Photochemical Regulation Of Reactive Oxygen Species And Mechanism Research On The Removal Of Organic Pollutants

The advanced oxidation process based on photocatalysis is regarded as one of the most promising technologies in environmental pollution control and food safety assurance which can directly utilize the energy of sunlight to generate a large variety of reactive oxygen species（ROS）and degrade residual pollutants in the environment.These abundant ROS play an appropriately crucial role in the degradation,detoxification,and continuous mineralization of pollutants.Although many researchers have done plenty work on the photocatalytic regulation of ROS and the corresponding mechanism research,the efficiency of photo-induced ROS generation remains to be improved.In addition,photo-induced ROS generation is often conducted simultaneously in multiple ways,and the interaction and conversion between different types of ROS lead to the increased complexity of ROS photochemical regulation,which makes the photochemical precise regulation of ROS difficult and brings huge challenges to deep understanding the mechanisms of distinct ROS-mediated degradation of organic pollutants.Taking into account of this,it is of fundamental importance to develop photochemical techniques to improve the efficiency of molecular oxygen activation（MOA）and realize the comprehensive enhancement and precise regulation of ROS,which benefits understanding the relationship between ROS and the photocatalytic degradation mechanism.Uncovering the mechanism of ROS-mediated pollutant degradation and detoxification will assist to deal with environmental pollutants more scientifically,accurately,and efficiently,thereby ensuring food safety and improving the ecological environment.Therefore,this paper aims to comprehensively reveal the photochemical regulation mechanism of the free state,adsorbed state,and in-situ induction of ROS in the heterogeneous photocatalytic process on the molecular level,which enhance the high-efficiency photocatalytic degradation and deep mineralization detoxification of organic pollutants and put forward a novel strategy of ROS-involved pollutant control and environmental remediation.The main research contents are as follows:1.The enhancement of MOA favors the generation of ROS.A synthetic strategy for the induction of oxygen vacancies by wet-chemical method was developed to realize the mild introduction and controllable construction of oxygen vacancies（OVs）on the surface of Bi₂MoO₆photocatalysts,revealing the value of the surface oxygen vacancies in activating molecular oxygen and removing ciprofloxacin organic pollutants.Hydrogen bonds within tetramethylethylenediamine（TMEDA）were employed to connect with the proton hydrogen on the surface of Bi₂MoO₆,which weakened the bond energy of the Bi-O bond and made it easier for the connected oxygen atoms to escape under the reducing atmosphere of ethylene glycol.The mild and controllable introduction of oxygen vacancies by adjusting the amount of TMEDA is available.The formed oxygen vacancies can cause the electron localization and create defect energy levels to accelerate MOA and the production of more ROS such as·OH,·O₂^-and ¹O₂,thus accelerating defluorination and piperazine rings breaking and detoxification of ciprofloxacin.The photocatalytic degradation of ciprofloxacin reached above 90%in 2 hours and the biological toxicity of the degradation solution almost completely disappeared after illumination for 6 hours.2.The enhancement of MOA is also dependent on the carrier’s transfer on the interface.The separation and migration of the carriers via constructing the Schottky junction by loading metallic Bi on Bi₂MoO₆were achieved.Theoretical calculations and experimental results paraded that excessive localized electrons around metallic Bi enable to accelerate the MOA.Moreover,the strong electronic interaction existed between metallic Bi and（001）facet exposed Bi₂MoO₆material creats a fast electron transfer channel in form of[Bi₂O₂]²⁺→Bi→[MoO₄]^2-which accelerates the separation and migration of photogenerated electrons and MOA.The results of the photoelectric property experiment and ROS test demonstrated that the Bi₂MoO₆with metallic Bi loading on the（001）facet benefitted the prolonged lifetime of photogenerated electrons,the strong separation of electron-hole pairs,and the enhancement of ROS including·O₂^-and ¹O₂.The photocatalytic sodium pentachlorophenate（Na PCP）degradation performance of Bi₂MoO₆with metallic Bi loading on the（001）facet was nearly 2.9 times.The analysis of the degradation mechanism clarified that a large amount of·O₂^-and ¹O₂produced by the activation of molecular