| China is one of major countries to produce chromate and the annual output of chromate was more than 160 thousand tons. However, a large amount of chromium-containing slag was discharged from chromate industries. The accumulated amount of chromium-containing slag was more than 4 million tons and 600 thousand tons are being discharged annually. Dissolvable hexavalent chromium (Cr(VI)) accounting for 0.3～1.5% of slag can be leached into soils and groundwater by rainfall and runoff, which incurs a significant risk to the environment and human health. At present, the accumulated soils polluted by chromium-containing slag reached up to 12.5 million tons. Chromium-containing slag heap sites are concerned as an important treatment object and the key technologies for remediating the polluted soils are urgently required in our country. Therefore, the characteristics of chromium (Cr) pollution at one chromium-containing slag site and Cr(VI) bioremediation in the contaminated soils were investigated in this dissertation.Mean concentrations of total Cr in the soils under the chromium-containing slag heap at one factory in Hunan province, in the vicinity of the slag heap and the agricultural land outside of the factory were 5.6, 7.6 and 5.0 times of the critical level of Secondary Environmental Quality Standard for Soil in China, respectively. The highest amount of total chromium in soil of these three areas was found at 40-60cm, 20-40cm and 0-20cm of soil depth respectively. Chromium was transported into the deep layer in soil profile. Fe and Mn oxides-bound chromium was the predominant fraction in the contaminated soils under the slag heap, while residual chromium was the main fraction in the soils in the vicinity of the slag heap. Mean contents of water soluble Cr(VI) in top soils under and in the vicinity of the slag heap were 137.5and 30.1 times of that in the unpolluted soils, respectively. According to the Tolerance Limit of Chromium in Foods, the occurance rates of exceeding over the critical level for the above-ground part of the celery, cabbage and lettuce on the farmland outside of the factory were 50%, 100% and 75%, respectively. Drinking water was heavily polluted and 50% of the samples exceeded the Cr(VI) Standard for Drinking Water Quality.The populations of three soil microflora were severely affected by chromium contamination. The populations of bacteria, fungi and actinomycetes decreased by 89.9%,99.8% and 99.9% as compared with that of the control site. The populations of bacteria, fungi and actinomycetes were all negatively correlated with the contents of total Cr and water soluble Cr(VI). Incubation experiment also indicated that hexavalent chromium inhibited the growth and decreased the diversities of soil culturable bacteria, fungi and actinomycetes. Actinomycetes was the most sensitive to Cr pollution. The activities of catalase and polyphenol oxidase in soils were not obviously depressed by chromium pollution and alkaline phosphatases activity was slightly suppressed by chromium pollution. However, Cr(VI) significant inhibited dehydrogenase activity, revealing that dehydrogenase activity could be used as a biological indicator for the chromium pollution.Four chromium-resistance strains were isolated from the contaminated soil under the chromium-containing slag heap. Only one strain exhibited a strong Cr(VI) reduction ability, which can completly reduce 500mg/L Cr (VI) within 24 h. Asynchrony of Cr(VI) reduction and cell growth was observed in this study. Moreover, Cr(VI) reduction ability of cells was independent on their resistance to Cr(VI). Scanning electron microscopy (SEM) was used to observe the morphologies of bacteria before and after Cr(VI) reduction. The results showed that the cells were rod with a flagellum at the terminal of bacteria. The discernible cluster of amorphous substances were bound to the terminal of the cells after reducing Cr(VI). Elemental composition and oxidation state of the chromium in the final product were verified by energy dispersive X-ray (EDAX) and X-ray photoelectron spectroscope (XPS) analysis, revealing that Cr was the major element that existed in trivalent state of chromium hydroxides under alkaline condition. Biochemical charasteristics and 16S rDNA sequencing showed that the chromate-reducing strain BB was a species of Pannonibacter phragmitetu. Based on the optimization of culture medium composition and growth conditions and ability of Cr(VI) reduction, the bioremediation of Cr-contaminated soil can be achieved by adding culture medium in soils to stimulate the activity of Pannonibacter phragmitetu. The optimal condition for the Cr(VI) reduction by Pannonibacter phragmitetu was 5g glucose and 5g yeast extract per kg soil at 30℃and the ratio of soil and water was 1:1. Under the optimal condition, 92% of total Cr(VI) in soil contaminated by chromium-containing slag heap was removed and water soluble Cr(VI) was completely removed at 4 days. Meanwhile, the removal of exchangeable Cr(VI) and carbonate-bound Cr(VI) reached up to 89% at 5 days and 84% at 10 days, respectively.The remediation of Cr-polluted soil was contributed to Cr(VI) reduction by Pannonibacter phragmitetu. Soil organic matter, iron oxides and magnesium oxides did not involve in Cr(VI) reduction in soils. The microbial metabolites and extracellular enzyme have no capability of reducing Cr(VI). The reduction of Cr(VI) by Pannonibacter phragmitetu was a direct reduction catalyzed by its intracellular enzyme. Furthermore, this intracellular enzyme was an NADH-dependent reductase. Cr(VI)-reducing enzyme was not induced by Cr(VI) but constitutively expressed in Pannonibacter phragmitetu.
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