(15) Levy, G. C., Nelson, G. L., "Carbon-13 Nuclear Magnetic Resonance for Organic Chemists", Wiley-Interscience,New York, N.Y., 1972.
(16) Johnson, L. F., Jankowski, W. C., "Carbon-13 NMR Spectra", Wiley, New York, N.Y., 1972. (17) Horsley, W., Sternlicht, H., Cohen, J. S.,J . A m , Chem. SOC.,92, 680 (1970). (18) Kemp, W., "Organic Spectroscopy", Wiley, New Yo&, N.Y., 1975. (19) Sadtler Res. Labs., Sadtler Stand. Spectrum No. 6245 (D-glucose), 1969.
Received for reuieu; August 29, 1977. Accepted February 21, 1978. f k ~ e a r c hsupported in part by the Kearney Foundation of Soil Science and in part with federal funds from the Environmental Protection Agency under grant number R804516010. The contents of this paper do not necessarily reflect the views and policies of EPA, nor does mention of trade names or commercial products constitute endorsement or recommendation for use. T h e Bruker W H 90-D-18 NMR spectrometer was supported by Bio-medical Sciences Grant No. 5 SO5 RR07010-09 from the National Institute of Health and by NSF @ant " 0 . MPS75-06138 t o the Department of Chemistry, Uniuersity of California, Riuerside.
Comparison of Associations of Different Hydrocarbons with Clay Particles in Simulated Seawater Philip A. Meyers" and Terrence G. Oas' Department of Atmospheric and Oceanic Science, The University of Michigan, Ann Arbor, Mich. 48109
Some aspects of the association of hydrocarbons and smectite clay in simulated seawater were investigated using NaCl solutions in laboratory experiments. Both n-eicosane and n-eicosene displayed identical association behaviors with this clay. Association increased linearly with increasing hydrocarbon concentration in water. The amount of n-alkane associated with smectite increased with carbon chain length from C17 to C28, reaching a maximum of 70% removal from water. This may be due to decreased accommodation in water as hydrocarbon chain length becomes larger. The level of association of aromatic hydrocarbons was generally low, and isoalkanes were more effectively removed from water than n-alkanes of the same number of carbons. A process contributing to the removal of petroleum hydrocarbons from natural waters is the association of these compounds with settling particles and eventual burial in sediments. Because crude petroleum and refined petroleum fractions consist of mixtures of hydrocarbons, selective interactions during sorption can lead to different amounts of removal of various components. The result of such selective association would be that certain petroleum fractions would preferentially become incorporated into sediments while others would remain in the overlying water. Furthermore, the reverse of this process, selective desorption, would be likely to enhance any fractionation that occurs during sorption because the more readily desorbed components will also be the more soluble and therefore less easily sorbed ones. This has been indicated both by laboratory experiments ( I ) and by field data ( 2 ) . In view of the probable importance of hydrocarbon-settling particle interactions in transferring petroleum components from water to sediments, we initiated a number of laboratory experiments designed to compare relative amounts of association of different hydrocarbons by a common clay mineral. The results of these studies are reported here and are relevant to predicting fractionation of petroleum components in marine waters. Experimental The laboratory experiments in this study were conducted using a procedure adapted from one developed by Meyers and Present address, Department of Oceanography, Florida State University, Tallahassee, Fla. 32306. 934
Environmental Science & Technology
Quinn ( 3 , 4 )for fatty acid-mineral association experiments. Seawater was simulated with a distilled water solution containing 34 g NaCl per kg of solution. The pH of this solution was not adjusted. The resulting solution pH of 6-7 may have enhanced association by about 10%relative to typical seawater of p H 8, assuming hydrocarbon association decreases with increasing p H in a way like that of fatty acids ( 4 ) .Hydrocarbons dissolved in 50 pL of benzene or ethyl acetate were introduced into 950-mL volumes of NaCl solution in separatory funnels and dispersed by vigorous shaking for at least 30 s. The amount of solvent used in each experiment was kept constant to minimize any effect upon hydrocarbon adsorption. Fifty mg of sodium smectite from the same lot utilized by Meyers and Quinn ( I , 3 , 4 ) for previous experiments were dispersed in 10 mL distilled water to achieve maximum swelling and then added to the hydrocarbon-NaC1 solution mixtures. After making the volume up to 1 L, the mixtures were again vigorously shaken for a t least 30 s. A minimum settling period of 2 days a t ambient