Relationship between bioavailability and hydrophobicity: Reduction of the uptake of organic...

Environmental Toxrcology and Chemrstry, Vol 9 , pp 715-724, 1990 Printed in the USA Pergamon Press plc

0730-7268/90 $3 00 + 00 Copyright 0 1990 SETAC

RELATIONSHIP BETWEEN BIOAVAILABILITY AND HYDROPHOBICITY: REDUCTION OF THE

UPTAKE OF ORGANIC CHEMICALS BY FISH DUE TO THE SORPTION ON PARTICLES

S. MARCA SCHRAP" and ANTOON OPPERHUIZEN Research Institute of Toxicology, Environmental Toxicology Section, University of Utrecht,

3508 TD Utrecht, The Netherlands

(Received 20 July 1989; Accepted 25 October 1989)

Abstract -The uptake of five chlorinated benzenes and three polychlorinated biphenyls by guppies (Poecilia reticulafa) from a sediment suspension has been investigated. To examine the availability of these chemicals, the uptake from water has been compared to that from a sediment suspension. In the two experiments, i.e., with and without sediment being present, the total amount of the test compounds was the same. The only difference was that the chemicals were purely dissolved in the water in one system and partly sorbed on the sediment in the other.

For all five chlorobenzenes bioconcentration factors are found to be reduced when fish are exposed in a sediment suspension. The reduction increased with increasing hydrophobicity of the compounds. A relationship is found between the amount sorbed on the sediment and the reduction of the bioconcentration factor. The same relationship was found from results of studies reported in the literature.

This study also showed that small amounts of sediment can be present in the intestines of the fish when the fish are exposed in a sediment suspension. Depending on the affinity of the chemical for the sediment, the concentrations of the test compounds measured in the whole fish can be significantly influenced.

Keywords - Bioavailability Hydrophobic compounds Fish Bioaccumulation

.

INTRODUCTION

For many persistent organic chemicals it has been shown that they can accumulate to high concentrations in aquatic organisms relative to the concentrations in their environment. Uptake from water [l-51 as well as from food [1,6-81 has been demonstrated. Because of the hydrophobic nature of these chemicals, not only accumulation in aquatic organisms but also sorption on soils and sediments seems to be an important process that determines the fate of these compounds in the aquatic environment [9-1 I].

In many studies it has been demonstrated that the accumulation of chemicals in organisms can be affected by the presence of particles in the aqueous phase, such as sediment, humic acids and other dissolved organic materials (DOM).

In most studies a reduction of the uptake of the chemicals by the presence of particulate matter has been found [12-161. Relationships have been found between the binding of the compounds to dissolved organic material and the reduction of the uptake of

*To whom correspondence may be addressed

some polycyclic aromatic hydrocarbons in Ponto poreia Hoyi [I71 and Daphniu [18,19] and a tetrachlorodibenzo-p-dioxin and Crungonyx laurentianus [20]. In addition, reduction of the toxicity of some synthetic pyrethroids to fathead minnows [21] and some cationic polyelectrolytes to Duphnia and fathead minnows [22] has been found when clay or dissolved organic carbons were present in the aqueous phase. In all these studies it was suggested that a reduction of the uptake of the chemicals is caused by a reduced availability of the compound due to the sorption on particles.

However, not all experiments are as conclusive as described in previous studies. For instance, in a study with Duphniu the uptake of methylcholanthrene was observed to be increased by humics, while in the same study a reduction of the uptake of some other polycyclic aromatic hydrocarbons was found [23].

In other studies an increase rather than a reduction of the uptake by the presence of sediment in the aqueous phase was reported for some aquatic invertebrates (benthic amphipods [24] and Chirono- mus tentuns [25]) and a hexachlorobiphenyl among others. Also in studies with yellow perch [26], fat-

715

716 S. M. SCHRAP AND A. OPPERHUIZEN

head minnow 1271, Leiostomus xanthurus 1281 and bluegill sunfish [29] an increased uptake of polychlorobiphenyls, hexachlorobenzene and benzo[a]- pyrene among others, was found by the presence of sediment.

