Equilibrium study of adsorption of L-cysteine by natural bentonite

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 NH (super +) (sub 3) moiety. The pH dependence suggests that these sites may be silanolate groups ( identical with Si-O (super -) ). 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 (Delta H degrees , Delta S degrees , and Delta G degrees ) indicate that the adsorption processes were exothermic.