Method for multi-stage repair of chlorinated groundwater pollution based on carbon dioxide nanobubbles

Abstract

The application discloses a method for repairing chlorinated groundwater pollution based on carbon dioxide nanobubbles, and the method is characterized in that nZVI, H2O2 and EDDS reagents are injected in a time sequence at different stages, and CO2 nanobubbles are used as a multifunctional medium throughout the process. The CO2 nanobubbles not only continuously remove the passivation layer through in-situ acidification, thereby fundamentally promoting the reduction effect of nZVI on chlorinated organic matter, but also couple a subsequent Fenton oxidation process, thereby greatly promoting the complete oxidation effect of toxic intermediates, effectively inhibiting the conversion of the final reaction product Fe 2+ Into secondary minerals, and controlling the risk of blockage from the source. Through the design of in-situ generation of catalyst Fe 2+ And the final removal of iron ions, the application maximizes the dosage of exogenous chemicals and the final residual, overcomes the environmental risks that may be introduced by traditional technologies or cause permanent physical damage, and realizes efficient, complete and sustainable repair of chlorinated organic pollutants in groundwater. 3+ ​

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of groundwater pollution remediation, in particular to a method for multi-stage remediation of chlorinated groundwater pollution based on carbon dioxide nanobubbles. BACKGROUND

[0002] Among the organic pollutants in groundwater, chlorinated hydrocarbons such as trichloroethylene (TCE), tetrachloroethylene (PCE), and carbon tetrachloride (CTC) have become a problem in water pollution control due to their strong persistence and toxicity. Chlorinated hydrocarbon pollutants contain chlorine atoms in their molecular structure. These compounds are not only difficult to biodegrade, but also can exist in groundwater for a long time, posing a potential threat to the ecosystem and human health.

[0003] In the removal of chlorinated hydrocarbon pollutants, chemical precipitation is often used to remove heavy metal pollutants, but the removal effect on organic pollutants is limited. Biological methods use microorganisms for degradation, but their application range is limited and the reaction time is long. When using nanometer zero-valent iron reduction method for removal, there are technical problems such as rapid decay of reaction activity, incomplete degradation leading to accumulation of toxic intermediates, and easy formation of reaction byproducts to block the aquifer; in order to ensure efficient remediation of chlorinated groundwater pollution, a multi-stage enhanced disposal method is needed. SUMMARY

[0004] The present application provides a method for multi-stage remediation of chlorinated groundwater pollution based on carbon dioxide nanobubbles, which solves the technical problems of maintaining high reduction activity of nZVI, intermediate byproducts Fe 2+ utilization, deepening the oxidation-reduction process, and effectively inhibiting the formation of final reaction products Fe 3+ and blocking the aquifer.

[0005] To achieve the above purpose, the technical scheme adopted by the present application is as follows:

[0006] The method for multi-stage remediation of chlorinated groundwater pollution based on carbon dioxide nanobubbles comprises the following specific steps:

[0007] Step one, connect the dosing mixing pool, nanobubble generator, CO2 bubble mixing pool and recharge pump in sequence through the pipeline; wherein the gas source of the nanobubble generator is CO2 source, and the recharge pump is connected to the contaminated area in the chlorinated groundwater through the recharge pipe;

[0008] Step two, add nZVI reagent in the dosing mixing pool in the first stage; CO2 nanobubbles are continuously released to gradually reduce the pH value of the groundwater environment to the weak acid range, continuously strip the passivation layer on the surface of the nZVI particles and promote the anodic dissolution of Fe 0 ;

[0009] Step three, after sufficient reaction, the pollution area is rich in high-activity Fe 2+ ; when the reduction rate in the first stage is monitored to slow down and the dissolved Fe 2+ concentration reaches a peak sufficient to catalyze the reaction, H2O2 solution is added to the dosing mixing tank to enter the second stage;

[0010] Step four, Fe 2+ produced 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;

[0011] Step five, after the completion of the second stage reaction, the third stage is started immediately, and EDDS solution is added to the dosing mixing tank;

[0012] CO2 nano-bubbles stably maintain the pH value of the groundwater environment in the weak acid range, effectively inhibit the hydrolysis of Fe 3+ produced in the second stage, and maintain 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.

