Organic xenobiotics removal in constructed wetlands, with emphasis on the importance of the support matrix

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Journal

of

Hazardous

Materials

j o ur na l h o me p a g e : w w w . e l s e v i e r . c o m / l o c a t e / j h a z m a t

Review

Organic

xenobiotics

removal

in

constructed

wetlands,

with

emphasis

on

the

importance

of

the

support

matrix

A.V.

Dordio

_,

_A.J.P.

_Carvalho

a_{ChemistryDepartment,UniversityofÉvora,RuaRomãoRamalho,59,7000-671Évora,Portugal}

b_{IMAR–MarineandEnvironmentalResearchCentre,UniversityofÉvora,RuaRomãoRamalho,59,7000-671Évora,Portugal} c_{CQE–ÉvoraChemistryCentre,UniversityofÉvora,RuaRomãoRamalho,59,7000-671Évora,Portugal}

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•Substrateofwetlandsplaysamajorroleinremovalofnon-biodegradablepollutants. •Optimizationmaybeattemptedbyselectionofmaterialswithhighsorptioncapacity. •Claymineralsaregenerallyefficient,widelyavailableandcheapoptions.

•Agriculturalwastesandby-productshavebeengainingincreasingpopularity. •Availablelab-scaleassaysmuststillbecomplementedwithfull-scalefieldassays.

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Articlehistory:

Received8December2012

Receivedinrevisedform7February2013 Accepted4March2013

Available online 14 March 2013 Keywords:

Adsorption

Constructedwetlandsystems Organicxenobiotics Supportmatrix Wastewatertreatment

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Constructedwetlands(CWs)areincreasinglypopularasanefficientandeconomicalalternativeto con-ventionalwastewatertreatmentprocessesforremoval,amongotherpollutants,oforganicxenobiotics.In CWs,pollutantsareremovedthroughtheconcertedactionoftheircomponents,whosecontributioncan bemaximizedbycarefulselectionofthosecomponents.Specificallyfornon-biodegradableorganic pol-lutants,thematerialsusedassupportmatrixofCWscanplayamajorrolethroughsorptionphenomena. InthisreviewtheroleplayedbysuchmaterialsinCWsisexaminedwithspecialfocusontheamountof researchthathasbeenconductedtodateontheirsorptionpropertiesrelativelytoorganiccompounds. Whereavailable,thereportsontheutilizationofsomeofthosematerialsonpilotorfull-scaleCWsarealso recognized.Greatestinteresthasbeendirectedtocheaperandwidelyavailablematerials.Amongthese, claysaregenerallyregardedasefficientsorbents,butmaterialsoriginatedfromagriculturalwasteshave alsogainedrecentpopularity.Mostavailablestudiesarelab-scalebatchsorptionexperiments,whereas assaysperformedinfull-scaleCWsarestillscarce.However,theavailablelab-scaledatapointstoan interestingpotentialofmanyofthesematerialsforexperimentationassupportmatrixofCWstargeted fororganicxenobioticsremoval.

© 2013 Elsevier B.V. All rights reserved.

Contents

1. Introduction... 273

2. Constructedwetlandsystems... 273

2.1. RemovaloforganicxenobioticsinCWS... 274

2.1.1. MainprocessesfororganicxenobioticsremovalinCWS... 274

2.1.2. Bioticprocesses... 274

2.1.3. Abioticprocesses... 275

2.2. ComponentsofCWS... 276

3. Supportmatrix... 277

3.1. Selectionofsupportmatrix... 277

3.2. Commonsupportmatrixmaterials... 278

∗ Correspondingauthorat:ChemistryDepartment,UniversityofÉvora,RuaRomãoRamalho,59,7000-671Évora,Portugal.Tel.:+351266745343. E-mailaddress:avbd@uevora.pt(A.V.Dordio).

0304-3894/$–seefrontmatter © 2013 Elsevier B.V. All rights reserved.

3.3. Materialsfortheactiveretentionoforganics ... 278

3.3.1. Activatedcarbon... 278

3.3.2. Clay-basedmaterials... 283

3.3.3. Zeolitesandothersiliceousmaterials... 284

3.3.4. Industrialandagriculturalwastesandby-products... 285

3.3.5. Generalfinalremarks... 286

4. Conclusion ... 286

References... 287

1. Introduction

Theenormousand rapiddevelopmentofchemical and agro-chemical industries during thelast century hasresulted in the releaseofalargenumberofchemicalcompoundsintothe envi-ronment.Infact,alotofdifferentorganiccompoundsarecurrently usedinthedailylifeofhumanbeingsorresultfromhuman activi-tiesandmanyofthesearefrequentlybeingdetectedinnumerous environmentalmonitoringstudies[1–9].Pollutionofsoilsand sur-faceorgroundwatersoccursasaresultofbothpointanddiffuse sources,theimproper useand disposalor accidentalrelease of chemicalsintotheenvironment.

Aquaticecosystems are especially vulnerable because water bodies are frequently used, directly or indirectly, as recipients of potentially toxic liquids and solids from domestic, agri-cultural and industrial wastes [10,11]. Some of the primary classes of pollutants which may accumulate in surface waters, ground water, soil and sediments are nutrients, organic con-taminants(inparticularxenobioticcompounds)and metalsand metalloids.

Overthelastdecadesincreasingattentionisbeingturnedto asetofharmfulorganicchemicalsmostly composedof xenobi-otics[4–6].Mostofthesepollutantsarechemicalsubstancesthat persist in the environment, can bioaccumulate throughout the foodchain,andmaybetoxictobioticcommunities,thusposing arisktohumanhealthandtheenvironment[5–7,12].Thelistof suchorganicxenobioticsincludespolycyclicaromatic hydrocar-bons(PAHs),BTEX(benzene,toluene,ethylbenzeneandxylene), petroleumhydrocarbons,polychlorinatedbiphenyls(PCBs), chlori-natedsolvents,explosives,dyes,pharmaceuticalsandpersonalcare products,phenoliccompoundsandpesticides[1,3–7,12].Manyof thesesubstancescanbetransportedoverlongdistances,canbe dis-tributedintodifferentcompartmentsoftheenvironment,andalso undergoavarietyofreactionsandtransformations[5].Becauseof themanycompetinginteractions,thefateofsuchpollutantsisnot easytopredictand,inmanycases,theirecotoxicologicaleffectsare difficulttoassess[5].Nevertheless,possiblesynergisticeffectsthat maypotentiatetheirtoxicityandtheestablishedpossibilityof long-rangetransportof thesesubstances toregionswhere theyhave neverbeenusedorproduced,withalltheconsequentthreatsthey mayposetotheenvironmentofthewholeglobe,hasmotivated theinternationalcommunitytocall,atseveraloccasions,forurgent globalactionstobetakenwiththeaimofreducingthereleaseof someofthesechemicals[13,14].

Inrecentyears,numerousstrategiesandtechnologieshavebeen developed for water and wastewater treatmentor remediation ofcontaminatedareas.Someoftheadvancedwastewater treat-menttechnologiesthat havebeenevaluatedfor theremovalof organicxenobiotics include advancedoxidative processes, acti-vated carbon adsorption, membrane filtration and membrane bioreactors[7,15–21].Despitethesometimeshighremoval effi-cienciesattained,mostoftheseprocessesarenot,however,widely usedmainly for reasonsof cost effectiveness[7,21–24]. Conse-quently,thereisagrowingneedforalternativeorcomplementary treatmentprocessesforremovingorganicxenobioticsfromsoils,

natural watersandwastewatersthathave higherefficienciesat reasonablecostsofoperation/maintenance.

Phytotechnologieshavebeensuccessfullyusedforremovalof many organic xenobiotics fromcontaminated soils, waters and wastewaters[21,24–33].Thistypeofapproachattemptstoexploit theabilityofplantstoadsorb,uptakeandconcentrateormetabolize organicxenobiotics,aswellastoreleaserootexudatesthatenhance biotransformationandmicrobialdegradationofthoseorganics.The implementationofphytotechnologiessuchasconstructedwetland systems(CWS)isbecomingapopularoption[33–39].These sys-temsareincreasinglybeingusedtoprovideaformofsecondary, tertiaryorquaternarytreatmentforwastewaters,andhavealready beenusedwithsuccesstoremoveseveralorganicxenobioticsof variousclassesfromcontaminatedwaters[36–53].

Inthispaperwepresentareviewonresearchworkcarriedout overthelatestyearsthatcanbeusefulforthetaskoffindingsuitable andmoreadequatematerialstobeusedastheimportant compo-nent ofCWSthatisthesupportmatrix.Thefocusisessentially centeredonthosesystemsespeciallytargetedfortheremovalof organicxenobioticsfromwastewaters.Aswillbeexplained fur-ther ahead, thesupport matrix is a crucial component of CWS whose carefulselectioncanleadtosignificantenhancementsof theefficienciesofthesesystems.Theseenhancementsare espe-ciallyrelevantandlonglastingiftheCWSareusedastertiaryor quaternary treatmentsystems, asis mostoftenthe caseinthe removalofnon-biodegradableorganicsindomesticwastewaters. Givenitsimportance,then,thecompositionofthesupportmatrix isaprimarypointofCWSoptimization.

Manyaspectsofthesupportmatrixcharacteristicsthatare rel-evantfortheCWSperformanceareconsideredinthisreviewbut sorptivepropertiesgenerallyhaveaprominentrole andreceive morecoverage.Inthisregard,thevastmajorityofthestudieswere not conducted in fullscale CWS but in simplerlab-scalebatch adsorptionassays, wherethepropertiesofthematerialscanbe bettercharacterizedundercontrolledconditions.Inmanyofthose studieswhichareaddressedinthisreviewthefocusisnotexactly theutilizationofthematerialsinCWS.However,whendeciding thecompositionofthesupportmatrixinthedesignofaCWS,the selectionofmaterialsmayinpartbebasedontheresultsofthese works.

2. Constructedwetlandsystems

Constructedwetlandsystemscanbedefinedasadesignedand man-madestructurecomposedofwatersaturated bedsplanted withemergentand/orsubmergentvegetationthatsimulates nat-uralwetlandsforhumanuseandbenefits[35,54–57].

Constructed wetlands can be classified into different types based onseveralof theircharacteristics. Oneclassification sys-tem defines four types based on the dominant plant species

[35,57–59]: (1) floating macrophytes (e.g. Eichhornia crassipes, Lemna minor),(2) floating-leafmacrophytes(e.g.Nympheaalba, Potamogetongramineus),(3)submergedmacrophytes(e.g.Littorella uniflora, Potamogeton crispus), and (4) emerged rooted macro-phytes(e.g.Phragmitesaustralis,Typhalatifolia).Anothercommon

classification divides wetlands according to their hydrology

[35,57,60]:(1)freewatersurface(FWS)wetlandsorsurfaceflow (SF)wetlands,(2)horizontalsubsurfaceflowwetlands(HSSF),(3) verticalsubsurfaceflowwetlands(VSSF),and(4)hybridsystems.

