Study On The Accelerated Aging Of Microplastics And The Interaction Of Aged Microplastics With Pharmaceuticals And Iron Red Pigment

Since the first proposal in 2004,microplastics（MPs）have drawn increasing attention in recent decades due to their small size and high abundance in the environment.MPs can adsorb organic pollutants from the surroundings,and affect their environmental fate.Moreover,MPs adsorbing pollutants may be ingested by organisms,and additional toxicity can be induced.In the environment,MPs are subject to various weathering processes including ultraviolet（UV）and heat radiation,physical abrasion and chemical oxidation,etc.These processes potentially change the physicochemical properties of MPs and cause the polymer chain scission and oxidation,following the release of organic products into the surroundings.Various types of additives would be incorporated into commercial plastics to achieve the desired properties for the intended use.These chemicals are likely to be released into the environment during weathering processes of MPs because most of the additives are physically mixed with polymers.In addition,aging process can affect the interaction between MPs and organic chemicals,and thus affect the adsorption and desorption behaviors of MPs.However,there is little information regarding the effects of weathering process on the surface properties and the release of MP-derived products and additives,the adsorption and desorption toward environmental pollutants.In addition,current problem is much too low weathering rate of MPs in the environment,which greatly restricts the understanding of long-term weathering behavior and environmental fate of MPs.Therefore,it is necessary to seek for laboratory technologies that can simulate and accelerate the weathering process of MPs in the environment.Based on above considerations,the paper evaluated the feasibility of laboratory technologies for simulating and accelerating the aging process of MPs by comparing the changes in physicochemical properties between advanced oxidation processes（AOPs）（Fenton,heat-activated K₂S₂O₈ and photo-Fenton）and natural weathering processes,and investigated the changes in physicochemical properties and the dynamic evolution of organic compounds released from aging process.Then,we explored the release kinetics of iron red pigment during light irradiation of pigmented polyethylene（PE）and polypropylene（PP）MPs and revealed the potential effects of iron red pigments on the aging behavior of MPs.In addition,we studied the adsorption mechanisms between pristine and aged polystyrene（PS）MPs toward pharmaceuticals,and probed the critical role of aging compounds in adsorption process.The desorption mechanisms of pharmaceuticals from pristine and aged PS MPs under simulated seawater,gastric and intestinal conditions of marine organisms were investigated.The main findings are following:（1）We firstly evaluated the feasibility of AOPs（Fenton,heat-activated K₂S₂O₈and photo-Fenton）to simulate and accelerate the aging process of MPs,and investigated the physicochemical properties and the structures of organic compounds released from aging process.By comparing the surface morphology and chemical compositions of MPs between AOPs and natural weathering processes,the AOPs could be used as an alternative to simulate and accelerate the aging process of MPs.The correlation models of aging properties of MPs,i.e.carbonyl index（CI）and oxygen-to-carbon（O/C）ratio over exposure time were developed.For PE MPs,the linear and exponential correlations were observed for CI and O/C versus aging time,respectively,while for PS MPs,the exponential correlations were observed,indicating that combining CI and O/C could synthetically quantify the surface aging degree of MPs.The model is the first attempt to clarify the relationships between aging time and surface properties of MPs,which is helpful for understanding MP pollution more holistically.TOC,excitation-emission matrices and HPLC/MS analyses of filtrates from the aging process of MPs showed that the organic molecules containing benzene rings were released from MP aging,but with the extended aging period,the benzene rings might be opened and the organic products might be re-adsorbed on MPs.These organic molecules containing benzene ring are mainly as hydroxylated and carboxylated organic molecules.