Repairing agent for repairing hexavalent chromium polluted soil-underground water and technology thereof

Abstract

The invention relates to the field of polluted soil-underground water treatment and remediation, and discloses a remediation agent which comprises the following components in percentage by mass: 20-60% of ammonium polysulfide; 5%-20% of a pH regulating component; 5%-25% of a redox potential control component; the pH regulating component is selected from a phosphate buffer, an acetate buffer, magnesium oxide, calcium carbonate and a combination of the phosphate buffer, the acetate buffer, the magnesium oxide and the calcium carbonate; the oxidation-reduction potential control component is selected from nanoscale zero-valent iron, micron zero-valent iron, slow-release organic acid, ferrous sulfide and a combination of the nanoscale zero-valent iron, the micron zero-valent iron and the slow-release organic acid. By taking ammonium polysulfide as a core reduction component, efficient reduction potential energy can be provided, rapid and thorough reduction of hexavalent chromium in a polluted environment is ensured, the risk that reduced Cr (III) is re-oxidized into Cr (VI) under the change of environmental conditions is fundamentally inhibited, the durability and reliability of the remediation effect are remarkably improved, and secondary pollution is effectively avoided.

Description

Technical Field

[0001] The present invention relates to the field of contaminated soil-groundwater treatment and restoration, and in particular to a restoration agent and a process thereof for restoring hexavalent chromium-contaminated soil-groundwater. Background Art

[0002] Hexavalent chromium (Cr(VI)) is widely recognized as a priority pollutant worldwide due to its high toxicity, high mobility, and carcinogenic, teratogenic, and mutagenic effects on organisms. It poses a serious ecological and health threat to soil and groundwater environments. While traditional Cr(VI) remediation technologies can achieve a certain degree of conversion or removal of the pollutant, they generally face multiple challenges in practical application, including efficiency, durability, and environmental compatibility. Their limitations are particularly prominent when dealing with complex and changing environmental media.

[0003] Existing chemical reduction methods, such as the use of reducing agents such as ferrous sulfate or sodium sulfite, mainly achieve remediation purposes by reducing Cr(VI) to trivalent chromium (Cr(III)), which has lower toxicity and significantly reduced mobility. However, these technologies show high sensitivity to environmental conditions in application. The efficiency and thoroughness of the reduction reaction are often heavily dependent on the specific pH value and redox potential (Eh) range. For example, many reducing agents exhibit optimal activity under acidic conditions, but when the environmental pH value tends to neutral or alkaline, their reducing ability will decrease significantly, resulting in a slowdown or even stagnation of the reaction rate, which in turn affects the comprehensiveness of the remediation. This makes it difficult to continuously maintain optimal reaction conditions in actual complex and heterogeneous soil or groundwater environments, thereby limiting its effectiveness in field applications.

[0004] In addition, the long-term stability of the reduced product, trivalent chromium, is another major challenge for existing technologies. Although the toxicity of Cr(III) is much lower than that of Cr(VI), if the environmental conditions after remediation change adversely, such as an increase in the Eh value due to good ventilation or groundwater level fluctuations, the reduced Cr(III) may be re-oxidized to highly toxic Cr(VI), causing the remediation effect to rebound and form the so-called "secondary pollution". Existing technologies generally lack effective mechanisms to stabilize Cr(III) in the long term, so that it can be solidified in the medium, thereby avoiding this potential reoxidation risk. Therefore, although the conversion of Cr(VI) may be achieved in the early stage, the lack of long-term effective control of the Eh value makes the durability of the remediation a significant bottleneck. Summary of the Invention

[0005] In response to the shortcomings of the existing technology, the present invention provides a remediation agent and process for repairing hexavalent chromium-contaminated soil and groundwater, which solves the problems existing in the existing hexavalent chromium pollution remediation technology, such as low reduction efficiency, poor stability of reduction products leading to secondary pollution, sensitivity to environmental conditions and insufficient durability of effect.

[0006] To achieve the above objectives, the present invention is implemented through the following technical solutions: a repair agent, comprising, by mass percentage: Ammonium polysulfide: 20%-60%; pH regulating component: 5%-20%; Redox potential control component: 5%-25%.

[0007] The above technical solution aims to solve the technical problems of low reduction efficiency, poor stability of reduction products leading to secondary pollution, sensitivity to environmental conditions and insufficient durability of effects in existing hexavalent chromium pollution remediation technologies, thereby achieving efficient, stable and environmentally friendly remediation of hexavalent chromium.

[0008] Preferably, the molecular formula of the ammonium polysulfide is (NH4)2S x , where x is an integer from 2 to 9.

[0009] Preferably, the pH regulating component is selected from phosphate buffer, acetate buffer, magnesium oxide, calcium carbonate and combinations thereof.

[0010] Preferably, the redox potential controlling component is selected from nanometer zero-valent iron, micrometer zero-valent iron, slow-release organic acid, ferrous sulfide and combinations thereof.

[0011] Preferably, the composition further comprises a synergistic auxiliary component selected from biochar, chitosan, xanthan gum, ferrous sulfate, and nitrifying bacteria, and the total content of the synergistic auxiliary component is 15.11% to 68% by mass percentage; Of which: Biochar: 10%-30%; Chitosan: 1%-10%; Xanthan gum: 0.1%-2%; Ferrous sulfate: 5%-15%; Nitrifying bacteria: 0.01%-1%.