[0013] Further, the dosing mixing tank is connected with a water supply source, and the water source added in the dosing mixing tank is a Newtonian liquid; the water supply source includes in-situ groundwater, reclaimed water or lake and reservoir water, and the particle size of suspended particles in the water supply source is not more than 2 mm;

[0014] 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%.

[0015] Further, a mixing device is further arranged in the dosing mixing tank, which uniformly mixes the added nZVI reagent, H2O2 solution and EDDS solution with water; the mixing device is a magnetic stirrer.

[0016] Further, in step two, nZVI reagent, i.e. nano zero-valent iron reagent, is added to the dosing mixing tank; after mixing in the dosing mixing tank, nZVI particles-CO2 nano-bubbles are generated after passing through the nano-bubble generator and then enter the CO2 bubble mixing tank, and then enter the pollution area through the recharge pump.

[0017] Further, in step two, when the nZVI particles enter the contaminated area, under the action of CO2 nano-bubbles, the nZVI particles are first affected by the electrostatic repulsive force at the gas-liquid interface, the surface charge state of the particles is adjusted due to the negative charge of the nano-bubbles, and the van der Waals attraction between the particles is partially offset, so that the particles are suspended and dispersed.

[0018] Further, in step two, under the continuous action of CO2 nano-bubbles, 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.

[0019] Further, before the first stage starts, it is confirmed that the site contains chlorine-containing target pollutants and the background pH value is neutral or alkaline;

[0020] After being injected into the contaminated area, 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 acid range, thereby continuously stripping the passivation layer on the surface of the nZVI particles and promoting the anodic dissolution of Fe 0 .

[0021] 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 CO2 nano-bubbles is dynamically controlled, and the injection rate is controlled by stably maintaining the groundwater environment pH value at 5.5.

[0022] Further, when the H2O2 solution is added in the second stage, the molar ratio of H2O2 solution to Fe 2+ is controlled to be between 1:1 and 10:1, and the injection is carried out in batches and at a low flow rate; at this time, CO2 nano-bubbles are continuously injected to stably maintain the groundwater environment pH value at 5.5 and induce micro-turbulence effect, thereby increasing the generation rate and utilization efficiency of hydroxyl radicals.

[0023] Further, 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, CO2 nano-bubbles are continuously injected to stably maintain the groundwater environment pH value at 5.5.

[0024] The beneficial effects of the present application are:

[0025] 1) The present application realizes efficient, thorough and sustainable remediation of chlorinated organic pollutants in groundwater by injecting nZVI, H2O2 and EDDS reagents in different stages in a time sequence, and using CO2 nanobubbles as a multifunctional mediator throughout; The present application does not pre-physically coat nZVI, but creatively introduces CO2 nanobubbles as a multifunctional mediator throughout; The advancement of this multi-stage enhancement method lies in: it not only continuously removes the passivation layer through in-situ acidification, thereby fundamentally improving the reduction effect of nZVI on chlorinated organic matter; Furthermore, it couples the subsequent Fenton oxidation process, thereby greatly improving the thorough oxidation effect of toxic intermediates; Finally, the system can also effectively inhibit the conversion of the final reaction product Fe 2+ Into secondary minerals, thereby controlling the risk of plugging from the source; 3+

[0026] 2) By using the core mediator CO2 nanobubbles, first, the weakly acidic environment provided by the core mediator CO2 nanobubbles can dynamically dissolve the passivation products on the surface of nZVI, thereby maintaining its high reduction activity and strengthening the reduction process; second, when hydrogen peroxide (H2O2) is introduced, the optimal pH environment created by CO2 nanobubbles ensures that the Fenton reaction catalyzed by Fe 2+ Can be efficiently carried out, thereby strengthening the deep oxidation process; finally, at the end of the reaction, the weakly acidic environment improves the solubility of Fe 3+ , thereby 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, thereby effectively inhibiting the rapid generation of secondary minerals such as Fe(OH)3, thereby ensuring the long-term permeability of the aquifer; This innovative multi-stage coupling method realizes the dual improvement of remediation efficiency and process safety, thereby exhibiting significant application advantages;

