Research and Application of Green Technology Based on Microbially Induced Carbonate Precipitation (MICP) in Mining: A Review

Abstract

Microbially induced carbonate precipitation (MICP), as an eco-friendly biomineralization technology, has opened up an innovative path for the green and low-carbon development of the mining industry. Unlike conventional methods, its in situ solidification minimizes environmental disturbances and reduces carbon emissions during construction. This article reviews the research on MICP technology in various scenarios within the mining industry, summarizes the key factors influencing the application of MICP, and proposes a future research direction to fill the gap of the lack of systematic guidance for the application of MICP in this field. Specifically, it elaborates on the solidification mechanism of MICP and its current application in the solidification and storage of tailings, heavy metal immobilization, waste resource utilization, carbon sequestration, and field-scale deployment, establishing a technical foundation for broader implementation in the mining sector. Key influencing factors that affect the solidification effect of MICP are discussed, along with critical engineering challenges such as the attenuation of microbial activity and the low uniformity of calcium carbonate precipitation under extreme conditions. Proposed solutions include environmentally responsive self-healing technologies (the stimulus-responsive properties of the carriers extend the survival window of microorganisms), a one-phase low-pH injection method (when the pH = 5, the delay time for CaCO₃ to appear is 1.5 h), and the incorporation of auxiliary additives (the auxiliary additives provided more adsorption sites for microorganisms). Future research should focus on in situ real-time monitoring of systems integrated with deep learning, systematic mineralization evaluation standard system, and urea-free mineralization pathways under special conditions. Through interdisciplinary collaboration, MICP offers significant potential for integrated scientific and engineering solutions in mine waste solidification and sustainable resource utilization.

1. Introduction

With the rapid development of technology and industrialization, global demand for natural resources has exhibited exponential growth. This large-scale mineral resource exploitation is accompanied by high energy consumption and high carbon emissions, while generating a large amount of solid waste such as tailings, coal gangue, and smelting slag [1,2]—posing a series of environmental and safety problems [3]. Exploring green and low-carbon technologies to handle solid waste is of great significance [4,5]. Microbially induced carbonate precipitation (MICP) technology, with its environmentally friendly solidification mechanism, demonstrates unique advantages in terms of environmental compatibility, process controllability, and green low-carbon characteristics [6,7]. This technology directs the metabolism and chemical reactions of microorganisms to drive the participation of calcium sources and urea in the mineralization process, ultimately achieving the targeted precipitation of calcium carbonate (CaCO₃). In mining area management, the engineering efficacy is manifested through two mechanisms: the carbonate precipitation pore-filling effect and the particle-bridging effect. These mechanisms significantly enhance the structural compactness, impermeability, and stability of tailings sand [8], while inhibiting the geochemical migration of heavy metals and the particle diffusion induced by wind erosion and dust [9,10]. Concurrently, microbial metabolism facilitates carbon dioxide (CO₂) sequestration through carbonation reactions. However, significant gaps remain in current research on the application of MICP technology in the mining industry; adaptability to multiple scenarios has not been systematically summarized, key influencing factors lack thorough analysis, and future development directions have yet to be clearly defined. Thus, this paper conducts a systematic review of MICP applications in mining, addressing the absence of systematic guidance for multi-scenario applications in this field.

As a versatile bio-cementation method, MICP technology exhibits unique engineering advantages for safe waste disposal and resource utilization. Microbial-induced calcium carbonate solidifies tailings sand, enhancing the mechanical properties of tailings dams. The biological mineralized cement layer formed by the consolidation of calcium carbonate dust has remarkable wind erosion resistance and water permeability resistance [11,12]. For the ecological restoration of mined-out areas, MICP-modified tailings slurry is injected through pipelines to fill the void areas, enabling long-term immobilization of heavy metals such as lead and zinc [13]. The particle size distribution of tailings sand closely resembles that of concrete sand. When MICP-modified tailings are incorporated into concrete, they form sustainable building materials, reducing CO₂ emissions during concrete production, and also consume a large amount of solid waste. The MICP–coal gangue composite material serves as a building material, solving the treatment challenges associated with coal gangue and providing a new approach for the large-scale treatment of solid waste [14,15].

In carbon sequestration, MICP technology facilitates the transformation of CO₂ from air and industrial flue gas into CaCO₃ precipitation through microbially induced mineralization and carbonation [16]. For waste resource utilization, MICP solidifies solid waste to produce bio-cement. Additionally, mining-process-derived waste powder and sand are processed into recycled aggregates or roadbed materials, thereby reducing non-renewable resource extraction and advancing resource circularity [17,18]. In heavy metal immobilization, studies have indicated that a microorganisms achieve simultaneous immobilization of heavy metals in multiple valence states through the adsorption mechanism of extracellular polymers and the co-precipitation mechanism with carbonates [19]. Under the optimal conditions (25 °C, pH 7.0, 1.0 g CaCl₂), Cr(III) can be effectively removed, with a removal rate close to 100% [20]. Compared with conventional remediation methods, MICP employs in situ solidification to avoid disturbance of the waste materials and reduces the carbon emission intensity of construction [21,22]. Its carbonate crystal structure immobilizes multiple valence states of heavy metals. Currently, MICP has been successfully applied in engineering fields such as slope reinforcement [23], tailings sealing [24], desertification control [25], concrete repair [26], and heavy metal pollution control [27].

In response to the pressing demands for safe waste disposal and resource utilization in mining areas, MICP technology, as a representative of eco-friendly microbial mineralization technology, demonstrates significant potential for green and low-carbon environmental governance of mining areas due to its distinctive biochemical mechanisms, as illustrated in Figure 1. Based on a comprehensive analysis of 157 peer-reviewed publications (retrieved from Web of Science Core Collection, Scopus, Science Direct, SpringerLink, and CNKI using keywords “MICP”, “mining waste”, “tailings”, and “heavy metals”, with low-relevance exclusions), this study systematically analyzed the solidification mechanisms of MICP, as well as its laboratory research, field applications research, key influencing factors, and improvement methods for promotion and application in relevant scenarios of the mining area. The bottlenecks encountered in the scaling application of this technology in the management of mining area wastes were discussed, as well as the future research directions. This study is intended to provide a theoretical framework and technical roadmap for the scaling application of MICP technology in the efficient immobilization of waste and resource utilization.

2. Solidification Mechanism

The research group led by Whiffin at Murdoch University, Australia, introduced MICP technology for the solidification of loose sand particles, and after biological cementation treatment, the strength of the sand increased to eight times that of unsolidified sand [33]. The core mechanism relies on microbial metabolic activities and chemical reactions. Microbially secreted urease catalyzes the mineralization process, while facilitating the precipitation of CaCO₃ within soil pore spaces through reaction pathways such as urea hydrolysis. Primary, MICP pathways include urea hydrolysis, denitrification [34], iron reduction [35], and sulfate reduction [36], as shown in Figure 2. Among these, urea hydrolysis is the most widely adopted due to its simplicity, fast reaction kinetics, and high solidification efficiency [37]. In the urea hydrolysis pathway, urease produced by bacteria is used to decompose urea into CO₃²⁻ and NH₄⁺. At the same time, the pH of the solution increases as a result of ammonium release. Under alkaline conditions, CO₃²⁻ combines with Ca²⁺ on the cell surface to form CaCO₃ precipitation. CaCO₃ adheres to the surface of tailings sand particles and fills the interparticle voids.

While current research on MICP predominantly focuses on oxygen-rich surface environments, in practical engineering scenarios such as subsurface heavy metal immobilization and mine backfilling for ecological restoration, the challenge of achieving microbial mineralization under anoxic conditions remains a critical concern. Studies have demonstrated that under anoxic conditions, the MICP pathway based on denitrification yields greater CaCO₃ precipitation compared to other pathways [38]. Denitrifying bacteria, being anaerobic, reduce NO₃⁻ into N₂ under anoxic conditions. This process also releases alkaline metabolic byproducts, resulting in an elevated pH environment that facilitates the combination of CO₃²⁻ and Ca²⁺ in the solution to form CaCO₃ precipitates [39]. From an environmental engineering perspective, the denitrification pathway offers distinct advantages over urea hydrolysis. The N₂ produced by the denitrification reaction is a major component of air. In contrast, the ammonia nitrogen by-product generated by the urea hydrolysis reaction is prone to causing ammonia nitrogen pollution. Jin et al. [34] utilized denitrifying bacteria to solidify tailings in an anoxic environment, achieving a uniaxial compressive strength (UCS)value of 1.01 MPa. Under aerobic conditions, bio-solidified samples exhibited UCS values ranging from 0.5 to 1.2 MPa. This pathway provides effective technical support for underground engineering, having achieved a comparable or superior solidification effect relative to urea hydrolysis while mitigating the risk of ammonia nitrogen pollution risks.

