Treatments of alkaline non-cyanide, alkaline cyanide and acidic zinc electroplating wastewaters by

13 Pages·2017·1.44 MB·English

ProcessSafetyandEnvironmentalProtection 105 (2017) 373–385
ContentslistsavailableatScienceDirect
Process Safety and Environmental Protection
journal homepage: www.elsevier.com/locate/psep
Treatments of alkaline non-cyanide, alkaline
cyanide and acidic zinc electroplating wastewaters
by electrocoagulation
M. Kobyaa, E. Demirbasb,∗, F. Ozyonarc, G. Sirtbasa, E. Gengecd
aDepartmentofEnvironmentalEngineering,GebzeTechnicalUniversity,41400,Gebze,Turkey
bDepartmentofChemistry,GebzeTechnicalUniversity,41400,Gebze,Turkey
cDepartmentofEnvironmentalEngineering,CumhuriyetUniversity,Sivas,Turkey
dDepartmentofEnvironmentalProtection,KocaeliUniversity,Kocaeli,Turkey
a r t i c l e i n f o a b s t r a c t
Articlehistory: Treatmentsofalkalinenon-cyanide,alkalinecyanideandacidiczincelectroplatingrinse
Received23August2016 wastewaters were investigated in an electrocoagulation (EC) reactor using Fe plate elec-
Receivedinrevisedform1 trodes. This is the first study involved with removals of zinc and cyanide together from
November2016 threedifferentwastewatersintheliterature.Theeffectsoftheoperatingparametersnamely,
Accepted26November2016 initialpH(pHi),currentdensityandoperatingtimeontheremovalefficiencieswereeval-
Availableonline2December2016 uated.Theremovalefficienciesandoperatingcostsweredeterminedas99.8%forZnand
0.74D/m3 atapHof7,80A/m2 and60minforalkalinenon-cyanide,99.9%forZn,99.9%
Keywords: forCNand1.72D/m3atapHof9.5,60A/m2and60minforalkalinecyanide,and99.9%for
Zincplatingwastewater Znand2.26D/m3 atapHof8,80A/m2 and60minforacidiczincelectroplatingwastew-
Alkalinenon-cyanide aters,respectively.Moreover,toxicitytestwasconductedtoobtaininformationaboutthe
Alkalinecyanide toxiceffectoftherawandtreatedwastewaters.Thetoxicityresultsindicatedthatallthe
Acidiczinc rawwastewaterscontainedhardlytoxiceffect(EC50 foracidic,alkalinecyanideandalka-
Electrocoagulation linenon-cyanidewere0.62,5.25and3.38).Ontheotherhand,thetreatedwastewaterwas
Operatingcost non-toxic.ThisstudyrevealedthattheECprocesswithFeelectrodewasveryeffectivefor
Ironelectrode removalofzincandcyanideionsfromdifferentzincelectroplatingrinsewastewaters.
©2016InstitutionofChemicalEngineers.PublishedbyElsevierB.V.Allrightsreserved.
1. Introduction duringrinsingprocesses.Additionally,batchdumpingspentacidand
cleaningsolutionscontributetothecomplexityofwastetreatment.
Metalplatingisoneofthemajorchemicalprocessesthatdiscardlarge Theindustrialuseofzinchasincreasedbecauseoftheextensive
amountsofharmfulandtoxicwastewaters.Platingwastewaterscon- use of zinc and zinc salts in the electroplating and metal finishing
tainheavymetals(Cd,Co,Cr,Cu,Ni,Pb,andZn,etc.),cyanide,oil,grease industries.Overtheyears,differentprocesseshavebeendevelopedfor
andsuspendedsolidsatlevelsthatmightbeconsideredhazardousto applyingzinccoatings(Hosseinietal.,2016).Inparticular,electrode-
theenvironmentandcouldposeriskstopublichealth(Hosseinietal., posited zinc is used extensively in automotive and other industries
2016;Bojicetal.,2009).Heavymetalsandcyanide,inparticular,are asaprotectivecoatingforlargequantitiesofsteelwires,strips,sheets,
ofgreatconcernduetotheirtoxicity.Becauseofthehightoxicityand tubesandotherfabricatedferrousmetalparts.Zincdepositsoffergood
corrosivenessofplatingwastestreams,platingfacilitiesarerequiredto protectionanddecorativeappealatlowcost(SchlesingerandPaunovic,
treatwastewaterpriortodischarge.Themetalplatingprocesstypically 2010).Themostwidelyusedzincplatingiscategorizedasalkalinenon-
involvesalkalinecleaning,acidpickling,plating,andrinsing.Copious cyanide zinc, alkaline cyanide zinc and acidic zinc or chloride zinc
amountsofwastewateraregeneratedthroughthesesteps,especially (Schlesinger and Paunovic, 2010; Naik and Venkatesha, 2002, 2003).
Alkalinenon-cyanidezincplatingbathscomposeofzinc(sodiumzin-
cate,6–23g/L),causticsoda,waterconditioner,andorganicadditives.
Alkalinecyanidezincplatingbathsconsistofzinccyanide(7.5–34g/L),
sodiumcyanideandproprietaryadditives.Acidiczincbathscontain
∗ Correspondingauthor.Fax:[phone]. zincchloride(15–38g/L),potassiumchloride,ammoniumchlorideor
E-mailaddress:[email protected](E.Demirbas). boric acid and proprietary additives. In the zinc electroplating pro-
http://dx.doi.org/10.1016/j.psep.2016.11.020
0957-5820/©2016InstitutionofChemicalEngineers.PublishedbyElsevierB.V.Allrightsreserved.
374 ProcessSafetyandEnvironmentalProtection 105 (2017) 373–385
cess,townwaterisgenerallyusedinthecleansingrinseofsolvent, realalkalinenon-cyanide,alkalinecyanideandacidiczincelectroplat-
alkaline and acid while deionized water is employed for the plat- ingrinsewastewatersbytheECprocessandprovidecomparisonsofthe
ing rinse and final rinse (Schlesinger and Paunovic, 2010; Naik and resultswiththeliteraturebasedontheoptimumoperatingconditions
Venkatesha,2002,2003).Thewaterintherinsingbathsgetscontami- andtheoperationalcost.Hence,theeffectsofexperimentaloperat-
natedduringthecleaningorplatingprocessduetothe‘drag-out’from ingparameterssuchasinitialpH,currentdensity,andoperatingtime
previousprocessbaths.Generally,zincconcentrationsvaryfrom0.112 onthezincandcyanideremovalefficiencieswereevaluatedtodeter-
to252mg/Linelectroplatingrinseeffluents(EPA,1985;Zouboulisetal., minetheoptimumoperatingconditions.Theoperatingcostsofthe
2005).Ontheotherhand,effluentsfromindividualoperationsatelec- treatmentprocessaccordingtotheelectrodeandenergyconsumptions
troplatingandmetalfinishingplantsgenerallycontainfrom1to3% werealsocalculatedforeachwastewater.Thetoxicitylevelsweremea-
(0.005–150,000mg/L)ofcyanide(EPA,1985;Dashetal.,2009).Theelec- sured.ThesludgeremainedaftertheECprocesswascharacterizedby
troplatingeffluentsusuallycontainmetal-ionconcentrationssuchas scanningelectronmicroscope(SEM),energy-dispersiveanalysisofX-
Zn,Fe,andNiandanionslikecyanidemuchhigherthanthepermis- rays(EDAX),X-raydiffractometer(XRD)andFouriertransforminfrared
siblelevels.Asexpectedinzincelectroplatingrinsewastewater,zinc spectroscopy(FTIR).
