On this the list, biochar attained from biomass via pyrolysis process can be used
as a potential exogenous mediator to participate in PCB bioleaching (Wang et al.
2016). Wang et al. (2016) observed a better efficiency in Cu bioleaching using
biochar participation compared to bioleaching without biochar. They also obtained a
leaching time that was reduced by one-third. A modified cathode using N 2 -doped
carbon nanotubes can be used for bioleaching of copper from PCBs to accelerate
EET (Brandl et al. 2001). To acquire excellent bioleaching, electrochemical potential
can be exposed. During sulfide mineral bioleaching, input of negative and positive
potentials can be done to promote the growth and activity of A. ferrooxidans. When
applying positive potential, a significant amount of oxygen is produced due to the
electrolytic conditions and insufficient ferrous iron caused by the presence of stable
ferric iron, which inhibits microbial activity (Vincke et al. 2001). Alternatively,
negative potential enhances biomass production and activity because it is conducive
to the reduction of ferric iron to ferrous iron that is needed for microbial catalysis
(Zhang and Forssberg 1997). Also, it can be expected that the bioleaching microorganisms multiply rapidly in the exponential growth stage. Another appreciable
benefit is that the precipitation of ferric iron can be suppressed under conditions of
negative potential.
Additionally, bioleaching can be enhanced by the presence of metal ions. In this
case, the adsorption of metal ions on mineral surfaces changes metabolic behavior
through modification of electrochemical properties (Garcia-Moyano et al. 2007; Van
der Zee et al. 2003). Various studies have documented that the presence of various
metal ions, such as Hg
2+ , Ag
+ , Cu
2+ , and Bi
+
, has great influence on the electrochemical mechanisms of bioleaching. These metal ions help in the generation of
metal sulfides that dissolve sulfides either by galvanic action or replacement in a
crystal lattice. Miller et al. (1981) showed that Ag
+ ions can improve Cu bioleaching
from chalcopyrite, which resulted from the formation of a porous Ag 2 S with very
low adhesion. Thermodynamically, Ag 2 S is more stable than chalcopyrite. Additionally, chalcopyrite can form electrochemical pairs and act as an anode to dissolve
Cu. However, the effect of catalytic activity of specific metal ions on e-waste
bioleaching will be different than for sulfides. This is because sulfides and e-waste
have different potentials, which will affect the galvanic interaction between the metal
ions and e-waste. The effect of metal ions on the bioleaching of e-waste needs further
investigation (Choi et al. 2004).
6 Future Perspectives on Bioleaching
In this chapter, the diversity of acidophiles in acidic environments and their applications in e-waste bioleaching are concisely discussed. Extreme acidophilic microorganisms show great potential in bioleaching applications. Therefore, the use of
acidophiles and extreme acidophiles in bioleaching should be one of the most
important research areas in the future. From the current reports, various aspects
need to be developed for bioleaching of e-wastes. They are:
302
S. Venkatesa Prabhu et al.
as a potential exogenous mediator to participate in PCB bioleaching (Wang et al.
2016). Wang et al. (2016) observed a better efficiency in Cu bioleaching using
biochar participation compared to bioleaching without biochar. They also obtained a
leaching time that was reduced by one-third. A modified cathode using N 2 -doped
carbon nanotubes can be used for bioleaching of copper from PCBs to accelerate
EET (Brandl et al. 2001). To acquire excellent bioleaching, electrochemical potential
can be exposed. During sulfide mineral bioleaching, input of negative and positive
potentials can be done to promote the growth and activity of A. ferrooxidans. When
applying positive potential, a significant amount of oxygen is produced due to the
electrolytic conditions and insufficient ferrous iron caused by the presence of stable
ferric iron, which inhibits microbial activity (Vincke et al. 2001). Alternatively,
negative potential enhances biomass production and activity because it is conducive
to the reduction of ferric iron to ferrous iron that is needed for microbial catalysis
(Zhang and Forssberg 1997). Also, it can be expected that the bioleaching microorganisms multiply rapidly in the exponential growth stage. Another appreciable
benefit is that the precipitation of ferric iron can be suppressed under conditions of
negative potential.
Additionally, bioleaching can be enhanced by the presence of metal ions. In this
case, the adsorption of metal ions on mineral surfaces changes metabolic behavior
through modification of electrochemical properties (Garcia-Moyano et al. 2007; Van
der Zee et al. 2003). Various studies have documented that the presence of various
metal ions, such as Hg
2+ , Ag
+ , Cu
2+ , and Bi
+
, has great influence on the electrochemical mechanisms of bioleaching. These metal ions help in the generation of
metal sulfides that dissolve sulfides either by galvanic action or replacement in a
crystal lattice. Miller et al. (1981) showed that Ag
+ ions can improve Cu bioleaching
from chalcopyrite, which resulted from the formation of a porous Ag 2 S with very
low adhesion. Thermodynamically, Ag 2 S is more stable than chalcopyrite. Additionally, chalcopyrite can form electrochemical pairs and act as an anode to dissolve
Cu. However, the effect of catalytic activity of specific metal ions on e-waste
bioleaching will be different than for sulfides. This is because sulfides and e-waste
have different potentials, which will affect the galvanic interaction between the metal
ions and e-waste. The effect of metal ions on the bioleaching of e-waste needs further
investigation (Choi et al. 2004).
6 Future Perspectives on Bioleaching
In this chapter, the diversity of acidophiles in acidic environments and their applications in e-waste bioleaching are concisely discussed. Extreme acidophilic microorganisms show great potential in bioleaching applications. Therefore, the use of
acidophiles and extreme acidophiles in bioleaching should be one of the most
important research areas in the future. From the current reports, various aspects
need to be developed for bioleaching of e-wastes. They are:
302
S. Venkatesa Prabhu et al.