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)center dot 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(H2PO4)(2) a (TM) H2O, KH2PO4, and K2HPO4 significantly decreased NH4OAc-extractable Cd (plant-available form) concentrations with increasing application rates. Compared to other phosphate materials used, K2HPO4 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)(2)HPO4 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 K2HPO4 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.