Speciation of uranium in contaminated ground water at Rifle, CO

1. Speciation of uranium in contaminated ground water at Rifle, CO by Nikki Peck

2. The Problem • 2/3 of DOE sites have uranium-contaminated ground water • Estimated4x1012 L of contaminated ground water • Excavation of contaminated soil ineffective [U] ≤ 50 mg/L [U] ~ 0.17 mg/L MCL: 0.044 mg/L EPA limit: .03 mg/L Oak Ridge, TN Rifle, CO

3. biogenic uraninite 500 nm Uranium contamination and speciation • Speciation: chemical/physical form, oxidation state, local molecular structure • U(VI) very soluble, very toxic • U(IV) orders of magnitude less soluble • Attempt to sequester uranium from ground water by reducing U(VI) into U(IV) U(VI) + 2 e-U(IV)

4. Bioremediation technique: acetate stimulation CH3COO− + UO2++ +H2O + NH4+UO2(s)+ H+ + HCO3− Inject: electron donor (acetate, ethanol) Groundwater flow Stimulate microbial growth in acetate plume Develop metal-reducing conditions U(VI) U(IV)

5. Microbial metal reduction • Anaerobic bacteria like Geobacter use metallic ions like we use oxygen • Acetate acts as an electron donor, stimulating growth and inducing anoxia • Microbes reduce electron acceptors like iron, sulfate and, of course, uranium!

6. But the question is… WHAT FORM OF URANIUM FORMS IN THE FIELD?

7. Uraninite CH3COO− + UO2++ + H2O + NH4+ = UO2(s) + H+ + HCO3− • Uraninite: least soluble form of nonmetallic U • Produced by metal-reducing bacteria in pure cultures BUT… Is uraninite actually the product of bioreduction in the field? U O FT(Х(k)•k3) R Uraninite

8. Rifle, CO • Site of a former uranium mill • Excavated under UMTRA, but ground water remains contaminated with 0.17 mg/L U

9. Rifle, CO • Many wells drilled into soil to allow access to aquifer

10. In situcolumns • Rifle U concentration is very low, making spectroscopy challenging • Need a method of adding U to allow for spectroscopyon sediment samples • Solution: in situ sediment columns! • Concentrate U in field conditions

11. Effluent pump Influent Pump In situ columns U(VI) ac solution RGW Reactor

12. In situ columns

13. In situcolumns: well deployment

14. XAS: X-Ray Absorption Spectroscopy • XAS consists of X-ray Absorption Near Edge Spectroscopy (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) XANES EXAFS

15. XANES: determining oxidation state • U(VI) vs. U(IV) shifts edge by ~3 eV • Fit linear combination of known U(VI) and U(IV) XANES spectra to find percentage 7% U(VI) 93% U(IV)

16. EXAFS: P101 Sediments

17. EXAFS: P102 Sediments

18. EXAFS: P101 & P102 P102 EXAFS P101 EXAFS

19. Not uraninite! • Actual data vs. Uraninite U O FT(Х(k)•k3) R Rifle well P102 sediment Uraninite

20. Not uraninite! • Actual data vs. Uraninite U O FT(Х(k)•k3) R Rifle well P102 sediment Uraninite

21. What does this tell us? • Clearly, the product of bioremediation is not uraninite • Models that apply to pure bacteria cultures do not hold for in situresults! CH3COO− + UO2++ + H2O + NH4+ = UO2(s) + H+ + HCO3−

22. What does this tell us? • Clearly, the product of bioremediation is not uraninite • Models that apply to pure bacteria cultures do not hold for in situresults! CH3COO− + UO2++ + H2O + NH4+ = UO2(s) + H+ + HCO3−

23. So what is it? • Obtain greater resolution to identify local structure more precisely • Understand speciation over time—does it change? • How stable is this reduced uranium?

24. Acknowledgements Special thanks to… • Department of Energy • SLAC SULI Program • My mentor, John Bargar • Fellow SULI members • Patricia Fox and Jim Davis at the USGS • Jose Cerrato from WUStL • Sung-Woo Lee and Carolyn Sheehan from OHSU • Marc Michel and Mike Massey • Many, many more!

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