微波制备改性的三乙烯四胺氧化石墨烯

Carbohydrate Polymers 131(2015)280–287Contents lists available at ScienceDirectCarbohydratePolymersj o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /c a r b p olMicrowave preparation of triethylenetetramine modified graphene oxide/chitosan composite for adsorption of Cr(VI)Huacai Ge ∗,Ziwei MaCollege of Chemistry and Chemical Engineering,South China University of Technology,Guangzhou 510640,Chinaa r t i c l ei n f oArticle history:Received 13February 2015Received in revised form 4June 2015Accepted 6June 2015Available online 16June 2015Keywords:Graphene oxide Modified chitosan Microwave Adsorption Cr(VI)a b s t r a c tA novel triethylenetetramine modified graphene oxide/chitosan composite (TGOCS)was successfully synthesized by microwave irradiation (MW)method and compared with one prepared by conventional heating.This composite was characterized by FTIR,XRD,SEM,BET and elemental analysis.Adsorption of Cr(VI)on the composite was studied.The experimental results indicated that the product obtained by MW had higher yield and uptake than one obtained by the conventional and uptake of TGOCS for Cr(VI)was higher than that of the recently reported adsorbents.The effects of various variables on adsorption of Cr(VI)by TGOCS were further researched.The highest adsorption capacity of 219.5mg g −1was obtained at pH 2.Adsorption followed pseudo-second-order kinetic model and Langmuir isotherm.The capacity increased as increasing temperature.The adsorbent could be recyclable.These results have important implications for the application expansion of microwave preparation and the design of new effective composites for Cr(VI)removal in effluents.©2015Elsevier Ltd.All rights reserved.1.IntroductionChromium (VI)(Cr(VI))has been commonly used in a number of industrial processes,such as leather tanning,electroplating,metal polishing,paint manufacturing,and textile coloring (Bhattacharya,Naiya,Mandal,&Das,2008;Li et al.,2013;Ouaissa,Chabani,Amrane,&Bensmaili,2013).Due to its high toxicity and bioac-cumulation,the Cr(VI)from effluents must be removed.Various methods of removing Cr(VI)have been developed,such as chem-ical precipitation (Carlos,Violeta,&Bryan,2012),adsorption (Hu et al.,2011;Huang,Yang,&Liu,2013),electrodeposition (Golder,Samanta,&Ray,2011),membrane systems (Gherasim &Bourceanu,2013),and ion exchange process (Rengaraj,Joo,Kim,&Yi,2003).Among these methods,adsorption is one of the most economically favorable and a technically easy method (Hu et al.,2011).Chitosan (CS),a bio-adsorber,is a biocompatible polysaccha-ride obtained from deacetylation of chitin (Ge &Wang,2014).It can chemically or physically entrap various metal ions due to the presence of amine and hydroxyl groups that can serve as the chelating and reaction sites (Aydın &Aksoy,2009;Ge &Fan,2011;Repo,Koivula,Harjula,&Sillanpää,2013;Wang &Ge,2015).Therefore,chitosan presents as a very promising starting mate-rial for chelating resins (Kandile &Nasr,2009).Several metals are∗Corresponding author.Tel.:+862087112900;fax:+862022236337.E-mail address:chhcge@ (H.Ge).preferentially adsorbed in acidic media while chitosan can dis-solve in acid condition.To overcome this problem,chitosan must be chemically modified with different crosslinking reagents,such as epichlorohydrin and glutaraldehyde (Ge &Huang,2010;Ngah,Endud,&Mayanar,2002).However,the adsorption capacity of crosslinked chitosan would be largely reduced due to the con-sumption of amine groups and hydroxyl groups after chemical modification.Hence,the crosslinked chitosan must be further mod-ified to improve the adsorption performance (Ge,Chen,&Huang,2012;Wu,Li,Wan,&Wang,2012;Zhang,Xia,Liu,&Zhang,2015).Graphene,which can be prepared from the low cost material graphite,is intensively investigated as adsorbents for heavy metal ions (Chowdhury &Balasubramanian,2014;Jabeen et al.,2011).Graphene oxide (GO)obtained by the oxidation of graphene con-tains a wide range of oxygen functional groups both on the basal planes and at the edges of GO sheets,such as –COOH,and –OH.These functional groups are essential for the high sorption of heavy