Figure 2. Different pathways of MICP reaction [40].

Figure 2. Different pathways of MICP reaction [40].

Beyond urea hydrolysis and denitrification, MICP technology can also utilize alternative microbial metabolic pathways such as sulfate reduction and iron reduction to achieve the solidification of waste materials. In an anaerobic environment, sulfate-reducing bacteria gradually decompose sulfate into S²⁻ and CO₃²⁻, and then CO₃²⁻ combines with Ca²⁺ to form CaCO₃ precipitation. The sulfate reduction pathway is particularly suitable for solidifying acidic waste and immobilizing heavy metal contaminants. However, its by-product H₂S is toxic and corrosive and therefore requires treatment, which complicates in situ immobilization applications. In contrast, iron-reducing bacteria reduce Fe³⁺ to Fe²⁺ through heterotrophic iron reduction. This process consumes H⁺ during electron transfer, leading to environmental alkalization and inducing CaCO₃ precipitation. Meanwhile, the Fe²⁺ generated during the iron reduction reaction can form FeCO₃ to enhance the solidification effect, giving this pathway a unique advantage in contaminant solidification. However, the sulfate reduction pathway carries the risk of secondary sulfide pollution, while the effectiveness of iron reduction pathway is limited by the low migration efficiency of Fe³⁺ in porous media. These two issues remain major bottlenecks, which restrict their widespread engineering applications.

3. Laboratory Research

3.1. Tailings Solidification and Storage

Microbially induced CaCO₃ precipitation can effectively fill the voids between tailings particles and form bridges between adjacent tailings sand particles, thereby significantly improving the stability of tailings dams and enhancing the interparticle cohesion. Simultaneously, under the alkaline conditions generated by microbial metabolism, CO₃²⁻ reacts with heavy metal ions present in the tailings to form metal carbonate precipitates. This process immobilizes the heavy metals in crystalline form within the tailing’s matrix, substantially reducing their leaching and migration risk. He et al. [41] applied MICP–CaO coupling technology to solidify copper–nickel tailings and to passivate heavy metals. Their research found that calcite solidified the tailing particles, reducing the permeability of the tailings. The passivation rates of Cu and Ni reached 78.8% and 78.1%, respectively. The MICP technology can simultaneously immobilize both the tailings and the heavy metals within the tailings, including metal ions such as Fe³⁺, Zn²⁺, Pb²⁺, Cu²⁺, and Cd²⁺. The UCS of the lead–zinc tailings after solidification reached a maximum of 0.93 MPa [42]. Niu et al. [29] demonstrated that the maximum compressive strength of the MICP-treated uranium mill tailings reached 1680.1 kPa (Figure 3a). The sedimentation blocked the pores, and a large amount of U(VI) and Rn were sealed inside the solidified body, reducing the pollution of harmful elements to the environment. Moreover, they discovered that after adding metakaolin and Bacillus subtilis, the solidification cycle of MICP was shortened by 50%, and the fixation rates of U and Rn were increased. This was because the pore volume and surface area of the uranium tailings decreased, resulting in more sedimentation, which enhanced the ability of MICP to solidify the tailings and seal heavy metals. These studies indicate that MICP technology and its modified approaches can effectively solidify various types of tailings, significantly enhance their mechanical properties and impermeability, and efficiently immobilize multiple heavy metal ions, demonstrating its promising potential in tailings management and resource utilization.

MICP offers a novel approach for recovering rare earth resources from ion-adsorption-type leaching waste solutions and treating rare earth tailings, demonstrating broad application prospects. Wang et al. [43] discovered that rare earth ions are immobilized through carbonate precipitation. Their study elucidated the interaction mechanism between metal ions in solution and microbial surfaces and further demonstrated that the addition of Ca²⁺ during the MICP process enhances urease activity and the removal efficiency of La³⁺. Currently, due to limitations in extraction technologies, leachates and tailings derived from rare earth mining often retain residual rare earth elements and heavy metal ions. Li et al. [44] found that the indigenous microbial strain U1 effectively adsorbed rare earth elements in mine wastewater (pH = 4) and induces calcite precipitation to co-immobilize these elements. After MICP treatment, over 99% of rare earth elements in the leachate were recovered, while a significant portion of heavy metal ions were simultaneously immobilized. In the field of tailings treatment, MICP technology is used to immobilize rare earth tailings, thereby mitigating the migration risks of toxic elements into surrounding environments and enabling future resource recovery. Using strain K-1, isolated from rare earth tailings, they achieved significant improvement in the mechanical strength of the treated tailings (as shown in Figure 3b) with a maximum compressive strength of 1.73 MPa and minimal loss of rare earth content before and after solidification [27]. Wang et al. [45] enhanced the original solidification technology and proposed a more economical method. They utilized carbonic anhydrase bacteria and endogenous calcium ions to solidify rare earth slags and pollutants. This approach reduced ammonium nitrogen by-products during mineralization and lowered costs. Microscopic analysis revealed that CaCO₃ grew and aggregated in the slag’s pores, thereby cementing the mineralized particles, enhancing strength and erosion resistance, and inhibiting the leaching of hazardous elements such as Cu²⁺, Pb²⁺, and U(VI).

The MICP technology has made significant progress in the treatment of tailings. Microorganisms induce the precipitation of CaCO₃, which fills the gaps between the particles of tailings, thereby improving the compressive strength and structural stability of the tailings. In the microbially induced alkaline environment, CO₃²⁻ ions react with heavy metal ions to form stable precipitates, effectively reducing their migration risks. This approach provides a new approach for rare earth tailings management and resource recovery—not only reinforcing the physical integrity of tailings and preventing the migration of Th and U, but also immobilizing residual heavy metals such as Pb, Cd, Cu, and Zn, and facilitating the efficient recovery of rare earth elements from leachates.

3.2. Heavy Metal Immobilization

Primary mechanisms for immobilizing heavy metals in soil include the formation of metal carbonate precipitates through the reaction of metal ions with CO₃²⁻, as well as the adsorption and coprecipitation of heavy metals by CaCO₃. The MICP technology is applied to contaminated soils, utilizing microbially induced carbonate precipitation to immobilize Cd and Pb. The results showed that after MICP treatment, Cd and Pb in the soil mainly existed as CdCO₃ and PbCO₃, and their extractable decreased by 52.6% and 41.77%, respectively [46]. Research found that the minimum inhibitory concentration (MIC) of Cd for Sporosarcina pasteurii is between 0.03 and 0.06 mM [47]. Once the concentration exceeds the MIC, the bacteria form spores that can germinate in the presence of nutrients, and the resulting bacteria can then immobilize Cd in the contaminated farmland through mineralization reactions. Ji et al. [48] found that after MICP treatment, the exchangeable Cd and Zn in the urban sludge-amended soil decreased by 31.02% and 6.09%, respectively, while the carbonate-bound fractions of Cd and Zn increased by 6.63% and 13.09%, respectively. As shown in

Table 1. Removal rate of metal in MICP-treated AMD with Sporosarcina sp. UB5 [49].

Parameters	Detection Limit	Control (mg/L)	T1 (AMD–Urea Sporosarcina sp. UB5)	Removal Rate (%)
Ag	0.025	0.038 ± 0.002	<0.025	100
Al	0.025	19.166 ± 1.144	0.222 ± 0.007	99
As	0.01	0.163 ± 0.018	<0.01	100
Ca	0.05	1546.222 ± 76.036	352.500 ± 10.135	77
Cd	0.005	1.236 ± 0.032	0.030 ± 0.001	98
Co	0.025	0.034 ± 0.00	<0.025	100
Cr	0.025	<0.025	Non detected	-
Cu	0.025	1.917 ± 0.083	0.300 ± 0.005	84
Fe	0.025	148.056 ± 5.245	0.078 ± 0.035	99
Zn	0.025	90.200 ± 8.479	0.848 ± 0.007	99
Mn	0.025	13.770 ± 0.431	4.560 ± 0.0094	67
Pb	0.025	0.042 ± 0.005	Non detected	100

, by using the strains isolated from tailings to immobilize the toxic metals such as Cu, Fe, and Cd in acidic mine water, the removal rate of these metals exceeded 67% [49]. Sujiritha et al. [50] revealed that Arthrobacter creatinolyticus could effectively immobilize Cr(VI) in contaminated soil, and further concluded from aqueous solution experiments that 82.21% of chromium in the water solution was coprecipitated with CaCO₃.