[0012] A process for preparing a repairing agent comprises the following steps: S1: Preparation of ammonium polysulfide; S2: preparing pH control components; S3: preparing redox potential control components; S4: mixing the ammonium polysulfide, the pH regulating component, the redox potential controlling component, and the synergistic auxiliary component.

[0013] Preferably, when the redox potential controlling component in step S3 is nanometer zero-valent iron or micrometer zero-valent iron, the particle size of the nanometer zero-valent iron is 50 nm to 100 nm, and the particle size of the micrometer zero-valent iron is 1 μm to 10 μm.

[0014] Preferably, the mixing process in step S4 is carried out at a temperature of 5° C. to 30° C., and the mixing time is 15 minutes to 120 minutes.

[0015] Preferably, when the pH regulating component in step S2 is a phosphate buffer, it is prepared by dissolving sodium dihydrogen phosphate and disodium hydrogen phosphate in water, with a total concentration of 5% to 20%.

[0016] A remediation agent is used for remediating hexavalent chromium-contaminated soil and groundwater.

[0017] The present invention provides a remediation agent and process for remediating hexavalent chromium-contaminated soil and groundwater, which has the following beneficial effects: 1. The repair agent of the present invention uses ammonium polysulfide as the core reducing component, which can provide efficient reduction potential to ensure the rapid and thorough reduction of hexavalent chromium (Cr(VI)) in the polluted environment. In addition, a pH regulating component and an oxidation-reduction potential (Eh) controlling component are introduced. The pH regulating component can maintain the pH of the repair area within the optimal range of Cr(VI) reduction and trivalent chromium (Cr(III)) precipitation. Eh controlling components, such as slow-release zero-valent iron or ferrous sulfide, can construct and maintain a moderate reducing microenvironment for a long time. This dual dynamic control mechanism fundamentally suppresses the risk of reduced Cr(III) being reoxidized to Cr(VI) under changing environmental conditions, significantly improves the durability and reliability of the repair effect, and effectively avoids the occurrence of secondary pollution.

[0018] 2. The ammonium ions released by the ammonium polysulfide of the present invention during the reaction process can be used as a nitrogen source to supplement soil nutrients. The introduction of biochar can effectively improve the physical structure of the soil, improve its air permeability and water holding capacity. It also provides a good attachment and reproduction environment for soil microorganisms (such as nitrifying bacteria), thereby promoting soil nitrogen circulation and improving soil fertility levels. In addition, ferrous sulfate not only assists in the reduction of Cr (VI) and the solidification of Cr (III), but the iron element it releases is also a trace nutrient necessary for plant growth. The combined effect of these components helps to optimize the physical and chemical properties of the soil in the repair area, provides more favorable conditions for the normal growth of plants, and thus promotes the natural recovery of damaged ecosystems. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a preparation process step diagram of the present invention. DETAILED DESCRIPTION

[0020] The following will clearly and completely describe the technical solution of the present invention in conjunction with the accompanying drawings. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0021] Please see the attached Figure 1 , an embodiment of the present invention provides a repair agent, comprising, by mass percentage: Ammonium polysulfide: 20%-60%; the molecular formula of ammonium polysulfide is (NH4)2S x , where x is an integer from 2 to 9.

[0022] pH regulating component: 5%-20%; selected from phosphate buffer, acetate buffer, magnesium oxide, calcium carbonate and combinations thereof.

[0023] Redox potential control component: 5%-25%; selected from nano zero-valent iron, micron zero-valent iron, slow-release organic acid, ferrous sulfide and combinations thereof.

[0024] The invention also includes a synergistic auxiliary component selected from biochar, chitosan, xanthan gum, ferrous sulfate, and nitrifying bacteria, and the total content of the synergistic auxiliary component is 15.11% to 68% by mass percentage; Of which: Biochar: 10%-30%; Chitosan: 1%-10%; Xanthan gum: 0.1%-2%; Ferrous sulfate: 5%-15%; Nitrifying bacteria: 0.01%-1%.

[0025] A process for preparing a repairing agent comprises the following steps: S1: Preparation of ammonium polysulfide; S2: preparing a pH regulating component. When the pH regulating component in step S2 is a phosphate buffer, the pH regulating component is prepared by dissolving sodium dihydrogen phosphate and disodium hydrogen phosphate in water, with a total concentration of 5% to 20%; S3: preparing a redox potential controlling component. When the redox potential controlling component in step S3 is nano-zero-valent iron or micron-zero-valent iron, the particle size of the nano-zero-valent iron is 50 nm to 100 nm, and the particle size of the micron-zero-valent iron is 1 μm to 10 μm. S4: mixing ammonium polysulfide, a pH regulating component, an oxidation-reduction potential controlling component, and a synergistic auxiliary component. The mixing process in step S4 is carried out at a temperature of 5° C. to 30° C., and the mixing time is 15 minutes to 120 minutes.

[0026] Example 1: Multifunctional repair agent This example is intended to demonstrate the preparation of an intelligent remediation agent comprising ammonium polysulfide, a pH regulating component, an Eh controlling component, and a variety of synergistic auxiliary components, and its application in simulated hexavalent chromium contaminated soil. In the formula, the content of the core reducing agent ammonium polysulfide ((NH4)2S4) is 20%, the pH regulating component (composed of 5% sodium dihydrogen phosphate and 3% disodium hydrogen phosphate, a total of 8%), and the redox potential (Eh) controlling component nano zero-valent iron (nZVI) are both 5%. In addition, synergistic auxiliary components are added: biochar 10%, chitosan 1%, xanthan gum 0.1%, ferrous sulfate 5%, nitrifying bacteria 0.01%, and the balance is deionized water.

