Biofilm growth is insufficient to retain large buoyant microplastics in constructed wetlands

Microplastics (MPs) are emerging contaminants, with wastewater treatment plants (WWTPs) as principal hotspots for their release into downstream systems, including constructed wetlands (CWs), a nature-based solution for water treatment. While non-buoyant MPs readily settle, buoyant MPs risk bypassing CWs and entering aquatic environments. Biofilm formation could influence MP transport by altering buoyancy, promoting sinking, and enhancing MP retention, yet its role in CWs remains unknown. This study, for the first time, quantifies the effects of MP polymer type, particle characteristics, exposure time, and seasonality on biofilm colonisation and its impact on terminal rising velocities of initially buoyant MPs in a UK-based CW receiving partially treated wastewater. Polypropylene (PP), expanded polystyrene (PS), and low-density polyethylene (LDPE) particles (3-5 mm) in spherical, beaded, and film shapes were incubated in situ over 12 months. Sampling followed two approaches: (1) a rolling bi-monthly schedule to capture seasonal variation, and (2) a long-term deployment with subsets retrieved every two months. Biofilm biomass was quantified by crystal violet staining, surface characteristics were captured by scanning electron microscopy (SEM), and terminal rising velocity experiments measured buoyancy changes. Biofilm growth showed strong seasonality, with peak biomass in late spring showing up to a 1972 % increase compared to winter. Despite widespread colonisation, changes in terminal rising velocity were minimal and largely non-significant (p < 0.05), indicating that biofilm formation alone is insufficient to retain initially buoyant MPs in CWs. These findings are crucial for deriving MP transport models and challenge assumptions that biofilm-induced density changes drive MP retention in CWs.