Evolving Biofilm Morphology Controls Microplastic Transport in Porous Media

Zhongyu Shi, Yinuo Noah Yao 

Microplastics (MPs) entering soils and subsurface environments interact with biofilms that continually reshape pore geometry and local flow pathways, yet biofilm effects on MP transport are only studied under fixed conditions. Here, we used a saturated microfluidic porous medium to directly image biofilm development and track fluorescent polyethylene MPs at different stages of biofilm growth. Changes in biofilm morphology were quantified from transmitted-light intensity distributions, while particle trajectories were used to determine microplastic retention times and trapped fractions. Streamer-rich biofilms resulted in spatially distributed MP pathways, broader retention-time distributions, and greater particle trapping. In contrast, the bioclogged porous medium resulted in connected preferential flow paths (PFPs) that enabled rapid MP transport and reduced particle-biofilm interactions. Across all conditions, the streamer-associated component weight was positively correlated with both the mean normalized retention time (Spearman $r_s=0.92$, $p=0.0002$) and trapped fraction (Spearman $r_s=0.87$, $p=0.0012$). In contrast, neither the mean normalized retention time nor the trapped fraction showed a significant relationship with the total biofilm-associated optical fraction. These results show that MP transport depends strongly on biofilm morphology rather than biofilm accumulation alone. Biofilm development can therefore shift MP transport from spatially distributed and delayed transport under streamer-rich conditions to rapid transport through a focused PFP. This morphology-dependent behavior, therefore, imposes significant challenges in predicting MP retention and downstream migration in biologically active porous media.