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9

Binding of Nonpolar Pollutants to Dissolved Organic Carbon Environmental Fate Modeling

Gail Caron1 and I. H. Suffet

Environmental Studies Institute, Drexel University, Philadelphia, PA 19104

Nonpolar compounds associate with organic carbon in the environment. The interaction between pollutants and dissolved organic carbon in natural waters is not as well defined as that between pollutants and sedimentary organic matter. The limitations of experimental techniques and extraction and concentration procedures are partially responsible for the incomplete description of pollutant-DOC (dissolved organic carbon) interactions. Despite the lack of complete understanding of the phenomenon, the association of nonpolar compounds with natural DOC can exert a significant influence on their environmental partitioning. Mathematical models of environmental behavior should include dissolved organic carbon in both overlying and sedimentary interstitial waters as compartments for equilibrium partitioning.

NUMEROUS PHYSICAL, CHEMICAL, AND BIOLOGICAL PROCESSES act upon organic chemicals that are released into the environment. The interaction of these factors determines the ultimate environmental fate of pollutant compounds, as well as the hazard they pose to living organisms. To assess the risk associated with a released chemical, it is necessary to understand how the compound will behave in the environment. In view of the large

1Current address: U.S. Environmental Protection Agency, Region 3, 841 Chestnut Street, Philadelphia, PA 19107

0065-2393/89/0219-0117$06.00/0 © 1989 American Chemical Society

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In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

118 AQUATIC HUMIC SUBSTANCES and ever-increasing number of organic chemicals being produced, experimental study of individual compounds is an impossible task.

Considerable effort is currently being directed toward developing mathematical models to accurately predict the environmental distribution of organic chemicals. Simple compartmental models such as the quantitative water, air, and sediment interactive (QWASI) fugaeity model of Mackay et al. (I) and the chemical equilibrium partitioning and compartmentalization (CEPAC) model of McCal l et al. (2) predict the environmental distribution of pollutants from physical-chemical properties of the compound that determine its affinity for various media. More complex models add the consideration of transformation reactions and transport processes.

The various environmental transport processes are poorly understood, especially for compounds associated with dissolved humic materials in the environment. We have a new approach to the modeling of hydrophobic organic pollutant behavior in the aquatic environment, in which dissolved humic materials play an important role.

Binding of Nonpohr Organic Compounds to Sedimentary Organic Carbon The association of nonpolar organic pollutants with soils and sediments has been studied extensively and identified as a major process affecting the environmental fate and distribution of these compounds. The binding of nonpolar organic compounds to sedimentary organic carbon is important background information related to the association of these compounds to dissolved humic materials.

The distribution of hydrophobic organic compounds between aquatic sediments and the overlying water column has typically been viewed as a surface adsorption phenomenon and, as such, has been studied with batch sorption isotherm techniques. Adsorption isotherms of nonpolar organic compounds on a number of soils and sediments are linear over a wide range of equilibrium solute concentrations (3-5). This behavior can be expressed as

C sed = &p X (1)

where C s e a and are sorbed and dissolved concentrations of a compound, respectively; and K p is the distribution, or partition, coefficient describing the ratio of the equilibrium concentration of a compound in the sediment to its equilibrium concentration in the water.

A number of studies have shown that the binding of nonpolar organic compounds to natural sediments is highly correlated with the organic carbon content of the solid material. Because of the important influence of organic

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9. CARON & SUFFET Binding of Nonpolar Pollutants to DOC 119

carbon, sediment-water distribution coefficients are often normalized to the organic-carbon fraction of the sediment (fj) by the expression

Kœ = -f- (2) Joe

For a given compound, the magnitude of is relatively constant among sediments (6, 7). K œ values, therefore, provide good predictions of sorptive behavior. The value of constants for describing the distribution of organic compounds between sedimentary organic carbon and water is further enhanced by the fact that values can be closely correlated with a chemical's octanol-water partition coefficient (K o w ) and water solubility (3, 6, 8). values that have not been experimentally determined thus may be estimated from measured K o w values for the same compound.

Lambert (9) and Chiou et al. (3, 4) have proposed that the association between nonpolar compounds and the organic carbon fraction of sediments, soils, and natural waters is better described as a liquid-liquid partitioning phenomenon than as a surface adsorption process. An organic-matter partitioning process is supported by a number of observations, including

1. linear sorption isotherms to near aqueous saturation concentrations of nonpolar organic substances, with no evidence of isotherm curvature at the higher concentration range; isotherm curvature at higher concentrations is predicted by adsorption theories;

2. small temperature effects on solute sorption;

3. absence of competition in experiments using binary solute systems; and

4. data covering seven orders of magnitude in which sediment-water partition coefficients were inversely proportional to aqueous solubility and well correlated to octanol-water partition coefficients.

