KINETICS AND PRODUCTS OF THE TIO2 PHOTOCATALYTIC DEGRADATION OF PYRIDINE IN WATER

Pyridine (Pyr), a noxious chemical whose ring is part of the chemical structure of many pesticides, is more rapidly eliminated in water by photocatalysis over TiO2 than benzamide, whose nucleus has also a relatively low electron density and whose extent of adsorption is equivalent. Hydroxylation of Pyr first occurs only at position 2. Beside acetate and formate, whose formation from Pyr is derived from ion chromatography analysis, seven aliphatic intermediates with one to five C atoms are identified by GC-MS. They all contain one or several C=O groups, and wherever the N atom subsists, it is as an amide, i.e., with an unchanged oxidation number. For relatively high initial Pyr concentrations (16.5 mmol/L), dipyridyl and carbamoyl pyridine isomers are also detected as intermediates, at much lower concentrations, however, than that of 2-hydroxypyridine; this shows the existence of coupling reactions. Within an UV irradiation period about 2.5 times as long as that necessary to eliminate Pyr, organic nitrogen is almost entirely mineralized, mainly into NH4+ ions that are very slowly oxidized to nitrate.