Environ. Scl. Technol. 1992, 26, 2 186-2 191
Press: Columbus, OH, 1985; pp 643-661. (8) Ball, J. C.; Greene, B.; Young, W. C.; Richert, J. F. 0.; Salmeen, I. T. Environ. Sei. Technol. 1990,24,890-894. (9) Paputa-Peck, M. C.; Marano, R. S.; Schuetzle, D.; Riley, T. L.; Hampton, C. V.; Prater, T. J.; Skewes, L. M.; Jensen, T. E.; Ruehle, P. H.; Bosch, L. C.; Duncan, W. P. Anal. Chem. 1983,55,1946-1954. (10) Alfheim, I.; Lofroth, G.; Moller, M. Environ. Health Perspect. 1983,47,227-238. (11) Ball, J. C.; Foxall-VanAken, S.; Jensen, T. E. Mutat. Res. 1984,138,145-151. (12) Ball, J. C.; Salmeen, 1. T.; Morris, S. M. Environ. Mol. Mutagen. 1989,13,100-106. (13) Maron, D. M.; Ames, B. N. Mutat. Res. 1983,113,172-215.
(14) Rosenkranz, H. S.; Mermelstein, R. In Nitrated Polycyclic Aromatic Hydrocarbons; White, C . M., Ed.; Dr. Alfred Huethig: New York, 1985; pp 267-297. (15) Massaro, M.; McCartney, M.; Rozenkranz, E. J.; Anders, M.; McCoy, E. C.; Mermelstein, R.; Rosenkranz, H. S. Mutat. Res. 1983,122,243-249. (16) Bell, J. C.; Young, W. C.; Wallington, T. J.; Japar, S. M. Environ. Sci. Technol. 1992,26, 397-399. (17) Ames, B. N. Science 1979,204,587-589. (18) Schuetzle, D.; Jensen, T. E.; Ball, J. C. Environ. Znt. 1985, 11, 169-181.
Received for review April 16,1992.Revised manuscript received July 6, 1992. Accepted July 8,1992.
Influence of Surfactants on Vapor-Liquid Partitioning Michael A. Anderson
Department of Soil and Environmental Sciences, University of California, Riverside, California 9252 1 Surfactants have been shown to significantly affect the solubility and sorption characteristicsof organic pollutants, though little work has evaluated their influence on the partitioning of organic chemicals between the liquid and vapor phases. This paper presents results from an investigation of vapor-liquid partitioning of common subsurface contaminants in the presence of two synthetic surfactants and humic acid. The presence of surfactants significantly altered equilibrium vapopliquid partitioning, resulting in substantial reductions in apparent Henry's of benzene, toluene, and o-xylene. Humic constants (H*) acid exerted little influence on H*of benzene and toluene, but did yield some modest reductions for o-xylene. A simple three-phase model, which partitions chemical mass to vapor, aquated, and surfactant-associated phases, accurately reproduced observed gas concentrations over the wide range of surfactant concentrations used in the study. Introduction There exists at present a great deal of interest in the fate and persistence of organic chemicals within the environment. As a result of spills, improper disposal practices, and leaking storage tanks, nonionic "hydrophobic" organic chemicals (HOCs) are common contaminants within soils and surface waters and groundwaters (1-3). Humic substances and other forms of dissolved organic matter within natural waters have been found to increase the apparent solubility of HOCs (4-6) and to decrease HOC sorption to soil solid phases (7). Synthetic surfactants have also been observed to significmtly increase the apparent solubility of HOCs (8, 9). This increased solubility in the presence of surfactants is the basis for one strategy for the remediation of HOCcontaminated soils and aquifers (particularly when present as a nonaqueous-phase liquid or NAPL) (10). The low solubility of many petroleum-based contaminants limits the mass partitioned to the aqueous phase. As a result, standard pump-and-treat methods are generally ineffectual for the remediation of contaminated aquifers (11). Socalled surfactant washing takes advantage of the increased apparent solubility of HOCs in the presence of surfactants (12,13).By flushing a contaminated soil or aquifer with a surfactant solution in which HOCs are appreciably more soluble than in a simple water stream, considerably more contaminant mass is partitioned to the mobile phase. This 2186
