electrokientic treatment of sludge sewage

Electrokinetic enhancement removal of heavy metals fromindustrial wastewater sludgeChing Yuana,*,Chih-Huang WengbaDepartment of Civil and Environmental Engineering,National University of Kaohsiung,No.700,Kaohsiung University Road,Nan-Tzu District,Kaohsiung City 811,Taiwan,ROCbDepartment of Civil and Ecology Engineering,I-Shou University,No.1,Sec.1,Hsueh-Cheng Road,Ta-Hsu Hsiang,Kaohsiung County 840,Taiwan,ROCReceived 17June 2005;received in revised form 20February 2006;accepted 20February 2006AbstractAn enhanced electrokinetic process for removal of metals (Cr,Cu,Fe,Ni,Pb,Zn)from an industrial wastewater sludge was per-formed.The electrokinetic experiments were conducted under a constant potential gradient (1.25V cm À1)with processing fluids of tap water (TW),sodium dodecylsulfate (SDS)and citric acid (CA)for 5days.Results showed that metal removal efficiency of heavy metals for EK-TW,EK-SDS and EK-CA systems are 11.2–60.0%,37.2–76.5%,and 43.4–78.0%,respectively.A highest metal removal performance was found in EK-CA system.The removal priority of investigated metals from sludge by EK process was found as:Cu >Pb >Ni >Fe >Zn >Cr.The results of sequential extraction analysis revealed that the binding forms of heavy metals with sludge after electrokinetic process were highly depend upon the processing fluid operated.It was found that the binding forms of metals with sludge were changed from the more difficult extraction type (residual and sulfate fractions)to easier extraction types (exchangeable,sorbed,and organic fraction)after treatment by electrokinetic process.Results imply that if a proper treatment technology is followed by this EK process to remove metals more effectively,this treated sludge will be more beneficial for sludge utilization afterwards.Before it was reused,the risk associated with metals of more mobile forms to the environment need to be further investigated.The cost analysis was also evaluated for the investigated electrokinetic systems.Ó2006Elsevier Ltd.All rights reserved.Keywords:Citric acid;Electrokinetic process;Heavy metal;Sludge;Surfactant1.IntroductionSludge derived from industrial wastewater treatment process generally accumulates appreciable amounts of met-als,such as cadmium,chromium copper,lead,nickel,and zinc.The toxicity of metal in the environments is critically dependent on its chemical forms.The principal forms of metal in the sludge are normally associated with soluble,precipitated,and co-precipitated oxides,adsorbed,and biological residues (Alibhai et al.,1985).The identification of metal forms in the sludge can be divided into exchange-able,sorbed,organic carbonate,and sulfide fractions by sequential extraction procedure (Oake et al.,1984;David-son et al.,1994).It has been caused considerable attention for heavy metal-laden sludge due to the high desire for environmentally safe and effective disposal/reuse.Once the metals are included in the ecological cycle,it can cause chronic illness due to metal accumulation in the bodies of living organisms (Sengupta,1999).Hence,removal of heavy metal from industrial sludge before disposal/reuse is a necessary step to achieve a sustainable sludge treat-ment.A process optimization for metal-laden sludge treat-ment should achieve two goals:(1)effectively remove heavy metals from sludge,and (2)potently convert the binding form of metals with sludge to an easier extractable form for further treatment.0045-6535/$-see front matter Ó2006Elsevier Ltd.All rights reserved.doi:10.1016/j.chemosphere.2006.02.050*Corresponding author.Tel.:+88675919178;fax:+88675919376.E-mail address:caroline@.tw (C.Yuan)./locate/chemosphereChemosphere xxx (2006)xxx–xxxThe electrokinetic(EK)process has been demonstrated to be successful and cost-effective in removing inorganic contaminants from soil in many bench-andfield-scale studies(Acar and Alshawabkeh,1993;Kim and Kim, 2001;Virkutyte et al.,2002;Gent et al.,2004;Altin and Degirmenci,2005).Few researches were found for metals removal from sludge by electrokinetic process(Kim et al., 2002).This process involves the application of an electrical field across a porous medium(sludge)to induce the move-ment of electrolyte solution