Environ. Sei. Technol. 1986, 20, 574-580
(2) Louw, R.; Dijks, J. H. M.; Mulder, P. Recl.: J. R. Neth. Chem. SOC. 1984,103 (lo), 271-274. (3) Louw, R.; Dijks, J. H. M.; Mulder, P. Chem. Ind. (London) 1983, 759-760. (4) Manion, J. A.; Mulder, P.; Louw, R. Environ. Sci. Technol. 1985,19, 280-282. (5) Catwell, H. M.; Titani, T. J. Am. Chem. SOC. 1933, 55, 1363-1376. (6) Smyser, S. H. F.; Smallwood,H. M. J.Am. Chem. SOC. 1938, 60,3498-3499. (7) Polanyi, M.; Cremeer, E.; Curry, M. Phys. Chem. B 1933, 23, 445. (8) Vance, J. E.; Baumann, W. C. J. Chem. Phys. 1938,6,881. (9) Clark, D. T.; Tedder, J. M. Trans. Faraday SOC. 1966,62, 393-398. 10) Clark, D. T.; Teddar, J. M. Trans. Faraday SOC. 1966,62, 405-411. 1 Arnold, S. J.; Kimbel, G. H.; Snelling, D. K. Can. J. Chem. 1974,52, 271-280. Arnold, S. J.; Kimbel, G. H.; Snelling, D. K. Can. J. Chem. 1975,53, 2149-2425. Combourieu, J.; Le Bras, G.; Paty, C. 14th Int. Symp. Combust., 14th 1972, 485-492. Chari, S. M.S. Thesis, New Jersey Institute of Technology, Newark, NJ, 1981. Bradley, J. N.; Whytock, D. A.; Zalesi, T. A. J. Chem. SOC., Faraday Trans. 1 1976, 72, 2284-2288. Westenburg, A. A.; DeHass, N. J. Chem. Phys. 1975, 62, 3321. Costes, M.; Donthe, G.; Destriaa, M. Chem. Phys. Lett. 1979, 61, 588. Gaisinovich, M. S.; Ketov, H. N. Russ. J. Inorg. Chem. (Engl. Transl.) 1969, 14, 1218-1220. Levenspiel, 0. “Chemical Reaction Engineering”, 2nd ed.; Wiley: New York, 1972. Poirier, R. V.; Carr, R. W., Jr. J. Phys. Chem. 1971, 75, 1593-1601. Chang, S. H. Ph.D. Dissertation, New Jersey Institute of
Technology, Newark, NJ, 1985. (22) Chuang, S. C. M.S. Thesis, New Jersey Institute of Technology, Newark, NJ, 1982. (23) Semeluk, G. P.; Bernstein, R. B. J. Am. Chem. SOC. 1957, 79, 46-49. (24) Benson, S. W.; O’Neil, H. E. “Kinetic Data on Gas Phase Unimolecular Reactions”, U.S. Department of Commerce, 1970, NSRDS-NBS 21. (25) Darwent, B. deB. “Bond Dissociation Energies in Simple Molecules”. U.S. Department of Commerce, 1970, NBS. NSRDS-NBS 31. (26) Zeleznik, F. J. Ind. Eng. Chem. 1968, 60 (6), 27-56. (27) Smith, W. R. Ind. Eng. Chem. Fundam. 1980,19 (l),1-10. (28) Meissner, H. P.; Kusik, G. L.; Dalgell, W. H. Ind. Eng. Chem. Fundam. 1969,8 (4), 659-665. (29) Virk, P. S.; Chambers, L. E.; Wosbcke, H. N. Adu. Chem. Ser. No. 131; American Chemical Society: Washington,DC, 1972; pp 237-258. (30) Harris, S. J.; Weiner, A. M. Combust. Sci. Technol. 1983, 32, 267. (31) Frendlich, M.; Taki, S. Combust. Flame 1983,54, 81. (32) Weissman, M.; Benson, S. W. Int. J. Chem. Kinet. 1984, 16, 307-333. (33) Froment, G.F. Chem. Eng. Sci. 1981,36, 1271-1282. (34) Shah, Y. T.; Stuart, E. B.; Sheth, K. D. Ind. Eng. Chem. Process Des. Dev. 1976, 15, 518-524. (35) Hougen, A. D.; Watson, K. M. ‘‘Chemical Process Principles, Part 11”;Wiley: New York, 1947. (36) Froment, G. F.; Bischoff, K. B. “Chemical Reactor Analysis and Design”; Wiley: New York, 1979. (37) Van Dammer, P. S.; Narayanan, S.; Froment, G. F. AIChE J. 1975,21, 1065-1073. Received for review October 31, 1983. Revised manuscript received May 21,1985. Accepted January 21,1986. This research was partially supported through Grant 81 R 0014 01 from the U S . Environmental Protection Agency.
