Mechanism and Kinetics of the OH-Radical Intervention during Fenton Oxidation in the Presence of a Significant Amount of Radical Scavenger (Cl^-)

Abstract

Orange II, the 4-(2-hydroxy-1-naphthylazo)benzenesulfonic acid Na-salt, was taken as a model for the oxidation of organic compounds in photoassisted Fenton process in the presence of Cl--anion in solution. The HO• radicals seem to originate due to the photolysis of Fe(OH)2+ complexes in solution, whereas the Cl2-• radical was a product of the photolysis of the FeCl2+ complexes. The rate constants for the reaction of HO• and Cl2-• radicals with Or II were determined by laser kinetic spectroscopy: kOH•+OrII = 6.0 × 109(M s)-1 and kCl2•+OrII = 3.7 × 107 (M s)-1. A significant decrease in the rate of decoloration was observed for Orange II upon addition of Cl- (10 mM), but further addition of Cl- only marginally affected the latter reaction rate. Chlorinated hydrocarbons are observed as the products of Orange II oxidation in photoassisted and dark Fenton processes in the presence of Cl- anions. Light irradiation was observed to decrease the amount of chlorinated organic products (AOX) in solution as compared to dark Fenton processes. The results obtained are shown to be in agreement with the radical reaction theory for the particular case of the Fenton reaction. Kinetic modeling of the Orange II decoloration was carried out taking into consideration the rate constants found for the reaction kHO•+OrII and kCl2•+OrII.