Effect Of Simulated Acid Rain Or EDTA Or In Combination On Release, Migration, And Transformation Of Extraneous Metals In Red Soil

The contamination of soils by heavy metals is one of the most serious globle environmental problems and poses a great threat to ecological environment and human health. Pb has been widely used all over the world for centuries and does great harm to human health, for which the world trends to lead-free technologies. The application of lead-free technologies will increase the consumption and disposal of Pb's alternative rare metals, including silver (Ag), bismuth (Bi), indium (In), antimony (Sb), and tin (Sn). The remediation of these metals raises a new problem. Ethylenediamine tetraacetate (EDTA) is a chemical agent that is widely used both in in-situ and ex-situ remediation of soil heavy metal pollution. Acid rain is one of the outstanding environmental concerns in tropical and subtropical regions of China, and it may affect the chemical behaviors of heavy metal in soil by changing the pH. Red soil, widely distributed in acid rain striken areas of China, was used in the experiment. The soil was artificially polluted with Pb and its alternative rare metals, including Ag, Bi, In, Sb and Sn. The experiment was carried out with self-designed soil leaching columns to investigate the concentration characteristics of those metals in the leachate, soil solution and soil, and their species in soil. The objective of this study was to illustrate the effect of simulated acid rain or EDTA or in combination on release, migration, and transformation of extraneous metals in red soil in terms of providing scientific lines of evidence for remediation and evaluation and of heavy metal contaminated soil. The results were as follows:(1) The concentrations of Ag, Bi, In, Pb, Sb, Sn in natural red soil were within the range of soil background values in China. The metal enrichment ratios were all less than 2, which belongs to low heavy metal accumulated areas. Compared with Pb, its alternative rare metals (Ag, Bi, In, Sb, Sn) were more stable in red soil and were more difficult to be extracted from soil.(2) Simulated acid rain promoted the release and migration of Bi, In and Pb, and their migration was down to 8 cm in the soil profile. Simulated acid slightly promoted the Sb release. But no enhancement of migration was observed in this process. It had little effect on the release or migration of Ag and Sn, either.(3) EDTA significantly affected the release of Bi, In and Pb, and had effect on the Sb release to some extent. However, little effect on release of Ag and Sn was observed. With the influence of EDTA, Ag, Bi, In, Pb, Sb and Sn migrated to 10 cm,20 cm,34 cm,30 cm, 14 cm and 14 cm, respectively.(4) The combined action of acid rain and EDTA obviously promoted the release of Bi, In and Pb, and enhanced the release of Sb to some extent. The release of Ag and Sn was little affected by this combination. Under the combined action of acid rain and EDTA, Ag, Bi, In, Pb and Sb migrated to 10 cm,22 cm,34 cm,22 cm and 8 cm, respectively.(5) The mobility of metals was in the order of Pb> Bi≥In> Sb> Sn> Ag. The individual and combined effect of acid rain and EDTA increased the environmental risk of metals by increasing the relative distribution of exchangable form. The individual effect of EDTA on the release, migration and transformation was the greatest. Among Pb's alternative rare metals, Bi and In posed higher risk than others.