Environ. Sci. Techno/. 1982, 16,93-98
(8) Huckabee, J. W.; Elwood, J. W.; Hildebrand, S. G. In “The Biogeochemistry of Mercury in the Environment”;Nriagu,
J. O., Ed.; Elsevier/North-Holland Biomedical Press: New York, 1979; pp 277-302. (9) Neyman, J.; Scott, E. L. In “Optimizing Methods in Statistics”;Rustagi, J. s.,Ed.; Academic Press: New York,
1971; pp 413-30.
Received for review April 2,1981. Accepted September 29,1981. Primary financial support was provided by contract EY-76-C09-0819 between the University of Georgia and the U.S. Department of Energy.
Sorption of Amino- and Carboxy-Substituted Polynuclear Aromatic Hydrocarbons by Sediments and Soilst Jay C. Means”
Center for Environmental and Estuarine Studies, Chesapeake Biological Laboratory and Department of Chemistry, University of Maryland, Solomons, Maryland 20688 Susanne G. Wood’
Institute for Environmental Studies, University of Illinois, Urbana, Illinois 61801 John J. Hassett and Wayne L. Banwart
Department of Agronomy, University of Illinois, Urbana, Illinois 61801 The sorption of 2-aminoanthracene, 6-aminochrysene, and anthracene-9-carboxylic acid on 14 sediment and soil samples exhibiting a wide range of physicochemical properties was studied. The equilibrium isotherms were linear, and the Freundlich partition coefficients (K,) for each compound were found to be highly correlated with the organic carbon content of the soil/sediment tested. No other significant correlations with soil/sediment properties were observed. The sorption constants (K ), when normalized to organic carbon content of the substrate (K,), could be predicted within a factor of 2-3 from the octanol-water partition coefficients or water solubilities of the compounds using equations developed in earlier studies. However, both equations tended to underestimate the K , values for the two amines tested. The amount of deviation from predicted sorption was highly correlated with the % organic carbon/ % montmorillonite clay ratio of the substrates. Experimental values of the water solubility and octanol-water partition coefficients for the three compounds are reported.
H
Introduction Polynuclear aromatic hydrocarbons (PAHs) are ubiquitous products of the combustion of carbon-based substances. In addition to the purely aromatic and alkylsubstituted PAHs, a wide variety of PAHs exist which are substituted with polar functional moieties such as primary amino groups or carboxyl groups. Like the normal PAHs, these substituted PAHs are of concern environmentally because many individual compounds within this group have been demonstrated to cause mutations and certain types of cancer (1,2).In one recent report, the aminoPAHs were identified as the most mutagenic fraction in a variety of synthetic fuels (1). Although the sources and distribution of PAHs in the environment have been studied extensively (3-7), the transport and the fate of substituted PAHs in sedi~
~~
‘This paper is Contribution No. 1230 of theCenter for Environmental and Estuarine Studies of the University of Maryland. t Current address: Illinois State Natural History Survey, 239 Natural Resources Studies Annex, Champaign, IL 61820. 0013-936X/82/09 16-0093$01.25/0
ment/water systems are poorly understood. In addition, the physical and chemical properties of the substituted PAHs which govern their interactions with substrates have not been characterized or quantitated. In water/sediment and water/soil sytems, sorption is recognized as one important factor in the determination of the fate of organic molecules (8). Karickhoff et al. (9) and Means and coworkers (10-18) have described the sorption behavior of a number of hydrophobic molecules on sediments and soils. The compounds included normal PAHs, nitrogen and sulfur heterocyclic PAHs, and some substituted aromatic compounds (Le., l-naphthol, acetophenone, benzidine). These studies suggested that the sorption of hydrophobic molecules (benzidine excepted) was governed by the organic carbon content of the substrate. Furthermore, the solubility of the compound and its solvent-partitioning characteristics (e.g., octanol-water partition coefficient, KOw)were found to be significantly correlated to the sorption constant K,, which is derived from the linear partition coefficient K pand the organic carbon content of the sorptive substrate. Conversely, the sorptive behavior of these compounds was found to be independent of substrate pH, cation-exchange capacity (CEC), textural composition, or clay mineralogy. We concluded that sediment/soil K , values could be reliably predicted from either the K , or the water solubility of the compound and from the organic carbon content of the individual substrates. Benzidine, a fairly polar aromatic molecule, did not exhibit the same type of sorptive behavior (16) but instead yielded curvilinear isotherms. This result suggested multiple mechanisms of sorptiqn. Further investigation indicated that, although the fractiop of benzidine which existed in a neutral form sorbed to organic matter in accordance with the above relationships, the sorption of charged forms was observed to be independent of organic matter and dependent upon sediment surface area. As might be expected, the sorption of benzidine was largely controlled by the pH of the aqueous phase in the isotherm mixtures. These data suggested that the sorption of substituted PAHs might be 4 multimechanistic process which would not be amenable to the predictive relationships outlined above (13, 14).
