Environ. Sci. Technol. 2002, 36, 790-796
Supercritical Fluid Extraction: An Innovative Tool for a Fly Ash-like Model Support D. BROCCA,† M. LASAGNI,‡ E. COLLINA,‡ M . T E T T A M A N T I , ‡ A N D D . P I T E A * ,‡ B.P. 10236-06, Ouagadagou 06, Burkina Faso, Africa, and Dipartimento di Scienze dell’Ambiente e del Territorio, Universita` degli Studi di Milano-Bicocca, Piazza della Scienza 1, 20126 Milano, Italy
The performance of the supercritical fluid extraction (SFE) technique to obtain a new and more appropriate model support for PCDD/F formation studies was investigated. To characterize fly ash and model supports and relate their chemical-physical properties, surface area and pore size were determined. To evaluate the influence on reactivity of the different model supports with respect to raw fly ash, a kinetic study of the thermal behavior of dibenzofuran (DF) was performed. Rate constants as well as the activation and thermodynamic parameters for the different systems were also compared. The model support obtained from SFE was very similar to raw fly ash from the structural, physicalchemical, and kinetic points of view.
Introduction During the combustion of municipal solid waste (MSW), a rather complex scheme of chemical reactions takes place (1) and involves different classes of organic micropollutants. As a result, particulate matter from the boiler and the air pollution control equipment contain hazardous contaminants such as polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs). To reduce the environmental impact of solid waste thermal treatments, a great number of studies concerning the formation and destruction reactions of PCDDs and PCDFs (2) and their mechanisms (3-6) have been carried out. Usually, the studies are performed (i) directly on fly ash and (ii) on model systems. In i, it was shown (7) that two mechanisms take place on fly ash surface: formation from precursors (i.e., chloroaromatic molecules such as polychlorobenzenes and polychlorophenols) or de novo synthesis (starting from more or less complex non-chlorinated molecules, native carbon included, and a chlorine source such as Cl2, HCl, or inorganic chlorides). In both mechanisms, many authors (8, 9) suggest that fly ash plays a fundamental role in the formation of PCDDs/DFs and may act as a catalyst too. At least in the second mechanism, oxygen is essential. In ii, model systems are used to gain specific information on the thermal behavior and reactivity of single compounds or their mixture. Here, to avoid interference, the ideal support in the model systems should not contain extractable organic compounds while the chemical and physical characteristics of the support surface should reproduce those of the real fly * Corresponding author phone: +39-02-64482823; fax: +39-0264482890; e-mail:
[email protected]. † B.P. 10236-06. ‡ Universita ` degli Studi di Milano-Bicocca. 790
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ash sample as closely as possible. Generally, silica gel (10) or thermally treated fly ash (3, 11) were used as model supports. The fly ash thermal treatment (TT) is simple and widely employed to prepare a support. However, it takes a long time (3 h) and high temperature (650 °C); moreover, at the end of the thermal treatment, the structure of the sample matrix is changed and it is no longer representative of the real system. In this work, we propose the use of supercritical fluid extraction (SFE) to obtain new and more realistic model systems for de novo PCDD/F formation studies. SFE is typically used in analytical chemistry as an alternative method to liquid-solid extraction, such as Soxhlet or sonication, for the recovery and analysis of organic micropollutants in environmental samples (12-15). The advantages of SFE as compared to Soxhlet are the rapidity, the selectivity, and the effectiveness of the extraction process; furthermore, hazardous organic solvents are not involved. A first goal of this work was to compare supports from TT and SFE with the real matrix. Because of the possible role of fly ash as a catalyst in the formation reaction of chlorinated micropollutants in the MSW incinerators, we studied the changes in structural and morphological fly ash properties due to high pressure during SFE or high temperature during TT. To characterize the fly ash and relate its chemical-physical properties to its behavior in terms of activity and selectivity, the fly ash surface area and the pore size (BrunauerEmmett-Teller, BET, isotherm) were measured. For fly ash morphology, scanning electron microscopy (SEM) analysis was employed. With the same equipment, the qualitative analysis of the chemical elements on the fly ash surface was performed. Furthermore, because of the important role of chlorine in the formation of PCDD/Fs (3, 16, 17), the change in chlorine amount due to the treatments was also investigated. A second goal of this work was to investigate the influence on reactivity of the model supports prepared by SFE and TT in comparison to that of the real samples. To do this, a kinetic study of the thermal behavior of the target compound dibenzofuran (DF) mixed with the three supports was performed. DF, the parent compound of PCDFs, was chosen as representative of organic compounds since it is often found in MSW fly ash (18, 19) and because of its toxicological inactivity. The rate constants and the activation or thermodynamic parameters for the different systems were also compared.
Experimental Section Reagents and Materials. DF (Janssen purity 99+%) was used without any further treatment. Two fly ash samples, FA4 and FA5, were collected at different times from the electrostatic precipitator hoppers of a MSW incinerator (Milan, Italy). Each sample was homogenized using a ball mill (Retsch, model S1) operating with three 10 mm diameter balls and two 20 mm diameter ones at 80 rpm for 25 min. It was then dried. In the following, these samples used for the experiments without further treatment are termed “raw fly ash”. The initial total organic carbon content (TOC0) of FA4 and FA5 was respectively 2080 ( 35 and 1830 ( 35 ppm. Model Support from Thermal Treatment. The raw FA4 or FA5 sample was heated for 3 h at 650 °C in a closed muffle furnace (Heraeus M110), inner volume 9 L, with oxygen present in great excess to completely eliminate the carbon. 10.1021/es015575q CCC: $22.00
2002 American Chemical Society Published on Web 01/12/2002
TABLE 1. Specific Surface Areas, Monolayer Volume, and Total Pore Volume from BET Analyses surface area sample
m2 g-1
FA4 TTFA4 SFEFA4 FA5 TTFA5 SFEFA5
3.02 0.56 3.47 2.67 0.70 3.40
a
% variationa -81 +15 -74 +27
monolayer vol cm3 g-1 0.693 0.129 0.797 0.613 0.162 0.781
% variationa -81 +15 -74 +27
total pore vol cm3 g-1 113 35 149 111 50 157
% variationa - 69 +32 -55 +41
Calculated as 100 × (TTFAi - FAi)/FAi or 100 × (SFEFAi - FAi)/FAi.
The residual total organic carbon (TOC) was 50 ( 50 ppm, i.e., a value in the detection limit of the TOC instrument. In the following, these supports used without further homogenization will be indicated as thermal treated fly ash (TTFA4 and TTFA5). Model Support from SFE. After a series of preliminary runs, each raw 5 g of FA4 or FA5 sample was supercritically extracted with CO2 (ISCO SFX 1220) in two successive steps: in a static mode (10 min) and then in a dynamic mode (20 min). CO2 modified with 1 mL of methanol directly spiked onto the sample was used at a temperature of 70 °C and at a pressure of 7500 psi. As shown in ref 20, with these conditions it was possible to remove the same organic compounds as Soxhlet extraction. To determine the residual TOC value, each sample was then degassed to desorb the CO2 residual from the extraction (20). In the following, these supports used without further homogenization will be indicated as supercritical fluid extracted fly ash (SFEFA4 and SFEFA5). The residual TOC was 1930 ( 25 and 1710 ( 30 ppm, respectively. Model Mixtures. DF-FAi (i ) 4, 5), DF-SFEFAi, and DFTTFAi mixtures were prepared (10) by directly mixing about 32.6 mg of DF and 20.0 g of each support. The initial TOC content due to DF (TOCDF0) was about 1500 ppm. Total Organic Carbon. TOC was measured using a Dohrmann instrument assembled with the standard module (DC-90), the purgeable organics module (PRG-1), and the sludge sediment sampler accessory (S/SS). Details of the analytical method have already been reported (21-23). BET Analysis. Specific surface area and specific pore volume (Table 1) as well as pore size distribution of the raw fly ash and of the derived model supports were measured with a Coulter SA 3100 instrument. Surface area was determined by the “five-point BET plot” method (24) where the relative pressure (p/p0) values (p and p0 being the pressure of the vapor and the saturation vapor pressure of the adsorbent gas, respectively) of five BET points ranged from 0.05 to 0.2. For the analysis of the pore size distribution, a radius range between 3 and 200 nm was investigated. Samples (0.7-5.0 g) were degassed at a temperature of 250 °C for 90 min at a reduced pressure of 10-2 Torr. Then, nitrogen was employed as adsorbent gas at the temperature of -195 °C and at the operating pressure of 810 Torr. As an example, Figure 1 shows the FA4 pore size distribution plot. SEM Analysis. To study the effect of the thermal and supercritical fluid treatments on the macroscopic structure of the surface of fly ashes, raw, SFE, and TT fly ashes were examined using a SEM analysis. The analyses were carried out with a Cambridge Stereoscan 150 microscope equipped with a tungsten filament (current ) 3 A) operating with a 20 kV accelerating voltage. A few milligrams of each sample was coated using gold targets to increase the low conducibility of the material. Several
