Synchrotron Spectroscopic Studies of the Reaction of Cleaved Pyrite ( {FeS2}) Surfaces with Cr(VI) Solutions

Pyrite is one of the most common sulfide ores, and the separation of valuable sulfide minerals from it has been an area of considerable interest for a long time. This extraction has led to a large quantity of pyrite waste, typically remaining in mine tailings piles which can interact with oxygen and surface water. The oxidation of pyrite under these conditions leads to the commonly known environmental problem of acid mine drainage, with acidification of surface waters, and the release of potentially toxic metals remaining within the pyrite matrix. A microscopic understanding of this oxidation process is extremely important and has been the aim of a number of studies. We apply the methods of synchrotron based surface science to this problem, utilizing surface sensitive photoemission and X-ray absorption spectroscopy to study the surface species present on the pyrite surface at the initial stages of oxidation. We have reacted pyrite surfaces with solutions containing chromate. Chromium exists in solution in two principal valence states, trivalent Cr(III) and hexavalent Cr(VI). Hexavalent chromium is itself considered an environmental problem due to its high toxicity and solubility, and thus mobility, whilst trivalent chromium is much less toxic and relatively insoluble. Hexavalent chromate is a strong oxidizing agent, and will react rapidly with the pyrite surface allowing the identification of oxidized iron and sulfur surface species. The possibility of using pyrite as a means of reducing chromate, and at the same time using chromate to passivate the pyrite surface to further oxidation through the buildup of a non-reactive iron-chromium (oxy)hydroxide layer will be investigated. The work was performed on rods cut from a natural pyrite single crystal from the Logroño region of Spain. The rods were then fractured over a reaction vessel, producing a fresh (100) surface for each experiment. The pyrite surfaces were reacted with 50 μM Cr(VI) solutions for 5 minutes at pHs of 2, 4, and 7. Additional studies were performed at pH 7 with 50 μM Cr(VI) for durations of 1 minute to 38 hours, and with 5 mM Cr(VI) for 30 seconds. Photoemission and X-ray Absorption Near Edge Structure (XANES) spectroscopies were performed under UHV conditions at beamline 10-1 at the Stanford Synchrotron Radiation Laboratory (SSRL). These studies show that chromate is an effective oxidizing agent for the pyrite surface and that oxidation products can be trapped on the pyrite surface under a reduced chromium layer. Sulfur 2p photoemission identified sulfate, sulfite and zero-valent sulfur species on the pyrite surface. The features due to sulfate and sulfite are broad and were fitted using a single broad envelope which may include contributions from disorder and varying degrees of hydration. Iron 2p photoemission and L-edge XANES showed that very minor amounts of iron (III) remain on the surface indicating that this species is rapidly lost into solution. The chromate was reductively sorbed onto the pyrite surface forming a (oxy)hydroxide layer which ultimately passivates the surface towards further chromium reduction or pyrite oxidation. We utilized synchrotron based PES and XAS to show that chromate is reductively sorbed at the pyrite surface, and that the pyrite oxidation products can be observed under a chromium (III) containing layer. These preliminary results indicate that reacting pyrite with Cr(VI) may play the dual role of reducing chromate and passivating pyrite surfaces towards oxidation.