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Article

Performance and Mechanism of Zn-Contaminated Soil through Microbe-Induced Calcium Carbonate Precipitation

by
Wei Xing
1,
Feng Zhou
1,
Rui Zhu
1,2,3,4,*,
Xudong Wang
1 and
Tingzhu Chen
5
1
School of Transportation Engineering, Nanjing Tech University, Nanjing 211816, China
2
Key Laboratory of Geohazard Prevention of Hilly Mountains, Ministry of Natural Resources of China, Fuzhou 350002, China
3
Key Laboratory of Geohazard, Fujian Province, Fuzhou 350002, China
4
State Key Laboratory of Simulation and Regulation of Water Cycle in River Basin, China Institute of Water Resources and Hydropower Research, Beijing 100048, China
5
Nanjing Jiangbei New Area Public Utilities Holding Group Co., Ltd., Nanjing 210061, China
*
Author to whom correspondence should be addressed.
Buildings 2023, 13(8), 1974; https://doi.org/10.3390/buildings13081974
Submission received: 15 May 2023 / Revised: 20 July 2023 / Accepted: 28 July 2023 / Published: 2 August 2023
(This article belongs to the Special Issue Sustainable and Low-Carbon Building Technology: Education, Design, and Practice)
Browse Figures
Versions Notes

Abstract

:
Zn is a toxic heavy metal that seriously endangers human health and ecological stability. For a long time, traditional remediation techniques have been used to remediate Zn-contaminated soil prone to other problems such as secondary contamination. In recent years, due to the great danger posed by Zn pollution, there has been an increasing interest in applying eco-friendly and sustainable methods to remediate Zn-contaminated soil. Therefore, in this study, microbially induced calcium carbonate precipitation (MICP) technology was used to bioremediate zinc ions by transforming ionic heavy metals into insoluble solid-phase minerals. Through the unconfined compressive strength (UCS) test, direct shear (DS) test, and penetration test (PT), the results showed that the unconfined compressive strength of the treated specimens increased by 187.2~550.5%, the cohesion increased significantly compared with the internal friction angle of specimens, and the permeability coefficient can be reduced by at least one order of magnitude. During the treatment of Zn pollutants, the mobility of heavy metal zinc ions was significantly reduced, the percentage of exchangeable state Zn content was significantly reduced, and the leaching concentration of zinc ions in Zn-contaminated soil was reduced to about 20 mg/L, which was significantly lower than the limit in the standard (100 mg/L). These results were further confirmed by scanning electron microscope (SEM) and X-ray diffraction (XRD) analyses, which indicated coprecipitation of calcium carbonate (CaCO3) and ZnCO3. The microbial solidification/stabilization of Zn-contaminated soil was most effective when the curing age of 28 d, the cementation solution concentration of 1 mol/L, and the cementation solution ratio of 1:2. Therefore, the bio-immobilization of zinc ions by MICP has the potential for application as a low-cost and eco-friendly method for heavy metal remediation.

