LAUSR

Abstract In this study, we investigated the oxidation kinetics of two structurally different dyes, C.I. Reactive Blue 21 (B21) and C.I. Reactive Yellow 15 (Y15), using H2O2, with NaOH or UV-light as the catalyst. The kinetics experiments were conducted in the temperature range of 30–60 °C for 120 min and measurements were taken by spectrophotometry. Some important parameters in the oxidation reaction were evaluated (amount of 30% H2O2, initial pH and NaOH concentration) and under optimal conditions decolorization efficiencies of >99% were obtained for B21 at 60 °C (1 mL of 30% H2O2/UV or 2 mL of 30% H2O2/NaOH). In the case of Y15, the degradation efficiency reached only 86 and 13% (60 °C, 3 mL of 30% H2O2) for the NaOH and UV-catalyzed reactions, respectively. The kinetic behavior followed a pseudo-first-order model for both dyes and the activation parameters (Ea, ΔH#, ΔG# and ΔS#) for the base and UV-light catalyzed oxidation reactions were determined. The results reported herein demonstrate that, for the two reactive dyes, the kinetic behavior is highly dependent on the molecular structure and the catalyst applied. Under the same experimental conditions, B21 (a copper II complex) and Y15 (a pyrazole derivative) presented kobs values of 37.3 × 10−3 and 0.9 × 10−3 min−1, respectively. The lower efficiency values obtained for Y15 suggest that the pyrazole ring confers additional stability to the molecule toward oxidation, based on the results of the UV–Vis and 1H NMR spectroscopy analysis performed after the experiments.