ContentslistsavailableatScienceDirect
Journal
of
Hazardous
Materials
j o ur na l ho me p a g e : w w w . e l s e v i e r . c o m / l o ca t e / j h a z m a t
Application
of
aqueous
two-phase
systems
for
the
development
of
a
new
method
of
cobalt(II),
iron(III)
and
nickel(II)
extraction:
A
green
chemistry
approach
Pamela
da
Rocha
Patrício,
Maiby
Cabral
Mesquita,
Luis
Henrique
Mendes
da
Silva,
Maria
C.
Hespanhol
da
Silva
∗GrupodeQuímicaVerdeColoidaleMacromolecular,DepartamentodeQuímica,CentrodeCiênciasExataseTecnológicas,UniversidadeFederaldeVic¸osa,Av.P.H.Rolfss/n,Vic¸osa, MG36560-000,Brazil
a
r
t
i
c
l
e
i
n
f
o
Articlehistory: Received16April2011
Receivedinrevisedform28June2011 Accepted16July2011
Available online 6 August 2011 Keywords:
Aqueoustwo-phasesystems
Liquid–liquidextractionwithoutorganic solvent
Cobalt Iron Nickel
a
b
s
t
r
a
c
t
WehaveinvestigatedtheextractionbehaviorofthemetallicionsCo(II),Fe(III)andNi(II)asafunction of the amount of potassium thiocyanate used as an extracting agent, using the following aque-oustwo-phasesystems(ATPS):PEO+(NH4)2SO4+H2O,PEO+Li2SO4+H2O,L35+(NH4)2SO4+H2Oand
L35+(Li)2SO4+H2O.Metalextractionfromthesalt-richphasetothepolymer-richphaseisaffectedby
thefollowingparameters:amountofaddedextractant,pH,andthenatureoftheelectrolyteandpolymer thatformstheATPS.MaximalextractionpercentageswereobtainedforCo(II)(99.8%),Fe(III)(12.7%)and Ni(II)(3.17%)whentheATPSwascomposedofPEO1500+(NH4)2SO4+H2Ocontaining1.4mmolofKSCN
atpH4.0,providingseparationfactorsashighasSCo,Fe=3440andSCo,Ni=15,300.However,whenthe
sameATPSwasusedatpH2.0,themaximalextractionpercentagesforironandnickelwere99.5%and 4.34%,respectively,withSFe,Niequalto4380.Theproposedtechniquewasshowntobeefficientinthe
extractionofCo(II)andFe(III),withlargeviabilityfortheselectiveseparationofCo(II)andFe(III)ionsin thepresenceofNi(II).
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Nickelandcobaltareimportantmetalsbecausetheyare exten-sivelyemployedincommerceandindustry[1].Theyareusedinthe fabricationoftheelectrodesofrechargeablebatteries.These bat-teriesareusedmainlyascomponentsofelectro-electronicitems suchascellphonesandportablecomputers[2–4].
The worldwide demand for electro-electronic equipment is growingdrasticallyduetotheimportanceofinformation technol-ogy.Asaresult,therehasbeenasharpincreaseintheproduction ofrechargeablebatteries[5,6],whoseinternalcomponents con-tributetoenvironmentalpollutionwhendisposed[5].Therefore, forthebenefitof theenvironment, efficientrecyclingprocesses forthemetalliccontentofthesematerialsmustbedevelopedto avoidthedischargeofdangerousresidues[7–9].Economic advan-tagescanalsobeobtainedfromthehighaddedvalueofsuchmetals [10,11].
Therecoveryofnickel and cobaltfromelectronicwasteand natural resources is usually achieved by applying pyrometal-lurgical or hydrometallurgical processes [12,13]. However, the
∗ Correspondingauthor.Tel.:+553138992175;fax:+553138993065. E-mailaddress:[email protected](M.C.H.daSilva).
pyrometallurgical route consumes a large amount of energy, and pollutinggasesareemitted duringtheprocess,which may causeconsiderabledamagetohumanhealthandtheenvironment [2,8,14].
Thehydrometallurgicalrouteconsistsofthedissolutionofthe desired metals bymeansof alkalineor acidleaching. Afterthe leachingstep,thegeneratedmetalsolutionscanbefurther submit-tedtotheprocessesofseparation,namelychemicalprecipitation, solventextractionandelectrodeposition[15,16].
