Method for strengthening desorption of floe organic matter in fluorine-containing wastewater by nanobubbles

Abstract

The application discloses a method for strengthening desorption of floe organic matters in fluorine-containing wastewater by nano-bubbles, and belongs to the technical field of sludge treatment. First, coconut shell powder is immersed in a ferric sulfate solution and calcined to prepare magnetic porous biochar, and then the magnetic porous biochar is hydrophobically modified by myristic acid to obtain hydrophobically modified magnetic porous biochar. The hydrophobically modified magnetic porous biochar is combined with nano-bubbles to construct a sludge activation treatment system. Subsequently, after the backflow sludge is concentrated, the backflow sludge is placed in a nano-bubble reactor, ozone is injected to make the organic film on the surface of floe desorb, and then the hydrophobically modified magnetic porous biochar is added to capture organic fragments in liquid phase and form magnetic core aggregates. After the magnetic core aggregates are removed through magnetic separation, the activated floe slurry is vacuum degassed for reuse. Through the synergistic effect of nano-bubble oxidation stripping, hydrophobically modified magnetic porous biochar adsorption and magnetic separation removal, the active sites on the surface of floe are recovered, the flocculating agent dosage is reduced, and the demand of a fluorine-containing wastewater treatment process for floe activation and reuse and operation economy is met.

Description

Technical Field

[0001] This invention belongs to the field of sludge treatment technology, specifically a method for enhancing the desorption of organic matter from fluoride-containing wastewater flocs using nanobubbles. Background Technology

[0002] Fluoride-containing wastewater treatment typically employs coagulation and sedimentation. This involves adding coagulants such as aluminum or iron salts to cause fluoride ions to react with metal ions, forming insoluble fluorides or coprecipitates, which then separate into solids and liquids through flocculation and sedimentation. To improve flocculation efficiency, a portion of the settled sludge is often recycled back to the upstream reaction zone, utilizing mature metal hydroxide flocs as nuclei to induce rapid growth of subsequent flocs. However, during long-term circulation, this recycled sludge continuously adsorbs organic matter such as humic acid and fulvic acid, as well as inorganic impurities like calcium and magnesium, leading to gradual floc aging. The active sites on the surface of aged flocs are masked by organic matter, and the pore channels are blocked by impurities, resulting in decreased fluoride removal efficiency and deteriorated floc settling performance.

[0003] In recent years, micro- and nanobubble technology has been introduced into the field of sludge activation treatment due to its unique oxidation and mechanical stripping capabilities. By using microjets and hydroxyl radicals generated during bubble collapse to strip the aged layer on the floc surface, the organic film undergoes oxidative chain breakage and desorbs from the floc surface, exposing active sites and providing a new approach for the activation treatment of aged flocs. However, after nanobubbles oxidize and strip the organic film on the surface of aged flocs, the stripped organic fragments remain dispersed in the liquid phase. Driven by the concentration gradient, they re-adhere to the floc surface, causing the active sites to be masked again. Simultaneously, the lack of effective means to capture and separate organic fragments in the liquid phase results in high residual pollutant concentrations in the reused flocs, making it difficult to restore fluoride removal activity.

[0004] Therefore, how to effectively capture and separate organic fragments in the liquid phase after floc treatment with nanobubbles, thereby restoring the active sites on the floc surface and meeting the comprehensive requirements of the process for defluorination efficiency, while maintaining the continuous operation of the sludge return process and the reuse form of floc slurry, is a core technical problem that urgently needs to be solved in this field. Summary of the Invention

[0005] The purpose of this invention is to provide a method for enhancing the desorption of organic matter from flocs in fluoride-containing wastewater using nanobubbles. This method involves introducing hydrophobically modified magnetic porous biochar into a nanobubble sludge activation treatment system, thereby constructing a sludge activation treatment system that combines organic debris capture and magnetic separation removal. Then, combined with nanobubble oxidation stripping, magnetic separation, and vacuum degassing processes, it achieves effective desorption of the organic film on the surface of aged flocs and removal of organic debris. This method not only effectively restores the active sites on the surface of the returned flocs but also reduces the dosage of metal flocculants, meeting the comprehensive requirements of fluoride-containing wastewater treatment processes for defluorination efficiency, continuous sludge return operation, and operational economy.

