Incorporation of trivalent actinides into calcite: A time resolved laser fluorescence spectroscopy (TRLFS) study

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Geochimica et Cosmochimica Acta 72 (2008) 464–474 www.elsevier.com/locate/gca

Incorporation of trivalent actinides into calcite: A time resolved laser fluorescence spectroscopy (TRLFS) study M. Marques Fernandes

a,*

, T. Stumpf a,b, T. Rabung a, D. Bosbach a, Th. Fangha¨nel b,c

a Forschungszentrum Karlsruhe, Institut fu¨r Nukleare Entsorgung, P.O. Box 3640, D-76021 Karlsruhe, Germany Physikalisch-Chemisches Institut, Ruprecht-Karls-Universita¨t Heidelberg, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany c European Commission, Joint Research Center, Institute for Transuranium Elements, P.O. Box 2340, 76125 Karlsruhe, Germany

b

Received 17 July 2006; accepted in revised form 9 October 2007; available online 30 Ocotber 2007

Abstract In order to characterize and quantify the substitution of Ca(II) by Cm(III) (coordination, charge compensation), homogeneous Cm(III) coprecipitated calcite was synthesized in a mixed-flow-through experiment. Two sets of experiments were conducted at pH 8.1 and at pH 12.5. At pH 8.1 two calcites, a calcite with a low Cm3+ concentration (LCMpH8.1) and a calcite with a high M3+ (Gd3+ and Cm3+) concentration (HCMpH8.1) were grown and investigated by time resolved laser fluorescence spectroscopy. The Cm(III) emission spectra of LCMpH8.1 and HCMpH8.1 show the same Cm(III) fluorescence signals for two Cm(III) species; Cm(III) species (1) with a peak maximum at 606.2 nm and Cm(III) species (2) with a peak maximum at 620.3 nm. Cm(III) species (1) has a mean lifetime of s = 386 ± 40 ls and Cm(III) species (2) has a mean lifetime of s = 1874 ± 200 ls. A lifetime of 386 ls correlates with 1.3 water molecule in the first coordination sphere of the Cm ion whereas a lifetime of 1874 ls indicates the total loss of the Cm(III) hydration sphere. According to the fluorescence emission peak position and the fluorescence emission lifetime, Cm(III) species (1) is identified as a surface sorbed species whereas Cm(III) species (2) is identified as a Cm(III) incorporated into the calcite lattice. Cm(III) fluorescence emission spectra of Cm(III) doped calcite grown at pH 12.5 (LCMpH12.5) show the same peak maxima which are found for LCMpH8.1 and HCMpH8.1 grown at pH 8.1 but an additional emission band at 608.2 nm (3) is found, which can be assigned to a further Cm(III) species. Fluorescence emission lifetime measurements show that this Cm(III) species (3) has a lifetime of s = 477 ± 25 ls, which correlates with 0.9 water molecules in the first coordination sphere. Cm(III) species (3) is suggested to be a CmOH2+ incorporated species.  2007 Elsevier Ltd. All rights reserved.

1. INTRODUCTION Demonstrating the geochemical long term safety of a nuclear waste repository requires a molecular level understanding of the actinides behavior in the geosphere. In particular the interaction of radionuclides with minerals (adsorption, structural incorporation) strongly affects their mobility and sequestration. Here, we will focus on the inter-

*

Corresponding author. Fax: +41 56 310 35 65. E-mail address: [email protected] (M.M. Fernandes).

0016-7037/$ - see front matter  2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2007.10.015

action of trivalent actinides with calcite, with the special focus on the structural incorporation. Calcite (CaCO3) is an omnipresent mineral phase in many host rock formations which are discussed as potential hosts for a nuclear waste repository. Furthermore, many waste repository concepts include cement based components and Calcite is one of the major secondary alteration products formed during the degradation of cement over geological timescales. Numerous studies on metal ion sorption/uptake by calcite have been performed including M1+,2+,3+,4+ (Mucci and Morse, 1983; Okumura and Kitano, 1986; Pingitore

