Graphene oxide based carbon composite as adsorbent for Hg removal:Preparation,characterization,kinetics and isotherm studies

Tayebeh Esfandiyari,Navid Nasirizadeh *,Mohammad DehghaniMohammad Hassan Ehrampoosh

1 Department of Environmental Health Engineering,Shaheed Sadoughi University of Medical Sciences,Yazd,Iran

2 Young Researchers and Elite Club,Yazd Branch,Islamic Azad University,Yazd,Iran

1.Introduction

Mercury(Hg)is one of the most harmful environmental contaminants,which gains high harmfulness even at low concentrations[1].It is non-biodegradable and tends to cumulate in living tissues,posing a direct danger to human health[2].It is quite dangerous to human and living things,leading to a variety of issues in the spectrum of health,include neurological disorder,disablement,blindness,and genetic defects[3].In order to reduce the risks associated with metal pollutants,it is essential that wastewaters be treated before being released to the environment[4].Hence,the removal and recovery of heavy metal ions from industrial waste water have been a noteworthy topic in most industrialsubdivisions[5].The severaltechnologies,such as chemicalprecipitation[6],liquid extraction[7],ionic exchange[8],membrane systems[9],and adsorption[10,11],have been developed over the years.The adsorption procedure has attracted extensive attention due to its simplicity and low cost.Numerous kinds of adsorbents,such as activated carbon[1], fly ash[12],Carbon nanotubes[13],microorganisms[14],industrial and agricultural byproducts[15]and clay minerals[16],have been applied for the elimination of heavy metal ions from wastewater.

On the other hand,making use of this kind of materials,their disorderly and unstable structure,the lack of model predictability and their incapability to eliminate the contaminants in trace concentrations may be,for very acute applications,a weakness of these materials.Therefore,study on the development of novel technical and highly specific materials is of great concern[17].

The graphene oxide(GO),as an ideal two-dimensional material,is characterized by a great specific surface area(2630 m2·g?1),determined from the monolayer carbon structure[18].In this sense,GO is the best nonporous adsorbent,and sorption generally occurs on its planar surface(outside surface),escaping from intraporous diffusion like that in porous carbon materials but only determined by external diffusion since powdered GO with randomly aggregated structures has open ion channels,which results in a short period to reach an adsorption equilibrium[19].

It is well known that sol–gel process is used in the fields of materials science and ceramic engineering,in which a colloidal solution(sol)is converted to an integrated network(gel)of either discrete particles or network polymers[20].The method allows the facile synthesis of stable composite with improved efficiency and performance at ambient conditions[21].A lot of work has been done about the synthesis of nanocomposites by sol gel.For example,Wanget al.[22]produced a Chitosan–silica composite for the elimination of Congo red from aquatic environment.Wuetal.[23]prepared a thiocyanato-functionalized silica gel as selective adsorbent of cadmium(II).Mercaptopropyl-coated cobalt ferrite(CoFe2O4)magnetic nanoparticles as a suitable and effective adsorbent for Hg2+ions were developed by Viltuzniket al.[24].In continuation with the previous study,we have provided the multifunctional(conductive–hydrophobic)textiles using a simple and cost effective sol–gel method[25].

In this study,graphene oxide–carbon composite(GO–CC)was synthesized by sol gel technique and used as an adsorbent to remove of Hg2+from aqueous solution.Physicochemical properties of prepared adsorbent were characterized by FESEM,EDX and BET analysis.The operational parameters including pH of the solution,ratio of GO in adsorbent matrix(wt%),adsorbent dosage(g·L?1)and contact time(min)were investigated to understand the adsorption process and also thermodynamic and isotherm parameters were determined.

2.Materials and Methods

2.1.Materials

Graphite(KS-10),Potassium trimethylsilanolate(PTMS)(see chemical structure in scheme S1),Sulfuric acid,Nitric acid,Methanol and HCl were supplied from Merck(Germany)Company.Mercury nitrate monohydrate Hg(NO3)2,was purchased from Degussa(Germany).Potassium chlorate was obtained from Sigma Aldrich.All chemicals were used as received without further purification.GO was prepared according to the Staudenmaier method[26](see Supplementary Material).

