x Abu Dhabi, UAESunday 23 July 2017

What nitrogen did for soil, carbon may do for water

A form of carbon discovered two decades ago could be the key to meeting the UAE"s soaring demand for fresh water

Graphene molecules give graphite its lubricating properties.
Graphene molecules give graphite its lubricating properties.

Carbon is a chemical paradox - at once capable of being the hardest substance in the known universe and a silky smooth lubricant.

Now a third form of carbon, discovered 20 years ago, may prove the answer to a pressing global need - and one of major interest to the UAE: the demand for fresh water.

On a planet that any visiting alien would more likely name "Water" than "Earth", the idea of water shortage can seem absurd: there's more than 100 million tonnes of H2O for every person on the planet.

The problem, of course, is that H2O has a habit of mixing itself with other stuff unsuitable for human consumption - notably salt. That leaves barely three per cent of the water on Earth drinkable.

Not that liberating fresh water is a particularly complex problem. Every schoolchild learns how to warm salty water and collect the resulting moisture as it condenses. Somewhat more sophisticated methods have been found for desalinating water on the scales needed for whole cities, but they all have a catch: they require energy.

That might not seem a problem for energy-rich nations like the UAE, but it is still not cheap: the Emirates spends more than US$7bn (Dh25.7bn) a year on desalination.

And it brings other problems, such as greenhouse gas emissions and reduced energy security.

Still, as the saying goes, you cannae change the laws of physics. But scientists are now discovering that those same laws can spring big surprises at microscopic scales - with huge implications for the search for ways of making the oceans drinkable.

The discoveries centre on a honeycomb-like arrangement of carbon atoms known as graphene. As its name suggests, it is graphene that gives graphite its lubricating properties, with weakly bonded sheets of graphene sliding smoothly over one another.

But in 1991, scientists identified another arrangement of graphene, in which the sheets are curled up to form tubes of molecular dimensions.

These carbon "nanotubes" have many unusual properties, including a strength and stiffness far exceeding that of steel. But it is the tube-like arrangement of the carbon atoms that looks set to revolutionise the problem of desalination.

The most widely used method today is reverse osmosis, in which water molecules are separated from the salt by forcing them through a membrane. It is less energy intensive than heat-based methods, but producing a decent flow of desalinated water is still hard work, requiring pressures of around 500 tonnes per square metre - making even this form of desalination pretty expensive.

It would all be so much easier and cheaper if water could be cleansed without so much physical effort. That requires a conjuring trick akin to pulling a car out of a hat. But now it seems it may be possible - if the "hat" is made of carbon nanotubes.

The current issue of Physics World carries a report of new studies of the effect of carbon nanotubes on the flow of water - or, to be more precise, water molecules.

That is a crucial distinction, as effects that can be ignored at the level of water spewing from a pipe become all-important at the molecular level.

And according to the report's author, Professor Jason Reese, a fluid mechanics expert at the University of Strathclyde in Scotland, these effects can have dramatic consequences for the flow of water through membranes.

Computer simulations have revealed that water molecules can flow along carbon nanotubes at rates up to several thousand times higher than predicted using conventional fluid flow theory.

Put simply, the carbon nanotubes exert intermolecular forces that act like turnstiles at sports stadiums, compelling a surging crowd to form into a nice, orderly line. At the same time the nanotubes exclude the hefty and unwanted sodium and chlorine-based components of salty water.

The result is virtually frictionless flow of purified water through the nanotubes - with huge reductions in the energy needed to force them through.

So great is the promise of nanotubes that teams in several countries are now racing to put the theory to the test in real life.

They include a group led by Dr Maryam Tofighy at the Iran University of Science and Technology, Tehran, which has recently reported success in filtering water using carbon nanotubes at one tenth the pressure normally required.

Others are studying the use of carbon nanotubes to turbo-charge other desalination methods. One of the most promising is membrane distillation. It can purify water at everyday temperatures and pressures, making it even more attractive than reverse osmosis.

Until now, the process has struggled to produce good flow-rates of purified water, prompting a team at the New Jersey Institute of Technology in the US to see if carbon nanotubes might help.

The team has found that by embedding the nanotubes in the distillation membrane, their ability to bring order to the chaotic flow of water molecules simultaneously boosts both the flow-rate and the level of purity achieved.

There are still some technical problems to be overcome - not least mass-producing nanotubes with the precise characteristics needed for them to work their magic. Even so, the sense of excitement about recent developments is palpable.

Around a century ago, the world faced the threat of mass starvation as the surging global population outstripped food supplies.

Then chemists found a way of creating the fertilisers needed to make the land more productive, by capturing the virtually limitless supplies of nitrogen in the very air we breathe.

A century on, chemists may be on the brink of saving the world again, with another demonstration of their genius for bending the building blocks of matter to their will.

 

Robert Matthews is visiting Reader in Science at Aston University, Birmingham, England.