0 1982 Amerlcan Chemical Society
Envlron. Scl. Tecpnol., Vol. 16, No. 2, 1982 93
Table I. Characteristics of Soils and Sediments CEC, sample pH ( l : l ) bmequiv/100 g EPA-B2 EPA-4 EPA-5 EPA-6 EPA-8 EPA-ga EPA-14= EPA-15 EPA- 18 EPA-20a EPA-21 EPA-22 EPA-23 EPA-26 a
6.35 7 -79 1.44 7.83 8.32 8.34 4.54 7.79 1.76 5.50 7.60 1.55 6.70 7.75
Soils; all others are sediments,
3.72 23.72 19.00 33.01 3.72 12.40 18.86 11.30 15.43 8.50 8.33 8.53 31.15 20.86
organic carbon, %
sand, %
clay, %
silt, %
1.21 2.07 2.28 0.72 0.15 0.11 0.48 0.95 0.66 1.30 1.88 1.67 2.38 1.48
67.5 3.0 33.6 0.2 82.4 7.1 2.1 15.6 34.6 0.0 50.2 26.1 17.3 1.6
18.6 55.2 31.0 68.6 6.8 17.4 63.6 35.1 39.5 28.6 7.1 21.2 69.1 42.9
13.9 41.8 35.4 31.2 10.7 15.6 34.4 48.7 25.8 71.4 42.1 52.7 13.6 55.4
2.0 40.1 25.1 60.8 6.1 16.3 13.8 10.1 29.1 21.9 2.8 14.8 57.6 37.1
Equilibrium pH of 1:l (water/substrate) slurries.
In the present study, we have investigated the detailed sorptive behavior of three substituted PAH compounds with varying chemical and structural characteristics on a group of 14 sediments and soils. Linear partition coefficients (K,)and K, values for each compound on each sediment/soil are reported. The relative influence of chemical and physical properties of the compounds and substrates on the sorptive process is discussed. The reliability of the K,, and water-solubility parameters for predicting the sorption of substituted PAHs is evaluated. Water solubilities and octanol-water partition coefficients for the three compounds were determined experimentally. Experimental Section Collection and Characterizationof Samples. The soil and sediment samples used in this study were collected from sites along the Missouri, Ohio, Wabash, Illinois, and Mississippi Rivers (see Figure 1 in ref 13). Fourteen samples were selected for the sorption studies following the detailed characterization described below. Each soil/sediment sample was analyzed for a number of physical and chemical properties including reaction pH, cation-exchange capacity, percent total nitrogen, percent organic carbon, and texture and clay mineralogy using methods outlined in Means et al. (10). The complete characterization was reported in our earlier paper (13). Water Solubilities and Octanol-Water Partition Coefficientsof Selected Substituted PAHs. The water solubilities and octanol-water partition coefficients of 2-aminoanthracene (2-AA) (M, 193.25, mp 238-241 "C), 6-aminochrysene (6-AC) (M, 243.31, mp 209-212 "C), and anthracene-9-carboxylic acid (9-CA) (M, 222.24, decomposes 214 "C) were determined by using the procedures outlined by Means et al. (10). In each determination, the final data were corrected for trace impurities by using methods outlined by Wood and Means (17). Reagents and Chemicals. The three substituted PAHs studied were obtained in pure form from Aldrich Chemical Co. Each pure compound (99+%) was tritiated by using the BF3-3H,P04 procedure described by Hilton and OBrien (19). The individual radiolabeled compounds were purified by microdistillation followed by preparative thin-layer chromatography. In each case, the final materials were assayed for purity and specific activity by liquid scintillation counting and gas chromatography. The compounds 2-aminoanthracene (97.1% pure; specific activity, 24.9 mCi/mmol), 6-aminochrysene (99.4%, pure; specific activity, 55.2 mCi/mmol), and anthracene-9-carboxylicacid (97.4% pure; specific activity, 0.928 mCi/mmol) were further purified by preparative thin-layer chromatography. 94
montmorillonite clay, %
Envlron. Sci. Technol., Vol. 16, No. 2, 1982
Table 11. Water Solubilities and Octanol-Water Partition Coefficients for Three Substituted PAHs water solubility, compd KOwa 1.30 i: 0.159 1 3 400 t 930 2-aminoanthracene 0.155 i: 0.018 96 600 i: 4200 6-aminochrysene anthracene-985.0 t 1.9 1 3 0 0 i 180 carboxylic acid a
Octanol-water Dartition coefficient.
