Materials Science and Engineering A391(2005)121–123Adsorption behaviors of heavy metal ions onto electrochemicallyoxidized activated carbonfibersSoo-Jin Park∗,Young-Mi KimAdvanced Materials Division,Korea Research Institute of Chemical Technology,P.O.Box107,Yusong,Daejeon305-600,South KoreaReceived4March2004;received in revised form17August2004;accepted27August2004AbstractIn this work,the effect of electrochemical oxidation treatments of activated carbonfibers(ACFs)was studied in heavy metal adsorption behaviors.As a result,the acidic or basic anodic treatment led to increases of Cr(VI),Cu(II),and Ni(II)adsorptions,which could be attributed to the oxygen-containing functional groups of the ACF surfaces.©2004Elsevier B.V.All rights reserved.Keywords:Activated carbonfibers;Electrochemical treatments;Surface properties;Heavy metal ions;Adsorption1.IntroductionAs compared with conventional granular or powder activated carbons,activated carbonfibers(ACFs)have been widely used as an excellent adsorbent because of their large surface area,microporous character,and high adsorption/desorption rate[1].Also,the microstructure of ACFs is developed during activation,and influenced by many factors,such as the degree of activation and the conditions used for carbonization[2].The adsorption/desorption rate of carbonaceous adsorbents is greatly depended on not only microporous structure but also surface properties[3].Gen-erally,the electrochemical oxidation treatment of carbon in the electrolytes used can produce microstructure and surface changes of the carbon surfaces.The advantage of electro-chemical oxidation treatment is obtaining a relatively large number of oxygen-containing functional groups on ACF surfaces[4].In this work,ACFs are modified by electrochemical ox-idation treatment with acidic or basic electrolyte to obtain oxygen-containing functional groups,and the effect of elec-∗Corresponding author.Tel.:+82428607234;fax:+82428614151.E-mail address:psjin@krict.re.kr(S.-J.Park).trochemical oxidation treatment on ACFs is studied in the context of heavy metal adsorption behaviors.2.ExperimentalThe pitch-based ACFs(bundle type,Kureha)were washed with deionized water and dried for24h at80◦C(untreated-ACFs).All other chemicals were purchased in analytical grade purity from Aldrich Chemical Co.and used as re-ceived.The ACFs were subjected to electrolytic reaction in the aqueous solutions of10wt.%H3PO4(A-ACFs)and NH4OH(B-ACFs),whereby negative ions were attracted to the surface of the ACFs acting as an anode,thereby modi-fying the ACF surfaces.A cathode graphite plate was also submerged in the electrolyte solution,and the conditions of the surface treatment were processed in an electro-bath at7A for10min.The treated ACFs emerging from the electrolytic cell were dried for6h at110◦C.The surface properties of ACFs were measured by Boehm’s titration.The specific sur-face area and the pore structure were evaluated from nitro-gen adsorption data at77K(Micromeritics,ASAP2010)[5].0.05g of the ACFs was placed in contact with150ml solution of20ppm concentrations of Na2CrO4·4H2O,CuCl2·2H2O, and NiCl2·6H2O.The single bottle was sealed with paraffin0921-5093/$–see front matter©2004Elsevier B.V.All rights reserved. doi:10.1016/j.msea.2004.08.074122S.-J.Park,Y.-M.Kim /Materials Science and Engineering A 391(2005)121–123Table 1Surface functionalities of ACFs by Boehm titrationOxygen-containing functional groups (meq/g)CarboxylicLactonic Phenolic Untreated-ACFs 34070A-ACFs 40230140B-ACFs48070film and then shaken.The adsorbed amount of heavy metal ions was measured by inductively coupled plasma-atomic emission spectrometer (ICP-AES,Jovin-Yvon Ultima-C).3.Results and discussionNumerous studies on surface functionalities of carbon are already described in the literature.The difference of the sur-face functionalities on basic ACFs and acidically treated ACFs are determined by Boehm’s titration and listed in Table 1.The three bases used in the titration are regarded as approximate probes of acidic surface functionalities accord-ing to the scheme NaHCO 3(carboxyl),Na 2CO 3(carboxyl and f-lactone),NaOH (carboxyl,f-lactonic,and phenolic).As seen in Table 1,various oxygen-containing functional groups,i.e.,carboxylic,lactonic,and phenolic groups are induced in the ACF surfaces by electrochemical oxidation treatment The porous textural parameters of the original and the modified samples are obtained,as listed in Table 2.The total pore volume and the micropore volume of elecrochemically treated ACFs are decreased.This is due to the increase of oxygen-containing functional groups,which are attributed to the block of the micropores.Also,the BET’s specific sur-face area is decreased by 19%for A-ACFs