An improved groundwater remediation agent and its preparation method

Abstract

This invention discloses an improved groundwater remediation agent and its preparation method. The groundwater remediation agent is granular, and each particle has a multi-layered structure. The outer layer of the particle is encapsulated nano-zero-valent iron, the middle layer is an isolation layer, and the inner layer is crystalline or amorphous sodium percarbonate. The improved remediation agent of this invention has slow-release properties. For groundwater contaminated with chlorinated hydrocarbons, the zero-valent iron first dechlorinates the chlorinated hydrocarbons. As the process proceeds, the hydrogen peroxide released from the sodium percarbonate, catalyzed by the ferrous iron formed during dechlorination, oxidizes and eliminates the chlorinated hydrocarbons and intermediate products.

Description

Technical Field

[0001] This invention relates to the field of groundwater remediation technology, specifically to an improved groundwater remediation agent and its preparation method. Background Technology

[0002] Due to their excellent thermal and chemical stability, chlorinated solvents have been used in industrial and daily production applications, such as solvents, dry cleaning agents, heat transfer fluids, and flame retardants. The widespread use of chlorinated hydrocarbons has created an opportunity for them to enter groundwater as pollutants. Because they are difficult to degrade rapidly through natural processes and have a relative density greater than water, they easily migrate vertically in groundwater and accumulate in aquitards, allowing for long-term release and persistent pollution, posing a serious challenge to groundwater remediation.

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

[0004] Because nano-zero valent iron is prone to aggregation and oxidative deactivation, researchers have adopted various technical measures. Chinese patent application CN202211498643.4 encapsulates nano-zero valent iron within oil droplets of a vegetable oil emulsion to enhance the reactivity of pollutants with the nano-zero valent iron; Chinese patent application CN202310737818.0 synthesizes cyclodextrin polymers from cyclodextrin monomers, then immobilizes nano-zero valent iron and trace amounts of copper within 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. To improve its reactivity release; Chinese patent application CN202311108797.2 uses core-shell structured sulfide nano-zero valent iron to overcome the passivation of internal zero-valent iron; Chinese patent application CN202311367057.0 uses metal sulfides and citrates for modification and carbon materials for loading to improve the dispersibility and sustained release ability of nano-zero valent iron; Chinese patent application CN201611062038.7 uses candle-wrapped zero-valent iron activator to achieve sustained release effect to activate persulfate.

[0005] Researchers have also conducted extensive research and made numerous improvements regarding the use of sodium percarbonate as an oxidizing agent for chlorinated hydrocarbons. 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, significantly improving the degradation rate; Chinese patent application CN202211120844.0 uses porous materials to pre-adsorb oxidants as the core material, and sodium alginate, polyacrylamide, ferric chloride, aluminum chloride, calcium chloride, etc. as the outer shell slow-release oxidants for chlorinated hydrocarbon remediation; Chinese patent application CN202411839574.8 uses fly ash-Al-MOF loaded with sodium percarbonate and fly ash-Al-MOF loaded with acidic ferrous sulfate in combination for organic pollution remediation; 202411838694.6 discloses a method for remediating petroleum hydrocarbons by catalyzing sodium percarbonate with zeolite-magnetic iron oxide.

[0006] The aforementioned patents disclose numerous methods for improving nano-zero-valent iron and methods for repairing with sodium percarbonate, primarily targeting single reduction or oxidation systems. Chinese patent application CN201910283513.0 describes a slow-release system formed by mixing percarbonate, paraffin, iron-based activator, and complexing agent. It utilizes paraffin as a slow-release agent to create a slow-release system that combines reduction and oxidation functions by mixing nano-zero-valent iron and sodium percarbonate. However, since the oxidant and reductant are uniformly mixed in the slow-release agent, the release of the oxidant may affect the reduction effect of the reductant in practical applications, with the oxidation system being the dominant one. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention provides an improved groundwater remediation agent and its preparation method. The groundwater remediation agent adopts a layered structure, in which encapsulated nano-zero valent iron is bound to the outer layer of sodium percarbonate with slow-release capabilities. This allows the agent to undergo reduction and oxidation reactions with chlorinated hydrocarbons sequentially, fully utilizing the reducing power of zero valent iron and the catalytic effect of the divalent iron generated after reduction on hydrogen peroxide, thereby achieving a synergistic remediation effect.

[0008] This invention discloses an improved groundwater remediation agent, wherein the groundwater remediation agent is granular, and each particle has a multi-layered structure; wherein,

[0009] The outer layer of the particles is encapsulated nano-zero valent iron (nZVI);

[0010] The middle layer of the particles is an isolation layer;

[0011] The inner layer of the particles is crystalline or amorphous sodium percarbonate (2Na2CO3•3H2O2).

