**1. Introduction**

Improving soil properties has become inevitable when finding available places with soils of considerable strength is difficult. Instead of finding areas with soil of good geotechnical properties, soil improvement using soil stabilization techniques in the desired location seems preferable. Soil stabilization focuses on the improvement of the soil's bearing capacity and the reduction of settlement and deformation [1–4]. Ground improvement is most effectively addressed through soil stabilization. Researchers have tested various techniques of stabilization, and some have been vastly implemented in the field, specifically in the past four decades. Land with good soil performance is becoming scarce due to population growth. Therefore, using techniques to improve the performance of existing soil has become necessary. Among the soil stabilization methods, mechanical and chemical stabilization are widely acclaimed. Mechanical stabilization involves the process of densifying the soil mass by expelling air voids with nominal variation in water content for better

performance, whereas chemical stabilization involves amending the soil with additives to achieve the desired density, reduce permeability, or improve soil strength [5].

Chemical stabilization utilizes cementitious materials like lime, asphalt, and chemicals such as silicates and polymers, and Portland cement. These affect the chemical form of the soil matrix, improving its geotechnical behavior [6]. Chemical stabilization has attracted greater attention due to its effectiveness in soil improvement using traditional binders with a calcium base, like lime, fly ash, and cement, or novel stabilizers, like acids, salts, lignosulfonates, enzymes, petroleum emulsions, resins, and polymers [7]. In this method, the additives must be mechanically mixed with the soil in its natural state. With the influence of a specific chemical stabilizer on the site, the additive mixed must be properly distributed in the soil mass to ensure its effectiveness [8].

Stabilization techniques also include physical methods wherein soil is reinforced to achieve more strength and reduced settlements using reinforcing bars, strips, grids, fibers, and sheets [9]. Due to the rapid population growth, the scarcity of land for construction has increased, leading to construction activities on problematic soils [10]. Using stabilization techniques for problematic soils ensures the safety of structures built on them by improving soil performance against loading. Therefore, soil stabilization serves vital purposes in civil engineering. Apart from increasing the soils' strength, reducing their permeability, improving their bearing capacity, and filling voids, contaminant remediation has also been used to reduce the hazardous effects of pollutants (heavy metals) present in the soils due to anthropogenic activities. Heavy metal contamination in soils is a threat because heavy metals intrude in the food chain and cause hazardous effects [11]. Many techniques are being developed to reduce or recover heavy metals from polluted sites. Physical and chemical methods are proven to be effective in removing a wide spectrum of pollutants. However, the process consumes a lot of energy and may require extra effort to reach the desired level of heavy metal removal [12]. The use of soil stabilization techniques has been proven to be useful in both geotechnical and geoenvironmental applications.

Biologically mediated soil modification is also an emerging trend in soil stabilization. Mitchell and Santamarina [13] explored the possibility of using the biological components of rocks and soils to trigger interests in biological applications in geotechnical engineering. Dejong et al. [14] considered their work as the first of its kind. They also quoted the National Research Council of USA [15] regarding the biogeotechnical field being an important research area in the 21st century. Ants and termites amend soils, making tunnels water-resistant; this shows that biogeotechnical processes happen naturally [16]. Nature has always been an inspiration to humans for exploring possibilities of reaping benefits by replicating natural phenomena. One similar attempt is made in soil stabilization by domesticating microbes to improve soil performance. Mineralization through microbes such as bacteria, fungus, and algae is observed in nature [17], and the mineralization process by using bacteria has various applications in engineering [18]. Bacterial intrusion called microbial-induced calcite precipitation (MICP) in soil treatment improves soils' geotechnical properties through the precipitation of calcium carbonate (CaCO3), binding soil grains together [19]. Soil stabilization through microbes, which precipitate CaCO3, is applied to different soil types like liquefiable soils [20], sand [21–23], sandy soil [24], and tropical residual soils [10]; it is used for the remediation of porous media [25] and the restoration of calcareous stone materials [26]. MICP is even used to seal rock fractures [27], treat wastewater [28], and reduce beach sand erosion [29].

