Alleviation of Reverse Salt Leakage across Nanofiber Supported Thin-Film Composite Forward Osmosis Membrane via Heat-Curing in Hot Water

Electrospun nanofiber with interconnected porous structure has been studied as a promising support layer of polyamide (PA) thin-film composite (TFC) forward osmosis (FO) membrane. However, its rough surface with irregular pores is prone to the formation of a defective PA active layer after interfacial polymerization, which shows high reverse salt leakage in FO desalination. Heat-curing is beneficial for crosslinking and stabilization of the PA layer. In this work, a nanofiber-supported PA TFC membrane was conceived to be cured on a hot water surface with preserved phase interface for potential “defect repair”, which could be realized by supplementary interfacial polymerization of residual monomers during heat-curing. The resultant hot-water-curing FO membrane with a more uniform superhydrophilic and highly crosslinked PA layer exhibited much lower reverse salt flux (FO: 0.3 gMH, PRO: 0.8 gMH) than that of oven-curing FO membrane (FO: 2.3 gMH, PRO: 2.2 gMH) and achieved ∼4 times higher separation efficiency. It showed superior stability owing to mitigated reverse salt leakage and osmotic pressure loss, with its water flux decline lower than a quarter that of the oven-curing membrane. This study could provide new insight into the fine-tuning of nanofiber-supported TFC FO membrane for high-quality desalination via a proper selection of heat-curing methods.

. Schematic diagram of the lab-scale FO setup.

Optimization of the hot-water-curing condition
In order to explore the hot-water-curing condition for fabricating the PA active layer of nanofiber supported TFC membrane with optimum FO performance, the effects of curing temperatures (60, 70, 80, 90 ℃) and curing time (1,2,4,6,8,10 min) on the pure water fluxes and reverse salt fluxes were investigated. Theoretically, raising heat-curing temperature can accelerate the diffusion of monomers to the reaction zone, and promote the motion and crosslinking of PA chains. However, overlong heat treatment under high temperature would lead to the deformation, damage and delamination of the PA active layer on support. According the results in Figure S2a, the curing temperature exhibited an obvious impact on the salt rejection of w-TFC. It seemed that there are significant defects in PA cured on hot water below 80 ℃, as the as-prepared w-TFC showed much severer reverse salt leakage due to the incomplete polymerization with insufficient curing temperature. Furthermore, when curing at higher temperature (90 ℃), the obtained PA layer could be quite dense and thus showed decreased water flux. Additionally, as seen in Figure S2b, insufficient or excessive curing time could cause defects in hot-water-curing PA layer, and the proper curing time within 2-6 min range would be favorable for better FO performance. Based on the comprehensive consideration on the trade-off between water flux and reverse salt leakage, all the mentioned hot-water-curing w-TFC membranes in this paper were prepared conducting the curing parameter (80 ℃, 4 min) as default.

Nanofiber supported PEI/TMC-PA TFC FO membrane
Typically, the MPD/TMC-PA film has inadequate hydrophilicity and rough ridgevalley morphology that is easily subjected to foulants blocking [1]. As it has been reported that, hyperbranched macromolecule polyethyleneimine (PEI) could be applied as a possible alternative to MPD, to fabricate loose and quite smooth hydrophilic nanofiltration (NF)-like PA active layers of TFC membranes with potential of high water permeability and low fouling propensity for FO application [2,3]. However, the oven-curing PEI/TMC-PA active layer possessing frequent defects, especially when prepared on the nanofiber support, hardly overcome the tradeoff between water flux and salt leakage. It normally showed severe salt leakage, or had to match with special draw solutes but showed relatively low water flux [4][5][6][7].
Herein, PEI was also employed to evaluate the "defect-healing" potential of hot-water-curing for the fabrication of the PEI/TMC-PA TFC membrane. The effects of adopting different heat curing methods on the morphology, hydrophilicity and FO performance of nanofiber supported PA layer synthesized by interfacial polymerization of PEI and TMC was further investigated.

Preparation process of the nanofiber supported PEI/TMC-PA TFC FO membrane
2% (w/v) PEI (Polyethyleneimine, Mw = 70000 g/mol, 50% aqueous solution, supplied by Aladdin, Shanghai, China) aqueous solution was used as an alternative monomer to MPD to saturate the PAN nanofiber support for 4 min, and then reacted with 0.2% (w/v) TMC for 1 min to form a loose NF-like PA active layer on the nanofiber membrane, and then performed with heat curing in oven or on hot water at 80 ℃ for 4 min.
DI water and 0.5 M MgCl2 were used as the feed solution and draw solution respectively to test the FO performance of the PEI/TMC-PA TFC membranes. And the other operation conditions were the same as that of MPD/TMC-PA TFC membranes.

FO performance of the nanofiber supported PEI/TMC-PA TFC FO membrane
As shown in Figure S5, the hot-water-curing PEI/TMC-PA TFC membranes exhibited comparable water flux, and quite lower reverse salt flux with excellent selectivity than the oven-curing TFC, which indicates the distinctive advantage of hot-water-curing over oven-curing in preparation of the PEI/TMC-PA active layer of TFC membrane with superior FO performance. Moreover, the formed PEI/TMC-PA possesses quite smooth morphology and superhydrophilicity ( Figure S6, S7), which can be able to effectively avoid the blocking and accumulation of pollutants, compared to the ridge-valley microstructures of the MPD/TMC-PA layer. Therefore, with hot water as the heat curing medium, PEI would be a good alternative as the aqueous monomer for interfacial polymerization thus achieving the aim of fabricating high-performance TFC FO membranes based on nanofiber support possessing favorable surface morphology and property for antifouling and cleaning. Simultaneously, with proper selection of novel draw solutes [8][9][10], heat curing would be a facile way to tune the surface properties and FO performance of nanofiber supported PA-TFC membranes for specific applications.