Mineral Surfaces and Humic Substances: Partitioning of Hydrophobic Organic Pollutants
The influence of aqueous chemistry on the sorption reactions of three polycyclic aromatic hydrocarbons (PAHs) with Suwannee River humic substances and with an inorganic fused silica surface was examined using fluorescence techniques. By varying the pH, ionic strength and composition of the background electrolyte, the influence of solution chemistry on carbon-normalized partition coefficients and surface area-normalized reaction quotients and adsorption constants was observed. The humic substances examined in this study were present as freely dissolved species and as organic coatings on colloidal-sized aluminum oxide particles.
Binding of the PAHs by dissolved and adsorbed humic material was complete within 3 minutes for all samples; several reactions with dissolved humic substances appeared to be equilibrated within 20 seconds. The association of a PAH compound with Suwannee River humic material appeared to depend on the size of the solute molecule and its ability to fit into hydrophobic cavities in the humic structure. The adsorption of humic and fulvic acid onto alumina decreased their ability to bind perylene. The ability of adsorbed humic substances to bind perylene was dependent on the type of surface complex which was formed between the alumina surface and the humic material. A major effect of solution chemistry was to alter the adsorption mechanisms of the humic substances, which in turn dictated the types of surface complexes formed.
The adsorption of perylene to a nonporous silica surface required 2½ to 4 hours to reach equilibrium. An apparent equilibrium was attained in 60 to 90 minutes for desorption reactions. Quantitative recovery of adsorbed perylene was observed after a 5- to 10-minute methanol extraction. The observed adsorption rates were correlated with the fugacity of perylene and the surface speciation of fused silica. From long-term adsorption data it appears that two different processes are operative in the adsorption of perylene to the fused silica surface. The nature of these processes [...]
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