Environ. Sci. Technol. I 9 9 4 28, 980-984
COMMUNICATIONS I n Sifu Chemical Speciation of Uranium in Soils and Sediments by Micro X-ray Absorption Spectroscopy Paul M. Bertsch' and Douglas B. Hunter
Division of Biogeochemistry, University of Georgia, Savannah River Ecology Laboratory, Drawer E, Aiken, South Carolina 29802 Stephen R. Sutton, Saga BaJt, and Mark L. Rivers
Department of Geophysical Sciences, University of Chicago, and Department of Applied Sciences, Brookhaven National Laboratory, Upton, New York 11973
Introduction The ability to develop adequate models for predicting the fate of inorganic contaminants in both surface and subsurface environments is highly dependent on accurate knowledge of the partitioning of these constituentsbetween the solid and solution phases and ultimately on the capability to provide molecular-level information on chemical species distributions in both of these phases. Furthermore, the development of environmentally sound yet cost-effective remediation strategies requires an understanding of the chemical speciation of the contaminants within the soil or sediment matrix in which they are contained. Owing to analytical difficulties, traditional methods for determining speciation of metals in soils and sediments have relied heavily on indirect chemical extraction techniques, in which various phases within the matrix are operationally defined and inferences on chemical associations are generated (1-3). This approach is less than satisfying, since it is not clear what specific reactions take place during the chemical extraction of the operationally defined phases nor what artifacts, such as contaminant element redistributions, may be introduced during and/or following the extraction of a given phase (2). Commonlyemployed in situ speciation methods (e.g., PIXE, EDX, XPS) can potentially avoid such limitations, but these methods have often not been sensitive enough to detect contaminant elements of interest, they require modifications of the sample, or the analysis must be conducted under conditions that may alter chemical species distributions, thus introducing other potential artifacts and complicating data interpretation (3). X-ray absorption spectroscopy (XAS) utilizing synchrotron radiation has been used in recent years to characterize bonding environments of contaminant metals and metalloids associated with well-defined monomineralic phases (4-10). These studies have demonstrated the utility of XAS for investigating the structure of adsorbed species at environmental concentrations and under environmentally realistic conditions. All of these investigationshave,
* Author to whom all correspondence should be addressed; Telephone: (803) 725-2472; FAX: (803) 725-3309; E-mail address:
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however, been conducted using conventional XAS beam lines in which the synchrotron-generated X-ray beam is in the millimeter to centimeter size range, thus requiring that the samplesbeing examined are isolated homogeneous phases. Other recent investigations have demonstrated the ability to conduct XAS experiments utilizing the synchrotron-based X-ray microprobe capabilities of beam line X-26A at the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory and of a nondedicated X-ray microprobe at the Photon Factory in Tsukuba, Japan (11-13). In the investigationsconducted on beam line X-26A at NSLS, Cr oxidation states in 200pm regions within olivine grains from lunar basalt and in nuclear waste forms were inferred using X-ray absorption near-edge structure (XANES) spectroscopy. I t has been demonstrated previously that a minimum beam size of approximately 2 pm is achievable utilizing the microanalytical capabilities of this beam line, which provides the ability to investigate metal speciation on a microscale within heterogeneous samples (11). It is also possible to simultaneously collect a full synchrotron X-ray fluorescence (SXRF) spectrum on the identical region in which a XANES spectrum is generated, thus providingadditional information on elemental distributions within the microregion of interest. There has been substantial interest recently in chemical speciation and species transformations of uranium in contaminated soils, sediments, and nuclear wastes, both from the standpoint of predicting its mobility to and within subsurface environments and for developing effective strategies for remediating contaminated sites (14-1 7). Oxidation state is a fundamental property of U speciation that greatly influences U solubility and, thus, mobility. Whereas U(V1) forms soluble complexes in most surface water and groundwater, U(1V) forms highly insoluble solid phases such as uraninite (UOz(c)) (18, 19). Recently published studies have demonstrated direct microbial reduction of U(V1) to U(IV), and it has been suggested that this process could be potentially utilized as an in situ biological remediation strategy. Other U remediation methods currently under consideration include chemical extraction to remove U from contaminated soils and sediments (20). Developingeffective chemical extraction 0013-936X/94/0928-0980$04.50/0
0 1994 American Chemical Society
procedures to remove U from contaminated soils and sediments will require considerable knowledge about the chemical speciation of the contained U. In this investigation, we exploited the microanalytical capabilities of beam line X-26A at Brookhaven National Laboratory to collect XANES and SXRF spectra on localized (50-300-pm) regions within a number of U-contaminated soils and sediments. This provided specific information on U oxidation states, qualitative information on U-bonding environments, and information on associated elemental distributions. As far as we are aware, this study represents the first published application of spatially resolved XAS to a contaminant associated with a heterogenous soil or sediment matrix.
