Method for improving microorganism sand consolidation strength based on modified plant fibers

Abstract

The invention belongs to the related technical field of microbiological treatment of soil, and discloses a method for improving microbiological sand consolidation strength based on modified plant fibers. Natural plant fibers are subjected to alkali treatment modification, so that non-cellulose components on the surfaces of the natural plant fibers are removed, the roughness and the specific surface area are remarkably improved, the interface friction force and the mechanical connection capacity between the fibers and sand particles are enhanced, more nucleation sites are provided for microorganisms to induce calcium carbonate precipitation, and the content of calcium carbonate is increased. The strength of the solidified sandy soil can be effectively improved, and the biomineralization period is greatly shortened. Meanwhile, after the modified fibers are doped into the sandy soil, a three-dimensional interlaced fiber network structure can be formed in the material in combination with a microorganism induced calcium carbonate deposition (MICP) technology, and the cohesiveness among sand grains and the mechanical property of the overall structure are effectively improved. According to the solidification method, the solidification strength of the sand column is remarkably improved, the technology is simple, green and environment-friendly, and good engineering adaptability and popularization value are achieved in the field of sand solidification.

Description

Technical Field

[0001] The present invention belongs to the technical field related to microbial treatment of soil, and more specifically, relates to a method for enhancing the sand-fixing strength of microorganisms based on modified plant fibers. Background Art

[0002] With the acceleration of the urbanization process and the continuous expansion of infrastructure construction, construction land is becoming increasingly tense. How to efficiently reinforce poor sandy soil foundations with insufficient bearing capacity and loose structures has become one of the main research directions in modern geotechnical engineering. In recent years, the technology of microbial-induced calcium carbonate precipitation (hereinafter simply referred to as MICP) has received extensive attention as a new green, environmentally friendly, and low-energy geotechnical reinforcement method. This technology generates carbonate ions through the metabolic activities of urease-producing bacteria, reacts with calcium ions in the environment or provided externally to form calcium carbonate crystals, which fill the soil pores to achieve cementation, thereby enhancing the overall strength and impermeability of sandy soil, and having broad application prospects in the fields of foundation treatment, anti-seepage plugging, and stabilization of heavy metals in soil and water.

[0003] At present, in order to further improve the mechanical properties of MICP-solidified soil, the introduction of fiber reinforcement technology into the MICP system has become a research hotspot. Fibers can form inter-particle bridging and network structures in sandy soil, improve the mechanical properties and anti-deformation ability of the soil structure, enhance the toughness and stability of solidified sandy soil, and are particularly suitable for environments with the risk of foundation liquefaction, becoming a key supplementary means to balance the strength and toughness of geotechnical materials.

[0004] In the application process of some technologies for sandy soil reinforcement, a relatively high proportion of traditional cementitious materials such as cement is used as the matrix. Although it has certain compressive and impermeable properties, the overall carbon emission level is relatively high, and the green environmental protection performance is insufficient, which is not conducive to the development of low-carbon engineering materials. For the pure biological mineralization reinforcement technology system, the existing solutions have achieved a certain degree of improvement in the consolidation strength of sandy soil by using plant fibers and microbial technology in combination. However, the current technical solutions mainly use fibers as the attachment and loading carriers of microorganisms, failing to fully exert the reinforcing and toughening effects of the fibers themselves, nor fully utilizing the biochemical reaction process of microbial mineralization to further improve the mechanical properties of sandy soil, and it is difficult to meet the engineering requirements in special geological environments such as foundation reinforcement and slope stability.

