Chromium-polluted aquifer in-situ layered injection remediation method based on response curved surface

Abstract

The invention provides a chromium-polluted aquifer in-situ layered injection remediation method based on a response curved surface. The method comprises the following steps that a prediction model is constructed based on a response curved surface method, optimal agent parameters are predicted, pilot test verification is conducted based on the predicted optimal agent parameters, and the optimal pilot test agent injection amount is determined; establishing a relational expression between the agent injection concentration and the initial concentration of the underground water Cr (VI) based on the theoretical agent dosage, the adsorption capacity of the water-containing medium to the agent and the model prediction result; the optimal pilot plant test agent injection amount is the total injection amount of all the injection points, and according to the relational expression, the agent injection concentration of all the injection points in the chromium pollution site is determined. The method disclosed by the invention can be used for accurately regulating and controlling medicament parameters in actual engineering application, improving the remediation efficiency, shortening the construction period and saving the capital cost, fully remedying the chromium-polluted aquifer and reducing the risk of'yellowing 'of underground water in the later period, and is suitable for in-situ remediation construction application of the large-scale polluted aquifer.

Description

Technical Field

[0001] This invention relates to the field of chromium pollution remediation technology, and in particular to a method for in-situ layered injection remediation of chromium-contaminated aquifers based on response surface methodology. Background Technology

[0002] Cr(VI) is a class of inorganic pollutants with direct toxic effects and carcinogenic risks. In nature, it exists in the form of oxyacid anions. Cr(VI) is soluble in water, stable in aquatic bodies, not easily adsorbed and fixed by soil, and highly mobile. Under anaerobic conditions, it can be reduced to Cr(III). Cr(III) is easily trapped by soil and has weaker mobility in groundwater. Cr(VI) is approximately 100 times more toxic than Cr(III) and is more easily absorbed by the human body, accumulating in the body.

[0003] Currently, direct injection technology is mostly used in in-situ groundwater remediation. However, the successful application of in-situ groundwater injection technology relies on sufficient contact between the target contaminant and the injected agent. When the aquifer at the contaminated site is thick, groundwater remediation in only a single layer can easily lead to uneven agent diffusion and insufficient remediation, and may result in a "yellowing" phenomenon in the later stage (due to the release of hexavalent chromium trapped inside the aquifer, resulting in a rise in the Cr(VI) concentration in the groundwater, which appears yellow).

[0004] The dosage of chemicals used in in-situ injection remediation is often determined based on laboratory results, aquifer thickness and porosity, and safety factors. However, the interaction between chemical concentration and injection volume is not considered, and the accuracy of the safety factor is insufficient. This leads to problems such as low precision and large errors in chemical parameters during actual remediation, making it difficult to guarantee the remediation effect. Meanwhile, the use of orthogonal experimental design to determine chemical concentration and injection volume is also limited in its application in practical site remediation due to the large number of experiments, long cycles, high costs, and difficulty in identifying the optimal values ​​for continuous factors.

[0005] Response surface methodology (RSM) is an optimization method that combines statistical experimental design with mathematical modeling. It explores the mathematical relationship between influencing factors and response output through multiple quadratic regression equations. While widely used in the optimization of pollution control systems, it is mostly at the laboratory level. Its optimization results still face key limitations in practical groundwater remediation applications, such as environmental heterogeneity, the long-term effectiveness of remediation, practical feasibility, and economic viability. Furthermore, actual contaminated sites have large spatial scales and heterogeneous pollutant concentrations. Response surface optimization results based on field pilot tests are still affected by regional pollutant concentrations. Directly applying these results to a site-wide chemical dosing strategy may fail to match the actual remediation needs of the entire site, potentially leading to decreased accuracy in chemical dosing, secondary pollution, or reduced remediation efficiency, ultimately failing to guarantee remediation effectiveness. Summary of the Invention

[0006] This invention proposes an in-situ, layered injection remediation method for chromium-contaminated aquifers based on response surface methodology. A predictive model is constructed using response surface methodology to establish the relationship between reagent parameters and the initial concentration of Cr(VI) in groundwater. In practical engineering applications, this method allows for precise control of reagent parameters, improved remediation efficiency, shortened construction cycles, and reduced costs. It effectively remediates chromium-contaminated aquifers while also reducing the risk of subsequent groundwater "yellowing," making it suitable for large-scale in-situ remediation of contaminated aquifers.