Experimental Section The synchrotron X-ray fluorescence microprobe on beam line X-26Aat the National SynchrotronLight Source (Brookhaven National Laboratory, NY) was used in its normal configuration (21,22)with the addition of a silicon, channel-cut (111)monochromator on the incident radiation. The beam size was adjusted to between 50 X 50 and 300 X 300 pm with tantalum shutters operated via computer-controlled motorized stepping micrometer. XANES spectra were collected at 0.3 eV step increments over a 120 evenergyrange (relative to 17163eV) extending from about 50 eV below to approximately 100 eV above the U LIIIabsorption edge. The count rate at each incident step was recorded for between 2 and 20 live s such that the total counts at the absorption maximum were 10 000. The U LIII fluorescence X-rays were measured under ambient atmospheric conditions with a Si(Li) energy dispersive detector having an area of 30 mm2mounted at 90° to the incident beam and 1 cm from the sample. Soil and sediment samples (-200-pm-thick) were mounted on Kapton adhesive windows within cardboard sample holders that were attached to 5 X 5 cm slide mounts. An exception was for the sand fraction samples, which were deposited as a single layer on Kapton tape such that single (100-300 pm) or small clusters (50-100 pm) of grains could be examined. A uraninite (UOz(c))sample was employed as the U(1V) reference material, and UOS and a reagentgrade uranyl acetate were used as the U(V1) reference materials for these investigations. Edge positions are expressed relative to the pure UOz(c) sample, which was arbitrarily set to 0 eV. Standards used to generate the calibration curve were prepared by mixing appropriate proportional amounts by weight of each phase and then homogenizing them in a grinding mill prior to mounting. The finely ground homogenized standards were then applied to Kapton adhesive as a thin film and mounted to cardboard sample holders as described above. The absorption edge was defined as the half-height (precisely determined by the derivative) of the XANES spectrum after pre-edge baseline subtraction and normalization to the maximum above-edgeintensity. The absorption edge positions of all standards and samples were referenced against the edge position of the UOz(c) standard, which was rerun at least every fourth sample. Mixed standards prepared by dilution with sodium acetate to a final concentration of U ranging from 200 to 2000 pg 8 1 (2 wt %) were ground, mixed, and pressed into 13mm diameter by 1mm thick discs using a Wilmad evacuating die under vacuum at a pressure of 9 t. No significant differences in XANES amplitudes or slopes of standard curves were
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Figure 1. Calibration curve (P = 0.987) of relative absorption edge energies of the U(IV) and U(V1) end members and three proportional (by weight) mixtures of the end members. Values represent the means of six determinations for the end members and the 50150 mixture and triplicate determlnatlons for the 7 5 / 2 5 and 25/75 mixtures. Error bars reprsent fl SE.
observed between these samples or the pure standard mounts, suggesting that any effects of self-absorption are minimal. Additionally, preliminary analysis of the XANES spectra of a number of mineral samples provided by the Smithsonian Institution including gummite (NMNH R13671) [y-U031, uranophane (NMNH R11015-17) [Ca(UOz)z(Si030H)2], and bassetite (NMNH R11137) [Fe(U02)2(P04)21revealed that the energy of the U LIII edge position is little influenced by matrix effects, i.e., the position is dependent on the oxidation state and is not sensitive to changes in local bonding environments. Six successive independent determinations of the U LIIIedge for both the U(1V) and U(V1) mineral phases provided standard errors of between *0.15 and 0.32 eV, which is in the range of that recorded for repetitive measurements on the contaminated soil or sediment samples examined. Sediment and soil samples were separated into three fractions by sieving and centrifugation: sand fraction, defined as C 300 but > 50 pm; silt fraction, defined as C 50 but > 2 pm; and clay fraction, defined as C 2 pm.
