Microbially Induced Carbonate Precipitation Process Research Articles

Synergistic mechanisms of humic acid and biomineralization in cadmium remediation using Lysinibacillus fusiformis

AbstractHeavy metal pollution, particularly cadmium, poses severe environmental and health risks due to its high toxicity and mobility, necessitating effective remediation strategies. While both microbially induced carbonate precipitation (MICP) and humic acid adsorption are promising methods for heavy metal mitigation, their combined effects, particularly the influence of humic acid on the MICP process, have not been thoroughly investigated. This study explores the interaction between humic acid and MICP, revealing that humic acid significantly inhibits the MICP process by reducing urease activity, with the 10% humic acid treatment resulting in a 23.8% reduction in urease activity compared to the control. Additionally, while higher concentrations of humic acid did not significantly reduce cadmium ion concentrations, they did result in a slight increase in organically bound cadmium, indicating an interaction that could alter metal speciation in the soil. These findings provide important insights into the mechanisms by which humic acid affects MICP, offering a foundation for optimizing combined remediation approaches. Future research should aim to fine‐tune the balance between MICP and humic acid to enhance the overall efficiency of cadmium remediation strategies. This study contributes to the development of more effective and sustainable methods for addressing cadmium contamination.

Two factors facilitate the cost-effective and harmless cementation of rare earth slags through MICP technology: Carbonic anhydrase bacteria and endogenous calcium ions

Microbially induced carbonate precipitation (MICP), a solidification/stabilization technique that doesn't interfere with recycling and can reduce the migration of pollutants, has been employed to cement rare earth slags (RES). Currently, the MICP process of the urease/carbonic anhydrase pathway has attracted considerable attention. However, the MICP process of the urease pathway may produce high concentrations of ammonia, which can cause secondary contamination; the addition of a calcium source (calcium chloride) may increase the cost of the treatment process. In this study, two factors, carbonic anhydrase bacteria and endogenous calcium ions (Ca²⁺), facilitated the MICP technology to cost-effectively and harmlessly cement RES and stabilize contaminants. The findings indicated that strain Z1 could utilize endogenous Ca²⁺ to cement the RES, thereby enhancing its unconfined compressive strength, reducing the disintegration rate, radiation dose, and wind erosion intensity, and facilitating the stabilization of heavy metals and radionuclides. In comparison to the CK group, the leaching concentrations of Cu2+, Pb2+, Zn2+, Cd2+, Th(Ⅳ), and U(Ⅵ) in the C1 group were reduced by 42.37 %, 20.20 %, 18.77 %, 22.53 %, 12.32 %, and 23.66 %, respectively. The treatment effect can be further enhanced by adding exogenous Ca2+. This study's findings can help reduce the potential environmental risks in the long-term sequestration of RES, while also providing technical support for the sustainable development of the rare earth industry.

AC-assisted microbially induced carbonate precipitation for sand reinforcement: An experimental study

Microbially induced carbonate precipitation (MICP) is a promising method for transforming natural soils into a rock-like material, enhancing soil strength and creating an environmentally friendly engineered geomaterial for load-bearing purposes. Applying alternating current (AC) for enhancing precipitation including changing the crystalline form of the calcium carbonate precipitates appeals as a possible solution to break the upper limit of the unconfined compressive strength (UCS) of bio-treated specimens. To assess the viability of AC-assisted MICP, a series of experiments were designed and conducted under various combination of conditions. The UCS, calcium carbonate content and permeability of the bio-fabricated specimens were obtained to evaluate the treatment effectiveness of AC-assisted MICP. The results demonstrate that the UCS of the sand column exhibits a linear increase with the applied voltage from 10 V to 30 V (i.e., electric field strength from 0.91 V/cm to 2.73 V/cm). The UCS value of the bio-specimen reaches 9.4 MPa after 3 treatments at a concentration of 1.00 mol/L, a voltage of 30 V, and a frequency of 100 Hz. With the assistance of an AC electric field, the adverse impacts caused by high chemical concentrations in the MICP process can be mitigated. We report that a more uniform distribution of the calcium carbonate content of the treated specimen is obtained under an optimal AC frequency of approximately 100 Hz in the current series of experiments. The induced ion vibration under the action of AC results in a change in crystalline form and an increase in the amount and uniformity of crystals precipitated on the surface of the soil grains, supported by X-ray diffraction (XRD) patterns and scanning electron microscope (SEM) images. For reference, the energy consumption and the cost for increasing the UCS of the bio-treated specimen to 5 MPa is estimated at 375.86 kWh and 676.55 HK$ per cubic meter, respectively. The findings from our experimental investigation and analysis provide compelling evidence that utilising AC electric field holds great potential for achieving an enhanced treatment effect of MICP and hence a stronger bio-soil.

