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Electrochemical Method for Remediation of Arsenic-Contaminated Nickel Electrorefining Baths Jenny Weijun Wang, Dorin Bejan, and Nigel J. Bunce* Department of Chemistry, University of Guelph, Guelph, Ontario, Canada N1G 2W1
At only ppm concentrations, arsenic is incompatible with the deposition of pure nickel at the cathode of a nickel electrorefining bath. We report an electrochemical method for the removal of arsenic. At a Ti/IrO2 anode, chloride ion in the bath is oxidized to HOCl, which effects the chemical oxidation of As(III) to As(V). HOCl also reoxidizes any As(III) that might be formed by back-reduction at the pure nickel cathode, preventing cathodic reduction of As(III) to elemental arsenic or to the toxic gas arsine, AsH3. If, alternatively, a pure copper cathode is employed, the cathode potential can be controlled to electroseparate copper from nickel during the remediation of the bath, while achieving complete oxidation of As(III). After pH adjustment to ∼4, arsenic is removed with added Fe(III) as a chemisorbed arsenate-ferrihydrite precipitate, leaving the bath essentially free of both iron and arsenic. Introduction Electrorefining involves dissolving an impure metal from the anode of an electrolytic cell while depositing the pure metal at the cathode.1 Impurities accumulate in the electrolyte as the anode dissolves, eventually reaching concentrations at which they also deposit at the cathode, degrading the purity of the product. The electrolyte must then be purified or discarded. Crude nickel typically contains 94-96% Ni, together with copper, cobalt, arsenic, and lead.2 Although concentrations of As up to 30 g/L (0.4 M) can be tolerated in Cu electrorefining,3 contamination of a Ni cathode commences at As concentrations >50 ppm (0.7 mM). The explanation is that Cu(II) is intrinsically more readily reduced than As(III) in an acidic solution [E°(Cu2+/Cu) ) +0.34 V; E°(HAsO2/As) ) +0.25 V], whereas As(III) is more easily reducible than Ni2+ [E°(Ni2+/Ni) ) -0.26 V].4 Borbely et al. removed Co, Fe, As, and Pb from a synthetic Cu-free Ni electrolyte in an electrochemical reactor.5 At high current density at the cathode, little deposition of metal occurred, but reduction of H+ left a high concentration of OH- near the cathode, and the metallic impurities precipitated as hydroxides. Gronningsaeter and Hommeren added finely divided Ni powder to Ni electrolyte containing Cu, As, Fe, and Co.6 After removal of Cu by cementation and filtration, freshly prepared iron(III) hydroxide was added to sorb As, Pb, and Co, which were removed by filtration. Renzoni proposed bubbling air through a spent Ni electrolyte containing Cu, Pb, Fe, As, and Co impurities, at pH 4-5, while electrolyzing at an inert anode.7 The patent claims that at this pH air would simultaneously oxidize Cu+ to Cu2+, and Fe2+ to Fe3+, but the instability of Cu+(aq) suggests that this analysis cannot be correct. As and Pb were oxidized by Cl2 and precipitated by adsorption to additional iron hydroxide. Cu was removed by cementation with Ni metal, followed by filtration. More Cl2 was then passed through the * To whom correspondence should be addressed. Tel.: (519) 824-4120 ext. 53962. Fax: (519) 766-1499. E-mail: nbunce@ uoguelph.ca.
