Evaluation of Rare Earth Element Mine Sites for Environmental Impact

Abstract

Rare earth elements are critical elements in the modern economy. Mining of rare earth elements has significantly intensified in the last several decades and studies of the environmental impact are in their infancy. In trace amounts, rare earth elements may support plant growth and development. At greater concentrations, rare earth elements are increasingly recognized as having a degree of mammalian toxicity; however, the mammalian toxicity potential may not be as acute as that for some heavy metals. The toxicity of rare earth elements requires detailed research to showcase toxicity thresholds for a wide range of ecosystem health. This study reveals case studies demonstrating that investigators rely on pollution indices, which do indicate that mining and ore processing possess environmental challenges. Further research has been identified to evaluate pollution indices for rare earth elements, especially concentrating on their biological availability.

1. Introduction

The rare earth elements (REE) are the 14 elements comprising the lanthanide series: cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu) [1]. Lanthanum (La) is associated with rare earth elements because of its periodic table position. Frequently, scandium (Sc) and yttrium (Y) are grouped with the lanthanides given their similar chemical reactivity. The lanthanide series are elements characterized as having one or more electrons in the 4f electronic orbitals for their ground state configuration [1, 2]. Promethium is absent in the environment because promethium-145 (¹⁴⁵Pm) decays via electron capture into neodymium-145 (¹⁴⁵Nd), with a half-life of 17.7 years.

The typically trivalent rare earth elements have considerable ionic bonding character [1]. Cerium may have a valency of +3 or + 4, whereas europium may have a valency of +2 or + 3 [1]. The rare earth elements show a decrease in their ionic radii on progression from La to Lu, the so-called “lanthanide contraction”. The “lanthanide contraction” supports greater chemical affinity for hydrolysis and chelate/complex stability with an increase in atomic number [1]. The LREE are the light rare earth elements, comprising of the elements from La to Eu, and the HREE are the heavy rare earth elements, comprising of the elements from Gd to Lu.

Rare earth element concentrations in soils, sediments, and other earth materials are dependent on their mineral assemblages, with rare earth element concentrations typically ranging from 0.1 to 100 mg kg⁻¹. Scandium concentrations across the earth’s crust are in the range from 16 to 30 mg Sc kg⁻¹, with mafic and argillaceous materials showing greater scandium concentrations. Typical soil scandium concentrations are from 0.8 to 28 mg Sc kg⁻¹. Yttrium concentrations average from 20 to 30 mg Y kg⁻¹ across crustal materials, where yttrium concentrations in soil range from 7 to 200 mg Y kg⁻¹ [3]. The Oddon-Harkins rule states that an element with an even atomic number has a greater concentration than the next element in the periodic table. The rare earth elements typically follow the Oddon-Harkin rule. The Post-Archean Australian Average Shale (PAAS), North American Shale Composite (NASC), and Upper Continental Crust (UCC) reflect the Oddon-Harkin rule (Table 1).

Element	PAAS	NASC	K-P	UCC
	mg/kg
La	38.2	32	26.1	3.1
Ce	79.6	73	48.7	63
Pr	8.83	7.9	7.6	7.1
Nd	33.9	33	19.5	27
Sm	5.55	5.7	4.8	4.7
Eu	1.08	1.24	1.2	1.0
Gd	4.66	5.2	6.0	4.0
Tb	0.774	0.85	0.7	0.7
Dy	4.68	5.8	3.7	3.9
Ho	0.991	1.04	1.1	0.83
Er	2.85	3.4	1.6	2.3
Tm	0.405	0.5	0.5	0.3
Yb	2.82	3.1	2.1	2.0
Lu	0.433	0.48	0.3	.31

Table 1.

REE abundances for various parent materials.

PAAS is Post-Archean Australian Average Shale, NASC is North American Shale Composite, and UCC is the upper continental crust. PAAS and NASC reported in McLennan [4, 5], K-P reported in Kabata-Pendias [3], and UCC reported in Rudnick and Gao [6].

