Study Of Degradation Of Diclofenac In Water By Ru/Fe Modified Electrode In Bioelectrochemical System

As a refractory persistent organic compound,diclofenac（DCF）residue in water poses a potential threat to human health and water environment safety.In recent years,it has attracted much attention,while it is necessary to develop an efficient degradation technology to reduce DCF in water.Since dechlorination and deep mineralization are the two keys to degrade DCF,these two problems have not been well solved at present.In this paper,taken DCF as the target pollutant,a technical strategy of anodic reduction of DCF and deep mineralization of its dechlorination products with cathode was proposed by using bioelectrochemical system（BES）.A BES was constructed with a Ru/Fe modified electrode for DCF degradation.The structure and morphology of Ru/Fe modified electrode were analyzed by various instrument characterization methods.The electrochemical performance of Ru/Fe modified electrode was investigated by means of electrochemical tests.High-throughput sequencing was used to explain the microbial community distribution of each modified electrodic biofilm.The degradation intermediates of DCF were determined by liquid-mass spectrometry.The potential ecotoxicity of the reaction solution during DCF degradation was investigated by microcystis aeruginosa inhibition test.The contributions of free radicals during DCF degradation were analyzed by EPR and quenching experiments.Through measuring and analysis of macro genomics and metabonomics,the responses to enzyme activity and microbial community structure and functional genes and the microbial metabolic pathway of the bioelectrode of BES under the stress of DCF was systematically discusses.So as to provide technical support and theoretical basis for rapid degradation of DCF by Ru/Fe modified bio-electrode BES,at the same time,it also provides a case and reference for rapid degradation of POPs by Ru/Fe modified bio-electrode BES.A microbial fuel cell（MFC）was constructed with a Ru/Fe-modified-anode prepared by reduction and coating for enhancing DCF degradation.The results showed that Ru⁰and Fe⁰were uniformly distributed on the surface of the carbon felt,and the Ru/Fe particles were of alloy structure.The charge transfer rate of Ru/Fe modified electrode was 1.43 times of carbon felt electrode.Ru/Fe modified electrode improved the anodic polarization performance,and made the maximum output power of Ru/Fe-MFC reach 0.600 W m^-2,which was 2.68 times of CF-MFC.The optimum operating conditions of Ru/Fe-MFC degradation process were as follows:the external resistance was 500Ω,the carbon source was 500 mg L^-1sodium acetate and the initial concentration of DCF was 5 mg L^-1.Under these conditions,the DCF degradation process by Ru/Fe-MFC conformed to the pseudo-first-order kinetic model,and the degradation kinetic constant k_obswas 0.711d^-1,which was 2.21 times of CF-MFC.However,the DCF degradation process was negatively correlated with coulombic efficiency of Ru/Fe-MFC.With the increase of initial DCF concentration from 0 to 20 mg L^-1,the Coulomb efficiency of Ru/Fe-MFC decreased from 27.50%to 21.34%.Ru/Fe-modified-anode accelerated the enrichment of electro-active bacteria and DCF-degrading bacteria such as Geobacter,Clostridium,Sedimentibacter,Pseudomonas and Desulfovibrionaceae.Five degradation intermediates were identified,and two possible degradation pathways of DCF were deduced.The main degradation pathway was the stepwise dechlorination of DCF under the synergistic reaction of Ru/Fe catalytic reduction and functional bacteria reduction,and eventually converted to 2-anilinophenylacetate（APA）.The results of Ru and Fe dissolution and cyclic DCF degradation indicated that Ru/Fe modified anode was capable of good stability.A double-chamber MFC with Ru/Fe-modified-biocathode was constructed for simultaneous mineralization of APA and denitrification.The surface of Ru/Fe modified electrode was rough and uneven,on which Ru⁰and Fe⁰particles were uniformly distributed.The charge transfer impedance（23Ω）of Ru/Fe modified biocathode was 0.14 times of carbon felt biocathode（164Ω）,indicating the electrical conductivity of Ru/Fe modified biocathode was better.Ru/Fe modification increased the open-circuit cathode potential from-0.11 V to 0.15 V,which significantly improved the polarization phenomenon of biocathode.The maximum output power of Ru/Fe-modified-biocathode-MFC was 0.18 W m^-2,which was4.92 times of carbon-felt-biocathode-MFC.The optimum degradation conditions were as follows:ammonia concentration was 50 mg L^-1,initial APA concentration was 5 mg L^-1,external resistance was 500Ωand carbon source Na HCO₃concentration was 500 mg L^-1.Under these conditions,the APA degradation efficiency of Ru/Fe-modified-biocathode-MFC was as high as 98.60%,which was significantly higher than that of other modified cathode-MFC,indicating both APA mineralization and denitrification efficiency.The APA degradation process conformed to the pseudo-first-order kinetic model,and APA degradation kinetic constant,the maximum removal efficiencty of TOC,ammonia and TN were 2.15d^-1,59.70%,99.20%and 44.56%,respectively.As APA concentration increased from 0 to 20 mg L^-1,the Coulombic efficiency of Ru/Fe-modified-bicathode-MFC decreased from 56.54%to48.29%.?OH was the main free radical in the APA degradation process.The redox reaction of Ru/Fe might affect the signal transduction and environmental adaptation function of the microorganism in Ru/Fe-modified-biocathode.The microbial community of Ru/Fe-modified-biocathode was dominated by Nitrosomonas at the genus level,and a variety of APA degrading bacteria,nitrifying bacteria and