Environ. Sci. Technol. 1992, 26, 469-478
Laboratory Investigations of the Partitioning of Organochlorine Compounds between the Gas Phase and Atmospheric Aerosols on Glass Fiber Filters Wllllam E. Cotham*,+and Terry F. Bidlemant-f Department of Chemistry and Biochemistry, Marine Science Program, and Belle W. Baruch Institute for Marine Biology and Coastal Research, University of South Carolina, Columbia, South Carolina 29208
w An experimental system was designed to equilibrate urban air particulate matter on a glass fiber filter (GFF) with gas-phase organochlorine (OC) pesticides and polychlorinated biphenyls (PCBs) at controlled temperature, humidity, and vapor concentrations. Particle-loaded filters (PLFs) were backed up by clean filters (CFs) to correct for gaseous semivolatile organic compound (SOC) adsorption to the filter matrix. Analysis of the CF indicated the presence of a complex mixture of alkanes and other organic compounds in addition to the OC pesticides introduced in the experiment. Gaseous OC adsorption was very low when two CFs were used in series instead of a PLF followed by a CF. Our interpretation is that organic compounds present on the PLF bleed off and are adsorbed by the backup CF, where they act as a stationary phase to cosorb the gaseous OCs. This artifact limits the use of filter-adsorbent samplers for estimating gas/particle distributions in the atmosphere. Relative humidity in the 30-95% range had little effect on OC adsorption to the particles. Experiments at 0-30 "C yielded heats of desorption, which ranged from 72 to 130 kJ mol-l.
Introduction Many organic contaminants, including organochlorine (OC) pesticides, heavy alkanes, polycyclic aromatic hydrocarbons (PAHs), polychlorinated dibenzofurans and dibenzodioxins, and polychlorinated biphenyls have vapor pressures between about 10" and 10-I P a and have come to be classed as semivolatile organic compounds (SOCs). SOCs distribute themselves to various degrees between the gas and particle phases in the atmosphere primarily according to their vapor pressure and the amount of surface area available for adsorption. High-volume sampling of SOCs is commonly performed by pulling air first through a filter which retains atmospheric aerosols followed by an adsorbent trap to collect gas-phase compounds. Using the amounts of SOCs found on the adsorbent trap and the filter, one can define operational estimates of the gas-phase and particle-phase concentrations in the atmosphere. How closely their ratio represents the true gas/particle split is uncertain. Variations in SOC concentration and temperature are likely to occur during the 24+-h collection period, which may lead to blowoff losses or adsorption gains to the particles on the filter. Other factors that may produce sampling artifacts are variations in particulate matter properties (e.g., aerosol concentration, carbon content, surface area, and size distribution) and adsorption of gaseous SOCs to the filter matrix (1-5). The influence of relative humidity on aerosol adsorption properties is also not known. Diffusion denuder samplers show promise for overcoming some of the above mentioned problems (5-8), but their development has been hindered by their relatively low air sampling rate. Despite the drawbacks associated with the high-volume system, field and laboratory experiments (1)have provided 'Department of Chemistry. tMarine Science Program and Belle W. Baruch Institute for Marine Biology and Coastal Research. 0013-936X/92/0926-0469$03.0010
insight to some of the factors influencing SOC gas adsorption to atmospheric particulate matter on glass fiber filters (GFFs). Some preliminary conclusions are that (1) the interaction between nonpolar SOC gases and aerosols is governed primarily by the saturation subcooled liquid vapor pressure (PoL)rather than the solid-phase vapor pressure of the SOCs, (2) heats of desorption (AHD)are approximately 8-17 kJ mol-l greater than the heats of vaporization (A",)from the pure liquid phase, and (3) some SOCs associated with particulate matter may be "nonexchangeable" and not in equilibrium with the gas phase. Interestingly, in many cases the experimental gas/particle distributions agree relatively well with the JungePankow model developed over a decade ago to predict the reversible adsorption of gases to aerosols. This model, originally formulated by Junge (9) and later thoroughly reviewed by Pankow (IO), is based on a linear Langmuir isotherm and relates the adsorption of a compound to the concentration of aerosol surface area (e, cm2 air) and poLof the sorbate according to the following equation: 4 = C e / @ o L + ce) (1) where 4 is the fraction of total atmospheric concentration which is adsorbed to the aerosol and c is a parameter which depends upon the sorbate molecular weight, surface concentration necessary for monolayer coverage, and AHDAHv. The value for 4 can also be calculated from
4 = 1/[1 + C,/C,(TSP)I
(2)
Convenient units are as follows: C = ng of SOC m-3 of air, C, = ng of adsorbed SOC pg- B of particles [note a different definition of C, than used by Ligocki and Pankow ( 2 ) ]and , total suspended particles (TSP) = pg of particles m-3 of air. Substituting into eq 1 gives log C,/C, = log P O L + log (TSP)/cO (3)
If c is approximately constant within a class of compounds, a plot of log C,/C, vs log poLwill have a slope of unity and an intercept of log (TSP)/cB (2). A detailed discussion of the parameters which make up the c term are found in ref 10. If c is not constant and/or sampling artifacts are present, slope # 1. Pankow and Bidleman discussed how sampling artifacts or a nonconstant c may affect the slope (1,lI). This paper reports laboratory experiments to elucidate how POL and sampling variables are related to the apparent distribution of SOCs between the gas phase and atmospheric aerosols on GFFs. The objective was to investigate temperature and humidity effects on C,/C, and the adsorption of gaseous nonpolar SOCs to the GFF matrix. These experiments were performed using laboratory-generated gas-phase OC pesticides and aerosols collected on GFFs in Chicago during February 1988. Adsorption of gaseous SOCs to particles on GFFs was studied by Foreman and Bidleman (3). Their apparatus was designed to simulate a high-volume sampler but was operated at a controlled temperature with constant SOC gas concentrations. These experiments required the use
0 1992 American Chemical Society
Environ. Sci. Technoi., Vol. 26, No. 3, 1992
469
1-1 ILTER HOLDER
CHAMBER
Figure 1.
