A microbial soil conditioner for soil chromium pollution remediation and preparation method thereof

Abstract

The present invention relates to the technical field of soil pollution control, specifically a microbial soil conditioner for soil chromium pollution remediation and a preparation method thereof, comprising lactic acid, citric acid, starch, L-lysine, sodium tripolyphosphate, lanthanum nitrate, fruit shell, microbial agent, and bone meal. The microbial agent component is a mixture of Bacillus lovreid, Bacillus polymyxa, Bacillus subtilis and modified polylactic acid. When the microbial soil conditioner is used for chromium-contaminated soil, Bacillus polymyxa and Bacillus subtilis can adsorb hexavalent chromium ions, and accelerate the reproduction of Bacillus lovreid through a quorum sensing system, thereby improving the remediation efficiency. Acrylic acid in modified polylactic acid enhances the stability of signal molecules and promotes synergy between microorganisms. Polylactic acid enzyme and esterase accelerate the degradation of modified polylactic acid, and its product, as a microbial carbon source, enhances microbial growth metabolism, forms a dynamic remediation cycle, and improves soil remediation and ecological restoration capabilities.

Description

Technical Field

[0001] The invention relates to the technical field of soil pollution control, and specifically is a microbial soil conditioner for repairing chromium pollution in soil and a preparation method thereof. Background Art

[0002] With the rapid development of industrialization and urbanization, soil pollution is becoming increasingly serious, especially chromium pollution. Chromium is a harmful heavy metal, and long-term accumulation in the soil will cause great harm to the ecological environment and human health. Therefore, it is very important to effectively control and improve soil chromium pollution.

[0003] Chromium mainly comes from industrial wastewater, waste gas emissions and solid waste dumping. These pollutants accumulate in the soil, resulting in excessive chromium content, which in turn affects soil quality and ecosystem balance. Chromium pollution inhibits the growth of crops and reduces the yield and quality of agricultural products. At the same time, it is transmitted through the food chain, posing potential hazards to human health, such as causing skin diseases, respiratory diseases, etc.

[0004] Chromium in soil exists mainly in the form of trivalent chromium (Cr(III)) and hexavalent chromium (Cr(VI)). Hexavalent chromium is highly soluble and mobile, and is more easily absorbed by plants, which is more harmful to the environment and human health. Hexavalent chromium is about 100 times more toxic than trivalent chromium. It is a strong mutagenic substance that can induce lung cancer and nasopharyngeal cancer. Short-term exposure to large doses of hexavalent chromium can cause acute rhinitis, red and swollen eyes, stomatitis, respiratory inflammation and acute gastroenteritis. In severe cases, it may lead to multiple organ failure. Long-term exposure to hexavalent chromium can lead to intestinal tumors.

[0005] The current status of soil chromium pollution remediation technology shows that there are many ways to deal with this environmental problem, but each has its own advantages and limitations. For example, although chemical reduction is currently the most widely used remediation technology, its long-term stability and safety issues such as chromium residue and reoxidation after soil remediation still need to be paid attention to. In contrast, microbial remediation technology has attracted much attention due to its advantages such as environmental friendliness, high cost-effectiveness and simple operation. In particular, the use of microbial agents has shown great potential in the remediation process.

[0006] In summary, soil chromium pollution remediation technology is developing in a more environmentally friendly and efficient direction, and the use of microbial agents plays an increasingly important role. Future research needs to focus on developing new microbial agents, optimizing remediation conditions, and evaluating long-term environmental safety to achieve sustainable remediation of soil chromium pollution. Summary of the invention

[0007] (1) Technical issues to be solved

[0008] The purpose of the present invention is to provide a microbial soil conditioner for soil chromium pollution remediation and a preparation method thereof, which can significantly improve the remediation efficiency of soil chromium pollution, restore soil ecological functions, and ensure the safety of agricultural products and human health.

[0009] (2) Technical solution

[0010] To achieve the above-mentioned purpose, on the one hand, the present invention provides a microbial soil conditioner for soil chromium pollution remediation, which is composed of the following raw materials in parts by weight: 20-30 parts of lactic acid, 20-30 parts of citric acid, 10-25 parts of starch, 15-20 parts of L-lysine, 15-25 parts of sodium tripolyphosphate, 0.1-1 part of lanthanum nitrate, 20-25 parts of fruit shells, 16-32 parts of microbial agents, and 5-20 parts of bone meal.

[0011] The microbial agent component is a mixture of 5 to 10 parts of Bacillus lovreyi, 5 to 10 parts of Bacillus polymyxa and 5 to 10 parts of Bacillus subtilis.

[0012] Furthermore, the microbial agent component also includes: 1 to 2 parts of modified polylactic acid;

[0013] The modified polylactic acid is generated by graft copolymerization of polylactic acid and acrylic acid, and the surface of the modified polylactic acid has hydroxyl, carboxyl and ester functional groups.

[0014] Furthermore, the preparation method of the modified polylactic acid comprises:

[0015] S11. Add the dehydrated L-lactic acid, diphenyl ether and stannous chloride to a three-necked flask and stir thoroughly. Evacuate with a circulating water pump and heat to 150-200°C at a rate of 10°C / min for 3h.