Behavior of Aliphatic Hydrocarbons in Coastal Seawater: Mesocosm Experiments with [’‘C]Octadecane and [14C]Decanet Stuart G. Wakeham* and Elizabeth A. Canuel Chemistry Department, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543
Peter H. Doering Marine Ecosystems Research Laboratory (MERL), University of Rhode Island, Kingston, Rhode Island 0288 1
[14CiOctadecane and [14Cidecane were used as model in ecosystem experiments to investigate the behavior Of hydrocarbons in coastal seawater. Under summer conditions, both octadecane and decane were removed from the water column primarily by biodegradation. Mass balances calculated after t h e 2.5week experiments showed t h a t 71432% of the radiolabeled hydrocarbon introduced into the ecosystem had been ~ removed only ~ 1 5 % mineralized t o 1 4 ~ 0Volatilization of t h e hydrocarbons. The experimental results were applied t o predict t h e fate of these compounds in Narragansett Bay. Residence times of about 1 day would be expected in summer. Introduction T h e fate a n d persistence of xenobiotics in t h e aquatic environment are of major concern. Volatile organic comt Woods Hole Oceanographic Institution Contribution No. 5893.
574
Environ. Sci. Technol., Vol. 20, No. 6, 1986
pounds (VOC) comprise one group of xenobiotics a n d include a variety of low molecular weight compounds of synthetic (chlorinated C1- and C2-hydrocarbons and chlorinated benzenes) and petrogenic (C6-C18-aliphatic hydrocarbons, benzenes, a n d naphthalenes) origins. Two
approaches have been used to assess the sources, transport mechanisms, and removal processes that control distributions of VOC in both fresh water a n d seawater. Field studies provide data on spatial a n d temporal distributions of a n d allow a f m n e w o r k for predicting or modeling their sources a n d fates (1-4). Laboratory and controlled ecosystem experiments provide an evaluation of the parameters used in the models (5-12). W e have been conducting radiotracer experiments in controlled experimental ecosystems (mesocosms) t o study t h e biogeochemistry of VOC in coastal seawater (12). [14C]Octadecane and [14C3decane were used as model comDounds to investigate the fate of aliphatic hydrocarbons in a n estuary s u c h as Narragansett Bay, Rhode
voc
Island.
0013-936X/86/0920-0574$01.50/0
0 1986 American Chemical Society
A simple one-box model can describe the mass balance of any chemical species in a well-mixed body of water with constant flow and volume V such as an estuary (or lake) where Mi, and Moutare physical transfer rates across the boundaries, and J and R represent in situ production and removal rates, respectively. If the input and output rates for water are equal, Le., V = constant, if J = 0 for compounds of nonbiogenic origins, and if all processes are assumed to be adequately described by first-order rate constants, then where k, is the rate constant for water flow, C, is the mean concentration in the inflowing water, C, is the concentration at time t, and k, is the removal rate constant. The purpose of our mesocosm experiments is to evaluate lz, under a set of conditions realistic for temperate estuaries such as Narragansett Bay; other parameters must be determined in field studies. Experimental Section The mesocosm experiments were conducted at the Marine Ecosystems Research Laboratory (MERL) located on the shores of Narragansett Bay at the University of Rhode Island. The mesocosm was a 5.5 m high, 1.8 m diameter fiberglass tank with a capacity of 13.3 m3 (13,14). The tank was filled with Narragansett Bay seawater and its associated planktonic and microbial communities several days prior to each experiment. To simulate tidal mixing in the bay, the tank was mixed 4 times daily for 2 h. Our experiments were run in batch (without water flow through) and without sediments. The octadecane experiment was begun on Sept 28,1982, and lasted for 17 days. Six hundred seventy microcuries of n-[l-14C]octadecane(Amersham Corp., 30 mCi mmol-l) was dissolved in 5 mL of acetone, and this solution was mixed into 1.5 L of seawater in a separatory funnel. The seawater solution was then introduced into midtank through a Teflon tube during a mixing cycle. We also introduced unlabeled octadecane, such that the initial tank water column concentration was about 0.5 pg L-l (concentrations of alkanes in upper Narragansett Bay are in this range ( 4 ) ) ,and Freon-12 (dichlorodifluoromethane (F-12); see ref 15 for procedure) as a gas-exchange tracer. During the experiment, the average tank temperature was 16 f 2 OC. On May 2, 1983, the tank was spiked with 650 yCi of n-[ l-14C]decane (Amersham; 30 mCi mmol-l), unlabeled decane, and F-12. The fate of decane was followed for 16 days with an average water temperature of 14 f 2 OC. All samples were collected during mixing to ensure that the tank water column was homogeneous. Samples to be returned to Woods Hole Oceanographic Institution for processing (volatile/dissolved 14C,intermediate metabolite 14C,unlabeled alkane, and Freon-12 as will be described) were poisoned with HgC12and transported in glass bottles without headspace and in coolers at about 4 OC; all other samples were processed immediately at MERL. During each experiment, 14Cactivity was traced in several operationally defined pools of the ecosystem. Analytical details are given in Wakeham et al. (121,but briefly the following measurements were made to estimate the 14Cactivity in the various pools. (1)Total 14C activity in the mesocosm was determined directly by liquid scintillation counting (LSC) of unfractionated water samples. Two 4-L water samples were collected during mixing, and 2 X 10-mL samples from each
