Hydrothermal Synthesis of Cellulose Nanocrystal-Grafted-Acrylic Acid Aerogels with

15 Pages·2017·4.3 MB·English

polymers
Article
Hydrothermal Synthesis of Cellulose
Nanocrystal-Grafted-Acrylic Acid Aerogels
with Superabsorbent Properties
XuehuaLiu1,RueYang2,MingcongXu1,ChunhuiMa1,WeiLi1,2,3,* ,YuYin1,
QiongtaoHuang2,YiqiangWu3,JianLi1andShouxinLiu1,*
1 KeyLaboratoryofBio-BasedMaterialScienceandTechnologyofMinistryofEducation,NortheastForestry
University,HexingRoad26,Harbin150040,China;[email protected](X.L.);
[email protected](M.X.);[email protected](C.M.);[email protected](Y.Y.);
[email protected](J.L.)
2 Post-DoctoralResearchCenter,YihuaLifestyleTechnologyCo.,Ltd.,Shantou515834,China;
[email protected](R.Y.);[email protected](Q.H.)
3 SchoolofMaterialsScienceandEngineering,CentralSouthUniversityofForestryandTechnology,
Changsha410004,Hunan,China;[email protected]
* Correspondence:[email protected](W.L.);[email protected](S.L.);
Tel./Fax:[phone](W.L.);[phone](S.L.)
(cid:1)(cid:2)(cid:3)(cid:1)(cid:4)(cid:5)(cid:6)(cid:7)(cid:8)(cid:1)
(cid:1)(cid:2)(cid:3)(cid:4)(cid:5)(cid:6)(cid:7)
Received:12September2018;Accepted:17October2018;Published:19October2018
Abstract: In this work, we applied a fast and simple method to synthesize cellulose nanocrystal
(CNC) aerogels, via a hydrothermal strategy followed by freeze drying. The characteristics and
morphologyoftheobtainedCNC-g-AAaerogelswereaffectedbythehydrothermaltreatmenttime,
volumeofaddedAA(acrylicacid),andthemassfractionoftheCNCs. Theformationmechanism
oftheaerogelsinvolvedfreeradicalgraftcopolymerizationofAAandCNCswiththecross-linker
N,N(cid:48)-methylene bis(acrylamide) (MBA) during the hydrothermal process. The swelling ratio of
the CNC-g-AA aerogels was as high as 495:1, which is considerably greater than that of other
polysaccharide-g-AAaerogelssystems. Moreover,theCNC-g-AAaerogelsexhibitedanexcellent
methylblue(MB)adsorptioncapacityandtheabilitytoundergorapiddesorption/regeneration.
ThemaximumadsorptioncapacityoftheCNC-g-AAaerogelsforMBwasgreaterthan400mg/g.
ExcellentregenerationperformancefurtherindicatesthepromiseofourCNC-g-AAaerogelsasan
adsorbentforapplicationsinenvironmentalremediation.
Keywords: cellulosenanocrystals;hydrothermal;aerogels;superabsorbent
1. Introduction
Developmentsinindustryandcommercehaveraisedtheproblemofdyecontamination,whichis
characterizedbyahighorganicmattercontent,resistancetobiodegradability,andcomplexbiological
toxicity[1]. Dyecontaminationraisesseriousenvironmentalconcernwithpotentialdangerstofood
safetyandaquaticlife[2]. Variousgroupshavetriedtodevelopmoreefficientmethodsofremoving
dye pollutants. Dye pollution removal is conventionally achieved by photochemical degradation,
membrane filtration, flocculation, biologicaloxidation, andchemical precipitation [3,4]. However,
these technologies are complex and expensive; hence, the design and development of alternative
low-costadsorbentshasbeenthefocusofmanystudies.
Cellulose nanocrystals (CNCs) are a renewable material with properties including good
biodegradability,mechanicalstrength[5],flexibility[6],easilymodified,andbiocompatibility[7,8].
Thehighstrength,dimensionalanisotropy,andnaturalsourcingofCNCs,haveattractedconsiderable
Polymers2018,10,1168;doi:10.3390/polym10101168 www.mdpi.com/journal/polymers
Polymers2018,10,1168 2of15
research interest for their use as functional and renewable reinforcing agents for polymer
composites [9,10]. The rigid and flexible mechanical properties of CNCs show great potential
for various applications and have motivated research on preparing CNC aerogels for different
fields[11–13].ThefabricationofCNC-basedaerogelscanavoidissuesrelatedtocellulosegelshrinkage,
capillarytension,andrupturingindryingprocessesowingtocapillarypressure[14]. Functionalized
CNCaerogelscan exhibit multiple responsesaccordingtochangesin externalconditions, suchas
temperature,pH,andpressure[15,16]. Thus,CNCaerogelscanbeusedinbiologicalapplicationsand
asstrainedmaterials,adsorbentmaterials,hydrophobicmaterials,andconductivematerials. Thereare
alsoidealmaterialsforapplicationsinenvironmentalmodification[17–19].
Highly porous CNC-based aerogels with many hydroxyl groups are widely regarded as
superabsorbent materials [20]. The unique properties of these materials include high strength,
high surface area, and tunable surface chemistry, which enable interactions with other polymers.
Inparticular,functionalpolymericmaterials,containingastronghydrophilicgroupintroducedbygraft
copolymerizationreactions,arewidelyusedinpolysaccharidereactionsystemsbecauseoftheirhigh
efficiencyandcompatibility. CNC-basedaerogelscanbepreparedbyafreechaingraftpolymerization
withpoly-methacrylicacid,whichismodifiedbyfunctionalizingthecarboxylicgroup[21].CNC-based
aerogelscanalsobepreparedbyfreeradicalpolymerizationofpoly(N-isopropylacrylamide)(PNIPAM)
and grafted by copolymerization of poly (acrylic acid) (PAA), to achieve functionalization by
cooperativehydrogenbondinginteractionsbetweenPNIPAMorPAAandtheCNCs[16,22]. Moreover,
CNCs,whicharegraft-copolymerizedwithAA(acrylicacid)givehydrogelswithaswellingratiofar
greaterthanthatofotherpolysaccharidesystemscombinedwithAAhydrogels,atthesameinitial
reactantratio[23,24].
Recently,hydrothermalprocedurestoproducehydrogelsfromcellulosicbiomasshavereceived
attentionowingtotheirinherentadvantagesintermsofliquidchemicalreactionsandtheirsimplicity[25].
Compared with reactions at ambient temperature and pressure, the straight forward hydrothermal
methoddevelopedhererequireslessreagentstopromotethereactiontocompletion. Ourgrouphas
preparedfluorescentCNC/carbondothydrogelsbyaone-stephydrothermalmethod[26].Theshape
andchemicalstructureoftheproductscanalsobecontrolledbythehydrothermalmethod.Hydrothermal
methodsareconsideredtobehighlyeffectiveforfabricatingCNC-basedhydrogels[27,28].Theseresults
encouragedustouseahydrothermalstrategytopromoteagraft-copolymerizedreactiontosynthesize
aCNC-basedaerogelsuperabsorbent.
