Observation of abnormal solubility behavior of aromatic hydrocarbons in seawater

Marine Chemistry, 17 (1985) 277--284

277

Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands O B S E R V A T I O N OF A B N O R M A L S O L U B I L I T Y BE H A V IO R OF AROMATIC HYDROCARBONS IN SEAWATER BRIAN G. WHITEHOUSE Department of Oceanography, Dalhousie University, Halifax, Nova Scotia B3H 4J1 (Canada)

(Received June 25, 1984; revision accepted July 8, 1985) ABSTRACT Whitehouse, B.G., 1985. Observation of abnormal solubility behavior of aromatic hydrocarbons in seawater. Mar. Chem., 17: 277--284. The Setschenow and McDevit--Long equations are applied to aromatic hydrocarbons in seawater by using solute surface area and recently available solubility data to evaluate the Setschenow constant. It is demonstrated that this approach avoids a previously encountered problem with the McDevit--Long equation while also pointing out fundamental theoretical discrepancies. Compounds that do not fit the presented semi-empirical relationship are o f interest as t h e y m a y exhibit abnormal partitioning behavior in seawater. Using this approach it is suggested that 1,2-benzanthracene and benzo(a)pyrene exhibit abnormal solution behavior in seawater. INTRODUCTION Aqueous solubility is one of several factors that may influence the cycling o f aromatic h y d r o c a r b o n s (AH) in an aquatic environment. The d e t e rmin atio n o f the aqueous solubility of an aromatic h y d r o c a r b o n is n o t a simple task. This analytical obstacle is reflected by the paucity of data available, and has led to t he d e v e l o p m e n t o f various m e t h o d s for estimating the solubility o f h y d r o c a r b o n s in distilled water. Unfortunately, attempting to predict AH partitioning in t he marine envi ronm ent using distilled water solubility can result in significant error (Whitehouse, 1984). A logical approach to this problem is to estimate seawater solubility from pure c o m p o n e n t data and the empirical solubility data for AH in pure water. This is by n o means a new approach. It u n d o u b t e d l y began with Setschenow (1889), wh o studied the solubility of CO 2 (g) in various single salt solutions. His entirely empirical relationship of log (So~S) = k C s

(1)

has since been successfully applied to a variety of So = the distilled water solubility of the solute, solute in the salt solution, Cs = molarity of k = constant. The Setschenow equation requires that both order to evaluate the Setschenow constant, k. If 0304-4203/85/$03.30

systems. In this equation S = the solubility of the the salt solution, and solubilities be known in the constant, k, could be

© 1985 Elsevier Science Publishers B.V.

278 evaluated from pure c o m p o n e n t parameters, then the equation would be considerably more valuable. This is what McDevit and Long (1952) att e m p t e d to do. They assumed 'that the only role of the nonpolar solute is simply to o c c u p y volume and thus to modify the ion--water interactions in this simple fashion' (McDevit and Long, 1952, p. 1775). Gordon and Thorne (1967a) demonstrated that the Setschenow equation applies to multi-electrolyte solutions, with the salinity effect being an additive function of the individual inorganic salts. Their application of this finding to naphthalene in seawater at 25°C (Gordon and Thorne, 1967b) advanced the application of the McDevit and Long equation to various hydrocarbons in seawater (Sutton and Calder, 1974; Aquan-Yuen et al., 1979; Whitehouse, 1983). These studies agreed with McDevit and Long's conclusion that for low salt concentrations the theoretical value of k agrees qualitatively, but not quantitatively with empirical values of k. T h e y also concluded that the discrepancy was probably due to uncertainties in the values of the solute and ion volumes. One of the more successful methods that is available for correlating solubility to pure c o m p o n e n t parameters is that which uses hydrocarbon surface area (Hermann, 1972; Valvani et al., 1976; Yalkowsky and Valvani, 1979). In addition to demonstrating a correlation, these authors also present theoretical methods for determining surface area. The success of the surface area-to-solubility correlation leads to the hypothesis that the Setschenow constant, k, could be determined from a consideration of hydrocarbon surface area rather than hydrocarbon volume. This hypothesis, together with the recent publication of sufficient AH seawater solubility data (Whitehouse, 1984) to allow an empirical evaluation of the salt constant for seawater, provides an opportunity to re-evaluate this semi-empirical approach. RESULTS The Setschenow equation indicates that a plot of log (So~S) versus Cs should yield a straight line of slope k and zero intercept. The results of such plots are shown in Fig. 1. These plots indicate that all of the AH studied b y Whitehouse (1984), except 1,2-benzanthracene, satisfy the Setschenow equation. A discussion of 1,2-benzanthracene is reserved for later. The slope and intercept data resulting from the plots of the five other c o m p o u n d s are presented in Table I. The data indicate that for a given c o m p o u n d the Setschenow constant, k, is independent of temperature. Since k is apparently independent of temperature, it may be determined by combining all o f the data for a given compound. This results in a much smaller value for t0.gs than that indicated in Table I. Figure 1 suggests that some of the plots have non-zero intercepts. Table I indicates that, except for 2-methylanthracene at 8.8, 12.9, and 17.0°C,

