A soil remediation and improvement method

Abstract

The present invention provides a soil remediation and improvement method, including using a biosurfactant to regulate the crystal phase of the mineralization product biocalcium carbonate induced by Bacillus pasteurianus, so that it is transformed from calcite to vaterite. The filtrate after preparing the biocalcium carbonate is used with biochar activated by phosphoric acid, magnesium oxide, etc. to prepare a biochar / magnesium ammonium phosphate composite material, and then the biocalcium carbonate and the biochar / magnesium ammonium phosphate composite material are successively applied to the soil around the mining area contaminated by harmful metal elements. The present invention has a simple process, low cost, no secondary pollution, and has the function of repairing and improving the soil around the mining area contaminated by heavy metals / rare earth elements.

Description

Technical Field

[0001] The invention belongs to the field of soil repair and improvement, and specifically relates to a soil repair and improvement method. Background Art

[0002] Mineral resources are an important material basis for human survival and development, but their exploitation and utilization will bring serious negative impacts on the environment. The main manifestations are: the direct discharge of mining wastewater and mineral processing waste liquid, as well as the stacking and leaching of waste rock and tailings, which lead to the accumulation of a large amount of harmful metal elements in the soil around the mining area, threatening the health of animals, plants and humans, while causing serious soil degradation and crop yield reduction. Therefore, it is urgent to find a technology that can effectively repair and improve the contaminated soil around the mining area.

[0003] At present, the main methods for remediating soil contaminated by harmful metal elements in mining areas include material stabilization technology, plant remediation technology, microbial remediation technology, etc. Due to the drought and little rain in the northwest region, the soil is poor and seriously salinized, making it difficult to promote and apply plant remediation in the region. Adsorption material stabilization technology and microbial improvement technology have the advantages of low cost and green and pollution-free, and have shown good application prospects in soil remediation and improvement in the region.

[0004] Microbially induced carbonate precipitation (MICP) technology has become a hot topic in the field of soil remediation in recent years because of its dual functions of stabilization and microbial remediation. Its principle is to hydrolyze urea through urease produced by microorganisms, change the pH of the system, produce carbonate ions, and then precipitate CaCO3 in the presence of a calcium source. The mineralized product calcium carbonate (CaCO3) formed can stabilize harmful metals in contaminated soil through three mechanisms: adsorption, precipitation conversion, and release of carbonate ions, effectively reducing the mobility of harmful metals in the soil. Because Bacillus pasteurianus has higher urease activity than other strains, the main strain used in MICP technology is Bacillus pasteurianus. In the soil remediation application of this technology, the current main application method is to add bacterial agents, urea and calcium chloride directly to the soil. Since calcium ions have a strong ion exchangeability with metal ions in the soil, directly adding calcium chloride will increase the desorption of harmful metal elements in the soil. In addition, the ammonia produced by the decomposition of urea will form a complex with copper ions, enhancing its mobility. Excessive ammonia will also escape into the air and cause secondary pollution, which greatly limits the application of this technology in soil remediation.

[0005] Moreover, the product induced by MICP technology is calcite-type calcium carbonate. Vaterite and calcite belong to different crystal phases of calcium carbonate crystals. Compared with calcite, vaterite often has a hollow or porous structure, micro-nano particle size, and has higher hydrophilicity, specific surface area and biocompatibility. When added to the soil, it has stronger adsorption stability for harmful metal elements. Therefore, it is necessary to develop a method to regulate Bacillus pasteurianus to induce the transformation of calcium carbonate crystals from calcite to vaterite.

