Reduction of U(VI) by Fe(II) in the presence of hydrous ferric oxide and hematite: Effects of solid transformation, surface coverage, and humic acid

Fe(II) was added to U(VI)-spiked suspensions of hydrous ferric oxide (HFO) or hematite to compare the redox behaviors of uranium in the presence of two different Fe(III) (oxyhydr)oxides. Experiments were conducted with low or high initial sorption density of U(VI) and in the presence or absence of humic acid (HA). About 80% of U(VI) was reduced within 3 days for low sorbed U(VI) conditions, with either hematite or HFO. The {Fe³⁺} in the low U(VI) experiments at 3 days, based on measured Fe(II) and U(VI) and the assumed presence of amorphous UO_2(s), was consistent with control by HFO for either initial Fe(III) (oxyhydr)oxide. After about 1 day, partial re-oxidation to U(VI) was observed in the low sorbed U(VI) experiments in the absence of HA, without equivalent increase of dissolved U(VI). No reduction of U(VI) was observed in the high sorbed U(VI) experiments; it was hypothesized that the reduction required sorption proximity of U(VI) and Fe(II). Addition of 5 mg/L HA slowed the reduction with HFO and had less effect with hematite. Mössbauer spectroscopy (MBS) of ⁵⁷Fe(II)-enriched samples identified the formation of goethite, hematite, and non-stoichiometric magnetite from HFO, and the formation of HFO, hydrated hematite, and non-stoichiometric magnetite from hematite.