Fouling and retention behavior of micro- and nano-plastics in gravity-driven membrane filtration

Micro- and nano-plastics (MPs/NPs) pose potential risks to the environment. Gravity-driven membrane filtration offers a potential solution for plastic retention, but the size-dependent fouling behavior remains insufficiently understood. This study presents a mechanistic analysis of micro- and nano-plastic fouling in gravity-driven membrane filtration by integrating particle-size effects, operation height, classical fouling models, and XDLVO theory. Commercial PVDF membranes were used to filter PS beads with particle sizes (20–500 nm) under operation heights of 15–45 cm. Fouling extent and mechanisms are primarily governed by particle size, while operation height affects fouling type. For large particles, both membrane pore entry and surface accumulation led to rapid flux decline, followed by stabilization, with retention consistently above 95%. In contrast, the retention rate for small plastics (20–50 nm) remained below 10% but increased with operation height. Notably, for plastics with sizes comparable to the membrane pores (50–200 nm), retention rates initially increased but decreased at higher operation heights (45 cm), due to pore-clogging and secondary pore formed by spherical particles, enhancing size exclusion. However, excessive driving pressure disrupted pore stacking, reducing retention efficiency. Larger particles required lower driving pressures for optimal pore blockage. These findings offer insights for selecting membrane pore sizes and operation pressures in plastic pollution management, and guide predictions of membrane fouling behavior.