| China is one of major countries to produce chromate and the annual output of that was more than 160 thousand tons. However, a large amount of chromium-containing slag was discharged from chromate industries, and the problems of mismanagement, serious aging equipment, workshop equipment leaking seriously and other reasons resulted in wastes without treatment and soil pollution seriously. Therefore monitoring the chromium from the soil samples which were collected in a chromium-contaminated site, finding out the pollution of chromium in the soil, and a further ecological health risk assessment were carried out using Hakanson potential ecological risk index method and US EPA Health Risk Assessment Model. Secondly, a Cr(VI) reduction bacteria NO.1 strain which has relatively high reduction ability was isolated and screened and the optimized reduction conditions. In addition, the bacteria was made of engineering bacteria to do the research on the in situ remediation of contaminated soil and analyse effects of environmental factors on the engineering bacteria remediation. Finally, the reduction mechanism of NO.1 strain was analysed preliminarily.The average total chromium concentration and Cr(VI) concentration were 127.96 ~ 3369.17 mg·kg-1 and 0.95 ~ 508.68 mg·kg-1 in a industrial factory in Chongqing, and those of production area were highest with 5.52 times of evaluation standard for soil environmental quality of exhibition land(provisional)(HJ350-2007) caused serious pollution due to the perennial production of chromate. Through the study of the vertical migration pattern of soil profile, we found the total chromium and hexavalent chromium content in soil profiles at research areas decreased with increasing soil depth, and chromium of chromium slag leaching can migrate into the soil at the depth of 12 ~ 16.0 m.Based on the investigation of ecological risk analysis, the ecological risk assessment result showed that potential ecological risk coefficient was between 1.35 ~ 35.46, the average of that was 10.32 and the potential ecological risk was light in the soil. Health risk assessment indicated that the non carcinogenic risk for children exceeded the acceptable value of 1 by oral ingestion of soil particles and drinking contaminated groundwater, while that for adults was greater than 1 by oral ingestion, inhalation of of soil particles and drinking contaminated groundwater. The carcinogenic risk for both children and adults exceeded 10-6 by inhalation of soil particles with a great risk harmfulness.By streak plate method six chromium-resistance strains were isolated from the chromium-contaminated soil. Only one strain showed a strong Cr(VI) reduction ability, which can basically reduce 150 mg·L-1 Cr(VI) within 48 h. Asynchrony of Cr(VI) reduction and bacteria growth was observed in this study and Cr(VI) reduction ability of bacteria was independent on their resistance to Cr(VI). Biochemical characteristics and16 S rDNAsequencing indicated that the chromate-reducing strain NO.1 strain was a species of Bacillus cereus.Reduction mechanism of NO.1 strain to Cr(VI) is its extracellular secretions, cell membrane fragments and the role of intracellular secretions. Compared with other components, extracellular secretion has a better Cr(VI) reduction ability, which could reduce Cr(VI) basically, membrane fragments comes second. Experiment results found that extracellular secretion include some Cr(VI) reductase and metabolites that can reduce Cr(VI). Membrane fragments may exist some fimctional groups which involved incoordination reaction with Cr(VI) to reduce water soluble Cr(VI), while intracellular secretion reduction is reductase.Based on the optimization of culture medium composition and growth condition, the bioremediation of chromium contaminated soil can be carried out by adding culture medium in soil. The optimal condition for the Cr(VI) reduction was 5 g·kg-1 glucose, 10 g·kg-1 nitrogen source at 30 ?and the ratio of soil and water was l:1. 90.0% of total Cr(VI) in chromium contaminated soil was removed at 4 days under the optimal condition. Application research of in situ remediation of chromium contaminated soil used rice husk, wheat bran, rice bran as bacteria vector showed that reducing effect on the polluted soil significantly improved with the increase in the proportion of rice husk. Under natural conditions, the higher temperature, the better the remediation effect of engineering bacteria on chromium contaminated sites. And alkaline soil is advantageous to repair polluted soil with good soil stability. |
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