Transformations and Speciation of Iodine in the Environment as a Result of Oxidation by Manganese Minerals

The fate and transport of iodine in the environment is contingent upon the presence of manganese oxides and the geochemical controls they exert. The oxidation of iodide by manganese oxides, mainly α-Mn2O3, was studied in the pH range 4-6. In the case of α-Mn2O3, the oxidation of iodide (I-) was observed to stop at iodine (I2), rather than fully oxidizing to iodate (IO3-). The oxidation reaction followed an observed second order kinetics and increase of ionic strength incurred a decrease in the oxidation of I- to I2. Additionally, the calcium sorption on α-Mn2O3 does not influence the oxidation of I-, indicating that cation sorption does not interfere with the sorption of I- at the anion vacancy active sites on the mineral surface. The formation of I2 in aqueous systems is important as it may lead to different reaction pathways for the fate and transport of iodine in the environment, such as sorption of I2 on natural substrates, as well as the volatilization of I2 into the atmosphere, or the formation of iodinated organic compounds. The formation of I2 from I- is most extensive under acidic conditions. The results of the present study indicate a strong geochemical control of manganese oxides over the oxidation and subsequent fate of iodine in the environment, which needs to be considered for studies related to iodine mobility or remediation of iodine impacted systems.