Method and system for repairing chlorohydrocarbon polluted underground water based on slow-release repairing agent

Abstract

The invention discloses a method and a system for repairing chlorohydrocarbon polluted underground water based on a slow-release repairing agent. Potassium persulfate and sodium persulfate are compounded according to the ratio of 4: 1, and sodium bentonite is added to prepare homogeneous suspension slurry; 32% NaOH is independently prepared; the two are mixed on line through a static mixer, GeoProbe direct-pushing high-pressure injection is carried out, and progressive injection is carried out from top to bottom at equal depth intervals of 0.5 m in the same vertical channel. The system is of an integrated structure of double-tank independence, online mixing and direct-push injection, wherein the integrated structure is formed by connecting a medicament tank 1, a medicament tank 2, a static mixer and GeoProbe in series. Bentonite enhances the stability of the suspension slurry and forms a clay-oxidant composite slow-release structure, so that the problem that potassium persulfate cannot be injected is solved; the effective concentration of the compound oxidizing agent is maintained for more than 4 months and is obviously superior to that of single sodium persulfate; and single-hole multi-layer injection reduces drilling holes and inhibits back diffusion. The in-situ remediation agent has efficient degradation and long-acting remediation capabilities, and is suitable for in-situ remediation of low-permeability stratums and persistent pollutants.

Description

A method and system for remediating chlorinated hydrocarbon-contaminated groundwater based on slow-release remediation agents. Technical Field

[0001] This invention belongs to the field of groundwater pollution remediation technology, specifically relating to a method and system for remediating chlorinated hydrocarbon-contaminated groundwater based on slow-release remediation agents. It is applicable to in-situ chemical oxidation remediation of low-permeability formations, low-concentration diffuse groundwater plumes, and persistent organic pollutants that are difficult to degrade. Background Technology

[0002] Chlorinated hydrocarbons are widely used in industrial applications as solvents, chemical raw materials, and drinking water disinfectants. However, typical chlorinated hydrocarbons such as carbon tetrachloride and trichloroethylene have been proven to have hepatotoxicity, carcinogenicity, and persistent environmental pollution characteristics. Once they enter groundwater, they become persistent pollutants that are difficult to degrade.

[0003] Traditionally, in-situ chemical oxidation (ISCO) of persulfates is a technique for the in-situ remediation of groundwater plumes contaminated with chlorinated hydrocarbons, utilizing the direct oxidation mechanism of persulfates (sodium persulfate, potassium persulfate) or the free radical oxidation reaction mechanism under activated conditions. In engineering applications both domestically and internationally, sodium persulfate is the primary chemical oxidizing agent. Sodium persulfate has high water solubility and can rapidly achieve high concentrations in the groundwater media of the target remediation area (the recommended minimum concentration is 25 g / L), facilitating rapid remediation of areas with medium to high concentrations of contamination.

[0004] However, the low persistence of sodium persulfate is a significant drawback in its applicable scenarios, including: 1. the continuous release of organic pollutants due to matrix diffusion-reverse diffusion in low-permeability formations; 2. low-concentration diffused groundwater plumes; and 3. scenarios involving pollutants with low water solubility that are slowly released into groundwater. These scenarios all place higher demands on the long-term effectiveness of the oxidant.

[0005] Although potassium persulfate possesses natural properties of low water solubility and slow release, its extremely low solubility in water makes it prone to sedimentation and stratification when formulated into a suspension, failing to form a stable injection medium. Therefore, it has long been considered unsuitable for in-situ high-pressure injection processes. In engineering practice, the use of potassium persulfate as an in-situ injection agent has been largely abandoned, replaced by the highly water-soluble sodium persulfate.

[0006] Furthermore, the generally accepted use of sodium-based bentonite in in-situ chemical oxidation is as a seepage barrier material or drilling lubricant, such as for constructing underground vertical impermeable walls and sealing contamination plume migration pathways. Those skilled in the art generally believe that bentonite, upon absorbing water and swelling, significantly reduces formation permeability. Adding it to oxidant suspensions for in-situ injection would exacerbate the risk of formation blockage and hinder agent delivery; therefore, the use of bentonite should be avoided in in-situ injection projects. This long-standing perception has created a technical bias, hindering the development of other functionalities for bentonite in oxidant formulations.

[0007] In the prior art, Liu Fei et al. from China University of Geosciences (Beijing) disclosed an agent for immobilizing and reducing benzene series compounds, its preparation method, and its application (CN114180666A). This agent includes bentonite and persulfate ions. It utilizes the adsorption and immobilization effect of bentonite combined with the advanced oxidation of persulfate ions to immobilize benzene series pollutants in situ and reduce their concentration. It should be noted that this technical solution aims to use bentonite as a fixation medium for pollutants to prevent their migration. No composite slow-release structure is formed between bentonite and persulfate. Furthermore, this solution explicitly states that no physical or chemical activation of substances containing persulfate ions is required. This differs fundamentally from the technical path, mechanism of action, and application scenarios of this invention, which uses alkali activation to stimulate sulfate free radicals and utilizes bentonite as an oxidant carrier to achieve slow release.

[0008] In summary, there is currently no method or system that can simultaneously solve multiple technical challenges, such as the inability to inject potassium persulfate in situ, the short effective period of sodium persulfate alone, the significant back-diffusion effect in low-permeability formations, and the technical bias of avoiding the use of bentonite in in-situ injection. Summary of the Invention

[0009] (a) The technical problem to be solved by the present invention

[0010] This invention aims to overcome the above-mentioned shortcomings of the prior art and provides an integrated solution to the following technical problems:

[0011] 1. Solve the technical problem that potassium persulfate cannot be used for in-situ high-pressure injection due to its extremely low water solubility and the easy sedimentation and stratification of the suspension, so that its slow-release properties can be used in engineering.

[0012] 2. To address the problem that the single in-situ injection of sodium persulfate has a short duration of effect (1-2 months) and cannot meet the requirements for back diffusion inhibition and long-term remediation in low-permeability formations, the effective concentration of persulfate in groundwater will be maintained for more than 4 months.

[0013] 3. To address the problem of continuous release of pollutants caused by matrix diffusion-reverse diffusion in low-permeability formations, the reverse diffusion path is physically cut off by optimizing the injection sequence.

[0014] 4. Overcome the technical prejudice of those skilled in the art that bentonite is not suitable for in-situ injection and will aggravate formation blockage, and transform the function of bentonite from a blockage agent that should be avoided to a suspension stabilizer and a slow-release reinforcing carrier, so as to realize its dual functional application.

