The hydroxyl radical, •OH, is one of the most reactive free radicals and plays significant roles in the oxidative degradation of organic pollutants and the electron transfer of inorganic ions in natural and engineered environmental processes. To quantitatively determine the contribution of •OH to oxidative reactions, a specific scavenger, such as tert-butyl alcohol (TBA), is usually added to eliminate •OH effects. Although TBA is commonly assumed to transform •OH into oxidatively inert products, this study demonstrates that utilizing TBA as an •OH scavenger generates the secondary peroxyl radical (ROO•), influencing the oxidation of transition metals, such as Mn. Although ROO• is less reactive than •OH, it has an extended half-life and a longer diffusion distance that enables more redox reactions, such as the oxidation of Mn2+(aq) to MnIV oxide solids. In addition to promoting Mn2+(aq) oxidation kinetics, TBA can also affect the crystalline phases, oxidation states, and morphologies of Mn oxide solids. Thus, the oxidative roles of •OH in aqueous redox reactions cannot be examined simply by adding TBA: the effects of secondary ROO• must also be considered. This study urges a closer look at the potential formation of secondary radicals during scavenged oxidative reactions in environmental systems.
Keywords: Mn2+ oxidation; Tert-butyl alcohol; photochemistry; secondary peroxyl radical; •OH scavenger.