and the transport of soluble contaminants toward the electrodes.As the EK process starts,the movement of H+ions generated from electroly-sis of H2O at the anode advances through sludge mass and toward the cathode,also the migration of charged ions is toward the opposite electrodes.Removal of contaminants from porous media by EK process is accomplished by the mechanisms of electrolysis of water,electroosmosis,elec-trophoresis and electromigration(Acar and Alshawabkeh, 1993).The electromigration,electroosmosis,and electro-phoresis are the principal mechanisms responsible for the electrokinetic removal of metals from sludge and soils for neutral and charged processingfluid(Acar and Als-hawabkeh,1993;Puppala et al.,1997;Kim et al.,2002). During electrokinetic process,the contaminants migration in the soil is simultaneously achieved by phenomena such as sorption/desorption,precipitation,and dissolution (Puppala et al.,1997).Among the mechanisms investi-gated,electrolysis of water caused the pH around the cath-ode areas rises to greater than11,whereas around the anode area it falls to below2(Reddy and Shirani,1997; Weng and Yuan,2001;Altin and Degirmenci,2005).Con-sequently,the movement of contaminants in the cathode areas is hindered by precipitation,whereas they are acceler-ated due to dissolution and desorption reactions in the anode area(Weng and Yuan,2001).To enhance the reme-diation efficiency of this process,acid solutions including citric acid,acetic acid or complexing substances such as ethylenediamine tetra acetic acid(EDTA)were added in the EK systems(Yang and Lin,1998;Altin and Degirm-enci,2005).Also,to overcome the decrease of current den-sity resulted from using acid electrolyte,an ion charge membrane was used to assist electrokinetic performance (Kim et al.,2005).However,the high operation cost might become a major drawback in application.Surfactants have been mostly used as additives in phase separation processes for remediation of organic com-pounds contaminated soil because its asymmetric structure will enhance the aqueous solubility and mobility of the organic compounds(Brown et al.,1999;Harwell et al., 1999;Chu and Kwan,2003).As such,several researches reported enhanced electrokinetic remediation efficiency of hydrophobic organic compounds in soil by introducing surfactant recently(Reddy and Saichek,2003;She et al., 2003;Yuan and Weng,2004).Moreover,researches were shown that the surfactant could be used to enhance the heavy metals desorption from soil and sludge(Doong et al.,1998;Mulligan et al.,2001).It might largely because of the hydrogen bonding and electrostatic forces between surfactants and metals.The free cations will also be precip-itated with anionic surfactant monomers and separated from the aqueous phase(Stellner and Scamehorn,1989; Jafvert and Heath,1991).Thereby,Sawada et al.(2003) used humic acid,which possessed surfactant characteris-tics,to enhance the amount of Cu(OH)2removed from soils three times larger than those in the absence of humic acid.It might result from the alteration of soil surface properties by humic acid.Kaya and Yukselen(2005)were reported that the zeta potential of soils was decreased by anionic surfactant in EK process,which was essential to prevent precipitation of heavy metals and surfactant during the treatment and improve remediation efficiency.Those results were just displayed an initial view point of surfac-tant application in electrokinetic removal of metals.How-ever,few literatures were found to deeply investigate the electrokinetic removal of metals by surfactant in soil,not even to say in sludge.This objective of this study was to investigate the enhancement of electrokinetic removal of a metal-laden sludge with two types of processingfluid(surfactant and acid solution).A sequential extraction procedure was per-formed to determine the binding forms of heavy metal with sludge and cost analysis was also evaluated.2.Materials and methods2.1.Sludge sampleThe sludge sample was collected right after a belt press dewatering machine from an industrial wastewater treat-ment plant in southern Taiwan.The basic properties of this sludge sample are summarized in Table1.The moisture content of sludge was69.7%,which