**10. Future research needs**

*Rare Earth Elements and Their Minerals*

Er(CO3)2

Ytterbium

YbCO3

*3.85 × 10<sup>−</sup><sup>3</sup>*

**Table 9.**

Yb(CO3)2

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

*Simulation of the partitioning of selected rare earth elements between carbonate complexes and fulvic acid in* 

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

**Species Percent of species**

<sup>−</sup> 83.9

<sup>+</sup> 15.7

<sup>−</sup> 82.2

Fulvic acid-Yb+ 1.3

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

+

The simulation of the REE partitioning between carbonate complexes and fulvic

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

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

and REE(CO3)2

+

<sup>−</sup> declined and increased, respectively, on progression with

mol kg<sup>−</sup><sup>1</sup>

<sup>−</sup> were the most

declining with increas-

 *mol/kg. Ionic strength was estimated at* 

<sup>−</sup> increasing with atomic

) at

estimated to be present at 99% of the total Yb concentration.

number. The REE concentrations of REE(CO3)

*All rare earth element concentrations are initially set as 0.31 × 10<sup>−</sup><sup>7</sup>*

*stream waters in equilibrium with ordered dolomite and hydroxyapatite.*

extensive species, with the concentration of REE(CO3)

ing atomic number and the concentration of REE(CO3)2

The REEs were simulated in the presence of phthalic acid (10<sup>−</sup><sup>3</sup>

**14**

number.

REECO3

+

and REE(CO3)2

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

increasing atomic number.

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 species.

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 element composition.
