Pyrite dissolution reactions

To further illustrate how the basis-swapping algorithm can be used to balance reactions, we consider several ways to represent the dissolution reaction of pyrite, FeS2. Using the program RXN, we retrieve the reaction for pyrite as written in the llnl database... 

Fig. 14.3. Variation in the concentrations of aqueous species involved in the dissolution reaction of pyrite, for the reaction path shown in Figure 14.2.

Under anoxic conditions in the subsurface, precipitation/coprecipitation, sorption, and dissolution reactions in hydrothermal fluids commonly involve realgar (AsS or AS4S4), arsenopyrite (FeAsS), arsenian pyrite (FeS2), and especially orpiment (AS2S3). Orpiment dissolves in reducing and low H2S hydrothermal waters at temperatures up to at least 300 °C as shown in the following reaction (Webster and Nordstrom, 2003, 110) ... 

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. 

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... 

Even neglecting the question of the precise steps that make up the overall reaction, our calculations are a considerable simplification of reality. The implicit assumption that iron in the fluid maintains redox equilibrium with the dissolved oxygen, as described in Chapter 7, is especially vulnerable. In reality, the ferrous iron added to solution by the dissolving pyrite must react with dissolved oxygen to produce ferric species, a process that may proceed slowly. To construct a more realistic model, we could treat the dissolution in two steps by disenabling the Fe++/Fe+++ redox couple. In the first step we would let pyrite dissolve, and in the second, let the ferrous species oxidize. 

Table 1 Important weathering reactions in order of ease of chemical weathering and solubility, which goes along with the reaction rate of the mineral dissolution, except for bacterial mediated pyrite oxidation [9, 10]...

Dissolution of pyrite can also be mediated by bacteria. Weathering reaction listed for silicates is elementary only... 

The results of various attempts to explain the provenance of the mature Whittle waters in terms of the above precursor waters and mineral phases are outlined below. Assuming that Whittle recharge water is the sole precursor, several attempts were made to model the evolution of the mature Whittle water by reaction of the Whittle recharge waters with various minerals. Where the mineral suite included calcite, dolomite, gypsum, pyrite and K-jarosite, but not ankerite, the resulting models were of poor credibility they invoke coupled precipitation of calcite (although dissolution is far more likely) and substantial dissolution of... 

Iron frequently has been postulated to be an important electron acceptor for oxidation of sulfide (58, 84,119, 142, 152). Experimental and theoretical studies have demonstrated that Fe(III) will oxidize pyrite (153-157). Reductive dissolution of iron oxides by sulfide also is well documented. Progressive depletion of iron oxides often is coincident with increases in iron sulfides in marine sediments (94, 158, 159). Low concentrations of sulfide even in zones of rapid sulfide formation were attributed to reactions with iron oxides (94). Pyzik and Sommer (160) and Rickard (161) studied the kinetics of goethite reduction by sulfide thiosulfate and elemental S were the oxidized S species identified. Recent investigations of reductive dissolution of hematite and lepidocrocite found polysulfides, thiosulfate, sulfite, and sulfate as end products (162, 163). 

In addition to a better understanding of the reaction of sulfide with ferric oxides and its role in pyrite formation, a more exact definition of the term reactive iron is critical. Does reactive iron mean a different iron oxide fraction for bacterial dissolution (e.g., weathering products such as goethite or hematite) than for reaction with sulfide (e.g., reoxidized lepidocrocite) In other words, is there a predigestion of ferric oxides by bacteria that allows a subsequent rapid interaction of sulfide with ferric oxides ... 

Reaction 6.16 results in the pH of the waters, in the absence of carbonate precipitation, being buffered at higher pH values (e.g., Ben-Yaakov, 1973). It is, therefore, reasonable to expect that the effectiveness of sulfate reduction in producing carbonate dissolution or precipitation may depend in part on the availability of reactive iron. This conclusion has been demonstrated in a study of the pore water geochemistry of aluminosilicate and carbonate-rich sediments from Kaneohe Bay, Hawaii. The aluminosilicate sediments contain abundant pyrite whereas the pyrite content of the carbonate sediments in this bay is low. Pore waters collected from the aluminosilicate sediments have higher pH values than those collected from the carbonate-rich sediments. This observation is a result of the pH values in the pore waters of the aluminosilicate sediments being buffered at... 

Digestion. The influence of ultrasound on the dissolution kinetics of phosphate rock in HNO3 solutions [11] and variables affecting it (viz. particle size, reaction temperature, acid concentration, amplitude of US power) were studied by Tekin [12]. The term dissolution in the presence of auxiliary energy and an acid seems inappropriate in this case as the process is more like a true digestion. Another case in point is the dissolution of pyrite ores in acid and Fe2(S04)3 solutions, which is improved by 30% with respect to the absence of US energy [13]. 

Reactions involving mackinawite and an oxidized sulfur species have been repeatedly shown to lead to pyrite formation (e.g., Bemer, 1969 Rickard, 1969, 1975). In addition, Wilkin and Bames (1996) and Penning et al. (2000) have shown that pyrite formation is exceptionally rapid when the mackinawite is pre-oxidized (e.g., exposed briefly to air) prior to the experiment. Based partly on X-ray photoelectron and Auger spectroscopy results of pyrrhotite oxidation (Mycroft et al., 1995), Wilkin and Bames (1996) hypothesized that this oxidative exposure initiates an iron-loss pathway similar to Equation (13). In sulfidic solutions, Fe(II) oxyhydroxides, shown as a product in this reaction, would not accumulate, but instead would undergo reductive dissolution by a reaction similar to Equation (14) ... 

The groundwaters are also predominantly oxidizing with low dissolved iron and manganese concentrations. There is no indication of reductive dissolution of iron oxides or of pyrite oxidation. Under the arid conditions, silicate and carbonate weathering reactions are pronounced and the groundwaters often have high pH values. Smedley et al. (2002) found pH values typically in the range of 7.0-8.7. 

Hence, each mole of sulfur produces 2H or, stated alternatively, each mole of pyrite produces four moles of H. Most nonsulfide minerals will react with acid to some extent, and if the effect of the mineral dissolution is to decrease the acidity of the original solution, then the mineral contributes NP, the amount of which is relative to the acidity consumption effected by calcite in the reaction... 

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