Simulation of Reduction of Cr(VI) by Fe(II) Produced Electrochemically in a Parallel-Plate Electrochemical Reactor

Transcription

1 E /2009/1566/E96/9/$25.00 The Electrochemcal Socety Smulaton of Reducton of Cr(VI) by Fe(II) Produced Electrochemcally n a Parallel-Plate Electrochemcal Reactor Sean Rayman* and R. E. Whte**,z Department of Chemcal Engneerng, Unversty of South Carolna, Columba, South Carolna 29028, USA A model s presented for the reducton of hexavalent chromum n a parallel-plate electrochemcal reactor va a homogenous reacton between CrVI and FeII generated at the ron anode. The effects of the space velocty of the feed soluton, the concentraton of supportng electrolyte, the dstance between the electrodes, and the cell potental on converson of CrVI to CrIII, are dscussed. Ths study ndcates that for reducton of CrVI usng FeII, the space velocty must be mantaned below 0.02 s 1 or the system becomes lmted by the rate of reducton of CrVI by FeII. Increasng the current densty by ncreasng the cell potental, ncreasng the amount of supportng electrolyte, and decreasng the dstance between the electrodes ncreases sngle pass converson of CrVI to CrIII; however, ncreasng the current densty also ncreases the specfc energy requred by the system The Electrochemcal Socety. DOI: / All rghts reserved. Manuscrpt submtted July 18, 2008; revsed manuscrpt receved November 25, Publshed Aprl 3, Hexavalent chromate, CrVI, wdely used for the producton of stanless steel, textle dyes, wood preservaton, and as antcorroson coatng, s hghly toxc to both anmals and plants. Concentratons of CrVI as low as 0.5 ppm n soluton and 5 ppm n sol can be toxc for plants; n contrast, the trvalent chromum s generally only toxc to plants at hgh concentratons and s a necessary mcronutrent for anmals. Currently, there are many ndustral stes n the world that have been contamnated wth CrVI due to neffcent contanment of process solutons contanng CrVI. In the U.S. there has been an ongong effort to clean up these stes by treatng the ground water on ste. 1 There are three ways to remove CrVI from the aqueous soluton: electroreducton, chemcal reducton, and electrochemcal reducton electrocoagulaton. Compared wth chemcal reducton, where an extra reducng agent such as ferrous salts s suppled from outsde the system to reduce an on, and electroreducton, where the on s reduced drectly at the cathode, electrocoagulaton uses electrcty to produce a reducng agent ferrous ons from an ron anode from a sacrfcal anode, and then the reducng agent s oxdzed va a homogeneous reacton. Several works have studed CrVI reducton. Fendforf and L, 2 Buerge and Hug, 3 and Sedlak and Chan 4 studed the chemcal reducton of CrVI n a batch reactor where the hexavalent chromate Na 2 CrO 4 was reduced by ferrous on FeCl 2 rapdly at room temperature. Rodrguez-Valdez et al. 5 expermentally studed the electrochemcal reducton of CrVI on a retculated vtreous carbon cathode. They expermentally show that converson of CrVI to CrIII ncreases at lower flowrates, lower ntal concentratons, and hgher potentals. Legrand, et al. 6 determned that the formaton of CrIII monolayers on green rust lmts the reducton of CrVI by permeable reactve barrers. Gheju and Lov 7 reported that the reducton of CrVI by ron metal occurs more rapdly at lower ph; however, as tme progresses, the ron metal surface becomes passvated. Parrsh and Newman 8,9 developed a two-dmensonal 2D model of a parallel-plate electrochemcal reactor PPER assumng that there was a dffuson boundary layer at each electrode and a wellmxed