All materials used for these studies were 99+% pure. All reagents used in these experiments were reagent grade, and certain solvents (e.g., 1-octanol, methanol) were redistilled in glass. The water used in all experiments was generated by a Milli-Q purifying system (Millipore). Sorption Isotherms. Batch equilibrium sorption isotherms (see Figure 1) were determined for five concentrations of each compound on each soil/sediment sample by using the techniques outlined in Means et al. (13). A soil/sediment to water ratio of 4 g to 40 mL was employed. Kinetic studies on each compound/substrate combination indicated that equilibrium was achieved in 20 h or less. The initial and final solution concentrations of the compound were determined by liquid scintillation counting, and the amount sorbed was calculated by difference. A mass balance for the 3H label was performed for each set of isotherms to verify that there were no losses of labeled material during the isotherm experiments. Techniques for these mass-balance determinations are discussed by Means et al. (10) and Wood and Means (17). It should be noted that studies of this type require handling of relatively large quantities of radioactive substances and compounds which are known or suspected carcinogens. Investigators are cautioned that experiments of this type should be conducted only in approved laboratory facilities with stringent supervision by personnel who have been trained specifically in the safe handling of radioactive and toxic compounds. Results and Discussion The physicochemical characterization of the 11 sediments and 3 watershed soils selected for this study revealed that the samples possessed a broad range of values for all parameters tested (Table I) (13). Matrix correlation of the major substrate characteristics (Table I) associated with sorption indicated that the 14 substrates were random with respect to all characteristics except for the % clay/ CEC interaction. The water solubilities and octanol-water partition coefficients of the three substituted PAH compounds used
7-
120, 140[
loot
AC
1I
-3
20
1
-2
-1
0 log
Cw(ng/rnl)
Anthracene-9-Carboxylic Acid
c&g&9
I O
F 002
,
1
t
2
-
3
4
S ( p g / ml)
Figure 2. Relationship between sorption constant (K), and the water solubiilty (S) of 22 compounds (TET = tetracene, HCB = hexachlorobiphenyl, DBA = dibenzanthracene, 3MC = 3-methylcholanthrene, DBC = dibenzocarbazole, DMB = 7,12-dimethylbenzanthracene, 9MA = 9-methylanthracene, AN = anthracene, 6-AC = 6-amlnochrysene, PY = pyrene, PHE = phenanthrene, BQ = biquinoline, DBT = dlbenzothlophene, MC = methoxychlor, 9AA = 9amlnoanthracene, ACR = acridine, 2MN = 2-methylnaphthylene, NA = naphthylene, 9-CA = anthracene-9-carboxylic acid, 1NA = 1naphthol, BZ = benzene, and AC = acetophenone). 0 = data from Karickhoff et ai. (9). 0 = data from ref 12- 75 and 18. 4 = data from this investigation.
