Bioprecipitation as a Bioremediation Strategy for Environmental Cleanup 25

2.3.1.2 Biological Precipitation

Bioprecipitation is an emerging bioremediation strategy. The process applies chemical precipitation

with the presence of microorganisms to catalyze the reaction. The microorganism activity typically

acts as the catalyst for the oxidative and reductive reactions. The reactions aim to generate

alkalinity or consume acidity to precipitate metal(loid)s (Johnson and Santos 2020). Through local

alkalinization or enzyme generated ligands, reactions can precipitate carbonate, hydroxide, sulfide,

phosphate, oxalate, etc. compounds (Kumar et al. 2013).

The metal(loid) in focus drastically impacts the type and efficacy of precipitation.

Metal(loid)s can react differently to the oxidative and reductive reactions. For example, some

metals (Fe, Mn) increase solubility under reductive conditions, while others (U⁶⁺, Cr⁶⁺) decrease

solubility (Gadd 2004). Further, the phase of the metal(loid) can impact its pH. Ferric iron (Fe³⁺)

in its amorphous form can generate alkalinity, while in soluble form it cannot (Johnson and Santos

2020). Therefore, the metal(loid) of focus should be analyzed to assess its chemical speciation and

establish chemical stability.

The most common form of bioprecipitation is Biological Sulfate Reduction (BSR). Sulfate

Reducing Bacteria (SRB) catalyze the dissimilatory sulfur reduction process, whereby oxidation of

an electron donor facilitates the reduction of sulfate (SO4

2–) to soluble sulfides (S2–) (Sánchez-Andrea

et al. 2014). The process aims to increase pH to precipitate metal(loid)s and reduce sulfate (Willis

and Donati 2017). It is a common remediation strategy for acidic wastewaters with metal(loid)

contamination (Sánchez-Andrea et al. 2014). This type of contamination is common among mining

industries, electroplating industries and tannery industries (Sahinkaya et al. 2017). Equations 2.3

and 2.4 demonstrate how chemically BSR can induce bioprecipitation to remediate contaminated

groundwater. Equation 2.3 uses formaldehyde (CH2O) as an electron donor to reduce sulfate from

acidic industrial groundwater to hydrogen sulfide. The hydrogen sulfide then reacts with other metal

divalent cations (M²⁺) to precipitate low solubility metal sulfides (MS), Eq. 2.4.

Reduction of Sulfate to Hydrogen Sulfide with the use of an Electron Donor (Sahinkaya et al. 2017,

Willis and Donati 2017)

–

2CH2 O + SO4

2– → H2S + 2HCO3

Eq. 2.3

Metal Precipitation from Hydrogen Sulfide (Sahinkaya et al. 2017, Willis and Donati 2017)

H2 S + M²⁺ → MS(s) + 2H⁺

Eq. 2.4

SRB species can be both heterotrophs and autotrophs (chemolithotrophs) found in anaerobic

environments (Barton et al. 2015, Hao et al. 2014). As of 2015, there were 59 genera and

220 species reported (Barton et al. 2015). However, the genera Desulfovibrio is the most highly

reported bacteria from bioreactor studies (Kiran et al. 2017). The microorganisms are classified

as complete oxidizers, incomplete oxidizers or both (Hao et al. 2014). There are over 75 energy

sources that promote SRB growth (Barton et al. 2015). Although trace metals, selenium (Se)

and molybdenum (Mo), are attributed to bacterial growth, the mechanisms linked to metal(loid)

reduction are not currently considered as related (Barton et al. 2003).

To assure microorganisms can facilitate bioprecipitation, they require some degree of metal

tolerance. The metal tolerance allows the microorganisms to flourish under normally toxic metal(loid)

contamination. This is achieved via metabolism, cell wall structure, Extra-cellular Polymeric

Substances (EPS), methylation, alkylation/dealkylation (Diels et al. 2006), intra-cellular and extra-

cellular sequestration, active transport effluent pumps, enzymatic detoxification, reduction in metal

sensitivity of cellular targets (Bruins et al. 2000, Diels et al. 2006) and exclusion by permeability

barrier (Bruins et al. 2000). Metal resistant genes have been found in both gram-positive and gram-

negative microorganisms (Abou-Shanab et al. 2007) and SRB species have also reported gram­