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Effects of Soil and Plant on Arsenic Accumulation by Arsenic Hyperaccumulator Pteris vittata L

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Permanent Link: http://ufdc.ufl.edu/UFE0013732/00001

Material Information

Title: Effects of Soil and Plant on Arsenic Accumulation by Arsenic Hyperaccumulator Pteris vittata L
Physical Description: Mixed Material
Copyright Date: 2008

Record Information

Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
System ID: UFE0013732:00001

Permanent Link: http://ufdc.ufl.edu/UFE0013732/00001

Material Information

Title: Effects of Soil and Plant on Arsenic Accumulation by Arsenic Hyperaccumulator Pteris vittata L
Physical Description: Mixed Material
Copyright Date: 2008

Record Information

Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
System ID: UFE0013732:00001


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Full Text













EFFECTS OF SOIL AND PLANT ON ARSENIC ACCUMULATION BY ARSENIC
HYPERACCUMULATOR Pteris vittata L












By

MARIA ISIDORIA SILVA GONZAGA


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


2006

































Copyright 2006

By

Maria Isidoria Silva Gonzaga

































To the memory of my father Henrique, whose love and inspiration have accompanied me
through the years; to my mother Ines who, together with my father, gave me all the
values that guide my life; and to my kids Jamile, Thomas and Genna, the best fruits of
my existence.















ACKNOWLEDGMENTS

I wish to thank my supervisory committee chair (Dr. Lena Q. Ma) for providing

valuable academic advices, critical support and guidance during my graduate study. I

thank my supervisory committee members (Jean-Claude Bonzongo, Nicholas B.

Comerford, Robert Stamps, and Roy Dean Rhue) for their assistance and advice. I also

wish to especially thank Dr. Nick Comerford for helping me apply to the University of

Florida and also for helping me and my family during our first few days in Gainesville.

Special thanks also go to Dr. Jean-Claude Bonzongo for his advisement and valuable

friendship.

I am grateful for the assistance and support of my husband, Dr. Jorge Santos, who

provided experimental and statistical advice and also helped in conducting my

experiments. I am grateful to Mr. Tom Luongo for his assistance with chemical analysis

and proofreading manuscripts; to Dr. Mristunjai Srivastava for his assistance in providing

fern plants and proofreading manuscripts; to Dr. John Thomas for his friendship and for

proofreading manuscripts; and Mr. Bill Reeve, for his friendship and help with chemical

analyses. I also wish to thank the members of the Biogeochemistry of Trace Metals

Laboratory (Gina Kertulius-Tartar, Abioye Fayiga, Joonki Yoon, Rocky Cao, Mike Tu,

and Autumm Wan), for what they have taught me. I thank Mrs. Marlin Pam for her help

and patience.

I would like to acknowledge the Brazilian institution (CAPES) for providing

financial support during my Ph.D. program. I am grateful to Dr. Paulo Gabriel Nacif (and









members of the Soil Chemistry Department of the Agronomy School, at the Federal

University of Bahia) for granting me permission to pursue my degree. I also thank Dr.

Joelito Rezende for all the encouragement and help, keeping me in the scientific life.
















TABLE OF CONTENTS

page

A C K N O W L E D G M E N T S ................................................................................................. iv

LIST OF TABLES ....................................................... ............ ....... ....... ix

LIST OF FIGURES ......... ......................... ...... ........ ............ xi

A B S T R A C T .............................................. ..........................................x iii

CHAPTER

1 IN TRODU CTION ................................................. ...... .................

2 LITERATURE REVIEW ........................................................................4

A rsenic ........................ ............ ...... .................... ...................... ....... 4
O ccurrence, A availability and Toxicity ........................................ .....................4
Remediation of Arsenic Contaminated Soils .................................................6
P hytorem edition ........................ ......................................................... ............... .. 7
Phytoextraction................................ ........... ...... .. ........ .. .. .. ........ .. 7
Phytostabilization ...................................... ........................... .8
Phytoim obilization .................................................................... .8
Phytovolatilization....................................... .. .... ... ............. ....
Phytoextraction of A rsenic .................................................................... ............... 9
H yperaccum ulator Plants ........... .. ....... .. ........... .................... ............... 13
Arsenic Hyperaccumulation by Pteris vittata ............................................. 16
Arsenic Species in Pteris vittata............................... ....................18
Arsenic Tolerance and Detoxification Mechanisms................ ..................19
Effects of Heavy Metals on Arsenic Hyperaccumulation..............................21
Effect of Phosphate on Arsenic Hyperaccumulation.......... ............... 22
Mycorrhizae Association and its Role in Arsenic Hyperaccumulation...............24

3 ARSENIC ACCUMULATION BY Pteris vittata L FROM ARSENIC
CON TAM IN A TED SOILS ............................................... ............................. 27

In tro du ctio n ...................................... ................................................ 2 7
M materials and M ethods ....................................................................... ..................29
Soil Collection and Characterization................... ...... ....................... 29
Experimental Design and Statistical Analyses............... ...............29









E xperim mental Set up .................................................. .............................. 29
Sam pling, D igestion and A analysis ........................................... .....................31
A rsenic Fractionation ....................... ...................... ........... ...............3 1
Single E xtractions........ ................................................................. .... .... .... 32
R results and D iscu ssion ................................. .............. ................ ..... .......... 33
Plant G row th........................ ....................................... 33
Arsenic Concentrations in the Frond Biomass ............................................ 36
Plant A rsenic R em oval from Soils ........................................... .....................38
Soil A rsenic D distribution ............... ....... ..... .............................................39
Comparison of Extractable Arsenic Using Single Extraction Methods .............44
Plant uptake and Arsenic Availability in Soils as Measured by Single
E x tractan ts............................. .................................................. ............... 4 4

4 EFFECTS OF PLANT AGE ON THE ARSENIC ACCUMULATION BY THE
HYPERACCUMULATION Pteris vittata L......................................................49

In tro du ctio n ...................................... ................................................ 4 9
M materials and M methods ....................................................................... ..................50
Plant Propagation .................................................. .. ....... .. ........ .. 50
Soil C haracterization ...................... .. .. ..................... .... .. ........... 50
E xperim ental D esign ........................................ ........................... .......... 1
C hem ical A naly sis......... .......................................................... .... ........52
D ata A n aly sis................................................ ................ 5 3
R results and D iscu ssion ................................. .............. ................ ..... .......... 53
A rsenic U ptak e ..............................................................53
Plant Biomass ................................................ 56
Phosphorus Distribution in the Plants ...................................... ............... 56
Concentrations of Other Nutrients in Plants..................................................59

5 ARSENIC ACCUMULATION AND ROOT CHARACTERISTICS OF Pteris
vittata L. AND Nephrolepis exaltata L ............................ ...... ...................... 63

Introdu action .......................... ............. .........................................................63
M materials and M methods ....................................................................... ..................66
Soil C haracterization ...................... .. .. ..................... .... .. ........... 66
E xperim ental Set up ................................................. ............................... 66
Root M easurem ent ............... .. .... .......... ...... ............ ... .... .. ............ 68
C hem ical A nalysis........... ........................................................ ...... .... .....69
A rsenic Fractionation .......................................... .. .. ...... ...............69
D ata A n a ly sis ................................................................................................. 7 0
R results and D discussion ............... ... ......... ................................ ...... .... .......... 70
Root Biomass, Root Length, Root Area, and Root Diameter ...........................70
Frond Biomass, Arsenic Uptake, Bioconcentration and Translocation .............72
Root Uptake Efficiency of Arsenic and Phosphorus................ ..................76
P lant N utrient U ptake ................... ...................................................................78
Arsenic Reduction and Distribution in the Soil ................................................79
Influence of Plants on Water-Soluble Arsenic, Soil pH and DOC................. 84









6 ARSENIC ACCUMULATION BY TWO HYPERACCUMULATOR Pteris
SPECIES FROM TWO ARSENIC CONTAMINATED SOILS..............................90

In tro du ctio n ...................................... ................................................ 9 0
M materials and M ethods ....................................................................... ..................9 1
Soil Collection and Characterization.................... ...... ...................... 91
C hem ical A naly sis........... ........................................................ .. .... ...... .94
D ata A n a ly sis ................................................................................................. 9 4
R results and D discussion ....................................................................................... .....94
Arsenic Accumulation and Distribution in the Ferns............... .... ........... 94
Plant Biomass of P. biaurita and P. vittata...................... .......................98
R hizosphere C hem istry ............................................. ............................ 100
E lem ental C om position ............................................. .............. ............105

7 CON CLU SION S ........... .. .... .... ... .... ................................ 111

LIST O F R EFEREN CE S ....... ............................................................. ............... 115

BIOGRAPHICAL SKETCH ......... ....... ............. ............. 132















LIST OF TABLES


Table pge

3-1 Selected chemical and physical characteristics of six arsenic contaminated soils...30

3-2 Arsenic sequential extraction procedure ...................................... ............... 32

3-3 The frond arsenic concentrations ofPteris vittata after the first, second and,
third harvest in six soils............. .... .................................................... .. .... ..... .. 37

3-4 Arsenic reduction (%) in each fraction in six soils as a result of plant arsenic
uptake by P. vittata............ ....... .................................. .................43

3-5 Concentrations of arsenic extracted by different extraction methods in six soils
before plant grow th. ......................................... ........................... 45

3-6 Percent of the arsenic taken up by P. vittata in each extracting method.................46

4-1 Plant biomass, bioconcentration factor and translocation factor ofP. vittata of
different ages after 8 weeks of growth in an arsenic-contaminated soil ................54

4-2 Macronutrients (K, Ca, and Mg) (g kg-1) and micronutrients (Fe, Zn, and Mn)
content (mg kg-1) in the fronds and roots of P. vittata of different ages.................61

5- 1 Selected properties of the soils used in this study ................ ............... 66

5-2 The root biomass (dw), root length, and root area of P. vittata and N. exaltata
after 8 weeks of growth in a As-contaminated soil (As-soil) and control soil.........71

5-3 The frond biomass (dw), arsenic accumulation, and bioconcentration and
translocation factors of P. vittata and N. exaltata .............................. ............... 76

5-4 The root uptake efficiency (RUE) as measured by As and P uptake (mg plant-)
by the fronds and roots of P. vittata and N. exaltata..............................................78

5-5 Nutrients concentrations (g kg-1) in the fronds and roots of P. vittata and N.
exaltata after 8 weeks of growth in an As-contaminated soil and a control soil. ....80

5-6 Arsenic distribution in different fractions in the bulk soil after 8 weeks of plant
g ro w th ...................................... ................................................... 8 1









5-7 Arsenic distribution in different fractions in the rhizosphere of P. vittata (PV)
and N. exaltata (NE) after 8 weeks of plant growth................. ......................82

6-1 Selected properties of the soils used in this study ....................................................92

6-2 Comparison of the arsenic concentration (mg kg-1), translocation factor (TF) and
bioconcentration factor (BF) of P. vittata and P. biaurita ................... ..............96

6-3 The macronutrients (P, K, Ca, Mg) (g kg-1) and micronutrients (Fe, Zn, Mn)
content (mg kg-1) in the fronds and the roots of P. vittata and P. biaurita ..........06
















LIST OF FIGURES


Figure pge

2-1 Factors that influence the efficiency of phytoextraction......................................... 11

2-2 Conceptual response of hyperaccumulator, indicator and excluder plants for
metal concentrations in the aboveground tissues. ............. ..................................... 15

3-1 Fond biomass ofPteris vittata after first, second and third harvests in six soils.....34

3-2 Arsenic accumulation by P. vittata after first, second and third harvests in six
s o ils .............................................................................. 3 8

3-3 Arsenic removed from the soil by Pteris vittata after three harvests.....................40

3-4 Arsenic concentrations (mg kg-1) in different fractions in soils before and after
plant growth............. ......... ...... ............................. .........41

4-1 Pteris vittata of 1.5, 4, 10, and 16 old months before transplant...........................51

4-2 Arsenic accumulation in the fronds and roots of P. vittata of different ages after
8 weeks of growth in an arsenic-contaminated soil. ..............................................54

4-3 The frond biomass of P. vittata of different ages before and after 8 weeks of
growth in an arsenic-contaminated soil......................................... ............... 57

4-5 Effect of plant age on phosphorus and arsenic distribution in the fronds of P.
vittata after 8 weeks of growth in an arsenic-contaminated soil..............................59

4-6 Effect of plant age on the phosphorus and arsenic distribution in the roots of P.
vittata after 8 weeks of growth in an arsenic-contaminated soil.............................60

5.1 Cross section of the rhizopot used in the study ...................................................67

5-2 The root length density (Lv) of P. vittata and N. exaltata as a function of root
diameter in a control soil (a) and an As-contaminated soil (b) .............................74

5-3 The root surface area density (Av) of P. vittata and N. exaltata as a function of
root diameter in a control soil (a) and an As-contaminated soil (b).........................75

5-4 Comparison of water-soluble arsenic in the bulk and rhizosphere soil of P.
vittata and N. exaltata after 8 weeks of growth in an arsenic-contaminated soil.....86









5-5 Comparison of the soil pH in the bulk and rhizosphere soil of P. vittata and N.
exaltata after 8 weeks of growth in an arsenic-contaminated soil. ..........................88

5-6 Comparison of dissolved organic carbon (DOC) in the bulk and rhizosphere soil
of P. vittata and N. exaltata ......... ....... ......... .................. ............... 89

6-1 Pteris biaurita and Pteris vittata growing in an arsenic-contaminated soil.............95

6-2 The frond and root biomass (mg kg-1 DW) of P. vittata and P. biaurita after 8
weeks of growth in two arsenic contaminated soils (Soil 1: 153; Soil2: 266)........99

6-3 The bulk soil and rhizosphere pH of P. vittata and P. biaurita after 8 weeks of
growth in two arsenic contaminated soils. .................................. .................101

6-4 The bulk soil and rhizosphere dissolved organic carbon (DOC) of P. vittata and
P. biaurita after 8 weeks of growth in two arsenic contaminated soils ...............102

6-5 The water-soluble P concentrations in the bulk soil and rhizosphere of P. vittata
and P. biaurita.......................... ..... .. ....... .......... 103

6-6 The water-soluble As concentrations in the bulk soil and rhizosphere of P.
vittata and P biaurita .......... ........................................ .............. .. .... ......... 104















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

EFFECTS OF SOIL AND PLANT ON ARSENIC ACCUMULATION BY ARSENIC
HYPERACCUMULATOR Pteris vittata L

By

Maria Isidoria Silva Gonzaga

May 2006

Chair: Lena Q. Ma
Major Department: Soil and Water Science

Phytoextraction using Pteris vittata L., an arsenic hyperaccumulator plant, could

be potentially applied to remediate arsenic-contaminated sites worldwide. Pteris vittata

grew well and took up large amounts of arsenic in six arsenic-contaminated soils with

different properties and different sources of arsenic contamination. However, efficiency

of the ferns in continually taking up arsenic from the soils decreased with time, and

varied with growing season. The arsenic concentrations in different chemical fractions

before and after plant uptake showed that P. vittata took up arsenic from the most

available and also from the less available pools in all soils. The change in arsenic

availability in the rhizosphere was evaluated by comparing P. vittata with a non-arsenic-

hyperaccumulator Nephrolepis exaltata. As expected, P. vittata removed more arsenic

from the soil than the non-arsenic-hyperaccumulator fern. Besides the efficient

detoxification mechanisms ofP. vittata, its more extensive and finer root systems, and its

greater capacity to change the soil pH and produce root exudates in the rhizosphere likely









contributed to the difference. Furthermore, the experiments with P. vittata of different

physiological ages revealed that younger plants (because of their higher metabolic and

growth rate) were more efficient than older plants in taking up arsenic. Therefore,

younger plants are recommended for phytoremediation. Comparing two arsenic

hyperaccumulators in the genus Pteris showed that arsenic hyperaccumulator ferns

diffred in their ability to take up arsenic when growing under the same soils and

environmental conditions. The concentrations of arsenic in both plants increased with the

soil arsenic concentration, however, P. vittata performed better, regardless of the arsenic

level in soils. After 4 weeks of growth, P. biaurita showed signs of stress and probably

would not have survived longer under those conditions, while P. vittata showed no

toxicity symptom. Our results showed that, in implementing an arsenic phytoextraction

project, many aspects related to the plant must be considered.














CHAPTER 1
INTRODUCTION

Arsenic contamination in the environment from both anthropogenic and natural

sources occurs in many parts of the world and is a global problem. Arsenic-contaminated

soils, sediments, and sludge are the major sources of arsenic contamination in food chain,

surface water, groundwater, and drinking water (Frankenberger and Arshad, 2002). Other

potential sources of arsenic contamination are the chemicals used extensively in

agriculture such as pesticides, insecticides, defoliants, wood preservatives, and soil

sterilants (Azcue and Nriagu, 1994).

In many areas, arsenic levels in the environment have exceeded the safe threshold

for human health. Epidemiological studies have documented various adverse effects of

arsenic on humans and animals. Conventional remediation technologies have been used

to clean up metal-contaminated sites because they are relatively insensitive to the

heterogeneity in contaminated matrix, and can function over a wide range of oxygen, pH,

pressure, temperature, and osmotic potentials (Cunningham et al., 1997). However, they

are expensive and time-consuming, often hazardous to workers, and produce secondary

wastes (Lombi et al., 2000). Of the disadvantages of conventional remediation methods,

cost is the primary driving force behind the search for alternative remediation

technologies, such as phytoremediation.

Phytoremediation is the use of plants and their associated microbes to reduce,

remove, degrade, or immobilize environmental contaminants in soil and water systems

and can be applied for both organic and inorganic pollutants (Salt et al., 1998).









