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    Implications of Mixture Characteristics on Humic-Substance Chemistry

    J. A. Leenheer, P. A. Brown, and T. I. Noyes

    U.S. Geological Survey, Denver Federal Center, Denver, CO 80225

    The chemistry of complex mixtures usually cannot be described as the sum of the characteristics of the individual components because of intermolecular interactions that modify component characteristics. A study of both inter- and intramolecular interactions in the Su-wannee River fulvic acid, by measurement of the dependence of den-sity on concentration in various solvents, indicated that polar functional groups (carboxyl, phenolic, hydroxyl, and ketone) were responsible for most of the molecular interactions in water. A two-stage fractionation of Suwannee River fulvic acid on silica gel resulted in 31 fractions that had variable molecular weight distributions and acid-group contents. Intramolecular interactions were determined to predominate over intermolecular interactions for the Suwannee River fulvic acid in all solvents at weight-volume concentrations less than 1%.

    HUMIC SUBSTANCES ARE HETEROGENEOUS with respect to the diversity of molecules that compose the mixture, and with respect to the diverse functional groups, structural units, and configurations that compose individ-ual molecules. Intermolecular interactions between molecular components in the mixture, or intramolecular interactions between organic functional groups within a molecule, may change chemical and physical properties of humic substances among various environments. Molecular interactions of humic substances are dependent on temperature, pH, ionic strength, type of solvation, degree of hydration (on drying), type of countercations, and concentration of the humic substance in solution. Environmentally signifi-cant phenomena that may be affected by molecular interactions of humic substances include organic-contaminant partitioning, trace-element inter-

    0065-2393/89/0219-0025$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.

  • 26 AQUATIC HUMIC SUBSTANCES

    actions, and rates of decomposition of humic substances in various environments.

    Humin and humic acid are reactive substances that form amorphous organic phases in soil, sediment, and water; types of interactions and ordering of these interactions into a membrane model are discussed by Wershaw and Marinsky (I). Fulvic acid exists as a solute in natural waters more commonly than does humic acid, and molecular interactions are controlled by the chemistry of water. A question frequently asked concerning fulvic acid is whether fulvic acid exists in water as a solute dissolved in an ideal solution, or does it interact to form nonideal molecular aggregates? If fulvic acid exists as a molecular aggregate in water, the chemistry of fulvic acid cannot be described as the sum of the characteristics of the individual components because of intermolecular interactions that modify component characteristics.

    This chapter wil l investigate the mechanisms of molecular interactions in a stream fulvic acid isolated from the origin of the Suwannee River at the Okefenokee Swamp in southern Georgia. Variations in solution densities and molecular-size distributions with solvents of differing polarity and concentrations were used to distinguish intermolecular from intramolecular interactions. A two-stage fractionation of the fulvic acid on silica gel was designed to disrupt heterogeneous molecular aggregates (if they exist) and obtain smaller-sized particles in solutions of the fractions that are more homogeneous than fulvic acid.

    Both polar and nonpolar interactions undoubtedly are responsible for the often unique chemical and physical properties of extremely complex mixtures that are called humic substances. Polar interactions such as hydrogen bonding will be emphasized in this chapter over nonpolar interactions because of the predominantly polar character of the Suwannee River fulvic acid. Molecular aggregates of fulvic acid caused by nonpolar interactions are not likely to form in natural waters, because concentrations of fulvic acid at natural concentrations are less than critical micelle concentrations of known detergents and because of the lack of significant nonpolar moieties in fulvic acid structure. However, polar interactions, both intramolecular and intermolecular, will increase the nonpolar character of fulvic acid, and they may increase partitioning of nonpolar contaminants into molecular structures of fulvic acid. The findings of polar-interaction mechanisms in fulvic acid also can be related to interactions between polar moieties in humin and humic acid structures. However, nonpolar interactions also should be recognized as relevant to humin and humic acid.