[0016] S12. After the reaction is completed, the obtained sample is distilled, the solvent is evaporated, and a certain amount of acetone is added to dissolve it, and then the obtained solution is precipitated in a large amount of water and filtered. The obtained polylactic acid is vacuum dried to constant weight to obtain a white powdery product;

[0017] S13. The polylactic acid was dissolved in tetrahydrofuran heated to 50°C, N2 was introduced, the mixture was stirred thoroughly, the temperature was raised to 66°C, constant temperature reflux was started, and acrylic acid and dibenzoyl peroxide were added in 4 portions within 2 minutes;

[0018] S14. After the reaction is completed, 50 mL of 95% ethanol solution is added to the reaction product solution, and after a white precipitate appears, the precipitate and the solution are poured into a sand core crucible with constant weight and filtered, and the precipitate is washed twice with 95% ethanol, and then the precipitate and the sand core crucible are placed in a 50 ° C constant temperature oven and dried to constant weight to obtain a crude graft copolymer;

[0019] S15. Pour the crude graft copolymer into a Soxhelt extractor, use acetone as solvent, extract for 16 hours, and reflux 5 to 6 times per hour to remove the homopolymer. The residue after extraction is dried in a 50°C constant temperature oven to constant weight, and the pure graft copolymer is obtained, which is the modified polylactic acid.

[0020] Furthermore, the preparation method of the microbial agent comprises:

[0021] S21. placing Bacillus lovrei, Bacillus polymyxa and Bacillus subtilis in a seed culture medium, culturing for 1 to 2 days at a temperature of 25 to 30° C. and a shaking speed of 100 to 200 r / min to obtain a corresponding seed solution;

[0022] S22. The seed solution is inoculated into the fermentation medium at a volume ratio of 4 to 8%, and cultured under shaking conditions of 25 to 33 ° C and a shaking speed of 100 to 150 r / min for 2 to 4 days to obtain the corresponding fermentation solution;

[0023] S23. The modified polylactic acid and the fermentation liquid are fully mixed in a mass ratio of 1:2.5, shaken at a temperature of 30°C and a shaker speed of 200 r / min, then placed in a constant temperature incubator at 30°C for fermentation for 48 hours, and finally dried at a constant temperature of 50°C to obtain the corresponding microbial agent.

[0024] Furthermore, the components of the seed culture medium are as follows: 10 g sucrose, 5 g peptone, 0.01 g FeSO4·7H2O, 0.5 g MgSO4·7H2O, and 1 L distilled water.

[0025] Furthermore, the ingredients of the fermentation medium are as follows: 20 g corn flour, 10 g sucrose, 2 g peptone, 2 g yeast extract, 2 g NaCl, 0.5 g MgSO4, and 1 L distilled water.

[0026] On the other hand, based on the same inventive concept, the present invention also provides a method for preparing a microbial soil conditioner for soil chromium pollution remediation, which is applied to the microbial soil conditioner for soil chromium pollution remediation, comprising the following steps:

[0027] S31. Preparation of modified polylactic acid;

[0028] S32. Preparation of microbial agents;

[0029] S33. Weigh each raw material according to the proportion, and then dry it. After the dried raw materials are fully mixed, add them to the air flow mill, control the fineness of the crushed material to be between 200 and 300 meshes, granulate the crushed material, and obtain a solid microbial soil conditioner.

[0030] The mechanism of action of the above raw material components is as follows:

[0031] The microbial agent contains three beneficial microorganisms, which can participate in the restoration process of soil chromium pollution, and can also promote plant growth and improve plant resistance to chromium. The microbial agent is composed of Bacillus lovrei, Bacillus polymyxa, Bacillus subtilis and modified polylactic acid.

[0032] Geobacter lovleyi is a microorganism that can reduce hexavalent chromium to trivalent chromium under anaerobic conditions through its special metabolic pathway. It uses the electron transfer protein in its respiratory chain to directly transfer electrons to dissolved hexavalent chromium and reduce it to trivalent chromium. This process can be carried out in an anaerobic environment and does not rely on oxygen. At the same time, the reductase (such as cytochrome c-type oxidoreductase) on the bacterial cell membrane directly reacts with hexavalent chromium to reduce hexavalent chromium to trivalent chromium. In addition, the bacterium can also bind to chromium ions in the soil through specific receptors on its cell wall and cell membrane, thereby adsorbing chromium ions to the bacterial surface. This adsorption can reduce the concentration of chromium in the soil solution, reducing its mobility and biological effectiveness.

[0033] Paenibacillus polymyxa can produce extracellular polysaccharides with high molecular weight and complex structure, which can combine with chromium ions in the soil by adsorption, complexation or precipitation, reducing the bioavailability and mobility of chromium. At the same time, the bacteria can secrete organic acids, such as citric acid and oxalic acid, which can form stable complexes with chromium ions and reduce the toxicity of chromium. In addition, the bacteria can also secrete plant growth regulators, such as indoleacetic acid, to promote plant root development and enhance plant tolerance to chromium stress.

[0034] Bacillus subtilis can provide negative charge sites through components such as peptidoglycan and teichoic acid on its cell wall, and adsorb chromium heavy metal ions in the soil through electrostatic adsorption, ion exchange and surface complexation, thereby reducing its mobility and bioavailability in the environment. At the same time, the bacteria can produce extracellular secretions such as organic acids under chromium stress conditions, which can form precipitates with chromium ions and reduce their biological toxicity. In addition, the bacteria can promote plant growth, increase the effective chromium content in the soil, and enhance the plant's ability to absorb and transport chromium, thereby improving the efficiency of plant restoration.

