**2. Rare earth element rock and primary-secondary mineral abundances**

Rock REE concentrations are predicated on rock type and source area. Most REE parent material compositions range from 0.1 to 100 mg/kg, thus REEs have moderate abundances. Typically, felsic's have greater REE concentrations than mafic's, with the LREE concentrations greater than the HREE concentrations. Similarly, argillaceous sediments have greater REE concentrations than limestones and sandstones.

The Oddo-Harkins rule states that an element with an even atomic number has a greater concentration than the next element in the Periodic Table. REEs typically obey the Oddo-Harkin rule. The PAAS, NASC, loess, and selected geochemical soil surveys usually reflect the Oddo-Harkin rule (**Table 2**).

Secondary minerals are (1) minerals formed after the rock enclosing the mineral was formed or (2) minerals that have chemically altered from primary minerals and have been transported. In some cases, REE are involved with isomorphic substitution or undergo adsorption reactions with phyllosilicates or oxyhydroxides. Precipitation reactions with fluoride, phosphate and carbonate may yield a variety of secondary REE minerals [6]. Cerianite (CeO2 ) may form in oxic soil environments [7, 8].

Clark [6] provided a listing of important REE-bearing minerals, including (i) fluorite (CaF<sup>2</sup> where Y and Ce replace Ca), (ii) allanite [(Ce,Ca,Y)<sup>2</sup> (Al,Fe2+,Fe3+) 3 (SO<sup>4</sup> ) 3 OH], (iii) sphene (CaTiSiO<sup>5</sup>


1 Reported in McLennan [4].

PAAS is Post-Archean Australian Average Shale, NASC is North American Shale Composite.

2 Reported in Kabata-Pendias [5].

(bdl) is below detection limit.

**Table 2.** Rare earth element abundances for various parent materials.

where Y and REE replace Ca), (iv) Zircon (ZrSiO<sup>4</sup> where Y and HREE replace Zr), (v) apatite (Ώ<sup>5</sup> (XO<sup>4</sup> ) 3 (F,OH,Cl); Ώ + =Ca,Be,Ce,Pb and Y and REE replace Ca), (vi) monazite ((CeLa)PO<sup>4</sup> ), (vii) xenotime (YPO<sup>4</sup> where REE replace Y), (viii) rhabdophane ((Ce,La)PO4 and REE replace La), and (ix) bastnaesite (LaREE fluorocarbonate).

The corresponding REE distribution from soil water extracts from the Menfro series closely

**Figure 2.** REE water extract concentration distribution in two paired soil profiles of the Menfro series (fine-silty, mixed,

superactive, mesic Typic Hapludalfs). (error bars are standard deviation). (Source: Data originally in [9]).

**Figure 1.** REE concentration distribution in two paired soil profiles of the Menfro series. (error bars are standard

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The hydrolysis of REE3+ species has been extensively investigated and numerous authors have published hydrolysis data [10–14]. For example, Eu3+ will undergo hydrolysis to

**4. Chemical reactivity of the rare earth elements in the soil** 

correspond to the whole soil REE distribution.

deviation). (Source: Data originally in [9]).

**4.1. REE hydrolysis and complexation reactions**

**environment**

## **3. Rare earth element soil abundances**

Rare earth element abundances in soils are influenced by (i) parent materials and organic matter contents, (ii) soil texture, (iii) pedogenic processes, and (iv) anthropogenic activities [5]. As with mineral assemblies, the soil LREE concentrations are generally greater than the soil HREE. Menfro soil series exists on uplands along the confluence of the Missouri and Mississippi Rivers (USA) and are developed in thick loess deposits. These well drained soils exhibit an A – E – Bt – C horizon sequence with acidification, Ca leaching and clay lessivage the dominant soil processes. The REE distribution shows that the light rare earth elements (La to Eu) are more abundant than the heavy REEs (Gd to Lu) and the distribution follows the Oddo-Harken rule. **Figures 1** and **2**

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**Figure 1.** REE concentration distribution in two paired soil profiles of the Menfro series. (error bars are standard deviation). (Source: Data originally in [9]).

**Figure 2.** REE water extract concentration distribution in two paired soil profiles of the Menfro series (fine-silty, mixed, superactive, mesic Typic Hapludalfs). (error bars are standard deviation). (Source: Data originally in [9]).

The corresponding REE distribution from soil water extracts from the Menfro series closely correspond to the whole soil REE distribution.

## **4. Chemical reactivity of the rare earth elements in the soil environment**

#### **4.1. REE hydrolysis and complexation reactions**

where Y and REE replace Ca), (iv) Zircon (ZrSiO<sup>4</sup>

**Table 2.** Rare earth element abundances for various parent materials.

Y 27 27 25

PAAS is Post-Archean Australian Average Shale, NASC is North American Shale Composite.

