Degradation of Methyl 2-Aminobenzoate (Methyl Anthranilate) by H2O2/UV: Effect of Inorganic Anions and Derived Radicals

Abstract

This study shows that methyl 2-aminobenzoate (also known as methyl anthranilate, hereafter MA) undergoes direct photolysis under UVC and UVB irradiation and that its photodegradation is further accelerated in the presence of H₂O₂. Hydrogen peroxide acts as a source of hydroxyl radicals (·OH) under photochemical conditions and yields MA hydroxyderivatives. The trend of MA photodegradation rate vs. H₂O₂ concentration reaches a plateau because of the combined effects of H₂O₂ absorption saturation and ·OH scavenging by H₂O₂. The addition of chloride ions causes scavenging of ·OH, yielding Cl₂·⁻ as the most likely reactive species, and it increases the MA photodegradation rate at high H₂O₂ concentration values. The reaction between Cl₂·⁻ and MA, which has second-order rate constantkCl2•−+MA= (4.0 ± 0.3) × 10⁸ M⁻¹·s⁻¹ (determined by laser flash photolysis), appears to be more selective than the ·OH process in the presence of H₂O₂, because Cl₂·⁻ undergoes more limited scavenging by H₂O₂ compared to ·OH. While the addition of carbonate causes ·OH scavenging to produce CO₃·⁻ (kCO3•−+MA= (3.1 ± 0.2) × 10⁸ M⁻¹·s⁻¹), carbonate considerably inhibits the photodegradation of MA. A possible explanation is that the elevated pH values of the carbonate solutions make H₂O₂ to partially occur as HO₂⁻, which reacts very quickly with either ·OH or CO₃·⁻ to produce O₂·⁻. The superoxide anion could reduce partially oxidised MA back to the initial substrate, with consequent inhibition of MA photodegradation. Fast MA photodegradation is also observed in the presence of persulphate/UV, which yields SO₄·⁻ that reacts effectively with MA (kSO4•−+MA= (5.6 ± 0.4) × 10⁹ M⁻¹·s⁻¹). Irradiated H₂O₂ is effective in photodegrading MA, but the resulting MA hydroxyderivatives are predicted to be about as toxic as the parent compound for aquatic organisms (most notably, fish and crustaceans).