andcyanideconcentrationsarehigherthanothers.Highconcentra-
tions of zinc can cause eminent health problems, such as stomach 2. Materialsandmethods
cramps,skinirritations,vomiting,nauseaandanemia(FuandWang,
2011).Short-termcyanideexposurecausesrapidbreathing,irritation
2.1. Characterizationsofzincelectroplatingrinse
andsoresonskin,tremors,andotherneurologicaleffectsandlong-
wastewaters
termexposuretocyanideleadstoweightloss,thyroideffects,nerve
damageanddeath(AdhoumandMonser,2002).Themaximumper-
Three different zinc electroplating rinse wastewaters were
mitted zinc and cyanide concentrations in industrial effluents after
thetreatmentaccordingtotheindustrialdischargestandardsandthe collectedfromalocalelectroplatingfactorylocatedinIstan-
receivingmediaare0.5and3mg/LforZnand0.01and0.2mg/Lfortotal bul.Acidiczincelectroplatingbathconsistedof70g/LZnCl2,
cyanide,respectively(TheTurkishWaterPollutionControlRegulation, 170g/LNH4Cl,proprietaryadditiveagents(10%),brightening
1988).Varioustechniqueshavebeenappliedforthetreatmentofheavy agent(3%),andZnmetalanode(purity>99.9%).Non-cyanide
metalsfromwastewater,suchaschemicalprecipitation(Blaisetal., alkaline zinc electroplating bath contained of 8–10g/L zinc
2008),adsorption(Zhangetal.,2016),ion-exchange(Silvaetal.,2008), metal anode, 140–170g/L water conditioning. The alkaline
electrodialysis(MahmoudandHoadley,2012),reverseosmosis(Petrinic
cyanide zinc plating bath included 45g/L ZnO, 70g/L NaOH,
et al., 2015) and electrocoagulation (Espinoza-Quinones et al., 2012;
Kobya et al., 2010a,b). The rest of treatment techniques except for 90g/LNaCN,0.004g/Lbrighteningagent,and0.004g/LAs2S3.
Afterthesebathswereusedinthefactory,thecharacteristics
electrocoagulation(EC)hasanumberofdisadvantagessuchashuge
oftheelectroplatingprocesseswastewatersweredetermined
spacerequirements,longoperatingtime,highoperatingcost,mem-
branefoulingandscaling,regenerationandperformancereduction, asCODof410mg/L,TOCof158.4mg/L,ironof0.643mg/L,apH
highlysensitivetothesolutionpH,additivechemicalreagentscaus- of12.3,conductivityof43.6mS/cm,andtotalZnof381mg/L
ing serious secondary pollution (Hosseini et al., 2016; Sancey et al., foralkalinenon-cyanidezincelectroplating,CODof570mg/L,
2011;Malakootianetal.,2010;Mahvietal.,2009).Recentresearchhas TOC of 107.2mg/L, iron of 0.682mg/L, a pH of 9.5, conduc-
shownthatECcanofferagoodopportunitytopreventandremedypol- tivityof7.6mS/cm,totalcyanideof135mg/LandtotalZnof
lutionproblems(VasudevanandOturan,2014;Chen,2004).ECconsists 175mg/Lforalkalinecyanidezincelectroplating,andCODof
ofaninsitugenerationofcoagulantsbyanelectricaldissolutionof
483mg/L,TOCof161mg/L,ironof1.282mg/L,apHof6.5,con-
FeorAlelectrodes.Thegenerationofmetalliccationstakesplaceat
ductivity of 20.4mS/cm and total Zn of 1477mg/L for acidic
theanode,whereasaH2gasproductionoccurstogetherwithhydroxyl zincelectroplatingwastewaters,respectively.
ionsreleasingatthecathode.Ferricoraluminumionsgeneratedby
electrochemicaloxidationofFeorAlelectrodemayformmonomeric
2.2. Experimentalset-upandprocedure
andpolymerichydroxylmetalliccomplexesdependingonthepHof
the aqueous medium, which has strong affinity or dispersed parti-
clesaswellascounterionstocausecoagulation(Kobyaetal.,2015; TheECexperimentswereconductedinabatchprocessusing
Chen,2004).Inconclusion,theformationofmetalhydroxideflocspro- 1LcapacityofanECreactormadefromPlexiglaswithdimen-
ceedsaccordingtoacomplexmechanism.Formedamorphoussuchas sionsof12×11×11cm(Fig.1).Iron(Fe)plates(purity>99.5%)
Fe(OH)3(s)occurs“sweepflocs”havinglargesurfaceareas.Theseflocs
areactiveinrapidadsorptionofsolubleorganicandinorganiccom-
poundsandtrappingofcolloidalparticlesandareeasilyseparatedfrom
aqueousmediumbysedimentationorH2flotation.Moreover,thispro-
cessischaracterizedbysimpleequipmentandeasyoperation,short
operatingtime,reducedornorequiredforadditionofchemicals,and
decreasedamountofsludge(Chen,2004).
ECprocessusingironandaluminumelectrodesshowedthesuc-
cessful removals of free cyanide (Moussavi et al., 2011), Cd and
Ni-cyanide (Kobya et al., 2010a,b), Zn-cyanide (Senturk, 2013), and
heavymetalssuchasCr,Cu,Mn,Ni,Pb,andZn(Al-Shannagetal.,2015;
Kobyaetal.,2015;Espinoza-Quinonesetal.,2012;Mansoorianetal.,
2012;HanayandHasar,2011;Dermentzisetal.,2011;AkbalandCamci,
2011;HeidmannandCalmano,2010;Huangetal.,2009;Arroyoetal.,
2009; Heidmann and Calmano, 2008; Kabdasli et al., 2009; Adhoum
etal.,2004)fromsyntheticandindustrialwastewaters.
Although there is considerable success for treatment of various
typesofwastewatercontainingcyanideandheavymetalsinthelit-
erature with the EC process, its application for the treatments of
differentzincelectroplatingwastewaters(alkalinenon-cyanide,alka-
linecyanideandacidiczincelectroplating)usingironplateelectrodes
hasnotbeenreportedyet.Thenovelfindingofthisstudywastotreat
Fig.1– Aschematicdiagramoftheexperimentalsetup.