[0007] The technical solution of this invention is implemented as follows: a method for in-situ layered injection remediation of chromium-contaminated aquifers based on response surfaces, comprising the following steps:

[0008] (1) Select a pilot test area within the chromium-contaminated site and determine the initial Cr(VI) concentration in the groundwater of the pilot test area; using the reagent injection concentration and reagent injection amount as independent variables, and the Cr(VI) concentration in the groundwater after reagent injection in the pilot test area as the response value, construct a reagent injection prediction model through central composite design and response surface analysis; based on the prediction model, predict the optimal reagent parameters, which include the optimal predicted reagent injection concentration and the optimal predicted reagent injection amount;

[0009] (2) The pilot-scale reagent injection concentration is determined by taking the largest integer not greater than the optimal predicted reagent injection concentration as the pilot-scale reagent injection concentration. A verification experiment is conducted in the pilot-scale test area to determine the optimal pilot-scale reagent injection amount and verify the rationality of the model prediction results.

[0010] (3) Using the optimal pilot-scale reagent injection rate from step (2) as a constant, the contaminated site is divided into zones according to the Cr(VI) concentration gradient. Based on the theoretical reagent dosage, the adsorption capacity of the reagent by the aqueous medium, and the model prediction results, the reagent concentration range for each remediation zone is determined. Within this range, experiments with two or more reagent concentration gradients are conducted to determine the reagent concentration values ​​for each remediation zone and to fit the data, establishing the relationship between the regional reagent injection concentration and the initial Cr(VI) concentration in the regional groundwater: Y CPS = a×ln(X+b)+c, where Y CPS X represents the regional reagent injection concentration, X represents the initial concentration of Cr(VI) in the regional groundwater, a is a positive constant, b is a constant, and c is the safety factor for reagent adsorption by the aquatic medium.

[0011] (4) Layout injection points in the chromium contaminated site, determine the initial concentration of Cr(VI) in the groundwater in the area where the injection point is located, substitute it into the relationship in step (3), take the obtained regional reagent injection concentration as the reagent injection concentration of the injection point, take the optimal pilot reagent injection amount as the total injection amount of the injection point, and carry out in-situ layered injection remediation of the injection point.

[0012] (5) Repeat step (4) to complete the in-situ layered injection remediation of all injection points in the chromium contaminated site.

[0013] Further, in step (4), the in-situ layered injection repair method is as follows: based on the pilot test results in step (2), taking into account the aquifer permeability, thickness and the diffusion of the repair agent, the aquifer is divided into layers and the injection depth of each layer is set; an in-situ direct-push injection device is used to inject the agent into each layer from top to bottom according to the set injection depth.

[0014] Furthermore, before proceeding to step (1), the following steps are required: to investigate the chromium-contaminated site, determine the degree of site contamination, identify the hydrogeological conditions, determine the spatial distribution and range of Cr(VI) and total chromium in the aquifer, analyze the migration patterns and boundary range of pollutants in the site, and identify the chromium-contaminated site to be remediated.

[0015] Based on the borehole columnar section, the lithological characteristics of each point within the block were identified. Combined with the theoretical dosage ratio of chemical reducing agents, laboratory small-scale tests were conducted to screen the most suitable agents and the optimal ratio. Simulated sand column experiments were carried out for the optimal ratio to verify the reduction efficiency of each agent for Cr(VI). The theoretical dosage of the agent was calculated from the laboratory small-scale test and the simulated sand column experiment.

[0016] Furthermore, in step (4), the method for setting up the injection points is as follows: based on the pilot test results in step (2), the remediation area of ​​each injection point is generalized to an ellipse, and the blank remediation area is reduced by using a plum blossom pile layout in the chromium contaminated site.

[0017] Furthermore, it also includes the following steps: (6) After in-situ layered injection repair at all injection points, the repair process is monitored, and injection points are added in areas where the repair effect has not reached the target value, until all groundwater samples at the site are below the repair target value.

[0018] Furthermore, it also includes the following steps: (7) Collect groundwater samples for follow-up monitoring for at least 1 year. If the pollutant concentration exceeds the standard, repeat step (6) until the test result is lower than the remediation target value.

