Microbially Induced Carbonate Precipitation Research Articles

Cyanobacterial Biocrust on Biomineralized Soil Mitigates Freeze–Thaw Effects and Preserves Structure and Ecological Functions

Biocrust inoculation and microbially induced carbonate precipitation (MICP) are tools used in restoring degraded arid lands. It remains unclear whether the ecological functions of the two tools persist when these methods are combined and subjected to freeze–thaw (FT) cycles. We hypothesized a synergetic interaction between MICP treatment and biocrust under FT cycles, which would allow both components to retain their ecological functions. We grew cyanobacterial (Nostoc commune) biocrusts on bare soil and on MICP (Sporosarcina pasteurii)-treated soil, subjecting them to repeated FT cycles simulating the Mongolian climate. Generalized linear modeling revealed that FT cycling did not affect physical structure or related functions but could increase the productivity and reduce the nutrient condition of the crust. The results confirm the high tolerance of MICP-treated soil and biocrust to FT cycling. MICP treatment + biocrust maintained higher total carbohydrate content under FT stress. Our study indicates that biocrust on biomineralized soil has a robust enough structure to endure FT cycling during spring and autumn and to promote restoration of degraded lands.

AN EXPERIMENTAL STUDY ON STABILIZING COHESIVE SOIL USING MICROBIALLY INDUCED CALCIUM CARBONATE PRECIPITATION

Bio-cementation is an environmentally friendly procedure grounded on microbial induced carbonate precipitation mechanism, occurring in environments abundant in calcium. In the effort to bolster soil stability, traditional urea is combined with bacteria to enhance the mechanical properties of soil. The research delves into the microbial mechanisms involved in microbially induced calcite precipitation, highlighting the role of urease-producing bacteria in facilitating calcium carbonate precipitation within the soil structure. This also entails pivotal laboratory and field experiments, evaluating the efficacy of microbially induced calcium carbonate in improving soil engineering characteristics, such as strength, permeability, and durability. Furthermore, this process can be employed across various soil types, offering adaptability in geotechnical applications. Additionally, we investigate the environmental consequences and sustainability aspects by underscoring its potential as an eco-friendly substitute for soil enhancement. Nonetheless, challenges such as optimizing bacterial activity, regulating the precipitation process, and ensuring even distribution of calcium carbonate within the soil necessitate further exploration. Based on the envisioned review, it contributes to the comprehension of MICP as a promising method for soil stabilization, providing insights for researchers, practitioners, and policymakers keen on sustainable and inventive approaches to geotechnical engineering. Key Words: Bio-geotechnical engineering, soil stabilization, Microbially induced calcium carbonate Precipitation, Soil improvement.

Enhancing microbial-induced carbonate precipitation (MICP) sand consolidation with alkali-treated jute fibers

Recent research has focused on reinforcing sand consolidated through microbial-induced carbonate precipitation (MICP) with alkali-treated fibers to enhance its mechanical properties and mitigate brittleness. This research investigated how modified fiber affected the microstructure and properties of MICP solid sand. The fiber content (0, 0.5, 1, 3, and 5%), pretreatment concentration (0, 1, 5, 10, and 20%), pretreatment time (0, 0.5, 1, 2, and 4 h), and pretreatment temperature (25, 35, 45, and 55 °C) required for the experiment were determined by MICP testing. The interactions between fiber, sand, and calcium carbonate(CaCO3) were analyzed by calcium carbonate content(CaCO3(%)), unconfined compressive strength (UCS), environmental scanning electron microscopy (ESEM), and X-ray diffraction (XRD). The specimen without added fiber had a UCS of 2.13 MPa, the UCS of the added fiber sample was 2.8 MPa, which was 31.46% more than that of the specimen without added fiber, and the UCS of the specimen with added alkali-treated fiber was 3.62 MPa, which was 70% more than that of the specimen without added fiber and 28.57% more than that of the added untreated fiber. The optimum content of jute fibers was 0.5%, and the optimum concentration of alkali treatment of jute fibers was 10% for one hour.

