Kinetics of Thiocyanate Formation by Reaction of Cyanide and Its Iron Complexes with Thiosulfate

Reactions between cyanide and compounds, which contain S–S bonds, in aqueous media result in formation of thiocyanate. In this work, we studied the kinetics of reactions of thiosulfate with free cyanide and its complexes under environmental conditions. Rates of reactions between cyanide species and thiosulfate decrease in the following order: CN− > HCN > [Fe(CN)6]3− > [Fe(CN)6]4−. However, at neutral and slightly acidic pH range, reaction of thiosulfate with iron-cyanide complexes outcompetes its reaction with free cyanide, which exists in equilibrium with complexed cyanide. At environmentally relevant conditions, the characteristic time of reaction between free cyanide and thiosulfate was found to be tens of thousands of years, while for iron-cyanide complexes it was found to be hundreds to millions of years. Examples of application of kinetic parameters for calculation of rates of cyanide consumption in industrial (coke oven wastewater) and non-polluted natural aquatic system (Delaware Great Marsh) are provided.