2 MOFs and Metal Oxides for Gas Adsorption and ......2 MOFs and Metal Oxides for Gas Adsorption and Environmental Applications 2.1 Introduction Acid gases like carbon dioxide (CO 2)

HaleemaSaleemandVikasMittal*,ThePetroleumInstitute(partofKhalifaUniversityofSci-enceandTechnology),AbuDhabi,UAE*Currentaddress:Bletchington,WellingtonCounty,Australia©2018CentralWestPublishing,Australia

2MOFsandMetalOxidesforGasAdsorptionandEnvironmentalApplications2.1IntroductionAcid gases like carbon dioxide (CO2) and hydrogen sulfide (H2S) representtwomostabundantharmfulgasesinthenaturalgasstreamswhichneedtoberemovedforachievingusefulnessofnaturalgas.Similarly,environmentalpol-lutionduetothereleaseofharmfulgasesfromdifferentindustrialandhouse-holdprocessesleadstoseveredegradationofthequalityofairandsignificanteffortsarerequiredtominimizepollutionlevelaswellastoremovethepollu-tantsfromtheenvironment.Someoftheharmfulmaterialsthatarepresentinour environment are H2S, COx, SOx, NOx, nitrogen containing compounds(NCCs), sulfur containing compounds (SCCs), volatile organic compounds(VOCs),dyes,pharmaceuticalsandpersonalcareproducts(PPCPs).2.1.1ClassificationofHazardousMaterialsDependingonthesources,themostplentifulhazardousmaterialscanbecate-gorized into two classes, namely naturally originating hazardous materialsandanthropogenic.Asubstantialquantityofnaturallyoccurringmaterialsareavailableundergroundaswellasinwater,soil,airandminerals.However,theanthropogenic materials generate from chemical reactions, combustion orfromtheeffluentoftoxicmaterials.Theglobalenergyneedissatisfiedbythenaturallyoriginatingmaterialssuchascoal,crudeoilandnaturalgas.Asmen-tionedearlier,naturalgascontainsharmfulgaseslikeCO2andH2Samongoth-ers, which require removal. In addition, these energy sources when burntgeneratehighamountsofharmfulgases to theatmosphere.Themajor toxicgases causing environmental air pollution are CO2,H2S, SOx,NH3,NOx, VOCsandotherhydrocarbons [1].Thegases likeSO2,CH4,O3andN2Oareconsid-eredasgreenhousegasesandtheejectionofthesegasesenhancestheozonelevelspresentinthetroposphere.VehiclesrelatedpollutantssuchasCH4,CO,NO2, SO2 aswell as carbon black also cause globalwarming. Currently, thecarbonbalanceoftheworldisconsideredasoneofthecriticalenvironmental

26 FunctionalNanomaterials&Nanotechnologies

problem,andhencethereductionofanthropogenicCO2releasehasnowbe-comeaverysignificantissue.VOCsarechemicalshavinghighvaporpressure,and are usually discharged from adhesives, resins, paints, solvents, etc. [2].Benzene, toluene, xylene and phenolics are some of the commonhazardousVOCs.Nitrogenorsulfurcontainingorganiccompoundsarenaturallyoriginat-ingspecies,andareavailableinfossilfuelsaswellasoilslikejetfuel,gasoline,crudeoil,heatingoilanddiesel.Thecombustionoffossilfuelsisalargesourceoftoxicreleases,whichcausesglobalwarming,greenhouseeffect,dangerousimpactonlivingspeciesandhazardousairpollution[3-5].Further,duetotheglobalindustrialadvancement,theseverityofwaterpol-

lutionhasbecomecritical,thus,posingsignificantchallengestothehealthoflivingbeings [6].Dissolvedheavymetal saltsororganicpollutantsareoftenpresentinthepollutedwater[7,8].Theorganicpollutantsexistinginthenat-uralwaterresourcescauseagreatdamagetotheaquatichabitatsandalsotothehumanhealth.Someofthetoxicpollutantsexistinginthewastewaterarebenzene,phenols,nitro-benzenes,chlorinatedbenzenes,phthalatesandchlo-rinatedphenols.Dyematerialsarecommonlyusedinplastics,textiles,paperandleatherindustries,andalargeamountofthesematerialsendsupinwaterasharmfulpollutant[9,10].Separationofthesedyesfromwastewaterisverycrucial,butchallenging.PPCPsarechemicalcontaminantsthatarepresentinwater,andarealsounsafeforlivingorganismsandhavetoberemoved[11].Thedisposalofheavymetal ions likechromium(Cr), lead(Pb),copper(Cu),arsenic (As), mercury (Hg), antimony (Sb), cadmium (Cd) and manganese(Mn)inprocessedwaterisalsohighlytoxictohumanbeingsaswellasecolog-icalsystems[12,13].2.1.2AdsorptionofHazardousMaterialsInthelastfewdecades,theporousmaterialshaveevolvedasgoodadsorbentsthat exhibit significant potential for the purification of air, gas and waterstreamsbyadsorptiveseparationofthecontaminants,andhenceagreatdealofresearchfocushasbeendevotedfortheexaminationandanalysisofthead-vancedporousmaterials[14].Forthedecontamination,adsorptionhasbeenregardedasthepreferabletechniqueduetoitslowcost,noorharmlesssec-ondaryproducts,designsimplicity,easeofoperationandefficientadsorbentregeneration.Thismethodisbasedontheporousadsorbentabilitytoselec-tively adsorb specificmaterials fromeither refinery streamsor from theat-mosphere. The adsorbent materials can be structurally tuned to generatesuitableporeshapeandsize,thus,providingefficientcontactwiththeadsorb-

MOFsandMetalOxidesforAdsorption 27

atematerials,thus,leadingtoeffectiveseparationofpollutantsduringthead-sorptionprocess.Dependingontheinteractionbetweentheporoussorbentsandtheadsorbates,theadsorbentscanbeclassifiedasphysicalandchemicaladsorbents [15]. With respect to physical adsorbents, the adsorbates aretrapped inside the adsorbent pores through weak van der Waals forces.Hence, the adsorbent material can be easily regenerated by physical treat-mentsorsimplesolventexchange.However,inthechemicaladsorbents,ab-sorptiontakesplacebythechemicalbondformationbetweentheadsorbentandtheadsorbate.Therefore,chemicaltreatmentsarerequiredfortheregen-erationof thespentadsorbents,whichmayalsooccasionally lead toperma-nentlossofadsorbentperformance.Thecapabilityoftheadsorptionprocessdepends on the selectivity for specific compounds, regenerability of adsor-bents,durabilityandadsorptionabilityoftheadsorbents.Fortheadsorptiveremoval of harmful compounds, different porous adsorbents like zeolites[16,17], activatedcarbons [18,19] ,mesoporousmaterials [20,21]andmetalorganic frameworks (MOFs) [22,23]havebeenstudied.For theeffectivead-sorption, specific pore geometry, porosity and adsorption sites are needed[24]. In addition to this, certain active species such as different functionalgroups(basicoracidic),metaloxides,metalions,polyoxometalatesandmetalsalts are generally included into the adsorbents for the selective removal ofharmfulcompoundsbyinteractionssuchashydrogenbonding,acid-baseandπ-πinteractions.Inthischapter,themainfocusistoelucidatetheroleofMOFsandmetalox-

ides for the adsorptive removal of hazardous gases like H2S, NH3, CO2, andotherhydrocarbonspresentintheatmosphereaswellasthedissolvedheavymetalsaltsortheorganicpollutantspresentinthepollutedwater.Beneficialcharacteristics of MOFs and metal oxides have been discussed that ensuretheireffectiveroleintheadsorptionofspecifictoxiccompounds.Theeffectoflinkers, openmetal sites, MOFmodification/functionalization, central metalionsandtheadsorbent/adsorbateinteractionsarealsoexplained.Further,theanalysisof adsorptionmechanisms suchas electrostatic interactions, hydro-gen bonding and acid-base interactions for the effective adsorption is alsopresented.Thus, thisreviewpresentsanup-to-datestudyof theoreticalpre-dictions,experimental resultsaswellasdevelopingconceptson theadsorp-tionabilityofdifferentMOFsandmetaloxides,withspecificemphasisontheremovalofharmfulgaseslikeCO2,H2S,NH3,andwatercontaminantsliketheheavymetalsandorganiccontaminants,amongothercontaminants. Inaddi-tiontothis,observationsintermsofbenefitsanddisadvantagesofemployingdifferent metal oxide frameworks as well as metal oxides have also been


pointed out, alongwith discussing in detail the thermodynamic and kineticanalysis.2.2MetalOrganicFrameworksMOFsconsistsoftwomajorcomponents,i.e.,anorganicmolecule(linker)anda metal ion or a group of metal ions. The linkers are generally di-, tri- ortetradendate ligands [25]. Examples of someMOFs are Cu-BTC,MOF-5 andCPO-27.TheporousMOFsareveryattractivebecauseoftheirversatileappli-cations like CO2 storage/adsorption [26], vapor adsorption [27], hydrogenstorage[28],chemicalseparation[29],drugdelivery[30]andsoon.Thesema-terialsarepreferredthanotherporousmaterialsduetotheirporefunctionali-ty,highandtunableporosity,porecompositionsandopenmetalsites.Fortheadsorption related applications, MOFs are promisingmaterials due to theircontrolledtuningoftheporesurface,whichcausestheselectiveadsorptionofcertainguestmoleculeswhichhavespecificfunctionalgroups.Thepropertiesof theMOFscanbe tuned forobtaining thedesiredperformance,byorderlyand intentionally tuning the functionalities and structures. Hence,many re-searchstudieshavebeencarriedoutontheadsorptionofdifferentliquidandgaseous components usingMOF, due to their beneficial features of pore ge-ometryandporosity[31].Dependingontheadsorbates,bothmodifiedaswellasvirginMOFscanbeusedfortheadsorptiveremovalofdifferenttoxicgasesandliquids.2.2.1AdsorptiveRemovalofHarmfulGasesUsing different MOFs, adsorption of various harmful gases/vapors like H2S[32,33],NH3[4],CO2[34,35],SO2[36],benzene[37],NO[38]havebeenstud-ied.Most of the aforementioned gases/vapors are either required to be re-moved from products or are ejected from different industries as waste by-products.Foracleanandsafeenvironment,efficientremovaloftheseharmfulgasesisveryimportant.Asmentionedearlier,MOFshaveadvantageslikehighporosityaswellaseasytunabilityofporeshapeandsize,frommesoporoustomicroporousscale,byalteringthenatureoforganicmoleculeandconnectivityof inorganic component [39].The characteristic examples include zirconiumbased MOFs namely UiO-68(Zr- terphenyldicarboxylate, Zr-TPDC), UiO-67(Zr-BPDC,Zr-biphenyldicarboxylate)andUiO-66 (Zr-benzenedicarboxylate,Zr-BDC) having high surface areas 4170, 3000 and 1187m2/g respectively[40].Thelargeenhancementinthesurfaceareaisobtainedbyvaryingtheor-


