Novel Approaches to Investigate The Oxidation Rate Of Fe(II) And The Role Of Fe(II)/Fe(III) Cycling On The Maintenance Of Reactive Oxygen Species In Aquatic Systems

The biogeochemical cycling of iron, oxygen, and organic carbon are inextricably linked through the intermediacy of Reactive Oxygen Species (ROS) brought about by the autoxidation of ferrous iron (Fe(II)). This dissertation presents laboratory and field-based studies to investigate the kinetics of Fe(II) oxidation and its role in the production of ROS at oxic/anoxic interfaces. The net oxidation of Fe(II) in natural waters is a complex process consisting of alternating cycles of Fe(II) oxidation and Fe(III) reduction processes that are ultimately terminated via precipitation as insoluble Fe(III) (oxy)hydroxides. This complicates kinetic measurements of individual reaction steps in the process. A critical hypothesis examined by this work is that the rate for the direct reaction of Fe(II) with dioxygen (O2) is comparable in magnitude to the reverse reaction of Fe(III) with O2- (108 M-1s-1). Competitive kinetic assays against a series of Fe(II)-binding ligands determined the bimolecular rate constant for Fe(II) oxidation to be at least 5.3(±0.2)×104 M-1s-1, with evidence pointing towards a true range of 107 – 109 M-1s-1. A multivariate experimental design was employed to evaluate the impact of factors that change the net rate and magnitude of ROS production during the oxidation of Fe(II). Laboratory-based measurements were validated in the field by measuring ROS production in an iron- and NOM-rich estuarine system. Two chemiluminescence-based methods (Acridinium Ester, MCLA) were modified for the real-time measurement of ROS in the presence of high levels of Fe(II) and Fe(III). Recommendations for reduction of artifacts arising from Fe-mediated catalysis and method-associated interferences (i.e. precipitation of Ca/Mg-hydroxides) are reported. Results suggest that Fe(II) catalytically initiates the production and consumption of ROS in estuarine systems, with strong evidence for Fe(II) oxidation as a primary source for ROS under aphotic conditions. Removal of Fe(II) from the system leads to the rapid depletion of ROS, suggesting that Fe(II) is not a significant sink for ROS with reference to microbially-mediated decay or reactions with organic scavengers