A new bacterial concentration method for large-scale applications of biomineralization

Bacterial suspension is an essential component of microbially induced carbonate precipitation (MICP)-based biocement and a large-scale production is required for field applications. In this study, a new bacterial concentration method is proposed to enable high concentration bacterial suspension to be produced to facilitate field work. By adding low concentration calcium to bacterial suspension, flocs are formed and bacterial cells are adsorbed on the flocs to achieve bacterial concentration. Compared to the traditional bacterial concentration method using centrifugation and freezing-drying method, the proposed method can concentrate a large volume of bacterial suspension without using special equipment. The feasibility of this method is verified by bacterial concentration tests, solution tests and sand column treatment tests. The results of both the solution test and the sand column treatment test show that the bacterial suspension concentrated by the proposed method can be effectively used for soil biocementation. There is a threshold calcium concentration that allows a complete bacterial concentration for the proposed method, and this threshold calcium concentration tends to increase linearly with the optical density of the cell suspension at a wavelength of 600 nm (OD600).

A non-classical crystallization mechanism of microbially-induced disordered dolomite

Non-classical crystallization pathways, involving the formation of complex precursors prior to nucleation, have been observed during the calcification process of invertebrate skeletons and shells. However, these pathways were rarely documented in microbially-induced carbonate precipitation. In this study, we presented experimental evidence demonstrating that a halophilic bacterium (Halomonas sp. strain JBHLT-1) catalyzed the biomineralization of disordered dolomite through an amorphous calcium-magnesium carbonate (ACMC) with a stoichiometry near that of dolomite. During the ACMC formation, strain JBHLT-1 significantly enhanced the incorporation of Mg2+ ions compared to abiotic ACMC that was synthesized without any microbial biomass. In the stage of ACMC crystallization, the presence of microbial biomass reduced the dissolution-reprecipitation rate and induced bias towards a solid-state reaction pathway. This observation was supported by the negligible loss of structural Mg2+ ions in the presence of cells of strain JBHLT-1. The microbially-induced growth of disordered dolomite appeared to follow a spherulitic growth mechanism, as evidenced by its hierarchical structure composed of coalesced nano-spheres, eventually growing into micron-sized spheroids and dumbbells with extended incubation times. Since disordered dolomite is considered as a crucial crystalline precursor of ordered sedimentary dolomite, the findings of this study have important implications for understanding the formation mechanism of natural dolomite.

Isolation, Identification, and Carbonate Mineralization Characteristics of a Newly Carbonic Anhydrase-Producing Strain.

Carbonic anhydrase-producing microorganisms can rely on their metabolism for carbon sequestration and carbonate precipitation, which is a relatively effective mode among the known microbially induced carbonate precipitation (MICP) methods. A newly carbonic anhydrase-producing strain was isolated from soil samples. 16S rDNA gene sequencing showed this strain had 99.18% sequence identity to Chryseobacterium gambrini. Various culture parameters (temperature, pH, rotational speed, inoculum size, and metal ions) were optimized for optimal microbial growth and CA activities. Optimal culture conditions were as follows: temperature of 30°C, pH 6-7, rotational speed 150rpm, and inoculum size 1%. It was observed that Co2+ and Mn2+ can improve CA activity with optimal concentrations of 0.02mM and 0.01mM, respectively. Furthermore, the introduction of CO2 for 15min daily leads to a 36% increase in the final production of biotic CaCO3, reaching 2.884g/L. Characterization of the mineralization precipitates was conducted to reveal the mechanism of the carbonic anhydrase-producing bacterium. Lastly, an analysis of the crystalline species and content of the biogenic CaCO3 was performed to lay the groundwork for future crystalline adjustments and to offer technical support for the application of the calcium method.

Feasibility of Microbially Induced Carbonate Precipitation to Enhance the Internal Stability of Loess under Zn-Contaminated Seepage Conditions

