Improved groundwater remediation agent and preparation method thereof

Abstract

The invention discloses an improved groundwater remediation agent and a preparation method thereof. The groundwater remediation agent is granular, and each particle is of a multi-layer structure; wherein the outer layer of the particle is coated nano zero-valent iron, the middle layer of the particle is an isolating layer, and the inner layer of the particle is crystalline or amorphous sodium percarbonate. The improved remediation agent disclosed by the invention has a slow release property, the chlorohydrocarbon is firstly dechlorinated by the zero-valent iron aiming at underground water polluted by the chlorohydrocarbon, and the hydrogen peroxide released by the sodium percarbonate completes oxidation elimination on the chlorohydrocarbon and an intermediate product under the catalysis of ferrous iron formed by dechlorination along with the proceeding of the process.

Description

Technical Field

[0001] The present invention relates to the technical field of groundwater remediation, and in particular to an improved groundwater remediation agent and a preparation method thereof. Background Art

[0002] Chlorinated solvents, due to their excellent thermal and chemical stability, were once used in industry and everyday production, such as solvents, dry-cleaning agents, heat transfer fluids, and flame retardants. The widespread use of chlorinated hydrocarbons has created opportunities for them to enter groundwater as pollutants. Because they are difficult to rapidly degrade through natural processes and have a relative density greater than that of water, they tend to migrate vertically in groundwater and accumulate in impermeable layers. This can lead to long-term release and persistent pollution, posing a significant challenge to groundwater management.

[0003] Reduction and chemical oxidation are two important remediation pathways for chlorinated hydrocarbons in groundwater. Nanoscale zero-valent iron (nZVI) is often used in the remediation of chlorinated hydrocarbon contamination due to its high electron capacity and strong reducing properties. Commonly used agents for oxidizing chlorinated hydrocarbons in groundwater include ozone, sodium persulfate, permanganate, ferrate, sodium percarbonate, and hydrogen peroxide. Sodium percarbonate is a promising oxidizing agent because it provides a relatively long-lasting release of hydrogen peroxide.

[0004] Because nano-zero-valent iron is prone to aggregation and oxidative inactivation, researchers have adopted a variety of technical measures. Chinese patent application CN202211498643.4 encapsulates nano-zero-valent iron in oil droplets of vegetable oil emulsion to increase the reaction activity between pollutants and nano-zero-valent iron; Chinese patent application CN202310737818.0 synthesizes cyclodextrin polymers from cyclodextrin monomers, and then fixes nano-zero-valent iron and trace copper in the cyclodextrin polymer network, thereby slowing down the aggregation and passivation of zero-valent iron; Chinese patent application CN201710232480.8 coats the surface of nano-zero-valent iron with magnesium hydroxide nanoparticles Improve its release of reaction activity; Chinese patent application CN202311108797.2 overcomes the passivation of internal zero-valent iron through sulfide nano-zero-valent iron with a core-shell structure; Chinese patent application CN202311367057.0 uses metal sulfide and citrate for modification, and uses carbon material for loading to improve the dispersibility and sustained-release ability of nano-zero-valent iron; Chinese patent application CN201611062038.7 uses candles to wrap the zero-valent iron activator to achieve a sustained-release effect to activate persulfate.

[0005] Researchers have also done a lot of research and improvements on the use of sodium percarbonate as a chlorinated hydrocarbon oxidizing agent. Chinese patent application CN201910537639.6 discloses a method for removing chlorinated hydrocarbons by adding FeSO4·7H2O, nano-zero-valent iron and sodium percarbonate to groundwater; Chinese patent application CN202010685979.6 uses natural polyphenols and soluble ferrous salts to activate percarbonate to degrade chlorinated hydrocarbons, greatly improving the degradation rate; Chinese patent application CN202211120844.0 uses porous materials to pre-adsorb oxidants as core materials, and sodium alginate, polyacrylamide, ferric chloride, aluminum chloride, calcium chloride, etc. as slow-release oxidants for the outer shell for chlorinated hydrocarbon remediation; Chinese patent application CN202411839574.8 uses fly ash-A1-MOF loaded with sodium percarbonate and fly ash-A1-MOF loaded with acid-containing ferrous sulfate, and uses them in combination for organic pollution remediation; 202411838694.6 discloses a method for repairing petroleum hydrocarbons with sodium percarbonate catalyzed by zeolite-magnetic iron oxide.