Aseriousproblemintheseparationofnickelandcobaltfrom aqueoussolutionsisthesimilarityinthepropertiesand charac-teristicsofthesemetals[4,17,18].Solventextractionisthemost efficientand widelyused techniquefor theseparation of these metallicionsfromaqueoussolutions[19–22].However,this tech-niqueinvolvestheuseoforganicsolventsthataremostlytoxic and/orflammable[23].
In recent years, alternative technologies for the separation and recoveryof metals have been implemented. In these new approaches, aqueous two-phase systems (ATPS) are proposed forthedevelopmentofenvironmentallysafetechniquesfor the extractionand/orrecoveryofmetals[24–26].Theseareefficient liquid–liquidextractionoperationsthathavehighextractionyields, low costand do notrequireorganicsolventsbecausethemain componentofthechemicalsystemsusediswater[27–33]. 0304-3894/$–seefrontmatter © 2011 Elsevier B.V. All rights reserved.
ATPSarespontaneouslyformedbymixingaqueoussolutions oftwochemicallydifferenthydrophilicpolymersorbycombining aqueoussolutionsofapolymerandanelectrolyteunderspecific thermodynamicconditions,therebypromotingtheseparationof twoimmiscibleaqueousphasesinequilibrium.Ingeneral,oneof thephasesisrichinonepolymer,whilsttheothercontainsmore electrolytesortheotherpolymer[34–38].
In this work, the extraction behaviors of the metallic ions Co(II), Fe(III) and Ni(II) were separately investigated with the following ATPS: L35+Li2SO4+H2O, L35+(NH4)2SO4+H2O,
PEO1500+Li2SO4+H2O and PEO1500+(NH4)2SO4+H2O, in the
presenceofKSCNasanextractingagent.Theinfluenceofsome parametersontheextractionpercentage(%E)ofthemetallicions wasalsoanalyzed,namelythechemicalstructureofthepolymer, thenatureoftheATPS-formingelectrolyte,thepHandtheamount ofextractantadded.
2. Experimental 2.1. Equipment
AWTWpH-meter(Wissenschaftlich-TechnischeWerkstätten, pH330i/SET)wasusedtomeasurethepHofallsolutions.TheATPS werepreparedbyweighingproperamountsofallcomponentsona ShimadzuAY220analyticalbalance,withprecisionof±0.0001g.A ThermoScientific(HerauesMegafuge11R)centrifugeanda Micro-quimica(MQBTC99-20)thermostaticbathwerealsousedinthe preparationof theATPS.Theconcentrations of cobalt,iron and nickelinthestandardsolutionsandthetopphaseoftheATPSwere determinedbyflameatomicabsorptionspectroscopy(AAS)witha VARIANAA-240spectrometer.Cobalt,ironandnickelwere deter-minedwithanair–acetyleneburneratwavelengthsof240.7,248.3 and232.0nmandlampcurrentsof7,5and4mA,respectively.The aperturewidthwas0.2nm.
2.2. Materialsandchemicals
Allchemicalswere of analytical gradeand used asreceived withoutfurtherpurification.TheATPSwerepreparedwiththe tri-blockcopolymerL35,HO–(EO)11(PO)16(EO)11–H,withanaverage
molarmass(MM)of1900gmol−1,orwithpoly(ethylene oxide) (PEO1500),withMM=1500gmol−1,bothacquiredfromALDRICH (Milwaukee, WI, USA). The chemicals (NH4)2SO4, Li2SO4·H2O,
H2SO4, KSCN, FeCl3·6H2O, CoCl2·6H2O and Ni(CH3COO)2·4H2O
werepurchased fromVETEC (Rio deJaneiro, Brazil). Deionized water(Milli-Q,Millipore)wasusedinthepreparationofall solu-tionsusedintheexperiments.
2.3. CompositionofATPS
Four different ATPS were tested in the extraction assays, as follows: L35+Li2SO4+H2O, L35+(NH4)2SO4+H2O,
PEO1500+Li2SO4+H2O, and PEO1500+(NH4)2SO4+H2O. Each
ATPS was prepared at the desired concentration, as shown in Tables1and2,andwasformedwiththeadditionof2.0gofthe copolymer (or polymer) solution containing different amounts of the KSCN extractant with 2.0g of the electrolyte solution containingthemetallicionataconcentrationof0.500mmolkg−1. For each amountofadded extractant,all respectiveATPSwere preparedintriplicate.