CWShavebeenusedtotreatavarietyofwastewatersincluding urbanrunoff,municipal, industrialandagriculturalwastewaters

[35–37,50,55–58,60–63].Infact,foralongtimenaturalwetlands havebeencreditedwithacapacity todepuratethewatersthat inundatedsuchareas.CWStakeadvantageofmanyofthesame processesthatoccurinnaturalwetlands,butdosowithinamore controlledenvironmentandcanbeoptimizedtopotentiallyattain greaterefficiencies.

Inthepast,CWShavebeenusedmainlyaswastewater treat-mentalternativesorcomplementarytotheconventionaltreatment fordomesticwastewatersofsmallcommunities.Thus,CWShave beenmostlyappliedintheremovalofbulkwastewaterpollutants suchassuspendedsolids,organicmatter,excessofnutrientsand pathogens.

Morerecently,CWSapplicationsfordealingwithmorespecific pollutants,suchas sometypes of organicpollutants, especially thosewhicharemorerecalcitranttotheconventionalprocesses usedinmostwastewatertreatmentplants(WWTPs),havebeen meetingalargerinterestandhavebeenthesubjectofanincreasing numberofstudies.Inmanyofsuchstudies,CWShavebeen prov-ingtobeefficientandcost-effectivesolutionsfortheremovalof someorganicxenobioticssuchaspesticides,dyes,explosives, phe-noliccompounds,petroleumhydrocarbons,pharmaceuticalsand hormones[36–38,40,42–52,64–71].

Ultimately,theoptimizationofCWSfortheremovalofmore specifictargetcompoundsrequiresabasicknowledgeofthe pro-cessesinvolvedintheremovalofthepollutantsandtheinteractions betweenthose and the CWS components. New trends in CWS researcharemovingtowardsthestudyofsuchprocessesand inter-actions,andfocusontheselectionandoptimizationoftheCWS componentsformorespecificapplications.

2.1. RemovaloforganicxenobioticsinCWS

TheexactpathwaysfororganicxenobioticsremovalinCWSare notyetcurrentlyknowninmuchdetail.Despitebeingclassified underacommondesignation,organicxenobioticscompriseaset ofchemicalcompoundswhichspanawidevarietyof characteris-ticsandthisclassis,infact,formedbyseverallargelyunrelated familiesofsubstances.Therefore,itisnotsurprisingtorealizethat manyverydiverseprocessesareresponsiblefortheirremoval.The degreetowhicheachprocesswillcontributetotheoverallremoval oftheorganicxenobioticsfromcontaminatedwatersinCWSisin turndependentonthephysico-chemicalpropertiesofthese com-pounds(e.g.,molecularstructure,polarity,pKa,water solubility, logKow,volatilityandchemicalstability),characteristicsofthe sup-portmatrix(e.g.,pointofzerocharge(pzc),organicmattercontent, mineralogicalcomposition,specificsurfacearea),theplantspecies, microbial populations,wastewater characteristics (e.g.,pH, dis-solvedorganicmatter(DOM),electrolytescomposition)aswellas otherenvironmentalconditions(e.g.,temperature,moisture, nutri-entavailability)[24,72].

A comprehensivedescriptionof organicxenobioticsremoval processesinCWSis,thus,notaneasytasktoaccomplishandthese wastewatertreatmentsystemshavebeen(and,tosomeextent,still are)frequentlyoperatedas“blackboxes”.Inthepast,muchofthe designofCWShasbeendonewithlittleknowledgeof(or consider-ationfor)therolesplayedbyeachcomponentandhowtheireffects couldbeenhancedandoptimized.However,overthemostrecent yearssomeaccumulatedknowledgehasbeenincreasinglyapplied intheconstructionandoperationofnewsystems,whichresultsina muchgreatervarietyofplantspecies,supportmatrixmaterialsand

constructedwetlanddesignsbeingseen,studiedandintroducedin newlysetupCWS[35,42,43,58,60,73–85].Thegoalsofthetarget contaminantstoremoveinthoseCWShavealsobecome increas-inglymoreambitious.

Major design parameters, removal mechanisms and treat-ment performanceshavebeenstudiedandreviewed byseveral researchers[35–37,56–58,60–62,73,86,87].

2.1.1. MainprocessesfororganicxenobioticsremovalinCWS Organic xenobiotics removal by CWSinvolves several inter-dependentprocesseswhich,underoneofthepossibleclassification criteria,maybedistinguishedasbiotic(carriedoutbyliving orga-nisms such as plants and microorganisms) or abiotic (physical orchemical)processes.Theseprocessesareessentiallythesame occurringinnaturalwetlandsandalsoidenticaltothose respon-sibleforthefateofxenobioticsintheenvironment[72].Theway inwhichCWSdifferfromanaturalsystemsuchasanatural wet-landisthattheeffectsofsomeoftheseprocessescanbeoptimized inthecontrolledenvironmentofaCWSinordertomaximizethe removalofpollutants(or,atleast,ofaspecificsetofcontaminants). Withthataim,abasicunderstandingofhowtheseprocesses oper-atein wetlandsisextremely helpfulforassessing thepotential applications,benefitsandlimitationsofCWS.

2.1.2. Bioticprocesses

CWS are essentiallybiological systems, considering that the biotic components (the wetland vegetation and the microbial populations)aretwoofthemajorcomponentsofCWS.Hence, bio-logicalprocessesnaturallyplayanimportantroleintheremovalof manypollutants(includingorganicxenobiotics)bythesesystems. Microorganismsusuallyplaythemainroleinthetransformation andmineralizationofnutrientsandorganicpollutants[35,58,82]. Inparticularwithrespecttotheremovaloforganicxenobiotics, althoughmicroorganismsmayalsoprovideameasurableamount ofuptakeandstorage,itistheirsetofmetabolicprocessesthatplays themostsignificantroleinthedecompositionofthesecompounds (oratleasttheonesthatarebiodegradable)throughthe transfor-mationofcomplexmoleculesintosimplerones.Biodegradationof organicchemicalsisassociatedprimarilywithheterotrophic bacte-riaandcertainautotrophicbacteria,fungiincludingbasidiomycetes andyeasts,andspecificprotozoa[35,72].

Biodegradationcanoccurunderbothaerobicandanaerobic con-ditions.Dependingontheoxygeninputbymacrophytesandthe availability ofotherelectronacceptors,thecontaminantsinthe wastewatercanbemetabolizedinvariousways.Highlyreduced conditionsarefavorableforthedegradationofsomechlorinated hydrocarbons[72].

Biodegradationofbenzene,octane,toluene,PAHsderivedfrom petroleum,andmostothermanufacturedorganiccompoundsis enhancedunderaerobiccondition,andPCBsdegradefasterunder moderatelyreducingconditions[72].

Microbialtransformationscaninvolvemorethanonetype of mechanismand,underdifferentconditions,severalproductscanbe derivedfromthesameinitialcompounddependingonthe environ-mentalconditions.Additionally,transformationscanbemediated byoneorganismorthroughcombinedeffectsofseveralorganisms

[72,82].

Theefficiencyandrateoforganicxenobioticdegradationand theextentofmicrobialgrowthduringdegradationishighlyvariable fordifferenttypesoforganiccompoundsandheavilyinfluencedby thexenobioticchemicalstructureaswell[88].Structurally sim-plecompoundswithhighwatersolubilityand lowadsorptivity are usually more similar to thenaturally occurringsubstances whichareusuallyusedasenergysourcesbythemicroorganisms and areeasily degraded.In contrast,xenobioticswithchemical structuresverydifferentfromthenaturallyoccurringcompounds

areoftendegradedslowlysincemicroorganismsdonotpossess suitable degrading genes. In these cases degradation by non-specificenzymesmaystilloccurbutataslowerratebynon-specific reactionswhichdonotsupportmicrobialgrowth(co-metabolism)

[89].

Xenobioticdegradationbythemicroorganismsisalsostrongly influencedbythesupportmediumwherethemicrobial popula-tionsgrow.Temperature,pH,oxygen,presenceoftoxicsubstances andnutrientsavailablewithintheCWSareexpectedtoplayavery importantroleintheremovalefficiency[72,81–83,90].

Microorganismswiththeabilitytodegradeawidevarietyof compounds,likebenzene,phenol,naphthalene,atrazine, nitroaro-matics,biphenyls,PCBsandchlorobenzoates,havebeenisolated and characterized [35,72,88]. Although simple aromatic com-poundsarebiodegradablebyavarietyofdegradativepathways, their halogenated counterparts are more resistant to bacterial attacksandoftenneedtheevolutionofnovelpathways[88].In particular,thepresenceofchlorineatomsinthearomaticcenteris knowntogreatlyretardtherateofdegradation.

Plantsalsohavearelevantroleinthebioticprocessesof organ-icsremovalinCWS,involvinganumberofbiologicalprocessesof whichmanydetailsstillremaintobefullyunderstood. Depend-ingonthepollutantproperties,organicsmaybedegradedinthe roots zone through the plant stimulation of microbial activity (phytostimulationorrhizodegradation)orbydirectuptakebythe plant,followedbydegradationwithintheirtissuesviatheirown enzymaticactivities(phytodegradation)andsequestration(in vac-uolesor boundtoinsolublecellularstructures)orvolatilization

[24–28,73,91–94].

Manyorganicpollutantscanbereadilytakenupbyplantsbut, asconsequenceof manyofthembeingxenobiotic, thereareno specifictransportersfor thesecompounds inplantmembranes. Therefore,organicxenobioticsmoveintoandwithinplanttissues viadiffusion(passiveuptake)throughcellwallsandmembranes

[27,28].Therateofuptakeandtranslocationwithintheplantsis highlydependentonthewatersolubilityandhydrophobicityofthe compounds(asexpressedbythelogKow).Amoderate hydropho-bicity,characterizedbyalogKowintherangeof0.5–3.5,isgenerally consideredtobeoptimalfor allowingtheorganicsubstance to moveeasilybetweenthelipidicandtheaqueousphases,without beingirreversiblytrappedinanyofeach[27,28,95].Suchgroupof compoundsincludesmostchlorinatedsolvents,BTEX,many pesti-cides,andshort-chainaliphaticchemicals.Alternatively,chemicals thataretoohydrophobic(logKow>3.5)maybecandidatesfor phy-tostabilizationand/orrhizospherebioremediationbyvirtueoftheir longresidencetimesintherootszone[27,28].

