Bacteria-Induced Calcite Precipitation for Engineering and Environmental Applications

engineering and environmental


Introduction
Bacteria-induced calcite precipitation (BCP) is the term for calcite synthesis at supersaturation circumstances and temperatures, which, because of the presence of bacterial cells and their metabolic activity, induce reactions that produce a variety of calcium carbonate polymorphs [1]. Organisms can secrete one or more metabolic products that react with divalent cations in their environment, resulting in mineral precipitation. Four fundamental components have the greatest impact on BCP, calcium carbonate content, dissolved inorganic carbon concentration, pH, and the availability of nucleation sites [2,3]. Te BCP technology can be employed to solve a variety of medicinal, environmental (heavy metals remediation, radionuclide remediation, and CO 2 sequestration), and engineering (biocementation and bioconsolidation) challenges [4]. More recently, BCP has gained attention for innovative technologies in self-healing in the feld of geotechnical engineering, earth science, and building materials. Te stabilization of soil by boosting its strength and decreasing its compressibility is one of the few well-known BCP uses in geotechnical and geoenvironmental disciplines. It is also used to fll the fssures in rock. Additionally, it is benefcial for lowering the porosity to reduce the seepage loss in hydraulic structures. Tis method can also be applied to other areas, such as soil erosion and landslides [5][6][7]. In terms of the geoenvironment, BCP is used to immobilize radionuclides and heavy metals in contaminated sediments and groundwater. Tis procedure can also be used to reduce the mobility of pollutants and limit their dispersal from sanitary landflls and sewage disposal sites [8][9][10]. Preservation of heritage monuments and artifacts made of limestone or other calcium carbonate-based materials is another application of BCP. Tis technology can also be used to restore mosaics and stone sculptures that have been damaged [11,12]. Similarly, BCP is employed to enhance the mechanical properties and decrease the porosity of concrete, and the concrete structure cracks can be repaired using this procedure [13,14].
Until now, many bacteria have been used to illustrate the molecular mechanisms involved in the calcite precipitation with diferent morphologies [1,[15][16][17]. Typically, the type of bacterium selected for BCP depends on the specifc function and environmental factors. Sporosarcina pasteurii [5], Bacillus spp. [18], Pseudomonas aeruginosa [19], and Arthrobacter spp. [20] are frequently used in BCP. Bacteria that produce biomolecules capable of altering the environment and promoting the precipitation of minerals are used in the biocementation process. Tese bacteria produce urease, which can result in the production of metabolites such as CO 2 and NH 3 , which can combine with Ca 2+ to form calcite [21]. Calcite precipitation by the bacteria mentioned above appears in monuments, caves, sediments, and constructions. Te function and structure of bioflm geometry and exopolymeric substances (EPS) explain how these bacteria nucleate Ca 2+ and synthesis minerals [22]. Calcium carbonate with diferent polymorphisms has been obtained. A few examples of the mineral carbonate produced by BCP mechanisms are calcite (CaCO 3 ), dolomite (CaMg(CO 3 ) 2 ), kutnahorite (CaMn(CO 3 ) 2 ), siderite (FeCO 3 ), hydrocerussite (Pb 3 (CO 3 ) 2 (OH) 2 ), magnesite (MgCO 3 ), otavite (CdCO 3 ), rhodochrosite (MnCO 3 ), strontianite (SrCO 3 ), hydrozincite (Zn 5 (CO 3 ) 2 OH 6 ), dypingite (Mg 5 (CO 3 ) OH 2 .5H 2 O), and witherite (BaCO 3 ) [23]. Te bicarbonate and carbonate ions aggregate as a result of bacterial activity in both alkaline environments and the lack of divalent cations, resulting in the formation of zeolite crystals. Soda lakes are examples of such zeolite synthesis as a result of numerous metabolic activities [24]. Because Ca 2+ is abundant in most soils and aquatic systems, calcium carbonate accounts for more than half of all known minerals carbonate on Earth, and Ca 2+ is involved in most cellular metabolic processes and bacterial activities [25]. Te word calcifcation was used for biomineralization until 1980. BCP is also known as the bacterial-derived calcite precipitation [24]. Subsequently, biocementation can be categorized into two crucial stages. In the frst step, bacteria create a bioflm on the surface, which acts as a matrix to hold minerals, then expand, and solidify. In the second step, bacteria create metabolic metabolites that can change the pH levels, increase Ca 2+ concentrations, and promote calcite precipitation. Tis process may eventually lead to the formation of a robust structure that can link the soil particles or other materials [26,27].
Several earlier studies explored BCP for the use in engineering, namely, building applications and the cementation of porous media. Few researchers have reviewed the role of BCP in enhancing and rehabilitating construction materials [28], in soil and porous media systems [29], strengthening sand [30], in construction materials, specifcally focusing on concrete [31], controlling hydraulics, consolidating porous media, treating construction materials, and remediating environmental problems [32]. In conclusion, the efcacy and afordability of biocementation by calcium carbonate formation can be used to strengthen and support the geotechnical and geoenvironmental disciplines. Additionally, bioprospecting is used to discover novel bacteria and methods, and biocementation is used to cause biomineralization and self-healing in concrete, as well as sequestered radioactive materials. Some of the most typical microbes used in BCP include S. pasteurii, Bacillus subtilis, and Pseudomonas putida, which are a economical, ecofriendly, and long-lasting substitute for the traditional processes.

A Brief History of Bacteria-Induced Calcite Precipitation
Te research towards BCP began in 1995. Te frst published BCP via Sporosarcina pasteurii ATCC6453 for controlling the leaching of groundwater contaminants in highly permeable channels by packing S. pasteurii ATCC6453 cultured with sand, which was achieved after 95 h with a reduction of 75% permeability [5]. Te noteworthy contributions to the BCP techniques in the feld of civil engineering are presented in a chronological order in Table 1. Te most commonly used bacteria are S. pasteurii, having the highest ureolytic activity and a greater calcite precipitation rate [36,54]. Te research mentioned above addresses a wide range of topics linked to biocementation and biomineralization, which involve using bacteria and other biological processes to strengthen and stabilize rocks, soils, and construction materials. Te important discoveries include the efectiveness of biocementation to stabilize sand and soil, improve brick durability, and plug rock pores. Using biocementation to sequester the radioactive materials is another fnding and the identifcation of new bacteria and biocementation techniques through bioprospecting. Overall, these works have enhanced the knowledge of biocementation and its potential application in engineering and construction. To date, various ureolytic bacteria have been isolated from their natural habitats, as shown in Table 2.
Te three bacteria S. pasteurii (K m : 26.2 mM and V max : 1.72 mM/min/mg protein at pH 7.7) [33], B. subtilis, and P. putida are the most frequently used ones in BCP. Te ability of these bacteria to release urease, which can hydrolyze urea to produce ammonium and increase the pH of the environment, and to encourage calcite precipitation, has been the subject of extensive research. Halomonas spp., B. licheniformis, and other bacteria have also been used in some studies. 2 Advances in Materials Science and Engineering Crystal-growth behavior [18] 2009 Acinetobacter (isolate B14) Limestone consolidant [11] 2010

