Conditions for homoaggregation of pristine polystyrene microplastic in aquatic environments

Abstract

This work experimentally investigates the homoaggregation potential of pristine polystyrene microplastics (PSMPs) with environmentally realistic shapes, morphologies, and dissimilar size. The experiments were performed in waters with varying ionic strengths, including deionized water (DI), aqueous NaCl solutions (50 and 200 mM), and natural riverine and marine waters. Environmentally realistic PSMPs exhibiting anisotropic sizes (0.5 – 130 μm), irregular shapes, and rough morphologies were obtained by a cryomilling procedure. Significant homoaggregation of PSMPs was observed only in DI, where the suspension was highly unstable, consistent with neutral zeta potential. In contrast, PSMPs suspended in NaCl solutions and natural waters displayed moderate stability. The results demonstrate that the aggregation of PSMP particles is primarily driven by physical processes, thereby transport step, which is augmented by PSMP properties (hydrophobicity, irregular shape and rough morphology, various sizes, MP concentration) and hydrodynamic conditions. The aggregates maintained their integrity and remained stable in DI, as well as after being transferred to NaCl solutions. Likewise, settling tests evidenced that homoaggregates did not break down while settling through column filled with solutions of ionic strength varying between 0 and 500 mM, showing that they might withstand natural water density stratification. These results provide new insight into the fate of pristine PSMPs in aqueous environment and contribute to improvements in laboratory research on PSMP suspensions. The observed behaviour contrasts with that of PS nanoparticles, highlighting the distinct aggregation dynamics of microparticles in different aqueous conditions.