**3. Distribution of rare earth elements in soils and earth materials**

REE concentrations in soils, sediments, and other earth materials are dependent on their mineral assemblage and source area, with REE concentrations typically ranging from 0.1 to 100 mg/kg. In general, felsics have greater REE concentrations and greater LREE/HREE ratios than mafics. As expected, fine-grained clastic sediments frequently exhibit 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. The REEs typically obey the Oddo-Harkins rule. The Post-Archean Australian average shale (PAAS), North American shale composite (NASC), selective representative soil collections, and selected geochemical soil surveys usually reflect the Oddo-Harkins rule [16–18] (**Table 1**).

Commonly occurring REE-bearing minerals include (i) fluorite (Ce replaces Ca), (ii) allanite (Ce), (iii) sphene (REE replace Ca), (iv) zircon (HREE replace Zr), (v) apatite (REE replace Ca), (vi) monazite ((CeLa) phosphate),

**5**

*1*

*2*

*3*

**Table 1.**

*Reported in McLennan [16].*

*Reported in Liang et al. [18].*

*Reported in Kabata-Pendias [17].*

−4.904 with r2

(dissolved) +0.624 with r2

*Review and Assessment of Organic and Inorganic Rare Earth Element Complexation in Soil…*

**Element PAAS1 NASC1 Soil2 Chinese soils3**

La 38.2 32 26.1 37.4 Ce 79.6 73 48.7 64.7 Pr 8.83 7.9 7.6 6.67 Nd 33.9 33 19.5 25.1 Sm 5.55 5.7 4.8 4.94 Eu 1.08 1.24 1.2 0.98 Gd 4.66 5.2 6.0 4.38 Tb 0.774 0.85 0.7 0.58 Dy 4.68 5.8 3.7 3.93 Ho 0.991 1.04 1.1 0.83 Er 2.85 3.4 1.6 2.42 Tm 0.405 0.5 0.5 0.35 Yb 2.82 3.1 2.1 2.32 Lu 0.433 0.48 0.3 0.35

**mg/kg**

(vii) xenotime (REE—phosphate), (viii) rhabdophane (Ce, REE—phosphate), and (ix) bastnaesite (REE fluorocarbonate). As with many mineral assemblies, the soil LREE concentrations are generally greater than the soil HREE; however, mineral assemblages featuring an abundance of zircon may differ in the LREE/HREE.

= 0.91] and the Kundulum River segment [suspended matter = 0.787

= 0.95] infer that the respective suspended and dissolved

**4. Rare earth element abundances in natural waters: river water**

*PAAS is Post-Archean Australian average shale; NASC is North American shale composite.*

*Rare earth element abundances for various parent materials.*

Natural waters include marine, river, lacustrine, and groundwater. Considerations for characterizing natural water REE concentrations include (1) the total REE concentration; (2) suspended minerals having adsorbed, occluded, or latticed REE; (3) organically complexed REE; and (4) soluble REE3+ and their hydrolytic and ion pair products. Liang et al. [18] cited literature references for river waters in China. 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-Harkins rule. These authors also compared rivers having either pristine and REE impaction because of REE mining activities (**Table 2**). The REE concentrations because of mining activity were intense, underscoring the environmental impact. Linear regression by the author of this manuscript of Liang et al. [18] river water data for dissolved and suspended REE load shows substantial correlation between the dissolved and suspended concentrations for all REEs. The linear relationship for the Yellow River segment [suspended matter = 317.86 (dissolved)

REE concentrations arise from similar chemical adsorption relationships.

*DOI: http://dx.doi.org/10.5772/intechopen.87033*


*Review and Assessment of Organic and Inorganic Rare Earth Element Complexation in Soil… DOI: http://dx.doi.org/10.5772/intechopen.87033*

*1 Reported in McLennan [16].*

*2 Reported in Kabata-Pendias [17].*

*3 Reported in Liang et al. [18].*

*PAAS is Post-Archean Australian average shale; NASC is North American shale composite.*

### **Table 1.**

*Rare Earth Elements and Their Minerals*

the lanthanide series.

ily anionic species, and it is expressed as.

concerning rare earth element hydrolysis.

[14], and Pourret et al. [15].

