Medieval alchemists searched in vain for the Philosopher’s Stone, which could turn base metals into gold. Now chemists think they’ve found something potentially far more valuable – a material able to make seawater drinkable.
The miracle substance is graphene, a form of carbon discovered a decade ago whose paradoxical properties have amazed scientists ever since.
In essence just carbon atoms arranged in sheets like chicken-wire, graphene is a mass of contradictions.
It is hundreds of times stronger than steel, yet stretchy. It’s a poor electrical insulator, but conducts heat better even than diamond. It hardly reacts to chemicals, but absorbs light so well even a single sheet is visible in daylight.
Now another of graphene’s weird abilities is making headlines – its relationship with water.
Graphene blocks the passage of all gases and liquids, making it perfect as a barrier coating. It’s also hydrophobic – “water-fearing” – so drops of water on its surface just sit there without spreading out.
But true to graphene’s paradoxical nature, all this changes if water molecules get into its structure.
In 2012, a team led by Prof Andre Geim of the University of Manchester, UK (who shared a Nobel for the co-discovery of graphene in 2010) showed that water will zip through sheets of graphene oxide billions of times faster than anything else. Molecules mixed in with the water, on the other hand, get left behind.
The explanation lies in the fact that, like graphene, water is a substance that defies convention. It turns out that the V-shaped H²O molecule can slip through the channels formed by sheets of graphene, and then close up the gap behind itself, blocking the path to other molecules.
Not surprisingly, this has led to huge interest in using graphene as the ultimate water filter.
That promise now seems to be paying off. Prof Geim’s team has just reported tests of graphene oxide laminates made up of lots of layers, and its “double act” with water seems to work amazingly well.
In the lab, the capillaries in the lattice ravenously sucks up the water, while leaving behind the stuff dissolved in it.
The team is now working on narrowing down the mesh size to the point where absolutely nothing gets through other than pure water. The goal is graphene-based water purification on an industrial scale.
The first products are likely to be graphene-based replacements for membranes in standard reverse osmosis plants.
This will make the most of the fact that graphene membranes can be far thinner, and thus more permeable, without compromising longevity.
Better still, being so thin and receptive to the passage of water means graphene won’t need the hefty pressures – and thus energy consumption – required by today’s methods.
And to cap it all, graphene itself is cheap and easy to make – just put some copper foil and carbon-rich methane in a furnace and heat. When they touch the foil, the methane molecules split apart, leaving their carbon behind to form into the chicken-wire arrangement of graphene.
A joint US-Saudi Arabian team based at the Massachusetts Institute of Technology has just unveiled a technique for creating the necessary perforated graphene sheets.
By blasting the pristine graphene with gallium ions, the team has already achieved a perforation level of around 5,000 billion holes just a few atoms across in area the size of a postage stamp.
That’s equivalent to creating holes the size of the dot on this “i” around 4mm apart in a sheet the size of metropolitan Abu Dhabi.
The next challenge now is to make the sheets large enough to allow huge volumes of contaminated water to pass through them.
According to a study by the MIT team reported in Energy and Environmental Science, the use of the thinner, more permeable membranes could cut desalination energy costs by as much as 46 per cent, and slash the number of pressure vessels needed by half.
One major technology company has already declared its interest in achieving all this – Lockheed Martin, the US-based defence contractor best known for aircraft and weapons systems.
Scientists at the company recently took out a patent on a material they call Perforene, a graphene-based membrane punched with molecular-sized holes.
Like the MIT membrane, these holes can be tuned in size and made small enough to desalinate water.
Desalination is not the only market being targeted by Lockheed Martin. They see opportunities for the use of the membranes in kidney dialysis machines, and also in cleaning out the chemicals used in the “fracking” of oil and gas reserves.
Yet there’s no doubt that the real prize is providing drinkable water to the estimated one billion-plus people living in arid areas such as the UAE, and in countries lacking reliable sanitation.
Access to drinking water is increasingly seen as a global security threat over the coming decades.
In 2012 a US National Intelligence Council report stated that “fresh water availability will not keep up with demand” in the absence of better management of water resources.
Along with nations such as the UAE, the report identified areas as diverse as central Spain, California and much of Australia as in a current state of “extreme water stress”, with demand far outstripping resources.
Back in the late 19th century, the world faced a similar crisis, this time over the ability of the world to feed itself. What was needed was a way of boosting food yields using nitrogen-rich fertiliser.
As with water, there was no shortage of nitrogen – the problem was that it was largely inaccessible, floating in the atmosphere.
Then chemists found a way of “fixing” atmospheric nitrogen and creating a limitless source of fertiliser, lifting the spectre of global starvation.
A century on, the alchemists of the 21st century are turning their attention to getting the world’s oceans to give us what we need. And the signs are their ingenuity will again succeed in averting a global crisis.
Robert Matthews is visiting reader in science at Aston University, Birmingham
