Cadmium sorption-desorption by soils in the absence and presence of phosphate

Batch experiments were carried out to determine the effect of phosphate anion (P) on the sorption-desorption of cadmium (Cd). The soils selected for the study were two kaolinitic Alfisols and two Andisols, which differ in their variable charge components. At the natural soil pH, Cd sorption in the absence of P was related to the net negative surface charge of the soils. Sorption of P increased the negative charge of the soils but the increase per unit amount of P added decreased with increasing P sorption. Both pH and soil characteristics influence the effect of P on Cd reactions. In the Alfisols, P and Cd were sorbed in similar amounts and the effect of increasing P rates on Cd sorption-desorption reactions was negligible. In the allophanic Andisols, whose capacity to bind P exceeds several times their capacity to retain Cd, the sorption of Cd was clearly enhanced with increasing P rates. In these soils, two distinct effects of P addition on Cd retention were observed. The first one arose from variations in the soil-suspension pH and the concomitant changes in soil surface charge. The second effect was directly related to the amount of sorbed P. Most of the Cd sorbed in the four soils was strongly bonded. In the Andisols, increasing P sorption on the surfaces restricted Cd desorption.