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The alteration of a depleted uranium (DU) disk in contact with a synthetic pore water, as a simulant for fertilized agricultural soil, was studied by exposing the DU to a calcium phosphate solution (2.5 × 10-3 M Ca, 1 × 10-3 M P). Within 12 months this contact resulted in the formation of a thin film of a secondary uranium mineral on the metallic DU. The reaction product was analyzed with both time-resolved laser-induced fluorescence spectroscopy (TRLFS) and infrared spectroscopy. Both techniques identified the reaction product on DU as a uranium(VI) phosphate phase, but only TRLFS provided its unequivocal identification as meta-autunite based on the positions of the fluorescence emission maxima at 487.8, 502.0, 523.6, 547.0, 572.1, and 600.6 nm and fluorescence lifetimes of 410 ( 15 and 3300 ( 310 ns. These results highlight the enhanced performance and sensitivity of the TRLFS technique for mineralogical characterization of thin surface films. Furthermore, they demonstrate that the dissolution of uranium from DU projectiles under the conditions described here is limited by the development and solubility of a meta-autunite secondary phase. The findings have helped clarify the interactions of DU ammunition with phosphate-rich soil-water.
DU has received more public attention following its use as ammunition, especially in the wars in Iraq and in the former Yugoslavia. Numerous investigations on the potential effects on human health and the ecological consequences from the use of DU in projectiles have been published recently (7-10). In vitro tests indicate that DU may be carcinogenic (11), nephrotoxic (12), genotoxic, and mutagenic (10). The response of native soil microbiota in terms of community composition to the presence of DU in an arid environment is reported in ref 13. The geochemical behavior of DU, including transport and immobilization in the near surface environment, has been the focus of several recent papers (14, 15). Potential risks for human health have triggered research on the geochemical behavior of DU in soil. The uptake of uranium by agricultural plants and incorporation in forage crops and its subsequent transfer into the human food chain are therefore of particular interest (16). Elementary uranium does not emit fluorescence, whereas U(VI) phases emit characteristic fluorescence signals and are thus detectable (17, 18). A characteristic laser fluorescence behavior in the case of U(IV) and U(V) phases is also well documented (19-21). In a long-term experiment dealing with weathering of uranium projectiles under natural conditions, spectroscopic analysis indicated the mineral sabugalite [Al(UO2)4(HPO4)(PO4)3 · 16H2O] on the surface of the metal (17). The samples used in the field experiments represent fired DU material, which was subsequently buried in a soil column and irrigated at weekly intervals. Baumann et al. (22) have previously used time-resolved laser-induced fluorescence spectroscopy (TRLFS) to provide spectroscopic evidence for the presence of meta-autunite on a pristine DU disk. However, despite extensive research into the environmental behavior of DU, the long-term effects on groundwater and drinking water as a result of finely dispersed DU projectiles in nature is still far from being understood. Since the fluorescence signals reported in ref 22 were extremely weak, we have performed additional experiments using infrared (IR) and a more advanced TRLFS system to study the weathering product on DU in well-defined conditions, mimicking a fertilized agricultural soil, i.e., pore water with a high content in calcium and phosphate.
Introduction
Experimental Section
In contrast to numerous studies on the environmental behavior of uranium compounds in nature (1-4), there are only a few investigations on the behavior of metallic uranium under environmental conditions. The use of depleted uranium (DU) in military projectiles has provoked discussions concerning the long-term environmental and human health impacts. DU is accumulated as a byproduct in the production of nuclear fuel in large amounts all over the world. It is used as counter or ballast weight in aircrafts, ships, and missiles, as well as in armorpenetration ammunitions and as cladding for armored vehicles (5). Uranium is the heaviest naturally occurring element, with a density of 19.1 g/cm3. It is approximately 1.7 times denser than lead. The uranium content of DU alloy comprises 99.8% 238U with 235U at only 0.2% and, therefore, emits approximately 60% of the R, β, and γ radiation found in natural uranium. In addition, its chemical toxicity has to be considered as in the case of other toxic heavy metals (6).