metal ions,and allows GO to participate in a wide range of bond-ing interactions (Guo et al.,2014;Zhang et al.,2014).However,GO is a nano-material with high dispersibility in aqueous solution (Cheng et al.,2013)and has the potential toxicity in environment (Sanchez,Jachak,Hurt,&Kane,2011).These problems may restrict the practical applications of GO as an adsorbent.To overcome these problems,various methods have been investigated,such as for-mation of ethylenediamine modified GO and magnetic graphene nanocomposites (Wang et al.,2014;Zhu et al.,2011).However,these methods revealed low adsorption capacity due to the reduced/10.1016/j.carbpol.2015.06.0250144-8617/©2015Elsevier Ltd.All rights reserved.H.Ge,Z.Ma /Carbohydrate Polymers 131(2015)280–287281adsorption area and oxygen-containing functional groups.Hence,the GO functionalized with magnetic cyclodextrin–chitosan was studied (Li et al.,2013).Recently,microwave irradiation (MW)as a means of chem-ical reaction has been widely applied in polymer synthesis due to higher conversion and shorter reaction times under MW than those of conventional heating (Ge &Luo,2005;Ge,Pang,&Luo,2006;Ge et al.,2012).In this work,microwave technology has been used to prepare a new chitosan-based composite.This work is to serve as not only an expansion of microwave irradiation in applica-tion of polymer synthesis,but also an expansion of chitosan-based nanocomposites as a high-efficient adsorbent for Cr(VI)removal.Considering that amine groups in triethylenetetramine could sig-nificantly enhance the adsorption ability,the triethylenetetramine modified graphene oxide/chitosan composite (TGOCS)was pre-pared by MW and compared with one prepared by conventional heating.The adsorption of TGOCS for Cr(VI)was systematically studied.2.Materials and methods2.1.MaterialsBiochemical reagent grade chitosan (degree of deacetyla-tion:90%,viscosity average molecular weight:400,000),standard reagent potassium dichromate,analytical grade triethylenete-tramine and graphite were purchased from China National Medicine Corporation Ltd.(Shanghai,China).All other agents were used on analytical grade and all solutions were prepared with dis-tilled water.2.2.PreparationGO was prepared from graphite powder by modified Hum-mers method (Chen,Chen,Bai,&Li,2013;Hummers &Offeman,1958).Graphite (1g),NaNO 3(0.5g)and KMnO 4(3g)were sequen-tially added into the stirred concentrated H 2SO 4solution (23mL)at 277K.After kept at below 293K for 1h,the mixture was vigorously stirred at 308K for 30min and slowly diluted with distilled water (46mL).The reaction temperature was rapidly increased to 371K and kept for 30min.Then,an additional 140mL of water was added,followed by a slow addition of 30%H 2O 2(15mL),turning the color of the solution from brown to yellow.The mixture was purified by centrifuging with 5%HCl and distilled water.The resulting solid (GO)was dispersed in distilled water by ultrasonic treatment for 3h.4g L −1GO aqueous dispersion was prepared by diluting with water and stored for the following preparation.Microwave preparation of TGOCS was done in a modified microwave oven equipped with a stirrer and constant-temperaturecirculating water (Ge &Huang,2010).Triethylenetetramine (5mL)was added to 4g L −1GO aqueous dispersion (50mL)under stirring at room temperature and the pH of the solution was adjusted to 8by adding dilute NaOH solution.The mixture was transferred to the microwave reaction system and was radiated for 15min at 343K under stirring.Then,epichlorohydrin (5mL)and CS (1g)were suc-cessively added and the mixture was radiated for 45min at 343K under stirring.After reaction,the mixture was cooled and filtered.The filter residue was successively washed with 5%HCl,ethanol and distilled water,and dried in vacuum at 333K.