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

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 (n) and formation constants for the La hydroxyl complexes (LaOH2+, La(OH)2+, and La(OH)30). From 25 to 50 °C, n ranges between ~1 and 2 at pH from 7.0 to 9.3. At 75 °C, 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 (βn°, n = 1 to 3) are derived for the reaction La3+ + nOH- = La(OH)n3-n, and fitted between 25 and 250 °C by combining the UV-Vis and literature solubility data. The resulting logβn° are expressed as function of temperature (T in Kelvin): logβ1° = -1.786 +0.0133T +1.049·103/T; logβ2° = -5.797 +0.0267T +2.713·103/T; logβ3° = 6.435 +0.0223T +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.本研究于25至75 ℃、近中性至碱性的含氢氧化钠水溶液体系中开展紫外-可见分光光度（UV-Vis spectrophotometric）实验，体系内镧（La）浓度范围为0至约0.23 mmol/kg且各不相同。采用显色指示剂间甲酚紫（m-cresol purple）测定原位pH，并推导得到镧羟基配合物（LaOH²+、La(OH)₂+及La(OH)₃⁰）的平均OH⁻配位数（n）与生成常数。在25至50 ℃区间内，当pH处于7.0至9.3范围时，n值约介于1至2之间；在75 ℃、pH为6.3至8.8的条件下，n值约介于1.5至3之间。上述结果表明，25至50 ℃区间内以LaOH²+与La(OH)₂+配合物为主要存在形式，而75 ℃时La(OH)₃⁰的占比显著提升。针对反应La³+ + nOH⁻ = La(OH)ₙ^(3-n)，本研究推导得到了其累积生成常数（βₙ°, n=1至3），并结合紫外-可见分光光度数据与文献报道的溶解度数据，在25至250 ℃区间内对其进行了拟合。所得logβₙ°可表示为温度（T，单位为开尔文）的函数： logβ₁° = -1.786 + 0.0133T + 1.049×10³/T； logβ₂° = -5.797 + 0.0267T + 2.713×10³/T； logβ₃° = 6.435 + 0.0223T + 512.7/T。将本次拟合得到的结果与采用Helgeson-Kirkham-Flowers状态方程（Helgeson-Kirkham-Flowers equation of state）得到的现有外推结果进行对比后发现，二者对镧水解行为的预测存在显著差异。因此，针对稀土元素（REE）水解行为的现有外推结果亟需更新。