Artistic impression of how stacked GO layers filter common salts from water to make it safe to drink.
Artistic impression of how stacked GO layers filter common salts from water to make it safe to drink.

Researchers from the University of Manchester believe that graphene oxide (GO) membranes could offer a simple means of filtering out unwanted salts and impurities from drinking water [Abraham et al., Nature Nanotechnology (2017) doi: 10.1038/nnano.2017.21].

Currently, hundreds of millions of people around the world do not have access to safe drinking water and supplies are under increasing strain from depletion, contamination with seawater, and pollution. As traditional desalination processes are energy intensive and environmentally damaging, an affordable means of extracting clean water from seawater or contaminated water is desperately needed.

Permeable membranes with sub-nanometer pores that filter out ions and impurities while letting water (or other liquids) through are attracting attention for these applications. Recently, carbon materials such as nanotubes and graphene have been hailed as promising candidates for membranes but are hampered by difficulties associated with producing these materials on the industrial scale needed for water filtration and desalination.

“Amazing properties of graphene such as fast water transport, high chemical stability, and the cost-effectiveness of preparation in large quantities make graphene-based membranes superior in comparison to other technologies,” explains Vasu Siddeswara Kalangi, one of the lead authors of the research.

The team, led by Rahul R. Nair, focused on GO, which works as a sieve because large molecules and ions cannot pass along the spaces in between its stacked sheets. But the filtering of ions is not just a question of geometry. When common salts dissolve in water, the ions acquire a ‘shell’ of water molecules. These water molecules have to be striped off before the ion can move through the interlayer spaces, creating an energy ‘cost’ or barrier. Simple water molecules, meanwhile, pass through regardless.

Until now, the drawback was that when GO is submerged in water, two or three layers of water molecules insert themselves into the interlayer spaces. This intercalation swells the gap, allowing ions and molecules to pass through and reducing the material’s selectivity.

Nair and his team found a simple way around this limitation by sandwiching stacked layers of GO between epoxy to restrict swelling when immersed in water. Cleverly, though, the researchers first exposed the GO layers to controlled levels of humidity to tailor the interlayer spacing. In this way, the team created membranes with interlayer spacing varying from 6.6–9.8Å (compared to 13.7Å for water-soaked GO). The physically confined graphene oxide (PCGO) membranes can successfully filter out common ions like Na+ and K+, while allowing water to pass through.

Moreover, the researchers found that incorporating graphene flakes into the stacked-layer laminates can also control the swelling of GO membranes. The hydrophobicity of graphene limits water intake into the stack and curtails swelling. The GO-Gr membranes achieved a salt rejection level of 97%, according to the team. The combination of ion-permeation suppression with fast water transport in PCGO membranes makes them an attractive proposition for water filtration and desalination believe the researchers, which could be scaled up to industrial levels.

“Our work shows that the efficiency of water filtration could be improved by using graphene-based membranes, which could potentially make drinking water cheap,” says another of the lead authors, Jijo Abraham.

Mainak Majumder of Monash University, Australia agrees that the new development in graphene-based membranes and the improvement salt rejection level is exciting.

“The novelty of this work lies in the identification of a new mechanism for salt rejection in sub-angstrom pores,” he explains. “The dehydration-based mechanism for ion transport in these pores − a mechanism observed in biological systems − hasn’t been clearly identified in artificial membranes experimentally before.”

However, Majumder cautions that there are many technical challenges − including the manufacture of the membranes – to overcome before the approach could become practically useful. The researchers’ salt-rejection hypothesis will also need rigorous testing before it will be fully accepted by the scientific community.

“Nevertheless, this work certainly pioneers this new insight and will inspire a splurge of activities looking at alternative ways to overcome the interlayer spacing problem in graphene,” he says.