Preparation of a tight ultrafiltration membrane containing bimetallic Co-Fe hydroxide nanoparticles for nitrophenol separation

To address the efficient separation of refractory small-molecule pollutants such as p -nitrophenol (PNP) from industrial wastewater, bimetallic Co-Fe hydroxide nanoparticles were synthesized via a reverse microemulsion method and incorporated into polymer matrices. Initial studies on hybrid membranes with polyvinylidene fluoride (PVDF) matrices demonstrated enhanced hydrophilicity (contact angle reduction from 77.7° to 64.9°) and antifouling performance (85 % flux recovery rate), but their large pore size (∼70 nm) limited PNP rejection (22.5 %). To overcome this problem, polysulfone (PSF) was selected as an alternative matrix substrate. By systematically adjusting the film-forming time (15–50 min) during phase inversion, tight ultrafiltration membranes with controlled pore structures were fabricated. Prolonged film-forming time (30 min) optimized the solvent-nonsolvent exchange kinetics of the obtained PSF-3 membrane, yielding a dense selective layer with a narrow pore size distribution (3–8 nm) and molecular weight cut-off. This membrane achieved a PNP rejection rate of 71 % compared to 5 % of the PVDF membrane through synergistic "size sieving-hydroxyl adsorption" while maintaining low salt rejection (<15 %) and high flux stability (<15 % flux decline rate over 6 h). Hydrophilicity (49.6° contact angle) and antifouling performance (85 % flux recovery rate after four cycles) were significantly enhanced by uniformly dispersed nanoparticles. This study highlights the critical role of film-forming time in tailoring membrane architecture and performance, providing a novel strategy for efficient small-molecule separation from saline wastewater.