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^VIO ₄^2-, 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.