Adsorption of lead and cadmium ions from aqueous solutions by tripolyphosphate-impregnated Kaolinite clay

The pretreatment of Kaolinite clay with tripolyphosphate (TPP) increased the cation exchange capacity (CEC) of Kaolinite clay from 13.45meq/100 g to 128.7meq/100 g. The equilibrium adsorption capacity of TPP-Kaolinite clay for Pb2+ and Cd2+ was 126.58 mg/g and 113.64 mg/g, respectively. The presence of Na- and Ca-electrolytes and with increase in their concentrations reduced the selectivity of TPP-Kaolinite clay for Pb2+ than Cd2+. TPP-Kaolinite clay showed higher selectivity for Pb2+ in the presence of these electrolytes and at all concentrations of these electrolytes used for the study. Binary mixtures of Pb2+ and Cd2+ in various concentrations caused a decrease in the adsorption capacity of TPP-Kaolinite for either metal ion. However, this may have caused the adsorption of Cd2+ onto high energy sites on the surface of the TPP-Kaolinite clay. Non-linear Chi-square model analysis of adsorption data using Langmuir, Langmuir-Freudlich, Freudlich, Toth and Temkin isotherms reveals that the adsorption of Pb2+ and Cd2+ by TPP-Kaolinite clay were best described by the Toth and Freudlich isotherms, respectively. At low concentrations (<= 500 mg/L) the adsorption of these metal ions showed better fits to the five models with Langmuir-Freudlich and Freudlich isotherms giving the best fits for Pb2+ and Cd2+, respectively. (c) 2006 Elsevier B.V. All rights reserved.