Kabata-Pendias [3] has compiled many studies observing REE abundances in mafic and felsic igneous rocks, sedimentary rocks (argillaceous, sandstones, and calcareous), and soils. Kabata-Pendias also compiled reference data on terrestrial plant species. Commonly occurring REE-bearing minerals include: (i) fluorite (Ce), (ii) allanite (Ce), (iii) sphene (REE), (iv) zircon (HREE), (v) apatite (LREE), (vi) monazite (Ce and La), (vii) xenotime (REE), (viii) rhabdophane (Ce and REE), and (ix) bastnaesite (REE) [4]. Mineral and soil assemblies typically show greater LREE concentrations than HREE; however, the LREE/HREE ratio may vary with zircon abundances. In a review, Van Gosen et al. [7] noted that many of the significant rare earth element deposits occur in carbonatites (carbonate igneous rocks). Peralkaline igneous systems, magmatic-magnetite–hematite bodies, and mafic gneiss-bearing xenotime-monazite deposits are also important sources for rare earth element extraction.

Van Gosen et al. [7], U.S. geological survey [8], Ramos et al. [9], Kim and Jariwala [10], and Van Veen and Melton [11] review the rare earth element available supply and known resources, concentrating on nations having substantial reserves: Australia, China, India, Malaysia, Russia, and the United States. China and Brazil have significant rare earth element ore extractions.

2. Rare earth elements and the modern economy

Rare earth elements are critical to scientific and industrial advancement, ranging from energy to information technologies. Some rare earth elements have unique applications, such as gadolinium as contrast agent essential for magnetic resonance imaging (Table 2). Other important rare earth element uses include (i) permanent magnets (neodymium-iron-boron), (ii) cell phones, (iii) electric motors for electric cars, (iv) steel making, and (v) phosphors (yttrium, cerium, lanthanum, europium, and terbium) Van Gosen et al. [7]. Additional applications may utilize an array of rare earth elements, such as yttrium, europium, dysprosium, and holmium, in the manufacture of lasers (Table 2) [7, 10].

Element	Usage
Scandium	Aluminum alloys, aerospace components
Yttrium	Lasers, computer displays, microwave filters
Lanthanum	Oil refining, hybrid-car batteries
Cerium	Oil refining, catalytic converters, and lens production
Praseodymium	Aircraft engines, carbon arc lights
Neodymium	Computer hard drives, cell phones, and high-power magnets
Promethium	Portable X-ray machines, nuclear batteries
Samarium	High power magnets
Europium	Lasers, computer displays, and optical electronics
Gadolinium	Magnetic resonance imaging contrast agents
Terbium	Solid-state electronics, sonar systems
Dysprosium	Lasers, high-power magnets, and nuclear reactor control rods
Holmium	Lasers, high-power magnets
Erbium	Fiber optics, nuclear reactor control rods
Thulium	X-ray machines, superconductors
Ytterbium	Portable X-ray machines, lasers
Lutetium	LED lightbulbs

Table 2.

Rare earth element usage in modern industries and technologies.

Source: Van Gosen et al. [7], Kim and Jariwala [10].

3. Rare earth elements and their influence on living organisms, including human health

Kabata-Pendias [3] documented rare earth element influences on plant physiology, noting several studies, reporting that the REE stimulates seed germination, root growth, nutrient uptake, biological nitrogen fixation, chlorophyll synthesis, and photosynthesis. Kabata-Pendias was careful to note that further research is required for conformation and whether the rare earth elements are considered plant essential elements. The influence of rare earth elements, because of their production, processing, and usage on mammalian health, including human health, has not been widely investigated [12]. Of the elements comprising the lanthanide series and their influence on human health, the elements Ce, La, Gd, and Nd have received the most scrutiny [13]. Compounding rare earth element research is that many chemicals involved in rare earth element mining, recovery, primary and secondary processing, and recycling are involved; thus, it is difficult to isolate the influence of rare earth elements on human health.

Industry and occupational health research are limited because ore extraction and refining are localized in only a few nations [12, 13]. Bioaccumulation of rare earth elements appears to be restricted near mine and ore processing sites. There is growing evidence of the adverse effect of gadolinium (Gd) on skin conditions and nephrogenic systemic fibrosis because of Gd’s use as an element in contrast agents used in magnetic resonance imaging [13]. Short-term exposure animal studies suggest rare earth element toxicity involves the liver, lungs, blood, and nervous systems. The global use of cerium oxide nanoparticles as a catalytic additive in diesel fuel may be an air and soil pollutant. Conversely, rare earth elements may confer beneficial antioxidant activity. In an extensive review, Rim [12] noted that the rare earth elements high redox potential supported oxidative stress, which may enhance diabetes, atherosclerosis, inflammatory conditions, high blood pressure, neurodegenerative diseases, and cancer. Rim [12] further noted that selective chemicals used for rare earth element ore extraction, processing, and manufacturing may contribute to the environmental impact. The combined toxicities of the rare earth elements may influence soil pH and influence the human toxic response.

