Environ. Sci. Technol. 1997, 31, 58-66
Effect of Hydrochloric Acid on Sampling and Analysis of Semivolatile Organic Compounds in Incineration Flue Gas. 2. Polycyclic Aromatic Hydrocarbons LIANG K. TAN* AND ALBERT J. LIEM Alberta Environmental Centre, P.O. Bag 4000, Vegreville, Alberta, Canada T9C 1T4
The effect of hydrochloric acid (HCl) at 0.17, 2.05, and 5.53 mg/L in incineration flue gas on the recoveries of 16 polycyclic aromatic hydrocarbons (PAHs) from XAD-2 adsorbent of the MM5 (Modified Method 5) train was investigated. At constant concentrations of moisture (14%), carbon dioxide (7%), and oxygen (10%), 2.05 and 5.53 mg/L HCl decreased the recoveries of benzo[a]pyrene, perylene, anthracene, and acenaphthylene. At 5.53 mg/L HCl, only 6, 8, 32, and 46% of these PAHs, respectively, were recovered from XAD-2. The recoveries of other PAHs were 88-112% with relative standard deviations of 3-13% at all acid concentrations in flue gas. Acid effect occurred primarily during sampling of flue gas, and a smaller acid effect occurred during sample processing, in which the XAD-2 containing HCl, PAHs, and moisture was Soxhlet extracted using dichloromethane. The hypothesis is that these PAHs formed cation radicals in the presence of HCl, which was followed by subsequent reaction with water to yield quinones. The extraction of PAHs from synthetic acidic condensate and acidic ethylene glycol of the MM5 train was not dependent on pH. Extractions from various acidities of these combined solutions gave recoveries of 76-102% with relative standard deviations of 4-13%. The implications of all the above-mentioned effects on the selection of surrogates and interpretation of analytical results for MM5 sampling and analysis are discussed.
Introduction The studies on the effect of hydrochloric acid (HCl) in incinerator flue gas on the sampling and analysis of all semivolatile organic compounds, i.e., chlorophenols (CPs), polycyclic aromatic hydrocarbons (PAHs), chlorobenzenes (CBs), polychlorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins (PCDDs), and polychlorinated dibenzofurans (PCDFs) are important when sampling is conducted upstream of the scrubber (1). The HCl concentration in the flue gas varies depending on the input of chlorinated organics into the incinerator. Yet, the effect of HCl in incinerator flue gas on the sampling and analysis of PAHs is not available in the literature. A Modified Method 5 (MM5) has been used for sampling (1-3) and analysis (1, 4-7) of semivolatile organic compounds in flue gas. In this method, flue gas is sampled, and a measured volume of the gas is passed through a cartridge containing XAD-2 (polystyrene) onto which organic compounds are adsorbed. The adsorbed compounds are then
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recovered by solvent extraction, separated by column chromatography cleanup, identified, and measured using a gas chromatograph with mass spectrometer detector (GC/MS). In part 1 of this study (1), the recoveries of 16 isomers of chlorophenols (CPs) from the adsorbent (XAD-2) of the MM5 train were reported. When the concentrations of moisture (14%), carbon dioxide (7%), and oxygen (10%) are kept constant, 0.17 mg/L HCl in flue gas 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. The CPs with more chloro substituents require more concentrated acid to desorb them from XAD-2. Based on the desorption behavior of CPs from XAD-2 in the presence of acid, the selection of surrogates for successful sampling of flue gas has been suggested (1). Also, a new extraction procedure for complete recoveries of CPs from XAD-2 has been proposed for flue gas containing HCl concentrations lower than 0.17 mg/L (1). This paper presents the results of investigating the effect of HCl in flue gas on the recoveries of 16 PAHs from XAD-2 of the MM5 train, conducted in a pilot incinerator. To clarify whether the HCl effect occurred during sampling of flue gas or during sample processing (in which the XAD-2 containing PAHs, HCl, and moisture was Soxhlet extracted), or during both processes, a separate investigation on the effect of HCl in the Soxhlet extraction of PAHs from XAD-2 was also carried out in the laboratory. To determine whether there were reactions between HCl and some PAHs, separate tests in the absence of XAD-2 (to maintain better contact between HCl and PAHs), with or without heating/reflux, were also carried out in dichloromethane, toluene, and hexane. Various solvents were used to find out if there was an ideal solvent that poorly solvated the reaction intermediates and hence prevented the occurrence of acid effect during Soxhlet extraction. In addition, the effect of HCl on the liquid-liquid extraction recoveries of PAHs from the condensate and ethylene glycol of the MM5 train compartments was also included in this study using the synthetic acidic condensate and synthetic acidic ethylene glycol solutions.