temperatures (around 20 "C) was used in all experiments and was sufficient for nearly all the clay to flocculate and settle. After this period the settled clay was drawn out the bottom of the separatory funnel and dried. Associated hydrocarbons were removed from the dried smectite by covering with 10 mL benzene/methanol, 1/1,held a t 45 "C 18 h followed by two 10-mL petroleum ether extractions. This extraction scheme gave quantitative recovery of n-eicosane. An internal standard of 100 pg n-tetracosane was added to the extraction mixture, and the solvent evaporated to dryness a t 25 "C. The residue was immediately redissolved in petroleum ether in preparation for analysis by gas-liquid chromatography. The extracted clay was collected on a preweighed Millipore HA filter, dried a t 60 "C for one week, and weighed. Analysis of the extracted hydrocarbons was done on a Hewlett-Packard 5711A flame ionization gas chromatograph equipped with 4 m X 2.1 mm i.d. columns packed with 3% SP-2100 on 80-100 Supelcoport (Supelco, Bellfonte, Pa.) operated isothermally in the range 150-300 "C depending upon the hydrocarbon being investigated. Quantitation was achieved using a Hewlett-Packard 3380 A electronic integrator. Controls were run on all procedures, and the reported results are corrected values. Eight replicate experiments using n-eicosane a t a concentration of 126 pg/L yielded a coefficient of variation of f8.0%. This value is assumed to be valid for all other hydrocarbons used in this study.
0013-936X/78/0912-0934$0 t.OO/O @ 1978 American Chemical
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Results Substantial amounts of some of the hydrocarbons added to the simulated seawater were removed by the settling clay in these experiments. Although the actual mechanism of removal is not known, it is probable that adsorption onto clay particle surfaces both before and during clay flocculation is important. Heats of adsorption of n-eicosane and of anthracene onto sodium smectite have been reported to be -2.1 and -9.1 kcal/mol, respectively ( I ) . These values are typical of physical adsorption. For this reason, we shall refer to the association processes described in this report as adsorption, even though other types of physical association may be contributing to the overall uptake of hydrocarbons from water by clay particles. Several series of experiments were performed using different hydrocarbons a t a concentration of 100 pg/L. The results of these experiments are listed in Table I. Both adsorption data and the percentage of the original amounts of hydrocarbons found on the settled clay are given. The relatively low adsorption of n-eicosane as compared to the value of 94% removal reported by Meyers and Quinn ( 1 ) is most likely due to differences in experimental procedure even though the same clay was used. It is evident that adsorption increases as molecular weight increases in n-alkanes, isoalkanes, and monoalkanes, but no similar direct relationship exists for polyunsaturated or aromatic hydrocarbons. It also appears that while adsorptions of n-alkanes and n-alkenes are similar a t a given carbon chain length, adsorption is greater for isoalkanes. Further evidence of the similarities in adsorptions of n alkanes and n-alkenes is given in Figure 1 in which are shown the results of experiments using concentrations ranging from 25 to 300 pg/L for n-eicosane and n-eicosene. The slopes of the plots of adsorption vs. concentration are identical up to a concentration of 125 pg/L. Above this level, an apparent discontinuity exists. For n-eicosene the rate of increase in adsorption is drastically different after experiencing a sudden increase. While only one point above a concentration of 100 pg/L exists for n-eicosane, it is in general agreement with the pattern found for n-eicosene. In addition to similarities in slopes and locations of discontinuities, n-eicosane and n eicosene both have essentially the same adsorptions over the concentration range of 25-100 Kg hydrocarbon per liter of simulated seawater. The close similarity in adsorption behavior exhibited by n-eicosane and n-eicosene is further demonstrated by the Freundlich adsorption isotherm shown in Figure 2. This plot is derived from the adsorption data displayed in Figure 1 and from calculated equilibrium concentrations. The latter are based upon the assumption that any hydrocarbon not adsorbed by 50 mg of clay remained in the simulated seawater. It is likely that a certain amount of hydrocarbon became attached to the inside of the separatory funnel, and some may have accumulated as a surface film. Although factors such as these may jeopardize the assumed equilibrium conditions, experimental conditions were made uniform for all experiments, with the exception of initial hydrocarbon concentration. Therefore, any error introduced by the assumption made in calculating these equilibrium concentrations should to a large degree cancel out when comparing data obtained within this study. In Figure 2,13 data pairs are plotted; 12 of these have been used to calculate a linear regression represented by the line drawn through them. The coefficient of linear correlation of this regression is 0.96. The 13th data pair is shown as a triangle. This point and two square points representing high levels of n-eicosene are the same three adsorption points which appear to form a separate and different line in Figure 1.In the