Incorporation of trivalent actinides into calcite: A TRLFS study

and Eastman, 1986; Davis et al., 1987; Terakado and Masuda, 1988; Dromgoole and Walter, 1990; Stipp et al., 1992; Paquette and Reeder, 1995; Zhong and Mucci, 1995; Tesoriero and Pankow, 1996; Piriou et al., 1997; Rimstidt et al., 1998; Reeder et al., 1999; Martin-Garin et al., 2003; Lakshtanov and Stipp, 2004; Curti et al., 2005; Zavarin et al., 2005) and even actinyl ions U(VI) (Curti, 1999; Reeder et al., 2001; Zavarin et al., 2005). Most of these studies were aimed at determining empirical partition coefficients. In a few studies the incorporation of metal ions has been spectroscopically characterized. In some cases thermodynamically favorable ‘‘solid solutions’’ with intermediate composition are formed (Mucci and Morse, 1983; Davis et al., 1987; Mucci, 1988; Dromgoole and Walter, 1990). Rare Earth elements (REE) with their electronic 4f configuration are considered as non-radioactive chemical analogs of 5f trivalent actinides (Am(III), Cm(III), Pu(III)). Several studies have been performed on the interaction of REE with calcite: on one hand empirical partition coefficients have been determined from co-precipitation experiments. On the other hand, REE and actinides containing synthetic calcite crystals have been studied spectroscopically to characterize the structural incorporation mechanism. It is presumed that the structural incorporation of trivalent actinides and lanthanides into calcite occurs on a calcite lattice site by the substitution of Ca2+. The size range covered by the sixfold coordinated trivalent actinides (Cm3+ 97 pm, Am3+ 98 pm) and lanthanides (Eu3+ 95 pm, Nd3+ 98 pm) includes that of Ca2+ (100 pm) (Shannon, 1976). In the calcite structure Ca2+ occupies a regular octahedron with a sixfold coordination to oxygen of six adjacent CO3 2 groups. An important question for any heterovalent substitution is the charge compensation mechanism to maintain electro neutrality of the host crystal. All partition coefficients for REE uptake by calcite which are published indicate a strong interaction of the metal ions with calcite. Nevertheless a significant variation of the partition coefficient was observed, mainly explained by different experimental conditions. Terakado and Masuda (1988) derived from batch experiments a partition coefficient of 10 for all REE. In a preliminary study Stipp et al. (2003) used the same batch technique to investigate the Eu3+ partitioning with calcite and derived a partition coefficient in the range 12–93. In this kind of experiment it is very difficult to correlate the partition coefficient with the composition of the solution because the solution does not remain constant throughout the experiment. In contrast to the method used by Terakado and Masuda, Zhong and Mucci (1995) used a constant addition experiment to derive partitioning data for a series of REE. This co-precipitation technique allows to establish steady-state conditions and to obtain a homogenous REE-containing calcite. For Eu(III), Zhong and Mucci (1995) found partition coefficients in the range 210–1390 depending on the Eu(III) concentration (10–170 nM). Lakshtanov and Stipp (2004) used the ‘‘constant addition’’ method and derived a partitioning coefficient of 770 ± 290 for Eu(III) with calcite.

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1.1. Spectroscopic characterization Although incorporation of trivalent actinides into the bulk structure of calcite may represent a very important retardation process, very little spectroscopic data are available. Extended X-ray absorption fine structure (EXAFS) studies of REE (Elzinga et al., 2002; Withers et al., 2003) coprecipitated with calcite in a batch-type synthesis show that the local oxygen coordination changes from six to sevenfold with increasing REE ionic radius (Elzinga et al., 2002; Withers et al., 2003). Stumpf and Fangha¨nel (2002) investigated the uptake of Cm(III) by calcite in a long-term batch-type experiment by time resolved laser fluorescence spectroscopy (TRLFS). They found two distinct coordination environments for Cm(III). Of the two Cm(III) species identified in calcite, one is partially hydrated (one H2O in the first coordination sphere), whereas the other is completely dehydrated. 1.2. Substitution mechanism Based on macroscopic sorption experiments as well as spectroscopic studies mentioned above various substitution mechanisms have been discussed but no unambiguous conclusion has been drawn. Zhong and Mucci (1995) observed a correlation between REE concentration and the Na+ partition coefficient in the calcite overgrowth. They suggest that Na+ might serve to balance the excess charge generated by the incorporation of trivalent REE in calcite by simple coupled substitution. Lakshtanov and Stipp (2004) investigated the co-precipitation of Eu(III) with calcite using a constant addition method. They proposed a substitution scheme with the end-members Ca3(CO3)3–Eu2x(CO3)3, where x represents a Ca2+ vacancy in the calcite lattice. For the sevenfold oxygen coordination in the case of larger trivalent REE obtained by EXAFS, Withers et al. (2003) suggested mechanisms like bidentate linkage of one carbonate group, or the presence of an OH/H2O species in addition to the six coordinating carbonate groups, which would imply for this latter mechanism an extreme lattice distortion. Curti et al. (2005) used an advanced thermodynamic ‘‘inverse’’ modeling approach to model the Eu/calcite solid solution aqueous solution (SSAS) system based on three independent sets of experimental data: (1) recrystallization at pH 13, (2) experimental data published by Zhong and Mucci (1995) and (3) by Lakshtanov and Stipp (2004). Apparently, only a ternary solid solution was able to model the experimental data consistently. This approach has led to the assumption that two different Eu3+ sites exist in the calcite crystal lattice. At low pH, Ca2+ is substituted by H+ and Eu3+. At higher pH, EuO(OH) is incorporated into the calcite lattice. The aim of this work is to investigate the incorporation mechanism of trivalent actinide Cm(III) into calcite, synthesized under constant and controlled conditions (in contrast to long-term batch experiments) by TRLFS. To characterize the different species involved in the co-precipitation process, three doped samples were synthesized at two different pH values and with different metal ion concentrations.

