Superhydrophobic Carbon Nanotube Network Membranes for Membrane Distillation: High-Throughput Performance and Transport Mechanism.

ABSTRACT

Despite increasing sustainable water purification, current desalination membranes still suffer from insufficient permeability and treatment efficiency, greatly hindering extensive practical applications. In this work, we provide a new membrane design protocol and molecule-level mechanistic understanding of vapor transport for the treatment of hypersaline waters via a membrane distillation process by rationally fabricating more robust metal-based carbon nanotube (CNT) network membranes, featuring a superhydrophobic superporous surface (80.0 ± 2.3% surface porosity). With highly permeable ductile metal hollow fibers as substrates, the construction of a superhydrophobic (water contact angle ∼170°) CNT network layer endows the membranes with not only almost perfect salt rejection (over 99.9%) but a promising water flux (43.6 L·m-2·h-1), which outperforms most existing inorganic distillation membranes. Both experimental and molecular dynamics simulation results indicate that such an enhanced water flux can be ascribed to an ultra-low liquid-solid contact interface (∼3.23%), allowing water vapor to rapidly transport across the membrane structure via a combined mechanism of Knudsen diffusion (more dominant) and viscous flow while efficiently repelling high-salinity feed via forming a Cassie-Baxter state. A more hydrophobic surface is more in favor of not only water desorption from the CNT outer surface but superfast and frictionless water vapor transport. By constructing a new superhydrophobic triple-phase interface, the conceptional design strategy proposed in this work can be expected to be extended to other membrane material systems as well as more water treatment applications.