REE3+ + yLn− = REE − Ly

usually reflect the Oddo-Harkins rule [16–18] (**Table 1**).

in the ionic radii on progression from La to Lu, the so-called lanthanide contraction. The "lanthanide contraction" occurs because of the incomplete electric field shielding by the f orbitals and increases in atomic number, supporting greater chemical affinity for hydrolysis and chelate/complex stability on progression across the lanthanide series [1]. The LREE are the light rare earth elements, comprised of the elements La to Eu, and the HREE are the heavy rare earth elements, comprised of the elements Gd to Lu. In some cases, The REEs have been partitioned as (i) the light REE (LREE includes La, Ce, Pr), (ii) the middle REE (MREE includes Nd, Sm,

The hydrolysis of REE3+ species has been extensively investigated. The primary thermodynamic literature featuring data involving REE3+ hydrolysis and inorganic complexation reactions include Baes and Mesmer [2], Hummel et al. [3], Smith and Martel [4], Schijf and Byrne [5], Luo and Byrne [6], Cantrell and Byrne [7], Gramaccioli et al. [8], Lee and Byrne [9], and Millero [10]. Klungness and Byrne [11] noted that REE hydrolysis is more stable with increasing atomic number across

Inorganic complexation of the REE elements involves coordination with primar-

where L<sup>n</sup><sup>−</sup> is an inorganic ligand with n ionic charge and y is the stoichiometric coefficient. For the lanthanide series, the dicarbonate complex becomes increasingly more stable with increasing atomic number [6, 7, 9]. Both hydrolysis and carbonate complexation show the expected increasing stability with increasing atomic number across the lanthanide series [12]. Aide [12] reviewed thermodynamic data

Common low-molecular-weight organic complexes include acetic acid, phthalic acid, oxalic acid, lactic acid, malic acid, and citric acid. Humus components typically include fulvic and humic acids. The seminal literature featuring thermodynamic data involving REE3+ organic complexation include Gu et al. [13], Dong et al.

REE concentrations in soils, sediments, and other earth materials are dependent on their mineral assemblage and source area, with REE concentrations typically ranging from 0.1 to 100 mg/kg. In general, felsics have greater REE concentrations and greater LREE/HREE ratios than mafics. As expected, fine-grained clastic sediments frequently exhibit 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. The REEs typically obey the Oddo-Harkins rule. The Post-Archean Australian average shale (PAAS), North American shale composite (NASC), selective representative soil collections, and selected geochemical soil surveys

Commonly occurring REE-bearing minerals include (i) fluorite (Ce replaces Ca),

(ii) allanite (Ce), (iii) sphene (REE replace Ca), (iv) zircon (HREE replace Zr), (v) apatite (REE replace Ca), (vi) monazite ((CeLa) phosphate),

**3. Distribution of rare earth elements in soils and earth materials**

(3−yn)

, (1)

Eu, and Gd), and (iii) the heavy REE (HREE includes Tb to Lu).

**2. Hydrolysis and complexation thermodynamic data**

**4**

*Rare earth element abundances for various parent materials.*

(vii) xenotime (REE—phosphate), (viii) rhabdophane (Ce, REE—phosphate), and (ix) bastnaesite (REE fluorocarbonate). As with many mineral assemblies, the soil LREE concentrations are generally greater than the soil HREE; however, mineral assemblages featuring an abundance of zircon may differ in the LREE/HREE.

## **4. Rare earth element abundances in natural waters: river water**

Natural waters include marine, river, lacustrine, and groundwater. Considerations for characterizing natural water REE concentrations include (1) the total REE concentration; (2) suspended minerals having adsorbed, occluded, or latticed REE; (3) organically complexed REE; and (4) soluble REE3+ and their hydrolytic and ion pair products. Liang et al. [18] cited literature references for river waters in China. 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-Harkins rule. These authors also compared rivers having either pristine and REE impaction because of REE mining activities (**Table 2**). The REE concentrations because of mining activity were intense, underscoring the environmental impact. Linear regression by the author of this manuscript of Liang et al. [18] river water data for dissolved and suspended REE load shows substantial correlation between the dissolved and suspended concentrations for all REEs. The linear relationship for the Yellow River segment [suspended matter = 317.86 (dissolved) −4.904 with r2 = 0.91] and the Kundulum River segment [suspended matter = 0.787 (dissolved) +0.624 with r2 = 0.95] infer that the respective suspended and dissolved REE concentrations arise from similar chemical adsorption relationships.