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M.M. Fernandes et al. / Geochimica et Cosmochimica Acta 72 (2008) 464–474

2. EXPERIMENTAL DETAILS The Cm-doped calcite crystals were synthesized by using a mixed flow reactor (MFR) under similar conditions as used by Zhong and Mucci (1995) (Fig. 1). MFRs’ are particularly useful because they allow crystal growth under constant and with defined hydrodynamic conditions (surface reaction controlled precipitation), saturation state, pH, ionic strength and temperature (25 ± 0.2 C). In a stirred MFR, the concentrations of the precipitating and co-precipitating components are expected to be constant during the experiment and a homogeneous precipitate should be obtained. Also, co-precipitation was conducted over extended periods of time (up to 4 weeks). The MFR reactor was fed by three independent solutions which were introduced separately by a peristaltic pump (0.333 ml/min) from reservoir bottles. The input solutions were pumped from the reservoir bottles to the reactor at a constant rate so that the concentrations of the precipitating and co-precipitating components reach steady-state and remain constant. The reactor has an internal volume of 45 ml. Input solutions entered and left the reactor through a 0.45 lm pore size hydrofoil Teflon membrane. Powder of natural calcite was used as seed material for calcite precipitation. The specific surface area of the seed calcite measured by the N2BET method was found to be 4 m2 g1. In case of seeded co-precipitation, it is assumed that growth occurs only on the crystallites. The general experimental conditions were the same for the three samples (closed system: no CO2 exchange, background electrolyte NaClO4, ionic strength I = 0.01). The following input solutions were used for Cm(III) co-precipitation experiments: (1) CaClO4, (2) an appropriate mixture of NaHCO3 and Na2CO3 and (3) a Cm-248 (t1/2 = 3.4 · 105 a) Gd(ClO4)3 resp. solution. Since the reservoirs feeding the reactor separately, no precipitation occurs before the solutions enter the reactor. The composition of the input solution defined the supersaturation of the system with respect to calcite and to Cm(III) pure solid phases. The Cm(III) stock solution consists of

97.3% Cm-248, 2.6% Cm-246, 0.04% Cm-245, 0.02% Cm247 and 0.009% Cm-244 in 1.0 mol L1 HClO4. Concentration of calcium, sodium and curium were measured by ICPMS. The steady state pH of the reacting solution was measured once a day and remained constant during the experiments. The steady-state calcite precipitation rate R has been determined as Rðlmol m2 min1 Þ ¼ 

F DC i Smi W seed

DCi, concentration difference between input and reacting solution of the element i; F, solution addition rate (L min1); S, specific reactive surface area (m2 g1); Wseed, mass of calcite seeds (g); ti, molar fraction of element i in the calcite overgrowth. The growth rate of the samples varied with metal ion concentration (Terjesen et al., 1961) from 0.75 to 1.35 lmol m2 min1. 2.1. Co-precipitation experiments at pH 8.1 Because of the very close physical and chemical properties, Am(III) is used as representative for trivalent lanthanides and actinides, particularly for Cm(III). For this reason speciation calculations were performed with PhreeqC (Parkhurst, 1995) using the Nagra/PSI Chemical Thermodynamic Database which includes the Am(III) NEA–TDB data (Hummel et al., 2002; Guillaumont et al., 2003). The degree of supersaturation (SI = log(IAP/Ksp), IAP, ion activity product; Ksp, solubility product) with respect to pure CaCO3 in the input solution was about 1.3– 1.4 (SI < 0 undersaturated, dissolution; SI = 0, equilibrium; SI > 0, supersaturation, precipitation). In case a thermodynamic favorable Cm(III) containing solid solution exists (negative excess Gibbs energy of mixing), the degree of supersaturation with respect of such a solid phase would be even higher. Furthermore, considering the precipitation conditions, it cannot be excluded that additional metastable phases form (e.g., pure Cm(III) compounds). If such metastable phases form, it seems unlikely that they transform

Fig. 1. Mixed flow-through reactor.

Incorporation of trivalent actinides into calcite: A TRLFS study

into thermodynamically more stable Cm(III) containing solid solution stable within the timeframe of our experiment. At pH 8.1 two Cm(III) doped calcite samples were synthesized. In the first experiment, the Cm(III) concentration in solution was about 3 · 1010 M Cm(III): sample LCMpH8.1 (Table 1). In this experiment performed at low Cm(III) concentration, the co-precipitation of pure Cm(III) solid phases like: Cm(OH)3, Cm2(CO3)3(cr), CmOHCO3(cr) can thermodynamically be excluded (Table 2a). In the second experiment, the total trivalent metal ion concentration in the input solution was increased about three orders of magnitude to 4 · 107 M by adding the non-fluorescent inactive and chemical analogue gadolinium (3 · 1010 M Cm3+ and 4 · 107 M Gd3+): sample HCMpH8.1. By increasing M3+ (Cm3+ and Gd3+) concentration, the tendency of precipitating additional M(III) containing solids increases too. In fact, speciation calculations which neglect the formation of solid solutions clearly indicate that the solution was supersaturated with respect to M(OH)3 and MOHCO3(cr) (Table 2b).

