**7. Results and discussion**

Soils of the Sharkey series (very-fine, smectitic, thermic chromic epiaquerts) have Ap-Bssg-Bssyg horizon sequences, and soils of the Lilbourn series (coarseloamy, mixed, superactive, nonacid, thermic aeric fluvaquents) have Ap-C horizon sequences. The Sharkey and Lilbourn soil series are composed of Holocene fluvial sediments from the ancestral Mississippi/Ohio rivers in southeastern Missouri (USA). The clayey-textured Sharkey soil series shows greater REE concentrations than the coarse-textured Lilbourn series, and both series exhibit appreciably greater than unity LREE/HREE concentration ratios. In general, the REE distributions obey the Oddo-Harkins rule. REE water extract concentrations are an approximate estimate of soil REE activity. As expected, the water extract concentrations for the Sharkey and Lilbourn soil series are approximately two to three orders of magnitude smaller than the aqua regia digestion extract concentrations


*Ap is the surface horizon and Bssg, Bssyg, and C4 are subsurface horizons. [unpublished soil data from the author of this manuscript].*

### **Table 5.**

*Soil rare earth element abundances for soil (mg kg<sup>−</sup><sup>1</sup> ) and water extract (μg kg<sup>−</sup><sup>1</sup> ).*

**Figure 1.**

*The relationship between total rare earth element concentrations (x-axis with units of mg kg<sup>−</sup><sup>1</sup> ) and water extractable rare earth element concentrations (y-axis with units of μg kg<sup>−</sup><sup>1</sup> ) for the Sharkey soil series.*

(**Table 5**). The REE distribution of the water extracts closely parallels the REE distribution of the aqua regia digestion distribution, inferring that (i) the REE release to water is influenced by the REE abundance regardless of atomic number and (ii) the water-absorbent partitioning is not strongly influenced by soil profile position (**Figure 1**).

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. As with the Sharkey and Lilbourn

**11**

*this manuscript].*

**Table 6.**

**Figure 2.**

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

*The relationship between total rare earth element concentrations (x-axis with units of mg kg<sup>−</sup><sup>1</sup>*

**Soil (mg kg<sup>−</sup><sup>1</sup>**

**Ap C1 C3 C5 C7 Ap C1 C3 C5 C7**

La 16.6 16.8 16.8 16.0 18.2 15.5 40.2 25.5 29.3 27.9 Ce 43.1 43.9 43.9 42.0 43.0 30.3 65.7 31.5 80.2 30.9 Pr 4.5 4.5 4.6 4.4 4.7 4.3 8.6 6.3 7.3 7.5 Nd 19.2 18.7 19.7 18.3 20.1 16.9 33.6 25.6 29.7 29.2 Sm 3.2 3.7 2.8 3.3 3.2 3.4 6.6 5.3 6.2 6.2 Eu 0.6 0.6 0.6 0.6 0.6 0.7 1.5 1.2 1.4 1.4 Gd 3.0 2.9 2.8 2.7 2.9 3.2 6.0 5.0 5.7 5.7 Tb 0.3 0.3 0.4 0.3 0.3 0.5 0.9 0.8 0.9 0.9 Dy 2.5 2.5 2.3 2.1 2.2 2.6 5.1 4.1 4.7 4.8 Ho 0.4 0.4 0.4 0.4 0.4 0.5 1.0 0.8 0.9 0.9 Er 1.2 1.3 1.2 1.1 1.2 1.5 3.0 2.2 2.5 2.5 Tm 0.1 0.1 0.1 0.1 0.1 0.2 0.4 0.3 0.3 0.3 Yb 0.9 1.0 0.9 0.8 0.8 1.3 2.6 1.8 2.1 2.0 Lu 0.1 0.1 0.1 0.1 0.1 0.3 0.5 0.3 0.3 0.3 *Ap is the surface horizon and C1, C3, C5, and C7 are subsurface horizons. [unpublished soil data from the author of* 

*extractable rare earth element concentrations (y-axis with units of μg kg<sup>−</sup><sup>1</sup>*

soil series, the release of REEs to the water is a function of REE abundance, regardless of atomic number. The regression slope for the Kaintuck soil series (**Figure 2**) is smaller than the corresponding Sharkey soil series (**Figure 1**), suggesting that the binding relationships involving REE release to water are slightly different (**Table 6**).

