Anal. Chem. 2000, 72, 3916-3921
Supercritical Fluid Extraction and Accelerated Solvent Extraction of Dioxins from High- and Low-Carbon Fly Ash I. Windal,† D. J. Miller,‡ E. De Pauw,† and S. B. Hawthorne*,‡
Mass Spectrometry Laboratory, University of Liege, B6c Sart-Tilman, B-4000 Liege, Belgium, and Energy and Environmental Research Center, University of North Dakota, Box 9018, Grand Forks, North Dakota 58202
This study investigates the replacement of Soxhlet extraction by supercritical fluid extraction (SFE) or accelerated solvent extraction (ASE) for the removal of dioxins from municipal waste incinerator fly ash. SFE is very matrix dependent; higher percent recoveries versus Soxhlet extraction can be obtained for low-carbon-level fly ash, but only a few percent of dioxins can be extracted from highcarbon-level fly ash. The addition of large quantities of toluene in the extraction cell prior to extraction of highcarbon fly ash improves the recovery of the lowest chlorinated dioxins (∼90%), but a maximum of 20% of the octachlorodibenzo-p-dioxins can be extracted. Since large quantities of toluene are needed to improve the recoveries, ASE with toluene was tested. Recoveries similar to Soxhlet extraction can be obtained in 2 h at 80 °C. Increasing the temperature to 150 °C increases the extraction rate and yields recoveries of ∼110-160% compared to 48-h Soxhlet extraction for all congeners for both low- and high-carbon fly ashes. These results question the choice of Soxhlet extraction as a reference method for dioxin determination. For several years, different analytical techniques have been developed to replace classical extraction techniques in an effort to reduce time and solvent consumption. Among the different techniques, supercritical fluid extraction (SFE) has several advantages including selectivity and reduction of time and of organic solvent consumption.1-7 The replacement of Soxhlet extraction by SFE for the removal of polychlorodibenzo-p-dioxins (PCDDs or dioxins) from fly ash has been investigated in our laboratory and results are described in a previous paper.8 * Corresponding author: (e-mail)
[email protected]. † University of Liege. ‡ University of North Dakota. (1) Janda, V.; Bartle, K. D.; Clifford, A. A. J. Chromatogr. 1993, 642, 283-299. (2) Chester, T. L.; Pinkston, J. D.; Raynie, D. E. Anal. Chem. 1998, 70, 301R319R. (3) Chester, T. L.; Pinkston, J. D.; Raynie, D. E. Anal. Chem. 1996, 68, 487R514R. (4) Stuart, I. A.; MacLachlan, J.; McNaughtan, A. Analyst 1996, 121, 11R28R. (5) Hawthorne, S. B.; Miller, D. J.; Burford, M. D.; Langenfeld, J. J.; EckertTilotta, S. J. Chromatogr. 1993, 642, 301-317. (6) McNally, M. E. P. Anal. Chem. 1995, 67, 308A-315A. (7) Camel, V.; Tambute, A.; Claude, M. J. Chromatogr. 1993, 642, 263-281.
