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Influence of activated carbon surface acidity on adsorption of heavy metal ions and aromatics from aqueous solution
Applied Surface Science 253 (2007) 8554–8559 www.elsevier.com/locate/apsusc
Influence of activated carbon surface acidity on adsorption of heavy metal ions and aromatics from aqueous solution Sanae Sato a, Kazuya Yoshihara a, Koji Moriyama a, Motoi Machida b,*, Hideki Tatsumoto b b
a Faculty of Engineering, Chiba University, Yayoi-cho 1-33, Inage-ku, Chiba 263-8522, Japan Graduate School of Engineering, Chiba University, Yayoi-cho 1-33, Inage-ku, Chiba 263-8522, Japan
Received 27 February 2007; received in revised form 14 April 2007; accepted 17 April 2007 Available online 21 April 2007
Abstract Adsorption of toxic heavy metal ions and aromatic compounds onto activated carbons of various amount of surface C–O complexes were examined to study the optimum surface conditions for adsorption in aqueous phase. Cadmium(II) and zinc(II) were used as heavy metal ions, and phenol and nitrobenzene as aromatic compounds, respectively. Activated carbon was de-ashed followed by oxidation with nitric acid, and then it was stepwise out-gassed in helium flow up to 1273 K to gradually remove C–O complexes introduced by the oxidation. The oxidized activated carbon exhibited superior adsorption for heavy metal ions but poor performance for aromatic compounds. Both heavy metal ions and aromatics can be removed to much extent by the out-gassed activated carbon at 1273 K. Removing C–O complexes, the adsorption mechanisms would be switched from ion exchange to Cp-cation interaction for the heavy metals adsorption, and from some kind of oxygen-aromatics interaction to p–p dispersion for the aromatics. # 2007 Elsevier B.V. All rights reserved. PACS : 81.05.Uw; 81.65.Mq; 81.65.Cf Keywords: Heavy metal; Aromatics; Carbon; Adsorption; Surface
1. Introduction Organic compounds and heavy metals can be found as contaminants in the water environment caused by the human activities in urban, industrial and mining areas. Numerous carbonaceous materials due to their easy handing and high operatability have been widely applied to waste water treatment and water purification to remove these hazardous materials [1,2]. It is generally known that adsorption capacity and affinity of these materials are determined by textural physical properties and surface chemical nature of carbons. While surface area and pore size distribution are usually classified into the physical properties, hetero atoms and compounds such as metal oxides composite of ash [3], nitrogen [4] and oxygen [5] in the carbon correlate with surface nature affecting the adsorption of organic compounds and heavy metal ions in aqueous solutions. Graphene layer Cp electrons of activated
* Corresponding author. Tel.: +81 43 290 3129; fax: +81 43 290 3129. E-mail address: [email protected] (M. Machida). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.04.025
carbon are strongly influenced by nitrogen and oxygen atoms [6,7], whereas effect of metal oxides on the Cp electrons can be hardly observed in the carbon [8]. In general oxygen atom is contained in the graphite structure to some extent depending on the carbon manufacturing process. Additional oxygen atoms can be introduced by oxidation using air, ozone, hydrogen peroxide and nitric acid resulting in formation of carboxyls, lactones and hydroxyls probably at the edge of the graphene layer. The carbon oxidation enhances the heavy metal adsorption in aqueous solutions but reduces the adsorption amount of organic compounds. On the contrary, non-oxidized carbon such as commercially available activated carbons exhibits excellent adsorption for organic compounds but poor performance for heavy metal ions. Consequently, the adsorption of heavy metal ions and organic compound is considered to take a trade-off relationship concerning the surface acidity. For example, the amount of phenol adsorption to activated carbons was significantly decreased with increasing surface acidity, and at the same time the adsorption kinetics was altered from diffusion control to adsorption control, indicating that adsorption sites of phenol on the carbon was changed by
S. Sato et al. / Applied Surface Science 253 (2007) 8554–8559
the oxidation in quality and quantity [9]. In the adsorption control, rate determining step of adsorption can be determined by collision with the adsorption sites on the surface. In our previous study the adsorption affinity to the basic sites of Pb(II) ions, namely Cp-cation interaction, was increased when C–O complexes were removed from the carbon surface by outgassing at 1273 K in helium flow [10], though ion exchange mechanism between lead(II) ions and carboxyl groups can be quantitatively operative for the oxidized carbon. In the present study, changes in adsorption amounts of heavy metals and aromatics were examined when the surface acidity of activated carbon was gradually varied changing the amount of C–O complexes. Adsorption isotherms of an aromatic compound for oxidized carbon and oxygen removed carbon were also examined to clarify the difference in adsorption mechanism. 2. Experimental 2.1. Adsorbents and their treatment Adsorbent used was commercially available granular activated carbon (GAC), namely Filtrasorb 400 (F400) made from coal, purchased from Cargon Mitsubishi Corporation, since good reproducibility could be obtained in the adsorption experiments for several aromatic compounds comparing with coconuts shell based GAC in the previous study [11]. The GAC was washed with hydrochloric acid and fluoric acid consecutively to remove ash in F400 containing around 7%, and then boiled in de-ionized water repeatedly more than ten times until pH of the aqueous solution was no longer changed. The deashed GAC was calcined in air at 773 K to confirm that the ash was thoroughly eliminated by the acid washing. The de-ashed GAC was oxidized with 8 M nitric acid worming at 363–368 K for 6 h, and allowed to cool in room temperature, and then the oxidized GAC was also rinsed with de-ionized water until the solution pH was reached a constant value. The oxidized GAC was still calcined at 623 K in air to decompose the nitric acid remaining in the carbon for 4 h, and allowed to cool to room temperature in a desiccator. The calcined oxidized GAC was referred to as GAC-Ox. Removal of the surface C–O complexes on GAC-Ox introduced by the nitric oxidation was carried out using outgassing in helium flow. One to three gram of GAC-Ox was placed in a quartz tube, and 99.9995% high purity helium gas was slowly introduced to the quartz tube to prevent the carbon from scattering toward the down flow direction. When the most of the inner tube volume was sufficiently replaced by helium gas, the tube temperature was increased to 383 K for 10 min and kept for 5 min in an electric furnace to remove the extra air and moisture remaining inside the activated carbon particle, and then the furnace temperature was raised up furthermore to the out-gassing temperature between 573 K and 1273 K, and kept at the desired temperature for 30 min. In the final step, the quartz tube was removed from the furnace and cooled down to room temperature, and kept helium flow for 5 min before contact with air. The outgassed carbon was stored in a desiccator. These out-gassed carbons were donated such as GAC-Ox-OG1073 indicating that
