Spectrochimica Acta Part A 72 (2009) 449–453
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Fluorescence properties of a uranyl(V)-carbonate species [U(V)O2 (CO3 )3 ]5− at low temperature Kay Grossmann ∗ , Thuro Arnold, Atsushi Ikeda-Ohno, Robin Steudtner, Gerhard Geipel, Gert Bernhard FZ Dresden-Rossendorf e. V., Institute of Radiochemistry, P.O. Box 510119, D-01314 Dresden, Germany
a r t i c l e
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Article history: Received 17 April 2008 Received in revised form 9 October 2008 Accepted 17 October 2008 Keywords: U(V) Uranyl(V)-carbonate Fluorescence Laser fluorescence spectroscopy
a b s t r a c t Fluorescence properties of a uranyl(V)-carbonate species in solution are reported for the first time. The fluorescence characteristics of the stable aqueous uranyl(V)-carbonate complex [U(V)O2 (CO3 )3 ]5− was determined in a frozen solution (T = 153 K) of 0.5 mM uranium and 1.5 M Na2 CO3 at pH 11.8 by time resolved laser-induced fluorescence spectroscopy (TRLFS). Two different wavelengths of 255 nm and 408 nm, respectively were used to independently of each other excite the uranyl(V)-carbonate species. The resulting U(V) fluorescence emission bands were detected between 380 nm and 440 nm, with a maxima at 404.7 nm (excitation with 255 nm) and 413.3 nm (excitation with 408 nm), respectively. It was found that by using an excitation wavelength of 255 nm the corresponding extinction coefficient was much higher and the fluorescence spectrum better structured than the ones excited at 408 nm. The fluorescence lifetime of the uranyl(V)-carbonate species was determined at 153 K as 120 s. TRLFS investigations at room temperature, however, showed no fluorescence signal at all. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Transport of toxic and radioactive heavy metals from contaminated soils into cultivated plants and animals may eventually enter via ingestion of food into the human body and, there cause harmful reactions to human health. Its migration behavior and bio-availability are strongly influenced by the binding form, the chemical speciation. Uranium is one of these potentially dangerous substances, which in enriched concentrations represents a major health hazard. In nature uranium contaminations are caused predominantly by uranium mining activities [1], but also to a minor extent by fired off depleted uranium (DU) projectiles, which on impact become pulverized and are transferred as finely dispersed powder into agriculturally used soil [2], or simply by the use of uranium-containing fertilizer [3]. Another potential danger arises from the possible leakages of high-level waste tanks and from geological repositories of radioactive wastes [4]. In soil uranium is in contact with inorganic and organic components, subsurface waters, microorganisms, plant roots, their respective exudates, and metabolites. All of them affect the speciation of uranium and hence its migration behavior. An understanding of the uranium speciation on a molecular scale would help to better assess its migration behavior in soil environments and
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[email protected] (K. Grossmann). 1386-1425/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2008.10.041
its bioavailability to plants. Uranium has various oxidation states (i.e., III–VI) and it is well established that its transport behavior strongly depends on its oxidation state. In nature, uranium typically occurs in the oxidation state VI or IV. Under oxidizing conditions, e.g. in surface waters and aerated groundwaters the uranium(VI) is commonly found in the UO2 2+ moiety which is coordinated in the equatorial plane with other ligands to form the respective uranyl(VI) species. Under reducing conditions, e.g. in subsurface waters uranium is commonly present in the tetravalent oxidation state (U(IV)). Here, the uranium(IV) atom is directly coordinated with respective ligands. U(IV) is only soluble at very low pH. At pH 2 and higher U(IV) is present in precipitated form, i.e. it is immobilized and is not migrating as dissolved species in the environment. In contrast to tetravalent uranium, U(VI) is much more soluble and may migrate in the environment in dissolved form via the water path, especially in the presence of carbonate, which