Energy-efficient desalination and water treatment technologies play a critical role in augmenting freshwater resources without placing an excessive strain on limited energy supplies. By desalinating high-salinity waters using low-grade or waste heat, membrane distillation (MD) has the potential to increase sustainable water production, a key facet of the water-energy nexus. However, despite advances in membrane technology and the development of novel process configurations, the viability of MD as an energy-efficient desalination process remains uncertain. In this review, we examine the key challenges facing MD and explore the opportunities for improving MD membranes and system design. We begin by exploring how the energy efficiency of MD is limited by the thermal separation of water and dissolved solutes. We then assess the performance of MD relative to other desalination processes, including reverse osmosis and multi-effect distillation, comparing various metrics including energy efficiency, energy quality, and susceptibility to fouling. By analyzing the impact of membrane properties on the energy efficiency of an MD desalination system, we demonstrate the importance of maximizing porosity and optimizing thickness to minimize energy consumption. We also show how ineffective heat recovery and temperature polarization can limit the energetic performance of MD and how novel process variants seek to reduce these inefficiencies. Fouling, scaling, and wetting can have a significant detrimental impact on MD performance. We outline how novel membrane designs with special surface wettability and process-based fouling control strategies may bolster membrane and process robustness. Finally, we explore applications where MD may be able to outperform established desalination technologies, increasing water production without consuming large amounts of electrical or high-grade thermal energy. We conclude by discussing the outlook for MD desalination, highlighting challenges and key areas for future research and development.

https://par.nsf.gov/servlets/purl/10058665

Membrane distillation at the water-energy nexus: limits, opportunities, and challenges

Design and fabrication of superwetting fiber-based membranes for oil/water separation applications

We investigated the factors that determine surface omniphobicity of microporous membranes and evaluated the potential application of these membranes in desalination of low surface tension wastewaters by membrane distillation (MD). Specifically, the effects of surface morphology and surface energy on membrane surface omniphobicity were systematically investigated by evaluating wetting resistance to low surface tension liquids. Single and multilevel re-entrant structures were achieved by using cylindrical glass fibers as a membrane substrate and grafting silica nanoparticles (SiNPs) on the fibers. Surface energy of the membrane was tuned by functionalizing the fiber substrate with fluoroalkylsilane (FAS) having two different lengths of fluoroalkyl chains. Results show that surface omniphobicity of the modified fibrous membrane increased with higher level of re-entrant structure and with lower surface energy. The secondary re-entrant structure achieved by SiNP coating on the cylindrical fibers was found to play a critical role in enhancing the surface omniphobicity. Membranes coated with SiNPs and chemically modified by the FAS with a longer fluoroalkyl chain (or lower surface energy) exhibited excellent surface omniphobicity and showed wetting resistance to low surface tension liquids such as ethanol (22.1 mN m(-1)). We further evaluated performance of the membranes in desalination of saline feed solutions with varying surface tensions by membrane distillation (MD). The engineered membranes exhibited stable MD performance with low surface tension feed waters, demonstrating the potential application omniphobic membranes in desalinating complex, high salinity industrial wastewaters.

Engineering Surface Energy and Nanostructure of Microporous Films for Expanded Membrane Distillation Applications

Among many other sustainable functional nanomaterials, nanocellulose is drawing increasing interest for use in environmental remediation technologies due to its numerous unique properties and functionalities Nanocellulose is usually derived from the disintegration of naturally occurring polymers or produced by the action of bacteria In this review, some invigorating perspectives on the challenges, future direction, and updates on the most relevant uses of nanocellulose in environmental remediation are discussed The reported applications and properties of nanocellulose as an adsorbent, photocatalyst, flocculant, and membrane are reviewed in particular However, additional effort will be required to implement and commercialize nanocellulose as a viable nanomaterial for remediation technologies In this regard, the main challenges and limitations in working with nanocellulose-based materials are identified in an effort to improve the development and efficient use of nanocellulose in environmental remediation

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Nanocellulose: Recent advances and its prospects in environmental remediation.

Membrane technology remains the most energy-efficient process for removing contaminants (micrometer-size particles to angstrom-size hydrated ions) from water. However, the current membrane technolo ...

