Phosphate Associated Cadmium Immobilization Mechanism Depending on the Original Concentration of Cd in Soil

Adsorption and precipitation of cadmium (Cd) could be dependent on rate of P addition and Cd level in soil. Therefore, the objective of this study was to examine how addition rate of P affect mechanisms of Cd immobilization such as adsorption and precipitation in different levels of Cd in soil. Arable soils were spiked with inorganic Cd (CdCl₂) to give a total Cd concentration of 10, 100, and 1,000 mg Cd kg -1 . Monopotassium phosphate (KH₂PO₄, MPP) was selected as phosphate material and mixed with the pretreated arable soil at the rates of 0, 800, 1,600 and 3,200 mg P kg-1. The mixture soils were incubated at 25°C for 8 weeks in dark condition. Soil pH decreased with increasing MPP addition rate in all levels of Cd but negative charge of soil increased, thereby reducing 1 M NH₄OAc extractable Cd. Soil solutions were undersaturated with respect to CdCO₃ and Cd₃(PO₄)₂ with all P addition rate in soil with low Cd level (≤ 100 mg Cd kg-1) but supersaturated in soil with high Cd level (1,000 mg Cd kg-1). From the above results, Cd solubility was controlled by precipitation of Cd minerals such as CdCO₃ and Cd₃(PO₄)₂ in soil with high Cd level but by Cd adsorption induced by increase in negative charge of soil with low level of Cd.