Role of Carbonate Speciation on the Oxidation Rate of Fe(II) in Aquatic Systems

Abstract

A unified model for Fe(II) speciation and oxidation by molecular oxygen is described. This model combines Pitzer's concept of specific interaction with classic ion pair formation theory to describe ferrous iron speciation under conditions typical of natural waters. Using this model, it was determined that ferrous carbonate complexes [Fe(CO3), Fe(CO3)2 2-, and Fe(CO3)(OH)-] dominate the speciation of Fe(II) in natural waters containing greater than 1 mM carbonate alkalinity. The speciation data were then utilized to evaluate the species-specific rates of Fe(II) oxidation by molecular oxygen for a range of media compositions and ionic strength. At pH values below 6.0, the oxidation rate of Fe(II) is well described in terms of the Fe2+ and FeOH+ species. However, for pH values above 6, the Fe(CO3)2 2- complex is the most kinetically active species. The combined speciation/oxidation model for Fe(II) was successful at predicting observed oxidation rates in lake water, well-defined salt solutions, and seawater.

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

“…Typically half-lives for the oxidation were around 90 min, consistent with extrapolation of the laboratory data of Millero and Sotolongo (1989) and Millero et al (1987) The lowest temperatures at which laboratory Fe(II) oxidation experiments have been reported in the literature is at 5 8C (Kuma et al, 1992(Kuma et al, , 1995Millero and Sotolongo, 1989;Millero et al, 1987) with most experimental data having been obtained at 25 8C. Similarly King and coworkers have shown the importance of carbonate speciation on the oxidation of Fe(II) by O 2 (King, 1998) and by H 2 O 2 (King and Farlow, 2000). In the present case, we could find no data for the speciation of Fe(II) carbonate complexes at low temperatures and thus more laboratory work is required to extend these key parameters to temperatures typically encountered in Polar oceans and in the deep sea.…”

“…The equilibrium inorganic speciation of Fe II in ambient seawater at 25 8C is strongly influenced by the ferrous carbonate complex, Fe(CO 3 ) (King, 1998). Organic complexation of Fe II in seawater is suspected (Croot et al, 2001), based on slower Fe II oxidation rates, but as yet no definitive evidence has been presented.…”

“…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).…”

Adsorption and surface oxidation of Fe(II) on metal (hydr)oxides

“…An important reason is the experimental challenge that is related to the work under reduced conditions, i.e. the oxidation rate of Fe(II) is very high in the experimental pH range of adsorption (Tamura et al, 1976;Emmenegger et al, 1998;King, 1998). Traces of oxygen will immediately lead to transformation of Fe(II) into Fe(III).…”

Evaluation of the Stability of Iron(II) Solutions by Precise Coulometric Titration with Electrogenerated Cerium(IV)

“…4 Many studies have considered the mechanism [5][6][7][8][9][10][11][12][13][14][15] involving the oxidation of iron(II) in nonaqueous solvents, 16 as well as seawater and other complicated systems. [17][18][19][20][21][22][23][24] However, such fundamental information of the stability of iron(II) is not enough, because a skillful analyst could obtain a relative standard deviation (RSD) of less than 0.1% as the repeatability of volumetric analyses.…”