Bioconcentration of lipophilic compounds by some aquatic organisms

ECOTOXICOLOGY

AND

ENVIRONMENTAL

SAFETY

11,

184- 197 ( 1986)

Bioconcentration of Lipophilic Compounds Some Aquatic Organisms

by

DARRYL W. HAWKER AND DES W. CONNELL SchoolofAustralianEnvironmental Studies,Grzjith University,Nathan,4111Queensland, Australia Received September IO, 1985 With nondegradable, lipophilic compounds having log P values ranging from 2 to 6, direct linear relationships have been found between the logarithms of the equilibrium bioconcentration factors, and also reciprocal clearance rate constants, with log P for daphnids and molluscs. These relationships permit calculation of the times required for equilibrium and significant bioconccntration of lipophilic chemicals. Compared with fish, these time periods are successivelyshorter for molluscs, then daphnids. The equilibrium biotic concentration was found to decrease with increasing chemical hydrophobicity for both molluscs and daphnids. Also, new linear relationships between the logarithm of the bioconcentration factor and log P were found for compounds not attaining equilibrium within finite exposure times. 0 I986 Academic mess, IIIC.

INTRODUCTION

Prediction of the bioconcentration of persistent chemicals from water by various aquatic organisms has important applications in the management of hazardous chemicals. The parameter usually used in assessing this behavior is the bioconcentration factor (Ks) which is defined as the ratio of the concentration of the chemical in the organism to the concentration in the surrounding water. This has been found to correlate well with the octanol/water partition coefficient (fl for nondegradable, lipophilic organic chemicals which have achieved equilibrium between the biotic and aqueous phases ( l-5). For example, Mackay (1) has found the following relationships with fish: K a(?&)= 0.048P or

log KWs) = log P - 1.320

ill PI

where K&,, is the bioconcentration factor attained at equilibrium. Up to the present time this relationship has been extensively investigated for fish only, and in particular, freshwater fish, although a recent study (6) has shown it to be applicable to marine species as well. One of the objectives of this present work was to determine whether similar relationships between KWrn) and P exist for other aquatic organisms utilizing existing experimental KWoD)values. In a previous paper (7) we used the above relationships and other data to derive the relationship between uptake and clearance rate constants (k, and k2, respectively) and P for fish. From this, the times required to achieve equilibrium and also significant bioconcentration are calculated. Another objective, in this present work, was to investigate similar relationships for biota, other than fish, and to compare any derived data with that obtained for fish.

0147-65

13186 $3.00

Copyri&t Q 1986 by Academic F’res, Inc. AU righfs of reproduction in any form -cd.

184

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185

THEORY Bioconcentration can be described as the ability of an aquatic organism to concentrate a chemical substance within itself from the surrounding water (8). If the organism and water are both considered as single homogeneous compartments or phases, then the bioconcentration process, for a nondegradable lipophilic compound, can be described by first-order kinetics as

dCB=kC dt

Iw

-kc 2B

where Ca is the biotic concentration, and C, the aqueous concentration of the chemical being bioconcentrated. Assuming C, as constant, the integrated form of Eq. [3] is CB = 2 cw(1 - e-kzt) and therefore, as t becomes larger, C&‘, approaches kl /k2, and at equilibrium Cw,,ICw = Wk2 = &cm), where CBcrn)is the biotic concentration at equilibrium. Mackay (1) found using existing data for fish that KB(mI could be predicted with reasonable precision from P according to Eq. [l] or 123. His theoretical derivation of this relationship applies to aquatic organisms generally and so equations taking a similar form, but with different empirically developed constants, would be expected for related groups of aquatic organisms, i.e. log Kwo3)= log(k,/k2) = a log P + b

