High-tech filter designs... this way for clean water!
New high-tech filter designs and technology developed by EU-funded researchers for use in salt-water purifiers could solve one of the most intractable problems on the planet - providing clean, affordable drinking water to millions who lack access.
© Vasily Merkushev - fotolia.com
Mastering new filter designs using tiny channels or nanocapillaries in salt-water purifiers can solve one of the most intractable problems on the planet providing clean, affordable drinking water to the parched millions. Graphene-based nanotechnology, a carbon-based wonder-material, developed by EU-backed projects, provides the all-important electric charge needed to filter out the salt ions, leaving fresh water.
For scientists in the EU-funded 2DSi project, it was an obvious choice which application of the technology they should focus on.
“Separating drinkable water from sea water with cheaper and scalable membrane materials is a great challenge, and any success in this direction will provide millions of people access to affordable, clean drinking water,” says Kalon Gopinadhan, a Marie Curie fellow based at the University of Manchester who led the 2DSi research.
And with around 780 million people currently lacking access to clean water, and diarrhoea caused by poor sanitation and water killing roughly 800 000 children under the age of five each year, that outcome could not come sooner.
But they faced a problem: the pores or capillaries in the filter membrane needed to be so small and materials at such sub-nano scale don’t have enough surface charge to carry out the electrolysis separating salt (ions) from water particles, leaving the pure, fresh water.
So 2DSi set out and succeeded in developing a new class of highly sensitive ion filters effectively by tricking van der Waals crystals or graphene oxide to behave in different, more amenable ways. The scientists used two-dimensional (2D) layered materials; a single layer of graphene and single layer of molebdnum disulfide (MoS2), which is commonly used in coatings and layering.
Peeling and doping
To shrink the capillaries but retain what the researchers describe as the “excellent monolayer-forming properties”, they tried different things. One approach was to combine mechanical peeling using crystals and lithographic techniques, literally to peel and stitch the coatings made in the lab to a graphene filter membrane. But the best results were obtained by doping or tricking graphite a type of silicate to form graphene oxide and gluing the layers together.
“Graphene nanochannels alone didn’t fully work,” explains Gopinadhan who collaborated with Jijo Abraham from Manchester University’s National Graphene Institute on this project, “The doped graphene-oxide combination worked better at separating the salt and water because there is already a charge in the capillary walls.”
But there were still challenges that had to be addressed.
“Graphene oxide actually swells in water, making the capillary sizes much larger than the hydrated size of the salt ions. So, we found that mechanically confining or containing these capillaries curbed the swelling and let the water pass better,” he adds.
Painting and decorating
It is still early days for the innovative techniques developed under 2DSi, and future lines of work could explore ways of “decorating” the graphene-only channels with charged particles that can be painted onto one of the layers, the research team reveals. The results are being demonstrated by commercial partners who have shown interest in the project including Lockheed Martin, the aerospace multinational.
On an individual level, the EU fellowship which targets cross-border, cross-sector research has helped to put Gopinadhan’s career on a firm professorship track.
“Working together on this project with the Physics Nobel laureate Andre Geim really raised the profile and quality of our research, which no doubt attracted the attention of prestigious publications like Nature and Nature Nanotechnology,” he says. “This fellowship also gave me the academic freedom to collaborate with other research groups such as that led by Rahul R Nair from the University of Manchester and led to excellent science. Without it, none of this would have been possible.”
The researcher is confident that 2DSi’s work will ultimately be scaled up to tackle the world’s clean water challenge.