Method for multi-stage remediation of chlorine-containing groundwater pollution based on carbon dioxide nanobubbles

Abstract

The invention discloses a method for multi-stage remediation of chlorine-containing groundwater pollution based on carbon dioxide nanobubbles, nZVI, H2O2 and EDDS reagents are injected in different stages in a sequential manner, the CO2 nanobubbles are used as a multifunctional mediator penetrating through all the time, the CO2 nanobubbles continuously remove a passivation layer through in-situ acidification, and the CO2 nanobubbles are used for repairing the chlorine-containing groundwater pollution through in-situ acidification. According to the present invention, the reduction effect of the nZVI on the chlorine-containing organic matter is fundamentally promoted, and the by-product Fe < 2 + > of the nZVI reaction is coupled with the subsequent Fenton oxidation process, such that the complete oxidation effect on the toxic intermediate product is substantially promoted, the final reaction product Fe < 3 + > can be effectively inhibited from being converted into the secondary mineral, and the blocking risk is controlled from the source. Through the design of in-situ generation of the catalyst Fe < 2 + > and final removal of iron ions, the dosage and final residues of exogenous chemicals are reduced to the maximum extent, and the environmental risk that exogenous pollution is possibly introduced or permanent physical damage is caused in a traditional technology is overcome; the efficient, thorough and sustainable remediation of the chlorine-containing organic pollutants in the underground water is realized.

Description

Technical Field

[0001] This invention relates to the field of groundwater pollution remediation technology, and in particular to a method for multi-stage remediation of chlorinated groundwater pollution based on carbon dioxide nanobubbles. Background Technology

[0002] Among organic pollutants in groundwater, chlorinated hydrocarbons such as trichloroethylene (TCE), tetrachloroethylene (PCE), and carbon tetrachloride (CTC) have become a major challenge in water pollution control due to their extreme persistence and toxicity. Chlorinated hydrocarbons contain chlorine atoms in their molecular structure, making them not only difficult to biodegrade but also prone to persisting in groundwater for extended periods, posing a potential threat to ecosystems and human health.

[0003] Chemical precipitation is commonly used to remove heavy metal pollutants from chlorinated hydrocarbons, but its effectiveness in removing organic pollutants is limited. Biological methods utilize microbial degradation, but their applicability is limited and the reaction time is long. Removal via nano-zero-valent iron reduction presents technical challenges, including rapid degradation of reactivity, incomplete degradation leading to the accumulation of toxic intermediates, and the formation of precipitates from reaction byproducts that can clog aquifers. Therefore, a targeted, multi-stage enhanced treatment method is needed to ensure efficient remediation of chlorinated groundwater pollution. Summary of the Invention

[0004] This invention provides a method for multi-stage remediation of chlorinated groundwater pollution based on carbon dioxide nanobubbles, which addresses the issues of maintaining high reduction activity and reducing intermediate byproduct Fe during the nZVI redox process. 2+ Utilize and deepen the redox process to effectively suppress the final reaction product Fe. 3+ And technical issues such as blocking aquifers.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: The method for multi-stage remediation of chlorinated groundwater pollution based on carbon dioxide nanobubbles comprises the following steps: Step 1: Connect the chemical mixing tank, nanobubble generator, CO2 bubble mixing tank and reinjection pump in sequence through pipelines; wherein, the nanobubble generator is supplied with CO2 gas, and the reinjection pump is connected to the contaminated area in chlorine-containing groundwater through a reinjection pipe; Step Two: In the first stage, nZVI reagent is added to the dosing and mixing tank; the continuous release of CO2 nanobubbles gradually lowers the pH of the groundwater environment to a slightly acidic range, continuously stripping the passivation layer on the surface of the nZVI particles and promoting Fe... 0 Anodic dissolution; Step 3: After the reaction is complete, the contaminated area is rich in highly reactive Fe converted from nZVI. 2+ The reduction rate slowed down and dissolved Fe was detected in the first stage.2+ When the concentration reaches the peak value sufficient to catalyze the reaction, H2O2 solution is added into the dosing mixing pool to enter the second stage; Step four, Fe 2+ generated in the first stage is used as a catalyst to trigger the Fenton reaction efficiently; wherein, the continuous injection of CO2 nano-bubbles forms carbonic acid in the reaction micro-environment, maintains the pH value at 5.5, and avoids the precipitation and inactivation of iron ions; the Fenton reaction is stably and continuously carried out until the pollutants are completely removed or the designed removal standard is reached; Step five, after the completion of the second stage reaction, the third stage is started immediately, and EDDS solution is added into the dosing mixing pool; The CO2 nano-bubbles stably maintain the pH value of the groundwater environment in the weak acidic range, effectively inhibit the Fe 3+ generated in the second stage from hydrolysis and maintain the soluble state of Fe 3+ ; the Fe 3+ is removed in the form of soluble complex by the injection of EDDS solution, thereby completing the multi-stage repair of the chlorinated groundwater pollution.