Laboratory gaslpartlcle equillbratlon apparatus.
of a full 20 X 25 cm particle-loaded GFF and were only done at one temperature (20 "C). The present design (Figure 1)has been miniaturized to use 47-mm-diameter GFF circles and is enclosed in a thermostated chamber, providing better control of temperature. S i c e fdter circles are cut from larger particle-laden GFFs, multiple experiments can be performed with the same particles. The ability to vary and accurately control temperature allows the determination of A",. A future paper will discuas the results of field investigations of gas/particle partitioning on GFFs and their relationship to laboratory findings.
Experimental Section Collection and Analysis of Aerosols. Particle samples were collected a t a TSP monitoring site in Chicago, IL,during February 16-20,1988. The station was on the roof of Washington High School in the Calumet area of South Chicago, a heavily industrialized area with steel mills, incinerators, coke ovens, and other large manufacturing facilities. Particle samples were obtained on 20 X 25 cm Gelman binderless GFFs using a conventional high-volume sampling system at 1.4 m3 min-I. All GFFs were baked at 420 O C overnight and sealed in aluminum foil before use. Two sets of particles were used in laboratory studies. They were collected during February 16, 1988,1330 to February 17,1988,1830 (particle set A) and February 18,1988,800 to February 20,1988,7:08 (particle set B). Between 1430 and 3850 m3 of air was sampled over a temperature range of -2 to 6 'C. Scanning electron micrographs (SEMs) of samples from particle-loaded and clean filters were made after coating with gold. Small sections from each particle-loaded filter and several blank filters were finely ground to a uniform color and consistency in an agate mortar and sent for total carbon (TC) analysis (Desert Analytics, Tucson, AZ). Gas/Particle Equilibration System and Experimental Procedure. Laboratory gas/particle partitioning 470
Envlron. Scl. Technd.. Vol. 26. No. 3. 1992
experimenta were performed by equilibrating particleladen GFFs with analyte vapors at constant temperatures in the apparatus shown in Figure 1. A prefiter assembly composed of a Teflon filter holder containing a bed of activated charcoal followed by a GFF provided particleand SOC-free air. The prefilter was followed by a heated water bubbler for maintenance of a constant humidity and a stainless steel mixing chamber where SOC vapors were introduced from generator columns composed of analytecoated glass beads (3). Flows through the columns were regulated using needle valves. A Teflon filter holder containing a 47-mm particle-loaded GFF cut from a 20 X 25 cm GFF was attached to the back of the mixing chamber. Since the edge of the GFF disk was covered by the filter holder, the actual diameter of the filter exposed to SOC gases was 43 mm. The loaded filter, containing 3.5-12.5 mg of particles, was backed up by a clean GFF disk to correct for SOC adsorption to the filter matrix (see detail, Figure 1). SOC vapor concentrations were monitored by polyurethane foam (PUF) plugs (3.5 cm long X 5 cm diameter) installed in a glass tube downstream of the fdter holder. After an initial 24-36-h prequilibration time, PUF plugs were changed periodically (every 2-4 h) and analyzed to obtain average C for that time interval. The apparatus, except for the bu%bler, flow gauge, and activated carbon filter, was contained in an insulated box 130 X 60 X 50 cm. Temperature was maintained using a Laude Model RMT 20 refrigerating circulator (Brinkman Instruments, Westbury, NY)connected to three copper-coil heat exchangers. Airflow through the system was provided by a hp oilless FL Pneumotive Model GH4VB vacuum pump (FL Industries, Monroe, LA). Flow rate and temperature were monitored using a Sierra Instruments Model 821-13-1 mass flowmeter (Sierra Instruments, Carmel Valley, CA) and two YSI series 400 thermistor probes with a Jenco Model 7002THH electronic thermometer (Jenco Electronics, Taipei, Taiwan). Relative humidity was
Table 11. log p " Values" ~
Table I. Analytes and Range of C, C,, ng m-3 particle set B particle set A n = 5-7) (n = 18-19)
compound
0°C
1% P O L , Pa 10°C 2OoC
LU-HCH HCB T-HCH 2,4',5-TCB cis-chlordane p,p'-DDE o,p'-DDT p,p'-DDT
-1.76 -2.00 -2.32 -2.71 -3.64 -3.88 -4.17 -4.79
-1.29 -1.54 -1.84 -2.18 -3.09 -3.29 -3.57 -4.16
-~ ~
compound (u-HCH HCB Y-HCH cis-chlordane p,p'-DDE o,p'-DDT p,p'-DDT
8-1200 49-1700 12-2000 29-1500 3-170 23-83 3-69
1.3-3100 1.3-2400 2.1-3100 1.2-820 1.4-390 0.2-610 2.0-440
monitored using a Vaisala Model HMP l l l Y humidity probe (Vaisala, Helsinki, Finland). Air was pulled through the system at 14-15 L min-' with a resultant face velocity across the filter of 10 m min-l. The pressure drop across the filter, 6 kPa, was in the range incurred during highvolume sampling with a GFF-PUF train. Most experiments were carried out with seven OC compounds: hexachlorobenzene (HCB), a- and y-hexachlorocyclohexane (a-and y-HCH), cis-chlordane, p,p'DDE, o,p'-DDT, and p,p'-DDT. Several experiments also included PCB congener 31 [2,4',5-trichlorobiphenyl (TCB)]. All OC pesticides were obtained from the U S . Environmental Protection Agency Pesticides and Industrial Chemicals Repository. The TCB was obtained from Ultra Scientific (North Kingstown, RI). Analytical Methods. PUF plugs were extracted (4-8 h) in a Soxhlet apparatus