[0035] Modified polylactic acid is a biodegradable polymer material with good biocompatibility. It can provide a physical barrier for microorganisms in microbial agents to prevent the death of microorganisms during storage and transportation. Modified polylactic acid can control the release rate of microorganisms by adjusting its molecular weight and polymer composition, ensuring that microorganisms continue to play a role in the soil and improving the remediation efficiency. Modified polylactic acid can form organic matter after degradation in the soil, which helps to improve soil structure and increase soil porosity and water retention.

[0036] Lactic acid is an organic acid that not only provides a nutrient source for microorganisms to activate and enhance their biological activity, promotes the reduction of hexavalent chromium to trivalent chromium, and reduces the bioavailability and toxicity of chromium, but also improves the physical and chemical properties of the soil, promotes plant root growth, and increases plant tolerance to chromium stress. It also reduces the risk of soil degradation and secondary pollution caused by unreasonable application, thus playing a key role in the microbial-assisted soil remediation process.

[0037] Citric acid is also an organic acid, similar to lactic acid. In microbial soil conditioners used for soil chromium pollution remediation, it can activate and enhance the activity of microorganisms, promote the reduction of hexavalent chromium by microorganisms, and reduce its toxicity. In addition, citric acid can also improve the pH and structure of the soil, increase soil fertility, promote plant growth, and improve the overall health and remediation efficiency of the soil ecosystem.

[0038] Starch is a polysaccharide. In microbial soil conditioners used for soil chromium pollution remediation, it can serve as a growth matrix for microorganisms, promoting the reproduction and activity of beneficial microorganisms. At the same time, the organic acids produced during starch degradation can change the soil pH, increase the bioavailability of heavy metal chromium in the soil, and promote its conversion into a plant-absorbable form. In addition, starch and its degradation products can also enhance soil structure, improve soil water retention and aeration, provide a more favorable growth environment for microorganisms and plants, and enhance the soil's natural ability to repair chromium pollution.

[0039] L-lysine is an amino acid that can serve as a nutrient source for microorganisms to promote their growth and activity, and enhance their ability to adsorb, transform, and fix chromium. At the same time, L-lysine may form a stable complex with chromium ions in the soil through its amino group, reducing the bioavailability and mobility of chromium and its potential harm to plants and soil ecosystems. In addition, the addition of L-lysine can also improve the physical and chemical properties of the soil, promote the development of plant roots, and increase plant tolerance to chromium stress, thereby playing a positive role in the microbial-assisted soil remediation process.

[0040] Sodium tripolyphosphate is a phosphate fertilizer that can provide the phosphorus source required for microbial growth, promote the activity and growth of microorganisms, thereby improving the removal efficiency of chromium in the soil and promoting soil ecological restoration. At the same time, it can also form a stable complex with chromium ions in the soil, thereby reducing the bioavailability and toxicity of chromium. In addition, sodium tripolyphosphate can also improve the physical and chemical properties of the soil, and enhance the soil's water retention capacity and nutrient supply.

[0041] Lanthanum nitrate is a lanthanum-containing compound. Lanthanum can be used as a trace element in the soil to provide necessary nutrients for the growth and metabolism of soil microorganisms, enhance the activity and reproduction of microorganisms, promote the fixation and transformation of chromium, and improve the physical and chemical properties of the soil, thereby enhancing the repair capacity of microorganisms in the soil, effectively reducing the biological effectiveness and environmental risks of chromium, and promoting the recovery of soil ecology.

[0042] Fruit shells are rich in organic matter and minerals, which can improve soil structure, increase soil porosity, and improve soil aeration and water retention. At the same time, the organic matter in fruit shells can provide nutrition for microorganisms and promote microbial activity, thereby accelerating the restoration process of soil chromium pollution. Due to its porous structure and large surface area, it can also effectively absorb chromium pollutants in the soil.

[0043] Bone meal is an organic fertilizer rich in minerals such as calcium and phosphorus, which can provide the nutrients needed for microbial growth. By enhancing the activity of microorganisms, bone meal can effectively promote the biotransformation and fixation of chromium, reduce the level of chromium pollution in the soil, and thus achieve soil repair and ecological restoration. In addition, the mineral components in bone meal help stabilize soil structure and improve soil fertility.

[0044] (3) Beneficial effects

[0045] Compared with the prior art, the present invention has the following beneficial effects:

[0046] 1. The microbial agent carrier adopts modified polylactic acid, which improves the toughness and strength of the material compared with polylactic acid, making it more durable in the soil environment, increasing the hydrophilicity of the material, improving the attachment and growth environment of microorganisms, and contributing to the activity of microorganisms. By adjusting the grafting ratio, the sustained release of microorganisms can be achieved, prolonging their effectiveness in the soil, and ensuring that the microorganisms continue to play a role in the remediation process.

[0047] 2. Bacillus lovrei, Bacillus polymyxa and Bacillus subtilis have good adaptability and survival ability, and can work synergistically to promote the transformation and fixation of chromium.

