Oxidation of Nanomolar Levels of Fe(II) with Oxygen in Natural Waters J. MAGDALENA SANTANA-CASIANO, † MELCHOR GONZA Ä LEZ-DA Ä VILA, † AND FRANK J. MILLERO* Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami, Florida 33149 The oxidation of Fe(II) by molecular oxygen at nanomolar levels has been studied using a UV-Vis spectrophotometric system equipped with a long liquid waveguide capillary flow cell. The effect of pH (6.5-8.2), NaHCO 3 (0.1-9 mM), temperature (3-35 °C), and salinity (0-36) on the oxidation of Fe(II) are presented. The first-order oxidation rates at nanomolar Fe(II) are higher than the values at micromolar levels at a pH below 7.5 and lower than the values at a higher pH. A kinetic model has been developed to consider the mechanism of the Fe(II) oxidation and the speciation of Fe(II) in seawater, the interactions between the major ions, and the oxidation rates of the different Fe(II) species. The concentration of Fe(II) is largely controlled by oxidation with O 2 and O 2 •- but is also affected by hydrogen peroxide that may be both initially present and formed from the oxidation of Fe(II) by superoxide. The model has been applied to describe the effect of pH, concentration of NaHCO 3 , temperature, and salinity on the kinetics of Fe(II) oxidation. At a pH over 7.2, Fe(OH) 2 is the most important contributing species to the apparent oxidation rate. At high levels of CO 3 2- and pH, the Fe(CO 3 ) 2 2- species become important. At pH values below 7, the oxidation rate is controlled by Fe 2+ . Using the model, log k i values for the most kinetically active species (Fe 2+ , Fe(OH) + , Fe(OH) 2 , Fe(CO 3 ), and Fe(CO 3 ) 2 2- ) are given that are valid over a wide range of temperature, salinity, and pH in natural waters. Model results show that when H 2 O 2 concentrations approach the Fe(II) concentrations used in this study, the oxidation of Fe(II) with H 2 O 2 also needs to be considered. Introduction The concentration of total dissolved iron (Fe(II) and Fe(III)) that can be present in seawater is influenced by redox conditions of the marine environment (1). Over the years a number of authors have studied the rates of oxidation of Fe(II) in different aqueous solutions to elucidate the behavior of Fe(II) in natural waters (2-16). The rates of oxidation of Fe(II) with O2 have been expressed as an apparent oxidation rate, independent of the mechanism describing the process, given by The brackets denote the total molar concentration. When the reactions are studied with an excess of O2, the reaction becomes pseudo-first-order (8): where k′ ) kapp[O2]. The most accepted mechanism to describe the Fe(II) oxidation with O2 in natural waters is the Haber-Weiss mechanism: At micromolar Fe(II) solutions, a 4:1 stoichiometry of Fe(II) oxidation by oxygen (8, 17) and a 2:1 stoichiometry for the oxidation by hydrogen peroxide (18) have been measured. At nanomolar Fe(II) concentrations, the rates for reactions 3-6 need to be considered. The superoxide (O 2 •- ) and OH • intermediates produced in the Fe(II) oxidation can be effective as oxidants to other reduced compounds (Br - , Cl - , HCO3 - , and dissolved organic matter) (11, 19, 20). The newly generated radicals can perform the role of OH • intermediates in reaction 6, as the radical derivatives are likely to also be extremely reactive (10, 11). Moreover, the back-reaction of Fe(III) with O2 •- (10, 13), the scavenging of O2 •- by nanomolar concentrations of inorganic Cu(II) present in solution (13), and the hydrolysis of Fe(III) to form insoluble Fe(OH)3 have been included in the latest models (13, 21). Rose and Waite (13) studied the Fe(II) oxidation in seawater at nanomolar concentrations at a fixed pH (8.09) and salinity (42.7) considering the above-mentioned processes. The oxidation rate constants are a strong function of pH and the speciation of Fe(II) (15, 18). The overall rate constants for reactions 3-6 are a function of the composition and physical-chemical properties of the solution. To define the oxidation of Fe(II) in seawater, it is necessary to know the effect these properties have on the oxidation rates before one can account for the effect of other competing compo- nents, in particular organic material. In this paper, we examine the oxidation of Fe(II) in seawater at nanomolar levels as a function of pH, HCO3 - concentration, temperature, and salinity by defining a kinetic model that include speciation changes valid over a wide range of experimental conditions. Measurements are also carried out in pure water and NaCl solutions with different bicar- bonate concentrations in order to define the role played by various Fe(II) species present in the solutions. A model was used to describe the rates of the reactions in terms of the species of Fe(II) in the solutions (Fe 2+ , Fe(OH) + , Fe(OH)2, FeCO3, and Fe(CO3)2 2- ). Experimental Section Stock solutions of Fe(II) (2 × 10 -3 and 4 × 10 -4 M) were prepared using ferrous ammonium sulfate hexahydrate (Fisher), acidified at a pH 2 with Suprapur HCl. The initial concentrations of Fe(II) were kept at 250 nM in the reaction vessel in most of the studies. To validate the model results, * Corresponding author phone: (305)421-4707; e-mail: fmillero@ rsmas.miami.edu. † Present address: Universidad de Las Palmas de Gran Canaria, Departamento de Quı ´mica, Facultad de Ciencias del Mar, 35017 Las Palmas de Gran, Canaria, Spain. d[Fe(II)]/dt )-k app [Fe(II)][O 2 ] (1) d[Fe(II)]/dt )-k′[Fe(II)] (2) Fe(II) + O 2 f Fe(III) + O 2 •- (3) Fe(II) + O 2 •- 9 8 2H + Fe(III) + H 2 O 2 (4) Fe(II) + H 2 O 2 f Fe(III) + OH • + OH - (5) Fe(II) + OH • f Fe(III) + OH - (6) Environ. Sci. Technol. 2005, 39, 2073-2079 10.1021/es049748y CCC: $30.25  2005 American Chemical Society VOL. 39, NO. 7, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 2073 Published on Web 02/09/2005