Anal. Chem. 1995, 67, 4571-4576
Subcritical Water Extraction of Polychlorinated Biphenyls from Soil and Sediment Yu Yaw, Sewn B~wadt,Steven B. Hawthome,* and David J. Miller
Energy and Environmental Research Center, University of North Dakota, Grand Forks, North Dakota 58202-9018
Two certi6ed reference materials, an industrial soil (CRM 481) and a river sediment (NIST 1939), were extracted by subcritical water at 50 atm and temperatures ranging from 50 to 300 "C. The extraction efficiency of PCBs is extremely dependent on water temperature, since the polarity (dielectric constant) of water can be dramatically lowered by raising the water temperature. While only traces of PCBs were extracted at 50 and 100 "C, quantitative PCB removal was reached at 250 and 300 "C. The extraction rate of subcritical water extraction is fast. For example, the largest quantities of all of the PCBs were extracted during the first 15 min at 250 or 300 "C (at liquid conditions). However, when the pressure was reduced to 50 atm at 300 "C (steam), the extraction was completed in less than 5 min. The recoveries reached by water extraction at 250-300 "C and 50 atm compare favorably to the certitled values for both samples. The construction of an extractiodcollection system that allows quantitative extraction of PCBs with collection in a few milliliters of organic solvent is described. COz has been the most common fluid used for supercritical fluid extra~tion,'-~ but it often fails to efficiently extract many organics from environmental solids since its polarity is often too low to obtain efficient extra~tions.j-~ Adding organic modifiers to pure COZhas been shown to increase SFE efficiencies of some organic pollutants from certain matrice~.~J-"However, since the influence of modifier on extraction efficiencies can be dependent on the sample matrix and target analyte, choosing a modifer for a particular application can require trying modifiers with different polaritie~.~.~ While NzO and CHClFz have been used to significantly improve extraction rates and efficiencies of a wide variety of analytes from many samples,9J2-16these pure fluids may not (1) Hawthorne, S. B. Anal. Chem. 1990,62, 633A (2) Chester, T. L.; Pinkston, J. D.; Raynie, D. E. Anal. Chem. 1992,64, 153R (3) Janda, V.; Bartle, K. D.; Clifford, A A J. Chromatogr. 1993,642, 283. (4) Bmadt, S.; Hawthorne, S. B. /. Chromatogr. A 1995,703, 549. (5) Yang, Y.; Gharaibeh, A.; Hawthorne, S. B.; Miller, D. J. Anal. Chem. 1995, 67, 641. (6) Langenfeld, J. J.; Hawthorne, S. B.; Miller, D. J.; Pawliszyn, J. Anal. Chem. 1993,65, 338. (7) Hawthorne, S. B.; Miller D. J. Anal. Chem. 1994,66, 4005. (8) Langenfeld, J. J.; Hawthorne, S. B.; Miller, D. J.; Pawliszyn, J. Anal. Chem. 1994,66, 909. (9) Onuska, F. I.; Terry, K. A J. High Resolut. Chromatogr, 1989,12, 357. (10) Wright, B. W.; Wright, C. W.; Gale, R W.; Smith, R D. Anal. Chem. 1987, 59. 38. (11) Wheeler, J. R.; McNally. M. E. J. Chromatogr. Sci. 1989,27, 534. (12) Hawthome, S. B.; Langenfeld, J. J.; Miller, D. J.; Burford, M. D. Anal. Chem. 1992,64, 1614. (13) Hawthorne, S. B.; Miller, D. J. Anal. Chem. 1987,59, 1705. (14) Hawthome, S. B.; Miller, D. J.; Langenfeld, J. J. J. Chromatogr. Sci. 1990, 28, 2.