S. pasteurii
Augmention and stimulation of biocementation [53] Advances in Materials Science and Engineering

Strategies, Merits, and Demerits of Bacteria-Induced Calcite Precipitation
Biostimulation and bioaugmentation are the two major categories of biological processes involved in BCP. In biostimulation, native bacteria are stimulated to proliferate by being exposed to an additional nutrient medium. When the exogenous bacteria and nutritional media are introduced into the soil to aid in their proliferation, this process is known as bioaugmentation. Te biostimulation strategy has signifcant limitations, including the uniformity of the treatment, and a longer duration is needed for the stimulation and proliferation of bacteria, despite the efcacy of its implementation in some research [10]. A few examples of successful biostimulation methods are strength improvement using soil column (5.1 ϕ × 10.2 ht·cm 2 ) with a reduction in the treated soil permeability [65], better cementation improvement in the cylindrical large-scale soil cylindrical tank (170 ϕ × 3 ht·cm 2 ) [66], calcite precipitation with CuCO 3 precipitation, which also remediates copper contamination [67], used in the improvement of the strength of the materials from the ground, etc. [68].
Although biostimulation is considered more suitable than bioaugmentation for various reasons, including the external addition of microbes, higher cost, nonuniform distribution over depth, and the likelihood that the bacterial numbers will decline if the conditions are unfavorable. Te BCP techniques using bioaugmentation approaches have undergone extensive laboratory research. Numerous processes, including urea hydrolysis [24], denitrifcation [69], iron reduction [10], and sulphate reduction [70], can be used to carry out the BCP process. Table 3 provides a summary of the merits and limitations of the biochemical mechanism related to bacteria-induced precipitation of calcite. Due to its excellent calcite precipitation efciency, urea hydrolysis is one of the methods that has been utilized most frequently. As a result of the low solubility of oxidizing substrates, which makes them difcult to dissolve, denitrifcation, iron reduction, and sulphate reduction have been less studied. Additionally, groundwater pollution with Cl − and NH 4 + ions  produced during hydrolysis of urea result from soil treatment with urease-derived BCP, which may also increase the risk to the nearby water and air associated with the increase in the pH level [71]. As a result, they need a large number of substrate solutions to produce adequate precipitation [10].

Bacteria-Induced Calcite Precipitation by Various Bacterial Metabolic Pathways
One or more processes, such as compaction, preloading, vibration, and chemical grouting, are frequently used in the soil strengthening techniques. Tese methods have been shown to enhance soil strength and other qualities in varying degrees. However, the above methods come at the expense of using a substantial amount of energy, either during their application, during the synthesis of the grouting material, or both. As seen in Figure 1, BCP employs the naturally existing bacteria to precipitate calcite, which binds soil particles together and increases strength.
Te main bacterial pathways seen to be involved in the formation of calcite precipitation via diferent mechanisms are photosynthesis [72], sulphate reduction [2], methane oxidation, nitrogen cycle, nitrate reduction [24], and urea hydrolysis [21]. After understanding the precipitation of calcite in the soil by the bacterial method, a comparable approach of precipitation without bacterial intrusion into the soil was explored by utilizing the enzyme that precipitates calcite. However, the size of bacteria for the process of calcite precipitation varies from 0.5-3 μm, soil pores smaller than 0.5 μm cannot hold them, so this problem is overcome by the enzyme-induced calcite precipitation (ECP). BCP is also sensitive to moisture and may dissolve; a regulated atmosphere must be maintained for healthy bacterial growth, culture, and enzyme synthesis. Because the circumstances for bacterial growth vary according to the species, including pH, temperature, and oxygen availability, BCP is also afected [71]. ECP is a technique for stabilizing the soil through the calcite precipitation that uses enzymes rather than bacteria, i.e., accomplished with no bacterial activity [73]. ECP is favored because the technique requires less monitoring. Because it is easier to apply and requires less maintenance than the BCP technique of soil treatment, the ECP method ofers a practical alternative to treat the soil. On the other hand, ECP can facilitate the calcite precipitation even in fne clays as the size is around 12 nm [71].

Photosynthesis.
In an aerobic or anaerobic environment, photosynthetic bacteria such as cyanobacteria will increase the pH of the surrounding environment, causing calcite precipitation (equation (1)). As these bacteria absorb CO 2 from the atmosphere without urea and carbon sources directly, the process is economical and simple [72]. Te study suggested that these bacteria can catalyze the carbonate mineralization reactions in marine and freshwater systems [74]. On the other hand, algae also absorb CO 2 in the environment through photosynthesis to form low-magnesium calcite and aragonite [72]. Te Magellan Seamount cluster's upper crust is rich in Coccolithophores, according to Wang et al. [75], who explored the factors that infuence the crust formation. Tis fnding suggests that Coccolithophores serve an integral part in the nucleation of the crust. According to Zhu et al. [76], Synechococcus spp. has huge possibilities for restoring concrete.

Bacteria Utilizing Methane
Oxidation. Te equation form (equations (2)-(4)) describes the bacterial enzymatic activity under aerobic conditions for the synthesis of carbonate from methane, carbonate ions, and CO 2 . In contrast, bicarbonate ions and carbonate minerals are produced in anaerobic environments by utilizing sulphate as an electron acceptor rather than oxygen (equations (5) and (6)). In anaerobic environments, methanogenesis is a process that autotrophic bacteria can use to initiate the calcite precipitation (equation (7)) [77]. Tis mechanism causes calcite precipitation in the oceans in large part [78].
Te previous research described that Methylocystis parvus was used and optimized for calcite precipitation in the methane oxidation process to produce the construction materials. Although this metabolic route is well described and performed in the research laboratory, adopting it in situ environmental settings requires additional research and analysis [79]. (12)) via urease (urea amidohydrolase; EC 3.5.1.5) and shown in Figure 2(a). Te urease-induced calcite precipitation is the simplest [21] and widely used method [26,27]. Te given urea functions as a primary nitrogen gas (N 2 ) source for various bacterial species during urea degradation [24].    Advances in Materials Science and Engineering 7

