Role of in situ electrogenerated reactive oxygen species towards degradation of organic dye in aqueous solution

Abstract

Although electrochemical oxygen reduction reaction (ORR) at glassy carbon (GC) electrode in aqueous solution involves the generation of several reactive oxygen species (ROS; e.g., O2•−, H2O2, OH•) the formation of hydroxyl radicals (OH•) is generally associated with the degradation of persistent organic contaminants in water. In this study, the route of degradation of a model organic dye, methylene blue (MB), by ROS at a GC electrode in aqueous solution is extensively investigated by employing a variety of radical scavengers such as 1,4-benzoquinone for O2•−, horseradish peroxidase for H2O2, and iso-propanol for OH• radicals using cyclic voltammetric technique. In addition to that effect of Fe(II) solution which facilitates the decomposition of H2O2 was also explored to generate more OH• radicals. Among the scavengers of ROS studied mainly iso-propanol inhibits the following chemical reaction, i.e., OH• radical produced in ORR from O2 through the consumption of electron was found to be the active radical for the degradation of MB. The mode of reaction of MB with ROS was further clarified by analyzing characteristics of the reaction mixture and products formed during bulk electrolysis that was carried by applying a constant potential at GC plate electrode. The pH of the solution decreased upon degradation and caused a consequent increase in conductivity. The experimental FT-IR spectrum of the degraded products were compared with theoretical ones for probable fragments of MB constructed using density functional theory (DFT) and the degraded products were identified as protonated amine residues.