0003-2700/95/0367-4571$9.00/0 0 1995 American Chemical Society
be acceptable to use on a routine basis, because CHClFz can contribute to ozone depletion, and N20 is an explosion hazard.12J6 Like COZ,water is an environmentally acceptable solvent and is cost effective, but it has not yet received particular attention as an analytical extraction fluid. At ambient temperature and pressure, water has a dielectric constant (E) of -80,17making it an extremely polar solvent, and therefore the solubility of nonpolar organics in ambient water is very poor. For example, polychlorinated biphenyls (PCBs) have solubilities ranging from -1 (dichlorobiphenyls) to only -0.0005 pg/mL (heptachlorobiphenyls) in ambient water.18 However, the dielectric constant of water is mainly dependent on temperature (the higher the temperature, the lower the E) and only slightly dependent on pressure (the lower the pressure, the lower the E). Thus, the dielectric constant of water can be dramatically decreased to less than 10 (similar to methylene chloride) at supercritical conditions (> 374 "C and > 218 atm),17 making it an extremely effective solvent for organic pollutant^.'^-^^ Therefore, supercritical water has been used for efficiently extracting organics from coal (or for cleaning coal)z6-28 and destroying or converting hazardous organics from a wide variety of wastes into harmless byproducts by supercritical water o x i d a t i ~ n . ~The ~ . ~application ~ - ~ ~ of supercritical water for sample preparation in analytical chemistry is limited by at least two practical factors. First, supercritical water is corrosive and therefore can cause failure of the extraction cell. Second, the temperature (374 "C) required by supercritical water is likely too high for the extraction of some organics because of thermal (15) Sauvage, E.; Rocca, J.-L.; Toussaint, G. J. High Resolut. Chromatop, 1993, 16, 234. (16) Sievers, R E.; Hansen, B. Chem. Eng. News 1991,69 (29), 2. (17) Haar, L.; Gallagher, J. S.; Kell, G. S. National Bureau of Standards/National Research Council Steam Tables;Hemisphere Publishing Corp.: Bristol, PA, 1984. (18) Erickson, M. D. Analytical Chemistry of PCBs; Buttenvorth Publishers: Stoneham, MA, 1986. (19) Shaw. R. W.; Brill, T. B.; Clifford, A A; Eckert, C. A; Franck, E. U. Chem. Eng. News 1991,69, 26. (20) Jain, V. K. Environ. Sci. Technol. 1993,27, 806. (21) Franck, E. U.; Rosenmeig, S.; Christoforakos, M. Bey. Bunsenges. Phys. Chem. 1990,94, 199. (22) Uematsu, M.; Franck, E. U. J. Phys. Chem. Ref:Data 1980,9, 1291. (23) Heger, K.; Uematsu, M.; Franck, E. U. Bey. Bunsenges. Phys. Chem. 1980, 84, 758. (24) Gupta, R. B.; Panayiotou, C. G.; Sanchez, I. C.; Johnston, K P. AIChEJ. 1992,38, 1243. (25) Kocher, B. S.; Azzam, F. 0.;Lee, S. Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1993,339-342. (26) Deshpande, G . V.; Holder, G. D.; Bishop, A A; Gopal, J.; Wender, I. Fuel 1984,63, 956. (27) Kershaw, J. R /. Supercrit. Fluids 1989,2, 35. (28) Hu, H.; Guo, S.; Kurt, H.J. Fuel Chem. Technol., 1992,20, 82. (29) Thomason, T. B.; Modell, M. Hazard. Waste 1984,1, 453. (30) Hirth, T.; Franck, E. U.Bey. Bunsenges. Phys. Chem. 1993,97, 1091. (31) Li, L.; Gloyna, E. F.; Sawicki, J. E. Water Environ. Res. 1993,65, 250. (32) Krajnc, M.; Levec, J. Appl. Catal. B: Enoiron. 1994,3, L101.
