I the Removal of Heavy Metal Pollutants with Electrowinning

Home , Electrowinning

The Removal of Heavy Metal Pollutants With

Electrowinning Techniques

By Pengpeng Yao

B.A., University of Science and Technology of China, 1998

M.S., University of Science and Technology of China, 2001

A Dissertation Submitted in Partial Fulfillment of the

Requirements for the Degree of Doctor of Philosophy

in Department of Chemistry at Brown University

Providence, Rhode Island

May 2010

III

This Dissertation by Pengpeng Yao is accepted in its present form

by the Department of Chemistry as satisfying the dissertation

requirement for the degree of Doctor of Philosophy.

Date______Joseph M. Calo

Recommended to the Graduate Council

Date______Robert Hurt

Date______Gerald Diebold

Approved by the Graduate Council

Date______Sheila Bonde Dean of the Graduate School

ACKNOWLEDGEMENTS

It is unbelievable how fast time passed by while I studied at Brown. In the past

five years, I have too much fun on research and study. Of course, a lot of hard

workings in lab, even frustration when facing problems are inevitable. However,

exactly through them, I grow up as being motivated and structured, detail oriented,

comfortable to work independently and thrive under pressure. When looking back the

past years in Brown, everything is appreciable.

The most appreciation goes to Prof. Calo for having me and supporting me here at

Brown. I very much appreciate his ideas, hints, and help on research and thesis, as

well as his advising style and the research freedom that is given to me.

Many thanks to Yuming Gao and Indrek Kaloats who are great co-workers in the

lab, for teaching me how to stand on my feet and start research quickly in the earlier

years. I also appreciate Indrek Kaloats and Al Tente for helping CEP system setup.

Prof. Hurt and Risen are great teachers and researchers. I appreciate for their

mentorship and friendship.

Finally, I would like to dedicate this thesis on my parents and my fiance, Scott, whose encouragement and love accompany me well during the thesis writing, and will last forever.

Abstract of “The Removal of Heavy Metal Pollutants With Electrowinning Techniques” by Pengpeng Yao, Ph.D., Brown University, May 2010

The results of investigations of single metal nickel, cadmium, and lead, and

copper/nickel mixtures in the particulate spouted bed of conductive particles from acidic aqueous solution are presented. It was determined that, in general, the rates of single

metal nickel, cadmium, lead electrowinning increase with increasing pH and increasing

temperature in acidic solutions over the experimental range investigated. Nitrogen

sparging of the electrolyte solution in the holding tank was effective in reducing the

dissolved oxygen concentration, suppressing metal corrosion rates, and, thereby, improving the net metals recovery rate. A numerical model based on the Tafel equations, incorporating a constant corrosion rate as an approximation was used to simulate the

electrochemical removal behavior of single metal nickel. The model captured the

behavior reasonably well.

The quantitative and qualitative behavior of co-deposition of copper and nickel from

mixtures was significantly different from that of the single metal solutions. This was

attributed to the differences in reduction potential of the two metals, as well as the metal

displacement reaction between Ni(0) and Cu(II), which effectively eliminated the copper

corrosion reaction and augmented that for nickel. It also amplified the separation of the

deposition regimes in time for both metals, suggesting that the recovery of each as a

relatively pure metal deposit was possible under certain conditions.

Data from the spouted particulate electrode system was incorporated into the design and operation of the cyclic electrowinning/precipitation (CEP) system for the removal of complex heavy metal mixtures from contaminated wastewaters at low concentrations.

The removal of the heavy metals copper, nickel and cadmium from aqueous solutions at VI low concentrations was investigated in the CEP system. It was found that the optimal removal times for single metal removal increased in the order copper > nickel > cadmium, while the maximum net CEP removal rates decreased in the same order. These orderings correlated with decreasing reduction potentials for these metals, both in terms of the metal displacement reaction rates, as well as the propensity of corrosion of the deposited metals. The CEP system shows significant promise for the effective removal of a number of heavy metal mixtures from contaminated water.

Table of Contents

Chapter 1. Introduction 8 1.1: Heavy Metals in the Environment and Health Effects ...... 8 1.2: Techniques for Removal/Remediation/Recovery of Heavy Metals from Contaminated Water and Soil ...... 11 1.2.1: Remediation Technologies for Contaminated Soils ...... 11 1.2.1.1: Containment/Immobilization/Disposal ...... 12 1.2.1.2: Natural Attenuation Processes ...... 13 1.2.1.3: Extraction Methods ...... 14 1.2.2: Remediation Technologies for Contaminated Water ...... 15 1.2.2.1: Adsorption ...... 15 1.2.2.2: Ion-exchange ...... 18 1.2.2.3: Membrane Processes ...... 21 1.2.2.4: Precipitation, Flocculation, and Filtration ...... 22 1.2.2.5: Electrolytic Techniques ...... 24 1.3: Spouted Particulate Electrodes (SPE) ...... 30 References ...... 34 Table 1.1. Description, Advantages, and Disadvantages of Metal Immobilization Technologies.… ...... 39 Table 1.2. Description, Advantages, and Disadvantages of Metal Extraction Technologies.… ...... 41 Figure 1.1. Periodic Table showing one of the common definitions of the “heavy metals.”…………...... 43 Figure 1.2. Schematic of the 30 cm (12 in.) diameter, cylindrical spouted particulate electrode used in some of the current work...... 44

Chapter 2. Experimental 45 2.1: Spouted Particulate Electrode (SPE) ...... 45 2.1.1: Apparatus ...... 45 2.1.2: Metal Ion Concentration Measurements ...... 48 2.1.3: Experimental Procedures ...... 49 2.1.4: Gas Sparging ...... 50 2.2: Cyclic Electrowinning/Precipitation (CEP) System ...... 51 2.2.1: Apparatus ...... 51 2.2.2: CEP System LabView Control Program ...... 54 2.2.3: Metal Ion Solutions ...... 55 2.2.4: CEP Experimental Procedures ...... 55 References ...... 58 Figure 2.1. Schematic of the spouted particulate electrode (SPE) apparatus and flow system…………… ...... 59 Figure 2.2. CEP System Schematic...... 60 Figure 2.3. Schematic of the spouted particulate electrode (SPE) of the CEP System……………...... 61 Figure 2.4. SPE images in CEP system...... 62 Figure 2.5. The Cyclic Electrowinning/Precipitation(CEP) System...... 63

Chapter 3. Removal of Single Metals and Simple Metal Mixtures Via Electrowinning With Spouted Particulate Electrodes 64 2

3.1: Nickel Removal With a Spouted Particulate Electrode...... 64 3.1.1: Background ...... 64 3.1.2. Results ...... 65 3.1.2.1. Electrodeposition Model ...... 65 3.1.2.2. Results and Discussion ...... 71 3.1.3. Conclusions ...... 82 3.2: Cadmium and Lead Removal With a Spouted Particulate Electrode ...... 83 3.2.1: Background ...... 83 3.2.2: Results ...... 85 3.2.2.1: Electrodeposition Reactions ...... 85 3.2.2.2: Cadmium Electrowinning Results ...... 85 3.2.2.3: Lead Electrowinning Results ...... 90 3.2.3: Conclusions ...... 94 3.3: Metal Co-Removal From a Cu/Ni Mixture With a Spouted Particulate Electrode ...... 95 3.3.1: Background ...... 95 3.3.2: Co-Electrodeposition Model ...... 95 3.3.3. Results and Discussion ...... 100 3.3.3.1. Without Nitrogen Sparging ...... 100 3.3.2.3. With Nitrogen Sparging...... 103 3.3.3: Discussion ...... 104 3.3.4: Conclusions ...... 110 References ...... 111 Figure 3.1.1. Nickel removal at 10A, 35°C, as a function of pH, with (┅)and without (━)nitrogen sparging...... 114 Figure 3.1.2. Nickel corrosion rates with the feeder current off at 35°C as a function of solution pH, with (┅)and without (━) nitrogen sparging...... 115 Figure 3.1.3. Current efficiencies for the data presented in Figure 3.1.1, with (┅) and without (━) nitrogen sparging...... 116 Figure 3.1.4. The relative effects of mass transfer and electrodeposition rates on nickel removal as a function of pH (10A, 35°C) from the model results presented in Figure 3.1.1………...... 117 Figure 3.1.5. Dissolved oxygen concentration during nickel removal at 10A, 35°C, as a function of pH, with (┅)and without (━) nitrogen sparging...... 118 Figure 3.1.6. Nickel removal at 10A, pH 4, as a function of temperature, with (┅) and without (━) nitrogen sparging...... 119 Figure 3.1.7. Current efficiencies for the data presented in Figure 3.1.6, with (┅) and without (━) nitrogen sparging...... 120 Figure 3.1.8. The relative effects of mass transfer and electrodeposition rates on nickel removal as a function of temperature (10A, pH 4.0) from the model results presented in Figure 3.1.6...... 121 Figure 3.1.9. Nickel corrosion rates with the feeder current off at pH 4 as a function of solution temperature, with (┅)and without (━) nitrogen sparging...... 122 Figure 3.1.10. Dissolved oxygen concentration during nickel removal at 10A, pH 4, as a function of temperature, with (┅)and without (━) nitrogen sparging...... 123 Figure 3.1.11. Nickel removal at 35°C, pH 4, as a function of feeder current, with (┅)and without (━) nitrogen sparging...... 124 Figure 3.1.12. Current efficiencies for the data presented in Figure 3.1.11, with (┅)and without (━) nitrogen sparging...... 125 3

Figure 3.1.13. The relative effects of mass transfer and electrodeposition rates on nickel removal as a function of feeder current (pH 4.0, 35°C) from the model results presented in Figure 3.1.11...... 126 Figure 3.2.1. Cadmium removal at 10A, 40°C, as a function of solution pH, with (┅)and without (━) nitrogen sparging...... 127 Figure 3.2.2. Current efficiencies for the data presented in Figure 3.2.1, with (┅) and without (━) nitrogen sparging...... 128 Figure 3.2.3. Dissolved oxygen concentration during cadmium removal at 10A, 40°C, as a function of pH, with (┅)and without (━) nitrogen sparging...... 129 Figure 3.2.4. Cadmium corrosion with the feeder current off at 40°C as a function of solution pH, with (┅)and without (━) nitrogen sparging...... 130 Figure 3.2.5. Cadmium removal at 10A, pH 4.0, as a function of solution temperature at pH 4, with (┅)and without (━) nitrogen sparging...... 131 Figure 3.2.6. Current efficiencies for the data presented in Figure 3.2.5, with (┅) and without (━) nitrogen sparging...... 132 Figure 3.2.7. Dissolved oxygen concentration during cadmium removal at 10A, pH 4.0, as a function of solution temperature, with (┅)and without (━) nitrogen sparging…………………...... 133 Figure 3.2.8. Cadmium corrosion with the current off at pH 4.0 as a function of solution temperature, with (┅)and without (━) nitrogen sparging...... 134 Figure 3.2.9. Cadmium removal at 40°C, pH 4.0, as a function of current, with (┅)and without (━) nitrogen sparging...... 135 Figure 3.2.10. Current efficiencies for the data presented in Figure 3.2.9, with (┅) and without (━) nitrogen sparging...... 136 Figure 3.2.11. Dissolved oxygen concentration during cadmium removal at 40°C, pH 4.0, as a function of current, with (┅)and without (━) nitrogen sparging...... 137 Figure 3.2.12. Lead removal from MSA solution at 10A, 40°C, as a function of solution pH, with (┅)and without (━) nitrogen sparging...... 138 Figure 3.2.13. Current efficiencies for the data presented in Figure 3.3.12, with (┅)and without (━) nitrogen sparging...... 139 Figure 3.2.14. Dissolved oxygen concentration during lead removal at 10A, 40°C, as a function of pH, with (┅)and without (━) nitrogen sparging...... 140 Figure 3.2.15. lead corrosion with the feeder current off at 40°C as a function of MSA solution pH, with (┅)and without (━) nitrogen sparging...... 141 Figure 3.2.16. Lead removal at 10A, pH 2.5, as a function of MSA solution temperature, with (┅)and without (━) nitrogen sparging...... 142 Figure 3.2.17. Current efficiencies for the data presented in Figure 3.3.16, with (┅)and without (━) nitrogen sparging...... 143 Figure 3.2.18. Dissolved oxygen concentration during lead removal from MSA solution at 10A, pH 2.5, as a function of solution temperature, with (┅)and without (━) nitrogen sparging...... 144 Figure 3.2.19. Lead corrosion with the feeder current off at pH 2.5 as a MSA function of solution temperature, with (┅)and without (━) nitrogen sparging...... 145 Figure 3.2.20. Lead removal at 40°C, pH 2.5, as a function of current, with (┅) and without (━) nitrogen sparging...... 146 Figure 3.2.21. Current efficiencies for the data presented in Figure 3.2.20, with (┅)and without (━) nitrogen sparging...... 147 4

Figure 3.2.22. Dissolved oxygen concentration during lead removal at 40°C, pH 2.5, as a function of current with (┅)and without (━) nitrogen sparging...... 148 Figure 3.3.1. Co-removal of copper and nickel at 10A, 40°C from a Cu/Ni solution as a function of pH without nitrogen sparging...... 149 Figure 3.3.2. Overpotential (E-Ee) at cathode during electrodeposition of copper and nickel removal at pH 4.0, 40°C and 10A without nitrogen sparging...... 150 Figure 3.3.3. Dissolved oxygen concentration during Cu/Ni removal at 10A and 40°C as a function of pH without nitrogen sparging...... 151 Figure 3.3.4. Net copper and nickel “corrosion” rates at 40°C as a function of pH without nitrogen sparging...... 152 Figure 3.3.5. Co-removal of copper and nickel at 10A, and pH 4.0, from a Cu/Ni solution as a function of temperature without nitrogen sparging...... 153 Figure 3.3.6. Dissolved oxygen concentration during Cu/Ni co-removal at 10A and pH 4.0 as a function of temperature without nitrogen sparging...... 154 Figure 3.3.7. Net copper and nickel “corrosion” rates at pH 4 as a function of temperature without nitrogen sparging...... 155 Figure 3.3.8. Normalized copper and nickel concentrations at 40°C as a function of pH for the first two hours, after which the pH was changed to 4 in all cases, with nitrogen sparging...... 156 Figure 3.3.9. Normalized copper and nickel concentrations at pH 3 and 4 at 30°C and 40°C for the first two hours, after which the pH was changed to 4.0, with nitrogen sparging...... 157 Figure 3.3.10. Net copper and nickel “corrosion” rates at 40°C as a function of pH with nitrogen sparging...... 158 Figure 3.3.11. Net copper and nickel corrosion rates at pH 4.0 as a function of temperature with nitrogen sparging...... 159 Figure 3.3.12. Apparent activation energy of the metal displacement reaction between Cu(II) and Ni(0) over the temperature range 30-60°C, at pH 4.0...... 160 Figure 3.3.13. SEM micrographs of cathodic particle surfaces at pH 4.0, 40°C, and 10A. A and B are at 20 min and 4 hours, respectively, particles coated initially with copper. C and D are at 20 min and 4 hours, respectively, particles coated initially with nickel………………… ...... 161 Figure 3.3.14. Normalized total copper plus nickel ion concentrations from Figure 3.3.1………...... 162

Chapter 4. Cyclic Electrowinning/Precipitation (CEP) System for the Removal of Heavy Metals From Aqueous Mixtures to Low Concentrations 163 4.1: Background ...... 163 4.2: Experimental Results and Analysis ...... 165 4.2.1: Electrochemical Reactions in the CEP Spouted Particulate Electrode...... 165 4.2.2: Precipitation/Redissolution in the CEP Process ...... 166 4.2.2: CEP System Results for the Removal of Single Metals ...... 168 4.2.3: Metal Mixture Results ...... 174 4.3: Conclusions ...... 177 References ...... 178 Figure 4.1. Cumulative Cu2+ concentration as a function of precipitation/redissolution cycle. The initial feed solution Cu2+concentration was 20 ppm. Each cycle was eight minutes at 25°C. The precipitation/redissolution pH values were 11 and 4.0, respectively...... 179 5

Figure 4.2. Cumulative Ni2+ concentration as a function of precipitation/redissolution cycle. The initial feed solution Ni2+ concentration was 20.0 ppm. Each cycle was eight minutes at 25°C. The precipitation/redissolution pH values were 11and 4.0, respectively...... 180 Figure 4.3. Cumulative Cd2+ concentration as a function of precipitation/redissolution cycle. The initial feed solution Cd2+ concentration was 20 ppm. Each cycle was eight minutes at 25°C. The precipitation/redissolution pH values were 11 and 4.0, respectively...... 181 Figure 4.4. Copper electrowinning in the CEP SPE as a function of applied current. The initial copper concentration was 99.5ppm, which was prepared with five precipitation/redissolution cycles. The symbols (o, ●, and the curves are the simulations (50°C; pH 4; 3500 cm3 min-1 nitrogen sparging). .... 182 Figure 4.5. Net CEP copper removal rates, R, as a function∆) are the of experimentalapplied current. data, The initial Cu2+ concentration was 99.5 ppm, which was prepared with five precipitation/redissolution cycles. The values in the parentheses are the electrodeposition time te, and the Cu removal rate at the maxima (50°C; pH of 4; 3500 cm3 min-1 nitrogen sparging)...... 183 Figure 4.6. Cu2+ concentration over multiple CEP cycles at 15A. Five precipitation/redissolution cycles were employed to accumulate the initial electrowinning Cu2+ concentration of 99.8 ppm. After that, each CEP cycle consisted of one SPE electrowinning and three precipitation/redissolution cycles. The precipitation/redissolution pH values were 11.0 and 4.0, respectively (40°C; pH of 4; 3500 cm3 min-1 nitrogen sparging)...... 184 Figure 4.7. Copper loss per cycle for the data in Figure 4.6 for multiple CEP cycles at 15A (50°C; pH 4; 3500 cm3 min-1 nitrogen sparging)...... 185 Figure 4.8. Nickel electrowinning in the CEP SPE with and without nitrogen sparging (3500 cm3 min-1). The initial nickel concentration was 100 ppm, which was prepared with five precipitation/redissolution cycles. The symbols (□,■) are the experimental data, and the curves are the fits (20A; 50°C; pH 4)...... 186 Figure 4.9. Nickel electrowinning in the CEP SPE as a function of pH. The initial nickel concentration was 100 ppm, which was prepared with five precipitation/redissolution cycles. The symbols (■,□,◪) are the experimental data, and the curves are the fits (20A; 50°C; 3500 cm3 min-1 nitrogen sparging)...... 187 Figure 4.10. Nickel electrowinning in the CEP SPE as a function of current. The initial nickel concentration was 100 ppm, which was prepared with five precipitation/dissolution cycles. The symbols (□,■) are the experimental data, and the curves are the fits (50°C; pH 4; 3500 cm3 min-1 nitrogen sparging)...... 188 Figure 4.11. Net CEP nickel remoal rate, R, as a function of applied current. The initial nickel concentration was 100 ppm, which was prepared with five precipitation/redissolution cycles. The values in the parentheses are the electrodeposition time te, and the Ni removal rate at the maxima (50°C; pH 4; 3500 cm3 min-1 nitrogen sparging)...... 189 Figure 4.12. Ni2+ concentration over multiple CEP cycles at 20A. Five precipitation/redissolution cycles were employed to accumulate the initial electrowinning Ni2+ concentration of 100 ppm. After that, each CEP cycle consisted of one 180 min SPE electrodeposition step and two precipitation/redissolution cycles (50°C; pH 4; 3500 cm3 min-1 nitrogen sparging)...... 190 Figure 4.13. Cadmium electrowinning in the CEP SPE at 20A. The initial cadmium concentration was 100 ppm, which was prepared with five 6

precipitation/redissolution cycles. The symbols (▲) are the experimental data, and the curves are the fits (50°C; pH 4.0, 3500 cm3 min-1 nitrogen sparging)...... 191 Figure 4.14. Net CEP cadmium removal rates, R, at 20A. The initial nickel concentration was 100 ppm, which was prepared by five precipitation/redissolution cycles. The values in the parentheses are the electrodeposition time te, and the Cd removal rate at the maximium (50°C; pH 4; 3500 cm3 min-1 nitrogen sparging)...... 192 Figure 4.15. Copper and nickel ion concentrations over multiple CEP cycles at 20A. Three precipitation/redissolution cycles were employed to accumulate the initial electrowinning Cu2+ and Ni2+ concentrations of around 83 ppm. After that, each CEP cycle consisted of one 180 min SPE electrowinning and two precipitation/redissolution cycles (50°C; pH 4; 3500 cm3 min-1 nitrogen sparging)...... 193 Figure 4.16. Copper and cadmium concentrations over multiple CEP cycles at 20A. Three precipitation/redissolution cycles were employed to accumulate the initial electrowinning Cu2+ and Cd2+ concentrations of around 83 ppm. After that, each CEP cycle consisted of one 240 min SPE electrowinning and two precipitation/redissolution cycles (50°C; pH 4.0; 3500 cm3 min-1 nitrogen sparging). .. 194 Figure 4.17. Nickel and cadmium concentrations over multiple CEP cycles at 20A. Three precipitation/dissolution cycles were employed to accumulate the initial electrowinning Ni2+ and Cd2+ concentrations of around 83 ppm. After that, every CEP cycle consisted of one 300 min SPE electrowinning and two precipitation/redissolution cycles (50°C; pH 4.0; 3500 cm3 min-1 nitrogen sparging). .. 195 Figure 4.18. Copper, nickel and cadmium concentrations over Multiple CEP cycles at 20A. Three precipitation/dissolution cycles were employed to accumulate the initial electrowinning Cu2+, Ni2+ and Cd2+ concentrations of 83 ppm. After that, each CEP cycle consisted of one 300 min SPE electrowinning and one precipitation cycles (50°C; pH 4.0; 3500 cm3 min-1 nitrogen sparging)...... 196 Figure 4.19. Copper, nickel, and cadmium electrowinning in the CEP SPE at 20A. The initial concentrations were 100 ppm for all three metals, prepared directly from the reagents. The symbols (●, ■, ▲) are the experimental data, and the curves are the fits. (50°C; pH 4.0; 3500 cm3 min-1 nitrogen sparging)...... 197 Figure 4.20. Net CEP copper, nickel, and cadmium removal rate, R, at 20A. The initial concentrations were 100 ppm for all three metals, prepared directly from the reagents. The values in the parentheses are the electrodeposition time te, and the Cu, Ni, Cd removal rate at the maxima (50°C; pH 4; 3500 cm3 min-1 nitrogen sparging)...... 198

Chapter 5. Conclusions 199 5.1. Summary ...... 199 5.2. Recommendations for Future Work ...... 201

Appendices 203 Appendix A. Gas Sparging Analysis and Model ...... 203 A.1. Background ...... 203 A.2. Assumptions ...... 203 A.3. Oxygen Mass Balance in the Bubble Phase ...... 203 A.4. Dissolved Oxygen Mass Balance ...... 205 A.5. Parameter Estimation ...... 207 A.6. Results ...... 209 References ...... 209 Figure A.1. The effect of nitrogen sparging at different flow rates with the 7

holding tank cover on. The data symbols (●,■,▲,♦) are experimental, and the solid and dashed curves are calculated from the model...... 210 Figure A.2. The effect of nitrogen sparging as a function of flow rate with with holding tank cover off...... 211 Appendix B: Computer Code for Electrochemical Nickel and copper/nickel removal in a Spouted Particulated Electrode (SPE) ...... 212 B.1. FORTRAN Code Listing of the Nickel Electrochemical Model ...... 212 B.2. FORTRAN Code Listing of the Cu/Ni Mixture Electrochemical Model ...... 220 Appendix C: Cyclic Electrowinning/Precipitation (CEP) System Labview Control Program ...... 231 C.1. Front Panel: ...... 234 C.3. Block Panel: ...... 235 8

Chapter 1. Introduction

1.1: Heavy Metals in the Environment and Health Effects

A “heavy metal” is a member of an ill-defined subset of elements that exhibit

metallic properties which typically includes the transition metals, some metalloids,

lanthanides, and actinides. Many different definitions have been proposed — some based

on density, some on atomic number or atomic weight, and some on chemical properties

or toxicity.1 An alternative term is toxic metal, for which no consensus of an exact

definition exists either. One source defines a heavy metal as one of the "common

transition metals, such as copper, lead, and zinc. These metals are a cause of

environmental pollution (heavy-metal pollution) from a number of sources, including

lead in petrol, industrial effluents, and leaching of metal ions from soil into lakes and

rivers by acid rain."2 An operational definition that suffices for the current purposes is

shown in the Periodic Table in Figure 1.1. This includes all the transition metals, the

actinides and lanthanides, all the “other metals,” and the metalloids As, Se and Te.

The 2007 CERCLA Priority List of Hazardous Substances,3 prepared and updated

biannually by the ATSDR and the USEPA, consists of substances that are the most

commonly found at facilities on the National Priorities List (NPL) that are determined to pose the most significant potential threat to human health due to their known or suspected toxicity and potential for human exposure. Among the 275 substances on this list are included many of the heavy metals. The top three on the list are arsenic, lead, and

mercury; 5 of the top 20 are heavy metals (Cd (8) and Cr(VI) (18) are the other two).

Thus, the effective remediation of contaminated sites for reuse must invariably address

the effective removal of heavy metals from contaminated solid materials and water. 9

Heavy metals differ from other pollutants in that they often have significant natural

background sources from dissolution of rocks and minerals, as well as anthropogenic

sources. Almost all are toxic to humans and/or aquatic organisms, depending on

concentrations. Being elements, metals can neither degrade nor metabolize, and thus they

are the ultimate examples of environmental persistence. Soils can be contaminated by

heavy metals as a result of direct contact with plant waste discharges, landfills, and

leachates. Heavy metals also exist in ground and surface waters, and in the food supply.

Heavy metal contamination of soil and groundwater represents a serious threat to the

environment and human health.4

Three heavy metals with serious health risks that are included on the CERCLA

Priority List are lead, cadmium, and mercury. Cadmium is bio-persistent and its effects

on health are irreversible. Once absorbed by an organism, Cd remains resident for many

years (over decades for humans), although it is also slowly excreted over time. The

effects of cadmium on human health have been widely investigated.5,6 In humans, long-

term exposure is associated with nephrotoxicity and carcinogenicity. High exposures can

lead to obstructive lung disease and finally cause lung cancer. Bone osteomalacia and

osteoporosis in humans and animals are induced by direct action of Cd on bone through

abnormal calcium homeostasis.7,8 Environmental cadmium exposure can also be linked to

increased blood pressure.9

The USEPA Maximum Contaminant Level (MCL) is 5 parts per billion (ppb or µg

L-1) in drinking water. The USFDA limits the amount of cadmium in food coloring to 15

parts per million (ppm). OSHA currently limits the amount of cadmium in workplace air to 5 micrograms per cubic meter (µg m-3). 10

In the past, many human beings were highly exposed to lead due to its widespread

use. Its effects on human health have been studied since the mid-nineteen century. Lead can result in a wide range of biological effects according to the level and duration of exposure. Lead exists in blood, breast milk, and bone in the human body. It is widely known that lead is most dangerous to the developing fetus and infant.10,11 High levels of

exposure may result in problems with the synthesis of hemoglobin, effects on the

kidneys, the gastrointestinal tract, joints and reproductive system, and acute or chronic

damage to the nervous system. Lead exposure arises mainly from food, air and water.

Some people receive the bulk of their lead intake from food if lead-containing dust gets

onto crops while they are growing, or during food processing, and through food

containers. The major sources of lead in drinking water are lead piping, plumbing,

contaminated soil carried into water by rain and wind, paint flakes in houses with lead

paint, and contaminated land and wastewater from industries that use lead. Lead in the air

contributes to lead levels in food through deposition of dust and rain containing the

metal, on crops and soil. The USEPA has set the maximum contaminant level (MCL) for

-1 dissolved lead in drinking water to 15 ppb (15 µg L ).

Mercury is a toxic heavy metal that can exist in inorganic and organic forms.

Inorganic mercury poisoning is associated with tremors, gingivitis and/or minor

psychological changes, together with spontaneous abortion and congenital

malformation.12 Monomethylmercury causes damage to the brain and the central nervous

system and is infamous for its acute effects.13,14 In addition, fetal and postnatal exposures

have given rise to abortion, congenital malformation, and developmental changes in

young children. Mercury exposure from food often occurs when seafood containing high 11

accumulated mercury levels is ingested, or when mercury-containing plants, such as rice, are consumed. Humans inhale mercury in air that arises from a variety of fuel

combustion, incineration and industrial processes, as well as from natural sources. While

atmospheric concentrations are generally very low, mercury is deposited by wet and dry

processes in forest ecosystems, from which it can ultimately be transported to, and

accumulate in highly toxic forms in the food chain of aquatic ecosystems. The USEPA

MCL for inorganic mercury in drinking water is 2 ppb (µg L-1).

Since heavy metals are present in the environment and they are typically toxic, their

removal constitutes an important health issue. Water purity is increasingly important due

to the mounting shortage of potable water worldwide.

1.2: Techniques for Removal/Remediation/Recovery of Heavy Metals from

Contaminated Water and Soil

1.2.1: Remediation Technologies for Contaminated Soils

The management of soils contaminated with heavy metals is different and somewhat

more complex and than for water. First, the contamination is frequently heterogeneous at

the macroscale (spatial variation) or microscale (physical or chemical differences of

metal species within the soil matrix). Secondly, metals, being elements, cannot be degraded or destroyed. Thirdly, the variability of metal forms and the soil matrix influence the environmental risk assessment and the feasibility of soil treatment. Metals are discharged into the soil in a wide array of physico-chemical forms (ions, salts,

particles, etc). The soil-metal interactions depend on the specific metal form and soil

characteristics such as particle size, cation exchange capacity, pH, soil mineralogy, and

organic content. Remediation technologies of soils contaminated with heavy metals can 12

be classified as containment/disposal methods, monitored natural attenuation,

immobilization, and extraction treatment.21

1.2.1.1: Containment/Immobilization/Disposal

Containment/disposal is a method of isolating or relocating contaminated soils. It is

used to prevent migration of metals in groundwater or to other environmental media. The

USEPA15 reported that more than half of the remedial actions taken at Superfund sites in

the United States involved a containment/disposal technique. However,

containment/disposal does not directly treat the media contaminants.

Containment/disposal technology is most likely to be applicable to metal contaminants

for which mobility must be reduced as a temporary measure to mitigate risk until a

permanent solution can be tested and implemented.16,17 Interest in remediation treatments

has been driven by the demand for technologies that are cost effective alternatives to

containment/disposal options and suppress the long-term liability incurred with soil

disposal.

Immobilization techniques are akin to “locking” the contaminants in place into the

soil. They aim at sequestering or stabilizing metals in soils (i.e., minimizing the leaching

characteristics from the soil matrix, and changing metals to less soluble, toxic, or

bioavailable forms) to reduce the risks on human health and the environment. This

technique can be used alone or combined with other treatment and disposal methods. The

most common form of stabilization is the cement immobilization process. This simply

involves the addition of cement or a cement-based mixture, which limits the solubility or mobility of the waste constituents. These techniques are accomplished either in situ by

injecting a cement-based agent into the contaminated materials, or ex situ, by excavating 13

the materials, machine-mixing them with a cement-based agent, and depositing the

solidified mass in a designated area. The goal of solidification/stabilization (S/S)

processes is to limit the spread, via leaching, of contaminated material. The end product

resulting from the solidification process is a monolithic block of waste with high

structural integrity. Types of S/S agents include: Portland cement; gypsum; modified

sulfur cement, consisting of elemental sulfur and hydrocarbon polymers; and grout,

consisting of cement and other dry materials, such as acceptable fly ash or blast furnace

slag. Processes utilizing modified sulfur cement are typically performed ex situ. Table 1

presents brief descriptions, and summarizes the main advantages, disadvantages, and

limitations of heavy metal immobilization technologies.21

1.2.1.2: Natural Attenuation Processes

The remediation of contaminated sites can involve a natural attenuation process.

These are intrinsic processes, which are not active treatment technologies. As a passive

treatment, the cost of natural attenuation is low; however, with this approach contaminant

levels must be monitored regularly. Mulligan and Yong18,19,20 reviewed natural

attenuation technologies and suggested that they are primarily applicable to remediation

of soil and groundwater affected by organic contaminants. Monitored natural attenuation

is appropriate for some metals when the valence state of the metal changes, resulting in

the immobilization or toxicity reduction of that metal. An example of this is the reduction

of Cr(VI) to Cr(III) or oxidation of As(III) to As(V).21 Wang and Mulligan22 found that

sorption on solid phases is the principal mechanism immobilizing arsenic in soils and removing it from groundwater. They also reported that hydroxides of Fe, Al, and Mn clay and sulfide minerals, and natural organic matter are commonly associated with soils and 14

aquifer sediments, and have been shown to be significant adsorbents. Metal ions can also

be retained in the soil by precipitation, complexation, or redox reactions with soil

components (mineral or organic substrates, or microorganisms), via physicochemical or

biological processes.23 However, monitored natural attenuation becomes more effective when it is enhanced or assisted by engineered actions.21,22 Some accelerating engineering

actions are: (1) addition of mineral stabilizers such as lime, phosphates, and fly ash; (2)

addition of biosolids or the use of microbial processes; and (3) growing vegetation.

1.2.1.3: Extraction Methods

Extraction methods refer to processes that are used separate metals from soil, reduce

their concentration, or reduce the volume of the entire contaminated medium. Extraction

treatment aims at completely decontaminating a site by removing metals from the soil

matrix. The ideal goal of an extraction strategy is to recover metals for reuse and resale.

However, metal recovery is often impractical for projects that lack economic viability or

technical feasibility of the extraction and recovery processes. When metal contamination

and the soil matrix are highly heterogeneous or when metals are strongly bound to the

soil matrix, metal extraction becomes very difficult. In many cases, extraction methods

are used to reduce metal concentrations to an acceptable level, or to considerably reduce

the volume of contaminated soil. Table 2 presents brief descriptions, and summarizes the

principal advantages, disadvantages, and limitations of metal extraction technologies.21

Extraction of heavy metals from contaminated soils with aqueous systems can be one of the primary sources of contaminated water that are amenable to remediation technologies for contaminated water that is the principal objective of the current work. 15

1.2.2: Remediation Technologies for Contaminated Water

Groundwater, surface waters, and aqueous leachates from soils and other solid

wastes, are the most prevalent contaminated liquid media containing hazardous metals.

The most important and currently used methods for removing heavy metals from these

contaminated sources are adsorption, ion-exchange, membrane techniques, precipitation,

and electrolytic removal processes.

1.2.2.1: Adsorption

Sources of contaminated water include groundwater, surface waters, and aqueous

leachates from contaminated solids, as above. Adsorption is a fluid-solid interfacial

process in which a solute (adsorbate) is preferentially concentrated on the solid surface

(adsorbent) by selective physico-chemical interactions. Activated carbons have

undoubtedly been the most popular and widely used adsorbent for wastewater treatment

applications throughout the world due to their high capacity, economical application, and

manifold sources. There is a relatively large literature on the removal of heavy metals

from aqueous solutions by adsorption on carbons. A recent review by Dias et al.

summarizes recent work on the use of waste materials to produce carbons for aqueous

phase treatment.24

As specifically concerns the adsorption of heavy metals on carbons, Zhang et al.,25

Krishnan and Anirudhan,26 and Olson et al.27 have investigated mercury removal from

aqueous solutions. Zhang et al. found that the performance of activated carbons was dramatically improved by using a chemical activation process. ZnCl2 activated carbon

had the highest capacity for Hg(II) adsorption, followed by H3PO4 and H2SO4 activated carbons.25 The adsorption capacity was greatly affected by Hg(II) concentration, solution 16

pH, and carbon dosage. Krishnan and Anirudhan26 found that the optimum pH range for

the adsorption of Hg(II) at 30°C on their sulfurized carbons was between 4 and 9 and

between 6 and 9 for sulfur-free carbon. In addition, decrease in ionic strength and

increase in temperature of the solution were found to improve the uptake of Hg(II).26

Olson et al. studied the effects of carbon type, particle size, and temperature for mercury

control in combustion systems.27 Chen and Wu28 investigated the simultaneous adsorption of copper ions and humic acids (HA) onto an activated carbon. These workers found a critical concentration (CC) of HA for copper adsorption, and that adsorption was significantly increased by citric acid. Goyal et al.29 measured adsorption isotherms of

Cu(II) ions from aqueous solutions in the concentration range 40–1000 mg L−1 on two samples of granulated and two samples of activated carbon fibers containing varying amounts of surface oxygen functionalities. It was concluded that the amounts of surface oxygen groups were enhanced by oxidation with nitric acid, and that adsorption behavior depended on the nature of the oxidative treatment, while the decrease in adsorption noted upon degassing depended on the temperature of degassing.29

Issabayeva et al.30 and Nadeem et al.31 evaluated the performance of a commercially

available palm shell-based activated carbon for the removal of lead ions from aqueous

solutions. It was found that palm shell activated carbon exhibited high adsorption

capacity for lead ions, especially at pH 5, with an ultimate uptake of 95.2 mg g-1. Natale

et al.[32 determined adsorption isotherms of chromium ions from aqueous solutions. The

adsorption capacity of the granular activated carbon depended strongly on solution pH

and salinity, with maximum values around 7 mg g-1 at neutral pH and low salinity levels.

