| The environment problem resulted by heavy metals is concerned widely, especially chromium (Cr) because of its complexicity of environmental behaviors including its toxicity and speciation. Using a chromium slag field of Tianjin as example, transport and transformation of soil Cr were investigated to provide the basic techniques and principles for soil heavy metal restoration. It was found that the total Cr content in the site soil was as high as20,000mg/kg, which made it impossible for plant growth. Therefore, it was essential to remove Cr and reduce the toxical stress for possible ecological restoration. Based on the analysis of Cr content and speciation, a strategy was presented mainly focused on the assessment of removal efficiency of different leaching agents (including acid leaching), especially the effects on the changes of fractions and speciation of Cr. Further, baterial strains with Cr tolerance were isolated by enrichment culture for the disposal of leaching eluent. The main results are outlined as follows:1) According to the different locations and layers of soil profiles, the contents of soil total Cr and Cr(VI) were far beyond the soil environmental quality standards (mg/kg): total Cr content (19,702.4±349.8mg/kg) was over65times higher than the standards, while Cr(VI) content was6,000times higher. The contents of the different fractions of Cr was water soluble fraction (F0)> Iron-manganese oxide combined fraction (F3)> residue fraction (F5)> exchangeable fraction (F1)> carbonate bound fraction (F2)> organic matter-oxidate bound fraction (F4). Soil bacteria existed only in the layers of IV, V of profile A and soil microbial diversity was extremely low.2) Five leaching agents, i.e., distilled water, citric acid, ammonium acetate, citrate and EDTA-Na2, were applied to remove the soil Cr with an initial content of19,702.4mg/kg). The Cr contents in the leaching eluents were11051.3,11119.6,11418.0,11082.1,10,494.7mg/kg, respectively. There were no significant differences between the different leaching agents. Along with the increase of leaching time, the Cr contents in the leaching eluents decreased significantly.After6times of leaching, the Cr contents did not vary significantly.3) The removal rate that water leaching decreased total Cr of soil was34.3%. Water leaching changed the fractions of Cr forms in F0, F1and F2significantly. However, there were no variations in F3, F4and F5. The content of Cr fractions followed the order:F3> F5≥F4,F0>F1,F2.4) Soil acidification procedure reduced soil pH (soil pH from alkaline into neutral) and the total contents of Cr significantly. The initial content of total Cr was7042.8mg/kg and the final content drop down to6396.1～5119.8mg/kg. The removal rate was50.6～60.4%, so the removal rate of soil(CK) was33.2%～40.7%. Analysis Showed that soil acidification change the Cr fraction completely. The content of F0increased, while the contents of F1, F2, F3, F4and F5decreased, among which the contents of F3and F5decreased the most. The best culture conditions of the acidification were21d and2.0mol/L5) One native bacterial strain named AV2A was isolated, which had strong resistance and reducibility to Cr. Its best pH condition for reduction was9-10. The bacterium AV2A could reduce696.4mg/kg Cr(Ⅵ) in leaching eluent with a removal rate of50.0%. Further research should be conducted to test the efficiency and properties of the strain. |
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