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increased solubility directly enhances the efficiency of the pump-and-treat technique. Surfactant washing may also mobilize residual NAPLs by reductions in interfacial tension (10). For many HOCs, the partitioning of mass between the aqueous and vapor phases is also an important factor affecting overall persistence (14). The equilibrium distribution of chemicals between the aqueous and vapor phases is described by Henry's law, which in the nondimensional form is written C, = HC, (1) where C, is the concentration in the gas (or vapor) phase (mg/L), C, is the concentration in the aqueous phase (mg/L), an%H is the Henry's constant (dimensionless). Alternate strategies for the remediation of volatile HOCcontaminated soils and waters are explicitly based upon this tendency for many chemicals to partition significant quantities of mass to the vapor phase (e.g., vacuum extraction, vapor stripping) (10, 15). Though the influence of natural dissolved organic matter (DOM) and surfactants on the solubility and sorption of HOCs is now well-established, the influence of DOM and surfactants on vapor-phase concentrations and apparent Henry's constants (i.e., ratio of vapor concentration ta total liquid concentration, here denoted H*) has not been extensively investigated. Nicholson et al. (16) reported no change in H* of selected trihalomethanes in distilled and natural waters to which humic acid was added. Yurteri et al. (17) observed that H* for trichloroethylene and toluene varied unpredictably as a function of chemical composition of aqueous phase. Callaway et al. (18) found that vapor-phase concentrations of chloroform and trichlorethylene were reduced relative to water under very high humic acid solution concentrations (10 wt %). This work presents results from an evaluation of the equilibrium partitioning of common subsurface contaminants between surfactant and humic acid solutions and the vapor phase. Three-phase Partition Model A three-phase model, similar to that presented by Garbarini and Lion (19) for liquid-solid-vapor systems, was developed. The model partitions chemical mass between vapor, aquated, and surfactant-associated phases. It was first assumed that the distribution of chemical
0013-936X/92/0926-2186$03.00/0
0 1992 American Chemical Society
between the aquated and surfactant-associated phases can be described by S = KC,, (2) where S is the amount of chemical partitioned to unit m w of surfactant (mg/mg)and K is the linear solute-surfactant partition coefficient (L/mg). The total liquid-phase concentration, C1, and aquated concentration (i.e., the concentration of dissolved chemical not associated with surfactant) are related by c1 = c,, + (3)
sx
where X is the concentration of surfactant (mg/L). Substitution of eq 2 into eq 3, upon rearrangement, yields (4) c,, = C1/(1 KX)
+
Thus, the aquated or “free” chemical concentration can be determined from the total liquid-phase concentration. Assuming then that only the aquated form participates in liquid-vapor exchange, Henry’s law (eq 1)can be rewritten using eq 4 as c, = HC1/(1 + KX) (5) For this study, predicted gas-phase concentrations were calculated from known mass addition to a closed system by a form of the mass balance equation Mt = v,c, + v,c, (6) rewritten using eq 5 as
Mt
cg
=
v, + [V1(1 + K X ) / H ]
(7)
where Mt is the total mass of chemical added to the system and VI and V, are the liquid and gas volumes (L), respectively.
