A method for fixing heavy metals based on biochar

Abstract

The present invention discloses a method for fixing heavy metals based on biochar, which comprises the following steps: (1) mixing biochar with contaminated water or contaminated soil to allow the biochar to capture heavy metals; (2) mixing a bacterial solution containing urease bacteria with the biochar with heavy metals captured in step (1) to allow the biochar to capture the urease-producing bacteria in the bacterial solution; and (3) mixing a binding liquid with the biochar with urease-producing bacteria captured in step (2), allowing the mixture to react with the binding liquid, wherein the binding liquid comprises calcium chloride and urea. The present invention improves the efficiency and long-term effectiveness of the biochar fixation of heavy metals and reduces the risk of secondary pollution after the biochar fixation of heavy metals.

Description

Technical Field

[0001] The present invention belongs to the technical field of heavy metal treatment, and specifically relates to a method for fixing heavy metals based on biochar. Background Art

[0002] Over the past few decades, heavy metal pollution in water, air, and soil has been widely reported. Through human production and daily activities, such as mining, smelting, waste incineration, and automobile exhaust, heavy metal pollutants are transferred to the biosphere, where organisms depend for survival. This has become a crucial part of the geochemical cycle of heavy metal pollutants. Humans can obtain heavy metals from nature through various pathways, but they generally lack the ability to biodegrade them, leading to their accumulation in the body and further health problems. Therefore, heavy metal pollutants pose a significant threat to both humans and the ecological environment. Controlling heavy metal pollution in water and soil is urgent, and the search for effective and long-term control methods has attracted the attention of scientists and engineers worldwide.

[0003] In the context of sustainable development, achieving long-term, effective biochar remediation not only reduces national investment costs for heavy metal pollution control but also aligns with green and environmentally friendly development, achieving both ecological and economic benefits. Currently, solidification / stabilization technology is the most widely used. This technology originated in the late 1950s, initially for sludge treatment and later for soil remediation. Since the 1990s, it has gained popularity as a heavy metal remediation technology in countries such as the United States, Canada, the United Kingdom, France, and the Netherlands. Since 2017, China's heavy metal remediation market has flourished, with solidification / stabilization technology leading the way. The effectiveness of this technology is highly dependent on the performance of the additives used to fix the metals in the soil. Biochar, due to its high affinity for heavy metals, low carbon footprint, and low cost, has become one of the most popular additives. A large portion of biochar's heavy metal fixation occurs through physical adsorption and cation exchange. These fixed heavy metal ions are easily bioavailable, have poor stability, and are significantly affected by environmental pH. Therefore, further improving the efficiency and long-term effectiveness of biochar for heavy metal fixation has been a research challenge in this field. Summary of the Invention

[0004] In response to the above technical problems, the present invention provides a method for fixing heavy metals based on biochar, which improves the efficiency and long-term effectiveness of biochar fixing heavy metals and reduces the risk of secondary pollution after biochar fixing heavy metals.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

[0006] A method for fixing heavy metals based on biochar, comprising the following steps:

[0007] (1) mixing biochar with contaminated water or contaminated soil to allow the biochar to capture heavy metals;

[0008] (2) mixing the bacterial solution containing urease-producing bacteria with the biochar having captured heavy metals in step (1), so that the biochar captures the urease-producing bacteria in the bacterial solution;

[0009] (3) mixing the cementing liquid with the biochar containing urease-producing bacteria captured in step (2), and reacting the urease-producing bacteria with the cementing liquid, wherein the cementing liquid comprises calcium chloride and urea.

[0010] Among them, microbial induced calcium carbonate precipitation (MICP) technology is used to improve the stability of biochar in fixing heavy metals.

[0011] Preferably, in step (3), the ratio of the weight of the biochar to the volume of the binder is 1 g: (200-400) mL, preferably 1 g: (250-300) mL. In a specific embodiment, the ratio of the weight of the biochar to the volume of the binder is 1 g: 300 mL.

