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 fluoresce nce quenching. Sources included isolated humic acids, fulvic acids, and com bined humic and fulvic fractions from soil, peat, and freshwater as well as Aldrich humic acid. Each of the humic materials was characterized by eleme ntal composition, ultraviolet absorbance at 280 nm, molecular weight, and f or 19 samples, composition of main structural fragments determined by C-13 solution-state NMR. The magnitude of the K-oc values correlated strongly wi th the independent descriptors of aromaticity of humic materials, including atomic H/C ratio, absorptivity at 280 nm, and three interdependent C-13 NM R descriptors (C-Ar-H,C-R,C- Sigma C-Ar, Sigma C-Ar/Sigma C-Alk) Statistica l comparison of humic sources grouped by the origin revealed that binding a ffinities were best predicted by the C-13 NMR descriptors, with a slight pr evalence of Sigma C-Ar/Sigma C-Alk ratio, while molecular weight was the po orest predictor. The latter produced either direct or inverse significant c orrelation with the K-oc values depending upon the origin and/or fractional composition of the grouped humic materials.