The Soil Chemistry of Cerium with an Emphasis on the Formation of Ion-Adsorption Rare Earth Element Deposits

Abstract

Cerium is an important rare earth element that has numerous and essential technological applications, as do many of the rare earth elements. Given that the rare earth elements do not exist as abundant and highly concentrated ore deposits, considerable research has been devoted toward their economically feasible extraction and subsequent processing. Ion-adsorption rare earth element deposits are emerging sources for rare earth element extraction and processing, including cerium. This manuscript presents a brief introduction to the soil thermodynamics of rare earth element hydrolysis, complexation, and adsorption onto phyllosilicates. These intrinsically critical thermodynamic-based activities govern rare earth element mineral weathering, species mobility, bioavailability, and suitability for specific extraction protocols. Ion-adsorption rare earth element deposits and their formation are discussed to provide options for subsequent research involving resource utilization, conservation, and environmental protection.

1. Introduction

The rare earth elements (REE’s) are 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). Frequently, lanthanum (La) is included because of its position in the periodic table and its trivalent oxidation state. Yttrium (Y), a transition metal with atomic number 39, has an electronic configuration of [Kr] 4d¹5s². Yttrium is frequently included with the rare earth elements because of its chemical similarity, and its natural occurrence is closely aligned with that of the heavy rare earth elements. Promethium’s presence in the natural environment is virtually nonexistent because of radioactive decay (half-life is 2.62 years) [1]. The light rare earth elements range from La to Eu, whereas the heavy rare earth elements range from Gd to Lu.

The Lanthanide series has ground state electronic configurations with the 4f electronic orbitals having at least one electron. Given cerium with an atomic number of 58, the corresponding electron configuration is [Xe] 4f¹ 5d¹ 6s², and the electron configuration for the trivalent species (Ce³⁺) is [Xe] 4f¹. The Ce⁴⁺ species has an electron configuration of [Xe]. The lanthanide contraction is attributed to the reduced ability of the inner-shell electrons to shield the outer-shell electrons; thus, with an increasingly charged nucleus, the greater electron attraction affords a decreasing atomic radius [1]. Figure 1 illustrates the progressive reduction of the atomic radii with increasing atomic number for lanthanum and the rare earth elements.

Figure 1.

The ionic radius for six coordination of the rare earth elements.

The rare earth elemental distributions vary across igneous, metamorphic, and sedimentary rocks, features attributed to diverse mineral compositions. Considering sedimentary rocks, argillaceous rocks typically exhibit greater rare earth abundances than sandstones; however, the variances are sufficiently large to limit comparisons (Figure 2). Similarly, sandstones typically have greater rare earth abundances than carbonaceous rocks [2]. For all these rock types, the light REE distribution typically exhibit greater elemental abundances than the heavy REE. The light and heavy REE’s typically reside in discretely different mineral assemblages.

Figure 2.

The comparison of the rare earth element abundances for argillaceous, sandstones, and calcareous rocks (source [3]).

Numerous minerals have cerium and other rare earth elements as lattice constituents. Some of the more abundant Ce-bearing minerals include (i) cerianite (CeO₂), (ii) fluorite (CaF with Ca replacement with Y and Ce), (iii) allanite ((CaCe)(AlAlFe²⁺)O[Si₂O₇][SiO₄](OH)), (iv) apatite (Ca₅(PO₄)3(Cl,OH,F) with REE substitution for Ca), (v) monazite (Ce,La,Nd,Th)(PO₄,SiO₄), (vi) xenotime (YPO₄ with Y substitution with Dy, Er, Tb, and Yb), (vii) rhabdophane ((CeLa)PO₄), and (viii) bastnaesite (CaREE which is a fluorocarbonate) [2, 3, 4]. Many rare earth element-bearing minerals occur because of isomorphic substitution during magma cooling.

The Portageville soil (fine, smectitic, calcareous, thermic Vertic Endoaquolls) is a clayey textured soil formed in Mississippi River alluvium. The rare earth element distribution of this soil series reflects the Oddon-Harkins rule, where for each element with an even atomic number, the next element in the periodic table, with an odd atomic number, generally has a smaller abundance. The light REEs typically are exceedingly more abundant than the heavy REEs (Figure 3).