[0012] Preferably, in step (3), the binder is a mixed solution of calcium chloride and urea, the concentration of calcium chloride is 0.05-0.5 mol / L, and the concentration of urea is 0.25-0.5 mol / L. The binder is an aqueous solution, and the solute is composed of calcium chloride and urea. A relatively high urea concentration and a moderate calcium chloride concentration are suitable binder concentrations, which can encourage microorganisms to form a relatively ideal carbonate precipitation protective shell on the surface of biochar. In a specific embodiment, the calcium chloride concentration is 0.3 mol / L and the urea concentration is 0.5 mol / L. Under these concentration conditions, the MICP technology plays the greatest role.

[0013] Preferably, in step (3), the biochar is immersed in the binder solution and placed in a constant temperature shaking incubator at 25-35°C for shaking. The total shaking time and the resting time are 12-24 hours. The speed of the constant temperature shaking incubator should be controlled to 100-150 rpm, the temperature is preferably 30°C, and the shaking time is controlled to 12 hours to ensure that the biochar uniformly captures the bacteria. It is further explained that the activity of Bacillus pasteurii is most ideal at 30°C. If other bacterial species are selected, the temperature during the soaking process needs to be adjusted according to their characteristics. After the shaking is completed, the biochar is allowed to rest for 12 hours to ensure that the bacteria are firmly attached to the biochar, the biochar fully captures the urease-producing bacteria in the bacterial solution, and the biochar is retained in place. The total shaking time and the resting time are less than or equal to 24 hours. It is further explained that the shaking and resting time can be appropriately adjusted according to the characteristics of the biochar and the type of heavy metal, but the total treatment time should not exceed 24 hours to prevent a significant decrease in bacterial activity and affect the subsequent microbial-induced calcium carbonate precipitation effect. Alternatively, in step (3), the binder solution is sprayed into the contaminated soil treated in steps (1) and (2), stirred and allowed to stand for no more than 24 hours before proceeding to the subsequent steps.

[0014] Preferably, in step (2), the urease-producing bacteria in the bacterial solution are selected from Sporosarcina pasteurii, and the liquid culture medium of the bacterial solution includes ammonium sulfate, yeast extract powder and tris(hydroxymethylaminotoluene). Specifically, the liquid culture medium includes 10 g / L ammonium sulfate, 20 g / L yeast extract powder and 15.73 g / L tris(hydroxymethylaminotoluene). After the bacterial strain is inoculated into the liquid culture medium at a volume fraction of 1 to 5%, it is aerobically cultured for 24 hours at 30°C and 200 rpm in a constant temperature shaking incubator. After 24 hours of aerobic culture, the bacterial solution must be immediately used for the soaking process. The initial conductivity of the bacterial solution is 9.6 mmol / L / min, and the initial OD600 value is 1.1. After the soaking process, the conductivity of the bacterial solution is 9.9 mmol / L / min, and the OD600 value is 1.2. It is further explained that a moderate concentration of heavy metals can ensure good bacterial activity. If the heavy metal concentration is too high, it may seriously affect the bacterial activity and even cause bacterial death. If necessary, the heavy metal concentration needs to be diluted before the bacterial solution is soaked.

[0015] Preferably, in step (2), the biochar containing heavy metals is removed from the contaminated water body and soaked in the bacterial solution. The speed of the constant temperature shaking incubator should be controlled to 100-150 rpm, the temperature is preferably 30°C, and the shaking time is controlled to 12 hours to ensure that the biochar uniformly captures the bacteria. It is further explained that the activity of Bacillus pasteurianus is most ideal at 30°C. If other bacterial species are selected, the temperature during the soaking process needs to be adjusted according to their characteristics. After the shaking is completed, the biochar is allowed to stand for 12 hours to ensure that the bacteria are firmly attached to the biochar, the biochar fully captures the urease-producing bacteria in the bacterial solution, and the biochar is retained in place, and the sum of the shaking time and the standing time is less than or equal to 24 hours. It is further explained that the shaking and standing time can be appropriately adjusted according to the characteristics of the biochar and the type of heavy metal, but the total treatment time should not exceed 24 hours to prevent a significant decrease in bacterial activity and affect the subsequent microbial-induced calcium carbonate precipitation effect. Alternatively, in step (2), the bacterial solution is sprayed onto the contaminated soil treated in step (1), stirred, and allowed to stand for no more than 24 hours before proceeding to step (3).

