A method for urease-induced calcium carbonate precipitation and solidification of sandy soil

Abstract

The present invention relates to the technical field of rock and soil reinforcement, and in particular to a method for urease-induced calcium carbonate precipitation and solidification of sandy soil. The present invention crushes plant seeds containing urease to obtain urease-containing powder; mixes the urease-containing powder with sandy soil to obtain mixed sandy soil; injects a binder into the mixed sandy soil to form calcium carbonate to solidify the sandy soil; the binder is an aqueous solution containing urea and calcium ions. The method provided by the present invention can not only solve the problems of difficult extraction and storage of urease, but also increase the thickness of the solidified layer and improve the mechanical properties, and the method is simple and easy to implement, low cost and risk-free.

Description

Technical Field

[0001] The invention relates to the technical field of rock and soil reinforcement, and in particular to a method for urease-induced calcium carbonate precipitation and solidification of sandy soil. Background Art

[0002] The arsenic sandstone area is located at the junction of Shanxi, Shaanxi and Inner Mongolia. Its main components are quartz, calcium montmorillonite, potassium feldspar and calcium carbonate. Due to its poor cementation between particles and poor water stability, a large amount of sediment flows into the Yellow River every year, becoming one of the main sources of coarse sand in the Yellow River, and soil erosion is extremely serious. Improving the water stability of arsenic sandstone is of great significance to the environmental governance and resource utilization of the arsenic sandstone area.

[0003] Plant urease-induced calcium carbonate precipitation is a new type of soil reinforcement technology. This method has strong environmental adaptability, short curing cycle, green environmental protection and low cost. At present, this technology widely adopts the method of extracting crude urease solution from plant seeds and then pouring it from the soil surface. This method is easy to use, but it also has some defects, such as: the urease extraction process is cumbersome, the urease after extraction has high requirements for storage conditions, the calcium carbonate is unevenly distributed, the soil curing layer is thin and the mechanical properties are poor. In addition, the size of urease is about 12nm, which is very easy to lose from the soil, and calcium carbonate also lacks nucleation sites, so it is unevenly distributed.

[0004] In response to the problem of lack of nucleation sites and poor mechanical properties of urease, domestic and foreign scholars have proposed adding admixtures, such as milk powder, lignin, sucrose and biopolymers. Although these methods are effective, the cost of admixtures is quite high, which is not conducive to cost control. In response to the problem of thin solidification layer and uneven distribution of calcium carbonate, some scholars have proposed a low pH injection method, which is to delay the precipitation of calcium carbonate by reducing the activity of urease, so that calcium carbonate is evenly distributed. However, the low pH injection method requires a large amount of acid solution to lower the pH value, and the acid solution is highly corrosive, so the risk of use is relatively high. Summary of the invention

[0005] In view of this, the purpose of the present invention is to provide a method for urease-induced calcium carbonate precipitation and solidification of sandy soil. The method provided by the present invention can not only solve the problems of difficult extraction and storage of urease, but also increase the thickness of the solidified layer and improve the mechanical properties. Moreover, the method is simple, easy to operate, low-cost and risk-free.

[0006] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0007] The present invention provides a method for urease-induced calcium carbonate precipitation and solidification of sandy soil, comprising the following steps:

[0008] crushing the urease-containing plant seeds to obtain urease-containing powder;

[0009] Mixing the urease-containing powder with sand to obtain mixed sand;

[0010] Pouring a cementing liquid into the mixed sand to form calcium carbonate to solidify the sand;

[0011] The binder is an aqueous solution containing urea and calcium ions.

[0012] Preferably, the particle size of the urease-containing powder is ≤0.15 mm.

[0013] Preferably, the urease-containing plant seeds include one or more of sword beans, soybeans, watermelon seeds, broad beans and lentils.

[0014] Preferably, the mass ratio of the urease-containing powder to sand is 1:(10-50).

[0015] Preferably, the concentrations of urea and calcium ions in the binder are independently ≤3 mol / L.

[0016] Preferably, the molar ratio of urea to calcium ions in the binder is 1:1.

[0017] Preferably, the sandy soil comprises arsenic sandstone and / or aeolian sand.

[0018] Preferably, the binder is poured in two or more times.

[0019] Preferably, the time interval between two adjacent perfusions is 8 to 24 hours.

