Low-pressure driven electrospun membrane with tuned surface charge for efficient removal of polystyrene nanoplastics from water

Nanoplastics (NPs) exert notably negative impacts to the environment. To effectively remove NPs from water, surface charge of a highly porous electrospun polyacrylonitrile (PAN) membrane was precisely tuned in a wide range through a layer-by-layer (LbL) assembly protocol. Three modified membranes (M-, M0 and M+) with enhanced mechanical strength and hydrophilicity respectively displaying negative, neutral and positive charges at the physiological pH (7.2) were prepared by alternatively assembling three layers of polyethylenimine (PEI) and poly(acrylic acid) (PAA) on the hydrolyzed electrospun PAN membrane in different conditions. The polystyrene (PS) microspheres in different sizes (50 nm, 100 nm and 500 nm) were selected as the model NPs. Even the 50 nm NP was much smaller than the membrane pores, it was effectively removed by the M+ membrane (89.9%) from water through a dead-end filtration driven by a very low pressure (0.6 psi). Interestingly, the selectivity of M+ to the 50 nm PS NP fell into the ultrafiltration range, whose permeability (861 L/m(2).h) however was higher than the conventional microfiltration membranes. The electrostatic attraction contributed notably additional selectivity to M+ for NP retention, which however did not lead to obvious irreversible membrane fouling because of the improved membrane surface properties. The membrane prepared through a facile and scalable protocol in this study proposed an efficient mute to relieve the NP pollution in water.