Bridging the Transuranics with Uranium(IV) Sulfate Aqueous Species and Solid Phases

Abstract

Isolating isomorphic compounds of tetravalent actinides (i.e., Th IV , U IV , Np IV , and Pu IV ) improve our understanding of the bonding behavior across the series, in addition to their relationship with tetravalent transition metals (Zr and Hf) and lanthanides (Ce). Similarities between these tetravalent metals are particularly illuminated in their hydrolysis and condensation behavior in aqueous systems, leading to polynuclear clusters typified by the hexamer [M IV 6O4(OH)4] 12+ building block. Prior studies have shown the predominance and coexistence of smaller species for Th IV (monomers, dimers, and hexamers) and larger species for U IV , Np IV , and Pu IV (including 38-mers and 70-mers). We show here that aqueous uranium(IV) sulfate also displays behavior similar to that of Th IV (and Zr IV ) in its isolated solid-phase and solution speciation. Two single-crystal X-ray structures are described: a dihydroxide-bridged dimer (U2) formulated as U2(OH)2(SO4)3(H2O)4 and a monomer-linked hexamer framework (U-U6) as (U(H2O)3.5)2U6O4(OH)4(SO4)10(H2O)9. These structures are similar to those previously described for Th IV . Moreover, cocrystallization of monomer and dimer and of dimer and monomer-hexamer phases for both Th IV (prior) and U IV (current) indicates the coexistence of these species in solution. Because it was not possible to effectively study the sulfate-rich solutions via X-ray scattering from which U2 and U-U6 crystallized, we provide a parallel solution speciation study in low sulfate conditions, as a function of the pH. Raman spectroscopy, UV-vis spectroscopy, and small-angle X-ray scattering of these show decreasing sulfate binding, increased hydrolysis, increased species size, and increased complexity, with increasing pH. This study describes a bridge across the first half the actinide series, highlighting U IV similarities to Th IV , in addition to the previously known similarities to the transuranic elements.

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

“…The charge and electronegativity of An­(IV) ions favor the olation products that are usually ill-defined structures and lead to amorphous precipitates unless stabilized through the presence of an oxygen donor or halogen ligands. − Both mechanisms have been prosed based on experimental observation via the characterization of the respective An­(IV) dimer products. − Two dimeric Pu­(IV)-complexes shown in Figure can be taken as an example of this. The first was reported by Hixon and co-workers where two Pu­(IV) centers are connected by an oxo anion consistent with an oxolation product (Figure a) .…”

Snapshots of Ce70 Toroid Assembly from Solids and Solution

“…37 The M6 cluster is contained within most M IV -oxoclusters, as well as in fluorite-type MO2; but it is not possible to simply build MO2 (and intermediate size clusters such as M38 [38][39][40][41][42][43][44] ) from M6-units, since the M6-units share M2 edges in the larger assemblies. In fact, M6 is shown to co-exist in solution with monomers, dimers and hexamers, [45][46] likely in dynamic equilibrium. This demonstrates that models of metal-oxo cluster building block approaches such as M6  [intermediates]  MO2 are oversimplified; and ultimately, scientists still lack control over the assembly processes in the M IV -oxocluster families.…”

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

“…[52][53][54][55][56][57][58][59][60][61][62] Both mechanisms have been prosed based on experimental observation via the characterization of the respective An(IV) dimer products. [63][64][65] Two dimeric Pu(IV)-complexes shown in Figure 3 can be taken as an example of this. The first was reported by Hixon and co-workers where two Pu(IV) centers are connected by an oxo anion consistent with an oxolation product (Figure 3a).…”