Search

The synthesis, characterisation and crystal structure of a novel U5+ (dominant) brannerite of composition U1.09(6)Ti1.29(3)Al0.71(3)O6 is reported, as determined from Rietveld analysis of high resolution powder neutron diffraction data. Examination of the UTi2-xAlxO6 system demonstrated the formation of brannerite structured compounds with varying Al3+ and U5+ content, from U0.93(6)Ti1.64(3)Al0.36(3)O6 to U0.89(6)Ti1.00(3)Al1.00(3)O6. Substitution of Al3+for Ti4+, with U5+ charge compensation, resulted in near-linear changes in the b and c unit cell parameters and the overall unit cell volume, as expected from ionic radii considerations. The presence of U5+ as the dominant oxidation state in near single phase brannerite compositions was evidenced by complementary laboratory U L3 edge and high energy resolution fluorescence detected (HERFD) U M4 edge X-ray Absorption Near Edge Spectroscopy. No brannerite phase was found for compositions with Al3+ / Ti4+ > 1, which would require U6+ contribution for charge compensation. These data expand the crystal chemistry of uranium brannerites to the stabilisation of dominant U5+ brannerites by substitution of trivalent cations, such as Al3+, on the Ti4+ site

Significance: Uranium (U) biogeochemical behavior is constrained by redox transformations. In anoxic environments, soluble hexavalent U is reduced and immobilized as tetravalent U. During abiotic U reduction, the formation of tetravalent U oxide (UO2) has been demonstrated and the persistence of an intermediate (pentavalent) valence state invoked. However, despite decades of study, there is little insight into the molecular mechanistic details of UO2 formation. Here, we show the formation of transient nanowires composed of randomly oriented UO2 nanoparticles followed by rearrangement into ordered UO2 nanoclusters. We also evidence the persistence of pentavalent U on the magnetite surface. These findings have implications for uranium isotopic fractionation, nuclear waste, and uranium remediation. Abstract: Uranium (U) is a ubiquitous element, present in the Earth’s crust at ~2 ppm. In anoxic environments, soluble hexavalent uranium (U(VI)) is reduced and immobilized. The underlying reduction mechanism is unknown but is likely of critical importance to explain variability in isotopic fractionation depending on the reducing agent and the chemical conditions. Here, we tackle the mechanism of reduction of U(VI) by the mixed-valence iron oxide, magnetite (Fe3O4). Through a combination of high-end spectroscopic and microscopic tools, we demonstrate that the reduction of U(VI) proceeds first through surface-associated U(VI) to form pentavalent U, U(V). U(V) persists on the surface of magnetite and is further reduced to tetravalent UO2 in the form of nanocrystals (~1-2 nm) arranged at random orientations in nanowires that extend hundreds of nanometers from the magnetite surface. Through re-orientation of the nanoparticles and their coalescence into larger nanoparticles, the nanowires collapse after several weeks to generate ordered UO2 nanoclusters. Thus, this work provides evidence for a transient U nanowire structure that may have implications for uranium isotope fractionation as well as for molecular-scale understanding of nuclear waste temporal evolution and the reductive remediation of uranium contamination.

The fire performance of magnesium potassium phosphate cement (MKPC) binders blended with fly ash (FA) and ground granulated blast furnace slag (GBFS) was investigated up to 1000°C using X-ray diffraction, thermogravimetric analysis and SEM techniques. The FA/MKPC and GBFS/MKPC binders dehydrate above 200°C to form amorphous KMgPO4, concurrent with volumetric and mass changes. Above 1000°C, additional crystalline phases were formed and microstructural changes occurred, although no cracking or spalling of the samples was observed. These results indicate that FA/MKPC and GBFS/MKPC binders are expected to have satisfactory fire performance under the fire scenario conditions relevant to the operation of a UK or other geological disposal facility.