Adsorption behavior of copper ions on graphene oxide–chitosan aerogel

Adsorption behavior of copper ions on graphene oxide–chitosan aerogelBaowei Yu a,Jing Xu a,Jia-Hui Liu b,c,Sheng-Tao Yang a,*,Jianbin Luo a,Qinghan Zhou a,Jing Wan a,Rong Liao a,Haifang Wang b,**,Yuanfang Liu b,ca College of Chemistry and Environment Protection Engineering,Southwest University for Nationalities,Chengdu610041,Chinab Institute of Nanochemistry and Nanobiology,Shanghai University,Shanghai200444,Chinac Beijing National Laboratory for Molecular Sciences,College of Chemistry and Molecular Engineering,Peking University,Beijing100871,ChinaIntroductionGraphene adsorbents have attracted great research interestrecently due to the fantastic properties and potential in watertreatment[1,2].For the applications in water purification,graphene and its derivatives possess remarkable inherent advan-tages.Graphene is of single-layer structure,enabling that all atomsare surface atoms.Thus,the available surface area of graphene iseven higher than that of carbon nanotubes(CNTs).The adsorptionkinetics on graphene is generally faster than on the traditionaladsorbents.The producing cost of graphene adsorbents is lowerthan other high-performance adsorbents,such as CNTs and resins,while the adsorption capacities are comparable.In addition,graphene adsorbents can treat multiple pollutants at same time.Graphene shows high adsorption capacity for dyes,organicpollutants,heavy metals,and so on[3–6].In recent years,theadsorption capacity and behaviors of graphene adsorbents havebeen investigated on different pollutant models[1–8].Among the graphene adsorbents,graphene oxide(GO)showsthe highest performance in treating heavy metal ions and cationicdyes[9–12].However,the separation of GO after adsorbingpollutants is very difficult.GO disperses very well in water,thatrequires high speed centrifugation to precipitate.Obviously,thehigh speed centrifugation is unpractical for water treatment,dueto the large quantity of waste water.A possible solution is theattachment of magnetic nanoparticles to GO,and the separationcould be achieved by magnetic separation[13–15].But suchmethod will increase the producing cost and reduce the adsorptioncapacity significantly.Thus,we need to explore new technology tofacilitate the separation of GO after adsorption while keepingsimultaneously the high adsorption capacity.Another drawback of GO adsorbents is that GO might loss thesurface area during the drying.The reduced surface area leads tothe decrease of adsorption capacity.Many studies have shown thatthe over stacking of graphene sheets inhibited the adsorption ofpollutants on GO adsorbents[16–18].To avoid the excess stacking,new formulations of GO adsorbents are in need of development.The most promising formulations are aerogel and sponge[19–22].Herein,we reported the fabrication of GO–chitosan(CS)aerogelby lyophilization for the removal of Cu2+,where the solid GO–CSaerogel could be easily separated from environment afteradsorbing pollutants.GO was aggregated by CS to facilitate theseparation,and the composite was formulated as aerogel to avoidover stacking of GO sheets.GO–CS aerogel was found to be a goodadsorbent of Cu2+with a maximum adsorption capacity of2.54Â101mg/g.The adsorption kinetics was slow and could bewell described by the pseudo-second-order model.The adsorptioncapacity of GO–CS was larger at higher pH,lower ionic strengthJournal of Environmental Chemical Engineering1(2013)1044–1050A R T I C L E I N F OArticle history:Received29June2013Received in revised form6August2013Accepted12August2013Keywords:Graphene oxideChitosanAerogelAdsorptionHeavy metalA B S T R A C TGraphene oxide(GO)–chitosan(CS)composite was lyophilized to prepare GO–CS aerogel for Cu2+removal,then the separation of adsorbents after adsorption was easily achieved byfiltration or lowspeed centrifugation.GO–CS was a good adsorbent of Cu2+with a large adsorption capacity of2.54Â101mg/g according to the Langmuir model.The adsorption kinetics was well described by thepseudo-second-order model with a k2of4.14Â10À3minÀ1.The intraparticle diffusion model wasadopted to reveal the diffusion mechanism.Higher pH,lower ionic strength and higher temperaturebenefited the adsorption.The thermodynamics parameters at303K were calculated as D G ofÀ3.89kJ/mol,D H of3.46kJ/mol and D S of2.42Â101J/mol/K.The adsorption identity was physisorption andapparently driven by the increase of randomness.The implications to the application of grapheneadsorbents in the decontamination of heavy metals are discussed.