Thermodynamic data sulfate species

The water analyses were coded and then processed with the computer program WATEQ2. This program was modified in several ways to handle acid mine waters (a) the Eh could be calculated from the Fe VFe activity ratio or vice versa, (b) several sulfate minerals were added,(c) metal sulfate and hydroxide complex constants were carefully evaluated and included, and (d) Mn, Cu, Zn and Cd species were added since they are major constituents for several of the water samples. These modifications and the evaluated thermodynamic data are described by Ball, Jenne and Nordstrom in this symposium (32). 

Table IE. Summary of Revised Thermodynamic Data. V Sulfate Species ...

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. 

The situation, when a sparingly soluble soUd is placed in water, is much more complicated in principle, because of the successive reactions taking place spontaneously in the aqueous phase. This fact is obvious for carbonates, sulfates, where the dissolved amoimt cannot be calculated directly from the characteristic thermodynamic data (i.e., from the solubility products) because additional reactions take place simultaneously in the liquid phase depending on its composition, which influence the activity of dissolved species and, therefore, the amoimt of dissolved material. For example, a common carbonate, calcite (CaC03), with intermediate solubdity dissolves as bicarbonate [Ca(HC03)2] at favorable conditions with respect to pH and partial pressure of CO2 or the dissolved amount of a moderately soluble salt, gypsiun... 

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