Comparative Study of Uranyl(VI) and -(V) Carbonato Complexes in an Aqueous Solution

Abstract

Electrochemical, complexation, and electronic properties of uranyl(VI) and -(V) carbonato complexes in an aqueous Na2CO3 solution have been investigated to define the appropriate conditions for preparing pure uranyl(V) samples and to understand the difference in coordination character between UO22+ and UO2+. Cyclic voltammetry using three different working electrodes of platinum, gold, and glassy carbon has suggested that the electrochemical reaction of uranyl(VI) carbonate species proceeds quasi-reversibly. Electrolysis of UO22+ has been performed in Na2CO3 solutions of more than 0.8 M with a limited pH range of 11.7 < pH < 12.0 using a platinum mesh electrode. It produces a high purity of the uranyl(V) carbonate solution, which has been confirmed to be stable for at least 2 weeks in a sealed glass cuvette. Extended X-ray absorption fine structure (EXAFS) measurements revealed the structural arrangement of uranyl(VI) and -(V) tricarbonato complexes, [UO2(CO3)3]n- [n = 4 for uranyl(VI), 5 for uranyl(V)]. The bond distances of U-Oax, U-Oeq, U-C, and U-Odist are determined to be 1.81, 2.44, 2.92, and 4.17 A for the uranyl(VI) complex and 1.91, 2.50, 2.93, and 4.23 A for the uranyl(V) complex, respectively. The validity of the structural parameters obtained from EXAFS has been supported by quantum chemical calculations for the uranyl(VI) complex. The uranium LI- and LIII-edge X-ray absorption near-edge structure spectra have been interpreted in terms of electron transitions and multiple-scattering features.

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

“…The edge jump slightly shifted from 17.164 to 17.163 keV (Figure 10), and the spectra were similar to those reported previously [15,16]. The solution was light yellow before the electrolysis and colorless and transparent afterwards, which is typical of Table 1) [15,16,47]. The fits are shown as dashed lines in Figure 11a and b, and these results agree with the previous reports [15,16].…”

“…The solution was light yellow before the electrolysis and colorless and transparent afterwards, which is typical of Table 1) [15,16,47]. The fits are shown as dashed lines in Figure 11a and b, and these results agree with the previous reports [15,16]. The electrode potential of − 0.95 V was high enough to form UO …”

“…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].…”

“…To prepare the reduced products, electrochemical bulk electrolysis was combined with UV-Visible spectroscopy or XAFS measurements of UO + 2 [14][15][16][17] and U…”

Voltammetric Study of Uranyl–Selenium Interactions. Part 2. Uranyl(V) Selenate Complex

“…During the last decade an increasing interest in complex formation of uranyl(V) could be observed. [14][15][16][17][18][19][20][21] According to the resulting conclusions, both UO 2 2+ and UO 2 + can often be coordinated by the same type of ligand but in the latter case stability constants are lower. Therefore such a possibility should be taken into account when voltammetric data from uranyl(VI)-selenate system are treated.…”