[51

where a and b are the empirically developed constants. This expression is usually used in the logarithmic form since this gives more relevant correlation coefficients (1) and can be satisfactorily presented in a graphical form. The ratio of the uptake and clearance rate constants is directly proportional to P” which suggests a possible relationship between kl and k2, separately, and P. Early studies indicated that with fish the magnitude of k2 decreased as the compounds being bioconcentrated became more hydrophobic (i.e., P increasing) (9, 10). In accord with this it has been shown theoretically that a linear relationship should exist between l/k2 and P for fish and in fact, aquatic organisms generally (11). Using existing data from several fish species we have found (7) a correlation of the form log ; = x log P + y where x and y are constants. An expression of this type should be applicable to aquatic organisms generally, and the bioconcentration factor KwrnJis related to P according to Eq. [5], thus the relationship between the uptake rate constant, k, , and P can be shown to be log k, = (a - x)log P + (b - y).

[71

The bioconcentration process, as described by Eq. [4], takes an infinite period of time to achieve an equilibrium situation at which stage CB(,,/C,,. = k, fk2. We can, however, calculate the finite time interval required for the biotic concentration to

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reach effective equilibrium at 99% of that value attained at equilibrium. equilibrium, CB(~) = 0.99CB(,,, and this time period is then &, and

At effective

C B&q) = 0.99 E c, . ( 1

From Eq. [4]

thus 0.99 = 1 - e+b and tw = 4.605 $

or

log tq = log 6 + 0.663.

2

Substituting

2

PI

Eq. [6] into the expression above log tes= x log P + (y + 0.663).

PI

The time period needed to reach equilibrium is therefore controlled only by the magnitude of the clearance rate constant or the related parameter P. Similarly, if a significant bioconcentration is defined as a biotic concentration 1% of that attained at equilibrium, i.e., Cauj = 0.0 1CB(~), then the time required to achieve this concentration t, can be derived in an analagous manner. When significant bioconcentration has occurred k, C B(s)= & C,(l - e-b”“) = O.OlCry,, thus 0.01 = 1 - e-k2ta and 0.01 = k2ts

or

,=O.Ol~. 2

Taking logarithms

of both sides of the equality log t, = log ; - 2.00 2

and therefore log t* = x log P + (y - 2.00). EXPERIMENTAL Table 1 contains experimental wide range of persistent organic values of a-endosulfan, (u-HCH, the literature, but were calculated to the equation log P

DATA

AND

WI

RESULTS

values of log kl , log l/kp, log P, and log I&,) for a compounds with some species of molluscs. The P and 3,4,2’-trichlorobiphenyl could not be found in from their aqueous solubility ( 1, 12- 15) according = -0.862 log S + 0.710

[Ill

where S is the liquid, or supercooled liquid solubility (mol 1-l). It is noteworthy that the table includes data on a diverse range of compounds: polyaromatic hydrocarbons

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187

BIOCONCENTRATION

TABLE 1 BIOACCUMULATIONANDRATE

log k, Compound cu-Hexachlorocyclohexane y-Hexachlorocyclohexane Heptachlor epoxide cu-Endosulfan Dieldrin Endrin

(hr-‘)

0.58 0.50 1.38 1.09 1.31 1.50 DDD 1.72 Benzo[a]pyrene 2,3Dichlorobiphenyl 3,4,2’-Trichlorobiphenyl 2,5,2’S’-Tetrachlorobiphenyl 2,3,2’,5’-Tetrachlorobiphenyl 2,3,4,2’,5’-Pentachlorobiphenyl 2,4,5,2’,4’,5’-Hexachlorobiphenyl Aldrin DDT Dieldrin Endrin Heptachlor y-Hexachlorocyclohexane Methoxychlor Dieldrin Endrin Fenitrothion y-Hexachlorocyclohexane Heptachlorepoxide Fenvalerate Chlordane Dieldrin Kepone Dieldrin Heptachlor DDT DDD

CONSTANTDATAFOR MOLLUSCS

log Wz W

log KN,,

log P

Organism Ref.

