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Adsorption of chromium on activated carbon

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Title:
Adsorption of chromium on activated carbon
Creator:
Kim, Jung Ik, 1943-
Publication Date:
Language:
English
Physical Description:
xi, 196 leaves : ill. ; 28 cm.

Subjects

Subjects / Keywords:
Adsorption ( lcsh )
Carbon, Activated ( lcsh )
Chromium -- Toxicology ( lcsh )
Environmental Engineering Sciences thesis Ph. D
Dissertations, Academic -- Environmental Engineering Sciences -- UF

Notes

Thesis:
Thesis--University of Florida.
Bibliography:
Bibliography: leaves 190-195.
General Note:
Typescript.
General Note:
Vita.
Statement of Responsibility:
by Jung I. Kim.

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Full Text
ADSORPTION OF CHROMIUM ON ACTIVATED CARBON
By
JUNG I. KIM
A DISSERTATION PRESENTED TO THE GRADUATE COUNCIL
OF THE UNIVERSITY OF FLORIDA
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE
DEGREE OF DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA 1976




ACKNOWLEDGMENTS
I wish to extend my sincere appreciation to my committee
chairman, John Zoltek, Jr., for his incessant guidance and ,.nderstanding. Special thanks are extended to Professor T. de S. Furman, who sets a standard for all engineers to strive for. Appreciation is extended to Dr. P. L. Brezonik and Dr. D. 0. Shah for being members. The many forms of help extended by Dr. Paul Urone are gratefully remembered.
My deepest gratitude is expressed to my parents. Had it not
been for their guidance and encouragement, this dissertation would never have been made possible. I wish to dedicate this thesis to them.
ii




CONTE NTS
ACKNOWLEDGMENTS i
LIST OF TABLES v
LIST OF FIGURES vi
ABSTRACT i
CHAPTER
1 INTRODUCTION 1
1-1 General Background 1
1-2 Chromium in the Environment 1
1-3 Standards for Chromium Removal 2
1-4 Chromium Removal 4
1-5 Purpose of This Research 5
2 CHROMIUM, ITS INDUSTRIAL USES, TOXICITY AND
METHODS OF REMOVAL FROM WATER 7
2-1 Industrial Application of Chromium 7
2-2 Toxicity of Chromium 10
2-3 Techniques for Removing Cr(VI) from Water 16
3 LITERATURE REVIEW 25
3-1 Chemistry of Chromium 25
3-2 Adsorption Phenomena 30
3-3 Nature of Activated Carbon 35
3-4 Adsorption Phenomena and Activated Carbon 37
3-5 Adsorption of Cr(VI) on Activated Carbon 45
4 EXPERIMENTAL APPARATUS AND PROCEDURES 46
4-1 Feed Solution 46
4-2 Activated Carbon Preparation 46
4-3 Stock Solutions 48
4-4 Experimental Equipment 49
4-5 Analytical Equipment and Techniques 50
4-6 Experimental Procedures S6
iii




CHAPTER
5 EXPERIMENTAL RESULTS AND DISCUSSION 58
5-1 Batch Studies 58
5-2 Column Studies 145
6 ENGINEERING APPLICATIONS 166
6-1 General Remarks 166
6-2 Process Design for Cr(VI) Removal
by Activated Carbon 168
6-3 Disposal of Spent Activated Carbon 173
7 ECONOMICS OF Cr(VI) REMOVAL BY ACTIVATED CARBON 174
8 CONCLUSIONS AND RECOMMENDATIONS 178
8-1 Principal Theoretical Findings 178
8-2 Suggested Future Research 180
APPENDICES 182
REFERENCES 190
BIOGRAPHICAL SKETCH 196
iv




TABLES
TABLE
2-1 Reported Cases of Chrome Ulceration in United Kingdom During the Period 1930-1972 11
2-2 Concentration of Chromium in Body Tissues With and Without Known Exposures to Chromium 12
4-1 Characteristics of Finished Effluent of Gainesville Water Treatment Plant 47
4-2 Physical Properties of Filtrasorb 400 48
5-1 Results of Column Performance for Varying Cr(VI) Concentrations 160
5-2 Effects of Acids Used for pH Control on the Adsorptive Capacity of Activated Carbon
Columns for Chromium 161
6-1 Dimension of a Full-Size Carbon Adsorption Column 171
7-1 Cost of Chemicals Used in Cr(VI) Removal by Activated Carbon Adsorption 175
7-2 Estimated Capital and Operating Costs for a
0.144 mgd Carbon Adsorption Column for Cr(VI)
Removal 176
7-3 Costs of Treatment of a Cr(VI) Waste Stream by Several Different Processes 177
v




FIGURES
FIGURE
3-1 Solubility of Cr(OH)3 as a Function of pH 29
3-2 A Distribution Diagram for the Various Cr(VI) Species as a Function of pH 32
3-3 Some Reactions of Flurescein-Type Lactones 40
3-4 (a) The Chromene-Acid Reaction; (b) Hydrolysis of the Carbonium Ion 43
4-1 Filtering Apparatus 52
4-2 Schematic of the Column Experiment
Configuration 54
5-1 CrT Concentration Remaining in Solution as
a Function of Initial pH 60
5-2 Cr(III), Cr(VI), and CrT Concentration
Remaining in Solution as a Function of
Initial pH 62
5-3 Cr(III) Precipitation with Activated Carbon
and Cr(III) Removal in the Presence of
Activated Carbon 65
5-4 CrT Concentration Remaining in Solution at
Different Activated Carbon Dosages as a
Function of Initial pH 68
5-5 Cr(VI) Concentration Remaining in Solution
at Different Activated Carbon Dosages as a
Function of Initial pH 70
5-6 Cr(III) Concentration Remaining in Solution
at Different Activated Carbon Dosages as a
Function of Initial pH 72
5-7 CrT Removal as a Function of Activated Carbon Dosage and pH 76
5-8 Cr(VI) Concentration Remaining in Solution at
Different Activated Carbon Dosages and
Different Initial pH Values 78
vi




FIGURE
5-9 Cr(III) Concentration Remaining in Solution
at Different Activated Carbon Dosages and
Different Initial pH Values 80
5-10 Relationship Between pH Rise and Activated
Carbon Dosage 83
5-11 Cr(III) Concentration Remaining in Solution
as a Function of Equilibrium pH 85
5-12 Proton and CrT Adsorption as a Function of
Initial Cr(VI) Concentration 89
5-13 Adsorbed Chromium as a Function of Initial
Proton Concentration 91
5-14 CrT Concentration Remaining in Solution as
a Function of Initial pH 94
5-15 Chromium and Potassium Adsorption at Varying
Dosages of Activated Carbon 97
5-16 Freundlich Isotherms of CrT Adsorption at an
Initial Proton to Cr(VI) Molar Ration of 1.0 100
5-17 Freundlich Isotherms of CrT Adsorption at a
Fixed Initial Cr(VI) Concentration of 100 pM 103
5-18 CrT Removal as a Function of Contact Time at
Different pH Values 105
5-19 Concentration of Cr(III) Formed as a Function
of Contact Time at Different pH Values 107
5-20 Cr(VI) Concentration Remaining in Solution as
a Function of Contact Time at Different pH
Values 109
5-21 Total Chromium Remaining in Solution as a
Function of pH 113
5-22 Cr(III) and Cr(VI) Concentrations Remaining in
Solution as a Function of Initial pH 115
5-23 Percentage Cr(VI) Removal by Activated Carbon
at Various Ionic Strength Solutions of Two
Different Salts 117
5-24 Cr(VI) and Cr(III) Concentrations Remaining in
Solution as a Function of the Ionic Strength
of Calcium Chloride 120
5-25 Concentrations of EDTA-Cr and EDTA Remaining in
Solution as a Function of the Equilibrium pH 122
5-26 Concentrations of NTA-Cr and NTA Remaining in
Solution as a Function of the Equilibrium pH 124
vii




F I GURE
5-27 Rates of Adsorption for Various Acids in the
Acid-Acid Salt System 127
5-28 CrT Concentration as a Function of Initial pH 130
5-29 Effect of Salt Concentration on Adsorption
Capacity for Chromic Acid 133
5-30 Titration of 100 ml of 10- M KHCrO4 with
0.001 M NaOH 136
5-31 Desorption of Chromium from Activated Carbon
at Different p1 Values 138
5-32 The Chromene-Chromic Acids Reactions 143
5-33 Total Chromium Concentration in the Column
Effluent vs. Dimensionless Empty-Bed Volume
Throughput at Several Different pH Conditions 147
5-34 Cr(VI) Concentration in the Column Effluent vs. Dimensionless Empty-Bed Volume Throughput
at Several Different pH Conditions 149
5-35 Cr(III) Concentration in the Column Effluent vs. Dimensionless Empty-Bed Volume Throughput
at Several Different pH Conditions 151
5-36 Total Chromium Concentration in the Column Effluent vs. Dimensionless Empty-Bed Volume
Throughput 154
5-37 Cr(VI) Concentration in the Column Effluent vs.
Dimensionless Empty-Bed Volume Throughput at
Several Designated Residence Times 156
5-38 Dependence of Column Breakthrough Capacity on Retention Time 158
6-1 Schematic of the Activated Carbon Column Process for Hexavalent Chromium Adsorption 170
viii




Abstract of Dissertation Presented to the
Graduate Council of the University of Florida
in Partial Fulfillment of the Requirements for the
Degree of Doctor of Philosophy
ADSORPTION OF CHROMIUM ON ACTIVATED CARBON
By
Jung I. Kim
March 1976
Chairman: John Zoltek, Jr.
Major Department: Environmental Engineering Sciences
A study was conducted to investigate the feasibility of remnoving chromate from water by activated carbon. The first phase dealt with batch studies to investigate the theoretical aspects of the interaction of chromate with activated carbon. The second
phase involved continuous column studies Using synthetic aqueous chromate solutions.
The interaction of chromate with activated carbon was found to
be complex because of the capability of activated carbon for adsorbing chromate as well as for reducing it to Cr(III) compounds. Adsorption and reduction were a function of the initial chromnate concentration and the initial pHi. Adsorption was maximal when the hydrogen ion and chromate were present in solution in an equimolar
ix




concentration. Activated carbon adsorbed chromate as chromic acid.
When the chromate concentration was present in solution in excess of the hydrogen ion concentration, decreased adsorption was observed. Hydrolytic adsorption took part in the overall adsorption. In a solution where the chromate concentration was below the hydrogen ion concentration, adsorption was hindered by reduction of chromate
to Cr(III) compounds. Cr(III) compounds were not removed to any great degree by activated carbon. In order to increase adsorption
of chromnate on the activated carbon, it was required to adjust the pH of solution such that the hydrogen ion concentration was equal. to the chromate concentration.
The extraction tests with organic solvents indicated the predominant chemical interaction of chromic acid with activated carbon. The extremely high adsorptive capacity of activated carbon
for chromate was due to the strong chromic acid activated carbon interaction through chemical bonding, along with the tendency of chromic acid to stabilize itself by formation of polyacids within the activated carbon.
Adsorption of chromate was for the most part complete in a
2-hr contact time, and a significant amount of chromate was adsorbed within 10 min of contact.
At a chromate to proton ratio of 1.0 and a column feed of 104 mg/l chromate as Cr, no chromium was detected in the column
effluent up to the throughput of S20 empty-bed volumes. It was found that residence time was a very important parameter in terms
of increasing -the column capacity.
x




Desorption studies indicated that alkali was a better
regenerant than acid, but 100 percent desorption was not possible. In the case of alkali desorption the desorbed chromate concentration was high enough for reuse.
Cost studies were made on a 0.144 mgd single carbon column
designed for treating a waste stream containing 104 mg/l chromate as Cr. The estimated operating costs were 74/1,000 gal without chromate recovery and 37/1,000 gal with chromate recovery.
xi




CHAPTER 1
INTRODUCTION
1-1 General Background
The toxic effects of many metals on man and his environment has been well recognized for many centuries and have attracted considerable attention during the last decade. The uncontrolled industrial use of heavy metal applications has resulted in adverse effects on man and his environment to such an extent that further industrial use of these toxic metals would begin to endanger our health and enjoyment of life. As population growth in conjunction with higher living standards begins to create a huge demand for water in both industrial and domestic uses, the recycling of water is considered to be the only solution to the water shortage problems in the future.
Chromium has been widely used in industry for many years. Recently, concern has been focused on the possibility of long-tern disturbances of our ecology by hexavalent chromium discharged by industries, as well as the short-term effects of its toxicity on municipal wastewater treatment processes. 1-2 Chromium in the Environment
The element chromium is never found in a free state in nature. The principal ore is chromite (FeO Cr 203 ) .1,2 The United States
1




2
imports virtually all of its chromite, because United States chromite ores that are commercially available are insignificant
3
in relation to the United States' requirements. In 1972 domestic consumption was 1.14 million short tons of chromite ore, or 353 thousand short tons of chromium. Of the chromite consumed, the metallurgical industry used 63.8 percent, the refractory
4
industry 19.6 percent, and the chemical industry 16.6 percent.
Chromium occurs in seawater at a concentration of 0.05 p/.
The average concentration of chromium found in United States surface waters was 9.7 pg/l. 6, The chromium content of twenty-four municipal water supplies was found to range from 1 to 40 pg/l.8 Results of a 2-yr survey of 163 public drinking water supplies in the United States showed a mean concentration of 3.3 pg/l with a range of 0.3 to 40 jig/l.9 Chromium concentrations in municipal wastewaters vary so widely from location to location because of contribution from industries that average value would have little meaning.
Chromium can have an oxidation state from -1 to +6. Only
Cr(III) and Cr(VI) are stable and are therefore present in natural
10
waters. Although chromium may exist in both the hexavalent and the trivalent state, it occurs mostly as Cr(VI) in potable water II, 12
supplies because of prevailing aerobic conditions.
1-3 Standards for Chromium Removal
Because of the toxic nature of Cr(VI) the USPHS Drinking Water Standards allows a maximum Cr(VI) concentration of 0.05 mg/l. The




3
limit of 0.05 mg/l for Cr(VI) was based on the lowest amount analytically determinable in 1946 when the drinking water standards were established. A concentration of 0.05 mg/l was believed to be sufficiently low to have no adverse effect on health.13 No limit has been set, however, for the less toxic Cr(III). According to Water Quality Criteria revised in 1972,7 the recommended concentrations of chromium as total Cr for designated uses were as follows:
(1) freshwater aquatic and wildlife 0.05 mg/l, (2) livestock drinking water 1.0 mg/l, (3) public water supplies 0.05 mg/l, (4) agricultural use (for continuous use) 0.1 mg/l, (5) marine aquatic life and wildlife 0.01 x 96 hr-LC50, and (6) for oyster harvesting
0.01 mg/l.
Realizing the magnitude of the problems and the necessity of cooperation, a coordinated industrial-municipal-regional approach to water pollution control began to surface. Many states and cities have enabling legislation or ordinances regulating the discharge of certain materials into lakes and streams. In many states, for example, a concentration of not more than 0.5 mg/l for Cr(VI) is the maximum allowable discharge into storm sewers by an electroplating industry. The Environmental Protection Agency (EPA) recently has proposed effluent limitations, guidelines, and new source performance standards. According to its recommendations, existing plating industries are subjected to limits in the discharge of Cr(VI). The limits call for a maximum discharge rate into navigable waters of
8 mg/m2 per day, single day maximum, and 4 mg/m2 for a 30-day average




4
by 1 July 1977 for existing plants, and 4 mg/sq m single day maximum and 2 mg/sq m for a 30-day average for new sources. These limitations were based on the Best Practicable Control Technology Currently Available (BPCTCA). The goal for 1 July 1983 is to have no discharge from electroplating industries through the application of the Best Available Technology Economically Achievable for re15
covery and reuse of water (BATEA).
Cr(VI) concentrations of typical untreated wastes are metal plants', bright dip wastes 10,000-50,000 mg/l, pickle bath or plating 60 mg/l; leather industry wastes 40 mg/l; cooling tower 12
blowdown waters 10-60 mg/l. The magnitude of the problem is exemplified by the fact that more than 30 billion gal of water is required annually to dilute 35 mg/l to 0.05 mg/l of Cr(VI) for one moderately sized cooling water system.
1-4 Chromium Removal
Chromium can be removed from water by methods such as reduction and precipitation, ion exchange, or reverse osmosis. Chemical reduction followed by precipitation has been most widely used. A wide variety of reducing agents may be used for the reduction of Cr(VI) to Cr(III). The choice is based on cost and availability, and convenience in each individual application. The rate of reduction is pH dependent, requiring low pH values for fast reduction. The reduced Cr(III) is then precipitated by raising the pH value to about 8.5, which is the pH for minimum solubility of Cr(III).




The certainty of successful operation is the intrinsic merit for this method. However, major problems with this treatment lie in the necessity of a relatively large settling tank for the precipitation of the highly hydrated and voluminous chromium hydroxides, and the difficulty of safe disposal of the sludge. 5Since the
recent advent of ion exchange -resins capable of withstanding the oxidizing power of chromate, chromium removal by ion exchange has received considerable attention. This method does not present a sludge disposal problem, and there is the advantage of reclamation of Cr(VI). 51,7However, ion exchange treatment does have the disadvantages of being poorly selective in choosing chromate over other anions and of having a rather critical flow rate necessary for efficient removal of Cr(VI). 5Other advanced wastewater treatment techniques such as reverse osmosis, evaporation, and ion flotation may be useful for Cr(VI) removal when employed in relatively small-sized treatm-nt facilities, but they are not likely to be used for large wastewater treatment plants. 1-5 Purpose of This Research
It was the purpose of this research to find the mechanisms responsible for the adsorption of Cr(VI) by activated carbon and to further develop techniques for practical applications. Activated carbon has been extensively used for removal of various impurities such as organics, color, and odor from water. Recent research indicated that activated carbon is capable of adsorbing, to a varying




6
degree, some of the heavy metals. Once the chemistry of Cr(VI) adsorption by activated carbon is established, economical means of removing Cr(VI) may be realized, since the activated carbon process is inexpensive compared to many other treatment processes.




CHAPTER 2
CHROMIUM, ITS INDUSTRIAL USES, TOXICITY, AND
METHODS OF REMOVAL FROM WATER
2-1 Industrial Application of Chromium
The largest use of chromium in the United States during 1972 was in the metallurgical field, utilizing 63.8 percent of the total chromium consumed. The stainless steel industry used about 60 percent of the metallurgical use of chromium, with the remaining 40 percent having been shared by other steel industries. The refractory industry used 19.6 percent of chromium, with the major use of these refractories having been in the steel industry. Chrome refractories are neutral or sometimes considered basic in character and have properties of a high melting point and moderate thermal exp mansion. 1, 2, 4, 8, 18 Although chromium consumption is relatively small (16.6%) in the chemical industry, this is where the most serious wastewater problems are present. The major uses of chromium chemicals produced in the United States have been classified as of 1960: metal finishing (corrosion control) 35 percent, pigment and allied products 25 percent, leather tanning and textiles 17 percent, chemical products 7 percent, other uses 6 percent. 19 Sodium dichromate is the primary chemical from which many other chromium compounds are manufactured. Sodium dichromate is manufactured by the calcination of a mixture of chrome ore
7




8
(chromite), sodium'carbonate, and lime, followed by a water leach to yield sodium chromate.
4(FeO:Cr203) + 8Na2 CO3 + 702 = 8 Na2CrO4 + 2Fe2 03 + 8C02 (2-1)
Calcium salts and iron oxide are precipitated out and removed by pH adjustment. Sodium chromate thus produced is acidified with sulfuric acid to obtain sodium dichromate and sodium sulfate as a by-product.
2Na2CrO4 + H2So4 =Na2Cr207 + H20 + Na2SO4 (2-2)
Sodium sulfate is removed by concentrating it by an evaporation process.
Two types of chrome plating are widely used in decorative electroplating. Chromium is usually coated to a thickness of 0.000010.00002 in. over an electro-deposited nickel for providing corrosion and abrasion resistance. Hard plating (0.001-0.01 in. of thickness) is used to provide wear resistance and a low coefficient
of friction. The plating bath is composed of chromic acid and sulfuric acid at a ratio of approximately 100 to 1 by weight. The sulfuric acid acts only as a catalyst. Florosilicate catalysts are also sometimes added to the bath.2' 1
Sodium dichromate mixed with sulfuric acid is widely used in
the bright dipping of brass and copper to remove oxide scales.




9
Dichromate is also extensively used in the metal-finishing field to provide corrosion protection and decorative effects. A metal is immersed in a dichromate solution under conditions intermediate
between corrosion and passivation. Chromic acid is also used to anodize metals, such as aluminum, to produce a protective oxide coat.
One of the most important uses of chromium compounds is in
corrosion inhibition in recirculation water systems. Chromate is known as an anodic inhibitor and prevents removal of the ionized metal from the metallic phase by providing a very thin layer on the metal surface. The concentration of chromate required to inhibit corrosion in a system varies depending on the ions in the solution, the temperature, and the pH.'19
Chromates and dichromates are a major component in waterborne preservatives and fire-retardant preservatives. Out of 13.8 million pounds of water-borne preservatives used in the American wood industry during the year 1963, 10.4 million pounds contained chromate compounds. The chromate reacts with the wood extractives and other preservative salts to produce an almost
insoluble complex while simultaneously rendering toxicity to wood-destroying fungi.1
Wide use of chromium compounds is to be found in the pigment industry. Chrome oxide green (Cr 20 3 ) chrome yellow (PbCrO 4), and zinc yellow (ZnCrO 4) are the most widely used chromium pigments.




10
In textile processes, chromates are used as a mordant to fast the colors on cotton and wool, and as an oxidizing agent for dyes on cotton and other textiles.20
The leather industry has been using chromium as basic chromium sulfate for tanning light leathers. The mechanism of chrome tanning is not well understood, but it is believed to be due to the coordination of chromium with the carboxyl groups of collagen. The chrome-treated leather possesses many superior properties, such as a high hydrothermal stability and shrinking temperature
2
as well as a resistance to bacterial action. 2-2 Toxicity of-Chromium
2-2-1 Toxic effects on humans. Chromium is present primarily in trivalent or hexavalent states in the natural environment. Cr(VI) is known to be more toxic than Cr(III) by a factor of about 100.21 Pure metallic chromium is believed to be biologically inert and exerts no harmful effects on the human body.20 The toxicity of soluble chromic salts is of a low order, and insoluble oxide and phosphate are believed to be nontoxic.
Cr(VI) is believed to produce lung cancers, since a more than normal incidence of lung tumors had been reported among workers in chromate manufacturing plants. 18, 22, 23 Table 2-1 shows reported incidences of chrome ulceration in the United Kingdom from 1930 to 1972.24 Table 2-1 indicates that most incidences came from workers in the chromium plating industries. Chromates are




TABLE 2-1 Reported Cases of Chrome Ulration in United Kingdom
During the Period 1930-1972
Manufacture Dyeing and Chrome Chrome (Bichromates) Finishing Tanning Plating
1930 6 15 5 57 12 95
1950 36 100 7 143
1959 100 85 7 192
1960 181 1 106 9 298
1961 82 1 1 121 7 211
1962 39 86 5 130
1963 35 146 19 200
1964 34 1 217 10 262
1965 34 127 6 167
1966 27 104 2 133
1967 42 1 83 5 131
1968 24 117 5 146
1969 26 1 91 3 121
1970 16 1 67 5 89
1971 24 59 6 89
1972 67 61 2 130
also known to cause irritation dermatitis and sensitization derma25
titis. Asthma, fever, nephritis, and eczema are also believed
5
to be caused by allergic reaction to chromates. Water containing
7.8 mg/l of chromium from chromate bath wastes was blamed for
eczema among workers in Czechoslovakia.5 Dusts and fumes generated
from the chromate process were found to cause caustic action on
mucous membranes resulting in perforation of nasal septum as well 25
as ulcers (chrome holes) on skin.
Table 2-2 shows chromium concentrations in body tissues from
chromate workers and those of normal tissues taken from subjects
with no known exposures to chromium. It appears that chromium
tends to accumulate in the respiratory areas such as lungs,
tracheobronchial lymph nodes, and bronchus. The American Conference




12
TABLE 2-2 Concentration of Chromium in Body Tissues With and
Without Known Exposures to Chromium20
Range of Concentrations Range of Concentrations Tissue Reported in Normal Tissues Reported in Tissues
Tissue) from Chromate Workers (g/100 g Wet Tiss) (g/100 g Wet Tissue)
Lungs 0-33 130-9,887
Lung tumor 0-1,658
Metastatic tumors 2-100
Tracheobronchial
lymph nodes 0-1 12-7,590
Bronchus 95-386
Trachea 0-32
Nasal septum 287
Larynx 21
Kidney 0-9.6 0-211
Liver 1-11 0-159
Spleen 0-98 0-91
Abdominal lymph
nodes 1 4-80
Stomach 0-5 4-11
Intestines 10 4-5
Bladder 3-226
Heart 0-20
Muscle 0-8 0-19
Pancreas 21 8-36
Thyroid 43 24-53
Adrenal 0-41 5-76
Brain 0-4 0-5
Bone 5 0-292
Cartilage 6
Hair 31
Skin 5
of Governmental Industrial Hygenists has set threshold limit values 25
(TLV) for chromic acid and chromate of 0.1 mg/cubic meter of air.
Chromate can penetrate the body through damaged skin. Twelve deaths
were reported because of external use of an antiscabetic ointment 26
in which a preparation of chrome was substituted in place of sulfur.
Analysis of lung tissue from workers who had been exposed to chromium




13
in the chromate chemical industry revealed that large amounts of acid-soluble chromium, as well as insoluble chromium, were re20
tained in the lung tissue for many years after exposure. Although sufficient data have been collected which clearly indicate deleterious effects of chromium on human health, there does not appear to be a clearly defined "no effect" level. It was suggested that a concentration of 0.05 mg/l, with an average intake of 2 liters of water per day, would not be hazardous to human health.7
2-2-1 Toxic effects on aquatic life. The susceptibility of organisms to chromium depends largely upon the type of organisms and their physical and nutritional condition at the time of ex27, 28
posure. 2 Generally the larger animals tend to be somewhat more resistant. Bluegills were found to be able to tolerate a 29
45 mg/l of Cr level for 20 days in hard water. The 96-hr lethal concentration (LC) for 50 percent mortality and safe concentrations for Cr(VI) were 33 and 1.0 mg/1 for fathead minnows, 50 and
0.6 mg/l for brook trout, and 69 and 0.3 mg/l for rainbow trout. Even though Cr(VI) is generally believed to be more toxic than Cr(III), toxicity of Cr(III) to these species was found to be about the same as Cr(VI).
Pickering and Henderson conducted an experiment on toxicity of chromium to four species of warm-water fish. The experimental results indicated that Cr(III) was significantly more toxic to each species than Cr(VI). The soft-water 96-hr TL (Median Tolerance
in




14
Limit) values ranged from 3 to 7 mg/l for trivalent chromium, and from 18 to 118 mg/l for hexavalent chromium. Zarafonetis and Hampton27 studied effects of chromium on two species of algae. Their data show that Cr(VI) concentrations of 10 and 20 mg/i as K2Cr207 had no apparent effect on the growth of Chlorella pyrenoidosa, but strongly inhibited the growth of Chiamydomonas reinhardi. The net photosynthesis by a natural algal population was inhibited by 20 and 50 pg/l of Cr(VI). From these results they suggested reexamining the present federal water pollution standard for Cr(VI) of 50 pg/l for drinking water supplies.
A long-term study of oyster mortalities in water containing 10 to 20 pg/l chromium indicates cumulative toxicologic effects. Nearly three quarters of the oyster deaths occurred in the warm period of May through July, when physiological activity might be expected to be greatest. Raymont and Shields31 investigated toxic effects of chromium in the marine environment and concluded that the toxic effects of the different chromate salts on Nereis (worms) was not significantly different. Heavy mortality was recorded at the concentration of 2 to 10 mg/l Cr in 2 to 3 weeks, and the long-term threshold concentration of Cr was found to be just below the 1 mg/l level. Radioactive Cr51 was used to investigate the 51Z
location in the body of the Nereis where the greatest adsorpt*Dn occurred. Both the gut and body wall appeared to be sites for adsorption of Cr. Considerable adsorption was found throughout the body in the absence of any food intake.31 Hill and Fromm32 found




that exposure of trout to Cr(VI) at the levels of 0.02-0.2 mg/i for a week caused a significant elevation of plasma cortisol.
2-2-3 Effects of chromium on the activated sludge waste treatment process. Effects of chromium on the activated sludge process have long been a concern in the wastewater treatment field. Jenkins and Hewitt33 found that chromium at the concentration of 10 mg/l as K2CrO4converted a good effluent to realtively moderate
34
quality. Spencer observed that biological activity in activated sludge ceased when the chromium concentration ranged from 3.5 to 67.6 mg/i as Cr. Jenkins and Hewitt35 reported later that the plant effluent BOD values were markedly decreased when sewage 36
contained 0.7 mg/l of Cr. In contrast, Ross found that an activated sludge plant at Richmond, Indiana, was able to treat, without much difficulty, 1 mg/l as Cr of chromate waste. Monk37 stated that 26 mg/l of chromium might be permitted without losing treatment efficiency to any great extent. Cr(VI) was found capable of inhibiting the growth nitrifying bacteria.33'35 Placat38 indicated that inhibition of nitrifying bacteria by very low concentrations of Cr(VI) would be a useful tool for arresting unwanted nitrification in BOD bottles. Microscopic examination of activated sludge that received chromate waste revealed the almost complete 39
disappearance of filamentous bacteria such as sphaerotilus, and an overall decrease in the number of stalked ciliates. Ingols and Fetner28 stated that chromate caused a bulking sludge, indicating a favorable growth of molds over the growth of the zoogleal masses.




