Degradation And Transformation Of Typical Hydrophobic Emerging Pollutants In Bioelectrochemical System

Triclosan(TCS)is a typical emerging hydrophobic contaminant in the environment.It is often used as a broad-spectrum antimicrobial agent in various detergent products.In recent years,TCS has been frequently detected in different environmental media.TCS became research hot spots due to the characteristics of widespread distribution,persistence,and refractory.It is a threat to aquatic organisms,mammals,humans,and soil ecosystem.Therefore,the degradation of TCS has attracted much attention.At present,wastewater treatment plants are still lack of special treatment links for the removal of TCS and other hydrophobic emerging contaminants.However,the degradation rate of traditional anaerobic biological treatment process suffered from the bacteriostatic effect,low efficiency,and by-products.Bioelectrochemical systems(BES),which combine biodegradation and electrochemical redox,have made some progress in the transformation and degradation of refractory pollutants,and may support for the deep removal and safe transformation of emerging hydrophobic contaminants such as TCS.This work mainly discussed the process of enhancing TCS degradation and transformation in BES,and clarified the key factors affecting the removal efficiency,degradation mechanism and transformation pathway in BES.(1)The results showed that BES effluent as the inoculation source had rapid start-up period,excellent electrochemical performance and better TCS degradation efficiency.In the two experimental groups,the internal resistance of the reactor was lower and the biofilm activity was higher.The overall efficiency of the two groups was better than the control group inoculated with domestic wastewater.The degradation rate of TCS was significantly higher than the control group(41.59±2.03% and 50.01±2.21%).The peak time of TCS reduction in directional membrane hanging group and reverse membrane hanging group moved to-0.476 V and-0.482 V compared with the peak time of control group(-0.785 V),and the peak current of TCS reduction peak in directional membrane hanging group and reverse membrane hanging group increased by nearly one order of magnitude compared with the control group.The biofilm thickness(163±6.32 μm)and cell activity(0.636)of the reverse membrane group were the highest,which were significantly higher than those of the natural membrane group.Microbial community structure and function were analyzed and found that control group electrode with domestication and TCS degradation experiments in BES still keep high biodiversity,but the relative abundance of bacteria related to electron transfer and transformation of TCS degradation function was low.It indicated that the selection of BES for the enrichment of functional microbial may be not enough,and choose the appropriate inoculum can effectively improve the function of microbial abundance,thus improving the overall efficiency.(2)Based on the optimized reactor start-up strategy,three single-chamber BES,namely direct operation group,electrode reverse group and anaerobic control,were employed to investigate the effect of initial TCS concentration of,applied voltage and buffer system on TCS treating performance.The results show that the applied voltage over 1 V or the initial concentration of TCS over 5 mg/L had negative effects on the BES,and the buffer capacity and conductivity of the buffer system will affect the final degradation rate of TCS in the reactor.When the applied voltage,initial TCS concentration,and the buffer system were fixed at 0.8 V,1 mg/L,and 50 m M PBS,respectively,the degradation efficiency of TCS in the electrode reversal group,the direct operation group and the anaerobic control group all reached the highest level,and the degradation rate of TCS in the electrode reversal group was the highest(90.51±1.87%).The degradation rate of TCS in the direct operation group was81.26±2.03%,and the degradation rate in the anaerobic control group was the lowest,only51.96±2.17%.The results of mass spectrometry showed that the concentration of TCS was effectively reduced with BES treatment.In the electrode reversal group and the direct operation group,TCS transformation was mainly through two pathways: reductive dechlorination and hydrolysis,and no obvious oxidation products were detected.Under pure anaerobic conditions,it is difficult to provide sufficient reduction conditions thus only TCS primary dechlorination products were detected,and subsequent reactions seemed be inhibited.Complete dechlorination of TCS can be achieved in bioelectrochemical system.(3)The degradation rate of TCS in the cathode chamber and anode chamber of the electrode reversal group reached 85.04±2.01% and 81.54±2.13%,respectively,and76.56±1.97% and 81.68±2.09% respective in the cathode chamber and anode chamber of the direct operation group.The complete dechlorination process occurred in the cathode chambers of both groups and the dechlorination products could be further converted into small molecules through hydrolysis reaction,which was similar to the transformation process of TCS in the single-chamber BES.In the anaerobic control group,relatively simple products were detected.TCS could only complete the first stage dechlorination reaction and could not achieve complete dechlorination.After biological anode reverse into biological cathode electrode,microbial community composition on species level changed slightly while the proportion of microbes in their respective electrode changed greatly.The biofilm of biological anode and biological cathode which were after the reversal contains substantial actinomycetes,which accounted for 44.21% and 20.68%,respectively.Typical electrochemical bacteria,Geobacter,in biological anode(21.01%)and biological cathode which were after the reversal(44.08%)were predominant member.This indicated that the source of inoculation and the start-up mode of the electrode will have a significant effect on the formation and enrichment process of the microorganism on the electrode surface.The start-up strategy of BES and critical parameters were optimized in this experiment.At the same time,the contribution rates of TCS degradation and the TCS transformation mechanism of cathode and anode were analyzed in dual-chamber BES.The outcomes of this work are excepted to provide theoretical and technical supports for future research and promote the BES technology closer to the practical application.