The actual physical mechanism of the reaction between nonpolar organic compounds and natural organic matter is still a matter of controversy. The terms sorption and partitioning wil l , therefore, be used loosely in this chapter.

A number of workers have attempted to describe the association between nonpolar organic compounds and humic material on a molecular level. Schnitzer and Khan (10) proposed that the humic polymer consists of an aromatic core to which peptides, carbohydrates, metals, and phenols are attached. This proposed structure is an open network, and it has been sug-

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120 AQUATIC HUMIC SUBSTANCES gested that organic molecules are trapped inside the spaces of the humic structure.

Freeman and Cheung (JJ) picture humic material as highly branched polymer chains that form a three-dimensional randomly oriented network. Interconnections between the chains prevent the network from dissolving in liquids. Instead, liquids may be absorbed, and absorption is typically accompanied by swelling of the network to form a gel. Freeman and Cheung suggested that humic substances bind organic chemicals by a process of incorporation into the humic gel structure, and that the binding of hydrophobic compounds is controlled by the relative affinity of the compound for the aqueous and gel phases.

At present, it is not known which of the proposed structures best describes the molecular configuration of naturally occurring humic material. Further research is necessary in this area.

Relatively recent evidence indicates that dissolved organic matter in natural waters can, like sedimentary organic carbon, "sorb" or bind nonpolar organic compounds. Dissolved organic carbon is composed largely of dissolved humic material. The binding of nonpolar organic chemicals with dissolved organic carbon (DOC) can be described by an equilibrium distribution coefficient, Kdoc, where

Cdoc — Kdoc X C a q (3)

where is the concentration of the chemical associated with the D O C at equilibrium.

Dissolved organic carbon in natural waters must be considered as a separate environmental compartment in a model of pollutant behavior. Mathematical models developed to date have not included the nonpolar-organic-pollutant-DOC interaction. Where D O C concentrations are high, this interaction can exert an important influence on the environmental behavior of nonpolar organic materials, especially those with a strong tendency to bind to dissolved humic substances.

Systems that contain naturally high levels of D O C include bogs, swamps, and interstitial waters of soils and sediments. Interstitial water (porewater) is formed by the entrapment of water during sedimentation, which isolates it from the overlying water. Porewater is considered to be in equilibrium with the sedimentary solid phase and separate from the overlying water column, or bulk water (12, 13). Dissolved organic carbon concentrations in sedimentary porewater can exceed 100 mg/L, whereas overlying surface waters typically contain less than 5 mg /L of D O C (14).

For modeling purposes, a kinetic boundary can be hypothesized at the sediment-water interface, as illustrated in Figure 1. The hypothesized boundary would describe conditions in lakes, reservoirs, and slow-moving streams, where the rates of dispersion and difiusion between the sediment and water column are orders of magnitude slower than those within the

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WATER COLUMN

C <r aq doc

Sedimentation

Diffusive flux of Overlying water

_ Kinetic Boundary Resuspension

Diffusive flux of pore voter

C <r-pw • s *

•» C

sed

pwdoc

SEDIMENT

Figure 1. Hypothetical kinetic boundary at sediment-water interface.

separate phases. The sediment and its associated porewater can then be treated as separate environmental compartments in an equilibrium model. The equilibrium reactions occurring in such a model are presented in List I.

Measurements of Binding of Nonpohr Organic Pollutants to DOC

The interaction between nonpolar organic pollutants and D O C is not as well understood as that between nonpolar organic pollutants and sedimentary organic carbon. Several experimental difficulties are responsible for the l imited understanding of pollutant-DOC binding. The development of adequate methods for studying the binding of nonpolar organic compounds has been

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122 AQUATIC HUMIC SUBSTANCES List L Equilibrium Relationships for Proposed Model

= K2 = Csed/Cpwdoc K3 = C IC

^ pwdoc' ^ pw K4

K s = Γ1 IC 0 pwdoc' ^ aq K 6 = Cdoc^aq

K 7 = C s e d / C a q

K 8 = ^ sed ̂ doc K 9

= CseJ(C pwdoc + ^ pw) Κ10

: C s e < J / ( C a q -f* C d o c ) = C s e d / C

where C s e d is the concentration bound to sedimentary organic matter C p w is the free concentration dissolved in porewater C pwdoc is * n e concentration bound to porewater D O C

is the free concentration dissolved in water column C d o c is the concentration bound to water column D O C C w is the C a q + C d o c

difficult. A number of methods have been tried, but none has proven suitable for studying all nonpolar organic compounds (15,16). These methods include gel permeation chromatography (17), ultrafiltration (18), reverse-phase liquid chromatography (19), equilibrium dialysis (20), solubility methods (21, 22), and gas-phase partitioning (23-25).

