Chemosphere, Vol.18, Nos.9/10, pp 1883-1893, 1989 0045-6535/89 $3.00 + .00 Printed in Great Britain Pergamon Press plc

QUANTIFYING THE SORPTION OF ORGANIC CHEMICALS ON SEDIMENTS.

S. MARCA SCHRAP I'2 and ANTOON OPPERHUIZEN 2'*

1 Laboratory of Environmental and Toxicological Chemistry,

University of Amsterdam, Nieuwe Achtergracht 166,

1018 WV Amsterdam, The Netherlands.

2 Department of Basic Veterinary Sciences, Environmental Toxicology Section

University of Utrecht, P.O. Box 80176, 3508 TD Utrecht,

The Netherlands.

ABSTRACT The use of a reference compound to quantify the sorption of nonpolar organic chemicals is proposed. This is because organic carbon normalized sorption coefficients (Koc) do appear to be dependent on the type of sediment, and are thus not generally applicable to characterize the sorption properties of chemicals. Therefore, in this paper the hypothesis that nonpolar chemicals sorb in a constant ratio, independent of the sediment, has been investigated. Evidence for this hypothesis is shown with data from the literature. This enables one to compare sorption properties of nonpolar compounds on different sediments, if the differences between the sediments are normalized with a reference chemical rather than with the organic carbon content. Sediments with an organic carbon content of less than 0.1% seem to be unsuitable, because the compounds do not sorb mainly on the organic carbon, but also on other parts of the sediment. Sorption coefficients of compounds with aqueous solubilities in the ~g per liter range or octan-l-ol water partition coefficients of more than i0 - are strongly influenced by the experimental techniques used. For these compounds the sorption coefficients measured by different techniques are less comparable. To enable comparison of sorption coefficients of hydrophobic chemicals, the use of a chlorobenzene as a reference compound in sorption experiments is suggested.

* TO whom correspondence may be addressed.

1883

1884

Introduction

Sorption of organic chemicals on sediments plays an important role in

controlling their fate in the aquatic environment.

A linear sorption isotherm is often used to describe the sorption of

nonpolar organic compounds, assuming that sorption of these compounds can be

seen as a partitioning between the sediment and the water (!,~):

c s = ~.c w (11

Here C s (g/kg) and C w (g/L) are the concentrations of the compound on the

sediment and in the water at equilibrium, respectively and ~ (L/kg) is the

sorption coefficient.

Because it is assumed that hydrophobic compounds will sorb mainly to the

organic carbon of the sediment, the sorption coefficient is usually

normalized to the organic carbon fraction by:

Koc = ~ / foc (21

where Koc is the organic carbon normalized sorption coefficient and foc the

organic carbon fraction of the sediment. These normalized sorption

coefficients are commonly used to quantify the distribution of organic

chemicals between the aqueous phase and the sediment. It is hereby assumed

that Koc values are chemical specific and independent of the nature of the

sediment, because of the commonly presupposed uniformity of the organic

carbon of all sediments. Although for many compounds there is a good

correlation between these normalized sorption coefficients and for instance

octan-l-ol/water partition coefficients (Kd,oct) (~), it is known that the

organic carbon fraction of the sediment cannot be treated as a well defined

organophilic phase. Clay content (4), clay species and the nature of the

organic carbon (~) are also factors that influence the sorption capacity of

the sediment. So Koc values are not independent of the nature of the

sediment and therefore cannot be used as the unique parameter which

quantifies the distribution of organic compounds between water and sediment.

In this paper we propose the use of a reference compound to quantify

the sorption of organic chemicals on sediments. Hereby it is assumed that

nonpolar organic chemicals sorb mainly to the organic carbon of a sediment.

The sorption capacity of this organic carbon depends on the sediment's

composition, and so influences the amount of the compound which is sorbed.