[0019] The beneficial effects of this invention are:

[0020] This invention uses the response surface methodology to construct a model in the in-situ injection repair system. This not only solves the problem of optimizing agent parameters under interactive effects, but also completes continuity analysis with fewer experimental points to determine the optimal parameter values. It is simple to operate and highly accurate, and has the advantages of providing effective decision-making basis while shortening the experimental cycle and reducing costs.

[0021] Furthermore, by dividing contaminated sites into concentration zones, and establishing a relationship between reagent parameters and the initial Cr(VI) concentration in groundwater based on theoretical reagent dosage, the adsorption capacity of reagents by aqueous media, and model predictions, precise control of reagent parameters can be achieved. This addresses the problems of insufficient contact between reagents and contaminants and incomplete remediation caused by inaccurate reagent parameters in traditional methods. It also reduces the risk of secondary pollution from high dosages and avoids the problem of reagents being adsorbed by aqueous media due to excessively low concentrations, thus hindering sufficient contact with contaminants. This approach is highly targeted and applicable to in-situ remediation of actual contaminated sites. The precise dosing of reagents is more suitable for practical applications, ensuring site remediation effectiveness while providing technical reference for the remediation of other contaminated sites, and has broad prospects for widespread application.

[0022] Based on actual chromium-contaminated sites, this invention focuses on aquifer pollution. It divides the aquifer into layers and uses multi-depth injection within a single borehole to complete the layered injection remediation of the aquifer. This ensures the full reaction of the reagents, reduces the risk of groundwater "yellowing", and maximizes the remediation effect.

[0023] Compared to other technologies, the direct-push in-situ layered injection remediation technology for groundwater is flexible and simple to operate, with minimal disturbance. It allows for real-time adjustment of the injection location based on monitoring data, saving construction time, space, and financial costs while being more targeted and long-lasting, making it worthy of widespread application. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0025] Figure 1 A schematic diagram of an in-situ direct-push injection system;

[0026] Figure 2 Three-dimensional response surface plots and contour plots showing the interaction of reagent parameters on the Cr(VI) removal effect;

[0027] Figure 3 This is a graph showing the change in Cr(VI) concentration in the monitoring well during the pilot-scale injection process.

[0028] Figure 4 The image shows the repair effect of the pilot-scale test.

[0029] Figure 5 Layout diagram of injection points;

[0030] Figure 6Comparison chart showing the effects of groundwater remediation at different stages of the remediation construction. Detailed Implementation

[0031] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0032] The chromium-contaminated site involved in this invention was originally a chromium-containing waste storage site. While the waste has been completely disposed of, Cr(VI) in the waste continues to dissolve and leach out with surface water due to rainwater erosion and leaching. Over a long period, this pollution has spread to the soil and groundwater through rainwater runoff, surface runoff, and vertical infiltration. The investigation revealed a thick aquifer at the site, and single-layer injection could easily lead to uneven reagent diffusion, incomplete reaction, and subsequent "yellowing" of the groundwater. Therefore, this invention addresses the issue of in-situ layered injection remediation targeting the aquifer.

[0033] The method for in-situ layered injection remediation of chromium-contaminated aquifers based on response surface includes the following steps:

[0034] Step 1: Conduct an investigation of the chromium-contaminated site, determine the degree of contamination, identify the hydrogeological conditions, determine the spatial distribution and range of Cr(VI) and total chromium in groundwater, analyze the migration patterns and boundary ranges of pollutants in the site, and identify the chromium-contaminated site to be remediated.

[0035] Step 2: Determine the optimal ratio of each reducing agent for complete repair of Cr(VI) through laboratory-scale experiments; conduct simulated sand column experiments based on the optimal ratio to verify the reduction efficiency of each reducing agent for Cr(VI), the specific methods are as follows:

[0036] 1. Indoor measurement of site hydrogeological parameters;

[0037] 2. Based on the theoretical addition ratio of chemical reducing agents, a static batch experiment was set up to select the agents. FeSO4, Na2S2O4 and CPS solutions were added according to the molar ratio with Cr(VI) of 1:1, 2:1, 3:1, 4:1 and 5:1 respectively. The mixture was kept at a constant temperature and shaken. The Cr(VI) concentration was sampled and analyzed at regular intervals. The repair effect of each reducing agent was compared and analyzed.

