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 H&sub2;O&sub2;. Hydrogen peroxide acts as a source of hydroxyl radicals (·OH) under photochemical conditions and yields MA hydroxyderivatives. The trend of MA photodegradation rate vs. H&sub2;O&sub2; concentration reaches a plateau because of the combined effects of H&sub2;O&sub2; absorption saturation and ·OH scavenging by H&sub2;O&sub2;. The addition of chloride ions causes scavenging of ·OH, yielding Cl&sub2;·- as the most likely reactive species, and it increases the MA photodegradation rate at high H&sub2;O&sub2; concentration values. The reaction between Cl&sub2;·- and MA, which has second-order rate constant k C l 2 • - + M A = (4.0 ± 0.3) × 10&sup8; M-1·s-1 (determined by laser flash photolysis), appears to be more selective than the ·OH process in the presence of H&sub2;O&sub2;, because Cl&sub2;·- undergoes more limited scavenging by H&sub2;O&sub2; compared to ·OH. While the addition of carbonate causes ·OH scavenging to produce CO&sub3;·- ( k C O 3 • - + M A = (3.1 ± 0.2) × 10&sup8; 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 H&sub2;O&sub2; to partially occur as HO&sub2;-, which reacts very quickly with either ·OH or CO&sub3;·- to produce O&sub2;·-. 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 SO&sub4;·- that reacts effectively with MA ( k S O 4 • - + M A = (5.6 ± 0.4) × 10&sup8; M-1·s-1). Irradiated H&sub2;O&sub2; 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).