was determined by the lost weight fraction at a temperature of105°C for over-night.The ash content was14.4%,which was determined as the residual fraction after combusted in furnace at 800±50°C.The organic content was50.1%,which was determined as the lost fraction at a temperature of430°C. The metal content in the sludge was determined by wet extraction method.Sludge sample(2g)was digested with 20ml of HNO3/HCl solution(1:3,v/v)in a250ml Teflon beaker at95°C for4h.The concentration of Cr,Cu,Fe, Ni,Pb,and Zn in the digested solution was analyzed by means of atomic emission spectrophotometer with induc-tively coupled plasma source(ICP,Perkin Elmer Optima 2000DV,USA)at a wavelength of268,324,260,463, 220,and428nm,respectively.As shown in Table1,up to 53000mg kgÀ1was found for Fe in sludge and a concentra-tion range of2400–15000mg kgÀ1was found for other metals.2.2.ProcessingfluidsSodium dodecylsulfate(SDS)and citric acid were selected as the tested processingfluids in this study.The2 C.Yuan,C.-H.Weng/Chemosphere xxx(2006)xxx–xxxconcentrations of SDS and citric acid were0.024M,and 0.4M for EK experiments,respectively,which was calcu-lated as3.0times of critical micelle concentration of SDS and1.4times of equivalents of metals involved(except Fe).The SDS is an anionic surfactant with a dodecyl group in the hydrophobic moieties and a sulfate group in the hydrophilic moieties,which is characterized as binding counterions on the micelle’s surface.Since SDS is biode-gradable by soil and/or aquatic microorganisms(Swisher, 1970),it is friendly used for subsurface remediation.Citric acid is an effective metal complexing regent and it performs a low pH environment in solution.As a consequence,metal precipitation would be hindered and dissolution reaction is become dominant.The other professingfluid was tapwater,which mainly consists of anions of SO2À4and ClÀas well as cations of Ca2+and Mg2+.The equivalent of anions and cations in tap water were2.26meq,respec-tively,which was followed charge balance relationship. The properties of processingfluids are summarized in Table1.2.3.Sequential extractionTo determine the binding forms of heavy metals with sludge before/after EK treatment,a sequential extraction procedure followed after Oake et al.(1984)was conducted in this research.Five solutions of1.0M KNO3,0.5M KF, 1.0M Na2P2O7,0.1M Na2EDTA,and6.0N HNO3were used in the extraction to representfive types of metal bind-ing:exchangeable,sorbed,organic,carbonate,and sulfate, respectively.The difference between the quantity of metal recovered by the sequential extraction and total metals concentration determined by HNO3/HCl digestion method (TEPA,2001)was defined as residual fraction.2.4.EK experimentsThe EK experiments were conducted in an acrylic cell (Fig.1)of4.2cm(u)·22cm(L),consisting of three com-partments:cathode reservoir(5cm in length),anode reser-voir(5cm in length),and sludge specimen chamber (12cm in length).Afiberglassfilter paper(0.45l m,Table1Characteristics of experimental materialsI.SludgeCharacteristics Values Metal contents Values(mg kgÀ1) Moisture content(%)69.7Cr6600Ash(%)14.4Cu4600 Combustible(%)15.9Fe53000 Organics(%)50.1Ni2500pH 6.82Pb10500Zn15100II.Tap waterAnions Values(meq)Cations Values(meq)SO2À41.14Ca2+ 1.32ClÀ 1.06Mg2+0.61NOÀ30.06Na+0.30K+0.03Total 2.26Total 2.26pH7.79III.SDS and citric acidCharacteristics Sodium dodecyl sulfate a(SDS)Citric acid aCharge Anionic surfactant NeutralFormula C12H25SO4Na C6H8O7ÆH2OMolecular weight(g/mol)288210cmc(mM)8.0–pH7.70 2.45Hydrophile–lipophile balance40–Water solubility10%in water Complete miscibleCompany Sigma,USA Nihon Shiyaku Industries Ltd.,JapanC.Yuan,C.