Langmuir 1996,11,2041-2048
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X-ray Absorption Spectroscopic Studies of Cadmium and Selenite Adsorption on Aluminum Oxides Charalambos Papelis*>? Environmental Engineering and Science, Department of Civil Engineering, Stanford University, Stanford, California 94305-4020
Gordon E. Brown, Jr., and George A. Parks Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115
James 0.Leckie Environmental Engineering and Science, Department of Civil Engineering, Stanford University, Stanford, California 94305-4020 Received August 25, 1994. I n Final Form: March 10, 1995@ To enhance our understanding of trace element partitioning at oxide-water interfaces, we studied the local coordination environment of cadmium(I1)and selenite complexessorbed on aluminum oxides,in situ, using X-ray absorption spectroscopy (XAS).The adsorbents included porous, high surface area transition aluminas (ALCOACP-5and (2-33)and the minerals corundum(a-Al203)andgibbsite (y-N(OH)s). Cadmium sorption densities ranged from 1.2 to 12.0 pmol/m2and selenite sorption densities ranged from 1.2 to 4.6 pmollm2. X-ray absorption fine structure (XAFS) analysis of the selenite spectra indicates that Se is coordinated to three 0 atoms at 1.69A, regardless of substrate, and that the complexes are mononuclear. XAFS analysis of low sorption density cadmium complexes suggests that cadmium is coordinated to six oxygensat 2.33A and that the complexesare mononuclear. Analysis of ahigh total cadmium concentration M) gave 0 first neighbors at 2.35 A and Cd second neighbors at 3.84 A, suggesting the sample formation of a disordered cadmium hydroxide or cadmium hydroxocarbonate precipitate. Absence of Al second neighbors in the selenite and the low sorption density cadmium samples is probably caused by the low backscattering amplitude of Al and thermal and static disorder effects. These results, in combination with sorption isotherm data, suggest that, under the conditions studied, cadmium and selenite diffuse into the pores of the transition aluminas and sorb as mononuclear complexes. These results have significant implications for the fate of trace elements in subsurface environments and the remediation of waters and groundwaters.
Introduction Macroscopic investigations alone cannot be used to identify the sorption mechanism of ion partitioning a t the mineral-water interface.l Spectroscopic techniques are needed to probe the local environments of sorbed complexes and to provide information about the structure and bonding of ions a t the mineral-water interface. X-ray absorption spectroscopy (XAS) has been used to study the local environment in a variety of earth materials including crystalline solids, glasses, melts, and cations and oxyanions sorbed at mineral-water interfaces. X-ray absorption fine structure (XAFS),a type of XAS sensitive to the local coordination environment of the absorbing element, can yield estimates of interatomic distances between a n X-ray absorbing central atom and its nearest neighbors and estimates of the average number and type of these neighbors. Introductions to XAFS were published by Brown2 and T ~ O .Brown ~ , ~ and c o - w o r k e r ~ published ~~~-~ reviews ofXAS applications in the study of earth materials.
* To whom all correspondence should be addressed. + Present address: Desert Research Institute, Water Resources Center, University of Nevada System, P.O. Box 19040, Las Vegas, Nevada 89132-0040. Abstract published in Advance ACS Abstracts, May 15, 1995. (1) Sposito, G. In Geochemical Processes a t Mineral Surfaces; Davis, J. A., Hayes, K. F., Eds.;ACS Symposium Series 323;AmericanChemical Society: Washington, D.C., 1986; pp 217-228. (2)Brown, G. E., Jr.; Calas, G.; Waychunas, G. A.; Petiau, J . In Spectroscopic Methods in Mineralogy and Geology; Hawthorne, F. C., Ed.; Reviews in Mineralogy 18; Mineralogical Society of America: Washington, D.C., 1988; pp 431-512. @
Structural information obtained from XAFS (distances and coordination numbers between a central atom and its nearest neighbors) can be used to propose a sorption mechanism for ion partitioning at the mineral-water interface. For example, on the basis of the closeness of the approach of a n ion to the surface, we can distinguish between inner- and outer-sphere coordination comp l e x e ~ In . ~ addition, ~~ XAFS can yield information about the type of surface complex formed, which often depends on the relative and absolute concentration of adsorbate and adsorbent. At low surface coverages mononuclear complexes may be formed which are being replaced by polynuclear complexes at higher surface c o ~ e r a g e s . l ~ - ~ ~ (3)Teo, B. K.; Joy, D. C. EXAFS Spectroscopy, Techniques and Applications; Plenum Press: New York, 1981; p 275. (4) Teo, B. K. EXAFS: Basic Principles and Data Analysis; Springer-Verlag: New York, 1986; p 349. ( 5 ) Brown, G. E., Jr. In Mineral-Water Interface Geochemistry; M. F. Hochella, M. F., Jr., White, A. F., Eds.; Reviews in Mineralogy 23; Mineralogical Society of America: Washington, D. C . , 1990; pp 309363. (6) Bassett, W. A.; Brown, G. E., Jr. Annu. Rev. Earth Planet. Sci. 1990, 18, 387-447. (7) Brown, G. E., Jr.; Parks, G. A. Reu. Geophys. 