Environ. Sci. Technol. 2002, 36, 4633-4641
Thermal Stabilization of Chromium(VI) in Kaolin Y U - L I N G W E I , * ,† S H U - Y U A N C H I U , † HSIEN-NENG TSAI,† YAW-WEN YANG,‡ AND JYH-FU LEE‡ Department of Environmental Science, Tunghai University, Taichung City, 407 Taiwan, and Synchrotron Radiation Research Center, Hsinchu, 300 Taiwan
Reduction of Cr(VI) by heating may be a useful detoxification mechanism for thermal immobilization. Using X-ray absorption spectroscopy, the change of speciation of chromium in 105 °C dried 3.7% Cr(VI)-sorbed kaolin further heated at 500, 900, or 1100 °C was studied. The 105 °C dried 3.7% Cr(VI)-sorbed kaolin sample was prepared by mixing 1.5 L of 0.257 M CrO3 solution (pH 0.71) with 0.5 kg of kaolin powder for 48 h, and then the slurry was heated (dried) at 105 °C until a constant weight was reached. The toxicity characteristic leaching procedure method was used to determine the percentage of leached chromium from all heated samples. In all 500-900 °C heated Cr(VI)-sorbed kaolin samples, Cr2O3 transformed from the hydrated Cr(VI) by a 4-h heat application was identified by the X-ray absorption near edge structure and extended X-ray absorption fine structure (EXAFS) spectroscopy as the key species that is leaching-resistant due to its low solubility. For the 1100 °C heated Cr(VI)sorbed kaolin sample, the Fourier transform of its EXAFS spectrum indicates that the intensity of the peaks at 2.45 (Cr-Cr shell of Cr2O3) and 5.00 Å (Cr-Cr and Cr-O shells of Cr2O3) without phase shift correction is either relatively smaller or disappearing, compared with that of the 500-900 °C heated Cr(VI)-sorbed kaolin samples. It is suggested that chromium octahedra were bridged to silica tetrahedra and incorporated in minerals formed at 1100 °C, such as mullite or sillimanite, since these phases were detected by XRD. Cr of this form is not easily leached.
Introduction The two oxidation states of chromium, Cr(III) and Cr(VI), have extremely different chemical and environmental properties. The determination of chromium oxidation state and speciation in various matrixes is critical because Cr(VI) is very mobile and hazardous to human health through inhalation, skin contact, and ingestion, yet Cr(III) exists as a less mobile precipitate in the natural environment and is an important micronutrient (1-3). Chromium(VI) compounds have been considered as one of the 17 chemicals associated with the greatest threat to human health by the U.S. Environmental Protection Agency (USEPA) (4). Heavy metal contaminants in soils or wastes can generally be remedied with four technologies: (i) extraction with suitable chemical reagents, (ii) concentration to the vicinity * Corresponding author. Phone: (+) 886 4 359 1368. Fax: (+) 886 4 359 6858. E-mail:
[email protected]. † Tunghai University. ‡ Synchrotron Radiation Research Center. 10.1021/es0114761 CCC: $22.00 Published on Web 09/20/2002
2002 American Chemical Society
of an in situ electrode with an applied voltage, (iii) solidification with cement- or polymer-based materials, or (iv) immobilization with a high-temperature slag process. These remediation technologies have been developed to reduce either the content or the mobility of heavy metals in soils or wastes. Our previous publications have shown that a major portion of both lead and cadmium compounds can be effectively immobilized in soil and in some inorganic sorbents through the application of low-temperature heating at 200700 °C (5-7). The leaching percentage of Pb2+ and Cd2+ was considerably reduced after thermal treatment. This suggests a potentially cost-effective remedial technology because the heat is readily available, simply from a heat exchanger across the flue gas duct or boiler steam of an industrial boiler/ furnace system in most industries (5-7). Volatility and extractability of common radionuclide contaminants from the heated hardened portland cement was studied (8). Aluminosilicates and silica sand, which are quite abundant in the earth’s upper crust (9), have proven able to capture heavy metals such as Cr, Pb, Cd, and Ni at an elevated temperature (10-14). Reduction of the mobility of cadmium or lead compounds using solid sorbents such as SiO2 and kaolin at high temperatures (>700 °C) proves to be effective under flue gas conditions (11, 12). The vapor of CdO or PbO reacts with various