Analytical Chemistty, Vol. 67, No. 24, December 15, 1995 4571
instability. In fact, supercritical water (with added oxidizing agents) is frequently used to cause degradation of hazardous organics as mentioned above.lg Fortunately, the dielectric constant of water can also be lowered substantially at relatively mild pressure and temperatures as low as 200-300 0C.17,33For example, the dielectric constant of liquid water is -27-29 at 250 "C (pressures from 50 to 350 atm), which is similar to those of ethanol ( E = 24) and methanol ( E = 33). (Note that the dielectric constant has very little dependence on pressure as long as sufticient pressure is available to maintain the liquid state.'? Thus, the solubilityof low-polarity organics can be dramatically increased by heating the water under moderate pressure. While little data are available for organics in subcritical water, the solubility of benzo[elpyrene has been reported to increase from 4 ng/mL in ambient ~ a t e I to 3 ~10 wt %35 (-25 million-fold) in hot water at 350 "C and -100 bar. We have previously demonstrated that water at 250 "C is an efficient extraction fluid for PAHs ranging in molecular weight from 128 to 276 am^.^^ PAH recoveries were shown to be highly dependent on water temperature but independent of extraction pressure (as long as sufficient pressure is used to maintain the liquid state). The purpose of the present study was to investigate the ability of subcritical water to extract PCBs from soil and sediment. Experiments were performed using liquid water at 250 "C (50 atm, density = 0.80 g/mL, E = 27),17liquid water at 300 "C (100 atm, density = 0.72 g/mL, E = 2O),I7 and steam (50 atm, density = 0.22 g/mL, E = l).17 The extraction system reported earlier was improved to include a collection cooling loop and a needle valve to control extraction flow rates. The extraction of PCBs from soil and sediment using water at 50 (or 100) atm and temperatures ranging from 50 to 300 "C and the influence of extraction time on extraction efficiencies of PCBs are described. EXPERIMENTAL SECTION Samples. Two real-world samples available as certiiied reference materials were selected for the determination of PCB extraction efficiencies: an industrial soil (CRM 481, Community Bureau of Reference, BCR, Brussels, Belgium),36and a river sediment (NISTSRM 1939, National Institute of Standards and Technology, Gaithersburg, MD). Both samples contained environmentally aged or "native" (not spiked) PCBs. The concentrations of the individual PCB congeners in the soil sample range from a few to -100 pglg, while the individual PCB concentrations are -100 times lower in the sediment sample. Extractions of the contaminated soil and sediment used 50- and 100-mg samples, respectively, placed at the outlet end of the extraction cell. The void volume of the cell was filled with precleaned (first using acetone, then n-hexane) white quartz sand (Aldrich, Milwaukee, WT). For the collection efficiency studies, the extraction cell was first filled with precleaned sand, and 100 pg of Arochlor 1254 (in 10 pL of methylene chloride) was spiked onto the sand. The cell was sealed immediately after spiking. Subcritical Water Extractions. All extractions were performed in a manner analogous to conventional supercritical carbon dioxide extraction. HPLC-grade water (Fisher Scientific, Fair Lawn, NJ) was first purged for 1-2 h with nitrogen to remove (33) Hawthorne, S. B.; Yang, Y.; Miller, D. J. Anal. Chem. 1994,66,2912. (34) Mackay, D.; Shiu, W. Y. J. Chem. Eng. Data 1977,22,399. (35) Sanders, N. D. Ind. Eng. Chem. Fundam. 1986,25,171. (36) Bswadt. S.;Johansson, B.; Wunderli, S.; Zennegg, M.; de Alencastro, L. F.; Grandjean, D. Anal. Chem. 1995,67, 2424. 4572
Analytical Chemistry, Vol. 67, No. 24, December 15, 1995
Shut-off valves
Pre-heat coil
r Extract Ion water Col l a c t Ion sol vent
Figure 1. Schematic diagram of subcritical water extraction using cooling/collection system.
