Antonio Mark R, Ellis Ross J, Estes Shanna L, Bera Mrinal K
Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, IL 60439, USA.
Phys Chem Chem Phys. 2017 Aug 16;19(32):21304-21316. doi: 10.1039/c7cp03350h.
X-ray and electrochemical studies of organic phases obtained by the extraction of tetravalent cerium, Ce(iv), from aqueous nitric acid (3 M) with tri-n-butyl phosphate (TBP) in n-dodecane reveal a tetranuclear Ce(iv) structural motif. This finding is consistent with the results of previous liquid-liquid extraction (LLE) studies that implicate the aggregation of (Ce-O-Ce) dimers into multinuclear Ce(iv)·TBP solvates. The organic solution structures elaborated here for the Ce(iv)-HNO-20% TBP-n-CH system are correlated with multiscale phenomena-from the atomic level of the cerium coordination environment to the supramolecular scale of solute aggregates-in the organic phases, which are of relevance to the PUREX (Plutonium Uranium Reduction EXtraction) process. The combination of XANES, EXAFS, and SAXS results indicate the presence of tetranuclear cerium(iv)-oxo core structures in each of the organic phases investigated. In addition to the use of X-ray spectroscopy and scattering for direct metrical details about the organic phase solute speciation, three-phase-electrode differential pulse voltammetry (DPV) of the third phase reveals a wave attributable to Ce(iv) reduction. The electrode potential is consistent with values for the reduction of Ce(iv) in (Ce-O-Ce) dimers in aqueous electrolytes. The Ce(iv) coordination chemistry of the organic solvates is independent of the bulk phenomenon of phase splitting, namely third phase formation. The local, molecular environment of Ce in the organic phase before splitting is identical to those in the two organic phases (the dense third phase and the light phase) after splitting. SAXS data are consistent with the formation of small spherical reverse micelles with core diameters (approx. 6 Å) that can accommodate a tetranuclear Ce(iv) oxo-cluster solvate of TBP. Sticky sphere modeling of the SAXS data for the organic phases with low cerium concentrations (<0.14 M) is consistent with the presence of randomly- and homogenously-dispersed micelles in combination with short-range percolated, associated micelles. At high cerium concentrations (approx. 1.5 M) in the third phase, the SAXS modeling is consistent with correlated, long-range percolated micellar aggregates. The presence of strong inter-micellar interactions (-3 to -5kT) in all organic phases of the Ce(iv)-HNO-TBP-n-CH LLE system suggests that the phenomena of phase splitting and third phase inversion are due to liquid precipitation that is dependent solely on the concentration of the tetranuclear Ce solvate.
通过用正十二烷中的磷酸三丁酯（TBP）从硝酸水溶液（3M）中萃取四价铈（Ce(IV)）得到的有机相的X射线和电化学研究揭示了一种四核Ce(IV)结构 motif。这一发现与先前液液萃取（LLE）研究的结果一致，该研究表明（Ce-O-Ce）二聚体聚集成多核Ce(IV)·TBP溶剂化物。这里阐述的Ce(IV)-HNO-20% TBP-n-C₁₂H₂₆体系的有机溶液结构与有机相中的多尺度现象相关——从铈配位环境的原子尺度到溶质聚集体的超分子尺度——这些现象与PUREX（钚铀还原萃取）过程相关。XANES、EXAFS和SAXS结果的结合表明在所研究的每个有机相中都存在四核铈(IV)-氧核结构。除了使用X射线光谱和散射来获取关于有机相溶质形态的直接测量细节外，第三相的三相电极差分脉冲伏安法（DPV）揭示了一个可归因于Ce(IV)还原的波。电极电位与在水性电解质中（Ce-O-Ce）二聚体中Ce(IV)还原的值一致。有机溶剂化物的Ce(IV)配位化学与相分裂的整体现象无关，即第三相的形成。分裂前有机相中Ce的局部分子环境与分裂后两个有机相（致密的第三相和轻相）中的环境相同。SAXS数据与形成核心直径约为6 Å的小球形反胶束一致，这些反胶束可以容纳TBP的四核Ce(IV)氧簇溶剂化物。对低铈浓度（<0.14M）的有机相的SAXS数据进行粘性球建模，与随机且均匀分散的胶束以及短程渗透、相关胶束的存在一致。在第三相中高铈浓度（约1.5M）下，SAXS建模与相关的长程渗透胶束聚集体一致。Ce(IV)-HNO-TBP-n-C₁₂H₂₆ LLE体系的所有有机相中存在强的胶束间相互作用（-3至-5kT），这表明相分裂和第三相反转现象是由于仅依赖于四核Ce溶剂化物浓度的液体沉淀所致。
