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United States, Peru, Russia, Mexico, and India
Figure 1.1 Schematic showing the pathway of AMD formation, its dispersion into the environment and entrance into the food chain: The destruction of natural vegetation in search of mineral resources exposes large surface areas to weathering effects. Due to the presence of sulfide materials in these mine tailings, AMD products are formed which can be washed away into nearby streams. Aquatic life suffers the consequences due to an increase in mortality rate; meanwhile, these polluted waters can be used for irrigation and thus will bioaccumulate in plants. Once in the food chain, human lives are affected in the process.
1.2 Rare-Earth Element Occurrence in Acid Mine Drainage
1.2.1 Acid Mine Drainage Generation and Effects
Although the formation of AMD has been historically attributed to mine tailing dams containing sulfide-bearing materials, it can occur naturally in an environment that exposes hefty volumes of sulfide-bearing materials to air and water [16]. However, of the different sulfide ore deposits shown in Table 1.1, pyrite is the most common and by far the most abundant sulfide mineral [17]. Using pyrite as an example for the generation of AMD, the oxidation process is represented by different reactions (1–3) [18]. Pyrite is oxidized to sulphate ions
The formation of Fe2+ is solubilized by the oxidation process and subsequently oxidized to ferric iron (Fe3+) and is the rate-determining step of the overall reaction (1.2).
Ferric cations produced can also oxidize additional pyrite into ferrous ions, and the net effect of these reactions is to produce H+, which increases the acidity of the influent and maintains the solubility of the ferric iron (1.3).
The pH value of AMD is as low as 2–4 and will naturally enhance the rate of dissolution of potentially toxic elements (PTEs), resulting in the tailing dam containing a high content of metal(loids), including sulfate ions in solution. Once leached into the lotic system (river), it will destroy their bicarbonate buffering system and enhance the rate of dissolution of metal ions, which could persist for hundreds of years once initiated, forming an age-long pollution stream with low pH [5, 19]. Consequently, the design lifespan of civil infrastructures such as water reticulation networks and bridges within this environment are shortened, caused by corrosion when metal oxides are formed [20]. In addition, the mortality rate of the aquatic organism within this inhospitable environment increases once these toxic metals accumulate in their organs [21]. More so, when this polluted water enters the irrigational system, the toxic metals accumulate in plants and indirectly enter the food chain. The growth of plants is distorted because, at low pH, plant nutrients such as nitrogen, phosphorus, and potassium necessary for their growth are immobilized, and the calcium and magnesium content becomes deficient. Meanwhile, in the food chain, these toxic metals bioaccumulate and cause deleterious health effects in humans (Table 1.2) due to their non-biodegradability [22, 23].
Table 1.2 Potential toxic elements and their effect in humans.
| Elements | Recommended levels in surface or groundwater (ppm) | Health-related issues in humans | Reference |
| Aluminum (Al) | 2.9 | Aluminum exposure is a risk factor for the development or acceleration of the onset of Alzheimer’s disease (AD) in humans. | [33] |
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