U(VI) interactions with carbonates: Spectroscopic studies Richard J. Reeder Department of Geosciences and Center for Environmental Molecular Science State 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 Respiration Weathering of carbonate minerals Carbonate speciation is pH dependent Why is this important? U(VI) has strong affinity for CO32-

UO22+ aqueous speciation in carbonate solutions Utot = 1 M, PCO2 = 10-3.5 bar, 25 oC

Influence of dissolved carbonate on U(VI) sorption: ferrihydrite U(VI) adsorption on ferrihydrite From Waite et al. (1994) Adsorption edges at: low pH (4-5) high pH (8-9) pH < 5 UO22+ dominant pH 5-8 Hydroxyl species pH >8 Carbonate species Uranyl carbonate complexes have 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 Calcite (R3c) Aragonite (Pmcn)

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)

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.

Mechanisms of Metal Uptake at the Mineral-Water Interface Surface precipitation Adsorption Co-precipitation

Experiment: Characterize U(VI) sorbed at calcite surface in situ using EXAFS and luminescence spectroscopies U(VI) Sorption Isotherm on Calcite 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 weeks pH 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 Ca surface sites pH 8.3

U(VI) Solubility Limits near Calcite Saturation (pH 8.3) Initial calcite saturation Calcite saturation maintained (UO2CO3)

Selected EXAFS Results for U(VI) Sorption on Calcite (pH 8.3) 4- 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

Intensity (cps) Time (msec) Time-resolved luminescence spectroscopy: Single uranyl species at lowest U concentration Additional species appears at higher U concentrations 10 M U(VI) Blue: single exp.  = 150 ± 20 s Black: double exp. 1 = 580 ± 240 s 2 = 125 ± 30 s 100 Intensity (cps) 50 Decay kinetics: Best fit with two exponentials 0.5 1.0 1.5 2.0 2.5 Time (msec)

Resolution of component spectra using short and delayed “gates” Distinct spectra indicate at least two uranyl species present

Identification of “delayed gate” spectrum This species resembles aqueous UO2(CO3)34- Possibly sorbed Ca2UO2(CO3)3

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 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

Time-resolved Luminescence Spectroscopy of CaCO3 Phases Exc. 420 nm LN2 Different uranyl species in polycrystalline calcite and aragonite Both exhibit single exponential decay kinetics Single uranyl species in each