Oxidation of Ferrous Iron

There are two overall oxidation reactions for ferrous ion below pH 3.5 (cf. Stumm and Morgan 1981 Morel 1983 Morel and Hering 1993). Below pH = 2.2 

The empirical rate law for Fe oxidation under these acid conditions (rate law 1, Fig. 2.9) is 

Above pH = 4, the rate of Fe oxidation is related to the overall reaction 

Wiersma and Rimstidt (1984) measured the rate of the overall reaction 

When molecular oxygen is the oxidizing agent, oxidation of sulfides and sulfur compounds by thiobacteria, which are all aerobic, can speed up oxidation rates by many orders of magnitude over the inorganic rates (Karamenko 1969). 

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Ehrenreich A, F Widdel (1994) Anaerobic oxidation of ferrous iron by purple bacteria, a new type of photo-trophic metabolism. Appl Environ Microbiol 60 4517-4526. 

Iron incorporation into mammalian ferritins is thought to involve the following steps (Crichton, 2001) (1) Uptake of Fe2+ into the protein shell, most probably through the hydrophilic three fold channels. (2) Oxidation of ferrous iron by the dinuclear ferroxidase... 

Heising S, Schink B (1998) Phototrophic oxidation of ferrous iron by Rhodomicrobium vanniellii strain. Microbiology 144 2263-2269... 

Straub KL, Benz M, Schink B, Widdel F (1996) Anaerobic, nitrate-dependent microbial oxidation of ferrous iron. App Environ Microbio 62 1458-1460... 

Ahmad AR, Nye PH. 1990. Coupled diffusion and oxidation of ferrous iron in soils. I. Kinetics of oxygenation of ferrous iron in soil suspension. Journal of Soil Science 41 395-409. 

Davidson, W. Seed, G. (1983) The kinetics of the oxidation of ferrous iron in synthetic and natural waters. Geochim. Cosmochim. Acta 47 67-79... 

Misawa,T Hashimoto, K. Shimodaira, S. (1973a) Formation of Fe(ll)-Fe(lll) intermediate green complex on oxidation of ferrous iron in neutral and slightly alkaline sulphate solution. J. inorg. nucl. Chem. 35 4167-4174... 

Cook G.B. and Cooper R.F. (1990) Chemical diffusion and crystalline nucleation during oxidation of ferrous iron-bearing magnesium aluminosilicate glass. /. Non-Cryst. Solids 120, 207-222. 

Further oxidation of ferrous iron (Fe2+) to ferric (Fe3+) may take place, followed by hydrolysis and precipitation of Fe3+-oxyhydroxides ... 

Pore-water profiles are frequently interpreted according to this concept. For example, White et ah (35) described a conceptual model of biogeo-chemical processes of sediments in an acidic lake (cf. Figure 4). They discussed the numbered points in Figure 4 as follows Diffusion of dissolved oxygen across the sediment-water interface leads to oxidation of ferrous iron and to an enrichment of ferric oxide (point 1). Bacterial reductive dissolution of the ferric oxides in the deeper zones releases ferrous iron (point 2). The decrease in sulfate concentration stems from sulfate reduction, which produces H2S to react with ferrous iron to form mostly pyrite in the zone below the ferric oxide accumulation (point 3). 

In alkaline solution oxidation of ferrous iron is fairly rapid,8 but certain acids retard the reaction. Ferrous sulphate, for example, in the presence of free sulphuric acid, is very stable in air. Concentrated hydrochloric acid assists the oxidation, as also do traces of certain substances, such as platinic and cupric chlorides, palladium nitrate, etc.9... 

Figure 4. Eh-pH diagrams of the system CU-H2O-CO2 Cu2+ = 10 5 M. (a) PC02 = 0 atm (b) PC02 = 10 3 5 atm (equivalent to that of air) (c) Pco2 = 1 atm. Traces A and B illustrate experimental aeration of a geochemical environment strongly influenced by oxidation of ferrous iron in the absence (A) and in the presence (B) of excess CaCO (as limestone...

Vinogradov has pointed out that with the appearance of the biosphere somewhere on the verge of 3-10 yr ago, there was a major upheaval in the evolution of the Earth. Oxidizing processes were abruptly accelerated, a nitrogen atmosphere arose in which carbon dioxide predominated over methane, and free carbon was oxidized to CO2. After the carbon was oxidized or at the same time as that process, there began oxidation of divalent iron (at — 10 ), which led to subsequent wholesale deposition of the sediments of the Precambrian BIF. Free carbon in equilibrium with the atmosphere appeared only after complete oxidation of ferrous iron compounds in the hydrosphere and on the land surface. 

In the thermodynamic analysis it has already been mentioned that oxidation of the sulfide ion is possible at lower partial pressure of oxygen than in the case of oxidation of ferrous iron, and both these reactions are irreversible in the presence of even mere traces of free O2. Therefore the suggestion of Cloud and Licari (1968) and Cloud (1969), that primitive organisms could have used electron donor reactions of the type ... 

Volumetric Methods.—Chromium is most easily estimated in solutions containing chromate or diehromate by volumetric methods. These generally depend either on the oxidation of ferrous iron or on the liberation of iodine from potassium iodide. 

BenzM., Brune A., and Schink B. (1998) Anaerobic and aerobic oxidation of ferrous iron at neutral pH by chemoheterotrophic nitrate-reducing bacteria. Arch. Microbiol. 169, 159-165. 

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