Use of rise husks to improve the efficiency of MICP-based soil improvement technique

Biogrouting has been identified as the best alternative approach for the conventional cement and chemical grouts. Among the various types of biogrouting techniques, microbial induced carbonate precipitation (MICP) has been recently recognized as a promising pathway for producing biogrout material, hence densifying the weak soils. In MICP, calcium carbonate (CaCO3) biocement is produced through biochemical reactions induced by the urease enzyme. The efficiency and productivity of MICP can be further improved by introducing various additives, including fibers, organic polymers, powders, waste materials, etc. In this research, rice husk was sustainably incorporated into the MICP process, and (i) the effect of the rice husk content, (ii) the concentration of the cementation solution and (iii) the treatment time on the unconfined compressive strength (UCS) of the sandy soil were investigated at the laboratory-scale. Cementation solutions with higher concentrations exhibited a higher UCS values than solutions with lower concentrations. Interestingly, the content of rice husk had a marked influence on the UCS, and the optimum rice husk content was 15%. Compared to that of the control sample (0% rice husk), a threefold greater UCS was obtained for 15% of the rice husk content sample. The outcomes further suggested that longer treatment with lower concentration of cementation solution was the best combination for achieving effective cementation within the sand matrix. Rice husk acted as the template for CaCO3 crystals to nucleate while promoting the efficient formation of CaCO3, indicating that the rice husk is an excellent additive to improve the efficiency of the MICP biocementation in a sustainable way.

Trends and opportunities for greener and more efficient microbially induced calcite precipitation pathways: a strategic review

Microbially induced carbonate precipitation (MICP) is extensively used for soil stabilisation, sand erosion control, bioremediation and concrete repair. However, detailed bibliometric studies on MICP are scarce, particularly regarding industrial feedstocks, such as urea for ureolytic bacteria and calcium chloride, and the associated ammonia gas production, which poses challenges for large-scale applications. This study reviews 857 articles from the Scopus database (2000–2024), using the VOSviewer software to visualise data on publications, citations, authors, countries, journals, trending keywords and research topics. Significant global collaboration is evident, led by China and the USA. The most cited papers discuss ureolytic microorganisms enhancing soil properties, the basic MICP process, reducing concrete damage, improving soil and groundwater systems and the impacts of MICP on urea hydrolysis and ammonia production. Key terms such as ‘calcium carbonate’, ‘ammonia’ and ‘Sporosarcina pasteurii’ are central to MICP research. Co-occurrence analysis highlights thematic clusters such as microbial cementation and geological features, emphasising the multidisciplinary role of MICP in tackling environmental problems. This review highlights the need to mitigate environmental impacts, particularly those of ammonia production and energy use, in ureolytic MICP. Life-cycle assessments reveal that while ureolytic MICP reduces carbon dioxide emissions, it often consumes more energy, necessitating alternative nutrient sources and cost-effective practices for large-scale applications.