1. Introduction

Zn pollutant accounts for half of the heavy metal pollutant emissions, which is a typical inorganic pollutant in soils. Compared with other pollutants, soil heavy metal pollution is hidden and enriched, difficult to degrade in the environment, and widely distributed in nature. The increasing severity of heavy metal pollution in soil has led to soil fertility degradation, reduced crop yield, and decreased quality, seriously affecting environmental quality and sustainable economic development, and threatening human food safety [1,2,3]. The urban soil has a more serious problem of excessive Zn content, which is concentrated in the mining, smelting, coal, and chemical industries around the cities [4]. Many researchers have studied the problem of heavy metal-contaminated soil and formed a series of technical methods, including chemical treatment, physical treatment, and bioremediation [5,6] (Table 1). However, traditional treatment methods often cause secondary pollution, slow response, limited scope, and other issues. Therefore, it is necessary to conduct research on eco-friendly remediation techniques for contaminated soils.
Microbial-induced calcium carbonate precipitation (MICP) is a promising technique for soil improvement [7]. In the field of geotechnical engineering, MICP has become a hot topic as it is considered to be a “green” and “eco-friendly” means of soil stabilization. It can simultaneously achieve the dual goals of controlling contamination and improving soil performance [8,9]. In recent years, many researchers have conducted research on the remediation of heavy metal-contaminated soils treated by MICP. For example, Chen et al. [10,11] conducted an in-depth study on the microbial remediation of Pb2+ and Cd2+, using the excellent electrical conductivity of biochar to react with the phosphorus solubilizing bacteria (PSB), which further accelerated the mineralization and immobilization of heavy metal ions and enhanced the remediation effect of the strains on them. Mwandira et al. [12] used Sporosarcina pasteurii to remediate lead-contaminated sandy soils sampled from mining and smelting operations in Zambia, where lead contamination has caused brain and central nervous system damage and even death in many people. Fortunately, the MICP method not only significantly reduced the concentration of lead ions, but also increased the unconfined compressive strength of the contaminated sandy soil to 2.8 Mpa, improving the properties of the soil. Song et al. [13] found that MICP treatment would be more effective in stabilizing zinc ions in fly ash which was collected from a municipal solid waste-to-power plant in Shenzhen, China, and that the compressive strength of the treated specimens reached 1.46 MPa. Chu et al. [14] conducted tests on microbial remediation of copper-contaminated sandy soils and showed that 90% of the copper ions were removed from the contaminated soils. The permeability coefficient of the contaminated sandy soils was significantly reduced after remediation. Zhang [15] found that a number of heavy metal ions in thermal power plant soils greatly exceeded the local soil pH value, endangering the ecosystem. Thus, the contaminated soils met the contamination concentration limits through a microbially treated fly ash-soil mixture test. Li et al. [16] investigated the biomineralization of Zn-contaminated soils by six different bacterial strains. They showed the removal of 88% zinc ions from the soil after the MICP treatment. Zhu et al. [17] treated nickel-contaminated soils with MICP which were collected near a battery factory in Shanghai, China. They found that the nickel ions in the soil were converted from an exchangeable to a stable biomineral identified as calcite, sphalerite, and nickel carbonate. The above studies analyzed the solidification effects and influencing factors of heavy metal-contaminated soils treated by MICP. However, the treatment mechanism of Zn-contaminated soils is still not clear, especially the studies about the effect of cementation solution concentration and its ratio on the physical and mechanical properties of Zn-contaminated soils treated by MICP are very scarce in the literature.
Therefore, this paper intended to conduct a series of laboratory tests to investigate the effects of different curing ages, cementation solution concentration, and its ratio on the mechanical properties, permeability properties, chemical forms of zinc ions, and toxic leaching capacity of Zn-contaminated soils treated by MICP, and to clarify the microstructural evolution law and stabilization mechanism of microbially solidified Zn-contaminated soil. This study provides a scientific reference for the application of microbial technology in the treatment of Zn-contaminated soil sites for foundation reinforcement and environmental management.
Table 1. Traditional treatment methods [5].
Table 1. Traditional treatment methods [5].
MethodsPrincipleApplicationDisadvantages
Physical methodsHeat treatmentThe contaminated soil is heated to high temperature, causing the heavy metals to evaporate.This method was used to degrade heavy metals from wastewater sludge [18].Such high temperatures lead to more leaching of the metals and also greater loss of humus.
Vitrification technologyHeating contaminated soil to a high temperature and then rapidly freeze, to form solids through glass transition.In Japan, this method has been used to reduce radioactive waste from its nuclear power plants [19].It is generally applied to a small, heavily contaminated area.
Chemical methodsChemical extraction and oxidationUsing chelating agents to extract heavy metals from soil.Application of such method in a wastewater plant in Henan Province, China, resulted in increased efficiency of heavy metal solidification [20].The transport of the chelating agents in the fine-grained soil is hindered.
Soil amendments (chemical fixation)Use of chemical additives to reduce cation charged metals to an acceptable limit of discharge.Four arctic and five subarctic sites with different soil characteristics were selected for the study, and the associated organic contaminants were subjected to alkaline hydrolysis [21].Generation of secondary waste and slow degradation of sludge.
Biological methodsPhytoremediationBased on the plant’s ability to degrade pollutants.Soil samples were collected from Ogun State and the Mn, Zn, Cu ions in soils were removed [22].It is a slower process and plants need to be treated centrally.

2. Materials and Methods

2.1. Test Materials

The test soil material, shown in yellow, was gathered from the slope of a cutting canal in northern Xinjiang, China. In accordance with the Standard of Geotechnical Testing Methods (GBT50123-2019) [23], the basic physical and mechanical properties such as water content, density, and liquid-plastic limit of soil materials were measured by methods such as drying method and combined determination of liquid plastic limit method (Table 2). Through granule analysis test [23], it was found that the content of fine particles (<0.075 mm) in the soil material was 21.1%, ranging from 15% to 50%, and the fine particles were mainly composed of silt particles (0.075 to 0.005 mm). Therefore, the soil belonged to the silt. In addition, Zn was used as a contaminant, and Zn-contaminated soil was treated manually by adding Zn solution [24,25].
The bacteria used were Sporosarcina pasteurii (ATCC 11859) [26], which was green, non-toxic, and could produce urease with high activity, so it was often chosen as a catalyst [27,28,29]. The bacteria provided an alkaline environment for the formation of calcium carbonate crystals through urea hydrolysis [30]. Based on the recommended culture medium from the source of the bacterial strain, as well as multiple ratio adjustment tests within the research group, the proportion of the culture medium that could be obtained contained 20 g/L nutrient broth plus urea, and the pH was adjusted to 8.0 using NaOH, it was named NB. The cementation solution consisted of CaCl2 and urea. According to the existing research results [31,32], mixing Sporosarcina pasteurii in nutrient broth containing urea, supplemented with different calcium sources (CaCl2, C4H6CaO4, Ca(NO3)2, CaO), lasted 7 days, during which bacterial growth, urease activity, calcite production, and pH were measured by electric conductivity method [33] and spectrophotometer [34]. CaCl2 was found to be the best calcium source for bacterial growth rate, and urease activity. Therefore, CaCl2 was selected as the calcium source to provide urea and calcium salt solution to MICP. The cementation solution allowed the bacteria to grow and multiply, and to remain active even when separated from the culture medium. The liquid culture method was used to inoculate bacteria into the culture medium, some details would be described in “(2) MICP gelling solution”. In the experiment, different cementation solution concentrations and their ratios (CaCl2:urea) were set in order to study the influence of the cementation solution on the stabilization effect of Zn-contaminated soil.