using petroleum ether. GFFs were refluxed with dichloromethane (4-8 h). The extracts from the PUFs and GFFs were concentrated using a rotary vacuum evaporator and nitrogen blowdown. Filter extracta were transferred into hexane or isooctane during this step. Solvents were chromatographic grade. For sample cleanup, extracts were treated with concentrated H2S04. Analysis was done on a Varian 3700 gas chromatograph with a 63Ni ECD using a 25 m X 0.2 mm i.d. bonded-phase fused-silica column (0.3-pm film thickness; 5% phenyl, 95% dimethyl polysiloxane; Hewlett-Packard or SGE Corp.). Samples were injected using splitless mode (1-2-pL volume; after a 30-5 delay, the split valve was opened) with the column at 90 "C. After a l-min hold, the oven was programmed at 6 "C mi& to a final temperature of 270 "C. Other GC conditions: carrier gas, hydrogen at 20-40 cm s-l; makeup gas, nitrogen at 20 mL m i d ; injector temperature, 240 "C; and detector temperature, 320 "C. Chromatographic data were collected and processed by a Hewlett-Packard 3390A or Shimadzu Chromatopac CR3A integrator. GC-FID analyses were carried out on a Carlo Erba Fractovap Series 4160 gas chromatograph using splitless injection with the column held at 90 "C for 1min and ramped at 10 "C mi& to 270 "C. A 12 m X 0.2 mm fused-silica column with a 0.33-pm film thickness (100% dimethylpolysiloxane, Hewlett-Packard, Avondale, CA) was used. Test compounds were quantified by external standards, using peak areas. A spike recovery study of a set of OC compounds (HCHs, chlordanes, and DDTs) from PUF indicated >85% recovery for all compounds. Results and Discussion Particle Set A. GFF circles cut from particle set A were equilibrated with gaseous OCs at 20 "C and ambient laboratory relative humidity, which ranged from 30 to 70%. Five to seven experiments were performed over a wide range of C, concentrations as shown in Table I. This set of experiments was primarily used to test the apparatus and explore SOC adsorption to the GFF matrix. To partly correct for this effect, C, was calculated by subtracting the
-0.861 -1.11
-1.40 -1.69 -2.57 -2.74 -3.01 -3.57
30°C -0.459 -0.702 -0.985 -1.24 -2.09 -2.23 -2.49 -3.03
OVapor pressure for OCs calculated from data given in ref 12; vaDor Dressure for 2,4',5-TCB calculated from data given in ref 13.
amount of OC found on the back filter from the amount found on the front filter. Adsorption isotherms of six OCs for which C, was varied by 1-2 orders of magnitude are displayed in Figure 2. Slopes of these isotherms ranged from 0.82 to 1.1 with r2 values between 0.89 and 0.96. Slopes of near-unity for these isotherms indicate that Cg/Cp ratios were fairly constant over the range of C, used in the experiments. Figure 3 shows a log-log plot of C /Cp vs p o Lfor the analytes under study. The subcooled liquid vapor pressures were computed from gas chromatographic retention data (12). log poLvalues at 0, 10,20 and 30 "C are given in Table 11. The slope of unity is consistent with that predicted by eq 3 and is an indication that, at least under laboratory conditions, atmospheric particles on GFFs behave according to the Junge-Pankow model. OC Adsorption to GFF Matrix. A separate but important part of the laboratory investigation was to explore the adsorption properties of the GFF matrix. There have been several investigations of the ability of glass and quartz fiber filters to sorb gaseous organic carbon ( 4 , 5 , 1 4 , 1 5 ) . From these studies, it appears that adsorption of gaseous SOC to glass or quartz fiber filters is substantial and is dependent upon sampling duration, filter type, and the face velocity of air across the filter. Under certain conditions, as much as 50% of the total organic carbon collected on quartz fiber filters was accounted for by adsorbed gas. As discussed above, adsorption gains to the filter matrix may cause errors in the calculation of C,/C . Ligocki and Pankow (2) suggested using a backup G%F as a correction for this adsorption artifact and we also used this method, but this assumes the backup filter adsorbs SOC to the same extent as the front GFF. To better understand uptake of gaseous SOCs by filters, equilibration experiments were performed using two clean GFFs. Figure 4 displays the amount of OCs adsorbed to front and back GFFs for all analytes, normalized to C The first set of bars (average of six or seven experiment8 represents the amount of OCs found on particle-loaded front filters and clean back filters. The second set of bars (average of three experiments) represents the results obtained using both a clean front and a back GFF. The difference clearly indicates that clean GFFs have little affinity for gaseous OCs under these experimental conditions and that there is enhanced adsorption of ar-alytes to the "clean" GFF behind the particle-loaded filter. Breakthrough of particles to the back filter does not appear likely after examination of the back GFF compared to the front GFF by SEM (Figure 5 ) . Also, McDow and Huntzicker ( 4 ) found no elemental carbon on back-up quartz fiber filters (QFFs). A more likely hypothesis for the enhanced adsorption is that organic material from the particles on the front GFF or gases adsorbed to the front GFF itself are stripped off in the clean airstream. Some Environ. Sci. Technol., Vol. 26, No. 3, 1992 471
-2
1
-2.5
a-HCH
-3
-3.0
-4
-3.5
-5
-4.0
1
0
3
2
1.5 -1
7-HCH
-2
g
I
2.5
2.0
