Microbially Induced Calcium Carbonate Precipitation Process Research Articles

Improvement Schemes for Bacteria in MICP: A Review

Biomineralization is a common phenomenon in nature, and the use of microbial-induced calcium carbonate precipitation (MICP) technology for engineering construction is a successful attempt to utilize natural biological phenomena, which has become a hot topic of current research. There are many factors affecting MICP, such as bacterial properties and external environmental factors. Many scholars have carried out a lot of research on these factors, but even under appropriate conditions, the MICP process still has the problem of low efficiency. According to different engineering, the tolerance and effect of bacteria in different environments are also different. At the same time, the cultivation and preservation of bacteria will also consume a large amount of raw materials, which is far more significant than the cost of engineering construction. The efficiency and cost limit the large-scale application of this technology in practical engineering. In response to these problems, researchers are exploring new ways to improve the efficiency of MICP technology. Based on the bacteria used in MICP, this paper explores the mechanism of bacteria in the process of MICP and reviews the improvement of bacteria from the perspective of efficiency improvement and economy.

Role of bacteria on bio-induced calcium carbonate formation: insights from droplet microfluidic experiments

Microbially induced calcium carbonate precipitation (MICP) has emerged as a promising solution for geotechnical issues. However, the role of bacteria in the formation of calcium carbonate (CaCO3) remains incompletely understood. In this study, a droplet microfluidic chip was developed to observe the growth process of calcium carbonate and bacterial behaviour during the MICP process under various bacterial density conditions. Scanning electron microscope was then utilised to analyse the calcium carbonate morphology, and Raman spectroscopy was employed to identify calcium carbonate polymorphs. Nucleation within microspaces showed a stochastic nature. Within the droplets where crystals formed, all crystals manifested as cubic calcite. Higher bacterial density led to the formation of larger and more irregularly shaped crystals, with crystal size showing a significant correlation with urease activity. In droplets where no crystals formed, higher bacterial density and urease activity resulted in the precipitation of amorphous calcium carbonate on the bacterial surface. However, this precipitation pattern differed from the formation of monocrystalline calcium carbonate. The results demonstrate that bacteria act primarily as urease secretors to regulate crystal growth during the MICP process, while their role as nucleation sites for crystals remains debatable. This study provides new insights into the bio-induced calcium carbonate formation mechanism.

The geochemical and thermodynamic characteristics of waste sand reinforced by microbially induced calcium carbonate precipitation

To achieve efficient utilization of waste sand and make it a recyclable resource, the waste sand was reinforced by microbially induced calcium carbonate precipitation (MICP). Scanning Electron Microscopy (SEM)–Energy Dispersive Spectrometer (EDS) and Fourier Transform Infrared Spectroscopy (FTIR) were performed to determine the mineral morphologies and elemental compositions. The results of SEM showed that rhombohedral and dumbbell-shaped minerals filled the pores of the sand column, and the elemental compositions were C, O, Ca, Al, and P. Various organic functional groups were discovered by FTIR. Mineral compositions were analyzed by X-ray diffraction (XRD). The results showed that the mineral components were calcite and aragonite, and the crystallinity of calcite improved with the increase in the bacterial concentrations. Stable carbon isotope analyses showed that the sand columns at different bacterial concentrations ranged from − 18.9 ‰ to − 21.4 ‰, which were more negative than chemical calcite with − 10.9 ‰. The mechanical properties of compression strength and splitting tensile strength proved that MICP could enhance the strength of sand columns. Thermodynamic characteristics were carefully investigated using thermogravimetric analysis from 50 °C to 1000 °C, which showed that the activation energy and thermal stability of the sand columns reinforced by MICP increased. Therefore, this study provides important insights into the process of MICP, which has good spontaneity, ecological performance, and low energy consumption. It is conducive to the construction of ecological civilization and the requirements of green development, and it has important engineering significance.

