Calcium uranyl(VI) oxyhydrates and uranates are structurally related U(VI)-phases featuring uranium oxo-polyhedral sheets, with calcium ions occupying the interlayer. Both coordination environments appear throughout the nuclear fuel-cycle as alteration products, colloids, and sorption complexes. However, concerted studies spanning the aqueous precipitation mechanisms of uranyl(VI) oxyhydrates, their solid-state transformations, and structural relationships with uranates, have hitherto remained largely unexplored.
A series of calcium-based uranyl(VI) oxyhydrates were precipitated via alkalisation of aqueous precursor solutions in titration and batch reactions. The bulk stoichiometric ratio of calcium to uranium (Ca/U) of precipitates was varied by modifying precursor stoichiometry, reaction temperature, or extraction pH. The rate of precipitation and its dependency on temperature was quantified in-situ using a quartz crystal microbalance. Novel insight was revealed on the mechanisms influencing nucleation and growth, by determining associated kinetic barriers as a function of precursor-Ca/U.
Remarkably, as the bulk precipitate Ca/U increased from ~⅛ to unity, there was a transition from crystalline Becquerelite to primary or secondary amorphous phases, with uranate-like coordination environments. Formation of the latter was driven by solution alkalinity, and comprises a poorly-ordered matrix with occlusions of Ca2+-rich nano-clusters. A congruency limit lies Ca/U of ~1.5 Ca/U, whereupon discrete Portlandite crystallises.
Solid-state transformation of all Ca2+-U(VI)-phases studied involved dehydration, dehydroxylation-decarbonation, and desorption processes. Associated kinetic barriers were catalysed by higher Ca2+-contents, and was reflected by reaction enthalpies for dehydration and desorption. Crystalline Becquerelite (~⅛ Ca/U) underwent amorphisation-crystallisation via partial egress of interlayer calcium, followed by reduction of β-UO3 to form a novel intercalation compound Ca0.18.α-U3O8. The endmember uranates Ca3U11O36, CaU2O7, Ca2U3O11, and CaUO4 crystallised from amorphous precursors with higher bulk Ca/U (~⅓, ~½, ~⅔, ~1), where Ca3U11O36 is a novel compound that is isostructural to (Pb/Sr)3U11O36. Nucleation and growth became predominant in the presence of Ca2+-rich occlusions. A higher Ca2+-loading facilitated the progressive ingress of interlayer-Ca2+, inducing a concerted axial compression in uranyl(VI) oxo-polyhedra towards the uranate-like coordination environment.
