Pyrite dissolution

Pyrite Oxidation. The oxidation of Fe(ll) minerals by Fe3+ is also of importance in the oxidation of pyrite by 02. This process is mediated by the Fe(II)-Fe(III)system. Pyrite is oxidized by Fe3+ (which forms a surface complex with the pyrite (cf. formula VI in Fig. 9.1) (Luther, 1990). The rate determining step at the relatively low pH values encountered under conditions of pyrite dissolution is the oxygenation of Fe(II) to Fe(III) usually catalyzed by autotrophic bacteria (Singer and Stumm, 1970 Stumm-Zollinger, 1972). Thus, the overall rate of pyrite dissolution is insensitive to the mineral surface area concentration. Microbially catalyzed oxidation of Fe(II) to Fe(III) by oxygen could also be of some significance for oxidative silicate dissolution in certain acid environments. 

Secondary U and REE minerals include autunite, Ce-phosphate, and Ld-Nd phosphates. The geochemical behaviour can be explained through pyrite oxidation that increases acidicity and releases sulphate and Fe(III), that would allow oxidative dissolution of the U ore, possibly precipitating uranopilite. When the pH increased at sites more distant from pyrite dissolution, U(VI) was hydrolysed and eventually co-precipitated with Fe3+-oxyhydroxides. 

The oxidation reactions are dependent on the microbial reactions with the end result of accelerating the transformation of FeS2 to ferrous sulfate, and thus equation (1) represents the overall reaction stoichiometry. Other reactions provide possible mechanistic pathways for the microbial pyritic dissolution. 

Model the oxidation of pyrite with different oxygen supphes (0.0, 0.001, 0.005, 0.01, 0.05, 0.1, 0.3, 0.6, 1.0 mol), and show the changes graphically for the two major elements building up from pyrite dissolution as well as for the pH value of the ground water given in Table 30. 

Figure 3. Diagram of a section through the cell wall of Acidithiobacillus ferrooxidans modified from Blake et al. (1992) showing the relationship between iron oxidation and pyrite dissolution. OM =outer membrane, P = periplasm, IM = inner or (cytoplasmic) membrane, cty = cytochrome, pmf = proton motive force. Passage of a proton (driven by proton motive force) into the cell catalyzes the conversion of ADP to ATP. Ferrous iron binds to a component of the electron transport chain, probably a cytochrome c, and is oxidized. The electrons are passed to a terminal reductase where they are combined with O2 and to form water, preventing acidification of the cytoplasm. Ferric iron can either oxidize pyrite (e.g. within the ore body) or form nanocrystalline iron oxyhydroxide minerals (often in surrounding groundwater or streams).

The fixed fugacity path (Fig. 12.4) differs from the previous calculation (in which the fluid was closed to the addition of oxygen) in that pyrite dissolution continues indefinitely, since there is an unlimited supply of oxygen gas. Initially, the reaction proceeds as... 

Lalvani SB, Deneve BA, Weston A (1991) Prevention of pyrite dissolution in acid media. Corrosion 47 55-61... 

Fig. 6. Scanning Electron photomicrographs illustrating key mineral textures in core samples. A, Plagioclase surface showing dissolution textures (DH-3/-484m) B, plagioclase surface coated by kaolinite (DH-3/-484m) C and D, biotite and iron oxyhydroxide mineral (DH-3/-484m) E, pyrite dissolution texture (DH-4/80m) ...

The water sampled from the fracture zone at DH-4/80m was calculated to be either supersaturated or under-saturated with respect to pyrite, depending upon the redox potential Eh) used in the calculations. If the water is undersaturated with respect to pyrite, then this mineral will dissolve. The etch pits seen on surfaces of pyrite crystals indicate that such dissolution could be occurring. However, the amount of SO in the groundwater is insufficient to balance the amount of dissolved Fe " present. Therefore, pyrite dissolution cannot contribute all the Fe + dissolved in the water. 

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