Ligand-Specific Dissolution of Iron Oxides in Frozen Solutions

Abstract

The dissolution of iron oxides (mainly goethite) induced by various ligands was investigated in aqueous and frozen solutions. While all the previous works on the dissolution of iron oxides reported higher iron dissolution rate in ice than in water, this work found that some ligands like ascorbic acid and citric acid suppress the iron dissolution in ice on the contrary. The extent of dissolution in water and ice highly varied depending on the kind and concentration of ligands, pH, and the kind of iron oxides. While most solutes are thought to be freeze-concentrated in the ice grain boundary region upon freezing, some ligands that exhibit the negative effects on the iron oxide dissolution in ice are proposed to be preferentially incorporated in the ice bulk and not freeze-concentrated. It was estimated that more than 50% of the total ascorbic acids are trapped in the ice lattice (pH 3) whereas a majority of iodide ions (>97%) are present in the ice surface (grain boundary) region. As a result, ascorbic acid molecules become less accessible to the iron oxide particles concentrated in the ice grain boundary and hence a suppressed dissolution of iron oxide in ice was observed. In addition, hydroxylamine enhanced the iron oxide dissolution in ice at pH 3 but suppressed it at higher pH. Theoretical calculations confirmed that the neutral form of hydroxylamine is preferentially incorporated in the ice lattice but its protonated form is more likely to be present on the ice surface region, which is in good agreement with the pH-dependent dissolution of goethite by hydroxylamine. The temperature-dependent goethite dissolution study found that ascorbic acid showed no sign of the freeze concentration of the ligand whereas hydroxylamine and iodide showed a clear sign of the freeze concentration of the ligands.