Iron-catalyzed photochemical transformation of benzoic acid in atmospheric liquids: Product identification and reaction mechanisms

Abstract: This study investigated iron-catalyzed photochemical oxidation of benzoic acid (BA), one of the major photodegradation products of petroleum hydrocarbons, under sunlight or monochromatic light irradiation in a wavelength range of 254–419nm. The photochemical degradation of BA in the absence of iron (III) occurred at irradiation wavelengths below 300nm. The photochemical transformation of BA in the presence Fe(III) was observed at both 254, 350, 419nm and under solar irradiation. The half-life for the photodegradation of BA (100μM) was 160±20min in the presence of 20μM Fe(III) at pH 3.20 on sunny August days at noon time. The degradation rate increased with increasing concentration of Fe(III). The reaction products were separated and identified using capillary electrophoresis (CE), gas chromatography/mass spectrometry (GC/MS) and UV-Visible spectrophotometry. The major reaction products were 2-hydroxybenzoic, 3-hydroxybenzoic and 4-hydroxybenzoic acids. Hydrogen peroxide (H2O2) and Fe(II) species were also formed during the photochemical reactions. The proposed reaction mechanisms include the photoexcitation of Fe(III) hydroxide complexes to form Fe(II) ions and hydroxyl radicals (OH ) that attack ortho, meta and para positions of BA to form corresponding monohydroxybenzoic acids and H2O2. The monohydroxybenzoic acids formed further react with hydroxyl and surperoxide radicals (HO2 − /O2 − ) to yield dihydroxybenzoic acids in atmospheric water droplets. [Copyright &y& Elsevier]