isotherm plot, these points may again form a nearly horizontal line and may possibly be distinct from the other data pairs. The smectite adsorptions of nine saturated and monounsaturated straight-chain hydrocarbons are compared in Figure 3. These data were obtained from experiments in which hydrocarbon concentrations were 100 pg/L. Although there is some scatter in the data, the coefficient of linear correlation of the regressed line is 0.80. This is significant at the 99% level according to Student's T-Test. This figure illustrates that alkanes and monalkenes behave similarly in degree of uptake by smectite clay. I t also shows that the amount of these hydrocarbons removed from water increases as carbon chain length increases. Discussion The lack of agreement between the n-eicosane adsorptions found in this study and those reported earlier ( I ) points out
Table 1. Adsorptions of Different Hydrocarbons from Saline Solutions at 25 'C. Experimental Conditions: 100 pg Hydrocarbon per Liter of Water; 50 mg Smectite Hydrocarbon
n-Alkanes n-Octadecane n-Eicosane n-Octacosane Unsaturates n-Eicosene n-Docosene Squalene lsoalkanes 2-Methyl hexadecane 2-Methyl heneicosane Aromatics Durene Acenaphthene Pyrene Anthracene
%
Adsorption (gg/mg)
Adsorbed
0.74 0.72 1.41
38 36 70
0.83 1.04 1.14
41 52 57
C22H46
0.95 1.87
48 92
C10H14 C12H10
0
0 0
C16tijo C14H10
0.39 0.90
19 46
Formula
Cl8H38 C20H42 C28H58
C20H40 C22H44 C30H50 C17H36
1.5 h
E,
1.0
--E
-k4 0.5 2
0
E
2.0
0 + a
EICOSANE
0 Q
1.5
0 li: 1.0
a v)
0.E
0
0
100
INITIAL
200 300 CONCENTRATION (ygrn/l)
Figure 1. Adsorption of n-eicosane and n-eicosene by smectite clay from NaCl solutions having different initial concentrations of hydrocarbon Volume 12, Number 8, August 1978
935
that comparison of separate investigations must be done with caution. Adsorption experiments are sensitive to small differences in such variables as temperature, clay preparation, equilibration times, and recovery procedures. While it is difficult to compare absolute adsorptions obtained from unrelated studies, it is probably valid to consider relative amounts of adsorption as indicative of actual processes occurring outside of the laboratory. Earlier work has suggested that solubility of hydrocarbons in water may be an important factor in determining the amount of each compound removed by mineral particles ( I ) . Because. solubility decreases as molecular weight increases within a homologous series of compounds, larger amounts of higher molecular weight hydrocarbons would be expected to become adsorbed onto settling particles than of lower molecular weight compounds. The adsorption data from this study for straight-chain, monounsaturated, and branched hydrocarbons listed in Table I follow this pattern. Also, the
0.1 10 EQUILIBRIUM
100 CONCENTRATION
1000
(ygm/l)
Figure 2. Freundlich adsorption isotherm of n-eicosane and n-eicosene Fifty mg smectite clay and 1-L NaCl solutions used
h
E 1.5 0 +-
t
1
!=
b a
-
1.0
Z
Q I-
h
iT
e ALKANES
n
rn ALKENES
2 0.5 Q
CARBON
CHAIN
LENGTH
Figure 3. Adsorption of n-alkanes and n-alkenes of differing carbon chain lengths Fifty mg smectite clay and initial concentrations of 100 pg hydrocarbon in I-L NaCl solution used
936
Environmental Science & Technology
increase in adsorption with increasing chain length shown in Figure 3 agrees with this concept. Thus, there appears to be some sort of inverse relationship between water solubility and adsorption of hydrocarbons. However, the concentrations of hydrocarbons used in these experiments were between 25 and 300 pg/L, well above the solubilities of n-alkanes in distilled water measured by McAuliffe (5), Sutton and Calder ( 6 ) ,and Button ( 7 ) .Furthermore, hydrocarbon solubility is depressed by the presence of dissolved inorganic salts ( 6 , 8 ) .Therefore, it is not possible for the saturated and monounsaturated hydrocarbon levels used in this study to represent true solution. Instead, these levels more realistically represent some type of micellar accommodation of hydrocarbons in the water. Peake and Hodgson (9) describe the limits of such accommodation of n-alkanes in distilled water to be above the