*7.2.3. Biological oxidation methods*

The use of microorganisms in the degradation of cyanide in tailing ponds has often been found to be potentially inexpensive and environmentally friendly compared to conventional chemical and physical processes [23, 71, 72]. Enzymatic activities associated with certain species of bacteria, fungi and algae are known to oxidise cyanide to less toxic cyanate [20, 73, 74]. Aerobic and anaerobic passive biological treatment processes are cost‐effective alternatives to conventional cyanide treatment strategies since they do not need external energy, chemicals and routine maintenance. However, they suffer limitations such as the need for warmer climates (>10°C), large space and long retention times. **Figure 1** shows a flowchart for the aerobic and anaerobic oxidation of cyanides and thiocyanides in gold tailing ponds. Common passive biological treatment processes comprise engineered wetlands containing substrate or a mixture of organic and inorganic compounds like manure, straw, saw dust and limestone [22]. In anaerobic wetlands, bacterial oxidation of cyanides and thiocyanides to sulphates, carbonates and ammonia occurs as illustrated in Eqs. (17) and (18).

$$\rm{\bf C\heartsuit}^{-} + \rm{\bf 2H}\_{2}\rm{O} + \rm{\bf 0.5O}\_{2} \rightarrow \rm{\bf N\dot{\bf}}\_{3}^{-} + \rm{\bf O\dot{\bf}}^{\rm{\bf}} + \rm{\bf CO}\_{3}^{2-} \tag{17}$$

$$\text{\bf SCN}^{-} + 2\text{H}\_{2}\text{O} + 2.5\text{O}\_{2} \rightarrow \text{N}\text{H}\_{4}^{+} + \text{HCO}\_{3}^{-} + \text{\bf SO}\_{4}^{2-} + \text{H}^{+} \tag{18}$$

**Figure 1.** Biological cyanide degradation processes in gold tailings.

The ammonia produced by the aerobic processes provides nutrients for microbial growth and the resultant uptake, sorption, conversion and/or precipitation of cyanides, thiocyanates, sulphates and nitrates by microorganisms [74]. The metals released during the oxidation of cyanide metal complexes are removed from gold tailings by chemical precipitation and/or adsorption on bacterial biofilm. Ammonia also undergoes further oxidation in a two‐step nitrification process (Eqs. (19) and (20)).

$$\text{N}\text{H}\_4^\* + \text{1.5O}\_2 \rightarrow \text{NO}\_2^- + \text{2H}^+ + \text{H}\_2\text{O} \tag{19}$$

Review of the Impact on Water Quality and Treatment Options of Cyanide Used in Gold Ore Processing http://dx.doi.org/10.5772/65706 235

$$\rm{NO}\_{2}^{-} + \rm{O.5O}\_{2} \rightarrow \rm{NO}\_{3}^{-} \tag{20}$$

Sulphates undergo anaerobic reduction to sulphides (Eq. (21)). This process is effected by sulphate‐reducing bacteria [75]. The sulphide produced is precipitated by metal ions resulting in its removal from aqueous tailings.

( ) ( ) - - -- - + ®+ + 2 2 2 SO 2C H O lactate S 2CO 2C H O acetate 43 4 33 2 4 2(21)

Several factors influence the biodegradation of cyanide in gold tailings. The most important environmental factors influencing biological treatment include pH, temperature, oxygen levels and nutrient availability. Enzymes that degrade cyanide are generally produced by mesophilic microorganisms, often isolated from soil, with optimum operating temperatures of between 20 and 40°C [34, 43, 76–79]. The availability of nutrient carbon has been found as a limiting factor in the biodegradation of metal‐cyanide complexes [75].

Highly acidic and basic conditions have adverse effects on cyanide‐degrading microorganisms since bacterial and fungal growth is optimal at pH 6–8 and 4–5, respectively [80]. Cyanide‐ degrading enzymes have optimum operating pH between 6 and 9. Concentrations of cyanide ions in water or slurries have an impact on the survival and growth of microorganisms. For instance, high cyanide concentrations have been reported to be toxic to *Klebsiella oxytoca* by damaging the nitrile‐degrading enzyme, nitrile hydratase [81].