Inthisstudy,weproposedasimplemethodtofabricateCNC-g-AAaerogelsviaahydrothermal
treatmentfollowedbyfreezedrying. Weexaminedtheswellingperformanceoftheobtainedaerogels
for the water absorption and the adsorption properties of the CNC-g-AA aerogels for methylene
blue(MB).
2. MaterialsandMethods
2.1. Materials
The CNCs were prepared from commercial bleached kraft softwood pulp (85% International
StandardizationOrganizationbrightness)astherawmaterial,whichwasprovidedbyapaper-making
factoryinHeilongjiang,China. Analytical-gradesulfuricacidwaspurchasedfromTianjinKermel
ChemicalReagentCo.,Ltd. (Tianjin,China). Tetramethylethylenediamine(TMEDA),N,N(cid:48)-methylene
bis(acrylamide)(MBA),andammoniumpersulfate(APS)werepurchasedfromAladdinReagentCorp
China(Chengdu,SichuanProvince,China). TheAAandMBwerepurchasedfromTianjinFuchen
ChemicalReagentFactory(Tianjin,China). Allchemicalreagentswereofanalyticalgrade.
2.2. PreparationofCNC-g-AAHydro/Aerogels
AnaqueousCNCssuspensionwaspreparedbyasulfuricacidhydrolysismethod,aspreviously
reported[29,30]. Bleachedkraftsoftwoodwasfirstmilledandthenpassedthrougha0.5-mmscreento
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Polymers 2018, 10, x FOR PEER REVIEW 3 of 15
ensureauniformparticlesize. Then,themilledpulpwashydrolyzedinsulfuricacid(64wt%;8.75mL
oafcisdu (lf6u4r wicta %ci;d 8s.7o5lu mtioLn opf esurlgfurarmic aocfidp usloplu)atito4n5 p◦eCr gforram30 omf pinuwlpi)t hatv 4i5g o°rCo ufosrs 3ti0r rminign. wTihthe cveigllourloouses
ssutisrprienngs. ioTnhew aceslltuhleonsed isluustepden, scieonnt riwfuags etdh,ewn adshileudte,da,n cdendtirailfyuzgeedd,a gwaainshstedd,i satinldle ddiwalaytzeerdu natgialitnhset
edxistetirliloerdw waatteerrb uenctaiml tehen eeuxttrearliobre wfoaretefru brethcaermues ne.eutral before further use.
TThhee CCNNCC--gg--AAAA hhyyddrrooggeellss wweerree pprreeppaarreedd bbyy aa ffrreeee rraaddiiccaall ggrraafftt ccooppoollyymmeerriizzaattiioonn ooff CCNNCCss aanndd
AAAA iinn tthhee pprreesseennccee ooff aa ccrroossss--lliinnkkeerr ((MMBBAA)) aanndd aa rreeddooxxi ninitiitaiattoorrs syysstteemm( A(APPSS//TTMMEEDDAA)).. AA 1100 mmLL
ppoorrttiioonn ooff tthhee pprreeppaarreedd CCNNCC ssoolluuttiioonn ((11..55 wwtt %%)) wwaass ppllaacceedd iinn aa 110000 mmLL tthhrreeee--nneecckkeedd flflaasskk,, wwhhiicchh
wwaass tthheenn ppllaacceedd iinn aa 2255 ◦°CC wwaatteerr bbaatthh aanndd ssttiirrrreedd wwiitthh aa mmaaggnneettiicc ssttiirrrreerr.. AA 00..005500 gg ppoorrttiioonn ooff AAPPSS
aanndd 00..005500 mmLL ooff TTMMEEDDAA,, aass aann iinniittiiaattoorr,, wweerree aaddddeedd ttoo tthhee ssoolluuttiioonn aanndd tthhee mmiixxttuurree wwaass ssttiirrrreedd ffoorr
1100 mmiinnt otog genenereartaeter ardaidciaclasl.sT. hTehreeraefatefrt,ear, car ocsrso-slisn-klienrk,eMr,B MAB(0A.0 (800.0g8)0a ngd) aancde rata cinervtaoilnu mvoeloufmAeA ofw AerAe
awdedreed a,dfodlelodw, efodllboywceodn tbinyu coounstsintiurroinugs fsotirr2rinhg. Tfhoer r2e ahc.t iTohnes orleuatcitoinonw saosltuhteionnp lwacaesd tihnena Tpelflaocend-l iinne da
sTteafilnolnes-lsinsteede lsatuationclelassv es(t1e0e0l mauLt)oacnladvhe ea(1te0d0 tomtLh)e danesdi rehdeatetemdp etroa ttuhree .dTehseirheydd rtoetmhepremraatlurreea. ctTiohne
whyadsrpoethrfeorrmmael drefaocrtioans pweacsi fipcertfiomrme.edT hfoer oab stpaienceifdic gtiemlsew. Tehree owbatashineedd igneldsi wstielrleed wwasahteedr itno dreismtiollvede
uwnarteearc ttoe dresmubosvtea nucnerse.acted substances.
TThheerreeaafftteerr,, tthhee aass--pprreeppaarreedd hhyyddrrooggeellss wweerree ffrroozzeenn iinn lliiqquuiidd nniittrrooggeenn.. AAfftteerr ccoommpplleettee ffrreeeezziinngg,,
tthhee ffrroozzeenn ssaammpplleess wweerree ssuubbjjeecctteedd ttoo ffrreeeezzee--ddrryyiinngg ((SScciieennttzz--1100NN,, NNiinnggbboo,, ZZhheejjiiaanngg PPrroovviinnccee,, CChhiinnaa))
ttoo oobbttaaiinn pprriissttiinnee CCNNCCss--gg--AAAA aaeerrooggeellss.. TThhee rreessuullttiinngg CCNNCC--gg--AAAA hhyyddrroo//aaeerrooggeellss aarree rreeffeerrrreedd ttoo aass
CCNNCC--gg--AAAA--rr--mm--nn,, wwhheerree rr ddeennootteess tthhee AAAA ccoonntteenntt,, mm ddeennootteess tthhee hhyyddrrootthheerrmmaall tteemmppeerraattuurree,, aanndd nn
ddeennootteess tthhee hhyyddrrootthheerrmmaall ttiimmee.. FFuurrtthheerrmmoorree,, wwee pprreeppaarreedd ppuurree hhyyddrroo//aaeerrooggeellss,, wwhheerree tthhee CCNNCCss
ssoolluuttiioonn wwasasr eprelapcleadcewd itwhditihs tidlliesdtilwleadte rwuantdere rucnondderit ioconnsdoift2io.0nms LofA A2.,0a mhyLd rAotAhe, rma ahlytedmroptehreartmurael
otefm12p0er◦aCtu,raen odf a12tr0e a°Ctm, eanntdt iam tereoaftm8ehnt( Dti-mg-eA oAf -82 -1h2 (0D-8-gh-AydAr-o2/-1a2e0ro-8g ehlys)d.roH/aereeriong,etlhs)e. sHweerelliinn,g thoef
CswNeCll-ign-gA oAf -C2-N12C0--g8-AgeAl-p2e-r1f2o0r-m8 agnecl epeinrfdorismtiallnecdew ina tdeirstisillsehdo wwanteinr Fisi gshuorew1n. in Figure 1.

