Enhancement Of Azo Dyes Decolourization And CO₂ Conversion In Bioelectrochemical Systems

Bioelectrochemical system(BES),as a novel bioelectrochemical technology,could be applied for power recovery,recalcitrant contaminants degradation,and value added chemicals generation.In recent years,BES has shown great potentials for azo dyes removal and CO2 fixation,however,there still have several challenges to overcome,such as low efficiency of azo dyes decolourization and CO2 reduction.In this thesis,various improvement approaches such as parameter optimization and electrode modifications have been investigated for azo dye removal and CO2 reduction in the BES,and moreover the possible mechanisms have also been explored.The main contents and results of this thesis are as follows:1.The effect of cathode potential,dissolved oxygen(DO)concentration of catholyte and cathodic biofilm on mono-azo dye decolourization in the BES was investigated.The results showed that a more negative cathode potential was more beneficial for mono-azo dye removal in the BES.While DO concentration of catholyte and cathodic biofilm had a negative effect on mono-azo dye decolourization in the BES.The mono-azo dye decolourization efficiency increased from 0 to 94.90±0.01%with the decrease in cathodic potential from-0.2 to-0.8 V vs.Ag/AgCl.When DO concentration of the catholyte increased from zero to 5.80 mg L-1,the mono-azo dye decolourization efficiency decreased from 87.19±4.73 to 27.77±0.06%.On the other hand,RSM(response surface methodology)was used to assess the effects of the influence parameters on the decolourization of diazo dye.Based on the results of RSM,the key parameters and the interactions of these parameters were elucidated.And a possible diazo dye decolourization mechanism in BES was proposed according to the results of UV-visible spectrophotometer,Fourier Transform Infrared Spectroscopy(FTIR)and LC-MS(Liquid Chromatography Electrospray Ionisation tandem Mass Spectrometry).Moreover,it also found that the mono-azo dye and diazo dye decolourization kinetics in the BES were well fit.to the pseudo-first kinetic model.And the reaction rate constant ki value of diazo dye decolourization was generally lower than those of mono-azo dyes removal in the BES.2.The influences of different molecular structures of azo dyes on chemical removal in the BES were investigated,including the positions and kinds of substituent groups.The results suggested that molecular structures of azo dyes had a significant effect on the azo dye decolourization in the BES.The electron-withdrawing substituent groups like sulfo group,carboxyl group and nitro group could be more preferred for azo dye removal,while the electron-donating substituent groups such as hydroxyl group had a negative effect on the removal of azo dye.Compared to meta and para positions,the carboxyl group at ortho position could promote the azo dye decolourization,while hydroxyl group at ortho position had a negative effect on the removal of azo dye.Both Cyclic Voltammetry(CV)and Electrochemical Impedance Spectroscopy(EIS)further supported these conclusions.Moreover,a quantitative structure-activity relationship(QSAR)model has been successfully developed to predict the effect of molecular structure on azo dye decolourization in the BES.3.The effect of temperature on acetate production by CO2 reduction in the BES with a mixed-culture biocathode was evaluated.The results showed that the optimum temperature for acetate production from CO2 reduction in the BES with a mixed-culture cathode was at the mesophilic condition(35?).The coulombic efficiency of acetate formation varied in a range of 14.50±2.20%to 64.86 ±2.20%when temperature was improved from 10? to 70? with a maximum of 64.86±2.20%at 55?.EIS results indicated that temperature had a remarkable influence on both ohmic and Interfacial charge-transfer resistances of the biocathode.Microbial analysis demonstrated that temperature had a significant impact on the diversity of microbial communities of the biofilm and suspensions in the biocathode,especially for acetogenic bacteria selectivity.4.Two redox mediators(AQDS and NR)were modified on the carbon felt by an electrochemically deposition method,respectively,which was adopted to develop mixed-culture biocathode for CH4 production from CO2 reduction in the BES.The results indicated that the modified biocathdes significantly improved CH4 production from CO2 reduction.EIS results showed that two redox mediators modified electrodes could significant decrease the ohmic and charge resistances of the reactor,which could increase the electrons transfer rate during CH4 production process.Moreover,according to the results of SEM,CLSM and microbial analysis,it found that the amount of biomass on the redox mediators modified electrodes was higher than that on the control electrode,especially for hydrogenotrophic and acetotrophic methanogens.On the other hand,CuNPs@rGO was successfully fabricated to catalyze electrochemical reduction of CO2 to formate production in the BES.The results showed that the coulombic efficiency of formate generation was up to 62.11±8.1 6%,which means that the CuNPs@rGO catalyst had a great selectivity towards formate production from CO2 reduction in the BES.