MnO₂-Induced Oxidation of Iodide in Frozen Solution

Abstract

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 (β-MnO 2 , γ-MnO 2 , and δ-MnO 2 ) 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 − /MnO 2 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.

Freezing-Enhanced Photoreduction of Iodate by Fulvic Acid

“…The majority of newly formed OICs in aqueous UV/IO 3 – /FA (80%), frozen UV/IO 3 – /FA (87.6%), and frozen UV/IO 3 – /Cl – /FA (84.6%) solutions were typical lignin-like compounds (O/C = 0.1–0.67 and H/C = 0.7–1.5). The primary structures of OICs produced from the frozen photoreduction of iodate are similar to those produced via the frozen oxidation of iodide, indicating their analogous iodination mechanisms. , About 67.4–70% of newly formed OICs were further classified as highly unsaturated and phenolic compounds by AI mod , in agreement with distribution characteristics of the OICs formed via the photoinduced or photocatalyzed oxidation of iodide . In addition, 101 species of OCCs (including 1 compound of CHO-I-2Cl) were also concurrently produced in frozen UV/IO 3 – /Cl – /FA solution (see Figure D).…”

“…For example, halocarbons were universally enhanced in sea ice relative to the underlying water . Our previous studies have proposed the potential mechanisms for OICs and/or RI production from iodide, which may contribute to an abiotic iodine cycle in some polar areas. − The present finding proposes a new photoreaction pathway leading to organohalogen compounds in frozen environments and may provide new mechanistic insight into the halogen cycle in the environment.…”

“…The enrichment of iodine compounds in the polar environment, aerosol, and sea ice has been reported, and some of them (e.g., RI and iodinated alkanes) can be transported into the atmosphere, undergoing various photochemical reactions and establishing a new iodine cycle . Our studies have proved that iodide could be the iodine source of RI or OICs during freezing-enhanced abiotic oxidation processes on a laboratory scale. − Herein, we further report that inert iodate can be transformed to RI or OICs via the NOM-sensitized reduction of iodate in a frozen solution. Iodate was mainly converted to iodide via the above processes, which could be subsequently converted to RI or OICs in natural environments or water treatment processes.…”

“…Over past decades, many environmental chemical reactions taking place in frozen solution have been found to be very different from their counterparts in aqueous solution, which offers valuable insights into understanding the chemical mechanisms behind distinctive reactions that occur in polar caps and the atmospheric boundary layer. − Various redox reactions are markedly accelerated in the frozen reaction media, which include the abiotic transformation of iodide to OICs by Fe­(III)/Mn­(IV) oxides. − The solar photochemistry can provide new paths to various chemical reactions occurring in frozen solution (e.g., iodide oxidation). ,− Gálvez et al revealed that the photolysis of frozen iodate salts in the presence of low water content (@100–253 K) serves as a source of RI. Most recently, Kim et al reported that freezing the iodate solution in the presence of various organic pollutants enhances the production of molecular iodine under dark condition, which indicates the accelerated reaction between iodate and organic substances in ice.…”

Freezing-enhanced chlorination of organic pollutants for water treatment

“… 27 ) and nitrite 28 have exhibited significantly improved efficiency in oxidizing organic pollutants under freezing conditions compared to in aqueous system. These accelerated reactions are not solely confined to homogeneous reactions in aqueous solutions; freezing has also proven to enhance the efficiency of heterogeneous reactions involving soils 29 and MnO 2 oxides 30 and photochemical reactions under irradiation. 31–33 The rate accelerations observed during the freezing process can be attributed to various physicochemical mechanisms that are associated with the formation and growth of ice crystals.…”