FFiigguurree 11.. PPhhoottooggrraapphhss sshhoowwiinngg sswweelllliinngg aanndd ssiizzee--cchhaannggee ooff CCNNCC--gg--AAAA--22--112200--88 ggeell iinn ddiissttiilllleedd wwaatteerr
oovveerr ttiimmee..
2.3. CharacterizationofCNC-g-AAAerogels
2.3. Characterization of CNC-g-AA Aerogels
FFTT--IIRR spsepcetcrtarao fCofN CC-Ng-CA-Ag-AaeAro gaeelsrowgeerlse mweearseu remdeoansuareFdo uroiner tar anFsofuorrimeri ntfrraanresdfosrpme ctirnofmraerteedr
(sipS1ec0t,rNomicoelteetr, (TiSh1e0rm, NoiSccoileent,t iTfihce,rWmaolt hSacimen,tMifAic,, UWSaAlt)hoavme,r MthAe,r aUnSgAe)o ofv40e0r –t4h0e0 r0acnmge− 1ofi n40tr0a–n4s0m00is csmion−1
mino tdrea.nsFmorismsioonrp mhooldoeg.i cFaolrs tmudoirepsh,owloegeixcaalm sitnueddietsh, ewCeN eCx-agm-AinAeda etrhoeg eClsNwC-itgh-AaAsc aanernoigneglse lwecittrho na
msciacnronsincogp eel(eScEtrMon, QmUicArNosTcAop2e0 0(,SFEEMI,,H QillUsbAoNroT,AO2R0,0U, SFAEI),. SHpielclsibmoernos, wOeRr,e UcoSaAte)d. Swpiethcigmoeldnsb ewfoerree
ScEoaMteodb sweritvha tgioonlds. Tbehfeo1r3eC SCEPM/ MobAsSerNvMatRioanns.a lTyhsies o13fCth CePsa/MmpAlSes NwMasRp earnfoarlymseisd oatf rtohoem staemmppleersa twuares
wpeitrhforamBerdu akte rroDomRX te-4m0p0esrpateucrtero wmiethte ar. BrSupkeecrt rDaRwX-e4r0e0 ascpqeuctirreodmewteitrh. Sape4c-tmram weMreA aScqpuriorebde wusitihn ga
a4-cmomm bMinAatSi opnroobfeC uPs,iMngA aS c,oamndbihniagthio-np oowf CerPp, MrotAoSn, danecdo huipglhin-pgomweetrh pordost.oAn dtoectaoluopfli8n0g0 msceatnhsodwse. rAe
atoctcaulm ofu 8la0t0e dscfaonrse wacehres aamccpulme.ulated for each sample.
Polymers2018,10,1168 4of15
2.4. SwellingCharacterizationofCNC-g-AAAerogelsinDistilledWater
The equilibrium swelling ratios of the CNC-g-AA aerogels were measured in distilled water
atroomtemperature. AportionoftheCNC-g-AAaerogels(W )wasthenpreparedandimmersed
1
in an excess of distilled water to achieve a state of equilibrium swelling. The weight gain of the
equilibriumsamples(W )wasmonitoredgravimetrically. Excesswateronthesurfaceofthehydrogels
2
wasremovedwithfilterpaper. Threereplicateswereperformedforeachcomposition. Theswelling
ratiowascalculatedbythefollowingEquation:
(w −w )
SR= 2 1 (1)
w
1
2.5. AdsorptionStudies
We selected the CNC-g-AA-2-120-8 aerogel for the adsorption experiments under different
experimentalconditions.Allexperimentswereperformedinglassvialsplacedinacontrolled-temperature
waterbathoscillatoroperatingat150rpm.TheMBsolutionsofdifferentconcentrationswereprepared
withdistilledwater. ThepHofthedyesolutionswasadjustedwith0.01MHClorNaOH.Afterdye
adsorptionequilibriumwasreached,wefilteredtheswollengelsandtheresidualconcentrationoffiltered
MB solution was measured with a UV-vis spectrophotometer (Evolution 600, Thermo Scientific Inc.,
Madison,WI,USA)andcalculatedfromtheabsorptionintensityat664nm.TheamountofadsorbedMB
wascalculatedbasedonthefollowingEquation:
v
q = (c −c ) (2)
e 0 e m
whereq (mg/g)istheamountoftheadsorbeddyeatequilibrium;c (mg/L)andc (mg/L)arethe
e 0 e
initialandresidualconcentrationsofthedyesolutionatequilibrium,respectively;V(L)isthevolume
ofthedyesolution;andm(g)isthemassoftheadsorbentused.
WeselectedCNC-g-AA-2-120-8aerogelforregenerationtestingunderconditionsofMBsolution
of 20 mg/L, pH = 8.0, adsorbent mass is 0.0050 g. Here, HCl (1 mol/L) was used as the eluent.
TheCNC-g-AA-2-120-8hydrogelwasloadedwithMBandtreatedwiththeeluent(50mL)for120min
atroomtemperature. Thedesorbedadsorbentwaswashed,freeze-dried,andpreparedforthenext
cycleofregenerationtesting.
3. ResultsandDiscussion
3.1. MorphologyofCNC-g-AAAerogels
The SEM images of D-g-AA aerogel and CNC-g-AA-r-m-n aerogels (Figure 2) showed three-
dimensional interconnected honeycomb open-cell structures. The cell wall surfaces of D-g-AA
aerogel,withoutaddedCNCs,weresmooth(Figure2a,b). FortheaerogelswithaddedCNCs,the
surface of the aerogel cell walls became wrinkled. Additionally, some web-like structures formed
on the cell walls, which likely improved the accessibility of water into the cell. The effects of the
hydrothermal treatment time might be related to the CNCs having many active –OH groups and
modified–COOHgroupsontheirsurfaces,whichaffectedthepolymerizationofthecompositesand
theformationofthethree-dimensionalpolymernetwork[31,32]. Forlongerhydrothermaltreatment
times,-CO-NH-inMBAtendedtocombinewithcarboxylgroups,whichcontributedtotheformation
of a three-dimensional honeycomb network [33]. This extended dislocation structure was caused
by vacancy motion at the nanoscale in the mixed liquid crystal lattice, which is conducive to the
formationofathree-dimensionalmacroporousstructureintheCNCsmixture[34,35]. Thistypeof
chemicalmodificationcanincreasethehydrophilicityofCNCsandtheircompatibilitywithhydrophilic
polymermatrices. Poresareregionswherewatercanpermeate,whichallowsinteractionsofexternal
compoundswiththehydrophilicgroupsofthegraftcopolymers[36].
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FFiigguurree 22.. SSEEMM iimmaaggeesso off( a(,ab,)bD) D-g--gA-AA-A2--122-102-80-a8e raoegreolg;e(cl;– f(c)–CfN) CCN-gC-A-gA-A-r-A12-r0--182a0e-8ro ageerlosgperlesp parreedpawreitdh
wdiiftfhe redniftfAerAencto AntAen tcso(n1t,e2n,t3s, a(1n,d 24, m3,L a);n(dg –4j) mCNL)C; -(gg-–AjA) -C2N-1C20--gn-AaeAro-2g-e1l2s0w-nit haedrioffgeerlesn twhiythd rdotihffeerrmenatl
htryedartmotehnetrmtimale tsre(a2t,m4,e8n,ta tnimde1s2 (h2), .4, 8, and 12 h).