Figure 3.

The rare earth element distributions of and a clayey textured soil (Portageville).

Many nations have rare earth element mining operations; however, several nations dominate the rare earth element production (Table 1). China is currently the world’s leading producer with 240,000 metric tons in 2023 and substantial proven and indicated reserves [5]. Brazil, Canada, and Vietnam have reserve capability to substantially increase production [5].

Nation	Production	Reserve
	metric tons	metric tons
Australia	18,000	5,700,000
Burma	38,000	not available
China	240,000	44,000,000
India	2900	6,900,000
Russia	2600	10,000,000
Thailand	7100	not available
United States	43,000	1,800,000
Brazil		21,000,000
Canada		14,000,000
Vietnam		22,000,000

Table 1.

Rare earth oxide mine production and indicated reserves (2023).

Source: [5].

The objectives of this manuscript are to provide a (i) brief review of rare earth element chemical and geological properties, (ii) review soil chemical properties such as hydrolysis, complexation and adsorption onto phyllosilicates and oxyhydroxides, and (iii) to explore ion-exchange type rare earth element deposits for rare earth element procurement.

2. Materials and methods

An aqua regia digestion was employed to obtain elemental abundances. Powdered 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 0.45 μm filtered, with analysis using inductively coupled plasma emission—mass spectrometry (ICP-MS). A hot water extraction involved equilibrating 0.5 g samples in 0.02 L distilled-deionized water at 80°C for 1 hour followed by 0.45 μm filtering and elemental determination using ICP-MS. For the aqua regia digestion and water extraction, selected samples were duplicated, and reference materials were similarly analyzed to assess analytical accuracy. The Sharkey, Portageville, and Kaintuck soil series were collected in Dunklin, New Madrid, and Jefferson Counties, Missouri.

3. Representative rare earth element soil distributions

The Kaintuck soil series (coarse-loamy, siliceous, superactive, nonacid, mesic Typic Udifluvents) consists of very deep, well-drained soils formed in loamy alluvium. The Kaintuck soil aqua regia digestion concentrations of the rare earth elements are presented in Figure 4, and the corresponding rare earth element water extraction concentrations are presented in Figure 5. The Sharkey soil series (very fine, smectitic, thermic Chromic Epiaquerts) consists of very deep, poorly, and very poorly drained, very slowly permeable soils in clayey alluvium. The Sharkey soil aqua regia digestion concentrations of the rare earth elements are presented in Figure 6, and the corresponding rare earth element water extraction concentrations are presented in Figure 7. In both soil series, the aqua regia digestion has cerium exhibiting the greatest concentration, followed by Nd and La. The light rare earth elements concentrations are substantially greater than the heavy rare earth elements concentrations. The Kaintuck soil series shows Nd, Y, La, and Ce having the greatest water extraction concentrations, whereas the Sharkey soil series exhibits cerium having the greatest water extract recovery. The Kaintuck soil series has a mean cerium concentration of 40 mg Ce kg⁻¹ (40,000 μg Ce kg⁻¹), whereas the corresponding cerium water extract concentration is approximately 3.2 μg Ce kg⁻¹. The Sharkey series presents similar rare earth element aqua regia digestion-water extraction concentration relationships. The large concentration differences between the aqua regia digestion and water extractable rare earth elements infer that a very substantial portion of the rare earth elements are less labile than the abundances associated with the water extraction pool.

Figure 4.

The aqua regia digestion concentrations of the rare earth elements of the Kaintuck soil series. Error bars are the standard deviation for 20 observations.

Figure 5.

The water extract concentrations of the rare earth elements of the Kaintuck soil series. Error bars are the standard deviation for 20 observations.

Figure 6.

The aqua regia digestion concentrations of the rare earth elements of the Sharkey soil series. Error bars are the standard deviation for seven observations.

Figure 7.

The water extract concentrations of the rare earth elements of the Sharkey soil series. Error bars are the standard deviation for seven observations.

4. Rare earth element hydrolysis with emphasis on cerium

Rare earth elements, typically exhibiting trivalent positive charges, undergo hydrolysis to reduce its charge polarization potential. Cerium hydrolysis for Ce³⁺ and Ce⁴⁺ are presented (Figures 8 and 9). The hydrolysis constants for Ce³⁺ are displayed in Eqs. 1, 2, and 3.

