on microbial growth. The time required for growth may be lower during the
production of spent medium, owing to lower lag phase period and higher growth
rate in the absence of metal-containing wastes.
Choice of these modes of bioleaching process operation depends on the type of
microbes chosen, inhibitory nature of the metals, and metal-microbe interaction
involved. The bioleaching efficiency can be maximized by adequate optimized
process conditions.
3.1 Biotic Factors Involved in Bioleaching
3.1.1 Nutritional Type of Microorganism
Microorganisms are classified into four major groups based on their electron, carbon,
and energy source requirements [30] as presented in Table 1. These comprise both
bacteria including actinomycetes and fungi (yeasts and molds). The chemotrophs are
well studied for their bioleaching abilities. Chemolithoautotrophs have the potential
to survive extreme environmental conditions. They are metal resistant and oxidize
metals into solution using oxygen from air as the final electron acceptor.
Chemolithoautotrophs derive energy from reduced inorganic chemicals (sulfides),
using atmospheric carbon dioxide as carbon source and inorganic hydrogen serve to
donate electrons. Most of the bioleaching members of the lithotrophs are sulfur and
iron oxidizers. The genera of bacteria effective for bioleaching of metals from their
surfaces are Acidithiobacillus [8, 22, 27], Leptospirillum [31], Acidianus,
Sulfolobus, etc. [15, 32]. Though lithotrophs are the preferred organisms for
bioleaching due to their low nutrient requirements, it can be applied only for
sulfur-containing compounds [20].
The contribution to the mode of nutrition for heterotrophs is provided by organic
compounds which serve as the source of carbon, energy, and electrons. The glycolytic pathway and the tricarboxylic acid cycle in the metabolism of organic compounds generate different types of organic acids that occupy a central position in the
heterotrophic bioleaching [33]. These organisms also have the ability to produce
Table 1 Nutritional classification of microorganisms
Nutritional type of
microorganisms
Energy source
Electron donor
Carbon source
Photolithoautotrophy
Light
Inorganic
hydrogen
Carbon
dioxide
Photoorganoheterotrophy
Light
Organic hydrogen
Organic
carbon
Chemolithoautotrophy
Inorganic
chemicals
Inorganic
hydrogen
Carbon
dioxide
Chemoorganoheterotrophy
Organic chemicals
Organic hydrogen
Organic
carbon
Process Engineering Aspects in Bioleaching of Metals from Electronic Waste
31
production of spent medium, owing to lower lag phase period and higher growth
rate in the absence of metal-containing wastes.
Choice of these modes of bioleaching process operation depends on the type of
microbes chosen, inhibitory nature of the metals, and metal-microbe interaction
involved. The bioleaching efficiency can be maximized by adequate optimized
process conditions.
3.1 Biotic Factors Involved in Bioleaching
3.1.1 Nutritional Type of Microorganism
Microorganisms are classified into four major groups based on their electron, carbon,
and energy source requirements [30] as presented in Table 1. These comprise both
bacteria including actinomycetes and fungi (yeasts and molds). The chemotrophs are
well studied for their bioleaching abilities. Chemolithoautotrophs have the potential
to survive extreme environmental conditions. They are metal resistant and oxidize
metals into solution using oxygen from air as the final electron acceptor.
Chemolithoautotrophs derive energy from reduced inorganic chemicals (sulfides),
using atmospheric carbon dioxide as carbon source and inorganic hydrogen serve to
donate electrons. Most of the bioleaching members of the lithotrophs are sulfur and
iron oxidizers. The genera of bacteria effective for bioleaching of metals from their
surfaces are Acidithiobacillus [8, 22, 27], Leptospirillum [31], Acidianus,
Sulfolobus, etc. [15, 32]. Though lithotrophs are the preferred organisms for
bioleaching due to their low nutrient requirements, it can be applied only for
sulfur-containing compounds [20].
The contribution to the mode of nutrition for heterotrophs is provided by organic
compounds which serve as the source of carbon, energy, and electrons. The glycolytic pathway and the tricarboxylic acid cycle in the metabolism of organic compounds generate different types of organic acids that occupy a central position in the
heterotrophic bioleaching [33]. These organisms also have the ability to produce
Table 1 Nutritional classification of microorganisms
Nutritional type of
microorganisms
Energy source
Electron donor
Carbon source
Photolithoautotrophy
Light
Inorganic
hydrogen
Carbon
dioxide
Photoorganoheterotrophy
Light
Organic hydrogen
Organic
carbon
Chemolithoautotrophy
Inorganic
chemicals
Inorganic
hydrogen
Carbon
dioxide
Chemoorganoheterotrophy
Organic chemicals
Organic hydrogen
Organic
carbon
Process Engineering Aspects in Bioleaching of Metals from Electronic Waste
31