Fate of tetravalent uranium under reducing conditions

The stimulation of microbial reduction of the soluble hexavalent U [U(VI)] to sparingly soluble tetravalent U [U(IV)] has been exploited as an in-situ strategy for the immobilization of uranium in contaminated aquifers. The success of this strategy rests on the low solubility of U(IV) phases that are formed as the product of microbial reduction. Recent research efforts have shown that these reduction products do not only consist of stable crystalline mineral phases such as uraninite, but are also associated with biomass such as non-crystalline U(IV) species. While the thermodynamic properties and the mechanisms and rates of mobilization of such biomass-associated U(IV) are unknown, experimental evidence seems to indicate that they are more mobile and may impact the efficacy of uranium bioremediation. Additionally, the identification of U(IV)-bearing colloids composed primarily of organic matter and iron in a wetland, suggests the importance of organic ligands in mobilizing U(IV). Finally, it was previously shown that biogenic ligands can accelerate the dissolution of crystalline uraninite and we suggest that they may also promote the mobilization of non-crystalline U(IV). In this context, the proposed work will address the kinetics and mechanisms of the mobilization of non-crystalline U(IV) by biogenic ligands and reduced humic substances and how they compare to those of uraninite. We will also probe the impact of ligands on the potential transformation of non- crystalline U(IV) to crystalline U(IV) species since this process is expected to impact the stability of U(IV). In order to establish tools to identify these processes in complex natural systems, we investigate the suitability of uranium isotope fractionation as a proxy for the mechanisms of U reduction, of mobilization of non-crystalline U(IV) induced e.g. by complexation with organic ligands, as well as of potential transformation of non-crystalline to crystalline U(IV). We propose that the quantification of isotope fractionation may also be used for the elucidation of the molecular mechanisms of these processes in field and microcosm studies. Finally, we will develop quantitative reactive transport models including kinetic processes and isotope fractionation that will be tested against column experiments that approximate the complexity of field scale uranium reduction and re-mobilization. This model will not only provide a useful tool for predicting uranium transport under reducing conditions, but also aide the interpretation of isotope fractionation along the flow path. The results of this research will deliver new insights into the mobility of U from underground sources of U contamination such as leaching from mine tailings or from unexploded depleted uranium ammunition in soil. We furthermore expect them, to provide valuable information that may be used to adjust the design of remediation strategies as well as to evaluate their sustainability.

The key findings of this work emphasize the ability of organic ligands present in the environment to increase dissolved uranium concentrations in groundwater, thus providing beneficial information to improve contaminated site characterization efforts and remediation strategies. Uranium (U) is a contaminant of concern in the environment, presenting threats to human health, partly due to its chemical toxicity. A long-established strategy to mitigate U concentrations in groundwater is to immobilize the U locally by harnessing native microbes and the addition of non-injurious chemicals (such as glucose) to facilitate the immobilization (by means of reduction). In doing such, the U is understood to be relatively stabilized as long as no oxygen enters the vicinity, which can re-dissolve the immobilized U (by means of oxidation). In our work we highlight the concerns posed by organic ligands to re-release immobilized U even in the absence of oxygen. Organic ligands can be found in the environment from both natural sources and synthetic (human-derived) processes. Examples of these include citrate, a ubiquitous organic ligand in the environment (used in food and cleaning supplies as well as present in natural waters and soils), N,N'-di(2-hydroxybenzyl)ethylene-diamine-N,N'-diacetic acid (HBED), a synthetic ligand commonly used in agricultural practices (notably in iron fertilizers), or natural humic and fulvic acids from soil forming processes. Results show that, depending on the form of immobilized U, ligands can be more or less effective at mobilizing U. Substantially higher dissolved U concentrations were found to come from more labile immobilized (reduced) U forms and at a faster rate. Furthermore, differing ligands will mobilize U to varied extents depending on a variety of chemical factors and generally increases in the ligand concentration correlate with increases in dissolved U concentrations. Experiments with humic substances procured from the Suwannee River, Florida, USA (humic acid and fulvic acid), which represent a natural source of organic matter with large environmental relevancy that additionally were shown to mobilize U to similar levels as other tested ligands. The effectiveness of organic ligands to mobilize U was shown to be affected by other metals which could be present in the environment, creating competition which results in diminished U mobilization in the presence of metals such as iron, calcium, zinc, and aluminum. Furthermore, it was shown that dissolved U concentrations could increase further when in the presence of both organic ligands and humic substances. The findings of this work provide meaningful insights into the stability of immobilized U in the environment (notably under oxygen-free conditions) in the presence of organic ligands. These results therefore yield valuable information for maintaining diminished U concentrations in drinking water sources where immobilized U sources are in the vicinity.