FIGURE 1. FA4 sample pore size distribution plot. electron micrographs were taken at different magnification levels. Here micrographs representative of the observed morphology are reported. Figure 2a,b shows micrographs of FA4, TTFA4, and SFEFA4 at two different magnifications, 500 and 2000, whereas Figure 3 shows those of FA5, TTFA5, and SFEFA5 at the magnification level of 2000. EDS Analysis. The electron dispersive spectrometry (EDS) gave information on qualitative surface elemental composition. It was performed on different points of each sample during the SEM analysis. The occurrence of chlorine and other elements in the raw, TT, and supercritically extracted fly ash is shown in Figure 4. Chlorine Analysis. A preliminary speciation analysis of chemical elements in raw fly ash samples, performed by X-ray photoelectron spectroscopy (XPS analysis; M-Probe Small Spot Esca, Surface Science Instruments) showed that inorganic chlorine is present as chloride. To quantify chloride present on fly ash, leaching tests coupled with EDS analysis of the solid before and after leaching were performed. These tests were carried out by washing 2 g of the sample two times with 50 mL of distilled water at room temperature. EDS results on FA4 (Figure 5) showed that the leaching treatment quantitatively removed chlorides from fly ash. The quantification of chlorides was performed by measuring the chloride content in the water from the leaching test (Fajans’ titration), and the results indicated that the percentage chloride content (( standard deviation) in FA4, TTFA4, and SFEFA4 was 18.8 (( 0.2), 16.9 (( 0.2), and 18.5 (( 0.3)%, respectively (weight percentage of the entire sample). Similar high chloride contents were already found in fly ash (25). Reaction Products in Air and Analytical Procedures. The reaction products of thermal treatment in air of DF-support mixtures or DF not supported on a matrix i.e., “free” DF (DFF), were determined by treating about 5 g of the reagent for 8 h at 150 °C in a closed fixed-bed reactor system (10) (reactor volume, 35 cm3). Then, nitrogen (100 mL/min) flowed through the reactor for 1 h, bubbling in a series of five impingers, the first three filled with cyclohexane and the following two with a saturated Ba(OH)2 solution (5). To determine how much carbon dioxide developed in the reaction, the two Ba(OH)2 solutions and a blank were titrated with 0.5 N HCl, but no CO2 was detected in any run. To identify organic products, cyclohexane traps were analyzed with a HP 5890 gas chromatograph (CP-Sil 8 CB column, Chrompack) coupled to a HP 5970 mass selective detector. The injector temperature was 280 °C. The oven temperature program was 1 min at 60 °C, then 20 °C min-1 to 280 °C, and VOL. 36, NO. 4, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. (a) SEM micrographs of FA4, TTFA4, and SFEFA4 at a magnification level of 500. (b) SEM micrographs of FA4, TTFA4, and SFEFA4 at a magnification level of 2000. 5 min at 280 °C. The results, both quantitative and qualitative, showed that the only reaction product was the DF itself (10). The residual TOC of the sample was also determined. Kinetic Runs. All the samples of DF-FAi, DF-SFEFAi, and DF -TTFAi (about 2 g) or DFF were thermally treated in air in a closed muffle furnace, internal volume 9 L. The temperature was determined by a thermocouple located at the top of the reaction vessel; isothermal temperature control was ( 3 °C. Oxygen (25 mmol) was present in excess. The kinetics of the model mixtures was followed by determining the TOC decrease in time; that of DFF was followed by determining the weight decrease of the sample. Experimental temperatures ranged from 40 to 150 °C. As also previously reported (5), no TOC variation was observed in preliminary runs performed on raw, TT, or SFE fly ash at 200 °C for 1500 min. So, the TOC decrease in time was due to DF volatilization. Details of TOC variation in time at different temperatures (expressed as TOCDF0, i.e., as the difference between the total and the support TOCs) for the about 50 overall kinetic runs are available as Supporting Information. All the reported TOC values are the mean value of a triplicate measure; relative error does not exceed 3%. Kinetic Data Processing. In any case, the TOC values, i.e., DF concentration, vs time data followed a first-order kinetics. The calculated rate constants are reported in Tables 792
