Interactions between a Hydrophobic Organic Chemical and Natural Organic Matter:  Equilibrium and Kinetic Studies

Abstract

Interactions between a well-characterized, purified, peat humic acid and pentachlorobenzene were studied in equilibrium and kinetic experiments. The kinetic experiments, performed with a gas-purge method, showed the presence of a fast-desorbing, labile fraction and a slow-desorbing, nonlabile fraction. Increased contact time did not change the measured continuous desorption curves. A desorption experiment with interrupted flow and the measured isotherm suggested nonlinear sorption behavior. A first-order model consisting of two parallel “sites” could describe the continuous desorption curves but failed to describe the adsorption isotherm and the desorption curve with interrupted flow. A Langmuir model with one site was able to describe the characteristics of the desorption experiments and of the adsorption isotherm, but it did not describe the observed nonlabile fraction. Expanding the model with an additional Langmuir or first-order site did not lead to a good description of the various experiments. We speculate that the nonlabile fraction is a consequence of a change of conformation of the humic acid after the binding of pentachlorobenzene. This structural change leads to entrapment of pentachlorobenzene within the humic acid structure.

Sorption of anthropogenic organic compounds by soil organic matter: a mechanistic consideration

“…3) because HSCAM has rigid structures, which should not be altered by cations. Furthermore, nonlinear isotherms of HOCs were reported for dissolved soil organic matter (Maxin and Kogel-Knabner 1995;Schlebaum et. al.…”

Aging effects on sorption–desorption behaviors of PAHs in different natural organic matters

“…In Schlebaum et al's study, results showed that part of pentachlorobenzene sorbed to a dissolved humic acid resisted desorption. And they proposed the likely mechanism that the non-labile fraction was a consequence of a change of conformation of the HA after the binding of pentachlorobenzene [17]. Ge et al observed a more compact conformation of humic substance assemblies after the sorption of naphthalene using atomic force microscopy [40].…”

“…Weber et al introduced a composite model, the distributed reactivity model for sorption by soils and sediments to characterize intrinsic heterogeneities in the properties and behaviors of soils and sediments [16]. A similar model was applied by Schlebaum in a study with humic acid (HA) [17]. These models were parallel models with discrete sites that may have different affinities for HOCs and different sorption mechanisms.…”

Assessing desorption resistance of PAHs in dissolved humic substances by membrane-based passive samplers

“…In Schlebaum et al's study, the results showed that part of pentachlorobenzene sorbed to a dissolved humic acid (HA) resisted desorption. They speculated that the nonlabile fraction was a consequence of a change of conformation of the HA after the binding of pentachlorobenzene [9]. Several different rate models have been used to describe desorption of HOCs from natural sorbents.…”

“…Time-integrated techniques have been used as infinite sink for desorbed compounds in the literature for time frames from a few hours to a few days. Examples of these methods include Tenaxaided desorption and equilibrium dialysis [5,9,[11][12][13]. Tenax extraction has advantages of high sorptive capacity and rapid adsorption from water.…”