[0016] Preferably, in step (1), biochar is added to the contaminated water body, and the ratio of the weight of the biochar to the volume of the contaminated water body is 1g: (150-300) mL, preferably 1g: 200 ml, and the mixture is shaken in a constant temperature shaking incubator for 24 hours, wherein the temperature is set to 25°C and the rotation speed is 250 rpm to ensure that the biochar reaches a fixed equilibrium and the biochar that reaches a fixed equilibrium is retained in place; or, in step (1), the biochar is evenly spread on the surface of the contaminated soil body, and the ratio of the weight of the biochar to the area of ​​the contaminated soil body is 1g: (0.4-0.8) m 2 Use a tiller to mix the biochar and contaminated soil evenly, and proceed to the subsequent steps after the balance is fixed.

[0017] Preferably, the biochar is a powder or granules obtained by pyrolysis and grinding of plant straw. Specifically, the biochar is prepared by pyrolysis and grinding of plant straw under high temperature conditions to obtain powder or granules. In this embodiment, agricultural waste rice straw is preferably used as the biochar raw material. The dried and crushed rice straw is heated at a rate of 10°C / min to a specified temperature (preferably 300-700°C) using a muffle furnace in an oxygen-deficient environment and maintained at a constant temperature for 1.5 hours. The resulting biochar is ground and passed through a 70-mesh sieve.

[0018] In a specific and preferred embodiment, the treatment of polluted water based on the above method is as follows:

[0019] S101, adding biochar to a polluted water body and shaking the water body to allow the biochar to capture heavy metals in the water body;

[0020] S102, removing the biochar from the water body, soaking the biochar in a bacterial solution containing Sporosarcina pasteurianus, and placing the biochar in a constant temperature shaking incubator at 25-35° C. for shaking and standing, wherein the sum of the shaking time and the standing time is less than or equal to 24 hours, so that the biochar captures Sporosarcina pasteurianus;

[0021] S103. Immerse the biochar in the binding liquid, place it in a constant temperature shaking incubator at 25-35°C for shaking, and the total shaking time and standing time is 12-24 hours, so that the pasteurian sporosarcina in the biochar reacts with the binding liquid, wherein the binding liquid is a mixed solution of calcium chloride and urea, the concentration of calcium chloride is 0.05-0.5 mol / L, and the concentration of urea is 0.25-0.5 mol / L.

[0022] In another specific and preferred embodiment, the treatment of contaminated soil based on the above method is as follows:

[0023] S201, evenly spreading biochar on the surface of the contaminated soil and stirring, so that the biochar captures heavy metals in the soil;

[0024] S202, spraying a bacterial solution containing Sporosarcina pasteuriana into the contaminated soil, stirring, and allowing to stand for no more than 24 hours, so that the biochar in the soil can capture the Sporosarcina pasteuriana;

[0025] S203. Spray the binder liquid into the contaminated soil and stir to allow the Sporosarcina pasteurii in the biochar to react with the binder liquid, wherein the binder liquid is a mixed solution of calcium chloride and urea, the concentration of calcium chloride is 0.05-0.5 mol / L, and the concentration of urea is 0.25-0.5 mol / L.

