Ground improvement method and ground improvement kit

Abstract

To provide a novel geological improvement method capable of forming a calcium inclusion such as calcite efficiently with a microorganism in soil and without generating a harmful byproduct, even under condition where oxygen deficiency and competition between microorganisms exist.SOLUTION: A method for improving the geology characterized by the formation of calcium carbonate in the soil using anaerobic fermentation comprises; (a) a step of adding, at least, powder or an aqueous solution of oxygenated organic matter and a powder or an aqueous solution of a calcium salt to the soil; (b) a step of generating carbon dioxide by fermenting the oxygenated organic matter in an anaerobic environment using microorganisms; and, (c) a step of forming calcium carbonate (CaCO3) in soil by a reaction with the calcium salt and the carbon dioxide.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a geological improvement method characterized by forming calcium carbonate in soil using anaerobic fermentation and a kit for the geological improvement.

Background Art

[0002] Conventionally, cement, polymers, etc. have been used to improve soft or highly water-containing geology and ground. However, since the spread from the injection port is limited, a ground excavation process such as a trench is required for large-scale implementation, and it has been difficult to apply in terms of cost.

[0003] On the other hand, calcite (CaCO3), which is a calcium carbonate mineral, is known to crosslink between particles in soil and contribute to ground strengthening, or to fill voids in rock and control permeability (for example, Non-Patent Document 1). In recent years, it has been becoming clear that the metabolic activity of microorganisms controls these mineral formations (for example, Non-Patent Document 2). In the biogrout technology that cementifies in situ using microbial metabolism in the ground, microorganisms that decompose urea with an enzyme called urease are injected together with nutrients containing urea to form calcite. However, the method using such urea-decomposing microorganisms is limited to application in an aerobic environment and has a problem that harmful ammonia is generated as a byproduct.

[0004] Also, hydroxyapatite (Ca 10 (PO4)6OH2) is known to have the ability to permanently fix harmful elements including radioactive elements and heavy metals. Since hydroxyapatite has low solubility near neutrality, research has been conducted to promote extensive mineral precipitation by injecting calcium into the groundwater environment together with a chelating agent such as citric acid and through the metabolism of decomposing the complex of calcium and the chelating agent by microorganisms. However, since the amount of calcium that can solubilize the chelating agent is small and the activity of microorganisms that decompose the complex is low, there are problems in application in the groundwater environment.

Prior Art Documents

Non-Patent Documents

[0005] [Non-Patent Document 1] Yasuhara et al., “Experiments and predictions of physical properties of sand cemented by enzymatically-induced carbonate precipitation” Soils and Foundations, vol.52, 3, 539-549, 2012 [Non-Patent Document 2] Zhu et al., “Carbonate Precipitation through Microbial Activities in Natural Environment, and Their Potential in Biotechnology: A Review”, Frontiers in Bioengineering and Biotechnology 4 (January), 2016 [Disclosure of the Invention] [Problems to be Solved by the Invention]

[0006] Therefore, an object of the present invention is to provide a novel ground improvement method capable of efficiently forming calcium-containing substances such as calcite without generating harmful by-products using microorganisms in soil even under conditions where oxygen deficiency and microbial competition exist. [Means for Solving the Problems]

[0007] As a result of intensive studies to solve the above problems, the present inventors have found that calcite can be efficiently formed by adding oxygen-containing organic substances and calcium salts to soil and performing fermentation under anaerobic conditions by microorganisms, and thereby the ground can be effectively improved. Further, it has been found that hydroxyapatite can be efficiently formed in soil simultaneously with calcite by such a method. Based on these findings, the present invention has been completed.

