Biodegradation Of Hexachlorocyclohexane Isomers By Sphingobiumindicum B90A And Its Bioremendiation Of Hexachlorocyclohexane Polluted Soil

Organochlorine pesticides (OCPs), a part of POPs(Persistent organic pollutants, POPs), is persistent, bioaccumulation, semi-volatile and high toxicity.Hexachlorocyclohexane(HCH) have been the most extensively used OCPs for public health and agricultural pests control. The insecticidal formation of a technical mixture of HCH predominantly consists of α-, β-, γ-and δ-isomers. Although the use of HCH has been discontinued for a long time, due to its lengthy persistence and toxicity, the problem of its residues still exists. Soil contamination with HCH poses a great threat to human health and the agricultural food quality, and that calls for an immediate action to remediate the contaminated sites. Microbial degradation and combined bioremediation have been suggested as a safe and cost-effective way to remediate HCH contaminated soils.In this paper, Sphingobium indicum B90A had been isolated and proved to be able to degrade HCH. This research was carried out under liquid and soil slurry cultures, and the conditions of degradation were also optimized.A bottle culture was used to investigate the effects of inoculum size, incubation temperature, pH, pesticides concentration and co-metabolic substrates on the degradation of mixed HCH isomers (α-, β-, γ-and δ-HCH) by S. indicum B90A in liquid. The results showed that with the increase of inoculum size from2%to10%, the degradation rate of HCH isomers by S. indicum B90A increased. The most optimum inoculum size was5%.&indicum B90A degraded the four kinds of HCH isomers with the optimal temperature of35℃, under4h incubation of different temperature (25、30、35、40℃). And it also shows the neutral and slightly alkaline pH range favored the biodegradation of mixed HCH isomers.The utilization of α-and β-HCH by S. indicum B90A was higher than that of γ-and8-HCH. In the culture solution of10mg·L-1mixed HCH, the degradation rates of α-, β-, γ-and δ-HCH by S. indicum B90A at30℃after72h were99.9%,86.1%,53.1%and33.0%, respectively. Associated with the increase of HCH concentration, the biodegradation rates of α-, β-, γ-and δ-HCH were all gradually declined. In the study of co-metabolic substrates, it was found that100mg·L-1glucose or50mg·L-1yeast extract added in the culture solution of10mg·L-1HCH could promote biodegradation of HCH by S. indicum B90A close to100%.The effects of water/soil ratio, exogenous additives and surfactants on the biodegradation of HCH by S. indicum B90A had been investigated by soil slurry culture experiments. Results indicated that S. indicum B90A degraded HCH well at all four different water/soil ratios and the ratio of10:4was chosen to increase the amount of polluted soil.As exotic carbon or nitrogen sources, rhamnose, glucose, yeast extract and soybean meal did no further biodegradation of HCH by S. indicum B90A in soil slurry cultures. The same thing happened when S. indicum B90A was additionally added to soil slurry cultures during biodegradation. That might be due to the adsorption of HCH to the soil particles, decreasing its bioavailability to the degradating bacteria.Results of surfactants studies showed that chemical surfactant Triton X-100and Tween80had almost no improvement on HCH biodegradation by S. indicum B90A under soil slurry cultures. Fermentation liquor consisted mainly of rhamnolipid was used as biosurfactant. It was found that23.6mg-L"1rhamnolipid in fermentation liquor slightly increased the bioavailability of HCH to S. indicum B90A. The degradation rate was93.6%, which is4.2%higher than that of control. But rhamnolipid content of118and472mg-L’1caused inhibition on the biodegradation of HCH.By using optimized condition of degradation in soil slurry culture, the residual concentration of HCH in bioremediated soil was2.31mg’kg"1, while the initial concentration was72.08mg·kg-1.96.8%of the HCH has been removed from the polluted soil. The toxicity of HCH polluted soil and bioremediated soil was investigated through ecological bioassay of Chinese cabbage and cucumber, by testing their seed germination and the inhibition rate of root elongation. Results showed that the inhibition effect on cucumber seeds had a reduction in bioremediated soil. Cucumber seed germination was75.4%in bioremediated soil, as in polluted soil it was only48.8%. In polluted soil, the inhibition rate of root elongation was23.94%, which was higher than in bioremediated soil (7.5%). However, for Chinese cabbage, the bioremediated soil has no toxicity to its seed germination and root elongation. And Chinese cabbage’s seed germination index was only 44.9%in polluted soil while it was100.0%after bioremediation. These results indicate that bioremediation is operable to HCH polluted soil.