Loess is widely distributed in Northwestern China and serves as the preferred engineering construction material for anti-fouling barriers. Heavy metal contamination in soil presents significant challenges to the engineering safety of vulnerable loess structures. Hence, there is an urgent need to investigate the impact of heavy metal ions on their percolation performance. In order to investigate the effectiveness of microbially induced carbonate precipitation (MICP) using Sporosarcina pasturii (CGMCC1.3687) bacteria in reducing internal seepage erosion, a saturated permeability test was conducted on reshaped loess under constant water head saturation conditions. The response of loess to deionized water (DW) and ZnCl2 solution seepages was analyzed by monitoring changes in cation concentration over time, measuring Zeta potential, and using scanning electron microscopy (SEM). The results indicate that the hydrolysis of Zn2+ creates an acidic environment, leading to the dissolution of carbonate minerals in the loess, which enhances its permeability. The adsorption of Zn2+ ions and the resulting diffusion double-layer (DDL) effect reduce the thickness of the diffusion layer and increase the number of free water channels. Additionally, the permeability of loess exposed to ZnCl2 solution seepage significantly increased by 554.5% compared to loess exposed to deionized water (DW) seepage. Following the seepage of ZnCl2 solutions, changes in micropore area ratio were observed, decreasing by 48.80%, while mesopore areas increased by 23.9%. MICP treatment helps reduce erosion and volume shrinkage in contaminated loess. Carbonate precipitation enhances the erosion resistance of contaminated loess by absorbing or coating fine particles and creating bridging connections with coarse particles. These research results offer new perspectives on enhancing the seepage properties of saturated loess in the presence of heavy metal erosion and the geochemical mechanisms involved.

Effect of Ba2+ on the biomineralization of Ca2+ and Mg2+ ions induced by Bacillus licheniformis.

The effect of barium ions on the biomineralization of calcium and magnesium ions is often overlooked when utilizing microbial-induced carbonate precipitation technology for removing barium, calcium, and magnesium ions from oilfield wastewater. In this study, Bacillus licheniformis was used to bio-precipitate calcium, magnesium, and barium ions. The effects of barium ions on the physiological and biochemical characteristics of bacteria, as well as the components of extracellular polymers and mineral characteristics, were also studied in systems containing coexisting barium, calcium, and magnesium ions. The results show that the increasing concentrations of barium ions decreased pH, carbonic anhydrase activity, and concentrations of bicarbonate and carbonate ions, while it increased the contents of humic acids, proteins, polysaccharides, and DNA in extracellular polymers in the systems containing all three types of ions. With increasing concentrations of barium ions, the content of magnesium within magnesium-rich calcite and the size of minerals precipitated decreased, while the full width at half maximum of magnesium-rich calcite, the content of O-C=O and N-C=O, and the diversity of protein secondary structures in the minerals increased in systems containing all three coexisting ions. Barium ions does inhibit the precipitation of calcium and magnesium ions, but the immobilized bacteria can mitigate the inhibitory effect. The precipitation ratios of calcium, magnesium, and barium ions reached 81-94%, 68-82%, and 90-97%. This research provides insights into the formation of barium-enriched carbonate minerals and offers improvements for treating oilfield wastewater.

Perspective of Hydrodynamics in Microbial-Induced Carbonate Precipitation: A Bibliometric Analysis and Review of Research Evolution

Microbial-induced carbonate precipitation (MICP) is a promising process with applications in various industries, including soil improvement, bioremediation, and concrete repair. However, comprehensive bibliometric analyses focusing on MICP research in hydrodynamics are lacking. This study analyses 1098 articles from the Scopus database (1999–2024) using VOSviewer and R Studio, identifying information on publications, citations, authors, countries, journals, keyword hotspots, and research terms. Global participation from 66 countries is noted, with China and the United States leading in terms of contributions. The top-cited papers discuss the utilisation of ureolytic microorganisms to enhance soil properties, MICP mechanisms, concrete deterioration mitigation, soil and groundwater flow enhancement, biomineral distribution, and MICP treatment effects on soil hydraulic properties under varying conditions. Keywords like calcium carbonate, permeability, and Sporosarcina pasteurii are pivotal in MICP research. The co-occurrence analysis reveals thematic clusters like microbial cementation and geological properties, advancing our understanding of MICP’s interdisciplinary nature and its role in addressing environmental challenges.

The Effect of Bacteria-to-Calcium Ratio on Microbial-Induced Carbonate Precipitation (MICP) under Different Sequences of Calcium-Source Introduction.

