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Low and ambient temperature XAFS study of U(VI) in solids and aqueous solution
Illllml Physica B 208&209 (1995) 167 168
ELSEVIER
Low and ambient temperature XAFS study of U(VI) in solids and aqueous solution H.A. Thompson*, G.E. Brown Jr., G.A. Parks aDepartment of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305-2115, USA
Abstract To evaluate the ability of XAFS spectroscopy to elucidate the structural environment of U(VI) at the solid-water interface, we have carried out an in-depth analysis of XAFS data from U(VI) solid and solution model compounds. Using the ab initio XAFS code F E F F 6, we have calculated phase-shift (q~)and amplitude (A) functions for fitting low (10 K) and ambient temperature data. F E F F 6 does a good job of reproducing experimental data, and is particularly valuable for providing tk and A functions for backscattering atoms whose spectral contributions are difficult to isolate from experimental data due to overlap of Fourier transform (FT) features. In solid-phase model compounds at ambient temperature, we are able to fit single-scattering (SS) contributions from neighbors out to 5.2 ~,. Multiple-scattering (MS) was assessed and found not to be significant above k = 3 A - 1. The use of shell-dependent disorder parameters (a 2) is essential and values have been constrained.
1. Introduction We are interested in using XAFS spectroscopy to determine the coordination environment of U(VI) at solid-water interfaces of environmental importance. This requires that the sensitivity of the technique to atomic neighbors beyond the first coordination shell (axial and equatorial O) be determined, which has not been done previously. The use of XAFS spectroscopy for studying uranyl environments is complicated by inherently large static disorder in equatorial neighbors. This results in overlapping FT features which can preclude the use of empirical q~ and A parameters for fitting data. Theoretical XAFS codes such as F E F F 6 [1] are thus attractive for providing ~b and A functions for neighboring atoms, but have yet to be thoroughly evaluated for use on uranyl structures. Finally, because ab initio calculation of shelldependent disorder parameters (a 2) has yet to be demon* Corresponding author.
strated effective for oxides, we require a basis for constraining a 2 values in order to fit XAFS data for unknowns. In this study we use experimental XAFS data for uranyl reference compounds of known structure and composition to establish XAFS "detection limits", evaluate the performance of F E F F 6, and constrain cr2 values, all for the uranyl environment.
2. Experimental Solid model compounds used include uranyl (di)acetate, uranyl nitrate hexahydrate, rutherfordine (uranyl carbonate), rubidium uranyl nitrate, uranophane (uranyl silicate), meta-autunite and meta-ankoleite (uranyl phosphates). Solids were mixed with boron nitride (BN) in a proportion to yield 65% absorption of the incoming beam and loaded in 0.5mm thick aluminum sample holders between Mylar windows. The solution model compound was 50mM aqueous uranyl nitrate at
0921-4526./95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 9 2 1 - 4 5 2 6 ( 9 4 ) 0 0 6 6 0 - 1
168
H.A. Thompson et aL /Physica B 208&209 (1995) 16~168
pH < 1.0, loaded into a 1.5 mm thick Teflon solution cell with Mylar windows. U r a n i u m Lm-edge XAS spectra ( ~ 17-18 keV; edge inflection 17 166 eV) were collected at SSRL ( ~ 3 GeV and 40-90 mA) using beam lines IV-2 (ambient temperature) and 11-3 (low T = 10 K). Low temperatures were maintained using an Oxford liquid He cryostat. The X-ray beam was unfocused and Si(2 2 0) monochromator crystals (~b = 90 °) were used. Spectra were collected in transmission mode using Ar-filled ionization chambers. Harmonic rejection was effected by 10-80% detuning of the incident beam. Three to five scans were collected for each sample. A uranium oxide standard was mounted between two ionization chambers downstream from the sample to provide a continuous energy calibration reference. Data reduction and analysis were accomplished using the E X A F S P A K programs of G. George (SSRL) in conjunction with F E F F 6. Eo was fixed at 17 180eV.