3.2. FT-IRandNMRCharacterizationofCNC-g-AAAerogels
3.2. FT-IR and NMR Characterization of CNC-g-AA Aerogels
FT-IRspectraofpureCNCs,D-g-AA-2-120-8aerogel,andCNC-g-AA-2-120-8aerogel,areshown
FT-IR spectra of pure CNCs, D-g-AA-2-120-8 aerogel, and CNC-g-AA-2-120-8 aerogel, are
inFigure3a.Inspectruma,peaksat1058,1427and3337cm−1areassociatedwithCNCs[37].Thepeaks
shown in Figure 3a. In spectrum a, peaks at 1058, 1427 and 3337 cm−1 are associated with CNCs [37].
at3337and1026cm−1correspondtotheO–Hstretchingvibrationanddeformationvibrationpeaks,
The peaks at 3337 and 1026 cm−1 correspond to the O–H stretching vibration and deformation
indicatingstronghydrogen-bondinginteractionsofCNCs[38]. Theregionoflowintensitybands
vibration peaks, indicating strong hydrogen-bonding interactions of CNCs [38]. The region of low
between1058and1427cm−1 suggeststhepresenceofC–OandC–Hbonds[39]. Inspectrabandc,
intensity bands between 1058 and 1427 cm−1 suggests the presence of C–O and C–H bonds [39]. In
weassignthepeakat1550cm−1 tothestretchingvibrationofN–H.Thesharpbandat1705cm−1
spectra b and c, we assign the peak at 1550 cm−1 to the stretching vibration of N–H. The sharp band at
isassignedtoasymmetricalstretchingvibrationofCOO−,whichindicatedthepresenceofCOO−
1705 cm−1 is assigned to a symmetrical stretching vibration of COO−, which indicated the presence of
groupsintheCNC-g-AA-2-120-8aerogelnetwork. TheseresultsconfirmedthattheAAmonomers
COO− groups in the CNC-g-AA-2-120-8 aerogel network. These results confirmed that the AA
weresuccessfullygraftedontothebackboneofCNCsandconfirmedourinterpretationoftheformation
monomers were successfully grafted onto the backbone of CNCs and confirmed our interpretation
ofthethree-dimensionalnetworkstructure.
of the formation of the three-dimensional network structure.
To further illuminate the structure of CNC, D-g-AA and CNC-g-AA, the samples were
To further illuminate the structure of CNC, D-g-AA and CNC-g-AA, the samples were
characterized by 13C CP/MAS NMR spectroscopy (Figure 3b). The spectra of CNC displayed the
characterized by 13C CP/MAS NMR spectroscopy (Figure 3b). The spectra of CNC displayed the
typicalsignalsfromcellulose,whichwereassignedasfollows: theC1(104.7ppm),C2/C3/C5(71.9
typical signals from cellulose, which were assigned as follows: the C1 (104.7 ppm), C2/C3/C5 (71.9
and 74.9 ppm), C4 (89.1 ppm), and C6 (65.6 ppm) peaks correspond to the carbon atoms of the
and 74.9 ppm), C4 (89.1 ppm), and C6 (65.6 ppm) peaks correspond to the carbon atoms of the
glucopyranoseringsinthecrystallineregions,whereastheC4(84.7ppm)peakcorrespondstothosein
glucopyranose rings in the crystalline regions, whereas the C4 (84.7 ppm) peak corresponds to
theamorphousareas[40]. ForCNC-g-AA-2-120-8,thepeakat177.5ppmisassociatedwithcarbon
those in the amorphous areas [40]. For CNC-g-AA-2-120-8, the peak at 177.5 ppm is associated with
atoms of carboxylic acid groups, the peak at 61.9 ppm for N–CH –CH –O– which supports the
carbon atoms of carboxylic acid groups, the peak at 61.9 ppm for N2–CH22–CH2–O– which supports
occurrenceofgraftingreactionbetweenthehydroxylgroupsofCNCandMBA.So,itisapparentthat
the occurrence of grafting reaction between the hydroxyl groups of CNC and MBA. So, it is
thepoly(acrylicacid)chainshavebeensuccessfullycross-linkedonCNCinthepresenceoftheMBA
apparent that the poly (acrylic acid) chains have been successfully cross-linked on CNC in the
cross-linker[41].
presence of the MBA cross-linker [41].
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Figure3. FT-IR(a)and13CNMR(b)spectraofCNC,D-g-AA-2-120-8aerogelsandCNCs-g-AA-2-
Figure 3. FT-IR (a) and 13C NMR (b) spectra of CNC, D-g-AA-2-120-8 aerogels and
120-8aerogels.
CNCs-g-AA-2-120-8 aerogels.
3.3. EquilibriumSwellingRatioinDistilledWater
3.3. Equilibrium Swelling Ratio in Distilled Water
TheequilibriumswellingratiooftheCNC-g-AA-r-120-naerogelswasalsorelatedtoAAcontent
andhyTdhreo tehqeurmiliablrriueamc tisownetlilmineg (Fraigtiuor eof4 at,hbe). CANsCth-eg-aAmAo-ur-n1t2o0f-na dadeerodgAelAs wwaass ianlcsroe arseeladtefrdo mto 1A.0A
toco4.n0temnLt ,anthde hsywderlolitnhgerrmatailo roefacthtieonC NtimCe-g (-FAigAu-rre-1 42a0,-b8).a Aerso gtheels aimncorueanst eodf fardodmed1 7A4Ato w4a9s5 .inAcrsetahseed
hyfrdormot h1e.0r mtoa l4r.0e amctLio, nthteim swewellaisngin rcarteiaos eodf tfhroem CN2Cto-g1-0AhA,-trh-e12s0w-8e lalienrgograetliso inocfrtehaesreeds ufrlotimng 1a7e4r otog e4l9s5.
