Process intensification of ZnO photocatalysis via electrochemical coupling for tartrazine degradation

Tartrazine (Tar) is a widely used azo dye that persists in wastewater, motivating the development of advanced treatment strategies. In this study, tartrazine degradation by ZnO-based photocatalysis (PC) was intensified through electrochemical coupling (PEC) under UV irradiation. ZnO was synthesized by thermal decomposition assisted by mechanical milling and characterized by thermogravimetric analysis (TGA), X-ray diffraction (XRD), Fourier transform infrared spectroscopy with attenuated total reflectance (FTIR-ATR), UV–Vis absorption and diffuse reflectance spectroscopy (UV–DRS) and field-emission scanning electron microscopy (FESEM) with energy-dispersive X-ray, confirming the formation of a structurally stable nanocrystalline wurtzite phase suitable for photocatalytic activation. Photocatalytic and EC systems were evaluated and optimized using a Box–Behnken design and response surface methodology. Conventional photocatalysis using ZnO showed limited tartrazine removal (<50%) after prolonged reaction times (12 h), with an apparent rate constant of 0.051 ± 0.003 h −1 . In contrast, the PEC, operated by applying an external electric potential between steel electrodes, achieved removal efficiencies exceeding 90% within short reaction times under optimized conditions (ZnO dosage: 7.23 mg; initial tartrazine concentration: 11.09 mg/L; applied potential: 10.21 V). PEC enhanced charge separation, suppressed electron-hole recombination, and sustained reactive oxygen species generation according to the proposed degradation pathway. Kinetic analysis revealed a more than 170-fold increase in the apparent degradation rate constant compared to conventional photocatalysis, reaching up to 9.00 ± 0.25 h −1 under acidic conditions (pH 5). These results support the potential application of ZnO-based PEC in advanced wastewater treatment, particularly for effluents from textile, pharmaceutical, and food industries containing tartrazine.