U(VI) interactions with carbonates:Spectroscopic studies

Richard J. Reeder

Department of Geosciences andCenter for Environmental Molecular ScienceState University of New York at Stony Brook

Collaborators: E. Elzinga, D. Tait, D. Morris

Support from NSF, DOE, Actinide Facility at ANL

Dissolved carbonate in environmental solutions

Derived from: Atmospheric CO2

RespirationWeathering of carbonate minerals

pH

3 4 5 6 7 8 9 10 11 12

% s

pec

ies

0

2e-5

4e-5

6e-5

8e-5

1e-4

H2CO3 HCO3-

CO32-

Why is this important? U(VI) has strong affinity for CO32-

Carbonate speciation is pH dependent

UO22+ aqueous speciation in carbonate solutions

Utot = 1 M, PCO2 = 10-3.5 bar, 25 oC

From Waite et al. (1994)

U(VI) adsorption on ferrihydrite

Influence of dissolved carbonate on U(VI) sorption: ferrihydrite

pH < 5 UO22+ dominant

pH 5-8 Hydroxyl species

pH >8 Carbonate species

Adsorption edges at:

low pH (4-5) high pH (8-9)

Uranyl carbonate complexeshave low sorption affinity

How does U(VI) interact with calcium carbonate?

• Potential binding sites at surface CO3 groups

• Calcium carbonate is moderately soluble (Ca2+, CO32-)

• Dissolved CO32- stabilizes aqueous uranyl complexes

Aragonite (Pmcn)Calcite (R3c)

Calcium carbonate-saturated solutions

Total dissolved carbonate (and Ca) depend on pH and PCO2

U(VI) aqueous speciation in calcium carbonate systems

Formation of Ca2UO2(CO3)3(aq) species favored in calcite-

equilibrated solutions (Bernhard et al., 1996, 2001)

15 M total U(VI) in calcite suspension

pH

5 6 7 8 9 10

U(V

I) s

pec

ies

(m)

0.0

2.0e-6

4.0e-6

6.0e-6

8.0e-6

1.0e-5

1.2e-5

1.4e-5

1.6e-5

Ca2UO2(CO3)3 UO2(CO3)3-4 UO2(CO3)2-2 UO2CO3 (UO2)2CO3(OH)3- UO2OH+ UO2(OH)2 UO2+2

U(VI) in Calcium Carbonate Phases

• Up to 1 wt.% U(VI) in calcite formed in leach tests of Portland cement-type grout (Fuhrmann et al., 2005)

• U(VI) in calcite formed in Hanford subsurface associated with releases of uranium waste (Wang et al., 2005)

• Synthetic U(VI) co-precipitation samples contain up to 1 wt.% U (Reeder et al., 2000)

• Natural CaCO3 minerals contain up to 300 ppm U (IV, VI)

Importance of Uranium Uptake by Carbonates

• Geochemical tracers (petrogenesis, diagenesis)

• Proxy for paleo-climate, paleo-ocean chemistry

• Role in geochemical cycles

• Potential for sequestration

Calcite is a highly effective sorbent for many metals.

Adsorption Co-precipitationSurface

precipitation

Mechanisms of Metal Uptake at the Mineral-Water Interface

Experiment: Characterize U(VI) sorbed at calcite surface in situ using EXAFS and luminescence spectroscopies

-3

-2.5

-2

-1.5

-1

-0.5

-5.5 -4.5 -3.5 -2.5

log (solution [U] in M)

log

(s

orb

ed

U i

n m

ole

/kg

)

Experimental conditions for sorption experiment

• Calcite: surface area ~10 m2/g (~2 m size)

• Calcite suspension pre-equilibration:

log P(CO2) = -3.5, 20–22 ºC, 4 weekspH 7.4–8.3, I = 0.0015–0.0025 m

• Total U(VI): 5 M–5 mM (added w/ and w/o CO3)

• Sorption equilibration – 24, 48, 72 h

• Wet pastes extracted for EXAFS, luminescence

U(VI) Sorption Isotherm on Calcite

pH 8.3

Ca surface sites

Total U(VI) (M)

0 1000 2000 3000

Sat

ura

tio

n in

dex

-3

-2

-1

0

1

2 Schoepite

-UO2(OH)2

Rutherfordine

Supersat.

