Water-dispersible few-layer graphene flakes for selective and rapid ion mercury (Hg2+)-rejecting membranes

Abstract

Mercury (Hg) is a global highly toxic pollutant released by both anthropogenic and natural sources. Hg decontamination is of the utmost importance for human and ecosystem protection. Here, we propose a novel graphene-based membrane capable of performing rapid and highly selective Hg²⁺-rejection from water. Functionalized graphene flakes are produced by a non-oxidative, room-temperature and post processing-free “green” method to simultaneously exfoliate graphite into single-/few-layer graphene (SLG/FLG) flakes in water and functionalize them with cationic rhodamine 6G (R6G) via a physisorption process (aromatic ring π–π stacking). The rhodamine 6G-functionalized graphene (R6G-FG) membrane shows a low-density (<0.5 g cm⁻³) packed laminar structure, where R6G molecules act as spacers between the SLG/FLG flakes. The presence of hydrophilic micro/nanodomains in this low-density structure results in a water permeation rate as high as 789.6 L m⁻² h⁻¹ bar⁻¹ (for an 80 μm-thick membrane, R6G-FG mass loading of 3.58 g m⁻²). Meanwhile, the R6G-FG complexes perform as ion-selective nano-traps for Hg²⁺, showing almost complete rejection (>99%) for a filtered solution volume normalized to the R6G-FG mass superior to 3 L g⁻¹. The selective rejection capability of the R6G-FG membrane is ruled by competitive adsorption of metal ions and positively charged R6G molecules with different affinity onto the negatively charged graphene surface. Lastly, a washing treatment in alkaline conditions is also proposed for membrane regeneration and reuse. The rationalization of the working mechanism of the R6G-FG membrane is promising for eliminating the “permeability–selectivity trade-offs” often tackled by laminar two-dimensional material membranes.