A multi-stage leaching system and method for hexavalent chromium contaminated soil leaching remediation

Abstract

The application provides a multistage leaching system and method for hexavalent chromium contaminated soil leaching repair, and belongs to the technical field of multistage leaching methods. The multistage leaching system and method for hexavalent chromium contaminated soil leaching repair comprises a leaching unit including a leaching tank body, a porous spray pipe network arranged at the bottom of the leaching tank body, and a MOF-based leaching agent storage tank connected with the porous spray pipe network through an acid and alkali resistant pipeline. The MOF-based leaching agent storage tank stores a composite leaching agent. A filtering unit includes a MOF@activated carbon composite column, and the inlet of the MOF@activated carbon composite column is connected with a flow guide groove through a pipeline. A regeneration unit includes a hydrochloric acid elution tank and a regeneration liquid storage tank, and the regeneration liquid storage tank is connected with the backwashing inlet of the MOF@activated carbon composite column through a pipeline. The regeneration liquid storage tank is communicated with the MOF-based leaching agent storage tank through a pipeline. A fluid driving assembly includes a peristaltic pump and a manual valve arranged on the pipeline, and solves the problems of low repair efficiency and high secondary pollution risk existing in traditional leaching repair technology.

Description

Technical Field

[0001] This invention belongs to the technical field of multi-stage leaching methods, specifically, it relates to a multi-stage leaching system and method for the leaching remediation of hexavalent chromium contaminated soil. Background Technology

[0002] Soil hexavalent chromium pollution, due to its high toxicity and recalcitrant nature, has become a major challenge in global environmental governance. Leaching remediation technology, as one of the mainstream methods, utilizes physicochemical interactions (such as complexation, dissolution, and adsorption) between leaching agents and hexavalent chromium in the soil to transfer the pollutant from the solid phase to the liquid phase, thereby achieving separation and removal. A typical leaching system usually includes a leaching agent preparation unit, a spraying execution unit, a solid-liquid separation unit, and a wastewater treatment unit. Its core lies in disrupting the binding equilibrium between hexavalent chromium and soil particles through chemical action, thereby increasing the mobility of the pollutant. In acidic soil environments, hexavalent chromium is more likely to combine with hydrogen ions to form soluble chromate ions. Traditional leaching systems often enhance the dissolution effect by increasing the amount of acidic leaching agents (such as hydrochloric acid and sulfuric acid) or chelating agents (such as EDTA). However, these systems are usually designed based on a crude "single-stage leaching-end-of-pipe treatment" model, lacking a deep adaptation to the soil-pollutant interaction mechanism, making it difficult to balance remediation efficiency and environmental safety.

[0003] Traditional systems often employ single-stage spraying processes, resulting in short contact times and uneven action between the leaching agent and the soil. To maximize removal rates, excessive amounts of reagents are required, and unreacted reagents are carried into the wastewater treatment unit with the leachate. This not only increases costs but can also lead to soil compaction or secondary water pollution due to reagent residues. Traditional solid-liquid separation units (such as sedimentation tanks and filter presses) rely on gravity or mechanical pressure for separation, which is ineffective at retaining nanoscale hexavalent chromium complexes. Multiple leaching cycles are often required to meet standards, extending treatment cycles to weeks or even months. Traditional systems lack real-time monitoring and control capabilities for soil characteristics (such as pH and organic matter content) and leaching parameters (such as temperature and flow rate), making it difficult to address the dynamic changes in hexavalent chromium speciation in acidic soils. Traditional wastewater treatment systems often employ a "chemical reduction + precipitation" end-stage process, consuming large amounts of reducing agents (such as sodium sulfite) and alkalis (such as lime). Furthermore, the treated wastewater is difficult to reuse, resulting in significant water waste. Summary of the Invention

[0004] In view of this, the present invention provides a multi-stage leaching system and method for the remediation of hexavalent chromium contaminated soil, which solves the problems of low remediation efficiency and high risk of secondary pollution in traditional leaching remediation technologies.

[0005] This invention is implemented as follows:

[0006] This invention provides a multi-stage leaching system for the remediation of hexavalent chromium-contaminated soil, comprising:

[0007] Rinsing unit: includes rinsing tank, the bottom of which is provided with a porous spray pipe network, which is connected to a MOF-based rinsing agent storage tank through acid and alkali resistant pipes; the MOF-based rinsing agent storage tank stores a composite rinsing agent, and the bottom of the rinsing tank is provided with an inclined guide channel.

[0008] Filtration unit: includes MOF@activated carbon composite column, whose inlet is connected to the flow channel through a pipe, and whose outlet is connected to the regeneration unit;

[0009] Regeneration unit: includes a hydrochloric acid elution tank and a regeneration solution storage tank. The hydrochloric acid elution tank stores 0.1 mol / L hydrochloric acid solution and is connected to the backwash inlet of the MOF@activated carbon composite column through a pipeline. The regeneration solution storage tank is connected to the MOF-based eluent storage tank through a pipeline.

[0010] Fluid drive assembly: includes a peristaltic pump and a manual valve mounted on a pipeline, the peristaltic pump being used to drive the flow of rinsing agent and eluent.

[0011] MOF (Metal-Organic Frameworks) are crystalline porous materials with a periodic network structure, formed by the self-assembly of metal ions or clusters with organic ligands through coordination bonds. The NH2-UiO-66 used in this scheme is an amino-functionalized MOF material with the chemical composition Zr6O4(OH)4(NH2-BDC)6 (where NH2-BDC is 2-aminoterephthalic acid). It possesses a high specific surface area, tunable pore structure, and abundant amino functional groups, enabling it to remove hexavalent chromium from soil through both coordination complexation and physical adsorption.

[0012] The modular design of the rinsing unit, filtration unit, and regeneration unit achieves a closed-loop process of "rinsing agent spraying - hexavalent chromium adsorption - reagent regeneration," with a rinsing agent recycling rate of ≥85%. The guide channel design at the bottom of the rinsing tank, combined with the inclined bottom surface, allows the leachate to automatically collect and flow into the filtration unit, avoiding manual intervention and improving system operating efficiency. The combination of acid and alkali resistant pipelines and fluid drive components ensures stable reagent delivery, making it suitable for hexavalent chromium remediation in acidic soils (pH≤5).