3.0
I
1
cis - chlordane
-3
-2 m = 082 b = -479
W
r
PI - 4
u
3.5
L 0.5
I
I
I
I
1.0
1.5
2.0
2.5
b = -425
r
= 0.97 I
3.5
3.0
-3-
w 0
-
1.0 -1
4
2.0
1.5
2
=097
I
I
2.5
3.0
,
3.5
I
-2
-2
’
-3
0.0
0.5
1.0
1.5
2.0
Log
2.5
-3
I 0.0
I
0.5
I
1 .o
I
1.5
I 2.0
cg
Figure 2. Adsorption isotherms for compounds of interest. I
I
lo6 a-HCH
V
a
lo5
-
‘in
lo4 -
lo3
1
-4
1
-3
-2
-1
0
Log PoL Figure 3. Equation 3 plot for OCs on particle set A at 20 O C , ambient relative humidity (30-70%) (m = 1.02, b = 6.74, r 2 = 0.98.)
of this material migrates to the clean back filter where it is adsorbed and then acts as “stationary phase”, causing enhanced adsorption of the gaseous OCs introduced in the experiment. The results of the GD-FID analysis of the filter extracts are shown in Figure 6. By use of an average response factor for a set of n-alkanes, 20 times more extractable organic material (41pg) was found on the “clean” filter (B) 472
Environ. Sci. Technol., Voi. 26, No. 3, 1992
behind the particle-loaded filter (A) than on the filter (C) from the clean front/clean back experiments (2 pg). The classic unresolved complex mixture (UCM) pattern seen in the chromatograms indicates that this material is probably hydrocarbons. In his study of the adsorption of gaseous organic carbon, McDow (16) analyzed the thermally desorbable organic matter on particle-loaded and backup QFFs using GC-MS. On the particle-laden filter, McDow observed n-alkanes from CI6to CZ5superimposed on a typical UCM pattern. The backup filter showed a similar pattern but was enriched with slightly more volatile (CL4-C2,) n-alkanes. McDow’s findings are in agreement with the FID patterns in Figure 6, which shows material from similar volatility ranges. These results support the stripping hypothesis since one would expect the higher volatility SOCs to migrate faster than lower volatility organic matter. It is important to note here that clean, particle-free air was passing over the filters in the present experiments, which may enhance the stripping effect over that which may occur under field conditions. Zhang and McMurray have recently reported data which suggest that this is indeed the case (17). Also, the particles used in these experimenta were collected at temperatures near 0 “C and the stripping experiments were performed at 20 OC. This temperature differential may also enhance the observed stripping effect. McDow’s work however, was performed using ambient air and temperatures, which indicates or-
t
T
[
cis-chlordane
1.0 r
10
0.8
o.6
HCB
t I
2
0
10
35 30
7-HCH
T
6t i 80 loo
1 I
250
pip'-DDE T
Back Filter
500
c
F
o tTp ’ - D D T
I
400 300 200
100
0
Flgure 4. Comparison of OC sorption between particle-loaded GFFs, their clean backup GFFs, and clean GFFs alone.
ganic gas adsorption occurs under field conditions as well. It is interesting to note that if one assumes a specific surface area for GFFs to be 1.76 m2 g-l ( 4 ) ,then a 43-mm effective diameter GFF (-0.1 g) has a surface area of 0.176 m2. The particles on the filter have a surface area of only 0.035 m2, assuming 3.5 mg of particles with a specific surface area of 10 m2 g-l(17). This exercise indicates that the filter matrix in these experiments has -5 times more surface area than the exposed particles. Thus, on the basis of pure physical adsorption, the GFF should overwhelm the particles for the amount of sorbed OC compounds. This is not the case however; indeed the clean GFFs appear to have little affinity for OCs used in this study (Figure 4). The above discussion and evidence suggests that hydrocarbon-likematerial is preferentially adsorbed to GFFs compared to the OC compounds. However, this is hard to explain since aliphatic hydrocarbons are nonpolar. Indeed, McDow (16) has stated that the presence of neutral hydrocarbons on the backup filter indicates that at least some fraction of organic gas adsorption is due to physical adsorption and not acid-base-type reactions, which are known to occur for some inorganic gaseous species. One hypothesis is that polar organic SOC such as long-chain
fatty acids and fatty alcohols, which are well-known components of atmospheric organic matter (19-21), are chemisorbed to the silanol groups found on the GFF surface. Their long lipid tails then act as a hydrophobic layer enhancing the adsorption of hydrocarbons and nonpolar SOCs. However, attempts by McDow (16) to detect monocarboxylic acids on the filters were inconclusive due to blank problems or sensitivity limitations. This “enhanced adsorption” problem complicates Cg/Cp measurements since quantities of OCs adsorbed onto a back filter may not be indicative of those on the front filter. However, it is probable that the amount of OCs found on the backup GFF does not overestimate the actual adsorption to the particle-loaded filter matrix since one would expect at least as much hydrocarbon-like material to be present on the front filter. With this in mind, back filter subtraction as a correction for GFF matrix adsorption is used for all laboratory equilibration experiments to at least partially correct for the adsorption artifact. McDow and Huntzicker (4) have performed more detailed experiments which indicate that this method does in fact only partially correct for adsorption and suggest a more elaborate method for correction involving a Teflon membrane filter ahead of two QFFs. Envlron. Scl. Technol., Vol. 26, No. 3, 1992
473
r.
noun 5.
