Molecular Dynamics Simulations of Heteroaggregation of Colloidal Plastic Particles

Nano- and microplastics are a growing threat for the environment, especially in aqueous habitats. To assess the influence on the ecosystem and provide possible solutions, it is necessary to investigate the "fate" of microplastics in environmental conditions. Microplastics are surrounded by a higher concentration of natural suspended particulate matter (SPM), which can cause microplastics aggregation due to favorable interactions. The heteroaggregation of microplastics with SPMs plays a key role in determining their fate in the environment. Among several other factors, the effect of the shape of microplastics and flow conditions of the water on the heteroaggregation of microplastics is not well understood. To fill this knowledge gap, we perform Molecular Dynamics simulations of heteroaggregation of different geometries of microplastic particles (cube, plate, cylinders, rod, rounded cube, sphere) with smaller spherical SPMs. We show that the morphology of the microplastic has a strong impact on the aggregate structure. Microplastics with mostly smooth surfaces (e.g., sphere, rounded cube) aggregate into compact structures with a high number of bridged neighbors with weak SPM bridges. Microplastics with edges and corners (e.g., cube, plate) aggregate into fractal structures with fewer bridged neighbors with strong SPM bridges. We also perform Multiparticle Collision Dynamics simulations to understand the behavior of aggregates under shear flow, which can break or restructure the aggregates. We show that the critical shear rate at which the aggregates break is much larger for spherical microplastics caused by their compact aggregate structure compared to the fractal aggregates of cubes and plates.