To explore the effects of the introduction order of calcium sources and the bacteria-to-calcium ratio on the microbially induced calcium carbonate precipitation (MICP) product CaCO3 and to achieve the regulation of CaCO3 crystal morphology, the mineralisation products of MICP were compared after combining bacteria and calcium at ratios of 1/9, 2/9, 3/9, 4/9, 5/9, and 6/9. A bacterial solution was combined with a urea solution in two calcium addition modes: calcium-first and calcium-later modes. Finally, under the calcium-first addition method, the output of high-purity vaterite-type CaCO3 was achieved at bacteria-to-calcium ratios of 2/9 and 3/9; under the calcium-later addition method, the output of calcite-type CaCO3 could be stabilised, and the change in the bacteria-to-calcium ratio did not have much effect on its crystalline shape.

Urban biomining: lithium recovery from spent batteries through multi-step bioprocesses

Multi-step design to evaluate the bio-recovery of lithium from spent batteries was studied. The first step consisted of lithium extraction from spent batteries, using bacterial and fungal acid extrolites. The second step explored lithium recovery in the form of carbonate salts by using MICP (Microbial Induced Carbonate Precipitation) bacteria from Sporosarcina species. For lithium extraction (Step 1) sulfuric acid produced by sulphur oxidizing bacteria Acidithiobacillus thiooxidans was evaluated for its capacity to leach lithium. Extraction with biogenic sulfuric acid and with fungal bio- produ-cts (from Aspergillus sp. and Simplicillum sp. isolated at our facilities) were compared with commercial sulfuric acid. For biorecovery processes (Step 2), two type strains of Sporosarcina sp. were tested due to their capacity to precipitate lithium carbonate. Results showed fungal bioextracts gave a lithium leaching yield close to 60% and a global recovery yield of 27%. These observations are reported for the first time and lay the foundations for continuing the study and scaling up of this combined process for lithium recovery.

Microbial-induced carbonate precipitation effectively prevents Pb2+ migration through the soil profile: Lab experiment and model simulation

Due to the inappropriate disposal of waste materials containing lead (Pb) and irrigation with sewage containing Pb, the migration of Pb2+ within the soil profile has been extensively investigated. The conventional Pb2+ block method is challenging to implement due to its complex operational procedures and high construction costs. To address this issue, this study introduces the microbial-induced carbonate precipitation (MICP) technique as a novel approach to impede the migration of Pb2+ in the soil profile. Soil acclimatization with urea resulted in an increased proportion of urease-producing microorganisms, including Bacillus, Paenibacillus, and Planococcaceae, along with heightened expression of urea-hydrolyzing genes (UreA, UreB, UreC, and UreG). This indicates that urea-acclimatized soil (Soil-MICP) possesses the potential to induce carbonate precipitation. Batch Pb2+ fixation experiments confirmed that the fixation efficiency of Soil-MICP on Pb2+ exceeded that of soil without MICP, attributed to the MICP process within the Soil-MICP group. Dynamic migration experiments revealed that the MICP reaction transformed exchangeable lead into carbonate-bound Pb, effectively impeding Pb2+ migration in the soil profile. Additionally, the migration rate of Pb2+ in Soil-MICP was influenced by varying urea amounts, pH levels, and pore flow rates, leading to a slowdown in migration. The Two-site sorption model aptly described the Pb2+ migration process in the Soil-MICP column. This study aims to elucidate the MICP biomineralization process, uncover the in-situ blocking mechanism of MICP on lead in soil, investigate the impact of Pb on key genes involved in urease metabolism, enhance the comprehension of the chemical morphology of lead mineralization products, and provide a theoretical foundation for MICP technology in preventing the migration of Pb2+ in soil profiles.

Deterioration phenomenon of Pb-contaminated aqueous solution remediation and enhancement mechanism of nano-hydroxyapatite-assisted biomineralization