16
2-3 Techniques for Removing Cr(VI) from Water
Virtually any advanced wastewater treatment techniques can be
applied to chromium removal. The choice depends on costs, reliability; and perhaps the degree of removal designated by state or local regulatory agencies.
Some techniques for removal of Cr(VI) are (1) reduction of Cr
(VI) to Cr(III) and precipitation of Cr(III) as chromium hydroxide,
(2) ion exchange, (3) evaporation, (4) reverse osmosis, (5) Precipitation of Cr(VI) by lead or barium, (6) ion flotation, (7) liquidliquid extraction, (8) activated sludge process, (9) adsorption on activated carbon. Among the methods listed above, only two processes, reduction and precipitation and ion exchange, are currently widely used.
2-3-1 Reduction of Cr(VI) to Cr(III) and precipitation of Cr(III) as chromium hydroxide. Cr(VI) is first reduced to the Cr(III) by the addition of a reducing agent, with proper adjustment of pH, followed by precipitation of the reduced chromium. Various reducing agents can be used. These include ferrous iron, sulfur dioxide, sodium(bi)sulfite, sodium or other metal sulfides, metallic iron, and zinc. The rate of reduction is pH dependent, requiring a low pH, frequently lower than 3, for a fast reaction. The acid requirements for the reduction of Cr(VI) depend upon the acidity of the original waste, the pH of the reduction reaction, and the type of reducing agents used.17 The reducing potential of reducing agents is also an important factor to be considered. For example, an excess




17
of about 25 times the stoichiometric amount is required for the reduction of Cr(VI) to Cr(III) when ferrous sulfate is used as a reducing agent because ferrous sulfate has a low reduction potential. The result is an excess of Fe(OH)3 sludge along with Cr(OH)3 that must be removed.21 Negative radical reducing agents such as sodium sulfite and sulfur dioxide have advantages over positive radical reducing agents such as Zn, Fe, and Cu because no extra sludge is produced along with the Cr(OH)3. This advantage is more important when the recovery of chromium oxide is to be considered. Once reduction of Cr(VI) to Cr(III) is complete, the pH of the solution is raised to reduce the solubility of Cr(III) by introducing an alkali. Cr(III) is amphoteric, being dissolved in both acid and alkali. It has a minimum solubility of about 0.1 mg/l at around pH 8.5. It is therefore essential to choose a reducing agent which is effective in reducing Cr(VI) at high pH values so that less acid is needed to first lower the pH and so that less alkali is needed to then raise the pH. A major difficulty with this treatment process is the sludge disposal problem of the precipitated hydroxides. Chromium hydroxides are in a colloidal state and tend to highly hydrated and voluminous, containing as much as 80 percent'water by volume. A large settling basin is required to prevent the carrying-over of hydroxides into the effluent.
2-3-2 Ion exchange. Ion exchange involves a reversible interchange of ions between a solid phase and a liquid phase. This




18
process does not present a sludge problem and has the advantage of reclamation of Cr(VI). An anion exchange resin in the chloride or sulfate form is used to exchange chromate ions in the solution to be treated. One of the following reactions may occur depending
on the prevailing pH conditions:
1. In neutral water
2RCI + CrO42- = R2CrO4 + 2C- (2-3)
2. In an acidic condition where dichromate is the species
2RCI + Cr2072- R2Cr207 + 2CI- (2-4)
3. In an acidic condition with a high chromate concentration
R2Cr207 + H2CrO = R2Cr3010 + H20 (2-5)
R2Cr 3010 + H2CrO4 =R2Cr4013 + H20 (2-6)
Acidifying the chromate solution has a twofold effect: first in-creasing the exchange capacity by a factor of two or more, as shown in the above reactions, and secondly, increasing the
5
selectivity of the resin for chromate over other foreign ions. Regeneration may be accomplished with a sodium-hydroxide sodiumchloride mixture and then followed by NaCl to restore the exchange resin to the chloride form. The addition of sodium hydroxide in the regenerant solution is considered to be necessary to convert acid chromate back to the neutral chromate, which is less
5
strongly adsorbed and therefore more easily replaced by chloride. The recovered chromate can be sent either to a cation exchanger




19
to recover Cr(VI) as chromic acid or can be further concentrated by using evaporative systems. It can frequently be reused without further concentration in many areas because of the high Cr(VI) concentration of up to 10 percent. Some of the disadvantages of the ion exchange process are Cl) the need of maintaining a critical flow rate, which if exceeded even temporarily, results in incomplete exchange and leakage of chromate (in this case the column's capacity is still far from exhausted) (2) the limited capacity of the ion exchange system requires relatively large installations to provide the exchange capacity needed between regeneration cycles, (3) disposal of the regenerated material, C4) slow deterioration of resin with use and possible contamination of water, CS) selectivity consideration of the resin for chromate over foreign anions present. 5,15
2-3-3 Evaporation. Evaporation is a well-established industrial process to separate solid from solution. In plating industries evaporation has been used successfully for recovering plating chemicals and water from plating waste effluents. There are numerous modifications of the evaporation process that may be employed.
In order to minimize the amount of energy required for evaporation, multiple-effect evaporation, vapor- compression evaporation, and multistage flash evaporation are the most widely used methods.
In multiple-effect evaporation, water evaporated at a given
pressure and high temperature is fed to a second compartment where additional water is evaporated at a lower temperature. The




20
evaporation of water is carried out in successive stages. A number of effects can be provided until optimum efficiency is reached.
In vapor- compression evaporation, water is evaporated at atmospheric pressure. The vapor is then compressed to raise the pressure of steam and is returned to the heating side of the evaporator. A temperature difference between the compressed steam and influent in the evaporator is the driving force for heat exchange. No cooling water is required ii, this process as is in the multiple-effect evaporator. However, the high initial cost of this process is not economically justified except where no cooling water is available. 40
In multistage flash distillation the water is heated to the highest temperature and flashed into the evaporator. The steam thus produced is condensed to produce water. The remaining concentrated solution is flashed into an additional evaporator which is operated at a lower pressure than the preceding ones. 14, 40
Evaporation is a relatively expensive process requiring high capital and operating costs. However, a single-effect evaporator in conjunction with ion-exchange may be increasingly useful in the plating industry for recovery of chemicals.
2-3-4 Reverse osmosis. Reverse osmosis is a process used to
separate a solution into a concentrate and a more dilute solution by applying pressure across a membrane. The flow of water across the membrane is directly proportional to the net pressure differential as expressed by the following equation: 21 W = K W (dP (2-7)




21
where W = water flow rate, Kv = membrane water permeation constant, dp=applied pressure differential, and Tr = osmotic pressure differential. Most of the existing reverse osmosis units in the plating industry are for treatment of nickel plating solutions because of the suitability for handling nickel solutions and the reuse of expensive chemicals. However, laboratory and pilot plant studies have been performed for treatment of cyanide and chromiumcontaining wastewater. A limitation of the reverse osmosis process is that acidic or alkaline solutions, as well as highly concentrated solutions, cannot be successfully handled without prior
15
treatment.
2-3-5 Precipitation by lead or barium.. Lead and barium chromates are highly insoluble and therefore can be used for precipitation
of chromate in waters. However, the extreme toxicity of both metals, along with their high price, imposesa limitation on their wide application.5
2-3-6 Ion flotation. Ion flotation is principally a combination of mineral flotation and ion exchange processes. A surface-active
agent such as ethylhexadecyldimiethyl ammonium bromide is used as a chromate collector and is added to a chromium- containing solution. Inorganic groups of the collector dissociate in water and are replaced by chromate ions to form an insoluble surface-active compound. The newly formed compound tends to remain at the solution/
air interface when air is introduced into the solution, and can be separated from solution by skimming off the chromate froth.




22
As high as 98 percent removal of chromate, for an initial concentration of 10 mg/l as chromium, was reported in a laboratory
41
scale study. Ion flotation is a recently developed technique.
There are many areas to be investigated prior to large-scale operation. These include disposal or reclamation of chromate and once-used surfactant, and feasibility of the continuous flotation method. 5 Results of a pilot-plant scale ion flotation study were reported by Grieves et al. 42 They found optimum results at a molar feed ratio of surfactant to dichromate of 2.1 with 70 percent recycle. Optimum detention time was 85 min. The feed concentration of the dichromate was about 100 mg/l. The concentration of the chromium in a liquid volume less than I percent of the volume of the waste could be achieved at about 67 cents chemical cost per pound of chromium.
2-3-7 Liquid-liquid extraction. Liquid extraction, sometimes
called solvent extraction, is the separation of the constituents by another insoluble liquid. The following chemical reactions take place when a chromate solution is reacted with a tertiary amine:
2- +
2R 3 N + Cr 2 0 7 + 2H = (R 3 NH) 2 Cr 2 0 7 (2-8)
Both R 3 N and (R 3 N14) 2 Cr') 0 7 are soluble in kerosene or other solvents, but almost insoluble in water. Chromate moves from the water to the organic phase, and chromium- containing organic solvent can be




23
easily separated from water by decantation. Chromium can be stripped out of the extract under alkaline condition.
(R3 NH)2C207 + 4NaOH = 2Na2CrO4 + 2R3 N + 3H 20 (2-9)
Both chromate and R3N can be reused. The treated water (called raffinate) is almost chromium free. Over 99 percent removal of chromate can be accomplished by this method.39
2-3-8 Activated Sludge Process. The conventional activated
sludge treatment process can result in the removal of heavy metals to a certain degree. 43' Stones44 reported in 1955 that 67
percent of the initial chromium of concentrations ranging from
0.17 to 0.56 mg/l as Cr was removed by this process. A 60 percent removal of chromium by the primary treatment process and 65 percent removal by the secondary treatment process was observed by 45
Oliver and Gosgrove. A similar degree of chromium removal was 43
also reported by Tarvin. The mechanism of chromium removal in the activated sludge process was believed to be the reduction of Cr(VI) to Cr(III), followed by precipitation of Cr(III) in neutral or slightly alkaline condition that prevailed in wastewater.5,
2
Bacterial utilization of oxygen available in the CrO4 ion was believed to be responsible for the rapid reduction of Cr(VI) to Cr(III). A study showed that an increase in the suspended solids concentration in an activated sludge tank resulted in an increased removal of chromium.43 Adsorption of heavy metals to suspended




24
solids has been well documented in many references. 7,48,49,50,51
Incidences of digester failure caused by heavy metals have been reported. 52 3The precipitated Cr(III) in the activated sludge probably was transferred to the digesters along with the sludge. The low pH which was exerted during the acid production period in
the digester increased the solubility of Cr(III) so that it redissolved, thereby causing a toxic action which led to the decreased rate or even stopping of digestion.




CHAPTER 3
LITERATURE REVIEW
3-1 Chemistry of Chromium
Chromium is the twenty-fourth element in the periodic table
with an atomic weight of 51.996 based on the relative atomic mass
12 2 2
of C = 12. The electronic configuration of 52Cr is ls 2s
6 2 6 5 1
2p 3s 3p 3d 4s It is possible from its electronic configuration that chromium can have all the oxidation states from
-1 to +6. It can form a negative monovalent ion by taking up one electron to fill the 4s shell. It can also form six positive ions by losing electrons successively from the 3d shell. However, the chromium compounds with the +3 and +6 values are the only stable ones and are the predominant forms found in natural water.
The oxidation potential of Cr3+ to Cr(VI) depends on the pH and the ratio of species present. Typical reactions are
Cr3++ 4H 0 =H2CrO4 + 6H+ + 3e(H2 CrO 4)
= 1.335 0.1182pH + 0.0197 log (3-1)
(Cr
Cr3+ + 4H0 = HCr04 + 7H+ + 3e(HerO4)
E = 1.335 0.1379pH + 0.0197 log (Cr3+ (3-2)
25




26
3+ 2- +
2Cr +7H20 = Cr407 + 14H + 6e 2
(Cr207 )
E0 = 1.333 0.1379pH + 0.0098 log (Cr 23+2 (3-3)
(Cr )
3+ 2- +
Cr + 4H 2 0 Cr04 + 8H + 3e 2
(Cr042
E = 1.477 0.1579pH + 0.0197 log (Cr ) (3-4)
(Cr 3
-2- +
CrO2 + 2H20 = CrO42 + 4H+ + 3e
2 2 4
(Cr042 +)
E0 = 0.945 = 0.0788pH + 0.0197 log (3-5)
(Cr02 )
3- 2+ +
CrO3 + H20 = CrO4 + 2H + 3e 2
E = 0.359 0.0394pH + 0.0197 log (Cr042) (3-6)
(Cr03 3+)
Eqs. 3-1 through 3-4 are the reactions that take place under acidic
conditions, and Eqs. 3-5 and 3-6 are under alkaline conditions.54 3+
C2 is more stable than Cr(VI) at neutral and acidic pH values. Cr(VI) becomes more stable at a pH over 12. Cr(III) normally precipitates under a neutral or slightly alkaline condition with a minimum solubility of about 0.1 mg/1 at approximately pH 8.5. Al-30 55
though the equilibrium constant of Cr(OH) is approximately 10-30,55 the solubility of this chromium hydroxide is increased with either




27
decreasing or increasing pH. Solubility increase with an increase in pH is due to the formation of Cr(OH)4 and other polynuclear hydrolysis species. The hydrolysis of Cr(III) can be calculated
56
in the same manner as shown by Stumm and Morgan. Actual computations are given in Appendix 1, and a solubility diagram based on the results of these data is shown in Fig. 3-1.
Chromium has a marked tendency to form coordination compounds with water. The simple hydrated ion can be prepared at room temperature by addition of distilled water to a chromium salt such as nitrate, perchlorate, and fluroborate. Ions such as sulfate and chloride enter the complex through the displacement of coordinated water.
However, the rate of exchange of water with other anionic species is considered to be very slow due to the tightly bound inner sheath of water molecules.18'57 Although chelating agents such as ethylene-diamine-tetraacetic acid (EDTA) and nitrilo-triacetic acid (NTA) form a very stable complex with Cr(III), the rate of formation is so slow that boiling of the solution for about 15
58
min is required to insure completion of the reaction.
2
Hexavalent chromium in water forms oxo(CrO4 ) and hydroxo
(CrO 3OH-) complexes which are acids. Acidity of aquo metal ions results from the repulsion of the proton of H20 molecules by the positive charge of the metal ion. The acidity of aquo metal ions increases with the decrease of the radius and an increase of charge of the central ion. The strong acidity of Cr042- and CrO3OH- results from the high oxidation state of Cr(VI). Various species of Cr(VI)




FIGURE 3-1 Solubility of Cr(OH) as a Function of pH.
Only monomeric hydrolysis species are considered.




29
-D4
0
~6
8 +( )4
Cr(OH2 10- Cr(O H),, Cr+3 +2
CrOH
2 4 610 12
pH




30
may exist depending on the pH and the concentration of Cr(VI) ions. Distribution diagrams of hexavalent chromium species are shown in Fig. 3-2. These diagrams are based on numerical equations listed in Appendix 2. Chromic acid is a fairly strong acid in its primary dissociation that does not exist except in solution. It shows a marked tendency in very concentrated solutions to form polynuclear species (polyacids) through the elimination of water.
2H2CrO4 = H20 + H2Cr207 (dichromic acid) (3-7)
3H2CrO4 = 2H2 0 + H2Cr3010 (trichromic acid) (3-8)
4H 2CrO4 = 3H 20 + H 2Cr4 013 (tetrachromic acid) (3-9)
H CrO converts to H Cr 0 almost instantaneously, but the further
2 4 2 2 7
polymerization requires a measurable time. Salts derived from these polyacids are also known to exist.18'20'59'60 Chromic acid is a strong oxidizing agent, the oxidizing power increasing with decreasing pH.18
3-2 Adsorption Phenomena
Adsorption is a process of the interphase accumulation or con61
centration of substances at a surface or interface. The energy
of interaction at the interface can be interpreted as a composite function resulting from the sum of attraction and repulsion forces.




FIGURE 3-2 A Distribution Diagram for the Various Cr(VI)
Species as a Function of pH. At Cr = 1, 10-2 and 10 M. Activity coefficients of 1.0 were
assumed for all cases (after Stumm and Morgan,6).




32
0 Cr z 1 07
0
0123 5678910
-1 0- ~ C 27 '0
0
o -21
0123 4 5 6 7 8 9 1 100
-0 o72
0
0 -2
o 4Y 4
Li 2
01 234 5 67 8 910
pH




33
Forces responsible for adsorption are (1) nonpolar van der Waals attraction, (2) formation of hydration bonds, (3) ion exchange
(4) chemical interaction, (S) coulombic, or electrical forces. 56'62 The force between atoms and molecules is always attractive. London
suggested in 1930 that the positively charged nuclei and the negatively charged electrons in molecules (and atoms) oscillate with respect to each other, producing oscillating dipoles. The resultant force is known as the dispersion force, or the London van der Waals force. The van der Waals attraction force between two atoms is inversely proportional to the sixth power of distance over small distances.56',63
Adsorption of a solute onto a solid substrate may take place if either contains hydrogen bond donor groups and the other contains acceptor groups. However, hydrogen-bond adsorption would be greatly hindered if one of the bonding groups had too strong an affinity for water. 6
Ion exchange or electrostatic attraction may be responsible for the adsorption of organic ions onto solids. For example, silica and carbon, which are normally negatively charged, readily adsorb cationic dyes and surfactants. 62Multivalent ions
are attracted with greater force than monovalent ions toward a site of opposite charge on the surface of the adsorbent, and 'his selectivity tends to decrease with the increasing ionic strength of the solution. 56',61
Adsorption by means of a chemical reaction differs from physical adsorption in that it first requires considerably higher




34
activation energy for adsorption. Chemical reactions also proceed more rapidly at elevated temperatures than at low temperatures. 61 However, it is often difficult to draw a sharp line of demarcation to distinguish between physical and chemical adsorption. 61, 64
Generally adsorption is increased with decreasing solubility
of the solute in the solvent. The solubility of solute in solvent can be related to the solute-solvent bond.
The surface tension of a solution, as described by the Gibbs adsorption equation, can be written as follows: 61, 63
EC = I ( dy (3-10)
RT dln a
where EC = the excess surface concentration of solute, y interfacial tension, and a = activity of solute. For dilute solutions
the concentration can be used instead of the activity. In Eq. 3-10 a decrease in surface tension is brought about through the accumulation of solute at the interface. Many organic compounds have hydrophobic radicals indicating low affinity for the aqueous phase. They tend to stay away from the bulk of water, favoring being adsorbed on an adsorbent. Many inorganic ions are readily hydrated with water molecules and tend to remain in solution, making themselves less available for adsorption. 56 In general, adsorption is minimal for the charged species and maximal for the undissociated (neutral) species. 65 The degree of dissociation is




35
*dependent on pH. For amphoteric compounds the maximum adsorption occurs at the isoelectric point, when the compound becomes neutrally charged. 65
Solute polarity also has effects on adsorption. A polar solute has a tendency to be adsorbed preferentially by a polar adsorbent from a nonpolar solvent. The effect of polarity of a solute on adsorption is closely related to surface tension and solubility. Functional groups such as -OH, -SH, -COGH, -NH 2 or SO 3 Htend to render their compounds polar. Solution by water then results by formation of a hydrogen bond from hydrogen in the water molecules to a group bearing a negative charge. Interfacial tension as well as water solubility is therefore increased, requiring more work to bring solute molecules to the interface for adsorption. 3-3 Nature of Activated Carbon
Activated carbon has been widely used for removal of organic
pollutants from water and wastewater. These organic pollutants include wide varieties of substances such as taste- and odorproducing materials, and color-causing organic compounds.
The most characteristic property of activated carbon is its
extremely large surface area (500-2,500 m 2/g) Having a large surface area is very important since adsorption is an interfacial phenomenon. However, any interpretation of the adsorptive behavior
of activated carbon based solely on the large surface area is incomplete. Equal weights of carbons prepared from different raw




36
materials by different methods may have the same total surface
area yet behave differently as an adsorbent. Relative pore size distribution within the carbon is responsible partly for different adsorptive capacity. 66A molecule will not readily find its way
into a pore smaller than a certain critical diameter and will be excluded from smaller pores. 67Various functional groups are known
to exist on the carbon surface. Differences in adsorptive capacity may be attributed partly to these functional groups, which are determined to a large extent by the method of activation as well 66
as the type of material from which the carbon is prepared. Although chars, coke, and activated carbon are frequently termed amorphorus carbon, X-ray studies have indicated that they have microcrystal line characteristics. The microcrystallites are formed by two or more flat plates in a hexagonal lattice shape, stacked one above the other. Although the structure of the
crystallite is similar to that of graphite, differences between the two carbons exist in many ways. Impurities in activated carbon which are not found in graphite are believed to have a significant effect on the adsorption of organic substances. In activated carbon, because of either the preparation procedure or the starting material, the planes orient themselves in a more disorderly manner than in ideal graphite. 66Garten and Weiss 68 stated that the analogy with graphite as a model is poor and
they prefer to visualize stacks of flat, aromatic, and for the most part, heterocyclic planes crosslinked in a random fashion,




37
affecting both the'distance of separation of adjacent planes and the adsorptive properties of the carbon. Exceptionally high
oxygen content (2-25%), as well as substantial amounts of hydrogen in carbon, is. believed to play an important role in determining the chemical behavior of carbon. Garten and Wes regarded activated carbon as a complex organic polymer rather than an amorphous form of the element carbon.
The production of activated carbon involves carbonization.
This is normally carried out by heating the raw material in the absence of air at an elevated temperature of approximately 600 0C. Many metallic chlorides such as zinc, calcium, and magnesium chlorides are often added to increase the effectiveness of carbonization. 64Chars prepared by carbonization do not have large internal surface areas. The large surface areas of activated carbons result from the process -referred to as activation. Activation is commonly carried out by oxidizing a carbonaceous char with C02, steam, and air. Activation temperature has a profound effect on the properties of activated carbon. 3-4 Adsorption Phenomena and Activated Carbon
In 1929 Kruyt and De Kadt (Garten & Weiss 69J reported that carbon activated at a low temperature (around 400 0 C) was capable of adsorbing alkali but little acid, whereas carbon activated at a high temperature (800-1,000 0 C was able to adsorb acid but not alkali. It has been generally believed that the adsorption of




38
alkali on L-carbon (activated at a low temperature) is due to the presence of oxygen complexes on the surface of activated carbon. Numerous theories have been forwarded to explain alkali adsorption. The presence of carboxyl groups in activated carbon was suggested by Schweizer and Goodrich (Garten & Weiss68' 69) in 1944, and the presence of phenolic groups was suggested by Villare in 1947 (Garten & Weiss 69). Garten and Weiss69 concluded that the acidity of L-carbons principally originated from three functional groups: the phenolic group, lactone groups in association with phenol, and normal lactone groups. Some reactions of fluresceintype lactones (f-lactones)are graphically shown in Fig. 3-3. An aqueous suspension of an H-carbon (activated at a high tempera70
ture) was found to have an alkaline pH value. Burshtein and Frumkin (Garten & Weiss 68) in 1929 observed that an H-carbon, outgassed in a high vacuum, was unable to adsorb acid from an oxygen-free solution unless oxygen was introduced. A study by Garten and Weiss69 showed that adsorption of acid was dependent on the partial pressure of oxygen. The adsorptive capacity increased with increasing partial pressure of oxygen up to 20 mm Hg, beyond which adsorption became almost independent of partial pressure. Bretschneider (Garten & Weiss 69) studied the adsorption of hydrochloric acid on aerated and outgassed carbon. It was observed that the isotherms were parallel except in the region of low acid concentration. He concluded from this finding that part of the adsorbed acid was chemically adsorbed by oxides on the surface,




FIGURE 3-3 Some Reactions of Flurescein-Type Lactones.
(After Garten & Weiss69.)




40
0
COOH COO Na
C H N
12 2 O 'N a 0 H DI L. AC I D
O'CO
co
0 2 0
COOH




41
and the remaining reacted acid was adsorbed by physical adsorption. Kolthoff in 1932 detected hydrogen peroxide that was released by carbon when activated coconut charcoal reacted with sulfuric 68
acid. However, Garten and Weiss reported that the amounts of oxygen and acid adsorbed were found to be far greater than that of hydrogen peroxide liberated. The stoichiometric determination of the adsorption of acid and oxygen is very difficult. This is due to the capability of hydrogen peroxide to attack carbon and consequently to form acid groups, and partly due to the difficulty in distinguishing quantitatively between physically and
68, 69
chemically adsorbed acid.
Many theories on adsorption of acid on carbon were proposed by numerous scientists. These include the electrochemical theory, a theory involving the neutralization of surface oxide, and pure physical adsorption. However, none of these theories can explain all the phenomena pertaining to acid adsorption. Garten and Weiss69 proposed the presence of chromene (benzpyran) groups to explain acid adsorption. Chromene is readily oxidized at room temperature in the presence of acid to become carbonium as shown in Fig. 3-4a. The carbonium (or benzophrylium) ion is a weak base having a dissociation constant on the order of 1010, and is partly hydrolyzed by water to form chromenol as shown in Fig. 3-4b. The p.rtial hydrolysis of the carbonium ion to chromenol is probably responsible for the difficulty in desorbing all the adsorbed acid from an H-carbon. However, the acid taken up by means of the




FIGURE 3-4 (a) The Chromene-Acid Reaction, (b) Hydrolysis
of the Carbonium Ion. (After Garten & Weiss69.)




43
)HCI22
R H
(a)
cCR+IICF+ HO 0~ j +H
(b)




44
chromene reaction is only part of the acid totally adsorbed, since part of the adsorbed acid can be displaced by adding organic solvents such as toluene and phenol. According to Steenberg (Garten & Weiss 69) the proton is primarily adsorbed by means of physical forces and the anion is secondarily adsorbed in the double layer. The proton is adsorbed readily because of negatively charged carbon, but held back because of the anion of acid molecules which is not readily taken into the double layer. Increased adsorption of acid with increasing salt concentration (anion pressure) is consistent with Steenberg's interpretation on physical adsorption of acid by activated carbon. 72
Neither one of the mechanisms which were discussed briefly herein can successfully explain all the phenomena pertaining to acid adsorption by activated carbon. The chromene-acid reaction theory proposed by Garten and Weiss appears to be gaining more attention among scientists.
Oxygen present on the surface of the activated carbon affects the adsorptive properties of the carbon because it tends to increase the polarity of the surface. 66According to Coughlin and Ezra, 73adsorption of phenol was greatly reduced when virgin carbon was oxidized, whereas increased adsorption was observed with reduced carbon. Similar results were also reported by Kipling and Shooter (Snoeyink & Weber 66) when iodine was reacted
with carbon. The increase in the polarity of carbon tends to decrease adsorption of nonpolar sorbate on the carbon.




45
3-5 Adsorption of Cr(VI) on Activated Carbon
Ions of inorganic salts have been considered poor adsorbates
on activated carbon because the activated carbon favors adsorption of undissociated molecules over dissociated molecules. Consequently, most of the literature concerning adsorption with activated carbon deals with organic adsorbates.
Sigworth and Smith74 reported in 1972 that activated carbon exhibited a very good potential to adsorb Cr(VI). Other research found that chromate undergoes reduction during adsorption by activated carbon.64,68 A high degree of removal of chromium by 67
activated carbon was also reported by Culp and Culp and Argo and Culp.75 Smithson41 conducted a study of chromium removal by activated carbon with little theoretical elaboration.




CHAPTER 4
EXPERIMENTAL APPARATUS AND PROCEDURES
The experiments were divided into two phases: batch systems and column systems. Batch studies dealt primarily with the theoretical study of the mechanisms of removal of chromium by activated carbon. Column studies were designed to develop an activated carbon adsorption system with industrial application for removing chromium. 4-1 Feed Solutions
4-1-1 Synthetic feed solution for batch studies. All synthetic feed solutions were made with deionized water and reagent grade chemicals. All glassware was washed with tap water, rinsed with 1 N HNO3, rinsed with distilled water, and then rinsed with deionized water having a pH value of about 5.5. All glassware was oven dried and cooled before use.
4-1-2 Synthetic feed solution for column studies. All synthetic feed solutions for column studies were made with tap water supplied from the Gainesville water treatment plant. The composition of this tap water is given in Table 4-1. 4-2 Activated Carbon Preparation
The activated carbon used for this study was Filtrasorb 400,
manufactured by Calgon Corporation. This coal-base activated carbon
46




47
TABLE 4-1 Characteristics of Finished Effluent of Gainesville Water
Treatment Plant*
2+
Ca^ 46 mg/i
Mg2+ 16.6 mg/l
Total hardness 184 mg/l as CaCO3
Na 18.5 mg/l as CaCO3
Cl 10 mg/l
2
SO4 17 mg/l
NO3 as N 0.0 mg/l
Phenolphthalein alkalinity 0.0 mg/l as CaCO3
Methyl orange alkalinity 142 mg/l as CaCO3
2- 0
CO3 142 mg/1 as CaCO3
OH- 0.1. mg/1 as CaCO3
CO2 3.12 mg/l as CaCO3
Total dissolved solids (estimated
by conductivity) 224 mg/1
Iron, total 0.01 mg/l
F 0.48 mg/l
Color 3 APHA
pH 7.68 at 250C
Conductivity 375 -pmho/cm at 250C
*Tests were run on a sample collected 21 March 1975. SOURCE: Gainesville Water Treatment Plant.
was selected on the basis of its having a large surface area of
2
1,050 to 1,200 m /g and its ability to withstand abrasion. Physical
properties of this carbon are given in Table 4-2. For the batch experiments this commercially available carbon was ground and sieved to have a size range of 0.149 to 0.25 mm (US sieve sizes 100 to 60). It was then washed with distilled water and demineralized




48
TABLE 4-2 Physical Properties of Filtrasorb 400
Total surface area
(N2: BET method) 1,050-1,200 m2/g
Bulk density 25 lbs/ft3
Particle density wetted in water 1.3-1.4 g/cc
Pore volume 0.94 cc/g
Effective size 0.55-0.65 mm
US standard series sieve size
larger than no. 12 3% max. 5%
smaller than no. 40 1% max. 5%
Mean particle diameter 1.0 mm
Iodine number min. 1,100
Abrasion min. 80
Moisture 0.5%, max. 2%
Ash content* 5.5%
*Determined by the author. Measurement was made after incineration of activated carbon at 6000C for 10 hrs. SOURCE: Calgon Corp., Bulletin 20-2d (1973).
water until dust and fine particles were removed. It was dried in a drying oven at a temperature of 105C for 2 hrs and cooled at room temperature in a desiccator for storage. For the column experiments, Filtrasorb 400 was used directly without modification.
4-3 Stock Solutions
4-3-1 Hexavalent chromium solutions: potassium (di)chromate. Fisher Certified reagent grade in fine crystal form was dried at 1030C for 2 hrs and cooled in a desiccator to room temperature. A
0.01 M stock solution as Cr was prepared in deionized water.




49
Working solutions Were obtained by diluting this stock solution to the predetermined strength.
Chromic acid. Chronic anhydride (Cr0 3) from Fisher Chemical Co. was used to make a 0.01 M stock solution as Cr. This hygroscopic chemical was kept in a desiccator, without predrying in an oven, to avoid possible decomposition at an elevated temperature. The concentration of this stock solution was checked against a potassium dichromate solution by the s-diphenyl carbazide color development method.
4-3-2 Trivalent chromium solution. Chromium nitrate (Cr(N03)3.9H20), Baker analyzed reagent grade was used as a source material of
Cr(III). No pretreatment was performed for this chemical to avoid the possible loss of crystal water molecules incorporated with chromium nitrate. The stock solution was prepared by dissolving it in deionized water which was previously acidified with nitric acid.
4-3-3 EDTA-Cr chelate solution. A 0.01 M solution of analytic grade disodium salt of EDTA was mixed with a 0.01 M solution of
58
Cr(NO 3)3 at a pH of 4.3 and boiled for 15 min.
4-3-4 NTA-Cr chelate solution. Prepared in the same manner as EDTA-Cr chelate.
4-4 Experimental Equipment
4-4-1 Water bath with a shaking unit. Agitation for batch studies was provided by a water bath equipped with a shaking unit manufactured by Eberbach Corp., Ann Arbor, Michigan. This unit had a variable shaking speed control device.




