Assessment Of Toxicity Of Heavy Metal Contaminated Soils And Remediation

Close attentions have been paid to assessment and remediation of heavy metal polluted soils all over the world. In this paper, TCLP (toxicity characteristic leaching procedure) method developed in U.S.A. was introduced to evaluate the toxicity of a zinc/lead mining tailings polluted soil. Remediation of the polluted soil using phosphorus fertilizers single superphosphate (SSP), phosphate rock (PR) and calcium and magnesium phosphate (CMP) was carried out both in laboratory and field. The main results are as follows:1. Assessment of Toxicity of Heavy Metal Contaminated Soils by Toxicity Characteristic Leaching ProcedureThe contents of heavy metals (Cu, Zn, Pb and Cd) in soils near a lead-zinc mine in Shangyu, Zhejiang Province, China, were analyzed and their toxicity was assessed by TCLP, which was developed by US EPA. The TCLP method is a currently recognized international method for evaluation of heavy metal pollution in soils. The available levels of Cu, Zn, Pb and Cd were 8.2³6 mg·kg^-1, 23¹43 mg·kg^-1, 6.4¹367 mg·kg^-1 and 0.41².2 mg·kg^-1, respectively, while the international standard were 15 mg·kg^-1, 25 mg·kg^-1, 5 mg·kg^-1 and 0.5 mg·kg^-1, respectively. The results show that soils around the mine were polluted with heavy metals Cu, Zn, Pb and Cd, especially polluted by Zn and Pb, followed by Cd and Cu. Moreover, the heavy metals in the soils extracted by TCLP indicated that fluid 2 is more effective than fluid 1 in extracting the heavy metals from the polluted soils and there was a positive correlation between fluid 1 and fluid 2. The contents of available heavy metals by TCLP were correlated with contents of total heavy metals.2 Remediation mechanism for lead polluted soils using phosphorus fertilizersThe results indicated that water soluble lead decreased in the soil after application of three phosphorus fertilizers. The mechanism was discussed. The reaction between P fertilizers and P formed the chloropyromorphite which has the lowest solubility ofPb, leading to a great decrease in Pb toxicity in the soil. At the same application rate, the order for remediation of Pb-contaminated soil was: CMP >PR >SSP. The content of water soluble Pb decreased with soil pH. It may be relating to the phosphorus existing in soil.3 Reduction of heavy metal availability in soil using phosphorus fertilizers in relation to uptake of heavy metals by vegetablesThree phosphorus fertilizers not only can remediate the soil polluted by Pb, but also can remediate the soil polluted by copper, zinc and cadmium. There was a negative relationship betweens soil pH and water soluble Pn, Zn Cu and Cd and the correlation coefficients were - 0.307, -0.589, -0.712 and - 0.482, respectively. The contents of heavy metals in the vegetables could be reduced notably by the three phosphorus fertilizers, leading to remarkable decrease by 62% in Pb uptake by the vegetable by the highest application rate, compared to the CK. At the middle application rate, the remediation effectiveness for SSP was best. The remediation effectiveness for CMP at the low application rate was that the Pb content in the vegetable decreased by 28.7% compared to CK. The amount of heavy metals in vegetables was positively correlated with water soluble content of heavy metals in the soil. Therefore, It is strongly suggested that phosphorus fertilizers should be used in remediation of heavy metal contaminated soils.