Surface oxidation of pyrite as a function of pH

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(ll) and SO42- were released into solution; from XPS analyses, the pyrite surface presented O-H groups, an Fe-deficient composition Fe1-xS2, and iron(lll) (hydr)oxide traces. Whatever the pH, the sulfur of the FeS2 surface was mainly under the (-1) state oxidation. When the pH increased, Fe(ll) disappeared and the surface was covered with iron(lll) (hydr)oxides. This overlayer did not passivate the sample against further oxidation, and a decrease in pH was still observed.