Method for treating river channel sediment by microbial induced carbonate precipitation

Abstract

The application discloses a method for treating river channel sediment by microbial induced carbonate precipitation, comprising the following steps: preparing iron / calcium double-loaded functionalized biochar; fixing urease-producing functional bacteria on the biochar to form a composite microbial agent; injecting the composite microbial agent into the sediment and standing for colonization; and injecting a cementing liquid containing urea and calcium chloride in batches to complete the passivation-solidification treatment of the sediment. The application realizes efficient immobilization and activity maintenance of the microorganism through the functionalized biochar carrier, realizes directional crystallization and uniform cementation of calcium carbonate through liquid injection in batches, and forms a double heavy metal immobilization mechanism of adsorption enrichment and mineralization fixation. The unconfined compressive strength of the treated sediment reaches 178 kPa after 28 days, and the heavy metal leaching reaches the standard, so that the treated sediment can be used as an ecological slope protection or an artificial wetland substrate, and in-situ treatment and resource utilization of the contaminated sediment are realized.

Description

Technical Field

[0001] This invention relates to the field of environmental microorganisms and sediment resource utilization technology, and in particular to a method for treating river sediment by microbial-induced carbonate precipitation. Background Technology

[0002] With the rapid pace of industrialization and urbanization in my country, large amounts of polluted sediment are deposited in river channels. This sediment typically exhibits high water content, high organic matter content, and low mechanical strength, while also being enriched with heavy metal pollutants such as lead, cadmium, and copper. Traditional dredging-ex-situ disposal methods are not only costly and require large land areas, but also prone to generating secondary pollution and significant carbon emissions during the disposal process. Therefore, developing a technology capable of in-situ remediation of polluted sediment and its transformation into usable resources has significant environmental and economic value.

[0003] Microbially Induced Carbonate Precipitation (MICP) is a novel soil reinforcement and environmental remediation technology that has emerged in recent years. Its basic principle is to utilize urease-producing microorganisms to hydrolyze urea to produce CO3²⁻, which, in the presence of exogenous Ca²⁺, precipitates CaCO3, thereby cementing soil particles and fixing heavy metals. However, applying MIP technology to in-situ remediation of riverbed sediments faces three major challenges: free microorganisms have poor retention in the sediment's porous environment and are easily lost through diffusion with pore water, making it difficult to maintain an effective bacterial density; existing technologies either focus on heavy metal fixation or on solidification and enhancement, lacking a unified technical solution that synergizes both; and when natural minerals are used as the calcium source, calcium release is uncontrollable, resulting in uneven cementation effects.

[0004] To address these issues, researchers have explored various approaches. For example, CN117534272B discloses a method for reinforcing and remediating Pb-contaminated sludge using nano-SiO2 synergistic with MIP technology. This method utilizes nano-SiO2 and PAN-modified nano-SiO2 as adsorbents and nucleation sites, uniformly mixing the cementing solution and bacterial solution in a single step followed by circulating grouting to improve soil strength and reduce the effective Pb concentration. However, this technology is mainly applied to laboratory-scale sludge mixing and treatment. Its single-step mixing and circulating grouting process is difficult to directly apply to in-situ riverbed sediment remediation in open water environments. Furthermore, the nanomaterial carrier's primary function is heavy metal adsorption, failing to specifically address the retention and long-term activity maintenance of functional microorganisms in in-situ sediment, and also failing to achieve resource utilization of the treated sediment.

[0005] CN109457688B discloses a method for reinforcing coarse-grained soil based on MICP technology. It improves the retention of bacterial solutions in coarse-grained soil by adding thickeners such as agar or sodium alginate. However, it is mainly for coarse-grained soil with a particle size of 5 to 20 mm, which is much larger than the fine particles of riverbed sediment. Furthermore, the introduction of thickeners may affect the subsequent permeability and ecological function of the sediment.

[0006] CN110438974B discloses a microbial solidification kit and a method for in-situ solidification of calcareous sand. The method uses acetic acid to dissolve calcareous sand to obtain calcium source in situ. However, this method is only applicable to calcareous sand rich in calcium carbonate and is not suitable for ordinary riverbed sediment with high organic matter content and low calcium carbonate content.

[0007] Therefore, developing a new method that can simultaneously achieve in-situ microbial solidification, heavy metal passivation, and sediment cementation and solidification, and also utilize the resulting products as resources, has significant engineering value and environmental implications. Summary of the Invention

[0008] This invention solves the technical problems of easy loss of free microorganisms, difficulty in synergistic fixation of heavy metals and solidification of sediment under in-situ conditions of riverbed sediment, and uneven cementation effect. It proposes a method for treating riverbed sediment by microbial-induced carbonate precipitation, which achieves the technical effect of simultaneously passivating heavy metals and solidifying sediment, and transforming the stabilized products into ecological slope protection or wetland matrix materials.

[0009] To achieve the above objectives, the following technical solution is proposed: A method for in-situ resource recovery of riverbed sediment through microbial-induced carbonate precipitation and synergistic biochar treatment includes the following steps: Preparation of iron / calcium dual-loaded functionalized biochar; Urease-producing functional bacteria are immobilized on the iron / calc dual-loaded functionalized biochar to form a microbial and biochar composite agent. The microorganisms and biochar composite agent were injected into the target riverbed sediment and left to stand for 48 hours. After settling, a cementing solution containing urea and calcium chloride is injected into the same sediment area in stages. Each injection is allowed to stand for 48 hours to complete the passivation-solidification treatment of the sediment.

[0010] The above technical solution, employing iron / calcium dual-loaded functionalized biochar as an immobilization carrier, pre-immobilizes urease-producing bacteria onto porous biochar before injecting it into the sediment. This significantly improves the retention and anti-loss ability of functional microorganisms in the sediment pores, providing a stable source of bacteria for subsequent MICP reactions. The biochar carrier in the composite microbial agent provides a physical shelter for the bacteria, protecting them from pore water erosion and protozoan predation.

[0011] Preferably, the preparation method of the iron / calcium dual-loaded functionalized biochar includes: using agricultural straw as raw material, pyrolyzing it at 500°C under a nitrogen atmosphere to prepare basic biochar; impregnating the basic biochar in a mixed solution of FeCl3 and CaCl2 for loading; and then adjusting the pH to 10.0 with an alkaline solution to allow iron and calcium to be deposited on the surface and in the pores of the biochar in the form of hydroxides or oxides.