Experimental Section Chemicals. The purification of XAD-2 polystyrene (Terochem) has been mentioned previously (1). All solvents (nhexane, dichloromethane, and toluene) were of distilled-inglass grade. Sodium hydroxide, concentrated hydrochloric acid, and ethylene glycol were of certified ACS grade. Silica (Bio-Sil A, 100-200 mesh, Bio-Rad) was sequentially washed with two portions each of hexane and dichloromethane using a Bu ¨ chner funnel with suction (6, 7). The volume of solvent used for each wash was equal to twice the estimated volume of silica. It was dried at 50 °C for 1 h, then conditioned overnight at 225 °C, and stored in a bottle with a Teflon cap liner and used for that day only. Anhydrous sodium sulfate (Na2SO4) was prepared in the same manner as the silica (6, 7). Sixteen PAH compounds (listed in Table 1) were obtained from Ultra Scientific and Aldrich. Stock solutions of 1000, 500, or 200 µg/mL in toluene were prepared from each PAH compound. A working solution of PAHs mixture in toluene was prepared at 40 µg/mL. Surrogate solutions of chrysened12 (MSD Isotopes) were prepared separately in the same manner. The internal reference compound used in GC/FID or GC/MS measurements was 9,10-dibromoanthracene (Aldrich) at twice the concentration of the PAH target compound. Glassware. All glassware was treated as mentioned previously (1).
S0013-936X(96)00037-5 CCC: $14.00
1996 American Chemical Society
TABLE 1. Percent Recovery of PAH from XAD-2 of MM5 Train HCl in flue gas (mg/L)
retention time, min
PAHa
abbreviation
0.17
2.05
5.53
mean ( SDb
11.01 11.50 12.86 15.83 15.99 20.44 21.37 27.05
acenaphthylene 1,2-dihydroacenaphthylene fluorene phenanthrene anthracene fluoranthene pyrene chrysene-d12
ACL ACT FLR PHE ANT FLT PY CHR-d12
27.16 32.90 33.06 34.77 35.13 35.76 45.95 46.49 49.07
chrysene benz[e]acephenanthrylene benzo[k]fluoranthene benzo[e]pyrene benzo[a]pyrene perylene indeno[1,2,3-cd]pyrene dibenz[ah]anthracene benzo[ghi]perylene
CHR BeA BkF BeP BaP PRL IPY BahA BghiP
91 93 98 103 120 107 107 86d 82e 103 116 104 104 91 94 89 101 109
77 97 106 110 70 112 112 102d 104e 102 112 114 109 57 65 94 105 109
46 79 103 111 32 114 100 93d 99e 110 108 106 106 6 8 82 95 93
c 90 ( 9 102 ( 4 108 ( 4 c 111 ( 4 106 ( 6 94 ( 8d 95 ( 12e 105 ( 4 112 ( 4 108 ( 5 106 ( 3 c c 88 ( 6 100 ( 5 104 ( 9
ionization potentialf (eV)
half-wave potentialk (V)
log of basicity constantm
8.02 7.66g 8.48h 7.80 7.43 7.80 7.41
j 1.11 1.25 1.23 0.84 1.18 1.12
j 1.6n -2.4n -3.5 3.8 j 2.1
7.61 j j 7.43 7.12 7.00 j 7.57i j
1.13 j j j 0.76 0.63l j j j
-1.7 j j j 6.5 4.4 j 2.2 j
a 20 µg of each PAH was spiked on XAD-2 in MM5 train. Internal reference compound was 9,10-dibromoanthracene at retention time 26.91 min. Mean percent recovery and standard deviation for each PAH that was not affected by various HCl concentrations. c The recoveries of these PAHs were affected by HCl. d Data for surrogate in the test train. e Data for surrogate in the blank train. f Data from ref 15. g Data from ref 16. h Data from ref 17. i Data from ref 18. j Data were not found in the literature. k Data from ref 19. l Data from ref 20. m Data from ref 21. n Data from ref 22. b
Spiking of Surrogates and Target Compounds. The 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, glass wool was placed on top of the XAD-2. The cartridge was then incorporated into the MM5 train, which was immediately used for experiment. The amount of each PAH spiked on XAD-2 was equivalent to gas phase concentrations of 0.57-1.1 ppb, depending on the molecular weight. The emission guideline for PAHs was 5 µg/m3 in flue gas, which was equivalent to 0.4-0.7 ppb (8). MM5 Train. The schematic diagram of the MM5 train used in the experiments has been given in ref 1. A leak test was done following the same protocol (2, 3). Flue Gas Sampling in the Pilot Incinerator. Flue