[0006] In addition, the soil around the mining area often has serious soil degradation and soil barrenness, which limits vegetation recovery and reduces grain crop yields. Among the methods for solving soil degradation, the addition of modifiers is one of the effective measures to repair degraded soil and quickly improve the soil environment. However, adding a single material to the soil often does not have the effect of achieving double or multiple repair and improvement, and multiple materials need to be added to achieve the purpose. Biochar is widely used in the repair and improvement of heavy metal contaminated soil as a soil repair modifier, but because biochar will increase soil pH, it is difficult to promote and apply it in alkaline soils in the northwest region. Magnesium ammonium phosphate, as a fertilizer, can be added to the soil to improve soil fertility and promote plant growth. Based on the above, the idea of ​​the present invention is to use MICP technology to induce the production of calcium carbonate, and to adjust its crystal form by adding biosurfactants to transform it from calcite to vaterite, and to obtain biological calcium carbonate by filtration and separation. On this basis, porous biochar was prepared by activating biomass with phosphoric acid to improve its adsorption performance and neutralize its surface alkalinity, which was then added to the MICP filtrate. Magnesium ammonium phosphate was synthesized by adding magnesium oxide. The separated bio-calcium carbonate and biochar / magnesium ammonium phosphate composite materials were added to the soil to control the migration of harmful metal elements while achieving the purpose of increasing soil enzyme activity and fertilization (providing C, N, and P sources for the soil). Summary of the invention

[0007] In order to solve the above problems in the prior art, the present invention provides a soil remediation and improvement method, comprising the following steps:

[0008] (1) inoculating Bacillus pasteurianus into a culture medium that has been sterilized by high pressure, then adding a sterilized biosurfactant, calcium chloride, and urea, culturing the culture medium on a shaking table at a certain temperature and speed, filtering to obtain a precipitate and a filtrate, and drying and grinding the precipitate to obtain biological calcium carbonate;

[0009] (2) uniformly mixing straw powder prepared from straw with a phosphoric acid solution for activation treatment, pyrolyzing the mixture in a tubular furnace at a certain pyrolysis temperature, adding the pyrolysis product to the filtrate of step (1), adjusting the pH value with magnesium oxide, fully mixing and stirring the mixture, and filtering to obtain a biochar / magnesium ammonium phosphate composite material;

[0010] (3) applying the biological calcium carbonate to soil, and then stirring the soil to make it uniform;

[0011] (4) applying the biochar / magnesium ammonium phosphate composite material to the soil, and stirring the soil again to make it uniform;

[0012] (5) Stirring the soil at regular intervals, and repeating the stirring multiple times.

[0013] As a preferred embodiment, the culture medium components are: 15 g / L molasses, 10 g / L peptone, 10 g / L sodium chloride, and 0.5 g / L dipotassium hydrogen phosphate.

[0014] As a preferred embodiment, the biosurfactant is sophorolipid or rhamnolipid or trehalose lipid, and the concentration is 1-2×10 -3 mol / L.

[0015] As a preferred solution, the concentration of the phosphoric acid solution is 40% to 60%; and the pyrolysis temperature is 600°C to 800°C.

[0016] As a preferred embodiment, the straw includes one or more of sunflower straw, corn straw, and wheat straw.

[0017] As a preferred solution, the time is 5 to 10 days, and the stirring step is repeated 3 to 5 times.

[0018] Therefore, the soil improvement method of the present invention has at least the following beneficial effects:

[0019] 1. The present invention applies biosurfactant to microbial induced carbonate deposition technology, so that the main product of Bacillus pasteurianus is changed into vaterite, and the content of organic components in CaCO3 is increased to form an inorganic-organic hybrid structure, which is green and environmentally friendly and has no secondary pollution. In addition, biosurfactant has rich functional groups, which can be doped in calcium carbonate to enhance its adsorption of harmful metal elements. The biological calcium carbonate preparation method of the present invention has a simple process, can in-situ passivate harmful metal elements in the soil, has a strong repair ability to the soil, and has a long lasting effect.

[0020] 2. The present invention applies separately prepared bio-calcium carbonate to soil remediation, which not only avoids the problems caused by ammonia produced by hydrolysis of calcium chloride and urea when microorganisms, urea and calcium chloride are directly added to the soil in the prior art, but also makes it possible to prepare biochar / magnesium ammonium phosphate composite materials using ammonia produced by urea hydrolysis, thereby improving resource utilization and saving costs.

