**6. Rare earth elements and soil availability**

Tyler [33] reviewed the importance of REE in soils and plants in which he underscored the recent contributions of Chinese soil scientists in addressing REEs as plant promoting elements. Tyler acknowledged that the traditional definitions of plant essential nutrients may be challenged because of recent research involving the REEs and other elements. Pang et al. [34] documented the increasing use of REE-bearing fertilizers in China. More research needs to be performed to accurately assess whether any of the REEs are "plant essential" or simply supportive of plant growth and development.

Rare earth elements frequently have been shown to have greater concentrations in plant roots than leaves or above-ground woody tissue [35–37]. Li et al. [38] demonstrated that a 0.1 *M* HCl based extraction protocol effectively indicated REE plant availability. Lanthanum and to a lesser degree the other REEs exhibited root concentrations that were inversely proportional to the soil pH [36]. Using nutrient solutions, Gu et al. [39] demonstrated that sulfate inhibited REE uptake. Zhang et al. [40] reported that a mixture of malic acid and citric acid was effective in estimating REE plant availability. Cao et al. (200b) showed that water-soluble, 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.

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

Cation exchange and adsorption reactions involving cations and their hydrolytic products are dominant soil processes. Aide and Aide [9] reviewed REE reactions in the soil environment, including REE adsorption. Numerous studies cited in this review produced similar REE adsorption conclusions, including: (i) cation exchange reactions are largely associated with basal planar surfaces and pH-dependent silanol and aluminol reactions at edge positions, (ii) predominance of outer-sphere complexes occur at pH levels less than 4 and an increasing degree of inner sphere complexes at pH levels greater than 5, (iii) cation exchange was consistent with one electrostatic and non-specific site and one specific complexation site involving edge aluminol groups, (iv) REE affinity was reduced by increases in the ionic strength, (v) REE complexation affinity was greater at higher pH intervals. Conversely Tertre et al. [31] demonstrated the inner-sphere nature of aluminol sites on kaolinite and montmorillonite. Tang and Johannesson [32] noting that REE adsorption was more pronounced at greater pH intervals. At lower pH intervals, adsorption was attributed to REE3+ species whereas at greater pH intervals adsorption was attributed to REE3+ and REE-carbonate species. The adsorption

Tyler [33] reviewed the importance of REE in soils and plants in which he underscored the recent contributions of Chinese soil scientists in addressing REEs as plant promoting elements. Tyler acknowledged that the traditional definitions of plant essential nutrients may be challenged because of recent research involving the REEs and other elements. Pang et al. [34] documented the increasing use of REE-bearing fertilizers in China. More research needs to be performed to accurately assess whether any of the REEs are "plant essential" or simply

Rare earth elements frequently have been shown to have greater concentrations in plant roots than leaves or above-ground woody tissue [35–37]. Li et al. [38] demonstrated that a 0.1 *M* HCl based extraction protocol effectively indicated REE plant availability. Lanthanum and to a lesser degree the other REEs exhibited root concentrations that were inversely proportional to the soil pH [36]. Using nutrient solutions, Gu et al. [39] demonstrated that sulfate inhibited REE uptake. Zhang et al. [40] reported that a mixture of malic acid and citric acid was effective in estimating REE plant availability. Cao et al. (200b) showed that water-soluble,

constants increased regularly with an increase in REE atomic number.

**6. Rare earth elements and soil availability**

supportive of plant growth and development.

onto goethite as a pH function.

58 Lanthanides

**5. Exchange and adsorption reactions**

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 substantial REE plant uptake of grass grown in a Cambisol.

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 was consistently observed.

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, whereas the Eutrudepts represent 24 observations.

Selective, sequential extractions have been used to estimate REE plant uptake potential [40, 46–48]. Brantley et al. [49] reinforced the microbial component for REE availability, an area of research that is largely missing within the literature.

#### **7. Rare earth elements and soil development**

The importance of the REEs rests with their "signature", which may be defined as either the actual REE concentrations, when displayed by atomic number. Analysis of the REE signatures typically involves identifying evidence of fractionation, i.e., LREE and HREE ratios, La/Yb ratios, Nd/Sm ratios, and the presence of Ce or Eu anomalies. REE signatures have been compared to reveal (i) lithologic discontinuities [9], (ii) the presence of eolian or anthropogenic additions [50], (iii) estimates of the weathering intensities and elemental loss rates of soils [33], and (4) oxidation–reduction conditions in soil [9, 51]. Wang et al. [52] observed that greater soil water contents supported greater overland water flow, in which greater quantities of REE and P were transported. Similarly, Wu et al. [48] observed that apatite and calcium phosphate fertilizers altered the speciation and availability of selected REEs.