[0012] Through the above technical solutions, since iron and calcium hydroxides or oxides are loaded on biochar, the iron oxides can buffer the pH fluctuations in the anaerobic environment of the sediment, the loaded calcium provides an in-situ calcium source for the MIP reaction, and the porous structure and high specific surface area of ​​the biochar itself provide an ideal microenvironment for microbial attachment. Its oxygen-containing functional groups can also pre-adsorb heavy metals.

[0013] Preferably, the preparation method of the microbial and biochar composite agent is as follows: the iron / calcium dual-loaded functionalized biochar and the bacterial solution of urease-producing bacteria are mixed at a solid-liquid ratio of 1:10, and the mixture is allowed to stand for 6 hours to immobilize, so that the bacteria are attached to the biochar.

[0014] Through the above technical solution, by adopting a solid-liquid ratio of 1:10 and a 6-hour static immobilization period, sufficient adhesion time for the bacteria to the biochar is ensured, and the live bacteria adhesion density in the compound bacterial agent can reach 10 per gram of dry weight. 8 The concentration of CFU or higher ensures the initial bacterial count after the sediment is injected. The solution is slowly stirred every hour to redistribute the bacteria evenly, preventing uneven fixation caused by bacterial settling.

[0015] Preferably, the urease-producing bacterium is *Sporosarcina pasteurii*.

[0016] The above technical solutions, by using Bacillus pasteurellii, which has the characteristics of high urease activity and good environmental adaptability, ensure the reliability and reproducibility of the present invention in large-scale applications.

[0017] Preferably, the concentrations of urea and calcium chloride in the cementing solution are both 0.5 mol / L or 1.0 mol / L.

[0018] By employing the above technical solutions, different concentrations of cementing solutions are selected based on the degree of sediment contamination. When the total heavy metal content is below 500 mg / kg, 0.5 mol / L is used, and when the total heavy metal content reaches or exceeds 500 mg / kg, 1.0 mol / L is used. This achieves differentiated and precise control of the remediation intensity, avoiding material waste caused by over-remediation in low-contamination sediments. At the same time, it ensures sufficient CaCO3 generation in high-contamination sediments to achieve adequate encapsulation of heavy metals and effective sediment cementation.

[0019] Preferably, the bonding liquid is injected in 3 to 4 equal amounts, and the standing time after each injection is 48 hours.

[0020] By employing the above technical solution, and by using a strategy of injecting the cementing solution in 3 to 4 equal-volume stages, the rapid increase in CaCO3 supersaturation in the pore water caused by a single large injection, which would lead to the formation of non-cementing, loose precipitates, is avoided. This allows carbonates to crystallize and grow directionally along the contact points between sediment particles and the surface of the biochar carrier, forming effective cementing bridges and ensuring the mechanical strength of the treated sediment. The 48-hour interval between each injection provides sufficient time for microorganisms to completely hydrolyze the injected urea and drive the growth of CaCO3 crystals at the thermodynamically more stable particle contact points.

[0021] Preferably, when injecting the microbial and biochar composite agent, the injection depth is 20 to 40 cm below the bottom mud, the injection pressure is 0.2 to 0.5 MPa, and the total injection volume is 20 to 30 liters per square meter.

[0022] The above technical solution, employing a low-pressure, slow-speed injection method, ensures the uniform distribution of the compound bacterial agent in the sediment at the target depth. Simultaneously, it avoids high-pressure erosion that could damage the original structure of the sediment, facilitating the in-situ colonization of functional bacteria at the injection point. The injection depth covers both the main contaminated layer and the surface layer requiring reinforcement in the riverbed sediment, ensuring the effectiveness of the treatment.

[0023] As a preferred option, the process also includes the resource utilization of the treated sediment: when the 28-day unconfined compressive strength of the treated sediment reaches or exceeds 150 kPa and the heavy metal leaching concentration is lower than the Class IV water standard of GB 3838-2002, it is used as an ecological slope protection material or artificial wetland substrate.

[0024] Through the above technical solutions, clear mechanical and environmental requirements are set for the treated sediment, thus realizing the functional transformation of "polluted sediment → engineering materials → ecological carrier". At the same time, it solves the two problems of sediment disposal and the source of ecological revetment materials. The whole process is carried out in situ without secondary pollution. Compared with the dredging-ex-situ disposal solution, it can reduce the carbon emissions of the project by more than 50%.

[0025] Preferably, in the iron / calcium dual-load functionalized biochar, the iron loading is 5% to 8% and the calcium loading is 3% to 5% based on the dry weight of the biochar.

[0026] By controlling the iron and calcium loading within the above ranges through the above technical solutions, sufficient pH buffering capacity and in-situ calcium source supply capacity are obtained while ensuring that the biochar pores are not excessively blocked. At the same time, the surface functional groups of the loaded iron and calcium oxides can also pre-adsorb and enrich heavy metals such as Pb²⁺, Cd²⁺, and Cu²⁺, forming a progressive fixation path of "adsorption and enrichment first, then mineralization and fixation".

[0027] Preferably, the step of injecting the cementing solution in stages can avoid a sharp increase in the supersaturation of calcium carbonate in the pore water caused by a single large injection, and allow calcium carbonate crystals to grow directionally at the contact points of sediment particles and on the surface of biochar carrier, thereby achieving effective cementation.

[0028] Through the above technical solutions, the calcium carbonate generated each time is injected in stages, allowing sufficient time for nucleation and growth at the particle contact points. This results in a bridging cemented structure rather than a loosely accumulated sediment. The treated bottom mud has an unconfined compressive strength of over 150 kPa after 28 days, meeting the mechanical requirements of ecological slope protection and wetland substrate.

[0029] The beneficial effects of this invention are: First, by using iron / calcium dual-loaded functionalized biochar as a carrier, an ideal attachment surface and microecological niche are provided for Bacillus pasteurellii, significantly reducing the cell loss rate. Simultaneously, the loaded iron and calcium oxides buffer the pH and provide nutrient replenishment, maintaining the long-term activity of the functional bacteria. Experimental data show that using the composite bacterial agent of this invention, the viable bacterial density in the sediment reached 8.8 × 10⁸ after 28 days. 7 The CFU / g is 42 times that of the direct injection of free bacteria.

[0030] Secondly, the oxygen-containing functional groups and supported iron oxides on the surface of biochar provide sites for chemical adsorption and surface complexation, enabling pre-adsorption and enrichment of Pb²⁺, Cd²⁺, Cu²⁺, etc. The CaCO₃ generated by MICP then mineralizes and immobilizes the pre-enriched heavy metals through lattice substitution and physical encapsulation, forming a progressive immobilization path of "adsorption enrichment followed by mineralization immobilization." Experimental data show that, using the technical solution of this invention, the TCLP leaching concentration of Pb decreased from 8.42 mg / L to 0.48 mg / L, with an immobilization efficiency of 94.3%, an improvement of approximately 27% compared to using the MICP process alone.

