Bioreduction
of soluble U(VI) to sparingly soluble U(IV)
is proposed
as an effective approach to remediating uranium contamination. However,
the stability of biogenic U(IV) in natural environments remains unclear.
We conducted U(IV) reoxidation experiments following U(VI) bioreduction
in the presence of ubiquitous clay minerals and organic ligands. Bioreduced
Fe-rich nontronite (rNAu-2) and Fe-poor montmorillonite (rSWy-2) enhanced
U(IV) oxidation through shuttling electrons between oxygen and U(IV).
Ethylenediaminetetraacetic acid (EDTA), citrate, and siderophore desferrioxamine
B (DFOB) promoted U(IV) oxidation via complexation with U(IV). In
the presence of both rNAu-2 and EDTA, the rate of U(IV) oxidation
was between those in the presence of rNAu-2 and EDTA, due to a clay/ligand-induced
change of U(IV) speciation. However, the rate of U(IV) oxidation in
other combinations of reduced clay and ligands was higher than their
individual ones because both promoted U(IV) oxidation. Unexpectedly,
the copresence of rNAu-2/rSWy-2 and DFOB inhibited U(IV) oxidation,
possibly due to (1) blockage of the electron transport pathway by
DFOB, (2) inability of DFOB-complexed Fe(III) to oxidize U(IV), and
(3) stability of the U(IV)-DFOB complex in the clay interlayers. These
findings provide novel insights into the stability of U(IV) in the
environment and have important implications for the remediation of
uranium contamination.