Weathering Pathways Differentially Affect Colloidal Stability of Nanoplastics

Aggregation is the most fundamental process affecting the fate, transport, and risks of nanoplastics in aquatic environments. Weathering of nanoplastics alters their physiochemical properties and consequently, aggregation behavior. Here, we show that two weathering pathways, including UV-irradiation (the primary aging pathway in surface water) and sulfide-induced transformation (a commom process in anoxic environments) affect aggregation and colloidal stability of polystyrene (PS) nanoplastics differentially. Compared to sulfide-induced aging, UV-induced aging introduced more oxygen-containing functional groups on nanoplastic surface, even though significant amounts of O-functional groups formed during sulfide-induced aging, due to the hydroxyl radicals formed from the spontaneous oxidation of sulfide. Accordingly, UV-aged PS nanoplastics (PS-UV) exhibited a higher stability than sulfide-aged PS nanoplastics (PS-S) in a monovalent cation-dominated solution, due to enhanced electrostatic repulsion and weakened van der Waals attraction. However, stability of PS-UV was lower than that of PS-S in a divalent salt solution, due to bridging effects of divalent ions. The results underline the importance of comprehending the effects of diverse environmental weathering processes on nanoplastics hehaviors, particularly, those readily occur in anoxic environments but insufficiently investigate.