Energy and biogeochemistry: Nuclear fuel and weapons production have produced radionuclide and heavy metal contamination in terrestrial environments across the United States, threatening aquifers already under pressure by increasing land use and climate change. Global warming, another important consequence of energy production, is also a significant concern. For example, the thawing of permafrost is creating the potential for unprecedented releases of powerful greenhouse gases, including methane and CO2, to the atmosphere. The behavior of radionuclides, heavy metals, and carbon in terrestrial environments is mediated by microbial activity as well as by soil and sediment characteristics, such as the composition of reactive coatings on mineral grains. These biological and geochemical aspects intertwine at the molecular scale in rich and complex tapestries of “biogeochemical” reaction networks. In spite of their importance, we know relatively little in detail about these processes in subsurface environments and the participating species. This lack of knowledge, a result of the tremendous complexity of terrestrial systems, severely limits our ability to understand and predict radionuclide migration in aquifers, the stability of soil carbon, and the long-term impacts of land-atmosphere interactions.
Mission: The mission of the SLAC SFA program is to develop deeper understandings of the molecular- to pore-scale biogeochemical processes that control the behavior of uranium, metals, and carbon in subsurface environments. This behavior, in turn, is heavily influenced by reactions involving other so-called biogeochemical critical elements (BCEs), particularly iron and sulfur, as well as the mineral surfaces and sediments hosting these reactions. Through molecular-scale studies of uranium, carbon, iron, sulfur, and other BCEs, we are providing the fundamental scientific understanding required to wisely manage contaminated sites, develop aquifer remediation technologies, and improve predictive understanding of uranium and carbon cycling in soils and sediments.
Unique approach: We are a world leader in the use of synchrotron-based x-ray techniques to study biogeochemistry. Synchrotrons are unique in their ability to non-invasively study uranium and BCEs in their native sedimentary environments at environmentally realistic conditions in space and time at length ranging angstroms to meters. Synchrotron techniques we are using include x-ray absorption spectroscopy (XAS), x-ray microprobe (XRM), scanning transmission x-ray microscopy (STXM), high-energy x-ray scattering pair distribution functional analysis (X-PDF), and x-ray diffraction (XRD). Much of this research is conducted at our home laboratory, the Stanford Synchrotron Radiation Lightsource (SSRL). However, we are also using other DOE-funded lightsources, including the Advanced Light Source in Berkely, CA, and the Advanced Photon Source in Argonne, IL. Our group has deep expertise in soils, biogeochemical, and contaminant science, and we have pioneered the integration of genomic, microbiological, and geochemical techniques with synchrotron approaches to explore “native” reactions occurring in aquifers and soils.
Major research questions and impact: We are investigating the following major questions:
(i) What controls the products formed when bacteria mediate the reduction of hexavalent uranium (U(VI), the most common oxidation state in natural waters) to the much less soluble and more desirable form, tetravalent uranium (U(IV))?
We have studied the molecular-scale interactions between bacteria and uranium in controlled laboratory studies to better understand and predict the uranium products obtained when bacteria pass electrons to U(VI) to chemically reduce it to U(IV). This work has played a major role in establishing the importance of previously unknown noncrystalline forms of U(IV) in the laboratory and field, discovering the biogeochemical factors controlling its production, and measuring its stability.
(ii) What controls the behavior of uranium in the contaminated aquifer at the Rifle, Colorado legacy ore processing site?
Uranium at this site is tightly linked to naturally abundant sediment organic carbon (decayed plant materials). Bacterial degradation of this organic carbon is the “engine” and electrons are the “currency” (transferred from carbon to uranium, iron, and sulfur) that causes U(VI) in groundwater to become transformed into U(IV) and accumulate in the sediments. We are studying the mechanistic molecular-scale linkages between these coupled biogeochemical processes to develop a fundamental understanding of the steps involved. We are also studying the processes controlling oxidation and release of U(IV) from these sediments, which is believed to be responsible for prolonging the lifespan of the uranium groundwater plume at this and similar sites in the Western U.S.
(iii) What controls the stability of natural organic matter in soils?
We are studying the speciation of carbon and nitrogen in permafrost and boreal soils in collaboration with Oak Ridge National Laboratory and University of Lund. This work is helping to identify the major compound classes present in solid phase soil organic matter and how these classes change in time and space as a result of microbial degradation. This knowledge will help to improve the predictive accuracy of climate-land models.
(iv) What is the molecular-scale structure and reactivity of the environmentally ubiquitous poorly crystalline nano-iron oxyhydroxide, ferrihydrite?
Ferrihydrite is a highly reactive material that coats mineral grains in soils and aquifers, masking their properties. It is profoundly important to the metabolism of iron-reducing bacteria, which also help reduce U(VI) to U(IV). Ferrihydrite strongly sorbs U(VI) and can oxidize U(IV) back to U(VI). We have developed new structural models to explain the properties of this mysterious mineral and its reactivity in soils and aquifers.
Support: This program is funded by the Subsurface Biogeochemistry program within the U.S. Department of Energy, Office of Biological and Environmental Research, Climate and Environmental Sciences Division. Funding for SSRL is provided by the Department of Energy, Office of Basic Energy Sciences.
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