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