Fast kinetic spectroscopy, decoloration and production of H2O2 induced by visible light in oxygenated solutions of the azo dye Orange II

A fast kinetics study by pulsed laser spectroscopy allows a mechanism for O range II photobleaching in the presence of O-2 to be proposed. Evidence is presented that the hydroperoxy radical HO2. is the main oxidative species f or Orange II in oxygenated aqueous solution under visible light irradiation . The probability of reaction between O-2 and Orange II leading to radical formation(approximate to 5 x 10(-4)) is observed to be very low. The lifeti me and amplitude of the radicals anions and cations in solution are compare d in O-2-, air- and Ar-containing solutions. Singlet oxygen (O-1(2)) is sho wn 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 O-2 causes two simultaneous processes: photodegradation to more biodegra dable intermediates with CO2 evolution and photoinduced generation of perox ides in solution. The observed peroxide generation stops when initial Orang e II reaches a low concentration and does not occur from the intermediates during the photodegradation of Orange II. Direct electron transfer from exc ited Orange II to O-2 provides the driving force for the accelerated photod egradation of the dye. No electron transfer to O-2 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 v alue of - 1.54 V(NHE) could be estimated for the oxidation potential of the couple Orange II*/Orange(+.).