partially purified Ni electrolyte, after adding nickel carbonate to maintain the pH at 4-5, to oxidize and precipitate Co. Although a high degree of electrolyte purification was achieved, several settling and filtration steps were needed to remove all of the impurities. Current technology (Dr. Tao Xue, Inco Ltd., personal communication) employs H2S to precipitate Cu and As from a Ni refining electrolyte as CuS and As2S3. Disadvantages include the use of the highly toxic and odorous H2S and the difficulty of recovering As2S3, which is only stable over a limited range of conditions according to the Pourbaix diagram.8 If this sludge is disposed in a landfill, As2S3 may be oxidized microbially, thereby releasing As into the environment as As(III), which is more toxic and more mobile in the environment than As(V),9 along with the formation of acid drainage. Previous work in our laboratory10 suggested that As(III) can be oxidized to As(V) electrochemically, followed by removal of As from the contaminated bath by sorption to precipitated iron(III) hydroxide, to which As(V) binds more strongly than As(III).11-13 This precipitate is recommended by the U.S. Environmental Protection Agency for land-filling As-containing sludge.14 Because electrorefining is itself an electrolytic technology, we considered electrochemical oxidation to be convenient for producing Fe and As in the oxidized form needed for efficient coprecipitation. The steps envisioned for a technology were oxidation of As(III) to As(V), delivery of additional Fe into the electrolyte as in the method of Gronningsaeter and Hommeren,6 and increase of the pH of the electrolyte to precipitate Fe along with sorbed As. The present work employed both synthetic solutions and an authentic spent Ni electrolyte supplied by Inco Ltd. Materials and Methods Materials. Iron(II) sulfate heptahydrate was purchased from VWR (Mississauga, Ontario, Canada), iron(III) sulfate pentahydrate and nickel sulfate hexahydrate were purchased from Aldrich (Milwaukee, WI), and sodium hydroxide, anhydrous sodium sulfate, and 98% sulfuric acid were purchased from Fisher Scientific (Toronto, Ontario, Canada). Sodium arsenate heptahy-
10.1021/ie048769y CCC: $30.25 © 2005 American Chemical Society Published on Web 04/02/2005
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Figure 1. Principle of operation of the divided plug-flow reactor: Cl-free experiments.
drate and sodium m-arsenite were purchased from Sigma (Mississauga, Ontario, Canada), iron and arsenic standard solutions were purchased from Alfa Aesar (Ward Hill, MA), and Ar gas was purchased from BOC Gases (Mississauga, Ontario, Canada). Silver diethyldithiocarbamate, morpholine, and chloroform used for arsine detection were purchased from Aldrich. All solutions were prepared with deionized water. An authentic Ni refining electrolyte solution was obtained from Inco Ltd. at Thompson MB. The clear green solution was collected directly from the plating tank house and had not been subjected to any conventional treatment. Apparatus. All electrochemical experiments were controlled with an EG&G model 363 potentiostat/ galvanostat. The cathodes were a stainless steel plate (composition Fe:Cr:Ni ) 70:19:11), a copper plate, supplied by CFF Specialties (Hamilton, Ontario, Canada), and a nickel plate (99.9%) supplied by Aldrich. The anodes were IrO2-coated Ti and SnO2-coated Ti, purchased from Eltech Systems Corp. (Chardon, OH), and graphite (99%), purchased from Alfa Aesar. DuPont Nafion-424 cation-exchange membrane (CEM), used to divide the electrochemical reactor, was purchased from Electrosynthesis Co. (Lancaster, NY). The electrochemical reactors comprised two plug-flow reactors and two batch reactors. The Plexiglas-divided plug-flow reactor comprised two compartments, each having dimensions of 5.8 cm × 1.5 cm × 0.45 cm, separated by a DuPont Nafion-424 CEM. The principle of operation is presented in Figure 1. Separate solutions were passed through the cathode and anode compartments at equal flow rates of 1 mL/min, using a Masterflex Compact/Low Flow pump. The catholyte was dilute sulfuric acid (prepared by adding 1.0 mL of concentrated H2SO4 to 1 L of water, pH 1.8). The anolyte was a 10 ppm As(III) solution, prepared by diluting a 100 ppm As(III) stock solution (prepared from sodium m-arsenite) and adjusted to pH 2.0 with 10% H2SO4. The undivided plug-flow reactor differed only in that the membrane was removed and only the As(III) solution was passed through the reactor, at a constant flow rate of 1 mL/min. The anode was a grid of IrO2/Ti (∼7.5 cm2), and the cathode was a stainless steel plate (∼5 cm2). Cells were operated in the galvanostatic mode, with samples collected at convenient times and analyzed by a speciation cartridge and As test kit right away. After each experiment, the cell was taken apart and washed with deionized water and the electrodes were checked for fouling and electrodeposition. The open-batch reactor was a rectangular Plexiglas cell having dimensions of 3.4 cm × 3.0 cm × 5.0 cm. Its
Figure 2. Four-neck open-batch reactor for detection of arsine.
