Enhancing aeolian sand stability using microbially induced calcite precipitation technology

Abstract

This study investigates the effectiveness of Microbially Induced Calcite Precipitation (MICP) technology in enhancing the stability of aeolian sand. Applying MICP to desert sand samples from Kashi, Xinjiang, the results demonstrated significant structural stability and erosion resistance in treated soils during wind erosion tests. Particularly after 14 days of treatment, the soil samples exhibited optimal wind erosion resistance and surface crust strength. Additionally, the formation of calcite significantly improved the soil’s penetration strength and wind erosion resistance, with SEM analysis confirming that calcite “bridges” between soil particles enhanced inter-particle bonding. Environmental impact assessments indicated that MICP technology is not only environmentally friendly but also effectively reduces the risk of soil environmental pollution. These findings validate the potential application of MICP technology in enhancing the stability and environmental adaptability of aeolian sand.

Introduction

The South Xinjiang Kashi region, lying adjacent to the world’s second-largest mobile desert—the Taklamakan Desert—frequently suffers from sandstorms during the spring and summer months. These complex weather events, including blowing sand and dust storms, are caused by winds lifting loose soil from the ground, creating a veil of dust aerosols that envelop the surface and negatively impact the local population¹. In the 21st century, global warming is projected to exceed 2 °C², leading to increasingly loose soils in arid and semi-arid regions and making them more susceptible to extreme wind and sand weather. Wind erosion not only results in soil loss but also depletes soil fertility in semi-arid regions³, damages vegetation cover, and degrades ecosystems, directly affecting agricultural productivity. Vegetation, a crucial component of ecosystems, helps stabilize the soil and reduce wind erosion. However, the persistent erosion makes it difficult for vegetation to grow, exacerbating soil loss and ecological degradation in a vicious cycle. To combat the detrimental effects of wind erosion, extensive research has been conducted to understand the formation and control methods of sandstorms^4–6. The formation of and storms is influenced by various factors, including soil particle size distribution, soil moisture content, vegetation coverage, and wind speed. It is widely believed that maintaining soil stability is key to controlling wind erosion. Soil stabilization is a critical aspect of geotechnical engineering, addressing issues related to soil erosion, subsidence, and the overall structural integrity of built environments. Traditional soil stabilization methods, such as chemical additives and mechanical compaction, often face limitations including environmental concerns, high costs, and limited long-term efficacy^7,8. And these methods are difficult to implement in areas with frequent sandstorms.

Therefore, developing a new wind erosion control technology that is energy-efficient, durable, and suitable for field applications is crucial. In recent years, Microbially Induced Calcite Precipitation (MICP) has emerged as a promising biotechnological approach to soil stabilization. This innovative technique leverages the natural processes of ureolytic bacteria to induce calcite precipitation, thereby enhancing soil cementation and strength^9,10.

MICP technology utilizes bacteria, such as Sporosarcina pasteurii, which hydrolyze urea to produce carbonate ions. These ions subsequently react with calcium ions in the environment to form calcium carbonate (CaCO₃). The precipitated calcium carbonate acts as a cementing agent, binding soil particles together and improving the mechanical properties of the soil^11,12. This bio-cementation process is environmentally friendly and can be applied in situ, making it a sustainable alternative to traditional methods¹³. MICP has found applications in various fields. The fundamental mechanism of MICP involves the following reactions:

1

2

Microbial urease catalyzes the hydrolysis of urea, producing ammonium and carbonate ions. When the generated carbonate ions combine with introduced calcium ions, calcium carbonate precipitates are formed, using the bacteria as nucleation sites as illustrated in Fig. 1. This process fills the gaps between soil particles and acts as a binding agent, thereby enhancing soil strength.

Fig. 1.

MICP mechanism diagram.

Numerous studies have demonstrated the effectiveness of MICP in various soil types and environmental conditions. For instance, DeJong et al.⁹ reported that MICP-treated soils exhibited significant improvements in shear strength and load-bearing capacity. Research by Whiffin et al.¹⁰ further highlighted the potential of MICP to reduce soil permeability and enhance its resistance to erosion. Similarly, Cheng and Cord-Ruwisch¹⁴ demonstrated that MICP could improve the stiffness and strength of sandy soils, indicating its potential for broad applications in civil engineering.