Urease-Expressing Bacteria. Te calcite precipitation by bacteria is explained in an equation form (equations (8)-
According to a recent investigation, S. pasteurii can generate 98% calcite precipitation at pH 9 under identical conditions, whereas chemical processes can induce only 54% [24]. Numerous scientifc research and feld trials have shown that, using BCP via ureolysis can strengthen the soil, improve oil recovery, sequester CO 2 , dust suppression, sealing of subsurface barrier reservoir or pond, remediate construction materials, concrete, limestone, polychlorinated biphenyls, groundwater, heavy metals, and radionuclides [57,73]. Bioaugmentation, also known as ex situ ureolytic bacteria, must be introduced under harsh circumstances such as nutritional requirements, low moisture, high pressure, high alkaline or acidic environments, and high salt concentrations. Te stimulation procedure is more viable than augmentation since it is economical, less destructive to the environment, and has no detrimental impact on the ureolytic bacteria. Te ureolytic bacteria are aerobic, and the fundamental challenge with BCP through ureolysis is the limitation of bacterial activity in the absence of low oxygen. As a result, ureolytic activities, as well as calcite precipitation, are hampered in most BCP application processes, such as in oil reservoirs, concrete crack healing, underground soil strengthening, and soil remediation below groundwater, due to a lack of oxygen [80].
Although various laboratory and mini-BCP trials using the ureolytic procedure have yielded encouraging outcomes, the fnal products, ammonia and ammonium, are undesired and possibly toxic to the environment [81]. As a result, more work and expense are required to eliminate or reutilize ammonia [82,83]. A modest amount of ammonium, on the other hand, may be transformed into nitrate and then into nitrogen via nitrifcation and denitrifcation. However, the amount of ammonium produced and its entire use by nitrifying bacteria remains unknown [81]. In addition, several researchers have recently undertaken tests on denitrifcation-based carbonate precipitation for an efective BCP application [84]. BCP via ureolysis is only feasible for specifc uses, provided alternative paths are followed, such as the fnal products used as in situ fertilizer [85]. However, according to several studies, urea breakdown is the primary pathway to induce calcite precipitation, which results in ammonia release and the formation of unpleasant odors. After treating reinforced concrete, it is noteworthy that in a self-healing process, ammonia is produced when urea and calcium are combined, which might increase the risk of steel corrosion [24,86].

Bacteria Utilizing Sulphate Reduction.
Under anaerobic and anoxic conditions, sulphate is reduced by sulphatereducing bacteria, which produce bicarbonate ions (HCO 3 − ) and hydrogen sulphide (H 2 S). Te production of HCO 3 − and CO 2 − aids in the synthesis of calcium carbonate (equations (13)- (15)) and is shown in Figure 2(b). However, it is primarily determined by the behavior of H 2 S, which alters the pH of the system. For example, an oxygenic phototrophic sulphide bacteria degas H 2 S and oxidize sulphur (S 2− ) to sulphur (S), which increases the pH of the system pH and hence favors intracellular or extracellular biomineral formation [70]. Te autotrophic aerobic sulphide oxidizing bacteria, in contrast, can oxidize H 2 S to sulphate ions (SO 4 2− ) and generate sulphuric acid, which lowers the pH and hinders the precipitation of calcite. Recently, sulphate-reducing bacteria such as Citrobacter freundii ZW123 showed a positive efect on the biocementation process [87]. Similarly, Desulfovibrio vulgaris subsp. vulgaris ATCC29579 can remove the black crust deposits on marbles, which was comparatively better than the chemical cleaning method [88]. However, the BCP via the sulphur cycle is not appropriate for engineering applications because it is problematic to maintain the anaerobic conditions and H 2 S is also detrimental to the environment [1]. It is shown in the following chemical equations: 4.5. Bacteria Utilizing Nitrogen Cycle. Te nitrogen cycle involves 3 independent metabolic pathways, as depicted in the equation (equations (16)- (19)) as follows: amino acid ammonifcation, nitrate reduction, and urea degradation, each of which could enhance the calcium carbonate formation. Te bacterial metabolic process produces CO 2 and ammonia during the ammonifcation process of amino acids. Ammonia (NH 3 ) is hydrolyzed to produce ammonium (NH 4 + ) and hydroxide ions (OH − ). OH − raises the pH level in the system, while the production of bicarbonate ions via CO 2 promotes calcite precipitation. Alcanivorax borkumensis and M. xanthus are prevalent in all habitats and depend solely on amino acids for energy [89]. In the presence of Ca 2+ , bacteria serve as a nucleation site, generating distinct calcium carbonate polymorphs [90]. From lead-contaminated soil, Zhao et al. [91] discovered a leadresistant Brevibacillus laterosporus ZN5 bacterium that will sequester Pb from Pb-contaminated soil by combining the ammonifcation and nitrate assimilation processes.
4.6. Denitrifying Bacteria. Te denitrifying bacteria of nature regulate the amount of nitrogen by reducing nitrate by converting terrestrial nitrate (NO 3 ) to N 2 gas. Furthermore, the presence of bacteria in specifc sites, such as contaminated areas, landflls, and eutrophic lakes, benefts the Table  4: Characteristics of three diferent classes of carbonic anhydrases found in prokaryotic organisms [97].
Types of CA  Denitrifcation of nitrate happens in the presence of biological matter to yield alkalinity, CO 2 , and nitrogen during the denitrifcation process [69]. Meanwhile, CO 2 reacts with water, releasing bicarbonate ions. Calcium carbonate occurs in this highly alkaline atmosphere and in the presence of divalent cations [70]. Te equations below describe the denitrifcation process (equations (20)- (22)). Even though the byproduct nitrogen is less detrimental to the environment compared to the intermediate products of BCP denitrifcation, namely, nitrite (NO 2 − ) and nitrous oxide (N 2 O). Denitrifying microorganisms (Alcaligenes spp., Pseudomonas denitrifcans, Tiobacillus spp., Denitrobacillus spp., Micrococcus spp., and Spirillum spp.) are generally facultative anaerobes [24]. Te studies suggested that, except calcite precipitation, denitrifying bacteria also have the inhibitory efect on the corrosion of steel bars due to the generation of nitrite ions during the reaction [92]. In another study by Gao et al. [93], introduced denitrifcationbased BCP by a method called large-volume circulation in the sand. Te precipitation rate and the amount of calcite were 0.13% per day and 4.61% by weight. Te result concluded that, compared to the conventional adopted methods, precipitation rate was 5 times higher and had high efciency, which is shown through teh following chemical equations:

