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XAS of trace element coordination in natural sediments at ambient and cryogenic temperatures
Physica B 208&209 (1995) 309-310
ELSEVIER
XAS of trace element coordination in natural sediments at ambient and cryogenic temperatures P.A. O'Day a'*, S.A. Carroll b, G.A. Waychunas c, B. Phillips b aDepartment of Geology, Box 871404, Arizona State University, Tempe, AZ 85287, USA bL-219 Earth Sciences Division, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA c Center for Materials Research, Stanford University, Stanford, CA 94305, USA
Abstract The molecular coordination of trace levels of Zn (1-2 wt%) in natural sediments from an acid mine drainage and of V, Cr, and Mn (10-300 ppm) from weathered hydrothermal soils was examined by synchrotron X-ray absorption spectroscopy (XAS) at ambient and cryogenic ( ~ 10 K) temperatures. XAFS analyses of mine drainage sample spectra show that, with increased weathering, Zn coordination changes from a sulfide phase in its host mineral, sphalerite (ZnS), to coordination indicative of hydroxide phases. In samples with silica, carbonate, and iron-hydroxide phases (3-54 wt% Fe(OH)3), the local atomic Zn environment changes with the total amount of bulk iron hydroxide, but Zn is not incorporated into carbonate or silica phases. Analysis of pre-edge and K-edge spectral features of very low concentrations of V, Cr, and Mn in oxidized, kaolinite-rich soils from hydrothermal areas indicates reduced valence states for these ions (V 3 ÷'4+, Cr 3 +, Mn 2 +) and suggests trapping of metals under paleo-reducing conditions during clay formation.
1. Introduction
2. Experimental
Hazard assessment and remediation of toxic metal contamination in natural soils and sediments is often difficult because little is known about how metals are incorporated into solid phases. Metals may be mobilized or retarded depending upon their mode of incorporation in the solid. The application of XAS to determining local metal ion bonding in natural soils and sediments is complicated by the presence of multiple solid phases, amorphous and disordered phases, and multiple metal ion absorbers. In this study, we examine the feasibility of using XAFS and XANES to obtain structural information about the local atomic coordination of trace metals in untreated, complex natural sediments.
Zinc-bearing sediments (1-2 wt% Zn) were collected from the US Tri-state mining district. Clay-rich soils containing 10-300 ppm V, Cr, and Mn were collected from hydrothermal vent areas in central Iceland. Absorption spectra for dried, undiluted solid soils and sediments were collected at SSRL on beam line 11-3 at cryogenic temperature ( ~ 10 K) and on beam line IV-I at ambient temperature (3 GeV, 40-90mA). Zinc K-edge and EXAFS spectra (9400-10700 eV; k ~ 3 to 12-15 A-~) were collected at low T using an unfocused Si(220) crystal and a Stern-Heald-type fluorescence detector. Vanadium (5465 eV), Cr (5990 eV), and Mn (6540 eV) fluorescence absorption edges were collected at ambient temperature using an unfocused Si(1 1 1) crystal. Both transmission (using N2- and Ar-filled ion chambers) and fluorescence absorption spectra were collected for solid
* Corresponding author.
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 8 3 - 0
310
P.A. O'Day et al./Physica B 208&209 (1995) 309 310
model compounds (diluted with inert B(OH)3 ). Harmonic rejection was achieved by detuning the incoming beam by 30-50% of maximum intensity. Data reduction and analysis were done with the EXAFSPAK programs (G. George, SSRL). For Zn XAFS analysis, reference phase-shift and amplitude functions were generated using F E F F 6 [1] based on atomic clusters of known crystal structures (Zn Eo = 9670eV). F E F F 6 was used for XANES analysis of V, Cr, and Mn edges and reference compounds.
Analysis of the XANES spectra of V-, Cr- and Mnbearing hydrothermal clays indicates that these metals are present in reduced oxidation states (V 3 ÷ : ÷ , Cr 3 +, Mn 2 +). Analysis of XAFS spectra was not possible due to the very low concentration of these metals and interference in the XAFS region from K and Ln.m absorption edges of other trace elements. Comparison of the soil sample spectra to that of reference oxide compounds shows no large pre-edge features that would indicate oxidized cations and edge features are similar to those observed in oxide compounds with reduced cations [2,31.