During the process of immobilizing heavy metals in soil using MICP, urease decomposes urea to produce by-products such as ammonium ions and ammonia, which pose risks of secondary environmental pollution [51]. Ammonia can combine with metal ions to form soluble complexes, thereby interfering with the efficiency of heavy metal immobilization. Liu et al. [52] demonstrated that a higher molar ratio of urea to calcium source better promotes the immobilization of cadmium in soil and significantly reduces the exchangeable Cd content. To mitigate the environmental hazards caused by the by-products of the MICP reaction, Chen et al. [53] used sophorolipid-assisted MICP to prepare bio-CaCO₃ for immobilizing heavy metals in contaminated soil (Figure 4a). Their study showed that after MICP remediation, the contents of Pb and Cd in the soil decreased by 41.23% and 35%, respectively, and the vaterite-type biological CaCO₃ exhibited minimal impact on the pH of the remediated soil. Thise sophorolipid-amended MICP process developed by Chen et al. not only reduced by-product generation during mineralization, but also offered a new path for heavy metal immobilization in soil, effectively immobilizing the contaminants while minimizing environmental hazards.

MICP immobilizes heavy metals in wastewater through biomineralization. Microorganisms produce urease to hydrolyze urea into CO₃^2- and NH₄⁺. Metal ions are initially adsorbed onto microbial surfaces via electrostatic interactions and complexation with surface functional groups. These adsorbed ions subsequently react with CO₃²⁻ to form carbonate precipitates, facilitating the efficient removal of heavy metals. Feng et al. [54] demonstrated that MICP treatment of high-calcium papermaking wastewater achieved 98.3% calcium ion removal. Bacillus subtilis adsorbed free Ca²⁺ to induce CaCO₃ precipitation, and subsequent crystal growth forming microscale aggregates.

Figure 4. (a) Schematic diagram of MICP for immobilizing heavy metals in soil [53]. (b) Schematic diagram of MICP treatment of heavy metals in wastewater [55].

Figure 4. (a) Schematic diagram of MICP for immobilizing heavy metals in soil [53]. (b) Schematic diagram of MICP treatment of heavy metals in wastewater [55].

In addition, urea hydrolysis generated NH₄⁺, thereby providing a nitrogen source for the subsequent anaerobic treatment of the wastewater. Wang et al. [55] demonstrated that salt-tolerant strains achieved high contaminant removal efficiencies in wastewater treatment (Figure 4b), with nitrate, Ca²⁺, bisphenol A, and phosphorus removal rates of 94.47%, 83.04%, 100%, and 98.87%, respectively. Their research revealed a dual effect of bisphenol A; while inhibiting microbial urease production and altered CaCO₃ crystal structures, it simultaneously stimulated increased secretion of extracellular polymeric substances (EPS). Such EPS overproduction potentially enhanced metal ion immobilization efficiency through improved microbial biosorption capacity. Xue et al. [56] confirmed that urea hydrolysis rate and environmental pH critically govern Cu²⁺ immobilization efficiency. Both excessive and insufficient urea hydrolysis reduced the Cu²⁺ removal efficiency, while elevated pH promoted the formation of soluble copper-ammonia complexes, thereby increasing Cu²⁺ mobility and decreasing immobilization efficiency. Song et al. [57] applied MICP technology to treat Cd²⁺-contaminated wastewater, where cadmium was coprecipitated with CaCO₃. When the concentration of Cd²⁺ was less than 0.5 mmol/L, the removal rate of Cd²⁺ in the wastewater reached 90%. In contrast, when the concentration of Cd²⁺ exceeded 0.5 mmol/L, high Cd²⁺ concentrations inhibited microbial activity and reduced immobilization efficiency. To reduce secondary pollution from ammonia nitrogen byproducts, Song et al. [58] employed the strain UN-1 to immobilize Pb in wastewater via MICP technology. UN-1 possesses the ability to metabolize and transform ammonia nitrogen byproducts. Microscopic analysis revealed that lead was converted into carbonate precipitates, while the ammonia nitrogen concentration remained relatively low, thereby reducing secondary environmental pollution from mineralization byproducts. Collectively, these advancements demonstrate that MICP technology effectively immobilizes heavy metals in contaminated wastewater through biologically induced carbonate precipitation. By converting metal ions into stable mineral forms, MICP not only ensures the immobilization of heavy metals, but also enables the reuse of treated wastewater for industrial or agricultural irrigation, provided that residual metal concentrations meet regulatory standards. The NH₄⁺ formed during the mineralization process can promote plant growth, opening up a new path for the construction of a sustainable model for waste management and resource recycling.

3.3. Waste Resource Utilization

Solid waste materials such as coal gangue, tailings, and waste concrete powder are often stockpiled due to inherent limitations, including low mechanical strength and structural instability. MICP offers an effective solution to these issues by inducing CaCO₃ precipitation, which enhances compressive strength and reduces permeability through pore filling and particle bonding. The resulting bio-cement can serve as a sustainable construction material for road embankments and can effectively deal with underground goaf in mines, achieving the resource utilization of tailings [59]. Dhriyan et al. [60] demonstrated the feasibility of stabilizing pond ash using MICP. CaCO₃ precipitation fills material pores, significantly reducing permeability and enhancing geotechnical stability. The compressive strength of bio-cemented fly ash reached 1105 kPa, meeting the standards for use as embankment materials, thereby mitigating environmental pollution from fly ash stockpiling. This establishes a viable treatment approach for coal combustion by-products. MICP technology can be used to reduce coal dust pollution, employing calcite precipitation to create clogged interparticle pores, forming a hardened layer. After treatment, wind erosion resistance reached 84% [61]. Crucially, the calorific value of the CaCO₃ and coal mixture remained comparable to raw coal, confirming minimal impact on fuel performance. Holeček et al. [62] treated waste concrete fines using MICP, achieving a 15% reduction in porosity along with significant improvements in mechanical strength and durability. The findings demonstrate the potential for recycled concrete powder to be a sustainable construction aggregate. Zhuang et al. [17] used MICP to strengthen recycled coarse aggregates from construction waste sand and powder. Microbially induced calcite crystals filled cracks and pores, reducing the values of moisture content and crushing index (Figure 5). The waste eggshells and fan shells are efficient Ca²⁺ sources for MICP, and when used as calcium sources, they produced 1.4 gm and 1.7 gm of carbonate precipitation, respectively [63]. Furthermore, the cohesion of the fine particles in the waste concrete treated by MICP has increased, and the treated waste concrete can be used as a solid building material [64]. In summary, MICP enables the effective recycling of industrial solid wastes—such as coal gangue, fly ash, and concrete fines—into bio-cemented materials or construction aggregates. This not only conserves natural resources but also contributes to environmentally sustainable waste management in mining engineering and construction engineering.

3.4. Carbon Sequestration

The carbon sequestration potential of MICP stems from the biomineralization process, in which atmospheric CO₂ is assimilated and converted into CO₃²⁻ ions. These carbonate ions subsequently react with calcium ions to form stable CaCO₃ precipitates, thereby achieving long-term CO₂ sequestration. Chen et al. [31] proposed a new method for underground mining to absorb CO₂. Bacteria capture CO₂ from the air and generate carbonate through microbially induced carbonation reactions. Figure 6 uses the average value of formed calcite to estimate the carbon capture capacity. By comparison, C9 specimens demonstrate the highest precipitated carbonate amount of 8.7%, approximately 74% higher than reference samples (C0). This suggests that calcium nitrate was not able to further increase the carbon uptake of the concrete with the addition of bacteria. Cui et al. [65] applied MICP carbonation to treat municipal solid waste incineration fly ash. The carbonation process both enhanced CO₂ uptake and improved matrix strength through the formation of pore-filling CaCO₃, while reducing cement dependency to lower costs and achieve carbon sequestration. The synergistic effect of MICP technology is manifested in its multiphase sealing functions. While it seals CO₂, heavy metal ions combine with CO₃²⁻ to form heavy metal carbonates, thereby preventing their environmental migration. Bhukya et al. [66] demonstrated the effectiveness of MICP in subsurface CO₂ containment. MICP significantly reduced CO₂ leakage rates through abandoned wells by forming a mineralized barrier in deep aquifers. Wang et al. [67] further revealed the carbon sequestration benefits of MICP in wastewater treatment, where increased carbonate precipitation enhanced the overall carbon capture efficiency of the system. During repair, dissolved CO₂ in the crack pore moisture participates in mineralization, thereby concurrently contributing to carbon capture. Such dual functionality in structural reinforcement and carbon uptake demonstrates the MICP role as a low-carbon engineering approach.

