For thousands of years, people in the Middle East and South America have extracted water from the air to help sustain their populations. Drawing inspiration from those examples, researchers are now developing a lightweight, battery-powered freshwater harvester that could someday take as much as 10 gallons per hour from the air, even in arid locations. They say their nanofiber-based method could help address modern water shortages due to industrial pollution, droughts, and groundwater depletion.
"I was visiting China, which has a freshwater scarcity problem. There's investment in wastewater treatment, but I thought that effort alone was inadequate," Shing-Chung (Josh) Wong, Ph.D., said. Instead of relying on treated wastewater, Wong thought it might be more prudent to develop a new type of water harvester that could take advantage of the abundant water particles in the atmosphere.
Harvesting water from the air has a long history. Thousands of years ago, the Incas of the Andean region collected dew and channeled it into cisterns. More recently, some research groups have been developing massive mist and fog catchers in the Andean mountains and in Africa.
To miniaturize water generation and improve the efficiency, Wong and his students at the University of Akron turned to electrospun polymers, a material they had already worked with for more than a decade. Electrospinning uses electrical forces to produce polymer fibers ranging from tens of nanometers up to 1 micrometer—an ideal size to condense and squeeze water droplets out of the air. These nanoscale fiber polymers offer an incredibly high surface-area-to-volume ratio, much larger than that provided by the typical structures and membranes used in water distillers.
By experimenting with different combinations of polymers that were hydrophilic—which attracts water—and hydrophobic—which discharges water, the group concluded that a water harvesting system could indeed be fabricated using nanofiber technology. Wong's group determined that their polymer membrane could harvest 744 mg/cm2/h, which is 91 percent higher than similarly designed membranes without these nanofibers.
Unlike existing methods, Wong's harvester could work in arid desert environments because of the membrane's high surface-area-to-volume ratio. It also would have a minimal energy requirement.
"We could confidently say that, with recent advances in lithium-ion batteries, we could eventually develop a smaller, backpack-sized device," he said.
What's more, Wong's nanofiber design simultaneously grabs water and filters it. The electrospun fiber network can act as an anti-fouling surface, sloughing off microbes that could collect on the harvester's surface. So the water would be "clear and free of pollutants" and immediately drinkable once it's collected, he said.
Next, Wong hopes to obtain additional funding to build a prototype of the freshwater harvester. He anticipates that, once his team is able to produce the prototype, it should be inexpensive to manufacture.
(Source: American Chemical Society)