3. Results Analyses of XAFS spectra of Zn unknown sediment samples, precipitated Zn-hydroxides, and known model compounds (Fig. 1) show that Zn does not form pure, known carbonate or hydroxide phases in the acid mine samples, nor does it appear to sorb to quartz surfaces. In sediments from the mine tailings, Zn is present primarily in sphalerite (ZnS) as indicated by S and Zn backscatterers at 2.32 and 3.83/~,, respectively. With increasing distance from the tailings, Z n - O backscattering (N = 4, R = 1.97-2.00/~) is apparent in the spectra in addition to Zn-S. Second-shell backscattering from Fe and/or Zn at distances shorter than Z n - Z n backscattering in ZnS is also observed. The position and intensity of spectral features attributed to second-neighbor Zn and Fe change with bulk Fe(OH)3 concentration. There is no evidence for Zn coordination in carbonate phases or association with Si phases.
--
711Hl~O(exp
4. Conclusion X-ray absorption spectroscopy can provide valuable information on the local atomic coordination of environmentally sensitive trace elements in natural soils and sediments at low concentrations. In Zn-bearing mine drainage sediments, Zn liberated by weathering of ZnS is incorporated primarily into amorphous hydroxide phases rather than carbonate and silica phases. Zn coordination is sensitive to the bulk Fe(OH)3 concentration in the sediment. In clay-rich hydrothermal soils, reduced metals are apparently trapped in clays during their formation under reducing conditions and strongly retained despite subsequent weathering under highly oxidizing conditions. We find that accurate determination of metal coordination in multicomponent soils and sediments from XAFS requires analysis of a variety of known experimental reference compounds as well as quantitative fitting with theoretical reference functions from FEFF. Data collection at low T ( ~ 10 K) greatly enhanced spectral resolution, especially for backscattering atoms beyond the first coordination shell.
1~4~wxp
Acknowledgements -
20I
u
"H MB30~c~o
,
MB06lexn
U
~1/'~ II/:
r /"J
~", II'~I '~ sphalertte(fell)
We thank J.J. Rehr for access to FEFF6. PAO acknowledges support from the US National Science Foundation postdoctoral fellowship program. This work was done at SSRL which is operated by the US Department of Energy, Office of Basic Energy Sciences.
References Fig. 1. Normalized EXAFS spectra of Zn-bearing sediments (MB06, MB30, TC43) compared to spectra of precipitated Zn-hydrous-ferric oxide (Zn-HFO) and sphalerite (ZnS) model compound (theoretical).
[1] J.J. Rehr et al., Phys. Rev. Lett. 69 (1992) 3397. [2] J. Wong et al., Phys. Rev. B 30 (1984) 5596. [3] J. Garcia et al., J. de Phys. 47 C8 (1986) 49.
ELSEVIER
XAS of trace element coordination in natural sediments at ambient and cryogenic temperatures P.A. O'Day a'*, S.A. Carroll b, G.A. Waychunas c, B. Phillips b aDepartment of Geology, Box 871404, Arizona State University, Tempe, AZ 85287, USA bL-219 Earth Sciences Division, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA c Center for Materials Research, Stanford University, Stanford, CA 94305, USA
Abstract The molecular coordination of trace levels of Zn (1-2 wt%) in natural sediments from an acid mine drainage and of V, Cr, and Mn (10-300 ppm) from weathered hydrothermal soils was examined by synchrotron X-ray absorption spectroscopy (XAS) at ambient and cryogenic ( ~ 10 K) temperatures. XAFS analyses of mine drainage sample spectra show that, with increased weathering, Zn coordination changes from a sulfide phase in its host mineral, sphalerite (ZnS), to coordination indicative of hydroxide phases. In samples with silica, carbonate, and iron-hydroxide phases (3-54 wt% Fe(OH)3), the local atomic Zn environment changes with the total amount of bulk iron hydroxide, but Zn is not incorporated into carbonate or silica phases. Analysis of pre-edge and K-edge spectral features of very low concentrations of V, Cr, and Mn in oxidized, kaolinite-rich soils from hydrothermal areas indicates reduced valence states for these ions (V 3 ÷'4+, Cr 3 +, Mn 2 +) and suggests trapping of metals under paleo-reducing conditions during clay formation.
1. Introduction
2. Experimental
Hazard assessment and remediation of toxic metal contamination in natural soils and sediments is often difficult because little is known about how metals are incorporated into solid phases. Metals may be mobilized or retarded depending upon their mode of incorporation in the solid. The application of XAS to determining local metal ion bonding in natural soils and sediments is complicated by the presence of multiple solid phases, amorphous and disordered phases, and multiple metal ion absorbers. In this study, we examine the feasibility of using XAFS and XANES to obtain structural information about the local atomic coordination of trace metals in untreated, complex natural sediments.