However, for all nonpolar compounds, which have comparable sorption

mechanisms, this influence will be the same. Therefore, it is hypothesized

that these compounds will sorb in a constant ratio, independent of the

nature of the sediment. Then, if the sorption coefficient of the reference

compound on a specific sediment is known, the sorption coefficient of

another chemical for that sediment can be predicted from its sorption ratio

1885

to that of the reference compound. Additionally, sorption capacities of

different sediments can compared by using the reference chemical.

Select~0n of data

Sorption coefficients of nine nonpolar organic compounds selected

from the literature are listed in table I. All studies which reported

sorption coefficients of two or more chemicals have been reviewed. Neither

sediment selection nor selection of the method used has been made. In table

II characteristics of the sediments concerned are listed. A great variation

can be seen, for instance the organic carbon percentage of the sediments

range from less than 0.01% up to 54%.

Most sorption coefficients were measured in batch-systems. 0nly

Schwarzenbach and Westall (6) also reported sorption coefficients measured

in column experiments. The sorption coefficients for two polycyclic aromatic

hydrocarbons from Socha et al (Z) and for two polychlorobiphenyls from Baker

et al (8) and Oliver (26) were measured in natural water sediment systems.

Table 1. Sorption coe f f i c ien ts (Kp) and orgsnic no r l l l i zed sorpt ion coef f ic ien ts ( * ) (Koc) of nine nonpolsr organic

chemicals on various sediments. References in parentheses.

14 124 TCE TeCE naph phen pyr 2255pcb 224455pcb

665* (~) 2100" (9) 6.6 s (10) 12.9s(10) 1000" (9) 5900* (9) 46774* (D

280* (9) 885* (~) 3.0 s (1.~0) 10.4s(1._00) 400* (~) 5800* (9) 84000* (20)

850* (9) 1300" (9) 2.0 a (10) 5.8a(1_~0) 960* (9) 1400" (9) 1260 (16)

158.5"(~) 501.2"(2) 3261.9 c (22) 7524.3c(2z) 1300" (20) 38904* (Z) 5370 (17)

70.7 (10) 237.1 (10) 3389.7 d (22) 10388.8d(2__2.) 20 (16) 23000* (20) 67000 (18)

87 (14) 265 (14) 4.0 (19) 6.8 (19) 33.9 (17) 250 (16) 1158.2(21)

1.1 (6) 3.5 (6) 2.6 (19) 4.0 (19) 870 (18) 758 (17) 5.2(21)

4.4 (~) 14.5 (!) 0.6 (19) 0.9 (19) 8.5 (21) 12000 (18) 30500 (27)

1.1 (6) 2.5 (~) 0.3 (19) 0.6 (19) 0.13(21) 109.7(21)

1.1 (6) 2.4 {6) 0.1 c19) o.2 (19) 2390 (27) 0.9(21)

0.9 (6) 1.5 (6) 13000 (27)

6.0 (6) 7.6 (6)

?94328* (26) 3981072*(26)

48.9 b (15) 320b(15)

21.2 b (15) 200b(15)

79432 (8) 199526 ( | )

5045 (25) 25898 (Z5)

Koc-vaLues , organic carbon normalized sorpt ion coefficients.

a Freundtich coef f i c ien t (K), 1/n from 0.91 to 1.16

b Freundtich coe f f i c ien t (g) , 1/n from 0.92 to 1.26

c Freundlich coef f i c ien t (K), 1/n from 0.39 to 0.51

d Freundtich coef f i c ien t (g) , 1/n from 0.36 to 0.48

14:l ,4-dichlorobenzene; 124: l ,2,&-tr ichlorobenzene; TCE:tr ichloroethylene; TeCE:tetrach|oroethylene;

naph:naflhthatene; phen:phenanthrena; pyr:pyrena; 2255pcb:2,21,5,5'-tetrachtorobiphenyl; 224455pcb:

2 ,2 ' ,4 ,4 , ,5 ,5 i -hexach lorob ipheny l

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Table I I . Charac te r i s t i c s o f the sediments.

r e f . sediment o.¢. a sand

% %

(9) (9) (9) (10) (10) ( lO)

(~) (14) (6) (~)

(6)

(~) (~)

(15)

(15) (1) (zo)

(16)

( J ! )

(_7) (19) (19)

(19) (19)

(19)

(~.!)

