Feed-back action of nitrite in the oxidation of nitrophenols by bicarbonate-activated peroxide system

• 2-amino-4-nitrophenol and 4-nitrophenol undergo fast oxidation in the BAP system.
• The oxidation follows an autocatalytic path promoted by the elimination of nitrite.
• The in-situ generation of ROS led the degradation to aliphatic products.

In this study it was found that the nitrite anion greatly increases the oxidation rate of the substituted phenols by the bicarbonate-activated peroxide (BAP) system at ambient temperature. This feed-back effect was investigated in the BAP oxidation of 2-amino-4-nitrophenol and 4-nitrophenol, where the kinetic analysis showed that the rate determining step was the elimination of a nitro group as nitrite. Complementary oxidation experiments with 2-aminophenol in BAP system, performed in the presence/absence of sodium nitrite confirmed the catalytic role of this anion. High concentrations of nitrite/BAP prevented formation of polymeric species and favoured the degradation to aliphatic fragments such as 3-oxobutanoic and acetic acid. Indirect evidence suggested in-situ generation of reactive oxygen species (ROS), other than hydroxyl, peroxyl and carbonate radicals, during the nitrite/BAP oxidation. Formation of additional ROS – peroxynitrite and nitrosoperoxycarbonate – may account the synergistic action of nitrite/BAP system. The estimated value of the apparent rate constant of the autocatalytic step for 2-amino-4-nitrophenol (kapp = (4.97 ± 0.19) × 103 M−3 s−1) demonstrates that fast degradation of this oxidation-resistant phenolic dye is feasible even in ambient conditions. These findings can be exploited for developing clean, cost-effective methods for the removal of phenolic dyes as alternative to current advanced oxidation processes.

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