> soil horizons into their sand, silt and clay fractions, then determined the REE distribution using aqua regia digestion with ICP-MS. Selecting La for presentation, the La concentrations for the Ap through Btg2 horizons are rather evenly partitioned among the textural separates, with the clay separate showing slightly greater La abundance (**Figure 9**). The other REE elements show similar patterns. The Btg3 and Btg4 separates have greater La expression, especially for the sand separate. The Btg3 horizons are marked by a significant increase in sand-sized glabules

> **Figure 8.** The rare earth element distributions for the eluvial and illuvial horizons of the Alred soil series (Cape Girardeau

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County, Missouri, USA). Error bars are the standard deviation).

**Figure 9.** Rare earth element distribution by particle size for the Overcup soil series.

In a review of literature, Aide and Aide [9] reiterated numerous studies indicating REE migration in soil profiles. A summarization of the key REE soil transformations are (i) CO<sup>2</sup> and organic matter displace REEs as carbonate complexes and chelates in near surface horizons to support their accumulation in deeper soil horizons because of exchangeable, adsorption or precipitation reactions, (ii) HREE were enriched in the deeper soil horizons to a greater degree than the LREE, whereas other studies have indicated that the LREE were more readily transported to deeper soil horizons, (iii) apatite weathering supports the relatively rapid mobilization of the LREEs, whereas the weathering resistance mineral 'zircon' limits the mobilization of HREEs, (iv) similarities involving the REE signatures among the soil horizons and the host rock have been used to support arguments for parent material uniformity, whereas differences involving the REE signatures among the soil horizons and the host rock have been used to infer lithologic discontinuities (v) argillic (illuvial) horizons may have greater concentrations of LREE than the near-surface horizons (eluvial) inferring that phyllosilicate adsorption is an important soil process, (vi) crystalline Fe-oxyhydroxide and labile organic fractions accumulated HREEs than the LREEs, whereas the soil organic matter fraction representing humic acids and fulvic acids preferentially accumulated LREEs.

As an example, recent unpolished data from the authors of this manuscript follow. The Alred soil series (Loamy-skeletal over clayey, siliceous, semiactive, mesic Typic Paleudalfs) demonstrates differences in the rare earth element signatures to isolate lithologic discontinuities. The Alred series is a deep, well-drained collection of soils formed in cherty hillslope sediments (loess) and the underlying clayey limestone residuum. The eluvial (overlying loess mantle) and the illuvial (hill slope sediments derived from limestone residuum) differ significantly in their respective rare earth element concentrations, suggesting the REE differences are inherited (**Figure 8**).

The overcup series consists very deep, poorly drained, very slowly permeable soils that formed in alluvium (Fine, smectitic, thermic Vertic Albaqualfs). Aide (unpublished data) separated the Lanthanide Soil Chemistry and Its Importance in Understanding Soil Pathways: Mobility, Plant… http://dx.doi.org/10.5772/intechopen.79238 61

Selective, sequential extractions have been used to estimate REE plant uptake potential [40, 46–48]. Brantley et al. [49] reinforced the microbial component for REE availability, an area of

The importance of the REEs rests with their "signature", which may be defined as either the actual REE concentrations, when displayed by atomic number. Analysis of the REE signatures typically involves identifying evidence of fractionation, i.e., LREE and HREE ratios, La/Yb ratios, Nd/Sm ratios, and the presence of Ce or Eu anomalies. REE signatures have been compared to reveal (i) lithologic discontinuities [9], (ii) the presence of eolian or anthropogenic additions [50], (iii) estimates of the weathering intensities and elemental loss rates of soils [33], and (4) oxidation–reduction conditions in soil [9, 51]. Wang et al. [52] observed that greater soil water contents supported greater overland water flow, in which greater quantities of REE and P were transported. Similarly, Wu et al. [48] observed that apatite and calcium phosphate

In a review of literature, Aide and Aide [9] reiterated numerous studies indicating REE migration in soil profiles. A summarization of the key REE soil transformations are (i) CO<sup>2</sup>

organic matter displace REEs as carbonate complexes and chelates in near surface horizons to support their accumulation in deeper soil horizons because of exchangeable, adsorption or precipitation reactions, (ii) HREE were enriched in the deeper soil horizons to a greater degree than the LREE, whereas other studies have indicated that the LREE were more readily transported to deeper soil horizons, (iii) apatite weathering supports the relatively rapid mobilization of the LREEs, whereas the weathering resistance mineral 'zircon' limits the mobilization of HREEs, (iv) similarities involving the REE signatures among the soil horizons and the host rock have been used to support arguments for parent material uniformity, whereas differences involving the REE signatures among the soil horizons and the host rock have been used to infer lithologic discontinuities (v) argillic (illuvial) horizons may have greater concentrations of LREE than the near-surface horizons (eluvial) inferring that phyllosilicate adsorption is an important soil process, (vi) crystalline Fe-oxyhydroxide and labile organic fractions accumulated HREEs than the LREEs, whereas the soil organic matter fraction representing