Cw(ng/rnl)
400 r
'
I
003
004
cw(ys/rnl)
Figure 1. Representative sorption isotherms for 2-aminoanthracene, 6-aminochrysene, and anthracene-9-carboxylic acid.
in these studies are presented in Table 11. The three compounds span a range of approximately 2 orders of magnitude for water solubility (0.16-85 ppm) and approximately 2 orders of magnitude for KO, (1.3 X 103-9.5 X lo4). The water solubilities and the octanol-water partition coefficients determined in this study for 2-AA, 6-AC, and 9-CA are the first experimental values to be reported. The data obtained from the batch equilibrium sorption experiments for the three substituted PAH compounds on the sediments and soils are presented in Table 111. The linear Freundlich equilibrium constants (K,) were obtained as discussed in Means et al. (13). The partition constants (K,) and corresponding coefficients of determination (r2) for all compound/substrate combinations are contained in Table 111. For 2-aminoanthracene (2-AA), the Kp values range from 79 to 875 on the 14 substrates. These Kp values were regressed against the values of each of the substrate properties reported in Tables I. The only significant relationship found was that between Kp and % organic carbon. The K , value thus derived was 28 129 (1.2 = 0.871). Individual K, values from 2-AA on each sediment/soil are presented in Table 111.
The sorption of 6-aminochrysene (6-AC) yielded Kp values ranging from 573 to 3973, indicating much stronger sorption than was observed for 2-AA. The Kp values were highly correlated with organic carbon content of the substrates. No other significant correlations were observed. The KO,values calculated for the individual substrates tended to converge on an average of 162900 when the K, value for soil EPA-9 was omitted. This value was dropped because it was more than two standard deviations above the mean. Regression of Kp vs. % organic carbon for all 14 samples yielded a KO,value of 143355 (r2 = 0.944). In previous work, we have demonstrated that the K, is a unique constant for a variety of nonpolar compounds which is dependent upon the physical properties of the compound being sorbed and on the organic carbon content of the sorbent but independent of the other sorbent properties (10, 13, 14). We have demonstrated that a significant relationship exists between K, and KO, (13-15, 18)for the sorption of 22 nonpolar compounds by various soils and sediments (eq 1). log KO,= log KO, - 0.317
(r2 = 0.980)
(1)
The calculated K, values for 2-AA and 6-AC obtained by using eq 1 are 6468 and 46556, respectively. These values are both significantly lower than the observed regressed KO,values, which were 28 129 and 143335, respectively. These data suggest that, although the sorption of these aromatic amines is highly correlated with the organic carbon content of the substrates, the strength of the sorption is greater than can be accounted for based on hydrophobic association of neutral aromatic nuclei to sediment organic matter as was observed for neutral PAHs (10, 13). This increase in sorption was also observed for benzidine (16) and for one heterocyclic amine compound, acridine, studied by our group on the same set of substrates and reported in another paper (Koc= 12910 observed vs. 2020 calculated) (14). Hydrophobic sorption appears to be the result of a weak solute/solvent interaction rather than a strong sorbate/sorbent interaction. Chiou and co-investigators suggested that the solubility of a hydrophobic organic molecule should therefore be a good estimator of the organic-water partitioning coefficients (21). By inference, we concluded that the log solubilities ( S ) should be a reliable means of estimating log K, (13,14). Regression of the water solubilities kg/mL) and K, values Environ. Sci. Technol., Vol. 16, No. 2, 1982
95
Table 111. Equilibrium Sorption Constants for Selected Substituted PAHs on 14 Soil/Sediment Samples 2-aminoanthracene soil/sed EPA-B2 EPA-4 EPA-5 EPA-6 EPA-8 EPA-Sa EPA-14a EPA-15 EPA-18 EPA-20a EPA-21 EPA-22 EPA-23 EPA-26 mean a
% OCb
1.21 2.07 2.28 0.72 0.15 0.11 0.48 0.95 0.66 1.30 1.88 1.67 2.38 1.43
Kp 321.6 329.2 304.1 259.5 79.0 103.7 145.1 391.9 283.0 458.7 531.9 502.1 875.2 688.7
Soils; all others are sediments.
r2
0.996 0.986 0.989 0.946 0.943 0.943 0.922 0,999 0.899 0.964 0.984 0.970 0.972 0.985
KO, 26580 15904 13 336 36039 52659 94 276 30225 41 248 42878 35287 28 292 30069 36772 46537 33 500 i: 1 0 800‘
% organic carbon.
