New solar material could clean up drinking water

Providing clean water to soldiers in the field and to citizens around the world is essential and yet one of the world’s greatest challenges. Now, a new super absorbent and super light absorbent aluminum material developed with Army funds could change that.

With funding from the Army Research Office, an element of the Army Research Laboratory of the U.S. Army Combat Capabilities Development Command, researchers at the University of Rochester have developed a new aluminum panel that concentrates more efficiently solar energy to evaporate and purify contaminated water.

“The Army and its warriors run on water, so there is a particular interest in basic materials research that could lead to advanced technologies to generate drinking water,” said Dr. Evan Runnerstrom, ARO program manager. “The combined light-absorbing and super-absorbent properties of these aluminum surfaces can enable passive or low-power water purification to better support the warrior in the field.”

The researchers developed a laser processing technology that converts normal aluminum black, making it highly absorbent as well as super absorbent (absorbs water uphill against gravity). Then they applied this super absorbent and super absorbent aluminum for this solar water purification.

The technology featured in Sustainability of nature, uses a burst of femtosecond (ultrashort) laser pulses to etch the surface of normal aluminum foil. When the aluminum panel is immersed in water at an angle facing the sun, it draws a thin film of water upward onto the metal surface. At the same time, the blackened surface retains almost 100 percent of the energy it absorbs from the sun to quickly heat water. Finally, absorbent surface structures change the intermolecular bonds of water, further increasing the efficiency of the evaporation process.

“These three things together allow the technology to work better than an ideal device with 100 percent efficiency,” said Professor Chunlei Guo, professor of optics at the University of Rochester. “This is a simple, long-lasting, and inexpensive way to address the global water crisis, especially in developing countries.”

Laboratory experiments show that the method reduces the presence of all common contaminants, such as detergents, dyes, urine, heavy metals, and glycerin, to safe levels to drink.

The technology could also be useful in developed countries to alleviate water shortages in drought-affected areas and for water desalination projects, Guo said.

The use of sunlight for boiling has long been recognized as a way to eliminate microbial pathogens and reduce deaths from diarrheal infections, but boiling water does not remove heavy metals and other contaminants.

Solar based water purification; however, it can greatly reduce these contaminants because almost all impurities are left behind when the evaporating water becomes gaseous and then condenses and collects.

The most common method of solar-based water evaporation is volume heating, in which a large volume of water is heated but only the top layer can evaporate. Obviously, this is inefficient, Guo said, because only a small fraction of the heating energy is used.

A more efficient approach, called interfacial heating, places floating, absorbent, and multi-layered absorbent materials on top of the water, so that only the water near the surface needs to be heated. But all available materials have to float horizontally on the water and cannot look directly at the sun. Additionally, available absorbent materials quickly clog with contaminants remaining after evaporation, requiring frequent replacement of materials.

The aluminum panel that the researchers developed avoids these difficulties by removing a thin layer of water from the reservoir and directly on the surface of the solar absorber for heating and evaporation.

“Also, because we use a surface with open grooves, it is very easy to clean just by spraying it,” said Guo. “The biggest advantage is that the angle of the panels can be continuously adjusted to directly face the sun as it rises and then move across the sky before sunset, maximizing energy absorption.”