[0006] The objective of this invention can be achieved through the following technical solutions: This invention provides a method for enhancing the desorption of organic matter from flocculent fluoride-containing wastewater using nanobubbles, comprising the following steps: Step 1: The sludge generated from the coagulation and sedimentation process of fluoride-containing wastewater is discharged through the sludge discharge pipe at the bottom of the high-density sedimentation tank and introduced into the gravity thickener. After gravity sedimentation and concentration, concentrated floc slurry is obtained.

[0007] The water content of the above-mentioned concentrated flocculent slurry is 96-98%.

[0008] Step 2: Place the concentrated floc slurry in a nanobubble reactor, pressurize the slurry to 0.3-0.4 MPa and send it to a micro / nanobubble generator. Inject ozone to form nanobubbles. Return the mixed slurry containing nanobubbles to the reactor to enhance the desorption of the organic film on the surface of the aged flocs. Stir at 100-200 r / min for 5-10 min at 25-35℃. Add hydrophobically modified magnetic porous biochar and continue stirring at 200-300 r / min for 3-5 min at the same temperature. Then stir at 40-60 r / min for 3-5 min to obtain the magnetic core aggregate slurry.

[0009] In the aforementioned micro-nano bubble generator, the gas-to-water ratio of ozone to slurry is 0.2-0.5:1.

[0010] The concentration of nanobubbles in the above-mentioned mixed slurry containing nanobubbles is 10. 8 -10 9 per mL.

[0011] The dosage of the above-mentioned hydrophobic modified magnetic porous biochar is 1-2% of the dry weight of the flocs.

[0012] Furthermore, the specific preparation process of hydrophobically modified magnetic porous biochar is as follows: Myristic acid and a 10wt% methanol aqueous solution were placed in a reaction vessel and stirred at 25-35℃ for 20-40 min. Magnetic porous biochar was added and ultrasonically dispersed for 20-40 min. The mixture was then reacted at 70-75℃ for 6-8 h. After filtration, washing, and vacuum drying to constant weight, hydrophobic modified magnetic porous biochar was obtained.

[0013] The mass ratio of the above-mentioned myristic acid, methanol aqueous solution and magnetic porous biochar is 1:80-100:5-10.

[0014] Furthermore, the specific preparation process of magnetic porous biochar is as follows: The magnetic biochar precursor was placed in a tube furnace and heated to 550-600℃ under a nitrogen atmosphere. It was then calcined for 2-3 hours and cooled in the furnace before being ground and sieved to obtain magnetic porous biochar.

[0015] The heating rate in the aforementioned tube furnace is 5℃ / min.

[0016] Furthermore, the specific preparation process of the magnetic biochar precursor is as follows: Ferric chloride hexahydrate, ferrous sulfate, and deionized water were placed in a reaction vessel and stirred at 25-35℃ for 20-40 min to obtain a mixed iron salt solution. Coconut shell powder was added to the solution and ultrasonically dispersed for 20-40 min. A 1 mol / L NaOH solution was added to adjust the pH of the reaction solution to 10-10.2. The solution was aged for 20-22 h, filtered, washed, and vacuum dried to constant weight to obtain a magnetic biochar precursor.

[0017] The mass ratio of the above-mentioned ferric chloride hexahydrate, ferrous sulfate, and deionized water is 1:2.0-2.2:100-110. The solid-liquid ratio of the above-mentioned coconut shell powder and iron salt mixed solution is 1:8-12. Step 3: The magnetic core aggregate slurry is introduced into the magnetic separation device for magnetic separation. The magnetic core flocs are magnetically separated, retained, and removed to obtain activated floc slurry. The magnetically separated floc slurry is then introduced into the vacuum degassing device and degassed for 3-5 minutes to obtain degassed activated floc slurry. The activated floc slurry is then recycled to the front-end coagulation and sedimentation reaction zone.

[0018] The magnetic field strength of the aforementioned magnetic separation device is 0.4-0.6T.

[0019] The vacuum degree in the above-mentioned vacuum degassing device is -0.08 to -0.095 MPa.

