Environ. Sci. Technol. 1996, 30, 1053-1060
Effect of Hydrochloric Acid on Sampling and Analysis of Semivolatile Organic Compounds in Incineration Flue Gas. 1. Chlorophenols LIANG K. TAN* AND ALBERT J. LIEM Alberta Environmental Centre, P.O. Bag 4000, Vegreville, Alberta, Canada T9C 1T4
The recoveries of 16 isomers of chlorophenols (CPs) from the adsorbent (XAD-2) of the MM5 train are affected by hydrochloric acid (HCl) present in the incinerator flue gas. At 2.05 and 5.53 mg/L in flue gas, HCl caused desorption of CPs from XAD-2. The desorption is dependent upon the number of chloro substituents in the CPs. The Soxhlet extraction behaviors of CPs from XAD-2 in the presence or the absence of HCl were also studied in the laboratory. The results explain the previously reported low recovery of pentachlorophenol from XAD-2 when the flue gas contained none or very low concentrations of HCl. An extraction procedure for complete CPs recoveries from XAD-2 is proposed that involved two sequential extractions: first with dichloromethane and then with the addition of 1.0 mL of 1.0 M aqueous HCl on XAD-2 and followed by the subsquent extraction with dichloromethane. This procedure does not affect other semivolatile compounds on XAD-2 and leads to [13C6]pentachlorophenol surrogate recovery of 90 ( 6%. An extraction anomaly of CPs using dichloromethane from the combined synthetic acidic condensate and acidic ethylene glycol is presented. Toluene is suggested as the solvent for complete CPs extraction from the combined liquids. This acid effect led to a suggestion of using at least one surrogate isomer for each CP group on XAD-2 of the MM5 train.
Introduction Incineration is widely used for destroying principal organic hazardous constituents (POHCs) in wastes. Improper design or operation of incinerators can lead to incomplete destruction of POHCs and can produce other equally or more hazardous products of incomplete combustion (PICs). Sampling and analysis of organic compounds in flue gas are therefore important for assessing incinerator system performance. A “Modified Method 5” (MM5) has been developed for sampling (1, 2) and analysis (3-6) of semivolatile organic compounds in flue gas. In this method, flue gas is sampled
0013-936X/96/0930-1053$12.00/0
1996 American Chemical Society
downstream of the scrubber, and a measured volume of the gas is passed through a cartridge containing XAD-2, onto which organic compounds are adsorbed. The adsorbed compounds are then recovered by solvent extraction, separated by column chromatography cleanup or derivatization, identified, and measured using a gas chromatograph with mass spectrometer detector (GC/MS). However when sampling is conducted upstream of the scrubber, there are complications arising from the fact that incineration of chlorinated organic waste generates HCl. The HCl concentration in flue gas varies depending on the input of chlorinated organics into the incinerator. Therefore, it is important to investigate the effect of HCl on the MM5 sampling and analysis. The retention behavior of organic compounds on XAD-2 has been correlated to their breakthrough volume based on a gas chromatography experiment using the XAD-2 column (7, 8). This approach is limited to compounds with lower boiling points and in the absence of corrosive gases such as HCl. A laboratory evaluation of the MM5 sampling of simulated flue gas has been reported (9, 10) where water vapor, HCl, SO2, and NOx were added to the sample matrix. Of the 12 compounds studied, only pentachlorophenol and chlorobenzene belonged to the 45 semivolatile organic compounds specified in the Canadian emission guidelines for incinerators (11). Pentachlorophenol was not consistently recovered (0-147%) (9, 10). The effect of HCl on the recoveries of semivolatile organic compounds from XAD-2 (in a pilot plant and in the laboratory) and from condensate and ethylene glycol (in the laboratory) are investigated. All the semivolatile organic compounds specified in Canadian emission guidelines (11) were included in this study: chlorophenols (CPs), polycyclic aromatic hydrocarbons (PAHs), chlorobenzenes (CBs), polychlorinated biphenyls (PCBs), polychlorinated dibenzofurans (PCDFs), and polychlorinated dibenzo-p-dioxins (PCDDs). This paper presents only the results for CPs. The results for other compounds will be presented later.