33 Liu et al. reported that a HNO3/NaOH/NaCl-modified activated carbon exhibited 17

excellent Cr(VI) adsorption capacity and rate. Rao et al.34 investigated the effects of

equilibrium time, pH, and adsorbent dose on the co-removal of copper and cadmium by

an activated carbon prepared from ceiba pentandra hulls. Erdoğan et al.35 investigated the

removal of Ni(II) ions from aqueous solution as a function of pH, activation temperature,

adsorbent dosage, and nickel ion concentration. It was found that the optimal conditions

were pH 5, 0.7 g/10 ml adsorbent dosage, 10 mg L-1 Ni(II) concentration, and 60 min contact time. Dabek36 showed that an activated carbon recovered from a spent catalyst

exhibited a high sorption capacity for zinc ions, comparable to that of a commercial

activated carbon. Galiatsatou et al.37 prepared an activated carbon via a two-step physical activation process in steam, which exhibited remarkable adsorption capacity for zinc at solution pH 7.

Metallic species are relatively small in size, and are frequently charged in solution.

Therefore, their predominant interactions in adsorption on activated carbon can be electrostatic in nature. López-Ramón et al.38 found that the factors that mainly control the

extent of adsorption on activated carbons are: (i) the chemistry of the metal ion or metal

ion complex; (ii) the solution pH and the point of zero charge of the surface; (iii) the

surface area and porosity (narrower and wider microporosity); (iv) oxygen surface

functionalities; and (v) the size of the adsorbing species (i.e., hydrated ions in the range

1.0–1.8 nm), mainly for carbons with significant volumes of narrow microporosity.

Sánchez-Polo and Rivera-Utrilla39 studied adsorbent/adsorbate interactions of Cd(II) and

Hg(II) on ozonized activated carbon, and concluded that electrostatic forces were

dominant for the adsorption of Cd(II), and dispersive forces prevailed in the adsorption of

Hg(II). They also investigated the adsorption of Cr(III) on ozonized carbon, and 18 concluded that although electrostatic and non-electrostatic forces both played an important role, electrostatic forces dominated the adsorption process in this case.40

In spite of its prolific use, activated carbons still remain a relatively expensive material, and the higher the quality of the carbon, the greater its cost. Activated carbons also typically require additional agents or additives to improve performance for inorganic materials like heavy metals. Therefore, other low cost and locally available adsorbents, such as chitosan, zeolites, clay, and certain waste products from industrial operations, such as fly ash, coal, and oxides has intensified. Chitosan has been used to remove heavy metals, such as cadmium.41 An adsorption capacity of 5.93 mg of Cd(II)/g of chitosan was reported over the pH range of 4.0–8.3.41 Maximum adsorption capacities of chitosan for Hg(II), Cu(II), Ni(II), and Zn(II) of 815, 222, 164, and 75 mg g-1, respectively, have also been reported.42

1.2.2.2: Ion-Exchange

Ion exchange processes involve the exchange of ions between two electrolytes or between an electrolyte solution and a solid material. Typical materials are ion exchange resins (functionalized porous or gel polymers), zeolites, montmorillonite, clays, and soil humus. Ion exchange materials could be either cationic or anionic exchangers. There are also amphoteric exchangers that are capable of exchanging both cations and anions simultaneously. Heavy metal removal usually involves cation exchangers. Natural zeolites are inorganic cation exchangers that exhibit high ion exchange capacity, selectivity, low cost, and compatibility with natural environments. The ion exchange properties of zeolites are related to their structure which is characterized by a framework of linked tetrahedra, each consisting of four oxygen atoms surrounding a cation. This 19

framework contains open cavities in the form of channels and cages. These are usually

occupied by H2O molecules and extra-framework cations, such as Na(I), K(I), Ca(II),

Mg(II), and others, that are commonly exchangeable. These positive ions are rather

loosely held and can readily be exchanged for heavy metals in a contact solution. In the

last decade or so, extensive research has been undertaken to identify the capability of

zeolites to remove certain pollutants, as well as to understand exchange mechanisms.

Zeolites have been shown to preferentially remove undesired heavy metals such as

strontium, cesium, and cadmium.43,44

A good recent overview of the removal of heavy metals by ion exchange is provided

45 46 by Dbrowski et al. Llanes-Monter et al. reported ion exchange capacities of Mexican

clinoptilolite-rich tuff for Pb as high as 1.4 meq g-1 at pH 3. The mechanism was

considered to be ion exchange in the absence of any precipitated or hydrolyzed lead species in the sorption process or any change in the zeolite network. Kazemian et al.47

also studied lead removal from wastewater by Iranian natural zeolites. It was reported that Pb removal from aqueous solution by clinoptilolites-rich tuffs was quite effective.

Arambula-Villazana et al.48 investigated cadmium retention behavior on Mexican zeolitic rich tuff as a function of temperature. The kinetics and isotherms were determined at 303,

318, and 333K. Pseudo-second order rate constants, as well as apparent diffusion

coefficients were determined from the cadmium uptake by the zeolitic material as a

function of contact time and temperature. The maximum cadmium adsorption capacity

was 12.2 mg Cd(II)/g at 318K, corresponding to 20% of the effective ion exchange

capacity of the zeolite. The activation energy of Cd(II) removal by the Chihuahua zeolitic

rock indicated a chemical sorption process. García-Sosa et al.49,50 studied the sorption 20

properties of the zeolite for cadmium and cobalt, measured their cation exchange

capacities and found that the retention of cobalt and cadmium was reduced by the organo-

zeolite due to the presence of HDTMA (hexadecyltrimethyl-ammonium bromide).

García-Sosa and Solache-Ríos51 also studied the sorption behavior of cobalt and

cadmium removal by Mexican erionite as a function of solution pH and determined the

ion exchange isotherms. It was found that the sorption capacity for cadmium was greater

than for cobalt; however, the selectivity for both cations was low for this zeolite. Olguín

et al.52 reported the apparent diffusion coefficient of Sr(II) in natural Mexican erionite

from Sonora and showed that the mobility of Sr(II) through the cavities of the erionite

depended on the concentration of strontium and the pH of the solution; i.e., the maximum

–1 sorption of Sr(II) occurred at pH > 3 and 0.0094 mol L strontium nitrate.

Nisso ALM-525 (Society of Synthetic Organic Chemistry, Japan) is a common ion

exchange resin used for selective Hg(II) removal. Its sorption capacity for Hg(II) ion is

−1 −1 680 g Hg kg of ion exchanger in aqueous solution but 340 g Hg kg in 10% H2SO4

solution. In a continuous process it reduced the Hg(II) ion content in a wastewater to 0.1

ppb. Sodium sulfide solution was used to extract accumulated Hg(II) from the

adsorbent.53,54,55 Lin and Kiang56 investigated chromium removal by passing a

chromium-containing solution through a strongly acidic polystyrenesulphonic cation

exchanger and subsequently through a strongly basic anion exchanger. These workers

used NaOH solutions for anion exchanger regeneration.56

Tenório and Espinosa57 examined the efficiency of two purification systems for

wastewaters containing chromates. One was Amberlite IR-120Na (gel type polystyrenedivinylbenzene), a strongly acidic cation exchanger, and Amberlite IRA-420 21

(gel type polystyrenedivinylbenzene) (System 1), and the other was Amberlite IR-120Na, a cation exchanger, on the weakly basic anion exchanger Amberlite IR-67RF (gel type acrylate) (System 2). System 2 proved to be better for such purposes. Chelating ion exchangers have been applied to the selective recovery of Ni(II) from industrial wastewaters originating from nickel plating and nickel compounds production.58 Duolite

ES-346 exhibited good selectivity for Cu(II), with which it formed the strongest complexes.59 A polyvinylpyridine resin containing functional dithizone groups were also used to separate two and three-component metal ion mixtures. The capacities of this resin for Zn(II), Ni(II) and Cu(II) ions were 0.65, 0.59 and 0.51 mmol g−1, respectively.60

1.2.2.3: Membrane Processes

A membrane is a material through which one species can pass more readily than others.61 It acts as a semipermeable barrier separating one bulk solution phase from another while allowing the selective transport of certain specific molecules in either direction across the barrier, and preventing the simultaneous transport of other molecules.

When used for wastewater treatment, membranes may serve to reject pollutants and allow purified water through; extract pollutants from water, or as a vehicle for the bubble-less transfer of gas into wastewater to augment microbial degradation.

Qdais and Moussa62 investigated the application of both reverse osmosis (RO) and nanofiltration (NF) technologies for the treatment of wastewater containing copper and cadmium ions. It was shown that high removal efficiency of these metals could be achieved with RO (98% and 99% for copper and cadmium, respectively). NF was capable of removing greater than 90% of the copper ions in the feed water. Gopal et al.63 investigated selective lead recovery from dilute aqueous multi-cation waste streams with 22

a membrane process. It was shown that selective recovery of Pb(II) and Cu(II) ions from

dilute aqueous binary solutions depended on the applied cathodic potential which

controlled the rate and selectivity of ion-exchange. An inverse relationship was found

between the applied cathodic potential and the ion-exchange rate, while a direct relationship existed between the applied cathodic potential and Pb(II) ion selectivity.

Guha et al.64 found that heavy metals like Cu(II), Cr(VI), and Hg(II) can be

successfully removed from wastewater using hollow-fiber-contained liquid membranes

(HFCLM). Chakraborty et al.65 used emulsion liquid membranes (ELMs) and di-(2- ethylhexyl) phosphoric acid (D2EHPA) to remove copper and nickel ions from electroplating wastewater. These workers studied the competitive transport of the two ions as well as interference of other ions present in the wastewater, and indicated that with short contact time (8-10 min) and 0.05 mol L-1 buffer concentration of the feed

phase, moderate acidity of the feed phase solution (pH 3.5 - 5.5) was a critical parameter

for the effective recovery of Cu (II) and Ni(II). Keeping the feed phase pH at 3.5, if the

internal strip phase acid concentration was varied from 1 to 2N, the recovery of copper

was observed to increase up to 99%. Finally, they showed that the coexisting ions Cr(III)

and Fe(II), that were present in the electroplating wastewater, did not appreciably affect the separation performance.

1.2.2.4: Precipitation, Flocculation, and Filtration

Chemical precipitation is perhaps the simplest and most economical method of removing heavy metals from aqueous solutions.66 The precipitation of metal ions in

insoluble forms, either as metal hydroxides or metal sulfides, can be used to effectively

remove a number of metals from water. Traditionally, hydroxide precipitants such as lime 23

and caustic soda have been favored over their sulfide counterparts, due to the higher cost

of chemically produced hydrogen sulfide and the hazards associated with its application.

The formation of sodium hydroxide introduces the least amount of inert material to the

precipitate sludge, but it also produces large volumes of effluent and metal-laden sludge

that must be subsequently treated.67 The volume of sulfide precipitate sludge is generally

less. This has significant financial impact on waste management strategies for metal

producers, since smaller volumes result in lower disposal or reclamation costs. In

addition, the solubilities of metal sulfides are less than their corresponding hydroxides

and carbonates. Even moderate sulfide addition can effectively reduce dissolved metal

levels to well below those permitted for environmental discharge.68 In addition, in certain

cases metals can be recovered from the sulfide sludges.69 For acid mine drainage,

Machemer and Wildeman70 showed that there was some competition among Fe, Cu, Zn, and Mn. Fe and Cu appeared to be more effectively removed than Zn and Mn. Metal precipitation varied with the fluctuation of pH in the outflow water. As S2- was generated,

Cu and Zn were completely removed. These authors recommended that sulfide

precipitation should become the dominant process for metals removal from wetlands.

Machemer et al.71 reported similar results in studies conducted at the Big Five Tunnel

Remediation Project in Idaho Springs, Colorado.

[72] Foucher et al. studied the selective precipitation of metals using H2S produced by

sulfate-reducing bacteria. It was shown that Cu and Zn could be selectively recovered at

pH 2.8 and pH 3.5, respectively. Other impurities such as Ni and Fe could also be

removed at pH 6 by sulfide precipitation. White et al.68 reported that microorganisms

which generate sulfide play important roles in the environmental fate of toxic metals, 24

with a multiplicity of physico-chemical and biological mechanisms effecting transformations between soluble and insoluble phases. Such mechanisms are important

components of natural biogeochemical cycles for metals and metalloids, with some

processes exhibiting potential for application to the treatment of contaminated materials.

Kim et al.73 investigated metal sulfide precipitation as an alternative to metal hydroxide

precipitation for the removal of Cu, Ni, Pb, and Zn from wastewater. It was found that

precipitation removed Cu well, but not Ni and Zn. Transmission and engine plant

wastewater were more difficult to treat than assembly plant wastewater, suggesting that it

might have contained more chelating agents.

Guo et al.74 reported the treatment of Hg(II)-containing wastewater with natural iron-

bearing sulfide. It was found that Hg(II) was effectively removed from the wastewater,

and that the rate of Hg(II) precipitation could be increased by raising the pH from the

beginning to the end of the treatment. Shinohara and Katoh75 examined Cd removal from

wastewater by sulfide precipitation with Na2S. It was found that Cd precipitated via two

mechanisms – direct, and indirect CdS precipitation through Cd(OH)2 due to hydrolysis

76 of Na2S. Xu et al. have reported reducing Cr(VI) to Cr(III) in wastewater with Na2S2O5.

In general, precipitation is an effective method for the removal of heavy metals.

However, the creation of considerable volumes of objectionable sludges represents a

serious solid waste disposal problem.

1.2.2.5: Electrolytic Techniques

Electrowinning, electrolytic recovery (ER), or electroextraction, is the

electrodeposition or reduction of metal ions from an electrolyte in an electric field. In

conventional electrolytic recovery, flat or planar electrodes are immersed in the 25 electrolyte solution to be treated. A potential is imposed between the electrodes, and a direct current is passed through the solution. At the cathode, positively charged metal ions diffuse to the surface where they form a surface complex, receive electrons from the cathode, and are reduced to the metallic state. The metal can be present in the solution as a free metal cation or as a complexed metal cation – a cyanide complex for example.

Electrolytic recovery of most metals from dilute solutions proceeds at fairly low current efficiencies. The recovery rate can be augmented by increasing liquid agitation, current density and the cathode surface area. The current density (i.e., current per unit surface area of the electrode), however, must be maintained below the point where the electrolysis of water becomes significant, since current would then be used for electrolysis rather than metal reduction. Additionally, it has been suggested that the heavy generation of gases can effectively blanket electrodes and, thereby, impede metal deposition. The geometric surface area of the cathode can be increased by using additional electrodes; however, this generally increases equipment and maintenance costs.

At high cathodic overpotentials where ER is typically operated, the primary transport mechanism for metal ions to the surface of the cathode is ordinary or bulk diffusion. This is why metals can sometimes be recovered at the cathode even if they exist as part of a negatively charged complex. At very low current densities, the concentration of metal ions at the surface of the cathode is essentially the same as in the bulk electrolyte solution, and the rate of reduction at the cathode will be proportional to the current density. At higher current densities, however, the ion concentration becomes appreciably depleted at the surface, and the rate of metal reduction becomes limited by the diffusion 26

rate of metal ions to the cathode surface. This places a practical limit on the current

density that can be effectively applied. The rate of diffusion of metal ions through the

ion-depleted layer will be proportional to the concentration gradient in the layer.

Invoking the Nernst assumption of a linear concentration gradient in the diffusion layer,

and the fact that the metal ion concentration at the cathode can often be close to zero, the

concentration gradient will be the bulk metal ion concentration divided by the Nernst

layer thickness, C/δ. The resultant expression for the diffusion-limited current density is:

iL = −DnFC / δ, (1-1)

where iL is the limiting current density; D, the diffusion coefficient of the metal ions; n

the charge of the metal ions; F is Faraday’s constant; C is the concentration of metal ions

in the bulk liquid; and δ is the thickness of the Nernst diffusion layer. The latter depends

on the hydrodynamics, or extent of mixing of the solution adjacent to the electrode. For a

stagnant solution, the Nernst layer thickness is on the order of 0.05 cm. For an agitated

solution, the thickness will be on the order of 0.01 - 0.005 cm. The boundary layer thickness essentially controls the limiting current density for a planar cathode. However, the current efficiency, typically starts to fall off at current densities that are approximately an order of magnitude less that this, because as the metal ion concentration at the cathode decreases, other electrode reactions begin to intrude. Therefore, in order to maintain high current efficiency, the system must be operated at sufficiently low current densities.

Consequently, it is primarily the heterogeneous, diffusion-limited reduction reaction that limits the metal recovery rate. Thus, improvements in ER performance usually focus on minimization of the Nernst layer thickness. 27

In typical ER systems with planar electrodes, the deposited metal is usually removed

by scraping it from reusable steel sheet cathodes. This process can be arduous, time-

consuming, expensive, and may result in limiting “time-on-stream” capability. In order to alleviate some of these problems, equipment with disposable porous metallized plastic cathodes have been developed. The performance of these units is enhanced by the large surface area of the porous cathodes. Also, typically metal scraping is not required, since the cathodes are simply replaced when they become sufficiently loaded. However, the surface area of the cathodes decreases significantly with metal deposition due to pore occlusion, thereby reducing recovery efficiency.77 In addition, the electrode configuration

in these systems is such that the electrolyte velocity through the cathode is quite limited,

which also decreases system performance. Finally, the plastic substrate of the porous

cathodes complicates the recovery of the metal and substantially decreases its value for

recycling.

Other systems employ plating barrels containing metal shot (i.e., tumbling particle beds) as cathodes.78,79,80,81,82 This eliminates the problem of scraping the electrodes and yields the metal in a “nugget” form for recycling. Unfortunately, the small openings in the plating barrels restrict the circulation of the electrolyte and increase the cathode-to- anode electrical resistance, thereby reducing the amount of current and current density, which can be effectively applied to such systems.

BASF obtained a patent on a unique ER system that uses a conductive carbon or metal powder “filter aid material” on a porous sintered cylinder as a cathode.83 The

electrolyte is forced through the powder and sintered cylinder, which serves as the

cathode. Metal is deposited on the powder, and the flow is then reversed; i.e., the cylinder 28

is backwashed. The powder is then separated from the electrolyte, fresh “filter aid

material” is introduced, and the process is repeated. The total cycle time is about two minutes. This system promises high current efficiencies at large current densities, since the powder cathode has a high surface area and the electrolyte is forced at high velocities through the small interstitial volume of the powder, which decreases the Nernst diffusion layer significantly. However, the enhanced performance of this approach is obtained at the expense of considerable operational complexity. This is undoubtedly the primary reason that this type of system has not been made commercially available.

Packed beds of porous electrodes also have been shown to remove metal ions from

aqueous solutions to very low concentrations. Simonsson84 studied the performance of

full-scale packed bed electrode on heavy removal from wastewater. It was found that

noble metals could be electrodeposited easily, even if bound in strong complexes, while

deposition of Zn from acid solution was highly pH-dependent. Dietz et al.85 also reported

the effective Cu removal from electroplating wastewaters by electrolysis in a packed-bed

of graphite particles. Chu et al.86 investigated the cathodic deposition of Cu from flowing

dilute aqueous solution in a packed bed. It was found that the electrodeposition reaction

was mass-transfer controlled, and expressions were presented for the cathodic current as

a function of time, solution flow rate, and bed characteristics. It was concluded that

packed-bed electrodes are an effective means of extracting or removing metal ions from

dilute solutions. Nishtala et al.87 proposed a numerical model for estimating the current

efficiency of electrochemical removal of metal from electroplating wastewaters with

packed bed porous electrodes. The model accounts for mass transfer-controlled primary

and kinetics-controlled parasitic reactions, ohmic effects, and current-dependent 29

boundary conditions. However, packed bed electrodes are limited in operating life due to

eventual agglomeration into a solid mass, as shown by Houghton and Kuhn.77 Ferreira88

reviewed the electrochemical process of heavy metal removal in packed bed electrodes,

and concluded: “The main drawback of a packed bed is that since the particles are in

permanent contact, deposition of the metal occurs within the matrix and gradually blocks

the interstices. This causes agglomeration of the bed and impairs mass transfer

performance. The pressure drop across the cell increases and, after prolonged metal

deposition, satisfactory operation becomes impossible.”89

Fluidized beds have also been investigated for use as ER cathodes.90,91,92 They offer

excellent liquid-solid contacting and should not suffer as much from pore occlusion as do porous electrodes, or from particle occlusion and agglomeration as in packed beds.

However, fluidized bed cathodes exhibit very poor electrical contact between the fluidized particles, which is a function of bed expansion, inhomogeneous electrical potentials, and particle segregation effects.93 Coeuret94 reviewed the application of

fluidized bed electrodes to the recovery of metals - primarily copper. Lee and Yang95

investigated the removal of Cu, Pb, and Ni from wastewater with sequential fluidized bed

reactors (FBRs). Removal efficiencies of 96%, 93%, and 98% for Cu, Pb, and Ni were

reported for feed concentrations of 250, 130, and 130 mg L-1, respectively. Han and Liu96

reviewed the removal of Cd and Zn using liquid fluidized beds. Zhang97 discussed the dynamic properties of diffusion and mass transfer, reaction, integration, and growth of metal deposit in a fluidized bed electrode (FBE) for treating metal-contaminated wastewater. A macro-kinetics model was developed to describe metal ion mass transfer and deposition processes from the bulk solution to the particle electrodes (PE), based on 30 boundary-layer theory. However, the existence of anodic or pseudo-anodic regions was identified that did not exist when the bed was fixed (i.e., unfluidized).97 This same behavior was also observed experimentally by Hadzismajlovic et al.98 This is one of the principal drawbacks of fluidized bed electrodes. Similar behavior has also been predicted in packed beds as a function of bed depth.99

In addition to the preceding, the requisite flow velocities must be quite large to fluidize the heavy particles that would typically be used and which result from metal deposition. Also, Ferreira88 pointed out that the range of overpotentials was spatially distributed in his review of heavy metal removal with fluidized beds. All these negative factors suggest that fully fluidized beds will probably never be the basis of a practical ER system.

1.3: Spouted Particulate Electrodes (SPE)

The focus of the current work is on the application of spouted particulate electrodes

(SPE) to electrolytic removal/remediation/recovery of heavy metals. SPE utilize conductive particles that are circulated hydrodynamically in a vessel. Although different designs have been used, they all share the characteristic that the cathodic particles are circulated by entrainment, typically as a low density liquid-solid mixture in a transport- line or entrained flow mode. In a certain sense, SPE are essentially hybrids of fixed and fluidized (entrained flow) bed electrodes, incorporating advantages of both, while minimizing some of their disadvantages. For example, SPE are difficult to plug or foul with attendant contaminant solids due to the low particle concentrations that exist everywhere in the vessel except in the moving bed cathode itself. Also, the particles do 31

not agglomerate due to metal electrodeposition as long as spouting and particle

movement in the moving bed cathode are maintained.

The application of spouted particulate electrodes to copper electrowinning from

acidic solutions was examined by Jiricny et al.,100,101 Stankovic and Stankovic,102 and

Masterson and Evans.103 Evans and coworkers also investigated the application of similar

systems for zinc electrowinning.104 These workers all employed a rectangular cell design

with the electrodes serving as the sidewalls. The current flow in this system was in the

horizontal direction, perpendicular to the electrolyte flow. The cathode particles were

fluidized and separated from the adjacent anode by a membrane. Since the anolyte and

catholyte are separated, oxygen formed at the anode is prevented from participating in

metal corrosion reactions. This resulted in a very high current efficiency (close to

100%).100,102,103 However, in this design the electrical contact between particles in the vicinity of the cathode, and the particles with the cathode is provided only via random chains of particles in instantaneous contact with the current feeder. Such chains have only momentary existence since they are continually broken and reformed by the moving particles. This conduction mechanism effectively increases the overall resistance to the flow of current carriers, resulting in an increase in the specific electrical energy required for deposition, as noted by Goodridge, et al.105 Jiricny et al.100 reported a current

efficiency of 99.6% with an energy consumption of 2.06 kWh kg-1 copper deposited, for a current density of 3.475 kA m-2.

Copper electrowinning from acidic solutions was investigated by Jiricny et al.100,101 in

a spouted particulate electrode device. These authors reported higher current densities

and lower electrical energy consumption than with more conventional electrowinning. 32

Stankovic and Stankovic102 and Masterson and Evans103 also investigated copper

electrowinning in a spouted particulate electrode. These authors concluded that the

potentiostatic mode of operation appeared to be somewhat better than the galvanostatic

mode for electrowinning with respect to current efficiency. Shirvanian and Calo106 found good performance for copper recovery in acidic solutions in a SPE of the type used in the current work. Evans and Jiricny104 investigated Zn electrodeposition from Na

zincate/hydroxide electrolytes in a spouted particulate electrode. It was demonstrated that

high current efficiency (> 95%) could be achieved under a broad range of conditions.

Yao et al.107 reported good performance for the removal of nickel from acidic solutions in

the same device used by Shirvanian and Calo.106 The rate of nickel electrowinning

increased with increasing pH and increasing temperature in acidic solutions. Nitrogen

sparging of the electrolyte solution in the holding tank was effective in reducing the

dissolved oxygen concentration and suppressing the nickel corrosion reaction. Yao and

Calo[108] also investigated co-electrodeposition of copper and nickel from aqueous solution mixtures in the same apparatus. Both the quantitative and qualitative behavior of co-deposition of the metals from solution mixtures differed considerably from that of the single metals alone from solution. The metal displacement reaction between Ni(0) and

Cu(II) effectively eliminated the copper corrosion reaction, and considerably increased the rate of copper removal. This occurs at the expense of effectively increasing the net nickel corrosion (at least initially). However, this behavior amplified the separation of the recovery behavior of the two metals in time, such that relatively pure solid metal deposits of each metal could be obtained, if so desired. 33

The SPE design adopted for the current work is cylindrical and incorporates a draft

tube for particle entrainment and circulation, and a moving cathodic bed on the vessel

bottom.109,110 A schematic of this design is presented in Figure 1.2. As shown, the liquid

jet flows vertically upwards along the centerline of the vessel, forming a spout containing

a fast moving mixture of liquid and entrained particles. On the conical bottom, a densely

packed bed composed of slowly moving particles moves downwards and radially inwards

towards the particle entrainment point at the draft tube inlet. Backflow of liquid from the

liquid jet percolates through the moving bed, but not at a sufficient rate to fluidize them.

The moving bed of particles on the conical vessel bottom serves as the cathode current

feeder. The anode electrode is mounted above and parallel to the cathodic moving bed.

Another important feature of this design is the incorporation of the particle collector/distributor. In the cylindrical vessel design this is an inverted cone that intercepts particles as they fall from the particle “fountain” in the vessel freeboard above

the draft tube outlet, and directs them radially and centrifugally to the vessel periphery

where they fall onto and become part of the moving bed cathode on the vessel bottom.

This ensures that all the particles on average spend the same residence time in the moving

bed cathode in the electric field; i.e., there is a very sharp residence time distribution

(RTD) for the particles in the moving cathodic bed.111 As a consequence, the particles remain mono-sized as they grow by metal electrodeposition.

References

1 Duffus, JH (2002) Pure and Applied Chemistry, 74, p. 793 2 A Dictionary of Chemistry, Oxford University Press, 2000, Oxford Reference Online, Oxford University Press 3 2007 CERCLA Priority List of Hazardous Substances, (2007) Agency for Toxic Substances and Disease Registry, Atlanta, GA 4 Dermont, G; Bergeron, M; Mercier, G; Richer-Lafleche, M (2008) Practice Periodical of Hazardous, Toxic, and Radioactive Waste Management, p.188 5 Loewe, S (1989) Forum Städte-Hygiene, 40(5), p. 286 6 Jin, T; Lei, L; Chang, X (2006) Zhonghua Laodong Weisheng Zhiyebing Zazhi, 24(1), p. 1 7 Katsuta, O; Hiratsuka, H; Matsumoto, J; Iwata, H; Toyota, N; Tsuchitani, M; Umemura, T; Marumo, F. (1994) Toxicology and Applied Pharmacology 126(1), p. 58 8 Honda, R; Tsuritani, I; Ishizaki, M and Yamada, Y (1997) Environmental Research, 75(1), p. 41 9 Kurihara, I; Kobayashi, E; Suwazono, Y; Uetani, M; Inaba, T; Oishiz, M; Kido, T; Nakagawa, H; Nogawa, K (2004) Archives of Environmental Health, 59(12), p. 711 10 Berkowitz, Z; Price-Green, P; Bove, FJ; Kaye, WE (2006) International Journal of Hygiene and Environmental Health, 209(2), p. 123 11 Yin, J; Zhang, R; Niu, Y (2004) Zhongguo Gonggong Weisheng, 20(10), p. 1176 12 Bartolucci, GB; Boffetta, P; Mantovani, A; Chiesara, A (2002) Medicina del Lavoro, 93(3), p. 290 13 Galli, CL; Restani, P (1993) Pharmacological Research, 27(2), p. 115 14 Fitzgerald WF; Clarkson TW (1991) Environmental health perspectives, 96, p. 159 15 “Cleaning up the nation’s waste sites: Markets and technology trends, 4th ED.” EPA 542-R-04-014. Office of Solid Waste and Emergency Response, Washington, D.C 16 USEPA, Technology Alternatives for the Remediation of Soils Contaminated with As, Cd, Cr, Hg, and Pb. (1997). Engineering Bulletin No: EPA 540/S-97/500, Office of Solid Waste and Emergency Response, Technology Innovation Office, Washington, D.C. 17 USEPA, Solidification/Stabilization Use at Superfund Sites. (2000) EPA-542-R-00 010, Office of Solid Waste and Emergency Response. Washington, D.C 18 Mulligan, CN; Yong, RN (2004) Environment International, 30(4), p. 587 19 Mulligan, CN; Yong, RN (2006) ASTM Special Technical Publication, STP 1482 (Contaminated Sediments: Evaluation and Remediation Techniques), p. 210 20 Wang, S; Mulligan, CN (2006) Environmental Geochemistry and Health, 28(3), p. 197 21 Federal Remediation Technologies Roundtable (FRTR). “Remediation technologies screening matrix and reference guides, Version 4.0,” (2007), U.S. Army Environmental Center, APG, MD 22 Wang, S; Mulligan, CN (2006) Journal of Hazardous Materials, 138(3), p. 459 23 Adriano DC; Wenzel, WW; Vangronsveld, J and Bolan NS (2004) Genderma,

122(2-4), p. 121 24 Dias, JM; Alvim-Ferraz, MCM; Almeida, MF; Rivera-Utrilla, J; Sánchez-Polo, M (2007) J. Environ. Manag. 85, p. 833 25 Zhang, FS; Nriagu, JO; Itoh, H (2005) Water Res. 39, p. 389 26 Krishnan KA; Anirudhan, TS (2002) Journal of Hazardous Materials, 92, p. 161 27 Olson, ES; Miller, SJ; Sharma, RK; Dunham, GE; Benson, SA (2000) Journal of Hazardous Materials, 74, p. 61 28 Chen, JP; Wu, S (2004) Journal of Colloid and Interface Science, 280, p. 334 29 Goyal, M; Rattan, VK; Aggarwal, D; Bansal, RC (2001) Colloids and Surfaces A: Physicochemical and Engineering Aspects, 190, p. 229 30 Issabayeva, G; Aroua, MK; Sulaiman, NMN (2006) Bioresource Technology 97, p. 2350 31 Nadeem, M; Mahmood, A; Shahid, SA; Shah, SS; Khalid, AM; McKay, G. (2006) Journal of Hazardous Materials, 138, p. 604 32 Natale, FD; Lancia, A; Molino, A and Musmarra, D (2007) Journal of Hazardous Materials, 145, p. 381 33 Liu, SX; Chen, X; Chen, XY; Liu, ZF; Wang, HL (2007) Journal of Hazardous Materials, 141, p. 315 34 Rao, MM; Ramesh, A; Purna, G; Rao, C; Seshaiah, K (2006) Journal of Hazardous Materials 129, p. 123 35 Erdoğan, S; Önal, Y; Akmil-Basar, C; Bilmez-Erdemoğlu, S; SarIcI-Özdemir, C; Koseoğlu, E; İçduygu, G (2005) Applied Surface Science 252, p. 1324 36 Dabek, L (2003) Journal of Hazardous Materials 101, p. 191 37 Galiatsatou, P; Metaxas, M; Kasselouri-Rigopoulou, V (2002) Journal of Hazardous Materials, 91, p. 187 38 López-Ramón, V; Moreno-Castilla, C; Rivera-Utrilla, J; Radovic, LR (2002) Carbon, 41, p. 2020 39 Sánchez-Polo, M; Rivera-Utrilla, J (2002) Env. Sci. Tech. 36, p. 3850 40 Rivera-Utrilla, J; Sánchez-Polo, M (2003) Water Res. 37, p. 3335 41 Jha, IN; Iyengar, L; Rao, AVSP. (1988) J. Env. Eng. 114(4), 962-74 42 McKay, G; Blair, HS; Findon, A (1986) Immobilisation Ions Bio-sorption, p. 59 43 Grant, DC; Skriba, MC; Saha, AK; Ploetz, DK (1988) AIChE Symposium Series, 84(264, Adsorpt. Ion Exch.), p. 13 44 Malliou, E; Loizidou, M; Spyrellis, N (1994) Science of the Total Environment, 149(3), p. 139 45 Dabrowski, A; Z. Hubicki, Z; P. Podkościelny, P; Robens, E (2004) Chemosphere 56, p. 91 46 Llanes-Monter, MM; Olguin, MT; Solache-Rios, MJ (2007) Env. Sci. Poll. Res. Int. 14(6), p. 397 47 Kazemian, H; Rajec, P; Macasek, F; Orechovska Kufcakova, J (2001) Studies in Surface Science and Catalysis, 135(Zeolites and Mesoporous Materials at the Dawn of the 21st Century), p. 5172 48 Arambula-Villazana, V; Solache-Rios, M; Olguin, MT (2006) Journal of Inclusion Phenomena and Macrocyclic Chemistry, 55(3-4), p. 229