ganic linker.ThefollowingsectionsdescribethespecificuseofMOFsfortheremovalofvariousharmfulgases.CO2AdsorptionTheadsorptionaswellasdesorptionofCO2inMOFsareattributedtothevar-iationshappeningintheframeworkstructures,andhavealsobeendescribedusingthe“gateeffectmechanism”ora“breathingtypemechanism”.Inafur-ther study,Waltonetal. [41]added through thecomparisonofMonteCarlosimulationandexperimentaladsorptionofCO2forMOFIRMOF-1thatattrac-tiveelectrostaticinteractionsbetweentheCO2moleculesresultedintheunu-sualshapeoftheadsorptionisotherms(Figure2.1).

Figure2.1ComparisonofMonteCarlosimulations(GCMC)andexperimentaladsorptionisothermsforCO2inIRMOF-1.ReproducedfromReference41withpermissionfromAmericanChemicalSociety.For the application in flue streams, MOF adsorbents for the CO2 capture

shouldmeettherequirementssuchashighCO2adsorptionability,corrosionresistance, high thermal stability and good selectivity for CO2 over severalothercomponentspresentinfluegas.Itshouldalsobenotedthatmostofthe-seadsorbentsexhibitlittleadsorptioninthelow-pressureconditionsi.e.0.1–0.2bars,which is the realisticpressure rangeemployed for theCO2adsorp-tionfromfluegasstream[42].Further,areductionintheiradsorptioncapaci-ty is seen when it exposed to gaseous mixtures under dynamic conditions.


Hence,significanteffortsarerequiredfortheenhancementintheCO2adsorp-tioncapacityunderequilibriumconditions.Inthisstudy,themodificationsinMOFswasachievedinthreeaspectsnamely(1)metalions,(2)organiclinkersand(3)novelconsolidationofboth.AsshowninFigure2.2,thefunctionalizedMOFwassubjectedtocyclicgastreatments.Asmall0.75wt%changeintheweight of the sample was observed over repeated cycles, which confirmedthat the material was able to withstand cyclic exposure to the mixed gasstream.Theadsorptionperformancewasmaintainedoverthecyclesandthematerialcouldberegeneratedeffectively.For themetal ions, generating openmetal sites is an effectivemethod, as

thevacantcoordinationsitesatmetal ionsactasprimarysites for theguestmolecules [43]. Britt et al. [43] stated thatMOF loadedwith openMg sites,Mg-MOF-74(Mg2(2,5-dioxidoterephthalate))hadhighselectivity,easyregen-eration andexcellentperformance for theCO2 removal.Mg-MOF-74was ex-posed toagas streamconsistingof20%CO2 inCH4, and theadsorbentwas

Figure2.2GascyclingexperimentforthefunctionalizedMOFat30°C,with15%CO2inN2followedbyaflowofpureN2.ReproducedfromReference42withpermissionfromAmericanChemicalSociety. observed to captureCO2alone,withoutCH4.Theadsorbent retained89gofCO2/kgofmaterial prior tobreakthrough (2.23mmol.g-1).Dietzelet al. [44]studiedtheoutcomeofthemetalcenterontheCO2adsorptionabilityandse-lectivityusingaseriesof iso-structuralMOFsnamelyM-CPO-27s(M=Ni,Mn,Co,Mg,Zn).Itwasobservedthatathighpressure(50bar)and298K,theCO2adsorptionwas63wt%forMOFCPO-27(Mg)and51wt%fortheMOFCPO-


27(Ni).InanotherresearchbyCaskeyetal.[45],itwasobservedthatat1atm,theCPO-27(Co)exhibitedCO2adsorptionof30.6wt%,whereasCPO-27(Mg)showedanadsorptionof35.2wt%.TheCO2adsorptionabilityofCPO-27(Mg)was found to be superior as compared to the othermembers of the series,therebyrepresentingamajordevelopmentintheCO2adsorptionabilityoftheMOFs. In addition to this, for enhancing the performance, the unsaturatedmetalsitescouldbefunctionalizedusinggroupslikeamine.Coucketal. [46]provedtheefficientseparationofCH4andCO2gasesusingaminofunctional-izedMIL-53(Al).DuetothepresenceofquadrupolemomentinCO2,theseCO2molecules have a greater affinity for the NH2 groups. Breakthrough experi-ments confirmed that the CH4, which was weakly adsorbed, was able totransmitthroughtheMOFpackedcolumnduringtheadsorptionofCO2.Thus,atambientconditions,effectiveseparationofCO2andCH4couldbeachievedusingtheaforementionedMOFadsorbent.Fortheorganiclinkersmodification,Baeetal.[47]analyzedthefeasibility

ofutilizingMOFswhicharecarboranebasedorcoordinatedwithligandmix-ture.AhigherselectivityforCO2overCH4wasnoticedintheseMOFmaterials.Carboraneshaveseveralbeneficialmaterialproperties likethermalstability,rigidityandchemicalstability.InthepursuitofpreparingnovelMOFshavinghigh CO2 adsorption capacity as well as stability, the capability to generateMOFshavingdifferentorganiclinkersaswellasmetaljointsisconsideredtocontribute significant flexibility, alongwithparticular physical behavior andchemical functionalities. Recently, Yaghi et al. [48] prepared a new class ofMOFscalledzeoliticimidazoleframeworks(ZIFs),wheremetalatomslikeZnwerebondedwithnitrogenatomsbyditopicimidazolateorfunctionalizedImlinkstogeneratetheneutralframeworks.Thematerialshadgoodchemicalaswellasthermalstabilities(until500°C).InanotherstudybyPhanetal.[49],itwasstatedthatat0°C,1LofZIF-69hadtheabilitytostore82.6LofCO2.Fur-ther, ZIFs also exhibitedhigher selectivity thanotherMOF types for the ad-sorptionofCO2fromfluegases.ItwasfoundthatonlyCO2couldslipintotheZIFcages,whereasothergasmoleculescouldjustpassthroughthemwithoutanyobstruction.Huetal.[50,51]studiedtheeffectofadditionofalkalinemetalcationslike

Na+,Li+andK+intoapreviouslystudiedZr-MOFhavingexcellentCO2adsorp-tionproperties,byfacileneutralizationreactions.TheCO2adsorptioncapacityoftheMOFswasanalyzedemployingbreakthroughexperimentsandsorptionisotherms.Fromthebinarymixedgasbreakthroughexperiment, itwasseenthat out of the different Zr-MOFs, theMOF UiO-66 (Zr)-(COONa)2 exhibitedhighCO2/N2selectivityvalueof99.6.Thiswasconsideredtobehighestamong


all the previously obtained values under analogous testing conditions. TheMOFswerereportedtohavegoodCO2adsorptioncapacity,highwaterstabil-ityandexcellentCO2/N2selectivity.Further,theirpreparationcouldbescaledup very efficiently by utilizing commercially accessible reagents with batchreactorsaswellasenvironmentalfriendlysolventlikewater.Theresultsob-tainedinthestudyconfirmedthatthesynthesizedMOFsareveryefficientad-sorbents for the removalofCO2 from fluegas.Yangetal. [52]observeden-hancedCO2adsorptionwithadsorbentsattainedbysonicationormicrowaveheating.At298K, thesmallsizedandhomogenousparticlesofCPO-27(Mg),generatedbysonicationmethod,adsorbedalmost350mg.g−1ofCO2.Inaddi-tiontothis,thematerialsalsoexhibitedlargeisostericheatofadsorption.Ta-ble2.1alsodemonstratestheCO2adsorptioncapacitiesofsomeoftheMOFs.Table2.1ComparisonofCO2adsorptioncapacitiesofsomeoftheMOFadsorbentsSl.No.

Adsorbent

Temperature Pressure CO2adsorptioncapacity(wt%)

Reference

1 CPO-27(Mg) 298K 50bar 63wt% 1312 CPO-27(Ni) 298K 50bar 51wt% 1313 CPO-27(Co) Ambient

temperature1atm 30.6wt% 132

4 CPO-27(Mg) Ambienttemperature

1atm 35.2wt% 132

AdsorptionofSulfurContainingCompoundsTheuseoffossilresources,likepetroleum,naturalgasandcoal,generatessul-furcontainingspecieslikeSO2,H2S,mercaptan,thiopheneandthioether.The-se sulfur containing compounds are harmful to human health and the envi-ronmentandhencethedesulphurizationprocessesareneeded.Asmentionedearlier,MOFsarealsopromisingabsorbentsfortheremovalofsulfurcontain-ingcomounds.InastudybyAchmannetal.[53],itwasnotedthattheadsor-bentMOF-199 possessed high efficiency for the removal of sulfur from lowsulfurmodelfuelsandoil.TheMOFalsoexhibitedthemaximalsorptionratesas well as rapid sorption characteristics. The minor pores in the MOF-199providedbetter stabilization for Cu- adsorbed sulfur species by the vanderWaalsforce,andtherebycausedanexcellentsulfuradsorptionperformance.Inanotherstudy,Chavanetal.[54]analyzedtheH2Sadsorptionperformanceon the adsorbent Ni-MOF-74. The adsorbent exhibitedmaximal uptake and


higherH2S adsorption enthalpy,when compared to other adsorbents,whilepreservingitsoriginalporosityandstructure(Figure2.3).