1.44 1.50 1.85 1.69 1.89 1.78 2.24 2.74 2.03 1.99 -

2.03 2.00 3.23 2.78 3.20 3.28 3.96 3.08 3.79 3.87 4.04 4.43 4.68 3.66 3.94 3.24 3.09 3.41 1.60 3.18 3.39 3.43 2.11 2.38 2.93 3.67 3.70 3.70 3.84 3.90 3.93 4.68 4.68

3.98 4.81 5.39 5.07 5.48 5.34 6.02 6.42 5.02 6.03 6.09 5.81 6.37 7.75 5.66 6.19 5.48 5.34 5.38 4.81 4.68 5.48 5.34 3.38 4.81 5.39 6.20 6.00 5.48 6.08 5.48 5.38 6.19 6.02

Mussel Mussel Mussel Mussel Mussel Mussel Mussel Mussel Oyster Oyster Oyster Oyster Oyster Oyster Soft clam Soft clam Soft clam Soft clam Soft clam Soft clam Soft clam Oyster Oyster Mussel Mussel Oyster Oyster Oyster Oyster Oyster Oyster Oyster Oyster Oyster

(16) (16) (16) (16) (16) (16) (16) (17) (20) (20) (20) cm (20) (20) (18) (18) (18) (18) (18) (18) (18) (19) (19) (6) (6) (6) (6) (6) (6) (6) (6) (6) (6) (6)

and pesticides with mussels (Mytilus edulis) (6, 16, 17) and PCBs and pesticides with oysters (Crassostrea virginica) and the soft clam, Mya arenaria (6, 18-20). Plots of log KNooj versus log P for some molluscs are presented in Fig. 1 together with the regression line and, for comparison, the regressionline of fish (1). The regression line is represented by the equation log J&a01 = 0.844 log P - 1.235

(r = 0.832).

[I21

With daphnids, experimental values of log k, , log l/kz, log P, and log KRooj for some heterocyclic compounds (10, 2 1, 25), polyaromatic hydrocarbons (22, 23), and pesticides and plasticizers (24,32) are recorded in Table 2. All available I&,) values were

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I

0

Id?I’

1

2

I

I

3

4

I

5

I

6

I

7

,

8

log P FIG.

(16),

1.A plotof log Km,, against log P for molluscs showing theregression line(-) anddatapoints[Ref. 0, (20), l ; (18), A; (19), 0; (6), n ], together with theregression linefor fish(---).

utilized, except for some (32) which were excluded because ( 1) multiple bioconcentration factors differing significantly in magnitude were reported, (2) mixtures of compounds were used, or (3) the experiments had clearly not run long enough for a near equilibrium situation to be attained. The kz values were obtained directly from the cited references or were calculated from the biological half-life (tip = 0.693/k& The only available data not used were

TABLE 2 BIOACCUMULATION

Compound Isoquinoline Acridine Benz[a]acridine Dibenz[a,h]acridine Naphthalene Phenanthrene Anthracene 9-Methyl-anthracene Pyrene Benz[a]anthracene Petylene Anthracene a-Hexachlorocyclohexane DDT y-Hexachlorocyclohexane Dieldrin 1,2,4-Trichlorobenzene 3,5,6-Trichloro-2-pyridinol n,n-Dibutyl phthalate DG(2ethylhexyl)phthalate Phosmet Hexachlorobenzene

AND RATE CONSTANT DATA FOR DAPHNIDS

101~h

(hr-‘) 1.91 2.04 2.32 2.29 2.31 2.85 2.75 3.05 2.83 2.88 2.89 -

log l/h

log I&,)

log P

Organism

Ref.