[0026] The beneficial effects of the present invention are as follows:

[0027] In the present invention, biochar is first placed in a heavy metal-contaminated water body or soil body, and after reaching a fixed equilibrium, the biochar is treated with a bacterial solution containing urease-producing bacteria, and the biochar absorbs the bacterial solution. Finally, a binder solution is used to introduce urease-producing bacteria into the biochar that has fully fixed the heavy metals and fully absorbed the bacterial solution, thereby inducing the precipitation of carbonates in the micropores of the biochar, thereby reinforcing the heavy metals adsorbed in the micropores of the biochar, greatly improving the efficiency and long-term effectiveness of biochar in fixing heavy metals, and reducing the risk of secondary pollution after biochar fixes heavy metals. The biochar treated in the water body can be recycled and reused, and the biochar treated in the soil body can increase soil fertility, which is in line with the low-carbon and environmentally friendly ecological development concept. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the technical solution of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0029] Figure 1 A schematic diagram of a process for treating polluted water bodies according to an embodiment of the present invention;

[0030] Figure 2 A schematic diagram of a process for treating contaminated soil based on a method according to an embodiment of the present invention;

[0031] Figure 3 This is a schematic diagram of the actual state of the biochar at lead fixation equilibrium before and after MICP treatment in Example 1 of the present invention;

[0032] Figure 4 Graph showing the lead extraction test results of biochar without MICP treatment, treatment in Example 1, and treatment in Example 2. DETAILED DESCRIPTION

[0033] The preferred embodiments of the present invention are described in detail below in conjunction with the accompanying drawings so that the advantages and features of the present invention can be more easily understood by those skilled in the art. It should be noted that the description of these embodiments is used to help understand the present invention, but does not constitute a limitation of the present invention.

[0034] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains; the term "and / or" used herein includes any and all combinations of one or more of the associated listed items.

[0035] This embodiment provides a method for fixing heavy metals based on biochar, which uses microbial induced calcium carbonate precipitation (MICP) technology to improve the stability of biochar-fixed heavy metals. MICP technology uses microorganisms and calcium ions that are widely present in the environment as raw materials and has the advantage of being green and environmentally friendly. Its core is to use urease-producing bacteria (such as Pasteurella) isolated from natural soil environments to hydrolyze urea and react with Ca through a series of reactions (Formula (1-5)). 2+ Forming carbonates.

[0036] CO(NH2)2+2H2O→H2CO3+2NH3 (1)

[0037]

[0038]

[0039]

[0040]

[0041] The primary processes of MICP heavy metal immobilization include encapsulation and precipitation, which are significantly more effective than physical adsorption and cation exchange. MICP technology leverages microbial reactions to create a calcium carbonate surface layer on the biochar at equilibrium, creating a protective barrier that resists environmental fluctuations and achieves long-term heavy metal immobilization.

[0042] The above method comprises the following steps:

[0043] Step 1: Mix biochar with polluted water or soil to allow the biochar to capture heavy metals;

[0044] Specifically, plant straw is pyrolyzed and ground under high temperature conditions to obtain powder or particles to make biochar. In this embodiment, agricultural waste rice straw is preferably used as the biochar raw material. The dried and crushed rice straw is heated to a specified temperature (preferably 300-700°C) at a rate of 10°C / min using a muffle furnace in an oxygen-deficient environment and kept at a constant temperature for 1.5 hours. The obtained biochar is ground and passed through a 70-mesh sieve. The biochar is added to the contaminated water body, with the ratio of the weight of the biochar to the volume of the contaminated water body being 1g: (150-300)mL, preferably 1g: 200ml, and shaken in a constant temperature shaking incubator for 24 hours, wherein the temperature is set to 25°C and the rotation speed is 250rpm to ensure that the biochar reaches a fixed equilibrium, and the biochar that reaches the fixed equilibrium is retained in place.

[0045] If the soil is contaminated, the biochar should be evenly spread on the soil surface. The ratio of the weight of the biochar to the area of ​​the contaminated soil is 1g: (0.4-0.8)m 2 Use a tiller to mix the biochar and contaminated soil evenly, and proceed to the subsequent steps after the balance is fixed.