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the de-ashed GAC was oxidized by nitric acid, followed by heating in helium flow at 1073 K as a out-gassing temperature. The out-gassing treatment at 1273 K was carried out for the nonoxidized de-ashed GAC, denoting GAC-OG1273, to compare with the out-gassed carbon after oxidation. 2.2. Properties of the prepared adsorbents The analytical characterization of the prepared activated carbon was made by N2 adsorption–desorption at 77 K (Beckman Coulter Model 3100) and Boehm acid and base titration [12,13,14]. The BET surface area and pore size distribution were obtained from the N2 adsorption using the t-plots numerical analysis. Surface acidic functional groups and basic sites were determined using the Boehm titration. A half-gram of carbon and 15 mL of NaHCO3 (0.1 M), Na2CO3 (0.05 M) or NaOH (0.1 M) solution was mixed in conical flask, and agitated at 100 rpm for more than two days in room temperature. An aliquot of the solution for each sample was back titrated with HCl (0.1 M). The NaHCO3 neutralizes only carboxylic groups on the carbon surface, Na2CO3 does carboxylic and lactonic, and NaOH reacts with carboxylic, lactonic and hydroxyl groups. Accordingly, the difference between the groups neutralized by NaHCO3 and Na2CO3 becomes lactones, the difference between those neutralized by Na2CO3 and NaOH is hydroxyls. The same procedure was carried out for the mixture of 0.5 g of the carbons and 15 mL of HCl (0.1 M) solution to determine the basic sites of the carbon surface. The remaining HCl solution was titrated with NaOH (0.1 M) after neutralization. Neutralization points were fixed using methyl red solution indicator for weak base titrated with strong acid, and phenolphthalein solution for strong acid and strong base combination. 2.3. Adsorption measurements Cadmium(II) and zinc(II) were employed as heavy metal ions; cadmium(II) chloride and zinc(II) nitrate were dissolved into de-ionized water to prepare stock solutions of 8.9 mmol/L and 15.3 mmol/L, respectively. Likewise nitrobenzene and phenol were added to de-ionized water to prepare aromatics aqueous stock solution. High solubility phenol stock solution was adjusted to 106 mmol/L, while relatively low solubility nitrobenzene was 12.7 mmol/L approaching solubility limit. All chemicals used in the experiments were regent grade. The stock solution was diluted to desired concentration with deionized water. A 100-mg of the adsorbents was dosed to 50 mL of the aqueous solution, and agitated by 100 rpm for more than 2 days at 298 K. When adsorption isotherm for nitrobenzene was drawn beyond the initial concentration of the solubility limit, a part or all of the equilibrium solution was replaced by new 12.7 mmol/L stock solution. Consequently, the equilibrium solution concentration can go up to solubility limit repeating this procedure. Amounts of adsorption onto the adsorbents were calculated from difference between initial and equilibrium concentrations. The solution pH was measured by a portable pH meter (HORIBA Model D-51) for the initial and the equilibrium
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solutions, by which proton release form the adsorbent in proceeding the heavy metals adsorption was calculated. Metal ions concentration in the solution was determined using atomic absorption spectroscopy (AAS, Rigaku novAA 300). Aromatics contained in the solution were measured by UV–Vis spectrometer (SHIMADZU UV-2550). Prior to the AAS and the UV analysis, the solution was diluted with de-ionized water and a drop of HCl (1 M) to the external standard concentration range. The hydrochloric acid was added to stabilize the chemical species in the solution during the analysis for heavy metal ions and phenol to minimize analytical error. 3. Results and discussion 3.1. Properties of adsorbents Fig. 1 shows the variation of micro-pore and meso- and macro-pore surface area when the out-gassing temperature was increased from 573 K to 1273 K. No major difference in surface area can be observed by changing the out-gassing temperature. In contrast, there is much difference in surface acidic functional groups and basic groups/sites between oxidized and out-gassed activated carbons as is clearly seen in Fig. 2. In our preliminary study, basic sites determined by Boehm titration can be principally attributed to the Cp electrons on the graphene layer [10]. Acidic groups were decreased but basic sites were increased with rise in out-gassing temperature. Decrease in carboxyls was proportional to out-gassing temperature up to 873 K, and above the temperature they could be no more detected on the carbon surface. Lactones were slightly increased up to 773 K resulted from dehydration of carboxyls and hydroxyls, and then gradually decreased with increasing temperature, finally disappeared completely at 1273 K. Hydroxyl groups were also gradually decreased with temperature rising, but more than 0.5 mmol/g hydroxyls were remained on the carbon surface even if out-gassing was made at 1273 K. On the other hand, basic sites reflecting Cp density on the graphene layer were gradually increased with decreasing carboxyls and lactones which could be electron withdrawing functional groups. Based on the results the activated carbon
Fig. 1. Influence of out-gassing temperature in helium flow on meso & macroand micro-pore surface area of activated carbon. GAC-Ox-OG873; out-gassing of nitric acid oxidized activated carbon at 873 K in helium flow.
