Hydrogen peroxide as a key intermediate for hydroxyl radical generation during catalytic ozonation of biochar: Mechanistic insights into the evolution and contribution of radicals

Abstract

The catalytic ozonation of porous carbonaceous materials has shown great promise in the treatment of refractory organic contaminants via the formation of hydroxyl radicals. However, conflicting results have been reported regarding the formation mechanism of hydroxyl radicals and the contribution of concurrently produced reactive oxygen species (ROS). In this work, we used highly porous biochar (HPB700) as a model carbonaceous material for catalytic ozonation, and we studied the formation, evolution, and contribution of the ROS. The kinetics constant, utilization efficiency of ozone, and the Rct value for 1,4-dioxane degradation increased by factors of 14.7, 3.7 and 87.5, respectively, in the presence of HPB700. Free hydroxyl radicals were generated during HPB-ozone oxidation and were the sole ROS for pollutant degradation. Hydrogen peroxide formed as a key intermediate in the generation of hydroxyl radicals. We proposed that ozone adsorbed onto HPB700 and reacted with the surface hydroxyl and carbonyl groups to form hydrogen peroxide, which diffused into the liquid bulk to react with aqueous ozone to generate free hydroxyl radicals. The adsorption of the target pollutant was not a prerequisite step for efficient degradation. These findings may strengthen our understanding of hydroxyl radical formation in catalytic ozonation and clarify the role of different ROS in pollutant degradation.