Study On The Degradation Mechanism Of Trichlorophenol By Microbial Fuel Cells With Novel Photocatalytic Air Cathodes

As a refractory compound,2,4,6-Trichlorophenol（TCP）is widely used in the synthesis of pesticides and dye,so it is commonly found in industrial wastewaters from pesticide,pharmaceutical,paper and pulp industries.TCP is listed as a priority pollutant by the US Environmental Protection Agency due to its mutagenesis,carcinogenesis and teratogenesis.It is so hard to be biometabolized and transform that would easily affects the development and reproduction of aquatic organisms once exposed to the waterbody,and eventually threatens human health along the food chain through bioaccumulation.Coupling microbial fuel cells with photocatalysis can achieve the efficient degradation of refractory organic matter.Based on the scientific research so far,suitable coupling method can not only achieve the green and efficient treatment of pollutants,but also improve the MFC power generation efficiency and the photocatalytic efficiency simultaneously as both of which contain the electronic processes.Since the electroactive microorganisms mainly accumulate on the anode,we would like to perform photocatalytic coupling on the MFC cathode.In this thesis,a microbial fuel cell constructed with novel photocatalytic air cathode was built to achieve efficient degradation of TCP and long-time stable operation.The main results and conclusions are summarized as follows:（1）Photocatalytic materials N-TiO2 were prepared by sol-gel method and hydrothermal method.Material characterization and heterogeneous photocatalytic degradation experiments of the prepared photocatalytic materials was applied to select the best N-doping ratio which is N:Ti=4 for further use.The novel photocatalytic air cathode was made of diffusion layer,catalyst layer and N-TiO2 layer.PDMS was used as a gas diffusion layer as well as a binder.Catalyst layer and N-TiO2 layer was sequentially loads on the PDMS to form a grid-like staggered structure where catalyst layer was stainless steel mesh coated with conductive carbon material and N-TiO2 was lied in the grid holes of stainless steel mesh.The staggered structure could achieve a good conductivity and enable photocatalysis contact with the electrolyte.（2）Degradation efficiency and operating performance of the novel photocatalytic-bio air cathode MFC was analysed.The traditional air cathode MFC was set as control,the novel photocatalytic air cathode MFCs with different parameters was tested whether they could adjust to mature MFCs.The parameters included the loading of the carbon material and whether the photocatalytic material was loaded.With TCP as the target pollutant,the concentration gradients of 10 mg/L,30 mg/L,and 50 mg/L were used for domestication.Under each concentration,the degradation efficiency was determined.Light acclimation of 12h-intermettent was conducted for the groups loaded with photocatalysts at the same time,the voltage output curve could response to the intermittent light.By comparing the degradation efficiency,the mineralization and the long-term running performance,it was found that the loading of photocatalytic materials and light acclimation improved the TCP degradation efficiency of the novel photocatalytic-bio air cathode MFC.When the loading of the carbon material was 135 mg AC and 15 mg CB,TCP removel achieved 100%degradation of 50 mg/L TCP in 72 h which was 11.5%higher than that without photocatalysts and the degradation rate was increased from 0.319 h^-1 to 0.913 h^-1 by light illumination.After 130 days cultivation under the same conditions,the peak voltage of the novel photocatalytic cathode MFC decreased 17%,while the control group decreased 56%,and the water loss rate of the chamber in one cycle was 8.5%and 73.0%respectively.（3）The TCP degradation mechanism by the novel photocatalytic-bio air cathode MFC was explored.Cyclic voltammetry（CV）,Electrochemical impedance spectroscopy（EIS）tests and calculations of cathode oxygen diffusion rate was performed,meanwhile microbial community structure and intermediate products was analysed.Loading photocatalytic materials could reduce the oxygen diffusion rate of the cathode from 0.28 mg L^-1 to 0.67 mg L^-1 to hinder the oxygen transfer to anode to inhibit aerobic degradation process of the substrates and reduce the internal resistance of charge transfer during the substrates degradation process,ultimately balance the microbial community structure of the novel cathode.The efficient degradation of 2,4,6-trichlorophenol can be ascribed as two key factors:one was photogenerated electrons-hole and advanced oxygen active species generated on the photocatalytic layer of the novel cathode,the other one was the unique structure of the cathode combined with the domestication culture method to cultivate a microbial community structure with a high proportion of electrochemically active bacteria and synergistic bacteria that can degrade pollutants.The coupled system was capable of ring cleavage for complete mineralization of TCP and showed the combined degradation pathway of microbial process and photocatalytic process.In this coupled system,the photogenerated electrons might firstly react with the electron acceptor on the surface of the cathode and the holes might combine with the electrons transferred from the anode through the external circuit,which improved the photocatalytic degradation efficiency and MFC power generation efficiency simultaneously.As an external environmental factor,the loading of photocatalysts affects the distribution of microbial communities in the system,resulting the presence of electrochemically active bacteria at the cathode（55.2%）,and domesticated a variety of synergistic bacteria（including Thauera,Paracoccus,Arenimonas,Comamonas,Acidovorax,etc.）that can degrade aromatic pollutants.