Figure2.3H2O(left)andH2S(right)moleculesasarrangedintheCPO-27-Nichannels.ReproducedfromReference54withpermissionfromAmericanChemicalSociety.

InastudyreportedbydeVoordeetal.[55],DFTanalysisontheadsorption

behaviorofS-heterocyclic compoundsonMOF-74series (Mg,Cu,Ni,ZnandCo)wascarriedout.Itwasconfirmedthatthemetalionhadaremarkableef-fectontheMOFsadsorptionabilityandS-heterocyclicspeciesaffinity.Itwassuggestedthat theadsorbentNi-MOF-74offeredthebestdesulfurizationbe-havior. Hamon et al. [56] examined the efficiency of MIL-series e. g. MIL-100(Cr),MIL-53(Cr,FeandAl),MIL-101(Cr)andMIL-47fortheadsorptionofH2Sat303K.Thestudysuggestedthatthe–OHgroupspresentintheMIL-53adsorbentpromotedtheH2Sadsorption.InadifferentstudybyHamonetal.,[57] itwasreportedthat the–OHgroupsexisting intheMOFsperformedasproton donor, thereby increasing theH2S adsorption. The authors observedthatonH2Sadsorption,theMIL-47(V)structureremainedrigiduptoapres-sure of 1.8 MPa. On the other hand, theMIL-53(Cr) initially present in thelarge pore form (LP) switched to its narrow pore version (NP) at very lowpressure,andexhibitedasecondstructuraltransitionfromtheNPtotheLPathigher pressure. The authors observed that experimental and simulated ad-sorptionenthalpiesforH2Sdecreasedinthesequence:MIL-53(Cr)NP>MIL-47(V)>MIL-53(Cr)LP,alsodepictedinFigure2.4.


SeveralpreviousstudieshaveshowedthattheefficiencyofMOFscanbein-fluencedbythefactorssuchasmetalsions,substituentgroupsandcoordina-tivelyunsaturatedsites(CUS).Further,mostofthestudieswerebasedontheadsorptionof thiophene species from liquid fuel.Chenetal. [58]performedresearch on the desulfurization ability ofMOFs for the H2S,mercaptan andthioetherspecies.AsystematicDFTanalysison the influenceofMOFsmetalcentre(structuresaswellasmetalions)andorganicligandsontheadsorptionofH2S,CH3SCH3andCH3CH2SHwascarriedout.Itwasnotedthatthedesulfu-rizationabilityofMOF-74andMOF-199withopenmetalsite,wassuperiortothe other MOFs without coordinatively unsaturated sites. The observationconfirmedtheremarkableinfluenceofCUSonthesulfurcompoundsadsorp-tion. Here, IRMOF-3 containing amino groups exhibited the maximal sulfur

Figure2.4IllustrationsofthepreferentialarrangementsforH2Ssimulatedat303KinMIL-47(V)(a),MIL-53(Cr)NP(b)andMIL-53(Cr)LP(c)solids.ReproducedfromReference57withpermissionfromAmericanChemicalSociety.


capacity, succeeded by IRMOF-8. However, it was noted that the density ofCUSsitespresent in theZn-MOF-74 (6.16×10−3mol/g)wasgreater thanCu-MOF-199(4.96×10−3mol/g),andhencethesulfurremovalabilityofZn-MOF-74wasconfirmedtobelesserthanthatofCu-MOF-199.Fernandezetal.[59]studied theadsorptiveremovalof toxicacidgasesbyusing fluorinatedMOF(FMOF-2),whichwaspreparedusingzincnitratehexahydrateand2,2’-bis(4-carboxyphenyl) hexafluoropropane. TheMOFwas observed to be stable fortheH2SandSO2adsorption.WithFMOF-2,theestimatedweightcapacitiesforH2SandSO2were8.3%and14%respectively,at1barandroomtemperature.Datheetal.[36]studiedtheSO2adsorptionusingBa(CH3COO)2(orBa(NO3)2andBaCl2)-impregnatedCu-BTC.Theimpregnationacceleratedtheformationofsmallmicro-crystalsofbariumsaltsinCu-BTCpores.Also,thesectionalde-structionofthehoststructuretookplaceinBaCl2.Itwasalsonoticedthatatgreater temperature, theSO2adsorptionsurpassedthestoichiometricabilitybasedonBa2+ concentration.Accordingly, theexcessSO2adsorptionwasbe-causeofthechemicalbondingbetweentheSO2andmetalcations,therebyre-sultinginthegenerationofCu-sulfates.Hence,atlowertemperature,Cu-BTCperformedasanexcellenthostmaterialforobtainingverywelldispersedbar-ium salts. However, at higher temperature, the adsorbent Cu-BTC decom-posedandgenerateduniqueCu species thatperformedasSOx storage sites,thusproducingCu-sulfatesfinally.Thus,themainlimitationofthesebariumssalts impregnated Cu-BTC adsorbent is the irreversible SOx storage in theirstructure.AdsorptionofOtherHarmfulGasesFortheadsorptionofvarioustoxicgases, theopenmetalsitesofMOFshavebeenstatedrepeatedlyastheactivesites.Further,a largenumberofstudiesstatedthatthepropertiesofMOFscouldbecontrolledbytheloadingofactivemetals(Pd,CuorAg)oractivespecies(sulfatedzirconiaorpolyoxymetalates)orbygeneratingcomposites[60].Gloveretal.[61]demonstratedtheadsorp-tionofvarioustoxicgaseslikeSO2,NH3,octanevaporandCNClusingM-CPO-27(M=Zn,Ni,MgorCo)underhumidanddryconditions.Thebreakthroughresults confirmed that only under the dry conditions, the adsorbents withopenmetal siteswereable toadsorb theaforementioned toxicgases. Inhu-midconditions,theadsorptiveabilitydecreasedsignificantlyastheaggressiveadsorptionofwater starteddominating. In thecaseofNH3, the reduction intheadsorptioncapacitywasnotidentified,asitwasadsorbedsimilarlyunderbothhumidanddryconditions.Hence,itcouldbeconfirmedthatthepresence


ofwatervapourhadanegativeinfluenceontheadsorptionofdifferenttoxicgasesusingtheCPO-27typeadsorbents.PetitandBandosz[62]studieddifferentcompositesofMOFs(MIL-100(Fe),

MOF-5orCu-BTC)andgraphiticcompound(GO,graphiteorgraphiteoxide),underambientconditions,fortheadsorptionofNO2,H2SandNH3.Itwasob-servedthattheopenmetalsitesofMOFscoordinatedwithoxidegroupsofGO,whichcausedthegenerationofanewporespaceintheinterfacebetweentheMOFunitsandcarbon layers, to formcompositeshavingdefiniteproperties.WiththeGO/Cu-BTCcomposites,morethan50%(forH2S),12%(forNH3)and4% (for NO2) enhancement in the adsorption ability was observed. The in-creaseintheadsorptionabilityforthetoxicgasesseeninthecompositeswasattributed to the porosity generated in the interface where the dispersiveforcesweresubstantial.Brittetal.[63]studiedthepossiblesuitablenessofsixMOFs, consistingof2-amino terephthalate (IRMOF-3),diacetylene-1,4-bis(4-benzoicacid)(IRMOF-62),Zn4O(CO2)6grouplinkedbyterephthalate(MOF-5),benzene-1,3,5-tris(4- benzoate) (MOF-177), Cu-BTC and Zn2O2(CO2)2 chainslinkedby2,5-dihydroxyterephthalate(CPO-27),fortheadsorptionofdifferenthazardousgases/vaporsincludingNH3,chlorine,SO2,benzene,tetrahydrothi-ophene and ethylene oxide. The obtained results were correlatedwith BPLcarbon.ItwasseenthatdifferentfactorsliketheactiveadsorptionsitehavingspecificfunctionalgroupandopenmetalsitesofCu-BTCorCPO-27playedacritical role in deciding thedynamic adsorption ability of theseMOFs. Com-paredtotheotherMOFs,CPO-27exhibitedexcellentperformance,andthead-sorptionabilitywasalmost6timeshigherthantheBPLcarbon.Theimprovedadsorptionwasbecauseof the existenceof an immensely reactive5 coordi-nate Zn species, together with the reactive oxo-group present in CPO-27.However, inthecaseofallothergasesexceptCl2,theMOFCu-BTCexhibitedhigherefficiencythanBPLcarbon.ForNH3adsorptioninIRMOF-3,theexist-enceofNH2 in theMOFgreatly increased theadsorptionability,whencom-pared to MOF-5 or IRMOF-1. Prior to the breakthrough, the IRMOF-3 ad-sorbedNH3approximately71times,whencomparedtotheBPLcarbon,duetotheabilityofNH3toeasilyformthehydrogenbonds.CO, another toxic gas in the environment, has goodbinding capacitywith

themetal sites andMOFswithCUS caneasily adsorbCO fromgaseousmix-ture.KarraandWalton[64]conductedmolecularsimulationanalysisandob-servedthatat298K,Cu-BTCwasagoodadsorbentforCOoverN2andH2.Itwas proposed that the electrostatic interaction between the CO dipole andpartialchargeofCUSofCu-BTCcontrolledtheadsorptivebehavior.DifferentMOFmaterialscanalsobeusedsuccessfullytoadsorbNObyadsorptionpro-


cesses[65].ThegravimetricNOadsorptionontheopenmetalsitesofCu-BTCat196Kand1barwasexaminedbyXiaoetal.[65].Itwasobservedthatbyusing1gofCu-BTCalmost9mmolofNOwasadsorbed,whichwasremarka-blygreater thananyotherporoussolids for theNOadsorption (Figure2.5).Theadsorptionofbenzene (both liquidandvaporphase)wasalsoanalyzedby Jhung et al. [66] using MIL-101(Cr), chromium terephthalate adsorbent.16.7mmol.g−1ofbenzene(vaporphase)wasadsorbedbytheadsorbentMIL-101(Cr).Theobtainedvaluewasapproximately8.7,5.5andtwotimeshigherthan the H-ZSM-5, SBA-15 and a commercial activated carbon, respectively.DuetothehighporosityofMIL-101(Cr),itdisplayedhigherbenzeneadsorp-tion capacity than the pitch based activated carbon (12.4 mmol.g−1) with ahigher surface area of 2600–3600 m2 g−1. The potential utilization of MIL-101(Cr)inthevaporphaseadsorptionofVOCslikep-xyleneandethylacetatewasalsostudiedbyShietal.[67].