Figure 8.

Concentration distribution of Ce³⁺ hydrolysis species from pH 6 to 9.

Figure 9.

Concentration distribution of Ce⁴⁺ hydrolysis species from pH 4 to 9.

$$\left[Ce\left(OH\right)\right]\left[H+\right]=K_1\left[Ce\right]\left[H_{2}O\right]K_1=10^{-8.41}$$E1

$$\left[Ce{\left(OH\right)}_2\right]{\left[H+\right]}^2=K_2\left[Ce\right]{\left[H2O\right]}^2{K}_2=10^{-17.6}$$E2

$$\left[Ce{\left(OH\right)}_3\right]{\left[H+\right]}^3=K_3\left[Ce\right][{\left[H2O\right]}^3{K}_3=10^{-27.23}$$E3

Ce_T is the total analytical Ce aqueous concentration and was set at 0.001 molal. The pH intervals ranged from 4 to 9 in 0.2 pH units. The equilibrium solution’s low ionic strengths yielded activity coefficients that were approximately 1, and thus activities and concentrations were essentially equal. The activity of water was 1. Equation 4 is the mass balance for Ce species.

$$C{e}_T=\left[Ce\right]+\left[Ce\left(OH\right)\right]+\left[Ce{\left(OH\right)}_2\right]+\left[Ce{\left(OH\right)}_3\right]$$E4

Equation 5 was derived after the substitution of Eqs. 1, 2, and 3 into Eq. 4.

$$C{e}_T=\left[Ce\right]\left\{1+K_1/\left[H+\right]+K_2/{\left[H+\right]}^2+K_3/{\left[H+\right]}^3\right\}$$E5

The hydrolysis species distribution of Ce³⁺ {[Ce], [Ce(OH)], [Ce(OH)₂], and [Ce(OH)₃]} is presented in Figure 8, showing that Ce³⁺ is the dominant Ce species from less than pH 6 to approximately pH 8.2.

For the hydrolysis of Ce⁴⁺, the hydrolysis constants were K₃ = 10^–2.3 and K₄ = 10^–8.5, where the constants K₁ and K₂ are only relevant at pH levels considerably below pH 4 and typically outside of the natural pH interval of soils. Relevant rare earth element hydrolysis thermodynamic data have been tabulated [6, 7, 8, 9]. Bentouhami et al. [9] provided log ß values for the following hydroxo Ce³⁺ species: Ce(OH)²⁺ is −6.9, Ce(OH)₃ is −19.82, Ce(OH)₄¹⁻ is −28.9, Ce₂(OH)₃³⁺ is −15.9, and Ce₂(OH)₇¹⁻ is −44.7. Figure 9 illustrates that [Ce(OH)₃⁺] is dominant from pH 5 to pH 6.2, whereas [Ce(OH)₄] is the dominant species in neutral and alkaline regimes.

5. Rare earth element complexation

Rare earth element complexation involving anionic species may be expressed as follows:

$$RE{E}^{3+}+y{L}^{n-}=REEL{y}^{\left(3-yn\right)},$$E6

where Lⁿ⁻ is an inorganic ligand with a negative n charge and y is the stoichiometric coefficient. Common rare earth element anionic inorganic complexing species include nitrate, chloride, fluoride, sulfate, carbonate, and phosphate. Carbonate complexes (REE(CO₃)⁺) are more prevalent in the light rare earth elements, and dicarbonate complexes (REE(CO₃)₂⁻) are more prevalent in the heavy rare earth elements [10, 11, 12]. Luo and Byrne [13] documented the carbonate complexing behavior of the rare earth elements. For the Lanthanide Series, the dicarbonate complex becomes increasingly more stable with increasing atomic number. Millero [11] and Gramaccioli et al. [14] observed that rare earth elements-fluoride complexes obtained greater stability on transition from La to Lu. Banfield and Eggleton [15] emphasized rare earth elements-phosphate precipitates, especially apatites, in reducing soil rare earth element mobility.

