FLUORESCENCE QUENCHING OF POLYCYCLIC AROMATIC-COMPOUNDS BY HUMIC-ACID

Single- and multi-channel stationary and time-resolved fluorescence measurements on selected polycyclic aromatic compounds (PAC) were performed to obtain photo-physical information about fluorophore/soil interactions. The fluorescence properties of anthracene, pyrene and phenanthrene in the presence of Aldrich humic acid in water were investigated with stationary and time-resolved fluorescence measurements. For all three PACs, fluorescence quenching was dominated by static quenching processes; dynamic quenching played only a minor role. It was found that the static quenching of phenanthrene fluorescence was significantly less efficient than the fluorescence quenching of anthracene and pyrene. The Stern-Volmer analysis showed a significant effect of pH on static quenching of anthracene and pyrene fluorescence; e.g., at pH 2, quenching of pyrene fluorescence was approximately three times more effective than at pH 12. This effect could be explained by the pH-dependent macromolecular structure of humic acids. For the dynamic quenching of pyrene fluorescence, no pH dependence was discernible and a Stern-Volmer constant of K-SV = 3 x 10(-3) l mg(-1) was obtained. The experimentally determined Stern-Volmer constants for dynamic and static fluorescence quenching were taken to estimate an average molar mass of the humic acid used and to assess the pH-dependent organic carbon-normalized partition coefficients K-OC.