CT image-based simulation of microbially induced carbonate precipitation

Soil improvement efforts occasionally negatively impact the delicate balance of the natural environment and disrupt ecosystems. Thus, adopting ecologically sensitive techniques that can harmoniously safeguard the Earth is imperative. Recently, researchers have focused on microbially induced carbonate precipitation (MICP), an eco-friendly soil enhancement method that fosters increased interparticle cohesion and augments the mechanical strength of soil. The spatial distribution of calcium carbonate precipitation in MICP-treated soil can be observed using scanning electron microscopy and X-ray computed tomography (CT). However, since this precipitation occurs in the microscopic region, it cannot be directly observed over time. Therefore, in this study, we propose a simulation technique to replicate the MICP process using a voxel dataset obtained via X-ray CT analysis. The calculated patterns of calcium carbonate were distributed on the soil particles in the same manner as the spatial precipitation observed in the experiments. Notably, some precipitates bridged the soil particles, indicating that the proposed technique could morphologically estimate the calcium carbonate distribution through MICP. Evidently, the proposed technique could accurately simulate MICP phenomena. Furthermore, the results showed that the proposed technique can effectively express a multidiscliplinary phenomenon. Therefore, the study can inspire further studies that employ such techniques to investigate MICP.

Different calcium sources affect the products and sites of mineralized Cr(VI) by microbially induced carbonate precipitation

Microbially induced carbonate precipitation (MICP) is a common biomineralization method, which is often used for remediation of heavy metal pollution such as hexavalent chromium (Cr(VI)) in recent years. Calcium sources are essential for the MICP process. This study investigated the potential of MICP technology for Cr(VI) remediation under the influence of three calcium sources (CaCl2, Ca(CH3COO)2, Ca(C6H11O7)2). The results indicated that CaCl2 was the most efficient in the mineralization of Cr(VI), and Ca(C6H11O7)2 could significantly promote Cr(VI) reduction. The addition of different calcium sources all promoted the urease activity of Sporosarcina saromensis W5, in which the CaCl2 group showed higher urease activity at the same Ca2+ concentration. Besides, with CaCl2, Ca(CH3COO)2 and Ca(C6H11O7)2 treatments, the final fraction of Cr species (Cr(VI), reduced Cr(III) and organic Cr(III)-complexes) were mainly converted to the carbonate-bound, cytoplasm and cell membrane state, respectively. Furthermore, the characterization results revealed that three calcium sources could co-precipitate with Cr species to produce Ca10Cr6O24(CO3), and calcite and vaterite were present in the CaCl2 and Ca(CH3COO)2 groups, while only calcite was present in the Ca(C6H11O7)2 group. Overall, this study contributes to the optimization of MICP-mediated remediation of heavy metal contaminated soil. CaCl2 was the more suitable calcium source than the other two for the application of MICP technology in the Cr(VI) reduction and mineralization.

Effects of biochar and magnesium oxide on cadmium immobilized by microbially induced carbonate: Mobilization or immobilization in alkaline agricultural soils?

Microbially induced carbonate precipitation (MICP) is a promising technique for remediating heavy metal-contaminated soils. However, the effectiveness of MICP in immobilizing Cd in alkaline calcareous soils, especially when applied in agricultural soils, remains unclear. Biochar and magnesium oxide are two environmentally friendly passivating materials, and there are few reports on the combined application of MICP with passivating materials for remediating heavy metal-contaminated soils. Additionally, the number of treatments with MICP cement and the concentration of calcium chloride during the MICP process can both affect the effectiveness of heavy metal immobilization by MICP. Therefore, we conducted MICP and MICP-biochar-magnesium oxide treatments on agricultural soils collected from Baiyin, Gansu Province (pH = 8.62), and analyzed the effects of the number of treatments with cement and the concentration of calcium chloride on the immobilization of Cd by MICP and combined treatments. The results showed that early-stage MICP could immobilize exchangeable cadmium and increase the residual cadmium content, especially with high-concentration calcium chloride MICP treatment. However, in the later stage, soil nitrification and exchange processes led to the dissolution of carbonate-bound cadmium and cadmium activation. The fixing effect of MICP influence whether the MICP-MgO-biochar is superior to the MgO-biochar. Four treatments with cement were more effective than single treatment in MICP-biochar-magnesium oxide treatment, and the MICP-biochar-magnesium oxide treatment with four treatments was the most effective, with passivation rates of 40.7% and 46.6% for exchangeable cadmium and bioavailable cadmium, respectively. However, attention should be paid to the increase in soil salinity. The main mechanism of MICP-magnesium oxide-biochar treatment in immobilizing cadmium was the formation of Cd(OH)2, followed by the formation of cadmium carbonate.

