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Removal of Cr(VI) from aqueous solution by adsorption onto activated carbon


Bioresource Technology 80 (2001) 87±89

Short communication

Removal of Cr(VI) from aqueous solution by adsorption onto activated carbon
K. Selvi a,*, S. Pattabhi a, K. Kadirvelu b
a

Department of Environmental Sciences, PSG College of Arts and Science, Coimbatore 641 014, Tamil Nadu, India b Department of Environmental Sciences, Bharathiar University, Coimbatore 641 046, Tamil Nadu, India Received 3 October 2000; received in revised form 24 February 2001; accepted 6 March 2001

Abstract Activated carbon (AC) prepared from coconut tree sawdust was used as an adsorbent for the removal of Cr(VI) from aqueous solution. Batch mode adsorption studies were carried out by varying agitation time, initial Cr(VI) concentration, carbon concentration and pH. Langmuir and Freundlich adsorption isotherms were applied to model the adsorption data. Adsorption capacity was calculated from the Langmuir isotherm and was 3.46 mg/g at an initial pH of 3.0 for the particle size 125±250 lm. The adsorption of Cr(VI) was pH dependent and maximum removal was observed in the acidic pH range. Desorption studies were carried out using 0.01±1 M NaOH solutions. ? 2001 Elsevier Science Ltd. All rights reserved.
Keywords: Adsorption; Cr(VI); Agricultural solid waste; Adsorption isotherms; Desorption

1. Introduction Chromium is a priority metal pollutant introduced into the water bodies from many industrial processes such as tanning, electroplating, metal processing, paint manufacturing, steel fabrication and agricultural runo?. Chromium is also used in explosives, ceramics and photography. Chromium occurs in the aquatic environment as both trivalent [Cr(III)] and hexavalent [Cr(VI)] states. Hexavalent chromium, which is primarily present in the form of chromate ?CrO? ? and di4 chromate ?Cr2 O? ?, possesses signi?cantly higher levels 7 of toxicity than the other valency states, (Sharma and Forster, 1995). So, the removal of Cr(VI) from water and wastewater is important to protect the environment. The methods employed for the removal of Cr(VI) from wastewater include reduction, precipitation, ionexchange and solvent extraction. However, these treatment methods are not widely practised due to their high cost and low feasibility for small-scale industries. Adsorption is by far the most e?ective and widely used technique for the removal of toxic heavy metals from wastewater. The use of activated carbon (AC) for the adsorption of heavy metals was ?rst proposed by WaCorresponding author. Present address: c/o Mr. M. Krishnasamy, 134/22, Union Mill Road, Tirupur, Coimbatore (Dist) 641 601, India. E-mail address: selvi_krishna2001@yahoo.com (K. Selvi).
*

tonabe and Ogawa (1929). In recent years several investigators have concentrated their work on low cost, non-conventional adsorbents to achieve the economically feasible and e?ective treatment of wastewater containing Cr(VI). Various industrial solid wastes, agricultural by-products and similar materials have adsorption a?nity for heavy metals, (Pollard et al., 1992). AC derived from coconut shells for Cr(VI) removal was reported by Padaki Srinivas Rao et al. (1992). Alves et al. (1993) have used Pinus sylvestris bark as an adsorbent for the removal of Cr(III) from aqueous solution. Rice straw has been found to be an e?ective adsorbent for the removal of Cr(VI) from aqueous solution (Ali and Deo, 1992). Studies have been reported on the use of leaf mould (Sharma and Forster, 1994), AC from olive stone and almond shell (Candela et al., 1995), sphagnum moss peat (Sharma and Forster, 1993) and pyrite ?nes (Zoboulis et al., 1995) for the removal of Cr(VI). Mohammed Ajmal et al. (1998) have utilized sawdust as adsorbent for the removal of Cu(II) from wastewater. Kadirvelu et al. (2000) have recently reported the utilization of coconut tree sawdust carbon for the treatment of dyeing industry e?uents. The main objective of this work was to evaluate the adsorption capacity of AC from sawdust, for the e?ective removal of Cr(VI) from solution by varying agitation time, Cr(VI) concentration, carbon concentration, pH and desorption.

