Tailoring polyamide morphology of thin film composite membranes based on nanofoaming theory towards enhanced performance for water purification

Abstract

Thin film composite (TFC) polyamide reverse osmosis (RO) membranes have been widely employed in desalination and water reuse to alleviate global water shortage. The separation performance of TFC RO membranes can be primarily determined by the nanovoids-containing roughness structure of their polyamide layers. To enhance the membrane performance, this thesis proposed three interfacial polymerization (IP) strategies to tailor the polyamide roughness structure based on a novel nanofoaming theory.

According to the theory, the nanovoids-containing roughness is formed by generated gas/vapor nanobubbles due to interfacial degassing of the aqueous solution and interfacial vaporization of the organic solvents during IP. Presumably, applying more volatile co-solvents can enhance the generation of vapor nanobubbles. Correspondingly, we found two essential roles of co-solvents in IP: (1) directly enhancing interfacial vaporization with higher volatility and (2) increasing amine solubility in the organic phase, thereby indirectly promoting IP. A co-solvent favoring interfacial vaporization could create more extensive nanovoids, significantly improving membrane permeance. Moreover, the resulted membrane showed enhanced anti-fouling performance.

To further enhance the nanobubble generation, plasmonic nano-heating was integrated during IP to improve both interfacial degassing and vaporization. Specifically, we introduced silver nanoparticles to the IP interface to serve as nano-heat-generators with their strong plasmon resonance under light illumination. A rapid local heating was achieved at the interface to boost the reactivity of monomers, improve their local mass transfer, and facilitate interfacial degassing/vaporization. The resultant RO membrane, featuring highly crosslinked polyamide with extensive nanovoids, exhibits transcendent desalination performance, excellent anti-fouling ability, and highly effective removal of various contaminants found in different water sources.

Apart from nanobubble generation, surfactants were employed to enhance nanobubble retention by stabilizing the nanobubbles at the reaction interface. Accordingly, we obtained enlarged nanovoids when the surfactant was added below its critical micelle concentration (CMC). Both membrane permeance and selectivity were enhanced thanks to the enlarged nanovoids and reduced defects in polyamide. Increasing the concentration above CMC resulted in shrunken nanovoids and deteriorated performance due to decreased stabilization effect by micelle formation. Interestingly, lower fouling propensity was also observed for the surfactant-assisted membranes.

Due to the observation of better anti-fouling performance for all the nanofoamed membranes tailored by the three IP strategies, we provided further investigations on the role of polyamide roughness in membrane fouling. We compared fouling behaviors of a smooth polyamide membrane and a nanovoid-containing rough polyamide membrane. The smooth membrane experienced severer fouling due to its uneven flux distribution caused by ‘funnel effect’. In contrast, the lower fouling tendency of the rough membrane can be explained by: (1) the weakened ‘funnel effect’ thanks to the modified water transport pathway through the membrane by nanovoids; and (2) the decreased average localized flux over the membrane surface due to the nanovoid-enlarged effective filtration area.

Overall, this thesis proposes three IP strategies to regulate the polyamide formation for improved membrane performance based on the nanofoaming theory. The mechanistic insights can not only help to better interpret some controversial results in existing literature but also facilitate the future development of high-performance RO membranes.