4. Field Applications Research

With the continuous deepening of the fundamental research on MICP technology, its engineering applications are shifting from the laboratory scale to practical implementation in diverse and complex geological environments. Although fundamental research has systematically analyzed the unique advantages of MICP technology, such as improving soil performance and enhancing environmental adaptability, the complexity of field environments still remains a key challenge for technical reliability and large-scale feasibility. This section focuses on representative application scenarios, including mine rock mass reinforcement, aeolian sand stabilization, and the resource utilization of solid waste. By analyzing the multi-scale action mechanism and response patterns of the biomineralization in field experiments, this section provides theoretical support and technical references for large-scale biological solidification in mining.

Duan et al. [24] applied MICP to solidify gold mine tailings (Figure 7). After MICP and MICP–steel slag treatment, the tailings achieved average UCS values of 0.51 MPa and 0.71 MPa, respectively. The leaching decrease rates of Cu, Pb, Cr, and Zn in the tailings treated with MICP–steel slag were all above 70%. Kirkland et al. [68] transported the bacterial solution and the cementation solution to a depth of 698 m underground, applying MICP technology to seal fractures in the surrounding rock and reduce the permeability of the wellbore wall. Cao et al. [69] demonstrated through field experiments that MICP treatment resulted in the formation of a hardened crust on desert sand. When the applied bacterial solution concentration was 75%, soil loss decreased by 51.93% and wind erosion was reduced by 77.27%. Scanning electron microscope (SEM) images show that calcite formed a solidified layer through two deposition modes: pore filling and particle bridging. Long-term monitoring confirmed that the wind erosion resistance and surface durability of the sand dunes were significantly improved after MICP treatment, marking a substantial advancement from laboratory-scale studies to field applications in sand dune stabilization, and providing a new approach for ecological restoration in arid regions. Liu et al. [70] reported that MICP treatment enhanced soil stability and durability, with repeated treatments further improving erosion resistance.

Li et al. [71] prepared two types of cementation solutions, one using seawater and the other using freshwater, and found that slope erosion resistance was significantly improved after microbial treatment. In non-tidal areas, a continuous bio-solidification layer was formed. However, physical scouring caused by tidal action led to diminished solidification effectiveness and a notably shortened service life. SEM images showed that MICP treatment promoted sand particle aggregation and increased surface roughness, effectively protecting slopes from both wind and seawater erosion. Moreover, the seawater-based cementation solution formed a composite carbonate precipitate via the Mg²⁺ doping effect, resulting in significantly lower permeability and higher slope stability compared to the freshwater-based solution [72]. Ghasemi and Montoya [73] used the method shown in Figure 8 to treat the slope. The study showed that after MICP treatment, the impermeability and erosion resistance of the slope were significantly enhanced. In their study, the surface spraying method was proven more suitable for the large-scale MICP treatment of surface soil. Yang et al. [74] demonstrated that after the lead–zinc tailings sand was treated with MICP, the heavy metal ions were effectively immobilized by CaCO₃, and the compressive strength of the treated material reached the required standard for a road embankment material. As MICP technology expands its application from aeolian sand control to slope reinforcement and waste solidification in mining areas, the effectiveness of mineralization reactions is governed primarily by internal factors such as soil properties, urease activity, cementation solution concentration, and cementation solution composition, but also by external environmental factors such as heavy metal ions, oxygen availability, temperature, and pH. Studies have shown that the strength of the solidified body decreases over time, and extreme conditions such as high heavy metal ion concentrations, anaerobiosis, and high temperatures will accelerate the degradation of CaCO₃. Therefore, considering the challenges of long-term waste storage, it is necessary to implement periodic MICP reinforcement treatments to ensure the long-term safe disposal of waste.

5. Influencing Factors

5.1. Solid Waste Characteristics

The ways in which solid waste properties significantly affect the solidification effectiveness of MICP has become a key research focus in the field. Among these properties, permeability, pore structure characteristics, particle size, and particle gradation have been confirmed as key factors determining solidification effectiveness. When there are no soil particles present, a CaCO₃ solidification layer is formed within the solution, which encapsulates bacteria and reduces the urea hydrolysis rate [75]. Conversely, the addition of soil particles provides attachment sites for bacteria, thereby increasing the reaction rate of MICP. Xu et al. [76] found that samples with small particle sizes (0.25–0.5 mm) exhibited the highest bacterial adsorption efficiency at 99.88%, whereas the efficiencies for medium (0.5–1 mm) and larger (1–2 mm) particle sizes were 97.30% and 93.49%, respectively. With increasing surface area, the rate of MICP reactions improved, significantly enhancing the solidification efficiency. The deposition of CaCO₃ in the pore throats reduces the permeability of tailings sand, while the cementing effect between particles enhances interparticle cohesion. This significantly improves the stability, strength, and erosion resistance of tailings sand and prevents the migration of heavy metal ions. Yin et al. [77] reported that effective MICP solidification requires bacteria to pass smoothly through pore throats. When the pore throat diameter approaches or falls below the bacterial cell diameter (typically 0.4–5 μm), it results in uneven colony distribution, thereby reducing the uniformity of CaCO₃ in the waste matrix. For optimal MICP solidification effectiveness, studies indicate an optimal particle size range of 10–1000 μm [78]. Thus, pore throats must exceed the bacterial cell dimensions to ensure uniform microbial migration and distribution in the solid waste. The low porosity of copper tailings limits the depth of mineralization reactions, resulting in significantly lower compressive strength (0.54 MPa) compared to that of beach sand (1.85 MPa) [79]. Cheng et al. [80] observed that under fully saturated conditions, aggregated rhombic crystals formed on particle surfaces, whereas stable calcite formed at low saturation. Calcite formed under low saturation exhibits higher hardness, directly enhancing particle cohesion. Consequently, the lower initial saturation of tailings during MICP treatment promotes stronger interparticle cementing. The degree of tailings saturation governs solidification strength and dam stability by controlling CaCO₃ crystal morphology. Fine particles (<0.075 mm) exhibit low permeability, which impedes bacterial migration through pore throats. This induces surface pore blockage and decreases cementation efficiency [81]. Conversely, coarse particles (>2 mm) contain large pores, which limit bacterial adsorption sites due to reduced particle contact area. During solution injection, rapid seepage decreases bacterial adsorption rates on particle surfaces, ultimately compromising interparticle cohesion and tailings sand stability.

5.2. Bacterial Solution

Bacterial species play a central role in the MICP process, with their metabolic specificity directly determining the diversity of metabolic pathways [34]. Key factors such as bacterial strain, cell concentration, and urease activity significantly influence the MICP reaction rate, the spatial distribution of CaCO₃, and the resulting crystal morphology (including calcite, aragonite, and vaterite, as shown in Figure 9). These parameters ultimately determine bio-solidification effectiveness. The spatial distribution of CaCO₃ strongly correlates with cementation effectiveness in tailings. CaCO₃ filling the pores reduces permeability, and that bridging adjacent particles enhances tailings dam strength and stability, thereby sealing pore throats and immobilizing pollutants. Additionally, Saricicek et al. [82] demonstrated that bacterial consortia (Sporosarcina pasteurii and Bacillus licheniformis) induce the formation of more complex CaCO₃ crystals through synergistic metabolic activity, significantly enhancing solidification effectiveness compared to one-strain treatments.

To advance engineering optimization, researchers utilized in situ screening strategies for functional strain. Native strains exhibit superior environmental adaptability and solidification efficiency compared to exogenous bacterial strains (

Table 2. The influence of bacteria species on the solidification effect.

Bacteria	Test Sample	Solidification Effect	Reference
Bacillus sp. DW015	400 μmol/L Tb (III) Wastewater	The adsorption efficiency of Tb is above 90%.	[86]
Sporosarcina pasteurii		The adsorption efficiency of Tb is above 59.7%.
Lysinibacillus sp. DW018.	400 μmol/L Tb (III) Wastewater	The adsorption efficiency of Tb is above 98%.	[89]
XR1	Fine sandy soil	The peak strength of the solidified sand reached 887 kPa.	[88]
Sporosarcina pasteurii		The peak strength of the solidified sand reached 504 kPa.
Sulfate-reducing bacteria	Lead–zinc tailings sand	The maximum UCS of sample reached 0.22 MPa. It has a better effect in fixing SO42− in the tailings.	[36]
Sporosarcina pasteurii		The maximum UCS of sample reached 0.95 MPa.