heterotrophic denitrifying bacteria were enriched,such as Pseudomonas,Nitrospira,Nitrobacter,Paracoccus,Thermoonas,Dechloromonas and Clostridium＿sutra＿stricto＿1,indicating that Ru/Fe-modified-biocathode was capable of higher nitrification and denitrification activity than that of carbon-felt-biocathode.Eight kinds of intermediates were detected,and two degradation pathways of APA were proposed,among which hydroxylation of APA was the main pathway.The synergistic mechanism of simultaneous APA mineralization and denitrification was mainly redox reaction of Ru/Fe and supplemented by aerobic biodegradation.The results of Ru and Fe dissolution and cyclic APA degradation and TN removal indicated that Ru/Fe-modified-biocathode was capable of good stability.A sequential reduction-oxidation process for DCF degradation was proposed by adjusting anaerobic/aerobic conditions with Ru/Fe-modified-biocathode in a double-chamber BES.At the applied voltage of 0.6 V,the cathode potential and charge transfer impedance of Ru/Fe modified biocathode were-0.80 V and 26Ω,which were significantly lower than-0.66V and 110Ωof carbon felt biocathode,respectively.The maximum output current of Ru/Fe-modified-biocathode-BES was up to 0.60 m A,which was 2.31 times of carbon-felt-biocathode-BES.The optimum degradation conditions were as follows:the applied voltage was 0.6 V,initial APA concentration was 5 mg L^-1,carbon source was 500 mg L^-1Na HCO₃and initial concentration of ammonia was 50 mg L^-1.Under these conditions,the maximum degradation efficiency and DCF kinetic constants of Ru/Fe-modified-biocathode for 2d anaerobic reaction were 93.19%and 1.31d^-1,which were much higher than 21.95%and0.12d^-1of carbon felt biocathode,respectively.[H]and?OH were the main active species in Ru/Fe-modified-biocathodic reduction and oxidation processes,respectively.Degraded intermediates of DCF were first generated by[H]attacked under anaerobic conditions,further oxidized by microbes and?OH attacking during followed oxidation process,achieving69.56%of mineralization.The dominant bacteria in Ru/Fe-modified-biocathode included dechlorinators such as Hydrogenedensaceae and Dethiosulfatibacter,hydrogen-producing bacteria such as Lentimicrobiaceae,Anaerobineaceae and Bacteroidales,and heterotrophic denitrifiers such as Azoarcus and Pseudofulvirmonas.Thirteen intermediates were measured,and two degradation pathways were proposed,among which sequential reduction-oxidation of DCF was the main pathway.The results of Ru and Fe dissolution and cyclic DCF degradation indicated that Ru/Fe-modified-biocathode was capable of good stability.The synergistic catalytic reaction of Ru/Fe and cathodic microorganisms under sequential anaerobic-aerobic alternately was the main mechanism of DCF mineralization.After 4d of reaction,microcystis aeruginosa growth inhibition rate to reaction solution decreased from 22.87%to 8.03%,signifying a significant reduction in biotoxicity.Metagenomics and metabonomics were used to investigate the responses to the enzyme activity,microbial community structure and the relationship between functional genes and microbial metabolic pathways of biocathodes under DCF stress in bioelectrochemical systems.The activities of various enzymes in the bioanode were significantly inhibited under DCF stress.The average dehydrogenase activity decreased from 1.46 U/（mg prot）in the control group to 0.62 U/（mg prot）under DCF stress.The relative content of CYP450 in the bioanode decreased from 0.70%to 0.58%under DCF stress.The relative abundance of bioanode CAZy decreased significantly under DCF stress.Similarly,under DCF stress,the activities of various enzymes of the biocathode also decreased,and the average dehydrogenase activity of the biocathode decreased from 11.21 U/（mg prot）in the control group to 10.16 U/（mg prot）under DCF stress.The abundance of CAZy in the biocathode under DCF stress was lower than that in the control group,while the relative abundance of CYP450 in the biocathode decreased from 0.76%to 0.72%.It was worth noting that under DCF stress,the dehydrogenase activity of the biocathode（10.16 U/（mg Prot））was significantly higher than that of the bioanode（0.62 U/（mg Prot））,indicating that the biocathode was more resistant to DCF stress than the bioanode.Electroactive bacteria such as Geobacter and Clostridium in bioanode and obligate anaerobes such as Methanobacterium were more adaptable to DCF stress.Most of the heterotrophic bacteria,such as Propionibacterium,Dehalobacter and Tessaracoccus,were not easily adapted to DCF stress.For the biocathode,the relative abundance of Azoarcus,Methanobacterium,Hyphomicrobium,Mesorhizobium and Propionibacterium were significantly enriched under DCF stress.However,the relative abundances of Nitrosomonas and Clostridium were significantly reduced under DCF stress.Functional genes involved in the metabolism of Carbohydrates,Lipids,and Amino acids were significantly inhibited under DCF stress,leading to the accumulation of metabolites such as sugars in the bioanodes.Thus,the performance of organic matter consumption of bioanode was affected.On the contrary,the functional genes related to Carbohydrate metabolism in the biocathode were less affected by DCF stress,and even had a promoting effect,leading to the difficulty of accumulation of Carbohydrate metabolites in the biocathode.Metabolomics further revealed that the metabolic processes of microorganisms in the specific inhibitory bioelectrode,including the biocathode and the bionode,such as the metabolic pathways of Carbohydrates,Lipids and Amino acids,were all inhibited by DCF stress,but the inhibition of DCF stress on the biocathode was smaller than that on the bioanode.