Tunable sieving of ions using graphene oxide membranes

Abstract

Graphene oxide membranes show exceptional molecular permeation properties, with a promise for many applications. However, their use in ion sieving and desalination technologies is limited by a permeation cutoff of 9 Å, which is larger than hydrated ion diameters for common salts. The cutoff is determined by the interlayer spacing d 13.5 Å, typical for graphene oxide laminates that swell in water. Achieving smaller d for the laminates immersed in water has proved to be a challenge. Here we describe how to control d by physical confinement and achieve accurate and tuneable ion sieving. Membranes with d from  9.8 Å to 6.4 Å are demonstrated, providing the sieve size smaller than typical ions' hydrated diameters. In this regime, ion permeation is found to be thermally activated with energy barriers of 10-100 kJ/mol depending on d. Importantly, permeation rates decrease exponentially with decreasing the sieve size but water transport is weakly affected (by a factor of <2). The latter is attributed to a low barrier for water molecules entry and large slip lengths inside graphene capillaries. Building on these findings, we demonstrate a simple scalable method to obtain graphene-based membranes with limited swelling, which exhibit 97% rejection for NaCl.Selectively permeable membranes with sub-nm pores attract strong interest due to analogies with biological membranes and potential applications in water filtration, molecular separation and desalination [1][2][3][4][5][6][7][8] . Nanopores with sizes comparable to, or smaller than, the diameter D of hydrated ions are predicted to show enhanced ion selectivity 7,9-12 because of dehydration required to pass through such atomic-scale sieves. Despite extensive research on ion dehydration effects 3,7,9-13 , experimental investigation of the ion sieving controlled by dehydration has been limited because of difficulties in fabricating uniform membranes with well-defined sub-nm pores. The realisation of membranes with dehydration-assisted selectivity would be a significant step forward. So far, research into novel membranes has mostly focused on improving the water flux rather than ion selectivity. On the other hand, modelling of practically relevant filtration processes shows that an increase in water permeation rates above the rates currently achieved (2-3 L/m 2 ×h×bar) would not contribute greatly to the overall efficiency of desalination 8,14,15 . Alternative approaches based on higher water-ion selectivity may open new possibilities for improving filtration technologies, as the performance of state-of-the-art membranes is currently limited by the solution-diffusion mechanism, in which water molecules dissolve in the membrane material and then diffuses across the membrane 8 . Recently, carbon nanomaterials including carbon nanotubes (CNT)

Removal of heavy metals and pollutants by membrane adsorption techniques

“…This technology has the potential to revolutionize water filtration across the world, in particular in countries which cannot afford large scale desalination plants (Robinson 2017;Abraham et al 2017). Graphine oxide (GO) as adsorbent for the removal of heavy metals is getting more attention due to its high surface area, mechanical strength, light weight, flexibility and chemical stability (Gopalakrishnan et al 2015;Taherian et al 2013;Kyzas et al 2014) wrote a review and discussed the application of GO (particularly as magnetic particles) as an adsorbent for wastewater treatment such as heavy metals separation (mercury, cadmium, copper, chromium, arsenic) and also organics (antibiotics, dyes, i.e.…”

Graphene membranes for water desalination

“…To stabilize as well as minimize the hydration-induced swelling of GO channels, very recently, Abraham et al 76 suggested an innovative physical approach. They prevented swelling by exposing the GO membrane strips to a given humidity to attain a certain interlayer spacing and subsequently embedding the GO laminate in epoxy.…”

Activated Carbon, Carbon Nanotubes and Graphene: Materials and Composites for Advanced Water Purification

“…Molecular dynamic studies have been performed to assess the ability of nanoporous graphene to purify water of trihalomethanes [180]. Recent research has extended the idea of the laminar structure by confining the hydration related swelling of the layered graphene and flowing a solution parallel through the laminar structure [181]. This system has been specifically designed for the rejection of ions and desalination purposes however control over the pore diameter introduces the possibility for the filtration of different contaminant species.…”