[0005] For example, the patent "A method for microbial solidification-fiber reinforcement combined modification of sand" (CN 109594552 B) discloses a method that combines fiber reinforcement technology with MICP sand solidification technology. By incorporating basalt fiber, carbon fiber, or steel fiber into sand and then achieving sand reinforcement through microbial-induced calcium carbonate precipitation. The introduction of fiber effectively improves the toughness and residual strength of sand, compensating for the defect of easy fracture in traditional MICP-solidified sand. However, this method requires two rounds of cementation and solidification in a constant-temperature curing device with water bath heating, with a 7-day curing cycle. The curing process steps are complex, the cycle is long, and the curing efficiency is low. The highest unconfined compressive strength of the specimen after curing is 1237 kPa, and the improvement degree of the compressive strength still does not meet the requirements of general engineering applications. Using artificial fibers, the cost is relatively high. And no treatment is carried out on the fiber surface, and the interfacial bonding ability between the fiber, calcium carbonate precipitation, and the sand body needs to be further improved.

[0006] Therefore, it is urgent to further improve the mechanical properties and curing efficiency of microbial mineralization and fiber composite solidified sand. Based on giving full play to the strengthening and toughening characteristics of the fiber itself, optimize the rate and efficiency of the mineralization reaction process, reduce the dependence on traditional cementitious materials, so that the fiber-reinforced bio-mineralization material can obtain better engineering application effects and ecological environmental protection in the field of foundation reinforcement. Summary of the Invention

[0007] Aiming at the above defects or improvement requirements of the prior art, the present invention provides a method for enhancing the strength of microbial sand solidification based on modified plant fibers, aiming to introduce natural fibers with good tensile properties into sand as reinforcement materials, providing excellent carriers for the attachment and colonization of microorganisms in sand and calcium carbonate nucleation, thereby solving the technical problem of insufficient mechanical properties of sand cementation and solidification.

[0008] To achieve the above object, according to the first aspect of the present invention, a method for enhancing the strength of microbial sand solidification based on modified plant fibers is provided, including the following steps: S1: Stir and mix natural fibers and sand until the fibers are evenly distributed in the sand to obtain a mixed base material; S2: Load the mixed base material obtained in S1 into a mold and make a sand column by vibrating and compressing; S3: Activate and expand the culture of microbial bacterial powder with mineralization effect to obtain a bacterial solution; S4: Pour the bacterial solution in S3 into the sand column until it is completely immersed, and let it stand to wait for the bacteria in the bacterial solution to fully attach to the sand particles; S5: Prepare a cementing solution and pour the cementing solution into the sand column to complete one biological grouting; S6: Repeat steps S4 and S5 for multiple rounds of biogrouting (3 - 10 rounds). After the mineralization reaction is basically completed, demold, wash, and dry the sand column to obtain the fiber - microbial cementitious material.

[0009] Preferably, the natural fiber is one of sisal fiber, jute fiber, ramie fiber, flax fiber, bamboo fiber, and coconut shell fiber, the sandy soil is medium - grade sand, and the particle size of the medium - grade sand is 0.5 - 1.0 mm.

[0010] Preferably, the cementing liquid is a mixed solution of calcium chloride and urea with equal concentration and equal volume, and the concentration is 0.4 - 0.6 mol / L.

[0011] Preferably, in step S3, the strain used is Sarcina pasteurii, the culture medium is a liquid culture medium, and the components of the culture medium include 20 g / L of yeast extract, 10 g / L of ammonium sulfate, 12 μg / L of manganese sulfate, 24 μg / L of nickel chloride, and 1.4 g / L of sodium hydroxide, and the pH of the culture medium is 8.5 - 9.0.

[0012] Preferably, before step S1, cut the natural fiber. The natural fiber is sisal fiber, and soak the sisal fiber in an alkaline solution for modification treatment to remove fiber gum.

[0013] Preferably, the diameter of the sisal fiber is 0.3 mm. After cutting it to a length of 5 - 15 mm, soak its fiber in a 3 - 7wt% sodium hydroxide solution, let it stand for 4 h, wash it with deionized water, and dry it to obtain modified fiber.

[0014] Preferably, in steps S1 - S2, mix 100 g of dry medium - grade sand with 0.4 g of modified sisal fiber in a stirrer, then evenly pour it into a cylindrical mold, vibrate and compact it to a height of 60 mm. Measure the total pore volume by the water - saturation weight method. The pore volume of a 100 - g sand column is 30 mL. Place a perforated plate and a permeable stone at the top and bottom of the sand column respectively, and fix the bottom with a rubber stopper.

