Hydrothermal decomposition and oxidation of p-nitroaniline in supercritical water

The decomposition kinetics and nitrogen speciation of p-nitroaniline (pNA) were investigated at supercritical water conditions in the absence/presence of oxygen. A tubular flow reactor system was used at the temperature ranges 385-440 degrees C and 380-420 degrees C for hydrothermal (oxygen absent) and oxidation experiments, respectively, The destruction of pNA significantly occurred in the absence of oxygen, The hydrothermal decomposition rate had an activation energy of (1.59 +/- 0.25) x 10(5) J/mol and was first-order with pNA. The formation of oxygen (O-2,), carbon monoxide (GO), carbon dioxide (CO2,), and nitrogen (N-2,) indicated that the nitro group in the decomposed pNA drove oxidation in the absence of oxygen. Appreciable quantity of ammonia and traces of nitrite and nitrate were formed. Oxygen enhanced the pNA destruction, although its effect did not show explicitly in the oxidation rate expression. The activation energy of the oxidation was (1.28 +/- 0.35) x 10(5) J/mol. Also, in the presence of oxygen, ammonia was formed only in trace quantities. The reduction of ammonia level was attributable to the potential ammonia oxidation by the nitro group of the decomposed pNA. It follows from this result that nitro group-containing compounds may be cooxidized with other nitrogen containing substances to reduce the production of ammonia, nitrite and nitrate in supercritical water oxidation processes, (C) 1997 Elsevier Science B.V.