Molecular and geochemical basis of microbially induced carbonate precipitation for treating acid mine drainage: The case of a novel Sporosarcina genomospecies from mine tailings

Microbially induced carbonate precipitation (MICP) immobilizes toxic metals and reduces their bioavailability in aqueous systems. However, its application in the treatment of acid mine drainage (AMD) is poorly understood. In this study, the genomes of Sporosarcina sp. UB5 and UB10 were sequenced. Urease, carbonic anhydrases, and metal resistance genes were identified and enzymatic assays were performed for their validation. The geochemical mechanism of precipitation in AMD was elucidated through geo-mineralogical analysis. Sporosarcina sp. UB5 was shown to be a new genomospecies, with an average nucleotide identity < 95 % (ANI) and DNA-DNA hybridization < 70 % (DDH) whereas UB10 is close to S. pasteurii. UB5 contained two urease operons, whereas only one was identified in UB10. The ureolytic activities of UB5 and UB10 were 122.67 ± 15.74 and 131.70 ± 14.35 mM NH4+ min−1, respectively. Both strains feature several carbonic anhydrases of the α, β, or γ families, which catalyzed the precipitation of CaCO3. Only Sporosarcina sp. UB5 was able to immobilize metals and neutralize AMD. Geo-mineralogical analyses revealed that UB5 directly immobilized Fe (1–23 %), Mn (0.65–1.33 %) and Zn (0.8–3 %) in AMD via MICP and indirectly through adsorption to calcite and binding to bacterial cell walls. The MICP-treated AMD exhibited high removal rates (>67 %) for Ag, Al, As, Ca, Cd, Co, Cu, Fe, Mn, Pb, and Zn, and a removal rate of 15 % for Mg. This study provides new insights into the MICP process and its applications to AMD treatment using autochthonous strains.

Improving the thermal-mechanical performance of bio-treated backfill materials by addition of magnetic iron oxide nanoparticles (nano-Fe3O4)

The thermal conductivity of backfill materials directly affects the heat transfer efficiency between energy geo-structures and the surrounding stratum. Microbially induced carbonate precipitation (MICP) possesses great potential for improving the thermal conductivity of backfill materials. Magnetic iron oxide nanoparticles (i.e., nano-Fe3O4) have been proven to enhance bacterial biochemical activity by altering the permeability of bacterial biofilms, thus potentially improving the MICP process. It was supposed to enhance the thermal conductivity of backfill materials, allowing for applying energy geo-structures in arid environments. In this study, MICP in a solution environment was conducted to analyze bacterial urease activity and morphology of precipitation at varying nano-Fe3O4 contents. Additionally, sand columns treated with MICP and different nano-Fe3O4 contents were performed to obtain the thermal conductivity and unconfined compressive strength (UCS) through the transient plane source (TPS) method and uniaxial compression (UC) experiment. The mineral type, precipitation morphology, and microstructure were identified using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The mechanism of the effect of nano-Fe3O4 on bacterial urease activity and thermal-mechanical behaviors was also discussed. The results indicated that the nano-Fe3O4 could enhance bacterial urease activity and promote vaterite precipitation in the solution environment. Conversely, when applied to MICP-treated sand, nano-Fe3O4 could facilitate calcite formation. Increasing the nano-Fe3O4 content showed a positive correlation with increased thermal conductivity and UCS. Specifically, the optimal values of thermal conductivity and UCS increased by 2.42 times and 2.39 times, respectively, compared to MICP-treated specimens without nano-Fe3O4. Microstructure analysis revealed that calcite precipitation at the particle contact served a dual function: cementing particles, thereby improving the mechanical strength and simultaneously acting as a "thermal bridge" to enhance the thermal conductivity. Furthermore, this study provides a new perspective on utilizing magnetized bacteria to reinforce specific locations within rocks and soils in the presence of an external magnetic field.

