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*Author to whom all correspondence should be addressed. E-mail: krishna2604@sify.com (K.G. Bhattacharyya).

Adsorptive Accumulation of Cd(II), Co(II), Cu(II), Pb(II) and Ni(II) Ionsfrom Water onto Kaolinite: Influence of Acid Activation

Krishna G. Bhattacharyya* and Susmita Sen Gupta Department of Chemistry, Gauhati University,

Guwahati 781014, Assam, India.

(Received 9 September 2008; revised form accepted 15 April 2009)

ABSTRACT: Naturally occurring kaolinite and its acid-activated form wereused to separate the toxic metal ions Cd(II), Co(II), Cu(II), Pb(II) and Ni(II)from water by adsorption. Kaolinite was treated with 0.25 M H2SO4 to obtain theacid-activated form. Adsorption onto the calcined forms of the clays wasinfluenced by pH, the solution concentration of metal ions, the amount ofadsorbent employed, the interaction time and the temperature. The processfollowed second-order kinetics very closely. Isotherm-fitting procedures showedcompliance with the Langmuir and Freundlich equations suggesting theinvolvement of strong interactions. The Langmuir monolayer capacity showedsome increase between kaolinite and the acid-activated form [Cd(II): 9.9 and11.4 mg/g; Co(II): 11.2 and 12.1 mg/g; Cu(II): 9.2 and 10.1 mg/g; Pb(II): 11.1and 12.1 mg/g; and Ni(II): 10.4 and 11.9 mg/g]. The thermodynamics of the rateprocesses showed that the adsorption of Co(II), Cd(II) and Cu(II) ions wasendothermic and accompanied by an increase in both the entropy and the Gibbs’free energy. However, in contrast, the adsorption of Pb(II) and Ni(II) ions wasexothermic with a decrease in entropy and an appreciable decrease in the Gibbs’free energy. The results have established that kaolinite and its acid-activated formshow a good potential as adsorbents for Cd(II), Co(II), Cu(II), Pb(II) and Ni(II)ions from aqueous media.

INTRODUCTION

Adsorption onto solids leads to the accumulation of adsorbates on the solid surface, therebyeffectively removing them from aqueous and gaseous streams. This process has therefore a specialsignificance in pollutant removal. The increasing contamination of water by metal ions is agrowing environmental problem since such ions are non-biodegradable, with most of them beinghighly toxic and also probably carcinogenic. Such concentration of metal ions can be reduced orminimized by precipitation, ultrafiltration, electrode-deposition, solvent extraction, etc. (Chironet al. 2003; Kadirvelu and Namasivayam 2003), but these processes suffer from high cost,generation of secondary pollutants, poor removal efficiency and ineffectiveness at low metal ionconcentrations. In contrast, adsorption has been found to be an effective and economical methodwith a high potential for the removal, recovery and recycling of metal ions from wastewater(Chang et al. 2002).

The clay minerals in soil play the role of a natural scavenger by filtering out pollutants fromwater through both ion-exchange and adsorption mechanisms. Clays consist of extremely fine