Method for rapid regeneration of PFAS-containing waste particles activated carbon and recovery of fluorine resources

Abstract

The application discloses a method for quick regeneration of PFAS-containing waste granular activated carbon and recovery of fluorine resources, and relates to the technical field of environmental pollution control and resource recycling, wherein, through the coupling of an alkali metal / alkaline earth metal additive and a sealed temperature control constant temperature Joule heat treatment, multiple benefits of treatment efficiency improvement, fluorine resource directional recovery and activated carbon performance regeneration are simultaneously achieved in the disposal process of the PFAS-containing waste granular activated carbon compared with the prior art.The method directly generates Joule heat by using the resistance of the granular activated carbon itself, does not need to rely on the external heat transfer path of the furnace wall and the carrier gas, has a fast heating rate, and can complete the heat treatment process within seconds.Compared with traditional furnace roasting, the treatment period is greatly shortened, the treatment flux of the unit equipment is effectively improved, and the device occupation and operation energy consumption are reduced.

Description

Technical Field

[0001] This invention relates to the field of environmental pollution control and resource recycling technology, specifically to a method for rapid regeneration of PFAS-containing waste granular activated carbon and recovery of fluorine resources. Background Technology

[0002] Per- and polyfluoroalkyl substances (PFAS) have become emerging pollutants of global concern due to their extremely high carbon-fluorine bond energies, strong environmental persistence, bioaccumulation, and potential health risks. Granular activated carbon (GAC), with its large specific surface area and well-developed pore structure, is currently the mainstream technology for the adsorption and removal of PFAS in drinking water, groundwater, and industrial wastewater. However, the adsorption process only achieves the transfer of PFAS from the aqueous phase to the solid phase and does not destroy its molecular structure. When GAC adsorption becomes saturated or breakthrough occurs, high-concentration PFAS-containing waste GAC is formed. If not properly disposed of, the accumulated PFAS can easily be released again, causing secondary pollution.

[0003] Current treatment routes for PFAS-containing waste GAC mainly include solvent desorption, hydrothermal alkaline treatment, mechanochemical treatment, traditional furnace heat treatment, and Joule heat treatment. Among these: Solvent desorption technology achieves the desorption of PFAS from the surface of GAC by eluting with organic solvents. However, it only completes the phase transfer of pollutants and cannot directly destroy CF bonds. It does not produce recyclable inorganic fluorine products and generates a large amount of fluorine-containing organic waste liquid that requires subsequent treatment, resulting in high treatment costs.

[0004] Hydrothermal alkaline treatment (HALT) achieves PFAS degradation and partial GAC regeneration in a near-critical high-temperature, high-pressure, strongly alkaline aqueous phase. However, the reaction requires pressure-resistant and corrosion-resistant equipment, the reaction time is as long as tens of minutes to several hours, and a highly alkaline fluorine-containing liquid phase is generated, making subsequent management difficult.

[0005] Traditional furnace-type thermal treatment relies on external heat transfer through the furnace wall and carrier gas, resulting in long heating and holding times and low processing efficiency. Improper atmosphere and temperature control can easily lead to the formation of incomplete mineralization products such as short-chain PFAS, volatile organic fluorine compounds, and HF, which increase the burden on tail gas treatment after being lost with the carrier gas. Furthermore, prolonged high temperatures can cause GAC ablation and damage to the pore structure, significantly reducing adsorption performance. While alkali-assisted furnace thermal mineralization can increase the proportion of solid-phase inorganic fluorine, it still does not solve the core problems of low external heat transfer efficiency and long processing cycles.

[0006] Ultra-high temperature flash Joule heating technology can reach 2000~3000℃ in the sub-second range to achieve efficient mineralization of PFAS. However, excessively high temperatures can lead to excessive graphitization of GAC, destroying the original microporous structure and particle morphology, and losing its adsorption and recycling value. At the same time, inorganic fluorine is easy to volatilize and disperse and deposit on electrodes, reactor inner walls and other parts at high temperatures, making fluorine resource collection difficult and posing a high risk of secondary release.

[0007] Electromagnetic induction rapid thermal degradation indirectly raises the temperature by heating the reactor wall, rather than directly heating the activated carbon itself. Existing studies mostly evaluate the degradation effect based on the residual amount of PFAS, and have not constructed a complete process chain of organic fluorine mineralization, inorganic fluorine recovery and activated carbon regeneration.