Modern metallurgical and smelting activities discharge the lead-containing wastewater, causing serious threats to human health. Bacteria and urease applied to microbial-induced carbonate precipitation (MICP) and enzyme-induced carbonate precipitation (EICP) are denatured under high Pb2+ concentration. The nano-hydroxyapatite (nHAP)-assisted biomineralization technology was applied in this study for Pb immobilization. Results showed that the extracellular polymers and cell membranes failed to secure the urease activity when subjected to 60mM Pb2+. The immobilization efficiency dropped to below 50% under MICP, whereas it due to a lack of extracellular polymers and cell membranes dropped to below 30% under EICP. nHAP prevented the attachment of Pb2+ either through competing with bacteria and urease or promoting Ca2+/Pb2+ ion exchange. Furthermore, CO32- from ureolysis replaced the hydroxyl (-OH) in hydroxylpyromorphite to encourage the formation of carbonate-bearing hydroxylpyromorphite of higher stability (Pb10(PO4)6CO3). Moreover, nHAP application overcame an inability to provide nucleation sites by urease. As a result, the immobilization efficiency, when subjected to 60mM Pb2+, elevated to above 80% under MICP-nHAP and to some 70% under EICP-nHAP. The findings highlight the potential of applying the nHAP-assisted biomineralization technology to Pb-containing water bodies remediation. Environmental implicationInappropriate handling lead (Pb)-containing wastewater, induced by extensive metallurgical activities, threatens our living environments. The performance of traditional biomineralization, when subjected to high Pb2+ concentration, has been restricted since bacteria or urease is denatured. The nano-hydroxyapatite (nHAP)-assisted biomineralization technology was applied in this work for Pb immobilization. nHAP prevented the attachment of Pb2+ either through competing with bacteria and urease or promoting Ca2+/Pb2+ ion exchange. Further, nHAP conquered an inability to provide nucleation sites by urease. Moreover, nHAP application transformed lead apatite minerals into more stable carbonate-bearing minerals reducing the potential for Pb2+ to diffuse into surrounding environments.

Genomic characterization of a novel ureolytic bacteria, Lysinibacillus capsici TSBLM, and its application to the remediation of acidic heavy metal-contaminated soil

Soil heavy metal contamination is an essential challenge in ecological and environmental management, especially for acidic soils. Microbially induced carbonate precipitation (MICP) is an effective and environmentally friendly remediation technology for heavy metal contaminated sites, and one of the key factors for its realization lies in the microorganisms. In this study, Lysinibacillus capsici TSBLM was isolated from heavy metal contaminated soil around a gold mine, and inferred to be a novel ureolytic bacteria after phylogenomic inference and genome characterization. The urease of L. capsici TSBLM was analyzed by genetic analysis and molecular docking, and further applied this bacteria to the remediation of Cu and Pb in solution and acidic soils to investigate its biomineralization mechanism and practical application. The results revealed L. capsici TSBLM possessed a comprehensive urease gene cluster ureABCEFGD, and the encoded urease docked with urea at the lowest binding energy site (ΔG = −3.43 kcal/mol) connected to three amino acids threonine, aspartic, and alanine. The urease of L. capsici TSBLM is synthesized intracellularly but mainly functions extracellularly. L. capsici TSBLM removes Cu/Pb from the solution by generating heavy metal carbonates or co-precipitating with CaCO3 vaterite. For acidic heavy metal-contaminated soil, the carbonate-bound states of Cu and Pb increased significantly from 7 % to 16 % and from 23 % to 35 % after 30 days by L. capsici TSBLM. Soil pH improved additionally. L. capsici TSBLM maintained the dominant status in the remediated soil after 30 days, demonstrating good environmental adaptability and curing persistence. The results provided new strain resources and practical application references for the remediation of acidic heavy metal contaminated soil based on MICP.

Interaction of microorganisms with carbonates from the micro to the macro scales during sedimentation: Insights into the early stage of biodegradation

The assembly process of Organic Matter (OM) from single molecules to polymers and the formation process of Ca–CO3 ion-pairs are explored at the micro-scale, and then the relationship between OM and carbonate based on the results of microbially-induced carbonate precipitation (MICP) laboratory experiments is established at the macro-scale. Molecular dynamics (MD) is used to model the assembly of OM (a) in an aqueous solution, (b) on surfaces of calcite (10 1‾ 4) crystals and (c) on defective calcite (101‾ 4) crystal surfaces. From the MICP experiments, carbonate minerals containing abundant OM were precipitated and were characterized by Scanning Electron Microscopy (SEM), X-Ray Diffractometry (XRD) and Fourier Transform Infrared Spectroscopy (FTIR). The results of the MD show that OM is assembled into polymers in all three simulation systems. Although the Ca–CO3 ion-pairs and OM were briefly combined, the aggregation assembly of OM molecules and the precipitation of carbonate calcium are not related in the long run. The highly specific surface area of the defective calcite shows an increase in the adsorption of OM. The van der Waals forces, which are primarily responsible for controlling the assembly of OM molecules, increase with the degree of aggregation. According to the MICP experiments, OM is enriched on the mineral surfaces, and more OM is found at the steps of defective crystals with their larger surface areas. Through MD and MICP laboratory experiments, this work systematically describes the interaction of OM and carbonate minerals from the micro to the macro scales, and this provides insight into the interaction between OM and carbonates and biogeochemical processes related to the accumulation of OM in sediments.