Results and Discussion It is well-established that the energy of an X-ray absorption edge increases with increasing valence, resulting from the reduced shielding of the core electrons from the nucleus. This increase in the binding energy of the core levels is often manifested by shifts in pre-edge and boundstate edge features in a XANES spectrum that can be correlated to differences in the oxidation state of a cationic center (23). Such shifts in the position of the LIIIedge for U in glasses have been previously reported (24,25),but there has not been a systematic evaluation of the capability to infer U oxidation states from U-bearing mineral phases based on the LIIIedge position. Examination by the microXANES technique of samples containing both pure U(1V) (as uraninite) and U(V1) (as uranyl acetate and/or UOa) containing phases and physical admixtures prepared on a proportional weight basis suggests that a near-linear relationship (r2 = 0.987) exists between the proportional amounts of U(1V) and U(V1) in the physical admixtures and the central location of the edge position (defined by the half-height energy) (Figure 1). The -3.75-4.3 eV shift to lower energy in the edge position of tetravalent U containing UOa(c) relative to the U(V1) phases is accompanied by the absence of the shoulder [multiple Environ. Sci. Technol., Vol. 28, No. 5, 1994 981
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Relative Energy (eV)
Figure 2. Uranium LIll XANES spectra of the pure end member U(IV) [uraninite (U02(c))] (-)and U(V1) [U03(c)] (..e) standards and a 50/50 mixture (by wt) of the two (- -). The arrows accentuate differences typically observed in the above-edge broad MSR between U(IV) and U(V1) phases.
scattering resonance (MSR)] on the high-energy side of the main absorption feature (the maximum above-edge intensity) characteristic and diagnostic of U(V1)-containing phases (24, 25) (Figure 2). This latter region of the XANES spectrum for uranyl compounds is related to multiple scattering events in the direction of the linear U-0-U group involving electrons excited into the continuum (25). We have examined a number of reference U-containing mineral phases to date and found that there are no discernible matrix effects on the position of the absorption edge, i.e., the absorption edge energy appears to be predominately a function of oxidation state. Whereas variation in bonding environments are manifested as differences, albeit sometimes subtle, in the above-edge region of the XANES spectrum arising from multiple scattering events. The uranium-contaminated soils and sediments examined in this investigation were obtained from areas surrounding two former U material processing facilities of the US. Department of Energy: Fernald, OH (Fernald Environmental Management Project or FEMP; soil sample SP2-2ABC)and SavannahRiver Site (SRS),SC (sediment sample CS-1A). A detailed characterization of these samples was made and is reported elsewhere (20,261.The soils from the FEMP site were contaminated with U from both aqueous waste and from airborne particulates; whereas, all of the U in sediments collected at the SRS was discharged as aqueous waste. XANES spectra of the clay fractions from both the soil (SP2-2ABC)and the sediment (CS-1A) samples indicated that the contained uranium was predominately in the U(V1) form, based both on the position of the absorption edge and on the characteristic high-energy structural features typical of U(V1). There was no evidence of spatial variability of the U speciation within the clay fractions. This was expected as the beam size is much larger than the individual clay particles, and the samples were mounted uniformly within the Kapton windows. Since an X-ray absorption spectrum represents a populationweighted average of U in all possible coordination environments and oxidation states, we cannot rule out the possibility that minor amounts of the U present within these samples are in the reduced form. However, based 982
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on the relationship generated for the proportional mixtures of U(1V) and U(V1) phases (Figure l), it is likely that >90 % of the U associated with the clay fractions of these samples is in the hexavalent oxidation state. This observation has been consistent for alarge number of sizefractionatedsediment samplesthat we have examined from the SRS, even though some of these samples were collected in the seasonally reduced environment of wetland areas. Therefore, we have not observed evidence for either significant microbial or abiotic reduction of U in the presence of high organic-containing sediments under seasonally mild reducing conditions, as has been suggested for other environments and in laboratory studies (14,19). Uranium distributions within the sand fractions of the contaminated soil and sediment were found to vary considerably as a function of the spatial region examined. The measurement strategy used was to search the mount employing a 50-pm beam and to visually monitor the XRF spectrum on an oscilloscope reading until the U intensity