Oxidation of Ni(II)-cyano and Co(III)-cyano complexes by Ferrate(VI): effect of pH.

Free cyanide (CN(-)) and metal-cyanide complexes (tetracyanonickelate(II)), Ni(CN)4(2-) and hexacyanocobaltate(III)), Co(CN)6(3-) are common constituents of effluents of mining, coal gasification, and petroleum refining. This article presents the degradation of Ni(CN)4(2-) and Co(CN)6(3-) by ferrate(VI) (Fe(VI)O4(2-), Fe(VI)) in alkaline media. The effect of pH (9.0-11.0) and reactant molar ratios on the degradation of the cyanide complexes was investigated. The removal of Ni(CN)4(2-) ion in 200 min was found to be > 90% at pH 9.0; forming cyanate (NCO(-)) ions as the stoichiometric products ([Fe(VI)]:[Total CN(-)] = [Fe(VI)]:[NCO(-)] ≈ 1.0). The degradation efficiency decreased with an increase in pH from 9.0 to 11.0. Comparatively, the Co(CN)6(3-) ion could be degraded only up to 10% in 200 min at pH 9.0 and the final oxidized products were nitrite and nitrate ions. The oxidation efficiency of removing Co(CN)6(3-) did not vary significantly with pH. Fe(VI) consumptions as a result of the oxidation of free cyanide and metal-cyanides and their products are compared and discussed.