[0038] 3. Conduct simulated sand column experiments: Inject the remediation agent with the optimal molar ratio selected from static experiments into each soil column with the same Cr(VI) concentration from bottom to top. Based on the site survey results, set the injection flow rate to 0.25 mL / min. Monitor the Cr(VI) and total chromium concentrations at each outlet daily until the concentrations at each outlet meet the Class III groundwater standard. Calculate the amount of agent consumed in this process.

[0039] 4. Based on the results of laboratory-scale tests, calcium polysulfide was selected as the chemical remediation agent, and its reaction equation is as follows:

[0040] Reduction reaction: 10H + +2CrO4 2- +6e - →2Cr(OH)3↓+2H2O

[0041] Oxidation reaction: 3CaS5 → 3Ca 2+ +15S↓+6e -

[0042] Overall reaction: 10H + +2CrO4 2- +3CaS5=2Cr(OH)3↓+2H2O+3Ca 2+ +15S↓

[0043] Step 3: Select a pilot-scale test area within the chromium-contaminated site. Based on the results of the laboratory-scale test, conduct pilot-scale tests in the pilot-scale test area, using an in-situ direct-push injection device for chemical reduction remediation. Through central composite design and response surface methodology, construct a reagent injection parameter model and analyze its reliability and results. The specific methods are as follows:

[0044] 1. Determine the background value of groundwater in the pilot test area. The initial concentration of Cr(VI) in the groundwater of the pilot test area is 217 mg / L.

[0045] 2. Using Design-Expert 8.0 software, a central composite design was created, employing reagent injection concentration and injection volume as independent variables, and the groundwater Cr(VI) concentration after reagent injection in the pilot test area as the response value. Based on the laboratory-scale test, under the condition of a groundwater Cr(VI) concentration of 217 mg / L, the ranges of reagent injection concentration (A) and reagent injection volume (B) were determined to be 31.89–53.10 L / m³. 3 and 13.79~56.21m 3 The experimental design scheme for the response surface is shown in Table 1.

[0046] Table 1. Design parameters of the central composite structure

[0047]

[0048] 3. Construct an in-situ injection system and complete the experiment according to the central composite design. For example... Figure 1 As shown, the in-situ direct-push injection system includes an in-situ direct-push injection device, a reagent preparation system, and a water and electricity supply system. The reagent preparation system includes a reagent preparation tank and a temporary storage tank. The reagent preparation tank and the reagent tank are connected by a dosing pump. The dosing pump adds the repair agent to the reagent preparation tank. The reagent preparation tank is also equipped with a water inlet pipe with a flow meter. A transfer pump is connected between the reagent preparation tank and the temporary storage tank. The temporary storage tank is connected to the in-situ direct-push injection device through a high-pressure grouting pump, which is equipped with a pressure gauge.

[0049] The repair agent is pumped into the dosing tank by a dosing pump. Tap water enters the dosing tank through the inlet pipe, and the mixture is stirred evenly to obtain the set injection concentration. Then, it is pumped to the temporary storage tank by a delivery pump, and then pumped to the in-situ direct-push injection equipment by a high-pressure grouting pump.

[0050] 4. Data fitting: Through response surface analysis, a quadratic regression equation (Equation 1) is constructed to establish a prediction model.

[0051]

[0052] Where Y is the predicted result, i is the linear coefficient, j is the quadratic coefficient, b is the regression coefficient, k is the number of factors studied and optimized in the experiment, and e is the random error. Since this experiment only involves two independent variables, drug concentration and drug dosage, and k = 2, the following equation is further derived:

[0053]

[0054] Based on equation (2), the coefficients of the response equation are determined to obtain the prediction model, as shown in equation (3).

[0055] Y Cr(Ⅵ) =0.168-52.09A-38.61B+6.26AB+42.31A 2 +32.04B 2 (Equation 3)

[0056] Among them, Y Cr(Ⅵ) Here, A represents the predicted Cr(VI) concentration after injection, and B represents the injection concentration and injection volume.

