Microplastic-Contaminant Interactions: Influence of Non-Linearity and Coupled Mass Transfer.

Microplastic particles are ubiquitously detected in the environment. Despite intensive public and scientific discussions, their potential of transporting contaminants in rivers and oceans under environmental conditions is still under assessment. In this study, we measured sorption isotherms and kinetics in batch experiments using phenanthrene as typical hydrophobic wastewater contaminant and microplastic particles, differing in size and material. We observed a linear sorption isotherm for polyethylene, contrasted by nonlinear sorption of polyamide and polystyrene which could be described best by the Freundlich and Polanyi-Dubinin-Manes isotherms, respectively. We model sorption kinetics as a combination of external mass transfer governed by diffusion through an aqueous boundary layer and intraparticle diffusion within the plastic. Which of these processes controls the kinetics depends on the sorption strength, particle size, diffusion coefficients, and time. We used semi-analytical and numerical methods to simulate the coupled mass transfer for both linear and non-linear sorption. We successfully applied the semi-analytical model to polyethylene and the numerical code to polyamide and polystyrene, reproducing the measured kinetics and obtaining reasonable values for mass transfer and intraparticle diffusion coefficients. Subsequently, we used these coefficients to estimate the transport potential and relevant time scales for microplastic-bound contaminants under environmental conditions. This article is protected by copyright. All rights reserved.