### **Table 2.**

*Rare earth element concentrations documented for two Chinese rivers.*

River waters typically have greater REE concentrations than marine waters because of their suspended load and a greater abundance of dissolved organic material [19–22]. Dupre et al. [19] observed that the REEs were primarily associated with suspended inorganic and organic colloids. Garcia et al. [20] studied river waters in Argentina draining predominately granitic landscapes, showing that high-rainfall periods effectively reduced or "diluted" REE concentrations. Andersson et al. [21] proposed that organic colloidal materials were largely responsible for REE transport in boreal Swedish river waters and that the LREE were more abundant than the HREE. Ingri et al. [22] demonstrated that the La concentrations in Swedish boreal river waters were seasonal and were associated with organic and Fe-oxyhydroxide inorganic colloidal material.

Gurumurthy et al. [23] documented 3 years of river discharge across southwestern India and provided river water chemistry, including rare earth elements (**Table 3**). They observed that the rare earth elements showed higher concentrations during the monsoon season as opposed to the dry season, suggesting that soil leaching across the watersheds was important to the increased monsoonal river water concentrations. Cerium anomalies were observed, pH moderated adsorptiondesorption reactions, and the dissolved oxygen concentrations were important in regulating the seasonality of the Ce anomalies. Rare earth element complexation was not highly significant in influencing the rare earth element concentrations.

Neal [24] documented La, Ce, Pr, and yttrium (Y) concentrations in the upper River Severn catchments in Mid Wales. Over a 7-year interval, larger river water concentrations of La, Ce, Pr, and Y were associated with high-rainfall events and baseflow/return flow from land parcels having acidic soil pH values, suggesting that the surrounding terrestrial environment is important to REE river chemistry. Leybourne and Johannesson [25] described that the REE adsorption affinity for stream waters and sediments was pH-dependent, with deprotonation of surface

**7**

stream compositions.

*Documented in Gurumurthy et al. [23].*

**Table 3.**

*India.*

*Review and Assessment of Organic and Inorganic Rare Earth Element Complexation in Soil…*

**Element Bantwal Gurupur Mugeru Shanthimugeru** La 854 793 1213 1136 Nd 759 765 1130 1096 Ce 1986 1867 2698 2658 Pr 198 189 291 276 Sm 162 159 216 229 Eu 46 44 62 62 Gd 148 134 209 211 Dy 112 105 166 174 Ho 27 28 34 36 Er 65 61 90 100 Tm 11 19 14 16 Yb 57 57 74 89

**)**

hydroxyl groups favoring REE adsorption at more alkaline pH intervals. With increasing pH, the adsorption potential may permit REE fractionation, with the adsorption affinity greatest for the LREE, less for the MREE, and least for the HREE. In Sweden, Ohlander et al. [26] recorded Sm/Nd ratios in the weathering of granitic till, noting Sm/Nd differences in the upper eluvial soil horizons relative to the deeper less weathered till. Weathering intensity differences and secondary preferential placement of Nd in the deeper less weathered till influenced adjacent

*Rare earth elements discharge-weighted mean averages of rare earth elements in river waters from southwestern* 

**5. Rare earth element abundances in natural waters: groundwater**

The total rare earth element concentrations in groundwater may be partitioned into (i) dissolved or free ion species that may include hydrolysis products and inorganic complexes, (ii) low-molecular-weight organic ligands and moderate- to largemolecular-weight chelates (e.g., humic and fulvic acids (FA)), and (iii) clastic colloids (e.g., phyllosilicates and Fe-oxyhydroxides) [27–33]. Groundwater may frequently exhibit a seasonal range in total REE concentrations [27]. Dia et al. [27] documented REE, dissolved organic carbon (DOC), and trace metals in well waters from a French catchment, noting that spatially distinct groundwaters may be partitioned based on DOC content and other hydrologic variables. Ultrafiltration of the distinct groundwaters reveals that the REE concentrations in the organic-rich waters were more associated with organic colloids, whereas the REEs in groundwaters having small DOC concentrations were more associated with inorganic colloids. Similarly, Pourret et al. [28], working with the same catchment as Dia et al. [27], employed ultrafiltration techniques and species modeling using the humic ion-binding model VI to show that (i) the smaller REE concentrations in ultrafiltration waters were attributed to the removal of REE-bearing organic colloids and (ii) modeling suggests that the lanthanum complexes were dominated by humic acids (80%) and subordinately with fulvic

