There is no doubt that acid mine drainage (AMD) is one of the serious environmental problems in the world. Typically, AMD contains high concentrations of sulphate, iron, and many other metallic ions at low pH (Prasad and Henry, 2009). Its formation involves a series of complex chemical, biological, and electrochemical reactions (Zdun, 2001). It is, therefore, important that there is a proper understanding of the formation of AMD by evaluating the biogeochemical interactions and the sequences in these processes (Dold, 2014). In addition, a proper understanding of the chemistry is required if the means of controlling the environmental hazard or enhancing the reaction for commercial exploitation are to be devised (Lowson, 1982). Indeed, understanding the chemistry behind the AMD formation will help create more cost-effective prevention and remediation solutions. Therefore, this chapter discusses the chemistry of AMD formation in detail. First, the chapter focuses on the chemistry of AMD formation with respect to pyrite. Pyrite is chosen amongst many other sulphide minerals because the process of AMD production is made more clear by considering the reactions during the oxidation of pyrite (Simate and Ndlovu, 2014), and that pyrite oxidation is the main process responsible for the generation of AMD (Esparia, 2008). Second, the chapter will briefly discuss the oxidative dissolution of a selected number of other sulphide minerals.
Oxidation of Sulphide Minerals
The mining and excavation operations have already been discussed in Chapter 1. These operations allow the introduction of oxygen onto the mineral surface and thus initiating the oxidation of minerals which are normally in a reduced state (Banks et al., 1997). The most common family of such minerals is the sulphides. In principle, the oxidation of sulphide minerals occurs when the mineral surface is exposed to an oxidant and water, either in oxygenated or anoxic systems, depending on the oxidant (Blowes et al., 2003). In addition, microorganisms also indirectly play a role in the oxidation of sulphide minerals (Rossi, 1990; Acevedo et al., 2004; Brierley and Brierley, 2001; Nestor et al., 2001; Kodali et al., 2004; Adams et al., 2005; Ndlovu, 2008). Therefore, as a result of various interacting factors, the AMD generation process may be considered as being a complex process involving chemical, biological, and electrochemical reactions that are dependent on the conditions of the environment (Zdun, 2001).