[0015] (II) Technical Solution

[0016] 1. The technical solution corresponding to the method claims

[0017] This invention provides a method for remediating groundwater contaminated with chlorinated hydrocarbons based on a slow-release remediation agent, comprising the following steps:

[0018] S1: Preparation of compound oxidant suspension

[0019] Potassium persulfate and sodium persulfate were mixed at a mass ratio of 4:1 to form a persulfate oxidant. Water was added and stirred to prepare an oxidant suspension with a mass fraction of 35%. The dry matter mass of the persulfate oxidant was calculated as 1% w / w of the soil mass.

[0020] Add sodium-based bentonite at a rate of 6% of the total mass of the oxidant suspension. Stir at 500 rpm for 5 minutes, then stir at 1500 rpm until a homogeneous slurry is formed, thus creating a stable compound oxidant suspension.

[0021] Among them, potassium persulfate particles are dispersed in the layered silicate spatial grid structure formed after bentonite absorbs water and expands, forming a clay-oxidant composite slow-release structure.

[0022] S2: Alkali activator is prepared independently.

[0023] Prepare a 32% (w / w) alkali activator solution using NaOH; the dry matter mass of the alkali activator is calculated as 40% of the dry matter mass of the persulfate oxidant.

[0024] S3: Online Mixing and Conveying

[0025] The compound oxidant suspension obtained in S1 and the alkaline activator solution obtained in S2 are respectively delivered to a static mixer. After being rapidly mixed in the static mixer, they are immediately delivered to the GeoProbe direct injection unit with zero buffer.

[0026] S4: High-pressure direct injection

[0027] Using GeoProbe direct injection technology, the mixed repair agent is injected underground at an injection pressure of ≥3.5MPa and an injection speed of ≥20L / min.

[0028] S5: Inject at equal depth intervals from top to bottom.

[0029] On the same vertical injection channel, multiple injection depth points are set at equal depth intervals of 0.5m, progressively from top to bottom, and injection is completed at each depth point in sequence until the bottom of the contaminated layer is reached.

[0030] 2. The technical solution corresponding to the system claims

[0031] The present invention also provides an in-situ injection system for a sustained-release repair agent to implement the above method, characterized in that it comprises:

[0032] Chemical tank 1: It is a compound oxidant suspension preparation unit. Its inlet is connected to potassium persulfate source, sodium persulfate source, sodium bentonite source and water source respectively. It is equipped with a stirring device inside. Its outlet is connected to the first inlet of static mixer through the first conveying pipeline.

[0033] The compound oxidant suspension formed in the reagent tank 1 is a homogeneous mud medium containing sodium-based bentonite. Potassium persulfate particles are dispersed in the layered silicate spatial grid structure formed by bentonite, forming a clay-oxidant composite slow-release structure.

[0034] Chemical tank 2: This is the alkali activator preparation unit. Its inlet is connected to the NaOH source and the water source, and its outlet is connected to the second inlet of the static mixer through the second conveying pipeline.

[0035] Static mixer: This is an online mixing unit whose outlet is connected to the inlet of the GeoProbe direct injection unit via a third conveying pipeline; the outlet of the static mixer is directly connected to the inlet of the GeoProbe direct injection unit, without any intermediate buffer tank in between.

[0036] GeoProbe direct injection unit: This is a terminal execution unit, including a drill rod. The drill rod has a vertical injection channel that communicates with the feed port. Multiple injection outlets are arranged from top to bottom along the depth direction at equal depth intervals of 0.5m. The injection outlets communicate with the vertical injection channel and correspond to different depth positions of the same borehole.

[0037] Optionally, the system further includes at least one groundwater monitoring well located beside the injection point, with its well casing extending vertically downwards to the contaminated aquifer.

[0038] (III) Beneficial Effects of the Invention

[0039] The technical solution described in this invention is a holistic solution where functions support each other and have a synergistic relationship, rather than a simple superposition of multiple technical features. Each technical feature is interconnected and indispensable, collectively bringing about the following unexpected technical effects:

[0040] 1. For the first time, in-situ high-pressure direct injection of potassium persulfate was achieved, overcoming long-standing technical biases.

[0041] This invention, through the synergistic effect of sodium-based bentonite suspension stabilization and GeoProbe high-pressure direct injection technology (≥3.5MPa), enables potassium persulfate, a drug with good sustained-release properties but which cannot be injected, to become an in-situ injectable repair agent that can be engineered for application for the first time.

[0042] The following technical biases have long existed in this field: 1. Potassium persulfate has extremely low water solubility; after being prepared into a suspension, it severely settles and stratifies within minutes, failing to maintain a homogeneous state. This inevitably clogs injection lines and formation pores, making it unsuitable for in-situ high-pressure injection; 2. Bentonite is a plugging agent, not an additive, and its addition should be deliberately avoided in in-situ chemical oxidation injection projects, otherwise it will exacerbate formation blockage and hinder agent delivery. These biases have led those skilled in the art to abandon the research and application of potassium persulfate and bentonite in this technical direction for a long time.

[0043] This invention overcomes the aforementioned technical biases by transforming bentonite from an undesirable plugging agent into a suspension stabilizer and sustained-release reinforcing carrier, and potassium persulfate from a discarded agent into a long-acting sustained-release core component. This breakthrough is not a simple extension of existing technology, but a fundamental reversal of the choice of technical route, which complies with the explicit provisions of the Patent Examination Guidelines regarding the inventiveness of inventions that overcome technical biases.

[0044] 2. The synergistic effect of the dual functions of bentonite brings about a qualitative change in its slow-release performance.

[0045] This invention is the first to disclose and utilize the dual functions of sodium-based bentonite:

[0046] Function 1 (Suspension Stabilization): After the bentonite absorbs water and expands, it forms a high-viscosity colloidal network, which provides physical suspension support for the potassium persulfate particles. This allows the potassium persulfate suspension, which would normally settle and stratify within minutes, to maintain a homogeneous and stable state for more than 2 hours, meeting the time window requirements for engineering injection.

[0047] Function 2 (Slow-release carrier): The layered silicate spatial grid structure formed by bentonite encapsulates, adsorbs, and embeds potassium persulfate particles inside or on the surface of the grid. After groundwater enters the grid, it slowly dissolves potassium persulfate, extending the diffusion path and reducing the release rate, thus forming a clay-oxidant composite slow-release structure.

[0048] The aforementioned dual functions are not simply a combination of the known functions of bentonite. In the prior art, bentonite has only been used as a seepage barrier material; its functions as an oxidant suspension stabilizer and a slow-release structural carrier have never been revealed, let alone applied in in-situ chemical oxidation injection engineering. This invention is the first to realize the functional leap of bentonite from a physical barrier medium to an active pharmaceutical agent carrier.

[0049] 3. The synergistic effect of the 4:1 compound ratio brings the dual advantages of rapid start-up and long-term maintenance.

[0050] The 4:1 compounding ratio (potassium persulfate: sodium persulfate) determined through numerous experiments in this invention is strictly irreplaceable.

