Degradation Of Organic Pollutants In Shale Gas Fracturing Flowback Fluid By Cavitation Oxidation Of Micro-Nano Bubbles

As a clean and low-carbon unconventional natural gas strategic resource,China’s shale gas resources rank first in the world in terms of recoverable reserves,with huge development potential.During the development of shale gas,due to the special characteristics of its storage,it will produce fracturing flowback fluid with complex organic composition,high total salinity and difficult treatment,which is of high environmental risk.The treatment methods for organic pollutants in shale gas fracturing flowback fluid mainly include coagulation,biological methods and advanced oxidation methods,among which advanced oxidation methods are more efficient in degrading organic pollutants.Cavitation oxidation technology is an environmentally friendly advanced oxidation technology that generates micro-nano bubbles which have shown great application prospects in the field of water treatment due to their advantages such as small size,large specific surface area,and strong oxygen carrying capacity.Firstly,the characteristics of the micro-nano bubbles and the generation law of·OH produced by the self-developed micro-nano bubble cavitation oxidation reactor were studied.The size and Zeta potential of micro-nano bubbles are analyzed,and it was found that the addition of surfactant and inorganic salt further reduced the size of micro-nano bubbles.The effect of surfactant,p H,and inorganic salt on the oxygen-carrying capacity of micro-nano bubbles was analyzed.The effects of operating parameters of the micro-nano bubble cavitation oxidation reactor on the formation of·OH during the cavitation oxidation process were investigated.Secondly,hydroquinone（HQ）was selected as a typical pollutant in the simulated water sample of shale gas fracturing backflow fluid and treated by micro-nano bubble cavitation oxidation reactor.The effects of inlet pressure,gas flow rate,p H,and initial concentration on HQ degradation efficiency were investigated and the process parameters were optimized.The experimental results showed that the HQ degradation efficiency reaches 91.25%under the optimized process parameters.The results of free radical capture experiments indicate that·OH is the main active oxygen radical produced in the micro-nano bubble cavitation oxidation system.The possible intermediate products were analyzed by GC-MS during the degradation process of HQ,and the degradation path of HQ was speculated.·OH will first attack the H on the phenolic hydroxyl group of HQ,causing it to undergo dehydrogenation and change to benzoquinone.Under the action of high-temperature pyrolysis and free radicals,benzoquinone undergoes ring opening,demethylation,decarbonylation to intermediate products such asα-ketoglutaric acid,maleic acid,and eventually conversion to CO₂ and H₂O.Density functional theory calculation indicates that when the reaction energy barrier for the capture of H from the phenolic hydroxyl group by·OH is much smaller than the reaction energy barrier for the benzene ring,the reaction is easier to proceed.When·OH is added to the benzene ring C,there is no significant difference in the reaction energy barrier.Finally,a micro-nano bubble cavitation oxidation reactor in conjunction with persulfate oxidation was used to treat shale gas fracturing flowback fluid,and the degradation performance of the micro-nano bubbles combined with the persulfate oxidation system was investigated experimentally.The experimental results show that under the conditions of a reaction time of 120 min,the inlet pressure of 0.2 MPa,p H=3,and the PS dosage of 2.39 m M,the COD removal efficiency of fracturing backflow fluid reaches 82.45%.The results of the kinetic study show that the COD removal is following the pseudo-first-order reaction kinetics.The factors influencing the COD removal efficiency were in the following order:PS dosage>p H>inlet pressure and the interaction between various factors was significant.The micro-nano bubble synergistic persulfate oxidation degradation system has a good effect on the oxidation degradation of acid esters and benzenes in the actual water samples of shale gas fracturing backflow fluid,while the effect on long-chain alkanes in general.The thesis has 57 figures,20 tables,and 122 references.