ß2013Elsevier Ltd.All rights reserved.*Corresponding author.Tel.:+862885522269.**Corresponding author.Tel.:+862166138026.E-mail addresses:yangst@(S.-T.Yang),hwang@(H.Wang).Contents lists available at ScienceDirectJournal of Environmental Chemical Engineeringj ou r n a l h o me p a g e:w w w.e l se v i e r.co m/l oc a t e/j e c e2213-3437/$–see front matterß2013Elsevier Ltd.All rights reserved./10.1016/j.jece.2013.08.017and higher temperature.The thermodynamics study indicated that the adsorption was endothermic and apparently driven by the increase of randomness.The implications to the application of graphene adsorbents in the decontamination of heavy metals are discussed.Materials and methodsMaterialsGraphite,CS and CuSO4Á5H2O were purchased from Sinopharm Chemical Reagent Co.,Ltd.,China.Other chemicals were of analytical grade.GO(Fig.S1)was prepared following the modified Hummers method[23–25].To prepare GO–CS aerogel,GO was dispersed in deionized water(0.5mg/mL,pH6.0,200mL).Then,CS (1.0mg/mL,pH2.0,10mL)was added dropwise under vigorous stirring.The precipitation of brown composite was observed and the mixture was further stirred for another1h.The composite was washed with deionized water for three times and then lyophilized for3d(LGJ-10C,Xiangyi Co.,China).Separately,we also prepared the GO–CS*(higher CS content)sample by increasing the CS volume to20mL.The GO aerogel was obtained by direct lyophilization of GO suspension.GO–CS aerogel was characterized by transmission electron microscopy(TEM,JEM-200CX,JEOL,Japan),X-ray photoelectron spectroscopy(XPS,Kratos,UK)and infrared spectrometer(IR, Magna-IR750,Nicolet,USA).The surface area of GO–CS was determined by methylene blue method.Isothermal adsorptionTo investigate the isothermal adsorption,the solid aerogel was shaken in pollutant solution,which was widely used for graphene sponge,aerogel and hydrogel[16,18,22].We adopted24h to ensure the equilibrium,because the kinetics studies showed that 8h was enough to reach the equilibrium.Typically,8mL of Cu2+ (pH 6.0, 1.92–32.0mg/L)was added to5mg of solid GO–CS aerogel.The mixture was shaken(100rpm)at303K for24h on a thermostat shaker(CHA-S,Jintan Hankang Electronic Co.,China). The GO–CS aerogel stayed solid during the adsorption.And then the supernatants were collected after the centrifugation at 3000rpm for2min(TDZ4-WS,Pingfan Instrument and Menter Co.,China).The Cu2+concentration in the supernatant was determined by acetaldehyde-bis(cyclohexanone)oxaldihydrazone (BCO)method on a UV-Vis spectrometer(V1800,Mapada Instrument Co.,China).The details of BCO method were described in our previous report[17].Data were presented as mean Æstandard deviation(meanÆSD).Except mentioned specifically, the following adsorption experiments were performed following the same protocol.The equilibrium concentration(C e)was calculated referring to the calibration curve of Cu2+(Fig.S2).The equilibrium adsorption capacity(q e)was calculated by(C0ÀC e)/C GO–CS,where C0was the initial concentration of Cu2+and C GO–CS was the concentration of GO–CS.The adsorption data werefitted to both the Langmuir model(Eq.(1))and the Freundlich model(Eq.(2)).The maximum adsorption capacity(q m)was obtained from the Langmuir model.1 q e ¼1q mþ1bq m C e(1)ln q e¼ln K Fþ1ln C e(2)To compare the GO–CS with GO suspension,GO aerogel and GO–CS*aerogel(higher CS content),we performed the adsorptionexperiment at Cu2+concentration of19.2mg/L following the sameprotocol.The adsorption capacity of GO–CS,GO–CS*,GO suspen-sion and GO aerogel were evaluated.For GO suspension,thecentrifugation was performed at12,000rpm.Adsorption kineticsA mixture of GO–CS(5mg)and8mL of Cu2+(19.2mg/L,pH6.0)was incubated at303K for different time intervals(30–720min)and then centrifuged.The concentration of Cu2+(C t)at differenttime point was taken for calculating the adsorption capacity(q t)by(C0ÀC t)/C GO.The adsorption data werefitted by the pseudo-first-order model(Eq.(3)),the pseudo-second-order model(Eq.(4))andthe intraparticle diffusion model(Eq.