50
4-4-2 Circulator. The temperature in the water bath was controlled by recirculating water through an external circulator which had a cooling unit. The temperature was regulated with a thermostat. A Lauda Model K-2/R Brinkmann Instrument was used.
4-4-3 Filtering apparatus. A conventional millipore filtering unit was modified to facilitate simple and repetitive filtration operation. A 50-ml bent test tube was placed in a 500-ml erlenmeyer flask on which a millipore filter was mounted as shown in Fig. 4-1. A check valve was connected to the bottom of this tube so that the valve opens only upon breaking the vacuum, enabling filtrate to flow by gravity out for collection. The carry-over contamination during successive filtrations was greatly reduced because of the small size of the test tube used compared to the erlenmeyer flask.
4-4-4 Polystaltic pumps. Variable speed Buchler polystaltic
pumps, Model 2-6100, were used for column studies to deliver solutions to the columns.
4-4-5 Activated carbon columns. 50-ml burets were used as adsorption columns. The inside diameter of the columns was 1.15 cm. A schematic of the column testing system is shown in Fig. 4-2.
4-5 Analytical Equipment and Techniques
4-5-1 Total chromium determination. Total chromium concentration was determined by atomic adsorption on a Varian Techtron Model 1200. Readings were made at a wavelength of 357.9 nm.




FIGURE 4-1 Filtering Apparatus




MILLIPORE FILTER UNIT
STOPCOCK / ,,
TO <
ASPIRATOR <-ERLENMEYER FLASK
<--50ml TEST TUBE
-CHECK VALVE




FIGURE 4-2 Schematic of the Column Experiment Configuration




7,7,
Constant t temperature Feed solution
water bah with forced Water jacket
pumping
Burette packed with Polystaltic activated carbon
pumpEffluent Effluent
Affluent EfflIue nt




Acetylene-air flame with the fuel slightly rich was used in all cases except when oxidizing agents were present in the solution. Acetylene-air flame with the air rich was used in a solution containing oxidizing agents. The detection limit of this instrument was found to be 0.05 mg/l as Cr. Concentrations below this value were determined with a carbon rod atomizer, Varian Techtron Model 63. Chromium concentrations as low as 0.01 mg/l could be determined with this procedure.
4-5-2 Hexavalent chromium determination. Hexavalent chromium was determined by colorimetric analysis using s-diphenylcarbazide according to the Standard Methods.11 The absorbance was read on a Bausch and Lomb Spectronic 70 at a wavelength of 540 nm. A light path of 2 cm was used. The minimum detection limit was found to be 0.01 mg/l as Cr. Readings were made 10 min after color development, since intensities of color gradually decrease with time.
4-5-3 Trivalent chromium determination. Trivalent chromium was determined by subtracting the hexavalent chromium concentration from the total chromium concentration.
4-5-4 EDTA and NTA determination. EDTA and NTA were determined
by titration with 1 mM of ZnCl2 solution, using a zincon (2-carboxy2'hydroxy-5'sulfoformazyl-benzene) indicator. The basic principle behind this method is that Zn2+ ions chelate with EDTA and NTA instantly, while zincon does not react with the chelated zinc, but reacts with free Zn2+ ions added in excess of the stoichiometric amounts. The detection limit was found to be approximately
2 uM.




56
4-5-5 pH measurement. pH.was measured with a Coming Model 12 expanded scale pH meter. The pH of the solutions was measured prior to the addition of activated carbon and again measured at the end of the reaction time.
4-6 Experimental Procedures
4-6-1 Batch studies. Carefully measured amounts of activated carbon were added into prewashed 125-ml stopper erlenmeyer flasks which contained 100 ml of chromium solutions of predetermined concentration. The pH of the solutions was measured prior to the addition of activated carbon and again measured at the end of the reaction time, which unless otherwise stated was 24 hr. The reaction was carried out at 20 C in a water bath equipped with a shaker unit. The shaking speed was controlled so that the activated carbon particles always remained in suspension. After the reaction the activated carbon was filtered from the solution by passing the solution through a 0.45 micron millipore filter. The filtrate was collected in cleaned and dried plastic bottles for later determination of chromium. Deionized water was used for the sample preparations for the batch studies.
4-6-2 Column studies. Filtrasorb 400 activated carbon was used directly without any pretreatment. Carefully weighed carbon was washed with distilled water to remove fine dusts. The loss of weight by washing was found to be negligible. It was then boiled gently for a few minutes to displace air with water. This step was necessary




57
to avoid formiation-of air pockets in the column which would cause channeling of the feed solution. After cooling, the slurry of carbon was transferred into a 50-ml buret column. Glass fiber was placed on the bottom of the buret to prevent plugging of the outlet port by activated carbon particles. Unless otherwise stated, 16.5 g of activated carbon with a moisture content of approximately 1 percent were used in each column. This amount of
activated carbon provided a column with about 40 cm of bed depth and 20 cm of free board. The flow rate of feed solution was controlled with a polystaltic pump. Calibrations were made by collecting a given volume of column effluent in a noted time. The designated feed rate did not change by any measurable degree throughout each experiment.




CHAPTER 5
EXPERIMENTAL RESULTS AND DISCUSSION
5-1 Batch Studies
5-1-1 pH dependency of Cr(VI) adsorption. An experiment was
performed to investigate the effect of pH on the adsorption of Cr(VI) on activated carbon. Chromic acid, chromate, and dichromate will
hereafter be designated as CrCVI), and trivalent chromium compounds as Cr(III). A series of 20 liM Cr(VI) solutions having different pH values were prepared by diluting the dichromate stock solution with deionized water. The pH values were controlled by adding either nitric
acid or sodium hydroxide. The chromium concentrations remaining in solution after 24-hr contact with activated carbon were measured and are plotted against pH in Fig. 5-1. Chromium concentration was minimal at a pH of about 4.5, indicating maximum adsorption at this pH value. Fig. 5-1 is replotted in Fig. 5-2 showing the individual concentrations of Cr(III) and Cr(VI) for the same pH values. Comparison of Fig. 5-2 with Fig. 5-1 reveals that the pH-CrT curve is composed of three distinct portions: a broken line denoting CrT all in the form of Cr(VI), a solid line representing Cr equal to the
T
sum of Cr(VI) and Cr(III), and a dotted line indicating CrT in the form of Cr(III) only. It can be concluded from Fig. 5-2 that chromium is adsorbed best by activated carbon at the lowest
possible pH below which the reduction of Cr(VI) to Cr(III) takes
58




FIGURE 5-1 CrT Concentration Remaining in Solution as a
Function of Initial pH. The initial Cr(VI) concentration was 20 pM and the activated carbon dosage was 200 mg/l. Agitation was for 24 hrs at 200C. Potassium dichromate
was the source of Cr(VI).




[CrTJ REMAINING IN SOLUTION(uM) 0 0N) 0) 0
00
00




FIGURE 5-2 Cr(III), Cr(VI), and CrT Concentration
Remaining in Solution as a Function of
Initial pH. The upper U-shaped curve
(solid line) is reproduced from Fig. 5-1.




62
~20
(D
~16 C r(VI)
/
Slo- 0/
U) C r(III) 0
8 /o
L
0 2 4 861
INITIAL pH




63
place. Apparently adsorption was hindered by formation of Cr(III). At a pH of 4 or less, all remaining chromium was in the Cr(III) form, and its concentration increased with decreasing pH.
5-1-2 Adsorption of Cr(III) by activated carbon. Fig. 5-3
shows the results of experiments where a 10 PM solution of Cr(III) was reacted in the presence and absence of activated carbon. The pH was controlled by adding either NaOH or HNO3. Under the test conditions with no activated carbon, the soluble Cr(III) began to precipitate out of solution at pH values greater than 6. Thereafter the Cr(III) concentration in solution was continuously decreased with increasing pH values and reached 10 pg/l at pH 8.2. The 0.0 activated carbon dosage line in Fig. 5-3 shows the values
of Cr(III) on the ordinate to be greater than what was calculated from a pH-Cr(OH)3 (s) solubility relationship. The values from Fig. 5-3 were found, however, to be low when compared with Fig. 3-1, where all the monomeric hydrolysis species were taken into account. Many polynuclear hydroxo complexes of Cr(III) have been known to form upon addition of Cr(III) ions to water. These complexes are kinetic intermidiates in the slow transition from free Cr(III) ions to solid precipitates.56 Hunt and Taube (Rubin 57) found experimentally that the ion [Cr(H2 0)6 3 exchanged water with the bulk solvent very slowly, with a half life of 40 hrs. The solubility curve shown in Fig. 5-3, represented by the line of zero dosage of activated carbon, was constructed on the basis of a 24-hr reaction time, and therefore may not represent the true




FIGURE 5-3 Cr(III) Precipitation with Activated Carbon
and Cr(III) Removal in the Presence of Activated Carbon. The initial Cr(III) concentration was 10 pM as Cr. The adsorption of
Cr(III) appears to be related to its solubility. Agitation was for 24 hr at 200C.




65
10
150mg/I.AC
o 8- O.Omg/l AC
D
-1 500 mg/l AC
0
cun) 6
z
4 7
F MA L p H




66
equilibrium solubility-pH relationship. The Cr(III) remaining in solution in the presence of activated carbon was always lower
than that of the corresponding pH values in the absence of activated carbon. This indicates adsorption of Cr(III) on the surface of activated carbon. From the similar shapes of curves in
Fig. 5-3 it can be concluded that the adsorption of Cr(III) is related to its solubility. Activated carbon does not appear to
adsorb Cr(III) ions to any substantial degree at pH values below
5.0.
5-1-3 Effects of activated carbon dosage on adsorption and reduction. In order to study how adsorption was related to the concentration of activated carbon applied, a series of tests with varying activated carbon dosages was conducted under otherwise identical experimental conditions. The results of the experiment are plotted in Fig. 5-4. It can be seen from the figure that adsorption may be divided into three distinct types according to pH:
first, increase in adsorption proportional to the carbon dosage at a pH of about 4 and higher; secondly, adsorption independent of the carbon dosage in the vicinity of pH 3.7; and third, a decrease in adsorption with increased activated carbon dosage in the low pH region. The ordinate in Fig. 5-4, CrT, was separated into
Cr(VI) and Cr(III), and replotted in Figs. 5-5 and 5-6. When the pH values were greater than 4.8 and when all the Cr remaining in solution was Cr(VI), adsorption was a direct function of the carbon dosage. Apparently adsorption was related only to the number




FIGURE 5-4 CrT Concentration Remaining in Solution at
Different Activated Carbon Dosages as a
Function of Initial pH. The initial concentration of Cr(VI) was 20 UM and agitation was for 24 hr at 200C. Potassium dichromate was the source of Cr(VI).




68
20
~16
>10 2 mg/I 3- ARBO
&12 100o mg/I.
UL-j CARBON
98 500 mg/l
CARBON
:D4-0
w
0 24 68 10
INITIAL pH




FIGURE 5-5 Cr(VI) Concentration Remaining in Solution
at Different Activated Carbon Dosages as a Function of Initial pH. The initial Cr(VI) concentration was 20 pM and agitation was
for 24 hr at 200C. Potassium dichromate
was the source of Cr(VI).




70
20
16
o 250 mg/1 CARBON
12- 100mg/1
CARBON
0
D 8 -500mg/il
CARBON ED
4- 0
-Li
0 2 4 6 8 10
INITIAL pH




FIGURE 5-6 Cr(III) Concentration Remaining in Solution
at Different Activated Carbon Dosages as a
Function of Initial pH. The curves represent
the concentration that was reduced from Cr(VI) and was not adsorbed. The initial Cr(VI) concentration was 20 pM and agitation was for 24
hr at 200C. Potassium dichromate was the
source of Cr(VI).




72
10
<-500 mgld CARBON
LU2 6- ,250 mg/l CARBON
c4
ii 1O0mg/1---- CARBON Q2
23 4 5 6 7
INITIAL pH




73
of active sites on-the surface of the activated carbon. In the low pH region, 3.8 or less for activated carbon dosages of 250 mg/l, all the chromium remaining in solution was in the Cr(III) form. At low pH values, activated carbon favored reduction of Cr(VI) to Cr(III) over adsorption. The degree of reduction was in proportion to the activated carbon dosage. The reduced Cr(III) was not removed to any appreciable extent at low pH and remained in solution. This greater reduction of Cr(VI) to Cr(III) explains why activated carbon is a poor adsorbent of chromium at low pH values.
At about pH 3.6 the amount of chromium removed from solution was approximately constant, independent of activated carbon used within a reasonable carbon dosage. It can be noted from Fig. 5-4 that for the initial pH 3.6, approximately 2.5 pM was the lowest possible concentration remaining in solution regardless of activated carbon dosage when the initial concentration of Cr(VI) was 20 pM. Figs. 5-4 and 5-5 also indicate that the pH value for maximum adsorption tends to increase with increased activated carbon dosage. This phenomenon appears to be closely interrelated with the availability of Cr(VI) for adsorption. For example, at the initial pH 4.4 there was no Cr(VI) found in solution after the end of the reaction with an activated carbon dosage of 500 mg/l. However, about 2 iiM of Cr(VI) was present at the corresponding pH value with a carbon dosage of 100 mg/i.




74
It can be generally concluded from the data presented so far that the interaction of Cr(VI) with activated carbon is a complicated phenomenon because of the reduction of Cr(VI) to Cr(III) by activated carbon, simultaneously with various degrees of adsorption of Cr(VI) and Cr(III) on the activated carbon. Perhaps the most important factor is pH. The magnitude of adsorption and reduction is significantly governed by pH.
In order to investigate the optimum pH for the best adsorption, along with factors affecting adsorption efficiency, a series of solutions each containing 100 pM of Cr(VI) was reacted with different activated carbon dosages and different initial pH values. The results obtained from this experiment are summarized in Figs. 5-7, 5-8, and 5-9. Fig. 5-7 shows removal steadily increased with activated carbon dosage as pH decreased from 5.0 to 4.0. As the pH decreased below 4.0 adsorption peaked and then declined with carbon dosage. From Fig. 5-8 it can be seen that as
the pH decreased from 5.0 to 2.5 the amount of Cr(VI) remaining in solution steadily decreased for increasing dosages of activated carbon. The Cr(VI) that remained in solution resulted from the original Cr(VI) either not being adsorbed on the activated carbon or not being reduced to Cr(III). Fig. 5-9 shows the percentage of Cr(III) remaining in solution plotted against increasing dosages of activated carbon at various pH values. At pH 4.0 or greater, very little Cr(III) remained in solution, indicating that it was either adsorbed on the activated carbon or was not




FIGURE 5-7 CrT Removal as a Function of Activated Carbon Dosage and pH. The initial Cr(VI)
concentration was 100 pM as Cr. Agitation was for 24 hr at 200C. Chromic acid
was the source of Cr(VI).




D OpH 4.0
O
0
C)
80o-pH 4.5
pH5.
__ o ---.. pH 3.4
O
0
40o
0
w 1
C)
S20 -pH 2.5
0
0 200 400 600 800 1000 1200 1400
ACTIVATED CARBON DOSE (mg/I)




FIGURE 5-8 Cr(VI) Concentration Remaining in Solution at Different Activated Carbon Dosages
and Different Initial pH Values. The initial chromic acid concentration was
100 pM as Cr and agitation was for 24 hr at 200C.




t
o
0
a9 80
z
(_9
z 60
<
0- pH 5.0
40
-pH 45
A
LUL
TH 4.0
- 20\Ld pH 3.4
W pH 2.5
C)
0 200 400 600 800 1000 1200 1400
ACTIVATED CARBON DOSE (mg/1)




FIGURE 5-9 Cr(III) Concentration Remaining in Solution at Different Activated Carbon Dosages and
Different Initial pH Values. The initial chromic acid concentration was 100 PM as Cr.
The curves represent the concentration of Cr(III) that was reduced from Cr(VI) and
was not adsorbed on the activated carbon or filtered on 0.45 micron millipore filters.
Agitation was for 24 hr at 200C.




1 100 0O
0
80
z
60
- pH 2.5
00
LLJ
40
U
- pH 3.4
20
00
LA/ pH 4.0
00
0 200 400 600 800 1000 1200 1400 1600
ACTIVATED CARBON DOSE (mg/I)




81
produced by reduction of Cr(VI). The critical pH appeared to be
4.0. Maximum removal of CrT was observed at this pH and adsorption was increased proportionally with amounts of activated carbon applied.
5-1-4 pH variation during adsorption of Cr(VI) and its effect on Cr(VI) removal. In Fig. 5-10 the final pH values are plotted against activated carbon dosage for several different pH values. An increase in pH was always observed upon the addition of activated carbon to a Cr(VI) solution. It can be seen from Fig. 5-10 that when the initial pH was between 4.0 and 5.0, the final pH exceeded 6.0, the value depending on the amounts of activated carbon used. This phenomenon leads to an important question regarding the true adsorption of Cr(VI) by activated carbon. Cr(VI), once reduced, can be either adsorbed on activated carbon or precipitated out as the final pH reaches 6 or higher. The solubility of Cr(III) is greatly reduced in a neutral or slightly alkaline solution. Under this environment adsorption of Cr(III) on activated carbon would be enhanced. Therefore it is difficult to quantitatively calculate the total amount of Cr(VI) reduced during the period of reaction or to calculate the percentage of the reduced Cr(III) which is actually consumed by adsorption. Fortunately this problem is only confined to a narrow region of initial p1'. To illustrate this clearly, Cr(III) in Fig. 5-2 was replotted against final pH in Fig. 5-11 as Curve 1. The results of the reaction of 20 PM solution of Cr(III) with 200 mg/l of activated




FIGURE 5-10 Relationship Between pH Rise and Activated
Carbon Dosage. Solutions of 100 VM of Cr(VI)
at different pH values were reacted with
various dosages of activated carbon. Final pH's were plotted against activated carbon
dosage. Agitation was for 24 hr at 200C.
Chromic acid was the source of Cr(VI).




83
8
'I !
7 -IN TIALpH: 5.0 4.5
4.0
I 6
-
4 3.4
L
Q
3
2.5
200 400 600 800 1000
ACTIVATED CARBON DOSAGE (mg/)




FIGURE 5-l Cr(III) Concentration Remaining in Solution
as a Function of Equilibrium pH. Activated
carbon dosage was 200 mg/i and agitation was for 24 hr at 200C. Curve 1 shows Cr(III) reduced from Cr(VI). The initial potassium dichromate concentration was 20 pM as Cr.
Curve 2 shows Cr(III) remaining after contact
with activated carbon. The initial Cr(NO3)3
concentration was 20 11M as Cr.




85
240
INTIAL PH 16
212E8
4
23 4 5 67
EQUILIBRIUM pH




86
carbon from a separate experiment are also included in this figure as Curve 2. By comparing the two curves it can be confirmed from Curve 1 that for the Cr(VI) concentration of 20 -M and activated carbon dosage of 200mg/l, reduction of Cr(VI) to Cr(III) was nonexistent at the initial pH of 4.7 or higher. Adsorption of reduced Cr(III) did not take place below the initial pH of 4.1. Furthermore, Cr(III) produced in small amounts by reduction at pH values between 4.1 and 4.7 tends to remain in solution, since at this pH range Cr(III) was not adsorbed to any great extent by activated carbon as shown from Curve 2, Fig. 5-11.
From Fig. 5-4 the critical pH was 4.7 for the 20 pM initial Cr(VI) concentration and a high activated carbon dosage. It is seen, however, from Fig. 5-7 that this initial critical pH shifted from 4.7 to about 4.0 as the initial Cr(VI) concentration increased from 20 to 100 pM. Finding this critical initial pH value for any given concentration of Cr(VI) is extremely important because adsorption is greatest at this pH. Ineffective adsorption results above the critical pH because of poor adsorption of Cr(VI) on the activated carbon, whereas below the critical pH, reduction hinders adsorption.
5-1-5 1:1 proton to Cr(VI) molar ratio adsorption. In order to determine the critical pH for adsorption, a series of experiments were performed where the initial pH and the ratio of initial (Cr(VI) to activated carbon were held constant while the initial
concentration of Cr(VI) was changed. The results are shown in




87
Fig. 5-12. Total chromium adsorption increased to a maximum with increasing pH up to an initial Cr(VI) concentration of 300 pM, after which adsorption decreased. Fig. 5-12 indicates that the total amount of chromium adsorbed was a direct function of the initial proton activity in the solution and was maximum at a mole ratio of proton to Cr(VI) of 1.0. In all cases the proton activity was measured with a pH meter and was very closely equal to the proton concentration due to the low ionic strength of all the experiments. When the Cr(VI) concentration exceeded the proton activity (initial Cr(VI) greater than 300 -pM), adsorption of CrT exceeded the number of protons available. This adsorption is known as hydrolytic adsorption and will be discussed in a later section of this chapter. In order to study the effect of the proton to Cr(VI) ratio on adsorption, a series of experiments were conducted where the initial proton to Cr(VI) ratio was set equal to 1.0 at a constant Cr(VI) to activated carbon ratio. The results are shown in Fig. 5-13. The chromium was introduced by two methods: the first using chronic anhydride; the second using potassium dichromate and controlling initial pH with hydrochloric acid. When chromic anhydride was dissolved in an aqueous solution,
the proton concentration was equal to the Cr(VI) concentration, resulting in a 1 to 1 proton to Cr(VI) molar ratio. The result3 shown in Fig: 5-13 indicate Cr(VI) adsorption was almost independent of pH as long as a 1 to 1 proton to Cr(VI) molar ratio condition was met. This finding leads to the conclusion that




FIGURE 5-12 Proton and CrT Adsorption as a Function of Initial Cr(VI) Concentration. The initial
pH was 3.52 (H = 300 pM), the initial Cr(VI) to activated carbon ratio was 0.33 V mole/mg, and potassium dichromate was the source of Cr(VI) with hydrochloric acid
used for pH control. Agitation was for 24 hr at 200C.




100
Total Chromium 80 A
0
CL60
0
l.-40
C-)
~20
0
0 100 200 300 400 500 600
INITIAL Cr (M) CONCENTRATION (,uM)




Full Text

PAGE 1

ADSORPTION OF CHROMIUM ON ACTIVATED CARBON By JUNG I. KIM A DISSERTATION PRESENTED TO TI-IE GRADUATE COUNCIL OF 11-IE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIRH!~NTS FOR TI-I E DEGREE OF DOCTOR OF PHILOSOPHY UNTVERSITY OF FLORIDA 1976

PAGE 2

ACKNO\\/LEDGMEN'i'S I wish to extend my sincere appreciation to my committee chairman, John Zoltek, Jr., for his incessant guidance and 1 .nder standing. Special thanks are extended to Professor T. de S. Furman, who sets a standard for all engineers to strive for. Appreciation is extended to Dr. P. L. Brezonik and Dr. D. 0. Shah for being members. The many forms of help extended by Dr. Paul Urone are gratefully remembered. My deepest gratitude is expressed to my parents. Had it not been for their guidance and encouragement, this dissertation would never have been made possible. I wish to dedicate this thesis to them. ii

PAGE 3

CONTENTS ACKNOWLEDGMENTS LIST OF TABLES LIST OF FIGURES ABSTRACT CHAPTER 1 2 3 4 INTRODUCTIO N 1-1 General Background 1-2 Chromium in the Environment 1-3 Standards for Chromium Removal 1-4 Chromium Removal 1-5 Purpose of This Research CHRO MIUM, ITS INDUSTRIAL USES, TOXICITY AND METHODS OF REMOVAL FROM WATER 2-1 Industri al Application of Chromium 2-2 Toxicity of Chromium 2-3 Techniques for Removing Cr(VI) from Water LITERATURE REVIEW 3-1 Chemistry of Chromium 3-2 Adsorption Phenomena 3-3 Nature of Activated Carbon 3-4 Adsorption Phenomena and Activated Carbon 3-5 Adsorption of Cr(VI) on Activated Carbon EXPERI~IBNTAL APPAR~TUS fu~O PROCEDURES 4-1 Feed Solution 4-2 Activated Carbon Preparation 4-3 Stoc k Solutions 4-4 Experimental Equipment 4-5 Analytical Equipment and Techniques 4-6 Exp eri m ental Procedures iii ii V vi i:i<: 1 1 1 2 4 5 7 7 10 16 25 25 30 35 37 45 46 46 46 48 49 so 56

PAGE 4

CHAPTER 5 6 7 8 EXPERIMENTAL RESULTS AND DISCUSSION 5-1 Batch Studies 5-2 Column Studies ENGINEERING APPLICATIONS 6-1 General Remarks 6-2 Process Design for Cr(VI) Removal by Activated Carbon 6-3 Disposal of Spent Activated Carbon ECONOMICS OF Cr(VI) REMOVAL BY ACTIVATED CARBON CONCLUSIONS AND RECmfM EN DATIONS 8-1 Principal Theoretical Findings 8-2 Sugge s ted Future Research APPENDICES REFERENCES BIOGRAPHICAL SKETCH iv 58 58 145 166 166 168 173 174 178 178 180 182 190 196

PAGE 5

TABLE 2-1 2-2 TABLES Reported Cases of Chrome Ulceration in United Kingdom During the Period 1930-1972 Concentration of Chromiwn in Body Tissues With and Without Known Exposures to Chromium 4-1 Characteristics of Finished Effluent of 4-2 5-1 5-2 6-1 7-1 7-2 7-3 Gainesville Water Treatment Plant Physical Properties of Filtrasorb 400 Results of Column Performance for Varying Cr(VI) Concentrations Effects of Acids Used for pH Control on the Adsorptive Capacity of Activated Carbon Columns for Chromium Dimension of a Full-Size Carbon Adsorption Column Cost of Chemicals Used in Cr(VI) Removal by Activated Carbon Adsorption Estimated Capital and Operating Costs for a 0.144 mgd Carbon Adsorption Column for Cr(VI) Removal Costs of Treatment of a Cr(VI) Waste Stream by Several Different Processes V 11 12 47 48 160. 161 171 175 176 177

PAGE 6

FIGURE 3-1 3-2 FIGURES Solubility of Cr(OH) 3 as a Function of pH A Distribution Diagram for the Various Cr(VI) Species as a Function of pH 3-3 Some Reactions of Flurescein-Type 3-4 4-1 Lactones (a) The Chromene-Acid Reaction; (b) Hy drolysis of the Carbonium Ion Filtering Apparatus 4-2 Schematic of the Column Experiment Configuration 5-1 5-2 5-3 5-4 5-5 5-6 5-7 5-8 Crr Concentration Remaining in Solution as a Function of Initial pH Cr(III), Cr(VI), and CrT Concentration Remaining in Solution as a. Function of Initial pH Cr(III) Precipitation with Activated Carbon and Cr(III) Removal in the Presence of Activated Carbon CrT Concentration Remaining in Solution at Different Activated Carbon Dosages as a Function of Initial pH Cr(VI) Concentration Remaining in Solution at Different Activated Carbon Dosages as a Function of Initial pH Cr(III) Concentrat i on Rema i ning in Solution at Different Activated Carbon Dosages as a Function of Initial pH CrT Removal as a Function of Activated Carbon Dosage and pH Cr(VI) Concentration Remaining in Solution at Different Activated Carbon Dosages and Different Initial pH Values vi 29 32 40 43 52 54 60 62 65 68 70 72 76 78

PAGE 7

FIGURE 5-9 5-10 5-11 5-12 Cr(III) Concentration Remaining in Solution at Different Activated Carbon Dosages and Different Initial pH Values Relationship Between pH Rise and Activated Carbon Dosage Cr(III) Concentration Remaining in Solution as a Function of Equilibrium pH Proton and CrT Adsorption as a Function of Initial Cr(VI) Concentration 5-13 Adsorbed Chromium as a Function of Initial 5-14 5-15 5-16 5-17 5-18 5-19 5-20 5-21 5-22 5-23 5-24 5-25 5-26 Proton Concentration CrT Concentration Remaining in Solution as a Function of Initial pH Chromium and Potassium Adsorption at Varying Dosages of Activated Carbon Freundlich Isotherms of CrT Adsorption at an Initial Proton to Cr(VI) Molar Ration of 1.0 Freundlich Isotherms of CrT Adsorption at a Fixed Initial Cr(VI) Concentration of 100 M CrT Removal as a Function of Contact Time at Different pH Values Concentration of Cr(I II) Formed as a Function of Contact Time at Different pH Values Cr(VI) Concentration Remaining in Solution as a Function of Contact Time at Different pH Values Total Chromium Remaining in Solution as a Function of pH Cr(III) and Cr(VI) Concentrations Remaining in Solution as a Function of Initial pH Percentage Cr(VI) Removal by Acti va.ted Carbon at Various Ionic Strength Solutions of Two Different Sal ts Cr(VI) and Cr(III) Concentrations Remaining in Solution as a Function of the Ionic Strength of Calcium Chloride Concentrations of EDTA-Cr and EDTA Remaining in Solution as a Function of the Equilibrium pH Concentrations of NTA-Cr and NTA Remaining in Solution as a Function of the Equilibrium pH vii 80 83 85 89 91 94 97 100 103 105 107 109 113 115 117 120 122 124

PAGE 8

FIGURE 5-27 5-28 5-29 5-30 5-31 5-32 5-33 5-34 5-35 5-36 5-37 5-38 Rates of Adsorption for Various Acids in the Acid-Acid Salt System CrT Concentration as a Function of Initial pH Effect of S a lt Concentration on Adsorption Capacity for Chromic Acid Titration of 100 ml of 104 M KHCr0 4 with 0 .001 M NaOH D e sorption of Chromium from Activated Carbon at Different pH Values The Chromene-Chromic Acids Reactions Total ChTomium Concentration in the Column Effluent vs. Dimensionless Empty-Bed Volume Throughput at Several Different pH Conditions Cr(VI) Concentration in the Column Effluent vs. Dimensionl e ss Empty-Bed Volume Throughput at Several Different pH Conditions Cr(III) Concentration in the Column Effluent vs. Dimensionless Empty-Bed Volume Throughput at Several Different pH Conditions Total Chromium Concentration in the Column Effluent vs. Dimensionless Empty-Bed Volume Throughput Cr(VI) CoTicentration in the Column Effluent vs. Dimensionless Empty-Bed Volume Throughput at Several Designated Residence Times Dependence of Column Breakthrough Capacity on Retention Time 6-1 Schematic of the Activated Carbon Column Process for Hexavalent Chromium Adsorption viii 127 130 133 136 138 143 14 7 149 151 154 156 158 170

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Abstract of Dissertation Presented to the Graduate Council of the University of Florida in Partial Fulfillment of the Requirements for the De gree of Doctor of Philosophy ADSORPTION OF CHROMIUM ON ACTIVATED CARBON Chairman: John Zoltek, Jr. By Jung I. Kim March 1976 Major Department: Environmental Engineering Sciences A study was conducted to investigate the feasibility of remov ing chromate from water by activated carbon. The first ph a se dealt with batch studies to investigate th e theoretical aspects of the interaction of chromate with activated carbon. The second phase involved continuous column studies using synthetic aqueous chromate solutions. The interaction of chromate with activated carbon was found to be complex because of the capability of activated carbon for ad sorbing chromate as well as for reducing it to Cr(III) compounds. Adsorption and reduction were a function of the initial chrom a te concentration and the initial pH. Adsorption was maximal when the hydrogen ion and chromate were present in solution in an equimolar ix

