Electrochemically Enhanced Anaerobic Dechlorination Of 2,3,4,5-tetrachlorobiphenyl Dechlorination:Disclosing The Important Factors

Polychlorinated biphenyls(PCBs)are typical persistent organic pollutants in the environment.The anaerobic reductive dechlorination of PCBs in soils and sediments has attracted much attention.The use of biostimulation to enhance reductive dechlorination has been widely investigated for the remediation of PCBs-contaminated sites;however,this method is greatly affected by the various biogeochemical conditions and its regulation is complicated.Therefore,exploration of efficient and controllable bioreduction methods have become a research hotspot.This study aims to demonstrate the enhanced anaerobic dechlorination of PCB 61(2,3,4,5-tetrachlorobiphenyl dechlorination)by electrochemical stimulation,and disclose the key factors affecting the dechlorination efficiency of the bioelectrochemical system.To achieve this,the effects of the level of the applied potential,the extent of the electrolysis time,and the type of surfactant on the kinetics of PCB 61 dechlorination were investigated and the variations in the microbial communities and their electrochemical characteristics in response to these different factors were analyzed.First of all,we conducted a series of experiments varying in the value of the applied potential.The results showed that the stimulation of a potential of-0.7 V(vs.SCE)enabled the highest removal efficiency,with 59.05% achieved after 24 weeks,larger than 32.22% associated with the open-circuit reactor.The bigger the overpotential,the higher the reduced dechlorination rate of the PCB 61 within the investigated potential range(i.e.,-0.1,-0.3,-0.5 and-0.7 V).Analysis of the bacterial composition suggested significant community shifts in response to variations in treatment conditions.The electric stimulation significantly increased the abundance of electrochemical active bacteria like Geobacter and Ignavibacterium,and then enhanced the dechlorination efficacy.Methanosarcina and Methanosaeta are the dominant bacteria in closed circuit conditions,and the enhanced dechlorination process in the system may be achieved through the combined action of electrochemically active bacteria and methanogens.Then,we studied the time-dependent bioelectrochemical dechlorination of PCB 61 in the presence of different surfactants,and investigated the interactions among the electrode,microorganisms and contaminants.The results showed that the application of a cathodic potential(i.e.,-0.7 V)significantly improved the rate and extent of PCB 61 dechlorination compared to the open-circuit scenario(without electrical stimulation),and the addition of an external surfactant further increased the dechlorination,with Tween 80 exerting more pronounced effects than rhamnolipid.The experimental results of kinetic degradation,high-throughput test analysis,and electrochemical characterization confirmed that the bacterial composition of the biofilms and the bioelectrochemical kinetics of the BERs were found to be time-dependent and to vary considerably with the incubation time and slightly with the coexistence of an external surfactant.Excellent correlations were observed between the dechlorination rate and the relative abundance of Dehalogenimonas,Dechloromonas,and Geobacter,the dechlorination rate and the cathodic current density recorded from the chronoamperometry tests,and the dechlorination rate and the charge transfer resistance derived from the electrochemical impedance tests,with respect to the 120 day-operation.