Abstract: Considering that chelant-enhance washing may increase the activity of residual metals in soil, the risk reduction of heavy metal-contaminated soil by integrated soil washing with immobilization was investigated. Ethylenebis (oxyethylenenitrilo) tetraacetic acid (EGTA) and triple super phosphate (TSP) were used as washing and stabilization agent, respectively, to remediate Cu-, Zn-, Pb-and Cd-contaminated yellow brown soil. The effects of EGTA dosage, washing time, liquid-to-solid ratio and TSP dosage on heavy metal removal rate and leaching concentration were explored. The relationship between the environmental risk reduction rate and EGTA dosage, liquid-to-solid ratio and TSP dosage was fitted by response surface method (RSM). The remediation performance was evaluated by the environmental risk assessment method involving heavy metal content in soil, leaching concentration and toxicity. The results show that with EGTA enhanced-washing, the elution rate of Cu and Cd was relatively higher and the leaching concentration of Zn and Cd was significantly reduced. By increasing the washing time, the removal rate of Cu, Zn, Cd was improved, while the leaching concentration declined. However, the leaching concentration of Pb was increased. With the increase of liquid-to-solid ratio, the removal rate of Zn and Pb was increased, while that for Cd was decreased and for Cu was firstly increased and then decreased. Meanwhile, the leaching concentration of Cu, Zn and Cd was decreased while for Pb was increased. The leaching concentration of Pb was significantly reduced by the passivation of TSP. The total environmental risk reduction rate (β) had a significant quadratic relationship with the EGTA dosage, liquid-to-solid ratio and TSP dosage. The model results are similar to the verification experiment results when the liquid-to-solid ratio, EGTA, TSP and β were 10, 1.0 g·L^-1, 2% and 62.80%, respectively, which indicate that the model has good simulation and prediction ability. The environmental risk of Cu, Zn, and Cd could be significantly reduced by increasing the EGTA dosage and liquid-to-solid ratio. The passivation of TSP possessed a positive effect on the environmental risk reduction of Pb. In addition, the synergistic effect on β was observed between EGTA dosage and liquid-to-solid ratio, as well as EGTA and TSP dosage.

Key words: soil heavy metal, washing, immobilization, environmental risk, response surface methodology (RSM)