[0012] As a further improvement of the present invention, the encapsulated nano-zero valent iron is nano-zero valent iron encapsulated in sodium hydroxymethyl cellulose (CMC).

[0013] 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 disposed from the inside out; wherein,

[0014] The material of the first coating layer includes one or more of silicates, sulfonates, acrylic maleic acid copolymers, sodium carbonate, sodium sulfate, and soluble magnesium salts;

[0015] The material of the second wrapping layer is polyvinyl alcohol;

[0016] The material of the third coating layer mainly includes sodium chloride and sodium carbonate, and also contains polyacrylamide, magnesium silicate and sodium silicate.

[0017] This invention also discloses an improved method for preparing a groundwater remediation agent, comprising:

[0018] 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;

[0019] Step 2: Prepare sodium percarbonate using a liquid-phase method;

[0020] Step 3: Sodium percarbonate particles are dried using a fluidized bed. During the drying process, a coating agent is sprayed in to coat the particles evenly, so as to obtain sodium percarbonate with an isolation layer.

[0021] Step 4: Place the sodium percarbonate with the isolation layer into a coating machine, wet it, add the encapsulated nano-zero valent iron, mix evenly, and dry at low temperature to obtain the improved groundwater remediation agent.

[0022] As a further improvement of the present invention, in step 1...

[0023] The iron raw material for preparing nano-zero valent iron is ferrous chloride or ferrous sulfate;

[0024] The reducing agent required for preparing nano-zero ferrous iron liquid is sodium borohydride or hydrazine hydrate;

[0025] The chosen packaging material is sodium hydroxymethyl cellulose.

[0026] As a further improvement of the present invention, step 2 includes:

[0027] Sodium percarbonate is produced by reacting hydrogen peroxide with a saturated soda ash solution or soda ash slurry in an aqueous solution.

[0028] As a further improvement of the present invention, in step 2...

[0029] The molar ratio of hydrogen peroxide to sodium percarbonate is 1.5-2, preferably 1.55-1.8;

[0030] In the preparation 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.

[0031] As a further improvement of the present invention, in step 3...

[0032] In a fluidized drying state, a coating liquid including one or more of silicates, sulfonates, acrylic maleic acid copolymers, sodium carbonate, sodium sulfate, and soluble magnesium salts is sprayed onto the surface of sodium percarbonate particles to complete the preparation of the first coating layer.

[0033] Under fluidized drying conditions, a polyvinyl alcohol solution is sprayed onto the surface of the first coating layer to complete the preparation of the second coating layer;

[0034] 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, ultimately yielding sodium percarbonate with an isolation layer.

[0035] As a further improvement of the present invention, in step 4...

[0036] The wetting solution used was a dilute sodium hydroxymethyl cellulose solution.

[0037] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0038] This invention optimizes the design of the reagent structure. The isolation layer can initially prevent sodium percarbonate from participating in the reaction, allowing nano-zero ferric 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 ferrous iron formed after the reduction reaction, performs Fenton oxidation on pollutants in groundwater, eliminating pollutants or intermediates.

[0039] This invention addresses the issue of the easy deactivation of nano-zero valent iron by employing CMC encapsulation, achieving both protection and sustained-release properties. For sodium percarbonate, by adding specific multilayer isolation, its sustained-release properties are modulated, thereby achieving a coordinated reduction-oxidation reaction. Attached Figure Description

[0040] Figure 1 This is a schematic diagram of the structure of the improved groundwater remediation agent disclosed in this invention.

[0041] In the picture:

[0042] 1. Sodium percarbonate; 2. Isolation layer; 3. Encapsulated nano-zero valent iron. Detailed Implementation

[0043] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0044] The present invention will now be described in further detail with reference to the accompanying drawings:

[0045] like Figure 1 As shown, this invention provides an improved groundwater remediation agent, wherein the groundwater remediation agent is granular, and each particle has a multi-layered structure; wherein,

[0046] The outer layer of the particles is encapsulated nano-zero valent iron 3; wherein, the encapsulated nano-zero valent iron is nano-zero valent iron encapsulated by sodium hydroxymethyl cellulose;

[0047] The intermediate layer of the particles is an isolation layer 2; wherein, the isolation layer includes a first coating layer, a second coating layer and a third coating layer arranged from the inside to the outside; the material of the first coating 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 coating layer is polyvinyl alcohol; the material of the third coating layer mainly includes sodium chloride and sodium carbonate, and also contains polyacrylamide, magnesium silicate and sodium silicate.

[0048] The inner layer of the particles is crystalline or amorphous sodium percarbonate 1.