Improvement of the Sand Quality by Applying Microorganism-induced Calcium Carbonate Precipitation to Reduce Cement Usage

This research determines the Microbially Induced Calcium Carbonate Precipitation (MICP) process utilized by the bacteria found in Thailand. Many researchers typically use the high-efficiency MICP bacteria to precipitate calcium carbonate. However, it is only available in some countries, leading to a high import expense. Therefore, the methodology for using the bacteria capable of producing calcium carbonate in Thailand was investigated. The five pure bacteria strains are obtained from the Thailand Institute of Scientific and Technological Research (TISTR), i.e., Proteus mirabilis TISTR 100, Bacillus thuringiensis TISTR 126, Staphylococcus aureus TISTR 118, Bacillus sp. TISTR 658 and Bacillus megaterium TISTR 067. To screen urease production, the bacteria were spread on Christensen's Urea Agar (UA) slant surface via a colorimetric method. All bacteria strains can produce urease enzymes by observing the color changes in the UA. Berthelot's method was used to determine the urease activity. The result shows the bacteria's urease activity: 2389, 1989, 1589, 789, and 589 U/ml, respectively. These directly lead to calcium carbonate production: 3.430, 3.080, 2.590, 1.985, and 1.615 mg/ml, respectively. Despite the bacteria in this research having a low precipitation efficiency compared to the strain used in many research studies, they can improve sand stabilization in 7 days. Proteus mirabilis TISTR 100 was the most stable and effective strain for the MICP process in Thailand. Hence, this research reveals the ability of the local bacteria to bond with the sand particle. Briefly, the improvement of the MICP process in sand stabilization can be improved to reduce imported expenses. In addition, the MICP process can reduce the use of cement in sand stabilization work.

The impact of seawater ions on urea decomposition and calcium carbonate precipitation in the MICP process

As an environmentally friendly biotechnology, the microbial induced calcium carbonate precipitation (MICP) provides a new approach to repair concrete cracks. Most existing studies focused on finding a suitable environment for bacteria to induce as much calcium carbonate as possible, but the effect of actual concrete service environment on the repair of concrete cracks was usually ignored. This work compared the repair quality of cracked specimens via MICP with the original Sporosarcina (Sp.) pasteurii strain and salt-tolerant Sp. pasteurii strain in the deionized water and simulated seawater. Then the adverse effects of seawater ions on the MICP process was investigated. The results showed that the permeability coefficient of repaired specimens was 175.7 % higher, and the bond strength between the calcium carbonate and concrete surface was lower in the simulated seawater as compared with the deionized water environment. The high concentration of sodium, chloride, magnesium, and sulfate ions in the seawater led to the decrease of the urea decomposition rate and calcium carbonate yield rate, which further induced the decreased super-saturation of calcium carbonate and the conversion of the calcium carbonate crystal form from calcite to vaterite. Besides, the salt-tolerant strain with a high salt adaptability showed a higher repair ability in the simulated seawater as compared with the original strain.

Application of microbially induced calcium carbonate precipitation (MICP) process in concrete self-healing and environmental restoration to facilitate carbon neutrality: a critical review.

Soil contamination, land desertification and concrete cracking can have significant adverse impacts on sustainable human economic and societal development. Cost-effective and environmentally friendly approaches are recommended to resolve these issues. Microbially induced carbonate precipitation (MICP) is an innovative, attractive and cost-effective in situ biotechnology with high potential for remediation of polluted or desertified soils/lands and cracked concrete and has attracted widespread attention in recent years. Accordingly, the principles of MICP technology and its applications in the remediation of heavy metal-contaminated and desertified soils and self-healing of concrete were reviewed in this study. The production of carbonate mineral precipitates during the MICP process can effectively reduce the mobility of heavy metals in soils, improve the cohesion of dispersed sands and realize self-healing of cracks in concrete. Moreover, CO2 can be fixed during MICP, which can facilitate carbon neutrality and contribute to global warming mitigation. Overall, MICP technology exhibits great promise in environmental restoration and construction engineering applications, despite some challenges remaining in its large-scale implementation, such as the substantial impacts of fluctuating environmental factors on microbial activity and MICP efficacy. Several methods, such as the use of natural materials or wastes as nutrient and calcium sources and isolation of bacterial strains with strong resistance to harsh environmental conditions, are employed to improve the remediation performance of MICP. However, more studies on the efficiency enhancement, mechanism exploration and field-scale applications of MICP are needed.