[0006] The above-mentioned patents disclose a large number of methods for improving nano zero-valent iron and repairing with sodium percarbonate, mainly targeting single reduction or oxidation systems; Chinese patent application CN201910283513.0 mixes percarbonate, paraffin, iron-based activator and chelating agent to form a sustained-release system, and uses paraffin as a sustained-release agent to combine nano zero-valent iron and sodium percarbonate to make a sustained-release system with both reduction and oxidation functions. However, since the oxidant and the reducing agent are evenly mixed in the sustained-release agent, the release of the oxidant may affect the reduction effect of the reducing agent in actual applications, and the oxidation system is mainly used. Summary of the Invention

[0007] In response to the deficiencies in the prior art, the present invention provides an improved groundwater remediation agent and preparation method. The groundwater remediation agent adopts a layered structure, in which encapsulated nano-zero-valent iron is combined with an outer layer of sodium percarbonate with sustained-release capability; the agent undergoes reduction and oxidation reactions with chlorinated hydrocarbons in sequence, fully utilizing the reducibility of zero-valent iron and the catalytic effect of divalent iron generated after reduction on hydrogen peroxide, thereby achieving a synergistic remediation effect between the two.

[0008] The present invention discloses an improved groundwater remediation agent, which is in granular form, and each granule has a multi-layer structure; wherein, The outer layer of the particle is coated with nano-zero-valent iron (nZVI); The middle layer of the particle is the isolation layer; The inner layer of the particles is crystalline or amorphous sodium percarbonate (2Na2CO3•3H2O2).

[0009] As a further improvement of the present invention, the encapsulated nano zero-valent iron is nano zero-valent iron encapsulated by sodium carboxymethylcellulose (CMC).

[0010] As a further improvement of the present invention, the isolation layer includes a first wrapping layer, a second wrapping layer and a third wrapping layer arranged from the inside to the outside; wherein, The material of the first wrapping layer includes one or more of silicate, sulfonate, acrylic acid maleic acid copolymer, sodium carbonate, sodium sulfate, and soluble magnesium salt; The material of the second wrapping layer is polyvinyl alcohol; The material of the third wrapping layer mainly includes sodium chloride and sodium carbonate, and also contains polyacrylamide, magnesium silicate and sodium silicate.

[0011] The present invention also discloses a method for preparing an improved groundwater remediation agent, comprising: Step 1: preparing coated nano-zero-valent iron by liquid-phase reduction method; wherein the coating is added before the liquid-phase reduction reaction, and the nano-zero-valent iron is coated on the surface of the nano-zero-valent iron after it is generated; Step 2, preparing sodium percarbonate by liquid phase method; Step 3: Drying the sodium percarbonate particles in a fluidized bed, spraying a coating agent during the drying process to uniformly coat the particles to obtain sodium percarbonate with an isolation layer; Step 4: Place the sodium percarbonate with the isolation layer into a coating machine, add the coated nano zero-valent iron after wetting, mix evenly, and dry at low temperature to prepare an improved groundwater remediation agent.

[0012] As a further improvement of the present invention, in step 1, The iron raw material for preparing nano zero-valent iron is ferrous chloride or ferrous sulfate; The reducing agent required for preparing nano-zero-valent iron liquid is sodium borohydride or hydrazine hydrate; The coating of choice is sodium hydroxymethylcellulose.

[0013] As a further improvement of the present invention, step 2 includes: Sodium percarbonate is produced by reacting hydrogen peroxide with a saturated soda ash solution or soda ash slurry in an aqueous solution.

[0014] As a further improvement of the present invention, in step 2, The molar ratio of hydrogen peroxide to sodium percarbonate is 1.5-2, preferably 1.55-1.8; In the preparation process of sodium percarbonate, one or more of polyacrylamide, magnesium silicate and sodium silicate are used as stabilizers, and sodium chloride is used as a salting-out agent.

[0015] As a further improvement of the present invention, in step 3, In a fluidized drying state, a coating liquid comprising one or more of silicate, sulfonate, acrylic acid-maleic acid copolymer, sodium carbonate, sodium sulfate, and soluble magnesium salt is sprayed onto the surface of the sodium percarbonate particles to complete the preparation of the first coating layer; In the fluidized drying state, a polyvinyl alcohol solution is sprayed onto the surface of the first wrapping layer to complete the preparation of the second wrapping layer; In the fluidized drying state, sodium chloride mother liquor is sprayed onto the surface of the second coating layer to complete the preparation of the third coating layer, so as to finally obtain sodium percarbonate with an isolation layer.

[0016] As a further improvement of the present invention, in step 4, The wetting liquid used is dilute sodium hydroxymethyl cellulose solution.