PAGE 10

concentration. Activated carbon adsorbed chromate as chromic acid. When the chromate concentration was present in solution in excess of the hydrogen ion concentration, decreased adsorption was observed. Hydrolytic adsorption took part in the overall adsorption. In a solution where the chromate concentration was below the hydrogen ion concentration, adsorption was hindered by reduction of chromate to Cr (III) compounds. Cr (III) compounds were not removed to any great degree by activated carbon. In order to increase adsorption of chromate on the activated carbon, it was required to adjust the pH of solution such that the hydrogen ion concentration was equal to the chromate concentration. The extraction tests with organic solvents indicated the pre dominant chemical interaction of chromic acid with activated car bon. The extremely high adsorptive capacity of activated carbon for chromate was due to the strong chromic acid activated carbon interaction through chemical bonding, along with the tendency of chromic acid to stabilize itself by formation of polyacids within the activated carbo~. Adsorption of chromdte was for the most part complete in a 2-hr contact time, and a significant amount of chromate was ad sorbed within 10 min of contact. At a chromate to proton ratio of 1.0 and a column feed of 104 rng/1 chromate as Cr, no chromium was detected in the column effluent up to the throughput of 520 empty-bed volumes. It was found that residence time was a very important parameter in terms of increasing the column capacity. X

PAGE 11

Desorption studies indicated that alkali was a better regenerant than acid, but 100 percent desorption was not possible. In the case of alkali desorption the desorbed chromate concen tration was high enough for reuse. Cost studies were made on a 0.144 rngd single carbon colwnn designed for treating a waste stream containing 104 rng/1 chromate as Cr. The estimated operating costs were 74/1,000 gal without chromate recovery a~d 37/1,000 gal with chromate recovery. xi

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CHAPTER 1 INTRODUCTION 1-1 General Background 1be toxic effects of many metals on man and his environment has been well recognized for many centuries and have attracted considerable attention during the last decade. The uncontrolled industrial use of heavy metal applications has resulted in adverse effects on man and his environment to such cin extent that further industrial use of these toxic metals would begin to endanger our health and enjoyment of life. As population growth in conjunction with higher living standards begins to create a huge demand for water in both industrial and domestic uses, the recycling of water is considered to be the only solution to the water shortage prob lems in the future. Chromium has been widely used in industry for many years. Re cently, concern has been focused on the possibility of long-term disturbances of our ecology by hexavalent chromium discharged by industries, as well as the short-term effects of its toxicity on municipal wastewater treatm e nt processes. 1-2 Chromium in the Environment ---------The element chromium is never found in a fr ee state in n a ture. The principal ore is chromite (FeO Cr O") 1 2 The United States 2 .) 1

PAGE 13

2 imports virtually all of its chromite, because United States chromite ores that are commercially available are insignificant in relation to the United State~ requirements. 3 In 1972 domestic consumption was 1.14 million short tons of chromite ore, or 353 thousand short tons of chromium. Of the chromite consumed, the metallurgical industry used 63.8 percent, the refractory industry 19.6 percent, and the chemical industry 16.6 percent. 4 Chromium occurs in seawater at a concentration of 0.05 /1. 5 The average concentration of chromium found in United States sur / 6, 7 face waters was 9. 7 g 1. The chromium content of twenty-four 8 municipal water supplies was found to range from 1 to 40 g/1. Results of a 2-yr survey of 163 public drinking water supplies in th e United States showed a mean concentration of 3.3 g/1 with a range of O. 3 to 40 g/1. 9 Chromium concentrations in municipal wastewaters vary so widely from location to location because of contribution from industries that average value would have little meaning. Chromium can have an oxidation state from -1 to +6. Only Cr(III) and Cr(VI) are stable and are therefore present in natural waters. 10 Although chromium may exist in both the hexavalent and the trivalent state, it occurs mostly as Cr(VI) in potable water . 11, 1 2 supplies because of prevailing aerobic cond1t1ons. 1-3 Standards for Chromium Removal Because of the toxic nature of Cr(VI) the USPHS Drinking Water Standards allows a maximum Cr(VI) concentration of O 05 mg/1. The

PAGE 14

3 limit of 0.05 m g /1 for Cr(VI) was based on the lowest amount analytically determinable in 1946 when the drinking water standards were established. A concentration of 0.05 mg/1 was believed to be sufficiently low to have no adverse effect on health. 13 No limit has been set, however, for the less toxic Cr(III). According to Water Quality Criteria revised in 1972, 7 the recommended concen trations of chromium as total Cr for designated uses were as follows: (1) freshwater aquatic and wildlife 0.05 mg/1, (2) livestock drink ing water 1.0 mg/1, (3) public water supplies 0.05 mg/1, (4) agri cultural use (for continuous use) 0.1 mg/1, (5) marine aquatic life and wildlife 0.01 x 96 hr-LC 50 and (6) for oyster harvesting 0.01 mg/1. Realizing the magnitude of the problems and the necessity of cooperation, a coordinated industrial-municipal-regional approach to water pollution control began to surface Many states and cities have enabling legislation or ordinances regulating the discharge of certain materials into lakes and streams. In many states, for example, a concentration of not more than 0.5 rng/1 for Cr(VI) is the maximum allowable discharge into storm sewers by an electroplating industry. The Environmental Protection Agency (EPA) recently has proposed effluent limitations, guidelines, and new source performance standards. According to its recommendations, existing plating in dustries are subjected to limits in the discharge of Cr(VI). The limits call for a maximum discharge rate into navigable waters of 2 2 8 mg/m per day, single day maximum, and 4 mg/m for a 30-day average

PAGE 15

4 by 1 July 1977 for existing plants, and 4 mg/sq m single day maxi mum and 2 mg/sq m for a 30-day average for new sources. These limitations were based on the Best Practicable Control Technology Currently Available (BPCTCA). The goal for 1 July 1983 is to have no discharge from electroplating industries through the application of the Best Available Technology Economically Achievable for rel 5 covery and reuse of water (BATEA). Cr(VI) concentrations of typical untreated wastes are metal plants', bright dip wastes 10,000-50,000 mg/1, pickle bath or plating 60 mg/1; leather industry wastes 40 mg/1; cooling tower 1 2 blowdown waters 10-60 mg/1. The magnitude of the problem is exemplified by the fact that more than 30 billion gal of water is required annually to dilute 35 mg/1 to 0.05 mg/1 of Cr(VI) for one moderately sized cooling water system. 1-4 Chromium Removal Chromium can be removed from water by methods such as reduction and precipitation, ion exchange, or reverse osmosis. Chemical reduction followed by precipitation has been most widely used. A wide variety of reducing agents may be used for the reduction of Cr(VI) to Cr(III). The choice is based on cost and availability, and convenience in each individual application. The rate of reduc tion is pH dependent, requiring low pH values for fast reduction. The reduced Cr(III) is then precipitated by raising the pH value to about 8.5, which is the pH for minimum solubility of Cr(III).

PAGE 16

5 The certainty of successful operation is the intrinsic merit for this method. However, major problems with this treatment lie in the necessity of a relatively large settling tank for the precipi tation of the highly hydrated and voluminous chromium hydroxides, 5 and the difficulty of safe disposal of the sludge. Since the recent advent of ion exchange resins capable of withstanding the oxidizing power of chromate, chromium removal by ion exchange has received considerable attention. This method does not present a sludge disposal problem, and there is the advantage of reclamation f ( ) s,16,17 h d h h o Cr VI However, ion exc ange treatment oes ave t e disadvantages of being poorly selective in choosing chromate over other anions and of having a rather critical flow rate necessary for efficient removal of Cr(VI). 5 Other advanced wastewater treatment techniques such as reverse osmosis, evaporation, and ion flotation may be useful for Cr(VI) removal when employed in rela tively small-sized treatment facilities, but they are not likely to be used for large wastewater treatment plants. 1-5 Purpose of This Research It was the purpose of this research to find the mechanisms re sponsible for the adsorption of Cr(VI) by activated carbon and to further develop techniques for practical applications. Activated carbon has been extensively used for removal of various impurities such as organics, color, and odor from water. Recent research indi cated that activated carbon is capable of adsorbing, to a varying

PAGE 17

6 degree, some of the heavy metals. Once the chemistry of Cr(VI) adsorption by activated carbon is established, economical mcilllS of removing Cr(VI) may be realized, since the activated carbon process is inexpensive compared to many oth er treatm e nt processes.

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CHAPTER 2 CHROMIUM, ITS INDUSTRIAL USES, TOXICITY, ANO METI-IODS OF REMOVAL FROM WATER 2-1 Industrial Application of Chromium The largest use of chromium in the United States during 19 72 was in the metallurgical field, utilizing 63.8 percent of the total chromium consumed. The stainless steel industry used about 60 percent of the metallurgical use of chromium, with the re maining 40 percent having been shared by other steel industries. The refractory industry used 19.6 percent of chromium, with the major use of these refractories having been in the steel industry. Chrome refractories are neutral or sometimes considered basic in character and have properties of a high melting point and moderate h 1,2,4,8,18 h h h t ermal expansion. Alt oug c romium consumption is relatively small (16.6%) in the chemical industry, this is where the most serious wastewater problems are present. The major uses of chromium chemicals produced in the United States have been classified as of 1960: metal finishing (corrosion control) 35 per cent, pigment and allied products 25 percent, leather tanning and textiles 17 percent, chemical products 7 percent, other uses 6 1 9 percent. Sodium dichromate is the primary chemical from which many other chromium compounds are manufactured. Sodium dichromate is manufactured by the calcination of a mixture of chrome ore 7

PAGE 19

8 (chromite), sodium carbonate, and lime, followed by a water leach to yield sodium chromate. (2-1) Calcium salts and iron oxide are precipitated out and removed by pH adjustment. Sodium chromate thus produced is acidified with sulfuric acid to obtain sodium dichTomate and sodium sulfate as a by-product. (2-2) Sodium sulfate is removed by concentrating it by an evaporation process. Two types of chrome plating are widely used in decorative elec troplating. Chromium is usually coated to a thickness of 0.0000i0.00002 in. oveT an electro-deposited nickel for providing corro sion and abTasion resistance. Hard plating (0.001-0.01 in. of thickness) is used to provide wear resistance and a low coefficient of friction. The plating bath is composed of chromic acid and sulfuric acid at a ratio of approximately 100 to 1 by weight. The sulfuric acid acts only as a catalyst. Florosilicate catalysts h 2, 19 are also sometimes a
PAGE 20

9 Dichromate is also extensively used in the metal-finishing field to provide corrosion protection and decorative effects. A metal is immersed in a dichromate solution under conditions intermediate between corrosion and passivation. Chromic acid is also used to anodize metals, such as aluminum, to produce a protective oxide coat. One of the most important uses of chromium compounds is in corrosion inhibition in recirculation water systems. Chromate is known as an anodic inhibitor and prevents removal of the ionized metal from the metallic phase by providing a very thin layer on the metal surface. The concentration of chromate required to in hibit corrosion in a system varies depending on the ions in the solution, the temperature, and the pH. 19 Chromates and dichromates are a major component in water borne preservatives and fire-retardant preservatives. Out of 13.8 million pounds of water-borne preservatives used in the American wood industry during the year 1963, 10.4 million pounds contained chromate compounds. The chromate reacts with the wood extractives and other preservative salts to produce an almost insoluble complex while simultaneously rendering toxicity to d d f 19 woo estroying ungi. Wide use of chromium compounds is to be found in the pigm e nt industry. Chrome oxide green (Cr 2 0 3 ), chrome yellow (PbCr0 4 ), and zinc yellow (ZnCr0 4 ) are the most widely used chromium pigments.

PAGE 21

10 In textile processes, chromates are used as a mordant to fast the colors on cotton and wool, and as an oxidizing agent for d 1 1 20 dyes on cotton an ot1er text1 es. The leather industry has been using chromium as basic chromium sulfate for tanning light leathers. The mechanism of chrome tanning is not well understood, but it is believed to be due to the coordination of chromium with the carboxyl groups of collagen. The chrome-treated ieather possesses many superior properties, such as a high hydrotherm a l stability and shrinking temperature 2 as well as a res:i:stance to bacterial action. 2-2 Toxicity of Chromium 2-2-1 Toxic effects on humans. Chromium is present primarily in trivalent or hexavalent states in the natural environment. Cr(VI) is knoTI to be more toxic than Cr(III) by a factor of about 100. 21 Pure metallic chromi~m is believed to be biologi20 cally ine rt and exerts no hannful effects on the human body. The toxicity of soluble chromic salts is of a low order, and in soluble oxide and phosphate are believed to be nontoxic. Cr(VI) is believed to produce lung cancers, since a more than normal incidence of lung tumors had been reported among workers } f plants. 18 22 23 Table 2-1 shows rein c irorna.te rnanu actur1ng ported incidences of chrome ulceration in the United Kin gdom from 1930 to 1972. 24 Table 2-1 indicates that most incidences came from workers in the chromium plating industries. Chromates are

PAGE 22

11 TABLE 2-1 Reported Cases of Chrome Ul~fration in United Kingdom During the Period 1930 1972Manufacture Dyeing and Chrome Chrome Others (Bichromates) Finishing Tanning Plating Total 1 930 6 1 5 5 57 1 2 95 1950 36 100 7 143 1959 100 8 5 7 192 1960 181 1 1 06 9 298 1961 82 1 1 1 21 7 211 1962 39 86 5 1 30 1 963 35 146 19 200 1 964 34 1 2 1 7 1 0 262 1965 34 1 27 6 1 67 1 966 27 1 04 2 1 33 1 967 42 1 83 5 1 3 1 1968 24 11 7 5 146 1 969 26 1 91 3 1 21 1 970 1 6 1 67 5 89 197 1 24 59 6 89 1 972 67 61 2 1 30 a l so known to cause irritation dermatitis and sens i tization derma ti t is 25 Asthma, fever nephritis and eczema are als o be l ieved to be caused by allergic reaction t o chromates. 5 Water con t aining 7 .8 mg/1 of chromium from chromate bath wastes was blamed for eczema among workers in Czechos l ovakia 5 Dusts and fumes generated from the c hromate process were f ound t o cause caus t ic action o n m ucous membranes resulting in perforation of nasa l septum as well 2 5 as ulcers (chrome ho l es) on skin T ab l e 2 2 shows chromium concentrations i n body t issues from chroma t e workers and those of normal tissues taken from subjects w it h no known exposures to chromium. It appears t hat chromium t ends to accumulate in the respiratory areas such as l ungs, tracheobronchial lymph nodes, and bronchus. The American Conference

PAGE 23

12 TABLE 2-2 Conc e ntr ation of Chromium i n Body Tis s ues With and Without Known Exposures to Chromiu m 20 Tissue Lungs Lung tumor Metastatic tumors Tracheobronchial lymph nodes Bronchus Trachea Nasal septum Larynx Kidney Liver Spleen Abdominal lymph nodes Stom a ch Intestines Bladder Heart Muscle Pancreas Thyroid Adrenal Brain Bone Cartilage Hair Skin Rang e of Concentrations Reported in Normal Tissues (g/100 g Wet Tissue) 0-33 0-1 0-9.6 1-11 0-98 1 0-5 10 0-8 21 43 0-41 0-4 5 Range of Concentrations R e port ed in Tissu es from Chrom a te Workers (g/100 g Wet Tissue) 130-9,887 0-1,658 2-100 12-7,590 95-386 0-32 287 21 0-211 0-159 0-91 4-80 4-11 4-5 3-226 0-20 0-19 8-36 24-53 5-76 0-5 0-292 6 31 5 of Governmental Industrial Hygenists has set threshold limit values ( L ) f / f 25 TV or chromic acid and chrom a te of 0.1 mg cubic m e ter o air. Chromate can penetrate the body throu gh dam aged skin. Twelve death s were reported because of external use of an antiscabetic ointment 26 in which a prepaTation of chro me was substituted in place of sulfu r Analysis of lung tissu e from wor ke rs who had been exposed to chrom ium

PAGE 24

13 in the chromate chemical industry revealed that large amounts of acid-soluble chromium, as well as insoluble chromium, were re20 tained in the lung tissue for many years after exposure. Although sufficient data have been collected which clearly indicate deleterious effects of chromium on human health, there does not appear to be a clearly defined "no effect" level. It was suggested that a concentration of 0.05 mg/1, with an average intake of 2 liters of water per day, would not be hazardous to human heal th. 7 2-2-1 Toxic effects on aquatic life. The susceptibility of or ganisms to chromium depends largely upon the type of organisms and their physical and nutritional condition at the time of ex2 7, 2 8 posure. Generally the larger animals tend to be somewhat more resistant. Bluegills were found to be able to tolerate a 45 mg/1 of Cr level for 20 days in hard water. 29 The 96-hr lethal concentration (LC) for 50 percent mortality and safe concentra tions for Cr(VI) were 33 and 1.0 mg/1 for fathead minnows, SO and 0.6 mg/1 for brook trout, and 69 and 0.3 mg/1 for rainbow trout. Even though Cr(VI) is generally believed to be more toxic than Cr(III), toxicity of Cr(III) to these species was found to be about the same as Cr(VI). Pickering and Henderson 30 conducted an experiment on toxicity of chromium to four species of warm-water fish. The experimental results indicated that Cr(III) was significantly mor e toxic to each species than Cr(VI). The soft-water 96-hr TL (Median Tolerance 111

PAGE 25

14 Limit) values ranged from 3 to 7 mg/1 for trivalent chromiwn, and I 27 from 18 to 118 mg 1 for hexavalent chromium. Zarafonetis a nd H a mpton studied effects of chromium on two species of algae. Their data show that Cr(VI) concentrations of 10 and 20 mg/1 as K 2 Cr 2 0 7 had no apparent effect on the growth of Chlorella pyrenoidosa, but strongly inhibited the growth of Chlamydomonas reinhardi. The net photosynthesis by a natural algal population was inhibited by 20 and 50 g/1 of Cr(VI). From these results they suggested ree x amining the present federal water pollution standard for Cr(VI) of 50 g/1 for drinking water supplies. A long-term study of oyster mortalities in water containing 10 to 20 g/1 chromium indicates cumulative toxicologic effects. Nearly three quarters of the oyster deaths occurred in the warm period of M a y through July, when physiological activity might be expected to be greatest. Raymont and Shields3 1 investigated toxic effects of chromium in the marine environment and concluded that the toxic effects of the different chromate salts on Nereis (worms) was not significantly different. Heavy mortality was recorded at the concentration of 2 to 10 mg/1 Cr in 2 to 3 weeks, and the long-term threshold concentration of Cr was found to be just below the 1 mg/1 level. Radioactive Cr 51 was used to investigate the location in the body of the Nereis where the greatest adsorpt ~ Jn occurred. Both the gut and body wall appear e d to be sites for ad sorption of Cr. Considerable adsorption was found throu g hout the 31 32 body in the absence of any food intake. Hill and Fromm found

PAGE 26

15 that exposure of trout to Cr(VI) at the levels of 0.02-0.2 mg/1 for a week caused a significant elevation of plasma cortisol. 2-2-3 Effects of chromium on the activated sludg~ waste treat ment process. Effects of chromium on the activated sludge process have loni been a concern in the wastewater treatm en t field. Jenkins and Hewitt 33 found that chromium at the concentration of 10 mg/1 as K 2 Cr0 4 converted a good effluent to realtively moderate quality. 34 Spencer observed that biological activity in activated sludge ceased when the chromium concentration ranged from 3.5 to 6 6 / k d J. 35 h 7. mg 1 as Cr. Jen ins a n lewitt reported later tat th e plant effluent BOD values were markedly decreased when sewa ge 36 contained 0. 7 mg/1 of Cr. In contrast, Ross found that an activated sludge plant at Richmond, Indiana, was able to treat, without much difficulty, 1 mg/1 as Cr of chromate waste. Monl 7 stated that 26 mg/1 of chromium might be permitted without losing treatment efficiency to any great extent. Cr(VI) was found capable f 33,35 38 d. o inhibiting the growth nitrifying bacteria. Plac at in icated that inhibition of ni trifying bacteria by very low con centrations of Cr(VI) would be a useful tool for arresting unwanted nitrification in BCD bottles. Microscopic examination of activated sludge that received chromate waste revealed the almost complet e 3 9 disappearance of filamentous bacteria such as spha ero tilus, and an overall decrease in the numb er of stalked ciliates. 28 Ingels and Fetner stated that chromate caused a bulking sludge, indicating a favorable growth of molds o ve r the growth of the zoogleal masses.

PAGE 27

16 2-3 Techniqu es for R e m o v i n g Cr(VI) from W a ter Virtually any advanced wastewater treatment techniques can be applied to chromium removal. The choice depends on costs, reliability; and perhaps the degree of removal designated by state or local regu latory agencies. Some techniques for removal of Cr(VI) are (1) reduction of Cr (VI) to Cr(III) and precipitation of Cr(III) as chromium hydroxid e (2) ion e x change, (3) evaporation, (4) reverse osmosis, (5) Preci pitation of Cr(VI) by lead or barium, (6) ion flotation, (7) liquid liquid extraction, (8) activated sludge process, (9) adsorption on activated carbon. Among the methods listed above, only two processes, reduction and precipitation and ion exchange, are currently widely used. 2-3-1 Reduction of Cr(VI) to Cr(III) and precipitation of Cr(III) as chromium hydroxide. Cr(VI) is first reduced to the Cr(III) by the addition of a reducing agent, with proper adjustment of pH, followed by precipitation of the reduced chromium. Various reducing agents can be used. These include ferrous iron, sulfur dioxide, sodium(bi)sulfite, sodium or other metal sulfides, metallic iron, and zinc. The rate of reduction is pH dependent, requiring a low pH, frequently lower than 3, for a fast reaction. The a cid requir e ments for the reduction of Cr(VI) depend upon the acidity of the original wa s te, th e pH of the reduction reaction and th e typ e of 1 7 reducing agents used. The reducing pot e ntial of reducing age nts is also an important factor to be considered. For example, an e x cess

PAGE 28

17 of about 25 times the stoichiometric amount is required for the reduction of Cr(VI) to Cr(III) when ferrous sulfate is used as a reducing agent because ferrous sulfate has a low reduction poten tial. The result is an excess of Fe (OH) 3 sludge along with Cr(Orl) 3 h b 21 d. tat must e removed. Negative ra 1cal reducing agents such as sodium sulfite and sulfur dioxide have advantages over positive radical reducing agents such as Zn, Fe, and Cu because no extra sludge is produced a.long with the Cr(OH) 3 This advantage is more important when the recovery of chromium oxide is to be considered. Once reduction of Cr(VI) to Cr(III) is complete, the pH of the solution is raised to reduce the solubility of Cr(III) by intro ducing an alkali. Cr(III) is amphoteric, being dissolved in both acid and alkali. It has a minimum solubility of about 0.1 mg/1 at around pH 8.5. It is therefore essential to choose a reducing agent which is effective in reducing Cr(VI) at high pH values so that less acid is needed to first lower the pH and so that less alkali is needed to then raise the pH. A major difficulty with this treatment process is the sludge disposal problem of the pre cipitated hydroxides. Chromium hydroxides are in a colloidal state and tend to highly hydrated and volm1inous, containing as much as 80 percent water by volume. A large settling basin is required to prevent the carrying-over of hydroxides into the 5 effluent. 2-3-2 Ion exchan ge Ion exchange involves a reversible inter change of ions betwe e n a solid phase and a liquid phase. This

PAGE 29

18 process does not present a sludge probl e m and has the advantage of reclamation of Cr(VI). An anion exchange resin in the chloride or sulfate form is used to exchange chromate ions in the solution to be treated. One of the following reactions may occur depending on the prevailing pH conditions =5 1. In neutral water (2-3) 2. In an acidic condition where dichromate is the species (2-4) 3. In an acidic condition with a high chromate concentration R Cr 0 2 3 10 + H 0 2 (2-5) (2-6) Acidifying the chromate solution has a twofold effect: first increasing the exchange capacity by a factor of two or more, as shown in the above reactions, and secondly, increasing the 5 selectivity of the resin for chromate over other foreign ions. Regeneration may be accomplished with a sodium-hydroxide sodium chloride mixture and then followed by NaCl to restore the exchange resin to the chloride form. The addition of sodium hydroxide in the regenerant solution is considered to be necessary to convPrt acid chromate back to the neutral chromate, which is less .d s strongly adsorbed and therefore more easily replaced by chlor1 e. The recovered chromate can be sent either to a cation exchanger

PAGE 30

19 to recover Cr(VI) as chromic acid or can be further concentrated by using evaporative systems. It can frequently be reused without further concentration in many areas because of the high Cr(VI) con centration of up to 10 percent. Some of the disadvantages of the ion exchange process are (1) the need of maintaining a critical flow rate, which if exceeded even temporarily, results in incom plete exchange and leakage of chromate (in this case the column's capacity is still far from exhausted), (2) the limited capacity of the ion exchange system requires relatively large installa tions to provide the exchange capacity needed between regenera tion cycles, (3) disposal of the regenerated material, (4) slow deterioration of resin with use and possible contamination of water, (5) selectivity consideration of the resin for chromate f 5, 15 over oreign anions present. 2-3-3 Evaporation. Evaporation is a well-established industrial process to separate solid from solution. In plating industries evaporation has been used successfully for recovering plating chemicals and water from plating waste effluents. There are numer ous modifications of the evaporation process that may be employed. h order to minimize the amount of energy required for evaporation, multiple-effect evaporation, vapor-compression evaporation, and multistage flash evaporation are the most widely used methods. In multiple-effect evaporation, water evaporated at a given pressure and high temperature is fed to a second compartment where additional water is evaporated at a lower temperature. The

PAGE 31

20 evaporation of water is carried out in successive stages. A number of effects can be provid e d until optimum efficiency is reached. In vapor-compression evaporation, water is evaporated at atmos pheric pressure. The vapor is then compressed to raise the pressure of steam and is returned to the heating side of the evaporator. A temperature difference between the compressed steam and influent in the evaporator is the driving force for heat exchange. No cooling water is required ir1 this process as is in the multiple-effect evaporator. However, the high initial cost of this process is not economically justified except where no cooling water is available. 40 In multistage flash distillation the water is heated to the highest temperature and flashed into the evaporator. The steam thus produced is condensed to produce water. The remaining concen trated solution is flashed into an additional evaporator which is 14, 40 operated at a lower pressure than the preceding ones. Evaporation is a relatively expensive process requiring high capital and operating costs. However, a single-effect evaporator in conjunction with ion-exchange may be increasingly useful in the plating industry for recovery of chemicals. 2-3-4 Reverse osmosis. Reverse osmosis is a process used to separate a solution into a concentrate and a more dilute solution by applying pressure across a membrane. The flow of water across the membrane is directly proportional to the net pressure differ. 21 ential as expressed by the following equation: W = K ( dp ,r) w (27)

PAGE 32

21 where W = water flow rate, K = membrane water permeation constant, w dp = applied pressure differential, and 1r = osmotic pressure differential. Most of the existing reverse osmosis units in the plating industry are for treatment of nickel plating solutions be cause of the suitability for handling nickel solutions and the reuse of expensive chemicals. However, laboratory and pilot plant studies have been performed for treatment of cyanide and chromium containing wastewater A limitation of the reverse osmosis process is that acidic or alkaline solutions, as well as highly concen trated solutions, cannot be successfully handled without prior 1 5 treatment. 2-3-5 Precipitation by lead or barium. Lead and barium chromates are highly insoluble and therefore can be used for precipitation of chromate in waters. However, the extreme toxicity of both metals, along with their high price, imposesa limitation on their wide 5 application. 2-3-6 Ion flotation. Ion flotation is principally a combination of mineral flotation and ion exchange processes. A surface-active agent such as ethylhexadecyldimethyl ammonium bromide is used as a chromate collector and is added to a chromium-containing solution. Inorganic groups of the collector dissociate in water and are re placed by chromate ions to form an insoluble surface-active com pound. The newly formed compound tends to remain at the solution/ air interface when air is introduced into the solution, and can be separated from solution by skimming off the chromate froth.

PAGE 33

22 As high as 98 percent removal of chromate, for an initial con centration of 10 mg/1 as chromium, was reported in a laboratory 41 scale study. Ion flotation is a recently developed technique. There are many areas to be investigated prior to large-scale operation. These include disposal or reclamation of chromate and once-used surfactant, and feasibility of the continuous flotation 5 method. Results of a pilot-plant scale ion flotation study were 42 reported by Grieves et al. They found optimum results at a molar feed ratio of surfactant to dichromate of 2.1 with 70 per cent recycle. Optimum detention time was 85 min. The feed concen tration of the dichromate was about 100 mg/1. The concentration of the chromium in a liquid volume less than 1 percent of the volume of the waste could be achieved at about 67 cents chemical cost per pound of chromium. 2-3-7 Liquid-liquid extraction. Liquid extraction, sometimes called solvent extraction, is the separation of the constituents by another insoluble liquid. The following chemical reactions take place when a chromate solution is reacted with a tertiary amine: (2-8) Both R N and (R NH) Cr O are soluble in kerosene or other sol vents, 3 3 2 2 7 but almost insoluble in water. Chromate moves from the water to the organic phase, and chromium-containing organic solvent can be

PAGE 34

23 easily separated from water by decantation. Chromium can be stripped out of the extract under alkaline condition. (2-9) Both chromate and R 3 N can be reused. The treated water (called raffinate) is almost chromium free. Over 99 percent removal of chromate can be accomplished by this method. 39 2-3-8 Activated Sludge Process. The conventional activated sludge treatment process can result in the removal of heavy metals d 43, 44, 45 44 to a certain egree. Stones reported in 1955 that 67 percent of the initial chromium of concentrations ranging from 0.17 to 0.56 mg/1 as Cr was removed by this process. A 60 percent removal of chromium by the primary treatment process and 65 per cent removal by the secondary treatment process was observed by 45 Oliver and Gosgrove. A similar degree of chromium removal was also reported by Tarvin. 43 The mechanism of chromium removal in the activated sludge process was believed to be the reduction of Cr(VI) to Cr(III), followed by precipitation of Cr(III) in neutral or slightly alkaline condition that prevailed in wastewater. 35, 43, 46 Bacterial utilization of oxygen avail ab le in the Cr0 4 2ion was believed to be responsible for the rapid reduction of Cr(VI) to Cr(III). A study showed that ~n increase in the suspended solids concentration in an activated sludoe tank re:ml ted in an increased 0 removal of chromium. 43 Adsorption of heavy metals to suspended

PAGE 35

24 47 48 49 so 51 solids has been well docwnented in many references. ' Incidences of digester failure caused by heavy metals have been 5 2 5 3 h d ( I I) h reported. Te precipitate Cr I int e activated sludge probably was transferred to the digesters along with the sludge. The low pH which was exerted during the acid production period in the digester increased the solubility of Cr(III) so that it re dissolved,thereby causing a toxic action which led to the decreased rate or even stopping of digestion.