total volume was 51 mL, but only 40 mL of electrolyte was used in any experiment. The cathodes were a Ni plate (1.4 cm × 5.2 cm and 3.4 cm × 3.8 cm), a stainless steel plate (1.5 cm × 3.3 cm), and a copper plate (3.2 cm × 3.8 cm). The anodes were a graphite rod with a radius of 0.3 cm, iron plates with different dimensions, and grids of Ti/IrO2 or Ti/SnO2 (1.5 cm × 3.5 cm). Before each experiment, the cathodes were abraded with sandpaper and washed with dilute sulfuric acid, soap, and deionized water. The anodes were washed with dilute sulfuric acid and deionized water. This reactor was operated galvanostatically with a synthetic Ni electrolyte and either galvanostatically or potentiostatically with an authentic electrolyte. Samples were withdrawn at convenient times and analyzed for As(III) and total As. The closed-batch reactor (Figure 2), which was cylindrical (diameter 5.0 cm and height 9.5 cm), with a fourneck cover, was used only to determine whether arsine was formed during electrolysis. To obtain the same current density as that in the open-batch reactor, 70 mL of electrolyte was used, giving a depth of 3.8 cm, immersing the same area of the electrodes as that in the open-batch reactor. The possible formation of arsine during electrolysis was checked by sweeping the product gases out of the electrolyte with argon through a test tube containing silver diethyldithiocarbamate and morpholine in chloroform (Figure 2). The pale yellow solution gave a red coloration if arsine was present.15 Electrolysis continued until arsine was detected, up to a maximum of 2 h. Analysis. Residual metal concentrations were analyzed by inductively coupled plasma atomic emission spectroscopy (Thermo Jarrell Ash, model Atomscan 16), which offered high sensitivity, a large linear range, and multiple-element analysis. Wavelengths used to analyze As, Co, Cu, Fe, and Ni were 189.042, 228.616, 324.754, 259.940, and 221.647 nm, respectively. Each sample was analyzed in triplicate, with each measurement consisting of an average of four readings; an average of the three was reported. Analysis of total As employed a test kit (Quick) purchased from Industrial Test Systems, Inc. (Roch Hill, SC). Addition of zinc converts inorganic As to AsH3, which gives a light yellow to dark brown color with a mercuric bromide test pad; concentrations in the range of 0-500 ppb are determined by comparison with a color chart. Speciation of As(III) and As(V) was achieved with disposable As speciation cartridges packed with 2.5 g of selective aluminosilicate adsorbent, purchased from Stevens Institute of Technology (Hoboken, NJ). At pH
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4-9, As(V) is present as H2AsO4-, which adsorbs selectively to the aluminosilicate, while HAsO2 is not retained. Analysis of the raw solution with the Quick test kit affords “total As”; analysis of the filtrate affords As(III), and As(V) is obtained by the difference.16 Results and Discussion A rapid method was required to distinguish between As(III) and As(V) in the presence of up to 60 g/L of Ni2+. The colorimetric method17 in which As(V) is detected selectively as its blue complex with molybdate ion is unsuitable for analyzing the bright green Ni electrolyte. Other published methods