Assessing the reactivity of Fe(II) sorbed on smectite surface towards U(VI) reduction

The reactivity of Fe(II) sorbed on clay edges towards U(VI) reduction is investigated at pH 6.0 using Fe-free synthetic montmorillonite (MONT), Fe-poor natural montmorillonite (Fe-MONT) and Fe-rich natural nontronite (NAu-2). Partial reduction of U(VI) by sorbed Fe(II) only occurred on MONT but neither on Fe-MONT nor NAu-2. In the former case, the formation of a hydrated uranyl silicate (soddyite)-like phase is demonstrated by U L III -edge EXAFS spectroscopy. 57 Fe Mössbauer spectrometry reveals that in this case 10% of the total sorbed Fe(II) was oxidized before- and an additional 6% after-addition of U(VI). The lack of reactivity towards U(VI) of Fe(II) sorbed on Fe-MONT or NAu-2 is likely due to inter-valence charge transfer (IVCT) between surface Fe(II) and structural Fe(III). The present study demonstrates that the reactivity of clay sorbed Fe 2+ towards U(VI) strongly depends on the nature of clay and correspondingly on the oxidation state of sorbed Fe. Graphical abstract