Surface and interface analysis of iron sulfides in aqueous media using X-ray photoelectron spectroscopy of fast-frozen dispersions

Abstract Iron sulfides are important minerals and materials, whose surface properties are largely determined by the interface with aqueous phases. Here, natural pyrrhotite (Fe9S10) and pyrite (FeS2) were ground, reacted in water and acidic non-oxidative and oxidative ferric chloride and sulfate solutions, then centrifuged, fast-frozen and examined with X-ray photoelectron spectroscopy at –160 oC (cryo-XPS) in comparison with the samples thawed in vacuum, dried in vacuum or air and measured at room temperature (RT). The interfacial layers were composed mainly of ferric (hydr)oxides and aqueous iron salts as the reaction products rather than reactants; RT-XPS found the products modified due to dehydration, further reactions and redistribution over the surface. The cryo-XPS revealed plentiful surface Fe-S centers with various spin and oxidation states at pyrrhotite and pyrite, particularly in water, missed in conventional XPS. The surfaces of pyrrhotite and, much less, pyrite are enriched with polysulfide anions -S-Sn-S-, which partially volatilize at RT both under non-oxidative and oxidative conditions; the number n can be larger than 6 at pyrrhotite. It was concluded, nevertheless, that elemental sulfur insignificantly contributes to the spectra under the conditions employed. Pros and cons of the technique in the case study of iron sulfides are discussed.