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Hickey, R. F.; Vanderweilen, J.; Switzenbaum, M. S. Biotechnol. Lett. 1987, 9(1), 63-66. Tholozan, J. L.; Samain, E.; Grivet, J. P. FEMS Microbiol. E d . 1988,53, 187-191. Zinder, S. H.; Cardwell, S. C.; Anguish, T.; Lee, M.; Koch, M. Appl. Environ. Microbiol. 1984, 47, 796-807. Kugelman, I. J.; McCarty, P. L. J.-Water Pollut. Control Fed. 1965,37,97-116. DeBaere, L. A.; Devocht, M.; VanAssche, P. Water Res. 1984,18, 543-548. Barker, H. A. Bacterial Fermentations; Wiley: New York, 1956. Sparling, R.; Daniels, L. J . Bacteriol. 1986,168,1402-1407.
(32) Hungate, R. E. Arch. Mikrobiol. 1967, 59, 158-164. (33) Boone, D. R.; Johnson, R. L.; Liu, Y. J . Appl. Environ. Microbiol. 1989, 55, 1735-1741. (34) Thiele, J. H.; Zeikus, J. G. Appl. Enuiron. Microbiol. 1988, 54, 20-29. (35) Zeikus, J. G.; Kerby, R.; Krzycki, J. A. Science 1985,227, 1167-1173.
Received for review March 5,1991. Revised manuscript received July 29, 1991. Accepted August 19, 1991. This research was supported by the National Science Foundation (Grant ECE841650).
EXAFS Spectroscopic Study of Neptunium(V) Sorption at the a-FeOOHIWater Interface Jean-Marie Combes,+ Catherine J. Chisholm-Brause,' Gordon E. Brown, Jr., * and George A. Parks
Aqueous and Surface Geochemistry Group, School of Earth Sciences, Stanford University, Stanford, California 94305-2 1 15 Steven D. Conradson, P. Gary Eller, Ines R. Trlay, David E. Hobart, and Arend Meljer
Isotope and Nuclear Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545
rn The sorption of aqueous Np(V) on crystalline a-FeOOH (goethite) has been studied at the molecular level by synchrotron-basedX-ray absorption spectroscopy. Sodium nitrate solutions (pH 7.2, ionic strength 0.08 M) having a M were reacted with Np(V) concentration of 1.3 X high surface area goethite (50 m2/g),resulting in -95% uptake of the Np and a sorption density of mol of Np/g of goethite. Np LIII-edgeX-ray absorption spectra were collected in the fluorescence-yieldmode on wet pastes after removal of -90% of the solution phase by centrifuging. Comparison of spectra from the sorption sample with Np LIIr spectra from crystalline Np02, a 1.3 mM Np(V) solution, and U(V1) LIII spectra from crystalline uranyl nitrate and triacetate and U(V1) sorbed onto ferric hydroxide gel shows that Np(V) sorbs as isolated neptunyl (Np02+)groups and is not multinuclear under the conditions of our experiment. Least-squares modeling of the EXAFS spectrum of the Np/goethite sorption sample shows two oxygen shells around Np consisting of two oxygens at 1.85 f 0.02 A and about five oxygens at 2.51 f 0.03 A, consistent with a distorted pentagonal-bipyramid complex. The only significant difference between Fourier transforms of the EXAFS spectra of the Np/goethite sorption sample and the Np solution sample is a weak feature at -3 A in the former. We have tentatively identified this feature as a Np-Fe second-neighbor pair correlation, which suggests that Np(V) sorbs at the goethitelwater interface under these solution conditions as an inner-sphere complex. Our results rule out sorption as a simple, ordered neptunium oxide or hydroxide or as a coprecipitate with an iron oxide-hydroxide phase. Introduction The toxicity, potential mobility, and half-life of 237Np render it one of the most important radionuclides to be stored in high-level nuclear waste repositories such as the one proposed at Yucca Mountain, NV ( I ) . Its transport properties in local groundwaters and its partitioning onto +Presentaddress: St. Gobain Recherche, 39, quai Lefranc, 93304 Aubervilliers, France. 2 Present address: Isotope and Nuclear Chemistry Division, Los Alamos National Laboratory, Los Alamos, NM 87545. 376