is present in high concentrations in air and in groundwater systems. With carbonate ions (CO3 2− ) uranium forms various types of complexes. Trivalent and tetravalent uranium carbonato complexes are expected to be insoluble in underground conditions [5] and hence are not considered for the hydrogeological assessment. However, hexavalent uranium carbonato complexes show a high mobility due to their high solubility. The carbonato complex of uranium is one of the most essential species not only for the migration study on radioactive waste repositories but also for investigation of the remediation of closed uranium mining activities [6]. Carbonato complexes of hexavalent uranium (i.e., uranyl(VI) ion, UO2 2+ ) in an
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aqueous solution have been well investigated [7]. In contrast, there is much less information available on pentavalent uranium (i.e., uranyl(V) ion, UO2 + ) carbonato complexes because UO2 + in nature is generally regarded as being unstable in aqueous solution and disproportionates (2UO2 + → UO2↓ + UO2 2+ , [8]) readily to uranium(IV) and uranyl(VI) species, particularly at pH values below neutral [9 and reference therein]. Aqueous uranyl(V) species in acidic solution are only stable in a very narrow pH range of pH 2–3 and only for a short time of approximately 1 h [10]. In fact, the only known stable aqueous uranyl(V) complex is a carbonate species formed in a basic aqueous solution with a high carbonate content (pH > 11) [11]. Its importance in the uranyl(VI)/uranyl(IV) redox reaction catalyzed by mineral surfaces has recently been discussed by several researchers [12–14]. Microbes are particularly well-known for their ability to reduce U(VI) to U(IV) [15–24]. In these studies, uranyl(V) as an intermediate of this redox reaction is not mentioned at all. Only two microbiologically orientated studies mention pentavalent uranium as an intermediate of the U(VI) redox reaction. Großmann et al. [25] observed in a very recent study short-lived U(V) intermediates in multispecies biofilms indicating that U(V) may temporarily be stabilized in biofilm environments during the microbially mediated U(VI) redox reaction. In another X-ray absorption study by Rentschaw et al. [26] on the subsurface metal-reducing bacterium Geobacter sulfurreducens U(V) was also observed as an intermediate of the reduction of U(VI) to U(IV). Fluorescence properties of U(V) in a 2-propanol perchlorate solution were identified by laser fluorescence spectroscopy [27] using 255 nm as excitation wavelength. In such a solution U(V) is stable for a short time of approximately 1 h. The highest extinction coefficient for uranium(V) is realized with an excitation wavelength of 255 nm [10], which is between the optimal excitation wavelengths for U(IV) at 245 nm [28,29] and U(VI) at 266 nm [30]. Absorption studies of tetravalent, pentavalent and hexavalent uranium in solution have been performed by numerous scientists. However, only fluorescence spectra of tetravalent [28,29] and in particular hexavalent aqueous [31,32] and adsorbed uranium(VI) species [33,34] have been reported. Suitable excitation wavelengths to detect the U(V) fluorescence were suggested by two recent studies. The first one is by Nagai and co-workers [35] and studies the redox equilibrium of UO2 2+ /UO2 + in molten NaCl–2CsCl. They found an absorption peak for U(V) at 395 nm. The second one is by Grossmann et al. [25] and studies uranium(V) and (VI) particles in a multispecies biofilms. Here, the U(V) fluorescence was observed by using an excitation wavelength of 408 nm. For a better tracking of the U(VI) reduction process to U(IV) in living systems it is important to have a spectroscopic tool for the determination of the U(V) intermediate. In this work it was intended to study the fluorescence properties of U(V). For this the stable aqueous uranyl(V) carbonate complex [U(V)O2 (CO3 )3 ]5− was chosen. Since pure U(VI)-carbonates do not show any fluorescence properties at room temperature, because of quenching effects [36], it could be assumed that U(V) carbonates behave similar and the laser fluorescence investigations were carried out at room temperature and 153 K (−120 ◦ C). For the U(V) excitation two excitation wavelengths of 255 nm and 408 nm were applied.
