Study On Adsorption Characteristics And Numerical Simulation Of Cadmium Transport In Soils

Cadmium (Cd) is one of the mostly polluting heavy metal. First, to characterize the adsorption property of cadmium, cadmium adsorption isotherms in soils were determined by the batch experiment, and the adsorption parameters were obtained; Second, miscible displacement experiments were carried out in saturated homogeneous soil columns under steady-state water flow, and the dynamics of flux concentration of cadmium leached in soils were measured. The adsorption characteristics of cadmium in three textured soils were described by Freundlich isotherms, whereas cadmium transport was described as a convection-dispersion process. The traditional Freundlich isotherms were compared with the two-species Freundlich isotherms, the effects of the organic matter content (OM) and pH were considered, and adsorption parameters affecting movement of cadmium in soil were gained.As for mathematical modeling, the deterministic models were adopted to fit and predict the breakthrough curves of cadmium in soils. Transport parameters of displacement experiment were calculated by tracer (chloride) displacement. The breakthrough curves of cadmium through the soils were simulated by the convection-dispersion equation according to the local equilibrium assumption, and cadmium transport in soils was fitted by the nonequilibrium models (including the two-site kinetic sorption model and the two-region model). Then, the factor of affecting cadmium transport was analyzed. It was shown that the two-site kinetic sorption model and the two-region model provided a useful approach to quantify cadmium transport more efficiently.The other part of their paper was to evaluate the risk of heavy metal (Cadmium) movement in the field. First, the experiment of cadmium sorption was carried out in a 15mX 15m plot at Yongledian, Tongxian county, in Beijing and 36 different soil samples were taken from the field plot according to the 3mX3m grid size. Using the two-site kinetic sorption model and the two-region model, cadmium transport in field soils was simulated, the average of the numerical solution of Cd transport at every soils sample was obtained and the spatial distribution of Cd concentration was computed.In a word, pH and OC can affect adsorption characteristics of cadmium in soils, so the two-species Freundlich model can improve the fitted precision on adsorption isotherms. Nonequilibrium model can simulate the cadmium transport in saturated soils under steady flow condition. The spatial distributions of cadmium transport in the field under sewage irrigation were investigated.