Toward Optimizing Microplastic Transport Models through Biofouling Experiments

Biofilms occur on all microplastics and are therefore increasingly being included in freshwater numerical microplastic transport models. However, parametrizations of biofilm growth in these models lack experimental validation. Here, we provide empirically supported guidelines and advice for the implementation of biofouling in freshwater numerical microplastic transport models. For a diverse set of microplastics the influence of polymer type, size, and shape on biofilm development was measured in a lab experiment using both mass-based and image-based approaches. We demonstrate how different analytical techniques provide distinct yet complementary insights into biofilm development. Our key finding is that for a model run duration of less than four months which uses pristine microplastics as the input, biofouling only significantly (change of >50%) affects the microplastic vertical velocity for particles less than 10 μm or with a density close (±50 kg/m³) to the fluid density. Biofouling can be ignored for simulations that run for several days and microplastic diameters of over 400 μm. For the first four months, biofilm growth can be simplified by using either a linear model or an exponential model. For the case of linear biofilm growth, we measured a net growth rate of 0.061-0.170 μm per week.