were placed in 10 mL of Aquasol (New England Nuclear) containing 0.3 mL of phenethylamine to minimize loss of 14C02. Total 14Cactivity includes volatile (dissolved) 14C, 14Cassociated with nonvolatile intermediate metabolies, particulate 14C,and 14C02,without differentiating between the pools. (2) Volatile (dissolved) 14C-labeledalkane was measured by closed-loop stripping of 200-500 mL of unfiltered tank water. Filtration was found to cause loss of alkane both by volatilization and by adsorption onto the filter material. The purged 14C-labeledalkane was trapped on activated charcoal (16),after which the charcoal was eluted with 6 mL of toluene and the 14Cactivity determined by LSC. Activities obtained by this procedure were corrected to account for incomplete recoveries of octadecane (50%)and decane (85%) which had been determined previously by stripping unlabeled alkane standards from seawater. We assume that stripping for 1h recovers primarily the dissolved alkane pool and that this pool is amenable to volatilization. However, we could be overestimating the volatile/dissolved pool for two reasons. First, stripping may recover some portion of a pool of metabolites if their Henry's law constants favor partitioning into the gas bubbles. Second, if an equilibrium exists between dissolved and particulate alkane pools, then removal of dissolved alkane by stripping will result in release of alkane from particles, and this alkane is then available to be stripped. At present we have no data to address either question quantitatively. (3) We estimated the 14Cactivity of polar intermediate metabolites (e.g., ketones, alcohols, and acids) by extracting stripped water samples with toluene and counting the extracts. This extraction could recover some 14Cactivity from particulate matter, but we believe the contribution from particulate 14Cto be small (e.g., Table I). (4) Particulate 14Cwas determined by filtering 2 L of tank water through Gelman A/E glass-fiber filters of 0.3pm nominal pore size and counting the activity on the filter. Particulate 14Cincludes label abiotically sorbed onto particles as well as label biotically incorporated into living cells, although some bacteria may not have been efficiently collected. (5) 14C02produced by mineralization of the 14C-labeled alkane was determined by stripping 500 mL of acidified (pH 2) tank water, passing the gas through a charcoal trap to remove any stripped alkane, and collecting the 14C02 in an absorber solution (Oxifluor-C02,New England Nuclear) for LSC. Recoveries of 14C02were found to be >95% by using NaH14C0,and the charcoal trap removed about 98% of any volatilized alkane. (6) Unlabeled alkane was determined by closed-loop stripping and trapping on activated charcoal. The charcoal was extracted with 15 pL of CS2and the alkane determined by glass capillary gas chromatography ( 4 ) . (7) Freon-12 in the tank water were determined by flame ionization gas chromatography of the headspace of water samples. Absolute concentrations of F-12 were not determined, and relative changes in the amount of F-12 in the water were determined by FID detector response/area units. Results Octadecane. Changes in distribution of 14Cactivity in the various pools in the water column of the mesocosm (given as pCi tank-l) and for F-12 are shown over the time course of the octadecane experiment in Figure 1. The trend for unlabeled octadecane was the same as that for ['4C]octadecane and thus is not shown. Total 14Cactivity decreased slowly at a rate of about 0.03 day-l over the Environ. Sci. Technol., Vol. 20, No. 6, 1986
575
Table I. Mass Balances and Total Counts Measured for Single Days for Isotope Added as [14C]Octadecane(670 pCi Introduced) and [14C]Decane (660 pCi Introduced)
Octadecane
dissolved (volatile) particulate (sorbed + biomass) intermediate metabolites COZ sum total direct counts
day 0 (2 h) pCi %
pCi
%
pCi
%
pCi
%
374 300 10 16 700 670
192 220 45 32 490 650
39 45 9 7
60 100 15 260 435 450
14 23 3 60
10 24 22 430 486 484
2 5 5 88
day 8
day 3
53 43 1 3
day 17
Decane
dissolved (volatile) particulate (sorbed + biomass) intermediate metabolites
co2
sum total direct counts
day 0 (2 h) pCi %
pCi
%
pCi
%
pCi
%
305 213 36 40 594 650
23 150 50 310 533 600
4 28 10 58
12 85 40 403 540
2 16 7 75
10 53 32 463 558 560
2 10 6 82
51 36 6 7
course of the experiment (total direct counts in Figure l), yielding a half-life of about 23 days. Decreases in both dissolved 14C activity and particulate 14C activity were faster. The dissolved 14Cactivity decreased at a rate of 0.36 day-l (t1/2 = 2 days) for the first 6 ways of the experiment followed by a slower loss at 0.18 day-l (tl/2 = 4 days) for the remainder of the experiment. Particulate 14C activity decreased at 0.15 day-' (tip = 5 days) over the entire experiment. The decreasing dissolved 14Cactivity reflects loss from this pool via a combination of volatilization, sorption onto particulate matter, and degradation, while loss of activity from the particulate pool would be due to desorption and degradation. The rapid increase in 14C02activity between days 2 and 7 resulted from intense mineralization of [14C]octadecane. The observation that total 14Cactivity in the tank decreased only slowly can be attributed primarily to the buildup of 14C02after the first few days of the experiment. C02 is lost from the water column only slowly (about 0.005 day-l (17))by gas exchange because of its high solubility as HCOc. Loss of F-12 from the water column was at a rate of about 0.07 day-l (t1/2= 10 days). The form in which the label was distributed in the mesocosm changed considerably during the experiment. This can be seen by comparing activities for the major pools at different times (Table I). For example, 2 h after introducing the [14C]octadecane,53% of the 14Cactivity existed in the volatile/dissolved pool. Particulate 14C accounted for about 43% of the summed total, 3% of the activity was present as 14C02,and 1% was in the intermediate metabolite pool. Following a lag or induction period of about 2 days [14C]octadecanewas rapidly mineralized, resulting in increasing 14C02activities at the expense of both volatile (dissolved) and particulate 14C. By day 8, 60% of the 14Cactivity in the tank water was as 14C02,and by the end of the experiment (day 17) 88% of the label was 14C02.At no time during the experiment did the 14Cactivity in the intermediate metabolite pool represent more than about 10% of the total 14Cactivity in the water column. Only a few percent of the activity was left as dissolved or particulate at the end of the experiment. It is interesting to note that the ratio of particulate 14Cto dissolved 14Cshifted from 0.8 at the start of the experiment to 2.4 on day 17. This might be taken as suggesting that the dissolved form is more readily lost from the water 576
Envlron. Sci. Technol., Vol. 20, No. 6, 1986
day 3
day 7
day 14
570
f4C- Octadecane
' W
L o
io
TOTAL i4C DIRECT COUNTS
-
'
5oo
io
t
?
