Pyrite oxidation: a state-of-the-art assessment of the reaction mechanism

Abstract

The oxidation of pyrite to release ferrous iron and sulfate ions to solution involves the transfer of seven electrons from each sulfur atom in the mineral to an aqueous oxidant. Because only one or, at most, two electrons can be transferred at a time, the overall oxidation process is quite complex. Furthermore, pyrite is a semiconductor, so the electrons are transferred from sulfur atoms at an anodic site, where oxygen atoms from water molecules attach to the sulfur atoms to form sulfoxy species, through the crystal to cathodic Fe(II) sites, where they are acquired by the oxidant species. The reaction at the cathodic sites is the rate-determining step for the overall process. This paper maps out the most important steps in this overall process.

Introduction

The oxidation of pyrite, the most abundant of all metal sulfide minerals, is the dominant process giving rise to the acidification of natural waters. Whether the source of the pyrite is shale or other rock with substantial accessory iron sulfide mineralogy, or dumps of waste material from a mining operation, the weathering of this pyrite can result in the acidification of large tracts of stream, river, and lake systems and the destruction of living organisms. Where anthropogenic influences have been involved, this is termed acid mine drainage (AMD), whereas the more general case is termed acid rock drainage (ARD). There is now a very substantial literature dealing with all aspects of AMD and ARD. Reviews and more general articles on this subject include those by Alpers and Blowes 1994, Banks et al 1997, Evangelou 1995, Evangelou and Zhang 1995, Gray 1996, Jambor and Blowes 1994, Jambor and Blowes 1998, Keith and Vaughan 2000, Nordstrom and Alpers 1999, and Salomons (1995).

In spite of many decades of research, the key controls of mechanisms and hence rates of the oxidation of pyrite remain poorly understood. This is largely because the processes of aqueous oxidation, which are relevant here, involve a complex series of elementary reactions. Basolo and Pearson (1967) pointed out that elementary steps of redox reactions almost always involve the transfer of only one electron at a time so that the oxidation of monosulfide minerals (e.g., sphalerite, galena) to release sulfate must require as many as eight elementary steps, and the oxidation of disulfides (e.g., pyrite and marcasite) must require up to seven elementary steps, depending on how elementary steps are defined. This process is further complicated by the fact that the minerals are semiconductors and the reactions are electrochemical in nature. This means that electrons can move from one part of the mineral to another so that the various reactions happen at different sites. Furthermore, the semiconducting properties of sulfide minerals such as pyrite are in turn critically dependant upon the precise composition of the particular pyrite sample or even the zone or region of a particular sample. Thus, as further discussed below, subtle differences in stoichiometry influence electrical properties and may in turn significantly affect reactivity. This is certainly the case for electrochemical oxidation processes; however, the situation regarding chemical oxidation is less clear. As discussed below, there are differences in rates of oxidation for pyrite samples from different sources, although grain size (and hence surface area) differences may exert a greater control. Further studies are needed to resolve these issues.

The question that we wish to address in this article is how we can dissect the process of aqueous oxidation to reveal each of the elementary reaction steps and hence determine the key controls of reaction mechanisms and rates. Drawing upon our and our collaborators’ research and upon other published material, we present our ideas in answer to this question in ways that have not previously been laid out in the literature. The emphasis here is on pyrite (FeS₂) oxidation, with some brief discussion of the oxidation of pyrrhotite (Fe_1-xS), as it is the iron sulfides that dominate natural systems. However, the principles developed below should apply to most, if not all, sulfide minerals.