**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

acids (20%). Inorganic complexes were of greater importance in groundwaters having low DOC concentrations. Omonona and Okoghue [31] showed REE concentrations from Nigerian aquifers, demonstrating the region's water REE chemical diversity (**Table 4**).

Adsorption reactions involving the REEs and aquifer materials are instrumental to understanding REE water concentrations and transport [34–41]. Rabung et al. [34] performed batch adsorption experiments involving Eu3+ on Ca-montmorillonite and Na-illite and showed Eu outer-sphere complexes at pH levels less than pH 4 on illite, whereas no outer-sphere complexes were observed with montmorillonite. For pH levels greater than pH 5, inner-sphere complexes were formed for both minerals. Coppin et al. [29] showed that lanthanide adsorption on smectite and kaolinite was pH and ionic strength dependent and demonstrated increased adsorption at higher ionic strengths near pH 5.5. At lower ionic strengths, REE adsorption onto smectite was weakly pH-dependent from 3 to pH 6, whereas REE adsorption was increasingly greater above pH 6. Kaolinite showed increased REE adsorption with increased pH. At the greater ionic strength, the heavy REEs exhibited greater adsorption, a feature consistent with lanthanide contraction.

Cteiner [42] observed monazite (NdPO4) reactivity at low ionic strengths to estimate the influence of Cl<sup>−</sup>, HCO3 <sup>−</sup>, SO4 <sup>2</sup><sup>−</sup>, oxalate, and acetate on monazite solubility. At pH levels ranging from 6.0 to 6.5, Nd (oxalate) was the dominant species, followed by Nd3+ and NdSO4 + . Davranche et al. [37, 38] demonstrated that REEs and humic acid complexes frequently dominate soil aqueous systems, especially in nearneutral pH levels and at greater dissolved organic carbon concentrations. Pourret et al. [43] observed the strong competitive interaction between humic acids and carbonates for REE complexation, especially at increasing pH levels. Similarly, Wu et al. [36] described the strong competition involving EDTA and humic and fulvic acids, which effectively inhibited lanthanum adsorption onto goethite.

Cation exchange and adsorption reactions involving cations and their hydrolytic products are dominant soil processes, including (i) multi-site cation exchange reactions, (ii) adsorption reactions with increasing degree of inner-sphere complexes


**9**

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

at pH levels greater than pH 5, (iii) REE affinity being reduced by increased ionic strength, and (iv) REE complexation affinity being greater at higher pH intervals. Davranche et al. [38] provided adsorption data on hydrous ferric oxides with REEs and REE-humate complexes. REE-humate complexes do not dissociate upon adsorption, with binding presumed to be anionic adsorption involving the humate portion of the complex. Pourret et al. [15] employed ultrafiltrate techniques to investigate La, Eu, and Lu synthetic humic acid complexation and modeled the datasets with the humic ionbinding model to demonstrate that the quantity of REE bonding increases with pH. The intensity of the REE-humic acid binding approached 100% near pH 4 for the highest

especially dicarbonate speciation were effective competing anions in alkaline media

An aqua regia digestion was employed to obtain a near total estimation of elemental abundance associated with all but the most recalcitrant soil chemical environments. Aqua regia does not appreciably degrade quartz, albite, orthoclase, anatase, barite, monazite, sphene, chromite, ilmenite, rutile, and cassiterite; however, anorthite and phyllosilicates are partially digested. Homogenized samples (0.75 g) were equilibrated with 0.01 L of aqua regia (3 mole nitric acid/1 mole hydrochloric acid) in a 35°C incubator for 24 hours. Samples were shaken, centrifuged, and filtered (0.45 μm), with a known aliquot volume analyzed using induc-

A hot water extraction was performed to recover only the most labile or potentially labile fractions. 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. In the water extract and the aqua regia digestion, selected samples were duplicated, and known reference materials were

Using Minteq software [44] chemical speciation may be estimated from an internal Minteq thermochemical data for specified pH intervals. Establishing a reasonably constant ionic strength using the background solution chemistry [NO3, Cl, NH4, Ca, K, Mg, Na, SO4, PO4] of subsurface tile-drainage effluent from the David M. Barton Agriculture Research Center [Missouri, USA], activity coef-

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

ficients were calculated using the Debye-Huckel equation at 25°C.

with the effectiveness of carbonate complexation increasing from La to Lu.

). Rare earth complexes involving carbonate and

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

humic acid concentration (20 mg L<sup>−</sup><sup>1</sup>

tively coupled plasma mass spectrometry (ICP-MS).

employed to guarantee analytical accuracy.

**7. Results and discussion**

**6. Materials and methods**

### **Table 4.**

*Rare earth element concentrations from selected aquifers in the Gboko area, Nigeria.*

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

at pH levels greater than pH 5, (iii) REE affinity being reduced by increased ionic strength, and (iv) REE complexation affinity being greater at higher pH intervals. Davranche et al. [38] provided adsorption data on hydrous ferric oxides with REEs and REE-humate complexes. REE-humate complexes do not dissociate upon adsorption, with binding presumed to be anionic adsorption involving the humate portion of the complex. Pourret et al. [15] employed ultrafiltrate techniques to investigate La, Eu, and Lu synthetic humic acid complexation and modeled the datasets with the humic ionbinding model to demonstrate that the quantity of REE bonding increases with pH. The intensity of the REE-humic acid binding approached 100% near pH 4 for the highest humic acid concentration (20 mg L<sup>−</sup><sup>1</sup> ). Rare earth complexes involving carbonate and especially dicarbonate speciation were effective competing anions in alkaline media with the effectiveness of carbonate complexation increasing from La to Lu.