ProcessSafetyandEnvironmentalProtection 105 (2017) 373–385 375
with dimensions of 5.0×7.3×0.3cm were used as the sac- C = i×tEC×Mw (2)
rificial electrodes. In each batch, four Fe plate electrodes electrode z×F×v
(twoanodesandtwocathodes)spacedby0.5cmwereplaced
verticallyintheECreactorandconnectedatmonopolarpar- whereUiscellvoltage(V),iiscurrent(A),tECistheoperating
allelmode.Theelectrodesweredippedintheelectroplating time(min)andvisthevolume(m3)ofwastewater(0.85L),Mw
wastewatertoadepthof8cmyieldingatotaleffectiveelec- ismolecularmassofiron(56.8g/mol),zisthenumberofelec-
trode surface area of 219cm2. All chemicals used in the EC trontransferred(2)andFisFaraday’sconstant(96,487C/mol).
experiments were of analytical grade. All runs were per- AccordingtotheTurkishmarketinMarch2016,pricesforelec-
formedwith0.85Lofwastewaterataconstanttemperature tricalenergywere0.095D/kWh,andpriceforFeelectrodewas
and300rpm(HeidolpMR3000D).Thesolutionwasconstantly 0.85D/kg, respectively. Prices of chemicals used for adjust-
stirredtoreducethemasstransportoverpotentialoftheEC mentofadesiredpHwere0.73D/kgforNaOH,0.29D/kgfor
reactor.TheelectrodeswereconnectedtoadigitalDCpower H2SO4.TheoperatingcostfortheECprocesswascalculated
supply(Agilent6675Amodel;30V,6A)operatedatgalvanos- withthefollowingequation:
taticmode.
Before each run, the impurities on the electrode OC=˛×Cenergy+ˇ×Celectrode+(cid:3)×Cchemicals (3)
surfaces were removed by mixing hydrochloric acid-
hexamethylenetetramine aqueous solution (Kobya et al., 2.5. Toxicitytests
2015). The current was held constant at desired values for
eachrunandtheexperimentwasstarted.Thesamplestaken ToxicitytestswereperformedaccordingtotheInternational
from the EC reactor at the different operating times were Standard Method (ISO) 21338 water quality-kinetic determi-
filteredusing0.45-(cid:2)mporesizefilterandthen,totalzincand nationoftheinhibitoryeffectsofsediment,othersolidsand
cyanide concentrations were determined. At the end of the coloredsamplesonthelightemissionofVibriofischeri(kinetic
run, the electrodes were washed thoroughly with water to luminescentbacteriatest)(ISO,2010).Luminescencewasmea-
removeanysolidresiduesonthesurfacesanddried.Thelost suredusingahighperformanceSiriusluminometerandlight
amountofanodeweightwasdeterminedbysubtractingthe outputwasrecordedautomaticallybyFB12software(Berthold
weightoftheelectrodesbeforeandaftertheexperiment.In detectionsystems,Germany).Priortomeasurement,freeze-
addition,sludgegeneratedaftertheECexperimentwasdried driedV.fischeriwerere-hydratedinreagentdiluent(2%NaCl)
inanoven(Memmert,Germany)at105◦Cfor24h. at 4◦C for at least 30min and then stabilized at 15◦C for
approximately 1h in a dry cooling block. NaCl content and
2.3. Analyticalprocedure pHofsampleswereadjustedto2%and7.0±0.2.Thesamples
weresubsequentlydilutedwith2%NaClsolutiontoobtaina
Zincandcyanideanalyseswereconductedbytheprocedures dilutionseries(1:2,1:4,1:8,1:16,1:32and1:64).Toxicitymea-
describedinStandardMethods(APHA,1998).Cyanideconcen- surementswereperformedbyinitiallyplacing300(cid:2)Ldiluted
trationinthesamplewasdeterminedbypyridine–barbutiric sampleintoluminometercuvettes(Sarstedt55.476)andincu-
acid method using a UV spectrophotometer (PerkinElmer bating at 15◦C for 10min. Following introduction into the
Lambda 35 UV/vis spectrophotometer, USA), zinc concen- Siriusluminometer,300(cid:2)Lbacterialsuspensionswereauto-
tration was measured with an Inductively Coupled Plasma maticallyinjectedintothesample,andbioluminescencewas
OpticalEmissionSpectrometer(ICP-OES,PerkinElmerOptima measured, and then repeated after 30min so that the rela-
7000DV).Theaccuracyofthesemeasuredvaluesforcyanide, tionship between end point toxicity and peak toxicity could
zinc, COD and TOC was estimated around 1%. pH and con- be elucidated. A correction factor was applied based on the
ductivityofthesamplesbeforeandaftertheECprocesswas response obtained from the non-toxic reference sample (2%
measuredbyapHmeterandaconductivitymeter(HachLange NaCl).Theinhibitionpercentage,EC50(theconcentrationthat
HQ40). The experiments were repeated three times and the causes50%reductionofbacteriarelativetocontrol)andEC20
average data was reported. The morphologies of the sludge valueswerecalculatedaccordingtotheISOstandardmethod
were characterized by the SEM (Philips XL30S-FEG). Crystal (11348-3,1998).
phasesoftheprecipitateswerecharacterizedusingtheXRD
(Rigaku2000D/maxwithCuK(cid:3)-radiation,(cid:2)=0.154nmat40kV 3. Resultsanddiscussion
and40mA).TheFTIRspectrumofthesludgewascollectedin
therangeof4000–650cm−1onaBioRadFTS175Cspectropho- 3.1. Treatmentofalkalinenon-cyanidezincrinse
tometer. wastewater
2.4. CostanalysisofEC InitialpH(pHi)onthetreatmentprocessinfluencesthesurface
chargeoforganicandinorganicpollutantsfromthewastew-
In this preliminary investigation, the operating cost (OC) of ater. The species from different pollutants can be formed
the treated electroplating rinse wastewater (OC, D/m3) can as a result of this and pHi can modify the species of coag-
becalculatedbyconsideringthreeparametersasmajorcost ulant and stability of various hydroxide species (Heidmann
items (Kobya et al., 2015) namely, the amounts of energy and Calmano, 2008; Kobya et al., 2010a,b). In this study, the
(Cenergy,kWh/m3)andelectrodematerial(Celectrode,kgFe/m3) effectofpHi(5–8)onthezincremovalfromthealkalinenon-
consumptions,andchemicalsconsumed(C ,kg/m3)in cyaniderinsewastewaterwasinvestigatedatacurrentdensity
chemicals
theECprocess.Thefollowingequationsareusedtocalculate of80A/m2 andanoperatingtimeof60min.Fig.2showsthe
CenergyandCelectrodefromtheFaraday’slaw initialandresidualzincconcentrationsasafunctionofoperat-
ingtimeatdifferentpH values.Zincconcentrationsreduced
i
Cenergy= i×Uv×tEC (1) f2r.o3m8m3g4/5L.1(99to.3%5.)55atmagp/LH(9o8f.64,%f)roamt a21p5H.7itoof05.,40frmomg/L3(2999..58%to)
i
376 ProcessSafetyandEnvironmentalProtection 105 (2017) 373–385
350 200
(a) Initial pH (b) j (A/m2)
i
300 5.0 20
mg/L) 250 67..00 mg/L)150 4600
n ( 8.0 n ( 80
o o
ati 200 ati
entr entr100
c c
n 150 n
o o
nc c nc c
Residual zi 15000 Residual zi 50
0 0
0 5 10 15 20 25 30 35 40 45 50 55 60 65 0 10 20 30 40 50 60 70
Operating time (t , min) Operating time (t , min)
EC EC
Fig.2– Effectsof(a)initialpH and(b)currentdensityonthetreatmentofnon-cyanideZnelectroplatingrinsewastewater.