at a particular location increased significantly. If sufficient sensitivity was attained at the region located (as determined by the SiLi detector response),a XANES spectrum was collected. The beam size was then progressively increased to 100, 150, 200, and finally 300 pm, and the process was repeated at each step. XANES spectra were collected in 22 individual regions for the soil (FEMP) sample and in 10 regions for the sediment (SRS) sample, with full SXRF spectra also being collected for many of the regions examined. Two types of populations were readily discernible in different regions examined by this analysis; one where the regions were highly enriched in U and depleted in other elements commonly found in soils and sediments (e.g., Fe and Mn), and the other where the U was less concentrated and co-associated with regions relatively enriched in Fe and Mn. For the FEMP soil sample, the U-rich regions were often relatively enriched in Ca, Cu, and Zn, while for the SRS sediment these regions were highly enriched in Ni. For both of these samples, the population of discrete highly enriched U regions was far less prevalent (-25% of the observations) than the regions having diffusely distributed U with high Fe and Mn concentrations. Previous SEM/EDX studies on the FEMP soil sample also demonstrated both highly concentrated zones of U co-associated with only C and P and more diffuse regions enriched in U that were more highly associated with Al, Si, Mn, and Fe (20). It is likely that these highly enriched regions represent discrete Ucontaining phases either originally deposited as a component of the source term or subsequently precipitated as a secondary phase within the soil, while the zones of lower U concentration are likely representative of U either coprecipitated with or sorbed to the iron, aluminum, and manganese oxyhydroxide phases inherently present. The XANES spectra for various regions within the sand fraction from the FEMP soil sample provided evidence for zones of varying oxidation state. Two of the 22 regions examined had absorption edges at about 4.0 and 4.3 eV below the U(V1) standard and lacked the characteristic dominant high-energy, multiple scattering post-edge feature in the XANES spectra that is diagnostic of U(V1). These features resemble the XANES of the U(IV) standards, suggesting regions containing pure U(IV) species (Figure 3). Other regions examined had absorption edges and XANES features characteristic of predominantly U(VI) species (Figure 3), while yet other regions
I
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Figure 3. Uranium Llll XANES spectra of two distinct regions within mounts of the sand fraction of the Fernald soil sample. Arrows accentuate the differences in both edge position and the post-edge region associated with broad MSR.
Figure 4. Uranium Llll XANES spectra of two distinct regions within mounts of the sand fraction of a Savannah River Site sediment sample. Arrows accentuate the differences in the post-edge region associated with the broad MSR.
had absorption edges indicative of a mixture of U(1V) and U(V1) oxidation states. The latter case was only observed for XANES spectra generated with larger beam sizes, ca. >150 Fm. In fact when operating a t a beam size of -300 pm, we were unable to locate any regions of the pure or predominantly U(1V) phases, suggesting that as the beam size increased, more U(V1) was contributing to the weighted average of the XANES spectra. This is consistent with the previous SEM/EDX analyses of these samples, where the discrete U phases were found to exist as 10100-pm particles (20). Thus, it is entirely likely that the regions identified in the FEMP soil sample with intermediate-edge positions represent areas having finely divided discrete phases of U(1V) surrounded by a large number of U(V1)-containingparticles or surface-associated U(V1) phases. Another important observation was that, as the beam size increased for the sand fraction mounts, the signal to noise sometimes decreased. This is likely the result of decreasing the importance of the localized area containing high U concentrations and, therefore, diluting the U signal by increasing the fluorescence X-ray contribution from other elements in the bulk sample. This is important since the bulk U concentration of the FEMP sand fraction, for example, was reported to be 1070 pg g-1 (20), which is close to or a t the detection limit often reported for adsorbed metals on many conventional XAS beam lines and was the highest U concentration examined in this study. Because the U is localized within the sample, it is possible to generate high-quality XANES spectra utilizing the X-ray microprobe capability at bulk sample concentrations at least 2 orders of magnitude below this level within a reasonable collection time. The various regions examined within the sand fraction of the SRS sediment sample did not display any differences in the energy of the edge position, with the central-edge positions being consistent with hexavalent U species (Figure 4). There were, however, qualitative differences often observed in the post-edge structure arising from scattering events in the continuum, suggesting differences in U-bonding environments within the regions examined. These differences were particularly evident between regions associated with what is interpreted as discrete U-containing phases and regions where the U appears to be sorbed to oxyhydroxide phases (inferred from elemental