Metabolismofpesticideshasalreadybeenextensivelystudied formanyyears[29,96,97]whilethemetabolismofnonagricultural xenobioticssuchastrichloroethylene(TCE),TNT,glyceroltrinitrate (GTN),PAHs,PCBs,pharmaceuticalsandotherchlorinated com-poundshasalsobeendealtwithmorerecently[28–30,98–101].It wasshownthatmostofthesecompoundsaremetabolizedbutonly afewchemicalsappeartobefullymineralized.Itmaybeamatter ofconcernthatsomeofthesemetabolitesproducedbyplantsmay bemoretoxicthantheoriginalcompounds,thusmakingplantsless attractivecomparedwithmicroorganisms,whichtotallydegrade organicpollutants[25].

Theeffectofstimulatingthedevelopmentofmicrobial popu-lations and enhancing their activity is another role of major importanceplayedbyplants.Infact,largermicrobialpopulations aresustainedintherhizospherethaninthebulksoilbya vari-etyofproductssecretedbyplants(exudates,mucigels,deadcell material,etc.)[28,60,82] whichevidenceshow important these interactionsbetweenCWScomponentscanbe.Someplant exu-datesmay also provide catalytic processes that can contribute tothe degradation of compounds, in addition tothe microbial

processes.Boththesephytostimulationorrhizodegradation pro-cesses are enhanced by large and dense root systems, which consequentlyhavehigherlevelsofdegradingenzymes[28],and, hence,thesearefavorablecharacteristicsinplantsusedfor phy-toremediation.Classesoforganiccompoundsthataremorerapidly degradedintherhizospherethaninbulksoilincludePAHs,total petroleumhydrocarbons,chlorinatedpesticidesaswellasother chlorinatedcompoundslikePCBs,explosivessuchasTNTandRDX, organophosphateinsecticides,andsurfactants[28,102,103].

Thecapacityofsomeplantspeciestorelease oxygenaround theirrootstipshasacrucialroleoffavoringthedevelopmentof aer-obicmicroorganismsandtheirgenerallymoreefficientdegradative processes.Insomecasesitcanalsopromoteoxidativechemical processes [58] and, thus, themostsensitiveareas onthe roots surfaceareprotectedbythisoxidativefilmfrombeingdamaged bytoxicwastewatercomponentsintheanoxic,usuallyextremely reduced,rhizosphere[60].

2.1.3. Abioticprocesses

Abioticprocessesinvolvedintheremovalofpollutantsfrom wastewater comprise a wide range of physical and chemical phenomena. Themostimportant processof thiskindoccurring inCWS,atthesurfaceofplantsrootsandofthesolidmedia(in thosesystemswherethereisasubstrate,e.g.SSF-CW)oratbiofilms developedonthosesurfaces,issorption,resultinginashort-term retentionorlong-termimmobilizationofcontaminants.Inaddition tophysicalsorption,ionexchangeinvolvingmineralcomponentsof thesolidmatrixcanalsocontributetotheremovalofsome particu-larclassesofpollutants.Theoccurrenceofthistypeofprocesseswill bestronglyinfluencedbythetypeofmaterialswhichcomposethe substrateoftheCWSbed.Thechemicalcharacteristicsofthissolid matrixdeterminesitscapacitytosorbpollutants[72]butretention inabioticcomponentsisalsoafunctionofseveralcharacteristics ofthewastewater(suchasitsDOMcontent,pHandelectrolytes composition)aswellasofthepollutantitself(e.g.pKa,solubility, logKow,etc.).Theextentofsorptioneffectscan,however,belimited intimeasthesurfacesbecomesaturatedwiththeadsorbates. Satu-rationofthesorptioncapacityofthesupportmatrixwilldependon theamountsofthesubstancespresentintheliquidandtheoverall complexityofthesolutioncomposition(i.e.thechemicalvarietyof substances).Inthecaseofpre-treatedwastewater(whentheCWS is usedin atertiaryorquaternarystage)boththeamounts and varietyofthepollutantshavebeensignificantlyreducedafterthe previoustreatmentstages,whichmayallowforalongerlife-time oftheadsorbentmedia.Fortheefficiencyofsorptionprocessesa goodhydraulicconductivityofthesupportmatrixisalsocrucial sincetheoccurrenceofoverlandflowsandpreferentialchanneling mustbepreventedandagood,uniformcontactofthe wastewa-terwiththeCWSmediamustbeassured[35,57].Otherimportant effectthatmayimpacttheperformanceofsorptionprocessesisthe naturaldevelopmentofbiofilmsoverthemedia’ssurface.

Other common abiotic processes such as hydrolysis, pho-todegradation, redox reactions and volatilization can also con-tribute,invariedextents,totheremovalofsomeparticularclasses ofcompounds,dependingontheirspecificproperties.However, withtheexceptionofsorption,abioticprocessesarenot,in gen-eral,majorremovalprocessesformostorganiccompoundsbecause eithertheconditionsinCWSorthepropertiesofthecompounds arenotadequate.Forexample,photodegradationcanonlybe sig-nificantinSFsystems(seebeginningofSection2),providedthat plantdensityisnottoohigh(suchthatitdoesnotcausetoomuch shade overthewatersurface)andonlyfor compoundsthatare photosensitive.InSSFsystemsphotodegradationdoesnotoccurin appreciableextentasthewaterlevelisbelowthesolidmatrix sur-faceand,therefore,exposureofthepollutantstosunlightisvery limitedinthistypeofsystems.Theprocessofvolatilizationisalsoof

modestimportanceasmostorganicxenobioticshavelowvolatility. WhereCWSareusedasatertiarytreatmentstageafter conven-tionalsecondarytreatmentinaWWTP,organiccompoundsdonot suffer,inmostcases,appreciablehydrolysiseither,astheyhave alreadybeensubjectedtosuchtypeofprocessesduringsecondary treatmentstage.Therefore,theorganicxenobioticspresentinCWS influentsarethosethathaveresistedsuchhydrolysisprocessesor theyarethetransformationproductsofthosesubstancesthatdid notresistit.

2.2. ComponentsofCWS

ACWSisbasicallycomposedofthree maincomponents:the supportmatrix(thebedsubstrate),thevegetationandmicrobial populations.Itisbytheconcertedactionofalltheseinterdependent components(throughthevarietyofchemical,physicaland biologi-calprocessesthathavebeenenumeratedanddescribedbefore)that thedepurationofwastewaterscanbeachievedinthesewastewater treatmentsystems.

PlantsplayseveralimportantrolesinaCWSwithbothadirect andindirectinfluenceintheglobaltreatmentefficiencyand, there-fore,acarefulselectionofthemoresuitablespeciestouseineach systemisanimportantdesigndecision.Someofthoseroles (includ-ingthecontributionofthemajorprocessesprovidedbyplantsas describedinSection2.1.2)are[24,35,58,59,87]:

-toprovidethefiltrationoflargedebris;

-tosupplysurfaceareafordevelopmentofmicroorganismsandto stimulatetheirgrowth(aidedbyexudatesreleasedthroughthe roots);

-topromotehydraulicconductivityofthesupportmatrix (exten-siverootsandrhizomeshelppreventclogging);

-to transport and release oxygen through the roots (which increasesaerobicdegradationandnitrification);

-andtodiminishthewastewaterpollutantsload(byadsorption, phytodegradationand/oruptakeofnutrientsandorganic com-pounds).

Forthepurposeofphytoremediation,oneoftheprimarycriteria intheselectionofplantspeciesistheirtolerancetothepollutants toxicityandtheextentoftheirroleinloweringthepollutants con-centrationsinwaterthroughthevarietyoftheprocesseswhich havealreadybeendescribed.

WhenintendedtobeusedinCWSotherdesirable characteris-ticsofplantsare[35,56,58,104]:ecologicaladaptability(thechosen plantsshouldposenorisktothenaturalecosystemsurrounding); tolerancetolocalconditionsintermsofclimate,pestsanddiseases; tolerancetothehypertrophicwater-loggedconditionsofwetlands; readypropagation,rapidestablishmentandgrowth,goodroot sys-temdevelopmentandperennialdurationratherthanannual.Other additionalobjectives(such asecological, aesthetic,recreational, andeconomic)ofwetlanddevelopmentsmayalsoaffectthechoice oftheplants[58,104].

Inadditiontospeciesselection,otheraspectsrelatedwiththe CWSdesign,suchasplantdensityandlayoutofthespecimens(e.g. thewayspecimensofdifferentspeciesmaybeintermixedwhen plantedinthebeds),maybeimportantandprobablyshouldbe carefullyplannedastheirinfluencemayrange fromsubtle dif-ferencesinthesystembehaviortomorepronouncedimpactsin theoverallefficiency[35,77].In particular,thecyclesof vegeta-tiveactivityofsomespeciesinadditiontovariationsofclimate conditionsmayleadtosignificantseasonalchangesinthesystem efficiency,whichinsomecasesmaybemitigatedbyusing polycul-turesofvegetation[35].

ThevegetationofaCWSisusuallyformedexclusivelyby macro-phytes (the type of plants that are more common in natural

wetlands). Aquatic macrophytes seem to be especially resis-tant to the toxicity of a great variety of organic pollutants at concentrationsnormallyencounteredintypicalwastewater com-positions.Inaddition,numerousstudieshaveshownthecapability ofmanymacrophytespeciestoreducetheaqueousconcentrations of variousorganicxenobiotics suchasexplosives[40,105–107], petroleum hydrocarbons [36], pesticides [108–110] and more recentlyonsomepharmaceuticals[100,111–115].

SeveraldifferentspeciesofmacrophytescanbeusedinCWS dependingonthetypeofCWSdesign(SForSSF)anditsmodeof operation(continuousorbatch),loadingrate,wastewater char-acteristics and environmental conditions. However,despite the factthatthedominantspeciesofmacrophytevarieslocally,the mostcommonlyusedinSSF-CWSintemperatezonesarethereeds (Phragmitesspp.)andthecattails(Typhaspp.)[35,56,57,104,116]. InEurope,Phragmitesarethepreferredplantsforthesesystems. Phragmiteshaveseveraladvantagessinceitisafastgrowinghardy plantandisnotafoodsourceforanimalsorbirds[56].However,in somepartsoftheU.S.theuseofPhragmitesisnotpermittedbecause itisanaggressiveplantandthereareconcernsthatitmightinfest naturalwetlands.Inthesecasescattailsorbulrush(Scirpusspp.) canbeused[116].