1989,27,519-533. (8)Hayes, K. F.; Roe, A. L.; Brown, G. E., Jr., Hodgson, K. 0.;Leckie, J. 0.; Parks, G. A. Science 1987,238, 783. (9) Brown, G. E., Jr.; Parks, G. A.; Chisholm-Brause, C. J . Chimia 1989,43,248-256. (10) O'Day, P. A,;Brown, G. E., Jr.; Parks, G. A. InX-ray Absorption pp Fine Structure: Hasnain.. S. S.,. Ed.:. Honvood: New York, 1991;. .260262. (11)Chisholm-Brause, C. J.; Brown, G. E., Jr.; Parks, G. A. InX-ray Absorption Fine Structure; Hasnain, S. S., Ed.; Honvood: New York, 1991; pp 263-265.
0743-746319512411-2041$09.00/0 0 1995 American Chemical Society
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Adsorption of trace elements on oxide surfaces can be used to treat industrial wastewaters and contaminated groundwaters. From an engineering perspective, large porous oxide particles with high internal surface area could be appropriate for treating large volumes of wastewater, but the rate of trace element removal could be limited by mass transfer. To increase our understanding of contaminant partitioning a t the mineral-water interface in large, porous oxide particles, we collected cadmium and selenite XAFS data using porous transition aluminas (ALCOA CP-5 and C-33). An introduction to the formation, structure, and properties oftransition aluminas was given by Wefers and Misra16and summarized b~Pape1is.l~ In addition to the transition aluminas, XAFS data were collected using synthetic, nonporous gibbsite and corundum to evaluate the effect of substrate and porosity on surface complex formation. It was hoped that XAFS studies using transition aluminas would provide information complementary to X-ray photoelectron spectroscopic(XPS)studies and would allow us to distinguish adsorption from surface precipitation. From XPS studies average surface coverages of the adsorbents could be obtained.l7Js Current XPS spectrometers, however, do not have the spatial resolution required for detection of clusters and surface precipitates, nor can they produce elemental maps, especially for materials as complex as the transition aluminas studied. On the basis of a combination of XPS studies17J8and the X A S studies reported here, we propose that intraparticle diffusion followed by adsorption is the predominant sorption mechanism in large porous transition aluminas under the conditions studied.
Experimental Section Materials and Methods. For the cadmium XAS studies, only transition aluminas (CP-5 and C-33) were used (specific surface areas of 200 and 110 mVg, respectively; the complete physicochemical characterization of these adsorbents was presented e1sewherel7J9). The experimental protocol for sorption experiments was described in detail e1~ewhere.l~ The solid (0.43 and 0.77 g/L for CP-5 and C-33, respectively) was equilibrated in a 500-mL reactor in 0.01 M sodium nitrate before adsorbate addition. Two total cadmium concentrations were used, 1.0 x and 1.0 x M. Cadmium and background electrolyte solutions were prepared from ACS reagent-grade cadmium nitrate and sodium nitrate salts. After adsorbate addition, the solution was gradually titrated, in air, from the starting pH (5) to pH 9 to maximize cadmium uptake. Sorption densities ranged from 1.2 to 12 pM/m2. During the course of the titration, the formation of a white precipitate was observed in the high cadmium concentration sample (1.0 x M) above pH 7.5. The specific experimental conditions are listed in Table 1. The samples were equilibrated for at least 24 h at pH 9; the solid was then allowed to settle, and most ofthe supernatant was discarded. The solid was then resuspended, transferred to 35-mL centrifuge tubes, and centrifuged at 3000 rpm for 15 min. The supernatant was removed, and the remaining wet paste was transferred to Teflon sample holders with Mylar windows. (12) Roe, A.L.; Hayes, K. F.; Chisholm-Brause,C. J.; Brown, G. E., Jr.; Parks, G. A.; Hodgson, K. 0.;Leckie,J. 0.Langmuir 1991,7,367373. (13) Chisholm-Brause,C. J.; O’Day, P. A,; Brown, G. E., Jr.; Parks, G. A. Nature 1990,348,528-531. (14)O’Day, P. A.;Brown, G. E., Jr.; Parks, G. A. J.Colloid Interface Sci. 1994,165, 269-289. (15) O’Day, P. A.; Parks, G . A.; Brown, G. E., Jr. Clays Clay Miner.