[0031] Third, a two-stage process of pre-injection of compound microbial agent followed by multiple injections of cementing solution was adopted, overcoming the problems of easy in-situ loss of microorganisms and non-directional precipitation of calcium carbonate, ensuring the directional growth of cementing material at particle contact points. After treatment, the unconfined compressive strength of the bottom mud reached 178 kPa after 28 days, meeting the strength requirements of ecological slope protection materials, while the permeability coefficient decreased from 5.2 × 10⁻⁻⁻⁶. 5 cm / s increased to 4.6 × 10⁻4 cm / s is beneficial for plant root growth and water transport when used as a wetland substrate.

[0032] Fourth, the entire construction process is completed within the river channel, eliminating the need for dredging and transportation, thus avoiding secondary pollution and large-scale carbon emissions. The treated sediment can be directly used for ecological slope protection and wetland substrate construction, realizing the functional transformation of polluted sediment into engineering materials and ecological carriers. Detailed Implementation

[0033] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific application examples.

[0034] Experimental materials and equipment: The main equipment and models used in this specific embodiment are listed in Table 1, and the main reagents and their sources are listed in Table 2.

[0035] Table 1 Main Equipment and Models Electric constant temperature drying oven DHG-9076A Shanghai Jinghong Experimental Equipment Co., Ltd. High-speed universal pulverizer DF-15 Wenzhou Dingli Medical Device Co., Ltd. Tubular furnace OTF-1200X model Hefei Kejing Materials Technology Co., Ltd. Vacuum filtration device GM-0.33A Jinteng Solvent Filter conductivity meter DDSJ-308A type Shanghai Instrument & Electronics Scientific Instruments Co., Ltd. Specific surface area analyzer ASAP 2460 Micromeritics, Inc. (USA) constant temperature incubator DNP-9052 type Shanghai Yiheng Scientific Instruments Co., Ltd. UV-Vis spectrophotometer UV-2600 model Shimadzu Corporation Constant temperature shaking incubator ZQZY-70 type Shanghai Zhichu Instrument Co., Ltd. High-pressure steam sterilizer YXQ-LS-50A type Shanghai Boxun Industrial Co., Ltd. Vortex oscillator VORTEX-5 Haimen Qilinbell Instrument Manufacturing Co., Ltd. Microwave digester Mars 6 CEM Corporation, USA Inductively Coupled Plasma Emission Spectrometer Optima 8000 PerkinElmer, USA <![CDATA[CO2 Gas Volume Tester]]> Type 2900 YSI Company, USA Microcomputer-controlled electronic universal testing machine C45.504 MTS Systems, Inc. Permeameter TST-55 type Nanjing Soil Instrument Factory Co., Ltd. pH meter PHS-3C type Shanghai Instrument & Electronics Scientific Instruments Co., Ltd. low-pressure plunger pump XB-10 Ningbo Xindu Hydraulic Machinery Co., Ltd. GPS-RTK i90 model Shanghai Huace Navigation Technology Co., Ltd. Table 2. Main reagents and their sources <![CDATA[FeCl3·6H2O]]> Superior Pure Sinopharm Chemical Reagent Co., Ltd. <![CDATA[CaCl2]]> Analytical Pure Sinopharm Chemical Reagent Co., Ltd. NaOH Analytical Pure Sinopharm Chemical Reagent Co., Ltd. HCl Analytical Pure Sinopharm Chemical Reagent Co., Ltd. <![CDATA[H2O2]]> Analytical Pure Sinopharm Chemical Reagent Co., Ltd. <![CDATA[HNO3]]> Superior Pure Sinopharm Chemical Reagent Co., Ltd. High-purity nitrogen 99.999% Nanjing Special Gases Plant Co., Ltd. Deionized water Resistivity ≥ 18 MΩ·cm Lab-made Bacillus pasteurellus freeze-dried powder ATCC 11859 American Type Culture Collection Yeast extract LP0021B British Oxoid Company ammonium sulfate Analytical Pure Sinopharm Chemical Reagent Co., Ltd. urea Analytical Pure Sinopharm Chemical Reagent Co., Ltd. Tris buffer T1503 Sigma-Aldrich, Inc. Sodium chloride Analytical Pure Sinopharm Chemical Reagent Co., Ltd. Sodium hydrogen phosphate Analytical Pure Sinopharm Chemical Reagent Co., Ltd. Potassium dihydrogen phosphate Analytical Pure Sinopharm Chemical Reagent Co., Ltd. <![CDATA[Nano SiO₂]]> Particle size 30 nm, S104869 Aladdin Reagent Company 3-Aminopropyltriethoxysilane Analytical Pure Sinopharm Chemical Reagent Co., Ltd. formaldehyde Analytical Pure Sinopharm Chemical Reagent Co., Ltd. 1-(2-pyridinazo)-2-naphthol Analytical Pure Sinopharm Chemical Reagent Co., Ltd. Anhydrous ethanol Analytical Pure Sinopharm Chemical Reagent Co., Ltd. Toluene Analytical Pure Sinopharm Chemical Reagent Co., Ltd. Nutritious meat broth M1168 Oxoid, Inc. (USA) Acetic acid Analytical Pure Sinopharm Chemical Reagent Co., Ltd. Preparation Example 1: The specific preparation method for iron / calcium dual-loaded functionalized biochar is as follows: (1) Preparation of basic biochar: Rice straw provided by a farm in Huai'an City, Jiangsu Province was used as raw material. First, the straw was collected by rice harvester, and weeds and stones were removed manually. About 10 kg of straw was taken, washed three times with deionized water to remove surface dust and impurities, and then dried in an electric constant temperature drying oven at 80 degrees Celsius for 12 hours until constant weight. The dried straw was crushed to a particle size of no more than 5 mm using a high-speed universal crusher, and the crushed material was collected and placed in sealed plastic bags for later use.

