Department of Civil & Environmental Engineering & Earth Sciences, University of Notre Dame, 301 Stinson-Remick, Notre Dame, IN, 46556, USA.
Karlsruhe Institute of Technology, Institute for Nuclear Waste Disposal, P.O. Box 3640, Karlsruhe, 76021, Germany.
Chemosphere. 2024 Sep;364:143233. doi: 10.1016/j.chemosphere.2024.143233. Epub 2024 Aug 31.
We investigated the binary Cm-citrate system using time-resolved laser fluorescence spectroscopy (TRLFS), parallel factor analysis (PARAFAC), and quantum chemical calculations. Evidence collectively suggests the stepwise coordination and deprotonation of citrate alcohol groups in Cm-cit complexes with two bound citrate moieties upon increasing pH, which is supported by a bathochromic shift in emission spectra, an observed increase in lifetime measurements, and lower energy minima for citrate alcohol involvement versus hydrolysis of the Cm-citrate species. Our PARAFAC results agree with a 3-component model for the Cm-citrate system and offer pure component decompositions, yielding fraction species across the studied pH range that have a correlated slope = 1 as a function of pH. For the first time, evidence of ternary Ca-Cm-citrate complexes was revealed by TRLFS with increasing calcium concentration at fixed pH. The formation of these ternary complexes was substantiated with density functional theory (DFT) calculations on simple model systems of the complexes. Shared citrate carboxylate groups between calcium and curium were proposed for all three ternary Ca-Cm-cit complexes based on DFT-determined Ca-O and Cm-O distances. Moreover, we found that the ternary complex with both alcohol groups deprotonated is most stable when it shares both two carboxylate and two alcohol groups between Ca and Cm. The presence of shared functional groups highlights the enhanced stability of these ternary complexes. Additional work is warranted to further constrain the stoichiometry, stability constants and dependence on ionic strength of these complexes for purposes of thermodynamic modeling of repository settings.
我们使用时间分辨激光荧光光谱法（TRLFS）、平行因子分析（PARAFAC）和量子化学计算研究了二元 Cm-柠檬酸体系。综合证据表明，随着 pH 值的增加，柠檬酸醇基团在 Cm-柠檬酸配合物中逐步配位和去质子化，具有两个结合的柠檬酸部分，这得到了发射光谱红移、寿命测量观察到的增加以及柠檬酸醇参与相对于 Cm-柠檬酸物种水解的能量最小值降低的支持。我们的 PARAFAC 结果与 Cm-柠檬酸体系的 3 组分模型一致，并提供了纯组分分解，在研究的 pH 范围内产生分数物种，其斜率与 pH 呈相关关系=1。首次通过 TRLFS 揭示了随着固定 pH 下钙浓度的增加而形成的三元 Ca-Cm-柠檬酸配合物的证据。通过对配合物的简单模型体系进行密度泛函理论（DFT）计算，证实了这些三元配合物的形成。基于 DFT 确定的 Ca-O 和 Cm-O 距离，提出了所有三种三元 Ca-Cm-柠檬酸配合物中柠檬酸共享的羧酸酯基团。此外，我们发现当两个羧酸酯和两个醇基团在 Ca 和 Cm 之间共享时，同时去质子化的三元配合物最稳定。共享官能团的存在突出了这些三元配合物的增强稳定性。需要进一步的工作来进一步约束这些配合物的化学计量、稳定性常数和对离子强度的依赖，以便对储存库环境进行热力学建模。
