Uranium Release from Contaminated Sediments
at the IFRC site in Rifle, CO
J.A. Davis1, P.M. Fox1, M.B. Hay1, K.M. Campbell1,
S.P. Hyun2, K.F. Hayes2, A.D. Peacock3,
K.H. Williams4, and P.E. Long5
1 U. S. Geological Survey, Menlo Park, CA 2 University of Michigan, Ann Arbor, MI
3 Microbial Insights 4 LBNL, Berkeley, CA 5 PNNL, Richland, WA
Groundwater Flow and Mixing
+ Reactive Processes
Field Scale Reactive Transport Model
Risk Assessment
Remediation strategies; understanding of
natural attenuation
Requires conceptual model and
calibrated parameters
Acetate
Injection
Gallery 1st row 2nd row 3rd row
Monitoring Wells
GW
Flow
U(VI) Sulfate
Fe(II)
Sulfate
Reactive Transport Modeling:
Steve Yabusaki, PNNL
Primary Chemical Species:
U(VI), Ca, Mg, Fe(II), pH,
HCO3-, SO4
2-, and acetate
Terminal Electron Acceptors:
Fe(III), SO42-, and U(VI)
Products: Fe(II), S2-, HCO3-, and U(IV)
Bacterial Growth: DMRB and SRB;
exopolymer production
Solid Phase Reactions: Bioreduction of Fe(III)-bearing minerals
Precipitation and dissolution of iron sulfide and carbonate minerals
S2- reaction with Fe(III)-bearing minerals
Ion exchange
U(VI) and Fe(II) adsorption
Precipitation of U(IV) – Entire Rifle team
Abiotic reduction of U(VI) by adsorbed Fe(II)
Abiotic oxidation of U(IV) by Fe(III) minerals
Ambient and
biostimulated conditions
Little Rusty Experimental Plot
Little Rusty location: non-
stimulated experiments
Colorado River
Biostimulated Experiments:
Winchester, Big Rusty
Little Rusty
Experimental Plot
U(VI) desorption at
the field scale
Port Depth
1 12 ft bgs
2 15
3 18
4 21
5 24
7 25
Surface and Aqueous Complexation Modeling
>FeOH + UO22+ + H2O >FeOUO2OH + 2H+
+
2Ca2+ + 3HCO3-
Ca2UO2(CO3)3(aq) + 3H+
Kads
KCa2UO2CO3 (Dong and Brooks, 2006)
Rifle baseline U(VI) sorption study
Conduct batch and column experiments with air-dried background sediments
under oxic conditions in artificial groundwater solutions; Primary
experimental variables: LR Groundwater:
pH: 7.2-7.9 7.1-7.4
pCO2: 0.05-2.6% 2-5%
U(VI) concentration: 0.03-6.3 (μM) 0.15-0.4 μM
Ca: 3-6.5 mM 3-4 mM
<2 mm Rifle Background-A Sediment Composite
Sample collected 1 mile upgradient (below water table)
56% quartz; 20% plagioclase; 15% K-feldspar
67 moles Fe/g sediment
1.35E-9 moles extractable U(VI) (0.02M (bi)carbonate solution, pH 9.5)
<2 mm Little Rusty Sediment Composite
Sample collected near LR gallery (below water table)
44 moles Fe/g sediment
1.39E-9 moles extractable U(VI) (0.02M (bi)carbonate solution, pH 9.5)
Kinetics of Bicarbonate Solution U(VI) Extraction
0.02M (bi)carbonate solution, pH 9.5 (Kohler et al., 2004)
Old Rifle U(VI)
equilibrium surface
complexation model
Hyun et al., 2009
Reaction Log K
Ss(OH)
2+UO
22+ = S
s(OH)OUO
2+ + H+ 8.03
Sw(OH)
2+UO
22+ = S
w(OH)OUO
2+ + H+ 3.55
Ss(OH)
2 + UO
22+ + 2H
2CO
3 = S
s(OH)OUO
2(CO
3)23- + 5H+ -15.2
Sw(OH)
2 + UO
22+ + 2H
2CO
3 = S
w(OH)OUO
2(CO
3)23- + 5H+ -20.1
<2mm composite surface area: 4.1 m2/g
Sand fraction surface area: 1.2 m2/g
Log dissolved U(VI) (moles/L)
Log dissolved U(VI) (moles/L)
Log K
d (
ml/
g)
Log K
a (
ml/
m2)
Test of Model: U(VI) on Sand-Sized Fraction 95% by weight; repeated wet sieving and sonification
Test of Model: Little Rusty U(VI) Adsorption Field Data
Hyun et al., 2009
Depth, ft
bgs
Dissolved
U(VI),
μM
<2mm,
surface
area (m2/g)
% <2mm
by weight
Carbonate
extractable
U(VI), nmol/g
Model predicted U(V)
adsorption, nmol/g
15-17.5 0.18 4.5 36 1.06 1.25
17.5-23.5 0.17 3.3 80 0.49 0.39
23.5-25.5 0.14 2.6 58 0.28 0.18
LR
Composite
NA 4.8 100 1.39 NA
Column packed with AGW with no alkalinity
Pore volume 26 mL; flow 1.9 mL/hr
5.2 pore volume pulse
of Br/HCO3 solution
Elution of U(VI) from LR sediment with 0.04 M NaHCO3 solution at pH 7
81% of labile U(VI) desorbed
77 hr stop-flow
