SURFACE REACTIONS OF LOW-MOLECULAR-WEIGHT ORGANICS WITH SOIL COMPONENTS

Abstract

The nature of the interaction between organic molecules and surfaces depends upon the chemistry of the compound and the properties of the surface. In the case of clay minerals, processes which may play a part in adsorption of organic molecules are: cation exchange, protonation at the mineral surface, coordination with metal exchange ions, several kinds of hydrogen bonding, physical forces, and entropy effects. The hydration status of the clay mineral surface may be an important factor in determining whether or not a nonionic yet polar organic species is adsorbed, since there is a competition for adsorption sites between water and other polar molecules. The surfaces of pure clay minerals are hydrophilic due to the solvation of metal exchange ions, and may thus not adsorb organic molecules which have low solubility in water. On the other hand, clay-organic complexes such as clays where the exchange sites are saturated with alkylammonium ions may become hydrophobic and in turn organophilic wherein they have the ability to adsorb molecules which are sparingly soluble in water. Organic matter itself adsorbs small organic molecules by a variety of mechanisms similar to those described for clays. In addition, portions of the organic matter complex are relatively hydrophobic and in turn organophilic so that molecules which are hydrophobic may be adsorbed.