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Equilibrium study of adsorption of L-cysteine by natural bentonite

Department of Chemistry, Isfahan University, Isfahan, I.R. Iran 81746-73441

^* E-mail: h.faghihian{at}sci.ui.ac.ir

(Received 13 September 2008; revised 29 October 2009)

L-cysteine (Cys) was adsorbed by bentonite (Ben) from aqueous solutions of variable concentrations and pHs. The raw materials and the adsorbed molecules (Ben-Cys) were characterized using X-ray diffraction (XRD), infrared spectroscopy (IR), thermal analysis methods (TG, DTG and DSC). The results indicated that the adsorption process depended strongly on pH, temperature and concentration of amino acid. Cysteine is specifically adsorbed on bentonite surface sites, probably through its moiety. The pH dependence suggests that these sites may be silanolate groups (Si-O^–). It is presumed that ion exchange interaction governs the adsorption process. The experimental data of adsorption isotherms obtained and analysed with the Langmuir model showed that this model fitted the adsorption data. Calculated thermodynamic parameters (H°, S°, and G°) indicate that the adsorption processes were exothermic.

KEYWORDS: bentonite, amino acid, cysteine, adsorption, isotherm, thermodynamics