Recent advancements in the understanding of microbial behavior and calcite precipitation mechanisms have paved the way for optimizing MICP processes for large-scale applications. Studies by van Paassen et al.¹⁵ have explored the field-scale implementation of MICP, showcasing its viability and effectiveness in real-world scenarios. Al-Thawadi¹⁶ also investigated the role of different bacterial species in MICP, suggesting potential for optimizing microbial consortia for enhanced performance.

Given the issues with methods such as plant cultivation in terms of cost, complexity of implementation, and survival rates in dry and water-scarce environments, the MICP method holds more promise. Despite these promising results, the application of MICP in soil stabilization faces several challenges. These include optimizing bacterial growth conditions, ensuring the uniform distribution of the bio-cementation agent, and addressing potential impacts on soil microbiota and local ecosystems^15,17. Addressing these challenges requires a multidisciplinary approach, integrating insights from microbiology, geotechnical engineering, and environmental science^18,19.

Currently, many studies were focused on soil instability caused by drought cracking of clay soils^20–23, but not much research has been done on desert soils^24,25, especially in the Kashi region of Xinjiang. This study was aimed at evaluating the effectiveness of MICP (Microbially Induced Calcite Precipitation) under simulated local conditions. To achieve this, a series of wind tunnel experiments were conducted, simulating varying wind speeds and erosion of bio-treated soil. The study assessed the wind erosion resistance of MICP-treated soils through a comprehensive analysis of factors such as curing period, volume of bacterial solution, soil retention ratio following wind erosion, and penetration strength. Furthermore, the formation of calcium carbonate was also analyzed using infrared scanning and XRD (X-ray diffraction) technology. The insights gained from this research offer valuable scientific evidence to support sand control initiatives in the region.

Materials and methods

Sample collection

The soil samples were collected from a desert not far from Markit County, Kashi prefecture, as shown in Fig. 2. According to the “Standard Test Methods for Geotechnical Engineering” (GB/T 50123 − 2019), the test results were presented in Table 1. The soil was classified as fine sand, characterized by uniform particle size and poor gradation. The particle size distribution was shown in Fig. 3.

Fig. 2.

Sampling site of aeolian sand.

Table 1.

Basic properties of desert sand.

GS	CU	CC	ρdmax (g/cm3)	ρdmin (g/cm3)	d10 (mm)	d30 (mm)	d60 (mm)
2.59	1.66	0.98	1.59	1.39	0.098	0.125	0.163

Fig. 3.

Grain size distribution of aeolian sand.

Bacteria and cementation solutions

In this study, Sporosarcina pasteurii (ATCC 11589), a urease-active strain procured from Shanghai Bioresource Collection Center, was used for MICP treatment. The bacteria, in the form of freeze-dried powder, was cultivated on specially prepared LB agar slants (Luria-Bertani). The formulation of LB liquid medium (1 L) was as follows. We dissolved 10 g of tryptone, 5 g of yeast extract, 10 g of NaCl, and 15 g of agar in 950 ml of ddH₂O. The pH was adjusted to 7.0 with 1 M NaOH and made up to 1 L. Then it was sterilized at 121 °C for 20 min under high pressure and stored at 4 °C. Additionally, 20 g/L of urea that had been sterilized by filtration was added. After successful activation, the inoculated LB liquid medium was cultured in a constant temperature incubator at 25 °C with a stirring speed of 180 rpm for 48 h. The bacterial solution (BS) had an OD₆₀₀ value of 0.287 after being diluted 5 times. According to the formula²⁶:

3

Approximately 7.838 × 10⁷ bacteria are present in 1 ml of the bacterial solution, meeting the experimental requirements. The cementation solution (CS) is prepared from a 1 mol/L urea and 1 mol/L calcium chloride solution. The BS and CS are then filled into graduated spray bottles for later use.