[0015] Preferably, in step S6, the fiber - microbial cementitious material is: in every 90 - 110 g of sandy soil, there are 0.36 - 0.44 g of modified sisal fiber, 150 - 210 mL of bacterial solution, 600 - 840 mL of cementing liquid, and 20 - 30 mL of water.

[0016] Preferably, in step S4, the bacterial solution is a bacterial solution containing 0.03 - 0.05 mol / L concentration of calcium chloride. After the bacterial solution submerges the sand column, seal the bottom rubber with a stopper and wait for 2 h to allow the bacteria to fully adhere to the sand particles.

[0017] Preferably, each round of biogrouting includes 1 bacterial inoculation and 4 cementing fluid groutings, with a total of 3 - 10 rounds of biogrouting.

[0018] Generally speaking, compared with the prior art through the above technical solutions conceived by the present invention, the method for improving the microbial sand consolidation strength based on modified plant fibers provided by the present invention mainly has the following beneficial effects: 1. The present invention uses natural fibers instead of artificial fibers as reinforcement materials. The natural fibers form a three-dimensional skeleton structure penetrating the interior of the specimen during the curing process, playing a role in bridging, traction, and crack restriction between sand particles, increasing the friction angle of sand particles, hindering the slip of sand particles, delaying the failure process, and thus improving the mechanical properties of cemented and solidified sand, breaking through the bottleneck of insufficient toughness of traditional MICP technology.

[0019] 2. Through the alkali treatment modification of natural fibers, the present invention removes non-cellulose components such as pectin and exposes more cellulose structures, playing a core role in significantly increasing roughness and specific surface area, providing more sites for microbial attachment and calcium carbonate nucleation, thereby promoting the precipitation amount of calcium carbonate and the raw material utilization efficiency, improving the mechanical strength and density of solidified sand, and the maximum increase in the mass ratio of calcium carbonate attached to the fiber surface is about 30.8%.

[0020] 3. The preferred biogrouting method of the present invention can further enhance the mechanical properties and curing efficiency of microbial mineralization and fiber composite solidified sand. Description of the Drawings

[0021] Figure 1 are microscopic images of bacteria mineralizing and depositing calcium carbonate on the fiber surface under different modification degrees.

[0022] Figure 2 is the infrared spectrogram of sisal fiber before and after modification (alkali treatment).

[0023] Figure 3 is the SEM image of sisal fiber without alkali treatment under a 500-fold microscope.

[0024] Figure 4 is the SEM image of sisal fiber without alkali treatment under a 1000-fold microscope.

[0025] Figure 5 is the SEM image of 7% alkali-treated sisal fiber under a 1000-fold microscope.

[0026] Figure 6 is the SEM image of 7% alkali-treated sisal fiber under a 2000-fold microscope.

[0027] Figure 7 is the mass ratio of calcium carbonate on the fiber surface attached to the fiber surface under different modification degrees. Detailed implementation manners

[0028] In order to make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0029] Based on the principle of microbially induced calcium carbonate precipitation (MICP), the present invention decomposes urea into ammonium ions and carbonate ions in an environment containing urea and a calcium source through urease-producing bacteria, and then reacts with calcium ions to generate calcium carbonate precipitation. The urease-producing bacteria are adsorbed on the sandy soil, and calcium carbonate is deposited on the surface and in the gaps of the sand grains, realizing the cementation and solidification of the soil body to achieve the purpose of improving the soil strength, and further enhancing the mechanical properties and stability of the sandy soil.

[0030] The generation principle of calcium carbonate precipitation is as follows: ; The following are the specific implementation manners of the present invention: Example 1 The influence of the incorporation of sisal fiber on MICP, namely the blank control, sisal fiber (MICP-sisal fiber combined reinforcement of quartz sand) 1) Preparation of sand samples Take 100 ± 0.1 g of quartz sand, respectively set a control group without adding sisal fiber (specimen 1) and an experimental group adding 0.4% by mass of sisal fiber (specimen 2), and stir them evenly. Pour the sandy soil evenly into a cylindrical PVC mold with a height of 11 cm and a diameter of 3.75 cm, and vibrate the sand body to a height of 6 cm, and number the samples.