    Experimental Methods and Materials The Suwannee River was sampled at its origin at the outlet of the Okefenokee Swamp near Fargo, Georgia, during November 1983. Onsite, 8104 Lof water were processed through a filtration and column-adsorption system that fractionated and isolated

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

  • 2. LEENHEER ET AL. Implications of Mixture Characteristics 27

    organic solutes into hydrophobic-neutral, strong hydrophobic fulvic acid, weak hydrophobic fulvic acid, hydrophilic acid, and hydrophilic-neutral fractions (2). The theory and chemical significance underlying these operational fractionation procedures are given in previous reports (3-7). Most (94.6%) of the fulvic acid was contained in the strong hydrophobic fulvic acid fraction; 382.4 g of this fraction was isolated from water and used in the study described in this chapter. The strong hydrophobic fulvic acid fraction was 66% of the dissolved organic-carbon concentration, 38.4 mg/L of the Suwannee River water.

    Molecular-weight distributions of the Suwannee River fulvic acid were estimated by the degree of polydispersity determined by the ratio of M w (weight-average molecular weight) to Mn (number-average molecular weight). M w and M n were determined by equilibrium centrifugation. Equilibrium-ultracentrifugation experiments were conducted on an ultracentrifuge (Beckman L8-70 M, equipped with a Beckman Prep UV scanner) that assayed solute concentrations at 280 nm throughout the ultracentrifuge cells during the run. A four-phase titanium rotor (AnF, 5.7-cm radius) was used at speeds of 20,000 to 40,000 rpm.

    Equilibrium between sedimentation and molecular diffusion of the solutes was virtually complete between 24 and 48 h of centrifugation. Analytical ultracentrifu-gation cells were constructed of aluminum bodies with centerpieces of Kel-F (for aqueous 0.2 M KC1 solvent) or aluminum (for organic solvents). Each cell had a sample and reference compartment with quartz windows in the centerpiece that contained 300 L of solution. The immiscible fluorocarbon oil (FC-43), used to determine the bottom of the solution in the cell, was used only for aqueous solvents and not for organic solvents that partially solubilized the oil. The M w and M n values were determined from the equilibrium-ultracentrifugation curves by the method of Lansing and Kraemer (8).

    Solution densities of the Suwannee River fulvic acid at 20 C were determined by pycnometry (9) in the solvent used for equilibrium ultracentrifugation. Pycnom-eters of 2-mL capacity gave results accurate to 0.02 g/mL for the samples at 1-2% w/v solute concentrations; 10-mL pycometers were used at the 0.4-1% solute concentrations. Partial molar volume or apparent molar volumes (for nonideal solutions) were calculated as the reciprocal of the solution density. All concentrations of humic substances in this chapter are expressed on a weight-per-solution-volume basis.

    Solvents used for density determinations included distilled water, aqueous 0.2 M KCl, tetrahydrofuran (freshly distilled), dioxane (freshly distilled), N,N-dimethylformamide (dried over calcium hydride and distilled), acetonitrile, and glacial acetic acid. Equilibrium ultracentrifugation was conducted with aqueous 0.2 M KCl, tetrahydrofuran, and glacial acetic acid. Both tetrahydrofuran and glacial acetic acid have small dielectric constants, so that the carboxyl group does not ionize to produce negative charges on fulvic acid solutes. These negative charges interfere with solute sedimentation during centrifugation. Tetrahydrofuran was preferred for use as a solvent for centrifugation because of its minimal density and viscosity; however, glacial acetic acid was used when samples were not soluble in tetrahydrofuran. Fulvic acid ionization was suppressed by 0.2 M KCl for centrifugation of samples dissolved in water.

    Fractionation of fulvic acid on silica gel was conducted with 100-200-mesh activated silica (ICN Biochemicals). The first-stage silica fractionation used a 2-L bed volume of silica contained in a 50-mm-i.d. X 950-m m glass column. The column was condi

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