3916 Analytical Chemistry, Vol. 72, No. 16, August 15, 2000
For the extraction of dioxins from fly ash, SFE is very matrix dependent, and high recoveries (e.g., 140% versus Soxhlet extraction) can be obtained as well as very low recoveries, especially for fly ash with high carbon content. When activated carbon is used in conjunction with lime for fume purification in the incinerator, the carbon content of fly ash is in the range 8-12%. When only lime is used for the fume purification, the carbon content is about 1-3%. SFE is very efficient for the low-carboncontent fly ash samples, but with the same conditions, recoveries of dioxins extracted from high-carbon-content fly ash are very low.8 In this study, attention is focused on one kind of fly ash (described as fly ash C in the previous paper), which contains 8.4% carbon. Different conditions are tested to improve the recovery with SFE and to understand extraction mechanisms. Since high quantities of toluene are required in SFE to increase the recovery, accelerated solvent extraction (ASE) using toluene was chosen as an alternative extraction method. ASE is a recent method that uses an organic solvent at relatively high pressure and temperatures above the boiling point.9-16 Despite the increasing use of this method in laboratories, there are still few systematic investigations of the effect of the different parameters: temperature, pressure, and flow rate. A complete study of ASE of dioxins from fly ash is presented here. The most abundant isomers of tetrachlorodibenzo-p-dioxins (TCDD or tetrachlorodioxins), pentachlorodibenzo-p-dioxins (PeCDD or pentachlorodioxins), hexachlorodibenzo-p-dioxins (HxCDD or hexachlorodioxins), heptachlorodibenzo-p-dioxins (HpCDD or heptachlorodioxins), and octachlorodibenzo-p-dioxins (OCDD or octachlorodioxins) were determined. The method was optimized with one kind of fly ash characterized by a high content of carbon and tested on five (8) Windal, I.; Eppe, G.; Gridelet, A.-C.; De Pauw, E. J. Chromatogr., A 1998, 819, 187-195. (9) Richter, B. E.; Jones, B. A.; Ezzel, J. L.; Porter, N. L.; Avdalovic, N.; Pohl, C. Anal. Chem. 1996, 68, 1033-1039. (10) Bautz, H.; Polzer, J.; Stieglitz, L. J. Chromatogr., A 1998, 815, 231-241. (11) Schantz, M. M.; Nichols, J. J.; Wise, S. A. Anal. Chem. 1997, 69, 42104219. (12) Heemken, O. P.; Theobald, N.; Wenclawiak, B. W. Anal. Chem. 1997, 69, 2171-2180. (13) Lou, X.; Janssen, H.-G.; Cramers, C. A. Anal. Chem. 1997, 69, 1598-1603. (14) Saim, N.; Dean, J. R.; Abdullah, M. P.; Zakaria, Z. Anal. Chem. 1998, 70, 420-424. (15) Vandenburg, H. J.; Clifford, A. A.; Bartle, K. D.; Zhu, S. A. Anal. Chem. 1998, 70, 1943-1948. (16) Saim, N.; Dean, J. R.; Abdullah, M. P.; Zakaria, Z. J. Chromatogr., A 1997, 791, 361-366. 10.1021/ac9914972 CCC: $19.00
© 2000 American Chemical Society Published on Web 07/14/2000