Figure2.5Adsorption(filledsymbols)anddesorption(opensymbols)isothermsat196K(squares)and298K(circles)fornitricoxideonHKUST-1.ReproducedfromReference65withpermissionfromAmericanChemicalSociety.2.2.2AdsorptiveEliminationofContaminantsfromWastewaterVariousMOFshavebeenemployedfortheadsorptionofdifferenthazardouscontaminants from water. The adsorption occurs through different mecha-nismsliketheelectrostaticinteractions,hydrogenbonding,acid-baseinterac-


tions, hydrophobic interactions and π-π interactions. One of the substantialbarrier for the application of several types ofMOFs is thewater instability(bothinvapourandliquidphase).ThisinstabilityofMOFsinwaterisacriticalproblemandmany research studies have been carried out to increase theirwater stability [68]. Computational aswell as experimental studies showedthatseveralMOFslikeMIL-101-V,MOF-5experienceddegradationby liganddisplacement, when thesematerials were exposed towater [69,70]. On theotherhand,MOFslikeUiO-66[71],ZIF-8[72]MIL-101-Cr[73],MIL-100(Fe,Al,Cr)[74,75]hadgoodwaterstabilityandweresuitablefortheadsorptionapplicationinwaterorwaterpurification.Severalmethodslikeincorporationofwaterrepellentfunctionalgroups[76],compositeformation[77]andfluor-ination[78]havebeenachievedtoimprovethewaterstability[79].Haqueetal.[80]reportedtheadsorptiveseparationofdyeusingMOFs.The

authorsutilized two typesof porousCrbasedMOFs, namelyMIL-53-Cr andMIL-101-Cr, fortheadsorptionofmethylorange(MO),ananionichazardousdye, from the aqueous solution. It was noticed that the performance of theMOFswas superior to theactivated carbon.Theadsorptionkinetic constantandtheadsorptioncapacityofMIL-101-Crwerehigher thanthatofMIL-53-Cr,therebyportrayingthesignificanceofporesizeaswellasporosityfortheadsorption.On the other hand, a better adsorptionwas seen after the func-tionalization of MIL-101-Cr with ethylenediamine (ED) and protonated ED(PED).Generally,MOexists in thesulfate form,andhence itexhibitsgreaterelectrostaticinteractionwithanadsorbenthavingpositivecharge.Itwasob-servedthatthepositivechargedispersionenhancedintheorderMIL-101-Cr<ED-MIL-101-Cr<PED-MIL-101-Cr.Ontheotherhand,thepositivechargeonPED-MIL-101-CrreducedwithenhancingthesolutionpH,duetothedeproto-nationof theprotonatedadsorbent. Inanother study,Haqueetal. [81]ana-lyzed theadsorptionofmethyleneblue(MB-cationic)andMO fromaqueoussolution,usingNH2-MIL-101-Al,anNH2-functionalisedMOF.Theultimatead-sorptionabilityofNH2-MIL-101-Al,fortheremovalofMBwasobservedtobe762±12mg/g at 30 °C,whichwashigher thanmost other porousmaterialsandMOFs.However,byusingnon-aminefunctionalizedframework(MIL-101-Al),loweradsorptioncapacitywasnoticed(195mg/g).Thus,itsuggestedthattheelectrostatic interactionsbetweenthecationicMBandtheaminogroupsof theMOF favored the adsorption.Haque et al. [82] also reported that theiron terephtahalateMOF (MOF-235) could adsorbboth the cationic (MB) aswellasanionic(MO)dyesfromthewastewater.Here,theadsorptionkineticconstant and adsorption capacity of MOF-235 were higher as compared tothat of the activated carbon. The higher adsorption of MB or MO was ex-


plainedonthebasisoftheelectrostatic interactionsbetweentheadsorbentsanddyes.MBexisted inpositive formandMOexisted innegative form,andhence electrostatic interactions were observed with the adsorbents havingnegative (chargebalancinganion)chargesaswellaspositive frameworkre-spectively.After theequilibrationwithMOF-235,MBandMOadsorptionsatvaryingpHvaluesweredetermined.ByincreasingthepHoftheMOsolution,it was observed that the quantity of adsorbed MO reduced, as the positivecharge density reduced with increasing pH. However, the quantity of ad-sorbedMBincreasedwithpHincrease,asthenegativechargeconcentrationincreasedwithpHincrease.Inanotherstudy,Lengetal.[83]analyzedthead-sorptionbehavior of uranineusingMIL-101-Cr andobserved the aforemen-tioned type of interactions. The adsorptive removal ability of uranine usingMIL-101-Crwasfoundtobe126.9mg/g,whichwasmuchhigherthantheac-tivated carbon (17.5 mg/g). This was due to the electrostatic interactions,whichwereexplainedbythezetapotentialanalysis.ThezetapotentialofMIL-101-Cr was observed to be around 21.1 mV, and zeta potential reduced to12.0mVaftertheuranineinteraction.Chenetal.[84]studiedthexylenolor-ange(XO)adsorptiononMIL-101-Crandobservedsignificantadsorptionabil-ityofthematerial.ItwasobservedthattheenhancementinthepHofthesolu-tion decreased the amount of adsorbed XO. No adsorption was observed,whenthepHofthesolutionattained12.Thus,aftertheanalysisofzetapoten-tial, the authors concluded that the interactions between the adsorbent anddyeplayedakeyroleintheadsorptionprocess.Hydrogenbondingalsohasamajorroleinadsorptionofharmfulmaterials

using MOFs. Liu et al. [85] examined the adsorption of phenol and para-nitrophenol (PNP) from aqueous solution, using different MOFs (NH2-MIL-101-AlandMIL-100-Fe,Cr).Duringthephenoladsorption,limitedaswellassimilaradsorptionabilitywasexhibitedbyall the threeMOFs.On theotherhand, NH2-MIL-101-Al exhibited very high adsorption of PNP, which wasabout1.9 timesand4.3 timesgreater than theMIL-100-Cr andMIL-100-Fe,respectively. The significant adsorption ability of NH2-MIL-101-Al was be-causeofthehydrogenbondingbetweenthePNPandNH2groupsinNH2-MIL-101-Al,as illustratedinFigure2.6.Xieetal. [86]examinednitrobenzenead-sorptionfromwastewaterusingtwotypesofaluminumbasedMOFs,namelyMIL-68-Al(1130±10mg/g)andCAU-1(970±10mg/g).Theadsorptionval-uesweregreaterthantheexperimentalvaluesreportedforotherporousma-terials.ThetwoMOFshadµ-OHgroupsinAl-O-Alunits,whichplayedacrucialroleinattaininghigheradsorptionbythegenerationofhydrogenbondingbe-tweenthenitrogenatominnitrobenzeneandµ-OHgroupsofMOFs.


Figure2.6MechanismofPNPadsorptiononNH2-MIL-101-Albyhydrogenbonding.ReproducedfromReference85withpermissionfromAmericanChemicalSociety.Khan et al. [87] studied the adsorption of phthalic acid fromwater using

ZIF-8 and otherMOFs likeNH2-UiO-66,MIL-53-Cr, UiO-66,NH2-MIL-100-Cr,MIL-100-CrandMIL-100-Fe.WhencomparedtootherMOFs,theZIF-8exhib-itedhigheradsorptionathighpH.Thismightbeduetotheelectrostaticinter-actionsbetweentheZIF-8(positivechargedsurface)andPA2-aswellasH-PA-ions.Further,itwasalsoobserevdthattheaminefunctionalizedMOFsexhib-ited higher adsorption of phthalic acid at lower pH, than theMOFswithoutNH2group.AtlowpH,thephthalicaciddidnotdeprotonate,andtheacid-baseinteraction between the basic sites and phthalic acid exhibited dominancy.Tongetal.[88]studiedtheeffectofframeworkmetalionsintheadsorptionofdyes usingMIL-100-Fe/Cr. The adsorbentsMIL-100-Cr andMIL-100-Fe hadidenticalporevolumes(0.75cm3/gand0.76cm3/g)aswellassurfaceareas(1760m2/gand1770m2/g).Ontheotherhand, itwasnoticedthatboththeMOFs exhibited different behavior for the adsorption of MB and MO fromwastewater. For the adsorption ofMO, the absorbentMIL-100-Fe displayedexcellent adsorption (1045.2mg/g), when compared to the adsorbentMIL-100-Cr (211.8mg/g).Due to the greater binding energyofwatermoleculeswiththemetalsitesofMIL-100-Cr,theaggressiveadsorptionofwaterandMOwas more outstanding at the MIL-100-Cr surface. The adsorption of watermolecules occurred at the apertures of hexagonal and pentagonalwindows,whichlimitedtheaccessibilityofMIL-100-CrcagesforMOtoahigherdegree.SizeselectiveadsorptionisalsonoticedduringadsorptionusingMOFs.Wangetal.[89]analyzedthesizeselectiveadsorptionofdyeshavingdifferentsizes,where neutralMOFswere used for the adsorption. TheMOFs had face cen-teredcubictopologieswith3-dimensionalframeworks,andwereusedforthe


adsorptionofthreedifferentdyes,i.e,MB,MOandrhodamine(RB).Itwasno-ticedthatMBandMOwereadsorbed,however,RBwasnotadsorbedduetoits largersizethatblockedthetransmissionthroughthesmallerwindowsoftheMOFs.Presenceofuraniuminwaterisoneofthemostseriousmeansofhazardto

humanhealth[90].Adsorptionisconsideredasthebestmethodfortheura-niumremovalfromwaterduetoitsdesignflexibility,lowcostandtoxicpollu-tant insensitivity. Feng et al. [91] examined uranium adsorption capacity oftheMOFHKUST-1fromaqueoussolution.Themostadvantageousadsorptionconditionswere fixed alongwith the adsorbent dosage (0.03g), solution pH(at6)aswellastheagitationtime(2h). Itwasnoticedthatwhentheinitialconcentration of uraniumwas less than 200mg/g, the temperature had novisible effect on the adsorption. However, when the initial concentration ofuranium exceeded 200mg/g, a greater temperature favored the adsorptionprocess.At318K, theadsorbentHKUST-1displayedanexcellentadsorptioncapacity,atinitialuraniumconcentrationof800mg/LandsolutionpHvalueof6.Further,theendothermicandimpulsivenatureoftheadsorptionprocesswereindicatedbythethermodynamicsofHKUST-1/uraniumsystem.2.3MetalOxides