PAGE 36

CHAPTER 3 LITERATURE REVIEW 3-1 Chemistry of Chromium Chromium is the twenty-fourth element in the periodic table with an atomic weight of 51.996 based on the relative atomic mass of C 12 = 12. The electronic configuration of 52 Cr is ls 2 2s 2 2p 6 2 6 5 1 f 3s 3p, 3d 4s It is possible rom its electronic configuration that chromium can have all the oxidation states from -1 to +6. It can form a negative monovalent ion by taking up one electron to fill the 4s shell. It can also form six positive ions 10 by losing electrons successively from the 3d shell. However, the chromium compounds with the +3 and +6 values are the only stable ones and are the predominant forms found in natural water. 3+ The oxidation potential of Cr to Cr(VI) depends on the pH and the ratio of species present. Typical reactions are Cr 3 + + 41-1 2 0 = H CrO + 61-1+ + 3e2 4 (1-1 2 CrO 4 ) E 0 = 1.335 0.1182pl-I + 0.0197 log ccr3 +) 3 + + Cr + 41-1 2 0 = HCro; + 71-1 + 3e (HCrO 4 -) E 0 = 1.335 0.1379pH + 0.0197 log eel+) 25 ( 3-1) (3-2)

PAGE 37

26 3 + 2 + 2Cr + 7H 2 0 = Cr 4 0 7 + 14H + 6e 2(Cr2 0 7 ) E 0 = 1.333 0.1379pH + 0.0098 log -3 -+2 (Cr ) 3 + 2 + Cr + 4H 2 0 = Cr0 4 + SH + 3e 2(Cr04 ) E 0 = 1.477 0.1579pH + 0.0197 log--cc/ +) + 2f\0 2+ = Cr0 4 + 4H +. 3e 2+ E 0 = 0.945 (Cr0 4 ) = 0.0788pH + 0.0197 log---E 2(Cr0 4 ) = 0.359 0.0394pH + 0.0197 log --3 -+ (Cr03 ) (3-3) (3-4) (3-5) (3-6) Eqs. 3-1 through 3-4 are the reactions that take place under acidic conditions, and Eqs. 3-5 and 3-6 are under alkaline conditions. 54 3+ C.i. is more stable than Cr(VI) at neutral and acidic pH values. Cr(VI) becomes more stable at a pH over 12. Cr(III) normally pre cipitates under a neutral or slightly alkaline condition with a minimum solubility of about 0.1 mg/1 at approximately pH 8.5. Alh 10 -30 ,ss tough the equilibrium constant of Cr(OH) is approximately 3 the solubility of this chromium hydroxide is increased with either

PAGE 38

27 decreasing or increasing pH. Solubility increase with an increase in pH is due to the formation of Cr(OH) 4 and other polynuclear hydrolysis species. The hydrolysis of Cr(III) can be calculated 56 in the same manner as shown by Stumm and Morgan. Actual computations are given in Appendix 1, and a solubility diagram based on the results of these data is shown in Fig. 3-1. Chromium has a marked tendency to form coordination compounds with water. The simple hydrated ion can be prepared at room temperature by addition of distilled water to a chromium salt such as nitrate, perchlorate, and fluroborate. Ions such as sulfate and chloride enter the complex through the displacement cf coordinated water. However, the rate of exchange of water with other anionic species is considered to be very slow due to the tightly bound inner 1 8, 5 7 sheath of water molecules. Although chelating agents such as ethylene-diamine-tetraacetic acid (EDTA) and nitrilo-triacetic acid (NTA) form a very stable complex with Cr(III), the rate of formation is so slow that boiling of the solution for about 15 58 min is required to insure completion of the reaction. 2Hexavalent chromium in water forms oxo(Cr0 4 ) and hydroxo (Cr0 3 0H-) complexes which are acids. Acidity of aquo metal ions re sults from the repulsion of the proton of H 2 0 molecules by the posi tive charge of the metal ion. The acidity of aquo metal ions increases with the decrease of the radius and an increase of charge of the central ion. 2The strong acidity of CrO 4 and Cr0 3 OH results from the high oxidation state of Cr(VI). Various species of Cr(VI)

PAGE 39

FIGURE 3-1 Solubility of Cr(OH) as a Function of pH. Only monomeric hydrolysis species are con sidered.

PAGE 40

29 -4 -2 0 Cr(OH)J (S) 2 u l9 4 0 _J I 6 8 10 Cr(OH)4cr+3 +2 CrOH 2 4 6 8 10 12 pH

PAGE 41

30 may exist depending on the pH and the concentration of Cr(VI) ions. Distribution diagrams of hexavalent chromium species are shown in Fig. 3-2. These diagrams are based on numerical equa tions listed in Appendix 2. Chromic acid is a fairly strong acid in its primary dissociation that does not exist except in solu tion. It shows a marked tendency in very concentrated solutions to form polynuclear species (polyacids) through the elimination of water. (37) (3-8) (3-9) H 2 Cr~ converts to H 2 Cr 2 0 7 almost instantaneously, but the further polymerization requires a measurable time. Salts derived from h d k 1 s 2 o, s 9 6 o Ch d t ese polyac1 s are also nown to exist. rom1c ac1 is a strong oxidizing agent, the oxidizing power increasing with d H 1 s ecreas1ng p 3-2 Adsorption Phenomena Adsorption is a process of the interphase accumulation or con61 centration of substances at a surface or interface. The energy of interaction at the interface can be interpreted as a composite function resulting from the sum of attraction and repulsion forces.

PAGE 42

FIGURE 3-2 A Distribution Diagram for the Various Cr(VI) Species as a Function of pH. At CrT = 1, 102 and 104 M. Activity coefficients of 1.0 w e re 56 assumed for all cases ( after Stumm and Morgan )

PAGE 44

33 Forces responsible for adsorption are (1) nonpolar van der Waals attraction, (2) formation of hydration bonds, (3) ion exchange 5 6 6 2 ( 4) chemical interaction, (5) coulombic, or electrical forces. The force between atoms and molecules is always attractive. London suggested in 1930 that the positively charged nuclei and the nega tively charged electrons in molecules (and atoms) oscillate with respect to each other, producing oscillating dipoles. The resultant force is known as the dispersion force, or the London van der Waals force. 1he van der Waals attraction force between two atoms is inversely proportional to the sixth power of distance over 5 6, 63 small distances. Adsorption of a solute onto a solid substrate may take place if either contains hydrogen bond donor groups and the other contains acceptor groups. However, hydrogen-bond adsorption would be greatly hindered if one of the bonding groups had too strong 62 an affinity for water. Ion exchange or electrostatic attraction may be responsible for the adsorption of organic ions onto solids. For example, silica and carbon, which are normally negatively charged, readily adsorb cationic dyes and surfactants. 62 Multivalent ions are attracted with greater force than monovalent ions toward a site of opposite charge on the surface of the adsorbent, and his selectivity tends to decrease with the increasing ionic strength of the solution. 56 61 Adsorption by means of a chemical reaction differs from physi cal adsorption in that it first requires considerably higher

PAGE 45

34 activation energy for adsorption. Chemical reactions also proceed 6 1 more rapidly at elevated temperatures than at low temperatures. However, it is often difficult to draw a sharp line of demarca. d. h b h 1 d h 1 d 61 64 tion to istinguis etween p ysica an c emica a sorption. Generally adsorption is increased with decreasing solubility of the solute in the solvent. The solubility of solute in sol vent can be related to the solute-solvent bond. The surface tension of a solution, as described by the Gibbs b f 61, 63 adsorption equation, can e written as allows: E C = 1 d y RT (dln a) ( 3-10) where E = the excess surface concentration of solute, y = inter c facial tension, and a= activity of solute. For dilute solutions the concentration can be used instead of the activity. In Eq. 3-10 a decrease in surface tension is brought about through the accumu lation of solute at the interface. Many organic compounds have hydrophobic radicals indicating low affinity for the aqueous phase. They tend to stay away from the bulk of water, favoring being adsorbed on an adsorbent. Many inorganic ions are readily hydrated with water mol e cules and tend to remain in solution, making h 1 56 I 1 d t ernse ves less available for adsorption. n genera a sorption is minimal for the charged species and maximal for the un dissociated (neutral) species. 65 The degree of dissociation is

PAGE 46

35 dependent on pH. For amphoteric compounds the m a ximum adsorption occurs at the isoelectric point, when the compound becomes neu65 trally charged. Solute polarity also has effects on adsorption. A polar solute has a tendency to be adsorbed preferentially by a polar adsorbent from a nonpolar solvent. The effect of polarity of a solute on adsorption is closely related to surface tension and solubility. Functional groups such as -OH -SH -COOH -NH 2 or S0 3 H tend to render their compounds polar. Solution by water then results by formation of a hydrogen bond from hydrogen in the water molecules to a group bearing a negative charge. Interfacial tension as well as water solubility is therefore increased, requiring more work to bring solute molecules to the interface for adsorption. 3-3 Nature of Activ a ted Carbon Activated carbon has been widely used for removal of organic pollutants from water and wastewater. These organic pollutants in clude wide varieties of substances such as tasteand odor producing materials, and color-causing organic compounds. The most characteristic property of activated carbon is its extremely large surface area (500-2,500 m 2 /g). Having a large sur face area is very important since adsorption is an interfacial phenomenon. However, any interpretation of the adsorptive behavior of activated carbon based solely on the large surface area is in complete. Equal weights of carbons prepared from different raw

PAGE 47

36 materials by different methods may have the same total surface area yet behave differently as an adsorbent. Relative pore size distribution within the carbon is responsible partly for different 6 6 adsorptive capacity. A molecule will not readily find its way into a pore smaller than a certain critical diameter and will be d f 67 f exclude rom smaller pores. Various unctional groups are known to exist on the carbon surface. Differences in adsorptive capacity may be attributed partly to these functional groups, which are determined to a large extent by the method of activation as well as the type of material from which the carbon is prepared. 66 Al though chars, coke, and activated carbon are frequently termed amorphorus carbon, X-ray studies have indicated that they have microcrystalline characteristics. The microcrystallites are formed by two or more flat plates in a hexagonal lattice shape, stacked one above the other. Although the structure of the crystallite is similar to that of graphite, differences between the two carbons exist in many ways. Impurities in activated carbon which are not found in graphite are believed to have a signifi cant effect on the adsorption of organic substances. In activated carbon, because of either the preparation procedure or the starting material, the planes orient themselves 1n a more d d h 6 6 G cl 1,r 6 B isorderly manner than 1n 1 eal grap 1te. arten an veiss stated that the analogy with graphite as a mod e l is poor and they prefer to visualize stacks of flat, aromatic, and for the most part, heterocyclic planes crosslinked in a random fashion,

PAGE 48

37 affecting both the distance of separation of adjacent planes and the adsorptive properties of the carbon. Exceptionally high oxygen content (2-25%), as well as substantial amounts of hydrogen in carbon, is believed to play an important role in determining the chemical behavior of carbon. Garten and Weiss 68 regarded activated carbon as a complex organic polymer rather than an amorphous form of the element carbon. The production of activated carbon involves carbonization. This is normally carried out by heating the raw material in the absence of air at an elevated temperature of approximately 6oo 0 c. Many metallic chlorides such as zinc, calcium, and magnesium chlorides are often added to increase the effectiveness of carb 64 h db b d on1zat1on. Cars prepare y car on1zat1on o not have large internal surface areas. The large surface areas of activated car bons result from the process referred to as activation. Activation is commonly carried out by oxidizing a carbonaceous char with CO 2 steam, and air. Activation temperature has a profound effect on the properties of activated carbon. 3-4 Adsorntion Phenomena and Activated Carbon In 1929 Kruyt and De Kadt (Garten & Weiss 69 ) reported that caro bon activated at a low temperature (around 400 C) was capable of adsorbing alkali but little acid, whereas carbon activated at a high temperature (800-l,000C) was able to adsorb acid but not alkali. It has been generally believed that the adsorption of

PAGE 49

38 alkali on L-carbon (activated at a low temperature) is due to the presence of oxygen complexes on the surface of activated carbon. Numerous theories have been forwarded to explain alkali adsorp tion. The presence of carboxyl groups in activated carbon was 6 8 6 9 suggested by Schweizer and Goodrich (Garten & Weiss ) in 1944, and the presence of phenolic groups was suggested by Villare in 69) 69 1947 (Garten & Weiss Garten and Weiss concluded that the acidity of L-carbons principally originated from three functional groups: the phenolic group, lactone groups in association with phenol, and normal lactone groups. Some reactions of flurescein type lactones (f-lactones)are graphically shown in Fig. 3-3. An aqueous suspension of an H-carbon (activated at a high tempera70 ture) was found to have an alkaline pH value. Burshtein and Frumkin (Garten & Weiss 68 ) in 1929 observed that a n H-carbon, outgassed in a high vacuum, was unable to adsorb acid from an oxygen-free solution unless oxygen was introduced. A study by Garten and Weiss 69 showed that adsorption of acid was dependent on the partial pressure of oxygen. The adsorptive capacity increased with increasing partial pressure of oxygen up to 20 mm Hg, beyond which adsorption became almost independent of partial pressure. Bretschneider (Garten & Weiss 69 ) studied the adsorption of hydro chloric acid on aerated and outgassed carbon. It was observed that th e isotherms were parallel except in the region of low acid concentration. He concluded from this finding that part of the adsorbed acid was chemically adsorbed by oxides on the surface,

PAGE 50

FIGURE 3-3 Some Reactions of Flurescein-Type Lactones. (After Garten & Weiss 6 9 .)

PAGE 51

40 COOH + COONa

PAGE 52

41 and the remaining reacted acid was adsorbed by physical adsorption. Kolthoff 71 in 1932 detected hydrogen peroxide that was released by carbon when activated coconut charcoal reacted with sulfuric d 68 acid. However, Garten an Weiss reported that the amounts of oxygen and acid adsorbed were found to be far greater than that of hydrogen peroxide liberated. The stoichiometric determination of the adsorption of acid and oxygen is very difficult. This is due to the capability of hydrogen peroxide to attack carbon and consequently to fonn acid groups, and partly due to the diffi culty in distinguishing quantitatively between physically and chemically adsorbed acid. 68 69 Many theories on adsorption of acid on carbon were proposed by numerous scientists. These include the electrochemical theory, a theory involving the neutralization of surface oxide, and pure physical adsorption. However, none of these theories can explain all the phenomena pertaining to acid adsorption. Garten and Weiss 69 proposed the presence of chromene (benzpyran) groups to explain acid adsorption. Chromene is readily oxidized at room temperature in the presence of acid to become carbonium as shown in Fig. 3-4a. The carbonium (or benzophrylium) ion is a weak base having a dissociation constant on the order of 1010 and is partly hydro lyzed by water to form chromenol as shown in Fig. 3-4b. The p~r tial hydrolysis of the carbonium ion to chromenol is probably re sponsible for the difficulty in desorbing all the adsorbed acid from an H-carbon. 69 However, the acid taken up by means of the

PAGE 53

FIGURE 3-4 (a) The Chromene-Acid Reaction, (b) Hydrolysis of the Carbonium Ion. (After Garten & Weiss 69 .)

PAGE 54

43 D OC/H )(Oil // '--R OR I CJ I\ R H (a) ( b)

PAGE 55

44 chromene reaction is only part of the acid totally adsorbed, since part of the adsorbed acid can be displaced by adding organic sol vents such as toluene and phenol. According to Steenberg (Garten & Weiss 6 9 ) the proton is primarily adsorbed by means of physical forces and the anion is secondarily adsorbed in the double layer. The proton is adsorbed readily because of negatively charged carbon, but held back because of the anion of acid molecules which is not readily taken into the double layer. Increased adsorption of acid with increasing salt concentration (anion pressure) is consistent with Steenberg's interpretation on physical adsorption of acid by b 12 activated car on. Neither one of the mechanisms which were discussed briefly herein can successfully explain all the phenomena pertaining to acid adsorption by activated carbon. The chromene-acid reaction theory proposed by Garten and Weiss appears to be gaining more attention among scientists. Oxygen present on the surface of the activated carbon affects the adsorptive properties of the carbon because it tends to in crease the polarity of the surface. 66 According to Coughlin and E 73 d f h d d h zra, a sorpt1on o p enol was greatly re uce wen virgin carbon was oxidized, whereas increased adsorption was observed with reduced carbon. Similar results were also reported by Kip1 d 66 d. d 1ng an Shooter (Snoeyink & Weber ) when 10 1ne was reacte with carbon. The increase in the polarity of carbon tends to de crease adsorption of nonpolar sorbate on the carbon.

PAGE 56

45 3-5 Adsorption of Cr(VI) on Activat e d Carbo~ Ions of inorganic salts have been considered poor adsorbates on activated carbon because the activated carbon favors adsorption of undissociated molecules over dissociated molecules. Consequently, most of the literature concerning adsorption with activated carbon deals with organic adsorbates. Sigworth and Smith 74 reported in 1972 that activated carbon ex hibited a very good potential to adsorb Cr(VI). Other research fou.~d that chromate undergoes reduction during adsorption by 6 4, 6 8 activated carbon. A high degree of removal of chromium by 67 activated carbon was also reported by Culp and Culp and Argo 7 5 41 and Culp. Smithson conducted a study of chromium removal by activated carbon with little theoretical elaboration.

PAGE 57

CHAPTER 4 EXPERIMENTAL APPARATUS AND PROCEDURES The experiments were divided into two phases: batch systems and column systems. Batch studies dealt primarily with the theoretical study of the mechanisms of removal of chromium by activated carbon. Column studies were designed to develop an activated carbon adsorp tion system with industrial application for removing chromium. 4-1 Feed Solutions 4-1-1 Synthetic feed solution for batch studies. All synthetic feed solutions were made with deionized water and reagent grade chemicals. All glassware was washed with tap water, rinsed with 1 N HN0 3 rinsed with dis tilled water, and then rinsed with de ionized water having a pH value of about 5.5. All glassware was oven dried and cooled before use. 4-1-2 Synthetic feed solution for column studies. All synthetic feed solutions for column studies were made with tap water supplied from the Gainesville water treatment plant. 1he composition of this tap water is given in Table 4-1. 4-2 Activated Carbon Preparation The activated carbon used for this study was Filtrasorb 400, manufactured by Calgon Corporation. This coal-base activated carbon 46

PAGE 58

47 TABLE 4-1 Characteristics of Finished Effluent of Gainesville Water Treatment Plant* Ca 2+ 46 mg/1 2+ 16.6 mg/1 Mg Total hardness 184 rng/1 as + mg/1 Na 18.5 as Cl 10 mg/1 so 4 217 mg/1 N0 3 as N 0.0 mg/1 Phenolphthalein alkalinity 0.0 mg/1 as Methyl orange alkalinity 142 mg/1 as co 3 2142 mg/1 as OH0.1 mg/1 as CO 2 3 .1 2 mg/1 as Total dissolved solids (estimated by conductivity) 224 mg/1 Iron, total 0.01 mg/1 F 0.48 mg/1 Color 3 APHA pH 7.68 at 25C Conductivity 375 mho/cm *Tests were run on a sample collected 21 March 1975. SOURCE: Gainesville Water Treatment Plant. CaC0 3 CaC0 3 CaC0 3 CaC0 3 CaC0 3 CaC0 3 CaC0 3 at 25C was selected on the basis of its having a large surface area of 2 1,050 to 1,200 m /g and its ability to withstand abrasion. Physical properties of this carbon are given in Table 4-2. For the batch experiments this commercially available carbon was ground and sieved to have a size range of 0.149 to 0.25 mm (US sieve sizes 100 to 60). It was then washed with distilled water and demineralized

PAGE 59

48 TABLE 4-2 Physical Properties of Filtrasorb 400 Total surface area (N 2 : BET method) Bulk density Particle density wetted in water Pore volume Effective size US standard series sieve size larger than no. 12 smaller than no. 40 Mean particle diameter Iodine number Abra~ion Moisture Ash content* 2 1,050-1,200 m /g 25 lbs/ft 3 1.3-1.4 g/cc 0.94 cc/g 0.55-0.65 mm 3% max. 5% 1% max. 5% 1.0 mm min. 1,100 min. 80 0.5%, max. 5.5% 2% *Determined by the author. Measurement was made after incinera.:. tion of activated carbon at 600C for 10 hrs. SOURCE: Calgon Corp., Bulletin 20-2d (1973). water until dust and fine particles were removed. It was dried in 0 a drying oven at a temperature of 105 C for 2 hrs and cooled at room temperature in a desiccator for storage. For the column ex periments, Filtrasorb 400 was used directly without modification. 4-3 Stock Solutions 4-3-1 Hexavalent chromium solutions: potassium (di)chromate. Fisher Certified reagent grade in fine crystal form was dried at 103C for 2 hrs and cooled in a desiccator to room temperature. A 0.01 M stock solution as Cr was prepared in deionized water.

PAGE 60

49 Working solutions ~ere obtained by diluting this stock solution to the predetermined strength. Chromic acid. Chromic anhydride (Cr0 3 ) from Fisher Chemical Co. was used to make a 0.01 M stock solution as Cr. This hygro scopic chemical was kept in a desiccator, without predrying in an oven, to avoid possible decomposition at an elevated tempera ture. The concentration of this stock solution was checked against a potassium dichromate solution by the s-diphenyl carbazide color development method. 4-3-2 Trivalent chromium solution. Chromium nitrate (Cr(N0 3 ) 3 H 2 0), Baker analyzed reagent grade was used as a source material of Cr(III). No pretreatment was performed for this chemical to avoid the possible loss of crystal water molecules incorporated with chromium nitrate. The stock solution was prepared by dissolving it in deionized water which was previously acidified with nitric acid. 4-3-3 EDTA-Cr chelate solution. A 0.01 M solution of analytic grade disodium salt of EDTA was mixed with a 0.01 M solution of Cr(N0 3 ) 3 at a pH of 4.3 and boiled for 15 min. 58 4-3-4 NTA-Cr chelate solution. Prepared in the same manner as EDTA-Cr chelate. 4-4 Experimental Equipment 4-4-1 Water bath with a shaking unit. Agitation for batch studies was provided by a water bath equipped with a shaking unit manu factured by Eberbach Corp., Ann Arbor, Michigan. This unit had a variable shaking speed control device.

PAGE 61

so 4-4-2 Circulator. Th e temperature in the water bath was con trolled by recirculating water through an external circulator which had a cooling unit. The temperature was regulated with a thermostat. A Lauda Model K-2/R Brinkmann Instrument was used. 4-4-3 Filtering apparatus. A conventional millipore filtering unit was modified to facilitate simple and repetitive filtration operation. A SO-ml bent test tube was placed in a 500-ml erlen meyer flask on which a millipore filter was mounted as shown in Fig. 4-1 A check valve was connected to the bottom of this tube so that the valve opens only upon breaking the vacuum, enabling filtrate to flow by gravity out for collection. The carry-over contamination duri~g successive filtrations was greatly reduced because of the small size of the test tube used compared to the erlenmeyer flask. 4-4-4 Polystaltic pumps. Variable speed Buchler polystaltic pumps, Model 2-6100, were used for column studies to deliver solu tions to the columns 4-4-5 Activated carbon columns. SO-ml burets were used as ad sorption columns. The inside diameter of the columns was 1.15 cm. A schematic of the column testing system is shown in Fig. 4-2. 4-5 Analytical Equipment and Techniques 4-5-1 Total chromium determination. Total chromium concentra tion was determined by atomic adsorption on a Varian Techtron Model 1200. Readings were made at a wavelength of 357.9 nm.

PAGE 62

FIGURE 4-1 Filtering Apparatus

PAGE 63

S TOPC OC K TO --~ ASPIRATOR tE--tv11LLIPORE FILTER UNIT ~-ERLENMEYER FLASK --CHECK VALVE u, N

PAGE 64

FIGURE 4-2 Schematic of the Column Experiment Configuration

PAGE 65

Constant temperature water bath with forced pumping Polystaltic pump Effluent Effluent Feed solution Wa.ter jacket Burette packed with activated carbon

PAGE 66

55 Acetylene-air flam e with the fuel slightly rich was used in all cases except when oxidizing agents were present in the solution. Acetylene-air flame with the air rich was used in a solution containing oxidizing agents. The detection limit of this instru ment was found to be 0.05 mg/1 as Cr. Concentrations below this value were determined with a carbon rod atomizer, Varian Techtron Model 63. Chromium concentrations as low as 0.01 mg/1 could be determined with this procedure. 4-5-2 Hexavalent chromium determination. Hexavalent chromium was determined by colorimetric analysis using s-diphenylcarba11 zide according to the Standard Methods. The absorbance was read on a Bausch and Lomb Spectronic 70 at a wavelength of 540 nm. A light path of 2 cm was used. The minimum detection limit was found to be 0.01 mg/1 as Cr. Readings were made 10 min after color de velopment, since intensities of color gradually decrease with time. 4-5-3 Trivalent chromium determination. Trivalent chromium was determined by subtracting the hexavalent chromium concentration from the total chromium concentration. 4-5-4 EDTA and NTA determination. EDTA and NTA were determined by titration with 1 mM of ZnC1 2 solution, using a zincon (2-carboxy2'hydroxy-5'sulfoformazyl-benzene) indicator. The basic principle behind this method is that Zn 2 + ions chelate with EDTA and NTA instantly, while zincon does not react with the chelated zinc, but reacts with free Zn 2 + ions added in excess of the stoichio metric amounts. The detection limit was found to be approximately 2 M.

PAGE 67

56 4 5-5 J2!i. measurement. pH was measured with a Corning Model 12 expanded scale pH meter. The pH of the solutions was measured prior to the addition of activated carbon and again measured at the end of the reaction time. 4-6 Experimental Procedures 4-6-1 Batch studies. Carefully measured amounts of activated carbon were added into prewashed 125-ml stopper erlenmeyer flasks which contained 100 ml of chromium solutions of predetermined concentration. The pH of the solutions was measured prior to the addition of activated carbon and again measured at the end of the reaction time, which unless otherwise stated was 24 hr. The re action was carried out at 20c in a water bath equipped with a shaker unit. The shaking speed was controlled so that the activated carbon particles always remained in suspension. After the reaction the activated carbon was filtered from the solution by passing the solution through a 0.45 micron rnillipore filter. The filtrate was collected in cleaned and dried plastic bottles for later deter mination of chromiwn. Deionized water was used for the sample preparations for the batch studies. 4-6-2 Column studies. Filtrasorb 400 activated carbon was used directly without any pretreatment. Carefully weighed carbon was washed with distilled water to remove fine dusts. The loss of weight by washing was found to be negligible. It was then boiled gently for a few minutes to displace air with water. This step was necessary

PAGE 68

57 to avoid formation of air pockets in the column which would cause channeling of the feed solution. After cooling, the slurry of carbon was transferred into a SO-ml buret column. Glass fiber was placed on the bottom of the buret to prevent plugging of the outlet port by activat~d carbon particles. Unless otherwise stated, 16.5 g of activated carbon with a moisture content of approximately l percent were used in each column. This amount of activated carbon pruvided a column with about 40 cm of bed depth and 20 cm of free board. The flow rate of feed solution was con trolled with a polystaltic pump. Calibrations were made by collect ing a given volume of column effluent in a noted time. The desig nated feed rate did not change by any measurable degree throughout each experiment.

PAGE 69

CHAPTER 5 EXPERIMENTAL RESULTS AND DISCUSSION 5-1 Batch Studies 5-1-1 E!:!_ dependency of Cr(VI) adsorption. An experiment was performed to investigate the effect of pH on the adsorption of Cr(VI) on activated carbon. Chromic acid, chromate, and dichromate will hereafter be designated as Cr(VI), and trivalent chromium compounds as Cr(III). A series of 20 M Cr(VI) solutions having different pH values were prepared by diluting the dichromate stock solution with deionized water. The pH values were controlled by adding either nitric acid or sodium hydroxide. The chromium concentrations remaining in solution after 24-hr contact with activated carbon were measured and are plotted against pH in Fig. 5-1. Chromium concentration was minimal at a pH of about 4. 5, indicating maximum adsorption at t i 1is pH value. Fig. 5-1 is replotted in Fig. 5-2 showing the individual concentrations of Cr(III) and Cr(VI) for the same pH values. Com parison of Fig. 5-2 with Fig. 5 1 reveals that the pH-CrT curve is composed of three distinct portions: a broken line denoting CrT all in the form of Cr(VI), a solid line representing Cr equal to the T sum of Cr(VI) and Cr(III), and a dotted line indicating CrT in the form of Cr(III) only. It can be concluded from Fig. 5-2 that chromium is adsorbed best by activated carbon at the lowest possible pH below which the reduction of Cr(VI) to Cr(III) takes 58

PAGE 70

FIGURE 5-1 CrT Concentration Remaining in Solution as a Function of Initial pH. The initial Cr(VI) concentration was 20 Mand the activated carbon dosage was 200 mg/1. Agitation was for 24 hrs at 20c. Potassium dichromate was the source of Cr(VI).

PAGE 71

60 20 2 3 z 016 I:) _J 0 12 ti) z (9 z 8 z <( 2 4 w 0:: ,....., ttLl 0 2 4 6 8 10 INITIAL pH

PAGE 72

FIGURE 5-2 Cr(III), Cr(VI), and CrT Concentration Remaining in Solution as a Function of Initial pH. The upper U-shaped curve (solid line) is reproduced from Fig. 5-1.

PAGE 73

62 ,,,-.... 2 20 :i. ...__, (9 z 16 z Cr(VI) o/ <{ / / 2 / w 10 6 0:: 0/ I 'lI I u C r(III) o/ o6 8 I I ,,,-.... rJ > I ...__,, I L u I r(II I)+ C r(VI) 4 C:J p .. ,,,-.... L S:d 0 2 4 6 8 10 INITIAL pH

PAGE 74

63 place. Apparently adsorption was hindered by formation of Cr(III). At a pH of 4 or less, all remaining chromium was in the Cr(III) form, and its concentration increased with decreasing pH. 5-1-2 Adsorption of Cr(III) by activated carbon. Fig. 5-3 shows the results of experiments where a 10 M solution of Cr(III) was reacted in the presence and absence of activated carbon. The pH was controlled by adding either NaOH or HN0 3 Under the test conditions with no activated carbon, the soluble Cr(III) began to precipitate out of solution at pH values greater than 6. There after the Cr(III) concentration in solution was continuously de creased with increasing pH values and reached 10 g/1 at pH 8.2. The 0.0 activated carbon dosage line in Fig 5-3 shows the values of Cr(III) on the ordinate to be greater than what was calculated from a pH-Cr(OH) 3 (s) solubility relationship. The values from Fig. 5-3 were found, however, to be low when compared with Fig. 3-1, where all the monomeric hydrolysis species were taken into account. Many polynuclear hydroxo complexes of Cr(III) have been known to form upon addition of Cr(III) ions to water. These com plexes are kinetic intermidiates in the slow transition from free Cr(III) ions to solid precipitates. 56 Hunt and Taube (Rubin 57 ) 3+ found experiment a lly that the ion (Cr(H 2 0) 6 ] exchanged water with the bulk solvent very slowly, with a half life of 40 hrs. The solubilit y cur v e shown in Fig. 5-3, represented by the line of zero dos a ge of activated c a rbon, was constructed on the basis of a 24-hr reaction time, and therefore may not represent the true

PAGE 75

FIGURE 5-3 Cr(III) Precipitation with Activated Carbon and Cr(III) Removal in the Presence of Acti vated Carbon. The initial Cr(III) concentra tion was 10 Mas Cr. The adsorption of Cr(III) appears to be related to its solu bility. Agitation was for 24 hr at 20c.

