State-of-the-art review of soil erosion control by MICP and EICP techniques: Problems, applications, and prospects

Response mechanism of extracellular polymers in the remediation of chromium pollution by carbonate mineralizing bacteria

This study examines the adaptability of carbonate mineralizing bacteria in Cr(iii)-contaminated environments with varying Cr(iii) concentrations and their response mechanism via EPS. Cr(iii) removal efficiency declined with concentrations exceeding 1000 mg L-1, while the removal amount continued to rise, indicating strong Cr(iii) tolerance in the bacterium. Analysis of dynamic changes in EPS revealed a significant increase in production, with polysaccharides and proteins playing key roles in Cr(iii) binding. A notable increase in mannose in the monosaccharide composition of EPS suggests its involvement in Cr(iii) binding. Moreover, alterations in the protein secondary structure, such as a reduction in α-helix content and an increase in β-sheet and random coil structures, may enhance EPS interaction with Cr(iii). These findings demonstrate that EPS contributes to heavy metal remediation not only through its polysaccharide components but also through changes in protein structure, offering a new theoretical foundation for Cr(iii) bioremediation.

Strength and water retention behavior of loess stabilized with guar gum and fiber under dry and wet cycles

To improve the mechanical and water-retention behavior of loess and reduce the erosion failure caused by dry-wet cycles, the applicability of guar gum (GG) biopolymer and basalt fiber in the solidification of loess is investigated. The addition of GG can enhance the compressive strength and disintegration resistance of loess. When the GG content is 0.5%, 1.0%, and 2.0%, the compressive strength of stabilized loess increased by 30.15%, 67.85%, and 124.8%, respectively. The shear strength of GG-fiber stabilized loess is obviously higher than that of specimens without GG, and the higher the GG content, the stronger the shear resistance. The dry-wet cycles have a significant degradation effect on untreated and GG-fiber stabilized loess. After 8 dry-wet cycles, the cohesion and internal friction angle of the specimen containing 2.0% GG decreased by 45.90% and 10.74%, respectively. As the GG content increases, the water-retention capacity of stabilized is enhanced, but the dry-wet cycles have a significant deterioration effect. Furthermore, the soil water characteristic curves prediction model for GG-fiber stabilized loess is established by considering the effect of dry-wet cycles and GG content, and the prediction results are basically consistent with the measured data (R2 = 0.92). This study confirmed the feasibility of applying guar gum and basalt fiber to improve soil strength, water stability, and water-retention capacity, and provided a basis for engineering construction and soil erosion control in the loess area.

Research on the Strength Properties and Microscopic Mechanism of Loess Stabilized by the Combined Use of MICP Technology and Plant Straw

There are many drawbacks in traditional loess-strengthening technology. MICP (microbially induced calcium carbonate precipitation) technology provides a new approach to loess management, but there are few studies on loess solidification and a lack of engineering application research and verification. This study investigated the strength and microscopic mechanisms of loess solidified by the application of MICP technology combined with plant straw. The permeability conditions of loess for MICP technology were derived, and multiple sets of experiments were conducted using specific loess, Bacillus pasteurii, cementing solution, plant straw, and other materials. The experiments explored shear strength, unconfined compressive strength, microscopic properties, plant growth adaptability, and factors affecting bacterial growth. The results indicated that within the temperature range of 25-35 °C, the concentration and urease activity of Bacillus pasteurii were significantly affected by temperature, with the highest bacterial concentration observed at 30 °C. During scaled-up cultivation, increasing the inoculation ratio prevented a significant decrease in the urease activity of individual bacterial strains, and a 1% inoculation ratio was generally sufficient to meet the experimental requirements. When the loess density was 1.7 g/cm3 and 1.8 g/cm3, the cohesive force and internal friction angle in the experimental groups with added bacterial solution were increased by approximately 30% and 50% and 15% and 5%, respectively, indicating that MICP technology can significantly enhance the shear strength of loess.