*Soil rare earth element abundances for the Kaintuck soil series (mg kg<sup>−</sup><sup>1</sup>*

**8. REE simulations involving inorganic and organic complexation**

Background electrolyte concentrations were obtained from tile-drainage water at the David M. Barton Agriculture Research Center of Southeast Missouri State University. The background total elemental concentrations (mol kg-water<sup>−</sup><sup>1</sup>

)

*).*

*) and water* 

**)**

*) for the Kaintuck soil series.*

*) and associated water extract (μg kg<sup>−</sup><sup>1</sup>*

**) Water (μg kg<sup>−</sup><sup>1</sup>**

*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*

### **Figure 2.**

*Rare Earth Elements and Their Minerals*

**Soil (mg kg<sup>−</sup><sup>1</sup>**

**10**

**Figure 1.**

*this manuscript].*

*Soil rare earth element abundances for soil (mg kg<sup>−</sup><sup>1</sup>*

**Table 5.**

position (**Figure 1**).

*The relationship between total rare earth element concentrations (x-axis with units of mg kg<sup>−</sup><sup>1</sup>*

(**Table 5**). The REE distribution of the water extracts closely parallels the REE distribution of the aqua regia digestion distribution, inferring that (i) the REE release to water is influenced by the REE abundance regardless of atomic number and (ii) the water-absorbent partitioning is not strongly influenced by soil profile

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. As with the Sharkey and Lilbourn

**Sharkey soil series Lilbourn soil series**

Ap Bssg Bssyg Ap Bssg Bssyg Ap C4 Ap C4

La 24.9 24.4 24.9 43 50 41 15.5 16.8 54.5 51.1 Ce 51.1 47.6 50.8 96.3 106 86.8 29.9 34.9 554 57.6 Pr 6.4 6.1 6.2 12 15 12 3.7 4.4 13.4 14.2 Nd 25.7 24.6 24.3 51 63 49 13.8 17.0 52.8 55.9 Sm 5.0 4.9 4.7 12 15 11 2.5 3.1 11.3 13.0 Eu 1.1 1.1 1.1 3 3 2 0.5 0.7 2.4 2.9 Gd 4.9 4.8 4.6 11 14 9.6 2.1 2.8 10.8 12.0 Tb 0.7 0.7 0.6 2 2 1 0.3 0.4 1.4 1.7 Dy 3.8 3.9 3.7 7.9 10 6.9 1.57 2.12 7.3 8.4 Ho 0.7 0.7 0.7 2 2 1 0.3 0.4 1.5 1.6 Er 1.9 1.9 1.8 5 6 4 0.8 1.1 4.3 4.5 Tm 0.3 0.3 0.2 1 1 0 0.1 0.1 0.6 0.6 Yb 1.4 1.4 1.3 4 5 3 0.6 0.9 3.9 4.1 Lu 0.2 0.2 0.2 1 1 0 0 0.1 0.6 0.6 *Ap is the surface horizon and Bssg, Bssyg, and C4 are subsurface horizons. [unpublished soil data from the author of* 

**) Soil (mg kg<sup>−</sup><sup>1</sup>**

*) and water extract (μg kg<sup>−</sup><sup>1</sup>*

*).*

**) Water (μg kg<sup>−</sup><sup>1</sup>**

**)**

**) Water (μg kg<sup>−</sup><sup>1</sup>**

*extractable rare earth element concentrations (y-axis with units of μg kg<sup>−</sup><sup>1</sup>*