-H.Weng/Chemosphere xxx(2006)xxx–xxx347mm,GS25,Advantec,Japan)was used to separate the sludge from processingfluid at each end of the cell.Two sets of graphite rod electrodes(0.64cm in diameter,AGKSP, Union Carbon Co.,USA)were installed at each side of sludge specimen and right behind thefilters.The processing fluid was initially placed into both anode and cathode reser-voirs and replenished in the anode reservoir every half day. Three EK experiments were repeated two times under a constant potential gradient of1.25V cmÀ1for5d with tap water,SDS,and citric acid as processingfluids,respec-tively.The electric current,reservoir pH,concentrations of heavy metals,and the quantity of electroosmosticflow were monitored during the test periods.After EK treatment,the sludge specimen was removed from the cell and sectioned equally intofive segments.Sludge pH and residual metal profiles along the sludge specimen with sequential extrac-tion procedure were determined at the end of each test. 3.Results and discussionThe experimental results are summarized in Table2and further discussed in the following sections.3.1.Electroosmosis permeabilityIn EK process,the movement of electrolyte solution dri-ven by an electricalfield is considered to be the major mechanism leading to the removal of metals from the sludge,which carries the removed metals toward the elec-trodes.The movement of electrolyte solution,i.e.,electro-osmoticflow,Q e(ml dÀ1),for a cylindrical soil core is directly proportional to the applied electric potential gradi-ent,i e(V cmÀ1):Qe¼K eÁi eÁAð1Þwhere A(cm2)is cross-section area of soil core and K e (cm2VÀ1sÀ1)is electroosmosis permeability.The experi-mental results showed that the direction of electroosmotic flow was from anode side toward cathode side.It was ob-served that the amount of electroosmoticflow was in-creased quickly in thefirst3-day and however sharply decreased in the latter.The total electroosmoticflow was 57.5,78.0,62.0ml for EK-TW,EK-SDS,and EK-CA sys-tems,respectively(Table2).The result implied that EO permeability in the initial treatment period was high,and then it was lowed in the later owing to the clogging of soil pore by metal oxide precipitations.After EK treatment,the K e values calculated according to Eq.(1)are listed in Table 2.A relatively higher electroosmosticflow was found in the EK-SDS system(K e=1.0·10À5cm2VÀ1sÀ1),which was in agreement with Shapiro and Probstein(1993)and Lang-eman(1994).The rather high K e value can be attributed to the micelle formation of SDS.A similar K e values of7.7·10À6cm2VÀ1sÀ1and8.3·10À6cm2VÀ1sÀ1were found in tap water system(EK-TW)and citric acid system (EK-CA),respectively.3.2.Reservoir/sludge pHs and current densityThe cathode reservoir pHs were about9.8–12.0in both EK-SDS and EK-TW systems,whereas2.0–2.5for EK-CA system.And the pHs in anode reservoir were about1.5–2.5 for all EK systems.These values were in good agreement with thefinding of many EK studies(Alshawabkeh et al., 1999;Yang and Long,1999;Weng and Yuan,2001;Weng et al.,2003)and this variation of reservoir pH is attributed to the electrolysis of water.In EK-CA system,the cathode reservoir pH was not affected by OHÀdue to strong acidity of citric acid.The generation of H+and OHÀunder an applied elec-tricfield will result in movement of acid and basic fronts in EK cell and will change the sludge pH drastically during EK process(Kim et al.,2002;Reddy and Chinthamreddy, 2003).The sludge pH profiles along EK cell for three EK systems are shown in Fig.2a.The acid front generated at anode reservoirflushed across the sludge specimen,conse-quently lowering the sludge pH value from6.8to2.1–5.9in the anode area for all cases.At the cathode side,the migra-tion of OHÀadvances toward the anode and the concentra-tion differences of OHÀbetween the reservoirs and sludge would make the sludge pH hardly decrease even though the H+ion was continuously swept into this region.The result indicates that sludge pHs near the cathode were around7.8–8.2for EK-SDS and EK-TW systems.How-ever,in EK-CA system,the sludge pH near cathode was still as low as2.5due to relatively low pH value in the res-ervoirs.Virkutyte et al.(2002)reported that the metal-hydroxide precipitation was at a minimum level if the pH value is below4.5.It appears that would be active across the whole EK cell in EK-CA system,whereas such reactionTable2Experimental results of EK systemsSystem a Processingfluid b K e(cm2VÀ1sÀ1)Electroosmoticflow rate(ml)Total powerconsumption(kWh tonÀ1)Metal removal efficiency c(%)Cr Cu Fe Ni Pb ZnEK-TW Tap water7.7·10À657.512229(1.0)35(1.0)19(1.0)60(1.0)11(1.0)21(1.0) EK-SDS SDS(0.024M) 1.0·10À578.014040(1.4)41(1.2)39(2.1)77(1.3)51(4.6)37(1.8) EK-CA Citric acid(0.4M)8.3·10À662.015553(1.8)43(1.2)60(3.2)78(1.3)59(5.4)67(3.2)a All EK systems were operated under a constant potential gradient of1.25V cmÀ1for5d.b Concentration of SDS=cmc of SDS·3.0;Concentration of citric acid=equivalents of metals involved(except Fe)·1.4.c The value in the parentheses is the ratio of metal removed in EK systems compared to that in EK-TW system.4 C.Yuan,C.