[0027] During the preparation process, sulfur powder and liquid ammonia are first precisely weighed and reacted at 0°C for 3 hours at a sulfur-to-liquid ammonia molar ratio of 1:4. Excess ammonia is then evaporated to produce a 25% ammonium polysulfide ((NH4)2S4) solution. Next, sodium dihydrogen phosphate and disodium hydrogen phosphate are dissolved in an appropriate amount of deionized water to prepare a pH buffer concentrate. Nano-zero-valent iron is prepared using the sodium borohydride reduction of ferric chloride. Specifically, 10 grams of ferric chloride is dissolved in 100 milliliters of deionized water, and a 2 mol / L sodium borohydride solution is slowly added dropwise with vigorous stirring. After the reaction, centrifugation and washing are performed to produce a 20% nZVI slurry. Various synergistic auxiliary components are also pretreated: biochar is ground to 0.5-1.5 mm, chitosan is prepared as a 2% dilute acetic acid solution, xanthan gum is prepared as a 1% aqueous solution, and ferrous sulfate is prepared as a 20% aqueous solution. Nitrifying bacteria are directly used as commercially available freeze-dried powder. To prepare the final mixture, add an appropriate amount of deionized water to the mixing tank. Under nitrogen, slowly add the ammonium polysulfide solution, pH buffer concentrate, and nano-zero-valent iron slurry in sequence while stirring (150 rpm). After stirring for 20 minutes, chitosan, xanthan gum, and ferrous sulfate solution are added in sequence. Finally, add the biochar powder and continue stirring for 30 minutes. The mixing temperature is maintained at 20°C. Nitrifying bacteria freeze-dried powder is added in appropriate proportions just before use.

[0028] In the simulation application phase, we first mixed uncontaminated soil with potassium dichromate solution to prepare simulated soil with a Cr(VI) contamination concentration of 180 mg / kg. The dosage of the remediation agent was precisely calculated to ensure that the molar ratio of total sulfur to Cr(VI) reached 2.5:1, and the liquid remediation agent was mixed with the soil using an in-situ stirring method, with the mixing depth set to 0.5 meters. During the application process, pH and Eh sensors were installed in the remediation area to monitor the pH and Eh values ​​in the soil pore water in real time. The monitoring data will be transmitted back to the control system. When the pH value deviates from the range of 6.0-7.0, dilute acid (such as 0.1MHNO3) or dilute alkali (such as 0.1MNaOH) solution will be added through the auxiliary injection point for precise control. When the Eh value increases and approaches +80mV, a small amount of nano zero-valent iron slurry will be added in time to ensure that the Eh value is stable between -80mV and +30mV. In the later period, samples will be taken regularly to analyze the residual amount of Cr(VI), total chromium content and stability of Cr(III) in the soil.

[0029] Example 2: Optimized formulation for specific pollution scenarios This example is intended to demonstrate the flexibility and optimization of the present invention's formulation when addressing groundwater contamination, with the content of each core component leaning toward the mid-to-high end of the claimed range. The remediation agent formula includes: 45% ammonium polysulfide ((NH4)2S3); 15% embedded magnesium oxide microparticles as a pH control component; and 20% biochar-loaded ferrous sulfide (FeS / Biochar), a redox potential (Eh) control component. In addition, 1% xanthan gum and 5% chitosan are added as synergistic auxiliary components, with the balance being deionized water.

[0030] The preparation process is as follows: First, the molar ratio of sulfur to liquid ammonia is precisely controlled to be 1:3 to prepare a 50% ammonium polysulfide ((NH4)2S3) solution. Secondly, nano-magnesium oxide powder (particle size 500nm) is prepared into slow-release microparticles by starch coating method, with a coating rate of 15%. Next, biochar (particle size 0.2-0.8 mm) is immersed in a mixed solution of FeCl2 and Na2S, and FeS is loaded on the surface of biochar by co-precipitation method. The loading amount is controlled at 20% of the mass of biochar, and then washed, dried, and ground. Finally, deionized water is added to the stirring tank, and ammonium polysulfide solution, xanthan gum solution, and chitosan solution are added in turn. After stirring evenly, embedded magnesium oxide microparticles and FeS / Biochar composite materials are slowly added, and the stirring speed is 200rpm for 1 hour, and the temperature is controlled at 15°C.

[0031] In the simulated groundwater remediation application, we configured simulated groundwater containing 8 mg / L of Cr(VI). The remediation agent slurry was continuously injected into the simulated groundwater flow field using a peristaltic pump through the injection well method. The injection rate was adjusted according to the groundwater flow rate and pollutant concentration to ensure that the molar ratio of total sulfur to Cr(VI) reached 3.5:1. During the remediation process, online pH and Eh probes and Cr(VI) concentration sensors were installed in the downstream monitoring wells for continuous monitoring. This embodiment focuses on the slow-release characteristics of the heavy components, and the pH and Eh regulation mainly rely on the slow release of embedded magnesium oxide and FeS / Biochar. If extreme deviations occur, it may be considered to pump in a small amount of acid, alkali or additional reducing agent for fine-tuning.

