exchange

process. Thomas (1944) developed an equation applicable to

cation transport in columns of synthetic ion exchanger,

assuming that the exchange mechanism obeyed reversible,

second-order, reaction kinetics. Rible and Davis (1955)

used chromatographic theory of ion exchange to investigate

cation exchange and transport in soil columns, but neglected

the effect of hydrodynamic dispersion during transport.

Assuming chemical equilibrium between cations in solution

and on the exchange phase, Lapidus and Amundson (1952)

proposed a model for ion transport such that exchange

followed a linear isotherm, with hydrodynamic dispersion





156


v


OX


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0

0


9
0
0
Ox
SO


-i


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8 *


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ro




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Emm

0 i r-I

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> 4 o0
tp m
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L 4=
0 go

In w rl
I-r
fa k


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1"'1"'1


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o .2 I T I I I .




96


0

+ +
U ( +
0 T+ (0 0

1n

CU V



ta)

1 0
CU _W 'I -I





a 0'4 0o


x> e


y 0 orc
a 3RI1 -r
0 >
X ND O^^




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x x
x -o -P 0 r0

X 4g 4 I E

X +CK


3 0 X+ 3a
X + 0

OD e N o0 in I 0 P4

/+o o o o o o tte




150


effect of soil organic matter. Nontreated topsoil contained

more organic matter and amorphous material than subsoil

(refer to Table 2-2). The variable-charge sites

attributable to organic matter or amorphous material tended

to saturate with a single basic cation. Therefore, the

replacement efficiency of H+ for cations in the topsoil

should have been greater than for subsoil (Chatterjee and

Marshall, 1950; Wiklander and Andersson, 1972).

Concentrations of Al determined in the column effluent

were assumed to be in the Al3+ form. Al3+ concentrations in

the first collected sample of effluent were approximately 1

mmole(+) L1 for K-subsoil and Ca-subsoil columns. This

relatively high effluent concentration was attributed to the

salt effect on exchangeable Al 3+. For Mg-subsoil, the Al3+

concentration was as high as 6.67 mmole(+) L-1. For all

columns, concentrations of Al3+ in consecutive effluent

samples consistently decreased. Figs. 4-9, 4-10 and 4-11

gave data for K-subsoil, Ca-subsoil and Mg-subsoil columns,

respectively. There was no effect of different input

solution pH on Al effluent concentration for all

pretreated subsoil columns.

Concentrations of Al3+ in the first collected effluent

sample for K-topsoil and Ca-topsoil columns were as high as

24.5 and 2.45 mmole(+) L-1, respectively. These high

concentrations were also attributed to the salt effect. The

elution of Al3 was different for Mg-topsoil, in that a

concentration peak occurred at about 8.0-8.2 pore volumes.













+ +


m




a a


II


9:
OR
cs
0 ,
Ci
ck


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ig





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0 0 u 0 ~~~ ~ ~ ~ M U*- 0 R 4U4 O)8AT41 l




169


Table 4-7 Concentrations of cations in solution
and exchange phases for K-subsoil after
leaching with pH 4.9 HC1 solution

Depth Ca Mg K Na Al Sum
(cm) Solution phase
mmole (+) L


1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
11.0
12.0
13.0
14.0
15.0
16.0
17.0
18.0
19.0
20.0


0.027
0.027
0.207
0.267
0.031
0.257
0.437
0.398
0.317
0.308
0.428
0.459
0.424
0.608
0.828
0.571
0.689
0.889
0.629
0.790


0.008
0.010
0.030
0.029
0.012
0.040
0.049
0.046
0.049
0.054
0.059
0.082
0.054
0.079
0.099
0.062
0.079
0.099
0.079
0.118


0.318
0.441
0.187
0.161
0.496
0.344
0.424
0.365
0.394
0.408
0.464
0.496
0.403
0.465
0.454
0.263
0.427
0.562
0.546
0.556


0.448
0.368
0.921
1.150
0.965
1.300
1.600
1.410
1.530
1.750
1.790
2.400
1.690
2.250
2.790
1.690
2.420
2.880
2.030
2.680


0.007
0.000
0.021
0.000
0.004
0.003
0.000
0.004
0.037
0.000
0.004
0.000
0.002
0.011
0.000
0.000
0.000
0.000
0.000
0.000


0.808
0.846
1.366
1.607
1.508
1.944
2.510
2.224
2.327
2.520
2.746
3.437
2.573
3.413
4.171
2.586
3.615
4.430
3.284
4.144


Depth Ca Mg K
(cm) Exchange
mmole(+)
1.0 1.280 0.226 9.790
3.0 1.200 0.165 13.900
5.0 1.150 0.062 13.700
7.0 1.410 0.062 15.500
9.0 1.330 0.041 14.100
11.0 1.360 0.062 13.600
14.0 1.270 0.062 14.500
16.0 1.410 0.062 15.100
18.0 1.240 0.062 14.000
20.0 1.330 0.062 14.800


Na Al


Sum


phaje
Kg soil
0.739 3.614 15.649
0.772 0.901 16.938
0.652 0.678 16.242
0.696 0.812 18.480
0.685 0.756 16.912
0.707 0.645 16.374
0.652 0.623 17.107
0.696 0.645 17.913
0.717 0.689 16.708
0.739 0.645 17.576




161


1 a a g g I a 1 4 a I J I I I I s

U
cr i


V 0. -I
01) 0 0 a


Sr-4 >
4Jl =

T T 04
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0 0
110 1
So 00o
+ oD 41
0






0 0
or

O0a
*0 E to)
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(o + W


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I I I I 14











I certify that I have read this study and that in my
opinion it conforms to acceptable standards of scholarly
presentation and is fully adequate, in scope and quality, as
a dissertation for the degree of Doctor of Philosophy.




R. Ma sell, Chairma
Professor of Soil Science


I certify that I have read this study and that in my
opinion it conforms to acceptable standards of scholarly
presentation and is fully adequate, in scope and quality, as
a dissertation for the degree of Doctor of Philosophy.




R. D. Rhue
Associate Professor
of Soil Science


I certify that I have read this study and that in my
opinion it conforms to acceptable standards of scholarly
presentation and is fully adequate, in scope and quality, as
a dissertation for the degree of Doctor of Philosophy.




E. A. Hanlon
Assistant Professor
of Soil Science


I certify that I have read this study and that in my
opinion it conforms to acceptable standards of scholarly
presentation and is fully adequate, in scope and quality, as
a dissertation for the degree of Doctor of Philosophy.




C. C. Hsu
Professor of
Engineering Sciences




136


Table 4-4 Soil parameters for mixed soil columns after
leaching with HC1 solutions


Parameter


Mixed-cation topsoil


Mixed-cation topsoil


Input solution pH 3 3.9
Bulk density (MG M ) 1.68
Volumetric water 0.36
3-3
content(M M)
Dispersion 2 3.03*10
coefficient M h )
Pore velocity (M ) 0.027
Pore volume (L) 0.079
Column length (M) 0.20
Input concentration -0.125
of H (mmole(+) L )
Total amount of H 0 253
applied (mmole(+) L )
Total no. of pore 23
volume collected


4.9
1.66
0.36

3.03*10~4

0.030
0.800
0.20
0.0125

0.026

26


Parameter Mixed-cation subsoil Mixed-cation subsoil

Input solution pH 3 3.9 4.9
Bulk density (MG M ) 1.47 1.47
Volumetric water 0.43 0.44
3-3
content (M M)
Dispersion -1.85*10 1.85*10
coefficient (M h )
Pore velocity (M h ) 0.0253 0.0248
Pore volume (L) 0.0951 0.965
Column length (M) 0.20 0.20
Input concentration -0.125 0.0125
of H (mmole4+) L )
Total amount of H 0 267 0.024
applied (mmole(+) L )
Total no. of pore 22.3 22.0
volume collected


The cation compositions of pretreated and mixed soils

are listed in Table 4-4 and concentrations of exchangeable

cations for nontreated Cecil topsoil and subsoil are given


in Table 4-5.




143


is called a "salt effect" (Wiklander, 1975). As mass flow

occurred, the acid displacing solution mixed with the native

soil solution by hydrodynamic dispersion. Acidity caused by

the salt effect was gradually leached, resulting in

increased pH of the soil solution and effluent once more.

From about 5 to 25 pore volumes of effluent the pH ranged

from about 6.0 to 6.6, indicating strong buffering by the

soil. The buffering reflects that H ions which were

partially removed from the solution phase either by ion

exchange, dissolution of clay minerals, reactions involving

aluminum species, or a combination of the three. The

initial effluent concentrations of Mg2+ from Mg-topsoil or

Mg-subsoil were relatively higher than Ca2+ from Ca-topsoil

or Ca-subsoil and than K+ from K-topsoil and K-subsoil. The

cause of these differences was uncertain.

The effect of input solution pH upon concentrations of

eluted cations in column effluent showed that the lower pH

of input acid solution resulted in approximately the same

quantities of cations being leached as obtained at the

higher pH. This phenomenon was found for all pretreated

topsoil and subsoil columns, but was most obvious for the

topsoil (Figs. 4-3, 4-4, and 4-5). Concentrations of

cations in the effluent from pretreated subsoil columns are

given in Figs. 4-6, 4-7, 4-8 for K-subsoil, Ca-subsoil, and

Mg-subsoil columns, respectively. A greater effect of input

solution pH upon cation elution was found for pretreated

topsoil than for subsoil. This can be explained by the




128


soil solution, will cause more leaching of soil cations, and

will lower the content of exchangeable cations.

Materials and Methods

Preparation of Pretreated Soil

Cecil topsoil and subsoil materials used in this study

were obtained from the same site as described in chapter

two. Cylindrical columns used to house the soil samples

were constructed from -acrylic plastic with of 0.3 m length

and 0.0832 m inside-diameter. The top and bottom endplates

obtained from commercially-available Tempe cells (Soil

Moisture Equipment Co., Santa Barbara, California) were

fitted to each column. Soil was held in each column by a

fine nylon mesh and a Whatman no.42 filter paper. Each

column was then placed in a vertical position and soil was

sequentially packed by adding increments of soil and tapping

the side of the column until it was full in order to obtain

the desired average bulk density. The endplates were

mounted onto ends of the column in order to hold the soil in

place. Using the procedure as stated, three soil columns

were prepared for topsoil and another three for subsoil.
-1
Stock solutions of 10 mmole(+) L1 KC1, MgC12 and CaCl2 were

prepared, respectively. Three topsoil columns were then

saturated with 1 mmole(+) L KC1, MgCl2 and CaCl2 diluted

stock solutions by application to the bottom of each soil

column, respectively, in a vertical position. A

four-channel peristaltic pump as described in chapter two

was used to deliver each of the stock solutions at a rate of










I I '


00


0

O
0
0


S I


S I


0

o
0

(Peqdos)


S I


0 0

uc o D=re J I


S I


0
ATlnb3


I
O


41
4-4

Cr

c 0

* r-l


0 o



Ur




-He

CO

4J
o
414 r-




.. >





r0 0
Cri




0 a)












s-I
^ -H
> c


to r1
u Q)






CN


t3
ren
P4-


ID

0


('j

0*




218


Mansell, R.S., S.A. Bloom, H.M. Selim, and R.D. Rhue. 1986.
Multispecies cation leaching during continuous
displacement of electrolyte solutions through soil
columns. Geoderma 38:61-75.

McLean, E.O. 1965. Aluminum. p.978-998. In Methods of
Soil Analysis. Part 1. C.A. Black (ed.). American Society
of Agronomy, Madison, WI.

McFee, W.W. 1980. Sensitivity of soil regions to long term
acid precipitation. p.495-505. In Atmospheric Sulfur
Deposition: Environmental Impact and Health Effects. D.S.
Shriner, C.R. Richmond, and S.E. Linberg (eds.). Ann Arbor
Science Publishers, Ann Arbor, MI.

Mollitor, A.V., and D.J. Raynal, 1982. Acid precipitation and
ionic movements in Adirondack forest soil. Soil Sci. Soc.
Am. J. 46:137-141.

Persaud, N., and P.J. Wierenga. 1982. A differential model
for one-dimensional cation transport in discrete homoionic
ion exchange media. Soil Sci. Soc. Am. J. 46:482-490.

Pinder, G.F. 1973. A Galerkin finite-element simulation of
ground-water contamination on Long Island, New York. Water
Resour. Res. 9:1657-1670.

Pinder, G.F. and W.G. Gray. 1977. Finite Element Simulation
in Surface and Subsurface Hydrology. Academic Press, New
York.

Price, H.S., C.J. Cavendish, and R.S. Vangar. 1968.
Numerical methods of higher order accuracy for
diffusion-convection equations. Soc. Petroleum Eng. Jour.
8:293-303.

Reiniger, P., and G.H. Bolt. 1972. Theory of chromatography
and its application to cation exchange in soil. Neth. J.
Agric. Sci. 20:301-313.

Reuss, J.O. 1983. Implication of the calcium-aluminum
exchange system for the effect of acid precipitation on
soil. J. Environ. Qual. 12:591-595.

Reuss, J.O., and D.W. Johnson. 1985. Effect of soil
processes on the acidification of water by acid
deposition. J. Environ. Qual. 14:26-31.

Reuss, J.O., and D.W. Johnson. 1986. Acid Deposition and
Acidification of Soil and Water. Springer-Verlag, New
York.





196


Table 4-27 Charge balance
Mg-subsoil

Mg-subsoil

H+ Ca2+

Initial cations 0.093
Total input 0.125
Final # 0.044
solution phase
Final # 0.237
exchange phase
Total 0.021 0.041
output in effluent


Mg-subsoil

H CaC2+

Initial cations 0.094
Total input 0.026
Final # 0.037
solution phase
Final # 0.263
exchange phase
Total 0.01 0.154
output in effluent


of cations for columns of


pH 3.9 (mmole(+))

Mg2+ K

30.164 0.093

0.013 0.015

2.518 0.077

22.959 0.043


pH 4.9 (mmole(+))

Mg2+ K

30.380 0.094

0.010 0.020

2.956 0.075

22.438 0.035


#: undetermined


Na+

0.006

0.148

0.099

0.041


Na

0.063

0.108

0.111

0.039


Al3+

0.436

0.001

0.725

0.062


Al3+
Al3

0.044

0.029

0.602

0.035







Two columns designated topsoil and subsoil were

prepared carefully by uniformly packing air-dried Cecil

topsoil and Cecil subsoil, respectively. Soil columns were

positioned vertically and placed between two wooden boards

supported by threaded steel rods to hold the soil and rings

in place. A Rainin model Rabbittm peristaltic pump was used

to introduce solutions at predetermined and calibrated rates

into the bottom of these columns. Effluent from the top of

the column was collected by an ISCO model 430 fraction

collector. Soil columns were initially saturated with 0.01

mmole(+) CaCl2 from the bottom of the columns. Periodic

checks for Ca concentration in the effluent were made,

until steady state was reached with respect to Ca2+

concentrations of input and output solutions. Soil exchange
2+
sites were then assumed to be saturated with Ca2. Miscible

displacement was initiated by adding a 0.01 mmole(+) L-1

MgCl2 exchanging solution into the bottom at constant flux.

When predetermined total amounts of exchanging solution had

been added to a given column, the flow was terminated. The

pore volume for each soil column was obtained as the weight

difference between wet and air-dried soil columns obtained

before and after each experiment. Pore-volume corrections

were made for the volume of entrapped solution inside the

two endplates. The columns were equilibrated overnight

before being dissected to ensure that soil exchange and

solution phases were of equilibrium. A centrifuge method

was used to separate the soil-solution phase from the





167


I I U U U U U


c
S-
e
jr


I I


. 0. i


U .. I .