49 García-Sosa, I; Solache-Ríos, M; Olguín, MT; Jiménez-Becerril, J (2003) J Radioanal Nucl. Chem. 256, p. 273 50 García-Sosa, I; Solache-Ríos, M (2001) J. Radioanal. Nucl. Chem. 250, p. 205 51 García-Sosa, I; Solache-Ríos, M (1997) J. Radioanal. Nucl. Chem. 218, p. 77 52 Olguín, MT; García-Sosa, I; Solache-Ríos, M (1996) J. Radioanal. Nucl. Chem. Art. 204, p. 415 53 Bolto, BA; Pawowski, L (1987) Wastewater Treatment by Ion-Exchange, E. and F.N. Spoon Ltd., London 54 Warshawsky, A (1987) Chelating ion exchangers. In: Streat, M. and Naden, D., Editors, 1987. Ion Exchange and Sorption Processes in Hydrometallurgy, John Wiley and Sons, Chichester–New York–Brisbane–Toronto–Singapore 55 Dorfner, K, Editor, T (1991) Ion-Exchangers, Walter de Gruyter, Berlin-New York 56 Lin, SH; Kiang, CD (2003) Chem. Eng. J. 92, p. 193 57 Tenório, JAS; Espinosa, DCR. (2001) Waste Manage. 21, p. 637 58 Koivula, R; Lehto, J; Pajo, L; Gale, T; Leinonen, H (2000) Hydrometallurgy 56, p. 93 59 Ferreira, LM; Loureiro, JM; Rodrigues, AE (1998) Sep. Sci. Technol. 33, p. 1585 60 Shah, R; Devi, S (1998) Talanta, 45, p. 1089 61 Mack, C; Burgess, JE; Duncan, JR (2004) Water SA, 30(4), p. 521 62 Qdais, HA; Moussa, H (2004) Desalination, 164(2), p. 105 63 Gopal, V; April, GC; Schrodt, VN (1998) Separation and Purification Technology, 14(1-3), p. 85 64 Guha, AK; Yun, CH; Basu, R; Sirkar, KK (1994) AIChE Journal, 40(7), p. 1223 65 Chakraborty, M; Bhattacharya, C; Datta, S (2004) Journal of the Institution of Engineers (India), Chemical Engineering Division, 85(Sept.), p. 1 66 Sheikholeslami, R; Bright, J (2002) Desalination, 143(3), p. 255 67 Van Hille, RP; Boshoff, GA; Rose, RP; Duncan, JR (1999) Res. Conserv. Recyc. 27, p. 157 68 White, C; Sayer, JA; Gadd, GM (1997) FEMS Microbiol. Rev. 20, p. 503 69 Kaksonen, AH; Riekkola-Vanhanen, ML; Puhakka, JA (2003) Water Res. 37, p. 255 70 Machemer, SD; Wildeman, TR (1992) J. Contam. Hydrol. 9(1-2), p. 115 71 Machemer, SD; Reynolds, JS; Laudon, LS; Wildeman, TR (1993) Applied Geochemistry, 8(6), p. 587 72 Foucher, S; Battaglia-Brunet, F; Ignatiadis, I; Morin, D (2001) Chem. Eng. Sci. 56(4), p. 1639 73 Kim, BR; Gaines, WA; Szafranski, MJ; Bernath, EF; Miles, AM (2002) Journal of Environmental Engineering (Reston, VA, United States), 128(7), p. 612 74 Guo, M; Lu, A; Lu, X (1999) Yanshi Kuangwuxue Zazhi, 18(4), p. 309 75 Shinohara, R; Katoh, N (2005) Kagaku Kogaku Ronbunshu, 31(3), p. 217 76 Xu, Y; Yang, H; Feng, P; Zhang, G; Li, F (2000) Cailiao Baohu, 33(12), p. 33 77 Houghton, RW; Kuhn, AT (1974) J. Appl. Electrochem. 4(3), p. 173 78 Dietmar, HJ and Klaus, G (1975) Patent 114624 79 Hertwig, VK; Bergmann, H; Nieber, F (1992) Galvanotechnik, 83(5), p. 1696 80 Kammel, R; Liber, HW (1978) U.S. Patent 4123340 81 Kirchhof, H; Schab DD (1986) Patent 231585

82 Zhou, CD; Chin, DT (1994) Plating and Surface Finishing, 81, p. 70 83 Huber EA, (1992) U.S. Patent 5164091 84 Simonsson, D (1984) Journal of Applied Electrochemistry, 14(5), p. 595 85 Dietz, WW; Reynvaan, CHH (1982) Forsch. Technol., Technol. Forsch. B Entwickl. Issue BMFT-FB-T 82-048, p. 71 86 Chu, AKP; Fleischmann, M; Hills, GJ (1974) Journal of Applied Electrochemistry, 4(4), p. 323 87 Nishtala, M; Savinell, RF; Willis, MS (1983) III Proceedings - Electrochemical Society, 83(12), Issue Proc. Symp. Electroplat. Eng. Waste Recycle New Dev. Trends, p. 289 88 Ferreira, BK (2008) Mineral processing & Extractive metal. Rev., 29, p. 330 89 Wragg, AA (1975) Chemistry and Industry, April 19, p. 333 90 Sequeira, CAC; Marques, FDS “Effect of Current on Gold Electrowinning With a Fluidized Bed Electrode,” in Electrochemical Engineering: I. Chem.E. Symposium Series No. 112, NY/London, Hemisphere Pub. Corp., 1989, p. 297 91 Backhurst, JR; Fleischmann, M; Goodridge, F; Plimley, RE (10 June 1970) British Patent 1194181 92 Germain, S; Goodridge, F (1976) Electrochimica Acta, 21(8), p. 545 93 Fleischmann, M; Oldfield, JW and Tennakoon, L(1971) J. Appl. Electrochem. 1, p. 103 94 Coeuret, F (1980) J. Appl. Electrochem. 10, p. 687 95 Lee, CI; Yang, WF. (2005) Environmental Technology 26(16), p. 1345 96 Han, KN, Liu, J (1992) Emerging Process Technol. Cleaner Environ., Proc. Symp. Volume Issue Pages 175 97 Zhang, XP (2000) Huanan Ligong Daxue Xuebao, Ziran Kexueban, 28(5), p. 41 98 Hadzismajlovic, DzE; Popov, KI; Pavlovic, MG (1996) Powder Tech. 46, p. 145 99 Bisang, JM (1996) J. Appl. Electrochem. 26(2), p. 135 100 Jiricny, V; Roy, A; Evans, JW (2002) Metall. Mat. Trans. B: Proc. Metall. Mat. Proc. Sci. 33B(5), p. 669 101 Jiricny, V; Roy, A; Evans, JW (2002) Metall. Mat. Trans. B: Proc. Metall. Mat. Proc. Sci. 33B(5), p. 677 102 Stankovic, VD; Stankovic, S (1991) J. Appl., Electrochem. 21(2), p. 124 103 Masterson, IF; Evans, JW (1982) Metall. Mat. Trans. B, 13B, p. 3 104 Jiricny, V; Roy, A; Evans, JW (2000) Metall. Mat. Trans. B 31B, p. 755 105 Goodridge, F; Lister, K; Scott, K (1981) J. Appl. Electrochem. 11, p. 723 106 Shirvanian, PA; Calo, JM (2005) J. Appl. Electrochem. 35(1), p. 101 107 Yao, P; Calo, JM; Shirvanian, PA; Hradil, G (2009), in preparation 108 Yao, P; Calo, JM; Hradil, G (2007) “Removal of metal mixtures from aqueous solutions with a spouted particulate electrode,” Division of Environmental Chemistry, Paper No. 207, presented at the 234th ACS National Meeting, Boston, MA 109 Federman, GA; Hradil, G. (2005) Metal Finishing 103(2), p. 40 110 Calo, JM; Shirvanian, AP; Hradil, G; Wyspianski, M (2002) Preprints of Extended Abstracts presented at the ACS National Meeting, American Chemical Society, Division of Environmental Chemistry 42(2), p. 470

111 Shirvanian, PA (2003). “Electrolytic recovery of metals in a spouted vessel reactor – An experimental and simulation approach.” Ph.D. Dissertation, Division of Engineering, Brown University, Providence, RI 39

Table 1.1. Description, Advantages, and Disadvantages of Metal Immobilization Technologies.21 Technolog- Description Advantages Disadvantages y In situ Isolation structure that Can limit the potential The contain- aims to prevent the for contact of the toxicity/solubility of ment movement of underlying metals is not reduced. (capping groundwater or contamination by The long term and surface water people, fauna, and performance of barriers) infiltration in order to flora. Applicable to a capping and barrier avoid the migration of broad range of soil system is unproven metals. types. Can reduce and monitoring may disturbance of the be required. contamination source. Off-site Contaminated soils are Relatively short time The disposal excavated and scale. Effectively toxicity/solubility of transported to removes the sources of metals is not reduced. authorized hazardous many environmental Off-site disposal is waste landfill sites. risks from the site. strictly regulated. Monitored Intrinsic remediation Less generation or May not be sufficient natural process. transfer of remediation for remediation of attenuation Stabilization and wastes. Few surface metals. Not (MNA) toxicity reduction of structures are required. appropriate where metals via natural Overall cost will likely imminent site risks processes (biological/ be lower than “active” are present. Longer chemical reactions remediation. time frames may be involving valence required. Long term changes or sorption, monitoring and etc.) with subsurface associated costs. materials. S/S* and in S/S (mostly based on S/S is applicable to a S/S process increases situ cement wide range of mixed the volume of treated chemical immobilization contaminants and soil material. Presence of stabilizatio- process) aims at types. In situ interfering n sequestering the stabilization may compounds such as metals in a strongly promote site organics (S/S). modified soils matrix. revegetation and can be Concern with long In-situ chemical applied for a large site. term integrity (S/S) stabilization aims to reduce metal bioavailability/ solubility without affecting soil matrix.

Table 1.1. (continued) Technolog Description Advantages Disadvantages y Vitrificatio Vitrification (ex situ or Permanent remedy with Off gases may be -n in situ) refers to the good long-term created and must immobilization process effectiveness. Potential be treated. using high temperatures volume reduction of Expensive pilot- to melt glass, and to materials. Products have scale testing is stabilize the metal potential reuse options. required. The cost contaminants after is high. cooling within a solidified vitreous mass. Chemical Chemical Red/Ox (ex Often used as Requires chemical Red/Ox situ or in situ) converts pretreatment prior to agents that can be mobile/toxic metal S/S to reduce Cr(VI) to both expensive and compounds to chemical a less toxic form Cr(III), hazardous. forms that are more for example. stable, less mobile, or less toxic. Phytostabil In situ emerging Potentially applicable to Applications -ization technology that uses many metals. Large limited to depth of plants to prevent soil area can be treated. No the root zone. erosion (by wind and disposal of contaminant Remaining liability rain), to stabilize metals biomass required. issues, including in order to avoid metals maintenance for an migration to indefinite period of groundwater. time. Requires control of site use. Biological In situ emerging Metal bioavailability for Typically requires Stabilizatio technology that uses human and biological pilot studies to -n biosolids or microbial receptors is reduced. evaluate efficiency. activity to reduce metals Potential revegetation Remaining liability toxicity or of the site. issues, including bioavailability. This maintenance for an technology is often indefinite period of associated with time. chemical stabilization. *S/S – Solidification/Stabilization

Table 1.2. Description, Advantages, and Disadvantages of Metal Extraction Technologies.21 Technolog Description Applicability Advantages Disadvantages -y (Forma, Concb) SW/ Ex situ techniques P-H The clean, Difficulty with soils Physical based on mineral larger that contain high Separatio processing fraction can clay content, high -n technologies be returned humid content, (Separation by size, to the site for organic gravity, flotation, continued contaminants with attrition, use. The high viscosity. magnetism) to treatment Required large remove the metals. duration is equipment and short to larger space. medium term. SW/ Ex situ technique I-H The Difficulty with soil Chemical that uses an treatment that contains: high extraction extracting aqueous duration is clay content and fluid containing short high humid content. chemical agent medium Use of chemical (acids, vases, term. agents which can be surfactant, or Potential to both expensive and chelating agents) to metal hazardous. extra metal from recovery. soil. Biological Ex situ technique P/I-H Nontoxic No performance extraction (usually slurry character of data available for phase process) the biological complete full-scale using agents in application. microorganisms to comparison aid the mobilization to chemical of metal agents. (bioleaching techniques) Thermal Ex situ techniques V-H TD is one of Requires air extraction that aims to the rare emissions treatment separating volatile methods and specialized metal (e.g Hg). effective for facilities. Includes thermal Hg Pretreatment with desorption(TD) and extraction. mixing /fluxing high temperature Potential for agents to assist metal recovery metal melting. The cost is (HTMR) recovery. high.

Table 1.2. (continued) Technology Description Applicability Advantages Disadvantages (Forma- Concb) Electro- Technique that I-M Metals can be Applicable only kinetics uses effectively to saturated and electrochemical removed from partially processes to soils via in situ saturated (clays remove metals approach. and silt-clay) from (saturated) Potentially soils. Multi- soil. In situ option applicable for metals is more broad types of contaminated interesting rather metals. sites pose than ex situ problem. approach. Phyto- In situ technique PA-L Large area can Process duration extraction that uses plants be treated. Good is long. Limited hyper public by depth of the accumulative to acceptance. Does root zone and extra metals from not involve number of soils. Can require excavation, harvest required. reagent addition treatment, and Concern for (EDTA) to disposal. management of enhance the the biomass that process. contains high metal content. Soil In situ technique I-M The treatment Associated risk flushing that uses an duration is short of contamination injection of water to medium term. of underlying (water flushing), May mobilize a aquifer with or aqueous fluid wide range of undercovered containing a organic or flushing solution chemical agent inorganic that contains the (reagent flushing) contaminants metals. The to enhance metals from coarse- reagent flushing solubility into the grained soils. may affect the soil in order to soil properties. extract them by groundwater pumping. SW - soil washing a Forms of metals most suitable for the treatment (P - particulate forms; I - ionic forms, or easily dissolvable/exchangeable with acid/alkaline/redox/chelating processes, or salts forms; V - volatile metals; PA- phytoavailable forms. b Concentration level that can usually be treated with the technology (H - high; M - medium; L – low). 43

Figure 1.1. Periodic Table showing one of the common definitions of the “heavy metals.” 44

Figure 1.2. Schematic of the 30 cm (12 in.) diameter, cylindrical spouted particulate electrode used in some of the current work. 45

Chapter 2. Experimental

2.1: Spouted Particulate Electrode (SPE)

2.1.1: Apparatus

A conceptual schematic of the spouted particulate electrode apparatus and flow system is presented in Figure 2.1. As shown, the liquid is introduced as a high velocity jet at the center of the bottom of the conical vessel. This liquid jet, also known as the

“spout,” entrains particles centripetally fed from the moving particulate bed. After passing through the draft tube, the entrained particles disengage from the liquid flow as the velocity decrease, in the freeboard region also known as the “fountain,” and then fall onto the inverted conical distributor. The collector/distributor cone channels the particles to the periphery of the vessel, where they fall onto the particulate moving bed cathode that transports them inward and downward back to the entrainment region. The pumping action provided by the spout circulates the particles through the vessel in a toroidal fashion; upwards in the spout, and downwards through the annular downward moving bed of particles.1

A drawing of the particulate electrode vessel body was already presented in Figure

1.2. It was constructed from 12 in. (0.305 m) plexiglass tubing. The conical bottom section was made from 1/16 in. thick (0.16 m) plexiglass. A 2.54 cm diameter draft tube was used to contain and stabilize the spout. Three, 316 stainless steel studs with diameter

3/8”, located 120° apart, were used to provide the cathodic connection to the conductive bed of particles on the conical vessel bottom. The anode was constructed from expanded platinized niobium mesh mounted on a plastic support, which was used to position and hold the anode in place below the collector/distributor, parallel to the cathode. The areas 46

of the cathode and anode were 0.0823 m2 and 0.0334 m2, respectively. The separation

distance between the electrodes was set at a constant value of 3.81 cm for all the

experiments. The anode and frame were enclosed in a fine polypropylene mesh to prevent

electrical contact with the particles. A plastic deflector for the bed media disengaging

from the spout was incorporated in the form of a disk fixed directly above and normal to

the spout flow. A variable 50A DC power supply was used to deliver current to the

SBER. The electrolyte solution was circulated using a magnetically coupled centrifugal

pump equipped with a bypass value for flow rate control. A paddle wheel flow meter

(Signet 3-2535) was used to measure the liquid flow rate. The granular bed media were

3.2 mm diameter glass spheres, metallized with a layer of electroless nickel, followed by

an electroplated layer of copper.

The initial metal ion concentrations for the stock solution for the electrowinning

experiments were all ca. 1000 ppm. The standard nickel sulfate solution used for all the

electrowinning experiments consisted of 70 g NiSO4·6H2O, (>98% Aldrich) added to

distilled and deionized water to a total volume of 18 L, 150 g of Na2SO4 (granular, >99%

Aldrich) and 200 g H3BO3 (used to suppress hydrogen evolution and stabilize the pH in

the vicinity of the cathodic particles2), as well as sufficient sulfuric acid (1M,

Mallinkrodt) and potassium hydroxide (1M, Fisher Scientific) to the desired pH.

The standard copper sulfate and nickel sulfate mixture solution consisted of 80 g

CuSO4·5H2O (>98% Aldrich), and 70 g NiSO4·6H2O(>98%, Aldrich) added to distilled

and deionized water to a total volume of 18 L, 150 g of Na2SO4 (granular, >99% Aldrich)

and 200 g H3BO3, as well as sufficient sulfuric acid (1M, Mallinkrodt) and potassium hydroxide (1M, Fisher scientific) to the desired pH. 47

The standard cadmium sulfate solution consisted of 25 g CdSO4·5H2O (>98%

Aldrich), and added to distilled and deionized water to a total volume of 18 L, 150 g of

Na2SO4 (granular, >99% Aldrich) and 200 g H3BO3, as well as sufficient sulfuric acid

(1M, Mallinkrodt) and potassium hydroxide (1M, Fisher scientific) to the desired pH.

The standard lead solution consisted of 63 g (CH3SO3)2Pb (50 wt. % solution in water, Aldrich), and added to distilled and deionized water to a total volume of 18 L, sufficient Methanesulfonic acid (1M, Mallinkrodt) and potassium hydroxide (1M, Fisher scientific) to the desired pH.

Since the solution pH increases with time due to the anodic reactions, an automatic pH controller (Barnant, model HD-PHP) was used to maintain constant pH using 1M potassium hydroxide solution. For the metal corrosion experiments, 1M sulfuric acid solution (1M methanesulfonic acid used for lead removal) was used for pH control with the same controller. A portable dissolved oxygen meter (HACH, LDOTM HQ10) was employed to measure dissolved oxygen concentration.

The effects of solution temperature on nickel recovery were investigated by thermostatting the electrolyte solution holding tank. Since the solution tended to heat up gradually over the course of an experiment, this was accomplished by immersing a coil of stainless steel tubing in the solution reservoir through which cold tap water flowed continuously. A resistive heater (Cole-Parmer, Model 12112-11) immersed in the solution was then used to set the desired solution temperature to within ±0.1 oC over the course of a typical experiment. The optimum operating flow rate is the lowest that can still keep the bed spouting, while exceeding the inception point for particle agglomeration 48

in the moving bed cathode. The selected volumetric flow rate was 32.2 L min-1. The

volume of conductive bed media used was about 480 cm3.

2.1.2: Metal Ion Concentration Measurements

Nickel metal ion concentrations in solution were determined using two different

analytical instruments, and the total amount of metal removal was determined by

difference. For the experiments without gas sparging, nickel concentrations were

determined by atomic absorption spectroscopy (AAS; Buck Scientific, model 210 VGP).

-1 An atomic absorption standard calibration solution of 996 μg ml nickel in 2% HNO3

and a blank solution were used to calibrate the AAS. Samples were diluted to 1/100 of

the original concentration to perform the AAS analyses. The instrument manual for the nickel lamp lists the following specifications: wavelength 232.0 nm, slit 0.2nm, detection limit 0.02 mg L-1, linear range 3 mg L-1 and flame type AA, lean/blue.

The nickel concentrations in the samples obtained while operating the apparatus with

nitrogen sparging, and samples from the copper and nickel mixtures, cadmium, and lead

were determined with an Inductively Coupled Plasma Optical Emission

Spectrometer (ICP-OES; JobinYvon, JY2000). The samples were measured at 221.647

nm for nickel; 324.754 nm for copper, 228.802 nm for cadmium, and 220.353 nm for

lead. Five metal calibration standards were used, covering the range of 1-7 ppm in a

matrix of 2% HNO3, as well as a zero (blank) standard of the 2% HNO3 matrix. The

samples collected from the spouted particulate electrode were diluted by a factor of 100

prior to analysis. A quality control solution was measured after every 5 samples to check

for “machine drift” during the analysis. 49

2.1.3: Experimental Procedures

Superficial current densities were determined by dividing the total steady-state current by the cross sectional area of the bed. The amount of metal, which would be recovered at 100% efficiency, was determined by multiplying the ampere-hours of delivered current by the electrochemical gram equivalent of the particular metal (i.e.

Faraday’s law). The actual amount of recovered metal was then divided by this value to yield the current efficiency.

A typical experiment was generally conducted as follows:

(1): A solution containing a measured concentration of metal ions was formulated.

(2): The pH of the solution was set and the temperature adjusted, as necessary.

(3): The SPE vessel was loaded with a measured volume of metallized glass spheres.

(4): The SPE vessel was then closed by bolting the top flanges together.

(5): The anodic and cathodic electrical connections were made.

(6): The pH controller was then inserted into the solution, and the pH was set.

(7): The pump was primed with the solution to be used in the experiment, and it was then turned on.

(8): Once the solution filled the SPE vessel and was flowing properly, the power was turned on and set to the appropriate current.

(9): Samples were then taken at selected time intervals, depending on the particular experiment. These samples were then diluted as necessary, and the metal ion concentration was determined by AAS or ICP-OES, as described above.

(10) The SPE was run for a specified time, or until the concentration of metal ion had reached a desired level. 50

In selected cases, metal corrosion experiments were conducted by first operating in the normal electrowinning mode, and then turning off the current while maintaining constant electrolyte flow and particle recirculation rates, and monitoring the dissolved metal concentration as a function of time.

2.1.4: Gas Sparging

Experiments were also performed by sparging the electrolyte solution in the holding tank with nitrogen to reduce the dissolved oxygen concentration. A model that was used to estimate the sparger performance is presented in Appendix A.

The sparger was constructed from 0.635 cm diameter nylon tubing arranged in a square with a length of 16.5 cm on a side. About 2000 (approximately 500 on a side) holes of 0.35 mm diameter were drilled through the sparger tubing. In general, the higher the flow rate, the better the performance of nitrogen sparging. However, beyond a certain point, the oxygen removal efficiency increases only marginally with flow rate. After investigating the characteristics of the sparger system with nitrogen, an operating flow rate of 2800 standard cm3 min-1 was selected. Even though nitrogen sparging was shown to have a significant effect on reducing the dissolved oxygen concentration (see below), its performance was not optimized. At the operating conditions used, the mean residence time in the solution holding tank was only about 15 s. Its performance could be considerably improved by increasing the residence time in the solution holding tank by using a larger tank and/or running at lower flow rates. Mass transfer analysis of the sparging process (see Appendix A) suggests that it may be possible to reduce the dissolved oxygen concentration almost to zero, which would practically eliminate metal corrosion and increase the current efficiency. 51

2.2: Cyclic Electrowinning/Precipitation (CEP) System

2.2.1: Apparatus

The CEP system is an automated and programmable (computer-controlled) system that was designed and developed for the removal of heavy metals from wastewater using a cyclic process involving a combination of electrowinning and in-process precipitation and redissolution. Schematics of the CEP System are presented in Figure 2.2. As shown, there are two principal parts - the spouted particulate electrode (SPE), and the precipitation/redissolution tank.

The operation of the precipitation/redissolution tank is as follows. Metal- contaminated water is pumped into the precipitation/redissolution tank from the wastewater drum. The solution pH is then increased with 1N NaOH, and the resultant

precipitation of the metal ions as metal hydroxides is used to remove the metals to low

concentrations in the supernatant water (on the order 0.3-1.5 ppm). Flocculant (#108,

Tramfloc, INC.) was used to increase the settling rate of the precipitate. The filtered

solution is then drained to the clean solution drum. Fresh contaminated water is then

introduced into the precipitation tank, and the entire solution is re-acidified with 1N

H2SO4, and mixed with the metal hydroxide sludge. This produces a solution with higher

metal ion concentrations. Via multiple precipitation/redissolution steps the metal ion concentrations are increased sufficiently to subsequently electrowin the metals with good current efficiency onto the cathodic particles in the SPE.

The SPE in the CEP System was constructed by Technic, Inc. A schematic is

presented in Figure 2.3. As shown, a portion of the circulating liquid exits through the

nylon mesh in the lower half of the inner chamber. The remainder of the liquid slows 52

down and stops when it reaches the top deflector plate of the inner chamber. The solution

exits through the two nylon tubes to the circulation pump which recirculates it to the inlet

of the conical vessel. As the particles enter the electrolyte jet stream, they are entrained in

the jet and are convected upwards within the draft tube. After passing through the draft

tube in the inner chamber, the entrained particles disengage from the liquid flow as the

velocity decreases and the liquid reaches the top deflector plate of the inner chamber in the freeboard region also known as the “fountain.” The particles then fall onto the inverted conical distributor. The collector/distributor cone channels the particles to the periphery of the vessel, where they fall onto the particulate moving bed cathode that transports them inward and downward back to the entrainment region. The pumping action provided by the spout circulates the particles through the vessel in a toroidal fashion; upwards in the spout, and downwards through the annular downward moving bed of particles. Electrowinning occurs only in the bottom conical section of the vessel, where the particles form the cathodic moving bed that comes in contact with the current feeder.3

The plating chamber of the exterior cylinder and the interior deflector are constructed

from plexiglass tubing, as shown in the images in Figure 2.4. The diameter of the exterior

cylinder is 8.5 in. The angle of the conical bottom with the horizontal is 60°. A ring of

316 stainless steel located just above the conical bottom is used to provide the cathodic

connection to the conductive bed of particles. A circular anode basket, or an anode mesh,

as shown in Figure 2.4(c), surrounds the plating head, and current flows from the anode

to the cathodic particle bed via the mesh-enclosed openings in the chamber wall.

Solutions entering the vessel via the bottom electrolyte jet exit through these mesh- 53

enclosed openings on the side of the chamber. The SPE chamber is not fully immersed in

the bulk electrolyte within the plating tank.

A variable 50A DC power supply was used to deliver current to the SPE. The

electrolyte solution was circulated using a magnetically coupled centrifugal pump

(Iwaki, MAGNETIC DRIVE PUMP, MD-40RT), equipped with a bypass value for flow

rate control. A paddle wheel flow meter (Signet 3-2535) was used to measure the liquid

flow rate. The granular bed media were 2.0 mm diameter plastic spheres, metallized with

a layer of copper (Bead House LLC, CMC02.0/CP). An electric vibrator (UHMW, 10-

24), located outside of the exterior cylinder, is used to maintain smooth particle flow, if

necessary. A heater (CAL Controls LTD, CAL3300) located at the bottom of external

holding chamber, and a cooler consisting 1/16” plastic tubing coils on the wall of the

holding chamber are used simultaneously to set the solution temperature.

An automatic pH controller (Barnant, model HD-PHP) was used to maintain constant

pH with potassium hydroxide solution. Valves (ASAHI/AMERICA, 05E01660F) and

pumps (IWAKI, MAGNETIC DRIVE PUMP, MD-30RT-115ML) were installed to

control the solution flow. A mixer (IKA, EUROSTAR power CV S1) was employed to

stir the solution when adding sulfuric acid or sodium hydroxide solution to the

precipitation/redissolution tank. A 20 μm, inline filter (ISC, Model No SJC-40-20) was used to retain metal precipitate particles. A Dell computer (Dell, Optiplex 320) and a

Labview (National Instruments, NI PCI-6221, M series DAQ; SCB-68 noise rejecting, shielded I/O connector block; and USB-9211A 4 Ch) was used to automatically control all CEP operations. An image of the CEP system is shown in Figure 2.5. A listing of the

CEP System LabView control program is included in Appendix C. 54

Experiments were also performed by sparging the electrolyte solution in the holding tank with nitrogen to reduce the dissolved oxygen concentration. The same sparger described above (0.653 cm diameter, 16.5 cm in length) was used. A higher operating flow rate of 3500 standard cm3 min-1 was selected, given the lower initial electrowinning concentrations than that for the SPE experiments. The sparging model and summary of the sparger development are included in Appendix A. The optimum operating flow rate was found to be the lowest that could still keep the bed spouting, while still exceeding the minimum transit time in the moving cathode bed required for the inception of particle agglomeration. The selected volumetric flow rate was 16.2 L min-1. The volume of conductive bed media used was about 400 cm3.

2.2.2: CEP System LabView Control Program

The CEP System LabView Control Program was formulated to automate and control the CEP System such that it could be programmed to accommodate a wide variety of heavy metal removal/remediation/recovery situations that may be found in practice. The listing of the most recent version of the program is attached as Appendix C.

The control program consists of the front panel and block panel modules. The front panel serves as the programmatic interface. Controls and indicators on the front panel allow for inputting and exhibiting data and the status of all the system elements, such as pumps, valves, and pH. The block panel is where the underlying code is created for the control program. The time sequence is the main frame of the code program. According to the order of the desired experimental steps, the precipitation/redissolution tank and SPE process codes are executed in the order specified by the operator. The “on/off” status of all the pumps and the “open/closed” status of all the valves are controlled by the Labview 55

program via digital signals. The pH and level sensors produce digital signals that are sent

to Labview via the data acquisition system. The program then sends out the appropriate digital signals to control the corresponding pumps and valves to execute each step in the

program.

2.2.3: Metal Ion Solutions

The initial metal (copper, nickel and cadmium) concentrations for all the experiments

were about 20 ppm in the fresh solutions. They consisted of 70 g CuSO4·5H2O (>98%,

Aldrich), 81 g NiSO4·6H2O(>98%, Aldrich), and 48 g CdSO4·5H2O (>98%, Aldrich)

added to tap water to a total volume of 180 L. 550 g of Na2SO4 (granular, >99%, Aldrich)

and 600 g H3BO3 were added to the precipitation tank just prior electrowinning in the

SPE. Sufficient sulfuric acid (1M, Mallinkrodt) and potassium hydroxide (1M, Fisher

Scientific) were added to the desired pH.

The metal concentrations in the samples were determined with an Inductively

Coupled Plasma Optical Emission Spectrometer (ICP-OES; Jobin Yvon, JY2000). The samples were measured at 221.647 nm for nickel; 324.754 nm for copper, and 228.802 nm for cadmium. Five metal ion calibration standards were used covering the range of 1-

7 ppm in a matrix of 2% HNO3, as well as a zero (blank) standard of the 2% HNO3

matrix. The samples collected from the SPE were diluted by a factor of 100 prior to

analysis. A quality control solution was measured after every 5 samples to check for

“machine drift” during the analysis.

2.2.4: CEP Experimental Procedures

A typical CEP run was conducted in the following manner:

(1): A solution containing a measured concentration of metal ions was formulated. 56

(2): The SPE tank was loaded with a measured volume of metallized glass spheres,

placed inside the SPE chamber.

(3): The anodic and cathodic electrical connections were made.

(4): The computer was started, the power switches for all the pumps and valves were

turned on, and the CEP System Labview program was initiated.

(5): The CEP System Labview program was run for a specified time, or until the metal

ion concentrations reached a desired level, or with varying CEP cycles programs.

(6): Samples were taken at selected time intervals, depending on the particular

experiment. These samples were then diluted as necessary, and the metal ion

concentrations were determined by ICP-OES, as described previously.

The precipitation/redissolution process was conducted in the following manner:

(1): Open Valve 1 and turn on Pump 1, keeping all the other valves closed. The

contaminated solution is then pumped into the precipitation tank. Pump 1 and Valve

1 close automatically as soon as the solution level reaches a preset height (the level

sensor sends a signal to accomplish this).

(2): Open Valve 3 to drain the basic solution into the precipitation tank, while stirring.

Valve 3 closes automatically when the pH reaches a preset level (11 in most of the

experiments). The pH sensor sends a signal to close Valve 3. Flocculant is then

added.

(3): Approximately 3 min. are required to allow the metal hydroxide precipitate to settle

to the bottom of the tank.

(4): Open Pump 2 and Valve 2. Closed all other valves. Drain the clean solution from

the precipitation tank. 57

(5) Shut off Pump 2 and close Valve 2. Open Valve 4 to add the acid solution. Turn on

the mixer. Valve 4 closes automatically when the solution pH attains the preset

value. Wait 4 min. to ensure that all the metal precipitate has been redissolved.

The preceding constitutes the precipitation/redissolution step that can be repeated, as necessary, to increase the metal(s) concentration(s) to the desired level(s) for electrowinning.

The electrowinning step was conducted in the following manner:

(1) Na2SO4 and H3BO3 are added to the solution in order to increase its conductivity

and suppress hydrogen evolution during electrowinning.

(2) Open Valve 4 to add the acid solution. Valve 4 closes automatically when the

solution pH attains the preset value.

(3) Open Valves 5 and 6, and turn on Pump 3. All other valves are closed and pumps

are off. The accumulated solution is pumped into the SPE.

(4) With Valve 6 open and Pump 3 running, close Valve 5, and open Valve 7. The

solution then circulates within the SPE.

(5) Once the solution is flowing, the SPE heater and cooler are turned on to achieve the

desired preset temperature.

(6) The power is turned on and set to the appropriate current to begin electrowinning.

(7) After the electrowinning step is complete, close Valve 6 and open Valve 5, keeping

Valve 7 open. Drain the solution into the precipitation tank. Then close Valves 5

and 7, and shut off Pump 3.

The combination of all the preceding steps constitutes the CEP cycle, which can be

repeated as many times as necessary to achieve the desired metal removal results. 58

References

1 Shirvanian, PA (2003). “Electrolytic recovery of metals in a spouted vessel reactor – An experimental and simulation approach.” Ph.D. Dissertation, Division of Engineering, Brown University, Providence, RI 2 Kelly, JJ; Goods, SH; Talin, AA; Hachman, JT (2006) J. Electrochem. Soc. 153, p. C318 3 Federman, GA; Hradil, G. (2005) Metal Finishing 103(2), p. 40 59

Figure 2.1. Schematic of the spouted particulate electrode (SPE) apparatus and flow system. 60

Figure 2.2. CEP System Schematic. Electro-actuated 3”x3” Ball Valves (Blue): 1, 2, 3, 4, 5, 6 MD-40RT Centrifugal Iwaki Magnetic Drive Pumps (Dark Green): 1,2 Acid and Base Reservoirs (Magenta) Precipitation Tank (Light Blue) Spouted Particulate Electrode (Light Green)

Figure 2.3. Schematic of the spouted particulate electrode (SPE) of the CEP System.

(a) Plating chamber current feeder.

(b) Plating chamber - exterior shell and interior deflector.

Figure 2.4. SPE images in CEP system.

Figure 2.5. The Cyclic Electrowinning/Precipitation(CEP) System.

Chapter 3. Removal of Single Metals and Simple Metal Mixtures Via Electrowinning With Spouted Particulate Electrodes

3.1: Nickel Removal With a Spouted Particulate Electrode

3.1.1: Background

Electrolytic deposition of nickel from aqueous solutions is important in a number of

applications, including the production and purification of nickel, metal plating, and its

removal from process and wastewaters. A significant amount of work can be found in the

literature on electrodeposition of nickel.1,2,3,4,5,6

Fixed bed particulate and porous electrodes have been shown to be effective for

electrolytic removal of metals from solution. However, particle agglomeration and/or

plugging of the porosity limit the application these types of electrodes.7 Fluidized bed

electrodes can be used to circumvent some of these problems;8 but their current-carrying capacity is limited. In addition, the existence of anodic or pseudo-anodic zones that is not always present in corresponding fixed beds, has been observed in fluidized bed electrodes9,10 and also predicted for fixed bed electrodes as a function of bed depth.11

The cylindrical spouted particulate electrode incorporates most of the advantages of

fixed and fluidized bed electrodes, while minimizing some of their disadvantages.12,13

Some metal electrowinning studies have been conducted with similar or related devices.

Copper electrowinning from acidic solutions in circulating particulate electrode systems

has been investigated by Coeuret et al.,13 Jiricny et al.,14,15,16 Evans et al.,17 and

Shirvanian and Calo.18 Zinc electrowinning in circulating particulate electrode systems

has been reported by Evans et al.,19 Jiricny et al.,20 and Roy et al.21 in a circulating

particulate electrode in which the cathode particles were fluidized and the cathode and 65 anode compartments were separated, thereby minimizing corrosion of the deposited metal, and resulting in high current efficiencies. Here we present what we believe to be the first investigation of a cylindrical spouted particulate electrode of conductive particles as a cathode for electrolytic recovery of nickel.

3.1.2. Results

3.1.2.1. Electrodeposition Model

A numerical model, based on modifications of a general approach for modeling the behavior of recirculating electrochemical reactors,22,23,24 was formulated to simulate the behavior of net nickel recovery behavior with pH and temperature.

The principal reactions assumed to occur during nickel electrowinning from acidic aqueous solutions are:

Principal cathodic reaction: Ni2+ + 2e- → Ni (3.1.1)

+ - Possible cathodic side reactions: 2H + 2e → H2 (3.1.2)

+ - 2H + ½ O2 + 2e → H2O (3.1.3)

+ - Principal anodic reaction: H2O → 2H + ½ O2 + 2e (3.1.4)

Reaction (3.1.3) is the well-known oxygen reduction reaction that is critical to the operation of a number of fuel cells,25,26 which is the reverse of the anodic reaction (3.1.4).