[0013] Based on the above technical solution, the multi-stage leaching system for remediation of hexavalent chromium contaminated soil of the present invention can be further improved as follows:

[0014] The top of the rinsing tank is equipped with a spray head, which is connected to a porous spray pipe network. The spray pressure of the spray head is controlled at 0.2-0.5MPa by a pressure reducing valve.

[0015] The beneficial effects of adopting the above-mentioned improved scheme are as follows: the interconnected design of the spray head and the porous spray pipe network enables uniform distribution of the leaching agent; the spray pressure is controlled at 0.2-0.5 MPa by a pressure reducing valve, ensuring that the leaching agent penetrates into the soil pores (pore size 10-50 μm), and the hexavalent chromium removal rate is increased by 15-20%. The pressure-controllable spraying method avoids damage to the soil structure, and the organic matter loss rate is ≤5%, which is superior to traditional high-pressure leaching technology (loss rate ≥10%).

[0016] Furthermore, the composite column adopts a series structure, including a primary adsorption column and a secondary adsorption column. The primary adsorption column is filled with MOF@activated carbon particles, and the secondary adsorption column is filled with pure MOF particles. The packing density of the composite column is 0.8 g / cm³. 3 The composite column is made of polytetrafluoroethylene.

[0017] MOF@activated carbon particles have a particle size of 1-2 mm, while pure MOF particles have a particle size of 0.5-1 mm. A primary adsorption column is packed with MOF@activated carbon particles, utilizing the high specific surface area of ​​activated carbon (1000-1500 m²). 2 The PTFE (polytetrafluoroethylene) adsorption column rapidly adsorbs most of the hexavalent chromium. The pure MOF particles in the secondary adsorption column achieve high-precision removal through amino complexation, reducing the outlet hexavalent chromium concentration to below 0.1 mg / L (national standard limit 0.5 mg / L). The PTFE column is resistant to strong acids (pH ≤ 1) and strong alkalis (pH ≥ 14), making it suitable for hydrochloric acid elution during the regeneration stage (0.1 mol / L hydrochloric acid, pH ≈ 1).

[0018] Furthermore, a stirrer is installed inside the regenerated liquid storage tank, with a stirring speed of 100-200 rpm, for mixing the regenerated liquid with the supplemented NH2-UiO-66 powder.

[0019] The beneficial effects of adopting the above-mentioned improved scheme are as follows: the stirrer mixes the regenerated solution and the supplemented NH2-UiO-66 powder at a speed of 100-200 rpm, which can ensure uniform dispersion of the agent, avoid precipitation and agglomeration, and ensure that the MOF concentration in the regenerated solution fluctuates within ±5%, thus guaranteeing the effectiveness of recycling. Mechanical stirring does not require electric drive (it can be manually or hydraulically driven), making it suitable for repair sites without power, thereby reducing energy consumption costs.

[0020] Furthermore, the connecting pipes between the rinsing tank, the MOF-based rinsing agent storage tank, the MOF@activated carbon composite column, the hydrochloric acid elution tank, and the regenerated liquid storage tank are all made of PVDF material, with an inner diameter of 25-50mm.

[0021] The beneficial effects of adopting the above-mentioned improvement scheme are as follows: PVDF material pipes (inner diameter 25-50mm) have strong chemical corrosion resistance and can resist long-term erosion by hydrochloric acid washing solution (0.1mol / L) and acidic rinsing agents, with a service life of ≥5 years, reducing maintenance costs by 30% compared with traditional PVC pipes (service life ≤2 years).

[0022] The pipe's inner diameter is designed to ensure that fluid resistance loss is ≤10%, maintaining stable system flow.

[0023] This invention provides a multi-stage leaching method for remediation of hexavalent chromium contaminated soil, comprising the following steps:

[0024] S1: Primary leaching: MOF-based leaching agent is transported from the MOF-based leaching agent storage tank to the porous spray pipe network of the leaching tank by a peristaltic pump, and the contaminated soil is sprayed at a flow rate of 0.5-1L / min for 2-4 hours. The leachate flows into the MOF@activated carbon composite column through the guide channel.

[0025] S2: Secondary filtration: The leachate is sequentially passed through a primary adsorption column and a secondary adsorption column, and is adsorbed under gravity or pressure of 0.1-0.2 MPa at an adsorption temperature of 25-30℃.

[0026] S3.: Three-stage regeneration: When the hexavalent chromium concentration at the outlet of the MOF@activated carbon composite column exceeds 0.1 mg / L, start the hydrochloric acid elution tank and backwash the composite column at a flow rate of 0.3 L / min for 2 hours. The eluent enters the regeneration solution storage tank. Add sodium sulfite reducing agent to the regeneration solution storage tank, stir for 30 minutes, and then let it stand to precipitate. After adjusting the pH to 6-7, the supernatant is returned to the MOF-based eluent storage tank for recycling.

[0027] The technical advantages of the multi-stage leaching method for remediation of hexavalent chromium-contaminated soil provided by this invention are as follows: The first-stage leaching removes 80-85% of water-soluble hexavalent chromium from the soil through the dual action of complexation and adsorption of MOF-based leaching agents; the second-stage filtration utilizes a composite column to adsorb residual hexavalent chromium, improving the overall removal rate; the third-stage regeneration, through sodium sulfite reduction and pH adjustment, ensures that the leaching agent recycling rate is ≥85%, reducing costs compared to traditional processes. Gravity or low-pressure (0.1-0.2 MPa) adsorption treatment can reduce energy consumption, and the adsorption temperature is controlled at 25-30℃, requiring no additional heating and suitable for ambient temperature environments.

[0028] Based on the above technical solution, the multi-stage leaching method for remediation of hexavalent chromium contaminated soil according to the present invention can be further improved as follows:

[0029] Furthermore, the amount of sodium sulfite reducing agent added in step S3 is determined based on the concentration of hexavalent chromium in the eluent, the molar ratio of reducing agent to hexavalent chromium is 2-3:1, and the amount of sodium sulfite reducing agent added is 0.1-0.5 mol / L.

[0030] The beneficial effects of adopting the above-mentioned improved scheme are as follows: controlling the molar ratio of reducing agent to hexavalent chromium at 2-3:1 ensures that hexavalent chromium is completely reduced to trivalent chromium (reduction rate ≥99%), avoiding secondary soil pollution (such as sulfite residue) caused by excessive reducing agent. Adding the agent quantitatively as needed can reduce reagent costs.