[0020] The beneficial effects of this invention are: 1. In this invention, a method for enhancing the desorption of organic matter from flocs in fluoride-containing wastewater using nanobubbles is employed. Nanobubbles oxidize and break down the organic film on the surface of aged flocs through hydroxyl radical oxidation and microfluidic mechanical shearing, exposing the masked active sites of metal hydroxides. Hydrophobically modified magnetic porous biochar, through its hydrophobic surface provided by the long-chain alkyl group of myristic acid, captures the desorbed organic fragments in the liquid phase via hydrophobic interactions. Simultaneously, the high specific surface area of ​​the porous biochar provides physical retention sites, causing the organic fragments to aggregate on the magnetic particles and be removed as a whole through magnetic separation. The synergistic effect of these components improves the accessibility of the active sites in the flocs, meeting the comprehensive requirements of fluoride removal efficiency and operational economy in fluoride-containing wastewater treatment processes.

[0021] 2. In the method of enhancing the desorption of organic matter from flocs in fluoride-containing wastewater using nanobubbles, this invention utilizes the synergistic effects of nanobubble treatment, hydrophobic modified magnetic porous biochar addition, magnetic separation, and vacuum degassing to improve the reuse of activated flocs. The oxidation and mechanical action of nanobubbles desorb the organic film on the surface of aged flocs, allowing organic fragments to enter the liquid phase. The hydrophobic modified magnetic porous biochar, through its hydrophobic surface and porous structure, adheres the dispersed organic fragments to the surface of magnetic particles, forming magnetic core aggregates, preventing the organic fragments from re-adhering to the floc fragments. Magnetic separation, under the action of a magnetic field gradient, traps and removes the magnetic core aggregates, allowing the organic fragments to be removed as a whole along with the magnetic particles. Vacuum degassing removes residual microbubbles, eliminating the air flotation effect and preventing air resistance interference caused by residual gas during reuse, ensuring the stable operation of activated flocs in the coagulation and sedimentation reaction zone. The sequential action of these steps maintains the accessibility of active sites in the activated flocs during reuse, thereby reducing the dosage of flocculant while ensuring defluorination efficiency.

[0022] 3. In the method of nanobubble-enhanced desorption of organic matter from flocs in fluoride-containing wastewater, this invention uses coconut shell powder as raw material to prepare magnetic porous biochar, which is then modified hydrophobically and used as a functional additive. This realizes the application of biomass-based materials in the treatment of fluoride-containing wastewater. At the same time, by reactivating and reusing the flocs, the dosage of metal flocculants is reduced, thereby reducing the amount of aluminum salt sludge generated and the subsequent treatment load. This meets the requirements of green, low-carbon and economical operation of fluoride-containing wastewater treatment processes. Detailed Implementation

[0023] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0024] Example 1: This example provides a method for enhancing the desorption of organic matter from fluoride-containing wastewater flocs using nanobubbles, using hydrophobically modified magnetic porous biochar, prepared through the following steps: S1: Ferric chloride hexahydrate, ferrous sulfate, and deionized water were placed in a reaction vessel at a mass ratio of 1:2:100 and stirred at 200 r / min for 20 min at 25 °C to obtain a mixed iron salt solution. Coconut shell powder was added to the solution, with a solid-liquid ratio of 1:8 g / mL between the coconut shell powder and the iron salt solution. The mixture was ultrasonically dispersed for 20 min, and a 1 mol / L NaOH solution was added to adjust the pH of the reaction solution to 10. The mixture was sealed and aged for 20 h. After filtration, the solid was washed with deionized water until the final wash solution was neutral. The solid was then vacuum dried at 60 °C to constant weight to obtain a magnetic biochar precursor.

[0025] This step involves Fe 3+ with Fe 2+ Under alkaline conditions, a co-precipitation reaction occurs, generating in situ magnetic iron oxide particles which are loaded onto the surface and pores of coconut shell powder to obtain a magnetic biochar precursor.

[0026] S2: The magnetic biochar precursor was placed in a tube furnace and heated to 550°C at a heating rate of 5°C / min under a nitrogen atmosphere. It was then calcined at this temperature for 2 hours. After cooling in the furnace, it was ground and sieved through a 50-mesh sieve to obtain magnetic porous biochar.

[0027] This step involves the pyrolysis and carbonization of cellulose, hemicellulose, and lignin in coconut shell powder to form a porous carbon framework. The pre-loaded Fe3O4 magnetic phase is stably retained in the carbon matrix at high temperature, resulting in magnetic porous biochar.