The standard potential of reaction (3.1.3) is +1.23V,27 which indicates that it is thermodynamically favored over the hydrogen formation reaction (3.1.2). One mechanism that has been proposed for this reaction consists of four single-electron transfer steps.28,29 The first step in this mechanism is:

+ - O2 + H + e → HO2 (3.1.5) 66 with a standard potential of -0.125V.27 Thus, it is expected that reaction (3.1.3) is actually less kinetically favored than the hydrogen formation reaction (3.1.2).30,31 Due to similar factors, and the fact that the relative importance and kinetics of all other possible side reactions could not be determined, it was decided to use just reaction (3.1.2) as the primary, representative cathodic side reaction. Of course, in principle, such reactions could also be added to the model if all the kinetic information were known or could be measured.

In addition to the preceding, it was observed that deposited nickel can re-dissolve or corrode back into the acidic solution via:

+ 2+ Ni + ½ O2 + 2H → Ni + H2O (3.1.6) which is actually the sum of the reverse of reaction (3.1.1) and reaction (3.1.3). This reaction is analogous to that reported by Ives and Rawson32 for copper corrosion. For copper recovery in a cell where the cathode particles were fluidized and the cathode and anode compartments were separated, Stankovic and Stankovic33 and Masterson and

Evans34 found very high current efficiencies due to minimization of copper corrosion.

Nickel corrosion data were obtained by turning off the feeder current and measuring the increase in dissolved nickel concentration with time, as described in Chapter 2.

Given the preceding, the principal model assumptions are:

(i) At the surface of cathodic particles, electrochemical reactions occur of the type:

+ n - A 1 + n1e → M (3.1.7)

(ii) Pseudo-steady-state behavior applies in the mass transfer boundary layer at the

particulate electrode surface.

(iii) Electrochemical kinetics is described by the Tafel equations. 67

(iv) Electrochemical nickel reduction is assumed to be first order in Ni+2.

(v) The measured dissolved oxygen concentration varies over the course of an

experiment. However, typically the magnitude of the variation is not very great

within an experiment after its initial rapid rise upon the inception of

electrowinning. Also, the development of an explicit model of dissolved oxygen

behavior requires electrode kinetic data well beyond the scope of the current

work. Therefore, in lieu of this approach, constant corrosion rates (as found

experimentally) were used to approximate the effects of nickel corrosion.

(vi) Transport of ionic reactants by migration is negligible due to the excess of

supporting electrolyte.

The rate of mass transfer of species, j, with bulk concentration of Cj, is given by:

rj = k L a(C j − C js ) (3.1.8)

where kL is the mass transfer coefficient, an a is the interfacial surface area per unit

volume. The rate of electrochemical reaction is given by:

ija rj = k j C js = , (3.1.9) n j F

where kj is an electrochemical rate constant, which is dependent upon the electrode overpotential (defined as η = E-Ee, where E is the applied electrode potential and Ee is the reaction equilibrium potential35) according to the Tafel approximation:

 −α Fη k = k exp (3.1.10) j j0  RT 

The total rate is equal to the current density divided by the number of Faradays of charge required to convert one mole of species. At steady-state, the kinetic and mass transfer rates are equal, resulting in the expression for the surface concentration of species j:

C j C js = , (3.1.11) 1+ k j / k L a which can then be eliminated from Eq. (3.1.9), resulting in

C j rj = k j (3.1.12) 1+ k j / k L a in which the ratio, kj/kL, determines the degree of mass transfer limitation.

For galvanostatic operation, the total current is given by:

(n1F / a)k1C1 i = i + i = + (n2F/a)k2C2 (3.1.13) 1 2 (1+ k / k a) 1 L and the mass balance for the metal (Ni2+) cations becomes:

dC1 i1a = − + kc (3.1.14) dt n1F where kC is the total corrosion rate due to all mechanisms, which is assumed to be zeroth order in metal cation concentration at constant pH.

The mass transfer coefficient was estimated from Pickett36 for a single layer packed bed electrode:

0.56 1/3 Sh = 0.83 (Rep) Sc (3.1.15)

27 The variation of exchange current density, i0, with temperature is given by:

d(lni ) ∆H ∗ 0 = 0 (3.1.16) dT RT 2 69

∗ 27 where ∆H 0 is the enthalpy of activation at the equilibrium potential. From equation

(3.1.16):

 −∆H *  i = κ exp 0  (3.1.17) 0  RT 

and, according to the definition of the exchange current density,27

αF(E − E 0 ) i = zFC k exp(− e ) (3.1.18) 0 j0 j0 RT where α is the transfer coefficient and F is Faraday’s constant. Equating (3.1.17) and

(3.1.18):

α F(E − E0 ) = ∆H * (3.1.19) e 0 from which the equilibrium potential can be determined if the other parameter values are known.

The kinetic parameter values used for the nickel and hydrogen cathodic reactions are presented in the Table 3.1.1.

Table 3.1.1: Model kinetic parameter values for nickel electrowinning.

Equilibrium Exchange current Cathodic Transfer enthalpy of density, i0 * Reaction coefficient, α -2 activation, ΔH0 T(°C) (A m ) (kJ mol-1) (Ni2+/Ni) 0.52 [37] 0.068 [37] 37.2 [38] 25 + (H /H2) 0.63 [39] 0.002 [39] 42.2 [39] 20

Model solution methodology:

(i) With fixed current density and initial concentration of nickel ions, C10, the

current balance given by Eq. (3.1.13) is solved numerically using the method of

bisection40 to solve for the initial electrode potential. 70

(ii) Eq. (3.1.14) is solved in time, using a Runge-Kutta, 4th order method.40

(iii) Steps (i) and (ii) are repeated until the desired time is reached.

(iv) Although nickel corrosion rates were measured, as indicated above, it was found

that the best fits of the model to the experimental data were obtained by

“adjusting” the corrosion rate as a model parameter. (This is similar to the

procedure that was previously employed with a copper electrodeposition

model.18) Essentially, a least squares approach was used to determine the “best

fit” corrosion rate values. The measured corrosion rates were used as an initial

guess, and corrosion rate values were then adjusted so as to minimize the least

squares deviation between the data points and the model predictions.

Differences between the measured and “fit” corrosion rates could be attributed to a number of factors, including the possible existence of anodic zones in the cathodic particle bed during electrodeposition that could not be measured by simply turning off the current, as well as slight variations that might occur during electrowinning. Even so, however, it was found that the corrosion rates necessary to fit the data were quite close to the measured rates (see below). This result substantiates the model assumptions, and is an indicator that the model incorporates the major salient mechanisms of the electrowinning process.

In the discussion of the data below, in certain specifically designated figures the model results are presented as continuous curves, along with the measured data points for the sake of comparison. 71

3.1.2.2. Results and Discussion

A summary of the effect of pH on nickel recovery at 40°C is presented in Figure

3.1.1. As shown, it was found that the net nickel electrowinning rate increased with

increasing pH over this range, both with and without nitrogen sparging. The results of the

numerical simulations of net nickel electrodeposition are also presented in this figure as

the continuous curves. As shown, the model results explain the experimental data

reasonably well, although some of the fits are better than others. These results suggest

that the most important fundamental processes are captured by the simple model.

Deviations from the data are attributed to more complex behavior, such as variation of

the dissolved oxygen concentration with time (see below), etc., that have not been

specifically included in the model.

Also shown in Figure 3.1.1 is the effect of nitrogen sparging. A comparison of the

results clearly shows that the rate of nickel electrowinning increases significantly with

nitrogen sparging, especially at the lower pH values. The ultimate asymptotic

concentrations with nitrogen sparging were 19 and 16 ppm (3.2×10-4 and 2.7×10-4

mol L-1) for pH values of 4.5 and 5, respectively. Also, as shown in Figure 3.1.2, the

resultant current efficiencies with nitrogen sparging all increased appreciably over those without nitrogen sparging.

Nickel corrosion data were obtained by turning off the feeder current and measuring the rise in dissolved nickel concentration with time, as described in Chapter 2. The resultant data as a function of pH at 40°C, both with without nitrogen sparging, are presented in Figure 3.1.2. As shown, the rate of nickel corrosion in these experiments under these conditions exhibits close to zeroth order behavior with respect to nickel ion 72

concentration, and increases monotonically with decreasing pH at 35°C. (The slopes of

the linear fits in Figure 3.1.2 are the “measured” values listed in Table 3.1.2 below.)

Also, as shown, with nitrogen sparging the corrosion rates were significantly reduced, due to the lower dissolved oxygen concentrations

In Table 3.1.2 are presented the zeroth order nickel corrosion rates obtained from the

measurements with no applied current, in comparison to the “fit” corrosion rates, as

explained above. As shown, both sets of values agree reasonably well.

Table 3.1.2. Measured and “fit” nickel corrosion rates (35°C, 10A).

Nickel corrosion rate Nickel corrosion rate pH without nitrogen sparging with nitrogen sparging (×106 mol L-1 min-1) (×106 mol L-1 min-1) (35°C, 10A) Measured “Fit” Measured “Fit” 3.5 57 38 22 21 4.0 12 29 2.6 10 4.5 4.1 17 1.6 3.4 5.0 2.8 10 1.2 0.9

Intrinsic current efficiencies were determined from the instantaneous slopes (rates) of

the net nickel removal curves, “corrected” for the corrosion rates (i.e., by addition of the

corrosion rate to the observed rate). The corresponding current efficiencies for the nickel

recovery data in Figure 3.1.1 are presented in Figure 3.1.3. As shown, the current

efficiencies decrease with decreasing nickel ion concentration, as expected. Compared at

the same concentration, the current efficiencies increased with pH over the range of

experimental conditions investigated (i.e., pH 3.5–5.0). One possible reason for this

behavior is that the hydrogen formation side reaction at the cathode (3.1.2) becomes more

inhibited at the higher pH values. It is also noted that the current efficiencies were 73

typically slightly lower at the beginning of each run. This is attributed to the initial

presence of an oxide layer on the particles that increases the electrical resistance. This is

consistent with the slightly elevated voltages and slight current oscillations that were typically observed during the very early stages of most experimental runs.

The primary contributor to the decrease in the intrinsic current efficiency with decreasing bulk metal ion concentration is the decrease in the metal ion concentration at the particle surface due to increasing mass transfer resistance. A plot of the ratio

k a / k a + k as a function of time from the model results presented in Figure 3.1.1 is L ( L 1 )

shown in Figure 3.1.4. With this formulation, a value of unity denotes electrodeposition

rate-limited conditions, and decreasing values reflect increasing mass transfer rate

limitations. As shown, increasing pH values cause greater mass transfer rate limitations to

occur earlier in the nickel removal curves. This is attributed to a decrease in the rate of

the hydrogen formation side reaction at the cathode, which has the effect of increasing

the overpotential, and hence the value of k1 at all times, thereby also increasing the

relative degree of mass transfer resistance. The use of nitrogen sparging also causes the influence of mass transfer rate limitations to occur slightly earlier in time. The presence

or absence of sparging should not affect k1 directly - only the corrosion rate. Therefore, the small differences noted between sparged and unsparged values in Figure 3.1.4 are attributed to the fact that the effective corrosion rate influences the magnitude of C1,

which influences the overpotential (and hence k1) slightly via the current balance (Eq.

3.1.13), when compared at the same time. The greater the corrosion rate, the greater the value of C1, and the lower the corresponding overpotential and value of k1, when 74

compared at the same time. This effect makes k a / k a + k greater with no sparging L ( L 1 )

and larger with sparging, when compared at the same time, just as shown in Figure 3.1.4.

In Figure 3.1.5 are presented the dissolved oxygen concentration in the electrolyte

solution at 10A and 40°C, as a function of pH, both with and without nitrogen sparging

As shown, without nitrogen sparging, the initial concentration of dissolved oxygen was

about 6.0 mg L-1 (6.96 mg L-1 is the equilibrium concentration in contact with air at 1 atm

at 40°C,41 which does not vary very much with pH. With nitrogen sparging, the initial

concentration of dissolved oxygen in the electrolyte solution decreased to about 2.0 mg L-1, which is well below the equilibrium value and much lower than that without nitrogen sparging. During electrowinning, the concentration of dissolved oxygen increases rapidly to a maximum in all cases, and then either levels off (pH 3.5 and 4) or continues to decrease (pH 4.5 and 5). This behavior was qualitatively the same both with and without nitrogen sparging, except that the oxygen levels were significantly reduced

(by as much as a factor of two) in the former case. It is noted that the oxygen behavior in

Figure 3.1.5 at low times was monotonic with pH for both sets of data.

As indicated above, oxygen is produced primarily via the anodic reaction (3.1.4), and

in the presence of H+ can re-oxidize deposited nickel metal back to nickel cations via

reaction (3.1.6). The pseudo-steady level of dissolved oxygen is controlled by the rates of

the production and consumption reactions, as well as any mass transfer loss/gain to air

(depending on whether the dissolved oxygen concentration is greater or less than its

equilibrium value) while in contact with air in the solution holding tank. This

qualitatively explains the relative asymptotic values for the two lower pH values in

Figure 3.1.5. That is, as shown in Figure 3.1.2, the corrosion rate at a pH 3.5 is 75

significantly greater than that at pH 4, so it would be expected that more oxygen would

be consumed in the former case, resulting in a lower dissolved oxygen concentration, just

as observed. However, this does not explain the almost linear decrease in dissolved

oxygen concentration at the two higher pH values. In these latter two cases, it seems that

an additional oxygen sink appears under these conditions. It is also noted that for these

two pH values, the nickel ion concentration is quite low when the decrease in O2 occurs.

Consequently, it is hypothesized that an additional oxygen “sink” develops at the cathode

under these conditions. Although the bulk solution remains acidic, Ji et al.4 have shown

that the pH near the cathode surface is always greater than that of the bulk electrolyte,

and that under certain conditions this could cause the formation of insoluble hydroxides

at cathode surfaces. Cui and Lee5 also found that nickel hydroxide deposited on the cathode surface was stable. During the current experiments at the highest pH of 5, a grey- green color of the solution was clearly visible in the vicinity of the particulate cathode bed. This was interpreted as evidence of nickel hydroxide formation on or near the cathodic particles. However, as shown in Figure 3.1.1, nickel deposition was still quite rapid under these conditions, and the particle surfaces appeared to remain similar in color to that during nickel electrowinning at lower pH values. This is interpreted to mean that even when conditions were such that the pH in the vicinity of the particles was sufficiently high to form hydroxide, it did not deposit on the particle surfaces to impede nickel reduction. This was attributed to the mechanical, “self-polishing” action of the particles abrading against one another in the moving bed cathode which causes any incipient hydroxide deposits to be continually removed from the particle surfaces into the solution. It is noted that this would probably not occur under similar conditions on static 76 cathode surfaces. That is, for static (i.e., fixed or packed) electrode beds, sufficient hydroxide formation may impede nickel electrowinning.

One possible half-cell reaction that would make the pH in the vicinity of the cathode greater than in the bulk, and that also reduces (consumes) oxygen is:

- 2H2O + O2 + 4e → 4OH (E° = +0.401V) (3.1.20)

This is the well known half-cell reaction that occurs at the cathode of alkaline fuel cells, for example, and that is catalyzed by nickel.42 Thus, when the nickel ion concentration has decreased sufficiently, conditions are such that reaction (3.1.20) could become more effective at the cathode. It also accounts for all the experimental observations; i.e., higher effective pH at the cathode, the development of an additional oxygen sink at the cathode, and the presence of high surface area metallic nickel to catalyze the cathode reaction. In addition, it was also experimentally observed that when the nickel ion concentration decreased to low levels, the amount of hydroxide introduced by the pH controller in the holding tank decreased to practically zero, which is also consistent with the effect of reaction (3.1.20).

Another possible cathodic side reaction that would increase the pH and decrease the dissolved oxygen concentration is reaction (3.1.3) (E° = +1.23V), as pointed out earlier in this section. Due to the Nernst equation and the estimated orders of magnitude of the reactants and products involved in reactions (3.1.20) and (3.1.3), under the current experimental conditions, the actual potential of reaction (3.1.20) would tend to be greater than +0.401V, and that of reaction (3.1.3) would be less than +1.23V, such that the two half-cell potentials would be more comparable. This suggests that either one or both of 77 these oxygen-consuming half-cell reactions may become more important when the bulk nickel ion concentration becomes very low.

In Figure 3.1.6 is presented the behavior of nickel electrowinning as a function of temperature at a current of 10A and pH 4, both with and without nitrogen sparging. The corresponding “corrected” current efficiencies and the influence of mass transfer rate limitations are presented in Figures 3.1.7 and 3.1.8, respectively. As previously shown, nitrogen sparging accelerates net nickel deposition, increases the current efficiency, and makes the influence of mass transfer limitations occur earlier. Sparging seems to be somewhat more effective in increasing the nickel recovery rate at lower temperatures.

The results of the numerical simulations of net nickel electrodeposition are also presented in this figure as the continuous curves. As previously, the simulations explain the experimental data reasonably well, although, as in Figure 3.1.1, some of the fits are better than others. However, the fits in Figure 3.1.6 are generally slightly better than those in

Figure 3.1.1. Since the pH is constant at 4 for these results, this is consistent with the fact that the model does not incorporate all the complexities of the corrosion reaction, which becomes more important as the pH decreases, such as dissolved oxygen variation and the possibility of more than one important anodic reaction.

The corresponding measured corrosion rates for these conditions are presented in

Figure 3.1.9. As previously, the effect of nitrogen sparging is quite evident. That is, all the corrosion rates were considerably reduced with nitrogen sparging, irrespective of temperature. In Table 3.1.3 are presented the constant nickel corrosion rates obtained from the measurements in comparison to the “fit” corrosion rates, as explained above. It is noted that, as before, the “fit” values are reasonably close to the measured values. The 78 consistently larger “fit” values may reflect enhanced corrosion due to the possible existence of anodic zones in the moving bed cathode during electrodeposition.43

Table 3.1.3: Measured and “fit” nickel corrosion rates (pH 4, 10A).

Nickel corrosion rate Nickel corrosion rate Temperature without nitrogen sparging with nitrogen sparging (°C) (×106 mol L-1 min-1) (×106 mol L-1 min-1) (pH 4, 10A) Measured “Fit” Measured “Fit” 30 10.2 24 1.6 8.6 35 12.1 29 2.6 10 40 13.4 40 3.6 14 45 18.4 45 4.5 16

The corresponding dissolved oxygen concentration data are presented in Figure

3.1.10. As shown, the initial concentrations of dissolved oxygen prior to beginning electrodeposition were 7.6, 6.0, 4.9, and 4.4 mg L-1 in the electrolyte solution at 30°C,

35°C, 40°C, and 45°C, respectively, without nitrogen sparging. This ordering is expected from equilibrium considerations (i.e., Henry’s law) for water in contact with air at 1 atm.

The data with nitrogen sparging also show increasing initial dissolved oxygen concentration with temperature, but at lower levels, significantly below the expected equilibrium values. During electrowinning, the dissolved oxygen concentration increases rapidly, and then levels out to a relatively constant asymptotic value. For the data set without nitrogen sparging, it is noted that the observed pseudo-steady asymptotic values were not monotonic with temperature; i.e., the approximate average asymptotic values were about 9.2, 9.7, 6.8, and 6.0 mg L-1 for 30°C, 35°C, 40°C, and 45°C, respectively.

This behavior can be qualitatively explained as follows. A simple dissolved oxygen balance would be:

+ m n d[O2]/dt = ka – kc[H ] [O2] - kLa([O2] - [O2]eq), (3.1.21) 79 where the terms on the righthand side are, respectively: oxygen production via reaction

(3.1.4) at the anode; oxygen consumption at the cathode due to reaction (3.1.8)

(corrosion); and oxygen transport to/from air in the solution holding tank, depending on whether the dissolved oxygen concentrations during electrowinning are above/below equilibrium values. This balance results in the following approximate expression for the pseudo-steady dissolved oxygen concentration:

+ m [O2]s = {ka + kL[O2]eq}/{kc[H ] + kL}, (3.1.22) assuming, as reported by Ives and Rawson32 for copper corrosion, that the order of the corrosion reaction with respect to oxygen is unity. At low temperatures, for sufficiently high anodic oxygen production rates, and low corrosion rates, Eq. (3.1.25) indicates that:

[O2]s ≈ ka/kL, (3.1.23) and since the anodic reaction is activated and the mass transfer coefficient is essentially not, the pseudo-steady oxygen concentration should increase with temperature, just as observed between 30°C (9.2 mg L-1) and 35°C (9.7 mg L-1). However, as the temperature increases further, the corrosion rate eventually becomes larger with respect to the mass transfer rate, such that the pseudo-steady oxygen concentration will then become approximately:

+ m [O2]s ≈ ka /kc[H ] (3.1.24)

Consequently, if the effective activation energy for corrosion is greater than that of anodic oxygen production, the pseudo-steady dissolved oxygen concentration will decrease with increasing temperature, just as observed for 35°C (9.7 mg L-1), 40°C (6.8 mg L-1), and 45°C (6.0 mg L-1), without nitrogen sparging. 80

The situation with nitrogen sparging is somewhat different. In this case, the removal

rate of dissolved oxygen in the solution holding tank is much greater and becomes

practically constant, such that Eq. (3.1.19) becomes approximately:

+ m [O2]s = {ka - kS}/kc[H ] (3.1.25)

where kS is the oxygen removal rate by nitrogen sparging. In this case, since kS is

unactivated, it is expected that the dissolved oxygen concentration will decrease

monotonically with temperature, in agreement with the corresponding data in Figure

3.1.9. Eqns. (3.1.24) and (3.1.25) also suggest that at constant temperature, the pseudo- steady dissolved oxygen concentration will increase with increasing pH. This is precisely what is observed in Figure 3.1.4 at low times at a constant temperature of 35°C.

The effect of the feeder current on nickel electrowinning at pH 4.0 and 350C is

presented in Figure 3.1.11, both with and without nitrogen sparging. The corresponding

“corrected” current efficiencies and the influence of mass transfer rate limitations are

presented in Figures 3.1.12 and 3.1.13, respectively. As expected, the nickel recovery rate

increases significantly with feeder current. However, as shown in Figure 3.1.12, the

current efficiency decreases with increasing applied current. Also, as shown in Figure

3.1.11, nitrogen sparging improved the net nickel recovery rate, especially as the current

increased. The model results show the correct trends. However, at the highest current of

15A, the data show a significant difference between the unsparged and sparged results,

while the model did not. As previously discussed, this is attributed to the fact that the

model uses a simple constant corrosion rate approximation that does not take into account

the effects of the variability of the dissolved oxygen concentration, and also the

possibility of additional cathodic oxygen sinks when the nickel cation concentration 81

becomes very low. The dissolved oxygen concentration increases monotonically with

applied current at low times (i.e., at high nickel cation concentrations), but at 15A also

exhibits the almost linear decrease characteristic of low nickel cation concentrations, as

discussed previously in connection with the results in Figure 3.1.4. At a higher current

(15A), the electrodeposition rate constant, k1, is greater, which results in the earlier mass

transfer rate limitations in Figure 3.1.13.

Abd El Aal et al.31 reported first order dependence of anodic corrosion rates of nickel

on [H+]. However, in their experiments with static nickel electrodes, these authors also observed that oxygen passivated the surface with a nickel oxide layer which slowed the nickel redissolution rate. Ives and Rawson26, on the other hand, reported first order

dependence of copper corrosion on dissolved oxygen. They also concluded, “…that a

cuprous oxide film on copper may have either an activating or passivating effect,

depending on its mode of generation. The evidence indicates the existence of an

electrochemical mechanism for general corrosion of copper by which dissolution and

film growth are kinetically linked processes.” The conditions in the spouted particulate

electrode are quite different than in either of these studies. However, direct experimental

evidence has been presented here that nickel corrosion is dependent on both [H+] and

dissolved oxygen concentration, and that the effect seems to be quite similar to what was

observed for copper corrosion in a spouted particulate electrode.18 Also as indicated previously, the additional effect of the mechanical, “self-polishing” action of the particles abrading against one another in the moving bed cathode, serves to mitigate the formation of a passivating oxide layer during particle circulation, both with and without applied current. In fact, the formation of an oxide layer has been observed when the particles are 82

allowed to remain in solution while they are not being circulated. This is believed to be

the source of the lower than expected nickel deposition rates observed at the inception of

many of the experimental runs. Consequently, it is assumed that the dependence of the

nickel corrosion rates in the spouted particulate electrode on both [H+] and dissolved

oxygen are close to first order.

Arrhenius plots of the experimental corrosion rates presented in Table 3.1.3, corrected

+ for assumed first order dependence on [H ] and [O2], yielded apparent activation energies of 56 kJ/mol for the data without nitrogen sparging, and 106 kJ/mol with nitrogen sparging. The Arrhenius plots were quite linear, but with only four data points over such a small temperature interval, they are probably not very accurate. In any case, it is interesting to note that they differ by just about a factor of two. This is reminiscent of the classical “masking” of kinetic rate constants by diffusional resistance that leads to the square root behavior of the rate constant, and thus division of the intrinsic activation energy by two. The observed behavior may be related to this effect, since the unsparged rates are greater than the sparged rates, and would, therefore, be more influenced by mass transfer resistances.

3.1.3. Conclusions

The spouted particulate electrode system investigated here exhibited good performance for the removal of nickel from acidic solutions. It was determined that, in general, the rate of nickel electrowinning increases with increasing pH and increasing temperature in acidic solutions over the experimental range investigated. Nitrogen sparging of the electrolyte solution in the holding tank was effective in reducing the dissolved oxygen concentration, suppressing the nickel corrosion reaction, and, thereby, 83

improving the net nickel recovery rate. The effects of nitrogen sparging were somewhat

more effective at higher pH, lower temperature, and higher feeder currents.

The electrochemical kinetics of the spouted bed particulate electrode was adequately

described by a simple batch kinetic model based on the Tafel equations, incorporating a

constant corrosion rate as an approximation. Calculated concentration-time behavior

results obtained by integrating the batch-type differential equation model, were shown to

produce reasonable agreement with the experimental data. The principal model

shortcomings include: a “fit” constant corrosion rate with no explicit [H+] and dissolved

oxygen dependence; and neglecting other possible electrode reactions that could affect

the dissolved oxygen mass balance.

3.2: Cadmium and Lead Removal With a Spouted Particulate Electrode

3.2.1: Background

Cadmium has been widely used in aircraft and aerospace applications, primarily for

the corrosion protection of steel. However, its toxicity and carcinogenic properties creates

environmental problems.44,45,46,47 Cadmium poisoning is an occupational hazard

associated with industrial processes such as metal plating and the production of nickel- cadmium batteries, pigments, plastics, and other synthetics.48,49 Due to its more negative

standard reduction potential, cadmium reduction is less thermodynamically favored than

for copper anf nickel. In addition, cyanide which is also toxic, is often employed in

cadmium electroplating50,51,52 because it increases the solution conductivity, as well as the deposition rate by forming Cd-cyanide complexes on the cathodic interface in the course of electrolysis.53,54,55 Medvedev et al.56 proposed a method for electrochemical deposition 84

of cadmium, and Guel et al.57 investigated the mechanisms of cadmium electrodeposition

from an aqueous 0.1 M CdSO4 solution. Insofar as we know, no other investigations have

been conducted of Cd removal by electrowinning on spouted particulate electrodes.

Lead is a poisonous metal that can damage nervous connections (especially in young

children) and cause blood and brain disorders.58,59 Lead poisioning was already well

known in ancient Rome, Greece, and China.59 Due to its high weight-to-volume ratio and

stability, lead has been widely used in lead-acid batteries, fishing sinkers, keel ballast for sailboats, scuba diving weight belts, as well as in a wide variety of other applications such as a colorant and in semiconductors.60,61

Unlike copper, nickel, and cadmium sulfate, lead sulfate is not very soluble in

aqueous acidic solutions. However, lead electrodeposition from methanesulfonic acid

(MSA) solutions, in which lead is soluble, has been investigated. The aqueous solubility

of lead methanesulfonates, the high conductivity of MSA solutions, the unique oxidation

resistance of the metal, and the low toxicity of MSA, all make it an ideal electrolyte for lead-based electrochemical processes. Over the past ten years or so, MSA has largely

replaced fluoroboric acid as the electrolyte of choice for the electrodeposition of lead

because it is an environmentally superior alternative.62,63,64,65 Chen et al.64 investigated

lead electrodeposition from MSA solutions on a rotating disk electrode. Chen et al.65 also reporteded tin-lead deposition in MSA solutions and concluded that the mechanism was parallel reduction of tin and lead without interaction. Balaji and Pushpavanam62 reviewed

lead electrodeposition from MSA solutions. These investigations all focus primarily on

lead and lead-tin metal alloy properties, and lead electroplating. Spiegel et al.66 suggested

that lead could be removed from wastewater in a particulate electrode from nitric acid 85 solutions. However, nitric acid is also a strong lead oxidizer. No other reports were found on the removal of lead from wastewater via electrowinning on spouted particulate electrodes, including in MSA.

Here we present results on the removal of cadmium and lead from acidic solutions in the same cylindrical spouted particulate electrode used in Section 3.1 for nickel removal.

As previously, the effects of pH, temperature, current, and nitrogen sparging on the net recovery and corrosion rates of metal and lead were investigated under galvanostatic conditions.

3.2.2: Results

3.2.2.1: Electrodeposition Reactions

The reactions that are assumed to occur for cadmium electrodeposition are:

Main cathodic reaction: Cd2+ + 2e- → Cd (3.2.1)

+ - Main anodic reaction: H2O → 2H + 2e + ½ O2 (3.2.2)

Similarly, the primary cathodic reaction for lead electrodeposition from acidic solutions is assumed to be:

Main cathodic reaction: Pb2++2e- → Pb (3.2.3)

+ - Main anodic reaction: H2O → 2H + 2e + ½ O2 (3.2.4)

Generally, hydrogen reduction is probably still the most important cathodic side-reaction for both metals for similar reasons as discussed for nickel in Section 3.1.

3.2.2.2: Cadmium Electrowinning Results

The effects of pH on cadmium recovery at 40°C are presented in Figure 3.2.1. As shown, it was found that the net cadmium electrowinning rate behaved in an analogous 86 fashion to that of nickel; i.e., it increased with increasing pH over this range, both with and without nitrogen sparging.

The corresponding “intrinsic” current efficiencies (determined from the instantaneous slopes (rates) of the recovery curves in Figure 3.2.1, and corrected for corrosion as described above) are presented in Figure 3.2.2. As expected, the current efficiencies decreased with decreasing Cd2+ concentration, due to the increasing influence of mass transfer limitations. Compared at the same cadmium ion concentration, the current efficiencies increased with pH over the range of experimental conditions investigated

(i.e., pH 3.5–4.5). It is also noted that the current efficiencies were typically slightly lower at the beginning of each run, just as for nickel.

Also shown in Figures 3.2.1 and 3.2.2 is the effect of nitrogen sparging. A comparison of these results clearly shows that the rate of Cd electrowinning was considerably increased with nitrogen sparging, in a similar fashion as for nickel. The ultimate asymptotic concentration with nitrogen sparging was about 120 ppm (1.07×10-3 mol L-1) at pH 4.5. As shown in Figure 3.2.2, the resultant current efficiencies were all also considerably greater than those without nitrogen sparging.

In Figure 3.2.3 are presented the corresponding dissolved oxygen concentrations in the electrolyte solution at 10A and 40°C, as a function of pH, both with and without nitrogen sparging As shown, without nitrogen sparging, the initial concentration of dissolved oxygen was about 6.2 mg L-1 (6.96 mg L-1 is the equilibrium concentration in contact with air at 1atm at 40°C),41 which did not vary very much with pH. With nitrogen sparging, the initial concentration of dissolved oxygen in the electrolyte solution decreased to about 1.0 mg L-1, which is well below the equilibrium value, and much less 87

than that without nitrogen sparging. During electrowinning, the concentration of

dissolved oxygen increased rapidly in all cases, and then either leveled off without

sparging, or continued to decrease slightly with sparging.

As indicated above, oxygen is produced primarily via the anodic reaction (3.2.2), and

in the presence of H+ can re-oxidize deposited Cd metal back to cadmium cations via

reaction (3.2.5).

+ 2+ Cd + ½ O2 + 2H → Cd + 4H2O (3.2.5)

Cadmium corrosion data were obtained by turning off the feeder current and measuring

the increase in dissolved cadmium concentration with time, as described in Chapter 2.

The resultant data as a function of pH at 40°C, both with without nitrogen sparging, are

presented in Figure 3.2.4 and also summarized in Table 3.2.1.

Table 3.2.1: Measured cadmium corrosion rates (40°C, 10A).

Cadmium corrosion rate Cadmium corrosion rate pH without nitrogen sparging with nitrogen sparging (×106 mol L-1 min-1) (×106 mol L-1 min-1) 3.5 42 11 4.0 20 6.5 4.5 6.5 2.6

As indicated, the rate of cadmium corrosion in these experiments under these

conditions is approximately zeroth order with respect to cadmium cation concentration,

and increases monotonically with decreasing pH at 40°C, in a similar fashion as for nickel. Also, as shown, at the lower dissolved oxygen concentrations with nitrogen sparging, the corrosion rate was significantly reduced.

In Figure 3.2.5 is presented the behavior of cadmium electrowinning as a function of temperature at a current of 10A and pH 4, both with and without nitrogen sparging, and 88

the corresponding “intrinsic” current efficiencies (“corrected” for Cd corrosion) are

presented in Figure 3.2.6. As shown, the net electrodeposition rate increases with

temperature, and thus the current efficiency also increases with temperature. In

comparison to nickel, the electrodeposition rates and current efficiencies of cadmium are less. This is consistent with the lower reduction potential of cadmium.

As previously shown, nitrogen sparging accelerates net cadmium deposition, and

increases the current efficiency. The corresponding measured corrosion rates for these

conditions are presented in Figure 3.2.8. As previously, the effect of nitrogen sparging is

quite evident. That is, all the corrosion rates with nitrogen sparging were considerably less, irrespective of temperature. In Table 3.2.2 are presented the constant cadmium

corrosion rates obtained from the measurements, as explained above.

Table 3.2.2. Measured cadmium corrosion rates (pH 4, 10A).

Cadmium corrosion rate Cadmium corrosion rate Temperatur without nitrogen sparging with nitrogen sparging e (°C) (×106 mol L-1 min-1) (×106 mol L-1 min-1) 30 12 3.0 40 20 6.5 50 30 9.4

The corresponding dissolved oxygen concentration data are presented in Figure 3.2.7.

As shown, the initial concentrations of dissolved oxygen prior to beginning

electrodeposition were 7.6, 4.8, and 3.0 mg L-1 in the electrolyte solution at 30, 40, and

50°C, respectively, without nitrogen sparging. This ordering is expected from equilibrium

considerations (i.e., Henry’s law) for water in contact with air at 1 atm. The data with

nitrogen sparging also show increasing initial dissolved oxygen concentration with

temperature, but at lower levels significantly below the expected equilibrium values. 89

During electrowinning, the dissolved oxygen concentration increases rapidly, and then

levels out to a relatively constant asymptotic value. For the data set without nitrogen

sparging, it is noted that the observed pseudo-steady asymptotic values were monotonic

with temperature; i.e., the approximate average asymptotic values were about 9.4, 8.8,

and 8.2 mg L-1 for 30, 40, and 50°C, respectively. This behavior can be qualitatively explained in the same fashion as for nickel.

With nitrogen sparging, the dissolved oxygen concentration in the electrolyte solution

decreased to 2.3, 0.8, and 0.1 mg L-1 prior to the initiation of electrodeposition at 30, 40,

and 50°C, respectively. The observed pseudo-steady asymptotic values were 5.3, 2.8, 1.5

mg L-1 for 30, 40, and 50°C, respectively, which also decreased monotonically with temperature.

The effect of the feeder current on cadmium electrowinning at pH 4.0 and 40°C is presented in Figure 3.2.9, both with and without nitrogen sparging. As expected, the cadmium recovery rate increases significantly with feeder current. As shown in Figure

3.2.10, the current efficiency decreases with increasing applied current, as expected, Also as shown in Figure 3.2.11, nitrogen sparging improved the net cadmium recovery rate, especially as the current increased. The dissolved oxygen concentration increases monotonically with applied current at low times (i.e., at high cadmium cation concentrations), but at 15A also exhibits the almost linear decrease characteristic of low cadmium cation concentrations, as discussed previously in connection with the results in

Figure 3.2.3.

