Anal. Chem. 1998, 70, 1228-1230
Decomposition of Acenaphthylene by Ultrasonic Irradiation E. Leonhardt and R. Stahl*
Institut fu¨ r Chemische Technik, Forschungszentrum Karlsruhe GmbH, Postfach 3640, D-76021 Karlsruhe, Germany
Polycyclic aromatic hydrocarbons were extracted from a soil sample using ultrasound and dichloromethane-, cyclohexane-, and toluene-water mixtures. It was found that when dichloromethane is used as an extractant, acenaphthylene reacts with the solvent. Several chlorinated and oxygenated derivatives were identified. The results show that chlorinated solvents should be avoided because of their sonolytic decomposition. Particularly unsaturated nonaromatic compounds might react with intermediate decomposition radicals of the solvent. Polycyclic aromatic hydrocarbons (PAH) are environmental pollutants often resulting from incomplete combustion or hightemperature pyrolytic processes. They represent a risk to the environment because PAH have been found to be mutagenic or carcinogenic and resistant to degradation.1 As nonpolar, lipophilic compounds, PAH are adsorbed on solid particles such as soil, fly ash, or river sediments.2,3 The U.S. Environmental protection Agency (U.S. EPA) has included 16 PAH in its pollutant list and has promulgated regulations for their monitoring in wastewater.4,5 The absolute concentrations of selected PAH serve as indicators for environmental contamination.6 Thus, the identification and quantification of these compounds are an important task. The sample preparation is a time-consuming extraction step which has to be done very carefully. For the extraction of PAH from solid samples, Soxhlet extraction,7-9 supercritical fluid extraction,10-13 accelerated solvent extraction,14 and sonication processes15,16 have been used. Some authors document the reproducibility and
Figure 1. Schematic diagram of setup for ultrasound extraction (a - b ≈ 10 mm).
efficiency of ultrasonic extraction.17-18 On the other hand it is well-known that imploding cavitation bubbles inside a liquid concentrate the acoustic energy in a small volume, resulting in high pressures and temperatures for a very short time.19,20 Solvent radicals formed during the cavitation might react with some of the PAH. The sonolytical decomposition of PAH may be useful as a means for their destruction.21 These facts indicate that the application of ultrasound for sample preparation is affecting the quantification of these compounds. The intention of this paper was to investigate the decomposition of PAH during their ultrasonic extraction from solid samples.
(1) Manahan, E. S. Fundamentals of Environmental Chemistry; Lewis Publishers: London, 1993. (2) Hoffman, E. J.; Mills, G. L.; Latimer, J. S.; Quinn, J. G. Environ. Sci. Technol. 1984, 18, 580-587. (3) Broman, D.; Colmsjo¨, A.; Ganning, B.; Na¨f, C.; Zebu ¨ hr, Y. Environ. Sci. Technol. 1988, 22, 1219-1228. (4) Keith, L. H., Telliard, W. A. Environ. Sci. Technol. 1979, 13, 417. (5) Bedding, N. D.; McIntyre, A. E.; Perry, R. J. Chromatogr. Sci. 1988, 26, 606. (6) Grimmer, G. GIT Fachz. Lab. 1992, 1, 12-21. (7) Ko ¨rdel, W.; Wahle, U. Pilotprojekt zur Entwicklung eines allgemeingu ¨ ltigen Analysenschemas fu ¨ r organische Chemikalien im Boden; Wilhelm Dostall KG: Esschweiler, Germany, 1990. (8) Hoenzlaer, B. GIT Fachz. Lab. 1990, 9, 1053-1057. (9) Marvin, C. H.; Allan, L.; McCarry, B., E.; Bryant, D. W. Int. J. Environ. Anal. Chem. 1992, 49, 221-230. (10) Hageman, K. J.; Mazeas, L.; Grabanski, C. B.; Miller, D. J.; Hawthorne, S. B. Anal. Chem. 1996, 68, 3892-3898. (11) Hawthorne, S. B. Anal. Chem. 1990, 62, 633A-642A. (12) Hawthorne, S. B.; Miller, D. J. Anal. Chem. 1987, 59, 1705-1708. (13) Hawthorne, S. B.; Miller, D. J. J. Chromatogr. 1987, 403, 63-76. (14) Ho ¨fler, F.; Ezzell, J.; Richter, B. LaborPraxis 1995, 4, 58-62.
(15) Nondek, L.; Kruzilek , M.; Krupicka, Sˇ . Chromatographia 1993, 37, 381386. (16) Vogt, N. B.; Brakstad, F.; Thrane, K.; Nordenson, S.; Krane, J.; Aamot, E.; Kolset, K.; Esbensen, K.; Steinnes, E. Environ. Sci. Technol. 1987, 21, 3544. (17) Manoli, E.; Samara, C. Chromatographia 1996, 43, 135-142. (18) Kicinski, H. G.; Adamek, S.; Kettrup, A. Chromatographia 1989, 28, 203208. (19) Henglein, A. Advances in Sonochemistry; JAI Press: Greenwich, CT, 1993; Vol. 3, pp 17-83. (20) Suslick, K. S.; Doktyez, S. J. Advances in Sonochemistry; JAI Press: Greenwich, CT, 1990; Vol. 1, pp 197-230. (21) D’Silva, A. P.; Laughlin, S. K.; Weeks, S. J.; Buttermore, W. H. Polycyclic Aromat. Compd. 1990, 1, 125-135.
