U(VI) Sorption and Reduction Kinetics on the Magnetite (111) Surface

Abstract

continuous batch-flow conditions. We observed, in real-time and in situ, adsorption and reduction of U(VI) and subsequent growth of UO 2 nanoprecipitates using atomic force microscopy (AFM) and newly developed batch-flow U L III -edge grazing-incidence x-ray absorption spectroscopy near-edge structure (GI-XANES) spectroscopy. U(VI) reduction occurred with and without CO 3 present, and coincided with nucleation and growth of UO 2 particles. When Ca and CO 3 were both present no U(VI) reduction occurred and the U surface loading was lower. In situ batch-flow AFM data indicated that UO 2 particles achieved a maximum height of 4-5 nm after about 8 hours of exposure, however, aggregates continued to grow laterally after 8 hours reaching up to about 300 nm in diameter. The combination of techniques indicated that U uptake is divided into three-stages; (1) initial adsorption of U(VI),(2) reduction of U(VI) to UO 2 nanoprecipitates at surface-specific sites after 2-3 hours of exposure, and (3) completion of U(VI) reduction after ~6-8 hours. U(VI) reduction also corresponded to detectable increases in Fe released to solution and surface topography changes.Redox reactions are proposed that explicitly couple the reduction of U(VI) to enhanced release of Fe(II) from magnetite. Although counter intuitive, the proposed reaction stoichiometry was shown to be largely consistent with the experimental results. In addition to providing molecularscale details about U sorption on magnetite, this work also presents novel advances for collecting surface sensitive molecular-scale information in real-time under batch-flow conditions. 3

Retracted Article: Simultaneous sorption and reduction of U(vi) on magnetite–reduced graphene oxide composites investigated by macroscopic, spectroscopic and modeling techniques

“…As shown in (Fig. 6,11 Structural Fe(II) was identied as a stronger reducing agent with E 0 ¼ À0.65 V to À0.34 V. 37 The kinetics results revealed that reduction rates of U(VI) on M-rGO were slower than removal rates. It can be seen that the sorption process was the predominant U(VI) removal mechanism by M-rGO.…”

“…4,5 More importantly, as the naturally occurring minerals composed of Fe(II) and Fe(III), magnetite was evidenced to reduce U(VI) to U(IV) by the oxidation of Fe(II). 11 However, the magnetic particles are difficult to use in continuous ow system because they are instability and susceptible to agglomeration and oxidation. [8][9][10] Further evidence indicated that the redox transition of U(VI) occurred aer U(VI) was adsorbed on the surface of magnetite.…”

Combined effects of Fe(II) and oxidizing radiolysis products on UO2 and PuO2 dissolution in a system containing solid UO2 and PuO2

“…The reason for the initial decrease is most probably reduction of U(VI) to U(IV) by Fe(II), which has been observed before in a number of studies [24][25][26] for both solid Fe(II) phases as well as Fe 2+ in solution.…”

Uranium isotopes fingerprint biotic reduction

“…It has been suggested to proceed via the transfer of a single electron to U(VI), producing U(V), followed by the disproportionation of U(V) to U(IV) and U(VI) (29,30). However, there is also evidence that direct reduction to U(IV) occurs and is accompanied by the release of Fe(II) into solution (31). The discrepancy in reduction mechanisms observed among studies is likely due to the fact that control on the extent of U(VI) reduction is wrought by the combination of two competing factors: the Fe 2+ /Fe 3+ ratio at the surface of iron minerals and the U coordination environment (32).…”