Scanning electron micrographs of a particle-loaded OFF (2500X) and its backup GFF (SOOOX).
Table 111. Comparison of Laboratory CJC. to Previous Results compound e-HCH HCB I-HCH cis-chlordane p,p'-DDE o,p'-DDT p.p'-DDT
set A (19% TCv 1.7) X 105
(4.7
(6.7 I 2 . 1 ) X (1.7 0.8) X (2.3 0.7) X (8.3 1.8) x (3.9 1.3) X (1.1 0.3) X
* *
106 105 10' 103 l(yl 103
set B (13% TC'P
(7.7 1.1) x 105 (9.3 i 1.7) X 105 (2.2 0.31) X 105 (3.6 I 1.1) x 104 (1.3 0.39) X 10' (6.5 1.8) x 103 (2.1 0.43) X 103
*
previous workd (12-19% TC') (2.3 (6.8 (8.1 (1.0
0.4) X 105 2.9) X 105 1.6) X 10' 0.1) x 10' (4.5 I 1.2) x 103 (0.56
0.18) X 103
"At 20 'C, ambient relative humidity (RH): 3C-70% RH set A; 30-5570 set B; not measured for Columbia 'TC, total carbon 88 percent of TSP. 'Chicago, 47-mm GFFs. dReference 3. Columbia 20 X 25 em GFF.
Particle Set B. Particle set B was used to determine AHn of OCs from a e m b on tiltera, and to study the effect of relative humidity on C,/C,. Equilibration experiments were performed at 0,10,20, and 30 "C. The range of C, for particle set B experiments is shown in Table I. During initial runs at 0 and 10 'C, the relative humidity inside the apparatus was raised to 8C-100% by the cooling of laboratory air. Therefore, all experiments were conducted with a humidifying bubbler to raise the relative humidity to >95%. Comparative runs were made at 20 "C with and witbout the bubbler to explore the effect of high and ambient relative humidity on adsorption and to compare adsorption characteristics with particle set A. Figure 7 shows eq 3 plots for OCs on particle seta A and B, at 20 'C and ambient humidity (30-70% for set A, 30-55% for set B). Particle set B showed slightly less affinity for the OCs than set A, a trend also seen in field adsorption data with these particles (22). Statistical analysis showed that regression fits are different at the 99% confidence level. Table I11 compares CJC, for Chicago urban air particulate matter to data obtained earlier in our laboratory (3). The latter were carried out with aerosols from Columbia, SC, in the summer of 1985 using full-size 20 X 25 cm GFFs and a face velocity of 10 m mi&. Agreement between the three sets of data is within a factor -4 for most compounds. 474
Emiron. Scl. Technci.. Vci. 26. No. 3. 1992
Plots were constructed using eq 3 at four temperaturea (all runs a t constant relative humidity of 295%; P = 0.97-0.98). The slopes for all runs (0,10,20, 30 "C) were very close to unity, (1.03,0.973,0.999,0.948).The intercepts, which contain information about the adsorption properties of the aerosols, were very close for experiments at 0, 10, and 20 "C, (6.86,6.91,6.93). The intercept at 30 "C was slightly higher a t 7.18, an indication that the aerosols have different adsorption characteristics at this temperature. As can be seen from Figure 8, the regression line at 30 "C is higher than at the other temperatures. If AHD - AHv is constant, then as a first approximation, C,/C, values for individual compounds should 'slide" up or down the game regression l i e as temperature and thus vapor pressure changes. Since they intercept is slightly temperature dependent (IO,ll),the above statement is not strictly correct, but between 0 and 30 "C, the differences in they intercepts caused by temperature is negligible. Statistical analysis indicates that the regressions for 0,10, and 20 ' C are not different a t the 95% confidence level; however, the 30 "Cline is signiiicantly different from the others. This indicates that the affiity of the particles for the OCs has decreased over that displayed at the other temperatures. A possible explanation for this anomaly may be the stripping effect discussed above. Since these aerosols were collected at near 0 O C , exposing them to a
107
0
Particle Set A
. - . _. - , 3
P a r t i c l e Set B
-3
-4
B
-2
-1
0
Figure 7. Equation 3 plots for particle sets A and B at 20 "C, ambient humidity. (Set A: see Figure 3. Set B: m = 0.94, b = 6.86, f 2 = 0.98.) I n some cases, error is less than the size of the symbol. lo8
v
e
I
107
C
lo8
-
lo5
-
I
-h
"
io4
-
lo3 -
d12
m
d12-CI
10'
1
-5
-4
-3
-2
-1
0
Figure 8. Comparison of eq 3 plots for particle set B at 0, 10, 20, and 30 "C.