regon between the two electrodes. Whte et al. 10 developed a 2D mathematcal descrpton of a PPER for the electrownnng of copper on a cathode, relaxng Parrsh and Newman s assumpton of a well-mxed regon. Alkre and Lsus 11 developed a PPER model assumng a well-mxed regon lke Parrsh and Newman that ncluded multple homogenous reactons n whch water was reduced to form OH and Br was oxdzed, and both electrochemcally generated speces were reactants n some of the homogenous reactons. Mader et al. 12 smplfed the PPER model by assumng that * Electrochemcal Socety Student Member. ** Electrochemcal Socety Fellow. z E-mal: whte@engr.sc.edu the axal rate of change of the concentraton of every speces was constant, the one-step approxmaton. The one-step approxmaton greatly reduced computatonal tme. Mader and Whte n a later paper 13 modeled a PPER wth a separator and a sngle homogenous equlbrum reacton usng the prevously developed one-step approxmaton for the Zn/Br 2 cell. Nguyen et al. 14 showed that when the dstance between the electrodes s small compared to the length of the electrodes, mass transport n the axal drecton s domnated by convecton, allowng the axal dffuson and mgraton terms to be neglected whle mantanng the accuracy of the model. Nguyen et al. 15 modeled a process n whch a contnuously strred tank reactor and PPER were coupled, allowng a homogenous reacton to take place outsde of the PPER. Coleman et al. 16 modeled a PPER wth a separator accountng for gas evoluton and multple electrochemcal reactons takng place at both electrodes. Prasad et al. 17 present a PPER model for the electrochemcal reducton of ntrates and ntrtes, whch s based on the boundary-layer model of Parrsh and Newman. Georgadou, 18 usng fnte-dfference formulas, showed that at hgh Reynolds numbers, Re = 1200, the convecton n the axal drecton lmted the concentraton gradents normal to the electrodes to small dstances from the electrodes. Hcks and Fedkw 19 studed expermentally and theoretcally the electrolyss of acetate to ethane usng a PPER wth bulk turbulent flow wth dffuson boundary layers at each electrode. Drake et al present a steady-state and a transent two-phase PPER model for the Smmons process, assumng that there s a no-slp boundary condton between the lqud and vapor phases. Jha et al expand Drake s PPER model of the Smons process to nclude both potentostatc and galvanostatc operaton, nonsteady-state operaton, and slp between the lqud-vapor boundary layer. Nether Drake nor Jha provde concentraton dstrbutons normal to the bulk flud flow n the lqud or vapor phases. Ths paper presents a mathematcal model for a PPER that combnes dffuson and mgraton throughout the entre subdoman, multple homogenous reactons, and the oxdaton of the anode to form one of the homogenous reactants for the frst tme. The chemstry presented n ths paper s the reducton of CrVI to CrIII by FeII as gven by Fendorf. Model The proposed PPER s schematcally shown n Fg. 1. The contamnated soluton s fed from the left sde at x = 0. In order to mprove the conductvty of the aqueous soluton, NaCl supportng electrolyte s added to the feed soluton. The model s based on the followng assumptons: 1. Steady-state condtons exst no tme dependence. 2. Isothermal condtons exst. 3. Gas evoluton and precptaton effects are gnored. 4. The dlute soluton theory apples. 27

