The measurement and interpretation of sorption and desorption rates for organic compounds in soil media

Sorption controls the physical and biological availability of chemicals in soil. Most organic molecules undergo primarily weak physisorption interactions and the driving force for sorption is the hydrophobic effect. Sorption and desorption rates, therefore, are governed mainly by molecular diffusion through the fixed interstitial pores of particle aggregates and through the three-dimensional pseudophase of soil organic matter. Retardation in the fixed pore system is due to tortuosity, chromatographic adsorption to pore walls, and, in the smallest pores, steric hindrance. Soil organic matter, which has the strongest affinity for most organic compounds, may exist in rubbery and glassy phases and retards sorption and desorption by its viscosity and by the presence of internal nanopores, which detain molecules and may sterically inhibit their migration. Soot carbon and/or ancient organic matter may be present in some soils but their roles are yet unclear. Desorption rates are correlated with the size and shape of the diffusant. Hysteresis is commonly observed but a satisfactory explanation for it has yet to emerge. Mathematical models based on bond energetics, driving force theory, diffusion, and stochastic analysis are discussed. These models have been used to describe batch experiments and have been coupled to advection-dispersion transport equations for use in flowing water systems. Diffusion models are the most realistic but also the most difficult to apply because diffusion is highly dependent on the geometry and composition of the sorbent. Soil heterogeneity impedes the mechanistic interpretation of rates. Particles span an extremely wide range of sizes. The appropriate diffusion length scale is of ten uncertain. The diffusion coefficient is expected to be concentration dependent in any diffusing medium in which sorption is nonlinear. Furthermore, the diffusant may alter the structure of soil organic matter. Bioavailability can be rate limited by desorption. Cells are believed to access only dissolved molecules, but organisms may affect sorption kinetics indirectly by steepening the concentration gradient or by altering soil properties through bioactivity. Coupled sorption-biodegradation models are necessary whenever nonequilibrium conditions prevail during exposure. Models coupling Monod or first-order biodegradation kinetics with "two-site," driving-force, or diffusion models have been employed. Some have been used in conjunction with the advection-dispersion transport model. © 2000 Academic Press.