[0046] In this embodiment, lead, which is highly harmful, difficult to control, and has a wide range of pollution, is selected as the heavy metal to be fixed. The lead-contaminated solution contains Pb(NO3)2 and NaNO3. The concentration of Pb(NO3)2 is preferably controlled at 3 to 20 mmol / L, and the concentration of NaNO3 is preferably controlled at 0.01 mol / L. In actual application, the composition and concentration of heavy metal pollution shall be based on the actual pollution situation. The ability of biochar to fix heavy metals is affected by raw materials, pyrolysis temperature, type of heavy metal ions, etc. It is necessary to select suitable biochar to fix heavy metals to ensure that the biochar achieves a good heavy metal capture effect, so as to smoothly carry out the subsequent steps of MICP treatment.

[0047] Step 2: mixing the bacterial solution containing urease-producing bacteria with the biochar containing heavy metals captured in Step 1, so that the biochar captures the urease-producing bacteria in the bacterial solution;

[0048] In this embodiment, the urease-producing bacteria in the bacterial solution are selected from Sporosarcina pasteurii, and the liquid culture medium used in the bacterial solution includes 10g / L ammonium sulfate, 20g / L yeast extract powder and 15.73g / L tris(hydroxymethyl)aminotoluene. After the strain is inoculated into the liquid culture medium that has been fully sterilized at high temperature at a volume fraction of 1-5%, it is aerobically cultured for 24 hours at 30°C and 200rpm in a constant temperature shaking incubator. After 24 hours of aerobic culture, the bacterial solution must be used for the soaking process immediately. The initial conductivity of the bacterial solution is 9.6mmol / L / min, and the initial OD 600 The conductivity of the bacterial solution after the soaking process was 9.9mmol / L / min, and the OD 600 The value is 1.2. It further illustrates that a moderate concentration of heavy metals can ensure good bacterial activity. If the heavy metal concentration is too high, it may seriously affect bacterial activity or even cause bacterial death. If necessary, the heavy metal concentration needs to be diluted before the bacterial solution is immersed.

[0049] The heavy metal-captured biochar was removed from the contaminated water, soaked in a bacterial solution, and then shaken in a constant-temperature shaker at 25-35°C. The speed should be controlled between 100 and 150 rpm, the temperature preferably at 30°C, and the shaking time should be controlled for 12 hours to ensure uniform bacterial capture. Furthermore, the activity of Bacillus pasteurii Sporosarcina is optimal at 30°C. If other bacterial species are used, the soaking temperature should be adjusted according to their characteristics. After shaking, the biochar was allowed to rest for 12 hours to ensure that the bacteria were firmly attached to the biochar, that the biochar fully captured the urease-producing bacteria in the bacterial solution, and that the biochar was retained in place. The total shaking and resting time should be less than or equal to 24 hours. Furthermore, the shaking and resting times can be adjusted appropriately based on the biochar's characteristics and the type of heavy metal involved, but the total treatment time should not exceed 24 hours to prevent a significant decrease in bacterial activity, which could affect the subsequent microbial-induced calcium carbonate precipitation.

[0050] If the soil is contaminated, the bacterial solution must be sprayed into the contaminated soil treated in step 1, stirred, and allowed to stand for no more than 24 hours before proceeding to the subsequent steps.

[0051] Step 3: Mix the cementing liquid with the biochar containing urease-producing bacteria captured in step 2, and let it stand to react, so that the urease-producing bacteria and the cementing liquid react.

[0052] The binder is a mixture of calcium chloride and urea solutions, with the concentration of the calcium chloride solution ranging from 0.05 to 0.5 mol / L and the concentration of the urea solution ranging from 0.25 to 0.5 mol / L. Furthermore, a high urea concentration and a moderate calcium chloride concentration are the optimal binder concentrations, encouraging microorganisms to form an ideal carbonate precipitate protective shell on the biochar surface. The optimal calcium chloride concentration is 0.3 mol / L and the urea concentration is 0.5 mol / L, at which the MICP technique performs best.