PAGE 76

65 10 2 :3_ z 8 O.O mg /1 A C 0 J,::> _J 500 mg/I 0 CJ) 6 z {.!) z 4 <( w 0:: 2 t=1 i... (.) 0 4 5 6 7 8 9 FfN A L pH

PAGE 77

66 equilibrium solubility-pH relationship. The Cr(III) remaining in solution in the presence of activated carbon was always lower than that of the corresponding pH values in the absence of activated carbon. This indicates adsorption of Cr(III) on the surface of activated carbon. From the similar shapes of curves in Fig. 5-3 it can be concluded that the adsorption of Cr(III) is related to its solubility. Activated carbon does not appear to adsorb Cr(III) ions to any substantial degree at pH values below 5.0. 5-1-3 Effects of activated carbon dosage on adsorption and re duction. In order to study how adsorption was related to the con centration of activated carbon applied, a series of tests with varying activated carbon dosages was conducted W1der otherwise identical experimental conditions. The results of the experiment are plotted in Fig. 5-4. It can be seen from the figure that ad sorption may be divided into three distinct types according to pH: first, increase in adsorption proportional to the carbon dosage at a pH of about 4 and higher; secondly, adsorption independent of the carbon dosage in the vicinity of pH 3. 7; and third, a de crease in adsorption with increased activated carbon dosage in the low pH region. The ordinate in Fig. 5-4, CrT, was separated into Cr(VI) and Cr(III), and replotted in Figs. 5-5 and 5-6. When pH values were greater than 4.8 and when all the Cr remaining in solution was Cr(VI), adsorption was a direct function of the car bon dosage. Apparently adsorption was related only to the number

PAGE 78

FIGURE 5-4 CrT Concentration Remaining in Solution at Different Activated Carbon Dosages as a Function of Initial pH. The initial concen tration of Cr(VI) was 20 M and agitation was for 24 hr at 20c. Potassium dichro mate was the source of Cr(VI).

PAGE 79

68 20r---r----.----,-----..--.. 16 2 32 :) 8 (1) _J :J 4 (25 w 0 100 mg/I CARBON 2 4 6 INITIAL pH 500mg/l CARBON 8 10

PAGE 80

FIGURE 5-5 Cr(VI) Concentration Remaining in Solution at Different Activated Carbon Dosages as a Function of Initial pH. The initial Cr(VI) concentration was 20 Mand agitation was for 24 hr at 20c. Potassium dichromate was the source of Cr(VI).

PAGE 81

7 0 20 ,,........ 2 3 16 r--, ,,........ > ....__ L 12 1oomg1I u CARBON 2 :J 8 0::: OJ _J :J 4 0 w 0 2 4 6 8 10 INITIAL pH

PAGE 82

FIGURE 5-6 Cr(III) Concentration Remainir.g in Solution at Different Activated Carbon Dosages as a Function of Initial pH. The curves represent the concentration that was reduced from Cr(VI) and was not adsorbed. The initial Cr(VI) con centration was 20 Mand agitation was for 24 hr at 20c. Potassium dichromate was the source of Cr(VI).

PAGE 83

.,-._, 2 :.1.. ..__., r--o _..._ ..__., L u L..J _J :) 10 8 6 72 f-500 mg;! CARBON le--" ----250mg/l CARBON 100mg/I--"7 CARBON 0 2 w 3 4 5 INITIAL pH 6 7

PAGE 84

73 of active sites on the surface of the activated c a rbon. In the low pH region, 3. 8 or less for activated carbon dosages of 250 mg/1, all the chromium remaining in solution was in the Cr(III) form. At low pH values, activated carbon favored reduc tion of Cr(VI) to Cr(III) over adsorption. The degree of reduc tion was in proportion to the activated carbon dosage. The re duced Cr(III) was not removed to any appreciable extent at low pH and remained in solution. This greater reduction of Cr(VI) to Cr(III) explains why activated carbon is a poor adsorbent of chromium at low pH values. At about pH 3. 6 the amount of chromium removed from solution was approximately constant, independent of activated carbon used within a reasonable carbon dosage. It can be noted from Fig. 5-4 that for the initial pH 3.6, approximately 2.5 M was the lowest possible concentration remaining in solution regardless of activated carbon dosage when the initial concentration of Cr(VI) was 20 M. Figs. 5-4 and 5-5 also indicate that the pH value for maximum adsorption tends to increase with increased activated carbon dosage. This phenomenon appears to be closely interrelated with the availability of Cr(VI) for adsorption. For ex a mple, at the initial pH 4.4 there was no Cr(VI) found in solution after the end of the reaction with an activated carbon dos a ge of 500 mg/1. However, about 2 M of Cr(VI) was present at the corresponding pH value with a carbon dosage of 100 mg/1.

PAGE 85

74 It can be generally concluded from the data presented so far that the interaction of Cr(VI) with activated carbon is a com plicated phenomenon because of the reduction of Cr(VI) to Cr(III) by activated carbon, simultaneously with various degrees of ad sorption of Cr(VI) and Cr(III) on the activated carbon. Perhaps the most important factor is pH. The magnitude of adsorption and reduction is significantly governed by pH. In order to investigate the optimum pH for the best adsorption, along with factors affecting adsorption efficiency, a series of solutions each containing 100 M of Cr(VI) was reacted with different activated carbon dosages and different initial pH values. The results obtained from this experiment are swnrnarized in Figs. 5-7, 5-8, and 5-9. Fig. 5-7 shows removal steadily in creased with activated carbon dosage as pH decreased from 5.0 to 4.0. As the pH decreased below 4.0 adsorption peaked and then de clined with carbon dosage. From Fig. 5-8 it can be seen that as the pH decreased from 5.0 to 2.5 the amount of Cr(VI) remaining in solution steadily decreased for increasing dosages of activated carbon. The Cr(VI) that remained in solution resulted from the original Cr(VI) either not being adsorbed on the activated carbon or not being reduced to Cr(III). Fig. 5-9 shows the percentage of Cr(III) remaining in solution plotted against increasing dosages of activated carbon at various pH values. At pH 4.0 or greater, very little Cr(III) remained in solution, indicating that it was either adsorbed on the activated carbon or was not

PAGE 86

FIGURE 5-7 C~r Removal as a Function of Activated Carbon Dosage and pH. The initial Cr(VI) concentration was 100 Mas Cr. Agitation was for 24 hr at 20c. Chromic acid was the source of Cr(VI).

PAGE 87

2 100 =:> pH 4.0 2 0 0::: I 80 u -pH 4.5 _J pH 5.0 I 0 60 1-LL 3.4 0 "-l _J <( > 40 0 w 0:: l20 "' 2.5 z w u 0::: w !l. I 200 400 600 800 1000 1200 1400 ACTIVATED CARBON DOSE ( mg/I)

PAGE 88

FIGURE 5-8 Cr(VI) Concentration Remaining in Solution at Different Activated Carbon Dosages and Different Initial pH Values. The initial chromic acid concentration was 100 Mas Cr and agitation was for 24 hr at 20c.

PAGE 89

z 0 100 ----;..__--------..----,------r----r---, r::, _J 0 U) 80 z <.9 z z 60
PAGE 90

FIGURE 5-9 Cr(III) Concentration Remaining in Solution at Different Activated Carbon Dosages and Different Initial pH Values. The initial chromic acid concentration was 100 Mas Cr. The curves represent the concentration of Cr(III) that was reduced from Cr(VI) and was not adsorbed on the activated carbon or filtered on 0.45 micron millipore filters. Agitation was for 24 hr at 20c.

PAGE 91

zlOO 0 r :) _J 0 80 U) z (9 z 60 z <:.( ""> .. w a: 40 ..---t=i ----\..... u I20 Z w 0 0::: w Q_ -pH 3 4 pH 4.0 200 400 600 800 1000 1200 1400 1600 ACTIVATED CARBON DOSE (mg/ I) 00 0

PAGE 92

81 produced by reduction of Cr(VI) The critical pH appeared to be 4.0. Maximum removal of CrT was observed at this pH and adsorption was increased proportionally with amounts of activated carbon applied. 5-1-4 .E!:!_ variation during adsorption of Cr(VI) and its effect on Cr(VI) removal. In Fig. 5-10 the final pH values are plotted against activated carbon dosage for several different pH values An increase in pH was always observed upon the addition of activated carbon to a Cr(VI) solution. It can be seen from Fig. 5-10 that when the initial pH was between 4.0 and 5.0, the final pH exceeded 6.0, the value depending on the amounts of activated carbon used. 'This phenomenon leads to an important question re garding the true adsorption of Cr(VI) by activated carbon. Cr(VI), once reduced, can be either adsorbed on activated carbon or pre cipitated out as the final pH reaches 6 or higher. The solubility of Cr(III) is greatly reduced in a neutral or slightly alkaline solution. Under this environment adsorption of Cr(III) on activated carbon would be enhanced. Therefore it is difficult to quantita tively calculate the total arnoWlt of Cr(VI) reduced during the period of reaction or to calculate the percentage of the reduced Cr(III) which is actually consumed by adsorption. Fortunately this problem is only confined to a narrow region of initial p To illustrate this clearly Cr(III) in Fig. 5-2 was replotted against final pH in Fig. 5-11 as Curve 1. The results of the reaction of 20 M solution of Cr(III) with 200 mg/1 of activated

PAGE 93

FIGURE 5-10 Relationship Between pH Rise and Activated Carbon Dosage. Solutions of 100 M of Cr(VI) at different pH values were reacted with various dosages of activated carbon. Final pH's were plotted against activated carbon dosage. Agitation was for 24 hr at 20c. Chromic acid was the source of Cr(VI).

PAGE 94

83 8r-----.---,------,-----r---7 IN \ TIAL pH: 5.0 4.5 4.0 I 6 (L 2 ::) 5 (1) _J ::) 4 3.4 G w 3 2.5 200 400 6CO 800 1000 ACTIVATED CARBON DOSAGE (mg/l)

PAGE 95

FIGURE 5-11 Cr(III) Concentr-ation Remaining in Solution as a Function of Equilibrium pH. Activated carbon dosage was 200 mg/1 and agitation was for 24 hr at 20c. Curve 1 shows Cr(III) re duced from Cr(VI). The initial potassium di chromate concentration was 20 Mas Cr. Curve 2 shows Cr(III) remaining after contact with activated carbon. The initial Cr(N0 3 ) 3 concentration was 20 Mas Cr.

PAGE 96

85 20.-----,----.---:--=~-----~ .1 16 2 12 3 8 4 2 INTIAL pH 0 INTIAL p7 4.7 3 4 5 6 7 E(;)UIL!BR!UM pH

PAGE 97

86 carbon from a separate experiment are also included in this figure as Curve 2. By comparing the two curves it can be confirmed from Curve 1 that for the Cr(VI) conc,entration of 20 M and activated carbon dosage of 200 mg/ 1, reduction of Cr(VI) to Cr(III) was nonexistent at the initial pH of 4.7 or higher. Adsorption of re duced Cr(III) did not take place below the initial pH of 4.1. Furthermore, Cr(III) produced in small amounts by reduction at pH values between 4.1 and 4.7 tends to remain in solution, since at this pH range Cr(III) was not adsorbed to any great extent by activated carbon as shown from Curve 2, Fig. 5-11. From Fig. 5-4 the critical pH was 4. 7 for the 20 M initial Cr(VI) concentration and a high activated carbon dosage. It is seen, however, from Fig. 5-7 that this initial critical pH shifted from 4.7 to about 4.0 as the initial Cr(VI) concentration increased from 20 to 100 M. Finding this critical initial pH value for any given concentration of Cr(VI) is extremely important because ad sorption is greatest at this pH. Ineffective adsorption results above the critical pH because of poor adsorption of Cr(VI) on the activated carbon, whereas below the critical pH, reduction hinders adsorption. 5-1-5 1:1 proton to Cr(VI) molar ratio adsorption. In order to determine the critical pH for adsorption, a series of experiments were performed where the initial pH and the ratio of initial (Cr(VI) to activated carbon were held constant while the initial concentration of Cr(VI) was changed. The results are shown in

PAGE 98

87 Fig. 5-12. Total chromium adsorption increased to a maximum with increasing pH up to an initial Cr(VI) concentration of 300 M, after which adsorption decreased. Fig. 5-12 indicates that the total amount of chromium adsorbed was a direct function of the initial proton activity in the solution and was maximum at a mole ratio of proton to Cr(VI) of 1.0. In all cases the proton activity was measured with a pH meter and was very closely equal to the proton concentration due to the low ionic strength of all the ex periments. When the Cr(VI) concentration exceeded the proton activity (initial Cr(VI) greater than 300 M), adsorption of CrT exceeded the number of protons available. Tnis adsorption is known as hydrolytic adsorption and will be discussed in a later section of this chapter. In order to study the effect of the pro ton to Cr(VI) ratio on adsorption, a series of experiments were conducted where the initial proton to Cr(VI) ratio was set equal to 1.0 at a constant Cr(VI) to activated carbon ratio. The re sults are shown in Fig. 5-13. The chromium was introduced by two methods: the first using chromic anhydride; the second using potassium dichromate and controlling initial pH with hydrochloric acid. When chromic anhydride was dissolved in an aqueous solution, the proton concentration was equal to the Cr(VI) concentration, resulting in a 1 to 1 proton to Cr(VI) molar ratio. The result, shown in Fig: 5-13 indicate Cr(VI) adsorption was almost inde pendent of pH as long as a 1 to 1 proton to Cr(VI) molar ratio condition was met. This finding leads to the conclusion that

PAGE 99

FIGURE 5-12 Proton and CrT Adsorption as a Function of Initial Cr(VI) Concentration. The initial pH was 3. 52 (H = 300 M). the initial Cr(VI) to activated carbon ratio was O. 33 mole/mg, and potassium dichromate was the source of Cr(VI) with hydrochloric acid used for pH control. Agitation was for 24 hr at 20c.

PAGE 100

80 z 0 r0... 60 0::: 0 (f) 0 <( ._ 40 z w 0 a: w 0... 20 0 0 100 [H] 00 \0 200 300 400 500 600 INITIAL Cr (fil) C0NCENTRATI0N(,uM) f

PAGE 101

FIGURE 5-13 Adsorbed Chromium as a Function of Initial Proton Concentration. The initial proton to Cr(VI) molar ratio was equal to 1.0, and the initial Cr(VI) to activated carbon ratio was 0.50 mole/mg.Agitation was for 24 hr at 20c.

PAGE 102

2000----------------::l 1600 .. 0 w m 1200 0::: 0 (J) 0 \D <( Chromate 800 ::) z O 0:: I 400 0 _J
PAGE 103

92 adsorption of Cr(VI) takes place while the Cr(VI is in the chromic acid form, either as f\ Cr0 4 or as H 2 Cr 2 0 7 Both forms con tain a 1 to 1 proton to Cr(VI) molar ratio as bichromate or di chromate ions. The slight difference in shape between the two straight lines in Fig. 5-13 was due to the foreign anion (Clion) in the potassium dichromate case, competing with HCrO for ad4 sorption on activated carbon. The proton was also taken up by activated carbon along with Cl to satisfy the overall electro neutrality of the solution. The undesirable tie-up of H+ with Cl_ has a twofold effect on Cr(VI) adsorption on activated carbon. First, the adsorbed acid (in this case hydrochloric acid) occupies the active sites on the activated carbon which otherwise are available for chromic acid. Second, the adsorbed acid creates a deficiency in the proton to Cr(VI) ratio. For these reasons a pH value slightly lower than that corresponding to [Cr(VI)]/[H+] = 1.0 s indicative of the best pH for the adsorption of chromium when the + solution containing ions other than H and HCr0 4 is to be treated. This phenomenon is illustrated in Fig. 5-14. The best adsorption of Cr(VI) was at a pH close to 3.7 for Cr(VI) when initially in the Cr0 3 form, whereas 3.55 was the pH for best adsorption of Cr(VI) when initially in the K 2 Cr0 4 At a 200 M concentration of initial Cr(VI), 3.7 was the pH where the proton concentration was equal to the Cr(VI) concentration.

PAGE 104

FIGURE 5-14 CrT Concentration Remaining in Solution as a Function of Initial pH. The initial Cr(VI) concentration was 200 Mas Cr. The activated carbon dosage was 500 mg/1 and agitation was for 24 hr at 20c. Nitric acid was used for pH control in the potassium dichromate case.

PAGE 105

.,.......,_ 2 .:J. 80 ..._,, z 0 I. :) 60 _J 0 V) \0 z (9 40 z z <( 20 w 0:: ,-., L 3.0 3.2 3.4 3.6 3.8 4.0 4.2 u L..J INITIAL pH

PAGE 106

95 5-1-6 Hydrolytic adsorption. When a solution contains chromate in excess of protons, activated carbon adsorbs Cr(VI) in excess of available H+. To investigate the mechanisms which are responsible for this type of adsorption, a series of different concentrations of activated carbon were added to chromate solutions prepared with only K 2 Cr 2 0 7 and deionized water. Consequently the solutions had very low proton to Cr(VI) ratios. Results of the experiments are presented in Fig. 5-15. It can be seen from Fig. 5-15 that activated carbon did not adsorb Cr(VI) and potassium in an equal molar ratio even though the dichromate solution was in the KHCr0 4 form. Carbon adsorbed substantially more Cr(VI) than K. This highly favorable adsorption of Cr(VI) over potassium ions was caused by the following hydrolysis: (5-1) This type of hydrolysis was not likely to occur by itself but was probably produced by continuous removal of H 2 Cr0 4 from solution because of favorable adsorption of Cr(VI) by activated carbon. The small amounts of potassium were adsorbed either as KHCr0 4 or K 2 Cr0 4 The KOH produced reacted with unadsorbed KHCr0 4 to fonn K 2 Cr0 4 which in turn was hydrolyzed as follows: = 2KOH + H CrO 2 4 (5-2)

PAGE 107

FIGURE 5-15 Chromium and Potassium Adsorption at Varying Dosages of Activated Carbon. The initial proton to Cr(VI) molar ratio was signifi cantly less than 1. 0. The initial Cr(VI) concentration was 100 M, and the initial Cr(VI) concentration was equal to the potassium concentration. The initial pH was 5.18 and agitation was for 24 hr at 20c.

PAGE 108

97 l 00 r----r----~--.,.----...---80 z Q Ia.. 0:: 60 Chromium 0 CJ) 0 <( Iz 40 w u a:: w 0.. 20 Potassium0 '----...1.--..t~=~---1------L----' 0 200 400 600 BOC 1000 ACTIVATED CARBON DOSE (mg/I)

PAGE 109

98 The amounts of Cr(VI) adsorbed by hydrolytic adsorption may be calculated from the following mass balance equation: Cr(VI)hy = Cr (VI) d (KHCr0 4 + K 2 Cr0 4 ) d Cr(VI) ta a nyh (5-3) where the subscripts used in the above equation are ad= adsorbed, hy = hydrolytically adsorbed, tad= total adsorbed, and nhy = non hydrolytically adsorbed. Nonhydrolytically adsorbed Cr(VI) was equal to the initial H+ in solution. To exemplify an actual calcu lation the adsorption curves in Fig. 5-15 are used. At a carbon dosage of 500 mg/ 1, the total adsorbed Cr(VI) was 70 i.iM ( 70% ad sorption), of which adsorption of Cr(VI) as KHCr0 4 accounted for 3 i.iM as shown in the lower curve in Fig. 5-15. Nonhydrolytic ad sorption was approximately 6 i.tM, which was equal to the initial hydrogen ion concentration of the solution. Therefore, from Eq. 5-3, 61 M of total adsorbed Cr(VI) can be accounted for by hydrolytic adsorption. Hydrolytic adsorption is a costly process requiring an excess amount of activated carbon. It can be seen from Pig. 5-15 that 100 mg/1 of activated carbon was required to remove only 80 per cent of Cr(VI) when the initial Cr(VI) was 100 i.iM. 5-1-7 Chromic acid adsorption isotherms. Isotherm curves were + prepared for solutions having different ratios of Cr(VI) to H. Fig. 5-16 shows Freundlich isotherms plotted at a proton to Cr(VI) molar ratio of 1.0, and Fig. 5-17 shows similar plots at Cr(VI) to

PAGE 110

FIGURE 5-16 Freundlich Isotherms of CrT Adsorption at an Initial Proton to Cr(VI) Molar Ratio of 1.0 Agitation was for 24 hr at 20c. o = pH 4.0, ~=pH 3.7, D = pH 3.3.

PAGE 111

30r-----.-------,--------------------20 0 S=0.100 cr 0 50 / >< ,--..... (J) --(./) o.504 Q) 10 S=0.106 Cr 0 8 2 0 0 ...__,,, l/) 6 w 0.504 ~S=0.0S-8 Cr 4 i 3 Q_ =:) 2 3 4 6 8 10 CrT (M) x 10 5 20 40

PAGE 112

FIGURE 5-17 Freundlich Isotherms of CrT Adsorption at a Fixed Initial Cr(VI) Concentration of 100 M. The dashed line is reproduced from the top line of Fig. 5-16. Agitation was for 24 hr at 20c.

PAGE 113

~o X _..._. CJ) ---
PAGE 114

103 proton ratio not equal to 1.0. The three parallel lines in Fig. 5-16 indicate a similar type of reaction took place. The differences in slope and shape between curves in Fig. 5-17 indicate that dis tinctly different mechanisms of adsorption were occurring as the initial proton to Cr(VI) molar ratio varied. 5-1-8 Rate of adsorption and reduction. Figs. 5-18 through 5-20 summarize the results of the reaction of a 20 M solution of Cr(VI) with 350 mg/i of aciivated carbon under different initial pH values. Chromiwn concentrations remaining in solution were measured at different time intervals. It is seen from Fig. 5-18 that adsorption of chromium was nearly complete after the contact time of 2 hrs. It is also noted that total chromium adsorption reached peaks and declined steadily with time at initial pH values of 3.45 and 2.5, indicating a process of desorption once adsorption was complete. From Figs. 5-19 and 5-20 the desorbed species was found to be Cr(III) because Cr(VI) was not found in solution after a 2-hr contact time. Apparently adsorbed Cr(VI) left the activated carbon as Cr(VI) was slowly reduced to Cr(III) inside activated carbon. The reduction was the result of acid adsorption other than chromic acid when H+ was present in excess of a concentration equal to Cr(VI). The adsorbed acid caused the surface of the activated carbon to have an acidic nature under which the reduction of adsorbed Cr(VI) occurred. Fig. 20 shows that the rate of reduction is pH dependent requiring a low pH for rapid reduction.

PAGE 115

FIGURE 5 18 CrT Removal as a Function of Contact Time at Different pH Values. Activated carbon dosage was 350 mg/1, and the initial Cr(VI) concentration was 20 M. The tempera ture was 20c. Potassium dichromate was the source of Cr(VI).

PAGE 116

100 r.t0 so LL pH 3.4 0 _J 60 0 2 w f--' 0:: 0 Ul f40 z I ~ w B u pH 2.5 f5 20 (l_ 0 4 8 12 16 20 24 CONTACT Tl ME ( hr)

PAGE 117

FIGURE 5-19 Concentration of Cr(III) Formed as a Function of Contact Time at Different pH Values. The initial potassium dichromate was 20 Mas Cr. The activated carbon dosage was 350 mg/1 and the temperature was 20c.

PAGE 118

20 ......... 2 :J 16 pH 2.5 0 w 2 12 cc 0 I-' LL 0 ---I ,--, 8 ,.._ -..._,, L u pH 3.45 LJ 4 pH 4.5 0 4 8 12 16 2 24 CONTACT TIME (hr)

PAGE 119

FIGURE 5-20 Cr(VI) Concentration Remaining in Solution as a Function of Contact Time at Different pH Values. The initial Cr(VI) concentration was 20 M. Potassium dichromate was the source of Cr(VI). The activated carbon dosage was 350 mg/1 and the temperature was 20c.

PAGE 120

,_ 220r----------------3 z Q15 I=> _J o V) 12 z (9 z 8 z <( w 4 n:: > ...._.. L U 0 L-1 4 8 12 16 20 24 CONTACT TIME (hr)

PAGE 121

llO 5-1-9 Oxidation of Cr(III) to Cr(VIJ by activated carbon. It has been discussed in an earlier section of this chapter that under the acidic condition, activated carbon is capable of re ducing Cr(VI) to Cr(III). Activated carbon also can oxidize Cr(III) to Cr(VI) unde~ alkaline conditions. Activated carbon was -5 able to oxidize a 2.05 x 10 M solution of Cr(III) at a pH of -6 4 12.92, and a 5 x 10 M solution at a pH of 11.92 when a 2 x 10M solution of Cr(III) was reacted for 24 hrs with 1,000 mg/1 of activated carbon. Blank tests were also run to insure that oxidation of Cr(III) to Cr(VI) was indeed caused by activated carbon and not simply by the presence of oxygen. No Cr(VI) was found in the blank solution of Cr(III) in the presence of oxygen only. The increase in oxidation capacity with increasing pH is closely related to the oxidation potential of chromium. Electrochemically, Cr(VI) is a more stable form of chromium under alkaline conditions and less stable under acidic conditions. The opposite is true for Cr(III). Both are quite stable at neutral pH. 5-1-10 Effects of oxidizing agents on Cr(VI) adsorption by activated carbon. The fact that adsorption of chromium is a maxi mum at the pH where reduction begins to occur has been discussed. It was the purpose of this phase of the study to try to increase the adsorptive capacity of activated carbon for Cr(VI) by curbing the reduction of Cr(VI) to Cr(III). A number of oxidizing agents such as hydrogen peroxide, potassium permanganate, potassium persulfate, ammonium persulfate, and potassium periodate

PAGE 122

111 were added to Cr(VI) solutions and reacted with activated carbon. Results of these tests were similar to the earlier tests, and no increase in adsorptive capacity was observed. The oxidizable portion of activated carbon appears to be impor tant in Cr(VI) adsorption. In order to further investigate this effect, activated carbon was pretreated with potassium persulfate to oxidize the active sites so that reduction of Cr(VI) by activated carbon would be minimized. It can be seen from Fig. 5-21 that the activated carbon treated with an oxidizing agent modified its characteristics in such a way that adsorptive capacity was less dependent on pH values, resulting in almost identical adsorptive capacities over a wide pH range. Active sites in activated carbon which were destroyed (oxidized) by the oxidizing agent appeared to be the location where reduction and adsorption took place. It can be seen from Fig. 5-22 that the activated carbon treated with potassium persulfate lost much of its reducing ability as compared with virgin activated carbon. 5-1-11 Effects of salts on Cr(VI) adsorption. In order to in vestigate effects of salts on the chromium removal by activated carbon, activated carbon was reacted with a series of 100 M Cr(VI) solutions having different salt concentrations. Test re sults presented in Fig. 5-23 show that salt concentrations up to SO and 100 times greater than the initial Cr(VI) concentration decrease the adsorptive capacity of activated carbon by 5 and 10 percent respectively. The decreased adsorption was due primarily

PAGE 123

FIGURE 5-21 Total Chromium Remaining in Solution as a Function of pH Open circles repre sent the reaction of 500 mg/1 of fresh activated carbon with 100 M of Cr(VI) for 24 hr at 20c. Open triangles repre sent identical experimental conditions with activated carbon pretreated with 0.001 M of potassium persulfate at pH 3.75.

PAGE 124

113 _:1 OOr------r----~----,----r------. 2 36 so ::> _J ~60 z ~40 z <( 2 20 w cc ,......, u L.J ~FRESH CARBON 3.0 3.5 4.0 4.5 5.0 INITIAL pH

PAGE 125

FIGURE 5-22 Cr(III) and Cr(VI) Concentrations Remaining in Solution as a Function of Initial pH. Open circles represent the reaction of 500 mg/1 of fresh activated carbon with 100 M of Cr(VI) for 24 hr at 20c. Open triangles represent the identical experi mental conditions with activated carbon pre treated with 0.001 M of potassium persulfate at pH 3.75.

PAGE 126

llS 1CX)r---,-----,-----,------,,---2 80 ::::t 0 w60 2 0:: 0 LL 40 ........., L 20 0 FRESH CARBON 3.0 3.5 4.0 4.5 5.0 INITIAL pH

PAGE 127

FIGURE 5-23 Percentage Cr(VI) Removal by Activated Carbon at Various Ionic Strength Solutions of Two Different Salts. Activated carbon dosage was 500 mg/1, and the initial Cr(VI) concentration was 100 Mat pH 3.75. Agitation was for 24 hr at 20c.

PAGE 128

117 100r-----.-----r----r-----.---fb so LL o _J i 60 0 w o:: 40 fz w 20 w (l_ 0 NaCl 2 4 6 8 10 SALT CONCENTRATION (mM)

PAGE 129

118 to unadsorbed Cr(VI) as shown in Fig. 5-24. It can be seen from Fig. 5-24 that the reduction of Cr(VI) was greatly reduced with increasing salt concentrations. 5-1-12 Adsorption of chelated Cr(III) on activated carbon. Hexavalent chromium docs not exist as a free metal ion. It coordinates with oxygen to fonn Cr(VI)-oxygen complexes. Chelation of Cr(VI) with chelating agents is therefore not possible. Trivalent chromium has a strong tendency to form coordination compounds with molecules or anions containing free pairs or electron (bases). A series of experiments was performed to study the effects of chelated Cr(III) adsorption on activated carbon. Two chelating agents, EDTA and NTA,were used for these experiments. For the purpose of comparison EDTA and NTA were studied individually. Results of these experiments are plotted in Figs. 5-25 and 5-26. From the figures it can be seen that chelated chromium adsorbs Cr 3 + differently than free ions or the separate chelating agents. The low adsorption at neutral pH, when compared to the results shown in Fig. 5-11, indicates that chelated Cr(III) was soluble in this pH range. Attempting to increase adsorption of Cr(III) on activated carbon by means of chelation with EDTA or NTA appears to be effective at pH values below 6. However, this technique has an inherent drawback in that Cr(III)-EDTA or Cr(III)-NTA complexes are thermodynamically stable but kinetically slow to form, requiring elevated temperature to increase the reaction rate.

PAGE 130

FIGURE 5-24 Cr(VI) and Cr(III) Concentrations Remaining in Solution as a Function of the Ionic Strength of Calcium Chloride. The activated carbon dosage was 500 mg/1, and the initial Cr(VI) concentration was 100 Mat pH 3.75. Agitation was for 24 hr at 20c.

PAGE 131

120 25 ,,-... 2 ::!. .......... (!) 20 z z <( 15 2 1 W e::: r-, 10 _.... > .......... L u c6 5 ,,.--.. ~ L u L-J 0 2 4 6 8 10 CaCl2 CONCENTRATION (mM)

PAGE 132

FIGURE 5-2S Concentrations of EDTA-Cr and EDTA Remaining in Solution as a Function of the Equilibrium pH. The initial concentrations were 20 Min all cases. Activated carbon dosage was 200 mg/1. Agitation was for 24 hr at 20c. The Cr(III) curve is reproduced from Fig. 5-11.

PAGE 133

122 20 2 .:J.. ......_.. z 0 16 er: Iz 12 w u z 0 u 8 2 ::) er:: m 4 _J ::) G 02 w 4 6 10 12 EQUILIBRIUM pH

PAGE 134

FIGURE 5-26 Concentrations of NTA-Cr and NTA Remaining in Solution as a Function of the Equilibrium pH. The initial concentrations were 20 Min all cases. Activated carbon dosage was 200 mg/1. Agitation was for 24 hr at 20c. The Cr(III) curve is reproduced from Fig. 5-11.