-1.26 -0.33 0.24

0.38 1.48 2.55

1.82 3.30 4.45

(10) (10) (10)

-0.22 0.27 0.23 0.84 0.46 0.84 0.86 -4.3 x 1o-3 -

3.54 2.12 2.51 2.96 3.66 3.43 4.00 3.85 2.88 2.40 4.15 2.91 3.54 2.15 1.34 3.70 3.72

5.60 3.30 4.45 4.45 5.56 4.90 5.60 6.06 4.45 3.98 6.19 4.81 5.48 4.02 3.21 5.15 5.11

0.78 3.05

2.83 5.50

Pulex Pulex Pulex Pulex Pulex Pulex Pulex Pulex Pulex Pulex Pulex Pulex Pulex Pulex Pulex Pulex Pulex Pulex Pulex Pulex Pulex Pnlex

(W

(21) (22)

(11) (11) (11) (11) (11) (11) (23) I;;; (32) r:;; (32) (32) I:;; (32)

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values of k2 with daphnids for perylene (log P = 6.56) and benz[a]anthracene (log P = 6.11) which were of the same magnitude as that for 9-methyl anthracene (log P = 5.56). It was concluded that the rate constants for perylene and benz[a]anthracene were too small for precise measurement so they were excluded from the correlation. A plot for log J&o?) versus log P is shown in Fig. 2 and the equation for the regression line is log KB(mJ = 0.898 log P - 1.3 15

(r = 0.962).

]131

Plots of log I/k* and log P for both molluscs and daphnids are shown in Fig. 3. Both demonstrated a high degree of linearity and the equations to the regression lines are Molluscs Daphnids

logi

= 0.540 log P - 0.983

log ; = 0.507 log P - 2.053

(r = 0.912)

1141

(r = 0.984).

[I51

DISCUSSION RELATIONSHIP

OF KB TO P

The regression Eqs. [ 121 and [ 131 for molluscs and daphnids, respectively, take the same form as Eq. [5] for the general relationship between log Kwooj and log P expected for aquatic organisms. This relationship has been previously described for fish, e.g. (1,2,4), together with one species of oyster, and mussel (6). These results demonstrate that it is also applicable for other aquatic organisms, in particular some molluscs and daphnids. The correlation coefficients indicate reasonable precision for the log KB values derived from use of Eqs. [ 121 and [ 131 for compounds with log P from ap proximately 2 to 6. With superlipophilic compounds (P > 106), log K&,) values are extrapolations, and the precision of their estimation is less certain. Figures 1 and 2 reveal that the slopes and particularly the intercepts for the three groups (fish, molluscs, and daphnids) are in general very similar. This is despite varying factors such as temperature, nature of lipid, lipid content, age, sex, and metabolic activity of the organisms (26-28). In addition the correlation within each group is

log P FIG. 2. A plot of log Km,, against log P for daphnids showing the regression line (-) and data points [Ref. (10,21,22), X; (23), 8; (24), H ; (32), 01, together with the regression line for fish (---).

190

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log P

FIG. 3. A plot of log I/k* against log P for molluscs [Ref.(la), A; (19),0; (17),0) anddaphnids[Ref. (10,22),X; (23),@I,togetherwith theregression linefor fish.

surprisingly good for such diverse species groups as mussels, oysters, and soft clams with molluscs. It is apparent, however, that the correlation between log KB and log P is poorer for the molluscs as compared to Daphnia pulex. These relationships will clearly not be applicable to all fish, molluscs and crustacea. For example, bioconcentration factors for the hard clam, Mercenaria mercenaria, are quite different to those of the soft clam, Mya arenaria, obtained under exactly the same conditions (18). The range of chemicals for which the relationships above are applicable is limited to those which are lipophilic, persistent, unionized, and nondegradable. However, there is a variation in the ability of different organisms to metabolize some types of compounds. If some groups of organisms have the capacity to metabolize certain types of compounds which other organisms cannot metabolize, then these relationships will only apply to the group which lacks this metabolic activity. For example, polaromatic hydrocarbons are degraded by fish but are unaltered by daphnids (21,25) and so the relationships, in this case, are applicable to daphnids only. RELATIONSHIP OF RATE CONSTANTSTO P

