Rhizosphere Iron and Carbon Influence on Uranium Redox Cycling in a Wetland Environment
EMSL Project ID
48616
Abstract
All mobile contaminants in the upper aquifer eventually resurface in wetland areas. Due to their unique redox gradients and elevated microbial and organic carbon levels, wetlands tend to immobilize or greatly slow down the mobility of several contaminants. Recent studies conducted at SRNL and Princeton have measured significant elevated uranium concentrations in the wetland rhizospheres (the sediment region chemically impacted by root exudates) compared to the corresponding bulk soil. Rhizosphere uranium exists as both U(IV) and U(VI) and is primarily associated with organic carbon and phosphate, and to a smaller extent iron. Research conducted at EMSL (Project 47898; FY14) with SRS wetland sediments, showed that compared to the bulk soil, the rhizosphere: 1) included highly ordered particles that formed a 5 to 10 millimeter layer around the root (X-ray Computed Tomography, HIM), 2) was enriched with ferrihydrite/lepidocrocite/OM-Fe complex (Mossbauer), 3) was enriched in Fe-nanoparticles and organic matter (TEM, HAADF-STEM), and 4) contained organic matter with a unique functionality (FT-ICR-MS). The objective of this FY15 project is to use the Radiochemical Annex to provide greater insight into the unique environment of the rhizosphere that promotes uranium accumulation, evaluating the uranium bonding environment and the distribution of uranium within the root, rhizosphere, and bulk sediment system. Sediment/plant samples will be collected from M-Area’s Tims Branch wetland, which is an SRS wetland contaminated with depleted uranium. Two types of subsamples will be created at SRNL for subsequent analysis: 1) aggregate subsamples (<1-g) collected as a function of distance from the rhizosphere, and 2) physically-undisturbed epoxied cross-sectioned and epoxied-traversed samples that include the rhizosphere, root, and bulk soil. Samples will be analyzed by Mossbauer, Cryo-TEM, radiological-XRD, SEM/TEM, and electron microprobe. Four sample locations (eight total samples) will be collected to increase our odds that we will collect samples with a sufficient uranium concentration to meet analytical detection limits. Results from this work will be used to explain existing results from our on-going DOE-OS/BER project that are being used to develop a numerical and conceptual model of how wetlands immobilize uranium and what environmental conditions, if any, may result in an episodic uranium releases. Data generated from the Radiochemistry Annex will be leveraged against on-going activities, including: long term (multiple years) uranium-greenhouse mesocosm experiments, long-term field Tims Branch studies to monitor seasonal changes of uranium concentrations, PCR analyses, and synchrotron X-ray microprobe mapping, XANES and EXAFS of uranium and iron. Together the Radiochemistry Annex and on-going studies will provide a multi-disciplinary explanation of an important component of the carbon and iron cycles on a natural contaminant immobilization process that is rarely accounted for in risk calculations.
Project Details
Project type
Special Science
Start Date
2014-11-01
End Date
2015-09-30
Status
Closed
Released Data Link
Team
Principal Investigator
Co-Investigator(s)
Team Members
Related Publications
Kaplan DI, RK Kukkadapu, JC Seaman, BW Arey, A Dohnalkova, S Buttner, D Li, T Varga, KG Scheckel, and PR Jaffe. 2016. "Iron Mineralogy and Uranium-Binding Environment in the Rhizosphere of a Wetland Soil." Science of Total Environment 569-570:53-64. doi:10. 1016/j. scitotenv. 2016. 06. 120