FeVI, FeV, and FeIV oxidation of cyanide: Elucidating the mechanism using density functional theory calculations

FeVIO42− (FeVI) is a promising oxidant/disinfectant and coagulant in treating water. FeVO43− (FeV) and FeIVO44− (FeIV) are intermediates in oxidations performed by FeVI and are highly effective in degrading recalcitrant contaminants. The performance of each ferrate requires a comprehensive understanding of the mechanism. This paper presents the use of density functional theory (DFT) to understand the oxidation of cyanide by FeVI, FeV, and FeIV. The sequence of steps in the oxidation process is the same for all three species. The electrostatic attraction of the positive H and C on HCN for two of the negative oxygen atoms on the ferrate is followed by the formation of a C-O bond with cleavage of an Fe-O bond. Subsequently, the formation of an H-O bond occurs with cleavage of the H-C bond, resulting in the formation of an NCO – ferrate complex. However, the energetics are different, with C-O bond formation being the rate determining step for FeVI while for FeV and FeIV it is the transfer of the H from C to O. Energy calculations describe the order of reactivity of ferrates with HCN as FeV > FeIV > FeVI, which agrees well with experimental results. © 2017 Elsevier B.V.