r 5 0 0
,0°7
A A
A
-
(Z/lCI) 5
b
Borro,uo FLOC
I
I
I
I
I
I
I
I
2
4
6
8
10
12
14
16
ro 1 5 18
Flgure 1. Distributions of "C activity in the octadecane experiment. Straight lines are regression fits to the data while curves serve only for visual clarity. Note that activity axes are logarithmic.
column by volatilization, sorption, and degradation and that the particulate form is relatively more stable and is lost more slowly. For most of the experiment, the summed total 14Cactivity (dissolved + particulate + intermediate metabolite C02)was remarkably similar to the activity obtained by direct counting of unfractionated (Table I). The greatest discrepancy was observed on day 3 when mineralization of octadecane became important. Decane. Results for the decane experiment are shown in Figure 2 and Table I, and trends are generally similar to those seen for octadecane. 14Cactivity determined by
+
c' -
7c
1 i
Table 11. Sorption Parameters for the Mesocosm Experiments
Decane
= =
f-12 8
500
.
T500
c
-I\
v.'
VOLATILE "C 0
5
7
3 is P
I
0
0
4
I00
50
50
0
5-2-83
i
j;_
100
4ol
I
PARTICULATE "C BOTTOM0 (17pC1) FLOC
,
2
4
6 -DAY-
8
10
12
14
,
IO
16
5-18-83
Figure 2. Distributions of I4Cactivity in the decane experiment. Note that the activity axes are logarithmic.
direct counting decreased a t about 0.02 day-' (tllz = 35 days). The initial decrease in dissolved 14Cactivity for decane was much faster than for octadecane and was about 1.3 day-' (tl = 0.5 day) over the first 2 days and correa rapid increase in l4CO2. However, between sponded days 4 and 14, the decrease was considerably slower, about 0.03 day-l (tllz = 23 days), as was the continued increase in l4CO2. The activity of the particulate 14Cpool decreased throughout the experiment at a rate of about 0.03 day-' (tl/z = 23 days). As in the previous experiment, F-12 was lost a t 0.07 day-l during the decane experiment. Changing distributions of 14Clabel during the decane experiment were for the most part also not greatly different from those during the octadecane experiment. Within 2 h after addition of [14C]decaneinto the tank, most of the label was in the dissolved pool (51%) and associated with particles (36%) both by sorption and by biotic uptake. However, there was no lag period before the [14C]decane was mineralized. After only 1 day, 14C02became the dominant 14Cpool (55%) and continued to increase, although much more slowly, until the end of the experiment.
wid
Discussion R e m o v a l R a t e C o n s t a n t s . In the mesocosm run in batch, k , = 0 so eq 2 may be simplified to (3) dCt/dt = -k,C, where k , = k , k , kb (4) where k , is the volatilization rate constant, k , is the sedimentation rate constant, and kb is the biodegradation rate constant (assuming no thermochemical or photochemical degradation). Since k , acts only on the concentration of the dissolved species Cl, k, acts only on the concentration in the particulate phase C,, and kb may affect the total concentration (both dissolved and particulate) C,, eq 3 C,, be rewritten as dC,/dt = -[f,k, + (1- f J k , + kb]Ct (5) where f i is the fraction of compound in the liquid phase and 1- f i the fraction in the solid phase. Equation 5 may be rewritten as dC,/dt = -(k,' k,' kb)C, (6)
+ +
+ +
seawater solubility at 15 OC," log KO,,' compound S, mol L-l L kg0,-' octadecane 7.9 X 3.5 X decane 2.3 x 10-3 F-12
fi' [om = 6 X lo4 kg L-l]
lo4 kg L-'1
0.17 0.77 1.0
0.56 0.95 1.0
5.9 4.7 1.0
[om = 1 X
a Estimated from ref 19. 'log KO, = -0.729 log S + 0.001 (from ref 18). c f f , = 1/(1 + lomlK,,).
