Air nanobubbles (ANBs) incorporated sandwich-structured carbon nanotube membranes (CNM) for highly permeable and stable forward osmosis

Abstract

The selective transport of water/ions through conventional forward osmosis (FO) membranes is largely impeded by solution-diffusion and internal concentration polarization (ICP). Herein, we report a novel air nanobubbles (ANBs) incorporated sandwich-structured carbon nanotube membrane (CNM) for highly permeable and stable FO desalination by taking advantage of the nanofluidic transport at the solid/liquid/vapor interface. Fluorinated multi-walled carbon nanotubes (F-MWCNTs) were assembled as the superhydrophobic interlayer between a hydrophilic cellulose acetate (CA) layer and a hydrophilic polyacrylonitrile (PAN) nanofibrous layer. The trapped ANBs in the superhydrophobic F-MWCNT layer crucially regulated the continuous water flow and effectively prevented salt diffusion. When tested with DI water as feed solution (FS) and 1 ​M NaCl as draw solution (DS), the ANBs incorporated sandwich-structured CNM achieved high water flux (158.0 ​L ​m⁻² ​h⁻¹) and ultralow reverse salt flux (0.4 ​g ​m⁻² ​h⁻¹) simultaneously, far beyond the state-of-the-art FO membranes. The PAN nanofibrous layer well protected the entrapped ANBs to allow a more durable FO performance. An ANBs-regulated nanofluidic flow model was proposed to elucidate selective water/salt transport mechanism. This work revealed the feasibility of ANBs incorporated membranes for osmosis-driven processes.