Pyrite, the most abundant metal sulfide at the surface of Earth, plays a key role in many processes such as acid mine drainage, redox cycling of metals at oxic-anoxic boundaries of lake bottom, and degradation of pollutants. The oxidation of pyrite was studied in batch experiments over a large range of pH (2.5-12), with trace oxygen. Surface analysis of the samples was performed using X-ray photoelectron spectroscopy (XPS). Speciation of the aqueous species was investigated by inductively coupled plasma atomic emission spectrometry (ICP-AES), ionic chromatography, and UV-vis spectrophotometry. The pyrite surface can drastically change with the pH, which was never at steady state and tended to reach an acidic value whatever the initial pH. For pH <4, Fe(II) and SO 4 2were released into solution; from XPS analyses, the pyrite surface presented O-H groups, an Fe-deficient composition Fe 1-x S 2 , and iron(III) (hydr)oxide traces. Whatever the pH, the sulfur of the FeS 2 surface was mainly under the (-I) state oxidation. When the pH increased, Fe(II) disappeared and the surface was covered with iron(III) (hydr)oxides. This overlayer did not passivate the sample against further oxidation, and a decrease in pH was still observed.

Sorption of mercury(II) onto well-characterized samples of pyrite was studied between pH 2 and 12 using X-ray photoelectron spectroscopy for surface analysis and extended X-ray absorption fine structure for surface speciation. In the presence of Hg, the surface oxidation of pyrite was strongly decreased. Even if the sorption capacity of pyrite for Hg was high, the sorption reversibility was possible by adding some strong ligands, such as I-, S2O3 2-, and CN-, to the aqueous phase. Spectroscopic studies showed the absence of Hg(0) and S(−II) and evidenced the formation of a surface complex between S(−I) and Hg(II). At low pH, ternary surface complexes ⋮S-I−Hg−OH or ⋮S-I−Hg−Cl were formed with the following distances:  R ⋮ S - Hg = 2.40 Å, R Hg - OH = 2.25 Å, and R Hg - Cl = 2.33 Å if Cl- was present. At high pH, the spectroscopic signals of Hg, S, or Fe decreased, possibly because of the presence at the surface of a solid solution constituted of Fe (hydr)oxides and surface complexes between Hg and both oxides and pyritic sulfur.