Effect of dissolved natural organic matter on the kinetics of ferrous iron oxygenation in seawater

We have investigated the kinetics of Fe(II) oxygenation in seawater in the presence of a variety of natural organic materials obtained from vegetation near Moreton Bay, Queensland. Natural organic matter (NOM) was observed mostly to accelerate Fe(II) oxygenation, but in some cases oxidation was retarded. We fitted a previously developed kinetic model to the experimental data to determine the critical rate constants, kf for the formation of Fe(II)−NOM complexes and kox for the oxygenation of the Fe(II)−NOM complexes, when assumed to be first order with respect to both the concentration of Fe(II) and the dissolved O2. Analysis of the critical model reactions indicated that the process is in general non-pseudo-first-order but approaches pseudo-first-order under certain conditions. These limiting conditions are rapidly approached, which makes it difficult to determine unique values of kf and kox from oxidation data alone. Both parameters varied considerably between the different samples of NOM, with the value of kox ranging from about 2 up to 1000 M-1 s-1 as compared with 13 M-1 s-1 for inorganic Fe(II) in seawater. Despite large assumptions, the values of kox calculated were consistent with a linear free energy relation.