). Studies experimentally demonstrated that artificially designed bacterial with complementary functions significantly enhance cementation effectiveness over one-strain treatments [86]. Clarà Saracho et al. [87] discovered that the morphology of CaCO₃ crystals depends on the initial bacterial concentration. The indigenous bacterium strain UN-1 immobilizes cadmium via CdCO₃ precipitation and degrades ammonium salts through coupled biogeochemical processes [28]. Wang et al. [88] demonstrated that MICP treatment with the novel strain XR1# yielded significantly higher CaCO₃ content, compressive strength, and ductility than treatment with Sporosarcina pasteurii. This highlights the potential of regional microbial resource development. Bian et al. [89] found that the efficiency of rare earth ion recovery increased with bacterial concentration (OD₆₀₀ 0.25–1), reaching nearly 100% recovery at OD₆₀₀ was 1 (Table 2). However, excessively high bacterial concentrations accelerate MICP reaction and may induce premature pore clogging, limiting solidification depth. Conversely, low-concentration bacterial solution suffers a loss of CaCO₃ due to repeated solution injection during treatment. This identified optimal bacterial concentration window provides a theoretical foundation for engineering strategies such as the graded-concentration injection approach to enhance treatment efficiency.

5.3. Cementation Solution

As the core reaction medium in MICP technology, the cementation solution supplies both the essential nutrients for bacterial metabolism and the calcium sources required for biomineralization. It regulates the deposition efficiency, distribution, volume, and crystal morphology of CaCO₃ precipitation, while also influencing urease activity. In previous studies, CaCl₂ and calcium acetate were commonly used as calcium sources for MICP reactions, but as calcium sources, they can be costly for large-scale applications. Furthermore, the Cl⁻ produced during the MICP reaction with CaCl₂ easily causes environmental harm [90]. Recent advancements have further expanded the repertoire of sustainable calcium sources, including biomass-derived oyster shells and concentrated seawater. By utilizing the concentrated Ca²⁺ and Mg²⁺ ions in seawater, this approach enables the formation of magnesium-containing carbonates alongside calcite. This method not only maintains solidification efficiency but also further improves the mechanical properties of the bio-solidified soil. Liang et al. [91] emphasized that Ca(CH₃COO)₂ and CaCl₂ significantly improved the shear strength of the samples and that Ca(CH₃COO)₂ performed better as a calcium source in terms of shear strength (Table 3). When using raw clam shells as the calcium source, the UCS and CaCO₃ content of the soil were higher than those when using Ca(NO₃)₂ and CaCl₂ as calcium sources, and the permeability coefficient was lower [92]. Yang et al. used concentrated seawater (with a Ca²⁺ concentration of 0.033 mol/L) as the calcium source, and after sample reinforcement, the maximum UCS was 635 kPa, the maximum CaCO₃ mass in the sample was 6.47 g, and the permeability decreased significantly, demonstrating that concentrated seawater can be a feasible calcium source for the MICP reaction [93].

As illustrated in Figure 10, when using seawater instead of freshwater, the sand columns are more tightly consolidated. This might be because the metal ions in seawater play an important role in transporting and fixing bacteria in the sample, promoting the uniform distribution of calcium carbonate [94], providing a feasible compositional source for the cementation solution in large-scale engineering projects such as MICP solidification of concrete waste sand and tailings. However, metal ions may inhibit bacterial and urease activity, which reduces the efficiency of CaCO₃ formation to some extent. Liu et al. [95] observed that the soluble calcium from calcareous sand served as the calcium source, and yielding a UCS of the solidified body reached a maximum of 2.458 MPa, which was higher than the 2.3 MPa of CaCl₂ (Table 3). Song et al. [96] pointed out that as the concentration of the cementation solution increased, the mass of sequestered CO₂ first increased and then decreased. Within a certain range of cementation solution concentration, the diffusion rate of calcium ions matched the dynamic process of urea hydrolysis, promoting the deposition of CaCO₃ along the solution seepage path, and achieving an optimal MICP solidification effect [97]. However, Zhang et al. [98] demonstrated an inverse relationship between CaCO₃ and cementation solution concentration. At elevated concentrations, MICP reactions proceed rapidly, leading to surface CaCO₃ precipitation and the formation of a dense barrier that inhibits subsequent biomineralization [99]. Conversely, if the concentration of cementation solution is too low, this leads to a decrease in the amount of Ca²⁺ participating in the MICP reaction, reducing the reaction rate and CaCO₃ content and affecting the solidification effect of waste materials such as tailings and coal dust.

Table 3. The influence of cementation solution on the solidification effect.

Bacteria	Cementation Solution	Test Sample	Solidification Effect
Sporosarcina pasteurrii [95]	Urea + CaCl2	Calcareous sand	UCS peak strength 2.3 MPa
	Urea + Soluble calcium		UCS peak strength 2.46 MPa
Sporosarcina pasteurii [100]	Urea + Ca(CH3COO)2	Strongly weathered phyllite	UCS peak strength 6.78 MPa
	Urea + Mg(CH3COO)2		UCS peak strength 6.13 MPa
	Urea + CaCl2		UCS peak strength 7.45 MPa
Sporosarcina pasteurii [101]	Urea + CaCl2	Industrial sand of 200–380 μm	UCS = 27.9 MPa
	Urea + Ca(NO3)2 4(H2O)		UCS = 27.6 MPa
	Urea + Ca(CH3COO)2 H2O		UCS = 26 MPa
Sporosarcina pas-teurii [91]	Urea + CaCl2	Granite residual soil	When the phase pressure is 100 kPa, the shear strength is 80.32 kPa.
	Urea + Ca(CH3COO)2		When the phase pressure is 100 kPa, the shear strength is 102.71 kPa.
	Deionized water		When the phase pressure is 100 kPa, the shear strength is 111.72 kPa.
Sporosarcina pas-teurii [92]	Urea + Raw clam shells	Sand column	The apparent porosity is 9.13%.
	Urea + Ca(NO3)2		The apparent porosity is 9.53%.
	Urea + CaCl2		The apparent porosity is 13.42%.

Table 3. The influence of cementation solution on the solidification effect.

Bacteria	Cementation Solution	Test Sample	Solidification Effect
Sporosarcina pasteurrii [95]	Urea + CaCl2	Calcareous sand	UCS peak strength 2.3 MPa
	Urea + Soluble calcium		UCS peak strength 2.46 MPa
Sporosarcina pasteurii [100]	Urea + Ca(CH3COO)2	Strongly weathered phyllite	UCS peak strength 6.78 MPa
	Urea + Mg(CH3COO)2		UCS peak strength 6.13 MPa
	Urea + CaCl2		UCS peak strength 7.45 MPa
Sporosarcina pasteurii [101]	Urea + CaCl2	Industrial sand of 200–380 μm	UCS = 27.9 MPa
	Urea + Ca(NO3)2 4(H2O)		UCS = 27.6 MPa
	Urea + Ca(CH3COO)2 H2O		UCS = 26 MPa
Sporosarcina pas-teurii [91]	Urea + CaCl2	Granite residual soil	When the phase pressure is 100 kPa, the shear strength is 80.32 kPa.
	Urea + Ca(CH3COO)2		When the phase pressure is 100 kPa, the shear strength is 102.71 kPa.
	Deionized water		When the phase pressure is 100 kPa, the shear strength is 111.72 kPa.
Sporosarcina pas-teurii [92]	Urea + Raw clam shells	Sand column	The apparent porosity is 9.13%.
	Urea + Ca(NO3)2		The apparent porosity is 9.53%.
	Urea + CaCl2		The apparent porosity is 13.42%.

5.4. Grouting Method

The grouting method critically influences MICP solidification of waste and resource utilization. By regulating the flow path and spatial distribution of the solution, it directly determines the spatial distribution characteristics of CaCO₃, which in turn affects the macroscopic mechanical and permeability properties of the soil. The grouting method profoundly affects the retention efficiency of microorganisms, the ion migration path, and the distribution of nucleation sites, thereby influencing the solidification effect. The most commonly used grouting methods include the injection method, surface spraying method, immersion method, and premixing method (Figure 11).

5.4.1. Injection Method

The one-phase injection method mixes the bacterial solution and the cementation solution and injects them simultaneously. This causes CaCO₃ to precipitate immediately, resulting in rapid blockage of the surface pores of the sample and a decrease in the uniformity of CaCO₃ distribution, thereby affecting the solidification effect of the deep soil. Shahrokhi-Shahraki et al. [105] confirmed that when the bacterial solution and the cementation solution are mixed, CaCO₃ precipitation is rapidly generated and blocks the near-surface pores of the sample. To address the uneven distribution of CaCO₃ caused by the one-phase injection method, Whiffin et al. [106] proposed the two-phase injection method, in which the bacterial solution and the cementation solution are injected into the soil in stages. The two-phase injection method eliminates the instantaneous blocking effect of premature CaCO₃ precipitation on the pore throats, and microbial pre-adsorption guides the directional deposition of CaCO₃ [107]. Huang et al. [108] pointed out that in the one-phase injection method, the maximum removal rate of Cd²⁺ was 38.47%, while in the two-phase injection method, the removal rate of Cd²⁺ reached almost 100%. Cheng et al. [109] employed a two-phase injection approach to solidify sand columns and reported a maximum UCS of the sample reached 300 kPa. Cheshomi and Mansouri [110] revealed that under intermittent injection conditions, the distribution of CaCO₃ was more uniform. Du et al. proved that multiple injections of low-concentration cementation solution were helpful for slope reinforcement, whereas excessive injection would cause CaCO₃ to be distributed on the surface. Compared with continuous injection, the intermittent injection of cementation solution significantly increased the calcite content in the deep layer.

