**6. Materials and methods**

*Rare Earth Elements and Their Minerals*

feature consistent with lanthanide contraction.

+

estimate the influence of Cl<sup>−</sup>, HCO3

followed by Nd3+ and NdSO4

(**Table 4**).

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

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

Cteiner [42] observed monazite (NdPO4) reactivity at low ionic strengths to

bility. At pH levels ranging from 6.0 to 6.5, Nd (oxalate) was the dominant species,

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

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

> **(μg L<sup>−</sup><sup>1</sup> )**

**Element Low High Mean**

La 0.33 42.85 6.83 Ce 0.73 85.15 6.83 Nd 0.36 36.51 6.18 Pr 0.09 9.25 1.55 Sm 0.05 5.47 1.04 Eu 0.00 0.50 0.07 Gd 0.06 3.61 0.81 Dy 0.00 2.08 0.49 Ho 0.00 0.38 0.09 Er 0.01 0.94 0.23 Tm 0.00 0.12 0.03 Yb 0.00 0.80 0.18

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

<sup>2</sup><sup>−</sup>, oxalate, and acetate on monazite solu-

. Davranche et al. [37, 38] demonstrated that REEs and

<sup>−</sup>, SO4

acids, which effectively inhibited lanthanum adsorption onto goethite.

**8**

**Table 4.**

*Source: Omonona and Okoghue [31].*

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 inductively coupled plasma mass spectrometry (ICP-MS).

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 employed to guarantee analytical accuracy.

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