Bioprecipitation as a Bioremediation Strategy for Environmental Cleanup 31

treatment, subsurface flow treatment (anoxic) (Faulwetter et al. 2009) or a surface flow (aerobic)

treatment (Kosolapov et al. 2004). The mode of operation refers to the feed mode (batch feed,

intermittent flow feed or continuous flow feed), hydraulic load rate and Hydraulic Retention Time

(HRT) (Faulwetter et al. 2009). The use of plant species to help mitigate pollution via the influence

of the redox condition is specific to wetlands (Faulwetter et al. 2009) and phytoremediation (Stephen

and Macnaughtont 1999). Phytoremediation can mitigate soil and groundwater pollution via five

primary mechanisms: phytostabilization, phytoextraction, phytovolatilization, rhizodegradation and

phytotransformation. Rhizodegradation, specifically, is applicable during wetland treatment, as the

plant-soil-microorganism system in the rhizosphere can degrade contaminants (Shmaefsky 2020).

Biofilters offer an additional method to remediate soil and groundwater via in-situ

bioprecipitation. The method consists of a filter media that allows microorganisms to attach and

multiply. The biofilter essentially acts as a surface for microorganism immobilization leading to

the development of a biofilm, which enables redox reactions necessary for bioprecipitation. There

are numerous organic and inorganic filter materials that can be used to facilitate biofiltration. In

keeping with the sustainable assessment of clean-up strategies, compost can be used as a viable,

environmental filter media. The compost media has nutrients for microorganism growth, good

water retention capacity for microorganism metabolism and good permeability for homogenous

distribution that are beneficial to the biofilter process (Pachaiappan et al. 2022).

MICP is typically applied to the field via injection, surface percolation or pre-mixing. The latter

two applications are in-situ treatment methods and therefore preferential. An issue experienced

with MICP remediation is the uniform distribution of the mixture (cementation solution and

microorganism) to adequately precipitate a CaCO3 matrix. An injection can lead to clogging around

the injection site, which inhibits its performance as a clean-up strategy. By reducing the injection rate,

the mixture may increase its reach. Surface percolation, however, has a better uniform distribution.

The mixture percolates through the soil via gravity. The depth of the required remediation should

be previously assessed as the permeability of soil impacts the depth reached by percolation (Mujah

et al. 2016).

Sorption and bioprecipitation are remediation mechanisms that typically occur simultaneously.

Sorption includes adsorption (accumulation to surfaces) or absorption (penetration into substances).

As mentioned earlier, sorption is part of the S/S technique aiming to immobilize metal(loid)

contaminants in-situ (LaGrega et al. 1994). Once contaminants precipitate from groundwater or pore

space, adsorption to soil particle surfaces is desirable. Again, considering sustainable remediation,

the use of an organic matter energy source during bioprecipitation can enhance the sorption capacity.

The carboxylic group of olive pomace, compost and leaves can facilitate sorption (Pagnanelli et al.

2009), improving immobilization.

These in-situ remediation methods offer a greener solution for industrial clean-up of soil and

groundwater. Although some greenhouse gas emissions and energy will be consumed to either drill

the injection wells or implement the reactive barrier, these strategies offer better sustainability to

ex-situ methods. However, it is important to note that these strategies are not viable for all types of

contamination. Often more intensive methods are required to achieve high degrees of metal(loid)

contaminant removal required by environmental and government agencies.

2.3.2.2 Ex-situ Bioprecipitation

Ex-situ bioprecipitation uses engineered structures (i.e., bioreactors) to treat soil. The operation

is considered an active treatment as it requires continuous chemical and/or biological additives

to facilitate biological precipitation. The process is highly demanding, requiring constant labor

requirements and monitoring. The operation has high capital and maintenance costs, where

contaminated soil is excavated, transported and treated using reactor design. The operation is less

sustainable than in-situ methods, however, it should be noted that the process is easily controlled

with better predictability and metal(loid) recovery (Kaksonen and Puhakka 2007, Kiran et al. 2017).