0960-8524/01/$ - see front matter ? 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 0 - 8 5 2 4 ( 0 1 ) 0 0 0 6 8 - 2

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2. Methods Coconut tree sawdust was collected from a saw mill near Pollachi, Coimbatore district, Tamil Nadu, India. It was dried in sunlight. The dried sawdust was mixed in a 1:0.5 (sawdust:H2 SO4 ) ratio with concentrated sulphuric acid. Then it was activated at 80°C for 12 h, in a hot air oven. This carbonized material was washed with double distilled water to remove the free acid. After washing, the AC was soaked in 1% sodium bicarbonate solution to remove the remaining acid. Then it was washed with distilled water, until the pH of the AC reached 5 ? 0:5. Washed AC was dried at 105°C and then sieved to the particle size 125±250 lm, which was used in this work. The characteristics of the AC are summarized in Table 1. A stock hexavalent chromium solution (1000 mg/l) was prepared in double distilled water, from potassium dichromate. Working solutions were prepared by diluting the stock solution with distilled water.

ugation and the supernatant was drained out. The adsorbent was given a gentle wash with water to remove any unadsorbed Cr(VI). 50 ml of 0.01±1 M sodium hydroxide solution was added to the chromium adsorbed AC and agitated for 180 min. The desorbed Cr(VI) in the solution was separated and analysed as before. All the experiments were carried out in duplicate and mean values are reported. The maximum deviation was 3%. 4. Results and discussion 4.1. E?ect of contact time and initial metal ion concentration The uptake of Cr(VI) from solution by AC increased with time and attained equilibrium in 180 min for all the concentrations studied. The equilibrium time was independent of initial Cr(VI) concentration. Increase in initial Cr(VI) concentration decreased the percent adsorption. The kad values at di?erent initial Cr(VI) concentration were calculated from the slopes of Lagergren plots and were 1:52 ? 10?2 , 1:01 ? 10?2 , 1:93 ? 10?2 and 1:40 ? 10?2 min?1 for initial Cr(VI) concentrations of 5, 10, 15 and 20 mg/l, respectively. Lee et al. (1995) have reported kad values of 5:87 ? 10?3 , 4:65 ? 10?3 and 4:58 ? 10?3 min?1 for copper coated moss for initial Cr(VI) concentrations of 5, 10 and 20 mg/l, respectively. 4.2. E?ect of AC dosage on Cr(VI) adsorption The percent adsorption increased with increase in AC dosage for all the concentrations studied. The percent removal decreased from 98.84 to 84.06 with increase in Cr(VI) concentration from 5 to 20 mg/l, respectively. 4.3. Adsorption isotherms In order to model the adsorption behaviour and calculate the adsorption capacity of AC, adsorption isotherms were studied. Both Langmuir and Freundlich isotherms were employed for Cr(VI) adsorption. Adsorption followed both isotherms. The adsorption capacity ?Q0 ? and energy of adsorption (b) were calculated from the slope and intercept of the Langmuir plot and found to be 3.46 mg/g and 0.47 l/ mg, respectively. The equilibrium parameter RL values between 0 and 1 for all the concentrations of Cr(VI), indicating favourable adsorption. The Freundlich isotherm was also used to explain observed phenomena. Kf and `n' calculated from the intercept and slope of the plots were 1.19 and 2.99, respectively. Mckay et al. (1980) reported that an n value of range 2±10 indicates favourable adsorption. The n

3. Batch mode adsorption experiments Batch mode adsorption studies were carried out by taking 50 ml of the Cr(VI) solution of the desired concentration (5±20 mg/l) and desired weight of AC in 100 ml conical ?asks. Then the conical ?asks were agitated at 150 rpm using a mechanical shaker. pH was adjusted using 0.1 N sulphuric acid or 0.1 N sodium hydroxide. After the equilibration time of 180 min, the adsorbent and adsorbate were separated by centrifugation at 3000 rpm and the supernatant was analysed colorimetrically with 1,5 diphenyl carbazide at 540 nm (APHA, 1985). The e?ect of AC dosage (50±750 mg/50 ml) was studied on initial concentrations of 5, 10, 15 and 20 mg/l for a contact time of 180 min. E?ect of pH on Cr(VI) removal was studied using 200 mg of AC and Cr(VI) concentration of 10 mg/l. Desorption studies were carried out as follows. The metal loaded AC was separated by centrifTable 1 Characteristics of AC Parameters Apparent density (g/ml) Ash content (%) pH (1% solution) Moisture content (%) Iron content (mg/g) Surface area ?m2 =g? Water soluble matter (%) Acid soluble matter (%) Decolourizing power (mg/g) Particle size ?lm) Calcium (mg/g) Sodium (mg/g) Potassium (mg/g) Value 1.15 9.721 2.95 0.84 0.08 486 1.82 8.07 27.0 125±250 90.0 31.0 3.0

