Abstract
The effect of divalent cations (Me: Ca2+ and Mg2+) on the ligand-exchange kinetics of Fe (III) -fulvic acid complex and desferrioxamine B (DFOB) was examined by spectrophotometrically measuring the rate of Fe (III) -DFOB complex formation at the various concentrations of Me found in natural water (0-50 mM) . A kinetic model for ligand-exchange incorporating the effects of Me was also developed. The model includes the following two pathways: (1) a disjunctive pathway where Fe (III) -fulvic acid complex initially dissociates followed by complexation of free Fe (III) by DFOB; and (2) an adjunctive pathway where DFOB directly associates with the Fe (III) -fulvic acid complex followed by dissociation of the ternary complex (DFOB-Fe (III) -fulvic acid) to form Fe (III) -DFOB. The ligand-exchange rates of both pathways were accelerated in the presence of Me. The enhanced rates were well explained by considering the ternary complex formation of Me and Fe (III) -fulvic acid complex.
