In-situ remediation method for underground water polluted by chlorinated organic compounds

Abstract

The invention discloses an in-situ remediation method for underground water polluted by chlorinated organics, which comprises the following steps: injecting a mixed solution containing a modified charcoal loaded nano zero-valent iron composite material, a buffer agent and a stabilizer into the underground water polluted by the chlorinated organics, arranging a cathode at the injection point of the mixed solution, and arranging an anode in the diffusion direction of pollution plumes, electrifying to form an electric field in the groundwater polluted area, and repairing groundwater in situ; the preparation method of the modified charcoal loaded nano zero-valent iron composite material comprises the following steps: S1, mixing a crushed biomass material, a modifier, ferrous sulfate and copper sulfate; s2, uniformly mixing the mixture obtained in the step S1 with water, and drying to constant weight; and S3, crushing the mixture obtained by drying in the step S2, performing anaerobic or anoxic atmosphere pyrolysis, and crushing again after the pyrolysis is completed, so as to obtain the modified charcoal loaded nano zero-valent iron composite material.

Description

Technical Field

[0001] This invention relates to the field of groundwater pollution remediation technology, specifically to a method for in-situ remediation of chlorinated organic pollutant-contaminated groundwater using an electric field-enhanced modified biochar-supported nano-zero-valent iron composite material. Background Technology

[0002] In industrial production processes, chlorinated organic solvents (such as trichloroethylene, tetrachloroethylene, 1,1,1-trichloroethane, and chloroform) are widely used in various fields, including metal degreasing, electronic cleaning, dry cleaning, pharmaceuticals, and chemical synthesis, due to their excellent solubility, chemical stability, and low cost. However, these solvents are generally highly toxic, difficult to biodegrade, and potentially carcinogenic. Once they enter the environment due to tank corrosion, pipeline leaks, operational errors, or illegal discharges, they can easily penetrate the vadose zone of the soil and seep into aquifers, causing widespread and deep-seated groundwater pollution. Because groundwater flows slowly and has weak self-purification capabilities, chlorinated organic pollutants can remain in the underground environment for a long time and continuously expand the pollution plume through migration and diffusion. This not only seriously threatens the safety of regional drinking water sources but may also indirectly harm human health through bioaccumulation in the food chain, posing a significant environmental risk.

[0003] For the remediation of groundwater contaminated with chlorinated organic compounds, in-situ chemical reduction technology has become one of the mainstream remediation strategies internationally because it can directly reduce and dechlorinate highly toxic chlorinated compounds into low-toxicity or even non-toxic small molecule products (such as ethylene and ethane), thus achieving complete mineralization or harmlessness of pollutants.

[0004] For example, the patent specification with publication number CN115636460A discloses a method for in-situ remediation of heavy metal-organic composite polluted groundwater using an electrochemically enhanced permeable reactive wall, which includes the following steps: (1) setting up a permeable reactive wall in the polluted groundwater area, with the reactive wall set vertically along the water flow direction, and anode and cathode set at the upper and lower ends of the reactive wall respectively; (2) filling the permeable reactive wall with three-dimensional electrode function with zero-valent iron modified carbon-based material particles as particle electrode materials; (3) applying an electric field at both ends of the reactive wall, and constructing an iron-carbon metal micro battery under the action of the electric field; (4) removing heavy metals in the groundwater through surface / internal adsorption, reduction and co-precipitation, and degrading halogenated hydrocarbon organics through dehalogenation reaction, thereby achieving in-situ remediation of composite polluted groundwater.

[0005] Nano-zero valent iron (nZVI) is considered the most promising reducing agent due to its extremely high specific surface area, strong reduction potential and good reactivity. It has shown significant dechlorination effects in laboratory research and some demonstration projects.

[0006] However, nZVI still faces significant challenges in practical applications: on the one hand, its surface readily reacts with dissolved oxygen and water molecules in groundwater, rapidly forming an iron oxide passivation layer, leading to a rapid decline in its reducing activity; on the other hand, due to their small size and high surface energy, nanoparticles easily aggregate in the aqueous phase, forming micron-sized aggregates. This not only significantly reduces the effective reaction interface but also severely limits their migration ability in porous media, making it difficult to distribute them evenly to the contaminated area, thus significantly weakening the overall remediation efficiency. Furthermore, the complex and variable groundwater environment, including pH, ionic strength, natural organic matter, and coexisting pollutants, can further exacerbate the deactivation and sedimentation of nZVI. Therefore, maintaining the reducing activity and migration of remediation materials in real aquifers while ensuring cost control is crucial for achieving efficient remediation of chlorinated organic contaminated groundwater. Summary of the Invention

[0007] To address the aforementioned technical problems and shortcomings in this field, the present invention provides an in-situ remediation method for groundwater contaminated with chlorinated organic compounds, which can solve the problem that existing zero-valent iron materials are difficult to maintain reducibility and mobility in the long term during in-situ remediation of groundwater contaminated with chlorinated organic compounds.