(5)).lnðq eÀq tÞ¼ln q eÀk1t(3)tq t¼1k2q2eþtq e(4)q t¼k i t1=2þC(5)Influence of pH and ionic strengthTo investigate the influence of pH on the absorption,the initialpH of Cu2+was adjusted to2–7using NaOH or HCl aqueoussolutions.We did not evaluated the adsorption under pH higherthan7,because Cu2+precipitates at a basic medium.At each initialpH,GO–CS(5mg)was mixed with8mL of Cu2+(19.2mg/L)for thecalculation of q e.To investigate the influence of ionic strength on the adsorption,GO–CS(5mg)was pre-mixed with NaCl solution(0–3.2mL,pH6.0,250mM),then3.07mL of Cu2+(pH6.0,50mg/L)was added,and the total volume was adjusted to8.0mL by adding water.Themixture was treated as described above for the calculation of q e.Adsorption thermodynamicsA mixture of GO–CS(5mg)and8mL of Cu2+(19.2mg/L,pH6.0)was incubated at different temperature(273–323K)for24h andthen centrifuged.The concentration of Cu2+(C e)in supernatant atdifferent temperature was taken for calculating the adsorptioncapacity(q e)by(C0ÀC e)/C GO.The adsorption data werefitted byEq.(6).The distribution coefficient K d was calculated as q e/C e.TheGibbs free energy(D G)was calculated accordingly.ln K d¼D HRTþD SR(6)Results and discussionCharacterization of GO–CS aerogelAs we discussed previously,GO–CS was formed by the non-covalent interaction between GO and CS[7].The fabrication of GOsheets with CS obviously resulted in a loosely packed structure ofGO–CS.During the lyophilization,the water molecules weresublimated to form the GO–CS aerogel.Lyophilization kept theporous structure of graphene,thus,produced aerogel[16,22].Thesurface area of GO–CS was345m2/g.The porous GO–CS aerogelwas not dispersible in water,thus could be separated byfiltrationB.Yu et al./Journal of Environmental Chemical Engineering1(2013)1044–10501045easily.The GO–CS sample could be removed from environment by filtration or low speed centrifugation.This would enable the facile applications of GO–CS in water treatment.It should be noted that the easy separation was a nature of solids,rather than the unique characteristics of GO–CS aerogel.To investigate the structure,we grinded GO–CS aerogel and then sonicated it in water for 1h.The small particulates were characterized under TEM.As shown inFig.1a,serious folding and stacking occurred in GO–CS.In contrast,free GO sheets only showed small wrinkles,and were generally flat (Figs.1c and S1).The majority of GO–CS were oxygen (32.0wt%)and carbon (61.4wt%)according to XPS.The nitrogen content of GO–CS was 0.379wt%,corresponding to about 5.1wt%of CS in GO–CS aerogel (see Supplementary data).In the IR spectrum,the carboxyl andFig.1.TEM images of GO–CS (a)and (b)and free GO (c);IR spectrum of GO–CS (d).Fig.2.Adsorption of Cu 2+on GO–CS at 303K.(a)Adsorption isotherm;(b)Langmuir model;(c)Freundlich model;and (d)adsorption of Cu 2+on different graphene adsorbents at 303K.B.Yu et al./Journal of Environmental Chemical Engineering 1(2013)1044–10501046hydroxyl groups were evidenced.The–COOH/–OH groups were indicated by the peak at3410cmÀ1(Fig.1b).The presence of C55O was indicated by the peak at1731cmÀ1.However,the typical glucopyranose rings peaks at1151cmÀ1and897cmÀ1were not observed in GO–CS,which might be due to the low CS content[7].Adsorption isothermThe adsorption isotherm of Cu2+on GO–CS aerogel was shown in Fig.2a.When the C e values reached at9mg/L,the adsorption capacities(q e)increased to peak,and then the q e values became constant from9to1.5Â101mg/L.At C e of1.48Â101mg/L,the q e value was2.39Â101mg/g.The adsorption data of Cu2+on GO–CS were wellfitted by the Langmuir model with a correlation coefficient R of0.993(Fig.2b). The maximum adsorption capacity(q m)was2.54Â101mg/g,close to the experimental number.The b value was0.273,indicating that the adsorption strength was not very strong.We alsofitted the data with the Freundlich model(R=0.989).The K F value was4.90mg/g (L/mg)1/n and n value was1.49.The small n value indicated the weak adsorption strength,consistent with the small b value of the Langmuir model.The1/n value was6.724Â10À1,indicating the surface was not very homogenous.We also compared the adsorption capacity of GO–CS aerogel with GO suspension,GO aerogel and GO–CS*(CS content was higher,namely6.1wt%)directly.As shown in Fig.2d,at initial concentration of 1.92Â101mg/L,GO suspension showed the highest adsorption capacity of4.07Â101mg/g.The adsorption capacity of GO suspension was consistent with our previous report [17],but higher than other studies[16,26].We attributed the difference to the different samples(although