[0020] Preferably, the curing is carried out at 20-40°C.

[0021] The invention provides a method for solidifying sandy soil by urease-induced calcium carbonate precipitation, comprising the following steps: crushing urease-containing plant seeds to obtain urease-containing powder; mixing the urease-containing powder with sandy soil to obtain mixed sandy soil; injecting a binder into the mixed sandy soil to form calcium carbonate to solidify the sandy soil; the binder is an aqueous solution containing urea and calcium ions.

[0022] The present invention eliminates the cumbersome urease extraction steps, saves storage costs, and avoids the problem of thin solidification layer and uneven distribution of calcium carbonate caused by the excessively fast precipitation rate of calcium carbonate blocking the infiltration channel. Specifically, the urease-containing powder is mixed with sandy soil, and the urease-containing powder is soaked in the sandy soil for a long time by a cementing liquid by utilizing the property that urease is soluble in water, so as to leach the urease in the urease-containing powder; the urease reacts chemically with the urea in the cementing liquid to produce an alkaline environment, and then the carbonate ions are generated to react with the calcium ions in the calcium chloride to promote the precipitation of calcium carbonate; the process of urease being leached is slow, and the cementing liquid has sufficient time to infiltrate into the deep layer during this process, and the urease has a low activity in the initial stage of leaching, and the speed of reacting and flocculating with the cementing liquid is slow, which further delays the precipitation rate of calcium carbonate and avoids the defect of calcium carbonate accumulating on the surface of the soil body; in addition, the urease-containing powder can improve the density of the sandy soil, and the calcium carbonate and the urease-containing powder cooperate to reinforce the soil body.

[0023] The present invention can independently determine the mixing amount of urease-containing powder and sand, thereby controlling the thickness of the solidified layer within a certain range; the quality of calcium carbonate generated in the arsenic sandstone solidified by the method of the present invention is slightly higher than that of the arsenic sandstone injected by the step-by-step injection method of crude urease solution, indicating that the urease extracted from the urease-containing powder by the cementing liquid has sufficient activity. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a flow chart of using urease to consolidate arsenic sandstone in Example 1;

[0025] Figure 2 The scanning electron microscope images of the arsenic sandstone solidification layer of Example 1 and Comparative Example 1 are shown. DETAILED DESCRIPTION

[0026] The present invention provides a method for urease-induced calcium carbonate precipitation and solidification of sandy soil, comprising the following steps:

[0027] crushing the urease-containing plant seeds to obtain urease-containing powder;

[0028] Mixing the urease-containing powder with sand to obtain mixed sand;

[0029] Pouring a cementing liquid into the mixed sand to form calcium carbonate to solidify the sand;

[0030] The binder is an aqueous solution containing urea and calcium ions.

[0031] Unless otherwise specified, the materials and equipment used in the present invention are all commercially available products in the art.

[0032] The invention crushes the plant seeds containing urease to obtain the urease-containing powder.

[0033] In the present invention, the urease-containing plant seeds preferably include one or more of sword beans, soybeans, watermelon seeds, broad beans and lentils.

[0034] The present invention has no special requirements for the pulverization process, as long as the particle size of the urease-containing powder after pulverization is ≤0.15 mm. In an embodiment of the present invention, the powder is pulverized into powder by a wall breaker and then sieved through a 100-mesh sieve. The present invention controls the particle size of the urease-containing powder within the above range, which is conducive to the full utilization of the urease in the plant seeds and makes the urease activity higher.

[0035] After obtaining the urease-containing powder, the present invention mixes the urease-containing powder with sand to obtain mixed sand.

[0036] The present invention has no special requirements on the type of the sand, and any sand that needs to be solidified can be used, such as arsenic sandstone and aeolian sand, which are not listed here. The present invention has no special requirements on the particle size of the sand, and loose sand can be used.

[0037] In the present invention, the mass ratio of the urease-containing powder to the sand is preferably 1: (20-50). In a specific embodiment of the present invention, the mass ratio of the urease-containing powder to the sand can be 1: 20, 1: 25, 1: 30, 1: 35, 1: 40, 1: 45 or 1: 50. Within the above range, the higher the mass ratio of the urease-containing powder to the sand, the higher the urease activity and the shorter the curing period.