U U U I U U U


0
?


o



a T OWW)


(1 O

o o 0

uoT euqueouoo


I


.t.


l i a l I I l l


O0)
*d


-,I >
ra
o 4


0u



Srd
a) c0+
-









En


0 H








P o
I 0
0 0





0
S-H
0







electrode. Organic matter content was determined by the

Walkley-Black method (Allison, 1965). Particle-size

distribution was determined by the pipette method (Day,

1965).

Exchangeable cation concentrations were determined

using neutral 1 M NH40OAc, by placing 5-g of 2-mm air-dried

soil in duplicate 50-ml centrifuge tubes, and adding 25 ml

of 1 M NH OAc to each. All tubes were stoppered and shaken

for 30 minutes. The tubes were then placed in a centrifuge

and spun at 2000 rpm for 10 minutes, with number 42 Whatman

filter paper being used to collect the supernatant in a

50-ml volumetric flask. The same procedure was repeated and

finally the solution was brought to a volume of 50 ml with 1

M NH ,OAc (Thomas, 1982).

Exchangeable Al was determined by 1 M KC1 extraction

of 10-g of air-dried soil in duplicate 50-ml volumetric

flasks, using 25 ml of 1 M KC1. The soil and KC1 were

mixed, allowed to stand for 30 minutes, and then transferred

to Buchner funnels fitted with number 42 Whatman filter

paper mounted on 250-ml vacuum flasks. An additional 125

ml of KC1 was added in 25-ml increments to give a final

volume of 150 ml (Thomas, 1982).

Total potential acidity as determined by BaCl2-TEA

extraction was obtained by placing 10-g soil samples in

duplicate 50-ml volumetric flasks. To each flask, 25 ml of

buffer solution was added, mixed well with the soil sample,

and allowed to stand for 1 hour. It was then transferred to













ACKNOWLEDGEMENT

The author wishes to express her sincere appreciation

to Dr. R. S. Mansell, chairman of the graduate supervisory

committee, for guidance, valuable discussions, and

suggestions throughout her entire course of study.

Sincere appreciations are also extended to Drs. E. A.

Hanlon, C. C. Hsu, W. W. McFee, and R. D. Rhue for their

valuable discussions and helpful suggestions made during

this work and their review of the manuscript.

Special appreciation is expressed to Dr. S. A. Bloom,

Scientific Programmer, for assistance and counsel in

plotting graphs, to Dr. C. C. Hsu for generously lending the

LSODE subroutine and to a friend, Mr. Fang-Yin Lee, for

providing x-ray analyses for each of the soil samples.

This research was supported in part by the U.S.

Environmental Protection Agency under Project A0457-06,

entitled "Effects of Acid Deposition on Ion Mobilities in

Selected Soils". Partial support by the Soil Science

Department is also greatly appreciated.

Deepest gratitude is expressed to the author's parents

for their support and to her husband Ming-Hsiung for his

understanding, consideration, and encouragement.


iii








relationship between C. and C.. Details of this analysis

for C. are given in the next section.

Equation for Instantaneous Cation Exchange

The reversible, equilibrium-controlled cation-exchange

reaction involving ion species A with valence z and ion

species B with valence z2 can be expressed as


z2 (ad) + 1 B(sol) = 1 B (ad) + 2 A(sol) [2-3]
The preference for soil exchange sites for competing cations

can be expressed in quantitative terms by the law of mass

action (Helfferich, 1962) as


-1 2
[B] [A]
K = -, [2-4]

[A] [B]1

where [A] and [B] denote activities of cation A and B

adsorbed onto soil exchange sites, respectively; [A] and [B]

denote activities of cations A and B in solution,

respectively; and K is the thermodynamic exchange constant

which is an indicator of the affinity of exchange sites for

ion B relative to affinity for ion A.

If MA and MB are given in units of mole per kg on the

exchange phase of ion species A and B, respectively, mole

fractions for ion species A and B are defined as


M
MAT
MT




105


Table 3-6 Concentrations of cations in solution and
exchange phases for subsoil after leaching
with pH 4.9 HC1 solution


Depth
(cm)


1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0


Ca


K Na Al Sum


Solution phase
mmole (+) L
0.084 0.033 0.116 0.551 0.010 0.794
0.072 0.029 0.087 0.515 0.020 0.723
0.067 0.029 0.060 0.224 0.016 0.396
0.080 0.033 0.097 0.365 0.024 0.599
0.094 0.040 0.084 0.407 0.036 0.660
0.082 0.037 0.063 0.372 0.031 0.585
0.107 0.041 0.055 0.350 0.018 0.571
0.080 0.033 0.061 0.228 0.020 0.422
0.108 0.043 0.067 0.315 0.016 0.548
0.126 0.053 0.101 0.393 0.014 0.687


Depth Ca Mg K Na Al Sum
(cm) Exchange phas?
mmole (+) Kg soil
1.0 2.200 0.913 0.271 0.174 6.883 10.441
2.0 3.590 1.190 0.261 0.157 6.616 11.814
3.0 4.190 1.230 0.281 0.191 6.282 12.174
4.0 4.590 1.330 0.279 0.065 5.671 11.935
5.0 4.640 1.320 0.315 0.122 6.194 12.591
6.0 4.840 1.300 0.330 0.148 5.938 12.556
7.0 4.790 1.370 0.350 0.157 6.093 12.760
8.0 5.190 1.390 0.363 0.178 6.004 13.125
9.0 5.240 1.440 0.427 0.209 6.049 13.365
10.0 4.990 1.370 0.379 0.178 5.871 12.788





210


about 5 pore volumes of effluent for nontreated soil, 3 pore

volumes for treated soil, and 3.5 pore volumes for mixed

soils. Between 5 to 30 pore volumes, effluent pH was in the

range of 6.0 6.5, but cation concentrations decreased in a

gradual manner. The H+ ions were obviously removed from the

soil solution at a fairly constant rate during the last 25

pore volumes of effluent as exchangeable cations were

leached from the soil.

During application of pH 3.9 and 4.9 HC1 solutions, the

total quantities of basic cation eluted in effluent were

0.286 and 0.238 mmole(+), respectively, from columns of

nontreated topsoil; 0.395 and 0.306 mmole(+), respectively,

from nontreated subsoil; 4.204 and 2.207 mmole(+),

respectively, from Ca-topsoil; 5.697 and 5.55 mmole(+),

respectively, from K-topsoil; 9.917 and 4.221 mmole(+),

respectively, from Mg-topsoil; 15.007 and 14.273 mmole(+),

respectively, from Ca-subsoil; 11.360 and 7.002 mmole(+),

respectively, from K-subsoil; 23.105 and 22.676 mmole(+),

respectively, from Mg-subsoil; 22.626 and 21.066 mmole(+),

respectively, from mixed subsoil; and 5.621 and 5.519

mmole(+), respectively, from mixed topsoil. Also, during

application of pH 3.9 and 4.9 HC1 solutions, total

quantities of aluminum were 0.110 and 0.017 mmole(+),

respectively, from nontreated topsoil; 0.001 and 0.001

mmole(+), respectively, from nontreated subsoil; 0.116 and

0.193 mmole(+), respectively, from Ca-topsoil; 2.910 and

2.518 mmole(+), respectively, from K-topsoil; 0.739 and




213


dominant basic cations was used as input CEC for the model,

however, the numerical simulation result gave a reasonable

prediction of the experimental data. The model proposed in

this study was shown to approximate the cation transport and

exchange processes during one-dimensional steady

displacement through columns of Cecil soil.

A ten-fold difference in H+ concentrations of HC1

solutions applied at constant flux to columns of nontreated,

treated, and mixed Cecil soil resulted in a less than

proportional difference in quantities of cations removed in

the effluent. This difference was noted from H+ and cations

added and recovered as a result of H interactions with and

dissolution of soil minerals. Therefore, further

investigation of effects of acid upon Cecil soil is needed

to determine dissolution rates for interlayered vermiculite

as well as for Al hydrous oxide, and for gibbsite.

Leaching of cations during acid application to Cecil

soil columns under laboratory condition presents an extreme

case compared to field conditions when periodic wetting and

drying of soil during rainfall events limit cation exchange

and transport with the water, and where cations released by

the dynamic weathering of soil minerals tends to offset

cation-leaching losses. The most deleterious effects of

acid deposition on the soil would be those cases where the

soil is of low CEC and of medium to high base saturation.

For most acid agricultural soils where lime and

fertilizer are applied with relatively high frequency, the








poorly-ordered alumino-silicates and hydrous oxides. A

third effect is a decrease in CEC as the pH drops. Aluminum

ions are mobilized by acid dissolution of soil components,

and thus strongly compete with other cations for soil

exchange sites.

Materials and Methods

Physical and Chemical Properties of the Soil

Cecil soil used in this study was obtained from a

forest site located near Clemson University in Clemson,

South Carolina. The exact location was reported earlier by

Dr. V. L. Quisenberry (Cassel, 1985), Agronomy Department,

Clemson University. Reported texture varied from site to

site, with the clay content of Ap horizon ranging from 6 to

38 percent depending upon the amount of subsoil mixing which

occurred subsequent to soil erosion. Clay content in the B

horizon ranged from 42 to 72 percent (Cassel, 1985). In

situ values of unsaturated hydraulic conductivity reported

(Cassel, 1985) for the 0-30 cm depth ranged from 1.70*10-4

to 4.51*10-2 (cm h-1), and those for the 30-60 cm depth

ranged from 4.32*10-4 to 4.6*10-1 (cm h-1 ), respectively.

In situ values of soil-water content for the 0-30 cm depth

ranged from 0.275 to 0.495 cm3 cm 3, and for the 30-60 cm

depth ranged from 0.409 to 0.560 cm3 cm3 (Cassel, 1985).

The Cecil soil used in this research is classified as a

Typic Hapludult. Topsoil and subsoil bulk samples were

obtained from depths of 0-30 and 30-60 cm, respectively.











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183


Table 4-16 Concentrations of cations in solution
and exchange phases for Mg-topsoil after
leaching with pH 3.9 HC1 solution

Depth Ca Mg K Na Al Sum
(cm)-- Solution phase
mmole (+) L
1.0 0.587 0.115 0.142 1.340 0.213 2.397
2.0 0.322 0.584 0.161 '0.164 0.024 1.256
3.0 0.067 0.387 0.106 0.848 7.860 9.268
4.0 0.569 0.370 0.215 0.861 0.078 2.093
5.0 0.192 0.704 0.115 1.410 0.053 2.474
6.0 0.215 0.629 0.299 1.280 0.367 2.790
7.0 0.657 0.398 0.197 2.260 0.178 3.690
8.0 0.354 0.432 0.141 1.680 1.350 3.957
9.0 0.399 0.388 0.186 1.890 1.860 4.723
10.0 0.491 0.372 0.075 1.660 0.173 2.771
11.0 0.595 0.652 0.179 4.050 0.053 5.529
12.0 0.539 0.559 0.197 2.300 1.630 5.225
13.0 0.437 0.580 0.230 2.490 4.170 7.907
14.0 0.503 0.991 0.321 3.310 21.200 26.325
15.0 0.439 0.457 0.127 1.530 0.058 2.611
16.0 0.255 0.431 0.117 1.570 0.138 2.511
17.0 0.403 0.536 0.216 1.620 10.900 13.675
18.0 0.411 0.497 0.106 1.640 3.750 6.404
19.0 0.527 0.454 0.157 1.730 5.570 8.438
20.0 0.517 0.872 0.199 1.870 21.000 24.458

Depth Ca Mg K Na Al Sum
(cm) Exchange phas
mmole (+) Kg soil
1.0 1.320 0.473 0.230 0.391 6.772 9.186
2.0 2.880 4.110 0.281 0.478 1.001 8.750
3.0 2.060 7.200 0.153 0.435 0.767 10.615
5.0 2.540 8.640 0.326 0.467 0.890 12.863
7.0 1.950 9.260 0.166 0.348 0.801 12.525
10.0 1.850 9.460 0.198 0.304 0.767 12.579
13.0 1.920 9.460 0.185 0.348 0.745 12.658
16.0 2.160 8.840 0.166 0.283 0.645 12.094
18.0 1.980 10.100 0.205 0.337 0.601 13.223
20.0 2.210 10.700 0.230 0.500 0.556 14.196








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181


Table 4-15 Concentrations of cations in solution
and exchange phases for K-topsoil after
leaching with pH 4.9 HC1 solution

Depth Ca Mg K Na Al Sum
(cm) Solution phase
mmole(+) L
1.0 0.407 1.330 0.426 2.050 0.006 4.219
2.0 0.292 1.620 0.463 0.761 0.012 3.148
3.0 0.182 1.420 0.478 0.946 0.016 3.042
4.0 0.139 0.259 0.511 0.528 0.016 1.453
5.0 0.135 2.120 0.744 1.150 0.014 4.163
6.0 0.212 2.710 0.700 1.230 0.018 4.870
7.0 0.355 1.050 0.565 0.487 0.016 2.473
8.0 0.208 0.449 1.140 1.580 0.019 3.396
9.0 0.164 2.420 0.770 0.661 0.009 4.024
10.0 1.150 1.460 0.919 1.500 0.018 5.047
11.0 1.380 0.518 1.170 1.720 0.012 4.800
12.0 0.436 0.992 1.080 0.763 0.012 3.283
13.0 0.120 0.062 0.721 0.380 0.011 1.294
14.0 0.092 0.045 0.733 0.283 0.012 1.166
15.0 0.144 0.089 1.040 0.430 0.014 1.717
16.0 0.103 0.037 0.967 0.274 0.023 1.404
17.0 0.045 0.052 1.040 0.215 0.022 1.374
18.0 0.076 0.059 1.110 0.215 0.076 1.536
19.0 0.112 0.089 1.170 0.313 0.020 1.704
20.0 0.501 2.100 0.944 1.050 0.039 4.634

Depth Ca Mg K Na Al Sum
(cm) Exchange phas
mmole (+) Kg soil
1.0 0.561 0.041 0.582 0.217 2.891 4.292
3.0 0.599 0.062 1.300 0.239 1.768 3.968
5.0 0.674 0.082 1.920 0.261 1.568 4.505
7.0 0.699 0.082 2.620 0.207 1.423 5.031
9.0 0.561 0.062 2.290 0.196 1.579 4.688
11.0 0.649 0.103 2.650 0.239 1.501 5.142
13.0 0.674 0.082 3.200 0.217 1.390 5.563
15.0 0.836 0.082 3.010 0.239 1.446 5.613
17.0 0.836 0.103 3.480 0.217 1.357 5.993
20.0 1.500 0.391 6.270 0.087 1.201 9.449





170


K-subsoil columns after application with pH 3.9 and 4.9

input HC1 solutions, respectively. In the solution phase,

cation concentrations were in the order Na+ > K+ Ca2+ >

Mg2+ > Al 3+, while for the exchange phase cation

concentrations were in the order K > Ca 2+ Al Na+ >

Mg2+ for both pH treatments. If one compared concentrations

of cations in the exchange phase in Tables 4-6 and 4-7 with

values in Table 4-4 for the initial K-subsoil, leaching loss

of K was significant, while leaching losses for Ca2+, Mg2+

Na+ and Al3+ were small. A similar trend was observed for

both pH treatments.

Tables 4-8 and 4-9 present concentrations of cations in

solution and exchange phases for Ca-subsoil columns after

application of pH 3.9 and 4.9 HC1 solutions, respectively.

Solution-phase concentrations of cations were of the order

Na > Ca2+ > Al3+ K+ > Mg2+ and, for the exchange phase,

concentrations of cations were Ca2+ > Al3+ > K+ = Na > Mg

for both pH treatments. If one compared cation

concentrations in the exchange phase in Tables 4-8 and 4-9

with values in Table 4-4 for the initial Ca-subsoil, the

concentrations of Ca were dramatically decreased by

leaching, but concentrations of other species such as Na
3+ 2+
and Al were actually somewhat increased. Mg was

slightly decreased for both pH treatments, and K was

increased for the pH 3.9 treatment but not for the pH 4.9

case. The explanation for increased concentrations of

certain species in the exchange phase is that K +, Na+ and




182


solution showed that very little exchangeable Al3+ existed

initially, however.