Phytoextraction, one of the strategies of phytoremediation, has attracted much attention

as an environmentally-friendly low-input remediation technique that uses plants that

extract heavy metals from the soil and accumulate them in the harvestable aboveground

biomass (McGrath et al., 2002). However, the effectiveness of a phytoremediation plan is

plant-dependent. Phytoextraction can be accomplished by using either tolerant high

biomass plant species or hyperaccumulator plant species. Plants native to the target area

should be considered, since they are adapted to the local climate, insects, and diseases.

Hyperaccumulators are plants that can take up and concentrate more than 0.1% of

a given element in their tissue (Brooks, 1998). Metal hyperaccumulation is a rare

phenomenon in terrestrial higher plants. The first arsenic hyperaccumulator, Pteris vittata

L., was identified by Ma et al. (2001), followed by Pityrogramma calomelanos L.

(Francesconi et al., 2002) and many other species of the Pteris genus such as P. cretica

L., P. longifolia L., P. umbrosa L., and P. argyraea L. (Zhao et al., 2002), and P.

quadriaurita L., P. ryiunkensis L. and P. biaurita (Srivastava et al., 2006).

Pteris vittata is a fast-growing fern and has the potential to produce relatively

high biomass while accumulating a large amount of arsenic in its aboveground tissue.

These characteristics and its ability to survive in different environments make this plant

an exceptional candidate for use in phytoremediation of arsenic-contaminated soils.

To increase the efficiency of phytoextraction of arsenic contaminated soils using

P. vittata, besides understanding its arsenic tolerance and detoxification mechanisms, it is

important to learn more about the biological processes involved, such as plant arsenic

uptake, plant nutritional requirements, rhizosphere processes and mobilization and






3


bioavailability of arsenic in the soil. Experiments included in our study had the following

objectives:

1. Evaluate the efficiency of P. vittata in extracting arsenic from different arsenic-
contaminated soils

2. Evaluate the importance of the physiological age of P. vittata on its arsenic uptake
and plant growth

3. Determine the characteristics of the root systems and changes in the rhizosphere of
P. vittata in comparison with a non-hyperaccumulator

4. Compare the efficiency ofP. vittata in arsenic phytoextraction in comparison with
Pteris biaurita, another arsenic hyperaccumulator.














CHAPTER 2
LITERATURE REVIEW

Arsenic

Occurrence, Availability and Toxicity

Arsenic concentrations range from below 10 mg kg1 in non-contaminated soils

(Adriano, 1986) to as high as 30,000 mg kg-1 in contaminated soils (Vaughan, 1993). In

rocks, arsenic is concentrated in magmatic sulphide minerals and iron ore. The most

important arsenic ores include arsenical pyrite or arsenopyrite (FeAsS), realgar (AsS),

and orpiment (As2S3).

Arsenic has been identified as a major toxic contaminant in many countries.

Various human activities have elevated the arsenic levels in soils such as the production

and the use of arsenical pesticides (fungicides, herbicides, and insecticides). In addition,

manufacture of arsenic-based compounds, smelting of arsenic-containing ores, and

combustion of fossil fuels have also contributed to arsenic contamination in soils, water,

and atmosphere (Azcue and Nriagu, 1994). An increase in industrialization has also lead

to an increase in the amount of arsenic present in biosolids. Arsenic deposits from the

atmosphere, runoff, and effluents of industries often increase the concentration of arsenic

in biosolids. Woolson (1983) reported a range of 0 to 188 mg kg-1 As dry weight of

biosolids. Biosolids are often disposed on land and may then increase arsenic

concentrations in the top 20 cm of soil by up to 0.15% (O'Neill, 1990).

Sorption onto soil particles is an important process for arsenic immobilization in

soils. Many studies have been devoted to arsenic sorption on well-characterized minerals









or soil particles (clay, oxides of Al, Fe, and Mn, calcium carbonates, and/or organic

matter) (Sadiq, 1997; Dobran and Zagury, 2005). The high affinity of arsenic for oxide

surfaces is well known and is affected by several biogeochemical factors such as pH,

redox potential, and competing ions (Adriano, 2001). The activity of arsenic in soil

solution is mostly controlled by surface complexation reactions on oxides/hydroxides of

Al, Mn, and especially Fe (Inskeep et al., 2002). The mobility of arsenic in soil is pH

dependent as arsenate is preferentially sorbed onto hydrous oxides in the pH range of 4 to

7; whereas, arsenite is preferentially sorbed for in the pH range of 7 to 10 (Pierce and

Moore, 1982). Frost and Griffin (1977) have shown that arsenic sorption onto kaolinite

and montmorillonite is pH dependent, while Lin and Puls (2000) confirmed that arsenate

is more strongly sorbed onto these minerals than arsenite. Furthermore, arsenate

adsorption onto humic substances reaches a maximum at pH 5.5, while arsenite

adsorption peaks at much higher pH of 8.5 (Thanabalasingam and Pickering, 1986).

The bioavailability, toxicity, and chemical behavior of arsenic compounds are

largely influenced by the form and speciation of arsenic. In natural systems, arsenic can

exist in four oxidation states: (-3), (0), (+3), and (+5). Arsenate [As(+5)] and arsenite

[As(+3)] are the main forms present in soils (Harper and Haswell, 1988). Arsenate

prevails under aerobic conditions and is somewhat less toxic and also less mobile than

arsenite (dominant form under anaerobic conditions) because arsenate sorbs more

strongly than arsenite onto minerals (Pierce and Moore, 1982). Generally, inorganic

arsenicals are more toxic than organic arsenicals while the trivalent oxidation state is

more toxic than the pentavalent oxidation state (Adriano, 2001).









Remediation of Arsenic Contaminated Soils

Many remediation techniques are available to address the problems in

contaminated sites. However, the relatively high capital expenditure, unsuitability for

large areas, and environmental disruption are some of the shortcomings of those

techniques. No single soil-remediation technique is suitable for all situations. Careful

investigation of the contaminated site characteristics, contaminant problem, treatment

options and treatment timeframe must be considered.

Current remediation methods for arsenic-contaminated soils include soil removal

and washing, physical stabilization, and/or the use of chemical amendments, all of which

are expensive and disruptive, with an average cost of $ 404,700 per ha (Raskin et al.,

1997). Following are some selected current remediation technologies for arsenic-

contaminated soils (USEPA, 2002a).

a. Capping: a hard cover is placed on the surface of a contaminated soil. It

is a simple method to reduce contaminant exposure. However, it does not

remove contaminants from the soil.


b. Solidification and stabilization: the contaminated soil is mixed with

stabilizers to reduce the arsenic mobility in a soil. It can be relatively

costly.


c. Vitrification: arsenic is chemically bonded inside a glass matrix forming,

silicoarsenates.


d. Soil flushing: uses water, chemicals, or organic to mobilize arsenic in a

soil and then flush it from the soil.









e. Phytoremediation/phytoextraction: uses plants to take up arsenic from

soil.


Ex situ remediation methods

f Excavation: physical removal and disposal of a contaminated soil in a

designated landfill. Even though it produces rapid results, excavation is

often expensive because of the operation, transport, and special landfill

requirements.


g. Soil washing/Acid extraction: based on suspension or dissolution of

arsenic in a water-based solution to concentrate the contaminant.


Phytoremediation

Phytoremediation includes several methods that use plants to either remove

contaminants or render them harmless in soil and water systems. It can be applied to

both organic and inorganic contaminants in soil and water (Salt et al., 1998). This

practice has been growing in popularity, because of its overall cost-effectiveness

(Watanabe, 1997; Salt et al., 1998; Kabata-Pendias and Pendias, 2001). The term

phytoremediation includes the following strategies.

Phytoextraction

Phytoextraction is the use of contaminant-accumulating plants, which are able to

extract and translocate contaminants to the harvestable parts. Phytoextraction is the most

effective strategy of phytoremediation, although technically the most difficult one. It

uses tolerant plants that concentrate soil contaminants in their aboveground biomass and

the contaminant-enriched biomass can then be properly disposed (Kramer, 2005).









Phytostabilization

Phytostabilization refers to the use of contaminant-tolerant plants for mechanical

stabilization of contaminated soils to prevent soil erosion and to reduce air-borne

transport and leaching of contaminants. It is used to provide a vegetation cover for a

contaminated site, thus preventing wind and water erosion (Kramer, 2005). Plants that

are suitable for phytostabilization have an extensive root system, provide a good soil

cover, are tolerant to the contaminants, and ideally immobilize the contaminants in the

rhizosphere. Arsenic-tolerant plants that may be potentially used for phytostabilization

purposes have been known for a long time (Rocovich and West, 1975; Porter and

Peterson, 1977; Benson et al., 1981).

Phytoimobilization

Phytoimmoblization is the use of plants to decrease the mobility and

bioavailability of contaminants by altering soil factors that lower contaminant mobility

by formation of precipitates and insoluble compounds and by sorption onto the roots.

Based on the chemical similarities between arsenic and phosphorus, there may be

formation of arsenic/lead compounds as shown for phosphorus/lead precipitates in the

rhizosphere ofAgrostis capillaris (Cotter-Howells et al., 1999). Other plant-mediated

processes of arsenic immobilization at the soil-root interface involve pH reduction and

oxidation of the root environment by 02 release from roots. Doyle and Otte (1997) found

accumulation of arsenic onto the iron plaque in the oxidized rhizosphere of salt marsh

plants, which may provide an effective immobilization and detoxification mechanism for

the plants.









Phytovolatilization

Phytovolatilization is the use of plants to volatilize contaminants and has been

demonstrated for Hg and Se. In the case of Hg, this was achieved by genetic

manipulation of plants (Rugh et al., 1996); whereas phytovolatilization of Se occurs

naturally in plants (Terry and Zayed, 1994). De Souza et al. (1999) demonstrated that

rhizosphere bacteria can enhance the Se volatilization and accumulation in plants.

Volatilization of arsenic is also known to occur in natural environments (Frankenberger

and Arshad, 2002), but rhizosphere studies have not been reported for the formation of

gaseous arsenicals enhancement at the soil-root interface. Available information on

arsenic volatilization for soil suggests that in the absence of plant roots, volatile

compounds account only for small proportions of total arsenic (Turpeinen et al., 1999).

Phytoextraction of Arsenic

Arsenic is a nonessential element for plants, and inorganic arsenic species are

generally phytotoxic. Under normal conditions, arsenic concentrations in terrestrial

plants are usually less than 10 mg kg-1 (Matschullat, 2000). Different plants contain

different levels of arsenic. The following plants contain arsenic in an increasing order:

cabbage (0.020 to0.050 mg kg-1) < carrots (0.040 to 0.080) < grass (0.020 to 0.160) <

potatoes (0.020 to 0.200) < lettuce (0.020 to 0.250) < mosses and lichens (0.26) < ferns

(1.3) (Matschullat, 2000).

An average toxicity threshold of 40 mg kg-1 was established for crop plants

(Sheppard, 1992). The chemical behavior of arsenic is largely similar to that of

phosphorus in soils and plants. In all plant species tested so far, arsenate is taken up via

the phosphate transport systems (Asher and Reay, 1979; Lee, 1992; Meharg and Macnair,

1992). As a phosphate analog, arsenate can replace phosphate in many biochemical









processes, thus disrupt phosphate metabolism in plants. For example, arsenate can

disrupt mitochondrial oxidative phosphorylation, thus the production of nucleotide

adenosine triphosphate, the main energy source for cells. This process is known as

arsenolysis, or the hydrolytic process whose first step is the replacement of arsenate for

phosphate (Oremland and Stolz, 2003). Unlike arsenate, arsenite reacts with sulfhydryl

groups of enzymes and tissue proteins, leading to inhibition of cellular function and death

(Meharg and Hartley-Whitaker, 2002).

The transfer of arsenic from soils to plants is low for most plant species. This is

due to several factors including: i) the restricted uptake by plant roots, ii) the limited

translocation of arsenic from roots to shoots, iii) arsenic phytotoxicity even at low

concentrations in plant tissues, and iv) the low bioavailability of arsenic in soil (Wang et

al., 2002).

Among phytoremediation methods, phytoextraction is the most suitable for

arsenic contaminated soils. It represents one of the largest economic opportunities for

phytoremediation. This is due to the size and scope of environmental problems

associated with arsenic contaminated soils, minimal environmental disturbance, public

acceptance and the competitive advantage offered by the plant-based remediation

technology (Raskin et al., 1997; Susarla, 2002).

After harvesting arsenic-enriched plants, the weight and volume of contaminated

material can be further reduced, transported, and disposed off site as hazardous material

(Salt et al., 1998; Ma et al., 2001). Successful application of phytoextraction to arsenic

contaminated soils depends on many factors, among which the concentration and









bioavailability of arsenic, the nutrient and water status of the soils and the capacity of the

plant to access the arsenic (Figure 2-1).

The arsenic bioavailability in soils is regulated by various physical, chemical and

biological processes and their interactions (Ernst, 1996). The physical characteristics of

soils have a great influence on the bioavailability of arsenic. For instance, soils with very

fine texture offer a physical resistance that may demand a lot of energy from the plant in

order to penetrate deeper layers. In this situation, arsenic may be inaccessible for plant

uptake.






Plant biomass / Plant specie



Contaminant bioavailability Contaminant concentration
Phytoreinediation

Uptake Rhizosphere processes



Soil characteristics Nutrient bioavailability

Figure 2-1. Factors that influence the efficiency of phytoextraction

Arsenic speciation and chemical conditions such as soil acidity and redox

potential can largely determine arsenic bioavailability (Wenzel et al., 2002). The soil

biota can strongly modify the chemical and physical conditions and processes, which

determine arsenic bioavailability. Rhizosphere interactions plays a key role in controlling

nutrient bioavailability to crop plants (Hinsinger, 2001) as well as in the understanding of

the processes occurring during phytoremediation (Lombi et al., 2001). The availability of









arsenic in a soil around the roots is strongly affected by root exudates and root

depositions mucilagee and border cells) but also by microbial activities (Lombi et al.,

2000). The processes occurring in the rhizosphere such as plant uptake, changes of pH

and redox potential, root exudation and etc. alter the chemical compositions of the soil-

root interface. It may also influence the bioavailability of arsenic in the soil and

consequently the efficiency of phytoextraction. For instance, under oxic soil conditions,

an increase in the rhizosphere pH could favor mobilization of labile and exchangeable

As(V), enhancing plant uptake (Wenzel et al., 2002). Also, nitrogen fertilization as

nitrate (NO3s) can potentially increase the rhizosphere pH, and thus possibly enhance

arsenic accumulation in plant tissues.

The symbiotic association of plant roots with mycorrhizal fungi in arsenic

contaminated soils can influence its bioavailability by exploiting a greater soil volume

and by solubilizing the arsenic. Therefore, the interactions of arsenic with soil matrix and

the ability of plants to continually accumulate and detoxify arsenic in their shoot system

are essential to the phytoremediation concept.

Phytoextraction can be accomplished by using either tolerant high biomass plant

species or hyperaccumulator plant species. The growth and remediation potential has

been assessed for various plants including cottonwood (Populus deltoides Bartr.), cypress

(Taxodium distichum L.), eucalyptus (Eucalyptus amplifolia Naudin, E. camaldulensis

Dehnh., and E. grandis Hill), and leucaena (Leucaena leucocephala L.), which are all

potential high biomass species. However, the use of hyperaccumulator plants has the

advantage of producing a more concentrated residue, reducing the final disposal of the

contaminant-rich biomass.









Unfortunately, most metal hyperaccumulator plants grow slowly with low

biomass, while plants that produce a high biomass are usually sensitive to high metal

concentrations. The energy costs of metal tolerance mechanisms are partially responsible

for this phenomenon (trade-off hypothesis). There are, however, exceptions (e.g. the

nickel hyperaccumulator Berkheya coddii and arsenic hyperaccumulator ferns),

indicating the capacity of a plant to accumulate and tolerate high metal concentrations in

shoots and to produce high amounts of dry matter is not always mutually exclusive

(Robinson et al., 1997; Ma et al., 2001).

Plant species used in the phytoextraction of arsenic should be able to thrive in a

contaminated site and at the same time accumulating a substantial amount of arsenic in

the aboveground tissues. They should also be responsive to agricultural practices

designed to enhance arsenic accumulation and to allow repeated planting and harvesting

of arsenic-rich biomass (Tu and Ma, 2002). However, continuous phytoextraction

depends on the natural ability of a plant to accumulate, translocate and tolerate high

concentrations of arsenic over the entire growth cycle (Garbisu and Alkorta, 2001) as

well as the arsenic bioavailability in the soil.

Hyperaccumulator Plants

Plants show several response patterns when growing in the presence of toxic

metals, such as tolerance, indicator and hyperaccumulation (Figure 2-2). Most plants are

sensitive even to low concentrations, others have developed resistance and a reduced

number behave as hyperaccumulators of a metal (Baker and Brooks, 1989; Schat et al.,

1999). Tolerant species are those that can grow in a soil with metal concentrations toxic

to most other plants. However, they are not necessarily indicators or hyperaccumulators,









as tolerant non-accumulators can exclude metals from entering the root tissue. Both

indicator species and hyperaccumulators are also tolerant (Assuncgo et al. 2001; Bert et

al. 2003). On the other hand, hyperaccumulators take up particularly high amounts of a

toxic element in their shoots during normal growth and reproduction cycle (Baker and

Whiting 2002). Metal resistance in species with exclusion strategy is frequently based on

reduced metal uptake into the roots, preferential storage of metals in the root vacuoles

and restricted translocation into the shoots. Hyperaccumulators, in contrast, take up more

metals, store a lower proportion of them in the root vacuoles, and export higher amounts

to the shoots.

