Effects of aqueous chemistry on the binding of polycyclic aromatic hydrocarbons by dissolved humic materials

Abstract

The influence of solution chemistry on the binding of three polycyclic aromatic hydrocarbons (PAHs) by well-characterized humic material (Suwannee River humic and fulvic acid) was examined by using fluorescence quenching techniques. Our experiments show that binding is complete within 3 min and that the fluorescence of PAH compounds associated with the humic substances is fully quenched as evidenced by quantum yields which approached zero for all systems. These observations validate the use of fluorescence quenching in determining partition coefficients. In NaCl solutions, the binding of PAHs by Suwannee River humic material generally decreased with increases in pH (constant ionic strength) and generally decreased with increasing ionic strength (fixed pH). The presence of Ca^(2+) yielded mixed results: at neutral to high pH values, it generally increased the binding of PAHs relative to that in NaCl solutions, while at low pH it generally had little effect on the binding. From the results of this study, it is hypothesized that the binding of a particular PAH compound by Suwannee River humic substances depends not only on the hydrophobicity of the PAH solute but also on the size of the solute molecule and its ability to fit into hydrophobic cavities in humic and fulvic material. This hypothesis is supported by the experimental observations above, as well as the failure of a Flory-Huggins partitioning (i.e., dissolution) model to consistently characterize the hydrophobic environment of the humic substances.

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