When it comes to selecting between microfiltration, ultrafiltration, nanofiltration, and reverse osmosis (RO), sometimes a choice must be made between the membrane with the highest level of filtration and the membrane with the highest level of productivity.
The pores make all the difference, said Sebastien Perrier, a professor with the department of chemistry at the University of Warwick in England.
“Micro- and ultrafiltration use pores that are a few nanometers in diameter and are typically used for the removal of larger particles. The presence of pores allows for high productivity with the application of low pressure,” explained Perrier. “Nanofiltration and reverse osmosis, on the other hand, do not work according to the principle of pores — separation takes place by diffusion through the membrane, which permits the removal of ions and salts. However, the pressure required is much higher in this case than the pressure required for micro- and ultrafiltration, with a lower productivity.”
Perrier, working with his team at the University of Warwick and a team led by Professor Kate Jolliffe at the University of Sydney in Australia, may have discovered a membrane technology that solves this dilemma.
The teams created tiny proteins coined “Janus nanotubes,” which can be used to make membranes with pores below one nanometer that only require the application of low pressure. Such small pores offer filtration down to the molecular level. “Since the membranes that use the Janus nanotubes would have pores of sub-nanometer size, they would permit the removal of salts and ions from water, and the use of pores would offer the possibility to work at low pressure and high productivity,” Perrier explained.
The Janus nanotubes have a tubular structure, with two different types of polymers attached that form a shell with two faces. This shape inspired the name ‘Janus’ after the Roman god with two heads.
When compared to current membrane technologies, these membranes are structured like microfiltration and ultrafiltration membranes, but the size of their pores allow them to also be used in applications that typically require nanofiltration and RO. The Janus nanotube technology has yet to be commercialized, but Perrier plans to work with engineers to build devices that use the technology, and then test it in real world conditions.
Along with making current methods of filtration more efficient, Perrier also sees potential for membranes created by Janus nanotubes to help solve water scarcity issues worldwide.
“The real advantage I can see is in the separation of salts, which would mean exciting applications in water desalination,” Perrier stated.
Perrier believes the potential applications for the Janus nanotubes, both in and outside the water sector, are many.
“The engineering of porous membranes is one of the applications we have been working towards, but there might be others — these materials are so new that it is hard to anticipate all their applications,” he said. “Beyond water-related functions, we are also anticipating applications that will range from the biomedical field, for drug delivery and sensors, to gas storage.”
To read more about membrane technology visit http://www.wateronline.com/industrysegment/water-membranes
Image credit: "Janus," © 2011 Thierry Ehrmann, used under an Attribution 2.0 Generic license: http://creativecommons.org/licenses/by/2.0/