[0006] Further, the dosing mixing pool is connected with a water supply source, and the water source added into the dosing mixing pool is a Newtonian liquid; the water supply source includes in-situ groundwater, reclaimed water or river and lake reservoir water; the particle size of the suspended particles in the water supply source is not more than 2 mm; The CO2 gas source is a 99.99% high-purity CO2 gas source; the nano-bubble generator is controlled at a gauge pressure of 0.4 MPa, and the gas-liquid ratio in the system is adjusted to 5.4%.

[0007] Further, a mixing device is further arranged in the dosing mixing pool, which uniformly mixes the added nZVI reagent, H2O2 solution and EDDS solution with water; the mixing device is a magnetic stirrer.

[0008] Further, in step two, the nZVI reagent, i.e. nano zero-valent iron reagent, is added into the dosing mixing pool; after mixing through the dosing mixing pool, the nZVI particles-CO2 nano-bubbles generated after passing through the nano-bubble generator enter the CO2 bubble mixing pool, and then enter the pollution area through the recharge pump.

[0009] Further, in step two, after the nZVI particles enter the pollution area, under the action of the CO2 nano-bubbles, the nZVI particles are first affected by the electrostatic repulsive force at the gas-liquid interface, the surface charging state is adjusted due to the negative charge of the nano-bubbles, the van der Waals attraction between the particles is partially offset, and the particles are suspended and dispersed.

[0010] Further, for step two, under the sustained action of CO2 nanobubbles, a local pressure gradient is formed near the nZVI particles, which promotes the detachment of the attached particles from the aggregates, so that the nZVI particles are further continuously suspended and dispersed.

[0011] Further, the method is started when it is confirmed that the site contains the chlorine-containing target pollutants and the background pH value is neutral or alkaline before the first stage starts. After being injected into the contaminated area, the CO2 nanobubbles are continuously released and dissolved in the water body to generate carbonic acid and bicarbonate ions, so that the local environment pH value gradually decreases to the weak acid range, thereby continuously stripping the passivation layer on the surface of the nZVI particles and promoting the anodic dissolution of Fe 0 .

[0012] Further, the dosage of the nZVI reagent is calculated according to the total amount of the chlorine-containing target pollutants, and the molar ratio is controlled to be between 10:1 and 100:1; and the injection amount of the CO2 nanobubbles is dynamically controlled, and the injection rate is controlled by stably maintaining the groundwater environment pH value at 5.5.

[0013] Further, when the H2O2 solution is added in the second stage, the molar ratio of the H2O2 solution to Fe 2+ is controlled to be between 1:1 and 10:1, and the H2O2 solution is injected in a batch and low flow rate manner; at this time, the CO2 nanobubbles are continuously injected to stably maintain the groundwater environment pH value at 5.5 and induce a micro-turbulent effect, thereby increasing the generation rate and utilization efficiency of the hydroxyl radicals.

[0014] Further, when the EDDS solution is added in the third stage, the molar ratio of the EDDS solution to the total dissolved iron is 1.1:1; at this time, the CO2 nanobubbles are continuously injected to stably maintain the groundwater environment pH value at 5.5.