Fig. 2. Variation of surface acidic groups and basic sites, determined by Boehm titration, as a function of out-gassing temperature. (*) Carboxylic groups; (~) lactonic groups; (&) hydroxyl groups; (*) basic sites.
used in the experiments thankfully changed only the surface oxygen functional groups by oxidation and out-gassing without a significant change in textural properties. 3.2. Influence of out-gassing temperature on heavy metals adsorption Fig. 3 shows changes in amounts of cadmium(II) and zinc(II) ions onto the activated carbons, and the ratio of the proton released, from GAC-Ox and their out-gassing counterparts to aqueous solution, to the equivalent metal ions adsorbed to them as a function of out-gassing temperature. The equilibrium pH
Fig. 3. Changes in amounts of adsorption of cadmium(II) (&) and zinc(II) (^) in the left hand vertical axis, and the corresponding ratio of proton released to metal ion adsorbed in the right hand axis, (&) and (^) for cadmium(II) and zinc(II), respectively, as a function of out-gassing temperature. [M2+]: molar concentration of metal ion of cadmium(II) or zinc(II). Initial concentration: 0.56 mmol/L for cadmium(II), 0.36 mmol/L for zinc(II).
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always ranged from 3.6 for oxidized carbon to 6.7 for out-gassed at 1273 K. The zinc(II) ions are always present as Zn2+ under the experimental conditions, while chemical species for cadmium(II) ions will take not only Cd2+ but also Cd(OH)+ by about 10% in maximum when the solution pH approaches 7.0. The adsorption amounts exhibit similar values for both cadmium(II) and zinc(II) ions though the initial concentrations were different between cadmium(II) and zinc(II) ions, revealing zinc(II) ions can be adsorbed onto GAC more than cadmium(II) ions. With increasing out-gassing temperature of GAC-Ox, the amounts of adsorbed cadmium(II) and zinc(II) ions decreased toward 973– 1073 K, especially sharp decline in the adsorption amounts was observed from 573 K to 973–1073 K. Afterwards, sharp regain in the adsorption amounts can be seen toward 1273 K. As also represented in Fig. 3, the ratio of the proton release to the equivalent metal adsorption showed constant values, a little less than 1.0, from GAC-Ox to GAC-Ox673, out-gassing temperature of 673 K, and then it was gradually decreased toward zero between 673 K and 973 K. In the higher out-gassing temperature from 973 K to 1273 K, the ratio of the proton release to the metal adsorption was remained zero, whereas sharp adsorption increase of cadmium(II) and zinc(II) ions was clearly seen. These results can be interpreted in terms of switching adsorption mechanism from ion exchange to Cp metal cation static interaction. In the lower out-gassing temperature up to 673 K, the metal ions can be adsorbed onto the carbon surface by ion exchange with carboxyl groups remained on the carbon surface. Since the number of proton release was a little less than that of equivalent metal ions adsorption, and as can be seen in Fig. 2, lactonic groups remained at higher temperature than carboxyls as also reported by S´wia˛tkowski et al. [15], lactonic groups might be contributed to adsorption in addition to carboxyl groups. Shim et al. pointed out that both lactones and carboxyls could be involved in the cupper and nickel ions adsorption on the carbon [16]. Rivera-Utrilla also observed that trivalent chromium ions could proportionally adsorbed onto both carboxylic and lactonic groups [17]. The amount of metal ions uptake and the proton release to metal adsorption ratio were gradually decreased toward 973 K accompanied by the decline in carboxylic groups. The successive sharp increase in metal ions uptake without any ion exchange with proton from 1073 K to 1273 K was resulted from Cp-cation static interaction [10,17]. When there are carboxyls and lactones on the carbon surface, Cp electron will be effectively shifted from the graphene sheet to the C–O functional groups, decrease in Cp cloud density, resulting in decrease in Cp-cation interaction. The significant decrease in Cp electron density by the C–O acidic groups formed on the edge of the graphene sheet will be possible, because wide area graphene sheet will be never formed at less than 2000 K [18], and the activation temperature for GAC manufacturing will be around 1000 K in most of the case including F400. However, once the C–O complexes are removed from the carbon surface by out-gassing at the higher temperature, Cp electron was turned back to the graphene layer, leading to the rise in Cpcation interaction. The sharp increase in the metal ions uptake, therefore, can be caused by the rise in Cp electron density. The increase in number of basic sites on the carbon by out-gassing
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Fig. 4. Changes in amounts of adsorption of nitrobenzene (*) and phenol (~) as a function of out-gassing temperature. Initial concentration: 7.93 mmol/L for phenol, 10.2 mmol/L for nitrobenzene.