[0051] Sodium persulfate component: High water solubility, dissolves and activates rapidly after injection, immediately generating high concentrations of sulfate free radicals to ensure rapid degradation of pollutants;

[0052] Potassium persulfate component: low water solubility, slowly released with the help of bentonite grid structure, and takes over to maintain the persulfate concentration in groundwater after the sodium persulfate concentration decays;

[0053] 4:1 critical ratio: When the potassium persulfate ratio is less than 4:1, the slow-release component is insufficient and cannot achieve long-term maintenance for more than 4 months; when the potassium persulfate ratio is greater than 4:1, the dissolved phase in the suspension is too small, the initial oxidation capacity is insufficient, and the suspension stability decreases.

[0054] This compound system and the bentonite sustained-release carrier function mutually support each other: without bentonite, potassium persulfate cannot be injected stably, nor can it form a long-lasting sustained release; without potassium persulfate, the effective period of sodium persulfate alone is only 1 to 2 months; without the 4:1 critical ratio, rapid start-up and long-term maintenance cannot be achieved simultaneously. The three constitute a complete technical whole that supports each other functionally.

[0055] 4. Top-down, equal-depth, spaced-interval injection structurally solves the backdiffusion problem.

[0056] The invention proposes a top-to-bottom progressive injection structure with a single vertical channel, 0.5m equal depth intervals, and a first-ever functional correlation between injection sequence and anti-diffusion suppression.

[0057] Top-down approach: Prioritize the remediation of the upper contaminated area, physically cutting off the migration path of pollutants from the lower layer to the upper layer, and preventing the deep remediation agent from being prematurely consumed by the pollutants in the upper layer.

[0058] 0.5m equal depth interval: uniform coverage, no treatment blind spots, avoiding repair dead corners caused by excessive injection spacing;

[0059] Single borehole, multi-layer injection: significantly reduces the number of boreholes (traditional single-hole single-point drilling requires several times more boreholes than this invention), reduces construction costs, and reduces formation disturbance.

[0060] This injection structure is not an optimization of the conventional construction sequence, but a specific solution for the anti-diffusion physical mechanism of low-permeability formations. Its technical effect cannot be achieved by conventional operations such as injecting deep layers first, then shallow layers, or injecting randomly.

[0061] 5. Quantifiable technical effects that are significantly superior to existing technologies.

[0062] This invention was pilot-scaled at an organically contaminated industrial site in Tianjin (pollutant volume 150 m³). 3 ) and a pilot test at an organic contaminated site in Nanjing (pollutant volume 400m³) 3 The technology achieved verifiable and significantly superior technical effects compared to existing technologies:

[0063] (1) Duration of effective persulfate concentration:

[0064] This invention: The concentration of persulfate in groundwater is maintained above 1 g / L for 4 months;

[0065] Sodium persulfate process alone: ​​its effectiveness lasts only 1-2 months in engineering applications;

[0066] The effect lasts more than twice as long, which is a qualitative breakthrough rather than a quantitative improvement.

[0067] (2) Pollutant removal efficiency (Tianjin pilot test, 6-month monitoring period):

[0068] Tetrachloroethylene: reduced from 640 μg / L to 65.5 μg / L (removal rate 89.8%).

[0069] Trichloroethylene: decreased from 67 μg / L to 5.6 μg / L (removal rate 91.6%).

[0070] Cis-1,2-dichloroethylene: decreased from 934 μg / L to 55.8 μg / L (removal rate 94.0%).

[0071] Vinyl chloride: decreased from 204 μg / L to 84 μg / L (removal rate 58.8%).

[0072] (3) Water quality compliance status:

[0073] After restoration, the groundwater quality met the Class IV water quality standard of the "Groundwater Quality Standard" (GB / T14848-2017).

[0074] The aforementioned technical effects were unforeseen by those skilled in the art prior to the application date. No existing literature has disclosed or implied that a combination of bentonite + potassium persulfate / sodium persulfate in a 4:1 ratio, high-pressure direct injection, and top-down equal-depth interval injection could maintain the effective concentration of persulfate in groundwater for more than four months, while simultaneously achieving efficient degradation of medium-to-high concentration chlorinated hydrocarbon pollutants.

[0075] (iv) Overall Declaration - The various technical features and functions support each other and constitute an inseparable overall technical solution.

[0076] Special note: The technical solution defined in this invention is a complete technical whole that is functionally mutually supportive and has a synergistic relationship, rather than a simple superposition or patchwork of multiple technical features.

[0077] The correlation and indivisibility of the various technical features are reflected in:

[0078] 1) Bentonite and potassium persulfate are interdependent: Without bentonite, potassium persulfate cannot be injected and its slow-release properties cannot be used in engineering; without potassium persulfate, bentonite can only be used as a thickener, cannot form a composite slow-release structure, and its effect lasts only 1 to 2 months.

[0079] 2) The 4:1 compound ratio and the bentonite slow-release carrier support each other: Without a 4:1 critical ratio, the slow-release components are insufficient or the initial oxidation capacity is insufficient, and rapid start-up and long-term maintenance cannot be achieved at the same time; without the bentonite grid structure, the potassium persulfate release rate is still relatively fast, and it is impossible to achieve a 4-month sustained effect.

[0080] 3) The top-to-bottom equal-depth interval injection structure matches the sustained-release characteristics: The sustained-release characteristics provide a sufficiently long operating window, making single-well multi-layer progressive injection possible; the top-to-bottom sequential physical blocking of the backdiffusion path allows the long-term efficacy of the sustained-release agent to be precisely applied to the target contamination layer, avoiding premature consumption by backdiffusion pollutants.

[0081] The absence of any one of the technical features will prevent the other features from working together to achieve the overall technical effect of being injectable, fast-starting, long-lasting, and inhibiting anti-diffusion. This is precisely the essence of the outstanding substantive features of this invention, and the fundamental reason why the principle of integrity must be adhered to in the evaluation of inventiveness. Attached Figure Description

[0082] Figure 1 is a schematic diagram of the connection between the core process of preparation and injection of the sustained-release repair agent of the present invention.

[0083] Figure 2 is a schematic diagram of the site in-situ injection engineering application layout described in this invention.

[0084] Figure 3 is a schematic diagram of the vertical operation of the injection point described in this invention. Detailed Implementation

[0085] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings. The embodiments of this invention include, but are not limited to, the following examples. Equivalent substitutions or modifications made based on the technical concept of this invention all fall within the protection scope of this invention.

[0086] The technical solution described in this invention is a holistic technical solution in which the functions of each component support each other and have a synergistic relationship. The following detailed description, through two pilot-scale engineering examples and two comparative experimental examples, combined with Figure 1 showing the connection diagram of the core process for the preparation and injection of the sustained-release repair agent, Figure 2 showing the layout diagram of the in-situ injection engineering application, and Figure 3 showing the vertical operation diagram of the injection points, provides a complete, clear, and feasible explanation of this invention.