[0015] The beneficial effects of the present application are embodied in: 1) The present application realizes efficient, complete and sustainable remediation of chlorine-containing organic pollutants in groundwater by sequentially injecting nZVI, H2O2 and EDDS reagents at different stages and using CO2 nanobubbles as a multifunctional medium throughout; the present scheme does not pre-physically coat the nZVI, but creatively introduces CO2 nanobubbles as a multifunctional medium throughout; the advancement of this multi-stage enhancement method lies in that it not only continuously removes the passivation layer through in-situ acidification, thereby fundamentally promoting the reduction effect of nZVI on chlorine-containing organic matter; further, it couples the subsequent Fenton oxidation process by considering the by-product Fe 2+ as a valuable resource, thereby greatly promoting the complete oxidation effect of the toxic intermediate product; finally, the system can also effectively inhibit the final reaction product Fe 3+Convert to secondary minerals, control the risk of plugging from the source; 2) The application utilizes the core mediator CO2 nanobubbles. Firstly, the weakly acidic environment provided by the CO2 nanobubbles can dynamically dissolve the passivation products on the surface of nZVI, maintain its high reduction activity, and strengthen the reduction process. Secondly, when hydrogen peroxide (H2O2) is introduced, the optimal pH environment created by the CO2 nanobubbles ensures the generation of Fe 2+ The catalytic Fenton reaction can proceed efficiently, strengthening the deep oxidation process. Finally, at the end of the reaction, the weakly acidic environment improves the solubility of Fe 3+ , creating conditions for the intervention of the green chelating agent ethylenediamine disuccinic acid (EDDS), thereby completely removing Fe 3+ in the form of a soluble complex and effectively inhibiting the rapid generation of secondary minerals such as Fe(OH)3, ensuring the long-term permeability of the aquifer. This innovative multi-stage coupling method realizes the dual improvement of repair efficiency and process safety, exhibiting significant application advantages. 3) The application not only strengthens the reduction process of nZVI through the activation of CO2 nanobubbles, ensuring efficient removal of parent pollutants, but also introduces hydroxyl radicals, a strong oxidizing species, through coupling Fenton reaction, strengthening the deep oxidation process of stubborn intermediates, realizing the dual guarantee of "selective reduction" to "non-selective strong oxidation", and ensuring the thoroughness of the repair; 4) The core mediator CO2 nanobubbles, oxidizing agent (H2O2, product is water), and chelating agent (EDDS, biodegradable) of the application are all environmentally friendly substances. Through the design of in-situ generation of catalyst (Fe 2+ ) and final removal of iron ions, the dosage and final residue of external chemicals are minimized, overcoming the environmental risks of potential introduction of external pollution or permanent physical damage in traditional technologies; Thus, the application solves the technical problems of maintaining the high reduction activity of nZVI, utilizing intermediate by-products Fe 2+ , deepening the redox process, effectively inhibiting the final reaction product Fe 3+ , and plugging the aquifer during the nZVI oxidation-reduction method. Other features and advantages of the application will be described in the subsequent specification, and some will become apparent from the specification, or will be understood by implementing the application; the main purpose and other advantages of the application can be achieved and obtained by the specific solutions indicated in the specification. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 is a schematic diagram of the treatment process of the method for repairing chlorinated groundwater pollution by carbon dioxide nanobubbles; Figure 2 is the Fe2+ Comparison chart of leaching effect; Figure 3 is a TCE removal effect chart of four reaction systems; Figure 4 is a dechlorination efficiency chart of four reaction systems; Figure 5 is a plugging inhibition efficiency chart of four reaction systems.

[0017] Reference signs: 1 - dosing mixing tank, 2 - dosing device, 3 - mixing device, 4 - nanobubble generator, 5 - CO2 bubble mixing tank, 6 - recharge pump. DETAILED DESCRIPTION

[0018] The technical solutions of the present application are described in detail below through examples. The following examples are merely exemplary and can only be used to explain and illustrate the technical solutions of the present application, but cannot be interpreted as a limitation on the technical solutions of the present application. EXAMPLE

[0019] Taking trichloroethylene (TCE) in groundwater as an example, a simulation experiment was carried out, and a one-piece nanobubble machine was used to drive a water flow of 1.8 L / min to pass through, with high-purity CO2 (99.99 %) as the gas source, and the gas-liquid ratio was 5.4 % to prepare nanobubble water. After 20 minutes of equipment circulation, the water sample was collected. Particle Metrix ZetaView nanoparticle tracking analyzer was used to test the bubble size, concentration and potential of the prepared bubbles. The measured bubble median diameter was 102.7 nm, the concentration was 1.9×108 per mL, and the Zeta potential was -16.6 mV.