A direct linear relationship between l/k2 and P for fish has been predicted and experimentally demonstrated for a limited range of compounds with one species (Carassius aura&s) (1 I). Similarly, Spacie and Hamelink (29) found an inverse linear correlation between log kz and log P. Extending both the range of compounds and number of species, we have shown a direct linear relationship between log l/k* and log P for fish (7) as (see Fig. 3) log t = 0.663 log P - 0.947. This takes the general form of Eq. [6] and provides a reasonable estimate of k2 for compounds with log P values Corn 2 to 6, and by extrapolation of kz for superlipophilic compounds. Log l/k2 is also inversely linear related to log P for some molluscs and for daphnids as demonstrated previously; see Eqs. [ 141 and [ 151 and shown in Fig. 3. The plots in Fig. 3 are extrapolations .for compounds possessing log P above 6 to

AQUATIC ORGANISM LIPOPHILIC BIOCONCENTRATION

191

6.5. In these cases, the clearance rate constant, k2, becomes too small to be measured by current techniques. Also there is an enhanced response of k2 to stereochemical factors in large molecules with high partition coefficients (30). Thus superlipophilic compounds may have k2 values smaller than predicted, and therefore l/k2 will be larger. Nonetheless, we believe that in most cases, these relationships provide a reasonable measure of the magnitude of kz . The expressions for the relationship of k, to P can be obtained using the general Eq. [7] with empirical constants derived from Eq. [ 121 to [ 151. Thus Daphnids

log kl = 0.391 log P + 0.738

[171

Molluscs

log k, = 0.304 log P - 0.252.

1181

These equations, together with that previously reported for fish (7), provide a method for calculation of k,, for these groups of organisms, from log P in the range 2 to 6. Values can be obtained for superlipophilic compounds by extrapolation. TIME PERIODTO

EQUILIBRIUM

AND SIGNIFICANTBIOACCUMULATION

For convenience, we have previously defined the time period required for the biotic concentration to reach 99% of its true equilibrium value as tw, the time required for attainment of effective equilibrium. This period is dependent only upon kz [Eq. [SJJ which in turn can be related to P [Eq. [9]]. Substituting the empirical constants derived from Eqs. [ 141 and [ 151 in Eq. [9] the following expressions were obtained relating tes to P: Daphnids

log b = 0.507 log P - 1.390

1191

Molluscs

1% tes = 0.540 log P - 0.320.

[W

These relationships together with that for fish (7) are shown in Fig. 4. Thus, knowledge of the P value for a given compound allows the calculation of tes for some fish, molluscs, and daphnids. It is immediately apparent that for any value of log P, equilibrium is reached quickest with daphnids, and successively more slowly with molluscs, then fish. Practical use can be made of these relationships by determining the log P above which equilibrium will not be attained. A representative organism can be selected from each of the three groups and the log P value derived from their maximum life spans. For example, the maximum recorded life span for the goldfish (C. uurutus) is 30 years (31) which indicates (from Fig. 4) that within the life span of the goldfish, equilibrium will only be achieved for chemicals with log P G 8.6. Similarly for the clam, Mya arenaria, whose maximum recorded life span is 8 years (3 l), the threshold log P value is 9.6. With the relatively short-lived Duphniu magna [ - 100 days (3 l)] the threshold log P is 9.4. The threshold log P values for the three representative organisms are approximately the same because the differing time spans for equilibrium attainment are balanced by the life spans. It is unlikely that this would be the case when all species of the three groups are considered because of their variable life spans. As well as tcs, it is important to know whether any significant bioconcentration has occurred within a given exposure period. We have arbitrarily considered significant bioconcentration to be a biotic concentration 1% of that attained at equilibrium, and

192

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AND CONNELL

FISH

t

Representative

/

/ MOLYSCS

log P

FIG 4. The relationship between the logarithm of the time period for effective equilibrium and log P also depicting threshold log P values above which effective equilibrium will not be attained for the representative organisms (Curussius aurutus, Myu urenuria, and Duphniu mugna).

the time period required for this is represented by ts. Substituting empirical constants derived from Eqs. [ 141 and [ 151 into Eq. [lo] the relationships between ts and P can be derived as Daphnids log t, = 0.507 log P - 4.053 [211 Molluscs

log ts = 0.540 log P - 2.983.

v21

Plots of log tsversus log P are found in Fig. 5 and include that for fish derived previously (7). As expected, the curves follow the same trends as those in Fig. 4. Using the same

FIG. 5. The relationship between the logarithm of the time period for significant bioconcentration and log P also depicting t&&old log P values above which significant bioconcentration will not occur for the representative organisms (Curussius aurutus, Myu arenariu, and Daphnia mugnu).

