Bioreduction of soluble U(VI) to
sparingly soluble U(IV) solids
was proposed as a remediation method for uranium contamination. Therefore,
the stability and longevity of biogenic U(IV) are critical to the
success of uranium remediation. However, co-occurrence of clay minerals
and organic ligands could potentially reoxidize U(IV) to U(VI). Herein,
we report a combined effect of Fe(III)-rich nontronite (NAu-2) and
environmentally prevalent organic ligands on reoxidation of biogenic
U(IV) at circumneutral pH. After 30 days of incubation, structural
Fe(III) in NAu-2 oxidized 45.50% U(IV) with an initial rate of 2.7
× 10–3 mol m–2 d–1. Addition of citrate and ethylenediaminetetraacetic acid (EDTA)
greatly promoted the oxidative dissolution of U(IV) by structural
Fe(III) in NAu-2, primarily through the formation of aqueous ligand-U(IV)
complexes. In contrast, a model siderophore, desferrioxamine B (DFOB),
partially inhibited U(IV) oxidation due to the formation of stable
DFOB-Fe3+ complexes. The resulting U(VI) species intercalated
into an NAu-2 interlayer or adsorbed onto an NAu-2 surface. Our results
highlight the importance of organic ligands in oxidative dissolution
of U(IV) minerals by Fe(III)-bearing clay minerals and have important
implications for the design of nuclear waste storage and remediation
strategies, especially in clay- and organic-rich environments.