Kinetics of Hydrolysis and Precipitation of Ferric Iron in Seawater

We have investigated the kinetics of iron hydrolysis and precipitation and dissolution of the solid formed via a novel chemical method, namely accessibility of iron to the fungal siderophore desferrioxamine B (DFB), with spectrophotometric detection of the ferrioxamine complex. Our results support a mechanism in which precipitation of dissolved inorganic iron in seawater is first order with respect to total (dissolved and precipitated) iron, with a second-order rate constant of (4.1 +/- 1.1) x 10(7) M(-1) s(-1) at pH 8.1. The rate of dissolution was first order with respect to the total iron concentration and the first-order rate constant decreased from 2.3 x 10(-4) s(-1) after aging for 1 min to 4.8 x 10(-6) s(-1) after aging for 1 week. The proposed reaction mechanism simulated a solubility limit condition in agreement with experimental observations, from which we calculated the solubility of ferric iron to be 1.2 x 10(-13) M when the precipitate had been aged for 1 week. This is approximately 2 orders of magnitude less than reported in previous studies, possibly due to the chemically based method for dissolved iron determination used here compared with traditional physical separation methods. Our results confirm that Fe(III) hydrolysis in seawater is fast and show thatthe precipitated solid is quite labile initially but rapidly becomes much less so, with important implications for sequestration by organic ligands such as siderophores.