In-situ method and device for remediation of hexavalent chromium contaminated groundwater

Abstract

The present application relates to the technical field of groundwater pollution remediation, and particularly relates to a method and device for in-situ remediation of hexavalent chromium contaminated groundwater, wherein the device comprises a multifunctional well group, a medicament supply system, an aeration system, a monitoring system and a control system; the multifunctional well group comprises a plurality of multifunctional wells; the medicament supply system is used for pretreating medicaments, modified composite reducing agents and composite functional bacteria liquid respectively; the control system is used for receiving monitoring data; the control system is also used for displaying monitoring data and equipment operating states, and setting operating parameters by a user; the control system is also used for automatically adjusting medicament dosing amount, aeration flow and aeration pressure according to monitoring data and operating parameters set by the user. Through the design of the multifunctional well group, the medicament supply system and the aeration system, the hexavalent chromium contaminated groundwater can be simultaneously subjected to chemical reduction, biological reinforcement and adsorption fixation, so that the three processes are coordinated to realize rapid, efficient and long-acting remediation of the hexavalent chromium contaminated groundwater.

Description

Technical Field

[0001] This invention relates to the field of groundwater pollution remediation technology, specifically to a method and apparatus for in-situ synergistic remediation of hexavalent chromium-contaminated groundwater. Background Technology

[0002] Groundwater is a precious freshwater resource on Earth and one of the most important water sources for human production and daily life. However, with the rapid development of industrial production (such as electroplating, metallurgy, chemical industry, and leather processing), the disorderly discharge of wastewater and waste containing hexavalent chromium has led to the pollution of large amounts of groundwater by hexavalent chromium. Hexavalent chromium (Cr(VI)) has strong oxidizing properties, high toxicity, and high mobility. It is easily soluble in water and can enter the human body through drinking water and skin contact. It has significant carcinogenic, teratogenic, and mutagenic effects and is listed as one of the priority heavy metal pollutants controlled by various countries, posing a serious threat to the ecological environment and human health. Currently, the remediation technologies for hexavalent chromium-contaminated groundwater are mainly divided into two categories: ex-situ remediation and in-situ remediation. Ex-situ remediation requires the extraction and treatment of contaminated groundwater, which has drawbacks such as large engineering workload, high energy consumption, high risk of secondary pollution, and significant site disturbance. Moreover, it is difficult to achieve complete remediation of deep groundwater pollution. In-situ remediation has become the mainstream technology for remediating hexavalent chromium-contaminated groundwater due to its advantages such as no need for excavation, minimal environmental disturbance, low remediation cost, and ability to remediate deep contamination.

[0003] Existing in-situ remediation technologies mainly include single remediation methods such as chemical reduction remediation, bioremediation, and adsorption remediation. Among them, chemical reduction remediation reduces hexavalent chromium to low-toxicity and insoluble trivalent chromium (Cr(III)) by injecting reducing agents (such as ferrous sulfate, calcium polysulfide, and zero-valent iron), but it has problems such as large agent consumption, remediation effect being easily affected by environmental factors such as groundwater pH and dissolved oxygen, and easy secondary release of reduction products, making it difficult to achieve long-term remediation. Bioremediation uses the metabolic action of microorganisms to reduce hexavalent chromium to trivalent chromium, which has advantages such as environmental protection and low cost, but the activity of microorganisms is easily limited by the concentration of pollution and environmental conditions, and the remediation rate is slow, making it difficult to be used for rapid remediation of groundwater contaminated with medium to high concentrations of hexavalent chromium. Adsorption remediation removes hexavalent chromium by injecting adsorbent materials (such as modified attapulgite and activated carbon), but the adsorbent materials are easily saturated and need to be replaced or regenerated regularly, increasing the remediation cost and operational complexity.

