Immobilization of trace metals by phosphates in contaminated soil near lead/zinc mine tailings evaluated by sequential extraction and TCLP

摘要

Purpose The combined contamination of Pb, Zn, Cu, Cd, and As in the soils near lead/zinc mine waste posed a potential threat to the surrounding environment. Mitigation methods are needed to reduce the environmental risk. The aims of this paper were to evaluate the feasibility and efficiency of different forms of phosphates in remediating combined contamination caused by multi-metals and arsenic near the lead/zinc mining tailings. Materials and methods The tested soil was taken from a clayey illitic thermic typic epiaquepts soil (depth of 0-20 cm) near a lead and zinc mine tailing, located in Shaoxing, Zhejiang Province, China. Four pure chemical reagents, K_2HPO_4, Ca(H_2PO_4)_2._2H_2O, Ca_3(PO_4)_2, and Ca_5(PO_4)_3OH, were added to the soil in solution form as the trace metal stabilizing additives at a P application rate equivalent to 2,283 mg P/kg- soil and P/Pb molar ratio of 1. Shifts in trace metal speciation were determined using sequential extraction procedures and relative toxicities were evaluated using the standard EPA tox-icity characteristic leaching procedure (TCLP). Results and discussion The addition of K_2HPO_4, Ca_3(PO_4)_2, Ca_5(PO_4)_3OH, and Ca(H_2PO_4)_2.2H_2O in the contaminated soil at the phosphorus application rate 2,283 mg P/kg-soil reduced Pb concentrations in TCLP extraction solution from 90.47 to 56.05, 83.80, 67.78, and 86.32 mg/kg (38.0, 7.36, 25.1, and 4.59% reduction), respectively. Sequential extraction analysis showed that phosphate treatments caused the transformation of easily available trace metal species to more stable forms. However, TCLP As in the soil increased from its initial value of 0.23 to 2.1, 0.70, 0.67, and 0.77 mg/kg, respectively, for the four treatments. The TCLP leachable As concentration of the K_2HPO_4-treated soil was about nine times of that from the untreated soil. Conclusions The addition of K_2HPO_4, Ca_3(PO_4)_2, Ca_5 (PO_4)_3OH, and Ca(H_2PO_4)_2.2H_2O were effective in reducing water soluble and exchangeable Pb, Zn, Cu, and Cd, and minimizing TCLP-extractable Pb, Zn, and Cu. The sequential extraction test and the TCLP indicate that Ca_5(PO_4)_3OH treatment has a higher potential in immobilizing Pb, Zn, Cu, and Cd, though a slight enhancement of As mobility, comparing with other phosphate treatments.