Transitionalmetaloxidenanoparticleshavebeenadequately researchedbe-causeoftheattractivebehaviorasadvancednanomaterialsinenvironmentalaswellasenergyareasduetotheirexcellentadsorptionperformance[92,93].Asmentionedearlier,harmfulgasesposeseriouschallengestotheprocessin-dustriesaswellasenvironment.Wastewater fromfoodprocessing,pharma-ceuticalandtextileindustrieshasconstantlybeenacrucialenvironmentalis-sue,asthecontaminantshaveadverseeffectsonaquaticlifeaswellashumanhealth. Several dyes as organic contaminants are stable and difficult to de-grade in conventionalwastewater treatmentmethods.Thus, establishing ef-fectiveandbeneficialadsorptionmethodsareneededforremovingtheorgan-iccontaminantspresentinthewastewater[94].Metaloxidebasedadsorbentsconfirmedtobeanefficientsolutionfortheremovalofawidevarietyofcon-taminants.Theseoxidesareavailableinvariousshapesandsizes,intheformof micro-particles, nanoparticles, nanocomposites and granules, therebydemonstrating distinct characteristics which are critical to their sorptionproperties.Severalresearchstudieshavebeencarriedoutduringthelasttwodecadesfordevelopingvariousoptimummethodsofpreparationofmetalox-ideadsorbents.Outof thedifferentmethods, thermaldecompositionandre-


duction, sol–gel processes and hydrothermal preparation [95] are themostpromisingones.2.3.1AdsorptionofCO2Inaddition to the removalofCO2 fromnaturalgas, the removalofCO2 fromthelargeCO2emissionsourceslikethermalpowerplantshasalsoasignificantroleforpreventingtheglobalwarming[96].FortheefficientremovalofCO2usingsolidadsorbents,higherselectivityofCO2againstothergaseslikenitro-genisveryimportant.ChemicaladsorptionofCO2isconsideredasoneofthemethodstoenhancetheselectivityoftheadsorbents.Themetaloxidesdisplayimproved selectivity due to the chemical adsorption between the CO2mole-culesandtheirsurfaces.ChemicaladsorptionalsoallowsthemetaloxidesforCO2 adsorption in the existenceofwater vapor.H2Oundergoesphysical ad-sorptionandhenceithaslimitedeffectontheCO2,whichisadsorbedchemi-cally.Thisbehaviorisdifferentfromzeolites,whicharenotedtohavehigherlossofCO2inthepresenceofwatervapor.TogetherwiththehighselectivityforCO2,thehighsurfaceareaoftheadsorbentisalsoveryimportantfortheefficient adsorption. The advancements in nanostructure control have facili-tatedthepreparationofoxideswithhighersurfaceareabygeneratingnano-particlesandporousstructures.Theutilizationofthesemethodsallowsadd-edenhancementintheCO2adsorptionabilityofthemetaloxides.Caietal.[97]exploredthesynthesisofmesoporousalumina(MA)andMA-

supported metal oxides. Aluminum isopropoxide, aluminum chloride, andaluminumnitratenonahydratewereusedasaluminumprecursors,whereasnickel,magnesium, iron, chromium,copper, cerium, lanthanum,yttrium,cal-cium,tinchlorides,ornitratesweremetalprecursors.Theauthorsobservedthat the use of aluminumnitrate nonahydrate for the synthesis led to largemesoporeswithsizerangingfrom∼7nmto16nm,improvedorderingoftheoxidesaswellas improvedadsorptionaffinitytowardCO2.Figure2.7showsthe temperature-programmed desorption (TPD) profiles of the <A and MAsupportedmetaloxidematerialsforCO2.Theprofilesexhibitedabroadpeak,whichindicatedthepresenceofawiderangeofbasicsitesonthesurfaceofthesematerials.Inanotherstudy,Leonetal.[98]studiedmagnesium-aluminumdoubleox-

ides derived from the thermal treatment of layered hydroxides for CO2 ad-sorption.Theauthorsanalyzed theeffectsof various factors, suchas the in-corporatedcation(KorNa),modeofadditionofprecursors, sonication,andcalcinationtemperature,andrelatedthesewiththeadsorptioncapacitybythe


Figure2.7CO2-TPDprofilesfor(a)MAsand(b)MAsupported-metaloxides.ReproducedfromReference97withpermissionfromAmericanChemicalSociety.useofthermogravimetryandcalorimetry.AsobservedinFigure2.8,theequi-libriumadsorption isothermsat6.7kPaand323Kof themixedoxides, rec-orded using microcalorimetry, exhibited the presence of combined chemi-sorption and physisorption behavior. It was observed that Langmuir trans-formationfitofthecurveswasachievedatlowcoverages,however,didnotfitthedataathigherextentofcoverage,thus,indicatingmultilayeradsorption.

Figure2.8CO2adsorptionisothermsat323Kand6.7kPa,measuredbymicrocalorimetryfor(⧫)1K723F,(⎕)2K723F,(∎)1KUS723F,and(∘)2KUS723F.ReproducedfromReference98withpermissionfromAmericanChemicalSociety.


Yoshikawa et al. [99] also studied the CO2 adsorption behavior of singlemetaloxides.CeO2,Al2O3, SiO2 andZrO2weregenerated, and the chemicallyadsorbedCO2amountwasmeasuredusingCO2pulse injection.Fromthead-sorptionanalysis, itwasobserved that theCeO2basedadsorbents exhibitedthe highest CO2 adsorption capacity. The temperature programmed desorp-tioninvestigationofCO2confirmedthattheCO2,adsorbedontheCeO2,gotde-sorbedmostlyat temperatures from323Kto473K,withthepeakat393K.Further, a very small amount of desorption remained till the temperaturereached723K.ForanalyzingtheCO2adsorptionaswellasdesorptionmech-anisms,theamountofCO2adsorbedonCeO2andthetemperaturedependencewereexaminedwiththehelpofFTIRspectroscopy.TheFTIRanalysisresultsconfirmedthatthehydrogencarbonateandbidentatecarbonatespecieswereessentiallyformedontheCeO2adsorbentsurface.Thelowestdecompositiontemperature was exhibited by the hydrogen carbonate species, which re-vealed that this species could be utilized to reduce the energy consumptionforthecaptureofCO2.Itwasalsonotedthatforincreasingthehydrogencar-bonate,treatmentwithwatervaporprovedtobeefficient.Astheexhaustgas-es fromthe thermalpowerplantsconsistofCO2andwatervapor, ithas theabilitytohelpthegenerationofhydrogencarbonatespecies.Hence,CeO2canbe considered as an efficient adsorbent for the CO2 adsorption in thermalpowerplants.Wangetal. [100]alsostudiedhigh-temperatureadsorptionofCO2onmixedoxides,whichwerederivedfromhydrotalcite-likecompounds.Theoxideswerecharacterizedwithvariousphysicochemicaltechniquessuchas x-ray diffraction analysis, Fourier transform infrared spectroscopy, ther-mogravimetric analysis andBET. Itwas observed that the generated oxideswereofeitherpericlaseorspinelphase,withaninterparticleporediameterof9.6-15.4nm.TheoxideswereobservedtoexhibithighCO2adsorptioncapabil-ityat350°C.Forinstance,mixedoxideCaCoAlOwasobservedtoadsorb1.39mmol/gofCO2(6.12wt%)fromagasmixture(8%CO2inN2)at350°Cand1atm.Therangeofadsorption forothermixedoxideswas0.87–1.28mmol/g(3.83–5.63wt%)ofCO2.Figure2.9alsodemonstratestheCO2adsorptiondatain the fixed bed reaction, in comparison with the simulated breakthroughcurves.In other studies for CO2 adsorption, Zhao et al. [101] synthesized MgO

nano/micro-particleswithvariousmorphologiesaswellasporousstructuresand studied theCO2 adsorptionabilityof theseporousmaterials. Songet al.[102]analyzed theCO2adsorptionkineticsofMgOadsorbentundervariousCO2partialpressuresandadsorptiontemperatures.TheadsorptionbehaviorsofMgOweresuccessfullypredictedbythepseudo-secondordermodels.The


