Eh-pH Relationships in the System Fe

An Eh-pH diagram for the Fe-02-C02-S-H20 system at 25°C is given in Fig. 12.19. Comparison withEh-pH diagrams for simpler systems in Figs. 12.10,12.11, and 12.16 shows notable differences. Most obvious is the stability field for pyrite, which lies roughly over fields for aqueous H2S and 

The lack of a stability field for siderite reflects the low carbonate and high sulfur concentrations chosen for the diagram, and the large sizes of both pyrite and goethite fields are consistent with the remarkable insolubilities of both minerals over a wide pH range. 

Environmentally, the most important boundary in Fig. 12.19 is the upper pyrite boundary with aqueous ferrous iron. The redox reaction is 

To plot this equation we must fix the ZS(aq) concentration and that of SFe(aq) at boundaries with Fe solids. We will assume IS(aq) = 10 mol/kg, and LFe(aq) = 10 mol/kg. With these substitutions the boundary equation is 

For comparison purposes, it is instructive to derive the similar boundary for freshly precipitated, metastable FeS(am). The reaction is 

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