hydrometallurgy termed as “biohydrometallurgy.” The benefits of this technique are
the flexibility of the organism to acclimatize to extreme conditions, being inexpensive, easier, and environmental friendly [8], though a relatively slow process. The
different mechanisms involved in biohydrometallurgy for metal recovery are
bioaccumulation, bio-oxidation, reduction, bioleaching, bioprecipitation,
biosorption, bioflocculation, etc. These processes occur individually or in combinations or can be employed as one of the steps in extraction of metals from
e-wastes [9].
2 Bioleaching of E-Waste for Metal Recovery
Bioleaching is the microbe-mediated transformation or mobilization of metals from
the ores or metal wastes into the lixiviant [10]. The abiotic factors that affect
bioleaching are pH, temperature, substrate concentration, oxygen requirements,
etc. The microbial genera, its nutritional type, inoculum size, metal resistance, and
adaptability contribute to the biotic factors of bioleaching [11].
A primary requirement for the microbe is to acclimatize itself to the metalcontaining e-waste. To establish this, the organisms express the cryptic metalresistant gene clusters present in the plasmids or chromosomal DNA [12]. Further,
metal resistance is also attained through an active efflux of metal ions or entrapment
by metal chaperones [13]. Three main principles [14] that govern the microbial
leaching of metals from e-waste are:
1. Redoxolysis accounts for metal solubilization through oxidation and reduction
reactions. Under aerobic and acidic conditions, the microorganism oxidizes
ferrous to ferric ions that act as oxidants for the insoluble metal forms. Additionally, ferric ion catalyzed sulfuric acid formation in case of metal sulfides and
dissimilatory reduction of ferric to ferrous ion result. This takes part in the next
cycle of reaction [15, 16].
2. Complexolysis is a consequence of the secretome of the organism complexing
with the metals. The extracellular metabolites of the microbial cells either act as
ligands, aid in chelation, or directly complex with the metals. Siderophoremediated iron chelation, peptides binding the metals, and carboxylate anions
from the carboxylic acids complexing with the metals churn out soluble forms
of metal complexes [17].
3. Acidolysis is the generation of inorganic and organic acids and dissolution of
metals from the surface. The inorganic acid produced by the microorganisms is
usually sulfuric acid that plays a role in solubilizing metal sulfides. Another group
of organisms excretes organic acids like citric, glutamic, and oxalic acids. The
metals are displaced from their surfaces by the protons from these carboxylic
acids [15].
Bioleaching of metals from the solid substrate is accomplished by contact mode
through chemotaxis and cell attachment to preferred sites of imperfection on the
Process Engineering Aspects in Bioleaching of Metals from Electronic Waste
29
the flexibility of the organism to acclimatize to extreme conditions, being inexpensive, easier, and environmental friendly [8], though a relatively slow process. The
different mechanisms involved in biohydrometallurgy for metal recovery are
bioaccumulation, bio-oxidation, reduction, bioleaching, bioprecipitation,
biosorption, bioflocculation, etc. These processes occur individually or in combinations or can be employed as one of the steps in extraction of metals from
e-wastes [9].
2 Bioleaching of E-Waste for Metal Recovery
Bioleaching is the microbe-mediated transformation or mobilization of metals from
the ores or metal wastes into the lixiviant [10]. The abiotic factors that affect
bioleaching are pH, temperature, substrate concentration, oxygen requirements,
etc. The microbial genera, its nutritional type, inoculum size, metal resistance, and
adaptability contribute to the biotic factors of bioleaching [11].
A primary requirement for the microbe is to acclimatize itself to the metalcontaining e-waste. To establish this, the organisms express the cryptic metalresistant gene clusters present in the plasmids or chromosomal DNA [12]. Further,
metal resistance is also attained through an active efflux of metal ions or entrapment
by metal chaperones [13]. Three main principles [14] that govern the microbial
leaching of metals from e-waste are:
1. Redoxolysis accounts for metal solubilization through oxidation and reduction
reactions. Under aerobic and acidic conditions, the microorganism oxidizes
ferrous to ferric ions that act as oxidants for the insoluble metal forms. Additionally, ferric ion catalyzed sulfuric acid formation in case of metal sulfides and
dissimilatory reduction of ferric to ferrous ion result. This takes part in the next
cycle of reaction [15, 16].
2. Complexolysis is a consequence of the secretome of the organism complexing
with the metals. The extracellular metabolites of the microbial cells either act as
ligands, aid in chelation, or directly complex with the metals. Siderophoremediated iron chelation, peptides binding the metals, and carboxylate anions
from the carboxylic acids complexing with the metals churn out soluble forms
of metal complexes [17].
3. Acidolysis is the generation of inorganic and organic acids and dissolution of
metals from the surface. The inorganic acid produced by the microorganisms is
usually sulfuric acid that plays a role in solubilizing metal sulfides. Another group
of organisms excretes organic acids like citric, glutamic, and oxalic acids. The
metals are displaced from their surfaces by the protons from these carboxylic
acids [15].
Bioleaching of metals from the solid substrate is accomplished by contact mode
through chemotaxis and cell attachment to preferred sites of imperfection on the
Process Engineering Aspects in Bioleaching of Metals from Electronic Waste
29