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2-5; to estimate the goodness of fit, the degree of freedom (df), the parameters standard errors (σ), and the determination coefficients (R2) are also reported. The activation and thermodynamic parameters calculated using Arrhenius and Eyring equations are reported in Tables 6 and 7; the goodness of fit is evaluated on the basis of the standard deviation of the least-squares regression parameters as well as the determination coefficients. To verify whether two activation or thermodynamic parameters were significantly different, a statistical procedure for comparing slopes and intercepts of the regression lines was applied at 95% confidence level (26).
Discussion The first goal of this work was to compare supports from TT and SFE with the real matrix. The structural and morphological characteristics of raw fly ash and derived model supports were investigated by BET, SEM, EDS, and Fajans’ analyses. The results of the BET analysis show (Table 1) that the surface area, the monolayer volume, and the total pore volume of TTFA4 and TTFA5 are smaller than those of FA4 and FA5 (negative % variation in Table 1). Moreover, the TTFA surface area is close to the instrumental limit of detection (0.5 m2/g); all the samples have a substantially nonporous structure. As the experimental total pore volume
FIGURE 4. EDS qualitative analysis of (a) FA4, (b) TTFA4, and (c) SFEFA4. FIGURE 3. SEM micrographs of FA5, TTFA5, and SFEFA5 at a magnification level of 2000. is very low too, it seems possible to conclude that the volume measured is due to the interstitial zones rather than to the structural pores. It has already been shown (27) that the thermal treatment removes the organic compounds and the native carbon from raw fly ash; these losses together with the related thermal shock cause a significant packing of the bulk structure with a subsequent decrease of the interstitial areas. On the contrary, the surface area, the monolayer volume, and the total pore volume of SFEFA4 and SFEFA5 are greater than those of FA4 and FA5 (positive % variation in Table 1): the increase in total pore volume may be due to the removal of the extractable organic compounds from the raw fly ash, while the structure seems to remain substantially unchanged, as shown by the small increase in surface area and monolayer volume. The analysis of the FA pore size distribution (see Figure 1 for FA4 experimental data) shows that most pores are mesopores with about 3 nm diameters. There is a monotone fall in pore volume as diameter increases. As shown in Figures 2 and 3, raw FA as well as TTFA and SFEFA samples mainly consist of fine particles, usually smaller than 2 µm. However, while the morphology of the SFEFA appears to be substantially the same as that of raw fly ash, the TTFA samples are more compact.
EDS analysis (Figure 4) indicates that the raw and SFE fly ash surface is rich in chlorine, calcium, potassium, sulfur, silicium, and some metals (Zn, Al, Au, Ca, Ti, Fe, Cu). Moreover, thermogravimetric analysis (27) has shown that sublimation processes are already active at a temperature close to that of the thermal treatment. Thus, all our characterizations clearly indicate that the SFE treatment does not significantly change the raw fly ash structure or morphology. The model support obtained is very close to raw fly ash and thus more representative of the real samples in comparison with the TT one. The second goal of this work was to investigate the effect of different model supports on dibenzofuran reactivity. To this purpose, a kinetic study of the thermal behavior of DF supported on raw, TT, and SFE fly ash as well as of DF not supported on a matrix, i.e., DFF, was performed and compared to previously reported results on the DF-SiO2 system (10). GC/MS analysis of the reaction products shows that, in any case, the only product is the reagent itself found in the organic solvent. During the reaction, DF is thus removed from the solid mixture without any chemical change: the overall reaction is a physical change from the solid phase to the gas phase and follows a first-order kinetics (Tables 2-5). Theoretically, the transformation can be either a sublimation or a solid-liquid phase transition followed by evaporation, i.e., a two-step process. The DF 85 °C melting point and 273 °C boiling point suggest that the latter cannot take place in the temperature range explored. VOL. 36, NO. 4, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 5. EDS chlorine analysis of FA4 (a) before and (b) after the leaching test.
