The Formation Mechanism Of Environmentally Persistent Free Radicals And Reactive Oxygen/Nitrogen Species In Photoaging Microplastics And Their Potential Toxicity

Currently,microplastics(MPs)have become a class of pollutants that are ubiquitous and continuously produced in oceans,terrestrial water bodies,soils and atmospheric media.Due to thier small size and wide distribution,MPs can be directly or indirectly ingested and swallowed by aquatic organisms,terrestrial organisms,and even humans,and accumulate in organs or tissues,which can cause direct physical damage to organisms.The release of toxic plasticizers(such as phthalates),additives(such as bisphenol A and polybrominated diphenyl ethers,etc.),and monomer substances in plastics can interfere with the nervous system,affect reproductive development,or induce genetic aberrations.In addition,MPs in the environment can absorb a variety of heavy metals and organic pollutants,transfer them to the organism,causing more serious multiple toxicity.Therefore,the study on the environmental behavior and ecological risks of MPs has become the research focus in the environmental fieldDue to the long-term exposure to the environment,MPs can be further fragmented by the sunlight irradiation,biodegradation,water power and wind,and decompose into smaller particles,leading to the significant changes of their morphology,structure and properties,which ultimately influence the environmental behavior and ecological risks of MPs.The current ecological risks of aging microplastics are mainly focused on their ability to carry and adsorb pollutants.However,there is a lack of in-depth understanding of the toxic effects caused by changes in the molecular structure and physical and chemical properties of microplastics.In particular,environmental risk substances that are newly generated during the aging process of MPs and are different from the parent molecules may be an important factor that has been overlooked.Therefore,a new type of pollutant,namely environmentally persistent free radicals(EPFRs),which may be formed on the photoaging MPs,was studied in this study.The formation process of EPFRs in the photoaging MPs,the generation mechanism of active species and the change of their oxidative potential(OP)were investigated.Moreover,the cytotoxicity results were used to clarify the potential mechanism of the toxicity induced by photoaging MPs.This provides effective information for the accurate assessing the environmental risks of MPs.The main research contents and results of this paper are as follows:(1)The potential formation of EPFRs on four kinds of hydrocarbon MPs in the environment,including polystyrene(PS),phenol-formaldehyde resin(PF),polyethylene(PE),and polyvinyl chloride(PVC),was explored under simulating sunlight.Surprisingly,several EPFRs,which are considered to be a type of emerging contaminants,were detected on the irradiated PS and PF,rather than PE and PVC by electron paramagnetic resonance(EPR)spectroscopy.This had an important relationship with the benzene ring structure of MPs.Depending on the photo-aging duration time,the characteristic g-factors of the EPFRs produced on PS and PF were 2.0044-2.0049 and 2.0043-2.0044,respectively.The generated EPFRs on PS and PF decayed rapidly at the initial stage,and then slowly disappeared with the elapsed aging time.The half-life of EPFRs on PS and PF was 13.33 h and 4.86 h,respectively.Analyses by attenuated total reflectance-fourier transform infrared spectroscopy(ATR-FTIR),X-ray photoelectron spectroscopy(XPS),nuclear magnetic resonance(NMR)and gel permeation chromatography(GPC)suggested that the MPs might experience chemical chain scission,O2/H2O addition,free radicals generation and EPFR formation under the light irradiation.Accompanying with the formation of EPFRs,reactive oxygen species,such as O2·-and ·OH,were also observed.(2)This study investigated the formation mechanism of reactive oxygen species(ROS)by simulated solar light irradiated MPs and their subsequent roles in the phototransformation of MP.In this study,the photodegradation of MPs,utilizing polystyrene microplastic(PS-MP)as a model,was investigated under irradiation with simulated solar light for as long as 150 d.A large amount of reactive oxygen species(ROS),including O2·-,1O2,H2O2 and·OH,were detected in the PS-MP suspension by spin trapping and chemical probe methods,which displayed significant relationships with the generated EPFRs.Distinct photoaging of PS-MP was observed with increased surface roughness and decreased particle size.However,these photoaging effects were significantly inhibited by ROS quenchers,suggesting that the generation ROS played a vital role in the PS-MP phototransformation.In addition,ROS induced formation of more oxidative functional groups on the PS-MP,thus enhancing the negative surface potential and the stability of PS-MP in water.This study elucidated the mechanism of formation of ROS by simulated solar light irradiated MPs and their subsequent roles in the phototransformation of MP,thus expanding current knowledge on the fate of MPs in aquatic environments.(3)The formation of EPFRs and reactive oxygen/nitrogen species(ROS/RNS)of nitrogen-containing microplastics(N-MPs)under light was studied.For the first time,we found that persistent aminooxy radicals(PAOR)and reactive nitrogen species(RNS)were formed on nitrogen-containing microplastics(N-MPs)under the light irradiation under light.After photoaging,an anisotropic triplet with g-factor of?2.0044,corresponding to PAOR,was detected on polyamide(PA)without benzene ring rather than amino resin(AmR)by electron paramagnetic resonance and confirmed by density functional theory calculations.The generated amine oxide portions on the photoaged PA,including hydroxyamine and nitrone,etc,were identified using pyrolysis-gas chromatography/mass spectrometry,which was considered to be the main structural basis/precursors of PAOR.Surprisingly,reactive nitrogen species(RNS)were also observed on the irradiated PA.The generated ·NO due to photolysis of nitrone groups simultaneously reacted with O2 and O2·-to yield NO2 and peroxynitrite,respectively,which were responsible for peroxyacyl nitrates(PAN)and CO3·-formation.(4)In order to evaluate the environmental risk of photoaging MPs,the effects of photoaging on the OP and cytotoxicity of MPs were investigated.No matter PF-MP,or the nitrogen-containing PA-MP,the OP levels increased with the irradiation time.The contents of the produced conjugated carbonyls,ROS and PF-bound EPFRs due to light irradiation increased as well,and displayed significant correlations with the OP(Spearman r>0.6,p<0.05).Besides,significantly higher oxidative potential(OP)and reductive potential(RP)were observed for the aged PA than AmR,attributing to the abundant RNS,organic hydroperoxides and PAN,and strong ability to transfer electrons from PAOR,respectively.The photoaged PF-MP distinctly increased the cellular ROS and reduced the cell viability of human lung epithelial adenocarcinoma cells(A549).The cytotoxicity of PF-MP showed a similar trend with the OP level in PF-MP,suggesting that the produced active species induced the in vitro toxicities.These results highlighted the adverse health effects of photoaging MP.In summary,the simulated sunlinght induced the formation of EPFRs on MPs,and the generation of EPFRs is significantly related to the molecular structure of MPs.The EPFRs are more likely to form on MPs with the benzene ring structure.The elemental composition of MPs also affects the type of EPFRs.Along with the generation of EPFRs,the various RS are also detected on photoaging MPs,including ROS and RNS,which are important related to the EPFRs of microplastics,as well as emerging functional groups and compounds after photoaging.In addition,the production of ROS during photoaging plays an important role in the phototransformation of MP in water.These active components further lead to the enhancement of OP and cytotoxicity of photoaging MP.In short,this work provides a new perspective on the risk assessment of MPs in the environment.