Nanocellulose-Enabled Membranes for Water Purification: Perspectives

We fabricated, characterized, and tested novel fibrous aerogel membranes in direct contact membrane distillation (MD) to elucidate the effects of a model high-porosity membrane material on MD performance. Unsupported bacterial nanocellulose aerogels exhibit higher porosity, thinner fibers, and lower bulk thermal conductivity than any previously reported MD materials. Modeling and experiments demonstrate that these material properties confer significantly higher intrinsic membrane permeability and thermal efficiency than symmetric PVDF phase inversion membranes with lower porosity. Development of macroporous fibrous membranes with aerogel-like porosity and thermal conductivity (>98% and <0.03 W m–1 K–1, respectively) in thinner-film formats may further improve MD flux.

Bacterial Nanocellulose Aerogel Membranes: Novel High-Porosity Materials for Membrane Distillation

Fabrication of a dual-layer (CA/PVDF) hollow fiber membrane for RO concentrate treatment

The invention relates to a hollow fiber type composite nano-filtration membrane and a preparation method thereof. The hollow fiber type composite nano-filtration membrane comprises a hollow fiber microporous base membrane adopted as a supporting layer, a polysulfone transition layer, and a polyamide composite layer, wherein the polysulfone transition layer is positioned inside membrane holes of the hollow fiber microporous base membrane, and the polyamide composite layer is positioned inside membrane holes of the polysulfone transition layer. The preparation method comprises: adopting a thermally induced phase separation method to obtain an asymmetric microporous base membrane, adopting an impregnation phase transformation method to obtain the polysulfone transition layer in the micro holes, and adopting pressure control and an interface condensation polymerization reaction to obtain the polyamide composite layer. The hollow fiber type composite nano-filtration membrane provides a sodium chloride salt solution entrapment rate of more than 90%, a divalent salt ion entrapment rate of more than 95%, and a pollutant entrapment rate of more than 99% under 0.2-0.8 MPa, such that the hollow fiber type composite nano-filtration membrane has characteristics of high strength, washing resistance, large flux and the like, wherein the pollutants have a molecular weight of 300-200,000 Daltons.

Hollow fiber type composite nano-filtration membrane and preparation method thereof

Violet phosphorus, as a kind of two-dimensional material, has become an excellent candidate for optoelectronic devices as it makes up for the drawbacks of black phosphorus applications. This study explores the structural and electrical properties of violet phosphorus under high-pressure conditions. Two structure phase transitions are revealed by the spectra and electromagnetic transport measurements. At $\ensuremath{\sim}8$ GPa, a structural transition from monoclinic to A7 phase is observed with the appearance of superconductivity. The transition from the A7 phase to the simple cubic phase starts at $\ensuremath{\sim}13$ GPa and completes after $\ensuremath{\sim}30$ GPa at low temperatures. In addition, the band gap of violet phosphorus decreases linearly with the increase of pressure until it disappears after the transition to the A7 phase. The anisotropic upper magnetic field of superconductivities in A7 and simple cubic phases showed the relative crystal orientations of the high-pressure phases compared with initial violet phosphorus crystal phase. 

Pressure-tuned structure and electric transport properties in violet phosphorus

Van der Waals heterostructures have recently attracted great interest of the scientific community due to their rich exotic physical properties and extensive application prospects. Therefore, we conducted pressure-dependent Raman and photoluminescence spectroscopic studies on MoS2–WS2 heterostructures in different twist angles (24.5 and 54°). Thus, it was confirmed that as the interlayer interaction increases under pressure, an electronic phase transition and a structural phase transition due to layer sliding are observed at ∼1.8 and ∼3.8 GPa in the HS-24.5° structures, while no phase transition is observed in the HS-54° structures. As a result of a larger tunable interlayer space in HS-24.5° structures, optical properties of HS-24.5° structures are more pressure-sensitive than those of the HS-54° structure. It is expected that this work will help comprehensively establish the correlation between the interlayer interactions and optical properties of vdW HSs at the atomic level. Understanding this correlation is crucial for the development of new excitonic devices. 

Chen-Kai Li

Papers

Bacterial Nanocellulose Aerogel Membranes: Novel High-Porosity Materials for Membrane Distillation

Fabrication of a dual-layer (CA/PVDF) hollow fiber membrane for RO concentrate treatment

Hollow fiber type composite nano-filtration membrane and preparation method thereof

Pressure-tuned structure and electric transport properties in violet phosphorus

Tuning of Interlayer Interaction in MoS2–WS2 van der Waals Heterostructures Using Hydrostatic Pressure