Environ. Sci. Technol. 1997, 31, 430-437
Class-Selective Extraction of Polar, Moderately Polar, and Nonpolar Organics from Hydrocarbon Wastes Using Subcritical Water YU YANG, STEVEN B. HAWTHORNE,* AND DAVID J. MILLER Energy and Environmental Research Center, University of North Dakota, Campus Box 9018, Grand Forks, North Dakota 58202
The polarity of water drops dramatically when heated under enough pressure to maintain the liquid state. For example, the dielectric constant () of water is 80 at ambient temperature, but drops to ∼30 at 250 °C. Thus, low temperature water can be used to extract polar organics, while higher temperature water will extract moderately polar and nonpolar organics. Four samples (a soil, a catalyst, and two sludges) were extracted by subcritical water at different temperatures and pressures. At lower temperatures (50-150 °C), phenols and BTEX (benzene, toluene, ethylbenzene, and xylenes) were quantitatively extracted by liquid water, while PAHs (polycyclic aromatic hydrocarbons) and alkanes were not extracted. At 250 or 300 °C, liquid water effectively extracted PAHs, but the high molecular weight alkanes (e.g., >C20) were still not extracted. The quantitative extraction of the high molecular weight alkanes was achieved only by super-heated steam (250 and 300 °C at 5 atm) extractions. Class-selective extractions of phenols, alkylbenzenes, PAHs, and alkanes were achieved by simply changing water temperatures (50-300 °C) or pressures (5-100 atm). The recoveries of all of the target analytes achieved by subcritical water extraction compare favorably (typically 90-120%) to those of conventional organic solvent extractions.
Introduction The desire to reduce the use of hazardous organic solvents for analytical- and process-scale separations has led to the investigation of alternative fluids. Supercritical CO2 has gained increased popularity; however, the polarity of supercritical CO2 is often too low to effectively extract many organics from environmental solids (1-3). Furthermore, even though the polarity of CO2 can be changed by controlling its density, the dielectric constant of CO2 only ranges from 1 to 1.6 (4), so that a class-selective extraction of polar versus nonpolar compounds is difficult to achieve. Thus, many recent studies in supercritical fluid extraction (SFE) have focused either on adding organic modifiers to increase the solvent polarity of CO2 (2, 5-7) or on searching for other extraction fluids (711). A second “environmentally-friendly” solvent, water in its supercritical state, has been effective for extracting organics from a wide variety of matrices because of its low dielectric constant and strong solvent strength (12, 13). However, the * To whom correspondence should be addressed. E-mail:
[email protected]; phone: 701-777-5256; fax: 701777-5181.
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use of supercritical water has been limited by the high temperature (>374 °C) and pressure (>218 atm) requirements as well as its corrosivity (14-21). Since the dielectric constant () of water is very high ( ) 80) at ambient temperature and pressure, ambient water is an extremely poor solvent for low polarity organics (e.g., solubility at room temperature is ∼3 ng/mL for benzo[ghi]perylene and 8 ng/mL for n-octadecane) and only a moderately good solvent for polar organics (e.g., 80 mg/mL for phenol). Fortunately, the attractive solvent properties of supercritical water (i.e., low dielectric constant) can be achieved at much milder temperature and pressure conditions. For example, the dielectric constant of water is lowered to 27 at 250 °C (at pressures >40 atm to maintain the liquid state), which makes liquid water a powerful solvent for nonpolar organics. For example, the solubility of benzo[e]pyrene in liquid water has been reported to increase by a factor of ∼25 million from 4 ng/mL at ambient conditions (22) to 1 × 108 ng/mL at 350 °C and ∼170 atm (23). Since water temperature greatly changes its solvent polarity, it should be possible to perform class-selective extractions from waste solids. For example, low temperature water should extract more water-soluble organics (e.g., phenol), while higher temperature water should extract less soluble organics such as polycyclic aromatic hydrocarbons (PAHs). We have previously demonstrated that subcritical water (both liquid and steam) is an efficient extraction fluid for chlorophenols, PAHs, polychlorinated biphenyls (PCBs), and n-alkanes (21, 24). The extraction of spiked semivolatile organics from sand reported earlier (21) showed that water efficiently extracted the more polar components such as chlorophenols at 50 °C, while most of the PAHs and n-alkanes remained in the matrix. When the water temperature was increased to 250 °C, all of the PAHs ranging in molecular weight from 128 to 276 were quantitatively removed by hot liquid water, but the high molecular weight n-alkanes were still not extracted until the pressure was reduced to generate steam. This extraction behavior for different classes of organics may be useful, since chlorophenols and PAHs are toxic contaminants and can be removed by hot liquid water, while alkanes are not regarded as toxic and are not generally removed by hot liquid water. The purpose of the present study is to determine the feasibility of using subcritical water to selectively extract polar from nonpolar organic pollutants and aromatic from aliphatic hydrocarbons from soil, petroleum waste sludges, and a spent catalyst. The effects of water temperature, pressure, and flow rate on extraction selectivity are discussed, and the extraction/ analyte collection system is described.
