IMX Seminar Series - Nanoscale mechanism of uranium reduction by magnetite

Uranium (U) is a ubiquitous element in the Earth’s crust and its biogeochemical behavior is largely constrained by its redox transformation from soluble uranium hexavalent species (U(VI)) to sparingly soluble tetravalent species ((U(IV)). U(VI) reduction by mineral phases has been shown to produce crystalline U in the form of U(IV)O2, but also to form persistent pentavalent U (U(V)). Magnetite (Fe3O4) is an Fe(II)-bearing iron oxide and experimental studies have shown that the co-precipitation of U(VI) and magnetite resulted in the formation of a stable U(V) coordination in the iron oxide mineral phases [1]. A study [2] reported the formation of single U oxide nanocrystals (1-5 nm) followed by the formation of nanowires that extended away the magnetite surface. Over time, the nanowires collapsed into ordered UO2 nanoclusters. Numerous questions arise from the transient formation of uranium oxide nanoparticle nanowires. The most salient are: (a) why do these nanowires form? (b) why do they persist? and (c) why do they collapse? The current hypothesis is that the nanoparticles harbor pentavalent uranium (as mixed valence uranium oxides) that is slowly reduced further to tetravalent uranium. Thus, the formation and persistence of nanowires is linked to that of U(V). Here, we present O K-edge and U N-edge electron energy loss spectroscopy spectra from individual uranium oxide nanoparticles within the nanowires in order to characterize the valence state of individual nanocrystals by comparing their fine structure to references mixed valence oxides
measured under the same conditions.
The mechanism that emerges at the scale of individual nanoparticles (1-5 nm) is the initial reduction of U(VI) to U(V) at the magnetite surface, producing mixed valence oxides UO2+x that self-assemble into nanowires. These nanowires are stable as long as no further reduction occurs but reduction to UO2 results in the collapse of nanowires into nanoclusters. The reduction of U(VI) by magnetite represents an example of heterogeneous reductive precipitation that, due to the properties of uranium, can be resolved at the near atomic scale and reveal the complexity of electron transfer from mineral to metal.
[1] Pidchenko et al. Environ. Sci. Technol., 51, 2217–2225 (2017).
[2] Pan et al., Nat. Commun., 11, 4001 (2020)

Bio: Rizlan Bernier-Latmani is a geo-microbiologist with interests in geochemical and microbial processes in the environment, particularly microbially-mediated transformations in subsurface environments and in the mammalian gut. She studied in the United States and came to Switzerland as a tenure-track assistant professor in 2005. She is now a full professor in the Environmental Engineering Institute. Her work spans molecular-scale mechanistic understanding to the interrogation of field-scale processes and makes use of a large array of tools ranging from spectroscopy and microscopy to meta-omics and sampling soil and water in remote areas. Today, she will talk about her work on uranium reduction by the iron oxide magnetite and its surprising mechanism.