(1.37±0.04)g of com-posite was obtained and named as TGOCS.The probable prepared routes were given in Fig.1.As comparison (1.21±0.04)g of composite was prepared by conventional heating and named as C TGOCS.Except for using oil-bath heating instead microwave radiation,all the conditions and processes of the preparation were same as the above microwave preparation.2.3.CharacterizationFourier-transform infrared (FTIR)spectra were recorded on a Bruker FTIR spectrometer (Tensor 27,Germany)using KBr pellets.The morphology was examined by scanning electron microscope (SEM)(LEO 1530VP,Germany).The surface of the sample was coated with gold to be observed and photographed.X-ray diffrac-tion (XRD)patterns were determined with a Rigaku diffractometer (D/max-IIIA,Japan).The specific surface area was measured by N 2adsorption at 77.15K using a Micromeritics surface analyzer (Tri-star 3000,USA).Elemental analysis was done on a Bruker element analyzer (Vario ELCube,Germany).2.4.Adsorption experimentsThe adsorption of Cr(VI)was done using batch method.Batch adsorption experiments were conducted by placing 20mg of adsor-bent with 50mL of Cr(VI)aqueous solutions (200mg L −1)at pH 2in 250mL conical flasks.The flasks were agitated at 180rpm using a mechanical rotary shaker at 303K for 2h to reach adsorption equilibrium.The concentration of Cr(VI)in the solution was deter-mined spectrophotometrically at 540nm using diphenyl carbazide as the complex agent.The adsorption capacities were calculated as follows:q e =(c 0−c e )Vm(1)where q e is the equilibrium adsorption capacity (mg g −1),c 0and c e are the initial and equilibrium concentration of Cr(VI)in the liquid phase (mg L −1),respectively.V is the volume of the solution (L)and m is the mass of adsorbent(g).Fig.1.The probable prepared routes of TGOCS.282H.Ge,Z.Ma/Carbohydrate Polymers131(2015)280–287The initial pH of the solution was adjusted by adding either 0.1mol L−1NaOH or0.1mol L−1HCl.To determine sorption kinet-ics,the initial test solution with pH2was sampled at various time intervals.The adsorption thermodynamics was determined at different temperatures(303K,313K,323K and333K).In order to obtain the adsorption isotherms,solutions with various initial Cr(VI)concentrations(16–206mg g−1)at pH2were treated at 303K.2.5.Regeneration and reuseThe adsorption was done in50mL of200mg L−1Cr(VI)solu-tion at pH2with20mg of TGOCS at303K for2h.Afterfiltration, the TGOCS was immersed in50mL of1mol L−1KOH or HCl aque-ous solution and agitated at303K for2h.Then,the TGOCS was removed from the solution and washed with water.The adsorbent was reused in the next cycle.The adsorption-regeneration cycles were repeated forfive times with the Cr(VI)uptake analysis.3.Results and discussion3.1.CharacterizationThe FTIR spectra of CS,GO,C TGOCS and TGOCS were shown in Fig.2(a).The major bands of CS could be assigned as fol-lows:3419cm−1(–OH and–NH2stretch),2876cm−1(–CH stretch), 1644cm−1(amide band),1617cm−1(–NH2bend),1382cm−1(–CH bend),1105cm−1(C–O stretch),and897cm−1(pyranoid ring stretch)(Ge&Wang,2014).The major bands of GO could be assigned as follows:3431cm−1(–OH stretch),1724cm−1(C O of COOH),1624cm−1(COOH asymmetry stretch)and1401cm−1 (COOH symmetry stretch)(Kumar,Kakan,&Rajesh,2013).For TGOCS and C TGOCS,their spectra were similar and showed the major bands of GO and CS.However,the COOH peak of GO at 1724cm−1and the–NH2group peak of CS at1617cm−1disap-peared and new amide peak at1522cm−1appeared.Hence,the products TGOCS and C TGOCS prepared by microwave and conven-tional methods had similar structures which could be crosslinked by epichlorohydrin with–NH2groups of CS and–NH2groups of graphene oxide-triethylenetetramine monoamide(as shown in Fig.1).The XRD patterns of CS,GO,C TGOCS and TGOCS were depicted in Fig.2(b).The XRD pattern of CS represented the distinct crys-talline peaks at12.8◦and20◦.The distinct crystalline peak of GO appeared at12.4◦.For C TGOCS,the peak at20◦decreased and the peak at about12.8◦became unapparent.For TGOCS,the peaks at 20◦and12.8◦disappeared almost.The crystallinities of GO,CS, C-TGOCS and TGOCS calculated from the peak areas were83.8%, 91.1%,61.4%and50.6%,respectively.The decrease in crystallinity of the composite should be attributed to the deformation of the strong hydrogen bond in original chitosan due to the reaction of amine groups with the grafted GO.This implied that the compos-ites were substantially more amorphous than chitosan