CHOICE OF SILICA AS STANDARDIZED MATRIX FOR PHOTODEGRADATION STUDIES OF THE PESTICIDE PHENMEDIPHAM ON ADSORBED PHASE

Silica is often used as support for the studies of photodegradation of pesticides on adsorbed phase. But, sometimes the silica may catalyze chemical degradations of the adsorbed product and the photochemical re sults are disturbed by chemical transformations. Different silica have been studied to select a convenient matrix and to determine the preva iling physico-chemical parameter of the reactivity of this material. T he study was performed with phenmedipham as pesticide model. The natur e, the accessibility of impurities and the pH give the best informatio ns to prevent a catalytic activity of silica on phenmedipham. The use of alkaline silica as support leads to the degradation of phenmedipham , whereas the pesticide is stable on neutral and acid silica. The indu ced mechanism by alkaline silica is different from the mechanism of th e photodegradation. On alkaline silica, the main product of degradatio n is the N,N'-di-m-tolylurea whereas under irradiation the methyl -N-( 3-hydroxyphenyl) carbamate and the m-toluidine are produced.