[0008] The specific technical solution is as follows: An in-situ remediation method for groundwater contaminated with chlorinated organic compounds includes: injecting a mixture containing modified biochar-supported nano-zero-valent iron composite material, buffer, and stabilizer into the groundwater contaminated with chlorinated organic compounds; setting a cathode at the injection point of the mixture; setting an anode in the direction of pollution plume diffusion; and applying electricity to form an electric field in the groundwater contaminated area to remediate the groundwater in situ. The preparation method of the modified biochar-supported nano-zero-valent iron composite material includes the following steps: S1, mixing crushed biomass material, modifier, ferrous sulfate and copper sulfate; the modifier includes one or more of carboxymethyl cellulose, sodium alginate, chitosan and guar gum; the mass ratio of biomass material to ferrous sulfate is 1:1.0~1.5; the mass ratio of biomass material to copper sulfate is 1:0.02~0.03; S2, after uniformly mixing the mixture obtained in step S1 with water, dry it to constant weight; S3, after the mixture obtained from drying in step S2 is crushed, it is subjected to pyrolysis in an oxygen-free or oxygen-deficient atmosphere. After the pyrolysis is completed, it is crushed again to obtain the modified biochar-supported nano-zero-valent iron composite material.

[0009] The present invention places the cathode near the drug injection point (high concentration pollution point) and the anode downstream of the pollution plume to help enhance the reduction effect at the high pollution point, and accelerates the migration of nano zero-valent iron particles downstream through electrophoresis.

[0010] This invention uses a one-step pyrolysis method to synthesize modified biochar-supported nano-zero-valent iron composite material, which has good reducibility and dispersibility. The composite material is injected into groundwater, and an electric field is applied to the injection area and its surroundings. The electric field enhances the material's reduction ability and improves its expansion performance, thereby achieving efficient remediation of groundwater contaminated with chlorinated organics.

[0011] This invention adds a certain amount of copper sulfate to ferrous sulfate. By introducing a second metallic copper, it serves two purposes: first, by depositing it on the iron surface to modify it and reduce the formation of agglomerates; and second, by forming a microcouple with the iron to promote electron transfer and inhibit the oxidation of iron.

[0012] The buffer preferably includes one or more of phosphates (e.g., sodium phosphate), pyrophosphates, and bicarbonates (e.g., sodium bicarbonate). The concentration of the buffer in the mixture is preferably 15-30 mM. The purpose of adding the buffer in this invention is to provide a suitable pH environment for the nano-zero-valent iron reaction, while a higher addition amount provides necessary buffering for cathodic alkalization caused by subsequent electric field application.

[0013] The stabilizer preferably includes one or more of carboxymethyl cellulose, chitosan, and humic acid. Adding a certain amount of stabilizer can further reduce agglomeration and enhance the migration ability of the composite material. The concentration of the stabilizer in the mixture is preferably 1-2 g / L.

[0014] In the mixture, the concentration of the modified biochar-supported nano-zero-valent iron composite material is preferably 15~25 g / L.

[0015] The cathode is preferably a graphite electrode.

[0016] The anode is preferably a graphite electrode or a titanium electrode with an iridium tantalum oxide coating.

[0017] The electric field strength is preferably 0.75~1.75V / cm.

[0018] The electric field is preferably generated under a constant voltage.

[0019] The electric field is preferably operated via intermittent power supply. Furthermore, the intermittent power supply is preferably a cycle of 6-12 hours of power on and 6-12 hours of power off.

[0020] The present invention employs a constant voltage + intermittent power supply method, which helps to avoid water electrolysis caused by local overpotential and continuous power supply, thereby reducing energy consumption and extending electrode life.