[0031] 2) Preparation of bacterial solution and cementation solution Prepare YE-NH4 medium, and each 1 L of the medium contains 20 g of yeast extract, 10 g of ammonium sulfate, 1.4 g of sodium hydroxide, 12 μg of manganese sulfate, and 24 μg of nickel chloride. Put the prepared medium into an autoclave for high-temperature and high-pressure sterilization at 121 °C for 20 min. After cooling, place it in a super clean bench for ultraviolet sterilization for 15 min, and then carry out bacterial inoculation. After the inoculation is completed, place the bacterial solution on a shaker with a rotation speed of 160 rpm and a temperature of 30 °C for bacterial activation. After 12 - 18 h, the obvious turbidity of the bacterial solution indicates successful activation, and the bacterial solution required for biogrouting is obtained. At the same time, the concentration and urease activity of the bacteria in the successfully activated bacterial solution are measured using a spectrophotometer and a conductivity meter respectively.

[0032] Prepare the cementing liquid. Each 1 L of the cementing liquid contains 30 g of urea and 55.5 g of calcium chloride, that is, the concentration of the cementing liquid is 0.5 M.

[0033] 3) Biological grouting treatment Prepare a bacterial solution containing 0.03 mol / L of low-concentration calcium chloride. Use a syringe to inject 30 mL of the mixed bacterial solution into the top of the specimen at one time. Wait until the sand column is completely immersed in the bacterial solution, and let it stand for 2 h to allow the bacteria to fully adhere to the sand particles. After draining the waste liquid, inject the prepared cementing liquid into the sand column specimen. After standing and reacting for 12 h, supplement 30 mL of the cementing liquid again. A total of 4 times of cementing liquid are injected in each round of grouting.

[0034] Repeat the above steps 5 times.

[0035] 4) Sampling and testing After completing 5 rounds of biological grouting, demold the sand column specimen, wash it 3 times with deionized water to terminate the MICP reaction, dry it in an oven at 60 °C to obtain a biocemented sand column. Conduct an unconfined compressive strength test and a calcium carbonate content test on the solidified sand column.

[0036] For the unconfined compressive strength test, use a strain-controlled triaxial apparatus to conduct the unconfined compressive strength test. The load accuracy is ±0.5%, and through the displacement control method, the loading rate is 0.5 mm / min. Continuously load the specimen until it is crushed.

[0037] For the determination of calcium carbonate content, use the damaged specimen and determine the calcium carbonate content by hydrochloric acid pickling method. Specifically, cut out the middle part of the sand column and crush it into powder. After collection, dry it in an oven at 105 °C for 24 h until it reaches a constant weight. Weigh a certain mass of the sand body after grouting with a beaker, add an excessive amount of 5% hydrochloric acid to the beaker for reaction until no bubbles are generated. After washing with clean water, dry it in an oven at 105 °C for 24 h until it reaches a constant weight, and record the weight. The mass difference of the sand column before and after pickling is the content of calcium carbonate generated in the biocemented sand column.

[0038] Comparative Example 1 (CN109594552B) Different from Example 1, the fiber is artificial fiber and the dosage is 1.2%.

[0039] Comparative Example 2 Different from Comparative Example 1, the dosage is 4%.

[0040] The test results are shown in Table 1: ; From the test results, it can be seen that compared with the traditional MICP solidification technology, incorporating sisal fibers to synergistically solidify sandy soil with MICP can significantly increase the compressive strength of the sand column by 38.4% and simultaneously increase the calcium carbonate content by 0.43%. It can be proven by Example 1 that the method of incorporating sisal fibers to synergistically solidify sandy soil can improve the mechanical properties of the material and the calcium carbonate mineralization yield.