[0020] In combination Figures 1 to 5 , the method for repairing chlorinated groundwater pollution based on carbon dioxide nanobubbles is further described, and the specific steps are as follows: Step one, connect the dosing mixing tank 1, nanobubble generator 4, CO2 bubble mixing tank 5 and recharge pump 6 in sequence through the pipeline; wherein the nanobubble generator 4 is supplied with CO2 gas source, and the recharge pump 6 is connected with the contaminated area of chlorinated groundwater through the recharge pipe; the contaminated area in this embodiment is constructed in the laboratory by a column reactor with a height of 28 cm and a diameter of 5 cm. The reactor is filled with quartz sand with a particle size of 1-2 mm, and the porosity of the reactor is 25.51%. Before filling, the quartz sand is cleaned and dried to remove impurities and ensure the accuracy of the experimental results.

[0021] In this embodiment, the dosing mixing pool 1 is connected with the water source, and the water source added in the dosing mixing pool 1 is Newtonian liquid. The water source includes in-situ groundwater, reclaimed water or river and lake water, and the particle size of suspended particles in the water source is not more than 2 mm. The gauge pressure of the nano-bubble generator 4 is controlled at 0.4 MPa, and the gas-liquid ratio in the system is adjusted to 5.4%. The nZVI reagent, H2O2 solution or EDDS solution is respectively filled in the dosing device 2. The mixing device 3 is further arranged in the dosing mixing pool 1, which uniformly mixes the added nZVI reagent, H2O2 solution and EDDS solution with water. The mixing device 3 is a magnetic stirrer.

[0022] Step two, the first stage is to add the nZVI reagent in the dosing mixing pool 1. The continuous release of CO2 nano-bubbles makes the pH value of the groundwater environment gradually decrease to the weak acidic interval, continuously strips the passivation layer on the surface of the nZVI particles and promotes the anodic dissolution of Fe 0 . The molar ratio of nZVI to TCE is controlled between 10:1 and 100:1.

[0023] Before the first stage starts, it is confirmed that the site exists the chlorinated target contaminant and the background pH value is neutral or alkaline. The injection amount of CO2 nano-bubbles is dynamically controlled, and the injection rate is controlled by stably maintaining the pH value of the groundwater environment at the recharging site at 5.5.

[0024] In this embodiment, the nZVI reagent, i.e. nano zero-valent iron reagent, is added in the dosing mixing pool 1. After mixing in the dosing mixing pool 1, the nZVI particles-CO2 nano-bubbles are generated by the nano-bubble generator 4, enter the CO2 bubble mixing pool 5 and then enter the contaminated area through the recharging pump 6.

[0025] In this embodiment, when the nZVI particles enter the contaminated area, the nZVI particles are first affected by the electrostatic repulsive force at the gas-liquid interface under the action of CO2 nano-bubbles. The surface charging state of the particles is adjusted due to the negative charge of the nano-bubbles, the Van der Waals attraction between the particles is partially offset, and the particles are suspended and dispersed. Under the continuous action of CO2 nano-bubbles, a local pressure gradient is formed near the nZVI particles, which promotes the detachment of the adhered particles from the aggregates, so that the nZVI particles are further continuously suspended and dispersed.

[0026] In this embodiment, after recharging into the contaminated area, the CO2 nano-bubbles are continuously released and dissolved in the water body to generate carbonic acid and bicarbonate ions, so that the local environment pH value gradually decreases to the weak acidic interval, thereby continuously stripping the passivation layer on the surface of the nZVI particles and promoting the anodic dissolution of Fe 0 .

[0027] Step three, after 60 min of reaction, the contaminated area is rich in high-activity Fe 2+; in the first stage, the reduction rate slows down and dissolved Fe 2+ When the concentration reaches a peak sufficient to catalyze the reaction, H2O2 solution is added to the dosing mixing pool 1 to enter the second stage.

[0028] Step four, Fe 2+ generated in the first stage is used as a catalyst to trigger the Fenton reaction efficiently; wherein the continuous injection of CO2 nanobubbles forms carbonic acid in the reaction microenvironment, maintaining the pH value at 5.5 to avoid the precipitation and inactivation of iron ions; the Fenton reaction is stably and continuously carried out until the pollutants are completely removed or the designed removal standard is reached.

[0029] In this embodiment, when the H2O2 solution is added in the second stage, the molar ratio of H2O2 solution to Fe 2+ is controlled between 1:1 and 10:1, and the injection is carried out in a batch and low flow rate manner; at this time, the continuous injection of CO2 nanobubbles stably maintains the pH value of the groundwater environment at 5.5 and induces micro-turbulent effects, thereby increasing the generation rate and utilization efficiency of hydroxyl radicals.

