Photocatalytic Degradation of Ni(II)-Cyano and Co(III)-Cyano Complexes

Cyanide and metal–cyanide complexes are common constituents of effluents of mining, petroleum refining, and coal gasification. This paper presents the photocatalytic degradation of free cyanide, Ni(II)-cyanide (Ni(CN)4 2−), and Co(III)-cyanide (Co(CN)6 3−) complexes in aqueous TiO2 suspensions. The effect of pH (9.5–12.0), TiO2 loading (0.1–2.0 g/l), and the airflow rate (0.5–2.0 l/min) in a photoreactor on the degradation of the cyanide complexes was investigated. Free cyanide fully converted to cyanate (NCO−) under alkaline conditions. The maximum removal of the Ni(CN)4 2− ion in 180 min was found to be 90 %; forming CN− and NCO− ions as the major and minor products, respectively. Comparatively, the Co(CN)6 3− ion could be degraded only up to 30 % in 180 min. The schemes of the photocatalytic oxidation of cyanides are briefly described. The possible causes for differences in degradation of Ni(CN)4 2− and Co(CN)6 3− are also discussed. Optimum conditions for efficient removal of Ni(CN)4 2− and Co(CN)6 3− ions separately and in mixtures are given.