Sorption of cobalt on bentonite

Abstract

Sorption of Co on bentonite has been studied by using a batch technique. Distribution coefficients (K _d ) were determined for the bentonite-cobalt solution system as a function of contact time, pH, sorbent and sorbate concentration and temperature. Sorption data have been interpreted in terms of Freundlich, Langmuir and Dubinin-Radushkevich equations. Thermodynamic parameters for the sorption system have been determined at three different temperatures. The positive value of the heat of sorption, ΔH ⁰=22.08 kJ/mol at 298 K shows that the sorption of cobalt on bentonite is endothermic, where as the negative value of the free energy of sorption, ΔG ⁰=−10.75 kJ/mol at 298 K shows the spontaneity of the process. ΔG ⁰ becomes more negative with an increase in temperature which shows that the sorption process is more favourable at higher temperatures. The mean free energyE∼7.7 kJ/mol for sorption of cobalt on bentonite shows that ion-exchange is the predominant mode of sorption in the concentration range of the metal studied i.e. 0.01 to 0.3 mol/dm³. The presence of some complementary cations depress the sorption of cobalt on bentonite in the order of K⁺>Ca²⁺>Mg²⁺>Na⁺. Some organic complexing agents and natural ligands also affect the sorption of cobalt. The desorption studies with ground water at low cobalt loadings on bentonite show that about 97% metal is irreversibly sorbed.