[0053] The ratio of the weight of biochar to the volume of the binder solution is 1g:(200-400)mL, preferably 1:300. The biochar, after capturing bacteria in step 2, is soaked in the binder solution and placed in a constant-temperature shaker at 25-35°C. The speed of the shaker should be controlled to 100-150 rpm, or even lower, to ensure that calcium carbonate is firmly precipitated on the biochar. The temperature is preferably 30°C to ensure bacterial activity. Furthermore, Sporosarcina pasteurii is most active at 30°C. If other bacterial species are used, the soaking temperature should be adjusted according to their characteristics. The total shaking time and resting time is 12-24 hours. If the bacterial activity on the biochar is strong or the binder solution is fast, the shaking and resting times can be shortened. In a specific embodiment, Sporosarcina pasteurii is used, the urea concentration is controlled at 0.25 mol / L, the calcium chloride concentration is controlled at 0.3 mol / L, the shaking time is controlled to 12 hours, and the resting time is 12 hours.

[0054] In soil-contaminated environments, the bacterial solution and binder solution must be sprayed onto the soil surface in sequence, and then mixed evenly with the contaminated soil using a tiller. Once the biochar is bonded, the calcium carbonate precipitate formed on the surface effectively protects the biochar from environmental changes, especially acid rain, thereby enhancing the long-term effectiveness of the biochar in fixing heavy metals. Furthermore, biochar can improve soil fertility, and the cementing effect of calcium carbonate can also increase soil strength.

[0055] Furthermore, the bacterial solution in step 2 and the cementing solution in step 3 may cause heavy metals immobilized on the biochar to migrate into the solution, posing a certain contamination risk. Controlling the cementing solution concentration is one way to reduce this risk. In known implementations, the lowest lead migration rate can be controlled to 5.72%.

[0056] In summary, the above method can be used to treat polluted water, remove heavy metals from it, and fix heavy metals in biochar through MICP enhancement. Figure 1 As shown, the treatment process of polluted water based on the above method specifically includes:

[0057] S101, adding biochar to the polluted water body and shaking it to allow the biochar to capture heavy metals in the water body;

[0058] S102, removing the biochar from the water body, soaking the biochar in a bacterial solution containing Sporosarcina pasteuriana, and shaking the biochar in a constant temperature shaker at 25-35° C., and allowing the shaking time and the standing time to stand for less than or equal to 24 hours, so that the biochar captures Sporosarcina pasteuriana;

[0059] S103. Soak the biochar in a binder, place it in a constant temperature shaking incubator at 25-35°C for oscillation, and the total oscillation time and standing time is 12-24 hours, so that the pasteurian sporosarcina in the biochar reacts with the binder, wherein the binder is a mixed solution of calcium chloride and urea, the concentration of calcium chloride is 0.05-0.5 mol / L, and the concentration of urea is 0.25-0.5 mol / L.

[0060] Based on the above method, polluted water bodies can be treated to remove heavy metals and fix the heavy metals in biochar.

[0061] like Figure 2 As shown, the treatment process of polluted water based on the above method specifically includes:

[0062] S201, evenly spreading biochar on the surface of the contaminated soil and stirring, so that the biochar captures heavy metals in the soil;

[0063] S202, spraying a bacterial solution containing Sporosarcina pasteuriana into the contaminated soil, stirring, and allowing to stand for no more than 24 hours, so that the biochar in the soil can capture the Sporosarcina pasteuriana;

[0064] S203. Spray the binder liquid into the contaminated soil and stir to allow the pasteurized bacteria in the biochar to react with the binder liquid. The binder liquid is a mixed solution of calcium chloride and urea, wherein the concentration of calcium chloride is 0.05-0.5 mol / L and the concentration of urea is 0.25-0.5 mol / L.

[0065] The above scheme is further described below with reference to specific examples. It should be understood that these examples are intended to illustrate the basic principles, main features, and advantages of the present invention, and the present invention is not limited in scope by the following examples. The implementation conditions used in the examples can be further adjusted according to specific requirements. The implementation conditions not specified are generally those used in routine experiments. In the following description, unless otherwise specified, all raw materials were purchased commercially or prepared according to conventional methods in the art.