Microfluidic chips for microbially induced calcium carbonate precipitation: Advantages, challenges, and insights

Microbially induced calcium carbonate precipitation (MICP) has garnered significant attention as a biomineralization process with diverse applications spanning from construction to environmental remediation. To propel MICP research and deepen our comprehension of MICP mechanisms, microfluidic chips have emerged as potent tools offering precise control over environmental parameters and real-time observations. Herein, we explore the benefits and challenges associated with employing microfluidic chips as a platform for investigating MICP. The advantages of microfluidic chips lie in their capacity to create controlled microenvironments conducive to emulating specific conditions crucial for MICP. The high-throughput nature of these devices accelerates experimentation by facilitating simultaneous testing of various microbial strains and nutrient compositions. Throughout the MICP process, observations were made on the behaviors of both bacterial cells and CaCO3 cementation. The inherent reduction in reagent consumption offered by microfluidics is both cost-effective and environmentally friendly. However, scaling up from microscale findings to practical applications necessitates careful consideration. Fully replicating the three-dimensional complexity and heterogeneous structures of the soil matrix, which influence microbial behavior, mineral distribution, and overall precipitation dynamics, using microfluidic chips remains challenging. Additionally, certain environmental complexities, including macroscopic soil components such as organic matter and various particle types, which significantly affect microbial activities and mineral precipitation patterns, may be difficult to replicate in microfluidic setups. However, microfluidic chips stand as invaluable tools for advancing MICP research. By addressing the advantages and disadvantages outlined here, researchers can harness the capabilities of microfluidic systems to unravel the intricacies of MICP, ultimately bridging the gap between fundamental understanding and real-world applications.

A Numerical Bio-Geotechnical Model of Pressure-Responsive Microbially Induced Calcium Carbonate Precipitation

The employment of Microbially Induced Calcium Carbonate Precipitation (MICP) is of increasing interest as a technique for environmentally sustainable soil stabilisation. Recent advancements in synthetic biology have allowed for the conception of a pressure-responsive MICP process, wherein bacteria are engineered to sense environmental loads, thereby offering the potential to stabilise specific soil regions selectively. In this study, a 2D smart bio-geotechnical model is proposed based on a pressure-responsive MICP system. Experimentally obtained pressure-responsive genes and hypothetical genes with different pressure responses were applied in the model and two soil profiles were evaluated. The resulting model bridges scales from gene expression within bacteria cells to geotechnical simulations. The results show that both strata and gene expression–pressure relationships have a significant influence on the distribution pattern of calcium carbonate precipitation within the soil matrix. Among the evaluated experimental genes, Gene A demonstrates the best performance in both of the two soil profiles due to the effective stabilisation in the centre area beneath the load, while Genes B and C are more effective in reinforcing peripheral regions. Furthermore, when the hypothetical genes are utilised, there is an increasing stabilisation area with a decreased threshold value. The results show that the technique can be used for soil reinforcement in specific areas.

The optimized microbial-induced calcium carbonate precipitation process to fabricate underwater superoleophobic mesh for efficient oil-water separation

