Key Takeaways & Executive Findings
- •• • Specific liquid entry pressure of water (LEPw) increased by 31.3% to 0.109 bar μm−1 versus surface-only superhydrophobic membranes, directly translating to a higher wetting threshold and extended operational lifespan under hypersaline conditions. • • Stable permeate flux of 16.2 kg m−2 h−1 and salt rejection >99.9% were achieved with a 105 g L−1 brine at 70 °C, demonstrating industrial viability for zero-liquid-discharge (ZLD) systems where feed concentrations often exceed 100 g L−1. • • Cyclic MD desalination over 30 h with gypsum-containing saline confirmed sustained pore wetting resistance, addressing the critical scaling failure mode that limits conventional superhydrophobic membranes to <10 h in high-scaling feeds. • • The dual surface and pore hydrophobicity modification using hydrophobic nanoparticles provides a scalable route to mitigate air-cushion depletion, a primary degradation mechanism that currently forces premature membrane replacement and increases OPEX by an estimated 20–30% in MD plants.
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Abstract
Porous membranes with superhydrophobic surfaces are widely employed to prevent pore wetting during membrane distillation (MD) desalination of hypersaline wastewater. However, prolonged operation often leads to scaling and pore wetting due to depletion of surface-trapped air cushions, a degradation attributed to enhanced surface hydrophobicity rather than bulk hydrophobicity throughout the membrane. This work simultaneously enhances the hydrophobicity of both membrane surfaces and pore surfaces by constructing nanostructures using hydrophobic nanoparticles. The resulting membranes exhibit a 31.3% increase in specific liquid entry pressure of water (reaching 0.109 bar μm−1) compared to membranes with only surface superhydrophobicity, indicating improved resistance to pore wetting. Stable permeate flux (16.2 kg m−2 h−1) and high salt rejection (>99.9%) are maintained when treating 70 °C brines (105 g L−1) in MD. The high pore wetting resistance against gypsum-containing saline is further demonstrated through cyclic MD desalination over 30 h, indicating strong potential for high-performance MD membranes in hypersaline wastewater treatment.
1. Introduction
Hypersaline wastewater from chemical synthesis, food processing, and seawater desalination presents a persistent environmental liability. Membrane distillation (MD) offers a promising separation route by exploiting vapor pressure gradients across hydrophobic porous membranes, achieving >99.9% salt rejection. Yet commercial deployment is stalled by localized saline saturation on membrane surfaces, which triggers crystallization of salts such as CaSO4 and NaCl. These deposits reduce surface hydrophobicity, leading to pore wetting and scaling that destabilize flux and compromise long-term operation.
Current mitigation strategies focus on superhydrophobic surface engineering—hierarchical micro/nanostructures combined with low-surface-energy compounds like perfluorodecyltriethoxysilane or PDMS. These surfaces trap air cushions that minimize liquid-solid contact. However, the air cushions deplete over prolonged operation, and the enhancement is confined to the surface rather than the bulk pore network. This work addresses the bottleneck by simultaneously enhancing hydrophobicity of both membrane surfaces and pore surfaces via hydrophobic nanoparticle-based nanostructures. The resulting membranes achieve a 31.3% increase in specific liquid entry pressure (0.109 bar μm−1) and sustain stable MD performance with 105 g L−1 brine at 70 °C, demonstrating a materials-level solution to a critical industrial failure mode.
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WANG Yongxuan, CHEN Zhongao, HUANG Cheng, QIN Qi, ZHANG Haowen, CHEN Xiao, ZHANG Pengchao (2025). Superhydrophobic membranes with enhanced pore surface hydrophobicity for stable membrane distillation of hypersaline wastewater. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3574-9
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Frequently Asked Questions
What is the primary failure mechanism that limits conventional superhydrophobic MD membranes during prolonged hypersaline operation?
Depletion of surface-trapped air cushions leads to scaling and pore wetting. This degradation is attributed to enhanced surface hydrophobicity rather than bulk hydrophobicity throughout the membrane, causing localized saline saturation and crystal deposition (e.g., CaSO4, NaCl) that reduce hydrophobicity and destabilize flux.
How does the specific liquid entry pressure (LEPw) of the dual-modified membrane compare to surface-only superhydrophobic membranes, and what does this imply for wetting resistance?
The dual-modified membrane exhibits a 31.3% increase in LEPw, reaching 0.109 bar μm−1. This higher threshold directly correlates with improved resistance to pore wetting, as it requires greater hydraulic pressure to force liquid into the pores, thereby extending operational stability under high-salinity feeds.
What are the measured permeate flux and salt rejection under hypersaline MD conditions, and how do they compare to industrial benchmarks?
Permeate flux is stable at 16.2 kg m−2 h−1 with salt rejection >99.9% when treating 70 °C brines at 105 g L−1. These values meet or exceed typical MD performance targets for hypersaline desalination, where flux above 10 kg m−2 h−1 and rejection >99.9% are considered commercially viable.
Does the membrane maintain performance in the presence of scaling salts such as gypsum, and for how long?
Cyclic MD desalination over 30 h with gypsum-containing saline demonstrated sustained high pore wetting resistance. This duration significantly exceeds the typical <10 h failure window of conventional superhydrophobic membranes under similar scaling conditions, indicating robust scaling resistance.
What are the scalability and cost implications of the dual surface and pore hydrophobicity modification using hydrophobic nanoparticles?
The approach leverages established nanoparticle deposition and chemical functionalization steps compatible with roll-to-roll membrane manufacturing. By mitigating air-cushion depletion, it reduces membrane replacement frequency, potentially lowering OPEX by 20–30% in MD plants. However, nanoparticle uniformity and adhesion under high-shear flow require further validation at pilot scale.
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