Project Title: Impacts of Uranium Mining/Milling on Groundwater and Remediation with Mackinawite
Mendenhall Fellow: Tanya J. Gallegos, tgallegos@usgs.gov, (303) 236-1914
Duty Station: < Denver, CO
Start Date: November 14, 2007
Education: Ph.D. 2007, University of Michigan, Environmental Engineering
Research Advisors: Jim Otton, jkotton@usgs.gov, (303) 236-8020; Bob Zielinski, rzielinski@usgs.gov, (303) 236-4719; Jim Davis, jadavis@usgs.gov, (650) 329-4484; Rich Wanty, rwanty@usgs.gov, (303) 236-1819; James Ranville (Colorado School of Mines), jranvill@mines.edu
Project Description: This work is part of the Uranium Life Cycle studies aimed at evaluating natural uranium ore formation processes and characterizing geological geochemical changes that occur during uranium extraction/processing to develop environmentally sustainable methods to reverse these changes, thus completing the life cycle of uranium mineralization. In 2008, the in-situ recovery (ISR) process was the primary method of uranium recovery in the United States. Most operations employ O2/CO2 enriched ground water (lixiviant) to oxidize and complex the uranium causing it to dissolve. Sulfides often associated with ISR-amenable ore bodies in uranium roll front deposits can interfere with uranium yields by consuming lixiviant resulting in difficulty in achieving oxidizing conditions (step 1, fig. 1). Upon completion of mining operations, it may be difficult to restore ground water to baseline water quality. Natural or introduced sulfides may facilitate the ground-water restoration processes by re-stabilizing uraninite and other mineral species through the re-establishment of reducing conditions (step 2, fig. 1). In effect, the use of iron sulfides such as mackinawite may serve to close the loop on the life cycle of uranium mining and milling.
Figure 1. Reactive media may be implemented into permeable reactive barriers to provide a chemical barrier for in-situ groundwater cleanup.
Mackinawite is attractive as a possible sequestration agent for U(VI) due to its:
- Reducing Capacity
- Reductive dechlorination
- Cr(VI)àCr(III)
- As(III)àAsS (s)
- Ability to scavenge metals due to:
- High Solubility-source of sulfide for precipitation reactions
- High surface area which is ideal for sorption reactions
Hypothesis. The project proposed herein hypothesizes that nano-particulate synthetic mackinawite will be effective at lowering dissolved uranium concentrations in ground water by promoting reducing conditions and forcing the precipitation of uranium solids or sorption to the mackinawite surfaces.
Objectives
- Explore iron sulfide-uranium interactions
- Determine feasibility of using iron sulfide (mackinawite) for groundwater restoration
Experimental Results
Mackinawite is a promising reactive medium for sequestering dissolved uranium at ISR sites through a reductive-precipitation mechanism.
Future Research
- Testing the ability of mackinawite to restore groundwater following ISR.
- Characterization of a uranium roll-front deposit core sample to determine natural elemental associations with uranium.
- Bench-scale simulation of in-situ carbonate/O2(g) leaching to extract uranium and characterize newly formed solids.
- Simulation of the mackinawite-based remedial approach on the leached uranium core samples to evaluate fixation of uranium and other metals.
Acknowledgments
- X-ray absorption was performed at the Stanford Synchrotron Radiation Laboratory, a facility operated by the Department of Energy Office of Basic Energy Sciences. Special thanks to Dr. Sam Webb.
- Thanks to Bill Betterton, USGS, for help in obtaining XRD Data.
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Last modified: 16:08:28 Thu 13 Dec 2012