Hyperaccumulators are plants that can take up and concentrate greater than 0.1%

of a given element in their tissue (Brooks, 1998). Metal hyperaccumulation is a rare

phenomenon in terrestrial higher plants. To date, over 400 plant species have been

identified as metal hyperaccumulators, representing <0.2% of all angiosperms (Brooks,

1998; Baker et al., 2000; Ma et al.' 2001). Approximately two-thirds of the known

hyperaccumulators are Ni accumulators. This is because of the widespread occurrence of

Ni-rich ultramafic (serpentine) soils and the long history of geobotanical studies of

ultramafic floras. Plant species that are able to hyperaccumulate Cd, Co, Cu, Pb, Zn, and

As are much less numerous.

Hyperaccumulator plants have evolved internal mechanisms that allow them to

take up and tolerate high concentrations of metals that would be toxic to other organisms

(Lasat, 2002). They are adapted to the particular environmental conditions of their habitat

and high metal accumulation may have contributed to their defense against herbivores

and fungal infections (Martens and Boyd, 2002).









The definition of a hyperaccumulator has to take into consideration not only the

metal concentration in the aboveground biomass, but also the metal concentration in the

soil. Both bioaccumulation factor (BF) and translocation factor (TF) have to be

considered while evaluating whether a particular plant is a hyperaccumulator (Ma et al.,

2001). The term BF, defined as the ratio of metal concentration in plant biomass to that in

the soil, has been used to determine the effectiveness of plants in removing metals from

soil. The term TF, defined as the ratio of metal concentrations in the shoots to those in

the roots of a plant, has been used to determine the effectiveness of plants in translocating

metals from the roots to the shoots (Tu and Ma, 2002). Therefore, an arsenic

hyperaccumulator plant should have BF > 1 and TF > 1 as well as total accumulation >

1,000 mg kg-1 arsenic in plant biomass.



Hyperaccumulator




q Excluder


Indicator





Metal Cone in soil
Figure 2-2. Conceptual response of hyperaccumulator, indicator and excluder plants for
metal concentrations in the aboveground tissues in relation to increasing metal
concentrations in a soil. (Adapted from Ghosh and Singh (2005).

While some plants can survive in an environment containing high concentrations

of metals, they may not show an ability of hyperaccumulating the metals. For example,

Agrostis tenuis concentrated 3, 470 mg kg-1 of arsenic when growing in a soil containing









as high as 26, 500 mg kg -1 of arsenic. Even though the plant arsenic concentration was

very high, it can not be characterized as an arsenic hyperaccumulator. This is because the

BF and TF were both lower than one.

Arsenic Hyperaccumulation by Pteris vittata

Arsenic hyperaccumulators were discovered only recently and the majority of

them are fern species in the Pteris genus. Ma et al. (2001) reported the first known

arsenic hyperaccumulator plant, Pteris vittata L, commonly known as Chinese Brake

fern. The plant was found on a site contaminated with the wood preservative chromated

copper arsenate in Central Florida. Afterwards, other arsenic hyperaccumulator ferns

have been identified in Florida and elsewhere (Francesconi et al., 2002; Zhao et al., 2002;

Srivastava et al., 2005).

Other non fern plants have also been reported as "arsenic hyperaccumulators"

(>1,000 |tg g-1 As) growing on mine wastes from various sites in the United Kingdom

(Porter and Peterson, 1977) and on smelter wastes in northeast Portugal (De Koe, 1994).

However, though accumulating large amounts of arsenic, these plants do not concentrate

arsenic, i.e. arsenic concentrations in the plant are lower than those in the soil. Hence,

they should not be classified as arsenic hyperaccumulators.

Although there are a number of ferns known to accumulate arsenic, P. vittata is by

far the most studied arsenic hyperaccumulator plant. It is native from China (Nelson,

2000) but is widespread in the old world, occurring from Europe to Asia. In the U.S., this

fern grows in the southeast and southern California (Ma et al., 2001). In Florida, P.

vittata is one of the only three naturalized exotic ferns (Nelson, 2000). Pteris vittata ferns

are very diverse and have survived in great numbers and adapted to both a vastly









changing environment and competition from seed plants. They are widespread thriving in

both temperate and tropical climates (Matschullat, 2000). Also, the distribution of P.

vittata is controlled by its requirement of a well-drained alkaline substrate exposed to

abundant sunshine (Ma et al., 2001).

Pteris vittata has an extraordinary capacity not only to tolerate and take up arsenic

but also translocate large amounts of arsenic to the fronds, with frond concentrations

reaching levels up to 100-fold greater than the soil concentrations. In addition, this plant,

with its high biomass, is an easy-to-grow, vigorous perennial that is resistant to disease

and pests, and exhibits a high arsenic accumulation rate. While natural arsenic

concentrations in most plants seldom exceed 1 mg kg-1 (Porter and Peterson, 1977), after

6 weeks of growth P. vittata accumulated 438-755 mg kg-1 As in a uncontaminated soil

and 3,525 to 6,805 mg kg-1 As in a contaminated soil (Ma et al., 2001). It also removed

26% of the added arsenate after 18 weeks of growth (Tu and Ma, 2002). Liao et al.

(2004) showed that, within seven months, P. vittata could extract up to 7.8 % of the

arsenic from a soil containing 64 mg kg-1 As.

The ability of P. vittata to take up high concentrations of arsenic and sequester it

into aboveground portions when grown on either uncontaminated or arsenic-enriched

soils implies that the fern has highly effective arsenic scavenging mechanisms. Another

interesting characteristic of the ferns is their pereniality, which allows for successive

cuttings of the aboveground biomass while growing in contaminated soils.

An intriguing observation was made by Tu et al. (2002). In a pot experiment, they

observed that addition of 50 mg kg-1 arsenate to a sandy soil increased the fern biomass

by 107%. After 12 weeks of growth, P. vittata produced more aboveground biomass in









soils containing 50 and 100 mg kg-1 arsenic compared to the control. The increase in

plant biomass may be due to the arsenic-enhanced phosphate concentrations in the soils.

However, when the soil arsenic concentration reached 200 mg kg-1, there was a slight

decrease in fern biomass. Another interesting fact is that arsenic has been shown to leach

from P. vittata fronds as they senesce (Tu et al., 2003). This may pose a potential

drawback to the use of P. vittata in phytoremediation of arsenic contaminated soils, as the

arsenic may be returned to the soil.

To date, the only non-Pteris fern to exhibit the ability of arsenic hyperaccumulation

is Pityrogramma calomelanos (Francesconi et al., 2002). Its fronds accumulated 2,760 to

8,350 mg kg-1 arsenic when growing in soil containing 135 to 510 mg kg-1 arsenic.

Interestingly, the fronds with the greatest arsenic concentration were collected from the

ferns growing in the lowest arsenic concentration in the soil (135 mg kg-1 As)

(Francesconi et al. (2002). In the case of P. vittata, studies have shown that the arsenic

concentration in the fronds increase with increasing arsenic concentration in the soil.

Arsenic Species in Pteris vittata

The form of arsenic accumulated by plants is important in determining its

suitability to remediate arsenic contaminated sites, because the arsenic-rich fronds will

need to be disposed properly to avoid further contamination (Ma et al., 2001). Pteris

vittata has been shown to be capable of taking up both inorganic and organic arsenic

species including arsenate, arsenite and monomethylarsonic acid (MMA), concentrating

up to 93% of the arsenic in the fronds (Ma et al., 2001; Kertulis et al., 2005). Research on

arsenic hyperaccumulation by P. vittata showed that arsenic exists in the plant mostly as

inorganic species, and up to 94 % of the arsenic in the fronds is present as arsenite.

Similar results were observed when arsenic was supplied to the ferns in several different









forms (Zhang et al., 2002; Tu et al., 2003; Kertulis et al., 2005). Regardless of the arsenic

species supplied to the fern, >90% of the total arsenic in the roots is present as arsenate,

versus approximately 94% arsenite in the fronds. In both studies, low concentrations of

organic arsenic were found in the fern. Similarly, in a study involving P. calomelanos,

most of the arsenic in its fronds was present as arsenite. Only trace amounts of

monomethylarsonate (MMA) and dimethylarsinate (DMA) were found in a few samples

(Francesconi et al., 2002).

The uptake rates of arsenite and arsenate by P. vittata roots may not be equal,

because the species are taken up through different mechanisms. For instance, Wang et al.

(2002) found that arsenate was taken up more quickly by P. vittata than was arsenite,

especially in the absence of phosphate. The authors suggest that this is due to arsenate

being assimilated via phosphorus-suppressible uptake system in the roots.

Arsenic Tolerance and Detoxification Mechanisms

Plants often contain trace concentrations of contaminants of concern. At low levels,

plants can usually metabolize or dispose of these compounds without significant injury.

Generally, at high concentrations in soil or water, plants are not able to metabolize

contaminants. However, some plants can survive and even grow well when they

accumulate high concentrations of toxic elements as is the case of the hyperaccumulator

plants. In ferns like P. vittata, arsenic is taken up at high rates and at concentrations

proportional to the arsenic concentrations in the growth media at least up to a certain

point where arsenic availability becomes a limiting factor and its arsenic concentration

exceeds its detoxification ability (Ma et al., 2001; Zhang et al., 2002; Wang et al., 2002;

Kertulis et al., 2005).









The fact that P. vittata could survive in a soil spiked with 1,500 mg kg-1 arsenic and

concentrate 2.3 % of arsenic in its biomass indicates that it is equipped with efficient

mechanisms for detoxifying accumulated arsenic. Mechanisms employed by plants to

detoxify metals include chelation, compartmentalization, biotransformation and cellular

repair (Salt et al., 1998). For example, heavy metals are generally transported and

deposited in a vacuole as metal chelates. Baker et al. (2000) reported that the solution

concentration of free metal ions taken up by plants into their tissues is reduced greatly

when they are chelated by specific high-affinity ligands (like oxygen-donor, sulfur-donor,

or nitrogen-donor ligands). Sulfur-donor ligands (like metallothioneins and

phytochelatins) form highly stable complexes with heavy metals, because sulfur is a

better electron donor than oxygen. In fact, the reduction of arsenate to arsenite is

catalyzed by glutathione (GSH) in microbes (Rosen, 2002). However, this has not been

demonstrated in P. vittata.

The formation of arsenite-tris-thiolate complexes has been demonstrated both in

vivo and in vitro by electrospray ionization mass spectroscopy (Schmoger et al., 2000)

and x-ray absorption spectroscopy (Pickering et al., 2000) with the thiolate derived from

either GSH or phytochelatins (PCs). The majority of the reports on arsenic toxicity in

plants show a clear role for PCs in the detoxification of arsenic (Schmoger et al., 2000;

Hartley-Whitaker et al., 2001, 2002; Schat et al., 2002). Reina et al. (2005) demonstrated

that PCs were the most abundant thiols in white lupin under higher arsenic exposure

levels than other plants could tolerate. Together, GSH and PCs were able to complex the

majority of arsenic in the shoots. However, the role of PCs seems to be minor in arsenic

hyperaccumulator ferns (Zhao et al., 2003; Raab et al., 2004; Zhang et al., 2004).









As for arsenic hyperaccumulators P. vittata and P. calomelanos the reduction of

arsenate to arsenite inside plant cells occurs as well (Ma et al., 2001; Francesconi et al.,

2002). This reduction of arsenate inside the plant cells is intriguing because arsenite is

more toxic than arsenate. Additionally, P. vittata was shown to have only 4.5% of its

arsenic completed with phytochelatins, as a glutathione-arsenite- phytochelatin complex

(Zhao et al., 2003). In a study by Raab et al. (2004), the arsenic hyperaccumulator, P.

cretica, had only 1% of its arsenic completed with phytochelatins. The conclusion

reached in both studies was that the phytochelatins may act as shuttles for transporting

arsenic in a non-toxic form through the cytoplasm and into the vacuoles. However,

arsenic complexation with phytochelatins by itself does not account for the high efficient

detoxification mechanism in arsenic hyperaccumulator ferns, suggesting a novel

mechanism of arsenic tolerance in P. vittata.

Arsenic detoxification in microorganisms includes methylation and

biotransformation. Some bacteria enzymatically reduce arsenate to arsenite by Ars C, and

the arsenite is then pumped out by the membrane protein Ars B (Cai and Ma, 2003).

None of these mechanisms were identified in the ferns. Therefore, an important gap in the

arsenic hyperaccumulation mechanism is how the ferns rapidly translocate arsenic from

the roots to the fronds, and still able to survive the exceedingly high concentrations of

arsenite in the fronds, which may perturb the cellular function by disrupting the

sulfhydryl groups of proteins.

Effects of Heavy Metals on Arsenic Hyperaccumulation

Many contaminated sites contain a variety of contaminants. So another important

point to be considered in phytoremediation is the ability of arsenic hyperaccumulator

plants to grow in soils contaminated with other heavy metals. The ability of P. vittata to









survive in soil contaminated with Cu and Cr is confirmed by the fact that this fern was

first discovered in a soil contaminated with CCA (chromated-copper-arsenate). The total

arsenic in that soil was 131 mg kg-1, Cr was 40.6 mg kg-1, and Cu was 8.30 mg kg1.

Fayiga et al. (2004) studied the effects of Cd, Ni, Pb, and Zn on the arsenic

accumulation by P. vittata. They observed the concentrations of heavy metals (50 and

200 mg kg-1) negatively affect the growth of the fern. Still total plant biomass increased

by 12-fold after 8 weeks of growth.

However, the efficiency of arsenic accumulation by the fern depended on the

concentration of the metals in the soil (Fayiga et al., 2004; An et al., 2006). In the study

of Fayiga et al. (2004), arsenic uptake decreased with an increase in the metal

concentration except in Pb-treated soils. Though effective in taking up arsenic, P. vittata

had a limited capability to take up other metals. Arsenic transfer factors ranging from

15.8 to 46.1, indicating that P. vittata was able to effectively translocate arsenic in the

presence of other metals. Caille et al. (2004) reported that P. vittata grew poorly in a soil

heavily contaminated with As, Cu, Pb and Zn, probably as a result of Zn and Cu toxicity.

The concentrations of Zn and Cu in the plants were well above the toxicity thresholds of

100-500 mg kg-1 Zn and 20 mg kg-1 Cu, respectively (Kabata-Pendias and Pendias, 2001).

Phytotoxicity of Zn and Cu not only decreased plant growth, but also arsenic uptake by

P. vittata, resulting in a negligible phytoextraction of arsenic from the soil.

Contamination of multiple metals or metalloids, particularly at high concentrations, thus

presents a challenge for phytoremediation.

Effect of Phosphate on Arsenic Hyperaccumulation

The competitive effect of arsenate and phosphate in soils (Livesey and Huang,

1981; Manning and Goldberg, 1997; Smith et al., 2002; Tu and Ma, 2003) and their









interactions in plants (Asher and Reay, 1979; Meharg and Macnair, 1992; Fourquerean

and Cai, 2001) have been demonstrated. Basically, the uptake of arsenate and phosphate

by plants has been reported to be competitive (Tu and Ma, 2003). Moreover, arsenate

interferes with phosphate metabolism causing toxicity in plants. On the other hand,

phosphate may be able to alleviate arsenate toxicity by improving phosphate nutrition

(Sneller et al., 1999). Thus, the presence of phosphorus in contaminated soils plays a role

in the phytoextraction process.

Tu and Ma (2003) studied the influence of phosphate on the arsenic

phytoextraction by P. vittata. They observed that, at low and medium arsenate levels (50

to 200 mg kg-1, respectively), phosphate had slight effect on arsenate accumulation by

and growth of P. vittata. However, phosphate substantially increased plant biomass and

arsenate accumulation by alleviating arsenate phytotoxicity at high arsenate levels (400

mg kg-1). The authors suggested a minimum P/As molar ratio of 1:2 in soil solution for

effective removal of arsenic by the plant.

Fayiga et al. (2006) reported that addition of phosphate rock in a soil spiked with

arsenic and other metals increased plant arsenic uptake, from 608 to 1,046 mg kg-1 in the

fronds. This shows the ability of phosphate rock to aid in arsenic accumulation by P.

vittata in a multi-metal system by reducing the toxic effects of the metals on the fern;

thereby, enhancing plant arsenic uptake. Boisson et al. (1999) also reported an increase in

arsenic uptake by plants (Zea mays cv. Volga and Phaseolus vulgaris cv. Limburgse

vroege) after applying hydroxyapatite to a soil contaminated with Zn, Pb, Cu, Cd and As.

They suggested that the increased phosphate concentration in the soil solution might be









responsible for increased plant uptake, since phosphate can displace adsorbed arsenate

from soils and increase the availability of arsenate (Smith and Naidu, 1998).

In a chromated-copper-arsenate contaminated sandy soil, addition of a large

amount of rock phosphate (15 g kg-1) also increased arsenate uptake by P. vittata (Cao et

al., 2003). Therefore, phosphorus additions, in the first place, enhanced plant growth, and

secondly, mobilized exchangeable arsenic resulting in an increased total arsenic uptake.

Despite the appeal of increased arsenic availability from application of soil

amendments such as phosphorus fertilizers for the purpose of phytoremediation, it is

environmentally paradoxical in that the benefit of enhanced arsenic removal from soils

may be offset by the risk of increased arsenic leaching into ground water. Therefore, the

rate of arsenic uptake should be higher than the rate of arsenic release in the soil. In this

regard, a very extensive root system is a valuable characteristic in a hyperaccumulator

plant.