[0066] Example 1

[0067] This example uses MICP technology to perform surface treatment on rice straw biochar with lead fixation equilibrium, and the specific steps are as follows:

[0068] 1) Immobilization of lead by rice straw biochar: 0.1 g of rice straw biochar was added to a centrifuge tube containing 20 mL of a solution of 5 mmol / L Pb(NO3)2 and 0.01 mol / L NaNO3. The tube was shaken at 25°C and 250 rpm for 24 h. After the fixation equilibrium was reached, the tube was centrifuged and the supernatant was removed.

[0069] 2) Soaking biochar in bacterial solution: Soak the biochar obtained in 1) with 30 mL of bacterial solution of Sporosarcina pasteurii that has been inoculated for 24 hours. Oscillate the mixture of bacterial solution and biochar at 150 rpm in a constant temperature shaking incubator at 30°C for 12 hours, and then let it stand for 12 hours to ensure that the biochar fully captures the bacteria in the bacterial solution. Remove the supernatant.

[0070] 3) Soaking biochar in cementing solution: Immediately soak the biochar obtained in 2) with 30 mL of cementing solution (urea concentration is 0.5 mol / L, calcium chloride concentration is 0.3 mol / L), and place the mixture of cementing solution and biochar in a constant temperature shaking incubator at 30°C and oscillate at 100 rpm for 12 hours. The mixture is then allowed to stand for 12 hours to ensure that the microorganisms can fully and stably react with the components in the cementing solution.

[0071] After the biochar is soaked in the binder fluid, a calcium carbonate surface barrier is formed on the surface of the biochar, which helps to improve the biochar's ability to resist environmental changes. Figure 3 Schematic diagram of the actual state of rice straw biochar at lead fixation equilibrium before and after MICP treatment in this example.

[0072] A simplified continuous extraction method was used to extract lead from the dried biochar samples obtained above in a stepwise manner to determine the contents of exchangeable lead, acid-soluble lead, and stable non-bioavailable lead in each biochar. In the first step, 8 mL of 0.5 mol / L MgCl₂ solution (pH = 7.0) was added to 0.1 g of biochar and shaken at room temperature for 20 minutes to extract exchangeable lead. In the second step, 8 mL of 1 mol / L NaOAc solution (pH = 5.0) was added to another 0.1 g of biochar and shaken at room temperature for 5 hours to extract exchangeable and acid-soluble lead. The supernatant of the extraction solution was collected, diluted, and acidified, and the lead concentration in the solution was determined using inductively coupled plasma optical emission spectrometry (ICP-OES). The first step yielded the exchangeable lead content (q₁) (mg / g); the second step yielded the total exchangeable and acid-soluble lead (q₂). The difference between q₁ and q₂ was the acid-soluble lead content. The content of non-bioavailable lead can be determined by the total fixed amount Q e Calculated by subtracting q2. Figure 4 is the change in the proportion of each form of lead on biochar before and after MICP treatment, from which we can judge whether the stability of lead fixed in biochar is improved after MICP treatment.

[0073] The test results show that: Figure 4 As can be seen, the stability of lead fixed in the biochar was significantly improved after the biochar was treated with MICP according to the above steps. The proportion of lead in each form changed as follows: exchangeable lead decreased from 1.8% to 0.2%, acid-soluble lead decreased from 94.7% to 7.5%, and stable lead increased from 3.5% to 92.3%.

[0074] Example 2

[0075] This example differs from Example 1 in the binder solution. Specifically, this example uses a binder solution with a urea concentration of 0.25 mol / L and a calcium chloride concentration of 0.1 mol / L to soak the biochar. The urease-producing bacteria, biochar type, and heavy metal species used are the same as in Example 1. The biochar heavy metal fixation and MICP treatment steps, as well as the testing methods for the proportion of each form, are also identical to those in Example 1.

