28

Bioremediation for Sustainable Environmental Cleanup

monovalent cation, i.e., K⁺, Na⁺, NH4

+, Ag+, or H3O+), schwertmannite (Fe16O16(SO4)2(OH)6 • nH2O,

where n is between 10 and 12), goethite (FeO (OH)), hematite (Fe2O3) and scorodite (FeAsO4 • 2H2O).

The pH, temperature, and overall chemistry of solution influence the precipitate formed (Sahinkaya

et al. 2017).

2.3.1.3 Microbially Induced Calcite Precipitation (MICP)

MICP is another form of bioprecipitation. However, unlike the earlier discussed oxidative and

reductive bioprecipitation processes, MICP is not easily influenced by redox reactions (Achal et al.

2011, Xiangliang 2009). This form of biological precipitation aims to immobilize the contaminant

via cementation. Microorganisms are used to facilitate the hydrolysis of urea creating carbonate

(CO3

2–) and ammonium (NH4

+), Eq. 2.5. The NH4

+ ions increase pH to ameliorate precipitation

(Achal et al. 2013a,b), and a cementation solution with calcium (Ca²⁺) ions is introduced to

precipitate calcium carbonate (CaCO3), Eq. 2.6. Other metal divalent cations (M²⁺) from solution

are also able to precipitate metal carbonate (MCO3) compounds, Eq. 2.7.

Urease Hydrolysis (Mwandira et al. 2022)

CO(NH2)2 + 2H2 O → CO3

2– + 2NH4

+

Eq. 2.5

Calcium Carbonate Precipitation (Mwandira et al. 2022)

Ca²⁺ + CO3

2– → CaCO3

Eq. 2.6

Metal Carbonate Precipitation (Mwandira et al. 2022)

M²⁺ + CO3

2– → MCO3

Eq. 2.7

The aim of this process is to immobilize the precipitates through the formation of a cement

matrix, whereby precipitates form bridges between soil particles. Since the crystals precipitate

out of the soil-groundwater system and clog the pore spaces, the soil properties are altered. The

permeability, porosity, stiffness, shear strength, unconfined compressive strength, microstructure

and shear wave velocity are all impacted by MICP (Mujah et al. 2016).

The main theory behind MICP remediation is solidification/stabilization (S/S). S/S is a strategy

that immobilizes the soil and groundwater contaminants by using additives that alters the physical

properties (i.e., solidifies/entraps the contaminant) and/or chemical properties (i.e., transforms

the contaminant to a less toxic, less mobile form), respectively (LaGrega et al. 1994, Sharma

and Reddy 2004). Historically, S/S used cements, pozzolans, thermoplastic materials or organic

polymers to achieve contaminant entrapment (Sharma and Reddy 2004), however MICP offers a

biological approach to reach S/S remediation. The mechanisms involved in S/S to remediate soil and

groundwater include macroencapsulation, microencapsulation, adsorption, absorption, precipitation

and detoxification (LaGrega et al. 1994). Through MICP, sorption can cause bioprecipitated

CaCO3 crystals and other MCO3 compounds to bond to soil particle surfaces (LaGrega et al. 1994,

Xiangliang 2009) via electrochemical bonds, such as van der Waal’s forces or hydrogen bonds

(LaGrega et al. 1994). This can aid the development of the cement matrix, which offers a more

sustainable approach to S/S remediation.

The most common microorganism used for MICP is the Bacillus species (Achal and Pan 2011).

To improve MICP performance a desirable microorganism should be high urease producing with

high metal tolerance. Again, these organisms can be gram-positive or gram-negative, however

gram-positive bacteria are more reactive (Beveridge and Fyfe 1985, Levett et al. 2020). The

negative charge of the bacterial cell wall can attract Ca²⁺ ions causing MICP on the gram-positive

and gram-negative cell walls (Achal and Pan 2011). Microorganism EPS can also affect MICP.