Environmentally friendly chitosan/spent coffee ground composites with continuous hydrophobically gradient of porous...

Activated carbon and other carbon-based materials are common photothermal materials, but their traditional production methods are energy-intensive, driving the search for low-energy alternatives. This study investigates spent coffee grounds (SCG) as a sustainable photothermal material. Rich in melanin, SCG was combined with chitosan (CS) to create a photothermal composite via hydrolysis and photo-crosslinking. The resulting CS/SCG composite demonstrated significant photothermal performance and was further optimized by introducing a wettability gradient using a simple alkyl chain grafting technique. This wettability gradient enhanced water absorption and evaporation efficiency by maintaining an optimal temperature distribution across the composite. In photothermal water evaporation tests, the CS/SCG composite with a wettability gradient achieved the highest evaporation rate of 1.57 kg/m²h. Life cycle assessment revealed a significant reduction in carbon emissions (0.707 kg CO₂ eq per gram of composite) compared to traditional, energy-intensive bio-based photothermal materials. These findings suggest that CS/SCG composites offer an eco-friendly, cost-effective, and scalable solution for photothermal water evaporation, promoting waste valorization and environmental sustainability.