Effect of adsorption on sulfate leaching

Movement of sulfate (S) through repacked columns of soil with high and low S adsorption capacity was studied in the laboratory. Differences in movement could be explained by the shape of adsorption isotherms which were determined from batch-type experiments. 35S was found to be a suitable tracer to study the movement of applied 32S. For step-function changes between 0, 3.12, and 6.24 mol m-3 in the concentration of the applied S solution, different breakthrough curves (BTC) were found for the rising and falling limbs. Also the higher the concentration of S in the applied solution, the earlier the added S appeared in the effluent. The BTC obtained agreed well with those calculated from numerical solution of the dispersion-convection equation, assuming a Freundlich adsorption isotherm and instantaneous reversible adsorption. However, after leaching of a pulse of S added to the surface soil, the observed S distributions in soil columns were not well described by theory. The discrepancies were largest when the soil containing the added pulse of S was incubated prior to leaching. Possible reasons for this lack of agreement are discussed