2.2. Specimen Preparation

(1)
Contaminant incorporation
Manual preparation of Zn-contaminated soil: First, the Zn solution and the test soil were weighed according to the designed concentration (the initial zinc ions concentration was set at two times the Chinese Soil Environmental Quality Level 3 standard (Zn ≤ 500 mg/L), which was 1000 mg/L). Then, the soils were mixed uniformly at optimum moisture content for uniformity and wrapped soils in black plastic bags. After that, the soils were left at 25 °C temperature and 95% relative humidity for 10 d. At the end of the curing period, the soils were dried for 24 h and grounded through a 0.5 mm sieve to produce Zn-contaminated soil materials (Figure 1).
(2)
MICP gelling solution
The MICP gelling solution, a mixture of bacteria and cementation solution, was used to prepare contaminated soil treated by MICP. The process was as follows: (a) The culture medium was autoclaved at the temperature of 121 °C for 2 h. As the culture medium cooled to room temperature and used a pipette to inoculate the bacteria into the culture medium, with an inoculation ratio of 1/100 (V bacteria/V culture medium) [35]. (b) Then the inoculated medium was cultivated in a constant temperature oscillation incubator at a speed of 180 rpm and a temperature of 30 °C. The urease activity, 22–30 mmol/L urea hydrolyzed·min−1, was measured by the electric conductivity method [33], OD600 = 0.8–1.2, was determined by the optical method using a spectrophotometer (model: UV-190i) [34]. In order to separate the supernatant and bacterial cells, the bacterial suspensions were centrifuged at a speed of 8000 rpm for 10 min. (c) According to the test scheme, the cementation solution and its specific ratio were preset, which were 0.25 mol/L, 0.5 mol/L, 1 mol/L, 1.5 mol/L, and 1:1, 1:1.5, 1:2, and 1:2.5, respectively. (d) Finally, the cementation solution was dissolved in the medium, and then all the isolated bacteria were stripped into each medium and mixed for preparing the MICP gelling solution (Figure 2).
(3)
MICP treatment
According to the Standard for Geotechnical Test Methods (GBT50123-2019) [23], microbially solidified contaminated soil specimens were prepared by the mixing method. It is noted that in order to ensure a consistent moisture content of the specimen, the MICP gelling solution added during the preparation process was equal to the amount of pure water added. The optimum moisture content and dry density of the specimens were 12.8% and 1.60 g/cm3, respectively. During the preparation process, specimens with a height of 80 mm and a diameter of 39.1 mm, a height of 20 mm and a diameter of 61.8 mm, and a height of 40 mm and a diameter of 61.8 mm were used for the unconfined compressive strength (UCS) test, direct shear (DS) test and penetration test (PT), respectively (Figure 3). Additionally, the prepared specimens were wrapped in cling film and placed in a constant temperature and humidity environment (a temperature of 25 °C and a relative humidity of 95%) for 3, 7, 14, and 28 d, respectively [36,37].

2.3. Test Methods

When the specimen reached the target curing age, a series of tests were conducted (Figure 4) as follows: (1) In accordance with ASTM D4219-08 [38], the UCS tests were performed using an unconfined compression instrument at an axial strain rate of 1 mm/min. The loading would stop as the strain of specimens reached 10%. (2) The DS tests were performed according to ASTM D3080-11 [39], which had a shear rate of 0.8 mm/min in the tests. The tests would stop as the shear displacement reached 6 mm. (3) Referring to ASTM D2434-19 [40], the specimens were vacuum-saturated for PT tests. The variable head PT was performed with a permeameter, which kept the head height between 20 and 40 cm. The continuous measurement of the initial head height and the corresponding time for each specimen was 3–6 times. (4) Tessier method [41] was used to determine the chemical forms of Zn pollutants in turn: exchangeable state and carbonate bound state. (5) According to ASTM method 1311 [42], parallel specimens for the UCS test were selected for Toxicity Characteristic Leaching Produce (TCLP) test using an atomic absorption spectrometer. The sodium acetate buffer solution (pH range 4.93 ± 0.05) was chosen as the leaching solution. According to the Hazardous Waste Identification Standards–Leaching Toxicity Identification (GB 5085.3-2007) [43], the criteria for leaching toxicity of Zn-contaminated soil specimens were established. If the leaching concentration of zinc ions did not exceed 100 mg/L, the specimens were considered to be in compliance with environmental requirements. It should be noted that the above tests were conducted in parallel, and the test results obtained were the average data of parallel tests, which reduced the test error.
Additionally, the soil blocks taken from the UCS test specimens were prepared for microstructural studies. For scanning electron microscope (SEM) tests, the specimens were freeze-dried and sprayed with gold before tests for a clear microstructure shape. The structural characteristics of specimens were observed by scanning electron microscope (JEOL-6510) to investigate the pore variation characteristics of specimens. For X-ray diffraction (XRD) tests, the chemical composition of specimens was analyzed by an X-ray diffractometer, which has a diffractometer-rated power of 3 KW and a scanning angle of 5°~80°. The microstructural characteristics obtained in SEM tests and XRD tests made for revealing the stabilization mechanism of Zn-contaminated soils treated by MICP.