Experimental Section Chemicals. XAD-2 polystyrene (Terochem) was purified by thorough washing with deionized water and then by washing with methanol on a Bu ¨ chner funnel with suction. It was further cleaned under reflux with methanol for 2 days and with dichloromethane for at least 1 day to ensure that the resin was free of target compounds. The clean XAD-2 was dried in a cartridge using a gentle stream of warm nitrogen (40-50 °C), and 30 g of it was used immediately for flue gas sampling. All solvents (n-hexane, dichloromethane, isooctane, and toluene) were of distilled-in-glass grade. Acetic anhydride, potassium carbonate, sodium hydroxide, concentrated hydrochloric acid, and ethylene glycol were of certified ACS grade. Sixteen isomers of chlorophenols and three phenols with bromo and fluoro substituents were obtained from Ultra Scientific and Aldrich. Their names are listed in Table 1. Unless otherwise specified, all of these phenols were abbreviated as CPs throughout the text. A stock solution of 1000 µg/mL in toluene was prepared for each isomer. Working solutions of CP mixtures in n-hexane were
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TABLE 1
Percent Recovery of CP from XAD-2 of MM5 Train HCl in flue gas (mg/L) CPa 4-chloro2,3-dichloro2,4-dichloro3,5-dichloro3,4-dichloro2,6-dichloro-
0.014
94 89 87 99 99 84 92 ( 7 3,4,5-trichloro102 2,3,5-trichloro91 2,3,4-trichloro97 2,4,5-trichloro94 2,3,6-trichloro89 2,4,6-trichloro91 94 ( 5 2,3,4,5-tetrachloro100 2,3,5,6-tetrachloro94 2,3,4,6-tetrachloro100 98 ( 4 pentachloro106 95 (96)b [13C6]pentachloro2,6-dichloro-4-fluoro- 86 2,6-dibromo-4-fluoro- 105 2,4,6-tribromo107
pKa
2.05
5.53
lit.c
63 60 67 66 69 74 67 ( 5 73 73 77 78 81 86 78 ( 5 83 77 93 84 ( 8 91 86 (83)b 73 90 92
16 34 36 43 43 52 42 ( 7 46 46 46 50 63 64 53 ( 9 55 69 87 70 ( 16 89 89 (80)b 34 66 82
9.37 7.71 7.90 8.25 8.62 6.78
lit.d
7.55 8.25 6.43 6.97 6.72 5.80 5.99 5.64 7.04 5.03 5.22 6.62 4.74
a 5.0 µg of each CP isomer was spiked on XAD-2 prior to the sampling of flue gas. b Data in parentheses are the surrogate recoveries from the blank trains. c Literature data from ref 18. d Literature data from ref 19.
prepared at 10 or 40 µg/mL. Surrogate solutions of [13C6]pentachlorophenol (Cambridge Isotope Laboratories) were prepared separately in the same manner. The internal reference compound used in GC/ECD and GC/MS measurements was p-dibromobenzene (Aldrich). Glasswares. Pyrex glass wool and Teflon boiling chips and stirring bars were rinsed with n-hexane and then dichloromethane. All glasswares were sequentially cleaned with detergent, deionized water, acetone, n-hexane, and dichloromethane. They were then baked in an oven at 150 °C overnight. Spiking of Surrogates and Target Compounds. Working solutions of surrogates and/or target compounds were spiked on the purified XAD-2 in an adsorbent cartridge (125 µL of 40 ng/µL CPs in hexane, 500 µL of 40 ng/µL PAHs in toluene, 250 µL of 20 ng/µL CBs/PCBs in hexane and 100 µL of 1.0 ng/µL PCDDs/PCDFs in toluene). After spiking, glasswool was placed on top of the XAD-2. The cartridge was then incorporated into the MM5 train and the train was immediately used for experiment. The amounts of CPs spiked on XAD-2 were equivalent to gas phase concentrations of 0.15 (estimated based on pentachlorophenol) to 0.31 ppb (estimated based on monochlorophenol) per CP isomer. In the emission guidelines for incinerators (11), anticipated emissions for CPs under good incineration performance was 1 µg/m3 in flue gas, which was equivalent to 0.1-0.2 ppb. The above method of spiking on XAD-2 was designed for the study of the effect of HCl on the retentions or the recoveries of these compounds from XAD-2. Another phenomenon, the effect of HCl on the adsorption of CPs in gas phase onto the XAD-2 was not included in this study because it required spiking of CPs into the hot flue gas prior to the MM5 train with the probability of being
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FIGURE 1. Schematic diagram of MM5 sampling train. The control box (CB) maintains the flow rate of flue gas from the stack (S). The gas is drawn through a probe (PR) and passed through the condensor (C) and then a cartridge containing XAD-2. The condensate (COND) is collected, and the flue gas passes through an impinger containing ethylene glycol (EG), an empty impinger (E), and an impinger containing silica gel (SG) prior to contact with control devices in the CB and being discharged through an exhaust (EX). P is a pump to circulate cool water for the condensor. B is an icebath.