[0021] 3. The present invention mixes a certain proportion of biochar activated by phosphoric acid, the filtrate produced in the biological calcium carbonate stage, and ammonium magnesium oxide to react to prepare a biochar-loaded magnesium ammonium phosphate composite material, which has the following outstanding advantages: (1) the ammonia in the filtrate remaining after the preparation of biological calcium carbonate is used to provide ammonia ions for the preparation of magnesium ammonium phosphate, thereby improving the utilization of ammonia and reducing costs; (2) phosphoric acid is used to activate biomass to prepare porous biochar, increase the pore structure of biochar, make it have better adsorption effect, and better improve soil structure and improve soil water and fertilizer utilization efficiency; (3) phosphoric acid activates biomass and converts it into thermal phosphoric acid, which is used as a phosphorus source for magnesium ammonium phosphate, thereby improving the utilization of phosphoric acid, reducing costs, and saving the step of mixing biochar and magnesium ammonium phosphate. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1a It is a chemically synthesized calcium carbonate.

[0023] Figure 1b The biological calcium carbonate prepared by adding sophorolipids according to the soil improvement method of the present invention.

[0024] Figure 2a This is a SEM image of biological calcium carbonate prepared according to the soil improvement method of the present invention without adding a biosurfactant.

[0025] Figure 2b This is a SEM image of biological calcium carbonate prepared by adding sophorolipids according to the soil improvement method of the present invention.

[0026] Figure 2c This is a SEM image of biological calcium carbonate prepared by adding rhamnolipid according to the soil improvement method of the present invention.

[0027] Figure 3 XRD diagram of biological calcium carbonate prepared by adding different concentrations of sophorolipids according to the soil improvement method of the present invention.

[0028] Figure 4a This is the SEM image of ordinary biochar.

[0029] Figure 4b This is a SEM image of the biochar / magnesium ammonium phosphate composite material prepared according to the soil improvement method of the present invention. DETAILED DESCRIPTION

[0030] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the accompanying drawings.

[0031] Prepare the culture medium, the composition of which is molasses 15g / L, peptone 10g / L, sodium chloride 10g / L, potassium dihydrogen phosphate 0.5g / L, and after high-pressure sterilization, dispense into different Erlenmeyer bottles, then add sterilized biosurfactant, calcium chloride and urea on the clean bench, and culture in a shaker at 30°C and 150rpm. After 7 days of culture, filter to obtain the precipitate and filtrate, store the filtrate for later use, dry and grind the precipitate, and put the ground biocalcium carbonate into a sample bag and mark it. Add biosurfactant as a template to regulate the morphology and crystal form of calcium carbonate.

[0032] In the biological calcium carbonate prepared according to the method of the present invention in FIG1 , the concentration of sophorolipid added is 1×10 -3 mol / L.

[0033] As shown in Figure 2, after adding the biosurfactants sophorolipids and rhamnolipids, the morphology of calcium carbonate is obviously spherical, which is consistent with the morphological characteristics of vaterite calcium carbonate.

[0034] Figure 3 The XRD patterns show the intensity of calcite and vaterite in the biological calcium carbonate prepared with the above-mentioned different concentrations of sophorolipids. It can be seen that without adding sophorolipids, the main diffraction peaks of biological calcium carbonate are at 2θ / (°)=24.8°, 29.28°, 39.34°, 43.74°, 55.76°, and the main crystal form under this condition is calcite; at 2×10 -3 Under the condition of 1×10 -3 The main crystal form under the condition of 1.5 mol / L concentration of sophorolipids is also vaterite. After comparison, it was found that the crystal form of calcium carbonate gradually changed from calcite to vaterite, indicating that sophorolipids can regulate the crystal form of biological calcium carbonate and is conducive to the formation of vaterite calcium carbonate.

[0035] Corn stalks (which may also be sunflower stalks, wheat stalks and other similar stalks) were washed, naturally air-dried, crushed and passed through a 100-mesh sieve to obtain the test stalk powder, which was then bagged for later use. A certain amount of corn stalk powder was mixed with an equal weight of 40% phosphoric acid solution for activation treatment. The mixture was pyrolyzed in a tube furnace at 600°C in an oxygen-limited environment for 2 hours to obtain a pyrolysis product. The pyrolysis product was added to the filtrate stored after the above-mentioned preparation of biological calcium carbonate in a certain proportion, and the pH was adjusted to about 9 with light magnesium oxide. The mixture was fully mixed and stirred for a period of time, and filtered to obtain a biochar / magnesium ammonium phosphate composite material (BC / P). Thereafter, the morphology of the biochar / magnesium ammonium phosphate composite material (BC / P) was characterized using a scanning electron microscope (SEM). As shown in Figure 4, after loading magnesium ammonium phosphate, aggregated magnesium ammonium phosphate particles appeared on the surface of the biochar, and the biochar still had a developed void structure.