Tables 4-16 and 4-17 present concentrations of cations

in solution and exchange phases for Mg-topsoil columns which

received input HC1 solutions of pH 3.9 and 4.9,

respectively. Concentrations of cations in the solution

phase were in the order Al > Na > Ca2+ > Mg > K, but
2+ 2+ 3+
for the exchange phase were in the order Mg > Ca > Al

> Na > K Abnormally high solution concentrations of

Al3+ were observed for both treatments. Comparison of

concentrations of cations in the exchange phase with values

in Table 4-4 for original exchangeable cations in Mg-topsoil

showed that the concentration of Mg2+ decreased but the

other cations such as Ca 2+, K Na and Al increased after

leaching with acid. This may be explained by decomposition

of the minerals due to the effect the input acid solutions.

A comparison of the CEC after leaching with the CEC of

initial pretreated topsoil showed a significant decrease but

if one compared the CEC that resulted after leaching with

the CEC of the original untreated topsoil (Table 4-5), good

agreement occurred. A small decrease in CEC was found only

for the section of the soil column that received input acid

solution. Large concentrations of exchangeable Al were

found at the end of the columns receiving input acid

solution.

Mixed subsoil. Tables 4-18 and 4-19 present

concentrations of cations in solution and exchange phases





174


Table 4-10 Concentrations of cations in solution
and exchange phases for Mg-subsoil after
leaching with pH 3.9 HCl solution

Depth Ca Mg K Na Al Sum
(cm) Solution phase
mmole(+) L


1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
11.0
12.0
13.0
14.0
15.0
16.0
17.0
18.0
19.0
20.0


Depth
(cm)

2.0
4.0
5.0
7.0
9.0
11.0
13.0
15.0
18.0
20.0


0.362
0.472
0.267
0.364
0.327
0.359
0.377
0.349
0.324
0.487
0.514
0.589
0.509
0.589
0.574
0.444
0.576
0.556
0.474
0.464


0.078
0.119
0.103
0.144
0.115
0.123
0.123
0.140
0.128
0.152
0.144
0.136
0.148
0.156
0.169
0.111
0.136
0.111
0.136
0.132


0.187
0.243
0.139
0.164
0.114
0.159
0.079
0.113
0.132
0.132
0.104
0.212
0.169
0.180
0.182
0.157
0.175
0.155
0.153
0.088


1.130
1.800
0.785
1.280
0.991
1.110
1.330
1.510
1.390
1.770
1.680
2.410
1.680
1.520
1.980
1.440
1.690
1.720
1.480
1.610


K Na


0.003
0.001
0.002
0.003
0.008
0.003
0.002
0.003
0.004
0.000
0.000
0.002
0.002
0.002
0.001
0.004
0.004
0.002
0.010
0.001


1.761
2.635
1.296
1.955
1.555
1.754
1.912
2.115
1.978
2.541
2.442
3.349
2.508
2.447
2.906
2.156
2.581
2.544
2.253
2.295


Sum


Exchange phae
mmole(+) Kg soil
1.210 5.550 0.179 0.391 9.329 16.659
0.861 8.020 0.179 0.337 3.269 12.666
0.724 8.640 0.192 0.228 1.390 11.174
0.624 7.820 0.185 0.217 1.401 10.247
0.636 8.230 0.217 0.239 1.301 10.623
0.761 8.840 0.205 0.304 1.379 11.489
0.736 8.840 0.441 0.457 1.379 11.853
0.686 8.840 0.230 0.283 1.223 11.262
0.699 8.020 0.281 0.326 1.223 10.549
0.674 8.020 0.365 0.391 1.368 10.818




204


than untreated soil. Concentrations of the major saturating

cation in the column effluent usually were not proportional

to the H+ concentration of the applied input solution,

though concentrations of major cations were usually higher

in the effluent from columns receiving pH 3.9 solution as

compared to columns receiving pH 4.9 solution. Cation

concentrations in the solution and exchange phases of soil

columns were in general as expected, with cation

concentrations in the solution phase being in the order Na+

> K+ > Mg2+ > Ca2+, whereas concentrations in the exchange
3+ 2+ 2+ + +
phase were in the order Al > Ca > Mg > K > Na.

Exceptions to these orders, however, were observed for some

columns.

Although pretreatment of the soil eliminated much

uncertainty with respect to chemical analysis, observed

charge balances were larger between inputs and outputs for

all of the pretreated and mixed soils. As HC1 acid

solutions were applied to the columns, several complicated

soil chemical reactions occurred between the pH-independent

exchange sites as well as between newly formed exchange

sites and pH-dependent charge-surface sites. Further

investigation of the effects of acid upon Cecil soil is

needed in the future, in order to understand the mechanism

and dissolution rate of interlayer vermiculite upon acid

application. Addition of neutral salt during chemical

treatment of the soil increased the exchange capacity of the

soil, and subsequent application of HC1 acid solution to the




122


Assuming an annual rainfall of 120 cm yr- columns of

topsoil and subsoil received 1.16 and 1.11 times the annual

rainfall equivalent during a period of only 7 days in these

study, such laboratory conditions are extreme compared to

field conditions, and greatly enhanced leaching of basic

soil cations. In nature, rainfall occurs during periodic

events separated by sometimes long periods without rain.

Such periodical wetting and drying of the soil tends to

limit cation exchange and cation transport, and allows time

for chemical reactions that inhibit the extent of leaching

of basic cations. Under forested conditions, it is possible

that Cecil soil could be largely depleted of most of the

basic cations (Ca2+ and Mg 2+) on exchange sites of the

topsoil if the annual quantity of acid rain was applied

continuously over a period of a few weeks to a few months.

Under such extreme conditions, application of lime and

fertilizer might be necessary in order to maintain the

production of forest located on Cecil soil.
























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BIOGRAPHICAL SKETCH


Ko-Hui Liu was born on November 5, 1951, in Taichung,

Taiwan, Republic of China. After graduating from high

school she entered National Chung-Hsing University,

Taichung, Taiwan, where she received her Bachelor of Science

degree in 1974. She then taught high school in the city of

Taipei. In the fall of 1979 she enrolled as a graduate

student in the Department of Soil Science at the University

of Florida. After receiving her Master of Science degree in

1982, she remained to perform graduate study toward the

Ph.D. degree.

She is married to Ming-Hsinug Chen. She is a member of

Gamma Sigma Delta, the Honor Society of Agriculture, and is

also an associate member of Sigma Xi.


221




216


Cosby, B.J., R.F. Wright, G.M. Hornberger, and J.N.
Galloway. 1985b. Modeling the effects of acid deposition:
Estimation of long-term water quality responses in a small
forested catchment. Water Resour. Res. 21:1591-1601.

Day, P.R. 1965. Particle fraction and particle size
analysis. p.552-562. In Methods of Soil Analysis. Part 1.
C.A. Black (ed.). American Society of Agronomy, Madison,
WI.

Dixon, J.B., and S.B. Weed. 1977. Minerals in the Soil
Environment. Soil Sci. Soc. Am, Madison, WI.

Gaines, G.L., and H.C. Thomas. 1953. Adsorption studies on
clay minerals: A formulation of thermodynamics of
exchange-adsorption. J. Chem. Phys. 21:714-718.

Gast, R.G. 1977. Surface and colloid chemistry. p.27-70.
In Mineralsin the Soil Environment. J.B. Dixon (ed.).
Soil Sci. Soc. Am, Madison, WI.

Gear, C.W. 1969. The automatic integration of stiff ordinary
differential equations. p.187-193. In Information Process.
A.J.H. Morrel. (ed.). North Holland Publishing Co.,
Amsterdam.

Grim, R.E. 1968. Clay Mineralogy. McGraw-Hill, New York.

Helfferich, F.G. 1962. Ion Exchange. McGraw-Hill, New York.

Helfferich, F.G., and G. Klein. 1970. Multicomponent
Chromatography. Marcel Dekker, Inc., New York.

Hindmarsh, A.C. 1980. LSODE and LSODI, Two new initial value
ordinary differential equation solvers. ACM-SIGNUM
Newsletter 15:10-11.

Hindmarsh, A.C. 1981. ODE solver for use with the method of
lines. In Advances in Computer Methods for Partial
Differential Equations-IV. R. Vichnevetsky, and R.S.
Stepleman (eds.). IMASC. New Brunswick, N J

Hindmarsh, A.C. 1983. ODEPACK. A systematized collection of
ODE solvers. p.55-63. In Scientific Computing. R.S.
Stepleman and M. Carver (eds.). IMACS. North-Holland
Publishing Co., Amsterdam.

Hsu, C.C. 1976. The use of splines for the solution of
boundary-layer equations. Tech. Rep. AFFDL-TR-75-158. Air
Force Wright Aeronautical Laboratories, WPAFB.

Hsu, C.C., and A. Liakopoulos. 1981. A finite
element-differential method for a class of compressible









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TABLE OF CONTENTS

Page

ACKNOWLEDGEMENTS........................................ iii

LIST'OF TABLES............................. ........... vii

LIST OF FIGURES........................................ x

ABSTRACT................... ............................ xv

CHAPTER

I INTRODUCTION .................................. 1
Description of Problem ...................... 1
Hypotheses .................................. 2
Objectives .................................. 3

II MODEL SIMULATIONS OF BINARY CATION EXCHANGE AND
TRANSPORT IN COLUMNS OF CECIL SOIL............. 5
Introduction ....................... ........ 5
Theory ................ .................... 9
Transport Equation ........................ 9
Equation for Instantaneous Cation
Exchange ............................. 11
Transport with Ion Exchange .............. 15
A Galerkin Finite-Element Numerical Method... 18
Finite-Element Method ...................... 18
Interpolation Function .................... 19
Method of Weighted Residuals ............... 23
Computational Methods ..................... 25
Material and Methods ........................ 28
Soil ............ .............. ....... 28
Column Experiment ......................... 31
Cation Exchange Isotherms from Soil
Columns ............................... 34
Results and Discussion ...................... 34
Soil Properties ........................... 34
Verification of the Numerical Model ....... 36
Exchange Isotherm Curves for Columns of
Cecil Soil ......................... ..... 38
Exchange Selectivity Coefficients (Ks)
for Soil Columns ..................... .. 43
Model Sensitivity Analysis ................ 46
Conclusions ................................ 73




153








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O-I
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n
N. = E 6i a. .(z) j=2,3,....,n+l
3 i=l 1 ,

n
and B = E 6 b (z)
i=l

For the initial boundary-value problem given by

equation [2-22] that satisfies the boundary condition

[2-23b), the trial solution can be written as

C(x,t) = C(x,t) = "n1 N.(z) C (t) + Bl(z) v C
j=1 D

j=1,2,.....,n+l [2-42]

Substituting equation [2-42] into equation [2-22] and

applying the method of weighted residuals one obtains

L o aC a2C 3C
0 N (x)[ (1 + -- F) t D --- + v ] dx = 0
ax [2-43]

Equation [2-42] can be explicitly rewritten as

L a n+1 C. v aCf
f (1+ -- F) Nm(x)[ j1 Nj(z) at- + B(z) ] dx


L n+1 2 2B1(z)
= 0 Nm 1 D 3 c(t) + DC dx


L n+1 SN +B1(z)
JO m j 1 x C(t) + vCf x ] dx '[2-44]

where N are weighting functions. Since there are n+1

unknown nodal values for the dependent variable C(x,t), a

set of n+1 weighting functions must be chosen for N .

Therefore, using Galerkin's method, the weighting functions







solutions with two different pH values .......... 140

4-2 Breakthrough curves for pH in the effluent from
Ca-subsoil columns which received input HC1
solutions with two different pH values............ 141

4-3 Breakthrough curves of K from K-topsoil columns
which received input HC1 solutions with two
different pH values ............................... 144

4-4 Breakthrough curves of Ca2+ from Ca-topsoil
columns which received input HC1 solutions with
two different pH values........................... 145

4-5 Breakthrough curves of Mg2+ from Mg-topsoil columns
which received input HC1 solutions with two
different pH values .............................. 146

4-6 Breakthrough curves of K+ from K-subsoil columns
which received input HC1 solutions with two
different pH values ............................. 147
2+
4-7 Breakthrough curves of Ca from Ca-subsoil columns
which received input HC1 solutions with two
different pH values .............................. 148

4-8 Breakthrough curves of Mg2+ from Mg-subsoil columns
which received input HC1 solutions with two
different pH values ................................ 149

4-9 Breakthrough curves of Al3+ from K-subsoil columns
which received input HC1 solutions with two
different pH values .............................. 151

4-10 Breakthrough curves of Al3+ from Ca-subsoil columns
which received input HC1 solutions with two
different pH values .............................. 152

4-11 Breakthrough curves of Al3+ from Mg-subsoil columns
which received input HC1 solutions with two
different pH values .............................. 153

4-12 Breakthrough curves of Al3+ from K-topsoil columns
which received input HC1 solutions with two
different pH values .............................. 155

4-13 Breakthrough curves of Al3+ from Ca-topsoil columns
which received input HC1 solutions with two
different pH values .............................. 156

4-14 Breakthrough curves of Al3+ from Mg-topsoil columns
which received input HC1 solutions with two
different pH values ............................. 157


xiii





132


Darcy velocity was within 3 % for most columns. Large

differences with regard to the dispersion coefficients

between soil columns were assumed absent due to the small

variations in bulk density and Darcy flux.

Method for Dissection, Extraction and Chemical Analysis of
Soil Columns

After liquid flow was terminated, each column was

equilibrated overnight to insure that the whole system was

at equilibrium. The column was placed in the vertical

position, and the tape on the outside of the column was

removed carefully. As the outflow endplate was removed, a

piece of parafilm was placed over the soil cross section and

a small-diameter steel wire was used to slice the

consecutive rings of the column. Each section of soil then

was carefully placed onto another piece of prepared parafilm

before being packed into a prenumbered small centrifuge tube

with a predrilled small hole in the bottom. A Whatman no.42

filter paper was cut and placed inside the tube over the

hole in the bottom of the tube. Then each small tube was

transferred into a large-size centrifuge tube with a glass

bead separating the extraction solution from the small

centrifuge tube, and spun at 4000 rpm for 30 minutes. The

soil sample was then removed from the small centrifuge tube

and placed into a plastic weighing boat. The weight of the

wet soil was recorded, soil in the weighing boat was

air-dried, and the air-dry weight was recorded. The

concentrations of basic cations in the exchange phase were








The equivalent fractions (Ci ) for cations 1 and 2 in the

exchange phase are given by

Ci
C = ------- 3-3]

C. + C

where i and j refers to cations 1 or 2, C. (or C.) is the

concentration of exchangeable cation 1 or 2 in moles of

positive charge (equivalents per Kg of soil), and C. is the

solution-phase concentration (mmole(+) L ) of cation 1 or

2. The magnitude of Ks indicates the relative preference of

exchange sites for cation 2 over cation 1. If z1 = z2 and

Ks = 1 the exchange sites show no preference for either of

two ions, whereas Ks < 1 indicates that cation 1 is adsorbed

preferentially, and Ks > 1 indicates that cation 2 is

preferred by the exchange sites.

Effects of Acidification

Continuous addition of acid solution to a soil tends to

increase total acidity of the soil and to decrease pH. The

extent to which soil pH is decreased by addition of acid is

greatly determined by the buffering capacity of a given

soil. A number of associated effects are also observed upon

addition of acid to soil. One of the more important effects

is the loss of basic cations such as Ca2+, Mg2+, K and

Na Basic cations are leached with co-anions, such as

bicarbonate, chloride, nitrate, sulphate, or organic anions.