dissolved oxygen and then filled into an Isco Model 260D syringe pump. The pump was operated in the constant pressure mode (or in the constant flow mode for steam extraction) to supply water through the l/l&.-o.d. (0.020 in. i.d.) stainless steel tubing (including a 4 m preheating coil placed inside the Hewlett-Packard Model 5730A GC oven) to the extraction cell. Empty HPLC columns (30 mm long, 4.6 mm id.) from Keystone Scientific (Bellefonte, PA) were used as extraction cells for all extractions. Extraction cells were mounted vertically in the 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 l / 1 ~ x '/Isin. Parker tubing union, which was itself connected to flow restrictors to provide a water flow rate of -1 mL/min. Two methods were used for controlling water flow. First, restrictors constructed from 50-pm-i.d. x 1 k m lengths of fused silica tubing (Polymicro Technologies, Phoenix, Az) were used for sequential temperature and sequential time extractions to provide a water flow rate of -1 mL/min at 250 "C and 50 atm measured as liquid water at the pump.37 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 fused silica capillary tubing without the cooling loop was used as restrictors for these extractions, since this restrictor setup does not require a loop rinsing step after each sequential step of the extraction. With this restrictor, the collection solvent evaporated during the extractions at higher water temperatures (200-300 "C), and therefore the collection vial was cooled with ice water to avoid solvent evaporation. Since preliminary extractions demonstrated that the fused silica restrictor was subject to frequent plugging, a second flow control method utilizing a stainless steel needle valve was developed to replace the fused silica restrictor as a flow control device. As shown in Figure 1, the setup for the extractions using a needle valve was similar to that for the extractions using a fused silica restrictor, except that a Swagelok stainless steel needle valve (Swagelok Co., Solon, OH) was used to replace the fused silica restrictor as a flow control device. (Note that this approach works much better for water than for supercritical COa, since water does not vaporize with resultant cooling from the Joules-Thompson effect after depressurization like COz.) 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 (l/16 in. 0.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. (37) Yang. Y.; Hawthorne, S. B.; Miller, D. J. J. Chromatogr. A 1995,690, 131.
After assembling the extraction cell in the GC oven, the cell was pressurized with 50 or 100 atm of water. The inlet valve of the cell was open while the GC oven was heated to the desired temperature. Therefore, the water was kept at the pump pressure during the heatup time (typically -1,2,3,4,5, and 7 min for 50, 100, 150, 200, 250, and 300 "C, respectively). Collection of the extracted analytes was performed by inserting the outlet of the fused silica restrictor or the outlet tubing from the needle valve into a 22-mL glass vial containing 3 mL of collection solvent. Thus, the water percolated through the dense collection solvent (methylene chloride), allowing the extracted analytes to partition into the solvent. Three solvents with less density than water (nheptane, toluene, and cyclohexane) were also evaluated to determine PCB collection efficiencies. After extraction, two PCB internal standards (PCB-35 and PCB-169) were added into the collection vial. 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 less chlorinated biphenyls (which 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- to 3-mL aliquot of collection solvent, and the two organic solvent aliquots were combined for GC analysis. Because GC/ECD was used for PCB analysis in this study, the methylene chloride samples were evaporated to near dryness, and n-heptane was added for the final GC solvent. After extraction with water, selected sample residues were mixed with sodium sulfate (1:l) and again extracted by sonication (bath sonicator) for 16 h in 5 mL of methylene chloride and acetone (1:l) to determine the concentration of any remaining PCBs. Extract Analysis. Extracts were analyzed using an electronic pressure-controlled Hewlett-Packard 5890 I1 gas chromatograph (GC) equipped with two 'j3Ni electron capture detectors (ECDs) held at a temperature of 300 "C (purged with 60 mL/min of nitrogen) and a HP 7673A autosampler. Aliquots (1 pL) of the extracts were injected oncolumn into two parallel coupled columns, a 60 m x 0.25 mm, 0.25 pm 50% diphenyldimethylsiloxane DB-17 column (J&W Scientific) and a series combination of a 25 m x 0.25 mm, 0.25 pm 5%diphenyldimethylsiloxane SILS column (Chrompack) and a 25 m x 0.22 mm, 0.10pm l,7-dicarbacloso-dodecaboranedimethylpolysiloxane HT-5 column (Scientific Glass Engineering). The columns were installed in the GC oven together with a deactivated 2 m x 0.53 mm fused silica retention gap using a quick-seal glass tee (Restec Corp., Bellefonte, PA). The GC oven initial temperature was 90 "C, for 2 min; then increased at a rate of 20 "C/min to 130 "C; and then increased at a rate of 2 "C/min to 275 "C and retained for 5 min. Hydrogen linear velocity was -43 cm/s, held constant by the pressurecontrolled inlet throughout the whole temperature program (starting pressure, 1.7 bar at 90 "C). This choice of columns and GC conditions has previously been shown to give optimum separation of individual PCB congeners as well as organochlorine pesticide^.^^^^^
Quantitative measurements of PCBs were performed using peak heights after a 7-point multilevel calibration curve was obtained using the power fit calibration routine provided with the (38) Rahman, M. S.; Bmadt, S.; Larsen, B. j High Resolut. Chromatogr. 1993, 16,731. (39) Bmadt, S.; Skeje-Andresen, H.; Montanarella, L.; Larsen, B. Int. J Enoiron. Anal. Chem. 1994,513, 87.