Fig. 1. UV–vis spectra for 0.5 mM U(VI) in 1.5 M Na2 CO3 solution (pH 11.8) before and after electrolysis.
pound was dissolved in 1.5 M Na2 CO3 solution (pH 11.8) to give a concentration of 0.5 mM U(VI). Then bulk electrolysis was performed to reduce the prepared U(VI) solution to the U(V) by using Pt mesh working- and counter electrodes, and a Ag/AgCl reference electrode. A detailed description about this electrolysis has been given previously [38]. UV–vis absorption measurements were carried out using a Cary 5G UV-visible-NIR spectrophotometer (Varian, Inc.) before and after the electrolysis to confirm the oxidation states of U in the sample solution. The measured spectra are shown in Fig. 1. The initial sample of U(VI) exhibited characteristic absorption bands from 380 nm to 480 nm. These absorption bands corresponding to U(VI) disappeared and a broad and featureless spectrum was obtained after the electrolysis. This spectral change was identical with those ones observed in the previously-reported U(V) preparation studies [9,38], suggesting that U(VI) was successfully reduced to U(V). The U(V)/U(VI) ratio in the reduced sample was estimated to be 96/4 (M/M) from the absorbances at 448 nm by assuming that the U(V) species has no absorbance at this wavelength. No absorption bands characteristic for uranium(IV) were observed. All of the chemicals (except uranium compounds) used in this study were reagent grade and supplied by Merck KGaA. The structure of the synthesized U(V) carbonate species has already been determined by an extended X-ray absorption fine structure (EXAFS) study by Docrat et al. [39] and Ikeda et al. [38]. They showed that the carbonato complex of the uranyl(V) ion is identical with that of the uranyl(VI) ion and is found to be a tricarbonato complex [UO2 (CO3 )3 ]5− . Its structure is shown in Fig. 2.
2. Experimental 2.1. U(V) synthesis A starting material of Na4 [UO2 (CO3 )3 ] was synthesized from UO2 (NO3 )2 ·6H2 O (supplied by Lachema Ltd.) according to the procedure reported previously [37]. This uranyl(VI) carbonate com-
Fig. 2. Structure of the uranyl(V)-carbonate complex [U(V)O2 (CO3 )3 ]5− .
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Fig. 3. Fluorescence emission spectra of the frozen uranyl(V)-carbonate species [U(V)O2 (CO3 )3 ]5− using either 255 nm or 408 nm as excitation wavelength measured at 153 K.
2.2. Laser-induced fluorescence spectroscopy After the successful electrolysis step the obtained uranyl(V)carbonate solution was transferred into 3 ml polystyrene cuvettes (BI-SCP, Brookhaven Instruments, USA), sealed and quick-frozen in liquid nitrogen. Prior to the time resolved laser-induced fluorescence spectroscopy (TRLFS) measurements the solid iced sample was removed from the cuvette and positioned in a cold gas system (Kaltgassystem Typ: TG-KKK, KGW Isotherm, Karlsruhe, Deutschland). The TRLFS analyses were carried out as triplicates at 153 K (−120 ◦ C). The U(V) complex was excited from the ground state to the excited state either with a 255 nm or a 408 nm pulsed laser beam, obtained by frequency doubling of a 510 nm beam from a pulsed (10 Hz) OPO laser. The pulse power was typically 1 mJ per pulse and the pulse width was about 5 ns. The fluorescence lifetime and the respective emission spectra were measured using an intensified CCD camera (HORIBA Jobin Yvon GmbH) and a spectrograph (M270, HORIBA Jobin Yvon GmbH), respectively. Deconvolution of the emission spectra were performed using Origin 7.5 (OriginLab Corporation, Northampton, MA, USA) and Lorentzian–Gaussian functions for peaks with fitting parameters such as peak wavelength, peak height and full-width at half maximum. Corresponding fluorescence lifetime calculations were also carried out with Origin 7.5.