other fly ash samples containing different amounts of activated carbon. EXPERIMENTAL SECTION Fly Ash Samples. Fly ash samples were collected at the bottom of the electrostatic precipitator at different municipal waste incinerators (using different fume purification systems) and had a carbon content ranging from 1.6 to 12.7%. The carbon content was determined by total combustion. Pretreatment. The pretreatment of fly ash by HCl greatly improves the efficiency of dioxin extraction,17 and all fly ash samples were pretreated for 2 h with 1 M HCl (8 mL of HCl/g of fly ash, with magnetic stirring). Fly ash was separated from the HCl solution by centrifugation at 3000 rpm for 10 min and rinsed 3 times with freshwater (separated by centrifugation). The fly ash was then dried overnight in an oven at 30 °C and stored in a closed vessel. To facilitate comparison, a sufficient quantity of fly ash was prepared to allow Soxhlet extraction, SFE, and ASE with the same sample. Optimization of the SFE and ASE Parameters. SFE was carried out with an Isco model 210D pump and SFX-210 extractor (Lincoln, NE). Two grams of fly ash was mixed with 2 g of prewashed sea sand and placed in the extraction cell (10 mL). The flow rate of the CO2, modified by 10% toluene (SFE grade in a premixed cylinder from Scott Specialty Gases, Plumsteadville, PA), was adjusted to 1 mL/min with an Isco coaxially heated adjustable capillary restrictor set to 80 °C. The extractor temperature was set at 150 °C and the pressure at 400 bar for all SFE experiments. Either 1 or 7 mL of toluene (Optima grade, Fisher Scientific, Pittsburgh, PA) was added in the cell prior to extraction, and a static extraction of 1 or 3 h was performed prior to a 30min dynamic extraction. Extracted analytes were collected by placing the outlet of the restrictor into a 15-mL vial containing 10 mL of toluene. ASE was performed in the same way using the same extractor, with the pump filled with toluene instead of CO2. The extracting toluene was collected in a vial, prefilled with 10 mL of toluene, and placed in an ice bath. Temperature and pressure were set depending on the experiments, with the restrictor temperature at 80 °C. For kinetic measurements, the flow rate was set at 1.0 or 0.33 mL/min at the pump and the pressure adjusted to 400 bar by controlling the restrictor valve. For both SFE and ASE development, the extract was spiked after the extraction with 20 µL of a solution containing 3.3 µg/ mL tetrabromobiphenyl (AccuStandard Inc., New Haven, CT) and 44 µg/mL hexabromobiphenyl (AccuStandard Inc.) in toluene. The extract was concentrated to 100 µL under reduced pressure using a rotary evaporator. Triplicate Soxhlet extractions were performed on replicate 10-g samples of fly ash. The extracts were spiked with 100 µL of the solution containing 3.3 µg/mL tetrabromobiphenyl and 44 µg/mL hexabromobiphenyl in toluene, used in SFE and ASE experiments. The ratios between GC/MS peak areas for dioxins and for polybromobiphenyls for Soxhlet extracts were used to define the 100% recovery. For validation of the method, ASE was performed using the optimized conditions: 150 °C, 50 bar, flow of 1 mL/min, and 1 or (17) Kooke, R. M. M.; Lustenhouwer, J. W. A.; Olie, K.; Hutzinger, O. Anal. Chem. 1981, 53, 461-463.