Gel permeation chromatography and reverse-phase separation techniques are based on the theory that the fraction of a nonpolar organic compound bound to D O C will not be retained by the gel or reverse-phase column. DOC-sorbed compounds wil l be excluded from the pore spaces of gel permeation columns and wil l not bind to reverse-phase column material at p H levels above 5. Free compound will be retained in either column type. The amount of bound compound measured increases with increasing flow rate. This relationship suggests that measurements of equilibrium distributions may not be accurate because of rapid desorption of bound material (16, 19).

Dialysis and ultrafiltration methods rely on physical separation of free and bound forms with semipermeable membranes. Ultrafiltration techniques are limited to nonpolar organic compounds, which do not interact with the ultrafiltration membrane (15, 16, 19). Similarly, equilibrium dialysis is l imited to nonpolar organic compounds that wil l readily pass through the membrane. Dialysis membranes strongly adsorb some nonpolar compounds (19). Carter and Suffet (21) reported some discrepancies between Kdoc measured by dialysis and by other methods. However, for compounds that do not significantly interact with the membrane, equilibrium dialysis is a promising

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9. CARON & SUFFET Binding of Nonpohr Pollutants to DOC 123

method for measuring the binding of nonpolar organic compounds to D O C (15, 16, 20).

Solubility methods measure the effect of dissolved organic carbon on the apparent solubility of nonpolar organic compounds (21, 22). The difference in measured solubility in the presence and absence of D O C is attributed to binding of the compound to the dissolved organic matter. The advantage of solubility determination is that the technique is applicable to virtually all nonpolar organic compounds. However, there are disadvantages to the method. Binding constants are measured at saturation, which is normally much greater than concentrations found in the environment. The activity of the compound in the aqueous and organic phases may change as saturation is approached (15, 16, 20). Furthermore, the accuracy and precision of solubility measurements of very hydrophobic compounds are often adversely affected by dispersion of the compound rather than true dissolution and by the presence of suspended microcrystals in solution.

The dynamic coupled column liquid chromatographic technique of May et al. (26) is designed to eliminate these problems. The method is based on pumping water through a column containing glass beads coated with the compound of interest. Whitehouse (22) successfully applied the technique to study the effect of dissolved humic substances on the aqueous solubility of polynuclear aromatic hydrocarbons.

Gas-phase partitioning methods have been used to study the interaction of nonpolar organic compounds with D O C (16, 23-25, 27). The technique is based on the fact that, according to Henry's law, the vapor concentration of the compound under study is directly proportional to the freely dissolved concentration. The concentration of the compound in the vapor phase is measured and used to calculate the aqueous concentration. The procedure avoids the problem of incomplete phase separations, which is inherent in other methods. However, the procedure is limited to compounds with significant vapor pressures and is complicated by sorption of the compound to container walls (16).

In summary, a number of methods have been applied to the study of DOC-nonpolar-organic-compound interactions. Each method has its advantages and its drawbacks; no one method has proven applicable to all nonpolar organic compounds.

Uncertainties in Binding of Nonpohr Organic Compounds to DOC

Current information indicates that the mechanism of pollutant binding to D O C is similar to that of binding to the organic carbon fraction of soils and sediments (15, 16, 20). The evidence for this mechanistic similarity includes the observation of linear binding "isotherms" for nonpolar organic com-

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124 AQUATIC HUMIC SUBSTANCES pounds and D O C ; the absence of competition in multisolute systems; and the close agreement between values measured for a given compound with sediments and D O C .