279

.4o

2 - methylanthracene

.3o

phenanthrene

25 .30,

%

°

2o,

d

.15

.10. ooo o.oo

.~o

.3o

;,o

.;o

o[

.;o

1.00

ant hracene

.25

~

o o~

.2o

.4o '

.6o '

.'8o

I, 2 - benzant hracene

.80

.10

0 O0

" o.oo

.4o,

T

)o

,

.4o

.

./~o

.~o

J

0.00

2-ethylanthracene

o.oo

.6o

.~o

~ao

.'60

,

.8o

.

benzo(a) pyrene

.50 .30

~

-

i

.4o. .20'

. _ / ~

.10 .10"

0.00

o.oo

.2o

.4o C s (mole/l)

.6o

.ho

0.00

,.oo

.~o

.~o

.~o

.8o

C s (mole/l)

Fig. 1. Log (So~S) versus Cs. Approximate temperatures: (e) 4°C, ([3) 8°C, (T) 12°C, (~) 17 o C, ( @ ) 21 o C, ( 0 ) 25 o C (see Table I for precise temperature data).

2-ethylanthracene at 4.6°C, and benzo(a)pyrene at 8.0 and 12.4°C, the apparent non-zero intercepts are not significant. Regardless of the actual cause of the observed non-zero intercepts, it will be demonstrated that the information required from these plots is unaffected by this apparent anomaly. Using the data presented in Table I and hydrocarbon surface area data

280 TABLE I Log (So/S) versus Cs data from Fig. 1 (95% confidence) Compound

T (°C)

Slope (kobs)

Intercept

-R2

Phenanthrene

4.6 8.8 12.9 17.0 21.1 25.3

0.281 0.276 0.284 0.285 0.274 0.269

+- 0.090 + 0.071 + 0.043 + 0.043 + 0.048 + 0.066

-{-0.010-+ 0.035 --0.012 + 0.028 --0.014 + 0.016 --0.009 + 0.016 --0.020 + 0.018 --0.018 + 0.025

