Mechanism and Kinetics of Cyanide Decomposition by Ferrate

Abstract

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, nonprotonated, 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 À ) as a ligand. The produced cyanate is oxidized by an oxo ligand of ferrate and exogenous oxygen molecule to CO 2 and NO 2 À . 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 51.9 kJ mol À1 for nonprotonated ferrate, 44.4 kJ mol À1 for monoprotonated ferrate, and 41.4 kJ mol À1 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.Since cyanides are used to solubilize metal ions in basic solutions, cyanide compounds are involved in wastewater streams from processes such as gold refining, metal plating, and iron and steel manufacturing.1 The compounds are very toxic and must be removed from the wastewater prior to discharge. The most common method of cyanide destruction is alkaline chlorination using sodium hypochlorite at basic pH, as shown below.Unfortunately, this method has some disadvantages such as high chemical costs, formation of toxic intermediate, and chloride contamination. Several methods have been devised to meet growing needs for alternative methods for destroying cyanides: electrolytic decomposition, ozonation, electrodialysis, catalytic oxidation, reverse osmosis, ion exchange, genetic engineering application, and photocatalytic oxidation. From an X-ray analysis, ferrate was found to be tetrahedral in structure like in chromate and manganate. 21 An isotope labeling experiment of oxygen 14 and IR spectroscopy 22 demonstrated that ferrate ion remains monomeric in aqueous solution and that the four oxygen atoms of ferrate are equivalent and exchangeable with solvent water.From density functional theory (DFT) calculations, we have investigated the mechanism and energetics of alcohol oxidation 23,24 and alkane hydroxylation 25 by ferrate. These reactions are likely to be initiated by an H-atom abstraction from a C-H or O-H bond. Our calculated results demonstrated that the oxidizing power of the three active species increases in the order nonprotonated ferrate < monoprotonated ferrate < diprotonated ferrate. 24 However, diprotonated ferrate is unlikely to be a main oxidant in the reaction because the fraction of...

The History and the Current Revival of the Oxo Chemistry of Iron in its Highest Oxidation States: FeVI – FeVIII

“…Typical examples with inorganic reductants are the oxidation of iodide to iodine, arsenite to arsenate, or of sulfite/selenite to sulfate/selenate,, , , but also of hydrogen peroxide, alkali superoxides, hydrazines, hydroxylamines, azide, nitrite, hydrogen sulfide, cyanide,, thiocyanate, hexacyanoferrate(II), thiosulfate,, dithionite, chromium(III), and many others.…”

Oxidation of inorganic contaminants by ferrates (VI, V, and IV)–kinetics and mechanisms: A review

“…6 and 7). According to density functional theory (DFT), HFeO 4 À has a larger spin density on the oxoligands than does FeO 4 2À , which increases the oxidation ability of HFeO 4 À species (Kamachi et al, 2005(Kamachi et al, , 2008. This has been demonstrated by performing calculations on the reactivity of ferrate(VI) with several compounds (Kamachi et al, 2005(Kamachi et al, , 2008Ohta et al, 2001;Shiota et al, 2003).…”

The treatment of greywater from a restaurant by electrosynthesized ferrate (VI) ion

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