Theroleofplantsinthedirectdegradationanduptakeoforganic pollutants,inparticularxenobioticsofanthropogenicorigin,isnot asimportant astheone provided bytheaction of microorgan-isms,but inmanycasesitmaystill representaverysignificant contribution.Notwithstanding,themajorroleinthe biodegrada-tionofmanyofthesecontaminantsisunquestionablyplayedby microorganisms(throughtheirmetabolictransformations)which consequentlyformavitalpartofCWS[58].Populationsof microor-ganismsdevelopnaturallyinCWSandareexposedtosomesimilar factors as those affecting their development in WWTPs. These microbialcommunitiesareverydiverseandonecanfindanarray ofbacteria,fungi,protozoa,algae,etc.,amongthemicroorganisms that thrivein wetlands,which provideadequateenvironmental conditionsfortheirgrowthanddevelopmentbutwhose character-isticsarealsopronouncedlyinfluencedbythetypeofvegetation andthesupportmatrixmaterials[35,58,81–83].The characteris-ticsofthesemicrobialpopulationscanadditionallybemodified byinoculationoftheCWSwithstrainsthataremoresuitablefor thepurposeofthesystem.Thisdiversitypotentiatesthecapacity of microbial biofilmstocarry through anassortment of micro-bialactivitieswhichareessentialintheperformanceofaCWS.In particular,adiversemixtureofbothaerobicandanaerobic bacte-riaisusefulinthedegradationoforganicmatter,includingsome organicxenobiotics.Therefore,theremaybesomeinterestinthe characterizationofmicrobialcommunitiesinCWS.Unfortunately, theinformationavailableuptodateonthemicrobialcommunities developingindifferentsolidmediaandontheinfluencesduetothe wetlandvegetationisstillratherlimited.

Onefeatureoftheactionofthebioticcomponents (microor-ganisms andplants) isthat theiractivitiestypicallygo through highandlowcyclesthroughouttheyearandthereforethey intro-duceundesirableseasonalvariationsintheCWSperformance.To someextent,thismaypossiblybesmoothedbytheactionofsome processesoccurringattheabioticcomponent(supportmatrix).

TheprimaryfunctionofthesupportmatrixinaCWS,justasthe soilinnaturalwetlands,istoprovideaphysicalsupportwherethe plantrootsystemscanbeanchoredandtopresentasurfacefor theattachmentandgrowthofmicroorganisms[57,58].Inaddition, plantrootsandmicroorganismscanfindinthiscomponenttheir suppliesofwater,airandnutrientsaswellassomemoderation ofenvironmentalconditionswhichcanaffecttheirdevelopment, suchastemperatureorpH.Thematrixshouldalsoallowaneven infiltrationandflowofthewastewaterthroughthesystem.Beyond thissupportiverole,thematrixactsasafiltermediumthatwilltrap

particulatepollutantsandsorbsometypesofdissolvedpollutants. Somemineralmaterialsalsohaveionexchangecapacitywhichmay provideforsomeabilitytoretainpolarorioniccompounds.

In practice, every CWS should be constructed with a care-ful selection of the materials used as the support matrix, the typeofvegetationandmicrobialpopulation,whichmayprovean importantstepintheoptimizationoftheconstructedwetland per-formance.

3. Supportmatrix

Thesupportmatrixisavitalcomponentinasubsurfaceflow (SSF)CWSasitprovidesthelinkbetweenallthecomponentsand themaintreatmentprocessesoccurringwithinthesystem.The supportmatrix maybecomposedof severaldifferentmaterials, each ofwhich havingits own particularcharacteristicssuchas granulometricandhydraulic properties,mineralogical composi-tion,acid–baseandsurfacechargeproperties,contentoforganic matter,sorptiveproperties,amongothers. Thesepropertieswill influencethedevelopmentofthebiotawhichthesubstrate pro-videssupportfor.Infact,ithasbeenobservedthatsomeparticular materialsseemtobepreferablemediaforsomeplantspeciesand microorganismsstrains[81–83,90].Ontheotherhand,the devel-opmentandactivityofthebioticelementsmayovertimeaffectand modifythesupportmatrixcharacteristics.Thechemicalproperties ofthematerialsmaydeterminethetypeandstrengthofthe inter-actionsbetweenthesupportmatrixandthepollutantmoleculesat thesurfacesofthesolids.Fordifferenttypesofsupportmatrices, thecorrespondinglydifferentpartitioningofthepollutantbetween thewaterandthesolidscompartmentswillinfluencethepollutant behaviorintheCWSandtheprocessestowhichitwillbesubmitted. Whendesigninganewwastewatertreatmentwetlandsystem, thestageofselectingthesupportmatrixmaterialstobeusedin thebedsisanopportunitytooptimizeandsignificantlyenhance thesolidmatrixroleintheremovalofthepollutants,whichmay contributetoanoverallimprovementofthetreatmentsystem effi-ciency.Forexample,sorptionprocessesareusuallyseenasonly providingashort-timeretentionofpollutantsuntilsorption capac-ityofthesupportmatrixbecomessaturated.However,ithasbeen observed[38,42,117] thattheseprocesses maybeimportant to moderatetheseasonalvariationsoftheefficiencyofaCWSthatare duetothevariationsofthebioticcomponentsactivities,thereby providingforamorestableperformanceofthesystemsthroughout thewholeyear.

3.1. Selectionofsupportmatrix

Thechoiceofthesupportmatrixmaterialstobeusedinany given system should be determined in terms of their physical andchemicalcharacteristics.Chemicalpropertieswhichare deter-minedbythematerialscompositions(e.g.thedevelopmentofa surface charge due toacid-base properties) aswell asphysical characteristics(e.g.particlesizedistributions,porosity,effective particlesizeandhydraulicconductivity)areallimportantfactors thataffectthesystemperformance[35,57,79,80].Giventhe inter-dependencesbetweenallCWScomponents(e.g.,theinfluencethat thesupportmatrixmayhaveonthedevelopmentofplantsand microorganisms;thepHchangesduringanaerobicbiodegradation processesthatmaychemicallyaffect/degradesomematerials)the selectionofthiscomponentshouldnotbemadeindependentlyof theselectionsoftheothertwocomponents.Instead,thedesignof thewholesystemshouldbeapproachedasanintegrationofallthe parts,inviewofthesynergisticeffectsthatcanbecreatedbythe setofallcomponentstogether.

Themechanicalandchemicalresistanceofthematerials com-posingthesupportmatrixisafundamentalcharacteristicasthis component should withstand theconditions of operation(flow andchemicalcompositionofthewastewater)withoutsignificant degradationofitsmainproperties.Inaddition,itshouldnotrelease substancesthatmaybeasourceoftoxicityforthebiotic compo-nentsorthatmayultimatelycontaminatethetreatedwastewater. The flow ofliquid throughthewetland beddepends onthe hydraulicgradientalongitaswellasthehydraulicconductivity, sizeandporosityofthematerialsused.Hydraulicconductivityisa fundamentalcharacteristicofthesupportmatrixasitshouldallow foranevendistributionoftheinletflowandcollectionoftheoutlet flow.Alowhydraulicconductivitywillresultinshort-circuitflowof thewastewaterbetweeninletandoutletoverthewetlandsurface orthedevelopmentofchannelingwhichbothcontributetoapoor contactofthewastewaterwiththesystemandseverelyreduces theeffectivenessofthesystem[118].Asageneralrule,successful operationrequireshydraulicconductivitiesofapproximately10−3 to10−4ms−1[56–58].

Thebedporosity(definedastheratioofporewatervolumeto thetotalvolume;typicallytheporosityofacoarsesandorgravel mediaisintherange30–45%[35,56,58,116])providesthespace forwastewatertreatmentprocesses(physicochemicalaswellas biological)tooccur.Inaddition,air-filledporositymaybeinvolved ingasexchangeprocesseswhicharenecessarytoaeratethe rhizo-sphere(inwell-drainedsoils,oxygenentersthesoiland carbon dioxide leavesthe soil throughthe air-filledpores, whereas in floodedsystems,themajorityofgasexchangeoccursthroughthe plantsastheeffectiveporespaceforthemovementofoxygenand othergaseswithinthesolidmatrixisinverselyrelatedtothe per-centofwaterfilledpores).Oxygendiffusionand,consequently,the supportmatrixoxygencontentaffectboth plantsandmicrobial growth[35,57,119].

Poresizesdistributionsand air-waterrelationshipsare influ-enced by the particles sizes of the support matrix materials. Generally,asparticlesizesgetlarger,thevoidsizeincreasesbut thevoidratio(i.e.,totalvolumeofvoidsversusthetotalvolumeof solidandvoids)decreases.

Porosity also significantly influences hydraulic conductivity, with materials of higher porosity typically having a higher hydraulic conductivity (moreopenarea for theflow of water), althoughin somecasesa simplerelationshipmaynotbeeasily establishedbetweenthetwoproperties[35,57].

Thesizesofthesupportmatrixparticleshave adetermining effectonboththehydrauliccharacteristicsandtheporosityofthe media.Particlesizefractionsintherangeof0–30mmarenormally useddependingonthetypeofmaterial[57];forexample,gravel sizesaretypicallywithin0–12mm[57,58].Oftenadditionalcriteria mustbecompliedwith,suchas,forinstance,therequirementthat d10(effectiveparticlesize)isgreaterthan0.3mmorthatd60/d10 (uniformitycoefficient;d60andd10representtheparticlediameter forwhich60%and10%ofthesamplemass,respectively,arefiner thanasievemeshofthatsize)islargerthan4[57].

Forthedevelopmentofvegetationandmicrobialcommunities, thephysicochemicalpropertiesofthesupportmatrixintermsof pH,porosity,surfacearea,availabilityofnutrientsandorganic mat-tercontentisalsodeterminant.Organicsoilsorcarbonrichsupport materials(suchascompostforexample)canpromotemicrobially mediatedprocessesbyprovidingthesystemwiththenecessary sourceofcarbon.Likewise,porousmatricesmayenhancethe con-tributionofmicroorganismsbyprovidingalargersurfaceareafor treatmentcontactandbiofilmdevelopment.Thenatureand activ-ity ofthebiotic CWScomponentsis alsostronglycorrelatedto thesupportmatrixpH,whichhasa particularlyprofoundeffect onthemicrobial communitycomposition andthedevelopment ofplants.Differentgradientsofredox,nutrientsavailability,and

environmentalconditions,suchaspHandtemperaturecreate vari-ablenichesthroughout thewetlandsystems, inwhichdifferent biochemicalprocessestakeplacewhichcanbedeterminantforthe treatmentefficiency[81–83,90].

