Weathering pathways differentially affect colloidal stability of nanoplastics†

Abstract

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. Herein, we show that two weathering pathways, namely, UV irradiation (the primary aging pathway in surface water) and sulfide-induced transformation (a common process in anoxic environments), affect the aggregation and colloidal stability of polystyrene (PS) nanoplastics differentially. Compared to sulfide-induced aging, UV-induced aging introduced more oxygen-containing functional groups on the nanoplastic surface, although significant amounts of O-functional groups were formed during sulfide-induced aging, owing to hydroxyl radicals formed through the spontaneous oxidation of sulfides. Accordingly, UV-aged PS nanoplastics (PS-UV) exhibited higher stability than sulfide-aged PS nanoplastics (PS-S) in a monovalent cation-dominated solution because of enhanced electrostatic repulsion and weakened van der Waals attraction. However, the stability of PS-UV was lower than that of PS-S in a divalent salt solution considering the bridging effects of divalent ions. These results underline the importance of comprehending the effects of diverse environmental weathering processes on nanoplastics' behaviors, particularly those that readily occur in anoxic environments but are insufficiently investigated.