[0008] That is, in one aspect, the present invention relates to a ground improvement method, and more specifically, <1>A ground improvement method characterized by forming calcium carbonate in soil using anaerobic fermentation, comprising: (a) adding at least a powder or aqueous solution of an oxygen-containing organic substance and a powder or aqueous solution of a calcium salt into the soil; (b) generating carbon dioxide by fermenting the oxygen-containing organic substance in an anaerobic environment using microorganisms; and (c) forming calcium carbonate (CaCO3) in the soil by reacting the calcium salt with the carbon dioxide. <2>The method according to <1> above, wherein the oxygen-containing organic substance is an organic acid. <3>The method according to <2> above, wherein the organic acid is succinic acid, propionic acid or pyruvic acid. <4>The method according to any one of <1> to <3> above, wherein the microorganism is a microorganism having a gene encoding methylmalonyl-CoA decarboxylase. <5>The method according to any one of <1> to <4> above, wherein the microorganism is a microorganism belonging to the phylum Bacteroidetes. <6>The method according to any one of <1> to <5> above, wherein the microorganism is a microorganism belonging to the order Bacteroidales. <7>The method according to any one of <1> to <6> above, wherein the microorganism is a microorganism originally present in the soil or a microorganism added externally before step (b). <8>The method according to any one of <1> to <7> above, wherein the calcium salt is calcium halide. <9>The method according to any one of <1> to <8> above, wherein the concentration of the oxygen-containing organic substance is in the range of 1 to 100 mM. <10>The method according to any one of <1> to <9> above, wherein the concentration of the calcium salt is in the range of 100 to 5000 mM. <11>In the step (a), further adding a powder or aqueous solution of nitrate into the soil, thereby including causing a nitrate reduction reaction to proceed in the soil, the method according to any one of <1> to <10> above; and <12>In the step (a), further adding a powder or aqueous solution of phosphate into the soil, thereby including forming hydroxyapatite in the soil, the method according to any one of <1> to <11> above is provided.

[0009] In another aspect, the present invention also relates to a kit for geological improvement, and more specifically, <13>A powder or aqueous solution of an oxygen-containing organic substance; and a kit for geological improvement including a powder or aqueous solution of a calcium salt; <14>The kit for geological improvement according to <13> above, wherein the oxygen-containing organic substance is an organic acid; <15>The kit for geological improvement according to <14> above, wherein the organic acid is succinic acid, propionic acid or pyruvic acid; <16>The kit for geological improvement according to any one of <13> to <15> above, wherein the calcium salt is calcium halide; <17>The kit for geological improvement according to any one of <13> to <16> above, further including a powder or aqueous solution of nitrate; <18>The kit for geological improvement according to any one of <13> to <17> above, further including a powder or aqueous solution of phosphate; <19>The kit for geological improvement according to any one of claims 13 to 18, further including a microorganism having the ability to produce carbon dioxide by fermentation in an anaerobic environment using the oxygen-containing organic substance as a raw material; <20>The kit for geological improvement according to <19> above, wherein the microorganism is a microorganism having a gene encoding methylmalonyl-CoA decarboxylase; <21>The kit for geological improvement according to <19> above, wherein the microorganism is a microorganism belonging to the phylum Bacteroidetes; <22>The geological improvement kit according to <19> above, wherein the microorganism belongs to the order Bacteroidales; <23>The geological improvement kit according to any one of <13> to <22> above, wherein the concentration of the oxygen-containing organic substance is in the range of 1 to 100 mM; and <24>The geological improvement kit according to any one of <13> to <23> above, wherein the concentration of the calcium salt is in the range of 100 to 5000 mM is provided.

Advantages of the Invention

[0010] According to the present invention, calcite or hydroxyapatite can be efficiently formed by microbial fermentation in soil even under conditions where oxygen deficiency and microbial competition exist. As a result, by filling the gaps and cracks between particles in the ground with calcium carbonate, the ground strength can be increased and the porosity and permeability can be decreased. In addition, both the formation process and the product in the geological improvement method of the present invention hardly impose an environmental burden, and have the advantage of not generating harmful substances such as ammonia generated when conventional urea decomposition is used.

[0011] The present invention is particularly useful as a technique for strengthening soft ground that causes liquefaction and landslides, waterproofing highly permeable ground, and increasing the production of natural gas and oil from gas fields and oil fields.

Brief Description of the Drawings

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Mode for Carrying Out the Invention

[0013] Hereinafter, embodiments of the present invention will be described. The scope of the present invention is not limited by these descriptions, and other than the following examples, it can be appropriately modified and implemented without departing from the spirit of the present invention.

[0014] The geological improvement method of the present invention is a method characterized by forming calcium carbonate (calcite) in soil using anaerobic fermentation, and more specifically, includes the following steps (a) to (c): (a) A step of adding at least a powder or aqueous solution of oxygen-containing organic matter and a powder or aqueous solution of calcium salt into the soil; (b) A step of generating carbon dioxide by fermenting the oxygen-containing organic matter using microorganisms in an anaerobic environment; and (c) A step of forming calcium carbonate (CaCO3) in the soil by the reaction of the calcium salt and the carbon dioxide.