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Fig. 4. Peak deconvolution of the fluorescence signal (excited by 255 nm) of the uranyl(V)-carbonate species, [U(V)O2 (CO3 )3 ]5− showing five emission maxima at 386.0 nm, 393.3 nm, 404.7 nm, 419.6 nm, and 443.4 nm.
ing excitation wavelength, similar to the observations made in our study. The extinction coefficient, however was much higher and the spectra were better structured for the 255 nm excitation wavelength in comparison with the 408 nm excitation wavelength. A comparison of the U(V) fluorescence data presented in this study with results published by Steudtner et al. [27] for a meta-stable U(V) solution at pH 2.4 revealed that the fluorescence emission peak maxima for the uranyl(V)-carbonate solution is shifted to shorter wavelengths. Steudtner et al. [27] found a peak maximum at 440 nm. The peak maximum for the uranyl(V)-carbonate solution in our study was detected at 404.7 nm and 413.3 nm, respectively. This is in agreement with cryogenic laser fluorescence investigations of uranyl(VI)-carbonates [7]. In U(VI) systems the fluorescence emission bands for uranyl(VI)-carbonates are significantly, by approximately 15 nm, shifted to shorter wavelength compared to U(VI) hydrolyses species or U(VI) phosphates measured at room temperature. Fluorescence studies of organic matter under cryogenic conditions show also a blue shift of the emission maxima when compared to fluorescence studies at room temperature [42]. However, that does not imply that TRLFS measurements of uranium samples carried out under cryogenic conditions behave in the same way. Quite the contrary, measurements of uranium(VI) phases performed at room temperature and under cryogenic conditions show the same positions of their respective emission
3. Results and discussion The frozen uranyl(V)-carbonate solution were analyzed by TRLFS measurements using either 255 nm or 408 nm as excitation wavelength. The resulting fluorescence emission spectra of the frozen [U(V)O2 (CO3 )3 ]5− species for both excitation wavelengths are shown in Fig. 3. TRLFS measurements of the [U(V)O2 (CO3 )3 ]5− species at room temperature showed no fluorescence properties at all. To our knowledge no fluorescence of pentavalent uranyl carbonate in solution was previously reported. The different excitation wavelength leads to two similar but not identical emission signals. The resulting U(V) fluorescence emission bands were detected for both excitation wavelengths between 380 nm and 440 nm, with a maxima at 404.7 nm (excitation with 255 nm) and 413.3 nm (excitation with 408 nm), respectively. A similar effect of the shift in the positions of the fluorescence emission bands using different excitation wavelengths was observed by Sen et al. [40,41] in a study on coumarin 480. The emission maxima of coumarin 480 were red-shifted with increas-
Fig. 5. Time resolved laser-induced fluorescence spectra of the fluorescence decay of the uranyl(V)-carbonate species, [U(V)O2 (CO3 )3 ]5− , excited by 255 nm.
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systems. The realization that U(V) is also excited at 408 nm opens up new possibilities, since conventional laser fluorescence microscopy systems only operate with laser wavelengths greater than 350 nm. In this context, it may contribute to a better understanding concerning the chemistry and thus the migration behavior of uranium in biologically influenced ecosystems. This will contribute to new and more effective uranium removal strategies in bioremediation processes. Acknowledgments The authors thank Prof. Y. Ikeda (Tokyo Institute of Technology) and K. Takao (FZD) for fruitful discussions. We also thank the German Research Council (DFG) for financial support (project no. AR 584/1-1 and HE 2297/2-1). References Fig. 6. Determination of the fluorescence decay lifetimes (excited by 255 nm) of the uranyl(V)-carbonate species, [U(V)O2 (CO3 )3 ]5− . A fluorescence lifetime of 120 s at 153 K was calculated.
bands (own measurements). Uranium carbonates show fluorescence properties at low temperature but not at room temperature. However, when a fluorescence signal of carbonate is observed, this one occurs at lower wavelengths than previously reported fluorescence signals e.g. for uranium hydroxides. The fluorescence spectra with the higher fluorescence intensity, i.e. the sample excited with 255 nm, were peak deconvoluted. The deconvolution of the emission spectra, shown in Fig. 4 revealed that the fluorescence signal of the U(V) carbonate species, [U(V)O2 (CO3 )3 ]5− showed five emission maxima at 386.0 nm, 393.3 nm, 404.7 nm, 419.6 nm, and 443.4 nm. The main emission peak was located at 404.7 nm. Minor contributions of
