UV-Vis spectrophotometric determination of rare earth elements (REE) speciation at near-neutral to alkaline pH. Part II: hydrolysis of Er from 25 to 75 °C

Abstract

Aqueous speciation of rare earth elements (REE) controls their mobilization, fractionation, and enrichment in the natural waters. Geochemical modeling of their speciation is key to improve our understanding of the formation of economic mineral deposits, for developing mineral separation and mine tailing recovery technologies, and for characterizing the geochemistry of thermal water. However, our ability to predict the fate of REE in a wide pH and temperature range is limited by the scarcity of thermodynamic data for the REE hydroxyl complexes. In Part I of this study¹, the optical properties of m-cresol purple (mCP) were determined using UV-Vis spectrophotometry between 25 and 75 ℃ in order to develop a method for deriving the hydrolysis constants of erbium (Er). Here, UV-Vis spectrophotometry experiments were conducted as a function of temperature between 35 and 75 ℃ to determine the hydrolysis of Er in near-neutral to alkaline solutions using mCP as an in situ pH color indicator. The experiments were conducted with Er concentrations from 0 to ~0.253 mmol/kg in low ionic strength solutions (≤ 0.001 mol/kg). The average OH⁻ ligand number coordinated to Er³⁺ increases from 1.0 to 2.7 at 35 ℃ and from 1.5 to 3.2 at 75 ℃ over a pH range from 6.3 to 9.0. The measured speciation shows an increased predominance of Er(OH)₃⁰ over the Er(OH)²⁺ and Er(OH)₂⁺ species with increased temperature. The logarithmic values of Er hydrolysis constants (log*β_n^◦, n =1 to 3) were derived for the reaction Er³⁺ + nH₂O = Er(OH)_n^3-n + nH⁺ at each temperature of 35, 50, and 75 ℃: log*β₁^◦ of −7.02 ± 0.09, −6.19 ± 0.06, −5.43 ± 0.16; log*β₂^◦ of −14.39 ± 0.15, −13.98 ± 0.01, and −13.45 ± 0.24; log*β₃^◦ of −22.85 ± 0.15, −21.79 ± 0.14, and −20.32 ± 0.09. These equilibrium constants were fitted to a function to predict the hydrolysis constants of Er between 25 and 75 ℃, and show a systematic increase in the stability of REE hydroxyl complexes with higher ligand numbers with increased temperature.