degraded or reacted prior to reaching the MM5 train. The latter acid effect required a separate study using a different appraoch. MM5 Train. All components of the train were as previously stated in the literature (1, 2), except the filter was removed because of the absence of particulates in the flue gas (Figure 1). A leak test was performed as stated in the standard protocol (1, 2). Sampling was carried out at an isokinetic rate of 12 L/min, and the total volume collected was approximately 2800 L/sample. Flue Gas Sampling in the Pilot Incinerator. A description of the incineration research facility has been given previously (12). The pilot incinerator was used only as a flue gas generator. Measured rates (1.7, 19, and 56 mL/ min) of 1,2,4-trichlorobenzene were introduced into the feed line or after the flame of the primary burner to produce low, intermediate, or high HCl emission rates, respectively. The HCl concentration in the flue gas was measured by a continuous analyzer and by the standard impinger method with good agreement. The results were 0.014, 2.05, and 5.53 mg/L HCl in flue gas. Moisture (H2O), carbon dioxide (CO2), and oxygen (O2) levels in the flue gas were kept constant at 14, 7, and 10%, respectively. The incinerator was operated to generate flue gas with an absence of POHCs and PICs. A blank train was used to verify the absence of organic compounds generated, while a test train was used to determine the recoveries of spiked organic compounds from XAD-2. In the blank train, the XAD-2 was spiked with surrogates only; whereas in the test train, it was spiked with both surrogates and target
TABLE 2
HCl Contents in Sampling Train HCl in flue gas
XAD-2
mg/L
ppma
0.014 2.05 5.53
103 0.003 1260 0.20 3390 0.58
g
mol × 103 0.09 5.50 15.9
condensateb g
M
0.50 0.051 5.95 0.610 18.7 1.88
ethylene glycolb g
M
0.005 0.024 0.11
0.001 0.005 0.024
a Based on volume. b The volume of condensate and ethylene glycol collected after the sampling of flue gas was 270 and120 mL, respectively.
compounds. The amounts used for spiking were 5.0, 20, 5.0, and 0.1 µg (per isomer), respectively, for CPs, PAHs, CBs/PCBs, and PCDDs/PCDFs. The blank and test trains were operated simultaneously at the same sampling location, upstream of the scrubber, and under the same conditions. Organic compounds in the XAD-2 after sampling of flue gas were immediately Soxhlet extracted with dichloromethane overnight. Although the XAD-2 was wet, the concentrated raw extract did not contain an immicible aqueous phase. The concentrate was equally divided into four portions. Three separate 25% portions were used for the analysis of CPs, PAHs, and CBs/PCBs/PCDDs/PCDFs, while a 25% portion was reserved as a contingency measure. The portion for CPs was not dried with anhydrous sodium sulfate, instead it was directly derivatized as outlined below. Separate Laboratory Soxhlet Extraction Experiment with HCl in XAD-2. Soxhlet extractions of CPs with dichloromethane were conducted at different known quantities of HCl in XAD-2. This experiment was performed to better understand the effect of HCl on the Soxhlet extraction behavior of each CP isomer and to show that the
decreases of CPs overall recoveries from XAD-2 of the MM5 train were not caused by the HCl during Soxhlet extraction. Five micrograms of each CP isomer (identical to pilot plant experiment) was spiked on XAD-2 in the thimble of a Soxhlet apparatus. Aliquots of 0.5 mL of HCl solution of various concentrations were added to cover the range of 0.05-6.0 mmol. Glass wool was placed on top of the XAD-2, and CPs were then extracted with dichloromethane for either 4 or 16 h as indicated in the text. A 25% portion of the raw extract was derivatized and measured as described below. Separate Laboratory Liquid-Liquid Extraction Experiment with Synthetic Condensate and Ethylene Glycol. Various synthetic acidic condensate and ethylene glycol solutions were prepared by adding HCl to deionized water or ethylene glycol to simulate the corresponding solutions found in the MM5 train after sampling of flue gas. The volumes and HCl concentrations were based on the experimental data of flue gas sampling in the pilot incinerator (Table 2). An aliquot of CPs mixture (1.25 µg per isomer) was added to the synthetic condensate or the combined synthetic acidic condensate and acidic ethylene glycol prior to or after adjustment of the pH. Four sequential extractions (100, 75, 75, and 50 mL) were done in a separatory funnel using dichloromethane, n-hexane, or toluene. The combined organic phase was concentrated and then processed for derivatization. Derivatization of CPs. Aqueous 0.1 M potassium carbonate (K2CO3) was purified by three serial extractions with dichloromethane. The CPs extract was added to a 500-mL separatory funnel containing 100 mL of n-hexane that had been purified by three serial extractions with aqueous 0.1 M K2CO3. Three serial extractions (30, 20, and 20 mL) were carried out using aqueous 0.1 M K2CO3. An
FIGURE 2. Typical GC/ECD chromatograms obtained from the extraction of CPs from XAD-2 using dichloromethane at various conditions: (A) standard solution; (B) in the absence of acid and water; (C) in the presence of 10 mL of water; (D-F) in the presence of 0.05, 0.38, and 0.50 mmol of HCl, respectively. The peak labeled R is due to p-dibromobenzene, which served as internal reference. Peaks due to CPs in the order of increasing retention times are as listed in Table 3.
