MnO2-Induced Oxidation of Iodide in Frozen Solution

Metal oxides play a critical role in the abiotic transformation of iodine species in natural environments. In this study, we investigated iodide oxidation by manganese dioxides (??- MnO2, ??-MnO2, and ??-MnO2) in frozen and aqueous solutions. The heterogeneous reaction produced reactive iodine (RI) in the frozen phase, and the subsequent thawing of the frozen sample induced the gradual transformation of in situ-formed RI to iodate or iodide, depending on the types of manganese dioxides. The freezing-enhanced production of RI was observed over the pH range of 5.0???9.0, but it decreased with increasing pH. Fulvic acid (FA) can be iodinated by I???/MnO2 in aqueous and frozen solutions. About 0.8???8.4% of iodide was transformed to organoiodine compounds (OICs) at pH 6.0???7.8 in aqueous solution, while higher yields (10.4???17.8%) of OICs were obtained in frozen solution. Most OICs generated in the frozen phase contained one iodine atom and were lignin-like compounds according to Fourier transform ion cyclotron resonance/mass spectrometry analysis. This study uncovers a previously unrecognized production pathway of OICs under neutral conditions in frozen environments.

Automated summary

In this study, we investigated the oxidation of iodide by different types of manganese dioxides (??- MnO2, ??-MnO2, and ??-MnO2) in frozen and aqueous solutions. The results show that in frozen environments, a heterogeneous reaction produces reactive iodine (RI), which gradually transforms into iodate or iodide upon thawing depending on the types of manganese dioxides. Additionally, iodination of fulvic acid by I???/MnO2 was observed in both aqueous and frozen solutions, with higher yields of organoiodine compounds (OICs) obtained in frozen solution.