2 E97 = F RT, and = v avg L and substtuted nto Eq. 6 to obtan a materal balance wth dmensonless dependent and ndependent varables for the th speces Fgure 1. Schematc of a PPER and flud velocty profle. 5. Constant physcal and transport parameters exst. 6. The Butler Volmer equaton adequately descrbes the current densty of the electrochemcal reactons The Nernst Ensten equaton, u = D /RT, apples The flud s an ncompressble Newtonan flud n welldeveloped lamnar flow A small aspect rato S/L exsts between the wdth and length of the reactor S/L The materal balance equaton of the th speces s expressed as N = R 1 where the flux vector conssts of dffuson, mgraton, and convecton terms, respectvely N = D c z u c F + c v 2 For a system wth a total number of N speces, there are N materal balance equatons but N + 1 varables the concentraton of each speces c plus the soluton potental. The electroneutralty condton s used as the extra requred equaton N z c =0 3 =1 As mentoned above n assumpton 8, the velocty dstrbuton wthn the reactor s a well-developed lamnar flow and s gven as 10 v x =6v avg y S y2 S 2 and v y =0 5 In order for the Reynolds number to be greater than 2300, the average velocty of the soluton phase n a reactor wth a wdth of 91 cm gven n ths paper and an electrode gap of 3 cm, would be greater than 2 cm/s, 0.14 s 1. If the electrode gap was 0.3 cm, the average velocty would be less than 40 cm/s, 1.3 s 1, to ensure lamnar flow. In a 2D Cartesan coordnate system, the materal balance s derved by substtutng Eqs. 4 and 2 nto Eq. 1: 2 c D x 2 z F D RT xc +6v x avg y S y2 = R y S 2 c F z D RT yc Next, the followng characterstc quanttes are defned = S L, = x L, = y S, = c c,ref, x D 2 c y D z D = z D +6 v avg S2 c,ref R L S2 2 D 2 2 Usng assumpton 9, the terms wth the coeffcent of 2 can be elmnated, whch s equvalent to assumng that convecton domnates mgraton and dffuson n the axal drecton. 15 Applyng the one-step approxmaton 12 for the concentraton gradent n the axal convecton term yelds 6 2,feed D S z = 1 7 c,ref R 8 The one-step approxmaton approxmates / as =1, =0,/1 0 smplfed as /,feed. In essence, there s only one computatonal element n the axal drecton when the one-step approxmaton s appled. Mader 13 uses the one-step approxmaton for a PPER that contans a homogenous equlbrum reacton. By dong so, he s able to elmnate the source terms from the materal balance equatons. As shown later n ths work, the frst homogenous reacton s consdered to go to completon, and thus not all the source terms can be elmnated from the set of materal balances. Because the equlbrum condton cannot be assumed for the present work, the one-step approxmaton wll need to be relaxed n future work. The length of the reactor, L, used n ths work s 30.5 cm. In order to reman wthn the lmts of assumpton 9, the maxmum dstance between the electrodes, S, should be no more than 3 cm. Equaton 8 s now a smplfed materal balance for the th speces n one spatal dmenson wth the followng boundary condtons: at =0anode N * n, = S s + z = c,ref D,j n,j j n j F, for Fe 2+ 0, for the other speces at =1cathode N * n, = S s z = c,ref D,j n,j j n j F, for OH 0, for the other speces 9 10 and at both = 0 and = 1 the electroneutralty condton s satsfed z c,ref =0 11 The current that flows through the reactor s a result of the electrochemcal reactons that take place at the anode and cathode. At the anode, the only electrochemcal reacton consdered s the oxdaton of Fe 0 to Fe +2, descrbed by Fe Fe 2+ +2e At the cathode, the only electrochemcal reacton consdered s the reducton of H 2 OtoOH and H 2, descrbed by