Experimental Section The equilibrium partitioning of benzene, toluene, and o-xylene between the aqueous and vapor phases was evaluated via closed vessel equilibrations (20). Apparent Henry’s constants were determined for solutions of varying concentrations of technical-grade sodium dodecyl sulfate (SDS), Witconol SN70, an ethoxylated alcohol with an average molecular weight of 392 (Witco Corp., New York), and Aldrich humic acid (Aldrich Chemical Co., Milwaukee, WI). Fifty milliliters of 0, 3 X lo9, and 3 X 10-2 M SDS;2.5 x 10-4,2.5 x 10-3,7.5 x 10-3,2.5 x 10-2, and 7.5 X M SN70; and 30,60,125,250, and 500 mg of C/L of humic acid (adjusted to pH 8 with 0.01 M NaOH) was added to 120-mL serum bottles. Solutions were prepared in 0.03 M NaCl to provide for some control of solution ionic strength. Ionic strength is an important property affecting critical micelle concentration (cmc) and other properties of surfactant solutions (21). Seven microliters of benzene, toluene, or o-xylene was injected below the water surface, and the bottles were rapidly sealed with aluminum crimp caps and Teflon-lined septa. It was calculated that addition of this volume of chemical was below the solubility limit in water for all three chemicals (10). Equilibrations were performed in triplicate at ambient temperature (23 i 1 “C). Apparent solubilities of benzene, toluene, and o-xylene in SDS and SN70 solutions were determined by following a procedure analogous to that developed by Chiou et al. (4). Solutions of SDS and SN70 were placed in 35-mL serum bottles to which an amount of chemical greater than that required to saturate the solutions was added. The
chemical was injected below the surface of the solution, and the serum bottles were rapidly sealed with Teflonlined septa and aluminum crimp caps. Headspace was kept to a minimum (ca. 0.99 for linear regressions of apparent solubilities with SDS concentration. Such behavior was also noted by Kile et al. (9) for some petroleum sulfonate surfactants, which was ascribed to a partition equilibrium of the solute between an emulsified phase and water. Kile and Chiou (8) observed nearly linear increases in apparent solubilities in the presence of other nonhomogeneous surfactants as Environ. Sci. Technol., Vol. 26, No. 11, 1992
2187
00000
00000
n
SN70 SDS
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c
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3
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'
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SDS Concentration (M) Flgure 2. Apparent solublllty of benzene, toluene, and o-xylene versus SDS concentratlon. Error bars provided when standard deviatlons exceed symbol slte.
well, though they found a two-slope relationship for the apparent solubility of TCB and DDT in the presence of reagent-grade SDS which was attributed to uptake by monomeric and micellar surfactant constituents. Neutral oil constituents in a commercial linear alkylbenzenesulfonate (LAS) formulation were found to substantially increase organic solute solubility below the cmc (24). As a result, a more linear relationship between apparent solubility and surfactant concentration was noted for commercial-grade LA5 than for purified LAS. The technical-grade SDS used in this evaluation (likely a product of quality similar to that which might be used in largescale surfactant washing applications) apparently also contains oils and other constituents which allow for heterogeneous or successive micellization and approximately linear increases in apparent solubility with surfactant concentration. Evaluation of apparent solubility in the SN70 solutions was stymied by the formation of an additional liquid phase within the sealed vessels when an excess of chemical was equilibrated with the SN70 solutions. As a result, no apparent solubility data are provided for this surfactant. Apparent solubility of organic chemicals in the presence of humic substances has been demonstrated in previous work ( 4 ) and was not reassessed here. 2188
Envlron. Scl. Pechnol., Vol. 26, No. 11, 1992
0
10
10
-'
10
10
-'
SDS Concentration ( M )
Figure 1. Surface tension versus the logarithm of surfactant concentration.
3
-'
12
I
S u r f a c t a n t Conc ( M )
n
i.1
AAAAA
'c L
5000
Benzene Toluene CIODClCj o-xylene __ .._. Benzene ( f i t ) . . - .. Toluene ( f i t ) _ _ _ . _ o-Xylene __ (fit)
affab*
Figure 3. Observed and model-fltted equilibrium gas-phase concentrations of benzene, toluene, and o-xylene versus logarlthm of SDS concentration. Error bars provMed when standard devlatlons exceed symbol slze.
