Statistical Thermodynamic Description of Heteroaggregation between Anthropogenic Particulate Matter and Natural Particles in Aquatic Environments

Aggregation is a key process in understanding the fate and transport of anthropogenic particulate matter, namely, nanoparticles and microplastics, in aquatic environments. Recent research has subdivided aggregation into two processes: homoaggregation, where two like particles aggregate, such as two fragments of microplastic, and heteroaggregation, where two unlike particles aggregate, such as a nanoparticle and sediment. Of the two processes, heteroaggregation is generally assumed to be more important because anthropogenic particles are much less concentrated than their naturally occurring counterparts. This assumption remains largely untested in many natural settings, and most aggregation models discount the process of disaggregation entirely. To address these deficiencies, we created a statistical thermodynamic aggregation model to predict the steady-state size distribution of any two-particle system, accounting for homoaggregation, heteroaggregation, and disaggregation. The results of the model confirm that homoaggregation is likely a negligible process for the fate of nanoparticles and microplastics. However, the model predicts that heteroaggregation will be incomplete, with at least 10% of the nanoparticles or microplastics remaining unaggregated (i.e., monomeric form) even under favorable bonding conditions and large concentration disparities (i.e., surrounded by a far greater concentration of secondary particles). Our model also predicts that heteroaggregation is influenced by the magnitude of the enthalpy of bonding (Hb) between the anthropogenic particle and the larger natural particle. While these predictions require experimental verification, the implications of this study highlight the critical need to consider and carefully examine disaggregation in the context of heteroaggregation (and homoaggregation) of anthropogenic and natural (in)organic particles.