Phosphate-induced Immobilization Of Heavy Metals In Multi-metal Contaminated Soils

Chemical inactivation is a cost-effective method for remediation of metal contaminated soil, which is based on reduction of metal availability through formation of amendment -induced insoluble metal precipitates. Application of phosphorus-bearing materials in remediation of contaminated soil has been greatly receiving more attention, but it is mostly focused on Pb immobilization. However, real soil contaminated with metal is often a system of multi-metal combined pollution. Therefore, it has more practical significances to apply phosphorus-bearing materials in remediation of the multi-metal contaminated soils.The expriment was conducted by using phosphate rock (PR) and phosphorus fertilizer (TSP), alone or in combination, to induce immobilization of Pb, Cd, Zn and Cu in a multi-metal combined contaminated soil. The treatment effectiveness was evaluated through CaCl2 extraction, toxicity characteristic leaching procedure (TCLP), physiologically based extraction test (PBET), and column leaching expriment. The results show that:(1) All three P treatments (PR, TSP, and PR+TSP) were effective in reducing Pb concentrations in CaCl2, TCLP, and PBET extracts. The concentrations of Pb in CaCl2 and TCLP extracts were reduced by 20~80% and 62~86%, respectively. Effects of three P amendments on Pb immobilization followed a trend of TSP> PR+TSP> PR. Similarly, three P treatment also effectively immobilize Cd, with Cd in TCLP extract being reduced by 32%. As for Cu and Zn, the treatment effectiveness depends on the P-bearing material nature and evaluation methods. PR was effective in reducing Cu and Zn in CaCl2 and TCLP extracts. In spite of reduction of TCLP-Cu and -Zn, TSP treatment increased Zn and Cu concentrations in CaCl2 extract. PR+TSP treatment increased Zn and Cu concentrations in both CaCl2 and TCLP extracts.(2) As the soil column leachate increased, a breakthrough of P was observed at more than two pore volume of leachate in the TSP and PR+TSP treatments, but P leaching was low and remained unchanged in the PR treatment. All three P treatment reduced Pb leaching, bu the leaching of Cu and Zn were elevated in the TSP and PR+TSP treatments. The order of the total released amount of the heavy metals was Cu >Zn> Pb.(3) Immobilization of Pb was probably attributed to formation of Pb phosphate precipitate; while surface adsorption and complexation may be responsible for inactivation of Cu and Zn.(4) Overall, all three P treatments were effective in immobilizing Pb and Cd, but less for Cu and Zn; and even the availability of Cu and Zn was elevated by some P treatments. This case should be paid more attention and may be avoided by application of P-bearing materials in combination with other amendments, e.g., CaCO3...