[0004] To address the shortcomings of existing in-situ remediation technologies for hexavalent chromium-contaminated groundwater, such as low remediation efficiency, unstable effects, poor environmental adaptability, high costs, and susceptibility to secondary pollution, this invention provides a method and apparatus for the synergistic in-situ remediation of hexavalent chromium-contaminated groundwater. Through the synergistic effects of chemical reduction, bio-enhancing, and adsorption fixation, it achieves rapid, efficient, and long-term remediation of hexavalent chromium-contaminated groundwater, while reducing remediation costs, minimizing the risk of secondary pollution, and adapting to the remediation needs of hexavalent chromium-contaminated groundwater under different concentrations and environmental conditions. Summary of the Invention

[0005] To address the aforementioned issues, this invention provides a method and apparatus for the in-situ synergistic remediation of hexavalent chromium-contaminated groundwater. Through the design of a multifunctional well group, a reagent supply system, and an aeration system, chemical reduction, bio-enhancing, and adsorption fixation of hexavalent chromium-contaminated groundwater can be carried out simultaneously, enabling the three systems to operate in a coordinated manner and achieving rapid, efficient, and long-term remediation of hexavalent chromium-contaminated groundwater.

[0006] To achieve the above objectives, the technical solution of the present invention is as follows: an in-situ synergistic remediation device for hexavalent chromium contaminated groundwater, comprising a multifunctional well group, a reagent supply system, an aeration system, a monitoring system, and a control system.

[0007] The multi-functional well group consists of several multi-functional wells, which are vertically inserted into the contaminated groundwater aquifer. The bottom of each multi-functional well is covered with a permeable screen pipe, which is wrapped with a filter layer. The multi-functional wells are arranged in a quincunx pattern, with a spacing of 5-10m between adjacent wells. The layout of the multi-functional wells is determined based on the hydrogeological conditions of the site, and the radius of influence is 5-10m. Each multi-functional well is detachably connected to a sealing cap, which has an injection port, an aeration port, and a sampling port.

[0008] The reagent supply system is used to pretreat the reagents, modified composite reducing agents, and composite functional bacterial solutions respectively.

[0009] An aeration system is used to inject air into groundwater.

[0010] The monitoring system is used to collect and store groundwater remediation indicators in real time, forming monitoring data. The monitoring system includes several monitoring wells, each with several monitoring sensors fixedly connected inside. Each monitoring well has a permeable structure at the bottom. The monitoring wells are located at the center of the multi-functional well group and the edge of the contaminated area. The monitoring sensors include hexavalent chromium concentration sensors, pH sensors, and dissolved oxygen sensors. The monitoring data includes hexavalent chromium concentration, pH value, and dissolved oxygen concentration.

[0011] The control system is used to receive monitoring data; it also displays monitoring data and equipment operating status, and allows users to set operating parameters; the control system is also used to automatically adjust the dosage of chemicals, aeration flow rate and aeration pressure based on the monitoring data and the operating parameters set by the user.

[0012] Furthermore, the drug supply system includes several delivery units.

[0013] The delivery unit is used to precisely control the dosage of the reagent; the delivery unit includes a reagent storage tank, a metering pump and a delivery pipeline; the metering pump is connected to the reagent storage tank; one end of the delivery pipeline is connected to the metering pump, and the other end of the delivery pipeline is connected to the reagent injection interface of the multi-functional well; the delivery pipeline is equipped with valves and flow sensors.

[0014] Furthermore, the aeration system includes an air compressor, aeration pipes, and aeration heads. The air compressor is connected to the aeration interface of the multi-functional well through the aeration pipes. The aeration heads are located at the bottom of the multi-functional well, above the permeable screen pipes. A pressure flow meter is fixedly connected to the aeration pipes. The aeration system is used to control the operation of the pressure flow meter.

[0015] Furthermore, the control system includes a controller and a touch display screen. The controller is used to control the operation of the metering pump, air compressor, flow sensor, pressure sensor, hexavalent chromium concentration sensor, pH sensor, and dissolved oxygen sensor.

[0016] The technical principle of the above solution is as follows: Chemical remediation is achieved by injecting reagents into the aquifer through a multi-functional well. Under pressure injection, the reagents migrate along the pores of the aquifer, chemically remediating the entire contaminated area. The reagent supply system uses metering pumps, flow sensors, and valves to independently add pretreatment reagents, modified composite reducing agents, and composite functional bacterial solutions. The aeration system delivers air to the aeration heads at the bottom of the multi-functional well via an air compressor. The air enters the aquifer through permeable screens, increasing the dissolved oxygen concentration in the groundwater and providing aerobic metabolic conditions for the composite functional bacteria, accelerating the microbial reduction and transformation of hexavalent chromium. Fixed hexavalent chromium concentration, pH, and dissolved oxygen sensors within the monitoring well continuously collect groundwater remediation indicators, generating monitoring data. The control system automatically switches or combines reagent injection sequences and dosages, and adjusts aeration flow and pressure based on the real-time feedback of hexavalent chromium concentration, pH, and dissolved oxygen data from the monitoring well.

