derived from the oxidation of NADH
+ to release electrons to the cytochrome c,c 3
system which functions as a conduit for electron flow to M
n+ reductase (Lovley and
Philips 1994; Chirwa and Wang 1997; Mtimunye and Chirwa 2014).
The results obtained from the U(VI) reduction (Mtimunye and Chirwa 2014), Cr
(VI) reduction (Chirwa and Molokwane 2011), and reduction of other metallic
species (Cervantes and Silver 1992; Lloyd et al. 1999; Yong et al. 2002) showed
that the reduction pathway for most metals is non-specific and is closely related to
the sulfate reduction transmembrane electron shuttle.
6.2 Biological Oxidation
The detoxification of arsenic by oxidation of As(III) to As(V) is used here as an
example of the bioremediation through an oxidation reaction process. In this example, As(III) served as an inorganic electron donor for beneficial oxidation to less
toxic and immobile As(V). Thermodynamically, the conversion of As(III) to As
(V) is an exothermic reaction and could generate considerable amount of energy
ranging from – (254 to 468) KJ/mol for cell growth and metabolism (Dastidar and
Wang 2010; Wang et al. 2013).
The first heterotrophic As(III) oxidation was observed in a cow dip in
South Africa in 1918 (Green 1918), whereas an autotrophic As(III) oxidation
was observed in 1981 (Ilialetdinov and Abdrashitova 1981). Heterotrophic As
(III) oxidation may represent a detoxification reaction on the cell’s cytoplasmic
(inner) membrane, whereas autotrophic As(III) oxidation releases energy that is
used for CO 2 fixation and cell growth under both aerobic and anaerobic conditions
(Santini et al. 2000). In 2010, Dastidar and Wang reported that about 256 KJ/mol
energy can be generated during oxidation of As(III) to As(V) by a purified culture
of Thiomonas arsenivorans strain b6 (Dastidar and Wang 2010). Further studies
showed that about 467.95 KJ of energy could be generated in the process (Wang
et al. 2013). The redox conversion of As(III) in aqueous environment was
represented by the following equation:
As
3þ
! As
5þ
þ 2e
À
þ energy À 254 to 468
ð
Þ KJ=mol 3
ð Þ
The oxidation of As(III) could operate in competition with the oxidation of the
other organic and inorganic electron donors as energy sources. For example, nitrate
(NO 3
À ) and chlorate (ClO 3
À
) in solution can act as competitive electron donors to
As(III) (Sun et al. 2010). In the recent study, the use of As(III) as an electron donor
for reduction of Cr(VI) was demonstrated using a consortium of As(III) oxidizing
species isolated from a cow dip in Tzaneen, Limpopo Province (South Africa)
(Fig. 2.4) (Igboamalu and Chirwa 2017). The predominance of As(III) as an electron
donor for metabolic processes over other electron donors in the system has not been
investigated.
2 Advances in Bioremediation of Toxic Heavy Metals and Radionuclides in. . .
33
+ to release electrons to the cytochrome c,c 3
system which functions as a conduit for electron flow to M
n+ reductase (Lovley and
Philips 1994; Chirwa and Wang 1997; Mtimunye and Chirwa 2014).
The results obtained from the U(VI) reduction (Mtimunye and Chirwa 2014), Cr
(VI) reduction (Chirwa and Molokwane 2011), and reduction of other metallic
species (Cervantes and Silver 1992; Lloyd et al. 1999; Yong et al. 2002) showed
that the reduction pathway for most metals is non-specific and is closely related to
the sulfate reduction transmembrane electron shuttle.
6.2 Biological Oxidation
The detoxification of arsenic by oxidation of As(III) to As(V) is used here as an
example of the bioremediation through an oxidation reaction process. In this example, As(III) served as an inorganic electron donor for beneficial oxidation to less
toxic and immobile As(V). Thermodynamically, the conversion of As(III) to As
(V) is an exothermic reaction and could generate considerable amount of energy
ranging from – (254 to 468) KJ/mol for cell growth and metabolism (Dastidar and
Wang 2010; Wang et al. 2013).
The first heterotrophic As(III) oxidation was observed in a cow dip in
South Africa in 1918 (Green 1918), whereas an autotrophic As(III) oxidation
was observed in 1981 (Ilialetdinov and Abdrashitova 1981). Heterotrophic As
(III) oxidation may represent a detoxification reaction on the cell’s cytoplasmic
(inner) membrane, whereas autotrophic As(III) oxidation releases energy that is
used for CO 2 fixation and cell growth under both aerobic and anaerobic conditions
(Santini et al. 2000). In 2010, Dastidar and Wang reported that about 256 KJ/mol
energy can be generated during oxidation of As(III) to As(V) by a purified culture
of Thiomonas arsenivorans strain b6 (Dastidar and Wang 2010). Further studies
showed that about 467.95 KJ of energy could be generated in the process (Wang
et al. 2013). The redox conversion of As(III) in aqueous environment was
represented by the following equation:
As
3þ
! As
5þ
þ 2e
À
þ energy À 254 to 468
ð
Þ KJ=mol 3
ð Þ
The oxidation of As(III) could operate in competition with the oxidation of the
other organic and inorganic electron donors as energy sources. For example, nitrate
(NO 3
À ) and chlorate (ClO 3
À
) in solution can act as competitive electron donors to
As(III) (Sun et al. 2010). In the recent study, the use of As(III) as an electron donor
for reduction of Cr(VI) was demonstrated using a consortium of As(III) oxidizing
species isolated from a cow dip in Tzaneen, Limpopo Province (South Africa)
(Fig. 2.4) (Igboamalu and Chirwa 2017). The predominance of As(III) as an electron
donor for metabolic processes over other electron donors in the system has not been
investigated.
2 Advances in Bioremediation of Toxic Heavy Metals and Radionuclides in. . .
33