There may not be much water on the ground in arid parts of the world, but there still is some water vapor that can be drawn from the air. While so-called "fog nets" provide one way of doing so, scientists have now created a higher-tech alternative – it's a device that incorporates a salty hydrogel.
Developed by a team at Saudi Arabia's King Abdullah University of Science and Technology, the gel used in the device contains cheap, stable, non-toxic calcium chloride salt. Although this salt absorbs a great deal of water vapor from the air, it's a deliquescent salt, meaning that it dissolves into the water that it absorbs.
This means that when used on its own for absorbing water vapor, calcium chloride produces salty water that's unsuitable for drinking. In the new device, however, that salty water is retained within the solid polymer hydrogel. That gel (pictured below, being poured to set in a mold) also contains a small amount of carbon nanotubes, which are very effective at converting sunlight into heat.
As a result, the device is able to absorb water vapor from the air at night, storing it as liquid within the hydrogel. When the sun heats that gel the next morning, the heat causes fresh water to evaporate out of the gel in the form of vapor, that vapor condensing back into liquid on a gathering surface within the device. It can then be stored and consumed.
In a test of the technology, a simple prototype containing 35 grams of the hydrogel was left outdoors overnight, with the relative humidity sitting at around 60 percent. By the next morning it had absorbed 37 grams of water, most of which was harvested by the device after 2.5 hours of natural sunlight irradiation.
The team is now working on scaling up the system, and adapting it so that it can produce water continuously instead of in batches.
Source: KAUST