The global scarcity of freshwater remains a significant barrier to sustainable development, especially since most of the Earth’s water is saline. Traditional desalination methods, while effective, often demand high energy consumption and carry substantial financial costs. In the journal Biochar, researchers Sihui Wang, Jiaqi Yang, Aijie Wang, and Wenzong Liu explore a low-carbon alternative known as solar interface evaporation. This technology uses solar energy to heat only the surface of the water, which prevents wasting energy on heating the entire water source. While hydrogels have been popular for this purpose due to their ability to transport water, their ability to convert sunlight into heat has historically been a weak point. The researchers addressed this by incorporating biochar derived from sorghum straw into a specific type of hydrogel, creating a hybrid system that optimizes both heat and water management.

The central difficulty in solar evaporation is balancing three specific tasks: absorbing light, moving water to the surface, and managing how much energy is needed to turn that water into vapor. Many existing materials are good at one of these but fail at others. For instance, some materials absorb light well but cannot keep up with the water supply, while others have great water flow but let too much heat escape into the water below. By doping a polyzwitterionic hydrogel with biochar, the team created a structure that addresses all these issues simultaneously. The biochar acts as a powerful light absorber while the hydrogel provides the necessary plumbing to keep the surface wet. This combination prevents salt from crusting over the device, which is a common problem that usually stops these systems from working over long periods.

The researchers discovered that biochar does more than just turn the material black to soak up the sun. The chemical groups on the surface of the biochar interact with the polymer chains of the gel, which actually changes the internal structure of the material. These interactions create smaller, more uniform pores that help the device trap light more effectively by bouncing it around inside. This internal scattering increases the chance that the energy will be absorbed rather than reflected away. Furthermore, these surface groups have a fascinating effect on the water itself. They break down the strong bonds that usually hold water molecules together, turning what scientists call free water into intermediate water. Because this intermediate water is less tightly bound, it takes far less heat energy to trigger evaporation.

The results of these structural and chemical changes are significant for the field of water treatment. The hybrid evaporator achieved a stable light absorption rate of over 95 percent across a broad spectrum of light. During testing, the surface temperature of the hybrid material rose much higher than the water underneath it, proving that the heat was being kept exactly where it was needed. This efficient energy use allowed for an evaporation rate of 3.57 kilograms per square meter per hour under standard sunlight conditions, which is nearly two times the rate of the gel without biochar. This increase is a direct result of lowering the energy threshold for evaporation. By making the process more efficient at a molecular level, the researchers have shown that biochar is not just a simple additive, but a vital component for next-generation water purification technologies.

Source: Wang, S., Yang, J., Wang, A., & Liu, W. (2026). Heat loss and water transport capacity regulation in hybrid evaporators. Biochar, 8(97), 1-10.