Interaction of aminotriazole with montmorillonite and Mg-vermiculite at pH

The interaction of aminotriazole (AMT) at pH 4 on Wyoming montmorillonite (mainly with Na ions) and Mg-vermiculite has been studied by X-ray diffraction and infrared spectroscopy. The AMT is adsorbed on montmorillonite in the cationic form by cation exchange. The amount of pesticide adsorbed was 71 mEq/100 g, which comprises *91% of the CEC of this sample (78.2 mEq/100 g). Saturation was reached in 24 h, giving rise to a complex with basal spacing 12.5 Å. Vermiculite adsorbs 167 mEq/100 g, almost 20% greater than the CEC (141 mEq/100 g), and the basal spacing was stabilized at 13.68 Å after five weeks of treatment with AMT. A part of the AMT is adsorbed in cationic form, displacing a great part of the exchangeable Mg cations. The rest is adsorbed in molecular form by coordination to the Mg cations which remain in the interlamellar space, removing a great amount of water, and remaining in the interlamellar space of vermiculite after washing with water, probably because of a steric hindrance from the AMT cations adsorbed. The transport and evolution of contaminants in the soil environment are governed by a series of processes taking place in the soil which depend mainly on the type of soil, pH, the nature of the soil colloids, moisture content and environmental factors (Sánchez-Camazano & Sánchez-Martı́n, 1990). The content and nature of the clay fraction is of special interest, mainly due to its high surface area and, for some clay minerals, their large cation exchange capacity (Khünel, 1992; Kowalska et al., 1994). The physico-chemical properties of clays are particularly important in pesticide adsorption in soil. The reversibility of pesticide adsorption on clays is of fundamental importance, inasmuch as irreversible adsorption may produce permanent soil contamination as well as block a part of the soil CEC. For this reason, one aspect of clay-pesticide adsorption that should be considered is the mechanism by which the pesticide is held to the clay minerals (Dios-Cancela et al., 1992). The chemical properties of some pesticides change depending on the environmental conditions under which they are applied. The pesticide aminotriazole (3-amino-1,2,4-triazole), AMT, is used extensively as a non-selective herbicide which inhibits chlorophyll formation and regrowth from buds. Studies on the adsorption of AMT by montmorillonite at its solution pH, carried out by Russell et al. (1968) showed that the AMT molecule is protonated when adsorbed on montmorillonite surfaces to produce the aminotriazolium cation. On the other hand, Nearpass (1970) postulated that adsorption of AMT on montmorillonite was due to proton association and cation exchange, and no molecular adsorption occurred. However, Morillo et al. (1991) demonstrated that adsorption of AMT on montmorillonite under these conditions yields a pesticide-clay complex formed through interlamellar cations of montmorillonite which, in general, are not # 1997 The Mineralogical Society ClayMinerals (1997) 32, 307–313