3. Results and discussion At ambient temperature in the solids studied, detectable neighboring atoms include axial O (1.8oA), equatorial O (2.2 2.5 A), N (2.9 A), C (2.9~), Si (3.1 A), P (3.6 A), U (3.9-5.2 A), and distant O (4.2 A) atoms. Two distinct subshells of equatorial O atoms are discernible only when the difference in their respective distances from U (AR)t> r~/2Ak. Contributions from N, C, Si, P and distant O improve the fit of model compound spectra but would probably not be discerned if one did not know I
I
I
I
I
I
I
Table 1 Shell-dependent fit parameters for a range of uranyl structures Atom
N
R (,A)
or2 (,~2)
Oax O~q N C U U Odist
2 4 6 2 3 2 2 2 8
1.75 1 . 8 2 2.29-2.50 2.96 2.95 3.97 4.31 4.28
0.0011-0.0037 0.0030-0.0120 0.0068-0.0121 0.0036 0.0054 0.0055 0.0099
of their presence. Detection of distant O atoms is furthermore enabled only when their positions are reinforced by axial bonds to other U atoms. MS paths produce small contributions to spectra for some of the solid compounds (Fig. 1), but individual MS path contributions cannot be determined. MS analysis in uranyl systems of u n k n o w n geometry would therefore be exceedingly difficult. F E F F 6 does a good job of reproducing experimental data (Fig. 1). For analysis of unknowns, F E F F 4) and A functions produce fit results for N, R, and tr2 similar to those produced using empirical standards, when empirical standards are obtainable. On this basis we conclude that F E F F is particularly valuable for providing ff and A functions for neighbouring atoms whose spectral contributions are difficult to isolate from experimental data. Finally, we have determined ranges for shell-dependent a2 values in uranyl structures (Table 1). The value oftr 2 is dependent on the n u m b e r of atoms in a shell, the distance of those atoms from U, and the strength of the bonds to those atoms.
Acknowledgements This work was supported by the US D O E through a grant from Los Alamos National Laboratory ( # 9X42-6947E-1 to GEB and GAP) and a US National Science Foundation Graduate Research Fellowship (HAT). We thank the staff of SSRL for support during data collection. SSRL is supported by the US D O E and NIH. 4a
,
~
,
~
,
i
k (1~_])
,
,i.
I ,
1;
'
,.
References Fig. 1. Low temperature uranophane [Ca(UO2)2 (SiO3OH)2-5H20 ] spectra (--). (a) FEFF spectrum (..-) consisting of 36 SS and 37 MS paths. (a) demonstrates the ability of FEFF to model most EXAFS features, even with a global a 2 (0.003). (b) six SS-shell data fit (---) with shell-dependent a 2. Arrows denote a feature that is better fit when MS paths are included.
[1] J.J. Rehr, J. Mustre de Leon, S.I. Zabinsky and R.C. Albers, J. Am. Chem. Soc. 113 (1991) 5135.
ELSEVIER
Low and ambient temperature XAFS study of U(VI) in solids and aqueous solution H.A. Thompson*, G.E. Brown Jr., G.A. Parks aDepartment of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305-2115, USA
Abstract To evaluate the ability of XAFS spectroscopy to elucidate the structural environment of U(VI) at the solid-water interface, we have carried out an in-depth analysis of XAFS data from U(VI) solid and solution model compounds. Using the ab initio XAFS code F E F F 6, we have calculated phase-shift (q~)and amplitude (A) functions for fitting low (10 K) and ambient temperature data. F E F F 6 does a good job of reproducing experimental data, and is particularly valuable for providing tk and A functions for backscattering atoms whose spectral contributions are difficult to isolate from experimental data due to overlap of Fourier transform (FT) features. In solid-phase model compounds at ambient temperature, we are able to fit single-scattering (SS) contributions from neighbors out to 5.2 ~,. Multiple-scattering (MS) was assessed and found not to be significant above k = 3 A - 1. The use of shell-dependent disorder parameters (a 2) is essential and values have been constrained.
1. Introduction We are interested in using XAFS spectroscopy to determine the coordination environment of U(VI) at solid-water interfaces of environmental importance. This requires that the sensitivity of the technique to atomic neighbors beyond the first coordination shell (axial and equatorial O) be determined, which has not been done previously. The use of XAFS spectroscopy for studying uranyl environments is complicated by inherently large static disorder in equatorial neighbors. This results in overlapping FT features which can preclude the use of empirical q~ and A parameters for fitting data. Theoretical XAFS codes such as F E F F 6 [1] are thus attractive for providing ~b and A functions for neighboring atoms, but have yet to be thoroughly evaluated for use on uranyl structures. Finally, because ab initio calculation of shelldependent disorder parameters (a 2) has yet to be demon* Corresponding author.
strated effective for oxides, we require a basis for constraining a 2 values in order to fit XAFS data for unknowns. In this study we use experimental XAFS data for uranyl reference compounds of known structure and composition to establish XAFS "detection limits", evaluate the performance of F E F F 6, and constrain cr2 values, all for the uranyl environment.