at the higher temperature as seen in Fig. 2 will also be reflected by the regain of Cp electron density by removing the C–O complexes. 3.3. Influence of out-gassing temperature on aromatics adsorption Influence of out-gassing temperature on aromatics adsorption was depicted in Fig. 4. Amount of adsorption was gradually increased with rise in out-gassing temperature for both nitrobenzene and phenol. Comparing with heavy metals adsorption in Fig. 3, the opposite tendency was obtained up to the out-gassing temperature of 873–973 K for the aromatics adsorption, supporting that the adsorption sites on the carbons would be different between heavy metals ions and aromatics. On the contrary, at out-gassing temperature above 973 K, both heavy metals and aromatics adsorption were enhanced by the elimination of surface C–O complexes, revealing that the adsorption sites will be on the same Cp electrons on the graphene layer. For phenol adsorption to oxidized and outgassed activated carbons, the out-gassing is advantageous for adsorption capacity compared to the oxidation. The adsorption kinetics was found out to be clearly switched form diffusion control to collision control against surface adsorption sites by the oxidation of the carbon surface [9]. Qi et al. also pointed out that when oxidized activated carbon was used, the adsorption of phenolic compounds mainly took place on the external surface, and was controlled by adsorption with increase in total density of carboxylic and lactonic functional groups [19]. In addition, two-stepped shape adsorption isotherms were measured for the oxidized carbon [11,17]. In the present study, adsorption isotherms of nitrobenzene were examined for the oxidized and the out-gassed activated carbons as shown in Fig. 5. Though the adsorption isotherm of nitrobenzene obeys the Langmuir
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concentration of nitrobenzene. On the other hand, only the flaton orientation seems to be valid for the out-gassing activated carbon, because the p–p dispersion is mainly considered to work and the sum of the occupational molecular area at flat-on orientation calculated by the maximum amounts of nitrobenzene adsorption are close to B.E.T. surface area. From the experimental results, when the carboxylic and lactonic groups are present on the carbon, trade-off relationship is held between heavy metals ions and aromatics adsorption. In case of no or a very little C–O acidic groups on the activated carbon, significant amounts of accommodation on the carbon can be realized both for heavy metals ions and aromatics due to interaction with high density Cp electrons on the graphene layer. 4. Conclusions
Fig. 5. Adsorption isotherms of nitrobenzene at 293 K for oxidized (~, GACOx) and 1273 K out-gassed activated carbon (*, GAC-Ox-OG1273). Solid line: predicted by Langmuir isothermal equation by adsorption capacity and affinity of 4.9 mmol/g and 5.2 L/mmol, respectively.
isotherm for the out-gassed activated carbon, GAC-OxOG1273, it is like a Henry isotherm up to the saturation amount and remains plateau at higher solution concentration for oxidized carbon, GAC-Ox, revealing that there will be different adsorption sites between out-gassed and oxidized activated carbons. Furthermore, it is noteworthy that maximum adsorption amounts of nitrobenzene are almost the same for the both activated carbons, whereas it is generally known that they are different in the phenol adsorption as mentioned above. The Henry type isotherm and/or twostepped isotherms on the oxidized activated carbon were also observed for other aromatics such as bisphenol A, aniline and benzoic acid [11]. It is also widely recognized that aromatics adsorption onto out-gassed activated carbon is considered to be via interaction between the Cp on the carbon and the p electrons of aromatics, namely p–p dispersive force. However, attractive force between aromatics and C–O acidic complexes on the carbon has not been elucidated yet so far, through hydrogen bonding and/or electron donor–acceptor mechanism are proposed [20]. From a phenomenal point of view, the nitrobenzene adsorption for the oxidized activated carbon might be altered its adsorptive orientation on the carbon outer sphere from flaton to end-on adsorption in which not only carbon solute interaction but p–p interaction among the solutes are operative on the carbon depending on its concentration [21]; at first the adsorption orientation of nitrobenzene can be flat-on position around the C–O complexes at the edge of the graphene layer, and then the flat-on position is gradually switched to end-on position to increase the number of accommodation onto the carbon surface with rise in concentration, and in the final stage of the saturation on the carbon, all nitrobenzene can take endon position, resulting in no further adsorption can take place on the oxidized activated carbon even if the higher solution
Influence of chemical surface heterogeneity of activated carbon on the adsorption phenomena of dissolved heavy metal ions of cadmium(II) and zinc(II) and aromatics of phenol and nitrobenzene. The results can be summarized as follows: (1) By out-gassing oxidized activated carbons, acidic surface functional groups and basic sites could be altered without major change in textural properties of surface area and pore distribution. (2) Up to 973 K of out-gassing temperature, the heavy metals ions adsorption was decreased but the aromatics adsorption was enhanced with removal in carboxylic and lactonic groups. Above 973 K, adsorptive uptake was sharply improved both for the heavy metals ions and the aromatics. (3) Adsorption mechanism of heavy metals ions onto the oxidized activated carbon was switched from ion exchange with the acidic functional groups to the Cp-cation interaction around 973 K of out-gassing temperature. (4) Different adsorption isotherms for nitrobenzene adsorption were observed between the oxidized and the out-gassed activated carbons at 1273 K. The reorientation mechanism from flat-on to end-on adsorption of the adsorbate can be proposed for the oxidized carbon, whereas only the flat-on adsorption by p–p dispersion interaction was estimated to be valid for the out-gassed counterpart. (5) Introduction of C–O complexes to the activated carbon by oxidation seems to be advantageous only for heavy metals ions adsorption, but the complete removal of them by outgassing is favorable both for heavy metals and aromatics. Acknowledgements The authors thank Dr. Masami Aikawa of Kisarazu National College of Technology and Ms. Yoko Fujimura of Chiba Prefectural Environmental Research Center for their discussion and suggestion on the study. They are also grateful to Dr. Keiichi Nagao, the head of Safety and Health Organization Chiba University, for his financial support and thankful encouragement on our works.
