Kinetic modelling of the hematin catalysed decolourization of Orange II solutions

•Mechanistic models for hematin-catalysed decolourization of Orange II were validated.•Ineffective hematin-catalysed H2O2 dismutation was detected and simulated.•High Orange II levels caused rates depletion probable by blocking hematin activation.•Hematin inactivation by superoxide explained biphasic profiles during H2O2 dismutation.

A mathematical description of the kinetics of the hematin catalysed decolourization reaction of Orange II alkaline solutions was constructed and validated under the assumption that hematin mimics the action of peroxidases. The clean oxidant H2O2 was applied, however, hematin dismutated it, as catalases do, under reaction conditions. Thus, special attention was cared to the understanding of this ineffective side-reaction by the proposal of straightforward pathways concerning the catalatic or the pseudo-catalatic cycles. Model validations were implemented by a parametrization procedure of relevant rate constants under dynamic simulation fitting to selected experimental time-courses data. The peroxidatic coupled to pseudo-catalatic mechanism gave predictions closer to experimental findings in a wide range of conditions. The initial rate method for data processing was successfully applied for providing reliable initial rate constant guesses but also for the detection of unexpected rate depletion at high dye concentrations. This was considered in the model as unproductive dye coordination to hematin native state prior to H2O2 activation. The pseudo-catalatic pathway involves the production of superoxide and its coordination to hematin native state to afford inactive but regenerable ferrous porphyrin. Model underestimations of experimental data were interpreted as cooperative oxidation of Orange II molecules by superoxide whereas model deviations at high dye concentration (>400 mg/l) was assigned to further hematin catalysed oxidation of Orange II degraded products.

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