**7.2. Chemical, physical and biological methods**

Natural methods of cyanide attenuation have failed to produce effluents of acceptable quality. This has led to the development of numerous biological, physical and chemical treatment methods [37].

Among the methods used in removing cyanide from wastewater include photocatalysis [38], biotreatment [39], copper‐catalysed hydrogen peroxide oxidation [40], ozonation [33], electrolytic decomposition, alkaline chlorination [22], reverse osmosis, thermal hydrolysis and adsorption [41]. Most of these methods have limited applications due to the high cost, production of toxic residues and incomplete degradation of all cyanide complexes [42, 43]. However, biodegradation of aqueous cyanide ions is cheaper than chemical and physical methods [30].

### *7.2.1. Chemical oxidation methods*

### *7.2.1.1. Sulphur dioxide/air (INCO) process*

This process was developed by a Canadian company, Inco metal limited, in 1984 [44]. The process makes use of air and sulphur dioxide in the catalytic oxidation of free and complexed cyanide to cyanate [37, 45, 46] as shown in Eq. (6). The process is catalysed by aqueous copper (II) ions under controlled pH of 8–10. The pH is normally maintained by addition of lime.

$$\text{CON}\_2 + \text{O}\_2 + \text{CN}^- + \text{H}\_2\text{O} \xrightarrow{\text{Cu}^{\cdot 2}} \text{SO}\_4^{\cdot 2} + \text{OCN}^- + \text{2H}^+ \tag{6}$$

After completion of the oxidation process, previously metal ions complexed with cyanide such as Zn+2, Cu+2 and Ni+2 are precipitated as metal hydroxides. This process effectively treats cyanide in slurries and solutions.

### *7.2.1.2. Alkaline chlorination*

In this process, cyanide is oxidised by alkaline chlorine. The process converts all acid dissoci‐ able cyanide except for iron cyanide complexes and more stable metal‐cyanide complexes. The process is a two‐stage process. The first stage involves initial oxidation of free cyanide to cyanogens chloride followed by hydrolysis of cyanogens chloride to cyanate (Eqs. (7) and (8)) at pH 11.

$$\text{ClCN}^{-} + \text{Cl}\_{2} \rightarrow \text{ClCN} + \text{Cl} \tag{7}$$

$$\bullet \text{C} \text{lCN} + \text{H}\_2\text{O} \rightarrow \bullet \text{C} \text{N}^- + 2 \text{H}^+ + \text{Cl}^- \tag{8}$$

During the second stage, cyanate is further oxidised to hydrogen carbonate and nitrogen as shown in Eq. (9). The reaction occurs at pH 8.5.

$$2\text{Cd}\_2 + 2\text{OCN}^- + 6\text{OH}^- \rightarrow \text{N}\_2 + 2\text{HClO}\_3^- + 6\text{Cl}^- + 2\text{H}\_2\text{O} \tag{9}$$

The alkaline chlorination process is primarily applied in the treatment of cyanide solutions rather than slurries, which consume a lot of chorine.
