194K.Xieetal./SeparationandPuri cationTechnology76 (2010) 191–197

Extraction percentage (%)

leftafterD2EHPAaddedtothesystem,andtheequilibrium(8)infavorofTiOH2A4prevailed.ThusmoreandmoreTiOH2A4canleadtothecompleteextractionofTi(IV).

TRPOisasolvatingreagent,andthereforefavorscomplexa-tionwithneutralTiOSO4,whiletheanionexchangerN235favorscomplexationwithTiO(SO4)22 .SoitisreasonablethatfairlypoorextractionofTi(IV)(<20%)wasachievedwithN235.ItisworthyofnotethatFe(III)couldbeextractedwithgoodselectivityoverTi(IV)whenN235wasusedastheextractant.3.3.TRPO–PEG2000–Na2SO4TESandTRPO–PEG10,000–(NH4)2SO4TES

SodiumsulfateandPEG10,000wereusedasphase-formingsubstances.TheyofferedaglimpseattheeffectofanotherinorganicsaltandPEGwithhighermolecularweight10,000onthephaseseparationbehaviorofTESandthepartitioningbehaviorofTi(IV),Fe(III)andMg(II).FourdifferentTESsweredesignedasfollows:(1)25wt.%TRPO–15wt.%PEG2000–20wt.%Na2SO4,(2)25wt.%TRPO–15wt.%PEG2000–12wt.%Na2SO4,(3)25wt.%TRPO–15wt.%PEG2000–20wt.%Na2SO4–20wt.%H2SO4,(4)25wt.%TRPO–15wt.%PEG10,000–12wt.%(NH4)2SO4.Amongthem(1),(2)and(4)wereconductedatpH0.5.

Itisobservedthatsubstitutionof(NH4)2SO4withNa2SO4resultedinarelativelymoreviscousPEG-richmiddlephase.Thiscoincideswiththesalting-outabilityofthesalts[27,28],whichisNa2SO4>(NH4)2SO4.Na2SO4showstobemoreeffectiveinphaseseparationandthusatagivenconcentration,Na2SO4inducesmoreincompatibilitybetweenthebottomphaseandtheuppertwophases.Itisclearfromtheliteraturethatincreasingtheincom-patibilitybetweentwophasesincreasestheaf nityofasoluteforaparticularphase[29].SinceTi(IV)andFe(III)preferthetopphase,theextractionpercentagesincreaseastheincompatibilityincreases.BycontrastingTestnumber1inFig.3withitscounter-partinFig.1,anincreaseof15%orsowasfoundat20wt.%saltconcentrationandpH0.5.Interestingly,atalowerconcentration(12wt.%,seeTestnumber2inFig.3)ofNa2SO4,ETi(IV),tandEFe(III),tmaintainedquantitativeratherthandecreasedinresponsetolessincompatibilitybetweenthetopphaseandthebottomphase.Itindicatesthatthesalt-outpowerofthesaltshasalimitedin u-enceonthetransferofmetalionstoorganicextracatantsandotherfactorssuchascoordinationbetweenammoniumandmetalionscanalsobeassumedtoplayarole.

Besides,compared(1)with20wt.%Na2SO4to(2)with12wt.%Na2SO4,(2)gavealargermiddlephasevolume.Similarresultswereobtainedundertheconditionofahigheracidity(20wt.%H2SO4,(3)inFig.3)orahigherPEGmolecularweight(PEG10,000(4)inFig.3).Decreasingsalinity,increasingaciditytogetherwithincreas-ingPEGmolecularweightdirectlyin uencestheequilibriumofthesystemsandfurtherlythemetalions’partitionparameters.Under

120

1101009080706050403020100

1

2

Test number

Fig.3.PartitioningofTi(IV),Fe(III)andMg(II)intheTESscontaining(1)25wt.%TRPO–15wt.%PEG2000–20wt.%Na2SO4,(2)25wt.%TRPO–15wt.%PEG2000–12wt.%Na2SO4,(3)25wt.%TRPO–15wt.%PEG2000–20wt.%Na2SO4–20wt.%H2SO4,(4)25wt.%TRPO–15wt.%PEG10,000–12wt.%(NH4)2SO4.Alltestsexcept(3)wereconductedatpH0.5.

34

suchconditions,morewaterwasincorporatedintothePEG-richmiddlephase.Metalsaccompaniedfreewatertothemiddlephase,hence,ETi(IV),m,EFe(III),mandEMg(II),mbecamehigher.However,thereisapracticallimitonthechoiceofthesalinityandacidityofaqueoussolutionandPEGmolecularmassbecauseofnotenoughofagainindistributionratioandseparationfactor.AndtheuseofhighermolecularweightPEGssufferincreasedviscosityandexpense.3.4.Modi cationofthemiddlephaseofTRPO–PEG2000–(NH4)2SO4TES

N,N-dimethylfomamide,N-methylpyrrolidone,L62,L121andF127weretriedasmiddlephasemodi ersbecausetheycouldbemisciblewithPEG2000andenrichedintothemiddlephase.ItwasprospectedthatN,N-dimethylfomamideandN-methylpyrroli-donecoordinatewithFe(III)viaamideligandandtransferFe(III)tothemiddlephase,andthesepluronicsurfactantsalterthehydrophilic–lipophilicbalanceofthemiddlephasesandsubse-quentlyfacilitatetheloadingofmetalions.Thechemicalstructuresofthesemodi ersareshowninFig.4.Thehydrophobicityofthepolymer-richmiddlephaseincreasedbypartiallyreplacingPEG2000withthetriblockcopolymerformedbyacertainamountofpropyleneoxidemonomer,whichisahydrophobicsegment.

ItisobservedthattheuppertwophasesturnedintoaviscousemulsioninthepresenceofasmallamountofL121orF127.ItismostprobablybecausethelonghydrophobicPPOblocksof

the

Fig.4.Chemicalstructuresof(a)N,N-dimethylfomamide,(b)N-methylpyrrolidone,(c)L62,(d)L121and(e)F127.