3. Test Results

3.1. Mechanical Properties of Zn-Contaminated Soil

3.1.1. Stress–Strain Relationship

The types of axial compression damage of soils were usually divided into strain-hardening and strain-softening characteristics. On this basis, the stress–strain relationships of soils were classified into six types: strongly hardened, generally hardened, weakly hardened, strongly softened, generally softened, and weakly softened, so as to better describe the stress–strain curves of geotechnical materials [44]. Figure 5 presented the stress–strain relationship of Zn-contaminated soil treated by MICP subjected to different conditions. It could be seen that the stress–strain relationship of specimens showed the characteristics of softening type. With the improvement of microbial mineralization, the strain value corresponding to the peak among the curve increased gradually. Specifically, when the cementation solution ratio and curing age were constant (Figure 5a), the stress–strain relationship of specimens gradually changed from weak softening type to strong softening type with the increasing cementation solution concentration. It is noted that the stress–strain relationship had changed from the weakly softened type of the untreated specimen to the generally softened type of the treated specimens at an initial cementation solution concentration of 0.25 mol/L. As shown in Figure 5b, the stress–strain relationships of the specimens developed from weak softening type (uncured soil) to generally softening type (1:1.1, 1:1.5) and strongly softening type (1:2, 1:2.5), respectively, with the increase of cementation solution ratio. Similarly, Figure 5c showed a similar pattern of change, the stress–strain curve of the specimen changes from weakly softened to strongly softened with the increase in curing age. It indicated that the MICP treatment could transform the softening characteristics of soil stress–strain relationship from weak type to strong type, which was the realization of strength enhancement and deformation reduction for specimens.

3.1.2. Elastic Modulus

For the mechanical properties of soils, the elastic modulus is one of the important parameters. In general, the stress at 1% axial strain from the conventional unconfined compression strength test was found to be a good indicator of the elastic modulus [45]. Figure 6 presented the variation curves of the elastic modulus of Zn-contaminated soil treated by MICP subjected to different conditions. It could be seen that regardless of the cementation solution concentration and its specific ratio, the elastic modulus of specimens tended to increase and then decrease, while the elastic modulus of specimens showed an increasing trend with the increasing curing age. Specifically, when the ratio of cementation solution was constant, as the cementation solution concentrations were between 0.25~1 mol/L, the elastic modulus of specimens presented an upward trend until the cementation solution concentrations reached 1 mol/L, which had a significant increase of 72%. After that, the elastic modulus of specimens had some slight attenuation. For this phenomenon, it is because this exorbitant solution concentration would correspondingly inhibit the reaction rate of microorganisms and weaken the soil cementation degree, resulting in a decrease in the elastic modulus of specimens, which was consistent with that reported in [46].

3.1.3. Failure Strength

For the softening type of soil stress–strain relationship, the maximum axial stress was taken as the failure strength. To some extent, the variation characteristics of failure strength with the curing age, cementation solution concentration, and their ratio reflect the effects of MICP treatment on soil mechanical properties. Figure 7 presented the failure strength variation curves of Zn-contaminated soil treated by MICP subjected to different conditions. The failure strength of the specimens increased from 49.4 to 550.5% when the cementation solution ratio was constant, and its concentration was between 0.25 and 1 mol/L. During this process, the failure strength of specimens showed an upward trend, while it decreased slightly with a range of 7~23% when the concentration exceeded 1 mol/L. Therefore, it indicated that there was a threshold of cementation solution concentration during the MICP treatment. When the concentration exceeded the threshold, the treatment effect of specimens would be correspondingly weakened, which was also in line with existing studies [47,48]. Meanwhile, when the curing age was constant, the failure strength of specimens increased when the cementation solution ratio (CaCl2:urea) was from 1:1 to 1:2. As the curing age increased, the failure strength of specimens presented a special upward trend, which was a significant increase from 0 to 14 d and then stabilized with increasing age.
In summary, the failure strength of Zn-contaminated soils treated by MICP was most significant when the specimens had a curing age of 28 d, a cementation solution concentration of 1 mol/L, and a cementation solution ratio of 1:2.

