Study On Photocatalytic-Microbial Fuel Cell Coupling For The Removal Of Chlorinated Organics And Electricity Generation Mechanism

Microbial Fuel Cell（MFC）uses microbes attached to the anode as catalysts to generate electricity during wastewater treatment,which has shown great potential for application in solving the two key social problems including environmental pollution and energy shortage.However,it is difficult for MFC to treat refractory pollutants such as chlorinated organics,suffering from low treatment efficiency and power output.Photocatalysis,has great advantages in treating chlorinated refractory pollutants,but also faces problems such as easy recombination of photogenerated electron-hole pairs and difficult further mineralization of intermediate products.Combining the advantages of MFC and photocatalysis can enhance the degradation of pollutants together with increasing the power output.Dissolved Oxygen（DO）affects both the life metabolism of microbes and photocatalytic degradation,therefore,it is important to investigate the role played by DO in the coupled photocatalytic-MFC system to reveal the mechanism of bioelectrochemical-photocatalytic interaction.To solve the above problems,different photoelectrodes were prepared in this thesis and the corresponding photocatalytic-MFC coupling systems were constructed.The effect of different factors on the degradation and power output of the coupled systems were investigated using refractory chlorinated organics as the target pollutants;through microbial community analysis and active species detection,the changes of microbes in different systems were compared,and the degradation mechanisms of pollutants were inferred from the identification of intermediate products;in order to broaden the application scope of the coupled photocatalytic-MFC system,the mechanism of DO influence on the degradation ability,the composition of the biological community and the capacity of the system was explored.The above studies provide basic data for the practical application of MFC-photocatalytic system.The main research results are as follows.（1）A dual-anode photocatalytic-MFC coupled system（PMFC）based on single-chamber MFC with titanium dioxide modified photoanode/bioanode was constructed to study the removal of 2-chlorophenol（2-CP）by this coupled system.The synergistic removal efficiency of 2-CP（5 mg/L）in PMFC was 76.20%,higher than the sum of MFC（19.33%）and photocatalytic（49.2%）.This was mainly attributed to the strong oxidation of·OH and·O₂^-（O₂from air reoxygenated）generated by the photocatalytic process,and the improvement of the separation efficiency of photogenerated electron-hole pairs in the coupled system.The effect of DO on the PMFC system is twofold.On the one hand,the high DO concentration（initial concentration of 5.62 mg/L）reduced the current density of the system and decreased the 2-CP removal efficiency by inhibiting the activity of the electrogenic bacteria in the system;on the other hand,the DO could also interact with the photoanode to generate·O₂^-to improve the2-CP removal efficiency.Photocatalytic coupling reduced the biotoxicity of 2-CP and enhanced the ability of MFC to utilize a sole 2-CP substrate.Illumination enhanced the abundance of Geobacter sp.,PHOS-HE36 fam.and Pseudomonas sp.,accelerated the electron transfer,which increased the maximum power density by 15.3%.The detection of the unique intermediate product 1,2-dichlorobenzene in the PMFC system suggested that microbes modified the 2-CP site of action for photocatalytic radical degradation in the coupled system and modulated the photocatalytic degradation pathway.（2）To further investigate the mechanism of DO effect on the removal of 2-CP by the coupled photocatalytic-MFC system,a single-chamber aerobic photocatalytic-MFC coupling system was constructed with the DO domesticated electrogenic bacteria and loaded with black phosphorus/carbon nitride（·O₂^-as the main reactive radical）as the photocatalyst in the bioanode.With the DO concentration decreasing from 6.16 to 0.13 mg/L within 8 h,the removal efficiency of 300 mg/L 2-CP in the coupled mode was 67.1%,which was higher than that of anaerobic photocatalytic MFC（31.4%）,aerobic MFC（20.5%）and anaerobic MFC（17.7%）,while the pre-coupling of DO and illumination（8 h）also facilitated the subsequent MFC treatment.DO increased the abundance of Geobacter sp.and enriched the biomass.However,the excessive DO concentration and the·O₂^-generated by its interaction with illumination would damage the microbial activity.The illumination contributed to the increased abundance of chlorophenol-degrading bacteria Azospirillum sp.and the electrogenic bacterium Comamonadaceae fam.The cathodic oxidation-reduction reaction was promoted by illumination and DO and the maximum power density was increased from 134 to 255 m W/m².The microbial electron transfer pathway of anode was dependent on NADH dehydrogenase,succinate dehydrogenase and terminal oxidase.The degradation of 2-CP under anaerobic conditions was dependent on microbial dechlorination and photogenerated h⁺,while microbial oxidative ring opening and photogenerated·O₂^-under aerobic conditions.（3）Based on the results of oxygen tolerance of bioanode,a floating photocathode/bioanode photocatalytic-MFC coupling system（Photocathode-MFC,PC-MFC）was constructed with black phosphorus/titanium dioxide nanoribbons modified cathodes.The removal of hydroxychloroquine（HCQ,a drug used for the treatment of COVID-19）was investigated.The degradation efficiency of 100 mg/L HCQ in 8 h was 73.7%,which was higher than that of 69.5%for photocatalysis,25.6%for MFC and 9.6%for adsorption.The kinetic analysis revealed that the coupling of photocathodes contributed to the subsequent bioelectric treatment and achieved complete degradation of HCQ within 96 h,higher than that of MFC at51.1%.The PC-MFC photocathode resulted in a significant increase in the Pseudomonas sp.abundance and system biomass,while the abundance of the anode producing bacteria Geobacterales sp.was also enhanced.The photocatalytic coupling reduced the cathodic reaction barrier while promoting the electrochemical behavior of the bioanode,which increased the maximum power density by 16.2%.The electron transfer pathway of the system was dependent on NADH dehydrogenase,succinate dehydrogenase and terminal oxidase.The PC-MFC photocathode enhanced the reductive dechlorination degradation of HCQ and changed the degradation toward a pathway favorable for biotoxicity reduction and environmental friendliness.