[0049] This invention provides an improved method for preparing a groundwater remediation agent, comprising:

[0050] 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 generation. The iron raw material for preparing nano-zero valent iron is ferrous chloride or ferrous sulfate, and the reducing agent required for preparing nano-zero valent iron liquid is sodium borohydride or hydrazine hydrate. The selected encapsulating material is sodium hydroxymethyl cellulose.

[0051] Step 2: React 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; in the preparation 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.

[0052] Step 3: Sodium percarbonate particles are dried using a fluidized bed. During the drying process, a coating agent is sprayed in to coat the particles evenly, so as to obtain sodium percarbonate with an isolation layer.

[0053] Specifically, it includes:

[0054] In a fluidized drying state, a coating liquid including one or more of silicates, sulfonates, acrylic maleic acid copolymers, sodium carbonate, sodium sulfate, and soluble magnesium salts is sprayed onto the surface of sodium percarbonate particles to complete the preparation of the first coating layer.

[0055] Under fluidized drying conditions, a polyvinyl alcohol solution is sprayed onto the surface of the first coating layer to complete the preparation of the second coating layer;

[0056] In a 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. In the process of preparing sodium percarbonate, the composition of the mother liquor prepared in the first batch changes due to the addition of reagents. The main component changes from a single sodium chloride to a state in which sodium chloride and sodium carbonate are the main components, and other additives (polyacrylamide, magnesium silicate and sodium silicate) are also contained.

[0057] Step 4: Place the sodium percarbonate with the isolation layer into a coating machine, wet it with a dilute sodium hydroxymethyl cellulose solution, add the encapsulated nano-zero valent iron, mix evenly, and dry at low temperature to obtain the improved groundwater remediation agent. Example 1:

[0058] An improved method for preparing a groundwater remediation agent, the specific process of which is as follows:

[0059] S11. Preparation of encapsulated nano-zero-valent iron using liquid-phase reduction method:

[0060] Add 4000ml of water to a 5000ml glass reactor, boil and reflux under nitrogen protection to remove dissolved oxygen, then cool and set aside. Take 400ml of the solution and add 80g of NaBH4 to prepare a NaBH4 solution, set aside. Add 100g of sodium carboxymethyl cellulose (CMC) coating to the reactor and stir to dissolve. Add 560g of FeSO4•7H2O to the reactor and stir to dissolve. Then, under nitrogen protection, slowly add the NaBH4 solution dropwise. After the addition is complete, stir for 0.5-2 hours. Then, centrifuge to separate the supernatant, wash the precipitate with deoxygenated water, and dry under vacuum at 70℃ to obtain sodium carboxymethyl cellulose-coated nano-zero valent iron, which is then bottled and set aside for later use.

[0061] S12. Preparation of sodium percarbonate using a liquid-phase method:

[0062] 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. Take 1200ml of the mother liquor and add 400g of soda ash, stirring until a paste is formed. Set aside 900g of 27.5% hydrogen peroxide. Transfer the remaining mother liquor to a 5000ml glass reactor with cooling. Start the stirring and cooling system. Add the paste-like sodium percarbonate and hydrogen peroxide evenly to the remaining mother liquor and react. Control the reaction temperature at 10-20℃ during the reaction process. After the addition is complete, stir thoroughly for 15 minutes. Filter to obtain granular sodium percarbonate. Set aside the mother liquor.

[0063] S13. Preparation of sodium percarbonate with an isolation layer:

[0064] Start the fluidized bed with an inlet gas temperature of 70°C and add sodium percarbonate granules. When the outlet gas temperature reaches 35°C, begin the first coating layer. The weight ratio of sodium silicate, acrylic acid-maleic acid copolymer, sodium sulfate, and magnesium sulfate in the coating solution is 10:5:80:5. The coating solution is sprayed into the fluidized bed to coat the sodium percarbonate granules, which are then dried to remove moisture. The weight of the first coating layer should be controlled to be 5-7% of the weight of the dried sodium percarbonate. Subsequently, under fluidized drying conditions, a polyvinyl alcohol solution is sprayed in for the second coating layer, which should be 0.5-2% of the weight of the dried sodium percarbonate. The third coating layer is the mother liquor, sprayed in under fluidized conditions, and the weight of the third coating layer should be controlled to be 3-5% of the weight of the dried sodium percarbonate. The entire coating layer should be controlled to account for approximately 9%-11% of the total weight of the sodium percarbonate granules. After coating is completed, the sodium percarbonate granules with the isolation layer are cooled for later use.