[0036] Weigh 300 grams of the above-mentioned crushed straw material and spread it evenly in a corundum ark, placing it in the constant temperature zone of a tube furnace. Start the vacuum pump to evacuate to -0.1 MPa, then introduce 99.999% pure nitrogen gas to atmospheric pressure. Repeat this evacuation and nitrogen purging process three times to completely remove air from the furnace tubes. Set the heating program: increase the temperature from room temperature to 500 degrees Celsius at a rate of 5 degrees Celsius per minute, and maintain this temperature at 500 degrees Celsius for 2 hours, then allow it to cool naturally to room temperature. Maintain a nitrogen flow rate of 200 ml per minute throughout the pyrolysis process. Once the furnace temperature drops below 50 degrees Celsius, remove the pyrolysis product to obtain the crude basic biochar.

[0037] The crude basic biochar was ground and passed through a 60-mesh standard sieve, and the undersize was collected. 100 g of the sieved biochar was added to 1000 mL of 1 mol / L hydrochloric acid solution and magnetically stirred at 25°C and 150 rpm for 12 hours to remove ash and soluble salts. After stirring, solid-liquid separation was performed using a vacuum filtration device, and the solid phase was collected. The solid phase was repeatedly washed with deionized water, adding 500 mL of deionized water each time, stirring for 5 minutes, and then filtering. The conductivity of the washing solution was measured until it was below 50 μS / cm. The washed solid phase was dried in a 105°C electric thermostatic drying oven for 12 hours to obtain the basic biochar. The specific surface area of ​​the basic biochar was determined to be 235 m² / g, the total pore volume was 0.12 cm³ / g, and the average pore size was 2.8 nm.

[0038] (2) Iron / Calcium Dual-Load Functionalization Modification: Prepare a 0.5 mol / L FeCl3 solution: Weigh 135.15 g of FeCl3·6H2O, dissolve in deionized water, and bring the volume to 1 L. Prepare a 0.5 mol / L CaCl2 solution: Weigh 55.49 g of anhydrous CaCl2, dissolve in deionized water, and bring the volume to 1 L. Mix the two solutions at a volume ratio of 1:1 to obtain a FeCl3-CaCl2 mixed solution, wherein the concentrations of Fe³⁺ and Ca²⁺ are both 0.25 mol / L.

[0039] Take 20 g of the basic biochar prepared in step (1) above, add 1000 ml of FeCl3-CaCl2 mixed solution with a solid-liquid ratio of 1:50, and place it in a 2 L Erlenmeyer flask. Place the Erlenmeyer flask in a constant temperature shaking incubator and shake it at 25 degrees Celsius and 150 rpm for 24 hours to allow Fe³⁺ and Ca²⁺ to fully impregnate and adsorb onto the surface and pores of the biochar.

[0040] After shaking, the conical flask was transferred to a digital display constant-temperature magnetic stirrer, and 1 mol / L NaOH solution was slowly added dropwise at a rate of approximately 2 mL per minute to adjust the pH of the system to 10.0 ± 0.2. The pH was monitored using a pH meter, and the reaction was carried out at 60°C and 300 rpm for 4 hours. During the reaction, the solution gradually became turbid, and Fe³⁺ and Ca²⁺ were deposited on the surface of the biochar as hydroxides or oxides. After the reaction, the solid phase was separated using a vacuum filtration device. The solid phase was repeatedly washed with deionized water, adding 500 mL of deionized water each time, until the conductivity of the washing solution was below 50 μS / cm. The washed solid phase was then dried in a vacuum drying oven at 60°C for 12 hours to obtain iron / calcium dual-loaded functionalized biochar.

[0041] The elemental content of the obtained functionalized biochar was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES). Pretreatment method: 0.2 g of sample was weighed and added to 10 mL of a 3:1 mixture of nitric acid and hydrogen peroxide. The mixture was digested in a microwave digester at 180°C for 20 minutes, followed by dilution to 50 mL. The results showed that, based on the dry weight of the biochar, the iron loading was 6.2% and the calcium loading was 4.1%. Calculations showed that this loading was within the optimal range, i.e., 5% to 8% iron and 3% to 5% calcium.

[0042] Preparation Example 2: The preparation of a microbial and biochar composite inoculant includes the following steps: (1) Strain selection: Sporosarcina pasteurii was selected, purchased from the American Type Culture Collection (ATCC) with accession number ATCC 11859. The strain was purchased as a lyophilized powder and revived according to the ATCC instructions: the lyophilized powder was dissolved in 1 ml of sterile resuscitation solution in a clean bench, inoculated onto an agar slant, and incubated at 30°C for 48 hours. Gram staining and microscopic examination confirmed it to be a Gram-positive rod-shaped bacterium. The agar slant culture was stored at 4°C for later use and subcultured monthly.

[0043] (2) Bacterial culture: The liquid culture medium formula is as follows: yeast extract 20.0 g / L, ammonium sulfate 10.0 g / L, urea 20.0 g / L, Tris buffer 0.13 mol / L. Preparation method: Weigh all the above components except urea, add about 800 ml of deionized water, stir to dissolve, adjust the pH to 9.0 with 1 mol / L NaOH solution, and then make up to 1 liter with deionized water. Dispense into 500 ml Erlenmeyer flasks, each containing 200 ml of liquid. After sealing with sealing film, sterilize in an autoclave at 121 degrees Celsius for 20 minutes. Urea is prepared separately with deionized water to a stock solution of 400 g / L, filtered through a 0.22 μm filter membrane for sterilization, and stored at 4 degrees Celsius. When using, add it to the sterilized culture medium at a ratio of 1:50.

[0044] In a clean bench, two loops of the bacterial culture preserved on the slant in step (1) were scraped off with a sterile inoculation loop and inoculated into 250 ml Erlenmeyer flasks containing 100 ml of sterilized culture medium. Two ml of urea stock solution was added after each inoculation. The inoculated Erlenmeyer flasks were placed in a constant temperature shaking incubator and cultured at 30 degrees Celsius and 180 rpm for 24 hours. After the culture was completed, the OD of the bacterial culture was measured. 600The value was 1.52, with uninoculated culture medium as a blank control. Viable cell density was determined using the plate count method: the bacterial suspension was serially diluted and spread onto solid culture plates, incubated at 30°C for 48 hours, and then counted; the result was 4.8 × 10⁻⁶. 8 CFU / mL. Urease activity was determined by conductivity method: at room temperature, 5 mL of bacterial culture was added to 45 mL of 1.6 mol / L urea solution, and the conductivity change was recorded from 0 to 10 minutes using a conductivity meter. The rate of increase in conductivity per unit time was calculated, and each increase of 1 mS / (cm·min) was defined as 1 U. The measured urease activity was 5.8 U / mL.

