Hydrophilicity and surface charge modulation of Ti3C2T x MXene based membranes for water desalination.

Abstract

Lamellar membranes obtained by stacking 2D layers possess ample transport pathways due to their intricate network of interlayer gaps. This makes them suitable for molecular separation applications. However, controlling the surface chemistry of the nanochannels within the membrane to tune the desired transport properties of water and ions is challenging. Ti3C2T x has been considered for water desalination because of its hydrophilic surface and negative surface charge. Most of the studies of Ti3C2T x membranes have presented promising salt rejection values in forward osmosis mode, which is less practical for water purification. Here, we investigate two types of reverse osmosis MXene-based lamellar membranes consisting of Ti3C2T x nanosheets hybridized with (i) WS2 nanosheets and (ii) polyvinyl phosphonic acid (PVPA). When hydrophilic and flexible Ti3C2T x nanosheets are interleaved with softer and more hydrophobic WS2 nanosheets in 2 : 1 mass ratio, nano capillaries with Janus chemistry are created with comparable rejection to bare Ti3C2T x membrane and threefold higher permeance values. Further, we find that decorating Ti3C2T x nanosheets with anionic polymers improves salt rejection. Our Ti3C2T x /PVPA composite membranes reject ∼97% of divalent ions and ∼80% of monovalent ions with ∼0.2 Lm-2 h-1 bar-1 of water permeance when tested with brackish water, and exhibit significantly improved chlorine resistance and cost benefits over the commercial Toray membranes.