Aggregation and hetero-aggregation of polystyrene microplastics: Roles of surface functionalization, water chemistry,...

As a direct consequence of escalating plastic production and disposal, microplastics (MPs) are ubiquitous and globally threatening. When MPs form a suspension in water, their aggregation behavior, and hence the stability of the suspension, determines their transport and environmental fate. Surface functional groups (e.g., amino, carboxyl), whether inherent or environmentally acquired, dictate MPs' surface charge and consequently their aggregation behavior. MP aggregation is also notably influenced by water chemistry conditions including pH, ionic strength, cation type, and the presence of natural organic matter and clay colloids. In this work, we investigated how the aggregation of polystyrene microplastics (PSMPs) was influenced by surface functionalization - using unmodified, amine- (-NH₂), and carboxylate- (-COOH) modified spheres under diverse water chemistry conditions. By using these model plastic spheres, we isolated the mechanistic role of specific chemical moieties in governing particle stability. Aggregation was quantified via light absorbance, zeta potential (ZP) and hydrodynamic diameter (HDD), and theoretical analyzed using DLVO theory. Our results showed distinct degrees of stability among PSMP suspensions, with COOH-PSMP suspensions being the most stable and unmodified PSMP the least stable. ZP was found to be the dominant control; both highly positive and highly negative ZPs prevented aggregation. Water chemistry altered aggregation via ZP: high pH, low ionic strength, and monovalent cations generally increased stability for negatively charged PSMPs but promoted aggregation for positively charged NH₂-PSMPs. Kaolinite colloids typically did not interact with like-charged PSMPs, yet could enhance aggregation with oppositely-charged particles. Humic acid generally increased stability for unmodified PSMPs, but its effect proved complex, varying with PSMP type and cation concentration. Overall, our findings demonstrate that surface modification significantly impacts MP fate by modulating ZP. By establishing this mechanistic baseline, our study provides a framework for predicting how environmental aging alters the transport of complex microplastics in the real world.