The Aging Process Of Polystyrene Microplastics And Its Effect On Aqueous Phototransformation Of Cardiovascular Drugs

Microplastics（MPs）with wide occurrence in the environment become an emerging class of contaminants.Owing to the smaller size,MPs can affect the environmental fate of organic contaminants via adsorption,and induce additional toxicity to organisms.Ultraviolet（UV）irradiation is considered to be the most important process of the aging of polymers.During this period,MPs generate superoxide anion(O₂^·-),hydroxyl radical（·OH）and other reactive oxygen species（ROS）,which cause polymer chain scission and aging,potentially changing the physicochemical properties of MPs.In addition,pharmaceutical and personal care products（PPCPs）have been extensively detected in natural water,especially for cardiovascular drugs,long-term chronic exposure of which is expected to adversely affect aquatic organisms and even human beings.Photodegradation is known as an important abiotic transformation pathway of organic contaminants in the environment.Because of chromophore structures,PPCPs can undergo direct photolysis,and be subject to indirect phototransformation induced by ROS generated from photosensitizers.Considering the ROS produced by MPs in UV aging process and great opportunity to coexist with PPCPs in the same aquatic environment,it is highly possible that MPs will play important roles as photosensitizers in the photochemical process of organic contaminants,especially for aged MPs.Unfortunately,it remains unknown regarding the potential impacts of MPs on the photochemical transformation of organic pollutants in aquatic environment.In this paper,atorvastatin（ATV）and amlodipine（AML）,two types of widely used cardiovascular drugs were selected as typical organic contaminants,and polystyrene（PS）MPs with different aging degrees were obtained via UV aging process.Based on the aging effect of MPs on the adsorption behavior of the drugs,detailed mechanisms of MPs and its aging degrees in the photochemical transformation of organic contaminants in aqueous solution were explored.The main findings are as follows:（1）By using UV treatment to accelerate the aging process of PS MPs,the effects of aging process on the adsorption of ATV and AML on MPs in aquatic environment were evaluated.The results suggested that many more oxygen-containing functional groups,cracks and pores were generated on the surface of high-degree aged MPs,leading to the decreased particle sizes and increased specific surface area.By comparing the similar changes of MPs with natural aging process,500 W mercury lamp could be used to simulate the natural sunlight aging process of MPs in water,and the aging degrees of PS MPs could be characterized by carbonyl index（CI）.The adsorption kinetics of ATV and AML on MPs was well fitted to the pseudo-second-order kinetic model,and the adsorption capacity of PS for ATV decreased initially and then increased with the increase of aging degree,while AML adsorption continued to increase,suggesting different adsorption behaviors between the two drugs on MPs.As the smaller size of aged MPs and oxygen-containing functional groups generated on the surface might be the main factors changing the adsorption behavior and mechanism of MPs,the ATV adsorption on virgin and aged PS MPs relied on hydrophobic and hydrogen bonding,respectively,while AML adsorption on aged MPs was controlled by electrostatic interaction.（2）The phototransformation of ATV and AML mediated by MPs were studied,especially for the potential relationship between the aging degree of PS MPs and the photodegradation efficiency of drugs.The results showed that aqueous photolysis of ATV and AML under simulated sunlight was significantly facilitated by PS MPs,which was well-described by the pseudo-first-order kinetic model.As the aging degree increased（CI=0.07-0.61）,the photolysis rate constants of ATV and AML in aqueous solutions increased from 0.1701 and 0.0481 h^-1 to 0.3441 and 0.3747 h^-1,respectively,positively correlated to the aging degree of MPs.High MPs concentration or low solution p H was more conducive to the mediation of MPs for the photolysis of contaminants.Additionally,the removal rate of ambient concentration ATV under simulated sunlight was more significantly promoted by PS MPs,especially for the MPs with high aging degrees.Moreover,the aging process of MPs might also be facilitated because of the indirect photolysis of contaminants induced by MPs.（3）Different mechanisms of aged PS MPs in the photolysis of ATV and AML were revealed,and the effects of MPs on the profiles of ATV degradation products were also evaluated.ROS identification and quantification tests indicated that singlet oxygen（¹O₂）,triplet-excited state PS（³PS*）,·OH and O₂^·-generated from photosensitization of PS were contributed to the photodegradation of ATV,especially ¹O₂.Additionally,the role of ³PS*became more important in the photodegradation mediated by higher degree aged MPs because much more ¹O₂ was generated from ³PS*,while the contribution of·OH was reduced.The results implied that the indirect photolysis mechanism of ATV induced by PS was different from that of direct one,and the difference also existed between different aged MPs.Meanwhile,adsorption was one of the factors affecting the photolysis of contaminants,but not the dominant one.PS-mediated indirect photolysis of AML mainly occurred on the solid surface of MPs,enhancing its photolysis with the adsorption.Differently,ATV photolysis reacted both on solid and liquid phase.To a certain extent,the adsorption effect on ATV photolysis showed a trend of initial inhibition and later promotion with the increased aging degree.Moreover,PS MPs could affect the types and yields of ATV products by changing its photodegradation pathways,especially for aged ones.