Various studies suggest that when growth of microorganism was separated from the
bioleaching process, the time of targeted leaching was reduced (Mishra et al. 2008a).
Zhu et al. (2011) observed that the removal of non-metallic elements can alleviate
the toxic effects of e-waste on microbial growth. Other influencing factors such as
the type of e-waste, initial concentration of electron donors, and inoculum size are
not discussed in detail here.
5 Developing Efficient Methods for Bioleaching
5.1 Non-electrochemical Methods
The crucial bottlenecks in bioleaching processes are long reaction times and low
efficiency (Wang et al. 2018). These problems hinder practical success. Recently,
researchers have suggested some novel methods to improve bioleaching efficiency.
Isıldar et al. (2016) suggested a process using a two-stage bioleaching. In this
approach, acidophilic bioleaching microorganisms are cultivated in the absence of
e-waste (Vincke et al. 2001). Then, the e-waste is added during the exponential
growth phase of the microorganisms, and the culture is further incubated. In this
method, the toxic impact of e-waste on microbes can be avoided, and the microbes
can develop well (Okabe et al. 2007). Okibe and Johnson (2004) suggested a method
based on the reduction of the negative impacts of excessive organics in the growth
environment of the autotrophs. This method proposed development of a microbial
ecosystem with the synergistic effects of acidophilic heterotrophs and autotrophs.
Panda et al. (2017) observed that the microbial community leans toward better
performance when mixed cultures of microorganisms are used in bioleaching rather
than pure cultures. Peng et al. (2012) studied the influence of surfactants on
bioleaching. They observed that supplementation of the media with surfactants
increased the surface contact between electron donors and microbes’ cells. Tween80 is a good surfactant for stimulating microorganisms to metabolize elemental
sulfur (Xiang et al. 2010; Yang et al. 2009). During the bioleaching process, a
passivation layer is generally formed by precipitation of Fe
3+ (such as jarosite) and
S
. This passivation layer coats the e-waste and inhibits the free movement of
electrons. As a result, metal bioleaching is suppressed, and the capital costs increase
significantly (Mishra et al. 2008b; Zeng et al. 2010). The passivation layer should be
destabilized to overcome this issue. In this case, generation of a Fe(III)-precipitate
can be inhibited by controlling the pH of solution or by employing thermophilic or
extremely thermophilic autotrophs for oxidation of S
, which can considerably
accelerate the process (Aditiawati et al. 2009; Priya and Hait 2018).
300
S. Venkatesa Prabhu et al.
bioleaching process, the time of targeted leaching was reduced (Mishra et al. 2008a).
Zhu et al. (2011) observed that the removal of non-metallic elements can alleviate
the toxic effects of e-waste on microbial growth. Other influencing factors such as
the type of e-waste, initial concentration of electron donors, and inoculum size are
not discussed in detail here.
5 Developing Efficient Methods for Bioleaching
5.1 Non-electrochemical Methods
The crucial bottlenecks in bioleaching processes are long reaction times and low
efficiency (Wang et al. 2018). These problems hinder practical success. Recently,
researchers have suggested some novel methods to improve bioleaching efficiency.
Isıldar et al. (2016) suggested a process using a two-stage bioleaching. In this
approach, acidophilic bioleaching microorganisms are cultivated in the absence of
e-waste (Vincke et al. 2001). Then, the e-waste is added during the exponential
growth phase of the microorganisms, and the culture is further incubated. In this
method, the toxic impact of e-waste on microbes can be avoided, and the microbes
can develop well (Okabe et al. 2007). Okibe and Johnson (2004) suggested a method
based on the reduction of the negative impacts of excessive organics in the growth
environment of the autotrophs. This method proposed development of a microbial
ecosystem with the synergistic effects of acidophilic heterotrophs and autotrophs.
Panda et al. (2017) observed that the microbial community leans toward better
performance when mixed cultures of microorganisms are used in bioleaching rather
than pure cultures. Peng et al. (2012) studied the influence of surfactants on
bioleaching. They observed that supplementation of the media with surfactants
increased the surface contact between electron donors and microbes’ cells. Tween80 is a good surfactant for stimulating microorganisms to metabolize elemental
sulfur (Xiang et al. 2010; Yang et al. 2009). During the bioleaching process, a
passivation layer is generally formed by precipitation of Fe
3+ (such as jarosite) and
S
. This passivation layer coats the e-waste and inhibits the free movement of
electrons. As a result, metal bioleaching is suppressed, and the capital costs increase
significantly (Mishra et al. 2008b; Zeng et al. 2010). The passivation layer should be
destabilized to overcome this issue. In this case, generation of a Fe(III)-precipitate
can be inhibited by controlling the pH of solution or by employing thermophilic or
extremely thermophilic autotrophs for oxidation of S
, which can considerably
accelerate the process (Aditiawati et al. 2009; Priya and Hait 2018).
300
S. Venkatesa Prabhu et al.