Assessing the Role of Iron Sulfides in the Long Term Sequestration of Uranium by Sulfate Reducing Bacteria
EMSL Project ID
34917
Abstract
Uranium (U), originating from weapon manufacturing and nuclear energy production, is one of the most prevalent radionuclide contaminants at DOE sites. In situ microbial reduction of U(VI) into sparingly soluble solids (e.g., UO2(s)) has been found to be a potentially effective remediation technique for groundwater and soil contaminated with U. The stability of bio-reduced U(IV) and its potential re-oxidation have been recognized as an important topic that requires further investigation. When reduced U(IV) is subjected to oxidants such as oxygen or nitrate re-oxidation can result in the oxidative mobilization of the U(IV) to soluble U(VI). Therefore, maintenance of the reduced state is an important aspect in controlling the longevity of in situ microbial reduction-based U-treatment techniques at DOE sites. The potential for iron sulfide minerals to act as redox buffers to protect bio-reduced U(IV) from re-oxidation has been recognized. Therefore, this proposed research has as its overarching goal to assess the impact of iron sulfides produced under sulfate reducing conditions on the oxidative mobilization of reduced solid forms of U as a function of pH, calcium, carbonate, and oxidant concentration (e.g, oxygen, nitrate or nitrtite, Fe(III) solids). To accomplish this, we will: 1) identify and characterize biogenically- and chemically-generated iron sulfides and U phases produced under sulfate reducing conditions (SRC), 2) characterize of the changes in the reduced U and iron sulfide phases produced under SRC when subjected to the targeted oxidants under various concentration conditions in batch, CMFR and column reactors studies, and 3) evaluate of the impact of the types of solid phases produced under various SRC on the subsequent oxidative dissolution of U using state-of-the-art instrumentation at the EMSL at PNNL. Selected biogenically and synthetically produced iron sulfide solid phases (e.g., mackinawite, greigite, pyrrhotite, and pyrite) in the presence and absence of RIFLE sediments, U associations with these solids, and changes that result during oxidative exposure in various reactor configurations (Batch, CMFR, and Column) will be characterized using advanced instrumentation at the PNNL EMSL including Mossbauer spectroscopy and XPS, and microscopic tools such as SEM and TEM. Both Mossbauer and XPS spectroscopy will be used to track subtle or dramatic changes that may occur in the bulk and on the surface of iron sulfides under low oxidant concentrations and shorter exposure times. Previous Mossbauer and XPS studies have illustrated that mackinawite quickly forms a weathered layer of greigite upon exposure to oxygen, a result that would be difficult to establish by either method alone. Yet, it is the oxidized surface layers that likely control the relative rates of oxidation of U and iron sulfide phases. Microtextural characterization of changes in the U phases loaded on nano-scale iron sulfide minerals will be performed using HR-TEM. Changes in the location or morphology of U-associated minerals will be performed using SEM with EDXS. This study will help clarify the role of iron sulfides in the long term immobilization of reduced U solids.
Project Details
Project type
Large-Scale EMSL Research
Start Date
2009-10-05
End Date
2012-09-30
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members
Related Publications
Bi Y, SP Hyun, RK Kukkadapu, and KF Hayes. 2013. "Oxidative Dissolution of UO2 in a Simulated Groundwater Containing Synthetic Nanocrystalline Mackinawite." Geochimica et Cosmochimica Acta 102:175-190. doi:10.1016/j.gca.2012.10.032