(340c) Microplastics Under the Sun Behaving like out-of-Equilibrium Colloids

Microplastics (MPs) are increasingly understood as colloidal-scale pollutants whose surface properties evolve under environmental stressors. Here, we present experimental evidence showing how exposure to simulated sunlight dramatically alters the interfacial properties of polyethylene microplastics over just a few days. These changes include a transition from hydrophobic to hydrophilic surface character, increases in surface charge (as measured by electrophoretic mobility), and a twofold increase in the adsorption capacity for common pollutants like lead ions and malachite green. Notably, these shifts occur before any detectable morphological breakdown, underscoring the importance of chemical transformation over physical degradation in the early stages of MP aging. Our results reveal that MPs exhibit time-dependent interfacial evolution, departing from equilibrium behavior in ways that critically influence their fate in aquatic environments. Recognizing microplastics as colloids with dynamic, light-sensitive surfaces reframes key questions about their transport, accumulation, and interactions. This understanding is also critical for addressing the role of wastewater treatment plants where microplastics accumulate, transform, and are inadvertently redistributed. Recognizing MPs as colloids with evolving surface properties may inform the development of targeted removal strategies in these hotspots.