Kinetics of the ozone oxidation of Reactive Orange 16 azo-dye in aqueous solution

Reactive azo-dyes present environmental concerns since they are persistent organics which are difficult to remove with conventional treatment processes. In this study, the ozonation of Reactive Orange 16 dye, C.I. 177757 (synonym Remazol Brilliant Orange 3R) was investigated at different experimental conditions in a semi-batch ozone reactor (25–100 mg/L dye; 20–80 g/m3 NTP O3 gas (NTP: 0 °C and 1 atm); pH 2, 7, 11). Ozone was very effective to remove completely the colour within a short period of time (few minutes). High gas-phase ozone concentrations and low dye concentrations resulted in high decolourisation rates. The decolourisation of the dye has also improved as pH increased from 2 to 11. Direct ozone reactions are assumed to predominate and hydroxyl radicals formed as a result of ozone decomposition at high pH reacted mostly with the products of the main decolourisation reaction. The reaction was assumed second order and Danckwerts model was used to determine the values of its rate constant, k2, at different pHs. Values of k2 were 2.5 × 105, 3.2 × 105, and 1.4 × 106 L/mol s for pH values of 2, 7, and 11, respectively. The stoichiometric ratio of the reaction was found equal to 3 mol O3/mol dye and a degradation pathway was proposed.

► RO16 azo-dye is difficult to biodegrade and may present environmental and health concerns. ► The effect of the operating parameters on RO16 degradation was studied. ► Ozone was effective to degrade RO16 and the rate constant increased with increasing pH. ► The kinetic data were determined and a degradation pathway was proposed.