IIT Mumbai scientists develop solar evaporators similar to lotus leaf for brine treatment

To address the major breakthrough in freshwater scarcity around the world, scientists from the Indian Institute of Technology, Mumbai have developed a new material that can promote water desalination.

Researchers Swatantra Pratap Singh and Professor Aiswarya CL developed a double-sided superhydrophobic laser-induced graphene (DSLIG) evaporator that addresses several shortcomings of early evaporators and has the potential for large-scale applications.

Although the water on Earth is abundant, about 3% of it is fresh water, and even within it, less than 0.05% of the water is easily obtained. Removing salt (desalting) from seawater and salt water is seen as one of the solutions to this problem, and the researchers are committed to developing more efficient and faster desalination techniques. However, inland areas, desalination of brine (concentrated salt solution) is a big problem and the industry is looking for liquid emissions.

The researchers say the solar-based desalination method is considered ideal due to the reduction in carbon footprint. However, factors such as fluctuations in sunlight intensity and availability and the absorption rate of light greatly affect the efficiency and consistency of solar-based desalination techniques.

Mr Singh explained that in recent years, interface evaporation systems have become a promising approach. A key component of these systems is an evaporator made of materials that absorb solar energy and heat it. The evaporator placed on the water surface focuses the solar heat on a thin layer of water on the surface of the evaporator instead of heating the entire volume. This local heating minimizes heat loss and increases the efficiency of the seawater desalination process.

However, despite this advantage, problems with traditional solar desalination techniques can also affect the interface evaporators.

“The fluctuations in solar radiation can cause temperature changes on the surface of the evaporator. On cloudy days, the performance of the interface system stops due to the lack of solar energy. In addition, changes in daily solar radiation affect the evaporation process, which usually peaks around the solar intensity when the evaporation rate is highest.”

Another major challenge in the interfacial evaporation system is the deposition of salt crystals on the surface of the evaporator. Salt deposition on the surface prevents water from contacting the evaporator, so its efficiency decreases over time. This study aims to address both issues. In addition to solar heating, DSLIG evaporators can be heated using electricity (called Joule heating) as well.

By combining solar energy and electrical heating, the material can be used in sunlight. If it is small or there is no sunlight, electricity can be used to heat the evaporator and maintain a similar temperature, ensuring continuous performance. Furthermore, DSLIG has superhydrophobic properties, which means it repels water like a lotus leaf.

Due to its surface characteristics, the superhydrophobic material reduces the contact area between the water droplets and the material surface, causing the droplets to roll over it rather than wet.

“In seawater desalination applications, the superhydrophobic properties of dslig help prevent salt from dissolving in water, thereby sticking to the evaporator surface, thus maintaining efficiency. Over time, the main goal of our work is to create surfaces with super-poly content that exhibit a lotus effect that shows the ability to play a role in the Solar and Joule Heating functions,” explains Mr Singh.

The researchers made dslig by coating one side of another polymer LIG with a layer of polymer called polydehydefluoride (PVDF). Graphene is then engraved on the PVDF polymer side of the material using laser-based engraving technique. “The name of the material comes from the fact that it has two different sides formed by two different polymers and the manufacturing technology used. PES does not exclude water, but it is essential to prevent the evaporator from rupturing easily. If only PES is used, the final surface will get wet on both sides, but, in both cases of stability, pvDF can be performed with PVDF. The hydrophobic characteristics necessary for the effective evaporation process,” said the professor.

Laboratory tests show that DSLIG not only exhibits lotus leaf-like behavior, prevents salt deposition and desalination under electrical heating, but is also very effective in treating extremely concentrated salt solutions. This makes it an ideal candidate for treating other desalination sockets as well as brine discharge from industrial wastewater.

The researchers also show that DSLIG’s performance improves when multiple evaporators are stacked on top of each other. Despite its low carbon footprint, low toxicity and cost-effectiveness make DSLIG a potential candidate for large-scale sustainable desalination applications and industrial wastewater treatment. Mr Singh said further field testing is required before such large-scale applications are made.

“One of the major obstacles the team faces in ensuring and testing industrial readiness for this technology is the lack of funding. At the same time, we look forward to developing more such superhydrophobic materials that can utilize both solar and electricity and have greater efficiency,” Mr Singh added.