2 h of dynamic extraction. Prior to extraction, 10 µL of a standard solution of 2,3,7,8-chloro-substituted 13C12-labeled dioxins (U.S. EPA 1613 Labeled Compounds Solution, Campro Scientific, Veenendaal, The Netherlands) was added in the extraction cell on the surface of the fly ash. The internal standard solution contained 13C12-labeled dioxins 2,3,7,8 TCDD; 1,2,3,7,8 PeCDD; 1,2,3,4,7,8 HxCDD; 1,2,3,6,7,8 HxCDD and 1,2,3,4,6,7,8 HpCDD at a concentration of 100 ng/mL and OCDD at a concentration of 200 ng/mL. The extract was concentrated to 100 µL by rotary evaporation and analyzed by high-resolution GC/high-resolution MS (HRGC/HRMS). Results were compared to those obtained by 48-h Soxhlet extraction with toluene. GC/MS Analysis. (A) At Low Resolution. Extracts were analyzed with a Hewlett-Packard model 5973 GC/MS. Chromatographic separations were performed using a 30-m × 0.25-mm-i.d. HP-5MS (0.25 mm film thickness) column. Two microliters of the extract was injected in the splitless mode at an injector temperature of 300 °C. Temperature programming was 90 °C, held 0.5 min, increased to 200 °C at 40 °C/min, increased to 268 °C at 1.2 °C/min, increased to 320 °C at 20 °C/min, and then held for 2 min. Helium was used as carrier gas and the MS transfer line temperature was 280 °C. The mass spectrometer was operated in the electron impact ionization mode (70 eV) using selected ion monitoring and a source temperature of 230 °C. The two most abundant ions in the chlorine clusters of the molecular ion were recorded for each isomer of TCDD, PeCDD, HxCDD, HpCDD, and OCDD. Two ions of the tetrabromobiphenyl and two ions of the hexabromobiphenyl were also monitored. The ratio between the two ions of each isomer was used to confirm the identity of dioxins or polybromobiphenyls. A 20% deviation compared to the theoretical ratio was accepted. The following ions were monitored: m/z 319.8, 321.8, 467.7, and 469.7 from 9 to 17 min; m/z 355.8 and 357.8 from 17 to 25 min; m/z 389.8 and 391.8 from 25 to 34 min, m/z 423.8, 425.8, 625.5, and 627.5 from 34 to 44 min, and m/z 457.8 and 459.8 from 44 to 64 min. (B) At High Resolution. High-resolution GC/high-resolution MS analysis and the identification and quantification of dioxins are described in detail elsewhere.8 Briefly, HRGC/HRMS was performed using a VG-AutoSpec-Q high-resolution mass spectrometer (Fison Instruments, Manchester, U.K.) and a HewlettPackard (Palo, Alto, CA) 5890 series II gas chromatograph equipped with an SP 2331 capillary column (Supelco, Bellefonte, PA), 60-m × 0.25-mm i.d., 0.2-µm film thickness and, in some cases, a DB-5MSTID capillary column (J&W Scientific, Alltech Europe, Belgium), 30-m × 0.25-mm i.d., 0.25-µm film thickness, for the analysis of HpCDD and OCDD. The MS was tuned to a minimum resolution of 10 000 (10% valley) and was operated in a mass drift correction mode using perfluorokerosene to provide lock mass. RESULTS AND DISCUSSION Methods were developed using a representative fly ash, which had a high carbon content (8.4%), described as fly ash C in the previous study8 and in Table 3 of this paper. Recovery was defined based on triplicate 48-h Soxhlet extractions of 10-g samples. SFE. The SFE conditions of temperature (150 °C) and pressure (400 bar) were chosen based on previous results.8 Toluene was used as modifier because it is the best solvent for Analytical Chemistry, Vol. 72, No. 16, August 15, 2000
3917
Table 1. Effect of SFE (CO2/10% Toluene Modifier, 150 °C, 400 bar, Flow Rate of 1 mL/min) Conditions on the Recovery of Dioxins % recovery SFE vs Soxhlet (% RSD)a replicate addition of toluene
compdb
48 h Soxhlet concn, ng/g
1 mL toluenec 1 h staticd 30 min dyne
TCDD TCDD PeCDD PeCDD PeCDD HxCDD HxCDD HxCDD HxCDD HxCDD HpCDD HpCDD OCDD
4.5 (3) 2.5 (2) 12.2 (3) 8.3 (4) 7.4 (3) 6.4 (5) 36.2 (2) 24.3 (6) 8.4 (8) 6.4 (10) 45.3 (5) 61.8 (5) 77.6 (25)
25 (14) 17 (10) 10 (17) 8 (14) 3 (89) ndg 3 (18) 2 (19) ndg ndg 1 (44) 1 (44) ndg
7 mL toluenec 3 h staticd 30 min dyne
1st extrn, 7 mL toluenec 1 h staticd 30 min dyne
96 (5) 91 (8) 89 (7) 79 (10) 64 (6) 56 (6) 46 (6) 45 (6) 38 (19) 20 (26) 25 (8) 19 (16) 10 (26)
86 (7) 84 (8) 81 (9) 74 (9) 57 (2) 50 (7) 46 (3) 44 (3) 36 (8) 18 (13) 28 (10) 20 (7) 10 (3)
2nd extrn, 7 mL toluenec 1 h staticd 30 min dyne
3rd extrn, 7 mL toluenec 1 h staticd 30 min dyne
10 (44) 15 (36) 20 (27) 22 (21) 26 (9) 20 (9) 22 (9) 22 (9) 22 (4) 11 (6) 18 (11) 14 (14) 7 (14)
2 (87) 2 (173) 5 (34) 7 (33) 13 (31) 11 (36) 13 (29) 15 (32) 17 (32) 10 (23) 16 (21) 13 (17) 8 (8)
cumul % rec, × 7 mL toluenec 3 × 1 h staticd 3 × 30 min dyne
f3
98 (9) 100 (9) 106 (8) 102 (8) 95 (8) 81 (9) 81 (6) 81 (7) 75 (7) 40 (11) 62 (10) 47 (14) 25 (13)
a Percent recovery of dioxins by SFE versus Soxhlet. Relative standard deviation (% RSD) are based on triplicate experiments. b The most abundant isomers of each family were determined by low-resolution GC/MS as described in the text. c Quantity of toluene added in the extraction cell prior to SFE. d Static SFE time. e Dynamic SFE time. f Sum of the percent recoveries of the three consecutive SFE. g nd: not detected.
