Yu Bian ,Zhaoyong Bian ,*,Junxiao Zhang ,Aizhong Ding ,Shaolei Liu ,Lei Zheng ,Hui Wang ,*

1 College of Water Sciences,Beijing Normal University,Beijing 100875,China

2 College of Environmental Science and Engineering,Beijing Forestry University,Beijing 100083,China

Keywords:Activated carbon Oxygen-containing functional groups Cd(II)Adsorption mechanism

ABSTRACT The adsorption of aqueous cadmium ions(Cd(II))have been investigated for modi fied activated carbon(AC-T)with oxygen-containing functional groups.The oxygen-containing groups of AC-T play an important role in Cd(II)ion adsorption onto AC-T.The modi fied activated carbon is characterized by scanning electron microscopy,Fouriertransforminfrared spectroscopy(FT-IR)and X-ray photoelectron spectroscopy(XPS).The results ofbatch experiments indicate that the maximal adsorption could be achieved over the broad pH range of 4.5 to 6.5.Adsorption isotherms and kinetic study suggest that the sorption of Cd(II)onto AC-T produces monolayer coverage and that adsorption is controlled by chemical adsorption.And the adsorbent has a good reusability.According to the FT-IR and XPS analyses,electrostatic attraction and cation exchange between Cd(II)and oxygen-containing functional groups on AC-T are dominant mechanisms for Cd(II)adsorption.

1.Introduction

Heavy-metal contamination in aqueous environments is of great concern because of the toxic effects to the ecosystem,especially to humans in the form ofMinamata disease and itai-itaidisease.Cadmium,one of the most commonly used heavy metals,has been released into the environment by combustion of fossil fuels,mineralization,cement production,electroplating and manufacturing ofbatteries and pigments[1,2].Because cadmium ions are toxic even at very low concentrations,cadmium has been classi fied as a human carcinogen by the US National Toxicology Program.

To treat aqueous heavy-metal contamination,absorbents such as activated carbon(AC)are widely used due to their excellent absorbing ability[3].AC exhibits porous structure with large surface area.However,common AC does not have suf ficient functional groups to adsorb heavy metals economically[4].In the absence of functional groups,little or no adsorption occurs on ACs[5].Heavy-metaladsorption onto carbonaceous materials is considered to mainly take place at oxygen-containing functional groups,such as carboxylic and lactonic groups[6-9].Additionally,the amount of heavy metal ions adsorbed onto AC is not the stoichiometric one of the oxygencontaining functional groups.Huang et al.reported that oxidized ACs with 1.4 mmol·g-1oxygen-containing functional groups had the maximal cadmium sorption capacity of 51.02 μmol·g-1[10].Motoi et al.showed that the oxidized AC with 1.39 mmol·g-1oxygen-containing functional groups adsorbed 0.11 mmol·g-1cadmium[5].The poor relation between the amount of functional groups and the adsorption capacity indicates that AC adsorption process is complex.The interaction between oxygen-containing functional groups and heavy metal ions is not yet understood well.

The present work aims to analyze the effect of the oxygencontaining functional groups of AC in the adsorption of Cd(II).Strong oxidation acids such as nitric acid are used to increase the oxygencontaining functional groups on AC.Batch experiments are performed to evaluate the effect of contact time and initial pH value on the Cd(II)adsorption.The chemical nature of adsorption is characterized by Fourier transform infrared spectroscopy(FT-IR)and X-ray photoelectron spectroscopy(XPS)to propose the Cd(II)adsorption mechanisms onto AC.

2.Experimental

2.1.Materials

Commercial AC pellets were used as the adsorbent.All of the chemical reagents used in this study were of analytical grade.A Cd(II)stock solution of 1000 mg·L-1was prepared by dissolving 1000 mg cadmium powder in a nitric acid solution.Experimental solutions with desired concentrations were obtained by appropriate dilution of the stock solution.

2.2.Preparation of modi fied activated carbon

Approximately 15 g activated carbons were impregnated into an HCl solution(2 mol·L-1)atan impregnation ratio of 2 mlHClsolution/g activated carbon,stirred for 12 h.The resulting mixture was filtered and washed several times with distilled water until the pH reached a constant value.The product was dried and stored in dry,clean and wellclosed bottles;named as AC.The AC sample was mixed with a 200 ml solution of concentrated HNO3(10 mol·L-1)and then heated at 363 K for 3 h under stirring and re fluxing conditions.The resulting mixture was filtered and washed severaltimes with distilled water untilthe pHreached a constantvalue.Then,the productwas dried at383 Kto obtain the modi fied activated carbon(AC-T).

