Modeling the evolution of nanoplastic particle aggregation in aquatic systems

Nanoplastics (NPs) are defined as nanoparticles that are intentionally manufactured in this size range or that originate from the unintentional degradation of larger plastic fragments. Nanoplastics (NPs) are defined as nanoparticles that originate from the unintentional degradation of plastic that
breaks down into nanoscale particles. Because of their size, buoyancy, and surface properties, NPs are very mobile. In fact, they can be found in aquatic environments, soils, the atmosphere, and even in the human body. During travel across long distances, NPs undergo several processes, including advection, dispersion, and aggregation. If the first two are considered in conventional transport models, the latter is generally neglected. Aggregation consists of the formation of closely attached NPs that can reach the size of colloids, favoring settling and retention within the soil, thereby reducing the NP migration distance. Developing a model that accounts for aggregation is, therefore, a paramount for accurate transport prediction of NPs in the environment. 
Here we present a mechanistic model of NP aggregation over a broad range of conditions that resemble natural aquatic environments. The model combines the mass conservation equation of the population balance equation (PBE) with the constitutive equations based on the extended DLVO theory. The model was verified with data of the evolution of the hydrodynamic diameter of polyester NPs from the literature and used to predict the behavior of a variety of plastic materials such as polypropylene (PP), polyethylene (PE), and polyvinyl chloride (PVC). The model agrees very well
with the data, and no parameter fitting is required as it is based on the physical-chemical properties of the system, e.g., the zeta potential of the suspension. The results in general show that as pH or salinity increase NPs aggregation becomes more important; whereas, organic material inhibits aggregation. The change of the polymer type may affect the magnitude of the aggregation phenomenon but in all cases the effect of bio-geochemical properties change of the solution stays the same.