the Soxhlet extraction of dioxins from fly ash.17,18 In addition, the carbon present in fly ash has an aromatic structure and is mainly responsible for the strong adsorption of dioxins (compared to other fly ashes with a low carbon content). Toluene has an aromatic, planar structure, similar to that of dioxins, and competes very well for the matrix adsorption sites. In dioxin analysis, toluene is the best eluant to recover dioxins when activated carbon is used as adsorbent for liquid chromatography during the sample cleanup.19,20 Because of these properties, the CO2 was modified with 10% toluene for all SFE experiments. As shown in Table 1, SFE using toluene-modified CO2 did not yield high recoveries from the high-carbon fly ash, in contrast to the high recoveries previously reported for low-carbon fly ash.8 Several attempts were made to increase the recovery of dioxins by SFE, including adding a longer static extraction time (3 h) before the 30-min dynamic extraction, adding 1 or 7 mL of toluene in the cell before the static extraction as proposed by Friedrich et al.,21,22 and multiple extractions of the same sample. With these conditions, the recovery of the lowest chlorinated dioxins is greatly increased (90% for tetrachlorodioxins). Unfortunately, the extraction of most chlorinated dioxins remains too low under all SFE conditions tested (Table 1). ASE. Since larger quantities of toluene during SFE yielded better recoveries, ASE with pure toluene was tested. Optimization of Temperature and Pressure Parameters. The effect of ASE temperature was investigated at 80 °C, which is approximately the temperature of Soxhlet extraction with toluene, and 150 °C, which is the maximum temperature available (18) EPA Method 8280. In Test methods for evaluating solid waste physical/chemical methods-third edition proposed update package, U.S. EPA, Washington, DC, 1989. (19) van Bavel, B.; Jaremo, M.; Karlsson, L.; Lindstrom, G. Anal. Chem. 1996, 68, 1279-1283. (20) European Standard: EN 1948-2, 1996. (21) Friedrich, C.; Kleibohmer, W. J. Chromatogr., A 1997, 777, 289-294. (22) Deuster, R.; Lubahn, N.; Friedrich, C.; Kleibohmer, W. J. Chromatogr., A 1997, 777, 227-238.
3918 Analytical Chemistry, Vol. 72, No. 16, August 15, 2000
Figure 1. Recovery of dioxins extracted from fly ash by ASE at 400 bar with 1 mL/min toluene. (a) 80 (b) 150 °C. Recoveries are based on triplicate 2-h extractions at each condition. 100% recovery is defined by 48-h Soxhlet extractions with toluene.
with our extractor. Panels a and b of Figure 1 show recoveries achieved for the same high-carbon fly ash (discussed above) versus Soxhlet as a function of time for ASE performed at 80 and 150 °C, respectively, at a flow rate of 1.0 mL/min. At 80 °C, the extraction is much slower than at 150 °C and recovery after 2 h is ∼100% for tetra-, penta-, and hexachlorodioxins and lower for hepta- and octachlorodioxins (90 and 74%) (Table 2). Increasing the temperature to 150 °C not only accelerates the extraction but the recovery of all isomers is increased, about 130-145% for tetrato heptachlorodioxins and 180% for octachlorodioxins (Table 2), compared to 48-h Soxhlet extraction.
Table 2. Effect of ASE Conditions on the Recovery of Dioxins from High-Carbon-Level Fly Ash 80 °C, 400 bar, 2 h
150 °C, 400 bar, 2 h
150 °C, 50 bar, 2 h
compda
% rec vs Soxhletb
KDc
% rec vs Soxhletb
KDc
% rec vs Soxhletb
TCDD TCDD PeCDD PeCDD PeCDD HxCDD HxCDD HxCDD HxCDD HxCDD HpCDD HpCDD OCDD
101 (4) 103 (5) 101 (4) 103 (4) 99 (3) 103 (6) 106 (5) 103 (5) 102 (6) 89 (7) 93 (4) 87 (4) 74 (11)
6 6 9 9 11 15 15 15 22 33 29 35 62
137 (2) 135 (3) 131 (4) 136 (3) 130 (2) 149 (4) 145 (3) 140 (4) 152 (2) 146 (2) 145 (2) 148 (2) 179 (1)
1 1 2 2 2 2 2 2 2 3 4 5 9
133 (7) 134 (8) 131 (7) 137 (7) 133 (7) 146 (7) 142 (5) 140 (5) 149 (6) 145 (6) 146 (4) 145 (3) 182 (3)
a The most abundant isomers of each family were determined by low-resolution GC/MS as described in the text. b Percent recovery of dioxins by ASE (80 or 150 °C; 50 or 400 bar; solvent, toluene; flow rate, 1 mL/min; 2 h) versus 48-h Soxhlet extractions. Relative standard deviations (% RSD) are based on triplicate experiments. c KD values (fly ash/toluene partitioning coefficients) were determined as described in the text.