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put solution are thermodynamically even at very low Cm3+ concentration oversaturated with respect to Am(OH)3(cr): SI  1.5. In summary, three different Cm(III) containing calcites have been synthesized. TRLFS measurements were performed at room temperature using a pulsed Nd:YAG pumped dye laser system (Continum ND 6000, Powerlite 9030, dye: exalite 398, repetition rate: 10 Hz). The fluorescence emission was detected by an optical multichannel analyzer, which allows simultaneous detection of the spectral range covered by the respective grating. The system consists of a polychromator (Chromex 250is) with a 1200, 600 and 300 lines/mm grating. The excitation of Cm(III) occurred at 396.6 nm and the fluorescence emission spectra of Cm(III) were recorded in the 575–640 nm range, within a constant time window of 1 ms. The Cm(III) emission decay was recorded within a constant time window of 7 ms, the delay time between laser pulse and camera gating was scanned with time intervals of 100 ls. 3. RESULTS

2.2. Co-precipitation experiment at pH 12.5 Cm(III) calcite co-precipitation experiment was performed at pH 12.5. A pH of 12.5 is close to the pH of cement pore water solution and therefore co-precipitation experiments under these conditions are highly relevant for safety aspects of nuclear waste disposal. The same experimental set-up and conditions (no CO2 exchange, I = 0.01) as previously mentioned were used to grow homogenous Cm(III) doped calcite at pH  12.5 (LCMpH12.5) (Table 1). NaOH was added to the CaClO4 solution and the Na2CO3 solution to obtain the desired pH value. The calcium solution remained undersaturated with respect to portlandite Ca(OH)2 (log Ks = 5.18): SI  0.93. The saturation index (of the input solution) with respect to calcite was about 1.4. Under this highly alkaline conditions the in-

Table 1 Summary of the different synthesized calcite samples Calcite sample

Growth condition

LCMpH8.1

Low Cm3+ concentration at pH 8.1 (0.8 ppm Cm3+) High M+ concentration at pH 8.1 (908 ppm Gd3+ + 1.1 ppm Cm3+) Low Cm3+ concentration at pH 12.5 (0.7 ppm Cm3+)

HCMpH8.1 LCMpH12.5

Table 2 Solid phase speciation at pH8 (a) at low metal ion concentration in the input solution, (b) at high metal ion concentration in the input solution Phase

SI (a)

SI (b)

Am(OH)3 Am (CO3)Æ1.5 H2O AmOHCO3

1.32 3.40 1.57

1.77 0.34 1.52

The composition of the co-precipitated calcites was analyzed by ICP-AES (Ca2+) and ICP-MS (Cm3+) after dissolving (1 g L1) the synthesized calcite in 30% HNO3. The following concentrations were found: 0.8 ppm Cm(III) for low concentrated sample LCMpH8.1, 909 ppm M(III) (1.1 ppm Cm(III) + 908 ppm Gd(III)) for the high concentrated calcite sample HCMpH8.1 and 0.7 ppm Cm(III) for the calcite sample LCMpH12.5. The determined partition coefficient for HCMpH8.1 is about 911, which is close to the value obtained by Zhong and Mucci (1995). The formation of other crystalline phases or other calcium carbonate polymorphs like vaterite or aragonite could not be observed on the basis of powder XRD measurements, ATR-IR spectroscopy and thermal analysis (Fig. 2) (in the range of detection limit of these methods). The IR spectra (Fig. 2b) only shows vibration bands at 1395 cm1 (t3), 872 cm1 (t2) and 712 cm1 (t4) which are typical for calcite. 3.1. Spectroscopic characterization of the Cm(III) coprecipitated calcite samples at pH  8.1 The Cm3+ fluorescence emission spectra of the 6D7/ 8 2 fi S7/2 transition were obtained when exciting the most intense absorption maxima (396.6 nm) of Cm3+ (Carnall and Crosswhite, 1985). The fluorescence emission of the Cm3+ aquo ion shows a peak maximum at 593.8 nm (Fig. 3). Spectral shifts and splitting of emission bands compared to the aquo ion are used to identify molecular actinide species. A change in local coordination environment results in a change in the ligand field of the Cm(III) ion and thus in a shift of the fluorescence spectrum. Fig. 3 shows the fluorescence emission spectrum of Cm3+ aquo ion and the fluorescence emission spectra of the Cm(III) in LCMpH8.1 and in HCMpH8.1. Both samples show two emission bands at 606.2 nm (1) and 620.3 nm (2) or 619 nm (HCMpH8.1). The peak

