Effect of Hematite on HAP Immobilization of Soil Copper and Cadmium Under Flooding Conditions

Hydroxyapatite (HAP) is considered as an amendment for heavy metals contaminated soils, while The iron oxides in the soil have strong binding abilities with phosphate. However, it's not clear about the effects of iron oxides on the immobilization of heavy metals in soil. The effect of hematite on the immobilization efficiency of copper and cadmium by HAP were investigated through flooding experiments in this study. Results show that the addition of HAP-only and hematite-only increased soil pH and soil solution pH compared with the control. Hematite with high content promoted the reduction of phosphate, dissolved Cu²⁺ and Cd²⁺ in the soil solution in the presence of HAP. Meanwhile, soil available phosphorus was reduced by 12.9%-41.0%, and CaCl₂ extractable Cu and Cd were reduced by 24.9%-64.5% and 11.4%-39.3% respectively in HAP plus hematite treatments. Thus, hematite promoted the immobilization of Cu and Cd in the presence of HAP, and decreased the available P in soil under flooding conditions. The results of this study indicated that phosphorus-based materials can be applied to paddy soils in southern China for immobilizing heavy metals.