[Advances in Chemistry] Aquatic Humic Substances Volume 219 (Influence on Fate and Treatment of Pollutants) || Charge-Transfer Interaction Between Dissolved Humic Materials and Chloranil

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  • 12 Charge-Transfer Interaction Between Dissolved Humic Materials and Chloranil

    Michael E . Melcer, Margaret S. Zalewski, and John P. Hassett

    Department of Chemistry, State University of New York, College of Environmental Science and Forestry, Syracuse, NY 13210

    Marion A. Brisk

    City University of New York Medical School, New York, NY 10038

    Using UV-difference spectroscopy, we have established the electron-donating ability of humic materials and introduced a simple, direct method for measuring binding constants. Chloranil (2,3,5,6-tetrachloro-p-benzoquinone) was combined with Aldrich humic materials and two dissolved organic matter samples. These mixtures gave rise to shifts in the chloranil 290-nm absorption band. Spectra of such solutions versus a chloranil reference contained a negative peak at this wavelength due to the binding interaction. Binding with the Aldrich humic material continued to occur over a period of several weeks. Binding with the dissolved organic materials occurred on the order of a few hours. A charge-transfer mechanism can be postulated for the chloranil-humic system and for similar systems.

    DISSOLVED HUMIC MATERIALS BIND ORGANIC CONTAMINANTS that are re

    leased into the environment (1-8). Bound pollutants behave differently from freely dissolved molecules, with changes in chemical processes such as hydrolysis and photolysis rates (9, JO). Physical parameters of bound organic compounds in the water column are also affected. These parameters include an increase in their apparent aqueous solubility (II), a decrease in their partitioning onto particulate matter, and a reduction in their rates of

    065-2393/89/0219-0173$06.00/0 1989 American Chemical Society

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  • 174 AQUATIC HUMIC SUBSTANCES

    vaporization (8, 12). The bioavailability of organic pollutants to aquatic organisms is also altered (7, 13). Because the complexation of organic contaminants may markedly change their chemical and physical pathways in the environment, an understanding of the binding interaction is crucial in order to predict the fate of chemical pollutants. Binding constants have been determined for several organic molecules by measuring the effect of binding on some physical property of the molecule (e.g., solubility enhancement, dialysis, vaporization depression, and sorption). However, the nature of the interaction has not been elucidated.

    Lindquist (14,15) demonstrated electron-donating and -accepting properties of humic materials by analyzing the effect of p-benzoquirione, a known electron acceptor, and hydroquinone, an electron donor, on the visible spectrum of humic materials. Mathus (26) also reported electron donation by triazenes to dissolved organic material, detected by electron spin resonance (ESR) and IR shifts. Mudambi (10) found that the presence of humic material altered the photochemical action spectrum of mirex, a result implying an electronic interaction between the two materials.

    Humic materials contain both electron-rich and electron-deficient sites; this structure would account for their electron-donating and -accepting properties (17). Electron-donating structures (such as hydroquinones, ethers, alcohols, nitrogen-containing compounds, and phenolic moieties) can form electron donor-acceptor complexes (charge-transfer complexes) with suitable electron acceptors such as benzoquinones and many chlorinated compounds in aqueous solution. Our earlier results using UV-difference spectroscopy also support a charge-transfer interaction mechanism between humic materials and organic molecules (18). Here we wil l use this technique to compare the binding constants and kinetics of interaction of chloranil with humic materials from three sources.

    Experimental Methods Aqueous solutions of chloranil (2,3,5,6-tetrachloro-p-benzoquinone) were mixed with Aldrich humic acid and water samples from Labrador Hollow (a marshy pond in Tully, NY) and Limestone Creek (a stream in Fayette ville, NY). A humic acid stock solution was prepared by dissolving 2 g of Aldrich humic acid in 1 L of distilled water. This solution was stirred for 24 h and filtered through a glass fiber filter (Whatman, 934-AH) to remove suspended material. A solution containing 6.0 X 10 6 M chloranil with 2.7 mg of C / L of humic acid was prepared from the stock humic acid solution. Water from the two field sources was collected, filtered, diluted with distilled water, and mixed with chloranil such that the final solutions contained 3.0 mg of D O C / L and 5 " 6 M chloranil. A l l solutions were stored in the dark at room temperature.

