Kinetic Model for Fe(II) Oxidation in Seawater in the Absence and Presence of Natural Organic Matter

Abstract

A detailed kinetic model has been developed to describe the oxidation of Fe(II) in seawater in both the absence and the presence of natural organic material. Experimental data were collected using a luminol chemiluminescence-based method to measure Fe(II), assuming that both the inorganic and the organically complexed species were detected. In the absence of organic matter, the data were modeled based on the Haber-Weiss mechanism with the inclusion of a back-reaction of Fe(III) with superoxide and precipitation of Fe(OH)3. Both reactions were found to be significant using sensitivity analysis. When organic matter is present, the model was extended by organic complexation of Fe(II) and Fe(III) with the creation of a parallel oxidation pathway for Fe(II). Fe(II) oxidation at natural (nanomolar) concentrations was accurately predicted for a range of organic concentrations. The model also accounted for scavenging of superoxide by sub-nanomolar levels of dissolved copper and by organic matter when present. The presence of a relatively strong Fe(III) binding ligand was observed to significantly increase the rate of Fe(II) oxidation, while ultimately retaining most of the iron in the system in dissolved (organically complexed) form. The complexation reactions and reaction of inorganic and organically bound Fe(II) with oxygen were found to be critical reactions in the system, while Fe(III) hydrolysis became unimportant even at low organic concentrations. The superoxide radical was also observed to have a major role in the cycling of iron due to its ability to act as both an oxidant and a reductant. The model indicates that the rate constant for the reaction of Fe(II) with O2 has generally been underestimated in previous work and that the secondary oxidation of Fe(II) by H2O2 and subsequently OH* plays a relatively minor role in these systems.

Spatial and temporal distribution of Fe(II) and H2O2 during EisenEx, an open ocean mescoscale iron enrichment

“…The mechanism proposed for the oxidation of Fe II in seawater by King et al (1995) has been widely accepted for studies of iron speciation and redox cycling in natural systems (Emmenegger et al, 1998;King, 1998;King and Farlow, 2000;Rose and Waite, 2002;Voelker et al, 1997).…”

Role of iron species in the photo-transformation of phenol in artificial and natural seawater

“…Fe(II) in seawater is usually present as complexed species, such as FeCO 3 or FeCl + and, under oxic conditions at the pH of seawater, Fe(II) species are rapidly oxidized to Fe(III) by O 2 and H 2 O 2 , as Fe(III) is the thermodynamically stable form of iron in seawater, freshwater and most aqueous systems containing dissolved oxygen. The transformation of dissolved Fe(II) species to particulate Fe(III)oxyhydroxides is central in the cycling of iron in aquatic environments (Stumm and Sulzberger, 1992;Rose and Waite 2002). Particulate Fe(III) oxyhydroxides are formed by oxidation of Fe(II) at the oxic/anoxic boundary in coastal marine waters (Yao andMillero, 1995, Kuma et al 1998).…”

Simulated atmospheric processing of iron oxyhydroxide minerals at low pH: Roles of particle size and acid anion in iron dissolution

“…Iron is the fourth most abundant element on the Earth's crust and is mostly found in its thermodynamic stable oxidation state, Fe(III). Fe(III) is relatively insoluble in oxic pH 8 seawater, thus limiting the inorganic concentration to 0.1 nM; any inorganic iron above this concentration will form an iron oxide solid phase (4). Despite this limitation, ocean total iron concentrations range from 0.1-2 nM.…”