2.4. Liquid–liquidextraction
Aftermixingspecificamountsofthepolymerandsaltsolutions, theATPSweremanuallystirredforapproximately5minandthen centrifugedat20,257×gs−1 forafurther5mintoinducephase
1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0 20 40 60 80 100 % E
Amount of KSCN added / mmol
Fig.1. TheeffectoftheamountofKSCNaddedtotheATPStopphaseonthe%Eof Co(II),Fe(III)andNi(II)usingtheL35+(NH4)2SO4+H2OsystematpH1.0:(䊉)cobalt; ()iron;()nickel;[metal]=0.250mmolkg−1;T=25.0◦C.
separation. Then, theywereplaced inthe thermostatic bathat 25.0◦Cfor20mintoattainthermalequilibrium.Aliquotsfromthe topphasewerecollectedanddiluted,and theconcentrationsof themetallicionsweredeterminedintheflameAAS.Theextraction percentages(%E)ofallionswerethencalculatedwithEq.(1): %E= nTPM
nT M
×100 (1)
wherenTP
M isthenumberofmolesofthedesiredmetallicioninthe
topphaseandnT
M isthetotalnumberofmolesofmetallicionsin
thesystem.
2.5. Effectoftheamountofaddedextractant
The%EofCo(II),Fe(III)andNi(II)wasassessedbyadding differ-entamountsoftheKSCNextractanttothetopphaseofeachsystem, rangingbetween0and1.4mmol,withincrementsof0.2mmol. 2.6. EffectofpH
TheinitialpHofthedeionized water was5.30.The pHwas adjustedbyaddingH2SO4solution(18molL−1).Thepolymerand
saltsolutions werethen preparedatthedesiredconcentrations usingtheseacidicsolutionsassolvents,withpH1.0,1.5,2.0or4.0. 3. Resultsanddiscussion
3.1. Extractionbehavior
Theliquid–liquidequilibriumdataoftheATPSusedinthiswork aredescribedelsewhere[40],andthelargesttie-linelengths(TLL) ofeachATPSwereselected,correspondingtophaseswithvery dis-tinctintensivethermodynamicproperties,whichallowforhigher extractionpercentages[28].
Fig.1summarizesthe%EofthemetallicionsCo(II),Fe(III)and Ni(II)addedtotheL35+(NH4)2SO4+H2OsystematpH1.0asa
functionoftheamountofKSCNintheATPStopphase.Theresults indicatethatthemetallicionscannotbeextractedtothetopphase fromthebottomphasewithlowamountsofKSCN(≤0.2mmol). TheextractionofCo(II)andFe(III)onlyoccurswhentheamountof addedKSCNis0.4mmol,withyieldsashighas68.0%forCo(II)and 39.0%forFe(III),withinsignificantextractionofNi(II).Anysmall variationsintheamountofKSCNhigherthan0.4mmolabruptly
Table1
Concentrationsofpolymerandsaltinthetopphase,bottomphaseandoverallcompositionoftheATPSexaminedat25.0◦Cforthetie-linelengthtested.
ATPS Composition/%(w/w)
Overall Topphase Bottomphase
Salt Polymer Salt Polymer Salt Polymer
L35+(NH4)2SO4+H2O 10.66 29.92 1.21 57.16 20.11 2.68
L35+Li2SO4+H2O 10.03 27.60 0.50 54.25 19.55 0.95
PEO+(NH4)2SO4+H2O 17.11 30.44 1.76 56.04 32.45 4.84
PEO+Li2SO4+H2O 12.14 29.43 2.15 53.26 22.12 5.60
enhancesthe%E ofCo(II) andFe(III),andmaximalyieldswere
obtainedwith1.0mmolofKSCNforFe(III)and1.4mmolofKSCN
forCo(II)andNi(II).Thesemaximalvalueswere(99.7±0.4)%for
Co(II),(84.4±0.3)%forFe(III)and(15.3±0.3)%forNi(II).
Intheabsenceoftheextractingspecies(SCN−),allmetallicions
concentratedinthesalt-richphase.Thisbehavioriscausedby
spe-cific interactions ofthe metalliccations withthesulfate anion,
therebyformingacomplexspecies(Eqs.(2)and(3)).
Mm+(aq)+xSO42−(aq)M(SO4)x(2x−m)
− (aq) (2) K M(SO4)x(2x−m)− = M(SO 4)x(2x−m)−·[M(SO4)x (2x−m)− ] Mm+·[Mm+]·SO2−4 ·[SO2−4 ] x (3) InEq.(3),K
M(SCN)(2x−m)−x isthestandard thermodynamic
con-stantofformationofthemetal–sulfatecomplex,Xistheactivity
coefficientoftheionicspeciesX,and[X]istheconcentrationof suchspecies (X=Mm+ orSO
42− orM(SO4)x(2x−m)
−
).Thestability constantdependsontheelectronicstructureofthecentralmetal ion. The metal–sulfate complexes can bepreferentially formed inthefollowingorder:Fe(logK=5.38)[41]>Co(logK=2.50)>Ni (logK=2.40)[42].Themetalextractionefficiencyisinversely pro-portionaltotheconstantofformationofthecomplexifonlythe metal–sulfateinteractionsareconsidered.