[0048] 3. Compared with traditional soil remediation methods, the microbial soil conditioner used in the present invention has low cost and is easy to produce and apply on a large scale. The microbial soil conditioner provided by the present invention is easy to use and easy to operate, and is suitable for remediation of different types of soil chromium pollution. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 This is a microscope image of Bacillus lovrei in the microbial agent of Example 1 of the present invention;

[0050] Figure 2 This is a microscope image of Bacillus polymyxa in the microbial agent of Example 1 of the present invention;

[0051] Figure 3 This is a microscope image of Bacillus subtilis in the microbial agent of Example 1 of the present invention. DETAILED DESCRIPTION

[0052] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0053] The test equipment and preparations of the examples described below are as follows:

[0054] Electronic balance (Sartorius, Germany), electric heated constant temperature dryer (Foma Laboratory Equipment, Shanghai), UV spectrophotometer (JENWAY, UK), flame spectrophotometer (Shanghai Precision Scientific Instruments), pH meter (Shanghai Precision Scientific Instruments), rotary evaporator (Shanghai Darlow Scientific Instruments), pulverizer (Shandong Tianfang Machinery), magnetic stirrer (Shanghai Meiyingpu), Soxhelt extractor (Qingdao Juchuang); lactic acid and citric acid were purchased from Changsha Yufeng Huabo Instrument Co., Ltd., Bacillus lovrei was purchased from Beijing Biobowei Biotechnology Co., Ltd., Bacillus polymyxa was purchased from Shanghai Guyan Industrial Co., Ltd., Bacillus subtilis was purchased from Shanghai Guyan Industrial Co., Ltd., and chemicals and reagents were purchased from Sigma-Aldrich.

[0055] Example 1

[0056] The present embodiment discloses a microbial soil conditioner for soil chromium pollution remediation, which is composed of the following raw materials in parts by weight: 22 parts of lactic acid, 23 parts of citric acid, 20 parts of starch, 18 parts of L-lysine, 20 parts of sodium tripolyphosphate, 0.2 parts of lanthanum nitrate, 22 parts of fruit shells, 22 parts of microbial agents, and 10 parts of bone powder. The microbial agent component is a mixture of 8 parts of Bacillus lovreyi, 6 parts of Bacillus polymyxa and 7 parts of Bacillus subtilis. The microbial agent component also includes: 1 part of modified polylactic acid; the modified polylactic acid is generated by graft copolymerization of polylactic acid and acrylic acid, and the surface of the modified polylactic acid has hydroxyl, carboxyl and ester functional groups.

[0057] When the microbial soil conditioner is applied to chromium-contaminated soil, the microbial agents in it begin to play their unique functions. Under the protection of modified polylactic acid, the microorganisms quickly adapt to the soil environment and begin their work. First, Bacillus polymyxa and Bacillus subtilis effectively adsorb hexavalent chromium ions in the soil with the help of their cell walls and secreted extracellular polysaccharides. This adsorption not only reduces the bioavailability and mobility of chromium, reduces the harm of metallic chromium to the soil ecosystem, but also lays the foundation for the subsequent bioremediation process. At the same time, through the quorum sensing system, Bacillus polymyxa and Bacillus subtilis can produce and respond to signal molecules, such as N-acetyl homoserine lactone. These signal molecules accelerate the reproduction rate of Bacillus lovreyi. Bacillus lovreyi can reduce hexavalent chromium to trivalent chromium through its metabolic pathway. When the number of Bacillus lovreyi increases, the reduction rate of hexavalent chromium per unit time becomes faster, thereby improving the remediation efficiency of soil chromium pollution. In order to further enhance this repair process, acrylic acid is introduced into the modified polylactic acid. This chemical substance can form a protective layer on the polylactic acid molecular chain, which shields the signal molecules, reduces the contact between the signal molecules and other reactive substances, and slows down the degradation or decomposition reaction of the signal molecules, thereby improving the chemical stability of the signal molecules, allowing the signal molecules to continue to function in the environment, and providing a lasting communication mechanism for the synergistic effect of microorganisms. Under the action of Bacillus lovrei, polylactic acid enzymes can specifically hydrolyze the ester bonds in the modified polylactic acid, thereby initiating its degradation process. On the other hand, the esterases secreted by Bacillus polymyxa and Bacillus subtilis can accurately cut the ester bonds of the modified polylactic acid, significantly increasing the degradation rate of the modified polylactic acid. In addition, under the guidance of signal molecules, Bacillus polymyxa and Bacillus subtilis also synthesize polymerases. This process enhances the hydrophilicity of the modified polylactic acid, making the modified polylactic acid more likely to become the target of the enzyme, providing convenience for subsequent enzymatic hydrolysis and ensuring the efficient degradation of the modified polylactic acid. The degradation products of modified polylactic acid, such as lactic acid, can serve as carbon sources for microorganisms, promote their growth and metabolism, and thus enhance the sensing behavior of the microbial community. The enhancement of the sensing behavior of the microbial community will not only increase the concentration and activity of signal molecules, but also promote the interaction between microorganisms, forming a dynamic repair cycle, further improving the repair efficiency and ecological restoration capacity of contaminated soil.

[0058] In summary, through the application of microbial soil conditioners, combined with the special design of modified polylactic acid, and the synergistic effect between Bacillus polymyxa, Bacillus subtilis and Bacillus lovrei, the remediation process of contaminated soil was significantly accelerated, which not only effectively reduced the pollution of hexavalent chromium, but also promoted the recovery of soil ecosystems and the healthy growth of microbial communities. This method meets the requirements of environmental protection and sustainable development, ensures the safety and environmental protection of microbial agents in the soil remediation process, and provides a green and sustainable solution for soil remediation.

