Ferrate(VI) oxidation of zinc–cyanide complex

Zinc–cyanide complexes are found in gold mining effluents and in metal finishing rinse water. The effect of Zn(II) on the oxidation of cyanide by ferrate(VI) (FeVIO42-, Fe(VI)) was thus investigated by studying the kinetics of the reaction of Fe(VI) with cyanide present in a potassium salt of a zinc cyanide complex (K2Zn(CN)4) and in a mixture of Zn(II) and cyanide solutions as a function of pH (9.0–11.0). The rate-law for the oxidation of Zn(CN)42- by Fe(VI) was found to be -d[Fe(VI)]/dt=k[Fe(VI)][Zn(CN)42-]0.5. The rate constant, k, decreased with an increase in pH. The effect of temperature (15–45 °C) on the oxidation was studied at pH 9.0, which gave an activation energy of 45.7 ± 1.5 kJ mol−1. The cyanide oxidation rate decreased in the presence of the Zn(II) ions. However, Zn(II) ions had no effect on the cyanide removal efficiency by Fe(VI) and the stoichiometry of Fe(VI) to cyanide was approximately 1:1; similar to the stoichiometry in absence of Zn(II) ions. The destruction of cyanide by Fe(VI) resulted in cyanate. The experiments on removal of cyanide from rinse water using Fe(VI) demonstrated complete conversion of cyanide to cyanate.