Communities in some regions of the world lack easy access to clean fresh water, some due to their remote locations, some to insufficient or damaged infrastructure, and others to changing climate conditions. People in these regions often rely on alternate methods of gathering fresh water, such as harvesting rain, dew, vapor, and fog—but that water can be polluted and dangerous to use.
Now, an innovative update to a tried-and-true method of harvesting fog water can purify it, too. Researchers developed and tested how well a polymer-based coating on a metal mesh collected water that had been contaminated with organic pollutants. They found that not only did the coated mesh outperform existing fog harvesters, but also the coating purified the water by 91%, producing nearly pure water without requiring any power.
“We would not recommend it directly for drinking because we couldn’t reach up to 100% [purification],” said lead researcher Ritwick Ghosh, a scientist at the Max Planck Institute for Polymer Research in Mainz, Germany. “But, of course, this can be used for your vegetation or for any other water uses that we need every day, like for your washroom.”
Out of Thin Air
Where freshwater access is scarce, various collection devices can be used to harvest water from the atmosphere. Rain barrels collect rain, radiative cooling surfaces condense dew, and meshes collect fog. Passive devices that use no power can be especially useful in areas that lack electricity.
Fog harvesters are particularly effective in dry, mountainous areas with low rainfall, like Chile’s Atacama Desert and the Namib Desert along the southwestern African coast. Fog harvesters installed in Chile, Eritrea, Ethiopia, Guatemala, Morocco, and Nepal record daily harvests of thousands of liters of water.
But there is no guarantee that this water is usable. Air pollution is easily trapped in fog droplets, so fog-harvested water is often contaminated, especially near pollution centers such as fossil fuel plants and manufacturing zones.
Harvest and Clean, Simultaneously
Ghosh’s experiment was inspired by a series of poor air quality incidents around the Indian capital that some dubbed “the Great Smog of Delhi.” With so much smog in the air, the polluting aerosols were being trapped in fog droplets. Fog harvesters around Delhi were collecting the polluted water, which spurred Ghosh and his team to think of ways to passively rid this water of contaminants.
Ghosh and his colleagues tested new ways to help cleanse collected fog water of contamination through a chemical process called photocatalysis, wherein some metal oxides become chemically active when exposed to certain wavelengths of light, allowing them to break down other molecules.
In their experiments, the researchers coated a fine mesh made of metal with a nanoscale-thin layer of a titanium dioxide (TiO2) polymer. When activated by sunlight, the TiO2 reacted with the organic pollutants trapped in fog and allowed the fresh water to run off into a collector.
Their laboratory and field tests demonstrated that the polymer-coated mesh collected water as well as or better than uncoated meshes that have been deployed in the past, at a rate of about 8% collection efficiency. Moreover, the coated mesh simultaneously purified the water of organic contamination without the coating sloughing off into the collected water. The harvester produced water that was cleansed of up to 91% of organic contaminants, as well as treating it for diesel and bisphenol A (BPA), in just over an hour.
What’s more, “the titanium dioxide gets activated by the sunlight, so the good thing is you don’t need any energy,” Ghosh added. “And an interesting thing that we saw is that this effect can stay even after the sunlight is gone.” The coating on the mesh stays activated and can purify water even when it is overcast, as is common in foggy areas. This research was published in Nature Sustainability in August.
Serving Foggy, Polluted Areas
The researchers “have presented a groundbreaking design in this field, introducing a fog collector that can store photocatalytic power on sunny days and release it for water purification during fog collection,” commented Zuankai Wang, lead researcher of the nature-inspired engineering lab at Hong Kong Polytechnic University. Both the harvesting efficiency and the organic contaminant removal efficiency meet or exceed levels achieved by current technologies, added Wang, who was not involved with the research.
“I believe that the fog harvester holds tremendous value, particularly in regions where fog is frequent but air pollution is heavy,” Wang said. “This device has the potential to serve as a source of clean water for local communities by enabling concurrent fog collection and decontamination.…After all, in today’s increasingly severe air pollution, it is difficult to guarantee the quality of the fog.”
Ghosh said future experiments aim to shorten the time it takes to collect and purify the fog by testing other photocatalysts, to improve the purification efficiency, and to retain the photocatalytic “charge” for longer periods of time. They also hope to scale up the experiments to larger swaths of mesh to see how the device performs in conditions closer to real-world scenarios.
Wang hopes to see future iterations of the device use a more powerful photocatalyst to speed up the purification process. Nonetheless, he said, the technology is “a small step towards device improvement but a significant leap forward for practical applications.”
—Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer
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