1994,42,337-355. (16)Wefers, K.;Misra, C. 0xidesandHydroxidesofAluminum;Alcoa Technical Paper 19, Revised; Aluminum Company of America: Pittsburgh, PA,1987. (17) Papelis, C. Ph.D. Dissertation, Stanford University, Stanford, CA, 1992. (18)Papelis, C. Environ. Sci. Technol., in press. (19)Papelis, C.; Leckie, J. 0. To be submitted for publication in Colloids Surf.
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Table 1. Summary of Experimental Conditions for Cadmium XAFS Samples solid CP-5 c-33
concn (M) 1.0x 10-4 1.0 10-4 1.0 10-3
uptake (%)
pH 9.0 9.1 100 (ioo)ppt= 9.0 100
surface coverage (% monolayer) pWm2 10.0 1.2 10.1 1.2 100 12.0
A precipitate was observed during titration.
Table 2. S u m m a r y of Experimental Conditions for Selenite XAFS Samples surface coverage concn (M) uptake (%) pH (% monolayer) pWm2 1.0 10-4 99 5.2 10.0 1.2 1.0 10-3 39 5.5 39.1 4.6 (3-33 1.0 10-4 90 5.2 9.1 1.1 19 5.5 19.6 2.2 1.0 10-3 a-Al203 1.0 x 93 5.1 9.4 1.1 1.0 x 26 5.0 26.2 3.1 y-Al(OH)s 1.0 x 96 5.2 9.8 1.1 1.0 x 26 5.1 26.0 3.1
solid CP-5
In addition to the sorption samples, two types of liquid samples were also loaded into Teflon sample holders with Mylar windows: a 0.1 M Cd(N03)~solution to compare the spectra from sorbed and dissolved cadmium and supernatants from sorption samples (taken from the centrifuge tubes after centrifugation) to verify that dissolved cadmium did not contribute to the spectrum of sorption samples. Synthetic otavite (CdC03)obtained from CERAC Inc. (purity 99.9%) was used as a reference crystalline compound for the cadmium bonding environment. Otavite has the calcite structureZ0in which cadmium is coordinated to six first-neighbpr oxygens at 2.29 A and six second-neighbor cadmiums at 3.93 A. The identity and crystallinity of the synthetic otavite sample were confirmed by X-ray diffraction using a Rigaku difiactometer. For XAFS analysis, the crystalline sample was ground and mixed with boron nitride to produce a sample with ~ 3 0 % absorption of the incoming X-ray beam. The diluted otavite sample was loaded on a 0.5 mm thick aluminum sample holder and sealed with Mylar tape. Other possible reference compounds for the cadmium coordination environment includeB-Cd(OH)zand y-Cd(OH)z. Otavite was chosen for the following reasons. First, otavite is substantially less soluble than cadmium hydroxidezl and a likely precipitate when carbon dioxide is present in aqueous solutions, as was the case in this study. Second, the structure of y-Cd(OH12 is highly distorted, as will be discussed in a later section, and therefore not a very good choice for a reference. In addition, otavite and greenockite (P-CdS)are the most common cadmium minerals in natural environments. For the selenite XAS studies, synthetic corundum (0.3-pm micropolish a-AlzO3 obtained from Buehler Ltd., Lake Bluff, IL) and syntheticgibbsite (y-Al(OH)sobtained from ALCOA, ALCOA Parc, PA), as well as the CP-5 and C-33 transition aluminas, were used as sorbents. The specific surface areas of corundum and gibbsite were 15 and 11mVg, respectively. Corundum and gibbsite solid concentrations of 5.7 and 7.7 g/L, respectively, resulted in the same total adsorbent surface area used with transition aluminas. Selenite solutions were prepared from ACS reagent-grade sodium selenite. Sorption samples with two selenite concentrations (1.0 x and 1.0 x M) were prepared in 250-mL (corundum and gibbsite) or 500-mL (CP-5 and (2-33) reactors. Sample preparation procedures were the same as for cadmium sorption samples, except that after adsorbate addition (at pH 9) the solutions were gradually titrated to pH 5 to maximize selenite uptake. Sorption densities ranged from 1.1to 4.6 pWm2. The specific experimental conditions are listed in Table 2. (20) Borodin, V. L.; Lyutin, V. I.; Ilyukhin, V. V.; Belov, N. V. Sou. Phys.-Dokl. 1979,24, 226-227. (21) Baes, C. F.,Jr.; Mesmer, R. E. The Hydrolysis of Cations; Krieger: Malabar, FL, 1986; p 489.