all were claimed as GO)[27].Nevertheless,CS occupied some adsorption cites of GO and also the GO sheets stacked more or less during the lyophilization.Thus,a decrease was observed when GO was converted into GO–CS aerogel(2.16Â101mg/g).Increasing the CS content led to the decrease of adsorption capacity,as GO–CS*had the lowest adsorption capacity of1.42Â101mg/g.Although twice of CS solution was used during the preparation of GO–CS*,most excess CS was washed out according to the XPS analyses.We were surprised that the tiny increase of CS content in GO–CS*led to such big decrease of adsorption capacity.A possible explanation might be that the higher CS concentration during the preparation induced more serious folding of GO sheets,which blocked more adsorption cites.Once the folding was accomplished,the remaining CS was capable to keep the conformation of the folding sheets.Interest-ingly,GO–CS aerogel showed higher adsorption capacity than GO aerogel(1.49Â101mg/g).This might be attributed to the porous structure of GO–CS,where CS behaved as the support to avoid overFig.3.Kinetics analyses of the adsorption of Cu2+on GO–CS at303K.(a)Adsorption as a function of contact time;(b)the pseudo-first-order model;(c)the pseudo-second-order model;and(d)intraparticle diffusion model.Table1Maximum adsorption capacity(q m)of various adsorbents for Cu2+.Adsorbent q m(mg/g)Ref.Waste beer yeast 1.45[26]Active carbon 5.08[27]Activated sludge19.06[28]Lignin22.88[29]CNTs28.5[30]GO aerogel19.65[16]GO46.6[17]GO–CS aerogel25.4This studyB.Yu et al./Journal of Environmental Chemical Engineering1(2013)1044–10501047stacking of GO sheets during the lyophilization.GO suspension could hardly be used in practical applications.Aerogel or sponge should be the promising form of graphene in treating contami-nants.To this regard,the fabrication of GO–CS composite in the aerogel or sponge form was necessary in order to enlarge the adsorption capacity.In addition,GO–CS aerogel was very stable in water,whilst GO aerogel dissolved partially upon vigorous stirring.We also compared the q m of GO–CS with other adsorbents reported in the literature.The comparison was listed in Table 1.GO–CS aerogel could be ranked among the very effective adsorbents of Cu 2+,comparable to activated sludge,lignin,CNTs and other graphene adsorbents [16,17,28–32].Adsorption kineticsThe adsorption kinetics of Cu 2+onto GO–CS was shown in Fig.3a.The adsorption capacities increased fast in the first 5h,and then became slower.Pseudo-first-order and pseudo-second-order models were adopted to analyze the kinetics data (Fig.3b).The q e calculated from the pseudo-first-order model was 3.51mg/g,much lower than the experimental q e (2.16Â101mg/g).The R value was 0.948.The low q e,cal and small R suggested that the pseudo-first-order model was not ideal for describing the adsorption process of Cu 2+on GO–CS.This was consistent with the results in the literature,where pseudo-first-order model wasFig.4.Influence of pH (a)and ionic strength (b)on the adsorption of Cu 2+on GO–CS.Table 2Coefficients of the pseudo-first and second-order adsorption kinetic models and the intraparticle diffusion model.Pseudo-first-order model q e,exp (mg/g)k 1(min )q e,cal (mg/g)R2.16Â101 1.86Â10À33.510.948Pseudo-second-order modelq e,exp (mg/g)k 2(min À1)q e,cal (mg/g)R2.16Â1014.14Â10À3 2.08Â1010.9998Intraparticle diffusion model (stage 1)k i (mg/g min À0.5)C (mg/g)R5.69Â10À2 1.79Â1010.983Intraparticle diffusion model (stage 2)k i (mg/g min À0.5)C (mg/g)R3.00Â10À1 1.48Â1010.997Intraparticle diffusion model (stage 3)k i (mg/g min À0.5)C (mg/g)R4.28Â10À21.93Â1010.963Fig.5.Influence of temperature on the adsorption of Cu 2+on GO–CS.(a)Adsorption capacity at different temperature and (b)thermodynamics analysis.B.Yu et al./Journal of Environmental Chemical Engineering 1(2013)1044–10501048not applicable in describing the adsorption kinetics of pollutants on graphene adsorbents.In contrast,the pseudo-second-order model was much better(R=0.9998,Fig.3c).The experimental (2.16Â101mg/g)and calculated(2.08Â101mg/g)q e values showed very good consistence.The small rate constant of adsorption(k2) 4.14Â10À3minÀ1was compatible with the relatively slow adsorption process.The parameters of both models were listed in Table2.The intraparticle diffusion model was adopted to reveal the diffusion mechanism(Fig.3d).The data werefitted