In recent years, the excessive accumulation of plastic waste in marine ecosystems has raised pressing concern about the fate of microplastic particles (MPs; size < 5 mm), which are released from large plastic debris under physicochemical weathering (mechanical abrasion, UV irradiation). Depending on their density, MPs might drift on the sea surface (e.g., polyethylene), or settle to the seafloor (e.g., polystyrene). In all cases, they interact with marine snow and serve as hotspots for microbial activities. Biofilm-forming bacteria rapidly colonize the plastic surface and change the morphology (size, shape, fractal dimension) and the hydrodynamic properties of the particles. It is important to understand the mechanisms of biofilm association with MPs and quantify the concomitant effects on MP transport and the oceanic carbon balance. 

In this work, computational fluid dynamics (CFD) simulation is used to investigate the sinking behavior of biofilm-covered microplastics (BMPs) with realistic shape in a water column. The structure of individual BMPs is either acquired from imaging experiments with BMPs of riverine origin (Hawkesbury river, Australia) [1], or generated through computer simulation of biofilm dynamics [2]. Our CFD simulations are parameterized by the Reynolds and Darcy numbers that capture the combined effects of particle size, excess density, seawater viscosity and biofilm poroelasticity. At steady-state sinking, high-resolution finite element analysis provides the distribution of elastic stresses within the biofilm and the locations of maximum likelihood for fragmentation. The computed terminal velocity is compared with measurements from particle tracking velocimetry experiments [1], across a wide range of aggregate geometry and biological fraction.

References
[1] Nguyen TH, Tang FHM, Maggi F (2020). Sinking of microbial-associated microplastics in natural waters. PLoS ONE 15(2): e0228209.
[2] Kapellos, G.E., Alexiou, T.S. and Pavlou, S (2015). Fluid-biofilm interactions in porous media, Elsevier, pp. 207-238.
 

microplastics,biofilm,fragmentation,finite element model,Sinking Particles