Mycorrhizae Association and its Role in Arsenic Hyperaccumulation

Mycorrhizae have a well-documented role in increasing the plant uptake of

phosphorus (Smith and Read, 1997) and other poorly mobile elements, and are

recognized as an important component of bioremediation strategies for soils

contaminated with heavy metals (Khan et al., 2000). In fact, mycorrhizae are integral,

functional components of plant root systems and the fungi involved can play an important

role either in alleviating metal toxicity in plants or enhancing metal uptake. For instance,

Weissenhorn and Leyval (1995) reported higher concentrations of heavy metals in plants,

even resulting in toxic levels, due to arbuscular mycorrhizae (AM) colonization, while

Heggo et al. (1990) observed reduced metal concentrations in the shoots due to









mycorrhizal colonization. On the other hand, Galli et al. (1995) found no effects exerted

by AM fungi on metal concentrations in plant shoots and roots.

Since arsenic and phosphorus are chemical analogues, it is expected that

mycorrhizal symbiosis may be involved in arsenic uptake as well as P uptake by arsenic

hyperaccumulator plants. However, few studies have been done in this regard. Ying et al.

(2004) investigated the colonization and diversity of AM fungi associated with common

pteridophytes and found no defined relationship between mycorrhizae and plants under

field condition. Out of all arsenic hyperaccumulator ferns, only association of P. vittata

with mycorrhizae has been investigated (Al agely et al., 2005; Leung et al., 2006).

In a study on the effect of increasing levels of arsenic and phosphorus on P.

vittata infected with mycorrhizae, Al Agely et al. (2005) observed that the AM fungi not

only tolerated arsenic amendment, but its presence increased the frond biomass at the

highest arsenic application rate (100 mg kg-1). The AM fungi also increased arsenic

uptake across a range of phosphorus levels, while phosphorus uptake was generally

increased only when there was no arsenic amendment, suggesting an important role of

AM fungi in arsenic accumulation by P. vittata.

The percentage of mycorrhizal infection in P. vittata increased as soil arsenic

concentrations increased (Leung et al., 2006). The authors also reported an increase in

arsenic accumulation by P. vittata infected with mycorrhizae and improvement of the

nutrient status of the plants. The presence of mycorrhizae likely increased the amount of

P transporters at hyphae level and, consequently, the amount of arsenic taken up by the

plant. The presence of mycorrhizae also seems to be related to the formation of thiol like

glutathione (Schutzendubel and Andrea, 2002), a phenolic defense system. It is






26


hypothesized that the production of thiols by P. vittata, induced by the increased arsenic

concentration, alleviated the toxicity of arsenic (Cai et al., 2004) and the plant can readily

retain arsenic with the aid of mycorrhizae (Diaz et al., 1996). Therefore, it is possible that

P. vittata may derive some benefit from the symbiosis by forming low molecular weight

thiols.














CHAPTER 3
ARSENIC ACCUMULATION BY Pteris vittata L FROM ARSENIC
CONTAMINATED SOILS

Introduction

Arsenic contamination in soils, groundwater, and drinking water is a serious

concern as it can affect both human and animal health. The extensive use of arsenic

compounds such as pesticides, insecticides, defoliants, wood preservatives, and soil

sterilants in the past (Azcue and Nriagu, 1994) has left a legacy in the history of arsenic

contamination in the environment and a burden for the future generations.

For instance, pesticides containing arsenic trioxide were largely used in cattle dip

vats in the southeastern USA to eradicate cattle fever tick resulting in many soils

contaminated with arsenic (Thomas et al., 2000). The production of the wood

preservative containing chromated-copper-arsenate (CCA) represented 67 % of the

arsenic used in 1992. Besides the contamination of soils in the sites of operation, CCA-

treated wood can potentially contaminate soils through the leaching of arsenic from the

wood. Smelting and mining activities represent significant sources of arsenic

contamination because pyrometallurgical production processes lead to large emissions of

Pb, Zn, Cu, Cd and As (Boisson et al., 1999). Moreover, herbicides containing

monosodium methane arsonate (MSMA) are still used for weed control in Florida (Cai et

al., 2002).

The use of plants to remove arsenic from soils is a fairly new technology. It is an

efficient and less costly way to treat contaminated soils. Hyperaccumulator plants possess









highly efficient mechanisms to acquire and concentrate arsenic in the plants. However,

not all contaminated sites are suitable to be treated by means of phytoextraction. This is

because the arsenic concentration in a soil, its availability and the depth of arsenic

contamination can be limiting factors. Moreover, the plant to be used for this purpose

must be tolerant to arsenic, adapted to the local soil and climate characteristics and take

up a large amount of arsenic.

During phytoextraction process, several crops of hyperaccumulator plants may be

needed to sequentially reduce soil arsenic concentration (Raskin et al., 1994). However,

continuous phytoextraction depends on the natural ability of a plant to accumulate,

translocate and tolerate high concentrations of metals over the whole growth cycle

(Garbisu and Alkorta, 2001).

Pteris vittata is a fern species identified by Ma et al. (2001) as an efficient

arsenic-hyperaccumulator plant. Because it hyperaccumulates an extremely high level of

arsenic (up to 2.3%) in its aboveground biomass and it is easy to grow in a variety of soil

environments, this plant has the potential to clean up arsenic contaminated sites

nationwide, and potentially worldwide. Furthermore, the perennial nature of P. vittata

make the phytoextraction process even more cost-effective since no replanting after

harvest is needed. However, practical issues such as the time required to achieve a given

target level, the long term efficiency of the process, and the arsenic pools depleted by the

plant still need to be addressed.

The aim of this study was thus to 1) assess the efficiency of repeated harvests of

the fronds of P. vittata growing in soils with different sources of arsenic contamination,

and 2) investigate the effects of plant arsenic uptake on arsenic redistribution in soils.









Materials and Methods

Soil Collection and Characterization

Five arsenic contaminated soils plus a soil with naturally high arsenic (Marl soil)

were used for this study. The soils were: contaminated with arsenical insecticide (Avon

soil), arsenical wood preservative (CCA soil), arsenical pesticide (CDV soil), arsenical

herbicide (EDS soil), and mining activities (Mining soil). Selected properties of the soils

are shown in Table 3-1. The soils were collected, air-dried, and passed through 2 mm

sieve. They were analyzed for: CEC by the ammonium acetate method (Thomas, 1982),

organic matter content by the Walkley Black method (Nelson and Sommers (1982), soil

texture by the pipette method (Day, 1965) and soil pH in 1:2 soil: water. Concentrations

of P, Ca, Mg, and trace elements were determined using the EPA Method 3050.

Experimental Design and Statistical Analyses

Six soils with different sources of arsenic contamination were used to grow P. vittata.

Each treatment was replicated four times and pots without plants were included as

controls. The pots were arranged in a completely randomized design.

SAS software was used for all statistical analyses (SAS Institute, 1987). Means and

standard deviations of arsenic concentrations were calculated for different treatments.

Analysis of variance was used to assess significant differences among treatments.

Experimental Set up

Pteris vittata used for the experiments were of similar age and size. One-year-old

ferns were transferred (1 per pot) to 2- gallon-size plastic pots filled with 4 kg of arsenic

contaminated soil. The soils were thoroughly mixed with Osmocote, extended time-

release base fertilizer (N-P-K = 18-6-12) at a rate of 2 g kg-1 soil. The fertilization was









applied annually. After transplanting, the ferns were watered to 60 % of the field

capacity.

Table 3-1. Selected chemical and physical characteristics of six arsenic contaminated
soils


Soil characteristic Marl Avon CCA CDV EDS Mining
pH 7.85 6.70 7.00 6.76 6.70 6.75
CECa 26.6 4.50 4.40 16.8 22.8 12.0
OM (g kg-1) 1.80 19.6 11.0 26.5 28.0 4.20
Total As (mg kg-1) 22.2 26.5 110 211 640 214
Extractable Asc (mg kg-1) 22.7 14.8 58.8 45.9 499 22.5
Extractable P (mg kg-1) 0.36 31.9 24.8 19.2 96.0 8.07
Extractable Ca (mg kg-1) 21000 1892 2960 2040 1602 4920
Extractable Mg (mg kg-1) 120 28.8 130 64.8 336 63.6
Fe oxides (mg kg-l)b 3.96 509 1322 5577 1117 1626
Al oxides (mg kg-l)b 5.23 632 884 1163 1056 179
Sand (g kg-1) 401 866 882 840 851 807
Silt (g kg-1) 410 89 91 140 117 152
Clay (g kg-1) 189 45 27 20 32 41
a:CEC: cation exchange capacity (Cmolc kg-1)
b: Amorphous Fe and Al extracted by 0.2 M oxalic acid + ammonium oxalate solution
c: Extractable elements were obtained using Mehlich III solution.

The first harvest was performed in the month of October, 2003, four months after

the transplant. The second (April, 2004) and third (October, 2004) harvests were

performed in a six months interval from the first harvest and from each other. The plants

aboveground biomass was analyzed for dry biomass weight and total arsenic

concentration.









Sampling, Digestion and Analysis

The plant samples were dried in a 65C oven for approximately 48 hours, weighed

and then ground into powder through a 1-mm mesh screen using a Wiley Mill for

digestion. Also, soil samples collected together with the plant samples were air-dried and

analyzed.

The soil samples were digested according to the EPA Method 305 la and the plant

samples a modified EPA Method 3051 (Chen and Ma, 1998; Komar et al., 1999). One

blank, one Standard Reference Material from National Institute of Standards and

Technology (NIST), one duplicate and one spiked sample were included for every 20

samples. Soil and plant samples were digested with nitric acid using a Hot Block

digestion system (Environmental Express, Mt. Pleasant, SC; EPA Method 3051).

Approximately 0.5-1.0 g of air-dried soil or 0.1-0.5 g of dry plant sample were

mixed with 1:1 HNO3 and allowed to set for approximately 24 hours. They were heated

at 105C for 2 hours and then cooled for 3 minutes. The samples were mixed with 1 ml

of 30% H202 and placed on the block digester for 15 additional minutes. After the

second heating, the samples were cooled completely and diluted to a 50 mL volume with

distilled water. A filter cartridge was placed in the bottom of the digestion tube. The

digested samples were analyzed for As concentration with a SIMMA 6000 graphite

furnace atomic absorption spectrophotometer (GFAAS, Perkin-Elmer, Norwalk, CT)

using the EPA SW 846 method 7060 A.

Arsenic Fractionation

Since it is assumed that plant arsenic uptake will substantially reduce soil arsenic

availability, it is important to determine how plant arsenic uptake affects arsenic









redistribution in a soil. A sequential extraction procedure was used to account for the

changes in different soil arsenic-pools as a function of plant uptake. According to Wenzel

et al. (2001), soil samples were fractionated into five arsenic fractions with decreasing

availability: (N) non-specifically bound, (S) specifically bound, (A) amorphous hydrous

oxide-bound, (C) crystalline hydrous oxide-bound and (R) residual (Table 3-2). Samples

from each fraction, with the exception of the residual fraction, were centrifuged at 3,500

rpm for 15 min and 20C after each extraction and/or wash. The supernatants were

collected and filtered through Whatman 42 filter paper and analyzed for arsenic

concentration using GFAAS.


Table 3-2. Arsenic sequential extraction procedure
Fractions Extracting solution
N- Non-specifically- bound (NH4)2SO4 0.05M
S- Specifically-bound (NH4)H2P04 0.05M
A-Amorphous hydrous NH4-oxalate buffer
oxide-bound (0.2 M) pH 3.25
C-Crystalline-hydrous- NH4-oxalate buffer
oxide-bound (0.2 M) + ascorbic
acid (0.1M) pH 3.25


Extraction condition
4 h shaking, 20C
16 h shaking, 20C
4 h shaking, 20C


30 min in a water
basin at 96 +3 C in
the light


R-Residual HN03/H202 Hot block digestion 1:50
*SSR=Soil solution ratio

Single Extractions

Water-extractable arsenic was obtained using a 1:10 soil to deionized water ratio,

shaking for two hours and centrifuging at 3500 rpm for 15 min (Olsen and Sommers,

1982). Mehlich III method (Mehlich, 1984) was commonly used as a standard procedure

in soil testing to assess the amount of P available in the soil for growing plants. As a

phosphorus analogue, this method was chosen to correlate plant arsenic availability in


SSR*
1:25
1:25
1:25


1:25









soil as well. Mehlich III extractable arsenic was obtained using Mehlich III extracting

solution (0.2 M CH3COOH, 0.25 MNH4NO3, 0.015 MNH4F, 0.013 MHNO3 and 0.001

MEDTA) after shaking for 5 min. Arsenic was also extracted with 1.6 mM organic acids

(phytic acid -1.4mM + oxalic acid 0.2 mM), in a 1:20 soil to solution ratio, shaking for

24 hours and centrifuging at 3500 rpm for 15 min. The concentrations of organic acids

used in this study were similar to those reported by Tu et al. (2004) in the root exudates

ofP. vittata. All the extracts were filtered through Whatman 42 filter paper and analyzed

for arsenic concentration using GFAAS.

Results and Discussion

Plant Growth

The aboveground biomass production and the re growth capacity are important

factors in the phytoextraction of arsenic using perennial plants since multiple harvests

will be needed in order to remove the arsenic from the soil (Fayiga and Ma, 2005).

Despite the different soil properties and arsenic concentration among the six soils,

the ferns grew well in all six soils during the first four months of the experiment (June-

October 2003), without showing any toxicity symptom (Figure 3-1). The plants biomass

was uniform in all soils ranging from 24.8 to 33.5 g plant-', greater than the biomass data

reported for P. vittata growing in pot experiments (Tu and Ma, 2002; Fayiga et al., 2004).

It should be pointed out that the 12-month-old plants used in this study were small, with

5-6 fronds.

There was a significant reduction in the frond biomass production in the

subsequent harvests, except for the ferns growing in the Marl and Mining soils in the

third harvest. For all soils, there was a significant decrease in the frond biomass in the









second harvest (40, 81, 82, 84, 79, and 71 % for the Marl, Avon, CCA, CDV, EDS and

Mining soils, respectively), likely due to the seasonal effect (cooler climate from October

to April). For all soils, October harvests yielded larger plant biomass than the April

harvest, except for the Marl and EDS soils.


Marl -22 Avon -27 CCA 110 CDV 211 Mining- 214 EDS 640
Soil


Figure 3-1. Fond biomass ofPteris vittata after first, second and third harvests in six
soils. Means followed by the same letter within the same soil are not different
by the Duncan test at p < 0.05.

Due to the alkaline nature of the Marl soil, condition that favors the fern growth,

dry matter yield was comparable in all three harvests. On the other hand, plants

drastically failed to regrow in the EDS soil after the first harvest, probably because of the

extremely high solubility of arsenic in this soil (Table 3-1).

Even though P. vittata has shown to be very tolerant to arsenic exposition, Tu and

Ma (2002) reported that addition of 500 mg kg-1 As to a sandy soil reduced the fern









biomass by 64 %, a common symptom of arsenic toxicity (Kabata Pendias and Pendias,

2001). However, in their study, addition of 200 mg kg-1 As did not affect biomass

production, suggesting a much higher tolerance of P. vittata as compared to normal

plants. The difference in arsenic concentrations in the Avon, CCA and CDV soils (Table

3-1) did not influence the dry matter yield since all had the same trends among different

harvests (Figure 3-1). However, the better performance of the CCA soil was likely due to

the higher pH and Ca concentration, and also due to the fact that this was the soil where

P. vittata was found naturally growing (Ma et al., 2001).

Overall, the regrowth capacity of the plants was related to the temperature effect.

That is, the difference in plant performance between harvests is due to colder temperature

prevailing during December to April. According to Jones (1998), P. vittata prefers

warmer climate. In a field experiment, Kertulius (2005) also reported greater biomass

production when P. vittata plants were harvested in the months of October and

December. Another factor that, associated with the temperature, could also explain the

poor regrowth capacity of the plants after the first harvest was the clipping procedure. All

the fronds, including the coiled young fronds (fiddleheads), were removed in the first

harvest. This did not happen in the second harvest.

The effect of successive fern cultivation on plant biomass and arsenic removal by

P. vittata is relatively scarce and inconsistent. For instance, pot experiments using P.

vittata have shown that the amount of biomass harvested decreased after two or three

successive cuttings as well as the amount of arsenic extracted from a soil (McGrath et al.,

2002). However, Kertulius (2005) found positive results when growing P. vittata under

field conditions. Likely, the exploitation of a greater soil volume and the amount of









residue produced at field condition may account for some of the differences between the

studies in pot (McGrath et al., 2002) and field condition (Ketulius, 2005).

Arsenic Concentrations in the Frond Biomass

The total arsenic concentration in the soils used in this study varied from 22 mg

kg-1 in the Marl soil to 640 mg kg-1 in the EDS soil (Table 3-1). The arsenic concentration

in the EDS soil was greater than the arsenic level of 500 mg kg-1 used by Tu and Ma

(2002), which was spiked as arsenate. Despite the high arsenic level, the plants grew well

and took up arsenic from all soils.

The frond arsenic concentrations ranged from 166 to 6,151 mg kg-1 in the first

harvest, from 110 to 3,056 mg kg-1 in the second harvest, and from 162 to 2,139 mg kg-1

in the third harvest (Table 3-3). The highest arsenic concentrations in plants were

observed in the EDS (from 2139 to 6151 mg kg-1), CDV (477 to 1872 mg kg-1) and

Mining (423 to 1079 mg kg-1) soils (higher soil arsenic concentrations) and the lowest in

the Marl (110 to 166 mg kg-1) soil (the lowest soil arsenic concentration).

Even though the Marl and Avon soils had similar arsenic concentrations, the

plants took up more arsenic from the Avon soil.

There was no significant difference in the frond arsenic concentrations of P.

vittata in the first and third harvest for all soils, except for the CDV and EDS soils.

