Redox controls on uranium mobility: the role of natural organic matter transformations
EMSL Project ID
48570
Abstract
Natural organic matter (NOM) profoundly influences the cycles of metals in the subsurface in its role as electron donor for bacteria and as a complexing agent. In the proposed work, we investigate the impact of NOM degradation and composition on the fate and transport of the widespread groundwater contaminant uranium (U). The first goal of the proposed work is to link transformations of NOM to accumulation of solid phase U in the subsurface. Breakdown of large particulate or dissolved NOM by fermentative bacteria produces low molecular weight (LMW) compounds that are used by iron and sulfate reducing bacteria, which mediate reduction of soluble U(VI) to insoluble U(IV) under anaerobic conditions. A steady supply of LMW compounds in combination with anoxic conditions should lead to the removal of U (as U(VI)) from the aqueous phase (in which it is mobile) and to the incorporation of U (as (U(IV)) into the solid phase (in which it is immobile). We focus on redox interfaces where we expect U(IV) accumulation to be greatest due to a steady supply of LMW compounds diffusing in from the oxic zone (where O2-dependent oxidative enzymes are available to depolymerize NOM), and where O2 concentrations are low enough for microbes to utilize alternative electron acceptors (iron, sulfate). We will identify changes in functional group abundance, molecular size and composition in NOM fractions (particulate, bioavailable-dissolved and mineral-associated) along redox gradients (i) in diffusion limited flow-through reactors and (ii) in the field using techniques available at EMSL that provide detailed information on the molecular structure of NOM. Specifically, we propose to use high-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) in combination with a suite of solid- and liquid-state nuclear magnetic resonances (NMR) techniques. NOM transformations along the redox gradient will be linked to U distribution using XAS techniques available at the Stanford Synchrotron Radiation Lightsource (SSRL).
The second goal of the proposed work is to determine how NOM functional groups act as complexing ligands for U(IV), thereby controlling the speciation, hence reactivity of U. Although the mineral uraninite (UO2) is the expected product of U(VI) reduction in the laboratory, non-uraninite U(IV), which is expected to be more reactive towards re-oxidation, forms in the subsurface. NOM (particularly microbial biomass) contains phosphorus ligands that are expected to have a high affinity for U(IV). U(IV) speciation will be studied in batch reactors containing model NOM and in U-contaminated sediments. Electron microscopy techniques and nano-scale secondary ion mass spectrometry (NanoSIMS) available at EMSL provide the high spatial resolution required to identify which components of NOM and sediments - bacterial cells, plant material, minerals - are physically associated with U. In combination with spectroscopy (electron energy loss [EMSL] and X-ray absorption [SSRL]), we can identify which organic ligands complex U(IV) and how U(IV) complexes are distributed at the nano-scale.
The proposed work provides essential knowledge on how NOM controls radionuclide transformations and fate in subsurface environments, thereby improving predictive modeling of radionuclide transport. Also, this work will improve our process-based knowledge of subsurface carbon storage.
Project Details
Project type
Special Science
Start Date
2015-06-03
End Date
2015-12-31
Status
Closed
Released Data Link
Team
Principal Investigator
Co-Investigator(s)
Team Members
Related Publications
Bone S.E., J.B. Cliff, K. Weaver, C.J. Takacs, S.J. Roycroft, S. Fendorf, and J.R. Bargar, et al. 2020. "Complexation by Organic Matter Controls Uranium Mobility in Anoxic Sediments." Environmental Science & Technology 54, no. 3:1493-1502. PNNL-SA-142857. doi:10.1021/acs.est.9b04741
Bone SE, J Dynes, JB Cliff, III, and J Barger. 2017. "Uranium(IV) adsorption by natural organic matter in anoxic sediments." Nature Geoscience 114(4):711-716. doi:10.1073/pnas.1611918114