Many soil studies have been conducted to determine if soil rare earth element concentrations may have sufficient soil variability to influence environmental responses. In Brazil, Landim et al. [14] established soil rare earth element quality references and assessed their spatial distributions. The mean background concentrations in soils followed the abundance of the earth’s upper crust: Ce > La > Nd > Pr > Sm > Dy > Gd > Er > Yb > Eu > Tb > Lu. In the Piaui state in Brazil, the ∑REEs across the mesoregions were (i) southeast (263 mg kg⁻¹), (ii) north and central-north (90 mg kg⁻¹), and (iii) southwest (40 mg kg⁻¹) [14].

The effects of rare earth elements on aquatic biota are largely unknown [15, 16]. Gonzalez et al. [15] researched the sensitivity of aquatic organisms to REE’s. Lanthanide series toxicity increased with atomic number for Aliivibrio fischeri and Pseudokirchneriella subcapitata, whereas the lanthanide series sensitivity of Daphnia magna, Heterocypris incongruens, Brachionus calyciflorus, and Hydra attenuata were effectively equal across atomic number [15]. Comparison of environmental risk indexes indicated that lanthanide concentrations immediately downstream of wastewater treatment plants are most problematic. Malhotra et al. [16] reviewed previous studies to understand the bioavailability, accumulation, interaction, and toxicity criteria of rare earth elements with aquatic organisms. Previous studies demonstrated that rare earth elements frequently provide a stimulatory influence at low dosage, whereas adverse influences are observed at higher dosages.

In Canada, the exposure-response relationships of three native plant species (Asclepias syriaca, Desmodium canadense, and Panicum virgatum) and two commonly used crop species (Raphanus sativus, and Solanum lycopersicum L.) to lanthanum, yttrium, and cerium were evaluated [17]. Germination was influenced by Ce at lower pH values. All species showed growth inhibition with Ce addition. Growth inhibition was evident for A. syriaca and D. canadense with rare earth element addition [17].

Li, et al. [18] assessed the toxicity of lanthanum, after 3 to 4 weeks of exposure, to five representative soil invertebrates. Toxicity was related to (i) total lanthanum, (ii) 0.01 M CaCl₂-extractable lanthanum, and (iii) porewater lanthanum concentrations. Reduced growth of Isopod (Porcellio scaber) showed the most sensitive response. Reproduction issues involving earthworms (Eisenia andrei), enchytraeids (Enchytraeus crypticus), springtails (Folsomia candida), and oribatid mites (Oppia nitens) were observed at various elevated soil La concentrations. This study suggested that La may affect soil ecosystems at La concentrations slightly above natural background levels (6.6–50 mg La kg⁻¹).

In China, Zhou et al. [19] performed experiments involving dry grass landfilling, chicken manure broadcasting, and plant cultivation to reclaim a rare earth element mine. After 2 years of restoration, soil organic matter, available potassium, available phosphorus, and acid phosphatase activity were improved. Soil physical properties (bulk density, water holding capacity, pH, and electrical conductivity), nutrient availabilities, and enzyme activities after 5 years were either similar or less impacted than soil not impacted by rare earth mining activities.

4. Commonly used pollution indices

Pollution indices are calculated estimates of the degree of soil and sediment contamination, usually associated with heavy metals. Pollution indices are also employed to (i) assess soil quality and health, (ii) predict ecosystem sustainability, and (iii) discriminate between natural processes and anthropogenic processes to explain heavy metal distributions in soil profiles [20, 21]. Many of the pollution indices rely upon a proper selection of a geochemical background (GeoBase) [20, 21]. The geochemical background, if properly selected, will permit an estimation of the intensity of the heavy metal or rare earth element pollution.

The geochemical background (GeoBase) for an element estimates the natural variations in concentration in the surficial environment [22] or is a “measure that is used to differentiate between the concentration of the natural compound and the concentrations with an anthropogenic influence in a given environmental sample” [23]. Kowalska et al. [21] assessed 18 pollution indices and estimated their strength and weaknesses. Diwa [24], Elvira [25], Lawrence et al. [26], Barbieri et al. [27], Ghrefat et al. [28], and Gargouri et al. [29] provided additional information on interpreting the environmental impact of pollution indices values. In the Czech Republic, Weissmannová et al. [30] chronicled the potential ecological risk and human health risk assessment in soils influenced by coal mining and metal processing.