i
atapH of7,andfrom195.5to0.26mg/L(99.9%)atapH of8.0 wereexpectedtodecrease(i.e.,lowpHdoesnotfavorhydrox-
i i
(Fig.2(a)). idesandhydroxylionsformationandconsequentlyinhibits
The effect of operating time can be explained by the the EC procedure). In addition, the dominant zinc species
Faraday’s law (Eq. (2)), the increasing current and opera- couldaffecttheremovalefficienciesatvariousinitialpHs in
tion time caused an increase in the amount of dissolved Fig.3(Reichleetal.,1975).Thus,finalpH(pH)valuesafterthe
f
coagulantfromtheanode.Theamountofelectro-generated ECprocessweremeasuredas6.2forapH of5,6.9forapHof
i
iron species increased with the increase in the operating 6,8.8forapH of7,and9.2forapH of8.ThepHofthetreated
i i
time. It also resulted in increase in the amount of flocs samples increased when the pH was low due to the excess
i
whichwasmadeupofinsolublemonomericspecies,Fe(OH)3 ofhydroxylionsproducedatthecathode(Eq.(6)).Inthealka-
andpolymerichydroxylcomplexesnamely,FeOH2+,Fe(OH)+, line medium (pH>8), the final pH did not change markedly
2
FFee(2H(O2OH))442+(O,HF)+2e(,OFeH2)(−4H,2OFe)8((HO2HO))42+2+,anFed(FHe22O(H)362+O,)6F(eO(HH)224O+)d5(eOpHen)2d+-, bsuemcaeudsebythFeeg3+enioenrastgeednheyrdatreodxyaltiothnesaantothdeecfaotrhmoidnegwFee(rOeHco)3n(s-)
ing on redox conditions and pH of the aqueous medium. flocs.Moreover,Taqvietal.(2007)reportedthatthedominant
ThespeciesofmetallicirondependingonthefinalpH(pHf) specieswereZn2+ (∼90%),Zn(OH)+ (∼5%)andZn(OH)2 (∼5%)
of EC process in turn precipitate as Fe(OH)2, a variety of atapHof8;Zn(OH)2 (∼78%),Zn(OH)+ (∼9%)andZn2+(∼13%)
Fe(II/III)(oxy)(hydro)oxidesandFe(OH)3(Kobyaetal.,2010a,b). atapHof9;Zn(OH)2(∼93%)andZn(OH)−3 (∼5%)atapHof10.
These iron (oxy)(hydro)oxides have strong affinity for dis- Furthermore,hydroxylionscouldalsopartiallycombinewith
persedparticlesaswellascounterionstocausecoagulation. Zn2+ionstoformtheinsolubleZn(OH) atapHof9.2.There-
2(s)
Contaminants (Ox) in the solution are removed via adsorp- fore,thepositivezincionsatapHof<9couldencouragethe
tionandco-precipitationbytheHFOspeciesproducedinthe chargeneutralizationandinsolublezincspecieslikeZn(OH)
2(s)
ECprocess(Eqs.(4)and(5))(NoubactepandSchoner,2010): at pH 8–11. In the EC process, zinc species on the electro-
chemicallyproducedHFOs(Eqs.(11)and(12))inthesolution
FeOOH +Ox →[FeOOH−Ox] (adsorption) (4) wereexpectedduetotheamountofadsorptionincreasedwith
(s) (aq)
pH up to 8. The dominant zinc species in the solution were
(cid:2) (cid:3)
Zn2+andZn(OH)+,andthesespecieswereadsorbedonHFOs
nFex(OH)(y3x−y)+Ox(aq)→ (cid:5)Fex(OH)(y3x−y)(cid:6)n·Ox (Trivedietal.,2004).
(s) (5)
ChargedensityofthesurfaceparticlesdecreasedasthepH
(co-precipitation)
approachedthepointofzerochargeatapHof7–9(Dyeretal.,
2004; Nowack et al., 2001). As the surface charge decreased,
The precipitation reactions could promote both co-
electrostaticrepulsionbetweenparticleswasreduced,allow-
precipitationofzincionswithironandthesorptionofZn2+
ingthemtomoveclosertogetherandaggregate.Wecanregard
ontoiron-corrodedsurfaceswhichcontributetotheremoval
the adsorption as a specific replacement of weakly acidic
of metal ions from the solution. The operating time at a
protonsbyequatedZn2+ orZn(OH)+ ratherthanasagener-
current density proceeded sufficient formation of iron(oxy)
alized counter ion process. Dyer et al. (2004) and Deliyanni
hydroxides(HFOs,alsoknownasamorphousironoxyhydrox-
et al. (2007) indicated that adsorption capacity of Zn2+ ions
ide,amorphousferrichydroxide,orferrihydrite)andzincions
ontoHFOssuchasgoethite,lepidocrociteandakaganeitewas
are adsorbed on and these species are co-precipitated. HFO
2.44–142mgZn2+/gFe.Forthealkalinezincrinsewastewater,
is formed by rapid hydrolysis of Fe3+ solution. Amorphous
amountsofironcalculatedastheoreticalandexperimentalat
ironoxidewilltransformintomorestableoxideformssuch
differentinitialpHs(5–8),80A/m2 (1.656A)and60minwere
as goethite (˛-FeOOH), lepidocrocite ((cid:3)-FeOOH), or hematite
about 2.03g Fe/L. Experimentally dissolved iron dosages for
(˛-Fe2O3). pH of5–8intheECprocesswere2.010,2.010,1.332and0.666g
Theremovalefficiencyremainedalmostthesameafterthe i
Fe/L,respectively.TheamountsofremovedzincpermgFeor
operatingtimeof60minsincetheinsolublemonomericand
Faraday’s(adsorptioncapacity)atanoperatingtimeof60min
polymeric iron oxyhydroxides sufficiently adsorbed almost
were169.9mg/gor6465mg/Faraday(0.0484mg/C)forapH of5
allZn2+ inthebulksolution,orprecipitateasinsolublezinc i
and163.7mg/gor6229mg/Faraday(0.0467mg/C)forapH of6.
hydroxide. The residual zinc concentrations in the solution i
ProcessSafetyandEnvironmentalProtection 105 (2017) 373–385 377
Fig.3– Fractionsof(a)zincand(b)ironspeciesoverarangeofpHinaqueoussolution.
However,effluentzincconcentrationsintheECprocesswere 180
140
obtainedas2.65mg/LforapHiof7at40minand2.20mg/Lfor L) (a) Initial pH
cacwifaooaopanprl-HcspHsauriaidelolpaosucfHtfoir8epii7donaiotbtgaaafs3ntset80idhor1,mvne52re9e8iaEdn.s99nCp..f5dmIoenpmrcagrttiod/hinggvcs/ideeosgoulsrcryopssa.6rttsrbM0i1eioy8a,1en5lt,to0hwamitn1elhas7gaets/mhwdrtFeesaergworerE/arFesaCpdaetmtearapiaryroradcocn(iha0ocncyee.nl0aryst(4spsa05r.ai.(e60nTHcm8iimhetn2ioii5gegsdvs/mmmeeCwfd)gafeeef/ntboCrcanyert)l concentration (mg/L) 11118024600000 Residual cyanide concentration (mg/11246802000000 8899....0505i
andCalmano,2008;Escobaretal.,2006). nc 60 0 0 10 20 30 40 50 60 70
parIatmisetweerlilnkflnuoewncninthgathtethreemcuovrraelnetffidceiennsictyyoifstahneEimCpproorctaensst ual zi 40 Operating time (tEC, min)
d
because it determines the production rate of coagulant and esi 20
R
adjusts bubble production (Heidmann and Calmano, 2008).
