Total degradation of p-hydroxybenzoic acid by Ru-catalysed wet air oxidation: a model for wastewater treatment

Catalytic wet air oxidation using heterogeneous catalysts is a promising technology for treatment of wastewaters. Wastewaters from olive oil mills are difficult to clean because effluents contain high levels of phenolic compounds that cannot be removed by biological treatment or incineration. Therefore, we tested the catalytic wet air oxidation of p-hydroxybenzoic acid, as model molecule of olive oil mills wastewaters, over Ru-supported CeO2–TiO2 mixed oxides. We focussed on the Ce/Ti molar ratio because this ratio modifies the properties of the Ru catalysts. Supports were prepared by using the sol–gel method. The Ru catalysts were prepared by impregnation. Those products were characterized by N2 adsorption–desorption, X-ray diffraction, H2 chemisorption, transmission electron microscopy and inductively coupled plasma. Catalytic wet air oxidation of p-hydroxybenzoic acid was performed in a batch reactor at 140 °C and 50 bar of air. Our results show that the most efficient catalyst was Ru with a Ce/Ti ratio of 1/5, which converted totally p-hydroxybenzoic after 7 h of reaction without Ru leaching. We explain this finding by an enhanced dispersion of Ru due to better structural properties of the CeO2–TiO2 support.