*) and water* 

*) for the Sharkey soil series.*

*The relationship between total rare earth element concentrations (x-axis with units of mg kg<sup>−</sup><sup>1</sup> ) and water extractable rare earth element concentrations (y-axis with units of μg kg<sup>−</sup><sup>1</sup> ) for the Kaintuck soil series.*


### **Table 6.**

*Soil rare earth element abundances for the Kaintuck soil series (mg kg<sup>−</sup><sup>1</sup> ) and associated water extract (μg kg<sup>−</sup><sup>1</sup> ).*

soil series, the release of REEs to the water is a function of REE abundance, regardless of atomic number. The regression slope for the Kaintuck soil series (**Figure 2**) is smaller than the corresponding Sharkey soil series (**Figure 1**), suggesting that the binding relationships involving REE release to water are slightly different (**Table 6**).

## **8. REE simulations involving inorganic and organic complexation**

Background electrolyte concentrations were obtained from tile-drainage water at the David M. Barton Agriculture Research Center of Southeast Missouri State University. The background total elemental concentrations (mol kg-water<sup>−</sup><sup>1</sup> )


### **Table 7.**

*La and Yb equilibria in a simulated natural water environment where La3+ or Yb3+ and their hydrolysis products are permitted speciation reactions with fulvic acid, carbonate, phosphate, sulfate, chloride, and nitrate complexes at three pH intervals and permitting precipitation of calcite, dolomite, and hydroxyapatite.*

were (i) Ca2+ was 0.0032 mol kg−<sup>1</sup> , (ii) CO3 was 0.0079 mol kg<sup>−</sup><sup>1</sup> , (iii) Mg2+ was 0.0032 mol kg<sup>−</sup><sup>1</sup> , (iv) Na<sup>+</sup> was 0.0025 mol kg<sup>−</sup><sup>1</sup> , (v) NH4 + was 2.8 × 10<sup>−</sup><sup>6</sup> mol kg<sup>−</sup><sup>1</sup> , (vi) NO3 <sup>−</sup> was 0.00032 mol kg<sup>−</sup><sup>1</sup> , (vii) PO4 was 10<sup>−</sup><sup>4</sup> mol kg<sup>−</sup><sup>1</sup> , (viii) Cl<sup>−</sup> was 10<sup>−</sup><sup>3</sup> mol kg<sup>−</sup><sup>1</sup> , (ix) DOC 4.16 × 10<sup>−</sup><sup>6</sup> mol kg<sup>−</sup><sup>1</sup> , (x) SO4 <sup>2</sup><sup>−</sup> was 10<sup>−</sup><sup>4</sup> mol kg<sup>−</sup><sup>1</sup> , (xi) La3+ was (when simulated) 3.1 × 10<sup>−</sup><sup>7</sup> mol kg<sup>−</sup><sup>1</sup> , and (xii) Yb3+ (when simulated) was 1.04 × 10<sup>−</sup><sup>9</sup> mol kg<sup>−</sup><sup>1</sup> . The ionic strength was 0.0158 mol kg<sup>−</sup><sup>1</sup> . In this simulation, hydroxyapatite, dolomite, and calcite were permitted to precipitate as finite solids.

Lanthanum and ytterbium were simulated at pH 4, 6, and 8 to estimate hydrolysis, inorganic, and fulvic acid complexation. At pH 4, La3+ and LaHCO3 2+ are the dominant species (**Table 7**), whereas fulvic acid-La complex was estimated to be present at 0.35%. At pH 8, LaCO3 + and La(CO3)2 <sup>−</sup> are the dominant species, with

**13**

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

La 0.99 18.8 0.20 48.28 31.25 Ce 0.51 — 0.31 52.49 45.82 Nd 0.22 10.90 0.19 37.67 50.93 Sm 0.12 3.97 0.23 31.39 64.24 Gd 0.14 3.83 0.27 30.29 64.90 Dy 0.06 1.22 0.21 20.10 78.31 Er 0.04 1.52 0.16 14.81 83.45 Yb 0.03 0.53 0.20 15.47 83.25