Urease-catalyzed microbial and enzymatic carbonate precipitation for eco-friendly heavy metal remediation

Heavy metal contamination significantly threatens environmental and public health, necessitating effective and sustainable remediation technologies. This review explores two innovative bioremediation techniques: microbially induced calcium carbonate precipitation (MICP) and enzyme-induced calcium carbonate precipitation (EICP). Both techniques show promise for immobilizing heavy metals in laboratory and field settings. MICP utilizes the metabolic activity of ureolytic microorganisms to precipitate calcium carbonate, sequestering heavy metals such as lead, cadmium, and arsenic as stable metal-carbonate complexes. EICP, on the other hand, employs urease enzymes to catalyze calcium carbonate precipitation, offering greater control over reaction conditions and higher efficiency in environments unfavorable to microbial activity. This mini-review compares the mechanisms of MICP and EICP, focusing on factors influencing their performance, including enzyme or microbial activity, pH, temperature, and nutrient availability. Case studies illustrate their success in sequestering heavy metals, emphasizing their practical applications and environmental benefits. A comparative analysis highlights the strengths and limitations of MICP and EICP regarding cost, scalability, and challenges. This review synthesizes research to support the advancement of MICP and EICP as sustainable solutions for mitigating heavy metal contamination.

Biocementation beyond the Petri dish, scaling up to 900 L batches and a meter-scale column

Microbial-induced calcite precipitation (MICP), which leverages ureolytic microorganisms, has received significant attention during the past decade as a promising method for sustainable building and geoenvironmental applications. However, transitioning from lab-scale experimentation to volumes suitable for practical use poses challenges. This study addresses these obstacles by screening and analyzing over 50 strains sourced from (i) a natural environment in the canton of Ticino in Switzerland; (ii) microorganism banks; and (iii) an industry-scale bioreactor. Several ureolytic Sporosarcina species have been identified in the natural environment, and their ureolytic potential has been compared with that of other strains. A reference, banked microorganism yielded the highest ureolysis rate. When this latter strain was inoculated in 900 L batches and continuously cultivated at 5400 L, no contamination issues were observed, and the reference strain remained the dominant species. The produced culture, obtained under an optimized medium composition involving the circular valorization of NH4+, was subsequently used to induce the biocementation of a 650 kg column of 0-1 mm sand. The results reveal the successful stabilization of the whole mass, with undrained Tresca strength values ranging from 90 to 140 kPa. This research lays the groundwork for scalable MICP production, which is capable of meeting the demands of real-world building and geoenvironmental projects.

Sustainable improvement of granite sludge dust properties using microbially induced carbonate precipitation (MICP): strength enhancement, erosion prevention, and dust mitigation

Granite sludge dust (GSD), a significant byproduct of granite processing globally, poses severe environmental and public health challenges, with India alone generating 200 million tons annually. The conventional use of GSD in soil stabilization and construction materials is limited to 20-30%, underscoring the urgent need for sustainable repurposing solutions within the circular economy catering to broader bulk utilization. Unlike traditional techniques, repurposing granite dust using microbially induced calcite precipitation (MICP) offers a sustainable low-impact and eco-friendly ground improvement solution. It also reduces waste and associated environmental pollution. MICP leverages bacterial enzymes to catalyze urea hydrolysis, leading to calcite (CaCO3) precipitation stabilizing the solids matrix. This study evaluates the efficacy of MICP in strength enhancement of GSD enabling its repurposing in low-volume roads. To assess this, unconfined compressive strength (UCS), wetting and drying (WD) durability, and X-ray diffraction (XRD) tests were conducted. Additionally, to assess the efficacy of MICP in mitigation of both wind and rainfall-induced erosion of GSD from waste containments, percentage weight loss in wind tunnel tests along with air quality parameters PM2.5, PM10, and drip erosion tests were conducted respectively. MICP treatment with Bacillus megaterium resulted in significant strength gain of up to 1355 kPa UCS, suitable for low-volume pavement subbases, enhanced durability up to two wetting and drying cycles, substantial reductions in PM2.5 and PM10 levels due to wind erosion, and improved resistance to rainfall-induced erosion sustaining the 10-min test. This low-carbon-intensive technique endorses circular economy goals by transforming GSD into a sustainable construction material addressing waste management, infrastructure resilience, and environmental sustainability. Further, the surficial application of MICP contributes to eco-friendly infrastructure and pollution control of GSD storage facilities.