Table 111. Volatilization Parameters for the Mesocosm Experiments H a t 15 OC," D at 15 'C,* compound atm m3 mol-' om2 k,, day-' octadecane decane F-12
0.01 7 0.3
3.6 X lo4 4.2 X lo4 8.3 X lo4
0.03' 0.04' 0.07
k;, day-'
0.01-0.02d 0.03-0.04d
"Reference 19. 'Estimated from ref 20 and 21. Ckvdkane/kw.ll= Dalkans/DF.lS.k,' = f,k, for the range of f i in Table 11.
Our mesocosm experiments were run without sediments and with a mixing cycle to minimize sedimentation. Thus, we assuhe the k, = 0 so that dCt/dt = -(k;
+ kb)C,
(7)
Evaluation of the apparent volatilization rate constant k,' requires that f i and k , be determined experimentally. The fraction of compound in the liquid phase may be estimated from its partition coefficient KO, between natural organic matter and water. KO,,in turn, is dependent on the aqueous solubility S of the compound. A set of parameters describing sorption of alkanes onto natural sorbents has not, to our knowledge, been empirically determined (as has been done of a variety of chlorinated hydrocarbons and polycyclic aromatic hydrocarbons). However, to a first approximation, Kommay be estimated by a relationship described by Chiou et al. (18) (log KO, = -0.729 log S + 0.001), yielding values given in Table 11. The concentrations of suspended organic matter in the two experiments were about 1-6 mg L-l (measured particulate organic carbon concentrations were 0.5-3 mg L-l), and the resulting f i s range between 0.17 and 0.56 for octadecane and 0.77 and 0.95 for decane (Table 11). Henry's law constants for octadecane, decane, and F-12 are all (Table 111)so that volatilization of these three compounds is liquid film controlled. Therefore k , for octadecane and decane may be estimated from k , of F-12 and the ratio of alkane and F-12 molecular diffusion coefficients (Table 111). F-12 is neither biodegradable nor associated with particles (22,23) so its only removal is by volatilization. For F-12, k , is the rate constant observed for its loss from the tank, or 0.07 day-l. The ratio of Dodadecane:DF-lz is 0.43, and k , for octadecane is 0.03 day-'. Similarly, Ddecane:DFSl2 = 0.51, and k , for decane is 0.04 day-'. Applying the appropriate f i values from Table I1 yields k,' of 0.01-0.02 day-l and 0.03-0.04 day-l for octadecane and decane, respectively (Table 111). The biodegradation (or strictly mineralization) rate constant kb is estimated from the production of 14C02. Mineralization of [14C]octadecanewas not important for the first 2 days of the experiment but then quickly became the most important removal process. By use of a linear fit to approximate the 14C02activity data in Figure 1, a l4COZproduction rate constant of 0.66 day-l is estimated, Environ. Sci. Technol., Vol. 20, No. 6, 1986 577
n-OCTADECANE ( C~BHII
54"
)
1
n
- DECANE
( cj0 H
~
~
)
3 A Y '7
Flgure 3. Mass balance for the [14C]octadecaneexperiment calculated for days 1 and 17.
corresponding to a degradation rate constant for octadecane of about 0.66 day-l. After day 7, the apparent production of 14C02decreased to a rate constant of about 0.05 day-' or a degradation rate constant of 0.05 day-'. Note that this initial degradation rate is much faster than the loss rate observed for the dissolved 14Cpool (0.35 day-l) and must reflect degradation of [14C]octadecanefrom both dissolved and particulate pools. The decrease in degradation rate after day 7 might reflect the low concentrations of substrate remaining in the tank after the period of rapid mineralization. A similar analysis of the decane data suggests a decane degradation rate constant of about 1.2 day-' during the first day of the experiment and 0.04 day-l after day 4. Mass Balances. At the end of each experiment, a mass balance was calculated to show the overall fate of the model compounds in the mesocosms. The mass balances were calculated by using the activities remaining in each pool at the end of the experiments and using the removal rate constants derived above. Volatilization rate constants were applied only during the first several days of the experiment (about 5 for octadecane and 1 for decane) when there were significant 14Cactivities in the dissolved/volatile pools. No corrections were needed for water flow through. In the case of octadecane, sorption onto particulate matter was important immediately after introduction of the radiolabel so a balance was calculated for day 1. This distribution was then used as the starting point for the mass balance calculated for day 17. The final mass balance (Figure 3) shows that after about 17 days, about 15% of the [14C]octadecaneinitially introduced into the mesocosm had been lost by volatilization, while 2% remained dissolved in the water column. About 5% of the label remained associated with particles and 7 % with intermediate metabolites which were recovered after stripping. Clearly the most important removal process was mineralization, which removed 71 % of the [14C]octadecane. Most of the 14C02thus produced remained in the tank as H14C03-. The final mass balance for decane (Figure 4) shows that 82% of the [14C]decanewas mineralized, while only about 6% was volatilized. These values are slightly different from those for octadecane. The greater percentage mineralized may be due to more rapid degradation of decane than octadecane, either because of decane's shorter carbon chain length or because of differences in hydrocarbon degrading microorganisms in the two experiments. The lower percentage volatilized for decane reflects the fact that mineralization began immediately, that is, without the lag period seen in the octadecane experiment, thus quickly depleting the dissolved pool which alone is amenable to volatilization. MERL Mesocosms and Narragansett Bay. Our mesocosm data may be used to predict the residence time 578
Environ. Sci. Technol., Vol. 20, No. 6, 1986
D A Y 16
Figure 4. Mass balance for the [I4C]decane experiment at day 16.