(22)

(22) (26)

( i T )

(22)

Apiaon 0.11 4 Fu t te r ton 0.05 11 Oortmont 1.2 2 Acid peat 0.2-54 Acid humic top s o i l 0.2-54 Calcareous humic top s o i l

0.2-54 Woodburn s i t t l o a m 1.9 9 Char tes r i ve r sediment <8.5 c KB 1H 0.15 KS 1 0.73 KB 1H 0.08 Kaol in 0.06

~-AL203 <0.1 SiO 2 <0.01 I L t i t e cLay Woodburn s o i l Like super io r sediment

s i l t c lay carbo- CEC b nate meq/

% % % 100g

10 86 76 4.5 21 68 64 4.4 38 60 129 4,2

02 -71 0-89 0.08-3 2 .8-8 .0 0.3-71 0-89 0.08-3 2 .8 -8 .0

pH

0.3-71 0-89 0.08-3 2 .8 -8 .0 68 21 14

area sur face m2/g

4.9 4.4 3.2

12 120 500

3.1 16.2 10-20

combinat ion of Coarse s i l t f r a c t i o n of Ooe run sediment and Coarse s i l t f r a c t i o n of Hickory H i l t sediment 2.7813.27

M iss iss ipp i River Mc Cture, iL 1.5 1.6 42.91 55.4

Tamer Estuary sur face sediment 4.02

combinat ion of 17 sediments 0.15-2.38 7-75.6 1-69

Eagle Harbor 0.36-2.74 44.5 48.1 7.4 BLack soil ! 4.9 BLack s o i l IX 3.2 Gray s o i l 0.5 Brown s o i l I 0.4

Brown s o i l I I 0.1 F l i n t aqu i f e r 1.87 87 12 1 Borden aqu i f e r 0.02 98 1 1 Granular ac t i va ted carbon Westveco's (Covington,VA) WV*G Condie s i l t 0.004 $ t .CLa i r -+De t ro i t -N iaga ra r i v e r

7-13 Ac t i va ted carbon ( F i t t r s s o r b 400)

164 76 42

171

180

Granular ac t i va ted carbon ( F i l t r s a o r b 400)

20.9 7.7

1.2-33.0

998

per t . s ize #m

<2000 <2000 <2000 <2000 <2000

<2000

< 840 63-125 < 125 < 125

20-50

>75% 2.7-37.

<125 <125

< 74

- not g iven in re ference a a percentage of 1% equals a f r a c t i o n of 0.01 b Cat ion Exchange Capacity c in r e f . ( 1 4 ) : #X organic carbon w i l l be s l i g h t l y lees than h a l f o f combust ible Ioss"(=17.0)

1887

Reference compound

From table I it is clear that the measured sorption coefficients can

differ considerably for each compound, even when these are normalized to the

organic carbon fraction. This means that the Koc values are not independent

of the sediment used, in contrast to what is generally assumed. The

influence of the sediment on the sorption is i11ustrated in fig.1. Here the

logarithms of the sorption coefficients of 1,4-dichlorobenzene and 1,2,4-

trichlorobenzene from table I are plotted against the logarithms of the

organic carbon fraction of the sediments, using the logarithmic form of

equation 2:

log ~ = log Koc + log foc (3)

Normalized sorption coefficients (Koc) from table I have been converted,

where possible, to sorption coefficients (~) (equation 2) with the organic

carbon fractions of the sediments. Linear regression of log ~ and log foc

(n=9) resulted in correlation coefficients (r 2) of 0.909 and 0.929 for 1,4-

dichlorobenzene and 1,2,4-trichlorobenzene respectively. Although log ~ and

log foc correlated for both compounds, it is clear from fig. 1 that the

deviation of the data points from the regression line parallels for the two

test compounds. This parallel variation of sorption coefficients of two test

compounds suggests that the influence of the properties of each sediment on

the sorption is equal for both compounds, and thus that the ratio of

sorption coefficients will be a constant, i.e. independent of the sediment.