6-aminochrysene
KP 1735.5 3115.7 3972.5 1078.7 573.3 686.4 924.3 1292.2 1424.5 871.9 2616.0 1459.0 3923.3 1688.8
r2 0.942 0.979 0.963 0.988 0.999 0.995 0.972 0.985 0.999 0.990 0.997 0.876 0.941 0.967
anthracene-9-carboxylic acid KO,
143427 150519 174232 149817 382 185 624022 192553 136025 215835 67070 139149 87 363 164844 114108 162 900 t 74 oooc
‘ Without EPA-9.
KP 5.27 5.49 7.96 5.47 1.84 2.82 10.03 2.66 1.78 13.27 6.45 5.59 9.88 7.50
r2 0.877 0.741 0.976 0.868 0.940 0.976 0.841 0.330 0.988 0.956 0.997 0,990 0.728
Koc 436 265 349 760 1227 2564 2090 280 270 1021 343 335 415 507 517 t 304d
Without EPA-9 and -14.
of 22 compounds yielded the following linear relationship (10, 12-14, 18) (Figure 2): log KO,= -0.686 log S (pg/mL)
+ 4.273 (r2 = 0.933) (2)
The calculated K, values using eq 2 for 2-AA and 6-AC are 15 662 and 67 384, respctively. Although these values are higher than those calculated from eq 1, they are still much lower than the observed K , values for these two compounds. This further supports the hypothesis that there is an enhancement of the sorption of these two amines above that expected based on hydrophobic bonding but which is nevertheless associated with the organic carbon content of the substrate. A close examination of the K , values for 2-AA in relationship to the % organic carbon revealed an inverse relationship. Those substrates with higher organic carbon contents tended to give lower K , values (closer to the values predicted from eq 2), while low organic carbon containing substrates generally gave higher KO,values. Substrates EPA-8 and -9, which had a very low % organic carbon (0.15% and 0.11%,respectively), gave anomalously high K , values of 52 659 and 94 276, respectively. From these data, it appears that the amount of sorption of 2-AA is dependent primarily upon the amount of organic carbon in the substrate, as well as upon the relative coverage of the inorganic particle surfaces by the existing organic matter. This hypothesis is further supported by the fact that, while the solubilities of anthracene (9) and 2-AA differ by over 2 orders of magnitude, the K , values are essentially the same (Figure 2). In a previous paper, we reported that the % organic carbon/ % montmorillonite clay ratio was critical in interpreting the sorption of 1-naphthol on these same substrates (15). When observed values of K, of 2-AA on each substrate were plotted against the % organic carbon/ % montmorillonite clay ratio of the substrates (Figure 3, upper half), it was noted that at ratios below 0.1 there is a very strong inverse relationship, suggesting that clay mineralogy may be important in the sorption of 2-AA. Similar trends were observed for the data for 6-AC (Figure 3, lower half) although they were less pronounced and the relative positions of various substrates differed for the two compounds (2-AA vs. 6-AC). This may be due to steric hinderance effects of the larger 6-AC molecule. The mechanisms for enhanced sorption of 2-AA and 6-AC must involve one or more specific interactions of the 96
Environ. Sci. Technol., Voi. 16, No. 2, 1982
2-AA
Kdregressed)
.-
r
.l
c
.2
.3
.4
.5
.6
6-AC I I I I I
40OCi
II “a
Y
I I
- II II
-
-*II* _ _ _ - - K,,(calculated) - _ _ _ - _ - - I
I
*
.7
Parris has shown that amines may undergo both reversible and irreversible reactions with humates to yield a variety of products (22). Benzidine and other aromatic amines are known to react with components of the clay minerals (16,23,24). Reactions of these types may account in part for our observations. The third compound, 9-CA, exhibited linear isotherms with Kp values which ranged from 1.78 to 13.27 on the 14 substrates (Table 111). The Kp values were highly correlated with % organic carbon of the sediments. All three soils tended to give high Kpvalues. No other significant correlations were observed. The K , values calculated for the sediments tended to converge on an average value of 517 when the anomalously high values obtained on soils EPA-9 and -14 were dropped because they fell more than two standard deviations above the mean. Regression