Figure 3. Comparison of ASE extraction rates with the liquid-liquid extraction model: extraction of (a) tetrachlorodioxin, (b) hexachlorodioxin, and (c) octachlorodioxin at 80 and 150 °C. ASE was performed with toluene at 400 bar and a flow rate of 1 mL/min. Recoveries are based on triplicate 2-h extractions at each temperature. 100% recovery is defined by 48-h Soxhlet extractions with toluene.
Figure 2. Extraction of dioxins by ASE (150 °C, 400 bar) with different flow rates: (a) tetrachlorodioxin; (b) octachlorodioxin. Recoveries are based on triplicate 2-h extractions at each condition. 100% recovery is defined by 48-h Soxhlet extractions with toluene.
The effect of pressure at 150 °C was also investigated at 50 and 400 bar (Table 2). None of the recoveries were significantly different, indicating that the primary effect of pressure is to maintain the solvent in the liquid state during extraction. Effect of Flow Rate and Temperature on the Extraction Rate: Mechanistic Considerations. Investigations of the ASE extraction mechanism were performed to determine whether the limitation for the extraction is of a thermodynamic or a kinetic nature. The identification of the slow step of the extraction can be based on the effect of flow rate on the extraction rate as proposed earlier for SFE.23 In that report, the extraction process
was divided into two steps: the first step is described by kinetic SFE models and consists of the desorption of the analyte from the initial adsorption site and its transfer into the extraction fluid. This first step can be considered irreversible because the bond to this adsorption site can only be established after aging of the sample, or during the sample formation. If the extraction is limited by this desorption/kinetic step (including desorption and diffusion of the analyte through the matrix pores to the extraction solvent), increasing the flow rate has little effect on the extraction rate, as long as sufficient extraction fluid is used to maintain concentrations of the solutes that are well below saturation in the extractant fluid.23 The second step of the extraction is the elution of the analyte out of the cell. The elution can be compared to classical chromatographic elution, with a partitioning of the analytes between the extracting fluid and adsorption sites easily accessible (23) Hawthorne, S. B.; Galy, A. B.; Schmitt, V. O.; Miller, D. J. Anal. Chem. 1995, 67, 2723-2732.
Analytical Chemistry, Vol. 72, No. 16, August 15, 2000
3919
Table 3. Comparison of 48-h Soxhlet Extraction and 1-h ASE for the Extraction of the Toxic 2,3,7,8-Chloro-Substituted Dioxins from Different Kinds of Fly Ash Samples 2,3,7,8-chloro-substituted congener
fly ash C: 8.4% C Soxhlet, pg/gb ASE, % recc fly ash D: 12.7% C Soxhlet, pg/gb ASE, recc fly ash T: 8.5% C Soxhlet, pg/gb ASE, % recc fly ash A: 8.4% C Soxhlet, pg/gb ASE, % recc fly ash E: 2.1% C Soxhlet, pg/gb ASE, % recc fly ash F: 1.6% C Soxhlet, pg/gb ASE, % recc
total toxicity
TCDD
PeCDD
HxCDD
HpCDD
OCDD
pg of TEq/g (% RSD)
240 (6) 136 (1)
1750 (4) 130 (13)
10800 (18) 139 (12)
61900 (8) 137 (12)
77600 (9) 159 (10)
3620 (8) 5280 (8)
150 (6) 103 (11)
620 (2) 115 (4)
1630 (7) 132 (5)
8220 (11) 125 (1)
16300 (13) 153 (16)
1030 (4) 1180 (3)
440 (17) 153 (14)
3000 (6) 121 (13)
13500 (4) 149 (7)
84900 (13) 133 (16)
160000 (20) 160 (18)
5650 (6) 7480 (11)
25 (22) 109 (19)
160 (13) 120 (14)
970 (9) 119 (5)
7120 (10) 107 (7)
32900 (22) 107 (28)
360 (13) 420 (11)
67 (10) 124 (17)
260 (33) 117 (5)
2400 (21) 118 (10)
20100 (17) 106 (1)
41500 (25) 110 (5)
770 (18) 900 (2)
54 (13) 121 (22)
230 (19) 110 (12)
2150 (15) 106 (14)
17000 (13) 114 (12)
32900 (13) 102 (17)
670 (15) 740 (11)
a Total concentrations of the 2,3,7,8-substituted congeners in picograms of toxic equivalents per gram of fly ash (pg of TEq/g). Toxic equivalent factors used were tetrachloro (1), pentachloro (1), hexachloro (0.1), heptachloro (0.01), and octachloro (0.0001). Total pg of TEq/g were calculated by summing the TEq/g (i.e., the concentration of each congener multiplied by its TEF) for all 2,3,7,8-substituted congeners. b Concentrations and relative standard deviations (% RSD) are based on triplicate 48-h Soxhlet extractions. Analyses were performed by HRGC/HRMS as described in the text. c Percent recoveries compared to Soxhlet extractions were based on triplicate 1-h ASE extractions. Relative standard deviations are based on triplicate extractions. Analyses were performed by HRGC/HRMS as described in the text.
