Cadmium transport in an unsaturated stony subsoil monolith

Heavy metal mobility in soil can adversely affect our environment. To assess Cd mobility under experimental conditions that can be found in field soils, we investigated Cd transport in an undisturbed stony monolith of a calcareous subsoil. In this context, we evaluated different batch experiments to independently predict Cd transport. Simple (monocomponent) Cd isotherms and binary Ca-Cd cation-exchange isotherms were determined. The method used for determining the ion-exchange isotherm accounted for calcite dissolution. The column experiment was carried out under unsaturated conditions by keeping the lower end at a suction of 2 kPa. A 20 mM Cd pulse was applied during 19 d and the effluent concentration monitored for 91 d. The peak concentration of the effluent exceeded the Cd concentration, as computed with thermodynamic equilibrium models, by three orders of magnitude. During elution, one-third of the Cd remained in the column, indicating sorption hysteresis. Simulations with Freundlich isotherms and selectivity coefficients were capable of describing the sorption front. We modeled the hysteresis with a two-site model that included irreversible sorption so that both sorption and desorption fronts were described equally well.