inAcrse athseed hfyrdormot4h6ertom4a7l 8r.eHacetinocne ,twimees weleacst eindctrheaesCedN Cfr-ogm-A 2A t-o2 -1102 0h-,8 taheer oswgeellslifnogr draettiaoi loefd tahnea rleyssuisltoinfg
thaeeirrowgaetles rinancrdeaMseBda fdrsoomrp 4ti6o ntop 4r7o8p.e Hrteiensc.e, we selected the CNC-g-AA-2-120-8 aerogels for detailed
anaTlyhseist iomf etshereirq wuiaretedr faonrdth MeBC NadCs-ogr-pAtAio-n2 -p1r2o0p-8eratnieds.D -g-AA-2-120-8aerogelstoreachequilibrium
areshoTwhne intimFiegsu rree4qcu.iTrehde sfwore llitnhge raCtNioCo-fgt-hAeAC-2N-1C2-0g--8A Aa-nade roDg-egl-wAaAs-2a-lm12o0s-8t nianeerotigmeless atos grreeaatch
aseqthuaitliobfrituhme Dar-eg -sAhAow-2n-1 i2n0 -F8igauerroe g4ecl. aTthaeb ssowrepltliionng eraqtuioil iobfr ituhme .CWNeCa-gtt-rAibAu-taeerthoigsedl iwffaesr eanlcmeotsot tnhiene
intciomrpeso raast iognreoaft CaNs Cthsaitn otof tthhee gDel-gm-AatAri-x2,-w12h0i-c8h aperroomgoelt eadt tahbesfoorrpmtiaotnio neqoufialibproiruomus. Waned ahtotnriebyuctoem thbis
mdoirfpfehroelnocgey .toT thhee CinNcoCrsposuraptpioonr teodf CthNeCpso ilnymto etrhne egtwelo mrkatarnixd, pwrhoivcihd epdrosmiteostefdo rththe efoinrmitiaattiioonn ooff a
poployrmoeursi zaantdio nh,ownehyicchomopbe mneodrpuhpotlhoegyae. rTohgee lCsNtruCcst usurepapnodrttehdu tshceo pnotrliybmuteerd nteotwthoerakb asnordp ptiroonvcidapedac sitiytes
anfodr stwhee lilninitgiartaioten ooff tphoelyhmyderriozgaetilosn[4, 2w].hiIcnha odpdeintieodn ,utph ethper easeerongceelo sftrCuNctCursei nantdh ethhuysd croongterlibmuatetrdix to
intchree aasbesdortphteioanm coaupnatcsitoyf ahnydd srowpehlliilnicgg rraoteu posf ,twheh hicyhderongaebllse d[4f2a]s. tIenr aadndditeioasni,e trhdei pffruessieonnceo foaf qCuNeoCuss in
sotlhuet ihoyndinrotogetlh emhaytrdixro igneclremaasterdix th[4e3 ,a4m4]o.unts of hydrophilic groups, which enabled faster and easier
diffusion of aqueous solution into the hydrogel matrix [43,44].
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Polymers 2018, 10, x FOR PEER REVIEW 7 of 15

Figure 4. (a) Swelling ratio of CNC-g-AA-r-120-8 aerogels with different AA contents; (b) Swelli ng
rFaitgiou roef4 .C(Na)CS-wg-eAllAin-g2-r1a2t0i-ono faeCrNogCe-lgs -AhyAd-rro-1th2e0r-8maaellryo gterlesatwedit hfodri ffdeirfefnerteAntA tcimonetse; n(tcs); (Ab)dSsowrepltliionng
Figure 4. (a) Swelling ratio of CNC-g-AA-r-120-8 aerogels with different AA contents; (b) Swelling
craaptiaocoitfyC oNf CD--gg--AAAA--22--112200--n8 aaeerrooggeellssh aynddr oCthNeCrm-ga-lAlyAt-r2e-a1t2e0d-8fo aredrioffgeerlesn ctotmimpeasr;e(dc) wAidthso trhpet iaodnscoarppaticoitny
ratio of CNC-g-AA-2-120-n aerogels hydrothermally treated for different times; (c) Adsorption
toimfDe.- g-AA-2-120-8aerogelsandCNC-g-AA-2-120-8aerogelscomparedwiththeadsorptiontime.
capacity of D-g-AA-2-120-8 aerogels and CNC-g-AA-2-120-8 aerogels compared with the adsorption
3.4. Ftiomrme.a tionMechanismofCNC-g-AAAerogels
3.4. Formation Mechanism of CNC-g-AA Aerogels
3.4. FAAor pmprraootpipooonss eMedde mcmheaecnchhisaamnn iiossftt iCicc NppaCatt-hhgw-wAaaAyy fAfooerrr tothgheeel fsfo orrmmaattiioonn ooff CCNNCC--gg--AAAA aaeerrooggeellss iiss sshhoowwnn inin FFigiguurree 55. .
SulfateanionradicalsgeneratedfromAPSabstractedprotonsfrom–OHgroupsoftheCNCbackbone
Sulfate anion radicals generated from APS abstracted protons from –OH groups of the CNC
A proposed mechanistic pathway for the formation of CNC-g-AA aerogels is shown in Figure 5.
toformalkoxyradicals,whichresultedinactivecentersontheCNCsbackbonethatradicallyinitiated
backbone to form alkoxy radicals, which resulted in active centers on the CNCs backbone that
Sulfate anion radicals generated from APS abstracted protons from –OH groups of the CNC
polymerization. TheCNCsarerichin–OHgroups,whichinducestrongmolecularinteractionsinthe
radically initiated polymerization. The CNCs are rich in –OH groups, which induce strong
backbone to form alkoxy radicals, which resulted in active centers on the CNCs backbone that
nanocompositesystem[45]. AfterAAisaddedtothenanocompositestheAAbecomesconjugated
molecular interactions in the nanocomposite system [45]. After AA is added to the nanocomposites
radically initiated polymerization. The CNCs are rich in –OH groups, which induce strong
withtheCNCsandcontributesmoreeffectivebindingsites. Thepresenceofthecross-linkingreagent
the AA becomes conjugated with the CNCs and contributes more effective binding sites. The
molecular interactions in the nanocomposite system [45]. After AA is added to the nanocomposites
(MBA) results in the formation of a copolymer network, which comprises a chemical cross-linked
presence of the cross-linking reagent (MBA) results in the formation of a copolymer network, which
the AA becomes conjugated with the CNCs and contributes more effective binding sites. The
structure[46]. DuringthehydrothermaltreatmentoftheCNCswiththecomposites,thepresenceof
comprises a chemical cross-linked structure [46]. During the hydrothermal treatment of the CNCs
–pCreOseOn−ce, wofh tihche cisrofsosr-mlinekdinbgy rdeeapgreontto (nMatBioAn) orefs–uCltOs OinH thger foourpmsa,tlioocna loifz eas ctohpeonlyemgaetri vneetcwhoarrgke, wofhtichhe
wcoimthp trhiese cso am cphoesmiteicsa, lt hcero pssre-lsiennkceed osft r–uCcOtuOre−, [w46h]i.c Dh uisr ifnogrm theed hbyyd droetphreortmonaal ttiroena tomf e–CntO oOf Hth eg rCoNupCss,
polymernetworkandenhanceselectrostaticrepulsion,whichfavorsexpansionofthechainnetwork.
localizes the negative charge of the polymer network and enhances electrostatic repulsion, which
with the composites, the presence of –COO−, which is formed by deprotonation of –COOH groups,
However, owingtothechemicalcompositionoftheCNCmolecules, theattached–OHfunctional
favors expansion of the chain network. However, owing to the chemical composition of the CNC
localizes the negative charge of the polymer network and enhances electrostatic repulsion, which
groupsmightinducehydrogenbondformation,whichisanadditionalfactorthatimprovesadhesive
molecules, the attached –OH functional groups might induce hydrogen bond formation, which is an
favors expansion of the chain network. However, owing to the chemical composition of the CNC
properties[47].
additional factor that improves adhesive properties [47].
molecules, the attached –OH functional groups might induce hydrogen bond formation, which is an
additional factor that improves adhesive properties [47].

