## 5 Discussion  
### Acid Generation and Release  
The sulphide wastes left in place are an important point source of ultra-acidic waters and dissolved PTEs. In this drainage system, the acid-producing process is driven by the oxidative dissolution of pyrite upon exposure to air and water through a complex series of chemical and bacterially-mediated reactions [10,28,29]:  
\[\text{FeS}_{2}+7/2\text{O}_{2}+\text{H}_{2}\text{O}\rightarrow\text{Fe}^{2+}+ 2\text{SO}_{4}{}^{2-}+2\text{H}^{+} \tag{5}\]  
\[\text{Fe}^{2+}+1/4\text{O}_{2}+\text{H}^{+}\rightarrow\text{Fe}^{3+}+1/2\text {H}_{2}\text{O} \tag{6}\]  
\[\text{FeS}_{2}+14\text{Fe}^{3+}+8\text{H}_{2}\text{O}\to 15\text{Fe}^{2+}+ 2\text{SO}_{4}{}^{2-}+16\text{H}^{+} \tag{7}\]  
The large surface area of the crushed ore induces high rates of acid production that exceed the acid neutralization capacity of the soil minerals, thus declining the pH to ultra-acid values and enhancing the mobilization of PTEs. The pH of the leachates emanating from the sulphide heaps is in the range of 1.66-2.16 [30]. Accordingly, soil acidity is controlled by the amount of hydrogen ions derived from pyrite oxidation and subsequent Fe\({}^{3+}\) hydrolysis. Exchangeable acidity is negligible in the waste environment due to the lack or scarcity of constituents with cation exchange capacity, such as clay minerals and organic matter. The potentially available pool of acid cations on the exchangeable sites of the reactive soil particles tends to increase with distance from the source of sulphide oxidation due to the progressive development of cation exchange reactions. This explains why the difference between the pH\({}_{\text{(H2O)}}\) and pH\({}_{\text{(KCI)}}\) values becomes more pronounced in the soil cores recovered in the distal zone.  
In addition to hydrogen ions, high solute concentrations of Fe, sulphate, and PTEs are released into nearby surface waters and soil. Oxidative dissolution of minor base-metal sulphides present in the pyrite ore, such as chalcopyrite, sphalerite and galena, usually does not produce acid; however, if Fe\({}^{3+}\) is the oxidant acid is formed through the reactions [31,32]:  
\[\text{CuFeS}_{2}+16\text{Fe}^{3+}+8\text{H}_{2}\text{O}\rightarrow\text{Cu} ^{2+}+2\text{SO}_{4}{}^{2-}+17\text{Fe}^{2+}+16\text{H}^{+} \tag{8}\]  
\[\text{PbS}+8\text{Fe}^{3+}+4\text{H}_{2}\text{O}\rightarrow\text{Pb}^{2+}+ \text{SO}_{4}{}^{2-}+8\text{Fe}^{2+}+8\text{H}^{+} \tag{9}\]  
\[\text{ZnS}+8\text{Fe}^{3+}+4\text{H}_{2}\text{O}\rightarrow\text{Zn}^{2+}+ \text{SO}_{4}{}^{2-}+8\text{Fe}^{2+}+8\text{H}^{+} \tag{10}\]  
In addition to Cu, Pb and Zn, environmentally hazardous concentrations of As, Cd, Sb, Co, Se, and Tl occurring as isomorphic substitutions in the sulphide lattices are mobilized, thus increasing the amount of PTEs available in the environment. Although water quality data are insufficient to make a reliable determination of metal fluxes in the area, [PERSON] et al. [30] have estimated the average concentrations of Cu, Zn and As to be 325.7, 185.0 and 34.9 mg L\({}^{-1}\), respectively, which is comparable to values emerging from the aids, heap leach piles, waste rock dumps, and tailings of the IPB mine sites [33].  
As another result of soil acidification, dissolution of carbonates and hydrolysis of silicates susceptible to chemical weathering provide a reservoir of cations (K\({}^{+}\), Na\({}^{+}\), Ca\({}^{2+}\), Mg\({}^{2+}\) and Al\({}^{3+}\)), which when combined with SO\({}_{4}\)\({}^{2-}\) and Fe\({}^{2+}\) ions lead to the formation of a variety of secondary sulphate minerals. The interaction of estuarine water with soil also contributes to the supply of seawater cations, especially Na\({}^{+}\).