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Activated carbon prepared from biomass as adsorbent: elimination of Ni(II) from aqueous solution
Bioresource Technology 81 (2002) 87±90
Short communication
Activated carbon prepared from biomass as adsorbent: elimination of Ni(II) from aqueous solution K. Kadirvelu *, P. Senthilkumar, K. Thamaraiselvi, V. Subburam Department of Environmental Sciences, Bharathiar University, Coimbatore, Tamil Nadu 641 046, India Received 20 November 2000; received in revised form 12 May 2001; accepted 16 May 2001
Abstract Activated carbon (AC) prepared from waste Parthenium was used to eliminate Ni(II) from aqueous solution by adsorption. Batch mode adsorption experiments are carried out, by varying contact time, metal ion concentration, carbon concentration, pH and desorption to assess kinetic and equilibrium parameters. They allowed initial adsorption coecient, adsorption rate constant and maximum adsorption capacities to be computed. The adsorption data were modeled by using both Langmuir and Freundlich classical adsorption isotherms. The adsorption capacity Q0 calculated from the Langmuir isotherm was 54.35 mg Ni(II)/g of AC at initial pH of 5.0 and 20°C, for the particle size 250±500 lm. Increase in pH from 2 to 10 increased percent removal of metal ion. The regeneration by HCl of Ni(II)-saturated carbon by HCl, allowed suggestion of an adsorption mechanism by ion-exchange between metal ion and H ions on the AC surfaces. Quantitative recovery of Ni(II) was possible with HCl. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Activated carbon; Metal ion; Adsorption; Kinetics; Isotherms; pH; Desorption
1. Introduction Nickel(II) is present in the euents of silver re®neries, electroplating, zinc base casting and storage battery industries (Kadirvelu et al., 2000a). In India the acceptable limit of nickel in drinking water is 0.01 mg/l and for discharge of industrial wastewater is 2.0 mg/l (Kadirvelu, 1998). At higher concentrations, Ni(II) causes cancer of lungs, nose and bone. Dermatitis (nickel itch) is the most frequent eect of exposure to nickel objects, such as coins and costume jewelry. Conventional methods for the removal of Ni(II) from solution include chemical precipitation, ion exchange, ®ltration, chemical reduction, electrodeposition and adsorption on activated carbon (AC) (LaGrega et al., 1994). Cost-eective alternative technologies or adsorbents for the treatment of metal-contaminated wastewaters are needed especially in developing countries like India. Natural materials which are available in large quantities, or certain waste products from industries or agricultural operations, may have potential as inexpensive adsorbents. *
Corresponding author. Tel.: +91-422-8807-16; fax: +91-422-422387. E-mail address: [email protected] (K. Kadirvelu).
Due to their low cost, after these materials have been expended, they can be disposed without expensive regeneration. Generally, an adsorbent can be assumed as low cost if it requires little processing, is abundant in nature, or is a by-product or waste material from another industry. Of course improved adsorption capacity may compensate the cost of additional processing. Research has already been conducted on a wide variety of adsorbents. They include walnut shell, waste tea, Turkish coee, nut shell, exhausted coee (Orhan and Buyukgugor, 1993), sawdust (Zarraa, 1995), rice bran, soya bean and cotton seed hulls (Marshall and Johns, 1996), peat (Viraraghavan and Drohamraju, 1993), sorghum peat moss (Lo et al., 1995), and peanut hull carbon (Periasamy and Namasivayam, 1995) have been investigated to remove nickel (II) from wastewater. Reports have appeared on preparation of AC from biomass like soya bean, peanut, pecan and walnut shells (Marshall and Champagne, 1995), coconut shell, (Arulanandham et al., 1989) coirpith (Namasivayam and Kadirvelu, 1997a,b) and sawdust (Kadirvelu et al., 2000b). Parthenium was ®rst reported in India from Pune in Maharastra and is now known to be present throughout the country except for the arid zone of Rajasthan and
0960-8524/02/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 0 - 8 5 2 4 ( 0 1 ) 0 0 0 9 3 - 1
88
K. Kadirvelu et al. / Bioresource Technology 81 (2002) 87±90
adjoining southwest area of Haryana. Approximately two million hectares of land have been reported to be infested with this weed in the metropolitan cities of Mumbai, Calcutta, New Delhi and Bangalore alone. It is considered to be a native of the West Indies and tropical North and South America and Asia. It was suspected to have crept into India along with imported wheat from the USA under the PL-480 program (Tripathi et al., 1991). The main aim of this work was to evaluate the feasibility of using AC from Parthenium for the removal of Ni(II) from aqueous solution. The in¯uence of experimental conditions contact time, metal ion concentration, carbon concentration and pH were studied.
