Study On Mechanism And Solidification/Stabilization Treatment Of Pb/Zn Contaminated Soils And Waste Residues From Mining Tailing

In recent years, with the rapid development of China’s mining industry, the home and abroad has paid more and more attention at heavy metal pollution which caused by mining process. Mining generated a lot of waste residues and contaminated soils. The high content of heavy metals in the slag would cause the heavy metal pollution with the direct or indirect effects on the surrounding soils by acidified and rainfall leaching after weathering which caused by open dumps, Mining waste emissions and stacking will not only cause huge economic losses, but also induced significant geological and engineering disasters such as dump landslides, mudslides, and other tailings dam which have brought huge damage to the country and society. The soils contaminated by heavy mentals have a hidden potential and can’t be readily perceived. The contaminated soils are persistence and irreversible, it will be difficult to return to the original state and cure if soils have been contaminated. Therefore, in the present works which based on the characteristics of heavy metals pollution in mining areas, the stabilization/ solidification using cement ash as curing agent was used to immobilize the Pb and Zn waste residues and the stabilization/solidification using phosphate as amendment was used to immobilize the Pb and Zn contaminated soils.In the present work, the stabilization/solidification using cement and fly ash as curing agent and potassium dihydrogen phosphate as amendment was used to immobilize the Pb and Zn waste residues which from Hunan Dong Jiang lakes area historical. The efficiency of the process has been evaluated through leaching tests and the compressive strength under conditions of different proportions. The results show that the stabilizing effect of potassium dihydrogen phosphate slag of Pb is significant when the dosage of 4%.Although the concentration of Pb in TCLP leachate reduced significantly, which still couldn’t meet the requirements of the security landfill when using cement alone or cement, fly ash combination. Add cement alone, with the increase of compressive strength of the cured cement content is enhanced, and cement content to waste residues of 40% is appropriate; replace part of cement with fly ash treatment of heavy metal contaminated waste, fly ash and cement ratio of 2:8 is appropriate. For S/S of the waste residues, the optimum condition are following:potassium dihydrogen phosphate as amendment dosage of 5%, the curing agent residue ratio of 0.4:1 (fly ash and cement ratio of 2:8), At these conditions, the leaching concentrations of Pb, Zn from solidified body were 0.19 mg/L,0.60 mg/L, respectively, and the cured body of heavy metals Pb, Zn compressive strength of 2.01 MPa. The result data could meet the requirements of the security landfill. Potassium dihydrogen phosphate for heavy metal stabilization mechanism of the main reaction by the phosphate ion and lead ion generating phosphate precipitation; cement and fly ash cement curing mechanism mainly by the substance contained and Al, Si and other oxides of heavy metal ions adsorption, thus affecting the mobility of heavy metal ions.In the present work, the combined application of potassium dihydrogen phosphate, quick lime and potassium chloride was used to immobilize the Pb and Zn contaminated soils. The efficiency of the process was evaluated through leaching tests and Tessier sequential extraction procedure. The morphology and structure analyzed by X-ray diffraction (XRD) and scanning electron microscope (SEM) to revel the mechanism of stabilization. The results showed that the stabilizing efficiency of Pb contaminated soils was over 80% and the leaching concentrations of Pb, Zn were far below the threshold when the ratio of exogenous P and soil (mol/mol) was 2:1-4:1 and the dosing ratio of CaO was 0.1%-0.5% (w/w), Meanwhile, Pb and Zn in soil were transformed from the exchangeable fraction into residual fraction, which implied that the migration of Pb, Zn in soil could be confined by stabilization treatment. XRD and SEM analysis revealed that Ca-P-Pb precipitation、lead orthophosphate(PbHPO4、Pb3(PO4)2), pyromorphite (Pb-PO4-Cl/OH) and mixed heavy metal deposits (Fe-PO4-Ca-Pb-Zn-OH) as could be formed after solidification/stabilization in which Pb and Zn could be wrapped up to form a solidified composition and to prevent leaching.