[0087] Figure 1 shows the process flow structure, illustrating the entire process from the preparation of the slow-release persulfate oxidant suspension and the formulation of the alkali activator to mixing and injection, including component connections and material flow. The components are connected in series, arranged according to the process sequence, as follows: Reagent tank 1 is the core preparation unit, receiving three types of materials: 28% potassium persulfate, 7% sodium persulfate (comprising a compound oxidant at a 4:1 mass ratio), and 6% sodium bentonite. These three materials are mixed and stirred within reagent tank 1 to generate the compound oxidant suspension, which is the output end of this suspension. Reagent tank 2 is the alkali activator preparation unit, independent of reagent tank 1, and is the sole output end of the alkali activator. The static mixer is the intermediate mixing unit, connected to the outlet ends of reagent tank 1 and 2, serving as the sole point of convergence for the two materials. The compound oxidant suspension and alkali activator undergo rapid mixing upon entering the static mixer, and the resulting repair agent is output from the static mixer. The GeoProbe direct injection unit is the terminal execution unit, which is directly connected to the discharge end of the static mixer. It receives the mixed repair agent and delivers it to the injection target depth point, serving as the final action point of the repair agent.

[0088] Figure 2 shows the three-dimensional spatial structure of the site, combining vertical layering with a horizontal layout. It illustrates the location and spatial connections of various equipment, geological structures, and monitoring facilities involved in in-situ injection of groundwater contaminated sites. The core element is the coordination between the GeoProbe drilling operation and various site elements, as detailed below:

[0089] 1. Vertical stratigraphic layering: From the surface to the underground, the layers are, in order from top to bottom, miscellaneous fill, silty clay, silty clay, and silty sand. The contaminated area and injection operations are carried out in this vertical stratigraphic layer, and the injection points penetrate vertically through each contaminated layer.

[0090] 2. Core equipment GeoProbe drilling rig: Located above the surface fill, it serves as the power source and main operating unit for injection operations. The drilling rig's injection pipeline extends vertically downwards, passing through each layer of soil and directly connecting to the underground injection point, which is the core channel for delivering the repair agent.

[0091] 3. Reagent preparation and mixing unit: Reagent tank 1, reagent tank 2, and static mixer are all located on the ground surface, adjacent to the GeoProbe drilling rig; the discharge ends of reagent tank 1 and reagent tank 2 are connected to the static mixer, and the discharge end of the static mixer is connected to the injection pipeline of the GeoProbe drilling rig, forming a material transport chain of surface preparation-mixing-underground injection.

[0092] 4. Injection impact range: An irregular area is formed centered on the underground injection point, spreading to the surrounding soil and groundwater.

[0093] 5. Monitoring and Hydrological Elements:

[0094] Groundwater monitoring wells: Located on the side (horizontally) of the injection points within the site, with the well pipes extending vertically downwards to the groundwater level and contaminated layer, used to monitor indicators such as groundwater quality and oxidant concentration during the remediation process;

[0095] Groundwater flow direction: Marked in the form of arrows in water-bearing strata such as underground silt layers, showing the natural flow direction of groundwater, forming a spatial correspondence with injection points and monitoring wells, reflecting the impact of site hydrological conditions on remediation.

[0096] Figure 3 shows the vertical local structure of the injection point, focusing on the underground depth operation of a single injection point. It presents the multi-layer injection layout of the GeoProbe drilling rig at the same vertical point, with the core being a vertical injection structure with equal depth intervals. The specific relationships are as follows:

[0097] 1. The injection point is a single vertical channel: it is drilled vertically downwards from the ground surface by the GeoProbe drilling rig, penetrating the target repair depth range. The channel is an underground delivery channel for the repair agent and is directly connected to the injection pipeline of the surface drilling rig.

[0098] 2. Injection depth points are arranged vertically at equal intervals: On the vertical channel of the same injection point, injection depth points are set from top to bottom at a depth interval of 0.5m; the first injection depth point is located at 0.5m underground, and the depth points are 1.0m, 1.5m and so on down to reach the bottom of the contaminated layer. All depth points belong to the same vertical injection channel and share the drilling rig and material conveying pipeline on the surface.

[0099] 3. The operation logic is top-down progressive: the repair agent is injected sequentially from top to bottom through the same vertical channel according to the depth points. After each depth point is injected, the drilling rig pipeline continues to push down 0.5m to the next depth point until the injection of the entire depth range is completed.

[0100] Example 1: Pilot test at an organically contaminated industrial site in Tianjin

[0101] I. Site Overview and Pollution Characteristics

[0102] This embodiment was implemented in the pilot-scale stage of a remediation project for an organically contaminated industrial site in Tianjin. The target contaminated area has a planar dimension of 5 meters × 6 meters, a contamination depth of 2 to 7 meters underground, and a total volume of contaminated soil and groundwater of 150 cubic meters. The site's hydrogeological conditions are as follows: the surface is filled soil, followed by a layer of silty clay (approximately 1.5 meters thick), a layer of silty clay (approximately 2 meters thick), and a layer of silty sand (approximately 3 meters thick). The groundwater depth is approximately 1.8 meters, and the groundwater flows from northwest to southeast.

[0103] The target pollutants were tetrachloroethylene, trichloroethylene, vinyl chloride, and cis-1,2-dichloroethylene, which had complex compositions and high concentrations. The initial concentrations were: tetrachloroethylene 640 μg / L, trichloroethylene 67 μg / L, cis-1,2-dichloroethylene 934 μg / L, and vinyl chloride 204 μg / L.

[0104] II. Implementation Steps

[0105] S1: Preparation of compound oxidant suspension

[0106] Based on the amount of contaminated soil and soil density (1.6 g / cm³), and assuming the dry matter of persulfate oxidant accounts for 1% of the soil mass by weight, the total amount of dry matter of persulfate oxidant required is: 150 cubic meters × 1.6 tons / cubic meter × 1% = 2.4 tons.

[0107] Weigh out 1.92 tons of potassium persulfate and 0.48 tons of sodium persulfate at a mass ratio of 4:1.

[0108] Add water to reagent tank 1 to prepare an oxidant suspension with a mass fraction of 35%. The total amount of oxidant suspension required is: 2.4 tons ÷ 35% ≈ 6.86 tons.

[0109] Sodium-based bentonite was added to reagent tank 1 at a rate of 6% of the total mass of the oxidant suspension, i.e., 6.86 tons × 6% ≈ 0.41 tons.

