Australasian Science Magazine

By Stephen Luntz

Clean water for the 780 million people who don’t have it is a step closer following the publication in Nature Communications of work on plasma-treated carbon nanotubes as filtration devices.

“The large industrialised purification plants we see in other parts of the world are just not practical – they consume a large amount of energy and have high labour costs, making them very expensive to run,” says Dr Zhaojun Han of CSIRO Plasma Nanoscience Laboratories. “Small portable purification devices are increasingly recognised as the best way to meet the needs of clean water and sanitation in developing countries and in remote locations, minimising the risk of many serious diseases.”

However, current filtration methods require a two-stage process when confronted with salty water, with reverse osmosis to remove the salt and microfiltration or nanofiltration for disease-causing organic contaminants.

“Ultralong nanotubes have a very large surface area that is ideal for filtration,” says co-author Prof Ken Ostrikov. “Secondly, nanotubes are easy to modify, which allows us to tailor their surface properties through localised nanoscale plasma treatment.”

Han says the nanotubes demonstrated their capacity to remove both sorts of impurities at salt concentrations of 1000–7000 ppm. This is still well short of what is required to purify seawater, which is usually 35,000 ppm of salt.

Current filtration methods also require substantial amounts of energy, which is often in short supply in the same places that lack clean water. Han says that gravity alone might be enough to force water through the nanotubes, but his experiments used only small amounts of electricity to speed the process. Systems the size of teapots could be powered by batteries.

Although salt saturates the tubes, Han says they can be removed and regenerated multiple times, so it may be possible that filters could be produced at prices affordable in places where the need is greatest.

Computer modelling indicates that boron nitride nanotubes might be able to tackle even higher saline concentrations (AS, October 2009, p.9), but Han says he has not explored that option.