oxygen played a crucial role in the degradation process,which could promote the degradation and dechlorination process of Na PCP and the subsequent ring-opening mineralization,and finally achieved complete removal.This part of this work reveals the intrinsic mechanism of the crystal plane-dependent Bi deposition of Bi₂MoO₆materials to promote MOA.3.It is especially environmental significant to exactly regulate ROS and reveal different ROS-mediated degradation mechanisms of pollutants.The directional generation and precise regulation of photoinduced ROS were achieved by the strategy of the co-modification between oxygen vacancies and metallic Bi.Specifically,Bi loaded oxygen-deficient Bi₂MoO₆photocatalyst was fabricated by the solvothermal synthesis in one step.Characterization tests and theoretical calculations confirmed that oxygen vacancies and Bi cocatalyst could optimize the gap band of Bi₂MoO₆photocatalyst and provide the sites of MOA,which created an important basis for the precise regulation of ROS.The results of the active species test illustrated that the formation rate of·O₂^-and¹O₂on the modified catalyst was increased to two times of the original one,while the generation of interfering radicals（·OH）was completely inhibited.Subsequently,the·O₂^-involved hydroxylation dechlorination process and the ¹O₂induced direct dechlorination process were investigated by combining theoretical calculation and scavenger experiment.Finally,ECOSAR software was utilized to evaluate the toxicity of the dechlorination products and their subsequent main degradation intermediates.This work revealed the mechanism of·O₂^-and ¹O₂mediated dechlorination and detoxification process of Na PCP.4.Surface hydroxyl groups are important intermediate active species in MOA and water oxidation,but are often neglected due to their rarity and unclear transformation process.A novel strategy for surface hydroxyl modification by ammonia treatment was developed,and Bi₂MoO₆photocatalysts rich in surface terminated hydroxyl groups and bridging hydroxyl groups were successfully prepared,and the chemical roles of these surface hydroxyl groups on the pollutants degradation were investigated at the molecular level.Subsequently,the chemical configurations and properties of bridged hydroxyl groups and terminated hydroxyl groups on the surface of Bi₂MoO₆photocatalysts were determined by theoretical calculations and relative experiments.The synergistic mechanism of the bridged hydroxyl group and terminated hydroxyl group in the degradation of polychlorophenols was revealed by adsorption experiment and in-situ DRIFTS test.Specifically,bridged hydroxyl groups,as Lewis acids,enable to provide adsorption sites to promote the chlorophenol pollutants adsorbed on the interface of photocatalyst,while terminated hydroxyl groups acted as the reaction sites to accelerate the hydroxylation dechlorination and detoxification process.This work explored the generation and activation mechanism of surface hydroxyl groups and provided a novel way of photochemical degradation of water pollutants dominated by surface hydroxyl groups.5.Persistent free radicals（PFRs）appear in the process of pollutant migration and chemical transformation as new environmental intermediates.Due to the comparative migration ability and reactivity,PFRs become a factor that can be hardly ignored in the process of pollutant migration and transformation.Based on previous studies on the degradation mechanism of Na PCP,The in-situ generation and activation mechanism of PFRs during photochemical migration and transformation of Na PCP was investigaed.In-situ electron paramagnetic resonance spectroscopy（EPR）was employed to monitor the generation and conversion of PFRs during the photocatalytic degradation of Na PCP.Relative results show that Na PCP was firstly oxidized by holes and ROS to generate tetrachlorohydroquinone intermediate,and then further oxidized by holes to form tetrachlorosemiquinone radicals,which was stably adsorbed on the catalyst interface to become PFRs.Such PFRs are susceptible to acid-base environments,oxygen,sunlight,and carriers,and keep stable for hours in a constant environment.Meanwhile,free radicals-scavenger experiments illustrated that these in-situ generated PFRs also retain high reactivity and endowed the enhanced MOA to produce a variety of ROS including·OH,·O₂^-and ¹O₂,thus accelerating the degradation and deep mineralization of Na PCP.This work firstly explored the formation and migration of PFRs during the photochemical transformation of Na PCP and revealed the mechanism of PFRs induced ROS generation and photochemical deep mineralization of chlorophosphine pollutants.