Investigation On Adsorption/Desorption Behavior And Mechanism Between Aged Polylactic Acid Microplastics And PPCPs

Microplastics,with long-term existence and extremely slow degradation,become a serious problem to be solved in environmental pollution.Moreover,as a carrier,microplastics can enrich and migrate pollutants in water,therefore forming a compound pollution effect.After aging in the environment,the surface morphology,surface functional groups,hydrophobicity,crystallinity and other physical and chemical characteristics of microplastics may change,as a result,the adsorption/desorption behaviors between microplastics and pollutants in solution also may change.However,existing studies mainly focus on the adsorption behavior of nonbiodegradable microplastics and pollutants,but neglect the ecological toxicology and environmental risk of biodegradable microplastics coexisting with pollutants.In this paper,polylactic acid(PLA),a typical biodegradable microplastic,was selected to study aging behaviors and their adsorption/desorption behaviors between PLA and typical Pharmaceutical and Personal Care Products(PPCPs).Based on the aging methods and characteristics of PLA microplastics,the adsorption and desorption behaviors of aging PLA and PPCPs were studied.The main research results of this paper are as follows:(1)The typical antibiotic ciprofloxacin hydrochloride(CIP)was selected as the target pollutant,and the PLA microplastics were treated with the 254 nm UV lamp for 24 h continuous irradiation and(16+8)h alternate irradiation day and night,to explore the changes in the physical and chemical characteristics of PLA microplastics and the differences in the adsorption/desorption behaviors of CIP by UV aging under different exposure methods.The results showed that the degradation of the ester bond and hydroxyl group occurred after UV aging,and the microplastics were broken into smaller particle sizes,with obvious yellowing,the increased surface hydrophobicity,the higher crystallinity,the decreased carbonyl index,and the more surface negative charge.The adsorption/desorption capacity of PLA with a small particle size(45 μm)was higher than that of PLA with a large particle size(550 μm),which indicated that PLA has a higher environmental risk after decomposing and aging.The results of isothermal adsorption experiments indicated that the aging PLA showed higher adsorption performance than the original PLA due to the reduction of particle size.The aging effect of simulated day and night alternation aging PLA is more obvious than that of 24 h continuous irradiation aging PLA,and its adsorption and desorption effects of CIP are higher,which indicated that temperature change is an important factor affecting the aging characteristics of PLA and the adsorption/desorption behaviors of CIP.The original PLA microplastics adsorb CIP in a single layer,while the aging PLA microplastics have a multilayer adsorption process of CIP dominated by chemical adsorption,and physical adsorption exists simultaneously.Desorption experiments showed that the desorption hysteresis of CIP on PLA is obvious,indicating that the sorption–desorption process of CIP on PLA is irreversible.The desorption degree of the two kinds of Aged-PLA was higher than that of the Origin-PLA,which indicated that the Aged-PLA have a higher migration ability for CIP.(2)The environmental hormone bisphenol A(BPA)was selected as the target pollutant,ultraviolet lamps with 254 nm wavelength and 340 nm wavelength were used to continuously irradiate PLA microplastics for 24 h,to explore the changes in the physical and chemical characteristics of PLA microplastics and the differences of the adsorption/desorption behaviors of BPA by UV aging at different wavelengths.The results showed that the degradation of ester bond and hydroxyl group occurred after UV aging at 254 nm wavelength,and the microplastics were broken into smaller particle sizes,with obvious yellowing,the increased surface hydrophobicity,the higher crystallinity,the decreased carbonyl index,and the more surface negative charge.In contrast,PLA microplastics showed no color change after UV-340 nm aging,and only a small fraction of PLA cracked into smaller microplastics with the same hydrophilicity as the original PLA.The adsorption/desorption capacity of PLA with a small particle size(45 μm)was also higher than that of PLA with a large particle size(550 μm),because of the larger specific surface area and more adsorption sites.The fitting degree of the second-order kinetic model and Freundlich model indicated that the adsorption capacity of UV-340 nm Aged-PLA was mainly chemical adsorption.The adsorption capacity of BPA on UV-340 nm Aged-PLA was slightly higher than that of Origin-PLA,which was attributed to the reduction of particle size,but the adsorption capacity of BPA on UV-340 nm Aged-PLA decreased,which was related to the increase of crystallinity of Aged-PLA.With the higher crystallinity,the adsorption capacity of BPA(one of the hydrophobic organic compounds)also decreased.Desorption experiments showed that BPA was easily desorbed from PLA microplastics into the liquid phase,and the desorption process was irreversible,with obvious desorption hysteresis.The desorption degrees of BPA by the two kinds of Aged-PLA microplastics were higher than that of the Origin-PLA,indicating that the aged PLA had a higher migration ability to BPA.Meanwhile,the desorption Hysteresis Index(HI)of BPA was smaller than that of CIP,indicating that BPA was more easily desorbed from PLA than CIP.The desorption rate of BPA in the simulated intestinal fluid was higher than that in ultrapure water,indicating that PLA have a higher ecotoxicological risk in organisms.