Bicarbonate-Mediated Generation of H ₂ O ₂ , ¹ O ₂ , and OH ^* at the Hydrophobic Gas–Water Interface and Its Underlying Mechanism

Spontaneous generation of reactive oxygen species (ROS), such as H₂O₂, OH^*, and O₂^-*, is known to occur at aqueous aerosol microdroplets or gas microbubbles in bulk water. However, the suggested mechanisms of their formation remain contentious because they posit the breaking of highly stable water bonds. In addition, the primary ROS that is the source for other ROS has also not been established. Herein, we evaluate ROS generation in much simpler macroscopic gas bubbles in the bulk electrolyte. Through in situ studies with electrolytes containing different anions (CO₃^2-, HCO₃^-, SO₄^2-, H₂PO₄^-, and Cl^-), cations (Na⁺, K⁺, and Li⁺), and pH and sparged with gases (O₂, CH₄, N₂, and CO₂), we show that ROS generation occurs through the hitherto unrecognized activation of CO₂/HCO₃^-/CO₃^2- that is present inadvertently in all air-exposed aqueous system. In addition, ROS-specific scavenging studies show that H₂O₂ is the primary ROS produced that serves as the source for OH^*, O₂^-*, and ¹O₂ generation. The combined evidence suggests that the underlying mechanism may be much more complex than simple bond breakage; rather, it may be electrochemical in origin, with coupled sets of reduction/oxidation reactions occurring at the bubble/water interface.