FFiigguurree 55.. MMeecchhaanniissttiicc ddiiaaggrraamm ooff CCNNCC--gg--AAAA aaeerrooggeell ffoorrmmaattiioonn..
3.5. AdsorptionStudiesFigure 5. Mechanistic diagram of CNC-g-AA aerogel formation.
3.5. Adsorption Studies
33..55.. 1A.dSstourdpiteiosno SntuDdyieesA dsorption
3.5.1. Studies on Dye Adsorption
TheeffectsofthesolutionpHoftheinitiallypreparedMBsolutionsonMBdyeremoval(%)and
3.5.1.T Shteu defiefesc otsn oDf yteh eA sdoslourtpiotino np H of the initially prepared MB solutions on MB dye removal (%)
theresidualconcentration(C )oftheMBafteradsorptionbyourCNC-g-AA-2-120-8aerogels,are
e
asnhdo wtThnhe eirn eesFfifidegcuutasrl e oc6fo atn.hceHe nsetorrleau,ttiwioonen (upCsHee)d ooaff tt0hh.0ee 0 Mi5n0Bi-t giaafpltlyeor r ptairodenspoaorrfpettdhio eMnC BbN ys Cool-uugrt- iAConANs-C 2o--ng1 -2MA0-AB8 -ad2e-y1re2o 0gr-ee8ml aionevr5ao0lg e(m%lsL),
are shown in Figure 6a. Here, we used a 0.0050-g portion of the CNC-g-AA-2-120-8 aerogel in 50 mL
aonfda t6h0e mregs/idLuMal Bcosnocleuntitorant.ioWn h(Cene) tohfe thsoe luMtiBo nafpteHr awdasosrdpeticornea bsyed oufrro CmN8C.0-gt-oA3A.0-,2-t1h2e0r-a8 taioeroofgMelsB,
of a 60 mg/L MB solution. When the solution pH was decreased from 8.0 to 3.0, the ratio of MB
are shown in Figure 6a. Here, we used a 0.0050-g portion of the CNC-g-AA-2-120-8 aerogel in 50 mL

of a 60 mg/L MB solution. When the solution pH was decreased from 8.0 to 3.0, the ratio of MB
Polymers 2018, 10, x FOR PEER REVIEW 8 of 15
Polymers2018,10,1168 8of15
removal decreased from 98.4% to 20.6% and the Ce decreased from 0.8 to 39.7 mg/L. We attribute the
low adsorption ability at low pH to the CNC-g-AA-2-120-8 aerogels being immersed in an acidic
removaldecreasedfrom98.4%to20.6%andtheC decreasedfrom0.8to39.7mg/L.Weattribute
e
environment having a more positive net charge at its surfaces, which electrostatically repelled MB.
thelowadsorptionabilityatlowpHtotheCNC-g-AA-2-120-8aerogelsbeingimmersedinanacidic
To select a suitable amount of the adsorbent, different portions of (0.0010, 0.0020, 0.0030, 0.0040,
environmenthavingamorepositivenetchargeatitssurfaces,whichelectrostaticallyrepelledMB.
and 0.0050 g) of CNC-g-AA-2-120-8 aerogels were added to 50 mL of 60 mg/L MB solutions, and the
Toselectasuitableamountoftheadsorbent,differentportionsof(0.0010,0.0020,0.0030,0.0040,
MB removal results are shown in Figure 6b. The adsorption capacity for the MB dyes on
and0.0050g)ofCNC-g-AA-2-120-8aerogelswereaddedto50mLof60mg/LMBsolutions,andtheMB
CNC-g-AA-2-120-8 aerogels decreased from 719.8–244.7 mg/g as the dosage increased from 0.0010 to
removalresultsareshowninFigure6b.TheadsorptioncapacityfortheMBdyesonCNC-g-AA-2-120-8
0.0050 g. However, the MB removal rate increased from 57.6% to 97.9% as the adsorbent dosage was
aerogelsdecreasedfrom719.8–244.7mg/gasthedosageincreasedfrom0.0010to0.0050g. However,
increased from 0.0010 to 0.0050 g. The adsorption reached a swelling equilibrium when the
theMBremovalrateincreasedfrom57.6%to97.9%astheadsorbentdosagewasincreasedfrom0.0010
adsorbent dosage was 0.0040 g. We attribute this result to the greater surface area of the adsorbent
to0.0050g. Theadsorptionreachedaswellingequilibriumwhentheadsorbentdosagewas0.0040g.
and the availability of more active adsorptive sites as the amount of the CNC-g-AA-2-120-8 aerogel
Weattributethisresulttothegreatersurfaceareaoftheadsorbentandtheavailabilityofmoreactive
was increased.
adsorptivesitesastheamountoftheCNC-g-AA-2-120-8aerogelwasincreased.
The initial MB concentration and the contact time with the CNC-g-AA-2-120-8 aerogels affected
TheinitialMBconcentrationandthecontacttimewiththeCNC-g-AA-2-120-8aerogelsaffected
tthhee aaddssoorrppttiioonn ccaappaacciittyy aanndd rreemmoovvaall eeffffiicciieennccyy.. TThhee eexxppeerriimmeennttss wweerree ppeerrffoorrmmeedd aatt 2255 ◦°CC,, aatt ppHH == 88,,
with different initial concentrations (20–100 mg/L) and different contact times (20–450 min). The
withdifferentinitialconcentrations(20–100mg/L)anddifferentcontacttimes(20–450min).Theresults
results are shown in Figure 6c. The adsorption capacity clearly increased from 96.98 to 415.13 mg/g
areshowninFigure6c. Theadsorptioncapacityclearlyincreasedfrom96.98to415.13mg/gasthe
as the initial MB concentration was increased from 20 to 100 mg/L. This result was attributed to the
initialMBconcentrationwasincreasedfrom20to100mg/L.ThisresultwasattributedtotheinitialMB
initial MB concentration providing the necessary driving force to surmount the resistance between
concentrationprovidingthenecessarydrivingforcetosurmounttheresistancebetweentheaqueous
the aqueous and solid phases and the increase of the initial MB concentration improved interactions
andsolidphasesandtheincreaseoftheinitialMBconcentrationimprovedinteractionsbetweendye
between dye molecules and adsorbents, which provided heterogeneous adsorption sites [48,49].
moleculesandadsorbents,whichprovidedheterogeneousadsorptionsites[48,49].
