Mechanisms of oxidation of organosulfur compounds by ferrate(VI)

The oxidation of organosulfur compounds requires the transfer of oxygen atoms and is important in decontamination of chemical warfare agents, desulfurization of fossil fuel for high quality, deodorization of wastewater and sludge, and remediation of industrial effluents. The kinetics of the oxidation of organosulfur compounds (sulfur-containing amino acids, aliphatic and aromatic thiols, and mercaptans) by the environmentally-friendly oxidant, ferrate(VI) FeO42-, was quantitatively examined in this study using a kinetic model considering possible reactions among the species of ferrate(VI) and organosulfur compounds. The ratios of ferrate(VI) to the various organosulfur compounds for the one oxygen-atom transfer were 0.50 and 0.67 for Fe(II) and Fe(III) as final products, respectively. The second-order rate constants for the oxidation of organosulfur compounds by protonated ferrate(VI) HFeO4- ion were correlated with thermodynamic 1 − e− and 2 − e− reduction potentials in order to understand the mechanisms of the reactions. The oxidation of the compounds involved a 1 − e− transfer step from Fe(VI) to Fe(V), followed by 2 − e− transfer to Fe(III) as the reduced product (Fe(VI) → Fe(V) → Fe(III)). The 2 − e− transfer steps resulted in the formation of Fe(II) (Fe(VI) → Fe(IV) → Fe(II)). Conclusions drawn from the correlations are consistent with the experimentally determined stoichiometries and products of the reactions. The calculated half-lives for the oxidation were in the range of ms to s at a dose of 10 mg K2FeO4 L−1 and hence ferrate(VI) has a great potential in treating organosulfur compounds present in water and wastewater.

Research highlights
► Correlation of rates with thermodynamic 1 − e− and 2 − e− reduction potentials of organosulfur compounds (S) to understand the mechanisms of the reactions.
► 1 − e− transfer step (Fe(VI) → Fe(V)), followed by 2 − e− transfer (Fe(V) → Fe(III)), gives a stoichiometry of 0.67 (Fe(VI):[S]) while 2 − e− transfer steps (Fe(VI) → Fe(IV) → Fe(II)) have a stoichiometry of 0.50.
► Ferrate(VI) oxidizes most of compounds within ms to s time scale and can decontaminate chemical warfare agents and desulfurize of fossil fuel for high quality.