are based on separation of As(III) and As(V) based on the pKa difference18 between arsenic acid, pK1 ) 3.5, and arsenous acid, pK1 ) 9.2, using ion-exchange chromatography,19,20 ion-pairing high-perfomance liquid chromatography,21,22 or selective arsine generation23 and detection by atomic absorption spectroscopy or inductively coupled plasma. These methods are suitable for the analysis of traces of arsenic in aqueous samples rather than Ni electrolyte with its high concentrations of background substances, especially Ni2+. These methods are also time-consuming and require elaborate instrumentation. The commercial Quick arsenic test kit, along with disposable speciation cartridges, analyzed As(III) and As(V) conveniently. Possible interferences were Ni, Cu, and especially Fe, because iron(III) hydroxide precipitates at pH >4 and sorbs inorganic arsenic strongly.11,12 To test whether the test kit could tolerate these possible interferences, we prepared a synthetic Ni electrolyte containing 60 g/L of Ni(II), 500 mg/L of Cu(II), and 11 mg/L of Fe(II), spiked with 15 ppm As(III), 15 ppm As(V), or 7.5 mg/L of As(III) + 7.5 mg/L of As(V). Analysis by the speciation cartridge and arsenic test kit gave indistinguishable measured and nominal concentrations of both As(III) and As(V) after dilution by both 200× and 500×, but arsenic was underestimated by 1520% at 500× dilution at concentrations of Fe(II) or Fe(III) > 0.2 ppm after dilution (40 ppm true concentration). Experiments with Synthetic Solutions. Initial experiments were carried out in the divided plug-flow reactor, with a Ti/IrO2 anode and a stainless steel cathode. The anolyte was 40 g/L of Ni2+ and 10 ppm As(III) at pH 2, and 1 mL/min flow rate. The catholyte was dilute H2SO4. The extent of oxidation of As(III) increased with the current density, reaching 90% at j ) 13 mA/cm2. Oxidation under these conditions occurs directly at the anode, because Ti/IrO2 reacts directly by the “higher surface oxide” mechanism24,25 and because undissociated HAsO2 is unreactive with H2O2 that might be formed by electrooxidation of water.26 The total As concentration in the effluent remained constant at 10 ppm. In the undivided plug-flow reactor, a nickel foil cathode was used, but the other parameters were unchanged. The apparent efficiencies for oxidation of As(III) were somewhat lower: at j ) 6.7 mA/cm2, 80% of As(III) was oxidized in the divided reactor and 65% in the undivided reactor. Although back-reduction of As(V) was possible in principle, no As was lost from the solution as As0 or AsH3. Experiments with 40 g/L of Ni2+, 50 ppm As(III), and initial pH 2.2 were carried out in open-top-batch reactors using a nickel foil cathode and a sacrificial iron
Table 1. Concentrations of As(III), As(V), and Total As at Different Chloride Concentrations, Anodes, and Current Densities in the Undivided Batch Reactora
electrolyte
anode
0 M Cl0 M Cl0 M Cl0 M Cl0 M Cl0.1 M Cl1.0 M Cl-
IrO2/Ti IrO2/Ti IrO2/Ti SnO2/Ti graphite IrO2/Ti IrO2/Ti
anode current remaining As(III) total As current density (mg/L) (mg/L) (mA) (mA/cm2) 10 50 80 50 50 10 10
1.8 8.8 14 8.8 8.8 1.8 1.8
BDL BDL BDL BDL BDL BDL BDL
21.5 BDL BDL BDL 9.3 47.5 49.2
a Initial As(III) ) 50 mg/L, pH ) 2.4. BDL ) below detection limit.
