In Situ Bioremediation Of The DDT-contaminated Soils

DDT was classified as the persistent organic pollutants (POPs), with degradation half-life in soils from several years to more than ten years. Although DDT was banned for many years, it is still often detected in agricultural soils, and even the detection rate was as high as 100% in some regions. These DDT residues had not only an adverse affect on the soil ecosystem, but also posed a potential threat on human health through food chains. Some studies found that soil microorganisms play a major role in the degradation of DDT residues. Therefore, microbial degradation is considered to be an effective method to remove DDT residues from the contaminated soil. In order to establish an in situ bioremediation technology for the DDT-contaminated soils, based on the early isolated and identified the DDT-degrading bacterial strain Stenotrophomonas sp. DDT-1 and pollution survey of DDT residues in field soil, we carried out studies on the biodegradation of DDT residues by strain DDT-1 in indoor soil and field plot soil, and field demonstration application of strain DDT-1 preparation and its ecological safety assessment. The main results were as follows:DDTs residual concentrations were detected in a vegetable base located in Cixi, Zhejiang, China. The results showed the average levels of DDTs residues in three regions were as high as 0.21,0.19,0.18 mg/kg, respectively. Therefore, these three regions were selected to conduct field experiments.The degradation percentages of p,p’-DDT in indoor Huajiachi medium loam, Xiaoshan sandy soil, Jinhua heavy loam were 24.5%,21.0%,20.9%, respectively, which was 2.6,2.5,2.4 times higher than those in the corresponding un-inoculated soils, indicating that strain DDT-1 can enhance the degradation of p,p’-DDT in different types of soils. In the field plot experiments, the degradation percentage of DDTs in field plot treatments was 38.0% after 210 d for strain DDT-1 inoculation. In the field demonstration application, the average degradation percentage of DDTs was 39.6% in all inoculated soils, and those were 38.7% and 42.1% in open field soils and greenhouse soils, respectively, after 420 d for strain preparation application. The average degradation percentages of DDTs were 49.4% and 47.2% in greenhouse ploughing soil and open field ploughing soil, respectively, which was higher than the corresponding no-ploughing soils, indicating that strain DDT-1 preparation could enhance the degradation of DDTs, and furthermore mechanical ploughing could accelerate its degradation in soils.No significant differences were found in the catalase activity between inoculated soils and the corresponding un-inoculated soils throughout the bioremediation period, and soil neutral phosphatase activity in the inoculated soils was only slightly higher than those in the un- inoculated soils in the late period of bioremediation. The overall microbial activities (AWCD value) and the dominant population size (Simpson index) were higher in open field unplanted soil, open field planted soil, greenhouse no-ploughing soil, greenhouse ploughing soil, open field no-ploughing soil, open field ploughing soil compared with their corresponding un-inoculated soils in initial stage of strain preparation application, and then the overall microbial activities and the dominant population size decreased to the un-inoculated control level in the middle and late period of strain preparation application. However, the richness (Shannon index) and evenness (McIntosh index) of soil microorganisms was not significantly different between inoculated soil and the corresponding un-inoculated soil throughout the bioremediation period. The structural diversity of soil microbial communities was generally similar between inoculated soil and un-inoculated soil, and the relative abundance of several soil microbial genera might have some changes, and the relative abundance of Stenotrophomonas including strain DDT-1 showed a trend from an initial increase and then decrease. Therefore, the application of strain DDT-1 preparation was ecological safe in field soils.