Isolation and Characterization of Indigenous Ureolytic Bacteria from Mindanao, Philippines: Prospects for Microbially Induced Carbonate Precipitation (MICP)

Microbially induced carbonate precipitation (MICP), a widespread phenomenon in nature, is gaining attention as a low-carbon alternative to ordinary Portland cement (OPC) in geotechnical engineering and the construction industry for sustainable development. In the Philippines, however, very few works have been conducted to isolate and identify indigenous, urease-producing (ureolytic) bacteria suitable for MICP. In this study, we isolated seven, ureolytic and potentially useful bacteria for MICP from marine sediments in Iligan City. DNA barcoding using 16s rDNA identified six of them as Pseudomonas stutzeri, Pseudomonas pseudoalcaligenes, Bacillus paralicheniformis, Bacillus altitudinis, Bacillus aryabhattai, and Stutzerimonas stutzeri but the seventh was not identified since it was a bacterial consortium. Bio-cementation assay experiments showed negligible precipitation in the control (without bacteria) at pH 7, 8, and 9. However, precipitates were formed in all seven bacterial isolates, especially between pH 7 and 8 (0.7–4 g). Among the six identified bacterial species, more extensive precipitation (2.3–4 g) and higher final pH were observed in S. stutzeri, and B. aryabhattai, which indicate better urease production and decomposition, higher CO2 generation, and more favorable CaCO3 formation. Characterization of the precipitates by scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS) and attenuated total reflectance Fourier transform spectroscopy (ATR-FTIR) confirmed the formation of three carbonate minerals: calcite, aragonite, and vaterite. Based on these results, all six identified indigenous, ureolytic bacterial species from Iligan City are suitable for MICP provided that the pH is controlled between 7 and 8. To the best of our knowledge, this is the first report of the urease-producing ability and potential for MICP of P. stutzeri, P. pseudoalcaligenes, S. stutzeri, and B. aryabhattai.

Microbial induced carbonate precipitation for cadmium removal in flue gas from sludge incineration

Flue gas cadmium emission during sludge incineration damages to the health of humans and ecological environment due to its toxic, persistent, and bioaccumulation. Cadmium removal in flue gas based on microbial induced carbonate precipitation (MICP) was investigated using a denitrifying membrane biofilm reactor (DBR). Cadmium removal efficiency obtained 89.8%. Caenispirillum, Halomonas, Stappia, Thauera were the dominant aerobic denitrifiers, which carried twelve cadmium resistance genes (czcA, czcB, czcC, czcD, cadA, cadC, cmtR, cueR, copZ, ctpC, zipB, zntA), and expressed six cadmium resistant proteins (ZntA, CzcA, CtpA, CopZ, CopR and CopB) and ten denitrifying proteins(NarG, NarH, NarI, NapA, NapB, NirK, NirS, NorB, NorC, NosZ), those proteins regulated binding, transport, exportation of cadmium and denitrification. The upregulation of nitrate reductase and nitrite reductase indicated that cadmium could promote the reduction of NO3- to NO, as shown by an integrated metagenomic and metaproteomic sequencing. Cadmium speciation of biofilms, such as cadmium carbonate, increased in proportion, whereas others, such as organic matter-bound cadmium, decreased. The biological adsorption process would gradually yield to MICP process. The MICP stabilized cadmium in flue gas mainly by precipitating Cd through sodium succinate-induced carbonate precipitation. Flue gas cadmium could transform to cadmium carbonate bioprecipitate; carboxyl, hydroxyl and amine groups in cell surface and humic acid could complex cadmium to form Cd-EPS. Denitrifying bacteria could induce MICP process to stabilize cadmium in flue gas. This provides a green and low-cost DBR process as advanced treatment technology for remove cadmium from flue gas.

