Adsorption and Precipitation of Cadmium Affected By Chemical Form and Addition Rate of Phosphate in Soils Having Different Levels of Cadmium.

Previous studies have reported that P-induced immobilization of Cd in soils could be attributed to two primary factors: (1) precipitation of Cd in a variety of inorganic phosphate forms and (2) Cd²⁺ adsorption to soil particles induced by increase in pH and negative charge of soil. These studies were conducted in various conditions such as P materials, addition rates of P, and inherent Cd concentrations in soil. Although the number of studies have examined Cd immobilization by P in soils, determining exact mechanism of Cd immobilization in various conditions has not been examined in detail. Therefore, the objective of this study was to determine changes of Cd immobilization such as adsorption and precipitation in differently given conditions. An arable soils were spiked with inorganic Cd (CdCl₂) to give a total Cd concentration of 10, 100, and 1,000 mg Cd kg^-1. K₂HPO₄ and KH₂PO₄ were selected 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 ℃ for 8 weeks in dark condition. In 10 and 100 mg kg^-1 of Cd levels, both P materials similarly increased negative charge of soil and decreased extractable Cd concentration. In XRD pattern for Cd 1,000 mg kg^-1 level soil added with 3,200 mg kg^-1 of K₂HPO₄, a diffraction peak revealing to the presence of a novel Cd compound, Cd(H₂PO₄)₂. Immobilization of Cd might be mainly attributed to negative charge induced Cd adsorption in soil with relatively low Cd levels (< 100 mg kg^-1). Precipitation of Cd(H₂PO₄)₂ and formation of poorly crystallized or amorphous CdCO₃ and Cd₃(PO₄)₂ might be a dominant mechanism to immobilize Cd besides Cd adsorption in soil with relatively high Cd levels (1,000 mg kg^-1).