[0008] In summary, existing technologies cannot simultaneously achieve efficient PFAS mineralization, targeted enrichment and recovery of fluorine resources, and retention of activated carbon adsorption performance. They generally suffer from problems such as low treatment efficiency, high equipment requirements, dispersed fluorine resources, and insufficient resource utilization. There is an urgent need to develop a PFAS-containing waste GAC disposal technology that can achieve harmlessness and resource utilization in a second-level rapid processing manner. Summary of the Invention

[0009] To address the aforementioned shortcomings of existing technologies, this invention provides a method for the rapid regeneration of PFAS-containing waste granular activated carbon and the recovery of fluorine resources. This method utilizes alkali metal / alkaline earth metal additives coupled with a closed-loop, temperature-controlled Joule heating system to achieve efficient PFAS mineralization within seconds. Simultaneously, the generated inorganic fluorine is directionally retained in the GAC solid phase. Subsequently, fluorine resources are extracted through uniform water washing, and the adsorption performance of the GAC is restored concurrently. This constructs a complete process chain of "pollutant mineralization - fluorine resource recovery - activated carbon material regeneration," solving the problems of low processing efficiency, dispersed fluorine resources, and poor activated carbon regeneration effects in existing technologies.

[0010] To achieve the above-mentioned objectives, the technical solution adopted by this invention is as follows: A method for rapid regeneration of PFAS-containing waste granular activated carbon and recovery of fluorine resources is provided, comprising the following steps: S1: After uniformly mixing the waste granular activated carbon containing PFAS with the alkali metal / alkaline earth metal additive, the mixture is dried. The molar ratio of the positive charge equivalent of the metal cations in the alkali metal / alkaline earth metal additive to the initial total fluorine in the waste granular activated carbon is 1:1 to 4:1. PFAS-containing waste granular activated carbon includes PFAS-containing waste granular activated carbon generated in actual water treatment processes, as well as simulated waste granular activated carbon prepared by artificially loading PFAS. S2: Place the waste granular activated carbon loaded with alkali agent in a closed reaction chamber, and use the resistance effect of the granular activated carbon to generate Joule heat by passing an electric current through it, so that the material is rapidly heated to 400~800℃ and kept at a constant temperature for 2~30s, so that PFAS undergoes a mineralization reaction. The inorganic fluorine generated by mineralization is mainly retained in the solid phase of granular activated carbon. S3: Heat-treated granular activated carbon is added to ultrapure water for extraction. After solid-liquid separation, fluorine-containing leachate and defluorinated granular activated carbon are obtained. S4: The fluorine-containing leachate is treated to recover fluorine resources; the defluorinated granular activated carbon is washed and dried to obtain regenerated granular activated carbon.

[0011] Furthermore, PFAS include perfluorocarboxylic acids, perfluorosulfonic acids and their precursors; granular activated carbon is coal-based, coconut shell-based or wood-based granular activated carbon for water treatment.

[0012] Furthermore, the alkali metal / alkaline earth metal additive is an alkali metal additive / alkaline earth metal additive. Alkali metal / alkaline earth metal additives are one or more of the hydroxides, oxides, carbonates or chlorides of sodium, potassium, magnesium or calcium; The molar ratio of the positive charge equivalent of the metal cations in the alkali metal / alkaline earth metal additives to the initial total fluorine is 1.5:1 to 2.5:1.

[0013] Furthermore, the alkali metal / alkaline earth metal additives are NaOH, KOH, Ca(OH)2, Mg(OH)2, CaO, MgO or CaCl2.

[0014] Furthermore, in step S2, the sealed reaction chamber is a sealed quartz tube with copper electrodes inserted at both ends. The copper electrodes simultaneously serve the functions of conducting electricity, compressing the material, and sealing the reaction chamber, thus creating a closed reaction environment without continuous purging. An infrared thermometer is used to measure the material temperature in real time, and constant temperature conditions are maintained through power feedback control. After heat treatment, the quartz tube remains sealed, and inert gas is introduced outside the quartz tube for cooling. The inert gas does not enter the interior of the quartz tube to prevent leakage of fluorine-containing substances. The product is collected after the reactor cools to room temperature.

[0015] Furthermore, in step S3, the fluoride extraction by water washing is carried out by a combination of oscillation and ultrasound. After extraction, the solution is filtered through a 0.22μm aqueous filter membrane to achieve solid-liquid separation.

[0016] Furthermore, in step S4, the fluorine resource recovery adopts any of the following methods: When alkali metal additives are used, the fluorine-containing leachate is directly concentrated by evaporation and crystallization to obtain an enriched product containing alkali metal fluorides. When alkaline earth metal additives are used, a soluble alkaline earth metal salt corresponding to the fluoride is added to the fluoride-containing leachate to convert fluoride ions into insoluble alkaline earth metal fluorides and enrich them in the solid phase. After solid-liquid separation, washing and drying, the fluoride-enriched product is obtained.

