Effective remediation of cadmium and lead contaminated soils by a novel slow-release phosphate amendment

Phosphate is widely used to immobilize cadmium (Cd) and lead (Pb) in soils through the insoluble metal phosphate precipitation. However, an increase in the phosphorus content of the environment can cause new pollution. In this study, five slow-release phosphate amendments (SRPAs) were synthesized and their characteristics including BET, SEM, FTIR, swelling ratio, and thephosphorus release were determined. The results show that SRPA was a sphere with a network structure with a specific surface area of 5 to 7.18 m(2)/g andcontained phosphate, hydroxyl, carboxyl and other functional groups. Among five SRPAs, S3 sample showed good performance for phosphate release. Phosphate release from SRPA was well fitted with Ritger-Peppas model with constant n between 0.45 and 0.85, indicating that the phosphate release was in accordance with non-Fickian diffusion. As compared with monocalcium phosphate (MCP), SRPA application led to a lower concentration of water-soluble phosphorus in the soil sample and higher remediation efficiencies of Cd and Pb. The remediation efficiencies of water-soluble Cd and Pb in soil with SRPA were 97.1% and 97.9%, respectively. The remediation efficiencies of bioavailable Cd and Pb were 71.85% and 76.47%, respectively. The results of Tessier extraction showed that the exchangeable and carbonatebound fractions of Cd and Pb in the soil sample after SRPA application significantly reduced, while the residual fraction increased, indicating the stability of heavy metals increased.