The roles of clay minerals and/or organic ligands in the mobility of uranium: an insight for geological disposal of radioactive waste

Clay and clay rocks are being considered as possible barriers for nuclear waste disposal worldwide, due to their high adsorption capacity and limited hydraulic conductivity. Moreover, clay minerals have been regarded as one of the major Fe-containing phases in the Earth’s crust due to their ubiquitous occurrences in soil and sediments.  Hence, a solid understanding of uranium interactions with Fe(III)-bearing clay minerals is needed for the optimal design and long-term stewardship of uranium waste disposal. Besides clay minerals, metal-chelating ligands were found at appreciable concentrations in certain uranium-contaminated sites due to their prevalent use as decontaminant (e.g., complexing) agents in radioactive waste streams, which ultimately impacted the redox kinetics of U in proximity. Herein, we report a combined effect of Fe(III)-rich nontronite (NAu-2) and environmentally prevalent organic ligands on re-oxidation of biogenic UO2 at circumneutral pH. After 30 d incubation, structural Fe(III) in NAu-2 oxidized 45.50% UO2 with an initial rate of 2.68*10-3 mol*m-2*d-1. The addition of citrate and EDTA greatly promoted the oxidative dissolution of UO2 by structural Fe(III) in NAu-2, primarily through the formation of aqueous ligand-U(IV) complexes. In contrast, a model siderophore, DFOB, partially inhibited UO2 oxidation due to the formation of a stable DFOB-Fe3+ complex. The resulting U(VI) species intercalated into the NAu-2 interlayer, driving UO2 dissolution by keeping dissolved U(VI) concentrations low. Our results highlight the importance of organic ligands on the 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.