[0017] Furthermore, in step S4, the defluorinated granular activated carbon is washed multiple times with deionized water until the pH, conductivity, and fluoride ion concentration of the washing solution meet the standards. After washing, the granular activated carbon is dried under vacuum, inert atmosphere, or oven to obtain regenerated granular activated carbon. The single-point adsorption performance of the regenerated granular activated carbon for PFOA is maintained at no less than 99%.

[0018] Furthermore, the method employs a continuous process: the waste granular activated carbon loaded with alkali is laid on a conductive conveying carrier, and rapid heating, second-level constant temperature and cooling processes are sequentially completed in a closed reaction chamber. The residence time is controlled by the conveying speed and the length of the heating zone. The treated granular activated carbon continuously enters the water washing, solid-liquid separation and regeneration processes.

[0019] The beneficial effects of this invention are as follows: This invention, through the coupling effect of alkali metal / alkaline earth metal additives and closed-loop temperature-controlled Joule heat treatment, achieves multiple benefits compared to existing technologies in the disposal of PFAS-containing waste granular activated carbon, simultaneously improving treatment efficiency, targeted recovery of fluorine resources, and regeneration of activated carbon performance. This method utilizes the inherent resistance of the granular activated carbon to directly generate Joule heat, eliminating the need for external heat transfer paths via furnace walls and carrier gas. The rapid heating rate allows the heat treatment process to be completed within seconds, significantly shortening the processing cycle compared to traditional furnace roasting, effectively increasing the throughput per unit of equipment, and reducing the footprint and energy consumption of the equipment.

[0020] Under the synergistic effect of alkali metal / alkaline earth metal additives and a medium-temperature closed reaction environment, the inorganic fluorine generated by PFAS mineralization can be mainly retained in the activated carbon solid phase in the form of water-soluble fluorides, with a concentrated proportion of over 95%. This avoids the dispersion and deposition of fluorine-containing products on multiple components such as reactor electrodes and inner walls. Subsequently, only a uniform water washing of the activated carbon solid phase is required to extract fluorine resources, which significantly simplifies the fluorine resource collection process and reduces the difficulty of equipment cleaning and the environmental risk of secondary release of fluorine-containing residues.

[0021] This invention controls the heat treatment temperature in the medium-low temperature range of 400~800℃, which can effectively avoid excessive graphitization and pore structure damage of high-temperature activated carbon. After the treated activated carbon is washed and dried, the single-point adsorption performance of PFOA can be maintained at over 99%, which is basically equivalent to the adsorption capacity of fresh activated carbon, and has long-term recycling value.

[0022] This method uses a dry solid-phase reaction system, which does not require near-critical high-pressure aqueous phase or extreme temperature and pressure conditions. The pressure resistance and corrosion resistance requirements of the equipment are much lower than those of hydrothermal alkaline treatment technologies. Moreover, the closed reaction system does not have a continuous carrier gas supply, which greatly reduces the amount of fluorine-containing tail gas generated, and lowers the tail gas treatment load and secondary pollution control costs.

[0023] The overall process constructs a complete closed loop of "PFAS mineralization - fluorine resource recovery - activated carbon regeneration", which not only completes the harmless treatment of hazardous waste containing PFAS, but also recovers fluorochemical raw materials and regenerated activated carbon adsorbent materials, thus achieving both environmental and economic benefits. The process is highly modular, easy to achieve continuous scale-up, and has good prospects for engineering applications. Attached Figure Description

[0024] Figure 1 This is a graph showing the effect of different isothermal times on the total water-washable inorganic fluorine yield in Example 1. Figure 2 This is a comparison of the adsorption performance of regenerated activated carbon and original activated carbon in Example 2; Figure 3 The bar chart shows the distribution ratio of recovered inorganic fluorine in different parts under alkali-added and alkali-free conditions. Detailed Implementation

[0025] The specific embodiments of the present invention are described below to enable those skilled in the art to understand the present invention. However, it should be understood that the present invention is not limited to the scope of the specific embodiments. For those skilled in the art, various changes are obvious as long as they are within the spirit and scope of the present invention as defined and determined by the appended claims. All inventions utilizing the concept of the present invention are protected.

[0026] In the following examples and comparative examples, F400 type coal-based granular activated carbon artificially loaded with PFOA was used as simulated waste GAC, with a typical PFOA loading of 9.6 mg PFOA / g GAC; NaOH was an analytical grade reagent. Total water-washable inorganic fluorine yield refers to the percentage of inorganic fluorine extracted by water washing relative to the initial fluorine content in PFOA, used to characterize the degree of conversion of organic fluorine to extractable inorganic fluorine.