2. Methods 2.1. Adsorbent Parthenium (stem and leaves) was collected in and around Coimbatore, Tamil Nadu, India and chopped into small pieces and dried in sunlight and used for the carbon preparation. The dried Parthenium was used for AC preparation using ZnCl2 . Carbon was prepared from Parthenium at an impregnation ratio of 1:0.25 [ZnCl2 :Parthenium]. It was de®ned Weight of ZnCl2 added : Impregnation ratio Weight of dried Parthenium The dried Parthenium was digested in a boiling solution of anhydrous zinc chloride for 1 h then the remaining solution was drained o and the solid ovendried at 60°C overnight. This material was placed in a copper vessel closed with a tight lid and carbonized at 700°C for 1 h. After cooling, the excess of ZnCl2 present on the AC particles was leached out by immersing it in a 0.1 M HCl solution maintained at 80°C for 12 h. The carbon was repeatedly washed with hot distilled water until chloride had disappeared completely (tested by silver nitrate method). This material was subsequently oven dried at 80°C for 6 h and stored in plastic containers. The particle size of 250±500 lm was separated by sieving and used for adsorption studies. 2.2. Adsorbate A stock solution of 1000 mg/l of Ni(II) was prepared by dissolving commercially available NiSO4 6H2 O salt in 1% HNO3 solution to prevent hydrolysis formation. The stock solution was diluted with distilled water to obtain the required standard solutions.
2.3. Batch mode adsorption studies Batch mode adsorption studies were carried out at 20 1°C using 250 ml of metal ion solution containing the desired concentration and 250 mg of adsorbent in 500 ml conical ¯asks (Stirring speed 300 rpm). Samples were withdrawn after predetermined time intervals, and were separated and ®ltered using 0.45 lm cellulose acetate ®lters. Ni (II) was determined by atomic absorption spectrophotometer (Perkin Elmer-2280). All the experiments were carried out at an initial pH of 5.0 where the adsorption is signi®cant but below the pH where metal hydroxide precipitation occurs. The metal concentration retained in the adsorbent phase (qe , mg/g) was calculated by using the following equation. qe C0
Ce V =m;
1
where C0 and Ce are the initial and ®nal concentrations of metal ion in solution (mg/l), V is the volume of solution (ml) and m is the mass of adsorbent (mg). Adsorption isotherms were performed at 20 1°C, with initial concentrations of Ni(II) from 10 to 70 mg/l, a solution volume of 250 ml and an AC weight of 250 mg. The stirring time was 12 h. The eect of pH on percent removal was studied from pH 2 to 10 using 250 ml metal ion solution of 20 and 40 mg/l with 250 mg carbon. 0.1 M HCl or NaOH was used to adjust the pH. 2.4. Batch mode desorption studies After adsorption experiments, the metal ion loaded carbons were separated and washed with distilled water to remove unadsorbed metal ions on the AC surface. They were stirred with 250 ml of HCl of various concentrations ranging from 0.00325 to 0.1 M for 12 h. Metal ion concentrations were analyzed as before. All the chemicals used were of analytical reagent grade. All the experiments were carried out in duplicate and mean values are presented. Maximum deviation was 3.0%. 3. Results and discussion 3.1. Kinetic study The kinetic study for the adsorption of Ni(II) was conducted at optimum pH where only adsorption takes place. The adsorption equilibrium was reached at 90, 150, 210 and 240 min for 10, 20, 30 and 40 mg/l, respectively. The contact time required for all the concentrations of Ni(II) removal was very short. This result is interesting because equilibrium time is one of the parameters for economical wastewater treatment plant applications. According to these results, the stirring time
K. Kadirvelu et al. / Bioresource Technology 81 (2002) 87±90
89
Table 1 Kinetic constants of Ni(II) adsorption onto AC Ni(II) Concentration (mg/l)
Kad (min 1 )
r2
c l=mg= min 10
10 20 30 40
1.82 1.77 2.27 3.25
0.9980 0.9084 0.9784 0.9849