Harkes et al. [111] proposed the three-phase injection method based on the two-phase injection method, in which a fixative (0.05 mol/L CaCl₂ solution) is injected before injecting the bacterial solution or cementation solution. The three-phase injection method enhances the friction and adhesion between particles, reducing porosity and increasing the strength of the silt [112]. Notably, Lin et al. [113] treated the samples using the three-phase injection method, and the maximum UCS of the solidified body reached 30 MPa. In the MICP treatment project, increasing the number of injections was shown to improve the uniformity of CaCO₃ distribution, and increase the soil strength to 425 kPa [114]. Yu and Yang [115] demonstrated that with their composite injection strategy, the permeability coefficient of sand decreased from 0.32 cm/s to 4.85×10⁻⁴ cm/s and the UCS increased to 197.96 kPa. Compared with the one-phase injection method and the two-phase injection method, the solidification effect achieved with their composite strategy was significantly improved. Therefore, when the concentration of the bacterial solution and the cementation solution is too high and causes blockage of surface pores, such a composite injection strategy is more effective.

5.4.2. Surface Spraying Method

To address the problems of dust erosion and migration in coal mines, as well as soil erosion and water loss on slopes, biological reinforcement of surface soil has become a key control measure. Liu et al. [32] treated a clay slope model using the surface spraying method, forming a 2.5 ± 0.5 mm thick dense CaCO₃ hardened layer on the surface. SEM analysis showed that CaCO₃ crystals cement the soil particles through particle bridging effects and coating effects (Figure 12). Xie et al. [116] further confirmed the erosion resistance of the microbially solidified body through disintegration tests. The natural soil sample underwent severe disintegration at 5 s, whereas the soil treated by the surface spraying method only showed surface detachment within 960 s. The surface spraying method builds an erosion-resistant hardened layer on coal mine slopes through surface application and solution infiltration. Its core advantages include wind erosion resistance, water erosion resistance, and self-healing ability, potentially achieved through ongoing microbial activity and continuous carbonate precipitation to seal new cracks.

5.4.3. Immersion Method

The immersion method utilizes the capillary seepage effect of the porous mold wall to facilitate the seepage of the bacterial solution and cementation solution into the soil, promoting the uniform distribution of CaCO₃ along the seepage path. Cheng et al. [117] demonstrated that in the sand columns treated using this method, the CaCO₃ content reached up to 17.1% by mass, and the precipitated CaCO₃ effectively filled the particle pores and cemented the sand grains, significantly improving the mechanical properties of the solidified body. This method offers unique advantages, particularly in treating low-permeability tailings.

5.4.4. Premixing Method

The premixing method enables the in situ deposition of CaCO₃ precipitates within the pores of the waste material through the simultaneous mixing and stirring of the bacterial solution, cementation solution, and waste material, thereby significantly enhancing the structural stability. Yasuhara et al. [118] applied this method to solidify sandy soil. After MICP treatment, the permeability coefficient decreased by one order of magnitude to 1.5 × 10⁻³ cm/s, demonstrating that MICP effectively improves the seepage characteristics of the soil. Similarly, Oliveira et al. [119] premixed urease, cementation solution, and soil, and the stiffness of the soil increased by 167.6%. The premixing method demonstrates unique value in the solidification of surface materials, with the formed CaCO₃ hardened layer serving as an anti-erosion layer for tailings dams and slopes. However, the premixing method involves one treatment, which makes it impossible to provide subsequent maintenance. It is therefore mainly applicable to scenarios such as slope reinforcement, reinforcement of mined-out area back-fill, and tailings solidification.

5.5. Temperature

As the thermodynamic driving force for microbial metabolism and crystal nucleation, temperature regulates both bacterial activity and urease activity, thereby determining the reaction rate of MICP and the efficiency of waste solidification. Song et al. [96] reported that when the temperature increased from 35 °C to 55 °C, urease activity decreased and the size of CaCO₃ crystals decreased significantly. At 45 °C, MICP treatment achieved the best reinforcement performance for CO₂ sequestration. This finding aligns with the conclusion of Cheng et al. [120], who observed that both the biological solidification strength and CaCO₃ crystal size were significantly greater at 25 °C than at 50 °C. Peng et al. [121] conducted research and found that within the temperature range of 10–25 °C, the urease activity was positively correlated with the CaCO₃ content. This stems from the increase in temperature, which accelerates the hydrolysis rate of urea, enhances the solidification effect of MICP, and promotes the rate of heavy metal adsorption [122]. By contrast, under high-temperature conditions (70 °C), urease activity initially reaches its peak, but protein denaturation causes the premature termination of the MICP process. At low-temperature, bacterial activity declines, the mineralization cycle is extended, and CaCO₃ tends to distributed along the pore network. In contrast, high temperature accelerates the attenuation of bacterial activity, reduces the rate of urease-mediated urea hydrolysis, and causes the mineralization reaction to prematurely terminate. As shown in Figure 13, as the temperature rises, the unconfined compressive strength of the solidified body decreases [123]. Research highlights the significant influence of temperature on microbial behavior and the solidification performance of MICP, thereby providing a theoretical basis for optimizing MICP-based waste treatment strategies.

5.6. pH

As a critical factor influencing urease activity and CaCO₃ nucleation, pH plays a significant role in the microbial mineralization process and the structural characteristics of CaCO₃ crystals. Zehner et al. [124] elucidated the relationship between the three stages of CaCO₃ precipitation and pH. Specifically, in the first stage, a precipitate forms in the solution, and the pH rapidly increases to 8.2. In the second stage, the precipitate formed in the previous stage decomposes, and the pH drops to 8.1. In the third stage, the precipitate enters the growth stage, and the pH stabilizes at around 8.0. Kim et al. [125] found that the CaCO₃ content initially increased and then decreased as pH increased from 6 to 10. At pH = 7, the CaCO₃ content induced by S. saprophyticus and S. pasteurii peaked. Seifan et al. [126] observed a similar trend for crystal size; when pH increased from 10 to 12, the size of CaCO₃ crystals decreased from 20 μm to 0.2 μm (Figure 14). Current research reveals that although a pH above 10 accelerates urea hydrolysis, it reduces CaCO₃ crystal size. Conversely, within the pH range of 8–10, delayed reaction kinetics promote the formation of large CaCO₃ precipitation that enhance heavy metal adsorption efficiency. Based on these changes in reaction kinetics and crystal morphology under the abovementioned pH regulation, research concludes that when the pH is between 8 and 10, the solidification effect of the waste is the best.

6. Improvement Methods

6.1. Self-Healing Technology

Traditional MICP technology relies on external solution injection to achieve biomineralization, and its engineering application efficiency is limited by extreme environments and the difficulty of repairing internal structural damage. Therefore, researchers have proposed an environmentally responsive self-healing technology, which embeds microorganisms and nutrients in advance within the material to achieve damage-triggered self-healing [127]. The core strategies for self-healing include the following. (1) Using spores instead of bacteria: Spores can resist adverse conditions due to their unique structure. When the environment improves, they germinate in response to triggers and, in the presence of calcium sources, induce CaCO₃ precipitation. (2) Implementing a carrier protection mechanism by encapsulating active colonies within microcapsules or similar carriers: This causes mechanical or chemical triggers to controllably rupture the carriers, releasing the colonies in a targeted manner. Currently, this technology has gone beyond the traditional scope of concrete solidification and has been extended to the fields such as bio-concrete and underground structure self-healing. Xue et al. [128] were the first to propose and apply self-healing MICP to immobilize Pb²⁺. They screened Bacillus pasteurii in an extremely acidic environment and used spores to achieve nearly 100% immobilization efficiency. This advancement demonstrates that self-healing technology has enabled a shift from passive intervention to active response, offering solutions to ensure the long-term service performance of underground engineering and enable in situ remediation of contaminated sites.