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value of 2.99 for Cr(VI) suggests that the adsorbent is e?ective for Cr(VI) adsorption. 4.4. E?ect of pH on Cr(VI) removal In highly acidic media, the adsorbent surfaces might be highly protonated and favour the uptake of Cr(VI) in the anionic form, HCrO4 (Padaki Srinivas Rao et al., 1992). With increase in the pH, from 4.0 to 11.0, the degree of protonation of the surface reduced gradually and hence decreased adsorption was noticed. The highest adsorption occurred at pH 3.0 and was due to the reduction of Cr(VI) to Cr(III). A similar trend has been observed for the removal of Cr(VI) by leaf mould (Sharma and Forster, 1994), Fe(III)/Cr(III) hydroxide (Namasivaysam and Ranganathan, 1993), Fe(III) hydroxide (Aoki and Munemori, 1982) and ?y ash (Viraraghavan and Rao, 1991). 4.5. Desorption of Cr(VI) Desorption studies help to elucidate the mechanism of adsorption and recover the precious metals from the metal-loaded adsorbent. The percent of Cr(VI) desorption increased with increase in sodium hydroxide concentration from 0.01 to 0.1 M. pH e?ect and desorption studies indicated that ion-exchange was not predominant in the adsorption process. The other adsorption mechanism, namely chemisorption, seemed to be the major mode of adsorption. Acknowledgements The authors thank Dr. P. Sampath Kumar, Principal and Prof. D.K.P Varatharajan, Secretary, PSG College of Arts and Science, Coimbatore, for providing the facilities to carry out this work successfully. References
Ali, M., Deo, N., 1992. E?ect of pH on adsorption process of chromium (VI) with a new low-cost adsorbent. Ind. J. Environ. Protect. 12, 202±209.

Alves, M.M., Beca, C.G.G., Carvalho, R.G.D., Castanheria, J.M., Periera, M.C.S., Vasconcdos, L.A.T., 1993. Chromium (II) removal in tannery wastewater, ``Polishing'' by Pinus sylvestrius bark. Water Res. 27, 1333±1338. Aoki, T., Munemori, M., 1982. Recovery of Cr (VI) from wastewater with Iron (III) Hydroxide: I. Adsorption mechanism of Cr (VI) on Iron (III) hydroxide. Water Res. 16, 793±796. APHA, 1985. Standard Methods for the Examination of water and Wastewater, 16th ed. American Public Health Association, Washington, DC. Candela, M.P., Martinez, J.M.M., Macia, R.T., 1995. Chromium (VI) removal with activated carbons. Water Res. 29, 2174±2180. Kadirvelu, K., Palanivel, M., Kalpana, R., Rajeswari, S., 2000. Activated carbon from an agricultural by-product, for the treatment of dyeing industry wastewater. Bioresour. Technol. 74, 263± 265. Lee, C.K., Low, K.S., Kek, K.L., 1995. Removal of chromium from aqueous solution. Bioresour. Technol. 54, 183±189. Mckay, G., Otterburn, M.S., Sweeney, A.G., 1980. The removal of colour from e?uent using various adsorbents, III silica rate processes. Water Res. 14, 15±20. Mohammed Ajmal, Khan, A.H., Ahamad, S., Ahamad, A., 1998. Role of sawdust in the removal of Cu (II) from industrial wastes. Water Res. 32, 3085±3091. Namasivaysam, C., Ranganathan, K., 1993. Waste Fe(III)/Cr(III) hydroxide as adsorbent for the removal of Cr(VI) from aqueous solution and chromium plating industry wastewater. Environ. Pollut. 82, 255±261. Padaki Srinivas Rao, Shashikant, R., Munjunatha, G.S., 1992. Kinetic studies on adsorption of chromium by coconut shell carbons from synthetic e?uents. J. Environ. Sci. Health A 27 (8), 2227±2241. Pollard, S.J.T., Fowler, G.D., Sollar, C.J., Perry, R., 1992. Low-cost adsorbents for water and wastewater treatment. A review. Sci. Total Environ. 16, 31±52. Sharma, D.C., Forster, C.F., 1995. Column studies into the adsorption of chromium(VI) using sphagnum moss peat. Bioresour. Technol. 52, 261±267. Sharma, D.C., Forster, C.F., 1994. The treatment of chromium wastewater using the sorptive potential of leaf mould. Bioresour. Technol. 49, 31±40. Sharma, D.C., Forster, C.F., 1993. Removal of hexavalent chromium using sphagnum moss peat. Water Res. 27, 1201±1208. Viraraghavan, T., Rao, G.A.K., 1991. Adsorption of cadmium and chromium from wastewater by ?yash. J. Environ. Sci. Health A 26, 721±753. Watonabe, T., Ogawa, K., 1929. Activated carbon for purifying copper electrolytes. Chem. Abstr. 24, 1037. Zoboulis, A.I., Kydros, K.A., Matis, K.A., 1995. Removal of hexavalent chromium anions from solutions by pyrite ?nes. Water Res. 29, 1755±1760.


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