2.2.Preparation of GO–carbon composite

The proposed procedure for construction of graphene oxide–carbon composite(GO–CC)as adsorbentby the solgelprocess[25]is as follows:First,a medium(4.0 ml)containing ethanol/waterwith volume ratio 9:1 was prepared and the pH was adjusted to 4.5 using 10%HCl.Then,20 μl PTMS was added to the solution and agitated for 90 min to prehydrolyze the organosilane.After that,a 3.0 g mixture of the synthesized GO and graphite(mass ratio 1:3)was immersed in the solution and stirred for 90 min.This was followed by evaporation of solvent at 45°C to allow covalent bonding between the GO/silanol groups/graphite.The driedsample,i.e.cured adsorbents was scoured with the 50/50 vol%ethanol/water solution for several times until unreacted organosilane was eliminated and then dried at 50°C for 24 h.The dried sample was eventually well maintained in a desiccator for future use.

2.3.Instruments

A Buck Scientific atomic absorption spectrometer,model 210 VGP,was used for all absorption experiments.A mercury hollow cathode lamp(Westinghouse WL-22847)was applied as the light source,and its operating current was set to the value recommended by the manufacturer.The wavelength and bandwidth were adjusted to 253.7 nm and 0.7 nm,respectively.A Buck Scientific hydride vapor generator,model 1015,was applied for mercury generation.The mercury was reduced to metallic mercury through tin(II)chloride,and the mercury generatorwas worked outwith N2gas ascarriergas.The shape and surface morphology of the GO–carbon composites(GO–CC)were examined by a field emission scanning electron microscope(FESEM,Mira 3-XMU)equipped with EDX(Bruker 127 eV)under an acceleration voltage of 15 kV.The textural properties of GO–CC were also con firmed by N2adsorption/desorption isotherms at 77 K using a Belsorp analyzer(BEL Japan Inc.).The specific surface area(SBET)was determined by the Brunauer–Emmett–Teller(BET)technique.The textural features of GO–CC are shown in Table 1.The pore size distribution was determined by means of the Barrett–Joyner–Halenda(BJH)method(Fig.S1).The point of zero charge(pHpzc)for GO–CC was determined by a Malvern zetameter(Zetasizer 2000)(see Fig.S2).

Table 1Textural properties obtained by N2 adsorption/desorption studies

2.4.Absorption experiment

The capability ofthe GO–CC to adsorb of the mercury ion from aqueous solution was determined using a batch of mercury nitrate aqueous solutions of known concentration.All the working Hg2+solution(0.1 μg·g?1)was prepared using Hg(NO3)2and DI water.Batch absorption investigations were carried out by optimization the solution pH,adsorbent dose,GO proportion in adsorbent matrix,contact time using central composite design (CCD)(see design experiment part in Supporting Information File).The ranges of selected variables are presented in Table S1.

Brie fly,in optimum condition,an accurately weighed adsorbent(45.0 mg)was added to 10 ml of Hg2+(0.1 μg·g?1)placed in 100 ml capped Erlenmeyer flasks.After regulation ofpHwith 0.1 mol·L?1buffer phosphate solution at pH=6.6,the mixture was then agitated on a magnetic stirrer at 150 r·min?1for 95.0 min.The solution was immediately lastly filtrated.The Hg2+concentration in the ef fluents was then determined by cold vapor atomic absorption spectrometry(CVAAS)quipped with a hollow cathode lamp.Seven standard solutions were used to drawn each calibration curve.All measurements were approved outin triplicate and no results were established,ifthe standard deviation was more than 3%.

The removal ratio of Hg2+(Re,%)and the capacities for Hg2+(qe,mg·g?1)were calculated by Eqs.(1)and(2).

where,C0andCeare the initial and equilibrium Hg2+concentrations(μg·g?1).qeis the quantity of Hg(mg·g?1)adsorbed on unit mass of adsorbent at equilibrium.Vis the volume of solution(ml);andMis the mass of the adsorbent(g).

3.Results and Discussion

3.1.Characterization of adsorbent

Fig.1a and b displays a representative SEM image of a carbon composite(a)and GO based carbon composite(b).Both composites have vastly porous morphology.Many graphene sheets were observed after the addition of GO into carbon composite matrix,re flecting the folding nature wholly of the morphology.