[0032] Example 3: Solid-state remediation agent for enhancing soil improvement effect This example focuses on the preparation of a solid remediation agent and highlights its advantages in improving soil structure. The formula comprises: 60% solid powdered ammonium polysulfide ((NH4)2S5), a core reducing agent; 20% finely powdered calcium carbonate, a pH-regulating component; and 25% micronized zero-valent iron (mZVI), a redox potential (Eh)-controlling component. Synergistic auxiliary components include 30% biochar, 15% ferrous sulfate, 1% nitrifying bacteria (adsorbed on a carrier), and the balance, an inert carrier, bentonite, used to adjust the overall formula to 100%.

[0033] In terms of preparation steps, first, sulfur is reacted with liquid ammonia to precisely control the evaporation and crystallization conditions to obtain a solid powder of ammonium polysulfide ((NH4)2S5) with a purity of 95%. Micronized calcium carbonate is ground to a particle size of less than 10 microns, and the commercially available micron zero-valent iron powder with a particle size of 5 microns is surface passivated. The nitrifying bacteria liquid is adsorbed onto a diatomaceous earth carrier and freeze-dried to obtain a bacteria-containing powder. All solid components, including biochar, micronized calcium carbonate, micron zero-valent iron, ammonium polysulfide solid powder, ferrous sulfate, powder loaded with nitrifying bacteria, and an appropriate amount of bentonite, are mixed in a V-type mixer for 45 minutes to ensure full and uniform dispersion. The humidity is strictly controlled below 10% during the mixing process.

[0034] In the simulation application, we prepared a simulated surface soil with a Cr(VI) contamination concentration of 250 mg / kg. The prepared solid remediation agent was evenly spread on the surface of the contaminated soil. The spreading amount was calculated based on the Cr(VI) concentration to ensure that the molar ratio of total sulfur to Cr(VI) reached 4:1. Next, a small rotary tiller was used to plow and mix the soil with the remediation agent to a depth of 0.3 meters to ensure sufficient contact between the remediation agent and the soil. After the remediation agent is mixed with the soil, it relies on natural rainfall or artificial sprinkling to penetrate and take effect. We will periodically monitor changes in the soil pH, Eh value, Cr(VI) content, and soil organic matter content.

[0035] In order to fully demonstrate the significant advantages of the present invention in terms of dynamic regulation of pH and Eh and multi-component synergy, we designed the following series of comparative examples.

[0036] Comparative Example 1: This comparative example is based on the formula of Example 1, but the pH regulating component is intentionally removed. In this repair agent formula, the content of the core reducing agent ammonium polysulfide ((NH4)2S4) is slightly increased to 28%, in order to still provide a certain reducing ability in the absence of pH regulation; the Eh controlling component nano zero-valent iron (nZVI) content is 5%. The synergistic auxiliary components (biochar 10%, chitosan 1%, xanthan gum 0.1%, ferrous sulfate 5%, nitrifying bacteria 0.01%) and the remaining deionized water remain unchanged. The preparation steps are basically the same as in Example 1, except that the preparation and addition of sodium dihydrogen phosphate and disodium hydrogen phosphate are omitted. During the application process, auxiliary regulation of the pH value is no longer performed, and only its natural changes are monitored. This comparative example aims to reveal the indispensability of pH regulation in the efficient conversion of hexavalent chromium and the long-term stabilization of trivalent chromium.

[0037] Comparative Example 2: This comparative example is also based on the formula of Example 1, but the Eh control component is removed. The formula of the repair agent is adjusted to: core reducing agent ammonium polysulfide ((NH4)2S4) 25%; pH control component (sodium dihydrogen phosphate 5%, disodium hydrogen phosphate 3%, a total of 8%). The synergistic auxiliary components (biochar 10%, chitosan 1%, xanthan gum 0.1%, ferrous sulfate 5%, nitrifying bacteria 0.01%) and the remaining deionized water remain unchanged. In the preparation steps, the preparation and addition of nano zero-valent iron are omitted. In actual application, the dynamic monitoring and control link will no longer perform auxiliary control of the Eh value, but only monitor its natural changes. Through this comparative example, we can clearly see the key role of the Eh control component in inhibiting the reoxidation of trivalent chromium and achieving long-term and stable remediation of pollution.

[0038] Comparative Example 3: This comparative example is intended to simulate the repair effect of a traditional single chemical reducing agent in order to highlight the overall advantages of the multi-component synergy and dynamic regulation of the present invention. The formula of this repair agent is extremely simplified, containing only 60% of the core reducing agent ammonium polysulfide ((NH4)2S4), and the balance is deionized water. The preparation process is also simplified accordingly: only a 60% ammonium polysulfide ((NH4)2S4) solution needs to be prepared. In the application process, the repair agent is added using an in-situ stirring method, but no real-time monitoring and regulation of pH or Eh is performed, and no auxiliary components are added. After repair, only regular sampling is performed to monitor the Cr(VI) concentration and Cr(III) stability. This comparative example will intuitively demonstrate the superiority of the composite repair system of the present invention and the limitations that may be caused by the lack of key regulatory components.

[0039] Comparative Example 4: This comparative example is based on the formula of Example 2, but the sustained-release property in the Eh control component is removed. The formula of the repair agent is: 45% ammonium polysulfide ((NH4)2S3); 15% embedded magnesium oxide microparticles as the pH control component; and 20% ferrous sulfide (FeS powder) without biochar loading as the Eh control component. The synergistic auxiliary components (1% xanthan gum, 5% chitosan) and the remaining deionized water remain unchanged. The preparation steps are basically the same as those in Example 2, but the complex process of FeS-loaded biochar is omitted, and FeS powder is directly used for mixing. The application process is consistent with that of Example 2. By comparison, we will focus on observing the ability to maintain the Eh value in the initial and later stages of addition (such as several weeks later), thereby highlighting the key significance of the sustained-release property in ensuring the long-term stability of Cr(III).