[0038] The present invention has no special requirements for the mixing, and it only needs to mix the urease-containing powder and the sand evenly.

[0039] After the mixed sand is obtained, the present invention injects a binder into the mixed sand to form calcium carbonate to solidify the sand.

[0040] In the present invention, the binder liquid is an aqueous solution containing urea and calcium ions.

[0041] In the present invention, the calcium source providing the calcium ions is preferably a soluble calcium salt, and the soluble calcium salt may specifically be calcium chloride, calcium sulfate, calcium nitrate, or calcium acetate, which will not be listed here in full.

[0042] The present invention has no special requirements on the preparation process of the cementing liquid, and urea and a calcium source containing calcium ions can be directly dissolved in water.

[0043] In the present invention, the concentrations of urea and calcium ions in the binder are independently preferably ≤3 mol / L. The present invention does not make any special requirements on the molar ratio of urea and calcium ions, and any molar ratio known in the art can be used. In the present invention, the optimal molar ratio of urea to calcium ions is 1:1.

[0044] In the present invention, the cementing fluid is preferably poured in more than 2 times, specifically 2 times, 3 times, 4 times or more times, and most preferably 4 to 5 times. The present invention does not make special requirements on the pouring amount each time and the total pouring amount, and those skilled in the art can determine it according to actual needs. Before reaching the upper limit, the greater the pouring number and the pouring amount, the better the curing effect. As the best solution, when the pouring amount of the cementing fluid each time is equal to the pore volume of the sand after mixing, it can not only save the amount of cementing fluid, but also achieve a good curing effect.

[0045] In the present invention, the time interval between two adjacent perfusions is preferably 8 to 24 hours; in a specific embodiment of the present invention, the time interval between two adjacent perfusions can be 8 hours, 12 hours, 16 hours, 20 hours or 24 hours. The present invention promotes the full reaction of urease and cementing fluid by controlling the time interval between two adjacent perfusions within the above range.

[0046] In the present invention, the time from the first injection of the cementing solution to the completion of the injection is preferably no more than 7 days (because the urease activity in the soil will gradually inactivate after 7 days).

[0047] In the present invention, the curing is preferably carried out at 20-40°C (that is, after each infusion is completed, it is placed at 20-40°C for curing). In a specific embodiment of the present invention, the curing can be carried out at 20°C, 25°C, 30°C, 35°C or 40°C. The present invention is conducive to maintaining good activity of urease by curing at 20-40°C.

[0048] The present invention eliminates the cumbersome urease extraction steps, saves storage costs, and avoids the problem of thin solidification layer and uneven distribution of calcium carbonate caused by the excessively fast precipitation rate of calcium carbonate blocking the infiltration channel. Specifically, the urease-containing powder is mixed with sandy soil, and the urease-containing powder is soaked in the sandy soil for a long time by a cementing liquid by utilizing the property that urease is soluble in water, so as to leach the urease in the urease-containing powder; the urease reacts chemically with the urea in the cementing liquid to produce an alkaline environment, and then the carbonate ions are generated to react with the calcium ions in the calcium chloride to promote the precipitation of calcium carbonate; the process of urease being leached is slow, and the cementing liquid has sufficient time to infiltrate into the deep layer during this process, and the urease has a low activity in the initial stage of leaching, and the speed of reacting and flocculating with the cementing liquid is slow, which further delays the precipitation rate of calcium carbonate and avoids the defect of calcium carbonate accumulating on the surface of the soil body; in addition, the urease-containing powder can improve the density of the sandy soil, and the calcium carbonate and the urease-containing powder cooperate to reinforce the soil body.

[0049] The method for urease-induced calcium carbonate precipitation and solidification of sandy soil provided by the present invention is described in detail below in conjunction with the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0050] Example 1

[0051] according to Figure 1 The process shown is used to solidify the arsenic sandstone, and the specific steps are as follows:

[0052] (1) Preparation of bean powder: Put the white beans into a wall-breaking machine and grind them into powder, and then pass through a 100-mesh sieve.

[0053] (2) Preparation of cementing fluid: Mix urea and analytical grade calcium chloride solution at a concentration of 2 mol and a volume ratio of 1:1.

