Salts for grabbing water from air

Harvesting water from the air could provide much-needed freshwater supplies, even in conditions of relatively low humidity. Many techniques to gather some of that water already exist, from simple mist-catching fabrics to more complex solutions. Developing more effective systems could allow atmospheric water harvesting to make a much more significant contribution to water supplies, especially in remote and arid regions.

Researchers in China report on the development of a new and extraordinarily effective water harvesting material in the journal Materials Today Energy. Their solution is also simple, based on activated carbon fiber felt containing hygroscopic (water-attracting) salts.

“Our work shows that using conventional materials with some modifications can be as effective as new technology, and much cheaper,” says Ruzhu Wang, who leads the research team at Shanghai Jiao Tong University.

In search of an inexpensive and simple water-absorbent, the researchers turned to the readily available salts lithium chloride (LiCl), calcium chloride (CaCl2) and lithium nitrate (LiNO3). These were incorporated within layered activated carbon fibre felts with a variety of pore sizes and large internal surface areas where the air comes into contact with the hygroscopic salt. The process uses activated carbon, which has undergone high temperature chemical processing to create a highly microporous structure with a very large internal surface area.

Converting the activated carbon felt into the water-catching product is straightforward. The felt is soaked in a 20% solution of the salt in water, then removed to allow the excess solution to be filtered off by suction. The material is then thoroughly dried at 120 ^oC, leaving it ready for use.

As summer is the most critical season for water scarcity, tests were conducted in conditions mimicking summer temperatures and humidity levels in Iran. This is the country of origin of two of the researchers, Akram Entezari and Moitaba Ejeian, and is typical of regions where atmospheric water harvesting could make a significant contribution to tackling existing water scarcity problems.

The felts carrying lithium chloride proved the most effective, capturing up to 2.9 grams of water per gram of material. Even at relative humidity levels as low as 20% the material still harvested almost 1.2 grams of water per gram.

The authors point out that many hot and dry regions of the world, such as around the Persian Gulf, have atmospheric humidity levels as high as 70% due to the proximity of the sea. The system may therefore work at its highest efficiency in some regions with the greatest need for fresh water. Due to its high uptake of water even from air with humidity level as low as 20% it could also work in very dry arid regions.

To demonstrate the full cycle of operation, the team devised a simple water collection system in which release of the harvested water is powered by energy levels typical of sunlight. Water quality tests revealed the collected product contained acceptably low levels of salt ions, suitable for drinking water.

Wang points out that the system could readily be used in arid desert, disaster-hit areas, or regions disrupted by conflict, in addition to providing routine water supplies. He comments: “Air is everywhere, and, if that can provide fresh water everywhere there need be no fighting for water.”

Article details:

Entezari, A., Ejeian, M, & Wang, R. Z. “Extraordinary air water harvesting performance with three phase sorption,” Materials Today Energy (2019)