U(VI) sorption and reduction by Fe(II) sorbed on montmorillonite

The influence of surface bound Fe(II) on uranium oxidation state and speciation was studied as a function of time and pH (6.1-8.5) in U(VI)-Fe(II)-montmorillonite (Ca-montmorillonite) system under CO2-free, anoxic (O2 <1 ppmv) conditions. The results show a rapid removal of U(VI) from the aqueous solution within 1 h under all pH conditions. U LIII-edge X-ray absorption near-edge structure (XANES) spectroscopy shows that 96% of the total sorbed U(VI) is reduced at pH 8.5. However, the extent of reduction significantly decreases at lower pH values, in line with specifically sorbed Fe(II) decreasing. The reduction kinetics followed by X-ray photoelectron spectroscopy (XPS) during 24 h at pH 7.5 demonstrates the presence of partially reduced surface species containing both U(VI) and U(IV). Thermodynamically predicted mixed valence solids like U3O8/β-U3O7/U4O9 do not precipitate as verified by transmission electron microscopy (TEM) and extended X-ray absorption fine-structure (EXAFS) spectroscopy. This is also supported by the bicarbonate extractionresults. The measured redox potentials of Fe(II)/montmorillonite suspensions are controlled by the Fe(II)/ hydrous ferric oxide (HFO(s)) couple at pH 6.1 and by the Fe(II)/ γ-FeOOH(s) couple at pH 7.5. The U(VI) reduction mechanism is suggested by considering individual U(VI), U(IV) surface complexes with Fe(II) specifically sorbed on strong and weak sites of montmorillonite.