[0054] (3) Sand treatment: Mix the arsenic sandstone and the prepared bean powder in a mass ratio of 1:30, take 300g and put it into a transparent cylindrical mold of Φ50mm×H100mm, place a permeable gauze at the bottom of the mold to allow the binder to flow out naturally. Then spray the binder on the surface of the sand. The spraying amount is 70ml each time, spray once every 24 hours, and spray 4 times in total. The experiment was carried out at room temperature (25℃).

[0055] Comparative Example 1

[0056] Comparative sample of step-by-step grouting method for preparing crude urease solution:

[0057] (1) According to the experimental conclusions, the prepared sword bean powder was mixed with pure water in a mass ratio of 1:30 (when the mass ratio is 1:30, the calcium carbonate conversion rate is the highest and the solidification layer is the deepest). After stirring for 2 hours, the supernatant was taken and placed in a centrifuge for centrifugation at 3000 r / min for 15 minutes. The supernatant after centrifugation is the crude urease solution.

[0058] (2) 300 g of arsenic sandstone was placed in a transparent cylindrical mold of Φ50 mm × H100 mm, and the prepared urease solution was poured in from the surface, with a pouring volume of 75 ml (the same as the amount of sword bean powder in Example 1). After the pouring was completed, the cementing solution was poured in 2 hours later, and the amount of cementing solution was the same as in Example 1. The experiment was carried out at room temperature (25°C).

[0059] Performance Test:

[0060] Experiment 1: Testing of solidified layer volume, penetration resistance, and calcium carbonate generation

[0061] Calcium carbonate production: First, use deionized water to wash away the remaining binder in the test block. After drying, weigh the mass change before and after solidification, and take the difference in mass change as the calcium carbonate production.

[0062] Volume of solidified layer: Put the arsenic sandstone test block into water, wait for 24 hours, take the undisintegrated part as the arsenic sandstone solidified layer, and measure the volume of its solidified layer by drainage method.

[0063] Penetration resistance: Use the Adberg HLD digital push-pull force gauge (range 500N) and a pointed cone probe. Place the saturated solidified layer flat on the table and aim at the center of the solidified layer to measure the penetration resistance, thereby characterizing the strength of the solidified layer. The test results are detailed in Table 1.

[0064] Table 1 Test results of Example 1 and Comparative Example 1

[0065]

[0066] As can be seen from Table 1, the method of the present invention has great advantages in solidifying arsenic sandstone. The calcium carbonate generation, solidification layer volume and penetration strength are respectively increased by 9.6%, 88.8% and 145% compared with the crude urease solution step-by-step surface grouting method of Example 1.

[0067] Experiment 2: Field Emission Scanning Electron Microscope (SEM) Experiment

[0068] The cured arsenic sandstone layers of Example 1 and Comparative Example 1 were observed by field emission scanning electron microscopy. The results are as follows: Figure 2 shown. Figure 2 Figures a, c, and e are the microscopic morphologies of calcium carbonate in the arsenic sandstone solidified in Example 1 of the present invention at 200X, 500X, and 5000X, respectively. Figure 2 Figures b, d, and f are the microscopic morphologies of calcium carbonate in the arsenic sandstone solidified by the step-by-step grouting method on the surface of the crude urease solution in Comparative Example 1 at 200X, 500X, and 5000X, respectively.

