Oxidation of an Azo Dye in Subcritical Aqueous Solutions

Oxidation of aqueous solutions of a model azo dye pollutant (Orange II) was studied in a semibatch reactor operated at temperatures between 180 and 240°C and oxygen partial pressures from 10 to 30 bar. The dye concentrations were in a range (100 and 1000 mg/L) that one may encounter in an industrial wastewater stream. Orange II oxidation undergoes a parallel-consecutive reaction pathway in which it first decomposes thermally and oxidatively to aromatic intermediates and then via organic acids to the final product-carbon dioxide. The thermal and oxidative disappearance rates of Orange II were found to be first-order reactions with respect to the mother compound, while the oxidation rate of intermediates was found to be second order when lumped by means of total organic carbon. The rate of organic carbon disappearance in solution can be predicted by adding up the rate at which organic carbon in Orange II disappears and the rate of carbon disappearance in lumped intermediates. Both oxidation rates obey the first-order dependence with respect to partial pressure of oxygen. The activation energies of all three steps, i.e. thermal and oxidative decompositions of Orange II and lump oxidation, are found to be 90, 104, and 57 kJ/mol, respectively. The last activation energy suggests that some intermediates from the lump are oxidized directly to carbon dioxide. The results show the biodegradability of solutions increases with treatment time, but complete biodegradation with nonacclimated seed was not reached. Maximum biodegradability was reached in an experiment carried out at 200°C.