[0015] The soil targeted by the treatment method of the present invention is not particularly limited in terms of its type, state, etc., as long as it includes soil and rock. Typically, it can be soil in ground where liquefaction countermeasures or landslide countermeasures are required, ground where water stoppage is required, or soft ground where strength increase is required. For example, soft ground mainly composed of sandy soil, etc., where liquefaction and landslides are expected during earthquakes, dikes, seawalls, cliffs, embankments, landfill sites, etc., that have started to loosen due to long rain, heavy rain, crustal movement, etc., and highly permeable ground can be cited.

[0016] In step (a) of the ground improvement method of the present invention, a powder or aqueous solution of an oxygen-containing organic substance and a powder or aqueous solution of a calcium salt are added to the soil. The oxygen-containing organic substance used here may be a compound that is fermented by microorganisms to produce carbon dioxide. For example, it can be an organic acid, alcohol, saccharide, or protein. The organic acid is preferably a carboxylic acid having 2 to 10 carbon atoms, more preferably succinic acid, propionic acid, or pyruvic acid. In particular, succinic acid is a common acid generated from cellulose, which is the main component of terrestrial plants, and is also preferable as it may enhance nitrate reduction by underground microorganisms as an electron donor. The alcohol is, for example, ethanol. The saccharides include monosaccharides, disaccharides, and polysaccharides, and examples include glucose or sucrose.

[0017] The concentration (dissolved concentration) of the oxygen-containing organic substance in the aqueous solution can be appropriately changed according to the state of the soil, etc., but is preferably in the range of 1 to 100 mM, more preferably 5 to 50 mM. When the oxygen-containing organic substance is added in powder form, typically, it can be in the range of 1 to 500 g per 1 kg of soil. The aqueous solution or powder may contain other additives, etc., as the case may be.

[0018] The calcium salt used in step (a) can be one or more of calcium halide, calcium hydroxide, calcium acetate, calcium carbonate, etc. Preferably, it is calcium halide, more preferably calcium chloride (CaCl2). When the calcium salt is added in the form of an aqueous solution, the concentration of the calcium salt in the aqueous solution can be appropriately changed according to the state of the soil, etc., but preferably it is in the range of 100 to 5000 mM, more preferably in the range of 200 to 1000 mM. When the calcium salt is added in the form of a powder, typically, it can be in the range of 1 to 500 g per 1 kg of soil. The aqueous solution or powder may contain other additives, etc. as the case may be.

[0019] In step (a), the addition amount of the above two kinds of aqueous solutions added to the soil can be appropriately adjusted according to the amount and state of the soil, etc. When added in the form of an aqueous solution, typically, it is in the range of 0.001 to 1 liter per 1 kg of soil. When the calcium salt is added in the form of a powder, an aqueous solution of an oxygen-containing organic substance can be added in the above addition amount range, and at the same time, the powder of the calcium salt can be in the range of 1 to 500 g per 1 kg of soil as described above. Conversely, the same applies when the oxygen-containing organic substance is added in the form of a powder. In the case of soil rich in calcium, it is also possible to reduce the addition amount of the calcium salt.

[0020] Next, step (b) in the geological improvement method of the present invention is to ferment the oxygen-containing organic substance added in step (a) by microorganisms under an anaerobic environment, thereby generating carbon dioxide. As the microorganisms to be used, those capable of producing carbon dioxide by anaerobic fermentation using the oxygen-containing organic substance as a fermentation raw material can be widely used. When the oxygen-containing organic substance is an organic acid, for example, the microorganisms perform fermentation represented by the following reaction formula to produce carbon dioxide.

Equation

[0021] Here, anaerobic fermentation means that fermentation by microorganisms proceeds in an anaerobic environment or a hypoxic environment.

[0022] Examples of the microorganisms used in step (b) include microorganisms having a gene (mmdA) encoding methylmalonyl-CoA decarboxylase. More specifically, microorganisms belonging to the phylum Bacteroidetes, particularly microorganisms belonging to the order Bacteroidales, are preferred. These microorganisms having the mmdA gene have a function of removing carbon dioxide from methylmalonyl CoA and are particularly preferred in that they efficiently produce carbon dioxide when using organic acids such as succinic acid as oxygen-containing organic substances. In the method of the present invention, the use of these microorganisms is preferably determined by metagenomic analysis of the microbial community as shown in the examples described below.

[0023] The microorganisms used in step (b) can be microorganisms originally present in the soil or microorganisms added from the outside before step (b). Preferably, they can be added from the outside before step (b), and particularly, they can be added to the soil together with the oxygen-containing organic substance and the calcium salt in step (a).