Zinc-bearing sediments (1-2 wt% Zn) were collected from the US Tri-state mining district. Clay-rich soils containing 10-300 ppm V, Cr, and Mn were collected from hydrothermal vent areas in central Iceland. Absorption spectra for dried, undiluted solid soils and sediments were collected at SSRL on beam line 11-3 at cryogenic temperature ( ~ 10 K) and on beam line IV-I at ambient temperature (3 GeV, 40-90mA). Zinc K-edge and EXAFS spectra (9400-10700 eV; k ~ 3 to 12-15 A-~) were collected at low T using an unfocused Si(220) crystal and a Stern-Heald-type fluorescence detector. Vanadium (5465 eV), Cr (5990 eV), and Mn (6540 eV) fluorescence absorption edges were collected at ambient temperature using an unfocused Si(1 1 1) crystal. Both transmission (using N2- and Ar-filled ion chambers) and fluorescence absorption spectra were collected for solid
* Corresponding author.
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 8 3 - 0
310
P.A. O'Day et al./Physica B 208&209 (1995) 309 310
model compounds (diluted with inert B(OH)3 ). Harmonic rejection was achieved by detuning the incoming beam by 30-50% of maximum intensity. Data reduction and analysis were done with the EXAFSPAK programs (G. George, SSRL). For Zn XAFS analysis, reference phase-shift and amplitude functions were generated using F E F F 6 [1] based on atomic clusters of known crystal structures (Zn Eo = 9670eV). F E F F 6 was used for XANES analysis of V, Cr, and Mn edges and reference compounds.
Analysis of the XANES spectra of V-, Cr- and Mnbearing hydrothermal clays indicates that these metals are present in reduced oxidation states (V 3 ÷ : ÷ , Cr 3 +, Mn 2 +). Analysis of XAFS spectra was not possible due to the very low concentration of these metals and interference in the XAFS region from K and Ln.m absorption edges of other trace elements. Comparison of the soil sample spectra to that of reference oxide compounds shows no large pre-edge features that would indicate oxidized cations and edge features are similar to those observed in oxide compounds with reduced cations [2,31.
3. Results Analyses of XAFS spectra of Zn unknown sediment samples, precipitated Zn-hydroxides, and known model compounds (Fig. 1) show that Zn does not form pure, known carbonate or hydroxide phases in the acid mine samples, nor does it appear to sorb to quartz surfaces. In sediments from the mine tailings, Zn is present primarily in sphalerite (ZnS) as indicated by S and Zn backscatterers at 2.32 and 3.83/~,, respectively. With increasing distance from the tailings, Z n - O backscattering (N = 4, R = 1.97-2.00/~) is apparent in the spectra in addition to Zn-S. Second-shell backscattering from Fe and/or Zn at distances shorter than Z n - Z n backscattering in ZnS is also observed. The position and intensity of spectral features attributed to second-neighbor Zn and Fe change with bulk Fe(OH)3 concentration. There is no evidence for Zn coordination in carbonate phases or association with Si phases.
--
711Hl~O(exp
4. Conclusion X-ray absorption spectroscopy can provide valuable information on the local atomic coordination of environmentally sensitive trace elements in natural soils and sediments at low concentrations. In Zn-bearing mine drainage sediments, Zn liberated by weathering of ZnS is incorporated primarily into amorphous hydroxide phases rather than carbonate and silica phases. Zn coordination is sensitive to the bulk Fe(OH)3 concentration in the sediment. In clay-rich hydrothermal soils, reduced metals are apparently trapped in clays during their formation under reducing conditions and strongly retained despite subsequent weathering under highly oxidizing conditions. We find that accurate determination of metal coordination in multicomponent soils and sediments from XAFS requires analysis of a variety of known experimental reference compounds as well as quantitative fitting with theoretical reference functions from FEFF. Data collection at low T ( ~ 10 K) greatly enhanced spectral resolution, especially for backscattering atoms beyond the first coordination shell.
1~4~wxp
Acknowledgements -
20I
u
"H MB30~c~o
,
MB06lexn
U
~1/'~ II/:
r /"J
~", II'~I '~ sphalertte(fell)
We thank J.J. Rehr for access to FEFF6. PAO acknowledges support from the US National Science Foundation postdoctoral fellowship program. This work was done at SSRL which is operated by the US Department of Energy, Office of Basic Energy Sciences.
References Fig. 1. Normalized EXAFS spectra of Zn-bearing sediments (MB06, MB30, TC43) compared to spectra of precipitated Zn-hydrous-ferric oxide (Zn-HFO) and sphalerite (ZnS) model compound (theoretical).
[1] J.J. Rehr et al., Phys. Rev. Lett. 69 (1992) 3397. [2] J. Wong et al., Phys. Rev. B 30 (1984) 5596. [3] J. Garcia et al., J. de Phys. 47 C8 (1986) 49.