The degradation of methyl orange by OH radicals in aqueous environments: A DFT study on the mechanism, kinetics, temperature and pH effects

Abstract

Methyl orange (MO) is extensively used in the food, textile, leather, and paper industries. As with many industrial chemicals, while primary environmental effects are documented, the secondary effects from its natural degradation have not been fully established. This study employs quantum chemical calculations to investigate the reaction of MO with HO^• radicals that is a typical breakdown pathway in aqueous environments. It was found that the HO^• + MO reaction in water can occur via either hydrogen transfer, single electron transfer, or radical adduct formation mechanisms, resulting in a variety of metastable intermediates and products. The calculated total rate constant (k_total = 2.43×10¹⁰ M⁻¹ s⁻¹ at 298 K, pH = 9.2) is in good agreement with the experimental rate constant (k_Exp = (2.0 ± 0.3) ×10¹⁰ M⁻¹ s⁻¹at 298 K, pH = 9.2), thus confirming the accuracy of the calculation method. MO can degrade in natural water within the broad time range of 3.28 hrs to 56.6 years. The breakdown of methyl orange by HO^• radicals is more effective at high temperatures and basic pH levels.