Removal of nitrogenous compounds by catalytic wet air oxidation. Kinetic study.

Aqueous wastes containing organic pollutants can be efficiently treated by wet air oxidation (WAO), i.e. oxidation by molecular oxygen in the liquid phase, under high temperature (200-325 degrees C) and pressure (up to 150 bar). However, organic nitrogen can be relatively resistant to oxidation and can be harmful to the environment. In the course of treatment, organic nitrogen (N-Org) is converted into ammonia (NH(3)), while organic carbon (C-Org) is converted mainly into carbon dioxide (CO(2)). This can be done without catalysts. In the presence of Mn/Ce composite oxides, it is possible to transform ammonia into molecular nitrogen at a temperature close to 260 degrees C. The direct conversion of organic nitrogen into molecular nitrogen also can be achieved using the same catalyst. This paper discusses the results obtained during the treatment of nitrogenous compounds like aniline, nitrophenol, beta-alanine and ammonia. Laboratory investigations were conducted in a stirred batch reactor with Mn/Ce composite oxides as catalysts. Very limited amounts of nitrites and nitrates were observed with amines, but more significant quantities were found with nitro-compounds. The kinetics of oxidation of ammonia, organic compounds, and more particularly aniline, were investigated. The treatment of a real waste (process wastewater) was also investigated. The dependence of the transformation rate on various parameters (amount of catalyst, temperature, etc.) was established. The rates of oxidation are described by first-order kinetic laws with respect to the various nitrogen species (aniline, NH(3)). Several parallel pathways are considered for the transformation of organic nitrogen, amongst which is an interaction with the catalyst surface. The orders with respect to oxygen and catalyst are established.