[0024] In the present invention, when microorganisms are added from the outside and used in the soil, those stored in a freeze-dried state after being cultured in a factory, laboratory, etc. or those stored in a culture solution may be transported to the site of method implementation for use, or those cultured at the site of implementation may be used. Using those cultured at the site of implementation is advantageous in that bacteria and microorganisms can be used in the best state where they are at the peak of high activity, and the implementation cost can be reduced because management during transportation is not required.

[0025] Next, step (c) in the geological improvement method of the present invention is to react the carbon dioxide generated in step (b) with the calcium salt added in step (a) to form calcium carbonate (CaCO3) in the soil.

[0026] More specifically, as shown in the following reaction formula, first, the carbon dioxide generated in step (b) dissolves in the water in the soil to form carbonate ions (CO3 2- ); the bicarbonate ions react with calcium ions (Ca 2+ ) derived from calcium salts, resulting in the precipitation (calcification) of calcium carbonate (CaCO3).

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[0027] As a result, the generated calcium carbonate fills the spaces between particles and cracks in the ground, thereby increasing the ground strength and reducing the porosity and permeability, obtaining a ground improvement effect.

[0028] In a preferred embodiment of the ground improvement method of the present invention, in the above step (a), a powder or aqueous solution of nitrate is further added to the soil, thereby including allowing a nitrate reduction reaction to proceed in the soil. By such a nitrate reduction reaction, denitrification (NO3→N2) with nitrogen gas as a by-product occurs, and by increasing the pH of the surrounding soil, the above carbonate ions can be increased, which is beneficial in that the formation of calcium carbonate can be promoted. In addition, the nitrogen (N2) generated in the nitrate reduction reaction has an effect that it can prevent liquefaction by being taken into the spaces between particles in the ground. Typically, the nitrate reduction reaction can be represented by the following formula.

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[0029] As the nitrate added in step (a), nitrates of alkali metals or alkaline earth metals can be used, and preferably, it is sodium nitrate (NaNO3). When adding the calcium salt in the form of an aqueous solution, the concentration of the nitrate in the aqueous solution can be appropriately changed according to the state of the soil, etc., but preferably, it is in the range of 10 to 100 mM, more preferably in the range of 20 to 50 mM. When adding the nitrate in the form of a powder, typically, it can be in the range of 10 to 100 g per 1 kg of soil. The aqueous solution or the powder may contain other additives, etc. as the case may be.

[0030] In a further preferred embodiment of the geological improvement method of the present invention, in the above step (a), a powder or an aqueous solution of phosphate is further added into the soil, thereby forming hydroxyapatite in the upper soil. Hydroxyapatite (Ca 10 (PO4)6OH2) is known to have the ability to long-term fix harmful elements including radioactive elements and heavy metals by surface adsorption, etc. Therefore, by generating hydroxyapatite together with the above calcium carbonate (calcite), in addition to the above geological improvement effect by calcite, it is beneficial in that it can also serve as a countermeasure against groundwater pollution by radionuclides and heavy metals present in the soil.

[0031] As the phosphate added in step (a), phosphates of alkali metals or alkaline earth metals can be used, and preferably, it is sodium phosphate (NaH2PO4) or calcium phosphate. When adding the phosphate in the form of an aqueous solution, the concentration of the phosphate in the aqueous solution can be appropriately changed according to the state of the soil, etc., but preferably, it is in the range of 10 to 100 mM, more preferably in the range of 20 to 50 mM. When adding the phosphate in the form of a powder, typically, it can be in the range of 10 to 100 g per 1 kg of soil. The aqueous solution or the powder may contain other additives, etc. as the case may be.

[0032] In addition, when injecting phosphate into deep soil or generating hydroxyapatite, the pH in the initial soil is preferably in the weakly acidic range (pH 5 to 6.9), and thus an appropriate pH adjuster can also be added accordingly. Such pH adjusters can be those known in the art.

[0033] Optionally, in the geological improvement method of the present invention, after performing step (a) or (b), a step of stirring the soil may be performed. This is because stirring may be preferable for promoting the formation of calcium carbonate in step (c).

[0034] Examples of means for performing such stirring include stirring the soil using a backhoe, a stabilizer, a deep mixing machine, a concrete mixer, etc. In addition, a crawler-type self-propelled soil improvement machine equipped with a soil scraping and feeding mechanism, a discharging mechanism, a chemical mixing tank, etc. may also be used.

[0035] In another aspect, the present invention also relates to a geological improvement kit containing each powder or aqueous solution used in the above soil treatment step. Specifically, such a kit contains a powder or aqueous solution of an oxygen-containing organic substance; and a powder or aqueous solution of a calcium salt.

