Adventurers have long learned to make stills that use the Sun's heat to turn saltwater fresh to survive on desert islands, but they can barely quench their thirsts. Higher-tech desalination methods require vast amounts of energy, but one team think they have found a way to use sunlight in a much more efficient manner.
The new method uses metal-organic frameworks (MOFs), materials so staggeringly porous, a sugar cube-sized MOF has a surface area equal to a football field. Pore size varies with composition, allowing scientists to make MOF sieves or holding cages for specific molecules.
Professor Huanting Wang of Monash University added poly(spirppyran acrylate) (PSP) to an existing MOF to produce PSP-MIL-53, which can trap common salts. Better still, Wang and colleagues report in Nature Sustainability that when exposed to ultraviolet light, PSP-MIL-53's molecular cages release their captives, ready for reuse.
Wang poured water too salty to drink into a tube containing PSP-MIL-53 and was rewarded with water pure enough to meet drinking standards. He then let the UV light in and flushed the salt out with a much smaller amount of water, leaving the MOFs ready for re-use.
With fresh water sources increasingly depleted and rainfall often moving to places where it is less needed, many technologies are competing to offer drinking water. Wang's contender offers several advantages over the alternatives.
For one thing, electricity use is stunningly low. If the UV light to cause the MOFs to switch modes is provided by the Sun, the only demand is for pumping the water – a tiny fraction of what's required for multistage flash distillation or reverse osmosis. Where some desalinators are slow, Wang needs just a 34-minute cycle (30 minutes to trap the salt and 4 minutes to release it), and even this may be reduced with further work.
Some desalination technologies also require large-scale operations to be efficient, but Wang's could be suitable for a household with contaminated bore water or a city whose entire water supply is inadequate.
So far, Wang told IFLScience, the system works best with brackish water, rather than something as salty as the ocean. Although the team demonstrated the capacity to purify seawater, undesirably large volumes of water were required to wash the MOFs during the illuminated part of the cycle, something Wang intends to work on.
Further testing is also required on the long-term stability of MOFs, having so far only been used for 20-30 cycles. Wang added the team is yet to explore whether other pollutants, such as bacterial contamination, can be removed in the same way.
"Sunlight is the most abundant and renewable source of energy on Earth,” Wang noted in a statement. As it is said in a different context: “Sunlight is the best disinfectant.”