concentrations used in all the experiments employing n-alkanes, n-alkenes, and isoalkanes, with the single exception of n-octacosane. Thus, these concentrations are between true solution and the onset of droplet formation and may reasonably approximate levels of hydrocarbons that can occur in natural waters. On this basis, the relationship shown in Figure 3 indicates that adsorption of hydrocarbons is inversely related to the accommodation concentration and not so much to the true solution level. In natural seawater, accommodation of saturated hydrocarbons, but not aromatic hydrocarbons, is enhanced by the presence of surface-active dissolved organic matter (8). Therefore, it is possible that selectivity of adsorption of a mixture of hydrocarbons such as petroleum would be affected by dissolved organic matter normally present in seawater. Most mineral particles in the marine environment have organic matter associated with them. The uptake of fatty acids ( 4 ) and hydrocarbons ( I ) by marine sediments is depressed by the presence of this natural organic matter. Consequently, a further modification of the adsorption behavior described in this study is possible under natural conditions. Comparison of adsorption behaviors of closelv related hvdrocarbons reveals some interesting observations. As shown in Figures 1 and 2 , n-eicosane and n-eicosene behave identically with respect to adsorption by smectite clay. Evidently, the presence of one single bond between the first and second carbon atoms has no effect on uptake by mineral particles. This implies there is no difference in accommodation levels of these two hydrocarbons. In contrast, a substantial difference is found between the adsorption of n-docosene and 2methyl heneicosane (Table I), both C22 hydrocarbons. Apparently, methyl branching either decreases the accommodation levels of hydrocarbons in water or in some way contributes toward increased adsorption, thus enhancing association with smectite. In view of this last comparison, the adsorption behavior of the triterpene hydrocarbon squalene is especially interest(2,6,10,15,19,23-hexamethyling. This compound 2,6,10,14,18,22-tetracosahexaene) contains both methyl branching and unsaturation and has a principal carbon chain 24 atoms long. The amount of adsorption found for squalene using an initial concentration of 100 pg/L is 1.14 pg/mg clay n-alkane (Table I). This adsorption corresponds to that of according to the regression plotted in Figure 3, suggesting that the increase in adsorption expected from the presence of methyl branching is counterbalanced by multiple unsaturation in this hydrocarbon. Thus, the adsorption behavior of squalene is that expected from an n-alkane of the same overall C24 carbon chain length. This implies that multiple double bonds function to reduce adsorption of hydrocarbons, conceivably by increasing accommodation levels in seawater. However, the nonlinear spatial configuration of squalene ( I O ) may also be a factor in the adsorption behavior observed in this study.
The discontinuities in the plots of n-eicosane and n-eicosene adsorptions vs. concentration in Figure 1 may indicate that a major change occurs in the adsorption process above a concentration of 125 pg/L. Up to this point the percent removal from solution of both hydrocarbons is invariant at 38% of the initial amounts added. Above 125 p g b , percent removal suddenly increases to 71% at 150 pg/L and then gradually decreases at higher concentrations. This may signal a change from a monolayer adsorption process to a multilayer one. However, a more likely possibility is that the accommodation capacity of the simulated seawater solution is exceeded above 125 pglL. Peake and Hodgson (9) measured the accommodation of n-eicosane to be 350 yg/L at 20 “C in distilled water. Sutton and Calder (6) found the solubility of n-eicosane in seawater to be about 40% of the value in distilled water at the same temperature. Therefore, if the same reduction applies to accommodation, the maximum accommodation capacity of our solutions may have been between 125-150 pg of neicosane or n-eicosene per liter. Above this level, hydrocarbon particles may have adhered to the container walls, collected at the water surface, or become attached to smectite particles in such a way to produce the discontinuous results observed in Figures 1 and 2. Adsorptions of aromatic hydrocarbons given in Table I are below detection or lower than those of the other hydrocarbons listed. Only anthracene has substantial adsorption. These observations agree with those of Meyers and Quinn ( I ) and probably reflect the higher solution and accommodation levels of aromatic hydrocarbons suggested by Boylan and Tripp ( I 1 ). Because it has been shown that aromatic hydrocarbons form stable complexes with smectite clay in nonaqueous systems ( I 2 ) ,these results indicate the importance of solvation effects of water in adsorption of dissolved or micellar materials. Evidently, the combined polarity of water molecules is greater than the polarity of the clay surface; consequently, complex formation is not readily accomplished. Instead solvation of the aromatic hydrocarbons results in substantially higher solubilities of these compounds (13) than found for saturated hydrocarbons (5-8). These experiments provide information that allows speculation about the possible interaction of petroleum hydro-
