| Abstract
| - The use of equilibrium expressions for sorption tonatural particles in fate and transport models is ofteninvalid due to slow kinetics. This paper reviewsrecent research into the causes of slow sorption anddesorption rates at the intraparticle level and howthis phenomenon relates to contaminant transport, bioavailability, and remediation. Sorption kinetics arecomplex and poorly predictable at present. Diffusionlimitations appear to play a major role. Contendingmechanisms include diffusion through natural organicmatter matrices and diffusion through intraparticlenanopores. These mechanisms probably operate simultaneously, but the relative importance of eachin a given system is indeterminate. Sorption showsanomalous behaviors that are presently not wellexplained by the simple diffusion models, includingconcentration dependence of the slow fraction,distributed rate constants, and kinetic hysteresis.Research is needed to determine whether adsorption/desorption bond energies may play a role alongwith molecular diffusion in slow kinetics. The possible existence of high-energy adsorption sites bothwithin the internal matrix of organic matter and innanopores is discussed. Sorption can be rate-limitingto biodegradation, bioavailablity, and subsurfacetransport of contaminants. Characterization of mechanism is thus critical for fate and risk assessment.Studies are needed to measure desorption kineticsunderdigestive and respiratory conditions in receptororganisms. Conditions under which the constraint ofslow desorption may be overcome are discussed,including the addition of biological or chemical agents,the application of heat, and the physical alterationof the soil.
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