Anal. Chem. 1998, 70, 616-622
Extraction of Petroleum Hydrocarbons from Soil Using Supercritical Argon Shijiang Liang and David C. Tilotta*
Department of Chemistry, University of North Dakota, P.O. Box 9024, Grand Forks, North Dakota 58202-9024
The use of supercritical argon is described for the extraction of petroleum hydrocarbons from soil samples. Argon is an attractive solvent because it is inexpensive and inert. Additionally, it has a clear spectral window in the infrared region which makes it useful for on-line (i.e., directly coupled) experiments. Spiking studies conducted with gasoline, no. 1 fuel oil, and no. 5 fuel oil on sand, loam, and clay show that component recovery rates for argon supercritical fluid extraction (SFE) generally increase with increasing pressure and/or temperature. The highest recovery rates (and recoveries) were obtained for Ar SFE at 500 atm and 150 °C. Under these conditions, the components of the gasoline and no. 1 fuel oil spikes could be recovered in as little as 12 min. However, the no. 5 fuel oil components could not be quantitatively removed from the loam and clay matrixes even for extraction times as long as 100 min. We also show in this work that Ar SFE performs similarly to CO2 SFE for petroleum hydrocarbon contamination in real-world soil samples under moderate pressure and temperature conditions. Specifically, Ar SFE and CO2 SFE have similar recoveries and reproducibilities, but Ar SFE requires a slightly longer extraction time. Commonly used methods for the determination of petroleum hydrocarbon contamination in soil are modifications of EPA method 418.1, which use sonication or a Soxhlet apparatus for analyte extraction and either infrared spectrometry1 or gas chromatography with flame ionization detection2,3 for extract analysis. Regardless of the analytical method following the extraction, both modifications use Freon-113, which has been implicated as a cause of ozone depletion. Therefore, alternative methods are being sought for the determination of hydrocarbon contamination in environmental samples that reduce the need for this halogenated solvent. Supercritical fluid extraction (SFE) with CO2 has been shown to be an excellent alternative to conventional solvent extraction for the removal of hydrocarbon pollutants from solid samples.4-6 (1) U.S. Environmental Protection Agency. Methods for Chemical Analysis of Water and Wastes; EPA 600/14-79/020; Washington. DC, 1979. (2) State Water Resources Control Board. Leaking Underground Fuel Tank (LUFT) Field Manual; State of California, Sacramento, 1988. (3) American Society for Testing and Materials. Annual book of ASTM Standards; Philadelphia, 1984; Vol. 11.02. (4) Camel, V.; Tambute´, A.; Caude, M. J. Chromatogr. 1993, 642, 263-281.
616 Analytical Chemistry, Vol. 70, No. 3, February 1, 1998
It is fast (∼30 min), nonpolluting, and relatively simple to implement. Additionally, recent work has shown that CO2 SFE is generally applicable to soil samples that have been contaminated with petroleum hydrocarbons ranging from those found in gasoline to those in medium crude oil (i.e., ∼90% recovery) were only achieved from the sand matrix. Poor CO2 SFE recoveries were obtained from the loam and clay matrix spikes under these extraction conditions. Argon SFE is most likely poorer than CO2 SFE because of the lower density of the Ar fluid (in comparison to the CO2 fluid) under these conditions. The densities of the CO2 and Ar under the extraction conditions employed in this work are shown in Table 1. The differences in the extraction times for the two fluids become significant for n-alkanes greater than about C24. Thus, Ar SFE (without the use of a polarity modifier) is not effective for removing heavy hydrocarbons from soil. It should be pointed out, however, that a similar result has been documented for CO2 SFE.7,9 Extraction of Real-World Samples. As stated above, it is generally more difficult to extract native analytes than spiked ones. Thus, three real-world samples were obtained from local sources and extracted in order to examine the performance of supercritical argon as an extraction fluid for native analytes. Figure 5 shows chromatograms of the Ar SFE extracts from each of these samples. One of these samples was known to be contaminated with diesel fuel (approximate hydrocarbon range, C11-C22), and one sample was known to be contaminated with no. 2 fuel oil (approximately C11-C24 hydrocarbons). The source of the petroleum hydrocarbon contamination of the third sample was difficult to identify because the sample was significantly weathered (as evidenced by the lack of distinguishable n-alkane peaks in its extract chromatogram). However, the hydrocarbon distribution
Figure 6. Ar SFE recovery rate (cumulative TPH) of no. 2 fuel oil extracted from a real-world sample. SFE conditions are given in the figure.