An EDXanalysis con firms the presence ofseveralelements including C(74.31%),Al(0.70%),K(4.16%),O(16.30%),Cl(0.57%)and Si(3.95%)(Fig.1c)in the composite structure.The high BET surface area of the GO(280.7 m2·g?1)improved the morphology of the carbon composite and enhanced the surface area from 5.28 m2·g?1(carbon composite)to 174.3 m2·g?1(GO–CC).The great surface area of the GO–CC composite increases the binding sites for Hg2+ions in the solution.Pore volume average is 0.52 cm3·g?1and pore size is 1.21 nm,which corresponds to intra-and inter-agglomerate pores.

The pHpzcof the GO–CC was 3.8(see Fig.S2).Mostly,the surface of adsorbent is positively charged at pH＜pHpzc,and negatively charged at pH＞pHpzc.The cationic adsorption is preferred at pH＞pHpzc;consequently,as can be seen below,removal percentage was higher for the pH=5.0–7.0.

Fig.1.FESEM image of carbon composite(a)graphene oxide–carbon composite(b)prepared by sol gel and(c)elemental analysis of prepared composites.

3.2.Analysis of the model

The CCD plan(Tables S1 and S2)was used to assess the main interaction of four independent variables(contact time(A),solution pH(B),GO ratio in adsorbent(C),and adsorbent dosage(D)).The 30 experiments and their responses are presented in Table S2.To find the most important effects and interactions,analysis of variance(ANOVA)was calculated using Design of Experiment 7.0.0.1(Table S3).Ap-value less than 0.05 in the ANOVA table shows the statistical significance of an effect at 95%con fidence level.F-test was used to evaluate the statistical significance of all terms in the polynomial equation within 95%con fidence interval.Data analysis gave a semi-empirical expression of Hg2+removal percentage with the following equation:

In Eq.(3),all the variables(A,C,andD)had positive effects on Hg elimination,butthe value ofC＞A＞Dverified that the linear term in fluence ofCwas more significant than that ofAandD,representing that the GO ratio in adsorbent matrix was the main factor in fluencing on Hg2+removal.We also establish that the coefficient of interaction termC2was clearly higher than that of other coefficients;hence,the square of GO exhibits higher affinity to Hg2+.

3.3.Effect of variables on the adsorption process

Fig.2 represents the relevant fitted 3D plots for the design and illustrates the response surface plots of Hg2+removal(%)versus change in variables.These graphs were drawn for a given pair of factors at fixed and optimal values of other variables.The curvature natures of these graphs show their interaction.

Fig.2a,illustrates the effect of the contact time and GO ratio in composite on the removal percentage of Hg2+ions by GO–CC using a 0.88 g·L?1adsorbent dose at pH 6.5.The removal percentage of Hg2+improved with increasing contact time from 20 to 95 min.The absorption level of Hg2+for GO–CC(as 5.0 wt%GO was applied)was quick at the initial periods(0–60 min)due to the adsorption of Hg2+ions onto the outside surface,and subsequently it became gentle when Hg2+ions move in the inner surfaces.The same trend can be seen with lower intensity when used 2.55 wt%GO.This might have been caused by a large quantity of unoccupied surface binding sites that were accessible for Hg adsorption during the initial period.Though,after a certain time,the available sites were almost occupied and the residual vacant surface sites were difficult to be engaged due to the repulsive forces between the adsorbent molecules on the solid and bulk phases[1,13].

Fig.2.Response surface plot of a)contact time vs GO ratio in composite,when pH and adsorbent dose in constant at 6.5 and 0.88 g,and b)solution pH vs adsorbent dose on removal of Hg2+ion at constant values of 95 min as contact time and 2.6 wt%as GO ratio.(The blue color represents lowest and red indicates the highest removal efficiency.)

Moreover,it was observed thatthe improvementof Hg removalwas accompanied by an increase of GO ratio in composite matrix from 0.1 to 2.5 wt%,after which it starts decreasing.There is improved adsorption capacity of the GO–carbon composite due to more adsorption sites for Hg2+ions in the presence of graphene sheets on carbon composite[27].With further increase in the GO ratio in carbon composite,the percentage removalwas decreased from 75%to 50%at95 min.The phenomenon could be described as a result of a partial aggregation of GO at higher ratios,which results in a decrease in effective surface area for the Hg adsorption.The same findings were observed by Wuet al.and Shiet al.regarding the reduction of removal rate due to the increase of GO[28,29].

