metal surface forming biofilm [15]. The bacterial cells form a biofilm, by the
production of extracellular polymeric substances (slime) on the material. The metals
bind to the functional groups of the carbohydrates and proteins that constitute the
proteinaceous surface layers [18, 19]. On the contrary, the presence of metals in the
environment of the planktonic state cells triggers the production of various secondary metabolites including organic acids, enzymes, etc., for non-contact mode of
bioleaching [20]. The release of CO 2 leads to carbonic acid attack on solid surfaces
[21]. The increase in Fe
2+ to Fe
3+ conversion and vice versa in the solution is
expected to increase the redox potential and in turn bioleaching [22].
3 Process Engineering Aspects in Bioleaching
Bioleaching like other bioprocesses can be efficiently applied to recover metals from
e-waste in various scales of application. Bioleaching involves the use of microbes,
and hence the biotic factors to be considered for its implication include the type of
microorganism, the inoculum concentration, its growth rate, the cell genotype, the
ability of the organism to resist the heterogeneity and toxicity of the e-waste, etc.
Various abiotic factors like the physicochemical conditions, the e-waste loading, its
size and composition, nutrients required by the microorganism, etc. also affect the
bioleaching [11]. Better mixing and process control may be achieved in contained
systems such as bioreactors. Industrial scale bioleaching processes are carried out in
bioreactors which are better equipped with the phase contact mechanisms and
control systems. Based on the complete understanding of the principles and modes
of bioleaching by a suitable microbe and bioreactor design, the process can be
established by opting for either batch or continuous mode of operation.
One-step bioleaching [23] is where the microorganism and the e-waste metal
concentrates are simultaneously added and incubated for solubilization [24]. In this
method, (1) the microbial growth may be inhibited by the presence of leached metals
in solution, thus reducing the leaching rate; (2) microbes may take longer time for
acclimatization in the media containing leached metals, and thus lag phase would be
longer; and (3) the leached metals may get adsorbed/utilized or accumulated in the
cells, thus reducing the metal recovery [25].
Two-step bioleaching [23, 26] is the addition of metal-containing solids after the
precultured microorganism has attained the late exponential phase of growth
[27, 28]. This process would reduce the inhibitory effect of the metals on growth,
however, and may still lead to adsorption/accumulation of leached metals in the
cells, thus reducing recoverability. The lag phase of growth (first step) may also be
lesser.
Spent medium bioleaching [26] is the dissolution of metals from the solids in cellfree supernatant that contains the extracellular proteins and secondary metabolites
[29]. In this process, the extracellular metabolites and proteins which tend to be
secreted only in the presence of metals would not be available for any leaching
action. However, this process can also prevent the toxic or inhibitory effect of metals
30
M. Minimol et al.
production of extracellular polymeric substances (slime) on the material. The metals
bind to the functional groups of the carbohydrates and proteins that constitute the
proteinaceous surface layers [18, 19]. On the contrary, the presence of metals in the
environment of the planktonic state cells triggers the production of various secondary metabolites including organic acids, enzymes, etc., for non-contact mode of
bioleaching [20]. The release of CO 2 leads to carbonic acid attack on solid surfaces
[21]. The increase in Fe
2+ to Fe
3+ conversion and vice versa in the solution is
expected to increase the redox potential and in turn bioleaching [22].
3 Process Engineering Aspects in Bioleaching
Bioleaching like other bioprocesses can be efficiently applied to recover metals from
e-waste in various scales of application. Bioleaching involves the use of microbes,
and hence the biotic factors to be considered for its implication include the type of
microorganism, the inoculum concentration, its growth rate, the cell genotype, the
ability of the organism to resist the heterogeneity and toxicity of the e-waste, etc.
Various abiotic factors like the physicochemical conditions, the e-waste loading, its
size and composition, nutrients required by the microorganism, etc. also affect the
bioleaching [11]. Better mixing and process control may be achieved in contained
systems such as bioreactors. Industrial scale bioleaching processes are carried out in
bioreactors which are better equipped with the phase contact mechanisms and
control systems. Based on the complete understanding of the principles and modes
of bioleaching by a suitable microbe and bioreactor design, the process can be
established by opting for either batch or continuous mode of operation.
One-step bioleaching [23] is where the microorganism and the e-waste metal
concentrates are simultaneously added and incubated for solubilization [24]. In this
method, (1) the microbial growth may be inhibited by the presence of leached metals
in solution, thus reducing the leaching rate; (2) microbes may take longer time for
acclimatization in the media containing leached metals, and thus lag phase would be
longer; and (3) the leached metals may get adsorbed/utilized or accumulated in the
cells, thus reducing the metal recovery [25].
Two-step bioleaching [23, 26] is the addition of metal-containing solids after the
precultured microorganism has attained the late exponential phase of growth
[27, 28]. This process would reduce the inhibitory effect of the metals on growth,
however, and may still lead to adsorption/accumulation of leached metals in the
cells, thus reducing recoverability. The lag phase of growth (first step) may also be
lesser.
Spent medium bioleaching [26] is the dissolution of metals from the solids in cellfree supernatant that contains the extracellular proteins and secondary metabolites
[29]. In this process, the extracellular metabolites and proteins which tend to be
secreted only in the presence of metals would not be available for any leaching
action. However, this process can also prevent the toxic or inhibitory effect of metals
30
M. Minimol et al.