[0017] The above approach has the following beneficial effects: 1. This invention utilizes a synergistic approach combining chemical reduction, bioremediation, and adsorption fixation. Chemical reduction rapidly lowers the concentration of hexavalent chromium, creating suitable conditions for bioremediation. Bioremediation, through the metabolic activity of a composite of functional bacteria, continuously reduces residual hexavalent chromium to trivalent chromium, achieving deep remediation. Simultaneously, the soybean hull pyrolysis powder in the composite reducing agent possesses adsorption properties, adsorbing and fixing the reduced trivalent chromium and trace amounts of unreduced hexavalent chromium, preventing secondary release. The synergistic effect of these three methods balances remediation efficiency and long-term stability, resulting in a significant synergistic remediation effect. After remediation, the hexavalent chromium concentration can stably reach the Class III groundwater quality standard, demonstrating superior remediation performance compared to single remediation methods.

[0018] 2. Through the design of the control system, this invention can realize the automated control of agent dosing, aeration and monitoring, without the need for a lot of manual operation, thus reducing labor intensity; the monitoring system can provide real-time feedback on the repair effect, which can facilitate timely adjustment of repair parameters and ensure that the repair process is stable and efficient.

[0019] 3. This invention uses a quincunx pattern to arrange the multi-functional wells. The layout of the multi-functional wells is combined with the hydrogeological conditions of the site to determine the radius of influence, which is about 5-10m. The agent can be evenly diffused to the entire polluted area, avoiding blind spots in the remediation and improving the effect of sewage treatment.

[0020] Furthermore, an in-situ synergistic remediation method for hexavalent chromium-contaminated groundwater includes the following steps: S1, Contaminated Site Investigation and Monitoring: Determine the scope of pollution, pollution concentration and hydrogeological parameters, and deploy multi-functional wells and monitoring wells.

[0021] S2, Pretreatment: Inject pH adjuster to adjust the pH of groundwater to 6.0-8.0.

[0022] S3, Chemical Reduction Treatment: By injecting a modified composite reducing agent through a multi-functional well, the concentration of hexavalent chromium is reduced to below 1.5 mg / L.

[0023] S4, Bio-enhanced Remediation: Injecting compound functional bacterial solution and aerating it reduces the concentration of hexavalent chromium to below 0.05 mg / L.

[0024] S5, Post-remediation monitoring: Continuously monitor groundwater indicators for no less than 6 months.

[0025] Furthermore, the modified composite reducing agent is composed of soybean hull pyrolysis powder, nano-zero valent iron, and carboxymethyl cellulose, with the addition ratio of each component being 3-6:2-4:1-3.

[0026] Furthermore, the compound functional bacterial solution consists of *Pseudomonas schlegelii*, *Bacillus longiformis*, and *Bacillus cereus*, with a cell density ratio of 2-6:1-3:1, and an active bacterial cell density of 102.7 -10 10 per mL.

[0027] Furthermore, the pH adjuster is sodium bicarbonate at a concentration of 5-10 g / L; sodium percarbonate is also injected as an oxygen supply agent in the pretreatment step, with a dosage of 0.6-0.8 g / m³. 3 groundwater.

[0028] Furthermore, in S3, the injection rate of the modified composite reducing agent is 0.5-1.0 m. 3 / h.

[0029] Furthermore, in S4, the addition ratio of the compound functional bacterial solution to the modified compound reducing agent is 230-450mL:60-90g, the aeration time is 20-30min, and the continuous repair lasts for 7-15 days.

[0030] Beneficial effects: This method can rapidly reduce the concentration of hexavalent chromium, and the bio-enhanced remediation rate is higher than that of existing bioremediation technologies. The entire remediation cycle can be shortened to 20-30 days, which is significantly shorter than existing technologies. At the same time, this method is applicable to the remediation of shallow, intermediate and deep groundwater pollution, and has a wide range of applications. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the in-situ synergistic remediation device for hexavalent chromium-contaminated groundwater according to the present invention.

[0032] Figure 2 This is a schematic diagram illustrating the steps of the in-situ synergistic remediation method for hexavalent chromium-contaminated groundwater according to the present invention.