[0017] Compared with the prior art, the present invention has the following beneficial effects: Through the optimized design of the reagent structure, the present invention allows the isolation layer to initially isolate sodium percarbonate from participating in the reaction, allowing the nano-zero-valent iron to preferentially undergo a reduction reaction. As the reaction proceeds, the internal sodium percarbonate can release hydrogen peroxide, which, under the catalysis of the divalent iron formed after the reduction reaction, performs Fenton oxidation on pollutants in the groundwater, eliminating pollutants or intermediates. In view of the fact that nano zero-valent iron is easily deactivated, the present invention adopts CMC wrapping to achieve a protective effect while also having a sustained-release property. For sodium percarbonate, the sustained-release property is adjusted by adding a specific multi-layer isolation method, thereby achieving coordination of the reduction-oxidation ordered reaction. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is a schematic structural diagram of the improved groundwater remediation agent disclosed in the present invention.

[0019] In the picture: 1. Sodium percarbonate; 2. Isolation layer; 3. Encapsulated nano-zero-valent iron. DETAILED DESCRIPTION

[0020] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of the present invention.

[0021] The present invention will be described in further detail below with reference to the accompanying drawings: like Figure 1As shown, the present invention provides an improved groundwater remediation agent, which is in granular form, and each granule has a multi-layer structure; wherein, The outer layer of the particle is coated nano zero-valent iron 3; wherein the coated nano zero-valent iron is nano zero-valent iron coated with sodium hydroxymethyl cellulose; The middle layer of the particles is an isolation layer 2; wherein the isolation layer includes a first wrapping layer, a second wrapping layer and a third wrapping layer arranged from the inside to the outside; the material of the first wrapping layer includes one or more of silicates, sulfonates, acrylic acid maleic acid copolymers, sodium carbonate, sodium sulfate, and soluble magnesium salts; the material of the second wrapping layer is polyvinyl alcohol; the material of the third wrapping layer mainly includes sodium chloride and sodium carbonate, and also contains polyacrylamide, magnesium silicate and sodium silicate.

[0022] The inner layer of the particles is crystalline or amorphous sodium percarbonate1.

[0023] The present invention provides a method for preparing an improved groundwater remediation agent, comprising: Step 1: Prepare encapsulated nano-zero-valent iron using a liquid-phase reduction method; wherein the encapsulating material is added before the liquid-phase reduction reaction, and the nano-zero-valent iron is encapsulated on the surface of the nano-zero-valent iron after it is generated. The iron raw material for preparing the nano-zero-valent iron is ferrous chloride or ferrous sulfate, the reducing agent required for preparing the nano-zero-valent iron liquid is sodium borohydride or hydrazine hydrate, and the selected encapsulating material is sodium hydroxymethyl cellulose.

[0024] Step 2: reacting hydrogen peroxide with a saturated soda ash solution or soda ash slurry in an aqueous solution to generate sodium percarbonate; wherein the molar ratio of hydrogen peroxide to sodium percarbonate is 1.5-2, preferably 1.55-1.8; and in the preparation process of the sodium percarbonate, one or more of polyacrylamide, magnesium silicate, and sodium silicate are used as stabilizers, and sodium chloride is used as a salting-out agent.

[0025] Step 3: Drying the sodium percarbonate particles in a fluidized bed, spraying a coating agent during the drying process to uniformly coat the particles to obtain sodium percarbonate with an isolation layer; Specifically include: In a fluidized drying state, a coating liquid comprising one or more of silicate, sulfonate, acrylic acid-maleic acid copolymer, sodium carbonate, sodium sulfate, and soluble magnesium salt is sprayed onto the surface of the sodium percarbonate particles to complete the preparation of the first coating layer; In the fluidized drying state, a polyvinyl alcohol solution is sprayed onto the surface of the first wrapping layer to complete the preparation of the second wrapping layer; Under fluidized drying conditions, sodium chloride mother liquor is sprayed onto the surface of the second coating layer to complete the preparation of the third coating layer, thereby ultimately producing sodium percarbonate with an isolation layer. During the sodium percarbonate preparation process, after the mother liquor prepared for the first time is used, its components change due to the addition of reagents. The main component changes from single sodium chloride to sodium chloride and sodium carbonate, with the remaining additives (polyacrylamide, magnesium silicate, and sodium silicate) also present.