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minerals associated with such repositories must be understood at a fundamental level in order to reliably predict its hydrogeochemical behavior from the source repository, where the Np concentration is likely to be high, to the far field, where the Np concentration should approach very low background levels. As for other elements, partitioning of Np onto the solids in soils, sediments, and aquifers can occur by simple precipitation or coprecipitation, formation and coagulation of colloids, or sorption, i.e., adsorption, absorption, or surface precipitation ( I d ) . If adsorption is involved, the surface complex produced may be bound weakly, retaining its hydration sheath, or strongly bonded directly to the surface (6). Hysteresis in sorption/desorption cycles suggests that several modes of sorption are involved in partitioning in some systems (6). Distinction among alternative modes of partitioning is important for several reasons. Immobilization of contaminants requires strong, effectively irreversible sorption; thus, we must understand the sorption complex well enough to anticipate its stability with respect to changes in contaminant concentration, pH, salinity, the presence and concentrations of complexing ligands, and oxidation potential. If changes in any of these variables are encountered in either space or time at a breached repository or in the resulting contamination plume, we can expect order of magnitude.changes in partition coefficients. Kent et al. (7), for example, demonstrated that changes in pH and total carbonate can lead to changes of several orders of magnitude in the Kd for U(V1) sorbing on goethite (aFeOOH). Changes in total contaminant concentration can be shown to alter K i s by orders of magnitude if sorption complexes are multinuclear. Changes in & imply changes in retardation factors, thus in the rate and extent of dispersion (2). Ignorance of the composition and structure of sorption complexes precludes anticipation of many of these changes and thus increases the risk of seriously erroneous predictions about the fate of contaminants. Np02+and Np020H(aq) are the dominant species in oxidizing aqueous environments under neutral and acidic conditions (8,9); carbonato complexes may be important if dissolved carbonate is high. Np(V) sorbs onto a variety of synthetic and natural minerals with decreasing affinity in the sequence (10)
0013-936X/92/0926-0376$03.00/0
0 1992 American Chemical Society
calcite
> goethite >> MnOz = clays
Sorption is not, apparently, the simple ionic association of NpOz+with the surface. A study by Keeney-Kennicut and Morse (10) showed that desorption from goethite upon dilution was incomplete, suggesting that at least some of the Np is bonded more strongly than expected for simple monovalent ions. Their experiments with calcite provided evidence for slow precipitation of an unidentified Np compound at Np concentrations higher than -8 pM. Sorption processes may also involve changes in oxidation state (11,12). For example, Np is less mobile in anoxic environments than in oxic environments (13, 1 4 ) , suggesting that reduction produces species that are sorbed more strongly or precipitated. Iron and manganese oxides are present as coatings on some of the mineral and rock fragments comprising the tuff deposits at Yucca Mountain (15,16)and are relatively ubiquitous phases in the near-surface environment. They provide high area, reactive surfaces on which sorption of actinides from groundwaters is likely. In order to distinguish among possible sorption mechanisms of Np on such surfaces, a molecular-level description of the type of sorption complexes formed is required (17). Moreover, because such sorption reactions occur at solid/water interfaces, structural characterization should be done in situ, i.e., with the aqueous phase in contact with the oxide surface. Drying the sorption sample following reaction with a Np-containing aqueous solution introduces the possibility of altering or destroying the sorption complex, particularly when it is weakly bound to the oxide surface. Synchrotron-based X-ray absorption spectroscopy (XAS) is one of the few structure-sensitiveprobes that can provide direct molecular-level description, including average local structure and the identity of nearest neighbors, of metal sorption complexes in situ (18-20). Using current synchrotron radiation sources, XAS has the sensitivity necessary to observe and characterize Np sorption complexes at concentrations expected under near-field conditions; however, more intense X-ray sources will be required to characterize sorbed Np in far-field and uncontaminated environments.
