Environ. Sci. Technol. 2010, 44, 1974–1979
Chlorinated Aromatic Compounds in a Thermal Process Promoted by Oxychlorination of Ferric Chloride TAKASHI FUJIMORI,* MASAKI TAKAOKA, AND SHINSUKE MORISAWA Department of Urban and Environmental Engineering, Graduate School of Engineering, Kyoto University, Katsura, Nisikyo-ku, 615-8540, Kyoto, Japan
Received November 3, 2009. Revised manuscript received February 3, 2010. Accepted February 9, 2010.
The relationship between the formation of chlorinated aromatic (aromatic-Cl) compounds and ferric chloride in the solid phase during a thermal process motivated us to study the chemical characteristics of iron in a model solid sample, a mixture of FeCl3 · 6H2O, activated carbon, and boron nitride, with increasing temperature. Fe K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy revealed drastic changes in the chemical form of amorphous iron, consistent with other analytical methods, such as X-ray diffraction using synchrotron radiation (SR-XRD) and Fourier-transform infrared (FT-IR) spectroscopy. Atomic-scale evidence of the chlorination of aromatic carbon was detected by Cl-K X-ray absorption near edge structure (XANES) spectroscopy. These results showed the thermal formation mechanism of aromatic-Cl compounds in the solid phase with ferric chloride. We attribute the formation of aromatic-Cl compounds to the chlorination of carbon, based on the oxychlorination reaction of FeCl3 at temperatures in excess of ca. 300 °C, when the carbon matrix is activated by carbon gasification, catalyzed by Fe2O3, and surface oxygen complexes (SOC) generated by a catalytic cycle of FeCl2 and FeOCl. Chemical changes of trace iron in a thermal process may offer the potential to generate aromatic-Cl compounds in the solid phase.
Introduction Thermal processes are well-known as major anthropogenic sources of chlorinated aromatic (aromatic-Cl) compounds (1, 2), such as polychlorinated dibenzo-p-dioxins (PCDDs) and -furans (PCDFs), biphenyls (PCBs), and chlorobenzenes (CBzs). Aromatic-Cl compounds are known to be emitted from municipal solid waste incinerators (MSWI) (3, 4) and iron ore sintering processes (5, 6). Many researchers have suggested the homo- and heterogeneous formation of aromatic-Cl compounds from macromolecular carbons (7-9). Fly ash collected from the postcombustion zone of such as MSWIs and iron ore sintering plants has the highest concentration of aromatic-Cl compounds, and unburned carbon and chlorine sources in fly ash and surrounding oxygen are known to be essential factors for aromatic-Cl formation (7, 8). Trace metal chlorides in fly ash promote aromatic-Cl formation (10). Although copper chlorides are the most wellknown and studied promoters of aromatic-Cl compounds * Corresponding author e-mail:
[email protected]. media.kyoto-u.ac.jp. 1974
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(11, 12), iron chlorides also have strong potential, and a chlorination mechanism of carbon has been proposed (10, 13-15). Additionally, the amounts of iron in MSWI fly ash have been reported to be much greater than those of copper (15), and, clearly, iron is the main component of the iron ore sintering process (16). Thus, iron is thought to greatly contribute to the formation of aromatic-Cl compounds in fly ash. However, little direct evidence exists regarding the formation mechanism of aromatic-Cl compounds with iron chlorides at the atomic level. X-ray absorption fine structure (XAFS) spectroscopy has recently been used to monitor the redox change of copper (17, 18). The behavior of chlorine and the atomic environment of copper in fly ash were clarified in a previous study using XAFS (18). The chlorination mechanism of carbon by iron chlorides may be better described if the behaviors of iron and chlorine atoms are observed at the same time. In this study, we discuss the behaviors of Fe and Cl in fly ash and, based primarily on XAFS spectroscopy, provide basic information on the chlorination mechanism of carbon by iron chloride that produces aromatic-Cl compounds during thermal processes. Chemical forms of iron at various temperatures were analyzed by Fe K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. The Fe K-edge EXAFS technique is suitable for amorphous iron characterization, as explained in the present study. We also used Cl-K X-ray absorption near edge structure (XANES) spectroscopy to determine the behavior of chlorine.
