Influence of Fe and Zn Loading and Calcination Temperature on Sol–Gel-Derived TiO2 Photocatalysts for Food-Industry Effluent Treatment

Fe/Zn-modified TiO2 photocatalysts were synthesized by the sol–gel method to investigate the influence of Fe loading, Zn loading, and calcination temperature on the degradation of food-industry pollutants. A 23 factorial design combined with Response Surface Methodology was employed to optimize the synthesis parameters. The catalysts were characterized by N2 adsorption–desorption, SEM/EDS, X-ray diffraction coupled with Rietveld refinement and point of zero charge analyses. The materials exhibited mesoporous structures with type IV isotherms, while Fe/Zn modification altered the crystalline phase composition and surface charge of TiO2. Low metal loading stabilized the anatase phase, whereas higher Fe contents promoted the formation of hematite and rutile. Photocatalytic performance was evaluated through the discoloration and degradation of Red 40 and Tartrazine under natural sunlight and the degradation of cheese whey under artificial irradiation. F10Z2-400 exhibited the highest activity toward Red 40 (99.85% discoloration and 77.02% COD removal), whereas T-400 showed the best performance for tartrazine (86.25% discoloration and 87.61% COD removal). For cheese whey, F10Z10-400 achieved the highest degradation (41.01% COD removal). Reactive-species scavenging indicated that hydroxyl radicals made the predominant contribution to the discoloration of both dyes, followed by superoxide radicals and photogenerated holes. Kinetic analyses indicated that the Behnajady–Modirshahla–Ghanbery model best described the degradation process. RSM identified calcination temperature as the most influential synthesis parameter and showed that the effects of Fe and Zn loading were pollutant-dependent. No single catalyst formulation provided the best performance for all evaluated matrices.