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[9] D.M. Nevskaia, A. Santianes, V. Munoz, A. Guerrero-Ruiz, Carbon 37 (1999) 1065. [10] M. Machida, T. Mochimaru, H. Tatsumoto, Carbon 44 (2006) 2681. [11] M. Machida, M. Hayashida, Y. Kato, H. Tatsumoto, TANSO 224 (2006) 266. [12] H.P. Boehm, Carbon 40 (2002) 145. [13] H.P. Boehm, Carbon 32 (1994) 759. [14] H.P. Boehm, Adv. Catal. 16 (1966) 179. [15] A. S´wia˛tkowski, H. Grajek, M. Pakuła, S. Biniak, Z. Witkiewicz, Colloids Surf. A 208 (2002) 313. [16] J.-W. Shim, S.-J. Park, S.-K. Ryu, Carbon 39 (2001) 1635. [17] J. Rivera-Utrilla, M. Sanchez-Polo, Water Res. 37 (2003) 3335. [18] H. Marsh, R. menedez, Fuel Process. Technol. 20 (1988) 269. [19] J. Qi, Z. Li, Y. Guo, H. Xu, Mater. Chem. Phys. 87 (2004) 96. [20] M. Franz, H.A. Arafat, N.G. Pinto, Carbon 38 (2000) 1807. [21] R.W. Coughlin, F.S. Ezra, Environ. Sci. Technol. 2 (1968) 291.
Influence of activated carbon surface acidity on adsorption of heavy metal ions and aromatics from aqueous solution Sanae Sato a, Kazuya Yoshihara a, Koji Moriyama a, Motoi Machida b,*, Hideki Tatsumoto b b
a Faculty of Engineering, Chiba University, Yayoi-cho 1-33, Inage-ku, Chiba 263-8522, Japan Graduate School of Engineering, Chiba University, Yayoi-cho 1-33, Inage-ku, Chiba 263-8522, Japan
Received 27 February 2007; received in revised form 14 April 2007; accepted 17 April 2007 Available online 21 April 2007
Abstract Adsorption of toxic heavy metal ions and aromatic compounds onto activated carbons of various amount of surface C–O complexes were examined to study the optimum surface conditions for adsorption in aqueous phase. Cadmium(II) and zinc(II) were used as heavy metal ions, and phenol and nitrobenzene as aromatic compounds, respectively. Activated carbon was de-ashed followed by oxidation with nitric acid, and then it was stepwise out-gassed in helium flow up to 1273 K to gradually remove C–O complexes introduced by the oxidation. The oxidized activated carbon exhibited superior adsorption for heavy metal ions but poor performance for aromatic compounds. Both heavy metal ions and aromatics can be removed to much extent by the out-gassed activated carbon at 1273 K. Removing C–O complexes, the adsorption mechanisms would be switched from ion exchange to Cp-cation interaction for the heavy metals adsorption, and from some kind of oxygen-aromatics interaction to p–p dispersion for the aromatics. # 2007 Elsevier B.V. All rights reserved. PACS : 81.05.Uw; 81.65.Mq; 81.65.Cf Keywords: Heavy metal; Aromatics; Carbon; Adsorption; Surface
1. Introduction Organic compounds and heavy metals can be found as contaminants in the water environment caused by the human activities in urban, industrial and mining areas. Numerous carbonaceous materials due to their easy handing and high operatability have been widely applied to waste water treatment and water purification to remove these hazardous materials [1,2]. It is generally known that adsorption capacity and affinity of these materials are determined by textural physical properties and surface chemical nature of carbons. While surface area and pore size distribution are usually classified into the physical properties, hetero atoms and compounds such as metal oxides composite of ash [3], nitrogen [4] and oxygen [5] in the carbon correlate with surface nature affecting the adsorption of organic compounds and heavy metal ions in aqueous solutions. Graphene layer Cp electrons of activated
* Corresponding author. Tel.: +81 43 290 3129; fax: +81 43 290 3129. E-mail address: [email protected] (M. Machida). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.04.025
carbon are strongly influenced by nitrogen and oxygen atoms [6,7], whereas effect of metal oxides on the Cp electrons can be hardly observed in the carbon [8]. In general oxygen atom is contained in the graphite structure to some extent depending on the carbon manufacturing process. Additional oxygen atoms can be introduced by oxidation using air, ozone, hydrogen peroxide and nitric acid resulting in formation of carboxyls, lactones and hydroxyls probably at the edge of the graphene layer. The carbon oxidation enhances the heavy metal adsorption in aqueous solutions but reduces the adsorption amount of organic compounds. On the contrary, non-oxidized carbon such as commercially available activated carbons exhibits excellent adsorption for organic compounds but poor performance for heavy metal ions. Consequently, the adsorption of heavy metal ions and organic compound is considered to take a trade-off relationship concerning the surface acidity. For example, the amount of phenol adsorption to activated carbons was significantly decreased with increasing surface acidity, and at the same time the adsorption kinetics was altered from diffusion control to adsorption control, indicating that adsorption sites of phenol on the carbon was changed by
S. Sato et al. / Applied Surface Science 253 (2007) 8554–8559
the oxidation in quality and quantity [9]. In the adsorption control, rate determining step of adsorption can be determined by collision with the adsorption sites on the surface. In our previous study the adsorption affinity to the basic sites of Pb(II) ions, namely Cp-cation interaction, was increased when C–O complexes were removed from the carbon surface by outgassing at 1273 K in helium flow [10], though ion exchange mechanism between lead(II) ions and carboxyl groups can be quantitatively operative for the oxidized carbon. In the present study, changes in adsorption amounts of heavy metals and aromatics were examined when the surface acidity of activated carbon was gradually varied changing the amount of C–O complexes. Adsorption isotherms of an aromatic compound for oxidized carbon and oxygen removed carbon were also examined to clarify the difference in adsorption mechanism. 