3.1.4. Shear Strength

Figure 8 presented the shear strength characteristics of specimens treated by MICP subjected to different conditions. The cohesion and internal friction angle of specimens showed a tendency of increasing firstly and decreasing subsequently with the increasing cementation solution concentration and its ratio, while they kept growing with the increasing curing age. For quantitative analysis, the cohesion and angle of internal friction of specimens with the same cementation solution concentration were selected. It was found that the growth of cohesion was positively correlated with the curing age with a maximum growth of 84.4%, while the upward trend of internal friction angle was not obvious, whose increase was less than 10%. In this regard, it is speculated that the increase in calcium carbonate production promoted the cementation of soil particles, while its effect on the soil particle friction was not significant [49,50].

3.2. Permeability Properties of Zn-Contaminated Soil

Permeability Coefficient

Figure 9 presented the permeability coefficient of specimens treated by MICP subjected to different conditions. It could be seen that the curing age, cementation solution concentration, and its ratio had a significant influence on the variation characteristics of permeability coefficients. The permeability coefficients of specimens showed a tendency of decreasing firstly and increasing subsequently with the increasing of cementation solution concentration and its ratio. Besides, the permeability coefficient of specimens showed a downward trend with the increasing curing age. Overall, the permeability coefficient of Zn-contaminated soils treated by MICP was reduced by at least one order of magnitude.
In the meantime, the permeability coefficients of specimens subjected to the same cementation solution ratio decreased gradually when the cementation solution concentration increased from 0.25 to 1 mol/L, which had a range of 25.3~411.4%. It indicated that the MICP treatment could improve the impermeability of Zn-contaminated soils. The main reason for this phenomenon was that in the process of MICP, large-shaped and unevenly distributed microbial mineralization products were gradually generated between the specimen pores, which formed embolisms [51]. On this basis, these embolisms blocked the permeation pathways, leading to a significant permeability coefficient decrease of Zn-contaminated soil treated by MICP. Moreover, the permeability coefficients of the low-concentration specimens decreased more slowly than those of the high-concentration specimens. And there was a downward trend of its permeability coefficient when the curing age was up to 28 d. It was speculated that this phenomenon might be related to the crystal distribution characteristics of the specimen during the MICP treatment process. The permeation within the specimens with a lower cementation solution concentration tended to be more uniform [52].

3.3. The Stability Characteristics of Zn-Contaminated Soil

3.3.1. Zn Chemical Forms

Heavy metal ions in contaminated soils treated by MICP, mainly exist in carbonate bound state, exchangeable state. The carbonate-bound state of heavy metals is more stable, and their precipitation cannot be easily dissolved in changing environments. However, the exchangeable state of heavy metals is more active, and the free heavy metal ions have strong mobility and are easily extracted and utilized by biology, which is the main toxic form of heavy metals in soil. Contaminated soils treated by MICP gradually transformed the active chemical morphology of heavy metals into an inert state, thus reducing the toxicity of heavy metals.
Figure 10 presented the proportion of carbonate-bound Zn content among specimens treated by MICP subjected to different conditions. It could be seen that with the increasing of the cementation solution concentrations and its ratio, the proportion of carbonate-bound Zn content tended to increase firstly and then decrease slightly, while it kept growing with the increasing curing age. Specifically, as shown in Figure 10b. It could be found that when the cementation solution ratio (CaCl2:urea) was increased from the initial ratio of 1:1 to 1:2, the incremental catalyzed CO32− in the specimen was effectively combined with zinc ions to produce carbonate-bound Zn. This resulted in a large increase in the proportion of carbonate-bound Zn content. Increased to 1:2.5, the proportion of carbonate-bound Zn content in the specimens showed a slight decrease. This is because the excessive catalysis led to a decrease in the rate of binding of zinc ions to CO32−, which in turn weakened the immobilization effect of MICP. As a result, the proportion of carbonate-bound Zn content of the specimen accounted for the decrease compared to the former ratio (1:2).
Figure 11 presented the proportion of exchangeable Zn content among specimens treated by MICP subjected to different conditions. Combined with Figure 10, it was found that as the proportion of carbonate-bound Zn content increased in the specimens, the corresponding Zn content in its exchangeable state concomitantly decreased. There was a negative correlation between them. The decrease in the proportion of exchangeable Zn content reflected that the mobility of zinc ions in contaminated soil was reduced. The influence of abiotic factors (pH, temperature, free ion concentration) on the toxicity of heavy metals also weakened. To sum up, Zn-contaminated soil treated by MICP had a good stabilization effect.

