Mechanisms and Rates of U(VI) Reduction by Fe(II) in Homogeneous Aqueous Solution and the Role of U(V) Disproportionation

Abstract

Molecular-level pathways in the aqueous redox transformation of uranium by iron remain unclear, despite the importance of this knowledge for predicting uranium transport and distribution in natural and engineered environments. As the relative importance of homogeneous versus heterogeneous pathways is difficult to probe experimentally, here we apply computational molecular simulation to isolate rates of key one electron transfer reactions in the homogeneous pathway. By comparison to experimental observations the role of the heterogeneous pathway also becomes clear. Density functional theory (DFT) and Marcus theory calculations for all primary monomeric species at pH values ≤7 show for UO and its hydrolysis species UOOH and UO(OH) that reduction by Fe is thermodynamically favorable, though kinetically limited for UO. An inner-sphere encounter complex between UOOH and Fe was the most stable for the first hydrolysis species and displayed an electron transfer rate constant k = 4.3 × 10 s. Three stable inner- and outer-sphere encounter complexes between UO(OH) and Fe were found, with electron transfer rate constants ranging from k = 7.6 × 10 to 7.2 × 10 s. Homogeneous reduction of these U(VI) hydrolysis species to U(V) is, therefore, predicted to be facile. In contrast, homogeneous reduction of UO by Fe was found to be thermodynamically unfavorable, suggesting the possible importance of U(V)-U(V) disproportionation as a route to U(IV). Calculations on homogeneous disproportionation, however, while yielding a stable outer-sphere U(V)-U(V) encounter complex, indicate that this electron transfer reaction is not feasible at circumneutral pH. Protonation of both axial O atoms of acceptor U(V) (i.e., by HO) was found to be a prerequisite to stabilize U(IV), consistent with the experimental observation that the rate of this reaction is inversely correlated with pH. Thus, despite prevailing notions that U(V) is rapidly eliminated by homogeneous disproportionation, this pathway is irrelevant at environmental conditions.

Synthesis and Characterization of a Water Stable Uranyl(V) Complex

“…The complex 5 was independently prepared from the reaction of UI 4 and 2 equiv of K 2 dpaea and crystallographically characterized (Figure S36). These results indicated that the monomeric uranyl (V) complex undergoes disproportionation upon protonation of the uranyl oxo groups as suggested by previous computational studies …”

Nanoscale mechanism of UO2 formation through uranium reduction by magnetite

“…However, uncertainty about its presence and of its role in reduction pathways involving iron oxides remains. The reduction of U(VI) to U(IV) can occur via (a) two single-electron transfer steps, from U(VI) to U(V), and U(V) to U(IV), or (b) from U(VI) to U(V) followed by disproportionation of two U(V) to U(VI) and U(IV) 22 , 27 – 29 . Theoretical calculations reported the reduction from U(VI) to U(V) by aqueous Fe(II) to be facile 29 and demonstrated that the incorporation of U in solid phases widens the stability field of U(V) species in the reduction by magnetite 16 .…”

Density functional theory (DFT) calculations of VI/V reduction potentials of uranyl coordination complexes in non-aqueous solutions

“…In recent years, computational approaches have been employed as important tools to predict several essential properties of actinides relevant to nuclear waste management and safe disposal such as electronic structure and bonding 5 , hydration [6][7][8] , speciation [9][10][11][12] , redox chemistry [13][14][15][16][17][18][19][20] , spectral properties [21][22][23][24] , reaction mechanisms [25][26][27][28][29][30][31][32][33] and electron transfer reactions. [34][35][36][37][38][39][40][41] Penta-valent uranyl(V) ions are unstable and readily disproportionate to form U(IV) and uranyl(VI) complexes in aqueous solution. Disproportionation reaction of actinyl(V) ions in aqueous solution was computationally studied by Steele et al.…”