National Research Program (NRP)
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.
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.