Low molecular weight organic acid complexes with the rare earth elements include oxalic acid, citric acid, acetic acid, lactic acid, succinic acid, tartaric acid, formic acid, malic acid, and malonic acid, as well as naturally occurring higher molecular weight fulvic and humic acids [16, 17, 18, 19]. As with the inorganic rare earth elements complexes, organic rare earth elements complexes tend to show greater stability for the heavy rare earth elements than the light rare earth elements [10, 17].

6. Cation exchange and adsorption reactions

Phyllosilicate-rare earth elements interactions have been examined to estimate the presence of lithologic discontinuities in soil and rare earth element mobility on soil weathering [4]. Nd and Eu adsorption on K-illite reflected outer-sphere exchange reactions and pH-dependent silanol and aluminol reactions [20, 21, 22]. Europium adsorption on Na- and Ca-montmorillonite was consistent with cation exchange reactions and strong and weak pH-dependent complexation reactions [23, 24, 25]. These authors noted the presence of pH-dependent inner-sphere and outer-sphere adsorption sites. Tertre et al. [26, 27, 28] documented that REE³⁺ exchange involving montmorillonite and kaolinite was consistent with non-specific and specific complexation sites.

Coppin et al. [29] investigated rare earth element adsorption on Na-montmorillonite and kaolinite. Rare earth element adsorption was independent of electrolyte and dissolved CO₂ content; however, adsorption was dependent on clay mineralogy, pH, and ionic strength. Rare earth element adsorption increased at pH 5.5. The distribution coefficient (Kd) on montmorillonite declined slightly after pH 6. Stumpf et al. [30] observed Eu³⁺ adsorption onto smectite and kaolinite. At pH levels associated with soil environments inner-sphere Eu³⁺ surface complexes involving clay/Eu/carbonate were observed. Using regolith-hosted ion-adsorption deposits, Borst et al. [31] used synchrotron X-ray adsorption spectroscopy to indicate that REE‘s were adsorbed as outer-sphere hydrated complexes onto kaolinite. Zhenghua et al. [32] established La³⁺ adsorption isotherms on quartz, feldspar, kaolinite, goethite, and humic acid. The adsorption capacity increased in the order of humic acid > goethite ≈ kaolinite > feldspar ≈ quartz. More alkaline pH levels supported humic acid and goethite adsorption.

Investigating the Carrizo coarse-textured aquifer, Tang and Johannesson [33] documented the influence of carbonate ions and humic material activities on rare earth element adsorption as a function of CO₂, pH, ionic strength, and the initial rare earth element concentrations. They demonstrated that increased carbonate concentrations and increasingly alkaline pH levels decreased rare earth element adsorption. Subsequently, Tang and Johannesson [34] assessed the ability of soil organic matter to extract rare earth elements from coarse-textured coastal plain aquifer sediments. The recovery of rare earth elements was predicated on the ligand’s concentration and complexation capability. Rare earth element surface stability constants from Fe-oxyhydroxides dominate the equilibria for rare earth element adsorption behavior if the ligand activities are small; however, if the ligand concentration is greater, the rare earth element fractionation patterns reflect the ligand activity. Shan et al. [35] demonstrated that the soil adsorption of La, Ce, Pr, and Nd was influenced by citric, malic, tartaric, and acetic acids, with adsorption capacity correlated with increasing cation exchange capacities and with increasing pH. In general, citric, malic, and tartaric acids suppressed rare earth element adsorption when compared to acetic acid or Ca(NO₃)₂.

Selecting eight soil profiles across Germany, Mihajlovic et al. [36] determined aqua regia digestion, hydroxylammonium chloride, and hot water extractable rare earth element concentrations to assess the potential for rare earth element mobilization. Silty and clayey textured soils showed greater rare earth element abundances than sandy and peaty textured soils. The hydroxylammonium chloride-extractable rare earth element concentrations inferred that Al, Fe, and Mn oxides and clay minerals were largely responsible for rare earth element adsorption. The hot water extractable rare earth element contents were greater in soils enriched in soil organic materials.

