Real-time quantification of nanoplastics deposition in nanofiltration using laser-induced breakdown detection (LIBD)

Nanofiltration (NF) is an effective barrier for removing nanoplastics (NPs) from water. However, NPs can deposit on the membrane surface and remain even after backwash, altering surface properties and reducing filtration performance. In this study, laser-induced breakdown detection (LIBD) is coupled in-line with a bench-scale NF system to quantify the deposition and release of polystyrene (PS) particles and weathered NPs at environmentally relevant concentrations (1-500 µg/L; 10⁷-10⁹ particles/mL). Particle deposition during filtration and release during backwash were successfully determined in all experiments, supported by theoretical analysis of the interplay between hydrodynamic and intermolecular forces. At permeate fluxes higher than 100 L/m².h, 50-100% of PS particles were deposited by the end of filtration experiments, forming cake layers up to 9 particle diameters thick. In contrast, weak permeate drag forces corresponding to fluxes below 50 L/m².h (i.e., just beyond the critical flux) resulted in insignificant deposition. Backwash fluxes from 15 to 57 L/m².h exhibited negligible differences in the release of deposited particles owing to only a little increase in backwash drag force. Two mechanisms were observed for the release of NPs during backwash: i) complete release in small circular areas (diameter ≤2 µm) for thin deposits and ii) fracturing of the cake layer for thick deposits. For weathered NPs, irreversible deposition on the membrane surface was observed, although potential particle aggregation and polydispersity must be accounted for to obtain truly quantitative results. The successful quantification of particle deposition and release showcases LIBD as an effective method for fundamental investigations on NP transport during membrane filtration.