The Effect Of Coupling Between Fe(Ⅲ) And Sulfate Reduction On Pentachlorophenol Dechlorinated Dissipation Under Anaerobic Condition

Recently, with intensive human activities and the development of industry and agriculture, large amount of chloroorganics pollutants enter the environment. Soil acts as a carrier for many kinds of contaminants and remediation of soil pollution has attracted increasing attentions in recent years. As an effective and low-cost technology for contaminated soil, biological remediation method is rising in recent years in the world. Lower organochlorines are mineralized completely by aerobic oxidation degradation, while higher chlorinated organic compounds are firstly degraded by reductive dechlorination to lower chlorinated organic compounds, then oxidative degraded. Pentachlorophenol (PCP, C6Cl5OH) has a highest chlorine level which is the most difficult to degrade in chlorophenol that is adapted for reductive dechlorination.In this study, an anaerobic soil incubation experiment was conducted by controlling different Fe/S (8/1,3/1,2/1,1/1,1/2 and 1/3) ratios to investigate the coupling effect of iron reduction and sulfate reduction on dechlorination of PCP. The community structure and abundance of sulfate-reducing bacteria, dechlorinating bacteria (Dehalococcoides), and iron-reducing bacteria (Geobacter) were analyzed with Real-time PCR molecular method. The main results of this research are as follows:(1) During 112 days anaerobic incubation with no nitrogen source, there was low attenuation of PCP in general terms. The addition of SO42- inhibited PCP dechlorination, but the effect was not significant which might be attributed to the slight biotransformation of PCP. The concentrations of PCP were reduced rapidly after adding mineral medium and carbon source, which can be probably explained by the following two reasons. Firstly, anaerobic microorganism were fit enough to generate the ability to degrade chloroorganics through a long time domestication. The growth of dechlorinating bacteria and dechlorinated respiration was then accelerated by sufficient carbon resource and nitrogen resource. Secondly, the degradation of PCP was promoted by accepting electron during the electron transmission of iron oxidation-reduction reactions.(2) Soil suspension system was greatly afftected by mineral medium, especially nitrogen source. After adding nitrogen source, the intensity of sulfate reduction was enhanced by meeting the growing demand of C/N ratio of soil microorganism and improving the activity of soil microbe, finally affected biotic dissipation of PCP indirectly.(3) During the 40 days of nitrogen source induced anaerobic incubation, the residual concentration of PCP was different among seven Fe/S ratio treatments under established iron concentration. Compared with control, the positive effect on PCP dechlorination was found in three treatments (2/1,3/1 and 8/1), while the others were negative effect, which suggest that PCP degradation has relationship with the concentration of SO42-. When Fe/S ratio is between 8/1-2/1, the addition of SO42- can accelerate PCP degradation. This might be partly due to the promoting effect of Fe(II) on PCP dechlorination was stronger than the inhibiting effect of SO42-. The negative effect of S2-on microbial activities might be neutralized by precipitation of S2" with iron oxide. In addition, some sulfate-reducing bacteria can accelerate PCP degradation. In general, there are complex coupling effects among iron reduction, sulfate reduction and PCP dechloriantion. PCP degradation is increased with the SO42- concentration at the certain range. When exceed this range, the higher concentration of SO42-, the stronger inhibition effect on PCP degradation. This depends on the interactions between iron reduction and sulfate reduction in a Fe-S-PCP coexistent system under anaerobic environment.(4) Iron-reducing microorganism as a multi-functional microbial community in soil suspension of this research can not only reduce Fe(Ⅲ), but also has the functions to reduce SO42- and dechlorinate PCP. The amount of Fe(Ⅱ) was rapidly accumulated while the concentration of SO42- changed slightly during the 112 days anaerobic incubation with no nitrogen source. After the nitrogen source was induced, the production rate of Fe(Ⅱ) decreased while the reduction of SO4- increased. This may be a alternant phenomenon of electron acceptors. SO4- become electron acceptor of iron-reducing bacteria to support the continual growth of these microorganism as the concentration of Fe(Ⅲ) decreased.This study preliminarily verified the effect of coupling between Fe(Ⅲ) and sulfate reduction on PCP dechlorinated dissipation under anaerobic condition. We analyzed the change of bacteria community structure in soil suspension under different Fe/S ratio and found a certain range of Fe/S ratio can promote PCP degradation. This may help to provide a thought to enhanced bioremediation of PCP polluted soil.