Remediation Mechanisms Of Chemical Regulation In Heavy Metal Compound Contaminated Soil

The main purpose of this paper was to analyze and evaluate heavy metal pollution of mining wasteland soils and screen dominant species. The surveys were conducted in abandoned mining area in Tongkeng of Dachang and Hezhou, Guangxi province. Then, it was necessary to do the redistributions of Mn, Cu, Cd and Zn in soil. And the effects of extraction of soil heavy metals by using EDTA, citric acid and oxalic acid were studied. Meanwhile, the pot culture experiments were develop. Heavy metal contaminated soils in different grades from lead/zinc abandoned mining area was used to plant maize, and added different level of slaked lime, calcium-magnesium phosphate, mixture of slaked lime and calcium magnesium phosphate respectively into the pot soil. Then, the research studied on the differences of the maizes, which were in different physiological characteristics and different heavy metals content.The main results of the experiment were shown as follows:(1) The average contents of Pb, Zn, Mn, Cd and Cu were2164.58mg·g-1,4547.08mg·kg-1,2190.89mg·g-1,53.80mg·kg11and67.53mg·kg11in the11soil samples in Dachang abandoned mining area. The contents of elements except for Cu were higher than the soil values of Environmental Quality Standard III and the Soil Background Values of Guangxi. All the11soil samples were polluted seriously by heavy metals respectively. The single factor evaluation indicated that Cd and Zn were the most serious pollutants.(2) There were plants in21species,13families of21genera, in this investigation area. Among these21species of plants, Miscanthus floridulus s(Labill.) Warb. of Agrositidoideae, Cyperus rotundus Linn, of Cyperaceae, Salanum nigrum L.of Solanaceae and Ixeris sonchifolia, Bidenspilosa L, Erigeron Canadensis L.of Compositae, which had the important value and summed dominance ratio, were dominant species in the investigation area. By analyzing the heavy metal concentration of plant samples, it was found that Cd and Zn contents in the plant tissues were usually higher than normal. Centella asiatica (L.) Urban., Buddleja officinalis Maxim., Ixeris sonchifolia and Salanum nigrum L. had very high transfer ability for particular metals such as Cd, Pb and Zn. Erigeron Canadensis L. had high transfer ability for Cd, Pb and Cu. Miscanthus floridulus had very strong concentrates ability for Pb and Zn and resistant to drought and inadequacy of nutrients and its biomass was very high. Thus, these plants could be used as pioneer plants for revegetation of abandoned mining area. The enrichment coefficient and biological transfer coefficient>1in Centella asiaticaiL.) Urban of Umbelliferae. And it was showed that strong accumulation characteristics for Cd in aerial part of the plant. Centella asiatica (L.) Urban of Umbelliferae could be the phytoremediation of cadmium-contaminated soils.(3) The EDTA, citric acid and oxalic acid had desorption ability for the heavy metals such as Cd, Zn, Mn and Cu. The desorption efficiencies of Cd, Zn, Mn and Cu increased with increasing concentration of extraction solvent. In the condition of higher concentration of extraction solvent (≥40mmol·L-1), the desorption efficiencies of Cd, Zn, Mn and Cu increased. While the desorption velocity of Cd, Zn, Mn and Cu in the soils decreased with increasing action time. The kinetics of desorption experiment could be divided into two stages, which were the rapid desorption and the slow desorption. Diffusion of these heavy metals in contaminated soils of Dachang and Hezhou were quite different.(4) The Tessier sequence extraction in soil showed that the chemical forms of heavy metals in abandoned mining area of Dachang and Hezhou were different. Sequential fractionations of treated and untreated soil samples showed that EDTA was effective on removing the five forms of Zn and Cu. The exchangeable, carbonate and oxide forms of Cu and Mn were the same effect. But EDTA was ineffective on the organic and residual forms of test heavy metals in Dachang abandoned mining area soil. EDTA was effective in removing the exchangeable and oxide forms of Cd、Zn、Mn and Cu in Hezhou abandoned mining area soil. Citric acid was effective in removing the exchangeable, carbonate and oxide forms of Cd, Zn, Cu and Mn. The desorption ability of Oxalic acid for the heavy metals was very weak. Bioavaility of Cd, Pb, Cu and Zn in contaminated soil decreased after desorption by EDTA and citric.(5) The growth inhibition role of toxicity of heavy metals played on the Maize which grew in the heavy metal contaminated soils. When the contaminated and uncontaminated soil mixed in the ratio of4:1, maize germination was inhibited. When the ratio was3:2, the maize grew well with dark green leaves and there is no sign of salinization harm. The application of soil ameliorants such as slaked lime, calcium-magnesium phosphate, mixture of slaked lime and calcium magnesium phosphate could promote the growth of maize, increase the pH in soil and reduce the concentration of Zn, Cd and Pb in maize significantly.