speciation of uranium in contaminated ground water at rifle co n.
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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. [ U] ≤ 50 mg/L. [ U] ~ 0.17 mg/L

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Presentation Transcript
the problem
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

uranium contamination and speciation

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)

bioremediation technique acetate stimulation
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)

microbial metal reduction
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
But the question is…

WHAT FORM OF URANIUM FORMS IN THE FIELD?

uraninite
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

rifle co
Rifle, CO
  • Site of a former uranium mill
  • Excavated under UMTRA, but ground water remains contaminated with 0.17 mg/L U
rifle co1
Rifle, CO
  • Many wells drilled into soil to allow access to aquifer
in situ columns
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
in situ c olumns

Effluent pump

Influent Pump

In situ columns

U(VI) ac solution

RGW

Reactor

xas x ray absorption spectroscopy
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

xanes determining oxidation state
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 p102
EXAFS: P101 & P102

P102 EXAFS

P101 EXAFS

not uraninite
Not uraninite!
  • Actual data vs. Uraninite

U

O

FT(Х(k)•k3)

R

Rifle well P102 sediment

Uraninite

not uraninite1
Not uraninite!
  • Actual data vs. Uraninite

U

O

FT(Х(k)•k3)

R

Rifle well P102 sediment

Uraninite

what does this tell us
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−

what does this tell us1
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−

so what is it
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
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!