PhytoremediationinEngineeredWetlandsMechanismsandA

1878-0296©2010PublishedbyElsevierdoi:10.1016/j.proenv.2010.10.142Availableonlineat(2010)1315–1325*Correspondingauthor.Tel.:[phone];fax:[phone].E-mailaddress:[e-mail]rEnvironmentalInformationSciences2010AnnualConference(ISEIS)PhytoremediationinEngineeredWetlands:MechanismsandApplicationsB.Y.Zhanga*,J.S.Zhenga,andR.G.SharpbaFacultyofEngineeringandAppliedScience,MemorialUniversityofNewfoundland,St.John's,NL,A1B3X5CanadabAbydozEnvironmentalLimted,MountPearl,NL,A1N4S9CanadaAbstractEngineeredwetlandphytoremediationisanaestheticallypleasing,solar-driven,passivetechniqueusefulforcleaningupwastesincludingmetals,pesticides,crudeoil,polyaromatichydrocarbons,andlandfillleachatesandhasbecomeanincreasinglyrecognizedpathwaytoadvancethetreatmentcapacityofwetlandsystems.Thisreviewaddressesthemechanismsofphytoremediationinengineeringwetlandsystemswhenreducingloadsofvariouscontaminants,aswellastheapplicationofphytoremediationasanenvironmentallysoundtechnologyinengineeredwetlandsystemsinbothlaboratoryandfieldlevels,followedbyacasestudyoffullscaleapplicationinNewfoundland,Canada.Thereviewisexpectedtohelpaddmorecapacitytounderstandphytoremediationinengineeredwetlandsystems,andestablishaneffectiveframeworkforfurtherapplications.©2010PublishedbyElsevierLtd.Keywords:Constructedwetlands,EWs;Mechanism,Metals,Organiccompunds;Phytoremediation1.IntroductionPhytoremediationistheuseofplantstocleanuporcontrolmanykindsofpollutantsincludingmetals,pesticidesandoil[1].Thoseplantsalsohelppreventwind,rain,andgroundwaterfromcarryingpollutionawayfromsitestootherareas[2].Overthelasttwodecades,phytoremediationhasbecomeanincreasinglyrecognizedpathwayforcontaminantremovalfromwaterandshallowsoilsandisanaestheticallypleasing,solar-driven,passivetechniqueusefulforremediationofshallowplumeswithlowtomoderatelevelsofcontamination[2].Subsurfaceplumestraveldown-gradientandcanundergoremediationalongthewayprimarilythroughmicrobialattenuationreactions[3].Asplumesreachshallowerdepths,theyencountertherhizosphereofuplandplantcommunitieswhereinitialphytoremediationcanbegin[4].Eventually,groundwaterflowsoutcropandfeedsurfacewaterflows[5].Inthiszone,diffuseplumecontaminantsaremoreaccessibletophytoremediationactivityandthe1316B.Y.Zhangetal./ProcediaEnvironmentalSciences2(2010)1315–1325plantcommunitiesarebydefinitionwetlands[6].Wetlandsbytheirpositioningforshallowaccesstothesecontaminantplumesandtheircharacteristicallyhighproductivity[6]mayrepresentthelow-cost/high-valuecleanupsystemsenvisionedbyEPA[2].Plantsinanaturalwetlandprovideasubstrate(roots,stems,andleaves)uponwhichmicroorganismscangrowastheybreakdownorganicmaterialsanduptakeheavymetals[1].However,asaresultoftheexponentiallyincreasingdemandsofhumanexpansionandresourceexploitation,ithasbeenrecognizedthatnaturalwetlandecosystemscannotalwaysfunctionefficientlyfordesiredobjectivesandstringentwaterqualitystandards[7].Theseandmanyotherfactorshaveledtotherapiddevelopmentofconstructedwetlandsforwaste(especiallywastewater)treatment[7].Aconstructedwetland(CW)isanartificialmarshorswamp,whichhavebeendesignedandconstructedtoutilizethenaturalprocessesinvolvingwetlandvegetation,soils,andtheirassociatedmicrobialassemblagestoassistinwastetreatment[8].Itusuallyconsistsofanumberofindividualrectangularand/orirregularly-shapedbasins(cells)connectedinseriesandsurroundedbyclay,rock,concreteorothermaterials.Threetypesofcellsmaybeusedinaconstructedwetlandsystem(CWS):freewatersurface(FWS)cells,sub-surfaceflow(SSF)cells,andhybridcellsthatincorporatesurfaceandsubsurfaceflows[9].CWSshaveprovensuccessfulforremediatingavarietyofwaterqualityissues,withadvantagesoverthenaturalwetlands,butstillhavesomedisadvantages[10]:performanceofCWSmaybelessconsistentthaninconventionaltreatmentsduetotheenvironmentalchangesatdifferentseasons;thebiologicalcomponentsaresensitivetotoxicchemicals(e.g.,ammoniaandpesticides);andflushesofpollutantsorsurgesinwaterflowmaytemporarilyreducetreatmenteffectiveness.Engineeredwetlandsystems(EWSs)thusdesignedtotakeadvantageofordinaryCWSs,butdosowithinamorecontrolledway[11].Engineeredwetlands(EWs)arespecial,advanced,semi-passivekindsofCWsinwhichoperatingconditionsaremoreactivelymonitored,manipulatedandcontrolledinsuchamannerastoallowcontaminantremovalstobeoptimized[9].Atthesametime,coldweatheroperabilityisimprovedinEWs,asistheabilitytodealwithotherwiseadverseconditionsandrecalcitrantwastewaterssuchaslandfillleachatesandminedrainages.AllEWsareCWs,butnotallCWsareEWs[11].CWSsmaybe“engineered”inmanywaysasshowninTable1.WithEWs,competingreactionsthatarecarriedoutinthesamecells(oftenSSF)ofordinaryCWs(e.g.,aerobicnitrificationandanaerobicdenitrification)canbecarriedoutinseparateEWcells(Inthecaseofthenitrogenreactions,virtuallystoichiometricconversionofammoniatonitrate,andnitratetonitrogengascanbeachievedmuchmoreefficientlyinamuchsmallerEWS.)[12].WithEWs,manykindsofbiologicalandchemicalprocesssystems(e.g.,aerobicandanaerobicbioreactors,limesto