2. Experimental 2.1. Adsorbents and their treatment Adsorbent used was commercially available granular activated carbon (GAC), namely Filtrasorb 400 (F400) made from coal, purchased from Cargon Mitsubishi Corporation, since good reproducibility could be obtained in the adsorption experiments for several aromatic compounds comparing with coconuts shell based GAC in the previous study [11]. The GAC was washed with hydrochloric acid and fluoric acid consecutively to remove ash in F400 containing around 7%, and then boiled in de-ionized water repeatedly more than ten times until pH of the aqueous solution was no longer changed. The deashed GAC was calcined in air at 773 K to confirm that the ash was thoroughly eliminated by the acid washing. The de-ashed GAC was oxidized with 8 M nitric acid worming at 363–368 K for 6 h, and allowed to cool in room temperature, and then the oxidized GAC was also rinsed with de-ionized water until the solution pH was reached a constant value. The oxidized GAC was still calcined at 623 K in air to decompose the nitric acid remaining in the carbon for 4 h, and allowed to cool to room temperature in a desiccator. The calcined oxidized GAC was referred to as GAC-Ox. Removal of the surface C–O complexes on GAC-Ox introduced by the nitric oxidation was carried out using outgassing in helium flow. One to three gram of GAC-Ox was placed in a quartz tube, and 99.9995% high purity helium gas was slowly introduced to the quartz tube to prevent the carbon from scattering toward the down flow direction. When the most of the inner tube volume was sufficiently replaced by helium gas, the tube temperature was increased to 383 K for 10 min and kept for 5 min in an electric furnace to remove the extra air and moisture remaining inside the activated carbon particle, and then the furnace temperature was raised up furthermore to the out-gassing temperature between 573 K and 1273 K, and kept at the desired temperature for 30 min. In the final step, the quartz tube was removed from the furnace and cooled down to room temperature, and kept helium flow for 5 min before contact with air. The outgassed carbon was stored in a desiccator. These out-gassed carbons were donated such as GAC-Ox-OG1073 indicating that
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the de-ashed GAC was oxidized by nitric acid, followed by heating in helium flow at 1073 K as a out-gassing temperature. The out-gassing treatment at 1273 K was carried out for the nonoxidized de-ashed GAC, denoting GAC-OG1273, to compare with the out-gassed carbon after oxidation. 2.2. Properties of the prepared adsorbents The analytical characterization of the prepared activated carbon was made by N2 adsorption–desorption at 77 K (Beckman Coulter Model 3100) and Boehm acid and base titration [12,13,14]. The BET surface area and pore size distribution were obtained from the N2 adsorption using the t-plots numerical analysis. Surface acidic functional groups and basic sites were determined using the Boehm titration. A half-gram of carbon and 15 mL of NaHCO3 (0.1 M), Na2CO3 (0.05 M) or NaOH (0.1 M) solution was mixed in conical flask, and agitated at 100 rpm for more than two days in room temperature. An aliquot of the solution for each sample was back titrated with HCl (0.1 M). The NaHCO3 neutralizes only carboxylic groups on the carbon surface, Na2CO3 does carboxylic and lactonic, and NaOH reacts with carboxylic, lactonic and hydroxyl groups. Accordingly, the difference between the groups neutralized by NaHCO3 and Na2CO3 becomes lactones, the difference between those neutralized by Na2CO3 and NaOH is hydroxyls. The same procedure was carried out for the mixture of 0.5 g of the carbons and 15 mL of HCl (0.1 M) solution to determine the basic sites of the carbon surface. The remaining HCl solution was titrated with NaOH (0.1 M) after neutralization. Neutralization points were fixed using methyl red solution indicator for weak base titrated with strong acid, and phenolphthalein solution for strong acid and strong base combination. 2.3. Adsorption measurements Cadmium(II) and zinc(II) were employed as heavy metal ions; cadmium(II) chloride and zinc(II) nitrate were dissolved into de-ionized water to prepare stock solutions of 8.9 mmol/L and 15.3 mmol/L, respectively. Likewise nitrobenzene and phenol were added to de-ionized water to prepare aromatics aqueous stock solution. High solubility phenol stock solution was adjusted to 106 mmol/L, while relatively low solubility nitrobenzene was 12.7 mmol/L approaching solubility limit. All chemicals used in the experiments were regent grade. The stock solution was diluted to desired concentration with deionized water. A 100-mg of the adsorbents was dosed to 50 mL of the aqueous solution, and agitated by 100 rpm for more than 2 days at 298 K. When adsorption isotherm for nitrobenzene was drawn beyond the initial concentration of the solubility limit, a part or all of the equilibrium solution was replaced by new 12.7 mmol/L stock solution. Consequently, the equilibrium solution concentration can go up to solubility limit repeating this procedure. Amounts of adsorption onto the adsorbents were calculated from difference between initial and equilibrium concentrations. The solution pH was measured by a portable pH meter (HORIBA Model D-51) for the initial and the equilibrium
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solutions, by which proton release form the adsorbent in proceeding the heavy metals adsorption was calculated. Metal ions concentration in the solution was determined using atomic absorption spectroscopy (AAS, Rigaku novAA 300). Aromatics contained in the solution were measured by UV–Vis spectrometer (SHIMADZU UV-2550). Prior to the AAS and the UV analysis, the solution was diluted with de-ionized water and a drop of HCl (1 M) to the external standard concentration range. The hydrochloric acid was added to stabilize the chemical species in the solution during the analysis for heavy metal ions and phenol to minimize analytical error. 3. Results and discussion 3.1. Properties of adsorbents Fig. 1 shows the variation of micro-pore and meso- and macro-pore surface area when the out-gassing temperature was increased from 573 K to 1273 K. No major difference in surface area can be observed by changing the out-gassing temperature. In contrast, there is much difference in surface acidic functional groups and basic groups/sites between oxidized and out-gassed activated carbons as is clearly seen in Fig. 2. In our preliminary study, basic sites determined by Boehm titration can be principally attributed to the Cp electrons on the graphene layer [10]. Acidic groups were decreased but basic sites were increased with rise in out-gassing temperature. Decrease in carboxyls was proportional to out-gassing temperature up to 873 K, and above the temperature they could be no more detected on the carbon surface. Lactones were slightly increased up to 773 K resulted from dehydration of carboxyls and hydroxyls, and then gradually decreased with increasing temperature, finally disappeared completely at 1273 K. Hydroxyl groups were also gradually decreased with temperature rising, but more than 0.5 mmol/g hydroxyls were remained on the carbon surface even if out-gassing was made at 1273 K. On the other hand, basic sites reflecting Cp density on the graphene layer were gradually increased with decreasing carboxyls and lactones which could be electron withdrawing functional groups. Based on the results the activated carbon
Fig. 1. Influence of out-gassing temperature in helium flow on meso & macroand micro-pore surface area of activated carbon. GAC-Ox-OG873; out-gassing of nitric acid oxidized activated carbon at 873 K in helium flow.