Microbial-induced calcium carbonate precipitation (MICP) is an environmentally-friendly and easy operation technique for separating oil-water mixtures. However, the effect of environmental factors on the wettability and oil-water separation performance of MICP remains unclear. In this study, three primary factors including mineralization solution (MS) concentration, bacterial suspension (BS) concentration, and temperature were carefully investigated. Under these varying conditions, stainless steel meshes (SSM) were chosen as the substrates for the growth of CaCO3 precipitations. Experimental results demonstrate a strong dependence of the morphology and wettability of CaCO3 precipitations on these factors. Higher MS concentration and temperature contribute to enhanced hydrophilicity of CaCO3 coatings due to the increased surface roughness. The water flux of MICP-coated SSMs decreases with the increase of BS concentration, MS concentration, and temperature. The MICP-coated SSM obtained with MS concentration of 0.15 M, BS concentration of 0.1 × 108 cells/mL, and mineralization reaction at 25 ℃ for 24 hours demonstrated the optimal comprehensive performance, exhibiting underwater superoleophobicity with an underwater oil contact angle (UWOCA) exceeding 151°, an ultra-high oil intrusion pressure (> 3.0 kPa), ultrahigh flux (28,644 L m−2 h−1), and a preferable oil rejection rate (> 99.9%). Additionally, the as-prepared mesh exhibited superior anti-oil-fouling property and reusability, while concurrently demonstrating excellent chemical and thermal stability. This work contributes to advancing our knowledge of the influence of various environmental factors on the wetting characteristics of MICP and provides a feasible solution for the highly efficient and low-cost preparation of oil-water separation membranes based on the MICP process.

Increased Microbially Induced Calcium Carbonate Precipitation (MICP) Efficiency in Multiple Treatment Sand Biocementation Processes by Augmentation of Cementation Medium with Ammonium Chloride

The cementation medium for ureolytic microbially induced calcium carbonate precipitation (MICP) typically consists of urea and a calcium source. While some studies have augmented this basic medium, the effects of adding substrates such as ammonium chloride are unclear. The studies detailed in this paper sought to quantify the effect of the ammonium chloride augmentation of cementation medium (CM) on the process of MICP. An aqueous MICP study was initially carried out to study the effects of adding ammonium chloride to the urea–calcium cementation medium. This batch test also explored the effect of varying the concentration of calcium chloride dihydrate (calcium source) in the CM. A subsequent sand column study was undertaken, whereby multiple treatments of CM were injected over several days to produce a biocement. Six columns were prepared using F65 sand bioaugmented with Sporosarcina pasteurii, half of which were injected with the basic medium only and half with the augmented medium for treatment two onwards. Effluent displaced from columns was tested using ion chromatography and Nesslerisation to determine the calcium and ammonium ion concentrations, respectively, and hence the treatment efficiency. Conductivity and pH testing of effluent gave insights into the bacterial urease activity. The addition of 0.187 M ammonium chloride to the CM resulted in approximately 100% chemical conversion efficiency within columns, based on calcium ion measurements, compared to only 57% and 33% efficiency for treatments three and four, respectively, when using the urea–calcium medium. Columns treated with the CM containing ammonium chloride had unconfined compressive strengths which were 1.8 times higher on average than columns treated with the urea–calcium medium only.

Effect of particle size and gradation on compressive strength of MICP-treated calcareous sand

Calcareous sand is a type of biological sand deposited from the remains of marine organisms. Although widely used in coastal construction, the reinforcement of calcareous sand is necessary due to its natures of low bearing capacity and high compressibility. Aiming for this, an environmentally bio-grouting technique, microbially induced calcium carbonate precipitation (MICP) was applied to reinforce the calcareous sand. However, MICP treatment is in turn influenced by the properties of calcareous sand, such as particle size and gradation. To reveal the effects of particle size and gradation, the MICP-treated calcareous sands with various median sizes, uniformity coefficients or curvature coefficients, were conducted with the unconfined compressive strength (UCS) tests and a series of laboratory experiments. The results indicated that with the median size or uniformity coefficient increasing, the UCS of the MICP-treated specimens decreased; while with increasing curvature coefficient, the UCS exhibited a trend of increasing before decreasing. Further experiments reveal that particle size and gradation affect the MICP process mainly by changing the amounts and volume of pores. The current study demonstrates the respective effects of particle size and gradation on the treatment of MICP and provides a theoretical reference for reinforcing calcareous sand in marine engineering.

