Inhibition by phenolic antioxidants of the degradation of aromatic amines and sulfadiazine by the carbonate radical (CO3•−)

The carbonate radical CO₃•⁻ and the excited triplet states of chromophoric dissolved organic matter play an important role in the photodegradation of some easily oxidized pollutants in surface waters, such as the aromatic amines. Anilines and sulfadiazine are known to undergo back-reduction processes when their degradation is mediated by the excited triplet states of photosensitizers (triplet sensitization). Back-reduction, which inhibits photodegradation, means that phenols or the antioxidant (mostly phenolic) moieties occurring in the natural dissolved organic matter of surface waters reduce, back to the parent compounds, the radical species derived from the mono-electronic oxidation of anilines and sulfadiazine. Here we show that a similar process takes place as well in the case of substrate oxidation by CO₃•⁻. The carbonate radical was here produced upon oxidation of HCO₃⁻/CO₃²⁻ by either HO•, generated by nitrate photolysis, or SO₄•⁻, obtained by photolysis of persulfate. Back-reduction was observed in both cases in the presence of phenols, but at different extents as far as the details of reaction kinetics are concerned, and the occurrence of additional reductants might affect the efficacy by which phenols carry out the reduction process. In particular, when the carbonate radicals were produced by NO₃– photolysis in the presence of HCO₃⁻/CO₃²⁻, the numerical values of [PhOH]₁/₂ (the phenol concentration that halves the photodegradation rate of the substrate) were 2.19 ± 0.23 µM for aniline, 1.15 ± 0.25 µM for 3-chloroaniline, 1.18 ± 0.26 µM for 4-chloroaniline, and 1.18 ± 0.22 µM for 3,4-dichloroaniline. In contrast, when CO₃•⁻ was produced by photolysis of persulfate in the presence of HCO₃⁻/CO₃²⁻, the corresponding values were 0.28 ± 0.02 µM for aniline and 0.79 ± 0.10 µM for sulfadiazine. Back-reduction has the potential to significantly inhibit photodegradation by CO₃•⁻ and excited triplet states in natural waters, and to comparatively increase the importance of HO•-mediated degradation that is not affected by the same phenomenon.