[0028] S3: Myristic acid and a 10wt% methanol aqueous solution were placed in a reaction vessel and stirred at 200 r / min for 20 min at 25 °C. Magnetic porous biochar was added and ultrasonically dispersed for 20 min. The mixture was then reacted at 70 °C with the same stirring rate for 6 h. The mass ratio of myristic acid, 10wt% methanol aqueous solution and magnetic porous biochar was controlled at 1:80:5. After the reaction was completed, the mixture was cooled to room temperature, filtered, and the solid was washed twice with n-hexane. The solid was then vacuum dried at 60 °C to constant weight to obtain hydrophobically modified magnetic porous biochar.

[0029] This step utilizes the polar interaction between the carboxylic acid group at one end of the myristic acid molecule and the oxygen-containing functional groups such as hydroxyl and carbonyl groups remaining on the surface of magnetic porous biochar, while the long-chain alkyl group at the other end extends outward in a directional manner, forming hydrophobic microregions on the surface of the biochar, thus obtaining hydrophobically modified magnetic porous biochar.

[0030] Example 2: This example provides a method for enhancing the desorption of organic matter from fluoride-containing wastewater flocs using nanobubbles, using hydrophobically modified magnetic porous biochar, prepared through the following steps: S1: Ferric chloride hexahydrate, ferrous sulfate, and deionized water were placed in a reaction vessel at a mass ratio of 1:2.1:105 and stirred at 250 r / min for 30 min at 30 °C to obtain a mixed iron salt solution. Coconut shell powder was added to the solution, with a solid-liquid ratio of 1:10 g / mL between the coconut shell powder and the iron salt solution. The mixture was ultrasonically dispersed for 30 min, and a 1 mol / L NaOH solution was added to adjust the pH of the reaction solution to 10.1. The mixture was aged for 21 h, filtered, and the solid was washed with deionized water until the final wash solution was neutral. The solid was then vacuum dried at 70 °C to constant weight to obtain a magnetic biochar precursor.

[0031] S2: The magnetic biochar precursor was placed in a tube furnace and heated to 580°C at a heating rate of 5°C / min under a nitrogen atmosphere. It was then calcined at this temperature for 2.5 hours. After cooling in the furnace, it was ground and sieved through an 80-mesh sieve to obtain magnetic porous biochar.

[0032] S3: Myristic acid and a 10wt% methanol aqueous solution were placed in a reaction vessel and stirred at 250 r / min for 30 min at 30 °C. Magnetic porous biochar was added and ultrasonically dispersed for 30 min. The mixture was then reacted at the same stirring rate at 72 °C for 7 h. The mass ratio of myristic acid, 10wt% methanol aqueous solution and magnetic porous biochar was controlled at 1:90:8. After the reaction was completed, the mixture was cooled to room temperature, filtered, and the solid was washed three times with n-hexane. The solid was then vacuum dried at 70 °C to constant weight to obtain hydrophobically modified magnetic porous biochar.

[0033] Example 3: This example provides a method for enhancing the desorption of organic matter from fluoride-containing wastewater flocs using nanobubbles, using hydrophobically modified magnetic porous biochar, prepared through the following steps: S1: Ferric chloride hexahydrate, ferrous sulfate, and deionized water were placed in a reaction vessel at a mass ratio of 1:2.2:110 and stirred at 300 r / min for 40 min at 35 °C to obtain a mixed iron salt solution. Coconut shell powder was added to the solution, with a solid-liquid ratio of 1:12 g / mL between the coconut shell powder and the iron salt solution. The mixture was ultrasonically dispersed for 40 min, and a 1 mol / L NaOH solution was added to adjust the pH of the reaction solution to 10.2. The mixture was aged for 22 h, filtered, and the solid was washed with deionized water until the final wash solution was neutral. The solid was then vacuum dried at 80 °C to constant weight to obtain a magnetic biochar precursor.

[0034] S2: The magnetic biochar precursor was placed in a tube furnace and heated to 600°C at a heating rate of 5°C / min under a nitrogen atmosphere. It was then calcined for 3 hours and cooled in the furnace before being ground and sieved through a 100-mesh sieve to obtain magnetic porous biochar.

