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Atom-level filtration - thanks to graphene membranes

A porous graphene structure over a permeable substrate could be ideal sieves for water and gas filtration. Figure courtesy of Dr Wang Bo
A porous graphene structure over a permeable substrate could be ideal sieves for water and gas filtration. Figure courtesy of Dr Wang Bo

Pure graphene is impenetrable to even the smallest atoms but with a few adjustments it is giving rise to a new generation of permeable membranes, with possible applications from water filtration to reducing power station emissions.

At Imperial College London, UK, Dr Wang Bo is aiming to create graphene-based membranes that purify water and can be used in wastewater treatment, drinking water production, or seawater desalination. He says that they would separate water from unwanted substances faster than the membranes used in these processes today.

‘It has been reported that (porous) graphene membranes made of few graphene layers - less than 1 nanometre thick - allow water to pass three orders of magnitudes faster than conventional membranes,’ he said.

Marie Skłodowska-Curie grantee Dr Wang is working on the GRAPHENEHF project under the supervision of Professor Kang Li. Their project takes two approaches to creating the membranes.

The first is to create a membrane from graphene oxide by using acid to oxidise graphite, and then exfoliating flakes of graphene oxide to form a two-dimensional layer. Graphene oxide membranes are impermeable to all gases and liquids apart from water.

The second is to poke miniscule holes in sheets of pure graphene to create a tiny sieve which lets through only water molecules.

‘The most challenging issue is the stability of the membranes.’

Dr Wang Bo, Imperial College London, UK

Dr Wang says that one of the biggest obstacles to creating graphene membranes is the feasibility of mass production. Because graphene sheets are so thin and have to rest on a surface, they need to be scaled up in a way that keeps the structure stable.

Novel approach

The scientists use a novel approach – wrapping the membranes around hollow tubes made from ceramic or metal, which are less than 2 millimetres across.

‘Our current research of using hollow fibre geometry could help to tackle this problem (of mass production). Graphene-based membranes supported by our homemade inorganic hollow fibres have shown very much prolonged stabilities compared with other research, and that is a good sign if we were to push the product towards real applications.’

He says that one potential application could be to treat wastewater from the textile industry, which contains dyes whose molecular weights mean they get through current membranes. However, the dyes would be too large to permeate graphene-based membranes, so when coloured water was filtered through a graphene membrane, colourless water would come out.

There’s still work ahead before such membranes could be used in industry, however. ‘The most challenging issue is the stability of the membranes under harsh industrial operation conditions, for example, higher speed fluids, probably with abrasive particles, which could destroy the membranes easily,’ said Dr Wang.

Supporting graphene membranes by wrapping them around ceramic tubes could help solve the problem of scaling up production. Courtesy of Dr Bo Wang

Professor Peter Budd, from the University of Manchester, UK, which is a partner on the EU’s multi-million euro Graphene Flagship programme, says he believes that such niche industrial applications, where current membranes need improving, offer more commercial potential for graphene-based membranes than the area of desalination.

‘If you want to desalinate water there’s already a very good material out there,’ he said. ‘That’s in a sense the area in which it’s going to be most difficult, because you’ve got to displace an existing material. Graphene oxide membranes are a real possibility, they will work, but will they beat the competition? That’s where the question mark is at the moment.’

The idea of the Graphene Flagship is to help produce commercial applications for graphene by suppporting risky early stage research with EU funds. Prof. Budd believes the most promising applications are in industries in which membranes do not yet work well and so are not currently widely used.

‘Probably the biggest potential actually is (removing) carbon dioxide from natural gas (to decrease emissions). That’s where it may not take such a big shift in the economics to shift to larger-scale membrane systems.

‘The (current) adsorption technology has a really high energy cost. So if you can improve on that it will give a much better chance for carbon dioxide capture to be taken up. It's high risk but super high return.’

Prof. Budd’s team is working out how to combine small amounts of graphene with other materials to create selective membranes, whose properties can be tweaked to allow certain molecules through.

‘The scientific problem is how to improve the selectivity,’ he said. ‘We are working on composites which involve polymers with graphene or graphene oxide in various forms, and we can demonstrate that both in liquid separations and some gas separations that very tiny amounts can significantly change and potentially improve the performance.’

However, he says that there is a long way to go before engineers are persuaded to use graphene-based membranes in industrial processes, as the membranes will need to be proven to work much better than existing technologies, be reliable over the long-term and be easy to fit and use. 

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