Nature and distribution of chemical species on oxidized pyrite surface: Complementarity of XPS and nuclear microprobe analysis

The coupling of X-ray photoelectron spectroscopy XPS) and nuclear microprob e analysis (NMA) using resonant reactions O-16(alpha,alpha)O-16 and C-12(p, p)C-12 at 3.05 and 1.725 MeV, respectively, is particularly adapted to the characterization of thin oxidation layers onto pyrites. XPS pen-nits to det ermine both oxidation state and chemical environment of S and Fe. NMA gives an information about spatial distribution and chemical composition heterog eneity of oxidation products. Pyrites oxidized in acidic medium produce few solid components. Only Fe-II sulfate is detected on the oxidized pyrite su rface. In carbonate medium, oxidation layer is more complex, Iron is mainly with a (+II) oxidation state under siderite or Fe-II, sulfate form. As ill ustrated by the comparison of Fe3p and Fe2p(3/2) peaks, iron has an (+III) oxidation state to a minor extent under alpha -FeOOH and Fe-III sulfate for ms from the first oxidized pyrite layers. Sulfur oxidation induces intermed iate species (polysulfides and sulfoxyanions as S2O32- also evidenced in so lution) indicating that oxidation occurs at solid state. NMA has shown that oxidation occurs only on localized points of pyrite surface, with oxidatio n layers showing spatial distribution and thickness heterogeneities. (C) 20 01 Published by Elsevier Science B.V.