5.0 3.3 2.4 1.9
was ®xed at 6 h for the further experiments to make sure of reaching adsorption equilibrium. The adsorption rate constants (Kad ) were calculated by using the Lagergren equation. The results showed that the removal of Ni(II) followed ®rst-order reaction. The Kad values (Table 1) were comparable with previous reports for adsorption of Ni(II) onto AC (Kadirvelu, 1998). The initial adsorption kinetic coecient c (l/mg/min) was also computed using the following equation (Kadirvelu et al., 2000a) and presented in Table 1. dC V c ; 2 dt t!0 mC0 where C is the metal ion concentration at time (t), t is the time (min), V is the solution volume (l) and m is the AC weight (mg). 3.2. Adsorption isotherms Two models, Langmuir and Freundlich equations, were used to determine adsorption of Ni(II) onto AC. The adsorption capacity (Q0 ) and energy of adsorption (b) were determined from the slope and intercept of the Langmuir plot and found to be 54.35 mg/g and 0.057 (l/mg), respectively. The Freundlich isotherm was also used to explain observed phenomena. Kf and n values were calculated from the intercept and slope of the plot and were found to be 1.8759 and 1.478, respectively. The n value between 1 and 10 indicates favorable adsorption of Ni(II) onto carbon. 3.3. Eect of carbon concentration Increasing carbon concentration increased the percent removal. This was due to availability of more surface functional groups and surface area at higher carbon concentrations (Kadirvelu et al., 2000a). For quantitative removal of Ni(II) from 250 ml of 20 mg/l a maximum carbon concentration of 400 mg was required. 3.4. Adsorption mechanism In order to ®nd out the mechanism of adsorption of Ni(II) by AC, the pH eect was studied. For AC and
3
qe (mg/g) 5.48 8.77 14.08 19.0
initial Ni(II) concentrations, the ®nal pH value was higher than the initial pH, with a value which decreased when metal ion concentration increased. This might have been due to release of H ions from the AC surface which indicates an adsorption mechanism by ion-exchange (Kadirvelu et al., 2000a). Ni(II) removal by carbon increased with increase in pH from 2 to 10. In the pH range 7.0±10.0 the maximum adsorption was observed; this might have been due to partial hydrolysis of metal ion resulting in the formation of MOH and M OH2 (Zouboulis and Kydros, 1993; Singh and Rawat, 1997). M OH2 would have adsorbed to a greater extent than MOH on the AC. Low solubilities of hydrolyzed metal ion species might have been another reason for the maximum adsorption in the pH range mentioned above. In an acidic pH range metal ion would be present predominantly as M2 : at acidic pH there would be competition between H and M2 ion for adsorption at the ion-exchangeable sites of carbon, leading to a low removal of metal ion, as has been previously reported by several authors (Corapcioglu and Huang, 1987; Kadirvelu et al., 2000a). One of the conventional methods of removing metals from water is the precipitation of metals as hydroxides using alkali. This method has the limitations that the metals cannot be completely removed due to solubility of metal hydroxide. The eect of initial pH on metal ions removal was tested to ®nd out at which pH the metal hydroxide precipitation occurred. The eect of initial pH on metal ions removal led to the conclusion that when adsorption occurred at 5.0 for Ni(II) it occurred below the pH of precipitation and adsorption was the only mechanism of removing metal ion. 3.5. Desorption studies Desorption studies were carried out to con®rm the adsorption mechanism proposed above and to recover the metals from adsorbent. The quantitative percent recovery of metal ion indicated that regeneration of carbon was possible. This is further evidence that ion exchange is involved in the adsorption mechanism. Furthermore, the adsorption results observed are in agreement with those previously reported for other ACs (Kadirvelu and Namasivayam, 2000; Namasivayam and Kadirvelu, 1997b, 1999).