However,anincrementaladditionintheamountofpotassium thiocyanateaffecttheextractionbehaviorofthemetallicions,and themetalextractionpercentageswereobservedtoincrease.This isdue totheformationofdifferentanioniccomplexes between the metallic ion and the thiocyanate species and alsobecause oftheultimatepreferentialtransferofsuchcomplexesfromthe electrolyte-richphasetothephasethatismoreconcentratedin L35.Theformationprocessofthenewcomplexesandthestability constantarerepresentedbyEqs.(4)and(5),respectively: Mm+(aq)+xSCN−(aq)M(SCN) x(x−m)−(aq) (4) K M(SCN)(xx−m)− = M(SCN)(x−m)− x · [M(SCN)(x−m)x −] Mm+·[Mm+]·SCN−·[SCN−]x (5) where K
M(SCN)(x−m)−x is the standard thermodynamicconstant of
formationofthemetal–thiocyanatecomplex,Xistheactivity
coef-ficientoftheionicspeciesXand[X]istheconcentrationofsuch species(X=Mm+orSCN−orM(SCN)
x(x−m)
− ).
Theextractionofametalliccomplexintothetopphaseis gov-ernedbytwomainfactors:thecompetitionbetweentheanionic bindingagents(SCN−versusSO42−)forthemetalandthe
interac-tionoftheresultantcomplexwiththemacromoleculepresentin thetopphase.
ThestabilityconstantoftheFe–sulfatecomplex(logK1=4.04;
logK2=1.34) [41] is higher than that of the Fe–thiocyanate
complex(logK1=1.96;logK2=2.02;logK3=−0.41;logK4=−0.14;
logK5=−1.57; logK6=−1.51) [43], which does not enable the
extractionofFe(III)tothetopphase.However,aftertheformation of theM(SCN)x(x−m)− complexes, theyarespontaneously trans-ferredtothetopphaseasaresultofpolymerandanioninteractions. AstheconcentrationoftheM(SCN)x(x−m)−anionincreasesinthe topphase,itsconcentrationinthebottomphaseisreduced,which causestheequilibriumshownbyEq.(4)tobedisplacedtoward theformationofthemetal–SCN−complex.TheEO–M(SCN)x(x−m)− interactiondependsonthenatureofthecentralatom.daSilvaetal. [44] havedemonstrated thatthepartitionof the[M(CN)5NO]x−
anionisstronglyaffectedbythecentralatom.Thelowvaluesof %EfortheNi(II)ionsinthesesystemsmaybeattributedtothe for-mationofcomplexessuchas[Ni(SCN)4]2−,whichinteractweakly
withthepolymersegments[27].
AnimportantaspectthatmustbenotedisthatFe(III)and/or Co(II) maybeseparatedor pre-concentratedin thepresenceof Ni(II)iftheratioof[SCN−]/[Mn+]≥600.
Theresultsof this workaresimilartotheonesreportedby Shibukawaetal.[25],therebycorroboratingtheefficiencyofthe ATPS in the preferential extraction of some metallic ions [27]. Shibukawaetal.[25] preparedATPSwithPEOand Na2SO4 and
investigatedtheextractionbehaviorofmetallicionsinthe pres-enceof thiocyanate(SCN−)andiodide(I−)asextractingagents. Theyobserved%Evaluesashighas75%forCd(II)ionsandover90% forCo(II),Cu(II),Fe(III)andZn(II)ionswhenthiocyanatewasused. 3.2. EffectofpH
Fig.2showstheinfluenceofpHontheextractionpercentagesof Co(II),Fe(III)andNi(II),asafunctionoftheamountofKSCN,forthe PEO1500+(NH4)2SO4+H2Osystem.Theresultsdemonstratethat,
regardlessofthepH,onlyasmallamountofthemetallicionswas extractedwithlowamountsofKSCNadded.
Similar to the ATPSprepared withL35, the increase in the amountofKSCNfavoredtheextractionofCo(II) andFe(III)into thetopphase;however,thesameeffectwasnotobservedforNi(II)
Table2
ConcentrationsofpolymerandsaltinthestocksolutionsusedforthepreparationoftheATPS.