In the present study the uptake of some chlorobenzenes from a sediment suspension by guppies is investigated. In two sets of experiments, the uptake of chemicals by fish from a sediment suspension was compared with that by fish from a system where no sediment was present. In the first set of experiments the concentrations of the test compounds in whole fish from the system without sediment and fish from the sediment suspension were compared. In the second set of experiments the contribution of the presence of sediment in the intestinal tract of the fish was investigated.

MATERIALS AND METHODS Chemicals

As test compounds were used 1,2,3-tri-, 1,3,5- tri- (Merck, Germany), 1,2,3,4-tetra-, penta- (Al- drich, Milwaukee, WI) and hexachlorobenzene (BDH, England), 2,4,4'-tri-, 2,2'5,5'-tetra- and 2,2',4,4',5,5'-hexachlorobiphenyl. The polychlorinated biphenyls (PCBs) were available from previous studies [30]. All chemicals were more than 95% pure. Structures and purity of the PCBs were confirmed by GC-ECD, GC-MS and NMR analyses. n-Hexane (Janssen) was used as an organic solvent after redistillation.

Laboratory-bred male guppies (Poecilia reticulata) of about 150 mg and 18 mm in experiment I and about 300 mg and 24 mm in experiment I1 were used. Fish had a fat content of approximately 5%. A day:night cycle of 12:12 h was imposed with daylight fluorescent lamps. During the experiments the fish were fed with Tetramin@ fish food. No detectable background contamination by the test compounds was shown in the fish.

Sediment A natural lake sediment (Oostvaarders Plassen)

from The Netherlands was wet sieved (1 mm) and stored under water at 5°C. The sediment was oven dried at 200°C before use. The organic carbon content was determined to be 3% (w/w) (based on dry weight, 40% (w/w)). Background contamination was less than 10 ng/g for 1,2,3-tri-, 1,3,5-tri- and 1,2,3,4-tetrachlorobenzene, less than 1 ng/g for penta- and hexachlorobenzene, 2,4,4'-tri- and 2,2', 5,5'-tetrachlorobiphenyl and 3 ng/g for 2,2',4,4', 5,5'-hexachlorobiphenyl.

Chemical analyses Fish (two fish each sample) were killed by im-

mersion in liquid nitrogen and frozen until analysis. The fish sample was homogenized in a mortar and after addition of 50 ml water and 50 ml hexane heated under reflux for 90 min. The hexane layer was removed after centrifugation and concentrated under a gentle nitrogen stream to about 5 ml. This extract was eluted through silica-H,S04 and silica-NaOH columns to remove lipids and other polar compounds. The sample was concentrated again before analysis [I].

Spiking clean fish with 1 ml of a solution of the test compounds in hexane resulted in recoveries of >95% for all the used chlorobenzenes and chlorobiphenyls.

Water samples were extracted with hexane and concentrated under a gentle nitrogen stream to 1 ml. The samples were analyzed by GC-ECD. The sediment suspension samples were treated in the same way.

Spiking clean water or a clean sediment suspension (10 ml) with 1 ml of a solution of the test compounds in hexane resulted in recoveries of >95% for all the used test compounds.

The fish, water and sediment suspension samples were analyzed on a TRACOR 550 gas chro- matograph equipped with a linearized 63Ni electron capture detector (packed column, 2% Dexil 300 Chromosorb W HP 80-100 mesh, 2.4 m x 2 mm, argodmethane (95%/5%) as carrier gas). The detector was connected to a Shimadzu, Chrnmpack C-R2AX integrator. An external standard method was used for quantification. The detection limit was approximately 100 pg. Uncertainties in sample weight or volume and GC quantification yielded an estimate of + lo% error in fish, water and sediment suspension concentrations.

Experiments Experimental system. Water (2/3 Amsterdam

tap water and 1/3 demineralized water) was contaminated using a generator column as previously described 1311 and divided over two identical experimental vessels (Fig. 1). Each vessel and pump constituted a closed system with essentially no headspace for air. Oven-dried sediment was then added to one of the two vessels with a sediment: water ratio of 700 mg sediment (dry weight) per liter water. So the total amount of the test compounds in the two vessels was equal, whether sediment was present or not. To keep the sediment in suspension, the water was circulated by a pump as indicated in the figure. An air inlet provided for the oxygen demand. Forty-eight hours after the

Relationship between bioavailability and hydrophobicity 717

Fig. 1. Schematic representation of experimental system.

sediment was added, fish were added to both systems at the same time.

