Nitrite-Induced Activation of Iodate into Molecular Iodine in Frozen Solution

A new mechanism for the abiotic production of molecular iodine (I₂) from iodate (IO₃^-), which is the most abundant iodine species, in dark conditions was identified and investigated. The production of I₂ in aqueous solution containing IO₃^- and nitrite (NO₂^-) at 25 °C was negligible. However, the redox chemical reaction between IO₃^- and NO₂^- rapidly proceeded in frozen solution at -20 °C, which resulted in the production of I₂, I^-, and NO₃^-. The rapid redox chemical reaction between IO₃^- and NO₂^- in frozen solution is ascribed to the accumulation of IO₃^-, NO₂^-, and protons in the liquid regions between ice crystals during freezing (freeze concentration effect). This freeze concentration effect was verified by confocal Raman microscopy for the solute concentration and UV-visible absorption spectroscopy with cresol red (acid-base indicator) for the proton concentration. The freezing-induced production of I₂ in the presence of IO₃^- and NO₂^- was observed under various conditions, which suggests this abiotic process for I₂ production is not restricted to a specific region and occurs in many cold regions. NO₂^--induced activation of IO₃^- to I₂ in frozen solution may help explain why the measured values of iodine are larger than the modeled values in some polar areas.