Fig. 2. Variation of surface acidic groups and basic sites, determined by Boehm titration, as a function of out-gassing temperature. (*) Carboxylic groups; (~) lactonic groups; (&) hydroxyl groups; (*) basic sites.
used in the experiments thankfully changed only the surface oxygen functional groups by oxidation and out-gassing without a significant change in textural properties. 3.2. Influence of out-gassing temperature on heavy metals adsorption Fig. 3 shows changes in amounts of cadmium(II) and zinc(II) ions onto the activated carbons, and the ratio of the proton released, from GAC-Ox and their out-gassing counterparts to aqueous solution, to the equivalent metal ions adsorbed to them as a function of out-gassing temperature. The equilibrium pH
Fig. 3. Changes in amounts of adsorption of cadmium(II) (&) and zinc(II) (^) in the left hand vertical axis, and the corresponding ratio of proton released to metal ion adsorbed in the right hand axis, (&) and (^) for cadmium(II) and zinc(II), respectively, as a function of out-gassing temperature. [M2+]: molar concentration of metal ion of cadmium(II) or zinc(II). Initial concentration: 0.56 mmol/L for cadmium(II), 0.36 mmol/L for zinc(II).
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always ranged from 3.6 for oxidized carbon to 6.7 for out-gassed at 1273 K. The zinc(II) ions are always present as Zn2+ under the experimental conditions, while chemical species for cadmium(II) ions will take not only Cd2+ but also Cd(OH)+ by about 10% in maximum when the solution pH approaches 7.0. The adsorption amounts exhibit similar values for both cadmium(II) and zinc(II) ions though the initial concentrations were different between cadmium(II) and zinc(II) ions, revealing zinc(II) ions can be adsorbed onto GAC more than cadmium(II) ions. With increasing out-gassing temperature of GAC-Ox, the amounts of adsorbed cadmium(II) and zinc(II) ions decreased toward 973– 1073 K, especially sharp decline in the adsorption amounts was observed from 573 K to 973–1073 K. Afterwards, sharp regain in the adsorption amounts can be seen toward 1273 K. As also represented in Fig. 3, the ratio of the proton release to the equivalent metal adsorption showed constant values, a little less than 1.0, from GAC-Ox to GAC-Ox673, out-gassing temperature of 673 K, and then it was gradually decreased toward zero between 673 K and 973 K. In the higher out-gassing temperature from 973 K to 1273 K, the ratio of the proton release to the metal adsorption was remained zero, whereas sharp adsorption increase of cadmium(II) and zinc(II) ions was clearly seen. These results can be interpreted in terms of switching adsorption mechanism from ion exchange to Cp metal cation static interaction. In the lower out-gassing temperature up to 673 K, the metal ions can be adsorbed onto the carbon surface by ion exchange with carboxyl groups remained on the carbon surface. Since the number of proton release was a little less than that of equivalent metal ions adsorption, and as can be seen in Fig. 2, lactonic groups remained at higher temperature than carboxyls as also reported by S´wia˛tkowski et al. [15], lactonic groups might be contributed to adsorption in addition to carboxyl groups. Shim et al. pointed out that both lactones and carboxyls could be involved in the cupper and nickel ions adsorption on the carbon [16]. Rivera-Utrilla also observed that trivalent chromium ions could proportionally adsorbed onto both carboxylic and lactonic groups [17]. The amount of metal ions uptake and the proton release to metal adsorption ratio were gradually decreased toward 973 K accompanied by the decline in carboxylic groups. The successive sharp increase in metal ions uptake without any ion exchange with proton from 1073 K to 1273 K was resulted from Cp-cation static interaction [10,17]. When there are carboxyls and lactones on the carbon surface, Cp electron will be effectively shifted from the graphene sheet to the C–O functional groups, decrease in Cp cloud density, resulting in decrease in Cp-cation interaction. The significant decrease in Cp electron density by the C–O acidic groups formed on the edge of the graphene sheet will be possible, because wide area graphene sheet will be never formed at less than 2000 K [18], and the activation temperature for GAC manufacturing will be around 1000 K in most of the case including F400. However, once the C–O complexes are removed from the carbon surface by out-gassing at the higher temperature, Cp electron was turned back to the graphene layer, leading to the rise in Cpcation interaction. The sharp increase in the metal ions uptake, therefore, can be caused by the rise in Cp electron density. The increase in number of basic sites on the carbon by out-gassing
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Fig. 4. Changes in amounts of adsorption of nitrobenzene (*) and phenol (~) as a function of out-gassing temperature. Initial concentration: 7.93 mmol/L for phenol, 10.2 mmol/L for nitrobenzene.
