Study Of Modified Industrial Solid Wastes On The Stabilization Remediation Of Heavy Metals Contaminated Soil

In recent years,soil heavy metal contamination events happened frequently, heavy metal pollution in soil seriously affects food safety and land utilization, which may cause harm to human health through the food chain. In this study, three modified industrial solid wastes-- modified low-grade phosphate rock with soda dregs(MP), modified flue gases desulfurization residue(MF), and a mixture of MF and phosphoric acid(MFP) –were used as amendments for the solidification/stabilization remediation of Cd, Pb and Cu contaminated agricultural field in Weng yuan, Shao Guan City. The effects of the amendments on soil p H, contents of mineral nutrients of soil, heavy metals distribution, plant growth and the heavy metal concentration of plant were studied to evaluated the restorative effects on the remediation of heavy metal-contaminated agricultural soil with pot experiment, and the correlations between the factors are analyzed to discuss the remediation mechanism primary. What’s more, sorption/desorption experiment and X-ray diffraction(XRD) and scanning electron microscope(SEM) were used to further investigate the remediation mechanism of the three amendments. The main conclusions are as follows:The additon of the three amendments promoted soil quality significantly, and created a better environment for the growth of plant: all the three amendments improved soil p H and the content of available Ca, Mg, Si of soil significantly(P<0.05); The addition of MP improved the content of available P at the same time; the addition of MF and MFP increased the content of available K as well; Significant positive correlations were observed between soil p H and available Ca and Mg in soil. What’s more, the addition of the three amendments significantly decreased the exchangeable fraction and carbonate-bound fraction of heavy metal in soil, which are more harmful than other fractions. The exchangeable fraction Cd decreased up to by 31.9%, and the decrease of the exchangeable fraction turned into Fe and Mn oxides-bound fraction; 4‰ MP treatment decreased exchangeable fraction Pb up to by 56.39%, and the exchangeable fraction main shifted into Fe_Mn oxides-bound fraction and organic matter-bound fraction; The exchangeable fraction Cu decreased most by 34.26%, and the exchangeable fraction turned into Fe Mn oxides-bound fraction and residual fraction. Correlation analysis showed that the increased soil p H and elevated mineral nutrients(Si, Ca and P) contributed the most to the redistribution of heavy metals in soil. The addition of MP, MF and MFP reduced the adsorption of heavy metals by pakchoi, and the improve of soil quality and the less inhibition of heavy metals promoted the growth of pakchoi significantly.There were new materials found in the XRD maps and the surface morphology changed dramatically in the SEM graph after the sorption/desorption reactions between the amendments and heavy metals. Based on the XRD and SEM characterization of MP after absorbing heavy metals, it revealed that MP reacted with Cd to form Cd(OH)F(d=3.6307) and Cd(H2PO4)2(d=8.293); MP reacted with Pb to form Pb5(PO4)3OH, Pb5(PO4)3Cl and Ca8Pb2(PO4)6(OH)2 and the reaction was done in adsorption and ion exchange order; According to the XRD results of MF after absorbing heavy metals, MF reacted with Cd to form Cd CO3(d=3.7983,2.9562 and 1.8282 et.al) and Cd SO3(d=3.0769); MF reacted with Pb to form hydrocerussite(Pb3(CO3)2(OH)2), cerussite(Pb CO3) and anglesite(Pb SO4); MFP reacted with Pb to form dundasite(Pb2Al4(CO3)4(OH)8) and hydrocerussite(Pb3(CO3)2(OH)2). Although in the XRD results none Cd, Pb or Cu-containing silicate compounds was found, significant changes of MF and MFP surface morphology were observed before and after absorbing heavy metals in the SEM images, which indirectly indicated that some reactions indeed happened between silicate compounds of MF and heavy metals, the reaction time maybe to short to form stable crystal or noncrystal precipitates such as Si-Cd-Ca co-precipitate and Si-O-Pb co-precipitate were formed, or the heavy metals were wrapped by silicate constituents in MF and MFP which can not be tested by XRD.