[0045] (3) Preparation of compound bacterial agent: Take 10 g of iron / calcium dual-loaded functionalized biochar prepared in Preparation Example 1 and add it to a 500 ml sterile beaker. Take 100 ml of the bacterial solution prepared in step (2) above, with a solid-liquid ratio of 1:10, and slowly pour it into the beaker. Gently stir with a sterile glass rod to completely impregnate the biochar. Place the beaker in a 30°C constant temperature incubator and let it stand for 6 hours for immobilization. Stir slowly with a sterile glass rod every hour, about 5 times each time, to redistribute the suspended bacteria in the bacterial solution evenly and avoid uneven immobilization caused by the biochar settling to the bottom.

[0046] After immobilization, the mixture was poured into a vacuum filtration apparatus to remove excess free bacterial culture. 100 mL of sterile phosphate buffer (pH 7.4) was added to the filtered solid phase. The buffer was prepared with NaCl 8.0 g / L, KCl 0.2 g / L, Na₂HPO₄ 1.44 g / L, and KH₂PO₄ 0.24 g / L, using deionized water, and sterilized at 121°C for 20 minutes. The mixture was stirred and washed for 2 minutes, then filtered again, and washed twice to remove any loosely attached bacteria. The solid phase was collected as the microbial and biochar composite inoculum. The viable cell density in the composite inoculum was determined using the plate count method: 0.5 g of the composite inoculum was added to 4.5 mL of sterile phosphate buffer, and the mixture was vigorously shaken on a vortex mixer for 5 minutes to detach the attached bacteria. After serial dilution, the mixture was plate-coated and cultured. The viable cell density was counted to be 1.2 × 10⁻⁶. 8 CFU / g dry weight. The compound microbial agent should be stored at 4 degrees Celsius and used within 24 hours.

[0047] Preparation Example 3: The specific steps for preparing cementitious solutions of different concentrations are as follows: (1) Low-concentration cementing solution (0.5 mol / L): Take 10 liters of natural river water from the target river channel and filter it through a 0.45 μm filter membrane to remove suspended particulate matter. Weigh 300.3 g of urea and 555.0 g of anhydrous calcium chloride, dissolve them separately in about 2 liters of filtered river water, mix them after complete dissolution, and then make up to 10 liters with filtered river water. Stir for 3 minutes to mix evenly. The cementing solution should be used up on the same day it is prepared, and any remaining cementing solution should be disposed of. The pH of the natural river water in the target river channel is 7.2, the conductivity is 350 μS / cm, and the turbidity is 8 NTU.

[0048] (2) High-concentration cementing solution (1.0 mol / L) is prepared in the same way as the low-concentration cementing solution, but the amount of urea used is 600.6 g and the amount of anhydrous calcium chloride used is 1110.0 g. It is also diluted to 10 liters with filtered river water. This concentration is used for medium- to high-pollution sediments, that is, sediments with a total heavy metal content of 500 mg / kg or more. Example

[0049] In-situ treatment of moderately to highly polluted riverbed sediment, as detailed below: (1) Background of sediment: A river near the Binhai Industrial Zone in Keqiao District, Shaoxing City, Zhejiang Province, which was polluted by heavy metals, was selected as the treatment target. The river is about 8 to 12 meters wide, about 0.8 to 1.5 meters deep, and has a flow velocity of about 0.1 to 0.2 meters per second. On April 1, 2025, sediment samples were collected and tested. Five sampling points were set up in the middle of the river and 1 meter from the bank. 0 to 50 cm deep columnar sediment samples were collected using a columnar sediment sampler. After being mixed evenly, the samples were placed in sealed bags and stored at 4 degrees Celsius before being transported to the laboratory.

[0050] The test results are as follows: the sediment moisture content was 65%, the liquid limit was 42.5%, the plastic limit was 18.3%, the organic matter content was 8.5%, and the pH was 7.0. The total heavy metal content was: lead 420 mg / kg, cadmium 28 mg / kg, copper 150 mg / kg, zinc 82 mg / kg, chromium 54 mg / kg, nickel 32 mg / kg, arsenic 12 mg / kg, and mercury 0.5 mg / kg, totaling approximately 778.5 mg / kg, exceeding 500 mg / kg, classifying it as medium to high pollution sediment. The TCLP leaching concentrations were: lead 8.42 mg / L, cadmium 0.58 mg / L, and copper 1.23 mg / L, exceeding the Class IV water standard of GB 3838-2002.

[0051] (2) Compound bacterial agent injection: Construction was carried out on April 10, 2025. The weather was sunny that day, with an ambient temperature of 26 degrees Celsius and a water temperature of 18 degrees Celsius. A small engineering vessel was used to carry out the injection system. The injection system consisted of a 200-liter stainless steel storage tank, a low-pressure plunger pump, an injection pipe, and a high-pressure hose. The storage tank was equipped with a stirrer with a rotation speed of 60 revolutions per minute. The plunger pump had a maximum pressure of 5 MPa and an adjustable flow rate of 0 to 10 liters per minute. The injection pipe was made of 304 stainless steel, with an outer diameter of 25 mm, an inner diameter of 20 mm, and a length of 1.2 meters. The bottom end was sealed, and four 2-mm diameter outlet holes were opened every 10 cm on the side wall, for a total of 12 holes in 3 layers. The high-pressure hose had an inner diameter of 13 mm, a working pressure of 2 MPa, and a length of 15 meters. It was used to connect the storage tank, the plunger pump, and the injection pipe.

[0052] Injection points were arranged at 1.5-meter by 1.5-meter intervals along the target river section, totaling 60 points covering a treatment area of ​​135 square meters, in 10 rows by 6 columns. GPS-RTK was used for precise positioning, with a horizontal positioning accuracy of ±8 mm plus 1 ppm and an elevation accuracy of ±15 mm plus 1 ppm. Each point was marked with a wooden stake.

[0053] The microbial and biochar composite agent prepared in Example 2 was mixed with sterile water to form a suspension with a solid content of 12%. The sterile water was tap water sterilized at 121 degrees Celsius for 30 minutes under experimental conditions; filtered river water can be used in actual engineering. During preparation, 1300 liters of sterile water were added to a 1500-liter stainless steel storage tank. The stirrer was started, and 18.75 kg of the dry weight of the composite agent was slowly added. Stirring continued for 15 minutes until a homogeneous suspension was formed.

