Speciation and Structural Study of U(IV) and -(VI) in Perchloric and Nitric Acid Solutions

Abstract

In order to elucidate the uranium solution chemistry at the high HNO(3) concentrations typically employed for the reprocessing of spent nuclear fuels, speciation and complex structures of U(IV) and U(VI) are studied in aqueous HNO(3) solutions, as well as in HClO(4) solutions, by means of UV-visible-near-infrared and X-ray absorption spectroscopies and density functional theory calculations. In 1.0 M HClO(4), U(IV) exists as a spherical cation of U(4+), which is surrounded by 9-10 water molecules in the primary coordination sphere, while it forms a colloidal hydrous oxide, U(IV)O(2) x nH(2)O, at a lower acidic concentration of 0.1 M HClO(4). U(VI) exists as a transdioxo uranyl cation, UO(2)(2+), and forms a 5-fold pure hydrate complex of [U(VI)O(2)(H(2)O)(5)](2+) in 1.0 M HClO(4). With increasing HNO(3) concentration, the water molecules of the U(IV) and U(VI) hydrate complexes are successively replaced by planar bidentate coordinating nitrate ions (NO(3)(-)), forming dominant species of [U(IV)(H(2)O)(x)(NO(3))(5)](-) in 9.0 M HNO(3) and [U(VI)O(2)(NO(3))(3)](-) in 14.5 M HNO(3), respectively. The present multitechnique approach also suggests the formation of two intermediate U(VI) species, a 5-fold mononitrato complex ([U(VI)O(2)(H(2)O)(3)(eta(2)-NO(3))](+)) and a 6-fold dinitrato complex ([U(VI)O(2)(H(2)O)(2)(eta(2)-NO(3))(2)](0)), involving an increase in the total coordination number on the uranyl(VI) equatorial plane from 5 to 6 with increasing HNO(3) concentration. The presence of unidentate coordinate nitrato complexes or tetranitrato U(VI) complexes is less probable in the present HNO(3) system.

An in-situ X-ray absorption spectroelectrochemical study of the electroreduction of uranium ions in HCl, HNO3, and Na2CO3 solutions

“…Although UO 2 was formed in the 0.1 M HClO 4 [11], a black precipitate was not observed during the measurement.…”

“…The U 4+ coordination number reportedly decreased from 9 to 8 as the Cl − concentration was increased from 0 (ClO − 4 ) to 9 M [9]. In the presence of NO [11,16].…”

“…Hennig and co-workers studied the coordination sphere of U 4+ ions in various electrolytes using an X-ray absorption spectroelectrochemical cell [9][10][11]. The U 4+ coordination number reportedly decreased from 9 to 8 as the Cl − concentration was increased from 0 (ClO − 4 ) to 9 M [9].…”

“…Numerous XAFS studies of the coordination structure of uranium complexes have been conducted and reviewed [2][3][4][5][6][7]. The technique enables the identification of the valence states of uranium during the redox reactions from the X-ray absorption near-edge structure (XANES [8][9][10][11][12][13].…”

Sorption Characteristics of Uranium on Goethite and Montmorillonite under Biogeochemical Reducing Conditions

“…또한, 수용액상의 pH, 탄산염(carbonate) 농도 및 기타 화학적인 조건에 따라 형성될 수 있는 다양한 용존 우라늄복합체(aqueous uranium complexes) 및 그 들의 안정성 등에 관한 많은 연구가 수행되어 왔다 (Ollila et al, 1996;Ikeda-Ohno et al, 2009). 현 재까지 많은 학자들은 지하수를 따라 이동 가능한 우라늄을 대부분 우라늄(VI)로 생각하고 환경 실험 과 모델링 등을 수행하고 있다 (Silva and Nitsche, 1995).…”