14.4 Role of Microbes in Bioleaching of E-Waste

213

In the extraction of metal from PCBs, both Leptospirillum sp. and A. ferrooxidans

play a crucial role. Acidimicrobium ferrooxidans and Acidithiobacillus caldus are

active between temperatures of 25 and 55 ^∘C. The bioleaching process undertaken

by A. ferrooxidans occurs by direct and indirect mechanism. Lot of work has been

carried out under both types, but indirect leaching is considered to be more appro-

priate at industrial level due to the flexibility of the process [19]. All reactions taking

place under the bioleaching process occur at mild acidic conditions at ambient

temperature and pressure and are found to be useful in developing eco-friendly

technologies for metal extraction from waste PCBs.

14.4.2

Fungi

Although both bacteria and fungi produce organic acids, majority of the reports iden-

tify fungi as the major contributor to bioleaching process. Carbon source and energy

are needed by fungi, and they produce organic acids like gluconic, citric, oxalic,

etc. while growing on organic supplements [20]. Metal leaching by fungi is possi-

ble at a very low acidic pH compared to iron and sulfur bacteria, thereby reducing

threat to environment. During metal mobilization, addition of organic compounds

by iron-oxidizing bacteria favors better metal solubilization due to increase in con-

centration of ferric iron in solution form. Several fungi grow in existence of toxins at

very low pH and temperature. Organic acids (oxalic, gluconic acid, citric, and malic)

aid as lixiviant for base metal solubilization. They are produced by fungi such as

Aspergillus sp. and Penicillium sp. Acidolysis is a process of fungal attack on mineral

surfaces by producing proton which breaks the bonds followed by removal of metal

ions. Other processes such as complexolysis, redoxolysis, and bioaccumulation are

involved in the solubilization of metals through organic acids produced by fungi [21].

Fungi can thrive under different environmental conditions, and the filamentous

soil fungi are considered to be of great interest in bioremediation. Treatment using

biological methods allows the cyclization of the sediment after treatment and is con-

sidered to be economical. Purpureocillium lilacinus was used as a bioleaching agent

which resulted in oxalates formation in the culture filtrates as confirmed through

Fourier Transform Infra Red spectroscopy (FTIR) technique after treatment. To

accomplish bioaccumulation involves the transportation of the metal ions that are

soluble through accumulated dense elements by crossing the membrane of the

cell. This might be performed by functional groups in fungal mycelium by binding

to the metal ions. Penicillium sp. and Aspergillus sp. fungal strains accumulate

radionuclides and metals from outside atmosphere [22].

14.4.3

Actinobacteria and Cyanogenic Organisms

Actinobacteria strains from mining areas were isolated and found to have poten-

tial to bioaccumulation. High concentration of zinc and lead, and low or moderate

concentration of copper, cadmium, and chromium were identified by analyzing the

residues from mining areas. It was determined that about 59 actinobacteria iso-

lated were resistance to 50 heavy metals. Among these, 59 actinobacteria isolated,