1228 Analytical Chemistry, Vol. 70, No. 6, March 15, 1998
S0003-2700(97)01008-1 CCC: $15.00
EXPERIMENTAL SECTION Chemicals. All solvents used were analytical-grade chemicals obtained from Merck (Darmstadt, Germany). Water was prepared using a Millipore Q unit. Acenaphthylene and standards were purchased from Supelco GmbH.
© 1998 American Chemical Society Published on Web 02/10/1998
Figure 2. Total ion chromatogram of a soil extract after sonication for 60 min in dichloromethane-water mixture.
Instrumentation. Sonications at 20 kHz were done with a 400-W sonicator (UP400S Dr. Hielscher GmbH, Stuttgart, Germany) supplied with a titanium horn. (Operating conditions: 60% power amplitude, pulse length 40%.) Analysis of the extracts was performed with a Hewlett-Packard model 5988 GC/MS station using a HP5-MS capillary column. The chromatographic oven temperature was held at 40 °C during the first 3 min, ramped up to 300 °C within 18 min, and kept constant for 5 min. Sample Preparation. Soil. The contaminated soil sample from an old gasoline station in Germany was dried at room temperature for 20 days. Afterward it was pulverized in a crusher and sieved. The 0.06-0.2-mm fraction was used for extraction experiments. Soxhlet Extraction. Samples (∼0.5 g) were extracted with 50 mL of dichloromethane for a period of 6 h. The resulting extract was filtered through a 0.45-µm filter. Ultrasonic Extraction. Samples (∼0.5 g) were suspended in a mixture of 25 mL of solvent (dichloromethane, toluene, cyclohexane) and 25 mL of water. The reaction vessel had a total volume of 100 mL. It was fitted with two gas supplies for adjusting the gas atmosphere and a sampling tube. The vessel was closed during the experiments. The sonotrode was situated ∼1 cm from the bottom of the beaker. Further details are given in Figure 1. The whole vessel was immersed in an ice bath to keep the temperature constant. The actual temperature inside the vessel measured with a thermocouple was ∼8 °C. RESULTS AND DISCUSSION Figure 2 shows a typical total ion chromatogram from Soxhlet extraction of the soil sample used as reference to demonstrate the equivalence and accuracy of the ultrasonic extraction procedure. The identified PAH and their concentrations are given in Table 1. The other components, alkyl-PAH and some nitro-PAH, will not be discussed further. The ultrasonic extraction procedure was optimized by studying the amount of PAH desorbed as a function of the organic solvent and of the time. In general, the concentration of nearly all compounds reached a maximum after 30 min of sonication. Only acenaphthylene and acenaphthene
Table 1. Common PAH Found in a Soil Sample concentration, mg/kg ((1) sonication, 60 min in
compound
abbrev
1. acenaphthylene 2. acenaphthene 3. fluorene 4. phenanthrene 5. anthracene 6. fluoranthene 7. pyrene 8. benz[a]anthracene 9. chrysene 10. benzo[j] fluoranthene 11. benzo[k]fluoranthene 12. benzo[e]pyrene 13. benzo[a]pyrene 14. perylene 15. indeno[1,2,3-cd]pyrene 16. benzo[ghi]perylene
Acy Ace F Ph An Fl Py B[a]An Chry B[j]Fl B[k]Fl B[e]Py B[a]Py Pe I(1,2,3)Py B[ghi]Pe
dichloromethane- tolueneSoxhlet water water 20 169 368 1182 209 725 725 219 159 139 79 169 169 nqc 99 nq
21b 143 316 1010 198 642 645 220 162 132 88 153 167 nq 101 nq
21 170 375 1191 192 625 662 240 166 146 94 185 171 nq 107 nq
a Compound numbers correspond to the numbering of the peaks in the Figure 2. b For 10-min sonification. c nq, not quantified.