[0110] Turn on the agitator: first stir at 500 rpm for 5 minutes to initially mix the components; then stir at a high speed of 1500 rpm until the medium is a homogeneous slurry. At this point, the potassium persulfate particles are dispersed in the layered silicate spatial network structure formed after the bentonite absorbs water and expands, constituting a stable clay-oxidant composite slow-release structure. The suspension showed no significant settling after standing for more than 2 hours, and its stability meets the requirements for engineering injection.

[0111] S2: Alkali activator is prepared independently.

[0112] Based on the calculation that the dry matter of the alkali activator is 40% of the dry matter of the persulfate oxidant, the required dry matter of sodium hydroxide is: 2.4 tons × 40% = 0.96 tons.

[0113] To prepare a 32% sodium hydroxide solution, the total amount of solution required is: 0.96 tons ÷ 32% = 3.0 tons.

[0114] The preparation is completed independently in reagent tank 2, and the mixture is stirred evenly before use. Reagent tank 2 is completely independent of reagent tank 1, and there is no premixing pipeline between the two.

[0115] S3: Online Mixing and Conveying

[0116] As shown in Figure 1, the discharge ends of reagent tank 1 and reagent tank 2 are connected to the two inlets of a static mixer, respectively. The discharge end of the static mixer is directly connected to the inlet of the GeoProbe direct-push injection unit, without any intermediate buffer tank. The compound oxidant suspension and the alkali activator solution are rapidly mixed in the static mixer, and the mixed repair agent immediately enters the injection unit, realizing online mixing and delivery with zero buffering.

[0117] S4: High-pressure direct injection

[0118] The GeoProbe direct injection technology was used, with the injection pressure set at no less than 3.5 MPa and the injection speed controlled at no less than 20 liters / minute. In this embodiment, the actual injection pressure was maintained between 3.8 and 4.2 MPa, and the injection speed was controlled between 22 and 25 liters / minute.

[0119] S5: Inject at equal depth intervals from top to bottom.

[0120] As shown in Figure 3, the injection depth range for this site is from 2 meters to 7 meters underground, with the contaminated floor slab located at 7 meters underground. A total of 8 injection points are set up throughout the site, each on the same vertical injection channel, with equal depth intervals of 0.5 meters, arranged sequentially from top to bottom: the first injection point is located at 2.0 meters underground, followed by points at 2.5 meters, 3.0 meters, 3.5 meters, 4.0 meters, 4.5 meters, 5.0 meters, 5.5 meters, 6.0 meters, 6.5 meters, and 7.0 meters. Each injection point has a total of 11 injection depth points, for a total of 88 injection depth points throughout the site.

[0121] The operational logic is as follows: the remediation agent is injected sequentially from top to bottom through the same vertical injection channel, according to the depth points. After each depth point is injected, the drilling rig continues to push downwards by 0.5 meters to the next depth point, repeating the injection operation until the bottom of the contaminated layer is reached. This operational sequence can physically cut off the migration path of lower-layer contaminants to the upper layers.

[0122] III. Monitoring Plan

[0123] As shown in Figure 2, a groundwater monitoring well was installed next to the injection site, with the well pipe extending vertically downwards to below the contaminated aquifer (8 meters deep). Groundwater samples were collected on days 1, 7, 15, 30, 60, 90, 120, 150, and 180 after injection to detect the concentration of the target pollutant and the residual concentration of persulfate.

[0124] IV. Repair Results

[0125] The results of a six-month follow-up monitoring program showed that:

[0126] The tetrachloroethylene concentration decreased from an initial 640 μg / L to 65.5 μg / L, achieving a removal rate of 89.8%.

[0127] The trichloroethylene concentration decreased from 67 μg / L to 5.6 μg / L, with a removal rate of 91.6%.

[0128] The concentration of cis-1,2-dichloroethylene decreased from 934 μg / L to 55.8 μg / L, with a removal rate of 94.0%.

[0129] The vinyl chloride concentration decreased from 204 μg / L to 84 μg / L, with a removal rate of 58.8%.

[0130] After restoration, the groundwater quality in the area reached Class IV of the Groundwater Quality Standard (GB / T14848-2017).

[0131] Key performance indicators: The persulfate concentration in groundwater remains above 1 g / L for 4 months, while the effective period of a single sodium persulfate process in engineering applications is only 1 to 2 months. This invention extends the effective period by more than 2 times, representing a qualitative breakthrough.

[0132] Example 2: An organically contaminated site in Nanjing

[0133] I. Site Overview and Pollution Characteristics

[0134] This example was implemented in a remediation project for an organically contaminated site in Nanjing. The target contaminated area had a planar dimension of 10 meters × 10 meters, a contamination depth of 0 to 4 meters underground, and a total volume of contaminated soil and groundwater of 400 cubic meters.

[0135] The target pollutants were chloroform and vinyl chloride, which are persistent and difficult-to-degrade pollutants. The initial concentrations were 2560 μg / L for chloroform and 1840 μg / L for vinyl chloride.

[0136] II. Implementation Steps

[0137] S1: Preparation of compound oxidant suspension

[0138] Based on a soil mass weight ratio of 1%, the total dry matter required for persulfate oxidant is: 400 cubic meters × 1.6 tons / cubic meter × 1% = 6.4 tons.

[0139] Weigh out 5.12 tons of potassium persulfate and 1.28 tons of sodium persulfate according to the ratio of potassium persulfate to sodium persulfate of 4:1.

[0140] The total amount of 35% oxidant suspension prepared is: 6.4 tons ÷ 35% ≈ 18.29 tons.

[0141] Adding 6% sodium bentonite: 18.29 tons × 6% ≈ 1.1 tons.

[0142] The stirring process is the same as in Example 1.

[0143] S2: Alkali activator is prepared independently.

[0144] Sodium hydroxide dry matter: 6.4 tons × 40% = 2.56 tons.

[0145] To prepare a 32% sodium hydroxide solution: 2.56 tons ÷ 32% = 8.0 tons.

[0146] S3: Online Mixing and Conveying

[0147] Similar to Example 1, an online mixing process is adopted using a static mixer, with independent conveying of reagent tank 1 and reagent tank 2, and the discharge end of the static mixer is directly connected to the GeoProbe direct-push injection unit.

[0148] S4: High-pressure direct injection

[0149] Injection pressure ≥3.5 MPa, injection speed ≥20 L / min, same as in Example 1.

[0150] S5: Inject at equal depth intervals from top to bottom.

[0151] The contamination depth at this site ranges from 0 to 4 meters, with the contaminated floor layer located at a depth of 4 meters. A total of 26 injection points are set up throughout the site. Each point is located on the same vertical injection channel, with injection depth points set sequentially from top to bottom at 0.5-meter intervals: the first injection depth point is located at 0.5 meters, followed by points at 1.0 meter, 1.5 meters, 2.0 meters, 2.5 meters, 3.0 meters, 3.5 meters, and 4.0 meters. Each injection point has a total of 8 injection depth points, for a total of 208 injection depth points throughout the site.