[0065] S14. Preparation of improved groundwater remediation agents:

[0066] Sodium percarbonate with an isolation layer is placed in a coating machine, which is then started at low speed. A dilute sodium carboxymethyl cellulose (CMC) solution is sprayed in to wet the particles. Then, encapsulated nano-zero valent iron is added, mixed evenly, and low-temperature drying is started for preliminary drying. The above process can be repeated multiple times depending on the required ratio of encapsulated nano-zero valent iron to sodium percarbonate with an isolation layer. Finally, the product is transferred to a vacuum drying oven for further low-temperature drying and then sealed in bags for storage. Example 2:

[0067] An improved method for preparing a groundwater remediation agent, the specific process of which is as follows:

[0068] S21. Preparation of encapsulated nano-zero-valent iron using liquid-phase reduction method:

[0069] Add 4000ml of water to a 5000ml glass reactor, boil and reflux under nitrogen protection to remove dissolved oxygen, then cool and set aside. Add 40g of sodium carboxymethyl cellulose to the reactor and stir to dissolve it into solution A. Take 1500ml of solution A and add 130g of FeCl2 to prepare solution B. Take 2000ml of solution A and add 100g of NaOH to prepare solution C. Take 500ml of solution A and add 240g of hydrazine hydrate to prepare solution D. Transfer solution C to the reactor, start stirring, and simultaneously add solutions B and C to the reactor at a uniform rate. After the addition is complete, continue stirring for 2-4 hours. Then, centrifuge to separate the supernatant, wash the precipitate with deoxygenated water, and dry it under vacuum at 70℃ to obtain sodium carboxymethyl cellulose-coated nano-zero valent iron, which is then bottled for later use.

[0070] S22. Preparation of sodium percarbonate using a liquid-phase method:

[0071] The preparation method is the same as that for S12.

[0072] S23. Preparation of sodium percarbonate with an isolation layer:

[0073] The preparation method is the same as S13, except that the coating layers are as follows: The first layer is coated with sodium silicate, sodium carbonate, acrylic acid-maleic acid copolymer, sodium sulfate, and magnesium sulfate in a weight ratio of 5:20:5:55:5, and the weight of the first coating layer accounts for 4-6% of the weight of the dried sodium percarbonate; the second coating layer is coated with polyvinyl alcohol, and the weight of the second coating layer accounts for 1-2% of the weight of the dried sodium percarbonate; the third coating layer is coated with mother liquor, and the third coating layer accounts for 4-6% of the weight of the dried sodium percarbonate. The coating layers are controlled to account for approximately 9%-11% of the total weight of the sodium percarbonate particles.

[0074] S24. Preparation of improved groundwater remediation agents:

[0075] The preparation method is the same as that for S14.

[0076] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should 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 granular, and each particle has a multi-layered structure; wherein, The outer layer of the particles is encapsulated nano-zero valent iron, which is nano-zero valent iron encapsulated by sodium hydroxymethyl cellulose; The middle layer of the particles is an isolation layer; the isolation layer includes a first coating layer, a second coating layer, and a third coating layer arranged from the inside to the outside; wherein, the material of the first coating 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 coating layer is polyvinyl alcohol; the material of the third coating layer mainly includes sodium chloride and sodium carbonate; The inner layer of the particles is crystalline or amorphous sodium percarbonate.

2. A method for preparing the improved groundwater remediation agent as described in claim 1, characterized in that, include: 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; Step 2: Prepare sodium percarbonate using a liquid-phase method; Step 3: Sodium percarbonate particles are dried using a fluidized bed. During the drying process, a coating agent is sprayed in to coat the particles evenly, so as to obtain sodium percarbonate with an isolation layer. Step 4: Place the sodium percarbonate with the isolation layer into a coating machine, wet it, add the encapsulated nano-zero valent iron, mix evenly, and dry at low temperature to obtain the improved groundwater remediation agent.

3. The method for preparing the improved groundwater remediation agent as described in claim 2, characterized in that, 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 ferrous iron liquid is sodium borohydride or hydrazine hydrate; The chosen packaging material is sodium hydroxymethyl cellulose.

4. The method for preparing the improved groundwater remediation agent as described in claim 2, characterized in that, 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.

5. The method for preparing the improved groundwater remediation agent as described in claim 4, characterized in that, In step 2, The molar ratio of hydrogen peroxide to sodium percarbonate is 1.5-2; In the preparation 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.

6. The method for preparing the improved groundwater remediation agent as described in claim 2, characterized in that, In step 3, In a fluidized drying state, a coating solution including one or more of silicates, sulfonates, acrylic-maleic acid copolymers, sodium carbonate, sodium sulfate, and soluble magnesium salts is sprayed onto the surface of sodium percarbonate particles to complete the preparation of the first coating layer. Under fluidized drying conditions, a polyvinyl alcohol solution is sprayed onto the surface of the first coating layer to complete the preparation of the second coating 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, ultimately yielding sodium percarbonate with an isolation layer.

7. The method for preparing the improved groundwater remediation agent as described in claim 2, characterized in that, In step 4, The wetting solution used was a dilute sodium hydroxymethyl cellulose solution.

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