ABSTRACT
From the formula, it is clear that the oxidation produces not only sulfuric acid but also sulfate and iron-bearing secondary minerals, which precipitate at different pH values. Generally, copiapite forms when pH < 1.5, jarosite when pH is in the range of 1.5-2.8, schwertmannite when pH is in the range of 2.8-4.5, goethite when pH < 6, hematite when pH is from 7 to 8, and ferrihydrites when pH > 5 (Anderson 1994; Alpers et al. 1994; Bigham 1994; Fitzpatrick 2003; Montero et al. 2005). Thus, these secondary iron-bearing minerals can be used as indicators of pH conditions when they are formed. However, the pH values of soils/ sediments not only depend on how much acidity is generated from pyrite oxidation but also on the balance between acid productivity, acid-neutralizing capacity, and availability of water and oxygen (Harries 1997). Once the formation of ASS occurs, the products of acidity may react with surrounding minerals, such as carbonates including calcite, dolomite, and siderite, and it will lead to acid neutralization and the potential production of gypsum. Meanwhile, some aluminosilicate minerals in soils, such as k-feldspar and kaolin group minerals, may also participate in the neutralization process, consuming H+ and releasing Al3+ (Blowes et al. 2003), and generate some aluminum-bearing minerals under different pH conditions, for instance, gibbsite forms in the primitive state of aluminosilicate neutralization in near neutral conditions (Bigham and Nordstrom 2000) and dissolves by reacting with sulfuric acid when pH further decreases to the range of 5 to 4. When pH drops below 4, soluble aluminum sulfate, such as halotrichite, pickeringite, and alunogen, may form (Bigham and Nordstrom 2000). The depletion of pyrite from the ASS results in the reduction of acidity production; eventually, this will become lower than the buffering capability of the soils and will result in the formation of insoluble hydroxysulfates such as alunite and basaluminite when pH increases to 5 (Bigham and Nordstrom 2000). At a mine site, Kim et al. (2003) found that aluminum sulfate was precipitated in the pH range of 4.45-5.95 and aluminum ions were mostly removed from the mining drainage when pH was >5. Like the presence of iron-bearing secondary minerals, aluminum-bearing minerals also can be regarded as good indicators of pH conditions and various concentration of Al3+. Based on the possible abundant distribution of kaolin, alunite, and gibbsite minerals and their indicative nature, in addition to their apparent spectral features, these three minerals have been selected as good indicators of the potential presence of Al3+ toxicity in ASSs.