Frequently, employed pollution indices are briefly described in Table 3 and more fully described below.

Indices	Description
Geoaccumulation index (Igeo)	Igeo = log2[HMconc/(1.5 x GeoBase)], where HMconc is the concentration and GeoBase is the geochemical reference concentration.
Single pollution index (PI)	PI = HMconc/GeoBase
Enrichment factor (EF)	EF = [HMconc/LV]/GeoBase/LV], where LV (low variability reference) is the element concentration considered as not supplied or depleted.
Contamination factor (CF)	CF = sample mean concentration relative to preindustrial concentration.
Biogeochemical index (BGCI)	BGCI = soil metal concentration of O horizon relative to A horizon.
Sum of contamination (PIsum)	PIsum = ∑ PI, where each PI is PI = HMconc/GeoBase.
Nemerow pollution index (PInem)	PInem = {[(1/n)∑PI)^2 + PImax^2]/n}^0.5, where n number of metals sampled and PImax is the maximum PI value.
Pollution load index (PLI)	PLI = {∏ PI}^1/n (the harmonic mean of the PI’s
Average single pollution index	PIaverage = (1/n) ∑PI.
The vector modulus of pollution	PIvector = {(1/n)∑ PI^2}^0.5.
Multi-element contamination	MEC = {∑(HMconc}/n.
Degree of contamination	Cdeg = ∑CF (multiple elements)
Potential ecological risk factor	Eir = Ti x HMconc/GeoBase, where Ti is a factor of a particular heavy metal’s toxic reaction.
Hazard quotient (HQ)	HQ = element concentration/element concentration where no environmental effect was observed.

Table 3.

Frequently employed pollution indices.

The geoaccumulation index (Igeo) is determined from the elemental concentration and the GeoBase. The GeoBase is the concentration value of the element selected from a geochemical reference background, and it is critical to select an appropriate geochemical background, with various research investigations using PAAS, NASC, UCC, or local baselines perceived as preindustrial or non-impacted. The Igeo value and the corresponding pollution level is (i) less than or equal to 0 is not impacted, (ii) 0 to 1 is at most moderately impacted, (iii) 1 to 2 is moderately impacted, (iv) 2 to 3 is moderate to highly impacted, (v) 3 to 4 is highly impacted, (vi) 4 to 5 is high to very highly impacted, and (vii) 6 is very highly impacted. The single pollution index evaluates the degree of heavy metal or rare earth element accumulation in soil or sediment relative to a reference GeoBase. The single pollution index estimates the total amount of an element’s accumulation and does not indicate the bioavailability of the heavy metals. The single pollution index (PI) allows the comparison of sites or different soils over time. The enrichment factor (EF) estimates the heavy metal’s anthropogenic impact and is determined as EF = [HMconc/LV] of sample/GeoBase/LV] of background, where LV (low variability reference) is the selected element concentration considered as not supplied or depleted. Typically, Fe, Al, Ca, Ti, Sc, and Mn have been used as the reference element. If the EF value ranges from 0.5 to 1.5, the likelihood of anthropogenic activity is low. The selection of the GeoBase that reduces the metal variability is critical for assessment. If the EF value and the enrichment level are: (i) less than 1, there is no likelihood of element enrichment (impact), (ii) 1 to 3 is minor enrichment, (iii) 3 to 5 is moderate enrichment, (iv) 5 to 10 is moderately severe enrichment, (v) 10 to 25 is severe enrichment, (vi) 25 to 50 is very severe enrichment, and (vii) more than 50 is extremely severe enrichment.

The contamination factor (CF) estimates the preindustrial increase of heavy metals or rare earth elements in soil and sediment. CF is estimated as the mean of more than five samples for a particular heavy metal or rare earth element relative to the heavy metal concentration from preindustrial samples. The selection and evaluation of the quality of the preindustrial reference samples may be difficult. Appropriate ratings are: (i) less than 1 is low contamination (impact), (ii) 1 to 3 is moderate contamination, (iii) 3 to 6 is considerable contamination, and (iv) greater than 6 is considered high contamination [25]. The biogeochemical index (BGCI) is estimated as a ratio of the heavy metal or rare earth element concentration in the soil’s O horizon to that element’s concentration in the soil’s A horizon. If the BGCI has values greater than 1, then there exists an increased heavy metal or rare earth element adsorption in the O horizon. The BGCI is well suited for forest soils; however, the biogeochemical index lacks consideration of the heavy metal biological availability.