[0031] Furthermore, the contaminated soil in S1 is pretreated by screening before rinsing, passing through a 2mm sieve, and the moisture content is adjusted to 20-30%. The soil layer in the rinsing tank has a thickness of 30-50cm.

[0032] The beneficial effects of adopting the above-mentioned improved scheme are as follows: screening pretreatment removes stones (particle size > 2mm) and plant roots from the soil, avoiding clogging of the spray pipe network; adjusting the moisture content to 20-30% can optimize the soil pore structure, improve the permeability of the leaching agent, and increase the leaching efficiency of hexavalent chromium.

[0033] Furthermore, the preparation method of the MOF-based elution agent is as follows: NH2-UiO-66 powder is mixed with 0.01mol / L ammonium acetate solution at a concentration of 1-2g / L, stirred at 25℃ for 30min, and the supernatant is taken after standing for 10min.

[0034] The beneficial effects of adopting the above-mentioned improved scheme are as follows: a uniform eluent can be prepared by stirring at room temperature for 30 minutes without heating or ultrasonic assistance, with zero energy consumption and a simple and easy-to-promote preparation process; after standing for 10 minutes, the supernatant can be taken to remove undissolved MOF particles and prevent pipeline blockage.

[0035] The non-electrically driven preparation method is suitable for field restoration scenarios, reducing reliance on equipment.

[0036] Furthermore, the MOF concentration of the composite rinsing agent is 1-2 g / L.

[0037] Compared with existing technologies, the multi-stage leaching system and method for remediation of hexavalent chromium-contaminated soil provided by this invention have the following advantages: This invention achieves a significant improvement in remediation efficiency and economy through the innovative design of a three-stage closed-loop leaching system. The composite leaching agent based on MOF material possesses both complexing and adsorption functions. Combined with gradient treatment using tandem adsorption columns, the recycling rate of the leaching agent is greatly improved, reducing reagent consumption and lowering costs compared to traditional processes. The optimized structure of the leaching tank and spray network enhances the uniformity of leaching agent penetration in the soil. Combined with the high-precision adsorption capacity of the composite columns, the remediation cycle is significantly shortened. The regeneration unit achieves reagent recycling through chemical reduction and pH control, avoiding the energy consumption problems of electrochemical treatment. The remediated soil exhibits stable physicochemical properties and outstanding environmental friendliness. The modular design allows the system to adapt to different acidic soil conditions, effectively controlling equipment investment and operating costs, and combining engineering applicability with economic feasibility. Attached Figure Description

[0038] Figure 1 An example diagram of a multi-stage leaching system and method for the remediation of hexavalent chromium-contaminated soil;

[0039] Figure 2 Top view of a multi-stage leaching system and method for leaching remediation of hexavalent chromium contaminated soil;

[0040] Figure 3 Side view of a multi-stage leaching system and method for leaching remediation of hexavalent chromium contaminated soil;

[0041] Figure 4 Flowchart of a multi-stage leaching method for the remediation of hexavalent chromium contaminated soil

[0042] The attached diagram lists the components represented by each number as follows:

[0043] 10. Rinse tank; 11. Guide channel; 20. Porous spray network; 30. Hydrochloric acid elution tank; 40. Regenerated liquid storage tank. Detailed Implementation

[0044] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings.

[0045] like Figure 1 As shown, an example diagram of a multi-stage leaching system for remediation of hexavalent chromium-contaminated soil provided by the present invention is shown, comprising:

[0046] Rinsing unit: includes rinsing tank 10, with a porous spray pipe network 20 at the bottom of the rinsing tank 10. The porous spray pipe network 20 is connected to the MOF-based rinsing agent storage tank through acid and alkali resistant pipes. The MOF-based rinsing agent storage tank stores composite rinsing agent. The bottom of the rinsing tank 10 is inclined with a guide channel 11.

[0047] Filtration unit: includes MOF@activated carbon composite column, whose inlet is connected to the flow channel 11 through a pipe, and whose outlet is connected to the regeneration unit;

[0048] Regeneration unit: includes hydrochloric acid elution tank 30 and regeneration liquid storage tank 40. Hydrochloric acid elution tank 30 stores 0.1 mol / L hydrochloric acid solution and is connected to the backwash inlet of MOF@activated carbon composite column through a pipeline; regeneration liquid storage tank 40 is connected to MOF-based eluent storage tank through a pipeline.

[0049] Fluid drive components: including a peristaltic pump and a manual valve mounted on the pipeline, the peristaltic pump being used to drive the flow of the eluent and elution solution.

[0050] The specific configuration method for the fluid drive component is as follows:

[0051] In the entire multi-stage rinsing-wastewater circulation system, peristaltic pumps and manual valves are arranged in sections along acid and alkali resistant pipelines (such as PVDF material), forming a synergistic drive structure. The peristaltic pumps are selected based on chemical corrosion resistance (such as BT100M), with pump heads made of fluororubber, capable of withstanding the highly corrosive environments of rinsing agents (containing NH2-UiO-66 and ammonium acetate) and eluents (0.1 mol / L hydrochloric acid). The pump body connects to the pipeline via flanges or clamps, and its installation position must be lower than the liquid level in the MOF-based rinsing agent storage tank. The liquid level difference and the peristaltic action of the pump body create a stable fluid driving force, with a flow rate control range of 0.5-1 L / min, ensuring that the rinsing agent flows through the spray network or composite column at a uniform flow rate.

[0052] Manual valves, made of PVDF, are ball or butterfly valves and are installed at the inlet and outlet of the peristaltic pump and at the branches of the connecting pipes of each unit. For example, on the connecting pipe between the rinsing unit and the filtration unit, the valve controls the flow of the eluent; on the backwashing pipe of the regeneration unit, the valve works with the peristaltic pump to switch the flow direction of the hydrochloric acid eluent. The valve installation location must be convenient for manual operation, and the connection with the pipeline must ensure sealing performance (e.g., using O-ring seals) to prevent chemical leakage. The entire drive assembly forms a closed loop through the pipeline layout. The peristaltic pump provides the active driving force, and the manual valves assist in adjusting the flow rate and direction, together ensuring stable fluid transmission and precise control of process parameters within the system.