Figure2.9CO2sorptiondatainthefixed-bedreactorandsimulatedbreakthroughcurves.“M-”representspredicatedcurves.ReproducedfromReference100withpermissionfromAmericanChemicalSociety.material exhibited twostageprocessof adsorption,havinga fast initial stepsucceededbyasluggishsecondstep.Thefirststepwasduetothediffusionoffilm from the bulk gas phase to theMgO exterior,whereas the second slowstepwasattributedtothestronginter-particlediffusionresistanceduetothelargenumberofnarrowpores,thus,reducingthediffusionrateofCO2.Fromtheporesizeandtheporeareadistribution,itwasnotedthatthesurfaceareaofthegeneratedMgOprimarilylocatedinsmallporesunder3nm,wheretheCO2 molecules underwent strong diffusion resistance. The study confirmedthat theslowdiffusionofCO2 in thenarrowporesreducedtheMgOadsorp-tionkineticsinspiteofthefactthatnumerousporescontributedtolargersur-face area. Thus, it was revealed that an outstanding chemical adsorbentshould have greater surface area for chemisorption aswell as suitable poresizedistribution forpromoting thediffusionofCO2 in theadsorbentporousstructure.Kumaretal. [103]analyzedtheCO2adsorptionabilityofmesopo-rousMgO,whichactedasapromisingpre-combustionCO2adsorbent. Itwasobserved that at 350 °C temperature and CO2 pressure of 10 bar, almost96.96%ofMgOchangedtoMgCO3.InanotherstudybyVuetal.[104],meso-porousMgOadsorbentwassynthesizedusingaerogeltechniqueandanalysisoftheCO2adsorptionbehavioratintermediatetemperaturerangeof250–400°Cwascarriedout.At325°Cand120min,theMgOsampleexhibitedhigher


adsorptioncapacityof13.9wt%.Thus, itwasconfirmedthatMgOexhibitedhigherCO2adsorptionabilityaftermodificationandhadthepotentialofbeinganexcellentadsorbentforCO2removal.Yu-Dongetal.[105]examinedtheCO2adsorptionefficiencyofMgOadsor-

bentinthepresenceofwatervaporinafixedbed.Initially,theanalysisofCO2adsorptioncapacityofMgOadsorbentunderdryconditionwascarriedout.At100mL/minflowrate,ittookalmost14minfortheCO2concentrationoftheoutlet to reach 14.5%. A decrease in the time to reach the equilibriumwasseen with the increased flow rate and it took just 4 min for that of 300mL/mintoobtainequilibrium.Itwasfoundthatat50°C,withtheincreaseinrelativehumidityfrom30%to70%,theCO2adsorptionabilityofMgOadsor-bentenhancedfrom0.82mol/kgto3.42mol/kg.Ataconcentrationofwatervaporof8.547%v/vandatanadsorption temperatureof75°C, theCO2ad-sorptioncapacity reachedamaximumvalueof3.54mol/kg. Itwas reportedthattheCO2adsorptionefficiencyofMgOwasgreatlydependentonthepres-ence ofwater vapor.However, the time to obtain equilibrium increasedbe-causeofthecontrolleddiffusionofCO2arisingduetothecondensationofwa-tervapor.Theresultsobtainedfromthecycleexperimentsconfirmedthatthecondensedwater increasedtheaggregationofMgOparticles.Further, itwasalsonotedthatathighertemperature, i.e.,100°C,nowatervaporcondensa-tion tookplaceand theporous structure remained stableduring the cyclingadsorption.Eliminationoftheporousstructuredegradationandenhancementin theCO2 adsorption capacitywas observed at adsorption temperaturebe-tween100°Cand110°C.ThestudyconfirmedthatMgOadsorbentcouldbedevelopedasapromisingmaterialforCO2adsorptionfromwetfluegas.2.3.2AdsorptionofH2SAsmentionedearlier,H2Sisatoxic,corrosivegasandisoneofthecommonlyseen contaminants present in several industrially essential feedstocks likecoalgas,naturalgas,liquefiedpetroleumgas,jetfuelandnaphtha.H2Sisalsoadominantsourceofacidrain,astheoxidationofH2Scausestheformationofsulfur oxide. Further, it can also poison several industrial catalysts like thesupportednickel catalyst,which is employed in hydrocarbon steam reform-ing.Hence,theseparationofH2Sfromtheseliquidorgaseousstreamsisveryessential. H2S adsorption using differentmetal oxides or theirmixtures hasbeenexploredingreatdetailstoovercomethesechallenges.MetaloxidessuchasironoxidesarewidelyusedforH2Sadsorption.These

oxides separate H2S by forming iron sulfideswhich are insoluble in nature.


Thedifferentchemicalreactionsoccurringinthisprocessareasdescribedasfollows:

Fe2O3+3H2SàFe2S3+3H2OFe2S3+3/2O2àFe2O3+3SFor the adsorption process, iron oxide is commonly employed in a form

known as “iron sponge”. The iron sponge consists of iron oxide (Fe2O3 andFe3O4)impregnatedwoodchips,andcanbeusedinbatchaswellascontinu-ous systems. After the saturation, the sponge can be regenerated.However,after each regeneration cycle, the activity generally gets decreased by one-third[106].Incontinuoussystemoperation,theairisprogressivelyaddedtothegaseousstream,andtheregenerationofironspongeoccurssimultaneous-ly. The H2S removal rate is generally very high i.e., 2500 mg H2S/g Fe2O3.However, the iron sponge has some disadvantages like high operating cost,chemicallyintensiveprocessandproductionofcontinuouswastestreamthatmightbeeitherdisposedofasdangerouswasteor regeneratedexpensively.SomecommerciallygeneratedironoxidesystemslikeSulfatreat410HPRareavailable,which canproducenon-hazardouswaste.The adsorption capacityof Sulfatreat 410 HPR was reported to be 150 mg H2S/g adsorbent [106].Carnesetal.[107]reportedthatthemetaloxidereactivitiesdependonintrin-sic crystallite reactivity, crystallite size and the surface area. Further, itwasalso seen thatwhencompared to themicro-crystalline structures, thenano-crystallinestructuresexhibitedenhancedreactivitywithH2S.Thisisbecauseofthehighersurfaceareathatpromotesadsorption.Further,thepresenceofFe2O3enhancedthereactionduetotheformationofironsulfideswhichactedas the catalyst. Rodriguez andMaiti [108] reported that theH2S adsorptionabilityofametaloxidedependedontheelectronicbandgapenergy.MoreH2Sadsorptiontookplace,whenelectronicbandgapenergywasless.Thisisduetothefactthattheelectronicbadgapisnegativelyrelatedtooxidechemicalreactivity. However, the reactivity depends on the mixing of oxide’s bandswithH2Sorbitals.Inthecaseofbettermixing,theoxidesreactwellwithsulfurcontainingmolecules,thusformingmetalsulfides.ThiscausesthedissociationofH2Smoleculesandsulfur immobilizationinmetalsulfides. Inspiteof this,theapplicationofmetaloxidesforH2Sremovalhassomedrawbackslikehighcost, low selectivity, low separation effectiveness and lesser sorp-tion/desorptionrate.Athighertemperatures(200°C-400°C),zincoxide(ZnO)basedmaterials

areemployedforremovingH2Straces.Thisisbecause,ZnOhavebetterselec-


tivityforsulfidesthantheironoxides[109].Davidsonetal.[109]conductedresearchontheH2SremovalcapacityofZnO,anditwasobservedthattheZnOsurfacereactedwithH2Stoformazincsulfideinsolublelayer,thus,separatingtheH2Sfromthegasstream.Almost40%ofavailableH2Swasconvertedandthe reaction depicted in the equation below led to the H2S removal.ZnO+H2SàZnS+H2OManycommercialproductshavebeendevelopedemployingZnO,andtheul-timate sulfur loading on these oxides materials has been observed in therange 300-400mg S/g sorbent.Majority of research studies carried out tilldate for the advancement of solid H2S adsorbents focused on thematerialswhich are acceptable for higher temperatures (> 300 °C). Ca- and Fe-containingmaterials are among themost researched solidmaterials forH2Sremovalfromcoalgas.Thisisbecauseoftheirlowcostandhigherreactivity[110].However,duetothepresenceoflargeamountofH2andlargeCO/CO2ratio in thecoalgas, thesematerialshavesomedisadvantages.Forexample,theFe3O4isreducedtoFeOandFe,however,theFe3Cformedduringthepro-cessdecreasesthesulfuruptakeofthesolid[111]. Lewetal. [112]alsoob-served that ZnOwas superior to iron oxide due to its agreeable sulfidationthermodynamics. However, the sulfidation kinetics of ZnOwas slower thanironoxide.ThemajordisadvantageofusingZnObased solid adsorbents fortheapplicationofhotgascleaningisthedamagewhichoccursbecauseofthereductionofZnO toelementalZnwhich isvolatile.Thus, anumberofmetaloxideshave thepotential forefficientuseasgasadsorbents,however,manychallengesarerequiredtoberesolved.Several mixed metal oxides like Cu-Fe-O, Zn-Fe-V-O, Zn-V-O, Zn-Fe-Ti-O,

Zn-Ti-O, Cu-Mo-O and Cu-Mn-O have been examined for theH2S adsorptionapplications [113-116]. Itwasobserved that theregenerationabilityaswellasreactivitygotenhancedwhenthesematerialsweredepositedonanappro-priate support. Themixedmetal oxide ZnFe2O4 also exhibited goodH2S ad-sorptionabilityathighertemperaturerangeof500°C–700°C[117].Forhightemperaturefuelgasdesulfurization,Kobayshietal.[118]examinedthezincferritesilicondioxidecomposites.Zincferritewasobservedtoundergosulfi-dation in the reducing environment to form iron sulfides, zinc blende andwurtziteinthepresenceof500ppmofH2S.Inanotherstudy,Huangetal.[119]studiedtheadsorptionandreactionof

H2S on TiO2 rutile (110) and anatase (101) surfaces using periodic densityfunctionaltheory(DFT).TheauthorsobservedthatH2S,HS,S,andHprefer-


entiallyadsorbedattheTi5c,O2c,(Ti5c)2,andO2csites,respectively,ontheru-tilesurface,whereasthesesiteswereTi5c,(Ti5c)2,(-O2c)(-Ti5c),andO2c,respec-tively, on the anatase surface. Figure2.10 also shows the adsorbedH2S andthefragmentsHS,SandHontheTiO2rutilesurface.Novochinskiietal.[120]

Figure2.10GeometriesofadsorbedH2Sanditsfragments,HS,S,andH,ontheTiO2rutile(110)surface.ReproducedfromReference119withpermissionfromAmericanChemicalSociety.alsoreporteda functionalZnObasedadsorbent for low-temperatureH2Sre-movalfromsteam-containinggasmixtures[120].TheauthorsobservedverylowH2Soutletconcentrations(20partsperbillionbyvolume)overthemodi-fiedZnOadsorbent.Theauthorsobserved the sulfur-trapcapacity (which istheamountofH2Strappedbeforebreakthrough)tobedependentonfactorssuchasspacevelocity, temperature,steamconcentration,CO2concentration,and particle size. Figure 2.11 also demonstrates the performance of a ZnO-based trap at different concentrations ofwater vapor andH2S. The authorsobservedhigher trap capacitywhen theH2S inlet concentrationwashigher.Thetrapcapacitywasalsoobservedtodecreaseasafunctionoftemperature.PresenceofsteamwasnoticedtoinhibitthecaptureofH2Sbytheadsorbent


duetotheshiftingoftheequilibriumofthereaction.Furthermore,increasingthe CO2 concentration in the feed up to 12 vol%was also observed to de-creasesthecapacityofZnOforH2Sadsorption.

