Assessing the reactivity of Fe(II) sorbed on smectite clays: U(VI) reduction

The reactivity of Fe(II) sorbed on three Ca-exchanged smectite clays has been probed via U(VI) reduction at pH 6.0 (± 0.2) under CO2-free, anoxic (O2 <1 ppmv) condition. The clays varied with regard to structural Fe content from Fe-free (0 wt.%) montmorillonite (MONT), to Fe-poor (2.6 wt.%) montmorillonite (Fe-MONT), to Fe-rich (25.8 wt.%) nontronite (NAu-2). U LIII-edge XANES spectra showed no reduction of U(VI) in presence of Fe(II) sorbed on either Fe-MONT or NAu-2 after 72 h but a partial reduction (21%) on MONT after 24 h. U LIII-edge EXAFS spectra further showed the formation of a soddyite-like surface precipitate on MONT in presence of sorbed Fe(II) after 72 h. The Mössbauer data further reveals that 10 (± 2)% of the total sorbed Fe(II) was oxidized in presence of MONT before and an additional 6 (± 2)% after U(VI) addition. The mechanism of U(VI) reduction involves Fe(II) specifically sorbed on oxidized and reduced strong edge sites of MONT. The non-reactivity of Fe(II) sorbed on Fe-MONT and NAu-2 towards U(VI) reduction is likely linked to the inter-valence charge transfer (IVCT) between surface Fe(II) and structural Fe(III). The present study demonstrates that the reduction capacity of surface bound Fe strongly depends on the nature of clay and correspondingly on the oxidation state of the sorbed Fe. Thus Fe(II) sorbed clay may not be always considered as “universal reductant” for U(VI).