through the origin of Kp vs. % organic carbon for all 11 sediments yielded a K , value of 422 (r2 = 0.751). The values of K , predicted for 9-CA using eq 1 and 2 were 626 and 889, respectively. The close agreement of the observed value of K , with the calculated values suggests that the sorption of 9-CA to a variety of substrates is controlled by the neutral aromatic portion of the molecule rather than the polar carboxyl anionic moiety. This is consistent with the fact that the surface charge on soils and sediments tends to be negative and therefore the negatively charged carboxyl group on the 9-CA molecule would be expected to be oriented away from the particle surface. Of the 14 substrates tested, 9-CA tended to be sorbed most strongly to the 3 soils (EPA-9, -14, and -20) and to 1sediment (EPA-8) which had a very low organic carbon content (0.15%). No consistent pattern of substrate properties was observed which could explain the enhanced sorption of 9-CA on these four substrates. However, various combinations of low % organic carbon, low CEC, low pH, and high silt/clay content may contribute to the high observed values of K , in these cases. In the case of EPA-14 and -20, the low pH of the soils may result in partial protonation of the 9-CA present, thus reducing the effective solubility and enhancing sorption. The contrast of the physical properties and sorptive behavior of 2-AA and 9-CA is significant. Although they are similar in molecular size and volume and each exhibits a single but opposite charge in solution at circumneutral pHs, their solubilities and octanol-water partition coefficients differ by approximately 1order of magnitude and their sorptive behavior expressed as K , differs by approximately 2 orders of magnitude. In a previous paper, the effective chain length of a series of neutral PAHs was found to be important in determining the relative strength of sorption to a sorbent (13). In the case of these substituted PAH compounds, it is clear that the nature and charge of the polar functional groups contained in molecules of similar size can profoundly influence the sorptive properties of the molecules. In spite of the large differences observed, the predictive value of eq 1 and 2 (particularly of eq 2) remained reasonably high for substrates with high % organic carbon but was reduced for substrates with low organic carbon contents.
Summary and Conclusions The sorption of 2-aminoanthracene (2-AA), 6-aminochrysene (6-AC),and anthracene-9-carboxylicacid (9-CA) has been studied on 14 sediment and soil substrates. In all cases, the equilibrium isotherms were linear and the resulting partition constants (K,) for each compound were found to be significantly correlated with the organic carbon
content of the 14 substrates. The Kpvalues appeared to be independent of any other single substrate characteristics. The enhanced sorption of 2-AA and 6-AC on certain substrates, however, was well correlated with the % organic carbon/ % montmorillonite ratio, suggesting that these compounds may be interacting with the inorganic matrix of the sorbent when the ratio is low (CO.1). Water solubilities ( S ) and octanol-water partition coefficients (K,) were determined experimentally for each compound. Predicted sorption constants (K,) were calculated from S and K,, by using equations developed and reported in a previous series of papers (IO, 11,13,14). In general, the predicted KO,values were within a factor of 2-3 of the observed values for the three compounds on most of the 14 substrates. For these three compounds, the solubility of the compound was a better predictor of K,. The reliability of these predictive equations appears to be impaired for substrates with very low carbon contents. The fact that both predictive equations consistently underestimated K , values for the aromatic amines, 2-AA and 6-AC, suggests that the sorptive mechanisms active for this class of compounds need further study and that caution should be observed in applying these predictive relationships to the sorption of other members of this class of toxic chemicals.