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K. Kadirvelu et al. / Bioresource Technology 81 (2002) 87±90
Acknowledgements Author K.K. is grateful to Prof. Pierre Le Cloirec, Head of the Department, DSEE, Ecole Des Mines Nantes, Nantes, France for his help and encouragement. The author K.T. is grateful to CISR (Government of India) for the award of SRF (Ext.). References Arulanandham, A., Balasuramanian, N., Ramkrisna, T.V., 1989. Coconut shell carbon for the treatment of cadmium and lead containing wastewater. Met. Finish. 87, 51±55. Corapcioglu, M.O., Huang, C.P., 1987. The adsorption of heavy metals ontohydrous activated carbon. Water Res. 21, 1031±1044. Kadirvelu, K. 1998. Preparation and characterization of activated carbon from coirpith and its utilization in treatment of metal bearing wastewaters, Ph.D. Thesis, Bharathiar University, Coimbatore, Tamil Nadu, India. Kadirvelu, K., Namasivayam, C., 2000. Activated carbon from agricultural solid waste a metal adsorbent: Adsorption of Pb(II) from aqueous solution. Environ. Technol. 21, 1091±1097. Kadirvelu, K., Brasquet, B., Le Cloirec, P., 2000a. Removal of Cu(II) Pb(II) and Ni(II) by adsorption onto activated carbon cloth. Langmuir 16, 8404±8409. Kadirvelu, K., Palanivel, M., Kalpana, R., Rajeshwari, S., 2000b. Activated carbon from agricultural by-product for the treatment of dyeing industry wastewater. Bioresource Technol. 75, 25±27. La Grega, M.D., Buckingham, P.L., Evans, J.C., 1994. In: Hazardous Waste Management. McGraw-Hill, New York, p. 859. Lo, Y.S., Johnwase, D.A., Forster, C.F., 1995. Batch nickel removal from aqueous solution by sorghum moss peat. Water Res. 29, 1327±1332.
Marshall, W.E., Champagne, 1995. Agricultural by-product as adsorbents for metal ions in laboratory prepared solutions and manufacturing wastewaters. J. Environ. Sci. Health A 30, 241± 261. Marshall, W.E., Johns, M.M., 1996. Agricultural by-product as metal adsorbents: sorption properties and resistance to mechanical abrasion. J. Chem. Technol. Biotechnol. 66, 192±198. Namasivayam, C., Kadirvelu, K., 1997a. Activated carbon from coirpith by physical and chemical activation methods. Bioresource Technol. 62, 123±127. Namasivayam, C., Kadirvelu, K., 1997b. Agricultural solid waste for the removal of heavy metals: adsorption of Cu(II) by coirpith carbon. Chemosphere 34, 377±399. Namasivayam, C., Kadirvelu, K., 1999. Uptake of mercury (II) from wastewater by activated carbon from unwanted agricultural solid waste by-product: coirpith. Carbon 37, 79±84. Orhan, Y., Buyukgugor, H., 1993. The removal of heavy metals by using agricultural wastes. Water Sci. Technol. 28, 247±255. Periasamy, K., Namasivayam, C., 1995. Removal of Ni(II) from aqueous solution and nickel plating industry wastewater using an agricultural waste: peanut hull. Waste Manage. 15, 63±68. Singh, D., Rawat, N.S., 1997. Adsorption of heavy metal ion on treated an untreated low-grade bituminous coal. Indian J. Chem. Technol. 4, 39±41. Tripathi, B., Barla, A., Singh, C.M., 1991. Carrot weed Parthenium hysterphorus. Over view of the problems and strategy for its control in India. Indian J. Weed Sci. 23, 61±67. Viraraghavan, T., Drohamraju, M.M., 1993. Removal of copper nickel and zinc from wastewater by adsorption using peat. J. Envi. Sci. Health A 28, 1261±1276. Zarraa, M.A., 1995. A study on the removal of chromium (VI) from waste solutions by adsorption onto sawdust in stirred vessels. Adsorption Sci. Technol. 12, 129±138. Zouboulis, A.I., Kydros, K.A., 1993. Use of red mud for toxic metals removal: In the case of nickel. J. Chem. Technol. Biotechnol. 58, 95±101.