Mechanism of Carbonic Anhydrase Expressing Bacteria in Calcite Precipitation
Only a little research on the function of carbonic anhydrase (CA, EC 4.2.1.1) in the biocementation process has been conducted and has achieved more attention [94]. Earlier research reported that bovine CA (bCA) aided in the synthesis of calcium carbonate. Te studies found that, adding bCA accelerated the calcium carbonate precipitation rate; similar fndings were seen for the addition of bCA to brines.
Since bacterial CA plays a crucial role in the carbon cycle, the impacts of the CA of Bacillus spp. on calcite precipitation are crucially signifcant [95]. Tree classes of CA have been found in prokaryotic organisms [96], including many pathogens as well. CA has been divided into three distinct classes (α-, β-, and c-) and summarized in Table 4. Te reaction catalyzed by CA is shown in the form of equations (equations (23)-(25)) and shown in Figure 2(c). Te HCO 3 − causes a pH shift, which allows the divalent cations (for example; Ca 2+ ) to precipitate. Te formation of HCO 3 − alters the surrounding pH and the process proceeds to create calcium carbonate. If the solution contains a signifcant Ca 2+ concentration and HCO 3 − in the solution, CaCO 3 precipitates at the bacterial surface.
where CA is carbonic anhydrase. Te bacterial metabolic activity is important in the calcite precipitation in BCP, as it also serves as a calcite nucleation precursor [98]. Due to the presence of carboxyl (RCOOH) and phosphate (PO 4 3− ) groups, bacterial surfaces are negatively charged [99]. Tese negatively charged molecules serve as a nucleation site to induce mineral precipitation by binding to divalent cations (Ca 2+ , Mg 2+ , Cu 2+ , Mn 2+ , and Zn 2+ ), and trivalent ions (Fe 3+ ) from the solution [70]. Te bacterial cell surfaces acting as nucleation sites for calcite precipitation in the presence of cations are described in the equation form (equations (26)-(28)) as follows: where CA: carbonic anhydrase and Cell N : nucleated site in the bacterial cell surface. Te energy threshold for calcium carbonate synthesis is lowered as a result of bacterial nucleation, and this rate is comparatively higher than in the chemical precipitation [100]. Surprisingly, even metabolically inert bacteria such as Desulfovibrio desulfuricans precipitate calcite by providing heterogeneous nucleation sites [101]. Te EPS generated around the bacterium serves as a heterogeneous nucleation site for calcite precipitation in addition to the bacterial surface [83]. Furthermore, in solution, EPS can obstruct precipitation by trapping and lowering the saturation of divalent cations [102]. Although the metabolic pathways discussed above result in biomineralization in the form of calcite precipitation, these methods are not appropriate for engineering applications [24].

Signifcances of Calcite Precipitation via Carbonic
Anhydrase. Te conventional soil improvement technologies such as reinforcement, stabilization using admixture, and mechanical stabilization have been used to enhance the quality of soils [24]. Tese techniques use polypropylene fbres, nano-SiO 2 , industrial wastes like fuel fy ash, incineration of municipal solid waste, rice husk ash (RHA), lignin along with the rice husk, waste plastic in the form of fbers, alkali-activated agrowaste with Wollastonite fbers along with fuel ash derived from palm oil processing, natural pozzolana and lime, cement, lime, and RHA, a combination of perlite, and lime as additives. By utilizing the alternatives to cement and waste products like ash, which create an issue to the environment after being disposed of, these techniques of soil stabilization assure reduction in pollution to the environment [103]. However, these options are typically expensive, time-consuming, or hazardous to the environment, as these techniques leave fewer CO 2 footprint [71]. Recent fndings in the traditional soil improvement have led to a signifcant contribution, such as the work of Ashfaq and Moghal [104] investigating the efect of fy ash in terms of cost and carbon emission and concluded that the use of fy ash signifcantly reduces both. However, the analysis was carried out where fy ash as waste material and freely available, which may be inapplicable in many parts of the world. Te study highlighted by Amulya et al. [105] focused on the efect of granite sand (GS) and calcium lignosulfonate (CLS) to improve the California Bearing Ratio (CBR) of clay. Te study revealed that the optimal GSs were 35%, 34%, 33%, and 32% for dosages of CLS of 0.5%, 1.0%, 1.5%, and 2.0%, respectively. A carbon footprint analysis was performed on a typical pavement section according to the recommendations of the Indian Road Congress. Te use of GS and CLS as stabilizers is observed to reduce the carbon energy by 97.52% and 98.53% over the traditional stabilizers lime and cement at doses of 6% and 4% dosages, respectively. But the carbon emission analysis for cement and lime was taken from other study without considering the CBR values after modifcation. Te BCP is a potentially low-cost and environmentally friendly alternative to typical soil improvement processes. Although BCP via urease is a widely used method, this process has some drawbacks such as the production of ammonia and ammonium; therefore, more work and expense are required to eliminate or reutilize the ammonia. Secondly, urea is coupled with a calcium source in a self-healing technique and the generated ammonia raises the risk of steel corrosion [24,86]. Terefore, BCP via CA is a better choice as the catalytic rate is fast [94], and the fnal products are not toxic to humans or the environment.

Bacteria-Induced Calcite Precipitation Applications in Engineering and Environmental Fields
Te BCP technology can be employed to handle a wide range of environmental concerns, including heavy metal and radionuclide treatment, bioconsolidation, biocement, CO 2 sequestration, and other purposes [4]. Te concept of BCP is attracting many academicians and engineers due to its wide use in the feld of engineering. Te newly demonstrated and prospective BCP via Bacillus spp. processes such as biocementation for the production of innovative building materials and self-healing of civil construction materials [44], hydraulic control, environmental remediation, and cementation of porous media [32]. Controlling BCP by altering the factors that impact the saturation state to meet specifc technical goals was a major research priority. Many designed applications rely on the management of the pace and calcite precipitation distribution in situ, which is determined by the spatial and temporal fuctuations. As mentioned above, several Bacillus spp. can induce calcite precipitation by converting atmospheric CO 2 to bicarbonate through enzymatic activity that creates an alkaline condition through the cleavage of nitrogen sources (proteins and urea) [22]. It is worth noting in this regard that, calcite precipitation occurs only in alkaline environments (pH > 8).
When added to reinforced concrete, this biogenic calcium carbonate will not corrode steel or reduce pH levels [106].