As a linear increase in apparent solubility with SDS was noted, the relationship used by Kile et al. (8)to describe the organic solubility enhancement due to the petroleum sulfonate surfactants [in the same form as that proposed by Chiou et al. ( 4 ) to account for the solubility enhancement due to dissolved humic substances] was used S,'/S, = 1 C KX (8) where S," is the apparent solute solubility (mg/L) and S, is the solubility in water (mg/L). In the liquid-phase partitioning experiments of Kile et al. (9),solute concentrations in both water and surfactant solutions were expressed on a mass-mass basis such that dimensionless ICs were derived. For direct comparison with the results of work by Chiou and others, dimensionless K's were derived from Figure 2. Values of K of 190, 560, and 1280 for benzene, toluene, and o-xylene, respectively, were in relatively good agreement with reported KO,values of 132, 537, and 891 (10).Kile et al. (8) also noted good agreement between calculated K values for petroleum sulfonate surfactants and KO,values for 1,2,3-TCB and p,p'-DDT. Vapor-Phase Concentrations and Apparent Henry's Constants. Equilibrium vapor-phase concentrations for the three chemicals in the absence of surfactant were between 21 and 24 mg/L, with toluene being slightly higher than o-xylene and benzene. At constant added chemical mass to the vessels, vapor-phase concentrations were significantly reduced at moderate to high SDS concentrations (Figure 3). Vapor-phase concentrations were reduced most significantly for o-xylene, followed by toluene, and benzene. Apparent Henry's constants necessarily followed these same trends and decreased with increasing SDS concentration (Table I). Equation 7 was used to predict gas-phase concentrations in the presence of SDS. Values of K derived from the apparent solubility data yielded predicted gas concentrations that were significantly lower than those observed for all three compounds (not shown). Solute-surfactant partition coefficients Calculated from gas concentration data using a form of eq 7 were lower than those derived from apparent solubility data (Table 11). This observation suggests that solute partitioning to SDS is some function of both surfactant and chemical concentration. This postulate was further evaluated and is discussed in a subsequent section. Nonetheless, using fitted values of K (Table 11),the three-phase model was able to reproduce
30 I
Table 1. Apparent Henry’s Constants as a Function of Surfactant Concentration
I
A
I
I
surfactant concn (M)
SDS 0.274 f 0.003 0.256 f 0.013 0.268 f 0.012 0.278 f 0.008 0.208 f 0.006 0.130 f 0.009
3x
0.229 f 0.004 0.213 f 0.006 0.231 f 0,001 0.229 f 0.001 0.199 f 0.003 0.158 f 0.002
2.5 X 2.5 X 7.5 X 2.5 X 7.5 X
SN70 0.230 f 0.002 0.272 f 0.001 0.218 f 0.001 0.240 f 0.001 0.195 f 0.001 0.185 f 0.002 0.135 f 0.001 0.104 f 0.006 0.077 f 0.001 0.054 f 0.008
0.236 f 0.005 0.167 f 0.009 0.088 f 0.001 0.050 f 0.006 0.021 f 0.001
30b 60 125 250 500
Humic Acid” 0.210 f 0.005 0.254 f 0.001 0.214 f 0.003 0.247 f 0.002 0.213 f 0.002 0.245 f 0.002 0.217 f 0.003 0.248 f 0.001 0.215 f 0.003 0.239 f 0.004
0.212 f 0.008 0.199 f 0.013 0.177 f 0.012 0.180 f 0.002 0.168 f 0.002
0 1 X lo4 1X 3X 1x
p---- ---&--
apparent Henry’s constant f 1 s benzene toluene o-xylene 0.239 f 0.008 0.236 f 0.004 0.239 f 0.004 0.185 f 0.004 0.111 f 0.003 0.053 f 0.004
f&Mi-i? Benzene
-
AAAAA
-
\
Benzene ( f i t ) Toluene ( f i t ) o-xylene (fit)
~
.
o’,
Toluene
ooooo o-Xylene ...... ~
”
“U
-
-
0
0
10
10
-4
-’
10
-’
Flgure 4. Observed and model-fitted equllibrlum gas-phase concentrations of benzene, toluene, and o-xylene versus logartthm of SN70 concentration. Error bars provided when standard deviations exceed symbol slze. 30
I
I
A
A
*
Table 11. Solute-Surfactant Partition Coefficients Derived from Gas-Phase Concentration Data (Figures 3-5)”
a
10
-?