However, cooler climate conditions not only affected plant biomass production but also

plant arsenic accumulation. Fern frond arsenic concentrations were 34, 46, 51, 75, 61 and

50% lower in the second harvest compared to those in the first harvest for the Marl,

Avon, CCA, CDV, Mining and EDS soils, respectively (Table 3-3).









Table 3-3. The frond arsenic concentrations ofPteris vittata after the first, second and,
third harvest in six soils.
Soil As levels Frond arsenic (mg kg -1)

mg kg 1 Harvest 1 Harvest2 Harvest 3

Marl 22 166 a 110b 162 a

Avon 27 336 a 181 b 280 a

CCA 110 659 a 325 b 747 a

CDV 211 1872 a 477 b 715 b

Mining 215 1079 a 423.4 b 962 a

EDS 638 6151 a 3056 b 2139 b

Means followed by the same letter in a row and within the same soil are not different by
the Duncan test at P < 0:05 within the same soil.
The frond arsenic concentrations increased with soil arsenic concentrations, in all

three harvests with a correlation coefficient of 0.97-0.98. However, the ability of the

plants to remove arsenic from the soils reduced with time. The highest arsenic

concentration (6,151 mg kg-1) was observed in the plants growing in the EDS soil, which

also had the highest soil arsenic concentration, both total and extractable (638 and 498

mg Kg-1 As, respectively; Table 3-1).

The high organic matter content of the EDS soil likely plays a role in the

availability of arsenic. However, little information is known on arsenic reactions with

organic matter. In fact, it is suggested that soil organic matter contributes very little to the

arsenic sorption in soils due to their anionic nature (Livesey and Huang, 1981; Wenzel et

al., 2001a). Therefore, considering the sandy nature and arsenic concentration of the EDS

soil, phytoextraction might not be a good alternative for this soil, especially if there is any










risk of arsenic exposure. Besides the long time required to cleaning up, there is the risk of

potential arsenic leaching.

Plant Arsenic Removal from Soils

Pteris vittata had significantly higher arsenic accumulation (arsenic concentration

X plant biomass) in the first harvest as compared with the second and third harvest from

the Avon, CDV and EDS soils (Figure 3-2).Therefore, the first harvest accounted for

most of the arsenic removed from these soils. In the Marl, CCA and Mining soils, the

accumulation of arsenic in the first harvest was similar to that of the third harvest.


200
E Harvest 1 (October 2003)
[] Harvest 2 (April 2004)
160 O Harvest 3 (October 2004)


0 120


80


40


0
Marl 22 Avon 27 CCA- 110 CDV- 211 Mining 214 EDS 640

Total Arsenic (mg kg-1)
Figure 3-2. Arsenic accumulation by P. vittata after first, second and third harvests in six
soils. Values represent mean + standard deviation (n=4).

If the Mining soil is not considered, arsenic accumulation followed the trend

arsenic concentrations in the soils. The total arsenic removed by the plants varied from

1.71 to 4.54 mg pot- in the Marl soil and from 17.0 to 175 mg pot- in the EDS soil.

More espicificaly, the plants removed 10, 13.5, 42.7, 70.9, 53.3, and 210 mg arsenic per









pot from the Marl, Avon, CCA, CDV, Mining and EDS soils, respectively (Figure 3-2)

after the three harvests. Even though arsenic hyperaccumulation did occur in the ferns

growing in the EDS soil, the rate of arsenic removal was low relative to the amount of

soluble and extractable arsenic in this soil.

The percentage of arsenic removed from each soil by the fern uptake was used as

a parameter of relative comparison of the soils with a wide range of arsenic

concentration. For instance, the plants removed 8.2 % of arsenic from the EDS soil

(higher arsenic content) after the three harvests, while the plants growing in the Marl and

Avon soils (soils with the lowest arsenic content) removed approximately 12 % of the

soil arsenic. The lower percent of arsenic removal from the EDS soil as compared to the

Marl and Avon soils was likely due to the failure of the plant in regrowing after the first

harvest. Similarly, the percent of arsenic removed from the CCA soil (9.5 %) was greater

than from the Mining soil (5.8 %). In this case, despite the similar growth of the plants in

both soils, most of the arsenic in the Mining soil is unavailable to the plants.

In general, the accumulation of arsenic by P. vittata translates to arsenic reduction

of 6.4 to 13 % (Figure 3-3). In spite of the greater amounts of arsenic extracted from the

first and third harvest as compared to the second harvest, it was still difficult to project

the time required for soil arsenic reduction to meet the required limits because the plants

failed to regrow after the third harvest.

Soil Arsenic Distribution

Despite all the criticisms to the fractionation procedures, it is important in

environmental research because it may provide useful information on the reactivity,

mobility and availability of the elements (Wenzel et al., 2001).










16

14




10 -
8-

2 6

4

2

0
Marl 22 Avon 27 CCA- 110 CDV 211 Mining 215 EDS 638
Soil
Figure 3-3. Arsenic removed from the soil by Pteris vittata after three harvests. Bars
represent mean standard deviation (n=4).

Figure 3-4 shows arsenic distribution in the five fractions based on sequential

extraction before and after plant growth. Among the five fractions, the N and S fractions

are considered to be the most plant-available, whereas R the least plant-available. The

sum ofN and S fractions constituted 17.5, 22.0, 32.1, 21.7, 4.33 and 46.3 % of the

arsenic in the Marl, Avon, CCA, CDV, Mining and EDS soils, respectively (Figure 3-4).

As expected, arsenic in these soils was primarily associated with the A fraction, ranging

from 40.0 to 58.7 %, except for the Marl soil which had a substantial amount of the

arsenic (59.1%) associated with the C fraction and only 9.28 % associated with the N+S

fraction. In the Mining soil, the A and C fractions constituted 84.3 % of the total arsenic.

In the EDS soil, the majority of the arsenic was distributed among the first three fractions

in the order A > S > N, further explaining the high arsenic availability in this soil (499

out of 640 mg kg-1 total As was extractable As).






































ArAenic fraction


N S A C R
Arsenic fractiorm


Figure 3-4. Arsenic concentrations (mg kg-1) in different fractions in soils before and
after plant growth. Values represent mean + standard deviation (n=4). N=
non-specifically bound, S= specifically bound, A= amorphous hydrous oxide-
bound, C= crystalline hydrous oxide-bound and R= residual fraction.

Wenzel et al. (2001) fractionated arsenic in 20 arsenic contaminated soils and also

showed that most of the arsenic was associated with the amorphous fraction. The

different soils with different sources of arsenic contamination used in this study

confirmed that, regardless of the source or form of arsenic that enter the soil, arsenic

reacts with soil components and becomes less available. It seems that the predominant

reactions occur with hydrous Fe and Al oxides which mostly coat soil particles (Pierce

and Moore, 1980; Smith et al., 1998).









Plant growth influenced arsenic mobilization from all five arsenic fractions in the

soils, except the residual fraction, which is the most stable one. Among the fractions, the

A fraction contributed the most for the arsenic reduction (45.3, 48.0, 72.0 and 59.0 %) for

the Avon, CCA, CDV and mining soils, respectively (Table 3-4). This was not observed

for the Marl and EDS soils (Figure 3-4; Table 3-4). For instance, in the Marl soil, the

highest arsenic mobilization occurred in the C fraction, which was also the fraction with

the highest arsenic concentration. Even though the greatest arsenic mobilization in the

EDS soil occurred in the S fraction (46.4 %), both the N and A fractions also contributed

to a great extent. The treatments without plants showed that the arsenic mobilization was

primarily a result of plant growth, although some arsenic mobilization was also observed

in the absence of plants. This was probably due to wetting and drying cycles during the

experimental period.

Although arsenic has a high affinity to soil, when the environmental condition

changes, the mobility and even speciation of arsenic can change. For instance, the

presence of plants leads to the exudation of some organic anions into a soil, which

directly impacts the mobilization of arsenic in the soil.

Total arsenic concentration is not a good predictor of its bioavailability because

only arsenic dissolved in water can be transported to the roots and taken up by plants

(Ritchie and Sposito, 1995). Plants tend to first take up the most available fraction of

arsenic from the soils and as this pool becomes smaller, some of the arsenic from other

fractions will be slowly transformed to water-soluble fraction to reestablish their

equilibrium (McGrath et al., 2000).









Table 3-4. Arsenic reduction (%) in each fraction in six soils as a result of plant arsenic
uptake by P. vittata.
As removed
Soil (mg pot-) N1 S A C R
----------------------- % As ------------------------

Marl 9.93 13.9 9.65 3.47 62.32 10.6
Avon 13.5 20.9 27.8 45.3 4.71 1.28
CCA 41.9 15.0 16.3 47.9 16.2 4.60
CDV 72.1 5.60 18.9 71.6 1.85 2.03
Mining 50.0 1.17 7.18 58.7 26.9 5.98
EDS 210 28.0 46.4 21.2 3.26 1.56
1 N= non-specifically bound, S= specifically bound, A= amorphous hydrous oxide-
bound, C= crystalline hydrous oxide-bound and R= residual fraction.
2 the fraction with the highest arsenic reduction for a given soil was in bold font.

As plants take up more arsenic, there is less available arsenic present in the soils,

resulting in reduced available arsenic in soils and reduced plant arsenic uptake.

Therefore, it is reasonable to assume that, only arsenic in N fraction can be readily taken

up by the plant and arsenic in other fractions has to convert to N fraction before being

taken up by the plant. In other words, N fraction acts as both a source of arsenic for the

plant and a sink for arsenic in the soil, thus linking arsenic in the soil with the arsenic

taken up by the plant.

High proportion of the total arsenic in historically polluted soils is mostly present

in the residual fraction (Voigt et al., 1996; Kavanagh et al., 1997). Thus, most of the

schemes commonly used tend to underestimate arsenic availability (Gleyzes et al., 2001).

However, the present results show that the presence of arsenic in the residual fraction was

low, even in the soils where the presence of arsenic is of natural origin, such as Marl and

Mining soils. This could indicate that the scheme we used seems to be more suitable for

the study of arsenic fractionation in these polluted soils.









Comparison of Extractable Arsenic Using Single Extraction Methods

In this study, three single extraction methods were used in order to relate arsenic

availability in soils and plant uptake. The arsenic concentrations extracted with these

methods are shown in Table 3-5. Generally, the extractable arsenic followed the

descending order of Mehlich III > organic acids > water-soluble in all soils. When

calculated in percent of the total arsenic determined using the EPA 3050 method, the

water soluble arsenic, Mehlich III and organic acid extractable arsenic represented 1.62,

102 and, 28.2 % (Marl), 5.81, 54.8 and, 49.3 % (Avon), 5.32, 53.4, 12.1% (CCA), 0.06,

21.7 and, 18.9 % (CDV), 1.46, 10.5 and, 3.96 % (Mining) and 21.3, 78 and 38 % (EDS),

respectively. The Mehlich III was expected to extract more arsenic than the water and

the organic acids, irrespective the soil because of its chemical composition, a dilute acid-

fluoride-EDTA solution at pH 2.5. The result showed that the presence of low molecular

weight organic acids in soils can be potentially dangerous as it increase the mobility and

availability of arsenic in the soil. For instance, in the Avon and CDV soils, the organic

acid extractable arsenic was comparable to the Mehlich III extractable arsenic. Therefore,

the presence of typical plants that do not hyperaccumulate arsenic in arsenic

contaminated soils has the potential to mobilize arsenic from the soil since the exudation

of organic acids is a common phenomenon in the rhizosphere of the plants.

Plant uptake and Arsenic Availability in Soils as Measured by Single Extractants

Arsenic phytoextraction from contaminated soils depends, among other factors,

on arsenic bioavailability and the capacity of a plant to access arsenic. Arsenic

bioavailability is regulated by physical, chemical and biological processes and their

interactions, which makes the simulation of element availability to plants with chemical

extractants a difficult task (Carter, 1993). On the other hand, the total concentration of an









element is only important for evaluating the toxicity potential of soils, but they say little

about elemental bioavailability.

Table 3-5. Concentrations of arsenic extracted by different extraction methods in six soils
before plant growth.
Soil Arsenic extracting solutions
Water Mehlich III Organic acids Total
------------------------------- As (mg kg-1)----------------------
Marl 0.36 0.15 (1.6)a 22.7 1.70 (102) 6.26 0.89 (28) 22.2 4.11
Avon 1.57 + 0.05 (5.9) 14.8 + 0.88 (55) 13.3 0.05 (49) 27.0 + 1.00
CCA 5.85 0.29 (5.3) 58.8 + 5.02 (54) 13.3 1.03 (12) 110+ 1.73.
CDV 0.12 0.03 (0.1) 45.9 20.5 (22) 39.9 1.00 (19) 211 2.65
Mining 3.13 0.15 (1.5) 22.5 1.00 (11) 8.49 0.97 (4.0) 214 16.5
EDS 136 4.04 (21) 498 + 57.5 (78) 242 30.1 (38) 640 5.52
Results are mean standard deviations (n=3). a: Results in the brackets are percentage of
the total.
In this section, the relationship between the soil arsenic extracted by three

different methods and the arsenic taken up by the plants was evaluated.

Water-soluble arsenic grossly underestimated the plant arsenic uptake by 242%

(based on the average of five soils, excluding the CDV soil), whereas the Mehlich III

over-estimated by 72%. Therefore, neither water nor Mehlich III solution was good

indicators of the plant available arsenic. On the other hand, arsenic taken up by P. vittata

constituted 74% of the arsenic extracted using organic acids, only 26% underestimation.

Based on arsenic extracted with water, the arsenic taken up by P. vittata implies

that a greater amount of arsenic was mobilized from other pools over time. This was

particularly true for the CDV soil in which the amount of arsenic taken up by the plants

was over 15,500 times greater than the amount of water-extractable arsenic (Table 3-6).









Because low molecular weight organic acids used in this study are products of

root exudates, microbial secretions, and plant and animal residue decomposition in soils

(Stevenson, 1986; Zhang et al., 1997), and they are found in the rhizosphere ofP. vittata

(Tu et al., 2004), we hypothesized that the organic acids extractable arsenic would be a

good predictor of plant available arsenic in soils. Phytic acid and oxalic acid were the two

main low molecular weight organic acids identified in the root exudates ofP. vittata (Tu

et al., 2004). Furthermore, the role of oxalic acid in mobilizing mineral elements has been

widely reported (Jones, 1998).

In fact, in this study, the organic acid extractable arsenic was the best method to

predict plant arsenic uptake by P. vittata (Table 3-6).

Arsenic bioavailability in soils has been found to be enhanced by plant root

exudates (Dinkelaker et al., 1989; Ohwaki and Hirata, 1992), suggesting that root

exudates could be important in the mobilization of soil arsenic and arsenic accumulation

by a plant.

Table 3-6. Percent of the arsenic taken up by P. vittata in each extracting method.

Soil Water-Soluble Mehlich III Organic acids
Marl 836* 11 40
Avon 216 23 95
CCA 183 18 81
CDV 16475 45 46
Mining 435 60 159
EDS 39 11 22
Average 342# 28 74
*Obtained by dividing the amount of arsenic taken up by P. vittata by the amount of
water-soluble arsenic. A value of 836% indicates that P. vittata took up 736% more
arsenic than the amount of water soluble arsenic.
# excluding the CDV soil.









The percent of arsenic taken up by P. vittata according to the organic acids

extraction method varied with soils. The best correlation was found for Avon (95 %),

CCA (81 %) and Mining (159 %). In the Mining soil, P. vittata took up 59 % more

arsenic than predicted by the organic acid method. However, the plants took up 60, 54

and 78 % less arsenic than predicted for the Marl, CDV and EDS soils, respectively.

Tu et al. (2004) reported that P. vittata exuded 40-106% more phytic acid and

300-500% times more oxalic acid under arsenic stress than a non arsenic

hyperaccumulator, N. exaltata fern. It also mobilized significantly greater amounts of

arsenic from both arsenic minerals and a CCA soil (data not shown). Under the same

concentrations, phytic acid mobilized more arsenic from the soils than oxalic acid, likely

due to its stronger complexation capability and greater acidity (Tu et al., 2004).

Oxalic acid has been widely used as an extractant for plant available nutrients

from soil, including phosphate (Fransson, 2001; Sagoe et al., 1998) due to its typical

presence in plant root exudates (Pinton et al., 2001) and its capability of both proton

donation and ion complexation.

Arsenic extraction with water underestimated the availability of soil arsenic to P.

vittata in all soils except the EDS soil. This was because 21% of the arsenic was water

soluble in the EDS soil (Table 3-5) whereas the plant took up 39% water-soluble arsenic,

i.e., the amount of arsenic taken up by the plant was less than the amount of water-

soluble arsenic in the EDS soil. On the other hand, the concentrations of water-soluble

arsenic were low in all other soils. The amount of arsenic taken up by P. vittata after

three harvests was consistent with the fact that more arsenic was solubilized over time

from other pools (Figure 3-3).









Because Mehlich III extracts a large proportion of total soil arsenic, it

overestimated plant available arsenic. The amount of arsenic taken up by P. vittata was

lower than the amount of arsenic extracted using Mehlich III extractant. Only Mehlich III

extractable arsenic in the Mining soil was close to the amount of arsenic taken up by the

plant (60 %). Results from this study support the conclusion of McLaughlin (2002) that

indices of metal phytoavailability in soils that are based on the determination of the most

available metal pools (water-soluble and organic acids), while not perfect, appear to

better predict risk from metal contamination than do total metal concentrations or metals

removed from soils by strong extractants (Mehlich III). Therefore, organic acids might be

more suitable than water and Mehlich III to assess arsenic availability to P. vittata in the

soils used in this study.














CHAPTER 4
EFFECTS OF PLANT AGE ON THE ARSENIC ACCUMULATION BY THE
HYPERACCUMULATION Pteris vittata L.