Binders.
One of the most notable BCP applications is the development of an innovative solution to cement-based building materials. Te lifespan of reinforced concrete and concrete can be extended by using bioremediation, coating systems, porous fller, and self-healing systems, as illustrated in Figures 3(a) and 3(b) [107]. So, for the synthesis of binder materials to seal the cracks, a biocement using the BCP is a fnancially advantageous substitute for cement and chemical grout [28]. A study has revealed that for microbial growth, the cost was up to $9/m 3 when waste materials were used as a source of carbon, while for chemical grouting cost was up to $72/m 3 [108]. Te BCP improves concrete's resistance, fortifes the soil and mixture's strength, and fxes the fractures in the material [32]. On the other hand, sand, limestone bricks, and other cement-based materials have been bioconsolidated and biocementated to create unique building materials. Bacillus spp. biocementation capacity has been utilized to construct a biogrouting system in wellbore rocks and soil that has been employed to prevent leaks [42,109]. In BCP, calcium carbonate can clog pores and inhibit the ability of a material to restore the surface, absorb water and reduce probable erosion by protecting the calcite layer [28]. Te BCP improves the stifness and strength of the porous media matrix, in addition to reducing porosity and permeability [29] and helps prevent the entry of harmful substances like Cl − in concrete surfaces [28]. As a result, B. subtilis has been shown to produce a calcium carbonate layer on the exterior of materials subjected to corrosive environments, enhancing their durability [12]. Using BCP, a new surface treatment of concrete bricks and other identical absorbent materials increased the resilience and mechanical properties of the bricks by around 36% [110]. A study of Bacillus spp. CT5 concluded that, the BCP improved the durability of building structures by up to 36% and there was 6 times less water absorption [111]. In healing technologies, the endurance of bacteria in the presence of potentially harmful chemicals must be evaluated in advance.

Advances in Materials Science and Engineering
A few investigations of some novel self-healing concrete/ biocement mortars using Bacillus spp. are B. subtilis, B. cereus, and S. pasteurii. Tese bacterial strains improved the selfhealing of cracks (<0.5 mm), crack repairmen efect, biocement production and compressive strength of cement mortar. However, as earlier said, some researchers have used ureolytic bacteria and calcium chloride; however, urea breakdown releases ammonia as an unpleasant odor, and calcium chloride can be destructive to reinforced concrete structures as a result of corroding Cl − that corrodes [1]. Research using Bacillus spp. in the production of self-healing substances has shown an improvement in the mechanical and durability qualities of the concrete as well as a reduction in Cl − absorption [112]. Researchers have documented crack repair by BCP via S. pasteurii and other Bacillus spp. [4]. B. pseudofrmus DSM8715 and B. cohnii DSM630 were applied as self-healing agents for the development of sustainable concrete [13]. Bioremediation for restoring stone surfaces in heritage buildings [113]. BCP via carbonatogenic strains was applied for the protection and regeneration of limestone buildings and historical patrimony such as monuments and statues [114].

Soil Stabilization and Strengthening.
Te utilization of BCP to change or enhance the mechanical characteristics of unconsolidated porous media has received a great deal of attention. Tis strategy has been suggested to enhance soil strength, stabilize slopes as shown in Figure 3(c), and improve liquefable soils [115].
Calcite precipitated during BCP can bridge grain gaps, bind them frmly, and reduce porosity, reduce pore throat size, and permeability while increasing the strength and stifness of the soil [29]. Much of the efort to date has been spent on increasing precipitation efciency, optimizing the extent of treatment, and optimizing chemical usage to decrease feld application costs. For economic purposes, it is preferred to calcite precipitation across a distance and utilize as little reactant volume as possible in engineering felds such as soil or sand consolidation fortifcation [116]. While selective plugging may be benefcial in some technical applications, nonhomogeneous bacterial dispersal and nonhomogeneous precipitation may have the drawback of restricting the scope of treatment to near-injection-point plugging where substrates are available [117]. Te improvement of the soils was evaluated by comparing the Unconfned Compression Strength (UCS) of biotreated soils. Diferent researchers compared the UCS values of soil samples (Natural; without biotreatment and test; with biotreatment) were shown in Figure 4, and concluded BCP methods improve soil strength as calcite precipitates help to bind soil grains together [103].
Suggested precipitation control strategies include increasing the spatial distribution of enzymatic activity in cells, regulating the transit and reaction rates, and encouraging favorable saturation conditions in certain places [32]. BCP was applied for ground improvement, which was accomplished with rather modest fow rates. Tis study also noted that the uniform calcite distribution was facilitated by matching the rate of urea hydrolysis with the delivery of reactants [35]. Another in situ soil strengthening method was investigated in fne-grained sand and it was observed that bacterial homogeneous distribution over large sand bodies was facilitated by low-ionic-strength solutions [116]. To evaluate stabilization of dry silty clay soil, immersion experiments were used by mixing urease and appropriate substrates in a proportion equal to 80% of the liquid limit. In comparison to the unstabilized soil, the stabilized soil has a much lower mass loss during immersion as a result of the development of calcite connections between particles that restrict water permeability by occluding pores and precipitated minerals. Additionally, the stabilized samples did not crack, supporting the efciency of the suggested stabilization technique [122]. Fewer investigations have also performed on feld-scale biomineralization to reinforce liquefable soils. Due to restrictions on shearing ability, such soils feature loose granular soil deposits that are commonly found in saturated conditions [32]. Biomineralization treatments were applied to soils along the Snake River's coast, resulting in soils cemented with approximately 1% CaCO 3 near the surface and 1.8-2.4% calcite below 9 cm [115]. However, the data did not match the lab set specimens and observed less precipitate which was related to the feld study's lower technical ability of the calcium source. Nonetheless, signifcant amounts of calcite accumulated further beyond the injection point than closer to it [32]. One of the important variables that must be taken into consideration for sustainable BCP consumption is the cost of utilizing BCP in building materials in this feld. Currently, BCP may be treated with biostimulation without introducing any foreign bacteria due to its simplicity and afordability [24]. Previous experiments have demonstrated that more efcient cementation can lead to material and cost savings [123]. Large-scale engineering application may be hampered by the high cost of materials. As a result, recycled materials have been utilized relatively often in civil engineering during the previous 10 yrs. Te high cost and brittle behavior of biocemented calcareous sand might be improved by using materials like eggshells [24] and rubber fragments from used tires. BCP via S. pasteurii demonstrated that waste rubber particles enhance biocemented calcareous sand's brittle behavior by signifcantly reducing brittleness and reducing the rate at which dilatancy changes as stress increases [50].

