Preparation Of Polymeric Hydroxyl Ferric Phosphate And Application To The Remediation Of Lead, Cadmium Contaminated Soils

Abstract:The remediation of contaminated soil has become one of large concerns currently. It is needed to find a highly efficient and stable method to remedy the heavy metal-contaminated soils because of a large amount of contaminated soils in China. The key of chemical immobilization, as a cost-effective method for remediation of heavy metal contaminated soil, is to find a suitable amendment. Application of phosphorus-containing materials in remediation of contaminated soil has been proven effective in numerous studies. This experiment was conducted by synthesizing a polymeric hydroxyl ferric phosphate (PHFP) as an amendment, which applied to immobilize cadmium and lead in the contaminated soil.In this previous experiment, the PHFP was synthesized by using industrial by-products and slag, and then was modified to improve its ability to immobilize cadmium and lead in soils. The optimal synthesized conditions were studied in the experiments and the products were characterized. The optimal remediation parameters were determined by optimizing the fixed time, water content, fixative dosage, particle size of soils. The morphological changes of heavy metals in soil before and after fixation were studied by a tessier sequential extraction procedure. The long-term stability of the treated soil was evaluated by a simulated acid rain method. The main results were as follows:(1) The optimum synthetic conditions of PHFP were as follows: Fe/H2O2mole ratio was1:1.25, PO43-/Fe mole ratio was0.6, polymeric temperature was80℃, pH of the solution was8. The specific surface and particle size area of modified PHFP were13.44m2/g and less than1μm, respectively. The product was a high polymer materials by the analysis of Fe-Ferron method, FT-IR and XRD.(2) The optimal remediation conditions of Cd and Pb contaminated soils were obtained as follows:the modified PHFP was chosen as a fixative which was the most effective; the optimum remediation time was above42d; water content in soils was85%of maximum field capacity; the most appropriate dosages of the modified PHFP was4%(w/w); the particle size of soil was less than2.4mm. After the remediation, the removal rates of available Cd and Pb reached to46%and67%, respectively, and the removal rates of water soluble Cd and Pb were55%and64%, respectively.(3) Cd in contaminated soil mainly existed in water-soluble, exchangeable and carbonate bounded forms. Pb mainly existed in exchangeable, residual and Fe-Mn oxides bounded forms. With the addition of modified PHFP, the high bio-available Cd and Pb in the treated soil was converted to be more stable form. The available and water-soluble Cd and Pb content significantly decreased.(4) Simulated acid rain leaching method was carried out to evaluate the long term stability of Cd and Pb in the remedied soil. After10years simulated leaching, the largest cumulative release amount of Cd and Pb in remediated soil decreased from3.67mg/kg>16.30mg/kg to1.71mg/kg、8.03mg/kg, respectively, and the soil pH changed a little. The results indicated that the modified PHFP showed excellent immobilization effect on Cd and Pb in soils. Meanwhile, the dissolve of Cd and Pb under acid rain leaching significantly decreased, implying the present method can immobilize heavy metals in contaminated soils.