The core challenge of self-healing MICP technology lies in establishing a functional synergy between microorganisms and carriers. By encapsulating dormant strains, calcium sources, and nutrients within microcapsules, carriers can protect microorganisms under extreme conditions and enable their directional release. For instance, bio-concrete incorporating waste materials such as fly ash, tailings, and waste concrete powder enables autonomous repair. When mechanical stress induces cracking, the decomposition of carriers triggers microbial revival. These microbes then utilize calcium sources and urea hydrolysis products to precipitate CaCO₃ within the cracks, as illustrated in Figure 15 [129]. A pioneering experiment by Shivanshi et al. [130] demonstrated the feasibility of this technology. Using lightweight expansive clay as the carrier, they encapsulated Lysinibacillus sphaericus and Bacillus coagulans. The resulting bacterial–concrete mixture exhibited excellent self-healing performance, with all cracks between 0.1 and 0.5 mm fully healed. Son et al. [131] encapsulated microorganisms in microcrystalline cellulose carriers for concrete crack repair. Upon crack formation, the decomposition of the carrier triggered microbial activation, and mineralization resulted in an exponential reduction in crack hydraulic conductivity. After 28 days, the crack healing rate reached 91.1%. Du et al. [132] applied core–shell structured microbial protective agents to underground engineering side walls. Compared with untreated concrete, the crack spacing and length reduction rates in self-healing samples were 152.6% and 64.5%, respectively. Jiang et al. [133] developed sugar-coated microbial agents and demonstrated complete healing of 0.15–0.25 mm cracks within 3 days, and 0.25–0.4 mm cracks within 7 days. These advances confirm that the key to self-healing MICP technology lies in the synergy between microbes and carrier. The stimulus-responsive properties of the carriers significantly extend the survival window of microorganisms. Consequently, the application of this approach has expanded from concrete crack healing to broader applications such as tailings dam reinforcement and heavy metal immobilization, signifying a shift from reactive remediation to proactive control in bio-geotechnical engineering.

6.2. One-Phase Low-pH Injection Method

Tailings sand typically exhibits low porosity and permeability, resulting in preferential flow paths that hinder the uniform distribution of CaCO₃ during traditional solution injection. To improve the spatial uniformity of CaCO₃, researchers have proposed the one-phase low-pH injection method. Initial acidic conditions inhibit CaCO₃ precipitation. Subsequently, urea hydrolysis elevates pH to alkaline levels, triggering controlled mineralization. This strategy offers a novel approach to alleviating near-surface pore clogging issues. Figure 16 compares the time when precipitates appear in the solution after the bacterial solution and the cementation solution are mixed under different initial pH values. Whereas the bacterial solution with a pH of 8.5 mixed with the cementation solution produces a large amount of precipitate after 10 min, the bacterial solution with a pH of 5 mixed with the cementation solution shows precipitates in the mixed solution approximately 1.5 h later [134]. Cheng et al. [135] demonstrated that when the pH is below 5.5, the formation of flocculent CaCO₃ formation. Their analysis revealed that urease activity is negatively correlated with CaCO₃ is significantly suppressed. High urease activity accelerated the hydrolysis rate of urea, resulting in local explosive deposition of CaCO₃, with the maximum difference in UCS values reaching 1190 kPa. Conversely, low urease activity prolongs the appearance time of CaCO₃ precipitation, enabling gradual and uniform precipitation along flow paths. The pH affects the activity of microorganisms and urease, and with decreasing pH, microbial and urease activity declines. At the same time, the rate of urea hydrolysis also decreases, prolonging the appearance time of calcium carbonate precipitation, expanding the solution seepage area, making the distribution of calcium carbonate more uniform and improving the curing effect of the solidified body. Zhang et al. [136] demonstrated that bacterial solutions at pH = 4 enhanced the spatial uniformity of CaCO₃ distribution in the solidified samples, with UCS values significantly increased compared to controls. Furthermore, Zhang et al. [136] proved that acidic conditions (pH = 5) delay the appearance time of CaCO₃ precipitation (appearance time delayed by 1.5 h), promoting the penetration of cementation solution to deeper zones and forming homogeneous solidification networks. Under alkaline conditions (pH = 8.5), rapid CaCO₃ deposition causes near-surface pore clogging, resulting in a significant decrease in the content of CaCO₃ as depth increases. There findings confirmed that the one-phase low-pH injection method can effectively prolong the appearance time of CaCO₃ precipitation, promote its uniform distribution of CaCO₃ in the pores, and thereby significantly enhance the reinforcement effect [137].

6.3. Auxiliary Additives

Auxiliary additives can promote a more uniform distribution of CaCO₃ precipitation in tailings and contaminated soils, thereby significantly improving the mechanical properties and stability of tailings sand and enhancing the efficiency of microbial immobilization of heavy metal elements. Chen et al. [138] reported that the addition of polyethylene glycol significantly increased the compressive strength of tailings sand and enlarged the crystal size of CaCO₃ precipitates. They further found that heavy metal ions such as Cu²⁺, Mn²⁺, and Pb²⁺ were immobilized within carbonate precipitates (Figure 17). Further research indicated that Pb²⁺ ions existed in two carbonate forms, namely PbCa(CO₃)₂ and PbCO₃. Beyond heavy metal immobilization, Zhang et al. [139] demonstrated that the addition of activated carbon significantly improved mechanical properties of uranium tailings. This improvement was attributed to enhanced adsorption capacity, which extended bacterial retention and increased the efficiency of the MICP process. When the activated carbon content reached 1.5%, the stress peaked at 2976.91 kPa. Shu et al. [140] found that addition of modified fibers to sand columns resulted in a maximum UCS of 3.62 MPa. The average UCS of samples with fibers was 2.8 MPa, compared to 2.13 MPa for samples without fibers. Notably, CaCO₃ content initially increased and then decreased with increasing fiber content. This occurred because fibers occupy excessive pore spaces, hindering microbial migration, leading to surface pore clogging and reducing deep-layer CaCO₃ content. This study confirmed that modified fiber addition significantly enhanced the solidification effect of soils. These fibers provide more microbial attachment sites, thereby promoting the uniform distribution of CaCO₃. Beyond mechanical reinforcement, Li et al. [47] employed artificial humic acid, biochar, and MICP for the combined remediation of contaminated farmland, immobilizing Cd²⁺ in the form of carbonates, with a remediation efficiency of 94.7%. The addition of biochar alleviated the inhibitory effect of Cd²⁺ on urease activity. Its porous structure provided more adsorption sites for microorganisms, while artificial humic acid served as a nutrient source and promoted the aggregation of soil particles into stable aggregates.

Table 4. The influence of auxiliary additives on the solidification effect.

Auxiliary Additives	Content (%)	Bacteria	Waste	Performance
Montmorillonite	1, 3, 5, 7, 9	Bacillus pasteurii [141]	Cyanide tailings	Cr, Cu and Pb leaching concentrations were reduced by 87.18, 60.56, and 88.79%, respectively.
Activated MgO	1, 2, 5, 10	Sporosarcina pasteurii [30]	Zinc ions disrupted soil	This treatment resulted in a UCS of 1.196 MPa and a Zn2+ leaching concentration of 0.1414 mg/L.
Coal fly ash	3, 6, 9	Sporosarcina pasteurii [142]	Ottawa silica sands	The peak deviator stress increased by 144%, 154%, and 115% when the additions of coal fly ash were 3%, 6%, and 9%, respectively.
PVA fibers	0.8	Freeze-dried Bacillus Sp. [143]	Ottawa silica sand	UCS increased by 138%, STS increased by 186%, permeability decreased by 126%, and brittleness decreased by half.
Discarded facial mask fiber	0.2	Sporosarcina pasteurii [144]	ISO standard sand	MICP treatment improved the UCS and reduced the water weakening of sand columns.
Biochar	2, 3, 4, 5	Sporosarcina pasteurii [145]	Contaminated soil	The addition of biochar enhanced the efficiency of Cd2+ immobilization through MICP, resulting in the UCS of the samples being 3.06 times that of the untreated samples.
Natural hemp fibers	2.5	Urease producing bacteria [146]	Natural sand	The treatment improves the strength, cohesion and internal friction angle of the sand.
Reactive magnesium oxide cement	20	Bacillus cereus [147]	Phosphogypsum	The UCS was able to achieve 3.2 MPa and the permeability coefficient was reduced by two orders of magnitude.
Fly ash	5
Waterborne polyurethane	5, 10, 15, 20	Bacterial strain Klebsiella [148]	Uranium tailings	The peak deviatoric stress of the improved specimens increased by 45.7% and the elastic modulus increased by 231.3%.

summarizes studies on auxiliary additives such as montmorillonite, MgO, and fly ash for solidifying tailings and remediating contaminated soils.