[0057] 5. Reliability Analysis of Response Surface Model

[0058] The coefficient of determination R of this model 2 =0.9804, indicating a good model fit; the model's p-value is less than 0.0001, and the F-value is 70.21, proving that the model is highly significant and the prediction results are reliable; Adj R 2 and Pred R2 The difference is 0.1055, which is less than 0.2, proving that the model's predicted values ​​are in good agreement with the experimental values. The model's signal-to-noise ratio (Adeq precision) is 22.10 > 4, demonstrating the model's good effectiveness. In conclusion, the model's prediction and simulation of the response values ​​are reliable.

[0059] 6. Analysis of Response Surface Model Prediction Optimization Results

[0060] like Figure 2 As shown, a three-dimensional response surface plot and contour plot were plotted. Based on the model prediction results, under the condition that the Cr(VI) concentration in the pilot-scale test area is 217 mg / L, the injection concentration is 42.5 L / m². 3 The injection volume is 35m 3 At this time, the best repair effect can be achieved.

[0061] Step 4: Conduct validation experiments on the model parameters and analyze to determine the optimal reagent parameters.

[0062] The initial Cr(VI) concentration in the groundwater of the pilot test area was 217 mg / L. Based on the model optimization results, the pilot-scale reagent injection concentration was determined to be the largest integer not exceeding the optimal predicted reagent injection concentration, and the injection concentration was 42 L / m³. 3 Calcium polysulfide agents. For example... Figure 3 and 4 As shown, the confirmatory test results indicate that the injection volume reached 34m³. 3 At that time, monitoring point ZB14, located 5.78m downstream of the injection point, had been completely repaired; the injection volume reached 42m³. 3 At that time, the Cr(VI) concentration at monitoring point ZB13, located 4.80m upstream, began to decrease; when the injection volume reached 46m... 3 Subsequently, the Cr(VI) concentration at monitoring point QG04, located 6.98m downstream, showed a decreasing trend. In conclusion, the response surface model's prediction of the optimal reagent parameters is reasonable and feasible.

[0063] Step 5: Divide the contaminated site into layers, determine the depth and location of each layer for injection, optimize process parameters, and formulate an overall site layout plan for large-scale remediation within the site. The specific method is as follows:

[0064] 1. Aquifer layer classification. The investigation results from step one indicate that the aquifer lithology at the contaminated site is mainly fine sand and silt, with a thickness of 4m, a burial depth of 9-13m, and a permeability coefficient k of 9×10⁻⁶. -4 cm / s. Due to the thickness of the aquifer, it is divided into two layers: 9-11m and 11-13m. The nozzles are placed in the middle of each layer, at 10m and 12m.

[0065] 2. Determine the injection method. Adopt an injection sequence from shallow to deep to prevent the formation of dominant channels in the aquifer, which could result in a small diffusion range of the reagent and insufficient contact with Cr(VI), thus failing to achieve the desired effect.

[0066] 3. Determine the injection concentration and injection volume. Based on the Cr(VI) concentration in the contaminated site zones, reagent coefficients are set. Response surface model validation results indicate that in the area with an initial Cr(VI) concentration of 217 mg / L in groundwater, an injection concentration of 42 L / m³ is appropriate. 3 Calcium polysulfide reagent 34m 3 This achieves optimal remediation results. The contaminated site is divided into three concentration zones—low, medium, and high—based on the Cr(VI) concentration gradient (<20, 20–50, >50 mg / L). To ensure the diffusion range of the reagent, a 34m... 3 The injection volume was constant. Taking into account the theoretical dosage (calculated from laboratory small-scale tests and soil column tests), the adsorption of the agent by the water-bearing medium, and the model prediction results, the required agent concentration range for each remediation area was determined. Within this range, tests were conducted at three agent concentration gradients to determine the agent concentration values ​​for each area and fit the data. The relationship between the agent concentration and the initial concentration of Cr(VI) in the regional groundwater was established as shown in Equation 4-5. The injection volume was 34 m³. 3 :

[0067] When X ≥ 20 mg / L

[0068] Y CPS =7.56×ln(X-18.07)+1.97 (Equation 4)

[0069] When X < 20 mg / L

[0070] Y CPS = 0.83×ln(X+0.98)+1.97 (Equation 5)

[0071] Among them, Y CPS The concentration of the injected calcium polysulfide reagent is given, X is the initial concentration of Cr(VI) in the regional groundwater, and 1.97 is the safety factor for the adsorption of the reagent by the aqueous medium.

