Wet air oxidation of trinitrophenol with activated carbon catalysts: Effect of textural properties on the mechanism of degradation

Activated carbons (ACs), with different activation degrees, were obtained by chemical activation of olive stones and tested for the removal of 2,4,6-trinitrophenol (TNP) from aqueous solutions by catalytic wet air oxidation (CWAO). These materials were characterized using different techniques (e.g., N-2 and CO2 adsorption, temperature programmed desorption, immersion calorimetry and scanning electron microscopy). The non-catalytic oxidation of TNP in water, at 473 K and 0.7 MPa of oxygen partial pressure, is negligible, while the use of ACs as catalysts leads to complete removal of this organic compound. Competition between adsorption and catalytic oxidation reaction pathways, occurring simultaneously in the CWAO process, depends on the carbon porous texture. The extent of oxidation is given by the amount of nitrates formed during reaction. When a highly microporous carbon with large surface area (S-BET = 1530 m(2)/g) is used, all dissolved TNP is adsorbed and oxidation takes place in wide micropores. However, when using a carbon material with low surface area (S-BET = 121 m(2)/g) and low microporosity, TNP degradation will also occur on the external surface area, since there is enough TNP available in the bulk liquid media; in this case fast degradation takes place on the carbon macro-mesopores. Following that, the degradation mechanism is always heterogeneous but takes place in different ranges of porosity and depends on the amount of dissolved TNP. In addition, the surface chemistry of the carbon materials is modified during the CWAO process due to the presence of oxygen at the conditions employed. (C) 2010 Elsevier B.V. All rights reserved.