0
TheinitialZnconcentrationinthewastewateratapH of7
i
0 10 20 30 40 50 60 70
was195.5mg/L.Effectofcurrentdensityonthetreatmentof
the wastewater at a constant pH of 7 was shown in Fig. 4. Operating time (t , min)
i EC
Residualzincconcentrationsat60minwere5.43mg/L(97.2%)
at20A/m2,4.36mg/L(97.8%)at40A/m2,0.34mg/L(99.83%)at 180
140
wrEsct26(6Treeo000Coeamq,nmAcrbEpus4eho/lnriuee0mvenoen,maem1,cr2tl6dg1)e,spl.i0yy.socatZ0Z,sian3anaoiilncd0ttnnlnoyapcdgdsi0les/dcaa.rLf82eucidocs06lgahlrsresa3tiAoiFcottte4miret/helvcrpm0v,deeaogetrmldi2/edwtoLaesthiwnsnaoep(e9csx,eeceo9t3apdrcaev.ne6ntee9pwai5srds%va0l.iuaiue4mc.0r)tm5l,iae.eyat26edti.rpt10e6ent8t68dsgtirt5a0oe/geha.Llng/A5ateLirs,t/ar2emmeaot0ea1ntn,er8262eoe4n.60f0rf0r.Atgnm9om,tymw,erm6ieicn0hanaoae,ntneuatsrhmtd1dnneshe.astd0eeespra23nrly8en8eo1t0tc9cof.ightnAt.dCe/i5revL/elootmemeEhdnaaclCygee2--tt dual zinc concentration (mg/L) 111146802460000000 Residual cyanide concentration (mg/L)112468020000000 0 10 20Op(ebra)ti3n0g time (4tE0C, min)5j0 (A/m 6 202468)0000 70
processiseconomicallyaswellastechnicallyfeasibleforthe Resi 20
wastewater.theaveragevoltagesbetweenelectrodeswerevar-
0
iedas1.64–2.41VforapHiof5–8atconstantappliedcurrent 0 10 20 30 40 50 60 70
of1.656A(80A/m2)andanoperatingtimeof60min.Values
Operating time (t , min)
oftheenergyconsumptionsatpH of5–8duringtheECpro- EC
i
cessvariedfrom3.195to1.565kWh/m3.Atanoperatingtime
Fig.4– Effectof(a)pH and(b)currentdensityontreatment
i
of20minandapH of8,zincremovalefficiencyandenergy
i ofthealkalinecyanidezincelectroplatingrinsewastewater.
consumptionwere>99.9%and1.565kWh/m3,respectively.
Thecurrentefficiency(CE,%)ortheFaradicyieldisdefined
as the ratio of the experimental or actual electrode con-
sumption(Cexp)tothetheoreticalvalue(Ctheory).Itisalsoan
378 ProcessSafetyandEnvironmentalProtection 105 (2017) 373–385
Table1–Resultsforthetreatmentofalkalinenon-cyanidezincplatingrinsewastewater.
Parameter i U tEC Cenergy Celectrode Wsludge pHf OC
(A) (V) (min) (kWh/m3) (kg/m3) (kg/m3) (−) (D/m3)
pHi=5.0 1.656 1.64 60 3.195 2.010 2.258 6.21 2.06
6.0 1.656 1.95 60 3.799 2.010 2.783 6.91 2.11
7.0 1.656 2.20 40 2.857 1.332 1.981 7.82 1.44
8.0 1.656 2.41 20 1.565 0.666 1.563 8.19 0.74
j=20A/m2 0.414 1.11 60 0.521 0.520 2.125 7.56 0.49
40 0.828 1.54 60 1.501 1.031 2.343 8.12 1.02
60 1.242 2.20 40 2.143 1.030 1.971 7.62 1.08
80 1.656 2.41 20 1.565 0.666 1.563 8.19 0.74
important parameter for the EC process because it affects theincreaseinthecurrentdensitiesasexpected.Theoperat-
the lifetime of the electrodes. The theoretical mass of Fe ingcostswerecalculatedas0.49D/m3for20A/m2,1.02D/m3
(C ) was obtained by using the Faraday equation and for40A/m2,1.08D/m3for60A/m2and0.74D/m3for80A/m2.
theory
the experimental mass (Cexp) was obtained by the electrode Theminimumoperatingcostsforremovalefficienciesofzinc
massdifferencebeforeandaftertheexperiment.However,the over 98.5% and 99.8% were 0.74D/m3 at 80A/m2, 20min,
actual electrode consumption may be reduced or increased 17.51Faraday/m3 or 2337.9C/L and 1.353D/m3 at 60A/m2,
from this theoretical value depending upon the wastewater 50min,26.26Faraday/m3or3506.8C/L.Theamountsofsludge
characteristicsandoperationalconditionduetotheelectro- producedintheECprocessat20–80A/m2variedfrom2.125to
chemicalsideandchemicalreactionsinthesolution.CEwas 1.563kg/m3.Theoptimumconditionsforthewastewaterwere
calculatedas99.8%forapH of5,99.6%forapH of6,100.1%for a pH of 8, 80A/m2 and 20min. Effluent zinc concentration,
i i i
apH of7and100.2%forapH of8.C forapH of8wascal- removalefficiencyandoperatingcostforthetreatedwastewa-
i i theory i
culatedas0.666kg/m3at1.656Aand20min.Theexperimental terattheoptimumoperatingconditionswere2.64mg/L,98.6%
electrode consumptions at different pH for the wastewater and0.74D/m3,respectively.
i
werecalculatedas2.010kg/m3forapH of5,2.010kg/m3fora Theamountsofthesludgepertreatedwastewaterinm3
i
pHiof6at60min,and1.332kg/m3forapHiof7(tEC=40min), withrespecttopHi aftertheremovalwere2.258kg/m3 fora
respectively(Table1). pH of5,2.783kg/m3forapH of6,1.981kg/m3forapH of7,
i i i
Chargeloading(q)isdefinedasthechargestransferredin and 1.563kg/m3 for a pH of 8 (Table 1). The operating cost
i
the electrochemical reactions for a given amount of water of the treatment process is an important criterion to eval-
treated. It is calculated with the following equation (Kobya uate its applicability on an industrial scale. Operating costs
etal.,2016): were calculated as 2.06–0.74D/m3 at a pH range of 5–8. The
resultsindicatedthattheOCsincreasedalongwithenergyand
q(C/L) = i×tEC or q(F/m3)= i×tEC (6) electrodeconsumptionsintheECprocess.