[0036] The geological improvement kit can further contain a powder or aqueous solution of nitrate for the nitrate reduction reaction. In addition, it can also further contain a powder or aqueous solution of phosphate for hydroxyapatite formation.

[0037] The concentrations of the oxygen-containing organic substance and the salt in each aqueous solution are as described above, and the amounts of each powder and aqueous solution stored in the kit can be appropriately changed according to the amount of soil to be treated.

[0038] In addition, the geological improvement kit can further contain microorganisms capable of producing carbon dioxide by fermentation in an anaerobic environment using an oxygen-containing organic substance as a raw material. The details of such microorganisms are as described above.

Examples

[0039] The present invention will be described in more detail below with reference to examples, but the present invention is not limited thereto.

[0040] 1. Treatment of soil samples Sediment soil was collected from a pond in a botanical garden in Tokyo, and 5 ml of sediment and 50 ml of fresh water medium were added to a container filled with Ar gas. From the obtained slurry-like sample, 0.5 ml and 50 ml of fresh water medium were added to another container filled with Ar gas. CaCl2, NaNO3, succinic acid, and NaH2PO4 were added to the solution at the concentrations shown below to adjust soil samples (Sample 1 and Sample 2). As a comparative example, a sample of the collected sediment soil sterilized by an autoclave was prepared. Sample 1 and Comparative Example 1 with 25 mM succinic acid had an initial pH adjusted to ~5.5. Sample 2 and Comparative Example 2 with 50 mM succinic acid had an initial pH adjusted to ~6. [Table 1]

[0041] The adjusted soil samples were cultured at 30 °C for 7 days. In Samples 1 and 2, the formation of obvious white precipitates was observed after culturing. For the liquid phase of the samples, pH measurement with an electrode and measurement of cations, anions, and organic acids by ion chromatography and HPLC were performed. For the precipitated solid phase, analysis of the powder X-ray diffraction (XRD) pattern and analysis by a scanning electron microscope equipped with an energy-dispersive X-ray spectrometer (SEM-EDS) were performed.

[0042] 2. Various measurements The results of observing the changes in pH and the number of bacteria during culturing are shown in FIGS. 1 and 2, respectively. In the figures, Sample 1 is labeled as "Lmed_pH5.5 at 30 °C", Sample 2 is labeled as "Lmed_pH6 at 30 °C", the sterilized Comparative Example 1 is labeled as "Lmed_pH5.5 sterile", and Comparative Example 2 is also labeled as "Lmed_pH6 sterile".

[0043] From the results in Figure 1, it was found that in Samples 1 and 2, an increase in pH was observed during cultivation, indicating that nitrate reduction was proceeding due to the microorganisms in the samples. On the other hand, in Comparative Examples 1 and 2, no increase in pH was observed. Also, from Figure 2, it was found that in Samples 1 and 2, an increase in the number of bacteria was observed during cultivation, indicating that the metabolic activity of the microorganisms was proceeding.

[0044] The changes over time in the concentrations of succinic acid and propionic acid during cultivation are shown in Figures 3 and 4, respectively. As a result, it was confirmed that in Samples 1 and 2, propionic acid was produced in an amount equal to or greater than the consumption of succinic acid. From this, it was found that succinic acid was converted to propionic acid by the microorganisms, and carbon dioxide was being produced. On the other hand, in Comparative Examples 1 and 2, substantially no consumption of succinic acid was observed.

[0045] Next, XRD pattern analysis was performed to confirm the formation of calcite and hydroxyapatite. As a result, as shown in Figure 5, in Samples 1 and 2, peaks corresponding to calcite and hydroxyapatite were observed, confirming that calcite and hydroxyapatite were formed. On the other hand, in Comparative Examples 1 and 2, no peaks corresponding to calcite and hydroxyapatite were observed.

[0046] Furthermore, observation by SEM-EDS was performed. The backscattered electron image of the sample in which the solid phase of Sample 1 was collected on a filter is shown in Figure 6. From Figure 6, it was confirmed that fine particles considered to be calcite and hydroxyapatite were densely present around the microbial cells. Also, in the EDS spectrum (Figure 7) of the portion indicated by the circle in Figure 6, peaks of calcium and phosphorus elements were clearly observed, further supporting the formation of calcite and hydroxyapatite.