ATPS TLLa/%(m/m) Concentration/%(m/m) Reference
Polymer Electrolyte L35+(NH4)2SO4+H2O 57.67 59.84 21.32 [34] L35+Li2SO4+H2O 56.60 55.20 20.05 [35] PEO+(NH4)2SO4+H2O 59.69 60.88 34.21 [39] PEO+Li2SO4+H2O 51.67 58.86 24.27 [40] aTie-linelength.
1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0 20 40 60 80 100 % E
Amount of KSCN added / mmol
Co(II) 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0 20 40 60 80 100 % E
Amount of KSCN added / mmol
Fe(III) 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0 20 40 60 80 100 % E
Amount of KSCN added / mmol
Ni(II)
Fig.2.TheinfluenceofpHonthe%EofCo(II),Fe(III)andNi(II)withdifferentamounts ofKSCNaddedtotheATPStopphase:PEO1500+(NH4)2SO4+H2Osystemat(䊉)pH 1.0;()pH1.5;()pH2.0;()pH4.0;[metal]=0.250mmolkg−1;T=25.0◦C.
ions.BetweenpH1.0andpH4.0,nosignificantinfluenceofpHon theextractionofionswasdetected,exceptforFe(III),whichwas extractedwithayieldofonly(12.7±1.9)%atpH4.0.
For systems formed with L35+(NH4)2SO4+H2O, there was
almostnoinfluenceofpHontheextractionofCo(II),withamaximal %E of(93.8±1.0)%at pH1.5 and (99.7±0.4)%at pH1.0. How-ever,the%EforFe(III)rangedbetween(24.5±0.8)%atpH4.0and (98.3±2.0)%atpH1.5.ForNi(II),nosignificantinfluenceofpHon the%Ewasobserved.ForsystemsformedwithL35+Li2SO4+H2O,
variationsinpHdidnotsignificantlyaffecttheextractionofthe threemetallicionsexamined.
Similarly, there wereno significantdifferences in themetal extractionpercentagesfortheATPSPEO1500+Li2SO4+H2OatpH
1.0andpH2.0.However,atpH4.0,the%Eofallmetalswiththis ATPSwasreduced,withvaluesashighas(76.9±7.5)%forCo(II) and(14.5±0.3)%forNi(II).TheFe(III)ionscouldnotbeextracted becauseprecipitationoccurredatpH4.0.
3.3. Effectoftheelectrolyte
Fig.3isacomparisonofthe%EvaluesforCo(II)andNi(II)in systemsformedwithL35(59.84%,w/w)and(NH4)2SO4(21.32%,
w/w),andL35(55.20%,w/w)andLi2SO4(20.05%,w/w),bothinthe
presenceofKSCNasanextractingagent.
The maximal extraction percentages for Co(II) ions were (98.6±1.2)% when (NH4)2SO4 was used as electrolyte and
(99.6±0.3)%inthepresenceofLi2SO4.ForNi(II)ions,the
maxi-mal%Evalueswere(14.6±0.2)%with(NH4)2SO4and(45.9±0.2)%
withLi2SO4.
InFig.4,thesamecomparisonis shownforCo(II) andNi(II) insystemsformedwithPEO1500(60.88%,w/w)and (NH4)2SO4
(34.21%,w/w),and PEO1500(58.86%,w/w)andLi2SO4 (24.27%,
w/w), both in the presence of KSCN. The Co(II) ions couldbe extractedwithamaximalyieldof(99.4±0.8)%inthepresenceof (NH4)2SO4andamaximalyieldof(96.1±1.0)%whenLi2SO4was
used.ForNi(II)ions,themaximal%E valueswere(4.34±0.18)% with(NH4)2SO4and(19.7±0.5)%withLi2SO4.
TheATPScontainingLi2SO4aselectrolyteweremoreefficient
thanthosethatcontained(NH4)2SO4intheextractionofthemetals
examinedinthiswork.ThiseffectisevidentinFigs.3and4. The significant increase in the values of %E for lithium-containingATPSresultsfromthestronginteractionsbetweenLi+
cationsandtheethyleneoxideunits(EO)ofthemacromolecule. The cation–EO interactions induce the formation of a pseudo-polycation.Consequently,a positive chargedensityforms along themacromoleculesofboththePEO1500andL35polymers,and theanioniccomplexesinteractelectrostaticallywithsuch pseudo-polycations, thereby beingpreferentially transferred to thetop phaseoftheATPS[45].