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M.M. Fernandes et al. / Geochimica et Cosmochimica Acta 72 (2008) 464–474 Cm(III) aquo ion LCMpH8.1 HCMpH8.1

Normalized Intensity

Normalized Cm(III) Fluorescence Emission

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Fig. 3. Fluorescence emission spectra of the Cm(III) aquo ion (solid line), Cm(III) in LCMpH8.1 and HCMpH8.1 (dashed and doted line). υ4

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Temperature (°C) Fig. 2. (a) Normalized powder X-ray diffraction spectra of the synthesized Cm(III) coprecipitated calcite Cm:CaCO3. The same spectra for the two Cm:CaCO3. Dashed lines, reference data. (b) ATR-IR spectra of Cm(III) coprecipitated calcite Cm:CaCO3. The vibration bands can clearly be attributed to calcite. (c) Thermogram of Cm(III) coprecipitated calcite Cm:CaCO3.

deconvolution of both spectra show for both samples the same emission bands at 606.2 nm (1) and 620 nm (2). Figs. 5a and b show the peak deconvolution of the time dependant Cm(III) emission spectra in HCMpH8.1 and the extracted spectra of the pure components. The position of the emission bands shows a red shift of 12.4 nm and 26.2 nm, respectively, compared to the emission band of the Cm3+ aquo ion. This is due to the increase of the ligand field splitting as a consequence of the complexation of Cm(III) with calcite. The two emission bands can clearly be assigned to different Cm(III) species. The peak positions are close to the peak positions of the Cm(III) species

600 610 620 Wavelength (nm)

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Fig. 4. Fluorescence emission spectra of Cm(III) in calcite at different delay times: (a) HCMpH8.1 and (b) LCMpH8.

identified by Stumpf and Fangha¨nel (2002). Cm(III) species (1) has a peak maximum at 606.2 nm. This red shift compared to the Cm3+ aquo ion is in the range of typical shifts (600.6–607.1 nm) observed for Cm(III) adsorption on mineral surfaces like: clay minerals (Stumpf et al., 2001a; Rabung et al., 2004; Rabung et al., 2005), feldspar (Stumpf

Incorporation of trivalent actinides into calcite: A TRLFS study

469

Eu(III) carbonates solids (as chemical analogue to An(III); Runde et al., 2000), shows that the fluorescence emission spectra of these solid phases are significantly different from the emission spectra of Eu(III) in calcite (Systma et al., 1991; Piriou et al., 1997, Marques Fernandes, 2006).

Normalized Cm(III) Fluorescence Emission

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Fig. 5. (a) peak deconvolution of the time dependent Cm(III) emission spectra in HCMpH8.1 and (b) the extracted spectra of the pure components Cm(III) species.

et al., 2006) or c- and a-Al2O3 (Stumpf et al., 2001b; Rabung et al., 2004). The strong red shift of the fluorescence emission spectrum of the Cm(III) species (2) to a peak maximum at 620.3 nm is a clear indication for a strong change in the ligand field of Cm(III). Such an important red shift (620.9 nm) has also been observed by Tits et al. (2003) for Cm3+ structurally incorporated in CSH. In our experiment performed at low Cm3+ concentration (LCMpH8.1), the input solutions were undersaturated with respect to solid phases like Cm2(CO3)3(cr), CmOHCO3(cr) (Table 2a). In the experiment performed at high M3+ concentration (HCMpH8.1), the input solutions were supersaturated with respect to these discrete phases (Table 2b). Since the calcite samples prepared at high and low metal ions concentrations LCMpH8.1 and HCMpH8.1 show both the same emission bands and we could not observe other additional Cm(III) fluorescence emission bands in HCMpH8.1, there is no spectroscopic evidence for the presence of a pure Cm(III) solid phase. The fluorescence spectra of pure Cm(III) carbonates solids are not available. However, because of the strongly different crystalline systems of pure Cm(III) solid phases and calcite, we suppose that they should show different fluorescence features (emission spectra, lifetime). Indeed, a TRLFS investigation of pure