[0030] Step five, after the completion of the second stage reaction, the third stage is immediately started, and EDDS solution is added to the dosing mixing pool 1; CO2 nanobubbles stably maintain the pH value of the groundwater environment in the weakly acidic range, effectively inhibiting the hydrolysis of Fe 3 + generated in the second stage and maintaining the soluble state of Fe 3+ ; through the injection of EDDS solution, Fe 3+ is removed in the form of a soluble complex, thereby completing the multi-stage repair of chlorinated groundwater pollution.

[0031] In this embodiment, when the EDDS solution is added in the third stage, the molar ratio of EDDS solution to total dissolved iron is 1.1:1; at this time, the continuous injection of CO2 nanobubbles stably maintains the pH value of the groundwater environment at 5.5.

[0032] To analyze the dynamic changes of the repair process, liquid samples are collected and filtered every 2 h during the reaction process. The obtained samples are first monitored for Fe 2+The dynamic changes in concentration were used to characterize the activation leaching efficiency of nZVI and to confirm that its concentration reached the peak value required to initiate the Fenton reaction. Simultaneously, the concentration of TCE was detected using an Agilent 5977B gas chromatography-mass spectrometry system (equipped with an HP-Plot-Q column). The injection port temperature was set to 250 °C, and the oven temperature program was as follows: initial temperature 40 °C held for 0.5 min, increased to 50 °C at 5 °C / min, then increased to 65 °C at 15 °C / min, subsequently increased to 72 °C at 2 °C / min, and finally increased to 230 °C at 30 °C / min and held for 10 min. Cl was further determined using a Dionex ICS-1000 ion chromatograph. - Concentration was used to comprehensively evaluate the removal efficiency of TCE and the overall dechlorination effect. Furthermore, after the injection of EDDS, the change in solution absorbance over time needed to evaluate the effect of this step on Fe in the system. 3+ It inhibits precipitation and has anti-clogging properties.

[0033] To systematically evaluate the key role of CO2 nanobubbles in the entire process of nZVI activation, pollutant degradation, and iron ion stabilization, this experiment constructed a blank system, an H2O2 system, a DI (deionized water) system, a DI water-nZVI system, a regular CO2 bubble water-nZVI system, a regular CO2 bubble water-H2O2 system, a CO2 nanobubble water-nZVI system, and a CO2 nanobubble water-H2O2 system for comparison at different stages. Figures 2 to 5 As shown, the effects of Fe on the dispersion and activation of nZVI were verified through a comparative study of four systems: DI water, DI water-nZVI, ordinary CO2 bubble water-nZVI, and CO2 nano bubble water-nZVI. 2+ Leaching effect: The TCE removal effect, dechlorination efficiency and anti-clogging performance were verified by comparing four systems: blank, H2O2 system, ordinary CO2 bubble water-H2O2 system and CO2 nano bubble water-H2O2 system.

[0034] like Figure 2 As shown, Fe in the four reaction systems 2+ Leaching results showed that Fe 2+ The concentration of Fe in the DI water system gradually increased with reaction time under different treatment conditions, but the rate of increase varied significantly. 2+ The concentration remained essentially unchanged, indicating that the Fe in the solution... 2+ The main source is reactive leaching of nZVI. In a standalone DI-water-nZVI system, Fe... 2+ The slow increase in concentration indicates that its surface is prone to passivation, thus limiting its reactivity. After adding ordinary CO2 bubbles, in the ordinary CO2 bubble water-nZVI system, Fe... 2+The generation rate increased, indicating that the acidic environment generated by CO2 dissolution can promote the dissolution of the nZVI surface oxidation film, thereby enhancing its reactivity. In contrast, the CO2 nanobubble water-nZVI system exhibited the highest Fe 2+ concentration and the fastest rate of increase, indicating that nanobubbles have a more significant strengthening effect. This effect is attributed to the large specific surface area and high gas-liquid mass transfer efficiency of nanobubbles, which can continuously release CO2 and maintain a stable acidic environment, effectively inhibiting the generation of a passivation layer, thereby prolonging the effective life of nZVI. The comprehensive results show that CO2 nanobubbles can significantly enhance the reactivity of nZVI and the Fe 2+ leaching rate, verifying the application potential of this technology in enhancing groundwater remediation and improving pollutant removal efficiency.

