A clever combination of water extraction and treatment: Researchers have developed a specially coated metal mesh that can efficiently extract water from fog and purify it at the same time. So-called photocatalytic titanium oxide particles ensure the breakdown of substances from air pollution. Only temporary solar energy is required to activate this effect. According to the scientists, the new technology could also enable fog to be used with little effort in dry regions with heavy air pollution.
Just turning on the tap – this luxury is not available to many people in the world: The lack of availability of clean water is a big problem in many dry regions. Efficient use of all available water resources is therefore required there. This is how the air is already being tapped in some regions: where at least occasionally wafts of fog sweep through the landscape, the floating droplets can be caught and collected. Nets or grid systems are used for this purpose, on which the water is deposited and then runs into collection containers. On some days, up to several hundred liters of water can be obtained from a fog collector that is only a few square meters in size.
In more remote regions, the fog water is usually of drinking water quality. However, this is not the case in densely populated areas where this water resource is particularly in demand. Because there, the fog drops often accumulate with high levels of air pollution. What collectors collect in urban arid regions is therefore usually not clean enough to use for drinking or cooking. However, subsequent cleaning would again make the concept expensive and impractical.
Effectively collected and cleaned
The innovation of the international research team led by the Swiss Federal Institute of Technology Zurich (ETH) is now addressing this problem: their method ensures effective water extraction and purification at the same time. The key here is the special design of the collector material. It is a close-meshed mesh of metal wire coated with a mixture of polymers and titanium dioxide. The mesh size and the properties of the polymer material are designed in such a way that fog droplets accumulate particularly effectively and flow off into a collection container as quickly as possible so that the wind does not pick them up again.
However, the scientists have also adapted the wetting properties and thus the retention time of the water in the mesh to the cleaning function of the system. It is based on a special property of the integrated titanium dioxide. It acts as a chemical catalyst leading to the breakdown of organic pollutant molecules. The highlight is: This effect is based on the influence of light energy. Irradiation triggers a structural change in titanium dioxide molecules, which in turn causes the formation of highly reactive molecules – radicals – in their vicinity. These then cause the breakdown of organic pollutants into harmless substances.
Light provides the cleaning function
In this process, the titanium dioxide is not consumed, but can be regenerated again and again through renewed light activation. As the scientists report, their system benefits from the so-called photocatalytic memory of the substance: If titanium oxide is activated with UV light, it remains catalytically active for a longer period of time, even in the dark. Half an hour of sun is therefore sufficient to guarantee the function for 24 hours. As a result, the coating can also provide cleaning performance for fog traps at night or in twilight.
The scientists have already been able to prove that their system delivers what it promises: they have successfully tested the fog catcher in the laboratory and in a pilot plant in Zurich. A good water yield was shown compared to previous processes – despite the additional function. The evaluations of the cleaning performance showed that the mesh was able to break down 94 percent of the organic compounds that the team had added to the fog. Among the pollutants tested were fine diesel droplets and the notorious plasticizer substance bisphenol A. This means that their development is a successful step towards their goal, the developers sum up: “By combining mist collection with water treatment, it could can also be used in regions with air pollution, for example in densely populated metropolitan areas,” says lead author Ritwick Ghosh from ETH Zurich.
As the team concludes, the uses of the concept could even go beyond drinking water production: it could be used in industry to recover water that rises in the form of steam, for example from cooling towers. “It would make sense to capture some of this water and make sure it’s pollutant-free in case you want to return it to the environment,” says senior author Thomas Schutzius from ETH Zurich.
Source: Swiss Federal Institute of Technology in Zurich, specialist article: Nature Sustainability, doi: 10.1038/s41893-023-01159-9