A potential sustainable technique to entrap contaminants against rill erosion based on MICP

Health risks due to toxic metals (TM) contamination is a global concern. Toxic metals could be release to the environment due to soil erosion. The present study was aimed to prove the synergistic mechanisms of microbially induced calcium carbonate precipitation (MICP) on improving the strength properties of soil particles toward rill erosion, as well as immobilizing toxic metals into stable crystals. Hence, laboratory batch studies were conducted to evaluate the rill erosion stability as well as Cd, Pb and Zn removal ability based on bioprecipitation through MICP, using urease producing bacteria. Results revealed that the application of the strain Bacillus pasteurii PTCC1645 in MICP process of test model treated with 500 mM cementation solution showed a great potential to decrease sand lost to below 20% with the flow rate of 15 ml/min with highest strength (131 KPa) and the highest calcium carbonate content (0.84%). Moreover, the immobilization of the toxic metals in biotreated test models above 50 mM under the flow rate of 3 ml/min showed the efficiency of Pb, Zn and Cd ions by 100%. SEM proved the presence of calcite among the sand particles which could be attributed to the effectiveness of the strain. Energy-dispersive X-ray spectroscopy(EDS) analysis revealed the presence of toxic metals, while were transformed into carbonates by MICP of PTCC1645. Hence, toxic metals have stronger stability encountered with rill erosion and low toxicity compared with metal ions. The present study illustrated that the carbonate-biomineralization can offer an effective and eco-friendly approach to immobilize soluble toxic metals and that MICP may play an important role in metal bioconservation during rill erosion.

Rehabilitation of Porous Building Components and Masonry by MICP Injection Method

Microbial-induced calcium carbonate precipitation (MICP) is a novel approach that is already being applied in various areas of construction. The precipitated calcium carbonate can be used to reduce porosity and thus increase the durability of deteriorated building components. This study investigates whether MICP injections are suitable for building rehabilitation. Porous mortar test samples of recycled aggregate and parts of deteriorated masonry were prepared. The MICP injections were performed without pressure and with an injection pump. The treatment effect was investigated after MICP injection by testing the porosity, strength and microscopic evaluation. It can be observed that multiple MICP injections under pressure result in a reduction of the pore volume of porous mortar samples. The produced calcium carbonate precipitates in the pore space of the samples and increases the density by 1.59% and the weight by 7.56%, which also results in a 48.3% reduction of the capillary water absorption. The results of strength tests show an increase of 45.16% in flexural strength and 35.64% in compressive strength compared with the untreated mortar samples. In addition, the MICP process was investigated and the precipitation was characterised. The X-ray diffraction (XRD) of the precipitated calcium carbonate confirms that mainly calcite was formed, which was also found in the pore structure of the MICP-injected masonry after the microscopic analysis. Precipitated calcium carbonate could be detected especially near the injection spots.

Pore-scale spatiotemporal dynamics of microbial-induced calcium carbonate growth and distribution in porous media

The naturally occurring bio-geochemical microbial-induced calcium carbonate precipitation (MICP) process is an eco-friendly technology for rehabilitating construction materials, reinforcement of soils and sand, heavy metals immobilization and sealing subsurface leakage pathways. We report pore-scale spatiotemporal dynamics of the MICP process in porous media, relevant for reduced environmental risk by leakage during CO2 geological storage. Effects of hydrodynamics and supersaturation on the MICP with Sporosarcina pasteurii stains were studied using a high-pressure, rock-on-a-chip microfluidic device. Bacterial cell numbers and variation in cementation concentration controlled the crystal size and pore-scale distribution by influencing the local supersaturation. Local pore structure determined crystal nucleation, where low velocity regions tended to nucleate more crystals. CaCO3 crystallization was observed at subsurface pressure (100 barg) with a reduced sealing performance due to the low microbial activity from elevated pressure. We identify that hydrodynamics and supersaturation determine crystal nucleation and growth in porous systems, providing important experimental evidence for subsurface environmental applications and validation of upscaled MICP models.