[0053] In the above technical solution, a spray head is installed at the top of the rinsing tank 10. The spray head is connected to the porous spray pipe network 20. The spray pressure of the spray head is controlled at 0.2-0.5MPa by a pressure reducing valve.

[0054] like Figure 2 , Figure 3 As shown, the composite column adopts a series structure, including a primary adsorption column and a secondary adsorption column. The primary adsorption column is filled with MOF@activated carbon particles, and the secondary adsorption column is filled with pure MOF particles. The packing density of the composite column is 0.8 g / cm³. 3 The composite column is made of polytetrafluoroethylene.

[0055] A stirrer is installed inside the regenerated liquid storage tank. The stirring speed of the stirrer is 100-200 rpm, which is used to mix the regenerated liquid with the supplemented NH2-UiO-66 powder.

[0056] The connecting pipes between the rinsing tank, MOF-based rinsing agent storage tank, MOF@activated carbon composite column, hydrochloric acid elution tank and regenerated liquid storage tank are all made of PVDF material, with an inner diameter of 25-50mm.

[0057] The present invention provides a multi-stage leaching method for remediation of hexavalent chromium contaminated soil, comprising the following steps:

[0058] S1: Primary leaching: MOF-based leaching agent is transported from the MOF-based leaching agent storage tank to the porous spray pipe network of the leaching tank by a peristaltic pump, and the contaminated soil is sprayed at a flow rate of 0.5-1L / min for 2-4 hours. The leachate flows into the MOF@activated carbon composite column through the guide channel.

[0059] S2: Secondary filtration: The leachate is sequentially passed through a primary adsorption column and a secondary adsorption column, and is adsorbed under gravity or pressure of 0.1-0.2 MPa at an adsorption temperature of 25-30℃.

[0060] S3.: Three-stage regeneration: When the hexavalent chromium concentration at the outlet of the MOF@activated carbon composite column exceeds 0.1 mg / L, start the hydrochloric acid elution tank and backwash the composite column at a flow rate of 0.3 L / min for 2 hours. The eluent enters the regeneration solution storage tank. Add sodium sulfite reducing agent to the regeneration solution storage tank, stir for 30 minutes, and then let it stand to precipitate. After adjusting the pH to 6-7, the supernatant is returned to the MOF-based eluent storage tank for recycling.

[0061] Primary leaching: The contaminated soil is passed through a 2mm sieve and the moisture content is adjusted to 25%. It is then piled up to a thickness of 30cm in the leaching tank. MOF-based leaching agent is sprayed at a flow rate of 0.8L / min for 3 hours. The leaching liquid flows into the composite column through the guide channel.

[0062] Secondary filtration: The leachate is passed sequentially through the primary and secondary adsorption columns at a pressure of 0.15 MPa, with an adsorption temperature of 28°C, and the concentration of hexavalent chromium at the outlet is monitored in real time.

[0063] Three-stage regeneration: When the outlet concentration is >0.1 mg / L, backwash the composite column with 0.3 L / min of 0.1 mol / L hydrochloric acid for 2 h, and put the eluent into the regeneration tank; add reducing agent at a molar ratio of hexavalent chromium to sodium sulfite of 1:2.5, stir for 30 min and let stand, adjust the pH of the supernatant to 6.5, and return it to the storage tank for recycling.

[0064] The amount of sodium sulfite reducing agent added in S3 is determined based on the concentration of hexavalent chromium in the eluent. The molar ratio of reducing agent to hexavalent chromium is 2-3:1, and the amount of sodium sulfite reducing agent added is 0.1-0.5 mol / L.

[0065] The contaminated soil in S1 is pretreated by screening before leaching, passing through a 2mm sieve, and the moisture content is adjusted to 20-30%. The soil layer in the leaching tank has a thickness of 30-50cm.

[0066] The preparation method of MOF-based elution agent is as follows: NH2-UiO-66 powder is mixed with 0.01mol / L ammonium acetate solution at a concentration of 1-2g / L, stirred at 25℃ for 30min, and the supernatant is taken after standing for 10min.

[0067] The MOF concentration of the composite rinsing agent is 1-2 g / L.

[0068] The specific embodiments corresponding to the claims of this invention are described in detail below:

[0069] A specific embodiment of a multi-stage rinsing-wastewater recycling system and method based on metal-organic framework (MOF) adsorption includes a rinsing unit, a filtration unit, a regeneration unit, and a fluid drive component. Each unit is connected through acid and alkali resistant pipes to form a three-stage closed-loop rinsing system.

[0070] The rinsing unit includes a rinsing tank made of corrosion-resistant PP material, with a 5° incline at the bottom to facilitate leachate collection. A porous spray network, constructed of PVDF pipes, is laid at the bottom of the tank. The pipes have evenly distributed 1mm diameter spray holes spaced 10cm apart. The spray network connects to a 500L MOF-based rinsing agent storage tank via PVDF pipes. The tank contains a stirring paddle rotating at 150rpm to mix NH2-UiO-66 powder with a 0.01mol / L ammonium acetate solution, producing a composite rinsing agent with a MOF concentration of 1.5g / L. Spray heads are installed at the top of the tank, connected to the porous spray network. The spray pressure is controlled at 0.3MPa by a pressure reducing valve to ensure uniform spraying of the rinsing agent.

[0071] The filtration unit includes a MOF@activated carbon composite column, which adopts a series structure consisting of a primary adsorption column and a secondary adsorption column. The column body is made of polytetrafluoroethylene, with a diameter of 10 cm and a height of 1.5 m. The primary adsorption column is filled with MOF@activated carbon particles with a particle size of 1.5 mm and a packing density of 0.8 g / cm³. 3 The secondary adsorption column was filled with pure MOF particles with a particle size of 0.8 mm and a packing density of 0.8 g / cm³. 3 The inlet of the composite column is connected to the guide channel at the bottom of the rinsing tank via a PVDF pipe. The guide channel is 15cm wide, 10cm deep, and inclined at a 3° angle to ensure that the leachate flows smoothly into the composite column.

[0072] The regeneration unit includes a hydrochloric acid elution tank and a regenerated solution storage tank. The hydrochloric acid elution tank has a capacity of 200L and stores 0.1mol / L hydrochloric acid solution. The regenerated solution storage tank has a capacity of 500L and is equipped with an internal stirrer with a stirring speed of 150rpm. The hydrochloric acid elution tank is connected to the backwash inlet of the MOF@activated carbon composite column via a PVDF pipe, and the regenerated solution storage tank is connected to the MOF-based eluent storage tank via a PVDF pipe.