Thechemicalcharacteristicsofthesupportmatrixalso deter-mineitscapacitytoretainpollutantsbysorptionorionexchange

[119,120]. Sorption of non-polar organic pollutants onto sus-pendedsolids and onto thesupport matrix can in many cases bedescribedasa water-lipidpartitioningprocess, wherebythe substance is partitioned between the water and the organic matter fraction in the solid phase [119]. Organic matter rich materials such as some types of soil, compost and biosorb-ent materials such as agricultural wastes are suitable for the retention of this type of pollutants (provided that the poten-tialfor releaseof significantamountsofDOM is mitigated).On the other hand, sorption of polar or ionic pollutants is domi-natedbyelectrostaticinteractionsandionexchangephenomena. Materials with such characteristics can be found among sev-eraltypes of clays [36,121–125]. The largemajority of organic xenobiotic pollutants are non-polar, although some particu-lar classes like most pharmaceuticals are polar or ionizable

[119].

Therateandextentoftheseprocessesarereportedtobe influ-encedbyseveralfactors,suchasnatureandamountofclays,nature andamountofsoilorganicmatter,periodsofsubmergenceand drying,presenceofvegetationandthedevelopmentofbiofilms.

Ultimately,costisafundamentalfactordeterminingthe selec-tionofthematerialsforasupportmatrix.Thelocalavailability,ease ofaccessandlackofexpensivetreatmentarethemaindecisive conditionsthatinfluencethepriceofmostmaterials.

3.2. Commonsupportmatrixmaterials

A wide variety of materials have been used as

support matrices for CWS, the most frequent choices being gravel,crushedrock,sand(usuallyriversand), gravel/sand mix-turesandlocalsoil[35,56,57,116].Somecharacteristicsofafew ofthemostcommonmaterialsusedasCWSsupportmatrixare presentedinTable1.

Traditionally,CWShavebeenconstructedwithsoilasgrowth mediafor theplants. However,systemswithfinesandandsoil basedsupportmatricesingeneralwillhavelowhydraulic conduc-tivities,withalltheassociatedproblemsthathavebeenmentioned before.Coarsesandandgravelbasedsupportmatrices,ontheother hand,displayhigherconductivities(Table1).Therefore,presently adopteddesignguidelinesarebasedongravelorcoarsesandand insomecasesintermittentlyloadedverticalflowbedsinsteadof horizontalflowbeds[55,116].Mixturesofsandwithgravelhave alsobeenshowntoproducesubstrateswithappropriatehydraulic propertiesforCWS[60].

These moretraditional materials,however,simplyactas fil-tersforretaininglargerparticlesandassupportfordevelopment ofthebiota.Theircapacitytoretainpollutantmoleculesby sorp-tionis,ingeneral,negligible.Theirsubstitutionforothermaterials, whichmayplayamoreactiveroleintheremovalofthepollutants especiallythroughthecontributionofsorptionprocesses,hasbeen

increasinglyattemptedandreportedbymanystudiespublished overthelatestyears.

3.3. Materialsfortheactiveretentionoforganics

Recently,theapplicationofCWSfortreatingincreasinglymore specifictargets,suchasxenobioticorganiccompounds,hasspurred numerousattemptstooptimizethecontributionofeach compo-nenttotheoverallpollutantsremoval.Specificallyintheselection ofthesupportmatrixmaterials,studiesshouldallowtodistinguish thosethatareinerttothetargetpollutantfromthosethat effec-tivelyinteractwithit.Then,itmaybepossibletoreserveamore activeroleforthesubstratecomponentthanmerelytoserveas supportforthedevelopmentofvegetationandmicrobial popula-tions.

PioneeringworkontheCWSoptimizationofthesupportmatrix componentwasinitiallytargetedattheremovalofnutrientssuch asphosphorus.Naturalmaterialsaswellasvariousartificialmedia were tested in order to improve phosphorus removal in CWS, e.g.factory madelight-weightexpanded clayaggregates(LECA)

[79,126–128],shale[128,129],crushedmarble[79,130], diatoma-ceousearth,vermiculiteand calcite[79],zeolitesandlimestone

[128,131],furnaceslag[132]andflyash[132].Thevastvarietyof materialsthathavebeenstudiedforP-removalinCWShasalready beenthesubjectofseveralreviewworks[133–136].Ithasgenerally beenfoundthatmanyofthetestedmaterialshavethepotentialto enhanceP-removalinCWS,althoughsorptioncapacitysaturation aftersomeperiodofthebed’soperationhasbeenidentifiedasa majorproblem.

Subsequentworkhasbeenextendedtoseveralothertypesof pollutants,in particularfor theremovalof recalcitrantorganics forwhichbiodegradationdoesnotpresentaneffectivesolution. Numerousstudiesonthesorptionproperties ofvariedtypes of materials(commonnaturalmineralandorganicmaterials, modi-fiedorprocessednaturalmaterialsandnewlydevelopedsynthetic ones)haveguidedtheselectionofthosewithappropriatechemical characteristics.

Avarietyofmaterialsthathavebeenstudiedrelativelytotheir capacity tosorborganics,is presentedinTable2.Following,an overviewisgivenofthemostrepresentativetypesofmaterialsthat havebeenevaluatedespeciallyforsorptionoforganicpollutants fromaqueousmedia.Mostworkhasconsistedofbatchsorption studieswhichonlygiveapreliminaryindicationoftheefficiency (and,insomecases,alsothekinetics)ofthesorptionprocesses,but afewexperimentshavealsobeencarriedoutinCWSbeds.Inthe latterstudycases,thesuccessfulexperimentsmaygivean indica-tionofthecompatibilityofthematerialswiththedevelopmentand sustainabilityofthebioticCWScomponents.

3.3.1. Activatedcarbon

Activatedcarbonisoneoftheoldest,mostpopularandwidely usedadsorbents[137–139].Itscapacityandversatilityismainly duetoitsporoustexturethatprovidesitwithalargesurfacearea, itscontrollableporestructureanditsthermo-stability[140].Its chemicalnaturecanalsobeeasilymodifiedbychemicaltreatment inordertoenhanceitsproperties.

Table1

Characteristicsofcommonsupportmatrixmaterials[55,58].

Substratetype Effectivesize,d10(mm) Porosity(%) Hydraulicconductivity,Ks(ms−1)

Coarsesand 2 32 1.2×10−2

Gravellysand 8 35 5.8×10−2

Finegravel 16 38 8.7×10−2

Mediumgravel 32 40 11.6×10−2

Table2

Studiesofcompoundssorptionbyseveralnatural,modifiedandsyntheticmaterialsandindustrialandagriculturalwastesandby-products.

Materiala _{Pollutants Studytype(assay}

type/mixture type/solvent)b

Removalefficiency/sorptioncapacity Reference

Activatedcarbon(AC)

Activatedcarbon BisphenolA BSS/SCS/PW 382.12–432.34mgg−1 _[161]

” BTEX BSS/SCS/PW 215.71–275.74mgg−1 _[160] ” Dyes BSS/SCS/PW 0.11–0.27mmolg−1 [143] ” Dyes BSS/SCS/PWand CS/SCS/PW 198–412mgg−1 [141] ” Dyes BSS/SCS/PW ∼90%; 53␮molg−1_(313K) [144] ” Dyes BSS/MCS/PW 159–309.2mgg−1 _[146] ” Dyes BSS/SCS/PW 0.3–8mmolg−1 _[145] ” Explosives CS/SCS/PW 0.080–0.196mgg−1 [159] ” Pesticides BSS/SCS/PW 163.9mgg−1 [153] ” Pesticides BSS/SCS/PW 0.186mgg−1 _[154] ” Pesticides BSS/SCS/PW 909.1mgg−1 _[152] ” Pesticides BSS/SCS/PW 96.15–181.82mgg−1 _[151] ” Pesticides BSS/SCS/PW 156.3mgg−1_{(303K) [150]} ” Pharmaceuticals BSS/SCS/PW 28.5mgg−1 _[157] ” Pharmaceuticals BSS/SCS/PW 236–354mgg−1 _[158] ” Pharmaceuticals BSS/SCS/PW 139.2–393.4mgg−1(298K) [155] ” Pharmaceuticals BSS/SCS/1:9 methanol:water 139–295mgg−1 [156] ” Phenoliccompounds BSS/SCS/PW 149.25mgg−1 _[149] ” Phenoliccompounds BSS/SCS/PW 42.3191mgg−1_{(298K) [147]} ” Phenoliccompounds BSS/SCS/PW 50–80mgg−1 _[148] CommercialAC(CAC) andagro-waste basedAC(AWAC) Dyes BSS/SCS/PW 980.3mgg−1_(CAC) 143.2–472.1mgg−1(AWAC) [142] Clay-basedmaterials Bentonite Dyes BSS/MCS/PW 430mgg−1 [189] ” PAHs BSS/MCS/PW 94–95% [189] ” Pharmaceuticals BSS/SCS/PW 47mgg−1 [314] Organo–bentonite Dyes BSS/MCS/PW >99%; 868.1mgg−1 [191] ” Surfactants BSS/MCS/PW >99%; 980.8mgg−1 [191] Several organo-bentonite PAHs BSS/SCS/PW Kdmax=1.7–36.2Lg−1 [197] Kaolinite Pesticides BSS/MCS/PW Kd=0.039–0.052Lg−1 [193] ” Pesticides BSS/SCS/PW Kd=0.21Lg−1 [192] ” Pesticides BSS/SCS/PW Kd=0.009–0.041Lg−1 [194] ” Pharmaceuticals BSS/SCS/PW 3.1mgg−1 _[157]

” Pharmaceuticals BSS/MCS/PW PoorfitoftheLangmuirmodel [176]

” Phenoliccompounds BSS/SCS/PW 0.11–2.16mgg−1 _[198]

Montmorillonite Aromaticcompounds BSS/SCS/PW Kd=1.48–213Lg−1 [174]

” Aromaticcompounds BSS/MCS/PW Kd=0.03–4Lg−1 [124]

” Pesticides BSS/MCS/PW Kd=67–119Lg−1 [193]

” Pesticides BSS/SCS/PW Kd=0.18Lg−1 [192]

” Pesticides BSS/SCS/PW Kd=0.005–0.014Lg−1 [194]

” Pharmaceuticals BSS/SCS/PW 6.1mgg−1 _[157]

” Pharmaceuticals BSS/MCS/PW PoorfitoftheLangmuirmodel [176]

” Pharmaceuticals BSS/SCS/PW 0.88mmolg−1 [177]

Montmorillonite (cation-saturated andwithsurfactants)

Pharmaceuticals BSS/SCS/PW Kd=0.048–0.073Lg−1(K+andCa2+

saturated),Kd=0.705Lg−1(with

surfactant)

[175]

Montmorillonite Phenoliccompounds BSS/SCS/PW 0.07–0.62mgg−1 _[198]

Sepiolite Dyes BSS/SCS/PW 2.82–13.5␮molg−1 _[182]

Sepiolite(rawand calcined)

Dyes BSS/SCS/PW 12.1␮molg−1_{(raw),9.8–16.2␮molg}−1

(calcined)

