14.2 What Is Bioleaching?

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into the direct and the indirect mechanism and refers to oxidation and reduction

reaction. In the direct mechanism, bacteria leach the metal through redox reaction,

followed by solubilization of metals by enzymatic reactions. In Complexolysis, an

organic acid leaches metals through complex formation; this process occurs slowly

compared to acidolysis, and solubilization of metal ions is based on the complexing

capacity of a compound with which a complex is formed. Apart from organic acids,

metabolites such as siderophores which is a low molecular weight chelating agent

can form complex, solubilized metals such as chromium and magnesium [10].

14.2.2

Industrial Processes of Bioleaching

The industrial method consists of passing acidified water through a heap of the waste

material. To improve the efficiency of extraction, the leachate is then recirculated

again. The extraction procedure is done by four different methods: (i) Dump leach-

ing – a process where the waste, as a dump, is treated with the leaching solution for

extraction of the metal, (ii) Heap leaching – followed for fine grained ores. The ores

are heaped in large basins, and the solvent passed for extraction; (iii) Underground

leaching – followed in the case of abandoned mines or ore deposits in mines, where

the concentration of the metal of interest is too low for conventional processes. This

method is also called in situ leaching; (iv) Tank leaching – a process, where in,

unlike the earlier three processes, the ores are submerged with the extractant in

tanks. Though expensive, this method is quite efficient as compared to the earlier

procedures mentioned [10].

14.2.3

Factors Affecting Bioleaching

Several factors such as pH, temperature, oxygen, carbon dioxide, nutrient

availability, and microbial activity affect the biomining process. By altering the pH

value of bioleaching environment, the bacterial growth can be adjusted to positively

influence the leaching behavior and efficient solubilization. For example, the

bacterial oxidation of ferrous iron and sulfide requires pH value between 2.0 and

2.5. If the value is below 2.0, Thiobacillus ferrooxidans gets inhibited but addition

of acid will help T. ferrooxidans adapt to low pH [9]. The optimum temperature

required for ferrous iron and sulfide oxidation by T. ferrooxidans is between 28 and

30 ^∘C. A decrease in the metal extraction will occur at low temperature, but for few

metals such as cobalt and nickel, bacterial solubilization occurs at low temperature.

Mechanism of leaching by thermophilic bacteria occurs at peak temperature [10].

To achieve optimal growth of iron complexes, sulfur is combined with magnesium

and other salts [10]. Dissolution by chemolithoautotrophs requires inorganic com-

pounds for growth. 9K culture media consisting of magnesium and a combination

of salts are the widely used culture media in industries and laboratory. A few studies

show that bacterial activity and growth can be inhibited by certain inhibitors like

cellobiose, etc. To ensure high activity of leaching by the bacteria, there must be an