Environ. Sci. Technol. 2010, 44, 921–927
Spectroscopic Identification of Ternary Cm-Carbonate Surface Complexes M . M A R Q U E S F E R N A N D E S , * ,†,| T . S T U M P F , ‡,§ B . B A E Y E N S , † C. WALTHER,‡ AND M. H. BRADBURY† Laboratory for Waste Management, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland, Institut fu ¨ r Nukleare Entsorgung, Karlsruher Institut fu ¨ r Technologie, P.O. Box 3640, Karlsruhe, 76021, Germany, and Physikalisch-Chemisches Institut, Ruprecht-Karls-Universita¨t Heidelberg, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany
Received July 21, 2009. Revised manuscript received November 10, 2009. Accepted November 17, 2009.
The influence of dissolved CO2 on the sorption of trivalent curium (Cm) on alumina (γ-Al2O3) and kaolinite was investigated by time resolved laser fluorescence spectroscopy (TRLFS) using the optical properties of Cm as a local luminescent probe. Measurements were performed at T < 20 K on Cm loaded γ-Al2O3 and kaolinite wet pastes prepared in the absence and presence of carbonate in order to pictorially illustrate any changes through a direct comparison of spectra from both systems. The red-shift of excitation and emission spectra, as well as the increase of fluorescence lifetimes observed in the samples with carbonate, clearly showed the influence of carbonate and was fully consistent with the formation of Cm(III) surface species involving carbonate complexes. In addition, the biexponential decay behavior of the fluorescence lifetime indicated that at least two different Cm(III)-carbonate species exist at the mineral-water interface. These results provide the first spectroscopic evidence for the formation of ternary Cm(III)-carbonate surface complexes.
Introduction The fate of released radionuclides in geological environments is primarily controlled by sorption/desorption processes onto mineral surfaces. Oxides and clay minerals are important constituents of host rock formations potentially considered for repositories of radioactive waste. The sorption of metal ions is strongly dependent on ionic strength, pH, and the presence of complexing organic or inorganic ligands in the liquid phase. Clearly, sorption models based on spectroscopically validated uptake mechanisms and surface speciation have a considerably higher credibility and a much higher reliability in their application and lead to a reduction in uncertainties. A thorough understanding of the sorption mechanisms at a molecular level is needed in order to establish reliable thermodynamic sorption * Corresponding author e-mail:
[email protected] † Paul Scherrer Institut. | Present address: Laboratory for Waste Management, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland. ‡ Institut fu ¨ r Nukleare Entsorgung. § Ruprecht-Karls-Universita¨t Heidelberg. 10.1021/es902175w
2010 American Chemical Society
Published on Web 01/05/2010
databases capable of calculating the radionuclide retention in deep geological formations. Carbonate is ubiquitous in natural systems, and has a great affinity for complexing with actinides (1-3). The formation of strong aqueous carbonate complexes can potentially lead to a decrease in metal ion sorption and thus increase the migration rates of actinides. Some structural data on the sorption of trivalent actinides (An) and lanthanides (Ln) for carbonate free systems have been obtained by means of spectroscopic techniques. The sorption of Eu(III) and Cm(III) on montmorillonite, illite, kaolinite and γ-Al2O3 has been investigated by time resolved laser fluorescence spectroscopy (TRLFS) as a function of pH (4-9). The structure of Am(III) sorbed on smectite and kaolinite at various pH values was studied using combined TRLFS and X-ray absorption spectroscopy (XAS). For the dioctahedral clay minerals (illite, montmorillonite) TRLFS/ XAS allowed to distinguish between outer sphere (cation exchange) and inner sphere complexation (pH dependent surface complexation of Cm(III) to the amphoteric edge sites). Further, for all the above-mentioned systems the formation of ternary hydroxo complexes tOsCm(OH)n(H2O)5 with increasing pH was shown. For Ln/An/carbonate/clay minerals or (hydr)oxides systems, thermodynamic and surface species structural data are sparse. However, such data are absolutely essential, since argillaceous rock porewaters often contain quite high carbonate concentrations (10). It has been reported that the formation of ternary hydroxo carbonato surface complexes may contribute in surface sorption reactions (11-13). A TRLFS study by Stumpf et al. (2002) (12) postulated that Eu ternary carbonato surface complexes could be formed on smectite and