[0027] Example 1: Temperature-controlled isothermal Joule heat treatment at different isothermal times S1: Calculate the amount of NaOH to be added according to the Na:F molar ratio of 2.5:1. Weigh 0.0348g of NaOH into an agate mortar and grind it into a powder. Then mix it thoroughly with 1.0000g of F400 granular activated carbon loaded with PFOA and dry it in an oven at 60~70℃ for 12h.

[0028] S2: Weigh 0.1000g of the dried material to be treated and fill it into the center of a quartz tube with a length of 7cm and an inner diameter of 8mm; insert copper electrodes into both ends and press them tightly to achieve conductivity and sealing, so that a closed reaction chamber is formed inside the quartz tube, and adjust the total resistance of the system to 1.0~1.2Ω.

[0029] S3: Activate the temperature control and constant temperature Joule heating device to rapidly raise the material to 600℃. Set the constant temperature time to 2s, 5s, 10s, 15s, 20s, and 30s respectively. Use an infrared thermometer to measure the temperature in real time and maintain temperature stability through feedback control.

[0030] S4: After the treatment is completed, keep the quartz tube sealed and introduce high-purity nitrogen into the external protective reaction chamber for cooling. Nitrogen should not enter the interior of the quartz tube. After cooling to room temperature, take out the treated GAC sample.

[0031] S5: Transfer GAC to a 50mL centrifuge tube, add 40mL of ultrapure water, oscillate at 40rpm for 24h, and then sonicate for 15min; after filtration through a 0.22μm aqueous filter membrane, measure the concentration of inorganic fluorine in the filtrate and calculate the total water-washable inorganic fluorine yield.

[0032] Experimental results are as follows Figure 1 As shown, by Figure 1 It can be seen that the total water-washable inorganic fluorine yields were 58.2% and 61.3% at 2s and 5s of constant temperature, respectively, indicating that the conversion of organic fluorine to water-washable inorganic fluorine was not yet complete. When the constant temperature time was extended to 10-20s, the total water-washable inorganic fluorine yield increased to 83.0%-85.8%, reaching a relatively high and stable level. When the time was further extended to 30s, the yield dropped to 80.4%, showing no further improvement. Therefore, the optimal constant temperature time at 600℃ is 10-20s.

[0033] Example 2 Evaluation of the re-adsorption performance of regenerated GAC The GAC sample treated at 600℃ for 20s in Example 1 was washed multiple times with deionized water until the washing solution was nearly neutral and the fluoride ion concentration was below the detection limit. Then it was dried at 60℃ to constant weight to obtain regenerated GAC.

[0034] 0.05 g of regenerated GAC and 0.05 g of fresh F400 granular activated carbon were weighed separately, and 15.0 mL of PFOA solution with an initial concentration of 150 mg / L was added to each. After sealing, the mixture was incubated at 40 rpm for 24 h for adsorption. After adsorption, the concentration of residual PFOA in the solution was measured, and the removal rate and single-point adsorption capacity were calculated.

[0035] The results are as follows Figure 2 As shown, by Figure 2 The results show that the PFOA removal rate of the fresh F400 group was 99.51%, with a single-point adsorption capacity of 44.78 mg / g; while the PFOA removal rate of the regenerated GAC group was 99.23%, with a single-point adsorption capacity of 44.65 mg / g. Calculations indicate that the PFOA single-point adsorption performance retention rate of the regenerated GAC was 99.72%, which is essentially consistent with the adsorption capacity of fresh activated carbon, demonstrating excellent reuse value.

[0036] Comparative Example 1: Traditional muffle furnace roasting treatment Using the same batch of PFOA-containing waste GAC as in Example 1, 0.9g of the material to be treated was weighed for each group and placed in a covered high-temperature resistant crucible, which was then placed in a muffle furnace. The temperature was increased from room temperature to 600°C at a rate of 10°C / min, and then held at the set temperature for 30min.

[0037] After the treatment was completed, the sample was naturally cooled to room temperature. The same water washing method as in Example 1 was used to extract inorganic fluorine from the solid phase of GAC, and the results were compared with those of Joule heat treatment.

[0038] Experimental results: The inorganic fluorine yield of muffle furnace roasting was only 10.7%, far lower than the 84.4% of that obtained by Joule heat treatment at 600℃ for 15 seconds; and the holding time of traditional furnace treatment was as long as 30 minutes, which is 180 times that of Joule heat treatment. This proves that the method of the present invention significantly improves the degree of PFAS mineralization and the inorganic fluorine conversion efficiency while greatly shortening the processing time.