[0073] The fluid drive assembly includes a peristaltic pump and a manual valve mounted on the pipeline. The peristaltic pump, model BT100M, is used to drive the flow of the eluent and rinsing solution. The manual valve is a PVDF ball valve used to control the opening and closing of the pipeline.

[0074] The specific implementation steps of the method of the present invention are as follows:

[0075] First, the contaminated soil is pretreated. The acidic hexavalent chromium contaminated soil to be remediated is transported to the leaching site and screened using a 2mm sieve to remove stones, branches, and other debris. Then, the soil moisture content is adjusted to 25%. The treated soil is then evenly piled in the leaching tank to a thickness of 40cm, ensuring that the soil is loose to facilitate the penetration of the leaching agent.

[0076] Next, MOF-based elution agent was prepared. 400 L of 0.01 mol / L ammonium acetate solution was added to the MOF-based elution agent storage tank, followed by the slow addition of 600 g of NH2-UiO-66 powder. The stirrer was turned on and stirred at 25°C for 30 min to fully disperse the MOF powder in the ammonium acetate solution. After standing for 10 min, the supernatant was taken as the composite elution agent.

[0077] Then, primary leaching is performed. The peristaltic pump is turned on to deliver the MOF-based leaching agent from the storage tank to the porous spray network of the leaching tank, controlling the flow rate at 0.8 L / min and the spraying time at 3 hours. The leaching agent is evenly sprayed onto the surface of the contaminated soil, penetrating downwards through soil pores and undergoing complexation and adsorption reactions with hexavalent chromium in the soil, removing water-soluble hexavalent chromium. The leachate, carrying the dissolved hexavalent chromium, flows into the MOF@activated carbon composite column through a guide channel.

[0078] In the secondary filtration process, the eluent first enters the primary adsorption column, passing through at a flow rate of 8 BV / h under a pressure of 0.15 MPa. Utilizing the high specific surface area and adsorption capacity of the MOF@activated carbon particles, most of the hexavalent chromium is removed. It then enters the secondary adsorption column, passing through at a flow rate of 5 BV / h. The pure MOF particles further adsorb any remaining hexavalent chromium, reducing the outlet hexavalent chromium concentration to below 0.1 mg / L. The adsorption temperature is controlled at 28℃ and maintained naturally by ambient temperature, requiring no additional heating.

[0079] When the hexavalent chromium concentration at the outlet of the MOF@activated carbon composite column exceeds 0.1 mg / L, initiate three-stage regeneration. Open the manual valve of the hydrochloric acid eluent tank and backwash the composite column at a flow rate of 0.3 L / min for 2 hours. The hydrochloric acid solution will elute the hexavalent chromium adsorbed on the composite column, and the eluent will enter the regeneration solution storage tank. Add sodium sulfite reducing agent to the regeneration solution storage tank, determining the amount added based on the concentration of hexavalent chromium in the eluent, ensuring a molar ratio of reducing agent to hexavalent chromium of 2.5:1. Turn on the stirrer and stir for 30 minutes to allow the sodium sulfite to fully react with the hexavalent chromium, reducing it to trivalent chromium. Then allow the mixture to settle for 2 hours, allowing the precipitate to settle to the bottom of the tank. Take the supernatant and adjust the pH to 6.5 with a 5% sodium hydroxide solution, stirring continuously during the adjustment process to ensure pH uniformity. Finally, return the adjusted supernatant to the MOF-based eluent storage tank via a peristaltic pump for reuse.

[0080] Throughout the rinsing process, parameters at each stage were monitored regularly. The flow rate and spray pressure of the rinsing agent were recorded hourly, and the MOF concentration of the rinsing agent in the MOF-based rinsing agent storage tank was checked daily to ensure the concentration remained within the range of 1-2 g / L. If insufficient, NH2-UiO-66 powder was added promptly. The hexavalent chromium concentration at the outlet of the composite column was checked every 2 hours; when the concentration exceeded 0.1 mg / L, regeneration was initiated immediately. Simultaneously, the pH value of the solution in the regenerated solution storage tank was monitored to ensure the reduction reaction proceeded under suitable conditions.

[0081] For equipment maintenance, flush the PVDF pipelines weekly with deionized water to prevent MOF particles from depositing and clogging them. Check the peristaltic pump's operation monthly to ensure stable flow. Evaluate the performance of the MOF@activated carbon composite column quarterly; if the adsorption efficiency drops by more than 10%, replace the packed MOF particles and MOF@activated carbon particles.

[0082] In this embodiment, the design of a three-stage closed-loop leaching system achieves efficient recycling of the leaching agent, with a recycling rate of 86% and a hexavalent chromium removal rate of 97%. The treatment cost is 180 yuan / ton of soil, meeting the requirements of efficient, environmentally friendly, and economical remediation. This system requires no complex electrochemical equipment; it effectively remediates acidic hexavalent chromium-contaminated soil using only reagents and a stratification process, making it suitable for soil remediation projects at various industrial contaminated sites.

[0083] The method for determining the key parameters involved in this invention will be described in detail below.

[0084] Determination of MOF-based leaching agent concentration: Calculate the required MOF dosage based on the initial concentration and quantity of hexavalent chromium-contaminated soil. Generally, 0.5-1g of NH2-UiO-66 is required to remove 1mg of hexavalent chromium. Considering the average concentration of hexavalent chromium in the soil is 50mg / kg, and assuming 1500 tons of soil per acre, the required MOF amount is 37.5-75kg. Therefore, the leaching agent concentration is set to 1-2g / L to ensure sufficient MOF to react with the hexavalent chromium.

[0085] Determination of the amount of sodium sulfite reducing agent: The reaction equation between sodium sulfite and hexavalent chromium is Cr2O7. 2- +3SO3 2- +8H + =2Cr 3+ +3SO4 2- +4H₂O. According to stoichiometry, 1 mol of hexavalent chromium requires 3 mol of sodium sulfite. By detecting the concentration of hexavalent chromium in the eluent, the amount of hexavalent chromium can be calculated, and thus the amount of sodium sulfite to be added can be determined to ensure that the molar ratio of reducing agent to hexavalent chromium is within the range of 2-3:1, thus guaranteeing the complete reduction of hexavalent chromium.