Experimental Section Samples. Four real-world samples were selected for determining the selectivity of subcritical water extractions. One sample was a certified reference material, a highly contaminated soil from a wood treatment facility (U.S. EPA Certified PAH-contaminated soil, Lot No. AQ103; Fisher Scientific, Fair Lawn, NJ). The other three samples were a petroleum waste sludge, a crude oil tank bottom dewatered sludge, and a spent catalyst (experimental-spent resid NiMo/A1203 catalyst; Amoco Oil Company, Chicago, IL) that were mixed by hand and appeared to be homogeneous before use. All four samples contained PAHs and alkanes. In addition, the petroleum waste sludge contained phenol and alkylphenols, and the tank bottom dewatered sludge contained BTEX (benzene, toluene, ethylbenzene, and xylenes) and very high levels (∼6% w/w) of branched hydrocarbons (C10-C17). Extractions of the soil and catalyst used 200-mg samples, while the extractions of the two sludges used 400-mg samples. The void
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FIGURE 1. Schematic diagram of subcritical water extraction and collection system. volume of the cell was filled with precleaned (first using acetone and then n-hexane) white quartz sand (Aldrich, Milwaukee, WI). For the collection efficiency studies, the extraction cell was first fully filled with precleaned sand, and then the target analytes were spiked onto the sand. After spiking, the cell was sealed immediately and extracted. Subcritical Water Extractions. All water extractions were performed in a manner analogous to conventional supercritical fluid extraction as shown in Figure 1. HPLC-grade water (Fisher Scientific) was first purged for 1-2 h with nitrogen to remove dissolved oxygen and filled into an ISCO Model 260D syringe pump (ISCO, Lincoln, NE). The pump was operated in the constant pressure mode (or in the constant flow mode for steam extraction) to supply water to the extraction cell through a 1/16-in. o.d. (0.020 in. i.d.) stainless steel tubing including a 4-m preheating coil. The extraction cell and the preheating coil were placed inside of a Hewlett-Packard Model 5730A GC oven (Hewlett-Packard, Avondale, PA). Empty stainless steel HPLC columns (30 mm long × 4.6 mm i.d.) from Keystone Scientific (Bellefonte, PA) were used as extraction cells for all extractions. Extraction cells were mounted vertically in the gas chromatograph (GC) oven with the water flowing from top to bottom so that any extracted analytes were immediately swept from the cell. The outlets of the extraction cells were connected by stainless steel tubing to a 1/16 × 1/16 in. “Parker” tubing union, which was itself connected to flow restrictors to provide a water flow rate of ∼1 mL/min. Three methods were used for controlling water flow. First, restrictors made from 30 to 50 µm i.d. × 10 cm lengths of fused silica capillary tubing (Polymicro Technologies, Phoenix, AZ) were used for sequential temperature and sequential time extractions to provide a water flow rate of ∼1 mL/min measured as liquid water at the pump (25). Second, a HIP Model 15-11AF1 shut-off valve (High Pressure Equipment Co., Erie, PA) was used as a flow restrictor for low pressure (e.g., 5 atm) extractions or when the fused silica restrictor was plugged during the higher pressure extractions (e.g., 50 atm). The experiments for evaluating the influence of extraction temperature or time on extraction efficiency required a sequential collection of fractions during the extraction. Therefore, only either fused silica capillary tubing (the first method) or the HIP shut-off valve (the second method) without the cooling loop was used as a restrictor for these extractions, since this restrictor setup does not require a loop rinsing step after each sequential step of the extraction. With this restrictor setup, the collection solvent evaporated during the extractions at higher water temperatures of 200300 °C; therefore, the collection vial was cooled with ice water to avoid solvent evaporation. Since the flow rate was difficult to precisely control when using the HIP valve, a third flow control method utilizing a stainless steel needle valve was used as a flow control device for the nonsequential extractions.