[0054] The injection pipe was manually inserted vertically into the bottom sediment to a depth of 30 cm. The plunger pump was started, the pressure adjusted to 0.3 MPa, and the flow rate controlled at 2.5 liters per minute. 25 liters of the compound bacterial agent suspension were injected per square meter at each injection point. During injection, the injection pipe was slowly raised to cover a depth of 20 to 40 cm. The injection time at each point was approximately 10 minutes. Injection was completed at 60 points over 3 days, averaging 20 points per day. After each injection, the injection pipeline was thoroughly flushed with pure water for 3 minutes to prevent blockage. After injection, the river was allowed to remain stagnant for 48 hours. During this period, temporary cofferdams were set up upstream and downstream to maintain a flow velocity below 0.05 meters per second in this section of the river.

[0055] (3) Injection of cementing solution in stages: After the bacterial agent injection is completed and allowed to stand for 48 hours, starting on April 15, 2025, the high-concentration cementing solution (1.0 mol / L) prepared in step (2) of Preparation Example 3 is injected into the same injection point at the same depth, i.e., 20 to 40 cm. The total amount of cementing solution injected is 50 liters per square meter, injected in four equal stages, each time 12.5 liters per square meter, with an interval of 48 hours between each stage. The first injection is on April 15, 2025, the second on April 17, the third on April 19, and the fourth on April 21. The flow rate, pressure, and operation method of each injection are the same as those of the compound bacterial agent injection. After each injection, the injection pipeline is flushed with 5 liters of original river water at a flow rate of 2 liters per minute to remove residual cementing solution in the pipeline and prevent CaCO3 scaling and clogging due to pH increase. The original river water is filtered through a 0.45 μm filter membrane before use.

[0056] (4) Maintenance and Monitoring: The final injection of cementing solution was completed on April 21, 2025. The river section was maintained under still or slow-flowing conditions for 28 days from that date, ending on May 19, 2025. During the maintenance period, the temporary cofferdam remained in place, controlling the water level difference between upstream and downstream to be less than 5 cm and maintaining a flow velocity below 0.02 m / s. On the 7th day (April 28), the 14th day (May 5), and the 28th day (May 19) of the maintenance period, columnar sediment samples were collected at depths ranging from 0 to 50 cm, with 5 parallel samples collected on each sampling day. After sampling, the samples were transported to the laboratory at 4 degrees Celsius, and all tests were completed within 7 days.

[0057] The testing items and methods are as follows: The CaCO3 content was determined by the gas flow method: 5 grams of air-dried bottom mud sample that had passed through a 2 mm sieve was weighed and placed in a 250 ml conical flask. The volume of CO2 gas produced after adding excess 1:3 hydrochloric acid was recorded using a CO2 gas flow meter. The CaCO3 content was calculated based on the standard curve.

[0058] The TCLP leaching concentration of heavy metals was determined according to EPA Method 1311: the leaching agent was an acetic acid solution with pH 2.88, the solid-liquid ratio was 1:20, after 18 hours of inversion and shaking, the solution was filtered through a 0.45 μm filter membrane, and the concentrations of lead and cadmium in the filtrate were determined by inductively coupled plasma atomic emission spectrometry.

[0059] Unconfined compressive strength was determined according to GB / T 50123-2019 standard: the sediment sample was placed into a steel three-lobed mold with an inner diameter of 39.1 mm and a height of 80 mm, the dry density was controlled at 1.30 g / cm³, and after demolding, it was tested using a microcomputer-controlled electronic universal testing machine with a loading rate of 0.1 mm per minute. The peak pressure was recorded, and the unconfined compressive strength was calculated according to the formula qu = P / A, where A is the corrected cross-sectional area.

[0060] The permeability coefficient was determined according to the variable head permeability test method in GB / T 50123-2019, using a TST-55 permeameter.

[0061] The test results are listed in Table 3.

[0062] Table 3. Test results during the maintenance period of Example 1 <![CDATA[CaCO3 Content (%)]]> 5.2±0.6 7.8±0.5 8.9±0.4 Lead TCLP leaching concentration (mg / L) 2.15±0.23 0.96±0.11 0.48±0.06 Cadmium TCLP leaching concentration (mg / L) 0.19±0.02 0.08±0.01 0.04±0.005 Unconfined compressive strength (kPa) 98±8 142±11 178±14 Permeability coefficient (cm / s) <![CDATA[2.1×10⁻ 4 ]]> <![CDATA[3.5×10⁻ 4 ]]> <![CDATA[4.6×10⁻ 4 ]]> Table 3 shows that the CaCO3 content continuously increased with the extension of the curing time, reaching 8.9% on day 28. The heavy metal leaching concentration continuously decreased; on day 28, the TCLP leaching concentrations of lead and cadmium dropped to 0.48 mg / L and 0.04 mg / L, respectively, meeting the Class IV water standard of GB 3838-2002, with limits of 0.05 mg / L for lead and 0.005 mg / L for cadmium. The heavy metal fixation efficiency was 94.3% for lead and 93.1% for cadmium. The unconfined compressive strength reached 178 kPa on day 28, meeting the strength requirement of ecological slope protection materials, which is not less than 150 kPa. The permeability coefficient decreased from 5.2 × 10⁻ ... 5 cm / s increased to 4.6×10⁻ 4 cm / s indicates a shift in the sediment structure from dense to porous and permeable.

[0063] Comparative Example 1: The direct injection method of free bacteria without adding biochar differs from Example 1 only in that: iron / calcium dual-loaded functionalized biochar is not prepared and used, and an equal number of viable Bacillus pasteurellium bacterial solution is directly injected into the bottom mud in a free state. The cementing solution is also injected in 4 times. All other construction parameters are the same as in Example 1.

[0064] The specific operation is as follows: following the method in step (2) of Preparation Example 2, culture Bacillus pasteurellium culture to OD. 600 =1.52, viable cell density is 4.8×10 8 1.8 × 10¹¹ CFU of bacterial culture was collected from 375 liters of water and diluted to 1500 liters with filtered river water. This solution was injected into the sediment at the same locations and in the same manner as in Example 1. After standing for 48 hours, a 1.0 mol / L cementing solution was injected in stages, following the same injection procedure as in Example 1. After 28 days of curing, sampling and testing were conducted on May 19, 2025, in the same manner as in Example 1.

[0065] Comparative Example 2: The only difference between Comparative Example 2 and Example 1 in the single-injection cementitious solution method is that the cementitious solution is not injected in stages, but rather 50 liters of 1.0 mol / L cementitious solution per square meter is injected all at once after the inoculum has been injected and allowed to stand for 48 hours, without any interval standing time. All other conditions are the same as in Example 1. The same tests are performed after 28 days of curing.