TABLE 2. Calculated Rate Constants for DF-FA4 (kFA4) and DF-FA5 (kFA5) Mixturesa DF-FA4
T (°C)
kFA4 ( σ
(min-1)
40 (4.7 ( 0.2) × 10-4 45 (6.4 ( 0.2) × 10-4 50 (1.01 ( 0.02) × 10-3 60 (1.5 ( 0.1) × 10-3 75 (2.02 ( 0.07) × 10-3 87 (3.4 ( 0.1) × 10-3 100 (4.9 ( 0.2) × 10-3 120 (8.8 ( 0.2) × 10-3
TABLE 4. Calculated Rate Constants for DF-SFEFA4 (kSFE4) and DF-SFEFA5 (kSFE5) Mixturesa
DF-FA5 df 7 8 8 7 7 7 6 4
R
2
kFA5 ( σ
(min-1)
0.981 (1.04 ( 0.07) × 10-3 0.989 0.996 (2.5 ( 0.1) × 10-3 0.950 (4.6 ( 0.3) × 10-3 0.991 (1.21 ( 0.08) × 10-2 0.994 0.994 (4.1 ( 0.3) × 10-2 0.998 (8.3 ( 0.7) × 10-2
DF-SFEFA4 df
R
2
6 0.976 6 0.990 8 0.976 6 0.976 4 0.985 2 0.984
a Standard errors (σ), degrees of freedom (df), and determination coefficients (R 2) are also reported.
TABLE 3. Calculated Rate Constants for DF-TTFA4 (kTT4) and DF-TTFA5 (kTT5) Mixturesa DF-TTFA4
DF-TTFA5
T (°C)
kTT4 ( σ (min-1)
df
40 45 50 55 60 75
(1.38 ( 0.06) × 10-2 (2.1 ( 0.1) × 10-2 (3.3 ( 0.1) × 10-2 (6.5 ( 0.4) × 10-2 (1.2 ( 0.1) × 10-1
4 4 4 3 2
R2
kTT5 ( σ (min-1)
0.991 (3.8 ( 0.2) × 10-3 0.981 0.992 (8.0 ( 0.4) × 10-3 0.987 0.977 (2.10 ( 0.08) × 10-2 (6.2 ( 0.1) × 10-2
df
R2
7 0.989 7 0.981 5 0.993 3 0.999
kSFE4 ( σ
T (°C) 40 45 50 60 75 87 100 120 150
(min-1)
(1.10 ( 0.03) × 10-3 (1.48 ( 0.05) × 10-3 (1.57 ( 0.03) × 10-3 (2.1 ( 0.1) × 10-3 (4.5 ( 0.2) × 10-3 (9.3 ( 0.3) × 10-3 (3.26 ( 0.07) × 10-2 (5.8 ( 0.1) × 10-2 (9.4 ( 0.9) × 10-2
DF-SFEFA5 df 7 7 8 7 7 5 3 2 1
R
2
kSFE5 ( σ (min-1)
0.994 (1.70 ( 0.01) × 10-3 0.993 0.996 (2.8 ( 0.1) × 10-3 0.982 (3.2 ( 0.1) × 10-3 0.980 (9.0 ( 0.5) × 10-3 0.996 0.999 (1.44 ( 0.04) × 10-2 0.999 (7 ( 1) × 10-2 0.991 (1.3 ( 0.1) × 10-1
df
R2
6 0.980 7 0.990 7 0.991 6 0.983 5 0.995 2 0.908 1 0.994
a Standard errors (σ), degrees of freedom (df), and determination coefficients (R 2) are also reported.