[0033] The reference numerals in the accompanying drawings of the instruction manual include: 1. Chemical storage tank; 2. Chemical compounding tank; 3. Multifunctional well group; 4. Monitoring well; 5. Gas storage tank; 6. Air compressor; 7. Filter; 8. Safety valve; 9. Pressure flow meter; 10. Injection pump; 11. Flow regulating valve; 12. Metering pump; 13. Ball valve; 14. Control system; 15. Aeration pipeline; 16. Multifunctional well pipe. Detailed Implementation

[0034] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0035] In the description of this invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing the invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0036] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0037] The following detailed description illustrates the specific implementation method: Example 1:

[0038] like Figure 1 As shown, an in-situ co-remediation device for hexavalent chromium contaminated groundwater includes a multifunctional well group 3, a reagent supply system, an aeration system, a monitoring system, and a control system 14.

[0039] Multifunctional well group 3 includes several multifunctional wells, which are vertically inserted into the polluted groundwater aquifer. The bottom of each multifunctional well is covered with a permeable screen pipe, which is wrapped with a filter layer. The multifunctional wells are arranged in a quincunx pattern, with a spacing of 5-10m between adjacent multifunctional wells. The layout of the multifunctional wells is determined based on the hydrogeological conditions of the site, and the influence radius is 5-10m. The top of each multifunctional well is detachably fitted with a sealing cap, which has an injection port, an aeration port, and a sampling port.

[0040] The reagent supply system is used to pretreat reagents, modified composite reducing agents, and composite functional bacterial solutions respectively.

[0041] The chemical supply system includes several delivery units; the delivery units are used to precisely control the amount of chemical added; the delivery unit includes a chemical storage tank 1, a metering pump 12 and a delivery pipeline; the metering pump 12 is connected to the chemical storage tank 1; one end of the delivery pipeline is connected to the metering pump 12, and the other end of the delivery pipeline is connected to the chemical injection interface of the multi-functional well (the part of the delivery pipeline located inside the multi-functional well is named multi-functional well pipe 16); valves and flow sensors are installed on the delivery pipeline.

[0042] In this embodiment, the delivery pipeline is also connected to a drug compounding tank 2 as in the prior art. The metering pump 12 is located between the drug storage tank and the drug compounding tank 2, which is convenient for the operator to adjust the drug. The end of the drug compounding tank 2 away from the drug storage tank is also connected in sequence to a flow regulating valve 11 and an injection pump 10, which is convenient for the operator to control the delivery of the drug.

[0043] The aeration system is used to inject air into the groundwater. The aeration system includes an air compressor 6, an aeration pipe 15, and an aeration head. The air compressor 6 is connected to the aeration interface of the multi-functional well through the aeration pipe 15. The aeration head is located at the bottom of the multi-functional well, above the permeable screen pipe. A pressure flow meter 9 is fixedly attached to the aeration pipe 15. The aeration system is used to control the operation of the pressure flow meter 9.

[0044] In this embodiment, the air inlet end of the aeration pipe 15 is connected to a filter 7, which can filter out impurities in the gas and improve the purification quality. The aeration pipe 15 is also connected to a gas storage tank 5 in the prior art. The top of the gas storage tank 5 is connected to a safety valve 8, and the bottom of the gas storage tank 5 is connected to a ball valve 13 to ensure the feasibility and safety of gas transportation.

[0045] The monitoring system is used to collect and store groundwater remediation indicators in real time, forming monitoring data. The monitoring system includes several monitoring wells 4, each with several monitoring sensors fixedly bonded inside. Each monitoring well 4 has a permeable structure installed at its bottom (in this embodiment, the permeable structure is also a permeable screen pipe). The monitoring wells 4 are all located at the center of the multifunctional well group 3 and the edge of the contaminated area. The monitoring sensors include a hexavalent chromium concentration sensor, a pH sensor, and a dissolved oxygen sensor. The monitoring data includes hexavalent chromium concentration, pH value, and dissolved oxygen concentration.

[0046] The control system 14 is used to receive monitoring data; the control system 14 is also used to display monitoring data and equipment operating status, and to allow users to set operating parameters; the control system 14 is also used to automatically adjust the dosage of reagents, aeration flow rate, and aeration pressure according to the monitoring data and the operating parameters set by the user. The control system 14 includes a controller and a touch screen display. The controller is used to control the operation of the metering pump 12, air compressor 6, flow sensor, pressure sensor, hexavalent chromium concentration sensor, pH sensor, and dissolved oxygen sensor.

