Oxidation chemistry of major arsenic-bearing sulfides

4 Oxidation chemistry of major arsenic-bearing sulfides 3.7.4.1 Oxidation of pyrite and other Fe(II) sulfides 

Pyrite is the most common sulfide mineral. It is a major contributor to the formation of mine drainage and sulfate-rich natural runoff. The oxidation of pyrite and other Fe(II) sulfides (e.g. marcasite and pyrrhotite) involves both iron and sulfur, as well as any arsenic impurities. Activation energies suggest that surface reactions dominate the oxidation of pyrite (Lengke and Tempel, 2005). Furthermore, evidence from pyrites in coal and ore deposits suggests that arsenian pyrite is more susceptible to oxidation from weathering than low-arsenic pyrite (Savage et al., 2000, 1239). 

The oxidation of pyrite by oxygenated water is often represented by simplified reactions, such as the following example from Krauskopf and Bird (1995, 148)  

In reality, the oxidation of pyrite and other Fe(II) sulfides typically involves several intermediate reactions, which may be enhanced by microbial activity or various chemical species, such as bicarbonate (HCO3-) (Welch et al., 2000 Evangelou, Seta and Holt, 1998). The exact mechanisms of each intermediate reaction are often very complex and poorly understood (Rimstidt and Vaughan, 2003). Mostly likely, sulfide oxidizes in pyrite before iron. Fe(II) is then released into solution as shown by the following reaction involving oxygen and water (Gleisner and Herbert, 2002, 139-140)  

In the presence of aerated water, dissolved Fe(II) readily oxidizes to Fe(III) (Gleisner and Herbert, 2002, 139-140)  

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