kaolinite under air equilibrated conditions in the alkaline pH region. The evidence was based on the differences observed in the fluorescence spectra and fluorescence emission lifetimes of sorbed Eu(III) in the presence and absence of carbonate. XAS studies have also indicated that ternary carbonato surface complexes of U(VI) are formed on montmorillonite and hematite (11). In a recent study by Marques et al. (2007) (13), macroscopic sorption studies of trivalent Ln/An on montmorillonite were carried out to investigate the influence of carbonate on their uptake behavior. Sorption measurements were carried out as a function of pH in the presence of various carbonate concentrations. The measurements showed a pronounced decrease of sorption in the presence of carbonate. The experimental data could be successfully modeled with the 2 Site Protolysis Non Electrostatic Surface Complexation and Cation Exchange (2SPNE SC/CE) sorption model (14) only by including two additional surface complexation reactions forming tSSOAnCO3 and tSSOAnOHCO-3 surface species. If the modeling is correct then the clear implication is that two distinct trivalent actinide ternary surface complexes form. Even though pure aluminum (hydr)oxides rarely exist in nature, their surface properties are often taken as a model mineral representing the aluminol sites of clay minerals (15). TRLFS studies have shown similarities between Cm(III) sorbed onto clay minerals and γ-Al2O3 compared to Cm(III) sorbed onto silica (16). Similar positions of the fluorescence emission bands and fluorescence lifetimes suggest the formation of comparable inner sphere complexes (17, 18). In this work investigations were carried out on γ-Al2O3 as well as on the natural clay mineral kaolinite as a direct comparison. The aim of this study was to characterize the influence of carbonate on the sorption behavior of trivalent VOL. 44, NO. 3, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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actinides and to verify whether or not ternary Cm-carbonato complexes form at the surface of γ-Al2O3 and kaolinite using TRLFS.
Experimental Section Materials. The investigations were carried out on γ-Al2O3 and kaolinite (KGa-1). Prior to its use, the γ-Al2O3 (Degussa, aluminum oxide C) was purified according to the procedures reported by other authors (19, 20). Hydrous γ-Al2O3 particles have a specific surface area, as obtained from N2-BET analysis on a freeze-dried sample, of ∼119 m2 · g-1. Well crystallized, Georgia kaolinite KGa-1 was obtained from the Clay Minerals Society Source Clay Repository. Kaolinite was used without any pretreatment. KGa-1 has been extensively characterized (21, 22). The N2-BET surface area of KGa-1 is ∼10 m2 g-1 and its cation exchange capacity (CEC) is ∼20 meq · kg-1. Sample Preparation. In order to investigate the influence of carbonate on the sorption of Cm(III) on γ-Al2O3 and kaolinite, samples were prepared in the absence and presence of carbonate. Carbonate free samples were prepared in a glovebox under N2-atmosphere (O2 < 3 ppm) to avoid any carbonate contamination. Experiments in the presence of carbonate were carried out under atmospheric conditions in closed vessels. Solutions were prepared from analytical grade chemicals. For the experiments 248Cm was used (t1/2 ) 3.4 × 105 years). The isotopic composition of the Cm stock solution stored in concentrated HCl was 89.7% 248Cm, 9.4% 246 Cm, and less than 1% other Cm isotopes. Cm(III) sorption experiments on γ-Al2O3 and kaolinite (in the absence and presence of carbonate) were carried out at fixed ionic strength (I ) 0.1 mol · L-1) and at a solid to liquid (S/L) ratio of ∼2 g · L-1 in 20 mL polyethylene (HDPE) bottles (Zinsser Analytic) at 298 K. The alumina and kaolinite suspensions were equilibrated for 1 day prior to adding Cm. The total Cm concentration of the samples was fixed at 1.25 × 10-7 mol · L-1. The pH of the samples was adjusted to 6 by adding analytical grade 0.1 mol · L-1 NaOH in small steps. This precaution was taken in order to prevent colloid generation and the precipitation of solid phases. Once a pH of 6 was obtained, the dissolved carbonate concentration was set to 20 mmol · L-1 by adding aliquots of 1 mol · L-1 NaHCO3 and the pH was buffered to ∼8.4 corresponding to a pCO2 ∼ 10-2.5 bar. Speciation calculation showed that the Cm(CO3)2- complex is the dominant aqueous species under the given experimental conditions in the presence of carbonate. The formations constants used for CO32- complexation and for hydrolysis are in the same order of magnitude except for the CmCO32- complex, the corresponding formation constant log