The breakthrough that could transform the process of desalinating sea-water
Graphene oxide filters out salts much more efficiently and more cheaply than traditional desalination. By fine-tuning the process, it can even target viruses, Robert Mathews reports
There is a lethal irony about our planet. Despite its name, almost three quarters of the Earth is covered with water – but 97 per cent of it is undrinkable.
With more than a billion people lacking access to potable water any breakthrough in the creation of fresh, clean water is big news, especially in the UAE.
Since the 1960s, the region has been heavily dependent on desalination for its water supplies, with Abu Dhabi using more than a billion tonnes a year.
Researchers in the United Kingdom have unveiled a major advance that is being seen as potentially transforming the desalination process.
It centres on a carbon-based material that can filter seawater and strip it back to pure water.
Graphene oxide consists of carbon atoms arranged in sheets like a molecular mesh. In 2012, a team led by Prof Andre Geim, who shared a physics Nobel Prize in 2010 for creating graphene, discovered that water molecules can still slip through it, while impurities are stopped.
Prof Geim, of the University of Manchester, and his colleagues seized on the finding and set about making water filters made of sheets of graphene oxide.
Early experiments proved promising, with water zipping through the filter easily. This raised hopes that desalination could be achieved using relatively low pressures, thus cutting the high-energy costs of the process.
But the researchers ran into a snag. Almost as soon as they were immersed in water, the graphene oxide layers swelled up, eventually leaving enough space for other, larger chemicals to sneak through.
Now Prof Geim and his colleagues have found a solution. They expose the membranes to water vapour, wait for the swelling to reach an acceptable size and then lock the spacing in place with epoxy resin.
Explaining their method in the journal Nature, the team report they have not only solved the swelling problem but can tune the spacing to the precise size they need to filter out specific impurities.
Better still, they have found that the quirky properties of water now work in their favour. While narrowing the spacing dramatically reduces the rate at which impurities get through, it has virtually no effect on the flow of pure water.
The reason is that the impurities themselves have water molecules stuck on them, and can only pass through by releasing some.
But since the water molecules stick to the impurities more strongly than they do to each other, they still put up more resistance to passing through than pure water, so the impurities lose out.
In contrast, at the atomic scale of the channels, the pure water molecules line up like a train. Applying pressure on one side of the filter then leads to the whole train travelling through at high speed, giving more throughput for less energy.
To achieve all this, the team has exploited quantum effects that occur on atomic scales.
To give some idea of how small that is, the filter channels are separated by the equivalent of the width of a single hair between two sheets of paper the size of Abu Dhabi.
Even so, the team thinks there is hope that such membranes can be mass-produced cheaply, opening the way to desalination at a much lower cost.
It is not just seawater than can be stripped of impurities, either. The filter channels are so narrow they can also remove bacteria and even individual viruses from contaminated fresh water.
Such pathogens are responsible for a large proportion of the 1.7 billion cases of diarrhoeal disease each year, which kills 800,000 children – more than Aids, malaria and measles combined.
So has scientific ingenuity finally solved the paradox of getting enough water from a planet covered in the stuff? Not quite. For even if the financial cost falls dramatically, there are growing concerns that desalination may carry a big environmental price-tag.
The single biggest reason can be summed up in one number: 4 per cent – the average salinity of seawater. While it may not seem very big, it means that every tonne of seawater contains 40 kilograms of salty gunk. For Abu Dhabi’s desalination plants alone, that amounts to a 40 million tonne annual disposal problem.
Simply dumping it offshore makes no long-term sense – least of all around the Arabian Gulf. Relatively small, shallow and subject to intense evaporation, it already has a salinity close to 6 per cent in some areas.
Worse, by being connected to the Indian Ocean by the Strait of Hormuz, the water of the Gulf is recycled only about every eight years.
As this newspaper reported in 2009, the International Centre for Biosaline Agriculture in Dubai has long been concerned about the effect of brackish and polluted water from desalination plants in the region. Exactly what the environmental effect might be is unclear.
A 2013 report by the Pacific Institute in the US found a lack of research into the effect on marine life.
What little there is suggests there could be serious localised effects, as some species are affected by even slight changes in salinity.
The centre believes simply waiting and watching is not an option. It has set up the Integrated Agriculture-Aquaculture System, aimed at finding sustainable solutions to the discharge from desalination plants.
Starting with small-scale units operated by farmers, the system has identified varieties of fish, crops and biofuels that thrive in high-salinity conditions.
Field experiments suggests the results are scalable to large installations used in the region. If so, the result could be a new form of high-salinity agriculture that protects the marine environment from potential harm.
And that would be a regional success story everyone could drink to.
Robert Matthews is visiting professor of science at Aston University, Birmingham, UK
Updated: April 15, 2017 04:00 AM