Enhanced Anaerobic Dechlorination Of Polychlorinated Biphenyl By Electric Field Stimulation

Polychlorinated biphenyls(PCBs),present in soils and sediments,are typical persistent organic pollutants that are resistant to degrade by indigenous microorganisms.Biostimulation to enhance reductive dechlorination of PCBs existed in contaminated soils and sediments has attracted considerable attention,due to their serious threat to human health.The development of an efficient,cost-effective and performance-controllable biological method is highly desirable.This study proposes a bioelectrochemical method,in an effort to promote ananerobic dechlorination of PCBs.The main purposes include(i)the demonstration of the feasibility of using an electric field to stimulate reductive transformation of PCBs and the investigation of their reaction kinetic and pathway;(ii)the identification of microbial communities and the elucidation of their relationship with dechlorination rate of PCBs in the bioelectrochemical environment;(iii)the understanding of effects of extra organic electron donor or carbon source and surfactant on the electric field-stimulated PCBs anaerobic dechlorination.Our first effort was made to construt a bioanode-electrochemical system with the electrode poised at 0.2 V(vs.SCE)and organic compounds contained in inoculated sediment serving as the electron donor.The seed sediment was originated from an electronic waste recycling site in the province of Guangdong.It was expected that this poised anode potential can increase population of electrochemically active bacteria(EAB),which in turn can promote the abundance of dechlorinating bacteria responsible for PCBs' reductive transformation.The 110-day incubation of the bioanode enabled 58% transformation of the total 2,3,4,5-CB(PCB61)at the initial concentration of 100 ?M,larger than 24% obtained from the open-circuit mode,showing that the anaerobic dechlorination of PCBs can be enhanced by bioanode stimulation.The introduction of acetate to the bioelectrochemical reactor further improved reductive transformation to 82%.It was revealed that PCB61 dehalogenation mainly appeared at the double-flanked para-position(2,3,5-CB)and meta-position(2,4,5-CB),following pseudo-first order kinetics.Analysis of the bacterial composition showed significant community shifts in response to variations in the treatment;the existence of EAB like Geobacter,Shewanella,and Dysgonomonas in the closed-circuit can stimulate the growth of dechlorinating microorganisms like Dehalogenimonas,Anaeromyxobacter,and Sedimentibacter.Our second effort was devoted to identify the possibility of PCBs reductive transformation drived by cathodophilic,autotrophic microorganisms in the bioelectrochemical reactor with its cathode poised at-0.5 V(vs.SCE).The results showed that the reduction of PCB61 under either the open-circuit mode or the closed-circuit mode cannot proceed,when an autotrophic inorganic carbon source(i.e.,carbonate)was added during the 120-day inoculation process.This phenomenon was considered to be induced by the low biomass growth yield of relative dehalogenation bacteria.In contrast,when a hetrotrophic organic carbon source(i.e.,sodium acetate)was introduced to the biocathode chamber,the reduction of PCB61 under both cases substantially increased.The fraction of PCB61 reduction was about 25.4% under the open-circuit condition,and this value further rose to 49.8% in the closed-circuit bioelectrochemical reactor.It was interesting to note that the amendment of a surfactant,Tween 80 further increased PCB transformation,as evident from 71.7% and 42.5% achieved in relation to the closed-circuit mode and open-circuit mode,respectively.This effect can stem from the promoted solubility and mobility of PCBs by the use of Tween 80.In all cases,the dechlorination products including trichlorinated biphenyl of PCB23,PCB29 and dichlorinated biphenyl of PCB9 were detected.The microbial community analysis via the high-throughput sequencing technique revealed that the microorganisms available in the inoculated sediment can be classified into three categories including dehalogenating bacteria like Dehalobacter,Dehalogenimonas,Clostridium,Desulfomonile,Sedimentibacter;synergetic dechlorinating bacteria like Syntrophobacter,Desulfovibrio and fermentating bacteria like Syntrophus,Treponema,Anaerolinea.