Graphene Oxide Membranes for Water Isotope Filtration: Insight at the Nano- and Microscale

Abstract

Recent experimental studies have revealed the selective permeation properties of lamellar graphene oxide (GO) membranes as applied to filtration of water isotopes. In this work, we explore the molecular structures and diffusive dynamics of water isotopes in GO membrane nanochannels by employing ReaxFF reactive molecular dynamics simulations. The significance of isotope effects and their role in the interactions between light/heavy water and the functional groups of GO membranes are identified, and it is found that isotope separation is driven by a combination of phase change from liquid to vapor and kinetic fractionation due to the difference in isotope diffusivity. Our phenomenological model reveals that pervaporation mode and monolayer surface diffusion of water result in efficient isotope separation, while liquid-phase pressure-driven permeation is not an effective mass transport mode for isotope filtration. These observations suggest that there is great promise for GO membranes as a means for isotope filtration, expanding the application space of GO membranes beyond the established scope of filtration by size-exclusion and preferential adsorption mechanisms.

Liquid-phase water isotope separation using graphene-oxide membranes

“…It should be noted that proton exchange between isotopic water molecules occurs, [14,41]. In the separation process, both HOD and D 2 O will be enriched as increasing the D/H in water will increase the likelihood for D 2 O to form, although, at low concentrations, HOD will be more common [42].…”

“…Membrane-based separation has been investigated as an alternative (e.g., to try to reduce costs and to try to simplify the process), particularly pervaporation and membrane distillation that have shown enhanced isotope separation with lower energy consumption due to the combined vapor pressure effect, diffusion effect and selective adsorption [13]. Chmieleswski et al reported that for water vaporizing inside the pores of the membrane [14], the vapor pressure effect is largely responsible for D separation from H and the diffusion effect is mostly responsible for 18 O separation from 16 O [15]. Due to the reported relatively large 18 O separation from 16 O achieved with membrane distillation, researchers have directed their attention into improving the efficiency of this process [16,17] and producing hydrophobic, polymer based-membranes to enhance isotopic water separation [18].…”

“…Studies show that the affinity strength of the adsorption site is influenced by the ZPE of the isotopes and that heavier isotopes with a lower ZPE generally lead to preferential adsorption and slower desorption [8,12]. Studies have shown that H 2 18 O and D 2 O form stronger hydrogen bonds with oxygen functional groups in cellulose membranes or on G-O [14,32]. The difference in adsorption strength of water molecules could affect their surface diffusion on G-O depending on adsorption and desorption rates.…”

“…It is expected that the molecular diffusion mechanism is hindered as G-O loading increases (for the 5 mL G-O membrane), resulting in a decrease in the efficiency of 18 O enrichment when compared to D. This is most evident in the slope of the line increasing from 1.5 to 4.8 as the loading increased from 1 mL to 5 mL of G-O solution, suggesting that molecular diffusion is no longer the dominant isotope fractionation effect. The adsorption effect is reported to occur due to the difference in surface diffusion rates caused by the preference in adsorption and desorption interactions between the water isotopes and the oxygen functional groups on G-O (epoxy and hydroxyl groups) [8,14], and the lateral interactions with neighboring water molecules. In the hopping model, water molecules could migrate along the surface of G-O by hopping between energetically favorable adsorption sites if the water molecule has sufficient activation energy to overcome the energy barrier to jump to the neighboring site [30].…”

“…The interaction is stronger for heavier isotopes that have lower ZPE, which allows them to have greater adsorption enthalpies and require higher energy to desorb [8]. Saidi et al, have modelled the hydrogen bonding formed between D 2 O and H 2 O with epoxy and hydroxyl groups, and found that the hydrogen bond formed with D is stronger for both groups [14]. Similarly, theoretical calculations performed by Krinkin show that H 2 18 O is less likely to rupture its hydrogen bond than H 2…”