More generally:

~AI KpB: c (4)

Kp A and Kp B are the sorption coefficients or the normalized sorption Here

coefficients of a compound A and B, respectively, and C is a constant. The

logarithmic form of this equation is given in equation 5:

log Kp A = C' + log Kp B (5)

Also this relation is plotted for 1,4-dichlorobenzene and 1,2,4-trichloro-

benzene (figure 2). Lineair regression for this relation resulted in a

correlation coefficient (r 2) of 0.991 (n=9).

In table III the ratios of the sorption coefficients of 1,4-dichloro-

benzene and 1,2,4-trichlorobenzene for the different sediments are listed. A

constant value (3.2 ± 0.1) for all the sediments has been found, except for

those with an organic carbon content of less than 0.1%. In the latter case

the chemicals probably do not sorb mainly on the organic carbon, but also on

other parts of the sediment (~,6), and their sorption coefficients may

therefore not comparable. It was also found by Southworth and Keller (9) and

Schwarzenbach and Westall (6), that correlations between Koc and octan-l-ol-

1888

LOG Kp

O-

-1

o O e

mO

0 • []

- i -1' LOG FOC

f ig .1 Relationship between tog Kp and tog foc for 1 , ; -d ichtoro

benzene and 1,2,4-tr ichlorobenzene.

tog Kp = tog Koc + tog foc (see text equation 3)

Closed symbols for organic carbon percentage >0.1X, open

symbots for organic carbon percentage <0.1%

[ ] • 1,4-dichtorobenzene r2=O.909,(n=9) art data;

r2=0.912, (n=6) organic carbon of sediment >0.1¢

0 • 1,2,4-tr ichtorobenzene r2=0.929, (n=9) art data;

r2=0.913, (n=6) organic carbon of sediment • 0.1Z

124 LOG Kp

2 ,

1

O"

==

o 14 LOG Kp

f i g . 2 ReLetionship between tog Kp of 1,4-dichtorobenzene end

the log Kp of 1,2,4-tr ichtorobenzene.

tog Kp A = C' + tog Kp 8 (see tex t equation 5)

r2=O.991,(n=9) a l l data; r2>O.999,(n=6) organic

carbon of sediment • 0.1Z

• organic carbon • 0.1~

organic carbon < 0.1X

1889

water partition coefficients (Kd,oct) are insignificant when the organic

carbon percentage of the sediment is less than 0.1%. When only sediments

with an organic carbon percentage of more than 0.1% are used the correlation

coefficient of equation 5 (figure 2) for 1,4-dichlorobenzene and 1,2,4-tri-

chlorobenzene becomes >0.999, while for equation 3 (figure i) the

correlation coefficients become 0.912 and 0.913 for 1,4-dichloro-benzene and

1,2,4-trichlorobenzene respectively.

Tebte 111. Ratios of sorptton coeff icients. References in ~rentheses.

organic carbon of sediment • 0.1X

124114 124/naph TeCE/TCE phen/neph pyr/phen pyr/naph 224455pcb/2255pcb

3.2 (9_.) 2.1 ¢9_.) 2.0 (10) 5.9 (9) 1.2 (7) 65 (20) 5.0 (26)

3.2 (9) 2.2 (9) 3.5 (10) 14.5 (9) 3.6 (20) 63 (16) 6.5 (15)

3.2 (~.) 2.9 (10) 17.7 (20) 5.0 (16) 159 (17) 2.5 (8_)

3.4 (10) 2.3 (22) 12.5 (16) 7.1 (17) 77 (18)

3.0 (14) 3.1 (22) 22.4 (17) 5.6 (18) 136 (21)

3.2 (6.) 1.7 (19) 13.8 (18) 10.6 (21) 13 (27)

3.3 (6) 1.5 (19) 12.9 (21) 2.3 (27)

1.5 (19) 5.4 (27)

2.0 eL9)

2.0 ( !9)

3.2 ", 0.1 2.2 * 0.1 2.2 * 0.6 13.1:1:5.3 5.0 ~" 2.9 86:1:49 4.7 ", 1.6

organic carbon of sediment < @.IX

124114 124/naph

1.5 (9) 1.4 (9)

2.3 (6.)

