Dispersion of cadmium in columns of saturated sandy soils

Nonequilibrium adsorption is often assumed to contribute most to the apparent dispersivity of Cd in leached columns of soil. Equilibrium adsorption of Cd to heterogeneously distributed adsorption sites can, however, be shown to produce the same apparent dispersivity. The aim of this study is to resolve and quantify these dispersive processes. The dispersivity of Cd was estimated in small columns of a saturated, homogeneously packed, sandy soil by fitting a numerical solute transport model to breakthrough curves (BTC) of step increases in the concentration of dissolved Cd. The transport model assumed homogeneous and instantaneous adsorption (single-regional, equilibrium adsorption). The apparent dispersivity found for Cd was much larger than for Cl-. Comparable results were obtained when ETC of Cd from the literature were fitted to the equilibrium transport model. The experimental dispersivities of both Cd and Cl- were not significantly affected when the how rate was varied between 0.3 and 6 m d(-1). At these rates of how, the dispersivity of Cd appears to be strongly affected by the heterogeneous distribution of adsorption sites and much less by rates of adsorption and desorption. The main adsorption sites in the soil studied were small aggregates of organic matter within a matrix of larger particles of quartz. Numerical simulation of the effect of distribution of these adsorption sites on dispersivity with a multi-regional model resulted in dispersivities of Cd similar to those observed experimentally.