Environ. Sei. Techno/. 1989, 23, 961-965
(15) Reinhard, M.; Goodman,N. L.; Mortelmans,K. E. Environ. Sei. Technol. 1982, 16, 351-362. (16) Osburn, Q. W.; Benedict, J. H. J. Am. Oil Chem. SOC.1966, 43, 141-146. (17) Cain, R. B. In Microbial Degradation of Xenobiotics and Recalcitrant Compounds; Leisinger, T. et al., Eds.; Academic Press: London, England, 1981; pp 325-370. (18) Swisher, R. D. In Surfactant Biodegradation; Marcel Dekker Inc.: New York, 1987. (19) Baggi, G.; Beretta, L.; Galli, E.; Scolastico, C.; Treccani, V. In The Oil Industry and Microbial Ecosystems; Chater, K. W. A., Somerville, H. J., Eds.; Institute of Petroleum (Heyden & Son): London, England, 1978; pp 129-136. (20) Kravetz, L.; Guin, K. F.; Shebs, W. T.; Smith, L. S.; Stupel, H. Soap, Cosmet., Chem. Spec. 1982, 58, 34-42, 102B. (21) Owen, W. F.; Stuckey, D. C.; Healy, J. B.; Young, L. Y.; McCarty, P. L. Water Res. 1979, 13, 485-492. (22) Stephanou, E.; Reinhard, M.; Ball, H. A. Biomed. Environ. Mass Spectrom. 1988, 15, 275-282. (23) American Public Health Association In Standard Methods
for the Examination of Water and Wastewater, 15th ed.;
American Public Health Association: Washington, DC, 1981.
(24) Sawyer, C. N.; McCarty, P. L. In Chemistry for Environmental Engineering; McGraw-Hill: New York, 1978. (25) Knackmuss,H. J. In Microbial Degradation of Xenobiotics; Leisinger, T. et al., Eds.; Academic Press: London, 1981; pp 189-212. (26) Watson, G. K.; Jones, N. Water Res. 1977, 11, 95-100. (27) Pearce, B. A.; Heydeman, M. T. J. Gen. Microbiol. 1980, 118, 21-27. (28) Haines, J. R.; Alexander, M. Appl. Microbiol. 1975, 29, 621-625. (29) Schink, B.; Stieb, M. Appl. Environ. Microbiol. 1983,45, 1905-1913. (30) Patterson, S. J.; Scott, C. C.; Tucker, K. B. E. J. A m . Oil Chem. SOC.1970, 47, 37-41. (31) Swisher, R. D. J. A m . Oil Chem. SOC.1962,40,648-656. (32) Mohanrao,G. J.; McKinney, R. E. Znt. J. Air Water Pollut. 1962, 6, 153-168. ~~
Received for review May 31,1988. Revised manuscript received March 8, 1989. Accepted April 18, 1989. The work presented in this paper was supported by a grant from the National Science Foundation (CEE-81-17561).
A Simple Water/Octanol Partition System for Bioconcentration Investigations Darryl W. Hawker' and Des W. Connell
Division of Australian Environmental Studies, Griffith University, Nathan, Queensland 41 1 1, Australia By use of a simple shake flask system, the first-order water to 1-octanol transfer rate constants have been experimentally determined for a series of chlorobenzene congeners and n-alkanes, whose 1-octanol/water artition coefficients (KO,)range from approximately 10 to 106.s. The magnitudes of these rate constants are essentially independent of the octanol/water partition coefficient, indicating that aqueous-phase diffusion is the controlling factor for these solutes. Over a wider log KO,range, a curvilinear relationship would be expected between the logarithms of water to octanol transfer rate constants and log K, The logarithms of water to lipid transfer or uptake rate constants for aquatic organisms have been found to be a similar curvilinear function of log KO,. It follows that a correspondence also exists in relationships between the logarithms of octanol to water transfer rate constants and log KO,and the logarithm of clearance rate constants and log K ,. The essential similarities of mass transfer kinetics in abiotic and biotic partitioning systems are summarized and suggest that abiotic systems may be a useful alternative to biological indicators in some monitoring situations.