[0047] The specific implementation process is as follows: A landfill site was found to be contaminated with hexavalent chromium in its surrounding groundwater, with a contaminated area of ​​1200 m². 2 The pollution depth is 3-10m, the concentration of hexavalent chromium in the groundwater is 2.1-10.3mg / L, the pH value is 6.5-7.8, and the dissolved oxygen concentration is 1.5-2.2mg / L. The remediation device of this invention is used for in-situ remediation.

[0048] First, 18 multi-functional wells were laid out in a quincunx pattern in and around the polluted area, with an adjacent spacing of 7m. Then, 6 monitoring wells were laid out. The concentration of hexavalent chromium, pH value and dissolved oxygen concentration were collected by hexavalent chromium concentration sensor, pH sensor and dissolved oxygen sensor respectively.

[0049] Subsequently, sodium bicarbonate (concentration 6 g / L) was injected into the contaminated area through a multi-functional well to adjust the pH value of the contaminated area to 6.5-7.0; sodium percarbonate was also injected at a dosage of 0.6 g / m³. 3 Groundwater maintains dissolved oxygen concentration at 1.8-2.2 mg / L.

[0050] Subsequently, a modified composite reducing agent was injected through a multi-functional well at an injection rate of 0.6 m. 3 / h; After continuous injection for 2 days, the concentration of hexavalent chromium in monitoring well 4 decreased to below 1.4 mg / L. The modified composite reducing agent is composed of soybean hull pyrolysis powder, nano-zero valent iron, and carboxymethyl cellulose, with a component addition ratio of 5:2:1.

[0051] The compound functional bacterial solution was then injected through a multi-functional well; the ratio of bacterial solution to reducing agent was 280 mL: 65 g; aeration was maintained to keep the dissolved oxygen concentration above 2.0 mg / L, and the treatment continued for 8 days until the hexavalent chromium concentration dropped below 0.04 mg / L. The compound functional bacterial solution consisted of *Pseudomonas schlegelii*, *Bacillus longiformis*, and *Bacillus cereus*, with a cell density ratio of 3:1:1 and an active bacterial cell density of 10-1. 8 The strains listed above are all non-pathogenic bacteria commonly used in environmental remediation and readily available to the public. Those skilled in the art can obtain them through commercial channels or by isolating and screening from known environmental samples using well-known microbiological techniques; there are no barriers to acquisition.

[0052] The compound functional bacteria are selected from chromium-resistant and acid- and alkali-resistant strains, and the compound reducing agents have good environmental adaptability. They are suitable for the remediation of hexavalent chromium-contaminated groundwater with different pH values ​​(5.0-9.0) and different pollution concentrations (0.5-50 mg / L). They have good environmental adaptability and can be adapted to different types of contaminated sites such as industrial sites and landfills.

[0053] After six months of post-remediation monitoring, the concentration of hexavalent chromium remained stable and met the standard (in this embodiment, the standard for stable compliance was Class III in the Groundwater Quality Standard (GB / T 14848-2017)). No secondary pollution was observed, indicating a good remediation effect. The six-month continuous stable monitoring results demonstrate that, through the synergistic effect of chemical reduction, bioremediation, and adsorption fixation, particularly the long-term stabilization and fixation of chromium by soybean hull biochar and the continuous purification effect of functional microorganisms, the pollution plume was effectively controlled, completely overcoming the shortcomings of traditional chemical reduction methods, which are prone to rebound or secondary release. Based on this significant stabilization effect, it can be expected that this method will achieve long-term safety over a longer timescale.

[0054] The remediation device designed in this embodiment is easy to operate and highly automated. The control system 14 can realize the automated control of reagent dosing, aeration and monitoring, without the need for a lot of manual operation, thus reducing labor intensity. The monitoring system can provide real-time feedback on the remediation effect, which is convenient for timely adjustment of remediation parameters and ensures that the remediation process is stable and efficient. The multi-functional wells are arranged in a quincunx pattern, which can evenly diffuse the reagent to the entire contaminated area and avoid remediation blind spots.