Introduction

Arsenic is a ubiquitous trace constituent in soil and plants. However, natural and

anthropogenic activities have resulted in many areas polluted with arsenic. The extensive

contamination of soil and water by arsenic worldwide has renewed the interest in more

cost-effective remediation technologies such as phytoremediation/phytoextraction.

The potential use of hyperaccumulator plants such as Pteris vittata L. for the

phytoremediation of arsenic-contaminated soils has been reported (Komar et al., 1998;

Ma et al., 2001; Tu and Ma, 2002). Several desirable characteristics, such as the ability to

concentrate arsenic in the fronds, large biomass, fast growth, easy reproduction,

resistance to adverse soil characteristics and its perennial nature, make P. vittata suitable

for phytoremediation (Ma et al., 2001).

It is known that during the sigmoidal phase of a plant growth cycle, dry matter

and nutrient accumulation occur at a maximal rate, although the rate of nutrient uptake

varies with developmental conditions of a plant and is different for different nutrients

(Garcia et al., 2003). There exists a set of plant physiological characteristics that change

with plant age, which either isolated or combined with other factors influence biomass

production, and nutrient and contaminant accumulation. For instance, young roots are

generally considered to have higher nutrient uptake activity than old roots (Eshel and

Waisel, 1972; Mengel and Barber, 1974: Vegh, 1991; Yanai, 1994) and the root systems









at different growth stages may not have the same specific nutrient uptake activity (Chen

and Barber, 1990; Smethurst and Comerford, 1993). This is important because the

accumulation ability of a plant is highly influenced by the effectiveness of its root

system.

Little information is available about arsenic hyperaccumulation in plants at different

ages. Furthermore, no study has been reported for P. vittata of different ages growing in

soil conditions. Therefore, identifying the plant age that allows maximal arsenic

accumulation may play an important role in successful phytoextraction. The aim of this

study was to assess the influence of the physiological ages of P. vittata on its arsenic

removal from a contaminated soil.

Materials and Methods

Plant Propagation

Pteris vittata plants used in this study were propagated in our laboratory. An

assembly of cool-white fluorescent lamps supplied an 8-h photoperiod to the plants with

an average photon flux of 825 [[mol m-2 s-1

Soil Characterization

The soil used in this study (sandy, siliceous, hyperthermic grossarenic paleudult)

was collected from an abandoned chromated- copper-arsenate (CCA) wood preservation

site in north central Florida. The soil had pH (1:2 soil:H20) 7.30; cation exchange

capacity (Thomas, 1982) 4.4 Cmol(-) kg-1; soil organic matter (Nelson and Sommers,

1982) 11 g kg-'; total As 153 mg kg-1; 880 g kg-1 sand; 90 g kg-1 silt, and 30 g kg-1 clay.









Experimental Design

This greenhouse study, set up as completely randomized design, assessed the

influence of four physiological ages of P. vittata (1.5, 4, 10 and 16 months after

transplant from the sporophytic phase) (Figure 4-1) on the arsenic removal of P. vittata

from a contaminated soil. The plants of different ages were chosen based on their

availability. Each treatment had four replications. Three control pots, soil without

plants, were also included.

Air-dried soil (2.5 kg) was weighed into each pot and thoroughly mixed with 3.0 g

of Osmocote, extended time-release fertilizer (18-6-12) (Scotts-Sierra Horticultural

Products Co., Marysville, OH). After one week of equilibrium under field capacity, one

plant was transplanted into each pot. The pre-experimental frond and root biomass was

taken before transplantation. The plants were allowed to grow for 8 weeks. All plants

were watered throughout the study to keep the soil at approximately 70% of field

capacity.


Figure 4-1. Pteris vittata of 1.5, 4, 10, and 16 old months before transplant.









After 8 weeks of growth, plants were harvested and separated into roots and

fronds. Plant tissues were washed thoroughly with tap water, and then rinsed with

deionized water. The fronds and roots were oven-dried for 3 d at 650 C, weighed and

ground with a Wiley mill to pass through a 1mm mesh screen for chemical analysis. Soil

samples from each pot were collected from the rhizosphere and bulk soil. The

rhizosphere was defined as the soil attached to the roots. The rhizosphere soil was gently

shaken from the roots and sieved through 2 mm mesh screen to separate it from the roots.

Chemical Analysis

Plants were digested using a modified version of the EPA Method 3050A for the Hot

Block Digestion System (Environmental Express, Mt. Pleasant, SC). Analysis was

performed with a transversely heated, Zeeman background correction equipped graphite

furnace atomic absorption spectrophotometer (Perkin Elmer SIMMA 6000, Norwalk,

CT). The analysis of K, Ca, Mg, Zn, Cu, Fe and Mn was performed with a flame atomic

absorption spectrophotometer (Varian model FS 220, Australia) with the same digestate.

Quality control of arsenic analysis was assured by including the Standard Reference

Material 1547 (Peach Leaves). Total P analysis was carried out using a modified

molybdenum blue method (Carvalho et al., 1998). This involved reduction of the As in

digestates from As (V) to As (III) with L-cysteine to minimize its interference with

phosphate analysis. Tightly capped test tubes are incubated at 800C for 5 min to allow

complete reduction of arsenate into arsenite. P was determined, after cooling, by a

double-beam spectrophotometer (Shimadzu UVI60U, Shimadzu Corp., Columbia, MD).









Data Analysis

All results were expressed as an average of four replicates. Treatment effects

were determined by analysis of variance according to the General Linear Model

procedure of the Statistical Analysis System (SAS Institute Inc., 1987). Duncan test at a

5% probability was used for post-hoc comparisons in order to separate treatment

differences.

Results and Discussion

Arsenic Uptake

Arsenic uptake in P. vittata ranged from 3.98 to 6.18 mg plant1 in the fronds and

from 0.94 to 2.84 mg plant- in the roots (Figure 4-2). The As uptake in the fronds of 1.5-

month old P. vittata was 36% greater than those in the 4- and 16-month old plants and

similar to that of the 10-month old plants. On the other hand, the arsenic accumulation in

the roots of the 1.5-month old ferns was lower than the rest. Interesting to note that, even

though the 10-month old plants accumulated as much arsenic in the fronds as those in the

1.5-month old ferns, they accumulated more arsenic in the roots as compared to the plants

of other ages.

Transfer factor (TF) (As in fronds/As in roots) was calculated to evaluate the relative

effectiveness of plants in moving arsenic from the roots to the fronds. This characteristic

of P. vitatta in accumulating high arsenic concentrations in its fronds makes this plant

suitable for phytoextraction. The TF values for 1.5, 4, 10 and 16 months' old plants

were 3.20, 2.05, 1.63 and, 1.60, respectively (data not shown). The TF reduced with plant

age. The 1.5-month old plants were the more efficient in translocating arsenic from the

roots to the fronds.






54










~3--- S





r r
.5 4 10 16
Plsant I.r tage

Figure 4-2. Arsenic accumulation in the fronds and roots of P. vittata of different ages
after 8 weeks of growth in an arsenic-contaminated soil. Means with the same
letter for the fronds or the roots are not significantly different (P < 0.05) based
on Duncan's multiple range test.

Table 4-1. Plant biomass, bioconcentration factor and translocation factor ofP. vittata of
different ages after 8 weeks of growth in an arsenic-contaminated soil.
Plant growth Biomass DW Bioconcentration Translocation

stage (g pot-) Factor Factor

(months) Frond Root Frond Root

1.5 12.7 b 6.8b 2.9 a 0.9b 3.0 a

4 12.0b 8.8b 1.9b 1.1 ab 1.8b

10 20.7 a 16.0 a 1.9 b 1.3 a 1.6b

16 13.1 b 8.9b 2.0b 0.5 c 3.8 a

1Means (3 reps.) followed by the same letter within the column are not significantly
different (P < 0.05) based on Duncan's multiple range test.

Tu et al. (2004) also reported that young fern plants were more efficient in removing

arsenic from a hydroponic system than older ferns. Arsenic levels in the growth medium

seem to determine the extent of arsenic accumulation by plants of different age. For a









given plant, arsenic tends to be preferentially stored in young fronds at low arsenic level

and in old fronds at high arsenic level in the media (Tu and Ma, 2002; Tu and Ma, 2005).

The ability of young P. vittata to accumulate more arsenic may be related to several

factors. The concentration of glutathione, a sulfur-containing tripeptide thiol and a

precursor of phytochelatins, a very important antioxidant involved in plant detoxification

(Scott et al., 1993), seems to decrease with the plant age (Hatton et al. 1996).

In P. vittata, the glutathione concentration tends to increase with the arsenic

concentration in the plant (Cao et al., 2004). Elsewhere, it has also been reported that

plant contaminant absorption decreases as plant is more mature, hence tolerance increase

with age (Wilcut et al., 1989; Leah et al., 1995). Knuteson et al. (2001) reported that

four-week-old parrot feather and canna were more tolerant of simazine than two-week-

old plants. Additionally, the root system at different ages has different nutrient uptake

activities. The average uptake rate per unit of root decreases as a plant matures (Barber,

1984).

The fact that P. vittata ferns are very diverse, easy to adapt to different environments

and are widespread thriving in both temperate and tropical climates (Matschullat, 2000)

raises the concerns on using these ferns for phytoremediation of contaminated sites as

they are classified as type II invasive species. One aspect that contributes to this is their

life cycle and breeding system. The large quantities of spores produced and released by

the ferns are easily dispersed to long distances favoring colonization of wide ecological

niches (Bondada and Ma, 2002). Therefore, the use of younger plants besides being more

efficient in arsenic uptake could also minimize the risk of uncontrolled ferns propagation

since younger plants could grow longer up to the phase of spore maturation in a









contaminated site. The arsenic rich biomass could be harvested before the spores

maturation phase as compared with older plants, which produce spores earlier after being

transplanted in a field.

Plant Biomass

Arsenic hyperaccumulation efficiency depends on both plant biomass production

and arsenic concentration in the plant tissue (Baker et al., 1991; Salt et al., 1998; Ma et

al., 2001). Therefore, the biomass of P. vittata was determined for all plant ages. After 8

weeks of growth, the frond and root biomass of the 10-month old plants was greater than

the rest (Figures 4-3 and 4-4).

Because the initial differences in the frond and root biomass of the treatments, the

similarity of final root and frond biomass among plants of different ages indicated that

plant biomass output occurred at different rates. Therefore, the net biomass (final minus

initial biomass) for each plant age and for each plant part (frond and root) was calculated.

Even though the final biomass of the 10-month old plants was greater than the plants of

other ages, the rate of increase in biomass was greater in the 1.5-month old plants,

suggesting a higher metabolic activity.

Phosphorus Distribution in the Plants

As a P chemical analogue, arsenic is taken up by plants via the phosphate

transport system (Meharg and Hartkey-Whitaker, 2002). Phosphate concentrations in P.

vittata of different ages were determined to examine its relationship with arsenic

concentrations.

During vegetative growth phase, P contents in plants usually ranges from 97 to

161 mM of plant dry matter (Marschner, 2003).










25


o20
--
a
-15



-C
510
.5

a 5


SInitial find
Final frond
a


1.5 4 10 16
Plant growth stage

Figure 4-3. The frond biomass of P. vittata of different ages before and after 8 weeks of
growth in an arsenic-contaminated soil. Means with the same letter are not
significantly different (P < 0.05) based on Duncan's multiple range test.


1 1.


E


Plant growth stage


Figure 4-4. The root biomass of P. vittata of different ages before and after 8 weeks of
growth in an arsenic-contaminated soil. Means with the same letter are not
significantly different (P < 0.05) based on Duncan's multiple range test.


18

16

1 -
~14
-4
S12


8-





2 -

0-
6-


SInltial root
- Final root




--









In this study, P concentrations ranged from 140 to 183 mM in the fronds and from

97 to 124 mM in the roots, which were typical of most plants. Phosphate concentrations

in the fronds and roots increased with plant age (Figures 4-5 and 4-6). As for arsenic, its

concentrations in the roots increased with plant age whereas those in the fronds

decreased. Therefore, accumulation of both arsenic and P in the roots was the highest in

the 16-month old plants. On the other hand, the tissues of older P. vittata had lower

arsenic concentrations in the fronds and higher arsenic concentrations in the roots, as

compared with the younger plants. It is interesting to note that the molar ratios of P/As

stayed approximately one in the roots for all plant age (Figure 4-6) whereas those in the

fronds increased with plant age (Figure 4-5).

The effectiveness in arsenic hyperaccumulation by P. vittata is related to a series

of detoxification mechanisms which are important to mitigate arsenic toxicity in the roots

and fronds (Lombi et al., 2002). One of them is the ability to maintain lower

concentrations of arsenic in the roots and higher in the fronds, thereby minimizing the

potential damage to the roots. The lower arsenic concentrations in the roots of the

younger plants suggest a more effective arsenic removal process by P. vittata.

Another important factor is the ability ofP. vittata to manipulate phosphate in the

plant biomass. Tu and Ma (2003c) reported that P. vittata maintained greater amount of

phosphate in the roots than Nephrolepis exaltata, a non arsenic-hyperaccumulator fern. It

is hypotesized that the ability of P. vittata in maintaining higher concentrations of

phosphorus in the roots may constitute one of its mechanisms of arsenic tolerance (Tu

and Ma, 2003c).







59



200 7
-uP
4As



I ^-------- p
160

5


120





o

2

40




0 0
1.5 4 10 16
Plant growth stage (Month)

Figure 4-5. Effect of plant age on phosphorus and arsenic distribution in the fronds of P.
vittata after 8 weeks of growth in an arsenic-contaminated soil. Bars represent
standard deviations of four replicates.

Concentrations of Other Nutrients in Plants

The K concentrations in the fronds and in the roots of P. vittata were within the

normal range for most plants (Havlin et al., 2005). However, the K concentration in the

fronds of the 1.5-month old plants was higher than those in other ages (Table 4-2).The K

distribution in the fronds and the roots in plants of different ages was similar to that of

arsenic suggesting synergetic relationship of K with arsenic in the plants. Tu and Ma

(2005) reported that arsenic and K had similar distribution in the fronds of P. vittata and

speculated that K may function as a counterbalancing ion in these plants.












160 3









100
140




05
120

2



t 80 ----------------1 5


10 0
80 15





05
2 0 -----------------------------------


20


0 0
1.5 4 10 16
Plant growth stage (Month)


Figure 4-6. Effect of plant age on the phosphorus and arsenic distribution in the roots of
P. vittata after 8 weeks of growth in an arsenic-contaminated soil. Bars
represent standard deviations of four replicates.

The Ca and Mg distribution in the fronds and roots of P. vittata were similar to


that of P, and increased with plant age (Table 4-1). Also, the plants that accumulated


more arsenic (1.5- and 4-month old plants) in the fronds had lower Ca. Tu and Ma (2005)


reported that Ca concentrations in the fronds were inversely related to those of arsenic


and were higher in older fronds growing in a soil spiked with up to 30 mg kg-1 As. Due to


the very low mobility of calcium in the phloem, the growth of the young parts of the plant


is dependent upon the concurrent uptake of calcium.


Among other roles of calcium in plants, the ionic form of calcium is required in


the vacuole of plant cells as a couter-cation for inorganic and organic anions (Marschner,


2001). As arsenic forms compounds with calcium in soil and arsenic is hypotesized to be









stored in the cell vacuoles, it would be reasonable to assume that the calcium

concentration in the plant would increase with increasing arsenic concentrations in the

plant tissues. However, based on the the results of this study and the studies of Tu and

Ma (2005) and Fayiga and Ma (2005), that does not seem to be the case. In fact, the fact

that calcium concentration in the older plants increased while arsenic concentrations

decreased suggests a limited role of calcium in arsenic detoxification by the plants.

Table 4-2. Macronutrients (K, Ca, and Mg) (g kg-1) and micronutrients (Fe, Zn, and Mn)
content (mg kg-1) in the fronds and roots of P. vittata of different ages after 8
weeks of growth in an arsenic contaminated soil.
Elemental Frond growth stage (Month) Root growth stage (Month)

content
1.5 4 10 16 1.5 4 10 16

Potassium 18.8 a 14.2 b 15.6 b 15.3 b 8.70 ab 10.9 a 10.0 a 5.50 b

Calcium 13.5 b 15.5 b 20.4 a 20.7 a 21.5 a 20.0 a 23.4 a 23.8 a

Magnesium 2.80 b 3.40 b 4.10 a 4.20 a 2.10b 1.70 b 2.90 b 16.6 a

Iron 69.2 ab 75.0 a 58.0 bc 45.0 c 1235 b 1397 b 1669 b 5627 a

Zinc 43.6 a 37.0a 39.5 a 44.3 a 114 a 131 a 123 a 122 a

Manganese 15.1 bc 28.6 a 20.3 b 13.9 c 41.3b 40.5b 39.5b 70.4a

Means followed by the same letter in a row and within each plant part are not different
by the Duncan test at P < 0.05

The concentrations of Fe, Mn and Zn were within the range of most plants

(Havlin et al., 2005). While the Fe concentrations in the fronds of the 1.5-and 4-months

old plants were higher, the Fe concentrations in the roots of the 16-month old plants were

higher than those in other plant ages. Tu and Ma (2005) observed an inverse correlation

between arsenic and Fe in P. vittata, but they offered no explanation. Chen et al. (2003)

reported an unusual high concentration of Fe in the epidermis ofPteris nervosa roots,

indicating the formation of Fe plaques. Other studies with non hyperaccumulator plants






62


also showed the formation of Fe plaques and the inhibition of arsenic uptake in roots

(Otte et al., 1991; Colleen et al., 2002).














