Microplastics in Aquatic Systems: Dual Roles as Pollutant Carriers and Emerging Functional Materials for Water Treatment

Abstract Microplastics (MPs) are now widespread contaminants in aquatic environments, acting not only as long-lasting pollutants but also as surfaces that interact with many chemical and biological species. This review brings together recent advances in understanding the dual roles of MPs—their ability to transport pollutants and their potential use as engineered materials for contaminant removal. The discussion examines the physicochemical processes that control adsorption, desorption, and surface reactivity, showing how polymer type, particle size, oxidation, and biofilm growth shape their environmental behaviour. Special attention is given to developments in engineered and biofunctionalized MPs, including materials modified with catalytic, magnetic, or enzymatic components to improve pollutant capture and breakdown. Despite notable progress, major gaps remain: standardized testing protocols are still lacking, long-term environmental studies are limited, and the ecological impacts of engineered MPs are not well understood. This review highlights these gaps and identifies priorities such as developing harmonized testing methods, improving understanding of transformation pathways, and designing biodegradable or recyclable alternatives aligned with sustainability goals. By presenting a framework that clarifies when MPs act as pollutant carriers and when they can serve as functional materials, the review outlines the mechanisms behind their dual behaviour and the risk–benefit considerations needed for their responsible use in water treatment. Graphical Abstract