[0035] S3: Myristic acid and a 10wt% methanol aqueous solution were placed in a reaction vessel and stirred at 300 r / min for 40 min at 35℃. Magnetic porous biochar was added and ultrasonically dispersed for 40 min. The mixture was then reacted at 75℃ with the same stirring rate for 8 h. The mass ratio of myristic acid, 10wt% methanol aqueous solution and magnetic porous biochar was controlled at 1:100:10. After the reaction was completed, the mixture was cooled to room temperature, filtered, and the solid was washed four times with n-hexane. The solid was then vacuum dried at 80℃ to constant weight to obtain hydrophobically modified magnetic porous biochar.

[0036] Example 4: This example provides a method for enhancing the desorption of organic matter from fluoride-containing wastewater flocs using nanobubbles, comprising the following steps: Step 1: The sludge returned from the coagulation and sedimentation process of fluoride-containing wastewater is discharged through the sludge discharge pipe at the bottom of the high-density sedimentation tank and introduced into the gravity thickener. After gravity sedimentation and thickening, the water content of the thickened sludge is controlled to be 96% to obtain a thickened floc slurry.

[0037] Step Two: Place the concentrated floc slurry in a nanobubble reactor, start the circulation pump to pressurize the slurry to 0.3 MPa, and then send it to the micro / nanobubble generator. Simultaneously, inject ozone at a gas-to-water ratio of 0.2:1 to form nanobubbles. Return the mixed slurry containing nanobubbles to the reactor to enhance the desorption of the organic film on the surface of the aged flocs. Control the bubble concentration in the reactor to 10. 8 The mixture was stirred at 100 r / min for 5 min at 25 °C, and hydrophobically modified magnetic porous biochar was added at a dosage of 1% of the dry mass of the flocs. The mixture was stirred at 200 r / min for 3 min at the same temperature, and then stirred at 40 r / min for 3 min to obtain the magnetic core aggregate slurry.

[0038] Step 3: The magnetic core aggregate slurry is introduced into the magnetic separation device, and the magnetic field strength is controlled at 0.4T for magnetic separation. The magnetic core flocs are magnetically separated, retained and removed to obtain activated floc slurry. The magnetically separated floc slurry is introduced into the vacuum degassing device, and the vacuum degree is controlled at -0.08MPa for vacuum degassing for 3 minutes to obtain degassed activated floc slurry. The activated floc slurry is recycled to the front-end coagulation and sedimentation reaction zone.

[0039] Example 5: This example provides a method for enhancing the desorption of organic matter from fluoride-containing wastewater flocs using nanobubbles, comprising the following steps: Step 1: The sludge returned from the coagulation and sedimentation process of fluoride-containing wastewater is discharged through the sludge discharge pipe at the bottom of the high-density sedimentation tank and introduced into the gravity thickener. After gravity sedimentation and thickening, the water content of the thickened sludge is controlled to be 97% to obtain a thickened floc slurry.

[0040] Step Two: Place the concentrated floc slurry in a nanobubble reactor, start the circulation pump to pressurize the slurry to 0.35 MPa, and then send it to the micro / nanobubble generator. Simultaneously, inject ozone at a gas-to-water ratio of 0.4:1 to form nanobubbles. Return the mixed slurry containing nanobubbles to the reactor to enhance the desorption of the organic film on the surface of the aged flocs. Control the bubble concentration in the reactor to 10. 9 The mixture was stirred at 150 r / min for 8 min at 30 °C, and hydrophobically modified magnetic porous biochar was added at a dosage of 1.5% of the dry weight of the flocs. The mixture was stirred at 250 r / min for 4 min at the same temperature, and then stirred at 50 r / min for 4 min to obtain the magnetic core aggregate slurry.

[0041] Step 3: The magnetic core aggregate slurry is introduced into the magnetic separation device, and the magnetic field strength is controlled at 0.5T for magnetic separation. The magnetic core flocs are magnetically separated, retained and removed to obtain activated floc slurry. The magnetically separated floc slurry is introduced into the vacuum degassing device, and the vacuum degree is controlled at -0.09MPa for 4 minutes to obtain degassed activated floc slurry. The activated floc slurry is recycled to the front-end coagulation and sedimentation reaction zone.

