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