Kdoc values for nonpolar organic compounds associated with D O C are, like sedimentary values, highly correlated with K o w values of the compounds. However, some studies indicate that a large variability in Kdoc values for a given compound depends on the source of the dissolved organic carbon (19-21, 27, 28). Carter and Suffet (20, 21) found a range of K d o c values for D D T (l,l-dichloro-2,2-bis(p-chlorophenyl)ethane) with dissolved humic and fulvic acids from different sources. The largest differences were observed between humic and fulvic acids. One fulvic acid (Suwannee River fulvic acid) did not bind D D T at all. Attempts to correlate the extent of D D T binding to measurable characteristics of the dissolved humic materials were unsuccessful.

A possible explanation for this seemingly anomolous behavior may be found in the common practice of using base-extracted humic materials in the study of the binding of nonpolar organic compounds to D O C . At present, it is not known how the humic material is affected by the extraction and cleanup procedures used in laboratory studies. The chemical structure of humic material is not definitively known; it may be rather fragile and easily disrupted by the extractions.

Perhaps some humic materials are more fragile than others. In the work of Carter and Suffet (20,21) Suwanee River fulvic acid showed little tendency to bind D D T , in contrast to other humic materials used. Suwannee River fulvic acid was the only humic material in that study that was subjected to extensive cleanup procedures. This treatment may have further changed the molecular structure and binding ability of the material.

Humic substances in natural waters are mixtures rather than pure substances, and they vary in composition from one environment to another. If natural organic matter is a mixture, can absolute consistency be expected? More likely, there wil l be variability in the behavior of organic materials from different environments, as reported by Garbarini and Lion (27). In the face of such variability, caution should be used in analyzing data from studies using soil-derived humic material as a model for D O C in natural waters. Future research should be directed at determining whether the differences in reported Kdoc values are caused by differences in the organic matter itself, or are the result of isolation and extraction procedures or techniques used to study the DOC-organic-pollutant interaction.

Importance of DOC in Interstitial Waters Dissolved organic carbon is found in high concentrations in marine sedimentary interstitial waters (29). The study of freshwater sedimentary interstitial waters is relatively new, but available data indicate elevated D O C

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9. CARON & SUFFET Binding of Nonpohr Pollutants to DOC 125

concentrations in these porewaters as well (13, 14). D O C concentrations in the porewater of the Brandywine River, which was used in the work reported here, ranged from 15 to 30 ppm. Overlying-water D O C concentrations were 3 ppm or less. Interstitial-water D O C thus constitutes a significant environmental compartment to be considered in environmental models.

Some studies report that interstitial-water D O C will bind nonpolar organic compounds. Eadie et al. (30) found that polycyclic aromatic hydrocarbons (PAH) in porewaters of Lake Michigan sediments were associated with very fine particles, colloids, and humic substances. Brownawell and Farrington (31, 32) studied the partitioning of polychlorinated biphenyls (PCB) in sediments and interstitial waters of New Bedford Harbor, Massachusetts. P C B concentrations were highly elevated in the porewaters. The observed partitioning of PCBs between sediments and interstitial waters could not be explained if sediment-solution partitioning were the only process involved. A three-phase equilibrium model that includes PCBs associated with colloidal organic material in the porewater, in the dissolved phase, and sorbed to sedimentary particulate material adequately explained the observed P C B distribution.

"Five-Phase" Model of the Aquatic Environment Our equilibrium model views the abiotic aquatic environment as consisting of five "phases*' or compartments:

1. the overlying water column,

2. D O C in the water column,

3. interstitial water,

4. D O C in the interstitial water, and

5. sediment particles with their "coating" of organic carbon.

Nonpolar organic compounds released into the environment are distributed among these phases according to the equilibrium relationships summarized in List I. Several of the partition coefficients are combined equilibria, yielding apparent partition coefficients that appear anomalous if the individual compartments are not considered. For example, the distribution of a compound between interstitial water with its associated D O C and sedimentary particulate matter yields the apparent K<)C designated K 9 in the proposed model. Because of a considerable level of D O C in the porewater, this partition coefficient indicates decreased binding of a given compound to the sediment in the presence of porewater with respect to overlying water. Consideration of the interaction of the compound with D O C accounts for the apparently low partition coefficient.

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126 AQUATIC HUMIC SUBSTANCES The analytical scheme developed to test the interaction hypothesis is

diagramed in Figure 2. A modification of the generator column method of May et al. (26) was used to measure the solubility of the test compound in the overlying water and interstitial water compartments to determine chemical partitioning between the aqueous compartments and their associated dissolved organic carbon in these phases. Partitioning of a compound between the aqueous compartments and sedimentary solids was determined by using a column technique in which a spiked solution was passed through a column containing a measured amount of sediment. The experimental design includes passing the solution through a granular activated carbon (GAC) column to remove the D O C without fractionation. The sediment column apparatus is diagramed in Figure 3.