carbons with mineral particles in natural waters. Assuming that solubility and accommodation are related, it is likely that less soluble components of petroleum will be preferentially taken up by settling particles and carried to the underlying sediments. More soluble components will remain in the water column. Furthermore, additional fractionation could occur through removal of slightly soluble hydrocarbons from resuspended sedimented material as shown by Meyers and Quinn ( I ) . Therefore, while association onto settling particles and incorporation into bottom sediments may effectively cleanse water of many petroleum components, processes such as resuspension of settled particles could lead to release of hydrocarbons back into the water and thus be a source of chronic pollution long after the original source of petroleum has been removed. Acknowledgment We thank J. G. Quinn, J. K. Rosenfeld, and C. Sutton for reviewing this manuscript, and J. G. Oas for technical assistance. Literature Cited (1) Meyers, P. A., Quinn, J. G., Nature, 244,23-4 (1973). (2) Blumer, M., Sass, J., Science, 176,1120-2 (1972). ( 3 ) Meyers, P. A,, Quinn, J. G., Geochim. Cosmochim. Acta, 35, 628-32 (1971). (4) Meyers, P. A., Quinn, J. G., ibid., 37,1745-59 (1973). (5) McAuliffe, C. D., Science, 163,478-9 (1969). ( 6 ) Sutton, C., Calder, J. A., Enuiron. Sei. Technol., 8 , 654-7 (1974). ( 7 ) Button, D. K., Geochim. Cosmochim. Acta, 40,435-40 (1976). ( 8 ) Boehm, P. D., Quinn, J. G., ibid., 37, 2459-77 (1973). (9) Peake, E., Hodgson, G. W., J . Am. Oil Chem. Soc., 43,215-22 (1966). (10) Fieser, L. F., Fieser, M., “Advanced Organic Chemistry”, p 1012, Reinhold, New York, N.Y., 1961. (11) Boylan, D. B., Tripp, B. W., Nature, 230,44-7 (1971). (12) Doner. H. E.. Mortland. M. M., Science, 166.1406-07 (1969). (13) Eganhouse, R. P., Calder, J. A,, Geochim. Cosmochim. Acta, 40, 555-61 (1976).
Receiued for review Nouember 21,1977. Accepted February21,1978. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this work.
Optimum Control Strategy for Photochemical Oxidants Robert W. Bilger Department of Mechanical Engineering, University of Sydney, Sydney, N.S.W. 2006, Australia
The use of isopleth diagrams relating ozone formation to precursor concentrations for determining control strategy is discussed. Apical lines that give a worst NMHC-to-NO, ratio dividing the diagram into NMHC and NO, control regions are currently not soundly based and are not applicable to strategies where substantial control of oxidant is required. For substantial control, hydrocarbon control is the most effective over most of the area of interest. Precursor background levels and consideration of costs and administrative feasibility are included.
In recent years there has been a substantial international controversy about the best route to the control of photochemical oxidants. Photochemical oxidants are secondary pollutants formed from primary emissions of hydrocarbons and oxides of ni0013-936X/78/0912-0937$01.00/0 @ 1978 American
Chemical Society
trogen under the action of strong sunlight, and their control involves the choice of whether to control hydrocarbon emissions, nitrogen oxide emissions, or both. In 1971 the USA chose the hydrocarbon route with the promulgation of the Appendix J method ( I ) . In Japan a very stringent standard for NO2 (0.02 ppm for a 24-h average) was introduced in 1973 with the implication that this would also control photochemical oxidants ( 2 ) .In Australia initial control policy has been to reduce hydrocarbons while imposing sufficient control on oxides of nitrogen so that atmospheric concentrations will not increase ( 2 ) .In Europe Guicherit et al. (3, 4 ) had inferred from the available data that the NO, route is the best. This has been taken up by Daly ( 5 )in questioning Australian strategies. In 1976 the USA (6) adopted an alternative approach using the isopleth diagram relating ozone formed to precursor concentrations, thus allowing NO, control where appropriate and allowing the effect of NO, control to be taken into account where the determination of the degree Volume 12, Number 8, August 1978
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