[0035] Figure 2 It is clearly revealed that CO2 nanobubbles have a promoting effect on the activation of nano zero-valent iron. First, in the two control groups of DI water without nZVI and DI water with only nZVI, the Fe 2+ concentration is almost zero throughout the 120-minute reaction time, which strongly proves that the corrosion and dissolution process of nZVI itself is very slow. When ordinary CO2 bubbles are introduced into the system, the Fe 2+ concentration begins to increase significantly and stabilizes at about 20 μM after about 80 minutes. The most critical comparison is that the CO2 nanobubble water-nZVI experimental group exhibits the best performance. Its Fe 2+ generation rate is the fastest, and the final concentration is also the highest, stabilizing at more than 40 μM in the late reaction period, which is about twice that of the ordinary CO2 bubble group. This result strongly proves that, compared with ordinary CO2 bubbles, CO2 nanobubbles can more efficiently construct and maintain a weakly acidic microenvironment around the nZVI particles by virtue of their large specific surface area and high stability, thereby continuously and deeply activating nZVI, greatly accelerating the conversion of Fe 0 to highly active Fe 2+ , providing sufficient catalysts for subsequent Fenton reaction steps.

[0036] To verify the effect of the method described in the present application in the advanced oxidation stage, after 2 h in step four, comparative experiments were conducted on the degradation effect of TCE in different systems, and the results are as follows: Figure 3As shown in the figure, the data clearly demonstrates that the TCE concentration in the blank control system remained unchanged throughout the reaction cycle, confirming its chemical stability. While the system with H2O2 added alone could initiate a certain degree of degradation, the reaction rate was extremely slow, with approximately 20% of TCE remaining after 24 hours. This indicates that the oxidation efficiency of H2O2 is limited in the absence of effective catalysis and a suitable pH environment. In the ordinary CO2 bubble water-H2O2 system, the system using ordinary CO2 bubbles to assist H2O2 activated the Fenton reaction through an acidified environment, significantly increasing the degradation rate. However, it still required approximately 22 hours to achieve complete TCE removal, reflecting the limitations of traditional aeration methods in terms of mass transfer efficiency and pH control precision. In stark contrast to the above comparison groups, the CO2 nanobubble-H2O2 system used in this invention achieved complete TCE degradation within 10 hours, with a reaction rate more than doubled compared to the ordinary CO2 bubble system. The above results fully demonstrate that CO2 nanobubbles, with their large specific surface area and efficient mass transfer characteristics, can rapidly, uniformly, and precisely regulate and stabilize the pH value of the reaction microenvironment within the optimal range for the Fenton reaction, thereby maximizing the excitation of Fe. 2+ catalytic activity.

[0037] After 8 hours in step four, a comparative experiment was conducted to assess the dechlorination effects under different systems. According to... Figure 4 Cl shown - The concentration-time curve results show that the blank control system contains no Cl. - The formation was as expected. Cl- in the H2O2 control system alone... - The formation rate is slow, with a cumulative concentration of only about 22 μM within 24 hours, indicating limited dechlorination capacity. A control system using ordinary CO2 bubbles-H2O2 was used, and the Fenton reaction was promoted by acidification. - The generation rate was significantly accelerated, with the final concentration reaching approximately 24 μM, confirming the importance of pH control for the dechlorination reaction. However, this also reflects that conventional aeration methods still have room for optimization in terms of reaction kinetics. In stark contrast to the above systems, the CO2 nanobubble-H2O2 system used in this invention exhibits higher dechlorination efficiency. -The concentration rapidly increased within the first 10 hours of the reaction, reaching a plateau of approximately 30 μM. Both the initial reaction rate and the final product yield were significantly higher than in all control groups. This result strongly demonstrates that CO2 nanobubbles, through their efficient mass transfer capabilities and the precise construction and maintenance of the optimal pH microenvironment for the Fenton reaction, greatly promote the generation of highly oxidizing hydroxyl radicals, thereby achieving rapid and complete stripping of chlorine atoms from TCE molecules. This not only verifies the effective degradation of TCE but also highlights the fundamental technical advantages of this method in achieving deep mineralization of pollutants.

