Oxidation of Inorganic Compounds by Ferrate(VI) and Ferrate(V): One-Electron and Two-Electron Transfer Steps

Ferrate(VI) (Fe(VI)O(4)(-), Fe(VI)) and ferrate(V) (Fe(V)O(4)(3-), Fe(V)) have a high oxidizing power and upon decomposition form a nontoxic byproduct, Fe(III), which makes them environmentally friendly oxidants in water and wastewater treatment. The kinetics of the reaction between Fe(VI) and I(-) was determined using a stopped-flow technique. The second-order rate constants (k, M(-1) s(-1)) for the oxidation of I(-) and other inorganic compounds by protonated ferrate(VI) (HFe(VI)O(4)(-)) and ferrate(V) (Fe(V)O(4)(3-)) ions were correlated with thermodynamic reduction potentials to understand the reaction mechanisms. A linear relationship was found between log k and 1-e(-) reduction potentials for iodide, cyanides, and superoxide while oxy-compounds of nitrogen, sulfur, selenium, and arsenic demonstrated a linear relationship for 2-e(-) reduction potentials. The estimated limits for the reduction potentials of the couple are E(o)(Fe(VI)/Fe(V)) >or= 0.76 V and E(o)(Fe(VI)/Fe(IV)) >or= -0.18 V. Conclusions drawn from the correlations are consistent with the mechanisms postulated from stoichiometries, intermediates, and products of the reactions. Implication of the kinetic results in treatment using ferrate(VI) is briefly discussed.