Microporous Polyethersulfone Membranes Grafted with Zwitterionic Polymer Brushes Showing Microfiltration Permeance and Ultrafiltration Bacteriophage Removal

Virus removal from water using microfiltration (MF) membranes is of great interest but remains challenging owing to the membranes' mean pore sizes typically being significantly larger than most viruses. We present microporous membranes grafted with polyzwitterionic brushes (N-dimethylammonium betaine) that combine bacteriophage removal in the range of ultrafiltration (UF) membranes with the permeance of MF membranes. Brush structures were grafted in two steps: free-radical polymerization followed by atom transfer radical polymerization (ATRP). Attenuated total reflection Fourier transform infrared (ATR–FTIR) and X-ray photoelectron (XPS) verified that grafting occurred at both sides of the membranes and that the grafting increased with increasing the zwitterion monomer concentration. The log reduction values (LRVs) of the pristine membrane increased from less than 0.5 LRV for T4 (∼100 nm) and NT1 (∼50 nm) bacteriophages to up to 4.5 LRV for the T4 and 3.1 LRV for the NT1 for the brush-grafted membranes with a permeance of about 1000 LMH/bar. The high permeance was attributed to a high-water fraction in the ultra-hydrophilic brush structure. The high measured LRVs of the brush-grafted membranes were attributed to enhanced bacteriophages exclusion from the membrane surface and entrapment of the ones that penetrated the pores due to the membranes' smaller mean pore-size and cross-section porosity than those of the pristine membrane, as seen by scanning electron microscopy (SEM) and measured using liquid–liquid porometry. Micro X-ray fluorescence (μ-XRF) spectrometry and nanoscale secondary ion mass spectrometry showed that 100 nm Si-coated gold nanospheres accumulated on the surface of the pristine membrane but not on the brush-coated membrane and that the nanospheres that penetrated the membranes were entrapped in the brush-grafted membrane but passed the pristine one. These results corroborate the LRVs obtained during filtration experiments and support the inference that the increased removal was due to a combined exclusion mechanism and entrapment. Overall, these microporous brush-grafted membranes show potential for use in advanced water treatment.