Disentangling the Roles of Dissolved Oxygen, Common Salts, and pH on Spontaneous Hydrogen Peroxide Production in Water: No O, No HO.

Despite mounting evidence that the air-water interface or microdroplet geometry has no bearing on the spontaneous formation of hydrogen peroxide (HO) 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 HO 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 HO formation because of its enhanced fractionation at the air-water interface and ability to donate electrons. The studies also suggested that HO 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 HO assay kit fluorometry, to examine these claims. The experiments reveal that the air-water interface or microdroplet geometry does not drive HO formation, regardless of the presence of halide in water. The reduction of oxygen gas (O) at the solid-water interface produces HO (i.e., in the absence of O, no HO is formed regardless of the halide). This work explains the relative dependence of HO concentrations on halides based on their propensity to drive pitting corrosion (Cl > Br > I). As pits appear in the passivating layer, exposing the metal, HO is consumed during further oxidation. This work also disproves the claim that alkalinity drives HO formation, as it demonstrates that aluminum and titanium surfaces generate higher HO 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.