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.