As an example, recent unpolished data from the authors of this manuscript follow. The Alred soil series (Loamy-skeletal over clayey, siliceous, semiactive, mesic Typic Paleudalfs) demonstrates differences in the rare earth element signatures to isolate lithologic discontinuities. The Alred series is a deep, well-drained collection of soils formed in cherty hillslope sediments (loess) and the underlying clayey limestone residuum. The eluvial (overlying loess mantle) and the illuvial (hill slope sediments derived from limestone residuum) differ significantly in their respective rare earth element concentrations, suggesting the REE differences are inherited (**Figure 8**).

The overcup series consists very deep, poorly drained, very slowly permeable soils that formed in alluvium (Fine, smectitic, thermic Vertic Albaqualfs). Aide (unpublished data) separated the

and

research that is largely missing within the literature.

60 Lanthanides

**7. Rare earth elements and soil development**

fertilizers altered the speciation and availability of selected REEs.

humic acids and fulvic acids preferentially accumulated LREEs.

**Figure 8.** The rare earth element distributions for the eluvial and illuvial horizons of the Alred soil series (Cape Girardeau County, Missouri, USA). Error bars are the standard deviation).

soil horizons into their sand, silt and clay fractions, then determined the REE distribution using aqua regia digestion with ICP-MS. Selecting La for presentation, the La concentrations for the Ap through Btg2 horizons are rather evenly partitioned among the textural separates, with the clay separate showing slightly greater La abundance (**Figure 9**). The other REE elements show similar patterns. The Btg3 and Btg4 separates have greater La expression, especially for the sand separate. The Btg3 horizons are marked by a significant increase in sand-sized glabules

**Figure 9.** Rare earth element distribution by particle size for the Overcup soil series.

(nodules of Fe- and Mn-oxyhydroxides) and an abrupt increase in pH from an acidic to alkaline regime. Thus, oxidation-reduction and pH appear to be the controlling variables.

[10] Baes CF, Mesmer RE. The Hydrolysis of Cations. NY: John Wiley and Sons; 1976

Base. Wettingen, Switzerland: Nagra; 2008

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[11] Millero FJ. Stability constants for the formation of rare earth inorganic complexes as a function of ionic strength. Geochimica et Cosmochimica Acta. 1992;**56**:3123-3132 [12] Klungness GD, Byrne RH. Comparative hydrolysis behavior of rare earth elements and yttrium: The influence of temperature and ionic strength. Polydron. 2000;**19**:99-107 [13] Hummel E, Berner U, Curti E, Thoenen A. Nagra/PSI Chemical Thermodynamic Data

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[14] Smith R, Martell A. Critical Stability Constants, Volumes 1 and 4. Inorganic Complexes.

[15] Schijf J, Byrne RH. Stability constants for mono- and dioxalato-complexes of Y and the REE, potentially important species in groundwaters and surface freshwaters. Geochimica

[16] Cantrell KJ, Byrne RH. Rare earth element complexation by carbonate and oxalate ions.

[17] Lee JH, Byrne RH. Complexation of trivalent rare earth elements (Ce, Eu, Gd, Tb, Yb) by

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[21] Gu ZM, Wang XR, Gu XY, Cheng J, Wang LS, Dai LM, Cao M. Determination of stability constants for rare earth elements and fulvic acids extracted from different soils. Talanta.

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[23] Dong MW, Li WJ, Tao ZY. Use of the ion exchange method for the determination of stability constants of trivalent metal complexes with humic and fulvic acids II. Tb3+, Yb3+ and Gd3+ complexes in weakly alkaline conditions. Applied Radiation and Isotopes. 2002;

[24] Wu ZH, Luo J, Guo HY, Wang XR, Yang CS. Adsorption isotherms of lanthanum to soil constituents and effects of pH, EDTA and fulvic acid on adsorption of lanthanum onto

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#### **8. Future research needs**

Future research needs include; (i) understanding of the REE-microbiological interactions, especially in the rhizosphere, (ii) are the REE elements plant essential elements or growth promoting entities, (iii) more complex models (along with thermodynamic data) to better simulate the soil environment, and (iv) anticipate REE impacts to the soil environment because of increasing industrial REE utilization.