CHAPTER 5
ARSENIC ACCUMULATION AND ROOT CHARACTERISTICS OF Pteris vittata L.
AND Nephrolepis exaltata L

Introduction

For successful application of phytoextraction, the hyperaccumulator plants must

be metal-tolerant, adapt to soil and climate environment of their intended use, and

effectively absorb the metal from soils (Keller et al., 2003; Schwartz et al., 1999). Since

heavy metals are present in soils predominantly as insoluble forms, their bioavailability is

generally low. Therefore, the efficacy of phytoextraction largely depends on the

development of extensive plant root systems in a contaminated soil (Schwartz et al.,

1999).

Plant root systems perform many important functions including water and nutrient

uptake, anchorage to the soil, and the establishment of biotic interactions in a

rhizosphere. The characteristics of a root system are important in plant nutrient

acquisition, particularly for elements of low mobility such as phosphorus and arsenic.

Plant root systems can be characterized based on length, surface area, biomass and

their relationship to plant shoots (Van Noordwijk and Brouwer, 1991; Jungk, 2002). The

efficiency of roots to transfer metals from a soil to the shoots can be characterized by net

influx per unit root length, and root length per unit of shoot biomass. Structural root

properties, such as diameter of the roots and the formation of root hairs, may also exert an

influence on nutrient uptake from the immediate vicinity of a root cylinder (Waisel and

Eshel, 2002).









According to Ernst (1996), most hyperaccumulators have limited root systems.

However, in a number of studies, unexplained growth enhancements were observed in

plants subjected to a mild stress from toxic metals. For example, the root growth of

Betulapendula seedlings was induced by low concentrations of cadmium (Gussarsson,

1994). Also, enhanced root elongation, root biomass production, and root hair formation

were found in plants of a tolerant population of Silene vulgaris treated with lead chloride

(Wierzbicka and Panufnik, 1998). Jiang and Liu (1999) reported stimulated root growth

of Brassicajuncea plants with low concentrations of lead nitrate. Schwartz et al (1999)

and Whiting et al (2000) reported a preferential root development in the Zn

hyperaccumulator Thiaspi cearulences in the presence of Zn.

Little information is available on the root system ofP. vittata. The process by

which P. vittata mobilizes and takes up arsenic is not well known. Hyperaccumulator

species may release root exudates containing chelators to enhance heavy metal uptake,

translocation and resistance (Wenzel et al., 2003). The ability of a plant to exude large

quantities of dissolved organic carbon (DOC) and to change the rhizosphere pH may

enhance the arsenic bioavailability in soils, thereby increasing its arsenic uptake (Tu et

al., 2004). The fate and bioavailability of arsenic in the rhizosphere can be different from

that of the bulk soil (Fitz et al., 2003). Depending on plant and soil factors, rhizosphere

pH can be up to two units different from the bulk soil (Marschner, 2003). Factors

affecting the rhizosphere, such as pH, plant nutritional status, organic acids excretion, and

CO2 production by roots and rhizosphere microorganisms, are accountable for the

differences (Marschner, 2003).









Several attempts have been made to measure the bioavailability of metals in soils.

Tessier at al. (1979) used a sequential extraction technique for determining the labile

metal in soils. This approach is based on the fact that metal associated with various

geochemical phases varies in its chemical reactivity and bioavailability. The drawback of

the method is its non-selectivity and metal redistribution among geochemical phases

(Howard and Brink, 1999). Despite its limitation, fractionation provides an understanding

of the relative mobility and bioavailability of metals in soils (Fitz and Wenzel, 2002;

Krishnamurti and Naidu, 2002), because plant metal uptake or metal toxicity is related to

those fractions (Chlopecka and Adriano, 1996; Guo and Yost, 1998). Water-extractable

and exchangeable forms of metals are usually considered to be the most available to

plants (Petruzzelli, 1989).

This study was conducted aiming to evaluate and compare two fern species, an

As-hyperaccumulator (P. vittata) and a non As-hyperaccumulator (Nephrolepis exaltata),

in terms of: 1) root system characteristics (biomass, length, surface area and diameter); 2)

plant biomass and arsenic uptake; and 3) root uptake efficiency of arsenic; and 4) effects

of plant arsenic uptake on arsenic distribution and bioavailability in the rhizosphere and

bulk soils. The information obtained from this study should be useful for determining the

plant density and effective root depth ofP. vittata for phytoextraction purpose as well as

for providing the critical linkage between the ability of P. vittata in solubilizing soil

arsenic and arsenic hyperaccumulation.









Materials and Methods

Soil Characterization

The soils used in this study were collected (0-20 cm) from an abandoned

chromated-copper-arsenate (CCA) wood preservation site (As-contaminated soil) and

from a non-contaminated site (control soil) in Florida. The site was contaminated with

arsenic from using CCA wood preservative between 1951-1962 (Komar, 1999). The soil

analyses were performed as described in the Chapter 4 and summarized in Table 5-1.

Table 5- 1. Selected properties of the soils used in this study

Property As-contaminated soil Control soil
pH (1:2 soil/water ratio) 7.30 +0.01* 6.81 +0.02
Organic matter content (g kg-1) 11.0 +0.30 13.2 + 0.42
CEC** (cmol(+) kg-1) 4.40+0.02 5.60+0.04
Total As (mg kg-1) 101 +5.40 0.41+0.01
Water-soluble As (mg kg-1) 0.2 +0.02 0.003+0.001
Mechlic III extractable P (mg kg-1) 21.8 + 0.30 87.2 +0.77
Sand (g kg-) 882 + 4.32 892 +4.76
Silt (g kg-1) 91+0.40 75 +0.30
Clay (g kg-1) 27 +0.01 33 +0.02
Bulk density (g cm-3) 1.29 1.39
Values represent mean standard deviation;
*Cation exchange capacity

Experimental Set up

The study employed a completely randomized design in a 2 x 2 split plot scheme.

Two fern species (P. vittata and N. exaltata) were used as the main plot and the chemistry

of the rhizosphere and bulk soil as the sub-plot. Each treatment was replicated four times.









A known weight (2.5 kg) of air dried and sieved (2 mm) soil was poured into

plastic bags and thoroughly mixed with 3.0 g of Osmocote extended time-release base

fertilizer (18-6-12) (Scotts-Sierra Horticultural Products Co., Marysville, OH), and then

poured into plastic pots (2.5-L rhizopot).


The rhizopot (16 cm in height and 15 cm in diameter; Figure 5-1) was used to

grow the plants. Plastic frames (13 cm in height and 7 cm in diameter) covered with

nylon mesh cloth (mesh size 45 [im) were used to separate the rhizosphere from the bulk

soil in the rhizopot.


Figure 5-1. Cross section of the rhizopot used in the study.

Pteris vittata was propagated in our laboratory whereas N. exaltata was procured

from a nearby nursery (Milestone Agriculture, Inc., Apopka, FL). Efforts were made to

ensure visual uniformity across all plants. The average biomass per plant before the

transplant was 0.70 g for the fronds and 0.45 g for the roots.


45wm mesh
cloth +
plastic net









Root growth was limited within the nylon cloth in the central compartment (500 g

soil). One week before the study, the soil was set to equilibrate at water holding capacity.

One healthy fern of similar age with five to six fronds was transplanted into each pot.

The plants grew for 8 weeks in a greenhouse with an average night/day

temperature of 14/30C and an average photosynthetically active radiation flux of 825

[tmol m-2 s-1. The plants were watered throughout the study to keep the soil at

approximately 70% of its field capacity. At the end of the experiment, the ferns were

harvested and each plant was separated into roots and fronds. Rhizosphere and bulk soil

were collected and used for analysis of arsenic, soil pH and dissolved organic carbon

(DOC). Rhizosphere soil was defined as the soil attached to the roots, which was

removed from the roots by shaking gently. The rhizosphere soil was sieved to remove the

roots while keeping the roots intact as much as possible. The bulk soil was defined as the

soil outside the central compartment.

The plants were washed thoroughly with tap water, and then rinsed with distilled

water. The fronds were oven-dried for 3 days at 650C, weighed and ground using a

Willey mill to 60-mesh fineness for chemical analysis. The roots were immediately

separated and scanned to prevent dehydration. After scanning, the roots were also oven-

dried, weighed and ground for chemical analysis.

Root Measurement

The roots were washed with water, spread out on a transparent tray and then the

image was digitalized using a scanner (Envisions88005) at 400 dpi resolutions with

Vistascan software (UMAX Data system, Inc). When necessary, root samples were

divided into sub-samples for more accurate measurement. The digitalized images were









processed by the program GSROOTS (Guddanti and Chambers, 1993), which was

programmed to yield root length data of pre-defined diameter classes (<0.10; 0.10 < d <

0.25; 0.25 2.0 mm). Root

length density (Lv, root length per unit volume of soil) and root area density (Av, root

surface area per unit volume of soil) were calculated. Unit root length and unit root area

were used in all discussions unless otherwise specified. The root biomass was also

determined.

Chemical Analysis

Plants and soils digestion and analyses were performed as described in the

Chapter 4.

Rhizosphere and bulk soil samples were evaluated for water-soluble arsenic and

DOC in 1:4 soil to water ratio (Olsen and Sommers, 1982), obtained after shaking (1 h),

centrifuging (15 min at 3500 g) and filtering (0.45 |tm syringe filter). Arsenic in solution

was determined by the same method described above. The concentration of DOC was

measured using a TOC-5050A TOC analyzer (Shimadzu, Japan). Quality control of

arsenic analysis was included using Standard Reference Materials 1547 (Peach Leaves)

and 2710 (soil) (US NIST, MD).

Arsenic Fractionation

The improved sequential extraction procedure (Wenzel et al. 2001) was used to

fractionate arsenic into five operationally-defined fractions, including non-specifically

bound (N), specifically bound (S), amorphous hydrous oxide-bound (A), crystalline

hydrous-oxide-bound (C), and residual (R), as described in the Chapter 3.









Data Analysis

All results were expressed as an average of four replications. Treatment effects

were determined by analysis of variance according to General Linear Model procedure of

the Statistical Analysis System (SAS Institute Inc., 1987, Cary, NC). The Duncan test at

5% of probability was used for post-hoc comparisons to separate treatment differences.

Pearson correlation coefficients were calculated between the root characteristics

and different plant parameters.

Results and Discussion

Root Biomass, Root Length, Root Area, and Root Diameter

Plant roots are in direct contact with, and are affected by soil constituents in

addition to regulating many plant processes. Measurement of below ground plant

productivity may provide information on root growth, distribution, water and nutrient

supply and plant potential of removing contaminant (Van Noordwijk and Brouwer,

1991). Root-soil contact is an especially important factor for the uptake of less mobile

elements such as P and arsenic from a soil. However, the available information on the

root characteristics of P. vittata has been limited to the measurement of root biomass (Tu

at al., 2002; Tu and Ma, 2003; Liao et al., 2004).

Root characteristics including root biomass, length and surface areas were

determined for both plants. They are important for estimating plant water and nutrient

uptake (Zhuang et al. 2001). In both soils, P. vittata had a more extensive root system

than N. exaltata (Table 5-2). The root biomass of P. vittata was 3.0 and 2.4 times greater

than that ofN. exaltata in the As-contaminated soil and in the control soil, respectively.









The root biomass accounted for approximately 25% of plant biomass for both plant

species (Tables 5-2 and 5-3).

Several studies have reported that the presence of contaminants stimulated the

growth of plant roots (Gussarsson, 1994; Wierzbicka and Panufnik, 1998; Jiang and Liu,

1999). Other studies indicate a severe inhibition of root growth. For instance, Schwartz et

al. (1999) reported that Thiaspi caerulences growing in a soil spiked with Zn had reduced

root growth while it explored a large soil volume with enhanced Zn acquisition in an

unpolluted soil. However, the root growth of T. caerulescens, a Zn hyperaccumulator,

was higher than that of Thlaspi arvense, a non-hyperaccumulator, when grown in a Zn-

amended soil (Schwartz et al., 1999; Whiting et al., 2000).

Table 5-2. The root biomass (dw), root length, and root area of P. vittata and N. exaltata
after 8 weeks of growth in a As-contaminated soil (As-soil) and control soil.

Root biomass Root length Root area
Fern species (g dw pot-f) (cm cm-3) (cm cm-2
As-contaminated soil


P. vittata 1.8a 4.3a 1.3 a

N. exaltata 0.6b 1.1b 0.4 b
Control soil

P. vittata 2.2a 6.2 a 1.8 a

N. exaltata 0.9b 2.1 b 0.7 b
Means followed by the same letter in a column within the same soil are not significantly
different at p < 0.05 (n=4).

The root length and root area of P. vittata were 3.9 and 3.2 times greater than that

of N. exaltata in the As-contaminated soil, and were 2.9 and 2.6 times greater in the

control soil, respectively (Table 5-2). In our study, the root length and surface area were

lower in the As-contaminated soil than in the control soil for both fern species. Although









the overall chemical and physical characteristics of the two soils (Table 5-1) were similar,

the higher P content and low As content of the control soil must have accounted for the

better development of the plant roots in that soil.

Root diameter determines the extent of the soil volume that potentially supplies

nutrients and contaminants to plants. For both soils, P. vittata had a greater root length

for all classes of diameter than N. exaltata (Figure 5-2). The roots of 0.25 to 1.00 mm of

diameter comprised a major proportion of P. vittata roots as measured by root length. For

both soils and plant species, the roots < 0.50 mm diameter comprised over 40% of the

total root length, while those of 0.50 to 1.00 mm diameter contributed another 25%. The

fine roots (0.25 to 1.00 mm) ofP. vittata contributed more to the total root length than

did N. exaltata. This is important because fine roots allow the root system to exploit a soil

volume more effectively while coarser roots have greater transport capacity (Forde and

Lorenzo, 2001).

For all classes of root diameter, P. vittata had a greater root area than N. exaltata.

The roots with diameter >1 mm represented 67 to 71 % of the total root area for N.

exaltata, whereas they only accounted for 59 to 60 % for P. vittata (Figure 5-3). Based

on the root characteristics (root biomass, length and area) of the two fern species, P.

vittata presented a much greater potential to remove As and nutrients from soils than N.

exaltata regardless of the soil characteristics.

Frond Biomass, Arsenic Uptake, Bioconcentration and Translocation

Good biomass production and high arsenic accumulation ability are two of many

desirable treats of hyperaccumulator plants for a successful application of

phytoextraction. Although the two ferns species had similar frond biomass









(approximately 0.7 g dw plant-) before the experiment, P. vittata had a greater biomass

(5.5 -6.6 g) than N. exaltata (1.8 2.8 g) after 8-week of growth (Table 5-3). The frond

biomass of P. vittata was 3.1 and 2.4 times greater than that of N. exaltata in the As-

contaminated soil and in the control soil, respectively.

This observation confirms the faster growing characteristic ofP. vittata when

compared with other fern species, which is desirable for a plant to be used for

phytoremediation purpose. Other studies have shown that P. vittata can grow normally or

even have the growth stimulated in the presence of arsenic (Tu and Ma, 2002).


The amount of arsenic taken up by P. vittata (2.51 mg per plant) was 29 times

greater than that of N. exaltata (0.09 mg per plant) in the arsenic-contaminated soil

(Table 5-3). Tu and Ma (2004) found a 5.8 fold increase in arsenic accumulation of P.

vittata over N. exaltata in a hydroponic study. Huang et al. (2004) reported that P. vittata

and Pteris cretica, another arsenic hyperaccumulator, removed more arsenic from water

than N. exaltata. While 92% of the plant arsenic was accumulated in the fronds of P.

vittata, only 37% was in the fronds ofN. exaltata (Table 5-3).

The ratio between the contaminant concentrations in the tissue relative to that in

the soil (bioconcentration factor, BF) as well as the partitioning of the contaminant

between the fronds and roots (transfer factor, TF) were determined aiming to gain insight

of the arsenic accumulating capability of the plants.












> 2.00 iN. exalata
S1.00-2.00 9I BP. vitati

0.75-1.00

S0.50-0.75

S 0.25-0.50

o 0.10-0.25

<0.10

0 0.5 1 1.5 2 2.5 3 3.5 4
Lv (cm cm-3)




> 2.00 N. elta
di e i aN.aedsil(tt a
S1.00-2.00 ersn P. vittata

0.75-1.00

S0.50 0.753

0.25-0.50

S0. 10-0.25

<0.10

0 0.5 1 1.5 2 2.5 3 3.5 4

Lv (cm cln-3)




Figure 5-2. The root length density (Lv) of P. vittata and N. exaltata as a function of root
diameter in a control soil (a) and an As-contaminated soil (b) after 8 weeks of
growth. Bars represent standard deviations of four replicates.











>2JO0

1.00-2100

0.75-1.00

mg
: 0.150-0.75

z 0.25-050

S0.10-025

< 0.10


0 0.1 0.2 0.3 OA 0.5 06 0.7 O
Av (cin2 cm-3)


Si"l-"-"- 111111111111111111 I I IIIII--
,RRNMeIIeIIe~


ON. exaltata
[0 P. vittata


0.10-025

<0.10

0 0.1 02 0.3 0.4 0.5


Av (cmn2 cm-3)





Figure 5-3. The root surface area density (Av) of P. vittata and N. exaltata as a function
of root diameter in a control soil (a) and an As-contaminated soil (b) after 8
weeks of growth. Bars represent standard deviations of four replicates


1111 111111111111111111111111111111111 11111111111 I


ON. exaltata
[ P. vittata


> 2.00

1.00-2100

0.75-1JO0

050-0.75


I II III III II III 11 1 1 II I1 11111 1111111111111111111 1-


.- Ill...-..-l l


D


TMTM-

RIIIIIIIIIIIIIIflH









Table 5-3. The frond biomass (dw), arsenic accumulation, and bioconcentration and
translocation factors of P. vittata and N. exaltata after 8 weeks of growth in a
arsenic-contaminated soil and control soil.
Frond As Bioconcentration Transfer
Fern biomass (mg plant-) factor (BF) factor
species (g plant-) (TF)
Frond Root Frond Root
As-contaminated soil
P. vittata 5.5 a 2.30 a 0.21 a 1.7 a 0.5 a 3.6 a
N. exaltata 1.8b 0.03 b 0.06 b 0.1 b 0.4 a 0.2 b
Control soil
P. vittata 6.6 a 0.003 a 0.00 0.55 a 2.5 a 0.2 a
N. exaltata 2.8 b 0.000 b 0.00 0.03 b 0.3 b 0.1 b
Means followed by the same letter in a column within the same soil are not significantly
different at p < 0.05 (n=4).

