Kinetic aspects of sorption and desorption processes

Former investigations revealed that rates of sorption reactions are mostly faster than that of desorption reactions. Based on this phenomenon detailed laboratory tests on porous sediments were performed to determine the equilibrium distribution coefficients of radioactive Sr2+ and Cs+ as well as the corresponding rates of the sorption and desorption processes. It has been found that the rate constants of the sorption processes differ for the radionuclides used and depend on the chemical composition of the sediment water mixtures. In general, the rate of radionuclide sorption becomes faster with decreasing ion strength. In our test systems the rate constants for the respective sorption processes were up to 100 times faster than the corresponding desorption rate constants. Measured equilibrium distribution coefficients varied in the range of 0.1-1000 cm3/g. With these results the sometimes observed 'irreversible sorption' of radionuclides as well as the spreading or tailing of the effluent curves measured in flow-through column experiments appear to be due to a kinetic effect. Therefore an attempt is reasonable to discuss sorption and desorption processes by help of kinetic considerations. (author)