FLUORESCENCE LIFETIME MEASUREMENTS OF FLUORANTHENE, 1-NAPHTHOL, AND NAPROPAMIDE IN THE PRESENCE OF DISSOLVED HUMIC-ACID

The binding of hydrophobic organic solutes and nonionic pesticides with dissolved humic substances may significantly influence their behavior and fate in natural systems. The binding of fluoranthene, 1-naphthol, and napropamide [2-(1-naphthoxy)-N,N-diethylpropionamide] by an International Humic Substances Society reference soil humic acid in aqueous solutions was studied using fluorescence quenching and lifetime measurements. The quenching of fluoranthene, 1-naphthol, and napropamide fluorescence by the humic acid was adequately described by the Stern-Volmer equation. The effects of temperature ranging from 283 to 313 K on the quenching ratios was not significant, and the fluorescence lifetimes of these compounds did not shorten as a function of increasing humic acid concentrations. No significant increase in rotational correlation times,longer than probe rotation, was detected by time-resolved anisotropy measurements of napropamide and 1-naphthol in the presence of humic acid. Based on the above data, it was concluded that the quenching mechanism for these compounds by humic acid was primarily static, resulting from a partition-like interaction of these solutes with dissolved humic acid, Although the exact binding mechanisms cannot be ascertained from the data collected, it is clear that the quenched fluorophores were not fluorescent (e.g., dark complex) suggesting that dynamic quenching did not significantly contribute to fluorescent quenching in the presence of humic acid. The conditional binding constants (distribution coefficients) normalized to dissolved organic C (K-doc) for fluoranthene, napropamide, and 1-naphthol with the reference soil humic acid were 91.5, 23.2, and 20.1 L/g of C, respectively, consistent with the expected order based on the water solubilities of these compounds. Calculations using the conditional binding constants indicate that substantial fractions of these organic solutes may be associated with dissolved humic substances, suggesting that naturally occurring dissolved organic C may significantly influence the fate of these chemicals in natural systems.