3 E98 2H 2 O+2e H 2 +2OH In Eq. 10 and 11, the normal component of the current densty of electrochemcal reacton j can be determned usng the Butler Volmer equaton: 27 At the anode n,j = 0,j,ref p j exp a,j F RT V a a,0 j,ref F RT U q j exp c,j F RT V a a,0 F RT j,ref U 12 and at the cathode n,j = 0,j,ref p j exp a,j F RT V c c,0 j,ref F RT U where Table I. Transport propertes and reference concentraton. Speces q j exp c,j F RT V c c,0 F RT U j,ref 13 U j,ref = U j RT n j F s j ln c,ref 0 14 Tables I-IV provde the values of the constants and the range of values used for the parameters nvestgated n ths paper. A base case, represented n Fg. 2-5 as a bold gray lne, was establshed wth the followng parameters: v avg = 0.50 cm/s, V a = 0.60 V, S = cm, and ConcSE = 1.00E 7mol/cm 3. In the case of these smulatons, the concentraton of the supportng electrolyte, ConcSE, was changed by changng the concentraton of Cl n the feed soluton. The Na + concentraton was calculated by assumng that the electroneutralty held n the feed soluton. The concentraton of most speces remaned below 10 3 mol/cm 3 even at the electrodes, whch ndcates that the soluton s dlute, assumpton 4, and Table II. Knetc and thermodynamc propertes of electrochemcal reactons. Reacton j 10 5 D 10 7 c,ref cm 2 /sec 28 z mol/cm ,j,ref A/cm 2 a,j c,j n j U o j V Table III. Stochometry of the electrochemcal reactons. c,feed mol/cm 3 Na Adjusted to mantan electroneutralty of the feed Cl Fe H OH CrO Fe Cr Reacton Speces s j p j q j 1 Fe OH that the moblty of the ons s proportonal to the dffuson coeffcent, assumpton 7. Source Terms n the Materal Balance Equaton In the consdered system, nne onc speces are ncluded: Na +, Cl,Fe 2+,H +,OH, CrO 2 4,Fe 3+, and Cr 3+. All the onc speces except Na + and Cl are nvolved n the homogeneous reactons expressed as Eq below CrO Fe 2+ +4H 2 O 3Fe 3+ +Cr 3+ +8OH 15 H + +OH H 2 O Cr OH CrOH 3 Fe OH FeOH Fe OH FeOH 2 19 Fendorf and L 2 determned that CrVI s reduced by FeII accordng to the knetc rate expresson r 1 = k Cr Fe CrO where k Cr = mmol 0.6 mn 1 L The products of Reacton 1, Cr 3+,Fe 3+, and Fe 2+, react wth OH to form CrOH 3, FeOH 3, and FeOH 2 as descrbed by Eq , respectvely. For Eq , the knetc reacton rates are expressed as fast equlbrum reactons r 2 = k 2,f c H +c OH k 2,b c H2 O 3 r 3 = k 3,f c Cr 3+c OH 3 r 4 = k 4,f c Fe 3+c OH 2 r 5 = k 5,f c Fe 2+c OH k 3,b c CrOH3 k 4,b c FeOH3 k 5,b c FeOH2 where the backward and forward rate constants are related by the followng dssocaton constants K w = k 2,b k 2,f, K CrOH3 = k 3,b k 3,f, K FeOH3 = k 4,b k 4,f, K FeOH2 = k 5,b k 5,f For the purpose of ths work, the authors assumed that the dssoluton of the sold products was extremely slow compared to the formaton of the sold products,.e., the forward reactons progressed much faster than the backward reactons. Equatons allow for the formaton of solds n the soluton phase; however, t s assumed Table IV. Operaton condtons and other parameters. Parameter Value Unt T K S cm V a V v avg cm/s V c 0.0 V g/cm 3 K w mol/cm 3 2 K FeOH mol/cm 3 4 K CrOH mol/cm 3 4 K FeOH mol/cm 3 3 k Cr cm 3 /mol 0.6 /s k 2,b a cm 3 /mol/s k 3,b a cm 3 /mol 3 /s k 4,b a cm 3 /mol 3 /s k 5,b a cm 3 /mol 2 /s a Chosen arbtrarly.

4 E99 Fgure 2. Dmensonless concentraton profle of CrVI as current densty ncreases. that none of the sold partcles coagulate nsde of the reactor assumpton 3. By takng nto account the homogenous reactons, the source term n the materal balance equaton for each speces s specfed as R Na + =0 21 R H + = r 2 R OH =8r 1 r 2 3r 3 3r 4 2r 5 R CrO4 2 = r R Cl =0 22 R Fe 3+ = r 4 27 R Fe 2+ = 3r 1 r 5 23 R Cr 3+ = r 3 28 Fgure 3. Dmensonless concentraton profle of CrVI as concentraton of supportng electrolyte ncreases.