[0021] During the implementation of the in-situ remediation method of the present invention, the groundwater quality can be tested. If the remediation target is not achieved, the mixture is injected again and an electric field is continuously applied until the remediation target is achieved.

[0022] In step S1, the biomass material preferably includes one or more of the following: sawdust, coconut shell, walnut shell, and lotus root. Sawdust, coconut shell, walnut shell, and lotus root are chosen as biomass materials because they all possess high carbon content and high porosity. The biochar produced after pyrolysis of these raw materials has fewer impurities and a purer structure, making it suitable as a carrier for loading nano-zero-valent iron. By loading nano-zero-valent iron onto the biochar formed from the above biomass materials, the aggregation of nano-zero-valent iron will be significantly inhibited, its passivation will be delayed, and the migration of the composite material in groundwater will be enhanced.

[0023] In step S1, the particle size of the crushed biomass material is preferably 0.15~0.3mm.

[0024] The modifier in step S1 helps to promote uniform loading and anchoring of active components on the biochar surface and prevents agglomeration.

[0025] In step S1, the preferred mass ratio of the biomass material to the modifier is 1:0.4~0.6.

[0026] In step S2, the mass ratio of the mixture obtained in step S1 to water is preferably 1:10~15.

[0027] In step S2, the drying temperature is preferably 70~80℃.

[0028] In step S3, it is preferable to crush the mixture obtained by drying in step S2 to a particle size of 0.15~0.3mm and then perform pyrolysis in an oxygen-free or oxygen-deficient atmosphere.

[0029] In step S3, the preferred pyrolysis temperature is 800-900℃. Compared with traditional biochar pyrolysis, the present invention selects a higher pyrolysis temperature for two reasons: firstly, it helps to achieve a higher proportion of iron reduction; secondly, high-temperature pyrolysis promotes the high condensation of the aromatic structure of biochar, forming graphite-like microcrystals, which significantly improves electronic conductivity and promotes the reduction and degradation of chlorinated hydrocarbons by nano-zero valent iron.

[0030] In step S3, the pyrolysis time is preferably 2 to 3 hours.

[0031] In step S3, the particle size after pyrolysis and subsequent crushing is preferably 0.03~0.05mm. By crushing the material to a finer particle size after pyrolysis, the internal active sites can be further exposed, thereby improving the reaction efficiency. At the same time, the finer micron-sized particles also help to improve the diffusion performance of the material in groundwater.

[0032] This invention can solve the problems of easy aggregation, low reactivity, short lifespan, and poor stability of composite materials in complex groundwater environments of existing technologies, and achieve long-term, efficient, and green remediation of chlorinated organics.

[0033] Compared with the prior art, the beneficial effects of this invention are as follows: This invention utilizes a modified biochar-supported nano-zero-valent iron composite material with high activity and good migration properties to achieve effective remediation of chlorinated organic pollutants. Since agglomeration is a major limiting factor for the remediation effectiveness of nano-zero-valent iron materials in practical remediation projects, this invention focuses on improving this defect in material preparation and use. For example, it employs biochar support, adds bimetallic compounds, and injects the material with stabilizers, significantly improving its migration performance. This invention combines in-situ reduction remediation technology with electrokinetic remediation. The electric field enhances the migration and reduction capabilities of the modified biochar-supported nano-zero-valent iron. The high conductivity of the modified biochar-supported nano-zero-valent iron also results in a more uniform electric field distribution, optimizing the electric field distribution and enabling efficient remediation of chlorinated hydrocarbon-contaminated groundwater, demonstrating significant application value. Detailed Implementation

[0034] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Operating methods not specifically specified in the following embodiments are generally performed under conventional conditions or as recommended by the manufacturer.

[0035] Example 1: Lotus root was dried and then crushed to 0.15 mm. 10 g of the crushed lotus root, 4 g of carboxymethyl cellulose, 10 g of ferrous sulfate, and 0.2 g of copper sulfate were mixed together, and 250 mL of water was added. The mixture was stirred in a magnetic stirrer for 2 hours until fully combined, and then dried in an oven at 70°C until constant weight. The dried mixture was crushed to 0.15 mm and then placed in a muffle furnace for pyrolysis in an oxygen-free atmosphere at 800°C for 3 hours. The pyrolyzed mixture was crushed again to 0.05 mm to obtain 7.95 g of modified biochar-supported nano-zero-valent iron composite material.

