Effects of metal cations on sorption and desorption of organic compounds in humic acids

Sorption/desorption of select model organic compounds (naphthalene, phenant hrene, and alpha -naphthol) by cation-saturated humic acids (HA) was invest igated using a batch equilibration method. All sorption isotherms were nonl inear, with Freundlich exponents (N values) less than one. The N values for all sorption isotherms were nearly unchanged for Ca-HA, compared with untr eated HA (H-HA), but were substantially decreased for AI-HA, For a given so rbent, N values followed the order: naphthalene > phenanthrene > alpha -nap hthol. Hysteresis occurred in all systems except for naphthalene/H-HA and n aphthalene/Ca-HA systems. The hysteresis index, evaluated by the ratio of t he N values for desorption and sorption, followed the order: H-HA congruent to Ca-HA > Al-HA and naphthalene > phenanthrene > rr-naphthol. The smaller the hysteresis index, the greater the degree of hysteresis. There was a go od correlation (r(2) > 0.90) between hysteresis index and N values of sorpt ion isotherms, These results can be successfully interpreted in the context of the dual-mode sorption model. Furthermore, these results suggest that t ype of cations can significantly influence sorption and desorption of hydro phobic organic molecules in humic substances.