Understanding the role of turbulence and biofilm on low density microplastic dynamics: An experimental approach towards natural conditions

The fate of microplastics (MP) in rivers is controlled by particle properties, biological interactions, and hydrodynamics, yet mechanisms governing near-bed behavior of low-density MP remain unclear. Despite their buoyancy, low-density MP are frequently found in sediments, suggesting that turbulence-driven transport and benthic biofilms influence the near-bed transport and retention. Flume experiments quantified how turbulence modulates MP transport, biofilm contact, and MP-biofilm interaction at the sediment-water interface. Flume integrating water, sediment, and biofilm compartments used fluorescent polyethylene spheres (0.995 g cm⁻³; ∼50 µm) under controlled flows. Particle trajectories near a monospecific Pseudomonas aeruginosa biofilm were reconstructed via particle tracking velocimetry, while turbulence intensity was characterized by friction velocity (u = 0.0009, 0.0014 and 0.0024 m s⁻¹). Increasing turbulence significantly increased MP-biofilm encounters (p < 0.001), with median fractions reaching 3.3-4.3% of all observed MP at u ≥ 0.0014 m s⁻¹ , compared with 1.6% at 0.0009 m s⁻¹ . Most MPs contacting the biofilm originated within the viscous boundary layer, although up to ∼20% came from above under higher turbulence, reflecting a shift in transport pathways. Biofilm retention of MPs remained low (0.6-5.8%) and decreased slightly with turbulence, revealing a trade-off between delivery and attachment. These results indicate a two-step mechanism: turbulence delivers particles to near-bed zone, while biofilm properties govern retention. This coupled process helps explain the presence of buoyant MP in sediments and highlights the role of benthic biofilms in mediating MP exchange between water and riverbeds. The empirical relationships derived here can inform process-based transport models to improve predictions of MP fate and fluxes in fluvial systems.