Reaction of methylene blue with OH radicals in the aqueous environment: mechanism, kinetics, products and risk assessment

Abstract

Methylene Blue (MB) is an industrial chemical used in a broad range of applications, and hence its discharge is a concern. Yet, the environmental effects of its degradation by HO˙ radicals have not been fully studied yet. This study employs quantum chemical calculations to investigate the two-step degradation of MB by HO˙ radicals in aqueous environments. It was found that MB undergoes a rapid reaction with the HO˙ radical, with an overall rate constant of 5.51 × 10⁹ to 2.38 × 10¹⁰ M⁻¹ s⁻¹ and has a rather broad lifetime range of 11.66 hours to 5.76 years in water at 273–383 K. The calculated rate constants are in good agreement with the experimental values (k_calculation/k_experimental = 2.62, pH > 2, 298 K) attesting to the accuracy of the calculation method. The HO˙ + MB reaction in water followed the formal hydrogen transfer and radical adduct formation mechanisms, yielding various intermediates and products. Based on standard tests these intermediates and some of the products can pose a threat to aquatic organisms, including fish, daphnia, and green algae, they have poor biodegradability and have the potential to induce developmental toxicity. Hence MB in the environment is of moderate concern depending on the ratio of safe to harmful breakdown products.