The sum of contamination (PIsum) involves a suite of heavy metals or rare earth elements and is simply the sum of the individual PI values for the sampled heavy metals. The calculation of the PIsum must contain all of the relevant heavy metals. The selection of the geochemical database must be appropriate to the site assessment, typically employing local and preindustrial sampling. Similar to the BGCI, the Pisum does not consider heavy metal biological availability. The Nemerow pollution index (PInem) is defined as PInem = {[(1/n)∑PI)² + PImax²]/n}^0.5, where n is the number of heavy metals sampled, and PImax is the maximum PI value for all of the heavy metals. The usage of the appropriate geochemical database, baseline values, or threshold levels must be ascertained. The Nemerow pollution index directly reflects the soil or sediment environmental pollution and highlights the heavy metal having the greatest environmental presence or intensity of pollution.

The pollution load index (PLI) is calculated as {∏ PI}^1/n and is simply the harmonic mean of the PI’s for the analyzed heavy metals or rare earth elements. The pollution load index does not consider heavy metal biological availability and is based on the reliability of the PI values. The average single pollution index (PIaverage) is estimated as the average of the individual PI values. The average single pollution index does not consider heavy metal or rare earth element biological availability and is based on the reliability of the PI values. Appropriate ratings are: (i) less than 1.5 is very low contamination, (ii) 1.5 to 2 is low contamination, (iii) 2 to 4 is moderate contamination, (iv) 4 to 8 is high contamination, (v) 8 to 16 is considered very high contamination, and (vi) 16 or greater is considered extreme contamination [25]. The vector modulus of pollution index (PIvector) is estimated {(1/n)∑ PI²}^0.5. The vector modulus of pollution index does not consider heavy metal or rare earth element biological availability and is based on the reliability of the PI values.

The multi-element contamination index (MEC) is estimated as MEC = {∑(HMconc of element i/tolerable level for element i)}/n. The multi-element contamination index does not require an assessment of the variation in natural processes. The degree of contamination (Cdeg) is estimated as Cdeg = ∑CF. The degree of contamination does not require an assessment of the variation in natural processes.

The potential ecological risk factor (Eir) is estimated as Eir from an estimate of the element’s toxicity (Ti factor) and associated PI value, where the Ti has values of 1 for zinc (Zn), 2 for chromium (Cr), 5 for Nickel (Ni), Copper (Cu) and lead (Pb), 10 for arsenic (As), and 30 for cadmium (Cd). Ti values for the rare earth elements are not yet determined. The potential ecological hazard index (RI) is estimated as IR = ∑Eir. If the Eir risk values are: (i) less than 40, which implies low ecological risk, (ii) 40 to 80 implies moderate ecological risk, (iii) 80 to 160 implies appreciable ecological risk, (iv) 160 to 320 implies high ecological risk, and (v) more than 320 implies series ecological risk. If the IR risk values are: (i) less than 150, which implies low ecological risk, (ii) 150 to 300 implies moderate ecological risk, (iii) 300 to 600 implies high ecological risk, and (iv) more than 600 implies series ecological risk [24]. The hazard quotient (HQ) of a rare earth element is estimated as the rare earth element concentration relative to the rare earth element concentration, where no environmental effect was observed.

5. Case studies of rare earth elements environmental impact because of mining, processing, and utilization

Rare earth element mines differ from other mines, that is, rare earth element ores are not highly concentrated, and the quantity of tailings is frequently substantial compared to the quantity of rare earth elements recovered. Krasavtseva et al. [31] investigated heavy metals and rare earth elements at mine sites in the Kola Subarctic (Russia). Noting that finely dispersed materials from mine tailings exhibited from 1.5 to 3 times the heavy metal and rare earth element concentrations, they calculated pollution assessments using the (i) geoaccumulation index, (ii) enrichment factor, (iii) potential ecological risk factor, and the (iv) potential environmental hazard index. Increased mobilized heavy metal and rare earth elements were observed for mine tailing leachates. The heavy metal and rare earth element leachate concentrations were increased with (i) reduced pH levels, (ii) elevated organic carbon levels, and (iii) increased temperatures.