3.4. Effectofthehydrophobicityofthephases
Onlyfew reports foundin theliterature focus onthestudy of iontransfer in ATPS,but almostall refer tosystemsformed withPEO and differentelectrolytes. To thebest of ourcurrent knowledge,onlytwoinvestigationsdiscusstheeffectof hydropho-bicity in the partition of ions. Rodrigues et al. [27] examined theextractionbehaviorof Ni(II),Fe(III),Cd(II),Co(II), Zn(II) and Cu(II)asa functionof theamountof extractantaddedintothe ATPS formed with a triblock copolymer (L35) and Na2SO4 in
thepresenceofiodideorthiocyanate.Themolecularinteraction betweentheL35copolymerand theelectrolytesisless intense thanthatobserved intheATPSformedwithPEOandinorganic salts.This fact suggeststhat theextraction is moreefficient in PEO/saltATPSwhen bothbiphasicsystems areprepared inthe sameoverall composition.Thus,theextractionefficiencyofthe metallicionsishighlydependentonboththespeciestobe par-titionedandthechemicalstructureofthepolymerthatformsthe ATPS.
da Silva et al. [46] studied the partition behavior of [Fe(CN)5(NO)]2− and[Fe(CN)6]3−inATPSformedwithatriblock
copolymerorPEOandpotassiumphosphate.Twotriblock copoly-merswithdifferenthydrophobicities weretested:L35(EO50% and1900gmol−1)andF68(EO80%and8400gmol−1).Therelative magnitudeofthepartitioncoefficientfortheanionstestedfollowed
1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0 20 40 60 80 100 % E
Amount of KSCN added / mmol Co(II) (A) 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0 20 40 60 80 100 % E
Amount of KSCN added / mmol Ni(II)
(B)
Fig.3. TheeffectoftheATPS-formingelectrolyteonthe%EofthemetalswithdifferentamountsofKSCNaddedtotheATPStopphase;systemsformedwithL35atpH2.0: (A)Co(II)+(䊉)(NH4)2SO4or()Li2SO4;(B)Ni(II)+()(NH4)2SO4or()Li2SO4;[metal]=0.250mmolkg−1;T=25.0◦C.
1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0 20 40 60 80 100 % E
Amount of KSCN added / mmol Co(II) (A) 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0 20 40 60 80 100 % E
Amount of KSCN added / mmol Ni(II)
(B)
Fig.4. TheeffectoftheATPS-formingelectrolyteonthe%EofthemetalswithdifferentamountsofKSCNaddedtotheATPStopphase;systemsformedwithPEO1500atpH 2.0:(A)Co(II)+(䊉)(NH4)2SO4or()Li2SO4;(B)Ni(II)+()(NH4)2SO4or()Li2SO4;[metal]=0.250mmolkg−1;T=25.0◦C.
theorderofL35<F68<PEO.Thus,itisclearthatthepartition coef-ficientdependsonthehydrophilic-hydrophobicbalance(HLB)of thepolymerused.Thetypicaleffectobservedisthedecreaseinthe partitioncoefficientwhenhydrophobicityincreases.
From these studies, it could be verified that the effect of hydrophobicityofthephaserichinmacromoleculesonthe par-titionof ionsis veryhigh.Thepartition of[Fe(CN)5(NO)]2− and
[Fe(CN)6]3−isimpairedwithincreasinghydrophobicity,whilstthe
1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0 20 40 60 80 100 % E
Amount of KSCN added / mmol (A) 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0 20 40 60 80 100 % E
Amount of KSCN added / mmol (B)
Fig.5. TheinfluenceofthehydrophobicityoftheATPSphasesonthe%EofthemetalswithdifferentamountsofKSCNaddedtotheATPStopphase;systemsformedwith (NH4)2SO4atpH4.0:(A)Co(II)+(䊉)L35or()PEO;Fe(III)+()L35or()PEO;(B)Ni(II)+()L35or()PEO;[metal]=0.250mmolkg−1;T=25.0◦C.
opposite is observed for complexes with the general structure ofM(SCN)x(x−m)−, whichtend tobemorepartitioned whenthe hydrophobicityincreases.
Fig. 5 shows the results of the study on the effect of the hydrophobicityoftheATPStopphasesontheextractionefficiency ofCo(II),Fe(III)andNi(II),withvariousamountsofKSCNadded.
TheresultsindicatethepreferentialtransferofCo(II)ionsto thephasesenrichedwithPEO1500orL35.However,theextraction behaviorissignificantlydifferentfortheexaminedions,depending onthestructureofthemacromoleculesthatformtheATPS.
