Effect of soil solution composition on cadmium transport through variable charge soils

Cadmium (Cd) is of no known essential biological function and is toxic to plants and animals. Leaching of Cd through soil profiles has implications for both its accumulation in subsoil or contamination of ground-water. We measured adsorption and transport of Cd in the presence of Ca and Na salts of varying ionic strengths using batch and miscible displacement techniques. Two variable charge soils, an Oxisol and an Alfisol, were used in this study. The Alfisol, despite its lower clay content, showed an adsorption coefficient (K) that was four times higher than the clay-rich Oxisol. Such a difference in adsorption was attributed to the presence of 2:1 layer silicate minerals in the Alfisol and the consequent high surface negative charge density. Zn column experiments, a marked effect of ionic strength on the breakthrough curves (BTCs) of Cd was observed when the concentrations of NaNO3 were increased from 0.03 M to 0.15 M in the background solutions. This increase caused nearly four times faster movement of Cd through the Oxisol soil column. The effect of increasing Ca(NO3)(2) concentration on Cd adsorption and transport was relatively less pronounced than that recorded for the NaNO3 solution. In the Alfisol, the increase in Ca(NO,), ionic strength from 0.05 M to 0.25 M resulted in four-fold smaller adsorption coefficient. For both soils, the movement of Cd at a constant ionic strength was, however, an order of magnitude faster in the presence of Ca(NO,), than that in the presence of NaNO3. To describe the combined effect of ionic strength and pH, we used the K values calculated from the BTCs with CXTFIT model to develop a simple theoretical relationship between Cd adsorption and [H+], [Ca2+] and [Cd2+] in solution. The relation was found to be consistent with other published studies. The study demonstrated that Cd mobility in soils is strongly affected by the soil solution composition (ionic strength and type of cations). (C) 1998 Elsevier Science B.V. All rights reserved.