Control experiment. In a control experiment 15 fish are exposed to a sediment suspension of 700 mg sediment (dry weight) per liter for 17 d. No test compounds were present.

Experiment I . In the first experiment five chlorobenzenes (1,2,3-tri-, 1,3,5-tri-, 1,2,3,4-tetra-, penta- and hexachlorobenzene) were used as test compounds. The contaminated water was diluted before use to avoid lethal toxic effects to the fish during the exposure period. So the concentrations of the test compounds in the aqueous phase were chosen so that the total concentration of the chlorobenzenes in fish would not exceed 2 pmol per gram fish [32].

Two groups of 30 fish were exposed in two vessels, one with a contaminated sediment suspension (A.1) and one with contaminated water without sediment (B.1). During the exposure period of 290 h 11 fish samples were taken from both systems. At the same times 10 ml water or 10 ml sediment suspension samples were taken from vessels B.1 and A.1, respectively. At the end of the experiment a 750-ml water sample from vessel B.1 and two 200- ml sediment suspension samples, one from the upper and one from the lower part of vessel A.1, were taken.

To control the loss of the chemicals from the system by volatilization or degradation by microor- ganisms, a third vessel (C.1) only contained a contaminated sediment suspension. Simultaneous with the fish and water samples in experiment A.1 and B.1, 11 samples of 10 ml of the sediment suspension were taken from this system.

Experiment II. In the second experiment penta-

and hexachlorobenzene, 2,4,4‘-tri-, 2,2’,5,5’-tetra- and 2,2’,4,4’,5,5’-hexachlorobiphenyl were used as test compounds. In this experiment four fishes were added to each of the two vessels, a sediment suspension (A.11) and one vessel without sediment (B.11). After 3 d the fish from both experiments were sampled and dissected. Because of the small size of the guppies, all organs of the fish, includ- ing gills, liver, intestinal tract, etc., were pooled and hereafter referred to as the intestines of the fish. These intestines were analyzed separately from the “rest” of the fish.

Theory To describe the bioconcentration of hydropho-

bic organic chemicals from water by fish, a simple model with first-order kinetics is often used [2]:

Here, k l (L.kgg‘.dg’) and k2 (d-’) denote the uptake and elimination rate constants. According to this model the concentration changes of the chemicals in the fish can be expressed by:

with C, the dissolved concentration of the test compound in the water (pg.Lg’) and C, the concentration in the fish (pg.kg-’). At a steady-state situation, dCJdt = 0, the bioconcentration factor K, (L . kg-I) is defined by:

Analogous to this model, a model for the bioconcentration of chemicals in fish from a sediment suspension is given by:

1 I

r---------

I I I +

Here the uptake rate constant k; (L.kg-‘ .d-’) refers to the uptake of the chemical from the sedi-


ment suspension. The total concentration of the chemical in the sediment suspension (pg.L-’) is denoted by C;. So the change of the concentration of the chemical in the fish is given by:

The apparent bioconcentration factor, K,* (L . kg-’), is then given by the ratio of the concentration of the test compound in the fish and the total concentration in the sediment suspension at equilibrium:

K,* = Cf/CG. (4)

When it is assumed that only the dissolved compounds are available for uptake by the fish, an apparent reduced accumulation will be found for the fish in the sediment suspension. So the apparent bioconcentration factor K,* will be lower than K,, due to the sorption of the compound on the sediment, because K,* is based on the total concentration of the chemical in the sediment suspension.

The total concentration of the test compound in the sediment suspension (C;) is the sum of the dissolved amount of the compound per liter (C,) and the amount of the test compound bound to the sediment per liter, as given in Equation 5 [33]:

cv: = c, + C,.S. (5)

Here C, is the concentration of the test compound on the sediment (pg.kg-’) and S the sediment:water ratio (kg.L-’). At an equilibrium between the dissolved concentration of the test compound in the water (C,) and the concentration on the sediment (Cs), the sorption coefficient Kp (Lakg-’) is defined by:

Kp = CJC,. (6)

Assuming equilibrium between the concentrations of the test compounds in the water, sediment and fish, combining Equations 2, 4, 5 and 6 gives:

K, = K,“. (1 + Kp.S). (7)

This equation shows that the apparent bioconcentration factor K,* multiplied by (1 + K,.S) equals the “real” bioconcentration factor K,. The term (1 + Kp. S) is called the bioavailability correction factor, which is the reciprocal of the dissolved fraction of the test compound in the aqueous phase. When the sediment:water ratio (S) and the affinity of the chemical to the sediment (K,) are known,

the bioavailability correction factor, and so the reduction of the bioaccumulation, can be calculated.

