Biofilm-induced sinking of buoyant microplastics in a freshwater environment

The degree of microplastic dispersal and retention in lakes and oceans critically depends upon the microplastic particle’s density, which can change over time due to microbial growth (biofilm). This experiment tests the mechanism by which initially buoyant microplastics can be lost from the surface layers of a lake and become deposited in sediments. While buoyant microplastics do initially float in water, the growth of biofilm denser than water on the microplastic surface leads to an increase in particle density as a whole. This increase in density results in slower rise velocities of biofouled particles when they are mixed into the water column, and can even lead to sinking of biofouled particles. Both slower rise velocities and particle sinking would increase microplastic residence time in the water body. Through ex-situ experiments on irregularly-shaped polypropylene microplastic granules in an emulated lake environment under overcast light levels, we have found that biofouling alone is sufficient to increase microplastic particle density and lead to sinking for small particles (~125-212 µm) in 18 days and larger particles (1000-2000 µm) in 50 days. These differences in settling onset time would likely lead to size-fractionation of particle sedimentation, where smaller particles are deposited closer to their sources relative to larger particles. Using the measured values of biofilm-induced sinking rates of larger microplastics (1000-2000 µm) and lake residence times, we can describe the fraction of microplastics expected to become deposited after they enter lakes. Our results on terminal velocity change inherent to biofouling provide new information for microplastic transport modelling.