V F×V
3.2. Treatmentofthealkalinecyanidezinc
where q is the charge loading (C/L or F/m3). Iron dosages
electroplatingwastewater
were determined by charge loading in the EC process. The-
oretically, whenever 1 Faraday of charge passes through the
The residual concentrations after the EC process in the
circuit, 28g of iron is dissolved in the EC process. When
wastewateratpH of8.0,8.5,9.0,9.5and60A/m2weredeter-
the charge loading was low, the iron dosages were not suf- i
minedas1.60(99.1%),0.92(99.5%),0.35(99.8%)and0.25(99.9%)
ficient to remove all zinc ions from the wastewater, and
mg/Lforzincand0.90(99.3%),0.30(99.8%),0.20(99.9%)and
thus the zinc removal efficiency was not high. The zinc
0.12 (99.9%) mg/L for cyanide, respectively (Fig. 4(a)). The
removal efficiency increased with increase in both charge
results were met with the discharge standards for zinc and
loadingandcurrentdensity.However,thehighchargeload-
cyanideandthecorrespondingremovalefficiencieswereover
ing resulted in high energy and electrode consumptions, a
99.5% and 99.8%, at an operating time of 60min and a pH
prime concern of the operating cost. Therefore, the charge
of>8,respectively.Cyanideandzincremovalsfromthealka-
loading was needed to be optimized. The minimum charge
linecyanidezincrinsewastewaterduringtheECprocesswere
dosage required for the zinc removal efficiency of >98.5%
dependedonproducedironoxy(hydroxyl)speciesandpH.As
was >1753.4C/L at 20A/m2 (0.414A and 60min), 3506.8C/L i
thepH increasedfrom8.0to9.5,thentheresidualconcentra-
or 26.26F/m3 at 40A/m2 (0.828A and 60min), 3506.8C/L or i
26.26F/m3 at60A/m2 (1.242Aand40min),and2337.9C/Lor tionsdecreasedsignificantly.ValuesoffinalpHs aftertheEC
processincreasedto8.6,8.9,9.4,and9.6,respectively.
17.51F/m3at80A/m2(1.656Aand20min).Ontheotherhand,
Therearetwometalsources;Znfromwastewaterandiron
thezincremovalcapacitywascalculatedasthezincremoved
ionsproducedduringtheEC,whichcanreactwithcyanideto
per charge loading (Coulomb) or mg Fe (electrochemically formcyanidecomplexes,whenMez+andcyanidearemainly
dissolved iron concentration) at different current densities. inthesolutionasMe(CN)2−n(n=0–4)inthepHrangeof5–10.
Theremovalcapacitywas0.055mgFe/C(0.0971mgFe/CL)or n
Zinc-cyanide interaction at low pH occurs according to the
7.36g/Faradayatacurrentdensityof80A/m2 andanoperat-
followingequation(wherenvariesfrom2to4):
ing time of 20min. The zinc removal efficiency and effluent
zincconcentrationweredeterminedas98.7%and2.64mg/L.
Moreover, zinc removal efficiency of 98.6% was obtained as Zn2++n CN−→Zn(CN)−n(n−2) (7)
0.055mg/C(0.065mgFe/CL)or7.34g/Faradayat60A/m2 and
40min. According to the results, the calculated energy con- Themainmechanisminvolvedintheremovalofzincand
sumptionsforthetreatmentofthewastewaterincreasedwith cyanidefromthewastewatermightbeFeoxidationintofer-
ProcessSafetyandEnvironmentalProtection 105 (2017) 373–385 379
rent efficiency at higher pH values was found to be higher
i
thanthatoflowerpH.Thismassoverconsumptionmaybe
i
duetothechemicalhydrolysisofthecathode,butitcanalso
be explained by the “corrosion pitting” phenomenon which
causedholesandledpracticallytoametallicmetallossonthe
electrodesurface.Thesefindingswerereportedearlier(Kobya
etal.,2011).
The amounts of produced sludge after the EC process
were 2.118kg/m3 at a pH of 8.0, 2.281kg/m3 at a pH of 8.5,
i i
2.673kg/m3 at a pH of 9.0, and 2.845kg/m3 at a pH of 9.5.
i i
Voltages in the EC reactor at pH of 8.0, 8.5, 9.0, and 9.5
i
were measured as 1.27, 1.75, 1.96 and 2.23V, respectively.
In this case, the energy consumptions were calculated as
1.856kWh/m3 at a pH of 8.0, 2.557kWh/m3 at a pH of 8.5,
i i
2.864kWh/m3atapH of9.0,and3.258kWh/m3atapH of9.5.
i i
Fig.5– SpeciationofZn(II)intheZn(II)/CN−/OH−systemas Operating costs at pHi 8.0, 8.5, 9.0, and 9.5 were calculated
afunctionofpH(cyanide=5×10−4M). as 1.497, 1.605, 1.656, and 1.719D/m3, respectively. On the
otherhand,irondosageandadsorptioncapacityatanoperat-
rous ions (Eq. (8)) and concurrent water electrolysis on the ingtimeof60minwere1.380g/Land125.6mgZn/g(114.3mg
surface of anode, resulting in the generation of oxygen (Eq. CN/g)atapHiof8.0,1.551g/Land112.3mgZn/gFe(102.2mg
(9));(b)oxidationofferrousionsintoferriconesthroughreac- CN/gFe)atapHiof8.5,1.586g/Land110.2mgZn/gFe(100.1mg
tionwithoxygenmoleculesandsubsequentformationofiron CN/g Fe) at a pHi of 9.0, and 1.660g/L and 105.4mg Zn/g Fe
hydroxide/polyhydroxide/polyhydroxyoxideprecipitates(Eqs. (95.7mgCN/gFe)atapHi of9.5.Theminimumchargeload-
(10)and(11))dependingonthesolutionpH;and(c)interac- ingpHiof8.0–9.5accordingtothedischargestandardsofthe
tionofzincandcyanideionswithironprecipitates(Eq.(12)). cyanide–zincplatingwastewaterwas5260C/Lor39.39F/m3at
In summary, the surface complexation of zinc and cyanide 60minand60A/m2.
ionswithironprecipitatesformedmightbethepredominant The effect of current density at 20–80A/m2 and initial
mechanismsofzincandcyanideremovalsintheEC(Moussavi pHi 9.5 on treatment of the alkaline cyanide Zn electroplat-
etal.,2011). ing wastewater is shown in Fig. 4(b). The residual zinc and
cyanide concentrations after 60min were 4.80mg/L (97.3%)
Fes→Fe2(a+q)+2e− (attheanode) (8) and 2.20mg/L (98.4%) at 20A/m2, 0.85mg/L (99.5%) and
1.30mg/L (99%) at 40A/m2, 0.25mg/L (99.9%) and 0.10mg/L
H2O+2e−→2H4+1⁄2O2+2e− (9) (99.9%)at60A/m2 and0.16mg/L(99.9%)mg/Land0.10mg/L
(99.9%) at 80A/m2. Cyanide and zinc removal efficiencies
2Fe2(a+q)+3⁄2O2+3H2O→2Fe(OH)3(s) (6<pH<10) (10) increased with the increase in the current density because
moreironionspassedintothewastewateratahighercurrent
nFe(OH)3→Fen(OH)3n (11) densityandtheformationrateofironhydroxidesincreased.
