The Effect Of Sulfate Reduction On The Attenuation Of Polychlorinated Biphenyls In Paddy Soil

Polychlorinated biphenyls (PCBs), one of typical chlorinated organic compounds, are widespread with low concentration and high risk in paddy soil. Therefore, it’s of great importance to eliminate the PCBs pollution by the natural attenuation ability of paddy soil. Sulfate reduction process plays an important role in the sulfur cycle and the anaerobic degradation of organic pollutants in paddy soil. However, the effect and mechanism of sulfate reduction on the attenuation of PCBs in paddy soil is still unclear.In this study, by regulating the sulfate reducing conditions, patch anaerobic incubation experiments of paddy soil was conducted to explore the law of the attenuation of PCBs under sulfate reducing conditions. The community structure and abundance of sulfate-reducing bacteria (SRB) and dechlorinating bacteria was analyzed with PCR-DGGE and Real-time PCR. Two SRB culture was enriched respectively from PCBs contaminated environment samples. PCBs degradation by enrichment cultures were studied afterward. The main results of this research are as follows:(1) The oxidation reduction potential (Eh value) of soil solution in patch anaerobic incubation experiments were all under reducing condition (<200mV), and the Eh value of some arrangements could decrease to around-220mV in a period, during which sulfate reduction was dominant. Under sulfate-reducing condition, the removal efficiency of different chlorinated biphenyls in polluted soil and spiked soil was decreased with the increase of the number of chloride substituted. The enhanced sulfate reduction arrangement had a removal efficiency of total PCBs of36.8%and showed an advantage in decreasing penta-CB and hexa-CB in low concentration polluted soil (around2mg/kg dw). PCBs in high concentration spiked soil also decreased by more than20%under sulfate-reducing condition. Results showed that sulfate reduction promoted PCBs attenuation to a certain extent. There was no significant affect on the decrease of PCB congener31,116, and153after the amendment of acetate (carbon source) and molybdate (SRB inhibitor) in samples cultivated for105days. S8, one of sulfate-reducing products, was detected by GC-MS, which proved the reaction of sulfate reduction. However, no other PCB congener was detected, inferring no retention of dechlorinated intermediate product. (2) In PCBs spiked soil samples, flooded anerobic condition could slightly increase the Shannon diversity index of total bacteria, while different treatments under flooded condition did not influence the Shannon diversity index much. Sulfate may inhibit other microorganisms while molybdate showed to be a significant SRB inhibitor. Clone and sequence of the specific band on the dechlorinating bacteria DGGE gel of soil showed a clone highly identical to Chloroflexi, which was closely related to the transformation of chlorinated hydrocarbons and aromatic compounds. The bacteria represented by the clone are likely to participate in the degradation of PCBs in soil.(3) PCBs domestication after the enrichment of SRB from PCBs contaminated samples seriously harmed the SRB culture, which represented by no Aroclor1254degrading ability in following experiments. Maintaining the PCBs concentration during the process of enrichment of SRB from PCBs contaminated sediment was preferable. A PCBs-tolerated SRB culture was obtained and showed the degrading ability of PCB31,116, and153.In general, the study preliminarily verified the promotion of PCBs attenuation by sulfate reduction, analyzed the change of bacteria community structure in soils under different sulfate reducing conditions, and help to provide a theoretical basis to explain the mechanism of natural attenuation of PCBs and other chlorinated organic compounds in paddy soil.