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 4x1012 L of contaminated ground

water Excavation of contaminated soil ineffective

Oak Ridge, TN

Rifle, CO

[U] ≤ 50 mg/L [U] ~ 0.17

mg/LMCL: 0.044 mg/LEPA 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!

But the question is…

WHAT FORM OF URANIUM FORMS IN THE FIELD?

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

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 Influen

t PumpU(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)•

k3)

Rifle well P102 sediment

O U

UraniniteR

Not uraninite! Actual data vs. Uraninite

FT(Х

(k)•

k3)

Rifle well P102 sediment

O U

UraniniteR

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!

Questions?

Questions?

Questions?

Questions?

Questions?

Questions?

Questions?

Questions?

Questions?

Questions?

Questions?