A second effect is the displacement of cations from

weak-acid exchange sites such as occur on humus and on




202


Table 4-29 Charge balance of cations for columns of
mixed-cation topsoil

Mixed topsoil pH 3.9 (mmole(+))

H Ca2 Mg K Na Al

Initial cations 1.894 2.413 2.265 0.222 1.114
Total input 0.253
Final # 0.026 0.020 0.022 0.012 0.607
solution phase
Final # 2.025 0.650 0.268 0.070 0.883
exchange phase
Total 0.03 1.357 2.147 2.058 0.029 1.372
output in effluent

Mixed topsoil pH 4.9 (mmole(+))

H Ca2+ Mg2 K Na Al3+

Initial cations 1.871 2.385 2.238 0.220 1.101
Total input 0.026
Final # 0.024 0.029 0.024 0.009 2.282
solution phase
Final # 1.985 0.667 0.368 0.075 1.033
exchange phase
Total 0.033 1.548 2.030 1.905 0.023 0.621
output in effluent

#: undetermined







approximations. Their numerical simulations included

homovalent binary cation exchange with different values for

the cation exchange capacity and dispersion coefficient

parameters. Valocchi et al. (1981) extended this technique

to a two-dimensional (axially symmetric) model with the use

of isoparametric quadrilateral elements in order to solve a

ground-water contamination problem for an aquifer. They

concluded that, for a binary system, ion concentration

profiles consist of a single advancing exchange front while,

for a ternary system, two fronts may be separated by a

plateau zone. These phenomena were actually observed in

their field simulation of Na Mg2+ and Ca2+ breakthrough

curves for well-water downstream from an injection well.

Jennings et al. (1982) followed the approach of Rubin and

James (1973) and applied equilibrium interaction chemistry

speciationn, etc.) independently of the

convective-dispersive transport equation. A set of

algebraic chemistry equations was coupled to the mass

transport equation and a numerical solution was obtained by

the Galerkin finite-element method applied over the space

domain. A backward finite-difference approximation was used

for the time derivative, and a surface complexation sorption

model allowed the consideration of complex metal and ligand

species at soil surfaces. The sorption reaction was assumed

to occur under instantaneous equilibrium conditions. Using

concepts from multispecies ion chromatography as given by

Valocchi et al. (1981), and mobile-immobile water theory as












0 w a
4-4 'H

0 toa
0 4 0

0 04-)




0 +r 4

o /8
0 r. 40
O 4 00

n r. c

0 .0 .

0 0 E
0 4 *0 0
Sco
*S 4, (D


*4J..)C -



O1) ay l




VO(DO




0 0 w




I I I .
X 0 0

0 o W4 -'- S



:j
o Ic-SI
0kr
O XC'-4
I a I *I .1-
W (0 (U
'I


(T/ (+) Se ouuW)


UCT~eJ~auoUOo




148





0




an a'







I01 00

OEa
II |i o >


H- 0
< *- 3










0


0u
+ a .1-


0 |o> 4-






SU (D 0 0
>+














0 0
jC14


uoT((WejqUBDUO3


(T/ (+) ai owuj)







a 5.5-cm diameter Buchner funnel fitted with Whatman number

42 filter paper. Three 25-ml volumes of buffer solution

were then added to the suspension. These additions were

followed with 100 ml of replacement solution to give a final

volume of 200 ml. Also, a blank solution was prepared from

100 ml of buffer solution and 100 ml of replacing solution.

Two drops of bromocresol green indicator and 10 drops of the

mixed indicator were added. Finally, the solution was

titrated with 0.2 M HC1 to a repeatable endpoint in the

visual color range from green to purple. The same

procedure, using the same endpoint, was used for soil

leachates (Thomas, 1982).

Column Experiment

The cylindrical container for soil columns used in this

investigation was constructed by stacking 20 lathe-cut

plexiglass rings with 3.75 x 10-2 m inside diameter and

0.01-m length each. At the top and bottom of the soil

columns endplates consisting of a fixed, thin-plastic disc

with small holes evenly distributed over the surface were

placed in contact with the soil. On the opposite side of

each endplate two outlets were designed, one for inflow or

outflow purposes and a second for flushing the solution, if

necessary. The cylindrical rings were held together by

carefully wrapping water-proof, acid-resistant electric tape

around the circumference. A check for water leakage was

made prior to packing soil into each column.




104


Table 3-5 Concentrations of cations in solution
and exchange phases for subsoil after
leaching with pH 3.9 HC1 solution

Depth Ca Mg K Na Al Sum
(cm) Solution phase
mmole (+) L
1.0 0.048 0.013 0.122 0.141 0.008 0.332
2.0 0.055 0.021 0.152 0.154 0.013 0.395
3.0 0.115 0.033 0.247 0.313 0.004 0.712
4.0 0.122 0.033 0.192 0.248 0.004 0.599
5.0 0.085 0.029 0.136 0.239 0.010 0.499
6.0 0.078 0.026 0.125 0.231 0.009 0.469
7.0 0.112 0.037 0.178 0.313 0.006 0.646
8.0 0.092 0.029 0.162 0.267 0.007 0.557
9.0 0.094 0.037 0.290 0.278 0.009 0.708
10.0 0.096 0.040 0.270 0.240 0.007 0.652

Depth Ca Mg K Na Al Sum
(cm) Exchange phase
Smmole (+) Kg" soil


1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0


0.661
3.220
5.610
7.190
6.540
7.420
7.570
7.580
6.560
6.860


0.082
0.514
1.050
1.280
1.360
1.560
1.540
1.540
1.440
1.520


0.243
0.262
0.313
0.320
0.301
0.371
0.390
0.454
0.460
0.576


0.217
0.207
0.207
0.196
0.196
0.174
0.207
0.207
0.217
0.196


12.520
9.785
8.084
6.738
6.850
6.282
6.516
6.505
5.871
5.560


13.723
13.988
15.264
15.724
15.247
15.807
16.223
16.286
14.548
14.712















































































tI oe B>/ (+) atowEu


I c,


0
0 i-


a-4


0 U
00 *



C4.






00



I8 r-4
-)0
-D -ri
40
MC L
-H .0


m O


CE0









0 0

(o 3
r-4 X1-n







C14
*0)
o .




'OC f
0) (


uo; eG;ueouoo








which an analytical solution was available, by comparing the

model simulation with the analytical solution. Assuming that

the retardation factor R = (1+(o/e)F) = 1 and that the

selectivity coefficient Ks = 0, the appropriate transport

equation for this problem is:


aC 32C aC
R = D 2 -v ---- 2-56]
at ax ax

where the following initial and boundary conditions were

assumed:

C(x,0) = C.

9C
-D --- + vC = vC 0 5 t 5 to
ax x=0 x=0

ac
and ---- =0
ax x=L [2-57]

-1
C. = 0.000001 mmole(+) L1
1
and C0 = 0.01 mmole(+) L1 were used as chosen values.

Brenner (1962) has given an analytical solution to this

boundary-value problem as:


C C 1 n t n
= -- erfc( -- ) + exp( )
CO -Ci 2 u 2 u


1 0 Rx+vt
+- ( 1+a + -- ) exp(a) erfc(- )
2 R u


P a 0 R
+ T (1+--- --- + -- ) exp(p ----- 02
2 4 4R 4Dt





194


Table 4-25 Charge balance of cations for columns of
Ca-subsoil


Ca-subsoil

H Ca

Initial cations 17.900
Total input 0.271
Final # 0.050
solution phase
Final # 3.580
exchange phase
Total 0.003 14.926
output in effluent

Ca-subsoil
H+ 2+
H Ca

Initial cations 17.900
Total input 0.024
Final # 0.060
solution phase
Final # 3.500
exchange phase
Total 0.003 14.167
output in effluent

#: undetermined


pH 3.9 (mmole(+))

Mg2 K

0.033 0.066

0.006 0.020

0.011 0.088

0.029 0.026,


pH 4.9 (mmole(+))

Mg2+ K

0.033 0.066

0.006 0.016

0.016 0.061

0.020 0.045


Na

0.000

0.126

0.086

0.023


Na+

0.000

0.162

0.100

0.038


Al3+
Al+

0.066

0.058

0.321

0.012


Al3+

0.066

0.026

0.124

0.071





130


Concentrations of exchangeable cations in all of the treated

soils were determined by extraction with 1 M neutral NH4OAc

(Thomas, 1982).

Soil Column Preparation and Procedure for Displacing HC1
Solution Through Columns

Each soil column consisted of a stack of 1-cm thick

acrylic plastic (plexiglass) rings with 3.72 x 10-2 m

inside-diameter. Water-proof, acid-resistant electrical

tape was tightly wrapped around the outside of the rings to

insure that lateral leakage of water did not occur during

runs. Each column had dimensions of 0.2 m length and 0.0375

m inside diameter, giving a total internal volume of 2.21 x

10 m3. The soil was held in the columns by a fine nylon

mesh and a piece of Whatman no.42 filter paper placed over a

thin plastic disc with small holes distributed over its

surface in each of the inflow and outflow endplates. A

check for water leaks was made prior to packing the soil

into a given column. Each column was then placed in a

vertical position and sequentially packed by slowly adding

incremental quantities of soil and tapping the side of the

column until a desired soil bulk density was obtained.

After packing, the entire soil column was mounted between

two wooden boards for support and fastened with four

threaded steel rods to hold the soil and rings in place.

Duplicate soil columns were constructed for each chemically

pretreated and mixed soil. Columns included Ca-saturated

topsoil, Mg-saturated topsoil, K-saturated topsoil,




219


Rible, J,M., and L.E. Davis. 1955. Ion exchange in soil
columns. Soil Sci. 79:41-47.

Rubin, J., and R.V. James. 1973. Dispersion-affected
transport of reacting solutes in saturated porous media:
Galerkin methods applied to equilibrium-controlled
exchange in undirectional steady water flow. Water Resour.
Res. 9:1332-1356.

Sardin, M., R. Krebs, and D. Schweich. 1986. Transient
mass-transport in the presence of non-linear
physico-chemical interaction law: Progressive modeling and
appropriate experimental procedures. Geoderma 38:115-130.

Sposito, G. 1981. The Thermodynamics of Soil Solutions.
Oxford University Press, New York.

Thomas, G.W. 1982. Exchangeable cations. p.159-165. In
Methods of Soil Analysis. Part 2. A.L. Page, R.H. Miller,
and D.R. Keeney (eds.). American Society of Agronomy,
Madison, WI.

Thomas, G.W., and W.L. Hargrove. 1984. The chemistry of soil
acidity. p.3-57. In Soil Acidity and Liming. F. Adams
(ed.). American Society of Agronomy, Madison, WI.

Thomas, H.C. 1944. Heterogeneous ion exchange in a flow
system. J. Am. Chem. Soc. 66:1664-1666.

Ulrich, B., R. Mayer, and P.K. Khanna. 1980. Chemical
changes due to acid precipitation in a loess-derived soil
in central Europe. Soil Sci. 130:193-199.

Valocchi, A.J., R.L. Street, and P.V. Rolberts. 1981.
Transport of ion-exchanging solutes in groundwater:
Chromatographic theory and field simulation. Water Resour.
Res. 17:1517-1527.

van Beek, G.G.E.M., and G.H. Bolt. 1973. The relationship
between the composition of the exchange complex and the
composition of the soil solution. p.379-388. In Physical
Aspects of Soil Water and Salts in Ecosystems. A. Hadas,
D. Swartzendruber, P.E. Rijtema, M. Fuchs and B. Yaron
(eds.). Springer-Verlag, New York.

van Genuchten, M.Th. 1981. Non-equilibrium transport
parameters from miscible displacement experiments. USDA
Res. Report No. 119. U.S. Salinity Laboratory. Riverside,
CA

van Genuchten, M.Th., and W.J. Alves. 1982. Analytical
solution of the one-dimensional convective-dispersive





151


. U 5


. .S I


'o






C)
0)









Ul
4O J4


0


-o
Jw






E 0 Q..
u i










r-I



> 0
O H
0-q











La



M -Hq
*0c






rl
EZ,


o)

0

(t/ (+) StoWW)


T T
a a

I n


. t 1


1 < (

o C

uoTyejuqueouo


_


I .


CP


9r~




125


exchange sites in the soil result partially from broken

edges of the silica-alumina crystalline units, which give

rise to unsatisfied electrostatic charge and which is

balanced by exchangeable cations. Broken-edge charge is

often a source of variable charge. In kaolinite minerals,

broken edges are the major source for cation-exchange

capacity but, for vermiculite, broken edges account for only

a relatively small portion of the CEC. Isomorphous

substitutions of A13+ for Si4+, and of Mg2+ for Al3+, that

occur within the crystalline lattice structure of clay

minerals result in unbalanced electrostatic charge that is

normally balanced by exchangeable cations. Such charge is

referred to as permanent (pH-independent) charge.

Exchangeable cations resulting from lattice substitution are

found mostly on cleavage surfaces of vermiculitic clay

minerals (Grim, 1968). In general, the order of replacement

of exchangeable-cation species for soil exchange sites is

Na < NH4 < K < Mg2+ < Ca2+ < Al3.

For pH-dependent charge colloids such as hydrous Al and

Fe oxides, organic matter and some clay minerals, both

active and potential acidic groups occur along with basic

groups. Active acid groups are directly equilibrated with

soil-solution cations, whereas potential acid groups are

activated only by an increase in the soil pH. Basic groups

can acquire a positive charge by uptake of H+ ions from the

soil solution. The positive charge is balanced by anion

species such as Cl~, NO3 SO4 2- H2PO4 etc. Thus, such







vermiculites and interstratified forms of these minerals

typically have constant surface charge regardless of

solution concentration or pH (Gast, 1977). Colloids with

constant surface potential and variable surface charge are

in turn commonly found in soils which are extensively

weathered and dominated by sesquioxides of Fe and Al or 1:1

kaolinitic minerals. The surface charge density for

colloids of this type varies with pH and salt concentration

(Keng and Uehera, 1974). Soil organic matter also has

pH-dependent charge, arising from dissociation of phenolic

OH and carboxylic groups.