3.3.2. Toxic Leaching Characteristics

The Zn-contaminated soil treated by MICP had a certain leaching risk of the carbonate-bound Zn, which was usually stabilized in the soil. The leaching concentration of zinc ions detected by TCLP tests could further reflect the toxic leaching characteristics of Zn-contaminated soil treated by MICP. Figure 12 presented Leached concentration of zinc ions among specimens treated by MICP subjected to different conditions. It could be seen that with the increasing of the cementation solution concentrations and its ratio, the leaching concentration of zinc ions tended to decrease firstly and then rebound slightly, while it kept decreasing with the increasing curing age. In general, most of the specimens met the leaching toxicity criteria regardless of the curing age, cementation solution concentration, and ratio. Particularly, when the curing age reached 28 d, the leaching concentration of zinc ions had reached the specified limit (Zn ≤ 100 mg/L) without exception, which achieved the stabilization goal of Zn-contaminated soils.
As shown in Figure 12a, the leaching concentration of zinc ions presented a downward trend from the cementation solution concentration of 0.25 to 1 mol/L. The leaching concentration of zinc ions was close to 100 mg/L when the specimens had a cementation solution concentration of 1 mol/L and a curing age of 3 d. Since the curing age was up to 28 d, the leaching concentrations of zinc ions were about 20 mg/L, significantly below the limit in the standard (GB 5085.3-2007) [43]. However, there was a slight increase in the leaching concentration of zinc ions when the cementation solution concentration was up to 1.5 mol/L. The cause of this phenomenon might be that the interaction between calcium salt and heavy mental Zn would be weakened under the circumstance of an excessively high cementation solution concentration [53]. As shown in Figure 12b, the average decrease in leaching concentration of zinc ions corresponding to the ratios (CaCl2:urea) of 1:1, 1:1.5, 1:2, and 1:2.5 were 46.5%, 61.2%, 73.1%, and 70.2%, respectively. It indicated that the ratio of 1:2 had a good treatment effect on the leaching concentration of zinc ions.
In summary, the stabilization effect of zinc ions in the specimens was optimal and met the environmental requirements when the specimens had a curing age of 28 d, a cementation solution concentration of 1 mol/L, and a cementation solution ratio of 1:2.

4. Mechanism Analysis

4.1. Scanning Electron Microscope

To investigate the effect of MICP treatment on the internal structure of Zn-contaminated soil, the specimens with a cementation solution ratio (CaCl2:urea) of 1:2 and a curing age of 28 d were selected for analysis. Figure 13 presented the SEM images of specimens treated by MICP. It could be seen that the untreated specimens had a looser particle structure, while that of the treated specimens was denser. During the MICP treatment, the calcium carbonate crystals presented between the pores of the soil particles mainly played the role of filling the pores, while the carbonate crystal aggregated wrapped around the adjacent soil particles played the role of cementing the soil particles, and this part of the calcium carbonate crystals could improve the cohesion between the soil particles and significantly increase the strength of the soil. With the microbially induced mineralization reaction, due to the presence of zinc ions in the contaminated soil, the consumption of CO3− in the cementation solution was accelerated and the amount of carbonate production rapidly increased, and the dispersed small granular calcium carbonate crystals began to aggregate, and under room temperature conditions (25 °C), the calcium carbonate crystals were continuously converted into calcite, which had a better cementing effect [54].
In addition, the treatment effect of the specimens depended largely on the cementation solution concentration. Specifically, the protein and extracellular polymer matrix of bacterial epidermal cells controls the morphology of calcium carbonate precipitation. The cell surface absorbs Ca2+ from the environment, decomposes urea into CO32− and NH4+ by urease secreted inside the cell, and Ca2+ combines with CO32− to generate a large number of calcite crystal-bound soil particles. The volume shape of calcite increases with the cementation solution concentration [55]. When the cementation solution concentration increased to 1 mol/L, calcite crystals with regular flocculation shapes were obviously collected on the surface of soil particles. The crystals would have a larger shape and be more uneven than those of specimens with less than 1 mol/L. It helped to increase the mechanical strength of specimens and improved its own impermeability.
In summary, Zn-contaminated soil treated by MICP could generate calcium carbonate. These microbial mineralization products would adhere to soil particles and play a binding role, which was the main cause for the improvement of the physical and mechanical properties of Zn-contaminated soils treated by MICP.

4.2. X-ray Diffraction

In order to further analyze the micro-properties of Zn-contaminated soil treated by MICP, the specimens with a cementation solution ratio (CaCl2:urea) of 1:2 and a curing age of 28 d were selected for XRD tests analysis, as shown in Figure 14. It could be seen that the specimens had three different mineral compositions internally: A-SiO2, the dominant substance at various concentrations; B-Calcite, the main cementing mineral, which acted as a “bridge” and cemented the loose soil particles together [56]; C-ZnCO3, the bound state of zinc ion and carbonate ion in the specimen. The high peaks of the calcite diffraction peaks indicated a higher production while the weaker diffraction peaks of the zinc carbonate products.
During the MICP treatment, the zinc ions with positively charged in the Zn-contaminated soil leached into the MICP gelling solution and were adsorbed by the bacteria with a negative charge. With the cementation solution concentration increased, the diffraction peaks of calcite and zinc carbonate gradually increased, and the exchangeable zinc ions in the specimens were removed and stabilized in the carbonate-bound state. It was found that the increase of cementation solution concentration promoted the urea hydrolysis reaction, which led to the reaction in the direction of calcium carbonate crystallization direction. Specifically, when the cementation solution concentration was 1 mol/L, the high concentration could provide a sufficient calcium source for the growth of bacteria, resulting in the highest calcite production and the ZnCO3 composition in the specimens. It indicated that the stabilization effect of Zn-contaminated soil treated by MICP was significant at the concentration of 1 mol/L.

