A constant potential titration method for studying the kinetics of Cu2+ desorption from soil and clay minerals

The kinetics of Cu²⁺ desorption from a contaminated Malaysian padi soil, English Supreme kaolinite and Wyoming bentonite are studied using a constant potential titration method. This technique involves maintaining a constant Cu²⁺-sensitive electrode potential corresponding to a Cu²⁺ ion activity below that which would naturally be supported by the soil in suspension. This is achieved by the controlled addition of complexing agent. Desorption occurs in response to the artificially reduced solution activity and the rate of the desorption reaction is revealed by the rate of addition of the complexing agent.
The desorption results were fitted to three established kinetic equations, all of which showed reasonable agreement with the data. In particular, a two-constant rate equation demonstrated some consistency between different materials. Both first-order and 'diffusion' kinetic equations appeared to apply within specific titration conditions. Each equation is discussed in terms of its implied desorption mechanism and the inconclusive nature of such mechanisms for short term (< 1 hour) desorption reactions is illustrated.