（2）We further investigated the release kinetics of iron red（Fe₂O₃）pigment during aging process of iron red incorporated PE and PP MPs and the potential effects of iron red pigments on the aging behavior of MPs.Characterization of PE,PE-Fe₂O₃,PP,and PP-Fe₂O₃ at different irradiation time showed that the presence of iron red pigment has little effect on the aging rate of PE MPs,but significantly reduced the aging rate of PP MPs.Additionally,TOC analysis of the filtrates from aging process showed that almost no organic product was released from PE and PE-Fe₂O₃ MPs due to the lower aging degree,while high and similar amounts of aging products were produced from both PP and PP-Fe₂O₃ MPs.Aging process resulted in the release of dissolved iron ions from pigmented PE and PP MPs,but the different types of iron ions were formed among PE-ions,while Fe²⁺was the main form for PP-Fe₂O₃ MPs under irradiation,indicating the different role of iron red pigment in photo-aging process between PE and PP MPs.By measuring the filtrate p H before and after light irradiation and analyzing the iron ion species from the dark condition of pigmented MPs and from the sole Fe₂O₃ under light and dark systems,we speculated that the inhibitory effect of iron red pigments on the photoaging rate of PP MPs was mainly ascribed to the light shielding effect and the competitions of Fe₂O₃ for the electrons or ROS produced from polymer photo-chemical reactions.The study provides important information for understanding the aging property and environmental behavior of MPs in aquatic environments,which are helpful for comprehensively evaluating the ecological risks of co-existed MPs and organic pollutants（3）In addition,we investigated the adsorption behaviors between pristine and aged MPs toward pharmaceuticals（ciprofloxacin（CIP）,atorvastatin（ATV）and amlodipine（AML））,and probed the effect of aging compounds on adsorption behavior of MPs.Results showed that aging processes significantly changed the adsorption behavior of PS MPs via the oxygen-containing functional groups,the fragmentation and generated cracks.Aging process changed the adsorption mechanism of MPs,where the adsorption of pristine PS MPs toward pharmaceuticals relied on hydrophobic andπ-πinteraction,while for aged PS MPs,hydrogen bonding and electrostatic interaction controlled the adsorption.We further investigated the potential effect of aging-released organic products on the adsorption behavior of different aged PS toward pharmaceuticals.Results showed that the PS-derived intermediates in high concentrations（TOC of 10mg/L）significantly affected the adsorption of PS（5.0 g/L）for pharmaceuticals（10mg/L）mainly through electrostatic interaction between MPs and intermediates.However,the aging intermediates in low concentration（0.1 mg/L）exhibited no significant effect on the adsorption of pharmaceuticals（1.0 mg/L）on MPs（0.5 g/L of PS）.The findings revealed that the adsorption data from pristine MPs in laboratory studies may not well reflect the adsorption behavior of MP samples found in the environment.（4）Finally,we investigated the desorption mechanisms of pharmaceuticals from pristine and aged PS MPs under simulated seawater,gastric and intestinal conditions of marine organisms,and assessed the potential risks of MP-associated pharmaceuticals.Results showed that the desorption of pharmaceuticals from pristine and aged PS MPs was significantly increased in simulated gastric and intestinal conditions compared to that in simulated seawater.The increased desorption in stomach mainly depended on the solubilization of pepsin to pharmaceuticals and the competition for sorption sites on MPs viaπ-πand hydrophobic interactions.However,high desorption in gut relied on the solubilization of intestinal components（i.e.bovine serum albumin（BSA）and bile salts（Na T））and the competitive sorption of Na T since the enhanced solubility increased the partition of pharmaceuticals in aqueous phase.Aging process suppressed the desorption of pharmaceuticals because aging decreased hydrophobic andπ-πinteractions but increased hydrogen bonding and electrostatic interaction between aged MPs and pharmaceuticals,which became less affected by gastrointestinal components.The potential risk of MP-adsorbed pharmaceuticals was assessed by comparing the daily intake of MP-adsorbed pharmaceuticals and the acceptable risk level of pharmaceuticals to organisms.After being ingested,the MP-associated pharmaceuticals might pose low risks to organisms,consistent with the low risk of environmental concentrations of pharmaceuticals in previous reports.Moreover,the risk of pharmaceuticals adsorbed on aged MPs was not necessarily lower than that on pristine ones,although aging process decreased the desorption of pharmaceuticals from MPs.This could be explained by the higher adsorption capacities of pharmaceuticals on aged MPs.