Removal of 1,4-naphthoquinone by birnessite-catalyzed oxidation: Effect of phenolic mediators and the reaction pathway

This study investigated the birnessite (δ-MnO₂) catalyzed oxidative removal of 1,4-naphthoquinone (1,4-NPQ) in the presence of phenolic mediators; specifically, the kinetics of 1,4-NPQ removal under various conditions was examined, and the reaction pathway of 1,4-NPQ was verified by liquid chromatography–tandem mass spectrometry (LC–MS/MS). The removal rate of 1,4-NPQ by birnessite-catalyzed oxidation (pH = 5) was faster in the presence of phenolic mediators with electron-donating substituents (pseudo-first-order initial stage rate constant (k₁) = 0.380–0.733 h^–1) than with electron-withdrawing substituents (k₁ = 0.071–0.244 h⁻¹), and the effect on the substituents showed a positive correlation with the Hammett constant (Σσ) (r² = 0.85, p < 0.001). The rate constants obtained using variable birnessite loadings (0.1–1.0 g L⁻¹), catechol concentrations (0.1–1.0 mM), and reaction sequences indicate that phenolic mediators are the major limiting factor for the cross-coupling reaction of 1,4-NPQ in the initial reaction stages, whereas the birnessite-catalyzed surface reaction acts as the major limiting factor in the later reaction stages. This was explained by the operation of two different reaction mechanisms and reaction products identified by LC-MS/MS.