As shown in Figure 1, the setup for the extractions using a needle valve was similar to the extractions using a fused silica restrictor or HIP valve except that a Swagelok stainless steel needle valve (Swagelok Company, Solon, OH) was used to replace the fused silica restrictor (or the HIP valve) as a flow control device. Since hot water may damage the packing material of the needle valve, a cooling loop, cooled with room temperature water, made from a 40-cm length of stainless steel tubing (1/16 o.d.) was connected between the extraction cell and the needle valve. The loop and needle valve were rinsed with 3 mL of organic solvent (same as the collection solvent) after the extraction to recover any deposited analytes. After assembling the extraction cell in the GC oven, the cell was pressurized with water. Both the inlet and the outlet valve of the cell were opened while the GC oven was heated to the desired temperature. Therefore, the water was kept at the desired pressure during the heatup time, typically ∼1, 2, 3, 4, 5, and 7 min for 50, 100, 150, 200, 250, and 300 °C, respectively. The temperature of the effluent water (measured by a thermocouple placed in a “tee” fitting at the outlet of the extraction cell) lagged 1-2 min behind the oven temperature. For example, the effluent water temperature reached 250 °C only 1.5 min after the oven temperature. Collection of the extracted analytes was performed by inserting the outlet of the flow restrictor (flow control device) into a 22-mL glass vial containing 3 mL of collection solvent (methylene chloride). The water percolated through the collection solvent allowing the extracted analytes to partition into the solvent. After extraction, two internal standards, m-diethylbenzene and 1,2,3,4,5,6,7,8-octahydrophenanthrene, were added into the collection solvent. The water extract and collection solvent inside the collection vial were shaken for ∼30 min by a rotator, and then the collection solvent was removed. For the more polar analytes that have higher water solubility, quantitative transfer from the water to the collection solvent may not occur by this single step. Therefore, the water layer from each extraction was extracted with a second 2-3-mL aliquot of collection solvent, and the two organic solvent aliquots (and the cooling loop rinsings when collection method 3 was used) were combined for GC analysis. After extraction with water, selected sample residues were mixed with sodium sulfate (1:1) and again extracted by sonication (bath sonicator) for 16 h in 5 mL of methylene chloride and acetone (1:1) to determine the concentration of any remaining target analytes. The fresh samples of the spent catalyst and two sludges were also extracted by sonication under the same conditions used for the residue samples as described above to generate the reference values for the concentration of the individual contaminants. Extract Analysis. The PAH analysis was done by gas chromatography/mass spectrometry (GC/MS) (Hewlett Packard Model 5988) using a 25 m × 0.25 mm i.d. (0.17 µm film
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TABLE 1. Effects of Temperature and Pressure on Selective Extraction of PAHs versus Alkanes from Soil % removalb steam
liquid water
supercritical water
temp (°C) pressure (atm)
concna
cert (µg/g) ( SD
250 5
250 50
250 350
300 350
400 350
naphthalene phenanthrene chrysene benzo[ghi]perylene C14 C16 C18 C20 C22 C24 C26 C28 C30
24 ( 28 1450 ( 570 311 ( 63 -c -
