Comment on ‘‘Pyrite dissolution in acidic media’’ by M. Descostes,

In a recent paper, Descostes et al. (2004) present data on a series of pyrite oxidation batch experiments conducted near pH 2 and 3 where they demonstrate very well the non-stoichiometric dissolution/oxidation of pyrite in acidic solutions through an extensive laboratory investigation of S/Fe ratios. The authors utilized this data to propose a ‘mechanism’ of pyrite oxidation based on these data. They conclude that measured sulfur deficiencies are due to a set of reactions involving thiosulfate ðS2O3 Þ and tetrathionate ðS4O6 Þ as the primary argument for presenting a specific pathway controlling pyrite oxidation. Using data taken from current literature, we challenge these assumptions regarding thiosulfate and tetrathionate reactivity and redefine these results in the context of a more complex, and more consistent, representation of pyrite oxidation pathways. To avoid confusion concerning this discussion, we wish to clarify that while several points about the mechanism(s) (i.e., an elementary reaction described in its simplest, smallest increment) of specific steps have been presented, most of the points within this comment concern a description of the pathway(s) (i.e., an overall reaction that represents a number of steps) describing pyrite oxidation. Discrete definition of these terms may seem semantic; however, their proper use eliminates any potential misunderstanding of important intermediate steps which are necessary to ultimately understand the kinetics of the overall reaction under varying conditions. Herein, we will use the term pathway unless confident that the described reaction truly is an elementary reaction.