Assessing the Settling Velocity of Biofilm-Encrusted Microplastics: Accounting for Biofilms as an Equivalent to Surface Roughness

Abstract

While it is well established that a biofilm contributes to the sinking of plastics, the underlying mechanisms of how it influences the vertical transport of plastics have not been well explained. In this context, our study dives into the intricate effects of biofouling on the settling velocity (W s ) of microplastics (MPs) within the fluid. We adopt the perspective that the biofilm is a form of surface roughness impacting the drag coefficient (C d ) and vertical settling of MPs. By advancing the biofouling process model, we simulate the temporal variations of density and biofilm thickness of biofouled floating MPs, accounting for realistic parameters and assuming a layer-by-layer growth of biofilm on plastisphere surfaces. MPs of polyethylene (PE) exhibit a quicker initiation of descent compared to their polypropylene (PP) counterparts. Furthermore, leveraging computational fluid dynamics (CFD) simulation, the method to predict the C d of spherical MPs with surface roughness is established. By treating the thickness of the biofilm as roughness height, an explicit method to predict the W s of biofouled MPs is derived. The settling experiments for biofouled MPs conducted not only support the combination of the biofouling model and the explicit method to predict the W s of biofouled MPs but also enhance the prediction accuracy by introducing a ratio parameter C o to better relate the equivalent surface roughness height (k) to the biofilm thickness (σ), i.e., k = C o •σ, where the recommended value of C o for spherical PP and PE MPs is between 0.5 to 0.8. This study, thus, provides new insights into the dynamics of biofouled MPs in hydraulic ecosystems.

Natural Factors of Microplastics Distribution and Migration in Water: A Review

“…Recent studies have shown that the biofilm on the surface of microplastics can be used as a kind of surface roughness, and the sedimentation rate can be changed by changing the resistance coefficient of microplastics during the sedimentation process. It has also been pointed out that the multilayer structure and anisotropy of microbial biofilms are not exactly equivalent to the roughness of traditional particles [159]. The spectral absorption summit of the functional groups of PE plastics was exposed to seawater changes, producing an additional peak at 1700, which is believed to be mainly due to the C==O produced by the decomposition of microplastics [156].…”