SN70 Concentration ( M )
Lower ambient temperatures during equilibration and analysis are believed to be responsible for the overall lower apparent Henry’s constants calculated for the humic acid-chemical equilibrations. *In millierams of C/L.
chemical
solute-surfactant Dartition coeff SDS SN70 humic acid
benzene toluene o-xylene
60 130 435
165 355 1100
Q,
A
0
a
l
l
850
AAAAA 00000
Corrected to dimensionless form of Chiou et al. ( 4 , 8 , 9).
observed gas concentrations for all three chemicals over the entire range of SDS concentrations (Figure 3). Witconol SN70 was somewhat more efficient at reducing vapor-phase concentrations than SDS at equivalent surfactant concentrations (Figure 4) As a result, H* was correspondingly lower at similar surfactant concentrations (Table I). At the highest SN70 concentration, H* for o-xylene was reduced to less than 10% of that for water. Values of K for SN70 derived from observed gas concentrations were considerably higher than those for the shorter-chained anionic SDS (Table II). Using these values of K , the model again reproduced observed gas concentrations for the SN70 solutions (Figure 4). This supports the basic assumptions of the model (i.e., linear partitioning of solute between aquated and surfactant phases, and liquid-vapor partitioning related to aquated concentration rather than total liquid concentration). At the lower levels of humic acid (relative to the surfactants) used in this aspect of the study, no discernible decrease in benzene and toluene vapor concentrations was noted, though the gas-phase concentrations of o-xylene did decrease slightly with increasing humic acid concentration (Figure 5 ) . As a result, H*’s for benzene and toluene in the presence of humic acid were unchanged (Table I), The extent of solute-surfactant partitioning was below that for which K could be calculated. A solute-surfactant partition coefficient of 850 was calculated for o-xylene, a value intermediate between those for SDS and SN70 (Table 11). The apparent solubility experiments were anticipated to provide an independent measure of K used in the above
*
0
0
Toluene o-Xylene o-xylene
200
(fit) 400
600
Humic Acid Conc (mq C / L ) Flgure 5. Observed and model-fitted equlllbrium gas-phase concentratlons of benzene, toluene, and o-xylene versus humic acid concentration. Error bars provided when standard deviations exceed symbol size.
gas-phase concentration predictions such that gas concentrations could be predicted knowing only the total chemical mass added to the systems. As previously noted, however, values of K from SDS apparent solubility data were substantially higher than those estimated from gasphase concentration data. This discrepancy in the magnitude of K prompted an evaluation of H* and K under varying HOC liquid-phase M). concentrations at constant SDS concentration Apparent Henry’s constants in M SDS were, in fact, observed to decrease approximately linearly with increasing HOC liquid concentration (Figure 6). Solutesurfactant partition Coefficients for toluene and o-xylene derived from these data increased linearly with increasing HOC liquid concentration (Figure 7), though values of K derived from the gas-liquid equilibrations at the highest HOC mass were somewhat higher than those derived from apparent solubility data. Reports of increasing affinity (i.e., increasing K ) of chemicals for a sorbent with increasing partitioned mass are rather uncommon. Bruaseau (25)has recently reported an increase in the magnitude of sorption coefficients for Envlron. Scl. Technoi., Vol. 20, No. 11, 1992
2189
Benzene
+C
Toluene o-Xylene
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VI
*
0
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005
a a
Q
000' 0
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'
500
'
'
1000
'
' 1500
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2000
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2500
Liquid Conc (mg/l-) Flgure 6. Apparent Henry's constants for benzene, toluene, and o-xylene versus total liquid-phase concentratlon In M SDS. Error bars provlded when standard devlatlons exceed symbol slze. 1500
7-
!
