based on cell density and coordinated by the N-acylhomoserinelactone (AHL) as the
quorum-sensing molecule and results in the formation of biofilms. The formation of
biofilms is noteworthy in bioleaching, as the contact mechanism is vital in the
recovery of metals [18, 48, 49]. The interphase between the bacteria and the cell
comprises of the secretory compounds like the metabolites, membrane-associated
compounds, organic acids, and extracellular proteins. These compounds facilitate
the process by complex formation, protonation, or electrochemical reactions. Investigation on the biochemistry of iron oxidation is valuable in the construction of
bioleaching systems for large-scale applications. The energy conserving genes
function to regulate the respiratory pathway resulting in ATP synthesis followed
by the expression of rus operon for copper bioleaching [16, 50]. Other examples
include the organic acid production by fungi to leach metals [51]; licanatase, a
lipoprotein contributes to bioleaching in Acidithiobacillus thiooxidans [52]; etc.
3.2 Abiotic Factors that Influence Bioleaching of E-Waste
3.2.1 Physicochemical Properties for Bioleaching
The properties like pH, temperature, oxygen requirement, and bioleaching medium
composition are organism specific and based on the bioleaching system to be
employed in the process. The initial pH and the temperature to be maintained during
the bioprocess solely depend on the natural habitat of selection. Most of the wellstudied autotrophic strains are mesophilic, thermophilic, and acidophilic [53]. At
higher initial pH, the availability of protons is less and inhibits the microbial
oxidation process. Most of the acidophiles require low pH to grow and in the process
solubilize metals. A pH below the optimum values may cause static effects. Most of
the heterotrophs require approximate physiological pH/temperature for growth and
bioleaching. The leaching rate is proportional to the temperature but depends on
whether it is a thermophile or mesophile [11]. Reports on the effect of Fe
2+
concentration, pH [54], temperature, carbon source, energy source [55], and additional supplement [42] optimization for bioleaching of metals from e-waste demonstrate each of its role in the process. Supplementing the media with precursors of
cyanide synthesis, where cyanogenic strains are employed, enhances the bioleaching
of precious metals from e-waste [27, 40]. Oxygen is the terminal electron acceptor in
most of the strains involved in bioleaching and contributes in the redox reactions. In
open systems like heaps, the oxygen requirement is met by a network of pipes and
not homogeneously distributed. This limits the oxygen mass transfer and anaerobic
conditions persist in the interior of the heaps [56]. In engineered systems, the
aeration rate can be controlled desirably to achieve effective oxygen transfer for
bioleaching [31].
Process Engineering Aspects in Bioleaching of Metals from Electronic Waste
33
quorum-sensing molecule and results in the formation of biofilms. The formation of
biofilms is noteworthy in bioleaching, as the contact mechanism is vital in the
recovery of metals [18, 48, 49]. The interphase between the bacteria and the cell
comprises of the secretory compounds like the metabolites, membrane-associated
compounds, organic acids, and extracellular proteins. These compounds facilitate
the process by complex formation, protonation, or electrochemical reactions. Investigation on the biochemistry of iron oxidation is valuable in the construction of
bioleaching systems for large-scale applications. The energy conserving genes
function to regulate the respiratory pathway resulting in ATP synthesis followed
by the expression of rus operon for copper bioleaching [16, 50]. Other examples
include the organic acid production by fungi to leach metals [51]; licanatase, a
lipoprotein contributes to bioleaching in Acidithiobacillus thiooxidans [52]; etc.
3.2 Abiotic Factors that Influence Bioleaching of E-Waste
3.2.1 Physicochemical Properties for Bioleaching
The properties like pH, temperature, oxygen requirement, and bioleaching medium
composition are organism specific and based on the bioleaching system to be
employed in the process. The initial pH and the temperature to be maintained during
the bioprocess solely depend on the natural habitat of selection. Most of the wellstudied autotrophic strains are mesophilic, thermophilic, and acidophilic [53]. At
higher initial pH, the availability of protons is less and inhibits the microbial
oxidation process. Most of the acidophiles require low pH to grow and in the process
solubilize metals. A pH below the optimum values may cause static effects. Most of
the heterotrophs require approximate physiological pH/temperature for growth and
bioleaching. The leaching rate is proportional to the temperature but depends on
whether it is a thermophile or mesophile [11]. Reports on the effect of Fe
2+
concentration, pH [54], temperature, carbon source, energy source [55], and additional supplement [42] optimization for bioleaching of metals from e-waste demonstrate each of its role in the process. Supplementing the media with precursors of
cyanide synthesis, where cyanogenic strains are employed, enhances the bioleaching
of precious metals from e-waste [27, 40]. Oxygen is the terminal electron acceptor in
most of the strains involved in bioleaching and contributes in the redox reactions. In
open systems like heaps, the oxygen requirement is met by a network of pipes and
not homogeneously distributed. This limits the oxygen mass transfer and anaerobic
conditions persist in the interior of the heaps [56]. In engineered systems, the
aeration rate can be controlled desirably to achieve effective oxygen transfer for
bioleaching [31].
Process Engineering Aspects in Bioleaching of Metals from Electronic Waste
33