[0152] III. Repair Effect

[0153] The results of a 6-month follow-up monitoring show:

[0154] The vinyl chloride concentration decreased from 1840 μg / L to 64 μg / L, with a removal rate of 96.5%.

[0155] The concentration of chloroform decreased from 2560 μg / L to 114 μg / L, with a removal rate of 95.5%.

[0156] After restoration, the groundwater quality met the Class IV water quality standard of the "Groundwater Quality Standard" (GB / T14848-2017).

[0157] Persulfate concentration maintenance time: ≥4 months to maintain the effective concentration level.

[0158] Comparative Example 1: Feasibility Comparison of Potassium Persulfate Suspension Injection under Bentonite-Free Conditions

[0159] I. Experimental Objective

[0160] This study demonstrates the crucial role of sodium-based bentonite in the stability of potassium persulfate suspensions and addresses the long-standing technical bias in the field that bentonite is unsuitable for in-situ injection.

[0161] II. Experimental Design

[0162] Two sets of samples were prepared under laboratory conditions:

[0163] Experimental group: formulated according to Example 1, containing 6% sodium bentonite, potassium persulfate:sodium persulfate = 4:1, total solids content 35%;

[0164] Control group: Except for the absence of bentonite, all other components, proportions, and mixing processes were exactly the same.

[0165] III. Observation Indicators

[0166] Settling time of slurry: The endpoint is determined by the appearance of obvious solid-liquid stratification (the height of the upper clear liquid is >10% of the total height);

[0167] Injection simulation test: A miniature high-pressure injection device was used to test the flow stability of continuous injection for 30 seconds at a pressure of 1.0 MPa.

[0168] IV. Experimental Results

[0169] Experimental group (including bentonite): No significant settlement after standing for more than 2 hours, stable injection flow rate, suitable for engineering injection;

[0170] Control group (without bentonite): Severe sedimentation and stratification occurred within 5 minutes of standing, and the injection pipeline became blocked, making engineering injection impossible.

[0171] V. Conclusion

[0172] Without bentonite, potassium persulfate suspensions exhibit severe sedimentation and stratification within 5 minutes, failing to maintain a homogeneous state and rendering them completely unsuitable for in-situ high-pressure injection. This clearly demonstrates that bentonite is an indispensable key component in the technical solution of this invention, and its role cannot be replaced by conventional thickeners. Those skilled in the art, guided by existing technology that bentonite should be avoided for in-situ injection, lacked the motivation to add it to persulfate suspensions, and even less could they have foreseen its ability to simultaneously achieve the dual functions of suspension stabilization and sustained-release enhancement. This invention overcomes long-standing technical biases.

[0173] Comparative Example 2: Effects of different compound ratios

[0174] I. Experimental Objective

[0175] The results show that the 4:1 ratio of potassium persulfate to sodium persulfate has critical selectivity significance, and not any arbitrary ratio can achieve the dual technical effects of rapid start-up and long-term maintenance.

[0176] II. Experimental Design

[0177] With fixed total dry matter of oxidant, bentonite addition of 6%, suspension concentration of 35%, alkali activator ratio of 40%, and injection parameters, only the mass ratio of potassium persulfate to sodium persulfate was changed to set the following 5 ratio groups:

[0178] Ratio A (in this invention): Potassium persulfate: Sodium persulfate = 4:1;

[0179] Ratio B: Potassium persulfate: Sodium persulfate = 2:1 (potassium ratio reduced);

[0180] Ratio C: Potassium persulfate: Sodium persulfate = 6:1 (potassium ratio increased);

[0181] Ratio D: Potassium persulfate: Sodium persulfate = 1:0 (pure potassium persulfate);

[0182] Ratio E: Potassium persulfate: Sodium persulfate = 0:1 (pure sodium persulfate, existing technology).

[0183] III. Experimental Results

[0184] Ratio A (4:1): The suspension is stable, with an initial 7-day degradation rate of 85%, and a persulfate retention period of ≥4 months. Overall, it is rated as excellent.

[0185] Ratio B (2:1): The suspension is stable, with an initial 7-day degradation rate of 88%, and the persulfate retention period is about 2.5 months, which is insufficient.

[0186] Ratio C (6:1): The suspension is unstable (sedimentation is accelerated), the initial 7-day degradation rate is 62%, the persulfate retention period is about 4.5 months, and the initial activity is insufficient;

[0187] Ratio D (pure potassium): The suspension is severely unstable, with an initial degradation rate of 45% in the first 7 days. The persulfate effect lasts for about 5 months and it is not suitable for injection.

[0188] Ratio E (pure sodium): The suspension is stable, with a degradation rate of 92% in the first 7 days, and the persulfate effect lasts for about 1.5 months. This is an existing technology.

[0189] IV. Conclusion

[0190] Excessive potassium persulfate ratio (≥6:1): Reduced suspension stability, severely insufficient initial oxidation capacity, and inability to rapidly degrade high-concentration pollutants;

[0191] A low potassium persulfate ratio (≤2:1): insufficient slow-release components, significantly shortened duration of effect, and inability to achieve 4-month long-term effect maintenance;

[0192] Pure potassium persulfate: The suspension cannot be injected, making it completely impractical in engineering.

[0193] Pure sodium persulfate: Its effective period is only 1.5 months, which cannot meet the long-term remediation needs of low-permeability formations.

[0194] The 4:1 critical ratio selected through extensive experiments in this invention is the optimal combination that simultaneously meets the triple requirements of injection feasibility, rapid initiation, and long-term maintenance, and has significant and unexpected technical effects.

[0195] Summary of Invention Principles

[0196] Based on the above embodiments and comparative examples, the core principle of the technical solution of the present invention is a triple collaborative mechanism. The absence of any one of these mechanisms will prevent the overall technical effect from being achieved.

[0197] I. The Dual Functional Synergistic Mechanism of Bentonite

[0198] Suspension stabilization function: Bentonite absorbs water and expands to form a high-viscosity colloidal network, which provides physical suspension support for potassium persulfate particles. This allows the potassium persulfate suspension, which would normally settle and stratify within minutes, to maintain a homogeneous and stable state for more than 2 hours, making potassium persulfate an injectable agent for the first time.

[0199] Slow-release carrier function: The bentonite layered silicate spatial grid structure encapsulates, adsorbs, and embeds potassium persulfate particles. After groundwater enters the grid, it slowly dissolves potassium persulfate, extending the diffusion path and reducing the release rate, forming a clay-oxidant composite slow-release structure.

[0200] Both are indispensable: without bentonite, potassium persulfate cannot be injected; with only bentonite and no potassium persulfate, there is only thickening but no sustained release.