State of the art on factors affecting the performance of MICP treated fine aggregates

Microbially-induced calcium carbonate precipitation (MICP) is a promising technique for enhancing the mechanical and durability properties of fine aggregates used in the construction industry. However, several factors affect the success of MICP treatment, including chemical and physical factors, which can impact the treatment's effectiveness. This review paper comprehensively analyzes such factors influencing the effectiveness of MICP treatment. The mechanisms of the MICP process, including enzymatic and non-enzymatic pathways, are discussed in detail. The paper also highlights the challenges and limitations of the technique and identifies research gaps and future directions. This review provides valuable insights into the potential benefits and challenges of using the MICP technique for improving the performance of fine aggregates in concrete and will be of interest to researchers, engineers, and other professionals working in the construction industry.

High level of calcium carbonate precipitation achieved by mixed culture containing ureolytic and nonureolytic bacterial strains.

This study demonstrates a remarkably high level of microbial-induced calcium carbonate precipitation (MICP) using a mixed culture containing TBRC 1396 (Priestia megaterium), TBRC 8147 (Neobacillus drentensis) and ATCC 11859 (Sporosarcina pasteurii) bacterial strains. The mixed culture produced CaCO3 weights 1·4 times higher than those obtained from S. pasteurii, the gold standard for efficient MICP processes. The three strains were selected after characterization of various Bacillus spp. and related species for their ability to induce the MICP process, especially in an alkaline and high-temperature environment. Results showed that the TBRC 1396 and TBRC 8147 strains, as well as TBRC 5949 (Bacillus subtilis) and TBRC 8986 (Priestia aryabhattai) strains, could generate calcium carbonate at pH 9-12 and temperature 30-40°C, which is suitable for construction and consolidation purposes. The TBRC 8147 strain also exhibited CaCO3 precipitation at 45°C. The TBRC 8986 and TBRC 8147 strains are nonureolytic bacteria capable of MICP in the absence of urea, which can be used to avoid the generation of undesirable ammonia associated with the ureolytic MICP process. These findings facilitate the successful use of MICP as a sustainable and environmentally friendly technology for the development of various materials, including self-healing concrete and soil consolidation.

Enhanced Compressive Strength of the Bentonite-Amended Cement via Bio-Mineralization

Bentonite, a supplementary cementitious material for Portland cement, has greatly contributed to environmental sustainability. However, few studies have investigated mortar samples produced by substituting bentonite for cement, and cement strength may be adversely affected when cement is replaced with bentonite in larger proportions. Therefore, this paper investigates and discusses the effect of microbially induced calcium carbonate precipitation (MICP) on improving the strength of bentonite-amended cement. The bio-mineralization process of MICP was characterized by SEM-EDS, while the biominerals formed in bentonite-amended mortar were identified by FIIR and XRD analysis. The results showed that: at bentonite concentrations of 0%, 10%, 20%, 30%, and 40% in cement, the bacterial suspension and reaction solution enhanced the compressive strength of bentonite-amended cement by 17%, 20%, 79%, 78%, and 38%, respectively, after 28 days, compared to control specimens; With the increased bacterial concentration in the presence of the reaction solution, the strength of the bentonite-amended cement (20% bentonite) increased remarkably compared to the control specimen (without bacteria). When the bacterial concentration was OD600 2.0, the compressive strength of bentonite-amended cement (20% bentonite) increased by 80% after 28 days; MICP process has a great effect on improving the strength of bentonite-amended cement. It is a green and economical choice to use MICP to improve the strength of bentonite-amended cement.

Effect of microbially induced calcium carbonate precipitation treatment on the solidification and stabilization of municipal solid waste incineration fly ash (MSWI FA) - Based materials incorporated with metakaolin