by the analytes under these conditions. The elution is governed by two parameters: (1) the solubility of the analytes in the fluids if the fluid is saturated, double the flow rate, double the quantity of fluid, and double the quantity of analytes extracted during the same time; (2) the partitioning equilibrium of the analytes between the fluid and the matrixsthe analytes undergo different readsorption/desorption steps during the elution. In this case, doubling the flow rate also doubles the extraction rate.23 In our experiments, the solubility of dioxins in toluene is on the order of milligrams per liter at ambient temperature and the concentration is on the order of nanograms per gram fly ash. The solubility is expected to increase with higher toluene temperature and is probably not a limiting factor (the maximum possible concentration in the solvent is substantially below saturation). For the rest of the discussion, the second step will be referred to as the elution step, governed only by the partitioning equilibrium. For ASE of dioxins at 150 °C (400 bar), tripling the flow rate from 0.33 to 1.0 mL/min approximately triples the extraction rate for all congeners, as shown in panels a and b of Figure 2 for tetrachloro- and octachlorodioxins, respectively. These results demonstrate that the first desorption step is rapid and that the extraction is mostly limited by the second step, the elution process (i.e., the partitioning equilibrium). A very simple model can be used to illustrate this behavior. If we consider that the extraction rate is only governed by the partitioning coefficient (i.e., the initial desorption kinetics are rapid), the extraction can be more or less compared to a classical liquid-liquid extraction. The first fraction of solvent extracts x% of the analyte. This fraction is removed and a next fraction of solvent is added, which extracts the same percentage of the remaining analyte and so forth. The value of x can be adjusted for fractions of 1 mL of toluene in ASE so that the model curve and the experimental curve coincide. For example, for ASE of tetrachlorodioxins at 150 °C and a flow rate 3920 Analytical Chemistry, Vol. 72, No. 16, August 15, 2000
of 1.0 mL/min, the value of x was adjusted to 27% so that the two curves coincide, which means that the first milliliter of toluene extracts 27% of the tetrachlorodioxins initially present in 2 g of fly ash. The second milliliter of toluene extracts 27% of the remaining tetrachlorodioxins and so forth. In this way, the value of the partitioning coefficient can also be calculated: KD ) (concentration of analyte in the fly ash)/(concentration of analyte in the toluene), which is 1.4 for tetrachlorodioxins (if concentrations are expressed in mass/mass units). The same calculation can be made for all congeners of dioxin. The experimental and model curves coincide very well for all congeners. Panels a-c of Figure 3 show results obtained for tetra-, hexa-, and octachlorodioxins at 150 °C (flow rate of 1 mL/min) as examples. The values of KD of the different congeners are summarized in Table 2. As the degree of chlorination increases, the partitioning coefficient progressively increases; therefore, octachlorodioxins are extracted more slowly than tetrachlorodioxins. Despite the very simple and approximate model used, the results confirm the interpretation of the effect of flow rate, i.e., on the extraction rate: the extraction is mostly governed by the partitioning coefficient. Mechanisms are quite different if ASE is performed at 80 °C. The simple model of liquid-liquid extraction can also be used in this case to illustrate the behavior. At 80 °C, the ASE of tetrachlorodioxins (Figure 3a) can be divided into two parts: (1) During the 10 first min, the extraction is quite fast and the experimental curve and the model curve coincide very well. This part of the extraction is, therefore, limited by the elution process as for ASE at 150 °C. However, the partitioning coefficient is greater at 80 °C (Table 2), and the ASE is ∼2 times slower at 80 °C, compared to ASE at 150 °C. (2) When about two-thirds of the tetrachlorodioxins are extracted (after 10 min), the extraction is much slower. The experimental curve and the model curve