[0086] Determination of spray pressure: Experiments were conducted to test the penetration effect of the leaching agent and the degree of damage to the soil structure under different spray pressures. When the spray pressure was 0.2-0.5 MPa, the leaching agent could penetrate into the soil pores well, and the damage to the soil structure was relatively small. Taking all factors into consideration, the spray pressure was controlled within the range of 0.2-0.5 MPa, and 0.3 MPa was selected in this embodiment.

[0087] Determination of adsorption flow rate: Under laboratory conditions, the removal rate of hexavalent chromium at different flow rates was tested. When the flow rate was 5-10 BV / h, the removal rate of hexavalent chromium decreased with increasing flow rate. When the flow rate was 8 BV / h, the removal rate of the primary adsorption column reached 85%, and the removal rate of the secondary adsorption column reached 95%. Therefore, the flow rate of the primary adsorption column was determined to be 8 BV / h, and the flow rate of the secondary adsorption column was determined to be 5 BV / h.

[0088] Specifically, the principle of this invention is as follows: The core technology of this invention lies in utilizing the special structure and chemical properties of MOF materials to achieve efficient removal of hexavalent chromium. The three-dimensional channels and amino functional groups of NH2-UiO-66 can capture hexavalent chromium through a dual action of physical adsorption and coordination complexation. The composite leaching agent formed by it and ammonium acetate solution enhances the dissolution effect in acidic soil through hydrogen ion competition and electrostatic attraction. In the three-stage closed-loop system, the first-stage leaching achieves efficient dissolution of water-soluble hexavalent chromium, the second-stage filtration completes deep purification through gradient adsorption of the MOF activated carbon composite column, and the third-stage regeneration utilizes hydrochloric acid elution and sodium sulfite reduction to achieve reagent circulation. The inclined flow guide design of the leaching tank and the acid and alkali resistant piping system ensure stable control of process parameters and long-term operation of the equipment. This technology integrates the molecular recognition characteristics of MOF materials with engineered processes, forming a synergistic mechanism of "dissolution-adsorption-regeneration" throughout the entire process, breaking through the efficiency and cost bottlenecks of traditional leaching technology in acidic soil remediation.

[0089] The following provides a specific embodiment 1 of the present invention, and the specific implementation of each step in this embodiment 1 is described in detail below.

[0090] Remediation was carried out on a hexavalent chromium contaminated site (approximately 9666.7 square meters). The soil pH of this site was 4.2-4.8, and the average concentration of hexavalent chromium was 52 mg / kg, classifying it as a typical acidic contamination scenario. Before remediation, the contaminated soil underwent pretreatment. A 2mm aperture vibrating screen was used to remove stones, roots, and other debris. The moisture content was adjusted to 25% using a spray system to ensure uniform penetration of the leaching agent. The treated soil was evenly piled to a thickness of 40cm in a PP leaching tank (5m long, 4m wide, and 1.5m high). The bottom of the tank was inclined at 5° towards the guide channel, which was 15cm wide and 10cm deep. A PVDF porous spray network with 1mm aperture and 10cm spacing was laid at the bottom and connected to a 500L MOF-based leaching agent storage tank via flanges.

[0091] like Figure 4As shown, the first-stage leaching step is as follows: MOF-based leaching agent is prepared in a storage tank. 600g of NH2-UiO-66 powder is added to 400L of 0.01mol / L ammonium acetate solution, stirred at 150rpm for 30min, and allowed to stand for 10min. The supernatant is then collected to obtain a composite leaching agent with a concentration of 1.5g / L. A peristaltic pump (model BT100M, fluororubber pump head) is started to deliver the leaching agent to the spray network at a flow rate of 0.8L / min. The contaminated soil is uniformly sprayed through four PVDF spray heads installed at the top at a pressure of 0.3MPa for 3 hours. NH2-UiO-66 in the leaching agent forms coordination bonds with hexavalent chromium through amino groups (coordination constant logK = 4.2-4.8). Combined with the physical adsorption of the porous structure, water-soluble hexavalent chromium dissolves and flows into the guide channel with the leachate. During the rinsing process, the pH of the leaching solution is monitored in real time. When the pH drops below 5.5, 5% sodium hydroxide solution is added to the storage tank to adjust it to 6.5-7.0 to maintain the stability of the MOF material.

[0092] Secondary filtration step: The eluent enters the MOF@activated carbon composite column at a pressure of 0.15 MPa through a guide channel. The composite column adopts a series structure. The primary adsorption column (10 cm in diameter and 1.5 m in height) is filled with MOF@activated carbon particles with a particle size of 1.5 mm, with a packing density of 0.8 g / cm³. 3 Operating at a flow rate of 8 bed volumes per hour (BV / h), the activated carbon has a high specific surface area (1000-1500 m²). 2 The adsorption of most hexavalent chromium (g) is achieved; the effluent enters a secondary adsorption column (10 cm in diameter, 1.5 m in height), filled with pure NH₂-UiO-66 particles with a particle size of 0.8 mm and a packing density of 0.8 g / cm³. 3 Residual hexavalent chromium was deeply adsorbed at a flow rate of 5 BV / h. The adsorption temperature was naturally maintained at 28℃ by ambient temperature. Under these conditions, the adsorption capacity of the MOF material for hexavalent chromium reached 120-150 mg / g, ensuring that the outlet hexavalent chromium concentration was ≤0.1 mg / L.

[0093] Three-stage regeneration step: When the hexavalent chromium concentration at the outlet of the composite column exceeds 0.1 mg / L, start the 200L hydrochloric acid eluent tank and backwash the composite column for 2 hours at a flow rate of 0.3 L / min. The 0.1 mol / L hydrochloric acid solution breaks the coordination bond between MOF and hexavalent chromium, and the eluent (pH = 1.5-2.0) enters the 500L regeneration solution storage tank. Based on the hexavalent chromium concentration in the eluent, add sodium sulfite (reaction equation: Cr2O7) at a molar ratio of reducing agent to hexavalent chromium of 2.5:1. 2- +3SO3 2- +8H + =2Cr 3+ +3SO4 2-Add 4H₂O, turn on the stirrer and mix at 150 rpm for 30 minutes to completely reduce hexavalent chromium to trivalent chromium. Then let it stand for 2 hours to form a hydroxide precipitate of trivalent chromium. Take the supernatant and adjust the pH to 6.5 with 5% sodium hydroxide solution. Return it to the MOF-based eluent storage tank through a peristaltic pump, add 200g of NH₂-UiO-66 powder, and then recycle.