La), and (ix) bastnaesite (LaREE fluorocarbonate).

**3. Rare earth element soil abundances**

Oddo-Harken rule. **Figures 1** and **2**

(Ώ<sup>5</sup> (XO<sup>4</sup> ) 3

1

52 Lanthanides

2

(vii) xenotime (YPO<sup>4</sup>

Reported in McLennan [4].

Reported in Kabata-Pendias [5]. (bdl) is below detection limit.

where Y and HREE replace Zr), (v) apatite

),

(F,OH,Cl); Ώ + =Ca,Be,Ce,Pb and Y and REE replace Ca), (vi) monazite ((CeLa)PO<sup>4</sup>

**Element PAAS1 NASC1 Loess1 Soil2**

La 38.2 32 35.4 26.1 Ce 79.6 73 78.6 48.7 Pr 8.83 7.9 8.46 7.6 Nd 33.9 33 33.9 19.5 Sm 5.55 5.7 6.38 4.8 Eu 1.08 1.24 1.18 1.2 Gd 4.66 5.2 4.61 6.0 Tb 0.774 0.85 0.81 0.7 Dy 4.68 5.8 4.82 3.7 Ho 0.991 1.04 1.01 1.1 Er 2.85 3.4 2.85 1.6 Tm 0.405 0.5 bdl 0.5 Yb 2.82 3.1 2.71 2.1 Lu 0.433 0.48 bdl 0.3

**mg/kg**

Rare earth element abundances in soils are influenced by (i) parent materials and organic matter contents, (ii) soil texture, (iii) pedogenic processes, and (iv) anthropogenic activities [5]. As with mineral assemblies, the soil LREE concentrations are generally greater than the soil HREE. Menfro soil series exists on uplands along the confluence of the Missouri and Mississippi Rivers (USA) and are developed in thick loess deposits. These well drained soils exhibit an A – E – Bt – C horizon sequence with acidification, Ca leaching and clay lessivage the dominant soil processes. The REE distribution shows that the light rare earth elements (La to Eu) are more abundant than the heavy REEs (Gd to Lu) and the distribution follows the

where REE replace Y), (viii) rhabdophane ((Ce,La)PO4 and REE replace

The hydrolysis of REE3+ species has been extensively investigated and numerous authors have published hydrolysis data [10–14]. For example, Eu3+ will undergo hydrolysis to produce Eu(OH)2+, Eu(OH)<sup>2</sup> + , Eu(OH)<sup>3</sup> and Eu(OH)<sup>4</sup> − , having log K° constants log K11° = −7.64, log K12° = −15.1, log K13° = −23.7, log K14° = −36.2, respectively [13]. Nd and Yb hydrolysis speciation as a function of pH illustrates that the Nd3+ and Yb3+ species are the dominant species in acidic and near-neutral pH environments, whereas the Nd and Yb mono- and di-hydroxy species are the dominant species in alkaline and Nd(OH)<sup>3</sup> , Nd(OH)<sup>4</sup> − Yb(OH)<sup>3</sup> , and Yb(OH)<sup>4</sup> − are the dominant species in strongly alkaline pH environments (**Figures 3** and **4**). The hydrolysis speciation of any REE3+ species is like that of Eu3+, with a necessary understanding that the relative stabilities of the various REE hydrolytic species are more stable on transition with increasing atomic number across the Lanthanide series (**Table 3**).

Complexation of the REE elements involves coordination with primarily anionic species and typically is expressed as:

$$\text{REE}^{\text{3+}} + \text{yL}^{\text{n-}} = \text{REE} - \text{L}\_{\text{y}}^{\text{(3-yn)}}\text{y}$$

where Ln− is an inorganic ligand with n ionic charge and y is the stoichiometric coefficient. Common inorganic complexing species with REE3+ include NO<sup>3</sup> − , Cl<sup>−</sup> , F− , SO<sup>4</sup> 2−, CO3 2−, and HPO<sup>4</sup> 2−. Carbonate and dicarbonate complexes exist, with carbonate complexes more prevalent in the LREEs and dicarbonate complexes more prevalent in the HREE [11, 16, 17]. Luo and Byrne [18] documented the carbonate complexing behavior of the REE. Cantrell and Byrne [16] estimated that 86% of the La speciation existed as a dicarbonate complex, whereas 98% of the Lu speciation occurred as the dicarbonate complex. Thus, for the Lanthanide Series, the dicarbonate complex becomes increasingly more stable with increasing atomic number. For illustration purposes, the La speciation involving carbonate complexes of water in equilibrium

