[Advances in Chemistry] Aquatic Humic Substances Volume 219 (Influence on Fate and Treatment of Pollutants) || Binding of Nonpolar Pollutants to Dissolved Organic Carbon

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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 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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    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 121

    WATER COLUMN

    C

  • 122 AQUATIC HUMIC SUBSTANCES List L Equilibrium Relationships for Proposed Model

    = K2 = Csed/Cpwdoc K3 = C IC

    ^ pwdoc' ^ pw K4 K s = 1 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) 10

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

  • 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 conce...

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