Impact of electrolyte and natural organic matter characteristics on the aggregation and sedimentation of polystyrene nanoplastics

Nanoplastics are increasingly pervasive in ecosystems worldwide, raising concerns about their persistence and mobility in the environment. Our study focused on the interactions between polystyrene nanoplastics (PS NPs, D:~200 nm) and Natural Organic Matter (NOM) uniquely isolated from water bodies under different electrolyte and temperature conditions (i.e., effectively mimicking a wide range of environmental scenarios). The selected dissolved NOM (DOM) fractions of varied physical chemical characteristics and geographical origins include: the hydrophobic acid (HPOA) fraction from the South Platte River (SPR HPOA, USA), the biopolymer/colloid fractions from Cazaux Lake (CL BIOP, France), and the dissolved fraction of the biofilm recovered from a nanofiltration-fouled module at the Méry-sur-Oise drinking water treatment plant (NF BIOP, France). The biopolymers (NF BIOP and CL BIOP) clearly hindered PS NPs aggregation through steric effects, forming a protective eco-corona, enhancing PS NPs stability, and inhibiting sedimentation in the long term, compared to HPOA. The temperature impacted the homo and hetero-aggregation of PS NPs differently, illustrating the complex interplay between thermal effects and NOMs stabilizing interactions. Furthermore, the seldom-explored aspect of the sequential introduction of reactants into the solution during aggregation experiments (i.e., which simulates a realistic scenario: the transport of PS NPs from one aquatic system to another of different compositions) was also investigated. This study provides essential insights into the dynamic behavior of PS NPs in environmental matrices and crucial knowledge for predicting nanoplastic interactions in complex ecosystems.