X - r a y Absorption Spectroscopy The theory and experimental details of extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) spectroscopies are summarized in ref 21. Briefly, the important points of EXAFS analysis can be summarized as follows: For neptunium, the X-ray absorption coefficient increases sharply at 17600 eV, which corresponds to the Np LIIIabsorption edge and results from the absorption of X-ray energy and the transition of a neptunium 2p3/, electron to higher energy valence and continuum states. Fine structure above the Lm edge results from the interference between outgoing photoelectron waves (from electrons ejected to continuum states) and backscattered waves from neighboring atoms. When EXAFS data are converted to momentum space k, the fine structure is directly related to the sum of sinusoidal oscillations for each shell of neighboring atoms. The frequency of each oscillation depends on interatomic distance and is given by [2kRN,x + ‘ ~ ( k ) ~ , where ~ ] , RNpX is the bond length between neptunium and neighboring is the phase function specific to atom X and ‘P(k)Np-X neptunium and atom X (because the frequency is inversely related to the bond length, the EXAFS contribution from an iron atom in the surface of goethite at twice the distance of the first shell of oxygens around sorbed neptunium will have a frequency approximately twice that of the EXAFS
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contribution of the first shell). The amplitude of an EXAFS oscillation depends on the number and type of near-neighbor atoms and is given by a function which is unique for each atom pair, e.g., Np-0, Np-Np, or Np-Fe. This interdependence allows identification of neighboring atoms around an absorber like Np. After the p(k) values have been determined from “model”compounds of known structure, the average bond length and coordination number for a given coordination shell of Np can be determined by a least-squares fit of a theoretically calculated curve to the experimental EXAFS data. The measure of the goodness of fit is given by the minimization function value F, defined as F = ( C [ k 2 ( x , b- ~ ~ , ) ~ ] /where n ) ~ Xobs ’ ~ and , xdC are the observed and calculated E M S , respectively, and n is the number of data points. XAS has recently been used to study the local environment of Np in valence states 111, IV, and V in borosilicate glasses used to confine the high-level radioactive waste of fission products (22). XAS methods have also been used to identify ions coordinating adions and to measure distances between adions and surface or coordinating ions, in order to distinguish among alternative modes of sorption ( 1 4 2 0 ) . For example, inner- and outer-sphere complexes can be distinguished by the presence of cations from the solid surface (S) among secondneighbor atoms around the adion (A). The numbers and distances of second-neighbor S atoms depend on the attachment geometry of the sorbate (Le., should help to distinguish between monodentate and multidentate complexes). The presence of one or more second-neighbor A atoms suggests multinuclear sorption complexes. The presence of large numbers of A atoms among the second neighbors of an A adion suggests a precipitate, whereas large numbers of second-neighbor S atoms suggests a coprecipitate or diffusion (absorption) of the adion into the sorbent. Hayes et al. (19) completed the first in situ EXAFS study of selenate and selenite complexes chemisorbed on a-Fe00H (goethite) in 1987. Analysis of the Se K-edge EXAFS data showed that selenite forms a bidentate, inner-sphere complex whereas selenate forms an outer-sphere complex on this surface. These spectroscopic results provided a molecular interpretation for the strong binding of selenite and the relatively weak binding of selenate at the a-FeOOH/water interface observed in macroscopic solution experiments. Since then, we have successfully used EXAFS and XANES spectroscopies on very dilute, wet samples of Pb(I1) on 7-Alz03(23)and a-FeOOH (24), Co(I1) on yAlzO3, TiOz (rutile), Al2Si2O5(0H),(kaolinite), a-Si02(quartz) (25-27), and Co(I1) on CaC03 (calcite) (28). These studies have shown that Co(I1) and Pb(I1) adsorb to form inner-sphere surface complexes. The sorption complexes are isolated and mononuclear at adsorption densities corresponding to a small fraction of a monolayer coverage and include multinuclear complexes at higher loadings. XAS methods require sorbate concentrations (e.g., millimolar) higher than expected in most natural environments, but they do help to identify the types of complexes most likely to yield erroneous extrapolations to natural conditions, and they can be used to calibrate less direct methods such as photoacoustic absorption spectroscopy that can be used at lower concentrations. We are not aware of any definitive spectroscopic work that has characterized Np sorption sites or complexes at the molecular level. Using the iron oxide-hydroxide, goethite, as a generic, first-approximation model for both the crystalline and amorphous iron oxide-hydroxide phases in potential nuEnviron. Sci. Technol., Vol. 26, No. 2, 1992