Microbially induced calcium carbonate precipitation (MICP) has been considered as a potential treatment method for the solidification and stabilization of municipal solid waste incineration fly ash (MSWI-FA).The main obstacle for MICP treatment of MSWI-FA is the harsh environment which causes the bacteria fail to maintain their urease activity effectively, thus decreases the solidification effect and material properties. Currently, there is no research on blending metakaolin (MK) as a protective carrier for the bacteria into the MSWI-FA. The effect of the MICP process on the curing properties of MSWI FA-based cementing materials in the MK and MSWI-FA reaction system is largely unknown. In this study, different mixing ratios of MK were used to adjust the Ca/Si/Al ratio in the mixture, and the properties of the cementing material (MSWI-FA mixed with MK and water) and the MICP-treated material (MSWI-FA mixed with MK and bacterial solution) were investigated. This study contributes to find suitable additives to promote effect of MICP on the solidification of MSWI-FA and the improvement of material properties. The results showed when the mixing ratio of MSWI FA was 90 wt %, the MICP treatment was able to increase the compressive strength of the samples up to 0.99 Mp, and the compressive strength of samples reached 1.46 MPa, when the mixing ratio of MSWI FA was 80 wt %. Though the metakaolin did not show inhibitory effect on the urease activity, the compressive strength of the MICP-treated samples did not further show a significant increase when the mixture of MK was increased from 20 wt% to 30 wt%. Further investigation suggested that MICP activities of bacteria utilizing calcium sources could have an impact on the formation/deformation of calcium-containing hydration products in the reaction system, thus affecting the mechanical and chemical properties of MSWI based materials. MICP treatment is effective in the immobilization of certain heavy metals of MSWI FA, especially for Pb, Cd and Zn. This research shows the potential of using MICP to treat the MSWI fly ash, meanwhile, it is necessary to find suitable reaction system with the proper additives in order to further improve the properties of the MSWI FA based material in terms of mechanical performance.

Microdroplet-Based In Situ Characterization Of The Dynamic Evolution Of Amorphous Calcium Carbonate during Microbially Induced Calcium Carbonate Precipitation.

Amorphous calcium carbonate (ACC) plays an important role in microbially induced calcium carbonate precipitation (MICP), which has great potential in broad applications such as building restoration, CO2 sequestration, and bioremediation of heavy metals, etc. However, our understanding of ACC is still limited. By combining microscopy of cell-laden microdroplets with confocal Raman microspectroscopy, we investigated the ACC dynamics during MICP. The results show that MICP inside droplets can be divided into three stages: liquid, gel-like ACC, and precipitated CaCO3 stages. In the liquid stage, the droplets are transparent. As the MICP process continues into the gel-like stage, the ACC structure appears and the droplets become opaque. Subsequently, dissolution of the gel-like structure is accompanied by growth of precipitated CaCO3 crystals. The size, morphology, and lifetime of the gel-like structures depend on the Ca2+ concentration. Using polystyrene colloids as tracers, we find that the colloids exhibit diffusive behavior in both the liquid and precipitated CaCO3 stages, while their motion becomes arrested in the gel-like ACC stage. These results provide direct evidence for the formation-dissolution process of the ACC-formed structure and its gel-like mechanical properties. Our work provides a detailed view of the time evolution of ACC and its mechanical properties at the microscale level, which has been lacking in previous studies.

Improving microbially induced calcium carbonate precipitation effects by nacre extractions

In microbially induced calcium carbonate precipitation (MICP) process, it is the precipitated calcium carbonate that cements loose sand particles together to improve their mechanical properties. Seashell nacre composed of calcium carbonate is a natural product, which is worth researching for its remarkable hardness, strength and toughness. However, there has been no study that bridges this natural nacre mineralisation with MICP. Therefore, a precedent herein is established to modify the MICP process by way of the water-soluble matrix (WSM) extracted from nacre, where WSM contributes to the great mechanical properties of nacre. This study examines the effects of WSM with different concentrations on urease activity of bacteria and strength as well as the microstructure of bio-cemented sand samples. The results show that a small number of WSM (50 mg/l) can improve the average strength of bio-cemented sand samples by 1·5 times. This is because 50 mg/l WSM can significantly improve the urease activity. Thus, more calcium carbonate crystals are precipitated, and the higher unconfined compressive strength of bio-cemented sand samples is achieved. The microstructures are investigated by x-ray diffraction and scanning electron microscopy. Overall, this study is an unprecedented exploration imitating nacre that hopefully paves way for future studies.