[185]

Sepiolite(rawand activated)

Dyes BSS/SCS/PW 0.18mmolg−1(raw),0.41mmolg−1 (activated)

[186]

Sepiolite Dyes BSS/SCS/PW 155.5mgg−1 _[184]

” Dyes BSS/SCS/PW 70.6–110.2mgg−1 _[183]

Activatedsepiolite Dyes BSS/SCS/PW 4.0–6.5mgg−1 _[187]

Sepiolite Pharmaceuticals BSS/SCS/PW 36–43% [75]

Smectite(calcined) Dyes CS/SCS/PW 10.36mgg−1 _[190]

Smectites(saturated withK+_andCa2+₎

Phenoliccompounds BSS/SCS/PW 31–62%(K+_{),12–18%(Ca}2+_{) [125]}

Vermiculite Dyes BSS/SCS/SWW 72–106mgg−1 _[188]

Expandedvermiculite Pharmaceuticals BSS/SCS/PW 51–60% [75]

Expandedclay Organicmatter CWS/MCS/WW 88%BOD5;92%COD [46]

” Organicmatter CWS/MCS/WW 59.1%BOD5;52.3%COD [206]

Table2(Continued)

Materiala _{Pollutants Studytype(assay}

type/mixture type/solvent)b

Removalefficiency/sorptioncapacity Reference

” Organicmatter CWS/MCS/SWW andCWS/MCS/WW 65–93% [84] ” Pesticides BSS/SCS/PW 0.0099–0.0278mgg−1 _[74] ” Pharmaceuticals BSS/SCS/PW 0.0045–0.0123mgg−1 _[74] ” Pharmaceuticals BSS/SCS/PW, BSS/MCS/PW, BSS/MCS/WW 58–95%(SCS/PW),51–93%(MCS/WW) 0.023–0.033mgg−1(SCS/PW), 0.014–0.030mgg−1_(MCS/PW), 0.011–0.027mgg−1_(MCS/WW) [75] ” Pharmaceuticals CWS/SCS/WW 82.0%(unplanted) 92.5–94.5%(planted) [47]

” Pharmaceuticals CWS/MCS/WW 43–91%(unplanted,summer)

41–87%(unplanted,winter) 75–97%(planted,summer) 48–88%(planted,winter)

[42]

Organovermiculite Pesticides BSS/SCS/PW Kd=0.015–0.355Lg−1 [195]

Organoclays(various) Pesticides BSS/SCS/PW, CS/SCS/PW

66–100% [196]

Zeolitesandothersiliceousmaterials Natural(clinoptilolite), ZSM-5,bothwith surfactant BTEX BSS/SCS/90%water: 10%ethanol 0.171–0.296mmolg−1_(cli.), 0.036–0.192mmolg−1_(ZSM-5) [211] Natural(92% clinoptilolite)with surfactant BTEX BSS/SCS/PW Kd=0.012Lg−1(bilayerof surfactant)–0.013Lg−1(monolayerof surfactant) [210]

Unspecifiedtypewith surfactant BTEX CS/MCS/WW Kd=0.018–0.095Lg−1 [236] Natural(92% clinoptilolite)with surfactant Dyes BSS/SCS/PW 61–111mgg−1 _[213] Natural(92% clinoptilolite)with surfactant Dyes BSS/SCS/PW 25–35% [233] Natural(68.5% clinoptilolite) Dyes BSS/SCS/PW 16.4–19.9mgg−1 _[215]

DAZ-P,DAY-P Dyes BSS/SCS/PW <15% [230]

Natural(90% clinoptilolite)with surfactant Dyes BSS/MCS/PW 14.9–55.9mgg−1 _[146] Natural(92% clinoptilolite)with surfactant Dyes BSS/SCS/PW 61–111mgg−1 [214]

DAY PAHs

BSS/SCS/water-butanol 769.231mgg−1 _[231] Natural(90% clinoptilolite)with chlorosilanes PAHs BSS/SCS/PW 35–54% [209]

A,X Pesticides BSS/SCS/PW 55%max.(A),80%max.(X) [227]

Natural(80% clinoptilolite)with surfactant Pesticides BSS/SCS/PWand CS/SCS/PW 2.0–4.4␮molg−1 _[219]

Y Pharmaceuticals BSS/MCS/PW >90%(saturatedsolutions) [229]

Natural(80% clinoptilolite)with surfactant

Pharmaceuticals BSS/SCS/PW 0.07–0.16mmolg−1 [220]

Y,mordenite,ZSM-5 Pharmaceuticals BSS/SCS/PWand BSS/MCS/WW(in Y) 42–100mgg−1_{(Y-PW),26–32mgg}−1 (mor.-PW),16–26mgg−1_(ZSM-5-PW), 96–100%(Y-WW) [222] Natural(clinoptilolite), Beta,ZSM-5

Pharmaceuticals BSS/SCS/PW 1.0mmolg−1_{(Beta),0.07mmolg}−1

(ZSM-5),0.04mmolg−1(cli.)

[221]

Beta,ZSM-5 Phenoliccompounds BSS/SCS/PW 0.64–0.72mmolg−1(Beta) 0.22–0.55mmolg−1(ZSM-5)

[223]

SyntheticNa-Y, Ni/Na-Y

Phenoliccompounds BSS/SCS/PW 0.8–0.9mmolg−1_(Na-Y),

0.9–1.1mmolg−1_(Ni/Na-Y)

[228]

Natural(clinoptilolite), ZSM-5,bothwith surfactant

Phenoliccompounds BSS/SCS/90%water: 10%ethanol 0.082–0.126mmolg−1_(cli.), 0.020–0.100mmolg−1_(ZSM-5) [211] Y,Beta,mordenite, ZSM-5

Phenoliccompounds BSS/SCS/PWand CS/SCS/PW

85%(Beta),70%(ZSM-5),65%(Y),and 50%(mor.)

0.723mmolg−1_{(Beta),0.595mmolg}−1

(ZSM-5),0.553mmolg−1_(Y),

0.361mmolg−1_(mor.)

[225]

Table2(Continued)

Materiala _{Pollutants Studytype(assay}

type/mixture type/solvent)b

Removalefficiency/sorptioncapacity Reference

Natural(97% clinoptilolite)with surfactant Phenoliccompounds BSS/MCS/PW 41–80%; 0.7647–12.7065mgg−1 [212] Natural(92% clinoptilolite)with surfactant Phenoliccompounds BSS/SCS/PW Kd=0.011Lg−1 [210]

Natural(natroliteand clinoptilolite)both withandwithout cyclodextrin

Phenoliccompounds BSS/MCS/PW 32–42%(natr.),47–64%(cli.),47–54% (natr.w/CD),66–74%(cli.w/CD)

[208]

Natural(88% clinoptilolite)

Phenoliccompounds CWS/MCS/WW 74–90% [238]

Unspecifiedtype Organicmatter CWS/MCS/SWW 97–98.2%BOD5 [241]

Clinoptilolite Organicmatter CWS/MCS/WW 71.4–73.9%COD [240]

Natural(33% clinoptilolite)

Organicmatter CWS/MCS/WW 90–93%BOD5 [239]

OtherAromaticcompounds Natural(92% clinoptilolite)with andwithout surfactant Aniline BSS/SCS/PWand CS/SCS/PW 0mgg−1_{(nat.),2.36mgg}−1_{(mod.) [235]}

DAY Anilinederivatives CS/MCS/PW 1.2–1.6mmolg−1 _[232]

Natural(92% clinoptilolite)with surfactant Aniline BSS/SCS/PW Kd=0.004Lg−1(bilayerof surfactant)–0.005Lg−1(monolayerof surfactant) [210]

Natural(natroliteand clinoptilolite)both withandwithout cyclodextrin

Anilineandaniline derivatives BSS/MCS/PW 34–36%(natr.),45–49%(cli.),46–49% (natr.w/CD),65–67%(cli.w/CD) [208] NaP1,hydroxysodalite (HS)with(m)and without(u) surfactant

BisphenolA BSS/SCS/PW 1.4mgg−1(NaP1u),3.5mgg−1(HSu), 56.8mgg−1(NaP1m),114.9mgg−1 (HSm) [226] Natural(92% clinoptilolite)with andwithout surfactant Nitrobenzene BSS/SCS/PWand CS/SCS/PW 2.5mgg−1_{(nat.),3.25mgg}−1_{(mod.) [235]}

Othersiliceousmaterials

Expandedperlite Pesticides BSS/SCS/PW <5% [74]

” Pharmaceuticals BSS/SCS/PW <5% [74]

” Dyes BSS/SCS/PW 63–76%

55.55mgg−1

[246]

Perliteandexpanded perlite

Dyes BSS/SCS/PW 43%(raw),65%(expanded) [247]

Perliteandexpanded perlite

Surfactants BSS/SCS/PW 9.8␮molg−1_(raw)

89.3␮molg−1_(expanded) [248] ” Dyes BSS/SCS/PW ∼15–55%; Kd=0.18–0.42Lg−1 [251] Diatomite Dyes BSS/SCS/PW 5.92–117.75mgg−1;28.6–99.23% [250] ” Dyes BSS/SCS/PW 26.04mgg−1 [254] ” Dyes BSS/MCS/PW 47mgg−1 _[252] ” Dyes BSS/SCS/PW 94.46–137.0mgg−1 _[253] ” Otheraromatics BSS/SCS/PW 0.73mgg−1 _[257] ” Pesticides BSS/SCS/NW 55–95% [255] ” Phenoliccompounds BSS/SCS/PW 92mgg−1 _[256]

Industrialwastesandby-products

Flyash Dyes BSS/SCS/PW 3–240␮molg−1 _[262]

” Dyes BSS/SCS/PW 13.5–97.6␮molg−1 _[263] ” Dyes BSS/SCS/SW 20␮molg−1 [260] ” Organicmatter CWS/MCS/WW 47% [274] ” PAHs BSS/MCS/Several solvents 25–100% [266] ” PCBs BSS/MCS/Several solvents 0–30% [266] ” PCBs BSS/SCS/PW <97% [267] ” Pesticides BSS/SCS/PW 0.56–3.33mgg−1 [264] ” Petroleumhydrocarbons

(PAHandBTEX)

BSS/SCS/PW 0.10–108.4Lkg−1 [265]

” Phenoliccompounds BSS/MCS/PW 88.9–92.6% [258]

” Phenoliccompounds BSS/SCS/PW 6.24–15.67mgg−1_{(303K) [259]}

Bagasseflyash Phenoliccompounds BSS/SCS/PW 23.83mgg−1 _[269]

” Pesticides BSS/SCS//PWand

BSS/SCS/NW

Table2(Continued)