The addition of an extracting agent to the system favored the extraction of Co(II), with maximal yields of (99.5±0.8)% for the L35+(NH4)2SO4+H2O system and (99.8±0.2)% for the
PEO1500+(NH4)2SO4+H2Osystem.Thesameeffectcouldnotbe
observedforNi(II)andFe(III),whichcouldbeextractedwith max-imal yields of only (13.6±0.3)% and (24.5±0.8)%, respectively, when usingthe L35ATPS, and (3.17±0.22)% and (12.7±1.9)%, respectively,whenusingthePEO1500ATPS.Allmaximal extrac-tionpercentagesareaffectedwhen1.4mmoloftheKSCNextractant wasaddedtothetopphaseoftheATPS.
Furthermore,theseresultshighlightthatCo(II)ionscanbe sepa-ratedorpre-concentratedinthepresenceofNi(II)andFe(III)when usingbothATPS.However,theamountofKSCNrequiredforsuch separationisverydifferentineach system.IntheL35ATPS,the amountof KSCNmustbehigherthan0.4mmol,whilstthe sys-temformedwithPEO1500requiresmorethan0.9mmolofKSCN toproducereasonableextractionlevels.
This difference in behavior is justified by the enhanced hydrophobicityofthecopolymermacromolecule,whichfeatures propylene oxide (PO) units on its chain. Although the macro-molecules are solvated in the aqueous medium, there is less interactionbetweenthePOunitsandthewatermolecules,thereby favoringintermolecularinteractionsofthePOunitsthemselves. Thisisaresultoftheso-calledhydrophobiceffect.Abovethecritical micelleconcentration(CMC)oftheL35copolymer,self-assembled micelle aggregates appear, each one comprising a hydrophilic coronaformedbyEOunitsandahydrophobiccorecomposedof POunits.
Therefore,itisbelievedthatthiocyanateionsarecoordinated withCo(II)ionsbythenitrogenatomwiththepurposeof gener-atingtetrahedralcomplexessuchas[Co(NCS)4]2−.Itisknownthat
suchcomplexesdonotreadilyforminaqueoussolutionand, there-fore,mustbetransferredtotheATPStopphase,wherehydrophobic regionsexistasaresultofthecopolymeraggregationnuclei[27].
Rodriguesetal.[27]haveconfirmedthatthehydrophobicityof thetopphasecausedbythepresenceofL35micellessignificantly enhancesboththeextractionpercentageandtheselectivityofthe metallicions,corroboratingtheresultspresentedherein.
3.5. SeparationFactor(SM,N)
Theseparationfactor(SM,N)expressestheefficiencyof
separa-tionoftwospecies,MandN,inliquid–liquidextractionoperations [21].ItcanbegivenbyEq.(6):
SM,N=DDM
N (6)
whereDMisthedistributioncoefficientofspeciesMandDNisthe
distributioncoefficientoftheconcurrentspeciesN.Thedistribution coefficientofanygivenspeciesisexpressedbyEq.(7):
DM= 100%−E%E (7)
Fortheeffectiveseparationofthemetallicions,itisessential
thatthevaluesofSM,Nrangebetween103and104. Table
3 Separation factors SCo,Ni , SCo,Fe and SFe,Ni obtained for different ATPS as a function of pH. SAB SCo,Ni SCo,Fe SFe,Ni pH 1.0 pH 2.0 pH 4.0 pH 1.0 pH 2.0 pH 4.0 pH 1.0 pH 2.0 pH 4.0 L35 + (NH 4 )2 SO 4 (1.83 ± 0.05)10 3 (9.93 ± 0.38)10 2 (1.27 ± 0.04) 10 3 (9.54 ± 0.39) 10 1 (1.66 ± 0.07) (6.12 ± 0.29) 10 2 (1.92 ± 0.09) 10 1 (5.99 ± 0.31) 10 2 (2.07 ± 0.12) PEO + (NH 4 )2 SO 4 (1.43 ± 0.05)10 3 (3.66 ± 0.22)10 3 (1.53 ± 0.15) 10 4 (0.776 ± 0.014) (0.834 ± 0.013) (3.44 ± 0.73) 10 3 (1.84 ± 0.06) 10 3 (4.38 ± 0.26) 10 3 (4.43 ± 1.03)
TheATPSthatproducedthelargestdifferencesinthe%Evalues
werethoseformedbyammoniumsulfate((NH4)2SO4).The
corre-spondingvaluesofSCo,Ni,SCo,FeandSFe,Ni werethencalculatedat
differentpHs,andtheresultsareshowninTable3.TheSCo,Ni,SCo,Fe
andSFe,NivaluesobtainedfortheL35+(NH4)2SO4+H2OATPSatpH
2.0werecalculatedwiththeadditionof1.0mmolofKSCNtothe topphase.TheseparationfactorsobtainedforallotherATPSwere calculatedwiththeadditionof1.4mmolofKSCN.
