Biochar Hydrogels Boost Solar Water Purification EfficiencyBiochar Hydrogels Boost Solar Water Purification Efficiency
Materials scientists have long eyed solar interfacial evaporation, a process that uses direct sunlight at the water's surface to turn saltwater or polluted water into pure steam but without any contaminants. However, finding a material that excels at absorbing light, trapping heat, and pumping water to the surface has proven tricky.
A study from researchers at the Harbin Institute of Technology (Shenzhen), published in the journal Biochar, offers a solution. By infusing carbonized sorghum straw into a polyzwitterionic hydrogel, the team created a solar evaporator that speeds up water purification.
According to corresponding author Dr. Wenzong Liu:
"Solar interfacial evaporation is attractive because it uses clean solar energy directly at the water-air interface, but the material must do several things well at the same time. Our work shows that biochar can help hydrogels absorb light, manage heat, transport water, and activate water molecules through one integrated design."
Researchers performed pyrolysis, acid washing, ball milling, and sieving to prepare the sorghum biochar. Then, they added it to the hydrogel network, turning what was once a clear hydrogel into a black hybrid one capable of capturing over 95% of incoming solar energy across the broad light spectrum.
The team used scanning electron microscopy to look inside the hydrogel after adding biochar, and the structure had clearly changed. A dense, uneven network of microscopic pores had formed, rough enough that sunlight entering the material gets scattered around inside rather than reflecting straight back out. Because of that scattering, almost all the incoming light ends up absorbed instead of wasted. The pores do double duty, too: the same network draws water upward through capillary action, feeding it to the surface so it can evaporate.
Results showed that the hybrid hydrogel recorded a 41.1°C surface temperature, which was higher than the 29.3°C temperature of the bulk water beneath it. The gap in the two temperature readings indicates that heat remains concentrated at the evaporation interface and does not bleed into the water basin, resulting in a 3.57 kg m⁻² h⁻¹ evaporation rate, which is almost double what the untreated hydrogel managed.
Besides absorbing light and trapping heat, the biochar also changes the behavior of water molecules, causing the energy needed to evaporate water inside the hybrid hydrogel to drop to 877.79 J g⁻¹, lower than standard liquid water.
Dr. Liu said:
"The key finding is that biochar is not only a solar absorber. It also regulates the hydrogel's pore structure and the state of water molecules. This dual pathway explains why the evaporation performance improves so significantly."
Furthermore, the hybrid material achieved a saturated water content of 520% in saline tests. This result proved that it can maintain fast fluid delivery without clogging or degrading over time.
Read the full article here to learn more about the biochar-powered hydrogel.
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