*DOI: http://dx.doi.org/10.5772/intechopen.87033*

**Southwestern Indian rivers (pmol L<sup>−</sup><sup>1</sup>**

*Review and Assessment of Organic and Inorganic Rare Earth Element Complexation in Soil… DOI: http://dx.doi.org/10.5772/intechopen.87033*


### **Table 3.**

*Rare Earth Elements and Their Minerals*

**Dissolved (μg L<sup>−</sup><sup>1</sup>**

*Rare earth element concentrations documented for two Chinese rivers.*

River waters typically have greater REE concentrations than marine waters because of their suspended load and a greater abundance of dissolved organic material [19–22]. Dupre et al. [19] observed that the REEs were primarily associated with suspended inorganic and organic colloids. Garcia et al. [20] studied river waters in Argentina draining predominately granitic landscapes, showing that high-rainfall periods effectively reduced or "diluted" REE concentrations. Andersson et al. [21] proposed that organic colloidal materials were largely responsible for REE transport in boreal Swedish river waters and that the LREE were more abundant than the HREE. Ingri et al. [22] demonstrated that the La concentrations in Swedish boreal river waters were seasonal and were associated with organic and Fe-oxyhydroxide

**) Suspended matter (μg g<sup>−</sup><sup>1</sup>**

**Yellow Kundulum Yellow Kundulum**

La 0.10 140 33.16 86.16 Ce 0.22 152 68.26 139.3 Pr 0.034 16.1 8.47 15.8 Nd 0.095 52.0 28.7 52.0 Sm 0.054 6.91 5.79 6.61 Eu 0.018 1.52 1.31 1.20 Gd 0.028 7.22 6.29 4.31 Tb 0.006 0.88 0.82 0.97 Dy 0.08 3.80 3.39 1.87 Ho 0.014 0.18 0.77 0.68 Er 0.03 2.34 2.25 1.36 Tm 0.007 0.24 0.25 0.22 Yb 0.032 1.48 1.76 1.01 Lu 0.006 0.21 0.25 0.22

**)**

Gurumurthy et al. [23] documented 3 years of river discharge across southwestern India and provided river water chemistry, including rare earth elements (**Table 3**). They observed that the rare earth elements showed higher concentrations during the monsoon season as opposed to the dry season, suggesting that soil leaching across the watersheds was important to the increased monsoonal river water concentrations. Cerium anomalies were observed, pH moderated adsorptiondesorption reactions, and the dissolved oxygen concentrations were important in regulating the seasonality of the Ce anomalies. Rare earth element complexation was not highly significant in influencing the rare earth element concentrations. Neal [24] documented La, Ce, Pr, and yttrium (Y) concentrations in the upper River Severn catchments in Mid Wales. Over a 7-year interval, larger river water concentrations of La, Ce, Pr, and Y were associated with high-rainfall events and baseflow/return flow from land parcels having acidic soil pH values, suggesting that the surrounding terrestrial environment is important to REE river chemistry. Leybourne and Johannesson [25] described that the REE adsorption affinity for stream waters and sediments was pH-dependent, with deprotonation of surface

**6**

inorganic colloidal material.

*Documented in Liang et al. [18].*

**Table 2.**

*Rare earth elements discharge-weighted mean averages of rare earth elements in river waters from southwestern India.*

hydroxyl groups favoring REE adsorption at more alkaline pH intervals. With increasing pH, the adsorption potential may permit REE fractionation, with the adsorption affinity greatest for the LREE, less for the MREE, and least for the HREE. In Sweden, Ohlander et al. [26] recorded Sm/Nd ratios in the weathering of granitic till, noting Sm/Nd differences in the upper eluvial soil horizons relative to the deeper less weathered till. Weathering intensity differences and secondary preferential placement of Nd in the deeper less weathered till influenced adjacent stream compositions.
