Abstract
This paper reviews the studies on the adsorption of organic pollutants by clay minerals for a better understanding of the role of clays and the problem of environmental pollution.
Adsoprtion of organic pollutants takes place through various bonding mechanisms to the surface of clay minerals. Among these the most general mechanisms are: cationic exchange, ion-dipole and hydrogen bonding. Ionization of organic bases and acids is greatly affected by solution pH. Cationic species of organic compounds are much more important for the adsorption than the anionic species because of the negatively charged clay surface. Recent studies show that organic molecules can be protonated in the interlayer region of clays even in suspension. Non-ionizing molecules are mostly adsorbed by ion-dipole or hydrogen on the clay surface.
The charge density distribution on the clay surface is an important factor for adsorption when cationic divalent or multivalent organic species are adsorbed. Adsorption of diquat and paraquat clearly shows the difference between montmorillonite and vermiculite. The regularity of the hexagonal network of water molecules on the clay surfaces also affect the adsorption of organic compounds.
The desorption of organic pollutants can be explained on the basis of adsorption studies and is demonstrated by the studies of toxicity of pollutants to plantgrowth and microbial degradation.
Kaolin minerals also become important when organic pollutants are small molecules with high polarity. Kaolinite-formamide complex formed in the interlayer position reveals hydrogen bonding between the clay surface and formamide. Studies of naturally occurring clay samples with organic pollutants have been reported only slightly. More studies in this subject are necessary to understand the real nature of clayorganic pollutants and their interactions.
