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Topical Research: Uranium Interactions With Natural Organic Matter: Fundamental Biogeochemical Processes Controlling Complexation, Reactivity, Mobility, and Bioavailability in the Environment

The United States has substantial natural uranium (U) resources, primarily in the western states. These resources have been mined and milled historically for U and associated metal resources, resulting in numerous legacy sites with surface- and ground water contamination. Current mining of U is ongoing for domestic energy production, and mining is expected to continue into the future to support energy demand. Both conventional and solution mining have the potential to contaminate surface and ground water resources. The impacts of current and future U mining have widespread implications for regions such as the Grand Canyon, where a moratorium on U mining has been emplaced by the DOI until sufficient scientific information is available on the impact on biological and water resources. Ultimately, understanding the fundamental processes that control U mobility, reactivity, and bioavailability in water will help to identify the impacts of land use (U mining/milling), improve site management, and offer improved remediation strategies for legacy and current U mining, milling, and disposal sites.

One of the critical knowledge gaps in U biogeochemistry is the interaction between U and natural organic matter (NOM; Campbell et al., 2014). The ubiquity and structural complexity of NOM makes it an important component of U fate and transport, but the fundamental processes controlling U complexation and redox activity with NOM are relatively unconstrained. Uranium has a high affinity for NOM, both dissolved and particle-associated fractions, and NOM has been implicated in increased colloidal transport of uranium in natural waters. The mobility of NOM-associated U is complicated: complexation of U(VI) with dissolved NOM increases mobility in water, but association with solid-phase NOM can result in accumulation of U. In addition, NOM can be redox active, transferring electrons directly, acting as an electron shuttle, or serving as an electron donor for microbial U reduction. Microbial activity associated with NOM has also been hypothesized to play an important role in the deposition of some economically important U ore deposits. Complexation with NOM may also affect U bioavailability to multicellular (higher order) organisms as well, but there is a dearth of information on this topic. Common cations (e.g., iron and calcium) may also play an important role in U mobility through NOM-stabilized metal nanoparticles. Despite the importance of U-NOM interactions, the complexation and reactivity between different types of NOM and U have not been systematically quantified, even though these interactions are critically important to understanding ore deposit formation, U mobility and reactivity in surface and ground water, and U bioavailability. The purpose of this work is to characterize the fundamental chemical interactions between U and different types of NOM, to assess the redox activity of U and NOM, and to understand how U-NOM complexation affects bioavailability in aquatic macroinvertebrates. This work expands upon ongoing efforts to understand the influences of NOM on the biogeochemistry of metals (e.g., Hg, Zn, Cu) and the interaction of NOM with natural and anthropogenic nanomaterials, an emerging field of importance in the environmental geochemistry of natural waters (Aiken et al., 2011). This work addresses a broad fundamental research challenge relevant to the Environmental Health, Ecosystems, and Water mission areas.

Proposed Work

  1. U(VI) and U(IV) complexation by different fractions of characterized NOM. A library of well-characterized fractions of NOM from a wide range of natural waters has been assembled by Dr. Aiken, and will be used to systematically test the complexation of U by NOM. The effect of Fe and Ca (an important, competitive U ligand) on U complexation will be investigated. These experiments will determine whether stabilization of Fe colloids by NOM is an important process for U
  2. Redox activity of NOM and U. The redox activity of NOM will be assessed, particularly in the presence of Fe. Because the redox potential of U and Fe are similar, iron has been shown to facilitate both U reduction and oxidation, depending upon water composition and U complexation reactions. The effect of U-NOM complexation and U-NOM-Fe interactions will be evaluated.
  3. Microbial utilization of different fractions of NOM as an electron donor for U(VI) reduction. A common and well-characterized iron and U-reducing bacterium, Shewanella oneidensis CN32, will be used to evaluate the efficacy of the various NOM fractions to serve as an electron donor for U reduction, once the complexation of U-NOM has been well-characterized. Additional experiments will evaluate the effects of Fe and Ca on the rate and extent of U reduction by strain CN32.
  4. Effect of NOM on U adsorption on mineral surfaces. Adsorption affects the transport and possibly the dietary bioavailability of U. The effect of NOM on U adsorption to environmentally relevant minerals (e.g., iron oxides, clays) and potentially natural soils will be investigated.
  5. Bioavailability and bioaccumulation in aquatic macroinvertebrates. Using current tracer techniques, the underlying mechanisms governing the bioaccumulation processes of U will be characterized in controlled laboratory experiments using model aquatic organisms. Considering that bioavailability and chemical toxicity are closely linked to chemical speciation, the influence of hardness, pH and NOM on U bioaccumulation will be assessed.

This work addresses an important aspect of the water-energy connection through uranium biogeochemistry, and it also is relevant to understanding fundamental processes that affect other mining-related contaminants (e.g., As, Se, Cu, Zn, Cd, Hg, etc.) as well as natural and engineered nanoparticulate materials. In addition, by systematically linking the chemistry and biology of uranium-NOM interactions in controlled laboratory experiments, this work is the foundation for application to a wide variety of field applications that can be explored in subsequent research. This work is partially in support of the Toxics Program work in the Grand Canyon area. The results of this work will be directly applicable to the Grand Canyon effort, and the TRT will work closely with the Toxics project.

Team Members

References

Aiken, G.R., Hsu-Kim, H., and Ryan, J.N. (2011), Influence of dissolved organic matter on the environmental fate of metals, nanoparticles, and colloids, ES&T, v. 45, pp. 3196-3201

Campbell KM, Gallegos, TJ, and Landa ER (2014) Biogeochemical aspects of uranium mineralization, mining, milling, and remediation. Applied Geochemistry, in press.