Selenium speciation in framboidal and euhedral pyrites in shales.

Area of Science:

• Geochemistry
• Environmental Science
• Mineralogy

Background:

• The release of selenium (Se) from shales is poorly understood due to unknown occurrence, distribution, and speciation.
• Understanding Se behavior is crucial for predicting its environmental fate and impact.

Purpose of the Study:

• Investigate the occurrence, distribution, and speciation of Se in low Se-content shales.
• Determine the relationship between Se and associated elements (Fe, As, Cr, Ni, Zn).
• Elucidate Se speciation at the micro-scale (μ-scale).

Main Methods:

• Bulk characterization of shale samples.
• Sequential extraction techniques to isolate mineral fractions.
• Spatially resolved μ-focus spectroscopic analyses for micro-scale speciation.

Main Results:

• Selenium primarily correlates with the pyrite fraction in shales.
• Se speciation is dependent on pyrite morphology: Se(-II) substitutes S in euhedral pyrites.
• A discrete iron selenide (FeSex) phase associated with framboidal pyrite was identified.

Conclusions:

• The identified FeSex phase significantly enhances and controls Se weathering and release dynamics.
• Faster oxidation of the FeSex phase compared to lattice-substituted Se accelerates Se mobilization.
• This finding is critical for understanding Se biogeochemical cycling in shale environments.