Ligand-Specific Dissolution of Iron Oxides in Frozen Solutions

Abstract

The freezing-enhanced dissolution of iron oxides by various ligands has been recently proposed as a new mechanism that may influence the supply of bioavailable iron in frozen environments. The ligand-induced dissolution of iron oxides is sensitively affected by the kind and concentration of ligands, pH, and kind of iron oxides. While most ligands are thought to be freeze-concentrated in the ice grain boundary region along with iron oxides to enhance the iron dissolution, this study found that some ligands, such as ascorbic acid, suppress the iron dissolution in frozen solution relative to that in aqueous solution. Such ligands are proposed to be preferentially incorporated in the ice lattice bulk and not freeze-concentrated in the liquid-like grain boundary. The experimental analysis estimated that the ionized forms of ligands (e.g., iodide ions) are hardly present in the ice bulk region (<3%) and enhance the iron dissolution in frozen solution (relative to that in aqueous solution), whereas some neutral ligands (e.g., undissociated ascorbic acid) are significantly trapped in the ice bulk (>50%) and suppress the iron dissolution compared to the aqueous counterpart. The present results reveal that the ligand-induced dissolution of iron oxide in frozen solution is not always enhanced relative to aqueous solution but depends upon the kind of ligand and experimental conditions.

In situ Visualization of Ferrous Ions Dissolved from Pure Iron Wire into Thin Frozen Aqueous Solution Films by Combination of Microscopy and Image Processing

“…(13)(14)(15) The reductive reactions of metal oxides and their dissolution have also been reported to be accelerated in FCSs. (16)(17)(18) These results suggest that a frozen system deteriorates the properties and quality of metallic materials in contact with the system. Thus, it is essential to elucidate the corrosion behavior under such conditions to prevent the unintended corrosion of steel materials therein.…”

“…The most common approach to investigating the chemical reactions in an FCS in contact with a metal surface is to freeze mixtures of metals and aqueous solutions in test tubes, then thaw the resultant solutions for chemical analysis. (17,18) However, such a methodology can interrupt the fine time-resolved elution behavior. In addition, the contact area between FCSs and metal materials is unpredictable due to stochastic aspects of freezing phenomena, leading to less precise analysis of reaction rates when estimating reaction models.…”

Freezing Enhances Leaching of Ferrous Ions but Hinders Reductive Dissolution of Ferric Ions from Iron Oxides

“…4 Therefore, one of the important processes of iron dissolution is the reduction of ferric to ferrous ions by photoirradiation, 5,6 bacterial activities, 7 and coexistent organic matters. 8 In addition, because iron oxides are important engineering materials, which are used as recording materials, 1 catalysts, 9 adsorbents for water treatment, 10 drug delivery vehicle, 11 and so forth, their stability in various circumstances is of practical interest.…”

“…Ferric ions, which are thermodynamically most stable under an aerobic condition, readily form hydroxides or oxyhydroxides in an aquatic environment at pH > 1–2 in the absence of effective ligands, whereas ferrous ions are more soluble under the same condition . Therefore, one of the important processes of iron dissolution is the reduction of ferric to ferrous ions by photoirradiation, , bacterial activities, and coexistent organic matters . In addition, because iron oxides are important engineering materials, which are used as recording materials, catalysts, adsorbents for water treatment, drug delivery vehicle, and so forth, their stability in various circumstances is of practical interest.…”

“…These methods allow us to characterize solid iron materials and have been applied to environmental analyses. Interestingly, several studies indicated that the dissolution of iron oxides is enhanced by freezing. ,,, Jeong et al showed the freeze enhancement of the dissolution of iron oxides at pH 2–4 in the presence of carboxylic acids. They suggested that the enhanced dissolution was caused by complexation rather than reduction because the enhanced dissolution depended on ligand types and ferric ions were the dominant dissolved species.…”