Our planet is nothing if not ironic. Earth is covered in water – millions of trillions of liters and gallons of the stuff – and only 2 percent of it is drinkable. Of that, 99.5 percent is frozen or buried below the ground. And of what’s left – well, human-made climate change is taking care of that.
One piece of good news, however, is that water we can drink isn’t just confined to places like lakes, rivers, and rain. There’s almost 13 trillion tons of delicious H2O hidden in plain sight all around us – we just have to extract it from the air.
There are a few ways to do this, but most are either too inefficient or prohibitively expensive. But now, researchers in Saudi Arabia say they have a solution: a simple device that can harvest and store its own weight in water, and release it when warmed by sunlight.
The key to the prototype is the cheap, stable, eco-friendly, and nontoxic chemical compound calcium chloride. This salt is so good at absorbing water that it will literally dissolve if left in fresh air – a property known to chemists as deliquescence.
But while you might think a chemical that can harvest up to six times its own weight in water out of thin air would be perfect for this device, it can actually be a problem.
“The deliquescent salt [dissolves] itself by absorbing moisture from air,” explained Renyuan Li, a PhD student in the team behind the device. “[and] systems that use liquid sorbents are very complicated.”
To combat this, the team developed a way of storing the calcium chloride as a hydrogel – a special type of polymer that can hold vast amounts of water while remaining a solid. And with the addition of some tiny carbon nanotubes to let the water escape, the team were able to use a light source to reclaim almost 100 percent from the gel.
In a paper, published in Environmental Science and Technology, the team describe the results of their small, “easy-to-assemble-at-household” prototype. After just two-and-a-half hours in the sunlight, they report, the device can deliver 20 grams (0.7 ounces or about four teaspoons) of water. And to provide an adult’s minimum water requirement for a day – 3 kilograms (6.6 pounds, or four and one-quarter cups) – would cost about half a cent per day.
Along with its low cost and high water yield, the device has the advantage of working well even in regions with relatively low humidity – perfect for arid or drought-stricken regions. It also needs no electricity, meaning it can be used even in remote parts of the world.
“Water scarcity is one of the most challenging issues that threaten the lives of mankind,” the paper reports.
“This technology provides a promising solution for clean water production in arid and land-locked [and] remote regions.”