Current reverse osmosis membranes poorly remove low molecular weight neutral solutes and are vulnerable to degradation from oxidants used in water treatment. Here, we demonstrate a novel pressure-driven distillation process that allows for membrane-based desalination with ultrahigh selectivity and oxidation tolerance. This process operates similarly to reverse osmosis but uses hydrophobic air-trapping membranes with sub-100 nm pores. Under an applied hydraulic pressure exceeding osmotic pressure of feed water, a partial vapor pressure gradient is created across the membranes, allowing water molecules to evaporate on the feed side, diffuse through air gap, and condense on the permeate side. To demonstrate the process, superhydrophobic membranes with 20 nm pore size were fabricated via an optimized coating of fluorinated alkyl silanes on anodized aluminum oxide substrates. These membranes showed more than 99.8% rejection of sodium chloride under hydraulic pressures up to 34.5 bar. The membranes also exhibited 99.1%, 98.1%, and 95.8% rejections of boron, urea, and N-nitrosodimethylamine, respectively. Desalination performance of the membranes is unaffected after severe exposures to 1000 ppm chlorine for 36 h and 25 ppm ozone for 1 h. We also find that reducing thickness allows increasing water permeability without compromising solute rejection, enabling such membranes to circumvent the permeability-selectivity trade-off that constrains the conventional reverse osmosis membranes. Finally, we show that the separation strategy is scalable using large-area polytetrafluoroethylene membranes which exhibit mor5e than 98% salt rejection even after cleaning with 1000 ppm chlorine for 3 h to prevent fouling from biofilm-forming <i>Pseudomonas aeruginosa</i> bacteria. With such high selectivity and oxidation resistance, pressure-driven distillation allows the reduction of pre- and post-treatment steps in the treatment trains and paves the way for low-cost and energy-efficient water purification.

Pressure-driven distillation for ultra-selective and oxidation-resistant water purification

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ACS Fall 2022

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