Researchers Wring Clean Water From the Desert Air

Access to clean water is literally a matter of life and death. People in arid regions and subtropical zones alike face major problems just finding potable water. Diseases like cholera are spread by contaminated water, and the minerals dissolved in deeply buried desert groundwater salt the earth when we use that water on croplands. Water catchment and desalination is possible in humid environments with abundant energy, but what of the places where it’s dry and power doesn’t come cheap? What of the places where there are no power lines at all? What is a farmer in arid regions to do?

In the Atacama desert, the driest place on earth, it almost never rains. Cacti that live there evolved fine fibers to catch droplets out of the fog banks that roll off the ocean. Mimicking them, we devised the finest of nets, to catch droplets from fog banks and wick them into a reservoir. For places where it’s damp all the time, we’ve got dehumidifiers that exploit the principles of condensation and evaporation, capturing water from the air itself. But even these techniques fail when there’s little moisture in the air to catch, or no electrical infrastructure to run the compressor for the dehumidifier. To answer the need for a low-overhead water catchment solution, scientists from MIT and UC Berkeley turned to the power of zirconium crystals.

Wang Laboratory, MIT

The crystals in question here aren’t like amethyst points, nor polished-rose-quartz worry stones. The zirconium is part of a metal-organic framework (MOF): porous lab-grown crystals with tunable properties that depend on what they’re made of. They’re like an aquarium air stone, or a sponge made of crystals instead of flexible cellulose.

MOF crystals grow like Tinkertoys assemble: Metal atoms act as hubs, and the sticklike carbon backbones of organic compounds link the hubs together. By choosing different metals and organics, explains Robert Service in Science Magazine, chemists can “dial in” the properties of each MOF, controlling what gases bind to them, and how strongly they hold on. In this case, prior work by professor Omar Yaghi of MIT showed that a zirconium-based MOF was great at absorbing water from the air, even under extremely dry conditions. So he teamed up with colleague and professor Evelyn Wang of UC Berkeley to make a new application for zirconium MOF crystals that did one thing — absorbing water — and did it very well.

Image: V. Altounian/Science

Wang and her students designed a system that used a kilogram of zirconium MOF that had been into a fine crystalline powder, as a strategic move. Increasing the surface area makes the system more effective, because it provides more surfaces on which water droplets can condense. Service elaborates:

At night the chamber is opened, allowing ambient air to diffuse through the porous MOF and water molecules to stick to its interior surfaces, gathering in groups of eight to form tiny cubic droplets. In the morning, the chamber is closed, and sunlight entering through a window on top of the device then heats up the MOF, which liberates the water droplets and drives them—as vapor—toward the cooler condenser.

Sunlight entering the chamber creates a temperature differential, driving water droplets to the cooler side of the apparatus — the place with the powdered zirconium crystals — and the droplets caught in the zirconium powder migrate toward the cool condenser plate, from where they drip as liquid water into a reservoir. The system is so effective that even in air with only 20 percent humidity, which is about the relative humidity of the Sahara, a kilogram of zirconium MOF powder produced 2.8L of liquid water from a night’s worth of air.

The problem here is that deploying this specific MOF system at scale doesn’t work. Zirconium is expensive. But the researchers are working on a version that uses aluminum, which would be a whole lot cheaper. From there, it’s not far to the marketplace. The capability to passively wring water from the desert air using just metallic crystals would be a phenomenal boon to water-stressed areas worldwide.

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