L I
30 "C environment and a clean airstream would likely alter the equilibrium of any sorbed organics, causing them to be stripped from the aerosols or the front filter fibers. This process is probably also occurring at 0, 10, and 20 "C, but at a lower rate. The reduced adsorption at 30 "C may be analogous to reduced capacity in chromatography as stationary film thickness is decreased, and that any active sites exposed by stationary phase stripping are filled by water. One would expect to see a continuous change in adsorption as temperature increases, but this does not appear to occur. It may be that kinetics for stripping of this stationary phase at lower temperatures are such that the effect is less noticeable over the time of the experiments. Heats of Desorption. The heat of desorption for a compound can be calculated using Pankow's equation (10, 11):
log C,/C, = m/T
rlcLlcl Figure 8. FID chromatograms of filter extracts: (A) particle-loaded GFF; (B) "clean" backup OFF behlnd particle-loaded GFF; (C) front OFF from clean frontlciean back experiment; (D) standard mixture of n alkanes (1 = C,, 2 = C,, 3 = C ,,, 4 = CZ6,5 = C, 6 = CZg,7 = C,,, 8 = C31). The four large peaks in chromatogram C and their correspondlng peaks in A and B are PAH internal standards.
+b
(4)
Plotting log C,/C, against 1/T gives a straight line with slope, m, approximately equal to -AHD/2.303R. Construction of accurate eq 4 plots requires that the adsorption characteristics of the particles do not change with temperature. Unfortunately this seems to occur at 30 "C. Table IV shows AHDvalues calculated from the slopes of the plots in Figure 9 and also from slopes using only the 0, 10, and 20 "C points. The AHDvalue for p,p'-DDT is calculated from data at 10,20 and 30 "C since gas-phase Environ. Sci. Technol., Voi. 26, No. 3, 1992 475
lo8
lou
I
a-HCH
I HCB
10'
m
=
-4423 10'
b = 22
r
3.2 lo8
3.3
3.4
3.5
3.6
3.7
= 0.97
3.2
,
1
1o7
7-HCH
I
I
I
I
I
3.3
3.4
3.5
3.6
cis-chlordane
3.7
7
io7
PI
u
hD
u
b = 23
10'
\
r
= 0.97
1 3.2 3.3 3.4 3.5 3.6 3.7
103
10'
,
I
t
I
DDE
loU
r
10'
-
o,p'-DDT
-m
b = 21
10' 1 3.2
10'
-
io6
2 = b-6265 \ = 25
loe
=097 I
I
I
I
I
3.3
3.4
3.5
3.6
3.7
r
in1 I _3.2
=096 I
I
I
I
3.3
3.4
3.5
3.6
3.7
1
10' lo3
-6909 =
loz r 10'
27
= 0.94
I
I
I
I
3.2
3.3
3.4
3.5
3.6
-1
T
X 1000 (K)
Flgure 9. Equatlon 4 plots for OCs. I n some cases, error is less than the size of the symbol.
levels at 0 " C were below detection. When all temperature data are used, A",- AHvvalues for most OCs are about 2-3 times larger than those calculated without the 30 "C data. This is an artifact of the apparent change in adsorption characteristics at 30 "C(Figure 8). Actual A", - A,€€" values for all the OCs are probably closer to those determined without the 30 O C data; typical field results for OCSand PAHs fall between 8 and 16 kJ mol-l(23,24). Humidity Effects. There is no apparent effect of relative humidity on the adsorption of OCs to particle set B aerosols for ambient laboratory relative humidity (30-70%) compared to high relative humidity (95-100% with bubbler) at 20 O C (Figure 10 upper graph; regressions are not statistically different at 95% confidence interval). As depicted in the lower graph of Figure 10, eq 3 plots at 476
Environ. Sci. Technol., Vol. 26, No. 3, 1992
Table IV. Heats of Desorption (AH,) AHD,''
kJ mol-'
0-30 "C datab
compound
A",
A",-A",
r-HCH cis-chlordane p,p'-DDE o@'-DDT
85 f 5.5 92 f 5.7 97 f 11 100 6.5 110 f 6.2 120 f 7.9 130 f 13
16 24 27 21 23 31 39
p,p'-DDTc
*
f
SE
0-20 "C datab A.HD AHD-AHv 76 f 8.1 81 f 7.5 72 f 12 89 f 9.3 96 f 8.9 110 f 13
7.5 12 1.4 6.6 8.8 19
"These data from particle set B at 95-100% RH. * AHv values were calculated from slopes of log POL vs 1 / T plots (12). rJr, for pip'-DDT 20, and 30 O C data* using
___
0 80
high h u m i d i t y
O C
I 104
a
u
1 I
!3/
1 ,fl ,
lo3
-2
-3
-4
-1
0
\
M
u
. _ _ _ 0 30 . “C 0 30 “C
___
ambient humidity high h u m i d i t y
This sorption seems to be caused by organic material being stripped from the particle-loaded filter and plating on the backup filter, thus acting as a “stationary phase” to enhance the sorption of the OCs. The stripping phenomenon suggests that the particles on the front filter are coated with this material and that, at least for these Chicago aerosols, the OCs may be predominantly partitioning into a liquid film on the particles rather than adsorbing to the solid particle surface. Finally, obtaining accurate values for AH,,is difficult because the process of measuring them apparently changes the sorption properties of the particles. This is probably another manifestation of the proposed stripping phenomenon. It should be remembered that these results are for two sets of urban aerosols and might not be generally applicable to other urban or rural aerosols.