2.3.Characterization

Surface morphology and microcompositional analysis were conducted on a HITACHI-S4800 scanning electron microscope(SEM)equipped with an Oxford X-ray energy dispersive spectroscopy(EDS)system.The infrared transmittance spectra in the range between 400 and 4000 cm-1were collected using a Nexus 670 spectrometer in KBr pellet state.The spectrum of pure KBr pellets prepared under the identical conditions as those for sample pellets was subtracted to avoid the in fluence of water absorbed by KBr powder.The XPS spectra were obtained using an ESCALAB 250Xi physical electronics spectrometer.The photoelectron spectra were analyzed with a hemispherical mirror,assuring an energy resolution of approximately 0.5 eV.After 5 h in situ at10-7Pa vacuum,the sample surfaces were suf ficiently clean formeasurements.The binding energy in the range of-10 to 1350 eV and the core-level characteristic peaks for C 1s and O 1s were measured.The background was subtracted using Shirley's approximation.The concentrations of Cd(II)ions in batch experiments were determined using an AFS-230E atomic fluorescence spectrometer(AFS).

2.4.Batch experiments

2.4.1.Determination of the point of zero charge(pHpzc)

For determination of pHpzc,certain amount(30 mg)of adsorbent was suspended in 10 ml of 0.1 mol·L-1NaNO3solution,used as an inert/background electrolyte in a 50 ml stopper conical flask.The initial pH of the solution was adjusted to 3.0 using 0.01 mol·L-1HNO3or 0.01 mol·L-1NaOH.The suspension was allowed to equilibrate for 4 h at 200 r·min-1in a shaker bath at room temperature of(20 ±1)°C.After the equilibration time,the mixture was filtered and the final pH value of the filtrate was measured.This set of experiments was performed at a pH interval of 1.0.

2.4.2.Adsorption experiments

The batch adsorption experiments were performed with 30 mg AC-T and 10 ml aqueous Cd(II)solutions at desired concentrations and appropriate pH.The initial pH was adjusted by the addition of 0.01 mol·L-1HCl or 0.01 mol·L-1NaOH.Then,the suspensions were shaken in an oscillator at 200 r·min-1for 4 h to achieve adsorption equilibration.Then the residual aqueous Cd(II)concentrations in solutions were determined by AFS.The uptake amount of Cd(II)at equilibrium(qe,mg·g-1)is calculated by

where C0and Ceare the initial and equilibrium concentrations of Cd(II)(mg·L-1),respectively,V is the solution volume(L),and W is the mass of adsorbent used(g).

2.4.3.Desorption experiments

The dry and saturated AC-T was placed in 50 ml of 0.1 mol·L-1HNO3for 6 h with stirring at120 r·min-1.The liquid phase was filtered and analyzed by AFS(AFS-230E).The desorbed percentage is determined as follows.

The reuse assays were performed in cycles,starting with the adsorption and ending with the desorption of cadmium ions.

3.Results and Discussion

3.1.Characterization of AC and AC-T

Fig.1 shows the SEM pattern of original AC and AC-T.The surface of AC-T has less impurity and exhibits more pores.In addition,elemental analysis by SEM/EDS reveals the presence of 90.6%of C and 9.1%of O as the basic elements in AC,while with 65.6%of C and 33.9%of O in AC-T.It indicates that the nitric acid treatment favors the formation of oxygen-containing functional groups.

FT-IR is an important analytical technique to identify characteristic functional groups on carbon surfaces.In Fig.2,peaks from the hydroxyl group-OH and C-O-C of AC are at 1065 and 1184 cm-1,respectively[11].However,in the FT-IR spectrum of AC-T,strong peaks are at 1713 and 1613 cm-1,attributed to the stretching vibrations of-COOH and COO-,respectively[12,13].It indicates an increase in the oxygencontaining functionalgroups afterthe oxidation thatgenerates carboxyl and carboxylate groups.For the AC-T treated with aqueous cadmium ions,the intensity of the peaks at 1604 and 1401 cm-1increases.This could be attributed to an increase in the amount of carboxylate group(COO-)generated by the interaction ofcadmium ionswith the carboxyl group[7,14].

Fig.1.SEM micrographs of original AC(a)and AC-T(b).

Fig.2.FT-IR spectra of AC,AC-T and AC-T-Cd.

