Interaction of selenite with iron sulphide minerals: a new perspective

XAS-based studies of the interaction of selenite with FeS2 and FeS at circum-neutral pH have shown that selenite is reduced to solid state zero-valent Se and FeSex, respectively [1-4]. While these results were initially unexpected, the occurrence of dissolved, low oxidation state selenium species in presence of insoluble elemental Se, resulting from selenite reduction in pyrite containing clay systems, was astonishing. In addition, several authors encountered an identical Se phase solid phase in presence of pyrite, but none of them has been able to relate this phase to a specific mineral.
Correlating selenium redox chemistry with sulphide mineral oxidation pathways allowed to relate these observations to the different oxidation behaviour observed between acid-soluble and acid-insoluble metal sulphides [5].
Acid insoluble metal sulphides such as pyrite, molybdenite or tungstenite exhibit oxidative dissolution only. Upon six consequent one-electron oxidation steps, a thiosulphate anion is liberated (thiosulphate pathway). Acid soluble metal sulphides (troilite, mackinawite, sphalerite, etc.) can exhibit both non-oxidative dissolution, hence liberating sulphide species (H2S, HS-,S2-), and oxidative dissolution in presence of FeIII with the release of sulphide cations (e.g. H2S+), that spontaneously dimerize into disulphide species which can further react to form polysulphide (polysulphide pathway) and finally elemental sulphur.
While the end products resulting from Se(IV) reduction by acid-soluble iron sulphur minerals are fairly well known, both the solid and liquid phase products from the interaction of SeO32- with pyrite are poorly characterized. Although the solid phase reaction product in could not yet be assigned to a specific phase, it was clearly identified as a Se0 compound and trigonal (grey) selenium could be excluded as a canditate species.[4]
The presence of an unexpectedly high concentration of reduced, dissolved species in presence of pyrite, induced the formulation of a new pyrite-based reduction mechanism. Based on this mechanism, hypothesis was put forward about the identity of the unknown dissolved species. In addition, the new mechanism allows explaining all current experimental observations, more specifically the presence of this currently non-identified dissolved species and the unexpected relation between Se(IV) reduction and pH.[6]

[1] Breynaert, E., et al. (2008) ES&T. 42(10): 3595-3601.
[2] Scheinost, A.C. and Charlet, L. (2008) ES&T. 42(6): 1984-1989.
[3] Scheinost, A.C., et al. (2008) J. Contam. Hydrol. 102(3-4): 228-245.
[4] Breynaert, E., et al. (2010) ES&T. 44(17): 6649-6655.
[5] Rohwerder, T. and Sand, W.(2007) in Microbial Processing of Metal Sulfides 35-58.
[6] Kang, M., et al. (2011) ES&T. 45: 2704-2710.