Significance of Microporosity to Reactive

Transport Modeling at DOE Sites

James A. DavisU. S. Geological Survey

Menlo Park, CA

Collaborators:

USGS: Gary Curtis, Kate Campbell, Deb Stoliker, Patricia Fox

LBNL: Carl Steefel, Li Li, Ken Williams

Johns Hopkins University: Joanne Stubbs, Dave Elbert,

Linda Veblen, David Veblen

PNNL: John Zachara, Phil Long, Steve Yabusaki

Uranium-Contaminated DOE Sites

Naturita UMTRA Rifle UMTRA

O

U

2.02Å2.40Å

3.43Å

O

O

O

O

O

Fe

C

O

O

C

O

O

Fe

Fe

2.88Å

+

Reactive Transport Modeling

=

Hanford 300 Area

Reactive transport models are commonly based on the

continuum representation of porous media, in which the

physical, chemical, and biological variables describing the

system vary continuously in space.

Reactive Transport Modeling: Continuum Models

An REV has average values

of physical, chemical, and

microbiological variables

REV1 REV2

River

No-flow

boundary

Discretization of

modeling domainREV

An REV is “well mixed”.

There are no sub-grid

gradients, e.g., in physical

structure, chemical

concentrations, surface area,

or biological properties.

5-100 m

2 km

Naturita

UMTRA site:

Alluvial

Aquifer

Sediment

Texture

50% cobbles,

>6.4 cm;

15%

Cobble

Sand grain

Silt grain

Preferred

Groundwater

Flowpath

Physical heterogeneity within REVPore

Scale

Faster flow

Mass transfer

RTM simulations by L. Li and C. Steefel

Layer of fine-grained

sediment

Effect of Subgrid Physical

Heterogeneity with Local

Chemical Equilibrium

Slide 7

5.5 6.0 6.5 7.0 7.5 8.0

pH

10

9

8

7

6

5

4p

C

UO22+

UO2OH+

UO2(CO3)34-

UO2(CO3)22-

(UO2)2CO3(OH)3-

U(VI)tot = 1 x 10-6M; 430 ppm CO2

5 mM NaNO3Ca2UO2(CO3)3

o

1.78 mM Ca(NO3)2

UO2CO3o

CaUO2(CO3)32-

5.5 6.0 6.5 7.0 7.5

pH

10

9

8

7

6

5

4

pC UO2

2+

UO2(CO3)34-

UO2(CO3)22-

(UO2)2CO3(OH)3-

U(VI)tot = 1 x 10-6M; 2% CO2

5 mM NaNO3Ca2UO2(CO3)3

o

1.78 mM Ca(NO3)2

UO2CO3o

Calciteprecipitation

CaUO2(CO3)32-

Fox, Davis, and Zachara, 2006, GCA

Importance of

Ca2UO2(CO3)30 aqueous

species at all 3 DOE sites

At 2% pCO2 typical of groundwater

conditions at all 3 sites

At equilibrium with 430 ppm CO2

Naturita; Rifle: pH 7

Hanford: pH 7.5-8

0 5 10 15 20 25 30 35 40 45 50

Time (hr)

0

10

20

30

40

50

60

70

80

90

100

% U

(VI)

ad

so

rbed

Naturita uncontaminated

U(VI)tot = 10-6M

Ca(NO3)2 = 2.3 mM

NaNO3 = 2 mM

500 ppm CO2pH = 7.95

sediment (250-500 um) = 77 m2/L

0 5 10 15 20 25 30 35 40 45 50

Time (hr)

0

10

20

30

40

50

60

70

80

90

100

% U

(VI)

ad

so

rbe

d

U(VI)tot = 10-6M

430 ppm CO2

Ca(NO3)2 = 8.9 mM

NaNO3 = 5 mM

pH = 7.3

Fox, Davis, and Zachara, GCA, 70, 1379-1387, 2006

Quartz = 8 m2/L Adsorption/desorption of U(VI)

reaches equilibrium quickly in well-

stirred batch reactors with non-

porous single mineral phase with

Ca2UO2(CO3)30 as the predominant

aqueous species

Adsorption/desorption of U(VI)

approaches equilibrium slowly in

well-stirred batch reactors with

aquifer sediments from all 3 DOE

sites with Ca2UO2(CO3)30 as the

predominant aqueous species,

taking weeks to months to reach a

steady-state U(VI) concentration

(Example: Naturita adsorption)

