Spectrophotometric Study of Erbium(III) Speciation in Chloride Solutions from 25 to 75 °C

Accurate thermodynamic properties for REE aqueous complexes are crucial for predicting the mobility of rare earth elements (REE) in natural waters and for developing improved separation and recovery technologies. In this study, UV-vis spectrophotometric experiments were conducted at 25-75 °C using pH 1.5 of Er-HClO₄-bearing solutions with varying NaCl concentrations from 0 to 1.7 mol/kg. Under these experimental conditions, Er hydrolysis is negligible, and speciation is controlled by Er³⁺ and Er chloride complexes. The molar absorbance coefficient of Er³⁺ decreases with temperature, and the absorbance intensity gradually decreases with increasing m_Cl/m_Er ratios. The number of absorbing species include Er³⁺ and ErCl²⁺ even at the highest m_Cl/m_Er ratio of ∼34. The formation constant (β₁ ⁰) for the ErCl²⁺ species was derived and fitted between 50 and 250 °C with existing literature values, yielding the following equation: logβ₁ ⁰ = -22.4315 + 0.0345T + 3.9379 × 10³/T, where T is temperature in Kelvin. A comparison between this new fit and previous predictions indicates good agreement at 100-250 °C, whereas larger differences occur in the 25-75 °C temperature range due to a broad scattering of previous experimental literature data at 25 °C. The new fit shows a systematic decrease in logβ₁ ⁰ values from 25 to 100 °C, indicating that REE chloride complexes become less stable with increasing temperature. These updated formation constant data are important for modeling REE speciation at low temperatures relevant to REE sorption onto clay minerals and REE extraction technologies.