Removal of polystyrene nanoplastic beads using gravity-driven membrane filtration: Mechanisms and effects of water matrices

Nanoplastics (NPs) are emerging water contaminants and the smaller particle size of NPs further hinders the remediation in the aspects of energy consumption and removal efficiency. Due to the loose structure and tunable functionalities, nanofibrous membranes, with ultrathin fibers (73.3 ± 31.9 nm), were developed for NPs removal in a gravity-driven application. With an ultralow ΔP₀ of 353 Pa, greater than92 % of model polystyrene NPs (average size from 107 to 1450 nm) were removed at an average water flux of 109 Lm^-2h⁻¹. With advanced material characterization, the removal mechanisms were determined as: (1) size-exclusive membrane interception, which dominates the removal when d_NPs > d_pores; and (2) membrane adsorption of smaller NPs via electrostatic and hydrophobic interactions. The distinct removal mechanisms trigger different long-term performances on removal and water flux. Complex water matrices were also evaluated, where acidic condition and anionic surfactant were observed to hinder the removal. In addition, the nanofibrous membranes showed resistance towards inorganic scaling whereas organic foulants decreased the water flux by 15.6 % after 40 consecutive filtration runs. The fouled membranes can be regenerated in mild conditions with 5 % ethanol. Overall, nanofibrous membranes are effective in NPs removal with the high-removal efficiency, low-fouling tendency, and ease of regeneration.