The effectof the pH and adsorbentamounton the Hg2+ion removal efficiency from aqueous solution is demonstrated in Fig.2b when,time and graphene oxide are fixed at 95.0 min and 2.6 wt%respectively.The solution pH has been known as the most important factor governing metal adsorption on the adsorbent.Once pH increased from 3.0 to 6.6,removal efficacy for Hg2+harshly also enhanced.It has been verified that at acidic pH,adsorption capacity was low,due to the competition of Hg2+ions with H+ions[1,13].On the other hand,the high positive charge density on the GO surface at below pHzpc(3.8)was another reason.Hg2+ions in solution are theoretically existent as various species depending on the media pH.For example,Puanngam and Unob[30]reported that Hg2+ions exist as Hg2+,HgOH+and Hg(OH)2at different pHs.It was expected that when pH＜4,the dominantspecies in the Hg(II)solution would be Hg2+;while Hg(OH)+and Hg(OH)2species dominated at pH＞4.From the Fig.1b,the perceived lower Hg2+removal at acidic pH(pH＜pHpzc(3.8))was maybe due to(i)rivalry between Hg2+ions and H+or H3O+ions existing in the solution for the available sites on the GO–CC composite and(ii)protonation of the functional groups on GO sheets which induced the electrostatic repulsion with the Hg2+.When,the pH of the system increased(pH＞pHpzc),adsorbent surface charged as negative and further functional groups would be accessible for metal ion binding due to deprotonation;thus high absorption isobserved.Moreover,ithad been reported about silicates that the reaction of the hydroxylated Hg(OH)2with the functional group caused the formation of--Si--HgOH and the binding ability could be superior than that of the protonated adsorbents with Hg2+[1,3,13,31].The highest Hg2+removalwas achieved at pH of 6.6.Furthermore,Hg2+removal efficiency reduced with increment of pH from 7.0 to 10.0.When the pH value was greater than 10.0,the adsorption of Hg2+ions was practically not detected.It seems that the interaction between the Hg2+ions and the absorbent surface is electrostatic and can be used once more from composite with a mild acid washing.

As could be seen in Fig.2b,Hg2+removalwas improved slowly with the increase of the adsorbent dosage from 0.1 to 0.88 g·L?1.This can be attributed to the increasing surface area and accessibility of more adsorption sites,when the adsorbent dosage increased[1,13].However,the removal of Hg2+decreased as the adsorbent dosage increased upper than 0.9 g·L?1,since the system reached equilibrium at where the adsorption sites of adsorbent stayed unsaturated with the fixed Hg2+quantity in solution.

3.4.Adsorption kinetics

The pseudo- first-order model(Lagergren model[32])and pseudosecond order model(Ho and Mckay model[33])were applied to identify the process of Hg2+adsorption onto the GO–CC.

The first-order rate constant(k1)was calculated fromthe slopes and interceptsofplotsoflg(qe?qt)versust.Besides,by plottingt/qtversust,the second-order rate constant(k2)andqevalues were concluded from the slope and intercepts of the plots.The mentioned plots and the corresponding linear fittings are shown in Figs.S3 and S4 for the pseudo first and pseudo second order models,respectively.The kinetic adsorption rate constantsk1andk2are shown in Table 2,along with the experimental and calculatedqe.As it may be seen,the removal of Hg2+may be explained by the second-order kinetic modelunderthe experimental conditions.Hence,the model presents the best fit,presenting highR2values(＞0.99)for Hg adsorption and the calculatedqeagreed with the experimental data.This model declares that,the absorption rate is in fluenced by the amount of mercury adsorbed on the surface of GO–CC absorbent and the amount adsorbed at equilibrium.

3.5.Adsorption isotherms

In this study,the equilibrium data were adapted with two parameter isotherm modelsi.e.Freundlich,Langmuir,Temkin and Dubinin–Radushkevich(D–R).These models correlate the amount of solute adsorbed perunitmass ofadsorbent,qe(mg·g?1),to the adsorbate concentration in solution at equilibrium,Ce(mg·L?1).The isotherm constants for each model were calculated from the linearized Eqs.(4)–(7):

Table 2Kinetics parameters for Hg2+adsorption on GO based carbon composites

Here,theqmax(mg·g?1)is the maximum adsorption capacity.KL(mg?1)is the Langmuir constants related to the monolayer adsorption capacity and the adsorption free energy(KL=e?ΔG/RT).KF(mg·g?1·mg?1/n)is the Freundlich constant related to adsorption capacity andnis the heterogeneity factor.β (mmol2·J?2)and ε (–)are the D–R constants.The constant ε=RTln(1+1/Ce),Ris the gas constant,8.3145 J·mol?1·K?1andTis the absolute temperature in K.Bcorresponds to the heat of adsorption,which is equal toRT/bTandKTis the equilibrium binding constant(g?1).