86 100 99 78 100 100 100 100 100 100 100 100 100
95 99 98 92 98 87 34 11 3.6 2.7 2.7 0 0
95 99 98 91 86 22 5.2 5.1 3.7 1.4 0 0 0
98 99 99 100 100 97 90 81 69 37 15 9.3 0
100 100 100 100 100 100 100 100 100 100 100 100 100
a Concentrations and standard deviations certified by the supplier based on Soxhlet and sonication extractions. Ref 21 reports extraction efficiency data for 14 PAHs from this sample. b 100% removal is defined by the lack of any detectable species in the 16-h sonication extract of the residues after 15-min water extraction. All extractions were done in triplicate, and the RSDs were typically 10:1 (Tables 1 and 2). The effect of steam versus liquid water on the extraction selectivity and efficiency was compared at 250 °C using 15min extractions at two different pressures. Both 5 atm (steam) and 50 atm (liquid) water at 250 °C efficiently extracted the
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FIGURE 2. Effect of extraction time on the extraction efficiency of BTEX compounds (a), representative PAHs (b), and n-alkanes (c) from a tank bottom dewatered sludge using water at 250 °C and 50 atm. Percent removals are based on sonication of the residue after water extraction. alkylbenzenes and PAHs, while only the steam extractions performed at 250 °C and 5 atm yielded quantitative extractions of the alkanes as shown in Table 3. Additional extractions performed at 350 °C and 50 atm (steam) gave results essentially identical to those shown in Table 3 for 250 °C and 5 atm (steam). Also, increasing the pressure from 50 to 500 atm yielded similar results when performed at 250 °C. These results clearly demonstrate that temperature is the major factor controlling the selectivity of water extractions, while the only important effect of the extraction pressure is to maintain liquid or gas state. Influence of Water Flow Rate on Selectivity of Extraction. Since the extraction of the tank bottom dewatered sludge was not as selective as the extractions of the soil and petroleum waste sludge, different water flow rates were tested in an attempt to improve the extraction selectivity. Triplicate extractions were performed at 250 °C and 50 atm using 0.15, 0.5, and 1 mL/min. The extraction time used for each extraction was 15 min. The percent recoveries of BTEX, PAHs, and n-alkanes obtained using different flow rates are listed
TABLE 3. 15-Min Extraction of Tank Bottom Dewatered Sludge % recoveryb liquidc
steam
150 °C flow rate (mL/min)
concna (µg/g) ( SD
toluene ethylbenzene m,p-xylene o-xylene naphthalene phenanthrene pyrene chrysene benzo[a]pyrene benzo[ghi]perylene C12 C15 C18 C21 C24 C27 C30 TPHd
34 ( 3 5.6 ( 0.6 58 ( 1 20 ( 3 56 ( 6 24 ( 1 8.5 ( 0.7 17 ( 1 15 ( 1 1.4 ( 0.1 67 ( 3 85 ( 8 52 ( 2 43 ( 3 22 ( 1 8.2 ( 0.6 6.5 ( 1.2 59000 ( 3700
250 °C 1.0
98 111 106 111 85 31 27 18 12 22 25 26 23 23 12 3.6 3.7 30
liquid
95 111 107 114 108 108 117 107 94 98 93 87 62 53 50 49 43 94
94 83 96 93 99 110 105 105 87 91 92 102 100 91 99 106 117 105
0.15
0.5
105 91 101 114 94 75 59 35 16 10 19 17 10 11 11 11 12 15
111 102 112 115 110 115 101 70 42 30 53 34 26 22 23 29 26 54
a Values were obtained by 16-h sonications. b Recoveries with water based on triplicate extractions at each condition compared to sonication of fresh samples. % RSDs of the water extractions were typically 10-40% when recoveries were low and 5-20% when recoveries were high. c Pressure was 50 atm for liquid and 5 atm for steam. d Based on the total peak area of the FID chromatogram.