Cadmium and Selenite Adsorption on Aluminum Oxides Sodium selenite and sodium selenate were used as crystalline reference compounds. In sodium selenate the selenium atom is coordinated to four oxygens at 1.65 A.zz In sodium selenite the selenium atom is coordinated to three oxygens at 1.69 A.B The identity of both model compounds was confirmed by X-ray diffraction using a Rigaku powder X-ray diffractometer. XAS Experimental Strategy. In preparing samples for XAS examination, it is important to use high surface area sorbents, to achieve optimum uptake of the sorbate under study on the sorbent and to minimize contributions from ions in solution to the signal being analyzed. Contributions from ions in solution will be negligible if sorbate fractional uptake is at least 90% and at least 90% of the supernatant is removed. Under these conditions XAFS spectroscopy will yield information about the coordination environment of sorbed species. Low concentration of the species of interest in the path of the incident beam may yield a prohibitively low signal-to-noise ratio for data analysis. Factors affecting the lowest analyzable concentration are the energy ofthe absorption edge studied, the monochromatorcrystal used, the synchrotron X-ray flux available, and the type ofX-ray detection method used, among others. In general, fluorescence yield detection methods permit XAS studies of lower sorbate concentrations than transmission or electron detection methods. For example, sorbed selenite XAS analysis will yield a much higher signal-to-noise ratio than cadmium XAS analysis using the Si(220) monochromator because at the cadmium reference energy (26 711 eV) only approximately 25% of the total beam flux remains. Data Collection. X-ray absorption spectra were collected at the Cd K edge (26 711 eV) and the Se K edge (12 658 eV). All data were collected at the Stanford Synchrotron Radiation Laboratory (SSRL) using wiggler beam lines nos. 4-2 and 4-3 with Si(220) monochromators. The beam energy was 3.0 GeV, The wiggler and the beam current ranged from 20 to 90 d. field was 18kG. Upstream slits before the monochromator were set at 1 mm vertically and 20 mm horizontally. Spectra were collected in transmission mode for the solid reference compounds and in fluorescence mode for sorption samples and aqueous solutions using a Stern-Heald type detect0r.2~The sample holder was oriented at 45" to the incident beam for fluorescence measurements and at 90" for transmission measurements. Selenite and cadmium spectra were collected by detuning the monochromator by 35 and 5%, respectively, at the highest scan energy to minimize contributions from higher order harmonics in the incident beam. For each sample, multiple scans were averaged to increase the signal-to-noise ratio. Selenite sorption samples required only 2 or 3 scans; cadmium sorption samples required 5-6 scans to yield usable spectra. Data Analysis. A summary of the data reduction procedure will be given here. More detailed descriptions were given by Brown2 and T ~ o . The ~ , ~raw spectra were checked for discontinuities because of step losses in the incident beam, calibration, other "glitches", or excessive noise. Acceptable spectra were averaged to increase the signal-to-noise ratio. Background was subtracted by fitting a straight line in the pre-edge region and a cubic, three segment spline in the extended X-ray absorption fine structure (EXAFS) region. The background-subtracted spectra were normalized using tabulated McMaster atomic absorption fall-off coefficient^^^ followed by conversion from energy to frequency space using the photoelectron wave vector 12 and weighting by k3 to account for decreasing amplitude of the EXAFS oscillations with increasing K . The backgroundsubtracted, normalized, /$-weighted spectra were Fourier transformed using an unsmoothed window to obtain radial structure functions (rsf), which contain pair correlations between the central absorbing atom and neighboring atoms. Each peak in the rsf corresponds to a shell of backscatterers at a unique distance, uncorrected for phase shift, from the absorber. Individual peaks in the Fourier transform were back-transformed to (22) Mehrotra, B. N. Ph.D. Dissertation, Technische Hochschule Aachen, Aachen, Germany, 1973. (23) Lytle, F. W.; Greegor, R. B.; Sandstrom, D. R.; Marques, E. C.; Wong, J.; Spiro, C. L.; Huffman, G. P.; et al. Nucl. Instrum. Methods 1984,226, 542-548. (24) McMaster, W. H.; Del Grande, N. K.; Mallett,J. H.; Hubbell, J. H. Compilation of x-ray cross-sections III; UCRL-50174; U. S. Atomic Energy Commission: University of California, Livermore, 1969.
