(Aarabi et al. 2010). These microorganisms have a peculiar characteristic of oxidizing elemental sulfur (S
) or reduced-sulfur elements, ferrous iron (Fe
2+ ), and pyrite
to generate H 2 SO 4 and ferric iron (Fe
3+ ) as leaching agents. Using these leaching
agents, solid metals can be converted into solubilized ions in an aqueous reaction by
the actions of bioleaching microbes (Rawlings 2005). These processes are known as
bioleaching (Marra et al. 2018).
Acidithiobacillus ferrooxidans is a potential bioleaching bacterium that was
isolated from the drainage of acid mine waste by Colmer et al. in 1947 (Colmer
and Hinkle 1947). A. ferrooxidans was the first documented bioleaching microbe
that could oxidize ferrous iron into ferric iron to extract copper from pyrite. Biological extraction of sulfide minerals was soon established (Zhu et al. 2011). These
techniques are sustainable methods that require low cost and little energy while
remaining ecofriendly. Presently, bioleaching is not only the only way to extract
metals from sulfide minerals (Peng et al. 2012). Instead, it is extensively applied in
the recovery of metals from various solid wastes, such as e-wastes, batteries, and
catalysts. Recently, several investigations have furthered process development for
bioleaching of many valuable metals such as cobalt, vanadium, and indium. A
summary of selected extremophile microorganisms for bioleaching applications is
given in Table 14.1.
Even though bioleaching has been successful for extracting various sulfide
minerals, recovery of valuable metals from e-waste is still challenging (Vera et al.
2013). This is because there are significant differences between e-waste and sulfide
minerals based on their chemical and physical properties, as well as their molecular
structures (Jowkar et al. 2018). Moreover, e-waste contains harmful elements that
may inactivate bioleaching microorganisms. Hence, bioleaching processes applicable to sulfide minerals cannot be simply applied to e-waste. A set of pretreatments
are required including sieving, crushing, shredding, and sorting before any
bioleaching process (Zhu et al. 2011).
3 Pathways of Bioleaching Processes
From previous research, bioleaching of e-waste using extreme acidophiles can be
predominantly divided into pathways mediated by iron (Fe) and reduced sulfur (S),
as well as a mixed pathway that is S- and Fe-mediated (Watanabe et al. 2009).
Bioleaching mechanisms are unclear and still under investigation (Pathak et al.
2017; Pradhan et al. 2008). For each bioleaching pathway, there are two types of
mechanisms, “direct” and “indirect,” for treating sulfide minerals (Ziegler et al.
2009). Each bioleaching pathway may have a direct and/or indirect mechanism,
but each microorganism will at least have an indirect mechanism (Liu et al. 2011). In
the direct mechanism, transfer of electrons occurs directly between the sulfide
minerals and the cells of microorganisms. In this case, the cells should be attached
at the surface of the minerals (Havlik et al. 2010). Various microbes have been
recognized that contribute in direct mechanisms, for example, A. ferrooxidans
14 Heavy Metal Extraction from E-Waste Through Bioleaching: A Promising. . .
295
) or reduced-sulfur elements, ferrous iron (Fe
2+ ), and pyrite
to generate H 2 SO 4 and ferric iron (Fe
3+ ) as leaching agents. Using these leaching
agents, solid metals can be converted into solubilized ions in an aqueous reaction by
the actions of bioleaching microbes (Rawlings 2005). These processes are known as
bioleaching (Marra et al. 2018).
Acidithiobacillus ferrooxidans is a potential bioleaching bacterium that was
isolated from the drainage of acid mine waste by Colmer et al. in 1947 (Colmer
and Hinkle 1947). A. ferrooxidans was the first documented bioleaching microbe
that could oxidize ferrous iron into ferric iron to extract copper from pyrite. Biological extraction of sulfide minerals was soon established (Zhu et al. 2011). These
techniques are sustainable methods that require low cost and little energy while
remaining ecofriendly. Presently, bioleaching is not only the only way to extract
metals from sulfide minerals (Peng et al. 2012). Instead, it is extensively applied in
the recovery of metals from various solid wastes, such as e-wastes, batteries, and
catalysts. Recently, several investigations have furthered process development for
bioleaching of many valuable metals such as cobalt, vanadium, and indium. A
summary of selected extremophile microorganisms for bioleaching applications is
given in Table 14.1.
Even though bioleaching has been successful for extracting various sulfide
minerals, recovery of valuable metals from e-waste is still challenging (Vera et al.
2013). This is because there are significant differences between e-waste and sulfide
minerals based on their chemical and physical properties, as well as their molecular
structures (Jowkar et al. 2018). Moreover, e-waste contains harmful elements that
may inactivate bioleaching microorganisms. Hence, bioleaching processes applicable to sulfide minerals cannot be simply applied to e-waste. A set of pretreatments
are required including sieving, crushing, shredding, and sorting before any
bioleaching process (Zhu et al. 2011).
3 Pathways of Bioleaching Processes
From previous research, bioleaching of e-waste using extreme acidophiles can be
predominantly divided into pathways mediated by iron (Fe) and reduced sulfur (S),
as well as a mixed pathway that is S- and Fe-mediated (Watanabe et al. 2009).
Bioleaching mechanisms are unclear and still under investigation (Pathak et al.
2017; Pradhan et al. 2008). For each bioleaching pathway, there are two types of
mechanisms, “direct” and “indirect,” for treating sulfide minerals (Ziegler et al.
2009). Each bioleaching pathway may have a direct and/or indirect mechanism,
but each microorganism will at least have an indirect mechanism (Liu et al. 2011). In
the direct mechanism, transfer of electrons occurs directly between the sulfide
minerals and the cells of microorganisms. In this case, the cells should be attached
at the surface of the minerals (Havlik et al. 2010). Various microbes have been
recognized that contribute in direct mechanisms, for example, A. ferrooxidans
14 Heavy Metal Extraction from E-Waste Through Bioleaching: A Promising. . .
295