Tailoring morphology of polyamide thin film composite membranes with nanobubble chemistry for enhanced separation performance in desalination and water reuse

Abstract

Polyamide-based thin film composite (TFC) reverse osmosis (RO) membranes have been dominantly applied in desalination and water reuse to address worldwide water scarcity. The membrane separation performance is strongly dependent on polyamide roughness features and the associated nanosized voids. However, the formation mechanisms of these features and voids remain poorly understood in literature. The impacts of interfacial polymerization (IP) chemistry and substrate properties on polyamide formation remain controversial. This thesis aims to revisit and explore the exact IP/substrate-morphology-performance correlation based on a novel nanofoaming theory. Inspired by the recent findings that the nanosized voids are formed due to the degassing of CO2 nanobubbles during the IP reaction, we systematically investigated the role of carbonate chemistry, particularly the solubility of CO2 in the aqueous m-phenylenediamine (MPD) solution. “Ridge-and-valley” roughness features were obtained when the pH was between the two acidity constants of the carbonate system (i.e., 6.3 ≤ pH ≤ 10.3). Increasing pH over this range led to both increased water permeability and better rejection of various solutes, thanks to the simultaneously enhanced effective filtration area and crosslinking degree of the polyamide layer. Further increase of pH to 12.5 resulted in more disparate rejection results due to membrane hydrolysis. The impacts of organic solvents on formation of the nanovoids were further investigated. Compelling evidence was found that vaporization of the organic solvent contributes to nanovoids formation during the exothermic IP process. A series of alkane solvents with systematically varying vapor pressure were used to prepare TFC membranes. An organic solvent with higher vapor pressure generated more vapor during the IP reaction, which in turn resulted in larger size of the voids in the polyamide thin film and higher membrane water permeability. The impacts of MPD concentration (0.05-8.0 wt.%) on polyamide formation were subsequently deciphered by adopting a free-interface IP strategy to suppress the nanofoaming effect. The corresponding polyamide nanofilms had negligible nanovoids and monotonously increased film thickness, leading to decreased water permeance at higher MPD concentrations. In contrast, the conventional TFC membranes exhibited optimal water permeance at the intermediate MPD concentration of 2 wt.%, which results from the tradeoff between improved nanovoid formation (which promotes higher permeance) and increased film growth (which limits permeance) at higher MPD concentration. The exact roles of substrates were further dissected. TFC membranes were prepared on a series of polycarbonate substrates with cylindrical track-etched pores (PCTE) of well-defined pore size and several conventional substrates with random pores. Substrate porosity plays a critical role in membrane water permeance, while smaller pores with greater pore density are favored to improve membrane rejection. The TFC membranes prepared on conventional substrates exhibit better performance compared to the PCTE-TFC membranes, thanks to the simultaneously enhanced confinement and MPD storage effects. Overall, this thesis provides new angles to understand the roles of reaction conditions and substrate properties on polyamide. The mechanistic insights can favor better interpretations on some controversial observations in literature. The fundamental framework gained in this thesis further improves the nanofoaming theory, which can guide the future design and optimization of TFC membranes.