Desalination can provide vital water supplies, but the most common methods have significant drawbacks. Reverse osmosis forces water through a specialized membrane to remove salts, while thermal distillation uses heat to separate fresh water from seawater. Both methods can consume large amounts of energy, often require water treatment before and after processing, and produce a highly concentrated saltwater waste product known as brine.

When brine is pumped back into the ocean, it can increase local salinity and reduce oxygen levels, creating conditions that may harm marine life.

A solar-powered alternative to traditional desalination

Researchers at the University of Rochester have developed a different approach that could address several of these challenges at once.

The solar desalination system is designed to produce fresh water without chemical additives or pretreatment of incoming water. It also avoids producing liquid brine by extracting nearly all of the dissolved salts in solid form. The method is described in a paper published in Light: Science & Applications.

The research was led by Chunlei Guo, professor of optics and physics and senior scientist at the University of Rochester’s Laboratory for Laser Energetics.

At the center of the technology is a black-metal solar panel treated with a femtosecond laser. A femtosecond is an extremely short unit of time—one quintillionth of a second. Pulses from the ultrafast laser precisely modify the metal’s surface, creating microstructures that dramatically change how it interacts with light and water.

The laser treatment allows the metal to absorb nearly all incoming sunlight. It also helps water spread rapidly across the surface instead of forming droplets.

How the self-cleaning desalination surface works

Each panel contains a laser-treated active area that draws a thin layer of seawater across its surface. The black metal absorbs solar radiation, heating the water and causing it to evaporate.

As evaporation takes place, salts and other dissolved minerals are left behind. Instead of allowing those materials to accumulate in the area where evaporation occurs, the panel directs them toward untreated sections along the edges, known as passive areas.

This design addresses one of the biggest obstacles in solar desalination: salt buildup. When minerals form a hard layer over the active surface, they can block water movement, reduce efficiency and eventually cause the system to fail.

Why real seawater is difficult to desalinate

Previous solar desalination systems often performed well in laboratory tests using artificial seawater made primarily from water and sodium chloride.

When sodium chloride crystallizes as water evaporates, it tends to form a relatively granular, porous structure. Water can continue moving through the crystals, dissolving some of the accumulated salt and helping clean the surface.

Real seawater is much more complex. Along with sodium chloride, it contains magnesium, calcium and many other dissolved substances. Some of these compounds form hard, dense deposits that are far less porous.

The process resembles the mineral scale that builds up inside a shower head or teapot. However, the problem is more severe in desalination systems because seawater contains hundreds of times more dissolved salts than ordinary tap water.

As these deposits accumulate, water can no longer flow freely across the surface. Efficiency declines, and the system may eventually become clogged.

The coffee ring effect helps move salt away from active areas

To prevent stubborn mineral crusts, Guo’s team designed microscopic grooves in the black metal. These grooves help push salts and minerals away from the active evaporation zones.

The researchers also used a familiar physical phenomenon called the coffee ring effect. Anyone who has watched spilled coffee dry has seen this process: as the water evaporates, suspended particles move toward the edge of the droplet, leaving behind a dark ring.

“When you drop coffee on a surface, the water eventually evaporates, leaving a ring of concentrated coffee particles on the outer edge,” Guo says. “We use the same principle to advance the salt into the passive region.”

Instead of allowing minerals to collect where sunlight drives evaporation, the system directs them toward the panel’s edges.

The researchers tested the technology with real seawater from the Pacific, Atlantic and Indian oceans. During the experiments, the surface effectively purified itself while producing fresh water. Remaining salts were directed to a passive area, where they could later be collected.

Importantly, the accumulation of minerals did not reduce the panels’ desalination efficiency.

Can desalination waste become a source of useful materials?

The fate of the extracted salt is another potential advantage of the technology.

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 become a resource instead of a waste product. Some of it may be used as table salt, while more valuable minerals could also be recovered.

One important target is lithium, a key material in the lithium-ion batteries used by electric cars, smartphones, laptops and many other electronic devices.

In a related study published in Materials Chemistry Journal A, Guo and colleagues demonstrated that the same super-hygroscopic solar panels could be modified to separate lithium from other salts produced during desalination.

The researchers embedded hydrogen titanate nanoparticles into microscopic grooves in the black metal. These particles selectively separate lithium from a mixture of salts and minerals.

“Extracting lithium from the earth has proven to be very costly from an energy and environmental perspective, so extracting lithium directly from seawater could be a very important tool in the future,” Guo said.

Recovering lithium from salt left after desalination

Using samples from the Great Salt Lake, the researchers recovered about 50 percent of the lithium in the salt remaining after desalination.

The findings point to a possible future in which desalination plants do more than produce drinking water. They could also recover commercially valuable materials that would otherwise remain trapped in the waste stream.

A promising proof of concept—but scaling remains the challenge

The technology is still in its early stages. So far, researchers have demonstrated the superabsorbent desalination approach in proof-of-concept experiments using relatively small equipment.

However, Guo said the basic design is scalable in nature. If the system can be successfully developed at larger scales, it could help expand access to fresh water while creating a more sustainable source of valuable minerals.

The approach could ultimately address two major challenges at the same time: producing more fresh water for a growing global population and reducing the environmental burden associated with desalination waste and mineral extraction.

This research was supported by the National Science Foundation, the Bill & Melinda Gates Foundation and the World University Network. Guo’s colleagues at the Optics Institute who contributed to the research include principal investigator Subash Singh; alumnus Ran Wei ’24, a doctoral student; doctoral students Luheng Tang and Tainshu Xu; and Mingjiang Ma.