[0066] Comparative Example 3: The method for using a biochar carrier without loading iron or calcium was employed in Comparative Example 3, which followed the same preparation steps as Example 1. However, when preparing the functionalized biochar, the loading step for iron or calcium was omitted; instead, the basic biochar prepared in step (1) of Example 1 was used directly as the microbial carrier. The conditions for preparing the composite bacterial agent, injection, and fractional injection of the cementing solution were the same as in Example 1. The same tests were performed after 28 days of curing.

[0067] Comparative Example 4: The nano-SiO2 synergistic MIP method, Comparative Example 4, was conducted according to the methods described in Examples 1 and 2 of CN117534272B. However, since this patented technology is a laboratory-scale soil column test, adaptive adjustments are needed to make it suitable for in-situ riverbed sediment treatment. The specific operation is as follows: PAN-nano SiO2 was prepared according to the method described in paragraphs 0060 to 0061 of CN117534272B: Nano SiO2 was activated at 110°C for 1 hour. 500 mg of the activated nano SiO2 was weighed and placed in a 100 mL round-bottom flask. 30 mL of toluene was added, and the mixture was ultrasonically dispersed for 20 minutes. 2 mL of 3-aminopropyltriethoxysilane was added dropwise over 10 minutes. The mixture was stirred electromagnetically and refluxed at 110°C for 8 hours. The mixture was then vacuum filtered, washed sequentially with anhydrous ethanol and toluene, and dried under vacuum at 60°C to obtain APTES-modified nano SiO2. The above product was then added to 15 mL of formaldehyde, 2 mL of concentrated hydrochloric acid, and 400 mg of PAN. 80 mL of anhydrous ethanol was added, and the mixture was stirred electromagnetically and refluxed at 80°C for 5 hours. The mixture was then vacuum filtered, transferred to an Erlenmeyer flask, thoroughly washed with anhydrous ethanol, centrifuged to settle, and dried under vacuum at 70°C for 4 hours to obtain PAN-nano SiO2.

[0068] PAN-nano SiO2 and ordinary nano SiO2 were mixed with sieved dry sediment powder at a mass ratio of 0.01:0.01, and then mixed with nano SiO2 to obtain powder A. The same sediment as in Example 1 was used here, first air-dried and then ground through a 2 mm sieve. The cementing solution was prepared as follows: 2.0 mol / L CaCl2, 2.0 mol / L urea, with an additional nutrient broth of 3 g / L, NH4Cl 10 g / L, and NaHCO3 2.12 g / L, and the pH was adjusted to 6.0. A bacterial suspension was prepared: Bacillus pasteurellii, OD... 600Adjust to 1.0. Add the cementitious liquid to powder A, stir well, and let stand for 2 hours. Then add the bacterial suspension and stir well. The volume ratio of cementitious liquid to bacterial suspension is 4:1. The total amount added is controlled according to the optimum moisture content of the bottom mud of 15.1%. Fill the mixture into the soil column mold, cure at 30 degrees Celsius for 7 days, and then circulate grout with 0.5 mol / L cementitious liquid every 12 hours.

[0069] Since the in-situ riverbed conditions could not achieve constant temperature curing and circulating grouting at 30 degrees Celsius, and the river water temperature during construction was 18 to 22 degrees Celsius, this comparative example was conducted in the laboratory under the optimized parameters of CN117534272B, namely Ca²⁺ concentration 2.0 mol / L, curing time 14 days, and nutrient concentration standard multiplied by 1. Samples of treated bottom sediment were taken for testing to compare the treatment effect of this invention and the original patented technology on the same bottom sediment material.

[0070] Comparison of the effects of the examples and the comparative examples: The test results of all treatment groups after 28 days of curing are summarized in Table 4.

[0071] Table 4 Comparison of the effects of Example 1 and various comparative examples after 28 days of curing Blank control (unprocessed) <![CDATA[3.2×10 5 ±0.5×10 5 ]]> 1.2±0.2 8.42±0.52 0.58±0.04 68±9 <![CDATA[5.2×10⁻ 5 ±0.6×10⁻ 5 ]]> Comparative Example 1 (Free Bacteria) <![CDATA[2.1×10 6 ±0.3×10 6 ]]> 5.6±0.4 2.15±0.18 0.19±0.02 112±10 <![CDATA[8.3×10⁻ 5 ±0.9×10⁻ 5 ]]> Comparative Example 2 (Single Grouting) <![CDATA[1.8×10 6 ±0.4×10 6 ]]> 7.2±0.5 1.26±0.15 0.12±0.01 131±12 <![CDATA[3.2×10⁻ 4 ±0.4×10⁻ 4 ]]> Comparative Example 3 (Unloaded Biochar) <![CDATA[3.5×10 7 ±0.6×10 7 ]]> 7.5±0.6 0.95±0.10 0.08±0.01 152±13 <![CDATA[4.0×10⁻ 4 ±0.5×10⁻ 4 ]]> Comparative Example 4 (CN117534272B simulation) — 8.1±0.5 0.58±0.08 — 163±12 — Example 1 (Invention) <![CDATA[8.8×10 7 ±1.1×10 7 ]]> 8.9±0.4 0.48±0.06 0.04±0.005 178±14 <![CDATA[4.6×10⁻ 4 ±0.5×10⁻ 4 ]]> The results are discussed below: First, regarding microbial retention. As shown in Table 4, the viable cell density in Example 1 was 8.8 × 10⁻⁶. 7 The CFU / g was significantly higher than that of Comparative Example 1 (2.1 × 10⁻⁶). 6 The CFU / g was increased by approximately 42 times, demonstrating that the iron / calcium dual-loaded functionalized biochar carrier effectively protected functional microorganisms, enabling them to survive long-term in the sediment. Comparative Example 3, without the biochar carrier, had a viable cell density of 3.5 × 10⁻⁶. 7 The CFU / g of Example 1 was also significantly higher than that of Comparative Example 1, indicating that the biochar carrier itself has a protective effect on the bacteria. However, Example 1 was even higher than that of Comparative Example 3, indicating that the iron and calcium loading further promoted the survival of the bacteria by improving the microenvironment, i.e., buffering pH and providing micronutrients.

[0072] Second, regarding the heavy metal fixation efficiency, Example 1 achieved fixation efficiencies of 94.3% for lead and 93.1% for cadmium, both superior to Comparative Example 1's 74.5% and 67.2%, Comparative Example 2's 85.0% and 79.3%, and Comparative Example 3's 88.7% and 86.2%. This indicates that the dual mechanisms of biochar adsorption pre-enrichment combined with MIP mineralization fixation, and the mechanism of staged grouting to avoid amorphous CaCO3 precipitation, worked together. Comparative Example 4 achieved a lead fixation efficiency of approximately 93.1%, slightly lower than Example 1's 94.3%, and no data for cadmium were reported in Comparative Example 4.

