UV light holds promise for energy-efficient desalination

A team of UC Riverside researchers has uncovered a potential breakthrough in solar desalination that could reduce the need for energy-intensive saltwater treatment.

Led by Luat Vuong, an associate professor of mechanical engineering in UCR’s Marlan and Rosemary Bourns College of Engineering, the team has demonstrated for the first time how the highest frequencies of sunlight—specifically invisible ultraviolet (UV) light—can break the stubborn bonds between salt and water.

“To our knowledge, nobody else has yet articulated this deep UV channel for salt-water separation,” Vuong said. “UV light in the wavelength range of 300–400 nanometers is used for disinfection, but this deep UV channel, around 200 nanometers, is not well known. We may be the first to really think about how you can leverage it for desalination.”

While much work remains before practical applications are developed, the discovery provides a clear path for further research and innovation.

Published in ACS Applied Materials & Interfaces, the study by Vuong and her colleagues details how the team made a wick from aluminum nitride—a hard, white ceramic—to separate salt from water by harnessing specific light wavelengths that interact with salt water without heating the bulk liquid.

Unlike traditional solar desalination methods, which rely on dark materials to absorb heat and boil water, Vuong’s approach could bypass the need for thermal processes altogether.

The experiments involved placing pairs of ceramic wicks in an enclosed chamber, with each allowed to equilibrate or adjust to similar environmental conditions. Under UV light, evaporation rates of salt water increased significantly compared to control samples kept in the dark or exposed to red, yellow, or infrared light.

“Aluminum nitride is well suited for emitting UV light due to its crystalline structure,” Vuong explained.

The material may be triggering a process called “photon upconversion,” in which low-energy photons combine into a single high-energy photon. That upconverted photon delivers a more powerful punch, potentially strong enough to break the salt-water bonds.

If this upconversion process occurs without generating excess heat, which is yet to be determined, the approach could offer a non-photothermal alternative to traditional solar desalination systems that boil or heat salt water to produce vapor, which then condenses into fresh water.

Such solar systems also could reduce the heavy electricity demands of reverse osmosis systems, which use high-pressure pumps to force salt water through membranes. The system could also address the concentrated reverse-osmosis brine waste, which is toxic to marine life when discharged into waterways.

Other potential applications for the wicking approach may be for other waste management processes, harvesting minerals in extreme environments, or replacing “swamp” coolers with salt water evaporation systems.

Still, Vuong emphasized that further research is needed before aluminum nitride-based solar desalination systems can be engineered for widespread use.

“Other materials may be designed to be just as effective, but aluminum nitride is practical. It is inexpensive, widely available, non-toxic, highly hydrophilic, and durable,” Vuong said.

Moving forward, Vuong’s group is designing system architectures, fabrication processes, and spectroscopic tools to better understand and enhance light-driven evaporation.