FLUORESCENCE QUENCHING OF POLYCYCLIC AROMATIC-COMPOUNDS BY HUMIC-ACID

Single- and multi-channel stationary and time-resolved fluorescence me asurements on selected polycyclic aromatic compounds (PAC) were perfor med to obtain photo-physical information about fluorophore/soil intera ctions. The fluorescence properties of anthracene, pyrene and phenanth rene in the presence of Aldrich humic acid in water were investigated with stationary and time-resolved fluorescence measurements. For all t hree PACs, fluorescence quenching was dominated by static quenching pr ocesses; dynamic quenching played only a minor role. It was found that the static quenching of phenanthrene fluorescence was significantly l ess efficient than the fluorescence quenching of anthracene and pyrene . The Stern-Volmer analysis showed a significant effect of pH on stati c quenching of anthracene and pyrene fluorescence; e.g., at pH 2, quen ching of pyrene fluorescence was approximately three times more effect ive 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-Volm er constant of K-SV = 3 x 10(-3) l mg(-1) was obtained. The experiment ally determined Stern-Volmer constants for dynamic and static fluoresc ence quenching were taken to estimate an average molar mass of the hum ic acid used and to assess the pH-dependent organic carbon-normalized partition coefficients K-OC.