anode, with the idea of coprecipitating As with ferric hydroxide. The apparent current efficiency for Fe dissolution at the anode was 99-108%, with the values >100% being explained by flaking from the anode. No attempt was made to optimize the current efficiency for Ni deposition (85-91%), and proton reduction at the cathode (4-8%) caused a gradual increase in the pH of the solution. Although the pH of these solutions reached only 2.8-3.0 and no Fe(OH)3 was precipitated, the soluble arsenic concentration fell below the detection limit (0.1 ppm). Moreover, the use of a dissolving anode was wasteful; much higher concentrations of Fe were formed than previous work indicated were needed to remove all of the As.9 Non-Fe anodes were used in order to investigate how As was being lost from the solution. The disappearance of As(III) in the batch reactor at 50 mA applied current, pH 2, and 1 mL/min flow rate followed apparent firstorder kinetics at Ti/SnO2, Ti/IrO2, and graphite anodes. The rate constants were indistinguishable (Ti/SnO2, k ) 0.061 ( 0.006 min-1; Ti/IrO2, k ) 0.067 ( 0.007 min-1; graphite, k ) 0.065 ( 0.004 min-1) and increased with the current density: e.g., at Ti/IrO2 with 10, 50, and 80 mA applied currents, k ) 0.035 ( 0.001, 0.067 ( 0.007, and 0.105 ( 0.011 min-1, respectively. Although these current densities are probably greater than the limiting current density, the increase in the rate constant with j probably results from improved mass transfer through evolution of O2. Because As was removed from the solution (Table 1, entries 1-5), these rate constants represent loss of As(III) to both As(V) and other pathways. At pH 2, As(III) and As(V) exist as the neutral species HAsO2 and H3AsO4, which migrate equally easily to both cathode and anode. Although electrochemical reduction of As(III) and As(V) is inefficient,27,28 long-term electrolysis in the batch reactor permitted irreversible loss of As from the solution as either As0 or AsH3(g), which latter was readily detectable.15 Arsine was not observed under analogous flow conditions because the detention time in the cell was short and because electrochemical oxidation of As(III) to As(V) is much more efficient than reduction.27 Authentic Ni electrolytic baths contain ∼1 M chloride ion to prevent anode passivation. Experiments in the undivided batch reactor revealed that oxidation of As(III) was much faster when NiCl2 was present (Figure 3) and that Cl- inhibited the loss of As from the solution (Table 1, entries 6 and 7). Arsine was undetectable when the electrolyte contained Cl-, unless the reactor was neither stirred nor otherwise agitated, when traces of arsine could be detected. The explanation is that Cl- is
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Figure 3. Remaining As(III) concentration in the synthetic Ni electrolyte vs time for different Cl concentrations on a Ti/IrO2 anode at constant current 10 mA. Symbols: filled diamonds, 0 M Cl-; open squares, 0.1 M Cl-; open triangles, 1 M Cl-.
readily electrooxidized to HOCl, which oxidizes As(III) in the bulk solution (eqs 1 and 2). Any As(III) produced
Cl- + H2O f HOCl + H+ + 2eHOCl + HAsO2 + H2O f H3AsO4 + H+ + Cl-
(1)
Figure 4. Cu removal as a function of electrolysis time in the 40 mL and 1.0 L batch reactors as a function of charge passed per initial concentration of Cu: filled squares, 1 L reactor; filled diamonds, 40 mL reactor. Table 2. Changes in Concentration in Selected Metals after Electrolysis in the 1 L Undivided Batch Reactor (Cu Cathode, Ti/IrO2 Anode, and Applied Current 0.62 A) and after pH and Fe Adjustment after Electrolysis and Filtrationa
(2)