TheeffluentpHsandconductivityvaluesintheECweredeter-
CN−(aq)+Fen(OH)3n floc→CN−Fe precipitatecomplex (12) m40iAne/mdt2o,9b.e79a.n6dan5d.45m.0Sm/cSm/camt6a0t2A0/mA/2m,a2n,9d.69.a8nadn5d.25m.6Sm/cSm/cmat
at80A/m2.Thehighestcyanideandzincremovalefficiencies
The affinity of cyanide for metals in the solution
were obtained at 80A/m2 because the residual Zn concen-
resulted in the formation of different Zn(II)-cyanide com-
tration decreased more with the increase in operating time
plexes (Fig. 5). Yngard et al. (2007) indicated that the
predominant species in Zn-CN-OH− system were Zn(CN)2−, and current density. The amounts of produced sludge after
Zn(CN)−, Zn(CN) OH−, and Zn(CN) OH2− at pH 9.0–11. O4n the EC process were 1.013kg/m3 at 20A/m2, 1.932kg/m3 at
3 2 3 40A/m2,2.845kg/m3 at60A/m2,and2.135kg/m3 at80A/m2.
the other hand, monomeric and/or polymeric iron hydrox-
Valuesoftheotherexperimentalparametersat20,40,60and
ides led to significant improvement in zinc and cyanide
80A/m2were0.477kWh/m3(0.414Aand0.98V),1.393kWh/m3
removalsmainlyduetoco-precipitationandadsorptionlike
Fe(OH)3(s),Fe(OH)(2−z)(O)z(Zn)(s),and(cid:5)Fe(OH)2(s)∗Zn(CN)2n−n(cid:6)(s) (40.7.81248kAWahn/md31.4(13.6V5),63A.2a5n8dkW2h.4/2mV3)(f1o.2r4e2nAeragnyd, 02..52234Vk)g,/amn3d,
(Latkowska and Figa, 2007). The Zn(II)-cyanide complexes
1.075kg/m3, 1.658kg/m3, and 2.185kg/m3 for electrode con-
viaco-precipitationand/oradsorptionmechanismwerepre-
sumptions, 101.4%, 104.2%, 107.1%, and 105.9% for current
sentedinEqs.(13)and(14):
efficiencies and 0.49, 1.05, 1.72 and 2.31D/m3 for operating
(cid:4) (cid:5)
Fe(OH) +Zn(CN)2−n→ Fe(OH) ∗Zn(CN)2−n (13) costs. On the other hand, iron dosages at 20, 40, 60, and
2(s) n 2 n (s) 80A/m2intheECprocesswere0.503,1.085,1.523and2.135g
(cid:4) (cid:5) Fe/L,respectively.Theadsorptioncapacitiesforcyanideand
Fe(OH)3(s)+Zn(CN)2n−n→ Fe(OH)3∗Zn(CN)2n−n (s) (14) zinc removals from the cyanide zinc plating rinse wastew-
ater were calculated as 310.4mg CN/g (397.8mg Zn/g) at
The theoretical electrode consumption at pH 8.0–9.5, 20A/m2, 145.1mg CN/g (188.8mg Zn/g) at 40A/m2, 104.2mg
i
60minand1.242Awascalculatedas1.548kg/m3.Thepracti- CN/g(135.1mgZn/g)at60A/m2,and74.4mgCN/g(96.34mg
calelectrodeconsumptionsatpH of8.0,8.5,9.0and9.5were Zn/g)at80A/m2.CyanideandzincremovalsperCoulomb(C)
i
1.554,1.602,1.628and1.658kg/m3,respectively.Inthiscase, orFaraday’s(F)wereobtainedas0.208mgCN/Cand0.267mg
thecurrentefficienciesatpH 8.0,8.5,9.0,and9.5were100.4%, Zn/C(23638mgCN/Fand30295mgZn/F)at20A/m2,0.049mg
i
104.2%,107.1%and105.8%,respectively.Inaddition,thecur- CN/C and 0.063mg Zn/C (5521mg CN/F and 7191mg Zn/F)
380 ProcessSafetyandEnvironmentalProtection 105 (2017) 373–385
1600
1400 (a) Initia l 5pHi 1900000 (b) j (A/m2)
on (mg/L)11020000 678 n (mg/L) 780000 24680000
Residual zinc concentrati 246800000000 Residual zinc concentratio 123456000000000000
0 0
0 10 20 30 40 50 60 70 0 10 20 30 40 50 60 70
Operating time (t , min) Operating time (t , min)
EC EC
Fig.6– Effectof(a)pH and(b)currentdensityonthetreatmentofacidiczincelectroplatingrinsewastewater.
i
at 40A/m2, 0.0233mg CN/C and 0.0302mg Zn/C (2646mg forapH of6,3.122kg/m3forapH of7,and3.922kg/m3fora
i i
CN/Fand3428mgZn/F)at60A/m2,and0.0125mgCN/Cand pH of8.OperatingcostsatpH of5–8were2.120,2.170,2.279,
i i
0.0162mgZn/C(1415mgCN/Fand1834mgZn/F)at80A/m2. and2.284D/m3,respectively.
The required Faraday’s per m3 treated wastewater at 20, 40, Theeffectofcurrentdensityonthetreatmentoftheacidic
60,80A/m2andanoperatingtimeof60minwere13.13,26,26, zinc electroplating wastewater at a pH of 8 is shown in
i
39.39, and 52.52F/m3, respectively. The optimum conditions Fig. 6(b). Initial acidic zinc concentration in the wastewater
forthetreatmentofthecyanidezincrinsewastewaterinthe was 985mg/L. The residual zinc concentrations after 60min
ECprocesswereanoperatingtimeof60min,apH of9.5(orig- were98.5mg/L(90%)at20A/m2,65.1mg/L(93.4%)at40A/m2,
i
inalpHofthewastewater),andacurrentdensityof60A/m2. 40.8mg/L(95.9%)at60A/m2 and0.8mg/L(99.9%)at80A/m2.
Thehighestremovalefficiencywasobtainedat80A/m2 and
an operating time of 60min because the residual zinc con-
3.3. Treatmentofacidiczincrinsewastewater
centrationdecreasedwiththeincreaseintheoperatingtime
andcurrentdensityaftertheECprocess.Experimentallydis-
Fig.6(a)showstheresidualzincconcentrationsat0–60min, solvedirondosagesfor20,40,60,and80A/m2at60minwere
pH of 5–8 and 80A/m2 for the acidic zinc rinse wastewa-
i 0.501,1.036,1.429and2.116gFe/L,respectively.Theamounts
ter.Theresidualzincconcentrationsat60minreducedfrom
of adsorption capacity were calculated as 1769.5mg/g or
1476.8 to 495.2mg/L Zn (66.47%) for a pHi of 5, from 1266.3 67519mg/Faraday (0.595mg/C) for 20A/m2, 887.9mg/g or
to329.8mg/L(73.96%)forapHi of6,from1012.1to1.6mg/L 35031mg/Faraday (0.309mg/C) for 40A/m2, 660.8mg/g or
(99.84%) for a pHi of 7, and from 985 to 0.80mg/L (99.92%) 23972mg/Faraday (0.211mg/C) for 60A/m2, and 465.1mg/g
for a pHi of 8. The residual zinc concentrations decreased or 18740mg/Faraday (0.14mg/C) for 80A/m2. The current
with the increase in pHi values. Higher zinc removal effi- efficiencies changed from 97.07% at 20A/m2 to 102.50% at
ciencywasachievedathighercurrentdensityandoperating 80A/m2.Theaveragecellvoltagesfor20,40,60,and80A/m2
timeintheECprocess(Fig.6).Over99.5%ofZnremovaleffi-
were determined as 1.124, 1.427, 1.681, and 2.624V, respec-
ciencywasobtainedatapH of7.Valuesofconductivity(initial
i tively.Thevaluesofconductivity,operatingcostsandamounts
valuewas20.4mS/cm)andfinalpHsaftertheECprocesswere of sludge were 13.2mS/cm, 0.49D/m3 and 2.144kg/m3
13.6mS/cmand6.3atapHiof5,13.2mS/cmand6.9atapHi at 20A/m2, 12.6mS/cm, 1.04D/m3 and 2.825kg/m3 at
of6,11.8mS/cmand7.8atapHiof7,10.4mS/cmand8.8ata 40A/m2, 11.7mS/cm, 1.56D/m3 and 3.351kg/m3 at 60A/m2,
pHiof8. 10.4mS/cm,2.26D/m3and3.922kg/m3at80A/m2.Asseenin
Experimentally dissolved iron dosages at pH of 5–8 and
i theresults,theoptimumoperatingconditionsfortreatment
60minintheECprocesswere1.998,2.023,2.124and2.116g
oftheacidiczincelectroplatingwastewaterwereapH of8,
i
Fe/L, respectively. In this case, the amounts of adsorption 80A/m2 and 60min. The zinc removal efficiency and efflu-
capacitywere491.3mg/gor18691mg/Faraday(0.141mg/C)at
ent zinc concentration after the EC process were 99.9% and
a pH of 5, 463.1mg/g or 17832mg/Faraday (0.134mg/C) at a
i 0.80mg/L.