Test procedure and treatment methods

The soil samples were divided into 19 groups and naturally accumulated in plates with a average diameter of 19.5 cm and a depth of 1.9 cm. The inner volume of the plate is approximately 567 cm³, and the dry density of desert sand is 1.45 g/cm². Therefore, the mass of each sample is approximately 822 g. The reported results are based on the averages from three identical samples with similar experimental conditions. A hand-held sprayer was used to evenly spray the liquid onto the surface of the aeolian sand from a height of 30 cm. The first group remained untreated, while the next 3 groups were treated with 10, 15, and 20 ml of distilled water. The following 15 groups underwent MICP treatment and were divided into groups A, B, and C, with group A receiving 10 ml each of BS and CS, group B receiving 15 ml, and group C receiving 20 ml. The samples were sprayed in the following sequence: BS + CS + BS + CS. For example, to prepare samples containing 10 ml of each solution, 5 ml of BS was first sprayed onto the sample, followed by 5 ml of CS, another 5 ml of BS, and finally, 5 ml of CS. Each group was cured for 1, 3, 7, 14, and 21 days.

After the curing period, a simulated wind tunnel test was conducted. In this experiment, to ensure that the wind erosion of the soil samples closely mirrors real-life conditions, the wind was oriented at an angle close to natural wind erosion angles. The angle of wind erosion is crucial for obtaining accurate and applicable results. Typically, wind erosion processes in natural environments involve various angles, but studies have shown that angles close to parallel to the ground (low-angle wind erosion) more accurately reflect the primary characteristics of wind erosion in desert areas. Specifically, wind erosion angles within the range of 0° to 30° are most common because at these angles, the wind force maximally contacts the ground, leading to more pronounced detachment and transport of soil particles. In this experiment, the erosion angle was set at 30° to simulate common wind erosion phenomena in desert areas, such as dune formation and surface soil erosion, while ensuring that the wind at this angle most effectively erodes the soil samples, thus enabling a more accurate assessment of the impact of MICP treatment on soil erosion resistance. The wind tunnel design is shown in Fig. 4. The wind speed was 11 m/s, and after continuous wind erosion for 8 min, the mass retention ratio W before and after wind erosion was measured.

4

Fig. 4.

Schemic diagram of Wind tunnel.

In the evaluation, M₁ represents the mass before wind erosion, and M₂ represents the mass after wind erosion. The greater the mass retention, the stronger the wind erosion resistance of the treated aeolian sand. Subsequently, a micro penetrometer was used to measure the surface crust strength, and the EDTA titration method was employed to determine the amount of calcium carbonate in the crust. X-ray diffraction (XRD) was used to characterize the mineralogy of the treated samples in powder form. Additionally, to understand the changes in chemical bonds during the treatment process, Fourier transform infrared spectroscopy with attenuated total reflection (FTIR-ATR) was used to conduct infrared scans from 400 cm⁻¹ to 4000 cm⁻¹. Lastly, the microstructure of the aeolian sand was assessed using scanning electron microscopy (SEM) micrographs.

Results and discussion

The effectiveness of this method in stabilizing aeolian sand has been extensively validated through various engineering testing procedures^27–29. This section presents the test results and infers changes in the aeolian sand matrix state.

Wind erosion resistance

Experimental Results: Untreated and distilled water-treated soil samples were completely eroded by the wind, leaving no residue in the dishes. This indicates that untreated desert soil lacks sufficient wind erosion resistance. In contrast, the soil samples treated with microbially induced calcium carbonate precipitation (MICP) demonstrated significant wind erosion resistance after the erosion process. The Fig. 5 mass retention rate W varied with the duration of curing. This phenomenon can be attributed to the effective promotion of calcium carbonate precipitation between soil particles by the MICP treatment within a certain curing period, making the aeolian sand more stable and enhancing the overall structure and wind erosion resistance of the soil samples. Specifically, at 14 days, the cementing effect of the calcium carbonate precipitation from the MICP treatment reached an optimal state in all three groups, with the treatment method of 10 ml BS + 10 ml CS + 10 ml BS + 10 ml CS (Group C) showing the best performance, with W = 99.979%. This treatment significantly reduced the wind erosion of the soil samples. The subsequent decrease in W is possibly attributed to the extracellular polymeric substances (EPS), a high-molecular-weight substance primarily composed of polysaccharides, proteins, lipids, and nucleic acids, secreted by microorganisms. During the MICP process, EPS not only supports microbial adhesion and provides a reaction site but also enhances the precipitation of calcium carbonate and the cohesive force of soil particles^30–33, thereby improving the efficiency and effectiveness of the entire process. However, in the later stages, microbial death and the arid environment cause the previously hydrated EPS to lose moisture, becoming dry and fragile, potentially leading to a reduction in cementing strength and consequently a decrease in W. The wind tunnel experiments demonstrate that MICP technology can significantly improve the wind erosion resistance of desert aeolian sand.

