A membrane cascade for size-based separation and concentration of nanoplastics in environmental waters

The presence of nanoplastics (NP) in freshwater and seawater is a growing environmental concern, although little is known about their amount, structure or behaviour associated with natural organic matter (NOM). While efforts have been made to isolate them, previous protocols were not able to minimize losses during extraction, preventing reliable characterization. Additionally, NP size fractionation, crucial for understanding of their transport dynamics and toxicity, remains challenging. This study presents a new protocol using a cascade of microfiltration (MF) membranes to i) extract NP from natural waters, ii) separate them by size range, iii) concentrate each fraction, iv) maximize recovery rates. Filtration experiments were performed using fluorescent polystyrene nanoplastics (PS NPs) of various sizes to account for the size polydispersity of environmental NP and evaluate membrane selectivity. Coupled with comprehensive membrane characterization and fouling models, the results showed that cascading a 2 µm stainless steel grid, a 0.45 µm PVDF membrane and a 0.1 µm PES membrane successfully allowed to separate NP into three size ranges (< 50 nm; 50–500 nm; 50–2000 nm), while achieving repeatable recoveries of 52 % for 500 nm PS NPs, 95 % for 200 nm PS NPs and 93 % for 50 nm PS NPs. The cascade was also tested on real river water doped with PS NPs: selectivity and recovery of all size fractions could be maintained for NOM concentrations of 0.11–0.54 mg/L. The development of this filtration cascade will serve as a foundation for efficient extraction of environmental NP, and unveils new knowledge about the use of MF membranes in this prospect.