Pyrite (FeS 2) oxidation: A sub-micron synchrotron investigation of the initial steps

Abstract

Pyrite is an environmentally significant mineral being the major contributor to acid rock drainage. Synchrotron based SPEM (scanning photoelectron microscopy) and micro-XPS (X-ray photoelectron spectroscopy) have been used to characterise fresh and oxidised pyrite (FeS 2) with a view to understanding the initial oxidation steps that take place during natural weathering processes. Localised regions of the pyrite surface containing Fe species of reduced coordination have been found to play a critical role. Such sites not only initiate the oxidation process but also facilitate the formation of highly reactive hydroxyl radical species, which then lead the S oxidation process. Four different S species are found to be present on fresh fractured pyrite surfaces: S 2 2− (bulk) (4-fold coordination), S 2 2− (surface) (3-fold coordination), S 2− and S 0/S n 2− (metal deficient sulfide and polysulfide respectively). These species were found to be heterogeneously distributed on the fractured pyrite surface. Both O 2 and H 2O gases are needed for effective oxidation of the pyrite surface. The process is initiated when O 2 dissociatively and H 2O molecularly adsorb onto the surface Fe sites where high dangling bond densities exist. H 2O may then dissociate to produce OH radicals. The adsorption of these species leads to the formation of Fe-oxy species prior to the formation of sulfoxy species. Evidence suggests that Fe–O bonds form prior to Fe–OH bonds. S oxidation occurs through interactions of OH radicals formed at the Fe sites, with formation of SO 4 2− occurring via S 2O 3 2−/SO 3 2− intermediates. The pyrite oxidation process is electrochemical in nature and was found to occur in patches, where site specific adsorption of O 2 and H 2O has occurred. Fe and S oxidation was found to occur within the same area of oxidation probably in atomic scale proximity. Furthermore, the O in SO 4 2− arises largely from H 2O; however, depending on the surface history, SO 4 2− formed early in the oxidation process may also contain O from O 2.