Samples and Sample Preparation. Disks of DU, 0.5 mm in thickness and 25 mm in diameter, were obtained from a pristine British military tank shell (CHARM 3 penetrator). More details on the samples are provided in ref 23. After degreasing, the sample was placed in a propylene beaker and submerged in a solution of 20 cm3 containing 2.49 × 10-3 M calcium nitrate with 1.05 × 10-3 M ammonium phosphate, both reagents supplied by Merck. The pH of the solution was adjusted to an initial value of 6.0 using HNO3 and NH4OH. After a contact time of one year at room temperature the sample was taken out of the solution and rinsed with deionized water prior to the TRLFS and IR measurements. The pH value of the solution decreased during the experiment from pH 6.0 to pH 5.6. The uranium mineral samples autunite [Ca(UO2)2 (PO4)2 · 2-6H2O], meta-autunite [Ca(UO2)2(PO4)2 · 10-12H2O], and chernikovite [(H3O)2(UO2)2(PO4)2 · 6H2O] were used as reference materials. These were hand-sized museum samples with patches of epitaxial growth of the respective uranium mineral phases. In the case of chernikovite, the quantity of sample available was not sufficient to allow diffuse reflection infrared Fourier transform spectroscopy (DRIFTS) experi-
Spectroscopic Verification of the Mineralogy of an Ultrathin Mineral Film on Depleted Uranium NILS BAUMANN,* THURO ARNOLD, HARALD FOERSTENDORF, AND DAVID READ† Institute of Radiochemistry, FZ Dresden-Rossendorf, P.O. Box 510119, D-01314 Dresden, Germany
Received March 20, 2008. Revised manuscript received August 18, 2008. Accepted August 26, 2008.
* Corresponding author e-mail:
[email protected]. † Enterpris Ltd. and Department of Chemistry, University of Aberdeen, United Kingdom. 8266
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10.1021/es800801h CCC: $40.75
2008 American Chemical Society
Published on Web 10/15/2008
ments. Chernikovite DRIFT spectra are not known from the literature. DRIFT spectra obtained from a synthetic chernikovite sample provided no further information concerning the identification of the secondary mineral film on DU (data not shown). DRIFTS. The DRIFTS experiments were carried out using a Perkin-Elmer GX-2000 instrument equipped with an MCT (mercury-cadmium telluride) detector. The spectral resolution was 4 cm-1 in the frequency range from 4000 to 600 cm-1. Each spectrum represents the averaged sum of 512 interferograms. The samples were placed on a DRIFT auxiliary (Specac Ltd., U.K.) without further treatment. TRLFS. The TRLFS measurements were performed directly on the surface of the sample. In the case of the alteration product on the DU disk, the disk was rinsed with deionized water and dried for 1 h under ambient atmosphere and subsequently used for TRLFS measurements. The results were compared with blank measurements on an unaltered DU disk which had not been in contact with a calcium phosphate solution. These measurements were carried out under the same analytical conditions (starting time, laser energy). The TRLFS system consists of a Nd:YAG diode laser (“Inlite” by Continuum) with subsequent fourth harmonic generation providing a wavelength of 266 nm which was used for the excitation of the samples. Fluorescence radiation generated in this way was collected by a fiber optic cable coupled to the slit of a spectrograph (EG&G Princeton Applied Research, model 1236 OMA 0.5 m spectrograph). The fluorescence spectra were measured by a charge-coupled device (CCD) camera (model 7467-0008, Princeton Instruments, Inc., a division of Roper Scientific, Inc., 1024 intensified diodes) and recorded in the range from 446 to 617 nm. The exposure time of the camera was set to 2000 ns. In steps of 100 ns, the delay time for each following exposure after the excitation laser pulse was recorded and ranged from 30 to 10 030 ns. The average laser power was approximately 0.3 mJ. Every spectrum was measured three times. For each