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Biofilm formation reverses polymer-specific microplastic transport through saturated sediments
Summary
Scientists found that microplastics moving through sand and sediment don't all behave the same way—it depends on the plastic type, particle size, and whether bacteria have formed a slimy coating (biofilm) on them. Surprisingly, biofilms made regular plastic (PE) more likely to get stuck and stay put, while they helped biodegradable plastic (PLA) travel deeper into the ground. This matters because it means some microplastics may be more likely to build up near the surface—closer to water supplies and ecosystems we interact with—while others could slip further into groundwater, affecting where these particles end up and how they might reach our drin
Microplastic transport through sediments controls their long-term subsurface storage, remobilization potential, and transfer between aquatic and subsurface environments, yet the mechanisms governing their retention under environmentally relevant conditions remain unresolved. Here, we investigated the transport of polyethylene (PE) and polylactic acid (PLA) fragments, including pristine and biotically aged particles, through saturated sediments with different granulometries. Across all sediment types, mechanical straining imposed a size-selective barrier at the inlet, capturing ≥40% of particles within the first 7.5 cm and restricting downstream transport to fractions <150 µm. Beyond this interface, sediment grain size determines how particles travel: larger pores reduce physical straining, causing transport pathways to diverge based on polymer density. Biotic aging fundamentally altered these transport behaviours. Biofilm formation enhanced PE retention, reducing breakthrough by 65%, while promoted deeper transport of PLA suggesting that biofilm modify particle–sediment interactions based on polymer characteristics. Our findings highlight that predicting microplastic fate in sediment requires incorporating plastic types, sediment structures, particle size distributions, and biological transformations to better understand microplastic transport in aquatic systems.