[0073] Third, regarding mechanical properties, the unconfined compressive strength of Example 1 was 178 kPa, the highest among all treatment groups. This represents a 59% increase compared to Comparative Example 1, a 36% increase compared to Comparative Example 2, a 17% increase compared to Comparative Example 3, and a 9% increase compared to Comparative Example 4. This indicates that the synergistic effect of staged grouting with loaded biochar is optimal.

[0074] Fourth, regarding permeability. The permeability coefficient of Example 1 is 4.6 × 10⁻ 4 cm / s, compared to 5.2 × 10⁻ in the blank control. 5 The cm / s increased by nearly an order of magnitude, indicating that the treated sediment transformed from a dense clay-like structure into a more permeable granular structure, which is beneficial for water transport and root growth when used as a wetland substrate.

[0075] Resource utilization verification: Samples of the sediment from Example 1, cured for 28 days, were tested. The unconfined compressive strength after 28 days was 178 kPa, not less than 150 kPa; the lead TCLP leaching concentration was 0.48 mg / L, and the cadmium TCLP leaching concentration was 0.04 mg / L, both lower than the limits set by the Class IV water standard in GB 3838-2002. This sediment meets the dual requirements of ecological slope protection material and artificial wetland substrate.

[0076] The treated sediment from Example 1 was tested on a small-scale field: a 10-meter-long, 1:1.5 slope of exposed riverbank was selected, and a 30-centimeter-thick layer of the treated sediment was laid. Bermuda grass seeds were sown on the surface at a rate of 15 grams per square meter. After 30 days, the seed germination rate was approximately 85%, the coverage was approximately 60%, no obvious erosion marks were observed on the slope, and the unconfined compressive strength was retested at 162 kPa, considering slight attenuation after natural drying.

[0077] Meanwhile, the treated sediment from Example 1 was placed into 20 plastic flowerpots (300 mm in diameter and 500 mm in height) and put into an artificial wetland unit constructed beside the river. The wetland unit was continuously filled with water to a depth of 20 cm, with a hydraulic loading of 0.5 m³ / (m²·d). After 60 days of operation, the wetland effluent showed a COD of 25 mg / L, NH₃-N of 1.2 mg / L, and TP of 0.3 mg / L, meeting the Class B standard of GB 18918-2002. The sediment in the flowerpots did not show any disintegration or strength reduction, indicating its stability and effectiveness as a wetland substrate.

[0078] In summary, the method provided by this invention can simultaneously achieve the passivation and fixation of heavy metals and the cementation and solidification of the sediment under in-situ conditions. The treated sediment meets the performance requirements of ecological slope protection and wetland matrix, and has significant technological progress and practical value.

[0079] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, several improvements and equivalent substitutions can be made without departing from the spirit and principle of the present invention, and these improvements and equivalent substitutions should also be considered within the scope of protection of the present invention.

Claims

1. A method for treating riverbed sediment by microbial-induced carbonate precipitation, characterized in that, Includes the following steps: Preparation of iron / calcium dual-loaded functionalized biochar; Urease-producing functional bacteria are immobilized on the iron / calc dual-loaded functionalized biochar to form a microbial and biochar composite agent. The microorganisms and biochar composite agent were injected into the target riverbed sediment and left to stand for 48 hours. After settling, a cementing solution containing urea and calcium chloride is injected into the same sediment area in stages. Each injection is allowed to stand for 48 hours to complete the passivation-solidification treatment of the sediment.

2. The method for treating riverbed sediment by microbial-induced carbonate precipitation according to claim 1, characterized in that, The preparation method of the iron / calcium dual-loaded functionalized biochar includes: using agricultural straw as raw material, pyrolyzing it at 500℃ and under N2 atmosphere to prepare basic biochar; impregnating the basic biochar in a mixed solution of FeCl3 and CaCl2 for loading; and then adjusting the pH to 10.0 with an alkaline solution to allow iron and calcium to be deposited on the surface and in the pores of the biochar in the form of hydroxides / oxides.

3. The method for treating riverbed sediment by microbial-induced carbonate precipitation according to claim 1, characterized in that, The preparation method of the microbial and biochar composite agent is as follows: the iron / calcium dual-load functionalized biochar and the bacterial solution of urease-producing bacteria are mixed at a solid-liquid ratio of 1:10, and the mixture is allowed to stand for 6 hours to immobilize, so that the bacteria are attached to the biochar.

4. The method for treating riverbed sediment by microbial-induced carbonate precipitation according to claim 1, characterized in that, The urease-producing bacterium is *Sporosarcina pasteurii*.

5. The method for treating riverbed sediment by microbial-induced carbonate precipitation according to claim 1, characterized in that, In the cementing solution, the concentrations of urea and calcium chloride are both 0.5 mol / L or 1.0 mol / L.

6. The method for treating riverbed sediment by microbial-induced carbonate precipitation according to claim 1, characterized in that, The cementing liquid is injected in 3 to 4 equal amounts, and the standing time after each injection is 48 hours.

7. The method for treating riverbed sediment by microbial-induced carbonate precipitation according to claim 1, characterized in that, When injecting the microbial and biochar composite agent, the injection depth is 20-40 cm below the bottom mud, the injection pressure is 0.2-0.5 MPa, and the total injection volume is 20-30 L / m².

8. The method for treating riverbed sediment by microbial-induced carbonate precipitation according to claim 1, characterized in that, It also includes the step of resource utilization of the treated sediment: when the 28-day unconfined compressive strength of the treated sediment is ≥150 kPa and the heavy metal leaching concentration is lower than the Class IV water standard of GB 3838-2002, it can be used as ecological slope protection material or artificial wetland substrate.

9. The method for treating riverbed sediment by microbial-induced carbonate precipitation according to claim 2, characterized in that, In the iron / calcium dual-load functionalized biochar, the iron loading is 5% to 8% and the calcium loading is 3% to 5% by dry weight of the biochar.

10. The method for treating riverbed sediment by microbial-induced carbonate precipitation according to claim 1, characterized in that, The step of injecting cementing solution in stages can avoid the rapid increase in calcium carbonate supersaturation in pore water caused by a single large injection, and enable calcium carbonate crystals to grow directionally at the contact points of sediment particles and on the surface of biochar carrier, thereby achieving effective cementation.