[0094] Process monitoring and system maintenance: Record peristaltic pump flow rate, spray pressure, and inlet / outlet pressure difference of the adsorption column hourly; monitor MOF concentration in the eluent daily (maintained at 1-2 g / L) and hexavalent chromium removal rate. Flush the PVDF pipeline (25 mm inner diameter) with deionized water weekly to prevent MOF particle deposition and blockage; check the peristaltic pump seals monthly for wear and replace them promptly; perform performance tests on the composite column packing quarterly, replacing 1 / 3 of the packing if the adsorption capacity decreases by more than 15%. Optimize key parameters during the repair process using the following equations:

[0095] Equation for determining leaching agent concentration: Based on the initial concentration of hexavalent chromium in the soil (C0, mg / kg) and the soil mass (M, tons), calculate the required MOF dosage (W, g):

[0096] W = M × C0 × K1;

[0097] Where K1 is a coefficient (0.5-1.0 g / mg), in this example C0 = 52 mg / kg, M = 1200 tons, K1 = 0.9, and W = 56.16 kg is calculated. It is added to the storage tank in batches to maintain a concentration of 1.5 g / L.

[0098] Sodium sulfite addition equation: Calculate the amount of sodium sulfite (m, g) to be added based on the concentration (C, mol / L) and volume (V, L) of hexavalent chromium in the eluent.

[0099] m = C × V × 3 × 126.04 (molar mass of sodium sulfite);

[0100] In this embodiment, during a certain regeneration, C = 0.01 mol / L, V = 120 L, and the calculated m = 453.74 g. The actual amount added was 567 g at a molar ratio of 2.5:1.

[0101] After remediation, 36 soil samples were collected using a grid method (5m×5m). Testing showed an average hexavalent chromium concentration of 0.32 mg / kg, with a removal rate of 97.5%, meeting the screening requirements for Class II land use in GB36600-2018. The entire remediation cycle took 15 days, treating 1200 tons of soil. The leaching agent was recycled 8 times, consuming 48 kg of NH2-UiO-66 powder. Compared to traditional single-stage leaching processes, the agent cost was reduced by 42%, with a treatment cost of 180 yuan / ton, achieving zero wastewater discharge. The system forms a closed loop through PVDF pipes, polytetrafluoroethylene composite columns, and a peristaltic pump-ball valve drive assembly, relying solely on agent stratification and mechanical components, demonstrating high efficiency and economy in the remediation of hexavalent chromium pollution in acidic soils.

[0102] To better understand and implement this invention, Example 2 of a specific application scenario is provided below: A former electroplating plant site covers approximately 15 acres, and its soil has been severely contaminated with hexavalent chromium due to historical production activities. Testing revealed that the soil pH was 4.0-4.5, and the average hexavalent chromium concentration was 68 mg / kg, exceeding the screening value for Class II land use in GB36600-2018 by 12 times. The soil type is poorly permeable silty clay, and the groundwater level is 1.5-2.0 m deep. The engineering team adopted the multi-stage rinsing-wastewater recycling method based on MOF adsorption of this invention, designing a remediation area of ​​9666.7 m². 2 The average remediation depth is 2.5m, and it is expected to treat 24,166.75 tons of contaminated soil. The goal is to control the hexavalent chromium removal rate at ≥96%, and the cost is reduced by more than 40% compared with traditional processes.

[0103] The repair system consists of four modular PP rinsing tanks (10m × 8m × 2m) with a 6° incline at the bottom. The guide channels are 20cm wide and 15cm deep. A PVDF porous spray network (1.2mm aperture, 8cm spacing) is laid at the bottom of each tank, connecting to two 1000L MOF-based rinsing agent storage tanks. These tanks store a 1.8g / L composite rinsing agent made from NH2-UiO-66 and 0.01mol / L ammonium acetate solution. The filtration unit uses four parallel-series MOF@activated carbon composite columns: the first-stage column (15cm diameter, 2m high) is filled with 2mm MOF@activated carbon particles (packing density 0.85g / cm³). 3 A secondary column (12cm in diameter, 2m in height) is filled with 1mm pure NH2-UiO-66 particles (packing density 0.82g / cm³). 3 The overall processing flow rate is 5L / min. The regeneration unit includes a 500L hydrochloric acid elution tank (0.1mol / L hydrochloric acid) and a 1000L regeneration liquid storage tank (with a built-in 180rpm stirrer). A closed-loop system is formed by driving PVDF pipelines (32mm inner diameter) through a BT300M peristaltic pump (fluororubber pump head).

[0104] After being excavated and piled by a PC200-8 excavator, the contaminated soil was sieved through a 3mm screen and its moisture content was adjusted to 22%-25%. It was then layered in a tank (30cm layers per layer, 4 layers in total), with a 10cm quartz sand pad at the bottom. During the first-stage leaching, 800L of ammonium acetate solution and 1.44kg of NH2-UiO-66 powder were added to the storage tank and stirred for 40 minutes to prepare the leaching agent (pH=6.7). This agent was sprayed at a flow rate of 1.0L / min and a pressure of 0.4MPa for 4 hours, achieving a water-soluble hexavalent chromium leaching rate of 82% and a hexavalent chromium concentration of 55.76mg / L in the leachate. In the second-stage filtration, the leachate was subjected to a first-stage column (6BV / h flow rate) at a pressure of 0.18MPa to adsorb 85% of the hexavalent chromium, and then further treated by a second-stage column (4BV / h flow rate) until the outlet concentration was ≤0.08mg / L.

[0105] When the concentration at the outlet of the secondary column exceeds 0.1 mg / L, the tertiary regeneration is initiated: backwashing with hydrochloric acid at a flow rate of 0.4 L / min for 3 hours, the eluent (pH = 1.8) is transferred to the regeneration tank, and a reducing agent (reaction formula: Cr2O7) is added at a molar ratio of hexavalent chromium to sodium sulfite of 2.8:1. 2- +3SO3 2- +8H + =2Cr 3+ +3SO4 2- Add 4H2O), stir for 35 minutes, let stand for 2.5 hours, adjust the pH of the supernatant to 6.6, add 0.8 kg MOF powder, and reuse.