[0027] 3) The present application not only strengthens the reduction process of nZVI through the activation of CO2 nanobubbles, thereby ensuring efficient removal of the parent pollutants; but also introduces hydroxyl radicals, a strong oxidizing species, through coupling the Fenton reaction, thereby strengthening the deep oxidation process of recalcitrant intermediates, thereby realizing dual protection from "selective reduction" to "non-selective strong oxidation", thereby ensuring the thoroughness of remediation;

[0028] 4) The core mediator CO2 nanobubbles, oxidizing agent (H2O2, product is water) and chelating agent (EDDS, biodegradable) of the present application are all environmentally friendly substances; the catalyst (Fe 2+ ​) and the final removal of iron ions, which minimizes the amount of exogenous chemicals added and the final residue, overcoming the environmental risks that traditional technologies may introduce exogenous pollution or cause permanent physical damage;

[0029] Thus, the present application solves the technical problems of maintaining the high reduction activity of nZVI, inhibiting the oxidation of nZVI, inhibiting the side product Fe 2+ and effectively inhibiting the final reaction product Fe 3+ and blocking the aquifer. Other features and advantages of the present application will be set forth in the following description, and in part will become apparent to those skilled in the art from the description, or can be learned by practice of the present application. The main objects and other advantages of the present application can be achieved and obtained by the solutions specifically pointed out in the description. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 is a schematic diagram of the treatment process of the method for repairing chlorinated groundwater pollution by multi-stage carbon dioxide nanobubble;

[0031] Figure 2 is the Fe 2+ leaching effect comparison chart;

[0032] Figure 3 is the TCE removal effect chart in the four reaction systems;

[0033] Figure 4 is the dechlorination efficiency chart of the four reaction systems;

[0034] Figure 5 is the plugging inhibition efficiency chart of the four reaction systems.

[0035] Reference signs: 1 - dosing mixing tank, 2 - dosing device, 3 - mixing device, 4 - nanobubble generator, 5 - CO2 bubble mixing tank, 6 - recharging pump. DETAILED DESCRIPTION

[0036] The technical solutions of the present application are described in detail below through examples. The following examples are only exemplary and can only be used to explain and illustrate the technical solutions of the present application, but cannot be interpreted as a limitation of the technical solutions of the present application. EMBODIMENT

[0037] The simulation experiment was carried out with trichloroethylene (TCE) in groundwater as an example. The water flow of 1.8 L / min was driven by the integrated nanobubble generator, high-purity CO2 (99.99%) was used as the gas source, the gas-liquid ratio was 5.4%, and the nanobubble water was prepared. After the equipment was circulated for 20 minutes, the water sample was collected. The bubble size, concentration and potential of the prepared bubbles were tested by Particle Metrix ZetaView nanoparticle tracking analyzer. The measured bubble median diameter was 102.7 nm, the concentration was 1.9×108 / mL, and the Zeta potential was -16.6 mV.

[0038] 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:

[0039] 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 gas source of the nanobubble generator 4 is CO2 source, the recharge pump 6 is connected with the contaminated area of the chlorinated groundwater through the recharge pipe; the contaminated area in this embodiment is constructed in the laboratory as 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.

[0040] In this embodiment, the dosing mixing tank 1 is connected with the water source, and the water source added in the dosing mixing tank 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 nanobubble generator 4 is controlled at a surface pressure of 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 tank 1, which uniformly mixes the added nZVI reagent, H2O2 solution and EDDS solution with water; the mixing device 3 is a magnetic stirrer.

[0041] Step two, the nZVI reagent is added in the dosing mixing tank 1 in the first stage; the CO2 nanobubbles are continuously released to make the pH value of the groundwater environment gradually decrease to the weak acid interval, continuously strip the passivation layer on the surface of the nZVI particles and promote the anodic dissolution of Fe 0 . Among them, the molar ratio of nZVI to TCE is controlled between 10:1 and 100:1.

[0042] The first stage is started when the site is confirmed to contain chlorine-containing target pollutants and the background pH value is neutral or alkaline. 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 recharging site at 5.5.

[0043] In this embodiment, nZVI reagent, i.e. nano zero-valent iron reagent, is added to the dosing mixing pool 1. After mixing in the dosing mixing pool 1, nZVI particles-CO2 nanobubbles are generated by the nanobubble generator 4 and then enter the CO2 bubble mixing pool 5, and then enter the contaminated area through the recharging pump 6.