sorbent flakes at a temperature greater than 700 °C to form MO‚Al2O3, MO‚2SiO2, and MO‚Al2O3‚ 2SiO2 (M represents Pb or Cd). These reaction products are identified by X-ray diffraction (XRD) techniques (11, 12), and they are more acid-resistant (11, 12). However, although the XRD technique is capable of identifying crystalline compounds of sufficient concentration, it hardly works for the case of measuring amorphous, tiny crystalline, or low-concentration crystalline compounds. In addition, the XRD patterns characteristic of various compositions in a measured sample are usually poorly resolved if the measured sample has complex matrixes. This makes the identification of target compounds difficult. In contrast, the X-ray absorption spectroscopy (XAS) technique can often be used as a tool for the speciation of target element without the shortcomings of the XRD technique. Information on the heat-stabilized metals at the molecular level, to elucidate the mechanism of heavy metal capture, has so far been very limited. This information is helpful in understanding the mechanism of thermal stabilization of heavy metals, and it can be used to assess whether the captured heavy metals can be stabilized for the long term. Amorphous iron (hydr)oxides containing heavy metals were treated at 50, 600, and 900 °C to simulate their transformation caused by heat treatment before disposal or aging at a disposal site (15). XRD, SEM, and XAS techniques were employed in their study, and they concluded that heat treatment at 600 and 900 °C significantly reduced the capturing capacity for heavy metals (15). A recent report has addressed the importance of the role played by molecular environmental science that can provide information needed for a long-term solution of environmental remediation and waste management (16). To understand the mechanism of thermal treatment of chromium-containing soil and waste sludge generated by plating and tannery industries, it is important to determine the change of oxidation state and speciation of chromium induced by thermal treatment. While many environmental scientists are facing a challenge in developing methods for contaminant speciation, the XAS technique can afford direct, nondestructive measurement of oxidation state and speciation of chromium compounds. A K-edge XAS spectrum of VOL. 36, NO. 21, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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transition elements includes preedge, X-ray absorption nearedge structure (XANES) and extended X-ray absorption fine structure (EXAFS) regions. The preedge, XANES and EXAFS measurement for a specific element in a sample are carried out in the same experiment. Using Cr as an example, XANES can reveal Cr electronic properties, such as oxidation state, effective charge density, and coordination symmetry. Cr EXAFS provides local geometric structure including coordination number, interatomic distance, degree of disorder of outer-shell atoms, etc. The Cr(VI) preedge spectrum is characterized by a strong absorption peak due to the transition of Cr 1s core electrons to its 3d orbital. This transition is allowed due to the lack of a center of inversion symmetry in the CrO42- tetrahedral structure (17, 18). The preedge region of Cr(III), which is dramatically different from that of Cr(VI), is characterized by two small-intensity peaks. The normalized area of the preedge peak of chromium has been determined to be quantitatively proportional to the ratio of Cr(VI) to total Cr (18). Because kaolin is an important raw material for manufacturing industries of ceramics/firebrick/construction brick and because kaolin was previously found to be effective in stabilizing heavy metals, it might be of great practical application to the following: (i) use kaolin to sorb Cr(VI) from the wastewater stream of the plating industry and then detoxify the Cr(VI) by heating the Cr(VI)-sorbed kaolin; (ii) mix kaolin with Cr(VI)-containing industrial sludge, followed by thermal treatment to the mixture to detoxify and stabilize chromium. Note that ceramics and firebricks can contain chromium as an important constituent. Thus, the objective of this research is to investigate the previously unrevealed mechanism of thermal stabilization of chromium in kaolin by acquiring information with usage of XAS and other instrumental techniques.