[0040] Comparative Example 5: This comparative example is based on the solid repair agent formula of Example 3, but all synergistic auxiliary components are intentionally removed, aiming to verify the synergistic effect of these components in soil improvement and overall repair effect. The repair agent formula is simplified as follows: core reducing agent ammonium polysulfide solid powder ((NH4)2S5) 60%; pH regulating component micronized calcium carbonate 20%; redox potential (Eh) controlling component micron zero-valent iron (mZVI) 25%, and the balance is bentonite. The preparation steps are similar to those in Example 3, but the addition of biochar, ferrous sulfate and nitrifying bacteria is explicitly omitted. The application process is the same as that in Example 3. Through this comparative example, we can directly compare the degree of improvement in the physical and chemical properties of the soil after repair (such as aggregate structure, permeability, fertility) and the possible differences in the impact on crop growth conditions in the absence of these auxiliary components, in order to demonstrate its unique value in the restoration of soil ecological functions.

[0041] The purpose of this experiment is to explore the differences between the examples of the present invention and the comparative examples in terms of hexavalent chromium reduction efficiency and the stability of the reduced product trivalent chromium, so as to verify the synergistic effect of the pH / Eh dynamic control components and the composite system.

[0042] Experiment 1: Comparative experiment on hexavalent chromium reduction efficiency and trivalent chromium stability: Experimental Methods and Setup: During the experimental preparation phase, typical farmland soil was collected and prepared according to HJ25.1-2019, "Technical Specifications for Soil Environmental Monitoring." This involved air-drying, grinding, and sieving through a 2mm sieve to ensure soil sample homogeneity. Subsequently, the treated soil was thoroughly mixed with a potassium dichromate solution of a specific concentration to prepare simulated contaminated soil with a hexavalent chromium (Cr(VI)) concentration of 200 mg / kg to ensure uniform distribution of the contamination.

[0043] The experimental setup used a batch reactor, a glass beaker with a stirring mechanism. Three replicates were set up for each treatment group to improve the reliability of the experimental results. Accurately 500 grams of simulated contaminated soil was added to each reactor. The remediation agent was added as follows: Example 1: The repair agent was added according to its formula and the molar ratio of total sulfur to Cr(VI) was 2.5:1.

[0044] Comparative Example 1 (without pH regulating component): the repair agent was added according to the corresponding formula and the same molar ratio.

[0045] Comparative Example 2 (without Eh control component): the repair agent was added according to the corresponding formula and the same molar ratio.

[0046] Comparative Example 3 (single reducing agent): the reducing agent was added according to its specific formula and molar ratio.

[0047] All reactors were placed in a constant temperature incubator at 25 ± 2 ° C. To ensure sufficient contact between the remediation agent and the soil, an intermittent stirring strategy was adopted, stirring for 30 minutes every day.

[0048] The monitoring phase is ongoing, focusing on the following key indicators: Cr(VI) content: On days 0, 1, 3, 7, 14, 28, and 60 after the start of remediation, samples were taken and analyzed for residual Cr(VI) in the soil according to HJ680-2013 “Determination of hexavalent chromium in soil and sediments by alkaline digestion-spectrophotometry.”

[0049] pH and Eh values: At the above sampling time points, the pH value and redox potential (Eh value) of the soil slurry were directly measured using a portable pH / Eh meter.

[0050] Trivalent chromium stability (leaching test): 28 days after remediation, the remediation soil of each treatment group was selected and a leaching test was conducted according to GB5085.3-2007 "Identification Standard for Hazardous Wastes - Leaching Toxicity Identification" to measure the total chromium content in the leachate to evaluate the stability of trivalent chromium.

[0051] Table 1: Hexavalent chromium reduction efficiency and Eh / pH changes Example 1 demonstrated rapid and efficient reduction of hexavalent chromium, significantly reducing its concentration within a short period of time. This effect is primarily due to the robust reducing action of ammonium polysulfide, while the pH-controlling component maintains the soil pH within a range suitable for the reduction reaction. Furthermore, the Eh-controlling component creates and maintains a sustained reducing microenvironment, effectively preventing the reoxidation of the reduced trivalent chromium, ensuring its long-term stability and thus mitigating potential secondary contamination risks.

[0052] In comparison, the data performance of Comparative Examples 1 and 2 is somewhat insufficient. Due to the lack of pH control in Comparative Example 1, the pH value fluctuates greatly during the reaction, which directly leads to a decrease in reduction efficiency. This shows that even if there is a highly efficient reducing agent, its effectiveness is difficult to fully exert if the environmental conditions are not suitable. Although the initial reduction effect of Comparative Example 2 is acceptable, due to the lack of Eh control components, the Eh value gradually increases over time, and some trivalent chromium may be reoxidized, affecting the durability of the repair. This confirms that a single reduction action cannot provide a long-lasting and stable repair effect, and the lack of comprehensive environmental regulation may lead to a decay of the repair effect.

[0053] Comparative Example 3, which uses only a single reducing agent, has particularly significant limitations. While it can partially reduce hexavalent chromium, the efficiency is relatively low, and the pH and Eh values ​​fluctuate dramatically, making them difficult to effectively control. This results in a large amount of trivalent chromium failing to stabilize and solidify, posing a high risk of re-release.