[0044] 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 nanobubbles. The surface charge state is adjusted due to the negative charge of the nanobubbles, and the van der Waals attraction between the particles is partially offset, which promotes the suspension and dispersion of the particles. Under the continuous 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.

[0045] In this embodiment, after recharging into the contaminated area, CO2 nanobubbles are continuously released and dissolved in the water body to generate carbonic acid and bicarbonate ions, so that the local environmental pH value gradually decreases to the weakly acidic range, thereby continuously stripping the passivation layer on the surface of the nZVI particles and promoting the anodic dissolution of Fe 0 .

[0046] Step three, after 60 minutes of reaction, the contaminated area is rich in high-activity Fe 2+ converted from nZVI; when the reduction rate in the first stage is monitored to slow down and the dissolved Fe 2+ concentration reaches a peak value sufficient to catalyze the reaction, H2O2 solution is added to the dosing mixing pool 1 to enter the second stage.

[0047] 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 maintains the pH value at 5.5 by forming carbonic acid in the reaction microenvironment, avoiding the deactivation of iron ions due to precipitation; the Fenton reaction is stably and continuously carried out until the pollutants are completely removed or the designed removal standard is reached.

[0048] 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 a micro-turbulent effect, thereby increasing the generation rate and utilization efficiency of hydroxyl radicals.

[0049] Step five, after the completion of the second stage reaction, immediately start the third stage, add EDDS solution in the dosing mixing pool 1; CO2 nanobubbles will maintain the pH value of groundwater environment in the weak acidic range, effectively inhibit the Fe 3 + hydrolysis and maintain the soluble state of Fe 3+ ; through EDDS solution injection, Fe 3+ is removed in the form of soluble complex, thereby completing the multi-stage repair of chlorinated groundwater pollution.

[0050] In this embodiment, when 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, CO2 nanobubbles continue to be injected to maintain the pH value of the groundwater environment at 5.5.

[0051] To analyze the dynamic changes of the repair process, liquid samples were collected and filtered every 2 h during the reaction process. The obtained samples were first monitored for the dynamic changes of Fe 2+ concentration within 2 h using 1,10-phenanthroline method and Dionex ICS-1000 ion chromatograph, to characterize the activation leaching efficiency of nZVI and confirm that the concentration reaches the peak value to start the Fenton reaction. At the same time, Agilent 5977B gas chromatograph-mass spectrometer (equipped with HP-Plot-Q chromatographic column) was used to detect the TCE concentration, with the injection port temperature set to 250 ℃, and the oven temperature program as follows: initial temperature 40 ℃, holding for 0.5 min, increasing to 50 ℃ at 5 ℃ / min, then increasing to 65 ℃ at 15 ℃ / min, followed by increasing to 72 ℃ at 2 ℃ / min, and finally increasing to 230 ℃ at 30 ℃ / min and holding for 10 min. Further, Dionex ICS-1000 ion chromatograph was used to determine the Cl - concentration to comprehensively evaluate the removal efficiency of TCE and the total dechlorination effect. In addition, after injecting EDDS, the change of solution absorbance with time was also continuously monitored to evaluate the inhibitory effect of this step on Fe 3+ precipitation and the anti-blocking performance.

[0052] To systematically evaluate the key role of CO2 nanobubbles in the whole process of nZVI activation, pollutant degradation and iron ion stabilization, this experiment constructed blank system, H2O2 system, DI water (deionized water) system, DI water-nZVI system, ordinary CO2 bubble water-nZVI system, ordinary CO2 bubble water-H2O2 system, CO2 nanobubble water-nZVI system and CO2 nanobubble water-H2O2 system for comparison in different stages. Figures 2 to 5Fe 2+ leaching; TCE removal, dechlorination efficiency and plugging inhibition were compared by blank, H2O2, CO2-bubbled water-H2O2 and CO2-nanobubble water-H2O2 systems.