Materials and Methods All the chemicals used here were of reagent grade, and they were purchased from Riedel-de Hean except the glacial acetic acid that was obtained from Merck. The kaolin (AKIMA 35, Associated Kaolin Industries) is in crystalline form. The 3.7% (i.e., calculated on a dry basis) Cr(VI)-sorbed kaolin slurry was prepared by mixing 1.5 L of 0.257 M CrO3 solution (solution pH 0.71) with 0.5 kg of kaolin powder in a mixing bottle (Associated Design) that rotated end to end at a speed of 30 rpm for 48 h. Deionized water with resistivity of 18 MΩ/cm was used to prepare all the aqueous solutions. Asprepared slurry mixture was then heated and dried at 105 °C until it reached a constant weight. To immobilize the chromium, the 105 °C dried sample was thermally treated under stagnant air in an electrically heated oven at 500, 900, or 1100 °C for 4 h, respectively. These samples are termed as 3.7% Cr-105 °C, 3.7% Cr-500 °C, 3.7% Cr-900 °C, and 3.7% Cr-1100 °C, accordingly. For comparison, “neat” CrO3 reference compounds i.e., without kaolin support, were thermally treated in the same manner and the resultant products are denoted correspondingly as CrO3-105 °C, CrO3-500 °C, CrO3900 °C, and CrO3-1100 °C. XRD was used to assess the crystallite phase of aluminosilicates. BET surface area was measured via the N2 sorption method. A high-resolution scanning electron microscope/ energy-dispersive spectroscope (SEM/EDS) (XL-40FEG, Phillips) was employed to examine the morphology of the samples and to map the chromium distribution. The pH values of solid samples were determined according to USEPA SW-846 method 9045C, an electrometric procedure for measuring pH values in soils and waste samples that include solids, sludges, and nonaqueous liquids (19). Practically, the samples were mixed thoroughly with reagent water first, and the aqueous solution drawn was filtered and centrifuged before its pH value was determined (19). 4634
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The toxicity characteristic leaching procedure (TCLP) (20) was carried out to examine the extent of chromium leaching (i.e., mobility) from the samples. The TCLP was designated by the USEPA for determining the mobility of both organic and inorganic analytes in slurry, solid, and multiphasic matrixes under an acidic environment. In brief, the deliquid (i.e., via pressurized filtration) matrixes were extracted with acidic fluid 1 or 2 in bottle extractors that rotated perpendicularly at a speed of 30 rpm for 18 h. Both fluids were prepared from varied amounts of glacial acetic acid, 1.0 N NaOH, and reagent water. The pH value was 4.93 ( 0.05 for fluid 1 and 2.88 ( 0.05 for fluid 2. The procedure for choosing which fluid was to be used for TCLP extraction was as follows. A 5.00-g sample was added to 96.5 mL of deionized water, and the resultant mixture was stirred vigorously for 5 min before the pH determination was made. If the pH value was less than 5.0, fluid 1 would be used. Otherwise, an aliquot of 3.5 mL of 1.0 N HCl was further added to the solution, and this new solution was covered with a watch glass and heated at 50 °C for 10 min. The solution pH was remeasured at room temperature. If the solution pH was still greater than 5.0, fluid 2 would be the choice; otherwise fluid 1 would be used. Following this prescription, fluid 1 was always used for all the present samples. The selected fluid 1 was mixed with the solid sample in a weight ratio of 20:1 and the mixture was TCLP-extracted. After TCLP extraction, the mixture was pressure-filtered and the solution pH of the leachate was determined at this stage. To determine chromium concentration in the TCLP leachates, a flame atomic absorption spectrometer (FAAS) (Z-6100, Hitachi) was employed. The TCLP leachates were digested with HNO3 on a hot plate, filtered through a 0.4-µm glass-fiber filter before measurement. The daily constructed calibration curve for determining chromium concentration in the leachates was required to have a correlation coefficient from least-squares-fitting greater than 0.9995. The experimental procedure, from the step of drying or thermal treatment through TCLP leaching, for each CrO3-sorbed sample was carried out in two to four repetitions to give two to four TCLP leachates. Then each