Hydrological Controls on Acid Mine Drainage
Acid Mine Drainage Generation
AMD is generated via the oxidation of sulphidic geological minerals containing pyrite, pyrrhotite and chalcopyrite (Kocaman et al., 2016). The oxidation process is catalysed by bacteria that oxidize metals and sulphur, including: (1) Desulfovibrio desulfuricans, (2) Thiobacillus spp., and (3) Sulfolobus acidocalde- rius (Chen et al., 2016). Equations 1.1 to 1.4 depict a series of processes involved in the formation of AMD (Wolkersdofer, 2006). In Equation 1.1, the oxidation of sulphide minerals occurs in the presence of water and oxygen to release ferrous iron (Fe2+), sulphate and acidity or protons (H+). The Fe2+ generated in Equation 1.1 is then rapidly oxidised to Fe3+ in the presence of oxygen and protons according to Equation 1.2. Subsequently, the Fe3+ formed reacts with water to form ferric hydroxide (Fe(OH)3) and more acidity (Equation 1.3). Equation 1.4 shows the overall governing equation obtained by summing up Equations 1.1 to 1.3.
The protons generated during the formation of AMD account for the acidic pH of AMD. High Fe3+ and its oxyhydroxides (Fe(OH)3) generate the reddish brown colour of AMD. Equations 1.1 and 1.4 explain the origins of high concentrations of sulphates detected in AMD (Mungazi and Gwenzi, 2019). Highly acid pH conditions are also responsible for the release of other contaminants such as metals, metalloids, radionuclides, and rare earth elements (REEs). These other contaminants are released via two processes: (1) simultaneously during the oxidation of sulphide-bearing geological materials, and/or (2) dissolution induced by highly acidic conditions (Consani et al., 2017; Balci and Demirel, 2018; Skousen et al., 2019).
Overall, Equations 1.1 to 1.4 clearly demonstrate that hydrology plays a critical role in AMD generation in two ways: (1) on the one hand, the oxidation process is mediated by water, and (2) on the other hand, permanently saturated and submerged conditions restrict the oxidation process, because oxygen has a very low solubility and diffusivity in water. Permanently saturated or submerged conditions may occur in natural systems in cases where sulphidic geological materials occur below the groundwater table. Permanently saturated/submerged conditions can also be created artificially, and this underpins the application of wet/water covers for the prevention of AMD (Aubertin et al., 2016; Kama and Hettiarachchi, 2018) as discussed in Chapter 6. In most natural geochemical settings, the availability of oxygen, rather than water, is often considered the rate-limiting condition in AMD generation. Therefore, strong wetting and drying cycles associated with distinct wet and dry seasons experienced in water-limited environments such as the tropics are ideal for AMD generation. Such alternation of wet and dry or freeze/thaw conditions will promote oxidation and formation of AMD in pits, drill holes, old mine workings, waste dumps, and mine tailings (Jouini et al., 2020). The hydrochemistry of AMD formation depicted in Equations 1.1 to 1.4 shows that restricting water and oxygen is critical in the prevention of AMD generation (refer to Chapter 6 for more details).