**Percent of total REE**

 *mol kg<sup>−</sup><sup>1</sup>*

**<sup>+</sup> REE(CO3)2**

*. Total REE concentrations are* 

**−**

**REE REE3+ REE-phthalic REE(OH)2+ REE(CO3)**

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

*Concentration of phthalic acid (benzene-1,2-dicarboxylic acid) is 10<sup>−</sup><sup>3</sup>*

*REE complexation with phthalic acid at pH 8.3 in calcite-saturated water.*

**Species Percent of species**

<sup>+</sup> 56.8

<sup>+</sup> 51.6

<sup>−</sup> 43.9

<sup>+</sup> 40.7

<sup>+</sup> 30.9

<sup>+</sup> 30.8

<sup>+</sup> 20.6

<sup>+</sup> 15.3

<sup>−</sup> 78.3

<sup>−</sup> 64.3

<sup>−</sup> 61.7

<sup>−</sup> 53.6

<sup>−</sup> 35.8

Fulvic acid-La+ 5.6

Fulvic acid-Ce+ 2.9

Fulvic acid-Nd+ 5.2

Fulvic acid-Sm+ 7.0

Fulvic acid-Gd+ 3.9

Fulvic acid-Dy+ 0.8

Fulvic Acid-Er+ 0.5

*3.1 × 10<sup>−</sup><sup>7</sup>*

**Table 8.**

 *mol/kg.*

Lanthanum

LaCO3

La(CO3)2

Cerium

CeCO3

NdCO3

Nd(CO3)2

Samarium

SmCO3

GdCO3

DyCO3

Dy(CO3)2

Erbium

ErCO3

Gd(CO3)2

Dysprosium

Sm(CO3)2

Gadolinium

Ce(CO3)2

Neodymium

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


*Concentration of phthalic acid (benzene-1,2-dicarboxylic acid) is 10<sup>−</sup><sup>3</sup> mol kg<sup>−</sup><sup>1</sup> . Total REE concentrations are 3.1 × 10<sup>−</sup><sup>7</sup> mol/kg.*

### **Table 8.**

*Rare Earth Elements and Their Minerals*

−log (activity) (% speciation at given pH)

−log (activity) (% speciation at given pH)

**Lanthanum speciation**

LaSO4

LaNO3

LaH2PO4

LaHCO3

LaCO3

YbSO4

YbNO3

YbHPO4

YbHCO3

YbCO3

Yb(CO3)2

Yb(SO4)2

La(CO3)2

Ytterbium speciation

**12**

were (i) Ca2+ was 0.0032 mol kg−<sup>1</sup>

was (when simulated) 3.1 × 10<sup>−</sup><sup>7</sup>

mol kg<sup>−</sup><sup>1</sup>

present at 0.35%. At pH 8, LaCO3

, (iv) Na<sup>+</sup>

<sup>−</sup> was 0.00032 mol kg<sup>−</sup><sup>1</sup>

, (ix) DOC 4.16 × 10<sup>−</sup><sup>6</sup>

0.0032 mol kg<sup>−</sup><sup>1</sup>

mol kg<sup>−</sup><sup>1</sup>

(vi) NO3

**Table 7.**

1.04 × 10<sup>−</sup><sup>9</sup>

10<sup>−</sup><sup>3</sup>

, (ii) CO3 was 0.0079 mol kg<sup>−</sup><sup>1</sup>

, (x) SO4

, (v) NH4

+

mol kg<sup>−</sup><sup>1</sup>

<sup>2</sup><sup>−</sup> was 10<sup>−</sup><sup>4</sup>

, and (xii) Yb3+ (when simulated) was

was 2.8 × 10<sup>−</sup><sup>6</sup>

<sup>−</sup> are the dominant species, with

was 0.0025 mol kg<sup>−</sup><sup>1</sup>

*Fulvic acid was set at 0.082 mg/L with estimated properties using the Stockholm Humic Model.*