TCLP leachate was measured with the FAAS for chromium concentration in triplicate. XAS was used to determine both the oxidation state/ speciation of and the local structure around chromium species. The Cr K-edge (5989 eV) XAS experiments were carried out on the wiggler beam line, BL-17C, of the Synchrotron Radiation Research Center (SRRC) in Taiwan. The electron storage ring was operated at 1.5 GeV with a stored current of up to 200 mA. Room-temperature XAS spectra were acquired in either fluorescence or transmission mode. The fluorescence mode detection was applied to chromium silicate and “samples” (the term “samples” refers to all heated Cr(VI)-sorbed kaolin samples). The transmission mode was adopted for all the others. Data reduction of XAS spectra was carried out using a commercially available WinXAS software (21). One useful feature of this software is the capability to perform species quantification for a multicomponent mixture. The XANES region is defined from the absorption threshold to ∼50 eV beyond. The X-ray absorption in this energy region is due to the characteristic electronic transitions from occupied to unoccupied electronic states, which is sensitively dependent on the detailed electronic structures of the sample materials. As a result, this XANES region is a fingerprinting area that can be further utilized for species determination. The species quantification in the aforementioned program is achieved by simulating the experimental XANES spectrum through a linear combination of various reference spectra that are deemed present in significant amount (21). When the leastsquares fitting is finished, reference compounds showing
TABLE 1. BET Surface Area, Solid pH, Loss on Ignition (LOI), and Major Oxide Composition of Neat Kaolin type of mineral
BET surface area (m2/g)
solid pH
LOI (%)
kaolina
16
3.54
13.0
a
Compositions of various oxides: 49.4% SiO2, 36.4% Al2O3, 1.28% K2O, 0.94% Fe2O3, 0.33% Na2O, 0.30% TiO2, 0.24% MgO, and 0.03% CaO.
negative percentage or unreasonable energy shift are unphysical and hence discarded. A detailed description of the theory, application, and technique of XAS is beyond the scope of the present report; interested researchers are referred to a recent publication (22).
Results and Discussion Table 1 lists major properties of “neat kaolin” used in this present study. It has a BET surface area of 16 m2/g, a solid
TABLE 2. BET Surface Area, Solid pH, and Chromium Leaching Percentage of Heated Samples, and Solution pH of Their TCLP Leachates
temperature (°C)
BET surface area (m2/g)
105a 500b 900b 1100b
1.5 9.6 5.9 0.2
a
solid pH
chromium leaching (%)
solution pH
0.91 ( 0.04 4.23 ( 0.08 6.57 ( 0.08 6.96 ( 0.05
75.4 ( 6.87 1.07 ( 0.08 0.04 ( 0.00 0.03 ( 0.00
4.33 ( 0.01 4.94 ( 0.01 4.93 ( 0.00 4.92 ( 0.01
Dried at 105 °C for 3-5 days.
b
Heated at 500-1100 °C for 4 h.
pH of 3.54, and a loss of ignition of 13%, and the major oxide constituents are SiO2 (49.4%) and Al2O3 (36.4%). For the heated Cr(VI)-sorbed samples, Table 2 summarizes the relevant properties such as BET surface area, solid pH, chromium leaching percentage (by the TCLP method), and solution pH of their TCLP leachates. The surface area for the
FIGURE 1. ×10000 morphology (left column) and Cr mapping (right column) of heated 3.7% Cr(VI)-sorbed kaolin samples. VOL. 36, NO. 21, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. XRD pattern of heated neat kaolin. (Note the varied scale among different compartments.) 105 °C dried sample is reduced to 1.5 m2/g, as compared to 16 m2/g for the neat kaolin. The reduction is attributed to the sorption of Cr(VI) that might have effectively clogged up the pores of the kaolin. Additional thermal treatment of the 105 °C dried sample at four temperatures with each lasting for 4 h results in BET surface area of 9.6 (500 °C), 5.9 (900 °C), and 0.2 m2/g (1100 °C). The recovery of the BET surface area after 500 °C heating is perhaps due to the decomposition of the Cr(VI) oxyanion in which an oxygen release is accompanied by the reduction of Cr(VI) to Cr(III). The released oxygen will result in a BET surface area increase once the oxygen leaves the sample. The surface area decrease observed after 900 and 1100 °C heat treatments could result from the high-temperature sintering effect. The Cr(VI) reduction to Cr(III) is thermodynamically possible due to a decrease of Gibbs free energy. For example, the calcination of Cr(III)-containing hydrotalcites around 377-427 °C causes a collapse of the layered structure, forming chromate or chromate-like compounds such as MgCrO4 and NiCrO4 (23). As the temperature is increased further, Cr(VI) can be transformed into Cr(III) with removal of O2 (23). As indicated in Table 2, the 105 °C dried sample has a much lower solid pH of 0.91 than neat kaolin (pH 3.54). The 4636