[0054] Experiment 2: Soil physical and chemical property improvement and ecological effect evaluation experiment This experiment aims to deeply evaluate the actual effect of the repair agent of the present invention, especially the embodiment containing synergistic auxiliary components, in improving the physical and chemical properties of soil and promoting the restoration of soil ecological functions, and compare it with the control example lacking auxiliary components.

[0055] Experimental Methods and Setup: Initially, simulated contaminated soil with a Cr(VI) concentration of 150 mg / kg was carefully prepared according to GB / T36190-2018, "Sample Preparation for Determination of Soil Physicochemical Properties," to ensure consistent contamination across the samples. Subsequently, a series of small potted plants were set up, each containing five replicates, to ensure data reliability. Each pot was precisely filled with 2 kg of simulated contaminated soil.

[0056] The repair agent is applied as follows: Example 3: Strictly following the solid repair agent formula and addition ratio (total sulfur to Cr(VI) molar ratio of 3:1), the repair agent was evenly spread on the soil surface and plowed and mixed to a depth of 0.2 meters using a small machine to ensure full contact between the repair agent and the soil.

[0057] Comparative Example 5 (without synergistic auxiliary components): the same addition ratio and tillage method were used for treatment.

[0058] Blank control group: only contaminated soil was added without adding any remediation agent.

[0059] One week after the application of the repair agent, an indicator crop suitable for local growth (such as wheat or corn) was sown in each pot. Daily watering and light management were strictly uniform to ensure normal growth of the crops.

[0060] The key to the experiment is long-term and detailed monitoring, focusing on the following indicators: Soil physical and chemical properties: Sampling and analysis were carried out according to national standards before the start of remediation, 30 days and 90 days after remediation: pH value: according to HJ963-2018 "Glass electrode method for determination of soil pH".

[0061] Organic matter content: Based on HJ615-2011 "Determination of soil organic matter - Potassium dichromate oxidation titration method with external heating".

[0062] Soil bulk density and porosity: Based on NY / T1121.4-2006 "Soil physical and chemical analysis method - Determination of soil bulk density" and NY / T1121.3-2006 "Soil physical and chemical analysis method - Determination of soil porosity", respectively, to reflect the improvement of soil structure and permeability.

[0063] Crop growth indicators: When crops reach maturity (approximately 90 days after sowing), plant height, aboveground biomass, and root biomass are measured. Chromium accumulation in different parts of the crop is determined according to relevant plant analysis standards to assess the impact of remediation on plant growth and the presence of phytoremediation potential.

[0064] Soil microbial activity: If conditions permit, the activities of key enzymes such as soil urease and phosphatase can be further monitored, or high-throughput sequencing technology can be used to deeply analyze changes in the structure of soil microbial communities to reveal the recovery status of soil ecological functions at a deeper level.

[0065] Table 2: Soil physical and chemical properties and crop growth indicators: Analysis of Results: The treatment results of Example 3 showed that the soil pH remained stable within a range suitable for crop growth, the organic matter content continued to increase, the soil bulk density decreased significantly, and the porosity increased significantly. This was primarily attributed to the synergistic effect of biochar and ferrous sulfate, the synergistic auxiliary components. Biochar improves the soil's physical structure, increasing its permeability and water-holding capacity and providing a favorable habitat for beneficial microorganisms. The enhanced activity of these microorganisms promotes soil nutrient cycling and organic matter accumulation, thereby improving soil fertility.

[0066] In contrast, in Comparative Example 5, while chromium pollution may have been somewhat controlled in the absence of a synergistic auxiliary component, improvements in various soil physical and chemical indicators were significantly limited. Organic matter increased insignificantly, bulk density decreased modestly, and porosity increased only modestly. The blank control group showed no improvement in soil conditions, and crop growth indicators were the most subdued, further highlighting the severe inhibitory effect of pollution on soil ecosystems.

[0067] Therefore, the value of the remediation agent of this invention goes far beyond pollutant removal. Through the synergistic action of its multifunctional components, it not only efficiently reduces and stabilizes hexavalent chromium, but more importantly, its formulation can comprehensively improve the physical and chemical properties of the soil, promoting the recovery and vitality of the soil microecosystem.

[0068] Experiment 3: Comparative experiment on the diffusivity and durability of repair agents: This experiment is dedicated to an in-depth investigation of the diffusion ability of the remediation agent of the present invention in the groundwater environment and the persistence of Eh and pH values, focusing on verifying the key role of the slow-release Eh control component in ensuring long-term stable remediation.

[0069] Experimental methods and settings: The experimental apparatus consists of a transparent plexiglass quicksand box, approximately 100 cm (length) x 20 cm (width) x 40 cm (height). It is filled with clean quartz sand to simulate a groundwater aquifer. A water inlet is located at one end of the device, and an outlet at the other. Precise control is used to create a stable groundwater flow field.

[0070] The simulated contaminated groundwater was prepared with a Cr(VI) concentration of 10 mg / L, and its determination method strictly adhered to GB / T5750.6-2006, "Standard Test Methods for Drinking Water - Inorganic Non-metallic Indicators." This simulated contaminated groundwater was continuously and steadily pumped in through the water inlet.

[0071] The repair agent is injected as follows: An injection hole is set in the middle of the quicksand box, about 30 cm away from the water inlet.

[0072] Example 2: According to the liquid repair agent formula and addition ratio optimized for groundwater (total sulfur and Cr(VI) molar ratio of 4:1), the repair agent slurry was precisely injected into the groundwater flow field at a constant rate through a peristaltic pump.

