Oxygen and superoxide-mediated redox kinetics of iron complexed by humic substances in coastal seawater

Complexes with terrestrially derived humic substances represent one of the most reactive pools of dissolved Fe in natural waters. In this work, redox kinetics of Fe-humic substance complexes (FeL) in simulated coastal seawater were investigated using chemiluminescence techniques with particular attention given to interactions with dioxygen (O2) and superoxide (O2•−). Although rate constants of FeIIL oxidation by O2 (5.6−52 M−1·s−1) were 4−5 orders of magnitude less than those for O2•− (6.9−23 × 105 M−1·s−1), O2 is likely to outcompete O2•− for FeIIL oxidation in coastal seawaters where steady-state O2•− concentrations are generally subnanomolar. Rate constants for FeIIIL reduction by O2•− of 1.8−5.6 × 104 M−1·s−1 were also determined. From the balance of FeIIL oxidation rates and O2•−-mediated FeIIIL reduction rates, steady-state FeIIL concentrations were estimated to be in the subpicomolar to picomolar range, which is generally lower than measured in situ Fe(II) concentrations under relevant conditions. This suggests that (i) processes other than O2•−-mediated reduction (such as photochemical ligand-to-metal charge transfer) may be responsible for Fe(II) formation, (ii) the in situ ligands differ significantly from the humic substances used in this work, and/or (iii) the influence of other environmental factors such as pH and temperature on Fe redox kinetics may have to be considered.