[0069] Depend on Figure 2In a and c, it can be seen that the surface of the arsenic sandstone particles is not coated with calcium carbonate, but calcium carbonate cluster crystals are formed in the pores. These calcium carbonate cluster crystals form a bonding and filling skeleton between the arsenic sandstone particles. Since urease is leached by the cementing solution, it can react chemically with the cementing solution when leached and flocculate rapidly. After flocculation, the viscosity is enhanced, and the bean powder swells and shrinks the pores when it comes into contact with water, making it easier for the reaction liquid to remain inside the soil. The step-by-step injection method is to inject the urease solution and the cementing solution in a front-to-back order. When the urease solution is injected into the soil without the action of the cementing solution, the urease is easy to lose due to its small size. When the cementing solution is subsequently injected, the reaction rate is too fast due to the high activity of the urease, causing calcium carbonate precipitation to block the infiltration pores. In addition, free urease lacks calcium carbonate nucleation sites, so the calcium carbonate is scattered and cannot be aggregated into a whole. The cellulose, hemicellulose and starch in soybean powder are rich in polysaccharides, which can form an electrostatic match with calcium ions; at the same time, cellulose and hemicellulose have the characteristics of high specific surface area, which can provide space for the aggregation of urease and the precipitation of calcium carbonate, further promoting the formation of cluster crystals; the amino acids in soybean powder are rich in carboxyl groups, which can form a coordination effect with calcium ions. These substances can fully contact and react with the cementing liquid, which promotes the formation of calcium carbonate cluster crystals, while substances such as cellulose and hemicellulose cannot dissolve in water, so they cannot be extracted during the extraction of crude urease solution. Figure 2 In b and d, it can be seen that the surface of the arsenic sandstone particles is still not covered with calcium carbonate. Calcium carbonate is mainly deposited in the narrow pore throats between particles and forms cementation. There is no formation of calcium carbonate cluster crystals, and it is impossible to form a filling effect between the arsenic sandstone particles. Figure 2 From the figures e and f, we can see that the arsenic sandstone solidified by the two different methods both produces calcite, but Figure 2 The middle e is a block of calcite formed by rhombuses biting each other, and Figure 2 The middle f is a block of calcite that is stacked and interlocked in sheets. The calcium carbonate generated by the step-by-step grouting method of crude urease solution only plays a bonding role between particles, and because the calcium carbonate crystals generated by urease are small in size and lack nucleation sites, large pores cannot be effectively filled, resulting in low strength. The calcium carbonate generated by the solidified sand block prepared by the present invention avoids the above problems.

[0070] In the process of curing arsenic sandstone by the mixing method of embodiment 1, urease is pre-placed in the soil body with bean powder particles as carrier, and after the bean powder contacts the cementing solution, urease is leached thereupon. The process of urease being extracted is slow, which avoids the disadvantage of too fast precipitation rate of calcium carbonate caused by too high urease activity. In addition, the moment urease is leached can react with the cementing solution, and the reaction of the cementing solution and urease can cause the generation of alkaline environment, at which time the negatively charged urease can attract positively charged calcium ions, and the pre-generated calcium carbonate is used as a follow-up to promote the formation of calcium carbonate crystals. In addition, sword bean powder contains organic matter such as cellulose, hemicellulose, protein and starch, and cellulose is rich in polysaccharides that can form electrostatic matching with calcium ions, provide nucleation sites for calcium carbonate, and also has the characteristics of high specific surface area, which can provide space for the precipitation of calcium carbonate. Similarly, amino acids can also provide nucleation sites for the nucleation of calcium carbonate, and some amino acids can also protect urease activity and slow down the inhibitory effect of calcium ions on it. In a word, these organic matters play a positive role in the formation and distribution of calcium carbonate. The organic matter in sword bean powder may not be completely extracted in the traditional water extraction urease process.

[0071] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for urease-induced calcium carbonate precipitation and solidification of sandy soil, characterized in that: The following steps are involved: crushing the urease-containing plant seeds to obtain urease-containing powder; Mixing the urease-containing powder with sand to obtain mixed sand; Pouring a cementing liquid into the mixed sand to form calcium carbonate to solidify the sand; The binder is an aqueous solution containing urea and calcium ions.

2. The method according to claim 1, characterized in that The particle size of the urease-containing powder is ≤0.15 mm.

3. The method according to claim 1 or 2, characterized in that: The urease-containing plant seeds include one or more of sword beans, soybeans, watermelon seeds, broad beans and lentils.

4. The method according to claim 1 or 2, characterized in that: The mass ratio of the urease-containing powder to the sand is 1: (10-50).

5. The method according to claim 1, characterized in that The concentrations of urea and calcium ions in the binder are independently ≤3 mol / L.

6. The method according to claim 1 or 5, characterized in that: The molar ratio of urea to calcium ions in the binder is 1:

1.

7. The method according to claim 1, characterized in that The sandy soil includes arsenic sandstone and / or aeolian sand.

8. The method according to claim 1, characterized in that The cementing liquid is poured in more than 2 times.

9. The method according to claim 8, characterized in that The time interval between two adjacent perfusions was 8 to 24 hours.

10. The method according to claim 1, 8 or 9, characterized in that The curing is carried out at 20-40°C.

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