212

14 Recovery of Precious Metals from Electronic and Other Secondary Solid Waste

14.3.3

General Methods of E-Waste Treatment

E-waste treatment is important for safe disposal and also for recovery of met-

als from wastes generated. Traditional process for waste treatment includes

pyrometallurgical and hydrometallurgical methods for recycling waste materials for

useful applications. The three major steps in recycling of waste include: disassembly

(separating out hazardous or valuable components followed by special treatment),

upgrading (materials are prepared for refining process by mechanical processing),

and refining (materials that are recovered in the previous step is purified using

chemical processing) [18].

14.4

Role of Microbes in Bioleaching of E-Waste

Microbial agents involved in bioleaching are grouped into two types based

on requirement of metabolism as chemolithotrophs and chemoorganotrophs.

Chemolithotrophs also known as acidophiles survive at very low pH and are further

categorized into mesophilic, moderately thermophilic, and thermophilic based on

optimal growth temperatures. Chemolithotrophs utilize inorganic matter as energy

source and carbon source from carbon dioxide. Carbon is used as energy source

by organotrophs and chemolithotrophic autotrophs. The mode of action of these

microbes in solutions is as follows: initially, microorganisms bring about oxidation

and reduction reactions in the solution. This is followed by formation of acid from

either organic or inorganic route and ends with the leaching of metal from the

sulfide matrix [9].

14.4.1

Bacteria

Solubilization of metals is achieved by sulfur-oxidizing and iron-oxidizing organ-

isms. Iron- or sulfur-oxidizing bacteria such as the mesophilic aerobic and

chemolithotrophic microorganisms are widely used to transform metallic fractions

in e-waste to water-soluble phases. The bacteria from the genus Acidithiobacillus

have greater tolerance to heavy metals and dominate the research works done

in bioleaching of e-waste. The mesophilic autotrophs such as A. ferrooxidans,

A. thiooxidans, and Leptospirillum ferrooxidans have been used extensively in

leaching e-waste [9]. Many other species from different genera are known to play

a vital role in metal mobilizations. The widely distributed four phyla in leaching

of metals include Proteobacteria, Nitrospirae, Firmicutes, and Actinobacteria.

Based on optimum temperature of microbes, they are of three types: mesophiles,

moderate thermophiles, and thermophiles. Organisms belonging to the domain

Euryarchaeota and Archaea such as Acidianus, Metallosphaera, Sulfurisphaera,

Ferroplasma acidiphilum, Ferroplasma acidarmanus play a vital role. Among these,

a few can be grouped as oxidizer of iron and the others as sulfur oxidizers. Still

another group exists which oxidizes both iron and sulfur based on their preference

of substrate.