RESULTS AND DISCUSSION The fish in the control experiment showed no

remarkable behavior. Only a slight change of color of the fish was observed at the end of the experiment, and on the seventeenth day one fish died. The oxygen concentration in the sediment suspension was consistently >7 ppm.

In the first set of experiments, the uptake of five chlorobenzenes by the guppies was investigated (experiment A.1 and B.1). The concentrations of the test compounds in the water or sediment suspension were 125 pg/L for 1,2,3-tri-, 84 pg/L for 1,3,5-tri-, 60 pg/L for 1,2,3,4-tetra-, 21 pg/L for penta- and 1.5 pg/L for hexachlorobenzene. So the total amount of the test compounds in the two comparable systems was equal, whether sediment was present or not. The only difference was the form in which the test compound was present: dissolved in the water or partly bound to the sediment.

After about one week of exposure a change of the sediment was observed. This was only in the system where fish and sediment were present together (experiment A.1). The sediment adhered to the walls of the vessel and showed an inhomoge- neous suspension. However, the concentrations of the test compounds were not significantly different in the upper and lower parts of the vessel (data not shown). In experiment C.1, which is the sediment suspension without fish, no changes of the sediment were observed. The mass balance control of this system showed no losses of the chlorobenzenes from the system during the whole experimental period (data not shown).

During the exposure period a clear difference in activity between the fish from the two experimental vessels was observed. The fish from the system without sediment (B.1) showed lower activities than the fish from the sediment suspension (A.1). Fur- thermore, three fish died on the fourth day in B.1, whereas no mortalities were found in experiment A.I.

For both uptake experiments (A.1 and B.1) the ratios of the concentrations of the test compounds in the fish and the aqueous phase at the end of the uptake period were calculated. From the measured concentrations in the fish, it can be seen that for all test compounds an equilibrium between the concentrations in the fish and water was reached (Fig. 2). This was also confirmed by the measured concentrations of the test compounds in the water (results not shown). The values for K, (experiment


B.1, Equation 2) and K,* (experiment A.1, Equa- tion 4) are given in Table l.

As can be seen, the apparent bioconcentration factor K," is lower than K, for all five chlorobenzenes. As was suggested before, this reduction is due to the sorption of the chemical on the sediment, which results in a reduced bioavailability of the chemical for uptake by the fish. According to Equation I the K, to K,* ratio has to equal the bioavailability correction factor (1 + Kp.S). In addition to the K, to K," ratios, values for this factor are given in Table 1. Because sorption coefficients of the chlorobenzenes for the used sediment were not available, estimations were made from the octan-1-ol/water partition coefficients (Kd,oct) [34], according to: log KO, = log - 0.36 [35]. Here KO, is the organic carbon normalized sorption coefficient, which is defined as: Kp = KO,. f,, [35], where f,, is the organic carbon fraction of the sediment.

In Table 2 the same parameters and calculations are given for different organisms and test compounds from the literature. The sorption coefficients that were used to calculate the bioavailability correction factor (1 + Kp.S) were reported in the studies or otherwise calculated from the reported bound fractions or the octan-1-ol/water partition coefficients.

From Tables 1 and 2 it can be seen that the K, to K," ratios are approximately equal to the bioavailability correction factor (1 + K,.S). The most remarkable exception is the experiment with benzo- [alpyrene and bluegill sunfish [29], where a smaller ratio of the bioconcentration factors was found than was expected from the calculated bioavailability correction factor. In this study it is suggested by the authors that for benzo[a]pyrene a direct uptake from the ingested sediment contributes significantly to the total uptake.