Comparison of the efficiency of traditional MICP and two-step MICP method for immobilizing heavy metals in aquatic environments

The application of the microbially induced carbonate precipitation (MICP) method for remediating heavy metals (i.e., HMs) has recently garnered significant attention. Nevertheless, the inhibition of urease activity by toxic Cd2+, Pb2+, Zn2+, and Cu2+ poses a challenge for MICP-based remediation of HMs contamination. This study: (1) first performed the traditional MICP tests (in which the bacterial solution, urea solution, and HMs were mixed simultaneously), and investigated the toxic effect of HMs on the urease activity and the immobilization efficiency, (2) analyzed the toxicity and immobilization mechanism during the MICP process by combining the simulation and XRD tests, (3) conducted the two-step MICP tests (which initially mixed the bacterial solution and urea solution to promote urea hydrolysis, then added the HMs solutions for HMs precipitation) to improve the immobilization efficiency. The tube experiments and simulations were investigated in the HMs concentration range from 1 to 10 mmol/L. Indicators including ammonium concentration, HMs concentrations, and pH were measured/recorded during the tests. The results show that soluble HMs exhibit a concentration-dependent inhibition of urea hydrolysis during the traditional MICP process, resulting in a decreasing immobilization efficiency. The two-step MICP method can effectively immobilize almost the Cd2+ and Zn2+ when the initial urea hydrolysis period exceeds 1–2 h. In addition, a high immobilization rate of over 90% can be achieved for Cu-contaminated solutions at the optimal first-step reaction time. Compared with the traditional MICP procedure, the effective two-step MICP method exhibits more promising application prospects for the immobilization of soluble HMs in aquatic environments.

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.

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.

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.

Biogenic calcium improved Cd2+ and Pb2+ immobilization in soil using the ureolytic bacteria Bacillus pasteurii

Bioremediation based on microbial-induced carbonate precipitation (MICP) was conducted in cadmium and lead contaminated soil to investigate the effects of MICP on Cd and Pb in soil. In this study, soil indigenous nitrogen was shown to induce MICP to stabilize heavy metals without inputting exogenous urea. The results showed that applying Bacillus pasteurii coupled with CaCl2 reduced Cd and Pb bioavailability, which could be clarified through the proportion of exchangeable Cd and Pb in soil decreasing by 23.65 % and 12.76 %, respectively. Moreover, B. pasteurii was combined separately with hydroxyapatite (HAP), eggshells (ES), and oyster shells (OS) to investigate their effects on soil heavy metals' chemical fractions, toxicity characteristic leaching procedure (TCLP)-extractable Cd and Pb as well as enzymatic activity. Results showed that applying B. pasteurii in soil significantly decreased the heavy metals in the exchangeable fraction and increased them in the carbonate phase fraction. When B. pasteurii was combined with ES and OS, the content of carbonate-bound Cd increased by 114.72 % and 118.81 %, respectively, significantly higher than when B. pasteurii was combined with HAP, wherein the fraction of carbonate-bound Cd increased by 86 %. The combination of B. pasteurii and biogenic calcium effectively reduced the leached contents of Cd and Pb in soil, and the TCLP-extractable Cd and Pb fractions decreased by 43.88 % and 30.66 %, respectively, in the BP + ES group and by 52.60 % and 41.77 %, respectively, in the BP + OS group. This proved that MICP reduced heavy metal bioavailability in the soil. Meanwhile, applying B. pasteurii and calcium materials significantly increased the soil urease enzyme activity. The microstructure and chemical composition of the soil samples were studied, and the results from scanning electron microscope, Fourier transform infra-red spectroscopy, and X-ray diffraction demonstrated the MICP process and identified the formation of CaCO3, Ca0.67Cd0.33CO3, and PbCO3 in heavy metal-contaminated soil.

