电絮凝PretreatmentofBrackishWaterUsingDCElectr

Iran.J.Chem.&Chem.Eng.VoL21,No.1,2002PretreatmentofBrackishWaterUsingDC-Electrocoagulation-MethodandOptimizationDaneshvar*+,Nezameddin;Ashassi,HabibandRakhshaei,RohanDepartmentofAppliedChemistry,FacultyofChemistry,TabrizUniversity,Tabriz,51664,J.R.Iran.ABSTRACT:Electrochemicallygeneratedironcanremovemostcontaminantspresentinwaterandwastewater,byprecipitation,adsorption,electrostaticinteraction,andcomplexformation(generallycalledelectrocoagulation-EC).Inthepresentpaper,pretreatmentofbrackishwaterhavinghightotalhardness(TH),alkalinity(.4),SO/-andCrions,isstudiedbyapplicationofaDC-electricalcurrenttotheironelectrodes.Operatingparameterssuchaselectricalcurrentdensity,volumeflow,surfaceareaofelectrodes,distancebetweenthetwoelectrodes,watertemperature,aeratingflow,retentiontime,contacttimeofproducedsludgebeforeandaftertheprocess,additionofcoagulantaid(bentonite)andthecolloidaladditive(clayparticles)wereopti-mized.Theresultsofthisstudyforabrackishsamplewith11.8TH,5.6HC03-(.4),12.4SO/-,and23.5Clmeq/litindicatethattheprocessisabletolowermorethan%94the(TH),%92(.4),%50SO/-and%43Cr.KEYWORDS:Pretreatment,Brackishwater,Electrocoagulation,Electrochemicalremoval,Bentonite,ClayINTRODUCTIONTherapidgrowthofpopulationandtheconsequentshortageoffreshwatersources,hasmadetheimpor-tanceofwatersupplyfeltmoreandmoreseriously.Goodqualitywaterisneededfordrinkingandalsoforagricul-turalandmostindustrialpurposes.Thatiswhythemethodsforsupplyinghighqualitywaterespecially,drinkingwaterinthelargescale,areconsideredveryimportant.Thebrackishandbrinewatersthoughexpensivelyavailable,containhighquantitesofTH,AandTDSandthuscannotbeuseddirectly.Highconcentrationofions,*Towhomecorrespondenceshouldbeaddressed.+E-mail:[e-mail]1021-9986/02/1/138/$/2.80suchasCa2+,Mi+,Na+,SO/-,CI-andRC03-arethemostconsiderableaswellastheprevailingproblemsintheextentofchemicaltreatmentofbrackishorsaltywaters.Relatingtothedesiredquality,theoutputwaterinthepresentprocess,canbeuseddirectly,oritcanbepipedtomembraneplantsasreverseosmosis(RO)and/orelectrodialysis(ED),orionexchangeplants,forprovidinghighlypureandpotablewaterwithlesscostandproblems.Theresearchersbelievesthat,thisisaneffectivesteptowardstomakecheaperprocessofdes-alinationofbrackishandsaltywaters.TheROisa13ArchiveofSID(EC)asanewmethodofdesalinationofbrackishwaterhasbeenstu-diedandthetechnicalfeasibiltyhasbeencomfirmed[1].TheECnotonlycanefficaciouslyreducetheturbidityofwaterbutalsocanremovesomekindofionslikeCa2+,Mi+,HC03-,sOl-,retardandsoanexistinginthewaterthescaleinRO[2].TheECtreatmentprocesstobeusedistheCUREprocessdevelopedbyCleanUPandRecovery(CURE)Corporation,Denver,Colorado[3].TheECtreatmenthasbeenreportedasaninexpensivemethodforseparationofsolidsandoilsfromtarsandprocessingwastwaters[3].Theelectrochemicaletechnology(DC-EC)usedtotreattextilewastewaterandforremovingheavymetalsfromcontaminatedgroundwater[4,5].TheCUREelectro-coagulationsystemwasevaluatedforremovaloflowlevelstheradionuclidesuranium,plutoniumandameri-ciumaswellasothercontanimatsinwastewater[6].Thealternatingcurrentelectrocoagulation(AC-EC)processhasbeensuccessfullyappliedforeffectivecoagulationofultra-finesolidsuspension,metalionsandoilemulsions[7,8].TheeconomicaspectsofECprocesshavebeendiscussedforachievementtonewinexpensivemethodofwastewatertreatment[1,6,7].MechanismofremovingionsThemechanismsofcoagulationhavebeenstudiedveryextensivly[9.10].Inthismethodawidevarietyofaluminiumandironsaltswasused.IntheDC-ECmethodaluminiumelectrodeshavebeenusedwithoutaeration.Thismethodiselectrolysisprocess.Theironelectrodeswithaerationprefertousebecausetheyaremoreeconomicthanaluminium.InthismethodFe2+oxidizecompletelytoFe3+andalsocathodicreductionispossibleduetoareation.TheproducedFe2+fromanodeanaresultofaeratingoxidizeimmediatelytoFe3+.14Theanodereactionsare:(25°C)5(Fe~Fe2++2e)5Fe2++02+3H2O~5Fe3++5OH-+l/2H2t5Fe+o2+3H2O~5Fe3++50H-+1/2H2t+lOe(I)Thecathodereactionsare:(25°C)2H20+2e~H2t+20H-electrolysis4H20+202+8e~80Hcathodicreductionbyaeration2(3H2O+02+5e~1/2H2t+50H-)(11)Theoverallreaction(I+ll)isasbelow,andthatisasaresultofbothfactorofelectrolysisandareation[2,11,12].5Fe+302+9H2O~5Fe3++150H-+3/2H2tInthisconditionthevariousspeciesproducedsuchasFe(OHh,[Fe(OH)3(H20)3],FeOOHincludehighlychargedpolymericionsassameas,[F~(OH)2,(H20)4]4+,[Fe(OH)4]5+[2,10,11,13].TheoverallremovalmechanismsWastewatertreatmentterminologyoftenincludesadsorption,co-precipitation,surfacecomplexationandelectrostaticattraction.Frequentlytheprocessesoverlapanddistinctionbecomesdifficult.Forsakeofthisdis-cussionandtodescribewhytheproposedtreatmentschemeworks,twosimpletermswillbeused;pre-cipi-tationandadsorption.Adsorption