Formation Mechanism And Environmental Effects Of Persistent Free Radicals On Photoaging Micro(Nano) Plastics

Micro（nano）plastic（MNPs）pollution is increasingly becoming a global environmental problem,and photoaging is considered to be the main way to affect the changes of molecular structure and physical and chemical properties of MNPs.The formation and stabilization mechanism of environmental persistent free radicals（EPFRs）produced by MNPs during this transformation are poorly understood.The photoaging of MNPs in different environmental media is usually affected by many factors.For example,the photoaging process of MNPs in aqueous phase is affected by organic or inorganic components with photochemical activity.The influence of highly active chemicals induced by EPFRs produced by MNPs in the aging process on the environmental behavior of coexisting organic pollutants cannot be ignored.At the same time,this kind of photoaging MNPs with a large amount of reactive oxygen species（ROS）may have toxic effects on biological cells.Therefore,a systematic study of the factors affecting the photoaging process of MNPs is of great significance for the comprehensive assessment of their environmental behavior and ecological risks.Based on this,this paper systematically studies the formation mechanism and influencing factors of EPFRs in the photoaging process of MNPs.The effects of active substances induced by EPFRs on the degradation of organic pollutants and their ecotoxicity on macrophages on MNPs were investigated.Main conclusions of above work are as follows:（1）The formation mechanism of EPFRs of two kinds of microplastics with different structures under simulated high altitude and coastal ultraviolet radiation environment was discussed,and the relationship between the formation of EPFRs and light energy was analyzed.Represented by polystyrene（PS）and polyethylene（PE）,MPs was irradiated by UVA,UVB and UVC ultraviolet light sources.The results showed that EPFRs was detected on PS with benzene ring irradiated by UVC and UVB and PE without benzene ring irradiated by UVC,which may be due to the energy difference of different wavelengths.It shows that the formation of EPFRs is not only related to the chemical structure of MPs,but also to the energy of ultraviolet light.Short-wave UVC is the main band that affects the formation of EPFRs on the surface of MPs.When MPs was irradiated by shortwave UVC,tertiary alkyl radical,peroxy radical and tertiary alkoxy radical were mainly formed on PS,and alkoxy radical and keto radical were mainly formed on PE.（2）The effect of co-existing components in water environment on the photoaging process of nano-plastics was studied,and the interaction between dissolved organic matter（DOM）and nano-plastics and how to affect the photo-induced ROS production of nano-plastics were discussed.The experimental results show that the addition of DOM can obviously inhibit the photoreactivity of PSNPs in water environment,and the inhibition mechanism is mainly light shielding,but there is quenching effect in addition.Compared with FA,HA has stronger light shielding ability,which may be due to its larger molecular weight and more aromatic benzene ring structure.When DOM and PSNPs coexist,the concentration of singlet oxygen（¹O₂）and superoxide radical(O₂^·-)is lower than that of PSNPs and DOM alone.This is the first time to analyze the reaction mechanism of the interaction between PSNPs and DOM from the point of view of energy transfer and electron transfer,and to explore how DOM affects the photochemical activity of PSNPs in aqueous phase.（3）The effects of EPFRs on microplastics on the environmental behavior of coexisting organic pollutants in the environment were studied.Taking PSMPs as representative plastic and sulfamethazine degradation（SMT）as target pollutant,the effect and mechanism of PSMPs on SMT degradation induced by light were studied by experiment and theoretical calculation.It was found that the EPFRs produced by PSMPs under sunlight could stimulate the production of ROS,which significantly increased the degradation rate of SMT.The mechanism of PS promoting the degradation of SMT is mainly due to the production of hydroxyl radical（·OH）in the system,and·OH is the main ROS species affecting the oxidative degradation of SMT.Combined with the net charge distribution and bond energy analysis of SMT,it is inferred that the sulfonamide group of SMT,the pyrimidine heterocyclic group and the amino group of aniline are the reaction sites of·OH priority attack.It is proposed that the oxidative degradation pathway of SMT mainly includes S-N bond cleavage,SMILES rearrangement and SO₂ removal.（4）In order to evaluate the ecological risk of ROS production induced by photoaging nanoplastics carrying EPFRs,the toxic effects of PSNPs modified with different sizes and functional groups on mouse mononuclear macrophages（RAW264.7）were investigated.The main purpose of this study was to consider the toxic effect of PSNPs itself on photogenerated ROS.50 nm-PSNPs was proved to be more cytotoxic than 100-nm PSNPs,and NH₂-PSNPs caused stronger cytotoxicity than uncoated PSNPs and COOH-PSNPs.This is because NH₂-PSNPs contains strong electron donor groups or the degradation of the surface coating produces more ROS.The cell viability of PSNPs in the presence or absence of free radical scavenger（NAC）was compared.It was found that the cell viability of PSNPs increased after the addition of NAC,which indicated that PSNPs could induce cell toxicity by producing ROS.To sum up,the formation of EPFRs on MNPs is not only related to its molecular structure,but also to the energy of ultraviolet light.In addition,this study also emphasized the important role of organic matter in the photoaging process of EPFRs,and deeply studied the effects of ROS induced by MNPs on the environmental behavior of organic pollutants,and the production of these active components further enhanced the cytotoxicity.In short,this work provides theoretical support for a comprehensive assessment of the environmental behavior,toxic effects and risk assessment of aging MNPs in nature.