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low pH is derived from the strong acidic nature of the Cr(VI) solution used for sample preparation (pH 0.71). As the sample is heat-treated toward higher temperatures, the solid pH increases to 4.23-6.96 accordingly, consistent with the more reduction from strongly acidic Cr(VI) to less acidic Cr(III). Table 2 shows that the extent of Cr leaching from the heated Cr(VI)-sorbed kaolin is clearly temperature-dependent. Heating the sample at higher temperature results in markedly less leaching of chromium as determined by the TCLP method. For the 105 °C dried sample, the Cr leaching is 75.4% and is drastically reduced to 1.07% for the 500 °C heated sample, 0.04% for 900 °C heated sample, and 0.03% for the 1100 °C heated sample, respectively. While CrO3 is soluble in acid and water, its decomposition productsCr2O3s is only slightly soluble in acid and almost insoluble in water (24). Therefore, the low chromium leaching observed after heating at 500 °C and above is directly related to the transformation of Cr(VI) to Cr2O3. The even lower chromium leaching for the 900 and 1100 °C heated samples is partially caused by the sintering phenomenon. Last, we note that the solution pH values for all TCLP leachates fall between 4.33 and 4.94, close to the initial pH of 4.93 ( 0.05 for fluid 1 used in TCLP leaching tests.
FIGURE 3. XRD pattern of heated Cr(VI)-sorbed samples. (Note the varied scale among different compartments.) Figure 1 presents the ×10000 morphology (left column) and Cr mapping (right column) for the samples. The samples appear to exhibit a layered or flaky structure. The morphology shows that there are many more small discrete crumbs in the 105 °C dried and 500 °C heated samples than in the 900 and 1100 °C heated samples. The slab- or sliplike features found in the 900 °C heated sample appear to be quite different from those found in the 105 °C dried and 500 °C heated samples. It seems that some degree of sintering of kaolin takes place in the 900 and 1100 °C heated samples, as evidenced by the decreasing number and the blurring boundary of the discrete crumbs (the bottom two images in left column of Figure 1). The occurrence of sintering could account for a partial decrease of chromium leaching found for the 900 and 1100 °C heated samples. The mapping of Cr species (right column) indicates that no Cr cluster formation can be observed after various heat treatments. Figures 2 and 3 show respective XRD patterns for heattreated “neat kaolin” materials that contain kaolinite and muscovite and the heat-treated Cr(VI)-sorbed samples. Two common trends related to kaolin are clearly observed in both figures. First, the degree of crystallinity for these heated substances is in the following order: 105 °C heated substance > 500 °C heated substance > 900 °C heated substance ≈
1100 °C heated substance. Second, the intensity of the peaks characteristic of kaolinite-1Md decreases at higher heating temperature and completely vanishes at temperatures of g900 °C. Meanwhile, the characteristic peaks for muscovite1M,syn can always be detected for the 105-900 °C samples. However, for the highest temperature, 1100 °C, both kaolinite1Md and muscovite-1M,syn peaks disappear, but the peaks attributable to mullite,syn and sillimanite emerge, suggesting the occurrence of the transformation. For the Cr related species as presented in Figure 3, Cr2O3, transformed from CrO3 via heat treatment, can be detected for the 500-1100 °C samples. It is noted that the observation of Cr2O3 species coincides with the low Cr leaching percentage found for 5001100 °C samples. The left column of Figure 4 presents a series of XANES spectra together with their fitted spectra for various heattreated samples as well as CrO3 