Dual-Action κ-Carrageenan: A Novel Capping Agent and Dopant Source for Enhanced Photocatalytic ZnO Nanoparticles in Azo Dye and Cr(VI) Degradation

Metal oxide nanoparticles synthesized via green routes are increasingly recognized for their potential in water treatment applications due to their enhanced photocatalytic properties. This study presents a novel approach for the synthesis of zinc oxide nanoparticles (ZnO NPs) using κ-carrageenan derived from alkali-treated <i>Kappaphycus striatus</i> (Katunay itum) as a capping agent during ZnO NP formation and as a source of nonmetal dopants, carbon, and sulfur. The resulting carrageenan-mediated ZnO NPs (car-ZnO NPs) were thoroughly characterized using physicochemical techniques to evaluate their morphological, optical, and chemical properties. The findings revealed that κ-carrageenan incorporation significantly reduced the band gaps and crystallite sizes of ZnO NPs. Specifically, the 0.75 car-ZnO NPs exhibited the most uniform particle size distribution and demonstrated superior photocatalytic efficiency in degrading methylene blue (MB), methyl orange (MO), and hexavalent chromium (Cr(VI)) under UV light, outperforming unmodified ZnO NPs (0.00 car-ZnO). The enhanced photocatalytic activity is attributed to the narrowed band gap and effective doping with carbon and sulfur, which improved charge carrier separation and extended their lifetime. Additionally, car-ZnO NPs showed excellent photostability and high efficiency in repeated photocatalytic cycles, emphasizing their eco-friendly nature and practicality for water treatment. This study highlights the dual role of κ-carrageenan as a promising, sustainable material for synthesizing ZnO NPs with advanced photocatalytic properties, offering a green alternative for effective wastewater treatment and environmental remediation.