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clear waste repository sites, we have carried out the first in situ, synchrotron-based XAS study of the local structural environment of aqueous Np(V) sorbed onto aFeOOH (goethite). Our primary aim was structural characterization of the Np(V)/goethite surface complex, Experimental Section Goethite was synthesized using the method described by Atkinson et al. (29), washed with doubly deionized water, freeze-dried, and stored dry. Its identity and crystallinity were confirmed by X-ray diffraction. The surface area of the goethite was measured using the BET method and was found to be 50 m2/g. A stock Np(V) solution was prepared using the following steps: (1) ultrapure Np metal was dissolved in "OB, (2) NpO20H. xH20 was precipitated using NaOH of high purity, (3) the precipitate was washed thoroughly with ultrapure water (Le., H 2 0with a resistivity of >16 MO cm), (4) the washed Np020H.xH20was dissolved with HC104,and (5) an aliquot of the perchloric Np02+solution was diluted to the desired concentration with ultrapure H20. The resulting Np(V) stock solution had a pH of 2 and a Np concentration of 1.3 mM with a high oxidation-state purity (as determined by spectroscopic assay). Sorption sample preparation procedures must meet several requirements in order to permit useful X-ray absorption spectra to be collected (30): (1) the sorbed Np concentration in the sample volume must be >1 mM to obtain usable signal-to-noiseratios with currently available detectors and wiggler-magnet synchrotron radiation beam lines, (2) the concentration of Np remaining in solution and the volume of solution retained in the XAS sample without altering solution composition or drying must both be minimized to reduce the contribution of dissolved Np to the XAS spectrum, and (3) precipitation of Np-containing solids from solution must be avoided. Based on preliminary adsorption measurements, the initial Np(V) concentration, pH, and solidfliquid ratio were chosen to maximize the fraction of Np removed from solution by sorption while still meeting these requirements. Np sorption samples were prepared in air by equilibrating the Np stock solution with 1 g of goethitefl of solution in the following way. Aliquots of a NaN03 solution, goethite, the Np stock solution (1.3 X M), and a NaOH solution were mixed using a magnetic stirrer to produce a solution with an initial Np(V) concentration of 1.3 X M, an equilibrium solution pH of 7, and an ionic strength of 0.08 M. After equilibration, the measured pH M (dewas 7.2, the final Np concentration was 7 X termined by y-ray spectrometry), and the Np uptake was -95%, corresponding to a sorption density of mol of Npfg of goethite, sufficient to give measurable X-ray absorption. About 90% of the solution was removed by centrifuging. The resulting paste was placed in aluminum E M S cells with 2.5 X 25 mm sample compartments, and the cells were sealed with Mylar windows, yielding a sample thickness of -2.5 mm. An aliquot of the Np stock solution was mounted in a nylonfgold cell with an optical path length of 2.3 cm. Np LnI-edgeX-ray absorption spectra were collected at the Stanford Synchrotron Radiation Laboratory (SSRL) on wiggler beam line 7-3 using Si(220) monochromator double crystals. Higher energy harmonic reflections were rejected by detuning the primary beam by 50%. During data collection, the storage ring was operated at 3.3 GeV and at a beam current of 10-30 mA. Spectra of the aqueous Np solution and the sorption samples were collected in fluorescence mode using a Stern-Heald-type detector (31) and a SrC03filter between the sample and
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I-T-l-7
-30 -20 -10 0
10 20 30 40 50 60 70
Energy (eV)
Flgure 1. Np-LIII edge or XANES spectra in which the edge jumps (height from zero absorbance to average background level through EXAFS spectrum) have been normalized to 1: (a) crystalline NpO,; (b) 1.3 mM Np(V) stock solution (pH 7.2; ionic strength 0.08 M) (Ilght line); (c) Np(V) sorbed onto a-FeOOH (goethite) (heavy iine); (d) U(V1) sorbed onto ferric oxide gels [Figure I d after Combes (37)].The arrow points to the multipkcattering resonance which is characteristlc of the local geometry of neptunyl or uranyl groups. The energy scale of these spectra has been defined by setting the energy of the first inflection polnt of the near edge to 0 eV.
the Ar-filled ionization chamber. The spectrum of crystalline NpO, was collected in transmission mode using appropriate Ar/N2 gas mixtures in the incident and transmitted beam ionization chambers; this material was used as a model compound for determining empirical phase shift and amplitude parameters for the Np-0 and Np-Np absorber-backscatterer pairs. A well-characterized neptunium(V) hydroxide would have been a better model but was not available; the necessary scattering parameters for the Np-O and Np-Np ion pairs can be derived from NpOz in any case. EXAFS spectra were recorded in 1-eV increments from 17 350 to 17 450 eV and in 0.05-A-' increments at higher energies to 18 750 eV. Absorption-edge spectra from a Zr metal foil (K-edge energy 17998 eV) placed in front of a third h-filled ionization chamber were collected during each data scan to provide an energy calibration. The energy resolution for these experimental conditions is estimated to be 3-4 eV. The signal-to-noise ratio in all spectra was increased by averaging several (three to five) raw scans. Results and Discussion XANES Spectra. The overall shapes of the Np-LIII X-ray absorption near-edge structure spectra of the Np sorption and aqueous solution samples are significantly different from that of crystalline Np02, which has the fluorite structure type (Figure la-c). However, they are quite similar to those observed for U(V1) in uranyl nitrate and triacetate compounds and for U(V1) sorbed onto poorly ordered ferric oxides (32) (Figure Id) in terms of the intensities and positions of the main absorption edge at -+lo eV and the broad shoulder at 10-15 eV above the edge. A study of Lm edges of various uranium compounds (33) and a polarized XAS study of uranyl nitrate (34) show clearly that the broad resonance (shown by the arrow in Figure Id) located 10-15 eV above the main edge feature
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