The coordination chemistry of CmIII, AmIII, and AcIII in nitrate solutions: an actinide L3-edge EXAFS study

Abstract

Understanding actinide(III) (An^III = Cm^III, Am^III, Ac^III) solution-phase speciation is critical for controlling many actinide processing schemes, ranging from medical applications to reprocessing of spent nuclear fuel. Unfortunately, in comparison to most elements in the periodic table, An^III speciation is often poorly defined in complexing aqueous solutions and in organic media. This neglect – in large part – is a direct result of the radioactive properties of these elements, which make them difficult to handle and acquire. Herein, we surmounted some of the handling challenges associated with these exotic 5f-elements and characterized Cm^III, Am^III, and Ac^III using An^III L₃-edge X-ray absorption spectroscopy (XAS) as a function of increasing nitric acid (HNO₃) concentration. Our results revealed that actinide aquo ions, An(H₂O)_x³⁺ (x = 9.6 ± 0.7, 8.9 ± 0.8, and 10.0 ± 0.9 for Cm^III, Am^III, and Ac^III), were the dominant species in dilute HNO₃ (0.05 M). In concentrated HNO₃ (16 M), shell-by-shell fitting of the extended X-ray fine structure (EXAFS) data showed the nitrate complexation increased, such that the average stoichiometries of Cm(NO₃)_4.1±0.7(H₂O)_5.7±1.3^{(1.1±0.2)−}, Am(NO₃)_3.4±0.7(H₂O)_5.4±0.5^{(0.4±0.1)−}, and Ac(NO₃)_2.3±1.7(H₂O)_8.3±5.2^(0.7±0.5)+ were observed. Data obtained at the intermediate HNO₃ concentration (4 M) were modeled as a linear combination of the 0.05 and 16 M spectra. For all three metals, the intermediate models showed larger contributions from the 0.05 M HNO₃ spectra than from the 16 M HNO₃ spectra. Additionally, these efforts enabled the Cm–NO₃ and Ac–NO₃ distances to be measured for the first time. Moreover, the An^III L₃-edge EXAFS results, contribute to the growing body of knowledge associated with Cm^III, Am^III, and Ac^III coordination chemistry, in particular toward advancing understanding of An^III solution phase speciation.