Natural Organic Matter (NOM) is a heterogeneous mixture of various organic moieties and occurs ubiquitously in freshwater and marine environments.  NOM plays a critical role in a variety of important biogeochemical processes in natural waters including generation of reactive oxygen species (ROS), redox cycling of metals, and degradation of organic pollutants. We have undertaken a range of studies using Suwannee River Fulvic acid (SRFA) as the model NOM in both acidic and circumneutral pH environment with the aim to elucidate (i) the mechanism of generation and decay of ROS in irradiated SRFA solution and (ii) implications of ROS generation to iron redox cycling in irradiated SRFA solution. <br/>Our results show that irradiation of SRFA with simulated sunlight resulted in production of nanomolar concentrations of superoxide and hydrogen peroxide under both acidic and alkaline environment. Superoxide was produced on reduction of oxygen by photo-generated organic radicals while hydrogen peroxide was generated exclusively via uncatalysed disproportionation of superoxide. Under irradiated conditions, superoxide decayed through both uncatalysed disproportionation and an oxidative pathway that did not result in hydrogen peroxide production whereas hydrogen peroxide was a stable end product of SRFA photolysis. Our results further reveal that SRFA catalysed disproportion of superoxide which was observed to occur in non-irradiated SRFA solution was inhibited under irradiated conditions.<br/>We also investigated the photochemical Fe(III) reduction kinetics in irradiated SRFA solution. Our results show that ligand-to metal charge is the main pathway for Fe(III) reduction at low [Fe]:[SRFA]; however Fe(III) reduction by photo-generated superoxide also contributes at higher [Fe]:[SRFA].  This change in the Fe(III) reduction mechanism possibly occur due to variation in the strength of the Fe(III)SRFA complex formed and/or formation of relatively less photo-active iron oxides .<br/>

Photochemistry of natural organic matter (NOM) in natural waters: Implications to reactive oxygen species generation and redox cycling of iron

255th National Meeting (2018)

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