Enhancing the aging of polystyrene microplastics through a flow-through electrochemical membrane system: Mechanism of confinement effect

Microplastics (MPs) are emerging pollutants in aquatic environments that pose serious health risks, and traditional wastewater treatments are ineffective at removing them. In this study, a flow-through electrochemical membrane (F-T) system was developed to simultaneously separate and age polystyrene microplastics (PS-MPs) in water. Under membrane pressure, PS-MPs were tightly pressed onto the surface of membrane electrode, forming a confinement space. The confinement effect overcomes the short lifetime of free radicals and accelerates the aging process of PS-MPs. This study identified the optimal conditions for aging PS-MPs in the F-T system, characterized the material properties before and after aging, and analyzed the degradation intermediates. After 8 h of treatment, the oxygen-to-carbon ratio (O/C) of the PS-MPs following the F-T system was 2.00 times greater than that following the traditional flow-by (F-B) system. In addition, the unit energy consumption (kW·h/g) for the aging of PS-MPs in the F-T system was 645.19 kW·h/g, which was 3.70 times lower than the F-B system. Free radicals, especially O₂^•- and •OH, played a major role in PS-MPs aging. Free radicals attack the main chain of PS-MPs, leading to subsequent chain breakage, hydrogen abstraction, and rearrangement reactions. After aging in the F-T system, the crystallinity, molecular weight, and zeta potential of the PS-MPs significantly decreased. In addition, carbon-centered environmental persistent free radicals formed on the surface of the PS-MPs. This study presents an energy-efficient method for the remediation of MPs in water and suggests that the aging efficiency can be increased through confinement.