2.2 (6.)

1.7 (6.)

1.3 (6.)

phenlnsph pyrlphen pyrlnaph 224455pcb12255pcb

1.s (9) 5.8 (21) 40 (7 ! ) 9.4 (15)

6.9 (71) 5.1 (25)

14:1,&-dichLorobenzene; 124:1,2,4-trichtorobenzene; TCE:trichLoroethytene; TeCE:tetrechtoroethyiene;

naph:nephthaLene; phen:phenanthrene; pyr:pyrene; 2255pcb:2,2',5,5'-tetrechiorobipheny[; 224455pcb:

2,2,,4,4' ,5,5'-hexschtorobiphenyt

A constant ratio of the sorption coefficients is not only found for

1,4-dichlorobenzene and 1,2,4-trichlorobenzene on sediments with more than

0.1% organic carbon, also the ratio between the sorption coefficients of

trichloroethylene and tetrachloroethylene is found to be a constant (2.2 ±

0.6) (table III). This ratio varies more than that between 1,4-

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dichlorobenzene and 1,2,4-trichlorobenzene. It must be noted, however, that

the sorption coefficients of the ethylene compounds are very small, so the

variation in the sorption coefficient itself is relatively high.

Furthermore, the sorption coefficients of these compounds given by Friesel

et al. (i0) and Crittenden et al (22) are not measured as a linear sorption

coefficient, but as a Freundlich coefficient, which is defined by:

x/m = K Cw I/n (6)

Here x is the weight of sorbate taken up by a weight m of the solid, K the

Freundlich coefficient, C w the concentration of the compound in the water at

equilibrium and n an empirically determined constant, n being 1 for linear

sorption isotherms. For the chloroethylenes values of n range from 0.39 up

to 1.16.

Constant ratios of sorption coefficients are not only found for two

compounds of the same chemical class (chlorobenzenes, chloroethylenes), but

also for those of 1,2,4-trichlorobenzene and naphthalene (a polycyclic

aromatic hydrocarbon) (2.2 ± 0.i). For these compounds the ratio of the

sorption coefficients for the sediment with less than 0.1% organic carbon is

also an outlier.

The ratios of the sorption coefficients of phenanthrene and

naphthalene (13.1 ± 5.3), pyrene and phenanthrene (5.0 ± 2.9), pyrene and

naphthalene (86 ± 49) and 2,2',5,5'-tetrachlorobiphenyl and 2,2',4,4',5,5'-

hexachlorobiphenyl (4.7 ± 1.6) are also listed in table III. These ratios

vary more than those mentioned earlier. For these high hydrophobic chemicals

the measured sorption coefficients are strongly influenced by the experi-

mental techniques used. Because of the impossibility to completely separate

water and sediment, a part of the sediment remains in the aqueous phase. The

compounds will sorb on this so called 'third phase' and an enhanced

concentration of the compounds in the aqueous phase is measured

(8,11,12,13). Due to an overestimation of the concentration of the compound

in the aqueous phase, the experimental sorption coefficient will be an

underestimation of the 'real' sorption coefficient. This experimental

artifact will be especially important for the more hydrophobic compounds,

because of their higher affinity to the third phase (12,13). In most studies

no account has been taken on this third phase problem, so the sorption

coefficients measured in the various studies using different separation

techniques are not necessarily comparable. Nevertheless the ratios of the

sorption coefficients of these compounds vary less than the organic carbon

normalized sorption coefficients.

