Recycling of waste coffee grounds as a photothermal material modified with ZnCl₂ for water purification

The aim of this study was to develop a photothermal material modified with carbonization and ZnCl₂ impregnation and supported by polyvinyl alcohol (PVA) for water purification using the waste coffee grounds. Scanning electron microscopy (SEM) characterization of the prepared material revealed that a significant surface modification was achieved due to the carbonization and ZnCl₂ impregnation. X-ray diffraction analysis (XRD) pattern of the samples showed two broad peaks at 18.4° and 22.2°, this is due to the crystal planes of β-crystal phase structure, which indicates the existence of strong hydrogen bonds between the micro-structures and therefore less suspectable to chemical attack. Additionally, thermogravimetric analysis (TGA) result suggests a slight mass reduction between the temperature range of 65-75 °C implying the thermal stability of the prepared material. The produced modified material had a photothermal conversion efficiency of 74% and could produce vapor at a rate of 1.12 kg/m²h under 980 W/m² irradiation at 1 sun. A significant reduction in Cu²⁺ ion concentration (83%), turbidity (91%), total dissolved solids (TDS) (61%), microbial load (95.6%), and total hardness (41.2%) were achieved. Therefore, waste coffee grounds can be considered as a future eco-friendly and low-cost candidate for water purification.