Fig. 5.

Retention rate of MICP treated aeolian sand after wind erosion experiment. Group A receiving 10 ml each of BS and CS, group B receiving 15 ml, and group C receiving 20 ml.

Calcium carbonate content

The calcium carbonate content on the surface of the soil samples was determined at different time points using the ethylenediaminetetraacetic acid (EDTA) titration method to evaluate the efficiency and stability of the MICP process. The experimental results indicate that the amount of calcium carbonate reached its maximum after 14 days of treatment and remained stable in the subsequent period until 21 days. As shown in Fig. 6a, the calcium carbonate generation significantly increased from the first day to the 14th day, indicating a high deposition rate of calcium carbonate during the initial stage of the MICP process. The specific data is as follows: the calcium carbonate content for the distilled water-treated and untreated groups remained constant at 4.8% over the curing period. For the biotreated soil samples, the increase in the calcium carbonate content on the third day was 1.59%, 1.87%, and 2.49% for Groups A, B, and C, respectively. The calculation method for the increment of calcium carbonate is as follows: the mass of calcium carbonate in the samples after MICP treatment is subtracted from the mass of calcium carbonate in the untreated samples, and the result is then divided by the dry mass of the samples after MICP treatment. By the 14th day, the calcium carbonate content increased to 3.14%, 3.77%, and 5.09%, representing an approximately 100% increase from the third day. However, from the 14th day to the 21st day, the calcium carbonate generation remained relatively stable. This trend may be attributed to the depletion of nutrients in the bacterial solution, calcium ions in the coagulation solution, and urea, leading to bacterial die-off.

Fig. 6.

(a) Calcium carbonate content; (b) Crust strength. Group A receiving 10 ml each of BS and CS, group B receiving 15 ml, and group C receiving 20 ml. Net CaCO₃ content = (mass of calcium carbonate in the treated sample - mass of calcium carbonate in the untreated sample) / total mass of the treated sample.

This changing trend indicates that the experimental data obtained through EDTA titration during the MICP treatment of desert aeolian sand show a rapid accumulation of calcium carbonate within the first two weeks, followed by stabilization. These results provide important reference for optimizing the MICP treatment duration and suggest that the optimal time point for calcium carbonate deposition may be around 14 days. Prolonging the treatment duration beyond this point may not significantly increase the generation of calcium carbonate.

Surface strength of residual crust after wind erosion

Using a micro penetration tester, the surface strength of the soil samples was measured. In the wind erosion experiment for desert soil, the untreated soil samples and those treated with distilled water exhibited zero penetration strength after one day of indoor curing, indicating their inability to resist external forces during wind erosion. In contrast, the soil samples treated with microbially induced calcium carbonate precipitation (MICP) demonstrated a significant increase in penetration strength at various curing durations. Upon one day of curing, the surface crust of the samples broke after wind erosion, resulting in a surface strength of 0kP. However, starting from the third day, as shown in Fig. 6b, the penetration strength of the soil samples increased with the duration of curing. Groups A and B continued to increase their strength after 14 days, indicating that bacteria remained actively growing and metabolizing during the 14-day curing period, leading to continuous calcium carbonate deposition and the enhancement of soil cementation strength. In Group C, the strength peaked at 14 days and then decreased, suggesting that under the condition of 20 ml of bacterial solution, the bacteria reached optimal activity and metabolic product peak within 14 days, maximizing the soil strength at this time. Possible reasons for this phenomenon include: Nutrient depletion - rapid bacterial growth and metabolism in the early stage resulting in depleted nutrients (e.g., urea and calcium ions) and insufficient nutrient supply in the later stage. Bacterial gradual death or dormancy - high bacterial concentrations during curing may lead to bacterial death or dormancy due to space limitations, waste accumulation, etc., reducing calcium carbonate production. Calcium carbonate dissolution or structural damage - after the formation of high concentrations of calcium carbonate, environmental changes (such as pH fluctuations, humidity changes) may lead to partial dissolution or structural damage, thereby reducing soil strength. MICP-treated soil samples demonstrated substantial surface strength enhancement even after intense wind erosion, possibly attributed to the cumulative effect of calcium carbonate precipitation during the MICP process, enhancing inter-particle cementation and improving surface structural stability and penetration strength. The peak strength was attained at 14 days, reaching a rupture pressure of 63 kPa, and according to the wind pressure formula, this is equivalent to withstanding approximately 320.75 m/s wind speed. According to the Beaufort Wind Force Scale, a wind speed of force 12 ranges from 32.7 to 36.9 m/s. The treated soil surface can withstand a wind speed of 320.75 m/s, far exceeding the wind speed of force 12.