B
Introduction Physicochemical properties are being increasingly used to predict the environmental behavior of compounds that may be discharged into the environment. Of these properties, the 1-octanol/water partition coefficient (KO,)is particularly useful. It has been used with persistent lipophilic compounds for the evaluation of aquatic toxicity and for predicting the extent of partitioning processes such as bioconcentration by aquatic organisms and sorption onto soils and sediment (1-9). It is relatively easily measured by a variety of techniques such as reverse-phase thin-layer chromatography and high-performance liquid chromatography, shake flask, and generator column methods (10-14). A major focus for work up to the present has been the relationship between KO,and the bioconcentration factor 0013-936X/89/0923-0961$01.50/0
KB (the ratio between the concentration in the organism and the concentration in water) (e.g., ref 3). These values must be measured at equilibrium to obtain relevant data, and when expressed on a lipid weight basis, KB values are often very close to KO,values for K, between 102 and (15). In recent work, we have found apparent relationships between the overall first-order uptake and clearance rate constants for bioconcentration and log KO,,as well as between the logarithm of the time required to attain an effective equilibrium for the bioconcentration process and log KO,(16). Thus, the kinetics of the processes involved have a major influence on the values obtained for KB if equilibrium is not attained, as occurs in some situations (16, 17).
Simple abiotic transfer models have previously been considered analogous to the bioconcentration of compounds from water by aquatic organisms, since bioconcentration has been described as a partitioning between an organism's body fat and water (18). In addition, Sodergren (19) found that dialysis membranes filled with hexane concentrate via a passive solvent-solvent partitioning process in a manner qualitatively similar to biota. Thus, abiotic partitioning studies may be used to confirm bioconcentration mechanisms and to predict quickly and effectively relative bioconcentration potential of nondegradable contaminants. The present work was initiated with a view to investigating whether any correspondence or similarity exists in relationships between uptake and water to octanol and between clearance and octanol to water rate constants and Kow. Most previous studies on water/octanol transfer rate constants have involved relatively hydrophilic solutes and have not been extended to hydrophobic chemicals capable of significant bioconcentration. A series of n-alkanes and chlorobenzene congeners were chosen for investigation in this model system because they are two structurally different groups, but each provides an appropriate range and sequence of K , values that can be used to evaluate any correspondence.
0 1989 American Chemical Society
Environ. Sci. Technol., Vol. 23, No. 8. 1989
961
I
Theoretical Background In a two-phase octanol/water system, the flux ( N , mol s-') from the aqueous phase to the octanol phase can be described in terms of simple first-order kinetics or fugacity. Because the process is basically a mass transfer one however, the Whitman two-resistance model initially provides the simplest description of the interphase flux (20). By this approach
N = V,(-dC,/dt)
- C,/K,,)
= k,&(C,
(1)
where k, is the overall mass transfer coefficient (m s-'), A the interfacial area (m2),C , and C, the aqueous and octanol concentrations of the solute (mol m-3), V, the aqueous-phase volume, and KO, the octanol/water partition coefficient. In the initial stages of transfer from water to octanol, when most of the chemical is still in the aqueous phase, C, > C,/K,,, and therefore eq 1 can be approximated by V,(dC,/dt)
= -k,&C,
(2)
To relate the mass transfer coefficients to first-order rate constants, interphase flux can also be described by dC,/dt = k2C0 - kiC,
(3)
where kl is the water to octanol transfer rate constant and k2 the octanol to water transfer rate constant. Initially k2C, > D,. The overall water/octanol transport coefficient Do, is related to Do and D, by
\ o
-1 3
5
10 15 20 Time (s)~ 1 0 . ~
25 E=
Figure 2. Typical plot of log concentration in the aqueous phase (C,) against time in the shake flask experiments for 1,4dichlorobenzene.
Table I. Experimental k,,log kl,and log KO,Values for Chlorobenzenes and n -Alkanes substrate
104kl,
2.71 f 1.49 chlorobenzene 1.47 f 0.42 l,4-dichlorobenzene 1.03 0.47 1,3,5-trichlorobenzene 1,2,4,5-tetrachlorobenzene 0.96 f 0.37 0.80 f 0.17 n-octane 1.87 f 0.96 n-nonane 1.25 f 0.27 n-decane 0.83 0.38 n-undecane
*
*
1
>> koAZo
log kl log Kow ref -3.57 -3.83 -3.99 -4.02 -4.10 -3.73 -3.90 -4.08
2.98 3.38 4.02 4.51 5.18 5.70 6.22 6.74
24 24 24 24 24 a
a Q
'Extrapolated from data in ref 24.
reporting integrator. The concentrations of the stock aqueous solutions were determined by extraction into hexane, spiking with the appropriate internal standard, and subsequent analysis as above. Analysis for ChlorobenzeneCongeners. The octanol solutions were analyzed periodically by UV spectroscopy using a Varian Model 635-M spectrophotometer. The wavelength employed corresponded to a maximum absorption of the particular congener involved. The initial aqueous concentrations were similarly determined after extraction into hexane. All concentrations in octanol and hexane solutions were derived from individual Beer-Lambert plots.
