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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 environ-ment. The interaction between pollutants and dissolved organic car-bon in natural waters is not as well defined as that between pollutants and sedimentary organic matter. The limitations of experimental tech-niques and extraction and concentration procedures are partially responsible for the incomplete description of pollutant-DOC (dis-solved organic carbon) interactions. Despite the lack of complete understanding of the phenomenon, the association of nonpolar com-pounds 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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    In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

  • 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

    K = -f- (2) Joe

    For a given compound, the magnitude of is relatively constant among sediments (6, 7). K 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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    In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

  • 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

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