The above research indicates that the auxiliary additive–MICP synergy offers significant advantages over traditional processes. This synergy can alleviate the inhibitory effects of heavy metals on microorganisms by providing protective habitats, while further enhancing the compressive strength and impermeability of waste materials such as tailings and concrete waste powder. Zhang et al. [46] demonstrated that adding eggshells and oyster shells enhanced microbial immobilization efficiency. However, auxiliary additive parameters have strict optimal ranges. Specifically, regulating fiber length and additive mass ratio is critical to balancing the microbial migration constraint and mechanical improvement. The synergy among additives, microorganisms, and CaCO₃ provides a theoretical foundation for waste immobilization and development of auxiliary solidification agents for geotechnical applications.

7. Discussion and Suggestions for Future Research

As a novel method for soil and rock reinforcement, MICP technology has become a research hotspot in the field of mining area remediation, owing to its low ecological disturbance and high environmental adaptability. This article summarizes the theoretical exploration and practical progress of MICP technology for solid waste treatment and resource utilization by numerous scholars, covering research results from laboratory tests to engineering applications. The effectiveness of MICP in solidifying waste materials is influenced by multiple factors, including soil properties, bacterial solution, cementation solution, grouting method, temperature, and pH. Although MICP technology has made notable advancements in laboratory research, its engineering application still faces many challenges, including technical repeatability, adaptability, and long-term stability. Consequently, in-depth research and the refinement of relevant technologies are urgently needed to promote the widespread application of MICP in real-world engineering.

(1)

MICP technology based on multiple reaction pathways, such as urea hydrolysis, denitrification, iron reduction, and sulfate reduction, can develop adaptable process protocols tailored to geological environment conditions. These pathways enable targeted mineralization control in complex engineering scenarios by selecting appropriate reaction pathways, directionally acclimating functional microbial communities, and regulating the reaction environment. To address the attenuation of microbial activity in extreme environments, self-healing technology use microcapsules to encapsulate bacteria and spores, thereby extending the survival period of microorganisms. Facing the complex environmental governance demands of mines, the innovative development of MICP will focus on three cutting-edge directions. The first direction is (1) extreme environment solid waste solidification, where the development of acid-resistant, high-temperature-resistant, and anaerobic engineered strains [149], and directional acclimation of mutant strains that can utilize specific heavy metal ions (Cu²⁺, Zn²⁺) as cofactors, are essential to achieve in situ storage of mine tailings, wastewater, and geothermal reservoir pollutants. The second is (2) the use of solidified waste materials as sustainable materials to construct a microbial mineralization system, with red mud, fly ash, and concrete waste powder, among others, serving as raw materials. Finally, the third direction is the (3) establishment of long-term safety and stability prediction models for mineralized products and the development of urea-free mineralization pathways under special conditions [150], thereby promoting the advancement of MICP technology toward precision and low-carbon goals.

(2)

Auxiliary additives significantly enhance the solidification performance of pollutants. For instance, the skeletal structure of activated carbon not only optimizes the seepage pathways through its pore-filling effect but also provides additional microbial adsorption sites, thereby promoting the uniform distribution of CaCO₃ within the waste matrix. However, the geometric characteristics, dosages, and surface of additives such as activated carbon and reactive magnesium oxide must be carefully matched with the pore structure of the target waste to avoid issues such as stress concentration and impeded microbial migration. The current technical bottleneck lies in the efficient remediation of fine-grained tailings. Breakthrough directions focus on the development of nanoscale auxiliary additives, such as functionalized nanofibers and mesoporous silica particles [151], which regulate the distribution gradient of the solution in the fine-grained pore network and induce directional mineralization [152]. These materials will utilize the multi-scale synergistic effects of microorganisms, CaCO₃, and auxiliary additives in fine-grained tailings, providing new ideas for the stabilization of high-plasticity clay. Additionally, there is a lack of experimental research on MICP technology in the field of underground deep pollution barriers. Future research should prioritize the development of slow-release cementation solution formulations to extend the effective temporal window for mineralization reactions, thereby providing robust technical support for the long-term in situ immobilization of deep waste materials.

(3)

To address the issue of uneven CaCO₃ distribution caused by the low permeability characteristics of fine-grained tailings and clay slopes, researchers have proposed a one-phase low-pH injection method. Such a technique inhibits premature CaCO₃ flocculation and deposition by controlling the pH conditions, extends the diffusion range of the solution, and generates CaCO₃ after the solution has seeped uniformly, thereby shifting the mineralization reaction zone from surface deposition to deep-layer uniform distribution. In contrast, in the coarse-grained tailings system, the large-pore structures reduce bacterial adsorption, thereby diminishing the effectiveness of tailings solidification and heavy metal immobilization. The current solution optimizes the bacterial attachment interface by introducing auxiliary additives such as activated carbon and activated magnesium oxide, thereby reducing CaCO₃ loss. Future research should concentrate on elucidating the multi-scale dynamic evolution mechanisms, combining CT scanning [153,154], numerical simulation [155], and microfluidic technology to reveal [156], from a microscopic perspective, microbial behavior and crystal growth laws in waste material pores. Such a breakthrough will lay a theoretical foundation for the precise regulation of biomineralization in complex environments.

(4)

To break through the bottleneck of mineralization reaction differences faced by MICP technology in engineering applications, it is necessary to establish a multi-angled collaborative monitoring system to analyze the distribution characteristics of CaCO₃ in real time [157]. Based on the current research status, in situ real-time monitoring systems integrated with deep learning have the greatest potential for breakthrough in this field. Among these monitoring methods, approaches such as conductivity, biosensors, and sound waves have shown preliminary progress in monitoring the mineralization process. This system is expected to effectively address the challenge of achieving a balance between mineralization efficiency and environmental adaptability under complex environmental conditions. At the same time, technology transformation should advance in parallel with the standardization process, establishing a unified mineralization evaluation standard, including core performance indicators such as the safety assessment of bacterial strains, the uniformity of CaCO₃ distribution, and the recycling standards of waste. This will promote the transition of MICP technology from laboratory research to standardized engineering practice, providing full-cycle technical support for waste solidification and resource utilization.

8. Conclusions

(1)

As a green biological reinforcement method, MICP technology has demonstrated unique environmental advantages and controllable cementation effects in the field of mining area remediation. Such a technique precisely regulates the generation of CaCO₃ precipitates through microbial metabolic activities and chemical reactions such as urea hydrolysis, denitrification, and sulfate reduction, and achieves soil particle solidification through three methods: surface coating, pore filling, and particle bridging. These methods effectively enhance the strength and stability of tailings dams and slopes and prevent the migration of heavy metal ions. At present, MICP technology has made remarkable progress in laboratory research and field research.

(2)

Waste solidification and slope-reinforcement performance are jointly influenced by the intrinsic properties of soil and external environmental factors. Among these, solid waste characteristics, bacterial solution, cementation solution, grouting method, temperature, and pH are the key determinants of solidification effectiveness. The particle size distribution and pore structure of the soil directly affect microbial adsorption and migration pathways, thereby influencing the uniformity of CaCO₃ distribution within geotechnical materials. The bacterial and cementation solution are essential components of the MICP reaction. The type of bacteria and calcium source jointly determine the morphology of CaCO₃ crystals, while the activity of urease and the concentration of cementation solution jointly determine the mineralization reaction rate and the spatial distribution of CaCO₃. The grouting method directly affects the spatial distribution of CaCO₃ by regulating the seepage path of the solution, ultimately determining the macroscopic mechanical and permeation properties of the solidified body. Temperature and pH mainly regulate the solidification process by affecting bacterial activity, urease activity, and MICP reaction rate.

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MICP performance in waste solidification has been significantly enhanced through three key improvement methods. The first involves self-healing technology, which utilizes microcapsules to encapsulate dormant bacteria, enabling targeted, crack-triggered microbial repair. The second is the one-phase low-pH injection method, which creates an acidic environment to delay CaCO₃ precipitation and prevent surface blockage. Concurrently, auxiliary additives optimize waste material spatial structure, provide more microbial attachment sites, and significantly enhance the mechanical properties and stability of bio-solidified materials. Through the synergistic effect of biochemical regulation and material design, these improvement methods promote the development of MICP technology towards self-healing scenarios, deep solidification, and engineering applications in extreme environments.

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Although MICP technology has achieved remarkable progress in the research of mine waste solidification and resource utilization, its engineering promotion still faces many challenges. Future research urgently needs to focus on developing urea-free mineralization pathways, screening low-environmental-sensitivity strains, researching and developing nanoscale auxiliary additives, constructing multi-scale dynamic models, establishing in situ real-time monitoring systems integrated with deep learning, and setting systematic mineralization evaluation standard system. These breakthroughs will provide a biological solution that combines engineering reliability, environmental compatibility, and economic feasibility for global waste management.