[0042] Example 6: This example provides a method for enhancing the desorption of organic matter from fluoride-containing wastewater flocs using nanobubbles, comprising the following steps: Step 1: The sludge returned from the coagulation and sedimentation process of fluoride-containing wastewater is discharged through the sludge discharge pipe at the bottom of the high-density sedimentation tank and introduced into the gravity thickener. After gravity sedimentation and thickening, the water content of the thickened sludge is controlled to be 98% to obtain a thickened floc slurry.

[0043] Step Two: Place the concentrated floc slurry in a nanobubble reactor, start the circulation pump to pressurize the slurry to 0.4 MPa, and then send it to the micro / nanobubble generator. Simultaneously, inject ozone at a gas-to-water ratio of 0.5:1 to form nanobubbles. Return the mixed slurry containing nanobubbles to the reactor to enhance the desorption of the organic film on the surface of the aged flocs. Control the bubble concentration in the reactor to 10. 9 The mixture was stirred at 200 r / min for 10 min at 35 °C, and hydrophobically modified magnetic porous biochar was added at a dosage of 2% of the dry mass of the flocs. The mixture was stirred at 300 r / min for 5 min at the same temperature, and then stirred at 60 r / min for 5 min to obtain the magnetic core aggregate slurry.

[0044] Step 3: The magnetic core aggregate slurry is introduced into the magnetic separation device, and the magnetic field strength is controlled at 0.6T for magnetic separation. The magnetic core flocs are magnetically separated, retained and removed to obtain activated floc slurry. The magnetically separated floc slurry is introduced into the vacuum degassing device, and the vacuum degree is controlled at -0.095 MPa for 5 minutes to obtain degassed activated floc slurry. The activated floc slurry is recycled to the front-end coagulation and sedimentation reaction zone.

[0045] Comparative Example 1: The difference from Example 2 is that hydrophobically modified magnetic porous biochar is not added in step two, while the other steps remain unchanged, resulting in flocculent slurry.

[0046] Comparative Example 2: The difference from Example 2 is that in step two, commercially available coconut shell activated carbon of equal mass is used instead of the hydrophobically modified magnetic porous biochar prepared in S2, while the other steps remain unchanged, resulting in flocculent slurry.

[0047] Comparative Example 3: The difference from Example 2 is that in step two, magnetic porous biochar prepared by S2 of equal mass is used instead of hydrophobically modified magnetic porous biochar prepared by S3, while the other steps remain unchanged, and flocculent slurry is obtained.

[0048] The flocculent slurries obtained in Examples 4-6 and Comparative Examples 1-3 were subjected to performance tests, and the test results are shown in Table 1. The coconut shell powder purchased in the above embodiments is obtained by washing, drying, crushing and sieving commercially available coconut shells to 20-40 mesh.

[0049] The coconut shell activated carbon purchased in the above comparative example was produced by Fujian Xinsen Carbon Industry Co., Ltd., with a particle size of 200 mesh.

[0050] Turbidity test: Take the same volume of flocculent slurry from the example and comparative examples, stir evenly and let stand. After the mud-water interface stabilizes, take the supernatant and use a turbidity meter to measure the turbidity value (NTU) according to the standard GB / T 13200-1991 "Determination of Turbidity in Water".

[0051] Simulated fluoride-containing wastewater: Sodium fluoride, anhydrous sodium sulfate, and humic acid were dissolved in deionized water to prepare simulated fluoride-containing wastewater. The initial concentration of fluoride ions was controlled at 20 mg / L, the concentration of sulfate ions at 500 mg / L, the total organic carbon (TOC) at 100 mg / L (calculated as humic acid), and the pH value at 7.

[0052] Fluoride removal performance test: Equal volumes of the simulated fluoride-containing wastewater were taken and added to the flocculent slurry obtained in the examples and comparative examples at a sludge return ratio of 10%. Polyaluminum chloride (PAC) 100 mg / L was added to each group. After stirring and settling, the supernatant was collected. The fluoride ion concentration was determined using the ion-selective electrode method according to GB / T 7484-1987 "Determination of Fluoride in Water - Ion Selective Electrode Method". The fluoride removal efficiency was calculated using the formula η = (C0 - C...) / (C ... e η is the defluoridation efficiency, C0 is the initial fluoride ion concentration of the simulated fluoride-containing wastewater (20 mg / L), and C is the defluoridation efficiency. e The concentration of fluoride ions in the supernatant after each reaction is given.