### **7.3. Emerging technologies on cyanide remediation**

and routine maintenance. However, they suffer limitations such as the need for warmer climates (>10°C), large space and long retention times. **Figure 1** shows a flowchart for the aerobic and anaerobic oxidation of cyanides and thiocyanides in gold tailing ponds. Common passive biological treatment processes comprise engineered wetlands containing substrate or a mixture of organic and inorganic compounds like manure, straw, saw dust and limestone [22]. In anaerobic wetlands, bacterial oxidation of cyanides and thiocyanides to sulphates,


2  NH


4

The ammonia produced by the aerobic processes provides nutrients for microbial growth and the resultant uptake, sorption, conversion and/or precipitation of cyanides, thiocyanates, sulphates and nitrates by microorganisms [74]. The metals released during the oxidation of cyanide metal complexes are removed from gold tailings by chemical precipitation and/or adsorption on bacterial biofilm. Ammonia also undergoes further oxidation in a two‐step

<sup>+</sup> - + + ® ++

 NO

2  2H H O

2

(19)

2

 NH

2  OH

3

> HCO

2

3

2

 4  H

(17)

(18)

 CO

 SO

3

carbonates and ammonia occurs as illustrated in Eqs. (17) and (18).

CN

SCN

234 Water Quality

 2H O 0.5O

2

 2H O 2.5O

**Figure 1.** Biological cyanide degradation processes in gold tailings.

NH 1.5O

4

nitrification process (Eqs. (19) and (20)).

2

> Since the 1990s, research has focused on introducing cyanide treatment technologies aimed at reducing costs and producing environmentally friendly products.

> Carbon dioxide has been successfully used without a catalyst to replace SO2 as an inexpensive alternative to the SO2/air process [82].

> Wastewater containing free and complexed cyanides can be oxidised by ultraviolet radiation in the presence of a semiconductor catalyst such as titanium dioxide [33]. When the catalyst mixed in the wastewater is exposed to the sunlight, it generates a highly reactive hydroxyl radical oxidant. These radicals initially convert cyanide to cyanate. Photocatalysis partially dissociates ferricyanide and ferrocyanide complexes to free cyanide and iron hydroxide. Photocatalytic oxidation is effective in relatively clear solutions. In the presence of ozone, ultraviolet oxidation does not produce undesirable by‐products such as ammonia.

> Solid or liquid cyanide wastes can be thermally decomposed upon treatment at elevated temperatures and pressure in batch or continuous mode [83]. This process is capable of destroying all cyanide complexes. Cyanide hydrolysis occurs in two steps (Eqs. (22) and (23)) producing ammonia and carbonates.

$$\text{NaCN} + \text{NaOCl} \rightarrow \text{NaCNO} + \text{NaCl} \tag{22}$$

$$\text{2NaCNO} + \text{3NaOCl} \rightarrow \text{H}\_2\text{O} + \text{3NaCl} + \text{N}\_2 + \text{2NaHCO}\_3 \tag{23}$$

This cost‐effective process was developed in the early 1990s for the treatment of wastes containing high concentrations of cyanide (100,000 mg/L). Thermal reduction reduces cyanide concentration to approximately 25 mg/L, which can be further oxidised by conventional methods such as ozone or hydrogen peroxide to environmentally permissible levels.

Free cyanide and cyanide complexes containing waste can be treated by electrochemical oxidation. This is an economical and environmentally friendly technique of destroying cyanide. The process results in cyanide ions being destroyed at the anode as metals are deposited at the cathode [27, 84]. During electrolysis, cyanide is initially oxidised at the anode‐ producing cyanate ions, which are further decomposed to carbon dioxide and nitrogen gas at the cathode (Eqs. (24) and (25)).

$$\mathbf{Anode} \colon \mathbb{C}\mathbb{N}^- + \mathcal{D}\mathbb{H}^- \to \mathbb{O}\mathbb{CN}^- + \mathbb{H}\_2\mathbb{O} + \mathcal{D}\mathbf{e} \tag{24}$$

$$\text{Cathode}: \text{ 2CNO}^{-} + 4\text{OH}^{-} \rightarrow \text{2CO}\_{2} + \text{N}\_{2} + 2\text{H}\_{2}\text{O} + 6\text{e}^{-} \tag{25}$$