[0059] The preparation method of the modified polylactic acid comprises:

[0060] S11. Add the dehydrated L-lactic acid, diphenyl ether and stannous chloride to a three-necked flask and stir thoroughly. Evacuate with a circulating water pump and heat to 150-200°C at a rate of 10°C / min for 3h.

[0061] S12. After the reaction is completed, the obtained sample is distilled, the solvent is evaporated, and a certain amount of acetone is added to dissolve it, and then the obtained solution is precipitated in a large amount of water and filtered. The obtained polylactic acid is vacuum dried to constant weight to obtain a white powdery product;

[0062] S13. The polylactic acid was dissolved in tetrahydrofuran heated to 50°C, N2 was introduced, the mixture was stirred thoroughly, the temperature was raised to 66°C, constant temperature reflux was started, and acrylic acid and dibenzoyl peroxide were added in 4 portions within 2 minutes;

[0063] S14. After the reaction is completed, 50 mL of 95% ethanol solution is added to the reaction product solution, and after a white precipitate appears, the precipitate and the solution are poured into a sand core crucible with constant weight and filtered, and the precipitate is washed twice with 95% ethanol, and then the precipitate and the sand core crucible are placed in a 50 ° C constant temperature oven and dried to constant weight to obtain a crude graft copolymer;

[0064] S15. Pour the crude graft copolymer into a Soxhelt extractor, use acetone as solvent, extract for 16 hours, and reflux 5 to 6 times per hour to remove the homopolymer. The residue after extraction is dried in a 50°C constant temperature oven to constant weight, and the pure graft copolymer is obtained, which is the modified polylactic acid.

[0065] The preparation method of the microbial agent comprises:

[0066] S21. placing Bacillus lovrei, Bacillus polymyxa and Bacillus subtilis in a seed culture medium, culturing for 1 to 2 days at a temperature of 25 to 30° C. and a shaking speed of 100 to 200 r / min to obtain a corresponding seed solution;

[0067] S22. The seed solution is inoculated into the fermentation medium at a volume ratio of 4 to 8%, and cultured under shaking conditions of 25 to 33 ° C and a shaking speed of 100 to 150 r / min for 2 to 4 days to obtain the corresponding fermentation solution;

[0068] S23. The modified polylactic acid and the fermentation liquid are fully mixed in a mass ratio of 1:2.5, shaken at a temperature of 30°C and a shaker speed of 200 r / min, then placed in a constant temperature incubator at 30°C for fermentation for 48 hours, and finally dried at a constant temperature of 50°C to obtain the corresponding microbial agent.

[0069] The components of the seed culture medium are as follows: 10 g sucrose, 5 g peptone, 0.01 g FeSO4·7H2O, 0.5 g MgSO4·7H2O, and 1 L distilled water.

[0070] The ingredients of the fermentation medium are as follows: 20 g corn flour, 10 g sucrose, 2 g peptone, 2 g yeast extract, 2 g NaCl, 0.5 g MgSO4, and 1 L distilled water.

[0071] A method for preparing a microbial soil conditioner for soil chromium pollution remediation, the method comprising the following steps:

[0072] S31. Preparation of modified polylactic acid;

[0073] S32. Preparing microbial agents;

[0074] S33. Weigh each raw material according to the proportion, and then dry it. After the dried raw materials are fully mixed, add them to the air flow mill, control the fineness of the crushed material to be between 200 and 300 meshes, granulate the crushed material, and obtain a solid microbial soil conditioner.

[0075] Example 2

[0076] The present embodiment discloses a microbial soil conditioner for soil chromium pollution remediation, which is composed of the following raw materials in parts by weight: 20 parts of lactic acid, 20 parts of citric acid, 10 parts of starch, 15 parts of L-lysine, 15 parts of sodium tripolyphosphate, 0.1 parts of lanthanum nitrate, 20 parts of fruit shells, 16 parts of microbial agents, 5 parts of bone meal, and a mixture of 5 parts of Bacillus lovreyi, 5 parts of Bacillus polymyxa and 5 parts of Bacillus subtilis, wherein the microbial agent component further comprises: 1 part of modified polylactic acid; the modified polylactic acid is generated by graft copolymerization of polylactic acid and acrylic acid, and the surface of the modified polylactic acid has hydroxyl, carboxyl and ester functional groups.

[0077] The preparation methods of the modified polylactic acid and microbial agent in this embodiment are the same as those in Example 1. The preparation method of a microbial soil conditioner for soil chromium pollution remediation in this embodiment is the same as that in Example 1.

[0078] Example 3

[0079] The present embodiment discloses a microbial soil conditioner for soil chromium pollution remediation, which is composed of the following raw materials in parts by weight: 30 parts of lactic acid, 30 parts of citric acid, 25 parts of starch, 20 parts of L-lysine, 25 parts of sodium tripolyphosphate, 1 part of lanthanum nitrate, 25 parts of fruit shells, 32 parts of microbial agents, and 20 parts of bone powder. The microbial agent component is a mixture of 10 parts of Bacillus lovreyi, 10 parts of Bacillus polymyxa and 10 parts of Bacillus subtilis. The microbial agent component also includes 2 parts of modified polylactic acid; the modified polylactic acid is generated by graft copolymerization of polylactic acid and acrylic acid, and the surface of the modified polylactic acid has hydroxyl, carboxyl and ester functional groups.