3.1.1. Fluorescence decay measurements of Cm(III) Complementary to the energy of the fluorescence emission band, the lifetime of the excited state contains supplemental information about the complexation state. In principle, the fluorescence lifetime is controlled by quenching molecules in the first/second coordination sphere. H2O molecules are the most prominent fluorescence quencher (Kimura and Choppin, 1994; Kimura et al., 1996). This fluorescence quenching (radiationless deexcitation) is due to the vibronic coupling of excited Cm(III) with OH oscillators of bonded H2O molecules (t1(OH) = 3405 cm1). If no other quench processes (e.g., metal to ligand transfer) are present in the vicinity of Cm(III) the decay rate (kobs = s1; reciprocal lifetime) is proportional to the number of OH oscillators present in the first coordination sphere of Cm3+. In general, replacing those OH entities by other non-fluorescing ligands (e.g., carbonate complexation, Fangha¨nel et al., 1998a,b) leads to an increase of the fluorescence lifetime (decrease of the decay rate). Substituting D2O for H2O does not change the electronic structure of Cm3+ ion but leads to an significant decrease of the lifetime of Cm(III). In fact compared to OH vibronic, OD vibronics are too low in energy (t1(OD) = 2420 cm1) to act as an efficient quencher. The difference in the values of decay rates measured in pure D2O and in pure H2O is proportional to the number of H2O in the first coordination sphere of Cm(III) Dk obs ¼ k obs ðH2 OÞ  k obs ðD2 OÞ  nH2 O This correlation between the number of inner-sphere water molecules and lifetime of the excited state can be used to determine the hydration state of Cm(III). We applied the formalism developed by Horrocks and Sudnick (1979) and applied to Cm(III) by Kimura and Choppin (1994, 1996), recently recalculated by Stumpf (private communication), to calculate the number of associated hydration waters for Cm(III) species nðH2 OÞ ¼ 0:642 k obs  0:45ðuncertainty : 0:5 H2 OÞ n(H2O), the number of coordinated water molecules; kobs, observed decay rate (reciprocal lifetime) of the excited state (ms1). Without an OH-quencher in the first coordination sphere of Cm(III), the calculated decay rate is 770 s1 (Carnall and Crosswhite, 1985), this corresponds to a radiative lifetime (reciprocal decay rate) of 1.3 ms. This value is verified for the Cm3+ lifetime measurements in D2O. In contrast to that, the eight to ninefold hydrated Cm(III) aquo ion (in H2O) has a lifetime of 68 ls (Beitz et al., 1988). Figs. 4a and b show the Cm(III) fluorescence emission spectra of HCMpH8.1 and LCMpH8.1 at different delay times. The decrease of the intensity of Cm(III) species (1) in HCmpH8.1 shows that the position of the emission band of Cm(III) species (2) shifts to 620 nm, supporting that

M.M. Fernandes et al. / Geochimica et Cosmochimica Acta 72 (2008) 464–474

this Cm(III) species (2) is the same in both samples. By changing the delay time the ratio of the two Cm(III) species changes. This indicates that Cm(III) species (1) and (2) have different fluorescence emission lifetimes and that the shorter one can be attributed to Cm(III) species (1). The decay rates for the Cm(III) fluorescence emission of LCMpH8.1 and HCMpH8.1 show a biexponential decay (Fig. 6) which confirms the presence of at least two different Cm(III) species. Applying the correlation between inner-sphere water molecules and lifetime of the excited state, it follows that Cm(III) species (1) contains approximately one H2O molecule in the first coordination sphere, whereas Cm(III) species (2) has lost his complete hydration sphere. This is a clear indication that Cm(III) species (2) is incorporated into the calcite bulk structure. On the contrary, Cm(III) species (1) seems to be related to a surface sorbed actinide. However, inner sphere sorbed species typically contain five hydration waters. Two additional experiments were performed in order to investigate the nature of Cm(III) species in more detail. The fluorescence lifetimes of both Cm(III) species are summarized in Table 3. 3.2. Overgrowth experiments In order to verify that Cm(III) species (1) with a peak maximum at 606.2 nm is a surface sorbed Cm(III)/calcite species, calcite overgrowth experiments were performed. Fig. 7. shows the Cm(III) emission spectra of HCMpH8.1 before and after overgrowth. A clear increase can be observed for the emission band of Cm(III) species (2) after calcite overgrowth. We deduce that Cm(III) species (1) with a peak maximum at 606.2 nm is a surface sorbed Cm(III) species.

Cm(III) Fluorescence Intensity

LCMpH8.1 HCMH8.1

In order to ensure that the quenching of the fluorescence of the Cm(III) species (1) is due to OH-oscillations and not to another quencher contained in the calcite matrix, two batch-type experiments were performed, one in H2O and one in D2O, identical to the one performed by Stumpf and Fangha¨nel (2002). As O–D oscillators are less efficient in quenching the fluorescence lifetime than O–H, the lifetime of the Cm(III) species in D2O is expected to be increased. 107 mol L Cm3+ were added to an H2O and to a D2O, respectively (Aldrich Deuterium Oxide, 99.9 atom %) calcite suspension (1 g L1). Emission and fluorescence lifetime were measured after 24 h contact time. Emission spectra of Cm(III)/calcite in H2O and Cm(III)/calcite in D2O are presented in Fig. 8. Both spectra show the signal of the Cm(III) species (1) with a peak maxima at 606.2 nm. The Cm(III) fluorescence emission of the H2O and D2O samples show a mono-exponential (Fig. 9) decay behavior which confirms the presence of only one Cm(III)/calcite species. Cm(III) sorbed onto calcite in H2O exhibits a lifetime of 332 ± 8 ls. It follows that this Cm(III) contains 1.5 H2O molecules in the first coordination sphere. The actinide ion in the calcite D2O suspension exhibits a lifetime

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0

500 1000 1500 2000 2500 3000 3500 4000 4500 5000 Delay Time (µs)

Fig. 6. Time dependency of the fluorescence emission decay of Cm(III).

