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

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*βn◦, n =1 to 3) were derived for the reaction Er3+ + nH2O = Er(OH)n3-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.

本研究采用紫外-可见分光光度法（UV-Vis spectrophotometry），以间氯苯酚（mCP）作为原位pH颜色指示剂，在35~75℃的温度梯度下开展实验，以表征铒（Er）在近中性至碱性溶液中的水解过程。实验所用溶液为低离子强度体系（离子强度≤0.001 mol/kg），铒的初始浓度范围为0至约0.253 mmol/kg。在pH 6.3~9.0的区间内，35℃时与Er³+配位的OH−平均配位数从1.0升至2.7，75℃时则从1.5升至3.2。通过物种形态分析可知，随着温度升高，Er(OH)₃⁰相较于Er(OH)₂⁺与另一Er(OH)₂⁺物种的优势占比逐渐提升。针对反应Er³+ + nH₂O = Er(OH)ₙ^(3-n) + nH+，我们分别在35、50和75℃下推导得到了铒水解标准常数的对数值（log*βₙ^⊖，n=1~3）：log*β₁^⊖分别为-7.02±0.09、-6.19±0.06和-5.43±0.16；log*β₂^⊖分别为-14.39±0.15、-13.98±0.01和-13.45±0.24；log*β₃^⊖分别为-22.85±0.15、-21.79±0.14和-20.32±0.09。我们将上述平衡常数拟合为温度依赖函数，可用于预测25~75℃区间内铒的水解常数；结果表明，配位数更高的稀土元素（Rare Earth Elements, REE）羟基配合物的稳定性随温度升高呈系统性增强。