Microplastic-Contaminant Interactions: From Experimental Data to Environmental Implications

Abstract

Microplastic particles are ubiquitously detected in all environmental compartments. Despite intensive public and scientific discussions, their potential to transport contaminants in rivers and oceans is still under assessment. To consider such particle facilitated transport, this thesis aims to quantify the underlying sorption mechanisms and to develop a comprehensive mechanistic model with parameter values derived from experimental data. The developed models consider material characteristics, physico-chemical properties of chemical compounds as well as different types of sorption isotherms. The sorption kinetics was modeled as a combination of external mass transfer governed by diffusion through an aqueous boundary layer and intraparticle diffusion within the plastic particles. Which of these processes controls the kinetics depends on the sorption strength, particle size, diffusion coefficients, and time. Based on the determined sorption isotherm, a semi-analytical model was developed for linear sorption and a numerical approach was applied to simulate coupled mass transfer for the case of non-linear sorption. Both model types were successfully validated for several plastic types, allowing to accurately describe the measured kinetics. To broaden the scope and environmental relevance of this thesis, further experiments were performed. It was revealed that changing pH conditions and the presence of additional natural sorbents significantly influenced both equilibrium partitioning and desorption kinetics. Due to the combination of experimental and mechanistic modelling tools, it was possible to elucidate coupled mass transfer processes for different experimental and field settings. Eventually, it was ascertained that time scales observed under experimental conditions may not be transferred to field conditions without an appropriate mechanistic model accounting for coupled mass transfer and the specific boundary conditions. Appropriate hydrodynamic relationships coupled to a thorough mass transfer analysis can serve to assess the vector function of pollutant-loaded particles and to evaluate whether microplastics rather act as a passive sampler or show potential to facilitate long-range contaminant transport. Moreover, as the theoretical mass transfer considerations also apply to other suspended particles, well-defined microplastic particles are ideally suited to perform in-depth mass transfer studies and to act as surrogates for particles occurring in the environment, including microplastics in urban runoff and contaminated sediment.