[0080] The preparation methods of the modified polylactic acid and microbial agent in this embodiment are the same as those in Example 1. The preparation method of a microbial soil conditioner for soil chromium pollution remediation in this embodiment is the same as that in Example 1.

[0081] Example 4

[0082] The present embodiment discloses a microbial soil conditioner for soil chromium pollution remediation, which is composed of the following raw materials in parts by weight: 25 parts of lactic acid, 25 parts of citric acid, 18 parts of starch, 17 parts of L-lysine, 22 parts of sodium tripolyphosphate, 0.5 parts of lanthanum nitrate, 23 parts of fruit shells, 25 parts of microbial agents, and 12 parts of bone powder. The microbial agent component is a mixture of 8 parts of Bacillus lovreyi, 8 parts of Bacillus polymyxa and 8 parts of Bacillus subtilis. The microbial agent component also includes: 1 part of modified polylactic acid; the modified polylactic acid is generated by graft copolymerization of polylactic acid and acrylic acid, and the surface of the modified polylactic acid has hydroxyl, carboxyl and ester functional groups.

[0083] The preparation methods of the modified polylactic acid and microbial agent in this embodiment are the same as those in Example 1. The preparation method of a microbial soil conditioner for soil chromium pollution remediation in this embodiment is the same as that in Example 1.

[0084] Example 5

[0085] The present embodiment discloses a microbial soil conditioner for soil chromium pollution remediation, which is composed of the following raw materials in parts by weight: 28 parts of lactic acid, 27 parts of citric acid, 15 parts of starch, 19 parts of L-lysine, 18 parts of sodium tripolyphosphate, 0.8 parts of lanthanum nitrate, 21 parts of fruit shells, 19 parts of microbial agents, and 18 parts of bone powder. The microbial agent component is a mixture of 6 parts of Bacillus lovreyi, 6 parts of Bacillus polymyxa and 6 parts of Bacillus subtilis. The microbial agent component also includes: 1 part of modified polylactic acid; the modified polylactic acid is generated by graft copolymerization of polylactic acid and acrylic acid, and the surface of the modified polylactic acid has hydroxyl, carboxyl and ester functional groups.

[0086] The preparation methods of the modified polylactic acid and microbial agent in this embodiment are the same as those in Example 1. The preparation method of a microbial soil conditioner for soil chromium pollution remediation in this embodiment is the same as that in Example 1.

[0087] Control group 1

[0088] The difference between this embodiment and embodiment 1 is that this embodiment does not contain Diebacterium lovreyi.

[0089] The present embodiment discloses a microbial soil conditioner for soil chromium pollution remediation, which is composed of the following raw materials in parts by weight: 23 parts of citric acid, 20 parts of starch, 18 parts of L-lysine, 20 parts of sodium tripolyphosphate, 0.2 parts of lanthanum nitrate, 22 parts of fruit shells, 22 parts of microbial agents, and 10 parts of bone powder. The microbial agent component is a mixture of 6 parts of Bacillus polymyxa and 7 parts of Bacillus subtilis. The microbial agent component also includes: 1 part of modified polylactic acid; the modified polylactic acid is generated by graft copolymerization of polylactic acid and acrylic acid, and the surface of the modified polylactic acid has hydroxyl, carboxyl and ester functional groups.

[0090] The preparation methods of the modified polylactic acid and microbial agent in this embodiment are the same as those in Example 1. The preparation method of a microbial soil conditioner for soil chromium pollution remediation in this embodiment is the same as that in Example 1.

[0091] Control group 2

[0092] The difference between this embodiment and embodiment 1 is that Bacillus polymyxa and Bacillus subtilis are not contained.

[0093] The present embodiment discloses a microbial soil conditioner for soil chromium pollution remediation, which is composed of the following raw materials in parts by weight: 22 parts of lactic acid, 23 parts of citric acid, 18 parts of L-lysine, 20 parts of sodium tripolyphosphate, 0.2 parts of lanthanum nitrate, 22 parts of fruit shells, 22 parts of microbial agents, and 10 parts of bone powder. The microbial agent component is 8 parts of Bacillus lovreidii, and the microbial agent component also includes: 1 part of modified polylactic acid; the modified polylactic acid is generated by graft copolymerization of polylactic acid and acrylic acid, and the surface of the modified polylactic acid has hydroxyl, carboxyl and ester functional groups.

[0094] The preparation methods of the modified polylactic acid and microbial agent in this embodiment are the same as those in Example 1. The preparation method of a microbial soil conditioner for soil chromium pollution remediation in this embodiment is the same as that in Example 1.

[0095] Control group 3

[0096] The difference between this embodiment and embodiment 1 is that modified polylactic acid is not contained.

[0097] This embodiment discloses a microbial soil conditioner for soil chromium pollution remediation, which is composed of the following raw materials in parts by weight: 22 parts of lactic acid, 23 parts of citric acid, 20 parts of starch, 18 parts of L-lysine, 0.2 parts of lanthanum nitrate, 22 parts of fruit shells, 22 parts of microbial agents, and 10 parts of bone meal, wherein the microbial agent component is a mixture of 8 parts of Bacillus lovreyi, 6 parts of Bacillus polymyxa and 7 parts of Bacillus subtilis.

