Iron Geochemistry

In oxidized surface waters and sediments, dissolved iron is mobile below about pH 3 to 4 as Fe and Fe(lII) inorganic complexes. Fe(III) is also mobile in many soils, and in surface and ground-waters as ferric-organic (humic-fulvic) complexes up to about pH 5 to 6 and as colloidal ferric oxyhydroxides between about pH 3 to 8. Under reducing conditions iron is soluble and mobile as Fe(II) below about pH 7 to 8, when it occurs, usually as uncomplexed Fe ion. However, where sulfur is present and conditions are sufficiently anaerobic to cause sulfate reduction, Fe(H) precipitates almost quantitatively as sulfides. Discussion and explanation of these observations is given below. Thermodynamic data for iron aqueous species and solids at 25°C considered in this chapter are given in Table A12.1. Stability constants and A//° values computed from these data are considered more reliable than their values in the MINTEQA2 data base for the same species and solids. 

Cumulative formation constants of some iron aquocomplexes are listed in Tables 12.1 and 12.2. Examination of the tabulated data shows that Fe generally forms weak complexes or ion pairs (except 

TABLE 12.1 Cumulative formation constants for ferrous and ferric hydroxyl complexes written in the proton form (e.g., nFe + mH20 = Fe (OH) - + mH+) at 25°C  

Source Yatsimirskii and Vasilev (1966). See discussion in Chap. 3. Baes and Mesmer (1976). Macalady etal. (1990). 

In our discussion of aqueous species of iron, it is appropriate to first consider the Fe(II) and Fe(III) hydroxyl complexes (Table 12.1). To judge their importance, we will construct plots to show the fractional distribution of these complexes as a function of pH. The computational approach is as presented in Chap. 3. Solute activity coefficients are ignored. 

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Leaching and desorption of As from its associated mineral surfaces such as iron, aluminum and manganese oxides under the influence of the aquifer complex geochemistry, largely take part in its transport from sediment to aquifer pore-water. Adsorption has widely been considered as the retardation of As transport (Smedley 2003). 

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