and Cr2O3 reference compounds. The sharp preedge peak at 5993 eV is due to the dipole-forbidden 1s-3d transition and is a unique attribute of tetrahedral bonding geometry assumed by Cr(VI) oxidation state. As a result, the relative height of this preedge peak is proportional to the percentage of Cr(VI) in total chromium (17). A glance at the preedge feature already reveals the nearcomplete reduction of Cr(VI) for 500-1100 °C samples, and VOL. 36, NO. 21, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. Fitting of XANES (left column) and EXAFS spectra (right column) of heated Cr(VI)-sorbed samples with that of reference compounds (CrO3 and Cr2O3) (s, experimental; O, simulated). The percent of reference compounds resulting from XANES fitting is shown in each compartment of the left column. The top and bottom compartments are for reference compounds. Simulation of the EXAFS spectrum was based on a linear combination of percent reference compounds resulting from XANES fitting. the degree of reduction is dependent on the treatment temperature. The 105 °C dried spectrum bears more resemblance to CrO3 than to Cr2O3. In comparison, the spectra for the 500-900 °C samples and Cr2O3 are rather alike. The observation that Cr2O3 predominates in almost all 500-1100 °C heated samples agrees with the finding deduced from XRD data, as shown in Figure 3. The species quantification can be achieved by performing a least-squares fitting of the experimental XANES spectrum using a set of reference compounds selected on the basis of most probable chemistry route. The fitted percentage of reference compounds is shown in each compartment as well as summarized in Table 3 for convenience. For the 500 °C heated sample, only ∼3% Cr(VI) is left in the unreduced state. After heating at 900-1100 °C, the reduction of Cr(VI) is 100% complete, indicated also by the absence of a preedge peak. A near total reduction of Cr(VI) in the 500-1100 °C heated 4638
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TABLE 3. Weight Percent Distribution of Cr Species Derived from XANES Fitting for Cr-Sorbed Kaolin Thermally Treated at 105-1100 °C for 4 h sample identity Cr species
3.7% Cr-105 °C
3.7% Cr-500 °C
3.7% Cr-900 °C
3.7% Cr-1100 °C
CrO3 Cr2O3
98 2
3 97
0 100
a a
a
Not adequately fitted with CrO3 and Cr2O3 references.
samples corroborates the low chromium leaching found for these samples. However, it is cautioned that the spectra for the 1100 °C sample cannot be satisfactorily synthesized with common chromium oxides, and the regions where major discrepancy occurs are marked with circles in Figure 4. One
FIGURE 6. XANES (upper compartment) and EXAFS spectra (lower compartment) for the 0.1% Cr-1100 °C reference.
FIGURE 5. Fourier transforms of EXAFS spectra of reference compounds (CrO3 and Cr2O3) and heated Cr(VI)-sorbed samples. The vertical lines running through various compartments are intended for an easier comparison of peak position between various samples and reference compounds. The top and bottom compartments are for reference compounds. viable possibility is that part, at least, of Cr(III) might have reacted with kaolin to form non-Cr2O3 chromium compounds that are not easily leached out. The right column of Figure 4 shows the experimental EXAFS spectra and their simulated ones for the 105-1100 °C samples. The simulated EXAFS spectrum for each heated sample was obtained with the linear combination of reference compounds (CrO3 and Cr2O3) based on the percent(s) resulting from XANES fitting. Again, no good fit can be obtained for the 1100 °C heated sample with CrO3 and Cr2O3 references. In addition, the XANES spectra and their first derivatives for the reference compounds (CrO3 and Cr2O3) and heated Cr(VI)-sorbed samples are shown in Figure SI-1