[0055] Example 2: Unlike the embodiments described above, as Figure 2 As shown, an in-situ synergistic remediation method for hexavalent chromium-contaminated groundwater includes the following steps: S1, Contaminated Site Investigation and Monitoring: Determine the pollution range, pollution concentration and hydrogeological parameters, and deploy multi-functional wells and monitoring wells 4.

[0056] S2, Pretreatment: A pH adjuster is injected to adjust the groundwater pH to 6.0. The pH adjuster is sodium bicarbonate at a concentration of 5 g / L. Sodium percarbonate is also injected as an oxygenating agent in the pretreatment step, at a dosage of 0.6 g / m³. 3 groundwater.

[0057] S3, Chemical Reduction Treatment: A modified composite reducing agent is injected through a multi-functional well to reduce the hexavalent chromium concentration to below 1.5 mg / L (in this embodiment, the dosage ratio of the modified composite reducing agent components is soybean hull pyrolysis powder: nano-zero valent iron: carboxymethyl cellulose = 4:3:2; the injection rate of the modified composite reducing agent is 0.8 m). 3 / h.

[0058] S4, Bio-enhanced Remediation: Injecting and aerating the compound functional bacterial solution to reduce the hexavalent chromium concentration to below 0.05 mg / L (in this embodiment, the cell density ratio of Pseudomonas stearothermii, Bacillus lysine spp., and Bacillus cereus in the compound functional bacterial solution is 4:2:1, and the active bacterial cell density is 5×10⁻⁶). 8The ratio of compound functional bacterial solution to modified compound reducing agent was 300mL:75g, the aeration time was 25min, and the treatment lasted for 10 days.

[0059] S5, Post-remediation monitoring: Continuously monitor groundwater indicators for no less than 6 months.

[0060] The specific implementation method is as follows: The groundwater at the decommissioned site of an electroplating plant was contaminated with hexavalent chromium, covering an area of ​​800 m² and a depth of 5-12 m. The concentration of hexavalent chromium in the groundwater was 5.2-18.6 mg / L, the pH value was 7.2-8.5, and the dissolved oxygen concentration was 1.2-2.0 mg / L. The remediation method and device of this invention were used for in-situ remediation.

[0061] The specific steps are as follows: (1) Contaminated site investigation and monitoring: 12 multi-functional wells were set up in the contaminated area and its surroundings in a quincunx pattern, with an 8m spacing between adjacent multi-functional wells; 4 monitoring wells were set up, located at the center of the multi-functional well group 3 and the edge of the contaminated area respectively; groundwater indicators were monitored in real time using hexavalent chromium concentration sensor, pH sensor and dissolved oxygen sensor.

[0062] (2) Pretreatment: Sodium bicarbonate (concentration 8 g / L) was injected through monitoring well 4 to adjust the pH of the groundwater to 7.0-7.5; sodium percarbonate was injected as an oxygen supply agent at a dosage of 0.8 g / m³. 3 Groundwater maintains the dissolved oxygen concentration at 2.0-2.5 mg / L.

[0063] (3) Chemical reduction pretreatment: Modified composite reducing agent is injected under pressure through a multi-functional well at an injection rate of 0.8 m. 3 / h; After 3 days of continuous injection, the concentration of hexavalent chromium in monitoring well 4 decreased to below 1.2 mg / L.

[0064] (4) Bio-enhanced remediation: Inject composite functional bacterial solution into the multifunctional well; after injection, continuously inject compressed air for 25 minutes, and then continuously aerate to maintain the dissolved oxygen concentration at 2.0-2.5 mg / L. After 10 days of continuous remediation, monitor the concentration of hexavalent chromium in well 4 to decrease to below 0.03 mg / L.

[0065] (5) Post-remediation monitoring: After the remediation was completed, continuous monitoring was conducted for 6 months. The concentration of hexavalent chromium in the groundwater remained stable below 0.03 mg / L, and the pH value and dissolved oxygen and other indicators met the Class III standard in the "Groundwater Quality Standard" (GB / T 14848-2017). The remediation effect was stable.