AGW with 0.2 uM U(VI)
Reactive Transport Model with Equilibrium U(VI) Adsorption-Desorption
Transport calculations with RATEQ (Curtis, 2007)
Pore volumes Pore volumes
U(VI), μM
Data
Model
Pore volumes
Br (mM) Alkalinity (meq/L)
Ca (mM)
Pore volumes
Reactive Transport Model with Equilibrium U(VI) Adsorption-Desorption
and Ion Exchange: Column
Pore volumes Pore volumes
U(VI), μM
Data
Model
Pore volumes Pore volumes
Ca (mM) Mg (mM)
pH
Error due to
aqueous
chemistry
Reactive Transport Model with Equilibrium U(VI) Adsorption-Desorption,
Ion Exchange, and Mass Transfer: Column
Pore volumes
U(VI), μM
Pore volumes
Ca (mM) Log (SI) calcite
Pore volumes
Little Rusty Injection
1000 L injected at
18, 21, & 24 ft bgs Injectate Composition
NaBr 5.6 mM
NaHCO3 38 mM
pH 7.45
D2O 0.40 g/L
Uranium None
Little Rusty Breakthrough Curves 3m downgradient
21’ bgs 25’ bgs
VBr = 0.49 m/day VBr = 0.33 m/day
See poster by P. Fox!
Mixed cobbles, gravel
and sand at 25’ bgs
Clay-rich
sediment
at 21’ bgs
Reactive Transport Model with Equilibrium U(VI) Adsorption-Desorption
and Ion Exchange: Little Rusty Tracer Test 3m downgradient, 21’ bgs
Time, hr
pH
Br (mM)
Time, hr
U(VI), μM
Time, hr
Ca (mM)
Time, hr
Reactive Transport Model with U(VI) Adsorption-Desorption,
Ion Exchange, and Mass Transfer: Little Rusty Tracer Test 3m downgradient, 21’ bgs
Time, hr
U(V
I), μ
M
Time, hr
Time, hr
pH Alkalinity (meq/L)
Microbial Results By Well
Differences were observed by well and depth, however it is not known whether
these differences are due to spatial heterogeneity or treatment. Analysis of
the data is ongoing.
Biomass Composition
Activity
Nano and Strathmann (2006)
2009 goals: Fe(II) sorption, U(VI) reduction by adsorbed Fe(II)
• Competition of Fe(II) and U(VI) for sorption sites
• Fe(II) competes with Ca and Mg for ion exchange sites
• Rate of U(VI) reduction as a function of pH, Fe(II), Ca, and HCO3
- concentrations (how is U(VI)
reduction rate affected by aqueous speciation?)
Big Rusty Expt:
50-100 μM Fe(II)
0.2-1.0 μM U(VI)
Jeon et al. (2005)
Pure mineral phases
LR composite sediment
Little Rusty Ambient:
0-20 μM Fe(II)
0.1-0.2 μM U(VI)
• Continuous upgradient injection
of Fe(II) to maintain Fe(II)
concentrations during subsequent
U(VI) injection
• Inject of a pulse of Old Rifle
U(VI)-contaminated groundwater
– N2/CO2 purged to keep out
oxygen, proper pH
– Higher U(VI) concentrations
than background (1 mM)
– Similar scale to Little Rusty
(1000 L)
Continuous
injection of Fe2+
Inject
pulse of U(VI)
2009: River Rouge Tracer Test
• Will Fe(II) injection stimulate Fe-
oxidizers or are O2 and NO3
limiting?
Possible conceptual model for U “immobilization” at Rifle site
Cobble
Sand grain
Silt grain
Preferred
Groundwater
Flowpath
Faster
flow
Mass transfer
Reduced
sediment
zones
Transient delivery of
oxygen and nitrate
Natural bioreduction:
Reduced sediments rich
in FeSx, clays, organic
C, contains U(IV)
Conclusions
• An Old Rifle equilibrium U(VI) surface complexation model was developed that can describe sorption as a function of groundwater pH, carbonate alkalinity, and Ca and U(VI) concentrations
• The equilibrium surface complexation model scales well with surface area and weight abundance of <2 mm sediment to accommodate sediment textural heterogeneities
• Desorption of U(VI) from sediments is limited by diffusion-controlled mass transfer kinetics
• Development of a kinetic surface complexation model is underway but is complicated by exact descriptions of Ca ion exchange and oversaturation with respect to calcite
Thanks to the Rifle Team!
With assistance from: Kathy Akstin, Dick Dayvault, Lucie
N’Guessan, Steve Yabusaki, Tom Resch, Dave Traub, John
Bargar, Sarah Morris, Evan Arntzen, Linda Davis
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