Microplastic-Contaminant Interactions: From Experimental Data to Environmental Implications

Microplastic particles are ubiquitously detected in the environment. Despite intensive public and scientific discussions, an assessment of their potential to transport hydrophobic organic contaminants in rivers and oceans under environmental conditions is still needed. 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. Within batch experiments various types of microplastic particles and common wastewater contaminants were applied to investigate both sorption isotherms and kinetics. The subsequently developed models consider material characteristics, physico-chemical properties of chemical compounds as well as different types of sorption isotherms were considered. In order to determine the dominant mass transfer mechanisms they account for both diffusion within the particles and through an external aqueous boundary layer. Which of these two fluxes controls the kinetics depends on the sorption strength, particle size, diffusion coefficients, and time. For the case of linear sorption patterns, as observed for polyethylene, a semi-analytical model was developed. Experiments performed with polystyrene and polyamide, however, revealed non-linear sorption isotherms which required a numerical model to simulate the coupled mass transfer. Both model types were successfully validated, allowing to describe the measured kinetics and to obtain reasonable parameter values. To broaden the scope and environmental relevance of this thesis, further experiments were performed. Applying changing pH conditions, data and model implied that the partitioning is strongly dependent on the compound’s hydrophobicity and that neutral species contribute largely to sorption. Furthermore, the presence of additional natural sorbents significantly influenced both equilibrium partitioning and desorption kinetics. Due to the combination of experimental work with modelling tools, it was possible to elucidate that material properties have the largest impact on the kinetics whereas the shape of the sorption isotherms has only a minor influence. Contrary to common believe, it was demonstrated that for high partition coefficients, desorption kinetics is fast in batch experiments and controlled by an external mass transfer while for low partition coefficients caused e.g. by high organic carbon loads kinetics was slow and limited by intraparticle diffusion. Conversely, under environmental conditions an increasing sorption slows down kinetics which is the fundamental difference between laboratory and field conditions. Consequently, 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. Eventually, 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 longrange 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.