Evaporation is nature’s important way of cleaning water, leaving behind impurities like salts and metal ions. As scarcity looms amidst rising demand and persistent pollution, desalination technologies have been developed to extract salt and produce freshwater. However, these typically require large plants, use much energy and are costly, making them impractical for resource-limited regions.
This prompted Bofan Li, a Research Scientist at the A*STAR’s Institute of Sustainability for Chemicals, Energy and Environment (ISCE2), to take a creative approach to solar-driven evaporation. “We use sunlight to heat only the water-air interface,” she said. “Evaporating water mainly from the hot surface is more efficient than heating an entire tank, just as boiling only a thin film of soup would be faster than heating the whole pot.”
Collaborating with Nanjing Forestry University, Li and the team built evaporator systems using biomass materials like rattan and bamboo, which are locally available, renewable, low-cost or even discarded as waste. Their design strategy leveraged the plant materials’ porous structures that reduced heat loss, as well as their inner continuous channels that worked like drinking straws to move water upward to the surface for evaporation.
Through a laser-based process called catalytic laser-induced graphitisation, the researchers transformed the surface of bamboo waste into a highly photothermal layer that effectively absorbs light and converts it into heat. By arranging the bamboo pieces in an alternating hollow pattern, they reduced ‘dead zones’ where water vapour is trapped in the structure. Their design led to better evaporation rates compared to more densely packed patterns. Salt crystals were also removed more rapidly, washed away from the surface by the continuous flow through the open channels, rather than accumulating near the top.
Rattan, meanwhile, naturally had large channels for transporting water, but its surface reflected light. The team used laser processing to carbonise the surface, darkening it for improved absorption and heat conversion. This also created tiny hill-like protrusions, which helped catch more light, promoted evaporation and moved salt away from the surface.
“In this way, the material can do three jobs at the same time: absorb sunlight, deliver water and keep heat near the surface,” Li said. The researchers envision their system to be used as small, modular evaporator units that complement rather than replace traditional desalination plants, providing an extra supply of fresh water in resource-limited regions. “Our approach’s practical value lies in connecting water production with local resource use,” added Li.
Now setting their sights towards wastewater, Li and the team aim to mould surfaces that can maintain stable evaporation over long periods even when contaminants rapidly accumulate. They also hope to introduce resource recovery functions to the evaporator design, capturing valuable metals and ions from such wastewater in addition to extracting clean water.
The A*STAR-affiliated researchers contributing to this research are from the A*STAR’s Institute of Sustainability for Chemicals, Energy and Environment (ISCE2).