0.9702 0.9806 0.9933 0.9934 0.9909 0.9827

Anthracene

4.6 8.8 12.9 17.0 21.1 25.3

0.273 0.292 0.283 0.250 0.276 0.270

+ + ± + + +

0.019 0.072 0.038 0.064 0.076 0.120

--0.008 --0.011 --0.011 --0.005 --0.017 --0.013

+ 0.007 + 0.028 + 0.015 + 0.025 -+ 0.030 + 0.046

0.9986 0.9822 0.9945 0.9805 0.9777 0.9449

2-Methylanthracene

8.8 12.9 17.0 21.1 25.3

0.318 0.339 0.342 0.335 0.305

+ 0.083 -+ 0.088 + 0.069 + 0.051 + 0.071

+0.104 +0.074 +0.041 +0.022 +0.024

+ + + + +

0.032 0.034 0.027 0.020 0.028

0.9800 0.9804 0.9880 0.9932 0.9843

2-Ethylanthracene

4.6 8.8 12.9 17.0 21.1 25.3

0.349 0.362 0.356 0.344 0.302 0.313

+ + + + + +

0.066 0.075 0.050 0.031 0.046 0.069

+ 0.037 --0.005 +0.003 --0.018 --0.004 +0.014

+ + + + + +

0.026 0.030 0.019 0.012 0.018 0.027

0.9895 0.9873 0.9942 0.9976 0.9930 0.9859

Benzo(a)pyrene

8.0 12.4 16.7 20.9 25.0

0.198 0.107 0.187 0.190 0.157

-+ 0.223 -+ 0.124 + 0.176 + 0.157 + 0.145

+0.159 +0.110 +0.070 +0.016 +0.045

+ 0.086 +- 0.047 + 0.067 + 0.060 + 0.056

0.7264 0.7175 0.7920 0.8316 0.7973

a v a i l a b l e in t h e l i t e r a t u r e ( V a l v a n i e t al., 1 9 7 6 ; Y a l k o w s k y a n d V a l v a n i , 1976; Yalkowsky and Valvani, 1979), the following equation was evaluated log (S0/S)

= kC s

= A)~obsC~

(2)

w h e r e A is t h e h y d r o c a r b o n s u r f a c e a r e a a n d )~obs is t h e s a l t i n g c o n s t a n t f o r s e a w a t e r as d e t e r m i n e d f r o m t h e kob~ d a t a o f T a b l e I. I t h a s b e e n s u g g e s t e d t h a t t h e M c D e v i t a n d L o n g a p p r o a c h m a y i a i l in c e r t a i n c a s e s d u e t o f u n d a m e n t a l i n a d e q u a c i e s ( S a y l o r e t al., 1 9 5 2 ; A q u a n Y u e n e t al., 1 9 7 9 ) . S u c h a s t a t e m e n t is s u p p o r t e d b y t h e o b s e r v a t i o n o f 1 , 2 - b e n z a n t h r a c e n e e x h i b i t i n g salting-in w h e n o t h e r A H e x h i b i t e d saltingo u t in s e a w a t e r ( W h i t e h o u s e , 1 9 8 4 ) . T h e M c D e v i t a n d L o n g e q u a t i o n p r e d i c t s t h a t o n e o r t h e o t h e r c a n o c c u r in a g i v e n s a l t s o l u t i o n , b u t n o t b o t h .

281 Another experimental observation that suggests fundamental failure of the McDevit and Long equation is the observation that benzo(a)pyrene does not exhibit greater salting-out than other AH that had lower molar volumes (Whitehouse, 1984). The McDevitt and Long equation predicts increasing salting-out with increasing partial molar volume. Accordingly, hobs was evaluated without using the data of 1,2-benzanthracene and benzo(a)pyrene. The mean hobs value obtained from Table I using the remaining four compounds and utilizing all the data at all temperatures investigated is 1.41 x 10 -3 , with a probable error at the 95% confidence interval of -+ 0.04 x 10 -3. If the four curves having significant non-zero intercepts are omitted, the result is 1.40-+ 0.04 x 10 -3. Thus, the four curves having non-zero intercepts have no significant effect upon the value of hobs. Equation 2 now reduces to log

(So~S)

=

0.00141ACs

(3)

