| 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] |
| Arsenic (As) | 0.01 | Exposure to arsenic causes skin and internal organ cancers, impaired nerve function, kidney and liver damage, or skin lesions. | [34] |
| Copper (Cu) | 2.0 | Exposure to excess copper induces oxidative stress, DNA damage and reduced cell proliferation in the human body. | [35] |
| Iron (Fe) | 1.0–3.0 | Iron is an essential part of hemoglobin in humans, but its overload causes severe health problems such as liver cancer, diabetes, cirrhosis of the liver, heart diseases and infertility. | [36] |
| Manganese (Mn) | 0.5 | Manganese is an essential nutrient to the body, but in excess can also interfere with the normal function of the nervous system to induce motor and cognitive impairments as well as neuropsychiatric symptoms. | [37] |
| Lead (Pb) | 0.01 | Exposure to lead causes cardiotoxicity, neurotoxicity, nephrotoxicity, carcinogenicity and genotoxicity in humans. | [38] |
| Zinc (Zn) | 5.0 | Zinc is considered an essential mineral in humans as it is necessary to produce hundreds of enzymes throughout the body. The toxicity of Zn in humans differs significantly and varies from acute exposure to chronic exposure. Renal injury, ranging from asymptomatic hematuria to interstitial nephritis or acute tubular necrosis, has also been reported due to acute toxicity, while chronic exposure can lead to sideroblastic anemia and granulocytopenia, and myelodysplastic syndrome. | [39] |
| Rare earth elements (REEs) | Data not established and are currently unregulated in humans and environment | Despite their extensive application in electronics, exposure to these metals will cause dysfunctional neurological disorders such as reduced intelligence quotient (IQ) in children, bone alteration, genotoxicity and fibrotic tissue injury and antitesticular effects and male sterility in humans. | [40] |
Table 1.3 Different types of applications and the uses of REEs.
| Types of applications | REEs used and their functions | Reference |
| 1. Medical (La, Ce, Pr, Gd, Nd, Tb, Dy, Ho, Er, Tm, and Yb) | Lanthanum oxide nanoparticles can be used for magnetic resonance imaging (MRI).Cerium-doped lutetium orthosilicate is used to convert high-energy radiation to visible light for positron emission tomography (PET) imaging. This test is used to reveal tissue and organ function. Praseodymium oxide nanoparticles have been synthesized and used for cancer treatment as a radiotherapy technique.The treatment of skin cancer, as well as hair removal using a laser beam, was achieved by using Neodymium as crystals.The magnetic properties of Gadolinium are used to enhance MRI images of tumors and intravenous radio-contrast agents in MRI scans.A radioisotope Dy-165 has been employed in the treatment of rheumatoid knee effusions.Holmium-based solid-state lasers have been used for non-invasive medical procedures for treating cancers and kidney stones.Erbium-based lasers have been used in medical and dental practice.A radioisotope Tm-167 has been used as a power source in portable X-ray devices.A radioisotope Yb-176 can be used to produce Lu-177, which is known to be a promising radioisotope for medical applications. | [41] |
| 2. Telecommunication | Neodymium, terbium, and dysprosium are used in smart cell phones to enable them to vibrate. | [42] |
| 3. Electronics | Scandium (Sc) is used in electron-beam tubes in TV.Yttrium (Y) is used in the manufacture of capacitors, phosphors, microwave filters, glasses, oxygen sensors, radars, lasers and superconductors.Eu, Tb, Gd, and Ce are used in flat-screen displays. | [43] |
| 4. Automobile | La, Ce, Nd, and Pr are used as catalytic converters to efficiently control pollution in cars. | [44] |
| 5. Weaponry | Yttrium(Y) and Terbium (Tb) are used for laser targeting and weapons in combat vehicles. | [45] |
1.2.2 Rare-Earth Elements and Their Importance
Rare earth elements consist of the 15 lanthanide elements including yttrium (Y) and scandium (Sc) on the periodic table. These elements exhibit similarities in geochemical behavior because of their identical stable trivalent oxidation state (except Ce and Eu), systematic decrease in ionic radius and increasing atomic number [24]. Mineral deposits of REE typically occur in low concentrations as oxides or carbonates in a broad array of geologic formations in very few countries, and over 90% of REE production occurs in China [25]. This near-monopoly has created a conceivable handicap for other countries where REEs are not readily produced [10]. The demand for these REEs has increased tremendously over the years due to their extensive application in several scientific advancements (Table 1.3) owing to their unique magnetic, phosphorescent, and catalytic properties [26]. The extraction of these elements from their conventional ores is energy-intensive and alone is insufficient to satisfy the rising demand in the foreseeable future due to their strategic importance in modern technology [27, 28]. Consequently, it has prompted researchers, national governments, and private entities to develop possible techniques for recovering these elements from unconventional sources, such as AMD, to meet the rising demand. In the United States, a significant concentration of REE was found in precipitates formed during acid mine drainage treatment from coal tailings [10]. In Brazil, acid water generated in a uranium mine in the state of Minas Gerais was found to contain a total concentration of 126 ppm of REE significantly higher than acid waters generated from different mines worldwide [29]. Also, in the Guizhou province in southwestern China, the Xingren coalfield mine is reported to contain REE concentrations varying between 0.1 and 0.9 ppm [30]. Although the recovery of rare earth metals from AMD remains a great challenge as it is several orders of magnitude lower in this product than the conventional REE ores, it can be recovered using a low-cost nano-adsorbent if concentrated out of the liquid solution during the neutralization processes [31]. The emergence of nanotechnology has contributed