5. Conclusions

Currently, heavy metal contamination in soil is becoming an increasingly serious problem, and the MICP method is an emerging technology for geotechnical treatment. Through a series of experimental studies, the following conclusions were obtained.
(1)
Microbial-induced calcium carbonate precipitation treatment changes the stress–strain relationship of Zn-contaminated soil from weakly softened type to strongly softened type. The unconfined compressive strength of the treated specimens increased by 187.2%~550.5%. The cohesion of treated specimens presented a significant upward trend while the internal friction angle keeps relatively stable. Microbial-induced calcium carbonate precipitation treatment could strengthen the mechanical properties of Zn-contaminated soils.
(2)
The permeability coefficient can be reduced by at least one order of magnitude. Microbial-induced calcium carbonate precipitation treatments significantly reduced the leaching concentration of zinc ions in Zn-contaminated soils to about 20 mg/L, which was lower than the limit (100 mg/L) in the standard. The mobility of heavy metal Zn was significantly reduced, and the proportion of exchangeable Zn content substantially declined. Microbial-induced calcium carbonate precipitation treatment could enhance permeability properties and reduce toxic leaching capacity for Zn-contaminated soils.
(3)
For mechanical properties, impermeability properties, and toxic leaching capacity, the stabilization effect of contaminated soil treated by MICP would be most significant when the specimens had a curing age of 28 d, a cementation solution concentration of 1 mol/L and a cementation solution ratio of 1:2.
(4)
The main microbial mineralization product of Zn-contaminated soil treated by MICP is calcite. The increase in cementation degree is the main reason for the improvement of the physical and mechanical properties of treated Zn-contaminated soil. At the same time, the exchangeable zinc ions in the specimen were removed and stabilized in the carbonate-bound during the MICP process, which made the Zn-contaminated soil in compliance with environmental requirements.

Author Contributions

Investigation, T.C.; Conceptualization, R.Z., W.X. and F.Z.; methodology, R.Z.; validation, W.X. and X.W.; formal analysis, W.X.; investigation, F.Z.; data curation, W.X.; writing—original draft preparation, R.Z. and W.X.; project administration, R.Z.; funding acquisition, R.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Postgraduate Research & Practice Innovation Program of Jiangsu Province (SJCX23_0495), the Natural Science Foundation of Jiangsu Province (BK20220356), the Opening Fund of Key Laboratory of Geohazard Prevention of Hilly Mountains, Ministry of Natural Resources (Fu-jian Key Laboratory of Geohazard Prevention) (FJKLGH2023K005), and the Open Research Fund of State Key Laboratory of Simulation and Regulation of Water Cycle in River Basin (China Institute of Water Resources and Hydropower Research) (IWHR-SKL-KF202319).

Data Availability Statement

The data used to support the findings of this study are available from the authors upon request.

Conflicts of Interest

The authors declare that they have no conflict of interest to report regarding the present study.