[0036] In another embodiment, a certain amount of corn stalk powder is mixed with a 60% phosphoric acid solution by weight for activation treatment, and the mixture is pyrolyzed in a tube furnace at 700° C. in an oxygen-limited environment for 2 hours.

[0037] In another embodiment, a certain amount of corn stalk powder is mixed with a 50% phosphoric acid solution by weight for activation treatment, and the mixture is pyrolyzed in a tube furnace at 800° C. in an oxygen-limited environment for 2 hours.

[0038] It should be noted that during the preparation of biological calcium carbonate, urease hydrolyzes urea to produce a large amount of NH4 + Therefore, the filtrate obtained by the final filtration contains a large amount of NH4 + , these NH4 + It reacts with magnesium oxide and phosphoric acid to form magnesium ammonium phosphate.

[0039] The soil in this embodiment was taken from the surface soil around a mining area. After natural air drying, the coarse impurities were initially removed and passed through a 100-mesh sieve for standby use. The pH of the soil was 8.23, the organic matter was 27.04%, the Pb and Cd contents were 604.65 mg / kg and 11.34 mg / kg respectively, and the mobile state contents were 70.6 mg / kg and 3.08 mg / kg; both elements exceeded the risk screening values ​​of my country's farmland soil environmental quality standards (Pb and Cd were 240 mg / kg and 0.8 mg / kg respectively); the rare earth elements Ce and La contents were 628.34 mg / kg and 438.69 mg / kg respectively; the mobile state contents were 122.38 mg / kg and 68.56 mg / kg.

[0040] Then, the bio-calcium carbonate and biochar / magnesium ammonium phosphate composite material of the present invention are applied to the soil at a certain concentration ratio. 20 、M 35 、M 50The CK group was a blank control group without any soil remediation materials added; the M0 group was a control group of the prior art, in which microorganisms, urea and calcium chloride were directly added to the soil in a certain proportion at a concentration of 20 g / kg and mixed and cultured for a period of time; 20 、M 35 、M 50 In order to add the bio-calcium carbonate and biochar / magnesium ammonium phosphate composite material (BC / P) (the two are collectively referred to as "soil remediation improver") of the present invention, the addition concentrations are 20g / kg, 35g / kg, and 50g / kg, respectively, and the specific steps are as follows:

[0041] 1. Mix the bio-calcium carbonate of the present invention with the contaminated soil and stir them thoroughly;

[0042] 2. Apply the biochar / magnesium ammonium phosphate composite material (BC / P) of the present invention in an amount equal to that of biological calcium carbonate to the soil in step 1, and stir evenly again;

[0043] 3. Stir once every 5 days, repeat 3 times;

[0044] 4. Take samples, dry and grind the soil samples for later use.

[0045] Among them, the stirring interval and multiple repetitions are for the purpose of fully reacting the biological calcium carbonate and harmful metals. Theoretically, the longer the time and the more repetitions, the more complete the reaction. However, considering the cost and benefit, after experiments, the preferred interval is 5-10 days and the repeated stirring times are 3 to 5 times, which not only ensures that the biological calcium carbonate can effectively treat the harmful metals, but also avoids excessive time and high costs.

[0046] The effective contents of heavy metals and rare earth elements in the soil of each treatment group were determined, and the enzyme activity in the soil was determined at regular intervals. The results are shown in Tables 1, 2 and 3, respectively.

[0047]

[0048] Table 1 Available Pb and Cd contents in each treatment group

[0049]

[0050] Table 2 Effective Ce and La contents in each treatment group

[0051]

[0052] Table 3 Enzyme activity test results of each treatment group

[0053] Result analysis: The soil remediation and improvement method of the present invention can effectively reduce the effective Pb and Cd contents of heavy metals in the soil (see Table 1); the rare earth elements Ce and La contents (see Table 2), and the reduction ratio increases with the increase of the application ratio. Therefore, the soil remediation and improvement method of the present invention has important functions such as reducing the mobility and bioavailability of heavy metals and rare earth elements in the soil. In addition, it can be seen from the enzyme activity data (Table 3) that after adding the repair and improvement agent, the urease activity in the soil is enhanced and the hydrogen peroxide activity is reduced to a certain extent. Therefore, the soil remediation and improvement method of the present invention has a very obvious effect on soil fertility.

