Study On Mechanism Of Microbial Fuel Cell Combined With Electrocatalysis For Treatment Of Shale Gas Development Wastewater

The occurrence state of shale gas and the characteristics of the low permeability of gas reservoirs determine the use of drilling and hydraulic fracturing technology in the production process.However,in the development process of shale gas,a lot of shale gas fracturing wastewater,oil-bearing sludge and cuttings will be generated.The fracturing wastewater contains complex components such as high salt(including Cl^-and SO₄^2-),refractory complex organic matter and heavy metals.In addition to the above characteristics,oil-based drill sludge and cuttings also have high oil esters,and dissolved volatile and semi-volatile complex organic compounds（such as polycyclic aromatic hydrocarbons）.It is a serious threat to the ecological environment and personal safety.Microbial fuel cell（MFC）and electrocatalytic oxidation have received wide attention in the field of biodegradable organic wastewater,and the electric energy generated by the former can provide energy for other systems（microbial electrolytic cell,membrane bioreactor,etc.）.Electrochemical oxidation technology has the advantages of no secondary pollution,rapid and thorough reaction and environmental friendliness.Based on this,this paper proposes to prepare high-performance carbon composite materials and Ni alloy electrodes,and apply them to MFC for treatment of shale gas development wastewater（general term for flowback wastewater produced during the extraction process and waste oil-based sludge produced during the drilling process）,and use the energy recovered from MFC as the driving power of electrolysis method.The mechanism of MFC and electrocatalytic degradation of shale gas well development waste and the mechanism of MFC-EC coupling treatment of development waste were explored through the characteristics of electricity production,the changes of main pollutants（chemical oxygen demand（COD）,inorganic nitrogen,conductivity and p H）in the flowback waste/sludge and the changes of microbial community structure.The specific research content and main conclusions are as follows:（1）Preparation and electrochemical performance of novel biochar and rare-earth doped Ni-Mo-P alloy electrodesOne of the keys to improving electrocatalytic wastewater treatment efficiency is preparing high-performance anode and cathode.Oil-based drilling sludge（OBDS）and oil-based drill cuttings（OBDC）were converted into biochar materials by an environmentally friendly low-energy consumption method（carbonization followed by activation）,and the effect of hydrofluoric acid acidification was further studied.The results of biochar characterization show that the biochar composite prepared with sludge and biochar as the precursor has a large specific surface area and a high graphitization degree.Si O₂ in the composites plays an important role in supporting the structureA new Ni-Mo-P composite electrode modified with rare earth elements（light rare earth Nd and heavy rare earth Yb）was prepared.The effects of the current density of DC electrodeposition,the doping ratio of Yb and Nd,and the number of cyclic voltammetry（CVD）cycles on the electrode structure and electrochemical properties were investigated.The results showed that the electrode has the maximum performance and significant stability when the molar ratio of Yb:Nd=8:2 and 20 cycles number under CVD conditions（the electrode was named Ni-Mo-P-Yb8-Nd2（CVD））.The electrochemical properties and characterization results show that there is a good synergistic effect between rare earth elements（Yb,Nd）and Ni-Mo-P alloy.（2）Process optimization and mechanism of EC degradation shale gas development wastewaterUsing OSC as anode and Ni-Mo-P-Yb8-Nd2（CVD）as cathode,the reaction conditions of electrocatalytic treatment of shale gas fracturing flow-back（SGFW）fluid were optimized.The optimal conditions for electrocatalytic reaction were determined as follows:current density 60 m A/cm²,p H 9,Na Cl concentration 10 g/L.The COD removal rates of SGFW and OBDS can reach more than 87%and 97%,and the first-order reaction was confirmed by the kinetic study.The degradation effect of aliphatic hydrocarbons may be significantly better than that of aromatic hydrocarbons,and no long-chain intermediates were produced after the reaction.（3）The characteristics of shale gas flowback wastewater and OBDS treated by MFCThe effects of aged landfill leachate with different volume ratios and conventional aerobic sludge as cathode solutions on the treatment of SGFW by MFC were studied.The removal rate and COD of 25%V/V aged landfill leachate for anodic liquid and cathode liquid are 61.9±2.4%and 60.3±0.8%,respectively,and the maximum power density is 973.8 m W/m².The microbial community structure indicated that aging landfill leachate doping contributed to the selective enrichment of cathode biofilms for sustainable power generation and nitrifying bacteria.The bio-power generation of MFC based on the solid-liquid cooperative treatment of OBDS was studied by adjusting the mixing rate and adding OBDC.The results show that the output maximum power density is 671 m W/m²when the stirring rate is 100 rpm and the concentration of OBDC in the anode chamber is 2 g/L,which is 2.4 times higher than that of the control group.Electrochemical measurements and high-throughput sequencing results showed that the mixing rate combined with OBDC could activate microbial activity and reduce the overpotential loss of the anode system.Thus,the electrogenic bacteria with extracellular electron transport function（such as Proteobacteria,Bacteroidetes and Actinobacteria）and denitrifying bacteria（such as Bacilli,Anaerolineae and Rhodopseudomonas）were enriched.（4）Construction and operation mechanism of driving MFCs-EC coupling systemBased on the optimization conditions of MFC and EC,several microbial fuel cells（MFCs）and EC coupling systems are constructed,which can not only make full use of the electric energy generated by MFC but also realize the disposal of shale gas development waste through the coupling effect of zero energy consumption.The electrochemical characteristics of series and parallel MFCs are outstanding.The coupling system and substrate replacement significantly improved the electrical performance and the treatment effect of organic matter.In series mode,the electrode inversion is caused by substrate deficiency and the energy differences between batteries.The parallel mode shows good electrocatalytic activity,which can drive reversible redox reactions of EC.The power density in the parallel mode is the highest（4.98 W/m²,which is higher than the cumulative value of MFC applied to shale gas development wastewater alone）and there is almost no inversion phenomenon.The COD removal efficiency of OBDS and SGFW can reach 76%and 80%after the sequence treatment of MFC and EC.The diversity of microbial community was significantly increased compared with monotonic MFC,and hydrolytic bacteria from SGFW and OBDS and electrogenic bacteria from inoculants were mutually symbiotic in the system under weakly alkaline conditions,which made an important contribution to the expansion of organic matter degradation pathways and the operation efficiency of the coupled system.The degradation process of OBDS showed that aromatic hydrocarbons and straight-chain compounds were basically removed and no other intermediates were produced.