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Cyanides are used in a number of chemical synthesis and metallurgical processes (as simple salts or cyanide complexes). As a class, cyanides are highly toxic and must be destroyed or removed from wastewaters prior to discharge. The most common method for treating free or simple cyanide is alkaline chlorination. However, chlorination of cyanide results in highly toxic intermediates (e.g., cyanogen chloride) and, if organic material is present, chlorinated VOC’s. These compounds, together with the residual chlorine, create additional environmental problems. Consequently, there is a growing need for alternative, non-chlorine methods for destroying cyanides.
Peroxygen compounds such as hydrogen peroxide, peroxymonosulfuric acid (1), and persulfates (1) are effective alternatives to alkaline chlorination for destroying free and complexed cyanides. The choice of peroxygen system depends on the reaction time available, the desired products (cyanate, or CO2 and NH3), the types of cyanides being treated (free, weak acid dissociable, or inert), and the system economics.
Treatment with Hydrogen Peroxide
While hydrogen peroxide will oxidize free cyanide, it is common to catalyze the reaction with a transition metal such as soluble copper, vanadium, tungsten or silver in concentrations of 5 to 50 mg/L (2).