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aliquot of 0.5 mL of acetic anhydride was added to the combined aqueous phase in a 150-mL narrow neck Erlenmeyer flask and stirred vigorously for 5 min. A n-hexane/isooctane mixture (7 mL, 6:1) was added to the solution, and vigorous stirring was continued for 6 min. Deionized water was then added to bring the organic layer up to the neck of the flask. The acetylation product in n-hexane was transferred quantitatively into a 15-mL centrifuge tube. Rinsing was done at least eight times with small fractions of n-hexane. This combined solution was further concentrated to 1.0 mL under a gentle stream of nitrogen at 40 °C. Instrumentation. A Varian 6000 gas chromatograph equipped with a splitless injector and a 30 m × 0.2 mm i.d. × 0.33 µm film thickness of 5% phenylmethyl polysiloxane fused capillary (HP 5) column was directly coupled to an HP 5970 mass spectrometer ion source, with electron impact in selected ion monitoring mode (SIM). This integral system was employed for GC/MS conformation and quantitation. The temperature program was 90 °C for 1 min, to 220 °C at 6 °C/min, to 280 °C at 35 °C/min, and then held for 0.63 min. Parameter settings and the ion masses monitored within each window of the SIM program followed the previously established method (5). A Varian 3400 gas chromatograph with electron capture detector (GC/ECD) equipped with an autosampler was also used where confirmation was not necessary. The experimental conditions were similar to those stated above, except the temperature program was 60 °C for 1 min, to 140 °C at 20 °C/min, to 200 °C at 4 °C/min, and then held for 1 min. Test solutions run on the GC/ECD were diluted to 250 ng/mL. Quantitation. In the recovery study, quantitation of CPs was based on relative response factors for single calibrations with external standards. The standard solution was a mixture of equivalent spiked solutions with concentrations equal to those added on the XAD-2 or other liquid samples in the experiments. The standard solution was also treated for derivatization in the same manner as that of the sample.
Results and Discussion Water and HCl Contents in the MM5 Train Components. After sampling of flue gas containing 0.014, 2.05, and 5.53 mg/L HCl, water contents were measured by weight differences. The water contents in the MM5 train were as follows: 3-6 g in XAD-2, 270 g in condensate (COND), and 20 g in ethylene glycol (EG). The measured HCl contents are listed in Table 2. The data indicate that an increase of HCl content in these MM5 train components was proportional to the increase in the concentration of HCl in flue gas. Effect of HCl in the Flue Gas. The quantities of CP recovered from XAD-2 of the MM5 train after sampling reflect the “overall recoveries” whereas the quantities of CPs extracted from XAD-2 in a separate laboratory experiment (discussed in the following section) reflect only the “Soxhlet extraction recoveries”. In all experiments, the total CPs found in the blank train were negligible, only about 2% of the amounts spiked on the XAD-2 in the test train. The overall recovery data at various HCl concentrations in flue gas are listed in Table 1. The overall recoveries of the surrogate ([13C6]pentachlorophenol) in the blank and in the test trains were close to the recoveries for the corresponding target compound (pentachlorophenol). At 0.014 mg/L HCl in the flue
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gas, which serves as an upper limit model for scrubbed flue gas (11), all CP isomers were well recovered from XAD-2. The mean percent recovery from monochloro- to pentachlorophenols is 95 with the relative standard deviation of 6%. At higher HCl concentrations, the overall recovery values of some CP isomers were lower; the 4-chlorophenol was as low as 16% and pentachlorophenol was as high as 89%. The decrease in CP overall recoveries was not caused by reaction of CP with HCl. Phenol reacts via an intermediate of oxonium ion toward electrophilic substitution. It reacts with electrophilic reagents such as sulfuric acid or nitric acid, but not with HCl, which is a nucleophilic reagent. This is also supported by the results from separate laboratory experiments (discussed in the next section) showing that the more HCl added to XAD-2 containing CPs, the better the Soxhlet extraction recoveries of CPs. HCl lowers the recovery of CPs by desorbing them from the XAD-2 adsorbent during sampling of flue gas. This process works particularly well because the XAD-2 was wet from moisture in the flue gas. The decrease in CP overall recovery as the HCl concentration in flue gas is raised can be observed in Table 1. In this table, CPs are listed in the order of decreasing desorption. The adsorption of 4-chlorophenol on XAD-2 is affected most by the presence of acid. Among all CPs, 4-chlorophenol is the most polarized by the electron-withdrawing group of the chloro substituent in the para position (without any effect from the meta or ortho substituent). Solvation in water is then most favorable and, therefore, so