Tables 1 and 2 show that when K," equals K, the bioavailability correction factor equals unity. This is for compounds with a relatively low sorption coefficient or in experiments with low sediment:water ratios. At high sediment:water ratios or for compounds with a high affinity for the sediment, the large reduction of the bioconcentration factor correlates well with the high bioavailability correction factor. These data support the hypothesis that the apparent reduction of the bioconcentration factor is caused by a reduced availability of the compound due to the sorption on the sediment.

For experiment I of the present study (A.1 and B.1) the concentrations of the chlorobenzenes in the fish during the uptake period are shown in Fig- ure 2. No difference between the concentration of the chemicals in the fish from both experiments can be seen for 1,2,3-tri-, 1,3,5-tri-, 1,2,3,4-tetra- and pentachlorobenzene. No difference was expected either, because no significant difference between the bioconcentration factors K, and K * has been found (Table 1). For hexachlorobenzene, where a considerable apparent reduction of the bioconcentration factor has been observed, lower concentrations in the fish from the sediment suspension were expected. However, in Figure 2 the contrary is shown, i.e., the concentrations of hexachlorobenzene in the fish from the sediment suspension (A.1) during the whole exposure period are higher than in the fish from the vessel without sediment (B.1).

These increased concentrations of hexachlorobenzene in the fish from the sediment suspension can be caused by the presence of sediment in the intestines of the fish. In an earlier study with carp, it was reported that inspection of the intestinal tract indicated that a significant amount of sediment was ingested by the fish [36]. To find further evidence for this hypothesis, the second set of ex-

Table 1. Physical-chemical parameters and bioavailability calculations

Chlorobenzene log Kd,octa KOb (L.kg-') K,' (L.kg-l) K:' (L.kg-') K,/K,* (1 + 1,2,3-Tri 4.139 1.8 x lo2 1,900 1,400 1.4 1.1 1,3,5-Tri 4.189 2.0 x 102 3,000 2,800 1.1 1.1

Hexa 5.731 7.0 x 103 27,000 5,100 5.3 5.9

1,2,3,4-Tetra 4.635 5.6 X lo2 6,600 3,800 1.7 1.4 Penta 5.183 2.0 x 103 23,000 7,900 2.9 2.4

"Octan-1-ol/water partition coefficient. From ref. [34]. bSorption coefficient: K p estimated from Kd,OCt according to Karickhoff [35]: log KO, = log Kd,oct - 0.36 and Kp =

'Bioconcentration factor (K,) and the apparent bioconcentration factor ( K t ) , present study. dUsed sediment: water ratio (S): 7.0 x

~ o , . f o , ( f o c = 0.031,

kg.L-'.


40

20

c m . in fish ( u g l g )

T 0 a

0 A

8 b 0 A 0

A A &

A P A 3

A 0 A

04 1 0 200 4 00

cmc in fish (uglg)

time (h)

A 0

A

O 0 A A A 0 0

C

Q

I 0 200 400

time (h)

cmc. in fish (ug/g)

time (h)

periments was carried out. In these experiments (A.11 and B.II), the intestines of the guppies from the sediment suspension were examined for the presence of the sediment. In all fish a small amount of sediment was identified. In addition, the concentrations of the test compounds in the intestines and the rest of the fish were analyzed separately. The results for the fish from the sediment suspension and the fish exposed to water without sediment are shown in Figure 3. For this experi-

time (h)

cmc in fish (uglg)

time (h)

Fig. 2. Concentrations of the test compounds in the fish during the exposure. (Figs. 2a and 2e reprinted by permis- sion of Kluwer Academic Publishers from ref. [37].) (a) 1,2,3-trichlorobenzene; (b) 1,3,5-trichlorobenzene; (c) 1,2,3,4-tetrachIorobenzene; (d) pentachlorobenzene; (e) hexachlorobenzene. Fish exposed to a sediment suspension (experiment A.1, A) and exposed to water without sediment (experiment B.1, 0) . For details see text.

ment test compounds with a relatively high sorption coefficient were selected, so a significant increase of the concentration of the intestines of the fish, caused by the presence of sediment with sorbed compounds, could be expected. As can be seen in Figure 3, for all five test compounds these increased concentrations in the intestines of sediment suspension exposed fish were found, whereas the concentration of the rest of the fish did not change. These results, together with the observed

Table 2. Literature data of physical-chemical parameters and bioavailability calculations

