Sorption kinetics of metallic and organic contaminants on micro- and nanoplastics: remarkable dependence of the intraparticulate contaminant diffusion coefficient on the particle size and potential role of polymer crystallinity

We developed a mechanistic diffusion model to describe the sorption kinetics of metallic and organic contaminants on nano- and micro-plastics. The framework implements bulk depletion processes, transient fluxes, and fully adaptable particle/water boundary conditions, i.e. not only the typically assumed simple linear Henry regime, which is not applicable to many contaminant-particle situations. Thus, our model represents a flexible and comprehensive theory for the analysis of contaminant sorption kinetics, which goes well beyond the traditional empirical pseudo first or second order kinetic equations. We applied the model to the analysis of a large body of literature data on the equilibrium and kinetic features of sorption of a wide range of contaminants by diverse types and sizes of plastic particles. Results establish the paramount importance of sorption boundary conditions (Henry, Langmuir, or Langmuir-Freundlich) and reveal interesting and often overlooked sorption features that depend on the plastic particle size and the extent to which the target compound is depleted in the bulk medium. The greater degree of polymer crystallinity reported for smaller particles may underlie our findings that the intraparticulate contaminant diffusion coefficient decreases with a decreasing particle size. We establish a universal law to predict the sorption kinetics and diffusion of any compound within any plastic phase, which has far reaching importance across many domains relevant to the environment and human health.