and GO and TGOCS was more amorphous than C TGOCS.SEM graphs(20,000×)of CS,GO,C TGOCS and TGOCS were shown in Fig.2(c).The surfaces of CS had some holes and crevasses. The surfaces of GO showed wrinkle fabrics with someflakes.For the composites,their surfaces were similar to those of GO.However, the surfaces of TGOCS had more crevasses than those of C TGOCS.parison of Cr(VI)adsorptionThe adsorption of Cr(VI)on the conventional product C TGOCS, MW product TGOCS and reactants(CS and GO)was conducted by placing20mg of adsorbent in250mL conicalflasks with50mL of200mg L−1Cr(VI)solutions at pH2and303K.The uptake capacities were listed in Table1.The order of capacity was TGOCS>C TGOCS>CS GO.This might be attributed to that the adsorption of Cr(VI)on the adsorbent was partly by the electro-static attraction.At pH2,the amine group existed in the cation and Cr(VI)existed mainly in the HCrO4−anion.The action of active –COOH and–OH groups in GO for Cr(VI)was weak and the action of active amine groups in CS for Cr(VI)was strong.Hence,the capac-ity of CS for Cr(VI)was significantly higher than that of GO.As for TGOCS and C TGOCS,however,the graft of triethylenetetramine increased the amount of amine groups which led to the increase of uptake.The results of elemental analysis and surface area were also summarized in Table1.The N content and surface area of TGOCS were larger than that of C TGOCS.These might be the main rea-son why the uptake of TGOCS was higher than that of C TGOCS. In this preparation,the product obtained by MW had higher yield and uptake than one obtained by the conventional.Hence,the microwave preparation was a better method and adsorption of Cr(VI)on the TGOCS produced by MW was systematically studied in the following.3.3.Microwave mechanism of the preparationBased on the above characteristic results of the products obtained by the microwave and conventional methods,the struc-tures and morphologies of the products were similar.Hence,the probable prepared routes(as showed in Fig.1)should be simi-lar.The higher yield and uptake of the product obtained by the microwave should be related to the interaction of the reactants with microwave.Microwave energy was transferred directly to the reaction mixture,through the molecular dielectric interaction with the electromagneticfield,generating the heat throughout the entire mixture volume simultaneously(Singh,Kumar,&Sanghi, 2012).However,the conventional heating is the heat transfer pro-cess from the outer surface of the mixture to the inner.In our reactive systems,the active groups of reactants such as carboxylic and amine groups were polar.Microwave could interact with these polar groups which might reduce the active energies of reactions (Vergara,de Sarrionandia,Gondra,&Aurrekoetxea,2014).These might led to that the yield and N content of the product obtained by the microwave were higher than by the conventional.The higher N content of the microwave product would be favorable for the uptake of Cr(VI).3.4.Effect of pHThe effect of pH was studied in the pH range of1–7.As shown in Fig.3(a),the pH of solution strongly affected the adsorption performance of TGOCS.The adsorption capacity increased with the increase of the pH value till a maximum value at pH2and then decreased slowly with further increase of pH while decreased markedly with pH>6.This was because Cr(VI)existed mainly in the form of HCrO4−in present study(Hena,2010).In acidic solu-tion,the amine groups of TGOCS could form protonated cations. The extent of protonation of amine group would be reduced with rising pH.The amine group cations in TGOCS would be in favor of forming complexes with Cr(VI)anions by the electrostatic attrac-tion.These resulted in that the adsorption of Cr(VI)on TGOCS hada maximal value at pH2.3.5.Effect of contact time and kinetic studiesAdsorption kinetics was an important constant for the evalua-tion of a good sorbent.As shown in Fig.3(b),the Cr(VI)uptake on TGOCS was rapid in thefirst20min,contributing to about85%ofH.Ge,Z.Ma/Carbohydrate Polymers131(2015)280–287283Fig.2.