S KP Kc K,* E Test compound Organism Sediment (kg.L-') (L.kg-') (L.kg-') (L.kg-') K,/K,* (1 + K,.S) Ref. 2. Naphthalene Bluegill sunfish Humic acid (Aldrich) 2.0 x 2.0 x lo3 a 315 305 1 .0 1 .0 [291

[I51 2: Lindane Atlantic salmon Natural humic 6.6 x lo-' 7.6 x lozb 260 270 1 .o 1 .0 2,4,5-PCB Atlantic salmon Natural humic 6.6 x 6.8 x 6,880 4,600 1.5 1.4 I151 -a Kelthane Fathead minnow Clay 8.2 x 10-7 5.2 x 105' is,ooo 9,500 1.6 1.4 ~41 x z Anthracene Daphnia Yeast 6.7 x 10-5 5.4 x 103a 486 396 1.2 1.4 1131

1.9 x 10-4 498 211 2.4 2.0 0-

1 .0 [I81 P 3-Methylcholanthrene Daphnia Humic acid (Aldrich) 1.5 x 3.2 x lo5' 13,209 12,171 1.1 1.5 x 8,121 1.6 1.5 1.5 x 10-5 2,311 5.7 5.8 t

[I91 e. Benzo [a] pyrene Daphnia Natural humics 3.5 x 10-6 3.4 x 105" 4,510 3,902 1.2 2.2 1.8 x 10-5 2.2 x 105a 1,645 2.7 5.0 9

1.9 x 10-5 7.7 x 104" 1,939 2.3 2.5 a 1.9 x 10-5 9.2 x 104a 2,533 1.8 2.7 Y

~91 tr

2. "KP given in reference. Y

3 v)

0 18,900 14,500 1.3 1.4

6'

c. d

2.1 x 10-5 7.6 x 104a 2,063 2.2 2.6 rs

-

tr

Daphnia Yeast 2.5 x 10-5 2.2 x 105a 8,000 1,008 7.9 6.5 t131 ~ E Bluegill sunfish Humic acid (Aldrich) 2.0 x 1.6 x 2,657 225 11.8 33.0

cr See Table 1 for abbreviations.

'K, estimated from Kd,oct according to Karickhoff [35]: log KO, =log Kd,oct - 0.36 and Kp =KO,. f,, (see text) (Kd,,,t values are given in ref. [15], 3.24 and 6.19 for lindane and 2,4,5-PCB, respectively).

'KP calculated from the reported bound fraction.

722 S. M. Scrimp AND A. OPPERHUIZEN

Ointestines of fish rest of fish 'oolLil 50 'LU

B I1 A II A II

Fig. 3. Concentrations of the test compounds in the intestines and the rest of the fish. (a) pentachlorobenzene; (b) hexachlorobenzene; (c) 2,4,4'-trichlorobiphenyl; (d) 2,2',5,5'-tetrachlorobiphenyl; (e) 2,2',4,4',5,5'-hexachlorobiphenyl. Fish exposed to a sediment suspension (experiment A.11) and exposed to water without sediment (experiment B.11). For details see text.

sediment in the intestines of the fish, explain the increased concentration of hexachlorobenzene in the whole fish from Figure 2. For the lower hydrophobic chlorobenzenes no increase has been found. For these chemicals the presence of sediment in the fish does not cause a significant increase of the concentration in the whole fish, because of the lower affinity of the compounds to the sediment.

The presence of sediment in the intestines of the fish can also explain the large difference between the ratio of the bioconcentration factors (KJK,") and the bioavailability correction factor for benzo- [alpyrene and bluegill sunfish as is reported in Table 2. The presence of sediment with the sorbed test compound in the intestines of the fish leads to an overestimation of the concentration in the whole fish. So K,* will be overestimated (Equa- tion 4), and the K, to K," ratio is lower than was expected from the reduced availability of the test compound due to the sorption on the sediment. As shown in Table 2, an apparent reduction of a factor 33 was expected from the calculated bioavailability correction factor, while the K , to K,* ratio was only 11.8. However, in the experiments per- formed by the same authors with benzo[a]pyrene [13] and 3-methylcholanthrene [18] and Daphnia, the predicted (1 + K, S) and apparent reduction (KJK,") of the bioconcentration factor show no significant difference. A possible explanation can be given by the precautions that were taken by the authors. In these latter experiments the test animals were maintained for two hours in particle-free water to clear their gut before analysis. For the exper-

iments with the fish [29] no precautions nor an inspection of the intestines of the fish for the presence of sediment were reported.