with linear regression(R varied in0.963–0.997).The lines in the plot did not pass through the origin,so the adsorption involved both the intraparticle diffusion and surface diffusion.According to Table2,clearly,there were three stages in the adsorption.In thefirst stage,the C value was1.79Â101mg/g, indicating the presence of the boundary layer diffusion effect.The C value decreased to1.48Â101mg/g in the second stage,which indicated the diffusion was easier and resulted in slightly faster adsorption.Finally,the C value increased again to1.93Â101mg/g. The capacity of GO–CS was exhausted and the uptake rate was more controlled mainly by the rate at which Cu2+was transported from the exterior to the interior sites of GO–CS.Correspondingly, the adsorption was slowed down.The existence of multiple diffusion stages has been reported in graphene adsorbents[33]. However,it was not observed in GO suspension,because the kinetics was so fast that we failed to analyze the kinetics in our previous studies[11,17].The multiple stages of GO–CS indicated that the diffusion of Cu2+toward GO–CS changed during the adsorption.The adsorption kinetics of Cu2+on GO–CS was much slower than that on GO.This was reasonable that additional diffusion was required for Cu2+to reach the surface of graphene sheets in GO–CS, since GO–CS was not dispersible in water.In contrast,free GO suspension was single layered(Fig.S1).Thus,the diffusion of Cu2+ toward GO proceeded immediately.Therefore,the adsorption of Cu2+on GO reached the equilibrium within20min[17].The formation of GO3D structure(aerogel or hydrogel)also slowed the adsorption kinetics,as reported by other groups[16,22].Anyhow, the adsorption kinetics of Cu2+on GO–CS was still quick enough for practical application[34].Influence of pH and ionic strengthThe pH regulated the ionization of carboxyl groups.It therefore had significant influence on the adsorption performance of GO–CS. GO–CS adsorbed more Cu2+at higher pH values(Fig.4a).At Cu2+ concentration of1.92Â101mg/L,the adsorption capacity increased from2.30Â101mg/g at pH6.0to2.65Â101mg/g at pH7.We could not increase the pH to basic range,because Cu2+would precipitate. The value decreased to2.01Â101mg/g at pH2.The pH regulated adsorption was similar to that of GO,where higher adsorption capacity for methylene blue was observed at higher pH[11].Positively charged ions might adsorb on GO competitively, resulting in the decrease of adsorption capacity[11].In addition,the high ionic strength might inhibit the ionization of GO,which also led to the decrease of adsorption capacity.In this study,we found that the increase of ionic strength inhibited the adsorption capacity of GO–CS(Fig.4b).At Cu2+concentration of 1.92Â101mg/L,the adsorption decreased to1.34Â101mg/g at100mM Na+.Adsorption thermodynamicsThe influence of temperature was also concerned.As shown in Fig.5,the adsorption was promoted at higher temperature.This implied that the adsorption was endothermic.The adsorption thermodynamics parameters were calculated.At303K,the D G value wasÀ3.89kJ/mol.The negative D G suggested that the adsorption was spontaneous.The higher temperature makes the lower D G,implying that the adsorption became more favorable at higher temperature.Further,the D G was very small,manifesting the weak adsorption strength,that was consistent with the small b value in Langmuir model and small n value in Freundlich model.The D H value was 3.46kJ/mol, indicating that the adsorption was endothermic.The positive D S value(2.42Â101J/mol/K)suggested the adsorption increased the randomness at the solid/solution interface.ConclusionsGO–CS aerogel was prepared by lyophilization for Cu2+removal, and the separation of adsorbents after adsorption was easily achieved.GO–CS was good adsorbent of Cu2+with a large adsorption capacity of2.54Â101mg/g and relative weak binding strength.The adsorption kinetics was slower than that of GO and could be described by the pseudo-second-order model.Higher pH, lower ionic strength and higher temperature benefited the adsorption of Cu2+on GO–CS.The physisorption of Cu2+on GO–CS was endothermic and mainly driven by the increase of randomness.Hopefully,our results would benefit the ongoing research of graphene adsorbent.AcknowledgmentsWe thank Prof.Y.-P.Sun at Clemson University for his kind help. 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