Interaction of Hydrophobic Organic Compounds with Mineral-Bound Humic Substances

The sorption of hydrophobic organic compounds (HOC) on mineral-associated peat humic acid (PHA) was evaluated under different pH and electrolyte regimes. Relative size distribution measurements indicated that PHA was “coiled” in solution at high ionic strength (I) and elongated at low I. The sorption of PHA to hematite and kaolinite varied with I and electrolyte cation, suggesting that the configuration of the humic acid in solution influenced its structure on the mineral surface. The sorption maxima for PHA on kaolinite indicated that PHA occupies twice the mineral surface area at low I (0.005) as that observed at high I (0.1). HOC sorption to mineral-bound PHA in Na+ electrolyte was greater at lower I, indicating that humate structure was a plausible determinant of HOC sorption. Freundlich isotherms of dibenzothiophene on the PHA-coated kaolinite did not display unit slope, regardless of pH, I, or cation. Carbazole and anthracene displayed competitive behavior for sorption onto PHA-coated kaolinite. Collectively, the experimental observations indicate that hydrophobic adsorption rather than phase partitioning was the dominant mode of HOC binding