ENGINEERING FOR CHANGE
WASTE-FREE DESALTING
A new desalination approach produces freshwater without brine.
Written by Rob Goodier
RESEARCHERS AT THE UNIVERSITY OF ROCHESTER have developed a new solar-thermal desalination system that converts seawater into fresh water without chemical additives and without producing concentrated brine waste. Instead, the system separates solid salts and minerals from freshwater, according to “Additive-free and brine-discharge-free solar-thermal desalination with simultaneous complete mineral mining from ocean water,” recently published in Nature’s Light: Science & Applications.
Desalination is becoming increasingly important as freshwater supplies decrease around the world. The United Nations estimates that 2.2 billion people lack safely managed drinking water. Yet many existing desalination technologies incur high environmental and energy costs. This new technology is designed for lower costs and environmental footprint, and it opens options for easier recovery of lithium and other materials in seawater.
The technology relies on specially engineered black metal surfaces, a project led by Chunlei Guo, optics and physics professor at the university. These surfaces are etched with femtosecond lasers, making them highly absorbent to sunlight and strongly attractive to water. As seawater moves across the surface, solar energy drives evaporation and freshwater production.
The approach takes advantage of a familiar phenomenon known as the coffee ring effect.
“If you drop coffee on a surface, eventually the water evaporates, and there’s a ring left at the outer edge that is the concentrated coffee particles,” Guo said in a press statement. “We use that same principle to advance the salts to the passive region.”
Vials of seawater, Great Salt Lake water, nickel sulfate, copper chloride wastewater, and desalinated water, along with recovered salts. Photo: University of Rochester/J. Adam Fenster
Salt concentration of Atlantic Ocean water samples before (red bar) and after (blue bar) the solar-thermal desalination. Each error bar represents the standard error of mean calculated from at least 15 data points. Photo: Tang, et al.
Schematic of ABF-STIC integrated with a solar trackable condensation system. Photo: Tang, et al.
How It Works
The researchers etched microscopic grooves into thin aluminum sheets to create ‘superwicking’ black metal surfaces. The result performs several jobs at once. It pulls a thin film of seawater upward across the metal, absorbs nearly all incoming sunlight, and concentrates heat at the water’s surface where evaporation occurs. Because the water layer remains very thin, the system uses solar energy efficiently and produces water vapor while leaving dissolved salts behind. The researchers describe the material as a multifunctional platform that combines water transport, solar absorption, evaporation, and salt management in a single surface.
One of the challenges in current solar desalination technologies is the buildup of salt. Some systems work well using simple salt solutions, but struggle when exposed to seawater, which contains a range of dissolved minerals. These minerals can form hard deposits that clog surfaces and reduce performance.
A key innovation in the superwicking surfaces is how they prevent salt from accumulating where evaporation occurs. The researchers designed the etched grooves to draw water from the active evaporation surface toward surrounding passive regions, carrying concentrated salts with it. Crystals form at the edges rather than on the working surface. A second process, called salt creeping, then pushes the crystals farther outward as they grow. Together, these two mechanisms allow the evaporator to clean itself while continuously producing fresh water and collecting salts and minerals as solids, instead of generating liquid brine waste.
The researchers tested the system using water collected from the Pacific, Atlantic, and Indian Oceans. The surface remained clear while continuing to produce fresh water, according to the published report.
In a related study, the team demonstrated that the same platform can help separate lithium from other salts.
The solar-powered desalination device features laser-etched superwicking black metal (right). Unlike existing solar desalination systems (left), Professor Chunlei Guo’s design prevents salt and mineral buildup from clogging the surface. Photo: University of Rochester/J. Adam Fenster
Valuable Solids
The captured salts may also have economic value. In a related study, the researchers demonstrated that their platform can help separate lithium from other salts. Using samples from Utah’s Great Salt Lake, the team recovered about half of the lithium present in the residual salts.
“Mining lithium from the earth has proven to be very taxing from an energy and environmental standpoint, so pulling lithium directly from saltwater could be a very important future route,” Guo said.
The technology remains at the proof-of-concept stage, but Dr. Guo believes it is scalable and could eventually expand access to drinking water while tapping new sources of critical minerals.
This article was first published at Engineering for Change. Read the original version here.
Rob Goodier is news editor at Engineering for Change, a community working to prepare the international technical workforce to improve life for people and the planet. ASME is a founding partner of E4C.
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