[0047] 3. Metagenomic analysis As a result of analyzing the microbial community composition after cultivation by metagenomic analysis, in Sample 1 where calcite formation was observed, bacteria of the order Bacteroidales were dominant (Figure 8). In Figure 8, the microbial community composition obtained from the coverage of contigs containing the nucleotide sequences of 16S rRNA is described from the phylum to the genus level. The genome of the order Bacteroidales that was almost completely restored (~99%) was found to have a gene cluster that produces propionate and carbon dioxide by succinate fermentation (mmdA: a gene encoding methylmalonyl-CoA decarboxylase) (Figure 9).

[0048] On the other hand, in the sample where calcite formation was not prominent, bacteria of the order Xanthomonadales with denitrifying ability, rather than bacteria of the order Bacteroidales, were dominant (Figure 8), and it was found that many of the genes for producing propionate and carbon dioxide by succinate fermentation were missing in the genomes of the major restored microorganisms (Figure 9). As a result, it was found that microorganisms having mmdA such as the order Bacteroidales can form calcium carbonate through succinate fermentation.

Claims

1. A geological improvement method characterized by forming calcium carbonate in soil, comprising: (a) adding at least a powder or aqueous solution of an oxygen-containing organic substance and a powder or aqueous solution of a calcium salt into the soil; (b) generating carbon dioxide by metabolizing the oxygen-containing organic substance in an anaerobic environment using microorganisms; and (c) A step of forming calcium carbonate (CaCO ３ ) in the soil by reacting the calcium salt with the carbon dioxide wherein the oxygen-containing organic substance is an organic acid, the microorganism is a microorganism having a gene encoding methylmalonyl-CoA decarboxylase, the geological improvement method.

2. The method according to claim 1, wherein the organic acid is succinic acid, propionic acid or pyruvic acid.

3. The method according to claim 1 or 2, wherein the microorganism is a microorganism belonging to the phylum Bacteroidetes.

4. The method according to any one of claims 1 to 3, wherein the microorganism is a microorganism belonging to the order Bacteroidales.

5. The method according to any one of claims 1 to 4, wherein the microorganism is a microorganism originally present in the soil or a microorganism added externally before step (b).

6. The method according to any one of claims 1 to 5, wherein the calcium salt is calcium halide.

7. The method according to any one of claims 1 to 6, wherein the concentration of the oxygen-containing organic substance in the aqueous solution of the oxygen-containing organic substance is in the range of 1 to 100 mM.

8. The method according to any one of claims 1 to 7, wherein the concentration of the calcium salt in the aqueous solution of the calcium salt is in the range of 100 to 5000 mM.

9. The method according to any one of claims 1 to 6, wherein in step (a), the powder of the oxygen-containing organic substance is added in the range of 1 to 500 g per 1 kg of soil.

10. The method according to any one of claims 1 to 6, wherein in step (a), the powder of the calcium salt is added in the range of 1 to 500 g per 1 kg of soil.

11. The method according to any one of claims 1 to 10, further comprising adding a powder or aqueous solution of nitrate into the soil in step (a) to cause a nitrate reduction reaction to proceed in the soil.

12. The method according to any one of claims 1 to 11, further comprising adding a powder or aqueous solution of phosphate into the soil in step (a) to form hydroxyapatite in the soil.

13. A powder or aqueous solution of an oxygen-containing organic substance; and a powder or aqueous solution of a calcium salt, further comprising a microorganism having the ability to produce carbon dioxide by metabolism in an anaerobic environment using the oxygen-containing organic substance as a raw material, wherein the microorganism is a microorganism having a gene encoding methylmalonyl-CoA decarboxylase, a microorganism belonging to the phylum Bacteroidetes, or a microorganism belonging to the order Bacteroidales, A kit for geological improvement.

14. The kit for geological improvement according to claim 13, wherein the oxygen-containing organic substance is an organic acid.

15. The kit for geological improvement according to claim 14, wherein the organic acid is succinic acid, propionic acid, or pyruvic acid.

16. The kit for geological improvement according to any one of claims 13 to 15, wherein the calcium salt is calcium halide.

17. The kit for geological improvement according to any one of claims 13 to 16, further comprising a powder or aqueous solution of nitrate.

18. The kit for geological improvement according to any one of claims 13 to 17, further comprising a powder or aqueous solution of phosphate.

19. The kit for geological improvement according to any one of claims 13 to 18, wherein the concentration of the oxygen-containing organic substance in the aqueous solution of the oxygen-containing organic substance is in the range of 1 to 100 mM.

20. The kit for geological improvement according to any one of claims 13 to 19, wherein the concentration of the calcium salt in the aqueous solution of the calcium salt is in the range of 100 to 5000 mM.

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