The effects of solution temperature on MB removal by CNC-g-AA-2-120-8 aerogels are shown
TheeffectsofsolutiontemperatureonMBremovalbyCNC-g-AA-2-120-8aerogelsareshown
in Figure 6d. The MB adsorption capacity of the CNC-g-AA-2-120-8 aerogels decreased slightly with
inFigure6d. TheMBadsorptioncapacityoftheCNC-g-AA-2-120-8aerogelsdecreasedslightlywith
iinnccrreeaassiinngg tteemmppeerraattuurree,, ffrroomm 1155 ttoo 5555 ◦°CC aatt tthhee ssaammee iinniittiiaall MMBB ccoonncceennttrraattiioonn.. TThhee MMBB aaddssoorrppttiioonn
capacity slightly decreased from 97.74 to 96.57 mg/g as the solution temperature was increased from
capacityslightlydecreasedfrom97.74to96.57mg/gasthesolutiontemperaturewasincreasedfrom
1155 ttoo 5555 ◦°CC aatt aa MMBB ccoonncceennttrraattiioonn ooff 2200 mmgg//gg.. TThhee MMBB aaddssoorrppttiioonn ccaappaacciittyy ddeeccrreeaasseedd ffrroomm 447744..2222 ttoo
467.31 mg/g at a MB concentration of 100 mg/g. The effects of temperature on MB adsorption might
467.31mg/gataMBconcentrationof100mg/g. TheeffectsoftemperatureonMBadsorptionmight
be attributed to the exothermic nature of the adsorption reaction. [50]
beattributedtotheexothermicnatureoftheadsorptionreaction.[50]

FFiigguurree 66.. ((aa)) EEffffeeccttss ooff ssoolluuttiioonn ppHH oonn MMBB rreemmoovvaall,, ((bb)) eeffffeeccttss ooff aaddssoorrbbeenntt ddoossaaggeess oonn MMBB rreemmoovvaall,,
((cc)) eeffffeeccttsso offc ocnotnatcatctti mtiemaen danindi tiinalitMiaBl MsoBlu tsioolnutcioonnc ecnotnracetinotnraotnioMn Bonad MsoBrp atidosno,r(pdt)ioefnf,e c(tds)o effsfoelcutsti oonf
steomluptieorna ttuermepoenraMtuBrere omno MvaBl aretmdioffvearle antt dinififteiarlenMt Binciotinacl eMntBr actoionnces.ntrations.
Polymers 2018, 10, x FOR PEER REVIEW 9 of 15
3.5.2. Adsorption Kinetics
The effects of the MB initial concentration and adsorption time on the CNC-g-AA-2-120-8
aerogels are shown in Figure 6c. The value of qe increased rapidly during the initial stage of
adsorption and then increased further at a relatively slow adsorption rate, before finally reaching
equilibrium after approximately 260 min when the initial concentration was 100 mg/L. At an initial
concentration of 40 mg/L, the aerogel reached swelling equilibrium (qe) after 100 min, beyond which
there was almost no increase in adsorption. A higher initial concentration of the solution required a
longer equilibrium swelling time. We used a pseudo-first-order and pseudo-second-order equations
to analyze the mechanism of the adsorption process. These can be expressed in linear form as:
ln(q − q ) = lnq −k t, (3)
e t e 1
t 1 t
= (cid:3397) (4)
qt K q(cid:2870) q
(cid:2870) (cid:2915) (cid:2915)
where qe (mg/g) is the adsorption capacity of the CNC-g-AA-2-120-8 aerogels at equilibrium, K1
(min−1) is the rate constant of the pseudo-first-order model, and K2 (g /mg min−1) is the rate constant
ofP otlhyme eprss2e0u1d8,o1-0s,e1c16o8nd-order model. Equations (3) and (4) were used to fit the experimental data9, oafs1 5
shown in Figure 7a,b respectively. The kinetic parameters obtained are summarized in Table 1. For
all CNC-g-AA aerogels, the calculated correlation coefficients (R2) were closer to unity for the
3.5.2. AdsorptionKinetics
pseudo second-order kinetic model than for the pseudo first-order kinetic model. Therefore, these
TheeffectsoftheMBinitialconcentrationandadsorptiontimeontheCNC-g-AA-2-120-8aerogels
results indicated that the dye adsorption of the CNC-g-AA-2-120-8 aerogels was described well by a
areshowninFigure6c. Thevalueofq increasedrapidlyduringtheinitialstageofadsorptionand
pseudo-second-order kinetic model. e
thenincreasedfurtheratarelativelyslowadsorptionrate,beforefinallyreachingequilibriumafter
We used the intra-particle diffusion model Equation (5) to further determine the mechanism of
approximately260minwhentheinitialconcentrationwas100mg/L.Ataninitialconcentrationof
the nanocomposite gels for dye considering the diffusion of dye molecules:
40mg/L,theaerogelreachedswellingequilibrium(q )after100min,beyondwhichtherewasalmost
e
(cid:2869)
noincreaseinadsorption. Ahigherinitialconqce=ntkrai(cid:2870)ti(cid:3397)onCo fthesolutionrequiredalongerequilibr(iu5)m
(cid:2930)
swelling time. We used a pseudo-first-order and pseudo-second-order equations to analyze the
where Ki is the intra-particle diffusion rate constant (mg/g min1/2) and C is a constant related to the
mechanismoftheadsorptionprocess. Thesecanbeexpressedinlinearformas:
extent of the boundary layer effect. As shown in Figure 7c, the kinetic data also agreed well with the
Elovich model, further indicating the heterogeneous chemisorption character of the
ln(q − q ) = lnq −k t, (3)
CNC-g-AA-2-120-8 aerogels. Moreover, eintra-tparticle ediffu1sion of dye molecules played an
important role at the initial stage of adsorpttion, as1 suggetsted by the intra-particle diffusion model
= + (4)
fitting well with our experimental kinetic daqtta. K q2 q
2 e e
where q (mg/g) is the adsorption capacity of the CNC-g-AA-2-120-8 aerogels at equilibrium, K
Teable 1. Adsorption kinetic parameters for MB adsorption of CNC-g-AA-2-120-8 aerogels. 1
(min−1)istherateconstantofthepseudo-first-ordermodel,andK (g/mgmin−1)istherateconstant
2
of the pseudo-second-ordLeargemrgoredne-Fli.rsEt-Oqurdaetri oKninset(i3c )Maonddel (4) werPeseuusdeod-Setoconfidt-Othrdeere xKpineertiicm Meondtea l data,
Ca0s (smhgo/Lw) n inqe,Fexipg (umrge/g7) a,b re𝐪s𝐞p𝟏 ectiveKly1. × T10h−3e (1k/minient) ic parRa2m eters𝐪o𝐞b𝟐t ainedKa2 ×re 10s−u4 (mg/mmga mriizne) d in TRa2 ble 1.
20 97.6388 96.9829 21.95 0.9240 98.1354 19.00 0.9999
ForallCNC-g-AAaerogels, thecalculatedcorrelationcoefficients(R2)wereclosertounityforthe
40 194.8410 194.1255 8.31 0.9489 196.8505 5.50 0.9992
pse6u0 dosecon29d1-.o04r7d0e rkin2e9t0i.c58m00 odelthan9.0f0o rthepse0u.9d59o1 first3-0o2r.1d1e48r kineticmo1.d50e l. Therefo0r.e9,97t2h ese
res8u0l tsindica3t8e3d.01t4h0a tthe32d7y.9e17a3d sorption7.4o3f theCNC0.-9g91-0A A-234-11.22097-08 aerogelsw1.0a0s described0.w99e57ll by
100 473.2217 416.6342 12.42 0.8629 452.4887 0.35 0.9926
apseudo-second-orderkineticmodel.

Figure 7. (a) Lagergren-first-order kinetic model, (b) pseudo-second-order kinetic model and
(c)intra-particlediffusionmodelforadsorptionofMBontoCNC-g-AA-2-120-8aerogelsat25◦C.

