Publisher Summary Clays interact with many organic compounds to form complexes of varying stabilities and properties. These interactions are of great importance in nature and in industry. The clays in soils and sediments often have organic material with which they are intimately associated. This chapter describes the bonding mechanisms involved in clay-organic complexes, the nature of some clay-organic complexes and reactions, and the nature and importance of clay-organic complexes in soils. The dominant factors determining the nature of clay-organic interactions are the properties of the organic molecule, the water content of the system, the nature of the exchangeable cation on the clay surface, and the unique properties of the clay mineral structures. The exchangeable cations determine the surface acidity, and therefore the possibilities of protonation of the organic compound. Much remains to be learned regarding the clay-organic systems and the reactions taking place at this interface. In particular, it is quite likely that many reactions that have not yet been recognized are catalyzed by the clay minerals.

Clay-Organic Complexes and Interactions

Seven factors are known to influence the fate and behavior of pesticides in soil systems: (1) chemical decomposition, (2) photochemical decomposition, (3) microbial decomposition, (4) volatilization, (5) movement, (6) plant or organism uptake, and (7) adsorption. The phenomenon of adsorption-desorption directly or indirectly influences the magnitude of the effect of the other six factors. Adsorption, therefore, appears to be one of the major factors affecting the interactions occurring between pesticides and soil colloids.

Factors influencing the adsorption, desorption, and movement of pesticides in soil

Interlayer and Intercalation Complexes of Clay Minerals

Synthetic sodium bimessite, having a cation-exchange capacity (CEC) of 240 meq/100 g (cmol/kg) was transformed into Li, K, Mg, Ca, Sr, Ni, and Mn2+ cationic forms by ion exchange in an aqueous medium. Competitive adsorption studies of Ni and Ba vs. Mg showed a strong preference for Ni and Ba by bimessite. The product of Mg2+-exchange was buserite, which showed a basal spacing of 9.6 A (22°C, relative humidity (RH) = 54%), which on drying at 105°C under vacuum collapsed to 7 A. Of the cation- saturated bimessites with 7-A basal spacing, only Li-, Na-, Mg-, and Ca-bimessites showed cation exchange. Heating bimessite saturated with cations other than K produced a disordered phase between 200° and 400°C, which transformed to well-crystallized phases at 600°C. K-exchanged bimessite did not transform to a disordered phase; rather a topotactic transformation to cryptomelane was observed. Generally the larger cations, K, Ba, and Sr, gave rise to hollandite-type structures. Mn- and Ni-bimessite transformed to bixbyite-type products, and Mg-bimessite (buserite) transformed to a hausmannite-type product. Li-bimessite transformed to cryptomelane and at higher temperature converted to hausmannite. The hollandite-type products retained the morphology of the parent bimessite. The mineralogy of final products were controlled by the saturating cation. Products obtained by heating natural bimessite were similar to those obtained by heating bimessite saturated with transition elements.

S. B. Weed

Papers

The effect of adsorbent charge on the competitive adsorption of divalent organic cations by layer-silicate minerals