Literature Cited (1) Guerin, M. R.; Ho, C.-H.; Rau, T. K.; Clark, B. R.; Epler, J. L. Environ. Res. 1980, 23, 42. (2) Scott, T. S. “Carcinogenic and Chronic Toxic Hazards of Aromatic Amines”; Elsevier: Amsterdam, 1972. (3) TRW Systems and Energy, Inc. ”Carcinogens Relating to Coal Conversion Processes”; U.S.Energy Research and Development Administration: Oak Ridge, TN, 1976. (4) Sawicki, E.; Johnson, H. Mikrochim. Acta 1964,435-50. (5) Epler, J. L. In “Chemical Mutagens”; de Serres, F. J., Hollaender, A., Eds.; Plenum Press: New York, 1980; Vol. 6, p 239. (6) Pelroy, R. A.; Peterson, M. R. Enuiron. Sci. Res. 1979,15, 463-75. (7) Sharkey, A. G.; Schultz, J. L.; White, C.; Lett, R. Washington, DC 1976, U.S. Environmental Protection Agency Report No. EPA-60012-76-075. (8) Kipling, J. J. “Adsorption from Solution of Nonelectrolytes”; Academic Press: New York, 1965; Chapters 1 and 7. (9) Karickhoff, S. W.; Brown, D. S.; Scott, T. A. Water Res. 1979, 13, 241. (10) Means, J. C.; Hassett, J. J.; Wood, S. G.; Banwart, W. L. In “Polynuclear Aromatic Hydrocarbons”; Jones, P. W., Leber, P., Eds.; Ann Arbor Science: Ann Arbor, MI, 1979; p 327. (11) Means, J. C.; Hassett, J. J.; Wood, S. G.; Banwart, W. L.; Ali, S.; Khan, A. In “Polynuclear Aromatic Hydrocarbons: Chemistry and Biological Effects”; Barseth, A. J., Dennis, A. J., Eds.; Battelle Press: Columbus OH, 1980; p 395. (12) Hassett, J. J.; Means, J. C.; Banwart, W. L.; Wood, S. G.; Ali, S.; Khan, A. J. Enuiron. Qual. 1980, 9, 184. (13) Means, J. C.; Wood, S. G.; Hassett, J. J.; Banwart, W. L. Enuiron. Sci. Technol. 1980, 14, 1524. (14) Banwart, W. L.; Hassett, J. J.; Wood, S. G.; Means, J. C. Soil Sci., in press. (15) Hassett, J. J.; Means, J. C.; Banwart, W. L.; Wood, S. G. Soil Sci. SOC.Am. J. 1981, 45, 38. (16) Zierath, D.; Hassett, J. J.; Banwart, W. L.; Wood, S. G.; Means, J. C. Soil Sci. 1980, 129, 277. (17) Wood, S. G.; Means, J. C. submitted for publication in Anal. Chem. (18) Khan, A.; Hassett, J. J.;Banwart, W. L.; Means, J. C.; Wood, S. G. Soil Sci. 1979, 128, 297. (19) Hilton, B. D.; O’Brien, R. D. J. Agric. Food Chem. 1964, 12, 236. Environ. Sci. Technol., Vol. 16, No. 2, 1982
97
Environ. Sci. Technol. 1982, 16, 98-102
Bailey, G. W.; White, J. L. J. Agric. Food Chem. 1964,12,
7, 389.
234.
Chiou, C. T.; Freed, V. H.; Schmedding, D. W.; Kohnert, R. L. Environ. Sci. Technol. 1977, 11, 475. Parris, G. E. Environ. Sci. Technol. 1980, 14, 1099. Theng, B. K. G. Clay Miner. 1971,19, 383. Soloman, D. H.; Loft, B. C.; Swift, J. D. Clay Miner. 1968,
Received for review April 27,1981. Revised manuscript received August 26,1981. Accepted October 12, 1981. Support for this work was obtained from the U.S. Environmental Protection Agency Contract No. 68-03-2555.
Optimal Emission Control Strategies for Photochemical Smog Vlcente Costanzat and John H. Selnfeld"
Department of Chemical Engineering, California Institute of Technology, Pasadena, California 91 125
A study of certain aspects of the selection of reactive hydrocarbon and nitrogen oxide emission reductions for photochemical oxidant abatement is carried out. Optimal emission control paths are defined as those minimizing a total cost function consisting of control cost and ozone dosage contributions. Los Angeles County ozone air quality and control cost data are used to formulate an optimal emission reduction path. The analysis is presented primarily to provide insight into the factors involved in designing oxidant control strategies.
