Aging characteristics of polylatic acid microplastics and their adsorption on hydrophilic organic pollutants: mechanistic investigations and theoretical calculations

The extended persistence of microplastics (MPs) in aquatic habitats can result in the uptake and accumulation of various pollutants, thereby creating a serious risk to the ecosystem. The research explored how various weathering environments affect the physicochemical traits of polylactic acid (PLA) MPs and their capacity to adsorb common hydrophilic organic contaminants, such as benzoic acid (BA), sulfamethoxazole (SMZ), and sulfamethazine (SMR). The results showed that the ability to adsorb was affected by pH and was dependent on the pH of PLA as well as the pK values of the contaminants. Calculated DFT results were consistent with the actual adsorption capacity of PLA (SMX > BA > SMR). The main adsorption mechanisms included hydrophobic interaction, hydrogen bonding, and electrostatic attraction, with hydrophobicity predominating. Additionally, charge-assisted hydrogen bond (CAHB) and partition effect enhanced adsorption under specific conditions. Compared to virgin PLA, the adsorption capacity of aged PLA for hydrophilic organic pollutants generally improved, with APLA showing the most increase. The impact of oxygen concentration, surface area, and crystallinity on the adsorption ability of MPs was minimal, whereas surface charge became the primary physicochemical factor influencing the adsorption performance of aged PLA. This study would provide important theoretical references and data to deepen the understanding of the environmental behavior of PLA and its potential environmental risks.