mol kg<sup>−</sup><sup>1</sup>

lysis, inorganic, and fulvic acid complexation. At pH 4, La3+ and LaHCO3

+

, (vii) PO4 was 10<sup>−</sup><sup>4</sup>

Species pH 4 pH 6 pH 8

La3+ 7.02 (92.55%) 7.22 (60.62%) 8.50 (2%) LaCl2+ 9.55 (0.15%) 9.75 (0.095%) 11.10

<sup>+</sup> 7.75(0.01%) 10.66 (4.13%) 11.51 (0.34%)

2+ 9.86 (0.073%) 10.06 (0.047%) 11.32

2+ 10.66 (0.012%) 10.93 14.09

2+ 9.14 (0.389%) 7.52 (16.32%) 8.84 (0.63%)

<sup>+</sup> 11.07 7.45 (12.84%) 6.78 (57.87%)

LaOH2+ — 10.03 (0.05%) 9.31 (0.21%) FA2-La+ 8.97 (0.35%) 7.74 (5.93%) 7.27 (17.74%)

Species pH 4 pH 6 pH 8

Yb3+ 10.36 (12.72%) 11.98 (0.31%) 14.31 YbCl2+ 13.01 (0.015%) 14.03 16.93

<sup>+</sup> 11.18 (0.72%) 12.80 (0.017%) 14.91

2+ 13.49 15.12 17.42

<sup>+</sup> 13.61 13.31 15.50 YbPO4 15.09 12.79 (0.016%) 12.98 (0.01%)

2+ 12.28 (0.082%) 12.09 (0.14%) 14.45

YbOH2+ 13.60 13.22 13.55 FA2-Yb+ 9.06 (86.44%) 9.00 (98.69%) 8.99 (98.66%)

*La and Yb equilibria in a simulated natural water environment where La3+ or Yb3+ and their hydrolysis products are permitted speciation reactions with fulvic acid, carbonate, phosphate, sulfate, chloride, and nitrate complexes at three pH intervals and permitting precipitation of calcite, dolomite, and hydroxyapatite.*

<sup>+</sup> 13.33 11.14 (0.79%) 11.50 (0.33%)

<sup>−</sup> 18.62 12.62 (0.026%) 11.02 (0.99%)

<sup>−</sup><sup>1</sup> 13.90 15.52 17.41

<sup>−</sup> — 9.85 (0.052%) 7.22 (21.22%)

mol kg<sup>−</sup><sup>1</sup>

. The ionic strength was 0.0158 mol kg<sup>−</sup><sup>1</sup>

dominant species (**Table 7**), whereas fulvic acid-La complex was estimated to be

and La(CO3)2

hydroxyapatite, dolomite, and calcite were permitted to precipitate as finite solids. Lanthanum and ytterbium were simulated at pH 4, 6, and 8 to estimate hydro-

, (iii) Mg2+ was

, (viii) Cl<sup>−</sup> was

mol kg<sup>−</sup><sup>1</sup>

. In this simulation,

mol kg<sup>−</sup><sup>1</sup>

,

, (xi) La3+

2+ are the

*REE complexation with phthalic acid at pH 8.3 in calcite-saturated water.*



*All rare earth element concentrations are initially set as 0.31 × 10<sup>−</sup><sup>7</sup> mol/kg. Ionic strength was estimated at 3.85 × 10<sup>−</sup><sup>3</sup> mol/L. Fulvic acid was set at 0.082 mg/L with estimated properties using the Stockholm Humic Model.*

### **Table 9.**

*Simulation of the partitioning of selected rare earth elements between carbonate complexes and fulvic acid in stream waters in equilibrium with ordered dolomite and hydroxyapatite.*

the fulvic acid-La complex being estimated to be present at 17.7%. At pH 4, Yb3+ and fulvic acid-Yb are the dominant species (**Table 7**), with the fulvic acid-Yb complex showing 86% of the total Yb concentration. At pH 8, the fulvic acid-Yb complex was estimated to be present at 99% of the total Yb concentration.