5

In the equation, P represents pressure (Pa) with air density assumed as 1.225 kg/m³, and v represents wind speed (m/s). Therefore, theoretically, the soil surface treated with MICP can still resist wind forces much higher than those of a 12-level wind after 8 min of erosion by a 6-level wind.

Analysis of the MICP cementation mechanism

In order to compare the mineral compositions of the aeolian sand before and after solidification, and to assess the effect of the MICP cementation reaction on the aeolian sand, the sample’s crystalline phases were determined through X-ray diffraction (XRD) testing. Figure 7a,b showed the XRD spectra of loose aeolian sand and MICP-solidified aeolian sand. It could be seen from the figures that the mineral composition of loose aeolian sand is predominantly quartz with fewer diffraction peaks, while the MICP improved aeolian sand exhibits more characteristic diffraction peaks. Analysis using the Jade 6 software determined these as characteristic diffraction peaks of calcite, indicating that the precipitated crystals in the treated samples are primarily composed of calcite. In addition, the albite (NaSi₃AlO₈) in untreated samples, may be partly changed into orthoclase(KSi₃AlO₈)through only ion exchange in the process of MICP. While the occurrence of siderite (FeCO₃) after MICP process may be due to the carbonion produced by hydration of urea combining with ferrous ion that may be from the partly decomposing of chlorite in the sample. Figure 7c presents the FTIR test results of loose aeolian sand and MICP improved aeolian sand. From the infrared spectra of the aeolian sand, it can be observed that the two materials have essentially the same composition. However, the intensity of the absorption peaks of functional groups is stronger in the treated samples, indicating a higher number of functional groups. The peak near 1404 cm⁻¹ is attributed to the stretching vibration absorption of carbonate ions, predominantly due to the carbonate mineral components in the desert soil. As the main component of desert soil is quartz, characteristic infrared absorption peaks of quartz are present. The absorption peak near 1003 cm⁻¹ corresponds to the anti-symmetric stretching vibration of Si-O-Si, while the peaks at 874 cm⁻¹, 745 cm⁻¹, and 455 cm⁻¹ are attributed to the bending vibration and symmetric stretching vibration of Si-O, respectively. The main component of sand particles, which is consistent with the XRD test results. It can be concluded that the carbonate has been generated in the process of MICP.

Fig. 7.

XRD spectrum: (a) loose aeolian sand (b) MICP improved aeolian sand (c) FTIR spectrum.

Microstructure

Figure 8 illustrates the micro changes in the aeolian sand before and after treatment. Figure 8a depicts the aeolian sand sample before treatment, characterized by loose, porous features. The post-treatment samples typically demonstrate a denser arrangement of particles, with a layer of calcium carbonate deposits covering the surfaces and original morphology of the particles. These deposits appear in crystalline or granular forms (calcium carbonate crystals typically exhibit regular geometric shapes, such as rhombic or spherical, in SEM). As shown in Fig. 8b, the voids are partly filled with calcium carbonate, resulting in an overall reduction in porosity. The uniform distribution of calcium carbonate crystals indicates a relatively uniform and effective MICP process. In Fig. 8c, the “bridge”-like structures formed by calcium carbonate between the particles can enhance the overall strength of the soil. The contact points between particles become tighter and more stable due to the deposition of calcium carbonate. From Fig. 8c,d, the crystals are deposited, aggregated, and grown between adjacent particles, bonding adjacent particles into an integrated structure, transforming point contact between particles into surface contact, thus improving the overall stability of the sample.

Fig. 8.

SEM image: (a) loose aeolian sand; (b) calcium carbonate crystals; (c) bridge structure; (d) filling effect.