is desorption from XAD-2. In contrast, pentachlorophenol is the most resistant. In general, the higher the number of chloro substituents on the CPs, the more concentrated the acid required for its desorption from XAD-2. Also, the positions of the chloro substituents seem to determine the susceptibility of CP to desorption. Among the isomers of each group with the same number of chloro substituents, 2,6-dichlorophenol, 2,3,6-/2,4,6-trichlorophenols, and 2,3,4,6-/2,3,5,6-tetrachlorophenols are more difficult to desorb from XAD-2. These CPs have two chloro substituents that are ortho to the hydroxyl group. The ortho substituent in phenol has previously been considered to inhibit water solvating the oxygen and the hydrogen of the hydroxyl group through a hydrogen bond (13). CPs have also been categorized based on their hydrogen bonding affinity and dipole moments in the relation with their pKa (14). Overall recoveries of CPs at 2.05 and 5.53 mg/L HCl are inversely correlated to pKa values (r ) -0.86 and -0.90; slopes ) -0.60 and -12.5, respectively). These experimental results on the desorption of various CP isomers from XAD-2 by HCl in flue gas have not been reported in the literature and have important implications in the sampling and analysis of CP in incinerator flue gas. Effect of HCl on the Soxhlet Extraction of CPs from XAD-2. Although GC/MS was used, GC/ECD chromatograms (Figure 2) were chosen to better illustrate the CPs extraction behavior because of the different peak sensitivities of various CP groups based on their number of chloro substituents. The GC/ECD, however, cannot detect the 4-chlorophenol derivative because one chloro substituent at the para position caused this compound to have a very small electron capture coefficient (15). Therefore, its peak is not shown in Figure 2, panels A-F. Figure 2A is a chromatogram of a CP standard mixture that has been derivatized and to which an internal reference compound
TABLE 3
Percent Recovery of CP from Soxhlet Extraction of XAD-2 in the Presence of HCl retention time of CP derivative (min) GC/MS
GC/ECD
CPa
mean ( SDb
9.33 9.93 11.52 11.99 12.27 12.73 13.30 13.87 14.19 15.20 15.38 15.50 16.49 16.77 18.19 18.28 19.50 20.30 21.95
-c 6.46 7.35 7.62 7.80 8.10 8.49 8.91 9.15 9.95 10.10 10.20 11.07 11.35 12.73 12.83 14.14 15.01 16.97
4-chloro2,6-dichloro-4-fluoro2,6-dichloro2,4-dichloro3,5-dichloro2,3-dichloro3,4-dichloro2,6-dibromo-4-fluoro2,4,6-trichloro2,3,6-trichloro2,3,5-trichloro2,4,5-trichloro2,3,4-trichloro3,4,5-trichloro2,3,5,6-tetrachloro2,3,4,6-trichloro2,3,4,5-tetrachloro2,4,6-tribromopentachloro-
92 ( 3 94 ( 5 90 ( 6 90 ( 7 97 ( 2 91 ( 8 97 ( 2 94 ( 5 91 ( 4 92 ( 3 93 ( 3 95 ( 3 91 ( 2 101 ( 2 97 ( 2 96 ( 2 98 ( 2 96 ( 3 95 ( 4
a 5.0 µg of each CP isomer was spiked on XAD-2. b Mean and standard deviation based on four separate extractions of CPs from XAD-2 containing 0.50, 1.0, 2.0, and 3.0 mmol of HCl. c Cannot be detected by GC/ECD.
has been added (labeled with R). The dilution factor for this standard was the same as that of the samples in Figure 2, panels B-F. The amplitude scale was also kept the same in all of these chromatograms. The names of the peaks and their retention times are listed in Table 3. When there was no acid in the XAD-2, 72% of 4-chlorophenol was extracted after 4 h as measured by GC/MS. Only 1-2% of some dichlorophenols were extracted (Figure 2B). The remaining CPs were not extractable at all. Under this condition, toluene also could not extract CPs completely, and methanol or acetone were not attempted since they are not suitable for other target semivolatile organic compounds (PAHs, PCBs, PCDDs/PCDFs). The difficulty with low recovery of pentachlorophenol has been reported previously in the literature (5, 9, 10). An extension of the extraction time to 16 h improved the Soxhlet extraction recovery of 4-chlorophenol (94%) and dichlorophenols (0.9-35%). However, negligible amounts of higher chlorinated phenols were recovered. When there was 10 mL of water in the XAD-2 and without HCl (Figure 2C), 4-h extraction led to almost complete Soxhlet extraction recovery for the dichlorophenols except the 2,6-dichloro isomer (43%). In addition, 39-44% of the 3,4,5- and 2,3,4trichlorophenols were recovered; however, the 2,3,6-/2,4,6isomers as well as the tetra- and pentachlorophenols were not. When the extraction was carried out with the addition of dilute HCl on XAD-2 (0.05 and 0.37 mmol in Figure 2, panels D and E, respectively), CPs were increasingly recovered. With the addition of 0.50 mmol of HCl, the chromatogram of the extract (Figure 2F) is the same as that of the standard (Figure 2A). Similar acid dependence in the Soxhlet extraction recovery of the brominated or fluorinated phenols was also observed. Figure 3 shows the Soxhlet extraction recoveries of selected CPs as a function of the amount of acid on XAD-2. To avoid overcrowding, only the 4-chloro-, 3,4-dichloro-, 3,4,5-trichloro-, 2,3,4,6-tetrachloro-, and pentachlorophe-
FIGURE 3. Recoveries of CPs from XAD-2 as a function of the amounts of added HCl. The solvent is dichloromethane.