5 E100 Fgure 4. Dmensonless concentraton profle of CrVI as electrode gap ncreases. R CrOH3 = r 3 R FeOH3 = r 4 R FeOH2 = r Results and Dscusson The effects of the cell potental, concentraton of supportng electrolyte n the feed soluton, dstance between the electrodes, space velocty of the feed soluton on the current densty, converson of CrVI to CrIII, and specfc energy requred were nvestgated by numercally solvng Eqs at dfferent V a, C Cl,feed, S, and v avg. As the current densty s ncreased n the reactor by an ncrease n appled potental from 0.11 to 6.0 V, more FeII s generated at the anode, favorng the progresson of Reacton 15, Fg. 2. In addton, as the current densty ncreases, the flux due to mgraton of chromate toward the anode ncreases. The mgraton of chromate toward the anode and the consumpton of chromate by FeII mgratng away from the anode allows for the maxmum concentraton of chromate to be found close to the center of the reactor, as shown n Fg. 2. Fgure 2 also ndcates that as the current densty s ncreased, the over all concentraton of CrVI extng the reactor decreases. Usng Fg. 2, t s clear that for all but the hghest appled voltage, the power densty of the reactor s less than 10 2 W/cm 2, thus valdatng the assumpton of sothermal operatng condtons. Also, at an appled cell potental of 5.8 V, the current densty s 2.15 ma/cm 2. If all of the H 2 exted the reactor n the gas phase, the volumetrc flowrate of gas extng the reactor would be 0.35 cm 3 /s; thus, the frst part of assumpton 3 s reasonable. The current densty Fgure 5. Dmensonless concentraton profle of CrVI as space velocty ncreases.

6 E101 Converson of Cr(VI) S=2.50E-001 cm S=3.18E-001 cm S=4.50E-001 cm S=6.50E-001 cm S=8.50E-001 cm S=1.05E+000 cm S=1.25E+000 cm S=1.85E+000 cm S=2.05E+000 cm S=2.25E+000 cm S=2.85E+000 cm S=3.00E+000 cm τ s -1 Fgure 6. Converson of CrVI as a functon of space velocty as electrode gap ncreases. of the reactor can also be ncreased whle mantanng a constant appled potental by addng supportng electrolyte to ncrease the conductvty of the feed soluton, Fg. 3. The results shown n Fgure 3 ndcate that as more supportng electrolyte s added to the feed soluton, the peak concentraton of chromate n the reactor moves from the anode toward that cathode, because more of the current n the soluton s carred by the more-moble chlorde on. Yet another way to ncrease the current densty s to decrease the dstance between the electrodes, Fg. 4. Whle actvely changng the dstance between the electrodes whle the cell s n operaton s not currently practcal, the dstance between the electrodes should be consdered a desgn parameter. The results shown n Fg. 4 ndcate that at dstances between electrodes greater than 1 cm at the base space velocty of s 1, there s a well-mxed regon that exsts n the reactor. Fgures 2-4 llustrate that when the current densty ncreases, as a result of ncreasng the appled potental, ncreasng the Fgure 7. Current densty as a functon of concentraton of supportng electrolyte as the electrode gap ncreases.

7 E102 Fgure 8. Converson of CrVI as a functon of space velocty as concentraton of the supportng electrolyte ncreases. concentraton of the supportng electrolyte, or decreasng the dstance between the electrodes, the bulk concentraton of chromate s decreased. By ncreasng the dstance between the electrodes, S, or by ncreasng the space velocty,, the convecton term n Eq. 8 begns to domnate the dffuson and mgraton terms over much of the reactor, gvng rse to the sharp ncreases n chromate concentraton near the anode seen n Fg. 4 and 5. As the space velocty ncreases, Fg. 5, dffuson- and mgraton-domnated transport regons move toward the electrodes, where the convecton term becomes small. As the regon where dffuson- and mgraton-domnated transport becomes thnner, the rate of dffuson n that regon becomes much greater due to an ncrease of the concentraton gradent near the electrodes, decreasng the concentraton of FeII at the anode. Further, the rate of dffuson of OH toward the anode s much greater than that of CrO 4 2, resultng n favorable condtons for homogenous Reacton 5, Eq. 19, to consume most of the avalable FeII. Once enough of the OH ons have been consumed to slow down Reacton 5, Eq. 19, some FeII ons become avalable to react wth Fgure 9. Converson of CrVI as a functon of space velocty as current densty ncreases.