Flowing-water remediation simulation experiments of lead-contaminated soil using UCB technology

The flowing-water remediation of contaminated soil was investigated. Urease combined with biochar (UCB) technology was used to handle the Pb2+-contaminated sand column. The results showed that with the continuous increase of pore volume, the concentration of Pb2+ in the leachate undergoes three stages: slow growth, rapid growth, and steady state. With increasing seepage velocity, the concentration of Pb2+ in leachate increased slightly. The residual amount of each section of the sand column gradually decreased with increasing migration distance. The comparative results indicated that the UCB technology had a good solidification effect on Pb2+. This was due to urease-induced CaCO3 precipitation, cementation, and adsorption of Pb2+. Biochar provided more nucleation sites for urease, and some Pb2+ was adsorbed on its surface or diffused into the pores of biochar, or ions exchanged with functional groups on the surface of biochar, which effectively stabilized the free Pb2+.

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.

Microbially Induced Calcium Carbonate Precipitation Using Lysinibacillus sp.: A Ureolytic Bacterium from Uttarakhand for Soil Stabilization

Microbially induced calcium carbonate precipitation (MICP) is a soil remediation method that has emerged as a viable and long-term solution for enhancing soil mechanical qualities. The technique of MICP that has been extensively researched is urea hydrolysis, which occurs naturally in the environment by urease-producing bacteria as part of their fundamental metabolic processes. The objectives of the current study include screening and identifying native ureolytic bacteria from soil in Uttarakhand, optimizing growth factors for increased urease activity, and calcite precipitation by the bacteria using response surface methodology. Additionally, it was assessed how well the isolated bacteria in the medium biomineralized when using synthetic media and cheaper alternatives such as cow urine and eggshell as sources of urea and Ca2+, respectively. The isolated strain identified as Lysinibacillus sp. was found to be the very active strain after soil samples were screened for ureolytic bacteria. It was discovered that optimization studies with values of pH 8, urea concentration (0.8 M), inoculum concentration (3%), and incubation time (48 h) yielded a higher activity of 33.7 U/mL (threefold increase), and a higher calcium carbonate precipitation (enzyme activity: 10.96 U/mL, pH: 8.92, soluble Ca2⁺: 25.53 mM and insoluble Ca2⁺: 0.856 g). The calcite precipitation in broth media supplemented with ready-made substrates and alternative sources demonstrated a similar result of increased pH and ammonia release. Thus, the current study successfully paves the way for several possibilities to stabilize the slopy soils prone to landslides and erosion in Uttarakhand and pinpoint an economic approach through biomineralization.

Preparation of an environment-friendly microbial limestone dust suppressant and its dust suppression mechanism

The development of an efficient and environmentally friendly dust suppressant is crucial to address the issue of dust pollution in limestone mines. Leveraging the synergistic microbial-induced calcium carbonate precipitation (MICP) technology involving NaHCO3 and dodecyl glucoside (APG), the optimal ratio of the dust suppressant was determined through single-factor and response surface tests. The dust suppression efficacy and mechanisms were analyzed through performance testing and microscopic imaging techniques, indicating that the optimal ratio of the new microbial dust suppressant was 20% mineralized bacteria cultured for 72 h, 0.647 mol L-1 cementing solution, 3.142% NaHCO3, and 0.149% APG. Under these conditions, the yield of calcium carbonate increased by 24.89% as compared to when no NaHCO3 was added. The dust suppressant demonstrated excellent wind, moisture, and rain resistance, as well as curing ability. More calcite was formed in the dust samples after treatment, and the stable form of the dust suppressant contributed to consolidating the limestone dust into a cohesive mass. These findings indicate that the synergistic effect of NaHCO3 and APG significantly enhanced the dust suppression capabilities of the designed microbial dust suppressant.

