Impact of natural organic matter on H2O2-mediated oxidation of Fe(II) in coastal seawaters

Whereas the oxidation of inorganic Fe(II) by H2O2 in seawater has been well studied, the oxidation of Fe(II) complexes with natural organic matter (NOM) by this ubiquitous oxidant has received little attention. Suwannee River fulvic acid (SRFA), a proxy for terrestrial NOM, is shown to have a much smaller impact upon Fe(II) oxidation kinetics in seawater than the strong effect previously observed in freshwater conditions. However, the oxidation kinetics of Fe(II) in seawater and freshwater can be quantitatively described employing the same mechanistic kinetic model, except that the apparent formation constant of Fe(II)–SRFA complexes is substantially decreased under conditions representative of estuarine and river-influenced coastal waters. This implies that the same basic processes occur in both systems, with differences between Fe(II) oxidation kinetics in seawater and freshwater largely attributable to effects of ionic strength and matrix composition. This was confirmed with studies employing NaCl solutions with or without Mg2+/Ca2+ addition demonstrating that both ionic strength and divalent cations effect a decrease in the Fe(II)-binding affinity of SRFA. The impact of NOM upon iron redox transformation kinetics is therefore greatly influenced by changes in both ionic strength and the presence of cations able to compete with Fe(II) for binding sites.