Equilibrium Precipitation and Dissolution of Iron Cyanide Solids in Water

Abstract

Iron cyanide solids are complex coordination compounds that are produced and used in various commercial products and processes as well as clinical products. These solids are also present in spent potlining wastes associated with aluminum production sites and in areas of former manufactured gas plant (MGP) facilities as a result of disposal of cyanide-bearing materials in soils containing iron. In such cases, the concentration of cyanide in groundwater is governed by dissolution of the iron cyanide solids under various environmental conditions. Understanding of the precipitation and dissolution chemistry of iron cyanide solids is still in a nascent state, however. In this work, the equilibrium solubility behavior of iron cyanide solids was studied as a function of pH, pE, and other solution conditions, including those conducive to coprecipitation of iron cyanide solids and hydrous ferric oxide. Equilibrium solubility products were determined for reagent-grade Prussian Blue, and for synthesized Prussian Blue and Turnbull's Blue. Prussian Blue [Fe4(Fe(CN)6)3(s)] was shown to be the stable iron cyanide solid at higher pE conditions over the aqueous pE–pH range, while Turn-bull's Blue [Fe3(Fe(CN)6)2(s)] was identified as the stable iron cyanide solid at lower pE values. X-ray diffraction studies yielded the same crystalline structure for these iron cyanide solids. The Fe2+/Fe3+ ratios and, implicitly, the chemical compositions of the solids, were determined using Mössbauer spectroscopy. When the iron cyanide solids were precipitated in the presence of excess dissolved iron, a coprecipitate with hydrous ferric oxide appeared to form. The resulting solids exhibited equilibrium solubility higher than those of the pure-phase iron cyanides. The observed solubility behavior in these cases could be described by considering the formation of a solid solution. Key words: Cyanide; iron cyanide; precipitation; dissolution; solubility; Prussian Blue; Turnbull's Blue