-H.Weng/Chemosphere xxx(2006)xxx–xxxwould only be active near the anode area in both EK-TW and EK-SDS systems.Fig.2b shows the results of current density as a function of time.At the beginning of the test,the current density was high for each system and it decreased thereafter.As shown,after a3-day treatment,it decreased to a stable range of0.32–0.36mA cmÀ2and maintained for the rest of treatment time.This might result from clogging of soil pores by metal oxides precipitation.3.3.Heavy metals removalAs seen in Table2,the removal efficiency of metals were in the range of11–60%,37–77%,and43–78%for EK-TW, EK-SDS and EK-CA systems,respectively.A removal effi-ciency of50%or more was found for Zn in EK-TW system, Ni and Pb in EK-SDS system,and Cr,Fe,Ni,Pb,and Zn in EK-CA system.Moreover,the results were also indi-cated that0.11mol metals was removed from EK-CA sys-tem totally,which was equivalent to1.5and2.8times larger than that from EK-SDS and EK-TW systems, respectively.Thereby,citric acid can be considered as a rel-ative better processingfluid operated for metal removal in this research.As seen in Fig.2a,in EK-CA system,since the cathode solution was apparently neutralized by citric acid regardless of electrolytic reaction that could generate OHÀion,a lower sludge pH was found throughout the EK cell.Such a phenomenon resulted in avoiding genera-tion and transport of high concentration of OHÀions into soils and to enhance electrodeposition of metals(Puppala et al.,1997;Gent et al.,2004).In other words,the citric acid can be treated as electrode conditioning to enhance metal removal.Among the metals,nickel was the easiest removal metal, while Pb,Zn and Cu were the most difficult to be removed in EK-TW,EK-SDS,and EK-CA systems,respectively. The removal efficiency of Fe for EK-SDS and EK-CA sys-tems were around39–60%.However,high Fe removal is not beneficial for land utilization because Fe is an essential element for plant growth.It was indicated that the binding capacity of SDS with Pb was1.6times greater than Cu (Yuan,1999).In such a case,a relatively higher removal efficiency enhancement for Pb(4.6–5.4times)was found in EK-SDS and EK-CA systems as compared with EK-TW system.Although the K e value in EK-CA system was less than EK-SDS system,the metal removal performance of EK-CA system was actually better than EK-SDS sys-tem.It might result from the easier formation of free metal ion and citric–metal complex ions at low pH environment in EK-CA system.The residual fraction profiles of metals along the EK cell are shown in Fig.3.It was expected that a higher potential gradient will push concentration front of metals more toward to cathode end.As such,an accumulated phenom-enon would occur in a lower potential gradient system.A concentration accumulation of metals was most noted in the second section of soil specimen in EK-TW system (Fig.3a),which was at the position of0.3of normalized distance from anode to cathode along the soil chamber. This implied that the potential gradient was not high enough to carry metals out.However,such accumulation phenomenon can be dwindled by application of processing fluid rather than increasing potential gradient.As shown in EK-SDS system(Fig.3b)and EK-CA system(Fig.3c),the residual concentration was more evenly distributed along the EK cell.Due to the binding characteristics of SDS with metals and low electroosmotic resistance resulted from cit-ric acid,more metals were removed from sludge.3.4.The binding form of metals with sludge determined by sequential extractionResults of sequential extraction of heavy metals in sludge before/after EK treatment are plotted in Fig.4.A highest faction of50–96%were performed as residual form in raw sludge and a fraction of less10%was determined for most other binding forms,the weaker ones,for all heavy metals. This implied that the heavy metals were difficult to be extracted from sludge.As shown in EK-TW system,the fractions of residual form were decreased,except for Pb and Zn.Among otherfive binding forms,the organic frac-tion was largely increased as compared to that in raw sludge,such as63%for Cu and24%for Fe,which was approximately equivalent to10and60times greater than raw sludge.In EK-SDS system,the residual fractions wereC.Yuan,C.