0 200 400 600 800 1000 1200 1400

Time (hr)

0.00

0.04

0.08

0.12

0.16

0.20

0.24D

es

orb

ed

U(V

I),

mic

rom

ol/

L

Hanford 300 Area contaminated

sediment (sample SPP2-18)= 1530 m2/L

Desorbable U(VI) = 2.4 x 10-7M

Ca = 0.73 mM

0.17% CO2

pH = 8.1

Desorption of U(VI) approaches

equilibrium very slowly in well-

stirred batch reactors with Hanford

aquifer sediments (sample SPP2-18)

(Bond, Davis, and Zachara, 2008)

Flow interruption in column

experiments with sample

SPP2-18 show that the rate

of U(VI) desorption is

rate-limited

(Qafoku et al, ES&T, 2005)

Slide 10

Nanoporosity and surface areas of Hanford sample NPP1-16 (

Magnified Particle

Scale Showing

Intraparticle Pore

- - - - - - - - - - - - -

- - - - - - - - - - - - -

- - - -

- -

- - -

Temporally-variant bulk water

concentrations,

(H+, U(VI), Ca2+, HCO3-)b

Iron oxide

precipitateCalcite

HCO3-

HCO3-

AlO O

UO O

Ca2UO2(CO3)30

Local Chemical

equilibrium

Ca2UO2(CO3)30

Surface Charge Diffusive flux

Slide 12

Column Experiment: Pore Scale

Diffusive flux

Diffusion plus

dispersion?

0 200 400 600 800 1000 1200 1400

Time (hr)

0.00

0.04

0.08

0.12

0.16

0.20

0.24D

es

orb

ed

U(V

I),

mic

rom

ol/

L

Hanford 300 Area contaminated

sediment (sample SPP2-18)= 1530 m2/L

Desorbable U(VI) = 2.4 x 10-7M

Ca = 0.73 mM

0.17% CO2

pH = 8.1

Approximately 50% of U(VI) desorbs from intragranular porosity??

Bond, Davis, and Zachara, 2008, Adsorption in Geomedia II

Advection, dispersion, and diffusion of tritium out

of a column packed with Hanford sample NPP2-4

c) Elution stage 2

0

0.02

0.04

0.06

0.08

0.1

1.5 2 2.5 3 3.5

sf 1

sf 2

4 hrs

C/Co

Pore volumes

b) Elution stage 1

0

0.2

0.4

0.6

0.8

1

0 0.5 1 1.5 2

sf 1

2 hrs

d) Elution stage 3

0

0.001

0.002

0.003

0.004

0.005

0.006

3 3.5 4 4.5 5

sf 2

sf 3

16 hrs

e) Elution stage 4

0

0.0005

0.001

0.0015

0.002

0.0025

0.003

4.5 5 5.5 6 6.5

sf 3

sf 4

10.75 days

f) Elution stage 5

0

0.0005

0.001

0.0015

0.002

0.0025

0.003

0.0035

6 6.5 7 7.5 8 8.5

sf 4

sf 5

30 min

g) Elution stage 6

0.E+00

1.E-04

2.E-04

3.E-04

4.E-04

7.5 9.5 11.5

sf 5

sf 6

7.66 days

i) Elution stage 8

0.0E+00

4.0E-05

8.0E-05

1.2E-04

1.6E-04

2.0E-04

14 15 16 17

sf 7

h) Elution stage 7

0.E+00

2.E-04

4.E-04

6.E-04

8.E-04

1.E-03

12 12.5 13 13.5 14 14.5

sf 6

sf 7

4 hrs

Procedure: Pack

sample in

column; let sit for

4 months in water

with high HTO

Solid curve shows

model with HTO

diffusion from

two immobile

zones with a total

intragranular

porosity of 1.05%

Slide 14

Naturita sediment quartz grain coatings

Relative abundances of Al and Fe in grain coatings (μm thickness)

20 μm 20 μm

Grain coatings: Another type of microporosity?

Slide 15

Bright-field TEM images

showing needle-like

goethite (G) crystals

immersed in illite/smectite

clay matrix.

