Mechanisms of slow sorption of organic chemicals to natural particles

The use of equilibrium expressions for sorption to natural particles in fate and transport models is often invalid due to slow kinetics. This paper reviews recent research into the causes of slow sorption and desorption rates at the intraparticle level and how this phenomenon relates to contaminant transport, bioavailability, and remediation. Sorption kinetics are complex and poorly predictable at present. Diffusion limitations appear to play a major role. Contending mechanisms include diffusion through natural organic matter matrices and diffusion through intraparticle nanopores. These mechanisms probably operate simultaneously, but the relative importance of each in a given system is indeterminate. Sorption shows anomalous behaviors that are presently not well explained by the simple diffusion models, including concentration 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 along with molecular diffusion in slow kinetics. The possible existence of high-energy adsorption sites both within the internal matrix of organic matter and in nanopores is discussed. Sorption can be rate-limiting to biodegradation, bioavailablity, and subsurface transport of contaminants. Characterization of mechanism is thus critical for fate and risk assessment. Studies are needed to measure desorption kinetics under digestive and respiratory conditions in receptor organisms. Conditions under which the constraint of slow desorption may be overcome are discussed, including the addition of biological or chemical agents, the application of heat, and the physical alteration of the soil.