[0026] Step 4: Place the sodium percarbonate with the isolation layer into a coating machine, moisten it with a dilute sodium hydroxymethyl cellulose solution, add the coated nano zero-valent iron, mix well, and dry at low temperature to prepare an improved groundwater remediation agent. Example 1:

[0027] An improved method for preparing a groundwater remediation agent, the specific process of which is as follows: S11. Preparation of encapsulated nano-zero-valent iron by liquid phase reduction method: Add 4000ml of water to a 5000ml glass reactor. Under nitrogen, boil, condense, and reflux to remove dissolved oxygen from the water. Cool and set aside. Remove 400ml of the solution and add 80g of NaBH4 to prepare a NaBH4 solution. Set aside. Add 100g of sodium hydroxymethyl cellulose (CMC) to the reactor and stir to dissolve. Add 560g of FeSO4·7H2O to the reactor and stir to dissolve. Then, under nitrogen, slowly drip the NaBH4 solution. After addition is complete, stir for 0.5-2h. Then, centrifuge to separate the supernatant, wash the precipitate with deoxygenated water, and vacuum dry at 70°C to obtain the sodium hydroxymethyl cellulose-coated nano-zero-valent iron. Bottle and set aside.

[0028] S12, preparing sodium percarbonate by liquid phase method: Prepare 2000ml of a mother liquor containing 10% sodium chloride (as a salting-out agent). Add 3g each of polyacrylamide, magnesium silicate, and sodium silicate (as a stabilizer) to the mother liquor. Add 400g of soda ash to 1200ml of the mother liquor and stir until it becomes a paste. Set aside 900g of 27.5% hydrogen peroxide. Place the remaining mother liquor into a 5000ml cooled glass reactor. Activate the stirring and cooling systems. Evenly add the sodium percarbonate paste and hydrogen peroxide to the remaining mother liquor. Maintain the reaction temperature at 10-20°C. Stir thoroughly for 15 minutes after addition. Filter to obtain granular sodium percarbonate. Set aside the mother liquor.

[0029] S13, preparing sodium percarbonate with an isolation layer: The fluidized bed is started with an inlet air temperature of 70°C. Sodium percarbonate particles are added. When the outlet air temperature reaches 35°C, the first coating layer begins. The weight ratio of sodium silicate, acrylic acid-maleic acid copolymer, sodium sulfate, and magnesium sulfate in the coating liquid is 10:5:80:5. The coating liquid is sprayed into the fluidized bed, coated onto the sodium percarbonate particles, and dried to remove water. The weight of the first coating layer is controlled to be 5-7% of the weight of the dried sodium percarbonate after coating. Polyvinyl alcohol solution is then sprayed in during fluidized drying for the second coating layer, which weighs 0.5-2% of the dried sodium percarbonate weight. The third coating layer is the mother liquor, sprayed in during fluidization, which weighs 3-5% of the dried sodium percarbonate weight after drying. The total coating layer is controlled to account for approximately 9-11% of the total weight of the sodium percarbonate particles. After coating, the sodium percarbonate particles with the isolation layer are cooled and set aside.

[0030] S14. Preparation of improved groundwater remediation agent: Place the sodium percarbonate with an isolation layer into a coating machine, start the coating machine at low speed, and spray in a dilute sodium hydroxymethyl cellulose (CMC) solution to complete particle wetting. Then, add the encapsulated nano-zero-valent iron, mix evenly, and start low-temperature drying for preliminary drying. Depending on the required ratio of the encapsulated nano-zero-valent iron and the sodium percarbonate with an isolation layer, the above process can be repeated multiple times. Finally, the product is transferred to a vacuum drying oven for further low-temperature drying and then bagged and sealed for storage. Example 2:

[0031] An improved method for preparing a groundwater remediation agent, the specific process of which is as follows: S21. Preparation of encapsulated nano-zero-valent iron by liquid phase reduction method: Add 4000ml of water to a 5000ml glass reactor. Under nitrogen, boil, condense, and reflux to remove dissolved oxygen from the water. Cool and set aside. Add 40g of sodium hydroxymethyl cellulose to the reactor and stir to dissolve to form Solution A. Remove 1500ml of Solution A and add 130g of FeCl2 to form Solution B. Take 2000ml of Solution A and add 100g of NaOH to form Solution C. Take 500ml of Solution A and add 240g of hydrazine hydrate to form Solution D. Transfer Solution C to the reactor and start stirring. Add Solutions B and C simultaneously and uniformly to the reactor. Continue stirring for 2-4 hours after addition. Then, centrifuge to separate the supernatant, wash the precipitate with deoxygenated water, and vacuum dry at 70°C to obtain the sodium hydroxymethyl cellulose-coated nano-zero-valent iron. Bottle and set aside.

[0032] S22, preparing sodium percarbonate by liquid phase method: The preparation method is the same as S12.

[0033] S23, preparing sodium percarbonate with an isolation layer: The preparation method is the same as S13, except that the coating layers are modified as follows: the first coating layer comprises sodium silicate, sodium carbonate, acrylic acid maleic acid copolymer, sodium sulfate, and magnesium sulfate in a weight ratio of 5:20:5:55:5, with the weight of the first coating layer accounting for 4-6% of the weight of the dried sodium percarbonate particles; the second coating layer comprises polyvinyl alcohol, with the weight of the second coating layer accounting for 1-2% of the weight of the dried sodium percarbonate particles; and the third coating layer comprises mother liquor, with the weight of the third coating layer accounting for 4-6% of the weight of the dried sodium percarbonate particles. The coating layer is controlled to account for approximately 9%-11% of the total weight of the sodium percarbonate particles.

[0034] S24. Preparation of improved groundwater remediation agent: The preparation method is the same as S14.

[0035] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. An improved groundwater remediation agent, characterized in that: The groundwater remediation agent is in granular form, and each granule has a multi-layer structure; wherein, The outer layer of the particle is wrapped nano-zero-valent iron; The middle layer of the particle is the isolation layer; The inner layer of the particles is crystalline or amorphous sodium percarbonate.

2. The improved groundwater remediation agent according to claim 1, characterized in that The encapsulated nano zero-valent iron is nano zero-valent iron encapsulated by sodium hydroxymethyl cellulose.

3. The improved groundwater remediation agent according to claim 1, characterized in that The isolation layer includes a first wrapping layer, a second wrapping layer and a third wrapping layer arranged from the inside to the outside; wherein, The material of the first wrapping layer includes one or more of silicate, sulfonate, acrylic acid maleic acid copolymer, sodium carbonate, sodium sulfate, and soluble magnesium salt; The material of the second wrapping layer is polyvinyl alcohol; The material of the third wrapping layer mainly includes sodium chloride and sodium carbonate.

4. A method for preparing the improved groundwater remediation agent according to any one of claims 1 to 3, characterized in that: include: Step 1: preparing coated nano-zero-valent iron by liquid-phase reduction method; wherein the coating is added before the liquid-phase reduction reaction, and the nano-zero-valent iron is coated on the surface of the nano-zero-valent iron after it is generated; Step 2, preparing sodium percarbonate by liquid phase method; Step 3: Drying the sodium percarbonate particles in a fluidized bed, spraying a coating agent during the drying process to uniformly coat the particles to obtain sodium percarbonate with an isolation layer; Step 4: Place the sodium percarbonate with the isolation layer into a coating machine, add the coated nano zero-valent iron after wetting, mix evenly, and dry at low temperature to prepare an improved groundwater remediation agent.

5. The method for preparing the improved groundwater remediation agent according to claim 4, wherein: In step 1, The iron raw material for preparing nano zero-valent iron is ferrous chloride or ferrous sulfate; The reducing agent required for preparing nano-zero-valent iron liquid is sodium borohydride or hydrazine hydrate; The coating of choice is sodium hydroxymethylcellulose.

6. The method for preparing the improved groundwater remediation agent according to claim 4, wherein: The step 2 includes: Sodium percarbonate is produced by reacting hydrogen peroxide with a saturated soda ash solution or soda ash slurry in an aqueous solution.

7. The method for preparing the improved groundwater remediation agent according to claim 6, wherein: In step 2, The molar ratio of hydrogen peroxide to sodium percarbonate is 1.5-2; In the preparation process of sodium percarbonate, one or more of polyacrylamide, magnesium silicate and sodium silicate are used as stabilizers, and sodium chloride is used as a salting-out agent.

8. The method for preparing the improved groundwater remediation agent according to claim 4, wherein: In step 3, In a fluidized drying state, a coating liquid comprising one or more of silicate, sulfonate, acrylic acid-maleic acid copolymer, sodium carbonate, sodium sulfate, and soluble magnesium salt is sprayed onto the surface of the sodium percarbonate particles to complete the preparation of the first coating layer; In the fluidized drying state, a polyvinyl alcohol solution is sprayed onto the surface of the first wrapping layer to complete the preparation of the second wrapping layer; In the fluidized drying state, sodium chloride mother liquor is sprayed onto the surface of the second coating layer to complete the preparation of the third coating layer, so as to finally obtain sodium percarbonate with an isolation layer.

9. The method for preparing the improved groundwater remediation agent according to claim 4, wherein: In step 4, The wetting liquid used is dilute sodium hydroxymethyl cellulose solution.

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