Thermal treated zeolite as catalyst in heterogeneous catalytic ozonation – optimization of experimental conditions and micropollutants degradation

Raw and thermally pre-treated zeolites, denoted as zeolite-T, were examined as catalysts in the heterogeneous catalytic ozonation process. Their catalytic activity was evaluated by the degradation of p-chlorobenzoic acid (p-CBA) (initial concentration 4 μΜ), a typical biorefractory organic model compound. The results showed that the thermally pre-treated zeolite presented higher catalytic activity than the raw material. Rates of 99.3%, 98% and 94.5% degradation of p-CBA were achieved within 3 min of reaction/oxidation time, applying the zeolite T/O₃, the zeolite/O₃ and the single ozonation (O₃) procedures, respectively. After 15 min treatment time the concentration of p-CBA was found to be below the detection limit (0.025 μΜ) of the analytical method used (HPLC) for both catalytic systems, while the application of single ozonation reached 96% for the removal of p-CBA. The removal of p-CBA was also examined for different initial pH values commonly found in natural waters, as well as for different oxidation reaction temperatures. The optimum conditions were defined at the pH value 8 and 25^o C, during which the production of hydroxyl radicals in the aqueous phase were found to be increased. Furthermore, the degradation of two other common micropollutants (i.e. benzotriazole and carbamazepine), presented higher pK_a values (i.e. 9.04 and 16, respectively) than the p-CBA, as well as different reaction rate constants with ozone (i.e. 20 M^-1s^-1 and 3*10⁵ M^-1s^-1) was additionally evaluated. It was found that all these micropollutants can be sufficiently removed by the catalytic ozonation system in the presence of zeolite-T. However, it is worth noting that carbamazepine can be also easily removed by the application of even simple ozonation and that the presence of catalyst just decreased the respective oxidation time. On the other hand, benzotriazole presented a lower ozone degradation rate than the other micro-pollutants and during the early stage (i.e. after the 3^rd min) of the oxidation reaction it was found to be removed by 96.5%.