nols are shown. Other CP isomers behaved similarly. Initially, a slight amount of acid led to a large increase in the extraction of CPs. Complete extraction for all CPs was achieved at 0.50 mmol of HCl. From this amount of acid to 3.0 mmol of HCl, the extraction of CPs was complete, with mean Soxhlet extraction recoveries ranging from 90 to 101% (Table 3) and relative standard deviation less than 10%. At 6.0 mmol of HCl, the amounts of CPs extracted were not markedly reduced. Obviously, both water and HCl aid the extraction of CPs from XAD-2. Previous literature (16) has also reported that complete recovery of phenols from a column extraction containing XAD-2 was achieved by eluting dilute HCl solution but not by water alone. Parallel to the overall recoveries from XAD-2 of MM5 trains (discussed in the previous section), the higher the number of chloro substituents, the larger the amount of acid required to extract CPs from XAD-2. The position of the chloro substituents also determines the ease of extraction. This fact supports the above-mentioned explanation for the desorption of CPs from XAD-2 at intermediate and high levels of HCl in flue gas. Proposed Extraction Procedure for Recovery of CPs from XAD-2 of MM5 Train When Sampling after a Scrubber. The above investigations reveal the reason for the low or inconsistent recoveries of pentachlorophenol from XAD-2 reported in the literature (5, 9, 10), where the sampling of flue gas was carried out after the scrubber. The amounts of HCl and water in the flue gas could vary from one operation to another. Also, since the XAD-2 is preserved with sodium carbonate, it could be slightly basic, depending on how well the preservative is removed from XAD-2 prior to its use. When the flue gas is perfectly scrubbed, in the absence of HCl in flue gas, pentachlorophenol is well adsorbed on XAD-2 of the MM5 train. Then, in the absence of HCl on XAD-2, the subsequent Soxhlet extraction cannot recover the pentachlorophenol satisfactorily. CPs are acid extractable compounds. They require a slightly acidic medium for complete extraction from XAD2. This behavior, however, is the opposite of PAHs, CBs/ PCBs, and PCDDs/PCDFs (base/neutral extractable compounds), which can be extracted by dichloromethane without the addition of acid (17). Moreover, some PAHs have been shown to react with HCl on XAD-2 (17). All these semivolatile organic compounds can be present in
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TABLE 4
Percent Recovery of CP from Extraction of Synthetic Condensatea pH ) 10b
CPb 4-chloro3,4-dichloro3,5-dichloro2,4-dichloro2,3-dichloro2,6-dichloro3,4,5-trichloro2,3,4-trichloro2,4,5-trichloro2,3,5-trichloro2,4,6-trichloro2,3,6-trichloro2,3,4,5-tetrachloro2,3,4,6-tetrachloro2,3,5,6-tetrachloropentachloro2,4,6-tribromo2,6-dibromo-4-fluoro2,6-dichloro-4-fluoro-
pKa
dichloro- tolu- in in methane ene waterc waterd 94 88 81 76 70 36 93 79 60 45 25 15 80 29 10 37 86 84 21
75 65 58 44 30 29 73 38 15 12 5 5 64 8 3 26 72 69 4
9.37 8.62 8.25 7.90 7.71 6.78 7.55 6.97 6.72 6.43 5.99 5.80 5.64 5.22 5.03 4.74
8.25
7.04 6.62
pH ) im,a 2, 4, 6
dichloromethane 92 ( 7 104 ( 4 105 ( 6 94 ( 7 92 ( 7 92 ( 5 102 ( 3 96 ( 3 97 ( 7 95 ( 5 98 ( 2 92 ( 9 98 ( 5 97 ( 5 98 ( 6 98 ( 7 98 ( 6 97 ( 5 96 ( 3
a
The synthetic condensate was aqueous 1.88 M HCl (pH immeasurably low, abbreviated by im). b pH ) 10 was achieved by adding NaOH. Extraction was immediately carried out after spiking the CPs and mixing the solution. 1.25 µg of each CP isomer was spiked in the solution. c Literature data from ref 18. d Literature data from ref 19.
