Cyanide Removal from Industrial Wastewater: Ozone Cyanide Oxidation
Cyanide is used in mining, industrial processing and manufacturing applications. Wastewater from these applications may contain free cyanide, which must be removed before discharge. In many cases, cyanide levels for discharge must be brought below legal limits. One of the most effective ways of this is the use of Ozone or Ozone + UV.
Ozone, hydrogen peroxide, alkaline chlorination (either with chlorine, hypochlorite, or on-site generation of hypochlorite using electrochemical systems), and high-pressure thermal oxidation have all been used to treat cyanide-containing wastewater.
- Highly effective against all free and complex cyanide alone or in combination with UV light.
- Ozone generators only consume electricity.
- It does not form undesirable products such as chlorinated organics or ammonia.
- Does not require field purchase, storage or transportation of hazardous chemicals
- Reaction with ozone does not require high temperature or pressure
Ozone Removal of Cyanide from Industrial Wastewater
Ozone is one of the most powerful oxidising agents known, with an electrode potential of +1.24 V in alkaline solutions. The oxidation of cyanide with ozone is a two-step process similar to alkaline chlorination. According to the following equation, cyanide is oxidised to cyanate, and ozone is reduced to oxygen:
CN– + O3 → CNO– + O2
The cyanate is then hydrolyzed to bicarbonate and nitrogen in the presence of excess ozone and oxidised as follows:
2 CNO- + 3O3 + H2O → 2 HCO3- + N2 + 3O2
In a reactor system, the reaction time for complete cyanide oxidation is short, with typical retention times ranging from 10 to 30 minutes. The reaction in the second stage is much slower than the reaction in the first stage. The reaction is typically carried out in the pH range of 10-12, with a relatively constant reaction rate. Temperature has little effect on the reaction rate. The first reaction requires 1.8 – 2.0 lbs of ozone per pound of CN- to complete.
Cadmium, copper, nickel, zinc, and silver metal cyanide complexes are easily destroyed by ozone. Copper and nickel have a significant catalytic effect in the first stage reaction but can reduce the rate of the second stage reaction (oxidation of cyanate). Unless a suitable catalyst is added, iron, gold, and cobalt complexes are very stable and only partially oxidised. Ultraviolet light (UV oxidation) in conjunction with ozone can completely oxidise these complexes.
Metal complexes such as ferricyanide and ferrocyanide partially dissociate when exposed to UV light. Iron cyanide is photolyzed to form free cyanide and iron hydroxide. As previously stated, ozone then continues the oxidation process.
When combined with ozone, ultraviolet (UV) oxidation can completely oxidise all metal cyanide complexes. Because waste streams are passed through a light-transmitting chamber and exposed to intense UV light, UV oxidation is limited to relatively clear solutions. When UV is combined with ozone, OH• radicals are formed, which are powerful oxidising agents capable of oxidising iron cyanide complexes. Suitable light sources have wavelengths ranging from 200 to 280 nanometers (nm). In this band, ozone will absorb.
The absence of undesirable byproducts (e.g., ammonia) is a significant advantage of UV/ozone oxidation. The reactions of ozone in the presence of UV light are summarized by the following equations:ozoneapplications
O3 +|| hv → O2 + O
O + H2O → 2OH•
Citation: Spartan Environmental