[0036] A groundwater simulation tank experiment was conducted. The experimental group used a simulation tank with a length of 30cm, a width of 5cm, and a height of 20cm, filled with quartz sand. An inlet was placed at the lower part of one end of the tank, and an outlet at the upper part of the other end. A graphite electrode was inserted 5cm from the inlet as the cathode, and another graphite electrode was inserted 5cm from the outlet as the anode. Simulated contaminated groundwater containing 6mg / L chlorobenzene and 2g / L sodium chloride was accurately prepared, with the groundwater inflow rate set to 2.5mL / min, ensuring the entire device was filled with contaminated groundwater. 0.3g of modified biochar-supported nano-zero-valent iron composite material, 0.3mmol of sodium phosphate, and 0.02g of carboxymethyl cellulose were accurately weighed and added to 20mL of water, stirred thoroughly to obtain the prepared remediation agent. The remediation agent was injected into the vicinity of the cathode position of the simulation device using a syringe, and an electric field was applied to start operation. The distance between the two electrodes was 20cm, and a constant voltage of 35V was applied, with the device switching on and off alternately every 6 hours.

[0037] In addition, the experiment set up two control groups. Control group 1 was injected with the repair agent but not with electric field enhancement, and control group 2 was only given an electric field but not injected with the agent. All other aspects were the same as the experimental group.

[0038] The chlorobenzene concentrations in the effluent from the control and experimental groups are shown in Table 1. The changes in effluent chlorobenzene concentration show that in control group 1, the effluent chlorobenzene concentration gradually decreased over time, reaching a peak removal rate of 66.17% at 14 hours, but then rebounded, indicating that the removal effect of the remediation agent on chlorobenzene was weakening. In control group 2, the effluent chlorobenzene concentration stabilized around 8 hours, and its removal rate fluctuated between 65% and 75% thereafter. In the experimental group, the chlorobenzene concentration decreased rapidly after the start of operation, reaching a removal rate of over 90% within 10 hours, and maintaining a removal rate of over 95% for 20-24 hours. This indicates that the addition of the remediation agent and the synergistic enhancement of the electric field can produce better remediation effects on chlorobenzene-contaminated groundwater.

[0039] Table 1 Example 2: Walnut shells were dried and crushed to 0.3 mm. 10 g of the crushed walnut shells, 6 g of guar gum, 15 g of ferrous sulfate, and 0.3 g of copper sulfate were mixed together, and 450 mL of water was added. The mixture was stirred in a magnetic stirrer for 2 hours until fully combined, then dried in an oven at 80°C until constant weight. The dried mixture was crushed to 0.3 mm and then pyrolyzed in a muffle furnace at 900°C for 2 hours. The pyrolyzed mixture was crushed again to 0.03 mm to obtain 8.92 g of modified biochar-supported nano-zero-valent iron composite material.

[0040] A groundwater simulation tank experiment was conducted. The experimental group used a simulation tank 30cm long, 5cm wide, and 20cm high, filled with quartz sand. An inlet was placed at the lower part of one end of the tank, and an outlet at the upper part of the other end. A graphite electrode was inserted 5cm from the inlet as the cathode, and an iridium-tantalum oxide-coated titanium electrode was inserted 5cm from the outlet as the anode. Simulated contaminated groundwater containing 2mg / L trichloroethylene and 2g / L sodium chloride was accurately prepared, with the groundwater inflow rate set to 2.5mL / min, ensuring the entire device was filled with contaminated groundwater. 0.5g of biochar-supported nano-zero-valent iron composite material, 0.6mmol of sodium bicarbonate, and 0.02g of humic acid were accurately weighed and added to 20mL of water, stirred thoroughly to obtain the prepared remediation agent. The entire remediation agent was injected into the vicinity of the cathode position of the simulation device using a syringe. Simultaneously, an electric field was applied to start operation, with the distance between the two electrodes 20cm and a constant voltage of 15V applied. The device was switched on and off alternately every 12 hours.

[0041] In addition, the experiment set up two control groups. Control group 1 was injected with the repair agent but not with electric field enhancement, and control group 2 was only given an electric field but not injected with the agent. All other aspects were the same as the experimental group.

