StudyofReverseOsmosisSeparationandPermeationRatefo

StudyofReverseOsmosisSeparationandPermeationRateforSulfonatedPoly(2,6-dimethyl-l,4=phenyleneoxide)MembranesofDifferentIonExchangeCapacitiesAllHAMZA,GEETACHOWDHURY,1.MATSUURA,*andS.SOURIRAJANIndustrialMembraneResearchInstitute,DepartmentofChemicalEngineering,UniversityofOttawa,Ottawa,Ontario,Canada,K1N6N5SYNOPSISThereverseosmosisperformanceofthin-filmcompositemembranespreparedfromsul-fonatedpolyphenyleneoxidepolymerwithdifferention-exchangecapacitieswasstudiedusingvariouselectrolytesolutes.Theeffectofthesolventusedforthepreparationofthepolymersolutionforsurfacecoatingwasalsostudied.Itwasfoundthatthepreparationofthin-filmcompositemembraneswithhighselectivitiesandhighfluxeswaspossiblebyadjustingproperlytheion-exchangecapacityandthesolvent.Itwasalsofoundthatthemembraneperformancewasgovernedprimarilybytheion-exchangereactionbetweenthesolutecationandtheprotoninSO,HandtheDonnanequilibrium.01995JohnWiley&Sons,Inc.INTRODUCTIONIntherecentyearsanumberofchargedthin-filmcompositemembraneshavebeendeveloped.Thesemembranesexhibithighwaterfluxandareresistanttochlorine,highandlowpH,andhightempera-ture.','Theselectivityofthesemembranesispri-marilycontrolledbytheelectricchargeofthethinselectivelayeratthemembranesurface.Becauseofthethinnessoftheselectivesurfacelayer,thechargedcompositemembraneisalsofeaturedbyahigherfluxoraloweroperatingpressurethanthoseofneutralmembrane^.^,^Thisprinciplehasbeenappliedindustriallyforthedevelopmentofnano-filtrationmembraneswhoseperformancecharac-teristicsarebetweenthoseofreverseosmosisandultrafiltrationmembranes.Studiesontheperformanceofchargedcompositemembraneshavebeenreportedbyanumberofau-thor~.~-~Normally,themembranesarenegativelycharged,andelectrolytesoluteswithhigheranionicchargedensitiesand/orwithlowercationicchargedensitiesarerejectedmoreeffectively.Ontheother*Towhomcorrespondenceshouldbeaddressed.JournalofAppliedPolymerScience,Vol.58,613-620(1995)01995JohnWiley&Sons,Inc.CCC0021-8995/95/030613-08hand,therejectionofsoluteswithoutelectricchargeislesseffectiveand,asaresult,thechargedthin-filmcompositemembranecanrejectionsmuchsmallerthanthemembraneporeradiibutcannotrejectsolutemolecules,whenthesolutesdonotcarryanyelectricchargeandtheirsizesareassmallasions.6-8Littlehasbeenmentionedsofar,however,aboutthedependenceofthepermeationrateonthenatureoftheelectrolytesolutepresentinthefeedsolution.Wehavereportedrecentlythatthemembraneper-meationrateinthepresenceoftheelectrolytesoluteinthefeedmayincreasesignificantlyfromthatintheabsenceoftheelectrolytes01ute.~Studiesontheeffectoftheconditionsofthemembranepreparationonthemembraneperfor-mancearealsofewintheliterature.Huangetal.studiedthepreparationandperformanceofpoly-phenyleneoxidemembranesandattributedthechangeinperformancemostlytothechangeintheion-exchangecapacity(IEC)ofthepolymer.'0,TheobjectiveofthisworkistostudytheeffectofIECvaluesonthereverseosmosisperformanceofsulfonatedpolyphenyleneoxidemembraneswhenthemembranesareincontactwithelectro-lytesolutionsofdifferentvalencesandionicradii.Theeffectofthesolventusedforthepreparation613614HAMZAETAL.ofpolymersolutionsformembranecoatingisalsoreported.EXPERIMENTALMaterialsTheflat-sheetultrafiltrationmembraneusedasthesubstrateforthecompositemembranewaspolysul-fonemembraneprovidedgenerouslybyOsmonics,Inc.Polyphenyleneoxide[poly(2,6-dimethyl-1,4-phenyleneoxide)abbreviatedhereafterasPPO]polymerwasprovidedbyGeneralElectricCo.Chlo-rosulfonicacid,chloroform,andmethanolwerepro-videdbyBDHCo.PreparationofSulfonatedPolyphenyleneOxide(SPPO)PPOpolymerwasusedwithoutanyfurthertreat-ment.A10wt%PPOsolutionwaspreparedbydissolvingPPOpolymerinchloroform.Then,astoichiometricquantityofchlorosulfonicacidwasaddeddropwisetothePPOsolutionaccordingtothemethoddescribedinRef.12.Assulfonationpro-gresses,SPPOprecipitatedfromthesolutionsinceSPPOisnotsolubleinchloroform.Theprecipitatewasdissolvedinmethanolorachloroform-methanolmixtureandcastintoafilm.TheSPPOfilmwasthendriedandcutintosmallpieces,whichwerefurtherwashedthoroughlywithdistilledwateruntilthewashwatershowedapHvaluehigherthanfour.Thepolymerwasthendriedexhaustivelyfor2daysundervacuumatroomtemperature.TheIECoftheSPPOpolymerthuspreparedwasmeasuredbytheacid-basetitrationmethod.PreparationofCompositeMembranesTheskinsideofthesubstrateultrafiltrationmem-branewascontactedwitha1wt%SPPOsolution.ThesolventusedtopreparetheSPPOsolutionde-pendedontheIECvalueoftheSPPOpolymer,asindicatedinthefollowing:ForIECvalueof1.29,chloroform-methanolmixture(chloroform47wt%)ForIECvalueof1.41,chloroform-methanolmixtures(chloroform61or15wt%)ForIECvalueof1.98,chloroform-methanolmixture(chloroform56wt%)ormethanolForIECvalueof2.60,methanolThesolutiononthemembranesurfacewasdrainedbyholdingthemembranevertically,leavingathinlayeroftheSPPOsolution.Thecoatedlayerwasthendriedforhalfanhouratroomtemperature.Theabovecoatingprocedurewasrepeatedthreetimes,whichwasfollowedbydryingovernightunderambientconditions.Themembranessopreparedwereinhydrogenformandstoredindistilledwater.ReverseOsmosisExperimentThereverseosmosisexperimentswer