# Presentation: 224982

## 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 CO****2**
**	****	****Respiration**
**	****	****Weathering of carbonate minerals**

**Why is this important?    U(VI) has strong affinity for CO****3****2-**

**Carbonate speciation is pH dependent**

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

**UO****2****2+**** aqueous speciation in carbonate solutions**

**U****tot**** = 1 ********M, P****CO****2**** ****= 10****-3.5**** bar, 25 ****o****C**

## U(VI) adsorption on ferrihydrite

**From Waite et al. (1994) **

**U(VI) adsorption on ferrihydrite**

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

**pH < 5 UO****2****2+**** dominant**
**pH 5-8 Hydroxyl species  **
**pH >8  Carbonate species**

**Adsorption edges at:**
**       ****low pH (4-5)**
**       ****high pH (8-9)**

**Uranyl carbonate complexes**
**have low sorption affinity**

## How does U(VI) interact with calcium carbonate?

** ****Potential binding sites at surface CO****3**** groups**
** ****Calcium carbonate is moderately soluble (Ca****2+****, CO****3****2-****)**
** ****Dissolved CO****3****2-**** stabilizes aqueous uranyl complexes **

**Aragonite (Pmcn)**

## Calcium carbonate-saturated solutions

**Total dissolved carbonate (and Ca) depend on pH and P****CO2**

## U(VI) aqueous speciation in calcium carbonate systems

**Formation of Ca****2****UO****2****(CO****3****)****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 CaCO****3**** 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

**Adsorption**

**Co-precipitation**

**Surface**
**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

**Experimental conditions for sorption experiment**** **
**  ****Calcite: surface area ****~10 m****2****/g (~2 ********m size)**
**  ****Calcite suspension pre-equilibration:**
**	****log *****P*****(CO****2****) = -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 CO****3****)**
**  ****Sorption equilibration – 24, 48, 72 h**
**  ****Wet pastes extracted for EXAFS, luminescence**

**U(VI) Sorption Isotherm on Calcite **

**pH 8.3**

**Ca surface sites**

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

**Calcite saturation maintained**

**Initial calcite saturation**

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

**(UO****2****CO****3****)**

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

**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-**

## Intensity (cps)

**100**

**50**

**0**

**Intensity (cps)**

**2.5**

**2.0**

**1.5**

**1.0**

**0.5**

**Time (msec)**

**    ****10 ********M U(VI)**
**Blue****: single exp. **
**          ******** ****= 150 ****± 20 ********s**
**Black: 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**

## Resolution of component spectra using short and delayed “gates”

**Resolution of component spectra using short and delayed “gates”**

**Distinct spectra indicate at least two uranyl species present**

##  This species resembles aqueous UO2(CO3)34-

** ****This species resembles aqueous UO****2****(CO****3****)****3****4-**
** ****Possibly sorbed Ca****2****UO****2****(CO****3****)****3**

**Identification of “delayed gate” spectrum**

## Identification of “short-gate” spectrum

**Identification of “short-gate” spectrum**

** ****Short-gate species resembles the UO****2****-doped calcite**
** ****U(VI) possibly coprecipitated during sorption**

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

**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**

## Slide 20

## Time-resolved Luminescence Spectroscopy of CaCO3 Phases

** ****Different uranyl species in **
**    ****polycrystalline calcite and aragonite**
** ****Both exhibit single exponential **
**    ****decay kinetics**
** ****Single uranyl species in each**

**Exc. 420 nm**
**LN2**

**Time-resolved Luminescence Spectroscopy of CaCO****3**** Phases**