Sorption Dynamics of Organic Compounds in Soils and Sediments

The thermodynamics of sorption of organic compounds in soils and sediments has been studied extensively. This chapter shows that molecules in the sorbed state can possess a wide range of mobilities, from rapidly reversible with “free” states to highly immobile. It describes slow sorption kinetics mechanistically and discusses implications for environmental fate. In many studies, hysteresis, or nonsingularity has been observed, giving rise to sorption isotherms that follow different paths from the adsorptive and desorptive directions. The resistant fraction of sorbate approached 90% and declined with increasing particle size and decreasing organic matter content. The goal of any model is to describe an accurate mechanistic picture, ultimately at the molecular level, and to be able to predict events based on the known properties of sorbate, sorbent, and extrinsic factors. The chapter also discusses some mechanistic concepts and mathematical approaches to describing sorption kinetics that may help explain resistant sorption.