Effects of Surface Morphology on Mesoporous Silicon-Modified Nanofiltration Membranes for High Rejection Performances

A novel approach was developed in this work in which composite nanofiltration (NF) membranes were directly and efficiently fabricated with control of the membrane pore structure and surface morphology. The fabrication of mesoporous silicon-modified polysulfone blend membranes is achieved via a phase inversion method. The structural morphology, surface functional group analysis, elemental analysis, hydrophilicity, chargeability, and nitrogen pollutant (ammonia nitrogen, nitrate nitrogen, total nitrogen) rejection properties of the modified membranes were found to be dependent on the amount of mesoporous silicon incorporated. The combination of the mesoporous silicon framework layer can not only effectively improve the surface structure of the modified membrane with a narrow pore size distribution but also increase the rejection of nitrogen pollutants compared with pure NF membranes. The mesoporous material interlayer can absorb and store the aqueous amino solution to facilitate the subsequent interfacial polymerization as well as induce changes in the pore radius and surface structure. Compared with pure NF composite membranes, the modified blend membranes exhibit increased water permeation flux as high as 29.09 L m^-2 h^-1 at 0.2 MPa. The results show that the optimum doping amount of mesoporous silicon is in the range of 0.5-1.0%. Characterization studies demonstrated that the addition of mesoporous silicon leads to a decreased membrane pore size. Then the retention of nitrogen pollutants was enhanced because of a combination of hydrophilicity enhancement from the carboxylic and hydroxyl functional groups present in their surfaces leading to electrostatic repulsion between functional groups present in the membranes and the nitrogen pollutant molecules.