8 E103 Fgure 10. Specfc energy requred as a functon of converson as the electrode gap ncreases. CrO 2 4, decreasng the concentraton of CrO 2 4 near the anode. The same trend of an abrupt ncrease n chromate concentraton near the anode wth a much broader peak n the center of the reactor occurrng at large dstances between electrodes s llustrated n Fg. 4. Fgure 6 further llustrates that as the dstance between electrodes s ncreased for a gven space velocty, the converson of CrVI to CrIII the rato of the feed concentraton mnus outlet concentraton to the feed concentraton decreases. Lkewse, for a gven dstance between electrodes, as the space velocty decreases, the converson of CrVI to CrIII ncreases. Fgures 2 5 also llustrate that there are practcal space veloctes, less than 0.1 s 1, and electrode gaps, less than 1 cm, for whch the reactor s not well-mxed. Whle mantanng a constant appled cell potental, the concentraton of supportng electrolyte was vared wth dfferent electrode gaps to determne the effect on current densty. Based on the results llustrated n Fg. 7, the reducton of dstance between electrodes by a factor of 5 has a much greater effect on current densty than ncreasng the concentraton of supportng electrolyte by 4 orders of magntude. Whle t was not studed n ths work, there s a practcal lmtaton to how close together the electrodes can be placed. There s also a practcal desgn lmtaton n the space velocty of the reactor no matter how much supportng electrolyte s added to the system, Fg. 8 or how much the appled potental s practcally ncreased, Fg. 9. In order to mantan hgh sngle-pass converson, the space velocty of the reactor must be less than 0.02 s 1. Otherwse, at space veloctes greater than 0.02 s 1, the converson of CrVI to CrIII becomes reacton-rate-lmted usng the knetc expresson for the reducton of CrVI by FeII, as suggested by Fendorf. 2 The specfc energy requred by the reactor to reduce one mole of CrVI to CrIII, gven by V a V c L E = 32 C CrO4 2,feed C CrO4 2,outSV avg was calculated as a functon of appled cell potental at dfferent supportng electrolyte concentratons. The results dsplayed n Fg. 10 ndcate that between 0 and 3 V, the specfc energy requrements ncrease wth ncreasng amount of supportng electrolyte, wth the lowest energy requrement beng 57.5 J/mol of CRVI reduced. However, at cell potentals greater than 4 V, the concentraton of supportng electrolyte has lttle effect on the specfc energy requrements. Conclusons A model where dffuson and mgraton are allowed to occur over the entre subdoman has been developed, showng that there are varatons n concentraton profles n the entre lamnar subdoman. The current densty of the reactor can be ncreased by ncreasng the electrcal potental of the cell, ncreasng the amount of supportng electrolyte, or decreasng the dstance between the electrodes. By ncreasng the current densty, more FeII s produced from the anode. The converson of CrVI to CrIII can be ncreased by ncreasng the current densty as long as the converson s not reacton-lmted or by reducng the space velocty of process flud as long as there s enough FeII beng produced to reduce all the CrVI to CrIII. Increasng the amount of supportng electrolyte ncreases the amount of FeII produced at the anode, whch ncreases converson of chromate. Further, by ncreasng the amount of supportng electrolyte, the specfc energy of the system to convert a mole of CrVI to CrIII also ncreases. Thus, t s necessary to balance the gans of addng supportng electrolyte on converson wth the ncreased energy requrements that result n ncreased supportng electrolyte concentraton. Acknowledgment Ths work was supported n part by Flour Hanford, Inc. Unversty of South Carolna asssted n meetng the publcaton costs of ths artcle. Lst of Symbols c concentraton of speces, mol/cm 3 c,ref reference concentraton of speces, mol/cm 3 c supp concentraton of the supportng electrolyte NaCl, mol/cm 3 D dffuson coeffcent of speces, cm 2 /cm Ecell potental, V a V c V F Faraday s constant, C/mol n,j normal component current densty due to electrochemcal reacton j, A/cm 2 0,j,ref exchange current densty of reacton j at reference concentraton, A/cm 2

