Ferrate(VI) and ferrate(V) oxidation of cyanide, thiocyanate, and copper(I) cyanide

Cyanide (CN^-), thiocyanate (SCN^-), and copper(I) cyanide (Cu(CN)₄^3-) are common constituents in the wastes of many industrial processes such as metal finishing and gold mining, and their treatment is required before the safe discharge of effluent. The oxidation of CN^-, SCN^-, and Cu(CN)₄^3- by ferrate(VI) (Fe^VIO₄^2-; Fe(VI)) and ferrate(V) (Fe^VO₄^3-; Fe(V)) has been studied using stopped-flow and premix pulse radiolysis techniques. The rate laws for the oxidation of cyanides were found to be first-order with respect to each reactant. The second-order rate constants decreased with increasing pH because the deprotonated species, FeO₄^2-, is less reactive than the protonated Fe(VI) species, HFeO₄^-. Cyanides react 10³-10⁵ times faster with Fe(V) than with Fe(VI). The Fe(V) reaction with CN^- proceeds by sequential one-electron reductions from Fe(V) to Fe(IV) to Fe(III). However, a two-electron transfer process from Fe(V) to Fe(III) occurs in the reaction of Fe(V) with SCN^- and Cu(CN)₄^3-. The toxic CN^- species of cyanide wastes is converted into relatively non-toxic cyanate (NCO^-). Results indicate that Fe(VI) is highly efficient in removing cyanides from electroplating rinse water and gold mill effluent