Mechanism of permeance enhancement in mixed-matrix reverse osmosis membranes incorporated with graphene and its oxides

Abstract

• Mechanism of permeance enhancement in Gr/PA and GO/PA mixed-matrix membranes is investigated. • The hydrophobic nature of Gr leads to the lower interior transport resistance of water molecules. • Higher water solubility enhances the permeance of GO/PA MMMs. • The contributions of solubility and diffusivity for MMMs are quantitatively revealed. Fabricating mixed-matrix reverse osmosis (RO) membranes via incorporating nanocomposites is a promising strategy to enhance permeance performance without sacrificing the ion rejection. To better design such mixed-matrix membranes (MMMs), the transport mechanism of water molecules through them is urgently desired. Compared to the stiff inorganic nanoparticles, the flexible graphene (Gr) and its oxides (GO) attract much attention due to their better compatibility with aromatic polyamide (PA) matrix. In this work, nonequilibrium molecular dynamics simulations of water passing through highly cross-linked PA membranes, Gr/PA and GO/PA MMMs are performed to reveal the transport mechanism. The simulation results show that the MMMs exhibit higher permeance compared to the pure PA membranes. After analyzing the transport details of water molecules, we discovered that the permeance enhancement for two types of MMMs result from distinct factors. While the Gr/PA MMMs have lower interior transport resistance because of the hydrophobic nature of Gr, the permeance enhancement of GO/PA MMMs should be attributed to the higher solubility of water molecules into MMMs. Therefore, a proper hydrophilicity of RO membrane, which has lower interior transport resistance while having higher solubility, is expected for the optimized performance.