Kinetics of iron complexation by dissolved natural organic matter in coastal waters

In coastal waters, where conditions are variable and can change rapidly, the kinetics of iron complexation by NOM is a major factor affecting its speciation. In this study we have examined the kinetics of complexation of ferrous and ferric iron by terrigenous NOM in terms of formation rate constants and dissociation rate constants by employing competitive ligand methods with visible spectrophotometry for determination of the complexed iron. Rate constants for organic ferrous complex formation ranged from 500 to 7.5×10⁴ M⁻¹ s⁻¹, and rate constants for complex dissociation ranged from 1×10⁻⁶ to 3.6×10⁻³ s⁻¹. Formation rate constants for organic ferric complexes were comparable to those determined for strong iron binding ligands found in the open oceans, and from 2.1×10⁵ to 9.6×10⁷ M⁻¹ s⁻¹. Ferric complex dissociation data were fitted by a two ligand model, with rate constants for both ligand classes typically higher than those for the strong ligands in the open ocean. Rate constants varied from 2×10⁻⁴ to 4×10⁻³ s⁻¹ for the ‘weak’ ligand class, and from 1.0×10⁻⁶ to 1.3×10⁻⁴ s⁻¹ for the ‘strong’ class. All rate constants varied by several orders of magnitude between NOM samples of different origin, reflecting the highly variable composition of these substances. Calculated conditional stability constants for organic complexes with ferric species in seawater, Fe′, were generally similar to those measured in the open oceans. Conditional stability constants for the ferrous complexes were less than those for the ferric complexes by two to four orders of magnitude. There was a weak positive correlation between the conditional stability constants for ferrous and ferric complexes, suggesting that similar functional groups are involved in binding each of the two forms of iron. One of the samples of NOM had a conditional stability constant with Fe′ that was comparable with those for siderophores and similar strong iron binding compounds. These results suggest that in the absence of oxidants, complexes between ferrous iron and NOM may be relatively long-lived. Our work also suggests that organic complexes between iron and terrigenous NOM may be quite strong, and will have a major effect on iron solubility in coastal waters.