Theoretical Study On The Heterogeneous Formation Mechanism Of Environmentally Persistent Free Radicals In Atmospheric Particulate Matter And Advanced Oxidation

Environmental persistent free radicals（EPFRs）have become ubiquitous pollutants of great concern due to their long lifetime and unique toxicity.EPFRs in atmospheric particulate matter originate from a variety of heterogeneous catalytic processes,and the interaction of different components of particulate matter with aromatic molecules in combustion systems is one of the most important processes.However,the heterogeneous formation mechanism of EPFRs on particulate matter is not fully understood and appreciated.EPFRs themselves and their precursors can also form more harmful persistent organic pollutants.How to degrade or remove EPFRs-related pollutants has also become a critical issue.In this study,the heterogeneous reactions of aromatic compounds on the surfaces of common catalysts such as metal oxides and carbonaceous catalysts were investigated theoretically through first-principles computational methods.Computational simulations were performed to obtain detailed information on molecular adsorption configurations,surface active sites,which are harder to obtain in experiments.The possible dissociative adsorption reactions of aromatic molecules were investigated to explore the possible reaction pathways for their formation of EPFRs.The effects of common co-existing substances in the atmosphere on the EPFRs formation were investigated by adding ambient water and oxygen to the system.The differences in the mechanism of EPFRs formation on different catalyst surfaces were further investigated by electronic structure analysis.The non-homogeneous degradation mechanism of persistent radicals and their precursors in the liquid-phase system of advanced oxidation was investigated accordingly by incorporating an implicit solvent model.This study provides a theoretical basis for the heterogeneous formation of EPFRs in the atmosphere and provides a scientific reference for the control of EPFRs-related pollutants.The main contents and conclusions of the study include the following four aspects:1.Study on the EPFRs formation mechanism from phenol on γ-Al₂O₃（110）The heterogeneous interfacial reaction on metal oxide surfaces in combustion and thermal processes is the crucial source of environmentally persistent free radicals（EPFRs）.Al₂O₃ is one of the most abundant metal oxides in fine particulate matter（PM）encountered in combustion systems.In the present work,the detailed formation mechanism of EPFRs from phenol on theγ-Al₂O₃（110）surface with different hydration levels was investigated by periodic theoretical calculations.The results show that γ-Al₂O₃ has stronger catalytic activity for the formation of EPFRs compared with other metal oxides.Furthermore,the heterogeneity of the γ-Al₂O₃（110）surface has an impact on the formation energy barrier of EPFRs,even on the reaction route itself.This work is expected to foster a crucial re-examination and integration of conventional EPFRs formation mechanism already reported in the literature,providing a novel insights into the heterogeneous generation of atmospheric PM-associated EPFRs.2.Mechanism of EPFRs formation from 2-CP on ZnO surface and the effect of O₂ mediationPrevious studies have identified the mechanism of EPFRs formation on various transition metal oxide surfaces;however,there are many remaining doubts regarding the formation mechanism of EPFRs on ZnO.Despite their enduring stability,the formation and properties of EPFRs are influenced by various external factors,among which oxygen plays an important role in controlling the formation and stability of EPFRs.However,the exact mechanism by which oxygen affects EPFRs formation is not well understood.In this study,the direct formation mechanism of EPFRs on the pristine and defected ZnO surfaces was investigated by theoretical calculations.The indirect and direct effects of oxygen on the formation pathways of EPFRs on ZnO were examined by adding surface coexisting oxygen molecules to the system.Finally,we used charge population analysis to investigate the unique electron transfer mechanism for the EPFRs formation on ZnO.This study provides new insights into the formation mechanisms of EPFRs on metal oxide surfaces and is critical for understanding the potential environmental impacts associated with EPFRs.3.Study on the EPFRs formation mechanism from 2-CP on GOCarbonaceous components of atmospheric PM have recently aroused intensive interest,due to their critical role in the formation of environmentally persistent free radicals（EPFRs）.This study focuses on the heterogeneous formation of EPFRs on the graphene oxide（GO）,the dominant component of carbonaceous PM.First-principles calculations have been performed to investigate the interaction between 2-chlorophenol（2-CP）,an important precursor of EPFRs,and GO to explore the decomposition route of 2-CP to generate different types of EPFRs on GO.The results indicate that aromatic precursors tend to undergo homolytic fission rather than heterolytic splitting to form EPFRs induced by surface oxygen-containing groups and carbon defects on the carbonaceous PM.The formation mechanism of EPFRs over GO unveiled in this study,which represents dramatic differences from the conventional mechanism on the metal oxide surfaces,may account for a previously overlooked source of EPFRs encountered in atmospheric PM.4.Preliminary degradation mechanism of 2-CP in PDS-carbon nanotube（CNT）advanced oxidation systemCarbon-based persulfate（PS）advanced oxidation（PS-AOPs）have been increasingly favored for the removal of phenolic pollutants in wastewater.However,there are still many debates and questions about the underlying mechanisms of PS-AOPs on the surface of carbonaceous catalysts dominated by non-radical pathways.Theoretical calculations can help overcome the limitations of experimental methods and systematically elucidate the mechanism of non-radical oxidation of PS-AOPs driven by carbonaceous catalysts.In this context,the present study investigates the catalytic degradation mechanism of 2-CP in a PS-carbon nanotube advanced oxidation system using first-principles computational methods.The initial degradation intermediates of this organic pollutant are typical EPFRs,which can further influence or even participate in the catalytic degradation process as key intermediates.It is shown that the electron transfer mechanism from the pollutants to the complex formed on the PS-CNT surface via conductor shuttle occupies a significant position in this oxidation system.The introduction of more C=O and surface defects in the carbonaceous material-based PSAOPs system play a key role in the removal of organic pollutants.