Disproportionation of the Uranyl(V) Coordination Complexes in Aqueous Solution through Outer-Sphere Electron Transfer

Abstract

Among the linear actinyl­(VI/V) cations, the uranyl­(V) species are particularly intriguing because they are unstable and exhibit a unique behavior to undergo H+ promoted disproportionation in aqueous solution and form stable uranyl­(VI) and U­(IV) complexes. This study uses density functional theory (DFT) combined with the conductor-like polarizable continuum model approach to investigate [UO2]2+/+ to [UIVO2] reduction free energies (RFEs) and explores the stability of uranyl­(V) complexes in aqueous solution through computing disproportionation free energies (DFEs) for an outer-sphere electron transfer process. In addition to the aqua complex (U1), another three commonly encountered ligands such as chloride (U2), acetate (U3), and carbonate (U4) in aqueous environmental conditions are taken into account. For the U1 complex, the computed 1e– (V/IV) and 2e– (VI/IV) RFEs are in good agreement with experiments. The computed DFEs reveal that the presence of H+ is imperative for the disproportionation to take place. Although the presence of the alkali cations favors the disproportionation to some extent, they cannot fully make the reaction thermodynamically feasible. For the anionic complexes, the high negative charge does not allow for the formation of a cation–cation encounter complex due to Coulombic repulsion. Furthermore, an additional factor is the ligand exchange reaction which is also an energy-demanding step. Therefore, the current study examined the Kern–Orlemann mechanism and our results validate the mechanism based on DFT computed DFEs and propose that for the anionic complexes, an outer-sphere electron transfer is highly probable and our computed protonation free energies further support this claim.

High-Performance Material for the Effective Removal of Uranyl Ion from Solution: Computationally Supported Experimental Studies

“… 32 Adsorption studies could not be performed in alkaline conditions because the precipitation of polyanionic species and hydroxides formed under these conditions adversely affects adsorption. 33 Since the natural pH of uranyl is in the range of 4–5, it has been observed that this adsorption pH can be also used for this new material.…”

“…In addition, with an increasing pH, the substitution of polycationic species instead of the dominant UO 2 2+ cation at a low pH leads to increased adsorption . Adsorption studies could not be performed in alkaline conditions because the precipitation of polyanionic species and hydroxides formed under these conditions adversely affects adsorption . Since the natural pH of uranyl is in the range of 4–5, it has been observed that this adsorption pH can be also used for this new material.…”

Actinyl Adsorption and Reduction on Pyrite Surfaces: Insights from DFT Calculations

“…However, when the pH is increased, the pentaaquo complex undergoes a hydrolysis reaction and the successive removal of protons from the pentaaquo actinyl complex takes place. For the actinyl species in aqueous solution, to some extent, a mixture of several hydrolyzed species coexists within a pH range of 4–7. − In order to mimic neutral to basic conditions, we investigate the adsorption of neutral actinyl aqua complexes onto a semiconducting pyrite surface as an inner-sphere complex with bridging hydroxyl (−OH) groups. A geometry optimization of the neutral uranyl complex results in an extended water structure and formation of hydrogen bonds with one of the uranyl-bound (Figure ) hydroxyl groups and a water molecule.…”

“…For the bidentate actinyl adsorption, the calculated PCET energetics suggests that the PCET reaction becomes thermodynamically more favorable on moving across the actinyl series. The neutral actinyl aqua complexes correspond to higher pH conditions (pH 4–6), and at high pH, hydrolysis becomes more probable and equilibrium between various hydrolyzed species − is anticipated. We use neutral actinyl­(VI) aqua complexes as models for surface reductions by PCET pathways.…”

“…Actinyl ions are stable linear oxo cations, and the metal–oxo bonds are strong and fairly inert; therefore, the crystal chemistry of actinyl is rich. , The coordinating ligands, e.g., organic, carbonate, hydroxide, or water ligands, tend to alter the electronic environment of the central An atom (An = U, Np, Pu), which influences the redox chemistry of actinyl complexes in solution. − The solution-phase properties of actinyl species are complicated, and key factors such as pH, coordinating ligands, counterions, dissolved organic matter (DOM), and dissolved oxygen control the properties and speciation of these complexes. This leads to the formation of numerous species that are likely to coexist under certain conditions. − The mineral–actinide interface is quite complex on its own, and the pivotal catalytic role of semiconducting mineral surfaces on the chemical transformations of actinides requires a molecular-level understanding.…”