MECHANISMS OF SLOW SORPTION OF ORGANIC-CHEMICALS TO NATURAL PARTICLES

The use of equilibrium expressions for sorption to natural particles i n fate and transport models is often invalid due to slow kinetics. Thi s paper reviews recent research into the causes of slow sorption and d esorption rates at the intraparticle level and how this phenomenon rel ates to contaminant transport, bioavailability, and remediation. Sorpt ion 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 throu gh intraparticle nanopores. These mechanisms probably operate simultan eously, but the relative importance of each in a given system is indet erminate. Sorption shows anomalous behaviors that are presently not we ll explained by the simple diffusion models, including concentration d ependence of the slow fraction, distributed rate constants, and kineti c hysteresis. Research is needed to determine whether adsorption/desor ption 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, bioavailab lity, and subsurface transport of contaminants. Characterization of me chanism is thus critical for fate and risk assessment. Studies are nee ded to measure desorption kinetics under digestive and respiratory con ditions in receptor organisms. Conditions under which the constraint o f slow desorption may be overcome are discussed, including the additio n of biological or chemical agents, the application of heat, and the p hysical alteration of the soil.