Research On Microbial Electrochemical Systems For Disinfection,Nitrate Removal And Biosensor Of Toxic Carbon Monoxide Monitoring

An incredible development of microbial electrochemical systems and technologies has been made in past decades.Compared with traditional treatments,bioenergy shows the potential of overcoming the thermal dynamic,and simultaneously recovers energy or harvests value-added chemicals.It is a worthwhile technology as it meets the demand of reducing energy input for environment remediation.First,this thesis proposed an innovative Microbial Electro-Fenton System for water purification,combining Fenton Reaction and microbial electrolytic cell.Microbial electrolytic disinfection（MED）,taking E.coli as a pathogen model,mainly occurred through cathode reactions.Firstly,electrons produced from organic chemical oxidation were transferred to cathode via external circuit.Oxygen,as electron acceptor,could be reduced to hydrogen peroxide via imperfect reduction.This two-electron reduction process was enhanced and controlled by external electric bias.Ferrous ions were added into catholyte to trigger Fenton reaction and consequent chain reactions.Afterwards,E.coli were killed by reactive oxygen species（e.g.hydroxyl radicals）generated by Fenton reactions.The amounts of E.coli were dramatically reduced from 10⁷ CFU/m L to 660 CFU/m L.This study examined the effects of external electric bias,cathode aeration and Fe²⁺dosage.The results indicate 0.2 V was the optimal external voltage with-550 mV^-600 mV of cathode potential.The yield of hydrogen peroxide decreased no matter the voltage was either higher or lower than 0.2 V.The increase of external voltage would lead the increase of pH value.Similarly,the more severe cathode aeration,the higher pH,resulting in lower disinfection efficiency.Although stronger cathode aeration could enhance the oxygen reduction process,it should also be avoided as higher aeration rate would in turn increase the capital cost.Thus,29.8 mL/min of cathode aeration was set as the optimal condition.In terms of Fe²⁺addition as the―trigger‖of Fenton reaction.It was observed higher Fe²⁺leading to higher disinfection efficiency.However,side effect of disinfection was quite limited when Fe²⁺addition was higher than 0.3 mmol/L.The potential mechanisms were revealed by observation of cell formation via Field-Emission Scanning Electron Microscope.Fatal cell membrane destruction by·OH was identified as one potential mechanism of disinfection,evidenced by the holes and gaps on the cell membrane observed after MED.This damaged cellular construction would also excessively destroy the membrane-permeability and affect the intracellular chemical environment and physiological function.To my knowledge,there is no such research focusing on the Microbial Electro-Fenton System for disinfection.This study successfully testified the feasibility of bio-electro-Fenton process for pathogens inactivation,which offers insight for the future development of sustainable,efficient,and cost-effective biological water treatment technology.The second work of this thesis,was optimized the conditions of the biocathode denitrification in a microbial electrolytic cell.Experiments were conducted with pure-culture named Pseudomonas aeruginosa CP1.The strain was proved as denitrification bacteria in our previous work.Firstly,it was evidenced that constant direct current largely promoted both bacterial growth and denitrification efficiency.Denitrification efficiency with 10 mA of current was 50%higher than that obtained in open circuit,and no nitrite accumulation was observed.Differ from conventional denitrification process which required anaerobic condition,in this study,it was confirmed that the strain was able to denitrify in the presence of oxygen.Afterwards,Plackett-Burman design was employed for screening the important variables.C/N and current density was selected as most important factors via 13 batch experiments（R²=93.89%）.Further optimization was carried out by employing CCD-based Response Surface Methodology.A binary quadratic equation was obtained（R²=96.26%）.According to the equation,the theoretical maximum denitrification rate could be 21.37 mg（NO₃^--N）/（L·h）with 5.45 and 9.81 mA of optimum C/N and current density,respectively.Lastly,the CP1strain was inoculated into a lab-scale MEC.The denitrification efficiency was maintained at85%during 20 days operation time,and no nitrite in the outflow was detected.Admittedly,toxic gas nitric oxide was probably produced as intermediate during denitrification.In this part,P.aeruginosa CP1 was used to investigate the possibility of NO electrochemical reduction via a series of Cyclic Voltammetry（CV）.The reduction peak appeared at-820 mV,indicating the cathodic catalytic property of CP1.Further experiments had verified the CP1 could secrete soluble electron shuttles during the NO electrochemical reduction process as the peaks of NO reduction was in proportion of the square roots of scan rates,illustrating a typical―diffusion control‖reaction.It could be concluded that the CP1played a role in this process by producing e-shuttles by which the strain accepted electrons from the cathode,and then transferred the electrons to nitric oxide.Nitrous oxide was detected during the process which means nitric oxide was not directly reduced to nitrogen gas.The findings of this research were expected to provide a novel protocol as an alternative strategy for subsequent denitrification,expanding the field of wastewater and gas treatments.Biocatalyst is considered as a great alternative of noble metals because of its sustainable,costless and energy-saving property.It was the first time that proposed an electron pathway in the process of NO bio-electrochemical reduction.Other than the explorations above,this study also made efforts on the feasibility of toxic biosensor based on microbial fuel cell（MFC）for carbon monoxide monitor and forewarning.The toxicity of CO to some extent inhibited the electric generation of anode microbial community,leading to a slight voltage decrease.The level of voltage drop was related to the concentrations of CO.Learning from the results of experiments,a fine linear relationship was calculated in the CO concentration range of 10%to 70%（R²=0.9873）.Deep insight was obtained from further tests.It was found that 50 min to 1-hour sensing time was needed for credible monitoring.Furthermore,the higher level of CO concentration,the longer time it needed.To the best of my knowledge,it was the first attempt in the world to discover a MFC sensor for toxic gas monitoring.As discussed above,this research mainly focused on applications of microbial electrochemical technologies and systems,corresponding to different environmental issues.This proof of concept research is expected to explore an exciting new discipline at the nexus of microbiology and electrochemistry...