[0038] To confirm the key effectiveness of this invention in inhibiting iron ion precipitation and preventing physical blockage, the formation of Fe(OH)3 precipitate in different systems was evaluated by monitoring changes in solution absorbance. For example... Figure 5 As shown, the absorbance of the blank control system increased significantly over time, eventually reaching nearly 0.8. This indicates that without pH adjustment, iron ions in the system undergo extensive hydrolysis to form Fe(OH)3 precipitate, leading to a serious risk of clogging. While the ordinary CO2 bubble-H2O2 system and the H2O2 system alone can mitigate precipitation to some extent, the absorbance continues to increase steadily, indicating limited ability to suppress clogging. In stark contrast to all the above comparison groups, the CO2 nanobubble-H2O2 system used in this invention successfully suppressed the solution absorbance to an extremely low level below 0.15 throughout the entire monitoring period, with the growth trend almost stagnating. This decisive result strongly demonstrates that CO2 nanobubbles can construct and maintain a stable, weakly acidic microenvironment for a long time, effectively inhibiting Fe... 3+ The hydrolysis reaction prevented the formation of Fe(OH)3 precipitate.

[0039] This invention's method is applicable to groundwater pollution caused by high concentrations of aged chlorinated hydrocarbons such as TCE, PCE, and CTC in sites of industries such as chemical, pesticide, electronics manufacturing, and dry cleaning. It is also applicable to complex scenarios in industrial parks and landfills where groundwater simultaneously contains pollutants requiring both reduction and oxidation treatment. It is suitable for low-permeability aquifers such as clay and silt, or areas highly sensitive to changes in groundwater conditions due to geological structure or engineering requirements. It is applicable to the remediation and redevelopment of historically contaminated sites such as former chemical plant sites and decommissioned tank areas. This technology not only reduces pollutant concentrations to below the risk screening values ​​required for land redevelopment, but its characteristics of thoroughly degrading pollutants and actively removing reaction byproducts ensure the long-term stability of the remediation effect and the absence of adverse aftereffects. It effectively avoids long-term risks such as pollutant "rebound" and deterioration of site permeability, providing a reliable technical guarantee for the safe reuse of land. The method of this invention is also applicable to acute, high-concentration chlorinated organic pollutant leaks caused by tank leaks, transportation accidents, etc. It can respond quickly and significantly reduce the concentration of the main pollutants in a short period of time to control the further spread of the pollution plume. Subsequently, through deep oxidation and risk removal mechanisms, it can achieve rapid and thorough purification of the affected area, making it a powerful technical option for emergency repair and response.

[0040] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for multi-stage remediation of chlorinated groundwater pollution based on carbon dioxide nanobubbles, characterized in that, The specific steps are as follows: Step 1: Connect the chemical mixing tank, nanobubble generator, CO2 bubble mixing tank and reinjection pump in sequence through pipelines; wherein, the nanobubble generator is supplied with CO2 gas, and the reinjection pump is connected to the contaminated area in chlorine-containing groundwater through a reinjection pipe; Step Two: In the first stage, nZVI reagent is added to the dosing and mixing tank; the continuous release of CO2 nanobubbles gradually lowers the pH of the groundwater environment to a slightly acidic range, continuously stripping the passivation layer on the surface of the nZVI particles and promoting Fe... 0 Anodic dissolution; Step 3: After the reaction is complete, the contaminated area is rich in highly reactive Fe converted from nZVI. 2+ The reduction rate slowed down and dissolved Fe was detected in the first stage. 2+ When the concentration reaches a peak sufficient to catalyze the reaction, H2O2 solution is added to the dosing mixing tank to enter the second stage; Step 4: Using the Fe produced in the first stage 2+ As a catalyst, it efficiently triggers the Fenton reaction; the continuous injection of CO2 nanobubbles forms carbonic acid in the reaction microenvironment, maintaining the pH value at 5.5 and preventing iron ion precipitation and deactivation; the Fenton reaction proceeds stably and continuously until the pollutants are completely removed or the designed removal standard is reached; Step 5: After the second stage reaction is completed, immediately start the third stage by adding EDDS (ethylenediamine disuccinic acid) solution to the dosing and mixing tank; CO2 nanobubbles will stabilize the pH of the groundwater environment in the weakly acidic range, effectively inhibiting the Fe produced in the second stage. 3+ Hydrolysis occurs and Fe is maintained 3+ The soluble state of Fe; Fe is injected via EDDS solution. 3+ It removes chlorinated groundwater pollution in the form of soluble complexes, thereby completing multi-stage remediation.