**Figure 4.** Aqueous hydroxyl speciation of Yb(III) over a pH interval. The Yb speciation involved concentrations without recourse to activity coefficients and overall formation quotients from Baes and Mesmer [10]. The total Yb concentration

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**)2 log Oxalate3 log HPO4**

**<sup>1</sup> Infinite dilution stability constants log CO3 log (CO3**

La −8.5 6.82 11.31 5.87 4.87 Ce −8.3 6.95 11.50 5.97 4.98 Pr −8.1 7.03 11.65 6.25 5.08 Nd −8.0 7.13 11.80 6.31 5.18 Sm −7.9 7.30 12.11 6.43 5.35 Eu −7.8 7.37 12.24 6.52 5.42 Gd −8.0 7.44 12.39 6.53 5.49 Tb −7.9 7.50 12.52 6.63 5.54 Dy −8.0 7.55 12.65 6.74 5.6 Ho −8.0 7.59 12.77 6.77 5.64 Er −7.9 7.63 12.88 6.83 5.68 Tm −7.7 7.66 13.00 6.89 5.71 Yb −7.7 7.67 13.08 6.95 5.73 Lu −7.6 7.70 13.20 6.96 5.75

Y −7.7 6.66

Mono-oxalato complexation constants at infinite dilution from Schijf and Byrne [15].

Q11 is the overall formation quotient for a hydrolysis product, Ln(OH)2+.

Carbonate-bicarbonate, phosphate, fluoride reported in Millero [11].

**Table 3.** Hydrolysis and complexation constants for the La, REEs and Y.

was 10−6 M.

1

2

3

4

Baes and Mesmer [10].

**Element Log Q1,1**

**Figure 3.** Aqueous hydroxyl speciation of Nd(III) over a pH interval. The Nd speciation involved concentrations without recourse to activity coefficients and overall formation quotients from Baes and Mesmer [10]. The total Nd concentration was 10−6 M.

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**Figure 4.** Aqueous hydroxyl speciation of Yb(III) over a pH interval. The Yb speciation involved concentrations without recourse to activity coefficients and overall formation quotients from Baes and Mesmer [10]. The total Yb concentration was 10−6 M.


1 Q11 is the overall formation quotient for a hydrolysis product, Ln(OH)2+.

2 Baes and Mesmer [10].

produce Eu(OH)2+, Eu(OH)<sup>2</sup>

−

typically is expressed as:

and Yb(OH)<sup>4</sup>

54 Lanthanides

HPO<sup>4</sup>

was 10−6 M.

+

REE3+ + yL<sup>n</sup><sup>−</sup> = REE − L<sup>y</sup>

Common inorganic complexing species with REE3+ include NO<sup>3</sup>

, Eu(OH)<sup>3</sup>

di-hydroxy species are the dominant species in alkaline and Nd(OH)<sup>3</sup>

and Eu(OH)<sup>4</sup>

log K12° = −15.1, log K13° = −23.7, log K14° = −36.2, respectively [13]. Nd and Yb hydrolysis speciation as a function of pH illustrates that the Nd3+ and Yb3+ species are the dominant species in acidic and near-neutral pH environments, whereas the Nd and Yb mono- and

and **4**). The hydrolysis speciation of any REE3+ species is like that of Eu3+, with a necessary understanding that the relative stabilities of the various REE hydrolytic species are more stable on transition with increasing atomic number across the Lanthanide series (**Table 3**). Complexation of the REE elements involves coordination with primarily anionic species and

where Ln− is an inorganic ligand with n ionic charge and y is the stoichiometric coefficient.

**Figure 3.** Aqueous hydroxyl speciation of Nd(III) over a pH interval. The Nd speciation involved concentrations without recourse to activity coefficients and overall formation quotients from Baes and Mesmer [10]. The total Nd concentration

2−. Carbonate and dicarbonate complexes exist, with carbonate complexes more prevalent in the LREEs and dicarbonate complexes more prevalent in the HREE [11, 16, 17]. Luo and Byrne [18] documented the carbonate complexing behavior of the REE. Cantrell and Byrne [16] estimated that 86% of the La speciation existed as a dicarbonate complex, whereas 98% of the Lu speciation occurred as the dicarbonate complex. Thus, for the Lanthanide Series, the dicarbonate complex becomes increasingly more stable with increasing atomic number. For illustration purposes, the La speciation involving carbonate complexes of water in equilibrium