[0053] Dosage of the reagent: Take the same volume of the above simulated fluoride-containing wastewater and add the flocculent slurry obtained in the examples and comparative examples at a sludge return ratio of 10%. PAC (50, 80, 100, 120, 150, 200 mg / L) is added to each group according to the gradient concentration. After stirring, the mixture is allowed to settle and the supernatant is taken to determine the fluoride ion concentration. The minimum PAC dosage is determined with the effluent fluoride ion concentration ≤1.5 mg / L as the criterion.

[0054] Table 1. Performance Test Table of Flocculent Slurry Turbidity value of supernatant 32.8 17.5 21.6 112.5 298.4 79.3 Defluorination efficiency (%) 74.6 82.3 79.5 50.8 55.6 65.2 Polyaluminum chloride dosage (mg / L) 128 112 118 188 165 142 As shown in Table 1, the activated floc slurry treated in Examples 4-6 maintains low supernatant turbidity while exhibiting high defluorination efficiency and reducing reagent dosage. This indicates that the present invention uses nanobubbles to oxidize and peel off aged flocs, causing the organic film on the flocs to desorb and release active sites. Hydrophobically modified magnetic porous biochar captures organic fragments in the liquid phase through its hydrophobic surface, forming magnetic aggregates. Magnetic separation then separates the organic fragments along with the magnetic particles. Finally, vacuum degassing eliminates residual microbubbles, preventing gas resistance interference during reuse. The synergistic effect of these steps restores the surface active sites and cleanliness of the returned flocs, thereby improving their fluoride ion capture efficiency and reducing PAC dosage.

[0055] In Comparative Example 1, after removing the hydrophobically modified magnetic porous biochar, the turbidity was significantly higher than in the Example, the defluorination efficiency was significantly reduced, and the minimum PAC dosage was also greatly increased. This may be because although nanobubbles can peel off the organic film on the surface of aged flocs, they lack the capture and magnetic core aggregation effect of hydrophobically modified magnetic porous biochar on organic fragments. Therefore, they cannot remove the organic fragments as a whole through magnetic separation, causing the peeled-off organic fragments to re-adhere to the floc surface under the drive of the concentration gradient. The active sites are then masked again, and the defluorination activity of the flocs deteriorates.

[0056] In Comparative Example 2, commercially available coconut shell activated carbon was used instead of hydrophobically modified magnetic porous biochar. The turbidity was significantly higher than in the Example 1, and the defluorination efficiency and minimum PAC dosage were improved. This may be because while coconut shell activated carbon can adsorb some organic matter, it lacks magnetic response and cannot form magnetic aggregates, thus failing to be removed by magnetic separation. Furthermore, its good particle dispersion and small particle size result in a large amount remaining in the liquid phase after treatment, becoming suspended particles and further increasing turbidity. The adsorbed organic fragments were also not effectively removed.

[0057] In Comparative Example 3, the turbidity and defluorination efficiency were inferior to those of the Example. This may be because although the unmodified magnetic porous biochar has a magnetic response and can be removed from the system by magnetic separation, it lacks ordered long-chain alkyl hydrophobic groups on its surface due to the lack of hydrophobic modification. For organic matter such as humic acid, which has hydrophobic aromatic rings and alkyl chains as its main structures, the hydrophobic interaction is weak, and the organic fragment capture efficiency is insufficient. After magnetic separation, a considerable proportion of organic fragments remain free in the liquid phase and reoccupy the active sites of metal hydroxides, resulting in the surface cleanliness and defluorination activity of the activated flocs being inferior to those of the Example.

[0058] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention.