Synergistic biocementation: harnessing Comamonas and Bacillus ureolytic bacteria for enhanced sand stabilization

Biocementation, driven by ureolytic bacteria and their biochemical activities, has evolved as a powerful technology for soil stabilization, crack repair, and bioremediation. Ureolytic bacteria play a crucial role in calcium carbonate precipitation through their enzymatic activity, hydrolyzing urea to produce carbonate ions and elevate pH, thus creating favorable conditions for the precipitation of calcium carbonate. While extensive research has explored the ability of ureolytic bacteria isolated from natural environments or culture conditions, bacterial synergy is often unexplored or under-reported. In this study, we isolated bacterial strains from the local eutrophic river canal and evaluated their suitability for precipitating calcium carbonate polymorphs. We identified two distinct bacterial isolates with superior urea degradation ability (conductivity method) using partial 16 S rRNA gene sequencing. Molecular identification revealed that they belong to the Comamonas and Bacillus genera. Urea degradation analysis was performed under diverse pH (6,7 and 8) and temperature (15 °C,20 °C,25 °C and 30 °C) ranges, indicating that their ideal pH is 7 and temperature is 30 °C since 95% of the urea was degraded within 96 h. In addition, we investigated these strains individually and in combination, assessing their microbially induced carbonate precipitation (MICP) in silicate fine sand under low (14 ± 0.6 °C) and ideal temperature 30 °C conditions, aiming to optimize bio-mediated soil enhancement. Results indicated that 30 °C was the ideal temperature, and combining bacteria resulted in significant (p ≤ 0.001) superior carbonate precipitation (14-16%) and permeability (> 10- 6 m/s) in comparison to the average range of individual strains. These findings provide valuable insights into the potential of combining ureolytic bacteria for future MICP research on field applications including soil erosion mitigation, soil stabilization, ground improvement, and heavy metal remediation.

Ecofriendly solidification of sand using microbially induced calcium phosphate precipitation

This study introduces microbiologically induced calcium phosphate precipitation (MICPP) as a novel and environmentally sustainable method of soil stabilization. Using Limosilactobacillus sp., especially NBRC 14511 and fish bone solution (FBS) extracted from Tuna fish bones, the study was aimed at testing the feasibility of calcium phosphate compounds (CPCs) deposition and sand stabilization. Dynamic changes in pH and calcium ion (Ca2+) concentration during the precipitation experiments affected the precipitation and sequential conversion of dicalcium phosphate dihydrate (DCPD) to hydroxyapatite (HAp), which was confirmed by XRD and SEM analysis. Sand solidification experiments demonstrated improvements in unconfined compressive strength (UCS), especially at higher Urea/Ca2+ ratios. The UCS values obtained were 10.35 MPa at a ratio of 2.0, 3.34 MPa at a ratio of 1.0, and 0.43 MPa at a ratio of 0.5, highlighting the advantages of MICPP over traditional methods. Microstructural analysis further clarified the mineral composition, demonstrating the potential of MICPP in environmentally friendly soil engineering. The study highlights the promise of MICPP for sustainable soil stabilization, offering improved mechanical properties and reducing environmental impact, paving the way for novel geotechnical practices.

Medium optimization and dust suppression performance analysis of microbial-based dust suppressant compound by response surface curve method

At present, microbial dust suppressants based on microbial communities lack necessary systematic analysis of factors affecting dust suppression performance. Therefore, in this study, the response surface curve method was used to optimize the culture conditions for enrichment of urease-producing microorganisms from activated sludge. The results indicated that when urea = 9.67 g L-1, NH4Cl = 5.21 g L-1, and pH = 9.57, the maximum urease activity of urease-producing microbial community (UPMC) was 8.22 mM min-1. The UPMC under optimized culture conditions reached a mineralization rate of 98.8% on the 1st day of mineralization. Ureolysis is one of the biological mechanisms that trigger microbial mineralization with the consequent effect of dust suppression. The analysis of microbial community structure indicated that the urease-producing bacteria Sporosarcina sp. had the highest abundance at the genus level in the microbial-based dust suppressant compound. Jeotgalicoccus sp. plays an important role in improving and maintaining the stability of urease. In addition, the optimal UPMC had low pathogenicity, which is extremely attractive for the safe application of microbial dust suppressants.