Enhanced Inorganic Redox Reactions in Ice and its Environmental Implications

Abstract

This study is directed to understanding the redox reactions of inorganic compounds trapped in ice and related environmental implications. The redox chemical reactions in the environment are an important part in environmental chemistry and provide basic information for preserving and remediating the affected environment. The mobility, toxicity, bioavailability, and environmental fate of inorganic elements are influenced by their redox speciation. Chemical mechanisms and processes of inorganic species have been extensively investigated in aquatic environment but those in ice have rarely to our knowledge been studied despite their importance in the environment, especially in the cold regions (e.g., upper troposphere, permafrost, polar/high latitude environment, and midlatitudes during winter season) where freeze-thaw cycles take place.1. The formation of dissolved iron from iron oxide particles in ice was investigated as a new pathway for a bioavailable iron production. The dissolution experiments were carried out under dark condition (in both the absence and presence of various organic complexing ligands). In acidic pH conditions (pH 2, 3, and 4), the dissolution of iron oxides was greatly enhanced in the ice phase compared to that in water. The dissolved iron from iron oxides was mainly in the ferric form, which indicates that the dissolution is not a reductive process. The extent of dissolved iron was affected by the presence and kind of organic complexing ligands and the type of iron oxides.2. The formation of bio-available iron (Fe(II)aq) from the dissolution of iron oxide particles was investigated in ice phase under both UV and visible light irradiation. The photoreductive dissolution of iron oxides proceeded slowly in aqueous solution (pH 3.5) but was significantly accelerated in polycrystalline ice with subsequently releasing more bioavailable ferrous iron upon thawing. The enhanced photogeneration of Fe(II)aq in ice was confirmed regardless of the type of iron oxides [hematite (α-Fe2O3),, maghemite (γ-Fe2O3), goethite (α-FeOOH)] and the kind of electron donors. The ice-enhanced dissolution of iron oxides was also observed under visible light irradiation although the dissolution rate was much slower compared with the case of UV irradiation. The iron oxide particles and organic electron donors (if any) in ice are concentrated and aggregated in the liquid-like grain boundary region (freeze concentration effect) where protons are also highly concentrated (lower pH). The enhanced photodissolution of iron oxides should occur in this confined boundary region. We hypothesized that electron hopping through the interconnected grain boundaries of iron oxide particles facilitates the separation of photoinduced charge pairs. The outdoor experiments carried out under ambient solar radiation of Ny-Ålesund (Svalbard, 78°55′N) also showed that the generation of dissolved Fe(II)aq via photoreductive dissolution is enhanced when iron oxides are trapped in ice. These results imply that the ice/snow-covered surfaces and ice-cloud particles containing iron-rich mineral dusts in the polar and cold environments provide a source of bioavailable iron when they thaw. 3. We investigated the reductive dissolution of manganese oxides trapped in ice under dark and light irradiation (visible and UV) in comparison with their counterparts in aqueous solution. The reductive dissolution of synthetic MnO2, which took place slowly in aqueous solution, was significantly accelerated in ice phase both in the presence and absence of light. The formation of Mn(II)aq via photoreductive pathway gradually increased with decreasing pH both in aqueous and ice phase and the rate was faster in ice phase. The photoreductive dissolution of MnO2 in ice under visible light irradiation was also faster than its aqueous counterpart. The enhanced generation of Mn(II)aq in ice can be mainly explained by freeze concentration of electron donors, protons, and MnO2 in liquid-like ice grain boundaries. The outdoor solar experiment that was conducted obtained in Arctic region (Ny-Ålesund, Svalbard, 78°55′N) suggests that the observed phenomenon might be applicable to real environmental condition. The present results imply that the dissolution of natural minerals like manganese oxides can be enhanced in icy environments such as polar regions, upper atmosphere, and permafrost.4. We investigated (1) the reduction of hexavalent Cr(VI) (as chromate) by model organic acids (e.g., citric and oxalic acid) and (2) the simultaneous removal of Cr(VI) and As(III) (as arsenite) in ice phase in comparison with their counterparts in aqueous solution. The reduction of Cr(VI) by various organic acids (electron donors) was negligible in ambient aqueous solution, but was significantly accelerated in ice. The simultaneous reduction of Cr(VI) and oxidation of As(III) in ice phase proceeded stoichiometrically whereas their mutual conversion was insignificant in aqueous solution. The enhanced redox conversion of Cr(VI)/As(III) in ice is ascribed to the freeze concentration of both electron donors (e.g., organic acids, arsenites) and protons in the ice crystal grain boundaries. When the concentrations of both electron donors and protons were highly raised to an extreme, the removal rates of Cr(VI) in aqueous solution approached to those in ice. This specific combination of Cr(VI)/As(III) redox couple may provide an example that represents innumerable redox conversion reactions that could be greatly accelerated in ice/snow-covered or frozen environments.5. The formation of triiodide through iodide oxidation was investigated in ice phase under UV irradiation (λ > 300 nm) and dark condition. The oxidation of iodide proceeded slowly in aqueous solution but was significantly accelerated in polycrystalline ice even in the absence of UV irradiation. The enhanced oxidation of iodide in ice is owing to the freeze concentration of iodides, protons, and dissolved oxygen in the ice crystal grain boundaries. The outdoor experiments carried out under ambient solar radiation of the Antarctic region (King George Island, 62°13′S 58°47′W, sea level) also showed that the generation of triiodide via photooxidative process is enhanced when iodide is trapped in ice. The observed authentic redox transformation of iodide in ice phase suggests a previously unknown generation pathway for the considerable release of reactive halogen compounds to the atmosphere consequently influencing O3 and Hg depletion event, perturbation of OHx/NOx cycles, and DMS oxidation to form CCN.6. We studied reductive transformation of iron oxides to produce Fe(II)aq and simultaneous oxidation of iodide to I3- in ice phase under UV irradiation or dark condition. The reductive generation of Fe(II)aq from iron oxide and oxidation of iodide were negligible in ambient aqueous solution but significantly accelerated in ice phase both in the presence and absence of light irradiation, subsequently producing bioavailable ferrous iron and evaporable I3-. The enhanced reductive generation of Fe(II)aq and oxidative formation of I3- in ice were confirmed regardless of the type of iron oxides [hematite (α-Fe2O3), maghemite (γ- Fe2O3), goethite (α-FeOOH), lepidocrocite (γ-FeOOH), and magnetite (Fe3O4)]. We speculated that the enhanced redox production of Fe(II)aq and I3- in ice is owing to the freeze concentration of iron oxides, iodides, protons, and dissolved oxygen in the ice crystal grain boundaries. When the concentration of both iodides and protons were raised by 10-fold each, the formation of Fe(II)aq in water under UV irradiation was approached to those in ice. The outdoor tests were carried out under ambient solar radiation in winter season (Pohang, Korea: 36°N latitude) and also confirmed that the generation of Fe(II)aq via reductive pathway and triiodide via photooxidative process were enhanced in ice grain boundaries. This research implies that iron mineral dust and iodide trapped in ice or snow media (acidic aerosol, ice/snow on sea ice, icebergs, ice sheets, etc) can provide bioavailable iron and reactive iodine species when they thaw.