Comparison of phosphate materials for immobilizing cadmium in soil

A study was conducted to compare the effects of phosphate (P) materials in reducing cadmium extractability. Seven P materials (commercial P fertilizers - fused phosphate (FP), 'fused and superphosphate' [FSP], and rock phosphate [RP]; P chemicals-Ca[H₂PO₄]₂.H₂O, [NH₄]₂.HPO₄, KH₂PO₄, and K₂HPO₄) were selected for the test. The selected P source was mixed with Cd-contaminated soil at the rate of 0, 200, 400, 800, and 1,600 mg P kg^-1 under controlled moisture conditions at 70% of water holding capacity, then incubated for 8 weeks. FP, Ca(H₂PO ₄)₂.H₂O, KH₂PO₄, and K₂HPO₄ significantly decreased NH ₄OAcextractable Cd (plant-available form) concentrations with increasing application rates. Compared to other phosphate materials used, K ₂HPO₄ was found to be the most effective in reducing the plant-available Cd concentration in soil, mainly due to the negative charge increase caused by soil pH and phosphate adsorption. Contrary to the general information, FSP and (NH₄)₂HPO₄ increased Cd extractability at low levels of P application (<400 mg kg_-1), and thereafter Cd extractability decreased significantly with increasing application rate. RP scarcely had an effect on reducing Cd extractability. Ion activity products of CdHPO₄, Cd(OH)₂, and CdCO₃ analyzed by the MINTEQ program were significantly increased by K₂HPO ₄ addition, but the effect of Cd-P compound formation on reducing Cd extractability was negligible. Conclusively, the P-induced alleviation of Cd extractability can be attributed primarily to Cd immobilization due to the increase in soil pH and negative charge rather than Cd-P precipitation, and therefore, alkaline P materials such as K₂HPO₄ are effective for immobilizing soil Cd.