in Supporting Information. The first derivatives are presented to clearly show the differences between the XANES spectra. Figure 5 presents the Fourier transforms, without phase shift correction, of EXAFS spectra for reference compounds (CrO3 and Cr2O3) and heated samples. The vertical lines running through various compartments are intended for an easier comparison of peak position between various samples and reference compounds. The top and bottom compartments contain the spectra of reference compounds Cr2O3 and CrO3, respectively. The Fourier transforms of EXAFS spectra of the 500-900 °C heated samples are clearly very similar to that of the Cr2O3 reference. In contrast, the spectrum in the 3.7% Cr-1100 °C compartment does not match that of the Cr2O3 reference. The peak heights at 2.45 (Cr-Cr shell) and 5.00 Å (Cr-Cr and Cr-O shells) of the 1100 °C heated sample are either relatively smaller or disappearing, compared with that of the 500-900 °C heated samples. The interatomic distance between the central Cr atom and its first shell (Cr-O shell at 1.53 Å) in the 500-1100 °C heated samples is identical to that of the Cr2O3 reference; whereas the 1100 °C heated sample has a shorter interatomic distance between the central Cr atom and its second shell (Cr-Cr shell) than that of the 500-900 °C heated samples and the Cr2O3 reference. Therefore, it was speculated that the chromium in the 1100 °C heated sample might contain mixed forms of chromiumsCr2O3 and non-Cr2O3 Cr(III). To check the possibility of chromium vaporization during heating at various temperatures, the “neat CrO3” was heattreated in the same way as the Cr-impregnated kaolin samples were. The weight remained (denoted as RW) after each heating was measured and recorded. The percent weight loss, not including the loss of oxygen due to thermal decomposition of CrO3, was calculated according to the following expression:
wt % loss ) 1 - [(RW/weight of CrO3 prior to heating)(2 × 100/152)] where 100 and 152 are the respective formula weights of CrO3 and Cr2O3. It was found that the weight loss was either less than or equal to 2.3%. Please note that the EXAFS spectra for the 500-1100 °C heated “neat CrO3” indicate that almost VOL. 36, NO. 21, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 7. Fitting of the XANES (upper compartment) and EXAFS (lower compartment) spectra for the 3.7% Cr-1100 °C sample with that of Cr2O3 and 0.1% Cr-1100 °C reference compounds (s, experimental; O, simulated). The percent of reference compounds resulting from XANES fitting is shown in each compartment. Simulation of the EXAFS spectrum was based on a linear combination of percent reference compounds resulting from XANES fitting. all chromium in the 500-1100 °C heated “neat CrO3” was in the form of Cr2O3 (see Figures SI-2 and SI-3 in Supporting Information). The fact that all chromium in the 500-1100 °C heated “neat CrO3” was only in the form of Cr2O3 may indirectly support our previous speculation that the 3.7% Cr-1100 °C sample might have contained chromium as a mixture of Cr2O3 and non-Cr2O3 Cr(III). The non-Cr2O3 Cr(III) could be the reaction product(s) formed by the reaction between Cr2O3 and the aluminosilicate sorbent. To rigorously prove this speculation, it is necessary to acquire the non-Cr2O3 Cr(III) reference compound(s) in a high-purity grade. However, no such reference compound is commercially available. As a result, the following two steps were taken to propose some structural mechanism for the sequestration of chromium in the compound formed at 1100 °C. The first step involved the “synthesis” of the non-Cr2O3 Cr(III) reference compound(s) by heating a conceptually monolayer Cr(VI)-sorbed kaolin at 1100 °C for 4 h. This synthesis was speculated to form product(s) by the reaction of Cr2O3 with kaolin. This non-Cr2O3 Cr(III) reference will be used together with the Cr2O3 reference to fit the XAS spectrum for the 3.7% Cr-1100 °C sample. A dried 0.1% Cr(VI)-sorbed kaolin sample was prepared and heated at 1100 °C for 4 h to represent the non-Cr2O3 Cr(III) reference compound(s) based on the following reasoning. The BET surface area of kaolin employed in this study is 16 m2/g (see Table 1), and thus, the presumed maximum single-layer surface coverage of Cr(VI) oxyanion(s) onto the aluminosilicate surface was calculated to be equivalent to 0.77% chromium content in kaolin provided that the reactive site density (5.5 site/nm2) for chromium sorption onto silica (25) is relatively similar to that onto kaolin in this study. Figure 6 is the XANES (upper side) and EXAFS (lower side) spectra of the 0.1% Cr(VI)-sorbed sample heated at 1100 °C for 4 h. Although the spectral patterns of this reference compound are quite different from the corre4640
