Pressure-driven distillation using air-trapping membranes for fast and selective water purification

Abstract

Membrane technologies that enable the efficient purification of impaired water sources are needed to address growing water scarcity. However, state-of-the-art engineered membranes are constrained by a universal, deleterious trade-off where membranes with high water permeability lack selectivity. Current membranes also poorly remove low–molecular weight neutral solutes and are vulnerable to degradation from oxidants used in water treatment. We report a water desalination technology that uses applied pressure to drive vapor transport through membranes with an entrapped air layer. Since separation occurs due to a gas-liquid phase change, near-complete rejection of dissolved solutes including sodium chloride, boron, urea, and N -nitrosodimethylamine is observed. Membranes fabricated with sub-200-nm-thick air layers showed water permeabilities that exceed those of commercial membranes without sacrificing salt rejection. We also find the air-trapping membranes tolerate exposure to chlorine and ozone oxidants. The results advance our understanding of evaporation behavior and facilitate high-throughput ultraselective separations.

Water Desalination via Pressure-Driven Distillation with Chlorine-Resistant and Large-Area Polymeric Membranes

“…Notably, the water flux can be substantially increased by decreasing the thickness of the membrane hydrophobic layer from the current thickness of approximately 43 μm to values less than 500 nm. 12,26 The importance of thin hydrophobic layers in improving flux was also shown in our experimental work using custom-made ceramic membranes, where a 119 nm thick air layer allowed for water permeabilities exceeding 1.3 kg m −2 h −1 bar −1 .…”

“…Models validated in this study and recent experimental work indicate that making the air layer of the membrane thinner (less than 500 nm) will result in higher permeabilities that are competitive with those of current RO membranes. 12 Future development of industrial-scale desalination modules with areas greater than 10 m 2 in spiral wound, hollow fiber, or flat sheet configurations is also essential to enable PD systems with a high membrane packing density. 30,31 In these modules, hydrophobic membrane materials that do not rely on fluorine-based chemistry should also be explored to mitigate the potential release of perfluoroalkyl and polyfluoroalkyl substances.…”

“…Pressure-driven distillation (PD) is an emerging process that can achieve higher selectivity and oxidation resistance than current RO systems . PD uses applied hydraulic pressure to drive vapor transport through an air-trapping porous hydrophobic membrane.…”

“…Despite the potential benefits of PD in desalination and other separations, few studies have experimentally investigated the process, in part due to the challenge of operating air-trapping membranes at pressures above 5 bar without wetting. Those studies that have been conducted have gleaned important insights on the process, showing that PD is experimentally viable, that PD membranes can reach water permeabilities comparable to RO membranes, and that PD can highly reject boron, urea, and other contaminants. ,, However, prior studies have relied on expensive exotic materials, such as hydrophobic porous alumina and carbon fiber, with small membrane areas (less than 6.5 cm 2 ). Such materials are not representative of the scalable, flexible membranes typically required for large-scale desalination.…”

“…9−11 Pressure-driven distillation (PD) is an emerging process that can achieve higher selectivity and oxidation resistance than current RO systems. 12 PD uses applied hydraulic pressure to drive vapor transport through an air-trapping porous hydrophobic membrane. In operation, PD is similar to RO in that it uses applied pressure to drive water flow, and PD membrane modules could eventually directly replace RO modules in a water treatment train.…”