Insights into the Characteristics, Adsorption, and Desorption Behaviors of Polylactic Acid Aged with or without Salinities

Degradable plastics, the substitute for conventional plastics, are becoming the new pollutant in the ocean. However, studies investigating the impact of freshwater and seawater on the basic aging process of microplastics and involving degradable plastics are still lacking. To accurately compare the potential hazards of degradable microplastics aging in seawater and freshwater, the role of salinity in the ultra-violet ray (UV) aging process of degradable microplastics (MPs) was explored in this study. Polylactic acid (PLA) aged under UV and 0‰, 10‰, and 40‰ salinities was taken as the subject. The results showed that salinity combined with UV accelerated the aging process, and PLA had more holes and cracks on the surface, larger specific area (SBET), and significantly more oxygen-containing functional groups and negative charges. The SBET of PLA aged under UV and 40‰ salinity was 2.97±0.11, 33% more than that aged by UV only. Additionally, salinity increased the adsorption capacity of PLA. The adsorption quantity of oxytetracycline (OTC) and Cu2+ by 40‰ salinity-aged PLA were 4.822 and 9.606 mg/g, 30% and 32% more than that aged by UV only. There was a drop in the desorption rate of OTC but a rise in that of Cu2+ by PLA aged in salinities. Moreover, the desorption quantity and rate of pollutants on PLA in simulated intestinal fluid were greater than those in water, and PLA might have a higher carrier effect on pollutants after aging under salinity conditions. These results showed that the potential hazard of PLA existing in seawater may be more serious than that in freshwater. This study also revealed the effect of salinity, enabling the environmental risk assessment of degradable MPs to be more scientific and more comprehensive.