In China, Bai et al. [32] investigated six rare earth element mines for their environmental impact and conducted a cross-sectional comparison. Accurate resource and environmental carrying capacity (RECC) assessments are critical for ensuring that rare earth element exploration and extraction activities are appropriate and conducted with achieving multiple interests. The RECC assesses the ability of an ecosystem to resist external disturbances and maintain its original ecosystem services. The RECC considers multiple factors, including human activities, climate change, and energy structure and consumption. Except for the Bayan mine in inner Mongolia, the support index (evaluation of policies, inputs, and technologies to mitigate environmental impact) was greater than the pressure index (omission of policies, inputs, and technologies to mitigate environmental impact), implying limited environmental impact. The ratio of financial investment in pollution control was an important factor limiting environmental sustainability.

Wang and Liang [33] observed geochemical rare earth element fractionation involving the light and heavy rare earth elements in tailings of the Baotou mine in China. Using the NASC and PAAS values for normalization, the light rare earth elements showed greater normalized patterns. A map of the rare earth element PAAS normalized concentrations exhibited reduced concentrations at an increasing distance from the mine site. The total rare earth element concentrations (Σ REE) of surface samples varied from 156 to 57,000 mg kg⁻¹, with a mean of 4700 mg kg⁻¹. The enrichment factors for all of the rare earth elements displayed values suggesting very high to extremely high enrichment in the east, southeast, and south directions from the mine site, whereas enrichment factors in the northwest direction were indicative of only significant impact. In Baotou, China, Zhou et al. [34] investigated rare earth element concentrations in dust samples and subsequently expressed data using enrichment factors. The enrichment factors, when normalized to the local loess, indicated rare earth element contamination; that is, the igeo index indicated contamination and the HQ did not indicate contamination. In India, Humsa and Srivastava [35] investigated industrial waste from a titanium dioxide pigment industry. Soil resources in the impacted area demonstrated increased heavy metal (Fe, Cr, V, Ni, Cu, Zn, and Pb) concentrations, as well as increased concentrations of Sm, Tb, and Dy.

In Australia, Nkrumah et al. [36] investigated the rare earth element biogeochemical behavior in natural ecosystems to estimate their soil abundances. In addition to a slight HREE enrichment, key rare earth element concentrations were (i) Ce (2550 mg kg⁻¹), (ii) La (645 mg kg⁻¹), (iii) Gd (25 mg kg⁻¹), and Lu (1.5 mg kg⁻¹). Plant uptake showed variation among species, with broadleaf plants typically having greater rare earth element accumulation than greases. In Brazil, Cunha et al. [37] observed uranium-phosphate deposits and estimated the spatial rare earth element soil distribution. The soil rare earth element concentrations were closely correlated with the soil uranium and phosphate concentrations. In China, Zhao et al. [38] evaluated mine tailings from sites with and without phytoremediation. The Nd and Y hazard quotients (HQ) were determined, with the concentration baselines for Nd and Y established at which 10% soil root length inhibition was observed. The HQs for wheat (Triticum aestivum) and mung bean (Vigna radiata) were less than 1; however, the geo-accumulation index was variably distributed as (i) uncontaminated (Igeo <0), (ii) uncontaminated to moderately contaminated (0 < Igeo <1), (iii) moderately contaminated (1 < Igeo <2), and (iv) moderately to strongly contaminated (2 < Igeo <3). Wang and Liang [39] assessed the environmental impact of rare earth elements in soils surrounding the Bayan Obo Mine. The Bayan Obo deposit is estimated to contain more than 100 million Mt. of rare earth reserves containing monazite and bastnaesite ores. The sum of the rare earth element concentrations in surface soil horizons varied from 150 to 18,900 mg kg⁻¹. The distribution patterns of the individual rare earth elements were like those of the Bayan Obo ores.

MacDonald et al. [40] reviewed the development of pollution indices for freshwater ecosystems, noting that further research is warranted to guarantee accurate predictive environmental outcomes. Chamber [41] discussed “technologically enhanced naturally occurring radioactive material,” noting that monazite mining will produce waste material having ²³²Th. ²³²Th will alpha decay slowly to ²²⁸Ra, which will more rapidly decay by beta emission to ²²⁸Ac (half-life of 5.75 years). Where the activity is substantial, appropriate actions to protect the environment and personnel are warranted. Aide and Aide [42] discussed the use of rare earth elements in identifying and assessing soil lithologic discontinuities. Aide and Aide also demonstrated that Fe-Mn masses (pedogenic nodules) in selected alluvial soils preferentially accumulated Ce and revealed a positive Ce anomaly, suggesting that alternating conditions of oxidation–reduction were important for glaebule (Fe-Mn nodules) synthesis and Ce incorporation.