by back-reduction of As(V) is immediately reoxidized by HOCl. Experiments with Authentic Ni Electrolyte. The electrolyte provided by Inco Ltd. contained the following: Ni, 60 g/L; Cl, 53 g/L; Cu, 0.50 g/L; Co, 0.10 g/L; As, 18 mg/L; Fe, 12 mg/L; Pb, 2.5 mg/L. Of the 18 ppm total As, 15 ppm was initially present as As(III). In the flow system with a Ti/SnO2 anode, a Ni foil cathode, and current densities in the range of 2.6-13 mA/cm2, the total As concentration in the effluent was equal to the initial concentration of 18 mg/L, As(III) was below detection limits, and no arsine was detected. In a similar reaction in the open-batch reactor using 40 mL of an authentic electrolyte, with a Ti/IrO2 anode, a Ni foil cathode, and an applied current of 10 mA, complete conversion to As(V) was achieved in 6 min (3.6 C total charge), and no arsine was detected even when electrolysis was continued for 2 h (72 C). In the previous experiments, Cu was deposited onto both the anode and cathode, probably because Cu2+ was reduced to elemental Cu by cementation on Ti. This did not significantly affect the function of the anode because the deposited Cu could also corrode back into solution. The reduction potentials cited earlier for deposition of Cu2+ and Ni2+ suggested that Cu might conveniently be removed from the solution by electroseparation onto a Cu cathode while simultaneously using the anodic formation of HOCl at Ti/IrO2 to oxidize As(III). Using concentrations rather than activities in the Nernst equation, we calculated that 95% of Cu could be removed from the authentic Ni electrolyte at a potential 0.24 V vs SHE, whereas Ni would not deposit until the potential reached ENi ) -0.25 V. Under potentiostatic conditions, using a Cu plate cathode, a Ti/IrO2 anode, and Ni2+/Ni as a “secondary” reference electrode, a potential of 0.4 V vs Ni2+/Ni (0.15 V vs SHE) was applied to the Cu cathode. After 90 min of electrolysis (total charge Q ) ∫t0I dt ) 210 C), the concentration of Cu2+ declined from 503 to 30 mg/L (Figure 4). The concentrations of dissolved Fe and As were unaffected, but As(III) was below the limit of detection. In the more convenient amperostatic mode,
initial Ni bath As (mg/L) Co (mg/L) Cu (mg/L) Fe (mg/L) Ni (g/L) Pb (mg/L) a
19.8 99.9 497.8 6.6 59.2 2.5
after electrolysis 21.8 101.8 21.1 7.1 59.5 2.6
20.4 99.5 35.4 6.7 57.3 2.6
after filtration BDL 100.0 6.3 BDL 59.1 2.5
BDL 100.6 9.2 BDL 60.1 2.4
BDL 99.5 6.7 BDL 57.9 2.3
BDL ) below detection limit.
94% of the initial concentration of Cu was removed after the passage of 180 C at an applied current of 25 mA, slightly more efficient than the potentiostatic mode. The mass of Cu deposited on the cathode represented 87% recovery, but this is a low estimate because some powdery Cu deposited on the edge of the cathode and was lost during cleaning and drying. The process was scaled up from a 40 mL reactor to a 1 L reactor, with larger electrodes (Cu, 49.2 cm2; Ti/IrO2, 22.8 cm2) at the same current density. Under conditions that 96% of the Cu was removed from the solution in the 1 L reactor, 92% was recovered as the increased mass of the cathode, and the loss of Cu from the solution as a function of charge passed relative to the total amount of copper in the solution was similar for both reactors (Figure 4). The disadvantage of the above process was the mismatch between cathode and anode stoichiometry: in the 40 mL reactor, complete oxidation of As(III) (18 ppm) only required 4 C, while the reduction of 500 ppm Cu required >180 C. Instead, removal of As from 100 mL of the electrolyzed Ni electrolyte was achieved by adding Fe2(SO4)3‚5H2O (66 mg) and NaOH (175 mg). This raised the pH to 5, allowing precipitation of Fe(OH)3, after which the filtrate was analyzed for As, Co, Cu, Fe, Ni, and Fe by inductively coupled plasma (Table 2). Columns 3 and 4 of Table 2 were the results of repeated electrolyses at the 1 L scale, and columns 5-7 were the results of the repeated precipitation and filtration process on the same solution. Both As and Fe were below the limit of detection after electrolysis + filtration, Cu levels were lowered substantially, and Co, Ni, and Pb were unchanged. This is similar to the observations of Krause and Ettel,29 who also precipitated As from metal electroplating baths using Fe(III) as a material they described as ferric arsenate.