    The chloranil-humic acid mixtures were each analyzed by UV-difference spectroscopy. Difference spectra of the Aldrich humic acid mixtures were

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

  • 12. MELCER ET AL. Charge-Transfer Interactions with Chloranil 175 determined upon mixing and were repeated at various intervals for up to 32 days. Dissolved organic matter samples were analyzed at shorter intervals for 2 or 4 h from the time of mixing. Two sets of spectra were obtained for each solution: spectra of sample mixtures versus a chloranil reference of equal concentration and spectra of sample mixtures versus a humic material reference of equal concentration. A l l spectra were obtained with a spectrophotometer (Varian D M S 100) with sample and reference solutions in matched 1-cm cuvettes. The baseline was corrected with an internal baseline correction program referenced to a water blank. Al l spectra were scanned from 190 to 350 nm at a rate of 100 nm/min.

    Analysis of Difference Spectra Chloranil is a well-known electron acceptor in charge-transfer complexes (19-24). The 290-nm absorption band of free chloranil is sensitive to com-plexation; a red shift is typically observed with binding to an electron donor, presumably due to an increase in electron density that destabilizes the molecular orbitals involved in the transition. U V spectra of an aqueous chloranil (6.0 X 10 6 M) and Aldrich humic acid (2.7 mg of C / L ) solution referenced to a 2.7 mg of C / L of humic acid solution indicate that over a period of 32 days the 290-nm band broadens and shifts to lower energy (Figure 1). The

    190 290 350 ()

    Figure 1. Difference spectra of a humic-chloranil solution measured over 32 days after mixing. Sample: Aldrich humic acid (2.7 mg of C/L) and chloranil (6.0 x JO" 6 M). Reference: Aldrich humic acid (2.7 mg of C/L). (Reprinted

    with permission from ref. 18. Copyright 1987 Pergamon Journals Ltd.)

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

  • 176 AQUATIC HUMIC SUBSTANCES shift is consistent with a charge-transfer model in which an increase in the chloranil electron density results from complexation with humic molecules. The broadening and shifting of the 290-nm band indicates that the fraction of bound-unbound chloranil increases with time.

    The UV-difference spectra of the same chloranil-Aldrich humic solution compared to a 6.0 x 10"6 M chloranil solution (Figure 2) shows the same trend. Because of complexation of the chloranil to humic materials, the concentration of the free dissolved chloranil in the sample is less than the concentration of chloranil in the reference; a negative peak at 290 nm results. This negative peak is therefore proportional to the concentration of the bound chloranil. Thus, the increasing negative peak intensity indicates a growing bound-unbound ([Ch] b/[Ch] u) ratio with time (Figure 2).

    The negative peak intensity at 290 nm is also a function of the total chloranil concentration (18). As the total chloranil concentration is increased, the negative peak intensity also increases, a result indicating a rise in the

    BO 290 350 NM)

    Figure 2. Difference spectra of a humic-chforanil solution measured over 32 days after mixing. Sample: Aldrich humic acid (2.7 mg of C/L) and chloranil (6.0 x 10^6M). Reference: chloranil solution (6.0 X 10~6M). (Reprinted with

    permission from ref 18. Copyright 1987 Pergamon Journals Ltd.)

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

  • 12. MELCER ET AL. Charge-Transfer Interactions with Chloranil 177

    concentration of complexe! chloranil. The concentration of complexe! chloranil can be determined directly by measuring the negative peak absorbance and relating it to a Beer's law plot for chloranil in the concentration region of interest. The bound-unbound ratio can then be calculated by using equation 1:

    [h]b = Ab [Ch] u A 0 - Ab U

    where A 0 = absorbance associated with initial concentration before any complexation and Ab = absorbance of complexed chloranil after 4 days (intensity of negative peak).