Short communication
Activated carbon prepared from biomass as adsorbent: elimination of Ni(II) from aqueous solution K. Kadirvelu *, P. Senthilkumar, K. Thamaraiselvi, V. Subburam Department of Environmental Sciences, Bharathiar University, Coimbatore, Tamil Nadu 641 046, India Received 20 November 2000; received in revised form 12 May 2001; accepted 16 May 2001
Abstract Activated carbon (AC) prepared from waste Parthenium was used to eliminate Ni(II) from aqueous solution by adsorption. Batch mode adsorption experiments are carried out, by varying contact time, metal ion concentration, carbon concentration, pH and desorption to assess kinetic and equilibrium parameters. They allowed initial adsorption coecient, adsorption rate constant and maximum adsorption capacities to be computed. The adsorption data were modeled by using both Langmuir and Freundlich classical adsorption isotherms. The adsorption capacity Q0 calculated from the Langmuir isotherm was 54.35 mg Ni(II)/g of AC at initial pH of 5.0 and 20°C, for the particle size 250±500 lm. Increase in pH from 2 to 10 increased percent removal of metal ion. The regeneration by HCl of Ni(II)-saturated carbon by HCl, allowed suggestion of an adsorption mechanism by ion-exchange between metal ion and H ions on the AC surfaces. Quantitative recovery of Ni(II) was possible with HCl. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Activated carbon; Metal ion; Adsorption; Kinetics; Isotherms; pH; Desorption
1. Introduction Nickel(II) is present in the euents of silver re®neries, electroplating, zinc base casting and storage battery industries (Kadirvelu et al., 2000a). In India the acceptable limit of nickel in drinking water is 0.01 mg/l and for discharge of industrial wastewater is 2.0 mg/l (Kadirvelu, 1998). At higher concentrations, Ni(II) causes cancer of lungs, nose and bone. Dermatitis (nickel itch) is the most frequent eect of exposure to nickel objects, such as coins and costume jewelry. Conventional methods for the removal of Ni(II) from solution include chemical precipitation, ion exchange, ®ltration, chemical reduction, electrodeposition and adsorption on activated carbon (AC) (LaGrega et al., 1994). Cost-eective alternative technologies or adsorbents for the treatment of metal-contaminated wastewaters are needed especially in developing countries like India. Natural materials which are available in large quantities, or certain waste products from industries or agricultural operations, may have potential as inexpensive adsorbents. *
Corresponding author. Tel.: +91-422-8807-16; fax: +91-422-422387. E-mail address: [email protected] (K. Kadirvelu).
Due to their low cost, after these materials have been expended, they can be disposed without expensive regeneration. Generally, an adsorbent can be assumed as low cost if it requires little processing, is abundant in nature, or is a by-product or waste material from another industry. Of course improved adsorption capacity may compensate the cost of additional processing. Research has already been conducted on a wide variety of adsorbents. They include walnut shell, waste tea, Turkish coee, nut shell, exhausted coee (Orhan and Buyukgugor, 1993), sawdust (Zarraa, 1995), rice bran, soya bean and cotton seed hulls (Marshall and Johns, 1996), peat (Viraraghavan and Drohamraju, 1993), sorghum peat moss (Lo et al., 1995), and peanut hull carbon (Periasamy and Namasivayam, 1995) have been investigated to remove nickel (II) from wastewater. Reports have appeared on preparation of AC from biomass like soya bean, peanut, pecan and walnut shells (Marshall and Champagne, 1995), coconut shell, (Arulanandham et al., 1989) coirpith (Namasivayam and Kadirvelu, 1997a,b) and sawdust (Kadirvelu et al., 2000b). Parthenium was ®rst reported in India from Pune in Maharastra and is now known to be present throughout the country except for the arid zone of Rajasthan and
0960-8524/02/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 0 - 8 5 2 4 ( 0 1 ) 0 0 0 9 3 - 1
88
K. Kadirvelu et al. / Bioresource Technology 81 (2002) 87±90
adjoining southwest area of Haryana. Approximately two million hectares of land have been reported to be infested with this weed in the metropolitan cities of Mumbai, Calcutta, New Delhi and Bangalore alone. It is considered to be a native of the West Indies and tropical North and South America and Asia. It was suspected to have crept into India along with imported wheat from the USA under the PL-480 program (Tripathi et al., 1991). The main aim of this work was to evaluate the feasibility of using AC from Parthenium for the removal of Ni(II) from aqueous solution. The in¯uence of experimental conditions contact time, metal ion concentration, carbon concentration and pH were studied.
2. Methods 2.1. Adsorbent Parthenium (stem and leaves) was collected in and around Coimbatore, Tamil Nadu, India and chopped into small pieces and dried in sunlight and used for the carbon preparation. The dried Parthenium was used for AC preparation using ZnCl2 . Carbon was prepared from Parthenium at an impregnation ratio of 1:0.25 [ZnCl2 :Parthenium]. It was de®ned Weight of ZnCl2 added : Impregnation ratio Weight of dried Parthenium The dried Parthenium was digested in a boiling solution of anhydrous zinc chloride for 1 h then the remaining solution was drained o and the solid ovendried at 60°C overnight. This material was placed in a copper vessel closed with a tight lid and carbonized at 700°C for 1 h. After cooling, the excess of ZnCl2 present on the AC particles was leached out by immersing it in a 0.1 M HCl solution maintained at 80°C for 12 h. The carbon was repeatedly washed with hot distilled water until chloride had disappeared completely (tested by silver nitrate method). This material was subsequently oven dried at 80°C for 6 h and stored in plastic containers. The particle size of 250±500 lm was separated by sieving and used for adsorption studies. 2.2. Adsorbate A stock solution of 1000 mg/l of Ni(II) was prepared by dissolving commercially available NiSO4 6H2 O salt in 1% HNO3 solution to prevent hydrolysis formation. The stock solution was diluted with distilled water to obtain the required standard solutions.
