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Bioremediation for Sustainable Environmental Cleanup

positive and gram-negative microorganisms (Barton et al. 2015). To improve culture resistance a

mixture of species is often selected for bioprecipitation processes (Kiran et al. 2017).

In addition to BSR, SRB have also been found to reduce other metal(loid)s with direct or

indirect microorganism activity. SRB can directly reduce metal(loid)s (i.e., Cr, As, Al, Te and Sb)

to a less toxic, insoluble form or can indirectly reduce metal(loid)s via hydrogen sulfide produced

during BSR (Sánchez-Andrea et al. 2016, Willis and Donati 2017). After metal(loid) reduction,

many species are capable of precipitation. For example, uranium (U⁶⁺) can precipitate uraninite

(UO2), chromium (Cr⁶⁺) in the presence of ferric iron (Fe³⁺) can precipitate chromium hydroxide

oxide (CrO (OH)) and chromium sulfide (Cr2S3) and arsenic can precipitate arsenic sulfide (As2S3)

and arsenopyrite (FeAsS). Metal(loid) reduction using SRB activity is presented in Table 2.3. The

following electron donors were used as an energy source in some of the noted reactions: acetate

(CH3COOH), formate (CH2O2), lactate (C3H6O3) and pyruvate (C3H4O3).

Table 2.3. Reduction of Metal(loid)s with SRB (Adapted from/Barton et al. 2015, Lovley 1993, Sahinkaya et al. 2017).

Metal(loid)

Electron Acceptor

Formula

Iron

Fe³⁺

CH3 COO^– + 8Fe³⁺ + 4H2O → 2HCO3

– + 8Fe2+ + 9H+

CO2 H ^– + 2Fe³⁺ + H2O → HCO3

– + 2Fe2+ + 2H +

C3O3H5

– + 4Fe3+ + 2H2O → CH3COO– + HCO3

– + 4Fe2+ + 5H +

C3O3H3

– + 2Fe3+ + 2H2O → CH3COO– + HCO3

– + 2Fe2+ + 3H+

Fe⁰

Fe⁰ + 2H⁺ → Fe²⁺ + H2

Manganese

Mn⁴⁺

MnO2 → MnCO3 *

2H2S + MnO2 → MnS + S⁰ + 2H2O

Mn⁶⁺

Mn⁶⁺ → Mn²⁺*

Uranium

U⁶⁺

CH3COO^– + 4U⁶⁺ + 4H2O → 2HCO3

– + 4U4+ + 9H+

H2 + U⁶⁺ → 2H⁺ + U ⁴⁺

Selenium

SeO4

2–

4CH3COO^– + 3SeO4

2– → 3Se0 + 8CO2 + 4H2O + 4H+

CH3COO^– + H⁺ + 4SeO4

2– → 4SeO3

2– + 2CO2 + 2H2O

SeO3

2–

2H2S + SeO3

2– +2H+ → SeS2 + 3H2O

Chromium

Cr⁶⁺

2Cr⁶⁺ + 3HS ^– → 2S⁰ +2Cr³⁺ + 3H⁺

Mercury

Hg²⁺

Hg²⁺ → Hg⁰*

Cobalt

Co³⁺

Co³⁺ + H2S → CoS + 2H⁺

Palladium

Pd²⁺

Pd²⁺ → Pd⁰*

Nickel

Ni³⁺

Ni³⁺ → Ni²⁺*

Technetium

Tc⁷⁺

Tc⁷⁺ → Tc⁴⁺*

Tc⁷⁺ → Tc⁵⁺*

Vanadium

V⁵⁺

V⁵⁺ → V³⁺*

Molybdenum

Mo⁶⁺

Mo⁶⁺ → Mo⁴⁺*

Arsenic

Ar⁵⁺

3Ar2S3 + 3H2S → 2H2As3S6

– + 2H+

As⁵⁺ → As³⁺*

Gold

Au³⁺

Au³⁺ → Au⁰*

Au⁺

Au⁺ → Au⁰*

* Not a full formula. Showing the redox couple.

Electron donors provide a carbon and energy source for the reaction. The selection of

an appropriate electron donor is the ratio of Chemical Oxygen Demand (COD) and sulfate ion

concentration (SO4

2–), i.e., COD/SO4

2– is important. There is a correlation between the interaction

of SRB with carbon source and electron acceptor (Barbosa et al. 2014, Kiran et al. 2017). The COD

denotes oxygen content required to oxidize the organic material and the electron acceptor is SO4

2–

(Barbosa et al. 2014). However, COD is measured under aerobic conditions, and therefore does not