Fluorescence quenching of polycyclic aromatic compounds by humic acid

Abstract

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 × 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.