−

are the dominant species in strongly alkaline pH environments (**Figures 3**

(3−yn) ,

> − , Cl<sup>−</sup> , F− , SO<sup>4</sup>

, having log K° constants log K11° = −7.64,

, Nd(OH)<sup>4</sup>

−

2−, CO3

2−, and

Yb(OH)<sup>3</sup>

,

3 Carbonate-bicarbonate, phosphate, fluoride reported in Millero [11].

4 Mono-oxalato complexation constants at infinite dilution from Schijf and Byrne [15].

**Table 3.** Hydrolysis and complexation constants for the La, REEs and Y.

with typical atmospheric concentrations of CO<sup>2</sup> are displayed in **Figure 5**. Similarly, the REE-Phosphate complex distribution as a pH function for La is displayed and shows that La3+ and La(HPO<sup>4</sup> ) are the dominant species (**Figure 6**).

The hydrolysis, carbonate and EDTA ligand complex, and solubility products for La, Eu, and Lu (**Table 4**) show the expected trend of lanthanide contraction.

Millero [11] and Gramaccioli et al. [20] observed that REE-fluoride complexes obtained greater stability on transition from La to Lu. REE-phosphate precipitates have been implicated in limiting the mobility of the REE in soils and sediments [9].

#### **4.2. Reactions involving organic complexation**

Common organic complexes include: oxalic acid, malic acid and other low molecular weight organic acids and the semi-stable humus components fulvic and humic acids [15, 21–25]. Tyler and Olsson [26] reported that between 46 and 74% of the REEs extracted from the soil water of a Cambisol were associated with dissolved organic carbon. As with the inorganic REE complexes, organic REE complexes tend to show greater stability for the HREEs than the LREEs [15, 16].

Cteiner [27] observed monazite (NdPO<sup>4</sup> ) reactivity at low temperatures and low ionic strength to determine the influence of Cl− , HCO<sup>3</sup> − , SO<sup>4</sup> 2−, oxalate and acetate on solubility. At pH levels ranging from 6.0 to 6.5 Nd(oxalate) was the dominant species, followed by Nd3+ and NdSO<sup>4</sup> + . Gu et al. [21] independently proposed that organic materials may have multiple binding sites with a range of complexing bond strengths that strongly retain REE at low concentrations and provide non-specific REE retention at higher concentrations.

The role of dissolved organic matter and element mobility is an active area of research. In a plot experiment, the release of La, Ce, Gd and Y decreased gradually as the pH of the soil was

**Reaction log β, Ksp, Log K**

<sup>+</sup> −17.9

<sup>−</sup> −26.2

)]+ 5.00

)]+ 5.76

)]+ 6.02

**Table 4.** Selected constants involving lanthanum, europium and lutetium with hydroxide, carbonate and EDTA

− 14.48

16.23

18.19

La3+ + OH− = La(OH)<sup>2</sup> <sup>+</sup> \ −9.1

La(OH)3(s) = La3+ + 3OH− −20.3 Eu3+ + OH− = La(OH)2+ −8.4

Eu(OH)3(s) = Eu3+ + 3OH− −24.5 Lu3+ + OH− = Lu(OH)2+ −8.0 La(OH)3(s) = Lu3+ + 3OH− −25.1

La3+ + EDTA4− = [La(EDTA) ]

Eu3+ + EDTA4− = [Eu(EDTA)] <sup>−</sup>

Lu3+ + EDTA4− = [Lu(EDTA)] <sup>−</sup>

La3+ + 2OH− = La(OH)<sup>2</sup>

Eu3+ + 4OH− = La(OH)<sup>4</sup>

2− = [La(CO<sup>3</sup>

2− = [Eu(CO<sup>3</sup>

2− = [Lu(CO<sup>3</sup>

Source: Smith and Martell [14].

(ethylenediaminetetraacetate).

La3+ + CO3

Eu3+ + CO3

Lu3+ + CO3

**Figure 6.** Aqueous carbonate speciation of La(III). The La speciation involved concentrations with activity coefficients determined using by the Debye-Hückel equation. The phosphate carbonate complexes are located in Millero [11].

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**Figure 5.** Aqueous hydroxyl and carbonate speciation of La(III). The La speciation involved concentrations with activity coefficients determined using by the Debye-Hückel equation. The carbonate complexation constants from Luo and Byrne [18] and acid dissociation constants for carbonic acid and bicarbonate from Essington [19].

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**Figure 6.** Aqueous carbonate speciation of La(III). The La speciation involved concentrations with activity coefficients determined using by the Debye-Hückel equation. The phosphate carbonate complexes are located in Millero [11].



**Table 4.** Selected constants involving lanthanum, europium and lutetium with hydroxide, carbonate and EDTA (ethylenediaminetetraacetate).