Figure2.11PerformanceofaZnO-basedtrapatdifferentconcentrationsofwatervaporandH2S.ReproducedfromReference120withpermissionfromAmericanChemicalSociety.ForlowtemperatureH2Sremovalapplications,severalotherstudieshave

alsobeenreported[107,109,121,122].CarnesandKlabunde[107]examinedthe nano-crystallinemetal oxides which were generated using sol gel tech-nique.Itwasconfirmedthatatlessertemperaturerange(25°C-100°C),theactivityorderwasobservedtobeZnO>CaO>Al2O3>>MgO.Davidsonetal.[109]examinedtheadsorptionabilityofH2Susingdopedandhighsurfacear-eaZnOatlowertemperaturesof25°C-45°C,andobservedabout40%con-versionofH2S.Bairdetal. [123]examinedtheZnOdopedwithdifferentox-idesofFe,CoandCu,mixedCo–Znoxides,Co-Zn-Aloxides,Co–Feoxides togenerateH2Sadsorbentsatlowertemperature.Outofthemixedmetaloxides,Co-Zn oxide was reported to have the best H2S adsorption ability at roomtemperature.Xueetal. [124]generatedaseriesofpuremetaloxidesaswellasmixedmetaloxidesfromdifferenthydrousoxidesandhydroxy-carbonateprecursors.ItwasobservedthatadsorbentslikeCuO,Zn/Co,Zn/Ti/Zr,Zn/Aland Zn/Mn mixed oxides exhibited good H2S adsorption capacity at roomtemperature. The benefits of using hydroxycarbonate precursors included


gooddispersionandasynergyofmetalcomponentswhichwereexpectedtobecontinuedinthemixedmetaloxidesucceedingcalcination[125].Thehighdegreeofdispersionofmetal oxides is oneof themost attractive featureofmetal oxide based adsorbents for the low temperature performance. Somestudieshavealso focusedon the advancementof themixedmetal oxide ad-sorbents like Cu-Zn and Cu-Zn-Al for the H2S adsorption at lower tempera-tures.TheseCubasedmixedmetaloxidesareconsidered toeffectiveadsor-bentsforlowtemperatureapplicationsbecauseoftheirlowcostthantheZn–Comixedmetal-oxideadsorbents.However,theCubasedmixedoxides,whicharegenerallyreducedbyhydrogenbeforebeingusedasadsorbents,werealsoanalyzedfortheH2Sadsorptionandsomeotherorganicsulfurcompoundsformiddleorhigher temperaturesalso [126,127]. Jiangetal. [128]generatedaseries of Cu-Zn hydroxy-carbonate precursors by co-precipitation method,withchangingCu/Znmolarratios. Inadditiontothis, twoseriesofCu-Zn-Alhydroxy-carbonateprecursorswerepreparedwithvaryingmetalmolarratio,by co-precipitation aswell asmulti-precipitationmethods. The high surfaceareaCu-Zn-Almetaloxidesdisplayedremarkablesulfurremovalcapacitiesatthetemperaturerange25°C-100°C.ItwasobservedthatforH2Sadsorptionatlowtemperature,theCurichadsorbentsweremoresuitedthantheZnrichadsorbents.Theprobablereasonforthisbehaviorcouldbeduetothefastsul-fidation rate of CuO thanZnObecause of the lesser rearrangement of anionlattice.Polychronopoulouetal.[129]alsostudiedtheefficiencyofmixedmetalox-

ides Fe-Mn-Zn-Ti-O of differing composition, generated using sol gel tech-nique,fortheremovalofH2Sfromagaseousmixtureconsistingof7.5vol%CO2,1-3vol%H2O,25vol%H2and0.06vol%H2S,analyzedattemperaturerange25°C-100°C.ThechemicalinteractionbetweenthemixedmetaloxideandH2Swasstudiedand itwasobservedthat themolarratioofFe/Mnem-ployedinthepreparationofaforementionedmetaloxidesmostlydeterminedthe sizeandmorphologyof thegeneratedsolidparticles.Comparisonof thetemperatureeffectontheH2Sadsorptionforthesolgelgenerated5Fe-15Mn-40Zn-40Ti-O solid and the commercialnickelbased solidwasmade in thepresenceof1vol%and3vol%H2Ointhe feedstream. Inthe1vol%H2Osystem,thecommercialsolidappearedtobebetterthanthesolgetgeneratedmixedmetaloxideat25 °C -100 °C temperature range.For thecommercialsolid, the H2S adsorption enhanced by 141% by the increase in adsorptiontemperaturefrom25°C-100°C.Thepresenceof1-3vol%H2Ointhe7.5%CO2/25% H2/0.06% H2S/He gas mixture remarkably increased the H2S ad-sorptionat25°Cfor5Fe-15Mn-40Zn-40Ti-Osolid.


Inarecentstudy,in-situformedgraphene/ZnOnanostructuredcompositeswere employed for low temperaturehydrogen sulfide removal fromnaturalgas[130].Nanostructuredcompositesofgrapheneandhighlydispersedsub-20 nm sized ZnO nanoparticles (TRGZ) were successfully prepared via bycombiningfreeze-dryingandthermalannealingprocesses.Aseriesofcompo-sitionswithdifferentweight ratiosofZnOnanoparticleswerepreparedandusedasareactivesorbentinlowtemperaturehydrogensulfide(H2S)removalfromnaturalgas.ThecompositesorbenthavingaZnOmassratioof45.1wt%exhibitedasignificantlygreaterH2Sadsorptioncapacity(3.46mmol.g-1)thanthat of pure ZnO (1.06mmol.g-1), indicating that hybridization of ZnOwithgraphene significantly improved the H2S removal ability. Figure 2.12 alsoshowsthetransmissionelecrommicrographs(TEM)of(a)pristineZnOnano-particles,(b)TRGZ-1,(c)TRGZ-2,and(d)TRGZ-3graphene/ZnOnanohybrids.The authors suggested that the possible mechanism involved initial phy-sisorption of H2Smoleculeswith oxygenated functional groups of graphenewhichlatercanreachtothefinelydispersedactiveZnOnanoparticlesandgetchemisorbed through reactive adsorption. The EDX spectra of H2S treatedsamplesexhibitedastrongpeakforsulfurelementindicatingthereactivead-sorptionofH2Sonthesorbent.

Figure2.12TEMimagesof(a)pristineZnOnanoparticles,(b)TRGZ-1,(c)TRGZ-2,and(d)TRGZ-3graphene/ZnOnanohybrids.ReproducedfromReference130withpermissionfromRoyalSocietyofChemistry.


2.3.3AdsorptionofOrganicContaminantsAmongthedifferentphysicalmethodsemployedfordyeremoval,adsorptioniswidelyusedmethodandconsiderableattentionhasbeenpaidfortheappli-cationofnano-sizedadsorbentmaterialsbecauseoftheirhighersurfacearea.Inrecentyears,theutilizationofmagneticparticletechnologyforsolvingen-vironmental issues has also gained attention. Different types of iron oxideshavebeenstudiedforthedyeadsorption,duetotheirreactivesurface.Thesematerials enable faster magnetic separation just after the adsorption. Themagneticparticlescanbeemployedforadsorbingthecontaminantsfromgas-eousoraqueouseffluentsandcansubsequentlybeeasily removed fromthemedium.Luizetal.[131]generatedmagneticadsorbentsbycombiningthemagnetic

properties of ironoxides and adsorptionproperties of the activated carbon.Theironoxide/activatedcarboncompositeswereusedfortheremovalofvol-atileorganiccompoundslikephenol,chloroform,drimarenreddyeandchlo-robenzenefromtheaqueoussolution.Itwasnotedthattheexistenceofironoxidedidnotinfluencetheadsorptionefficiencyofactivatedcarbon.Inanoth-erstudybyKhosravietal.[132],ironoxidenano-sphereswerepreparedus-ing solvo-thermalmethod andwere employed as an efficient adsorbent foranionicdyeseparationfromtheaqueoussolution.Here,efficiencyofironox-idenano-spheres for the adsorptionof anionic dyesnamely reactiveorange(RO)andreactiveyellow(RY)wasanalyzed.ThemaximaladsorptioncapacityofRYandROwasobservedtobe25.0mg/gand32.5mg/g,respectively.Thekinetic analysis confirmed rapid adsorption rate onto nano-spheres, whichtookplacewithin10min.Theadsorptionprocesswasexothermicandtheop-timumpHwasabout4,i.e.,acidicsolution.Atthefinalstage,duetothemag-neticnatureoftheironoxidenano-spheres,smoothseparationfromsolutionbyamagnetcouldbeachieved.2.3.4HeavyMetalRemovalfromWater/WastewaterHeavymetalexposure,evenataverysmalllevel,isconsideredtobeadangerfor the living beings [133]. For the heavy metal separation from the wa-ter/wastewater,adsorptionmethodshavebecomeprominenttechniques.Outof thevariousadsorbentsavailable,nano-sizedmetaloxides (NMOs)are re-gardedaspromisingonesfortheheavymetalremovalfromaqueoussystems[134] .This is due to their greater surface areas aswell as higher activitiesgenerated by the size quantization effect. The NMOs offer numerous ad-


