Hydrolysis of Azinphosmethyl Induced by the Surface of Smectites

-The effect of layer charge and of the interlayer cations of smectites on the hydrolysis of azinphosmethyl (0,0-dimethyl S-((4-oxo-1,2,3-benzotriazine-3(4H)-yl)methyl) phosphorodithioate) in an aqueous medium was investigated. Ultraviolet spectroscopy was used for monitoring the hydrolysis process. Hydrolysis of the pesticide is catalyzed by Ca-hectorite (layer charge 0.216) but is not catalyzed by Ca-nontronite nor by Ca-montmorillonites with a layer charge above 0.216. The Mgand Cu-hectorites and Cu-montmorillonite with a layer charge of 0.264 also show catalytic activity. The catalytic activities of the Ca 2+ and Cu 2+ cations as exchange cations of the smectite and as salts are compared. In agreement with previously reported work, the results show that the hydrolysis of azinphosmethyl may involve the adsorption of the pesticide into the interlayer space of the smectites, forming a hidentate complex with the interlayer cations. This interaction must enhance the electrophilic nature of the phosphorus atom, thereby facilitating its nucleophilic attack by the OH ion and producing rupture of the P-S bond. Key Words--Hydrolysis, Adsorption, Azinphosmethyl, Smectite, Organophosphorus pesticides. I N T R O D U C T I O N Clay minerals are able to inactivate adsorbed pesticides by catalyzing their decomposition. In most cases, this degradation occurs through hydrolysis (Hance, 1970; Mortland, 1970; Theng, 1974). This ability of clays to convert toxic organic compounds into their corresponding nontoxic forms may be of great importance if it is taken into account that these minerals are a common component of the colloidal fraction of soils, and also that they are used in the practical formulation of pesticides. Finely-divided clays are often used as inert diluting agents to reduce the concentration of pesticides to levels appropriate for their use in agriculture, and also for improving their handling properties. The catalytic effect of kaolinite (Saltzman et al. 1976; Mingelgrin et al. , 1977), montmoril lonite (Mortland and Raman, 1967; Sfinchez-Camazano and SfinchezMartin, 1983; Pusino et al. , 1988) and some soils (Kontad e t al . , 1967, 1969; Konrad and Chester, 1969) in the hydrolysis of organophosphorus pesticides has been demonstrated. A previous work by the authors on different aspects of the adsorption ofazinphosmethyl by smectites (Sfinchez-Martin and Sfinchez-Camazano, 1984) included the study of the adsorption of this pesticide by different minerals from this group in an aqueous medium. In that study, occasional traces of azinphosmethyl hydrolysis were observed. Accordingly, it was thought of interest to investigate the effect of the layer charge and interlayer cations ofsmectite on the catalytic hydrolysis of this pesticide. Azinphosmethyl is the common name of the organophosphorus insecticide (0,0-dimethyl S-((4-oxoCopyright 9 1991, The Clay Minerals Society 1,2,3-benzotriazine-3(4H)-yl)methyl) phosphorodithioate). Its structural formula is: