Soil remediation method based on synergy of desulfurized gypsum and microorganisms
By using a synergistic method of desulfurized gypsum and microorganisms, desulfurized gypsum powder and slurry are prepared. Combined with chemical modification and biomineralization, the problems of the lack of long-term effects of desulfurized gypsum in improving saline-alkali land and the poor effect of remediating heavy metal contaminated soil are solved, and efficient soil remediation in arid or poorly drained areas is achieved.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- WUHAN UNIV OF TECH
- Filing Date
- 2026-03-10
- Publication Date
- 2026-04-21
AI Technical Summary
In existing technologies, desulfurized gypsum has a short-lasting effect when used to improve saline-alkali land, and its effect on remediating soil contaminated with heavy metals is not good. Furthermore, its application is limited in arid or poorly drained areas, posing environmental risks.
A method combining desulfurized gypsum and microorganisms was used to prepare desulfurized gypsum powder and slurry. Desulfurized gypsum source functional mineralizing bacterial solution was obtained through enrichment culture. Combining chemical improvement and biomineralization, salinization and heavy metal pollution were solved simultaneously.
It improves the remediation effect of salinized and heavy metal contaminated soils, reduces the cost of remediation materials, reduces reliance on abundant leaching water sources and complete drainage systems, broadens the scope of application, and has a long-lasting remediation effect.
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Abstract
Description
Technical Field
[0001] This invention relates to the field of soil remediation technology, and in particular to a soil remediation method based on the synergistic effect of desulfurized gypsum and microorganisms. Background Technology
[0002] Coal-fired power plants generate large amounts of desulfurization gypsum during flue gas desulfurization (FGD) processes. The storage and disposal of this gypsum not only occupies land but also poses environmental risks. Current technologies allow for the use of desulfurization gypsum in saline-alkali land remediation; however, its effectiveness is highly dependent on abundant leaching water and requires specific drainage conditions, limiting its application in arid or poorly drained areas. Furthermore, the remediation effect is not sustainable. In addition, salinization pollution often coexists with heavy metal pollution in the soil to be remediated, and desulfurization gypsum is ineffective in remediating soils contaminated with heavy metals.
[0003] Therefore, there is an urgent need to develop a new technical solution to address the aforementioned technical problems. Summary of the Invention
[0004] This invention provides a soil remediation method based on the synergistic effect of desulfurized gypsum and microorganisms, which can improve the remediation effect of salinized and heavy metal contaminated soils and avoid the environmental risks caused by the stockpiling of desulfurized gypsum.
[0005] In a first aspect, the present invention provides a soil remediation method based on the synergistic effect of desulfurized gypsum and microorganisms, comprising: Desulfurized gypsum powder and desulfurized gypsum slurry were prepared using desulfurized gypsum produced from coal-fired power plants as raw materials. The desulfurized gypsum powder was used as the initial bacterial source and inoculated into a prepared enrichment medium for enrichment culture and subculture to obtain a desulfurized gypsum source functional mineralization bacterial solution. The nitrogen source in the enrichment medium was urea. The desulfurized gypsum slurry is applied to the soil to be repaired, mixed evenly, and left to stand for a preset time period to obtain pre-improved soil. The desulfurized gypsum-based functional mineralizing bacterial solution is mixed with urea solution to obtain a remediation reaction solution. The remediation reaction solution is applied to the pre-modified soil to precipitate the target heavy metal ions in the pre-modified soil, thereby achieving the remediation of the pre-modified soil.
[0006] This invention provides a soil remediation method based on the synergistic effect of desulfurized gypsum and microorganisms. It can prepare desulfurized gypsum slurry and powder for soil remediation using desulfurized gypsum produced from coal-fired power plants as raw material. This not only avoids the environmental risks associated with desulfurized gypsum stockpiling but also reduces the cost of remediation materials. The introduction of a microbial-induced mineralization process reduces the dependence of desulfurized gypsum remediation on abundant leaching water and a complete drainage system, broadening its applicability in arid, semi-arid, or poorly drained areas. Combining the chemical remediation effect of desulfurized gypsum with the biomineralization effect of desulfurized gypsum-derived functional mineralizing bacterial solution simultaneously addresses the problems of salinization and heavy metal pollution. Furthermore, the desulfurized gypsum-derived functional mineralizing bacterial solution is directly screened and acclimatized from desulfurized gypsum, resulting in a bacterial agent with natural tolerance to high-calcium, high-salt, and heavy metal environments. This high compatibility with desulfurized gypsum raw materials improves the remediation effect and the durability of the remediation effect. Attached Figure Description
[0007] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.
[0008] Figure 1 This is a schematic flowchart of a soil remediation method based on the synergistic effect of desulfurized gypsum and microorganisms provided in an embodiment of the present invention. Detailed Implementation
[0009] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are some embodiments of the present invention, but not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.
[0010] Please refer to Figure 1 This invention provides a soil remediation method based on the synergistic effect of desulfurized gypsum and microorganisms, comprising: Step 100: Using desulfurized gypsum produced by coal-fired power plants as raw materials, desulfurized gypsum powder and desulfurized gypsum slurry are prepared respectively; Step 102: Desulfurized gypsum powder is used as the initial bacterial source and inoculated into the prepared enrichment medium for enrichment culture and subculture to obtain desulfurized gypsum source functional mineralization bacterial solution. The nitrogen source in the enrichment culture medium is urea; Step 104: Apply the desulfurized gypsum slurry to the soil to be repaired, mix it evenly, and let it stand for a preset time period to obtain pre-improved soil; Step 106: Mix the desulfurized gypsum source functional mineralizing bacteria solution with urea solution to obtain a remediation reaction solution. Apply the remediation reaction solution to the pre-modified soil to precipitate the target heavy metal ions in the pre-modified soil and achieve the remediation of the pre-modified soil.
[0011] In this embodiment of the invention, desulfurized gypsum produced by coal-fired power plants is recycled and utilized. Desulfurized gypsum powder and desulfurized gypsum slurry are prepared using the desulfurized gypsum as raw material. The desulfurized gypsum slurry is generated by mixing a portion of the desulfurized gypsum powder with water. Another portion of the desulfurized gypsum powder is selected, and Bacillus spores present in the desulfurized gypsum are used as the initial bacterial source. This is then inoculated into an enrichment culture medium for enrichment and subculturing to obtain a desulfurized gypsum-derived functional mineralizing bacterial solution. During the acclimation process, the high-salt, high-calcium environment of the desulfurized gypsum itself and the urea (nitrogen source) in the culture medium are used as selection pressures to screen for indigenous dominant bacterial groups (desulfurized gypsum-derived functional mineralizing bacteria) that can tolerate the gypsum environment and have high urease production activity. The desulfurized gypsum slurry and the desulfurized gypsum-derived functional mineralizing bacterial solution are then applied synergistically to the soil to be remediated. First, desulfurized gypsum slurry is applied to the soil to be remediated. The soil is then tilled to ensure even mixing of the slurry and the soil, and allowed to stand for 3-5 days to obtain pre-treated improved soil. Next, desulfurized gypsum-derived functional mineralizing bacteria solution is mixed with urea solution to obtain a remediation reaction solution, which is then evenly applied to the contaminated soil. The soil is then tilled again to ensure even distribution of the remediation reaction solution.
[0012] Specifically, after applying desulfurized gypsum slurry to the soil to be remediated, part of the desulfurized gypsum slurry... In the replacement While promoting elution, it also acts as a cationic bridge between clay particles and organic matter, promoting the cementation of single particles into micro-aggregates. Applying desulfurized gypsum slurry enhances the aggregation of mineral soil particles, increases soil mechanical stability and the number of water-stable aggregates, and the effect increases with the application rate. Furthermore, applying desulfurized gypsum slurry promotes the formation of new pores in the topsoil, increases soil permeability, and rapidly captures pollutants from the soil solution. This process generates tiny calcium carbonate crystal nuclei, rapidly lowering the soil pH (if the initial soil pH is too high (>9.5), it may lead to functional death after subsequent application; after settling, the soil pH can be reduced to 8-9, which is the optimal range for urease activity). After applying the remediation reaction solution, the previously applied desulfurized gypsum slurry has already lowered the soil pH and increased soil porosity, making it easier for bacteria to colonize the surface of soil particles. The bacteria secrete urease, which hydrolyzes urea to produce... and Microorganisms utilize the calcium source in desulfurized gypsum slurry and the carbonate ions produced by urea hydrolysis to induce the formation of calcite-type calcium carbonate precipitates in soil pores. They also simultaneously solidify heavy metal ions in the soil through co-precipitation and isomorphic substitution, thereby achieving the remediation of pre-modified soil.
[0013] The above process can be represented by the following chemical reaction formula: Ion exchange is performed by applying desulfurized gypsum slurry:
[0014] Urea hydrolysis:
[0015] Calcium carbonate precipitation and heavy metal fixation:
[0016] in, This represents a divalent metal cation, which can co-precipitate with calcium carbonate, and has an ionic radius close to... Heavy metal ions (e.g.) , , , and ) can be Isomorphic substitution, or inclusion through penetration into the gaps or defects of the crystal. Within the crystal lattice, heavy metals transform from soluble ions into insoluble forms, preventing their re-release into the environment. Furthermore, this immobilization form, supported by calcium carbonate, is insensitive to changes in the redox potential of the surrounding environment, thus enabling efficient heavy metal immobilization and maintaining long-term stability. The cell walls of functional mineralizing bacteria carry negative charges (carboxyl and phosphate groups), allowing them to adsorb positively charged metal ions like magnets. These adsorbed ions subsequently become nucleation sites for calcium carbonate crystal growth.
[0017] In one embodiment of the present invention, desulfurized gypsum powder is used as the initial bacterial source and inoculated into a prepared enrichment culture medium for enrichment culture and subculturing to obtain a desulfurized gypsum-derived functional mineralizing bacterial solution, comprising: Desulfurized gypsum powder was used as the initial bacterial source and inoculated into an enrichment medium with urea as the sole nitrogen source to obtain a primary enrichment solution. The primary enrichment solution was subcultured in an acclimatization medium containing desulfurized gypsum extract to obtain a desulfurized gypsum-derived functional mineralizing bacterial solution.
[0018] In this embodiment, the enrichment medium was prepared as follows: 5 g of soybean peptone, 15 g of casein peptone, 5 g of sodium chloride, and 20 g of urea (as the sole nitrogen source) were weighed. 900 g of distilled water was added, and the mixture was stirred until homogeneous. The pH was then adjusted to between 7 and 8. The prepared enrichment medium was sterilized at 120°C for 15 minutes at 0.1 MPa, and then 20 g of urea was added as the sole nitrogen source. If urea was added before sterilization, a low-temperature sterilization method was used to prevent urea decomposition at high temperatures. 10 g of desulfurized gypsum powder dried at 60°C was added to a conical flask containing 100 mL of the enrichment medium and cultured with shaking at 30°C and 150 rpm for 24-48 hours to obtain the primary enrichment solution. Select 1 mL of primary enrichment solution and inoculate it into fresh acclimatization medium at a ratio of 1:50 (the composition of the acclimatization medium is the same as that of the enrichment medium, and 1% desulfurized gypsum extract can be added to enhance the strain's tolerance to heavy metals). Perform subculture (preferably 2-3 consecutive subcultures) to obtain a composite functional bacterial solution with stable genetic traits and high metabolic activity. When the culture medium becomes turbid, with obvious precipitation and a strong, pungent ammonia odor (indicating that urea has decomposed to produce ammonia gas), and the pH value rises significantly (>8.5), it indicates that the activation and acclimatization of the desulfurized gypsum-derived functional mineralizing bacteria have been successful.
[0019] In one embodiment of the present invention, the preparation process of the desulfurized gypsum extract includes: After mixing desulfurized gypsum powder and deionized water according to a preset mass ratio, the mixture is subjected to a first shaking treatment to obtain a shaken mixture. The shaken mixture is subjected to solid-liquid separation treatment, and the supernatant is extracted as the desulfurized gypsum extract. The desulfurized gypsum extract includes calcium ions and sulfate ions dissolved from the desulfurized gypsum, as well as at least one target heavy metal pollutant ion dissolved from the impurities in the desulfurized gypsum.
[0020] In this embodiment, desulfurized gypsum produced from a coal-fired power plant is dried and pulverized to a predetermined fineness. A certain amount of desulfurized gypsum powder is weighed and placed in a clean container. Deionized water is added according to a preset solid-liquid mass ratio, and the mixture is stirred to initially disperse the powder. The mixture is transferred to a constant-temperature oscillation device and continuously oscillated at a set temperature and oscillation speed for a predetermined time to allow the soluble components in the desulfurized gypsum to fully leach out, resulting in a uniform oscillating mixture. After the oscillation treatment, the oscillating mixture is subjected to solid-liquid separation to remove solid residues. The resulting supernatant is collected and further filtered and sterilized to obtain a desulfurized gypsum extract. This extract contains calcium and sulfate ions dissolved from the desulfurized gypsum, as well as one or more target heavy metal pollutant ions dissolved from the impurities contained in the desulfurized gypsum.
[0021] In one embodiment of the present invention, the preset mass ratio of desulfurized gypsum powder to deionized water is 1:(5-20); The temperature conditions for the first vibration treatment are 20-30℃, the rotation speed is greater than or equal to 100rpm, and the duration of the first vibration treatment is 12-48 hours.
[0022] In this embodiment, when preparing the desulfurized gypsum extract, the mass ratio of desulfurized gypsum powder to deionized water is controlled between 1:5 and 1:20. If the mass ratio is lower than 1:5, the system will become too thick and difficult to mix; if the mass ratio is higher than 1:20, the extract concentration will be insufficient to provide effective microbial acclimation pressure. The extraction process is underway... Oscillation under conditions of speed ≥100rpm Hours. This temperature range ensures efficient dissolution and energy saving, while sufficient rotation speed prevents powder deposition and ensures adequate contact; Hours of oscillation allow the target ions to fully dissolve and reach a stable state.
[0023] In one embodiment of the present invention, the primary enrichment solution is subcultured and acclimatized in an acclimatization culture medium containing desulfurized gypsum extract to obtain a desulfurized gypsum-derived functional mineralizing bacterial solution, comprising: The primary enrichment solution was inoculated into the acclimatization medium at a volume ratio of 1:50. Adding desulfurized gypsum extract to the acclimatization culture medium to acclimate the microorganisms in the primary enrichment solution to the tolerance of the target heavy metal ions. The acclimatization culture medium with added desulfurized gypsum extract was placed in a preset temperature environment for a second shaking treatment; Repeat the steps from adding the primary enrichment solution to the acclimatization culture medium to placing the acclimatization culture medium with added desulfurized gypsum extract in a preset temperature environment for a second shaking treatment a preset number of times to obtain a desulfurized gypsum source functional mineralization bacterial solution.
[0024] In this embodiment, after obtaining the primary enrichment solution, 1 mL of the primary enrichment solution was inoculated into fresh acclimatization medium at a ratio of 1:50. The composition of the acclimatization medium was the same as that of the enrichment medium, with the addition of 1% desulfurized gypsum extract to enhance the strain's tolerance to heavy metals. The acclimatization medium with added desulfurized gypsum extract was placed in a preset temperature environment for a second shaking treatment. Two to three subcultures were performed for acclimatization to obtain a composite functional bacterial culture with stable genetic traits and high metabolic activity.
[0025] In one embodiment of the present invention, the optical density value of the desulfurized gypsum-derived functional mineralizing bacterial solution at a wavelength of 600 nm is between 1.0 and 1.5, and the urease activity of the desulfurized gypsum-derived functional mineralizing bacterial solution is between 10 and 15 U / mL.
[0026] In this embodiment, a spectrophotometer was used to measure the bacterial concentration. The optical density of the desulfurized gypsum-derived functional mineralizing bacterial solution at a wavelength of 600 nm needed to be controlled at approximately 1.0-1.5 to ensure sufficient bacterial biomass. A conductivity meter was used to detect urease activity. By measuring the rate of change in the conductivity of the bacterial solution, the urease activity was controlled at 10-15 U / mL (or the conductivity change was controlled within a specific range) to ensure the catalytic efficiency of the subsequent MIP reaction.
[0027] In one embodiment of the present invention, the target heavy metal ions include one or more of cadmium ions, lead ions, copper ions, and zinc ions.
[0028] In this embodiment, the method of the present invention is applicable to the remediation of common heavy metal pollution in soil, including ions corresponding to elements such as cadmium, lead, copper, and zinc. These ions can be simulated by desulfurized gypsum extract and effectively immobilized during subsequent microbial mineralization. The soil remediation method disclosed in this embodiment can simultaneously treat one or more of these heavy metal ions.
[0029] In one embodiment of the present invention, the particle size of the desulfurized gypsum powder is less than or equal to 0.075 mm; The preparation process of desulfurized gypsum slurry is as follows: desulfurized gypsum powder and water are mixed in a mass ratio of 1:(4-6), and then stirred to obtain a slurry with a solid content of 15%-25%.
[0030] In this embodiment, desulfurization gypsum (the main component of which is produced by a coal-fired power plant) is selected. The desulfurized gypsum (with a moisture content of approximately 10-15%) is placed in a drying device (forced air drying oven) and dried at 60℃±5℃ for 12-24 hours (to remove surface free water and prevent dehydration and phase inversion of dihydrate gypsum). It is then pulverized using a ball mill or pulverizer and passed through a 200-mesh standard sieve with a diameter of 0.075mm, collecting the powder that passes through the sieve. This powder is mixed with water at a mass ratio of 1:(4-6) (preferably 1:5), and mechanically stirred for 30 minutes to prepare a uniformly suspended desulfurized gypsum slurry with a solid content of 15%-25% (preferably 20%).
[0031] In one embodiment of the present invention, the concentration of the urea solution is 18-22 g / L and the pH value is 7.0-8.0.
[0032] In this embodiment, a urea solution with a molar concentration of 18-22 g / L (preferably 20 g / L) is prepared, and the pH value is adjusted to 7-8 (this can be achieved by adding a small amount of urea solution). (Solution adjustment). The effect of urea concentration includes: if the urea concentration is too high, the excessively high concentration of urea will be rapidly hydrolyzed by urease, releasing a large amount of... This leads to a sharp increase in soil pH, inhibiting bacterial activity; if the urea concentration is too low, Insufficient urea concentration leads to low calcite precipitation, preventing heavy metal fixation via co-precipitation and significantly reducing fixation efficiency. This inhibits bacterial growth and urease activity, and the low urea concentration cannot support long-term remediation needs. A urea concentration of 20 g / L optimally matches bacterial growth and urease activity, achieving the highest heavy metal fixation efficiency and avoiding a balance between substrate limitation and toxicity inhibition.
[0033] In one embodiment of the present invention, the volume ratio of desulfurized gypsum-derived functional mineralizing bacteria solution to urea solution in the repair reaction solution is 1:1.
[0034] In this embodiment, the functional mineralizing bacteria solution and urea solution are mixed at a 1:1 ratio and then uniformly applied to the contaminated soil. The soil is then tilled again to ensure even distribution of the liquid. This ensures that the microbial activity matches the urea hydrolysis rate, providing sufficient carbonate ions for the precipitation reaction while avoiding a sharp increase in pH due to excessively high local urea concentrations, which would inhibit bacterial activity. The reaction solution prepared in this ratio exhibits good reaction stability and heavy metal fixation efficiency.
[0035] It should be noted that, in this invention, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus.
[0036] Finally, it should be noted that the above are merely preferred embodiments of the present invention, used only to illustrate the technical solutions of the present invention, and are not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention are included within the scope of protection of the present invention.
Claims
1. A soil remediation method based on the synergistic effect of desulfurized gypsum and microorganisms, characterized in that, include: Desulfurized gypsum powder and desulfurized gypsum slurry were prepared using desulfurized gypsum produced from coal-fired power plants as raw materials. The desulfurized gypsum powder was used as the initial bacterial source and inoculated into a prepared enrichment medium for enrichment culture and subculture to obtain a desulfurized gypsum source functional mineralization bacterial solution. The nitrogen source in the enrichment medium was urea. The desulfurized gypsum slurry is applied to the soil to be repaired, mixed evenly, and left to stand for a preset time period to obtain pre-improved soil. The desulfurized gypsum-based functional mineralizing bacterial solution is mixed with urea solution to obtain a remediation reaction solution. The remediation reaction solution is applied to the pre-modified soil to precipitate the target heavy metal ions in the pre-modified soil, thereby achieving the remediation of the pre-modified soil.
2. The method according to claim 1, characterized in that, The process involves using the desulfurized gypsum powder as the initial bacterial source, inoculating it into a prepared enrichment medium for enrichment and subculturing to obtain a desulfurized gypsum-derived functional mineralizing bacterial solution, including: The desulfurized gypsum powder was used as the initial bacterial source and inoculated into an enrichment medium with urea as the sole nitrogen source to obtain a primary enrichment solution. The primary enrichment solution was subcultured in an acclimatization medium containing desulfurized gypsum extract to obtain a desulfurized gypsum-derived functional mineralizing bacterial solution.
3. The method according to claim 2, characterized in that, The preparation process of the desulfurized gypsum extract includes: After mixing desulfurized gypsum powder and deionized water according to a preset mass ratio, the mixture is subjected to a first shaking treatment to obtain a shaken mixture. The shaken mixture is subjected to solid-liquid separation treatment, and the supernatant is extracted as a desulfurized gypsum extract. The desulfurized gypsum extract includes calcium ions and sulfate ions dissolved from the desulfurized gypsum, as well as at least one target heavy metal pollutant ion dissolved from the impurities in the desulfurized gypsum.
4. The method according to claim 3, characterized in that, The preset mass ratio of the desulfurized gypsum powder to deionized water is 1:(5-20); The temperature conditions for the first oscillation treatment are 20-30℃, the rotation speed is greater than or equal to 100rpm, and the time for the first oscillation treatment is 12-48 hours.
5. The method according to claim 2, characterized in that, The process involves subculturing the primary enrichment solution in a culture medium containing desulfurized gypsum extract to obtain a desulfurized gypsum-derived functional mineralizing bacterial solution, comprising: The primary enrichment solution is inoculated into the acclimatization culture medium, wherein the volume ratio of the primary enrichment solution to the acclimatization culture medium is 1:
50. Desulfurized gypsum extract was added to the acclimation culture medium to acclimate the microorganisms in the primary enrichment solution to the target heavy metal ions. The acclimatization culture medium containing the desulfurized gypsum extract was placed in a preset temperature environment for a second shaking treatment. Repeat the steps from adding the primary enrichment solution to the acclimatization culture medium to placing the acclimatization culture medium containing the desulfurized gypsum extract in a preset temperature environment for a second shaking treatment a preset number of times to obtain a desulfurized gypsum source functional mineralization bacterial solution.
6. The method according to claim 3, characterized in that, The optical density value of the desulfurized gypsum-derived functional mineralizing bacterial solution at a wavelength of 600 nm is between 1.0 and 1.5, and the urease activity of the desulfurized gypsum-derived functional mineralizing bacterial solution is between 10 and 15 U / mL.
7. The method according to claim 1, characterized in that, The target heavy metal ions include one or more of cadmium ions, lead ions, copper ions, and zinc ions.
8. The method according to claim 1, characterized in that, The particle size of the desulfurized gypsum powder is less than or equal to 0.075 mm; The preparation process of the desulfurized gypsum slurry is as follows: the desulfurized gypsum powder and water are mixed in a mass ratio of 1:(4-6), and then stirred to obtain a slurry with a solid content of 15%-25%.
9. The method according to claim 1, characterized in that, The concentration of the urea solution is 18-22 g / L, and the pH value is 7.0-8.
0.
10. The method according to claim 1, characterized in that, The volume ratio of the desulfurized gypsum-derived functional mineralizing bacteria solution to the urea solution in the repair reaction solution is 1:1.