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0
Mzfnt-h Benzene
Toluene o-Xylene
AAAAA
om00
to increase with increasing solute concentration. The role of this phenomenon in the equilibrium airwater partitioning of more hydrophobic chemicals in natural waters can be inferred based upon data from available literature. As an example, the reduction in the apparent Henry's constant for 2,2',4,5,5'-PCB in a natural water is calculated by assuming a dissolved organic carbon (DOC) content of 10 mg/L, a value of K of 1.17 X lo4 (mass-mass basis) (4), a nondimensional Henry's constant of 1.16 X (26),and a total liquid concentration at its solubility limit in water of 1.1 X mg/L (4). From eq 4,an equilibrium gas concentration of 1.14 X lo-* mg/L is predicted, which would yield a value of H* of 1.04 X lo+, a reduction of 10%. For a biologically-rich or polluted water with a higher DOC content, for example, 100 mg/L, an equilibrium vapor concentration of 5.88 X mg/L is predicted. This higher DOC water would yield an apparent Henry's constant of 5.35 X a reduction of 54% in H*. Thus for most natural waters, minor reductions in apparent Henry's constants could be anticipated, though substantial reductions in H* can be expected for hydrophobic chemicals in natural or polluted waters and treatment streams with high concentrations of DOC or surfactants.
Acknowledgments I thank Ms. Michelle Kim and Ms. Margaret Resketo for their analytical assistance in this project. Registry No. Benzene, 71-43-2; toluene, 108-88-3; o-xylene, 95-47-6.
4 c
I
Literature Cited
-
01 0
f7p
/
500
1000
,
I
I
1500
I
2000
I
2500
L q u i d Conc (mg/L) Flgure 7. Solute-surfactant partition coefficients versus total llquldM SDS, derived from Figure 6 data. Error phase concentration In bars provlded when standard devlatlons exceed symbol size.
nonionic organic chemicals in binary-solute solutions over those derived from single-solute solutions. This phenomenon was attributed to an increase in the apparent organic carbon content of the sorbent (25). Likely of more importance for these surfactant solutions is a change in the micellar structure within the solution (21). An increase in aggregation may promote increased partitioning of solute to the surfactant phase, though additional work is needed to more fully evaluate this phenomenon.
S u m m a r y and Conclusions The presence of surfactants resulted in significant reductions in the apparent Henry's constants of benzene, toluene, and o-xylene. Humic acid had little effect on equilibrium vapor-liquid partitioning of benzene and toluene, but did result in some slight reductions in H*for o-xylene. Gas-phase concentrations were described by a simple three-phase model which partitions mass to vapor, aquated, and surfactant-associated phases. Vapor-liquid-phase partitioning was described by Henry's law with the aquated (i.e., nonsurfactant associated) HOC concentration used in lieu of the total liquid-phase concentration. Aquated and surfactant-associated components were described by a linear partition equation, though K was found 2190
Environ. Sci. Technol., Vol. 26, No. 11, 1992
Duffy, J. J.; Peake, E.; Mohtadi, M. F. Environ. Znt. 1980, 3, 107-120. Harris, G. R.; Garlock, L.; LeSeur, L.; Mesinger, S.; Wexler, R. J . Environ. Health 1982,44, 287-295. Mackay, D. M.; Roberta, P. V.; Cherry, J. A. Environ. Sci. Technol. 1985, 19, 384-392. Chiou, C. T.; Malcolm, R. L.; Brinton, T. I.; Kile, D. E. Environ. Sci. Technol. 1986, 20, 502-508. Boehm, P. D.; Quinn, J. G. Geochim. Cosmochim. Acta 1973, 37, 2459-2477. Chiou, C. T.; Kile, D. E.; Brinton, T. I.; Malcolm, R. L.; Leenheer, J. A. Environ. Sci. Technol. 