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k (A') Figure 1. Background-subtracted, K3-weightedEXAFS spectra for cadmium sorbed on transition aluminas (CP-5 and C-33), a 0.1M aqueous cadmium nitrate solution, and synthetic otavite. produce filtered E M S and to isolate specificcontributions from individual shells in the EXAFS. The number and identity of the backscattering atoms and the distances between absorber and neighboring atoms (corrected for phase shift) were determined by fitting the filtered EXAFS (with nonlinear, least-squares curvefitting techniques) using phase and amplitude parameters derived from crystalline reference compounds with known structure or from the0ry.~5
Results and Discussion (A) Cadmium Complexes Sorbed on Aluminum Oxides. The background-subtracted, normalized, k3weighted E M S spectra for the otavite reference compound, three sorption samples, and a cadmium nitrate solution are shown in Figure 1. Two frequencies are clearly visible in the otavite spectrum. The second frequency, superimposed on the main frequency, produces a "beat pattern" phich can be observed at approximately 6.2,7.9, and 9.7 A-I. The two frequencies in the EXAFS spectrum (frequency space) correspond to two distances of two distinct shells (in real space) surrounding the cadmium atom. The Fourier transform of the otavite spectrum is shown in Figure 2. A ycond-shell peak is clearly visible at approximately 3.6 A (not corrected for phase shift). Data analysis shows that the first peak correspgnds to six oxygens surrounding the cadmium atom at 2.29 A. The identity of the second peak was determined by back-transforming this peak only, followed by fitting the back-transformed EXAFS using theoretical McKale phase and amplitude parameter^.^^ Ths best fit was obtained assuming six cadmiums a t 3.93 A (distance and coordination number obtained from otavite structure refinement). Assuming any other atom or combination of atoms gave worse fits. This peak was then used to derive phase and amplitude parameters for the Cd-Cd pair. The EXAFS spectrum of the low sorption density sample of Cd sorbed on CP-5 (r= 1.2pM m-2) is shown in Figure 1, and the Fourier transform is shown in Figure 2. No beat pattern was visible in this spectrum. Only the peak corresponding to the oxygen first neighbors was present. The Cd-0 distance (2.33 f 0.02 A) and coordination number (5.1 f 1.0 oxygens) were determined by fitting the back-transformed first-shell peak using phase and (25)McKale,A. G.; Veal, B. W.; Paulikas, A. P.; Chan, S. K.; Knapp, G. S. J . A m . Chem. SOC.1988,110,3763-3768.
2044 Langmuir, Vol. 11, No. 6, 1995
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R + A (A) Figure 2. Radial structure functions for cadmium sorbed on transition aluminas (CP-5and C-33),a 0.1 M aqueouscadmium nitrate solution, and synthetic otavite.