spectrum 100 laser shots were averaged. For 1 time-resolved spectrum, 101 spectra at delay times (steps) starting from 30 ns and increasing to 10 030 nssin increments of 100 nsswere collected. All functions (time control, device settings, recording of the spectra, data storage) of the spectrometer were computer controlled. The spectroscopic data were evaluated with in-house software to obtain the fluorescence lifetimes (24). Generation of all graphics and the peak deconvolution of the single spectra obtained utilized the Origin v7.5 software (Origin Laboratory Inc.). The fluorescence decay function was determined on the basis of the fluorescence spectra at different delay times (100 nm steps). The biexponential decay function y ) y0 + A1 · e-(x-x0)/t1 + A2 · e-(x-x0)/t2 gave the best approximation, yielding two fluorescence decay times. TRLF spectra for five different spots (about 3 mm in diameter) on the DU disk were recorded. An optical filter (long-wave pass filter 10LWF-400-B, Newport) was used to retain all wavelengths smaller than 400 nm. In this way, the so-called laser dispersion peak was avoided. TRLFS measurements at low temperatures of 10 K carried out with a cryostat provided no improvement of the fluorescence signal. We undertook TRLFS measurements on five different spots on the DU disk, each 3 mm in diameter. Each measurement was consecutively denoted as DU_1, DU_2, etc.
Results and Discussion DRIFTS on DU. Figure 1 shows the IR spectra of the altered DU surface (Figure 1a) and the U(VI) minerals autunite (Figure 1b) and meta-autunite (Figure 1c), serving as reference spectra, in the spectral region between 1800 and 750 cm-1. In this region, characteristic bands are observed which were not present in
FIGURE 1. DRIFT spectra of the secondary mineral on depleted uranium (DU) (a) and natural samples of autunite (b) and meta-autunite (c).
TABLE 1. DRIFTS Spectral Data (cm-1) of Secondary Mineral on DU and Natural Uranium(VI) Phosphate Minerals secondary mineral on DU
autunite
meta-autunite
1633
1632 1494 1452 1403 1187 1118 1050 1026 912 825
1497 1457 1196 1121 1053 1000 923 826
1126 1026 915 819 808
the spectrum of the unaltered DU surface (data not shown). All spectra showed broad bands around 1630 cm-1 which are attributed to bending vibrations of residual water in the minerals and on the DU surface. In the spectral region between 1200 and 800 cm-1 overlapping bands are generally observed in uranium(VI) phosphate minerals representing modes of the phosphate groups and also of the uranyl ion (25, 26). A detailed interpretation of this spectral region is difficult due to the strong overlapping bands. Since the spectra of autunite and metaautunite were obtained directly from the surface of natural mineral phases, contributions from impurities to these spectra cannot be ruled out. Nevertheless, a comparison of the spectra clearly demonstrates a similar pattern of bands in all spectra between 1200 and 800 cm-1, which allows an unequivocal assignment of the secondary mineral phase formed on DU to a uranium(VI) phosphate phase. Therefore, the formation of oxide or carbonate phases can be clearly ruled out. The assignment of the secondary mineral phase on DU to a distinct uranium(VI) phosphate mineral is ambiguous. The spectral differences observed in the spectra of the natural mineral phases are not yet fully understood. According to recent investigations of uranyl micas (25), we assign the bands around 1120 and 1020 cm-1 to the antisymmetric stretching vibrations of the phosphate groups, whereas the bands around 915 cm-1 mainly reflect the ν3 mode (asymmetric stretch vibration) of the UO22+ ion. The spectrum of the secondary mineral on DU shows several spectral features, which are also observed in the spectrum of meta-autunite (see Table 1 and Figure 1a,c). There is a band around 1190 cm-1 which is not observed in the spectrum of autunite, suggesting it represents a characteristic spectral feature of natural meta-autunite. Additionally, several small bands are lacking in the spectrum of autunite around VOL. 42, NO. 22, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Lifetimes t (ns) from the TRLFS Signals from Altered DU Compared with Lifetimes Measured on Uranium(VI) Minerals Autunite, Meta-autunite, and Chernikovitea