Equation 3 should estimate the molar solubility of an AH in seawater at temperatures ~ 25°C from a knowledge of its molar distilled water solubility (So), surface area (A), and the molar seawater salt concentration (Cs). The application of Eq. 3 to a range of aromatic hydrocarbons is demonstrated in Table II, which compares all of the solubility data available in the literature for aromatic hydrocarbons in distilled water and seawater (McAuliffe, 1966; Gordon and Thorne, 1967b; Sutton and Calder, 1975; Eganhouse and Calder, 1976; Rossi and Thomas, 1981) to those predicted by Eq. 3. For compounds having more than one literature value, all data have been listed, thus indicating the significant disagreement in the literature solubility data. The averages of these values are generally in good agreement with Eq. 3. DISCUSSION It has been demonstrated that, in general, the McDevit--Long approach to the Setschenow constant is valid. The only A H to exhibit disagreement with Eq. 3 were 1,2-benzanthracene and benzo(a)pyrene. These two compounds have the lowest solubility and are the only A H investigated here that are suspected carcinogens. One can conclude from experimental observation (Whitehouse, 1984) and the plots presented in Fig. I that the aqueous solubility of benzo(a)pyrene is relatively insensitive to changes in salinity and temperature, and that there is a lack of significant response in the solubility of 1,2-benzanthracene to salt concentrations greater than approximately 8O/oo. Such observations are difficult to account for with the McDevit--Long approach. The unusual behavior of these two compounds raises a question of exceptions to the view of nonpolar solutes existing as unassociated molecules in aqueous solution. A speculative alternative is that some A H m a y exist in solution in association. For 1,2-benzanthracene the association m a y be

282 TABLE II AH solubility data: observed from literature and calculated (Eq. 3) Compound

A (•2)

Cs

log ~S 0/S)obs

(tool 1-1 )

(25vC)

Toluene

126.5

Ethylbenzene o-Xylene m-Xylene p-Xylene Isopropylbenzene

144.9 146.8 150.3 150.3 163.4

tert-Butylbenzene Naphthalene

176.8 155.8

Acenaphthene Biphenyl Pyrene

175.0 182.0 213.0

0.5732 0.5648 0.5648 0.5648 0.5648 0.5648 0.5648 0.5648 0.5648 0.5732 0.4762 0.4764 0.1495 0.1444 0.5732 0.5732 0.5732

0.083 0.150 0.165 0.118 0.140 0.150 0.186 0.070 0.144 0.150 0.109 0.105 0.023 0.023 0.120 0.190 0.160

log (So/S)calc

0.102 0.101 0.115 0.117 0.120 0.120 0.130 0.130 0.141 0.126 0.105 0.105 0.033 0.032 0.141 0.147 0.172

o f the s o l u t e - - i o n t y p e , as r e f l e c t e d by the intense salting-in u p o n the a d d i t i o n o f small a m o u n t s o f salt. This s t a t e m e n t is s u p p o r t e d in the lite r a t u r e in t e r m s o f various a r o m a t i c h y d r o c a r b o n - - i o n i n t e r a c t i o n s {Andrews a n d Keefer, 1 9 4 9 ; B o h o n and Claussen, 1951; Krishnan and F r i e d m a n , 1974; J a n a d o et al., 1983). A c c o r d i n g t o t h e o r y discussed in this r e p o r t , salting-in should o n l y o c c u r w h e n the solute has a higher t o t a l m o l e c u l a r p o l a r i z a t i o n t h a n t h e solvent, and t h e e l e c t r o l y t e ions are relatively large {Long and McDevit, 1952). However, such t h e o r y does n o t a c c o u n t f o r l y o t r o p i c salting-in (Bockris et al., 1 9 5 1 ) w h e r e b o t h the solute and t h e e l e c t r o l y t e ions are c o m p a r a t i v e l y large. T h e AH 1 , 2 - b e n z a n t h r a c e n e and b e n z o ( a ) p y r e n e w o u l d fit this description. This s t a t e m e n t has i m p o r t a n t implications with respect t o t h e a p p l i c a t i o n o f Eq. 3. It suggests t h a t this b r e a k d o w n o f Eq. 3 m a y n o t o c c u r f o r the smaller AH. A l t h o u g h t h e four-ring AH p y r e n e satisfied Eq. 3, it is possible t h a t this c o m p o u n d m a y a p p r o a c h a m i n i m u m solubility limit f o r the e q u a t i o n .

ACKNOWLEDGMENTS This research was f u n d e d b y grants to R.C. C o o k e f r o m Imperial Oil Ltd. and t o P.J. Wangersky f r o m the Natural Sciences and Engineering Research Council o f Canada.

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