Opportunity
Water scarcity is a critical global challenge, and desalination of seawater or wastewater is a promising solution to provide freshwater. Currently, semi-permeable membranes, particularly those based on two-dimensional (2D) nanomaterials like graphene oxide (GO) and MXenes, are widely used for desalination due to their energy efficiency and sustainability. However, these membranes face significant stability issues. GO laminates are prone to swelling in aqueous environments, which enlarges their interlayer spacing and leads to microstructural disintegration, degrading separation performance. Similarly, MXene membranes also exhibit swelling due to hydrophilic functional groups, causing irreversible degradation. While alternative approaches like physical confinement or chemical cross-linking have been proposed to address swelling, they often involve complex processes that increase costs and hinder practical application. Transition metal dichalcogenides (TMDs), such as molybdenum disulfide (MoS₂), offer better stability than GO or MXenes but suffer from another limitation: pristine MoS₂ laminates have densely packed layers with extremely narrow capillary channels (around 0.2 Å), which impede water permeation and make them unsuitable for efficient desalination. Thus, there is a pressing need to develop stable, high-performance TMD-based membranes with tunable nanochannels that enable both high water flux and excellent ion rejection for continuous water desalination.
Technology
This patent addresses the above challenges by disclosing a novel membrane composed of stacked layers of thio-group functionalized single-layer TMD nanosheets, specifically MoS₂. The innovation lies in covalently grafting thiol-group containing compounds—cysteine, 1-propanethiol, or 3-mercaptopropane-1,2-diol—onto sulfur vacancy sites of exfoliated MoS₂ nanosheets. This functionalization strategically enlarges the interlayer spacing to create a network of well-defined water permeation capillary passages with tunable widths ranging from about 4.5 to 5.5 Å. The process involves exfoliating bulk MoS₂ via lithium intercalation and sonication to produce single-layer nanosheets, followed by reacting them with the thiol compounds in aqueous or organic solutions. The resulting membrane exhibits precisely controlled characteristics: capillary width (e.g., 5.2 Å for 1-propanethiol, 5.1 Å for cysteine, 4.7 Å for 3-mercaptopropane-1,2-diol), surface wettability (contact angles of 64–90°), degree of functionalization (10–15%), low swelling (0.5–4.0%), and thickness (200–1300 nm). These tailored nanochannels are sufficiently wide to allow water molecules to pass through efficiently via forward osmosis (FO) but narrow enough to block hydrated ions (e.g., Na⁺, K⁺, Ca²⁺, Mg²⁺) based on steric hindrance. The covalent bonding ensures membrane stability, preventing the swelling and degradation seen in GO or MXene membranes, while the adjustable capillary width optimizes the trade-off between water permeability and ion selectivity.
Advantages
- High Ion Rejection: Achieves over 99% rejection of monovalent and divalent ions (e.g., Na⁺, K⁺, Ca²⁺, Mg²⁺) in forward osmosis.
- Superior Water Flux: Enables high water permeation rates (e.g., up to 54.1 mol m⁻² h⁻¹ for 1-propanethiol-functionalized membranes), outperforming many existing 2D material membranes.
- Excellent Stability: Exhibits minimal swelling (0.5–4.0%) compared to GO (~70%) or MXene (~27%) membranes, ensuring long-term structural integrity in aqueous environments.
- Tunable Performance: Capillary width and surface properties can be adjusted by selecting different thiol-group compounds (e.g., hydrophobic 1-propanethiol enhances flux, while hydrophilic cysteine improves wettability).
- Robust Durability: Maintains high performance over multiple filtration cycles (e.g., 11 cycles) and extended operation (100 hours), with resistance to chlorine exposure.
- High Selectivity: Demonstrates outstanding water/salt selectivity (up to 800 bar⁻¹), surpassing conventional GO and MXene membranes.
- Broad Compatibility: Effective for desalination of seawater, brackish water, and wastewater, and capable of rejecting organic dyes (e.g., Rhodamine B) with over 90% efficiency.
Applications
- Seawater Desalination: Producing freshwater from seawater using forward osmosis systems for municipal or industrial water supply.
- Wastewater Treatment: Removing ions and micropollutants from industrial or municipal wastewater to enable water reuse.
- Brackish Water Purification: Desalinating brackish water sources for agricultural or potable use in water-scarce regions.
- Portable Water Purification Devices: Integrating into compact, energy-efficient filtration units for emergency or remote area water supply.
- Industrial Process Water Treatment: Purifying water for use in manufacturing, cooling, or boiler systems to prevent scaling and corrosion.
- Dye and Organic Pollutant Removal: Filtering organic contaminants (e.g., dyes) from textile or chemical industry effluents.
- Hybrid Desalination Systems: Combining with other membrane technologies (e.g., reverse osmosis) to enhance overall efficiency and reduce energy consumption.