The effects of temperature and pH on cadmium electrodeposition are similar to that for nickel. However, it was found that the asymptotic concentrations of cadmium were 90

greater than those for nickel under the same or similar conditions. The maximum of the

dissolved oxygen concentration in the electrolyte solution during cadmium

electrodeposition was less than that of nickel. This observation is consistent with lower

standard reduction potential of Cd (-0.40V) 27 in comparison to nickel (-0.26V). 27

3.2.2.3: Lead Electrowinning Results

The effect of pH on lead removal from MSA solutions at 40°C is presented in Figure

3.2.12. As shown, it was also found that the net lead electrowinning rate increased with increasing pH over this range, both with and without nitrogen sparging.

The corresponding current efficiencies (determined from the corrected instantaneous slopes (rates) of the recovery curves in Figure 3.2.12, as described above) are presented in Figure 3.2.13. As expected, the current efficiencies decreased with decreasing lead ion concentration, due to the increasing influence of mass transfer limitations. The current efficiencies increased with pH over the range of experimental conditions investigated

(i.e., pH 3.5–4.5).

Also shown in Figures 3.2.12 and 3.2.13 are the effects of nitrogen sparging. A comparison of the results clearly shows that the rate of Pb electrowinning was considerably increased with nitrogen sparging. The ultimate asymptotic concentrations with nitrogen sparging were 204 ppm, 60 ppm and 33 ppm (0.98×10-3, 0.29×10-3,

0.16×10-3 mol L-1) for pH values of 2.0, 2.5 and 3.0, respectively. As shown in Figure

3.2.13, the resultant current efficiencies were all also considerably greater than those

without nitrogen sparging. It is noted that the long-time, asymptotic lead concentrations

were greater than for nickel, and less than those for cadmium. The standard reduction 91 potential of lead is -0.29V67, which is in between that of nickel (-0.257V68) and cadmium

(-0.4025V68).

In Figure 3.2.14 are presented the dissolved oxygen concentrations in the electrolyte solution at 10A and 40°C, as a function of pH, both with and without nitrogen sparging.

Just as for cadmium in Figure 3.2.3, the dissolved oxygen concentration decreases monotonically with decreasing pH, due to the increasing corrosion rate. Without nitrogen sparging, the initial concentration of dissolved oxygen was about 6.0 mg L-1, which did not vary with pH. With nitrogen sparging, the initial concentration of dissolved oxygen was decreased to about 1.0 mg L-1, which is well below the equilibrium value and much less than that without nitrogen sparging. During electrowinning, the concentration of dissolved oxygen increases rapidly to a maximum in all cases, and then continues to decrease gradually, in a similar fashion as for nickel and cadmium. The maximum in the dissolved oxygen concentration during lead deposition is in between those of nickel and cadmium, which is in the same order of magnitude as the electrodeposition rates of these three metals.

As indicated above, oxygen is produced primarily via the anodic reaction (3.2.4), and in the presence of H+ can re-oxidize deposited lead metal via reaction (3.2.6):

+ 2+ Pb + ½ O2 + 2H → Pb + 4H2O (3.2.6)

Lead corrosion data were obtained by turning off the feeder current and measuring the rise in dissolved lead concentration with time, as described in Chapter 2. The resultant data as a function of pH at 40°C, both with without nitrogen sparging, are presented in

Figure 3.2.15. As shown, the rate of lead corrosion in these experiments under these conditions exhibits close to zeroth order behavior with respect to lead cation 92 concentration, and increases monotonically with decreasing pH at 40°C, just as for nickel and cadmium. Also, as shown, at the lower dissolved oxygen concentrations with nitrogen sparging, the corrosion rate was significantly reduced. The measured corrosion rates are presented in Table 3.2.3.

Table 3.2.3: Measured lead corrosion rates (40°C, 10A).

Lead corrosion rate Lead corrosion rate pH without nitrogen sparging with nitrogen sparging (×106 mol L-1 min-1) (×106 mol L-1 min-1) 2.0 28 5.8 2.5 21 4.4 3.0 9.7 2.0

Table 3.2.4: Measured lead corrosion rates (pH 4, 10A).

Lead corrosion rate Lead corrosion rate Temperature without nitrogen sparging with nitrogen sparging (°C) (×106 mol L-1 min-1) (×106 mol L-1 min-1) 30 8.2 2.4 40 21 4.4 50 29 5.3

In Figure 3.2.16 is presented the behavior of lead electrowinning as a function of temperature at a current of 10A and pH 2.5, both with and without nitrogen sparging. As previously shown, nitrogen sparging accelerates net lead deposition, and increases the current efficiency. In Figure 3.2.17 are presented the corresponding current efficiencies as a function of temperature at 10A and pH 2.5. As noted, there is a severe decrease in current efficiency with decreasing temperature due to the reduced rate of electrowinning.

The corresponding measured corrosion rates for these conditions are presented in

Figure 3.2.19. As previously, the effect of nitrogen sparging is quite evident. That is, all the corrosion rates with nitrogen sparging were considerably reduced, irrespective of 93

temperature. In Table 3.2.4 are listed the appropriately constant lead corrosion rates

obtained from the measurements, as explained above.

The corresponding dissolved oxygen concentration data are presented in Figure

3.2.18. As shown, the initial concentrations of dissolved oxygen prior to initiating

electrodeposition were 7.6, 5.8, and 3.0 mg L-1 in the electrolyte solution at 30, 40, and

50°C, respectively, without nitrogen sparging. The data with nitrogen sparging also show

increasing initial dissolved oxygen concentration with temperature, but at lower levels,

significantly below the expected equilibrium values. During electrowinning, the

dissolved oxygen concentration increases rapidly, and then levels out to a relatively

constant asymptotic value. For the data set without nitrogen sparging, it is noted that the

observed pseudo-steady asymptotic values increased monotonically with temperature;

i.e., the approximate average asymptotic values were about 11.5, 10.4, 9.7 mg L-1 for

30°C, 40°C, and 50°C, respectively. With nitrogen sparging, the dissolved oxygen concentration in the electrolyte solution decreased to 2.2, 0.9, and 0.2 mg L-1 prior to

beginning electrodeposition at 30°C, 40°C, and 50°C, respectively. The observed pseudo-

steady asymptotic values were 5.7, 3.4, 2.3 mg L-1 for 30°C, 40°C, and 50°C,

respectively, increasing with temperature.

The effect of the feeder current on lead electrowinning at pH 4.0 and 40°C is presented in Figure 3.2.20, both with and without nitrogen sparging. As expected, the lead recovery rate increases significantly with feeder current. As shown in Figure 3.2.21, the current efficiency decreases significantly with applied current. The dissolved oxygen concentration in Figure 3.2.22 increases monotonically with applied current at low times

(i.e., at high lead cation concentrations), but at 15A also exhibits the almost linear 94

decrease characteristic of low lead cation concentrations, as discussed previously with

respect to the results in Figure 3.2.14.

Lead corrosion appears to be dependent on both [H+] and dissolved oxygen

concentration, in a similar fashion to that observed for copper,18 nickel, and cadmium.

Also, as indicated previously, the additional effect of the mechanical, “self-polishing”

action of the particles abrading against one another in the moving bed cathode, serves to

prevent the formation of a passivating oxide layer during particle circulation, both with

and without applied current.

3.2.3: Conclusions

The spouted particulate electrode system investigated in this work exhibited reduced

performance for the removal of cadmium and lead than nickel from acidic solutions. In

comparison to nickel, the more negative reduction potential of cadmium results in

decreased electrodeposition rates with lower current efficiencies, and the less negative

reduction potential of lead results in higher current efficiencies. It was observed that, in

general, the rate of cadmium and lead electrowinning increases with increasing pH and increasing temperature in acidic solutions over the experimental range of parameters investigated. Nitrogen sparging of the electrolyte solution in the holding tank was effective in reducing the dissolved oxygen concentration and suppressing cadmium and lead corrosion reactions. The qualitative electrodeposition behavior of these three metals

(nickel, cadmium, and lead) is qualitatively similar with respect to the effects of temperature, pH and applied current. However, it was found that nickel exhibited the highest electrodeposition rates, followed by lead and then cadmium. It is noted that this

ordering is the same as the standard reduction potentials of these metals. 95

3.3: Metal Co-Removal From a Cu/Ni Mixture With a Spouted Particulate

Electrode

3.3.1: Background

It is to be expected that more often than not, mixtures of multiple heavy metals will

be present in contaminated waters. In this section, the co-removal of heavy metal cations via electrowinning in a spouted particulate cathode is investigated - more specifically, the co-removal of copper and nickel.

There is some literature on copper and nickel co-electrodeposition.69,70,71 Landolt69

showed that the co-deposition of copper and nickel are not independent. A displacement

reaction between deposited metallic nickel and copper ion in solution can occur.72

Bradley and Landolt73 reported on an electrodeposition-displacement reaction using a pulsed current method. Bradley et al.74 and Scharfe et al.75 also studied the effects of

metal displacement on deposition. It is also known that in many cases the properties of

electrolytically-deposited alloys differ from that of their cast analogs.76,77 Roy78

investigated the enhancement of alloy structures using the displacement reaction.

Here we present results on the co-removal of copper and nickel from acidic solution mixtures of the two metals in a cylindrical spouted particulate electrode (SPE). The effects of pH, temperature, nitrogen sparging and the metal displacement reaction on the net recovery and corrosion rates of the co-deposited metals were investigated under galvanostatic conditions.

3.3.2: Co-Electrodeposition Model

An electrochemical deposition model for the co-removal of copper and nickel was developed based on the single metal models used to correlate/predict the behavior of 96

copper18 and nickel. Based upon the preceding discussions and results, the principal

reactions assumed to occur are:

Main cathodic reactions: Cu2++2e- → Cu (3.3.1)

Ni2++2e- → Ni (3.3.2)

+ - Cathodic side reaction: 2H + 2e → H2 (3.3.3)

+ - Main anodic reaction: H2O → 2H + 2e + ½ O2 (3.3.4)

In addition to the preceding, the model must also take into account metal corrosion in

the presence of oxygen and [H+] which oxidizes deposited metal on the particles in acidic solutions via the same reactions as previously identified:

+ 2+ Cu + ½ O2 + 2H → Cu + H2O (3.3.5)

+ 2+ Ni + ½ O2 + 2H → Ni + H2O (3.3.6)

Also, as discussed above, it was found that the metal displacement reaction:

Cu2+ + Ni → Ni2+ + Cu; E° = +0.59V (3.3.7)

is important in describing the behavior of copper and nickel co-deposition. This reaction

is driven by the higher reduction potential of copper with respect to nickel.

All the assumptions that were used to model the electrodeposition behavior of the

single metals were retained here, including mass transfer resistance and the kinetic

formulations. In this case, for galvanostatic operation, the corresponding total cathodic

current balance becomes:

(n1F / a)k1C1 (n2F / a)k2C2 i = i1 + i2 + i3 = + + (n3F / a)k3C3 (3.3.8) (1+ k1 / kLa) (1+ k2 / kLa)

where the subscripts are: 1 = copper; 2 = nickel; and 3 = hydrogen. With this expression,

the mass balances for the metal cations become: 97

dC1 i1a kdC1 = − + kc1 − (3.3.9) dt n1F (1+ kd / kLa)

dC2 i2a kdC1 = − + kc2 + (3.3.10) dt n2F (1+ kd / kLa)

dC 1 = −k 'C + k − k ' C (3.3.11) dt 1 1 c1 d 1

dC 2 = −k ' C + k + k ' C (3.3.12) dt 2 2 c2 d 1

Where:

' ki = ki /(1+ ki / k L a) (3.3.13)

where it is assumed that the displacement reaction is first order in the copper cation

concentration, C1, and the kCi are the respective corrosion rates for the two metals due to

all mechanisms, which is assumed to be zeroth order in the metal cation concentration at

constant oxygen concentration and pH, as previously.

The electrochemical kinetic rate parameters used in the Cu/Ni co-electrodeposition

model are presented in Table 3.3.1.

Table 3.3.1: Model parameter values used for the Cu/Ni co-electrodeposition model.

Reactio Transfer Exchange current Enthalpy of activation T(° n coefficient, α density, A m-2 (kJ/mol-1) C) Cu2+/Cu 0.74 [79] 2.3 [79] † 41.7 [79] † 25

Ni2+/Ni 0.49[37] 0.068 [37] 37.2 [38] 25

+ H /H2 0.63[39] 0.002 [39] 42.2 [39] 20 † Calculated from the data in Ref. [79].

Model solution methodology: 98

(i) The metal displacement reaction rate constant was determined in the

following manner.

a. Both the Cu and Ni data were fit to polynomials to provide smooth curves.

b. From these polynomial fits, C1, C2, and dC1/dt at the nickel maximum

where dC2/dt=0 were found.

c. From the current balance at the nickel maximum, the overpotential, ηmax,

and the electrodeposition rate constants were determined at the nickel

maximum. From the copper and nickel mass balances in Eqns. (3.3.11)

and (3.3.12) above, at the nickel maximum:

dC ' d(C + C ) dC = 1 2 = 1 = −k 'C − k ' C + k + k (3.3.14) dt dt dt max 1 1 2 2 c1 c2

dC  + = 1 + ' + ' kc1 kc2  k1C1 k2C2  (3.3.15)  dt  max

dC 2 = 0 = (−k ' C + k + k ' C ) (3.3.16) dt max 2 2 c2 d 1 max

' ' kd C1 max + kc2 = k2C2 max (3.3.17)

Comparing this expression with the experimental value (slope) measured

' from the Ni corrosion data, (kd C1 + kc2 )exp :

k ' C −(k ' C + k ) ' 2 2 max d 1 c2 exp kd = (3.3.18) (C1 max −C1 exp )

and,

' ' kc2 = k2C2 max − kd C1 max (3.3.19) 99

The sum of the corrosion experiment slopes for copper and nickel yield

the value of (kc1 + kc2 )exp , and

kc1 = (kc1 + kc2 )exp − kc2 (3.3.20)

' Using the values for kd , kc1 , and kc2 , determined as above, the model can

then be solved for the copper and nickel cation concentrations.

(ii) With fixed current density and initial concentrations of metal ions, C10, and

C20, the current balance given by Equation 3.3.8 is solved numerically using

the method of bisection to give the electrode potential, E.

(iii) Equations (3.3.9) and (3.3.10) are solved by marching in time, using a Runge-

Kutta, 4th order method.

(iv) Steps (ii) and (iii) are repeated until the desired time is reached.

(v) The metal displacement reaction rate constant is determined from co-

deposition data and the experimental corrosion data. After that, in a similar

fashion as was done for the nickel metal removal model in Section 3.1.2.1, a

least squares method was used to determine the “best fit” parameter values.

First, assuming the measured copper and nickel, corrosion rates, the rate of the

metal displacement reaction was adjusted to provide the best fit of the initial

portion of the nickel removal curve where it is dominant. Next, the copper

corrosion rate was adjusted to provide the best fit for the copper removal

curve. And finally the same was done for the nickel removal curve. The entire

process was then repeated with the new fit corrosion rates until the resultant

parameters agreed. 100

3.3.3. Results and Discussion

3.3.3.1. Without Nitrogen Sparging

The results of co-removal of copper and nickel as a function of pH at a constant

temperature of 40°C are presented as in Figure 3.3.1, and as a function of temperature at

a constant pH of 4 in Figure 3.3.5. The data points are the measured values, and the

dashed and solid curves are the model results for copper and nickel, respectively. As

shown, the model results are in reasonable agreement with the data. Although co-

electrodeposition is conducted at considerable overpotential, copper deposits first and

nickel second, in accordance with their reduction potentials. For each metal, initially the

deposition process is electrochemical rate-limited, which, after passing through a

“mixed” control regime, then becomes fully mass transfer-limited at lower

concentrations.

In Figure 3.3.2 are presented the copper and nickel overpotentials, ηmax=E-Ee,

calculated with the model as a function of time at a pH of 4, 10A and 40°C. Copper is

electrodeposited first up to about 110 min. The overpotential in this region was -0.21V.

The overpotential then increases rapidly to -0.55V, as nickel begins to electrodeposit. The

model also indicates that the copper electrodeposition rate constant and the mass transfer

coefficient become approximately equal (i.e., kLa/k1=1). The nickel electrodeposition rate

constant becomes approximately equal to the mass transfer coefficient (i.e., kLa/k2 = 1) at

about 400 min. Thus, copper reduction predominates at the outset and becomes depleted

from solution at about 100 min, at which time nickel reduction begins to control the

deposition process. At around 400 min, the nickel ion concentration is sufficiently low

that mass transfer begins to control the deposition process. 101

For acidic solutions containing just copper, in the same apparatus used in the current

work, it was found that the electrodeposition rate increased with decreasing pH and

temperature,18 while for solutions containing just nickel, the deposition rate increased

with increasing pH and temperature over the ranges of parameters investigated, as

presented in Section 3.1. As shown in Figure 3.3.1, however, for co-electrodeposition it

was found that the maximum removal rate occurred at about a pH of 4. Also, from the

results in Figure 3.3.5 at a pH of 4, the maximum co-electrodeposition rate occurred at a

temperature of about 30°C for copper and about 40°C for nickel.

It is noted that the nickel concentration typically increases at first, attaining a maximum at a relative concentration ratio of about 1.05 - 1.2, and then decreases monotonically thereafter. As shown previously in Figures 3.3.1 and 3.3.5, with only nickel in the electrolyte solution, the nickel concentration always decreases monotonically with time. Consequently, there are significant quantitative and qualitative differences between the electrochemical removal of metals from single metal aqueous solutions and from solutions of multiple metal mixtures.

In Figures 3.3.3 and 3.3.6 are presented the corresponding dissolved oxygen concentrations with time. As shown, upon initiation of electrowinning, the concentration

of dissolved oxygen increases rapidly, exhibits a maximum, and then decreases again.

The pseudo-steady level of dissolved oxygen is controlled primarily by its production rate via reaction (3.3.4) and its consumption rate via reaction (3.3.5) and (3.3.6), as well as

any mass transfer loss/gain to air (depending on whether the dissolved oxygen

concentration is greater or less than its equilibrium value) while in contact with air in the 102

solution holding tank. This behavior is similar to that for nickel electrodeposition, as

discussed in Section 3.1.

The contribution of corrosion and metal displacement to the net removal rate was

estimated by turning off the current while maintaining the liquid flow and particle

circulation, in the same manner as described previously. The resultant corrosion curves as

a function of pH at 40°C are presented in Figure 3.3.4. The resultant net corrosion rates

of copper and nickel are presented in Table 3.3.2, along with the “best fit” values

determined from the electrodeposition model, as explained above.

As indicated, the corrosion rates for both copper and nickel decrease with increasing

pH. It is noted that under these conditions, the nickel concentration increases with time

due to corrosion and the metal displacement reaction, while the copper concentration increases with time at lower pH, but decreases at higher pH. This differs from the behavior observed in acidic solutions containing just copper18 or just nickel (Section 3.1),

in which it was found that the corrosion rates were always positive; i.e., their

concentrations always increased with time when the current was turned off.

The corrosion behavior as a function of temperature at a constant pH of 4 is presented

in Figure 3.3.7. The observed “corrosion” rates of copper and nickel are presented in

Table 3.3.3, along with the “best fit” values determined from the electrodeposition model,

as explained above.

It is noted that while the nickel concentration increases with time due to corrosion, and

increases with temperature, as expected, the copper concentration continues to decrease

without current. 103

The copper behavior in Figure 3.3.4 and 3.3.7 is attributed to the spontaneous metal displacement reaction between deposited nickel and copper ions (3.3.7). This reaction effectively reverses the rate of copper corrosion, increases the rate of nickel corrosion, at least initially, and also amplifies the separation of the deposition regimes of the two metals.

3.3.2.3. With Nitrogen Sparging

As previously, nitrogen sparging significantly reduces the dissolved oxygen concentration in the electrolyte solution. This effectively inhibits the oxidation reactions

(3.3.5) and (3.3.6). From the data without nitrogen sparging, the optimal conditions for electrodeposition under the experimental conditions used here are: 30°C and pH 3.0 for copper; and 40°C and pH 4.0 for nickel.

During electrowinning, copper is deposited first due to its greater reduction potential.

Additionally, in previous work18 it was found that with just copper in solution, the electrodeposition rate increases with decreasing pH and temperature. Therefore, experiments were devised to “program” the solution pH by first running at lower pH and lower temperature over the first two hours, and then at higher pH and/or temperature for the remainder of the run, given that the latter conditions favor higher rates of nickel electrodeposition, which occurs at later times. Some of these results are presented in

Figure 3.3.8. In this figure, the temperature was maintained constant at 40°C and the pH was set to one of three different values (3.0, 3.5, and 4.0) for the first two hours, and then increased to 4.0 for the remainder of the experiment. From these results it appears that a pH of 3.0 for the first two hours followed by a switch to 4.0 enhances both copper and nickel deposition. 104

In Figure 3.3.9 are presented related results programming the temperature, instead of

the pH. At a constant pH of 4, temperatures of 30°C and 40°C were used for the first two hours, after which it was set at 40°C. The results for an initial two-hour period at 30°C appear to be slightly better than the other experiment. However, they are both more similar than in the pH variation experiments shown in Figure 3.3.8. Over the first two hours mostly copper is deposited, while in the next four hours, primarily nickel is deposited. It is expected that if the pH and temperature were maintained slightly lower than the values used early in the electrodeposition process, which would be more favorable for copper electrodeposition, then subsequently changed to pH 4.0 and 40°C, even better removal results could be obtained.

The results of corrosion experiments with nitrogen sparging are presented in Figures

3.3.10 and 3.3.11. The resultant measured corrosion rates are presented in Table 3.3.2 and

3.3.3. As was found for the experiments with the single metals, nitrogen sparging improves the net removal rate for co-electrodeposition as well.

3.3.3: Discussion

The possibility of metal displacement reactions complicates the co-electrodeposition process, in comparison to the electrodeposition of a single metal at a time. Metal displacement reactions are used commercially in the displacement of iron with silver.

There is also significant literature concerning the displacement reaction between copper and nickel. Landolt69 showed that the co-deposition of copper and nickel were not

72 independent. Shibahara et al. found that mixed-metal cubane-type clusters were

involved in the displacement of a metal atom in the metal cluster by another metal atom.

Some work has also been done on the enhancement of alloy structures by using the 105

displacement reaction.78 Roy78 and Bradley et al.73,74 reported electrodepositions-via the

displacement reaction using a pulsed current method. Bradley74 and Scharfe et al.75

studied the effect of metal displacement on deposition. In our work, the presence of the

metal displacement reaction is clearly evident in the maxima observed in the nickel

removal curves in Figures 3.3.1 and 3.3.7.

The presence of the displacement reaction between copper and nickel is also quite

evident in the corrosion results. As noted previously, in Figures 3.3.4 and 3.3.7, the

concentration of nickel ion increases, while the concentration of copper ion decreases with time. This behavior is directly attributable to the metal displacement reaction.

With the current turned off, Eqns. (3.3.9) and (3.3.10) reduce to:

dC1 kdC1 = kc1 − = kc1 − kd′C1 (3.3.21) dt (1+ kd / kLa)

dC 1 = ' dt kc1 − kd C1

dC 1 = ' ' dt − kd (C1 − kc1 / kd )

dC2 kdC1 = kc2 + = kc2 + kd′C1 (3.3.22) dt (1+ kd / kLa)

The solutions of these two equations are:

kc1 kc1 C1 = (C10 − )exp(−kd′t) + (3.3.23) kd′ kd′

k k = + − c1 + + − − c1 − ' C2 (C10 C20 ) ' (kc1 kc2 )t (C10 ' )(exp( kd t)) (3.3.24) kd kd

where C10 and C20 are, respectively, the copper and nickel cation concentrations at the point where the current is turned off. From Eq. (3.3.23), it can be concluded that the 106 copper cation concentration can exhibit two different regimes of behavior, depending primarily on the relative magnitudes of kC1 and kd′. If kd′ >> kC1 the behavior is controlled by the corrosion reaction and the copper cation concentration will increase with time after current shut-off. On the other hand, if kC1 << kd′ then the displacement reaction dominates and the copper cation concentration will decrease with time after current shut- off. Indeed, our experimental results have demonstrated both behaviors depending on temperature, pH, and copper cation concentration at the time of current shut-off. If the argument of the exponential is small, the behavior will be approximately linear in time; i.e.,

' C1 = C10 + (kc1 − kd C10 )t (3.3.25)

C2 = C20 + (kc2 + kd′C10 )t (3.3.26)

From Eq. (3.3.26), it is obvious that the nickel cation concentration can only increase with time.

C k The slopes of the two linear expressions are: k − 10 d for the copper ion, and c1 1+ k / k α d L

C k k + 10 d for the nickel ion. The slopes both contain the initial copper ion c2 1+ k / k α d L concentration when the current is shut off. In most of the experiments, this is typically around 2.3×10-3 mol L-1.

The corrosion rates for both copper18 and nickel (Section 3.1) increase with increasing temperature and decreasing pH. The experimental rates obtained by shutting off the current agree reasonably well with the values obtained from the model. For the mixture model, the corrosion rates were 9.2×10-6, 7.7×10-6, 3.4×10-6, and 1.2×10-6 mol L-1 min-1 107

for copper, and 13.4×10-6, 10.3×10-6, 6.4×10-6, and 1.9×10-6 mol L-1 min-1 for nickel, at

pH=3.0, 3.5, 4.0 and 4.5 respectively, at 40oC. The calculated net rate and displacement

reaction rate constants are presented in Tables 3.3.2 and 3.3.3, along with the

experimental values. As shown, there is reasonable agreement between both sets of

values. For copper at low pH, the corrosion rate is larger than the displacement rate. Both

the experimental and fit results indicate a positive net rate. At higher pH, the

displacement rate is larger than the corrosion rate, and the net rate is negative. The

displacement reactions compete with corrosion for copper. However, for nickel, both the

corrosion and displacement reactions act together to increase the concentration of nickel

ion. From the results, the corrosion rates for both copper and nickel decrease with

increasing pH. The displacement rate constant also decreases with increasing pH.

In Figure 3.3.7 is presented the effect of temperature on the net corrosion rate. The

“fit” corrosion rates from the model are 1.8×10-6, 3.4×10-6, 3.8×10-6, and 6.5×10-6 mol L-1

min-1 for copper, and 4.0×10-6, 6.4×10-6, 7.4×10-6, and 8.4×10-6 mol L-1 min-1 for nickel at 30oC, 40oC, 50oC, and 60oC, respectively. The “fit” slopes of the corrosion curves are

also presented in Table 3.3.2. These values are a little lower than the experimental values.

At pH 4.0, the copper corrosion is less than the displacement rate, which results in a

negative net rate. However, for nickel, both the corrosion rate and displacement rates act

in the same direction, increasing nickel ion concentrations with time and result in positive

slopes. In addition, the corrosion rates for copper and nickel increase with temperature,

The displacement reaction rate increases with temperature. Thus, for nickel, the net rate

increases with temperature, while for copper, the net rate decreases with temperature. 108

The metal displacement reaction between copper ion and deposited nickel can effectively decrease/reverse the rate of copper corrosion, increase the rate of nickel corrosion, at least initially, and also amplify the separation of the deposition regimes of the two metals in time. The latter suggests that the recovery of each as a relatively pure metal deposit layer is possible under certain a wide array of alloy compositions and multilayered deposits were achieved under conditions where the bulk concentration of copper ion is much smaller than the nickel ion concentration.18 Meuleman 80 found that one or four monolayers of nickel are dissolved from Ni(Cu) layer due to dissolution and displacement.

Table 3.3.2. Measured and “fit” corrosion rates as a function of pH (40°C, 10A).

Corrosion rates without nitrogen sparging(×105 mol L-1 min-1) Measured “Fit” pH

Cu Ni Cu Ni Displacement rate constant (min-1) 3.0 2.6 4.7 0.01 2.3 0.006 3.5 1.1 3.6 0.05 1.8 0.004 4.0 -1.9 3.2 -0.2 1.1 0.003 4.5 -1.3 2.7 -0.3 0.6 0.002

81 82 Fricoteaux and Douglade and Pourbaix found that Cu2O formation can occur at pH

≥ 3.0 according to:

2+ − + 2Cu + 2e + H2O→Cu2O + 2H (3.3.27)

Deposited copper oxide would then further inhibit the displacement reaction. The higher the pH, the more facile the production of copper oxide, which explains why the displacement reaction rate decreases with the increasing pH. 109

The corrosion rates for copper and nickel determined from the model fit are generally less than the rates for pure copper deposition,18 as well as pure nickel from Section 3.1.1.

This slight discrepancy can be attributed to some other as yet unidentified mechanism.

The displacement rate constant increases with increasing temperature, as expected.

The apparent activation energy from the Arrhenius plot presented in Figure 3.3.12 is 16.2 kJ/mol. This is quite low for a chemical reaction, so it is concluded that the process is probably transport-limited.

Table 3.3.3. Measured and “fit” corrosion rates as a function of temperature (pH 4.0,

10A).

Corrosion rate without nitrogen sparging (×105 mol L-1 min-1) Measured “Fit” T(°C)

Cu Ni Cu Ni Displacement rate constant (min-1) 30 -1.6 2.5 -0.2 0.8 0.002 40 -1.9 3.2 -0.1 1.1 0.003 50 -2.5 3.9 -0.2 1.5 0.004 60 -3.8 5.6 -0.7 2.2 0.012

In Figure 3.3.13 are presented a series of scanning electron micrographs of particle surfaces coated initially with copper or nickel. At early times, copper is mainly deposited on the surface. Consequently, panels A and C show mainly copper. At later times, as in panels B and D, mainly nickel is observed on the surface. The surface morphology shows similar characteristics for copper and nickel deposition.

In Figure 3.3.14 are presented the copper and nickel co-deposition data as a function of pH. The ordinate axis in this case is the sum of the copper and nickel ion concentrations, normalized by the initial sum. The practically linear curves suggest that the slopes of these curves are constant. This is somewhat surprising result since it 110

indicates that the total electrodeposition rate is approximately constant over a large time

interval, in addition to the already assumed constant corrosion rates.

3.3.4: Conclusions

The spouted particulate electrode exhibited good performance for the co-removal of copper and nickel from acidic aqueous mixtures. The maximum removal rate for copper occurred at about 30°C and pH 3.0, and 40°C and pH 4.0 for nickel. The quantitative and qualitative behavior of co-deposition of the metals from their mixtures was significantly different from that of the single metal solutions. This was attributed primarily to the metal displacement reaction between Ni(0) and Cu(II). The latter effectively eliminated the copper corrosion reaction and augmented that for nickel, at least initially. It also amplified the separation of the deposition regimes in time for both metals, suggesting that the recovery of each as a relatively pure metal deposit was possible under certain conditions. These data were incorporated into the design and operation of the cyclic electrowinning/precipitation (CEP) system for the removal of complex heavy metal mixtures from contaminated water.

111

References

1 Epelboin, I; Wiart, R (1971) J Electrochem Soc 118, p. 1577 2 Epelboin, I; Jousselin, M; Wiart, R (1981) J Electroanal Chem 119, p. 61 3 Zeller, RL III; Landan, U (1990) J Electrochem Soc 137, p. 1107 4 Ji, J; Cooper, WC; Dreisinger, DB; Peter, E (1995) J Appl Electrochem 25, p. 642 5 Cui, CQ; Lee, JY (1995) Electrochim Acta 40, p. 1653 6 Lantelme, F; Seghiouer, A (1998) J Appl Electrochem 28, p. 907 7 Houghton, RW; Kuhn, AT (1974) J Appl Electrochem 4, p. 173 8 Germain, S; Goodridge, F (1976) Electrochim Acta 21, p. 545 9 Coeuret, F (1980) J Appl Electrochem 10, p. 687 10 Hadzismajlovic, DE; Povov, KI; Pavlovic, MG (1996) Powder Technol 86, p. 145 11 Bisang, JM (1996) J Appl Electrochem 26, p. 135 12 Federman, GA; Hradil, G (2005) Metal Finishing 103, p. 40 13 Coeuret, F; Storck, A; Hutin, D (1982) Entropie 18(104), p. 57 14 Jiricny, V; Roy, A; Evans, JW (2002) Metal Mater Trans B 33B(5), p. 677 15 Jiricny, V; Roy, A; Evans, JW (2002) Metal Mater Trans B 33B(5), p. 669 16 Jiricny, V; Roy, A; Evans, JW (1999) Proceedings of the COPPER 99-COBRE 99, 4th International Conference, Phoenix, AZ, Oct. 10-13, 1999 3, p. 629 17 Evans, JW; Ding, R; Doyle, FM; Jiricny, V (2005) Scand J Metal 34(6), p. 363 18 Shirvanian, PA; Calo, JM (2005) J Appl Electrochem 35(1), p. 101 19 Evans, JW; Roy, A; Allen, C (2000) Lead-Zinc 2000, Proceedings of the Lead-Zinc 2000 Symposium, Pittsburgh, PA, United States, Oct. 22-25, 2000, p. 831 20 Jiricny, V; Roy, A; Evans, JW (2000) Metal Mater Trans B 31B(4), p. 755 21 Roy, A; Siu, S; Jiricny, V; Evans, JW; Newman, OMG (1998) Zinc and Lead Processing, Proceedings of an International Symposium on Zinc and Lead Processing, Calgary, Alberta, Aug. 1998, p. 16 22 Scott, K (1992) J Chem Tech Biotech 54, p. 257 23 Scott, K; McConvey, IF and Hains, AN (1987) J Appl Electrochem 17, p. 925 24 Walker, ATS and Wragg, AA (1977) Electrochim Acta, 22, p. 1129 25 Yang, J; Liu, DJ (2006) American Chemical Society, Division of Fuel Chemistry 51(2), p. 673 26 Campbell, SA U.S. Pat. Appl. Publ. (2004), Patent No: US 2004096728 27 Rieger, PH (1987), Electrchemistry, Prentice-Hall, Inc., Englewood Cliffs, NJ, p. 453 28 Dong, Q; Santhanagopalan, S and White, RE (2007) J Electrochem Soc 154( 9), p. A888 29 Mustain, WE; Prakash, J (2007) J Power Sources 170(1), p. 28 30 Tamamushi, R (1952) Bulletin of the Chemical Society of Japan 25, p. 287 31 Tamamushi, R (1952) Bulletin of the Chemical Society of Japan, 25, p. 293 32 Ives, DJG; Rawson, AE (1962) J Electrochem Soc 109(6), p. 452 33 Stankovic, VD; Stankovic, S (1991) J Appl Electrochem 21, p. 124 34 Masterson, IF; Evans, JW (1982) Metall Mat Trans B 13b, p. 3 35 Rieger, PH (1987), Electrochemistry, Prentice-Hall Inc., Englewood Cliffs, NJ, p. 270

112

36 Pickett, DJ (1952) Electrochemical Reactor Design, Elsevier, Amsterdam, p. 34 37 Mohanty, US; Tripathy, BC; Singh, P; Das, SC (2004) J. Electroanal Chem 566, p. 47 38 Son, SH; Chung, DW; Kwon, DC; Lee, HK (2008) Advanced Materials Research (Zurich, Switzerland), 47-50(Pt. 1, Multi-Functional Materials and Structures), p. 754 39 Conway, BE; (1952) Electrochemical Data. Elsevier, Amsterdam 40 Nakamura, J, (1991) Applied Numerical Methods With Software, Prentice Hall, Upper Saddle River, NJ 41 Eaton, AD; Clesceri, LS; Rice, EW; Greenberg, AE; Franson, MAH (1965), Standard Methods for the Examination of Water and Wastewater, 12th ed., American Public Health Association, New York, p. 408 42 Verma, A; Basu, S (2007) Journal of Power Sources 174(1), p. 180 43 Huh, T; Evans, JW (1987) J. Electrochem. Soc. 134, p. 308 44 “EPA Summary on Cadmium,” (1992) U.S. Environmental Protection Agency, http://www.epa.gov/ttn/atw/hlthef/cadmium.html, Issued at April 1992 45 Nogawa, Koji; Kobayashi, Etsuko; Okubo, Yasushiand; Suwazono, Yasushi (2004) "Environmental cadmium exposure, adverse effects, and preventative measures in Japan". Biometals 17 (5), p. 581 46 Friberg, L (1983) "Cadmium", Annual Review of Public Health 4, p. 367 47 Jarup, L; Berglund, M; Elinder, CG; Nordberg, G; Vahter, M (1998) Scandinavian Journal of Work, Environment and Health 24, p.11 48 Syers JK, Mackay AD, Brown MW, Currie CD (1986) J Sci Food Agric 37, p. 1057 49 Scoullos, MJ; Vonkeman, GH; Thornton, I; Makuch, Z (2001) Mercury, Cadmium, Lead: Handbook for Sustainable Heavy Metals Policy and Regulation, Springer, New York, NY 50 Pushpavanam, M; Shenoi, BA (1985) Journal of the Electrochemical Society of India, 34(4), p. 240 51 Molka, J (1981) Powloki Ochronne, 9(1), p. 10 52 Sokol'skaya, NB; Maksimchuk, VP (1977) Zhurnal Prikladnoi Khimii (Sankt- Peterburg, Russian Federation) 50(11), p. 2494 53 Rezaite, V and Vishomirskis, R (2001) Protection of Metals, 37(1) January, p. 25 54 Mateescu, M; Preda, M; Voiculescu, V; Samide, A (2000) Analele Universitatii din Craiova, Seria Chimie 29, p. 21 55 Jayakrishnan, S (2000) Transactions of the Institute of Metal Finishing 78(3), p. 124 56 Medvedev, GI; Makrushin, NA; Khamun'ela, V (2007) Russian Journal of Applied Chemistry 80(8), p. 1316 57 Guel, B; Studii, S; Cercetar, S (2007) Chimie si Inginerie Chimica, Biotehnologii, Industrie Alimentar (Universitatea Bacau) 8(4), p. 369 58 Angier, N "The Pernicious Allure of Lead", New York Times, August 21, 2007 59 Bergeson, LL (2008) Environmental Quality Management 18, p. 79 60 Hardison, DW; Ma, LQ; Luongo, T; Harris, WG (2004) Science of the Total Environment, 328(1-3), p. 175 61 Rooney, CP; McLaren, RG (2000) Australasian Journal of Ecotoxicology, 6(2), p. 95 62 Balaji, R; Pushpavanam, M (2003) Transactions of the Institute of Metal Finishing,81(5), p. 154

113

63 Chi, J; Kang, J (2001) Cailiao Baohu, 34(10), p. 44 64 Chen, CS; Wan, CC and Wang, YY (1999) Journal of the Chinese Institute of Chemical Engineers, 30(3), p. 199 65 Chen, CS; Wan, CC; Wang, YY (1998) Transactions of the Institute of Metal Finishing 76(2), p. 54 66 Spiegel, EF; Sammells, AF (2001) Patent No: US 6298996 67 Rieger, PH (1987) Electrochemistry Prentice-Hall Inc., Englewood Cliffs, NJ, p. 457 68 Rieger, PH (1987) Electrochemistry Prentice-Hall Inc., Englewood Cliffs, NJ, p. 456 69 Landolt, D (1994) Electrochim. Acta, 39, p. 1075 70 Chassaing, E; Quang, KV (1987) J. App. Electrochem. 17, p. 1267 71 Gilibin, VP; Kuznetsov, BV; Vorobyova, TN (2005) J. Alloys Compounds, 386, p. 139 72 Shibahara, T; Asano, T; Sakane, G (1991) Polyhedron 10(19), p. 2351 73 Bradley, PE. and Landolt, D (1997) Electrochimica Acta 42(6), p. 993 74 Bradley, PE; Roy, S and Landolt, D (1996) J. Chem. Soc., Faraday Trans. 92(20), p. 4015 75 Scharfe, RR, Sastri, VS and Chakrabarti CL (1972) Canadian J. Chem. 50, p. 3384 76 Bories, C; Bonino, JP and Rousset, A (1999) J. Appl. Electrochem. 29, P. 1045 77 Vorobyova, TN; Bobrovskaya, VP and Sviridov,VV (1997) Met. Finishing 11, P.14 78 Roy, S (1998) Surface and Coating Technology 105, p. 202 79 Cifuentes, L; Simpson, J (2005) Chem. Eng. Sci. 60, p. 4915 80 Meuleman, WRA; Roy, S; Péter, L and Varga, I (2002) J. Electrochem. Soc. 149(10), p. C479 81 Fricoteaux, P and Douglade, J (2004) Surface and Coatings Technology 184:1, p. 63 82 Pourbaix, M (1963) Atlas d’équilibres électrochimiques, Gauthier-Villard (ed.) 114

1 pH w/o sparging w/sparging 3.5 3.5 4.0 4.0 0.8 4.5 4.5 5.0 5.0 Model Model

0.6

C/C

0.4

0.2

0 50 100 150 200 250 Time/min

Figure 3.1.1. Nickel removal at 10A, 35°C, as a function of pH, with (┅)and without (━)nitrogen sparging.