The Cd immobilization mechanisms in paddy soil through ureolysis-based microbial induced carbonate precipitation: Emphasis on the coexisting cations and metatranscriptome analysis

Microbial induced carbonate precipitation (MICP) can immobilize metals and reduce their bioavailability. However, little is known about the immobilization mechanism of Cd in the presence of soil cations and the triggered gene expression and metabolic pathways in paddy soil. Thus, microcosmic experiments were conducted to study the fractionation transformation of Cd and metatranscriptome analysis. Results showed that bioavailable Cd decreased from 0.62 to 0.29 mg/kg after 330 d due to the MICP immobilization. This was ascribed to the increase in carbonate bound, Fe-Mn oxides bound, and residual Cd. The underlying immobilization mechanisms could be attributed to the formation of insoluble Cd-containing precipitates, the complexation and lattice substitution with carbonate and Fe, Mn and Al (hydr)oxides, and the adsorption on functional group on extracellular polymers of cell. During the MICP immobilization process, up-regulated differential expression urease genes were significantly enriched in the paddy soil, corresponding to the arginine biosynthesis, purine metabolism and atrazine degradation. The metabolic pathway of bacterial chemotaxis, flagellum assembly, and peptidoglycan biosynthesis and the expression of cadA gene related to Cd excretion enhanced Cd resistance of soil microbiome. Therefore, this study provided new insights into the immobilization mechanisms of Cd in paddy soils through ureolysis-based MICP process.

Understanding the formation and structure of bio-mineralization for self-healing of marine concrete: An experimental and thermodynamic approach

In marine engineering, infrastructures are facing serious issues of deterioration resulted largely from concrete cracking due to the hostile seawater environment. Microbial induced carbonate precipitation (MICP) is an effective and environmentally friendly way to achieve self-healing of concrete cracks. However, the formation and structure of MICP products for crack healing in seawater remain unclear, thus hindering the application of MICP in self-healing of marine structures. By means of experimental and thermodynamic approaches, this work first investigated the evolution of aqueous species as well as the phase assemblages and microstructures of bio-minerals in compound solutions simulating concrete cracks in seawater. The coupling effects of Mg2+ and bacteria on bio-mineralization kinetics result in significant discrepancy between experimental results and thermodynamic outputs in terms of phase assemblages in seawater, yet the morphology depends on Mg2+ rather than bacteria. A comparative analysis on healing products collected from real concrete cracks in seawater revealed a fast abiotic precipitation of brucite followed by the MICP process, which is more conducive to attain a high-efficient self-healing in marine environment.

Properties and mechanisms of steel slag strengthening microbial cementation of cyanide tailings

The advantages of microbial induced carbonate precipitation (MICP) as bio-cementation technology for tailings-solidification are under extensive investigation. In order to improve performance of bio-cementation, many strengthening materials were applied to the bio-cementation of tailings. Steel slag (SS) is a kind of industrial solid waste, its chemical composition and mineral composition are similar to cement, and it has a certain application prospect as an auxiliary cementing material. In this study, the properties and mechanism of SS strengthening MICP cementation of cyanide tailings (CT) were investigated. The results showed that Sporosarcina pasteurii growth is not inhibited by SS, and Sporosarcina pasteurii can promote the hydration reaction of SS, providing a suitable alkaline environment and Ca2+, promoting the production of more CaCO3 in the MICP process. When 200 mL of CT leachate was added 1.4 g SS (200–400 mesh), the adsorption of Cu, Pb, Zn, Cd, total cyanide (T-CN), and free cyanide (F–CN) reached 48.05%, 44.28%, 36.25%, 16.67%, 79.05%, and 67.20%, respectively. The maximum unconfined compressive strength（UCS） of the cemented body (with 5%, 150 mesh SS) was 1.97 MPa, which was 3.396 times as high as that without SS. The cemented body with the addition of SS (5%, 150 mesh) contained more carbonate bound Cu (2.75%), Pb (4.89%), Zn (5.37%), and Cd (5.75%), and less exchangeable Cu (3.65%), Pb (6.85%), Zn (2.27%), and Cd (4.42%) than that without SS. In summary, the addition of SS improved the UCS of cemented bodies and the stability of heavy metals and cyanide, reduced the environmental risks existing in the process of CT storage. Meanwhile, it also provides new ideas for resource utilization of industrial solid waste SS and improvement of mine filling materials.

