Disentangling the Roles of Dissolved Oxygen, Common Salts, and pH on Spontaneous Hydrogen Peroxide Production in...

Despite mounting evidence that the air-water interface or microdroplet geometry has no bearing on the spontaneous formation of hydrogen peroxide (H₂O₂) down to a 50 nM limit of detection, the debate persists. Recent studies have demonstrated connections between the spontaneous formation of hydroxyl (HO^•) radicals and H₂O₂ in sprayed microdroplets and the solution pH, dissolved salts, nebulizing gas, and gaseous environment. Among the halides, chloride (Cl^-), bromide (Br^-), and iodide (I^-), the studies found that Br^- dominated H₂O₂ formation because of its enhanced fractionation at the air-water interface and ability to donate electrons. The studies also suggested that H₂O₂ production at the air-water interface scales with the alkalinity of water. In response, this report employs a comprehensive set of techniques, encompassing nuclear magnetic resonance, potentiodynamic polarization, electron microscopy, and H₂O₂ assay kit fluorometry, to examine these claims. The experiments reveal that the air-water interface or microdroplet geometry does not drive H₂O₂ formation, regardless of the presence of halide in water. The reduction of oxygen gas (O₂) at the solid-water interface produces H₂O₂ (i.e., in the absence of O₂, no H₂O₂ is formed regardless of the halide). This work explains the relative dependence of H₂O₂ concentrations on halides based on their propensity to drive pitting corrosion (Cl^- > Br^- > I^-). As pits appear in the passivating layer, exposing the metal, H₂O₂ is consumed during further oxidation. This work also disproves the claim that alkalinity drives H₂O₂ formation, as it demonstrates that aluminum and titanium surfaces generate higher H₂O₂ concentrations in acidic and alkaline conditions, respectively. Taken together, this report provides alternate explanations for the observations that should help draw the debate toward its closure.