The REEs were simulated in the presence of phthalic acid (10<sup>−</sup><sup>3</sup> mol kg<sup>−</sup><sup>1</sup> ) at pH 8.3. In this simulation dolomite, calcite, and hydroxyapatite were permitted to precipitate (**Table 8**). The REE—phthalic acid complexes as a percentage of the total REE concentration—was greatest for La and declined with increasing atomic number. The REE concentrations of REE(CO3) + and REE(CO3)2 <sup>−</sup> were the most extensive species, with the concentration of REE(CO3) + declining with increasing atomic number and the concentration of REE(CO3)2 <sup>−</sup> increasing with atomic number.

The simulation of the REE partitioning between carbonate complexes and fulvic acid in stream waters in equilibrium with ordered dolomite and hydroxyapatite was performed (**Table 9**). The fulvic acid-REE complexes generally represented less than 10% of the total REE concentration. Conversely, the concentrations of REECO3 + and REE(CO3)2 <sup>−</sup> declined and increased, respectively, on progression with increasing atomic number.

## **9. Evolution of REE studies and needs**

Kautenburger et al. [45] demonstrated that (i) humic acid and (ii) humic acid with partially blocked phenolic OH and COOH groups supported different complex stability constants, showing that humic acids with a high concentration of strong binding sites can be responsible for increased REE mobility because of dissolved negatively charged metal-humate complexes. Marang et al. [46] investigated the competitive behavior of Cu and Ca on Eu binding with sedimentary humic acid. Copper2+ and Eu3+ were shown to exhibit direct competition with humic acid, whereas Ca2+ competition was indirect and attributed to simple electrostatic interactions. Sonke [47] evaluated complexation of river, coal, and soil humic acid binding of rare earth elements. Upon progression from La to Lu, the observed increase in complex stability is consistent with lanthanide contraction and supports the premise that organic matter outcompetes carbonate complexation, even in alkaline environments, and that REE fractionation in aquatic environments is common.

Aosai et al. [48] employed nanofiltration membranes to estimate organic colloids in deep groundwaters. Ramirez-Guinart et al. [49] observed soil sorption and desorption of Sm were predicated on the Sm concentration, with dilute Sm concentrations exhibiting higher sorption and reduced desorption. Sorption of Sm was influenced by

**15**

**Author details**

Southeast Missouri State University, Missouri, USA

\*Address all correspondence to: mtaide@semo.edu

provided the original work is properly cited.

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

Michael Aide

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

pH and soil organic matter solubility, and the soil phases of organic matter, presence

Our collective understanding of rare earth element activity in surface and groundwater requires a more fundamental examination of (i) REE partitioning within the aqueous phase, including complexation and adsorption reactions involving organic and inorganic colloids; (ii) partitioning involving REE in the aqueous phase and the surrounding solid phases constituting the river bed and aquifer skeleton; (iii) the influence of temperature, Eh (pe), pH, and ionic strength; and (iv) a greater and more accurate thermodynamic database of organic and inorganic

We also need a more significant database of rare earth element abundances in surface and groundwaters to gauge the extent of environmental impact and to serve as a reference for future REE environmental impact in water. Key areas of extensive groundwater and surface water across North and South America, Europe, Africa, and Asia have not received any preliminary documentation of their rare earth ele-

of carbonates, and clay separated were important predictors of Sm mobility.

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

**10. Future research needs**

species.

ment composition.

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

pH and soil organic matter solubility, and the soil phases of organic matter, presence of carbonates, and clay separated were important predictors of Sm mobility.