compared to the B-ACFs.This can be explained by that the specific surface area and the micropore volume of A-ACFs are decreased by the pore blocking of surface functional groups and by pore destroying of acidic electrolyte with these basic carbon ma-terials.Adsorption isotherms of Cr(VI),Cu(II),and Ni(II)from aqueous solutions of heavy metal ions in the time range 0–180min on the three samples of ACFs without adding any buffering reagent are shown in Figs.1and 2.Fig.1clearly shows that the initial adsorption rate of Cr(VI)ion on the ACFs increased rapidly,especially due to the molecular sieve structures of the ACFs.Also,the amount of Cr(VI)adsorbed is comparatively much larger than the amount of Cu(II)and Ni(II)adsorbed under similar conditions.This can be imag-ined that the high ionic radius of Cu(II)(0.70˚A)and Ni(II)Table 2Textural characteristics of electrochemically treated ACFsMicropore volume (cm 3g −1)Total pore volume (cm 3g −1)BET surface area (m 2g −1)Untreated-ACFs 0.787 1.0271944A-ACFs 0.2430.5601430B-ACFs0.6600.7811757Fig.1.Adsorption of Cr(VI)onto activated carbon fibers as a function of contacttime.Fig.2.Adsorption of metal ions onto activated carbon fibers as a function of contact time:(a)Cu(II)and (b)Ni(II).S.-J.Park,Y.-M.Kim/Materials Science and Engineering A391(2005)121–123123(0.69˚A)compared to that of Cr(VI)(0.52˚A)induces a quick saturation of adsorption sites because of steric over-crowding.It results in lower adsorption capacities for Cu(II) and Ni(II)than for Cr(VI)[6].The adsorption capacity for Cu(II)appears to be higher than that for Ni(II).It seems to be due to the difference of affinity between ACF surfaces and adsorbed ions.At the low pH,the dominant species of Cr(VI)in the solution are anionic species like HCrO4−, Cr2O72−,Cr4O132−,and Cr3O102−[7].Also,at the high pH,carbon behaves as an acid,whereas at the low pH values it behaves as a base[8].Thus,the chromate anions will be expected to interact more strongly with the electron accepter H+of surface functional groups of ACF surfaces.That is, there is a possibility of the existence of some active sites where negatively charged Cr(VI)can be adsorbed.At pH5,Cu(II)and Ni(II)are cationic species,such as, Cu2+or CuOH+and Ni2+or NiOH+[6].And they are suit-able to interact with negatively charged groups of ACFs.H+ ions compete with metal ions for the exchange sites in the system at pH5and the surface functional groups in aqueous solution produce H+ions,which are directed towards the liq-uid phase,leaving the carbon surface with negatively charged sites.Thus,the availability of relatively high concentration of negatively charged sites in case of the treated ACFs results in an increase in the adsorption of Cu(II)and Ni(II),as seen in Fig.2.As illustrated in Figs.1and2,the percentage adsorption of heavy metal ions increases with increasing the agitation reaction time and the adsorbent capacities decrease in the order B-ACFs>A-ACFs>untreated-ACFs,in spite of a de-creases of specific surface area,as seen in Figs.1and2.This is because at higher dose of adsorbent due to the increased oxygen-containing functional groups,more adsorption sites are available,causing higher removal of heavy metal ions. Also,A-ACFs show lower percentage adsorption of heavy metal ions than that of B-ACFs.It seems to be due to the destruction of micropores by acidic electrolyte,resulting in decreasing the specific surface area.These results can be ex-plained that the adsorption capacities of ACFs are greatly depended on not only microporous structure but also surface properties.4.ConclusionsIn this work,the oxidized ACFs were studied in the ad-sorption characteristics in terms of the microstructures and surface functional groups.In the results of XPS and BET, the specific surface area of the treated ACFs was decreased, whereas oxygen-containing functional groups of the treated ACFs were increased.As expected,the increased surface functional groups led to an increase of the adsorption of heavy metal ions.In case of A-ACFs,the adsorption of heavy metal ions was increased by increasing of oxygen-containing func-tional groups,in spite of a decrease of specific surface area by acidic electrolyte.References[1]A.Ahmadpour,D.D.Do,Carbon35(1997)1723.[2]Z.Yue,C.Mangun,J.Economy,P.Kemme,D.Cropek,S.Maloney,Environ.Sci.Technol.35(2001)2844.[3]S.J.Park,J.S.Shin,J.Colloid Interface Sci.264(2003)39.[4]C.Grogger,S.G.Fattakhov,V.V.Jouikov,M.M.Shulaeva,V.S.Reznik,Electrochim.Acta49(2004)721.[5]S.Brunauer,P.H.Emmett,E.Teller,J.Am.Chem.Soc.60(1938)309.[6]K.Kadirvelu,C.Faur-Brasquet,P.Le Cloirec,Langmuir16(2000)8404.[7]V.K.Garg,R.Gupta,R.Kumar,R.K.Gupta,Bioresour.Technol.92(2004)79.[8]A.¨Ozer,D.¨Ozer,J.Hazard.Mater.100(2003)219.。
