Sorption non-equilibrium during cadmium transport through soils

We studied the sorption behaviour of cadmium during its transport through laboratory soil columns packed with two different soils (a Spodosol and an Oxisol). Models based on both equilibrium and time-dependent sorption reactions were used to describe observed breakthrough curves (BTCs) of cadmium. Under batch conditions, the sorption of cadmium was essentially complete within 3 h for the Spodosol and within 24 h in the Oxisol. However, sorption in the Oxisol tended to increase at a very slow rate after 10 h up to 300 h. Simulations carried out using batch sorption data, which followed the Freundlich equation, failed to describe the observed cadmium BTCs satisfactorily. A sharp BTC front was expected due to the nonlinear nature of the sorption isotherm. However, a more dispersed front with significant asymmetry was observed in both soils, indicating the occurrence of non-equilibrium sorption during cadmium transport. The presence of non-equilibrium conditions was further supported by the observed decrease in Cd concentrations in the effluent fractions collected after stopping the flow through the columns. The Cd BTCs were adequately described only when the bi-continuum (two-site/two-region) approach of solute transport modelling was utilized. Only a small degree of asymmetry was noted in chloride BTCs for both soils, thus suggesting the presence of a small physical non-equilibrium component. It is concluded that the sorption non-equilibrium observed during Cd transport is likely to be due to both sorption related (chemical) and transport-related (physical) mechanisms.