in Table 3. Extraction at 0.15 mL/min effectively recovered the BTEX components, while the recoveries of all of the n-alkanes and high molecular weight PAHs only ranged from 10 to 20%; the low molecular weight PAHs having higher recoveries. Therefore, a selective extraction of BTEX versus n-alkanes and high molecular weight PAHs can be conducted using low water flow rate (0.15 mL/min). Please notice that the total volume of water used for the extraction using this low flow rate was only 2.25 mL as the extraction time was 15 min. While 1 mL/min water extracted 94% the bulk matrix branched hydrocarbons, only ∼15% of the branched hydrocarbons was extracted by water at 0.15 mL/min (Table 3). Therefore, the selectivity of extracting BTEX from the bulk branched hydrocarbons was ∼7:1. The recoveries of PAHs, alkanes, and TPH were increased by raising the flow rate to 0.5 mL/min, but only the low molecular weight PAHs achieved quantitative yield. Further increases in extraction efficiency of the high molecular weight PAHs (recoveries g94%), alkanes (recoveries ranging from 50 to 100% for up to ∼C20 and from 40 to 50% for alkanes larger than C20), and TPH (recovery ) 94%) occurred when the flow rate increased to 1 mL/min. The use of different flow rates (with otherwise identical extraction conditions) to determine whether extractions are limited by solubility or by kinetic limitations (e.g., diffusion of the analytes in the sample matrix) has been applied in detail for supercritical CO2 extractions (26). Based on the arguments in that previous report, the use of different flow rates is equivalent to changing sample size (with constant flow) in order to determine whether the extraction of a particular analyte is limited by solubility. The lack of effect of flow rate on the extraction of BTEX shown in Table 3 demonstrates that their extraction is not limited by their solubility at 250 °C and 50 atm, while the dependence on flow rate for PAHs and alkanes does indicate that higher solubility conditions may increase the extraction rates (note that the alkanes are easily extracted using steam at 250 °C as shown in Table 3). Although the extractions of the soil and petroleum waste sludge (Tables 1 and 2) as well as spiked samples and air particulate matter (21) demonstrated that most alkanes (e.g., >C20) could be left behind on the sample matrix under
conditions that would quantitatively extract all of the regulatory PAHs, the selectivity of extracting PAHs versus alkanes is only ∼2:1 for the tank bottom dewatered sludge. We are not certain why the extraction of PAHs versus the alkanes was not as selective as for our other samples, but we suspect it is related to the large mass of co-extracted matrix organics (i.e., the branched hydrocarbons). The very high levels (∼6% of the sample weight) of the branched hydrocarbons may act as a modifier, so that the liquid water can more effectively extract the high molecular weight alkanes than it does for a less-contaminated matrix. It should be noted, however, that the extraction was quite selective based on total sample mass. For example, extraction at 250 °C and 50 atm gave quantitative removal of all of the regulated PAHs, while only extracting 6% of the tank bottom sludge. Since the tank bottom sludge was ∼33% organic (on a dry basis), the selectivity of the extraction for PAHs was ∼5:1 as compared to the bulk matrix organics and approximately 15:1 for the extraction of PAHs to the total sample. Class-Selective Extraction of PAHs versus Alkanes from Spent Catalyst. The final sample tested was a spent catalyst that contained PAHs and a broad range (C7->C40) of alkanes. Initial extractions of this sample at sequentially higher temperatures (50 atm) showed very little extraction of PAHs until 200 °C and very little extraction of alkanes until 250 °C. Even at 250 °C, the recoveries of PAHs from the catalyst were substantially lower than from the soil, petroleum waste sludge, and tank bottom dewatered sludge samples discussed above. Therefore, additional extractions were performed at both 250 and 300 °C. As shown in Table 4, extractions at 250 °C performed sequentially with liquid water (50 atm, 15 min) followed by steam (5 atm, 15 min) gave poor recoveries for the PAHs with liquid water and only moderate recoveries with the steam (note that liquid water at 250 °C gave quantitative PAH recoveries for the other three samples discussed above). Note also that, while liquid water did not extract any significant alkanes, steam at 250 °C extracted >55% of the alkanes. When the extraction temperature was raised to 300 °C (15 min for liquid water extraction at 100 atm followed by 15-min extraction using steam at 5 atm), the PAH recoveries improved for the liquid water extraction (Table 4), but the sum of the liquid plus steam recoveries were not