of aliphatic hydrocarbons in Narragansett Bay and the relative importance of each removal process. In the following discussion, three assumptions are made. In the absence of any data on temporal variations in inputs into Narragansett Bay and only limited data on concentrations of aliphatic hydrocarbons in the bay ( 4 ) ,we assume that levels of hydrocarbons in the upper bay near Providence, RI, are similar to initial concentrations in our experiments and that degradation will not be substrate limited. More importantly, we assume that the bay water contains a microbial community that is already acclimated to such concentrations of hydrocarbons and therefore does not need to be induced to degrade the alkanes. Finally, the dissolved/particle partitioning in the bay is similar to that in the tanks; this depends primarily on the suspended particulate organic carbon, which is similar in both ecosystems. The residence time of the hydrocarbons in the bay will be the reciprocal of the sum of the rate constants for the various removal processes: where k, = the advective removal rate constant (day-I), k,' = removal rate constant for sedimentation (day-I), and k,' and kb are as discussed above. Each process has been evaluated as follows: (1) River inflow and tidal mixing result in a residence time for water in the bay of about 30 days (24). Thus, k, = 0.04 day-l. (2) Extrapolating volatilization rates from the MERL data to Narragansett Bay requires recognition of the widely differing hydrodynamics of the two systems. The stagnant boundary layer model (25,26)is useful in this comparison. Stagnant boundary layer film thicknesses on the order of 200-500 pm are common in the mesocosms ( 1 1 , 1 2 , 1 5 , 2 7 ) (200 and 215 pm for the octadecane and decane experiments, respectively) where 15-30 cm of tank wall extend above the water surface, thus reducing wind-driven turbulence considerably. In Narragansett Bay, boundary film thicknesses have been estimated to range between 60 and 180 pm (24). If we assume an average thickness of 100 pm
for the bay, then our MERL-derived volatilization rates must at least be doubled for Narragansett Bay, giving 0.02-0.04 day-l for octadecane and 0.06-0.08 day-l for decane, In reality, order of magnitude fluctuations in turbulence are likely in both the mesocosm and bay during storms which would increase short-term volatilization rates dramatically, The impact of storms on volatilization rates has been demonstrated in a MERL experiment involving [14C]toluene (12); during a storm the volatilization of toluene increased an order of magnitude above rates before and after the storm. (3) Our MERL experiments suggest that, at least in May and September (i-e.,summer), degradation of octadecane and decane, and presumably other aliphatic hydrocarbons as well, proceeds at rates of about 0.70-1.2 day-l and is the most important process removing these compounds from the water column. Degradation rates would be expected to be minimal in winter which was shown previously for toluene (12). (4) Assessing the removal of octadecane and decane by sedimentation is more difficult. In the mesocosm, sedimentation was assumed to be negligible because of the absence of sediments and benthos and because the mixing of the tank tends to keep particles in suspension. Narragansett Bay, however, does accumulate sediments, although over a wide range of sedimentation rates (0.001 cm year1 in lower and mid Narragansett Bay to 0.5 cm year-' in the upper bay (28)). Estimated residence times for particles in the bay are 2-14 days and are greatly dependent on location in the bay and season. The residence times of particulate alkanes must take into consideration the fraction of substance that is associated with particles f,, where f, = 1 - fi and may be estimated by Talkane = 7p&i&/fS. Assuming a median particle residence time of 7 days and f, of 0.83 for octadecane and 0.23 for decane (for particulate organic matter 6 mg L-' typical for upper Narragansett Bay), then we would estimate residence times of 8 and 30 days for particulate octadecane and decane, respectively, in the upper bay. Corresponding sedimentation rate constants k,' (where k,' = 1/ra1kane) would be 0.13 and 0.03 day-l for octadecane and decane. In lower Narragansett Bay, lower suspended organic matter loads (1 mg L-l) would lead to lower f, (0.44 and 0.05 for octadecane and decane, respectively), longer residence times (16 and 140 days, respectively), and lower sedimentation rate constants (0.06 and 0.01 day-l, respectively). These sedimentation rate constants are subject to several potential errors. Resuspension of particles enriched in sorbed hydrocarbon (Le., already saturated) might not sorb additional hydrocarbon from the water column so that the estimated k,' would be too high. Alternatively, resuspension of particles depleted in sorbed hydrocarbon might be able to sorb more hydrocarbon from the dissolved phase. And, finally, our estimates do not consider degradative removal of hydrocarbon from the particles. Considering the four removal processes discussed earlier, we would estimate ranges of ktotalof 0.82-0.91 day-' for octadecane and 1.31-1.35 day-l for decane. Residence times would be about 1.2 and 0.8 days, respectively. For both model compounds, mineralization is clearly the most important removal process, with the degradation rate constant accounting for about 80% of the estimated total removal rate constant of octadecane and about 95% of the total removal rate constant of decane. As noted earlier, the relative importance of each process may vary greatly depending on season, with degradation probably being minimal in winter at low water temperatures and volatilization and/or sedimentation becoming increasingly
