Mechanism and kinetics of cyanide decomposition by ferrate

The mechanism of cyanide oxidation by ferrate in water is discussed using DFT computations in the framework of the polarizable continuum model. The reactivity of three oxidants, monoprotonated, monoprotonated, and diprotonated ferrates is evaluated. This reaction is initiated by a direct attack of an oxo group of ferrate to the carbon atom of cyanide, followed by an H-atom transfer from cyanide to another oxo group to lead to an intermediate having cyanate (NCO^-) as a ligand. The produced cyanate is oxidized by an oxo ligand of ferrate and exogenous oxygen molecule to CO₂ and NO ₂^-. The initial C-O bond formation is found to be the rate-determining step in this reaction. The activation energy for the C-O bond formation is 51.9kJ mol^-1 for nonprotonated ferrate, 44.4 kJ mol ^-1 for monoprotonated ferrate, and 41.4 kJ mol^-1 for diprotonated ferrate, which indicates that the oxidizing power of the three oxidants is in the order of nonprotonated ferrate < monoprotonated ferrate < diprotonated ferrate. The general energy profile for cyanide oxidation by ferrate is downhill toward the product direction after the C-O bond formation, so cyanide is readily converted to the final products in water. The reaction kinetics of this reaction are analyzed from the calculated energy profile and experimentally determined pK_a values.