Density Functional Theory Study of Methylene Blue Demethylation as a Key Step in Degradation Mediated by Reactive Oxygen Species.

Methylene blue (MB), a widely used organic dye, poses significant environmental challenges due to its stability and persistence in aquatic ecosystems. This study employs density functional theory (DFT) to investigate the demethylation mechanisms of MB mediated by reactive oxygen species (ROS), a critical initial step in its photocatalytic degradation. Computational analyses reveal that demethylation is energetically favorable, particularly when mediated by hydroxyl radicals (OH) and hydroxyl ions (OH) with reaction energies of -154 kcal/mol and -214 kcal/mol, respectively. These pathways lead to the formation of key intermediates, such as Azure B, methanol (CHOH), and formaldehyde (CHO), which align with experimentally detected degradation byproducts. The study further demonstrates that the dissociation of hydrogen peroxide species (HO, HO, HO) plays a fundamental role in generating the ROS required for MB degradation. Potential energy surface analyses confirm that these ROS-driven processes are thermodynamically and kinetically viable. The findings provide a theoretical framework that bridges existing knowledge gaps in MB degradation, reinforcing the role of ROS in advanced photocatalytic systems and contributing to the optimization of wastewater treatment strategies. This work underscores the importance of integrating computational and experimental approaches to develop more effective strategies for the remediation of recalcitrant pollutants in wastewater.