Humic and Fulvic Acid Adsorption by Silicon and Aluminum Oxide Surfaces on Clay Minerals
1991
Schulthess, C. P. | Huang, C. P.
Natural (untreated) clays are most probably amorphous on their outermost surface layers due to the continuous and natural process of partial dissolution and reprecipitation of the clay components at the solid-aqueous interface. The pH-dependent adsorption on Al and Si oxides, mordenite, kaolinite, and montmorillonite of a humic acid (HA) and a fulvic acid (FA) was described as occurring on Al and Si sites. The surfaces of the clay minerals were modeled as mixtures of amorphous Al and Si oxides. The results showed a strong adsorption of the organics by the Al sites on the Al oxide and kaolinite, and a weak adsorption of organics by the Si sites on the Si oxide, mordenite, and montmorillonite. At low pH values, the Si sites on the Si oxide, mordenite, and montmorillonite adsorbed FA; these latter observations strongly suggest that the adsorption of FA by the interplanar spaces of expanding clays is driven by forces that can be studied using amorphous Si oxide as the adsorbent. Multivalent cations will form organo-metallic complexes that significantly increase adsorption, particularly on Si sites; exceptions were found with some FA-metal complexes, which were attributed to the degree of complexation. The implication of these observations is that, in natural systems, the adsorption of aqueous compounds is highly dependent on the type of (amorphous) surface present at the outermost layer of the solid phase in contact with the liquid phase.
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