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Uranium incorporation into magnetite and its behaviour during subsequent oxidation has been investigated at high pH to determine the uranium retention mechanism(s) on formation and oxidative perturbation of magnetite in systems relevant to radioactive waste disposal. Ferrihydrite was exposed to U(VI) aq containing cement leachates ( pH 10.5-13.1) and crystallization of magnetite was induced via addition of Fe(II) aq . A combination of XRD, chemical extraction and XAS techniques provided direct evidence that U(VI) was reduced and incorporated into the magnetite structure, possibly as U(V), with a significant fraction recalcitrant to oxidative remobilization. Immobilization of U(VI) by reduction and incorporation into magnetite at high pH, and with significant stability upon reoxidation, has clear and important implications for limiting uranium migration in geological disposal of radioactive wastes.

24In this study we used in-situ synchrotron-based energy dispersive X-ray diffraction 25 (EDXRD) to follow the transformation of ferrihydrite to hematite at pH ~ 8 and ionic strength 26 between 0.1 and 0.7. In addition, the effects of co-precipitated molybdenum (Mo) and 27 vanadium (V) on the transformation were evaluated both through EDXRD and X-ray 28 absorption spectroscopy (XAS). The transformation end-product in all experiments was 29 hematite with small amounts of goethite as an intermediate phase. XAS results revealed that 30Mo and V were initially adsorbed and co-precipitated onto/with ferrihydrite as molybdate and 31 vanadate ions, respectively. After the ferrihydrite transformed to hematite these metals were 32 sequestered into the hematite structure. The kinetic results showed that the presence of Mo 33 and V in the ferrihydrite structure had little to no effect on the kinetics of the ferrihydrite 1 transformation. The transformation however occurred ~ 30% faster at higher ionic strength. 2 3

In this work, we present the results from multi-length-scale studies of a Mn-Na-W/SiO2 and a La-promoted Mn-Na-W/SiO2 catalyst during the oxidative coupling of methane reaction. The catalysts were investigated from the reactor level (mm scale) down to the single catalyst particle level (μm scale) with different synchrotron X-ray chemical computed tomography techniques (multi-modal chemical CT experiments). These operando spatially-resolved studies performed with XRD-CT (catalytic reactor) and multi-modal μ-XRF/XRD/absorption CT (single catalyst particle) revealed the multiple roles of the La promoter and how it provides the enhancement in catalyst performance. It is also shown that non-crystalline Mn species are part of the active catalyst component rather than crystalline Mn2O3 / Mn7SiO12 or MnWO4.