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

Microplastic particles are ubiquitously detected in the environment. Despite intensive public and scientific discussions, their potential to transport contaminants in rivers and oceans under environmental conditions is still under assessment. In the present study we measured sorption isotherms and kinetics in batch experiments using phenanthrene (as a typical hydrophobic wastewater contaminant) and microplastic particles of different sizes and materials. We observed a linear sorption isotherm for polyethylene, in contrast to nonlinear sorption of polyamide and polystyrene, which could be best described by the Freundlich and the Polanyi-Dubinin-Manes isotherms, respectively. We modeled 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 nonlinear 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. Environ Toxicol Chem 2019;38:1635-1644. © 2019 SETAC.