DU_1 DU_2 DU_3 DU_4 DU_5 mean autunite meta-autunite
FIGURE 2. Time-resolved fluorescence spectra of altered DU as a function of the delay time. 1495, 1455, and 1050 cm-1, which may indicate the formation of a secondary phase which shows more specific spectral attributes of meta-autunite. However, since we have to take into account contributions from impurities to the IR spectra, this assignment still has to be proven by a complementary technique. Therefore, we applied TRLFS to try to distinguish between the uranium(VI) phosphate minerals. TRLFS on DU. Two types of information are provided by the fluorescence signal from uranium(VI) species: the position of the fluorescence emission bands and the fluorescence lifetime from the fluorescence signal (24). Primary features of the TRLF spectrum are the positions of the fluorescence emission bands, whereas the fluorescence lifetime is secondary, because it is dependent on the temperature of the experiment (27). The TRLF spectra obtained from altered DU after drying and presented here (Figure 2) are of considerably enhanced quality and show much more intensity than the spectrum reported previously (22). Furthermore, the laser dispersion peak at 532 nm is suppressed completely by an optical filter. Six distinct fluorescence emission maxima are clearly visible, and four of them are very sharp. Their peak maxima were observed at 487.8, 502.0, 523.6, 547.0, 572.1, and 600.6 nm. The secondary feature to identify fluorescence spectra of unknown phases is the fluorescence lifetime. The number of possible species is extractable from the time-resolved spectrum. All time-resolved fluorescence spectra of this study were fitted with a monoexponential, a biexponential, and a triexponential decay function. The correlation coefficients from a biexponential decay function of all measurements show an accuracy >0.986, two of them >0.999. In all cases, a monoexponential decay results in a significantly worse fit, with correlation coefficients between 0.96 and 0.98. Triexponential decay functions yielded unacceptably large scatter in the lifetimes. In two measurements an assumed triexponential decay function resulted in only two lifetimes (measurements DU_2 and DU_3). In the case where two lifetimes are assumed, those lifetimes are very reproducible; they were checked and proved in five measurements (Table 2). The two lifetimes from these five measurements are 500 and 2200 ns on average. We explain the two lifetimes with two different lattice sites for the uranium with different numbers of water molecules in the uranium coordination environment. These two lifetimes are much closer to the lifetimes measured on the mineral meta-autunite than on the minerals autunite and chernikovite (Table 2). The very short lifetime of 50 ns measured on DU after staying in contact with calcium phosphate solution and published in ref 22 can be explained by fast relaxation processes immediately after excitation of the sample. The long lifetime of 2200 ns presented here was not visible in the measurement published in ref 22, because the signal in 8268
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mean chernikovite
t1
t2
R2
350 ( 3 520 ( 5 500 ( 1 570 ( 1 570 ( 1 500 ( 90
1900 ( 10 2100 ( 6 2700 ( 4 2300 ( 2 2100 ( 1 2200 ( 300
0.9924 0.9859 0.9977 0.9994 0.9995
7800 ( 3
43000 ( 16
0.9990
400 ( 1 420 ( 1 410 ( 15
3500 ( 4 3060 ( 3 3300 ( 310
0.9986 0.9975
1300 ( 80
5500 ( 400
0.9948
a
R2 is the coefficient of correlation between the experimental points and the fit.
