Modeling cadmium transport in soils using sequential extraction, batch, and miscible displacement experiments

Asymmetric transport behavior of Cd in soils can result from sorption nonlinearity or rate-limited sorption, of which the relative significance is difficult to be discerned by transport modeling alone. Multiple approaches incorporating batch, sequential extraction, and miscible displacement experiments were therefore used in this study. Batch isotherm was used to indicate the range of Cd concentration where sorption became nonlinear. It is noteworthy that batch kinetics illustrated a shorter apparent equilibrium time of sorption at a higher Cd concentration, which presumably correlates with the significance of rate-limited sorption. In addition, sequential extraction results indicated that the relative contribution of rate-limited specific sorption to total sorption became negligible with increasing Cd concentration. Since the rate-limited specific sorption appears to be of low capacity, it is only important for sorption at low concentration. In miscible displacement experiments, Cd transport at 10-5 M could only be simulated using a nonequilibrium model, whereas a nonlinear model was necessary for describing Cd transport at 10-3 M. Therefore, this study demonstrated that transport behavior is predominantly influenced by rate-limited sorption at sufficiently low concentration, while by sorption nonlinearity at sufficiently high concentration. Additionally, sequential extraction can be a useful tool to illustrate the significance of rate-limited specific sorption and the corresponding chemical nonequilibrium transport behavior.