Ferrate(VI) Oxidation of Weak-Acid Dissociable Cyanides

Cyanide is commonly found in electroplating, mining, coal gasification, and petroleum refining effluents, which require treatment before being discharged. Cyanide in effluents exists either as free cyanide or as a metal complex. The kinetics of the oxidation of weak-acid dissociable cyanides by an environmentally friendly oxidant, ferrate(VI) (Fe^VIO₄²⁻, Fe(VI)), were studied as a function of pH (9.1–10.5) and temperature (15–45 °C) using a stopped-flow technique. The weak-acid dissociable cyanides were Cd(CN)₄²⁻ and Ni(CN)₄²⁻, and the rate-laws for the oxidation may be   −d[Fe(VI)]/dt = k [Fe(VI)][M(CN)₄²⁻]ⁿ where n = 0.5 and 1 for Cd(CN)₄²⁻ and Ni(CN)₄²⁻, respectively. The rates decreased with increasing pH and were mostly related to a decrease in concentration of the reactive protonated Fe(VI) species, HFeO₄⁻. The stoichiometries with Fe(VI) were determined to be: 4HFeO₄⁻ + M(CN)₄²⁻ + 6H₂O → 4Fe(OH)₃ + M²⁺ + 4NCO⁻ + O₂ + 4OH⁻. Mechanisms are proposed that agree with the observed reaction rate-laws and stoichiometries of the oxidation of weak-acid dissociable cyanides by Fe(VI). Results indicate that Fe(VI) is effective in removing cyanide in coke oven plant effluent, where organics are also present.