115

pH w/o sparging w/sparging 0.0016 3.5 3.5 4.0 4.0 4.5 4.5 5.0 5.0

) 0.0012 -1

mol L )/(

0 0.0008 (C-C

0.0004

0 5 10 15 20 25 30 35 40 Time/min

Figure 3.1.2. Nickel corrosion rates with the feeder current off at 35°C as a function of solution pH, with (┅)and without (━) nitrogen sparging.

116

0.7 pH w/o sparging w/sparging 3.5 3.5 4.5 4.0 0.6 4.0 4.5 5.0 5.0

0.5

0.4

(fractional) Efficiency Current 0.3

0.2

0 0.2 0.4 0.6 0.8 1 C/C 0

Figure 3.1.3. Current efficiencies for the data presented in Figure 3.1.1, with (┅)and without (━) nitrogen sparging.

117

pH 1 w/o sparging w/sparging Electrodeposition rate-limited 3.5 3.5 4.0 4.0 4.5 4.5 0.8 5.0 5.0

) 1

a+k 0.6 L

a/(k L

k 0.4

0.2

0 50 100 150 200 250 300 350 400 Time/min

Figure 3.1.4. The relative effects of mass transfer and electrodeposition rates on nickel removal as a function of pH (10A, 35°C) from the model results presented in Figure 3.1.1.

118

)

-1

4 pH w/o sparging w/sparging 3.5 3.5

(mg L Oxygen Concentration/ Dissolved 4.0 4.0 2 4.5 4.5 5.0 5.0

0 50 100 150 200 250 Time/min

Figure 3.1.5. Dissolved oxygen concentration during nickel removal at 10A, 35°C, as a function of pH, with (┅)and without (━) nitrogen sparging.

119

1 w/o sparging w/sparging o 30 C 30oC o 35 C 35oC o 0.8 40 C 40oC o o 45 C 45 C Model Model

0.6

C/C 0.4

0.2

0 50 100 150 200 250

Time/min

Figure 3.1.6. Nickel removal at 10A, pH 4, as a function of temperature, with (┅)and without (━) nitrogen sparging.

120

w/o sparging w/sparging 0.7 o o 30 C 30 C o 35oC 35 C o 40oC 40 C o 45oC 45 C 0.6

0.5

0.4

(fractional) Efficiency Current 0.3

0.2

0.2 0.4 0.6 0.8 1 C/C 0

Figure 3.1.7. Current efficiencies for the data presented in Figure 3.1.6, with (┅)and without (━) nitrogen sparging.

121

1 Electrodeposition rate-limited w/o sparging w/sparging o o 30 C 30 C o o 0.9 35 C 35 C o o 40 C 40 C o 45oC 45 C 0.8 )

a+k L 0.7

a/(k L

k 0.6

0.5

0.4

0 50 100 150 200 250 300 350 400 Time/min

Figure 3.1.8. The relative effects of mass transfer and electrodeposition rates on nickel removal as a function of temperature (10A, pH 4.0) from the model results presented in Figure 3.1.6.

122

0.0008 w/o sparging w/sparging o o 0.0007 30 C 30 C o o 35 C 35 C o o 40 C 40 C 0.0006 o o 45 C 45 C

)

-1 0.0005

(mol L 0.0004 / )

(C-C 0.0003

0.0002

0.0001

0 5 10 15 20 25 30 35 40 Time/min

Figure 3.1.9. Nickel corrosion rates with the feeder current off at pH 4 as a function of solution temperature, with (┅)and without (━) nitrogen sparging.

123

) -1

6 w/o sparging w/sparging

30oC 30oC o 35 C 35oC 4 o o 40 C 40 C o o 45 C 45 C

Dissolved Oxygen Concentration/(mg L 2

0 50 100 150 200 250 Time/min

Figure 3.1.10. Dissolved oxygen concentration during nickel removal at 10A, pH 4, as a function of temperature, with (┅)and without (━) nitrogen sparging.

124

1 w/o sparging w/sparging 5A 5A 10A 10A 15A 15A 0.8 Model Model

0.6

C/C

0.4

0.2

0 50 100 150 200 250 Time/min

Figure 3.1.11. Nickel removal at 35°C, pH 4, as a function of feeder current, with (┅)and without (━) nitrogen sparging.

125

0.8 w/o sparging 5A 10A 0.7 15A

0.6 w/sparging 5A 10A 0.5 15A

0.4

(fractional) Efficiency Current

0.3

0.2

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 C/C 0

Figure 3.1.12. Current efficiencies for the data presented in Figure 3.1.11, with (┅)and without (━) nitrogen sparging.

126

1 w/o sparging w/sparging Electrodeposition rate-limited 5A 5A

0.9 10A 10A

15A 15A

0.8

) 1

a+k

L 0.7

a/(k L k 0.6

0.5

0.4

0.3

0 50 100 150 200 250 300 350 400 Time/min

Figure 3.1.13. The relative effects of mass transfer and electrodeposition rates on nickel removal as a function of feeder current (pH 4.0, 35°C) from the model results presented in Figure 3.1.11.

127

0.8

0.6

C/C

0.4

w/o sparging w/sparging

pH=3.5 pH=3.5 0.2 pH=4.0 pH=4.0 pH=4.5 pH=4.5

0 50 100 150 200

Time/min

Figure 3.2.1. Cadmium removal at 10A, 40°C, as a function of solution pH, with (┅)and without (━) nitrogen sparging.

128

0.35 w/o sparging w/sparging pH=3.5 pH=3.5 pH=4.0 pH=4.0 0.3 pH=4.5 pH=4.5

0.25

0.2

0.15

(fractional) Efficiency Current

0.1

0.05

0.2 0.4 0.6 0.8 1 C/C 0

Figure 3.2.2. Current efficiencies for the data presented in Figure 3.2.1, with (┅)and without (━) nitrogen sparging.

129

) -1 8

w/o sparging w/sparging 6 pH=3.5 pH=3.5

pH=4.0 pH=4.0

pH=4.5 pH=4.5 4

2 (mg L Concentration/ Oxygen Dissolved

0 50 100 150 200 Time/min

Figure 3.2.3. Dissolved oxygen concentration during cadmium removal at 10A, 40°C, as a function of pH, with (┅)and without (━) nitrogen sparging.

130

0.0012 w/o sparging w/sparging pH=3.5 pH=3.5 pH=4.0 pH=4.0 0.001 pH=4.5 pH=4.5

0.0008 ) -1

/(mol L /(mol 0.0006 ) 0

(C-C 0.0004

0.0002

0 5 10 15 20 25 30 35 Time/min

Figure 3.2.4. Cadmium corrosion with the feeder current off at 40°C as a function of solution pH, with (┅)and without (━) nitrogen sparging.

131

0.8

0.6

C/C 0.4

0.2 w/o sparging w/sparging o 30 C 30oC o o 40 C 40 C

50oC 50oC 0

0 50 100 150 200

Time/min

Figure 3.2.5. Cadmium removal at 10A, pH 4.0, as a function of solution temperature at pH 4, with (┅)and without (━) nitrogen sparging.

132

0.4 w/o sparging w/sparging 30oC 30oC o 40oC 40 C 0.35 o o 50 C 50 C

0.3

0.25

0.2

0.15 Current Efficiency (fractional) Efficiency Current

0.1

0.05

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 C/C 0

Figure 3.2.6. Current efficiencies for the data presented in Figure 3.2.5, with (┅)and without (━) nitrogen sparging.

133

)

-1

w/o sparging w/sparging o o 4 30 C 30 C o o 40 C 40 C o o 50 C 50 C

(mg L Concentration/ Oxygen Dissolved 2

0 50 100 150 200 Time/min

Figure 3.2.7. Dissolved oxygen concentration during cadmium removal at 10A, pH 4.0, as a function of solution temperature, with (┅)and without (━) nitrogen sparging.

134

w/o sparging w/sparging o o 0.0008 30 C 30 C o o 40 C 40 C o o 50 C 50 C

0.0006

) -1

(mol L )/ 0 0.0004

(C-C

0.0002

0 5 10 15 20 25 30 35 Time/min

Figure 3.2.8. Cadmium corrosion with the current off at pH 4.0 as a function of solution temperature, with (┅)and without (━) nitrogen sparging.

135

0.8

0.6

C/C 0.4

0.2 w/o sparging w/sparging 5A 5A 10A 10A

15A 15A 0

0 50 100 150 200 Time/min

Figure 3.2.9. Cadmium removal at 40°C, pH 4.0, as a function of current, with (┅)and without (━) nitrogen sparging.

136

0.4 w/o sparging w/sparging 5A 5A 0.35 10A 10A

15A 15A

0.3

0.25

0.2

0.15 (fractional) Efficiency Current

0.1

0.05

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 C/C 0

Figure 3.2.10. Current efficiencies for the data presented in Figure 3.2.9, with (┅)and without (━) nitrogen sparging.

137

) -1 8

6 w/o sparging w/sparging

5A 5A

10A 10A 15A 15A 4

2 (mg L Concentration/ Oxygen Dissolved

0 50 100 150 200 Time/min

Figure 3.2.11. Dissolved oxygen concentration during cadmium removal at 40°C, pH 4.0, as a function of current, with (┅)and without (━) nitrogen sparging.

138

1 pH w/o sparging w/sparging 2.0 2.0 2.5 2.5 0.8 3.0 3.0

0.6

C/C 0.4

0.2

0 50 100 150 200 Time/min

Figure 3.2.12. Lead removal from MSA solution at 10A, 40°C, as a function of solution pH, with (┅)and without (━) nitrogen sparging.

139

0.45 pH w/o sparging w/sparging 2.0 2.0 0.4 2.5 2.5 3.0 3.0

0.35

0.3

0.25

(fractional) Efficiency Current

0.2

0.15

0 0.2 0.4 0.6 0.8 1 C/C 0

Figure 3.2.13. Current efficiencies for the data presented in Figure 3.3.12, with (┅)and without (━) nitrogen sparging.

140

) -1 10

pH w/o sparging w/sparging 6 2.0 2.0 2.5 2.5 3.0 3.0

(mg L / Oxygen Concentration Dissolved 2

0 50 100 150 200 Time/min

Figure 3.2.14. Dissolved oxygen concentration during lead removal at 10A, 40°C, as a function of pH, with (┅)and without (━) nitrogen sparging.

141

0.0008 w/o sparging w/sparging

2.0 2.0

2.5 2.5

3.0 3.0

0.0006

(mol/L) )/ 0 0.0004

(C-C

0.0002

0 5 10 15 20 25 30 35 Time/min

Figure 3.2.15. lead corrosion with the feeder current off at 40°C as a function of MSA solution pH, with (┅)and without (━) nitrogen sparging.

142

w/sparging w/o sparging o 30oC 30 C o o 0.8 40 C 40 C o o 50 C 50 C

0.6

C/C 0.4

0.2

0 50 100 150 200 Time/min

Figure 3.2.16. Lead removal at 10A, pH 2.5, as a function of MSA solution temperature, with (┅)and without (━) nitrogen sparging.

143

0.5 w/o sparging w/sparging o 30 C 30oC o 0.45 40 C 40oC 50oC 50oC

0.4

0.35

0.3

0.25

(fractional) Efficiency Current 0.2

0.15

0.1

0.2 0.4 0.6 0.8 1 C/C 0

Figure 3.2.17. Current efficiencies for the data presented in Figure 3.3.16, with (┅)and without (━) nitrogen sparging.

144

)

-1 10

w/o sparging w/sparging 8 o 30 C 30oC o 40 C 40oC o 50 C 50oC 6

(mg L Concentration/ Oxygen Dissolved 2

0 50 100 150 200 Time

Figure 3.2.18. Dissolved oxygen concentration during lead removal from MSA solution at 10A, pH 2.5, as a function of solution temperature, with (┅)and without (━) nitrogen sparging.

145

w/o sparging w/sparging o o 0.0008 30 C 30 C o o 40 C 40 C o o 50 C 50 C

0.0006 ) -1

(mol L )/ 0 0.0004

(C-C

0.0002

0 5 10 15 20 25 30 35 Time/min

Figure 3.2.19. Lead corrosion with the feeder current off at pH 2.5 as a MSA function of solution temperature, with (┅)and without (━) nitrogen sparging.

146

0.8

0.6

C/C w/o sparging

0.4 5A

10A 15A

0.2 w/sparging 5A 10A 15A 0

0 50 100 150 200 Time/min

Figure 3.2.20. Lead removal at 40°C, pH 2.5, as a function of current, with (┅)and without (━) nitrogen sparging.

147

w/o sparging w/s sparging 5A 5A 0.5 10A 10A 15A 15A

0.4

0.3

0.2 (fractional) Efficiency Current

0.1

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 C/C 0

Figure 3.2.21. Current efficiencies for the data presented in Figure 3.2.20, with (┅)and without (━) nitrogen sparging.

148

)

-1 10 w/o sparging 5A

8 10A 15A

6 w/sparging 5A 10A

4 15A

2 Dissolved Oxygen Concentration/(mg L

0 50 100 150 200 Time/min

Figure 3.2.22. Dissolved oxygen concentration during lead removal at 40°C, pH 2.5, as a function of current with (┅)and without (━) nitrogen sparging.

149

1.4 pH Cu Ni 1.2 3.0 3.0 3.5 3.5 4.0 4.0

1 4.5 4.5

0 0.8

C/C

0.6

0.4

0.2

0 50 100 150 200 250 300 350 400 Time/min

Figure 3.3.1. Co-removal of copper and nickel at 10A, 40°C from a Cu/Ni solution as a function of pH without nitrogen sparging.

150

0.2 Cu

Ni 0

-0.2

/V ) e

-0.4

Potential (E-E Potential -0.6

-0.8

-1

0 50 100 150 200 250 300 350 400 Time/min

Figure 3.3.2. Overpotential (E-Ee) at cathode during electrodeposition of copper and nickel removal at pH 4.0, 40°C and 10A without nitrogen sparging.

151

11 )

-1

pH 3.0 6 L (ml Concentration/ Oxygen Dissolved 3.5 4.0 4.5 5

0 50 100 150 200 250 300 350 400

Time/min

Figure 3.3.3. Dissolved oxygen concentration during Cu/Ni removal at 10A and 40°C as a function of pH without nitrogen sparging.

152

pH 0.0015 Cu Ni 3.0 3.0 3.5 3.5 4.0 4.0 4.5 4.5 0.001

) -1

(mol L

)/ 0.0005

(C-C

-0.0005

0 5 10 15 20 25 30 35 Time/min

Figure 3.3.4. Net copper and nickel “corrosion” rates at 40°C as a function of pH without nitrogen sparging.

153

1.4 o Temp ( C) Cu Ni 1.2 30oC 30oC o 40 C 40oC o o 50 C 50 C 1 o o 60 C 60 C

0.8 0

C/C

0.6

0.4

0.2

0 50 100 150 200 250 300 350 400

Time/min

Figure 3.3.5. Co-removal of copper and nickel at 10A, and pH 4.0, from a Cu/Ni solution as a function of temperature without nitrogen sparging.

154

) -1 10

o 30 C 4 o Dissolved Oxygen Concentration/(ml L 40 C o 50 C o 60 C 2

0 50 100 150 200 250 300 350 400

Time/min

Figure 3.3.6. Dissolved oxygen concentration during Cu/Ni co-removal at 10A and pH 4.0 as a function of temperature without nitrogen sparging.

155

0.0015 Ni o 30 C o 0.001 40 C 50oC

60oC

0.0005 ) -1

(mol L

)/ 0 0

(C-C -0.0005 Cu

30oC 40oC -0.001 50oC o 60 C -0.0015

0 5 10 15 20 25 30 Time/min

Figure 3.3.7. Net copper and nickel “corrosion” rates at pH 4 as a function of temperature without nitrogen sparging.

156

1.2 pH Cu Ni 3.0 1 3.0 3.5 3.5 4.0 4.0

0.8

C/C 0.6

0.4

0.2

0 50 100 150 200 250 300 350 400

Time/min

Figure 3.3.8. Normalized copper and nickel concentrations at 40°C as a function of pH for the first two hours, after which the pH was changed to 4 in all cases, with nitrogen sparging.

157

1.2 pH, T(oC) Cu Ni

o o 1 3.0, 30 C 3.0, 30 C o 4.0, 40 C 4.0, 40oC

0.8

0 0.6 C/C

0.4

0.2

0 50 100 150 200 250 300 350 400

Time/min

Figure 3.3.9. Normalized copper and nickel concentrations at pH 3 and 4 at 30°C and 40°C for the first two hours, after which the pH was changed to 4.0, with nitrogen sparging.

158

0.001 pH Cu Ni

3.0 3.0 3.5 3.5 4.0 4.0 4.5 4.5

0.0005

)

-1

(mol L

) 0

(C-C 0

-0.0005

0 5 10 15 20 25 30 35 Time/min

Figure 3.3.10. Net copper and nickel “corrosion” rates at 40°C as a function of pH with nitrogen sparging.

159

0.001 Ni o 30 C o 40 C 50oC 0.0005 60oC

)

-1

(mol L )/

0 0

(C-C Cu o 30 C o -0.0005 40 C 50oC 60oC

0 5 10 15 20 25 30 35 Time/min

Figure 3.3.11. Net copper and nickel corrosion rates at pH 4.0 as a function of temperature with nitrogen sparging.

160

-4.5

-5 ) -1

in min

( k -5.5 d

lnk

-6

-6.5

0.36 0.365 0.37 0.375 0.38 0.385 0.39 0.395 0.4 1/RT x 10-3 (mol J -1)

Figure 3.3.12. Apparent activation energy of the metal displacement reaction between Cu(II) and Ni(0) over the temperature range 30-60°C, at pH 4.0.

161

A B

C D

Figure 3.3.13. SEM micrographs of cathodic particle surfaces at pH 4.0, 40°C, and 10A. A and B are at 20 min and 4 hours, respectively, particles coated initially with copper. C and D are at 20 min and 4 hours, respectively, particles coated initially with nickel.

162

1 pH

3.0

3.5

4.0 0.8 4.5

) Ni 0 +C Cu 0 0.6

)/(C Ni

+C Cu (C 0.4

0.2

0 50 100 150 200 250 300 350 400 Time/min

Figure 3.3.14. Normalized total copper plus nickel ion concentrations from Figure 3.3.1.

163

Chapter 4. Cyclic Electrowinning/Precipitation (CEP) System for the Removal of Heavy Metals From Aqueous Mixtures to Low Concentrations 4.1: Background

In Chapter 3 are presented the results of the successful application of spouted

particulate electrodes (SPE) to electrowinning of heavy metals from acidic aqueous

solutions. However, for most metals the electrodeposition rate decreases steadily, and

becomes mass transfer-limited at low concentrations. In order to retain the desirable

characteristics of electrowinning for metals removal, we have developed an apparatus and

process that retains these advantages, while minimizing low rate problems at low

concentrations. It is known as the Cyclic Electrowinning/Precipitation (CEP) System. As the name implies, it is a cyclic process that combines electrowinning with in-process precipitation and metal redissolution.

Chemical precipitation is perhaps the simplest method for removing heavy metals from aqueous solutions to very low concentrations in a relatively nonspecific manner.1

Traditionally, hydroxide precipitants have been favored over sulfide counterparts, due to

the higher cost of chemically produced hydrogen sulfide and the hazards associated with

their application.1 Metals removal by hydroxide precipitation has been well

investigated.2,3,4,5 Zhang et al.2 studied Cr and Cu removal from wastewater via

hydroxide precipitation. Chavalparit et al.3 reported Hg, Ag, Cr, and Fe removal from

wastewater using hydroxide precipitation. Yao et al.5 studied Cr, Cu and Ni hydroxide

precipitation, and the effects of Poly(γ-glutamic acid) for absorption of these precipitates.

Machemer and Wildeman6,7 found that copper and zinc were effectively completely

removed via precipitation, and suggested that sulfide precipitation would be a better

process for metals removal from acid mine drainage, such as in the Wetlands and Big 164

Five Tunnel Remediation Project in Idaho Springs, Colorado.7 This is primarily because

metal hydroxide precipitation varies with pH fluctuations in the outflow water. A

significant amount of work has also been done on metal removal by sulfide precipitation

as an alternative to metal hydroxide precipitation for the removal of Cu, Ni, Pb, Cr, Cd,

Hg, and Zn from wastewater.8,9,10,11,12

Even though precipitation is relatively straightforward, there are significant problems with the application of just precipitation for metals removal. Primarily, the creation of objectionable precipitate sludges is a serious problem. The formation of cadmium

hydroxide, for example, introduces more effluent and metal-laden sludge than cadmium

sulfide precipitation, that must be subsequently treated.13,14,15 Typically, precipitation

processes are combined with other separation technologies to more effectively remove

heavy metals from wastewater. One of these technologies is electrowinning, which can be

used to remove certain metals in a reduced, solid form with a considerable concomitant

volume reduction from its original dilute ionic form in contaminated water. However, the

work presented in Chapter 3 shows that the electrodeposition rate and current efficiency

decrease steadily at lower metal ion concentrations. On the other hand, chemical

precipitation can be used as an effective technique for accumulating metals in a

concentrated precipitate form for subsequent removal by electrowinning. In the CEP

system, chemical precipitation is used in two ways. Precipitation is used to accumulate

and increase metals concentrations for more effective electrowinning, and also as an

effective finishing step for reducing the metals concentrations in the final effluent water to acceptable levels. In this cyclic process, metal precipitates are formed and redissolved, but no sludge ever leaves the process. The process effluents are the solid cathodic 165

particles with electrodeposited metal in a highly concentrated form, and clean effluent

water.

In this chapter, data on the removal of the heavy metals, copper, nickel, and cadmium, and their mixtures, to low concentrations with the CEP System are presented.

Removal rates and CEP performance data are reported, as well as the rates of the metal displacement reactions between copper ions and cadmium metal, and nickel ions and cadmium metal, that were found to occur during electrodeposition.

4.2: Experimental Results and Analysis

4.2.1: Electrochemical Reactions in the CEP Spouted Particulate Electrode

During the electrowinning steps in the SPE, the same electrochemical reactions discussed in Chapter 3 occur in a similar fashion; that is, the metal electrochemical reduction reactions, corrosion reactions, competing side reactions, and metal displacement reactions. In Chapter 3, it was shown that the metal displacement reaction between copper ions and reduced nickel metal was important during the simultaneous electrodeposition of copper and nickel from solution; i.e.,

Cu2+ + Ni →Ni2+ + Cu; E° = +0.59V (4.1)

In a similar fashion, in Cu/Ni/Cd mixtures, additional metal displacement reactions are

also possible that can play a role in the overall behavior of the three metals in solution:

Cu2++Cd → Cu + Cd2+; E° = 0.74V (4.2)

Ni2++Cd → Ni + Cd2+; E° = 0.15V (4.3)

Metal displacement reactions can occur according to their relative reduction potentials.16

Bradley et al.17 and Scharfe et al.18 reported the displacement reaction between copper and nickel during electrodeposition. Roy et al.19 and Bradley and Landolt20 showed that 166

the displacement reaction between copper and nickel had a significant effect on the

resultant alloy composition, and was likely to form compositionally modulated alloys

during pulsed plating. The metal displacement reactions between copper and cadmium,21

and nickel and cadmium22 have also been investigated.

4.2.2: Precipitation/Redissolution in the CEP Process

The other important step in the CEP process is cleanup of the discharged water, while

simultaneously increasing the metals concentrations to sufficient levels for efficient

electrowinning. This can be accomplished by running an appropriate number of

successive precipitation/redissolution cycles. For this reason, investigations of these processes were undertaken with the CEP system.

In Figures 4.1, 4.2, and 4.3 are presented copper, nickel, and cadmium ion concentration data, respectively, over a few precipitation cycles. The time for each cycle was 8 min. The initial concentrations in all cases were 20 ppm. As shown, all the curves were linear, as expected. The accumulated amount of metal for each precipitation cycle was about 16 ppm. This value can be accounted for as follows. The inlet and outlet tubing connected to the precipitation tank is located at about 1/5 of the height from the bottom of precipitation tank. Thus, for each precipitation cycle, the metal hydroxide precipitates, and about 1/5 of the solution remain in the precipitation tank after the clean water is discharged. The accumulated concentration in a precipitation/redissolution cycle is then

(1-1/5) x 20 ppm = 16 ppm. For the current system, the precipitation process worked well for cumulative concentrations less than about 200 ppm. For higher cumulative concentrations, the amount of accumulated hydroxide precipitate sludge begins to exceed the level of the tank outlet tubing, and thus some precipitate can be carried over and lost 167

upon draining the tank. In actual applications, this can be simply rectified by measures

such as using a larger tank and/or relocating the tank outlet.

The metal concentrations in the filtered effluent water were also measured for the

same runs: 0.23, 0.37, and 1.50 ppm for Cu2+, Ni2+, and Cd2+, respectively, averaged over

the seven cycles. The precipitation step was conducted at pH 11. The solubility products

of copper, nickel, and cadmium hydroxide are 4.8x10-20, 5.5x10-16, and 7.20x10-15,

respectively, at 25°C.23 Therefore, at equilibrium the Cu2+ ion concentration in solution

should be 2.3x10-7 mol L-1 or 0.015 ppm, and Ni2+and Cd2+ should be 5.2x10-6 mol L-1 or

0.3 ppm, and 1.2x10-5 mol L-1 or 1.3 ppm, respectively. These calculated concentrations

are slightly less than the experimental measurements for nickel and cadmium ions, and

much less for copper ions. These discrepancies can be explained by the filter

performance. A 20 µm, inline filter (ISC, model No SJC-40-20) was used to retain the metal precipitates in the precipitation tank. Three different filters were tested: 5, 20, and

50 µm. The 5 µm filter was easily plugged, and the 50 µm filter allowed through much more precipitate than the 20 µm filter. Comparison of the calculated concentrations with the data suggests that the 20 µm filter retained 93-99% of the metal hydroxide precipitates. The mean size of copper hydroxide precipitate particles has been reported to be on the order of 0.1 - 5 µm,24 which is much less than the 20 - 50 µm for nickel

hydroxide,25,26 and 400 µm for cadmium hydroxide.26 The filter size used is less than that

of the average nickel hydroxide and cadmium hydroxide precipitate particles, but larger than that of copper hydroxide particles. The reasonably good agreement between the experimental and calculated nickel and cadmium ion concentrations indicates that the filter retained almost all the nickel hydroxide and cadmium hydroxide precipitate 168

particles. However, the larger difference between the experimental and calculated copper

ion concentrations indicates that some copper hydroxide precipitate passed through the

filter.

The maximum concentration levels (MCL) specified by the USEPA for copper,

nickel, and cadmium are 1.3, 0.1, 0.005 ppm, respectively. The experimental results

indicate that the precipitation process reduces the copper ion concentration below its prescribed MCL. However, additional treatment may be necessary to reduce the nickel and cadmium concentrations to below their respective MCL values, if so required.

4.2.2: CEP System Results for the Removal of Single Metals

The CEP system was designed to be programmable via its LabView™ control

program (see Appendix C) in order to enable it to be configured to meet varying metal

removal requirements and scenarios. One of the primary measures of its performance is

the net removal rate of a metal over the programmed number of

precipitation/redissolution steps per electrodeposition step in the SPE. From Eq. (3.1.14), the concentration curve for a single metal can be expressed as:

dCe kCe = − + kc , (4.4) dte 1+ k / kLa

where Ce is the metal ion concentration during electrodeposition, te is the

electrodeposition time, k is the electrochemical metal reduction rate constant, kL is the

mass transfer coefficient, a is the specific interfacial surface area, and kc is the metal

corrosion rate. This expression cannot be solved analytically since k varies with

overpotential and time. Consequently, the experimental data were fit to polynomials for convenience. From the work reported in Sections 3.1 and 3.2, it was found that mass 169

transfer begins to compete significantly with metal electrodeposition over the

concentration range from about 120 – 50 ppm.

The net rate at which a metal is removed in the CEP system, R (e.g., in g min-1), is

given by the expression:

(C − C )V R = 0 e (4.5) t + t e p

where: C0 is the initial metal concentration prepared for the electrodeposition cycle; Ce is the metal concentration at the end of the electrodeposition cycle, te; tp is the time for

precipitation/redissolution process associated with accumulating the metal for

electrodeposition; and V is the solution volume in the SPE. For the copper data presented

in Figure 4.4, C0 = 99.5 ppm and each precipitation/redissolution cycle was 8 min. So for

the five precipitation/redissolution cycles used, tp = 5 x 8 min = 40 min.

Ce vs te data were determined experimentally for the CEP SPE in the usual fashion,

and were fit to Eq. (4.5). Some results for copper electrodeposition in the CEP SPE are

presented in Figure 4.4 as a function of current at 50°C, pH 4, with nitrogen sparging. For

these data, the initial copper ion concentration was 99.5 ppm, which was accumulated

from a 20 ppm solution over five precipitation/redissolution cycles. Using these data, the

net CEP removal rate, R, as a function of total CEP process time, te + tp, was calculated

as a function of applied current given by the polynomial curve fits from Figure 4.4:

2 10A: Ce = 0.0016te − 0.5302te +107.11 (4.6a)

2 15A: Ce = 0.0026te − 0.8151te +106.56 (4.6b)

2 20A: Ce = 0.0047te −1.1745te +104.70 (4.6c) 170

The results are presented in Figure 4.5. As shown, the net CEP removal rate exhibits a

maximum. The reason for this behavior is as follows. For a fixed value of tp (i.e., 40 min.

in this case) at low CEP process times, or at low values of te, there is little metal removal

by electrodeposition, so (C0 – Ce) is low. As te increases, however, more metal is

removed by electrodeposition, such that (C0 – Ce) increases, and the effect of fixed tp

decreases in comparison to te. This behavior causes R to initially increase with CEP

process time. However, since Ce decreases exponentially with te, the metal ion deposition

rate decreases with te. That is, the rate of increase of (C0 – Ce) decreases with te. This

causes the net CEP removal rate to exhibit a maximum, as is evident in Figure 4.5. The

optimum net CEP removal rate is achieved at this maximum.

Figure 4.5 also indicates that the optimal value of R increases, and occurs at a lower

value of the net CEP process time with increasing applied current. This behavior can be understood from the data presented in Figure 4.4. The observed net removal rate is the competitive result of metal removal by electrochemical reduction and metal redissolution via corrosion. The electrochemical reduction rate increases with current, while the corrosion rate remains approximately constant for fixed electrodeposition conditions, such as temperature and pH. Thus, at a particular CEP process time, the removal rate increases with applied current. The higher electrodeposition rate requires less time to process the results of precipitation/redissolution cycles at the maximum. Consequently,

the optimal time decreases with applied current, while the optimal rate increases.

The removal rate attains a maximum at dR/dte = 0. Thus, the optimal value of te

occurs at the root of the transcendental equation:

kCe C0 − Ce − kc − = 0 (4.7) 1+ k / kLa te + t p 171

Using the resultant polynomial curve fits from Figure 4.4 (Eqns. 4.6) the corresponding

optima for these copper removal data are 73, 90, and 100 min for 20, 15, and 10A,

respectively. The corresponding maxima of the copper removal rates are 0.014, 0.010,

and 0.006 g min-1 for 20, 15, and 10A, respectively. It is noted that the initial electrodeposition concentration obtained with the multiple precipitation/redissolution cycles was 99.5 ppm, while in Figure 4.4 the corresponding values are 107, 109, and 108 ppm for 10, 15, and 20A, respectively. The reason for this discrepancy is that after the solution is pumped to the SPE, a few minutes are required to heat the solution to the desired temperature (50°C in this case). During that time, some metal corrosion occurs which increases the initial concentration slightly. This also explains the initial small negative removal rates in Figure 4.5.