Co(II) ions can be separated from Ni(II) ions by using the PEO1500+(NH4)2SO4+H2O system at pH 4.0, because
in this case, SCo,Ni=(1.53±0.15)×104. If this ATPS is again
used at pH 4.0, one can separate Co(II) from Fe(III), with SCo,Fe=(3.44±0.73)×103.TheseparationofFe(III)andNi(II)can
beeffectedbyusingthePEO1500+(NH4)2SO4+H2OsystematpH
2.0,withSFe,Ni=(4.38±0.26)×103.
Based ontheresults listedin Table3, it is evident thatthe PEO1500+(NH4)2SO4+H2Osystemisresponsiblefortheselective
extractionofCo(II)andFe(III)inthepresenceofNi(II)andalsofor theselectiveextractionofCo(II)inthepresenceofFe(III). 4. Conclusion
An alternative technique that can replace traditional liquid–liquid extraction operations has been proposed in this work for the extraction and/or separation of Co(II), Fe(III) and Ni(II)ions. Thetechnique employsaqueoustwo-phase systems (ATPS)thatarebasedontheprinciplesofgreenchemistry,with goodefficiencyandeconomicalviability.TheATPSwereprepared witheithertheL35triblock copolymerorpoly(ethylene oxide), withanaveragemolar massof 1900gmol−1 and 1500gmol−1, respectively,andwitheitherammoniumsulfate((NH4)2SO4)or
lithiumsulfate(Li2SO4)astheelectrolyte.KSCNwasalsousedas
anextractingagent.Ithasbeendemonstratedthattheextraction efficiencyofthesemetalsisaffectedbythefollowingparameters: amount of added extracting agent, pH, type of electrolyte and thenatureoftheATPS-formingpolymer.TheuseofLi2SO4asan
ATPS-formingelectrolyteinstead of(NH4)2SO4 produces higher
extractionpercentages(%E)ofthemetallic ionsexamined, par-ticularlyincomparisonwithnickel.Forthismetal,thevaluesof %Evariedfrom(14.6±0.2)%to(45.9±0.2)%whentheL35system wasused,andfrom(4.34±0.18)%to(19.7±0.5)%,withsystems formed by PEO1500 at pH 2.0, when (NH4)2SO4 was replaced
withLi2SO4.Itcouldalsobeobservedthatwithloweramountsof
KSCN,theL35copolymercouldproduce%Evaluescloseto100% forCo(II)andFe(III).However,higherseparationfactorscouldbe obtainedwiththePEO1500+(NH4)2SO4+H2OsystematpH4.0
containing 1.4mmol of KSCN, namelySCo,Fe=(3.44±0.73)×103
andSCo,Ni=(1.53±0.15)×104,withmaximal%EforCo(II),Fe(III)
andNi(II)equalto(99.8±0.2)%,(12.7±1.9)%and(3.17±0.22)%, respectively.However,ifthesameATPSispreparedatpH2.0with thesameamountofKSCN, themaximalextractionpercentages were (99.5±0.8)% for Fe(III) and (4.34±0.18)% for Ni(II), with SFe,Co=(4.38±0.26)×103.The technique proposedin this work
hasproventobeefficientfortheextractionofCo(II)andFe(III), withgreat viability for theselectiveseparation ofCo(II) inthe presenceofNi(II)andFe(III)atpH4.0.Also,theselective separa-tionofFe(III)canbeeffectedinthepresenceofNi(II)atpH2.0, duetostronginteractionsbetweentheanioniccomplexandthe macromoleculesintheATPS.
Acknowledgements
TheauthorsarethankfultoFundac¸ãodeAmparoàPesquisa do Estado de Minas Gerais (FAPEMIG), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and Instituto
NacionaldeCiênciaseTecnologiasAnalíticasAvanc¸adas(INCTAA) for financial support. P.R.P. is thankful to the Coordenac¸ão de Aperfeic¸oamentodePessoaldeNívelSuperior(CAPES)fortheM.Sc. postgraduateresearchscholarship.M.C.M.acknowledgesCNPqfor theM.Sc.postgraduateresearchscholarship.
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