Results A typical experimental plot of In C, against time is shown in Figure 2. Using the regression line to calculate the slope of these plots, values of kl have been determined as the means of five individual experiments in all cases except for n-octane, where three determinations were made. These values were calculated for a series of n-alkanes (nCB_ll),and a series of chlorobenzene congeners (chlorobenzene, l,bdichlorobenzene, 1,3,5-trichlorobenzene, and 1,2,4,5-tetrachlorobenzene).Differing octanol volumes (15-50 mL) were used to facilitate analysis. Experimental kl and log k l values are found in Table I and are compared with log KO,values obtained or derived from the literature (24). It is apparent that the log kl values are relatively constant, even though the octanol/ water partition coefficient changes by almost 4 orders of magnitude. From eq 8, log klis constant when ( V,/Q,)K, >> V,/Qo and equal to log (Qw/Vw).Since the mean of the kl values for all solutes is 1.37 X lo4 s-l and V, is 5 X lo4 m3, Q,, the effective solute flow rate in the aqueous phase, is 6.85 X m3 s-l. Discussion The water to octanol transfer rate constant (k,)may be expressed in terms of aqueous phase volume, effective flow
D,
(10)
For the chlorobenzene and alkane solutes examined, the transfer process is dominated by the small aqueous phase transport coefficient. From eq 8 it would be predicted that k, would be linearly related to KO, in similar abiotic partitioning systems by for more hydrophilic solutes (lower K,,). A study of octanol/water partitioning rates of quaternary alkylammonium bromides by Kubinyi (25)showed k1 linearly related to KO,for solutes with log KO,< 0.56. In this case, transport is dominated by a small octanol phase transport coefficient. Overall then, depending on the KO,value of the solute, partitioning between octanol and water can be controlled by transport through either phase. This results in curvilinear relationships between log (l/k2) and log KO, as well as log kl and log KO,, and such relationships have previously been observed in abiotic systems by a number of workers (25-27). Mackay (21) found that bioconcentration by fish on a lipid basis could be described in terms of aqueous-phase and lipid-phase diffusion flow processes by the following equations for first-order uptake and depuration rate constants, respectively:
hi = K o w / ( V ~ Z ~ / Q+JVwL Z L / Q J J
(12)
l / k z = VLZL/QJ, + VLZL/Q~&
(13)
where VL and ZL are the volume and fugacity capacity constant of the lipid phase. Depending on the hydrophobicity of the solute, transfer is controlled by transport through lipid or through aqueous phases, again resulting in curvilinear relationships between log kl and log KO,,as well as log (l/kz) and log KO, (see Figure 1) (28). The above equations may be compared with eq 8 and 9 for the abiotic partitioning system. If octanol is a perfect model for biotic lipid and 2, = Z, then the equations become very similar indeed. The equations presented in Table I1 summarizing the essential similarities of biotic and abiotic partitioning systems are derived on the assumption that ZL = 2,. Recent work has suggested that this assumption may be invalid for extremely hydrophobic (log KO, > 6.5) compounds (29). Phase mass transfer coefficients may be related to phase diffusion coefficients di (m2s-l) by iti = di/Axi where Axi is the diffusion layer thickness (m). Since Qi = kiA, the water to octanol transfer rate constant can be expressed as
KOWA
kl = ~
w
~
~
~
w
+/ A x~ o / dw o ) ~
~
o
(14) w
For relative hydrophilic solutes, interphase transfer is Environ. Sci. Technol., Vol. 23, No. 8, 1989
963
Table 11. Summary of Uptake (k,)and Inverse Clearance ( l/k2) Rate Constants for Abiotic and Biotic Partitioning Systems, Based upon Fugacity and Diffusion abiotic Uptake ( k , )
KO,
-+-
QZwQozo
octanol or membrane diffusion control
K o ~ ~ o
TI "wy-zo A-
Ad, aqueous phase diffusion control Vwkw Clearance (l/&
-+Qwzw V
fugacity octanol or membrane diffusion control
J
O
VZO
VL~L VL~L -+-
Qozo
Q d w
QJo
VwAxo
-
Ad0 aqueous hase diffusion KowVwAxw controt' Ad,
under octanol phase diffusion control, and therefore, do
