Mechanism of cadmium immobilization in phosphate-amended arable soils

Abstract

AbstractLittle is known about the exact mechanism of cadmium (Cd) immobilization by phosphate (P) in arable soil containing low Cd concentration. This study was designed to describe this mechanism in detail. We determined the phosphorus (P) addition rate allowing Cd to precipitate as Cd minerals in Cd-contaminated arable soils and identified the main species of Cd minerals formed by the reaction of Cd and P, using the chemical equilibrium model MINTEQ. To determine the amount of Cd adsorption to soil adsorbed P, 0 –10,000 mgPL−1 of K2HPO4 solution was reacted with soil, then the P adsorbed soil was reacted with a CdCl2 solution (500 mg Cd L−1). Cadmium might not precipitate as Cd minerals such as Cd3(PO4)2 and CdCO3 with a recommended application rate of P fertilizer in field scale. Cadmium might be immobilized by Cd2+ adsorption instead of precipitation under a low P application system. Phosphate adsorption increased the negative charge of soil and Cd adsorption. The contributions of the increase in pH- and P-induced negative charges to the total increase in the soil negative charge were 93.2 and 6.8%, respectively. The increase in Cd adsorption caused by P adsorption was mainly attributed to the increase in pH-induced negative charge.