Unraveling the interactions between nanoplastics and biofilms on dense filtration membrane surfaces

Biofilms readily form on plastic surfaces across diverse environments. However, quantitative understanding of how polystyrene nanoplastics (PS NPls) integrate with biofilms at the cellular level and accumulate over time remains limited. Additionally, the influence of water chemistry, particularly natural organic matter (NOM) and ions-on PS NPl retention by biofilms is not fully resolved. In this study, trackable 200 nm palladium (Pd)-cored PS NPls were used to investigate dynamic NPl-biofilm interactions, including bio-association/co-pelleting, bio-adsorption, and bio-accumulation. Experiments were conducted under controlled high-exposure, membrane-based conditions designed for mechanistic tracking rather than to reproduce natural-water NPl concentrations. Results indicated that NPls can associate with bacterial cells and extracellular materials and be retained within biofilms, with higher bacterial densities promoting co-sedimentation in centrifugation assays. Short-term exposure led to limited retention on established biofilms, whereas 4-day co-development substantially increased biofilm-associated NPls. Biofilm composition, ionic conditions, especially Ca²⁺, and NOM matrix produced condition-dependent effects on NPl retention on fouled NF and reverse osmosis (RO) membranes. Short-term adsorption increased with NPl exposure concentration, whereas final accumulation during 4-day biofilm growth was not significantly different between 2 and 20 mg/L under the tested conditions. NPls did not measurably inhibit P. aeruginosa growth at concentrations up to 20 mg/L. This study provides mechanistic insight into NPl behavior in engineered membrane systems and informs future work on nanoplastic retention at biofilm-rich aquatic interfaces.