    The ratio of [Ch] b / [Ch] u for this system was determined to be 3.02. A unitless partition coefficient K d o c can be calculated from this ratio:

    - . _ _ L _ ( 2 ) *** " [Ch] u D O C W

    where D O C is the concentration of dissolved organic carbon in milligrams of C per milligram of H 2 0 .

    After 4 days, Kdoc was 1.2 10 5 in a 2.7 mg of C / L of humic acid solution; this result indicates that 30% of the total chloranil was bound. A Kdoc of 5 10 5 (60% bound) was calculated for the 6 X 10 ~ 6 M total chloranil-humic acid solution after 32 days. The Kdoc for the chloranil-humic acid complex appeared to remain constant 4-7 days after mixing. However, after approximately 7 days, negative peak intensity of the 290-nm valley began to increase slowly, leveling off again after approximately 15-20 days from the time of initial mixing. This two-step increase in binding may be due to conformational changes of the humic macromolecule as sites become occupied by chloranil. These conformational changes apparently allow for an increase in availability of binding sites. Zepp et al. (25) also noted a two-step binding process with organic molecules and sediment.

    Difference spectra of the Labrador Hollow (Figure 3) and Limestone Creek (Figure 4) chloranil mixtures versus a chloranil reference also show a negative peak at 290 nm. The K d o c for the Labrador Hollow dissolved organic matter (DOM) at 2 h and Limestone Creek D O M at 4 h were 9.7 10 4 and 1.1 10 5, respectively. A plot of In [Ch] l o t a l / [Ch] u versus time yields a pseudo-first-order rate constant (initial slope) for the complex formation (Figure 5). Values of this constant for the samples studied are: 0.26 h 1 (r = 0.99) for the Labrador Hollow sample; 0.17 h 1 (r = 0.99) for the Limestone Creek sample; and 0.0033 h" 1 (r = 0.98) for the Aldrich humic acid sample. As the DOM-chlorani l interactions proceed the plots show curvature that indicates that the first-order approximation is no longer valid. The first-order plot of the Aldrich humic acid-chloranil interaction

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  • 178 AQUATIC HUMIC SUBSTANCES

    2 hr

    1.3 hr

    0.6 hr

    0.3 hr

    0 hr

    Figure 3. Difference spectra of Labrador Hollow DOC mixed with chloranil over 2 h after mixing. Sample: Labrador Hollow DOC (3.0 mg of CIL) and

    chloranil (5.0 x 10 6 M). Reference: chloranil (5.0 x 10 6 M).

    clearly demonstrates the two-step process (Figure 6). Thus, aside from the differences observed for K d o c , the rate of complex formation is also dependent on the humic material source.

    Humic substances from different sources are structurally different (26). The concept of site-specific binding interaction may explain the different affinities of humic materials from different sources for D D T , as noted by Carter and Suffet (I). It would also explain the differences we found in Kdoc and kinetics of interaction. Both the availability and nature of the binding sites would determine the extent and rate of interaction between a charge acceptor and humic acids.

    Conclusions Difference spectroscopy of humic acids mixed with chloranil in aqueous solutions suggest the presence of binding sites for these organic molecules

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  • 12. MELCER ET AL. Charge-Transfer Interactions with Chloranil 179

    220 nm 290nm 350 nm

    Figure 4. Difference spectra of Limestone Creek DOC mixed with chloranil over 4 h after mixing. Sample: Limestone Creek DOC (3.0 mg of CIL) and

    chloranil (5.0 x 10 6 M). Reference: chloranil (5.0 X 10 ~6 M).

    on the dissolved humic materials. A charge-transfer or electron donor-acceptor mechanism appears to be responsible for the interaction between sites on the humic acids and the organic molecules. This model is supported by:

    previous studies of chloranil interaction with electron donors, with similar shifts of the 290-nm peak;

    a constant Kdoc for different concentrations of hydrophobe, which is consistent with complex formation;

    known presence of electron-donating groups on the humic acid molecule; these groups can act as binding sites for electron acceptors;

    slow formation of the complex (over a period of hours to days), which is characteristic of many charge-transfer complexes.

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