Enhanced Dechlorination Of Typical Chlorinated Organics Using Bio-electrochemical Systems And The Effects Of Concurrent Environmental Factors

Polychlorinated biphenyls(PCBs)and trichlorophenol(TCP)are typical persistent chlorinated organic pollutants,which are prevalent in soils/sediments and groundwater,representing a potential threat to the environment.Taking into account the limitations of the traditional biological treatment methods,the development of a more effective and controllable strategy is highly desirable.This study aims to demonstate the use of a bioelectrochemical technology to achieve enhanced reductive dechloriantion and invesitage the effects of concurrent environmental parameters.In specific,the reseach focuses include:(i)the demonstration of unsing an low-voltage electric field coupled with the addition of iron oxides to stimulate dechlorination of PCBs;(ii)the identification of the role of nitrate on bioelectrochemical dechlorination of 2,4,6-TCP;(iii)the unraveling of the response of microbial community structure to the coexisting environmental factors in the bioelectrochemical reactors(BERs).A BER was constructed with the working electrode poised at-0.5 V(vs.SCE)and iron oxides amended with the spiked PCBs in the inoculated sediments.PCB 61 was chosen as a model congener of PCBs,and hematite was selected as a typical iron oxide.The results showed that the addition of iron oxides can promote the dechlorination of PCB 61 under both the closed-circuit and open-circuit conditions,with pronounced effects displayed in the BERs.The pseudo-first-order kinetic rate constant of PCB 61 dechlorination showed positive correlation with adsorbed Fe(II)concentrations,indicated the role of the biogenic Fe(II)in mediating dechlorination in the system.High-throughput sequencing results showed that the dominant genus under the closed-circuit conditions were Geobacter and Desulfovibrio,and the prences of iron oxides can further promote their popoulation.The dominant genus enriched from the open-circuit reactors was Hydrogenophaga.Low-voltage electric field stimulation was also effective in enhancing the microbial dechlorination of 2,4,6-TCP,which was found to proceed with the ortho-dechlorination pathway rather than the para-dechlorination,eventually transformed to nonchlorinated endproducts(phenol).The coexistence of nitrate,which features a higher redox potential and can compete electrons with 2,4,6-TCP,resulted in the suppression of 2,4,6-TCP dechlorination.It was evident from the electrochemical impedance spectroscopy(EIS)results that high concentrations of nitrate were not favorable for the growth of biofilm on the electrode,as observed from the increased charge transfer resistance.The microbial community analysis revealled that dechlorinators such as W22 and Dechloromonas were dominant in the bioelectrochemical system with and without the addition of 20 mg L-1 nitrate.These speices were not distinctly dectected in the system with 80 mg L-1 nitrate,and denitrifiers such as Halomonas and Thauera was predominant.