(a)FT-IR spectra of CS,GO,C TGOCS,TGOCS and TGOCS-Cr(VI);(b)XRD curves of CS,GO,C TGOCS and TGOCS;(c)SEM graphs(20,000×)of CS,GO,C TGOCS and TGOCS.the ultimate adsorption amount for Cr(VI),and then augmented gradually.In present study,the adsorption equilibrium was achieved within about2h.The pseudo-first-order and pseudo-second-order models were used to investigate the adsorption mechanism.The linear forms of pseudo-first-order and the pseudo-second-order equations are expressed as follows(Ge et al.,2012):ln(q e−q t)=ln q e−k1t(2) tq t=1k2q2e+tq e(3) where k1(min−1)and k2(g mg−1min−1)are successively the pseudo-first-order and pseudo-second-order rate constants;q t is the amount adsorbed at time t(min),and q e denotes the amount284H.Ge,Z.Ma /Carbohydrate Polymers 131(2015)280–287Table 1The results of elemental analysis,surface area and uptake capacity of Cr(VI)for CS,GO,C TGOCS and TGOCS.Materialwt (%)Surface area (m 2g −1)q e (mg g −1)CNHCS 40.187.5117.776 6.20131.4GO99.570.0080.04511.95 3.653C TGOCS 36.21 6.0237.52110.58178.8TGOCS34.746.2297.47111.69216.9adsorbed at equilibrium,both in units of mg g −1.The values of k 1and q e (namely q e cal )are calculated from the slope and intercept of the linear fit of ln (q e exp −q t )versus t ,and q e exp is the exper-imental value of q e .The values k 2and q e can be calculated from the linear fit of t /q t versus t .The pseudo-first-order and pseudo-second-order kinetics models were shown in Fig.3(b).The results of kinetic constants and correlation coefficients (R 2)were listed in Table 2.Based on the R 2values and Fig.3(b),the pseudo-second-order model could give the best fit for the experimental data.The results connoted that the adsorption was a chemical process.The intraparticle diffusion model was also selected to fit the kinetic data and it can be formulated as (Ge &Fan,2011):q t =k i t 1/2+C(4)where k i (mg g −1min −1/2)is the intraparticle diffusion rate con-stant and C (mg g −1)is a constant.Values k i and C can be obtained by the linear fit of q t against t 1/2.The intraparticle model kinetics was also shown in Fig.3(b).Taken as a whole,the linear relation of q t versus t 1/2was not good.However,the linear relation was better in the initial 20min of Cr(VI)adsorption,and the constants were also listed in Table 2.The positive value of C indicated that the ini-tial stage of the adsorption process was governed by the boundary layer diffusion (Annadurai,Ling,&Lee,2008).The kinetic plots inFig.3(b)exhibited the two-stage linearity and the latter was the final equilibrium stage where the intraparticle diffusion slowed down.3.6.Effect of initial Cr(VI)concentration and adsorption isothermAdsorption isotherm was important to evaluate the adsorption capacity of TGOCS.Fig.4(a)showed the adsorption equilibrium isotherm of Cr(VI)on TGOCS.Obviously,the uptake of Cr(VI)on TGOCS increased as the initial concentration increased up to 120mg L −1thereafter uptake reached the ngmuir,Freundlich and Temkin isothermic models were used to fur-ther understand the adsorbate–adsorbent ngmuir,Freundlich and Temkin models can be represented as follows (Ge et al.,2012;Kumar et al.,2013):q e =q mK L c e 1+K L c e or c e q e =c e q m +1q m K L,R L =1(1+K L c 0)(5)q e =K F c 1/n eor ln q e =1nln c e +ln K F(6)q e =RT b Tln(K T c e )or q e =RT b Tln(K T )+RT b Tln(c e )(7)where c e (mg L −1)is the equilibrium concentration of adsorbate in solution and q e (mg g −1)is the amount adsorbed at equilibrium.K L (L mg −1)is the Langmuir constant,q m (mg g −1)is the maximum adsorption capacity for monolayer formation on adsorbent,R L is the separation factor.K F (mg g −1(L mg −1)1/n )is a constant represent-ing the sorption capacity and n is a constant depicting the sorption intensity.R (8.315J K −1mol −1)is the universal gas constant,T (K)is the absolute temperature,and b T is related to the heat of adsorp-tion.The Langmuir constants,q m and K L ,could be calculated from the linear fit of c e /q e versus c e .The Freundlich constants,K F and n ,4080120160200q e (m g g -1)pH(a)q (m g g t-1)t (min)(b)Fig.3.