Cation-Exchange Equilibria

The exchange reaction between an exchange-phase cation

1 with valence z1 and a cation 2 of valence z2 in solution

can be described by the equation


z2 C1 + z1 C2 = z1 C2 + z2 C1 [3-1]

For a reversible reaction such as equation (3-1) at

chemical equilibrium, one type of exchange selectivity

coefficient (Ks) can be expressed as



C 2
c1 2
Ks (- [3-2]

C1 C2





118


Table 3-12 Charge balance of cations for columns of subsoil

Subsoil pH 3.9 (mmole(+))
+ 2+ 2+ + + 3+
H+ Ca Mg K Na Al3

Initial cations 1.332 0.331 0.121 0.424 1.060
Total input 0.198
Final # 4 850 1.610 0.010 0.013 4.160
solution phase (*10 )
Final # 0.896 0.180 0.056 0.031 1.131
exchange phase
Total 0.006 0.230 0.090 0.040 0.030 0.001
output in effluent

Subsoil pH 4.9 (mmole(+))

H Ca2 Mg K Na Al

Initial cations 1.332 0.331 0.121 0.424 1.060
Total input 0.017
Final # 43730 1.950 4.150 1.950 1.080
solution phase (*10 )
Final # 0.670 0.200 0.049 0.024 0.933
exchange phase
Total 0.006 0.170 0.061 0.034 0.035 0.001
output in effluent

#: undetermined





135


Table 4-3 Soil parameters for chemically pretreated subsoil
columns after leaching with HC1 solutions

Parameter Ca-subsoil Ca-subsoil Mg-subsoil

Input solution pH 3.9 4.9 3.9
Bulk density (MG M ) 1.47 1.47 1.41
Volumetric water 0.42 0.42 0.44
content (M M )
Dispersion 2 -1.85*10 1.85*10- 1.85*10-
coefficient (M1 h~)
-1
Pore velocity (M h ) 0.023 0.021 0.024
Pore volume (L) 0.092 0.093 0.098
Column length (M) 0.20 0.20 0.20
Input concentration i0.125 0.0125 0.125
of H (mmole4+) L )
Total amount of H 0 271 0.024 0.271
applied (mmole(+) L )
Total no. of pore 23 23 21
volumes collected

Parameter Mg-subsoil K-subsoil K-subsoil

Input solution pH 4.9 3.9 4.9
-3
Bulk density (MG M ) 1.42 1.39 1.37
Volumetric watgr
content (M M ) 0.44 0.47 0.47
Dispersion 2 -1 -4
coefficient (M h J 1.85*10 1.85*10 1.85*10
Pore velocity (M h ) 0.0248 0.0232 0.0228
Pore volume (L) 0.0977 0.1041 0.1034
Column length (M) 0.20 0.20 0.20
Input concentration
of H mmole(+) L 0.0125 0.125 0.0125
Total amount of -
H applied mmole(+) L 0.024 0.271 0.026
Total no. of pore
volumes collected 21.1 19.3 18.3








ac. o ac a2C Bac
+ = D 2 v i=1,2,...n [2-1]
at e at ax2 ax

where C.(x,t) (mmole(+) L ) is the aqueous-phase
1-
concentration of species i, C. (mmole(+) Kg ) is the
2 -1
adsorbed-phase (exchange) concentration, D (m2 h 1) is the

dispersion coefficient, v (m h-1 ) is the average pore-water

velocity, a (Mg m -3) is the dry-soil bulk density, e

(m m ) is the volumetric water content, x (m) is distance

(downward) in the soil, and t (h) is time. The second term

on the left-hand side of equation [2-11 describes the time

rate of change of the exchange-phase concentration for ion

species i. If n cation species are considered, n equations

having the form of equation [2-1] must be solved

simultaneously subject to the following initial and boundary

conditions:


t = 0 and x 0 C. = C.
1 ib c
t 0 and x = 0 vCif = D + v C

ac. [2-2]
t 2 0 and x = L =0 [2-2
ax
-1
where Cib(mmole(+) L ) is the initial concentration of each

species i in the porous medium and Cif is the input solution

concentration for ion species i. Before solving equation

[2-1] subject to auxiliary equations [2-2], the time rate of

change of the adsorbed concentration (C.) in equation [2-1]

with respect to C. should be obtained as a functional




211


mmole(+), respectively, from Mg-topsoil; 0.012 and 0.071

mmole(+), respectively, from Ca-subsoil; 0.062 and 0.035

mmole(+), respectively, from Mg-subsoil; 0.015 and 0.010

mmole(+), respectively, from K-subsoil; 1.372 and 0.621

mmole(+) from mixed topsoil; and 0.017 and 0.020

mmole(+),respectively, from mixed subsoil. Therefore, the

total leaching of basic cations in the effluent from the

soil columns that received ten-fold different H+

concentrations differed by less than two-fold, indicating

that, in addition to ion exchange, chemical reactions such

as acid dissolution of soil minerals were important during

the leaching process. Higher concentrations of Al3+ were

observed in column effluent from nontreated topsoil than

from nontreated subsoil, but the concentrations of Al were

very low for all chemically pretreated and mixed subsoil

columns.

Distributions of cation concentrations on the exchange

sites revealed that, during displacement of pH 3.9 acid

solutions through the soil columns, base saturation

decreased from 52 to 11% for nontreated topsoil; from 68 to

29% for nontreated subsoil; from 99 to 27% for K-subsoil;

from 99 to 10% for Mg-subsoil; from 99 to 21% for

Ca-subsoil; from 85 to 79% for Ca-topsoil; from 76 to 15%

for K-topsoil; from 96 to 24% for Mg-topsoil; from 99 to 15%

for mixed subsoil, and from 86 to 38% for mixed topsoil.

During the period of pH 4.9 acid solution application, base

saturation decreased from 52 to 12 % for nontreated topsoil;





162




cu



i
O o
0 0



-HS
0

0O a
S0 0 o
a a -
4J 1 P

S- C

0O -r-

rO o
+ 0 ) e0


0 r--

cr > k
as -H


o at








0
(IE COD









V 9 0
-/ > c O




ao o

0 cc,-







m (U rt0 0
(-1-) 9 TO Oe u eouoo




189


Table 4-21 Concentrations of cations in solution and
exchange phases for mixed-cation topsoil
after leaching with pH 4.9 HC1 solution


Depth Ca Mg K
(cm) Solution
mmole(+)


1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
11.0
12.0
13.0
14.0
15.0
16.0
17.0
18.0
19.0
20.0


0.413
0.382
0.497
0.491
0.281
0.272
0.311
0.252
0.217
0.254
0.245
0.232
0.249
0.202
0.309
0.225
0.263
0.269
0.344
0.326


0.173
0.111
0.459
0.326
0.420
0.485
0.474
0.403
0.276
0.378
0.119
0.218
0.385
0.292
0.601
0.257
0.449
0.548
0.563
0.360


0.138
0.137
0.305
0.295
0.325
0.353
0.427
0.321
0.257
0.257
0.283
0.217
0.272
0.223
0.389
0.315
0.364
0.413
0.328
0.431


Na Al Sum
phase
L


0.170
0.096
0.136
0.146
0.073
0.067
0.162
0.089
0.150
0.074
0.122
0.080
0.055
0.089
0.083
0.123
0.086
0.112
0.112
0.271


Depth Ca Mg K Na
(cm) Exchange phase
mmole (+) Kg
2.0 5.140 1.760 0.647 0.217
3.0 6.040 1.910 0.729 0.200
6.0 5.190 1.790 0.998 0.209
8.0 5.990 1.970 1.110 0.200
10.0 5.840 1.790 1.040 0.191
12.0 5.240 1.810 1.030 0.235
13.0 4.740 1.720 0.941 0.130
16.0 5.040 1.710 1.120 0.226
18.0 4.690 1.780 1.190 0.209
19.0 6.190 1.950 1.220 0.217


11.300
0.600
41.800
20.800
30.500
32.600
38.500
31.400
17.300
3.270
1.350
10.200
33.100
29.800
64.400
16.300
33.100
66.000
69.300
18.900


12.194
1.326
43.197
22.058
31.599
33.777
39.874
32.465
18.200
4.233
2.119
10.947
34.061
30.606
65.782
17.220
34.262
67.342
70.647
20.288


Al Sum

soil
3.847 11.611
2.913 11.792
2.780 10.967
2.769 12.039
2.680 11.541
2.713 11.028
2.646 10.177
2.402 10.498
2.858 10.727
2.558 12.135




121


for both soils and were highly dependent upon the cation

concentrations in solution and on the exchange phase as well

as upon cation valence and upon the pH of the applied HC1

solution.

Greater quantities of basic cations were leached from

soil columns that received pH 3.9 solution than of pH 4.9.

Specifically, the total losses of divalent Ca 2+, and Mg2+

ions from the topsoil were 0.32 and 0.23 mmole(+),

respectively, during application of pH 3.9 and 4.9 HC1

solutions. For the subsoil these losses were only 0.12 and

0.08 mmole(+) for pH 3.9 and 4.9 HC1 treatments,

respectively. Application of HC1 solution definitely

accelerated leaching losses of cation nutrients from the

Cecil soil. Charge-balance errors ranged from -10 to -33%

and were likely due to undetermined H in solution and on

the exchange phase for the soil, as well as to negatively-

charged surfaces of Fe oxides, experimental error, soil

chemical reactions that act as sinks for applied H +, and

acid dissolution that provides a source for Al to enter

into exchange reactions though the Column of Cecil soil

received solution with a ten-fold difference in HC1

concentration, the total quantities of basic cations removed

differed by less than two-fold. Therefore, leaching by the

HC1 solution not only involved cation exchange between

monovalent, divalent, and trivalent ions, but also

apparently involved the dissolution of gibbsite and other

Al-bearing compounds (Sardin et al., 1986).







increments of air-dry soil until the total quantity of soil

required was obtained, the sides of the column being gently

tapped during packing to provide the desired bulk density.

The whole column was mounted vertically between two wooden

support boards and fastened with four threaded steeL,: rods

to give mechanical support. After the end of the

experiment, the pore volume of each column was obtained from

the mass difference between wetted soil columns and oven-dry

soil columns, with a correction being made for the amount of

solution held inside the endplates. Solutions of HC1 were

displaced through columns of Cecil topsoil and subsoil with

an average Darcy velocity of 1.06 ( 2%) cm h-1. Displacing

solutions were prepared from a HC1 stock solution with pH

was adjusted to either 3.9 or 4.9. A Rainin model Rabbit

peristaltic pump was calibrated and used to deliver solution

to the bottom of each soil column. The soil column was

initially air-dry, so transient flow occurred when the

experiments were started. Eventually (after displacement of

1 to 2 pore volumes of effluent) steady-state water flow was

maintained for each column. Effluent from the top of the

column was collected in equal increments of 11.05 ( 2%) ml,

using a fraction collector. These samples were stored in

refrigerator for later analysis.

Dissection of Soil Columns, Extraction, and Chemical
Analysis

After flow had been terminated, columns were

maintained in a vertical position overnight to ensure that




127


saturation and increases the ratio 6M/8H. Exchangeable

cation nutrients such as Ca 2+, Mg2+ and K are easily

replaced and lost by leaching. Trivalent Al3+ ions,

however, are much more resistant to removal by leaching,

with very concentrated salt solutions being required to

remove them from the exchange phase of the soil.

With continuous input of acid solution to soil, H ions

have been postulated to successively replace cations from

cation-saturated soil in two separate steps (Wiklander and

Andersson, 1972); this can be represented as:

MA + H+ -> HA + M+ [4-1]

where M is a given cation species (K Ca2+, Mg 2+), A is the

variable charge of the soil, HA is a weak acid group in the

soil and MA represents the soil saturated with cation M.

Due to the high bonding energy of H in HA, which produces a

high 6M/8H ratio, the exchange will be practically complete

before the beginning of the next step of cation desorption

MP + H -> HP M+ [4-21

where P is the permanent charge, MP is from isomorphous

substitution, and HP behaves as a strong acid. This means

that H is weakly bound and has a low replacing power for

ion species M. Natural occurrence of large quantities of

Al as an exchangeable ion in most acid soils results in a

decrease of the replacing efficiency of H+ for cation

species M. With continuous application of H ions in

solution, H+ and Al concentrations will increase in the







3 0 v 2
-[2p-a + -- --- + ( --3- ) ] exp(p)
2 R 2D

R(2L-x)+vt
erfc( ) [2-58]
u

where

0 = (2L-x+vt/R), u = 2(DRt)1/2

T = (4v2t/TtDR) 1/2 = v2t/D,

and a = vx/D, p = vL/D, n = Rx vt

The numerical solution was obtained using the Cecil

topsoil parameters presented in Table 2-1. The result is

shown in Fig. 2-1 for the case Ks = 0 and R = 1.

Nonpreferential ion exchange with selectivity coefficient

K = 1 and R = (1+(o ET)/(e CT)) was also performed and is

given in Fig. 2-2. Excellent agreement between numerical

and analytical solutions demonstrates that the computer

program functioned as designed for the case of transport of

inert solutes as well as for the case of non-preferentialiy

reactive cations.

Exchange Isotherm Curves for Columns of Cecil Soil

The Mg2+ exchange isotherms for Mg2+ -> Ca2+ binary

exchange on Cecil topsoil and subsoil are shown in Figs. 2-3

and 2-4, respectively. The magnitude of KMg-Ca indicates
2+
the relative preference of exchange sites for the Ca2+ and

Mg2+ species. The shape of experimental isotherms for the

Cecil soil was concave relative to the diagonal (for

normalized plots of equivalent fractions of Mg2+ in the

exchange phase plotted versus equivalent fractions of Mg2+




106


of subsoil after leaching with pH 3.9 and 4.9 HC1 solutions,

respectively. Concentrations of cations in the solution

phase were of the order Na+ > K Ca2+ > Mg2+ > Al3 and
3+ 2+ 2+ +
for the exchange phase of the order Al3+ > Ca2+ > Mg > Na

= K The orders of cation concentration in the solution

phase for the topsoil and subsoil were thus dissimilar, but

were similar for the respective exchange phases.

A comparison of exchangeable basic cations before and

after leaching were made for the topsoil and subsoil and the

two different treatments. Comparison of Table 3-2 with

Tables 3-3, 3-4, 3-5 and 3-6 revealed leaching losses of

basic cations and increased concentrations of Al3+ for both

soils during application of the HC1 solutions. Leaching

losses of basic cations and Al 3+, and final amounts of

exchangeable Al 3+, were not strictly proportional to the

input H concentration for either the topsoil or subsoil

columns, though greater losses of basic cations were

observed for the pH 3.9 treatment. The pH 3.9 treatment

resulted in slightly higher magnitudes of Al than did the

pH 4.9 treatment for in the topsoil and subsoil columns.

Estimated Selectivity Coefficients for Ion Pairs

Based on information given in Tables 3-3, 3-4, 3-5 and

3-6, binary selectivity coefficients as defined by equation

(3-2) were approximated for each pair of ion species. These

values are only approximations, since experimental

conditions involved multiple species of ions rather than a

simple binary system. Estimates of the selectivity






186


Table 4-18 Concentrations of cations in solution and
exchange phases for mixed-cation subsoil
after leaching with pH 3.9 HC1 solution


Depth
(cm)

1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
11.0
12.0
13.0
14.0
15.0
16.0
17.0
18.0
19.0
20.0


Ca Mg K Na Al Sum
-Solution phase
mmole (+) L
0.050 0.016 0.142 0.178 0.017 0.403
0.082 0.037 0.151 0.165 0.011 0.446
0.085 0.062 0.205 0.167 0.011 0.530
0.062 0.058 0.129 0.100 0.011 0.360
0.040 0.021 0.288 0.139 0.000 0.487
0.032 0.016 0.260 0.115 0.006 0.429
0.015 0.025 0.285 0.133 0.013 0.471
0.027 0.021 0.326 0.148 0.022 0.544
0.060 0.037 0.365 0.202 0.006 0.669
0.000 0.000 2.530 0.154 0.000 2.684
0.078 0.035 0.523 0.334 0.060 1.029
0.057 0.021 0.386 0.243 0.050 0.757
0.035 0.016 0.362 0.189 0.000 0.602
0.015 0.012 0.365 0.165 0.000 0.557
0.042 0.020 0.375 0.219 0.033 0.689
0.023 0.005 0.324 0.236 0.014 0.602
0.012 0.015 0.301 0.130 0.007 0.464
0.027 0.010 0.335 0.157 0.027 0.555
0.033 0.015 0.275 0.193 0.020 0.536
0.045 0.020 0.410 0.237 0.060 0.772


Depth Ca


K Na


Al Sum


(cm) Exchange phase
mmole(+) Kg soil
1.0 2.990 0.716 0.491 0.222 8.117 12.536
2.0 4.640 3.040 1.010 0.183 0.912 9.785
3.0 5.240 3.130 1.370 0.191 1.007 10.938
5.0 4.940 2.960 3.500 0.196 0.845 12.441
7.0 4.690 2.710 4.450 0.174 0.790 12.814
9.0 4.790 2.630 5.650 0.191 0.899 14.160
12.0 4.790 2.140 6.270 0.200 0.723 14.123
15.0 4.140 2.330 6.040 0.226 0.801 13.537
18.0 4.890 2.410 6.500 0.217 0.634 14.651
20.0 4.240 2.350 6.340 0.252 0.656 13.838





163


GT01
(TJo,


n I

o


(t/ (+) 9TOWW)


.c


'I

0 04
>i *
4J






-4-) -






Em






00
p *1 3


















0
S -4
Oa)
So U
0 0



Sr-
M ^
0) ^
> ^! a,












Er


uo qej;ueouoo


oqO





140


0
o


X 0

2 0
0


0


to
0
0 0
O*
O




O 2
02







0
02











0

20


0)0 1

m U

II
an



ox


. I 1 1 I I II I I I I I .1 2 I 1


. .A 1 *I


4>
.ri



0 3


ui*Va
0 10o

4 -4







0)
S ra
>I 0)




H4
4)

0 C 0.

H -4 -H

- w
0 1









$4
L 4
0 9




Ow
C




9Q I
ino .