Davis et al., GCA, 68, 3621 (2004)

Naturita sediment grain coatings

U(VI) diffusive flux

Hanford uncontaminated vadose

zone sample: C5001-67B

Hanford contaminated vadose

zone sample: NPP2-2

20 μm

20 μm

Coating consists of micron-sized

mineral fragments.

Coating has much finer texture,

with a fine-grained clay coating

several microns thick at the outer

ridge of the grain. Probably

influenced by infiltration of low and

high pH pond water containing high

concentrations of Al and Si.

Stubbs et al., 2008

metatorbernite

coating

Lithic fragment

Hanford contaminated vadose

zone sample: NPP2-2

Metatorbernite precipitate [Cu(UO2)2(PO4)2·8H2O] is encapsulated within coatings on

contaminated grains

Stubbs et al., 2008Slide 18

0 20 40 60 80 100 120 140 160 180 200

Time (hr)

0.00E+000

1.50E-008

3.00E-008

4.50E-008

U(V

I) r

ele

as

ed

(m

ole

s/L

)

No grinding

Grinding (mortar/pestle)

Hanford sample NPP2-4

250-500 μm size fraction

Artificial groundwater; pH 7.5; HCO3- = 4 mM

Hanford contaminated vadose

zone sample: NPP2-4

Hanford contaminated vadose

zone sample: NPP2-2

Backscattering image of a 60 μm

wide, fine-grained clay coating.

Outer edge of coating contains

very high concentrations of Zr

and U, presumably from cladding

waste. Electron microprobe WDS

linescans show gradients in U

concentration across the coating.

Stubbs et al., 2008

Zones of Natural Bioreduction in

Rifle Aquifer Sediments

Slide 21

50 g/L air-dried sediment

Extractions performed in air; pH 9.4; 17.2 mM total carbonate

U(V

I) e

xtr

act

ed (

mo

les/

g)

Time (hr)

Sodium carbonate extractions of Rifle

sediment samples (BKG-A and RABS)

Bi/carbonate – H2O

2

Bi/carbonate + 1% H2O

2

RABS total U = 1.76E-8 moles/g

U(V

I) e

xtr

act

ed (

mo

les/

g)

Time (hr)

Sodium carbonate extractions of RABS

sediment sample with H2O2

1% H2O2 added

No H2O2 added

Slide 23

Sodium carbonate extractions of BKG-A

sediment sample with H2O2

Time (hr)

U(V

I) e

xtr

act

ed (

mo

les/

g)

1% H2O2 added

No H2O2 added

- - - - - - - - - - - - - - - - - - -

- - - -

- -

- - -

Iron oxide

precipitate

Fe2+Al

O O

UO O

Ca2UO2(CO3)30

Ca2UO2(CO3)30

Diffusive flux

Acetate

Fe2+Fe2+

UO2(ppt)- - - - - - - - - - - - - - - - -

Slide 25

Acetate

Nanoparticulate UO2(s)

1020 1030 10351010 10251015

Field remediationField research site

Column studies

Spectroscopy

Batch studies

Total Surface Sites in System(Fine-grained porous media,0.1 moles sites/m3)

Experimental Upscaling

No bulk spatial

gradients in well-

mixed reactor

Chemical

gradients at pore

scale as f(flow)

and along

reaction fronts

Subsurface

heterogeneity; spatial

and temporal gradients

Lichtner and Kang, Water Resources Research, 43, W12S15, 2007

Multiscale Continuum Models

Primary Continuum (3-dimensional)

Sub-grid scale domains

(1-dimensional)

One of the largest problems for single continuum RTM at the field

scale is heterogeneities of physical, chemical, and biological properties

at the sub-grid scale and the non-linear scale dependence of coupled

processes. Intragranular pore space and mineral grain coatings may be

an important physical regime for many U-contaminated sites.

Conclusion

Research Priorities:

1) Multiscale continuum models (requires high performance computing)

2) Multiscale experimental studies

3) Methods for field characterization of significant parameters

4) Improved but simplified conceptual models for coupled processes (e.g.,

sorption and aqueous speciation)

5) Better understanding of parameter and conceptual model uncertainties

Cobble

Sand grain

Silt grain

Faster flow

Groundwater sampling: What

mixture of water is sampled?

Slide 12

Well

Batch/column Intermediate-scale studies Field-scale predictions

Need for multiscale experiments!

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