Post on 17-Dec-2015
The Problem 2/3 of DOE sites have uranium-
contaminated ground water Estimated 4x1012 L of contaminated ground
water Excavation of contaminated soil ineffective
Oak Ridge, TN
Rifle, CO
[U] ≤ 50 mg/L
[U] ~ 0.17 mg/L
MCL: 0.044 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
CH3COO− + UO2++ + H2O + NH4
+ UO2(s) + H+ + HCO3
−
Inject: electron donor (acetate, ethanol)
Stimulate microbial growth in acetate plumeDevelop 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!
UraniniteCH3COO− + UO2
++ + H2O + NH4+ = UO2(s) +
H+ + HCO3−
Uraninite: least soluble form of nonmetallic U Produced by metal-reducing bacteria in pure
culturesBUT…
Is uraninite actually the product of bioreduction in the field?
FT(Х
(k)•
k3)
O U
UraniniteR
Rifle, CO Site of a former uranium mill Excavated under UMTRA, but ground water
remains contaminated with 0.17 mg/L U
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
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)
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!
CH3COO− + UO2++ + H2O + NH4
+ = UO2(s) + H+ + HCO3
−
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!
CH3COO− + UO2++ + H2O + NH4
+ = UO2(s) + H+ + HCO3
−
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?
AcknowledgementsSpecial 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!