Materials and Methods Model Fly Ash. To determine the behavior of Fe and Cl, we prepared a model fly ash (MFA) using a mixture of ferric chloride, activated carbon (AC), and boron nitride (BN). Any organic compounds were removed from the AC by heating at 500 °C for 60 min under a 100% nitrogen stream. Preparation of Ferric Oxychloride (FeOCl). We made FeOCl from ferric chloride hexahydrate, according to Ryan and Altwicker (15). A quartz boat containing FeCl3 · 6H2O (3-4 g) was placed in the center of a quartz tube. The FeCl3 · 6H2O was then heated to 250 °C for about 30 min under a 50mL/min flow of nitrogen. The residue (dark black/red) in the quartz boat was rinsed with deionized water, followed by acetone, on filter paper three times. The product was dried in vacuo and identified as FeOCl by X-ray diffraction (Figure S1) and Fe K-edge EXAFS, by comparison with literature data (19). In Situ Fe EXAFS and Data Analysis. Using in situ Fe K-edge EXAFS spectroscopy, we detected the chemical structure of Fe at the atomic level in the MFA, which contained ferric chloride hexahydrate (FeCl3 · 6H2O), activated carbon (AC), and boron nitride (BN). The composition was 1.5% Fe, 2.9% Cl, and 10% AC, and the remaining 82.7% was almost entirely BN. After the MFA was ground, using a mortar and then an agate mortar for 10 min each, it was pressed into a disk. Fe K-edge EXAFS spectroscopy was performed using beamline BL01B1 at SPring-8 (Hyogo, Japan), with the MFA disk heated in a T-type in situ cell (17, 18) consisting of a glass cell, a mantle heater, and a temperature controller. The temperature of the sample was increased gradually from room temperature to 450 °C at a rate of around 5 °C/min, following the profile in Figure S2. A 10% O2 (90% N2) gas atmosphere was introduced from the inlet of the T-type cell at 50 mL/ min and exhausted from the outlet. The energy area from 6600 to 8700 eV of Fe K-edge EXAFS spectra could be measured in 2 min in quick-scan mode. EXAFS spectra of the MFA disk were collected in transmission mode with a 10.1021/es903337d
2010 American Chemical Society
Published on Web 02/19/2010
Si(111) monochromator. The spectra of reference materials, FeCl3, FeCl3 · 6H2O, FeCl2 · 4H2O, FeOCl, FeO, Fe2O3, Fe3O4, FeO(OH), Fe3C, and Fe, were measured to compare their spectral shapes and to identify major species, because an EXAFS spectrum can be used as a fingerprint reflecting the local environment of the iron. More reference irons should be studied to characterize iron forms in detail. However, number and species of iron compounds became restricted in case of the MFA. As the MFA and gas phase included five elements (Fe, Cl, O, H, and C), we selected chlorides, oxides, hydroxide, carbide, and metal of iron. These iron compounds express possible chemical forms of iron at each temperature. Species can be distinguished using the least-squares linear combination fit (LSF) technique, in which spectra of known reference species are fitted to the spectrum of an unknown sample. We used the LSF technique on the k3-weighted EXAFS spectrum to determine the major species using the SixPACK software (20) (ver. 0.63). The residual value reducedχ2 )
1 N-P
N
∑ (χ
obs i
- χifit)2
(1)
i)1
was used to evaluate the LSF for the experimental spectra (21). χiobs is the ordinate of the EXAFS spectrum measured from the sample at the ith energy point, χifit is the ordinate of the fitted EXAFS spectrum, N is the number of data points in the fitted wavenumber k range, and P is the number of fitted components. Principal components analysis (PCA) was used to determine the number and type of principal components in k-range 3-12 Å-1, and target transformation was then employed to identify the probable iron species in MFA during increasing temperature for the set of 10 reference compounds. Details of PCA and target transformation and fitting of EXAFS spectra were from Ressler et al. (22) and Manceau et al. (23), respectively. Cl K-Edge XANES. The Cl forms present after the MFA was heated were determined by measuring the Cl K-edge XANES spectra. A MFA contained FeCl3 (2% Fe and 3.8% Cl), AC (5%), and BN (remainder) and was ground using a mortar for 10 min. We then placed the MFA powder on a quartz boat in a quartz tube filled with 10% O2 (90% N2) at 50 mL/min and heated for 30 min in an electric furnace preheated to 200, 300, and 400 °C. After the heating procedure, MFA powder was sealed as quickly as possible and sent for the measurement of Cl K-edge XANES spectra, which was performed using BL-11B in the Photon Factory (Tsukuba, Japan). An in situ cell was not used in the Photon Factory because of physical restrictions of the device. Powdered MFA samples were mounted on carbon tape, and their XANES spectra were collected in total fluorescence yield (TFY) mode in a vacuum. X-ray absorption spectra