2. Experimental Solid model compounds used include uranyl (di)acetate, uranyl nitrate hexahydrate, rutherfordine (uranyl carbonate), rubidium uranyl nitrate, uranophane (uranyl silicate), meta-autunite and meta-ankoleite (uranyl phosphates). Solids were mixed with boron nitride (BN) in a proportion to yield 65% absorption of the incoming beam and loaded in 0.5mm thick aluminum sample holders between Mylar windows. The solution model compound was 50mM aqueous uranyl nitrate at
0921-4526./95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 9 2 1 - 4 5 2 6 ( 9 4 ) 0 0 6 6 0 - 1
168
H.A. Thompson et aL /Physica B 208&209 (1995) 16~168
pH < 1.0, loaded into a 1.5 mm thick Teflon solution cell with Mylar windows. U r a n i u m Lm-edge XAS spectra ( ~ 17-18 keV; edge inflection 17 166 eV) were collected at SSRL ( ~ 3 GeV and 40-90 mA) using beam lines IV-2 (ambient temperature) and 11-3 (low T = 10 K). Low temperatures were maintained using an Oxford liquid He cryostat. The X-ray beam was unfocused and Si(2 2 0) monochromator crystals (~b = 90 °) were used. Spectra were collected in transmission mode using Ar-filled ionization chambers. Harmonic rejection was effected by 10-80% detuning of the incident beam. Three to five scans were collected for each sample. A uranium oxide standard was mounted between two ionization chambers downstream from the sample to provide a continuous energy calibration reference. Data reduction and analysis were accomplished using the E X A F S P A K programs of G. George (SSRL) in conjunction with F E F F 6. Eo was fixed at 17 180eV.
3. Results and discussion At ambient temperature in the solids studied, detectable neighboring atoms include axial O (1.8oA), equatorial O (2.2 2.5 A), N (2.9 A), C (2.9~), Si (3.1 A), P (3.6 A), U (3.9-5.2 A), and distant O (4.2 A) atoms. Two distinct subshells of equatorial O atoms are discernible only when the difference in their respective distances from U (AR)t> r~/2Ak. Contributions from N, C, Si, P and distant O improve the fit of model compound spectra but would probably not be discerned if one did not know I
I
I
I
I
I
I
Table 1 Shell-dependent fit parameters for a range of uranyl structures Atom
N
R (,A)
or2 (,~2)
Oax O~q N C U U Odist
2 4 6 2 3 2 2 2 8
1.75 1 . 8 2 2.29-2.50 2.96 2.95 3.97 4.31 4.28
0.0011-0.0037 0.0030-0.0120 0.0068-0.0121 0.0036 0.0054 0.0055 0.0099
of their presence. Detection of distant O atoms is furthermore enabled only when their positions are reinforced by axial bonds to other U atoms. MS paths produce small contributions to spectra for some of the solid compounds (Fig. 1), but individual MS path contributions cannot be determined. MS analysis in uranyl systems of u n k n o w n geometry would therefore be exceedingly difficult. F E F F 6 does a good job of reproducing experimental data (Fig. 1). For analysis of unknowns, F E F F 4) and A functions produce fit results for N, R, and tr2 similar to those produced using empirical standards, when empirical standards are obtainable. On this basis we conclude that F E F F is particularly valuable for providing ff and A functions for neighbouring atoms whose spectral contributions are difficult to isolate from experimental data. Finally, we have determined ranges for shell-dependent a2 values in uranyl structures (Table 1). The value oftr 2 is dependent on the n u m b e r of atoms in a shell, the distance of those atoms from U, and the strength of the bonds to those atoms.
Acknowledgements This work was supported by the US D O E through a grant from Los Alamos National Laboratory ( # 9X42-6947E-1 to GEB and GAP) and a US National Science Foundation Graduate Research Fellowship (HAT). We thank the staff of SSRL for support during data collection. SSRL is supported by the US D O E and NIH. 4a
,
~
,
~
,
i
k (1~_])
,
,i.
I ,
1;
'
,.
References Fig. 1. Low temperature uranophane [Ca(UO2)2 (SiO3OH)2-5H20 ] spectra (--). (a) FEFF spectrum (..-) consisting of 36 SS and 37 MS paths. (a) demonstrates the ability of FEFF to model most EXAFS features, even with a global a 2 (0.003). (b) six SS-shell data fit (---) with shell-dependent a 2. Arrows denote a feature that is better fit when MS paths are included.
[1] J.J. Rehr, J. Mustre de Leon, S.I. Zabinsky and R.C. Albers, J. Am. Chem. Soc. 113 (1991) 5135.