Ligand-Specific Dissolution of Iron Oxides in Frozen Solutions

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.