[0073] Comparative Example 4 (Eh control component has no sustained-release property): the same formulation, dosage ratio and pumping rate were used for injection.

[0074] Blank control group: only the contaminated groundwater was continuously pumped in without adding any remediation agent.

[0075] Monitoring is a key component. Monitoring points are set up at varying distances downstream of the injection point (e.g., 10 cm, 30 cm, and 50 cm), along with micro-sampling ports and online pH / Eh probes. After the remediation agent is injected, water samples are drawn from each monitoring point at 0, 1, 3, 7, 14, 28, 60, and 90 days, and the online probe data is recorded.

[0076] Monitoring indicators of concern include: Groundwater Cr(VI) concentration: The Cr(VI) concentration in the water sample was determined according to GB / T5750.6-2006 “Standard test methods for drinking water—Inorganic non-metallic indicators”.

[0077] pH value and Eh value: obtained by real-time reading of online pH / Eh probe data.

[0078] Diffusion range of the repair agent: If the repair agent is colored, its color change in the quicksand box can be directly observed; if the color is not obvious, the actual diffusion range of the repair agent in the groundwater can be evaluated by analyzing the distribution of characteristic components such as sulfur or iron in the quartz sand.

[0079] Outlet water quality: Continuously monitor the Cr(VI) concentration at the outlet to reflect the overall remediation effect.

[0080] Table 3: Groundwater remediation effect and diffusivity: Analysis of results: Example 2 achieved excellent treatment results, with Cr(VI) concentrations rapidly and significantly reduced in a short period of time, and significant remediation results were still observed at locations far downstream of the injection point. Its core advantage lies in the ability to maintain the Eh value in a stable reduced state over a long period of time, while also keeping the pH value within an appropriate range. This is primarily due to the design of the slow-release Eh control component, which acts as a continuous energy supply system, continuously providing reducing power, effectively ensuring the long-term stability of trivalent chromium and preventing its re-oxidation.

[0081] Compared with Comparative Example 4, although its Eh value decreased in the early stage, it was not sustainable enough and rebounded quickly, causing the Cr(VI) concentration to rise. This clearly shows that the Eh control component, which lacks the slow-release property, has an effect like a short-term burst and cannot provide a lasting reduction environment. This further highlights that in groundwater remediation, only short-term conversion of pollutants is insufficient, and the key is to achieve long-term stable control. The data of the blank control group confirms that in the absence of intervention, the pollutant concentration remains basically unchanged, emphasizing the necessity of remediation technology.

[0082] Therefore, the advantages of the present invention in groundwater remediation are not only reflected in the rapid and efficient removal of pollutants, but also in its precise control of Eh and pH and the persistence of the effect. Through the introduction of rheological modifiers such as xanthan gum, the repair agent can more evenly penetrate and diffuse into various areas within the groundwater flow field, effectively solving the problem of uneven coverage of pollutants in traditional remediation methods. This ability of precise control and long-term maintenance enables the present invention to flexibly respond to complex groundwater pollution scenarios, and even deep pollution can be effectively repaired, showing its broad application prospects in groundwater pollution control.

[0083] Experiment 4: Long-term stability and environmental compatibility evaluation experiment: This experiment aims to conduct a comprehensive evaluation of the remediation agent of the present invention in a long-term actual environment, including the durability of its effects, and to deeply consider its potential impact on the surrounding soil and groundwater environment to ensure that the remediation process does not create new ecological burdens.

[0084] Experimental methods and settings: First, carefully select a site contaminated by hexavalent chromium, either in the soil or groundwater. Before the formal experiment, conduct a detailed investigation of the site characteristics, including the degree of contamination and geological and hydrological conditions. Ensure that all preliminary work complies with relevant national standards, including HJ25.1-2019, "Technical Specifications for Soil Environmental Monitoring."

[0085] The site was then divided into multiple areas: at least two remediation zones were set up, each treated with a remediation agent from a different embodiment of the present invention. A blank control zone, untreated, was also established to serve as a baseline for efficacy evaluation. Each remediation zone was to be no less than 20 square meters to ensure representative and reliable experimental results.

[0086] The application of the repair agent is carried out according to the respective process: Example 1 area: In-situ stirring repair is carried out according to the liquid repair agent addition process described in Example 1.

[0087] Example 2 area: If the site is groundwater contaminated, remediation is carried out according to the injection process described in Example 2.

[0088] Example 3 area: If the site is surface soil contaminated, the solid repair agent spreading and tillage process described in Example 3 is used for repair.

[0089] Blank control area: no repair treatment was performed.

[0090] The key to the experiment is long-term monitoring. After the restoration begins, regular monitoring should be conducted every one or three months, lasting at least six months to one year. The monitoring indicators and methods are as follows: Soil / groundwater Cr(VI) content: A core indicator. The measurement method is strictly in accordance with relevant national standards such as HJ680-2013 (for soil) or GB / T5750.6-2006 (for groundwater).

[0091] Soil / groundwater pH and Eh: Determine these key environmental parameters directly on-site.

[0092] Total chromium content and speciation analysis: In addition to the total chromium content (refer to HJ766-2015 "Determination of total chromium in soil and sediments by inductively coupled plasma atomic emission spectrometry"), the long-term stability and fixation effect of Cr(III) are evaluated by speciation analysis (such as sequential extraction method, refer to HJ907-2017 "Speciation analysis method for heavy metals in soil and sediments").