[0066] During the remediation process, the pyrolysis of soybean hulls decomposes the cellulose and hemicellulose within them into porous biochar. This biochar is rich in active groups such as hydroxyl and carboxyl groups, which can fix Cr(VI) through electrostatic adsorption and complexation. Simultaneously, it acts as a dispersing framework for the reducing agent, preventing agglomeration. It also possesses weak reducing properties, assisting in the reduction of Cr(VI) to Cr(III). The resulting aromatic carbon structure exhibits high stability and is not easily washed away by groundwater, extending the remediation duration. It adsorbs Cr(VI) anions, increasing the local concentration of pollutants and accelerating the reduction reaction. The composite reducing agent uses agricultural waste such as soybean hulls as raw materials, and the composite functional bacteria can naturally reproduce in the underground environment without continuous addition, reducing remediation costs. The entire remediation process generates no toxic or harmful substances and does not cause secondary pollution, aligning with the concept of green and environmentally friendly remediation.

[0067] Nano-sized zero-valent iron provides an active iron source and is the core functional component for reducing Cr(VI). It possesses extremely strong reducing activity, rapidly reducing highly toxic and highly mobile Cr(VI) to less toxic and sparingly soluble Cr(III). Its reduction efficiency is far higher than existing iron salts, and its larger nanoscale surface area and faster reaction rate make it suitable for rapid remediation of medium- to high-concentration pollution. The reaction generates iron (hydride) oxides and chromium iron hydroxide precipitates, achieving in-situ stable solidification of Cr(III) and preventing secondary leaching. Simultaneously, in synergy with soybean hull biochar, it can prevent nZVI aggregation and oxidation, improving its mobility and long-term effectiveness in groundwater.

[0068] Carboxymethyl cellulose (CMC) possesses a long-chain structure and numerous -OH and -COOH groups, which can create steric hindrance, stabilizing the composite agent system. Simultaneously, it enhances the migration ability of the agent in the porous media of the aquifer, acting as a dispersant, stabilizer, and thickener to prevent rapid sedimentation and aggregation of remediation agents in groundwater. It can improve the suspension of the agent, allowing it to migrate with groundwater and expand the remediation range. It also provides hydrophilic groups, enhancing the contact efficiency with groundwater and Cr(VI). Furthermore, it can complex some heavy metal ions, aiding in fixation.

[0069] This method can simultaneously perform chemical reduction, bio-enhanced treatment, and adsorption fixation on hexavalent chromium-contaminated groundwater, ensuring the coordinated operation of these three processes. Chemical reduction rapidly lowers the concentration of hexavalent chromium, creating suitable conditions for bioremediation. Bio-enhanced remediation, through the metabolic action of composite functional bacteria, continuously reduces residual hexavalent chromium to trivalent chromium, achieving deep remediation. Simultaneously, the soybean hull pyrolysis powder in the composite reducing agent possesses adsorption properties, adsorbing and fixing the reduced trivalent chromium and trace amounts of unreduced hexavalent chromium, preventing secondary release. The synergistic effect of these three processes balances remediation efficiency and long-term stability, resulting in a significant synergistic remediation effect. After remediation, the concentration of hexavalent chromium can stably reach the Class III groundwater quality standard, demonstrating a remediation effect superior to single remediation methods.

[0070] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A device for in-situ synergistic remediation of hexavalent chromium-contaminated groundwater, characterized in that, It includes a multi-functional well group (3), a chemical supply system, an aeration system, a monitoring system, and a control system (14). The multi-functional well group (3) includes several multi-functional wells. The multi-functional wells are vertically inserted into the polluted groundwater aquifer. The bottom of the multi-functional well is covered with a permeable screen pipe, and the permeable screen pipe is wrapped with a filter layer. The multi-functional wells are arranged in a quincunx pattern, and the distance between adjacent multi-functional wells is 5-10m. The radius of influence for the placement of multi-functional wells is determined based on the hydrogeological conditions of the site, and the radius of influence is 5-10m. Each multi-functional well has a detachable sealing cover at the top, and each sealing cover has an injection port, an aeration port, and a sampling port. A reagent supply system is used to pretreat reagents, modified composite reducing agents, and composite functional bacterial solutions separately; An aeration system is used to inject air into groundwater; The monitoring system is used to collect and store groundwater remediation indicators in real time, forming monitoring data; The monitoring system includes several monitoring wells (4), each of which is fixedly connected with several monitoring sensors. The bottom of each monitoring well (4) is equipped with a permeable structure. The monitoring wells (4) are located in the center of the multi-functional well group (3) and at the edge of the polluted area. The monitoring sensors include a hexavalent chromium concentration sensor, a pH sensor, and a dissolved oxygen sensor. The monitoring data includes hexavalent chromium concentration, pH value, and dissolved oxygen concentration; The control system (14) is used to receive monitoring data; The control system (14) is also used to display monitoring data and equipment operating status, and to allow users to set operating parameters; The control system (14) is also used to automatically adjust the dosage of the agent, the aeration flow rate and the aeration pressure according to the monitoring data and the operating parameters set by the user.

