Relationships between structure and binding affinity of humic substances for polycyclic aromatic hydrocarbons: Relevance of molecular descriptors

Partition coefficients for the binding affinities of pyrene, fluoranthene, and anthracene to 26 different humic materials were determined by fluorescence quenching. Sources included isolated humic acids, fulvic acids, and combined humic and fulvic fractions from soil, peat, and freshwater as well as Aldrich humic acid. Each of the humic materials was characterized by elemental composition, ultraviolet absorbance at 280 nm, molecular weight, and for 19 samples, composition of main structural fragments determined by C-13 solution-state NMR. The magnitude of the K-oc values correlated strongly with the independent descriptors of aromaticity of humic materials, including atomic H/C ratio, absorptivity at 280 nm, and three interdependent C-13 NMR descriptors (C-Ar-H,C-R,C- Sigma C-Ar, Sigma C-Ar/Sigma C-Alk) Statistical comparison of humic sources grouped by the origin revealed that binding affinities were best predicted by the C-13 NMR descriptors, with a slight prevalence of Sigma C-Ar/Sigma C-Alk ratio, while molecular weight was the poorest predictor. The latter produced either direct or inverse significant correlation with the K-oc values depending upon the origin and/or fractional composition of the grouped humic materials.