SORPTION NONEQUILIBRIUM DURING CADMIUM TRANSPORT THROUGH SOILS

We studied the sorption behaviour of cadmium during its transport thro ugh laboratory soil columns packed with two different soils (a Spodoso l 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 t he Oxisol. However, sorption in the Oxisol tended to increase at a ver y slow rate after 10 h up to 300 h. Simulations carried out using batc h sorption data, which followed the Freundlich equation, failed to des cribe 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 bo th soils, indicating the occurrence of non-equilibrium sorption during cadmium transport. The presence of non-equilibrium conditions was fur ther supported by the observed decrease in Cd concentrations in the ef fluent fractions collected after stopping the flow through the columns . The Cd BTCs were adequately described only when the bi-continuum (tw o-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-equilibri um component. It is concluded that the sorption non-equilibrium observ ed during Cd transport is likely to be due to both sorption related (c hemical) and transport-related (physical) mechanisms.