2. The method for multi-stage remediation of chlorinated groundwater pollution based on carbon dioxide nanobubbles as described in claim 1, characterized in that, The dosing mixing tank is connected to the water supply source, and the water added to the dosing mixing tank is a Newtonian liquid. The water supply source includes in-situ groundwater, reclaimed water, or river, lake, or reservoir water. The suspended particulate matter in the water supply source has a particle size of no more than 2 mm. The CO2 source is 99.99% high-purity CO2; the gauge pressure of the nanobubble generator is controlled at 0.4 MPa, and the gas-liquid ratio in the system is adjusted to 5.4%.

3. The method for multi-stage remediation of chlorinated groundwater pollution based on carbon dioxide nanobubbles as described in claim 2, characterized in that, A mixing device is also installed in the dosing and mixing tank. The mixing device will uniformly mix the added nZVI reagent, H2O2 solution and EDDS solution with water. The mixing device is a magnetic stirrer.

4. The method for multi-stage remediation of chlorinated groundwater pollution based on carbon dioxide nanobubbles as described in claim 3, characterized in that, In step two, nZVI reagent, i.e. nano zero-valent iron reagent, is added to the dosing and mixing tank. After mixing in the dosing and mixing tank, the mixture is then passed through a nano bubble generator to generate nZVI particles-CO2 nano bubbles, which then enter the CO2 bubble mixing tank and are subsequently pumped into the contaminated area through a reinjection pump.

5. The method for multi-stage remediation of chlorinated groundwater pollution based on carbon dioxide nanobubbles as described in claim 4, characterized in that, In step two, when the nZVI particles enter the contaminated area, under the action of CO2 nanobubbles, the nZVI particles are first affected by the electrostatic repulsion force at the gas-liquid interface. The surface charge state is adjusted by the negative charge of the nanobubbles, and the van der Waals attraction between the particles is partially canceled, which promotes the suspension and dispersion of the particles.

6. The method for multi-stage remediation of chlorinated groundwater pollution based on carbon dioxide nanobubbles as described in claim 5, characterized in that, In step two, under the continuous action of CO2 nanobubbles, a local pressure gradient is formed near the nZVI particles, which causes the attached particles to detach from the aggregates, allowing the nZVI particles to be further suspended and dispersed.

7. The method for multi-stage remediation of chlorinated groundwater pollution based on carbon dioxide nanobubbles as described in claim 6, characterized in that, The process begins when the presence of chlorine-containing target pollutants at the site is confirmed and the background pH is neutral or slightly alkaline before the start of the first phase. After being reinjected into the contaminated area, CO2 nanobubbles are continuously released and dissolve in the water, generating carbonic acid and bicarbonate ions. This gradually lowers the local pH value to a slightly acidic range, thereby continuously stripping the passivation layer from the surface of the nZVI particles and promoting Fe... 0 Anodic dissolution.

8. The method for multi-stage remediation of chlorinated groundwater pollution based on carbon dioxide nanobubbles as described in claim 7, characterized in that, The dosage of nZVI reagent is calculated based on the total amount of chlorine-containing target pollutants, and the molar ratio is controlled between 10:1 and 100:1; while the injection amount of CO2 nanobubbles is dynamically controlled, and the injection rate is controlled by maintaining the pH value of the groundwater environment at the recharge site at 5.

5.

9. The method for multi-stage remediation of chlorinated groundwater pollution based on carbon dioxide nanobubbles as described in claim 8, characterized in that, When H2O2 solution is added in the second stage, H2O2 solution reacts with Fe 2+ The molar ratio is controlled between 1:1 and 10:1, and CO2 nanobubbles are injected in batches at low flow rates. At this time, the continuous injection of CO2 nanobubbles stabilizes the pH value of the groundwater environment at 5.5 and induces microturbulence, thereby increasing the generation rate and utilization efficiency of hydroxyl radicals.

10. The method for multi-stage remediation of chlorinated groundwater pollution based on carbon dioxide nanobubbles as described in claim 9, characterized in that, When EDDS solution was added in the third stage, the molar ratio of EDDS solution to total dissolved iron was 1.1:1; at this time, the continuous injection of CO2 nanobubbles stabilized the pH value of the groundwater environment at 5.5.

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