Initial results using 2,2',4,4'-tetrachlorobiphenyl as the test compound indicated considerable binding to interstitial-water D O C . Water, sediment, and interstitial-water grab samples taken from the Brandywine River in southeastern Pennsylvania were used. D O C concentrations in the overlying water were below 3 mg/L, although porewater D O C concentrations ranged from 15 to 30 mg/L. The sedimentary organic carbon content was 1.2%.

Binding to D O C in the overlying water had no effect on the binding of the test compound to sediments in the Brandywine River. From the model, K 6 was undeterminable, and as a result, K 1 0 is equivalent to K 7 . Binding to D O C in the interstitial water, on the other hand, showed a significant effect. From the model, K 3 and Kx are combined to calculate K 9 , the apparent for the interstitial water. The mean value of the sediment-water partition coefficient, K ^ , for 2,2',4,4'-tetrachlorobiphenyl in the Brandywine River was 3.61 Χ 10 4. The apparent Κ Μ for tetrachlorobiphenyl in the presence of Brandywine River porewater was 6.98 x 10 3. This value indicates a considerable effect of porewater D O C on sediment-water partitioning.

The decreased sorption of tetrachlorobiphenyl to sedimentary material was attributed to binding of the compound to interstitial-water D O C . The distribution coefficient, (K3 of the model), represents the ratio of equilibrium concentrations bound to porewater D O C and freely dissolved in the porewater. For 2,2',4,4'- tetrachlorobiphenyl in the Brandywine River, was determined by solubility enhancement to be 1.63 Χ 10 5.

The distribution coefficient K p w d o c for 2,2',4,4'-tetrachlorobiphenyl bound to D O C in the interstitial water of the Brandywine River was approximately an order of magnitude greater than the binding constant of that compound to the organic carbon fraction of the sedimentary solid material. This observation confirms that D O C in the porewater of natural sediments can influence the environmental fate of nonpolar organic compounds to a considerable extent. Further research is needed to determine the applicability of the sediment-column experimental design to other compounds and sediment types.

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Filter overlying water through 0.45-μπι filter

Circulate overlying water through generator column

C M + CA

Filter overlying water through GAC to remove DOC

Circulate GAC-filtered overlying water through

generator column

Cdoc — Co,, + C d œ Cm

Load sediment-pore water mixture into pressure filtration apparatus

Circulate pore water through generator column

C 4- C V y pw T ^ pwdoc

Filter pore water through GAC to remove DOC

Circulate GAC-filtered pore water through generator

column

Cpwdoc C p w ~t~ ^ pwdoc "t" C nwHnc Cn

Figure 2. Analytical scheme for determining the concentration of compound partitioned in each environmental compartment.

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128 AQUATIC HUMIC SUBSTANCES

=0

τ >

Sediment Column

ppump ι τ i

Reservoir

Figure 3. Sediment column apparatus used for equilibration of aqueous phases with sedimentary particulate material.

Summary

Dissolved organic carbon in natural waters interacts with and influences the environmental behavior of nonpolar organic compounds. A number of methods have been developed to study and quantify this interaction. At present, no universally applicable technique has been defined. Further research is necessary to develop new methods that wil l overcome the experimental difficulties encountered with existing procedures.

The extent of binding of nonpolar organic compounds to D O C is a function of the octanol-water partition coefficient and aqueous solubility of the compound. Present data indicate that the magnitude of binding is a function of the humic material as well. It is not known whether there are indeed differences in the binding ability of D O C from different sources or whether the discrepancies result from alteration of the structure of natural organic matter during sampling, isolation, and extraction.

An experimental design using D O C as found in the environment (i.e., with no fractionation, extraction, or chemical alteration) was used to investigate the importance of DOC-pollutant interactions in the aquatic environment. Results of this research indicate that D O C in the interstitial water of natural sediments can significantly affect the behavior of hydrophobic compounds exhibiting high K o w values. Environmental models should in-

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elude this interaction in the prediction of the ultimate fate and transport of nonpolar pollutants.

Acknowledgment

We thank R. Lee Lippincott for help in the development of the equilibrium model presented in this chapter and for providing computer graphics.

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157-169.

RECEIVED for review November 5, 1987. ACCEPTED for publication July 19, 1988.

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