Innovative Construction
Material. An innovative construction material called "biobrick" has been developed as a building material. Biobrick helps both the public and the construction sector improve the durability and lowering carbon emissions, resulting in a pollution-free environment. Research focuses on creating a biobrick with calcite precipitation that is eco-friendly and economical compared to regular brick manufacture, which demands high combustion energy. Published data showed that bricks produced by bacteria can reduce approximately 800 million tonnes of carbon emissions per year [36]. Due to the pores and voids, regular bricks are susceptible to cracking over time. As a biobrick has an inherent capacity to heal itself, BCP has shown to be a revolutionary means of fxing these fractures or reinforcing bricks [10]. BCP can precipitate calcite continuously over the brick with a high impermeable layer, which increases the compressive strength and prevents the water into the brick, and automatically increases the brick durability [36]. In a study conducted to produce biobricks using urea from stabilized human urine, the results demonstrated higher compressive strength with an increase in the number of treatments with the highest compressive strength of 2.7 MPa [124]. Bricks treated with BCP after 28 d showed 83.9% improvement in compressive strength and 48.9% decrease in water absorption capacity when compared to the control specimen [125].

Carbon Dioxide Sequestration.
Te carbon cycle in which CO 2 in the atmosphere is converted to carbohydrates in plants through photosynthesis, plants consumed by man and animals, metabolized to CO 2 and other products, and fnally returned to the atmosphere. Some amount of this carbon pool is sequestrated from the cycle and deposited in soil, trees or in deep earth's crust. Tis deposited carbon pool from prehistoric times, has been removed in the form of coal and oils, as fossil fuels. Soil organic carbon that has been stored can be used for sustainable crop production and for possible carbon trading. Te chemical processes can directly convert CO 2 into soil-based inorganic carbon molecules such as calcite, aragonite, dolomite, magnesite, and MgCO 3 across geological time scales [98]. One method by which soil directly sequesters CO 2 from the atmosphere is through the weathering of the feldspar mineral to kaolin (Al₂Si₂O₅(OH)₄). Additionally, certain bacteria also employ chemosynthesis to use atmospheric CO 2 . Te existing CO 2 content in the earth's atmosphere is 400 ppm. As a result, there is an urgent need to limit CO 2 emissions into the atmosphere [3] as excess atmospheric CO 2 may be deposited in the soil. Tus, researchers suggested biomimetic CO 2 sequestration by employing an enzyme to reduce the amount of localized CO 2 [126]. Formation trapping, solubility trapping, and mineral trapping are the three strategies proposed by BCP to contribute to in situ CO 2 leakage. BCP may restrict permeability to reduce leakage possibilities through information trapping. In contrast to insolubility trapping, this technique may improve CO 2 storage as bicarbonate or carbonate by increasing the dissolved CO 2 in the subsurface. Lastly, in mineral trapping, the BCP may  (1): [118], (2) and (3): [119], (4) and (5): [120], and (6): [121].
improve the dissolved CO 2 precipitation as a mineral carbonate [127]. Calcite precipitation has been suggested to protect against supercritical CO 2 plug microfractures in the nearby well habitat and reduce the cap rock porosity, as shown in Figure 5(a) [128,129]. Te spatial extent and temporal efciency of precipitation should be managed in these types of systems. Studies conducted under environmental circumstances resulted in the development of injection techniques that encourage a more equal geographical dispersion of calcium carbonate [22]. In sand column reactors, pulse fow injection with short fuid injection followed by batch biomineralization period precipitated calcium carbonate in the fuent continuous fow injection. Furthermore, lowering the SI near the injection point, while active biomineralization decreased near injection point clogging [130,131]. BCP is presented as an option for dust particle consolidation [132]. S. pasteurii cells or urease and NH 4 + /Ca 2+ treatment solutions were sprayed over sand samples, which were subsequently submitted to wind erosion experiments. BCP is quite successful in dust management; however, its efectiveness depended on soil quality and grain size distribution, in addition to climatic factors such as temperature and humidity [133]. To generalize, BCP has been investigated for a variety of engineering applications that involve porous media, such as sand or soil, building subsurface barriers, closing aquaculture ponds, and dust suppression. Tese applications are frequently managed by varying the transport and reaction speeds to produce either homogeneous deposition or controlled deposition in specifc locations. A complex series of variables, mainly ambient circumstances, may considerably impact the treatment results when BCP is applied to a porous medium [32]. BCP used for dust suppression is shown in Table 5.

Subsurface Barriers.
Since the beginning in particular coastal locations, saltwater penetration into aquifers and groundwater resources has been a big challenge. However, to avoid salt-laden water from migrating into freshwater aquifers, the issue is frequently addressed by building tunnel dams or improving the artifcial recharge of freshwater resources, as shown in Figure 5(b). Te subsurface BCP barriers may provide a remedy to the aforementioned problems, but they must be able to precipitate calcite in saline conditions to be employed in such ecosystems [138]. To select the acceptable in situ conditions, several environmental parameters infuence BCP [117]. Notable results were acquired, including the fnding that anaerobic circumstances did not appear to limit short-term enzymatic activity once the cells were cultivated aerobically, which is consistent with the earlier fndings from a few similar studies [139]. Due to the increased cation and alkalinity supply, fulland half-strength seawater also enhanced the calcite precipitation rates. Tese fndings revealed the utility of BCP in the development of subsurface barriers to avoid salinization [117].
6.6. Impermeable Crusts. One interesting engineering usage of BCP, as shown in Figure 5(c) is the formation of crusts to prevent seepage from ponds or reservoirs into underlying soils or sands [32]. Following percolation, treatment with high concentrations of Ca 2+ and NH 4 + solutions resulted in a highly impenetrable crust on the sand's top, according to research on Bacillus spp. VS1. Reducing the seepage rate actually and bringing sand to a similar permeability range as properly compacted clay [140]. Calcite bridged the sand particles by using Sporosarcina pasteurii MTCC1761 to construct a biocemented crust. Te bioclogging process reduced the permeability and porosity of the calcite bridge, which had a 0.2 cm thick layer of water-impermeable crust. Te permeability tests were used to assess the efectiveness of seepage management and revealed that it decreased to 99% (three orders of magnitude) on the 7 th day with just a small change up to the 14 th day (100%). Similarly, Figure 6 depicts the maximum permeability reduction rate (%) predicted by various studies following soil biomineralization. the biocementation techniques are used to reduce the permeability and have the potential for applications, including seepage control in water retaining structures and lining the bases of water bodies [103].