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TABLE 4. Sequential Liquid Water and Steam Extraction of Organics from Spent Catalyst % removal, water/water + steamb original sample
ground sample
(µg/g)
250 °C 15 min
300 °C 15 min
300 °C 60 min
300 °C 15 min
300 °C 15 min
31 ( 1 165 ( 5 14 ( 1 2510 ( 67 8.1 ( 0.6 55 ( 2 64 ( 4 13 ( 1 11 ( 1 91 ( 10 308 ( 17 173 ( 6 572 ( 18 348 ( 12 320 ( 27 197 ( 8 125 ( 6 22 ( 3 8650
73/100 38/93 34/97 20/87 15/62 14/77 12/62 16/50 11/60 10/56 49/100 8/100 1/98 -d/94 -/82 -/74 -/70 -/57 4/90
95/100 86/99 81/98 84/97 47/71 62/87 57/82 47/71 49/73 45/67 84/100 55/100 18/100 11/99 10/95 4/88 -/73 -/66 21/95
100/100 99/100 98/100 100/100 98/100 97/100 94/100 93/100 91/100 90/100 100/100 100/100 62/100 12/100 10/99 7/92 5/82 -/76 40/97
99/100 100/100 99/100 100/100 100/100 99/100 98/100 98/100 94/100 93/100 100/100 100/100 66/100 28/100 33/100 13/100 9/100 -/100 49/100
96 (1) 120 (28) 154 (9) 98 (14) 137 (5) 96 (7) 91 (7) 126 (3) 81 (4) 113 (15) 71 (5) 91 (8) 99 (6) 94 (15) 100 (10) 137 (8) 123 (19) 151 (9) 102
concna naphthalene phenanthrene fluoranthene pyrene benz[a]anthracene chrysene benzo[b+k]fluoranthene benzo[a]pyrene indeno[1,2,3-cd]pyrene benzo[ghi]perylene C7 C10 C15 C20 C25 C30 C35 C40 total alkanes
% recoveryc
a Based on 16-h sonication. b Removals based on triplicate water extractions and 16-h sonication of the water extraction residues. Each extraction used liquid water (50 atm for 250 °C, 100 atm for 300 °C) for 15 or 60 min, followed by steam for 15 min (5 atm) at the same temperature. c Recoveries with water based on triplicate extractions at each condition compared to sonication. d Not detected (-).
substantially higher than the 250 °C extractions. As was the case at 250 °C, the 300 °C steam extractions extracted the bulk of the alkanes (Table 4), but the alkane removals were still much lower than those achieved using steam with the previous samples. The poor removals for the catalyst (compared to previous samples) might be caused by two reasons. First, the particle size of the catalyst (1-2 mm thickness) is so large that diffusion of the target analytes (from the interior of the matrix to water) takes longer than for the previous samples. Second, the organics may be tightly bound to active sites on the catalyst, therefore, making it harder to remove the organics from the catalyst. The higher recoveries achieved with 300 °C versus 250 °C indicate that the analytes were tightly bound to the catalyst matrix. However, since recoveries at 300 °C were still not quantitative, it appears that diffusion in the large particle of the catalyst limits the extraction efficiencies. If this is true, there are two ways to improve the PAH recoveries. One is to increase the extraction time, and the other is to reduce the particle size of the catalyst to achieve maximum surface area. Based on the discussion above, additional extractions were performed with liquid water at 300 °C and 100 atm for 60 min followed by sequential extraction for 15 min with steam at 300 °C and 5 atm. As shown in Table 4, extraction at 300 °C gave very good extraction efficiencies for the PAHs after 60 min (compared to the 15-min extractions listed in Table 4). In addition, while the PAHs were quantitatively removed from the catalyst with liquid water, the removal of alkanes >C20 was typically 80%) extracted the alkanes (Table 4). Note also that the 60-min extractions gave good PAH recoveries compared to 16 h of sonication (essentially identical to the results shown in Table 4 for the percent recoveries achieved with the ground catalyst). Grinding the catalyst was also effective in increasing extraction rates. The ground catalyst (ground with a mortar and pestle to a particle size