gases containing 0.17, 2.05, and 5.53 mg/L HCl (equivalent to 103, 1260, and 3390 ppm, respectively) were generated in the pilot incinerator by introducing 1,2,4-trichlorobenzene at various feed rates (1). [The lowest HCl concentration in flue gas printed in the previous paper (1) was 0.014 mg/L and is an error. It should be 0.17 mg/L throughout ref 1.] Moisture, carbon dioxide, and oxygen contents in the flue gas were controlled at 14, 7, and 10%, respectively, in all incineration experiments. The sampling of flue gas (isokinetic rate 12 L/min, total volume 2800 L/sample) with blank and test trains simultaneously at the same sampling location, upstream of the scrubber, and under the same conditions has also been explained in ref 1. The incinerator was operated to generate flue gas with an absence of principal organic hazardous constituents (POHCs) and products of incomplete combustions (PICs). A blank train, which was spiked with surrogates only, was used to verify the absence of organic compounds generated. A test train, which was spiked with both surrogates and target compounds, was used to determine the recoveries of spiked organic compounds from XAD-2. The amount of each organic compound, congener, or isomer in XAD-2 was 5, 20, 5, and 0.1 µg, respectively, for CPs, PAHs, CBs/PCBs, and PCDDs/PCDFs. Organic compounds on XAD-2 after sampling of flue gas were Soxhlet extracted overnight with dichloromethane. The raw extract was concentrated at 40 °C by a rotary evaporator
under reduced pressure. A 25% portion of the concentrated raw extract was used for PAH analysis. The dichloromethane was displaced with hexane in a centrifuge tube under a gentle stream of nitrogen at 40 °C. The 1-mL solution in hexane was used for the PAH column cleanup as outlined below. Water and HCl Adsorbed on XAD-2 of MM5 Train after Sampling of Flue Gas. The amounts of water adsorbed on XAD-2 of MM5 train after sampling of flue gases were 3-6 g, determined by weight differences (1). There were 0.09, 5.50, and 15.9 mmol of HCl adsorbed on XAD-2 of the MM5 train after sampling of flue gases containg 0.17, 2.05, and 5.53 mg/L HCl, respectively, determined by titrations of the XAD-2 washing waters (1). Separate Laboratory Soxhlet Extraction Experiment with HCl in XAD-2. The PAHs mixture at 20 µg each (identical to that of the pilot plant experiment) was spiked on XAD-2 in the thimble of a Soxhlet apparatus. Aliquots of 0.5 mL of aqueous HCl solution of 0.10-12.0 M were added to XAD-2, which corresponded to a range of 0.05-6.0 mmol. Glass wool was placed on top of the XAD-2, and PAHs were then extracted with dichloromethane. A 50% portion of the concentrated raw extract was used for analysis. The dichloromethane in this raw extract was displaced with hexane, and the 1-mL extract in hexane was processed by column cleanup and measured as described below. Laboratory Experiment with HCl without XAD-2. Two experiments were carried out as follows. (A) With heating/ reflux: PAHs mixture at 10 µg each was added into a boiling flask containing 300 mL of dichloromethane, hexane, or toluene. A 1-mL aliquot of water or aqueous HCl solutions of various concentrations were added to cover the range of 0-12.0 mmol. This mixture was refluxed overnight. The resulting solution was concentrated at 40 °C by rotary evaporator under reduced pressure to 3 mL. In the case where the solvent is dichloromethane or toluene, the solvent was first displaced with hexane prior to column cleanup as described below. (B) Without heating/reflux: After leaving 10 µg of PAHs and 12 mmol of aqueous HCl in 300 mL of dichloromethane at room temperature overnight, the volume of dichloromethane in the resulting solution was rotary evaporated under reduced pressure. The dichloromethane in this concentrate was also displaced by hexane prior to column cleanup.
