In a fog
Fog harvesting offers regions devoid of lakes and rivers another source for freshwater, but in urban centers, where water is often scarce, there is the added challenge of air pollution. Now, researchers have developed a simple way to simultaneously collect water from fog and remove harmful contaminants, an advancement that could help provide millions of people worldwide with access to safe drinking water.
The researchers demonstrated how a nanoengineered steel mesh with a special solar-powered coating can collect water droplets from fog, then treat the water to make it safe for drinking. The coating, a polymer consisting of titanium dioxide nanoparticles, has the unique ability to stay reactive once exposed to sunlight and remove pollution, rain or shine, around the clock. This fully passive, hybrid approach to collecting and treating water is “a first in its field,” according to Thomas Schutzius, assistant professor of mechanical engineering.
To remove these contaminants from the captured fog water droplets, the researchers looked to polymer coatings. Lead author Ritwick Ghosh, a scientist from the Max Planck Institute for Polymer Research (MPIP) and visiting researcher at ETH Zurich, had previously discovered that it was possible to treat, to a limited degree, contaminated fog by using mesh coatings containing photocatalytically active metal oxide nanoparticles such as titanium dioxide. Such coatings become reactive when exposed to sunlight and cause pollution molecules in fog droplets to decompose into harmless agents, making the collected water safe to drink. But these coatings required active and continuous ultraviolet lamp illumination to do the job.
In this latest study, Schutzius and Ghosh, working with Michael Kappl and Hans-Jürgen Butt from MPIP, took the next step to make treatment fully passive: They optimized the nanoparticle coating so that it could continue treating the water without requiring around-the-clock exposure to UV light.
“The key here is that we can make the surface reactive when it’s sunny, and it stays reactive even when it’s foggy or cloudy — exhibiting almost a capacitive-like behavior,” said Schutzius, describing the reactive coating’s ability to store charge, much like a battery, making it possible to effectively treat water regardless of weather conditions and time of day.