at the higher temperature as seen in Fig. 2 will also be reflected by the regain of Cp electron density by removing the C–O complexes. 3.3. Influence of out-gassing temperature on aromatics adsorption Influence of out-gassing temperature on aromatics adsorption was depicted in Fig. 4. Amount of adsorption was gradually increased with rise in out-gassing temperature for both nitrobenzene and phenol. Comparing with heavy metals adsorption in Fig. 3, the opposite tendency was obtained up to the out-gassing temperature of 873–973 K for the aromatics adsorption, supporting that the adsorption sites on the carbons would be different between heavy metals ions and aromatics. On the contrary, at out-gassing temperature above 973 K, both heavy metals and aromatics adsorption were enhanced by the elimination of surface C–O complexes, revealing that the adsorption sites will be on the same Cp electrons on the graphene layer. For phenol adsorption to oxidized and outgassed activated carbons, the out-gassing is advantageous for adsorption capacity compared to the oxidation. The adsorption kinetics was found out to be clearly switched form diffusion control to collision control against surface adsorption sites by the oxidation of the carbon surface [9]. Qi et al. also pointed out that when oxidized activated carbon was used, the adsorption of phenolic compounds mainly took place on the external surface, and was controlled by adsorption with increase in total density of carboxylic and lactonic functional groups [19]. In addition, two-stepped shape adsorption isotherms were measured for the oxidized carbon [11,17]. In the present study, adsorption isotherms of nitrobenzene were examined for the oxidized and the out-gassed activated carbons as shown in Fig. 5. Though the adsorption isotherm of nitrobenzene obeys the Langmuir
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concentration of nitrobenzene. On the other hand, only the flaton orientation seems to be valid for the out-gassing activated carbon, because the p–p dispersion is mainly considered to work and the sum of the occupational molecular area at flat-on orientation calculated by the maximum amounts of nitrobenzene adsorption are close to B.E.T. surface area. From the experimental results, when the carboxylic and lactonic groups are present on the carbon, trade-off relationship is held between heavy metals ions and aromatics adsorption. In case of no or a very little C–O acidic groups on the activated carbon, significant amounts of accommodation on the carbon can be realized both for heavy metals ions and aromatics due to interaction with high density Cp electrons on the graphene layer. 4. Conclusions
Fig. 5. Adsorption isotherms of nitrobenzene at 293 K for oxidized (~, GACOx) and 1273 K out-gassed activated carbon (*, GAC-Ox-OG1273). Solid line: predicted by Langmuir isothermal equation by adsorption capacity and affinity of 4.9 mmol/g and 5.2 L/mmol, respectively.
isotherm for the out-gassed activated carbon, GAC-OxOG1273, it is like a Henry isotherm up to the saturation amount and remains plateau at higher solution concentration for oxidized carbon, GAC-Ox, revealing that there will be different adsorption sites between out-gassed and oxidized activated carbons. Furthermore, it is noteworthy that maximum adsorption amounts of nitrobenzene are almost the same for the both activated carbons, whereas it is generally known that they are different in the phenol adsorption as mentioned above. The Henry type isotherm and/or twostepped isotherms on the oxidized activated carbon were also observed for other aromatics such as bisphenol A, aniline and benzoic acid [11]. It is also widely recognized that aromatics adsorption onto out-gassed activated carbon is considered to be via interaction between the Cp on the carbon and the p electrons of aromatics, namely p–p dispersive force. However, attractive force between aromatics and C–O acidic complexes on the carbon has not been elucidated yet so far, through hydrogen bonding and/or electron donor–acceptor mechanism are proposed [20]. From a phenomenal point of view, the nitrobenzene adsorption for the oxidized activated carbon might be altered its adsorptive orientation on the carbon outer sphere from flaton to end-on adsorption in which not only carbon solute interaction but p–p interaction among the solutes are operative on the carbon depending on its concentration [21]; at first the adsorption orientation of nitrobenzene can be flat-on position around the C–O complexes at the edge of the graphene layer, and then the flat-on position is gradually switched to end-on position to increase the number of accommodation onto the carbon surface with rise in concentration, and in the final stage of the saturation on the carbon, all nitrobenzene can take endon position, resulting in no further adsorption can take place on the oxidized activated carbon even if the higher solution
Influence of chemical surface heterogeneity of activated carbon on the adsorption phenomena of dissolved heavy metal ions of cadmium(II) and zinc(II) and aromatics of phenol and nitrobenzene. The results can be summarized as follows: (1) By out-gassing oxidized activated carbons, acidic surface functional groups and basic sites could be altered without major change in textural properties of surface area and pore distribution. (2) Up to 973 K of out-gassing temperature, the heavy metals ions adsorption was decreased but the aromatics adsorption was enhanced with removal in carboxylic and lactonic groups. Above 973 K, adsorptive uptake was sharply improved both for the heavy metals ions and the aromatics. (3) Adsorption mechanism of heavy metals ions onto the oxidized activated carbon was switched from ion exchange with the acidic functional groups to the Cp-cation interaction around 973 K of out-gassing temperature. (4) Different adsorption isotherms for nitrobenzene adsorption were observed between the oxidized and the out-gassed activated carbons at 1273 K. The reorientation mechanism from flat-on to end-on adsorption of the adsorbate can be proposed for the oxidized carbon, whereas only the flat-on adsorption by p–p dispersion interaction was estimated to be valid for the out-gassed counterpart. (5) Introduction of C–O complexes to the activated carbon by oxidation seems to be advantageous only for heavy metals ions adsorption, but the complete removal of them by outgassing is favorable both for heavy metals and aromatics. Acknowledgements The authors thank Dr. Masami Aikawa of Kisarazu National College of Technology and Ms. Yoko Fujimura of Chiba Prefectural Environmental Research Center for their discussion and suggestion on the study. They are also grateful to Dr. Keiichi Nagao, the head of Safety and Health Organization Chiba University, for his financial support and thankful encouragement on our works.
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