(a)The effect of pH on Cr(VI)adsorption by TGOCS;(b)adsorption kinetics of Cr(VI)onto TGOCS.Table 2Constants of kinetic and isotherm models for Cr(VI)adsorption on TGOCS.Kinetic model constantsIsotherm model constantsPseudo-first-orderq e cal (mg g −1)83.59Langmuirq m (mg g −1)219.5k 1(min −1)0.02460K L (L mg −1)0.5524R 20.9522R 20.9997Pseudo-second-orderq e (mg g −1)218.6FreundlichK F (mg g −1(L mg −1)1/n )78.86k 2(g mg −1min −1) 1.103×10−3N 3.939R 20.9996R 20.9178Intraparticle diffusion ak i (mg g −1)42.34Temkinb T (g kJ mg −1mol −1)88.31C (g mg −1min −1)21.34K T (L mg −1)26.52R 20.8796R 20.9828aLinear fitting results at the initial 20min.H.Ge,Z.Ma /Carbohydrate Polymers 131(2015)280–2872854080120160200240q e (m g g -1)c e (mg L -1)(a)q e (m g g -1)Temperat ure (K)(b)Fig.4.(a)Adsorption isotherms of Cr(VI)onto TGOCS;(b)effect of temperature on the adsorption of Cr(VI)by TGOCS.Table 3Adsorption comparison of TGOCS and reported studies on adsorbents for Cr(VI).No.Adsorbentq max (mg g−1)Reference 1TGOCS219.5This work2Cyclodextrin–chitosanmodified GO61.31Li et al.(2013)3Protonated crosslinked chitosan189.3Huang et al.(2013)4Ethylenediamine-modified cross-linked magnetic chitosan51.813Hu et al.(2011)5Cross-linked chitosan86.81Wu et al.(2012)6Ti–CTS171Zhang et al.(2015)7CD-E-MGO68.41Wang et al.(2014)8Chitosan22.09Aydın and Aksoy (2009)9Graphene nanosheets43Jabeen et al.(2011)can be obtained from the linear fit of ln q e versus ln c e .The Temkin constants,K T and b T ,can be obtained from the linear fit of q e versus ln c e .The isotherm constants were summarized in the Table 2and the three models were shown in Fig.4(a).The R 2values (in Table 2)and Fig.4(a)manifested that the Langmuir isotherm could give the best fit for the experimental data.The maximum adsorption capacity of Cr(VI)on the TGOCS from Langmuir model was 219.5mg g −1,which was higher than that of the recently reported adsorbents (listed in Table 3).The R L values were 0.008721–0.09908(0<R L <1),indicating that the adsorption processes were favorable.3.7.Thermodynamic studiesThe influence of temperature for the adsorption of Cr(VI)on TGOCS was given in Fig.4(b).The Cr(VI)uptake increased with increasing temperature.The thermodynamic parameters such as enthalpy change ( H ),entropy change ( S ),and Gibbs functionchange ( G )of the adsorption process were obtained from exper-iments using the following equations (Ge et al.,2012):lnq ec e=S R − H R 1T(8) G = H −T S(9)where q e ,c e ,R and T are the same as indicated above.The S and H values could be calculated from the linear fit of ln(q e /c e )versus 1/T .The calculated values of H and S were 22.02kJ mol −1and 42.26J K −1mol −1,respectively.The positive H indicated an endothermic nature for the adsorption process.The positive S showed the increasing randomness at the solid/liquid interface during the sorption of Cr(VI)onto TGOCS.Meanwhile,the values of G at 303K,313K,323K and 333K were −2.238,−2.761,−3.183and −3.606kJ mol −1,respectively.The negative G revealed the spontaneity of uptake process for Cr (VI).It was noteworthy that the value of H approached the general reaction enthalpy (≥40kJ mol −1)and the value of S was also large (Ge et al.,2012).These meant that the adsorption process should be predominantly chemical.3.8.Effect of adsorbent dosageThe effect of TGOCS dosage on the removal of Cr (VI)was con-ducted in 50mL of Cr (VI)solution with pH 2at 303K for 2h and the result was shown in Fig.5.Obviously,the removal percentage of Cr (VI)increased with the increase of adsorbent dosage.For the initial Cr(VI)concentrations of 80and 200mg L −1,98%removal for Cr(VI)required about 40mg and 100mg of TGOCS,respectively.These results indicated that the Cr(VI)in solution could be effec-tively removed when the dosage of TGOCS was about 10times amount of Cr(VI).3.9.Effect of coexisting ionsBy contacting 20mg of TGOCS with 50mL of Cr (VI)solution (200mg L −1)at pH 2and 303K for 2h,effects of coexisting salts (50mmol L −1of NaCl,KCl or K 2SO 4)were evaluated.The capac-ities in absence of coexisting salt,coexisting NaCl,KCl or K 2SO 4were 213.2,205.8,207.9and 72.27mg g −1,respectively.The result showed that the presence of Na +or K +had no