[0072] 4. Determine the injection flow rate. Appropriately increasing the injection flow rate of the reagent solution can induce a rupture reaction underground, creating a rapid diffusion channel for the reagent and increasing its effective range. However, excessive injection pressure can cause slurry seepage, leading to reagent loss. Generally, the injection flow rate can be selected within the following range:

[0073] Within 5 meters below the surface: 0.25–1.5 meters 3 / h;

[0074] 5-10m below the surface: 1-4m 3 / h;

[0075] Depths greater than 10m below the surface: This can be further increased as needed.

[0076] According to the pilot-scale test results, the lithology at a depth of 9–13 m below the surface is mainly silt and fine sand, and the injection velocity is 1–6 m / s². 3 At a flow rate of 1 m / h, there is no backflow of slurry. Therefore, the flow rate is set to 1–6 m / h. 3 / h, and adjust the injection flow rate in real time according to the actual situation.

[0077] 5. Layout of injection points and monitoring points: such as Figure 5 As shown, based on the pilot test results, the remediation area of ​​each injection point is generalized as an ellipse, with a remediation range of 5.3m upstream and downstream and 4.2m on both sides. A staggered layout of injection points is used within the site to reduce the remediation gaps. The spacing between injection points perpendicular to the water flow direction is 8.22m, and the spacing between injection points along the water flow direction is 8.30m.

[0078] 6. Based on the model results and actual site conditions, complete the layered injection construction of the aquifer. After the in-situ direct-push injection system is debugged, lower the nozzle to a depth of 10m below the ground and lock it in place, then inject 17m of reagent. 3 After the injection in this layer is completed, continue lowering the nozzle to 12m below ground level and injecting 17m of the agent. 3 After injection, the drill rod is removed and the borehole is sealed with bentonite. This step is repeated until all stratified injection points in the chromium-contaminated site are completed. Furthermore, any issues such as slurry backflow or injection hole blockage during construction are addressed promptly to ensure optimal results. Simultaneously, additional sampling points are added in areas where the remediation effect has not reached the target value until all groundwater samples from the site are below the remediation target value.

[0079] Step Six: Monitoring should be conducted before, during, and after the stratified injection process. Monitoring items should include key parameters of the construction process, target pollutants, injected chemicals, byproducts, the physicochemical properties of groundwater, and pollutants that may be generated during the remediation process. Specific details are as follows:

[0080] (1) Injection point, depth, flow rate, drug concentration, and cumulative injection volume and cumulative drug dosage at each injection point.

[0081] (2) Daily inspections should be carried out during the injection process. The inspection content should include the operation of the injection equipment and instruments, the surrounding environment, etc.

[0082] (3) Monitor water quality indicators such as groundwater level, pH value, conductivity, redox potential, and dissolved oxygen, and sample and test the concentration of Cr(VI) and total chromium in groundwater, the removal rate of Cr(VI) after reagent injection, and the concentration of by-products.

[0083] Step 7: Remediation is considered complete once the Cr(VI) concentration in the groundwater is below the control target value (0.05 mg / L). Groundwater samples will be collected and monitored for at least one year. If the pollutant concentration exceeds the standard, Step 5 will be repeated until the test results are below the remediation target value.

[0084] like Figure 6 As shown, after the remediation work, the Cr(VI) concentration in the monitoring wells around the injection point continued to decrease, reaching the control target value. Cr(VI) in the groundwater within the contaminated site was below the detection limit, and no "yellowing" phenomenon occurred. Before the work, nine out of nine groundwater monitoring points within the site exceeded the standard, with the highest Cr(VI) concentration at 410 mg / L. After the work was completed and during a one-year follow-up monitoring period, all nine monitoring points within the site met the standard, and the Cr(VI) concentration was below the detection limit. Furthermore, during the injection process at the points around the monitoring wells, the concentrations of Cr(VI) and total chromium decreased significantly, showing a similar decreasing trend. Cr(VI) in the groundwater was reduced to stable Cr(III) by calcium polysulfide and precipitated, indicating a good remediation effect and high efficiency.

