Aqueous Speciation of Tetravalent Actinides in the Presence of Chloride and Nitrate Ligands

Abstract

Speciation of hexachloride tetravalent uranium, neptunium, and plutonium species in aqueous media has been investigated using density functional theory in the presence of inner sphere ligands such as chloride, nitrate, and solvent molecules. All possible structures with the formula [An IV (Cl) x (H 2 O) y (NO 3 ) z ] 4−x−z (An = U, Np, and Pu; x = 0−6, y = 0−8, and z = 0−6) were considered to explore the speciation chemical space of each actinide. The nature of the mixed-ligand complexes present in solution is controlled by the concentration of free ligands in solution. A low chloride concentration is suitable to drive the speciation away from the highly thermodynamically stable hexachloride species. Furthermore, the formation of dimeric species can proceed through both olation and oxolation mechanisms. Oxolation is preferred for monomers that contain fewer water ligands, while olation becomes favorable for complexes with more water ligands.

Pertechnetate/perrhenate‐capped Zr/Hf‐Dihydroxide Dimers: Elucidating Zr−TcO4 Co‐Mobility in the Nuclear Fuel Cycle

“…Both TcO 4 − and ReO 4 − ligate Zr/Hf IV invariably, splitting the common Zr 4 /Hf 4 flat tetramer ( Figure 1F) to yield a dihydroxide‐bridged dimer, exemplified in four isostructural compounds determined by single‐crystal X‐ray diffraction; M 2 (OH) 2 (XO 4 − ) 6 (H 2 O) 6 (M=Zr IV or Hf IV ; X=Re VII or Tc VII , referred to later as Zr 2 Re 6 , Hf 2 Re 6 , Zr 2 Tc 6 and Hf 2 Tc 6 ; see SI for synthesis details). This dimer was recently predicted by computation to be the first step of Zr/Hf‐hydrolysis, [27] but not observed in an isolated and soluble form, prior to this current study. The Zr/Hf‐dimers (Figure 1F) were synthesized two ways; 1) By dissolving corresponding Zr/Hf oxyhydroxide in perrhenic/pertechnic acid, or 2) By co‐dissolving the corresponding oxychloride salt (Zr 4 /Hf 4 ) with perrhenic/pertechnic acid, followed by slow evaporation to yield crystals (see SI for more details).…”

Co-Crystallization of Plutonium(III) and Plutonium(IV) Diglycolamides with Pu(III) and Pu(IV) Hexanitrato Anions: A Route to Redox Variants of [PuIII,IV(DGA)3][PuIII,IV(NO3)6]x

“…The cation geometry is a tricapped trigonal prism with a point group around the plutonium metal center of D 3 h with three outer-sphere nitrate ions. Solution-state and computational studies have shown the tendency for the hexanitrato species to complex with lanthanides and actinides. − Solid-state studies by Kawasaki et al have also shown that hexanitrato species with the Ln 3+ ions can form in their presence of DGA complexes . Additionally, solid-state actinide hexanitrato species have also been reported in crystal structures containing Th 4+ , U 4+ , Np 4+ , Pu 4+ , Am 3+ , and Cm 3+ ions. , These studies have shown that there is potential to isolate the hexanitrato species for plutonium TMDGA complexes.…”

Trapping an Unexpected/Unprecedented Hexanuclear Ce(III) Hydrolysis Product with Neutral 4‐Amino‐1,2,4‐triazole

“…The ligands used to bind the M 6 O 8 cores are usually selected in such a way to help control the hydrolysis process and stop the formation of larger number of metal center aggerates [26] . These ligands control the hydrolysis process by taking up part of the coordination spheres of each metal enter leaving limited possibility for further [OH] − coordination and bridging [27] .…”

“…[14,21] The ligands used to bind the M 6 O 8 cores are usually selected in such a way to help control the hydrolysis process and stop the formation of larger number of metal center aggerates. [26] These ligands control the hydrolysis process by taking up part of the coordination spheres of each metal enter leaving limited possibility for further [OH] À coordination and bridging. [27] Typically this approach utilizes bulky ligands that coordinate the metal centers in a bridging manner which limits the interactions with water while also acting as a unit to direct the formation of the M 6 O 8 core.…”