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

The Problem  2/3 of DOE sites have uranium-contaminated ground water  Estimated 4x10 12 L of contaminated ground water  Excavation of contaminated soil ineffective Oak Ridge, TNRifle, CO [U] ≤ 50 mg/L [U] ~ 0.17 mg/L MCL: mg/L EPA limit:.03 mg/L

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) biogenic uraninite 500 nm U(VI) + 2 e -  U(IV)

Bioremediation technique: acetate stimulation CH 3 COO − + UO H 2 O + NH 4 +  UO 2 (s) + H + + HCO 3 − Inject: electron donor (acetate, ethanol) Stimulate microbial growth in acetate plume Develop metal-reducing conditions Groundwater flow U(VI)  U(IV)

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!

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

Uraninite CH 3 COO − + UO H 2 O + NH 4 + = UO 2 (s) + H + + HCO 3 −  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? FT(Х(k)k 3 ) O U Uraninite R

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

Rifle, CO  Many wells drilled into soil to allow access to aquifer

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

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

In situ columns

In situ columns: well deployment

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

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)

EXAFS: P101 Sediments

EXAFS: P102 Sediments

EXAFS: P101 & P102 P101 EXAFS P102 EXAFS

Not uraninite!  Actual data vs. Uraninite FT(Х(k)k 3 ) Rifle well P102 sediment O U Uraninite R

Not uraninite!  Actual data vs. Uraninite FT(Х(k)k 3 ) Rifle well P102 sediment O U Uraninite R

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 situ results! CH 3 COO − + UO H 2 O + NH 4 + = UO 2 (s) + H + + HCO 3 −

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 situ results! CH 3 COO − + UO H 2 O + NH 4 + = UO 2 (s) + H + + HCO 3 −

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?

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!

Questions?