[0054] The soil after using the soil remediation and improvement method of the present invention can effectively promote plant growth. The effect of the soil remediation and improvement method of the present invention is further illustrated by planting plants in the soil, which includes the following steps:

[0055] 1. Soil treatment: Take the above CK, M 20 、M 35 、M 50 1.5 kg of the cultivated soil from each of the four treatment groups was placed in a flower pot with a diameter of 15 cm and a height of 20 cm, and an appropriate amount of water was added to balance it for one week.

[0056] 2. Sowing: After the alfalfa seeds are disinfected and germinated, 50 full and uniform seeds are sown in each flower pot. Cover with 2 cm of soil after sowing. To ensure the soil moisture during the experiment, water the seeds regularly and quantitatively every day until the day before harvest.

[0057] 3. Harvest: The plants were harvested after 60 days of normal growth. Five representative plants were selected from each pot to measure their growth parameters, including the biomass of the aboveground part of the plant, as well as the plant height and root length of each plant.

[0058] Results: After 60 days of growth, the indicators of alfalfa in the corresponding pots of the four treatment groups increased to varying degrees. 50 The biomass indexes under the treatment reached the maximum value. 20 、M 35 、M 50 The plant height of the groups increased by 60.63%, 93.75%, and 143%, the root length increased by 47.27%, 59.09%, and 61.81%, and the aboveground biomass increased by 47.19%, 50.56%, and 53.93%, respectively. This indicates that the soil remediation improver obtained by the soil remediation method of the present invention can make alfalfa germinate and grow, has a significant growth-promoting effect on the plant height and root length of alfalfa, and has a more obvious effect on the biomass weight increase of the plant.

[0059] Therefore, the soil remediation and improvement method of the present invention has important functions such as promoting plant growth and effectively increasing plant biomass, solving the problem of improving soil quality, especially having a good remediation effect in the soil around the mining area contaminated by harmful metals, and has long-term development prospects.

Claims

1. A soil remediation and improvement method, characterized in that: The following steps are involved: (1) Inoculating Bacillus pasteurianus into a culture medium that has been sterilized by high pressure, then adding a sterilized biosurfactant, calcium chloride, and urea, culturing the culture medium on a shaker at a certain temperature and speed, filtering to obtain a precipitate and a filtrate, and drying and grinding the precipitate to obtain biological calcium carbonate, wherein the biosurfactant is a sophorolipid, a rhamnolipid, or a trehalose lipid, and the concentration is 1 to 2×10 -3 mol / L; (2) uniformly mixing straw powder prepared from straw with a phosphoric acid solution for activation treatment, pyrolyzing the mixture in a tubular furnace at a certain pyrolysis temperature to obtain biochar, adding the biochar to the filtrate of step (1), adjusting the pH value with magnesium oxide, fully mixing and stirring the mixture, and filtering to obtain a biochar / magnesium ammonium phosphate composite material; (3) applying the biological calcium carbonate to the soil and then stirring it evenly; (4) applying the biochar / magnesium ammonium phosphate composite material to the soil and stirring again; (5) Stirring the soil at regular intervals, and repeating the stirring multiple times.

2. The soil remediation and improvement method according to claim 1, characterized in that: The culture medium comprises: 15 g / L molasses, 10 g / L peptone, 10 g / L sodium chloride, and 0.5 g / L dipotassium hydrogen phosphate.

3. The soil remediation and improvement method according to claim 1, characterized in that: The concentration of the phosphoric acid solution is 40% to 60%; the pyrolysis temperature is 600°C to 800°C.

4. The soil remediation and improvement method according to claim 1, characterized in that: The magnesium oxide is light magnesium oxide.

5. The soil remediation and improvement method according to claim 1, characterized in that: The straw includes one or more of sunflower straw, corn straw, and wheat straw.

6. The soil remediation and improvement method according to claim 1, characterized in that: The time is 5 to 10 days, and the number of repeated stirring is 3 to 5 times.

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