Table 1. PCB Removal Efficiencies from Industrial Soil (CRM 481) Obtained by Sequential Extraction with Higher Temperatures
liquid water temperature ("C) water dielectric constant PCB-52 PCB-101 PCB-118 PCB-149 PCB-153 PCB-105 PCB-138 PCB-128 PCB-156 PCB-180 PCB-170
50
200 35
250 27
300
Cumulative Removal* (%) 0 15 36 71 0 4 16 53 0 3 10 49 0 2 11 51 0 1 9 4 8 0 0 14 52 0 1 9 48 0 1 8 50 0 0 6 4 7 0 0 4 4 3 0 0 5 4 5
100 94 92 93 92 92 92 93 90 90 91
100 100 100 100 100 100 100 100 100 100 100
70
100 56
150 44
steamu
-
1
At 50 atm, water is liquid at 250 "C and lower temperatures and steam at 300 "C. 100% removal is defined by the lack of any detectable species in the 1Sh sonication extract (using methylene chloride and acetone) of the residues after a 6@min (10 min at each temperature) water extraction.
HP Chem 3365 software. PCBs were calibrated in the concentration interval of 1.7-573 pg/pL on each of the two columns, and the lowest results obtained from the quantitation of the extracts were adapted as the value closest to the true value. Standards were injected after every fifth sample to determine deterioration of separation or drift. Extracts of the sediment sample were cleaned by acid silica before the analysis to avoid gradual column deterioration following oncolumn injection.40 RESULTS AND DISCUSSION
Influence of Water Temperature on Extraction Efficiencies. The effects of water temperature (at a constant prissure of 50 atm and a flow rate of -1 mL/min) on the extraction efficiencies of PCBs from industrial soil and river sediment are summarized in Tables 1and 2, respectively. These temperature experiments were done by sequential extractions of a single sample aliquot at six different temperatures, and the extraction time for each temperature was 10 min (therefore, the total time used for the extraction was 60 min). The 100%removal indicated in Tables 1and 2 is defined by the lack of any detectable species in the l&h sonication extracts (using methylene chloride and acetone) of the residues after the 60-min water extractions. As shown in Tables 1 and 2, almost no PCBs were extracted at 50 and 100 "C, and even at 150 "C, only 5-36% of the PCB congeners were extracted. However, as the temperature was increased to 200 "C, the removal of all of the PCBs (in 10 min) from both samples increased substantially, and the removal of the less chlorinated biphenyls was generally higher than 50%, while the removal of the highly chlorinated PCBs was normally lower than 50%at 200 "C (this phenomenon was especially clear for PCB removal from the sediment). The differences in removal which occurred between low and high molecular weight PCB congeners demonstrate that the less chlorinated biphenyls are easier to extract than the highly chlorinated biphenyls, which is consistent (40) Bmadt, S.; Hartonen, K; Hawthorne, S. B.; Miller, D. J.; Grobecker, K H. Proceedings of PittCon '95, New Orleans, LA, March 1995.
Analytical Chemistry, Vol. 67, No. 24, December 15, 1995
4573
Table 2. PCB Removal Efficiencies from River Sediment (SRM 1939) Obtained by Sequential Extraction with Higher Temperatures
liquid water temperature ("C) water dielectric constant PCB-28 PCB-52 PCB-101 PCB-118 PCB-149 PCB-153 PCB-105 PCB-138 PCB-128 PCB-156 PCB-180 PCB-170
50 70
100 56
150 44
200 35
Cumulative Removalb(%) 0 1 29 84 1 3 32 93 0 0 10 75 0 0 6 5 0 0 0 7 5 9 0 2 8 4 7 0 8 15 60 0 10 17 51 0 2 28 57 0 0 5 2 2 0 9 11 20 0 14 20 30
steama 250 27
300
th
+P m - m +PCB-52
+PCB-105
1
+PCB-149
+PCB-138
94 100
95 97 99 98 97 94 94 95 97 97
*PCB-170
100 100 100
100 100
100 100 100 100 100 100 100
a At 50 atm, water is liquid at 250 "C and lower temperatures and steam at 300 "C. 100%removal is defined by the lack of any detectable species in the 16h sonication extract (using methylene chloride and acetone) of the residues after a 60-min (10 min at each temperature) water extraction.