The lowest variations in the ratios of the sorption coefficients has

been found for those compounds (l,4-dichlorobenzene, 1,2,4-trichlorobenzene,

1891

trichloroethylene, tetrachloroethylene, naphthalene, table IiI) with a log

Kd,oc t value lower than 5 and a relatively high aqueous solubility (mg per

liter range, table IV). For these compounds the influence of the third phase

on the measured sorption coefficient is negligible (12). This is in

agreement with the conclusions of Chiou et al. (13) who found that the

effect of dissolved organic material on the enhancement of the concentration

of a compound in the aqueous phase is minimal for compounds such as 1,2,3-

trichlorobenzene and lindane which have aqueous solubilities of 18.0 and

7.87 mg/liter respectively (13).

Table IV. Log octan-l-ot water part i t ion coeff ic ients (Kd,oc t ) and aqueous so lub i l i t i es . References in parentheses.

compound Log Kd,oc t aqueous so lub i l i ty compound tog Kd,oc t aqueous so lubi l i ty

(mg/t) (moll)

14 3.38 (23) 30.9 (23) naph 3.35 (23) 30.6 (23)

124 3.98 (23) 46.1 (23) phen 4.57 (23) 1.18 (23)

TCE 0.68 (24) 1090 (10) pyr 5.18 (23) 0.135 (23)

1000 (19)

2255pcb 6.11 (5) 0.0265(23)

TeCE 0.51 (24) 160 (10)

140 (19) 224455pcb 6.57 (5) 0.001 (5)

14:1,4-dichlorobenzene; 124:1,2,4-tr ichtorobenzene; TCE:tr ichtoroethytene; TeCE:tetrachtoroethyLene;

neph:nephthsLene; phen:phenenthrene; pyr:pyrene; 2255pcb:2,2 ' ,5 ,5 ' - te t rachtorobiphenyt ; 224455pcb:

2,2, ,4 ,4, ,5 ,5 ' -hexachtoroblphenyt

The observations that the ratio of sorption coefficients of organic

chemicals on most sediments is a constant enables one to use the sorption

properties of one chemical in those cases as a reference to both: i.

quantify the sorption properties of other chemicals on a soil or sediment,

and ii. to compare sorption phenomena of chemicals on different soils and

sediments.

If sorption coefficients are normalized to that of a reference

chemical, they can be compared regardless of the sediment on which they were

determined. This is an important observation since neither sorption

coefficients nor organic carbon normalized sorption coefficients are

independent of the sediment properties. Although many chemicals can be used

as reference chemicals, the best results may be obtained for persistent

1892

hydrophobic chemicals with log Kd,oc t values less than 5 since for such

chemicals experimental sorption coefficients can be determined relatively

easily. Based on the data shown in table III, chlorinated benzenes for

example may be successful reference chemicals in future sorption

experiments. Whether or not they are also applicable in experiments with

sediments with less than 0.1% organic carbon is not fully clear at this

time.

Conclusions

The use of a reference compound to quantify the sorption of nonpolar

organic chemicals on sediments seems to be successful.

It seems that sediments with an organic carbon content of less than

0.1% can not be used.

Experimental sorption coefficients for chemicals with a log Kd,oc t

value higher than 5 or a low aqueous solubility (fig per liter range) appear

to be less suitable. For these chemicals the presence of a third phase in

the aqueous phase leads to an enhancement of the aqueous phase concentration

and therefore to an underestimation of the sorption coefficient. Less

hydrophobic compounds appear to be insensitive to this experimental artifact

and will sorb in a constant ratio.

The use of chemicals which are not sensitive to the third phase and

which have relatively high sorption coefficients, such as chlorobenzenes,

may be suitable reference compounds to quantify the sorption of organic

chemicals on sediments.

AcknowledGement

This work was supported by the Institute for Inland Water Management

and Waste Water Treatment, Ministry of Transport and Public Works, The

Netherlands.

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(Received in Germany 18 November 1988; accepted 24 February 1989)