Bioprecipitation as a Bioremediation Strategy for Environmental Cleanup 29

EPS is reported to impact the biofilm, cell adhesion, CaCO3 capture (Achal and Pan 2011) and

the CaCO3 mineralogy (Kawaguchi and Decho 2002). The CaCO3 mineralogy can be altered via

various polymorphs (i.e., calcite, vaterite and aragonite). The type of polymorph produced during

MICP can impact the stability of precipitates, where calcite is the most stable and desirable, while

aragonite and vaterite are less stable.

Other factors influencing the efficacy of MICP as a remediation technique are temperature,

bacterial concentration or density, pH, degree of saturation, concentration of cementation solution

and field application (Mujah et al. 2016). This bioprecipitation method offers a promising remediation

strategy. However, the long-term impact should be studied. Metal(loid) dissolution from redox and/

or pH changes could release contaminants into the soil and groundwater. Additionally, over time

metal(loid) leaching can occur from cracks, fissures or interstices formed in the cement matrix.

These defects can be caused by wind, erosion, wetting and drying cycles, hot and cold cycles, rain

and snow and/or other environmental elements.

2.3.2 Design

Bioprecipitation can be applied as a remediation solution in a variety of methods. The processes are

classified as in-situ or ex-situ based on where the operation takes place. If clean-up occurs at the

site, the operation is considered an in-situ process. However, if soil is extracted from the site and

transported for treatment, the operation is an ex-situ process. In addition to these operation models,

bioprecipitation often occurs either simultaneously or in sequence with other treatment methods.

This is based on the requirements of the project, i.e., future land usage and the desired level of

metal(loid) removal. This chapter will put emphasis on in-situ bioremediation strategies that are

passive or rely on natural attenuation processes. These strategies are innately more sustainable and

therefore, offer benefits as a clean-up protocol.

2.3.2.1 In-situ Bioprecipitation

In-situ bioprecipitation can take place as a natural or engineered process. These operations are

typically passive, as they aim to enhance the natural processes that occur. They typically consume

minimal energy, have little operation and maintenance requirements and are of low cost. Further,

the reduction in transport and the recycle of materials negates environmental impacts developed

with active processes (Hengen et al. 2014). However, these processes are harder to control and

metal recovery is difficult (Kaksonen and Puhakka 2007, Kiran et al. 2017). Overall, in-situ

bioprecipitation adheres to the social, economic and environmental demand expected by an eco­

friendly, sustainable remediation strategy. Table 2.4 provides information on various case-studies

demonstrating the efficacy of in-situ methods.

In-situ bioprecipitation can be achieved via injection wells. There are two ways to facilitate

remediation with injection wells. In the first, the wells are constructed to provide the reactants to

the contaminated zone. A mixture (including electron donors and microorganisms) is injected into

the well, which follows the groundwater flow path with the aim to precipitate and immobilize the

contaminant (Vanbroekhoven et al. 2008). The concentration of the electron donor during injection

should be tested since a temporarily high carbon content at the well may produce methane without

metal(loid) precipitation (Diels et al. 2005). The second, uses the pull-push-pull principle, in which

groundwater is extracted from the well, is mixed with additives and then is reinjected into the same

well (Janssen and Temminghoff 2004). In a pilot test using the latter method, BSR was achieved

whereby zinc concentrations were significantly reduced (40 mg/L to < 0.001 mg/L), however more

interesting was the longevity of BSR 5 wk post operation (Janssen and Temminghoff 2004). The

long-term stability of the metal-sulfide precipitates can be of concern (Miao et al. 2012), requiring

continuous monitoring.

Bioprecipitation can also be facilitated via permeable reactive barriers. A reactive barrier is

implemented in soil to cut across the groundwater flow. The barrier is implemented downstream of a