Statistical and machine learning analysis for the application of microbially induced carbonate precipitation as a physical barrier to control seawater intrusion

Seawater intrusion in coastal aquifers is a significant problem that can be addressed through the construction of subsurface dams or physical cut-off barriers. An alternative method is the use of microbially induced carbonate precipitation (MICP) to reduce the hydraulic conductivity of the porous medium and create a physical barrier. However, the effectiveness of this method depends on various factors, and the scientific literature presents conflicting results, making it challenging to generalise the findings. To overcome this challenge, a statistical and machine learning (ML) approach is employed to infer the causes for the reduction in hydraulic conductivity and identify the optimum MICP parameters for preventing seawater intrusion. The study involves data curation, exploratory analysis, and the development of various models to fit the input data (k-Nearest Neighbours – kNN, Support Vector Regression – SVR, Random Forests – RF, Gradient Boosting – XgBoost, Linear model with interaction terms, Ensemble learning algorithms with weighted averages – EnL-WA and stacking – EnL-Stack). The models performed reasonably well in the region where permeability reduction is sensitive to carbonate increase capturing the permeability reduction profile with respect to cementation level while demonstrating that they can be used in initial assessments of the specific conditions (e.g., soil properties). The best performing algorithms were the EnL-Stack and RF followed by XgBoost and SVR. The MICP method is effective in reducing hydraulic conductivity provided that the various biochemical parameters are optimised. Critical biochemical parameters for successful MICP formulations are the bacterial optical density, the urease activity, calcium chloride concentration and flow rate as well as the interaction terms across the properties of the porous media and the biochemical parameters. The models were used to identify the optimum MICP formulation for various porous media properties and the maximum permeability reduction profiles across cementation levels have been derived.

Investigation on the Shear Behavior and Mechanism of MICP-Treated Loess Soil

Microbially induced carbonate precipitation (MICP) has been utilized as a new method to improve loess soil strength. In this study, we investigated the influence of the main parameters on the shear strength of MICP-treated loess specimens. Initially, culture media with different formulas and pH values were examined to identify the most efficient medium for loess soil. To explore the shear behavior of MICP-treated loess under general stress levels, unconfined compressive strength (UCS) tests and triaxial tests relevant to the compression strength and vertical loads were performed on MICP-treated loess with different calcium sources, cementation concentrations, and curing periods. Subsequently, calcium chloride was selected as the optimal calcium source based on the ultimate strength of the MICP-treated loess. The effective cementation concentration in the loess soil was between 1.0 and 1.25 M. The ultimate strength of the MICP-treated loess was 3.6 times of the untreated loess. The stress-strain curves indicate that a higher cementing effect can be expected with an increase in the curing period. The formation process of calcium carbonate and the micromorphology of the MICP-treated loess samples were examined using scanning electron microscopy. In this study, we present an environmentally friendly technique for improving loess soil strength.

Study on the mechanism and stability of microbially induced struvite precipitation (MISP) for enhanced immobilization of uranium

The microbial induced carbonate precipitation (MICP) technique has been shown to immobilize a wide range of heavy metals. However, the MICP process for removing uranium-dominated composite heavy metals ignores the formation of substances such as uranyl carbonate, which can easily lead to uranium migration. Furthermore, the by-product ammonia is one of the bottlenecks limiting the application of MICP. In this work, the microbially induced struvite precipitation (MISP) method was used to enhance U immobilization while reducing by-product contamination. The results showed that the optimal uranium removal rate of the MISP process was increased by 82.98% compared to the MICP. Characterization results confirmed the co-precipitation of U with NH4+ in the form of uramphite and struvite, while the participation of U changed the mineral crystal shape. Stability experiments showed that the maximum release of U from struvite was only 1.27%, which is much lower than the minimum release of U from CaCO3. As a result, the MISP process both enhanced uranium immobilization efficiency, decreased the risk of uranium migration-release, and reduced the environmental impact of the by-product ammonia. These results provided a new perspective for studying U contamination remediation and its co-precipitation behavior in microbial mineralization.