With the understanding of calcite precipitation in the soil through the bacterial method, a similar method of precipitation without bacterial intrusion in the soil was attempted by directly using the urease enzyme, which precipitates calcite. This method of soil stabilization through the precipitation of CaCO3 with the use of enzymes instead of microorganisms is called enzyme-induced calcite precipitation (EICP) [30–33]. This method also has a wide range of engineering applications for soil treatment such as stabilizing slopes, avoiding erosion due to wind and water, reducing the scouring of soil, checking the seepage beneath levees, improving the bearing capacity of soil, tunnelling, and controlling seismic

settlement [34] and dust [35]. Further biostabilization of soils has also found its way in the remediation of contaminants. This review considers the available research studies conducted on the EICP and MICP techniques and discusses their applicability and challenges in geotechnical and geoenvironmental applications.

### **2. Overview of the Microbial-Induced Calcite Precipitation (MICP) and Enzyme-Induced Calcite Precipitation (EICP) Methods**

In MICP, precipitates of calcium carbonate are produced by a combination of dissolved calcium ions and urea produced by the urease bacteria after hydrolysis [36]. Due to the complexity of the cultivation of urease-producing microorganisms and the uncontrollability of enzymatic activities, urease activity is incited directly with enzymes, specifically with urease [37–39]. The enzyme-mediated precipitation of calcite is achieved without any bacterial activity. The EICP method is used to improvise the geotechnical properties of soils by using an aqueous chemical solution that precipitates calcite within soil voids. The precipitates help in roughening and binding soil grains and even in pore filling, thereby improving the strength and stiffness of the soils. The EICP method is also distinguished from MICP by its use of free urease instead of bacteria. Enzymes can be derived from microbes, fungi, and agricultural sources [40,41].

#### *Hydrolysis of Urea*

The process before the precipitation of CaCO3 in soil voids in biotreatment starts from urea hydrolysis initiated by the urease enzyme. During urea hydrolysis, the decomposition of urea leads to the formation of carbon dioxide and ammonia. The water in the system helps ammonia to dissolve and form hydroxide and ammonia ions. These ions create an environment that allows an increase in the solution's pH. Simultaneously, carbon dioxide dissolves in water and develops ions of bicarbonates and hydrogen due to the increased pH of the environment; carbonates are formed due to the reaction between bicarbonate and hydroxide ions, forming carbonate ions and calcium carbonate in the presence of calcium ions; the calcium carbonate formed is precipitated because of its low dissolution rate in water [42]. Urea hydrolysis can be imitated either through the urease produced by bacteria or directly by using the free urease enzyme. Therefore, the reactions that take place in the soil is common in both MICP and EICP, but these two methods differ in terms of the source that initiates the hydrolysis [43]. Equations (1) and (2) show the chemical reactions that represent urea hydrolysis, leading to the precipitation of CaCO3 [44].

$$\text{CO(NH}\_2\text{)}\_2 + 2\text{H}\_2\text{O} \overset{\text{Unapse}}{\underset{4}{\rightleftharpoons}} 2\text{NH}\_4^+ + \text{CO}\_3^{2-} \tag{1}$$

$$\rm Ca^{2+} + CO\_3^{2-} \rightarrow \rm CaCO\_3 \tag{2}$$

Figure 1 presents the EICP and MICP mechanisms. The precipitation of CaCO3 by microbes was tested with and without urea by Golovkina et al. [45], who inferred that two metabolic routes—autotrophic and heterotrophic—were responsible for the precipitation of CaCO3 in the soil. Precipitation with urea was initiated with the usual urea hydrolysis, and the urea-free medium also successfully precipitated calcite at low pH values with different bacteria strains.

**Figure 1.** Mechanisms of microbial-induced calcite precipitation (MICP) and enzyme-induced calcite Precipitation (EICP) in the CaCO3 precipitation.