*Hl
&4r


Suan 4iT.3 4o Hd


~


111





145


(I/ (+) aT IOWW)


uoT euausouo3


ra



40
u
a)



C




0 0
Sri
0)


04-
0 r--



0- 4




> i
o.c



44
> v+
I a




4-)
0 -l
0El *
0 > -0









. I.
U 0)
E '.


O o
> .co

0r O4
Lcm


Mr-
+ ^
<*< IM rl




> I-i







The activity coefficients rA and rB for the solution

phase may be calculated from the extended Debye-Huckel

equation as functions of ionic strength (I) as



-0.5085 z2 VI
r. = exp [2-91
1 1+0.328 a. .I


where z. is the valence of cation i (i = A, or i = B), a. is

the ion-size parameter for ion A or B, and I = 0.5 E(C.z. ).

Equation (2-3) then can be rewitten as

zz 2
[B] (rA MA)
K = [2-10]
S2 2
[A] (rB Mg)

When an exchange reaction is reversible, it is useful

to measure the Vanselow selectivity coefficient (Kv) as a

function of the exchange composition. Any variation in Kv

is related directly to that of the activity coefficient of

the components of the exchange phase. Kv is defined by

Sposito (1981) as

z2
Kv = A K [2-11]
"1

In equation [2-11] the exchange-phase activity

coefficients are generally not known for soils. However,

the thermodynamic formulations of Gaines and Thomas (1953)

can be used for calculation of the equilibrium constant and

exchange-phase activity coefficients, yielding









I -- I r *
0 04-
0o .5
HO H



0 Ua)

00
OH
0 (O -4
0r-i4

9 a) 4J
00

o 0 *

0 40 at


(a 0 )
4 kD
O HQ 0 (0







o 4 .0U
0 m EH




V C (O
o aH Q)








00
0-I .u 4


x .j (




0 m0


O <0*







-- -- I -I -


(t/ (+) 8TOww)u


U0T ;ej3U9oU0D




193


Table 4-24 Charge balance
Mg-topsoil

Mg-topsoil

H CaC2+

Initial cations 0.326
Total input 0.277
Final # 0.030
solution phase
Final # 0.756
exchange phase
Total 0.001 0.033
output in effluent

Mg-topsoil

H Ca2+C

Initial cations 0.326
Total input 0.025
Final # 0.033
solution phase
Final # 0.656
exchange phase
Total 0.001 0.035
output in effluent

# : undetermined


of cations for columns of


pH 3.9 (nmmole(+))

Mg2+ K

4.241 0.072

0.043 0.016

2.836 0.776

9.840 0.022


pH 4.9 (mmole(+))

Mg2+ K

4.241 0.072

0.044 0.020

0.320 0.082

4.128 0.040


Na

0.109

0.145

0.141

0.021


Na

0.109

0.146

0.141

0.017


Al3+

0.181

0.330

0.491

0.739


Al3+

0.181

1.341

0.330

0.849








once the Cl concentration of the effluent had reached that

of the input concentration. The Cl dispersion-coefficient

values were obtained by using the resulting Cl breakthrough

curves and a least-squares fitting procedure (van Genuchten,

1981) was used to fit an analytical solution (Brenner, 1962;

van Genuchten and Alves, 1982) to experimental breakthrough

data.

Cation-Exchange Isotherms from Soil Columns
2+ 2+
The cation exchange isotherm for Mg -> Ca exchange

was obtained from columns of Cecil topsoil and subsoil,

respectively. Concentrations of Ca2+ and Mg2+ in solution

and exchange phases as obtained by the centrifuge method

were used to determine the exchange isotherms. Tables 2-2

and 2-3 present concentrations of cations in the two phases

for Cecil topsoil and subsoil, respectively. CEC values for

each specific depth were obtained by taking the sum of the

basic and acidic cations. An average CEC value for the

whole column was used in this calculation. Total solution

concentrations of cations were assumed to have similar

magnitudes as those of the input solution. Concentrations

of Ca2+ and Mg2+ were determined using an atomic absorption

spectrophotometer (Perkin Elmer model 460).

Results and Discussion

Soil Properties

Soil parameters used in the column experiments are

presented in Table 2-1. General soil properties, such as




171


Table 4-8 Concentrations of cations in solution and
exchange phases for Ca-subsoil after
leaching with pH 3.9 HC1 solution

Depth Ca Mg K Na Al Sum
(cm) Solution phase
mmole (+) L


1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
11.0
12.0
13.0
14.0
15.0
16.0
17.0
18.0
19.0
20.0


0.385
0.385
0.417
0.349
0.313
0.531
0.442
0.858
0.359
0.409
0.339
0.404
0.344
0.806
0.489
0.614
0.430
1.190
0.851
0.873


0.053
0.043
0.063
0.056
0.043
0.045
0.066
0.095
0.045
0.058
0.049
0.037
0.054
0.078
0.058
0.058
0.067
0.099
0.103
0.115


0.161
0.210
0.214
0.191
0.214
0.137
0.152
0.243
0.320
0.165
0.107
0.334
0.265
0.148
0.141
0.171
0.125
0.637
0.246
0.217


0.918
1.240
1.300
0.835
1.260
1.580
1.120
1.960
0.978
1.420
0.861
0.939
0.874
1.360
1.230
1.280
1.330
3.150
1.430
2.280


0.013
0.391
0.018
0.374
0.125
0.172
1.230
0.022
0.578
0.856
5.130
0.450
1.630
0.045
0.178
0.767
0.465
0.027
0.061
0.022


1.530
2.269
2.011
1.805
1.955
2.465
3.010
3.178
2.280
2.908
6.486
2.164
3.167
2.437
2.096
2.890
2.417
5.102
2.691
3.507


Depth Ca


K Na


Al Sum


(cm) Exchange phas -
mmole (+) Kg soil
1.0 2.820 0.021 0.185 0.239 6.961 10.226
3.0 10.200 0.021 0.205 0.217 0.267 10.910
5.0 11.900 0.041 0.205 0.239 0.334 12.719
7.0 12.800 0.062 0.301 0.359 0.300 13.822
9.0 11.700 0.021 0.198 0.228 0.323 12.470
11.0 11.600 0.021 0.230 0.207 0.278 12.336
14.0 11.400 0.021 0.205 0.196 0.300 12.122
16.0 12.100 0.021 0.269 0.228 0.467 13.085
18.0 12.500 0.082 0.646 0.348 0.256 13.832
20.0 11.700 0.021 0.217 0.217 0.267 12.422







where n denotes the number of nodal points.

Substituting the trial function u into the linear

differential operator L for the exact solution u will result

in a residual R(x,t) as defined by



R(x,t) = L(u) = L ( [ 8 G (t) N.(t) ] ) 4 0 [2-26]
j=1

This residual is forced to zero in an average sense

over the entire domain D through the selection of

undetermined coefficients G (t). The G (t) values are

calculated by setting the weighted integral of the residual

to zero. In the Galerkin finite-element, weighted-residual

method, the shape functions are used as the weighting

functions and the resulting integral form is:

D R(x,t) Ni(x) dx = 0 [2-27]



Interpolation Function

The use of cubic-spline functions as interpolation

functions in the finite-element method has been successfully

applied in solving boundary-layer flow problems (Hsu, 1976;

Hsu and Liakopoulos, 1981). In the present study, the

unknown function C(x,t) at a given depth in a soil column

(profile) is represented by classical cubic splines. Since

classical cubic-spline interpolation functions provide

expression of C(x,t) as continuous functions with continuous

first and second derivatives, classical cubic-spline




100


The first few samples (within 0.5 pore volume) of

column effluent had low pH and high concentrations of

cations in each case. This phenomenon can be explained as a

salt effect (Wiklander, 1975; Reuss and Johnson, 1985).

Topsoil and subsoil columns initially contained 0.6 and 1.0%

by volume of water before the leaching experiment was

initiated, respectively. As constant-flux infiltration

progressed with time and as the wetting front advanced

through the soil, soluble salts were accumulated in the

moving front. Resulting high concentrations of cations in

the wetting front replaced part of the exchangeable H+ and

Al which decreased the pH of the soil solution. This

phenomenon is called a "salt effect". With continuous

application of dilute HC1, initially displaced H+ and A13+

were subsequently leached resulting in an abrupt rise of

effluent pH once more. Another cause for the higher pH of

column effluent than of input HC1 may have been removal of

the solution from contact with soil matrix. As the effluent

flowed from soil columns and was exposed to the atmosphere,

the CO2 partial pressure of the solution would decline and

the pH of the effluent would increase somewhat. During the

elution of 28 pore volumes, effluent pH was always observed

to be higher than pH of the input solution. As HC1

infiltrated the soil, H+ ions in the acid input solution

underwent cation exchange with basic cations located on soil

exchange sites. Larger amounts of displaced Al were found

from both topsoil and subsoil columns receiving pH 3.9 HC1




102


Table 3-3 Concentrations of cations in solution and
exchange phases for topsoil after leaching
with pH 3.9 HC1 solution

Depth Ca Mg K Na Al Sum
(cm) Solution phase
mmole(+) L
1.0 0.062 0.016 0.146 0.174 0.040 0.439
2.0 0.030 0.016 0.176 0.191 0.088 0.501
3.0 0.023 0.016 0.116 0.146 0.155 0.456
4.0 0.042 0.021 0.188 0.200 0.127 0.578
5.0 0.022 0.016 0.102 0.113 0.110 0.363
6.0 0.032 0.023 0.132 0.146 0.400 0.733
7.0 0.042 0.045 0.183 0.193 0.614 1.078
8.0 0.034 0.069 0.172 0.176 1.880 2.331
9.0 0.040 0.118 0.223 0.192 1.720 2.293
10.0 0.080 0.112 0.377 0.310 1.440 2.319

Depth Ca Mg K Na Al Sum
(cm) Exchange phae
mmole(+) Kg soil
1.0 0.262 0.021 0.122 0.217 7.850 8.472
2.0 0.412 0.041 0.166 0.239 9.096 9.954
3.0 0.374 0.021 0.141 0.217 16.857 17.610
4.0 0.661 0.041 0.179 0.217 8.451 9.549
5.0 0.786 0.041 0.147 0.065 9.040 10.079
6.0 0.936 0.021 0.122 0.152 8.084 9.314
7.0 1.350 0.062 0.160 0.337 7.639 9.548
8.0 1.530 0.082 0.160 0.326 7.405 9.504
9.0 1.200 0.144 0.166 0.348 6.672 8.530
10.0 1.140 0.288 0.198 0.391 6.916 8.933





172


Table 4-9 Concentratons of cations in solution and
exchange phases for Ca-subsoil after
leaching with pH 4.9 HC1 solution


Depth Ca Mg K Na
(cm) Solution phase
---- mmole (+) L


0.503 0.066
0.826 0.092
0.479 0.046
0.523 0.056
0.575 0.053
0.659 0.066
0.665 0.069
0.555 0.056
0.645 0.072
0.659 0.053
0.681 0.066
1.220 0.094
0.840 0.072
0.831 0.082
0.495 0.049
0.575 0.049
0.816 0.095
0.551 0.076
0.591 0.072
0.399 0.059


0.178
0.257
0.137
0.192
0.192
0.235
0.135
0.108
0.143
0.171
0.221
0.327
0.193
0.132
0.085
0.151
0.204
0.171
0.153
0.099


Al Sum


1.910 0.129
2.440 0.018
1.140 0.765
1.310 0.369
1.610 0.080
2.310 0.167
1.660 0.013
1.250 0.129
1.660 0.125
1.380 0.156
1.650 0.040
3.900 0.040
1.940 0.031
2.200 0.067
1.380 0.018
1.230 0.031
2.030 0.027
1.570 0.142
1.460 0.169
1.060 3.180


2.786
3.633
2.567
2.450
2.510
3.437
2.542
2.098
2.645
2.419
2.658
5.581
3.077
3.312
2.027
2.036
3.172
2.510
2.445
4.798


K Na


Al Sum


Exchange phas-
mmole (+) Kg soil
8.980 0.123 0.205 0.261 0.979 10.548
L0.400 0.041 0.173 0.304 0.289 11.207
L1.600 0.041 0.256 0.283 0.300 12.480
L0.500 0.041 0.198 0.326 0.289 11.354
L1.000 0.041 0.147 0.370 0.322 11.880
.1.500 0.041 0.160 0.391 0.334 12.426
9.610 0.041 0.192 0.348 0.311 10.502
.0.500 0.021 0.179 0.174 0.311 11.185
.0.500 0.041 0.166 0.196 0.300 11.203
.1.600 0.041 0.179 0.380 0.322 12.522


1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
11.0
12.0
13.0
14.0
15.0
16.0
17.0
18.0
19.0
20.0


Ca


Depth
(cm)

1.0
3.0 1
5.0 3
7.0 1
9.0 1
11.0 1
12.0
15.0 3
17.0 1
19.0 1







given by van Genuchten and Wierenga (1976), Mansell et al.

(1986) constructed a numerical model to simulate the

leaching of cations in soil columns. The mathematical

solution was numerically approximated using a

Crank-Nicholson finite-difference technique, and cation

transport was observed to be best simulated when selectivity

coefficients were given as functions of the equivalent

fractions of specific cations in the solution phase.

The first objective of this investigation was to

develop a finite-element computer model capable of

simulating exchange and transport of multiple species of

cations in soil columns. This computer model could then be

used by the soil scientists or foresters as a basic working

tool for predicting the movement of cations in soil. A

second objective was to experimentally determine selectivity

coefficients from cation concentrations in solution phase

and on exchange sites for soil columns. A third objective

was to evaluate the model as a means to simulate binary ion

exchange and transport in columns of Cecil soil by comparing

simulated with experimental data.

Theory

Transport Equation

The general form of the differential equation used to

describe one-dimensional, convective-dispersive transport of

each cation species i in uniform porous medium or soil under

steady water-flow conditions may be written as





173


Ca2+ occur in nonexchangeable forms such as between lattices

or interlayers of interlayer-hydroxy vermiculite

(M+(Mg,Fe)3(Si,Al)4010(OH)2), where M+ denotes cation

species located between lattices or interlayers (Dixon and

Weed, 1977; Bohn et al., 1985). A long period of leaching

with acid solutions will result in weathering of the

minerals, releasing nonexchangeable cations from within the

lattices or interlayers.

Tables 4-10 and 4-11 present concentrations of cations

in solution and exchange phases for Mg-subsoil columns after

application of pH 3.9 and 4.9 HC1 solutions, respectively.

