Study of molybdenum disulfide for nanofiltration application : stability and transport mechanism

Abstract

Nanofiltration (NF) membranes with high separation efficiency are greatly demanded in various fields, including organic solvent separation and water treatment. The prevailing commercially available NF membranes mostly are thin-film composite (TFC) membranes. Although TFC membranes possess many advantages such as ease of fabrication and a wide applicable pH range, they suffer from a trade-off effect. To overcome this trade-off effect, thin-film nanocomposite (TFNi) membranes have been developed by introducing nanomaterial interlayers into TFC membranes.

Among the various nanomaterials investigated for TFNi membrane preparation, two-dimensional (2D) materials have been extensively investigated. However, hydrophilic terminal groups of 2D nanomaterials such as graphene oxide (GO) can compromise the inherent stability of the TFNi membranes in polar solvents and varying humidity. To address this issue, TFNi membranes were fabricated using molybdenum disulfide (MoS2) nanosheets covalently modified with carboxyl (–COOH) and acylamino (–CONH2) groups. The resulting TFNi-MoS2-CONH2 membrane demonstrated the biggest permeance of 8.60 ± 0.23 L m−2 h−1 bar−1 for MeOH, with a high rejection of 87.1 ± 1.3% for Evans Blue (EB). More importantly, the TFNi membranes interlayered with MoS2-COOH and MoS2-CONH2 nanosheets demonstrated outstanding structural stability, maintaining ~90% permeance and unchanged rejection for EB in MeOH after 12 h drying.

Another approach to enhance both permeability and selectivity is the stacking of 2D lamellar membranes. Many studies have been conducted regarding the application of MoS2 membranes in organic solvent nanofiltration (OSN) due to their high stability. However, studies regarding the effect of interlayer spacing on solvent permeance for MoS2 membranes has been limited. In this thesis, MoS2 membranes were prepared by restacking of monolayer MoS2 nanosheets. These MoS2 membranes exhibited the biggest permeance of 34.68 ± 6.11 L m−2 h−1 bar−1 for Hexane among seven different types of solvents. The Hagen-Poiseuille equation was also employed to understand the relationship between the permeance for these solvents and the effective interlayer spacing. Our findings revealed significant variations in the effective interlayer spacing for 2D MoS2 membranes when processing solvents of different sizes. Furthermore, for 2D membranes with nanoconfined transport channels, the accurate estimate of the interlayer spacing is crucial to improve the precision of the Hagen-Poiseuille equation.

Furthermore, a more profound comprehending of the impact of interlayer spacing on ion transport in 2D membranes holds great promise for their application in desalination and metal ion recovery processes. However, research in this area is limited. In this thesis, we developed MoS2 membranes and tuned the interlayer spacing with varying hydrostatic pressure (1 to 4 bar). Adjusting the pressure allowed us to control the interlayer spacing, ranging from 1.70 nm (1 bar) to 1.42 nm (4 bar), resulting in water permeance ranging from 23 to 8 L m−2 h−1 bar−1. The membrane history test for water permeance demonstrated the effectiveness of the pressure tuning strategy for controlling interlayer spacing of the pressure-tuned MoS2 membranes. Additionally, the energy barrier of four salts transported through the pressure-tuned MoS2 membranes was measured, revealing an increase in the energy barrier as the interlayer spacing of the MoS2 membrane decreased.