[0053] As Figure 2 shown, Fe 2+ leaching in four reaction systems 2+ The results showed that Fe 2+ concentration remained almost unchanged in DI water system, indicating that Fe 2+ mainly came from the reaction leaching of nZVI. In the DI water-nZVI system alone, Fe 2+ concentration increased slowly, indicating that passivation easily occurred on the surface, thus limiting the reactivity. After adding CO2 bubbles, the Fe 2+ generation rate increased in the CO2-bubbled water-nZVI system, indicating that the acidic environment produced by CO2 dissolution could promote the dissolution of the nZVI surface oxide film, thereby enhancing its reactivity. In contrast, the CO2-nanobubble water-nZVI system showed the highest Fe 2+ concentration and the fastest rising rate, indicating that nanobubbles had a more significant strengthening effect. This effect was attributed to the larger specific surface area and higher gas-liquid mass transfer efficiency of nanobubbles, which could continuously release CO2 and maintain a stable acidic environment, effectively inhibiting the formation of a passivation layer, thereby prolonging the effective life of nZVI. The comprehensive results showed that CO2 nanobubbles could 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.

[0054] Figure 2 clearly revealed that CO2 nanobubbles had a promoting effect on the activation of nano zero-valent iron. First, in the DI water without nZVI and the DI water with only nZVI, the Fe 2+ concentration was almost zero throughout the 120-minute reaction time, which strongly proved that the corrosion and dissolution process of nZVI itself was very slow. When CO2 bubbles were introduced into the system, the Fe 2+ concentration began to increase significantly and stabilized at about 20 μM after about 80 minutes. The most critical comparison was that the CO2-nanobubble water-nZVI experimental group showed the best performance. Its Fe 2+generated at the fastest rate and the highest final concentration, which was stabilized at above 40 μM in the late reaction period, approximately twice that of the common CO2 bubble group. This result strongly proves that, compared with common CO2 bubbles, CO2 nanobubbles can more efficiently construct and maintain a weakly acidic microenvironment around nZVI particles by virtue of their large specific surface area and high stability, thereby continuously and deeply activating nZVI, greatly accelerating the transformation of high-activity Fe 0 to high-activity Fe 2+ , providing sufficient catalysts for subsequent Fenton reaction and other steps.

[0055] To verify the effect of the method described in the application in the advanced oxidation stage, comparative experiments were conducted on the degradation effect of TCE in different systems after step four was performed for 2 h, and the results are shown in Figure 3 The data in the figure clearly show that the TCE concentration of the blank control system did not change at all during the entire reaction period, confirming its chemical stability. Although the system with only H2O2 added can initiate a certain degree of degradation, the reaction rate is extremely slow, and about 20% of TCE remains after 24 hours, indicating that the oxidation efficiency of H2O2 is limited in the absence of effective catalysis and a suitable pH environment. In the common CO2 bubble water-H2O2 system, the common CO2 bubble assisted H2O2 system activates the Fenton reaction through an acidified environment, and the degradation rate is significantly improved, but it still takes about 22 hours to achieve complete removal of TCE, which reflects the limitations of traditional aeration methods in mass transfer efficiency and pH regulation accuracy. In sharp contrast to the above-mentioned comparison groups, the CO2 nanobubble-H2O2 system used in the application achieves complete degradation of TCE within 10 hours, with a reaction rate more than doubled compared to the common CO2 bubble system. The above results fully prove that CO2 nanobubbles, by virtue of their large specific surface area and efficient mass transfer characteristics, can quickly, uniformly and accurately adjust and stabilize the pH value of the reaction microenvironment to the optimal range of the Fenton reaction, thereby maximizing the catalytic activity of Fe 2+ .

[0056] After step four was performed for 8 h, comparative experiments were conducted on the dechlorination effect in different systems. According to the results of the change curve of Cl Figure 4 concentration with reaction time shown in - , no Cl - was generated in the blank control system, which was consistent with expectations. The Cl - generation rate of the H2O2 control system alone was slow, and the cumulative concentration was only about 22 μM within 24 hours, indicating that its dechlorination ability was limited. In the common CO2 bubble-H2O2 control system, the Fenton reaction was promoted through an acidified environment, and the Cl -The generation rate was significantly accelerated, and the final concentration reached about 24 μM, which confirmed the importance of pH regulation on the dechlorination reaction, but also reflected that there was still optimization space in the reaction kinetics of the conventional aeration mode. In sharp contrast to the above-mentioned system, the CO2 nanobubble-H2O2 system used in the present application showed higher dechlorination efficiency. - The concentration rapidly increased within 10 hours of the start of the reaction and reached a plateau of about 30 μM, with both the initial reaction rate and the final product generation amount being significantly higher than all the comparison groups. This result strongly proves that CO2 nanobubbles greatly promote the generation of highly oxidizing hydroxyl radicals by their high mass transfer capacity and precise construction and maintenance of the optimal pH microenvironment for the Fenton reaction, thereby achieving rapid and complete stripping of the chlorine atoms in the TCE molecule. This not only verifies that TCE is effectively degraded, but also highlights the fundamental technical advantage of the method in achieving deep mineralization of pollutants.