In the solution phase, concentrations of cations were of the

order Na+ > Ca2+ > K+ = Mg2+ > Al3+ but, for the exchanger

phase, concentrations of cations were Mg2+ > Al3+ > Ca 2+>

Na > K for both pH treatments. If we compare

concentrations of cations in the exchange phase in Tables

4-10 and 4-11 with values in Table 4-4 for initial

Mg-subsoil, the concentrations of Mg2+ were greatly

decreased by leaching but Al3+ concentrations were changed

only slightly, Ca2+ and Na+ concentrations were increased

and K+ was decreased. This change in cation concentrations

was found both for pH treatments 3.9 and 4.9. Usually,

cation species with the higher valence have a higher

affinity for exchange sites. Therefore, high concentrations

of divalent and trivalent species were observed in the

exchange phase compared to Na and K Cations with low

valence demonstrated lower affinity for exchange sites, such




157


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II


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0


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0


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^o

O

0


0 0
0



I I 1 1 I I 1 *AI I i X)


(D

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uoq;e1uqueouoo


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u

C.)

c
U o


0 >
3m


O TP>


N on
04
tn T
4-) 0.4
0 4-)




'H\ 0

<5
4-4 *H
a) 0




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-3 > o

wo



(U cU
L 40 M

In -4


I
S-r4


1-1-
*r
0


- *


1 e I I i


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I.





176


that the concentration of monovalent cations tended to be

higher in the solution phase. After leaching pretreated

subsoil with HC1 solution for about 20 pore volumes, the

initial cation species which had saturated the exchange

sites were observed to still be the most dominant

exchangeable cation in the soil as shown in Tables 4-6

through 4-11. The resultant CEC of the pretreated soil was

quite stable and had approximately the same quantity of

cations, but less than that for the nontreated soil, as

given in Table 4-5. Base saturation was greatly decreased

by continuous acid leaching from 90% initially to 55% after

application of acid for all cases. In the section of each

column which received acid input solution a high

concentration of exchangeable Al3+ was generally observed

and was attributed to poorly crystalline vermiculite or

hydrous oxides of Al being exposed as exchange sites.

Treated Topsoil. Tables 4-12 and 4-13 present

concentrations of cations in solution and exchange phases

for Ca-topsoil columns which received input HC1 solutions of

pH 3.9 and 4.9, respectively. Concentrations of cations in

the solution phase were in the order Na+ > Ca2+ > Mg2+ > K+

> Al3+ and for the exchange phase were Ca2+ > Al3+ > K

Na+ > Mg2+ for both treatments. If concentrations of

cations in the exchange phase for Tables 4-12 and 4-13 are

compared with values in Table 4-4 for original Ca-topsoil,

the cation concentrations for Mg 2+, K and Na tended to

increase during acid leaching but Ca and Al












0
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109


Table 3-8 Topsoil selectivity coeffients as determined
after leaching with pH 4.9 HC1 solution


K-->Ca
0.10667E-01
0.89593E-02
0.76123E-02
0.48023E-02
0.60545E-02
0.58130E-02
0.62203E-02
0.59719E-02
0.45946E-02
0.10347E-01


Na-->Ca
0.13705E-01
0.27394E-02
0.18604E-02
0.19921E-02
0.25930E-02
0.23563E-02
0.45791E-03
0.27082E-02
0.57266E-03
0.84928E-02


Na-->K
1.1335
0.55296
0.49436
0.64407
0.65443
0.63668
0.27132
0.67341
0.35304
0.90598


K-->Ca
0.70911E-01
0.73449E-01
0.78906E-01
0.59975E-01
0.53520E-01
0.48137E-01
0.18206
0.10106
0.26298E-01
0.59182E-01


Average selectivity values

0.11256 0.71042E-02 0.37478E-02 0.63197 0.75350E-01


Na-->Mg Al-->Ca Mg-->Al K-->Al Na-->Al
1 0.91103E-01 0.88631 0.38408E-02 0.11703E-02 0.17042E-02
2 0.22458E-01 0.01999 0.90790E-01 0.59979E-02 0.10141E-02
3 0.19284E-01 0.00557 0.16127 0.89009E-02 0.10754E-02
4 0.24879E-01 0.00323 0.15917 0.58600E-02 0.15657E-02
5 0.22922E-01 0.02439 0.59357E-01 0.30165E-02 0.84548E-03
6 0.19513E-01 0.00837 0.21037 0.48441E-02 0.12502E-02
7 0.13402E-01 0.00089 0.44839E-01 0.16450E-01 0.32855E-03
8 0.45829E-01 0.00121 0.17083 0.13278E-01 0.40550E-02
9 0.32777E-02 0.02230 0.23916 0.20856E-02 0.91769E-04
10 0.48577E-01 0.04832 0.11060 0.47880E-02 0.35605E-02

Average selectivity values

0.31125E-01 0.10206 0.12502 0.66392E-02 0.15491E-02


Mg-->Ca
0.15043
0.12198
0.09647
0.08007
0.11313
0.12076
0.03417
0.05909
0.17471
0.17483





14



In K = In K dCB [2-12]


An equilibrium-controlled cation exchange reaction is

defined as follows (Helfferich, 1962; Bolt, 1967):


z2 C1 + z1 C2 = z1 C2 + z2.C1 [2-13]

where 1 and 2 are exchanging species with z1 and

z2 valences, respectively. Concentrations (mmole(+) kg-1

soil) of each exchange species are given as C and

concentrations (mmole(+) L-1 ) of dissolved species are given

as C Therefore, an ion-exchange selectivity coefficient

Ks can be defined as


2 1

Ks 1 2 [2-14]
C1 C2


where


C
C = i, j = 1 or 2

c. + Cj



and C. is called the equivalent fraction of ion i on the

exchange phase.

This form is reasonably suited to describe the ion-

exchange phenomenon. The magnitude of Ks changes with the

cation concentrations in the solution phase and also with

the equivalent fractions of individual ions on the exchange







4-15 Breakthrough curves of cations in the effluent
from a mixed-cation topsoil column which received
pH 3.9 input HC1 solution ....................... 158

4-16 Breakthrough curves of cations in the effluent
from a mixed-cation subsoil column which received
pH 3.9 input HC1 solution ........................ 159

4-17 Breakthrough curves of K+ from mixed-cation topsoil
columns which received input HCl solutions with two
different pH values .............................. 161

4-18 Breakthrough curves of Ca2+ from mixed-cation topsoil
columns which received input HC1 solutions with two
different pH values ............................. 162

4-19 Breakthrough curves of K from mixed-cation subsoil
columns which received input HC1 solutions with two
different pH values .............................. 163

4-20 Breakthrough curves of Mg2+ from mixed-cation subsoil
columns which received input HC1 solutions with two
different pH values ............................ 164

4-21 Breakthrough curves of Al from mixed-cation topsoil
columns which received iAput HC1 solutions with two
different pH values ............................... 166

4-22 Breakthrough curves of Al from mixed-cation subsoil
columns which received input HC1 solutions with two
different pH values .............................. 167


xiv








respectively. The CEC for each section was obtained by

extraction with buffered 1 M NH4 OAc plus unbuffered 1 M KC1,

and then summing exchange-phase concentrations for Ca2+

Mg2+ K Na and Al3+. The CEC for the last set of

sections obtained from the soil column were used to

represent the CEC for experimental columns of subsoil and

topsoil, respectively. Column effluent pH values were below

pH 4.5 for both columns (Fig. 2-23). This acidity could be

coming from Al3+ which underwent exchange with Ca2+ and

Mg2. The pH values of the input solutions were 5.5 for

CaC12 and 5.7 for MgCl2.

Figures 2-13 and 2-14 demonstrate the dramatic model

sensitivity to the cation exchange capacity parameter.

The cation exchange capacity values used were 10.6 and 54.0

mmole(+) Kg1 soil for topsoil, and 17.0 and 70.0 mmole(+)

Kg-1 soil for subsoil, respectively. A large increase in

CEC value the simulated movement of the Mg2+

front to be retarded during miscible displacement relative

to the experimental data. This effect was greatest for the

finer-textured Cecil subsoil.

Calculated and observed data did not coincide exactly

for either the topsoil'or subsoil columns. Overall,

model-simulated curves for distributions of solution-phase

concentrations of Mg2+ with depth best described

experimental data when the smaller of the cation exchange

capacity values were used. This result was as expected,





192


Table 4-23 Charge balance of cations for columns of
K-topsoil


K-topsoil

H+ Ca 2+

Initial cations 0.219
Total input 0.348
Final # 0.036
solution phase
Final # 0.335
exchange phase
Total 0.009 0.001
output in effluent

K-topsoil pH

H+ CaC2+

Initial cations 0.217
Total input 0.029
Final # 0.026
solution phase
Final # 0.275
exchange phase
Total 0.008 0.233
outputin effluent

#: undetermined


pH 3.9 (mmole(+))

Mg2+ K

0.030 6.272

0.117 0.075

0.048 0.893

0.001 5.144


4.9 (mmole(+))

Mg2+ K+

0.029 6.233

0.078 0.069

0.040 0.990

0.044 5.167


Na

0.109

0.121

0.081

0.042


Na

0.108

0.068

0.077

0.098


Al3+

2.115

0.009

2.210

2.910


Al3+

2.102

0.002

0.584

2.518








III CATION LEACHING DURING CONTINUOUS DISPLACEMENT
BY AQUEOUS HYDROCHLORIC ACID SOLUTION THROUGH
COLUMNS OF CECIL SOIL .......................... 75
Introduction ................................. 75
Basic Theory ................................. 77
Surfaces of Soil Particles ................ 77
Cation Exchange Equilibria ................ 78
Effects of Acidification ................... 79
Materials and Methods ...................... 80
Physical and Chemical Properties of Soil... 80
X-Ray Diffraction ......................... 81
Column Preparation and Displacement
Procedure ............................... 82
Dissection of Soil Columns, Extraction, and
Chemical Analysis ....................... 83
Results and Discussion ....................... 85
Concentrations and pH of Column Effluent .. 87
Concentrations of Cations in Solution
and Exchange Phases ..................... 101
Estimated Selectivity Coefficients of
Ion Pairs ............................... 106
Charge Balance of Major Cations for
Topsoil and Subsoil ...................... 114
Conclusions.................................. 119

IV CATION LEACHING DURING CONTINUOUS DISPLACEMENT BY
HYDROCHLORIC ACID SOLUTION THROUGH COLUMNS OF
CHEMICALLY-PRETREATED CECIL SOIL ............... 123
Introduction ............................... 123
Cation Exchange Reaction ................... 124
Saturation Mechanisms for Soil Exchange Sites
with a Single Cation Species ............ 124
Mechanisms of H Replacement of
Exchangeable Cations on Soil Exchange
Sites ...................................... 126
Materials and Methods ....................... 128
Preparation of Pretreated Soil ............. 128
Soil Column Preparation and Procedure for
Displacing HC1 Solutions through Columns. 130
Method for Dissection, Extraction and Chemical
Analysis of Soil Columns ................ 132
Results and Discussion ...................... 133
Exchange Sites for Chemically Pretreated
Soil .................... ........ ..... 138
Effluent pH for Treated Soil Columns....... 138
Cation Concentrations in Effluent from
Treated Soil Columns ................... 139
Cation Concentrations in Effluent from
Chemically Pretreated Mixed Soil Columns. 154
Concentrations of Al in Effluent from Mixed
Soil Columns ........................... 165
Concentrations of Cations in Solution and




48


0

m* a
M r:
0
So .
0 :,o 0 w

30 4
f t6oa
1 ( o) wm


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r000 4JoC
CE 0 t- 0 p
L I c t o 4
s el (rl Q)



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L H(0 U U



sIsII 0 -a 00
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N 0 (a 't





50
44


(t/ (+) StouWuW)


uo Teu .jueouoo




164


I0)


II


0-0
o o a 0so
m* '. (


's
*H-
.0
C
O
3


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C.

3a


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> I



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ty,





Er
r



00
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E
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> 1

CM


uotUT.ejuuouoo


i


(r/ (O) arurur




184


Table 4-17 Concentrations of cations in solution
and exchange phases for Mg-topsoil after
leaching with pH 4.9 HC1 solution


Depth Ca Mg K
(cm) Solution
------ mole(+)
1.0 0.737 0.349 0.234
2.0 0.413 0.494 0.150
3.0 0.626 0.415 0.183
4.0 0.596 0.329 0.230
5.0 0.631 0.698 0.357
6.0 0.701 0.434 0.363
7.0 0.180 0.681 0.225
8.0 0.695 0.527 0.358
9.0 0.293 0.528 0.178
10.0 0.607 0.372 0.166
11.0 0.418 0.588 0.160
12.0 1.000 0.666 0.577
13.0 0.363 0.553 0.302
14.0 0.152 0.502 0.175
15.0 0.134 0.543 0.187
16.0 0.087 0.605 0.171
17.0 0.120 0.647 0.310
18.0 0.031 0.518 0.213
19.0 0.132 0.592 0.190
20.0 0.134 0.513 0.189


Na Al Sum
phase
L


1.740
2.350
1.550
2.000
2.070
2.490
1.680
1.970
1.940
1.380
1.720
3.990
2.440
1.100
1.280
0.848
1.520
1.210
1.270
0.751


Depth Ca Mg K Na
(cm) Exchange phase
--mmole (+) Kg
1.0 2.070 5.550 0.416 0.500
3.0 1.920 8.640 0.160 0.391
5.0 1.710 8.840 0.141 0.304
7.0 1.860 8.840 0.294 0.446
9.0 1.630 9.460 0.147 0.228
11.0 1.670 9.050 0.115 0.239
14.0 1.720 9.460 0.160 0.370
16.0 1.770 9.670 0.173 0.359
18.0 1.770 8.640 0.365 0.489
20.0 1.970 10.100 0.294 0.565


2.080
11.700
1.410
3.740
1.670
2.860
32.600
10.600
13.600
2.740
11.900
0.183
8.480
17.700
17.300
19.600
22.800
17.100
21.900
12.900


5.140
15.107
4.184
6.895
5.426
6.848
35.366
14.150
16.539
5.265
14.786
6.416
12.138
19.629
19.444
21.311
25.397
19.072
24.084
14.487


Al Sum

soil
2.335 10.871
0.956 12.067
0.923 11.918
0.767 12.207
0.812 12.277
0.689 11.763
0.634 12.344
0.712 12.684
0.623 11.887
0.656 13.585








I certify that I have read this study and that in my
opinion it conforms to acceptable standards of scholarly
presentation and is fully adequate, in scope and quality, as
a dissertation for the degree of Doctor of Philosophy.




W. W. McFee
Professor of Soil Science


This dissertation was submitted to the Graduate Faculty of
the College of Agriculture and to the Graduate School and
was accepted as partial fulfillment of the requirements
for the degree of Doctor of Philosophy.



August, 1987 --

Dean, CElege of Agriculture


Dean, Graduate School














































rq





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M 0 0 0
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4. (d 4-4
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S(U U

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small values of CEC gave reasonable predictions for the

cation distributions in the soil columns.

Dilute HC1 solutions with a pH of 3.9 and 4.9 were

applied at 1.0 cm h-1 Darcy flux to columns of air-dry soils

to simulate the effects of acid rain upon leaching of soil

cations. Soil columns were prepared using Cecil topsoil and

subsoil initially saturated separately with K Ca 2+, and

Mg2+, also using untreated and mixed-cation soils.

"Mixed-cation" soils were obtained by mixing equal masses of

K -, Ca -, and Mg saturated soils.

The initial effluent samples from acid-treated soil

columns were high in basic and acidic cation concentrations

and low in pH. Dramatic decreases in cation concentrations

and increases in pH were observed with increasing volume of

effluent up to about 3 to 5 pore volumes. Between 5 to 30

pore volumes of effluent, pH was in the range from 6.0 to

6.5, and cation concentrations decreased gradually. Soil

base saturation in the columns was drastically reduced after

acid application, especially for columns receiving pH 3.9

acid solution. Experimental results from the leaching

experiments indicated that total quantities of cations

removed by the acid at pH 3.9 were 1.2 to 2.0 times more

than those at pH 4.9.


xvi








dissolution of sesquioxides and clay minerals to yield Al3+

ions.

To understand the mechanisms of ion movement during

acid-rain deposition in forest soils, the interaction

between rainwater and soil must be carefully examined. In

particular, ion-exchange processes are of great importance

(Wiklander, 1975).