Introduction The determination of hydrocarbon and nitrogen oxide emission reduction levels needed to meet National Ambient Air Quality Standards (NAAQS) for ozone and nitrogen dioxide in major urban areas is a central problem in implementing the Federal Clean Air Act (1-4). The problem involves two major aspects, determining the relationship between hydrocarbon and nitrogen oxide (NO,) emissions and ozone (0,)and nitrogen dioxide (NO,) air quality and identifying available emission control measures. The approach that in many cases is being employed for relating emissions to air quality for photochemical smog in control strategy determinations is the US Environmental Protection Agency's EKMA (Empirical Kinetic Modeling Approach) method that is based on the relationship between peak ozone concentrations achieved during a multihour period and the initial concentrations of reactive hydrocarbons (RHCs) and NO, (5-8). Figure 1 shows ozone isopleths generated by the EKMA method, the use of which for determining emission reduction levels is described in the caption. A similar ozone-precursor relationship determined by Trijonis ( I ) from Los Angeles monitoring data is shown in Figure 2. The basic emission control problem is to select a path from a starting point on Figure 1 (or Figure 2) to the region representing the desired air quality goal, e.g., in Figure 1, the 0.12 ppm isopleth, while at the same time not violating the air quality standard for NOz. The purpose of this communication is to demonstrate some properties of optimal paths in the RHC-NO, plane. To do so requires that optimality be carefully defined, and we will propose a measure of optimality. I t is important to note that the analysis that we will present is not intended as a ready framework for the determination of optimal control strategies for photochemical smog. Rather, we hope to provide some insight into the factors entering into such a complex decision and, by demonstrating an example in detail, to show the possibility of applying classical techniques to this problem. This work is, of course, not the first on deter+EnvironmentalQuality Laboratory. 98
Environ. Sci. Technol., Vol. 18, No. 2, 1982
mining optimal air pollution control strategies. For additional material we refer the reader to some available literature on this subject (1-4,9-13). In particular, Bilger and Post (4,14,15) have also considered the use of isopleth diagrams relating ozone formation to precursor concentrations for determining control strategies.
Formulation of the Problem For the purpose of our discussion let us focus on Figure 2, the ozone-precursor relation expressed in terms of the number of days per year of violation of a given O3level, and RHC and NO, emission levels. Although our analysis will apply without conceptual change to Figure 1, the standard EKMA representation, we choose to use the data of Figure 2 because Trijonis (1)developed detailed control costs associated with the emission levels on the ordinate and the abscissa. The availability of control cost data is essential to our analysis. The isopleths shown in Figure 2 can be assumed to be obtained from a surface z = f(x,y) where z = number of days per year that hourly-average ozone concentrations exceeded 0.10 ppm, x = Los Angeles County RHC emissions (ton/day), and y = Los Aggeles County NO, emissions (ton/day). The function f can be approximated in principle by fitting a polynomial f(x,y) to the data used to construct the surface on the isopleths. The emission control problem is then as follows: If t represents time (in years), it is desired to determine a path a(t),@(t) that proceeds from (ao,Po), the emission levels at the starting point, Le., current air quality, to desired air quality. We use 4 )and @(t)to denote the emission reduction path in the x-y plane; thus, a(t)and @(t)refer to the RHC and NO, emissions, respectively. In Figure 1 desired air quality is represented by any point on the 0.12 ppm isopleth. One would also ordinarily require that the desired air qualtiy also satisfy the constraint that the NO, concentration not exceed the value corresponding to the NAAQS for NOz. That aspect will not be explicitly considered here, although it can in principle be treated through the introduction of a constraint on the terminal NO, concentration. In Figure 2 the goal is taken to be the line corresponding to 50 day/yr violations. We are interested in determining an optimal path; therefore, we must define the basis for judging optimality. In general, we define a cost or "performance index",J that consists of two components, J1and J2,J1representing the control costs and J2representing the costs associated with the air quality level. The optimal path will then be that path minimizing J. Let us consider the development of a form of the control cost portion, J1,of the overall performance index J. The control costs determined by Trijonis (1) are shown in Figure 3. The form of the control cost curves in Figure
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0 1982 American Chemical Society