β ∼ 8 exceeds the one for hydrolysis (log β ∼ 6.8)) by almost 1 order of magnitude. The contribution of the hydrolysis species becomes negligible because the free carbonate concentration (CO32-) is ∼2 orders of magnitude higher than the OH- concentration at pH 8.4. The aqueous speciation for Cm(III) in 20 mmol · L-1 NaHCO3 in the pH range 5-10 in 0.1 mol · L-1 NaClO4 is given in Supporting Information (SI) Figure S1. Metal hydrolysis and carbonate complexation constants were taken from the Am(III) NEA-TDB Chemical Thermodynamic Database as no selected NEA data for Cm are reported and because Am(III) and Cm(III) are often considered to be chemical analogues (3, 23). The pH of the carbonate free samples was then adjusted to the pH value of the carbonate containing samples in order to allow a direct comparison between the two systems. Samples were stored for at least 2 days and periodically shaken. (Sorption kinetics experiments performed for time periods between 2 and 60 days showed that equilibrium conditions were already reached after 2 days.) To ensure that the measured fluorescence signal was not due to aqueous 922
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Cm-carbonate complexes, the TRLFS measurements were carried out on Cm(III) loaded wet pastes. After equilibration, and immediately before the TRLFS measurements, the suspensions were centrifuged at 62 000 g for one hour. The supernatants were analyzed for Cm(III) and for pH measurements. The Cm(III) loaded pastes were transferred into a sealed copper cell. Methods. Time Resolved Laser fluorescence Spectroscopy (TRLFS). The TRLFS measurements (excitation spectra, emission spectra, lifetimes) were carried out on the Cm(III) loaded γ-Al2O3 and kaolinite wet pastes prepared in the presence (20 mmol · L-1 NaHCO3) and absence of carbonate. In order to reduce linewidths and hence facilitate discrimination of nonequivalent sites, samples were cooled to T < 20 K. Cm(III) is not only a good representative of the trivalent actinides, but also exhibits excellent fluorescent properties suitable for surface speciation measurements. The sensitivity of TRLFS for Cm(III) is extremely high and enables speciation studies in the nanomolar concentration range. The information provided by TRLFS is 2-fold: (i) changes in the ligand field are detectable from spectral shifts of the 6D7/2 f 8 S7/2 transition; the magnitude of the shift is correlated with the extent of covalent bonding to the ligands within the first coordination sphere of the ion (18) and, (ii) the analysis of the Cm(III) fluorescence decay rate allows the hydration state of the given Cm(III) species to be determined. If no other quench processes (e.g. metal-to-ligand transfer) are present in the vicinity of Cm(III), the rate of radiationless de-excitation is directly proportional to the number of H2O ligands in the inner coordination sphere of the Cm(III) ion. Replacing these entities by other ligands through aqueous complexation reactions (hydrolysis, carbonate complexation, etc.), sorption or incorporation on/in mineral phases, generally increases the fluorescence lifetime (18). According to the linear relationship developed by Kimura and Choppin (24), the number of associated hydration waters for Cm(III) is given by the following equation. n(H2O) ) 0.642kobs- 0.88(uncertainty: ( 0.5 H2O) where n(H2O) is the number of coordinated water molecules and kobs is the observed decay rate (reciprocal lifetime) of the excited state (ms-1). The efficiency of the energy transfer process is temperature-independent as long as the major fraction of water molecules remains in their vibrational ground state. Fluorescence measurements at low temperature increase the efficiency of the fluorescence intensity as the fluorescence increases with decreasing temperature. Furthermore cryogenic conditions improve the spectral resolution thus facilitating spectral analysis. Two different Cm(III) fluorescence excitation paths were considered in this study: indirect and direct excitation. The schematic energy level diagram of the Cm3+ describing the two excitation paths is given in the SI Figure S2. In the case of indirect excitation, the Cm3+ fluorescence emission spectra of the 6D7/2 f 8S7/2 transition is obtained upon excitation at 396.6 nm into the F level, that is, the most intense absorption band of Cm3+ (25). The 6D7/2 (A) level of each nonequivalent Cm(III) site is populated via non radiative relaxation and the transition between the ground state and lowest excited 6D7/2 level(s) gives rise to the Cm(III) fluorescence emission. In the case of direct excitation, the Cm3+ is excited to the 6 D7/2 state instead of the F state, which is the case in the indirect excitation, a very similar emission spectrum is observed due to the radiative relaxation of the 6D7/2 f 8S7/2 transition. However in case of direct excitation, a subset of ions with a unique transition energy can be selectively excited. By using an optical multichannel analyzer a complete emission spectrum at each excitation wavelength is obtained,