[0076] Results from MICP-treated biochar in this example showed that the stability of lead immobilized in the biochar was significantly improved. The percentage of lead in each form decreased as follows: exchangeable lead decreased from 1.78% to 1.5%, acid-soluble lead decreased from 94.73% to 39%, and stable lead increased from 3.49% to 52.5%. The improved long-term effectiveness of the biochar in Examples 1 and 2 is primarily attributed to the physical encapsulation and chemical buffering effects of the calcium carbonate surface barrier on the biochar.

[0077] In summary, compared with the prior art, the present invention has the following beneficial effects:

[0078] (1) The present invention has significant advantages in improving the long-term effectiveness of biochar in fixing heavy metals. By using the biological reaction process of urease-producing bacteria, a calcium carbonate surface layer with resistance to changes in the external environment is formed on the biochar, which overcomes the shortcomings of the existing biochar modification technology in terms of mechanical properties and insufficient biochar fixation stability. In addition, the biochar treated with MICP technology is not easily affected by the environmental pH value and has good fixation efficiency and long-term effectiveness.

[0079] (2) The present invention is a soil solidification technology that conforms to the concept of sustainable development. The urease-producing bacteria used are widely distributed in the soil in nature and will not cause pollution to the ecological environment. The raw material of biochar used is rice straw, which has a wide source and low cost. It also avoids the large-scale non-standard burning of agricultural waste to a certain extent, reducing energy consumption and carbon emissions.

[0080] (3) Compared with the existing biochar modification technology, the MICP enhanced biochar fixation technology adopted in the present invention can have a faster enhancement speed and a simpler enhancement process. Generally speaking, it only takes about 48 hours from capturing bacteria to completing calcium carbonate precipitation. After a short period of microbial treatment, the fixation of exchangeable and acid-soluble heavy metals on biochar can be enhanced, making the stable part of heavy metals dominate.

[0081] (4) The present invention can meet the needs of the treatment of heavy metal contaminated sites in soil and water bodies, with low construction difficulty, low equipment demand, low investment cost, long maintenance cycle, and obvious treatment effect.

[0082] The above embodiment is intended only to illustrate the technical concepts and features of the present invention and is a preferred embodiment. Its purpose is to enable those skilled in the art to understand the contents of the present invention and implement them accordingly. It is not intended to limit the scope of protection of the present invention. Any equivalent changes or modifications made in accordance with the spirit of the present invention are intended to be covered by the scope of protection of the present invention.

Claims

1. A method for fixing heavy metals based on biochar, characterized in that: The steps include: S101, adding biochar to the contaminated water, wherein the ratio of the weight of the biochar to the volume of the contaminated water is 1 g: (150-300) mL, and shaking to allow the biochar to capture heavy metals in the water; S102, removing the biochar from the water body, soaking the biochar in a bacterial solution containing Sporosarcina pasteuriana, and placing the biochar in a constant temperature shaking incubator at 25-35° C. for shaking and standing, wherein the sum of the shaking time and the standing time is less than or equal to 24 hours, so that the biochar captures Sporosarcina pasteuriana; S103. Soak the biochar in a binder, place it in a constant temperature shaking incubator at 25-35°C for shaking, and the total shaking time and standing time is 12-24 hours, so that the pasteurian sporosarcina in the biochar reacts with the binder, wherein the ratio of the weight of the biochar to the volume of the binder is 1 g: (200-400) mL, and the binder is a mixed solution of calcium chloride and urea, the concentration of calcium chloride is 0.05-0.5 mol / L, and the concentration of urea is 0.25-0.5 mol / L.

2. The method for fixing heavy metals based on biochar according to claim 1, characterized in that: In step (2), the bacterial strain is inoculated into the liquid culture medium at a volume fraction of 1-5%, and the bacterial solution is obtained by aerobic culture. The OD of the bacterial solution is 600 Value greater than or equal to 1.

1.

3. The method for fixing heavy metals based on biochar according to claim 1, characterized in that: The biochar is powder or granules obtained by pyrolysis and grinding of plant straw.

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