Acknowledgments ,‘Y lo4. l o 3 L. -4
??” I
-3
-2
-1
0
Flgure 10. (Top) Equation 3 plots for OCs on particle set B at 20 OC at high (>95%) and ambient (30-70%) relative humidity. (Ambient relative humidity: m = 0.94, b = 6.86, r z = 0.98. High relative humidity: m = 0.98, b = 6.85, r 2 = 0.98.) I n some cases, error is less than the size of the symbol. (Bottom) Equation 3 plots for OCs on particle set B at 30 O C for high (>95%) and ambient (-35%) relative humidity. (Ambient relative humidity: m = 1.04, b = 6.93, r 2 = 0.97. High relative humidity: m = 0.948,b = 7.18, r 2 = 0.98.)
30 “C for high relative humidity air do display decreased adsorption, indicating a humidity effect at this temperature. Even so, the effect is relatively small (factor of -3 in C / C , values) compared to that seen on soils. Chiou and khoup (25) have shown that humidity effects on the adsorption of low molecular weight organics to soil is most pronounced for soils with low organic carbon content ( N 2%), where sorption of organic vapors is strongly suppressed by water competition for active sites on the mineral. However, urban aerosols are not very much like soil; they have a higher carbon content (1530%) because they arise predominantly from combustion sources (26). It may be that gaseous OCs are “seeing”a predominantly organic stationary phase on aerosols and are partitioning into this film rather than sorbing to a solid aerosol surface. As such, water molecules would not affect OC sorption (at least at moderate humidities) to any large extent since there are no polar sorption sites for water molecules to fill. This hypothesis is supported by the apparent stripping of organic material to the back filter, the change in apparent sorption characteristics at 30 OC, and the lack of any major humidity effect between -30 and 100% RH. Further experiments in which particle-loaded filters are exposed to airstreams for varying lengths of time at different temperatures over a wider range of humidities may help elucidate this problem.
Conclusions Laboratory equilibration experiments have revealed several interesting things about the partitioning of OCs between the gas phase and particles on glass fiber filters. There appears to be a substantial adsorption artifact of OCs to GFFs used as backups for particle-loaded filters.
Thanks to Thomas Kowalski and George Allen of the Chicago Department of Consumer Services and Bob Hutton and Linus Whitaker of the Illinois EPA for their help in obtaining field samples. Also thanks to Dana Dunkelburger of the University of South Carolina for SEMs of filter specimens. Registry No. a-HCH, 319-84-6; HCB, 118-74-1; y H C H , 5889-9; cis-chlordane, 5103-71-9; p,p’-DDE, 72-55-9; o,p’-DDT, 789-02-6; p,p’-DDT, 50-29-3; TCB, 16606-02-3.
Literature Cited Pankow, J. F.; Bidleman, T . F. Atmos. Enuiron. 1991,25A, 2241-2249. Ligocki, M. P.; Pankow, J. F. Enuiron. Sci. Technol. 1989, 23, 75-83. Foreman, W. T.; Bidleman, T. F. Enuiron. Sci. Technol. 1987, 21, 869-875. McDow, S. R.; Huntzicker, J. J. Atmos. Environ. 1990,24A, 2563-257 1. Appel, B. R.; Cheng, W.; Salaymeh, F. Atmos. Enuiron. 1989,23, 2167-2175. Coutant, R. W.; Brown, L.; Chuang, J. C. Atmos. Enuiron. 1988,22,403-409. Lane, D. A.; Johnson, N. D.; Barton, S. C.; Thomas, G. H. S.; Schroeder, W. H. Environ. Sci. Technol. 1988, 22, 941-947. Eatough, D. J.; Benner, C. L.; Bayona, J. M.; Richards, G.; Lamb, J. D.; Lee, M. L.; Lewis, E. A.; Hansen, L. D. Enuiron. Sci. Technol. 1989, 23, 679-687. Junge, C. E. In Fate of Pollutants in the Air and Water Environments; Suffet, I. H., Ed.; Wiley: New York, 1977; Part I, pp 7-26. Pankow, J. F. Atmos. Environ. 1987,21, 1405-1409. Pankow, J. F.; Bidleman, T. F. Atmos. Environ., in press. Hinckley, D. A.; Bidleman, T. F.; Foreman, W. T.; Tuschall, J. R. J . Chem. Eng. Data 1990, 35, 232-237. Bidleman, T. F. Anal. Chem. 1984,56, 2490-2496. Appel, B. R.; Tokiwa, Y.; Kothny, E. L. Atmos. Enuiron. 1983,17, 1787-1796. Cadle, S. H.; Groblicki, P. J.; Mulawa, P. A. Atmos. Environ. 1983, 17, 593-600. McDow, S. R. Ph.D. Dissertation, Oregon Graduate Center, 1986. Zhang, X.; McMurry, P. H. Environ. Sci. Technol. 1991, 25, 456-459. Bidleman, T. F. Enuiron. Sci. Technol. 1988,22,361-367. Matsumoto, G.; Hanya, T. Atmos. Enuiron. 1980, 14, 1409-1419. Cautreels, W.; van Cauwenberghe, K. Atmos. Enuiron. 1978, 12, 1133-1141. Simoneit, B. R. T.; Mazurek, M. A. Atmos. Environ. 1982, 16, 2139-2159. Cotham, W. E.; Bidleman, T. F. University of South Carolina, in preparation. Environ. Sci. Technol., Vol. 26, No. 3, 1992
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Bidleman, T. F.; Billings, W. N.; Foreman, W. T. Enuiron. Sci. Technol. 1986,20, 1038-1043. Yamasaki, H.; Kuwata, K.; Miyamoto, H. Environ. Sci. Technol. 1982, 16, 189-194. Chiou, C. T.; Shoup, T. D. Environ. Sei. Technol. 1985,19, 1196-1200. Shah, J. J.; Kneip, T. J.;Daisey, J. M. J. Air Pollut. Control
ASSOC. 1985, 35, 541-544. Received for review June 18, 1991. Revised manuscript received August 26,1991. Accepted September 3,1991. This work was supported by a grant from the National Science Foundation. Contribution No. 909 of the Belle W. Baruch Institute.