associations determined by the collection of full XRF spectra, vide supra). Studies are currently underway to generate XANES spectra for a wide range of U mineral phases and for U sorbed to and coprecipitated with oxyhydroxide phases of Fe, Mn, and A1 and sorbed to a number of aluminosilicate clay mineral phases. These data may facilitate a more detailed interpretation, albeit still qualitative, of differences observed in the 'fingerprint' region of the XANES spectra. Furthermore, we are currently developing the capability to conduct spatially resolved extended X-ray absorption fine structure (EXAFS or XAFS) spectroscopy. XAFS can potentially provide much more detailed information on U-bonding environments within soils or sediments with little or no sample manipulation, provided that the chemical heterogeneity of U within the spatial region of interest is not too great or does not vary at a scale significantly smaller than the collimated beam. There were some spatially resolved differences in the edge positions of the silt fraction from the FEMP soil, although the samples were predominantly U(V1) species. The lower average-edge positions for these samples (1.02.0 eV below the U(V1) standard) are likely the result of some U(1V) particles being dispersed among the predominant U(V1) species within the sample on a smaller scale than the beam size utilized. We have previously found evidence for spatial variability in U concentrations within this sample utilizing an 8-pm beam size while operating in the white light XRF mode, thus providing some support for variability on a scale smaller than our current analytical capabilities when operating in the XAS mode. We interpret the presence of tetravalent U in the silt and sand fractions of the FEMP soil sample as a result of U(1V) being present in the source term as airborne particulates (201, rather than via the reduction of U(V1) in the soil. Consistent with this view is the absence of appreciable quantities (i.e., >lo%) of tetravalent U in the clay fraction of the FEMP soil which, because of the much higher surface area, might be expected to contain appreciable quantities of U(1V) if biotic or abiotic reduction processes were operative. As with the clay and sand fractions from the SRS sediment sample, there was no evidence for any measurable quantity of tetravalent U in the silt fraction, and the sample tended to be spatially Environ. Sci. Technol., Vol. 28, No. 5, 1994 983
homogeneous, again indicating that proposed biotic or abiotic reduction processes of U are not operating in these sediments. Anticipated increases in both beam flux and spatial resolution at the Advanced Photon Source, a thirdgeneration synchrotron facility currently being constructed at Argonne National Laboratory, should greatly enhance capabilities to examine metal-contaminated soils and sediments by spatially resolved XAS and SXRF. The information generated from such analyses will be critical for the validation of geochemical speciation models and the development of chemical intervention technologies or management strategies for waste site remediation. Acknowledgments
This research was partially funded by Contract DEAC09-76SR00819between the University of Georgia and the U.S. Department of Energy (P.M.B. and D.B.H.), by Contracts DE-FG02-92ER14244 (University of Chicago) and DE-AC02-76CH00016(Brookhaven National Laboratory) from the US.DOE Office of Basic Energy Sciences, and by Research Grants NAG9-106 (S.R.S.) and NSF EAR89-15699 (M.L.R.) from NASA, by the University of Chicago CARS Startup Grant, and by the State of Illinois Technical Challenge Grant. We are indebted to Dr. S. Y. Lee of the Environmental Sciences Division, Oak Ridge National Laboratory, for providing the samples from the FEMP site. We also wish to thank Dr. C. Strojan and Dr. S. Clark for reviewing an earlier version of this manuscript and Dr. P. Powhat from the Smithsonian Institution for graciously providing a number of reference U mineral phases. We also thank the staff of the NSLS for providing the X-ray beam. Literature Cited (1) Tessier,A.;Campbell,P. G. C.InMetalSpeciation;Kramer, J., Ed.; Lewis Publishers: Boca Raton, FL, 1988; pp 183199. (2) Belzile, N.; Lecomte, P.; Tessier, A. Environ. Sei. Technol. 1989,23,1015. (3) Tipping, E.; Hetherington, N. B.; Hilton, J.; Thompson, D. W.; Bowles, E. Anal. Chem. 1985,7,1944. (4) Hayes, K. F.; Roe, A. L.; Brown, G. E.; Hodgson, D. 0.; Leckie, J. 0.;Parks, G. A. Science 1987,238,783. (5) Chisholm-Brause, C. J.; Brown, G. E.; Parks, G. A. Physica B 1989,158,646.
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Received for review July 27,1993.Revised manuscript received October 15, 1993. Accepted January 27, 1994.