[0041] Example 2 The steps in this example are basically the same as those in Practical Case 1, with the differences being as follows: Before the step of filling sand and preparing specimens, the sisal fibers are modified: after cutting the sisal fibers to a length of 5 - 15 mm, they are immersed in a 3 - 7 wt% sodium hydroxide solution, left standing for 4 h, washed with deionized water and dried to obtain modified fibers. The specimens are respectively numbered as Specimen 3 (0% modified fibers), Specimen 4 (3% modified fibers), Specimen 5 (5% modified fibers), and Specimen 6 (7% modified fibers).

[0042] Among them, the density of the sisal fibers is 1.46 g / cm 3 , the fiber diameter is 0.3 mm, the tensile strength is 470 MPa, the fiber length is 5 - 15 mm, the texture is tough, and it has good wear and corrosion resistance.

[0043] Figure 1 are the microscopic images of calcium carbonate attachment on the fiber surface under different modification degrees. It can be seen from Figure 1 that a large amount of calcium carbonate crystals are deposited on the surface of the modified fibers (3%, 5%, 7% alkali treatment), while the amount of calcium carbonate deposition on the surface of the unmodified fibers (0% alkali treatment) is significantly less, indicating that the modification treatment of the fibers significantly promotes the nucleation and attachment of calcium carbonate.

[0044] From Figure 2 in the infrared spectrum analysis diagram of the modified sisal fibers, it can be seen that alkali treatment can effectively remove non - cellulose components such as hemicellulose, pectin, and lignin in plant fibers, expose more cellulose structures, improve the surface activity of the fibers, increase the surface roughness and specific surface area of the fibers, and provide a good contact surface for the subsequent combination of microorganisms and calcium carbonate crystals.

[0045] From Figures 3 - 6 of the scanning electron microscope images, it can be seen that there are significant differences in the fiber structures before ( Figure 3 and Figure 4 ) and after ( Figure 5 and Figure 6 ) modification. It can be seen that the fiber bundles are dispersed, the surface roughness increases, and the specific surface area increases, which is beneficial to the colonization of microorganisms and the attachment of calcium carbonate crystallization.

[0046] Figure 7 Quantitatively shows the attachmentFigure 1 The results showed that the fibers modified by alkali treatment could promote the adhesion of calcium carbonate on their surface and significantly increase the amount of calcium carbonate deposition. Among them, the 5% alkali-treated modified fibers had the best effect on promoting the precipitation of calcium carbonate. Compared with the unmodified fibers, the calcium carbonate mass ratio increased by about 30.8%.

[0047] The unconfined compression test and calcium carbonate content test results of the cured sand sample of Example 2 are shown in the following table: ; The test results show that compared with the unmodified fiber-MICP synergistic curing technology, the modified sisal fiber after alkaline treatment with sodium hydroxide and MICP curing sand can increase the compressive strength of the sand column by up to about 90.6%, and the calcium carbonate content can be increased by up to about 3.39%. The experiment proves that the method of modified sisal fiber and MICP curing sand can effectively improve the mechanical properties of the material and the amount of calcium carbonate deposition, thereby shortening the mineralization curing cycle.

[0048] In summary, the present invention provides a method for MICP-modified fiber synergistic solidification of sand, which increases the roughness and provides more nucleation sites by alkali treatment on the surface of modified fibers to promote the deposition of calcium carbonate, and increases the friction between the fibers and the sand, thereby enhancing the mechanical connection effect. The strength and calcium carbonate content of the solidified sand can be effectively improved, and the biomineralization cycle can be shortened. The structural strength and toughness of the biomineralized solidified sand can be enhanced, and the deposition efficiency of microbially induced calcium carbonate can be improved. The shortcomings of traditional MICP reinforcement materials such as low curing strength and slow deposition efficiency can be overcome, and the method has good engineering application prospects and ecological sustainability, and has good application prospects in the fields of foundation reinforcement and soft soil foundation treatment.