[0042] The trichloroethylene concentrations in the effluents of the control and experimental groups are shown in Table 2. The changes in trichloroethylene concentration in the effluent showed that the trichloroethylene concentration in the control group gradually decreased over time, reaching a maximum removal rate of 73.45% at 20 hours, but then rebounded, indicating that the removal effect of the remediation agent on trichloroethylene was weakening. The chlorobenzene concentration in the control group stabilized around 12 hours, and its removal rate fluctuated between 45% and 50% thereafter. In the experimental group, the trichloroethylene concentration decreased rapidly after the start of operation, reaching a removal rate of over 90% within 10 hours, and maintaining a removal rate of over 94% from 12 to 24 hours. This indicates that the addition of the remediation agent and the synergistic enhancement of the electric field can produce a better remediation effect on trichloroethylene-contaminated groundwater.

[0043] Table 2 Furthermore, it should be understood that after reading the above description of the present invention, those skilled in the art can make various alterations or modifications to the present invention, and these equivalent forms also fall within the scope defined by the appended claims.

Claims

1. An in-situ remediation method for groundwater contaminated with chlorinated organic compounds, characterized in that, include: A mixture containing modified biochar-supported nano-zero-valent iron composite material, buffer, and stabilizer is injected into groundwater contaminated with chlorinated organics. A cathode is placed at the injection point of the mixture, and an anode is placed in the direction of the pollution plume diffusion. Electricity is applied to create an electric field in the groundwater contaminated area, thereby remediating the groundwater in situ. The preparation method of the modified biochar-supported nano-zero-valent iron composite material includes the following steps: S1, mixing crushed biomass material, modifier, ferrous sulfate and copper sulfate; the modifier includes one or more of carboxymethyl cellulose, sodium alginate, chitosan and guar gum; the mass ratio of biomass material to ferrous sulfate is 1:1.0~1.5; the mass ratio of biomass material to copper sulfate is 1:0.02~0.03; S2, after uniformly mixing the mixture obtained in step S1 with water, dry it to constant weight; S3, after the mixture obtained from drying in step S2 is crushed, it is subjected to pyrolysis in an oxygen-free or oxygen-deficient atmosphere. After the pyrolysis is completed, it is crushed again to obtain the modified biochar-supported nano-zero-valent iron composite material.

2. The in-situ remediation method for groundwater contaminated with chlorinated organic compounds according to claim 1, characterized in that, The buffer includes one or more of phosphates, pyrophosphates, and bicarbonates.

3. The in-situ remediation method for groundwater contaminated with chlorinated organic compounds according to claim 1, characterized in that, The stabilizer includes one or more of carboxymethyl cellulose, chitosan, and humic acid.

4. The in-situ remediation method for groundwater contaminated with chlorinated organic compounds according to claim 1, characterized in that, In the mixture, the concentration of the modified biochar-supported nano-zero-valent iron composite material is 15~25 g / L, the concentration of the buffer is 15~30 mM, and the concentration of the stabilizer is 1~2 g / L.

5. The in-situ remediation method for groundwater contaminated with chlorinated organic compounds according to claim 1, characterized in that, The cathode is a graphite electrode; The anode is a graphite electrode or a titanium electrode with an iridium tantalum oxide coating. The electric field strength is 0.75~1.75V / cm; The electric field is generated under a constant voltage; The electric field operates by intermittent power supply; The intermittent power supply consists of a cycle of 6-12 hours of power on and 6-12 hours of power off.

6. The in-situ remediation method for groundwater contaminated with chlorinated organic compounds according to claim 1, characterized in that, In step S1: The biomass materials include one or more of the following: sawdust, coconut shells, walnut shells, and lotus root. The particle size of the crushed biomass material is 0.15~0.3mm.

7. The in-situ remediation method for groundwater contaminated with chlorinated organic compounds according to claim 1, characterized in that, In step S1, the mass ratio of the biomass material to the modifier is 1:0.4~0.

6.

8. The in-situ remediation method for groundwater contaminated with chlorinated organic compounds according to claim 1, characterized in that, In step S2: The mass ratio of the mixture obtained in step S1 to water is 1:10~15; The drying temperature is 70~80℃.

9. The in-situ remediation method for groundwater contaminated with chlorinated organic compounds according to claim 1, characterized in that, In step S3: The mixture obtained from drying in step S2 is crushed to a particle size of 0.15~0.3mm and then subjected to pyrolysis in an oxygen-free or oxygen-deficient atmosphere. The pyrolysis temperature is 800~900℃; The pyrolysis time is 2-3 hours; The particle size after pyrolysis and subsequent crushing is 0.03~0.05mm.

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