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

Abstract

The coupling of X-ray photoelectron spectroscopy (XPS) and nuclear microprobe analysis (NMA) using resonant reactions 16 O( α, α) 16 O and 12 C( p, p) 12 C at 3.05 and 1.725 MeV, respectively, is particularly adapted to the characterization of thin oxidation layers onto pyrites. XPS permits to determine both oxidation state and chemical environment of S and Fe. NMA gives an information about spatial distribution and chemical composition heterogeneity of oxidation products. Pyrites oxidized in acidic medium produce few solid components. Only Fe II sulfate is detected on the oxidized pyrite surface. In carbonate medium, oxidation layer is more complex. Iron is mainly with a (+II) oxidation state under siderite or Fe II sulfate form. As illustrated by the comparison of Fe3p and Fe2p 3/2 peaks, iron has an (+III) oxidation state to a minor extent under α-FeOOH and Fe III sulfate forms from the first oxidized pyrite layers. Sulfur oxidation induces intermediate species (polysulfides and sulfoxyanions as S 2O 3 2− also evidenced in solution) indicating that oxidation occurs at solid state. NMA has shown that oxidation occurs only on localized points of pyrite surface, with oxidation layers showing spatial distribution and thickness heterogeneities.