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Buildings 13 01974 g001
Figure 1. Zn-contaminated soil materials.
Figure 1. Zn-contaminated soil materials.
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Figure 2. Preparation of MICP gelling solution.
Figure 2. Preparation of MICP gelling solution.
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Figure 3. Test specimens: (a) UCS specimens; (b) DS specimens.
Figure 3. Test specimens: (a) UCS specimens; (b) DS specimens.
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Figure 4. Test instruments: (a) UCS instrument; (b) DS instrument; (c) TCLP instrument.
Figure 4. Test instruments: (a) UCS instrument; (b) DS instrument; (c) TCLP instrument.
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Figure 5. Typical stress–strain relations of specimens treated by MICP: (a) Stress–strain relationship with cementation solution concentration; (b) stress–strain relationship with cementation solution ratio; (c) stress–strain relationship with curing age.
Figure 5. Typical stress–strain relations of specimens treated by MICP: (a) Stress–strain relationship with cementation solution concentration; (b) stress–strain relationship with cementation solution ratio; (c) stress–strain relationship with curing age.
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Figure 6. Typical elastic modules variation curves of specimens treated by MICP: (a) The variation of elastic modulus with cementation solution concentration; (b) The variation of elastic modulus with cementation solution ratio; (c) The variation of elastic modulus with curing age.
Figure 6. Typical elastic modules variation curves of specimens treated by MICP: (a) The variation of elastic modulus with cementation solution concentration; (b) The variation of elastic modulus with cementation solution ratio; (c) The variation of elastic modulus with curing age.
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Figure 7. Typical failure strength variation curves of specimens treated by MICP: (a) The variation of failure strength with cementation solution concentration; (b) the variation of failure strength with cementation solution ratio; (c) the variation of failure strength with curing age.
Figure 7. Typical failure strength variation curves of specimens treated by MICP: (a) The variation of failure strength with cementation solution concentration; (b) the variation of failure strength with cementation solution ratio; (c) the variation of failure strength with curing age.
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Figure 8. Cohesion and internal friction angle of specimens treated by MICP subjected to different conditions: (a) cohesion; (b) angle of internal friction.
Figure 8. Cohesion and internal friction angle of specimens treated by MICP subjected to different conditions: (a) cohesion; (b) angle of internal friction.
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Figure 9. Permeability coefficient of specimens treated by MICP subjected to different conditions: (a) The variation of permeability coefficient with cementation solution concentration; (b) The variation of permeability coefficient with cementation solution ratio; (c) The variation of permeability coefficient with curing age.
Figure 9. Permeability coefficient of specimens treated by MICP subjected to different conditions: (a) The variation of permeability coefficient with cementation solution concentration; (b) The variation of permeability coefficient with cementation solution ratio; (c) The variation of permeability coefficient with curing age.
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Figure 10. The proportion of carbonate-bound Zn content among specimens treated by MICP subjected to different conditions: (a) The variation of carbonate Zn content proportion with cementation solution concentration; (b) the variation of carbonate Zn content proportion with cementation solution ratio; (c) the variation of carbonate Zn content proportion with curing age.
Figure 10. The proportion of carbonate-bound Zn content among specimens treated by MICP subjected to different conditions: (a) The variation of carbonate Zn content proportion with cementation solution concentration; (b) the variation of carbonate Zn content proportion with cementation solution ratio; (c) the variation of carbonate Zn content proportion with curing age.
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Figure 11. The proportion of exchangeable Zn content among specimens treated by MICP subjected to different conditions: (a) The variation of exchangeable Zn content proportion with cementation solution concentration; (b) the variation of exchangeable Zn content proportion with cementation solution ratio; (c) the variation of exchangeable Zn content proportion with curing age.
Figure 11. The proportion of exchangeable Zn content among specimens treated by MICP subjected to different conditions: (a) The variation of exchangeable Zn content proportion with cementation solution concentration; (b) the variation of exchangeable Zn content proportion with cementation solution ratio; (c) the variation of exchangeable Zn content proportion with curing age.
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Figure 12. Leached concentration of zinc ions among specimens treated by MICP subjected to different conditions: (a) The variation of Zn leaching concentration with cementation solution concentration; (b) the variation of Zn leaching concentration with cementation solution ratio; (c) the variation of Zn leaching concentration with curing age.
Figure 12. Leached concentration of zinc ions among specimens treated by MICP subjected to different conditions: (a) The variation of Zn leaching concentration with cementation solution concentration; (b) the variation of Zn leaching concentration with cementation solution ratio; (c) the variation of Zn leaching concentration with curing age.
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Figure 13. SEM images of specimens treated by MICP: (a) 0 mol/L; (b) 0.25 mol/L; (c) 0.5 mol/L; (d) 1 mol/L; (e) 1.5 mol/L.
Figure 13. SEM images of specimens treated by MICP: (a) 0 mol/L; (b) 0.25 mol/L; (c) 0.5 mol/L; (d) 1 mol/L; (e) 1.5 mol/L.
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Figure 14. XRD test results of specimens treated by MICP: (a) 0.25 mol/L; (b) 0.5 mol/L; (c) 1 mol/L; (d) 1.5 mol/L.
Figure 14. XRD test results of specimens treated by MICP: (a) 0.25 mol/L; (b) 0.5 mol/L; (c) 1 mol/L; (d) 1.5 mol/L.
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Table 2. Fundamental properties of soil material.
Table 2. Fundamental properties of soil material.
Soil PropertiesSilt
Specific gravity2.67
Liquid limit18.4%
Plastic limit12.3%
Optimum moisture content12.8%
Maximum dry density1.80 Mg/m3
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Xing, W.; Zhou, F.; Zhu, R.; Wang, X.; Chen, T. Performance and Mechanism of Zn-Contaminated Soil through Microbe-Induced Calcium Carbonate Precipitation. Buildings 2023, 13, 1974. https://doi.org/10.3390/buildings13081974

AMA Style

Xing W, Zhou F, Zhu R, Wang X, Chen T. Performance and Mechanism of Zn-Contaminated Soil through Microbe-Induced Calcium Carbonate Precipitation. Buildings. 2023; 13(8):1974. https://doi.org/10.3390/buildings13081974

Chicago/Turabian Style

Xing, Wei, Feng Zhou, Rui Zhu, Xudong Wang, and Tingzhu Chen. 2023. "Performance and Mechanism of Zn-Contaminated Soil through Microbe-Induced Calcium Carbonate Precipitation" Buildings 13, no. 8: 1974. https://doi.org/10.3390/buildings13081974

APA Style

Xing, W., Zhou, F., Zhu, R., Wang, X., & Chen, T. (2023). Performance and Mechanism of Zn-Contaminated Soil through Microbe-Induced Calcium Carbonate Precipitation. Buildings, 13(8), 1974. https://doi.org/10.3390/buildings13081974

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