Wet Air Oxidation of nitrogen-containing organic compounds and ammonia in aqueous media

Abstract

Treatment of toxic nitrogen-containing compounds is one of the major applications of the Wet Air Oxidation (WAO) processes. The aim of this paper is to review the literature dealing with the Catalytic Wet Air Oxidation (CWAO) of these nitrogenous compounds, mainly produced in chemical and pharmaceutical industries. Many studies deal with oxidation of aniline, often chosen as a model molecule of pollutant of dye industries. First, the results obtained with CWAO are compared with those obtained with other oxidation processes. Particular attention is paid to the selectivity towards organic by-products (specially, azo, nitroso and nitro compounds, phenolic compounds and carboxylic acids) as well as towards several inorganic forms of nitrogen (NH4+, N2, NO2−, NO3−). In a second part, the review focuses on the mechanism of chemical reactions that can explain the formation of the observed products. Usually, similar catalysts can be used for CWAO of oxygen-containing (phenol, carboxylic acids) and nitrogen-containing organic compounds. Ammonia is one of the most refractory by-product formed during catalytic WAO of the nitrogen-containing organic pollutants and is itself a pollutant. For this reason, recent reports about its oxidation by the CWAO process are finally reviewed. Very high selectivities to dinitrogen can be obtained on certain noble metal catalysts. As a rule, catalysts active and selective for ammonia oxidation are different (nature of active phase, support, etc.) from the solids proven to be the best catalysts for CWAO of organic compounds. Multifunctional catalysts are, thus, required for the treatment of nitrogenous organic compounds.