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Column Cleanup of PAHs. The column was a 15 cm × 1.2 cm Pyrex glass with a receiver head on top and a Teflon stopcock in the bottom. A glass wool plug was inserted into the tip above the stopcock. This column was packed with 2.5 g of silica and topped with 0.5 g of anhydrous sodium sulfate (6). Rinsing was done with 12 mL of hexane. Just as the solvent drained to the top of the sodium sulfate layer, the sample was transferred to the column using a disposable pipet. Three 1-mL hexane rinses of the centrifuge tube were added to the column, followed by two 5 mL of hexane. These eluents were discarded. Three 4-mL aliquots of toluene were added to the column, and the eluent was collected in a centrifuge tube. It was reduced to 1 mL under a gentle stream of nitrogen at 40 °C. The internal reference solution was added prior to volume adjustment. 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 of the synthetic condensate and ethylene glycol were 270 and 120 mL, respectively (1). The HCl concentrations in the condensate were 0.051, 0.610, and 1.88 M to represent the resulting condensate after sampling of flue gases at 0.17, 2.05, and 5.53 mg/L HCl, respectively. The HCl concentrations in the ethylene glycol were 0.001, 0.005, and 0.024 M to represent the resulting ethylene glycol after sampling of flue gases at the above three HCl levels, respectively. These values were based on the previously reported (1) experimental data of flue gas sampling in the pilot incinerator. A mixture of PAHs at 10 µg each was added to the acidic synthetic condensate or to the combined synthetic acidic condensate and ethylene glycol solution prior to or after adjustment of pH, as indicated in the text. Adjustment of pH was carried out by adding sodium hydroxide to these solutions in an ice-bath. A glass-coated magnetic stirring bar was also employed. Four serial extractions (100, 75, 75, 50 mL) were done in a separatory funnel using toluene. The combined organic phase was concentrated. If an aqueous phase formed after the concentration step, it was separated in a centrifuge tube by pipetting the toluene phase. Internal reference was added to the resulting extract in toluene, and the volume was adjusted to 1 mL. This solution was measured without column cleanup. Instrumentation. A gas chromatograph/mass spectrometer (GC/MS) with electron impact and selected ion monitoring mode (SIM) was used for identification and measurement of test solutions. The temperature program was 90 °C for 1 min, to 200 °C at 10 °C/min, to 280 °C at 5 °C/min, and then held for 23.1 min. A gas chromatograph with flame ionization detector (GC/FID) was also employed where confirmation was not necessary. The temperature program was the same as stated for GC/MS. The instrumentation for the two techniques have been reported in ref 1. Quantitation. In the recovery study, quantitations of PAHs were based on relative response factors for single point 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 column cleanup and concentration step in the same manner as that of the test solution.
Results and Discussion Effect of HCl in the Flue Gas. The quantities of PAHs recovered from XAD-2 of the MM5 train after sampling of flue gas reflect the “overall recoveries”, whereas the quantities of PAHs extracted from XAD-2 in a separate laboratory
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FIGURE 1. Typical GC/MS chromatograms: (A) PAHs standard; (B and C) are PAHs recovered from XAD-2 of MM5 trains after sampling of flue gas at 2.05 and 5.53 mg/L HCl, respectively. All chromatograms were displayed at the same scale. R denotes 9,10-dibromoanthracene, which served as the internal reference compound. The names of abbreviated peaks based on increasing retention times are listed in Table 1. experiment (discussed in the following section) reflect only the “Soxhlet extraction recoveries”. In all experiments, there were no target PAHs found in the blank train. The overall recoveries of the surrogate (chrysened12) in the blank and in the test trains were close to the recoveries for the corresponding target compound (Table 1). Figure 1 illustrates typical GC/MS chromatograms of PAHs that were recovered from XAD-2 after sampling of flue gas at various HCl concentrations. Figure 1A shows the chromatogram of the PAHs standard to which an internal reference compound 9,10-dibromoanthracene (labeled R) has been added. The dilution factor for this standard was the same as that of the samples in Figure 1B,C. The amplitude scale was also kept the same in all of these chromatograms. The names of the peaks are listed in Table 1 according to their increasing retention times. When the HCl concentration in flue gas was 0.17 mg/L, which served as a model for scrubbed flue gas, the overall recoveries of PAHs were 91-120%. At 2.05 mg/L HCl, the overall recoveries of PAHs from XAD-2 are shown in the chromatogram of Figure 1B. Several PAHs peaks, namely,
FIGURE 2. Linear regression lines of the PAHs recoveries and the amounts of HCl on XAD-2 obtained from laboratory Soxhlet extraction experiment and from MM5 train experiment. Group I consists of benzo[a]pyrene, perylene, anthracene, and acenaphthylene. Group II consists of the remaining PAHs.
FIGURE 3. PAHs recoveries after heating/reflux in dichloromethane as a function of the amount of HCl. Labels with numbers 1, 2, and 3 represent benzo[a]pyrene, perylene and acenaphthylene, respectively. Other PAHs are represented by symbols.
benzo[a]pyrene, perylene, anthracene, and acenaphthylene, were smaller than the corresponding peaks in the standard. The overall recoveries of these compounds were 57, 65, 70, and 77%, respectively (Table 1), and were smaller than those of the other PAHs. At 5.53 mg/L HCl, the overall recoveries of these compounds were greatly reduced. There were only