of Cl in different inorganic and organic reference compounds were collected to assist in the identification of the chemical state of Cl in MFA after heating (Figure S3). Polyvinyl chloride and the reference iron compounds (FeCl3 · 6H2O, FeCl3, FeCl2 · 4H2O, and FeOCl) were measured in total electron yield (TEY) mode in a vacuum. Cl K-edge XANES spectra of chlorobenzenes and chlorophenols were measured under atmospheric pressure by the conversion electron yield (CEY) method at BL-9A at the Photon Factory. Cl bound to inorganic, aromatic, or aliphatic carbon can be distinguished by the features of a Cl XANES spectrum, as reported previously (18, 24). As reflected in these spectral features, analyses were performed by a linear combination fit using reference materials of chlorine and REX 2000 software (ver. 2.5.5; Rigaku, Japan). In Situ Powdery X-ray Diffraction Using Synchrotron Radiation. Trace crystal structures in a MFA were determined by in situ powder X-ray diffraction using synchrotron radiation (SR-XRD). FeCl3 · 6H2O (containing 1% Fe and 2% Cl), 5% AC, and BN (remainder) in a MFA was added to a
FIGURE 1. Contour plot of the k3-weighted Fe K-edge EXAFS spectra (A) and the percentage of iron in different forms, calculated by linear square fitting (LSF) of Fe K-edge EXAFS spectra (B), at each temperature. quartz capillary column (0.5 mm in diameter) using a Pasteur pipet and sealed using a burner under an air atmosphere. We placed the capillary in a Debye-Scherrer camera for measurement of SR-XRD using BL02B2 in SPring-8 (Hyogo, Japan). The capillary was heated in N2 gas from room temperature to 500 °C, and we measured the SR-XRD pattern, from which crystal information was identified using the MDI Jade 6j software (Rigaku, Japan) contained within the International Centre for Diffraction Data powder diffraction file. FT-IR Spectroscopy. The carbon surface of and the chemical form of the iron in a disk of MFA were analyzed by Fourier transform infrared (FT-IR) spectroscopy (FT-IR8400, Shimadzu Co., Ltd.). The MFA contained ferric chloride hexahydrate and AC (1:1 weight) and was heated to 200, 300, and 400 °C for 30 min under 10% oxygen gas stream and 99.5% KBr. Total Organic Carbon. Total organic carbon (TOC) was measured as described previously (18).
Results and Discussion Chemical Form of Iron at Each Temperature. Analyzing the Fe K-edge EXAFS data set, we found evidence of dechlorination and oxidation processes of iron in the MFA upon heating, related to the formation of aromatic-Cl compounds in a thermal process, as discussed below. We selected the EXAFS region to analyze and characterize the chemical form of iron. The amplitude and interval of the oscillation structure of the Fe-EXAFS region showed more different oscillation features at each temperature than the XANES region in the normalized XAFS spectrum (see Figure S4). The k3-weighted Fe K-edge EXAFS spectra changed dramatically upon heating the MFA (Figure 1A), indicating dynamic change in the chemical environment around the Fe atom. We studied the chemical form of the iron by means of LSF analyses of the Fe K-edge EXAFS spectra. Although the Debye-Waller factor is known to affect the amplitude of EXAFS oscillations with temperature rising, peak positions and intervals of EXAFS oscillation which mainly characterize the iron compounds are not affected by the Debye-Waller factor. So, we concluded that analysis by using EXAFS oscillation is a useful method to characterize the iron in case of rising temperature. About six significant components were extracted from PCA of the 35 Fe K-edge EXAFS spectra between 3 and 12 Å-1. We determined the number of components using the minimum value of indicator (IND), defined by Malinowski (25), using SixPACK software (20). VOL. 44, NO. 6, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. The k3-weighted Fe K-edge spectra at room temperature (rt), ca. 300 °C, and 400 °C, and the spectra of six forms of iron. After PCA, probable iron species were identified by target transformation of 10 reference compounds as described in the Materials and Methods. Six model compounds were found to yield sufficient matches: specifically, four iron chlorides (FeCl3, FeCl3 · 6H2O, FeCl2 · 4H2O, and FeOCl), ferric oxide (Fe2O3), and ferric oxide hydroxide [FeO(OH)]. These iron references were adopted using a combination of SPOIL values, as defined in ref 23, the reducedχ2 value, and similarities in the EXAFS spectra between target and transformation, with help from the report of Slowey et al. (21). When the SPOIL value is 3.0) and reducedχ2 values (Table S1) and showed different EXFAS shapes (Figure S5). Fe had the largest reducedχ2 value, as shown in Table S1. Thus, we deselected Fe, FeO, Fe3O4, and Fe3C. On the other hand, the six selected iron references showed lower SPOIL (