In selected alluvial soils, Aide [37] demonstrated Ce preferential accumulation on pedogenic Fe-Mn glaebules in soils having alternating oxidation-reduction conditions. In Taiwan, Wu et al. [38] examined rare earth elements to infer their potential mobility upon dissolution of pedogenic Fe oxides. Rare earth element abundances were positively associated with increased soil free iron oxide contents. Compared to the bulk soil, Fe oxyhydroxides exhibited preferences for heavy rare earth elements over the light rare earth elements. In mangrove soil environments, Andrade et al. [39] documented rare earth element abundances were in the decreasing order of the light rare earth elements, medium rare earth elements, and heavy rare earth elements. Fractionation of the light rare earth elements over the heavy rare earth elements was greater when kaolinite was the dominant phyllosilicate, and preferential heavy rare earth element adsorption was associated with Fe-rich smectite environments.

7. Ion adsorption-type rare earth element deposits

Ion adsorption-type rare earth element deposits are global rare earth element sources composed primarily of heavy rare earth elements and yttrium and are typically formed in weathered granitic regolith overlying granitic rocks [31, 39, 40, 41]. These deposits are also termed regolith-hosted ion-adsorption deposits and laterite-hosted rare earth deposits. In China, Kang et al. [39] observed regolith-hosted ion adsorption deposits formed by subtropical weathering of igneous rocks and demonstrated that electrostatically held rare earth elements were preferentially associated with kaolinite-halloysite regoliths [30].

In a recent and extensive review, Wu et al. [40] provided a general overview of the weathering sequence that culminates with the formation of rare earth element ion-adsorption deposits. Primary minerals are frequently associated with felsic igneous rocks, in subtropical climates weather, and are transformed to phyllosilicates. The initially formed secondary phyllosilicate illite is subsequently transformed to montmorillonite and, eventually with more intense weathering, to kaolinite and halloysite. With soil evolution, phyllosilicate eluviation-illuviation fosters clay textural differences, with rare earth element horizon concentration differences partially attributed to co-eluviation. Weathering further increases the degradation of rare earth-bearing minerals, particularly the more readily weatherable minerals (titanite, allanite, gadolinite, bastnasite, apatite, and others). Apatite weathering may release appreciable Ce³⁺, which may be oxidized and recrystallized as cerianite (CeO₂). More weathering resistant minerals (zircon, monazite, and others) may remain as crystalline lattice structures. In southern Jiangxi Province, China, Li et al. [42] surveyed soils for their rare earth element contents and documented the following: (i) soil rare earth element abundances show spatial variation, (ii) concentrations of light rare earth elements are greater than the heavy rare earth elements concentrations, (iii) negative Ce anomalies and negative Eu anomalies were ubiquitous, and (iv) the soil rare earth element contents are primarily influenced by soil parent material inheritance.

Xiao et al. [43] focused on the adsorption thermodynamics of REEs on kaolinite to demonstrate that La, Nd, and Y adsorption conformed to Langmuir isotherms, and their saturated adsorption capacities were 1.73, 1.59 and 0.97 mg/g, respectively. Zhou et al. [44] investigated the sorption and desorption of Eu³⁺ on halloysite and kaolinite under slightly acidic conditions, noting that (i) at low NH₄⁺ concentrations, halloysite adsorption of Eu is less than kaolinite, (ii) halloysite has a stronger capacity for REE enrichment than kaolinite, and (iii) Eu adsorption is primarily inner-sphere.

In ion-adsorption type deposits, rare earth element leaching is frequently facilitated by humic material complexation and formation of carbonate and phosphate ion-pairs. The heavy rare earth elements form stronger complex associations; therefore, these species may leach more readily and deeper into the regolith, permitting a greater degree of rare earth element fractionation [31, 32, 33, 34]. Hydrolysis supports rare earth element adsorption as an exchangeable cation on phyllosilicates, again permitting the heavy rare earth elements to migrate into deeper regolith. The deeper regolith typically is less acidic, which supports the formation of more negatively charged sites along the phyllosilicate (primarily kaolinite) periphery, which may be represented as:

$$-SOH+O{H}^{-}\to S{O}^{-}+H_{2}O$$E7

$$-S{O}^{-}+RE{E}^{3+}\to -{\left[SO-REE\right]}^{2+},$$E8

where -[SO-REE]²⁺ represents REE complexation with aluminol and related functional groups.