To identify the coordination types between Cd(II)and functional groups,XPS spectra of C 1s and O 1s are presented in Fig.3.The C 1s spectra of carbon exhibit three types of peaks with differentbinding energy values:(C-I)C-C at 284.78 eV,(C-II)C-O(phenolic,alcohol and ether)at 285.97 eV,and(C-III)O=C-O(carboxyl and ester)at 288.60 eV[15].In addition,the O 1s spectra of samples are composed of two types of peaks with different binding energies:(O-I)C-OH or C-O-C at533.21 eV and(O-II)C=Oat531.83 eV[16].AC-T shows a signi ficant increase in the intensity of C-III peak and O-II peak compared with AC.The content of oxygen-containing functional groups such as carboxylgroup increasesduring the oxidation process.The chemicalnature of adsorption of Cd(II)onto AC-T is con firmed by XPS measurements after Cd(II)adsorption.The binding energy of O-I and O-II peaks shifts from533.21 and 531.83 eVto 533.43 and 532.04 eV,respectively.The slight increase could be attributed to the O atom donating its electron density to cadmium ion when interaction occurs[17].Additionally,some typical oxygen-containing functional groups in carbon materials are phenolic hydroxyl group and carboxyl group,which could be responsible for Cd(II)ion adsorption.Phenolic hydroxyl group can be represented by the behavior of O-I and C-II whose binding energy increases from 533.21 and 285.97 eV of AC-T to 533.43 and 286.31 eV of AC-T-Cd,respectively.The changes of O-II and C-III suggest the state of carboxyl groups.Compared with AC-T,AC-T-Cd exhibits blue shift on the binding energy of O-II and C-III on 532.04 and 288.86 eV,respectively.It indicates the formation of C-O-Cd and carboxyl-Cd species and the chemical interaction between Cd(II)and oxygen-containing functional groups represents the adsorption of Cd(II)onto AC-T[18].

Fig.3.XPS spectra of AC,AC-T and AC-T-Cd:C 1s spectra(a)and O 1s spectra(b).

3.2.Determination of the pHpzc

pHpzcis the pH value at which the surface charge of adsorbent is 0.The acid-base properties of AC,which can be re flected by pHpzc,play an important role when AC is used as adsorbent.As shown in Fig.4,the pHpzcvalues of AC and AC-T are approximately 3.3 and 3.0,respectively.It indicates that the nitric acid oxidation treatment increases the acidic property ofAC-T due to the formation ofoxygen-containing functional groups.

Fig.4.The pHpzc of AC and AC-T.

3.3.Adsorption of Cd(II)onto AC-T

3.3.1.Effect of pH on Cd(II)adsorption

The pH value of the solution affects the adsorption of metal ions because it affects the chemical properties of surface functional groups and the speciation of metal ions.Cd(II)species at various pH values determined with Visual MINTEQ(V 3.0)[19,20]are shown in Fig.5,existing as Cd2+,[Cd(NO3)]+,[Cd(OH)]+,Cd(OH)2(s),[Cd(OH)3]-or[Cd(OH)4]2-.At pH below 8.0,Cd2+is the main species of Cd(II)in aqueous solutions.Hence,the adsorption of metal ions onto AC-T is investigated at pH values from 2.0 to 8.0.

Fig.5.Cadmium species in the aqueous system as a function of pH.

Fig.6.Effect of initial solution pH on Cd(II)removal.

The effectof pHon the sorption is shown in Fig.6,with the metalremoval against the initial solution pH.The maximum Cd(II)uptake is 88%,achieved atpHof4.5-6.5.For the pHvalue less than 4.5,the removal of cadmium increases with pH.At the pH value higher than 6.5,the removal rate changes little.The negatively binding sites of AC-T with oxygen-containing functional groups take competition reactions with both Cd(II)ions and H+ions[21].Moreover,the pHpzcvalue of the oxidized AC isapproximately 3.0.The surface charge ispositive atpH＜3.0,while it is negative at pH＞3.0.When the pH value is less than 3.0,the active binding sites of AC-T become inactive for the Cd(II)adsorption because AC-T is protonated[17].However,increasing pH value decreases the protonation of acidic functional groups on the AC-T surfaces and thus enhances the Cd(II)adsorption via electrostatic attractions.At higher pH,Cd(OH)+,Cd(OH)2,[Cd(OH)3]-or[Cd(OH)4]2-species appear with a decreased electrostatic attraction and repulsion to the negatively charged binding sites of AC-T.Hence,high pH is unfavorable for the adsorption of Cd(II)onto AC-T.

3.3.2.Adsorption isotherms

Adsorption isotherms provide important parameters for designing an adsorption system.The aqueous Cd(II)adsorption isotherm on ACT is shown in Fig.7.

Fig.7.Cd(II)sorption isotherms by AC-T.