1987,21,1231-1234. Chin, Y. P.; Weber, W. J., Jr.; Eadie, B. J. Enuiron. Sci. Technol. 1990,24, 837-842. Kile, D. E.; Chiou, C. T. Environ. Sci. Technol. 1989,23, 832-838. Kile, D. E.; Chiou, C. T.; Helburn, R. S. Environ. Sci. Technol. 1990,24, 205-208. Mercer, J. W.; Cohen, R. M. J. Contam. Hydrol. 1990,6, 107-163. Travis, C. C.; Doty, C. B. Environ. Sci. Technol. 1990,24, 1464-1466. Ellis, W. D.; Payne, J. R.; McNabb, G. D. Treatment of contaminated solid with aqueous surfactants; EPA/ 60012-85/129; U.S.E P A Washington, DC, 1985. Abdul, A. S.; Gibson, T. L.; Rai, D. N. Ground Water 1990, 28, 920-926. Baehr, A. L. Water Resour. Res. 1987,23, 1926-1938. Johnson, P. C.; Kemblowski, M. W.; Colthart, J. D. Ground Water 1990,28, 413-429. Nicholson, B. C.; Maguire, B. P.; Bursill, D. B. Environ. Sci. Technol. 1984, 18, 518-521. Yurteri, C.; Ryan, D. F.; Callow, J. J.; Gurol, M. D. J. Water Pollut. Control Fed. 1987.59. 950-956. - . Callaway, J. Y.; Gabbita, K.V:; Vilker, V. L. Environ. Sci. Technol. 1984,18, 890-893. Garbarini, D. R.; Lion, L. W. Environ. Sci. Technol. 1985, 19, 1122-1127. Murphy, T. J.; Mullin, M. D.; Meyer, J. A. Environ. Sci. Technol. 1987,21, 155-162.
Envlron. Scl. Technol. 1992, 26, 2191-2198
(21) W e n , M. J. Surfactants and Interfacial Phenomenon,2nd ed.; John Wiley and Sons: New York, 1989; pp 170-206. (22) Huisman, H. F. K. Ned. Akad. Wet., Proc. Ser. B 1964,67, 388-395. (23) Anderson, M. A. Agron. Abstr. 1991, 237. (24) Chiou, C. T.; Kile, D. E.; Rutherford, D. W. Enuiron. Sci. Technol. 1991,25, 660-665. (25) Brusseau, M. L. Enuiron. Sci. Technol. 1991,25,1747-1752.
(26) Anderson, M. A,; Parker, J. C. Water,Air, Soil Pollut. 1990, 1-18.
Received for review April 3,1992. Revised manuscript receiued June 29,1992. Accepted July 8,1992. This research was partially supported by a grant from the Kearney Foundation of Soil Science.
Formation of Polybrominated Dibenzofurans during Extrusion of High-Impact Polystyrene/Decabromodiphenyl Ether/Antimony ( I I I)Oxide Ronald LulJk,*gtHarrle A. J. Govers,t and Laurent Nellssent
Department of Environmental and Toxicological Chemistry, University of Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands, and Eindhoven University of Technology, Centre for Polymers and Composites, P.O. Box 513, 5600 MB Eindhoven, The Netherlands Polybrominated dibenzofurans (PBDFs) are formed during extrusion of high-impact polystyrene (HIPS) containing the flame retardant system decabromodiphenyl ether/antimony(III) oxide (Br,,,DPO/Sb203). The process of formation of PBDFs was examined at an extrusion temperature of 275 "C. To get a better understanding of processes taking place in the polymer melt during extrusion) the thermal stability of polystyrene chains was also examined. It was found that the flame retardant system Br&PO/Sb203 shows a stabilizing activity toward thermal degradation of polystyrene chains. Temperature development during extrusion processing of HIPS/ Br1&PO/Sb2O3, simulated in a computer model, supports a mechanism of formation of PBDFs which is initiated by thermal degradation of polystyrene. Polystyrene degradation products, i.e., (po1y)styrene radicals, are stabilized by radical scavenging properties of the flame retardant decabromodiphenyl ether (Br&PO). Debromination of Brl&PO is combined with a ring-closure reaction forming PBDFs. It seems inevitable that PBDFs are formed during extrusion of polybrominated diphenyl ethers in addition polymers such as polystyrene. A tentative quantification of the yield of PBDFs in HIPS/BrloDPO/Sb203by RPHPLC showed an increase from 1.5 to 9 ppm for heptabromodibenzofuran and from 4.5 to 45 ppm for octabromodibenzofuran after four extrusion cycles.
