Spectrophotometric determination of the stability of La hydroxyl complexes at near neutral to alkaline pH from 25 to 75 °C

The hydrolysis of rare earth elements (REE) potentially controls their mobility during fluid-rock interaction in a broad range of pH and temperature conditions. However, there is still a lack of thermodynamic data for modeling accurately the stability of REE hydroxyl complexes in hydrothermal aqueous fluids. In this study, UV–Vis spectrophotometric experiments were conducted from 25 to 75 °C in near-neutral to alkaline NaOH-bearing aqueous solutions with varying lanthanum (La) concentrations (0 to ∼0.23 mmol/kg). The color indicator m-cresol purple was used to determine in situ pH and derive the average OH⁻ ligand number ($\overrightarrow{n}$) and formation constants for the La hydroxyl complexes (LaOH²⁺, La(OH)₂⁺, and La(OH)₃⁰). From 25 to 50 °C, $\overrightarrow{n}$ ranges between ∼1 and 2 at pH from 7.0 to 9.3. At 75 °C, $\overrightarrow{n}$ ranges between ∼1.5 and 3 at pH from 6.3 to 8.8. These results suggest the predominance of LaOH²⁺ and La(OH)₂⁺ complexes from 25 to 50 °C, and an increased predominance of La(OH)₃⁰ at 75 °C. The cumulative formation constants (β_n^°, n = 1 to 3) are derived for the reaction La³⁺ + nOH⁻ = La(OH)_n^3-n, and fitted between 25 and 250 °C by combining the UV–Vis and literature solubility data. The resulting logβ_n^° are expressed as function of temperature (T in Kelvin): logβ₁^° = −1.786 + 0.0133 T + 1.049·10³/T; logβ₂^° = −5.797 + 0.0267 T + 2.713·10³/T; logβ₃^° = 6.435 + 0.0223 T + 512.7/T. A comparison between these new fits and existing extrapolations using the Helgeson-Kirkham-Flowers equation of state indicates significant differences in the predicted hydrolysis of La. The latter extrapolations should therefore be updated for the hydrolysis of REE.