UV-based Aging Of Microplastics And Its Adsorption Performance For Phosphorus In Water Processes

Plastics are widely used around the world.After being used,a large amount of plastics are discharged into the environment.Plastics are transformed into microplastics（MPs）by the action of water,wind,gravel,etc.Microplastics have become an emerging pollutant due to their stable physical and chemical properties and their ability to exist in the water environment for a long time.After entering the aqueous environment,microplastics experience aging effects of various environmental factors,which cause changes in their properties,such as specific surface area and functional groups,and thus change the behavior of microplastics in water environment.Among the various factors,Ultraviolet light（UV）has the highest contribution to aging,and various ions also influence the aging process.Microplastics with large specific surface area,high migration capacity and hydrophobicity can be used as carriers to adsorb pollutants on the surface,thus changing the behavior of pollutants in the environment.After undergoing the aging process,the physicochemical properties of the microplastics change,which in turn changes the adsorption capacity of the contaminants.Therefore,in this thesis,three wavelengths of UVA,UVB and UVC and the corresponding UV/PS systems were used as aging methods to compare and study the aging effects of different systems on PE.We also investigate the effects of anions and natural organic matter（NOM）on aging,elucidate the adsorption mechanism of microplastics on phosphorus after aging,in order to provide a theoretical basis for an in-depth study of the aging mechanism of microplastics and the inter-relationship between microplastics and pollutants.The main conclusions are as follows.（1）PE microplastics showed increased roughness,cracks,and oxygen-containing functional groups after aging at three wavelengths of UV.FTIR and XPS patterns showed that oxygen absorption peaks appeared on the surface of aged PE,with carbonyl index（CI）increasing from 0.010 to 0.013（UVA）,0.021（UVB）,and 0.016（UVC）,respectively,and O/C increasing from 0.022 to 0.025,0.023 and 0.032.In terms of the combined results,among the three bands,UVC showed the highest aging of PE microplastics,and C=O was detected in XPS,while the other two bands did not have C=O absorption peaks.（2）The aging of PE microplastics by UV coupled with PS process（AOPs）revealed that the action of free radicals such as SO₄^·-,·OH,O₂^·-generated by UV activated PS and UV light were the key to PE aging.Compared with the action of UV alone,UV/PS damaged PE more deeply.Before aging,the PE surface was smooth with a small amount of cracks.With the increase of PS concentration,the PE surface showed cavities and collapse,and more serious flocculent layer and fragmentation.The specific surface area of aged PE also became larger,and when the PS concentration was 50 m M,the specific surface area increased 1.41-8.28 times.The analysis of chemical groups on the surface of aged PE revealed that four oxygen-containing functional groups,C—O,—OH,C=O,and O—C=O,appeared after aging,and the O/C increased accordingly from 0.022 to 0.041（UVA）,0.032（UVB）,and 0.045（UVC）.The aging degree was ranked as UVC>UVA>UVB.The composition of free radicals in each reaction system was examined by EPR technique,and four free radicals,SO₄^·-,·OH,O₂^·-and ¹O₂,were generated in all three systems,and the content of free radicals is related to the penetrating ability of UV light（UVA>UVB>UVC）,and the photon energy of UV light is UVC>UVB>UVA,therefore,the aging of PE by UV/PS is the result of the joint action of free radicals and UV light,and the aging of UV light is dominant.（3）The common ions and NOM in water environment had a great influence on the aging process of PE.The addition of Cl^-inhibited the aging process of both UVB and UVC systems,and the higher the concentration of Cl^-,the more obvious the inhibition effect.In the UVA/PS system,due to the activation of the generated Cl·,Cl₂^·-,the Cl^-promoted the aging process at low concentration,while the increasing concentration showed a tendency to hinder it.CO₃^2-hampered the aging of PE,and the mechanism of inhibition was the quenching of strong oxidizing radicals by CO₃^2-,which generated CO₃^·-,the redox potential of CO₃^·-was much lower than that of SO₄^·-,·OH.The influence of NOM on the aging process was divided into two aspects:1)the UV wavelength was 200-290 nm,FA had better absorption for short-wave particles,meanwhile,its internal chromophores caused light shielding effect,which hindered the aging of PE microplastics;2)when the wavelength was more than 290 nm,FA,as a photosensitizer,absorbed light and promoted the generation of ROS.By EPR analysis,the aging effect of NOM to enhance UVA and UVB systems was attributed to its promotion of·OH and SO₄^·-,and the number of free radicals in the UVC system did not fluctuate greatly,so it could be judged as the light shielding effect of NOM.（4）The changes in the physicochemical properties of the PE surface caused by aging led to a decrease in its adsorption performance on phosphorus（H₂PO₄^-）.The results of the adsorption kinetic fitting showed that the fit of the pseudo-secondary kinetic model was high,indicating that the adsorption processes of PE before and after aging were relatively complex and dominated by chemisorption.The adsorption capacity of phosphorus onto PE after aging of UVC/PS was the smallest,followed by UVA/PS and finally UVB/PS.It can be concluded that the higher the degree of aging,the weaker the adsorption capacity of PE.Langmuir model well described the isothermal adsorption process（R²>0.992）,so that the adsorption of H₂PO₄^-on PE was a monolayer chemically nonlinear process.Combining the aging characteristics as well as the adsorption properties,it was found that hydrogen bonding and electrostatic force were the main forces for H₂PO₄^-adsorption by PE.In addition,the adsorption was maximum at pH=3 and much greater than other pH conditions.The negative correlation between pH and adsorption was mainly attributed to the difference of charged condition of PE surface and H₂PO₄^-at each pH condition.