Figure 5. Chromatograms of Ar SFE extracts of real-world soil samples contaminated with petroleum hydrocarbons. SFE conditions are given in Table 5. Chromatographic conditions are given in the text.
of this “aged fuel” sample was approximately in the range of C13C28. Figure 6 shows the kinetic data for the extraction of the no. 2 fuel oil-contaminated sample for Ar SFE under four sets of conditions. The plots in Figure 6 have been expressed as the percent extraction of all chromatographable components (TPH) based on an exhaustive extraction of the same sample with CO2. For comparison purposes, Figure 7 shows similar data for CO2 SFE. As shown in Table 5, the concentration of TPH in this sample is on the order of 10 000 ppm. It should be noted that we use this soil sample as an example of a “worst case” because, of the two real-world samples that contain resolvable n-alkane data, this sample should yield the lowest Ar SFE extraction rates (because it is contaminated with a heavier oil). The Ar extraction data show, as expected, that the recoveries at a given extraction time generally decline with either temperature or pressure. Although these results are consistent with the spiking studies, the extraction of the real-world samples showed that the Ar SFE temperature was the more critical factor. This influence was not as pronounced for the CO2 SFE, as demonstrated in Figure 7. A comparison of the Ar SFE data to the CO2 SFE data shows that the CO2 extraction rates are faster at the same temperature and pressure conditions. It is interesting to
Figure 7. CO2 SFE recovery rate (cumulative TPH) of no. 2 fuel oil extracted from a real-world sample. SFE conditions are given in the figure.
note that a comparison of the extraction rates under similar density and temperature conditions (500 atm, 150 °C for the Ar and 300 atm, 150 °C for the CO2) shows that the CO2 SFE is still faster by ∼2× for this sample. Similar behavior was observed for the extraction of the diesel-contaminated sample and the “aged fuel”contaminated sample; however, the CO2 SFEs were faster only by about 20-30% (for these two samples). Figures 8 and 9 show detailed kinetic data for some of the n-alkanes extracted by both SC Ar and SC CO2 under conditions of 500 atm and 150 °C. It is obvious from the results shown in Figure 8 that it becomes difficult and/or time-consuming (i.e., >30 min) to extract n-alkanes greater than about C21 (for this sample). As expected, the CO2 is more efficient at extracting the higher molecular weight compounds, as evidenced by the faster extraction rates shown in Figure 9. Similar trends were also observed for the diesel fuel-contaminated sample. Analytical Chemistry, Vol. 70, No. 3, February 1, 1998
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Table 5. A Comparison of Ar SFE and CO2 SFE for Determining Total Petroleum Hydrocarbons (TPH) in Three Real-World Soil Samples (SFE Conditions: 500 atm and 150 °C) CO2 SFE timea
Ar SFE timea
sample
(min)
concn (ppm)b
(min)
concn (ppm)b
aged fuel (dist C13-C28) diesel (dist C11-C22) no. 2 fuel oil (dist C11-C24)
50 20 20
9628(6) 4380(7) 3780(10)
60 30 40
10288(11) 4348(8) 3540(3)
a Extraction time until chromatographable components could not be detected. b The numbers in parentheses are relative standard deviations (RSDs) determined from triplicate extractions.
Figure 9. CO2 SFE recovery rates of representative components from no. 2 fuel oil extracted from a real-world sample. SFE conditions: 500 atm and 150 °C. Representative n-alkanes are identified in the figure.
by the characteristics of the sample (e.g., the size and type of the particles) rather than the extractant.4
Figure 8. Ar SFE recovery rates of representative components from no. 2 fuel oil extracted from a real-world sample. SFE conditions: 500 atm and 150 °C. Representative n-alkanes are identified in the figure.
Table 5 shows a summary of the extraction results (i.e., TPH) for Ar SFE and a comparison to CO2 SFE under the same conditions for the three real-world samples. On the basis of the results from the spiking experiments that the extractions proceeded the fastest with the highest temperature and pressure, we employed argon extraction conditions of 500 atm and 150 °C. For comparison purposes, we also extracted these samples in CO2 under the same temperature/pressure conditions. As can be seen from Table 5, the extraction results with SC Ar (i.e., recovery times and TPH concentrations) compare favorably with those using SC CO2. However, the Ar SFE times are longer by about 1.2-2.0×, depending on the molecular weight distribution of the sample. Additionally, it should be noted that both the Ar SFE and the CO2 SFE required >30 min to extract the fuel components from the aged fuel sample under these conditions, most likely because of the higher molecular weight distribution of the contaminants in this sample and the difficulty of both fluids to overcome the matrix/analyte interactions. Nevertheless, SC Ar does a reasonable job of quantitatively extracting these hydrocarbons under these conditions. The reproducibilities of the determinations (relative standard deviations, RSDs) of the TPH in the real-world samples compare favorably for both the SC Ar and the SC CO2 and are typical for these kinds of samples. Generally, the RSDs in SFE are governed 622 Analytical Chemistry, Vol. 70, No. 3, February 1, 1998
CONCLUSIONS This work has shown that Ar SFE is a viable alternative to CO2 SFE for the removal of petroleum hydrocarbon contamination from soil samples. Argon is less expensive than CO2 and has a better optical transmittance in the IR region. Preliminary spiking studies show that pure SC argon works almost as well as pure SC carbon dioxide for extracting