XAD-2, and they all must be determined from the same sampling train. A single Soxhlet extraction of these compounds from XAD-2 with dichloromethane is not sufficient to recover all CPs; yet, a single Soxhlet extraction with acid may affect some of the PAHs. Therefore, at low HCl concentrations in flue gas, the following extraction procedure is suggested. Two sequential extractions should be carried out. First with dichloromethane, then a 1.0-mL aliquot of 1.0 M aqueous HCl is added on XAD-2 and followed by the subsequent extraction with dichloromethane. Two times the optimum amount of HCl is suggested (1.0 mmol) to ensure sufficient acid without affecting the CPs recoveries (Figure 3). The concentrated raw extract from each extraction should be divided into portions in the same manner. The portions for determination of CPs from the two extractions should be combined and used directly for derivatization without predrying with anhydrous sodium sulfate. This procedure has been tested in separate pilot incineration experiments, and the recovery of [13C6]pentachlorophenol from XAD-2 from five separate MM5 trains is 90 ( 6%. The above procedure is recommended for the following reasons. First, the pentachlorophenol surrogate recovery value is good. Second, there is no reaction of CPs with HCl during Soxhlet extraction as discussed above. Third, HCl is a gas, and it disappears during rotary evaporation of the raw extract under slightly reduced pressure. Fourth, had there been a slight amount of HCl dissolved in the wet concentrated raw extract, it would have been neutralized in the treatment with the first portion of 0.1 M K2CO3 during the derivatization process, thereby not affecting the derivatization product. Liquid-Liquid Extraction of CPs from Acidic Condensate and Acidic Ethylene Glycol. Distribution coefficients of CPs were strongly dependent on pH of the aqueous phase with pKa values of CPs in the 4.7-9.4 range
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FIGURE 4. Recoveries of CPs from liquid-liquid extraction as a function of pH. Dichloromethane is used as solvent, and the initial solution is aqueous 1.88 M HCl.
(18). Inorganic acid in the aqueous phase was also capable of the “salting out” (increasing the distribution coefficients) of phenols (20). Interestingly, in the relation to the MM5 train analysis, recovery values of CPs from acidic and neutral synthetic condensate using dichloromethane have been reported as 62-68% (10). Also, the same solvent recovered dichloro- and pentachlorophenols from the ethylene glycol of the train impinger at 48-55% and 50-64%, respectively (6). The analytical procedures in the literature often entailed the laborious sequential extractions of condensate and ethylene glycol at very low (5, 6, 9, 10), neutral (5, 6), and very high (5, 6, 9, 10) pH in order to completely recover all the semivolatile organic compounds (CPs, PAHs, CBs/PCBs, and PCDDs/PCDFs). It is important to investigate why such low CPs recoveries were obtained and whether the sequential extractions at various pH is necessary. In the present study, 270 mL of aqueous 1.88 M HCl was used to represent the condensate collected after the sampling of flue gas at 5.53 mg/L HCl (see Table 2, pH is immeasurably low). Table 4 shows the liquid-liquid extraction recoveries of CPs from this synthetic condensate, which was brought to pH ) 10 prior to the spiking of CPs. Dichloromethane and toluene were used as solvents. The purpose was to explain the incomplete extraction of various CP isomers. The data are listed based on the decreasing recoveries within each CP group of the same number of chloro substituents. In general, dichloromethane is a better solvent than toluene. The order of extractability of all CP isomers is the same with either dichloromethane or toluene. Similar behavior as investigated in the extraction of CPs from XAD-2 is observed. The higher the number of the chloro substituents, the more difficult the CP extracted from the basic solution. The ortho-substituted CPs are the least recovered. Based on grouping with the same number of chloro substituents on CPs, the order of the decreasing CPs recovery within a group agrees with the order of decreasing pKa as shown in Table 4. Previous investigations have shown (21) that the increasing distribution coefficients in a given system are in the order of fluoro- < chloro- < bromo- < iodophenols. The present results for the liquid-liquid extraction recovery of 2,4,6-tribromophenol relative to those of 2,4,6-trichlorophenol (with either dichloromethane or toluene) are in
TABLE 5
Percent Recovery of CP from Extraction of Combined Synthetic Condensate and Ethylene Glycol Using Various Solvents 0.051 M HCl in waterb + 0.001 M HCl in ethylene glycol CPa 4-chloro3,4-dichloro3,5-dichloro2,4-dichloro2,3-dichloro2,6-dichloro3,4,5-trichloro2,3,4-trichloro2,4,5-trichloro2,3,5-trichloro2,4,6-trichloro2,3,6-trichloro2,3,4,5-tetrachloro2,3,4,6-tetrachloro2,3,5,6-tetrachloropentachloro2,6-dichloro-4-fluoro2,6-dibromo-4-fluoro2,4,6-tribromo-
1.88 M HCl in waterc + 0.024 M HCl in ethylene glycol
dichloromethane
hexane
toluene
dichloromethane
hexane
toluene
15 15 23 59 60 74 46 ( 26 21 57 60 65 81 72 59 ( 21 57 66 64 62 ( 5 56 71 86 91 83 ( 10
1 4 12 92 86 99 59 ( 47 20 88 90 106 88 91 81 ( 30 102 96 96 98 ( 3 104 103 101 100 101 ( 2
85 107 110 109 107 105 108 ( 2 106 93 98 106 88 92 97 ( 8 101 90 109 100 ( 10 91 113 98 95 102 ( 10
57 55 56 92 84 91 76 ( 19 52 81 82 80 86 77 76 ( 12 69 68 59 65 ( 6 55 91 94 83 89 ( 6
2 6 16 82 78 84 53 ( 39 24 96 99 85 98 98 83 ( 30 104 106 97 102 ( 5 110 92 79 107 93 ( 14
88 107 101 95 96 97 99 ( 5 106 98 102 91 96 97 98 ( 5 106 104 92 101 ( 8 105 95 82 103 93 ( 11
pKa in ethylene glycold
in 75% water + 25% ethylene glycold
10.26
10.04
8.62
8.08
7.57
7.00
7.62
7.15
6.70
6.35
a 1.25 µg of each CP isomer was spiked in the combined synthetic condensate and ethylene glycol. b Synthetic condensate combined with synthetic ethylene glycol represents COND + EG collected after sampling of the flue gas at 0.014 mg/L HCl (pH is 1.5). c Same as in footnote b except after sampling of flue gas at 5.53 mg/L HCl (pH is immeasurably low). d Literature data from ref 22.
