Around the world, billions of people still lack reliable access to safe drinking water. The United Nations estimates that 2.2 billion people do not have safely managed drinking water, while regions from California to the Middle East increasingly rely on desalination plants to turn ocean water into fresh water.
Desalination can provide a critical water supply, but today’s most common methods come with significant drawbacks. Reverse osmosis forces water through specialized membranes to remove salt, while thermal distillation uses heat to separate fresh water from seawater. Both approaches can consume large amounts of energy, often require water treatment before and after the process, and generate a highly concentrated salty waste known as brine.
When that brine is discharged back into the ocean, it can increase local salinity and reduce oxygen levels, creating harmful conditions for marine organisms.
A Solar-Powered Alternative to Conventional Desalination
Researchers at the University of Rochester have developed a different approach that could help address several of those problems at once.
Scientists at URochester’s Institute of Optics created a solar thermal desalination system designed to produce fresh water efficiently without generating liquid brine or requiring chemical additives to pre-treat the incoming water. The method is described in a paper published in Light: Science & Applications.
The research was led by Chunlei Guo, a professor of optics and physics and a senior scientist at URochester’s Laboratory for Laser Energetics.
At the heart of the technology are solar panels made from black metal that has been treated with femtosecond lasers. A femtosecond is an extraordinarily short unit of time, equal to one quadrillionth of a second. Pulses from these ultrafast lasers can precisely alter a material’s surface, creating microscopic structures that dramatically change how it interacts with light and water.
The laser treatment makes the metal extremely effective at absorbing sunlight and also gives it superwicking properties, meaning water spreads rapidly across the surface instead of beading up.
How the Self-Cleaning Surface Works
Each panel contains a laser-treated active region that draws a very thin layer of seawater across its surface. The dark metal absorbs nearly all of the incoming solar radiation, heating the water and causing it to evaporate.
That evaporation leaves salts and other dissolved minerals behind. Rather than allowing those materials to accumulate where evaporation is taking place, the panel directs them toward untreated areas along its sides, known as the passive region.
This movement is important because salt buildup is one of the biggest challenges facing solar desalination systems. If minerals form a hard layer across the active surface, they can block water movement and eventually stop the system from working.
Why Real Seawater Is Much Harder to Desalinate
Guo says previous solar thermal desalination systems have often performed well in laboratory experiments using simplified artificial seawater made from water and sodium chloride.
When sodium chloride crystallizes as water evaporates, it tends to form a relatively grainy and porous structure. Water can continue moving through those crystals, helping dissolve accumulated salt and making the surface easier to clean.
Real seawater is far more complicated.
In addition to sodium chloride, it contains magnesium, calcium, and many other dissolved substances. Some of those compounds can crystallize into hard, dense deposits that are much less porous.
It is similar to the mineral scale that gradually builds up inside a shower head or tea pot. In a desalination system, however, the challenge is much greater because seawater contains hundreds of times more dissolved salts than ordinary tap water.
As those deposits accumulate, water can no longer flow freely across the surface, reducing efficiency and eventually clogging the system.
Using the Coffee Ring Effect to Move Salt
To overcome that problem, Guo’s team carefully designed microscopic grooves in the black metal so salts and minerals would be pushed away from the active region instead of forming a stubborn crust.
The researchers also took advantage of a familiar physical phenomenon called the coffee ring effect.
Anyone who has spilled coffee and watched it dry has probably seen it. As water evaporates, suspended particles move toward the edge of the droplet, leaving behind a dark ring.
“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,” says Guo. “We use that same principle to advance the salts to the passive region.”
Instead of allowing minerals to build up where sunlight is driving evaporation, the system guides them toward the edges of the panel.
The researchers tested the technique using actual seawater collected from the Pacific, Atlantic, and Indian Oceans. In those experiments, the surface effectively cleaned itself while producing fresh water, directing the remaining salts into the passive region where they could later be collected.
Importantly, that mineral buildup did not reduce the panel’s desalination efficiency.
Turning Desalination Waste Into Useful Materials
Another major advantage is what happens to the salt after the water is removed.
Rather than producing concentrated liquid brine that must be treated or discharged, the system extracts nearly 100 percent of the dissolved salts in solid form.
That material could potentially become a resource rather than a waste product. Some of it could be used as table salt, while more valuable minerals could also be recovered.
One particularly important target is lithium, a key material used in lithium-ion batteries that power electric vehicles, smartphones, laptops, and many other electronic devices.
In a related study published in the Journal of Materials Chemistry A, Guo and his colleagues demonstrated that the same superwicking solar panels can be modified to separate lithium from the other salts produced during desalination.
The researchers embedded nanoparticles made from hydrogen titanate into the microscopic grooves of the black metal. Those particles selectively isolate lithium from the mixture of salts and minerals.
“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,” says Guo.
Recovering Lithium From Saltwater
Using samples from Great Salt Lake, the team was able to recover about 50 percent of the lithium contained in the salts left behind after desalination.
That result points to a possible future in which desalination facilities could do more than produce drinking water. They might also recover commercially valuable materials that would otherwise remain trapped in waste streams.
The technology is still at an early stage. So far, the researchers have demonstrated the superwicking desalination approach in proof-of-concept experiments using relatively small devices.
Guo says the basic design is inherently scalable, however, and could eventually contribute both to expanding access to fresh water and to creating more sustainable sources of valuable minerals.
If successfully scaled, the approach could address two major challenges at the same time: producing more fresh water for a growing global population while reducing the environmental burden associated with both desalination waste and mineral extraction.
The research was supported by the National Science Foundation, the Bill & Melinda Gates Foundation, and Worldwide Universities Network. Guo’s colleagues from the Institute of Optics who contributed to the research include Senior Scientist Subash Singh, alumnus Ran Wei ’24 (PhD), PhD students Luheng Tang and Tainshu Xu, and Mingjiang Ma.