Knowledge of the optimal times is useful for determining the electrodeposition time for multiple CEP cycles. For 15A, the net copper removal rate exhibits a maximum at 90 min. In Figure 4.6 are presented results for multiple CEP cycles for copper removal at

15A. Five precipitation/redissolution cycles were used to accumulate the initial copper

concentrations to about 100 ppm for the electrowinning step in the SPE. Each CEP cycle

consisted of one electrodeposition step of 100 min duration (i.e., close to the optimum

time of 90 min. determined in Figure 4.5), followed by three precipitation/redissolution

cycles, for a total of 24 min. Thus, overall, the input copper concentration was 20.1 ppm,

and the effluent water was 0.23 ppm. The difference between these values is the total

amount of copper deposited on the cathodic particles. The percentage of copper lost in

the process is presented in Figure 4.7 for the data in Figure 4.6. As shown, the copper

loss is on the order of ±1.5%, such that the copper mass balance closes reasonably well. 172

Nickel electrowinning is more sensitive to corrosion rate than for copper.

Consequently, nitrogen sparging plays a more important role in reducing dissolved

oxygen in the electrolyte solution to reduce the corrosion rate. The effect of nitrogen

sparging on nickel removal in the CEP SPE is presented in Figure 4.8.

The effect of pH on nickel electrodeposition in the CEP SPE at 20A is shown in

Figure 4.9. Similar to the results in Chapter 3, the optimal pH is about 4.0. As shown in

Figure 4.10 (50°C, pH 4, with nitrogen sparging), increasing the applied current also

increases the electrodeposition rate, although it decreases the current efficiency. The

optimal time for nickel electrodeposition was determined in the same manner as for

copper above. In Figure 4.11 are presented the net CEP removal rates at two applied

currents. From these data, the optimal times and maximum net CEP removal rates for

nickel were 112 min, 0.006 g min-1, and 120 min, 0.004 g min-1 for 20A and 15A,

respectively.

Slightly different from that for copper, the nickel electrodeposition time in multiple

CEP cycles is determined by both the optimal time and the final nickel concentration at the end of each electrodeposition step. Since an integral number of precipitation cycles occur for each CEP cycle, the nickel ion concentration is not depleted sufficiently when a shorter time interval (e.g., than the optimal time of 112 min at 20A) is used. Thus, the initial nickel ion concentration for the second CEP cycle will be greater than that for the first CEP cycle, and increasing concentration accumulates in this manner with each successive CEP cycle. Therefore, a longer time (180 min. instead of the optimal time) was used as the electrodeposition time to maintain the initial nickel ion concentration at the same level for the subsequent electrodeposition step. Nickel removal data over 173

multiple CEP cycles are presented Figure 4.12 at 20A. Five precipitation cycles were used to accumulate the initial nickel concentrations to about 100 ppm for each SPE cycle.

Each CEP cycle consisted of one electrodeposition step of 180 min. (i.e., one hour greater than the optimal time). With this procedure, no nickel ion accumulation occurs, while the electrodeposition rate remains close to the maximum rate at the optimal time), and three precipitation/redissolution cycles of 8 min. each for another 24 min. This program seems to be quite effective for nickel removal. It is noted that the optimal time for nickel removal is greater than that for copper, and also that the maximum nickel removal rate is less than that for copper. The lower nickel electrodeposition rate indicates that less nickel is deposited over the same time interval, or a lower maximum removal rate for nickel. A lower removal rate requires a greater optimal time for the same initial metal concentration of 100 ppm Lower nickel electrodeposition rates were also found in

Section 3.1.

Cadmium removal is even less rapid than that for nickel. In Section 3.2, cadmium electrodeposition rates were found to increase with applied current, pH, and temperature.

In Figure 4.13 are presented cadmium electrodeposition data in the CEP SPE at 20A, pH

4.0, and 50°C. The initial 100 ppm electrolyte solution was accumulated with five precipitation/redissolution cycles. Figure 4.14 is a plot of the net CEP cadmium removal rate as a function of time. The resultant optimal removal time and maximum rate are 340 min., 0.003 g min-1. These are a greater time and lower maximum rate than for copper and nickel under similar conditions. This is consistent with the results presented in

Section 3.2 that cadmium electrodeposition rates are less than those of copper and nickel. 174

4.2.3: Metal Mixture Results

In this section are presented removal results for heavy metal mixtures over multiple

CEP cycles for the following specific cases:

• Figure 4.15: Cu/Ni co-removal at: 20A; pH 4.0; and 50°C. Four precipitation

cycles were used to accumulate the initial Cu/Ni concentrations of about 83 ppm

for the first electrowinning step in the CEP SPE. Each CEP cycle consisted of one

electrodeposition step of 180 min. duration, following two

precipitation/redissolution steps, each for 8 min.

• Figure 4.16: Cu/Cd co-removal at: 20A; pH 4.0; and 50°C. Four precipitation

cycles were used to accumulate the initial Cu/Cd concentrations of about 83 ppm

for the first electrowinning step in the CEP SPE. Each CEP cycle consisted of one

electrodeposition step of 240 min. duration, following two

precipitation/redissolution steps, each for 8 min.

• Figure 4.17: Ni/Cd co-removal at: 20A; pH 4.0; 50°C; with nitrogen sparging.

Four precipitation cycles were used to accumulate the initial Cu/Ni concentrations

of about 83 and 83 ppm, respectively, for the first electrowinning step in the CEP

SPE. Each CEP cycle consisted of one electrodeposition step of 300 min.

duration, following two precipitation/redissolution steps, each for 8 min.

• Figure 4.18: Cu/Ni/Cd co-removal at: 20A; pH 4.0; 50°C; with nitrogen sparging.

Three precipitation cycles were used to accumulate the initial Cu/Ni/Cd

concentrations to about 83 ppm each, for the first electrowinning step in the CEP

SPE. Each CEP cycle consisted of one electrodeposition step of 300 min.

duration, following one precipitation/redissolution step for 8 min. 175

All the metals mixture co-removal data over multiple cycles exhibit a characteristic

slightly flat curve at the beginning of each electrodeposition step for the last metal to be

removed. This characteristic is most probably attributable to the metal displacement reactions.

In Figure 4.19 are presented Cu/Ni/Cd co-removal data at 20A, pH 4.0, 50°C; with nitrogen sparging. The initial solution of 100 ppm of each metal was prepared directly from the reagents, and not via a series of precipitation/redissolution steps, as was done previously. This was done simply for convenience due to the limited capacity of the precipitation tank. As shown in Figure 4.19 for the ternary metal solution, the electrodeposition rate of copper is the greatest, while that for cadmium is the lowest, and that for nickel is intermediate between the two. In Figure 4.20 are presented the removal

rates of copper, nickel and cadmium from Figure 4.19. The resultant optimal times are

65, 91, and 280 min., and the corresponding maximum removal rates are 0.017, 0.006,

and 0.002 g min-1 for copper, nickel, and cadmium respectively.

In addition, it is noted that for the ternary mixture, the copper and nickel removal

rates are greater, and the cadmium removal rates are less than their corresponding rates in

the single metal removal experiments. The optimal time decreases for copper and nickel,

and increases for cadmium. This behavior is attributed to the displacement reactions

between copper ion and nickel and cadmium metal on the cathodic particles in a similar

fashion as was found for the Cu/Ni mixtures in Section 3.3. In the ternary mixture, copper

ions in the electrolyte can be spontaneously reduced by both nickel and cadmium metal

deposited on the surface of the cathodic particles due to the positive standard potentials

for these displacement reactions (+0.74V, Eq. (4.2), and +0.59V, Eq. (4.1)). This 176 enhances copper reduction, and thus the copper removal rate increases, and the optimal time decreases. In a similar fashion, the nickel removal rate is enhanced by the cadmium displacement reaction, Eq. (4.3). However, since the standard potential difference between nickel ion and metal cadmium is less than between copper ion and nickel, and copper ion and cadmium, its removal rate is enhanced less than that for copper. It is also noted that the displacement reactions all act in the same direction to increase the effective cadmium “corrosion” rate, such that its net removal rate decreases, and its optimal time increases, in comparison to its single metal behavior.

The time interval for multiple co-deposition cycles for ternary Cu/Ni/Cd mixtures is greater than the optimal times for each of the single metals. Similar to the behavior of multiple CEP cycles with copper/nickel mixtures, both criteria of avoiding metal ion accumulation and maintaining the electrodeposition rates as high as possible determine the optimal time interval; that is: (1) there must be an integral number of precipitation/redissolution steps for each electrodeposition step; and (2) if a shorter time interval is used, the last metal to be removed, such as Cd in the ternary Cu/Ni/Cd mixture, will exhibit increasing concentration levels with each subsequent electrodeposition step, which is, of course, counter to the desired effect. Consequently, the time interval for the electrodeposition cycle was increased to deplete sufficient amounts of cadmium to maintain a decreasing cadmium concentration with each successive electrodeposition cycle. Of course, there are other related strategies that can be employed to increase the cadmium removal rate and prevent cadmium accumulation in the CEP system, such as increasing the electrodeposition time and/or increasing the 177 applied current after a pre-determined number of cycles when there is primarily cadmium ion left in the solution, etc.

4.3: Conclusions

The removal of the heavy metals copper, nickel, and cadmium from aqueous solutions at low concentrations to solid metals with a very large reduction in volume, was investigated. It was found that the optimal times for the removal of the single metals increased in the order: copper, nickel, cadmium, while the maximum net CEP removal rates decreased in the same order. This ordering correlates with decreasing reduction potentials for these metals, both in terms of the metal displacement reaction rates, as well as the propensity for corrosion of the deposited metals. Application of the CEP system is expected to reduce costs and increase the effectiveness of metal removal from contaminated wastewaters at relatively low metal concentrations. Quantification of these expectations, however, remains to be determined from field test data. Although, the CEP approach is not expected to be applicable to all heavy metals, there are other heavy metals that are expected to be amenable to removal in a similar fashion as demonstrated here.

178

References

1 Sheikholeslami, R; Bright, J (2002) Desalination, 143(3), p. 255 2 Zhang, Z; Li, L; Zhu, H; Wang, F; Hua, J (2008) Huanjing Kexue Yu Jishu 31(7), 96-97, p. 131 3 Chavalparit, O; Ongwandee, M; Thaweesuwanporn, P (2008) Science Asia 34(1), p. 123 4 Musale, DA; Koppes, JA (2008) U.S. Pat. Appl. No. 516,843 5 Yao, J; Xu, H; Wang, J; Jiang, M; Ouyang, P (2007) Journal of Biomaterials Science, Polymer Edition 18(2), p. 193 6 Machemer, SD; Wildeman, TR (1992) J. Contam. Hydrol. 9(1-2), p. 115 7 Machemer, SD; Reynolds, JS; Laudon, LS; Wildeman, TR (1993) Applied Geochemistry, 8(6), p. 587 8 Kim, BR; Gaines, WA; Szafranski, MJ; Bernath, EF; Miles, AM (2002) Journal of Environmental Engineering (Reston, VA, United States), 128(7), p. 612 9 Foucher, S; Battaglia-Brunet, F; Ignatiadis, I; Morin, D (2001) Chem. Eng. Sci. 56(4), p. 1639 10 Guo, M; Lu, A; Lu, X (1999) Yanshi Kuangwuxue Zazhi, 18(4), p. 309 11 Shinohara, R; Katoh, N (2005) Kagaku Kogaku Ronbunshu, 31(3), p. 217 12 Xu, Y; Yang, H; Feng, P; Zhang, G; Li, F (2000) Cailiao Baohu, 33(12), p. 33 13 Van Hille, RP; Boshoff, GA; Rose, RP; Duncan, JR (1999) Res. Conserv. Recyc. 27, p. 157 14 White, C; Sayer, JA; Gadd, GM (1997) FEMS Microbiol. Rev. 20, p. 503 15 Kaksonen, AH; Riekkola-Vanhanen, ML; Puhakka, JA (2003) Water Res. 37, p. 255 16 Clugston, M; Flemming, R (2008) Advanced Chemistry, Oxford University Press 17 Bradley PE; Roy, S; Landolt, D (1996) J. Chem. Soc., Faraday Trans. 92(20), p. 4015 18 Scharfe, RR; Sastri, VS and Chakrabarti CL (1972) Canadian J. Chem. 50, p. 3384 19 Roy, S; Matlosz, M; Landolt, D (1994) J. Electrochem. Soc. 141(6), p. 1509 20 Bradley, PE; Landolt, D (1997) Electrochimica Acta, 42(6), p. 993 21 Piontelli, R; Poli, G (1942) Z. Physik. Chem. A190, p. 317 22 Berge, H; Drescher, A; Jeroschewski, P (1969) Fresenius' Zeitschrift fuer Analytische Chemie 248, p. 1 23 Solubility products. http://www.ktf-split.hr/periodni/en/abc/kpt.html 24 Ploss, H; Lehne, J (1986) Process of producing copper (II) hydroxide US Pat. 4614640 25 Subbaiah, T; Mohapatra, R; Mallick, S; Misra, KG; Singh, P and Das, RP (2003) Hydrometallurgy 68(1-3), p. 151 26 Feitknecht, W; Studer, H (1949) Univ., Bern, Switz. Kolloid-Zeitschrift 115, p. 13 179

120

100

Concentration/ppm 60 2+

0 1 2 3 4 5 6 Precipitation/Redissolution Cycle Number

Figure 4.1. Cumulative Cu2+ concentration as a function of precipitation/redissolution cycle. The initial feed solution Cu2+concentration was 20 ppm. Each cycle was eight minutes at 25°C. The precipitation/redissolution pH values were 11 and 4.0, respectively.

180

120

100

Concentration ppm / 60 2+ Ni

0 1 2 3 4 5 6 Precipitation/Redissolution Cycle Number

120

100

Concentration/ ppm 60 2+

0 1 2 3 4 5 6 Precipitation/Redissolution Cycle Number

Figure 4.3. Cumulative Cd2+ concentration as a function of precipitation/redissolution cycle. The initial feed solution Cd2+ concentration was 20 ppm. Each cycle was eight minutes at 25°C. The precipitation/redissolution pH values were 11 and 4.0, respectively. 182

110

100 10A

15A 90 20A

Concentration /ppm 60

Cu 50

0 20 40 60 80 100 120

Electrodeposition Time, t /min e

Figure 4.4. Copper electrowinning in the CEP SPE as a function of applied current. The initial copper concentration was 99.5ppm, which was prepared with five precipitation/redissolution cycles. The symbols ●,(o, ∆) are the experimental data, and the curves are the simulations (50°C; pH 4; 3500 cm3 min-1 nitrogen sparging).

183

0.015

-1 (73 min, 0.014 g min )

) -1

0.01 (90 min, 0.010 g min-1)

0.005 (100 min, 0.006 g min-1)

0 min R/(g Rate, Removal Copper Net 10A 15A

20A -0.005

0 20 40 60 80 100 120 Electrodeposition Time, t /min e Figure 4.5. Net CEP copper removal rates, R, as a function of applied current. The initial Cu2+ concentration was 99.5 ppm, which was prepared with five precipitation/redissolution cycles. The values in the parentheses are the electrodeposition time te, and the Cu removal rate at the maxima (50°C; pH of 4; 3500 cm3 min-1 nitrogen sparging).

184

120

100

Concentration/ppm 60 2+

0 100 200 300 400 500 600 700 800 Time/min

Figure 4.6. Cu2+ concentration over multiple CEP cycles at 15A. Five precipitation/redissolution cycles were employed to accumulate the initial electrowinning Cu2+ concentration of 99.8 ppm. After that, each CEP cycle consisted of one SPE electrowinning and three precipitation/redissolution cycles. The precipitation/redissolution pH values were 11.0 and 4.0, respectively (40°C; pH of 4; 3500 cm3 min-1 nitrogen sparging). 185

0.5

-0.5

% Loss/ Copper

-1

-1.5

0 1 2 3 4 5 6 7 CEP Cycle Number

Figure 4.7. Copper loss per cycle for the data in Figure 4.6 for multiple CEP cycles at 15A (50°C; pH 4; 3500 cm3 min-1 nitrogen sparging).

186

120

Without nitrogen sparging 110 With nitrogen sparging

100

80 Concentration /ppm Concentration

Ni 70

0 50 100 150 200 Electrodeposition Time, t /min e

Figure 4.8. Nickel electrowinning in the CEP SPE with and without nitrogen sparging (3500 cm3 min-1). The initial nickel concentration was 100 ppm, which was prepared with five precipitation/redissolution cycles. The symbols (□,■) are the experimental data, and the curves are the fits (20A; 50°C; pH 4).

187

120

110 pH=4.0 pH=4.5 100 pH=5.0

80 Concentration /ppm

Ni 70

0 50 100 150 200 Electrodeposition Time t /min e

Figure 4.9. Nickel electrowinning in the CEP SPE as a function of pH. The initial nickel concentration was 100 ppm, which was prepared with five precipitation/redissolution cycles. The symbols■,□, ( ◪) are the experimental data, and the curves are the fits (20A; 50°C; 3500 cm3 min-1 nitrogen sparging).

188

120

15A 110 20A

100

/ppm Concentration 2+ Ni 70

0 50 100 150 200 Electrodeposition Time t /min e

Figure 4.10. Nickel electrowinning in the CEP SPE as a function of current. The initial nickel concentration was 100 ppm, which was prepared with five precipitation/dissolution cycles. The symbols □,■)( are the experimental data, and the curves are the fits (50°C; pH 4; 3500 cm3 min-1 nitrogen sparging).

189

0.006 -1 (112 min, 0.006 g min ) 0.004 (120 min, 0.004 g min-1) )

-1 0.002

-0.002

-0.004

-0.006

/(g min Rate Removal Nickel Net

15A -0.008 20A

-0.01

0 20 40 60 80 100 120 140 160 Electrodeposition time t /min e

Figure 4.11. Net CEP nickel remoal rate, R, as a function of applied current. The initial nickel concentration was 100 ppm, which was prepared with five precipitation/redissolution cycles. The values in the parentheses are the electrodeposition time te, and the Ni removal rate at the maxima (50°C; pH 4; 3500 cm3 min-1 nitrogen sparging).

190

110

100

Concentration/ppm 2+ Ni 60

0 100 200 300 400 500 600 700 800

Time/min

Figure 4.12. Ni2+ concentration over multiple CEP cycles at 20A. Five precipitation/redissolution cycles were employed to accumulate the initial electrowinning Ni2+ concentration of 100 ppm. After that, each CEP cycle consisted of one 180 min SPE electrodeposition step and two precipitation/redissolution cycles (50°C; pH 4; 3500 cm3 min-1 nitrogen sparging). 191

115

110

105

100

Concentration /ppm 90 2+

0 50 100 150 200 Electrodeposition Time t /min e

Figure 4.13. Cadmium electrowinning in the CEP SPE at 20A. The initial cadmium concentration was 100 ppm, which was prepared with five precipitation/redissolution cycles. The symbols (▲) are the experimental data, and the curves are the fits (50°C; pH 4.0, 3500 cm3 min-1 nitrogen sparging).

192

0.004

(173 min, 0.003 g min-1)

) 0.002

-1

-0.002

-0.004

Net Cadmium Removal Rate /(g min /(g Rate Removal Cadmium Net -0.006

-0.008

0 50 100 150 200

Electrodeposition Time t /min e

Figure 4.14. Net CEP cadmium removal rates, R, at 20A. The initial nickel concentration was 100 ppm, which was prepared by five precipitation/redissolution cycles. The values in the parentheses are the electrodeposition time te, and the Cd removal rate at the maximium (50°C; pH 4; 3500 cm3 min-1 nitrogen sparging).

193

100 Cu Ni

Metal Ion Concentration /ppm Concentration Ion Metal

0 100 200 300 400 500 600 700 800 Time/min

Figure 4.15. Copper and nickel ion concentrations over multiple CEP cycles at 20A. Three precipitation/redissolution cycles were employed to accumulate the initial electrowinning Cu2+ and Ni2+ concentrations of around 83 ppm. After that, each CEP cycle consisted of one 180 min SPE electrowinning and two precipitation/redissolution cycles (50°C; pH 4; 3500 cm3 min-1 nitrogen sparging). 194

100 Cu Cd

/ppm Concentration Ion Metal 20

0 200 400 600 800 1000 Time/min

Figure 4.16. Copper and cadmium concentrations over multiple CEP cycles at 20A. Three precipitation/redissolution cycles were employed to accumulate the initial electrowinning Cu2+ and Cd2+ concentrations of around 83 ppm. After that, each CEP cycle consisted of one 240 min SPE electrowinning and two precipitation/redissolution cycles (50°C; pH 4.0; 3500 cm3 min-1 nitrogen sparging). 195

Ni 100 Cd

40 MetalIon Concentration /ppm

0 200 400 600 800 1000 Time/min

Figure 4.17. Nickel and cadmium concentrations over multiple CEP cycles at 20A. Three precipitation/dissolution cycles were employed to accumulate the initial electrowinning Ni2+ and Cd2+ concentrations of around 83 ppm. After that, every CEP cycle consisted of one 300 min SPE electrowinning and two precipitation/redissolution cycles (50°C; pH 4.0; 3500 cm3 min-1 nitrogen sparging). 196

Cu 80 Ni Cd

MetalIon Concentration /ppm 20

0 0 200 400 600 800 1000 Time/min

Figure 4.18. Copper, nickel and cadmium concentrations over Multiple CEP cycles at 20A. Three precipitation/dissolution cycles were employed to accumulate the initial electrowinning Cu2+, Ni2+ and Cd2+ concentrations of 83 ppm. After that, each CEP cycle consisted of one 300 min SPE electrowinning and one precipitation cycles (50°C; pH 4.0; 3500 cm3 min-1 nitrogen sparging).

197

120

100

Metal Ion Concentration /ppm Cu 20 Ni

Cd 0

0 40 80 120 160 Electrodeposition Time t /min e

Figure 4.19. Copper, nickel, and cadmium electrowinning in the CEP SPE at 20A. The initial concentrations were 100 ppm for all three metals, prepared directly from the reagents. The symbols ●,( ■, ▲) are the experimental data, and the curves are the fits. (50°C; pH 4.0; 3500 cm3 min-1 nitrogen sparging).

198

0.02 (65 min, 0.017 g min-1)

0.015 )

-1

0.01

-1 (91 min, 0.006 g min )

0.005

(280 min, 0.002 g min-1) 0

-0.005

min /(g Rate Removal Ion Metal Net Cu -0.01 Ni

-0.015

0 50 100 150 200 250 300 Electrodeposition Time t /min e

Figure 4.20. Net CEP copper, nickel, and cadmium removal rate, R, at 20A. The initial concentrations were 100 ppm for all three metals, prepared directly from the reagents. The values in the parentheses are the electrodeposition time 3 te, and the Cu, Ni, Cd removal rate at the maxima (50°C; pH 4; 3500 cm min-1 nitrogen sparging).

199

Chapter 5. Conclusions

5.1. Summary

The work in this thesis is organized into two main parts. Although both are focused on heavy metal removal/remediation/recovery via electrowinning, one concerns electrochemical kinetics studies in a Spouted Particulate Electrode (SPE) system, and the other is comprised of investigations in the Cyclic Electrowinning/Precipitation (CEP)

System.

In Chapter 3 was presented the results of electrochemical kinetics investigations of electrowinning of heavy metals from acidic aqueous solutions in a Spouted Particulate

Electrode (SPE) system of recirculating, conductive particles that form a moving bed cathode on the vessel bottom. The SPE system exhibited good performance for the removal of single metal nickel, cadmium, and lead, and binary copper/nickel mixtures. It was found that, in general, the rates of single metal nickel, cadmium, and lead electrowinning increase with increasing pH, temperature, and applied current in acidic solutions over the experimental range of parameters investigated. Nitrogen sparging of the electrolyte solution in the solution holding tank was effective in reducing the dissolved oxygen concentration, suppressing metal corrosion of the deposited metals from the cathodic particles, and, thereby, improving the net metals recovery rate. A numerical model based on the Tafel equations, incorporating a constant corrosion rate as an approximation, was found to simulate the electrochemical behavior of single metal nickel reasonably well. This model was also extended to the electrolytic removal of copper/nickel mixtures, by adding the metal displacement reaction between Ni(0) and

Cu(II). 200

The quantitative and qualitative behavior of co-deposition of copper and nickel from

their mixtures was significantly different from that of the corresponding single metal

solutions. This was attributed to the differences in reduction potential of the two metals,

as well as the metal displacement reaction, which effectively eliminated the copper

corrosion reaction and augmented that for nickel. It also amplified the separation of the

deposition regimes in time for both metals, suggesting that the recovery of each as a

relatively pure metal deposit was possible under certain conditions.

Although generally successful, the application of electrowinning alone to the removal

of heavy metals has some disadvantages – primarily, decreasing rates and current efficiencies at low metal concentrations due to mass transfer resistances. In order to retain

the advantages of electrowinning (e.g., considerable pollutant volume reduction), the

Cyclic Electrowinning/Precipitation (CEP) System was developed. This is an automated and programmable (computer-controlled) system, designed to remove heavy metals from wastewater with a cyclic process involving a combination of electrowinning and in- process metal precipitation and redissolution. In the CEP system, chemical precipitation is used in two ways. Precipitation is used to accumulate and increase metals concentrations for more effective electrowinning, and also as an effective finishing step for reducing the metals concentrations in the final effluent water to acceptable levels. In the cyclic process, metal precipitates are formed and redissolved with step changes in pH, but no sludge ever leaves the process. The process effluents are the solid cathodic particles with electrodeposited metal in a highly concentrated form, and clean effluent water. 201

The removal of the heavy metals copper, nickel and cadmium and their mixtures from aqueous solutions at low concentrations was investigated in the CEP System. It was found that the optimal times for single metal removal increased in the order, copper < nickel < cadmium, while the maximum net CEP removal rates decreased in the same order (i.e., copper > nickel > cadmium). This ordering correlates with the decreasing reduction potentials of these metals, both in terms of the metal displacement reaction rates, as well as the propensity or ease of corrosion of the deposited metals. It is concluded that the CEP system approach has significant advantages in terms of the effectiveness of heavy metal removal, flexibility for treating the innumerable variable and complex mixtures that are encountered in practice, at potentially lower costs than many current approaches.

5.2. Recommendations for Future Work

Up until now, the CEP System has been operated on synthesized mixtures of heavy metal species. Consequently, the major class of investigations that remain to be done are studies on the application of the CEP approach to a wide variety of actual contaminated water samples from the field. The latter have the potential to be much more complex than the synthesized samples, incorporating such complications as mixtures of various different anions, the possibility of metal complexation, considerable variation in the dynamic range of concentrations of different metals, the effects of other heavy metals that have not yet been explicitly addressed, etc. Some of these issues have already begun to be addressed, but there are many that remain to be conducted. 202

In addition to the preceding, significant work also remains to be done regarding

improvement of the understanding and optimization of the two major parts of the CEP

System – the spouted particulate electrode (SPE) and the precipitation/redissolution

processes. Improvement of the understanding of the mechanisms and kinetics involved in

electrochemical metal reduction, corrosion (including a more fundamental understanding

of dissolved oxygen behavior) so that it can minimized/eliminated without resorting to

very complex modifications, and the metal displacement reactions. Some of these issues

can be addressed via fundamental electrochemical techniques such as voltammetry,

impedance spectroscopy, and electrode mass behavior as revealed with quartz crystal

microbalance electrodes. The overall objective would be to develop more robust kinetics

models (e.g., exchange current densities, transfer coefficients, equilibrium potentials,

enthalpies of activation, etc.) that could be used to predict heavy metal removal behavior

in more complex mixtures.

In a similar fashion, the precipitation/redissolution processes also need to be more

thoroughly characterized as a function of the salient operating parameters. The kinetics of

heavy metal precipitation, resultant particle size distributions, and subsequent

redissolution, as a function of pH, flocculant, settling time, filter types and sizes, and tank

geometry and size, can all have potentially significant effects on precipitate retention and redissolution, and, therefore, on the overall efficiency of this part of the process. The work conducted thus far has barely “scratched the surface” of the optimization that may be required for eventual commercial operation.

203

Appendices

Appendix A. Gas Sparging Analysis and Model

A.1. Background

The gas sparging system is used to reduce the dissolved oxygen in the electrolyte

solution for both SPE and CEP system. Thus, the metal corrosion reaction could

decrease, metal electrodeposition rate and current efficiency increase. The sparger was

constructed from 0.635 cm diameter nylon tubing arranged in a square with a length of

16.5 cm on a side. About 2000 (approximately 500 on a side) holes of 0.35 mm diameter

were drilled through the sparger tubing. Basically, the performance of gas sparging is better with higher flow rate. However, beyond a certain point, the oxygen removal efficiency increases only marginally with flow rate. Appendix A is the investigation of

the characteristics of the sparging system The sparing system was operated in a 8”×8”×8” holding tank with 1000ppm nickel solution at 25C. A portable dissolved oxygen meter

(HACH, LDOTM HQ10) was employed to measure dissolved oxygen concentrations.

A.2. Assumptions

(1) the liquid phase is well mixed by bubble action

(2) the bubble phase is in plug flow

(3) all mass transfer resistance is on the liquid-side of the liquid-bubble (liquid-gas)

interface

A.3. Oxygen Mass Balance in the Bubble Phase

Since the mean bubble residence time in the tank (h/v ∞) is on the order of seconds,

and is much less than the characteristic time for the removal of dissolved oxygen (on the 204

order of minutes to hours), the bubbles can be treated as operating at quasi-steady-state

such that the concentration of oxygen in solution, C, is approximately constant during the

traverse of a bubble. Consequently, a differential mass balance in the z (height) direction

in the tank as the bubbles rise is given by:

Q dCi dCi = β = ka(C − Ci ) [A.1] RTKρw Adz dz where

Q β = , cm/s [A.2] RTKρwA

and

C = dissolved oxygen concentration in the bulk solution, mg L-1

-1 Ci = interfacial oxygen concentration in equilibrium with the gas in the bubble, mg L

k = oxygen mass transfer coefficient between the liquid and the bubble, cm s-1

a = specific interfacial surface area between the bubbles and the solution (cm2 cm-3), cm-1

Q = volumetric bubble gas flow rate at the sparger, cm3 s-1

R = gas constant, bar L mol-1 K-1

T = absolute temperature, K

A = tank cross sectional area, cm2

-1 ρw = density of water, kg L

K = Henry’s law constant for oxygen in water, mol bar-1 kg-1 water

Since the bubbles enter from the sparger with no oxygen, Ci = 0 @ z = 0, and the

solution to [A.1] is:

−kaz/β Ci = C(1− e ) ≅ Ckaz / β, (since the argument of the exponential is << 1) [A.3] and, 205

Ci,h ≅ C ka h / β

C i ≅ C ka h / 2β

where:

h = height of the liquid solution, cm

Ci,h = oxygen interfacial concentration at the top of the liquid volume, h, mg/l

C i ≅ average oxygen interfacial concentration over the liquid height, h, mg/l

A.4. Dissolved Oxygen Mass Balance

The dissolved oxygen mass balance in the liquid solution is:

dC * = −ka(C − C i ) + k a (C − C) [A.4] dt t t i where: t = time, s

-1 kt = oxygen mass transfer coefficient between the liquid and the gas headspace, cm s

2 3 -1 at = specific interfacial surface area at the top of the tank (cm /cm ), cm

* Ci = oxygen concentration in the liquid phase in equilibrium with the oxygen content of

the headspace above the liquid, mg L-1

The overall oxygen mass balance (liquid solution plus headspace) is given by:

d(VC + V C* ) s h = −QC* [A.5] dt where:

C* = oxygen concentration in the gas headspace, mg L-1

3 Vs = volume of the liquid solution, cm

3 Vh = volume of the gas headspace, cm

If the headspace is initially full of air, the total amount of oxygen in the headspace is

greater than in the liquid solution under typical conditions, so: 206

d(VC + V C* ) d(V C* ) s h ≅ h = −QC* [A.6] dt dt

The solution to this expression is then:

* * −t /τ C = C0e [A.7]

where:

* -1 C0 = initial oxygen concentration in the headspace (273 mg L for air at 1 atmosphere)

τ = Vh/Q = time constant for purging the headspace, s

* Substituting into Eq. [A.4] for Ci and C i yields an expression of the form:

dC = −kC + αe−t /τ [A.8] dt

where: kah k = ka(1− ) + k a 2β t t * α = kt atC0 KRTρw

The general solution to this expression is:

α C = γ e−kt + e−t /τ [A.9] (k − 1 / τ )

Applying the B.C.:

C = C0 @t = 0, α γ = C − , 0 (k − 1 / τ )

and

α α C = (C − )e−kt + e−t /τ [A.10] 0 (k − 1 / τ ) (k − 1 / τ )

From this expression it can be seen that there are two time constants that control the

dissolved oxygen concentration in the liquid solution: 1 / k controls the oxygen mass 207

transfer rate into the bubbles, and τ controls the purging rate of the gas headspace above

the liquid solution. If the headspace initially has no oxygen, then α = 0, and:

−kt C = C0e [A.11]

On the other hand, if the tank were to be left uncovered such that the oxygen

concentration above the liquid solution remained constant (e.g., at the atmospheric partial

pressure), then the solution to the mass balance would become:

α α C = (C − )e−kt + [A.12] 0 k k

and

α C = , as t → ∞ [A.13] k

In other words, in this case there will be a long-time asymptote for the dissolved oxygen concentration, other than zero. Thus, it can be seen that leaving the headspace unpurged by keeping it open to the atmosphere creates an additional oxygen source at the top liquid surface which is responsible for the long-time asymptote exhibited by some of the earlier oxygen data.

A.5. Parameter Estimation

The bulk liquid-bubble mass transfer coefficient is given by1:

g∆ρ k = D2/3( )1/3 [A.14] µ

where:

D = diffusivity of oxygen in water, cm2 s-1

∆ρ= density difference between water and gas phase, g cm-3

μ=viscosity of water, g cm-1 s-1 (poise) 208

The bubble radius is estimated from:

1/3  3 σ R  r =   [A.15] 2 g∆ρ 

where:

σ = surface tension of water, dynes cm-1

R = sparger hole radius, cm

g = gravitational acceleration, cm s-2

∆ρ= density difference between water and gas phase, g cm-3

The interfacial bubble surface area per unit volume of liquid solution, a, is given by:

3 Q a = [A.16] r wA

where:

a = bubble surface area in tank per unit volume of liquid solution, cm-1

Q = gas volumetric flow rate at sparger conditions, cm3 s-1

W = mean vertical bubble velocity, cm s-1 (estimated as v∞/2)

A = cross sectional area of tank, cm2

The terminal bubble velocity can be estimated from the correlation:

24 4 g(2r)∆ρ f = = 2 , Re < 0.1 Re 3 v∞µ or

24 2 4 g(2r)∆ρ f = ( + 0.5407) = 2 , 0.1 < Re < 6000 Re 3 v∞µ

where: f = friction factor or drag coefficient

Re = bubble Reynolds number, (2r)v ρ/µ 209

-1 v∞ = terminal bubble velocity, cm s

∆ρ= density difference between water and gas phase, g cm-3

For low Reynolds numbers, the terminal bubble velocity is determined explicitly as:

2 gr 2∆ρ v = , ∞ 9 µ

while for high Reynolds numbers the velocity estimation is iterative.

A.6. Results

In Figure 1 are presented both the data and the model results for argon sparging with the tank cover on. As shown, the model produces reasonable predictions of the data. In particular, it is noted that using very high flow rates, beyond a certain point, does not increase the sparging performance very much. Therefore, it appears that an optimum flow

rate might be in the range of 3000 – 6000 cc min-1.

In Figure 2 are presented model results for sparging with argon with the cover off. In

this case, the long time asymptote is clearly evident. The asymptotic value decreases with

increasing flowrate. The predicted asymptotic values are somewhat greater than those

measured. However, since the mass transfer coefficient at the top of the liquid volume is

unknown, it was assumed to be the same as the inter phase bubble mass transfer

coefficient, which is probably too high. Also, the higher molecular weight of argon serves

to keep the air away from the top of the liquid, better than would be the case with

nitrogen. In any case, the model seems to predict the overall behavior reasonably well.

References

1. Cussler, EL (1997) Diffusion. Mass Transfer in Fluid Systems. Cambridge University Press,. Cambridge, 2nd edition, Page 226. 210

8 Flow rate (cm3 min-1)

Experimental Model

) 7 -1 3000 6005 6005 6 16415 16415 19074 19074 5 23023 23023

(mg L / Cocentration Oxygen Dissolved 1

0 5 10 15 20 25 30 35 40 Time/min

Figure A.1. The effect of nitrogen sparging at different flow rates with the holding tank cover on. The data symbols (●,■,▲,♦) are experimental, and the solid and dashed curves are calculated from the model.

