From Selective Adsorption to Photocatalytic Removal: Complementary Cobalt Oxides for Pollutant-Oriented Treatment of Dye Mixtures

The development of photocatalysts for water treatment generally focuses on maximizing removal efficiency, whereas the role of material–pollutant interactions in determining pollutant-dependent removal behaviour remains comparatively underexplored. Herein, pristine Co3O4 and a CuCo2O4-based mixed oxide were comparatively investigated with two chemically distinct anionic dyes, methyl orange (MO) and Congo red (CR). Physicochemical characterization revealed differences between the materials, which were associated with distinct removal behaviours. Under solar irradiation, Co3O4 achieved 94% MO removal within 180 min at acidic pH, whereas CR removal showed a stronger adsorption contribution. Conversely, the CuCo2O4-based oxide exhibited near-complete CR removal through adsorption but limited photocatalytic contribution toward MO. After regeneration, substantial recovery of both photocatalytic and adsorption performance was observed. These complementary behaviours were exploited in sequential treatment of a binary MO/CR mixture, combining CR removal by adsorption over the CuCo2O4-based material with photocatalytic removal of the residual MO by Co3O4. A simultaneous one-pot configuration achieved comparably high final removal of both dyes, showing that temporal separation was not required for high overall removal under the investigated conditions. Overall, matching material functionalities to pollutant characteristics enables pollutant-oriented treatment strategies integrating complementary adsorption and photocatalytic functions.