2. The in-situ synergistic remediation device for hexavalent chromium-contaminated groundwater according to claim 1, characterized in that, The drug supply system includes several delivery units; The delivery unit is used to precisely control the dosage of the reagent; The delivery unit includes a drug storage tank (1), a metering pump (12), and a delivery pipeline; the metering pump (12) is connected to the drug storage tank (1); one end of the delivery pipeline is connected to the metering pump (12), and the other end of the delivery pipeline is connected to the drug injection interface of the multi-functional well; valves and flow sensors are provided on the delivery pipeline.

3. The in-situ synergistic remediation device for hexavalent chromium-contaminated groundwater according to claim 2, characterized in that, The aeration system includes an air compressor (6), an aeration pipe (15), and an aeration head. The air compressor (6) is connected to the aeration interface of the multi-functional well through the aeration pipe (15). The aeration head is located at the bottom of the multi-functional well, above the permeable screen pipe. A pressure flow meter (9) is fixedly connected to the aeration pipe (15). The aeration system is used to control the operation of the pressure flow meter (9).

4. The in-situ synergistic remediation device for hexavalent chromium-contaminated groundwater according to claim 3, characterized in that, The control system (14) includes a controller and a touch display screen. The controller is used to control the operation of the metering pump (12), the air compressor (6), the flow sensor, the pressure sensor, the hexavalent chromium concentration sensor, the pH sensor and the dissolved oxygen sensor.

5. A method for in-situ synergistic remediation of hexavalent chromium-contaminated groundwater, comprising the in-situ synergistic remediation apparatus for hexavalent chromium-contaminated groundwater according to any one of claims 1-4, characterized in that, Includes the following steps: S1, Pollution site investigation and monitoring: Determine the pollution range, pollution concentration and hydrogeological parameters, and set up multi-functional wells and monitoring wells (4). S2, Pretreatment: Inject pH adjuster to adjust the pH of groundwater to 6.0-8.0; S3, Chemical Reduction Treatment: By injecting a modified composite reducing agent through a multi-functional well, the concentration of hexavalent chromium is reduced to below 1.5 mg / L; S4, Bio-enhanced Remediation: Injecting compound functional bacterial solution and aerating it to reduce the concentration of hexavalent chromium to below 0.05 mg / L; S5, Post-remediation monitoring: Continuously monitor groundwater indicators for no less than 6 months.

6. The in-situ synergistic remediation method for hexavalent chromium-contaminated groundwater according to claim 5, characterized in that, The modified composite reducing agent is composed of soybean hull pyrolysis powder, nano-zero valent iron and carboxymethyl cellulose, with the addition ratio of each component being 3-6:2-4:1-3.

7. The in-situ synergistic remediation method for hexavalent chromium-contaminated groundwater according to claim 6, characterized in that, The compound functional bacterial solution consists of *Pseudomonas schlegelii*, *Bacillus longiformis*, and *Bacillus cereus*, with a cell density ratio of 2-6:1-3:1, and an active bacterial cell density of 102. 7 -10 10 per mL.

8. The in-situ synergistic remediation method for hexavalent chromium-contaminated groundwater according to claim 7, characterized in that, The pH adjuster is sodium bicarbonate at a concentration of 5-10 g / L; sodium percarbonate is also injected as an oxygen supplier in the pretreatment step, at a dosage of 0.6-0.8 g / m³. 3 groundwater.

9. The in-situ synergistic remediation method for hexavalent chromium-contaminated groundwater according to claim 8, characterized in that, In S3, the injection rate of the modified composite reducing agent is 0.5-1.0 m. 3 / h.

10. The in-situ synergistic remediation method for hexavalent chromium-contaminated groundwater according to claim 9, characterized in that, In S4, the dosage ratio of compound functional bacterial solution to modified compound reducing agent is 230-450mL:60-90g, the aeration time is 20-30min, and the continuous repair lasts for 7-15 days.

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