Abstract
The kinetics of the formation and dissociation of the Fe(III)-NOM (natural organic matter) complex in coastal waters was investigated using the competitive ligand method. The rate constants for the formation and dissociation of the Fe(III)-NOM complex were determined by fitting a model to observed data. The iron-binding capacity of NOM, which is required for the determination of the rate constants, was initially measured by the ion exchange method. The rate constants of the formation of the Fe(III)-NOM complex ranged from 2.6×106 to 7.3×106 M-1·s-1. A discrete ligand model for the dissociation kinetics of the Fe(III)-NOM complex yielded rate constants from 6.9×10-4 to 2.0×10-3 s-1 for weak-binding ligands and from 1.6×10-5 to 8.3×10-5 s-1 for strong-binding ligands, respectively. The kinetics of the Fe(III)-NOM complex in coastal water was then predicted using the estimated rate constants. The model prediction showed that the dissociation of the complexes is relatively slow with a half-life longer than 120 hr. The adsorption peak ratios of oxygen-containing functional groups of NOM calculated from FTIR spectra showed good correlation with both the iron-binding capacity and the rate constants, implying that the ratio is a useful factor for assessing the kinetics of organically complexed iron in coastal water.