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Although our work did not address the issue of remediation of Co from the Ni bath, this objective appears to be readily achievable based on the experience of Devuyst et al.,30 who reported an electrochemical method in which Co(II) was oxidized to Co(III) at the anode of a divided cell, followed by precipitation of Co(OH)3. In conclusion, complete oxidation of As(III) to As(V) was achieved by electrolyzing an authentic Ni electrolyte with simultaneous electroseparation of metallic Cu. Undivided electrochemical reactors were used, thereby eliminating the use of membranes that might be subject to fouling. Oxidation by electrolytically formed HOCl is more convenient than dosing the solution with Cl2, as proposed by Renzoni.7 Addition of Fe3+ to the electrolyzed solution and adjustment of the pH to 5 allowed quantitative removal of As(V) from the solution, in the form recommended for land disposal by the U.S. EPA.13 If desired, Fe(II) salts could also be used because air oxidation of Fe(II) is rapid at pH 5.31 pH adjustment was achieved in our hands by means of either NaOH or Na2CO3, although we note the use of nickel carbonate by Gronningsaeter and Hommeren.6 Acknowledgment We thank Dr. Tao Xue of Inco Ltd. for providing the authentic spent Ni plating electrolyte and the Natural Sciences and Engineering Research Council of Canada for financial support. Literature Cited (1) Pletcher, D.; Walsh, F. C. Industrial Electrochemistry; Blackie Academic & Professional: Glasgow, U.K., 1993; pp 231244. (2) Eteru, B. A.; Mozowayu, M. A.; Dobiyuisuto, E. O. Purification of Nickel Electrolyte. Canada Patent 1,197,490, 1984. (3) Houlachi, G. J.; Claessens, P. L. Arsenic removal from electrolytes with application of periodic reverse current. U.S. Patent 4,083,761, 1976. (4) Reduction potentials from: CRC Handbook of Chemistry and Physics, 64th ed.; Weast, R. C., Ed.; CRC Press: Boca Raton, FL, 1984; pp D156-D160. (5) Borbely, G. J.; Illis, A.; Brandt, B. J. Purification of Nickel Electrolyte by Electrolytic Oxidation. U.S. Patent 3,983,018, 1976. (6) Gronningsaeter, A. M.; Hommeren, B. G. Nickel Electrolyte Purification. Canada Patent 400,642, 1941. (7) Renzoni, L. S. Electro-refining of Nickel. U.S. Patent, 2.394,874, 1946. (8) Brookins, D. G. Eh-pH Diagrams for Geochemistry; SpringerVerlag: New York, 1988; pp 28 and 29. [There is an error in the original: HAsO4- should be HAsO42- and AsO4- should be AsO43-.] (9) Korte, N. E.; Fernando, Q. A review of arsenic(III) in groundwater. Crit. Rev. Environ. Control 1991, 21, 1-39. (10) Wang, J. W.; Bejan, D.; Bunce, N. J. Removal of arsenic from synthetic acid mine drainage by electrochemical pH adjustment and coprecipitation with iron hydroxide. Environ. Sci. Technol. 2003, 37, 4500-4506. (11) Ferguson, J. F.; Anderson, M. A. In Chemistry of Water Supply, Treatment, and Distribution: Chemical Formation of Arsenic in Water Supplies and their Removal; Rubin, A. J., Ed.; Ann Arbor Science: Ann Arbor, MI, 1974; pp 137-158.