Table 3 Fluorescence emission lifetimes of LCMpH8.1 and HCMpH8.1 Cm(III) species (1)

LCMpH8.1 367 ± 40 ls HCMpH8.1 406 ± 12 ls

n(H2O)

Cm(III) species (2)

n(H2O)

1.3 ± 0.5 2042 ± 200 ls 0.12 ± 0.5 1.2 ± 0.5 1706 ± 300 ls 0.07 ± 0.5

Normalized Cm(III)F luorescence Emission

Fig. 7. Fluorescence emission spectra of Cm(III) in calcite before (solid line) and after overgrowth (dashed line).

mean lifetimes: 386 +/- 40 μs and 1874 +/- 200 μs

Calcite sample

3.3. Spectroscopic study of the interaction of Cm(III) with calcite in H2O and D2O, respectively

Normalized Cm(III) Fluorescence Emission

470

Cm(III)/calcite in D2O Cm(III)/calcite in H2O

580

590

600

610 620 Wavelength (nm)

630

640

Fig. 8. Fluorescence emission spectra of Cm(III) in H2O (doted line) and D2O (solid) calcite suspension.

Normalized ln(Cm(III) Fluorescence Intensity)

Incorporation of trivalent actinides into calcite: A TRLFS study

from the local chemical environment of the Cm(III) species (1) found in the samples LCMpH8.1 and HCMpH8.1. This fluorescence maxima does not correlate with the emission band identified by Stumpf et al. (2004) for the interaction of Cm(III) with portlandite Ca(OH)2 (613.6 nm) nor with fluorescence emission band of Cm3+ co-precipitated with Gd(OH)3 (607.1 nm) (Tits et al., 2003).

Cm(III)/Calcite in H2O Cm(III)/Calcite in D2O

787 +/- 10 µs

332 +/- 8 µs

3.5. Fluorescence decay measurements of Cm(III)

300

600

900 1200 1500 1800 2100 2400 2700 3000 3300 Delay time (µs)

Fig. 9. Time dependency of the fluorescence emission decay of Cm(III)/calcite in H2O and D2O.

of 787 ± 10 ls. It follows that this species would contain 0.37 H2O molecules in the first coordination sphere. For an unquenched Cm(III) species, we would expect s P 1.3 ms. The shorter lifetime of 787 ls may be attributed to the presence of small amounts of H2O (e.g., sorbed at the calcite surface). Indeed small amounts of water can considerably influence the decay rate since deuterated water D2O exchanges rapidly with H2O. However, the strong increase of the lifetime confirms that the quenching can be attributed to the presence of H2O in the first Cm(III) coordination sphere. 3.4. Spectroscopic characterization of Cm(III) coprecipitated calcite sample at pH 12.5 Selected Cm(III) fluorescence emission spectra of LCMpH12.5 and LCMpH8.1 are presented in Fig. 10. The Cm(III) fluorescence spectrum of LCMpH12.5 shows two emission bands at 608.5 nm (3) and 618.9 nm (4). The two emission bands can clearly be assigned to two different Cm(III) species. The emission band at 608.5 nm (3) shows red shift about 2.3 nm when compared to Cm(III) species (1) detected in LCMpH8.1 and HCMpH8.1. This indicates that the local chemical environment of this Cm(III) species (3) found in the sample synthesized at pH 12.5 is different

608.2 nm (3)

618.9 nm (4)

620 nm

Delay Time 3001 µs 2001 µs 1001 µs 801 µs 501 µs 201 µs 1 µs

580

590

600

610 620 Wavelength (nm)

630

640

Fig. 11. Fluorescence emission spectra of Cm(III) in LCMpH12.5 at different delay times.

Cm(III)/calcite pH 12.5

ln(Cm(III) Fluorescence Intensity)

LCMpH12.5 LCMpH8.1

Fig. 11 shows the fluorescence emission spectra of Cm(III) in LCMpH12.5 at different delay times. By changing the delay time the ratio of the two Cm(III) species changes. This indicates that Cm(III) species (3) and (4) have different fluorescence emission lifetimes. The decrease of the intensity of Cm(III) species (3) shows that the position of the emission band of Cm(III) species (4) shifts to 620 nm. In Fig. 12 the fluorescence intensities are plotted as a function of the delay time. The fluorescence decay is biexponential which confirms the presence of two different Cm(III) species. Cm(III) species (3) with peak maxima at 608.5 nm shows a fluorescence emission lifetime of s1 = 477 ± 25 ls. This lifetime does not correspond to the lifetime determined for Cm(III) in a portlandite suspension

Normalized Cm(III) Fluorescence Emission

0

Normalized Cm(III) Fluorescence Emission

471

lifetimes: 477 +/- 25 µs and 1850 +/- 200 µs

580

590

600 610 620 Wavelength (nm)

630

640

Fig. 10. Fluorescence emission spectra of Cm(III) in LCMpH8.1 (dashed) and in LCMpH12.5 (solid).

