Spectroscopic investigations of uranyl reduction by Fe-bearing clays


Spectroscopic investigations of uranyl reduction by Fe-bearing clays

Chakraborty1, S.; Boivin, F. F.; Gehin, A.; Banerjee, D.; Scheinost, A. C.; Greneche, J. M.; Mullet, M.; Bardelli, F.; Charlet, L.

Ferrous iron in clay is a potential natural reductant in anoxic environments. The reduction of UVI by FeII could be an important pathway for the immobilization of uranium in natural subsurface environments as well as in high-level nuclear waste (HLW) repositories. In the present study, we employed three dithionite-bicarbonate-citrate (DCB) treated, Ca-exchanged, Fe-bearing clays viz. montmorillonite (FeSM), Fe-rich smectite (Swa-1) and nontronite (NAu-2) with varying Fe content, and studied the reactivity of structural and readsorbed (surface complexed) Fe(II) species with respect to U sorption and/or reduction at pH 6 in a CO2-free anoxic atmosphere (<1 ppm O2 (v/v)) by using XPS, EXAFS and Mӧssbauer spectroscopies. The surface complexed Fe(II) species on untreated, Ca-exchanged clays were not found to be reactive in uranyl reduction. All three partially (17-45%) reduced, Ca-exchanged clays remove U from solution with a fast rate (minutes-hours), followed by a slow (months) reduction step. EXAFS analysis showed that UVI forms a mononuclear bidentate surface complex with FeIII in the untreated, Ca-exchanged clays which is the pre-requisite of heterogeneous reduction of U by their analogue in DCB treated clays. The adsorbed U was present as partially reduced mixed valence state (UVI and UIV) after 15 and 30 days. After 3 months, XANES spectra showed a substantial increase of UIV (20-50%) at the expense of UVI in case of reduced (DCB treated) FeSM, Swa-1 and complete reduction occurred (100%) in case of reduced NAu-2. The U4f5/2 XPS spectra were deconvoluted into two components, a higher binding energy (393.2±0.2 eV) for UVI and a lower binding energy (391.5±0.2 eV) for UIV. Despite the presence of abundant structural and readsorbed Fe(II) species and favourable reducing condition, the slow reduction of U might be due to (1) the stabilization of Ca in the clay interlayer inhibiting the cation release into solution which is coupled to the electron transfer process (2) that not all structural FeII species are potentially reactive at pH 6 and accessible to U for coordination.

Keywords: Uranium; reduction; clays; EXAFS; XPS

  • Lecture (Conference)
    Goldschmidt Conference, "Challenges to Our Volatile Planet", 21.-26.06.2009, Davos, Switzerland
  • Geochimica et Cosmochimica Acta 73(2009)13, A205-A205

Permalink: https://www.hzdr.de/publications/Publ-12670
Publ.-Id: 12670