Ozone Oxidation of Cyanide for Cyanide Destruction (Removal)
(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.
Advantages of Ozone Oxidation
Oxidation of Cyanide with Ozone
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•
(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.
Advantages of Ozone Oxidation
- Extremely effective against all free and complexed cyanides either alone or in combination with UV light
- Does not form any undesirable by products such a chlorinated organics or ammonia
- Does not require the purchase, storage or handling of dangerous chemicals on site
- Ozone is produced on site from air using an ozone generator
- The reaction with ozone does not require high temperatures or pressures
Oxidation of Cyanide with Ozone
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•
Spartan Environmental Technologies
Air and Water Treatment
Air and Water Treatment
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Spartan Environmental
Technologies, L.L.C.
7383 Lauren J Dr
Mentor, OH 44060
USA
Phone: 800-492-1252
Fax: 440-368-3569
E-mail: info@
spartanwatertreatment.com
Technologies, L.L.C.
7383 Lauren J Dr
Mentor, OH 44060
USA
Phone: 800-492-1252
Fax: 440-368-3569
E-mail: info@
spartanwatertreatment.com