vantages like preferable sorption, higher capacities and faster kinetics to-wards theheavymetalspresent in thewateraswellaswastewater.Theex-amples forNMOs includenano-sized ferricoxides,aluminumoxides,magne-siumoxides,manganeseoxides,titaniumoxidesandceriumoxides.Thesema-terialsexistindifferentformssuchastubes,particles,etc.Ontheotherhand,whenthemetaloxidesizedecreasesfromthemicrometerleveltonanometerlevel, the enhancement in the surface energy causes poor stability. Hence,NMOsareprone toagglomerationbecauseof thepresenceofvanderWaalsforcesandotherinteractions,andtherebythehighadsorptioncapacityaswellasselectivityofNMOscansignificantlyreduceorevendisappear[135].Thus,theshapeandsizeoftheseadsorbentsarecriticalfactorsinfluencingtheirad-sorptionbehavior.Theutilizationofironoxidebasednano-materialsisconsideredtobevery

effective for the separation ofmetal contaminants fromwater, due to theirremarkable features such as small size, magnetic property, reusability andhighsurfacearea[136].Forinstance,ironoxidenanomaterialsexhibitexcel-lent adsorption ability for the separation of arsenic fromwater. Feng et al.[137] generated supermagnetic Fe3O4 coatedwith ascorbic acid, by hydro-thermalprocess.Thematerialhadasurfaceareaof179 m2/ganddiameter<10nm.Atroomtemperature, thematerialexhibitedsuperparamagneticbe-haviorandthesaturationmagnetizationattained40 emug−1,whichindicatedthatthesematerialsweresuitableforuseasanadsorbentfortheseparationofarsenic fromwastewater.TheultimateadsorptioncapacityofAs(III)andAs(V)wasfoundtobe46.06 mg/gand16.56 mg/grespectively.Theexcessiveapplicationofcopperintheindustriesleadstotheaccumula-

tionintheenvironment,therebypollutingthewater.Haoetal.[138]generat-ed a magnetic nano-adsorbent (MNP-NH2) by covalent binding of 1,6-hexadiamineontheFe3O4nanoparticlessurface,andanalyzeditsadsorptioncapacity for theremovalofcopper ions fromtheaqueoussolution.Differentfactors influencing the adsorption performance such as salinity, initial Cu2+concentration, temperature, amountofMNP-NH2, contact time andpHwereexamined.Themagneticnanoparticleswereable to separate98%of copperfromtapwateraswellasthepollutedwater.Itwasnoticedthattheadsorp-tionoccurredfasterandtheequilibriumwasattainedwithin5min.Thechem-ical sorptionwasnoticed tobe therate limitingstepof thesorptionmecha-nism,andtheultimateadsorptionabilitywasobservedtobe25.77mg.g−1at298KandpHvalue6.TheadsorbentMNP-NH2exhibitedgoodresuabilityaswellashigherstabilityundertheexperimentalconditions.Completedesorp-tionoftheCu2+ionswasachievedbyusing0.1mol.L−1HClsolutionfor<1min


andtheregeneratedMNP-NH2wasabletoretaintheoriginalmetaldischargelevel.Itwasobservedthattheadsorptionabilityofthenano-adsorbentMNP-NH2remainedconstant,andnochangeinthedesorptionabilityoccurreddur-ing15adsorption-desorptioncycles.InanotherstudybyNashaatetal.[139],theadsorptionmechanics,equilibria,thermodynamicsandkineticsof Pb(II)ionsontoFe3O4nano-adsorbentswereanalyzed.ItwasfoundthatFe3O4nano-adsorbentshadgreaterremovalefficiencyasPb(II)wasadsorbedfromwastewater in a very brief time and the equilibriumwas accomplishedwithin 30min. Further, the adsorption was dependent on temperature, pH and thePb(II) initial concentration. At pH value 5.5, themaximal removal of Pb(II)was obserevd. As the temperature and Pb(II) initial concentration was in-creased, the adsorption was also observed to enhance. The adsorption iso-therms were determined using the Freundlich and Langmuir models. ThethermodynamicsoftheadsorptionofPb(II)revealedtheimpulsenature,en-dothermic behavior and physisoprtion process. The regeneration aswell asthedesorptionanalysisconfirmedtherepeateduseofnano-adsorbentswith-out influencing the adsorption capacity. This confirmed the usefulness ofFe3O4 nano-adsorbents as inexpensive and effective adsorbents for the fastremovalandrecoveryofthemetalionsfromwastewater.The adsorptive behavior of nano-sized manganese oxides (NMnOs) are

considered tobe superiorwhencompared to itsbulkcounterpartdue to itsgreatersurfaceareaaswellaspolymorphicstructures[140].Inthepastsev-eraldecades,NMnOshavebeenusedforthesorptionofanionicandcationiccontaminantslikearsenate,phosphateandheavymetalionsfromnaturalwa-ter.Alumina(Al2O3)isalsoaconventionaladsorbentfortheremovalofheavymetalsandγ-Al2O3is regarded tobe superior toα-Al2O3 inadsorptionper-formance [141]. Nano-sized γ- Al2O3 adsorbents are generated using sol-gelmethod,andthematerialshavebeenusedassolidphaseextractionmaterialsfor the pre-concentration/separation of the tracemetal ions. In the case ofTiO2, itwasobservedthatthebulkandnano-TiO2differedinsurfaceacidity,catalytic reactivity and chemical behavior, based on their various surfaceplanes.FromtheBETanalysis,itwasseenthatthenominalparticlesizeswere8.3 and 329.8 nm, for nano-sized and bulk particles respectively [142]. Thespecific area of bulk particleswas found to be 9.5m2/g,whereas the nano-particles’ specific area was 185.5 m2/g. In a research study by Liang et al.[143],thenano-TiO2adsorbent,withBETsurfacearea208m2/ganddiameter10–50nmexhibitedadsorption capacityofCdandZnas7.9mg/gand15.3mg/g respectively at pHvalue9. The effects of contact time, pH, interferingionsandelutionsolutionontheadsorptionperformanceofnano-TiO2forthe


removal of Zn andCdwere examined. Thepresence of commonanions andcations(intheconcentrationrangeof100-5000mg/L)wasfoundtohavenosignificanteffecton themetaladsorption(Cd2+ ionsandZn2+of1.0mg/mL)underthespecifiedconditions.Manyresearchstudieshavealsoconfirmedtheroleofnano-structuredZnOtoeffectivelyremovetheheavymetalsfromvari-ousmedia[144].Inarelatedstudy,Leeetal.[145]generatedNZnOspowderusing the solution combustion method. The authors observed that as com-paredwithtwoTiO2powders,P25andtheotheronegeneratedbyhomoge-nousprecipitationatlowertemperature,theNZnOpowderdisplayedgreaterCu2+adsorptionfromthesolution.Table2.2 summarizes theheavymetalsadsorptioncapacitiesofdifferent

metaloxidesreviewedinthisstudy.Table2.2Heavymetaladsorptioncapacitiesofsomeofthemetaloxideadsorbents

Sl.No.

Adsorbent

Tempera-ture

pHvalue

Heavymetal

Adsorptioncapacity(mg/g)

Refer-ence

1 Super-magneticFe3O4coatedwithascorbicacid

roomtempera-ture

- As(III) 46.06 mg/g 115

2 Super-magneticFe3O4coatedwithascorbicacid

roomtempera-ture

- As(V) 16.56 mg/g 115

3 Magneticnano-adsorbent(MNP-NH2)

298K 6 Cu 25.77mg/g 116

4 Nano-TiO2adsorbent 9 Cd 7.9mg/g 1035 Nano-TiO2adsorbent 9 Zn 15.3mg/g 103

2.4SummaryandOutlookAdsorption represents an immensely useful technology for the removal ofharmfulgaseslikeH2S,CO2,SO2,NH3presentinthegasstocksandairaswellasseparationoforganiccontaminantsandheavymetalsfromthewastewater.Ithasattractedremarkableconsiderationinbothscientificexplorationaswellas commercial utilizations. Further, the development of state-of-the-art po-rousmaterialsfortheadsorptionprocessisrequiredtoenhancetheadsorp-tion capacity or to overcome the existing disadvantages such as small porevolumeorpore sizeof adsorbents. In this chapter, a comprehensive assess-mentontheuseofMOFsandmetaloxidesforgas/vaporandliquidphasead-sorptions has been presented. Further, focus has also been devoted to the


separation of a variety of contaminants like dyes and various heavymetalspresentinthewaterusingtheMOFsandmetaloxides.Asmentionedbefore,MOFs are promising adsorbents for the aforementioned applications due totheiruniquepropertieslikehighersurfaceareaaswellastunableporosities.Inadditiontotheseproperties,MOFsareabletoconsolidatefunctionalitiesbymeansofgraftingorloadingactiveorfunctionalspeciesandtherebyincreas-ingtheadsorptioncapacity.SeveralmodificationsinMOFs,whichweredoneinthreeaspectsnamely(1)metalions,(2)organiclinersand(3)novelconsol-idationofboth, forenhancingtheadsorptionabilitywerealsoanalyzed.Themetaloxidesexhibitenhancedselectivitybecauseofthepresenceofchemicaladsorptionbetweenthegasmoleculesandtheirsurfaces.Boththesurfacear-eaaswellasporesizeoftheadsorbentgreatlyaffectsthegasdiffusionintheadsorbent porous structure. Their reactivity depends on the crystallite size,intrinsiccrystallitereactivityaswellasthesurfacearea.Nanosizedmetalox-ides(NMOs)areconsideredtobeanexcellentadsorbentfortheheavymetalremovalfromaqueoussystems.InspiteofthehighadsorptioncapacityoftheMOFsandmetaloxides,thereareseveraleconomicalandtechnicalbarrierstopromote theusage of thesematerials for gas adsorption and environmentalapplications.Most of these challenges are associatedwithmaterial stability,remigration,hightemperatureperformance,durability,lossofadsorptionandselectivity,etc.However,withtheemploymentofadvancednanotechnologies,itisenvisagedthatmanyofthecurrentlyexistinglimitationswouldbeover-comeinduecourseoftime,whichwillpavethewayforthelargescaleappli-cationoftheseMOFsandmetaloxidematerialsforindustrialandcommercialapplications.References

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