Membrane distillation (MD) has been garnering increasing attention in research and development, since it has been proposed as a promising technology for desalinating hypersaline brine from various industries using low-grade thermal energy. However, depending on the application context, MD faces several important technical challenges that would lead to compromised performance or even process failure. These challenges include pore wetting, mineral scaling, and membrane fouling. This review is devoted to providing a state-of-the-art understanding of fundamental mechanisms and mitigation strategies regarding these three challenges. Guided by the fundamental understanding of each membrane failure mechanism, we discuss both operational and material strategies that can potentially address the three technical challenges. In particular, the material strategies involve the development of MD membranes with tailored special wetting properties to impart resistance against different types of membrane failure. Lastly, we also discuss research needs and best practices in future studies to further enhance our ability to overcome technical challenges toward the practical, sustainable, and scalable applications of MD.

We report in this study a scalable and controllable approach for fabricating robust and high-performance superhydrophobic membranes for membrane distillation (MD). This novel approach combines electro-co-spinning/spraying (ES2) with chemical vapor welding and enables the formation of robust superhydrophobic (r-SH) membranes that are mechanically strong, highly porous, and robustly superhydrophobic. Compared with superhydrophobic membranes obtained using surface deposition of fluorinated nanoparticles, the r-SH membranes have more robust wetting properties and higher vapor permeability in MD. MD scaling experiments with sodium chloride and gypsum show that the r-SH membrane is highly effective in mitigating mineral scaling. Finally, we also discuss the mechanism of scaling resistance enabled by superhydrophobic membranes with a highlight on the roles of the surface-bound air layer in reducing the crystal-membrane contact area, nucleation propensity, and ion-membrane contact time.

Mineral scaling constrains membrane distillation (MD) and limits its application in treating hypersaline wastewater. Addressing this challenge requires enhanced fundamental understanding of the scaling phenomenon. However, MD scaling with different types of scalants may have distinctive mechanisms and consequences which have not been systematically investigated in the literature. In this work, we compared gypsum and silica scaling in MD and demonstrated that gypsum scaling caused earlier water flux decline and induced membrane wetting that was not observed in silica scaling. Microscopic imaging and elemental mapping revealed contrasting scale morphology and distribution for gypsum and silica, respectively. Notably, while gypsum crystals grew both on the membrane surface and deep in the membrane matrix, silica only formed on the membrane surface in the form of a relatively thin film composed of connected submicrometer silica particles. We attribute the intrusion of gypsum into membrane pores to the crystallization pressure as a result of rapid, oriented crystal growth, which leads to pore deformation and the subsequent membrane wetting. In contrast, the silica scale layer was formed via polymerization of silicic acid and gelation of silica particles, which were less intrusive and had a milder effect on membrane pore structure. This hypothesis was supported by the result of tensile testing, which showed that the MD membrane was significantly weakened by gypsum scaling. The fact that different scaling mechanisms could yield different consequences on membrane performance provides valuable insights for the future development of cost-effective strategies for scaling control.

Membrane distillation (MD) is a thermal desalination process with the capability of harnessing low-grade waste heat to treat hypersaline brine. For this reason, MD has been actively explored as a promising technology for brine management and zero-liquid discharge (ZLD). The major and inevitable challenge with conventional hydrophobic MD membranes, however, is membrane scaling, i.e., the formation and deposition of mineral crystals on the membrane surface that eventually leads to process failure. By performing comparative MD experiments in this study, we show that a superhydrophobic membrane or gas purging can slightly alleviate gypsum scaling, but neither of them is an effective strategy for achieving sustained MD performance against gypsum scaling. However, the synergistic combination of both superhydrophobic membrane and periodic gas purging is extraordinarily effective in mitigating gypsum scaling in MD, enabling MD to concentrate a highly saline feed stream by 5-fold without suffering flux decline due to scaling that is always observed with a commercial hydrophobic membrane. Energy dispersive X-ray spectroscopy reveals the formation of crystal "anchors" inside the pores of the commercial hydrophobic membranes but not those of the superhydrophobic membrane, which explains the different effectivenesses of purging in mitigating scaling for the two membranes. The long-term flux stability offered with this scaling mitigation scheme is important for MD to be applied for brine management and ZLD.

To overcome the current scarcity of fresh water sustainably, new technologies will be required that produce potable water from a range of sources, including seawater and moisture from the atmosphere. Moreover, we must recover and reuse water from wastewater streams to reduce our global water footprint. To date, there remain significant concerns about the environmental/ecological impact, high energy consumption, and extensive maintenance costs of current technologies that might prevent their transition to more sustainable routes of potable water generation. One class of material that can enable low-energy water production is thermoresponsive polymers. Due to their unique phase behavior, production flexibility, and biocompatibility, these materials may allow for sustainable routes to fresh water in current and new technologies. In this Perspective, we specifically summarize the design and application of poly(N-isopropylacrylamide)-(PNIPAm-) based thermoresponsive microgels and hydrogels. In particular, we show how these materials have been used for water purification, including wastewater treatment, seawater desalination, and moisture harvesting from the atmosphere. Finally, we discuss the opportunities and challenges of transforming current thermoresponsive materials into practical water-related technologies.