ADSORPTION OF AQUATIC HUMIC SUBSTANCES ON COLLOIDAL-SIZE ALUMINUM-OXIDE PARTICLES - INFLUENCE OF SOLUTION CHEMISTRY

The adsorption of Suwannee River humic substances (HS) on colloidal-si ze aluminum oxide particles was examined as a function of solution che mistry. The amount of humic acid (HA) or fulvic acid (FA) adsorbed dec reased with increasing pH for all solutions of constant ionic strength . In NaCl solutions at fixed pH values, the adsorption of HA and FA in creased with increasing ionic strength. The presence of Ca2+ enhanced the adsorption of HA but had little effect on FA. For identical soluti on conditions, the amount (by mass) of HA adsorbed to alumina was alwa ys greater than FA. Adsorption densities for both HA and FA showed goo d agreement with the Langmuir equation, and interpretations of adsorpt ion processes were made from the model parameters. For FA, ligand exch ange appears to be the dominant adsorption reaction for the conditions studied here. Ligand exchange is also a major adsorption reaction for HA; however, other reactions contribute to adsorption for some soluti on compositions. At high pH, cation and water bridging become increasi ngly important for HA adsorption with increasing amounts of Na+ and Ca 2+, respectively. At low to neutral pH values, increases in these same two cations make hydrophobic bonding more effective. Calculations of HS carboxyl group densities in the adsorbed layer support the proposed adsorption reactions. From the adsorption data it appears that fewer than 3.3 HS-COO- groups per nm2 can be bound directly as inner-sphere complexes by the alumina surface. We propose that the influence of aqu eous chemistry on HS adsorption reactions, and therefore on the types of HS surface complexes formed, affects the formation and nature of or ganic coatings on mineral surfaces.