We assumed that plant root characteristics play a more important role in

determining BF than TF. The fronds (1.7) and roots (0.5) BF of P. vittata was 17 and 1.3

time greater than that ofN. exaltata in the arsenic-contaminated soil (Table 5-3).The fact

that P. vittata had a more extensive and more efficient root system may have contributed

to its greater BF as compared with N. exaltata. The transfer factor (TF=3.6) of P. vittata

was 18 folds greater than that of N. exaltata (Table 5-3). Huang et al. (2004) also

reported that N. exaltata was unable to translocate the absorbed arsenic from the roots to

the fronds, which was expected for a non-hyperaccumulator plant specie. The ability of

P. vittata to translocate arsenic from the roots to the fronds is considered to be one of the

tolerance and detoxification mechanisms involved in its arsenic hyperaccumulation.

Root Uptake Efficiency of Arsenic and Phosphorus

Root uptake efficiency (RUE), which is defined as elemental accumulation in

plant tissues (fronds or roots) per unit root biomass, length or area, can be used to









evaluate how efficient a plant is in taking up an element (Table 5-4). The arsenic RUE

for the fronds of P. vittata measured by root biomass, root length and root area was 15-23

times greater than that ofN. exaltata in both soils. This means that not only did P. vittata

have a greater root biomass but also a more efficient root uptake system for arsenic.

Because N. exaltata tended to accumulate arsenic in the roots, the differences in the

arsenic RUE for the roots between the two plants were smaller, 8-10 times in the control

soil and similar in the arsenic contaminated soil (Table 5-4).

Because arsenic is an analogue to phosphorus, we also determined the P RUE for

the fronds and roots of both plants. The P RUE in the control soil and for roots in the

arsenic contaminated soil was similar between the two plant species (Table 5-4).

However, the P RUE for the fronds was greater in N. exaltata than P. vittata in the

arsenic-contaminated soil.

These RUE results are rather interesting because they indicate that despite the

similarity between P and arsenic, both P. vittata and N. exaltata root systems tended to

remove P from the soil preferentially and more efficiently than arsenic.

Recently, it has been found that some natural hyperaccumulators proliferate their

roots positively in patches of high metal availability.In contrast, non-accumulators

actively avoid these areas, and this is one of the mechanisms by which

hyperaccumulators absorb more metals when grown in the same soil (Schwartz et al.,

1999; Whiting et al., 2000).

In the greenhouse study it was observed that most of the roots of P. vittata tended

to concentrate in the top 7 cm of the pot (data not shown). The observation gathered in

this study points to the need to evaluate the behavior ofP. vittata root system under field









conditions in an arsenic-contaminated soil as well as the effective root depth of the plant.

The answer to this question will help to determine the effective depth that the plant will

be able to clean up arsenic -contaminated soil.

Table 5-4. The root uptake efficiency (RUE) as measured by As and P uptake (mg
plant-) by the fronds and roots of P. vittata and N. exaltata per unit of root
parameters (root dry weight-RB, length- Lv, and surface area-Av) in a
As-contaminated soil and control soil.
Root As-contaminated soil Control soil
characteristics P. vittata N. exaltata P. vittata N. exaltata
Frond
As-RB* 1277 a 61 b 1.7 a 0.1 b
As-Lv** 552 a 31 b 0.6 a 0.04 b
As-Av* 1913 a 85 b 2.1 a 0.1 b
P-RB* 5400 b 10500 a 11600 a 9000 a
P-Lv** 2300 b 5600 a 3800 a 3600 a
P-Av*** 8200 b 14500 a 12500 a 10800 a
Root
As-RB 116 a 92 b 2.6 a 0.3 b
As-Lv 50 a 47 a 0.9 a 0.1 b
As-Av 176 a 124 a 3.2 a 0.3 b
P-RB 2400 a 1700 a 2900 a 2500 a
P-Lv 1100 a 900 a 1000 a 1100 a
P-Av 3700 a 2400 a 3200 a 3200 a
*: ug plant- (As or P)/g root; **: ug plant-1 (As or P)/cm cm-3 root; ***: ug plant- (As or
P)/cm2 cm-3 root.Means followed by the same letter in a row within the same soil are not
different at P < 0.05 (n=4).

Plant Nutrient Uptake

Balanced supply of essential nutrients is one of the most important factors in

increasing crop yields (Fageria, 2001). The frond P (1.8 -3.5 g kg-1), K (1.28 2.51g kg-

1), Ca (1.26-1.58 g kg-1), and Mg (2.8-4.8 g kg-1) concentrations of both fern species









(Table 5-5) were within the normal concentration range for most plant species

(Marschner, 2003). They were also similar to those reported by Tu and Ma (2005).

For the As-contaminated soil, the P and Mg concentrations in the fronds of N.

exaltata were greater than that of P. vittata (Table 5-5). Positive interaction between P

and Mg are expected since Mg is an activator of kinase enzymes and activates most

reactions involving phosphatase transfer (Faglia, 2001). In most plants, P is generally

concentrated in the upper parts or reproductive organs (Marschner, 2003). However, in

this study, the frond P concentration of P. vittata (1.8 g kg-1) was lower than that in the

roots (2.4 g kg-1) in the arsenic-contaminated soil. This result agrees with Tu and Ma

(2004) who found similar trend for P concentration in fronds and roots of P. vittata and

N. exaltata growing in a hydroponic solution. Arsenic hyperaccumulation by P. vittata

may be facilitated by its high arsenic influx rate and its high molar P/As ratio in the roots

resulting from both high arsenic TF and low P TF (Tu and Ma, 2004).

In the control soil, N. exaltata had a greater K and Mg concentration in the fronds

(1.7-1.9 fold) as well as Mg in the roots than P. vittata. Plant potassium content

influences the plant biomass and arsenic content (Komar, 1999). In the present study

neither plant biomass nor arsenic uptake of N.exaltata was related with the status of K in

the plant. It is unclear why N. exaltata took up more K in both soils, especially in the

fronds.

Arsenic Reduction and Distribution in the Soil

Whereas they caused no significant change in total arsenic concentration in the

bulk soil (Table 5-6), N. exaltata and P. vittata reduced arsenic concentrations in the

rhizosphere soil by 28.6 and 40.7% (Table 5-7).









Table 5-5. Nutrients concentrations (g kg-1) in the fronds and roots of P. vittata and N.
exaltata after 8 weeks of growth in an As-contaminated soil and a control soil.
Fern fronds roots

species P K Ca Mg P K Ca Mg

As-contaminated soil

P. vittata 1.8b 13.3a 15.8a 3.2b 2.4a 8.9a 17.3a 3.0a

N. exaltata 3.4a 25.1a 14.9a 4.8a 1.7a 7.8a 17.2a 2.8b

Control soil

P. vittata 3.5a 12.8b 15.0a 2.8b 2.8a 7.6b 16.0a 2.5a

N. exaltata 3.0a 21.7a 12.6a 4.4a 2.5a 10.3a 13.3a 2.6a

Means followed by the same letter in a column within the same soil are not significantly
different at p < 0.05 (n=4).

To achieve the arsenic reduction from 105 to 75.0 mg kg-1 for N. exaltata and 105

to 62.3 mg kg-1 for P. vittata in the rhizosphere soil, the soil mass required to contribute

the 0.09 and 2.51 mg arsenic (Table 5-3) constituted only 3 and 61g out of 2.5 kg,

respectively.

Similar to total arsenic, no change was observed for different fractions of arsenic

in the bulk soil with or without a plant (Table 5-6). Most of the arsenic was present in the

A fraction (61.5%) and C fraction (22.2%), contributing a total of 83.7% (Table 5-6). The

N and R fractions accounted for only 4.63 and 2.93%.

Unlike the bulk soil, the decrease in arsenic concentrations was observed across

all five arsenic fractions in the rhizosphere soil (Table 5-7). Nephrolepis exaltata and P.

vittata reduced arsenic concentrations in the A fraction (As-A) from 66.7 to 42.0 and 66.7

to 30.9 mg kg-', respectively, accounting for 76 and 67% arsenic decrease in the

rhizosphere.









Table 5-6. Arsenic distribution in different fractions in the bulk soil after 8 weeks of plant
growth.
As fractions No plant P. vittata N. exaltata As distribution

---------- mg kg ------------- ----%---

N** 5.00 a* 5.30 a 5.40 a 4.63

S 9.80 a 9.50 a 8.60 a 9.07

A 66.4 a 63.8 a 63.1 a 61.5

C 24.0 a 24.4 a 22.4 a 22.2

R 3.20 a 3.20 a 3.70 a 2.96

Sum 108 106 103 100

Total As 105 99 97

Recovery (%) 103 107 106

Means with the same letter in each row are not significantly different according to
Tukey HSD test at 5 % level.
Fractions N, S, A, C and R stand for non-specifically bound, specifically bound,
amorphous Al and Fe hydrous-oxide bound, crystalline hydrous-oxide bound, and
residual arsenic fraction;
*Based on the treatment with no plant.












Table 5-7. Arsenic distribution in different fractions in the rhizosphere of P. vittata (PV) and N. exaltata (NE) after 8 weeks of plant
growth.
As fractions Soil As concentrations % As in each fraction* As reductions*** % of total As
(mg kg-1) (mg kg-1) reductions***
No plant PV NE No plant PV NE PV NE PV NE

N**"" 4.30 a 2.0 b 4.10 a*1 4.63 4.18 6.03 2.30 0.20 4.38 0.62

S 10.7 a 3.9 c 6.50 b 9.07 8.16 9.56 6.80 4.18 13.0 12.9

A 66.7 a 30.9 c 42.0 b 61.5 64.6 61.8 35.2 24.6 67.0 76.2

C 16.0 a 9.7 b 13.7 ab 22.2 20.3 20.1 6.40 2.30 12.2 7.12

R 2.80 a 1.3 b 1.70 b 2.96 2.09 2.50 1.50 1.10 2.86 3.41
00
Sum 101 a 47.8 c 68.0 b 100 100 100 52.5 32.3 100 100

Total As 105 a 62.3 c 75.0 b 40.6 28.5

Recovery (%) 95.7 76.7 90.7


SMeans with the same letter in each row are not significantly different according to Tukey HSD test at 5 % level.
* Based on total arsenic of 101, 47.8 and 68.0 mg kg-1.
SBased on no-plant treatment for each fraction.
SBased on total reduction of 52.5 and 32.3 mg kg-1.
Fractions N, S, A, C and R stand for non-specifically bound, specifically bound, amorphous Al and Fe hydrous-oxide bound,
crystalline hydrous-oxide bound, and residual arsenic fraction;









Compared to N. exaltata, P. vittata removed more arsenic from each fraction. For

example, reductions in arsenic concentration in the A, S and C fractions were 24.6, 4.18

and 2.30 mg kg-1 for N. exaltata and 35.2, 6.80, and 6.40 mg kg-1 for P. vittata (Table 5-

7). However, plant uptake had little effect on the relative arsenic distribution among the

five fractions in the rhizosphere of two plant species, with 61.8-64.6% in the A fraction,

20.1-20.3% in the C fraction and 8.16-9.54% in the S fraction. This was because arsenic

in all fractions was reduced, with an average reduction of 52.7% and 27.0% for P. vittata

and N. exaltata, respectively (Table 5-7)

The results for arsenic distribution in different fractions obtained in our study

were consistent with the data reported by Wenzel et al. (2001) where they determined

arsenic fractionations in 20 Austria soils with differing levels of arsenic contamination

(96-2,183 mg kg 1). Arsenic in those soils was also mostly present in the A fraction

(42.3%) followed by C fraction (29.2%), accounting for a total of 71.5% or 12.2% less

than that obtained in our study. However, they reported concentration of 0.24 and 17.5%

in the N (most available) and R (least available) fractions. Comparatively, the soil used in

our study had 4.39% more arsenic in the most available fraction and 14.5% less arsenic in

the least available fraction, indicating that arsenic in the soil used in our study had greater

availability than in the soils from their study.

The decrease in arsenic concentration in the five arsenic fractions of the

rhizosphere soil did not follow arsenic availability according to the sequential extraction,

rather it was positively correlated to the arsenic concentrations in each fraction (r = 0.995

for both plants, n=4). This means that the fraction having the highest arsenic

concentration, i.e. the A fraction, had the greatest reduction in its concentration. The fact









that arsenic concentration in the N fraction, the most available, was lower with P. vittata

(2.0 mg kg-1) than N. exaltata (4.1 mg kg-1) was consistent with the greater ability of P.

vittata to take up arsenic. The results also imply that arsenic transfer from less-available

fractions to the N fraction was slower than the arsenic depletion by P. vittata. Fitz et al.

(2003) reported a much lower arsenic transformation in P. vittata rhizosphere in a soil

with a higher buffer capacity. The high rate of arsenic mobilization (reduction in the A

fraction) observed in the rhizosphere of P. vittata can also be attributed to the sandy

nature of the soil (88% sand) (Table 5-1), i.e. low ability to retain arsenic, which was

supported by low oxalate extractable Fe and Al (267 and 260 mg kg-1) (Table 5-1).

The mass balance of the total arsenic in the soil, as determined by the sum of the

fractions, was evaluated (Table 5-6 and 5-7). Good recoveries of total soil arsenic (103-

107% in the bulk soil and 77-96% in the rhizosphere) indicated that the difference

between the two methods ranged from 3 to 24%. Despite all the criticism attributed to

sequential extraction, the procedure was able to provide some insight into the

mobilization and availability of arsenic in contaminated soils. Additionally, the sum of

the fractions was in good agreement with the total arsenic values.

Influence of Plants on Water-Soluble Arsenic, Soil pH and DOC

The water soluble arsenic in the rhizosphere of P. vittata (0.7 mg kg-1) was two

times lower than that in the N. exaltata (1.4 mg kg-1), which was similar to the no plant

treatment (1.7 mg kg-1). Compared to the bulk soil, reduction in water-soluble arsenic in

the rhizosphere soil by P. vittata and N. exaltata were 58.9 and 17.6% (Fig 5-4). The

high water-soluble arsenic in the N. exaltata rhizosphere was consistent with the results

found for arsenic in the N fraction, the most available arsenic.









Concentration of water-soluble metal is a good indicator of bioavailability for

plant uptake in soils (McBride, 1994) since plants preferentially take up nutrients from

soil solution (Linehan et al., 1985). The depletion or the enrichment of an element in the

rhizosphere is determined by the capacity of a soil to replenish the soluble or

exchangeable forms of the element (Hinsinger, 1998).

Concentration of water-soluble metal is a good indicator of bioavailability for

plant uptake in soils (McBride, 1994) since plants preferentially take up nutrients from

soil solution (Linehan et al., 1985). The depletion or the enrichment of an element in the

rhizosphere is determined by the capacity of a soil to replenish the soluble or

exchangeable forms of the element (Hinsinger, 1998).

The supply of elements such as P and As to the plant rhizosphere is limited by

diffusion. When plant outpaces the soil supply capacity, the depletion occurs in the

rhizosphere, as it was the case of P. vittata (Table 5-7).

When the ability of the plant removes arsenic from the soil is lower than the

arsenic diffusion rate, arsenic accumulates in the rhizosphere, as it was the case of N.

exaltata (Table 5-7). Thus, lower water-soluble arsenic in the rhizosphere of P. vittata

was due to the higher ability of the plant to deplete the arsenic from the rhizosphere as

compared with N. exaltata. These results are consistent with the more extensive root

system of P. vittata, as well as with the arsenic reduction in its rhizosphere.

The changes in rhizosphere pH and DOC may have a great effect on soil arsenic

bioavailability. The influence of plant growth on soil pH was reflected by the pH

difference in the bulk and rhizosphere soil. Plants had no effect on the bulk soil pH for all

treatments (Figure 5-5).







86



2.5
Bulk soil
B Rhizosphere




1.5






0.5



0
P. vittata N. exaltata Control

Treatments

Figure 5-4. Comparison of water-soluble arsenic in the bulk and rhizosphere soil of P.
vittata and N. exaltata after 8 weeks of growth in an arsenic-contaminated
soil. Bars represent standard deviations of four replicates.

However, the rhizosphere pH of P. vittata (7.66) and N. exaltata (7.18) were 0.4

units higher and -0.13 lower, respectively, than that in the no plant treatment.


Youssef and Chino (1989) and Luo et al. (2000) also reported an increase of pH in

the rhizosphere of Triticum aestivum and Thlaspi cearulences. Yet Fitz et al. (2003)

found no change in the rhizosphere pH of P. vittata. Since the soil used in their

experiment had high carbonate content and high buffer capacity, their results were not

unexpected. The increase in soil pH observed in the rhizosphere of P. vittata in our study

was more likely due to the low buffer capacity of the sandy soil as well as the

cation/anion balance caused by high excretion of hydroxyl groups in response to high

absorption of arsenic. The pH buffering capacity of the soil and the initial soil pH are the

main factors determining the extent to which plant roots can change rhizosphere pH