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A fast kinetics study by pulsed laser spectroscopy allows a mechanism for Orange II photobleaching in the presence of O2 to be proposed. Evidence is presented that the hydroperoxy radical HO2 is the main oxidative species for Orange II in oxygenated aqueous solution under visible light irradiation. The probability of reaction between O2 and Orange II leading to radical formation (≈5×10-4) is observed to be very low. The lifetime and amplitude of the radicals anions and cations in solution are compared in O2-, air- and Ar-containing solutions. Singlet oxygen (1O2) is shown not to be present during the initial photobleaching stages of Orange II. Steady-state irradiation of Orange II in aqueous solution in the presence of O2 causes two simultaneous processes: photodegradation to more biodegradable intermediates with CO2 evolution and photoinduced generation of peroxides in solution. The observed peroxide generation stops when initial Orange II reaches a low concentration and does not occur from the intermediates during the photodegradation of Orange II. Direct electron transfer from excited Orange II to O2 provides the driving force for the accelerated photodegradation of the dye. No electron transfer to O2 was observed in the dark involving the ground state of the azo dye. The oxidation potential Orange II/Orange+ is determined by cyclic voltammetry to be 0.76 V(NHE) and a value of -1.54 V(NHE) could be estimated for the oxidation potential of the couple Orange II*/Orange+.
Bandara et al. (Fri,) studied this question.