Studies on the chemical form of cadmium contaminants in phosphate fertilizers

Greenhouse and laboratory experiments were conducted to characterize the chemical form or forms of Cd, which is a contaminant in phosphate fertilizers, in terms of plant availability and extractability from soil. Winter wheat (Triticum aestivum L.), cv. Nugaines, was grown on Mountview silt loam (limed to pH 5.5 and 7.5) treated with three commercial diammonium phosphate (DAP) products containing 48, 73, and 188 ppm Cd and reagent grade DAP alone (1 ppm Cd) or with added CdCl2 to supply 188 ppm Cd. Cadmium concentrations in wheat forage, grain, and straw were significantly increased with increasing Cd concentrations in DAP at both soil pH levels. Concentrations of Cd in plant tissues were higher from reagent DAP containing CdCl2 than from commercial DAP containing the same Cd concentration and also were higher in plant tissues from all fertilizer treatments on acid than on alkaline soil. Results of a second greenhouse experiment also showed that Cd concentrations in corn (Zea mays L.) forage from DAP products containing the same concentrations of Cd (131 ppm) decreased in the order: reagent DAP + CdCl2, commercial DAP, and reagent DAP + Cd3(PO4)2.In laboratory experiments, commercial DAP and superphosphate (TSP) containing 188 and 180 ppm Cd, respectively, and reagent DAP containing about the same concentrations of Cd added as several Cd salts were mixed with soil at a rate of 200 ppm P (0.19 ppm Cd). After incubation in moist soil at room temperature, levels of extractable Cd decreased in the order: DAP + CdCl2, commercial DAP and TSP, DAP + Cd(H2PO4)2, DAP + CdHPO4, and DAP + Cd3(PO4)2. Concentrations of Cd in extracts from each treatment in alkaline soil generally were lower than those in extracts from acid soil, followed the same trends with each experiment, and decreased in the order: 0.1 N HCl, 0.005 M DTPA, and deionized H2O. The decrease in level of extractable Cd in treated soil is in the same order as that of plant uptake results in the greenhouse experiments. Results therefore suggest that the chemical form of Cd contained in TSP and DAP is Cd(H2PO4)2, CdHPO4, or a mixture of these salts, which are Cd analogues of the P compounds in commercial P fertilizers.