which allows comparing the emission spectra for each of the excitation wavelengths. TRLFS measurements were performed using a pulsed excimer pumped dye laser system (20 Hz). The dye QUI, with an emission maximum at λ ) 380 nm, was used for the UV excitation (indirect excitation) of the most intense absorption maxima (396.6 nm) of Cm3+. Rhodamin B dye, with an emission maximum at λ ) 620 nm, was used for the direct excitation of the Cm(III) 6D7/2 f 8S7/2 transition. An optical multichannel system consisting of a polychromator with a 300, 600, and 1200 lines/mm grating and an intensified CCD camera was used to detect the fluorescence emission. The time-resolved detection was controlled via a PG200 delay generator. The fluorescence lifetime was measured by varying the delay time between laser pulse and camera gating with time intervals of 7-40 µs. For the low temperature spectroscopic measurements, the Cm(III) loaded pastes were transferred into an in house constructed copper cell with a sapphire window sealed with a Teflon disk and a total cell volume of 40 µL. The helium cooling system (Cryodyne Cryocooler model 22C, compressor 8200, CTI-Cryogenics, U.S.) comprised of a continuous closed-cycle refrigeration which allowed the copper sample holder to be cooled at the coldfinger down to approximately 17 K in a two-stage decompression step. The coldfinger with the sample holder was surrounded by a vacuum chamber with four quartz windows. The pressure at low temperatures was in the range of 10-4-10-5 mbar. A microcontroller-based auto tuning temperature controller (model 330-1X, Lake Shore, U.S.) with a silicon dioxide temperature sensor was used to control the temperature. A heater allowed the temperature to be adjusted between 17 and 370 K. For sample excitation, the laser beam was focused on the copper cell in the vacuum chamber and the fluorescence signal collected and directed via glass fiber into the monochromator.
Results and Discussion Cm Sorption onto γ-Al2O3 in the Presence and Absence of Dissolved Carbonate at T < 20 K. Indirect excitation. Figure 1a shows the fluorescence emission spectra of the Cm(III)/ γ-Al2O3 system in the presence (continuous line) and absence of carbonate (dotted line) when exciting at 396.6 nm. In both cases the emission spectra are rather broad, which is a typical feature of adsorbed Cm species (full width at half-maximum (FWHM) is ∼8.5 nm). When identical ions are present in an amorphous (i.e. glass) matrix or a very disordered arrangement when sorbed on a mineral, than a large amount of band broadening is normally observed. This “inhomogeneous broadening” of the transition mostly arises from the fact that Cm(III) ions occupy slightly different 6D5/2 levels, thus resulting in a continuous distribution of overlapping emission bands at different wavelengths. The recorded Cm(III) fluorescence emission maxima represents the center of gravity of the excited 6D7/2 level(s). In the absence of carbonate, the emission spectrum of Cm(III) exhibits a peak maximum at ∼601.5 nm. In the present study the maxiumum fluorescence intensity is detected at the same wavelength as reported by Stumpf et al. (2001) (6) and Rabung et al. (2006) (7) under similar pH conditions (pH ∼ 8.4) in the absence of carbonate. In the presence of carbonate the maximum of the fluorescence emission band is shifted to ∼605 nm, indicating a change in the first coordination sphere of Cm(III) compared to the carbonate free system. Such a red-shift is not unusual for Cm(III) surface complexes and may be a possible consequence of the formation of ternary Cm(III)-carbonate complexes at the γ-Al2O3 surface. The emission band maxima for Cm surface complexes published in the open literature are usually found at λmax e 607 nm (18). The decay rates of the Cm(III) fluorescence emission in both systems are shown in Figure