[0098] The preparation methods of the modified polylactic acid and microbial agent in this embodiment are the same as those in Example 1. The preparation method of a microbial soil conditioner for soil chromium pollution remediation in this embodiment is the same as that in Example 1.

[0099] Test verification: Weigh 2kg of uncontaminated soil, air-dry and crush, add 4L of 180mg / kg Cr(VI) standard solution, stir, air-dry and crush to make simulated contaminated soil. Take 8 plastic bottles, add soil samples that have passed through a 20-mesh sieve, and then add microbial soil conditioners of each experimental group and control group for remediation. During the remediation period, take out the plastic bottles regularly every 5 days to measure the soil weight and Cr(VI) concentration, and calculate the degradation rate of modified polylactic acid and the reduction rate of hexavalent chromium from these data.

[0100] To calculate the degradation rate of modified polylactic acid, the degradation amount of modified polylactic acid must first be determined. The weight loss method can be used to monitor the weight loss of the carrier by weighing, and the weight loss at each sampling is recorded to indirectly reflect the degradation amount. The degradation amount of modified polylactic acid measured after each sampling is divided by the initial amount of modified polylactic acid to obtain the degradation rate of modified polylactic acid.

[0101] The reduction rate of hexavalent chromium was calculated, and the content of hexavalent chromium in the initial soil and the content of hexavalent chromium after each sampling were measured.

[0102] Determination of hexavalent chromium in soil: Weigh 2.5g soil sample into a 250mL Erlenmeyer flask, add 50mL 0.28MNa2CO3+0.5M NaOH (digestion solution), add 0.4g MgCl2, then add 0.5mL 0.5M phosphate buffer (K2HPO4 / KHPO4), and stir for 5 minutes. Heat the sample to 90-95℃, continue heating at this temperature for 60 minutes, and stir continuously. Slowly cool to room temperature. Transfer all samples to the filtration device. The sample passes through a 0.45μm micro-membrane, and the Erlenmeyer flask is washed with deionized water. The filtrate and rinse solution are placed in a 250ml Erlenmeyer flask, and the filter paper is stored at 4℃. Slowly add 5.0M HNO3 dropwise to the filtrate (stirring continuously) and adjust the pH to 9.0±0.5. Transfer the sample solution to a 100mL volumetric flask, dilute to volume, and determine.

[0103] The change of hexavalent chromium can be obtained by subtracting the hexavalent chromium content after each sampling from the hexavalent chromium content in the initial soil, and the reduction rate of hexavalent chromium can be obtained by dividing the change of hexavalent chromium by the hexavalent chromium content in the initial soil.

[0104] Table 1 Example group, control group for soil chromium Pollution remediation for hexavalent chromium reduction rate

[0105] Group 5D 10D 15D 20D Example 1 34.23％ 56.97％ 89.52％ 92.49％ Example 2 22.54％ 40.84％ 60.61％ 76.34％ Example 3 30.68％ 49.66％ 71.58％ 85.88％ Example 4 27.82％ 47.39％ 64.65％ 79.03％ Example 5 28.19％ 48.25％ 64.59％ 81.92％ Control group 1 0.27％ 0.66％ 0.78％ 0.84％ Control group 2 8.14％ 13.58％ 14.57％ 15.63％ Control group 3 9.18％ 18.98％ 22.58％ 36.08％

[0106] Table 2 Degradation rate of modified polylactic acid in the embodiment group and the control group for soil chromium pollution remediation

[0107] Group 5D 10D 15D 20D Example 1 8.24％ 12.56％ 28.74％ 33.67％ Example 2 5.72％ 8.46％ 17.46％ 24.87％ Example 3 7.38％ 10.24％ 23.75％ 28.36％ Example 4 6.34％ 9.85％ 20.98％ 26.68％ Example 5 6.98％ 9.37％ 21.25％ 27.23％ Control group 1 3.11％ 4.12％ 5.62％ 7.46％ Control group 2 1.14％ 2.58％ 3.57％ 4.63％ Control group 3 0％ 0％ 0％ 0％

[0108] The reduction rates of hexavalent chromium in the embodiment group and the control group for soil chromium pollution remediation are shown in Table 1. From the data in Table 1, it can be seen that the microbial soil conditioner formula of the embodiment group is significantly better than the control group in the reduction rate of hexavalent chromium. This shows that the ingredients such as Bacillus lovreyi, Bacillus polymyxa, Bacillus subtilis mixture and modified polylactic acid added in the embodiments have a significant effect on improving the reduction efficiency of hexavalent chromium in soil. In the embodiment group, there are differences in the reduction rates of different embodiments, which may be related to the different ratios of microbial agents and modified polylactic acid and other ingredients in each embodiment. By optimizing the ratio of these ingredients, the role of microbial soil conditioners in the hexavalent chromium reduction process can be further exerted.

[0109] The degradation rates of modified polylactic acid for soil chromium pollution remediation in the example group and the control group are shown in Table 2. The degradation rate of modified polylactic acid in the example group was improved compared with that in the control group, which further confirmed the promoting effect of Bacillus lovrei, Bacillus polymyxa and Bacillus subtilis on the degradation process of modified polylactic acid. In the example group, the degradation rate of modified polylactic acid in Example 1 was increased most significantly, from an initial 8.24% to a final 33.67%. This significant improvement may be related to the specific formula ratio in Example 1, indicating that the degradation rate of modified polylactic acid can be further optimized by accurately adjusting the formula ratio.

