SCIENCE ADVANCES

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.

INTRODUCTION

Anthropogenic climate change and increasing water demands have led to more severe water scarcity, necessitating the use of nontraditional water sources including wastewater, seawater, and brackish water ( 1 – 3 ). Safe use of these sources requires water treatment systems that remove nearly all dissolved constituents from contaminated water. Membrane technologies, in particular reverse osmosis (RO), have emerged as the premier tools for water reuse and desalination due to their high energy efficiency, ease of operation, and compact design ( 4 – 6 ).

Despite their widespread implementation, RO systems have experienced longstanding limitations in performance related to membrane materials. Current polymeric salt-rejecting membranes are constrained by a trade-off where high permeability comes at the cost of decreased water-salt selectivity ( 7 ,  8 ). Membranes also routinely fail to remove harmful contaminants since low–molecular weight neutral species can pass through polymer membranes ( 9 – 11 ); contaminants that are poorly rejected by RO include boron, urea,  N -nitrosodimethylamine (NDMA), and 1,4-dioxane. Furthermore, the polymeric materials used in RO membranes are vulnerable to chemical oxidation, decreasing longevity, and precluding cleaning of the membranes with chlorine, ozone, and other disinfectants ( 12 ,  13 ).

The fundamental constraints of membranes used in current RO systems motivate the study of alternative separation processes for advanced water treatment. Distillation-based technologies, where separation relies on a gas-liquid phase change, have been used for millennia and maintain key advantages compared to RO. Since separation is accomplished via a phase change, distillation systems remove all low-volatility species from water, including those poorly rejected by RO ( 14 – 16 ). Distillation technologies can also operate with feedwaters containing harsh oxidants, solvents, and other chemicals ( 17 ).

Source

Taxonomy

• Membranes
• Membrane
• Membrane Aerated Biofilm Reactor (MABR)