[0057] To confirm the key effectiveness of the present application in inhibiting iron ion precipitation and preventing physical plugging, the formation of Fe(OH)3 precipitate in different systems was evaluated by monitoring the change in solution absorbance. As shown in Figure 5 The absorbance of the blank control system increased significantly with time, eventually reaching nearly 0.8, which indicates that in the absence of pH regulation, the iron ions in the system will hydrolyze to form Fe(OH)3 precipitate, causing a serious plugging risk. The ordinary CO2 bubble-H2O2 system and the H2O2 system alone can slow down the precipitation to some extent, but the absorbance still continues to grow steadily, showing that their ability to inhibit plugging is limited. In sharp contrast to all the above-mentioned comparison groups, the CO2 nanobubble-H2O2 system used in the present application successfully inhibited the solution absorbance to a very low level below 0.15 throughout the monitoring period, and the growth trend was almost stagnant. This decisive result strongly proves that CO2 nanobubbles can construct and long-term maintain a stable weakly acidic microenvironment, effectively inhibiting the hydrolysis reaction of Fe 3+

[0058] ​The method of the present application is suitable for groundwater pollution caused by high-concentration, aged chlorinated hydrocarbons such as TCE, PCE and CTC in the fields of chemical industry, pesticide, electronic manufacturing and dry cleaning. It is also suitable for complex scenarios in which pollutants requiring reduction treatment and pollutants requiring oxidation treatment coexist in the groundwater of industrial parks, waste landfill sites and other areas. It is suitable for low-permeability aquifers such as clay and silt, or areas that are highly sensitive to changes in hydrogeological conditions due to geological structure, engineering needs and other reasons. It is suitable for the remediation and redevelopment of historical pollution sites such as old chemical plant sites and decommissioned tank farms. The present technology not only reduces the concentration of pollutants to below the risk screening value required for land redevelopment, but also ensures the long-term stability of the remediation effect and the absence of adverse effects due to its characteristics of complete degradation of pollutants and active removal of reaction byproducts, effectively avoiding long-term risks such as pollution "rebound" and deterioration of site permeability, thereby providing reliable technical support for the safe reuse of land. The method of the present application is also suitable for acute, high-concentration chlorine-containing organic pollutant leakage events caused by tank leaks, transportation accidents and other reasons, and can quickly respond to reduce the concentration of the main pollutants significantly in a short period of time to control the further spread of the pollution plume; then through the deep oxidation and risk removal mechanism, it realizes the rapid and complete purification of the affected area, which is a powerful technical choice for emergency remediation and response.

[0059] The above merely describes the preferred embodiments of the present application, but the protection scope of the present application is not limited thereto, and any changes or substitutions within the technical scope disclosed by the present application should be covered within the protection scope of the present application.

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 value of the groundwater environment to the weakly acidic range, continuously stripping the passivation layer on the surface of the nZVI particles and promoting Fe... 0 Anodic dissolution; 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. In addition, the process will be initiated before the first phase begins, provided that the site contains chlorine-containing target pollutants and the background pH is neutral or slightly alkaline. 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 inducing microturbulence effect, thereby increasing the generation rate and utilization efficiency of hydroxyl radicals and avoiding 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; continuous injection of CO2 nanobubbles stabilizes the pH of the groundwater environment in the weakly acidic range, effectively inhibiting the Fe generated 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, 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.

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, 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.

6. The method for multi-stage remediation of chlorinated groundwater pollution based on carbon dioxide nanobubbles as described in claim 5, 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.

7. The method for multi-stage remediation of chlorinated groundwater pollution based on carbon dioxide nanobubbles as described in claim 6, 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 injection is carried out in batches at low flow rates.

8. The method for multi-stage remediation of chlorinated groundwater pollution based on carbon dioxide nanobubbles as described in claim 7, 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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