FIGURE 1. (a) Emission spectra of the 6D7/2 f 8S7/2 transition of the Cm(III)/γ-Al2O3 system in the absence (---) and presence (s) of carbonate and (b) the corresponding fluorescence decay profiles measured at T < 20 K under broadband excitation at 396.6 nm. 1b. For the carbonate free sample, the fluorescence lifetime follows a monoexponential decay behavior indicating that only one hydrated Cm3+ species exists. The measured lifetime is about τ1a ) 107 ( 2 µs, which in a good agreement with the value of ∼110 ( 2 µs found in the open literature for carbonate free Cm(III)/γ-Al2O3 suspensions under broadband excitation (6, 7). This lifetime correlates with the formation of an Cm(III) inner sphere complex at the γ-Al2O3 surface having ∼5 H2O molecules in the first hydration sphere. For the carbonate containing system, the fluorescence lifetime shows a biexponential decay behavior with measured lifetimes of τ2a ) 138 ( 3 µs and τ3a ) 418 ( 10 µs (Figure 1b). This behavior indicates the presence of at least two differently hydrated Cm3+ species corresponding to a coordination environment with ∼4 H2O and ∼1 H2O molecule(s) respectively. The increase in the lifetimes compared to the carbonate free system reflects the formation of inner sphere complexes in which additional water molecules have been excluded and/or been replaced by CO32- in the first coordination sphere of Cm(III). The presence of different Cm(III) species is further confirmed by the time dependent emission spectra. The peak maximum shifts from 605 to 606.5 nm VOL. 44, NO. 3, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Excitation spectra of the 6D7/2 f 8S7/2 transitions of Cm(III)/γ-Al2O3 system in the absence (---) and presence (s) of carbonate measured at T < 20 K. with increasing delay time confirming that the biexponential decay is due to the presence of at least two Cm(III) surface species with different hydration states. See SI Figure S3. Direct Excitation. The excitation spectra of Cm(III) sorbed onto γ-Al2O3 in the presence and absence of carbonate was obtained by scanning the excitation wavelength in 0.1 nm steps in the range of “the inhomogeneous broadened” 6D7/2 level (596-625 nm) and recording simultaneously the subsequent fluorescence emission as a function of the exciting wavelength. The integrated intensity of the emission band is taken as a measure of fluorescence intensity at a given excitation energy and plotted against the excitation wavelength. The measurements were carried out at constant laser energy of 1 mJ. Figure 2 shows the excitation spectra of the Cm(III) sorbed on γ-Al2O3 in the absence (dashed line) and presence of carbonate (continuous line). In both cases the excitation spectra are rather broad reflecting the continuous distribution of site energies, indicating that Cm(III) is present as sorbed species as already observed in the indirect excitation mode. This means that the inhomogeneous broadening is larger than the energy difference between the lowest excited 6D5/2 levels occupied by the Cm(III) ions. The excitation spectrum of the carbonate free system shows a maximum ∼602 nm. For the Cm(III)/γ-Al2O3/carbonate system, two maxima are observed in the excitation spectrum at ∼598 nm and ∼605 nm. Further, a considerable red-shift of the excitation spectrum in the presence of carbonate compared to the carbonate free system is observed, confirming a change in the ligand field of the Cm(III) ion. The noticeably different shape and red-shift of both spectra clearly indicate that distinct chemical environments exist for Cm(III) in the two cases and support the results obtained by the “indirect excitation” measurements. Figure 3a and b show the fluorescence emission spectra of the Cm(III)/γ-Al2O3/ carbonate sample obtained by (i) exciting directly (resonantly) at various wavelength within the inhomogeneously broadened excitation spectra (596-625 nm) and (ii) exciting indirectly through broadband excitation at λex ) 396.6 nm. It can be readily seen from Figure 3a that the envelope of all the fluorescence emission spectra obtained when exciting directly at various wavelengths reproduces very well the shape of the emission spectra obtained under broadband excitation at 396.6 nm. (Note that the FWHM of the spectrometer was controlled by setting the slit width to a value of ∼1 nm, as determined by measuring the spectra of a low-pressure neon 924