[0110] After the above effective tests, a microbial soil conditioner component for soil chromium pollution remediation in Example 1 of the present invention shows significant application effects on the reduction rate of hexavalent chromium and the degradation rate of modified polylactic acid. The microbial soil conditioner can effectively and efficiently exert the chromium removal efficiency and has practical application value for the remediation of soil chromium pollution. By optimizing the ratio of components such as the microbial mixture and modified polylactic acid, the effect of the soil conditioner in the process of soil chromium pollution remediation can be further improved, providing a feasible method for solving the problem of soil pollution.

[0111] Finally, it should be noted that: Although the present invention has been described in detail with reference to the aforementioned embodiments, it is still possible for those skilled in the art to modify the technical solutions described in the aforementioned embodiments, or to make equivalent substitutions for some of the technical features therein. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A microbial soil conditioner for soil chromium pollution remediation, characterized in that: The invention is composed of the following raw materials in parts by weight: 20-30 parts of lactic acid, 20-30 parts of citric acid, 10-25 parts of starch, 15-20 parts of L-lysine, 15-25 parts of sodium tripolyphosphate, 0.1-1 parts of lanthanum nitrate, 20-25 parts of fruit shells, 16-32 parts of microbial agents, 5-20 parts of bone meal, and 1-2 parts of modified polylactic acid; The microbial agent component is a mixture of 5-10 parts of Bacillus lovredoides, 5-10 parts of Bacillus polymyxa and 5-10 parts of Bacillus subtilis; The modified polylactic acid is produced by graft copolymerization of polylactic acid and acrylic acid, and the surface of the modified polylactic acid has hydroxyl, carboxyl and ester functional groups; The preparation method of the microbial agent comprises: S11. placing Bacillus lovrei, Bacillus polymyxa and Bacillus subtilis in a seed culture medium, culturing for 1 to 2 days at a temperature of 25 to 30° C. and a shaking speed of 100 to 200 r / min to obtain a corresponding seed solution; S12. The seed solution was inoculated into the fermentation medium at a volume ratio of 4 to 8%, and cultured under shaking conditions of 25 to 33 ° C and a shaking speed of 100 to 150 r / min for 2 to 4 days to obtain the corresponding fermentation solution; S13. The modified polylactic acid and the fermentation liquid were fully mixed in a mass ratio of 1:2.5, shaken at a temperature of 30°C and a shaker speed of 200 r / min, then placed in a constant temperature incubator at 30°C for fermentation for 48 hours, and finally dried at a constant temperature of 50°C to obtain the corresponding microbial agent.

2. A microbial soil conditioner for soil chromium pollution remediation according to claim 1, characterized in that: The preparation method of the modified polylactic acid comprises: S21. Add the dehydrated L-lactic acid, diphenyl ether and stannous chloride to a three-necked flask and stir thoroughly. Evacuate with a circulating water pump and heat to 150-200°C at a rate of 10°C / min for 3h. S22. After the reaction, the obtained sample was distilled, the solvent was evaporated and a certain amount of acetone was added to dissolve it, and then the obtained solution was precipitated in a large amount of water and filtered. The obtained polylactic acid was vacuum dried to constant weight to obtain a white powdery product; S23. The polylactic acid was dissolved in tetrahydrofuran heated to 50°C, N2 was introduced, the mixture was stirred thoroughly, the temperature was raised to 66°C, constant temperature reflux was started, and acrylic acid and dibenzoyl peroxide were added in 4 portions within 2 min; S24. After the reaction is completed, 50 mL of 95% ethanol solution is added to the reaction product solution, and after a white precipitate appears, the precipitate and the solution are poured into a sand core crucible with constant weight and filtered, and the precipitate is washed twice with 95% ethanol, and then the precipitate and the sand core crucible are placed in a 50 ° C constant temperature oven and dried to constant weight to obtain a crude graft copolymer; S25. Pour the crude graft copolymer into a Soxhelt extractor, use acetone as solvent, extract for 16 hours, reflux 5 to 6 times per hour to remove the homopolymer, and bake the residue after extraction in a constant temperature oven at 50°C to constant weight to obtain a pure graft copolymer, which is modified polylactic acid.

3. The microbial soil conditioner for soil chromium pollution remediation according to claim 1, characterized in that: The components of the seed culture medium are as follows: 10 g sucrose, 5 g peptone, 0.01 g FeSO4·7H2O, 0.5 g MgSO4·7H2O, and 1 L distilled water.

4. The microbial soil conditioner for soil chromium pollution remediation according to claim 1, characterized in that: The ingredients of the fermentation medium are as follows: 20 g corn flour, 10 g sucrose, 2 g peptone, 2 g yeast extract, 2 g NaCl, 0.5 g MgSO4, and 1 L distilled water.

5. A method for preparing a microbial soil conditioner for soil chromium pollution remediation, which is applied to prepare the microbial soil conditioner for soil chromium pollution remediation as claimed in any one of claims 1 to 4, characterized in that: The method comprises the following steps: S31. Preparation of modified polylactic acid; S32. Preparation of microbial agents; S33. Weigh each raw material according to the proportion, and then dry it. After the dried raw materials are fully mixed, add them to the air flow mill, control the fineness of the crushed material to be between 200 and 300 meshes, granulate the crushed material, and obtain a solid microbial soil conditioner.

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