TABLE 5. Calculated Rate Constants for Free Dibenzofuran (kDF)a T (°C)
kDF ( σ (min-1)
df
R2
57 68 75 87 100 125
(4.2 ( 0.2) × 10-3 (7.5 ( 0.4) × 10-3 (8.7 ( 0.3) × 10-3 (1.8 ( 0.1) × 10-2 (3.1 ( 0.1) × 10-2 (1.17 ( 0.08) × 10-1
7 3 7 3 5 3
0.983 0.993 0.990 0.989 0.989 0.985
a Standard errors (σ), degrees of freedom (df), and determination coefficients (R 2) are also reported.
a Standard errors (σ), degrees of freedom (df), and determination coefficients (R2) are also reported.
DF reactivity can be discussed by comparing the rate constants both on analogous or on different model supports. As also seen in Figure 6, where the Arrhenius plots are reported, the calculated k ratios at 83 °C of a model system named 4 to the corresponding one named 5 are smaller than 1 for raw (k(DF-FA4)/k(DF-FA5) ) 0.22) and supercritically extracted (k(DF-SFEFA4)/k(DF-SFEFA5) ) 0.76) fly ash whereas it is
greater than 1 for thermally treated fly ash (k(DF-TTFA4)/ k(DF-TTFA5) ) 7.7). To compare the reactivities of different systems, we chose DF-SiO2 as a reference; the rate constant ratios to k(DF-SiO2) are calculated at 83 °C, which is the temperature at which the two straight lines in the piecewise Arrhenius plot of the latter system cross (10). The ratios are about 380 and 50 for DF-TTFA4 and DF-TTFA5, respectively, whereas the values are in the 2-8 range for the other systems,
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TABLE 6. Activation (Using Arrhenius Equation) and Thermodynamic (Using Eyring Equation) Parameters for DF-FA4, DF-FA5, DF-TTFA4, and DF-TTFA5 Mixtures parameter (A/min-1)
ln Ea (kJ mol-1) R2 ∆Sq (kJ K-1 mol-1) ∆Hq (kJ mol-1) R2 ∆Gq (kJ mol-1)a % ∆Sq a
DF-FA4
DF-FA5
DF-TTFA4
DF-TTFA5
6.2 ( 0.6 36 ( 2 0.986 -(0.235 ( 0.004) 33 ( 1 0.990 121 ( 2 72.4
12 ( 1 50 ( 3 0.976 -(0.166 ( 0.006) 53 ( 2 0.994 115 ( 3 53.9
31 ( 2 93 ( 6 0.986 -(0.03 ( 0.02) 90 ( 6 0.985 100 ( 10 9.9
23 ( 1 73 ( 3 0.997 -(0.101 ( 0.009) 70 ( 3 0.996 108 ( 4 34.9
∆Gq at an average temperature of 373 K.
TABLE 7. Activation (Using Arrhenius Equation) and Thermodynamic (Using Eyring Equation) Parameters for DF-SFEFA4 and DF-SFEFA5 Mixtures, Free Dibenzofuran, and DF-SiO2 Mixturesa DF-SiO2 parameter
DF-SFEFA4
DF-SFEFA5
DF free
ln (A/min-1) Ea (kJ mol-1) R2 ∆Sq (kJ K-1 mol-1) ∆Hq (kJ mol-1)
12 ( 1 49 ( 3 0.969 -(0.190 ( 0.009) 46 (3 0.965 117 ( 5 60.7
11 ( 1 45 ( 4 0.969 -(0.20 ( 0.01) 42 ( 4
14 ( 1 53 ( 3 0.990 -(0.174 ( 0.008) 50 ( 3
R2 ∆Gq (kJ mol-1)d % ∆Sq a
Ref 10.
b
0.965 116 ( 5 63.9
0.988 115 ( 4 56.5
Low-temperature range, LTR. c High-temperature range, HTR.
d
50-75
°Cb
28 ( 4 101 ( 10 0.979 -(0.05 ( 0.03) 98 ( 10 0.978 118 ( 16 17.2
75-200 °Cc 2.1 ( 0.4 25 ( 1 0.986 -(0.273 ( 0.003) 22 ( 1 0.981 123 ( 3 82.5
∆Gq at an average temperature of 373 K.