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193

representative species (C. aurutus, M. arenavia, and D. magna) as before log P values above which significant bioconcentration will not occur within a lifetime are 12.6, 14.5, and 14.7, respectively. RELATIONSHIPOFC~TOPATSPECIFICEXPOSURE

TIMEPERIODS

Knowledge of uptake and clearance rate constants enables the calculation of the biotic concentration (Ca) and bioconcentration factor (Ka) following a specific exposure time for the species in the three groups investigated. This is important since most organisms will not be exposed for a life span as considered above. Since P is related to aqueous solubility (S) according to Eq. [ 111 [note that a different expression for S (mol 1-l) is needed depending on whether the compound is a solid or a liquid], and log KNrnj = log P - 1.32 for fish (l), then log CB(r,,)- log S = 1.OO log P - I .320.

~31

From Eq. [I l] logs=-1.160logP+0.824+logF

t24]

where F = eAsf’R(‘-TM’T) for solids. [TM is melting point (K) and T is the system temperature (K)]. The term F is equal to 1 for liquids. As an approximation ( 1, 14) A&/R is considered to be 6.79 for all compounds. Combining Eqs. [23] and [24] log &,)

= -0.160 log P - 0.496 + log F.

1251

Thus Cwrnj (in mol 1-l) has a dependence on the hydrophobicity of a chemical as measured by P. For liquids and supercooled liquids, there is an inverse linear relationship between log C&,) and log P. With molluscs and daphnids, the situation is similar. Using the molluscs as an example, log Kecoo,= 0.844 log P - 1.235, and therefore from Eq. [24] logC&=

-0.316logP-0.411

+logF.

WI

A similar equation can be derived by an analogous procedure for daphnids using Eq. [ 131. Plots of these relationships (for liquids or supercooled liquids only) are shown in Fig. 6. This means that compounds having increased lipophilicity exhibit lower equilibrium biotic concentrations and take longer to attain that concentration. If an uptake experiment is conducted over a specific time span some of the more lipophilic chemicals may not reach equilibrium, resulting in biotic concentrations which could be exceedingly small. In addition, considering possible reducexl membrane permeability due to size or shape, even lower concentrations may occur. The biotic concentrations would be less than those shown at equilibrium in Fig. 6 for the three groups of biota. Given these circumstances, the amount of some super-lipophihc compounds in biota would be below detection thresholds, and it would appear as if these compounds were not bioconcentrated at all. Sample Calculation

of CB after u SpeciJic Exposure

Time Period

Assume a mollusc of wet wt 500 g and density 1000 kg me3 is exposed to a solid chemical of MP 15O”C, with a P value of 10’ and mol wt 200 at its maximum aqueous solubility for 12 hr. From Eq. [26]

194

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AND CONNELL

I

I

I

I

1

1

1

2

4

6

6 100 P

10

12

14

FIG. 6. The relationship between log Cecm,and log P for fish, mokcs,

and daphnids.

Cwm)= 6.67 X 10e5 mol 1-i. Now CB = C&&l

- e-k”)

and utilizing Eq. [ 141 and values above, then CB = 3.67 X lo-’ mol 1-l. Using the molecular weight of the compound and the weight of the mollusc the biotic concentration is only 73 ppb. RELATIONSHIP

