Preparation of Small-Pore Ultrafiltration Membranes with High Surface Porosity by In Situ CO2 Nanobubble-Assisted NIPS

The fabrication of ultrafiltration (UF) membranes with a small pore size (<20 nm) and high surface porosity is still a great challenge. In this work, a nanobubble-assisted nonsolvent-induced phase separation (BNIPS) technique was developed to prepare high-performance UF membranes by adding a tiny amount of CaCO₃ nanoparticles into the casting solution. The phase inversion occurred in a dilute-acid coagulation bath to simultaneously generate CO₂ nanobubbles, which regulated the membrane structure. The effects of the nano-CaCO₃ content in the casting solution on the structure and performance of poly(ethersulfone)/sulfonated polysulfone (PES/SPSf) UF membranes were studied. The UF membrane prepared from a casting solution with 0.3% nano-CaCO₃ achieved a surface porosity of 12%, a pore diameter of 10.2 nm, and a skin-layer thickness of 80.3 nm. The superior structure of the UF membrane was mainly attributed to the in situ generation of CO₂ nanobubbles because the CO₂ nanobubbles were amphiphobic to water and solvents to delay the phase inversion time and acted as nanosize porogens. The produced membrane showed an unprecedented separation performance, achieving a pure water permeance of up to 1128 L·m^-2·h^-1·bar^-1, 2.5 fold that of the control membrane. Similarly, a high bovine serum albumin rejection of above 99.0% was obtained. The overall permeability and selectivity were better than those of commercial and other previously reported UF membranes. This work provides insight toward a simple and cost-effective technique to address the trade-off between pure water permeance and solute rejection of UF membranes.