[0201] II. The Initiation-Maintenance Relay Synergistic Mechanism of Compound Ratio

[0202] Sodium persulfate: Highly water-soluble, it dissolves and activates rapidly after injection, immediately generating a high concentration of sulfate free radicals to ensure the rapid degradation of pollutants;

[0203] Potassium persulfate: Low water solubility, it is slowly released with the help of the bentonite grid structure, and takes over to maintain the persulfate concentration in groundwater after the sodium persulfate concentration decays;

[0204] 4:1 critical ratio: After extensive experimental screening, it is the optimal balance point that simultaneously meets the triple requirements of injection feasibility, rapid start-up, and long-term maintenance.

[0205] The proportions cannot be replaced: too low potassium will result in insufficient duration of effect; too high potassium will result in insufficient initial activity and decreased suspension stability.

[0206] III. Backdiffusion blocking mechanism of top-down equal-depth interval injection

[0207] Top-down order: Prioritize the remediation of the upper contaminated area to physically cut off the migration path of pollutants from the lower layer to the upper layer, and prevent the deep remediation agent from being prematurely consumed by the pollutants in the upper layer.

[0208] 0.5-meter equal-depth interval: uniform coverage, no treatment blind spots, avoiding repair dead corners caused by excessive injection spacing;

[0209] Multi-layer injection in the same borehole: significantly reduces the number of boreholes, lowers construction costs, and reduces formation disturbance.

[0210] The operation sequence is irreversible: if the deep layer is injected first and then the shallow layer, the lower layer remediation agent will be largely consumed by the high concentration of pollutants in the upper layer during the upward diffusion process, and the deep layer remediation effect will be significantly reduced.

[0211] Industrial applicability

[0212] The method and system described in this invention have clear equipment connections, well-defined operating procedures, and readily available and widely sourced reagents. They have been successfully implemented in pilot-scale trials in Tianjin and Nanjing, two industrial contaminated sites with different hydrogeological conditions and pollution types, and have achieved remediation effects that are significantly superior to existing technologies.

[0213] This invention can be widely applied to: in-situ remediation of groundwater contaminated with chlorinated hydrocarbons; treatment of the continuous release of organic pollutants from low-permeability formations due to matrix diffusion-reverse diffusion; long-term treatment of low-concentration diffused groundwater plumes; and remediation of pollutants (such as polycyclic aromatic hydrocarbons) that are low in water solubility and slowly released into groundwater. It possesses significant industrial applicability.

[0214] Comparative Example 3: Effects of different injection sequences on repair efficacy and anti-diffusion inhibition

[0215] I. Experimental Objective

[0216] This proves that the "same vertical channel, 0.5m equal depth interval, and top-down progressive injection" method used in this invention is not a simple optimization of the construction sequence, but a specific technical means for the reverse diffusion mechanism of low-permeability strata. Its technical effect cannot be achieved by conventional operations such as bottom-up or random injection.

[0217] II. Experimental Design

[0218] An indoor two-dimensional sandbox simulation device was used, measuring 1.0 meter high, 0.8 meters wide, and 0.2 meters thick. The chamber was filled with three layers of media from top to bottom to simulate a typical low-permeability contaminated formation structure.

[0219] The upper layer is silty clay, 0.3 meters thick, with low permeability and an initial tetrachloroethylene contamination concentration of 500 micrograms per liter;

[0220] The middle layer is silt, 0.4 meters thick, with medium permeability and is easy to clean;

[0221] The lower layer is silt, 0.3 meters thick, with medium permeability, and an initial tetrachloroethylene contamination concentration of 500 micrograms per liter.

[0222] The groundwater flow direction is horizontal, and the flow velocity is set at 0.1 m / day. A single vertical injection channel is set at the center of the tank, with injection outlets at 0.2 m (upper layer), 0.5 m (middle layer), and 0.8 m (lower layer) along the depth direction, with vertical intervals of 0.3 m (a proportional reduction of the 0.5 m interval in the simulation project). The oxidant formula and injection parameters are the same as in Example 1.

[0223] Three sets of comparative experiments were set up, with each set repeated 3 times and the average value taken:

[0224] Experimental group (method of the present invention): The injection sequence is from top to bottom, that is, first inject the upper layer 0.2 meters, then inject the middle layer 0.5 meters, and finally inject the lower layer 0.8 meters.

[0225] Control group 1 (bottom to top): The injection sequence was from bottom to top, that is, first inject the lower layer 0.8 meters, then inject the middle layer 0.5 meters, and finally inject the upper layer 0.2 meters.

[0226] Control group 2 (random order): The injection order was randomly shuffled, that is, the injection was performed in the order of middle layer 0.5 meters, upper layer 0.2 meters, and lower layer 0.8 meters.

[0227] The monitoring indicators include:

[0228] 1. Changes in persulfate concentration in each layer: Samples were taken and tested at 1 hour, 1 day, 3 days, 7 days, 14 days, and 28 days after injection;

[0229] 2. Changes in tetrachloroethylene concentration in each layer: Samples were taken and tested before injection and at 7, 14, and 28 days after injection;

[0230] 3. Degree of back diffusion of the lower layer repair agent to the upper layer: The amount of persulfate ions detected in the medium at 0.2 meters above the lower layer injection point (mg / kg) is used as the back diffusion characterization value.

[0231] III. Experimental Results

[0232] 1. Duration of persulfate activity and uniformity of vertical distribution

[0233] Experimental group (from top to bottom): Within 28 days after injection, the persulfate concentration in the upper, middle, and lower layers remained stable within the range of 8–12 mg / L, with a uniform vertical distribution. After the upper layer pollutants were rapidly degraded, the consumption of remediation agents diffusing upward from the lower layer was minimal, and the oxidant concentration in each layer remained stable.

[0234] Control Group 1 (from bottom to top): After injection into the lower layer, persulfate rapidly diffused back into the upper layer. A high concentration of persulfate (more than 15 mg / L) was detected in the upper layer 3 days after injection. However, this part of the oxidant was largely consumed after reacting with the high concentration of pollutants in the upper layer. By the time the upper layer was injected, the concentration of persulfate had decreased significantly. After 28 days, the concentration of persulfate in the lower layer was only 2-4 mg / L, and the effective period was shortened by about 40% compared with the experimental group.

[0235] Control group 2 (random order): After the middle layer was injected first, the oxidant diffused to both the upper and lower sides at the same time, but the pollution sources in the upper and lower layers were not treated in time. After 7 days, the persulfate concentration in the middle layer decreased by more than 50%. When the upper and lower layers were injected subsequently, the oxidant was unevenly distributed in each layer. The persulfate concentration in some areas of the lower layer was less than 1 mg / L, resulting in a repair blind zone.