Introduction Polybrominated flame retardants are widely used in synthetic polymers to reduce inflammability. Due to their universality and minor influence on mechanical properties, polybrominated aromatic compounds have a broad application area. Despite technical advantages in applications, the use of polybrominated compounds in polymers has two disadvantages: First, formation of corrosive HBr during compounding (2' = 190-225 "C) and second, but more important, formation of polybrominated dibenzo-pdioxins (PBDDs) and -dibenzofurans (PBDFs) during thermal degradation (T= 350-800 "C).The formation of PBDDs and PBDFs has been shown during pyrolysis of pure brominated flame retardants (1-3)and during pyrolysis of polymers with brominated flame retardant additives (4, 5). Especially brominated diphenyl ethers (PBDPOs) are reactive precursors in the formation of University of Amsterdam. Polymers and Composites.
3 Centre for
0013-936X/92/0926-2191$03.0010
PBDDs and PBDFs (6, 7). Equation 1shows the structural similarity of PBDPOs and PBDFs/PBDDs. Formation of PBDFs consists of a ring-closure reaction with the elimination of a small molecule (Br2,HBr, or H2). An extra oxidation reaction is needed for the formation of PBDDs.
X ' + y ' i S
In this study the thermal behavior of PBDPOs during extrusion (melt compounding) of high-impact polystyrene (HIPS) was examined. HIPS is a blend of polystyrene and polybutadiene (6-12 wt %, dispersed in the polystyrene matrix) of which inflammability is reduced by the flame retardant system decabromodiphenyl ether/antimony(III) oxide (Br,oDPO/Sb203). From pyrolysis studies, it is known that mainly PBDFs are formed from PBDPOs during thermal degradation of HIPS/Br1,DPO/Sb2O3. The formation of PBDFs occurs during polymer degradation (2' = 350-400 "C) (8). Recently, formation of PBDFs during compounding of polymers with polybrominated flame retardants has been shown (9-11). Brenner et al. (12)measured PBDFs in the workplace area during compounding of poly(buty1ene terephthalate) (PBTP)/glass fiber/Br,oDPO/Sb203 polymer blends. They reported total amounts of 34 ng/m3 tetrabromodibenzofurans (Br,DFs), 143 ng/m3 pentabromodibenzofurans (Br,DFs), 554 ng/m3 hexabromodibenzofurans (Br,DFs), and about 200 ng/m3 heptabromodibenzofurans (BqDFs). PBDDs were also found but at much lower concentrations than PBDFs. Around the extruder head the amount of 2,3,7&substituted PBDF congeners was tentatively determined at 1.3 ng/m3 for Br,DFs and 2.6 ng/m3 for Br6DFs. Assuming equal toxicity of bromineand chlorine-substituted dioxins and furans (13,14),one can estimate the contribution of 2,3,7,84etrachlorodibenzo-p-dioxin equivalents (toxic equivalents or TEQs) using the toxic equivalence factor method (15). The amount of TEQs in the surroundings of the extruder head during extrusion of PBTP/glass fiber/Br1&PO/Sb2O3 was estimated at 330-910 pg/m3 (12). The tolerable daily
0 1992 American Chemical Society
Environ. Sci. Technol., Vol. 26, No. 11, 1992 2191
