Photo/photochemical oxidation of cyanide and metal-cyanide complexes: ultraviolet A versus ultraviolet C.

Degradation of free cyanide (CN(-)), weak-acid dissociable (WAD) (Zn(CN)4(2-), Cu(CN)3(2-)) and strong-acid dissociable (SAD) (Fe(CN)6(4-) cyanide complexes by photo and photochemical oxidation with ultraviolet (UV) light and H2O2 was investigated. The experiments were performed in batch reactors under ultraviolet A (UVA; 395 nm) and ultraviolet C (UVC; 254 nm) light; the degradation efficiency was followed in terms of free cyanide, complex and metal concentrations. UVC and UVA photo-oxidations were found to be equally effective in CN(-) and WAD degradation, while the degradation of the SAD complex was more difficult for both UV wavelengths, and UVC was more effective. The initial pH of the solution has influenced the degradation of all cyanide species and the optimum initial pH was evaluated as 10.5 for CN(-) and Cu(CN)3(2-); 12.0 for Zn(CN)4(2-) and 9.0 for Fe(CN)6(4-) degradation. Photochemical oxidation using H202 provided higher degradation at shorter durations with both UVA and UVC. Time-dependent variations in free cyanide and metal concentrations have indicated that metal-cyanide complexes are firstly degraded into metal and CN(-) ions, followed by oxidation of CN(-) ions, while metals in the system were partially removed as hydroxide precipitates. Therefore, depending upon the effluent requirements, the studied UV photo/photochemical oxidations were offered as either a pre-treatment method for the separation of metal and the cyanide, or as an oxidation technology to degrade especially WAD complexes and CN(-). Estimated operational cost of photo-oxidation by UVC was 1.6-2.5-fold higher than UVA degradation, although degradation times were close. In the photochemical oxidation with H2O2, the operational costs of UVC and UVA degradation were closer, owing to peroxide costs, but UVC was still more expensive.