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.