2.3. Batch mode adsorption studies Batch mode adsorption studies were carried out at 20 1°C using 250 ml of metal ion solution containing the desired concentration and 250 mg of adsorbent in 500 ml conical ¯asks (Stirring speed 300 rpm). Samples were withdrawn after predetermined time intervals, and were separated and ®ltered using 0.45 lm cellulose acetate ®lters. Ni (II) was determined by atomic absorption spectrophotometer (Perkin Elmer-2280). All the experiments were carried out at an initial pH of 5.0 where the adsorption is signi®cant but below the pH where metal hydroxide precipitation occurs. The metal concentration retained in the adsorbent phase (qe , mg/g) was calculated by using the following equation. qe C0
Ce V =m;
1
where C0 and Ce are the initial and ®nal concentrations of metal ion in solution (mg/l), V is the volume of solution (ml) and m is the mass of adsorbent (mg). Adsorption isotherms were performed at 20 1°C, with initial concentrations of Ni(II) from 10 to 70 mg/l, a solution volume of 250 ml and an AC weight of 250 mg. The stirring time was 12 h. The eect of pH on percent removal was studied from pH 2 to 10 using 250 ml metal ion solution of 20 and 40 mg/l with 250 mg carbon. 0.1 M HCl or NaOH was used to adjust the pH. 2.4. Batch mode desorption studies After adsorption experiments, the metal ion loaded carbons were separated and washed with distilled water to remove unadsorbed metal ions on the AC surface. They were stirred with 250 ml of HCl of various concentrations ranging from 0.00325 to 0.1 M for 12 h. Metal ion concentrations were analyzed as before. All the chemicals used were of analytical reagent grade. All the experiments were carried out in duplicate and mean values are presented. Maximum deviation was 3.0%. 3. Results and discussion 3.1. Kinetic study The kinetic study for the adsorption of Ni(II) was conducted at optimum pH where only adsorption takes place. The adsorption equilibrium was reached at 90, 150, 210 and 240 min for 10, 20, 30 and 40 mg/l, respectively. The contact time required for all the concentrations of Ni(II) removal was very short. This result is interesting because equilibrium time is one of the parameters for economical wastewater treatment plant applications. According to these results, the stirring time
K. Kadirvelu et al. / Bioresource Technology 81 (2002) 87±90
89
Table 1 Kinetic constants of Ni(II) adsorption onto AC Ni(II) Concentration (mg/l)
Kad (min 1 )
r2
c l=mg= min 10
10 20 30 40
1.82 1.77 2.27 3.25
0.9980 0.9084 0.9784 0.9849
5.0 3.3 2.4 1.9
was ®xed at 6 h for the further experiments to make sure of reaching adsorption equilibrium. The adsorption rate constants (Kad ) were calculated by using the Lagergren equation. The results showed that the removal of Ni(II) followed ®rst-order reaction. The Kad values (Table 1) were comparable with previous reports for adsorption of Ni(II) onto AC (Kadirvelu, 1998). The initial adsorption kinetic coecient c (l/mg/min) was also computed using the following equation (Kadirvelu et al., 2000a) and presented in Table 1. dC V c ; 2 dt t!0 mC0 where C is the metal ion concentration at time (t), t is the time (min), V is the solution volume (l) and m is the AC weight (mg). 3.2. Adsorption isotherms Two models, Langmuir and Freundlich equations, were used to determine adsorption of Ni(II) onto AC. The adsorption capacity (Q0 ) and energy of adsorption (b) were determined from the slope and intercept of the Langmuir plot and found to be 54.35 mg/g and 0.057 (l/mg), respectively. The Freundlich isotherm was also used to explain observed phenomena. Kf and n values were calculated from the intercept and slope of the plot and were found to be 1.8759 and 1.478, respectively. The n value between 1 and 10 indicates favorable adsorption of Ni(II) onto carbon. 3.3. Eect of carbon concentration Increasing carbon concentration increased the percent removal. This was due to availability of more surface functional groups and surface area at higher carbon concentrations (Kadirvelu et al., 2000a). For quantitative removal of Ni(II) from 250 ml of 20 mg/l a maximum carbon concentration of 400 mg was required. 3.4. Adsorption mechanism In order to ®nd out the mechanism of adsorption of Ni(II) by AC, the pH eect was studied. For AC and