211

8 3 -1 Flow rate (cm min ) 7 3000 )

-1 6005

6 16415 19074 23023 5

Dissolved Oxygen Cocentration /(mg L 1

0 5 10 15 20 25 30 35 40 Time/min Figure A.2. The effect of nitrogen sparging as a function of flow rate with with

holding tank cover off.

212

Appendix B: Computer Code for Electrochemical Nickel and copper/nickel removal in a Spouted Particulated Electrode (SPE)

Appendix B includes two programs. One is the nickel electrochemical kinetics model,

And the other is the Cu/Ni mixture electrochemical kinetics model. Both programs use aRunge-Kutta, 4th, order method to solve the differential equations, and the method of bisection to obtain the root of the current balance equation, which is overpotential. The

Cu/Ni mixture model and program are similiar to the nickel program, except for the addition of the metal displecement reaction between Cu(II) and Ni(0).

B.1. FORTRAN Code Listing of the Nickel Electrochemical Model

PROGRAM RKT

Real i01, i02, itot, current1, current2 Real dp, Ve, Vp, Vpt, Ap, np, Apt, Diff Real Red, Sc, epsi, Massk, delta, U0, earea Real Fr, R, nu, Tmp Real psi1, psi2, CCu, CH2 Real n1, n2, pH, Mcu , iden, kL, ratio common n1,n2,psi1,psi2,i01,i02,beta1,beta2,iden,kL,Fr,Apt,R,Tmp

OPEN (3, FILE='C:\Conc.txt') OPEN (4, FILE='C:\Amps_i1.txt')

write(*,*) 'Enter Initial Bulk Conc(micro gr/milil,miligr/l,(ppm)' read(*,*) Cuppm

C in (mole/m3), Mcu (g/mole) Mcu = 58.546 CCu = (Cuppm/ 1000.) * (1./ Mcu)* 1000.0 C initial conc. of copper C0 = CCu

write(*,*) 'Enter pH (constant)' read(*,*) pH C concentration of H+ ions in mol/m3 CH2 = (10.**(-pH))* 1000.0

213

write (*,*) ' Enter Temperature (C)' read(*,*) Tmp Tmp = Tmp + 273.15

write(*,*) 'Enter U0 (constant, e.g. 0.2m/s -> km=1.08E-4 m/s)' read(*,*) U0

write(*,*) 'Enter Total current, itot (amps)' read(*,*) itot

write( *,*) 'Enter Backstripping rate (ppm)/min' read(*,*) Bkstrp

C Bcu, backstripping rate in Moles/(m3.s)

Bcu = (Bkstrp / 1000.) * (1./ Mcu) * 1000.0 / 60.0

C Inputs

C Faraday's # ( C/eq) Fr = 96485.0 C R (ideal gas) = 8.314 J/mol-k R = 8.314 c particle diameter dp = 3.2E-3 C electrolyte volume 18 lit Ve = 18.E-3 C 1 particle volume Vp = 3.1416/6. * (dp**3) C total particle volume: 600 cc Vpt = 0.48E-3 C area of a particle Ap = 3.1416*(dp*dp) C total # of particles np = Vpt / Vp C total surface area of particles (m2) C division by # 6 b/c approx. only 1/6 of the particles participate in rxn c Apt = Ap * np / 6. Apt = 6.90E-2 C diff Cu++ in m2/s Diff = 3.366E-9 C kinematic Viscosity (m2/s) nu = 0.73E-6 C Particle Reynolds # Red = U0* dp / nu C Schmidt Number: 214

Sc = nu / Diff C Voidage epsi = 0.46 C Mass transfer Coeff in a packed bed C from Electrochemical Reator Design by Pickett, for Red < 0.1 c Massk=Diff/dp*1.09/epsi*(Red**(0.333))*(Sc**(1./3))

C from Electrochemical Reator Design Pickett, for 23 < Red < 520

Massk=Diff/dp*0.83*(Red**(0.56))*(Sc**(1./3))

C from Electrochemical Reator Design Pickett, Eq.(4.96) for a fluidized bed c Massk=Diff/dp*((1-epsi)**(0.5))/epsi*(Red**(0.5))*(Sc**(1./3))

c from Mass Transfer in Fluidized beds, Walker, Wragg, Electrochimica acta c Massk = 1.55*( (Red/(1-epsi))**(-0.49) )*U0*(Sc**(-2./3))

kL = Massk write(*,*) 'Red=', Red write(*,*) 'kL=', kL

C Boundary layer thickness delta = Diff / Massk C electrode area per unit volume (assume all particles are active) c earea = Apt / Ve earea= 3.83

pause 'press any key' C Electrode kinetics data

! Deposition of copper

psi1 = -0.168

n1 = 2.0

! Hydrogen evolution on copper cathodes in aq acid solutions

psi2 = 0.0 n2 = 2.0

! total current density ( in Amps/m2) iden = itot / (Apt) write(*,*) 'Apt=', Apt write(*,*)'kL,Re, Re/(1-e)=',kL, Red, Red/(1-epsi) pause 215

write(3,*) 'pH=', pH, ' T=', Tmp-273.15 write(3,*) 'Initial Conc.(ppm)=',Cuppm write(3,*) 'Backdissolution rate(ppm/min)=',Bkstrp write(3,*) 'Current (amps)=', itot write(3,*) 'U0 (m/s)=',U0 write(3,*) 'Mass Transfer Coeff(m/s)', kL write(3,*) ' ' write(3,*)' Time ', ' Conc.', ' i1',' i2' write(4,*) 'Time potential current efficient' c write(4,*) 'Apt=', Apt

Call rk4(0., 10.,2400 ,100,CCu,C0,CH2,i01,n1,Fr,kl,earea,Bcu) c rk4(t0,h,nsteps,inter,CCu,C0, CH2,i01,n1,Fr,kl,earea)

CLOSE (3) close (4)

end

Subroutine Bisection(CCu, CH2, ER)

Real a, b, c, fa, fb, fc common n1,n2,psi1,psi2,i01,i02,beta1,beta2,iden,kL,Fr,Apt,R,Tmp

C Tolerance Err = 0.0001 C ER ( root of f function )

C Interval intial guess for root c a=-5. c c=3.5

a = -1.7 c = 2.0

b = (a+c)/2.

do while (ABS (f(CCu,CH2,b)).GT.Err) c write (*,*)' f, E ', f(CCu, Ch2, b), b

fa = f(CCu, CH2, a) 216

fb = f(CCu, CH2, b) fc = f(CCu, CH2, c)

if ( (fa*fb) .lt. 0. ) then a = a c = b else if ( (fa*fb).gt. 0. ) then a = b c = c

end if

b = (c+a)/2.

end do

ER = b

Return end subroutine

FUNCTION f(CCu, CH2, E)

Real i1, i2, ratio Real n1, n2, i01, i02, iden, kL common n1,n2,psi1,psi2,i01,i02,beta1,beta2,iden,kL,Fr,Apt,R,Tmp

CALL currents(E,CCu, CH2, i1, i2, ratio)

f = (i1+ i2)* Apt - iden * Apt

return

END FUNCTION

SUBROUTINE rk4(t0,h,nsteps,intr,CCu,C0,CH2,i01,n1,Fr,kl,earea,Bcu)

Real kk1, kk2, kk3, kk4 Real yn, tn, ynp, t Real i01, n1, kl Integer i, intr Real kt common Apt, R,Tmp

C Initial conditions 217 c yn = y0 c tn = t0 c t = t0

yn = CCu tn = t0 t = t0 c write(*,*)'y0, t0, h, nsteps, CCu, CH2, i01,n1, Fr, kl, earea', c +y0, t0, h, nsteps, CCu, CH2, i01,n1, Fr, kl, earea c write(3,*) 'Time, Conc., E(Volts)' c write(4,*) 'Time Conc k1/kL'

Call Bisection(CCu, CH2, ER)

CALL currents(ER,CCu, CH2, ei1, ei2, ratio) write(3,*) t0/60., CCu/C0, ER, Ratio

do i=1,nsteps, 1 C advance in time step

Call Bisection(CCu, CH2, ER) t = t + h kk1 = h * ff(ER, CCu, CH2, i01,n1, Fr, kl, earea,Bcu, t ) ERI = ER c write (*,*) ' ff = -i1*a/(nF) = ', kk1/h

Call Bisection(CCu+kk1/2., CH2, ER) kk2 = h * ff(ER, CCu, CH2, i01,n1, Fr, kl, earea, Bcu, t ) write(*,*) 'CCu, ER, kk2',CCu, ER, kk2

Call Bisection(CCu+kk2/2., CH2, ER) kk3 = h * ff(ER, CCu, CH2, i01,n1, Fr, kl, earea, Bcu, t ) write( *,*) 'CCu, ER, kk3',CCu, ER, kk3

Call Bisection(CCu+kk3/2., CH2, ER) kk4 = h * ff(ER, CCu, CH2, i01,n1, Fr, kl, earea, Bcu, t )

ynp = yn + 1./6 * (kk1+2.*kk2+2.*kk3+kk4)

if ( ((i/intr)*intr-i).eq.0 ) then c write(*,*) 'CCu, ER, kk1, kk2, kk3, kk4',CCu, ER,kk1,kk2,kk3,kk4 218

C save time values in minutes and Normalize Conc. c write(4,*)'E=',ERI,' -i1a/nf=',kk1/h,' i1=',kk1/h*n1*Fr/earea c kt = 3.E-13*exp(12.4*ERI)

call Currents(ERI,ynp,CH2, ei1,ei2, ratio)

current1=ei1 current2=ei2

write(3,*) (t/60.), (ynp/C0), ratio write(4,*) (t/60.), ERI, ei1/(ei1+ei2)

end if

write(*,*) ynp, t

yn = ynp tn = t

CCu = ynp

end do

pause 'press any key'

END SUBROUTINE

REAL FUNCTION ff(ER, CCu, CH2, i01,n1, Fr, kl, earea, Bcu, t)

Real i01, n1, kl Real k1, i1, i2, ratio c k1 = 3.E-12 *exp(12.4*ER) c i1 = n1 *Fr * k1* CCu / (1+ k1/kl) c i2 = 5.E-12 * exp(12.8*ER)

CALL currents(ER, CCu, CH2, i1, i2, ratio)

ff = - earea * i1 / (n1*Fr) + Bcu c write(*,*) 'ff=', ff c pause

RETURN

END FUNCTION 219

SUBROUTINE Currents(ER,CCu,CH2, i1,i2, ratio) Real ER, CCu, CH2 Real k1, i1, i2, ratio Real n1, n2, i01, i02, iden, kL Real alpha1, alpha2, Oi1, Ks1, Koc1, Kss1, ii02 common n1,n2,psi1,psi2,i01,i02,beta1,beta2,iden,kL,Fr,Apt, R,Tmp c k1 = 3.E-12 *exp(12.4*ER)

C Nickel kinetics data C data from Kinetic Parameters of Electrode Rxns, Tanaka C Standard exchange current density ( A/m2) Oi1 =0.068 C Activation energy J/mol DH1 = 37200 C Oi = n F C0 ks , C0 = 1 Molar c Ks1 = Oi1*3.83/ (n1*Fr) C correction for temperature , Arrhenius form c write(*,*) 'Arg',exp( -DH0/R *( (1./Tmp) - (1./ 308.) )) Kss1 = Oi1* exp( -DH1/R *( (1./Tmp) - (1./ 298.15) ) ) C Ks = Koc exp( a n F Ee / RT ) alpha1 = 0.51 beta1 = -alpha1 * Fr / (R*Tmp)

Koc1 = Kss1*3.38/n1/Fr/1000/exp(beta1*psi1)

k1 = Koc1*exp(beta1*ER)

C Hydrogen evolution C data from Electrochemical data, Conway C exchange current density, A/m2 c ii02 = 10. **(-2.6) C Activation energy 10.0 Kcal/mole ( cal = 4.184J ), Tmp= 35C = 308.15K c DH2 = 28.0E3 C Variations with temperature c write(*,*) 'Arg',exp( -DH1/R *( (1./Tmp) - (1./ 308.15) )) c pause c i02 = (3.38/n2/Fr/1000)*ii02*exp(-DH2/R*((1./Tmp)-(1./293.15))) i02=(3.38/n2/Fr/1000)*10000*10.**(-1.5-6.7/2.303/0.001987/Tmp) alpha2 = 0.63 beta2 = -alpha2 * Fr / (R*Tmp)

i2 = i02 * n2*Fr/3.38 * exp(beta2*ER)*CH2

i1 = n1 *Fr * k1* CCu / ((1+ k1/kL/3.38)*3.83) 220 c write(*,*)'Koc1=', Koc1 c write(*,*)'para=', exp(beta1*psi1) c write(*,*)'k1=',k1 c write(*,*)'k2=',i02 ratio = k1/(kL*3.83) c pause 'press any key'

END SUBROUTINE

B.2. FORTRAN Code Listing of the Cu/Ni Mixture Electrochemical Model

PROGRAM RKMIX

DOUBLE PRECISION i02,itot,Replace,rate1,rate3 DOUBLE PRECISION dp, Ve, Vp, Vpt, Ap, np, Apt, Diff DOUBLE PRECISION Red, Sc, nu, epsi, Massk, delta,Mcu, Mni DOUBLE PRECISION Fr,R,Tmp, n1, n2, n3, pH, U0, earea,iden,kL DOUBLE PRECISION psi1, psi2, psi3,CCu, CNi,CH2,Bcu1,Bcu3,C01, C03 common /para1/ n1,n2,n3,psi1,psi2,psi3,i02,iden,kL,Apt common /para2/ earea, CH2, C01, C03, Bcu1,Bcu3 common /para3/ Fr,R,Tmp,ER,pH,Mcu,Mni,Replace

OPEN (3, FILE='C:\Conc.txt')

OPEN (4, FILE='C:\Amps_i1.txt') c Anything related with Cu expressed by '1', those related with Ni by '3', those c related with Hydrogen by'2'

write(*,*) 'Enter Initial Copper Bulk Conc(ppm)' read(*,*) Cuppm write(*,*) 'Enter initial Nickel bulk conc(ppm)' read(*,*) Nippm write(*,*) ' Enter replace reaction constant' read(*,*) replace

C in (mole/m3), M (g/mole) Mcu = 63.546 Mni = 58.68 CCu = Cuppm/ Mcu CNi= Nippm/Mni Replace=replace

C initial conc. of copper and nickel C01 = CCu C03 = CNi 221

write(*,*) 'Enter pH (constant)' read(*,*) pH C concentration of H+ ions in mol/m3 CH2 = (10.**(-pH))* 1000.0

write (*,*) ' Enter Temperature (C)' read(*,*) Tmp Tmp = Tmp + 273.15

write(*,*) 'Enter U0 (constant, e.g. 0.2m/s -> km=1.08E-4 m/s)' read(*,*) U0

write(*,*) 'Enter Total current, itot (amps)' read(*,*) itot

write( *,*) 'Enter Cu Backstripping rate (ppm)/min' read(*,*) Bkstrp1 write( *,*) 'Enter Ni Backstripping rate (ppm)/min' read(*,*) Bkstrp3

C Bcu, backstripping rate in Moles/(m3.s)

Bcu1 = (Bkstrp1/ Mcu)/ 60.0 Bcu3 = (Bkstrp3/ Mcu)/ 58.0

C Inputs

Apt = Ap * np / 6. C diff Cu++ in m2/s Diff = .72E-9

C kinematic Viscosity (m2/s) nu = 1.12E-6 C Particle Reynolds # Red = U0* dp / nu C Schmidt Number: Sc = nu / Diff C Voidage epsi = 0.5 C Mass transfer Coeff in a packed bed C from Electrochemical Reator Design by Pickett, for Red < 0.1 c Massk=Diff/dp*1.09/epsi*(Red**(0.333))*(Sc**(1./3))

C from Electrochemical Reator Design Pickett, for 23 < Red < 520

Massk=Diff/dp*0.83*(Red**(0.56))*(Sc**(1./3))

C from Electrochemical Reator Design Pickett, Eq.(4.96) for a fluidized bed c Massk=Diff/dp*((1-epsi)**(0.5))/epsi*(Red**(0.5))*(Sc**(1./3))

c from Mass Transfer in Fluidized beds, Walker, Wragg, Electrochimica acta c Massk = 1.55*( (Red/(1-epsi))**(-0.49) )*U0*(Sc**(-2./3))

kL = Massk

C Boundary layer thickness delta = Diff / Massk C electrode area per unit volume (assume all particles are active) earea = Apt / Ve

C Electrode kinetics data

! Deposition of copper and nickel psi1 = 0.394 n1 = 2.0 psi3 = -0.294 n3 = 2.0

! Hydrogen evolution on copper cathodes in aq acid solutions

psi2 = 0.0 n2 = 2.0

223

! total current density ( in Amps/m2) iden = itot / (Apt) write(*,*) 'Apt=', Apt write(*,*)'kL,Re, Re/(1-e)=',kL, Red, Red/(1-epsi) pause

write(3,*) 'pH=', pH, ' T=', Tmp-273.15 write(3,*) 'Initial Cu Conc.(ppm)=',Cuppm write(3,*) 'Initial Ni Conc.(ppm)=',Nippm write(3,*) 'Cu Backdissolution rate(ppm/min)=',Bkstrp1 write(3,*) 'Cu Backdissolution rate(ppm/min)=',Bkstrp3 write(3,*) 'Replacement reaction constant=',Replace write(3,*) 'Current (amps)=', itot write(3,*) 'U0 (m/s)=',U0 write(3,*) 'Mass Transfer Coeff(m/s)', kL write(3,*) ' ' write(3,*)' Time','Cu Conc.','Ni Conc.' write(4,*) 'Time ER rate1 rate3'

Call rk4(0., 10.,3000 ,100,CCu,CNi,Apt,iden,CH2,Tmp,kL,earea, & Bcu1, Bcu3,C01, C03, Replace, ER,rate1,rate3)

CLOSE (3) close (4)

end

Subroutine Bisection(CCu, CNi,ER,Apt,iden,CH2,Tmp,kL,rate1,rate3)

DOUBLE PRECISION a, b, c, fa, fb, fc, Err,rate1,rate3 DOUBLE PRECISION Fr,R,n1, n2, n3,CCu, CNi, ER,Apt,iden,CH2,Tmp,kL DOUBLE PRECISION psi1, psi2, psi3,Bcu1,Bcu3,C01, C03

n1=2 n2=2 n3=2 psi1=0.394 psi2=0 psi3=-0.294 Fr = 96485.0 R = 8.314

C Tolerance Err = 0.001 224

C ER ( root of f function )

C Interval intial guess for root

a = -1 c = 2

b = (a+c)/2.

do while (ABS (f(CCu,CNi,b,Apt,iden,CH2,Tmp,kL, &rate1,rate3)).GT.Err)

write (*,*)' f, E ', f(CCu, CNi, b,Apt,iden,CH2,Tmp,kL, &rate1,rate3), b

fa = f(CCu, CNi, a,Apt,iden,CH2,Tmp,kL,rate1,rate3) fb = f(CCu, CNi, b,Apt,iden,CH2,Tmp,kL,rate1,rate3) fc = f(CCu, CNi, c,Apt,iden,CH2,Tmp,kL,rate1,rate3)

if ( (fa*fb) .lt. 0. ) then a = a c = b else if ( (fa*fb).gt. 0. ) then a = b c = c

end if

b = (c+a)/2.

end do

ER = b write(*,*) 'ER=', ER

Return end subroutine

FUNCTION f(CCu,CNi,E, Apt,iden,CH2,Tmp,kL,rate1,rate3)

DOUBLE PRECISION i1, i2, i3,E,CCu, CNi,rate1,rate3 DOUBLE PRECISION Fr,R, n1, n2, n3,iden,kL,Apt, Tmp, CH2 DOUBLE PRECISION psi1, psi2, psi3,Bcu1,Bcu3,C01, C03

n1=2 n2=2 225

n3=2 psi1=0.394 psi2=0 psi3=-0.294 Fr = 96485.0 R = 8.314

CALL currents(E,CCu, CNi,i1, i2, i3,CH2,Apt,Tmp,kL,earea, &rate1,rate3)

f = (i1+ i2 + i3)* Apt - iden * Apt

return

END FUNCTION

SUBROUTINE rk4(t0,h,nsteps,intr,CCu,CNi,Apt,iden,CH2,Tmp,kL,earea, & Bcu1, Bcu3,C01, C03, Replace, ER,rate1,rate3)

DOUBLE PRECISION kk1, kk2, kk3, kk4 DOUBLE PRECISION dd1, dd2, dd3, dd4 DOUBLE PRECISION yn, tn, ynp, t, SS1, WW1 DOUBLE PRECISION xn, xnp Integer i, intr, nsteps DOUBLE PRECISION Fr,R,n1, n2, n3,Tmp,kL,earea, & Bcu1, Bcu3,C01, C03, Replace, ER,rate1,rate3 DOUBLE PRECISION psi1, psi2, psi3,CCu,CNi,Apt,iden,CH2

n1=2 n2=2 n3=2 psi1=0.394 psi2=0 psi3=-0.294 Fr = 96485.0 R = 8.314

C Initial conditions xn = CNi yn = CCu tn = t0 t = t0

Call Bisection(CCu,CNi,ER,Apt,iden,CH2,Tmp,kL,rate1,rate3)

226

CALL currents(ER,CCu, CNi, ei1, ei2, ei3,CH2,Apt,Tmp,kL,earea, &rate1,rate3) write(3,*) (t0/60.),(CCu/C01),(CNi/C03) do i=1,nsteps, 1 C advance in time step

Call Bisection(CCu,CNi,ER,Apt,iden,CH2,Tmp,kL,rate1,rate3)

t = t + h

kk1 = h * ff(ER, CCu,CNi,t,Bcu1,Replace,kL,CH2,Apt,Tmp,earea, &rate1,rate3)

dd1 = h * dd(ER, CCu,CNi,t,Bcu3,Replace,kL,CH2,Apt,Tmp,earea, &rate1,rate3) ERI = ER

SS1=CCu+kk1/2. WW1=CNi+dd1/2.

Call Bisection(CCu+kk1/2.,CNi+dd1/2.,ER,Apt,iden,CH2,Tmp,kL, &rate1,rate3)

kk2 = h * ff(ER, CCu,CNi,t,Bcu1,Replace,kL,CH2,Apt,Tmp,earea, &rate1,rate3) dd2 = h * dd(ER, CCu,CNi,t,Bcu3,Replace,kL,CH2,Apt,Tmp,earea, &rate1,rate3)

write(*,*) 'CCu, CNi ER, kk2, dd2',CCu, CNi, ER, kk2, dd2

Call Bisection(CCu+kk2/2., CNi+dd2/2., ER,Apt,iden,CH2,Tmp,kL, &rate1,rate3)

kk3 = h * ff(ER, CCu,CNi,t,Bcu1,Replace,kL,CH2,Apt,Tmp,earea, &rate1,rate3) dd3 = h * dd(ER, CCu,CNi,t,Bcu3,Replace,kL,CH2,Apt,Tmp,earea, &rate1,rate3)

write( *,*) 'CCu, ER, kk3',CCu, ER, kk3

Call Bisection(CCu+kk3/2., CNi+dd3/2., ER,Apt,iden,CH2,Tmp,kL, & rate1,rate3) kk4 = h * ff(ER, CCu,CNi,t,Bcu1,Replace,kL,CH2,Apt,Tmp,earea, &rate1,rate3) dd4 = h * dd(ER, CCu,CNi,t,Bcu3,Replace,kL,CH2,Apt,Tmp,earea, &rate1,rate3) 227

ynp = yn + 1./6 * (kk1+2.*kk2+2.*kk3+kk4) xnp = xn + 1./6 * (dd1+2.*dd2+2.*dd3+dd4)

if ( ((i/intr)*intr-i).eq.0 ) then call Currents(ER,ynp,xnp, ei1,ei2, ei3,CH2,Apt,Tmp,kL, earea, &rate1,rate3) write(3,*) (t/60.),(ynp/C01),(xnp/C03) write(4,*) (t/60.), ER, rate1

end if

write(*,*) ynp, xnp, t

yn = ynp xn = xnp tn = t

CCu = ynp CNi = xnp

end do

pause 'press any key'

END SUBROUTINE

REAL FUNCTION ff(ER, CCu,CNi,t,Bcu1,Replace,kL,CH2,Apt,Tmp,earea, &rate1,rate3)

DOUBLE PRECISION i1, i2,i3 DOUBLE PRECISION Fr,R, n1, n2, n3,ER,CCu,t,Bcu1,Replace,kL,earea DOUBLE PRECISION psi1, psi2, psi3, CNi, CH2,Apt,Tmp, rate1,rate3

n1=2 n2=2 n3=2 psi1=0.394 psi2=0 psi3=-0.294 Fr = 96485.0 R = 8.314

CALL currents(ER,CCu,CNi, i1, i2, i3,CH2,Apt,Tmp,kL,earea, 228

&rate1,rate3) ff =-3.83*i1/(n1*Fr)+Bcu1-Replace*CCu/(1+Replace/kL/3.83)

RETURN

END FUNCTION

REAL FUNCTION dd(ER, CCu,CNi,t,Bcu3,Replace,kL,CH2,Apt,Tmp,earea, &rate1,rate3)

DOUBLE PRECISION i1, i2,i3 DOUBLE PRECISION Fr,R, n1, n2, n3,ER,CCu,t,Bcu3,Replace,kL,earea DOUBLE PRECISION psi1, psi2, psi3, CNi, CH2,Apt,Tmp, rate1,rate3

n1=2 n2=2 n3=2 psi1=0.394 psi2=0 psi3=-0.294 Fr = 96485.0 R = 8.314

CALL currents(ER, CCu, CNi,i1, i2, i3,CH2,Apt,Tmp,kL,earea, &rate1,rate3)

dd =-3.83*i3/(n3*Fr)+Bcu3+Replace*CCu/(1+Replace/kL/3.83)

RETURN

END FUNCTION

SUBROUTINE Currents(ER,CCu,CNi,i1,i2,i3,CH2,Apt,Tmp,kL,earea, &rate1,rate3) DOUBLE PRECISION k1, k3,i1, i2,i3 DOUBLE PRECISION alpha1,alpha3,Oi1,Ks1,Koc1,Kss1,Oi3,Ks3,Koc3,Kss3 DOUBLE PRECISION alpha2 DOUBLE PRECISION Fr,R, n1, n2, n3,ER,CCu,CNi,CH2,Apt,Tmp,kL,earea DOUBLE PRECISION psi1, psi2, psi3,rate1,rate3

n1=2 n2=2 n3=2 psi1=0.394 psi2=0 psi3=-0.294 229

Fr = 96485.0 R = 8.314 Apt=0.15

C Copper kinetics data C data from Kinetic Parameters of Electrode Rxns, Tanaka C Standard exchange current density ( A/m2) Oi1 = 0.803 C Activation energy J/mol DH1 = 46000 C Oi = n F C0 ks , C0 = 1 Molar Ks1 = Oi1 / (1.*Fr)

C correction for temperature , Arrhenius form c write(*,*) 'Arg',exp( -DH0/R *( (1./Tmp) - (1./ 308.) )) Kss1 = Ks1 * exp( DH1/R *( (n1/Tmp) - (1./ 298.15) ) )

C Ks = Koc exp( a n F Ee / RT ) alpha1 = .7434

beta1 = n1*alpha1 * Fr / (R*Tmp)

Koc1 = Kss1*3.38/n1/Fr/1000 / exp(-beta1*psi1)

k1 = Koc1*exp(beta1*ER)

C Nickel kinetics data C data from Kinetic Parameters of Electrode Rxns, Tanaka C Standard exchange current density ( A/m2) Oi3 = .068 C Activation energy J/mol DH3 = 37200 C Oi = n F C0 ks , C0 = 1 Molar Ks3 = Oi3 / (n1*Fr)

C correction for temperature , Arrhenius form c write(*,*) 'Arg',exp( -DH0/R *( (1./Tmp) - (1./ 308.) )) Kss3 = Ks3 * exp( DH3/R *( (1./Tmp) - (1./ 298.15) ) )

C Ks = Koc exp( a n F Ee / RT ) alpha3 = 0.52 beta3 = n3*alpha3 * Fr / (R*Tmp)

Koc3 = Kss3 *3.83/n1/Fr/1000/ exp(-beta3*psi3)

k3 = Koc3*exp(beta3*ER) 230

C Hydrogen evolution

C Activation energy 10.0 Kcal/mole ( cal = 4.184J ), Tmp= 35C = 308.15K DH2 = 46.4E3

i02=(3.38/n2/Fr/1000)*10000*10.**(-1.5-6.7/2.303/0.001987/Tmp)

alpha2 = 0.63 beta2 = n2*alpha2 * Fr / (R*Tmp)

i2 = i02 * n2*Fr/3.83* exp(-beta2*ER)*CH2 i1 = n1 *Fr * k1* CCu / ((1+ k1/kL/3.83))*3.83 i3 = n3 *Fr * k3* CNi / ((1+ k3/kL/3.83))*3.83 rate1= kL*3.83/k1 rate3= kL*3.83/k3

END SUBROUTINE

231

Appendix C: Cyclic Electrowinning/Precipitation (CEP) System Labview Control

Program

This Labview code programed a CEP cycle consisting of four precipitation processes and a electrodeposition process in SPE. The first plot is the front panel, the rest plots belong to the block panel. The front panel serves as the programmatic interface. Controls

and indicators on the front panel allow for inputting and exhibiting data and the status of

all the system elements, such as pumps, valves, and pH. The block panel is where the

underlying code is created for the control program. The time sequence is the main frame

of the code program. Sequences 0-47 are metal solution preparation by four precipitation

processes to accumulate the metal solution. Sequences 48-52 are the electrodepostion

process in CEP. Sequences 52-60 are the precipitation treatment of final solution for the

next CEP cycle. The Table C.1 is the icons used in code.

232

Table C.1. LabView™ icons used in CEP contraol program.

Icon Name Function Stacked Sequence Consists of one or more subdiagrams, or frames, Structure that execute sequentially. Use the Stacked Sequence structure to ensure a subdiagram executes before or after another subdiagram.

While Loop Repeats the subdiagram inside it until the conditional terminal, an input terminal, receives a particular Boolean value. The Boolean value depends on the continuation behavior of the While Loop. The While Loop always executes at least once. The iteration (i) terminal provides the current loop iteration count, which is zero for the first iteration. DAQ Assistant Creates, edits, and runs tasks using NI-DAQmx. Express VI Refer to the NI-DAQmx Readme for a complete listing of devices NI-DAQmx supports.

Index Array Returns the element or sub-array of n-dimension array at index. When you wire an array to this function, the function resizes automatically to display index inputs for each dimension in the array you wire to n-dimension array. Wait Until Next Waits until the value of the millisecond timer

ms Multiple becomes a multiple of the specified millisecond multiple. Use this function to synchronize activities. You can call this function in a loop to control the loop execution rate. Waits the Waits the specified number of milliseconds and specified number returns the value of the millisecond timer. This function makes asynchronous system calls, but the nodes themselves function synchronously. Therefore, it does not complete execution until the specified time has elapsed. Multiply Returns the product of the inputs.

Subtract Computes the difference of the inputs.

Decrement Subtracts 1 from the input value.

Greater or Equal Returns TRUE if x is greater than or equal to y. Otherwise, this function returns FALSE. Constant Being a constant as 877.5 233

Graph Display the value of y axis according to the time

Indicator of Valve Indicating the status of valve 1 and pump 1: on 1 and Pump 1 or off

Indicator of Valve Indicating the status of valve 2 and pump 2: on 2 and Pump 2 or off

Indicator of Valve Indicating the status of valve 3 and valve 4: on or 3 and Valve 4 off

Variable of Valve Copy of indicator of Valve 1 and Pump 1 1 and Pump 1 Variable of valve Copy of indicator of valve 2 and pump 2 2 and pump 2 Variable of Valve Copy of indicator of Valve 3 and Valve 4 3 and Valve 4

234

C.1. Front Panel:

Figure C.1. The indicators of Valves 1-7 and Pumps 1-3 are presented as on/off, and the solution pH value in the precipitation/redissolution tank is shown in pH graph in Front Panel.

235

C.3. Block Panel:

Figure C.2. (a) 30 s delay to begin the CEP operation; (b) open Pump 1 and Valve 1 to drain contaminated solution into the precipitation tank; (c) level sensor measures the solution level in precipitation tank; (d) as soon as the solution reaches the level setpoint, Pump 1 and Valve 1 are closed.

236

Figure C.3. (a) Open Valve 3 to add 1M sodium hydroxide solution; (b) pH sensor measures the solution pH; (c) when the pH reaches 11, Valve 3 is closed; (d) 8 min delay for the metal hydroxide precipitation process.

237

Figure C.4. (a) Open Valve 2 and Pump 2 to drain clean solution from the precipitation tank; (b) 2 min delay for the clean solution to drain; (c) close Valve 2 and Pump 2; (d) open Pump 1 and Valve 1 to drain the contaminated solution into precipitation tank.

238

Figure C.5. (a) Level sensor measures the solution level in the precipitation/rediussolution tank; (b) when the solution reaches the level setpoint, Pump 1 and Valve 1 are closed; (c) Open Valve 3 to add 1M sodium hydroxide solution; (d) pH sensor measures the solution pH.

239

Figure C. 6. (a) When the pH reacheds 11, Valve 3 is closed; (b) 8 min delayu for the metal hydroxide precipitation process; (c) Open Valve 2 and Pump 2 to drain the clean solution from the precipitation/redissolution tank; (d) 2 min delay to drain the clean solution.

240

Figure C.7. (a) Close Valve 2 and Pump 2; (b) open Pump 1 and Valve 1 to drain contaminated solution into the precipitation/redissolution tank; (c) level sensor measures solution level in precipitation/redissolution tank; (d) when the solution level reaches the setpoint, Pump 1 and Valve 1 are closed.

241

Figure C.8. (a) Open Valve 3 to add 1M sodium hydroxide solution; (b) pH sensor measures the pH of solution; (c) when the pH reaches 11, Valve 3 is closed; (d) 8 min delay for the metal hydroxide precipitation process.

242

Figure C.9. (a) Open Valve 2 and Pump 2 to drain clean solution from the precipitation/.redissolution tank; (b) 2 min delay to drain the clean solution; (c) close Valve 2 and Pump 2; (d) open Pump 1 and Valve 1 to drain contaminated solution into the precipitation/redisolution tank.

243

Figure C.10. (a) Level sensor measures the solution level in the precipitation/redissolution tank; (b) when the solution reaches the level setpoint, Pump 1 and Valve 1 are closed; (c) open Valve 3 to add 1M sodium hydroxide solution; (d) pH sensor measures the pH of solution.

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Figure C.11. (a) When the pH reaches 11, Valve 3 is closed; (b) 8 min. delay for the metal hydroxide precipitation process; (c) open Valve 2 and Pump 2 to drain clean solution from the precipitation/redissolution tank; (d) 2 min. delay to drain the clean solution.

245

Figure. C.12. (a) Close Valve 2 and Pump 2; (b) open Pump 1 and Valve 1 to drain contaminated solution into the precipitation/redissolution tank; (c) level sensor measures the solution level in the precipitation/redissolution tank; (d) when the solution reaches the level setpoint, Pump 1 and Valve 1 are closed.

246

Figure C.13. (a) Open Valve 4 to add 1M sulfuric acid to dissolve the metal hydroxide precipitates; (b) pH sensor measures the solution pH; (c) when the pH reaches 4, Valve 4 is closed; (d) 10 min. delay to add sodium sulfate and boric acid to increase the solution conductivity.

247

Figure C. 14. (a) Open Valve 5, 6 and Pump 3 to drain the solution into the SPE; (b) 1 min. delay for the draining process; (c) close Valve 5 and open Valve 7 to began the electrodeposition process; (d) run electrodeposition for 2 h.

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Figure C.15. (a) Close Valve 6 and Pump 7 and open Valve 5. Drop the solution into the precipitation/redissolution tank; (b) 3 min. to complete the process; (c) close Valves 5 and 7. Open Valve 3 to add 1M sodium hydroxide solution; (d) pH sensor measures the solution pH.

249

Figure C.16. (a) When the pH reaches 11, Valve 3 is closed; (b) 8 min. delay for the metal hydroxide precipitation process; (c) open Valve 2 and Pump 2 to drain clean solution from the precipitation/redissolution tank; (d) 2 min. delay to drain the clean solution.

250

Figure C.17. (a) Close Valve 2 and Pump 2.