separate and the extraction rate is then mainly limited by the initial desorption step. The curve tends to be flat at ∼100% recovery as defined by Soxhlet extraction, but recoveries are substantially below those obtained with 150 °C to toluene. Similar observations on the mechanisms controlling the extraction of more highly chlorinated dioxins apply (Figure 3b and c); i.e., at 150 °C the extraction rate is controlled by the elution (KD -controlled) step, while at 80 °C, the extraction becomes increasingly controlled by the initial desorption step. In any case, extraction at 150 °C is substantially faster than at 80 °C. Confirmation of the Method Recoveries. The methods described above were developed using fly ash C, with analysis of the extracts by low-resolution GC/MS for the most abundant isomers of each family. However, only seven congeners of dioxins with chlorine atoms at positions 2,3,7,8 are toxic, and their determination is best performed with HRGC and HRMS. Therefore, to validate the ASE method for the toxic dioxin congeners, extractions of several fly ash samples were performed with 13Clabeled dioxins as internal standards and analysis of the extracts by HRGC/HRMS. The effects on health of the seven toxic isomers are considered to be similar, but the doses needed to obtain the same effects are different. To take into account the differences in toxicity levels, the concentrations of the toxic isomers (pg/g) are multiplied by a toxic equivalent factor (TEF) and results are then expressed in picograms of toxic equivalents per gram (pg of TEq/g).25 The 2,3,7,8-tetrachlorodioxin is the most toxic and is taken as reference with a TEF of 1. Figure 1b shows that, for the high-carbon-content fly ash, the extraction of all congeners is complete after 1 h, except for the octachlorodioxin. Since the toxic equivalent factor is only 0.0001 for this congener, its contribution to the toxicity of the (24) Handbook of Chemistry and Physics, 48th ed.; CRC: Cleveland, OH, 196768; p 33. (25) Van den Berg, M.; Birnbaum, L.; Bosveld, A. T. C.; Brunstrom, B.; Cook, P.; Feeley, M.; Giesy, J. P.; Hanberg, A.; Hasegawa, R.; Kennedy, S. W.; Kubiac, T.; Larsen, J.; van Leeuwen, F. X. R.; Liem, A. K. D.; Nolt, C.; Peterson, R. E.; Poellinger, L.; Safe, S.; Schrenk, D.; Tillitt, D.; Tysklind, M.; Younes, M.; Warn, F.; Zacharewsky, T. Environ. Health Perspect. 1998, 106, 775-792.
sample is very small. Therefore, a 1-h ASE extraction will yield essentially the same concentration expressed in pg of Teq/g as a 2-h extraction. Since one goal of this work was to minimize the extraction time, the method was validated using 1-h extractions at the same conditions (150 °C, 50 bar, flow rate of 1 mL/min), using 13C-labeled dioxins as internal standards and analysis of the extracts by HRGC/HRMS. The concentrations of the 2,3,7,8-chloro-substituted toxic congeners extracted from six different fly ashes (three with highcarbon and three with low-carbon contents) by ASE are compared to those obtained with the EPA 8280 method (48-h Soxhlet extraction) in Table 3. For all fly ash samples and for all congeners, 1 h of ASE yields systematically higher recoveries than 48 h of Soxhlet extraction. CONCLUSIONS ASE with toluene at 150 °C gives substantially higher recoveries of chlorinated dioxins from municipal incinerator fly ash than Soxhlet extraction, while ASE at 80 °C gives recoveries similar to Soxhlet extraction. Although SFE recoveries from low-carbon fly ash are also very high, SFE is not effective for high-carbon fly ash. At 150 °C, ASE extraction rates are controlled by fly ash/ toluene partitioning (i.e., chromatographic elution). In contrast, extraction rates at 80 °C are initially controlled by partitioning, but become limited by the rate of the initial desorption of dioxins from their sorption sites to the extractant toluene rather than by chromatographic elution. ACKNOWLEDGMENT The authors thank the “Region Wallonne” for financial support, the private industries for collecting fly ash samples used in this study, and the Center for Metallurgical Research for the determination of the carbon content of fly ash samples. Carol Grabanski is thanked for editing the manuscript. Received for review December 30, 1999. Accepted May 9, 2000. AC9914972
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