Soil Contaminants Remediated.
Heavy metals such as copper (Cu), cadmium (Cd), chromium (Cr), cobalt (Co), lead (Pb), arsenic (As), nickel (Ni), selenium (Se), zinc (Zn), mercury (Hg), antimony (Sb), and thallium (Tl) from industrial activities pose a major threat to the soil owing to their toxicity, nonbiodegradability, and persistent deposition [8]. Additionally, the heavy metals that accumulate in the soil create many health problems for humans and other living organisms [152]. Various traditional remediations such as adsorption, chemical precipitation, electrochemical treatment, evaporation, fltration, ion exchange, membrane technology, oxidation/reduction, and reverse osmosis [9] have been used to remove heavy metals from the contaminated environments. However, these treatment methods may not be long-term solutions and inefective in removing metals successfully besides being expensive and consuming large amounts of chemicals and energy [4,8]. In addition to the aforementioned techniques, many biological therapies, such as phytoremediation, bioaccumulation, biocoagulation, bioleaching, biosorbents, and bioimmobilization, have been developed recently to remove these metal pollutants from the polluted sites [153]. Tese techniques are also inefective, as the above techniques are costly, time-consuming, and cause immobilized or adsorbed heavy metals to be released back into the environment [4]. Many bacterial species may be able to immobilize metals by altering their redox state during bioremediation, decreasing their solubility. As a result, efective, afordable, and environmentally friendly removal of heavy metals is achieved through the use of alternative techniques such as BCP.
According to the previous studies, BCP can eliminate heavy metals from the environment [10]. To increase the efectiveness of the BCP process, multiple researchers have isolated metal-tolerant bacteria with enzymatic capabilities from diverse habitats. Heavy metal toxicity impacts the bacterial growth and efciency of this process [154]. By substituting appropriate divalent cations (such as Ca 2+ ) in the calcite lattice, heavy metals (for example, Pb 2+ ) may be absorbed into calcites and then converted from soluble to insoluble forms, as shown in the following equation [27,153]: Several researchers have reported the capacity of BCP for heavy metal remediation in the environment and few published bioremediations via BCP are given in Table 6.
Te strontium (Sr) is toxic to human health. For healthy adults, the level of Sr in soil up to 240 mg/kg is regarded as less hazardous and beyond this limit is highly risky. Sr can be transported and deposited through the wet deposition method in oil, coal, soil, and rocks as a form of isotopes 84 Sr, 86 Sr, 87 Sr, and 88 Sr as a result of anthropogenic and natural activities that release Sr into the atmosphere. Tis form of Sr prevents its atoms from decomposing naturally during biodegradation or hydrolysis [176]. Terefore, an alternative method such as BCP involves cleaning up the radionuclides safely from the environment and soil. Tis method stimulates bacteria to promote calcite precipitation, which in turn leads to promoting coprecipitation of radionuclides by substitution of Ca 2+ and formation of radionuclide carbonate minerals [177]. A few studies were conducted on strontium 90 ( 90 Sr). In living organisms, 90 Sr is soluble and highly toxic due to its long half-life (28.8 yrs). It can be readily passed through the food chain from contaminated soil or water [140]. Te mobility and carcinogenic efects of 90 Sr 2+ afect groundwater usability [178], and available conventional remediation techniques are both expensive and inefective [179]. A few examples of strontium-tolerant bacteria reported having the capability of BCP for the removal of 90 Sr in the environment are Sporosarcina pasteurii WJ2 [180], Halomonas spp. SR4 [180,181], and S. pasteurii : [147], (9): [148], (10): [149], (11): [150], and (12): [151]. [98]. Te strontium carbonate precipitation ( 90 SrCO 3 ↓) caused 90 Sr to replace Ca 2+ in the calcite crystal, stopping the spread of radioactive contamination [30].

Calcium Ions Polychlorinated Biphenyl Contaminants.
Polychlorinated biphenyls (PCBs)-containing oils that leak from equipment are a serious threat to both the environment and public health. Te successful methods for eliminating PCB-contaminated oil include solvent cleaning, hydroblasting, or sandblasting followed by encapsulation in epoxy coatings. However, these procedures are inefective since the oil keeps resurfacing [3,182]. Calcite precipitation, on the other hand, can form a layer capable of sealing PCB-contaminated regionsm as shown in Figure 7(a). Indeed, the processcovered regions revealed no leaching and a 1-5-order-ofmagnitude reduction in permeability [182]. High Ca 2+ concentrations (0.5-1.5 g/L) in industrial wastewater are problematic due to the formation of calcium, such as carbonate, phosphate, and/or orgypsum, which can cause scaling in pipes, boilers, and heat exchangers, as well as malfunctions in the anaerobic and aerobic reactors [21]. Te use of BCP for calcium removal from industrial efuents has been the subject of various investigations. Te BCP is a promising approach for eliminating inorganic pollutants from the environment. According to reports, alkalinity produced by bacteria in a normal upfow anaerobic sludge bed reactor was employed in a fuidizer sand bed calcifcation reactor to remove calcium from industrial efuents. Te biocatalytic calcifcation reactor was used in this method, where about 85-90% of the Ca 2+ precipitated as calcium carbonate and was efciently removed by sedimentation in the treatment reactor [183]. As a result, this provides an efcient, environmentally friendly, and easy approach to remove calcium from industrial efuents.

Dust Suppression.
Dust is harmful to humans and has historically been repressed by chemical application or watering, which may be challenging to control and may employ ecologically hazardous chemicals. BCP can also be used to suppress dust [32], as shown in Figure 7(b). Dust has harmful efects on the air quality, and various strategies have been used to lower dust production. Te techniques include the use of dust suppressants, water spraying, and the provision of windshield walls to prevent dust emission [184]. For a maximum of 4 h, water spraying can be used to manage dust. Since water only provides a temporary fx and requires frequent application, using it to reduce dust depletes water supplies [185]. Te chemically activated dust suppressants are also quite caustic and harmful to the environment. Te demand for environmentally friendly and sustainable dust management methods is high [184].

