Combined Effects of Fe(III)-Bearing Nontronite and Organic Ligands on Biogenic U(IV) Oxidation

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.