THE ROLE OF SORBED HUMIC SUBSTANCES ON THE DISTRIBUTION OF ORGANIC AND INORGANIC CONTAMINANTS IN GROUNDWATER

Mineral-bound humic substances modify inorganic surfaces in subsurface sediments, changing the nature and number of complexation sites for c ontaminants. Because of adsorptive enrichment, the reactive surface ar ea or site concentration contributed by mineral-bound humic substances can exceed that of dissolved or colloidal humic substances by two ord ers of magnitude. Mineral-bound humic materials may, therefore, provid e a major sink for the removal of contaminants in groundwater. The rea ctivity of the humic substance is primarily determined by the structur al and bulk chemical properties of the humic substance and the aqueous solution chemistry. Organic and inorganic contaminants sorb readily t o mineral-bound humic substances. The sorption of hydrophobic organic compounds increases as ionic strength decreases, is enhanced by divale nt cations, and displays non-linear isotherms and competitive adsorpti on behavior. Collectively, these results suggest that hydrophobic adso rption, rather than phase partitioning, is the primary sorption mechan ism for neutral organic molecules on these particle coatings. Mineral- bound humic substances augment, rather than change, the intrinsic comp lexation properties of mineral surfaces for metal cations. The degree of sorption enhancement promoted by mineral-bound organic material var ies strongly with pH and depends on the magnitude of the stability con stants between the metal cation and the humic substance, the strength and magnitude of adsorption of the humic substance by the mineral surf ace, and the extent of aqueous complex formation between the non-sorbe d humic substance and metal. The simplest sorption model for humate-mo dified surfaces is the linear additivity model (LAM). Sorption data fo r certain hydrophobic organic compounds and metal cations appear to co nform to this model.