0
10
4574
Analytical Chemistry, Vol. 67,No. 24, December 15, 1995
30 Time [min]
40
50
60
Figure 2. Effect of extraction time on the extraction efficiency of representative PCBs from an industrial soil using water at 250 "C and 50 atm. Percent removals are based on sonication of the residue after water extraction. Table 3. PCB Removals and Recoveries from Sediment (SRM 1939) Obtained by Water Extractions at 250 "C and 50 atm for 15 min
PCB with the decreasing water solubility at ambient conditions with molecular weight.Is A similar trend was reported when water was used to extract PAHs from a highly contaminated soil, where the recoveries of the low molecular weight PAHs (up to phenanthrene and anthqacene) were quantitative at 200 "C compared to those certified based on Soxhlet and sonication extraction, although the yield of the high molecular weight PAHs was only around 300/0.~~ Further increases in PCB removal for both samples were achieved by increasing the temperature to 250 "C, and the removals after 10 min at 250 "C were between 90 and 100%(based on sonication of the residue). Please note that, despite the fact that the water dielectric constant of 27 (at 250 "C and 50 atm) was still quite high compared to those of PCBs of ~ 5 , the 3 ~polarity of water was sufficiently reduced at 250 "C so that water was an effective extraction solvent. The 100%removal was reached at 300 "C for all of the individual PCB congeners. Influence of ExtractionTime on PCB Removals. As shown in Tables 1 and 2 by sequential extractions at different temperatures, the extraction of all of the PCB congeners was efficient at 250 and 300 "C (both at 50 atm) with the 10-min sequential extractions. To determine the optimal extraction time, the PCB extraction rates were determined for the industrial soil sample at 250 "C and 50 atm. As shown in Figure 2, the extraction of the tri- and tetrachlorobiphenyls (PCB-28 and PCB-52) was essentially complete after -10 min, and additional extraction time above 15 min (even up to 60 min) yielded no increase in removal. For the penta- (PCB-105) and some of the hexachlorobiphenyls (PCB149 and PCB-138),90%removal was reached after either 15 or 20 min. The removal of the hepta- and some of the hexachlorobiphenyls achieved in 15 min ranged from 70 to 78%. Figure 2 also demonstrates that the largest quantities of all of the PCBs were extracted during the first 15min and that increasing the extraction time to 60 min yielded only small increases in the amounts of PCBs that were extracted. In addition, if water extraction is performed for 60 min, -60 mL of water extractant will be collected after the extraction because of the 1mL/min water flow rate. This
20
2,3',5tri-CB/ 2,4,4'-tri-CB 2,2',3,5'-tetra-CB 2,2',5,5'-tetra-CB 2,2',4,5,5'-penta-CB 2,3',4,4',5penta-CB 2,2',3,4',5',6-hexa-CB 2,2',4,4',5,5'-hexa-CB 2,3,3',4,4'-penta-CB 2,2',3,3',4,4'-hexa-CB 2,3,3',4,4',5hexa-CB
no.
concn .. . ~ . ~ ~ @g/g) removala (%) recove@ (%)
(%RSD)
26 4.20e (6.9) 28 2.21e (4.5) 44 1.07e(11.2)
52 101 118 149 153 105 128 156
4.48d(1.3) 0.82d(1.2) 0.51d(2.0) ndg nd nd 0.1W (10)
nd 2,2',3,4,4',5,5'-heptaCB 180 0.16d(6.3)
(%RSD)b
(%RSD)b
>99 (99 ( ~ 1 ) >99 (99 (
