Treatment of olive oil mill wastewater by catalytic wet air oxidation: 3. Stability of supported ruthenium catalysts during oxidation of model pollutant p-hydroxybenzoic acid in batch and continuous reactors

Catalytic wet air oxidation of an aqueous solution of p-hydroxybenzoic acid chosen as a representative phenolic pollutant of olive mill wastewater was conducted over ruthenium catalysts supported on TiO2 or ZrO2 in a stirred batch autoclave and in a fixed bed reactor at 140 °C and 50 bar air. The objective was to examine the stability of these catalysts by conducting catalyst recycling experiments in batch reactor and long-term stability tests in trickle-bed reactor. A small deactivation was observed during the first recycling of ruthenium catalyst in batch experiments, which was totally reversible after catalyst reduction. A small deactivation was also observed at the start of the continuous runs. This moderate deactivation was attributed to an over-oxidation of the surface or subsurface of ruthenium particles. Then the catalysts proved to be very stable in ca. 50-day continuous runs. Noticeably, no leaching from the catalysts was observed at all under these operating conditions. Catalysts prepared on ZrO2 from ruthenium nitrosyl nitrate showed the highest performances.