Electrochemical degradation of tartrazine by persulfate electrolysis: Process optimization and mechanistic insights from DFT calculations

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Contamination of water resources by synthetic azo dyes represents a major environmental challenge, particularly due to their persistence, toxicity, and resistance to conventional treatment methods. In this study, the degradation of tartrazine (E102), a widely used anionic azo dye, was investigated by the Electro-Persulfate (EPS) process. The influence of the applied current intensity, persulfate concentration, and electrolysis time was systematically studied using kinetic analyses and then optimized using response surface methodology (RSM) based on a central composite design (CCD). The statistical model demonstrated excellent predictive power with a coefficient of determination R² = 94.37%. The identified optimal conditions were a current intensity of 337.4 mA, a persulfate concentration of 1.6 mM, and an electrolysis time of 21.5 min, achieving a maximum tartrazine removal efficiency of 98.01%. Density functional theory (DFT) calculations performed at the wB97XD/6-311 G(d,p) level in aqueous medium were used to elucidate the degradation mechanism at the molecular level. Analysis of frontier molecular orbitals, molecular electrostatic potential mapping, and Fukui function calculations identified N11, N12, and C20 as the preferential sites of radical attack. Three degradation pathways involving the cleavage of C-N bonds around the azo group have been proposed, leading to the progressive degradation of tartrazine into small molecules such as H₂SO₄, CO₂, H₂O, and N₂. The combination of experimental optimization and theoretical mechanistic study provides a comprehensive understanding of the EPS process and confirms its effectiveness as an innovative and sustainable solution for treating industrial effluents contaminated with dyes.