Study On Stabilization Remediation Of Soil Contaminated By Lead And Cadmium Using Phosphorus-bearing Materials

The improper mining and smelting of mineral resources is a major aspect of environmental heavy metal pollution, mining heavy metal area have wide area, multi-metal combined pollution, high content, large latent and fertility degradation in characteristics. Most current technology focused on the single heavy metal contaminated soil repair; for multi-metal combined pollution is not outstanding. Therefore research on multi-metal contaminated soil remediation technologies is of great significance. In this study, immobilizing agents for stabilization of lead and cadmium in multi-contaminated soils were screened、compounded and further used to search for the optimal combination of immobilizing agents, the technique parameters were optimized to obtain the highest remediation efficiency, the stabilization of lead and cadmium in the remedied soils was evaluated. The main results are as follow:（1） Based on phosphates, industrial by-products, clay minerals deactivator rapid screening to determine Ca（H₂PO₄）₂ and montmorillonite as the main stabilizer. The experimental results are as follows:Ca（H₂PO₄）₂ was the optimal immobilizing agent to remedy lead and cadmium contaminated soils. The concentrations of available lead and cadmium in the Ca（H₂PO₄）₂-remedied soils declined by 70.05% and 69.55%, respectively. Phosphorus-bearing materials combinations with montmorillonite can reduced the 50% amount of Ca（H₂PO₄）₂, The concentrations of available lead and cadmium in the remedied soils declined by 71.32% and 65.10%.（2） The optimal conditions for mining lead and cadmium soils immobilization were as follows:the dosage of Ca（H₂PO₄）₂:montmorillonite was 1:1. The dosage of stabilizer was 5%. Soil water content was 50% of maximun soil water capacity. The reactiontime was 60 days. Under the above optimal condition, the removal rates of available lead and cadmium reached up to 80.20% and 70.96%, respectively.（3） Tessier sequential extraction was used to assess the mobility of the heavy metal. Ca（H₂PO₄）₂ and their combinations with montmorillonite addition significantly reduce the lead of ion exchangeable, Carbonate-bound and Fe-Mn oxides bound, significantly increased the amount of lead’s residual state. It’s reduced the cadmium of ion exchangeable and Fe-Mn oxides bound, significantly increased the amount of cadmium’s organic bound states. The study prove phosphorus-based materials can significantly change the soil heavy metals by biological effective transition to organic and residual steady state.（4） Ca（H₂PO₄）₂-remedied soil, the stabilization soil’s pH was reduced by 1.43～3.29 units, their combinations with montmorillonite passivation soil, the stabilization soil’s pH was reduced by 0.67～2.57 units. The results of column leaching experiment show that:Concentrations of Lead and cadmium in leachate and deep-layer soil are not significantly elevated. All treatments can immobilize lead, retaining lead in the top-layer contaminated soil. Ca（H₂PO₄）₂ with montmorillonite composite materials can reduce migration of soil arsenic mine activation and P in leachate and deep-layer soil are not significantly elevated.（5） The XRD analysis showed that lead and cadmium in the Ca（H₂PO₄）₂-remedied soils formed insoluble substance. The XRD analysis showed that lead and cadmium in the Ca（H₂PO₄）₂ with montmorillonite-remedied soils formed insoluble substance Pb₃（PO₄）₂ and Cd₃（PO₄）₂. Montmorillonite and soil arsenic ligand complex reaction reduced PbAs₂O₆ content in the soil.