significant effect for the Cr(VI)adsorption.However,SO 42−had greater inhibitory effect than Cl −.This was because the sorbent (TGOCS)had posi-tive charge due to the protonation of amine groups (–NH 3+)in acid solution,causing the electrostatic attractions between anion and286H.Ge,Z.Ma /Carbohydrate Polymers 131(2015)280–287R e m o v a l p e r c e n t a g e (%)Absorbent dos age (m g)Fig.5.The effect of adsorbent dosage on the removal of Cr (VI)by TGOCS.The initialCr(VI)concentration was 80and 200mg L −1,respectively.sorbent.Therefore,the anions such as SO 42−and Cl −in the solution could result in a competitive adsorption with Cr(VI)(mainly in form of HCrO 4−)on the sorbent.In contrast,the electrostatic repulsion could occur between cation (Na +and K +)and the sorbent,leaving the adsorption site of sorbent still available for Cr(VI).3.10.Regeneration of TGOCSThe regeneration and reuse of TGOCS for Cr(VI)removal were also evaluated.1mol L −1NaOH or 1mol L −1HCl was tried to use as a solvent of desorption and the desorption percentages of Cr(VI)on TGOCS were 92.25%and 82.82%,respectively.Hence,1mol L −1NaOH was further selected as a solvent of desorption in adsorp-tion/desorption/regeneration cycles.After five cycles,the uptake of Cr(VI)on TGOCS was still maintained above 80%of initial uptake.This result suggested that TGOCS could be used repeatedly for the treatment of Cr(VI)in wastewater.3.11.Mechanism of Cr(VI)adsorptionFrom the results of the pH and coexisting ion studies,the active groups on the surface of TGOCS composite could be protonated in an acidic medium.The protonated surface had a stronger electrostatic attraction for chromium anions (HCrO 4−).On the other hand,the kinetic and thermodynamic studies indicated that the adsorption on TGOCS was a chemical process.This could be further confirmed by FTIR analysis.The FTIR spectra of TGOCS-Cr(VI)obtained after adsorption of Cr(VI)on TGOCS was also given in Fig.2(a).For com-parison with TGOCS,two new peaks at 930cm −1(Cr O stretch)and 750cm −1(Cr–O stretch)(Kumar et al.,2013)appeared and the peak (N–H and O–H stretch)was shifted from 3417cm −1to 3373cm −1.These indicated that the adsorption of Cr(VI)on TGOCS was a chemical process partly by electrostatic attraction.4.ConclusionsThe triethylenetetramine modified graphene oxide/chitosan composites (TGOCS and C TGOCS)were synthesized by microwave irradiation and conventional heating method.These composites were characterized by FTIR,XRD,SEM,BET and elemental analy-ses.The adsorption of Cr(VI)on these composites were studied and compared.The probable mechanism of the microwave preparation had been discussed.The results indicated that the product obtained by MW had higher yield and uptake than one obtained by the con-ventional.The microwave preparation was a better method than the conventional.The effects of various variables on the adsorptionof Cr(VI)by TGOCS were further discussed.The maximum uptakeof Cr(VI)with 20mg TGOCS at 303K was 219.5mg g −1at pH 2.The adsorption was a fast process which could reach 85%of maximal uptake in 20min.The adsorption followed the pseudo-second-order model and Langmuir isotherm.The Cr(VI)uptake increased with increasing temperature.The adsorption was an endothermic and spontaneous chemical process partly by electrostatic interac-tion.The TGOCS with 10times amount of Cr(VI)could effectively remove the Cr(VI)of solution and the composite could be recycled with 1mol L −1NaOH.Thus the composite may be promising for application in the removal of Cr (VI)from the wastewater.AcknowledgementsThe authors gratefully acknowledge the financial support from the National Nature Science Foundation of China (No.21077034).Additionally,students Na-na Wu and Zhao-xin Wang took part in some work on this research.ReferencesAnnadurai,G.,Ling,L.Y.,&Lee,J.F.(2008).Adsorption of reactive dye from anaqueous solution by chitosan:Isotherm,kinetic and thermodynamic analysis.Journal of Hazardous Materials ,152,337–346.Aydın,Y.A.,&Aksoy,N.D.(2009).Adsorption of chromium on chitosan:Optimiza-tion,kinetics 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