Limitation of Bacteria-Induced and Enzyme-Induced Calcite Precipitation
Although the BCP method has numerous advantages, an additional research is required to address the difculties associated with sustaining bacterial cultivation, environmental concerns, soil type, and consistent treatment of the soil mass before it is commercialized. Bacterial processes are often slower, much more complicated than chemical methods, and require expertise [147]. Considering that the aforementioned restriction does not apply to the ECP approach, so the ECP could be an alternative option. Te bacterial activity is entirely reliant on the parameters such as pH, temperature, the concentration of electron donors and acceptors, and the concentration and difusion rates of nutrients and metabolites. Furthermore, the economic challenges of using laboratory-grade nutrients in the potential application must be resolved. Secondary compounds generated during this procedure may be harmful and hazardous to both humans and soil microbes in high quantities, implying that BCP is not environmentally benefcial. Te treatment cost is also the major challenges faced by ECP, which is up to 98% [103]. However, groundwater pollution with Cl − and CaCO 3 precipitated that result from soil treatment with BCP/ECP may also increase the risk to the nearby water and air due to pH increase [71]. Te BCP maintains bacterial cultivation in the soils, which could require the approval from the relevant authorities. A regular inspection is required to check not to provide a threat to the environment by bacteria and bacterial metabolites to the people and animals. However, the ECP approach degrades with time, it may not have a long-term efect on the soil strength improvement as well as environment [186]. Te BCP method is also limited to the subsoil, and other areas of the soil may not be conducive to the bacterial growth. Bacterial cultivation was difcult in fne soil pores due to the bacterial size, but in the ECP method enzymes can easily accommodate up to 0.5 μm pore size soils. Due to these restrictions, alternative methods of calcite precipitation are sought after, and ECP turns out to be the best replacement to the BCP process. Te use of bacteria culture and storage for soil treatment requires a specifc environment in the soil mass [187]. As a result, further research to enhance the technique and decrease the undesirable byproducts is required before BCP may be used commercially.

Future Perspective
Te bacterial calcite precipitation has recently received much interest. Numerous patents have been submitted in the felds of biodeposition (Europe), concrete remediation (USA), biomineralization (China), and high-strength manufacture (Australia). One of the key problems for calcite precipitation technology is feld implementation; hence, converting the controlled laboratory work into feld applications is required to gain signifcant benefts from those patents. Te BCP is unquestionably an environmentally favorable method; therefore, the establishment of biostabilization techniques has shown to be sustainable, durable, and efcient in treating the soil. Tis has boosted the geotechnical performance of soil, such as reduction in porosity and permeability, improvement bearing capacity, control erosion, seepage, and liquefaction, slope stabilization, and contaminant remediation. However, the cost of production is now expensive. For example, the activity of bacteria is determined by various environmental variables such as pH, temperature, electron donor, acceptor concentration, nutrients, metabolite concentration, and difusion rate. Additionally, there are fnancial barriers that must be eliminated to use the laboratory-grade nutrient sources in feld applications. ECP would therefore be a preferable choice because it does not require technical expertise. Future research should focus on stabilizing the various enzymes that precipitate calcite. Biocementation can be the basis for novel approaches to geotechnical challenges. Introducing bacterial solutions into soil, ground, and porous rocks can lead to the formation of calcite, which holds the particles together. As a consequence, it represents a potential technique for ground improvement, tunneling, home underpinning, and other applications. Experts in the Netherlands (Deltares) are exploring various applications for ureolytic bacteria, such as highway construction and strengthening the ground beneath structures that may liquefy during earthquakes. With the aid of biocementation, a geotechnical revolution is being launched. A penetration depth of several meters is necessary to enable the commercial application of biocementation. Furthermore, the ability to control the degree of strength development, the low fow rate of injecting the various required solutions and bacteria in situ, the low cost, particularly by enhancing the bacteria from the natural ecosystem, the reuse of bacterial cells, relatively homogeneous cementation and permeability retention are required.

Conclusions
A bacterial enzyme, a suitable substrate, and divalent cations (Ca 2+ ) are used in the basic mechanism of bacterial-induced calcite precipitation (BCP) to attract negatively charged ions and precipitate calcite on the bacterial surface. Due to diferent aspects, BCP has drawn attention to self-healing technologies in the felds of geotechnical engineering, earth science, and building materials. Photosynthesis, sulfate reduction, methane oxidation, the nitrogen cycle, nitrate reduction, and ureolysis are the principal bacterial pathways believed to be involved in calcite precipitation via various methods. Te molecular pathways involved in calcite precipitation by urease (NH₂−CO−NH₂⟶CO 2 + NH 3 ) have been illustrated by the most bacteria, including S. pasteurii, B. subtilis, and P. putida. Bacteria that have negatively charged surfaces, bind to divalent cations or trivalent ions in the solution to act as a nucleation site for the minerals' precipitation. Among the bacteria mentioned above, S. pasteurii has the highest urease activity (K m : 26.2 mM) and precipitation rate. Although the use of BCP via urease is common, there are some drawbacks to this process, including the production of NH 3 , which is toxic and should be eliminated, and an elevated NH 3 will also increase the risk of steel corrosion. BCP via carbonic anhydrase (α-, β-, and c-) is, therefore, a preferred choice because of the fast catalytic rate and nontoxic end products for both humans and the environment. Similarly, the enzyme-induced calcite precipitation (ECP) solves the problem that bacteria (0.5-3 μm) are not being able to survive in soil pores smaller than 0.5 μm that stabilize the soil by precipitating calcite using enzymes and without the usage of bacteria. Binders, soil stability and strengthening, innovative construction materials, dust suppression, subsurface barriers, impermeable crusts, remediated soil contaminants, polychlorinated biphenyl calcium ions, and CO 2 sequestration are some examples of the newly established BCP applications. Te BCP methods improve the soil strength (43-930 kPa) and permeability decreases by 21-99%. Due to their higher CO 2 footprint, the conventional soil improvement technologies are expensive or environmentally hazardous. Terefore, BCP is an economical, ecofriendly, and long-lasting substitute for the conventional soil treatment procedures. Even though the BCP method has many benefts, more research is needed before it can be commercialized in order to address the challenges involved in maintaining bacterial cultivation, environmental concerns, soil type, and the pore size (0.5 μm), and consistent treatment of soil mass. Additionally, the bacterial processes are slower, more complex, require more expertise, and produce secondary compounds that could be harmful; therefore, those key problems should be addressed properly before the feld implementation. Since the ECP approach deteriorates over time, and there is a longterm impact on the improvement of soil strength, the treatment costs can be as high (up to 98%). A collaborative research efort that involves feld demonstrations, modeling, and explanation of the underlying mechanics of BCP has profted from and will continue to assist in the transfer of BCP-based innovations from the research laboratory to the feld.

Data Availability
Te data used in the fndings of this study are available from the corresponding author upon request.