[0093] Environmental compatibility indicators: Comprehensively evaluate the potential impact of the restoration process on the environment to avoid secondary pollution.

[0094] Soil heavy metal background values ​​and potential secondary pollution: Monitor the content of other heavy metals in the soil (such as nickel, copper, zinc, etc.) except chromium, and evaluate whether the remediation agent introduces new pollutants, referring to GB15618-2018 "Soil Environmental Quality Agricultural Land Soil Pollution Risk Control Standard (Trial)" or GB36600-2018 "Soil Environmental Quality Construction Land Soil Pollution Risk Control Standard (Trial)".

[0095] Groundwater quality: In addition to controlling Cr(VI), monitor S in groundwater 2- 、 Indicators such as GB / T14848-2017 Groundwater Quality Standard are used to evaluate the overall impact of the remediation agent on groundwater quality. Plant growth: Record ecological indicators such as the growth and coverage of surface plants in the restoration and control areas as direct evidence of ecological restoration.

[0096] Table 4: Long-term restoration effects and environmental compatibility Result analysis: The data from the long-term stability and environmental compatibility evaluation experiments strongly support the excellent performance of the repair agent of the present invention. In Examples 1, 2, and 3, during a monitoring period of up to 12 months, the concentration of Cr(VI) in the soil or groundwater was maintained at a low level, far below the relevant national standards. At the same time, the pH and Eh values ​​were also maintained within the ideal range, which directly confirmed the sustained reducing ability of ammonium polysulfide and the long-term action mechanism of the pH and Eh regulating components. The reduced trivalent chromium was shown to be extremely stable through morphological analysis and leaching experiments, reducing the risk of secondary pollution. These results show that the repair mechanism of the present invention is stable and long-lasting, and can effectively convert toxic hexavalent chromium into environmentally friendly trivalent chromium and firmly fix it in the soil or sediment.

[0097] In terms of environmental compatibility, monitoring data show that no new heavy metal pollutants were introduced during the restoration process, and the concentrations of various environmental indicators such as sulfide and ammonium nitrogen were also within a controllable range, and gradually returned to normal levels in the later period. This fully demonstrates that the present invention has low disturbance and good compatibility with the ecological environment while achieving efficient restoration. The continued high pollution state of the blank control group further highlights the necessity and superiority of the restoration technology of the present invention. In addition, the plant growth in the restoration area is good, and even improved compared to the blank control area. This confirms the suitability of the environment after restoration from a biological point of view, indicating that the ecological function of the contaminated site is gradually recovering.

[0098] Overall, this invention not only solves the immediate problem of hexavalent chromium contamination but also, through its synergistic mechanism and long-lasting performance, ensures long-term stability of the remediation effect, avoiding the environmental "aftereffects" associated with traditional remediation technologies. This comprehensive and sustainable remediation solution provides a practical and feasible technical path for the treatment of contaminated soil and groundwater. Its environmental friendliness and economic feasibility give it broad application potential.

[0099] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A repair agent, characterized in that Included by mass percentage: Ammonium polysulfide: 20%-60%; pH regulating component: 5%-20%; Redox potential control component: 5%-25%.

2. A repair agent according to claim 1, characterized in that: The molecular formula of the ammonium polysulfide is (NH4)2S x , where x is an integer from 2 to 9.

3. A repair agent according to claim 1, characterized in that: The pH regulating component is selected from phosphate buffer, acetate buffer, magnesium oxide, calcium carbonate and combinations thereof.

4. A repair agent according to claim 1, characterized in that: The redox potential controlling component is selected from nanometer zero-valent iron, micrometer zero-valent iron, slow-release organic acid, ferrous sulfide and combinations thereof.

5. A repair agent according to claim 1, characterized in that: The invention also includes a synergistic auxiliary component selected from biochar, chitosan, xanthan gum, ferrous sulfate, and nitrifying bacteria, and the total content of the synergistic auxiliary component is 15.11% to 68% by mass percentage; Of which: Biochar: 10%-30%; Chitosan: 1%-10%; Xanthan gum: 0.1%-2%; Ferrous sulfate: 5%-15%; Nitrifying bacteria: 0.01%-1%.

6. A process for preparing a repair agent, according to any one of claims 1 to 5, characterized in that: The following steps are involved: S1: Preparation of ammonium polysulfide; S2: preparing pH control components; S3: preparing redox potential control components; S4: mixing the ammonium polysulfide, the pH regulating component, the redox potential controlling component, and the synergistic auxiliary component.

7. A repair agent preparation process according to claim 6, characterized in that: When the redox potential controlling component in step S3 is nanometer zero-valent iron or micrometer zero-valent iron, the particle size of the nanometer zero-valent iron is 50 nm to 100 nm, and the particle size of the micrometer zero-valent iron is 1 μm to 10 μm.

8. A process for preparing a repairing agent according to claim 6, characterized in that: The mixing process in step S4 is carried out at a temperature of 5° C. to 30° C., and the mixing time is 15 minutes to 120 minutes.

9. A process for preparing a repairing agent according to claim 6, characterized in that: When the pH regulating component in step S2 is a phosphate buffer, it is prepared by dissolving sodium dihydrogen phosphate and disodium hydrogen phosphate in water, with a total concentration of 5% to 20%.

10. A use of a repair agent, according to any one of claims 1 to 5, characterized in that: Used in the remediation of hexavalent chromium contaminated soil and groundwater.

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