6. Observation of rare earth element soil distributions in missouri

6.1 Study area

Summers are hot and humid with a mean July temperature of 26°C and winter temperatures are mild with a mean January temperature of 2°C. The mean annual precipitation of 1.19 m is seasonally distributed, with greater rainfall in spring. Vegetation is either a mixed hardwood forest or land that has been cleared of vegetation, land-graded, and employed in row-crop agriculture.

6.2 Soils

The alred series (loamy-skeletal over clayey, siliceous, semiactive, mesic Typic Paleudalfs) consists of very deep, well-drained soils formed in cherty hillslope sediments and the underlying clayey residuum. The Alred has an A-E-Bt-2Bt horizon sequence. The rueter series (loamy-skeletal, siliceous, active, mesic Typic Paleudalfs) consists of very deep, somewhat excessively drained soils formed in colluvium and residuum from cherty limestone. The Rueter has an A-E-Bt-2Bt horizon sequence. The Menfro series (Fine-silty, mixed, superactive, and mesic Typic Hapludalfs) consists of very deep, well-drained soils formed in thick loess deposits on uplands. The Menfro has an A-E-BE-Bt-C horizon sequence. The Kaintuck soil series in Missouri (coarse-loamy, siliceous, superactive, nonacid, mesic Typic Udifluvents) are very deep and well-drained floodplain soils formed from loamy alluvium and have an Ap-C horizon sequence.

6.3 Protocols

An aqua regia digestion was employed to obtain a near-total estimation of elemental abundance associated with all but the most recalcitrant soil chemical environments. Aqua regia does not appreciably degrade quartz, albite, orthoclase, anatase, barite, monazite, sphene, chromite, ilmenite, rutile, and cassiterite; however, anorthite and phyllosilicates are partially digested. Homogenized samples (0.75 g) were equilibrated with 0.01 liter of aqua regia (3 mole nitric acid: 1 mole hydrochloric acid) in a 35°C incubator for 24 hours. Samples were shaken, centrifuged, and filtered (0.45 μm), with a known aliquot volume analyzed using inductively coupled plasma mass spectrometry (ICP-MS). Aide and Fasnacht [43] reviewed the application of the aqua regia digestion protocol.

7. Missouri rare earth element investigations

The Alred and Rueter soil profiles were completely characterized, with the rare earth element suite determined with aqua regia digestion. The ochric (A and E horizons) and illuvial (Bt and 2Bt) soil horizons were identified and compared to their rare earth element concentrations. For clarity, eluvial soil horizons occur where strong leaching of clay minerals, oxides, and organic material is observed, whereas illuvial soil horizons contain materials that have been transported downwards either in solution or suspension and subsequently deposited. The illuvial horizons, with their greater clay contents, show greater rare earth element abundances (Table 4). Paired t-test comparing ochric and illuvial rare earth element concentrations are significant (0.016 for Alred and 0.020 for Rueter).

	Alred		Rueter
Element	Ochric	Illuvial	Ochric	Illuvial
	mg kg^−1
La	16.9	34.6	1.2	6.2
Ce	41.9	85.2	10.1	29.7
Pr	3.7	8.3	0.5	2.0
Nd	13.7	33.3	2.1	9.1
Sm	2.4	6.5	0.4	2.3
Eu	0.3	1.3	0.1	0.5
Gd	2.0	6.4	0.3	2.4
Tb	0.2	0.9	0.1	0.4
Dy	1.2	5.7	0.2	2.5
Ho	0.2	1.2	0.1	0.5
Er	0.5	3.3	0.2	1.5
Tm	0	0.5	0	0.2
Yb	0.4	3.0	0.2	1.5
Lu	0	0.4	0	0.2

Table 4.

The rare earth element distribution of the Alred and Rueter series.

