Edge-charged functionalized graphene oxide nanoslit for water desalination and ion-sieving

Layered graphene oxides (GOs) have been widely used as membrane materials for desalination and ion-sieving. However, GO membranes still suffer from limitations and challenges. Edge modification of GO sheets can effectively regulate the permeation of edge channels and improve the separation performance of GO membranes. Herein, molecular simulations were conducted to investigate the water desalination and Na⁺/Ca²⁺ sieving for six types of GO nanoslits with varied edge functional groups, including single group (COOH, NH₃⁺, COO⁻) and mixed group configurations. It is demonstrated that edge functional groups have a significant impact on desalination and ion sieving. The water permeation across the GO slits is closely related to the functional groups through edge-induced interactions. The NH₃⁺-functionalized slit with positively charged groups displays superior desalination performance and higher Na⁺/Ca²⁺ selectivity, surpassing edge-carboxylated GO slits. The ion permeation and rejection were interpreted in terms of the free energy landscapes for ions passing, in which the differentiating ion-slit electrostatic interactions and ion hydration behavior play a critical role. Meanwhile, selective ion adsorption within the slits offers an additional action to the species permeation. The distinctive separation mechanisms for charged GO slits, revealed in this work, provide new functionalization strategies for GO membranes.