Hypotheses

The working hypothesis for this research is that acid

precipitation increases nutrient leaching from forest soils

in direct proportion to the H+ concentration of incoming

rainwater. Direct sampling of soil and soil solution at

specific forest sites to determine changes in ionic

composition of soil exchange and solution phases is very

time-consuming and expensive. More importantly, forest

soils also commonly generate H+ ions (Mollitor and Raynal,

1982, Reuss and Johnson, 1985) due to biological processes.

A need exists to provide foresters and soil scientists

with predictive tools for describing the leaching of

nutrient cations, such as Ca 2+, Mg 2+, K etc., through soil

profiles. The usefulness of such predictive tools for

planning purposes should include assessment of the benefits

and consequences of forest management and fertilizing.

Furthermore, such a tool could provide information help in

controlling groundwater quality. Predicting changes in

ionic composition of the soil solution and exchange phases





208


The use of cubic spline functions as shape functions enabled

employment of a Galerkin finite-element formulation over the

spatial and time domain. The resulting ordinary

differential equations were solved by a method based on

backward differentiation formulation (Gear, 1969).

Verification of the numerical model was performed using

an analytical model for nonreactive solute transport during

miscible displacement, where the retardation factor (R) is

assumed to be unity. Verification was also performed for

the case of non-preferential ion exchange with a selectivity

coefficient equal to unity and cation-transport retardation

R = 1 + [(a CT)/(9 CT)].

Sensitivity analyses for dispersion coefficient,

volumetric water content, bulk density, selectivity

coefficient and cation exchange capacity parameters were

performed for the numerical model with respect to the

experimental data from the columns of Cecil soil.

Sensitivity analysis showed cation exchange capacity to be

the most critical parameter in the model. Relatively small

values of CEC gave the best simulations for measured cation-

concentration distributions with depth in the solution and

exchange phases. Use of CEC values obtained from 1 M NH OAc

extraction resulted in discrepancies between observed and

predicted cation distributions within the columns.

Leaching of soil cations was investigated during

displacement of acid solutions under constant liquid flux

through columns of initially air-dry Cecil topsoil and








adsorbed phase for each section of every soil column. For

the centrifuge method, two different sizes of centrifuge

tube are required. A small hole was made at the bottom of

the smaller tubes (0.015-m diameter) and a number 42 Whatman

filter paper was cut and placed over that hole. Each column

section of soil was carefully transferred into a prenumbered

small centrifuge tube. Then, each small tube was placed

into a corresponding prenumbered larger centrifuge tube

(0.025-m diameter). A glass bead was placed between the

small and large tubes. After balancing pairs of tubes

within the centrifuge, they were spun at 4000 rpm for 30

minutes. Moist soil samples were quantitatively removed and

weighed. Weights of air-dry soil were also recorded. Each

dried sample was then ground and passed through a 2-mm

sieve. The exchanger phase concentrations of cations were

obtained by the aforementioned extraction method. A

correction was made for concentrations of cations in

residual solution in the exchange phase of the soil sample.

In addition, one topsoil column and one subsoil column

were prepared to permit determination of solute dispersion

coefficients (D). Soil columns were set u'p vertically for

flow experiments. A peristaltic pump was calibrated and

used to introduce distilled water into the bottom of each

soil column. After one day of leaching, a check for zero

Cl concentration was obtained, and the distilled water was

changed to 0.001 M KC1 input solution. The effluent was

fractionally collected, and the experiments were terminated











Ca 4)


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124


The first objective of this study was to determine the

cation exchange capacity for chemically-pretreated Cecil

topsoil and subsoil, and a second objective was to determine

the influence of input solution pH upon cation leaching

during continuous displacement of HC1 solution through

hand-packed columns of chemically-pretreated Cecil topsoil

and subsoil, respectively. A third objective was to

determine the distribution of cation concentrations in

solution and exchange phases of each pretreated soil column

after leaching with HC1.

Cation-Exchange Reaction

Saturation Mechanisms for Soil Exchange Sites with a
Single Cation Species

Most mineral soils contain a mixture of colloids having

constant and variable charge-surfaces (Dixon and Weed,

1977), even though one type might tend to dominate over

others. Soils located in the Southeastern United States

which are highly weathered have properties commonly

dominated by sesquioxides of Fe and Al with their

pH-dependent charge, by 1:1 kaolinite-type crystalline clay

minerals and by 2:1 interlayer-hydroxy vermiculite minerals.

Soil organic matter also provides an important contribution

to the pH-dependent charge of such soils.

When a neutral-salt solution (examples: KC1, MgCl2, or

CaCl2) is added to a soil it causes a cation-exchange

reaction involving the transfer of multiple species of

cations between solution and exchange phases. Cation-




115


where the first term (1) was obtained from the product of

the concentrations of input solution H+ (mmole(+) L1 ) and

the total volume of input solution (1). The second term (2)

was obtained as the cation exchange capacity (mmole(+) kg )

of a given soil times the weight of soil (kg) residing in

the column. The third term (3) was obtained by integrating

breakthrough curves of effluent pH and converting mmole(+).

The fourth term (4) was obtained by integrating

breakthrough curves for each cation species and then summing

for all species. The last term (5) accounted for remaining

cations in the solution and exchange phases. Total charge

of cations in the solution phase was taken as the sum of the

charge of each individual cation as obtained from the

product of its cation concentration, volumetric water

content (M3 M3 ) of the soil and total volume of soil (L).

In the same manner, the total charge of cations in the

exchange phase was taken as the sum of the charge of each

individual cation on the exchange phase obtained using the

concentrations of cations (mmole(+) kg1 ) as obtained from 1

M NH40OAc extraction multiplied by the weight of soil in the

columns (kg) to give the masses of individual cation

species.

Using the previously stated concept of charge balance,

the initial charge (total input of H+ ions and cations

initially present in the column) and final charge (total H+





188


Table 4-20 Concentrations of cations in solution and
exchange phases for mixed-cation topsoil
after leaching with pH 3.9 HC1 solution

Depth Ca Mg K Na Al Sum
(cm) Solution phae
mmole (+) L
1.0 0.202 0.078 0.151 0.115 0.100 0.646
2.0 0.167 0.062 0.155 0.183 0.139 0.706
3.0 0.205 0.074 0.182 0.159 0.094 0.715
4.0 0.227 0.128 0.208 0.193 0.295 1.051
5.0 0.279 0.152 0.223 0.196 1.070 1.920
6.0 0.319 0.267 0.313 0.198 13.800 14.897
7.0 0.277 0.197 0.228 0.154 2.300 3.156
8.0 0.342 0.276 0.205 0.124 9.060 10.007
9.0 0.317 0.230 0.220 0.150 1.530 2.447
10.0 0.414 0.415 0.292 0.152 23.000 24.273
11.0 0.409 0.341 0.290 0.157 10.800 11.997
12.0 0.392 0.317 0.385 0.148 7.800 9.042
13.0 0.384 0.247 0.294 0.187 1.150 2.262
14.0 0.359 0.346 0.278 0.141 14.000 15.124
15.0 0.387 0.354 0.504 0.152 13.400 14.797
16.0 0.334 0.235 0.340 0.182 0.757 1.848
17.0 0.374 0.420 0.336 0.117 23.100 24.347
18.0 0.394 0.337 0.344 0.159 12.100 13.334
19.0 0.334 0.296 0.267 0.100 10.000 10.997
20.0 0.357 0.284 0.331 0.109 9.440 10.521

Depth Ca Mg K Na Al Sum
(cm) --- Exchange phas
Immole (+) Kg soil
1.0 3.340 0.946 0.379 0.252 4.770 9.687
3.0 4.040 0.872 0.358 0.239 2.320 7.829
6.0 5.440 1.650 0.489 0.226 2.210 10.015
8.0 5.290 1.890 0.650 0.217 2.430 10.477
10.0 5.490 1.890 0.650 0.209 2.420 10.659
12.0 5.640 2.040 0.744 0.174 2.160 10.758
14.0 6.040 2.020 0.837 0.226 2.350 11.473
17.0 6.040 2.000 0.929 0.130 2.040 11.139
18.0 6.790 2.050 1.030 0.109 1.630 11.609
20.0 6.420 2.130 1.150 0.109 1.460 11.269





133


obtained extraction with neutral 1 M NH OAc (Thomas, 1982).

Unbuffered 1 M KC1 (Thomas, 1982) was used to obtain

exchangeable Al3+. Concentrations were corrected for

entrapped equilibrium solution near the exchange sites.

Concentrations of Ca2+, Mg2+, K and Na+ in the column

effluent, solution and exchange phases of each soil column

were analyzed by an atomic absorption spectrometer as

described in previous chapters. The Al was determined by

optical emission spectroscopy (inductively coupled argon

plasma, ICAP) on a unit located in the Soil Testing

Laboratory, Soil Science Department, University of Florida.

Results and Discussion

Initial exchangeable cations in nontreated topsoil and

subsoil are presented in Table 4-1. The soil parameters

used for the miscible displacement experiments are presented

in Tables 4-2, 4-3 and 4-4 for chemically-pretreated

topsoil, pretreated subsoil, and mixed topsoil and subsoil,

respectively.

From Tables 4-2 through 4-4, the deviation of the bulk

density, volumetric water content and pore-water velocity of

hand-packed treated topsoil and subsoil columns were within

3 %, respectively. Similar deviations for the bulk

density, volumetric water content and pore water velocity

were also found for hand-packed mixed subsoil and topsoil

columns.




220


solute transport equation. USDA Tech. Bull. 1661. U.S.
Salinity Laboratory. Riverside, CA

van Genuchten, M.Th., and P.J. Wierenga. 1976. Mass transfer
studies in sorbing porous media, I. Analytical solution.
Soil Sci. Soc. Am. J. 40:473-480.

Whittig, L.D. 1965. X-ray diffraction techniques for mineral
identification and mineralogical composition. p.671-698.
In Methods of Soil Analysis. Part 1. C.A. Black (ed.).
American Society of Agronomy, Madison, WI.

Wiklander, L. 1975. The role of neutral salts in the ion
exchange between acid precipitation and soil. Geoderma
14:93-105.

Wiklander, L., and A. Andersson. 1972. The replacing
efficiency of hydrogen ion in relation to base saturation
and pH. Geoderma 7:159-165.







and the total solution-phase concentration CT at any given

time and distance is the sum of the concentrations of ion

species present in the solution phase

CT = C1 + C2 [2-18]

If one further considers the simplified case where the

(input) solution has the same total normality CT as the

native solution (CT is constant), an explicit form for the

exchange isotherm can be obtained by substituting equations

[2-17] and [2-18] into equation [2-14]. This yields


C T
C = [2-19]


l/z1 22/21
where r = a / p, a= C C1, and K = Ks C 1

This simplification reduces the binary-exchange problem to a

single-species exchange problem (Rubin and James, 1973;

Valocchi et al., 1981). If we define a parameter F as



F -- [2-20]
aC s

1/z 1/z
where T = C K C and e = CT + (Ks 1)C1

the chain rule may be used to obtain


DC DC, aC aC
-- a I F [2-20]
at a3C at at

Therefore, using equations [2-14], [2-20] and [2-21],

equations [2-15] and [2-16] can be combined to give one

equation with one dependent variable C1 as given by





138


Exchange Sites for Chemically Pretreated Soil

Treating the Cecil soil with a specific cation tended

to increase the base saturation and CEC (Tables 4-1 and 4-5)

of the soil, with the largest effect being found for the

subsoil. The pretreated topsoil and subsoil materials were

non-homoionic with respect to cations in the exchange phase,

indicating that the soil exchange sites were not completely

saturated initially with any one specific ion. For example,

equivalent fractions of K+ in K-topsoil were 95 and 72% for

cases in which Al was excluded from and included in the CEC

calculation, respectively. Equivalet fractions of Mg2+ in

Mg-topsoil were 89 and 86% for similar cases, with

corresponding values of Ca2+ for Ca-topsoil 99 and 84%, 96

and 95% for K-subsoil, 99 and 98% for Mg-subsoil, and 99 and

95% for Ca-subsoil. Quantities of other basic cations in

the exchange phase were small compared with the dominant ion

for each of the pretreated soils.

The subsoil had very little exchangeable Al3+ following

the cation-saturation procedures. Saturation of topsoil

exchange sites was not as complete as it was for the

subsoil.

Effluent pH for Treated Soil Columns

Regardless of pH of the applied HC1 solution, effluent

pH of all columns of pretreated Cecil topsoil, subsoil, and

mixed-cation columns started at about 4.0-4.4 and then

quickly increased to about pH 6.7 within two pore volumes of

elution. Thereafter, readings fluctuated around pH 6.1-6.8




212


from 68 to 35% for nontreated subsoil; from 99 to 31% for

K-subsoil; from 99 to 11% for Mg-subsoil; from 99 to 21% for

Ca-subsoil; from 85 to 80% for Ca-topsoil; from 76 to 16%

for K-topsoil, 96 to 91% for Mg-topsoil, 99 to 16 % for

mixed subsoil; from 86 to 40% for mixed topsoil. As

expected, the distribution of cations on soil exchange sites

followed the order Al3+ > Mg2+ > Ca2+ > Mg2+ > K Na,

while for the solution phase the order was Na > K+ = Ca2+ >
2+ 3+
Mg > Al3. The most significant response to input HC1

solution occurred at the receiving end of the soil columns,

where larger values of exchangeable Al3+ were observed.

Base saturation drastically decreased for columns of soil

which received pH 3.9 input acid solution. Columns of soil

which were chemically pretreated showed a greater decrease

in base saturation than did nontreated soil.

Conclusions

In general, the numerical model for binary cation

transport during steady liquid flow agreed very well with

analytical solutions for simple cases. Experiments were

performed to generate data for evaluating the predictive

capacity of the numerical model. During steady liquid flow

through water-saturated columns of Ca-saturated Cecil

topsoil and subsoil, aqueous MgCl2 solutions were displaced

through the columns. When the overall measured CEC values

in the column were used as the input for the model,

simulated results overestimated the distributions of cation

concentrations in the columns. When the measured sum of








after leaching with pH 4.9 HC1 solution ..........111

3-11 Mass balance of cations for columns of topsoil ...117

3-12 Mass balance of cations for columns of subsoil ...118

4-1 Concentrations of exchangeable cations for
nontreated topsoil and subsoil ..................134

4-2 Soil parameters for chemically pretreated topsoil
columns after leaching with HC1 solutions ........134

4-3 Soil parameters for chemically pretreated subsoil
columns after leaching with HC1 solutions ........135

4-4 Soil parameters for mixed soil columns after
leaching with HC1 solutions ...................... 136

4-5 Concentrations of exchangeable cations for
pretreated and mixed topsoil and subsoil .........137

4-6 Concentrations of cations in solution and
exchange phases for K-subsoil after leaching
with pH 3.9 HC1 solution ......................... 168

4-7 Concentrations of cations in solution and
exchange phases for K-subsoil after leaching
with pH 4.9 HC1 solution .........................169

4-8 Concentrations of cations in solution and
exchange phases for Ca-subsoil after leaching
with pH 3.9 HC1 solution .........................171

4-9 Concentrations of cations in solution and
exchange phases for Ca-subsoil after leaching
with pH 4.9 HC1 solution .........................172

4-10 Concentrations of cations in solution and
exchange phases for Mg-subsoil after leaching
with pH 3.9 HC1 solution .........................174

4-11 Concentrations of cations in solution and
exchange phases for Mg-subsoil after leaching
with pH 4.9 HC1 solution ........................175

4-12 Concentrations of cations in solution and
exchange phases for Ca-topsoil after leaching
with pH 3.9 HC1 solution .........................177

4-13 Concentrations of cations in solution and
exchange phases for Ca-topsoil after leaching
with pH 4.9 HC1 solution .........................178

4-14 Concentrations of cations in solution and

viii