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FIGURE 8. Fitting of the XANES (top two compartments) and EXAFS (bottom two compartments) spectra of the 3.7% Cr-1100 °C sample with that of Cr2O3 and chromium silicate reference compounds (s, experimental; O, simulated). The percent of reference compounds resulting from XANES fitting is shown in each compartment. Simulation of the EXAFS spectrum was based on a linear combination of percent reference compounds resulting from XANES fitting. sponding spectra for the Cr2O3 reference in Figure 4, it should be noted that part of chromium in the 0.1% Cr-1100 °C sample might still be in form of Cr2O3. The upper compartment of Figure 7 shows XANES fitting of the 3.7% Cr-1100 °C sample with Cr2O3 and the 0.1% Cr1100 °C references. The fitting is successful, and the results indicate that the 3.7% Cr-1100 °C sample contains about one-third Cr2O3 and two-thirds 0.1% Cr-1100 °C reference. The lower compartment of Figure 7 presents the simulation of the EXAFS spectrum for the 3.7% Cr-1100 °C sample based on the percentages obtained by the corresponding XANES fitting. A close spectral match in k space between the 3.7% Cr-1100 °C and the linearly-combined reference spectrum is obtained. The slight deviation in the higher “k” range between the experimental and simulated spectra is due to the noise effect. The second step involved the synthesis of “chromium silicate” reference and its measurements of the XAS spectrum (see Figure SI-4 in Supporting Information). This spectrum was then used, together with that of Cr2O3, to fit the chromium XANES and EXAFS spectra for both 0.1 and 3.7% Cr-1100 °C samples by assuming that chromium silicate might have formed in these samples given the XRD detection of mullite
and sillimanite. Chromium silicate reference compound (see Figure SI-5 in Supporting Information for its XRD pattern) was synthesized according to the method described in a U.S. patent (26). Figure 8 shows the fitting results that suggest an existence of chromium silicate in these two Cr(VI)-doped kaolin heated at 1100 °C for 4 h. It indicates that chromium silicate accounts for 43% of total chromium in the 0.1% Cr-1100 °C sample versus 25% in the 3.7% Cr-1100 °C sample. This discrepancy is reasonable. Because the percent of monolayer chromium coverage in the 0.1% Cr-105 °C sample is presumed to be much greater than that in the 3.7% Cr-105 °C sample, chromium in the former sample would have a greater opportunity to contact and react with the kaolin surface. Data consistency between the fitting result obtained during the first step (Figure 7) and that during the second step (Figure 8) is satisfactorily met. The fitting result in Figure 7 for the 3.7% Cr-1100 °C sample, 34% Cr2O3 plus 66% 0.1% Cr-1100°C that contains 57% Cr2O3 and 43% chromium silicate (see the top compartment of Figure 8), can be translated as 34% Cr2O3 + (66%)(57%) Cr2O3 + (66%)(43%) chromium silicate or equivalent to 72% Cr2O3 + 28% chromium silicate. This translated result is close to the fitting result, 75% Cr2O3 + 25% chromium silicate as shown in the bottom compartment of Figure 8, obtained by directly fitting the 3.7% Cr-1100 °C sample with Cr2O3 and chromium silicate references.
Acknowledgments We thank the staff at SRRC for their assistance during the experimentation and the anonymous reviewers for their helpful comments. This project was sponsored by the National Science Council of Taiwan, ROC (NSC 89-2211-E029-008).
Supporting Information Available (i) Experimental XANES spectra and their corresponding derivative spectra for reference compounds (CrO3 and Cr2O3) and heated Cr(VI)-sorbed samples (Figure SI-1); (ii) fitting of XANES spectra and EXAFS spectra of heated “neat CrO3” with the reference compounds of CrO3 and Cr2O3 (Figure SI-2); (iii) Fourier transforms of EXAFS spectra of heated “neat CrO3” samples (Figure SI-3); (iv) XANES and EXAFS spectra of chromium silicate reference compound (Figure SI-4); and (v) XRD pattern for the synthesized chromium silicate hydrate
(Figure SI-5). This material is available free of charge via the Internet at http://pubs.acs.org.
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Received for review December 13, 2001. Revised manuscript received August 6, 2002. Accepted August 16, 2002. ES0114761
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