Photocatalytic degradation and transformation pathway analysis of Rhodamine B over cobalt oxide-decorated hollow carbon nitride in complex water matrices

A cobalt oxide-decorated hollow carbon nitride ( g -C 3 N 4 ) photocatalyst was synthesized and evaluated for the degradation of Rhodamine B (RhB) under ultraviolet (UV) irradiation, with particular attention to removal efficiency, matrix tolerance, and stability. Characterization showed that cobalt oxide species were finely dispersed on the g -C 3 N 4 framework, and the modified composite exhibited a narrower band gap and suppressed charge-carrier recombination relative to pristine g -C 3 N 4 . X-ray photoelectron spectroscopy further suggested interfacial Co-N/Co-O interaction and mixed cobalt valence states. The optimized catalyst achieved more than 95% RhB removal within 30 min under the selected conditions. It retained activity in the presence of common coexisting ions, maintained over 80% degradation efficiency after five reuse cycles, and remained effective after 28 days of storage. Effective degradation was also observed in lake water and paper mill effluent, and methyl violet was removed in these complex matrices, indicating the applicability of the catalyst to dye-containing wastewater. Mechanistic studies showed that •O₂ − and •OH were the dominant reactive species. Density functional theory and high-performance liquid chromatography coupled with quadrupole-time-of-flight tandem mass spectrometry (HPLC-Q-TOF-MS/MS) were further combined to identify preferential radical attack sites and elucidate the major transformation pathways of RhB. The degradation proceeded mainly through sequential de-ethylation, chromophore cleavage, and ring-opening reactions, ultimately yielding low-molecular-weight aromatic acids, including benzoic acid and o-phthalic acid. Toxicity estimation using the Toxicity Estimation Software Tool (T.E.S.T) model suggested that the final products were associated with lower ecological risk than the parent dye. These results indicate that cobalt oxide modification enhances the photocatalytic performance of g -C 3 N 4 for dye-containing wastewater treatment.