FIGURE 3. Fluorescence spectra of the mineral on DU, meta-autunite, autunite, and chernikovite. All spectra are normalized curves and are offset along the vertical axis. that former measurement was too weak (very low signal-tonoise ratio) and the detection system (diode array) was less sensitive than those used for the experiments of this work. The shape of the first and most intensive fluorescence spectra gained from the mineral film on DU was compared with the shapes of the first and most intensive fluorescence spectra gained from samples of the naturally occurring uranium phosphate minerals autunite, meta-autunite, and chernikovite (Figure 3). The greatest similarity is between spectra from the mineral on DU and spectra from meta-autunite. This conformance becomes obvious in the ratio of the height of the second and third peaksscounted from the left side. Both peaks have almost the same intensity in the spectra of meta-autunite and the secondary mineral detected on altered DU. In the spectra of autunite and chernikovite the second peak shows a significantly lower intensity than the third peak. This conformance is also visible in the positions of the peak maxima (Table 3). In summary, the surface of a DU disk, immersed in a calcium phosphate solution for one year, was investigated by TRLFS and Fourier-transform IR. First, our intention was to evaluate the capabilities of these two techniques when applied to this problem. Second, we wanted to validate the results presented in ref 22 since these results showed very weak fluorescence signals with a low signal-to-noise ratio. The IR spectra presented here clearly confirm that the mineral film from alteration is a uranium phosphate, since
TABLE 3. Wavelengths of Peak Maximum Positions (nm) for Measurements on DU and from Reference Minerals
DU_1 DU_2 DU_3 DU_4 DU_5 mean DU
first peak
second peak
third peak
fourth peak
fifth peak
sixth peak
488.98 489.78 487.78 486.14 486.14 487.78
502.23 501.92 502.05 501.92 501.88 502.05
523.50 523.86 523.63 523.68 523.81 523.63
546.81 547.12 546.99 546.94 546.94 546.99
573.66 572.95 572.07 572.6 572.11 572.07
603.88 599.19 600.65 601.45 601.09 600.65
autunite 489.56 504.84 524.56 547.87 574.28 602.11 meta-autunite 488.05 501.57 523.72 547.21 573.26 600.25 chernikovite 484.90 502.14 523.63 547.47 572.95 602.82
characteristic bands are observed in the spectral range between 1200 and 900 cm-1. However, an unequivocal determination of the U(VI) phase cannot be made since the vibrational spectra obtained may show contributions from specific mineral contaminants in the natural samples or from the metallic background of the DU, which interfere with the phosphate bands and, thus, alter relative band intensities. These effects make it very difficult to identify the mineral phase unambiguously. The excellent agreement between TRLFS spectra from altered DU and from the naturally occurring uranium mineral (fingerprint) allows the mineral film on DU to be unambiguously identified: The homologies of the shapes of the spectra from the altered DU sample and of the meta-autunite sample together with the respective lifetimes provide evidence for the formation of meta-autunite on the DU disk under the prevailing conditions. It is shown that TRLFS is able to clearly identify a uranium phosphate mineral formed as an ultrathin layer on the surface of DU. The results demonstrate the performance and sensitivity of TRLFSasaveryhelpfultoolintheinvestigationofenvironmentally relevant problems. It is also shown that meta-autunite can be formed from metallic uranium in calcium phosphate solutions and, therefore, has to be considered as a potential product of alteration processes occurring in agricultural soils at low temperatures. The newly formed mineral phase is a potential source for further dispersion of uranium. These findings provide the basis for an improved understanding of the migration behavior of uraniuminthenearsurfaceenvironment,anessentialprerequisite for estimating the health risks associated with the military and industrial uses of uranium.
Acknowledgments The uranium reference samples were provided by courtesy of Andreas Massanek, Mineral Collection of the TU Bergakademie Freiberg, Germany. We thank Karsten Heim for IR measurements.
Supporting Information Available Tablegivingacomparisonofthecalculationresultswhenassuming one, two, or three lifetimes from all five measurements on the altered DU sample and the corresponding lifetimes of autunite, meta-autunite, and chernikovite. This material is available free of charge via the Internet at http://pubs.acs.org.
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