An estimation of the amount of the sediment in the guppies of the present study has been made using the hexachlorobenzene results of experiment I . The total measured concentration in the fish (C;, pg hexachlorobenzene per kg fish) consists then of the amount taken up per kilogram fish (Cf, pg hexachlorobenzene per kg fish) plus the amount of sediment present in the fish (Sf, kg sediment per kg fish) multiplied by the concentration of the test compound on the ingested sediment (Ci, pg hexachlorobenzene per kg ingested sediment):

c; = Cf + c;.s,. (8)

The apparent bioconcentration factor is now defined by:

K,** = C,*/C:. (9)

Combining Equations 5 and 6, and then 2, 8 and 9 gives:

Sf = (K,**(l + K,.S) - Kc)/(C;/Cw). (10)

Because an equilibrium between the concentration of the test compound in the water and on the ingested sediment was assumed, Equation 10 can be written as:

Sf = (K,**(l + K , . S ) - K,)/Ki. (1 1)


From this equation an amount o f sediment of about 0.42 g per gram of fish was calculated, using the data presented in Table 1. Note that the values of K,* of Table 1 can be substituted for K,** from Equation 11, since both are defined as the ratio of the experimental concentration of the test compound in the fish and in the sediment suspension. In the same way a n amount of sediment in the fish of about 0.3% (w/w) of the fish weight was calculated for the bluegill sunfish of the experiment of McCarthy [29] (Table 2).

Although the latter calculation seems to be rea- sonable, the former, a sediment uptake of about 40% (w/w) of the body weight of the fish, is very unlikely. However, to calculate this amount of sediment that is present in the fish, the sorption coefficient for the ingested particles ( K J was assumed to be equal to K p . The latter was estimated by assuming a mean organic carbon content of the sediment of 3% (w/w). Although this will give a good estimation for the overall sorption coefficient, deviations have to be expected for selective parts of the sediment because of the nonhomogeneous nature of the sediment. In some studies it was suggested that the organic carbon content increases with a decreasing particle size [9,10]. When it is assumed that hydrophobic organic chemicals sorb mainly on the organic carbon of the sediment [9], higher sorption coefficients are expected for the smaller particles. Furthermore, it is likely that there is a selective uptake of especially small particles of the sediment by the fish. From Equation 11 it can then be seen that for a higher sorption coefficient for the ingested sediment (KL) a lower amount of the sediment will be found in the fish. For instance, when the organic carbon of the ingested particles increases with a factor five (15% (w/w) organic carbon), also the sorption coefficient for these ingested particles increases by a factor of five. The amount of sediment that is present in the fish decreases with a factor five to 8% (w/w) of the fish weight. On the other hand, the uptake of sediment can be different for different fish species, depending on for instance their feeding behavior. However, until now no information is available in the literature o n the selective uptake or the differ- ences between fish species.

CONCLUSIONS

From the results of the described experiments and the data from the literature, a n apparent reduction of the bioconcentration of hydrophobic chemicals in fish at the presence of suspended particles is caused by a reduced bioavailability due t o

the sorption of the chemicals on the particles. This reduction is only significant when large amounts of the chemical are sorbed relative to the dissolved fraction, that is, for chemicals with a high affinity for the suspended particles (high sorption coefficients) or in experiments with high partic1e:water ratios. The reduction can be quantified by the bioavailability correction factor when the sorption coefficient as well as the sediment:water ratio are known.

It seems that small amounts of sediment can be present in the intestinal tract of fish exposed to a sediment suspension. Depending on the amount of sediment in the fish and the affinity of the test compound for the sediment, the measured concentrations of the test compounds in the whole fish can be significantly influenced.

Acknowledgement-The authors thank T.H.M. Sijm for his useful comments on the manuscript. We thank The Institute for Inland Water Management and Waste Wa- ter Treatment, The Ministry of Transport and Public Works, The Netherlands, for supplying and characteriz- ing the sediment, and for their financial support of this study.

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