appear to need more time (60 min) to be desorbed. This may be because small molecules are adsorbed in narrow pores of the sample. The exchange of the organic solvent has no influence on the resulting final concentrations, which agree well with those obtained from Soxhlet application. Performed within 60 min, the ultrasonic extraction is faster than the Soxhlet method. Nevertheless, two problems appeared: in the case of cyclohexane, a persistent emulsion was formed. Centrifugation was necessary for phase separation, a fact that complicates the extraction procedure. In the case of dichloromethane-water mixtures, the concentration of acenaphthylene reached a maximum after ∼10 min of sonication, and thereafter, its concentration decreased significantly (Figure 3). For a detailed study of these phenomena, a 3 mM solution of acenaphthylene in dichlormethane was sonicated in the same way for 240 min. As Figure 4 shows, the Analytical Chemistry, Vol. 70, No. 6, March 15, 1998
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Table 2. Identified Products of 3 mM Acenaphthylene in a Dichloromethane-Water Mixture after 4 h of Sonication formula
yield,a %
Saturated 1,1,2-trichloroethane 1,1,2,2-tetrachloroethane 1,1,3,3-tetrachloropropane 2,3-dichloro-2-methylbutane
C2H3Cl3 C2H2Cl4 C3H4Cl4 C5H10Cl2
79.2 132.5 1.3 3.0
Unsaturated chlorethylene trichloroethylene 1,3-dichloropropene-1 1,2,3-trichloropropene-1 1,1,2,3-tetrachloropropene-1 pentachloropropene-1 1,1,3,4-pentachlorobutadiene-1,3
C2H3Cl C2HCl3 C3H4Cl2 C3H3Cl3 C3H2Cl4 C3HCl5 C4H2Cl4
6.2 50.9 33.4 17.5 16.9 13.7 10.4
product
Figure 3. Relative concentrations (Ci(t)/Ci(max) of some PAH as a function of ultrasound extraction time with dichloromethane-water mixtures (* the extraction curves for the other PAH are identical).
Polycyclic Aromatic 1-oxoacenaphthene C12H8O naphthalene-1-carboxylic acid-8C12H8O2 hydroxymethyllactone 1 chloro-8-hydroxyacenaphthene C12H9OCl 1 chloro-8-oxoacenaphthene C12H7OCl acenaphthenequinone C12H6O2 1,8-dichloroacenaphthene C12H8Cl2 1,8-naphthalic anhydride C12H6O3 1-formyl-8-carboxynaphthalene C12H8O3 a
Figure 4. Relative concentration of acenaphthylene (C0 ) 3 mmol) as a function of the sonication time in dichloromethane-water mixture under an atmosphere of air and argon.
concentration of acenaphthylene decreased within 120 min of pulsed sonication in air by a factor of ∼2. The identified derived organic products are given in Table 2. A mass-sensitive detector (HP 5890) and the NIST mass spectral and structure database were used for identification. Authentic standards were also analyzed. Additionally, a small amount of a black insoluble deposit was found which could not be analyzed. The products can be divided into two groups: (a) The first group contains saturated and unsaturated chlorohydrocarbons. They might be formed by various combinations of the radicals CH2Cl and •CHCl2 and the atoms H and Cl. These results agree very well with those already described in the literature.22-25 During the sonication, the pH of the aqueous phase decreased continuously, indicating the formation of HCl from the chlorinated reactants. The concentrations of chloride as well as of some small chlorinated hydrocarbons such as trichloroethane and tetrachloroethane were directly proportional to the amount of the ultrasonic energy input. (b) The second group is built-up byproducts based on chemical reactions of acenaphthylene. While the basic dicyclic (22) (23) (24) (25)
Alippi, A.; Cataldo, F.; Galbato, A. Ultrasonics 1992, 30, 148-151. Popa, N.; Ionescu, S. G. Roum. Chim. 1992, 37, 697-701. Hua, I.; Hoffmann, M. R. Environ. Sci. Technol. 1996, 30, 864-871. Pe´trier, C., Reyman, D.; Luche, J.-L. Ultrason. Sonochem. 1994, 1, S103S105.
1230 Analytical Chemistry, Vol. 70, No. 6, March 15, 1998
34.8 1.3 12.0 1.8 3.3 0.9 4.4 2.2
Rate of peak area to initial area of Acy.
aromatic structure is conserved, only the initial double bond undergoes various reactions leading to oxygenated and chlorinated products. Similar experiments under argon atmosphere instead of air showed a lower decomposition rate forming the same products, however. Even when only dichloromethane was used as the solvent for sonication, different degradation products were found. The oxygenated products are supposed to be built up in reactions with water. No such products appeared when toluenewater or cyclohexane-water mixtures were sonicated. Dichloromethane seems to have an intermediate function in these reactions. Detailed studies about the mechanisms and the kinetics are still under investigation. These results show that the use of chlorinated solvents in the ultrasonic solvent extraction of solid samples must be avoided. In particular, unsaturated nonaromatic compounds of the samples seem to react with intermediately formed solvent radicals. Assuming that it will be possible to identify the reaction mechanisms, there might be a chance for the application in waste destruction or synthesizing processes. ACKNOWLEDGMENT The authors acknowledge the financial support by the Otto Benecke foundation (Bonn). The assistance of Mrs. G. Schnabel in the gas chromatographic analytical work is gratefully appreciated.
Received for review September 11, 1997. December 15, 1997. AC9710083
Accepted
