Electrochemically Enhanced Microbial Dechlorination Of Typical Organochlorine Pollutants

Organochlorine pollutants are ubiquitous in the environment,which pose risks to the ecosystems and have adverse effects on human health.In situ remediation of organic chlorine pollution in the environments is of great concern.Recently,bioelectrochemical system has been developed to use electrons supplied by electrode to stimulate microbial dechlorination of organic chlorine pollutants.However,the bioelectrochemical dechlorination of organochlorine pollutants at low temperature remains unclear.Also,little is known about the effects of different electron shuttles on bioelectrochemical dechlorination.In this study,based on the investigation of organochlorine pollution in Taihu Lake,a typical shallow freshwater lake in the Yangzte River Delta,organochlorine pollution in Baiyangdian Lake,a shallow lake in North China,was comprehensively examed,including organochlorine pollution distribution characteristics and ecological risks.Then,2,4,6-trichlorophenol was selected as a representative organic chlorine pollutant,and the indigenous microorganisms of the sediment collected from Zhushan Bay,Taihu Lake was used to invesitage the dechlorination potential,based on its better behavior than Baiyangdian Lake sediment in degradation.From the perspective of simulated in-situ remediation,the feasibility of electrochemical stimulation of microorganisms for remediation of organochlorine pollution was discussed.Furthermore,the effects of natural electron humin and black carbon on bioelectrochemical system were investigated,and their enhancement on microbial anaerobic reductive dechlorination in bioelectrochemical system were evaluated.The main findings are as follows:(1)Organochlorine pesticides(OCPs)and polychlorinated biphenyls(PCBs)were detected in most surface sediments of Zhushan Bay and Baiyangdian Lake.The total OCPs concentrations ranged from 1.21～2.62 ng/g and 1.22 to 52.45 ng/g,respectively,while the total PCBs levels were from 19.63～84.05 ng/g and below detection limit to 37.61 ng/g,respectively.Compared with other rivers and lakes,the concentration of total OCPs in Zhushan Bay of Taihu Lake was relatively low and the concentration of total PCBs was at a high level.Meanwhile,the concentration of total OCPs and total PCBs in Baiyangdian Lake were both at a medium level.The risk assessment showed that the surface sediments of Zhushan Bay of Taihu Lake and Baiyangdian Lake had a moderate risk,which should be drawn more attention.(2)2,4,6-trichlorophenol could be dechlorinated using sediment in Zhushan Bay of Taihu Lake.Electrochemical stimulation(-0.279 V vs.SHE)was capable of enhancing the reductive dechlorination of 2,4,6-trichlorophenol,confirmed by the results when2,4,6-trichlorophenol was completely degraded by microbes with electrochemical stimulation,only 60% degradation was observed without electrochemical stimulation.The degradation capability remained after repeated addition of 2,4,6-trichlorophenol.Notably,at low temperatures,microbial-catalyzed 2,4,6-trichlorophenol was greatly inhibited without electrochemical stimulation(almost no degradation within 60 days).By contrast,2,4,6-trichlorophenol dechlorination triggered in bioelectrochemcial systems except for a longer lag time(29 days).(3)In cathode chamber,the degradation of 2,4,6-trichlorophenol was dominated by reductive dechlorination,and the dechlorination products 2,4-dichlorophenol,4-chlorophenol and phenol were detected in turn.2,4,6-trichlorophenol could also be degraded in the anode chamber and the main pathways of 2,4,6-trichlorophenol degradation may be via the formation of dimer or ring cleavage throuth oxidation.(4)The addition of 2,4,6-trichlorophenol could cause a decrease in relative abundance of certain microorganisms(ellin6067,sva0081 sedimentation group,bd1-7 clade,denitratisoma and desulfatiglans).Meanwhile,the relative abundance of Pseudomonas increased,indicating a selective enrichment of this genus.The relative abundance of Pseudomonas further increased after electrochemical stimulation.Pseudomonas was likely the dominant genus of dechlorinating bacteria in this study.(5)The electron shuttles(1.2g/L),humin(1.2g/L),black carbon as well as AQDS(0.5mmol/L),could facilitate the dechlorination of 2,4,6-trichlorophenol in bioelectrochemical system without changing the dechlorination pathways.Humin and black carbon exhibited good stability in bioelectrochemistry system after five rounds of2,4,6-trichlorophenol additions(2,4,6-trichlorophenol degraded totally within 6 days).But AQDS was not.Moreover,humin and black carbon are more environmentally friendly,less expensive and suitable in situ remediation.Compared with humin,black carbon not only acted as an electron shuttle to promote dechlorination,but also could adsorb2,4,6-trichlorophenol.(6)The addition of electron shuttles greatly shifted the microbial communities in the bioelectrochemical systems,and the effects of different electron shuttles were distinct.With the electrochemical stimulation,Pseudomonas was the dominant dechlorinating genus.In the presence of AQDS,the prevalent microorganisms in the bioelectrochemical system were Pseudomonas and Sulfuricurvum.Rhodobacter was the dominant microorganism in bioelectrochemical system with humin addition.In the presence of black carbon,the dominant genera highly similar to Dechloromonas may played a curcial role in dechlorination.