Claims

1. A method for enhancing the desorption of organic matter from flocculent fluoride-containing wastewater using nanobubbles, characterized in that, Includes the following steps: Step 1: The sludge returned from the coagulation and sedimentation process of fluoride-containing wastewater is discharged through the sludge discharge pipe at the bottom of the high-density sedimentation tank and introduced into the gravity thickener. After gravity sedimentation and concentration, concentrated floc slurry is obtained. Step 2: Place the concentrated floc slurry in a nanobubble reactor, pressurize the slurry to 0.3-0.4 MPa and send it to a micro-nanobubble generator, inject ozone to form nanobubbles, and return the mixed slurry containing nanobubbles to the reactor to enhance the desorption of organic film on the surface of aged flocs. Stir at 100-200 r / min for 5-10 min at 25-35℃, add hydrophobically modified magnetic porous biochar, and continue stirring at 200-300 r / min for 3-5 min at the same temperature, followed by stirring at 40-60 r / min for 3-5 min to obtain magnetic core aggregate slurry; Step 3: The magnetic core aggregate slurry is introduced into the magnetic separation device for magnetic separation. The magnetic core flocs are magnetically separated, retained, and removed to obtain activated floc slurry. The magnetically separated floc slurry is then introduced into the vacuum degassing device for vacuum degassing to obtain degassed activated floc slurry. The activated floc slurry is then recycled to the front-end coagulation and sedimentation reaction zone.

2. The method for enhancing the desorption of organic matter from fluoride-containing wastewater flocs using nanobubbles according to claim 1, characterized in that, The concentrated flocculent slurry described in step one has a water content of 96-98%.

3. The method for enhancing the desorption of organic matter from fluoride-containing wastewater flocs using nanobubbles according to claim 1, characterized in that, The specific preparation process of the hydrophobically modified magnetic porous biochar described in step two is as follows: Myristic acid and a 10wt% methanol aqueous solution were placed in a reaction vessel and stirred at 25-35℃ for 20-40 min. Magnetic porous biochar was added and ultrasonically dispersed for 20-40 min. The mixture was then reacted at 70-75℃ for 6-8 h. After filtration, washing, and vacuum drying to constant weight, hydrophobic modified magnetic porous biochar was obtained.

4. The method for enhancing the desorption of organic matter from fluoride-containing wastewater flocs using nanobubbles according to claim 3, characterized in that, The mass ratio of myristic acid, methanol aqueous solution and magnetic porous biochar is 1:80-100:5-10.

5. The method for enhancing the desorption of organic matter from fluoride-containing wastewater flocs using nanobubbles according to claim 3, characterized in that, The specific preparation process of the magnetic porous biochar is as follows: The magnetic biochar precursor was placed in a tube furnace and heated to 550-600℃ under a nitrogen atmosphere. It was then calcined for 2-3 hours and cooled in the furnace before being ground and sieved to obtain magnetic porous biochar. The heating rate in the tubular furnace is 5°C / min.

6. The method for enhancing the desorption of organic matter from fluoride-containing wastewater flocs using nanobubbles according to claim 5, characterized in that, The specific preparation process of the magnetic biochar precursor is as follows: Ferric chloride hexahydrate, ferrous sulfate, and deionized water were placed in a reaction vessel and stirred at 25-35℃ for 20-40 min to obtain a mixed iron salt solution. Coconut shell powder was added to the solution and ultrasonically dispersed for 20-40 min. A 1 mol / L NaOH solution was added to adjust the pH of the reaction solution to 10-10.

2. The solution was aged for 20-22 h, filtered, washed, and vacuum dried to constant weight to obtain a magnetic biochar precursor. The mass ratio of ferric chloride hexahydrate, ferrous sulfate, and deionized water is 1:2.0-2.2:100-110; The solid-liquid ratio of the coconut shell powder and iron salt mixture is 1:8-12.

7. The method for enhancing the desorption of organic matter from fluoride-containing wastewater flocs using nanobubbles according to claim 1, characterized in that, In step two, the gas-to-water ratio of ozone to slurry in the micro-nano bubble generator is 0.2-0.5:

1.

8. The method for enhancing the desorption of organic matter from fluoride-containing wastewater flocs using nanobubbles according to claim 1, characterized in that, The concentration of nanobubbles in the mixed slurry containing nanobubbles in step two is 10. 8 -10 9 per mL.

9. The method for enhancing the desorption of organic matter from fluoride-containing wastewater flocs using nanobubbles according to claim 1, characterized in that, The magnetic field strength of the magnetic separation device in step three is 0.4-0.6T.

10. The method for enhancing the desorption of organic matter from fluoride-containing wastewater flocs using nanobubbles according to claim 1, characterized in that, The vacuum degree of the vacuum degassing device mentioned in step three is -0.08 to -0.095 MPa, and the vacuum degassing time is 3-5 min.