The Langmuir,Freundlich,D–R and Temkin model constants were calculated respectively,from the plot between the values of(i)lgqeand lgCe,(ii)(1/qe)and(1/Ce),(iii)lnqeand ε2and(iv)qeand lnCe.

The adsorption isotherm graphs ofHg+2on the GO–CC was presented in Fig.S5.The relative parameters calculated from the Langmuir,D–R,Temkin and Freundlich models are listed in Table 3.A high correlation coefficient represents the good agreement between experimental and anticipated data.The value of correlation coefficient of Langmuir is greater(＞0.99)than the others(i.e.the Freundlich(0.981),D–R(0.856)and Temkin(0.941)).Hence,the Langmuir equation represents best fit of experimental data over other models,and these results are in agreement with the other previously reported works[34–42].

Table 3Isotherm parameters for Hg2+adsorption on GO–carbon composites

In addition,the calculated values ofQmare found to be 68.8 mg·g?1,which are very close to its experimental values.The capacity of GO CC adsorbent for Hg was compared with other adsorbents[34–42].As it can be seen in Table 4,the adsorption capacity of proposed composite is comparable with carbon based adsorbent reported in the previous publications.

3.6.Thermodynamic studies

Effect of temperature on Hg2+removal was also assessed at the Hg2+initial concentration of 800× 10?6and the temperature range of 303–333 K.The thermodynamic parameters likely,changes in standard Gibbs free energy(ΔG°),standard enthalpy(ΔH°),and standard entropy(ΔS°)were determined by the van't Hoff model,which supposed that the activity coefficient of the solutes used in the solution was unity[43].

Table 4Comparison of adsorption capacity against similar adsorbents

In the lately mentioned equations,kis the distribution adsorption coefficient,qe(mg·g?1)andCe(mg·L?1)are the adsorption capacity of adsorbent and adsorbate concentration at equilibration,Ris the universal gas constant(8.314 J·mol?1·K?1),andTis the absolute temperature(K).The diagram of lnk versus1/Tgets a straight-line with the slope of ΔH°(kJ·mol?1)and intercepts of ΔS°(kJ·mol?1·K?1)as presented in Fig.S6.The calculated values of thermodynamic parameters were represented in Table 5.The negative Gibbs free energy change ΔG°proves that the adsorption of Hg2+onto GO–CC was a thermodynamically possible and spontaneous process[27].The value of ΔG°is increasing with increment of the temperature,which is representing more favorable and spontaneous adsorption at a higher temperature[34].In addition,the positive value of the enthalpy ΔH°suggests that the Hg2+adsorbed onto GO based composite was endothermic in nature,and a higher adsorption capacity would be acquired with a higher temperature.The positive value of ΔS°re flected that the randomness improved at the solid–solute interface during Hg2+adsorption onto GO–CC,also the good affinity of GO–CC for Hg2+.

Table 5Thermodynamic parameters of Hg2+adsorption onto GO–CC

4.Conclusions

In the presentstudy,removalofHg2+from aqueous solutions by adsorption on the graphene oxide based carbon composite(GO–CC)was surveyed.The GO–CC was prepared using the sol gel technique.FESEM images clearly showed porous structure and graphene oxide sheets on the composites surface.The adsorption of Hg2+on the GO–CC increases with increment of the time and GO ratio in composite matrix,and achieves the highest value of 68.8 mg·g?1.In addition,the study showed that the best explanation for the experimental data of GO–CC for Hg2+from aqueous solution was given by the Langmuir isotherm equation,and the adsorption kinetics could be followed by the pseudo second order rate equation wonderfully.The thermodynamic parameters of adsorption include ΔG, ΔHand ΔSand were?1.89 kJ·mol?1(30 °C),14.71 kJ·mol?1,and 54.93 kJ·K?1·mol?1,respectively.As a result,the good adsorption capability made GO–CC a favorable candidate adsorbent for mercury removal from aqueous solutions.

Supplementary Material

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.cjche.2017.02.006.

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