Arsenopyrite, FeAsS (a derivative of the marcasite structure), is the most common arsenic-bearing mineral. Under oxidizing conditions, either occurring naturally or as a result of mining processes, the mineral produces arsenite (AsO33-), arsenate (AsO43-), and sulfate (SO42-), [1–3] thus contributing to the acidification of water as well as the release of soluble arsenic species. Despite the potential environmental and health hazards posed by the oxidative dissolution of arsenopyrite, the mineral has received far less attention in the laboratory than pyrite (FeS2), the most studied of the sulfide minerals.
In acidic environments, the rate of sulfide mineral dissolution is typically limited by the supply of ferric iron, Fe3+; however, in the presence of iron-oxidizing microorganisms, the supply of ferric iron is continuously replenished by microbial oxidation of the ferrous iron released from sulfide minerals. Despite the importance of oxidation by ferric iron in natural systems, many of the fundamental details of the oxidation of arsenopyrite by Fe3+ under acidic conditions still remain unclear. One critical issue is the stoichiometry of the reaction with respect to the sulfur species. The literature is divided about whether the majority of the sulfur from the mineral is released into solution as sulfoxy anions[5, 6] [in a scheme similar to eqn. (1)], or whether a substantial amount of the sulfur remains as insoluble elemental sulfur (S8) at the mineral surface[7, 8] [as shown in eqn. (2)]:
FeAsS + 11Fe(III) → 12Fe(II) + As(III) + S(VI) (1)
FeAsS + 5Fe(III) → As(III) + 6Fe(II) + S(0) (2)
Previous efforts to characterize the arsenopyrite surface by scanning electron microscopy (SEM) after leaching reactions with ferric iron found no evidence of elemental sulfur on the mineral surface.[5, 7] Other studies, using X-ray photoelectron spectroscopy (XPS), did not detect elemental sulfur on arsenopyrite surfaces that were exposed to mine wastewaters.
In contrast, Raman spectroscopic and chromatographic investigations of arsenopyrite samples oxidized in ferric iron solutions identified elemental sulfur as the major reaction product. Additionally, numerous studies of the oxidation of arsenopyrite in the presence of iron-oxidizing microorganisms have revealed the presence of significant quantities of elemental sulfur at the mineral surface. [10–14] Clearly, the distribution and abundance of sulfur oxidation products is still a matter of much debate.
One possible explanation for the diversity of findings regarding elemental sulfur on arsenopyrite is the chemical modification of the mineral surface during analysis. Both SEM and XPS are vacuum-based techniques which employ electrons and X-rays, respectively, capable of volatilizing high vapor pressure surface products such as elemental sulfur. Raman spectroscopy, which uses lower-energy excitation in the visible region of the spectrum and can be performed at ambient pressures, is well suited to investigations of elemental sulfur on sulfide mineral surfaces. The difficulties, however, of producing a calibration standard with a uniform dispersion of elemental sulfur on the mineral surface limit the application of this method to semi-quantitative studies.
A quantitative determination of the distribution of sulfur reaction products is necessary for mechanistic investigations. Fernandez, et al. measured elemental sulfur on electrochemically oxidized arsenopyrite samples by extracting the elemental sulfur in carbon disulfide and weighing the evaporative residues. Although the extraction method allowed them to quantitatively collect the elemental sulfur from the mineral surface, the lack of selectivity and the low sensitivity of the mass measurement severely limited the accuracy of the elemental sulfur determination.
The recent development of a new technique in our laboratory, involving the extraction of the elemental sulfur from the mineral surface with an organic solvent and subsequent analysis by HPLC, allowed us to make the first quantitative measurements of the abundance of elemental sulfur on sulfide mineral surfaces after reaction under oxidative conditions. We report here the determination of elemental sulfur at the arsenopyrite surface after reaction with acidic, ferric iron solutions. These quantitative measurements place restrictions on any proposed mechanism of arsenopyrite oxidation and rule out the possibility that elemental sulfur is solely a product of thiosulfate decomposition in acid.