more important depending on local conditions of winddriven turbulence and suspended particle load. How do the predictions from these radiotracer experiments compare with other studies of the fate of hydrocarbons in coastal environments? A series of experiments were previously conducted at MERL using chronic additions of no. 2 fuel oil added as an oil/water dispersion giving concentrations of hundreds of micrograms of oil per liter (10, 29-31). While no data on rates for removal processes were given, it was hypothesized that evaporative loss to the atmosphere was the major removal mechanism affecting aliphatic hydrocarbons. Estimates of volatilization accounted for 60-90% of the total hydrocarbon loss. A maximum of about 16% of the hydrocarbons were accumulated in the sediments. Biodegradation was variable, being negligible in winter but increasing in importance as the microbial communities became acclimated to the oil and as water temperatures increased with the onset of spring. Most biodegradation apparently occurred within the sediments. Another study (32)used tritiated crude oil in a 14-day controlled ecosystem experiment to simulate the fate of oil in the North Sea. The major removal process in this case was evaporation from a thick ( 5 mm) surface slick, accounting for 53% of the loss from the water column. Sedimentation and biodegradation were both minor, removing 1% and 0.05%, respectively, of the added oil. The low rate of degradation may readily be explained by the low water temperatures involved (0.5-2.5 O C ) and the apparent nutrient limitation of the microbial communities as the experiment progressed. Nearly 50% of the added oil remained in the slick. None of the above cited experiments are really comparable to our 14Ctracer experiments, primarily because of the much higher concentrations of added hydrocarbon (as oil). Furthermore, degradation and volatilization were estimated either by difference or by measuring concentration changes of selected compounds. On the other hand, our experiments demonstrate conclusively that, at least in summer, mineralization of aliphatic hydrocarbons in the water column may play the major role in controlling their concentrations. We would expect both volatilization and sedimentation to play minor roles.
Acknowledgments We thank J. Frithsen at MERL and J. W. H. Dacey at WHO1 for assistance. C. Lee, B. Brownawell, J. Farrington, and R. Schwarzenbach provided many valuable discussions on the data and manuscript. Registry No. Octadecane, 593-45-3; decane, 124-18-5.
Literature Cited (1) Schwarzenbach, R. P.; Molnar-Kubica, E.; Giger, W.; Wakeham, s. G. Enuiron. Sci. Technol. 1979,13,1367-1373. (2) Schwarzenbach, R. P.; Bromund, R. H.; Gschwend, P. M.; Zafiriou, 0. C. Org. Geochem. 1978, I, 93-107. (3) Gschwend, P. M.; Zafiriou, 0. C.; Mantoura, R. F. C.; Gagosian, R. B. Enuiron. Sci. Technol. 1982, 16, 31-38. (4) Wakeham, S. G.; Goodwin, J. T.;Davis, A. C. Can. J.Fish. Aquat. Sci. 1983, 40 (Suppl. 2), 304-321. (5). Dilling, W. L.; Tefertiller,N. B.; Kallos, G. J. Enuiron. Sci. Technol. 1975,9, 833-838. (6) Dilling, W. L.; Bredeweg, C. I.; Tefertiller, N. B. Enuiron. Sci. Technol. 1976, 10, 351-356. (7) Dilling, W. L. Enuiron. Sci. Technol. 1977, 11, 405-409. (8) Cohen, T.; Cocchio, W.; Mackay, D. Enuiron. Sci. Technol. 1978,12, 553-558. (9) Schwarzenbach, R. P.; Westall, J. C. Enuiron. Sci. Technol. 1981,15, 1360-1367. (10) Gearing, P . J.; Gearing, J. N. Mar. Enuiron. Res. 1982, 6, 115-132. Environ. Sci. Technol., Vol. 20, No. 6 , 1986
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Wakeham, S.G.; Davis, A. C.; Karas, J. I,. Environ. Sci. Technol. 1983, 17, 611-617. Wakeham, S.G.; Canuel, E. A,; Doering, P. H.; Hobbie, J. E.; Helfrich, J. V. K.; Lough, G. G. Biogeochemistry 1985, I, 307-328. Pilson, M. E. Q.; Vargo, G. A.; Gearing, P.; Gearing, J. N. In “Proceedings of the 2nd National Conference, Interagency Energy, Environment R & D Program”. U. S.Environmental Protection Agency, Washington, DC, 1977, Report 600/9-77-012, pp 513-516. Santschi, P. H. In “Marine Mesocosms: Biological and Chemical Research in Experimental Ecosystems”; Grice, G. D.; Reeve, M. R., Eds.; Springer-Verlag: New York, 1982; pp 63-80. Bopp, R. F.; Santschi, P. H.; Li, Y.-H.; Deck, B. L. Org. Geochem. 1981,3, 9-14. Grob, K.; Zurcher, F. J. Chromatogr. 1976,117,285-294. Rudnick, D., MERL, private communication, 1984. Chiou, C. T.; Porter, P. E.; Schmedding, D. W. Enuiron. Sci. Technol. 1983, 17, 227-231. Mackay, D.; Shiu, W. Y. J. Phys. Chem. Ref. Data 1981, 10, 1175-1199. Wilke, C. R.; Chang, P. Am. Znd. Chem. Eng. J. 1955, 1, 264-270. Hayduk, L. W.; Laudie, H. Am. Znd. Chem. Eng. J. 1974, 20, 611-615. Blake, D. A.; Mergner, G. W. Toxicol. Appl. Pharmacol. 1974,30, 396-407. Mergner, G. W.; Blake, D. A.; Helfrich, M. Anesthesiology 1975, 42, 345-351.