Materiala _{Pollutants Studytype(assay}

type/mixture type/solvent)b

Removalefficiency/sorptioncapacity Reference

Blastfurnacedust Pesticides BSS/MCS/PW 13–21mgg−1 _[315]

Blastfurnacedust Phenoliccompounds BSS/MCS/PW 9.5–13.8mgg−1 _[316]

Blastfurnaceslag Dyes BSS/SCS/PW 1.15mgg−1 _[317]

Blastfurnacesludge Dyes BSS/SCS/PW 1.3–2.1mgg−1 _[318]

” Pesticides BSS/MCS/PW 23–30mgg−1 [315]

” Phenoliccompounds BSS/MCS/PW 12.7–18.9mgg−1 [316]

Redmud Phenoliccompounds BSS/SCS/PW 4.127mgg−1 _[319]

Ricehuskash Phenoliccompounds BSS/SCS//PW 1.63–153␮molg−1 _[273]

Agriculturalwastesandbyproducts

Almondshell Dyes BSS/SCS/PW 97%;

20.5mgg−1_{(almondshellmixture)} [301]

Almondshell Dyes BSS/SCS/PW 51.02–76.34mgg−1 _[296]

” Dyes BSS/SCS/PW 29.0mgg−1 _[298] ” Dyes BSS/SCS/PW 32.6mgg−1 _[297] ” Hormones BSS/SCS/PW 90% [305] ” Pharmaceuticals BSS/SCS/PW 2.5mgg−1 [307] ” Phenoliccompounds BSS/SCS/PW 93% [290] ” Phenoliccompounds CS/SCS/PW >99.98% [291]

” Phenoliccompounds BSS/SCS/PWand

BSS/SCS/WW

91.36%(PW);85.54%(WW) [292]

Bananapeel Pesticides BSS/SCS/PW 90–99%;

14mgg−1

[284]

” Phenoliccompounds BSS/MCS/WW 689mgg−1 [293]

Chickpeahusk Pesticides BSS/SCS/PWand

CS/SCS/PW

7.7–25␮molg−1(BSS) [288]

Coconutfiber Pesticides BSS/SCS/PW 85.9–86.3% [285]

Coconutshellscharcoal Pesticides BSS/SCS/PW 92.4–95.2% [285]

Coconutshells Phenoliccompounds BSS/SCS/PW 205.8mgg−1 _[294]

Cork(granulated) PAHs BSS/MCS/PW >96%;0.016–0.049mgg−1 _[303]

Cork Pesticides BSS/SCS/PW 19.08mgg−1 [153]

Cork(granulated) Pesticides BSS/SCS/PW 80%;0.26–0.55mgg−1 [289]

Cork(granulated) Pesticides BSS/SCS/PW 0.136–0.303mgg−1 [154]

” Pharmaceuticals BSS/SCSand

MCS/PWandWW

0.06056–0.3668mgg−1_(SCS,PW)

8–63%(MCS,WW)

[85]

Corkbark Pharmaceuticals BSS/SCS/PW 0.99mgg−1 _[306]

Cork Phenoliccompounds

BSS/SCS/hydro-alcoholic solutions

9.8–31.2Lkg−1 _[295]

Hazelnutshell Dyes BSS/SCS/PW 0.214mmolg−1 [299]

” Dyes BSS/SCS/PWand

CS/SCS/PW

40.8–76.9mgg−1 _[300]

Pinebark Hormones BSS/SCS//PW >99% [305]

” Explosives BSS/SCS/WW 50–80%

1.68mgg−1

[304]

Pinebarkwith(M)and without(U) modification

PAHs BSS/SCS/PW 62.9–80.95%;2.21–3.73mgg−1(U) 39.2–95.48%;0.68–22.69mgg−1(M)

[302]

Pinebark Pesticides BSS/SCS/PW 10mgg−1 _[283]

” Pesticides CS/MCS/WW 72% [282]

Ricebran Pesticides BSS/SCS//PWand

NW

390␮molg−1_{(PW);94–99%(NW) [271]}

Ricebran Pesticides BSS/SCS//PWand

NW

82␮molg−1(PW);98–99%(NW) [287]

Ricehusk Pesticides BSS/SCS//PWand

NW

350␮molg−1_{(PW);97%(NW) [271]}

” Pesticides BSS/SCS//PWand

NW

25␮molg−1_{(PW);94–96%(NW) [287]}

” Phenoliccompounds CWS/MCS/WW 100%(planted),25–100%(unplanted) [311]

” Dyes BSS/SCS/PW ∼70%;

59␮molg−1(313K)

[144]

Woodsawdust Dyes BSS/SCS/PW 26.2–59.2mgg−1 [300]

” Pesticides BSS/SCS/PW 69.44mgg−1 _[153]

” Pesticides BSS/SCS/PW 78.5–80.5% [285]

Wood Pesticides BSS/SCS/PW 40–80%(untreatedwood) [286]

Woodcharcoal Organicmatter CWS/MCS/WW 95.5% [312]

a_{ZSM-5,DAZ-P,DAY,A,X;YandNaP1arecommonabbreviations/designationsforcertaintypesofzeolites,asisdetailedinthetext.}

b_{BSS—Batchsorptionstudies;CS—columnstudies;CWS—constructedwetlandssystems;SCS—single-componentsolutions;MCS—multi-componentsolutions;PW—pure}

water;WW—wastewater;SWW—syntheticwastewater;NW—naturalwater;CD—cyclodextrin. Activated carbon hasbeen proven to be an effective

adsor-bent for the removal of a wide variety of organic pollutants from aqueous media or gaseous environments. A small illus-trativesample of thetests that have been conducted tostudy

theadsorptionoforganicpollutantsbyactivatedcarbonis pre-sented in Table 2. Among the most commonly tested organic substances, several examples can be found including dyes

pharmaceuticals [155–158] and many other organic pollutants

[159–161].

Therearetwomostcommonphysicalformsinwhichactivated carbonsareused,namelyaspowderedorgranularmedia.Thereare otherforms thathaverecentlybeenattractingincreasing atten-tion,amongthemfibersmainlyobtainedfromisotropiccoaland petroleumpitch,clothsandfelts[162].

Almostanycarbonaceous materialmaybeusedasa precur-sorforthepreparationofactivatedcarbons,themainrequirement beingthatitpossessesahighcarboncontent.Inaddition,the selec-tionoftherawmaterialsisbasedmainlyonthefollowingcriteria

[162]:

• lowinorganicmattercontent,

• easeofactivation(e.g.,calcinedcokeisa“difficultmaterial”while woodcharisreadilyactivated),

• availabilityandlowcost, • lowdegradationduringstorage.

However,inpractice,commerciallyavailableactivatedcarbons areusuallyderivedfromnaturalmaterialssuchaswood,nutshells andfruitstones,bananastalks,sawdust,cork,ricehusk,coconut shell,straw,peat,bamboodust,charcoal,softcoal,lignite, bitumi-nouscoal,petroleumcoke,etc.[140–142,147–150,155,156].

Theuseofactivatedcarbon(ingranularform,duetohydraulic conductivityrequirements)asasupportmatrixforCWSdoesnot meetthe samepopularity asitsuseas filtermedium, sinceits effectivenessisnotashightojustifyitshighcost[163–165].In addition,sinceitisa lessspecificadsorbent,itscapacity is eas-ilysaturatedbythecomplexliquidmatrixofwastewatersandits regenerationisdifficultwhenusedasCWSbed.Infact,thebiggest barriertothegeneraluseofactivatedcarbonisitscostandthe diffi-cultiesassociatedwithregeneration[137,140].Activatedcarbonis quiteexpensiveandthehigherthequalitythegreaterthecost.The useofcarbonsbasedonrelativelyexpensivestartingmaterialsis unjustifiedformostpollutioncontrolapplications[166].Also,both chemicalandthermalregenerationofspentcarbonisexpensive, impracticalonalargescaleandproducesadditionaleffluentand resultsinconsiderablelossoftheadsorbent.

Alltheseshortcomingshaveledmanyresearcherstolookfor moreeconomicadsorbents,especiallyforwaterpollutioncontrol wherecostfactorsplayamajorrole.

3.3.2. Clay-basedmaterials

Claymineralsareoneofthemostcommonconstituentsofsoils. Itsubiquityinnaturemakesthisclassofmaterialsunsurprisingly oneofthemostextensivelystudiedgroupsofadsorbentsandalso anaturalchoiceforatleastoneofthesupportmatrixcomponents ofCWS.

Claysarebroadlydefinedasthosemineralsthatmakeupthe colloidfraction(<2␮m)ofsoilsandwhichmaybecomposedof mixturesoffine-grainedclaymineralsaswellasclay-sized crys-talsofotherminerals(e.g.quartz,carbonatesandmetaloxides). Inamorestrictdefinition,claymineralsarehydrousaluminium phyllosilicateswhichpossessalayeredstructure.Accordingtothe differencesin that structure theycanbeclassifiedas smectites (montmorillonite, saponite), mica (illite), kaolinite, serpentine, pylophyllite(talc),vermiculiteand sepiolite[167].Thestructure ofthesemineralsfeaturesanetnegativechargewhichisbalanced byexchangeablecations.Theseionscanbeeasilyexchangedwith somecationsfromtheliquidmedia[168,169].

Theamphiproticcharacterofsilanoland aluminolgroups in clayssurfacesis responsiblefora pH-dependent surfacecharge inclays(however,watermoleculesassociatedwithexchangeable cationsandclaysurfacesmayobscurethesechargedadsorption sites;sucheffectisdependentonthehydrationstrengthof the

exchangeableions[170–175]).Electrostaticinteractionswiththe surface andmechanismssuchascation exchange,cation bridg-ingwiththesurface,surfacecomplexation,andhydrogenbonding seem tobe involved in the capture of ionic and polar species from aqueous media [121–125,176]. In addition, the interlayer expandabilityofmanyofthesemineralsandthepresenceofwater moleculesassociatedwithexchangeablecationsinthese interlay-ersallowstheexchangeofthesehydratedionswithmuchlarger organicmoleculesandtheirintercalationbetweenthe aluminosil-icatelayers[123,177,178].

Because of this high potentialfor ion exchange and surface interactions,inadditiontotheextensivesorptioncapacities result-ing from their large surface areas (due to the sheet structure of theseminerals)claysact asnaturalscavengersof pollutants. Forsuchreasons,andaddingtotheirwideavailabilityand asso-ciated low cost, in recent years there has been an increasing interestinutilizingnaturalclaysfortheremovaloforganic con-taminantsfromaqueoussolutions(Table2).However,duetothe hydrophiliccharacteristicsoftheirsurfacesandcharges,naturally occurringclaysseemlesseffectivefortheadsorptionofanionic contaminants and hydrophobic or non-polarorganic pollutants

[179].