9 E104 Greek k Cr reacton rate constant for Reacton 1 k,b backward reacton rate constant of reacton k,f forward reacton rate constant of reacton K dssocaton constant of homogenous reacton K w water equlbrum constant, mol/cm 3 2 L electrode length, cm N* n,i the normal component of the dmensonless flux of speces p j anodc reacton order of speces n reacton j q j cathodc reacton order of speces n reacton j r homogeneous reacton rate of reacton, mol/cm 3 s R unversal gas constant, J/mol K s,j stochometrc coeffcent of speces n reacton j S electrode gap, cm SE Supportng electrolyte SHE. standard hydrogen electrode T temperature, K u moblty of speces, cm 2 mol/j s U j standard oxdaton potental of electrochemcal reacton j vs SHE, V U j,ref thermodynamc equlbrum potental of electrochemcal reacton j at the reference concentratons, V v velocty vector of the soluton, cm/s v avg average velocty of the electrolyte, cm/s V a anode potental, V V c cathode potental, V x axal coordnate, cm y normal coordnate, cm z charge number of speces a,j anodc transfer coeffcent for reacton j c,j cathodc transfer coeffcent for reacton j soluton potental, V rato of the gap between the electrodes to the electrode length dmensonless normal coordnate dmensonless concentraton of speces dmensonless soluton potental 0 solvent densty, g/l rato of the average soluton velocty n the axal drecton to the electrode length, space velocty, s 1 dmensonless axal coordnate References 1. J. Guertn, J. A. Jacobs, and C. P. Avakan, Chromum(VI) Handbook, p. 761, CRC Press, Boca Raton, FL S. E. Fendorf and G. L, Envron. Sc. Technol., 30, I. J. Buerge and S. J. Hug, Envron. Sc. Technol., 31, D. L. Sedlak and P. G. Chan, Geochm. Cosmochm. Acta, 61, F. Rodrguez-Valadez, C. Ortz-Exga, J. G. Ibanex, A. Alatorre-Ordaz, and S. Guterrez-Granados, Envron. Sc. Technol., 39, L. Legrand, A. E. Fgugu, F. Mercer, and A. Chausse, Envron. Sc. Technol., 38, M. Gheju and A. Iov, J. Hazard. Mater., 135, W. R. Parrsh and J. Newman, J. Electrochem. Soc., 116, W. R. Parrsh and J. Newman, J. Electrochem. Soc., 117, R. E. Whte, M. Ban, and M. Rable, J. Electrochem. Soc., 130, R. C. Alkre and J. D. Lsus, J. Electrochem. Soc., 132, M. J. Mader, C. W. Walton, and R. E. Whte, J. Electrochem. Soc., 133, M. J. Mader and R. E. Whte, J. Electrochem. Soc., 133, T. V. Nguyen, C. W. Walton, R. E. Whte, and J. V. Zee, J. Electrochem. Soc., 133, T. V. Nguyen, C. W. Walton, and R. E. Whte, J. Electrochem. Soc., 133, D. H. Coleman, R. E. Whte, and D. T. Hobbs, J. Electrochem. Soc., 142, S. Prasad, J. W. Wedner, and A. E. Farell, J. Electrochem. Soc., 142, M. Georgadou, J. Electrochem. Soc., 144, M. T. Hcks and P. S. Fedkw, J. Appl. Electrochem., 28, J. A. Drake, C. J. Radke, and J. Newman, J. Electrochem. Soc., 145, J. A. Drake, C. J. Radke, and J. Newman, Ind. Eng. Chem. Res., 40, J. A. Drake, C. J. Radke, and J. Newman, Ind. Eng. Chem. Res., 40, J. A. Drake, C. J. Radke, and J. Newman, Chem. Eng. Sc., 56, K. Jha, G. L. Bauer, and J. W. Wedner, J. Electrochem. Soc., 145, K. Jha, G. L. Bauer, and J. W. Wedner, J. Appl. Electrochem., 30, K. Jha, G. L. Bauer, and J. W. Wedner, J. Appl. Electrochem., 31, J. Newman and K. E. Thomas-Alyea, Electrochemcal Systems, p. 647, John Wley & Sons, Hoboken, NJ CRC Handbook of Chemstry, and Physcs, CRC Press, Boca Raton, FL G. Prentce, Electrochemcal Engneerng Prncples, p. 296, Prentce-Hall, New York 1991.