3
qe (mg/g) 5.48 8.77 14.08 19.0
initial Ni(II) concentrations, the ®nal pH value was higher than the initial pH, with a value which decreased when metal ion concentration increased. This might have been due to release of H ions from the AC surface which indicates an adsorption mechanism by ion-exchange (Kadirvelu et al., 2000a). Ni(II) removal by carbon increased with increase in pH from 2 to 10. In the pH range 7.0±10.0 the maximum adsorption was observed; this might have been due to partial hydrolysis of metal ion resulting in the formation of MOH and M OH2 (Zouboulis and Kydros, 1993; Singh and Rawat, 1997). M OH2 would have adsorbed to a greater extent than MOH on the AC. Low solubilities of hydrolyzed metal ion species might have been another reason for the maximum adsorption in the pH range mentioned above. In an acidic pH range metal ion would be present predominantly as M2 : at acidic pH there would be competition between H and M2 ion for adsorption at the ion-exchangeable sites of carbon, leading to a low removal of metal ion, as has been previously reported by several authors (Corapcioglu and Huang, 1987; Kadirvelu et al., 2000a). One of the conventional methods of removing metals from water is the precipitation of metals as hydroxides using alkali. This method has the limitations that the metals cannot be completely removed due to solubility of metal hydroxide. The eect of initial pH on metal ions removal was tested to ®nd out at which pH the metal hydroxide precipitation occurred. The eect of initial pH on metal ions removal led to the conclusion that when adsorption occurred at 5.0 for Ni(II) it occurred below the pH of precipitation and adsorption was the only mechanism of removing metal ion. 3.5. Desorption studies Desorption studies were carried out to con®rm the adsorption mechanism proposed above and to recover the metals from adsorbent. The quantitative percent recovery of metal ion indicated that regeneration of carbon was possible. This is further evidence that ion exchange is involved in the adsorption mechanism. Furthermore, the adsorption results observed are in agreement with those previously reported for other ACs (Kadirvelu and Namasivayam, 2000; Namasivayam and Kadirvelu, 1997b, 1999).
90
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Acknowledgements Author K.K. is grateful to Prof. Pierre Le Cloirec, Head of the Department, DSEE, Ecole Des Mines Nantes, Nantes, France for his help and encouragement. The author K.T. is grateful to CISR (Government of India) for the award of SRF (Ext.). References Arulanandham, A., Balasuramanian, N., Ramkrisna, T.V., 1989. Coconut shell carbon for the treatment of cadmium and lead containing wastewater. Met. Finish. 87, 51±55. Corapcioglu, M.O., Huang, C.P., 1987. The adsorption of heavy metals ontohydrous activated carbon. Water Res. 21, 1031±1044. Kadirvelu, K. 1998. Preparation and characterization of activated carbon from coirpith and its utilization in treatment of metal bearing wastewaters, Ph.D. Thesis, Bharathiar University, Coimbatore, Tamil Nadu, India. Kadirvelu, K., Namasivayam, C., 2000. Activated carbon from agricultural solid waste a metal adsorbent: Adsorption of Pb(II) from aqueous solution. Environ. Technol. 21, 1091±1097. Kadirvelu, K., Brasquet, B., Le Cloirec, P., 2000a. Removal of Cu(II) Pb(II) and Ni(II) by adsorption onto activated carbon cloth. Langmuir 16, 8404±8409. Kadirvelu, K., Palanivel, M., Kalpana, R., Rajeshwari, S., 2000b. Activated carbon from agricultural by-product for the treatment of dyeing industry wastewater. Bioresource Technol. 75, 25±27. La Grega, M.D., Buckingham, P.L., Evans, J.C., 1994. In: Hazardous Waste Management. McGraw-Hill, New York, p. 859. Lo, Y.S., Johnwase, D.A., Forster, C.F., 1995. Batch nickel removal from aqueous solution by sorghum moss peat. Water Res. 29, 1327±1332.
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