Spontaneous Production of H2O2 at the Liquid–Ice Interface: A Potential Source of Atmospheric Oxidants

We present experimental evidence for the spontaneous production of hydrogen peroxide (H2O2) at the liquid–ice interface during the freezing of dilute salt solutions. Specifically, sample solutions containing NaCl, NaBr, NH4Cl, and NaI at concentrations between 10–6 and 10–1 M were subjected to freezing–melting cycles and then analyzed for H2O2 content. The relationship between the production rate of H2O2 and the salt concentration follows that of the Workman–Reynolds freezing potential (WRFP) values as a function of the salt concentration. Our results suggest that H2O2 is formed at the liquid–ice interface from the self-recombination of hydroxyl radicals (OH·), produced from the oxidation of hydroxide anions due to the high electric field generated at the aqueous–ice interface under the WRFP effect. Furthermore, the involvement of O2 likely acting as an electron capturer could promote to produce more OH radicals and hydroperoxyl radicals (HO2·), thus enhancing the production of H2O2 at the liquid–ice interface. Overall, this study suggests a novel mechanism of H2O2 formation in ice via its spontaneous production at the liquid–ice interface, induced by the Workman–Reynolds effect.