OF NONEQUILIBRIUM

KB TO P

Although CNrnj decreases with increasing hydrophobicity for both the molluscs and daphnids, (see Fig. 6), Kr+,) increases because relative to P the aqueous solubility decreases at a faster rate than CR.+ If all correlatable compounds that satisfied the criteria of lipophilicity, stability, nonionizability, and nondegradability reached equilibrium and the KB values measured then a plot of log &++tl) versus log P would be linear (see Figs. 1 and 2). If, however, Ka values are measured after a limited period, then for some compounds, equilibrium and hence their maximum potential KB, i.e., KB(+ will not be attained. The higher the value of P the greater the deviation from the infinite exposure linear log &/log P relationship. We have earlier found that, for fish, this relationship eventually takes a new linear form with specific exposure time (7). With molluscs and daphnids, the situation is similar, as shown in Fig. 7 where log KB values, alter a OS-year exposure, are plotted against log P. These calculations are based upon Eqs. [ 121 and [ 131 and

where Kwexptljis that bioconcentration factor actually attained during the exposure time, and KB(ex,,tij/KB(,,,j is the fraction of the bioconcentration factor at equilibrium achieved. The log Kwcxptl)values follow the linear infinite time exposure (i.e., equilibrium attained) plot until log P = 5.9 (fish), 7.3 (molluscs), and 9.9 (daphnids). For compounds with higher log P values the exposure period of 0.5 years is insufficient to achieve equilibrium, and a new linear relationship with log P is eventually observed, since under these conditions only the first two terms of the exponential expansion are significant (see Fig. 7). The slopes and intercepts of the new relationship are different for

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FIG. 7. A plot of log Kwmtij against log P following a OS-year exposure period for fish, molluscs, and daphnids. The broken line represents KB values attained if equilibrium is established, i.e., KB(+

each biotic group, and depend only on the constants of the log K,,,-log P and log l/&-log P relationships, as well as the exposure time (33). It is interesting to observe that for chemicals not attaining equilibrium with any group, KBo values for daphnids are in fact greater than those for fish. But had equilibrium been attained, then the reverse would have been the case. Because of the possible differential response of kr and k2 to the steric effects of large molecules (usually with high log P values), it is possible that from experiments of finite exposure time measuring Ka directly, the log KB(Wptlj-log P plots could approach a parabola as has been proposed (34) and observed (35). CONCLUSIONS The linear correlation between log Ka and log P, which has been employed successfully for some fish species, is also applicable to some molluscs and daphnids over the log P range from 2 to 6. These equations are log&,,

= 0.844logP-

1.235 (molluscs)

log KWcoj = 0.898 log P - 1.315 (daphnids). However, it is unlikely that all fish, molluscs, and crustacea will conform to these relationships. Those fish, molluscs, and daphnids that do also exhibit a linear relationship between log l/k2 and log P log l/k2 = 0.540 log P - 0.983 (molluscs) log l/k2 = 0.507 log P - 2.053 (daphnids). These equations allow the development of expressions for k,, time to equilibrium (t& time for significant uptake (t,), and biotic concentration at equilibrium for these organisms. The expressions for tcs are 1% tea = 0.507 log P - 1.390 (daphnids) log t, = 0.540 log P - 0.320 (molluscs).

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AND CONNELL

Use of representative maximum life spans for a species within each of the biological groups considered, indicated that compounds with log P > 8.6, >9.6, and >9.4 would not achieve equilibrium in the life span of goldfish, the clam (M. arenaria), and D. magna, respectively. The biotic concentration at equilibrium declines with both mollusts and daphnids with increasing hydrophobicity (i.e., P). With superlipophilic compounds (log P > 6) the linear relationships between log I&) and log P previously observed generally don’t apply. This is due to the need for lengthy time periods to establish equilibrium. Exposure times are generally less than this leading to lower KB values than expected. New linear relationships are established which are dependent on exposure time. The principles discussed in this manuscript relating to the bioconcentration of persistent chemicals in water are not only applicable to the management of hazardous chemicals, but may be very useful in other kinds of safety evaluation involving drugs or chemicals. ACKNOWLEDGMENT The authors gratefully acknowledge the award of an Australian Marine Science and Technology Research grant to carry out this work.

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