-H.Weng/Chemosphere xxx(2006)xxx–xxx5decreased as compared to EK-TW system for all metals and,furthermore,less than 5%was found for Cu,Ni,and Pb.It was also found that the major binding form of heavy metals in sludge was shifted from residual to organic and sorbed after EK-SDS system treatment.A similar phenom-enon was also observed in EK-CA system,whereas,it was surprised to see that the exchangeable fraction was 6.1–66.7times greater than that in raw sludge and it became the major binding form of metals in sludge.It might largely resulted from the lowest sludge pH (Fig.2a)in EK-CA sys-tem and,consequently,the metal ions were desorbed from sludge and removed by electromigration to the cathode end.It reveals that the binding types of metals with sludge were changed from a more difficult extraction type (residual and sulfate fractions)to easier extraction types (exchange-able,sorbed,and organic fraction)by EK process.If a proper treatment technology is followed by this EK process to remove metals more effectively,this treated sludge will be more beneficial for sludge utilization afterwards.Before it is reused,the risk associated with metals of more mobile forms to the environment need to be further investigated.It was also concluded that the binding form of heavymetalsFig.3.The residual profiles of heavy metals in (a)EK-tap water system;(b)EK-SDS system;and (c)EK-citric acid system,respectively.6C.Yuan,C.-H.Weng /Chemosphere xxx (2006)xxx–xxxwith sludge after EK process would be highly dependent upon the processing fluid applied.Among these metals,a highest residual fraction (88–96%)was found for chromium in sludge both before and after treated by EK process.There was no signifi-cance effect on binding form alteration of Cr in three EK systems,whereas it showed a significant impact on other five metals.For Zn,the organic and exchanged forms increased sharply to 38%and 46%in EK-SDS and EK-CA system,respectively.Similarly,the major binding form after treatment was changed to organic form in EK-TW and EK-SDS systems and exchangeable form in EK-CA system.For Ni,Pb,and Cu,the sorbed form also became one of the major existed binding forms with sludge.From Fig.4,it was found that the removal priority of investigated metal form sludge by EK process was:Cu >Pb >Ni >Fe >Zn >Cr.This might be largely related to the ion mobility of metals.3.5.Cost analysisAccording to Ho et al.(1997),the overall expense for EK treatment was classified to four parts:35–40%for elec-trode construction,27–32%for electricity and materials,17%for labor,and 16%for licenses and other fixed costs.Among these,the expense of electricity and materials were considered as operational cost.Energy expenditure is cal-culated as follows:E u ¼P W ¼1WZVI d t ð2Þwhere E u =energy expenditure per unit weight of sludge (kWh ton À1);P =energy expenditure (kWh);W =weight of sludge (ton);V =voltage (V);I =current (A);t =time (h).In the tests of constant-voltage condition,the energy expenditure is directly related to the time integral of the current across the cell.The calculated energy consumption is 122,140,and 155kWh ton À1of sludge for EK-TW,EK-CA,and EK-SDS systems,respectively.It is apparent that the type of processing fluid utilized is one of the controlling factors with respect to energy consumption.Considering the metal removal performance,it is clear that energy ap-plied is not enough for completion of remediation.How-ever,this can be improved by increasing potential gradient or prolonging the treatment periods (Gent et al.,2004).By taking into account the quantity of metals re-moved,as shown in Table 3,the power consumption per mol of metal removed is 0.38,0.25,and 0.45kWh for the above-mentioned systems,respectively.Table 3summarizes the results of cost analysis of EK sys-tems with different processing fluids.The total metals removed were 3.87·10À2,7.26·10À2,and 1.1·10À1mol for EK-TW,EK-SDS,and EK-CA systems,respectively.Results showed that the costs including the expense of pro-cessing fluid and energy were 0.02,6.08,and 9.76USD per mol of metals removed for the three above-mentioned sys-tems,respectively.On account of 27–32%of overallcostFig.4.Mass fraction distribution of metals binding with sludge in EK systems.C.Yuan,C.