Visible-light driven photocatalytic degradation of tartrazine using Co@ZIF-8 monoliths

Metal-organic frameworks have emerged as promising materials for environmental remediation due to their high surface area, tunable porosity, and structural versatility. However, most reported MOF photocatalysts are synthesized as powders, which are prone to aggregation, and challenging to recover after use. Herein, monolithic cobalt-doped ZIF-8 was synthesized via a rapid sol-gel method for the degradation of tartrazine dye in aqueous solution. PXRD results confirmed the successful formation of the ZIF-8 framework with preservation of the sodalite topology after cobalt incorporation, while UV-Vis DRS revealed the emergence of visible-light absorption bands attributed to d-d transitions of tetrahedrally coordinated Co²⁺ ions. Tauc analysis indicated the presence of mid-gap electronic states (1.9 eV) associated with cobalt doping, enabling improved visible-light responsiveness. The Co@ZIF-8 monolith achieved 86.0 ± 1.6% degradation under optimal conditions (4 mg L^-1, pH 5, 45 mg, 150 min), outperforming pristine ZIF-8 (42.8 ± 2.1%). Dark adsorption control showed 31.9 ± 1.9% removal, confirming dye concentration on the monolith surface prior to photocatalysis. Kinetic studies indicated pseudo-first-order behaviour, while reusability tests showed retained activity after five cycles, with PXRD confirming framework integrity. These findings highlight the potential of monolithic Co-doped ZIF-8 as a stable and reusable photocatalyst for the treatment of dye-contaminated wastewater.