Degradation of Methyl 2-Aminobenzoate (Methyl Anthranilate) by H2O2/UV: Effect of Inorganic Anions and Derived Radicals
Résumé
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 constant kCl∙−2+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·− ( kCO∙−3+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 ( kSO∙−4+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).