Effects of Metal Cations on Sorption and Desorption of Organic Compounds in Humic Acids

Sorption/desorption of select model organic compounds (naphthalene, phenanthrene, and α-naphthol) by cation-saturated humic acids (HA) was investigated using a batch equilibration method. All sorption isotherms were nonlinear, with Freundlich exponents (N values) less than one. The N values for all sorption isotherms were nearly unchanged for Ca-HA, compared with untreated HA (H-HA), but were substantially decreased for Al-HA. For a given sorbent, N values followed the order: naphthalene > phenanthrene > α-naphthol. Hysteresis occurred in all systems except for naphthalene/H-HA and naphthalene/Ca-HA systems. The hysteresis index, evaluated by the ratio of the N values for desorption and sorption, followed the order: H-HA ≅ Ca-HA > Al-HA and naphthalene > phenanthrene > α-naphthol. The smaller the hysteresis index, the greater the degree of hysteresis. There was a good correlation (r2 > 0.90) between hysteresis index and N values of sorption isotherms. These results can be successfully interpreted in the context of the dual-mode sorption model. Furthermore, these results suggest that type of cations can significantly influence sorption and desorption of hydrophobic organic molecules in humic substances.