Oxidation of iodide and iodine on birnessite (δ-MnO2) in the pH range 4-8

The oxidation of iodide by synthetic birnessite (δ-MnO₂) was studied in perchlorate media in the pH range 4-8. Iodine (I₂) was detected as an oxidation product that was subsequently further oxidized to iodate (IO₃⁻). The third order rate constants, second order on iodide and first order on manganese oxide, determined by extraction of iodine in benzene decreased with increasing pH (6.3-7.5) from 1790 to 3.1 M⁻² s⁻¹. Both iodine and iodate were found to adsorb significantly on birnessite with an adsorption capacity of 12.7 μM/g for iodate at pH 5.7. The rate of iodine oxidation by birnessite decreased with increasing ionic strength, which resulted in a lower rate of iodate formation. The production of iodine in iodide-containing waters in contact with manganese oxides may result in the formation of undesired iodinated organic compounds (taste and odor, toxicity) in natural and technical systems. The probability of the formation of such compounds is highest in the pH range 5-7.5. For pH <5 iodine is quickly oxidized to iodate, a non-toxic and stable sink for iodine. At pH >7.5, iodide is not oxidized to a significant extent.