The binding capacity of humic substances of peat and hymatomelanic acids in relation to polyaromatic hydrocarbons (as illustrated by naphthalene)

Humic substances are polydisperse compounds with a nonstoichiometric composition. They are composed of an aromatic carbon skeleton, which leads to their high affinity to polynuclear aromatic hydrocarbons, but the mechanism of this interaction is still unclear. The article offers a description of the binding process of naphthalene based on the structure of the analyzed humic substances. It was found, based on data from electronic spectroscopy and infrared spectros-copy, that humic substances are enriched in polyaromatic moieties that form the basis of their structure. The obtained values of the colority coefficients (E450/E650) showed that humic substances extracted from terrestrial peat have the highest aromaticity; increased values of E450/E650 are observed for humic substances extracted from sphagnum peat, which indicates the low transformation of the original biological material. Increasing the colority coefficients for hymatomelanic acids indicates the reduction of polyconjugated fragments and decrease of the proportion of the peripheral part. A fluorescence quenching method was used to assess the degree of interaction of humic substances with naphthalene. The recorded spectra of naphthalene fluorescence characterize the complex nature of the interaction with humic substances. It was found that humic substances extracted from terrestrial and reed peats completely suppress naphthalene fluorescence, while the low molecular weight fraction of hymatomelanic acids has a minimum quenching effect, and the binding of them with naphthalene is minimal. The binding constant is a quantitative characteristic that allows evaluating the degree of the interaction of humic substances with naphthalene; it was calculated according to the Stern-Volmer equation. Humic substances extracted from terrestrial and reed peats have the highest values of the binding constants. Received values allow estimating the degree of sorption of naphthalene on humic substances. The highest degree of sorption observed is with the addition of a more concentrated solution of humic substances, which is especially noticeable for the terrestrial and reed peats. Their degree of sorption at maximum concentration is increased by 40% compared to the minimum. Thus, the affinity of humic substances with polynuclear hydrocarbons is determined by the source of their origin and the total content of hydrophobic molecules in the unfractionated preparation. The rate constant for the fluorescence quenching was estimated for a more complete understanding of the binding process and proof of the static nature of the quenching. It was experimentally established that the excess of the rate constant of values of 10 (L mol' s') indicates the static nature of the quenching that involves the formation of a complex between the quencher andfluorophore. Thus, based on the evidence found, it is arguable that the aromatic nucleus of humic substances provides the largest contribution to the interaction with naphthalene. It is provided by intermolecular hydrophobic interaction between the "aromatic core" of humic substances and polynuclear aromatic hydrocarbons by stacking interaction.

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