Synergistic adsorption and photocatalysis using coffee husk-derived ZnO/hydrochar for efficient caffeine removal
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Elsevier Ltd
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Caffeine is an emerging environmental contaminant due to its persistence and widespread presence in aquatic environments. Aligning with circular economy principles, this study utilizes coffee husk (CH), a major agro-industrial waste, to synthesize a bifunctional ZnO/hydrochar composite for combined adsorption and photocatalysis. The materials were prepared hydrothermally using varying CH amounts (0.25-1.0g), resulting in HZnO-x, composites when x indicates the final mass fraction of hydrochar. Characterization showed ZnO nanoflakes and nanorods (wurtzite phase) supported on hydrochar (11.4-40.6wt%). HZnO-0.4, with 40.6wt% hydrochar, exhibited a reduced bandgap (3.11eV) and superior adsorption. Optimal caffeine removal occurred at pH ∼6 and 0.6gL⁻1dosage. Adsorption dominated at low caffeine concentrations and high adsorbent dosages, achieving 96.5% and 80% removal for 5 and 10mgL⁻1, respectively. At lower adsorbent dosage (0.3gL⁻1), photodegradation became dominant (52%), while adsorption dropped to 42%. A synergistic effect enabled complete caffeine removal (90-100%) across 5-10mgL⁻1concentrations. In darkness, removal was primarily via adsorption; under light, ZnO functioned as a photocatalyst, generating reactive oxygen species (ROS) such as singlet oxygen and superoxide radicals. These species degraded caffeine into hydroxylated and ring-opened products. Post-treatment analyses confirmed the structural stability of ZnO and hydrochar, and HZnO-0.4 maintained performance over repeated cycles. This study demonstrates the potential of coffee husk-derived ZnO/hydrochar composites as sustainable, efficient materials for wastewater treatment. © 2025 Elsevier B.V., All rights reserved.
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Adsorption, Caffeine, Coffee Husk, Degradation, Hydrochar, Photocatalysis, Zinc Oxide, Coffee, Driers (materials), Industrial Economics, Oxygen, Photodegradation, Stability, Wastewater Treatment, Zinc Sulfide, Adsorbent Dosage, Agro-industrial Wastes, Aquatic Environments, Bi-functional, Circular Economy, Coffee Husks, Environmental Contaminant, Hydrochar, Synergistic Adsorptions, Zno, Adsorption, Caffeine, Ii-vi Semiconductors, Photocatalysis, Zinc Oxide