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The conventional interfacial polymerization (CIP) technique used for preparing thin-film composite (TFC) nanofiltration membranes typically requires a large amount of monomers during polyamide (PA) synthesis where most of the monomers are discarded after cross-linking. Thus, a new fabrication concept is proposed in this work to synthesize a PA layer via a mist-based interfacial polymerization (MIP) technique where only a small amount of aqueous solution is dispersed as mist. This approach also eliminates the rubber-rolling step in CIP. In addition to forming a thinner and looser PA structure, the piperazine solution required in the IP reaction is significantly reduced, that is, 17 times lower than that of CIP. The microdroplet dispersion approach in MIP could form a higher cross-linked PA due to the high polymerization interface besides forming a higher free-volume selective layer due to the disruption in the PA repeat structure. Our findings revealed that the newly developed mist-based TFC membrane could achieve 9.08 L/m2·h·bar pure water permeability and 97.2% Na2SO4 rejection coupled with a complete flux recovery rate. As a comparison, the conventional TFC membrane only attained 2.84 L/m2·h·bar and 95.7%, respectively. The MIP technique could also be potentially considered for developing a nanofiller-incorporated TFC membrane due to the absence of the rubber-rolling step.

In this work, several ultrafiltration (UF) membranes with enhanced antifouling properties were fabricated using a rapid and green surface modification method that was based on the plasma-enhanced chemical vapor deposition (PECVD). Two types of hydrophilic monomers—acrylic acid (AA) and 2-hydroxyethyl methacrylate (HEMA) were, respectively, deposited on the surface of a commercial UF membrane and the effects of plasma deposition time (i.e., 15 s, 30 s, 60 s, and 90 s) on the surface properties of the membrane were investigated. The modified membranes were then subjected to filtration using 2000 mg/L pepsin and bovine serum albumin (BSA) solutions as feed. Microscopic and spectroscopic analyses confirmed the successful deposition of AA and HEMA on the membrane surface and the decrease in water contact angle with increasing plasma deposition time strongly indicated the increase in surface hydrophilicity due to the considerable enrichment of the hydrophilic segment of AA and HEMA on the membrane surface. However, a prolonged plasma deposition time (>15 s) should be avoided as it led to the formation of a thicker coating layer that significantly reduced the membrane pure water flux with no significant change in the solute rejection rate. Upon 15-s plasma deposition, the AA-modified membrane recorded the pepsin and BSA rejections of 83.9% and 97.5%, respectively, while the HEMA-modified membrane rejected at least 98.5% for both pepsin and BSA. Compared to the control membrane, the AA-modified and HEMA-modified membranes also showed a lower degree of flux decline and better flux recovery rate (>90%), suggesting that the membrane antifouling properties were improved and most of the fouling was reversible and could be removed via simple water cleaning process. We demonstrated in this work that the PECVD technique is a promising surface modification method that could be employed to rapidly improve membrane surface hydrophilicity (15 s) for the enhanced protein purification process without using any organic solvent during the plasma modification process.

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