4 Influencing Factors for Bioleaching Processes
Several investigations have documented that the biomass and activity of the microorganisms associated with bioleaching are two potential factors that significantly
influence the rate of oxidation of reduced sulfur substances such as pyrite and Fe
2+ .
The temperature, microbial species, pH, and type of e-waste are the four principal
factors that can affect the biomass and microbial activity. Bioleaching is predominantly dominated by microbial communities or functional microbes. Wang et al.
(2009) studied bioleaching of Cu from PCBs (with 7.8 g/L of solids loading) using
two different microbial species. They observed that A. ferrooxidans showed a 99.0%
Cu bioleaching efficiency using Fe
2+ as the electron donor. However, A. thiooxidans
showed only a 74.9% bioleaching of Cu using S
as the energy source (Baker and
Banfield 2003; Macalady et al. 2007). Various experiments on bioleaching showed
that these two acidophilic communities had apparent bioleaching capabilities to
extract Cu from chalcopyrite (Gonzalez-Toril et al. 2003; Liang et al. 2010). Using
pyrite (FeS 2 ) and S
as the energy sources, a mixed culture of Sulfobacillus acidophilus and Sulfobacillus thermosulfidooxidans showed bioleaching efficiencies
80%, 82%, 90%, and 85% for Al, Ni, Cu, and Zn, respectively, after 12 days.
Another experiment carried out by Ilyas et al. (2013) using a mixed culture of
Thermoplasma acidophilum and S. thermosulfidooxidans showed bioleaching efficiencies of 80% for Zn, 80% for Ni, 85% for Cu, and 75% for Al after 18 days.
Another important parameter, temperature, influences the activities of
psychrotolerant, mesophilic, thermophilic, and extreme thermophilic microorganisms (Zhou et al. 2015). Various researchers showed that the efficiency of metal
bioleaching was considerably improved using thermophilic microorganisms due to
their higher growth temperatures and metal tolerance, as well as their remarkable
metabolic characteristics (Hose et al. 2000). Additionally, a high temperature destabilizes the passivation layer caused by the precipitation of S
or Fe
3+ that coats the
surfaces of e-waste and limits the rate of bioleaching (Li and Li 2014; Zhang and
Katayama 2012). Various studies have shown that a high pH hinders the growth of
acidophilic microorganisms and stimulates Fe
3+ precipitation. A lower initial pH can
produce a more favorable environment for chemical leaching, even though it is not
conducive to the metabolism of microbes (Isıldar et al. 2016).
Since e-waste contains hazardous substances, it adversely impacts microbial
growth. Hence, the high dosage of the e-waste leads to the lower efficiency in
metal bioleaching. Also, a high dosage of solid (e-waste) powder negatively influences the distribution of air and oxygen solubility (Okabe et al. 2007). Xiang et al.
(2010) pointed out that the rate of Cu bioleaching from PCBs decreased with an
increase in the solid-to-liquid ratio. The toxicity of heavy metal ions increases with
the dosage of these toxic solids. This could be an important reason for the reduced
efficiency of bioleaching. Since e-waste inhibits the activity and growth of microbes,
the bioleaching initiation time, that is, when e-waste is added, could be imperative.
14 Heavy Metal Extraction from E-Waste Through Bioleaching: A Promising. . .
299
Several investigations have documented that the biomass and activity of the microorganisms associated with bioleaching are two potential factors that significantly
influence the rate of oxidation of reduced sulfur substances such as pyrite and Fe
2+ .
The temperature, microbial species, pH, and type of e-waste are the four principal
factors that can affect the biomass and microbial activity. Bioleaching is predominantly dominated by microbial communities or functional microbes. Wang et al.
(2009) studied bioleaching of Cu from PCBs (with 7.8 g/L of solids loading) using
two different microbial species. They observed that A. ferrooxidans showed a 99.0%
Cu bioleaching efficiency using Fe
2+ as the electron donor. However, A. thiooxidans
showed only a 74.9% bioleaching of Cu using S
as the energy source (Baker and
Banfield 2003; Macalady et al. 2007). Various experiments on bioleaching showed
that these two acidophilic communities had apparent bioleaching capabilities to
extract Cu from chalcopyrite (Gonzalez-Toril et al. 2003; Liang et al. 2010). Using
pyrite (FeS 2 ) and S
as the energy sources, a mixed culture of Sulfobacillus acidophilus and Sulfobacillus thermosulfidooxidans showed bioleaching efficiencies
80%, 82%, 90%, and 85% for Al, Ni, Cu, and Zn, respectively, after 12 days.
Another experiment carried out by Ilyas et al. (2013) using a mixed culture of
Thermoplasma acidophilum and S. thermosulfidooxidans showed bioleaching efficiencies of 80% for Zn, 80% for Ni, 85% for Cu, and 75% for Al after 18 days.
Another important parameter, temperature, influences the activities of
psychrotolerant, mesophilic, thermophilic, and extreme thermophilic microorganisms (Zhou et al. 2015). Various researchers showed that the efficiency of metal
bioleaching was considerably improved using thermophilic microorganisms due to
their higher growth temperatures and metal tolerance, as well as their remarkable
metabolic characteristics (Hose et al. 2000). Additionally, a high temperature destabilizes the passivation layer caused by the precipitation of S
or Fe
3+ that coats the
surfaces of e-waste and limits the rate of bioleaching (Li and Li 2014; Zhang and
Katayama 2012). Various studies have shown that a high pH hinders the growth of
acidophilic microorganisms and stimulates Fe
3+ precipitation. A lower initial pH can
produce a more favorable environment for chemical leaching, even though it is not
conducive to the metabolism of microbes (Isıldar et al. 2016).
Since e-waste contains hazardous substances, it adversely impacts microbial
growth. Hence, the high dosage of the e-waste leads to the lower efficiency in
metal bioleaching. Also, a high dosage of solid (e-waste) powder negatively influences the distribution of air and oxygen solubility (Okabe et al. 2007). Xiang et al.
(2010) pointed out that the rate of Cu bioleaching from PCBs decreased with an
increase in the solid-to-liquid ratio. The toxicity of heavy metal ions increases with
the dosage of these toxic solids. This could be an important reason for the reduced
efficiency of bioleaching. Since e-waste inhibits the activity and growth of microbes,
the bioleaching initiation time, that is, when e-waste is added, could be imperative.
14 Heavy Metal Extraction from E-Waste Through Bioleaching: A Promising. . .
299