amplitude parameters derived from the otavite model compound. The results are listedin Table 3. These results are in good agreement with analysis of a 0.1 M cadmium nitrate solution. The EXAFS spectrum and Fourier transform of the solution sample are shown in Figures 1 and 2, respectively. The Cd-0 distance (2.32 f 0.02 A) and coordination number (7.3 f 1.0 oxygens) are indistinguishable from the Cd-0 distance and coordination number in the sorption sample within the accuracy of the technique ( f 0 . 0 2 A in distance, f 2 0 % in coordination number). These results suggest that the geometry of the cadmium ion does not change upon sorption; cadmium is octahedrally coordinated to atotal of six oxygens, including a few oxygens on the oxide surface and water molecules or hydroxide ions in the aqueous phase. Similar results were obtained from analysis of a sorption sample of cadmium on C-33(sorption density, 1.2pM m-2). The EXAFS spectrum and Fourier transform are shown in Figures 1 and 2, respectively. Analysis of the Cd-0 peak in the Fourier transform gave the same distance as in the CP-5 sorption sample (2.33A) and a slightly lower coordination number (4.7). As in the case of the CP-5 cadmium sorption sample, no second-shell peak could be observed in the Fourier transform. Absence of secondshell cadmium neighbors in these samples suggests that the sorbed complexes are probably mononuclear; the formation of a cadmium surface precipitate can probably be ruled out because in both cadmium hydroxide and cadmium carbonate the Cd-Cd distance is short enough to allow detection of the second shell (compare the second shell in the otavite model compound). Absence of a secondshell peak in the Fourier transform if cadmium atoms were located a t distances shorter than 4-5 A could only be explained by the formation of a highly disordered phase. The formation of mononuclear cadmium complexes on aluminas is consistent with recent studies of cadmium sorption complexes on goethite and ferrihydrite.26 These authors reported absence of multinuclear cadmium complexes for up to essentially monolayer coverage of goethite and fenihydrite, as long a s cadmium solubility was not exceeded. They attributed this behavior of cadmium to its low ability to hydrolyze and to the relatively high solubility of cadmium hydroxide.26 (26)Spadini, L.;Manceau,A.;Schindler, P. W.;Charlet, L. J . Colloid Interface Sci. 1994,168, 73-86.
There are three possible reasons why there is no evidence of aluminum second neighbors to cadmium ions from our XAS analysis. First, the distance between the central absorbing atom and the aluminum atoomon the surface could be too large (greater than 4-6 A) for the aluminum atoms to be detected by XAFS. A large Cd-Al distance would result if the sorbates maintained their hydration sheaths upon adsorption, thus forming the surface equivalent of aqueous, outer-sphere complexes. Inner- or outer-sphere surface complex formation has been correlated to the ionic strength dependence ofion sorption on mineral surface^.^^-^^ Sorption of ions forming weak outer-sphere complexes can be influenced dramatically by the concentration of the background electrolyte, whereas the sorption behavior of strongly binding ions, forming inner-sphere complexes, is essentially unaffected by background electrolyte concentration. The correlation between ionic strength dependence data and inner- or outer-sphere complexformation was consistent with XAFS data in the case of selenite and selenate sorption on goethite.s Absence of metal second neighbors was attributed to outer-sphere complex formation in the case of selenate adsorption on goethite. Selenite, however, was found to form inner-sphere complexes on goethite, and the Se-Fe distance was found to be 3.38 A, too short to accommodate the intervening water of a n outer-sphere complex.8 The sorption of 1.0 x M cadmium on 0.43 g/L CP-5 in three background electrolyte concentrations (0.1,0.01, and 0.001 M sodium nitrate) is shown in Figure 3. Sorption is essentially ionic strength independent. Our ionic strength dependence data, therefore, suggest that cadmium ions form inner-sphere complexes on transition aluminas. Our interpretation of cadmium sorption data is consistent with the recently reported formation of cadmium inner-sphere complexes on goethite and fernhydrite.26 A second possible reason that aluminum second neighbors were not detected (even if inner-sphere surface complexes were formed) is the low backscattering amplitude of low atomic number element^.^,^,^^ Aluminum (2 = 13) backscatters more weakly than iron (2 = 26); thus aluminum second neighbors are more difficult to detect than iron second neighbors. This may explain why iron second nearest neighbors could easily be detected on goethite and fenihydriteZ6but not on aluminas. The third possible reason why aluminum second neighbors were not detected, involves static and thermal disorder effects.2 Such disorder is simulated in XAFS analysis by the Debye-Waller factor, u2. Static disorder (interatomic distances varying over a certain range) will damp EXAFS oscillations by removing amplitude so that information about second and more distant shells may be lost. The defect spinel structure of the transition aluminas, with aluminum atoms randomly distributed in octahedral and tetrahedral sites, may contribute to a n increased static disorder factor, thereby decreasing the amplitude of Cd-A1 shells. Thermal (vibrational)disorder effects can be minimized by lowering the temperature. Experiments a t lower temperature (