Discussion

This article examined the feasibility of solidifying desert soil through a bio-augmentation method, utilizing small-scale indoor wind tunnel experiments. The findings revealed that the resulting crust significantly enhances both the strength and wind erosion resistance of the desert surface soil. This conclusion was substantiated by variations in key indicators, including the sample mass retention ratio, the surface crust strength of the sandy soil, and the mass of calcium carbonate produced, as well as micro-analyses. These results are consistent with the findings of other researchers^19,24,28.

In Fig. 5, a spraying volume of 20 ml for the microbial and binding solutions, coupled with a curing time of 21 days, yields the highest sample mass retention ratio, exceeding 95%. Conversely, when a spraying volume of 10 ml was applied, the mass retention ratio surpassed 90% after one day but experienced a sharp decline on the third day, followed by a gradual recovery, ultimately approaching the levels observed in the higher dosage groups by day 14. This phenomenon could be attributed to the rapid initiation of the calcium carbonate precipitation reaction by microorganisms, even with smaller volumes of microbial and binding solutions during the early experiment stages. Most of the initial precipitates accumulate in the surface layer of the sand, resulting in a thin, dense crust, which effectively resists wind erosion in the short term and accounts for its strong wind erosion resistance on the first day. However, the thin, uneven, and unstable thickness of this surface layer limits long-term structural stability, rendering it susceptible to external factors, such as wind erosion and temperature fluctuations, which can degrade the surface crust and sharply diminish overall wind erosion resistance by the third day. With the passage of time, the bacteria display heightened adaptability, as their activity revives and internal precipitates consolidate steadily. This progression leads to a gradual enhancement in strength beyond the seventh day.

In Fig. 6b, the penetration resistance of groups A and B continues to rise over time, while group C increases until the 14th day, subsequently declining. This variation may stem from the role of extracellular polysaccharides. Initially, the extracellular polysaccharides secreted by the bacteria act as nucleation sites for calcium carbonate crystals^29,30 and encapsulate these crystals due to their high viscosity and gel-like properties^31,32, resulting in rapid strength increases. However, as time progresses, the excess extracellular polysaccharides in group C may excessively encapsulate the calcium carbonate particles, inhibiting crystal aggregation and tight intercrystal bonding, which leads to a decrease in strength after 14 days. In contrast, the appropriate amounts (not excessive) of microbial and binding solutions in groups A and B do not present this issue, therefore continue to rise.

The Kashi region, located at the western edge of China’s largest desert, the Taklamakan Desert, faces significant social and economic threats from dust storms that occur for three to four months each year. This study proposes a novel approach to low-cost sustainable suppression or complete elimination of dust storms.

Nevertheless, due to funding and time limitations, only indoor experiments within 21days were conducted. Future field studies beyond 21days, if feasible, may have more significant implications. Furthermore, this research focused exclusively on the wind erosion resistance of microbial crusts. Investigating the combined effects of biological-plant crusts³³ and the influence of sand layer thickness on the movement characteristics of desert soil particles³⁴ may represent a valuable research direction in the future.

Conclusions

Through small-scale wind tunnel experiments utilizing microbial inoculation methods on lab desert soil samples, the following conclusions were reached:

• The bio-augmentation method is effective for stabilizing and suppressing dust in Taklamakan Desert sand.

• Within the study scope, when the application volumes of both the microbial and binding solutions were set at 20 ml (~ 0.67 L/m²), the mass retention ratio, penetration strength, and calcium carbonate production in the desert soil samples reach their maximum values, enhancing their resistance to wind erosion.

• X-ray diffraction (XRD) analysis revealed a significant increase in calcium carbonate content in samples treated with microbially induced calcite precipitation (MICP) over untreated samples, thereby confirming the efficacy of MICP in stabilization.

Author contributions

Conceptualization, methodology, writing—original draft, and figure work have been verified by J. L. Qu, W. Q. Qu. Formal analysis, writing—reviewing and editing have been done by J. Y. Yin, Z. Yibulayimu. Supervision, revision work has been done by J. L. Qu. All authors have reviewed the manuscript.

Data availability

All data generated or analysed during this study are included in this published article.

Declarations

Competing interests

The authors declare no competing interests.