Thermodynamics of copper adsorption-desorption by Ca-kaolinite

The temperature effect on Cu adsorption and desorption on kaolinite has been investigated at four temperatures (8 degreesC, 25 degreesC, 30 degreesC and 40 degreesC). The clay sample was saturated with Ca. Copper was sorbed from solutions containing eleven Cu concentrations between 0.0010 and 0.0211 M, equilibrated for 2 days. Cu adsorption decreased, but desorption increased with increasing temperature, indicating that adsorption is an exothermic process while desorption is endothermic. These conclusions are confirmed by the values of DeltaH(0) estimated for adsorption and desorption. The initial copper concentration is a significant factor influencing mainly the adsorption process, which is spontaneous (DeltaG(0) < 0) only for low initial Cu concentrations. The desorption process is spontaneous for all Cu concentrations. DeltaS(0) values have also been calculated. Experimental adsorption data were successfully fitted to the Freundlich isotherm and to the Gouy-Chapman model in order to express the process quantitatively.