Getting clean drinking water is a challenge in many parts of the world, most typically in the desert. Yet while these regions are defined by their arid conditions, even in the desert there is some humidity in the air that, if retrieved, can provide a valuable source of drinking water.
To collect water from the air requires a material on which droplets will condense and then be released. In 2017 Omar Yaghi and Evelyn Wang in the US and Saudi Arabia and colleagues showed that metallorganic frameworks (MOFs) could fit the bill. These materials – metal ions or clusters coordinated to organic ligands – typically form porous structures in one, two or three dimensions. Their high surface area had already attracted studies into applications in catalysis, gas separation and storage and even dehumidifying. However, despite the low capture rate and high energy cost of releasing captured water from alternative materials such as zeolites, use of MOFs in water harvesting is a relatively new idea.
Yaghi and Wang used the porous metal-organic framework MOF-801 – [Zr6O4(OH)4(fumarate)6]. While they chose this MOF to minimize energy consumption, some power was still needed, which their device soaked up from the Sun, an abundant resource in the sun-scorched desert. In addition the nightly temperature plummet in the desert would aid water absorption that can be released during the hot days.
This week Yaghi and colleagues report in Science Advances results from testing their device in genuine desert conditions in Arizona, as well as improvements on the prototype that eliminate the need for any external cooling source. The device collects around 200 ml of water per kilogram of MOF per day–night cycle, and with a cheaper aluminium-based alternative MOF (MOF-303) the water captured could double.
Of course, water shortages are not just an issue in arid areas – two-thirds of the world’s population are affected. Where humidity levels are high, fog capturing meshes can help. There have been material developments to encourage greater drop-off rates, but the real limiting factor with these devices seems to be aerodynamic deviations that prevent the droplets being captured in the first place. This week Maher Damak and Kripa K Varanasi at MIT in the US show that they can resolve aerodynamic deviation issues by charging the droplets. The device can retrieve more than 60 g of water from fog per hour at an energy consumption of 2 kWh/m3 – a lower energy cost than current reverse-osmosis desalination procedures.
The fog catcher could also be used to catch steam from the chimneys of power plants where, as Varanasi points out, for a typical 600-megawatt power plant it could capture 150 million gallons of water a year, representing a value of millions of dollars. Varanasi and Damak have already co-founded a company with the aim of commercializing the device.
This week was World Environment Day, and around the world people celebrated proactive habit changes that might help save the planet. Besides concerns over plastic pollution, probably the most cataclysmic effect of current human activity on the planet is projected climate change, and the resulting large-scale desertification – while the homes of vast swathes of the world’s population are plunged under the sea. With this gloomy prospect in mind, it’s worth thinking what can be done to ensure the demand for fog and water catchers like these doesn’t sky rocket.
