**2. Hydrolysis and complexation thermodynamic data**

The hydrolysis of REE3+ species has been extensively investigated. The primary thermodynamic literature featuring data involving REE3+ hydrolysis and inorganic complexation reactions include Baes and Mesmer [2], Hummel et al. [3], Smith and Martel [4], Schijf and Byrne [5], Luo and Byrne [6], Cantrell and Byrne [7], Gramaccioli et al. [8], Lee and Byrne [9], and Millero [10]. Klungness and Byrne [11] noted that REE hydrolysis is more stable with increasing atomic number across the lanthanide series.

Inorganic complexation of the REE elements involves coordination with primarily anionic species, and it is expressed as.

$$\text{REE}^{\text{3\*}} + \text{yL}^{\text{n-}} = \text{REE} - \text{L}\_{\text{y}}^{\text{(3-yn)}},\tag{1}$$

where L<sup>n</sup><sup>−</sup> is an inorganic ligand with n ionic charge and y is the stoichiometric coefficient. For the lanthanide series, the dicarbonate complex becomes increasingly more stable with increasing atomic number [6, 7, 9]. Both hydrolysis and carbonate complexation show the expected increasing stability with increasing atomic number across the lanthanide series [12]. Aide [12] reviewed thermodynamic data concerning rare earth element hydrolysis.

Common low-molecular-weight organic complexes include acetic acid, phthalic acid, oxalic acid, lactic acid, malic acid, and citric acid. Humus components typically include fulvic and humic acids. The seminal literature featuring thermodynamic data involving REE3+ organic complexation include Gu et al. [13], Dong et al. [14], and Pourret et al. [15].
