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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)42− and hexacyanocobaltate(III)), Co(CN)63- are common constituents of effluents of mining, coal gasification, and petroleum refining. This article presents the degradation of Ni(CN)42− and Co(CN)63- by ferrate(VI) (FeVIO42−, 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)42− 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)63− 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)63− 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.