(Spartan Technical Bulletin on Ozone Oxidation of Cyanide PDF)
Cyanide is used in a number of industrial applications, most notably metal plating. Wastewater from these applications can contain free cyanide which must be destroyed before discharge. In many situations, cyanide levels must be reduced to less than 1 ppm for discharge.
Treatment of cyanide bearing wastewater has been carried out with ozone, hydrogen peroxide, alkaline chlorination (either with chlorine, hypochlorite or on-site generation of hypochlorite using electrochemical systems), and high pressure thermal oxidation. For additional information on cyanide treatment you can download an US EPA document on cyanide treatment in metal finishing.
Ozone, with an electrode potential of +1.24 V in alkaline solutions, is one of the most powerful oxidizing agents known.
Cyanide oxidation with ozone is a two-step reaction similar to alkaline chlorination. Cyanide is oxidized to cyanate, with
ozone reduced to oxygen per the following equation:
CN– + O3 → CNO– + O2
Then cyanate is hydrolyzed, in the presence of excess ozone, to bicarbonate and nitrogen and oxidized per the following reaction:
2 CNO- + 3O3 + H2O → 2 HCO3- + N2 + 3O2
The reaction time for complete cyanide oxidation is rapid in a reactor system with 10 to 30 minute retention times being typical. The second-stage reaction is much slower than the first-stage reaction. The reaction is typically carried out in the pH range of 10-12 where the reaction rate is relatively constant. Temperature does not influence the reaction rate significantly. To complete the first reaction requires 1.8 – 2.0 lbs of ozone per lb of CN-.
The metal cyanide complexes of cadmium, copper, nickel, zinc and silver are readily destroyed with ozone. The presence of copper and nickel provide a significant catalytic effect in the stage one reaction but can reduce the rate of the stage two reaction (oxidation of cyanate). Iron, gold and cobalt complexes are very stable and are only partially oxidized, unless a suitable catalyst is added. Ultraviolet light (UV oxidation), in combination with ozone, can provide complete oxidation of these complexes.
UV light causes metal complexes such as ferricyanide and ferrocyanide to partially dissociate. In the case of iron cyanide the compound is photolyzed to free cyanide and iron hydroxide. Ozone then continues the oxidation as noted above.
UV oxidation, in combination with ozone, can completely oxidize all metal cyanide complexes. UV oxidation is limited to relatively clear solutions, since waste streams are passed through a light-transmitting chamber and exposed to intense UV light. UV in combination with ozone results in the formation of OH• radicals, which are strong oxidizing agents capable of oxidizing iron cyanide complexes. Suitable light sources emit in the range of 200 to 280 nanometers (nm). Ozone will absorb in this band. A major advantage UV/ozone oxidation is that no undesirable byproducts (e.g., ammonia) are generated. The following equations summarize the reaction ozone in the presence of UV light:
O3 +|| hν → O2 + O
O + H2O → 2OH•