Photoproduction of hydroxyl radicals from Fe(III)-hydroxy complex: a quantitative assessment

Photolysis of aqueous solutions of ferric perchlorate in the presence of methanol, benzene and 2-deoxy-D-ribose at low pH has been investigated using sunlight and UV light in order to quantitatively assess the involvement of hydroxyl radicals ((OH)-O-.) in the photolysis of Fe(III)-hydroxy complexes. Time-dependent formation of formaldehyde, phenol and thiobarbituric acid-reactive substance (TBA-RS) was observed in the presence of methanol, benzene and 2-deoxy-D-ribose, respectively. The observed quantum yields of formaldehyde, phenol and TBA-RS are 0.025,0.0098 and 0.0038, respectively, using UV light. A slightly lower quantum yields are obtained with sunlight owing to its lower intensity. The mechanistic aspects of these reactions are proposed based on the formation of (OH)-O-. from the photo-excited Fe(OH)(2+) complex and their subsequent reactions with methanol, benzene and 2-deoxy-D-ribose. The quantum yield of (OH)-O-. is calculated from the observed quantum yields of formaldehyde, phenol and TBA-RS based on the reported percentage contribution of (OH)-O-. in the formation of these products using radiation chemical techniques from the respective reactions. The calculated quantum yields of (OH)-O-. were in the range 0.018-0.025 and 0.014-0.018 with UV light and sunlight, respectively. The results obtained with the photolytic studies have been compared with those from radiation chemical studies. (C) 2001 Elsevier Science B.V. All rights reserved.