Table1.AdsorptionkineticparametersforMBadsorptionofCNC-g-AA-2-120-8aerogels.
Lagergren-First-OrderKineticModel Pseudo-Second-OrderKineticMode
qe,exp
C0(mg/L) (mg/g) q K1×10−3 R2 q K2×10−4 R2
e1 (1/min) e2 (g/mgmin)
20 97.6388 96.9829 21.95 0.9240 98.1354 19.00 0.9999
40 194.8410 194.1255 8.31 0.9489 196.8505 5.50 0.9992
60 291.0470 290.5800 9.00 0.9591 302.1148 1.50 0.9972
80 383.0140 327.9173 7.43 0.9910 341.2970 1.00 0.9957
100 473.2217 416.6342 12.42 0.8629 452.4887 0.35 0.9926
Weusedtheintra-particlediffusionmodelEquation(5)tofurtherdeterminethemechanismof
thenanocompositegelsfordyeconsideringthediffusionofdyemolecules:
1
qt =ki2 +C (5)
Polymers 2018, 10, x FOR PEER REVIEW 10 of 15
Figure 7. (a) Lagergren-first-order kinetic model, (b) pseudo-second-order kinetic model and (c)
intra-particle diffusion model for adsorption of MB onto CNC-g-AA-2-120-8 aerogels at 25 °C.
3.5.3. Adsorption Isotherm and Thermodynamics
Polymers2018,10,1168 10of15
We determined the MB adsorption onto the CNC-g-AA-2-120-8 aerogels as a function of the
equilibrium concentration of dye solution (Ce, mg/L). Langmuir and Freundlich isotherm equations
wewreh eurseedK tois dtehteerinmtirnae-p tahret iicsloethdeifrfmus pioanrarmateetceorsn astnadn tli(nmeagr/ fgittmining 1b/a2)seadn donC thisea twcoon isstoatnhterremla mtedodteolsth e
i
aree xsthenotwonf tihne Fbioguunrde a8rya,lba.y Terheef foebctt.aiAnsedsh ioswotnheinrmFi gpuarream7ce,ttehres kairnee tsicudmamtaaarilzsoeda girne eTdawbleel l2w. iTthheth e
LaEnlgomviuchir misoodtheel,rfmur rtheseurlitnsd siucagtginesgtethde ah seuterrpolguesn oefo uatsocmhesm oirs moroplteiocunlcehs aornac tthere oafdtshoerCbeNnCt -sgu-rAfaAc-e2, -t1h2e0 -8
effaeecrtos goefl sM.MB orreesoivdeura,li nvtarale-pnacretsi cwleedrief fcuosmionpaorfadbylee mtoo tlheceu mlesolpelcauyleadr adniaimmpetoerrt;a nhternoclee, atthteh eLiannitgiamlustiarg e
isootfhaedrmsosr pdteisocnr,ibaesss umgogneosltaeydebr yadthseoripnttiroan-p. aTrhtiec lLeadnigffmusuioirn ismoothdeerlmfi tmtinogdewl eilsl vwailtihd ofuorr emxponeroilmayeenrt al
adksoinrepttiicodna otna.to a surface with a finite number of identical and equivalent sites and can be expressed
in the following linear form:
3.5.3. AdsorptionIsothermandThermodynamics
C 1 C
(cid:2915) (cid:2915)
= (cid:3397) (6)
We determined the MB adsorption qontoqthekCNCq-g-AA-2-120-8 aerogels as a function of the
(cid:2915) (cid:2923) (cid:2896) (cid:2923)
equilibriumconcentrationofdyesolution(C ,mg/L).LangmuirandFreundlichisothermequations
where qm is the maximum dye adsorption ontoe adsorbent (mg/g) and KL is the Langmuir adsorption
wereusedtodeterminetheisothermparametersandlinearfittingbasedonthetwoisothermmodels
equilibrium constant (L/g). The Freundlich isotherm model is an empirical equation that is regularly
areshowninFigure8a,b. TheobtainedisothermparametersaresummarizedinTable2. TheLangmuir
applied to multilayer adsorption, with a non-uniform distribution of adsorption heat and affinities
isothermresultssuggestedasurplusofatomsormoleculesontheadsorbentsurface,theeffectsof
over the heterogeneous surface of absorbent. It can be expressed in the following linear form:
MBresidualvalenceswerecomparabletothemoleculardiameter; hence,theLangmuirisotherms
1
describesmonolayeradsorption. TheLangmuirisothermmodelisvalidformonolayeradsorption
L q =L k (cid:3397) L C (7)
(cid:2924) (cid:2915) (cid:2924) (cid:2890) n (cid:2924) (cid:2915)
onto a surface with a finite number of identical and equivalent sites and can be expressed in the
followinglinearform:
Table 2. Adsorption isotherm paramCeete=rs for1 MB+ adCsoerption of CNC-g-AA aerogels. (6)
q q k q
Langmuir e m L m Frendlich
whereq isthemaximumdyeadsorptionontoadsorbent(mg/g)andK istheLangmuiradsorption
m T L
equilibriumco(n℃st)a nt(KLL/ (gL)/.mTgh)e Frqemuanx (dmlicgh/gi)s otheRrm2 modelKisF anempnir icalequRa2 tionthatisregularly
appliedtomultilayeradsorption,withanon-uniformdistributionofadsorptionheatandaffinities
15 0.3467 724.6377 0.9903 169.8320 1.5524 0.9731
overtheheterogeneoussurfaceofabsorbent. Itcanbeexpressedinthefollowinglinearform:
25 0.3200 735.2941 0.9914 176.3358 1.5596 0.9635
35 0.2773 757.5758 0.9925 148.2559 1.5818 0.9731
1
45 0.2628 751.L87n9q7e =L0n.9k9F1+2 n1L5n3C.2e657 1.5139 0.9771 (7)
55 0.2681 729.9270 0.9980 150.1148 1.4996 0.9823
FigFuigreu r8e. (8a.)( La)anLgamngumiru ainrdan (db)( bFr)eFurneudnlidchli cishoitshoetrhmersm fosrf othret haedsaodrspotripotnio onf MofBM bBy bCyNCCN-gC--AgA-A-2A--122-01-280 -8
aeraoegroelgse alst adtifdfeifrfeenret ntetmtepmepraetruartuesr.e s.
Three therTmaboledy2.nAamdsiocr pptaioranmiseottheresrm mpuasrta mbee tceorsnfsoidrMerBeda ddsuorrpintigo nstoufdCiNesC o-gf -aAdAsoareprotgioenls .processes,
namely, enthalpy (∆H), and entropy (∆S). The values of ∆H and ∆S can be obtained by the van ’t Hoff
Langmuir Frendlich
equation as follows:
T(◦C) KL(L/mg) qmax(mg/g) R2 KF n R2
15 0.3467 724.6377 0.9903 169.8320 1.5524 0.9731
25 0.3200 735.2941 0.9914 176.3358 1.5596 0.9635
35 0.2773 757.5758 0.9925 148.2559 1.5818 0.9731
45 0.2628 751.8797 0.9912 153.2657 1.5139 0.9771
55 0.2681 729.9270 0.9980 150.1148 1.4996 0.9823