When it comes to desalinating salt water, two of the main options are thermal distillation and reverse osmosis. Thermal distillation involves boiling the water and collecting the resulting freshwater condensation, while reverse osmosis involves pressurizing the salt water and forcing it through a semipermeable membrane, which will allow water molecules to pass through, but not salt. Both of these methods, however, require a considerable amount of energy – not as environmentally sound as they could be, nor entirely practical for use in developing nations, where electricity isn't readily available. Now, however, a newly-developed membrane that incorporates carbon nanotubes could make desalination much quicker, easier and energy-efficient.
The patent-pending membrane, developed by New Jersey Institute of Technology chemist Somenath Mitra, utilizes another already-existing form of desalination known as membrane distillation. Sort of like a cross between thermal distillation and reverse osmosis, it involves heating the salt water then passing it through a tube made from a semipermeable membrane, which allows water vapor to pass through while not admitting salt molecules.
Because the water is only heated to 60-90C (140-194F) and isn't pressurized, membrane distillation uses less energy than either of the other two methods. Unfortunately, it can be difficult to achieve the right degree of permeability in the membrane, and even a temperature of 60C could still be difficult to attain in impoverished conditions.
In Prof. Mitra's new material, carbon nanotubes are immobilized in the membrane's pores. This reportedly results in much greater vapor permeation while keeping liquid water from clogging the pores, and it allows for higher flow rates while requiring lower temperatures – as compared to a regular membrane, it demonstrated the same level of salt reduction at a temperature that was 20C cooler, and at a flow rate that was six times higher.
"Together these benefits lead to a greener process which could make membrane distillation economically competitive with existing desalination technologies, and we hope could provide potable water where it is most needed," said Mitra.