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FIGURE 3. (a) Fluorescence emission spectra of the 6D7/2 f 8S7/2 transitions measured at T < 20 K for the Cm(III)-carbonate/ γ-Al2O3 system as a function of excitation wavelength (596-622 nm). The sharp bands (panels a and b) are the fluorescence emission resonant with the excitation; the broad sidebands are fluorescence emission from the lower lying 6D7/2 excited states. lamp.) The emission spectra obtained when exciting at λex < 599 nm are identical to the spectrum obtained under broadband excitation with a peak maxima ∼605 nm (Figure 3b). Indeed at these excitation wavelengths all the discrete energy levels are populated and participate to the fluorescence emission. The sharp feature appearing in the emission spectra at λex g 599 nm is the fluorescence line resonant with the excitation (resonant fluorescence line (RFL) (26)), that is, a subset of Cm(III) ions with a unique transition energy is excited and the position of this band shifts with the excitation wavelength (Figure 3b). The broad sideband at higher wavelength is due to the simultaneous fluorescence emission from the lowest level of lower lying 6D7/2 levels. The delay between the laser pulse and the camera gating was set to 10 µs in order to make sure that when exciting directly, the recorded signal is not influenced by the laser beam. The higher the excitation wavelength, the weaker the fluorescence
FIGURE 4. Fluorescence decay profiles of the 6D7/2 f 8S7/2 transitions obtained for the Cm(III)/γ-Al2O3 system in the absence and presence of carbonate measured at T < 20 K when exciting directly (resonantly) at various wavelength within the inhomogeneously broadened excitation spectra. contribution of the lower-lying energy levels. Since the energy difference between the lowest excited 6D7/2 states is not large enough, it is not possible to excite single levels without populating the energy levels below. In addition to the emission and excitation spectra, the fluorescence lifetime of the 6D7/2 f 8S7/2 transition of both systems was recorded. Figure 4 shows the decay profiles obtained after exciting at various excitation wavelengths and the derived lifetimes. The carbonate free sample shows the same monoexponential decay behavior for each of the excitation wavelengths, indicating that only one hydrated Cm(III) surface species exists. The measured lifetime is about τ1b ) 120 ( 2 µs, which in a good agreement with the value of ∼107 ( 2 µs found under broadband excitation. For the carbonate system, biexponential decay behavior is observed for each of the excitation wavelengths, indicating the presence of at least two Cm(III) surface species with different hydration shell. Exciting at the local maxima of the excitation spectrum at 598 or 605 nm (see Figure 2) shows the same decay behavior, indicating that the maximum in the excitation spectrum at 598 nm is not due to another Cm(III) surface species but corresponds to a “hot band”, that is, thermally populated energy level(s). The measured fluorescence lifetimes are τ2b ) 157 ( 5 µs and τ3b ) 425 ( 12 µs. The lifetimes measured are again in the same range as the ones obtained under broadband excitation and confirm the existence of at least two Cm(III) surface species with different hydration shells. From the clear spectroscopic differences between the carbonate free and the carbonate containing system and the fact that the Cm(CO3)n3-2n are the predominant aqueous species under the given experimental conditions it is concluded that the Cm(III) species identified in the presence of carbonate are ternary Cm(III)-carbonate surface complexes. Table 1 summarizes the fluorescence features obtained for the Cm(III)/γ-Al2O3 system. Cm Sorption onto Kaolinite in the Presence and Absence of Dissolved Carbonate at T < 20 K. The same types of spectroscopic measurements (excitation spectra, fluorescence lifetime) were carried out on the Cm(III)/kaolinite system in order to verify that the results obtained on γ-Al2O3
are comparable to clay minerals. Figure 5a shows the excitation spectra of the Cm(III)/KGa-1 in the absence (dotted line) and presence of carbonate (continuous line). In both cases the excitation spectra are rather broad, as expected for Cm sorbed on the kaolinite surface. The excitation spectrum of the carbonate free systems shows no pronounced maximum. On the other hand, the excitation spectrum of the carbonate system is red-shifted and has a noticeably different shape characterized by two local maxima at ∼598 nm and ∼606 nm. Clearly, these differences indicate that the presence of carbonate leads to the formation of other different surface species and are in very good agreement with the results obtained on the Cm(III)/γ-Al2O3 system. This statement is further corroborated by the results of the fluorescence lifetimes measurements. Figure 5b shows the decay of the emitting 6D7/2 level to the 8S7/2 ground state for both systems after exciting at various wavelengths. For the carbonate free sample, the same monoexponential decay behavior is observed for each of the excitation wavelengths, confirming that only one type of hydrated Cm species exists on the clay surface. The measured lifetime is about τ1c ) 93 ( 10 µs, which is slightly smaller than the value of ∼110 µs found in the open literature for carbonate free