pH of6,475.8mg/gor19241mg/Faraday(0.144mg/C)atapH
i i
of7,and465.1mg/gor18740mg/Faraday(0.14mg/C)atapH
i
of8.Energyandelectrodeconsumptionswerecalculatedas 3.4. TheeffectofEConthetoxicitylevelsof
4.384kWh/m3 and 1.998kg/m3 at a pH of 5, 4.773kWh/m3 wastewater
i
and2.023kg/m3 atapH of6,4.988kWh/m3 and2.124kg/m3
i
atapH of7,and5.112kWh/m3 and2.116kg/m3 atapH of Thetoxicitylevelsofwastewaterweremeasuredbythekinetic
i i
8, respectively. The cell voltage values at pH of 5–8 and an luminescentbacteriatest.Thelightemissionwasmeasured
i
appliedcurrentof1.656A(80A/m2)weremeasuredas2.250, and recorded from the moment of dispensing of the bacte-
2.450, 2.560 and 2.624V, respectively. According to the Fara- rialsuspensiontothesampleuntilthemaximumvaluewas
daylaw,thetheoreticalelectrodeconsumptionatpH of5–8 reachedandafteracontacttimeof30min.Theinhibitionby
i
was2.0644kg/m3(1.656Aand60min).Inthiscase,thecurrent asamplewasexpressedastheeffectiveconcentrations(EC20
efficienciesforpHi of5–8werecalculatedas96.78%,97.99%, andEC50)whichresultedin20%and50%oflightreductions
102.89% and 102.50%, respectively. The amounts of sludge comparedtothecontrolsample.Theratiosofinhibitoryeffect
aftertheECprocesswere2.152kg/m3forapH of5,2.690kg/m3 andeffectiveconcentrationvalueswerecalculatedbymeans
i
ProcessSafetyandEnvironmentalProtection 105 (2017) 373–385 381
(a) (b)
(c)
Fig.7– Theinhibitionratiosof(a)gammavalues,(b)rawwastewaterand(c)treatedwastewateratdifferentdilutionratios.
ofdilutionseries(Fig.7).Thegammavalues((cid:4),ratiooflight centration of the toxic compound is small and the bacteria
lost)werecalculatedusingEq.(15)foreachdilutionlevelofthe areusingitasnutrient(mainlysmallmolecularorganiccom-
testsampleafter30mintoevaluatetherelationshipbetween pounds).Iftheinductionwas>30%,itmeansthattherewere
concentrationandinhibitoryeffectbyusingalinearregression better conditions for the bacteria than the control sample.
technique TheseresultsshowedthattheECprocesspresentedavigorous
alternativeforthetreatmentoftoxicwastewater.
H
(cid:4)30= (100−t Ht) (15) 3.5. Analysesofsludge
where Ht is the inhibitory effect of a sample after a contact Scanningelectronmicroscope(SEM)wasusedtoanalyzethe
timeof30min.EC50forrawacidic,alkalinecyanideandalka- sludgegeneratedundertheselectedconditionstodetermine
line non-cyanide were calculated as 0.62, 5.25 and 3.38 by thesurfacecharacterizationinthisstudy.TheSEMimageof
means of the gamma values, respectively. Results for EC50 theECsludgewascomposedofagglomeratedfinepowder-like
showedthatalltherawwastewaterswerehardlytoxic.EC50 particlesatthemicronscaleandaheterogeneousmorphol-
value of cyanide was reported as 1.91mg/L (Marugán et al., ogy (Fig. 8(a–c)). Energy-dispersive analysis of X-rays (EDAX)
2012) which was lower than alkaline cyanide zinc and alka- wasusedtoanalyzetheelementalconstituentsofthesludge
linenon-cyanidezincelectroplatingwastewaters.EC50 value showninFig.8(d).TheEDAXanalysisprovidedthatthesludge
forametalplatingwastewaterbyChoiandMeier(2001)was wascomposedmainly27%ofzinc,46%ofiron,16%ofoxygen,
measuredas0.7mg/L.Theseobservationswereconsistentto 3%ofpotassiumand8%ofchloride.ItalsoconfirmedthatFe
otherstudiesintheliterature. andZnwereprecipitatedassludge(Fig.8(d)).
Moreover,negativeinhibitionvaluesweremeasuredforthe The infrared spectrum of the sludge also analyzed at
treatedwastewater.Thisphenomenoniscalledasinduction. wavenumbers between 4.000 and 650cm−1 is depicted in
Therearetwopossiblewaysforinduction.Ithappenswhen Fig. 9(a) The strong peaks at 3435–3401cm−1 were assigned
therearenutrientsforthebacteriathatinducethelightoutput to stretching vibration of hydroxyl groups indicating that
by giving better conditions for living and, second the con- hydroxidespeciesweretheprominentintheprecipitates.A
382 ProcessSafetyandEnvironmentalProtection 105 (2017) 373–385
Fig.8– SEMimageofsludgefor(a)acidicZn,(b)alkalinecyanideZn,(c)alkalinenoncyanideZnelectroplatingrinse
wastewatersand(d)EDAXsurfaceanalysisofthesludge.
sharpbandwasobservedat2066cm−1relatedtothecomplex couldbeduetocomplexationwiththeironhydroxideprecip-
of CN–Fe precipitates. Therefore, the cyanide was removed itates.
through complexation/interaction with the iron hydroxide XRD patterns of EC sludge obtained from alkaline non
precipitates.Theotherpeakswereassignedtostretchingfre- cyanide, alkaline cyanide and acidic zinc plating wastew-
quencyofZn–Oat1436–1269cm−1andFe–Oat744cm−1.The aters indicated the presence of the solid product. Sludge
sludgeanalysisindicatedthattheremovalsofZnandcyanide produced at the optimum conditions showed peaks con-