Researchers have developed a laser-engineered surface that separates water from dissolved salts during solar evaporation, overcoming one of the biggest obstacles to solar desalination: the build-up of mineral deposits that clog evaporators and reduce their efficiency.
Desalination is playing an increasingly important role in supplying drinking water in regions facing water scarcity, but conventional processes such as reverse osmosis and thermal distillation are energy intensive and generate highly concentrated brine. When returned to the sea, this waste stream can increase local salinity and reduce oxygen levels, affecting marine ecosystems.
Researchers from the University of Rochester in New York have developed a solar-thermal desalination system that uses a laser-textured metal surface to convert seawater into fresh water while directing dissolved salts away from the evaporation zone, helping to prevent the fouling that commonly limits the performance of solar desalination systems.
The work, published in Light: Science & Applications, uses black metal panels treated with femtosecond lasers to create a surface that is both highly light absorbing and strongly water-attracting. Water is drawn across the active region of the panel, where sunlight drives evaporation, while dissolved salts migrate towards untreated sections of the surface where they can be collected.
One of the main challenges for solar desalination systems is mineral build-up. While laboratory studies often use simplified salt solutions, real seawater contains a complex mixture of dissolved minerals, including magnesium and calcium compounds that can form hard deposits and block water flow.
To overcome this, the researchers engineered microscopic grooves into the panel surface and exploited the so-called “coffee ring” effect, in which particles carried by an evaporating liquid accumulate around its edges.
“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,” said Chunlei Guo, professor of optics and physics at the University of Rochester. “We use that same principle to advance the salts to the passive region.”
Tests using seawater collected from the Pacific, Atlantic and Indian oceans showed that the design enabled continuous freshwater production while moving salts away from the evaporation surface, allowing them to be collected without reducing the system’s performance.
Rather than producing a concentrated liquid brine stream, the researchers report that the system recovers salts in solid form. Besides common salt, they suggest that future systems could also recover higher-value minerals from suitable feedwaters, potentially improving the economics of desalination while reducing waste.
In a related study published in the Journal of Materials Chemistry A, the team demonstrated a method for selectively recovering lithium using the same superwicking surface. By incorporating hydrogen titanate nanoparticles into the laser-textured grooves, they were able to separate lithium from other dissolved salts.
Lithium recovery
Using water from Utah’s Great Salt Lake, the system recovered around 50% of the lithium remaining after desalination. The group note that this approach could provide an alternative route to obtaining critical battery materials from naturally lithium-rich brines, although further work will be needed to assess its commercial viability and suitability for large-scale deployment. “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,” said Guo. The technology has so far been demonstrated at proof-of-concept scale, but the researchers believe the approach is inherently scalable and could contribute to more sustainable freshwater production while reducing desalination waste and supporting recovery of valuable mineral resources.
The research was supported by the US National Science Foundation, the Bill & Melinda Gates Foundation and the Worldwide Universities Network.