Cm(III)/kaolinite suspensions (9). A possible explanation might be the quenching of the fluorescence lifetime by structural iron present in kaolinite (∼0.15 wt % Fe2O3). The quenching effect of structural iron present in different clay minerals on the fluorescence lifetime of Cm(III) outer-sphere complexes has been investigated by Hartmann et al. (8). For the carbonate containing system, biexponential decay behavior is observed for each of the excitation wavelengths. Exciting at the local maxima ∼598 nm or ∼606 nm shows the same decay behavior, indicating that the band in the excitation spectrum at 598 nm is a thermally populated band as observed for the γ-Al2O3 system. The measured fluorescence lifetimes are τ2c ) 147 ( 19 µs and τ3c ) 490 ( 27 µs, which are again in a good agreement with the previous results obtained on the Cm(III)/γ-Al2O3 system. The increased lifetimes and the biexponential decay behavior compared to the carbonate free system, reflects the formation of at least two different Cm(III) species on the kaolinite surface in the presence of carbonate. Cm(III) was used as a local luminescence probe to investigate the effect of aqueous phase carbonate complexation on the sorption of Cm(III) on γ-Al2O3 and kaolinite. TRLFS measurements performed on Cm(III)/γ-Al2O3 and Cm(III)/kaolinite, in the presence and absence of carbonate at a pH ∼8.4, allowed the influence of carbonate on the surface sorbed species of trivalent lanthanides/actinides on these two minerals to be characterized. The fluorescence features (red-shift and the shape of the excitation spectra, emission spectra, increase of the fluorescence lifetime) of the Cm(III)-carbonate systems differ strongly from those of the carbonate free systems indicating clearly different coordination environments for Cm(III) at the mineral surfaces. These features strongly indicate that ternary Cm(III)-carbonate surface complexes do form on γ-Al2O3 and kaolinite and hence contribute significantly to immobilization of trivalent actinides. Additional measurements at different pCO2 and/or different pH values are necessary to further characterize and quantify the nature of the different Cm(III) species. The application of other surface analysis techniques such as XAS are required to obtain structural information (coordination, bond distance) on the Cm(III)-carbonate surface complexes. Time-resolved laser fluorescence spectroscopy with Cm(III) represents a suitable method for the elucidation of geochemical reactions at the water-mineral interface on a molecular level. Based on this molecular level description thermodynamic data will be derived. This work will contribute VOL. 44, NO. 3, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Summary of the Fluorescence Maxima and Lifetimes of Cm(III) Sorbed on γ-Al2O3 in the Absence and Presence of Carbonatea indirect excitation λex ) 396.6 nm
direct excitation 596 < λex < 625 nm
Cm(III)/γ-Al2O3
emission max (nm)
decay lifetimes (µs)
n(H2O)
max (nm)
decay lifetimes (µs)
n(H2O)
no carbonate in 20 mmol · L-1 NaHCO3
∼601.5
τ1a ) 107 ( 2 τ2a ) 138 ( 2 τ3a ) 418 ( 10
5.1 ( 0.5 3.8 ( 0.5 0.7 ( 0.5
∼602
τ1b ) 120 ( 2 τ2b ) 157 ( 5 τ3b ) 425 ( 12
4.5 ( 0.5 3.2 ( 0.5 0.6 ( 0.5
a
∼605
∼605
[Cm]total ) 1.2 × 10-7 mol · L-1, pH ∼ 8.4, background electrolyte 0.1 mol · L-1 NaClO4 and S/L ratio ) 2 g · L-1.
was carried out in the framework of a EURATOM IntraEuropean Fellowship (EIF) and was cofinanced by the EU 6th Framework Integrated Project NF-PRO (contract number F16W-CT-2003-02389). Partial financial support was provided by the National Cooperative for the Disposal of Radioactive Waste, NAGRA (Switzerland).
Supporting Information Available Aqueous speciation of Cm(III) (Figure S1), schematic energy level diagram of the Cm3+ describing the two fluorescence excitation paths (Figure S2), and Figure S3 supporting the TRLFS results. This material is available free of charge via the Internet at http://pubs.acs.org.
Literature Cited
FIGURE 5. (a) Excitation spectra of the 6D7/2 f 8S7/2 transitions obtained for the Cm(III)/KGa-1 system in the absence (---) and presence (s) of carbonate and (b) the corresponding fluorescence decay profiles measured when exciting directly at various wavelength within the inhomogeneously broadened excitation spectra. significantly to an improved knowledge of the actinides/ mineral interface chemistry and will be beneficial for developing the safety cases for the disposal of chemically toxic and radioactive wastes.
Acknowledgments Sebastian Bu ¨chner from INE (Karlsruhe, Germany) is gratefully acknowledged for his technical assistance. This work 926
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