Kremer, J. N.; Nixon, S. W. In “A Coastal Marine Ecosystem; Simulation and Analysis”; Springer-Verlag: New York, 1978; p 217. Liss, P. S.; Slater, P. G. Nature (London) 1974, 247, 181-184. Broecker, W. S.; Peng, T.-H. Tellus 1974, 26, 21-35. Nixon, S.W.; Alonso, D.; Pilson, M. E. Q.; Buckley, B. A. In “Microcosms in Ecological Research”; Giesy, J. P., Ed.; National Technical Information Service: Washington, DC, 1980; DOE Symp. Ser. CONF-781011, pp 818-849. Santschi, P. H.; Nixon, S. W.; Pilson, M. E. Q.; Hunt, C. Estuarine Coastal Shelf Sci. 1984, 19, 427-449. Gearing, J. N.; Gearing, P. J.; Wade, T.; Quinn, J. G.; McCarty, H. B.; Farrington, J. W.; Lee, R. F. In “Proceedings of 1979 Oil Spill Conference (Prevention, Behavior, Control, Cleanup)”;American Petroleum Institute, Environmental Protection Agency, United States Coast Guard: Los Angeles, CA, 1979; pp 555-546. Gearing, P. J.; Gearing, J. N.; Pruell, R. J.; Wade, T. L.; Quinn, J. G. Environ. Sci. Technol. 1980, 14, 1129-1136. Wade, T. L.; Quinn, J. G. Mar. Environ. Res. 1980,3,15-33. Laake, M.; Tjessem, K.; Rein, K. Environ. Sci. Technol. 1984,18,641-647. Received for review May 13, 1985. Revised manuscript received December 2,1985. Accepted January 13, 1986. This research was supported by Grant NA81RAD00015 from the Office of Marine Pollutant Assessment, National Oceanic and Atmospheric Administration.
Characteristics of Atmospheric Organic and Elemental Carbon Particle Concentrations in Los Angeles H. Andrew Gray and Glen R. Cass” Environmental Engineering Science Department and Environmental Quality Laboratory, California Institute of Technology, Pasadena, California 9 1125
James J. Huntzicker, Emily K. Heyerdahl, and John A. Rau Department of Environmental Science, Oregon Graduate Center, Beaverton, Oregon 97006
A fine particle air monitoring network was operated in the Los Angeles area during 1982. It was found that carbonaceous aerosols accounted for typically 40% of total fine particle mass loadings at most monitoring sites. The ratio of total carbon (TC) to elemental carbon (EC) in ambient samples and in primary source emissions was examined as an indicator of the extent of secondary organic aerosol formation. It was found that TC to EC ratios at all sites on average are no higher than recent estimates of the TC to EC ratio in primary source emissions. There is little evidence of the sustained summer peak in the ratio of TC to EC that one might expect if greatly enhanced secondary organics production occurs during the photochemical smog season. The TC to EC ratio does rise by the time that air masses reach the prevailing downwind edge of the air basin as would be expected if secondary organics are being formed during air parcel transport, but the extent of that increase is modest. These results suggest that primary particulate carbon emissions were the principal contributor to long-term average fine aerosol carbon concentrations in the Los Angeles area during 1982.
Introduction Fine carbonaceous particulate matter is emitted from most combustion processes (1-4). These primary carbon 580
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particles consist of organic compounds accompanied by black nonvolatile soot comonents that have a chemical structure similar to impure graphite (5). The black portion of these particulate emissions, commonly referred to as elemental carbon, is a major contributor to visibility reduction in urban areas (6-11), and some investigators recently have suggested that light absorption by elemental carbon plays an important role in the earth‘s radiation budget (12-16). Organic aerosol components present in vehicular emissions (17)and in ambient samples (18)have been found to be mutagenic in the Ames test, and soots have been shown to be carcinogenic in experimental animal studies (19). As a result, there is considerable interest in the behavior of primary carbon particle concentrations in the atmosphere and in how those concentrations might be controlled. Primary emissions of carbonaceous aerosols are not regulated separately from the remainder of the urban aerosol complex. Routine monitoring programs in the United States do not provide data on aerosol carbon concentrations in the atmosphere, and data on the emissions of carbonaceous aerosols are sparse. As a result, information sufficient to support engineering studies of methods for controlling fine carbon particle concentrations is lacking. This control strategy development problem is
0013-936X/86/0920-0580$01.50/0
0 1986 American Chemical Society