[0236] 2. Pollutant removal efficiency

[0237] The removal rates of tetrachloroethylene in each layer 28 days after injection are as follows:

[0238] Experimental group (from top to bottom): top layer removal rate 92%, middle layer removal rate 88%, bottom layer removal rate 86%, overall average removal rate reached 88.7%.

[0239] Control group 1 (from bottom to top): upper layer removal rate 85%, middle layer removal rate 76%, lower layer removal rate 58%, overall average removal rate only 73.0%. The lower layer remediation effect was seriously insufficient, and the shortened effective period resulted in obvious deep-layer pollutant residues.

[0240] Control group 2 (random order): Top layer removal rate 78%, middle layer removal rate 82%, bottom layer removal rate 61%, overall average removal rate 73.7%. Repair was uneven across layers, with varying degrees of repair dead zones in both the top and bottom layers.

[0241] 3. Quantitative characterization of backdiffusion

[0242] Seven days after injection, the persulfate content in the medium 0.2 meters above the lower injection point was measured to characterize the cumulative amount of the lower repair agent diffusing upwards.

[0243] Experimental group (from top to bottom): Not detected. Due to the preferential remediation of the upper layer, the pollutant concentration has been significantly reduced, and the oxidation zone formed after the remediation of the upper layer simultaneously degrades the pollutants diffusing upward from the lower layer, effectively blocking the reverse diffusion path.

[0244] Control group 1 (bottom to top): Detection level was 12.6 mg / kg. During the upward diffusion of the lower layer remediation agent, it reacted with the high concentration of pollutants in the upper layer, resulting in a large consumption of oxidant and a significant shortening of the effective period of the lower layer.

[0245] Control group 2 (random order): detected amount was 8.3 mg / kg. After interlayer injection, the pollution sources in the upper and lower layers remained, back diffusion was not effectively controlled, and the loss of oxidant was between that of the first two groups.

[0246] IV. Conclusion

[0247] 1. The top-to-bottom injection sequence is a structural technical approach to block back-diffusion, rather than an optimization of construction habits. Prioritizing the remediation of the upper contaminated area physically cuts off the upward back-diffusion migration path of lower-layer pollutants, while preventing the lower-layer remediation agent from being prematurely consumed by upper-layer pollutants, thus achieving uniform remediation and long-term maintenance across all layers.

[0248] 2. Neither bottom-up nor random injection can achieve the same repair effect. Bottom-up injection results in a significant loss of the lower layer repair agent, a substantial decrease in the deep repair effect, and a reduction in the duration of effectiveness by more than 40%; random injection creates repair blind spots, reducing the overall removal rate by more than 15 percentage points compared to the method of this invention.

[0249] 3. The injection sequence and the sustained-release remediation agent provide functional synergy. The sustained-release characteristic provides a sufficiently long operating window, allowing for a smooth implementation of multi-layered, top-down injections; while the top-down injection sequence ensures that the long-term efficacy of the sustained-release agent can accurately target the contaminated layer, preventing premature consumption by back-diffusion pollutants. Both are complementary and indispensable.

[0250] This comparative example further confirms that the "same vertical channel, 0.5m equal depth interval, and top-down progressive injection" configuration defined in this invention is a specific solution for the reverse diffusion mechanism of low-permeability formations. Its technical effect cannot be achieved through conventional construction methods by those skilled in the art, and it has outstanding substantive characteristics.

[0251] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for remediating chlorinated hydrocarbon-contaminated groundwater based on a slow-release remediation agent, characterized in that, Includes the following steps: S1: Potassium persulfate and sodium persulfate are compounded at a mass ratio of 4:1 to form a persulfate oxidant. Water is added and stirred to prepare an oxidant suspension with a mass fraction of 35%. Sodium-based bentonite is then added and stirred at high speed to form a homogeneous and stable compound oxidant suspension. S2: NaOH is prepared into an alkali activator solution with a mass fraction of 32%. S3: The compound oxidant suspension obtained in S1 and the alkali activator solution obtained in S2 are respectively sent to a static mixer. After being rapidly mixed in the static mixer, they are immediately sent to the GeoProbe direct injection unit. S4: Using GeoProbe direct injection technology, the mixed repair agent is injected underground at an injection pressure of ≥3.5MPa and an injection speed of ≥20L / min; S5: On the same vertical injection channel, multiple injection depth points are set from top to bottom at equal depth intervals of 0.5m, and the injection at each depth point is completed in sequence until the bottom plate of the contaminated layer is reached.

2. The method according to claim 1, characterized in that, The dry matter weight of the persulfate oxidant in S1 is calculated as 1% w / w of the soil mass, and the amount of sodium-based bentonite added is 6% of the total mass of the oxidant suspension. The stirring speed is 500 rpm for 5 minutes, and then the mixture is stirred at high speed of 1500 rpm until it becomes a homogeneous slurry.

3. The method according to claim 1, characterized in that, The dry matter weight of the alkali activator described in S2 is calculated as 40% of the dry matter weight of the persulfate oxidant.

4. A sustained-release repair agent in-situ injection system for implementing the method of any one of claims 1-3, characterized in that, include: The reagent tank 1 is a compound oxidant suspension preparation unit. Its inlet is connected to a potassium persulfate source, a sodium persulfate source, a sodium bentonite source, and a water source, respectively. It is equipped with a stirring device inside, and its outlet is connected to the first inlet of a static mixer through a first conveying pipeline. The reagent tank 2 is an alkali activator preparation unit. Its inlet is connected to a NaOH source and a water source, and its outlet is connected to the second inlet of a static mixer through a second conveying pipeline. The static mixer is an online mixing unit. Its outlet is connected to the inlet of a GeoProbe direct injection unit through a third conveying pipeline. The GeoProbe direct injection unit includes a drill rod. The drill rod has a vertical injection channel communicating with the inlet. The drill rod has multiple injection outlets spaced along the depth direction. The injection outlets are communicating with the vertical injection channel.

5. The system according to claim 4, characterized in that, The compound oxidant suspension formed in the reagent tank 1 is a homogeneous mud medium containing sodium-based bentonite. Potassium persulfate particles are dispersed in the layered silicate spatial grid structure formed by bentonite, forming a clay-oxidant composite slow-release structure.

6. The system according to claim 4, characterized in that, The first, second, and third delivery pipelines are all high-pressure corrosion-resistant hoses. The outlet of the static mixer is directly connected to the inlet of the GeoProbe direct injection unit, without any intermediate buffer tank.

7. The system according to claim 4, characterized in that, The multiple injection outlets on the drill rod are arranged from top to bottom at equal depth intervals of 0.5m. All injection outlets are connected to the vertical injection channel inside the same drill rod and correspond to different depth positions of the same borehole.

8. The system according to claim 4, characterized in that, It also includes at least one groundwater monitoring well, which is located next to the injection point and whose well casing extends vertically downward to the contaminated aquifer.

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