-H.Weng /Chemosphere xxx (2006)xxx–xxx7being electricity and materials (Ho et al.,1997),the over-all cost can be preliminarily estimated to 0.07,20.6and 33.1USD per mol of metals removed for the three above-mentioned systems,respectively.Although the lowest treatment fee was found in EK-TW system,the lowest removal efficiency was a disadvantage for this sys-tem.For other two systems,it was found that the EK-SDS system was likely more cost-effective than EK-CA system.4.ConclusionsWith the experimental results of heavy metal removal from industrial wastewater sludge by three EK systems,the important conclusions have been summarized as follows:1.The pH in the reservoir maintained at 1.5–2.5near the anode and 9.8–12.1near the cathode (except for citric acid in the range of 2.0–2.5).The sludge pH near the anode was lowered to around 4.0–6.0due to an acidic front generated at anode reservoir flushed across the sludge specimen and the sludge pH near the cathode increased to approximately 7.8–8.2due to the OH Àproduction from water electrolysis.2.The results of sequential extraction analysis revealed that the binding form of metals with sludge after EK process treatment was changed from the resid-ual form,the most difficult extracted type,to the exchangeable,sorbed and organic forms,the easier extraction types.It implies that the sludge treated by EK process will be more beneficial for further utilization once a proper technology is followed by EK process to remove metals more effectively.Results were also showed that the metal removal efficiency by EK process was:Cu >Pb >Ni >Fe >Zn >Cr.3.The removal efficiency of metals were in the range of 11–60%,37–77%,and 43–78%for EK-TW,EK-SDS and EK-CA systems,respectively.It was shown that a best performance was found in EK-CA system,which was resulted from the water electrolysis reaction in the cathode was neutralized by citric acid to avoid genera-tion and transport of high concentration of OH Àions into soils and to hinder electrodeposition of metals.The accumulation phenomenon of metal residual along EK cell would diminish as using SDS and citric acid in EK system.4.The overall cost of this research can be estimated to 0.07,20.6and 33.1USD per mol of metals removed for EK systems with tap water,SDS,and citric acid as processing fluid,respectively.The lowest treatment fee was found in EK-TW system however the lowest removal efficiency was a major disadvantage for this system.It was found that the EK-SDS system was likely more cost-effective than EK-CA system.T a b l e 3C o s t a n a l y s i s o f E K s y s t e m sS y s t e m aC o n c e n t r a t i o n o f p r o c e s s i n g flu i d (M )(4)P r o c e s s i n g flu i d s c o s tE n e r g y c o s t(11)f(1)(2)(3)T o t a l m e t a l r e m o v e d c (m o l )(5)(6)(7)d(8)(9)(10)eT o t a l c o s t U S D m o l e o f m e t a l s r e m o v e d À1)T a p w a t e rS D S bC i t r i c a c i d b S u s p e n d e d v o l u m e (L )U n i t p r i c e (U S D k g À1)S u b t o t a l c o s t (U S D m o l e o f m e t a l s r e m o v e d À1)U n i t p o w e r c o n s u m p t i o n c(k W h m o l e m e t a l r e m o v e d À1)U n i t p r i c e (U S D k W h À1)S u b t o t a l c o s t (U S D m o l e o f m e t a l s r e m o v e d À1)E K -T W X ––3.87·10À20.4000.380.050.020.02E K -S D S –0.024–7.26·10À20.4159.46.070.250.050.016.08E K -C A––0.41.10·10À10.431.99.740.450.050.029.76aT h e d e n s i t y (D )a n d m a s s o f s l u d g e i n e a c h s y s t e m w a s 1.33g c m À3a n d 220g ,r e s p e c t i v e l y .bT h e m o l e c u l a r w e i g h t (M W )o f S D S a n d c i t r i c a c i d i s 288a n d 210g /m o l ,r e s p e c t i v e l y .cT h e d a t a w e r e d e r i v e d f r o m T a b l e 2.d (7)={{[(1),(2)o r (3)]·(5)}·M W o f p r o c e s s i n g flu i d Ä1000·(6)}Ä(4).e(10)=(8)·(9).f(11)=(7)+(10).8C.Yuan,C.-H.Weng /Chemosphere xxx (2006)xxx–xxx。