Toincrease theabilityof mineralclaystoremovenon-polar andanionicwaterpollutants,likemanyothernaturalmaterials, clayscanalsobemodifiedtoimprovetheirsorptionproperties. Thismodificationcommonlyconsistsofasimpleionexchangeof thenaturalinorganicinterlayercationswithcertainorganiccations suchasquaternaryammoniumcationsoflonghydrocarbonchains. Byintroducingcationicsurfactantmoleculesintotheinterlamellar spacethroughionexchange,thepropertiesofclaymineralsare enhancedtothoseoforganoclays[179–181].Theintercalationof acationicsurfactantbetweentheclaylayerschangesthesurface propertiesfromhighlyhydrophilictoincreasinglyhydrophobic.At sufficientloadingthesurfactantformsabilayerthatresultsina reversalofthechargeontheexternalsurfaceoftheclayadequate forretentionofanions,whileneutralspeciescanpartitionintothe hydrophobiccore.Inaddition,modificationofaswellingclaywith acationicsurfactantresultsinanincreaseofthebasalspacingofthe layerandexposureofnewadsorptionsites.Theseorganoclayshave beenextensivelyinvestigatedinrecentyearsforawidevarietyof environmentalapplications.

Asanalternativetothechemicalmodificationofclaysurfaces, somestudieshaveemployedlightweightclaymaterialsthatare basedonprocessednaturalclays.Thetypicalformofprocessing consistsofsomethermaltreatmentthathastheeffectofcausing interlayerhydrationwatertoquicklyvaporize(sometimes com-plementedwithanadditionaleffectofthethermalexpansionof injectedgasessuchasCO2)withaconsequentexpansionofthe sheetstructureofthemineralsandformationofporesand chan-nelsasthegasesescapethesoftenedheatedmaterials.Thisprocess yieldshighlyporousmaterials,withanincreasedaccessiblesurface area,whichcansubstantiallyimprovethesorbentcapacity.Someof themostcommonprocessedclaymaterialsthathavebeenusedare lightexpandedclayaggregates(LECA),expandedshale,expanded slateandexfoliatedvermiculite.

Intermsofhydraulicconductivityrequirements,particlesizes ofthesematerialscan,ingeneral,beobtainedinadequate distribu-tionswhichallowsomecontrolofthehydraulicpropertiesofthe medium.

Some of the most popular types of clay minerals (kaolin-ite,illite,montmorillonite,vermiculite,sepiolite,bentonite)have been tested to remove by adsorption a variety of organic pol-lutants suchas dyes [182–191], pesticides [74,192–196], PAHs

[124,189,197], pharmaceuticals [74,75,157,175–177], phenolic compounds[125,174,198]andotheraromatics[124,174,189,198]. In thevast majorityofthesestudiestheorganicsubstances are

removed most commonly from pure water solutions in batch adsorptionassays.

Clayshavebeenapopulartypeofmaterialsusedasthe sup-portmatrixofCWS,forreasonsnotexclusivelyrelatedwiththeir qualitiesas sorbents and low cost,but just becausetheyare a typical component of soils and, therefore, in many cases they arenaturallypresentinthesupportmatrixofCWSwhensoilis usedassubstrate.Recently,despitetheslightlyincreasedcostof production,processed claymaterialshave alsobecomepopular choicesforCWSsubstratesandinanycaseclay-basedmaterials provedtobeanadequatemediumforagoodperformanceofCWS

[42,46,47,83,84,126,165,199–206]. 3.3.3. Zeolitesandothersiliceousmaterials

The useofzeolitesand othernaturalsiliceoussorbents such asperlite,diatomite,silicaandglassfibersfororganicxenobiotics removalfromwastewaterisincreasingbecauseoftheirabundance, availabilityandlowcost.

Zeolitesarehighlyporousaluminosilicatesfeaturinga distinc-tivechannelorcage-likestructure.Theycaneitheroccurasnatural mineralsor can beartificially synthesized.More than 40 natu-ralzeolitesareidentifiedintheworld,andareavailableinlarge depositsinmanycountriessuchasGreece,UK,Italy,Mexico,Iran, andJordan[137].Themostabundantandfrequentlystudiedone isclinoptilolite,whileonlysixothertypesofnaturalzeolitesalso existinsufficientquantityandpuritytobeconsideredexploitable, namelymordenite,chabazite,erionite,ferrierite,phillipsite,and analcime[207].

Like in clays, thealuminosilicate frameworkof zeolitesalso hasanegative latticecharge which isbalanced by cationsthat canbeexchangeablewithcertaincationsinsolution.Thehighion exchangecapacityofzeolites,theirextensiveporositiesand spe-cificsurfaceareas,aswellastheirrelativelycheapprices,make naturalzeolites attractiveadsorbents[208–222].However, syn-theticzeolitesarealsowidelytested,especiallybecausethrough synthesis a useful control can be obtained over the Si/Alratio in the zeolite composition. A zeolite with a higher content of themore hydrophobicsiliconcanshowhigheraffinity for non-polarorganics[223,224].Someofthesyntheticzeolitesforwhich studiescanbefoundintheliteraturearethebeta(BEA)zeolite

[221,223,225],ZSM-5[211,221–223],hydroxysodalite[226], zeo-liteA(LTA)[224,227],zeoliteP[226],zeoliteX[224,227],zeoliteY

[222,224,228,229]anddealuminatedY(DAY)zeolite[230–232]. In fact, many unmodified naturalzeolites are notvery good adsorbents of anionic compounds [207] and, due to its sur-facehydrophilics,theyarepooradsorbents ofmostorganicsas well [214,226,233]. However, surface modification can change thesurfacefunctionality byaddingcationicsurfactantsorother hydrophobic groups thus making them applicable for adsorp-tionof variousanions andorganics. The mostcommonsurface modifiers are salts of a variety of quaternary amines such as hexadecyltrimethylammonium(HDTMA,themostpopular surfac-tant) [210–214,224,226,234–236], octadecyldimethylbenzylam-monium(ODMBA) [216–218], stearyldimethylbenzylammonium (SDBA)[219,224]orbenzyltetradecylammonium(BDTDA)[212]. Otherposibilitiesarecetylpyridiniumsalts[211,220],cyclodextrins

[208]andchlorosilanes[209].

Zeoliteshavebeenintensivelystudied(abriefsampleofthese studiesispresentedinTable2)andconsideredasviableoptions(in spiteofusuallyrequiringlow-costmodifications)totreat waste-water contaminated withtracequantities of organicpollutants

[237]. Among some of the organic compounds that have been observed to be successfully removed by zeolites are phenolic compounds[208,210–212,223,225,228,232,238],PAHs[209,231], BTEX[209–211,236],otheraromaticssuchasnitrobenzene, bisphe-nolA,anilineandanilinederivatives[208,210,226,232,235],dyes

[146,213–215,230,233],pesticides[219,227]andpharmaceuticals

[220–222,229].

Efficiencyofpollutantsremovalbymodifiedzeoliteshasbeen observed,inmostcases,asbeingproportionaltothesurfactant con-tentanditscoverageofthezeolitesurface[210,216,226],which showsthatthesurfactantisakeycomponentinzeoliteadsorption processes.AdsorptionbyzeolitesappearstobealsopH depend-entinmanycases[210,216,226,232].Thestateofionizationofthe adsorbateisnaturallydeterminantfortheextentofionexchange withzeolitecations(inwhichcaseacationicstateisfavored)or adsorption/partitiontothesurfactant(aneutral formis usually moreefficientlyremovedfromtheliquidinsuchcases).

TherehavebeenalreadysomestudiescarriedoutinCWSwhere zeoliteshavebeenusedatleastasoneoftheconstituentsofthe supportmatrix[117,238–242].Mostofthestudiesfocusonthe removalofnitrogen,phosphorusandunspecifiedorganics(organic matter)bysuchCWS,buttherehavebeenalreadysomeattempts toevaluatetheremovalofspecificorganicssuchasphenolic com-pounds[238].Theseassaystogetherwiththestudiesonadsorption oforganicsaspresentedinTable2pointtotheuseofzeolitesasa viableoptionforthesupportmatrixofCWSfororganicpollutant treatment.

Amongothernaturalsiliceousmaterials,perliteanddiatomite are probably the most commonly tested. Perlite is an inex-pensive and easily available material that consists of a glassy volcanic rockwitha highcontentof silica(typically>70%)and a relativelyhighamountof water.Asone ofits salient charac-teristics,perliteexpandswhenheatedtoasuitabletemperature. Attemperaturesbetween760and 1090◦C,whichis the soften-ingrange of perlite(sinceit is aglass),thewater containedin thecrude perliterock quicklyvaporizesand escapes the struc-ture of the material [243],thus causing perlite to significantly expand(10–15timesitsoriginalvolume)andformalightweight glasslike solid with a cellular structure and very large surface area.

Almostallperliteisconsumedinanexpandedform,although a small amountof unexpanded perlitehasbeen usedin a few applications.Expandedperliteismostlyusedasanexcellent fil-teraidandasa fillerinvariousprocessesand materialsdueto itschemicalinertnessinmostenvironments.Inaddition,itfinds someapplicationsinagricultureforgrowingofseedsand regular-izingofthesoil,thussuggestingsomepotentialfor useasCWS support matrix. Asa sorbent,due to thepresence of a signifi-cantamountofsilanolgroupsonitssurface,itfeaturesinteresting adsorptivequalitiesandithasalreadybeenstudiedforthe sorp-tionofseveraldifferenttypesoforganiccompounds,namelydyes

[244–247],pesticides[74],pharmaceuticals[74]andsurfactants

[248].

Diatomite,ordiatomaceousearth,isanaturallyoccurring sed-imentaryrockconsistingmainlyofthefossilizedskeletalremains of diatoms, a type ofhard-shelledalgae. Itschemical composi-tionis essentiallyamorphoussilica (typicallyover 80%of dried diatomite), although variable smallamounts of other materials (metal oxides,clays,salts(mainlycarbonates)andorganic mat-ter)mayalsobepresent.Diatomiteisaverylightweightmaterial thatiseasilyavailableandinexpensive.Duetoitswideporesize distribution,highpermeability,highporosity,smallparticlesize, largesurfaceareaandchemicalinertnessitisconsideredagood adsorbentwithahighadsorptioncapacity[249–252].Inaddition, diatomitehassurfacechargesduetothepresenceofionisable func-tionalgroups,especiallysilanolgroups,thatspreadoverthematrix ofthesilica,whicharesuitableadsorptionsitesforpolarorganic compounds.

Diatomitehasbeentestedasanadsorbentforseveraltypesof organics,namelydyes[250–254],pesticides[255],phenolic com-pounds[256]andotheraromatics[257].