Determination of cadmium ion desorption from non-uniform sediment particles

A model for heavy metal desorption from sediments with different grain sizes is described in this paper. Using the adsorption reaction kinetics equation and the mass conservation equation, formulae for the calculation of the residual quantity of heavy metal adsorbed on unit weight sediment and the dissolved heavy metal concentration were deduced for the case of desorption. Furthermore, formulae for calculating equilibrium conditions were also obtained. On the basis of these theoretically deduced formulae and through laboratory reactor modeling of artificially contaminated sediments with Cadmium in suspension, heavy metal desorption abilities from non-uniform sediments were studied. It was found that the experimental data agree with the theoretical results fairly well. This suggests that the formulae are valid and can be applied as the base for future quantitative studies. It was also noticed from the experiments that the adsorption-desorption ability of heavy metals to and from sediments is dependent on the content of active adsorption components in the sediment. The more the content of active adsorption components, the stronger the adsorption ability of the sediments and the weaker the desorption ability. The total amount of heavy metal desorbed from sediments is directly proportional to the concentration of suspended sediment particles. Thus, in practice, much attention should be paid to the coarse sediment particles with high content of heavy metals. Also when dredging heavy metal contaminated sediments, attention should be paid to control the suspended sediment concentration during the operation. Experiments and theoretical analysis of heavy metal desorption kinetics on sediments with different grain sizes indicate that there is no competition and interference among different grain sizes. The total desorption quantity from unit weight sediment is the sum of the desorption quantities from each grain size, and the content of each grain size in non-uniform sediment. This can be used to calculate the amount of heavy metal desorbed from sediments with different grain sizes in natural rivers.