Neodymium speciation in Cl-bearing aqueous solutions at 25-200 °C from in situ Raman spectroscopy

Rare earth elements (REE) are strategic resources due to their uses in high-tech and energy efficient devices. The speciation of REE with ligands such as Cl^- is important for REE transport in hydrothermal fluids and controls their mobility in ore deposits. Previous studies indicate that REE are mobilized in saline ore-forming fluids at >100-400 °C. Thermodynamic data for REE chloride complexes are largely based on solubility experiments, whereas most in situ Raman studies have been undertaken at ambient temperature. In this study, in situ Raman spectroscopy was used to measure the speciation of Nd in acidic solutions with variable chlorinity in fused quartz capillary cells from 25 to 200 °C. Raman bands specific to Nd-O and Nd-Cl bonds were identified near 350 cm^-1 and 230 cm^-1 respectively. Measurement of the Nd-O band in a Nd₂O₃-HClO₄ solution was used to derive a scattering coefficient of 4.8 ± 0.3 × 10^-3 for the Nd-O vibrational mode of the fully hydrated Nd³⁺ aqua ion. The logarithm of the species formation constant for NdCl²⁺ at zero ionic strength was determined to be 0.64 ± 0.03 at 25 °C and 1 bar, in good agreement with literature values derived using other indirect experimental methods. The log β₁ and log β₂ values derived at 100 to 200 °C also display generally good reproducibility with other experimental studies, indicating that Raman spectroscopy can be an accurate method for deriving thermodynamic constants for REE in hydrothermal solutions.