(12) Pierce, M. L.; Moore, C. B. Adsorption of arsenite and arsenate on amorphous iron hydroxide. Water Res. 1982, 16, 12471253. (13) Raven, K. P.; Jain, A.; Loeppert, R. H. Arsenite and arsenate adsorption on ferrihydrite: kinetics, equilibrium, and adsorption envelopes. Environ. Sci. Technol. 1998, 32, 344-349. (14) MSE Inc. Final ReportsArsenic Oxidation Demonstration Project, 1998. http://www.arsenic.org/PDF%20Files/Mwtp-84.pdf (accessed 01/2003). (15) Greenberg, A. E.; Eaton, A. D.; Clesceri, L. S. Standard Methods for the Examination of Water and Wastewater; American Public Health Association: Washington, DC, 1991; pp 36-40. (16) Meng, X.; Wang, W. Speciation of Arsenic by Disposable Cartridges. Third International Conference on Arsenic Exposure and Health Effects, San Diego, CA, July 12-15, 1998. (17) Johnson, D. L. Simultaneous Determination of Arsenic and Phosphate in Natural Waters. Curr. Res. 1971, 5, 411-414. (18) Van Muylder, J.; Pourbaix, M. In Atlas of Electrochemical Equilibria in Aqueous Solution: Arsenic; Pourbaix, M., Ed.; National Association of Corrosion Engineers: Houston, TX, 1974; pp 516-523. (19) Grabinski, A. A. Determination of Arsenic(III), Arsenic(V), Monomethylarsonate, and Dimethylarsinate by Ion-Exchange Chromatography with Flameless Atomic Absorption Spectrometric Detection. Anal. Chem. 1981, 53, 966-968. (20) Soto, E. G.; Rodriguez, E. A.; Mahia, P. L.; Lorenzo, S. M.; Rodriguez, D. P. Ion-exchange Method for Analysis of Four Arsenic Species and Its Application to Tap Water Analysis. Anal. Lett. 1995, 28, 2699-2718. (21) Hakala, E.; Pyy, L. Selective Determination of Toxicologically Important Arsenic Species in Urine by High-performance Liquid Chromatography-Hydride Generation Atomic Absorption Spectrometry. J. Anal. Atom. Spectrom. 1992, 7, 191-196. (22) Chana, B. S.; Smith, N. J. Urinary Arsenic Speciation by High-performance Liquid Chromatography/Atomic Absorption Spectrometry for Monitoring Occupational Exposure to Inorganic Arsenic. Anal. Chim Acta 1987, 197, 177-186. (23) Andreae, M. O. Determination of Arsenic Species in Natural Waters. Anal. Chem. 1977, 49, 820-823. (24) Comninellis, C.; Pulgarin, C. Electrochemical oxidation of phenol for wastewater treatment using tin dioxide anodes. J. Appl. Electrochem. 1993, 23, 108-112. (25) Stucki, S.; Kotz, R.; Carcer, B.; Suter, W. Electrochemical waste water treatment using high overvoltage anodes. Part II: Anode performance and applications. J. Appl. Electrochem. 1991, 21, 99-104. (26) Pettine, M.; Millero, F. J. Effect of metals on the oxidation of As(III) with HO. Mar. Chem. 2000, 70, 223-234. (27) Bejan, D.; Bunce, N. J. Electrochemical reduction of As(III) and As(V) in acid and basic solutions. J. Appl. Electrochem. 2003, 33, 483-489. (28) Tomilov, A. P.; Smetanin, A. V.; Chernykh, I. N.; Smirnov, M. K. Electrode Reactions Involving Arsenic and Its Inorganic Compounds. Russ. J. Electrochem. 2001, 37, 997-1011. (29) Krause, E.; Ettel, V. A. Solubilities and stabilities of ferric arsenate compounds. Hydrometallurgy 1989, 22, 311-337. (30) Devuyst, E. A.; Ettel, V. A.; Mosoiu, M. A. (Jpn. Kokai Tokkyo Koho). Purification of nickel electrolyte bath. Canadian Patent 1,197,490, 1985. (31) Snoeyink, V. L.; Jenkins, D. Water Chemistry; John Wiley and Sons: New York, 1980; p 384.
Received for review December 21, 2004 Revised manuscript received March 7, 2005 Accepted March 8, 2005 IE048769Y