Effect of a harsh circular environment on self-healing microbial-induced calcium carbonate materials for preventing Pb2+ migration

Microbially induced carbonate precipitation (MICP) is increasingly being explored for Pb-contaminated water bodies and soil remediation. However, the Pb-related precipitate resulting from the MICP process can possibly leach acid over time when subjected to harsh environments, causing serious threats to human health. In this study, for the first time, self-healing microbial-induced calcium carbonate (MICC) materials are proposed and applied to prevent Pb2+ migration where the Pb-related precipitate is acid leached after spore germination, spore-vegetative cell transformation, urease secretion, and urea hydrolysis, thereby producing spore-containing precipitation. This process was repeated five times to explore the effect of a harsh circular environment on self-healing MICC materials. Results indicated that Pb immobilization would have deteriorated if the inosine and trace elements had not been intervened during spore germination and spore-vegetative cell transformation, respectively. The spores and vegetative cells provided extra nucleation sites for Pb2+ and minerals to attach. The extracellular polymeric substances (EPSs) combined Pb2+ with functional groups and chemical bonds to prevent their migration to surrounding environments. The scanning electron microscopy–energy-dispersive X-ray spectroscopy (SEM–EDS) images also indicated that the cerussite mineral was precipitated prior to the calcite mineral because Pb2+ had more affinity to combine with CO32- and OH-. An immobilization efficiency of greater than 95% remained nearly the same after five cycles, while it reduced very quickly to less than 10% after three cycles when neglecting the self-healing MICC materials, thus highlighting their relative merits.

Application of electric fields to improve cementation homogeneity of biogrouted sands

AbstractThe inhomogeneity of cementation is a common problem in soil improvement with microbially induced carbonate precipitation process (MICP), which is caused by uneven spatial distribution of bio‐induced calcium carbonate precipitation. Electric field adjustment method was developed and adopted for improving cementation homogeneity of bio‐grouted sands in this research. Horizontal one‐dimensional biogrouting experiments were conducted in sand columns under 0.1 V/cm of direct current (DC) electric field. The column treated with DC electric field showed a significant improvement in homogeneity of calcium carbonate precipitation as compared to the control (without DC electric field). Spatial distribution of calcium carbonate in columns were affected by the properties of bacteria, whether suspended or attached. In general, the attached bacteria with uneven spatial distribution played an important role in the calcium carbonate precipitation when flow rate was regulated at 0.54 cm/min, while the suspended bacteria could also be attached upon times with consumption of cations during biogrouting. With the influence of electric field, suspended bacterial transport in porous media occurred in one direction to the target locations because of the electrophoresis at an average speed of 0.14 cm/h, while sodium ion was found to be moved from anode to cathode by electromigration approach. Electrolysis of water, on the other hand, resulted a slight fluctuation in pH, even though this variation did not show any significant effect on the metabolic activity of bacteria and microbial‐induced carbonate precipitation process. The redistribution of suspended bacteria and sodium ion in porous media provided a possible for attached bacteria to distribute uniformly, which could be considered as the key factor to influence spatial distribution of the bio‐induced calcium carbonate in application of electric field adjustment method.