Photochemical production of superoxide and hydrogen peroxide from natural organic matter

Irradiation of Suwannee River fulvic acid (SRFA) at pH 8.1 with simulated sunlight resulted in production of nanomolar concentrations of superoxide and hydrogen peroxide. SRFA contains a redox-active chromophore which reduced oxygen to yield superoxide upon photoexcitation. Hydrogen peroxide was generated exclusively via uncatalysed disproportionation of superoxide produced in this way. Superoxide decayed through both uncatalysed disproportionation and an oxidative pathway that did not result in hydrogen peroxide production, whereas hydrogen peroxide did not undergo further reaction to any discernible extent over the one-hour duration of irradiation. Singlet oxygen did not contribute substantially to production of superoxide or hydrogen peroxide but was found to play a critical role in controlling the mechanism and associated rate of superoxide decay in the irradiated solution. A kinetic model based on these observations is presented which provides an excellent description of the experimental results and is also consistent with observations from a wide range of other studies in which various aspects of SRFA redox chemistry and photochemistry have been investigated.