Ma. Schlautman et Jj. Morgan, ADSORPTION OF AQUATIC HUMIC SUBSTANCES ON COLLOIDAL-SIZE ALUMINUM-OXIDE PARTICLES - INFLUENCE OF SOLUTION CHEMISTRY, Geochimica et cosmochimica acta, 58(20), 1994, pp. 4293-4303
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.