Studies on stabilization effect of calcium dihydrogen phosphate (Ca(H2PO4)2) -on Lead(Pb)-contaminated Soil

Abstract

This study investigated the effect of Ca(H2PO4)2 on pH, leaching toxicity and speciations of soil before and after leaching on it. Different amounts of Ca(H2PO4)2 were added to Pb-contaminated soil and stabilized for 30 days. The changes of pH and leaching toxicity of Pb-contaminated soil were tracked during that period. The content of Pb in soil before and after leaching was also determined after 30 days of stabilization. Results showed that the pH of the Pb-contaminated soil didn’t change much with the addition of-Ca(H2PO4)2. When the amount of Ca(H2PO4)2 reached to 3 wt%, the leaching toxicity met the standard limiting level of groundwater class III of China. The change of leaching toxicity was found to be mainly affected by the water-soluble fraction and mild acid-soluble fraction of lead. The speciation experiments revealed that the changes on reducible, oxidizable, and residual fractions are significant, while there are only minor changes on the water-soluble and mild acid-soluble fractions. X-ray diffractometry (XRD) analysis showed that Pb9(PO4)6 and Pb2P2O7 substances were generated in the stabilized soil. The stabilization mechanism of Ca(H2PO4)2 was mainly attributed to the formation of insoluble Pb phosphate precipitates through interactions between the heavy metal Pb and the Ca(H2PO4)2. In such a way the active species of Pb in the soil can be successfully stabilized. Novelty statement At present, the leaching toxicity is currently used for the evaluation of stabilization effect of heavy metal contaminated soil. The speciation distribution of stabilized contaminated soil before and after leaching has rarely been studied, and the research on stabilizing contaminated soil after leaching is less. Therefore, this paper mainly studies the stabilization effect through the speciation changes of contaminated soil before and after leaching, providing a new idea and method for the evaluation of the stabilization effect of contaminated soil remediation. Ca(H2PO4)2 has no significance in pH of contaminated soil: 5.05＜pH＜5.5. The content of the water-soluble fraction and the mild acid-soluble fraction of Pb were availably reduced by Ca(H2PO4)2. The content of the water-soluble fraction and the mild acid-soluble fraction of Pb has no marked change before and after leaching. The stabilization mechanism of Ca(H2PO4)2 is through interaction between the Pb in the soil and phosphate to form insoluble substances of lead phosphate. Ca(H2PO4)2 has a good effect on the stabilization of lead-contaminated soil.