Adsorption and precipitation of cadmium affected by chemical form and addition rate of phosphate in soils having different levels of cadmium

Although a number of studies have examined cadmium (Cd) immobilization by phosphate (P) in soils, determining the exact mechanism of Cd immobilization in various conditions, including P chemical form, P rate, and inherent Cd concentrations in soil has not been examined. The objective of this study was to determine changes in Cd immobilization through adsorption and precipitation in different conditions. Arable soils were spiked with inorganic Cd to give a total Cd concentration of 10, 100, and 1000 mg Cd kg−1. K2HPO4 (DPP) and KH2PO4 (MPP) were selected and mixed with the pretreated arable soil at the rates of 0, 800, 1600 and 3200 mg P kg−1. In soils with 10 and 100 mg Cd kg−1, both P materials similarly increased negative charge of soil and decreased extractable Cd concentration. Using X-ray diffraction (XRD), a diffraction peak revealing the presence of Cd(H2PO4)2 was observed in soil with 1000 mg Cd kg−1 and 3200 mg P kg−1 soil. In addition, X-ray photoelectron spectroscopy (XPS) analysis and modeling for saturation index for Cd minerals proved that formation of CdCO3 and Cd3(PO4)2 occurred in soil having 1000 mg Cd kg−1 and addition of both DPP and MPP. Immobilization of Cd might be attributed to Cd adsorption in soil with relatively low Cd levels (<100 mg kg−1). Precipitation of Cd(H2PO4)2, CdCO3, and Cd-phosphate might be a dominant mechanism to immobilize Cd, besides Cd adsorption, in soil with relatively high Cd levels (1000 mg kg−1).