In the Southeastern United States, Foley et al. [45] observed weathered granite-derived regoliths that have rare earth elements. The rare earth element chondrite-normalized patterns show tetrad patterns that infer their magmatic and weathering histories. Sanematsu and Watanabe [46] observed that rare earth element weathering associated with acidic soil environments will exhibit downward rare earth element transport. The rare earth elements, complexed with humic substances and carbonate-bicarbonates, are subsequently adsorbed or precipitated as secondary minerals in these less acidic environments. The secondary minerals include halloysite-kaolinite and phosphates. Cerium was adsorbed onto Mn oxides or oxidized-precipitated as cerianite (CeO₂). Fenga et al. [47] noted that the rare earth elements were preferentially surface adsorbed on kaolinite, and that adsorption was pH dependent with the maximum adsorption around pH 10. The rare earth element speciation was determined to be REE(OH)₂⁺.

Yu et al. [48] investigated the granitoid bedrock of the Swedish Baltic Shield and focused on secondary mineral precipitates collected from fracture walls and groundwater samples. The data demonstrated that Ce(VI) was preferentially associated with Mn oxides in the upper portion of the fracture network. The authors inferred that repeated oxidation-reduction episodes were responsible for capturing Ce(VI). In a batch adsorption experiment in Madagascar, Berger et al. [49] investigated an intensely weathered soil profile and documented that the primary rare earth element minerals in the A horizon weather and illuviate and reconstitute as secondary rare earth element minerals in the developing B horizon. Cerium(IV) is associated with redox-sensitive elements (Mn, Fe) in B-horizon, and cerianite is preferentially accumulated. Li et al. [50] studied the Zudong deposit with heavy rare earth element adsorption onto halloysite and kaolinite in the deeper regolith. In rare earth element ion deposits, Yang et al. [51] noted that kaolinite and halloysite were the dominant phyllosilicates for rare earth element adsorption. The heavy rare earth elements were preferentially adsorbed at higher pH levels and greater ionic strengths.

In south China, Bao and Zhao [52] investigated rare earth element mineralization involving granite and acidic volcanic rocks, noting that granitic rock rare earth element mineralization is primarily controlled by the weathering resistance of rare earth accessory minerals. Well-developed weathering profiles typically showed three horizons: (i) a lateritic horizon, (ii) a weathered horizon, and (iii) the weathering front. Continuous leaching results in rare earth element accumulation, assuming that the erosion rate is appropriately limited. Exchangeable rare earth elements were recoverable with complexing agents. Cerium was enriched in the upper most layer, a feature attributed to Ce oxidation and cerianite formation or excessive clay and Fe and Al oxyhydroxide adsorption.

The surface mining and heap leaching of China’s unique ion-adsorption rare earth resources have caused severe environmental damage [53]. Liu et al. [54] investigated ion-adsorption rare earth mine sites and documented excessive concentrations of rare earth element in soils, streams, and sediments. Enriched middle rare earth element concentrations suggested leaching and Al and Fe oxyhydroxide adsorption, coupled with ligand complexation, were important processes in rare earth element fractionation. Moldoveanu and Papangelakis [55] determined that (i) rare earth elements adsorbed on phyllosilicates are recovered by leaching with simple salt solutions, (ii) the leaching efficiency decreased in the order Cs⁺ > NH₄⁺ > Na⁺ > Li⁺, and (iii) sulfate anions confer greater recoveries than chlorides.

8. Summary

Ion-adsorption rare earth element deposits are an area of interest given the increasing utility of rare earth elements in advanced technologies. The framework for understanding these potentially lucrative rare earth element resources is rapidly evolving. Our existing understanding of rock weathering, soil formation, and soil processes is appropriately being applied to support the evaluation of these resources for rare earth element provision and the development of viable and sustainable extraction and processing.

9. Further research aims critical to rare earth element supply

Technology procurement and intensification is predicated on the acquisition of rare earth elements, especially cerium. In accordance, future rare earth element research must focus on (i) furthering our understanding of REE relationships in soil, including hydrolysis, complexation, and adsorption and (ii) developing environmentally sustainable mining and processing protocols involving ion-adsorption rare earth element deposits. A more sophisticated understanding of ion-adsorption REE deposit formation supports future improvements in mining site classification and the provision of economically advantageous recovery and processing technologies.

Conflict of interest

The author has no conflict of interest.