agreement. The same agreement is observed with the increasing liquid-liquid extraction recoveries in the order of 2,6-dichloro-4-fluoro- < 2,6-dibromo-4-fluoro- < 2,4,6tribromophenols. Figure 4 illustrates the liquid-liquid extraction recoveries of CPs using dichloromethane from aqueous 1.88 M HCl synthetic condensate and those from higher pH by adding NaOH into this solution prior to the spiking of CPs. In this figure, the 2,3,6-, 2,4,6-, and 3,4,5-trichlorophenols are taken as examples. Other CP recoveries show the same pattern on pH dependency. The extraction recovery of CPs was complete at very low pH. The recovery then slightly decreased as the pH was raised to 6. But the mean percent recoveries averaged over pHs immeasurably low, 2, 4, and 6 were 92-105 with relative standard deviations of 2-10% (Table 4). The “inflection points” of the curves in Figure 4 were in the pH 6-9 range. At pH > 10, the recovery of CPs was low. These results lead the conclusion that CPs are well extracted using dichloromethane at any pH e 6. When ethylene glycol from the MM5 train was mixed with an equal volume of dichloromethane, the two liquids were hardly separated. Previous analytical procedures (5, 6) required the addition of large amounts of water (to 1.5 L) to facilitate the extraction using dichloromethane. In the present study, the synthetic acidic ethylene glycol was combined with acidic synthetic condensate (without the addition of water). Three solvents (dichloromethane, n-hexane, and toluene) were used to extract the CPs from the combined liquids. Very interesting extraction behaviors are seen from the results listed in Table 5 as compared to those results without ethylene glycol (Table 4). In the presence of ethylene glycol,
n-hexane and toluene (nonpolar solvents) were better solvents than dichloromethane. Toluene was the best among the three. It completely extracted all CPs from the combined condensate and ethylene glycol at both low and high HCl concentrations. Most CP isomers were not completely recovered using dichloromethane. Better recoveries of all CP isomers at a higher concentration of HCl was obtained with dichloromethane and n-hexane. The presence of ethylene glycol reversed the order of decreasing recoveries of all CP isomers, except for the bromo- or fluorosubstituted phenols. The pKa values of CPs in ethylene glycol were significantly higher than those in water (22). The addition of water into ethylene glycol decreased the pKa of CPs, but there was no considerable further decrease at 75% water content in the mixture (22). Data in Table 5 indicate that the order of decreasing recoveries within each CP group of the same number of chloro substituents is in the reversed order of their decreasing pKa values. Selective solvation of CP ions by water molecules is appreciably diminished in the mixture of water and ethylene glycol (22).
Conclusions The experimental results indicate the limitation of the MM5 method for sampling of incinerator flue gas containing high concentrations of HCl. At 14% moisture, 7% carbon dioxide, and 10% oxygen, 0.014 mg/L HCl does not affect the recoveries of CPs. However, higher concentrations of HCl (2.05 and 5.53 mg/L) cause desorption of some CPs from the XAD-2 adsorbent. The higher the number of chloro
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substituents on the CPs, the more concentrated the acid to desorb them from XAD-2. These results have important implications on the selection of surrogates. When only higher chlorinated phenol surrogates are used in XAD-2 of the MM5 train, acceptable recoveries may be obtained. But this does not necessarily prove successful sampling. Lower chlorinated CPs may be unknowingly desorbed during sampling, and hence false negative results may be obtained. It is recommended that at least one surrogate isomer should be used for each CP group. To be completely extracted from XAD-2, CPs require a slightly acidic medium; yet, some PAHs react with acid. Accordingly, a new extraction procedure is proposed when the flue gas contains low HCl concentrations (
