2005-JMS-The role of membrane surface charge and solute physico-chemical properties in the rejection

HPLC

JournalofMembraneScience249(2005)

227–234

Theroleofmembranesurfacechargeandsolutephysico-chemicalpropertiesintherejectionoforganicacidsbyNFmembranes

ChristopherBellona,J¨orgE.Drewes

EnvironmentalScienceandEngineeringDivision,ColoradoSchoolofMines,Golden,CO80401-1887,USAReceived12November2003;receivedinrevisedform17September2004;accepted24September2004

Availableonline21November2004

Abstract

Theobjectiveofthisstudywastoinvestigatethecontributionofelectrostaticinteractionstotherejectionofselectedorganicacidsbynano ltrationmembranes.Effectivemembranesurfacecharge(determinedbytwomethodsofzetapotentialmeasurements)oftwocommercialnano ltrationmembraneswasexaminedatdifferentpHvaluesandfeedwaterchemistries.Sixdifferentorganicacidswereselectedforthisstudyrepresentingtypicalphysico-chemicalpropertiesofemergingorganictracepollutants.Findingsofthisstudyindicatedthattherejectionofnegativelychargedorganicacidswaslargerthanexpectedbasedonstericexclusionandwasprimarilydrivenbythesurfacechargeofthemembraneandcorrelatedwiththedegreeofionizationofthesolute.IncreasingfeedwaterpHresultedinanincreasednegativesurfacecharge,anincreasedpercentageofsolutesinthedeprotonatedstateandanincreasedrejectionthroughelectrostaticrepulsion.Therejectionofthepharmaceuticalresidueibuprofen,anorganicacidwithhydrophobicproperties,wasalsopHdependent,butatpHvaluesbelowthepKaibuprofenadsorbedandpartitionedthroughthemembrane.AtpHabovethepKa,adsorptionofibuprofenwasminimalduetoanincreasedsolubilityandthedominantroleofelectrostaticrepulsion.Thepresenceofcalciuminthefeedwaterloweredtheeffectivemembranesurfacecharge(asdeterminedbyelectrophoreticmobilitymeasurements)ofbothmembranestested,however,rejectionofnegativelychargedorganicsolutesdecreasedonlyformembraneswithamolecularweightcut-offlargerthanthesolutemolecularweight.©2004ElsevierB.V.Allrightsreserved.

Keywords:Nano ltration;Electrostaticrepulsion;Membranesurfacecharge;Zetapotential;Organictracepollutants

1.Introduction

Apartialrejectionoforganicmicropollutantssuchaspes-ticides,endocrinedisruptingcompounds,orpharmaceuticalresiduesisbecominganimportantconcernduringwaterandwastewatertreatmentapplicationswherenano ltration(NF)orreverseosmosis(RO)treatmentisemployed.Therefore,understandingthefactorsdrivingtherejectionoforganicmicropollutantsisessentialinordertoassesstreatmentef cienciesapriori.PreviousstudiesontherejectionoforganicsbyNFandultra-lowpressurereverseosmosis(ULPRO)membranesreportedthattheretentionofsolutesdependsuponbothsoluteproperties(i.e.,size,polarityandcharge)andmembraneproperties(i.e.,poresize,chargeand

Correspondingauthor.Tel.:+13032733401;fax:+13032733413.E-mailaddress:jdrewes@mines.edu(J.E.Drewes).

hydrophobicity)[1–8].Recentstudiesontherejectionofor-ganiccompoundsbyROandNFmembraneshavereportedahighrejectionofnegativelychargedorganicsolutesasaresultofelectrostaticinteractionsbetweenmembraneandsolute[1,7,9].Investigations,however,onthemechanismsbehindtherejectionoforganicacids,especiallytheroleofmem-braneandsoluteproperties,islimited.OzakiandHuafang[9]andBergetal.[1]reportedthattherejectionoforganicacidsincreasedwithincreasingpHrelativetothesolutesdisassoci-ationconstant(pKa).Numerousstudiesfocusingontheroleofmembranesurfacechargehavereportedthatmostthin lmcompositemembraneshaveanegativechargeatneutralpHduetodeprotonatedacidicfunctionalgroupswhichareaddedduringthemanufacturingprocesstoincreasetheselectivityand uxofULPROandNFmembranes[1,9–15].MembranesurfacechargehasbeenfoundtovarybetweenmembranesandisdependentuponfeedwaterchemistryincludingpH,

0376-7388/$–seefrontmatter©2004ElsevierB.V.Allrightsreserved.doi:10.1016/j.memsci.2004.09.041