0

500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000

Delay Time (µs)

Fig. 12. Time dependency of the Cm(III) fluorescence emission decay in LCMpH12.5.

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s = 62 ± 8 ls (Stumpf et al., 2004) or for Cm(III) in Gd(OH)3 s = 56 ± 2 ls (Tits et al., 2003). For Cm(III) species (4) with peak maxima at 620 nm a fluorescence emission lifetime of s2 = 1850 ± 200 ls has been determined. The long fluorescence lifetime indicates that the excited state of the Cm(III) species (4) is not quenched by OH vibration. Applying the correlation between inner-sphere water molecules and lifetime of the excited state, a lifetime of 1874 ls indicates the total loss of the Cm(III) hydration sphere. The lifetime as well as the peak maximum of the fluorescence emission of Cm(III) species (4) are similar to the incorporated species Cm(III) species (2) determined in the samples grown at lower pH (LCMpH8.1 and HCMpH8.1). However, Cm(III) species (3) with a peak maximum at 608.4 nm seems to have a slightly longer lifetime as measured for the surface species Cm(III) species (1) with peak maximum at 606.2 nm. A fluorescence lifetime of 477 ls corresponds to 0.9 H2O molecules in the first coordination sphere. This species would be compatible to the incorporation of a CmOH2+ species since OH is considered to act independently and quenching less than H2O. However, we cannot exclude the possibility that Cm(III) species (3) could be a surface incorporated Cm(III) with still one OH molecule in the hydration sphere. However, the formation of an incorporated CmOH2+ is in good agreement with a model proposed by Curti et al. (2005). In alkaline solution at very low pCO2 Curti et al. suggested the incorporation of a M3+ oxy hydroxo species into the calcite lattice.

emission peak maximum as well as by a long fluorescence emission lifetime which indicates the total loss of the Cm(III) hydration sphere. The slow kinetics in batch-type experiments (Stumpf and Fangha¨nel, 2002), the extraordinary red-shift and the long fluorescence lifetime confirms that this Cm(III) species is incorporated into the calcite crystal. At pH 12.5 a third Cm(III)/calcite species is found: Cm(III) species (3) a with a peak maximum at 608.5 nm and a slightly different fluorescence emission lifetime than Cm(III) species (1). We suggest that this species could be a ‘‘CmOH2+’’ species incorporated into the calcite structure. Cm(III) has been used here as a molecular probe to study the structural incorporation of trivalent actinides elements into calcite. TRLFS allowed to investigate the reaction pathways for the Cm uptake by calcite via co-precipitation, which involves several species (Cm3+, Cm(OH)2+) in function of pH value. Clearly our results indicate that the Cm(III) co-precipitation with calcite is a rather complex process on molecular level. However, without spectroscopic identification of the various species and sites involved, thermodynamic data on aqueous solution solid solution equilibrium can not be derived. Further investigations, in order to elucidate the mechanism of charge compensation in this heterovalent substituted system are going on.

4. SUMMARY AND CONCLUSION

This work was co-financed by the Helmholtz Gemeinschaft Deutscher Forschungszentren (HGF) by supporting the Helmholtz-Hochschul-Nachwuchsgruppe ‘‘Aufkla¨rung geochemischer Reaktionsmechanismen an der Wasser/Mineralphasen Grenzfla¨che’’.

Our investigations led to the conclusion that three different molecular Cm(III)/calcite species exist: Cm(III)/calcite species (1) is a surface sorbed curium ion with one water molecule in the actinide coordination shell. Cm(III)/calcite species (2) is structurally incorporated into the bulk structure and presumably occupies a calcium lattice site. Cm(III)/calcite species (3) is also a structurally incorporated actinide ion species. However, it seems to be a Cmhydroxo species rather than a free Cm3+. The abundance of these species in calcite is controlled by the chemical conditions of the aqueous solution, preliminary the pH. At pH 8.1: Cm(III) species (1) with a peak maximum at 606.5 nm is found to be a surface incorporated Cm(III) species with one H2O molecule in the first coordination sphere. This was shown by the rapid kinetics (batch-type experiments performed in H2O and in D2O) of the interaction of Cm(III) with calcite crystallites in the batch experiment as well as by the moderate shift of the peak maximum of the Cm(III) fluorescence emission band and by the Cm(III) fluorescence lifetime which indicates the presence of one H2O molecule in the Cm(III) hydration sphere. Cm(III) species (2) with peak maxima at 620.3 nm is characterized by an extreme red shift of the fluorescence

ACKNOWLEDGMENTS

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