SEQUESTRATION OF HYDROPHOBIC ORGANIC CONTAMINANTS BY GEOSORBENTS

The chemical interactions of hydrophobic organic contaminants (HOCs) w ith soils and sediments (geosorbents) may result in strong binding and slow subsequent release rates that significantly affect remediation r ates and endpoints. The underlying physical and chemical phenomena pot entially responsible for this apparent sequestration of HOCs by geosor bents are not well understood. This challenges our concepts for assess ing exposure and toxicity and for setting environmental quality criter ia. Currently there are no direct observational data revealing the mol ecular-scale locations in which nonpolar organic compounds accumulate when associated with natural soils or sediments. Hence macroscopic obs ervations are used to make inferences about sorption mechanisms and th e chemical factors affecting the sequestration of HOCs by geosorbents. Recent observations suggest that HOC interactions with geosorbents co mprise different inorganic and organic surfaces and matrices, and dist inctions may be drawn along these lines, particularly with regard to t he roles of inorganic micropores, natural sorbent organic matter compo nents, combustion residue particulate carbon, and spilled organic liqu ids. Certain manipulations of sorbates or sorbent media may help revea l sorption mechanisms, but mixed sorption phenomena complicate the int erpretation of macroscopic data regarding diffusion of HOCs into and o ut of different matrices and the hysteretic sorption and aging effects commonly observed for geosorbents. Analytical characterizations at th e microscale, and mechanistic models derived therefrom, are needed to advance scientific knowledge of HOC sequestration, release, and enviro nmental risk.