[0049] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for enhancing the strength of microbial sand fixation based on modified plant fibers, characterized in that, It includes the following steps: S1: Stir and mix natural fibers with sandy soil until the fibers are evenly distributed in the sandy soil to obtain a mixed base material; S2: Load the mixed base material obtained in S1 into a mold, and make a sand column by vibrating and compressing; S3: Activate and expand the culture of microbial bacterial powder with mineralization effect to obtain a bacterial solution; S4: Pour the bacterial solution in S3 into the sand column until it is completely immersed, and let it stand to wait for the bacteria in the bacterial solution to fully adhere to the sand particles; S5: Prepare a cementing solution, and pour the cementing solution into the sand column to complete one-time biogrouting; S6: Repeat steps S4 and S5 for multiple rounds of biogrouting. After the mineralization reaction is completed, demold, wash, and dry the sand column to obtain a fiber-microbial cementitious material.

2. The method for enhancing the microbial sand fixation strength based on modified plant fibers according to claim 1, wherein The natural fiber is one of sisal fiber, jute fiber, ramie fiber, flax fiber, bamboo fiber, and coconut shell fiber. The sandy soil is medium-grade sand, and the particle size of the medium-grade sand is 0.5 - 1.0 mm.

3. A method for enhancing the microbial sand fixation strength based on modified plant fibers according to claim 1, characterized in that, The cementing solution is a mixed solution of calcium chloride and urea with equal concentration and equal volume, and the concentration is 0.4 - 0.6 mol / L.

4. A method for enhancing the strength of microbial sand fixation based on modified plant fibers according to claim 1, characterized in that, In step S3, the strain selected is Sporosarcina pasteurii, and the culture medium is a liquid culture medium. The components of the culture medium include 20 g / L of yeast extract, 10 g / L of ammonium sulfate, 12 μg / L of manganese sulfate, 24 μg / L of nickel chloride, and 1.4 g / L of sodium hydroxide. The pH value of the culture medium is 8.5 - 9.

0.

5. A method for enhancing the microbial sand fixation strength based on modified plant fibers according to claim 1, characterized in that, Before step S1, cut the natural fiber into sections. The natural fiber is sisal fiber, and soak the sisal fiber in an alkaline solution for modification treatment to remove fiber gum.

6. A method for enhancing the microbial sand fixation strength based on modified plant fibers according to claim 5, characterized in that, The diameter of the sisal fiber is 0.3 mm. After cutting it to a length of 5 - 15 mm, soak its fiber in a 3 - 7 wt% sodium hydroxide solution, let it stand for 4 h, wash it with deionized water and dry it to obtain modified fiber.

7. A method for enhancing the microbial sand fixation strength based on modified plant fibers according to claim 5, characterized in that, In steps S1 - S2, after mixing 100 g of dry medium-grade sand with 0.4 g of modified sisal fiber in a stirrer, pour it evenly into a cylindrical mold, vibrate and compact it to a height of 60 mm. Measure the total pore volume by the water saturation weight method. The pore volume of a 100 g sand column is 30 mL. Place a porous plate and a permeable stone at the top and bottom of the sand column respectively, and fix the bottom with a rubber plug.

8. The method according to claim 1, characterized in that The fiber-microbial cementitious material is: every 90 - 110 g of sandy soil contains 0.36 - 0.44 g of modified sisal fiber, 150 - 210 mL of bacterial solution, 600 - 840 mL of cementing solution, and 20 - 30 mL of water.

9. A method for enhancing the strength of microbial sand fixation based on modified plant fibers according to claim 1, characterized in that In step S4, the bacterial solution is a bacterial solution containing 0.03 - 0.05 mol / L concentration of calcium chloride. After pouring the bacterial solution to immerse the sand column, seal the bottom rubber plug with a stopper, and wait for 2 h to make the bacteria fully adhere to the sand particles.

10. A method for enhancing the strength of microbial sand fixation based on modified plant fibers according to claim 1, characterized in that, Each round of biogrouting includes 1 time of bacteria inoculation and 4 times of cementing solution grouting. A total of 3 - 10 rounds of biogrouting are required to reach densification.

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