Capillary-Flow Mechanism for Fugitive Emissions of Volatile Organics from Valves and Flanges: Model Development, Experimental Evidence, and Implications Sang J. Chol,+ Robert D. All, Mlchael R. Overcash, and Phooi K. Lim” Department of Chemical Engineering, North Carolina State University, Raleigh, North Carolina 27695-7905
Experimental and model results are presented to show that the so-called “nonleaker” emissions of volatile organics from valves and flanges probably proceed by a capillary mechanism. A capillary-flow model is formulated based on theoretical consideration and field data, and equations are derived which permit a test of the model under laboratory conditions. Model predictions are shown to be confirmed by an experimental study of emission from a valve. The results suggest some practical steps for reducing fugitive emissions of VOCs from valves and flanges: (1) the use of nonwetting and low surface energy packing and containment materials, (2) the use of a resilient, nonswelling or nonshrinking packing material, (3) the damping of vibrations and temperature or pressure cyclings, and (4) the application of a compressive stress just sufficient to produce small pore sizes in the packing but insufficient to harm the resiliency and life span of the packing. I. Introduction The fugitive emissions of volatile organic compounds (VOCs) from flow-control devices-such as regular valves, pressure-relief and safety valves, flanges, containment vessels, drains, and pumps and compressors-is an important concern under the SARA Title I11 regulations (1-3). Plant data indicate that emission losses from the regular valves constitute the largest portion-possibly as high as 70%-of the total emission losses from all flowcontrol devices (3-5). Although the average emission loss from a valve is less than that from any other devices other than the flanges, the large number of valves in a process unit boosts their cumulative losses to the highest proportion. The usual approach to controlling fugitive emissions is to rely on a monitoring and maintenance (M & M) program to spot and repair or replace the leakers (3). The leaker emissions are generally the result of broken seal joints that have characteristic punctured openings larger than 100 pm and an emittant concentration higher than 10000 ppm. One or two such punctured openings are sufficient to turn a device into a leaker, but these can usually be spotted and fixed by an M & M program. Leaker emissions may be modeled as laminar or turbulent flows for liquid emittants and as sonic and subsonic compressible flows for gaseous emittants. Equations describing these flows are available (6). Emissions also occur from the so-called nonleakers, which are devices that nevertheless leak slowly and im‘On leave from Kyungpook National University, Korea. 478
Environ. Sci. Technol., Vol. 26, No. 3, 1992
perceptibly. These nonleaker emissions are somewhat harder to detect and fix because their emittant concentrations are often substantially lower than the 10000 ppm cutoff level that delineates them from the leaker emissions (3, 4). The nonleaker emissions proceed through small pores with characteristic dimensions of 1 pm or less. Precise data on nonleaker emissions are scarce, but their cumulative effect could be quite large if a significant number of the nonleakers do, in fact, leak continuously and imperceptibly. In the face of increasingly stringent emission standards that are being put in place ( l ) there , is a need to go beyond the M & M program and control the low-level emissions. In addition to lowering the overall emission, a control of the low-level emissions may also forestall the development of nonleakers into full-scale leakers. Unfortunately, the control of the low-level emissions is seriously hampered by a poor understanding of the fundamental emission mechanisms. A flow-through-pipe mechanism has often been presumed in the analyses of fugitive emissions (7,8), but, as will be shown shortly, it is often erroneous and misleading. In this paper, we present theoretical considerations and experimental data in support of a capillary-flow mechanism as the principal mechanism for the nonleaker emissions of VOCs from valves and flanges. By extension, the capillary-flow mechanism may also apply to the nonleaker emissions from other static devices. We formulate the capillary-flow mechanism in section II-A and show in section II-B that the mechanism explains some important trends of field data for which the flow-through-pipemodel is inadequate. In section III-C, we present the results of experimental tests of the model under laboratory conditions. In section IV, we consider the implications of our findings with respect to emission control. Finally, in section V, we propose additional research to extend the present work to other static devices and to address some important issues which have not been treated in this study. II. Theoretical Foundation of the Capillary-Flow Mechanism (A) Theortical Formulation and Justifications. We consider the steady-state emission of a VOC through the packing material of a valve and the gasket material of a pair of flanges. We assume that the emission proceeds through N parallel, nonconnecting paths that cut across the barrier material. Each of the N parallel paths is assumed to contain at least one pinch point which, for simplicity, will be modeled as a cylindrical or rectangular-slit pore. The pinch points are defined as narrow contrictions that limit the flow of the VOC. They are preceded and
0013-936X/92/0926-0478$03.00/0
0 1992 American Chemical Society
