Short CommunicationRemoval of Cr(VI) from aqueous solutions by adsorption onto hazelnut shell activated carbon: kinetic and equilibrium studies
Introduction
Chromium and its compounds are widely used in plating, leather tanning, cement, dye, and photography industries producing large quantities of toxic pollutants which causes severe environmental and public health problem. It leads to liver damage, pulmonary congestion, oedema and causes skin irritation resulting in ulcer formation (Raji and Anirudhan, 1998). Its concentrations in industrial wastewaters range from 0.5 to 270,000 mg/l (Patterson, 1985). The tolerance limit for Cr(VI) for discharge into inland surface waters is 0.1 mg/l and in potable water is 0.05 mg/l (EPA, 1990). Conventional methods for the removal of Cr(VI) from wastewater include reduction, ion exchange, evaporation, chemical precipitation and adsorption on activated carbon (Patterson, 1985). Many reports have appeared on the development of low-cost activated carbon adsorption from cheaper and readily available materials for the removal of heavy metals from industrial wastewater (Babel and Kurniawan, 2003; Bailey et al., 1999). Studies on the adsorption of Cr(VI) by activated carbon and low cost materials such as dried powder marine algae (Lee et al., 2000), leaf mould, sphagnum moss peat, coconut fiber compost, maize cob, sugar beet pulp and cane bagasse (Sharma and Forster, 1993, Sharma and Forster, 1994a, Sharma and Forster, 1994b), activated groundnut husk carbon (Periasamy et al., 1991), coconut shell activated carbon (Alaerts et al., 1989), coconut shell, wood and dust coal activated carbons (Selomulya et al., 1999), used tyres carbon (Hamadi et al., 2001), have been reported in the literature.
Hazelnut shells are an agricultural waste product which is mainly used as fuel and its estimated reserves are approximately 3 × 105 tons/year in Turkey. Removals of Cr(III), Cr(VI), Cd(II), Zn(II) (Cimino et al., 2000; Ekinci et al., 1997) and of Ni(II) (Kobya et al., 2002; Demirbas et al., 2002) ions from aqueous solutions by hazelnut shell activated carbon (HSAC) were also reported. Cimino and co-workers obtained an adsorption capacity of Cr(VI) as 17.7 mg/g at pH 2.0. It appears that HSAC has got a great potential for the removal of Cr(VI) from wastewater solutions as compared to other adsorbents mentioned in the literature (Table 3).
In this study, the adsorption of Cr(VI) ions from aqueous solutions under different kinetic and equilibrium conditions is investigated in detail.
Section snippets
Adsorbent
Carbon for HSAC was obtained from species of Corylus avellana from Trabzon in Turkey. Preparation and characteristics of activated carbon from HSAC was reported (Demirbas et al., 2002).
Batch adsorption studies
A chromium sample was prepared by dissolving a known quantity of potassium dichromate (K2Cr2O7) in double distilled water and used as a stock solution. The batch adsorption experiments were performed on a rotary shaker using 150 ml screw-cap conical flasks at constant conductions (stirring rate 200 rpm, carbon
Adsorption dynamics
The kinetics of Cr(VI) adsorption on HSAC were analysed with four kinetic models which are pseudo first-order, pseudo second-order, Elovich and intraparticle diffusion (Hamadi et al., 2001; Kobya et al., 2002). The conformity between experimental data and the model-predicted values was expressed by the correlation coefficients (r2). A relatively high r2 value indicates that the model successfully describes the kinetics of Cr(VI) adsorption by HSAC. The experimental data fitted best to the
Conclusion
HSAC is an effective adsorbent for the removal of Cr(VI) from aqueous solutions. Adsorption of Cr(VI) is highly pH dependent and the best results are obtained in the pH range 1.0–2.0. At low pH a high percentage of Cr(VI) reduces to Cr(III) form. Chromium is adsorbed rapidly when lower concentrations are used. An increase in temperature from 293 to 323 K doubled the maximum adsorption capacity. The experimental data fitted well to the pseudo first-order kinetic model. The Langmuir isotherm
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