Effects Of Alternative Electron Acceptors (SO₄^2-, FeOOH) On Microbial Reductive Dechlorination Of Polychlorinated Biphenyls In Taihu Lake Sediments

Polychlorinated biphenyls (PCBs) are a class of typical persistent organic pollutants, widely spead in the soil, sediment, water and air. Particularly, sediment is regarded as one of the most important sinks of PCBs as well as an important source of PCBs. In situ microbial-catalyzed dechlorination is a potential alternative approach to remedy PCBs pollution in sediment. This study selected sediments which were from four main inflow rivers and Zhushan Bay of Taihu Lake Basin as the research object. Firstly, the basic physical and chemical properties of the sediments were investigated, and then the main influencing factors of microbial dechlorination were examined, the main work and conclusions are as follows:(1) Sixteen sediment samples were collected, and the samples were slightly acidic. The pH value of sediment samples were between 6.2 and 7.2. The contents of the moisture, total organic carbon (TOC), SO42- and Fe were determined by standard methods, and the average contents were 47.87%(21.01-62.71%),0.98%(0.36%-2.46%),1429.44 mg/kg (155.60-3876.00 mg/kg (dry weight)),30044.95 mg/kg (23405.84-36089.99 mg/kg (dry weight)), respectively. Moreover, the concentrations of Cr, Ni, Cu, Zn, As, Cd and Pb in sediment samples were 5.25 times,2.02 times, 4.69 times,2.18 times,2.37 times,1.62 times and 13.60 times higher than their background concentrations(the contents of heavy metals in soils of Jiangsu Province were taken as background value), respectively. This indicated a significant cumulative effect.(2) The raw sediments were taken as the inoculum when sediment environment was simulated to establish sediment microcosms. The three groups of experiments were constructed by adding PCBs, PCBs+FeOOH and PCBs+SCO42- respectively to the sediment microcosms. In the three experiment groups the dechlorination reaction started after 3 weeks',12 weeks'and 18 weeks'lag time, respectively. And the rates of dechlorination were 3.705 nmol/g·week,0.546 nmol/g·week and 0.103 nmol/g·week, respectively. After 24 weeks of incubation, on average, the chlorine per biphenyl (CPB) values dropped from 4.36±0.02 to 3.88±0.04,4.29±0.05,4.34±0.02, respectively. In the three experiment groups, the difference of the lag time, the rates of dechlorination and the effect of dechlorination showed that the alternative electron acceptors (FeOOH, SO42-) greatly inhibited PCB dechlorination in Taihu Lake sediments, and the inhibition effect of SO42 -was more significant than that of FeOOH. Therefore, in in-situ PCB remediation, the influcenes of alternative electron acceptors should be of great concern.(3) The preferred dechlorination pathways mainly belonged to meta-dechlorination in Taihu Lake sediments. After a time course of 24 weeks, in PCBs group, the average molar percentage of meta-dechlorination products was 47.08%, while the average molar percentage of para-and ortho-dechlorination products was only 12.52%. The dechlorination reaction of PCBs+FeOOH group and PCBS+SO42- group was still at the initial stage, here only the apparent dechlorination pathways of PCB 105 (234-34) and PCB5 (23-) were identified.In PCBs+FeOOH group, all of the dechlorination products were resulted from/wefo-dechlorination process. Moreover in PCBs+SO42-group, not only the meta-dechlorination product of PCB 105 (PCB66 (24-34)) was found, but also the para-dechlorination product of PCB 105 (PCB55 (234-3)) was also observed, which indicated that the presence of SO42- was able to favorr para-dchlorination pathways..(4) In the three experiment groups, PCB5 (23-) and dioxin-like PCB 105 (234-34) were firstly observed starting dechlorination.This indicated these PCBs were more likely to be degraded by microorganisms. In addition, another dioxin-like PCB congener PCB114 (2345-4), was also observed starting dechlorination firstly in PCBs+FeOOH experiment group and PCBS+SO42-experiment group. After 24 weeks of incubation, the TEQ of the three groups decreased by 94.00%, 30.00% and 14.00% respectively.The physical and chemical conditions of the sediment have a direct impact on the anaerobic dechlorination of the PCBs in sediments. The research results of this study are helpful to the development and application of the in-situ PCB remediation technology.