Bioprecipitation as a Bioremediation Strategy for Environmental Cleanup 27

accurately portray the organic carbon available to anerobic microorganisms (Neculita and Zagury

2008). The minimum theoretical value acceptable for organic degradation and BSR is 0.67 (Hao

et al. 1996, Kiran et al. 2017, Neculita and Zagury 2008).

Other noteworthy indicators for an electron donors’ performance are the carbon (C) to nitrogen

(N) ratio (C/N) and the Dissolved Organic Carbon (DOC) to SO4

2– ratio (DOC/SO4

2–) (Neculita

and Zagury 2008). The C/N ratio gives information about the capacity of biological degradation of

an electron donor (Reinertsen et al. 1984, Zagury et al. 2006). While the ratio does not necessarily

indicate the actual C and N available to the microorganisms (Reinertsen et al. 1984), a value of about

10 is generally accepted as a suitable substrate (Béchard et al. 1994, Neculita and Zagury 2008,

Reinertsen et al. 1984, Zagury et al. 2006). The DOC/SO4

2– ratio is like the COD/SO4

2– ratio but is

more easily quantified (Neculita and Zagury 2008).

An electron donor can be organic or synthetic. In keeping with the sustainable nature of

remediation, an organic energy is a natural, economic and socially acceptable additive to the

operation. Organic waste can be considered sewage sludge, animal manure, leaf mulch, wood chips,

sawdust and cellulose (Liamleam and Annachhatre 2007). The high carbon content of organic waste

is advantageous to BSR demonstrating high rates of sulfate reduction, specifically when wastes

are applied as mixtures (Hao et al. 2014, Liamleam and Annachhatre 2007). The COD/SO4

2– ratio

for organic wastes ranges from 1.6–5 (Kiran et al. 2017). In general, for all performance indicators

(i.e., C/N, COD/SO4

2–, DOC/SO4

2–), a higher ratio is linked to superior BSR (Neculita and Zagury

2008).

Molasses is the most cost effective and widely available electron donor for BSR (Janssen and

Temminghoff 2004, Liamleam and Annachhatre 2007), indicating its preferential use for sustainable

engineering. In addition to its use as an electron donor, it contains nutrients (i.e., P, K, Cl, amino

acids) for SRB growth (Janssen and Temminghoff 2004). The process aims to ferment molasses into

lactate which is used as the electron donor or carbon source (Liamleam and Annachhatre 2007).

However, SRB growth is inhibited at molasses concentrations greater than 5 g/L (Hao et al. 2014,

Janssen and Temminghoff 2004), and there is seemingly a link between the quantity applied and

the pH rise (Janssen and Temminghoff 2004). Further, partial decomposition creates high COD in

effluent (Hao et al. 2014).

Lactate is another organic substrate used as an energy source for BSR. Its use as an electron donor

is not temperature dependent, and does not impact its oxidizing ability (Liamleam and Annachhatre

2007). Both lactate and ethanol are considered optimal for SRB growth (Janssen and Temminghoff

2004), although, ethanol is not an organic substrate it is considered the most cost effective (Gibert

et al. 2002, Liamleam and Annachhatre 2007). Further, acetate is the most used energy source for

Fe³⁺ bioprecipitation (Lovley 1993). Other energy sources include hydrogen, formate, methanol,

propionate, butyrate, sugar and hydrocarbons (Liamleam and Annachhatre 2007). Zero-valent iron

has also been used to establish an anerobic environment for SRB growth by consuming oxygen

(O2) while generating hydrogen (H2) which acts as an electron donor (Pagnanelli et al. 2009). A

high value for Gibbs free energy is preferable for BSR to assure that sulfidogenic reactions prevail

over methanogenic reactions (Liamleam and Annachhatre 2007). However, the concentration of

the electron donor should be monitored as high carbon concentration can lead to methanogenic

conditions (Diels et al. 2005, 2006). A mixture of organic substrates is often used with success,

that can produce high rates of sulfate reduction (Liamleam and Annachhatre 2007). The cost of the

substrate should be kept in mind and adhere to the projects’ economic sustainability.

Bioprecipitation can also occur as an oxidative reaction. Ferrous iron (Fe²⁺) is transformed via

microbial oxidation into ferric iron (Fe³⁺) for precipitation. Microbial oxidation can take place as

an anaerobic process (using phototropic and nitrate-reducing microorganisms) and aerobic process

(using neutrophilic and acidophilic microorganisms).The phylum Proteobacteria is the most common

bacteria responsible for Fe²⁺ reduction (Kiskira et al. 2017). Zero-valent iron can oxidize to Fe²⁺ and

Fe³⁺ ions, help to facilitate the bioprecipitation process (Pagnanelli et al. 2009). From the oxidative

reaction, Fe³⁺ can precipitate ferric hydroxide (Fe (OH)3), jarosite (MFe3(SO4)2(OH)6, where M is a