PAGE 135

124 ........... 2 20----~~--r----.----,--, =t ---z 0 116
PAGE 136

125 5-1-13 Adsorption of Cr(VI) in comparison with other acids. --~ A family of curves is presented in Fig. 5-27 to show the + different rates of H adsorption in the presence of varying conjugate bases. More than 90 percent adsorption of all acids occurred within 1 hr. A very high adsorptive capacity for the chromic acid-potassium salt system was observed. This indicates a strong interaction between chromic acid and activated carbon. Direct comparison of adsorptive capacity of the chromic acid system with other strong acid systems is difficult. Unlike the mono(di)protic acids tested in this experiment, HCr0 4 and Cr 2 0 7 ions exist in different ratios depending on the prevalent pH and Cr(VI) concentration remaining in solution. From Fig. 5-27 it can be seen that the divalent H 2 S0 4 -K 2 S0 4 system had the lowest adsorptive capacity of the acids. Parks and Bartlett 76 reported on the decrease in activated carbon adsorptive capacity of sulfuric acid in the presence of sodium sulfate. This phenomenon can be accounted for by the following reaction. (5-4) According to the authors the above reaction took place to a con siderable degree and increased with increasing sodium sulfate con centration. Consequently, the higher the concentration of NaHS0 4 the fewer protons in the solution available for adsorption.

PAGE 137

FIGURE 5-27 Rates of Adsorption for Various Acids in the Acid-Acid Salt System. The initial acids and their potassium s a lt concentra tion were 200 Mand 0.01 N respectively Activated carbon dosage was 300 mg/1 and the reaction was for 24 hr at 20c.

PAGE 138

127 H 2 Cr0 4 KHCr0 4 16 2 :t ......., 0 12 HN0 3 -K N03 u
PAGE 139

128 In Cr(VI) adsorption by activated carbon, acid adsorption appears to be an important factor to be considered when acid is used to lower the pH. Several different acids were used to lower the pH of the chromate solutions in order to find the effect of these acids on the adsorptive capacity of activated carbon for Cr(VI). Fig. 5-28 summarizes the results of this study. This figure suggests that hydrochloric acid should be used for pH adjustment for Cr(VI) removal. The activated carbon had a lower adsorptive capacity for hydrochloric acid compared to nitric or sulfuric acid. Although sulfuric acid had the lowest adsorption of the three acids in an acid salt system (see Fig. 5-27), Fig. 5-28 indicates that sulfuric acid had the most adverse effect on Cr(VI) adsorption. This apparent anomaly sterns from the fact that the adsorptive capacity of activated carbon for sulfuric acid is apparently increased in the absence of common anion salts. When HCr0 4 ions are present in solution in an equal concentration with other anions, the adsorptive capacity for Cr(VI) relative to the other anions can be calculated by using Fig. 5-28. As an + example, assume a solution at pH 3.7 had H, Cl, and HCr0 4 in equal concentrations of 200 M. After a 24-hr contact time with activated carbon the Cr(VI) concentration would have decreased from 200 M to 22 M, indicating that activated carbon adsorbed + 178 M. The same amount of H was also assumed to be consumed, since Cr(VI) was taken up by activated carbon in the chromic acid form. The final pH was 5.43 for this experiment. This pH value

PAGE 140

FIGURE 5-28 CrT Concentration as a Function of Initial pH. pH was controlled by the addition of different acids shown. Activated carbon dosage was 500 mg/1 and agitation was for 24 hr at 20c. The initial potassium di chromate concentration was 200 Mas Cr.

PAGE 141

130 120.----,---_,...,.-----t--------2 3 100 z 0 13 80 0 V) z 60 (9 z z
PAGE 142

131 + was equivalent to a proton concentration of 3.7 M. H adsorbed in excess of the amount required to form chromic acid was con sidered to be taken up by activated carbon as HCl. Therefore H+ adsorbed as HCl accounted for 18.3 M, 200 M minus the H+ ad sorbed as chromic acid and minus the reamining H+ in solution. The molar ratio of adsorbed Cr(VI) to Cl (178/18.3) about 10 to 1. Note that the above calculation was based on the assumptions: (I) no reduction took place, (2) Cr(VI) was present as HCr0 4 -, (3) no hydrolytic adsorption took place, and (4) no salts were adsorbed. These assumptions are valid at pH 3.7 where all species exist in equal molar concentration of 200 M. Adsorption phenomena of chromic acid on activated carbon are more complex than would be expected from monoor diprotic 72 acids. Snoeyink and Weber found that the adsorption of acids was limited by anions because of the negative surface potential of activated carbon. An increase in salt concentration would create anion pressure for more acid adsorption. This anion effect did not appear to hold for chromic acid, at least for the low residual proton concentration in this study. Experimental results given in Fig. 5-29 show that the adsorption capacities increased as salt concentration was increased in the region of residual H+ -5 activity greater than about 3.5 x 10 M. However, below a re+ -5 sidual H activity of 3.5 x 10 M the opposite seemed to be true. This contradicting phenomenon may be resolved by considering that amounts of adsorbed Cr(VI) cannot be directly computed from

PAGE 143

FIGURE 5-29 Effect of Salt Concentration on Adsorption Capacity for Chromic Acid. The initial chromic acid concentration was 200M and agitation was for 24 hr at 20c.

PAGE 144

20 "'J" 0 15 0) -. (l) 0 E 10 ...._,, 0 1 -3 r---2.5X 10 K 2 cr 2 0 7 -3 ---5x10 K 2 Cr 2 O 7 2 3 4 5 H+ ACT! VTY x 1 o 5 6 7

PAGE 145

134 the pH change before and after the reaction. As described in the previous section, pH variation depends not only on the quantity of OH produced by hydrolytic adsorption, but also on the concentration of HCr0 4 which remains in solution. HCrO = is a weak 4 conjugate acid of Cr0 4 -. The pH can only successfully be predicted from titration curves similar to the curve shown in Fig. 5-30. This is not the case for other strong monoprotic acids and their salt systems such as HCl-KCl and HN0 3 -KN0 3 The amount of adsorbed acid can be directly calculated from pH change, regard less of the amounts of salt concentration in solution. 5-1-14 Chromium desorption. Desorption studies were performed at different pH values with various acids and sodium hydroxide. It was found that alkaline desorption proved to be more effective than any acid desorption. By analysis it was found that the chromium desorbed in alkaline pH regions was in a hexavalent form, whereas trivalent chromium was the predominant species in the case of acid desorption. Fig. 5-31 summarizes the desorption results. Sulfuric acid was found to be the best desorbing acid down to a pH of 0.5. However, at no time was more than 40 percent of the adsorbed chromium desorbed using any acid, even at pH values below zero Using sodium hydroxide, more than 80 percent chromium was desorbed during the 24-hr contact period of the experiment. Sulfuric acid was sup e rior to nitric acid as a desorbing agent, and nitric acid was a better desorber than hydrochloric acid. These results follow from the data presented in Fig. 5-28 on the

PAGE 147

136 PERCENT MOLE Cr04 FORMATION 0 50 100 10-=-----,-----r----r----,-,.---, 9 8 I n... 7 6 4 8 12 16 ml KOH

PAGE 148

FIGURE 5-31 Desorption of Chromium from Activated Carbon at Different pH Values. The activated carbon initially contained 170 M Cr per g of carbon. Agitation was for 24 hr at 20c.

PAGE 149

80 2 0 ICl. 60 n:: 0 en .... w c.,:i 00 0 I40 z w u 0:: w a. 20 O'----'---_.;.....a-.1...____,;;::.a::....;....a ___ -'-__ -'__ __._ ___ ...__--.-J -2 0 2 4 6 a 10 12 14 INITIAL pH

PAGE 150

139 effect of these acids on adsorption of Cr(VI) by activated car bon. In alkali and acid desorption, two disimilar mechanisms of desorption are involved. In alkali desorption a conjugate base of chromic acid is replaced by a hydroxyl ion at the carboniurn and alkali interface within the activated carbon. Newly formed chromate leaves the activated carbon surface 9~cause of the low adsorbility of this salt on the activated carbon. In the case of acid desorption, acid is adsorbed on the surface of the activated carbon, and Cr(VI) undergoes reduction within the activated carbon under this acid condition. The reduced Cr(III) leaves the activated carbon because it is not adsorbed to any appreciable amount. Acids once adsorbed on the surface of activated carbon are not readily desorbed. Difficulty in desorbing acids from acti vated carbon is believed to be the result of the conjugate anion 68,72 71 being tightly held by activated carbon. Kolthoff could partially remove an adsorbed mineral acid by the addition of h 68) d p enol or armyl alcohol. Steenberg (Garten & Weiss ma1nta1ne that adsorption of acid by carbon was a physical process. He found that the acid adsorbed from a 0.02 N solution was able to be displaced by shaking the carbon with a solution immiscible with water. A study by Snoeyink and Weber 72 indicated that not all the acid was taken up by means of the chromene reaction (chemical 69 adsorption theory) postulated by Garten and Weiss. Approximately

PAGE 151

140 55 percent of the acid adsorbed at an equilibrium pH of 3.5 was displaced by adding phenol to the solution. Apparently activated carbon adsorbs acids physically as well as chemically. ~ -in order to determine if the adsorbed chromic acid behaved similarly to other acids such as HCl and HN0 3 tests were run where organic solvents were used as desorbers. Phenol, chloroform, toluene, and ethanol were used to desorb the adsorbed Cr(VI) from activated carbon which had been contacted with 0.01 M of Cr(VI). The amount of activated carbon used was sufficient to bring the final Cr(VI) concentration below 20 M. None of the above solvents were capable of desorbing Cr(VI). Desorption with distilled water also proved to be extremely difficult. These tests show that the affinity of chromic acid to activated carbon is extremely strong, indicating that adsorption appears to be chemical rather than physical, and can be best described by the chromene type of reaction. The high affinity of carbonium for the OHion appears to be one of the reasons why alkaline desorption was more effective than any other method. 5-1-15 Mechanisms of Cr(VI) adsorption: the Cr(VI) form within activated carbon. It is difficult to qualitatively determine the final species of adsorbed chromium within activated carbon. De sorption techniques are unsuitable since reduction as well as oxidation of chromium takes place, depending on the pH at which desorption is performed Using a sol vent to desorb the adsorbed chromium also is unsuccessful due to the incapability of any tested solvents for displacing the sorbed chromium

PAGE 152

141 Some factors that indicate that Cr(VI) probably is the perma nent form of chromium residing within activated carbon are (1) be low pH 5 practically no Cr(III) is adsorbed on the activated car bon; (2) adsorption o f Cr(VI) is a maximum when the Cr(VI) to H+ molar ratio is 1. 0, ind.icating that adsorbed chromium is in the chromic acid form; (3) a small amount of Cr(VI) could be desorbed upon prolonged contact with distilled water; and (4) when a solu tion containing Cr(VI) in a low Cr(VI) to H+ molar ratio was reacted with activated carbon, adsorption was poor and most of the unadsorbed chromium was in a Cr(III) form. Chromene type of reaction of Cr(VI) with activated carbon. Sincer Bartell and Mille/ 7 published the first systematic study of acid adsorption by carbon, there has been considerable re search on this subject. However, the exact mechanisms of acid adsorption are not clearly understood. Interaction of acids with activated carbon are complex, and contradicting results are often reported. Among many mechanisms proposed by numerous scientists, d 6 9 the chromene-acid reaction suggested by Garten an Weiss in 1957 is widely accepted, since it explains more details of the overall reaction more satisfactorily than other existing theories. The chromene-chromic acid reaction is schematically described in Fig. S-32(a,b). The very high adsorptive capacity of activated carbon for chromic acid, relative to other acids, probably results from the formation of polynuclear species of chromic acid. Chromic acid shows a marked tendency to form polyacids in a

PAGE 153

FIGURE 5-32 The Chromene-Chromic Acids Reaction. (a) Reaction with monochromic acid. (b) Reaction with dichromic acid. (c) Reaction with polychromic acid.

PAGE 154

X) ,,R C 'H 143

PAGE 155

144 concentrated solution by eliminating water. The chemical steps are shown in Eqs. 3-7 through 3-9 in Chapter 3. A more highly polymerized molecule with a formula (CrO) with n greater than 4 n 4, may form depending on the concentration of chromic acid in aqueous solution. Toes~ polymerized molecules may react with chromene groups in activated carbon as shown in Fig. 5-32(c). The physical and chemical conditions within activated carbon are much different from that of the bulk solution. An extremely high chromic acid concentration prevails in a rather disturbed state of water molecules. Under this environment polymerization of chromic acid would be expected to be enhanced. By polymerization, hydrogen atoms in chromene can be displaced by l/2(Cr 0 3 1 ) inn n+ stead of a single anion as in HCl or HN0 3 thus increasing the capacity of activated carbon for chromic acid adsorption. As re ported by Shepherd and Jones 5 this is similar to the doubling ex change capacity of the anion resin for chromate when forming polychromate species. The possibility of chromic acid anhydride formation within the carbon cannot be ignored, particularly when activated carbon is saturated or near saturation with Cr(VI). Other theories such as physical, chemical, and electrostatic adsorption cannot be totally excluded. The difficulty in desorbing the sorbed Cr (VI) with sol vents which are highly reactive with activated carbon indicates that a good deal of Cr(VI) adsorption results from chemical adsorption, probably some kind of chromene chromic acid reaction.

PAGE 156

145 5-2 Column Studies 5-2-1 Factors affecting column breakthrough: E!:!_. A series of column tests were run at a constant retention time of 8.5 min for a feed solution containing 104 mg/1 (2 mM) of Cr(VI) under varying pH values. The retention time was the actual residence time that an average element of liquid spent inside the column. Its value was the void volume of the column divided by the column volumetric flow rate. From the results plotted in Fig. 5-33 it can be seen that the maximum CrT removal was at pH 2. 8. A pH of 2.7 was the pH where the molar Cr(VI) concentration was equal to the proton concentration. Unfortunately, no test was conducted at this pH. At pH values 3.1 and 3.4, Cr(VI) leakage was experienced almost instantly. No Cr(III) was detected in the column effluent. It can be seen from Figs. 5-34 and 5-35 that at the influent pH of 2.5 no Cr(VI) was found in the column effluent up to a throughput of approximately 400 empty-bed columns, and effluent chromium was all in the Cr(III) form. The reduction of Cr(VI) was accompanied by the production of carbon dioxide. Activated carbon was consumed in this process as expressed in the following chemical equation: aI-t + bHCrO 4 + cC (5-5) Carbon dioxide production was detrimental to Cr(VI) adsorption be cause it not only destroyed the activated carbon but also tended

PAGE 157

FIGURE 5-33 Total Chromium Concentration in the Colwnn Effluent vs. Dimensionless Empty-Bed Volume Throughput at Several Different pH Conditions. The feed solution had a Cr(VI) concentration of 104 mg/1. Potassium dichromate was the source of Cr(VI). The pH was controlled by adding sulfuric acid. Retention time was 8.5 min at 20c. Each column contained 16.5 g of activated carbon.

PAGE 158

100 -.. 0) 80 E ..---, c..t60 Ll z w 40 ::) LL LL w 20 0 100 pH 3.4 pH3.1 200 pH ~.B 300 400 500 EMPTY BED VOLUME THROUGHPUT 600

PAGE 159

FIGURE 5-34 Cr(VI) Concentration in the Column Effluent vs. Dimensionless Empty-Bed Volume Throughput at Several D1fferent pH Conditions. The feed solution had a Cr(VI) concentration of 104 mg/1. Potassium dichromate was the source of Cr(VI). The pH was controlled by adding sulfuric acid. Retention time was 8.5 min at 20c. Each column contained 16.5 g of activated carbon.

PAGE 160

100 ::::: 80 CJ) E 60 > L &d 140 z w =:> _J 20 w o 100 200 300 400 5 0 600 EMPTY BED VOLUME THROUGHPUT

PAGE 161

FIGURE 5-35 Cr(III) Concentration in the Column Effluent vs. Dimensionless Empty-Bed Volume Throughput at Several Different pH Conditions. The feed solution had a Cr(VI) concentration of 104 mg/1. Potassium dichromate was the source of Cr(VI). The pH was controlled by adding sulfuric acid. Retention time was 8.5 min at 200c. Each column contained 16.5 g of activated carbon.

PAGE 162

100 ,---.... -01 80 E ....__ r-, ---. =60 0 ...__ L Ll .._. V, .._. r40 z w =:) _j LL 20 LL w 100 200 300 400 500 600 EMPTY BED VOLUME THROUGHF UT

PAGE 163

152 to break loose an otherwise uniform carbon bed as gases rose up through the bed, creating channeling of incoming feed solution. Cr(III) concentrations in the column effluent appear to be in fluenced by column effluent pH values. This indicates the possi~ bility of Cr(III) precipitation on the carbon bed and Cr(III) re moval through a simple physical filtering process. As seen from Fig. 5-36 column effluent pH remained above 5.5 up to the break through point and then gradually dropped. The breakthrough point was defined as 5 mg/1 of total chromium with Cr(VI) no greater than 1 mg/1 in the effluent. The pH could be used as an indicator of the column performance at any time during the column run. Retention time. In Fig. 5-37 column effluent Cr(VI) concentra tions were plotted against the empty-bed volume throughput at several designated retention times. From Fig. 5-37 it can be seen that the retention time had a significant effect on the column chromium adsorptive capacity of activated carbon. The increase in adsorptive capacity with increasing retention time was probably due to the formation of time-dependent polynuclear complexed species. The breakthrough capacity of the column for a feed solu tion containing 104 mg/1 was found to be 0 125 g Cr per g activated carbon. The corresponding empty-bed throughput volume up to the breakthrough point was 520 at a retention time of 52 min. At a retention time of 12 min the column was able to treat 300 empty bed volwnes of feed solution before the effluent reached the breakthrough point. Breakthrough capacities at different retention times can be obtained from Fig. 5-38.

PAGE 164

FIGURE 5-36 Total Chromium Concentration in the Column Effluent vs. Dimensionless Empty-Bed Volume Throughput. The pH of the column effluent was also plotted against the empty-bed volume throughput. The column was run at 20c with a retention time of 23 min. The feed solution had a potassium dichromate concentration of 104 mg/1. The pH of the feed solution was controlled at 2.7 by adding hydrochloric acid. The column contained 16.5 g of activated carbon. D = pH values of column effluent, o = CrT, = Cr(VI).

PAGE 165

100 .......... ...___ 0) Eso ,-, c u ;Q 60 > '--' L &2 ~40 z w :=) CC 20 LL w < 100 200 300 400 500 EMPTY BED VOLUME THROUGHPUT 10 8 61 rz u, ..,. w 4:J _J LL LL w 2

PAGE 166

FIGURE 5-37 Cr(VI) Concentration in the Column Effluent vs. Dimensionless Empty-Bed Volume Throughput at Several Designated Residence Times. The feed solution had a potassium dichromate concentration of 104 mg/1 as Cr. The pH was controlled at 2.7 by adding hydrochloric acid. Each column contained 16.S g of activated carbon.

PAGE 167

1(X) -0) 80 E ,--, ; 60 L u L-1 r40 z w :) __J LL 20 LL w RETENTION TIME (1)-12 min (2)-23 min ( 3)-42 min (4 ) 52 min 0 100 200 300 400 600 700 EMPTY BED VOLUME THROUGHPUT

PAGE 168

FIGURE 5-38 Dependence of Column Breakthrough Capacity on Retention Time. This figure was plotted based on the experimental results from Figs. 5-34 and 5-37. Correla tion coefficient r = 0.95.

PAGE 169

0.16 2.5 >J........... u ........... 0.14 2.0 _c z ~o E <(CO ...._ UIY E <( 0.12 1.5 _, IU w l9 O> I=:) ...._ ..... OL <( (.n 00 0::: u 0.1 1.0 cc I~ 5 I0 <( __J 0.08 0.5 LL w 0::: Cl) 10 20 30 40 50 TRUE RETENTION TIME (min)

PAGE 170

159 Feed Cr(VI) concentration. The results of varying column feed concentrations are presented in Table 5-1. Table 5-1 also shows that the feed concentration did not appear to influence the Cr(VI) breakthrough capacity of activated carbon to any great extent. Acid used for .E!:!_ control. In order to find the effect of different acids on activated carbon column performance, three strong acids--hydrochloric acid, nitric acid, and sulfuric acidwere tested lillder otherwise identical conditions. In the case of pH control with sulfuric acid, the capacity of the activated carbon was found to be surprisingly low. From Table 5-2 it is seen that adsorptive capacity was reduced by as much as one third of the corresponding values found with nitric acid and hydro chloric acid used for pH adjustment. It has been discussed in Section 5-1-13 that reduced adsorptive capacity of activated car bon was observed with batch studies when sulfuric acid was used for the pH control. 5-2-2 Column regeneration. Caustic soda as well as sulfuric acid was used for regenerating spent activated carbon. When re generated with 0.5 N NaOH solution using 90 min of residence time, the effluent chromium concentration reached a peak of 10,000 mg/1 with pH values as low as 4.0 in the first 100 ml of spent regenerant. The low pH was due to neutralization of NaOP with chromic acid. After regeneration the activated carbon was rinsed with distilled water, 0.1 N l\S0 4 and followed once again by a distilled water rinse prior to a new column test.

PAGE 171

TABLE 5-1 Results of Column Performance for Varying Feed Cr(VI) Concentrations RetenEmpty-Bed Effluent Feed Proton tionVolume Chromium Percentage Chromium Solution Concen. Time Throughput Concen. Removal Loading (mg/1) (mM) (pH) (mM) (min) (dimensionless) Cr(VI) CrT Cr(VI) CrT (g Cr/g C) (mg/ 1) (mg/1) 104a 2 2.7 2 52 520 0.83 2.3 99.3 97.8 0.128 104a 2 2.7 2 49 392 0.52 5.7 99.5 94.5 0.091 260b 5 2.3 5 90 290 1.0 9.8 99.5 96.2 0.179 520a 10 2.0 10 87 147 2.7 43 99.5 91. 8 0 .180 NOTE: Each column contained 16.5 g of activated carbon. HN0 3 was used for pH adjustment. aPotassium dichromate was the source of Cr(VI). bChromic acid was the source of Cr(VI), and the carbon was regenerated with caustic soda. 0

PAGE 172

TABLE 5-2 Effects of Acids Used for pH Control on the Adsorptive Capacity of Activated Carbon Columns for Chromium RetenEmpty-Bed Effluent Feed Acids tion Volume Chromium Percentage Chromium Solution Used Time Throughput Cone. Removal Load pH (mg/1) (mM) (min.) (dimensionless) Cr(VI) CrT Cr(VI) CrT (g Cr/g C) (m /1 (rn /1) 104 2 HCl 17.3 378 3.4 7.28 96.7 93.0 0.091 2.7 104 2 HN0 3 17 .6 372 3.1 6.76 97.0 93.5 0.090 2.7 104 2 H 2 S0 4 16.7 254 3.8 10.4 96.3 90.0 0.061 2.7 I--' I--' NOTE: Each column contained 16.5 g of activated carbon.

PAGE 173

162 The acid washing was necessary to prevent an initial poor adsorbility of chromium that would have resulted from the still high pH condi tion after the initial rinse. The results of this study are pre sented in Table 5-1. The empty-bed volume throughput was 392 prior to the breakthrough point, indicating that activated carbon was able to adsorb 9.1 percent of chromium by weight and still main tained a column effluent Cr(VI) concentration below 1 mg/1. It can be shown from Table 5-1 that the adsorptive capacity of the re generated activated carbon for Cr(VI) was reduced by about 29 per cent that of fresh activated carbon. Strict comparison appears to be difficult due to the slightly different retention times em ployed in each run. The loss of adsorptive capacity was believed to result from the incomplete desorption of chromium and possible structural change within activated carbon during alkali regenera tion and acid washing. Acid regeneration with 0.5 N sulfuric acid solution using a 90-min retention time showed an average Cr(III) concentration of 2,440 mg/1 in the first 500 ml of spent regenerant and 1,500 mg/1 in the first 1,000 ml. Activated carbon used for the regeneration study contained 0.151 g of chromium per g of activated carbon. With acid regeneration,reduction of Cr(VI) to Cr(III) was accompanied by carbon dioxide gas production. The loss in the activated carbon from CO 2 formation was theoretically 0.173 g of carbon per g of Cr(VI) converted to Cr(III) However, other organic compounds present in activated carbon may also be subjected to oxidation by Cr(VI)

PAGE 174

163 It was felt that perhaps oxidation would enlarge the pore volume and increase the surface area, which in turn could increase the total adsorptive capacity of activated carbon. Unfortunately, this was not found to be the case. According to Hassler, 64 the action of an oxidizing agent on activated carbon involves a rather com plicated phenomenon and cannot be explained in this simple a manner. The ash content (inorganic constituents) of activated carbon is believed to be removed when washed with acids. 72 It was found that the decrease in adsorptive capacity of activated carbon for chromium was rather significant when activated carbon was re generated with acid. An acid-regenerated column operating at a retention time of 17 min was only able to treat 90 empty-bed volumes of a feed solution containing 104 mg/1 of Cr(VI). The virgin column treated 378 empty-bed volumes. The reduced adsorp tive capacity indicates that there were apparently some structural change caused by oxidation, such as destruction or disorder of the active sites on activated carbon. Oxygen content of the acti vated carbon would have been increased by oxidation. Increased oxygen content might render activated carbon properties and characteristics similar to the L-carbon (carbon activated at a low temperature). L-carbon tends to adsorb more alkali than acid. Iodine adsorption tests were performed on the regenerated activated carbon and on fresh activated carbon. Regeneration of activated carbon containing Cr(VI) was made by eluting nitric acid through it in such a way that the nitric acid concentration was progressively

PAGE 175

164 increased from 1 to 4 N to insure a high degree of desorption. The iodine test was conducted in accordance with the "Determina tion of the Iodine Number of Activated Carbon" by Pawlowski. 78 A decrease in iodine number by as much as 23 percent was found for the acid-regenerated activated carbon. Unfortunately, it was not possible to detennine to what degree the remaining chromium in the activated carbon affected the reduced iodine adsorption. The remaining chromium was, however, considered to be very small in quantity after such harsh treatment with acid. Kipling and 66 Shooter (Snoeyink & Weber ) indicated that chemisorption of oxygen may not be readily displaced by Cr(VI) when acid-regenerated activated carbon is reused for the adsorption of Cr(VI). In order to further investigate the nature of the chromium which could not be readily desorbed, the spent activated carbon was treated with 1 N NaOH until no further significant amount of chromium was desorbed. It was then subjected to heat treatment at 0 a temperature of 200 C for 2 hrs. The temperature was chosen so that chromic acid would not be either decomposed or transformed (mp = 197C) to a less stable form with respect to carbon. It was found that chromium which could not be desorbed by means of alkali desorption found its way out of the activated carbon when soaked in distilled water. There may have existed a stable double la; er film between the interface of chromic acid and the alkali medium within the activated carbon. In a quiescent state,molecular diffusion of chromic acid across this film may have been slow.

PAGE 176

165 This film may have been easily broken by the thermal energy supplied by heating, thus facilitating the neutralization of acid and alkali. Consequently, the salt (sodium chromate) that was produced was not adsorbable and was readily desorbed from activated carbon.

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6-1 General Remarks QIAPTER 6 ENGINEERING APPLICATIONS This study indicates that the application of activated carbon appears to be a very promising technique to remove Cr(VI) from water. Over 500 empty-bed volumes throughput of 104 mg/1 Cr(VI) can be achieved with a removal efficiency of over 95 percent prior to breakthrough. Most of the chromium present in the column effluent is in the Cr(III) form, which is known to be less toxic than Cr(VI). Considerably higher column performance could be achieved, depending on the incoming feed Cr(VI) concentration. In the chemical reduction method, pH as low as 2.0 should be maintained in order to insure a rapid reaction regardless of the concentration of Cr(VI) to be treated. For a large plant, the chemical reduction and precipitation method may require a large reaction and neutralization tank along with a huge settling basin. Sludge produced is mostly chromium hydroxide, a gelatinous sludge containing as much as 80 percent water. In an activated carbon colunm process most of the problems en countered in the chemical precipitation process can be eliminated. Optimum pH values for Cr(VI) adsorption vary depending upon the Cr(VI) concentration in the water. No pH adjustment may be 166

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167 necessary for a water having a low Cr(VI) concentration. The pH of the column effluent is between 5.5 and 7.5 during the operating period, requiring no pH adjustment prior to discharge or reuse. In addition, no precipitating-agent pollutant is discharged at the expense of Cr(VI) removal. Cr(VI) can be easily recovered by stripping the spent carbon with alkali. Although column studies indicate a great potential for future application of this process, the single most important factor appears to be control of retention time. Another important aspect to be considered in order to make this process more attractive for large-scale application is in the area of regenera tion. It was found in this study that although alkaline regenera tion was more effective than acid regeneration, complete desorp tion by this method was not possible. Thermal regeneration, which is a typical process of carbon regeneration for the removal of organic load in activated carbon, appears to be impractical because of the nonvolatility of chromium compotmds. Heat treatment, at a temperature of around 200c in the presence of alkali, was found to be very effective in desorbing much of the remaining chromium which was not completely removed by alkali treatment alone. It is felt that the activated carbon adsorption technique for Cr(VI) removal is most suitable for the treatment of water con taining a Cr(VI) concentration not more than 200 mg/1. Chemical reduction and precipitation methods may more effectively treat the water containing higher Cr(VI) concentrations.