**Figure 5.** Aqueous hydroxyl and carbonate speciation of La(III). The La speciation involved concentrations with activity coefficients determined using by the Debye-Hückel equation. The carbonate complexation constants from Luo and

Byrne [18] and acid dissociation constants for carbonic acid and bicarbonate from Essington [19].

with typical atmospheric concentrations of CO<sup>2</sup>

**4.2. Reactions involving organic complexation**

Cteiner [27] observed monazite (NdPO<sup>4</sup>

to determine the influence of Cl−

) are the dominant species (**Figure 6**).

Lu (**Table 4**) show the expected trend of lanthanide contraction.

, HCO<sup>3</sup> − , SO<sup>4</sup>

provide non-specific REE retention at higher concentrations.

limiting the mobility of the REE in soils and sediments [9].

La(HPO<sup>4</sup>

56 Lanthanides

LREEs [15, 16].

Phosphate complex distribution as a pH function for La is displayed and shows that La3+ and

The hydrolysis, carbonate and EDTA ligand complex, and solubility products for La, Eu, and

Millero [11] and Gramaccioli et al. [20] observed that REE-fluoride complexes obtained greater stability on transition from La to Lu. REE-phosphate precipitates have been implicated in

Common organic complexes include: oxalic acid, malic acid and other low molecular weight organic acids and the semi-stable humus components fulvic and humic acids [15, 21–25]. Tyler and Olsson [26] reported that between 46 and 74% of the REEs extracted from the soil water of a Cambisol were associated with dissolved organic carbon. As with the inorganic REE complexes, organic REE complexes tend to show greater stability for the HREEs than the

ranging from 6.0 to 6.5 Nd(oxalate) was the dominant species, followed by Nd3+ and NdSO<sup>4</sup>

Gu et al. [21] independently proposed that organic materials may have multiple binding sites with a range of complexing bond strengths that strongly retain REE at low concentrations and

are displayed in **Figure 5**. Similarly, the REE-

) reactivity at low temperatures and low ionic strength

2−, oxalate and acetate on solubility. At pH levels

+ . raised from strongly acidic to alkaline pH ranges [28]. Davranche et al. [29] demonstrated that REEs and humic acid complexes frequently dominate soil aqueous systems, especially in near-neutral pH levels and at greater dissolved organic carbon concentrations. Pourret et al. [30] observed the strong competitive interaction between humic acids and carbonates for REE complexation, especially at increasing pH levels. Similarly, Wu et al. [24] described the strong competition from EDTA, humic and fulvic acids influencing lanthanum adsorption onto goethite as a pH function.

exchangeable, and carbonate-organic fractions resulting from a selective-sequential extraction protocol were effective predictors of REE uptake in alfalfa (*Medicago sativa.* L). Wu et al. [25] isolated sap from xylem from non-hyperaccumulating REE plants to discover that aspartic acid, asparagine, histidine and glutamic acid were correlated with La and Y xylem transport. Tyler and Olsson [41] showed that the majority of the REE were 40–50% removed from the A and E horizons of a Swedish Haplic Podzol. In a subsequent investigation Tyler [42] performed a *Fagus sylvatica* growth study and demonstrated only incidental REE uptake, except for Eu which was preferentially accumulated, mostly likely as Eu2+. Soil liming has been shown to reduce REE concentrations in soil solution [43]. Tyler and Olsson [35] documented

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Aide (unpublished research) employed a 45 mμ filtered water leach extraction on a series of Endoaqualfs (poorly drained Alfisols) and Eutrochepts (somewhat poorly-drained Inceptisols) in southeastern Missouri to show REE availability (**Figure 7**). Cerium was consistently the most abundant REE leached from the soils, followed by La and Nd. The LREE had greater leachate concentrations than the HREE. REE compliance with the Oddo-Harkin's rule

Loell et al. [44] employed total and EDTA extractions to infer bioavailability and reported that Ce had the greatest total concentration and the lowest bioavailability, whereas Y had the highest availability expression. Using regression analysis, the REE bioavailability was a function of pH, clay content, organic carbon and the total REE concentration. Mihajlovic et al. [45] observed the vertical distribution of REE in marshland soils using selective sequential extractions and documented that the residual fraction exhibited the largest REE abundance, followed by the reducible fraction. They also reported that the LREE were more abundant than the HREE, that the HREE exhibited the greater tendency to leach because of complex formation and the HREE were relatively more abundant in the exchangeable/available fractions.

**Figure 7.** Soil water extract concentrations from two great groups in Missouri. The Endoaqualfs represent 27 observations,

substantial REE plant uptake of grass grown in a Cambisol.

was consistently observed.

whereas the Eutrudepts represent 24 observations.