Undersat.

w/o CO3

w/ CO3Calcite saturation maintained

Initial calcite saturation

U(VI) Solubility Limits near Calcite Saturation (pH 8.3)

(UO2CO3)

U L3-edge

R + (Å)

0 1 2 3 4 5 6 7 8

Fo

uri

er t

ran

sfo

rm m

agn

itu

de

0.0

0.1

0.2

0.3

U(VI):calcite

5 mM U(VI)

150 M U(VI)

15 M U(VI)

UO2(CO3)3 (aq)

Selected EXAFS Results for U(VI) Sorption on Calcite (pH 8.3)

Two types of EXAFS spectra (as seen in FT magnitude):

• Total U(VI) 500 M – single but broad equatorial peak• Total U(VI) 500 M – split equatorial peaks

4-

620600580560540520500480460

Emission Wavelength (nm)

No

rma

lize

d In

ten

sit

y 482

491

502510

523

532

547

100 M

20 M

10 M

5 M

Exc. 420 nm

100

50

0

Inte

nsit

y (

cp

s)

2.52.01.51.00.5

Time (msec)

10 M U(VI)Blue: single exp. = 150 ± 20 sBlack: double exp. 1 = 580 ± 240 s 2 = 125 ± 30 s

Time-resolved luminescence spectroscopy:

• Single uranyl species at lowest U concentration

• Additional species appears at higher U concentrations

Decay kinetics:

• Best fit with two exponentials

620600580560540520500480460

No

rmal

ized

Inte

nsi

ty

Emission Wavelength (nm)

10 M Sample; LN2 TempExc. 420 nm

delayed gate(0.700 --> 1.500 msec)

short gate(0.050 --> 0.100 msec)

482

503

524

546

533

511491

Resolution of component spectra using short and delayed “gates”

Distinct spectra indicate at least two uranyl species present

650600550500450

No

rma

lize

d I

nte

ns

ity

Emission Wavelength (nm)

484

502

523

548

482

502

523

547

484504

526

548

LN2 Temperatureexcite 420 nm

100 M sorbed sampledelayed gate

5 M sorbed samplefull gate

Uranyl in aragonite(triscarbonate monomer)full gate

• This species resembles aqueous UO2(CO3)34-

• Possibly sorbed Ca2UO2(CO3)3

Identification of “delayed gate” spectrum

650600550500450

492

512

533

493

513

534

Emission Wavelength (nm)

Inte

ns

ity

LN2 Temperature

100 M sorbed (short-gate)

U(VI) calcite (single xl)

Identification of “short-gate” spectrum

• Short-gate species resembles the UO2-doped calcite

• U(VI) possibly coprecipitated during sorption

What about U(VI) in Natural Calcium Carbonate Samples?

3 cm

Calcite speleothem, N. Italy (300 ppm U)

XRD, FTIR – only calcite in yellow band

20000

10000

0

Inte

nsi

ty (

cps)

600550500450

Wavelength (nm)

Spotl Calcite Long delay Short delay/short gate Full Gate

Time-resolved luminescence

• Double exponential decay kinetics two uranyl species

• Long gate – aragonite-like species

• Short gate – calcite-like species

What can we conclude ?

• At U(VI) < 10 M, uranyl carbonate complex adsorbs on calcite surface

• At U(VI) = 10–500 M, multiple sorbed uranyl species exist at calcite surface:

• One sorbed species is uranyl triscarbonate-like• Other may be a coprecipitate

• At U(VI) > 500 M, a surface precipitate forms

• Presence of multiple species may result in U(VI) retention with multi-phase behavior/kinetics

• Differences in experimental conditions for co-precipitation result in different local coordination of uranyl species.

• The use of complementary techniques (EXAFS and time-resolved luminescence) may provide better chance for characterizing complex environmental systems

Emission wavelength (nm)

450 500 550 600 650No

rma

lize

d l

um

ine

sc

en

ce

in

ten

sit

y

0

1Polycrystalline calcite

Aragonite

• Different uranyl species in polycrystalline calcite and aragonite

• Both exhibit single exponential decay kinetics

• Single uranyl species in each

Exc. 420 nmLN2

Time-resolved Luminescence Spectroscopy of CaCO3 Phases