Impact of coagulation characteristics on the aggregation of microplastics in upper-ocean turbulence

The dynamics and aggregation of microplastics in marine environments are investigated through high-fidelity direct numerical simulations with Lagrangian point-particle tracking. The properties of microplastics and biogenic particles, including size, density, and concentration, align with scenarios typical of seawater systems. The stickiness nature of microplastics, induced by biofilm formation (biofouling), is modeled through coagulation efficiency (stickiness parameter), which represents the probability of aggregation following a collision event. Two main aspects are at the core of the present work: analyzing the mechanisms of collision and coalescence between microplastics and biogenic particles, along with their spatial distribution, and characterizing the emerging aggregates. The results indicate that particles stickiness, concentration and (especially) size impact on the collision and coalescence rates. Furthermore, microplastics exhibit a strong tendency to accumulate near biogenic particles, leading to the creation of hetero-aggregates whose tendency to sink supports the general hypothesis of “missing microplastics”. Particularly, in cases where microplastics and biogenic particles are evenly concentrated, microplastics primarily contribute to the formation of aggregates. The stickiness mainly determines the most complex and large aggregates, which are less than 1% of the total.