[0085] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for in-situ layered injection remediation of chromium-contaminated aquifers based on response surface methodology, characterized in that, Includes the following steps: (1) Select a pilot test area within the chromium-contaminated site and determine the initial Cr(VI) concentration in the groundwater of the pilot test area; using the reagent injection concentration and reagent injection amount as independent variables, and the Cr(VI) concentration in the groundwater after reagent injection in the pilot test area as the response value, construct a reagent injection prediction model through central composite design and response surface analysis; based on the prediction model, predict the optimal reagent parameters, which include the optimal predicted reagent injection concentration and the optimal predicted reagent injection amount; (2) The pilot-scale reagent injection concentration is determined by taking the largest integer not greater than the optimal predicted reagent injection concentration as the pilot-scale reagent injection concentration. A verification experiment is conducted in the pilot-scale test area to determine the optimal pilot-scale reagent injection amount and verify the rationality of the model prediction results. (3) Using the optimal pilot-scale reagent injection rate from step (2) as a constant, the chromium-contaminated site is divided into zones according to the Cr(VI) concentration gradient. Based on the theoretical reagent dosage, the adsorption capacity of the reagent by the aqueous medium, and the model prediction results, the reagent concentration range for each remediation zone is determined. Within this range, experiments with two or more reagent concentration gradients are conducted to determine the reagent concentration values ​​for each remediation zone and to fit the data. A relationship is established between the regional reagent injection concentration and the initial Cr(VI) concentration in the regional groundwater: Y CPS = a×ln(X+b)+c, where Y CPS X represents the regional reagent injection concentration, X represents the initial concentration of Cr(VI) in the regional groundwater, a is a positive constant, b is a constant, and c is the safety factor for reagent adsorption by the aquatic medium. (4) Layout injection points in the chromium contaminated site, determine the initial concentration of Cr(VI) in the groundwater in the area where the injection point is located, substitute it into the relationship in step (3), take the obtained regional reagent injection concentration as the reagent injection concentration of the injection point, take the optimal pilot reagent injection amount as the total injection amount of the injection point, and carry out in-situ layered injection remediation of the injection point. (5) Repeat step (4) to complete the in-situ layered injection remediation of all injection points in the chromium contaminated site.

2. The method for in-situ layered injection remediation of chromium-contaminated aquifers based on response surface according to claim 1, characterized in that, In step (4), the in-situ layered injection repair method is as follows: Based on the pilot test results in step (2), the aquifer permeability, thickness and repair agent diffusion are considered, and the aquifer is divided into layers and the injection depth of each layer is set; an in-situ direct-push injection device is used to inject the agent into each layer from top to bottom according to the set injection depth.

3. The method for in-situ layered injection remediation of chromium-contaminated aquifers based on response surface according to claim 1 or 2, characterized in that, Before proceeding to step (1), the following steps are required: to investigate the chromium-contaminated site, determine the degree of site contamination, identify the hydrogeological conditions, determine the spatial distribution and range of Cr(VI) and total chromium in the aquifer, analyze the migration patterns and boundary range of pollutants in the site, and identify the chromium-contaminated site to be remediated. Based on the borehole columnar section, the lithological characteristics of each point within the block were identified. Combined with the theoretical dosage ratio of chemical reducing agents, laboratory small-scale tests were conducted to screen the most suitable agents and the optimal ratio. Simulated sand column experiments were carried out for the optimal ratio to verify the reduction efficiency of each agent for Cr(VI). The theoretical dosage of the agent was calculated from the laboratory small-scale test and the simulated sand column experiment.

4. The method for in-situ layered injection remediation of chromium-contaminated aquifers based on response surface according to claim 1 or 2, characterized in that, In step (4), the method for setting up injection points is as follows: Based on the pilot test results in step (2), the remediation area of ​​each injection point is generalized into an ellipse, and the blank remediation area is reduced by using a plum blossom pile layout in the chromium contaminated site.

5. The method for in-situ layered injection remediation of chromium-contaminated aquifers based on response surface according to claim 1, characterized in that, It also includes the following steps: (6) After in-situ layered injection remediation at all injection points, the remediation process is monitored. In areas where the remediation effect has not reached the target value, injection points are added until all groundwater samples at the site are below the remediation target value.

6. The method for in-situ layered injection remediation of chromium-contaminated aquifers based on response surface according to claim 5, characterized in that, It also includes the following steps: (7) Collect groundwater samples for follow-up monitoring for at least one year. If the pollutant concentration exceeds the standard, repeat step (6) until the test result is lower than the remediation target value.

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