[0106] Key parameters were calculated based on the following model: Leaching agent concentration was calculated as W = M × K (M is the total hexavalent chromium content in the soil, 1643.34 kg, K = 1.2 safety factor), requiring 1972.01 kg of MOF, prepared in 1096 cycles; Sodium sulfite dosage:

[0107] m=C×V×3×126.04×10 -3 ×K2;

[0108] Where C = 12.5 mmol / L, V = 720 L, K2 = 1.1, we get m = 3.63 kg; adsorption column flow rate (Corresponds to 6BV / h).

[0109] Post-remediation grid sampling (10m×10m) showed an average hexavalent chromium concentration of 0.29 mg / kg, a removal rate of 99.57%, soil pH of 6.8-7.2, and an organic matter loss rate of 4.1%. The leaching agent was circulated 12 times over a 35-day period, consuming 1980 kg of MOF (Metal-Oxide-Foil), with a reagent cost of 4.95 million yuan (43% lower than traditional processes), resulting in a unit cost of 205 yuan / ton. Further improvements could include adding 0.8% nanobubbles (80nm in diameter) to the leaching agent to increase the permeability of silty clay by 15%, or integrating PLC and LSTM models for dynamic parameter control. This example demonstrates how the coordination adsorption of MOF materials and a three-stage closed-loop process effectively addresses the problem of severe hexavalent chromium pollution in acidic soils, providing an engineering paradigm for similar sites.

[0110] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A multi-stage leaching system for hexavalent chromium-contaminated soil leaching remediation, characterized by, Comprising: Rinse unit: including a rinse tank, the bottom of which is provided with a porous spray pipe network connected to a MOF-based rinse agent storage tank through acid and alkali resistant pipes; the MOF-based rinse agent storage tank stores a composite rinse agent, the bottom of the rinse tank is provided with a flow guide groove, and the composition of the MOF-based rinse agent is: mixing NH2-UiO-66 powder with 0.01 mol / L ammonium acetate solution at a concentration of 1-2 g / L, stirring at 25°C for 30 min, and taking the supernatant after standing for 10 min; Filtering unit: including a MOF@activated carbon composite column, the inlet of which is connected to the flow guide groove through a pipe, and the outlet is connected to the regeneration unit; Regeneration unit: including a hydrochloric acid elution tank and a regeneration liquid storage tank, the hydrochloric acid elution tank stores 0.1 mol / L hydrochloric acid solution and is connected to the backwashing inlet of the MOF@activated carbon composite column through a pipe; the regeneration liquid storage tank is connected to the MOF-based rinse agent storage tank through a pipe, the composite column adopts a series structure, including a primary adsorption column and a secondary adsorption column, the primary adsorption column is filled with MOF@activated carbon particles, and the secondary adsorption column is filled with pure MOF particles, the filling density of the composite column is 0.8 grams per cubic centimeter, and the column body material of the composite column is polytetrafluoroethylene; Fluid driving assembly: including a peristaltic pump and a manual valve arranged on the pipe, the peristaltic pump is used to drive the flow of the rinse agent and the eluent.

2. The multi-stage leaching system for Cr(VI) contaminated soil leaching remediation according to claim 1, characterized in that, The top of the rinse tank is provided with a spray head, which is in communication with the porous spray pipe network, and the spray pressure of the spray head is controlled at 0.2-0.5 MPa by a pressure reducing valve.

3. The multi-stage leaching system for Cr(VI) contaminated soil leaching remediation according to claim 2, characterized in that, The regeneration liquid storage tank is provided with a stirrer, and the stirring speed of the stirrer is 100-200 rpm for mixing the regeneration liquid and the supplemented NH2-UiO-66 powder.

4. The multi-stage leaching system for Cr(VI) contaminated soil leaching remediation according to claim 3, characterized in that, The connecting pipes between the rinse tank, the MOF-based rinse agent storage tank, the MOF@activated carbon composite column, the hydrochloric acid elution tank and the regeneration liquid storage tank are all made of PVDF material, and the inner diameter of the pipes is 25-50 mm.

5. A multi-stage leaching method for the remediation of hexavalent chromium contaminated soil by leaching, characterized in that, A method for repairing a hexavalent chromium contaminated soil by using the multi-stage rinse system according to any one of claims 1-4, the method comprising the following steps: S1: primary rinse: the MOF-based rinse agent is transported from the MOF-based rinse agent storage tank to the porous spray pipe network of the rinse tank by the peristaltic pump, the contaminated soil is sprayed at a flow rate of 0.5-1 L / min, the rinse time is 2-4 h, and the leachate flows into the MOF@activated carbon composite column through the flow guide groove; S2: secondary filtration: the leachate passes through the primary adsorption column and the secondary adsorption column in sequence, and is subjected to adsorption treatment under the action of gravity or a pressure of 0.1-0.2 MPa, and the adsorption temperature is 25-30°C; S3: tertiary regeneration: when the outlet concentration of hexavalent chromium of the MOF@activated carbon composite column exceeds 0.1 mg / L, the hydrochloric acid elution tank is started, the composite column is backwashed at a flow rate of 0.3 L / min for 2 h, the eluent enters the regeneration liquid storage tank, sodium sulfite reducing agent is added to the regeneration liquid storage tank, stirring is performed for 30 min, and then the supernatant is returned to the MOF-based rinse agent storage tank after adjusting the pH to 6-7 for recycling.

6. The multi-stage leaching method for Cr(VI) contaminated soil leaching remediation according to claim 5, characterized in that, The addition amount of the sodium sulfite reducing agent in the S3 is determined according to the concentration of hexavalent chromium in the eluent, and the molar ratio of the reducing agent to hexavalent chromium is 2-3:1, and the addition amount of the sodium sulfite reducing agent is 0.1-0.5 mol / L.

7. The multi-stage leaching method for Cr(VI) contaminated soil leaching remediation according to claim 6, characterized in that, The contaminated soil in the S1 is pretreated by screening before leaching, and is passed through a 2mm screen, the moisture content is adjusted to 20-30%, and the accumulation thickness of the soil layer in the leaching tank is 30-50cm.

8. The multi-stage leaching method for Cr(VI) contaminated soil leaching remediation according to claim 7, characterized in that, The MOF concentration of the composite leaching agent is 1-2g / L.

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