Can bubble barriers retain microplastics? An evaluation using laboratory and hydrodynamic analysis of transport and retention

Abstract Microplastic (MP) pollution poses a growing threat to aquatic ecosystems. Air bubble barriers (BB) have emerged as a promising passive method for intercepting and retaining macroplastics, and potentially microplastics, before they reach marine environments or exit wastewater treatment plants. This study explores the hydrodynamic impact of a BB system and its potential to modify MP retention through flume-scale experiments with velocity field analysis and particle tracking techniques, conducted under turbulent, subcritical flow conditions (Re ≈ 4.7 × 10 3 , Fr ≈ 0.03). Using fluorescein as a conservative tracer and microplastic particles of varying buoyancy and size, we assess the BB’s performance across three BB air pressures (500, 750, and 1000 mbar). Tracer breakthrough curves showed that the BB induces both preferential flow paths and recirculation zones in the water column, indicating a clear influence of the BB on downstream mass and momentum transfer. These processes increased fluorescein residence time by up to 24%. Velocity contour analysis corroborates these observations, showing BB-induced upward convection and localised turbulence that redistributed flow velocities near the bubble stream. The upward flow aligned with naturally buoyant MP (low-density polyethylene and high-density polyethylene), which significantly enhanced their surface accumulation and reduced downstream recovery to less than 20%. In contrast, the BB was less effective for non-buoyant polystyrene (PS) particles. Smaller PS MPs (75–125 μm) exhibit minimal retention due to their low inertia and strong coupling with the flow field, resulting in downstream recoveries of 80% and similar behaviour to fluorescein. Mid-sized PS particles (200–400 μm) exhibited moderate interaction with BB-induced turbulence, whereas the largest particles (600–1000 μm) were governed mainly by gravitational settling, with the low recovery primarily linked to early-stage deposition rather than BB-mediated retention. Notably, velocity contours at higher pressures showed that strong upward convection near the BB can remobilise smaller non-buoyant MPs, offering potential for improving the retention of particles deposited in the sediments. The results highlight the BB’s capacity to significantly influence solute and particle transport through pressure-dependent modulation of the local flow field. While highly effective for large, buoyant MPs, its limitations for denser, smaller particles underscore the need for refined BB designs that optimise turbulent interactions, particularly vertical flow components. These findings advance our understanding of multiphase flow manipulation for environmental remediation and provide a mechanistic basis for improving BB systems to achieve comprehensive microplastic retention.