The Langmuir and Freundlich isotherms are adopted to describe the adsorption behavior of Cd(II)on the oxidized AC.The Langmuir isotherm equation is written as

where q is the amount of Cd(II)adsorbed per gram of adsorbent(mg·g-1),Ceis the equilibrium concentration of adsorbate in the bulk solution afteradsorption(mg·L-1),qmis the maximalsorption capacity at equilibrium(mg·g-1),and b is a constant related to the free energy of adsorption.

The Freundlich isotherm equation is written as

where q is the amount of Cd(II)adsorbed(mg·g-1),Ceis the equilibrium concentration of adsorbate in the bulk solution(mg·L-1),K and 1/n are characteristic constants for relative adsorption capacity(mg·g-1)and intensity of adsorption,respectively.

Fig.8 demonstrates that the adsorption process follows the Langmuir isotherm over the concentration range studied.The values of constants qmand b calculated from the slope and intercept,respectively,are presented in Table 1.The Freundlich plot is shown in Fig.9.Calculated values of K and 1/n are exhibited in Table 1.

Fig.8.Langmuir plot of Cd(II)sorption on AC-T.

Table 1 Langmuir and Freundlich constants

The results show that the experimental data are better fitted by the Langmuir model than by the Freundlich model.It indicates that the sorption of Cd(II)ions onto the oxidized AC is a monolayer of coverage.The qmvalue of Cd(II)sorption on the oxidized AC is 25.13 mg·g-1,while that on untreated AC is 16.20 mg·g-1.It shows the importance of oxygen containing functional groups in Cd(II)adsorption.Moreover,the n value(1.84)in the Freundlich modelis between 1 and 10,indicating that the adsorption is favorable under the conditions.

The maximum adsorption capacity of other adsorbents for Cd(II)are shown in Table 2.The experimental maximum adsorption capacity of the AC-T is higher.Therefore,Cd(II)adsorption with the AC-T is favorable.

3.3.3.Kinetic study for adsorption

Fig.10 shows the effectofcontacttime on the Cd(II)removalby AC-T.The adsorption rate is fastin the first5 min and the process reachesequilibrium at 20 min.The adsorption kinetic data are analyzed in terms of the pseudo- first-order and pseudo-second-order rate equations.The linearized equations are as follows.

Fig.9.Freundlich plot of Cd(II)sorption on AC-T.

Table 2 Adsorption capacity of different adsorbents for Cd(II)adsorption

Fig.10.Effect of contact time on the metal uptake by AC-T.

where k1(min-1)and k2(g·mg-1·min-1)are the rate constants of the pseudo- first-order and pseudo-second-order rate equations,respectively.

The kinetic data are analyzed in terms of the pseudo- first-order(Fig.11)and pseudo-second-order(Fig.12)rate equations,and the results are given in Table 3.The pseudo-second-order model has a higher correlation coef ficient,with a calculated qevalue close to the experimental value.Therefore,the pseudo-second-order model is an appropriate model to predict the kinetic behavior of Cd(II)adsorption onto AC-T.In this process,the chemical adsorption plays a major role in the interaction of Cd(II)ions with oxygen-containing functional groups of AC-T.

3.4.Adsorbent recycling

Fig.13 shows the percentages ofCd(II)adsorption and desorption on AC-T for three cycles.The first cycle gives 92%adsorption and 81%desorption with the initial concentration of 10 mg·L-1.The second and third cycles give 79%and 61%for adsorption and 63%and 52%for desorption,respectively.The decrease in the adsorption capacity may be the result of the cumulative effect of incomplete desorption process.After three cycles,the Cd(II)adsorption of AC-T is still good.

Fig.11.Pseudo- first-order kinetic model fitting the adsorption process.

Fig.12.Pseudo-second-order kinetic model fitting the adsorption process.

4.Conclusions

The study gives the Cd(II)adsorption capacity and adsorption process of AC-T.Nitric acid oxidation introduces oxygen-containing functional groups(-COOH)on carbon surfaces,and the functional groups play an important role in Cd(II)adsorption.The adsorption isotherms fit the Langmuir model better.The maximum sorption capacity under the experimental conditions is 25.13 mg·g-1.The sorption kinetics follows the pseudo-second-order rate equation and the calculated qevalue is in good agreementwith the experiments.Based on the study on initial pH,adsorption thermodynamics,adsorption kinetics and carbon content before and after Cd(II)adsorption,the Cd(II)adsorption on AC-T is mainly attributed to cation exchange and electrostatic attraction between Cd(II)species and oxygen-containing functional groups on the carbon surfaces.

Table 3 Constants and regression coef ficient for the kinetic models

Fig.13.Percentage of Cd(II)adsorbed-desorbed in the cycle.