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168 6-2 Process Design for Cr(VI) Removal by Activated Carbon A design was developed based on wastewater with a flow rate of 100 gal per min having a Cr(VI) concentration of 104 mg/1. A true retention time of 35 min was chosen. For this retention time, the breakthrough capacity was experimentally found to be 0.105 g Cr/g activated carbon from Fig. 5-38. A surface loading rate was 2 arbitrarily chosen to be 2 gpm/ft For the purpose of simplicity a single adsorption carbon column will be considered in this study. Actual calculations are found in Appendix 3. The results from these calculations are summarized in Table 6-1, and a schematic diagram of this process is presented in Fig. 6-1. The carbon column removes 1,848 pounds of Cr(VI) at the rate of 125 pounds per day until the breakthrough point is reached. The theoretical amount of caustic soda required to neutrali z e the adsorbed carbon is 2,843 pounds (1,848 x 40 x 2/52). In reality, however, a much ~reater amount of caustic soda would be required, for regeneration was estimated as 5,686 pounds, which was twice the theoretical requirement. About 17,000 gal of water would be required to make a 1 N regeneration solution This caustic SJlution could be delivered to the column at a surface loading ? rate of 1 gpm/ft-. About 17,000 gal of rinse water would be required prior to acid wash in order to remove the excess caustic soda. Even after alkali regeneration followed by rinsing with water, the pH of the regenerated activated carbon remains very high. An

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FIGURE 6.1 Schematic of the Activated Carbon Column Process for Hexavalent Chromium Adsorption

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pH 0~1~1 354Ib CONTROLL R STREAM H 2 s~ 0 1 OOGAL/ min Na OH ( 1 N 25.,600 GAL) '-----H2S04 (1N 18.,000 GAL) 1, 21 04 plm -cr+6 ~,-r,I I, RINSE WATER 3.0X 10 GAL HOLDING TANK 14 J'!-JGAL l MIXING TANK RINSE WATER--56,000 GAL Cr(VI) FREE -E'------.1 WATER C RECLAIMED Na2Cr04 \;' X 10 5 GAL CLARIFIER l 29 WITH A MIXING UNIT Cr(OH ) 3

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171 TABLE 6-1 Dimension of a Full-Size Carbon Adsorption Column and Chemical Requirements for Treating Chromate Wastewater (flow rate of 0 .144 MGD and waste strength of 104 mg/1) Bed volume Column diameter Bed depth Column height Amount of carbon required Acid requirement For the pH control For the acid wash Caustic soda requirement Amount of water required Frequency of regeneration 701. 8 ft 3 8 ft 14 ft 21 ft 17,600 lb 354 lb/day 3,900 lb 5,684 lb 109,000 gal 14.5 days acid wash step is therefore required to prevent initial difficulty in adsorption because of the prevailing high pH condition. An acid wash also has an additional advantage in that further Cr(VI) to Cr(III) would be brought about during the acid wash process. For the acid wash the amount of sulfuric acid required was estimated as 3,900 pounds. This amount was based on the acid re quired to neutralize the ~pent caustic regenerant. The water necessary for the acid wash was estimated as 9,600 gal. The sur face loading rate during acid wash would be 2 gpm/ft 2 The Cr(III) desorbed by acid washing can be precipitated out during a neutralization process. An additional rinse after acid wash is also required to wash out the excess acid present in the column, thus preventing reduction of Cr ( VI) to Cr(III) when the column is put back in operation. This last rinse could also pro vide a means of backwashing. Rinse water introduced to the bottom of the column would flow upward at about 25 gpm/ft 2 This flow rate

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172 is necessary to maintain 25 to 50 percent bed expansion. Approxi mately 75,000 gal of water would be needed for this step. Back washing would be helpful in keeping the column clean of solids and would thereby minimize head loss during operation. Spent regenerant solution could be almost completely neutralized by mixing it in a clarifier with the acid from the acid wash cycle. The settling of insoluble chromium hydroxide would be complete, since the detention time of this clarifier can be as long as 14 days until the next regeneration cycle. The amount of Cr(OH) 3 pro duced depends not only on the amount of the undesorbed Cr(VI) during alkali regeneration, but also on the effectiveness of the acid wash. Generally, only a small quantity of Cr(III) is produced compared to Cr(VI). The settled sludge composed mostly of Cr(OH) 3 may be either disposed of or reclaimed after dewatering. The supernatant of the neutralization step contains some chromate. This chromate solution may be reintroduced to the column after proper pH adjustment or may be destroyed by the conventional reduction process. A 0.3-million-gal holding tank would be neces sary to store wastewater for 2 days during the regeneration period or any unexpected down-time. The material for constructing the column should be corrosion resistant because of the acidic nature of the water to be treated. For small columns up to 12-ft diameter, shop-fabricated steel vessels with coal-tar epoxy internal coating would be most eco nomical. Above 12-ft diameter, field-fabricated concrete construc79 tion is competitive and offers lower maintenance costs.

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173 6-3 Disposal of Spent Activated Carbon It may be more economical in a small plant to dispose of the spent activated carbon instead of regenerating it Spent activated carbon may be either incinerated or buried. Prior to disposal of the spent activated carbon, the chromate may be reclaimed by eluting it with alkali until the chromate solution is too dilute to reuse.

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CHAPTER 7 ECONOMICS OF Cr(VI) REMOVAL BY ACTIVATED CARBON Calculations of the cost for Cr(VI) adsorption are based on the process design in Chapter 6. Chemical costs were based on the "Chemical Marketing Report" values for 10 November 1975 and are summarized in Table 7-1. Costs of activated carbon can vary in a wide range depending on the supplier and their adsorptive capacity. Therefore a preliminary study should be made prior to the selection of any particular type of activated carbon. The selection of acid for pH control of wastewater is an important parameter to be considered, since it affects the adsorptive capacity of activated carbon. Hydrochloric acid is better than sulfuric acid in this aspect. However, it is more expensive, more corrosive than sulfuric a~id, and its high water content makes it more difficult to handle. For this reason, sulfuric acid is considered in this cost calculation. Capital and operating costs listed herein have all been adjusted 3 to 1975 values. The estimated cost for a column size of 1,050 ft is $85,000. This cost includes the contractor, piping, valves, 21 storage tanks, building costs, and instrumentation costs. In addition, a clarifier ($10,000), a holding tank and mixing tank ($15,000), and activated carbon ($8,800) are required. The total construction cost is calculated to be $118,800. To this construction 174

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175 TABLE 7-1 Cost of Chemicals Used in Cr(VI) Removal by Activated Carbon Adsorption Sulfuric acida a Caustic soda, 76% Na 2 0 basis Chromic acid,a 99.75% Sodium chromate,a anhydride Activated carbonb $SO per ton $8 per 100 pounds $0.58 per pound $0.29 per pound $0.50 per pound a"Chemical Marketing Report'~ (10 November 1975). bEstimated. cost must be added engineering costs, legal expenses, administra tive costs, land acquisition, and interest expenses. These items are estimated to be 20 percent of the construction cost. 21 The total capital cost is then estimated as $142,560. The operating cost must include the cost of chemicals, labor, maintenance, and amortization. The net operating cost is the sum of these items minus the cost of recovered chromium. The estimated total costs are listed in Table 7-2. As can be seen from this table, the operating cost is 74 cents per 1,000 gal without chromium re covery and 37 cents per 1,000 gal with chromium recovery. These figures indicate that the operating cost could be reduced by al most half by means of Cr(VI) recovery. The operating cost depends to a great extent on the initial Cr(VI) concentration in the wastewater to be treated. This cost study was based on a waste stream containing 104 mg/1 of Cr(VI) as Cr. In most cooling tower blowdown water, the Cr(VI) concentration was found to be in the

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176 TABLE 7-2 Estimated Capital and Operating Costs for a 0.144 mgd Carbon Adsorption Column for Cr(VI) Removal CAPITAL COST Construction cost for column and its appurtenances Clarifier Holding tank and mixing tank Activated carbon Engineering, legal, administrative, and land cost Total capital cost OPERATING COST ($/day) Caustic soda Sulfuric acid Labor, 8 hrs ($3/hr) Supplies and maintenance capital cost) AmortizeG capital cost at Subtotal per 1,000 gal -5 (3 x 10 total 8% interest Reclaimed chromate based on SO percent recovery efficiency TOTAL OPERATING COST WITH CHROMATE RECOVERY PER 1,000 GAL $ 85,000 10,000 15,000 8,800 23, 760 $142,560 $ 31. 36 15 .58 24.00 4.28 31.25 $ 0.74 $ 52.98 $ 0.37 range of 10 to 60 mg/1, with 30 mg/1 being the most common. The operating cost for treating water which contains a low Cr(VI) concentration would be expected to be lower than the cost esti mates given above. This results from less frequent regeneration as well as being able to use a slightly higher loading rate. The process outlined herein appears to be best suited for treating a large quantity of water having a low Cr(VI) concentration. This type of water is costly to treat by conventional chemical treat ment processes.

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177 TABLE 7-3 Costs of Treatment of a Cr(VI) Waste Stream by Several Different Processes (flow rate 15 gpm) 4 1 REMOVAL ONLY Conventional method with Na 2 S 2 0 5 Ion exchange Carbon adsorption Liquid-liquid extraction Ion flotation Reagent Cost ($/lb Cr) 0.30 0.59 0.21 0.54 0.55 REMOVAL PLUS RECOVERY OF Cr(VI) Ion exchange Carbon adsorption 0.29 0.16 NOTE: Costs are based on 1971 values. Capital Costa ($) 8,000 8,500 5,500 8,500 4,500 10,000 7,000 Operating Costb ($/day) 10.00 12.50 8.00 11.00 10.50 a Based on a waste flow rate of 15 gal per min and a contaminant level of 100 mg/1. bBased on an 8-hr day, 250 work days per year, waste flow of 15 gal per min, and a contaminant level of 100 mg/1. Includes cost of reagent, labor, power, fuel, and maintenance at 5 percent per year. cincludes in addition to the items listed in Note b a credit for recovery based on chromium at $0.65/lb. Table 7.3 provides comparative costs for treating Cr(VI) containing water by several different methods. Table 7-3 indi cates that carbon adsorption is the most inexpensive method of removing Cr(VI).

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CHAPTER 8 CONCLUSIONS AND RECOMMENDATIONS 8-,,l Principal Theoretical Findings 8-1-1 Batch studies. (a) Activated carbon removes Cr(VI) as chromic acid, not as chromate. Therefore, protons must be present in order for the activated carbon to adsorb Cr(VI). (b) The optimum adsorption of Cr(VI) on activated carbon is at a proton to Cr(VI) molar ratio of 1.0. As long as this condition is met, adsorption is independent of the initial pH values. (c) At a Cr(VI) concentration higher than the initial proton concentration, an amount of Cr(VI) equal to the initial proton concentration is first adsorbed, with any remaining Cr(VI) being removed by hydrolytic adsorption. Hydrolytic adsorption is a costly chemical process requiring significant amounts of activated carbon compared to nonhydrolytic adsorption. (d) At a Cr(VI) concentration lower than the initial proton concentration, reduction of Cr(VI) to Cr(III) takes place, with reduced Cr(III) remaining in solution at low pH values. The de gree of reduction is a function of the initial proton to Cr(VI) ratio, and the amount of activated carbon in solution. The higher the proton to Cr(VI) ratio, the greater the reduction and the less adsorption. There is an optimum dosage of activated carbon 178

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179 in this reduction pH range for a given Cr(VI) concentration for maximum adsorption. This optimum dosage is inversely proportional to the proton concentration. (e) A portion of Cr(VI) is adsorbed on the activated carbon re gardless of pH. (f) Adsorption of Cr(VI) is for the most part complete in a 2-hr contact time, and a significant amount of Cr(VI) is adsorbed within 10 min of contact time. (g) Added acid for pH control competes to some extent with chromic acid for adsorption on the activated carbon. The degree of interference because of this undesirable competition varies with acids, with HCl, HN0 3 H 2 S0 4 being in the increasing order of interference. (h) The very high adsorptive capacity of activated carbon for Cr(VI), in comparison with other acids, appears to be related to the tendency of chromic acid to form undissociated polynuclear species at the high Cr(VI) concentrations within the activated carbon. (i) The final form of adsorbed chromium within the activated carbon is in the hexavalent state. (j) Activated carbon is not only capable of reducing Cr(VI) to Cr(III) at a low pH, but also can oxidize Cr(III) to Cr(VI) at a high pH. (k) Desorption of adsorbed Cr(VI) can be accomplished by either alkali or acid. The final form of desorbed chromium is

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180 Cr(VI) in the case of alkali desorption, and Cr(III) in the case of acid adsorption. (1) Although alkali is a more effective desorbing agent, com plete desorption is not accomplished. 8-1-2 Column studies. (a) A carbon column is able to remove Cr(VI) completely. (b) The breakthrough capacity is a function of retention time. The uptake at a feed concentration of 104 mg/1 and 52-min reten tion time was 0.125 g Cr/g carbon up to breakthrough. At a reten tion time of 14 min, the uptake was 0.077 g Cr/g carbon up to breakthrough. (c) Activated carbon loses much of its adsorptive capacity after regeneration with acid, probably because Cr(VI) residing within the activated carbon oxidizes carbon in the presence of acid, resulting in a change in the carbon structure originally present. 8-2 Suggested Future Research 8-2-1 Theoretical considerations. The studies presented in this dissertation can be extended to include several important aspects relating to increasing adsorptive capacity of the activated carbon. Adsorption studies with various types of commercially available activated carbons is certainly one area to be considered. Com parisons of these activated carbons in terms of surface area, activation temperature and method, and oxygen content will reveal

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181 valuable information on the selection of activated carbon. Hope fully these findings could provide suppliers with the knowledge required to manufacture the activated carbon best suitable for Cr (VI) adsorption. Another area of exploration lies in improving means of re generation of spent activated carbon and reclamation of Cr(VI). For example, cooking of spent activated carbon at approximately 200c in the presence of caustic soda appears to be very promising. No excess NaOH beyond the equivalent amount required for neutraliz ing adsorbed chromic acid is necessary, and recovered chromate can be reused without further pH adjustment or concentration. 8-2-2 Engineering considerations. A cost analysis for utilizing a carbon column for Cr(VI) removal indicates that this process has a potential for future large-scale development. Reclamation of Cr(VI) is one of the most important areas in terms of lowering the operating cost for this process. Successful application of this process appears to depend largely on reuse of activated car bon through development of more effective regeneration methods. Full evaluation of operating techniques and costs should be made on a pilot scale operation prior to any design of actual full scale operation.

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APPENDICES

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APPENDIX 1 HYDROLYSIS OF A Cr(III) SOLUTIO N IN EQUILIBRIUM WITH CHROMIUM HYDROXIDE The following equilibrium constants are known: 3+ H 2 0 2+ H+ Cr + = CrOH + log *IS = 4.01 cr0l++H 2 o Cr(OH) 2 + + H+ = log *K = 6.22 2 Cr(OH) 3 (s) 3 + = Cr + 30H log K = 30 27 so 3+ 4+ 2Cr + 2H 2 0 = Cr (OH) + 2 2 2H+ log 8 22 = -2.69 Cr(OH) 3 (s) + OH = Cr(OH) 4 log Ks 4 = -0.4 Cr(OH) 3 (s) + 3+ + 3H = Cr + 3H 2 0 log *K = 11. 7 so Cr(OH) 3 + H 2 0 = Cr(OH) 4 + + H log *K = -14.4 s (i) (ii) (iii) (iv) (v) (vi) (vii) where *K = equilibrium constant for protonated ligand reaction with elimination of proton, B = cumulative or gross constants, K = equilibrium constant for solid-liquid system K = equilibrium s so constant for solid-simple (uncomplexed) species considered The species distribution in the heterogeneous system of Cr(OH) 3 (s)-Cr(III ) can be calculated as follows: By combining (i) with (vi) we obtain 2+ log (CrOH ) = log *Kso + log *K 1 2 pH (viii) From (ii) and (viii) we obtain log (Cr(OH) 2 +) = log K 1 + log *K 2 + log Kso pH (ix) = 1.4 7 pH 183

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184 Eqs. vi and vii can be rewritten as 3+ log (Cr ) = log *K = 3pH so = 11. 7 3pH log (Cr(OH) 4 -) = log *K + pH S4 = -14.4 + pH (x) (xi) Eqs. viii through xi can be plotted in a double logarithmic dia gram.

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APPENDIX 2 DISTRIBUTION OF THE VARIOUS Cr(VI) SPECIES IN A HOMOGENEOUS SYSTEM The following equilibrium constants are known: H+ + CrO 2 = HCr0 4 log K 1 = 6.57 4 H+ + HCr0 4 = H 2 Cr0 4 log K 12 = -0.8 21-t + 2Cr0 2 2H 2 0 = Cr O + log B 22 = 14.66 4 2 7 + 2= HCr 2 0 7 H + Cr 2 0 7 log K 23 = 0.07 2HCr04 Cr 2 0 7 2+ H 0 log K22 1.52 = 2 (i) (ii) (iii) (iv) (v) In the homogeneous system the total Cr(VI) concentration is the sum of the concentration of each species. CrO 2 2+ 2(HCr 2 o;) CrT = + HCrO + 2(Cr 2 0 7 ) 4 4 (vi) = CrO 2 [1 + K H+ + 2B22 (H+)2 (Cr04 2-) ZK23 8 22. (H+/ (CrO 2 -h + 4 1 4 We can define successive distribution coefficients L 0 L 1 L 2 CrO 2 L 4 = (vii) 0 CrT Ll HCr0 4 = CrT (viii) 185

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186 2L2 2Cr 2 0 7 = CrT L3 2HCr 2 0 7 = CrT By combining (viii) with (vi), L can be redefined as 0 (ix) (x) The remaining distribution coefficients are also redefined with the help of (i) through (v). 11 = K 1 L 0 (H+) (xii) 12 = 2 + 3 2B 22 1 0 (H) (CrT) (xiii) 13 = 2 + 3 2B 22 K 23 1 0 (H ) (CrT) (xiv) Also, by definition the sum of all the distribution coefficients is unity. L + L + L + L = 1 O 1 2 3 (xv) Inserting (xi) through (xiv) into (xv) we obtain

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187 or (xvi) The above quadratic equation can be solved with respect to L for 0 a given CrT and for varying proton concentrations. Once L 0 is known, the remaining distribution coefficients can be computed readily by inserting L into (xii), (xiii), and (xiv). 0

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APPENDIX 3 Bed volume = 100 gal/min 52.5 min (superficial contact time) = 5,250 gal = 701 8 l Column diameter = (100 gal/min 2 gal/min/ft 2 ) 112 4/3.14 = 8 ft Bed depth = 701. 8 ft 3 /SO ti = 14 ft Column height = 14 ft + 7 ft (free board) Amount of water to be treated = 100 gal/min 1,440 min/day = 1.44 10 5 gal/day Amount of carbon required Amount of Cr to be adsorbed up to the breakthrough pointa Amount of Cr to be adsorbed dailyb = 5.45 10 5 1/day = 701.8 fl 25 lb/ft3 (buik density of the carbon = 17,600 lbs = 17,600 lbs X O .105 lb Cr/lb = 1,848 lbs carbon = 5.45 105 1/day 0.104 g Cr/1 lb/453.6 g = 125 lb/day 188

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Frequency of regeneration Amounts of acid required for the pH adjustment 189 = a ; b = 1,845 lbs 14.8 days 125 lb/day Amount of acid required depends on the chromate concentration as well as alkalinity of the water to be treated. Assume the pH and alkalinity of the water are 6.5 and 150 mg/1 as CaC0 3 respectively Acid required to neutralize al kalinity of the water is 5 5.45 10 1/day 0.003 N 49 g/N H 2 S0 4 lb/453 6 g = 177 lb/day Acid required to convert chromate into chromic acid (104 mg/1 of Cr(VI) solution at a pH of 6.5 contains chromate and bichromate in equal molar ratio) is 5.45 10 5 I/day (0.001N + 0.002 N) 49 g/N H 2 S0 4 16/453.6 g = 177 lb/day Total amount of H SO required is 2 4 354 lb/day.

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191 13. US Public Health Service. Drinking Water Standard, 1962. US Dept. of Health, Education, and Welfare, 1962. 14. Lund, H.F. Industrial Pollution Control Handbook. McGraw Hill Book Co., N.Y., 1971. 15. EPA. Development Document for Proposed Effluent Limitations Guidelines and New Source Performance Standards (Segment of the Electroplating Point Source Category). EPA-440/1-73-003. Aug. 1973. 16. Armstrong, D. T. "Chromic Acid Recovery by Ion Exchange at Grumman Aircraft," Sew. and Ind. Wastes 25:8,934(1953} 17. Eckenfelder, W.W., Jr. Industrial Water Pollution Control. McGraw-Hill Book Company, N.Y., 1966. 18. Kolthoff, I.M., and Elving, P.J. Treatise on Analytical Chemistry, Part II. vol. 8. Interscience Publishers, N.Y., 1963. 19. Herman, F.M.; McKetta, J.J., Jr.,; and Othmer, D.F. Kirk Othmer Encyclopedia of Chemical Technology. vol. 5. 2nd edi tion. Interscience, John Wiley & Sons, Inc., N.Y., 1964. 20. Udy, M.J. Chromium. Reinhold Pub. Corp., N.Y., 1956. 21. Liptak, B.G. Environmental Engineers Handbook. vol. 1. Chil ton Book Co., Radnor, Penn., 1974. 22. Davids, H.W., and Lieber, M. "Underground Water Contamination by Chromium," Wat. and Sew. Works 98,528(1951). 23. Wilber, G.C. Biological Aspects of Water Pollution. Charles C. Thomas Pub. Co., Springfield, Ill., 1969. 24. Royle, H. "Toxicity of Chromic Acid in the Chromium Plating Industries (I)," Envir. Res. 10:l,39(1975). 25. Olishitski, J.B., and McElroy, F.E. Fundamentals of Industrial Hygiene. National Safety Council, Chicago, 1971. 26. White, R.P. The Dermatergoses or Occupational Affections r f the Skin. 4th edition. H.K. Lewis and Co., London, 1934. 27. Zarafonetis, J .H., and Hampton, R.E. "Effects of Small Con centrations of Cr on Growth and Photosynthesis in Algae," Chem. Abst. 86560q, 81:15(1974).

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192 28. Ingols, R.S., and Fetner, R.H. "Toxicity of Chromium Com pounds under Aerobic Conditions," Jour. Wat. Poll. Con. Fed. 34:4,366(1961). 29. Doudoroff, P., and Katz, M. "Critical Review of Literature on the Toxicity of Industrial Wastes and Their Components to Fish," Sew. and Ind. Wastes 25:7,802(1953). 30. Pickering, Q.H., and Henderson, C. "The Acute Toxicity of Some Heavy Metals to Different Species of Warm Water Fishes," Air and Water Poll. Intl. Jour. 10:453(1966). 31. Raymont, J.E.G., and Shields, J. "Toxicity of Copper and Chromium in the Marine Environment," Adv. in Water Poll. Res. The Macmillan Co., N.Y., 1964. 32. Hill, C.W., and Fromm, D.O. "Response of the Internal Gland of Rainbow Trout (Salmo gaidneri) to Stress, II Int:l. Symp. en Comparative Endocrinology Proc. 11:1(1940). 33. Jenkins, S.H., and Hewitt, C.H. "The Effect of Chromate on the Purification of Sewage by Treating in Bacterial Filters," Jour. and Proc. Inst. Sew. Purif. 219(1940). 34. Spencer, J.H. "The Effect of Chromium Plating Wastes on Sewage Works and Sewage Treatment," Jour. and Proc. Inst. Sew. Purif. 17(1939). 35. Jenkins, S.H., and Hewitt, C.H. "The Effect of Cr Compounds on the Purification of Sewage by the Activated Sludge Process, Jour. and Proc. Inst. Sew. Purif. 222(1942). 36. Ross, W.E. "Industrial Waste Problems at Richmond Sewage Treatment Works," Sew. Works Jour. 18:3,586(1946). 37. Monk, H.E. "Chemical and Bacteriological Properties of Trade Wastes Containing Chromate Ions in Sewage Dilutions," Jour. and Proc. Inst. Sew. Purif. Part 1, 89(1939). 38. Placat, O.R.; Ruchhoft, C.C.; and Snapp, R.G. "Copper and Chromate Ions in Sewage Dilutions," Ind. Eng. Chem. 41,2238 (1949). 39. Edwards, G.P., and Nussberger, F.E. "Effect of Chromate Wastes on the Activated Sludge Process at the Tallmans Island Plant," Sew. Works Jour. 19:4,598(1947). 40. Clark, J.W., and Viessman, W., Jr. Water Supply and Pollu tion Control. International Textbook, Scranton, Penn., 1969.

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193 41. Smithson, G.R., Jr. An Investigation of Techniques for Re ITK:Jval of Chromium from Electroplating Wastes. Battelle Memorial Institute, Columbus, Ohio, 1971. 42. Grieves, R.B.; Ettelt, G.A.; Schadt, J.T.; and Bhattacharyva, D. "Dissolved-Air Ion Flotation of Industrial Wastes: Hexa valent Chromium," Proc. Ind. Waste Conf., 23rd, Kentucky Univ., Lexington, Kentucky, May 1968. 43. Tarvin, 0. "Metal Plating Wastes and Sewage Treatment," Sew. and Ind. Wastes 28:11,1371(1956). 44. Stones, T. "The Fate of Chromium During the Treatment of Sewage," Jour. Inst. Sew. Purif. Part 2, 345(1955). 45. Oliver, B.G., and Gosgrove, E.G., "The Efficiency of the Heavy Metal Removal by a Conventional Activated Sludge Treat ment Plant," Water Res. 8:11,869(1974). 46. Webber, J. "Effects of Toxic Metals in Sewage on Crops," Wat. Poll. Cont. 71,404(1972). 47. Bothner, M.H., and Carpenter, R. "Sorption-Desorption Reac tions of Mercury with Suspended Matter in the Columbia River." Intl. Atomic Ener. Sump., Seattle, Wash., 10-14 July 1972. 48. Chen, K.Y.; Young, C.S.; Jan, T.K.; and Rohatgi, N. "Trace Metals in Wastewater Effluents," Jour. Wat. Poll. Cont. Fed. 46:12,2663(1974). 49. Huch ins on, G.E. "Minor Metallic Elements in Lake Water," A Treatise in Limnology. Vol. 1. John Wiley & Sons, Inc., N.Y., 1957. SO. Brown, H.G. "Efficiency of Heavy Metals Removal in Municipal Sewage Treatment Plants," Envir. Letter 5, 103(1973). 51. Lamb, A., and Tollefson, E .L. "Toxic Effects of Cupric, Chromate, and Chromic Ions on Biological Oxidation," Water Res. 7,599(1973). 52. Regan, T .M., and Mercer, M.M. "Heavy Metals in Digesters Failure," Jour. Wat. Poll. Cont. Fed. 42:10,1832(1970). 53. Coburn, S.E. "Limits for Toxic Wastes in Sewage Treatment," Sew. Works Jour. 21:3,522(1949). 54. Pourbaix, M.J .N. Atlas of Electrochemical Equilibrium in Aqueous Solution~. English translation. Pergamon Press, Cebelcor, Brussels, 1964.

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194 55. Sillen, L.G., and Martell, A.E. Stability Constants of Metal Ion Complexes. Chem. Soc., London, 1964: 56. Stumm, W., and Morgan, J.J. Aquatic Chemistry. Wiley-Interscience, N. Y., 1970. 57. Rubin, A.J. Aqueous Metals. Ann Arbor Sci., Ann Arbor, Mich., 1964. 58. Meites, L. Ha. ndbook of Analytical Chemistry. McGraw-Hill Book Co., N. Y., 1964. 59. Mellor, J.W. A Comprehensive Treatise on Inorganic and Theo retical Chemistry. Vol. 6. Longmans, Green and Co., N.Y., 1931. 60. Thorne, P.C.L., and Roberts, E.R. Inorganic Chemistry. Inter science Publishers, Inc., N.Y., 1947. 61. Weber, W.J., Jr. Physicochemical Processes for Water Quality Contrc,l~ Wiley-Interscience, N.Y., 1972. 62. Osipow, L.I. Surface Chemistry. Robert E. Krieger Pub., Inc., Huntington, N.Y., 1972. 63. Glasstone, S., and Lewis, D. Elements of Physical Chemistry. Van Nostrand Co., Inc., N.Y., 1960. 64. Hassler, J.W. Purification with Activated Carbon. Chemical Pub. Co., Inc., N.Y., 1974. 65. Anderson, A.H. "The Pharmacology of Activated Charcoal," Acta. Pharmacol. Toxicol. J,199(1947). 66. Snoeyink, V. L. and Weber, W. J., Jr. "The Surface Chemistry of Active Carbon," Envir. Sci. Tech. 1:3,228(1967). 67. Culp, G.L., and Culp, R.L. New Concepts in Water Purification. Van Nostrand Reinhold Co., N.Y., 1974. 68. Garten, V.A., and Weiss, D.E. "The Ionand Electron-Exchange Properties of Activated Carbon in Relation to Its Behavior as a Catalyst and Adsorbent," Reviews of Pure and Applied Chem. 7 ,69(1957). 69. Garten, V .A., and Weiss, D .E. "A New Interpretation of the Acidic _. and Basic Structures in Carbons," Austral. Jour. of Chem. 10,309(1957).

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195 70. Garten, V.A., and Weiss, D.E. "The Quinone-Hydroquinone Character of Activated Carbon and Carbon Black," Austral. Jour. of Chem. 8,68(1955). 71. Kolthoff, I.M. "The Acid Constituent of Charcoal Properties of Active Charcoal Reactivated in Oxygen at 4QQOC," Jour. of Amer. Chem. Soc. 54,4473(1932). 72. Snoeyink, V .A., and Weber, J. W. "Reaction of the Hydrated Proton with Active Carbon," Adsorption in Aqueous Solution. Advances in Chemical Series, Vol. 79. Amer. Chein. Soc., 1968. 73. Coughlin, R.W., and Ezra, F.S. "Role of Surface Acidity in the Adsorption of Organic Pollutants on the Surface of Carbon," Envir. Sci. Tech. 12:4,291(1968). 74. Sigworth, E.A., and Smith, S.B. "Adsorption of Inorganic Compounds by Activated Carbon," Jour. of Amer. Water Works Assn. 64,386 (1972) 75. Argo, G.D., and Culp, L. C. "Heavy Metal Removal in Waste water Treatment Processes, Part 1," Wat. and Sew. Works 119: 8,62(1972). 76. Parks, L.R., and Bartlett, P.G. "The Effect of Inorganic Sal ts on the Adsorption of Inorganic Acids and Bases," Jour. of Amer. Chem. Soc. 49,1698(1927). 77. Bartell, F.E., and Miller, E.J. "Adsorption by Activated Sugar Charcoal," Jour. of Amer. Chem. Soc. 44,1866(1922). 78. Pawlowski, B. S. "Determination of the Iodine Number of Activated Carbon," Pittsburgh Activated Carbon Test Method. Calgon, April 1971. 79. EPA. Process Design Manual for Carbon Adsorption. Technology Transfer, Oct. 1973.

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BIOGRAPHICAL SKETCH Jung I. Kim was born 18 September 1943 in Chuchiang, Chi na. His family moved back to Korea during World War II. In March 1965 he entered the Inha Institute of Technology, where he majored in chemical engineering. He was graduated cum laude from the Inha Institute of Technology in March 1969. He then began research work at the Department of Chemical Engineering, Inha Institute of Technology, until August 1970. In September 1970 he began work toward the degree of Master of Science in en vironmental heal th engineering at the University of Alaska. After he received the Master of Science degree, he entered the Ph.D. program in the Department of Environmental Engineering Sciences at the University of Florida, in September 1972 After graduation the author will continue research work in the Department of Civil Engineering at Virginia Polytechnic Institute and State University. 196

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. I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. J. ( ~pl tek, ~r ,', Chairman Ass.j/stant Pr6fessor of Environmental Engineering Sciences I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and ~uality, as a dissertation for the degree of Doctor of Philosf ~.Y. TI'i\ tlJ '.)l('L tt 'i L T. de S. Furman Professor of Environmental Engineering Sciences I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. Patrick L. Brezonik Associate Professor of Environmental Engineering Sciences I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. Dinesh 0. Shah Associate Professor of Chemical Engineering

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This dissertation was submitted to the Graduate Faculty of the College of Engineering and to the Graduate Council, and was accepted as partial fulfillment of the requirements of the degree of Doctor of Philosophy. March 1976 Dean, Graduate School