Cadmium sorption and desorption in limed topsoils as influenced by pH: Isotherms and simulated leaching

The influence of pH on cadmium (Cd) sorption and desorption isotherms was studied in sandy and clayey soils of northern Germany. Seventeen topsoils from four long-term lime amendment experiment plots ranging in pH from 4.1 to 6.9 were collected, Batch adsorption isotherms demonstrated the strong influence of pH on Cd sorption behavior, Cadmium sorption increased,vith pH, even within a very narrow range of pH values (Delta pH approximate to 0.3), The experimental desorption isotherms, which were determined by widening the soil/solution ratio, differed markedly from the adsorption isotherms. Differences decreased when reaction time for desorption was prolonged. The data for Cd adsorption and desorption were pooled to derive the coefficients of a generalized Freundlich-type expression S = S(C-M) including soil pH in 0.01 M CaCl2 and organic C content (oc) (% by mass) (S = k* (H+)(a) oc(b) C-M). Sorption isotherms were used for simulations to predict vertical displacement of Cd in the field. The simulations using measured field mean concentration profiles S(z) as initial values, showed Cd adsorption isotherms to be appropriate for describing longer-term desorption equilibria.