https://www.nature.com/articles/s41565-020-00777-0

A large population of the world experiences water stress, while industrial activities and global climate change continue to threaten our precious fresh water resources. Utilization of alternative water sources including saline water and wastewater provides a potential solution. However, desalination, which is necessary to treat these alternative water sources, is highly energy intensive, especially for water sources with very high salinity.       

Membrane distillation (MD) is thermally driven membrane desalination process that can recover fresh water from hypersaline sources. In MD, a hydrophobic microporous membrane separates the saline feed water and the fresh permeate; a temperature difference across the membrane leads to evaporation of the feed water at the membrane surface and transport of the vapor through the membrane. In conventional MD, the temperature difference is provided by heating the feed water, an approach that has limited thermal efficiency, fresh water recovery and scalability.

In this study, we used a hexagonal boron nitride (hBN) coated stainless steel wire cloth (SSWC) to electrically heat the feed stream at the membrane/feed interface (Figure 1a, b). The SSWC is porous (Figure 1c), flexible, robust, low-cost, and has microscopically uniform high thermal and electrical conductivity. The hBN nanocoating has excellent thermal conductivity and chemical stability; it is also electrically insulating and impermeable (Figure 1b, c, d, e). It provides a perfect chemical and electrical barrier while allowing fast heat transfer. As such, the hBN coated SSWC (hBN@SSWC) can provide high electrothermal energy intensity (e.g., 50 kW/m²) and be highly resistant to chemical and electrochemical corrosion under hypersaline  conditions (up to 300 g L^-1 NaCl). When attached to a commercial PVDF membrane, hBN@SSWC enabled high performance desalination of hypersaline water with a power source of household frequency (50 Hz), simultaneously producing the highest reported module-scale water flux, single-pass water recovery, heat utilization efficiency, and a near-saturated brine.

The hBN@SSWC demonstrated excellent stability during long-term operation, with no (electro)chemical degradation or scraping of hBN@SSWC observed. Furthermore, the flexibility and porosity of SSWC allows large-scale, uniform growth of the hBN coating using existing chemical vapor deposition methods in a common tube furnace (Figure 1f). Using a large hBN-SSWC sample, we developed a novel spiral-wound SHMD module (Figure 1g), which further improved single-pass heat utilization efficiency and achieved extremely high desalination rate and throughput (Figure 1h).

The hBN@SSWC’s thin and porous structure, high corrosion resistance, and capability of generating high heat intensity may find broader applications in water and wastewater treatment as well as other industrial processes. For example, it can be used as catalyst support in thermal-catalytic filters to enhance catalytic reactivity; it can also be used to deliver uniform, high heat intensity for pyrolysis of refractory substances.  The synergistic combination of material and system design in this study demonstrates how the unique properties of nanomaterials, when strategically integrated into a process, can be utilized to address highly challenging engineering problems and overcome the limitations of conventional technologies.