The multiple pedons for the Menfro and Kaintuck soil series were characterized for rare earth element concentrations for each soil horizon in their respective soil profiles (Table 5). The Menfro soil is an Alfisol with ochric silt loam texture transitioning to a silty clay loam texture in the agrillic horizon. The Kaintuck soil series is an entisol with sandy loam soil textures througthout the soil profiles. The Menfro soil profiles have greater rare earth element concentrations, a feature attributed to their greater clay contents. The coefficient of variation for the rare earth element concentrations is greater for the Menfro soil pedon, a feature attributed to the discrete clay differences in the ochric and agrillic horisons when compared to the rather uniform texture distribution in the Kaintuck pedons.

	Menfro series		Kaintuck series
Element	Mean (mg kg^−1)	CV (%)	Mean (mg kg^−1)	CV (%)
La	25.8	13.6	17.2	6.0
Ce	43.9	12.6	43.7	3.1
Pr	6.2	15.5	4.6	4.9
Nd	23.2	17.4	19.4	4.9
Sm	4.4	20.3	3.2	9.0
Eu	0.8	23.7	0.6	<1
Gd	3.5	22.5	2.9	5.3
Tb	0.4	26.4	0.4	14.4
Dy	2.1	27.0	2.3	6.5
Ho	0.4	27.2	0.4	<1
Er	0.9	30.9	1.2	7.1
Tm	0.1	28.3	0.1	<1
Yb	0.8	32.0	0.9	5.6
Lu	0.1	136	0.1	<1

Table 5.

The mean rare earth element concentrations and coefficient of variations.

The Wilbur soil series (coarse-silty, mixed, superactive, mesic Fluvaquentic Eutrudepts) consists of very deep, moderately well-drained soils formed in silty alluvium. The four pedons are located across a 40 ha production field and were evaluated for morphology, routine physical and chemical characteristics, and rare earth element concentrations across all pedon horizons. Yttrium was included in the rare earth element analysis. As expected, the rare earth element distribution clearly is in accordance with the Oddon-Harkin rule (Table 6).

Element	Mean	Coefficient variation
	mg kg^−1	Percent
Y	12.1	21
La	27.3	7
Ce	57.3	7
Pr	6.9	8
Nd	25.9	9
Sm	4.7	10
Eu	0.9	15
Gd	4.1	14
Tb	0.5	18
Dy	2.7	17
Ho	0.5	21
Er	1.3	19
Tm	0.2	28
Yb	1.0	19
Lu	0.1	34

Table 6.

Mean and coefficient variation for the Wilbur soil series.

Total of 32 observations of four pedons.

The Wilbur pedons are distant from industry and mining, thus these pedons may be considered pristine. The rare earth element distribution is very uniform within and across pedons, features reflecting the pedon’s as belonging to the Inceptisol order and having little soil texture variation. The rare earth elements may be used in the GeoBase for the following pollution indices: (i) geoaccumulation index, (ii) single pollution index, (iii) The enrichment factor, (iv) the contamination factor, (v) the sum of contamination, (vi) the pollution load index, (vii) the average single pollution index, (viii) the vector modulus pollution index, (ix) the degree of contamination, and (x) the hazard quotient. If an estimate of the factor of the rare earth element’s toxic reaction becomes available, then (i) the potential ecological risk factor and (ii) the potential ecological hazard indexes can be calculated.

8. Prospectus concerning research needs

New rare earth element mines are in various states of development across the United States and in many other nations. Given that rare earth element mines and ore processing operations span a range of locations, the climate, and physical settings will vary. The environmental impact assessment must include (i) mine conditions, (ii) ore geochemistry, (iii) local geology and hydrology, and (iv) physiographic settings and geomorphology. Mine and processing sites may disturb or accentuate (i) surface water because of drilling fluids, acid, and neutral mine discharge water, and influence aquatic organisms, (ii) groundwater impacts because of mine pit lakes, evaporation ponds, (iii) air pathways involving fugitive dust, aerosols and chemical vapors, radioactivity, and (iv) tailing storage facilities [44]. The environmental assessment must consider (i) current and future construction workers/employees (ingestion and inhalation), (ii) traditional tribal lifeways, and (iii) on-site and off-site residents. The documentation of potential health effects requires a greater research emphasis. Pulmonary toxicity of inhaled rare earth elements, and expanded investigations, involving integrated risk information systems and provisional peer-reviewed toxicity values are necessary [44].

Pollution indices need to be verified for the rare earth elements. Additionally, substantial research in assessing the biological availability of the rare earth elements in ecosystems is warranted as the total rare earth element concentrations may not accurately describe the environmental risk.