FIGURE 6. Arrhenius plots for (a) DF-FA4 (FA4), (b) DF-FA5 (FA5); (c) DF-TTFA4 (TTFA4), (d) DF-TTFA5 (TTFA5), (e) DF-SFEFA4 (SFE4), (f) DF-SFEFA5 (SFE5), (g) DF-SiO2 (LT-SI, LTR, and HT-SI, HTR) mixtures and for (h) DF free (DF). DFF included. Thus, the reactivity confirms that, unlike the thermal treatment, SFE extraction does not significantly change the raw fly ash structure or morphology. To identify the operating steps of the reaction mechanism, the dependence of the rate constants from the temperature was determined (Tables 6 and 7). The basically constant value of the molar free energy of activation (∆Gq) confirms that there is only one operating reaction i.e., the DF phase transition. The ∆Gq is mainly determined by (i) the ∆Hq, the contribution of T∆Sq being lower than 30%. This is the case
of DF-TTFA4, DF-TTFA5, and DF-SiO2 in the LTR systems: the ∆Sq between -0.03 and -0.1 kJ K-1 mol-1 suggests a small structural change from the reagent to the transition state; ∆Hq in the 70-90 kJ mol-1 range seems to indicate a sublimation (∆Hsub ) 85.63 kJ mol-1 for DF (10)) or (ii) the ∆Sq, with a T∆Sq contribution over 80%. This is the case of DF-SiO2 system in the HTR: the ∆Sq ) -0.273 kJ K-1 mol-1 suggests a great structural change along the reaction coordinate toward the transition state, probably a solid-gas phase transition (∆Ssub ) 0.279 kJ mol-1 for DF (10)); the ∆Hq VOL. 36, NO. 4, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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) 22 kJ mol-1 indicates a subsequent diffusion step. The behavior of the other model systems is midway between these two extremes. The different behavior of the DF-SiO2 system in the LTR and HTR can be interpreted as follows: (i) in the LTR, the decrease of DF initial concentration (lower than 50%; see Table 3 in ref 10) is due to the sublimation of the reagent from the silica surface and the activation enthalpy is mainly needed for sublimation; (ii) in the HTR, the reagent sublimates inside the pores (rate-determining step) and then diffuses toward the support surface. This interpretation partially amends the previous one (10), but it is consistent with the new experimental data reported in Table 1. Because of the low TTFA surface area (average value, 0.63 m2 g-1) and total pore volume (43 cm3 g-1), the activation and thermodynamic parameters for TTFA systems, similar to that of DF-SiO2 in the LTR, can be explained with the reasonable hypothesis that all DF is on the TTFA surface. The increase of average FA and SFEFA surface area (3.1 ( 0.4 m2 g-1) and total pore volume (133 ( 24 cm3 g-1) with respect to TTFA and the comparison with SiO2 surface area (465 m2 g-1) and total pore volume (165 cm3 g-1, with an average pore diameter of 6 nm) lead us to conclude that the activation and thermodynamic parameters for DF-FA and DF-SFEFA systems indicate a progressive change from mechanism i to mechanism ii. All the characterizations indicate that the SFE treatment does not significantly change the raw fly ash structure or morphology. The matrix obtained is very close to raw fly ash from the structural, physical-chemical, and kinetic points of view and can be used to study PCDD/F formation from fly ash native carbon without interference of extractable organic compounds.
Acknowledgments We thank Massimo Ferri for his valuable work in the laboratory, the Fondazione Lombardia per l’Ambiente, and the Italian National Research Council for financial support (Grant CNR99.00022.MZ79).
Supporting Information Available Listing of TOC vs time data at different temperatures for FA4, FA5, TTFA4, TTFA5, SFEFA4, SFEFA5, and DF samples (Tables S1-S7). This material is available free of charge via the Internet at http://pubs.acs.org.
Literature Cited (1) Baukal, C. E. Heat Transfer in Industrial Combustion; CRC Press: Boca Raton, FL, 2000.
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Received for review June 21, 2001. Revised manuscript received November 6, 2001. Accepted November 20, 2001. ES015575Q