[0039] Comparative Example 2: Joule heat treatment without alkali metal / alkaline earth metal additives Two parallel experiments were set up, both of which were subjected to Joule heat treatment at 600℃ for 15s: the experimental group was treated with NaOH at a Na:F molar ratio of 2.5:1, while the control group was treated without any alkali metal / alkaline earth metal additives. The rest of the operation was the same as in Example 1.

[0040] After processing, the solid phase of GAC, the inner wall of the quartz tube, and the copper electrode plug were washed and extracted with water. The inorganic fluorine content of each part was determined, and the total water-washable inorganic fluorine yield and fluorine distribution ratio were calculated. At the same time, the residual PFOA content in GAC was determined.

[0041] Experimental results are as follows Figure 3 As shown, by Figure 3 It can be seen that in the group with added NaOH, the residual amount of PFOA was 0.51% of the initial amount, corresponding to a degradation rate of 99.5%; the total water-washable inorganic fluorine yield was 83.0%; among which, 95.8% of the recovered inorganic fluorine was retained in the solid phase of GAC, accounting for 2.2% and 1.9% in the quartz tube and plug, respectively, indicating that inorganic fluorine was highly concentrated in the solid phase product.

[0042] In the group without NaOH, the PFOA degradation rate was 99.0%, but the total water-washable inorganic fluorine yield was only 40.7%. Moreover, a large amount of inorganic fluorine was dispersed in the device components, with 81.7% distributed in the plugs and only 2.2% retained in the GAC solid phase, making subsequent recovery extremely difficult.

[0043] The above results demonstrate that alkali metal / alkaline earth metal additives are the core conditions for achieving efficient conversion of organic fluorine to water-washable inorganic fluorine and for directionally retaining inorganic fluorine in the GAC solid phase; the coupling of alkali agent and closed-loop temperature-controlled Joule heating is the key technology combination for achieving efficient recovery of fluorine resources in this invention.

Claims

1. A method for rapid regeneration of PFAS-containing waste granular activated carbon and recovery of fluorine resources, characterized in that, Includes the following steps: S1: The waste granular activated carbon containing PFAS is mixed evenly with alkali metal / alkaline earth metal additives and then dried. The molar ratio of the positive charge equivalent of the metal cations in the alkali metal / alkaline earth metal additives to the initial total fluorine in the waste granular activated carbon is 1:1 to 4:

1. S2: Place the waste granular activated carbon loaded with alkali agent in a closed reaction chamber, and use the resistance effect of the granular activated carbon to generate Joule heat by passing an electric current through it, so that the material is rapidly heated to 400~800℃ and kept at a constant temperature for 2~30s, so that PFAS undergoes a mineralization reaction. The inorganic fluorine generated by mineralization is mainly retained in the solid phase of granular activated carbon. S3: The heat-treated granular activated carbon is added to water for extraction, and after solid-liquid separation, a fluorine-containing leachate and defluorinated granular activated carbon are obtained. S4: The fluorine-containing leachate is treated to recover fluorine resources; the defluorinated granular activated carbon is washed and dried to obtain regenerated granular activated carbon.

2. The method according to claim 1, characterized in that, The PFAS includes perfluorocarboxylic acids, perfluorosulfonic acids and their precursors; the granular activated carbon is coal-based, coconut shell-based or wood-based granular activated carbon for water treatment.

3. The method according to claim 1, characterized in that, The alkali metal / alkaline earth metal additive is an alkali metal additive / alkaline earth metal additive. The alkali metal / alkaline earth metal additive is one or more of the hydroxides, oxides, carbonates or chlorides of sodium, potassium, magnesium or calcium. The molar ratio of the positive charge equivalent of the metal cations in the alkali metal / alkaline earth metal additive to the initial total fluorine is 1.5:1 to 2.5:

1.

4. The method according to claim 3, characterized in that, The alkali metal / alkaline earth metal additives are NaOH, KOH, Ca(OH)2, Mg(OH)2, CaO, MgO or CaCl2.

5. The method according to claim 1, characterized in that, The sealed reaction chamber in step S2 is a sealed quartz tube with copper electrodes inserted at both ends. The copper electrodes simultaneously perform the functions of conducting electricity, compressing materials, and sealing the reaction chamber. An infrared thermometer is used to measure the temperature of the material in real time, and a constant temperature is maintained through power supply feedback control. After the heat treatment is completed, the quartz tube is kept sealed, and an inert gas is introduced into the outside of the quartz tube for cooling. The inert gas does not enter the inside of the quartz tube. The material is discharged after cooling to room temperature.

6. The method according to claim 1, characterized in that, In step S3, the fluoride extraction by water washing is carried out by a combination of oscillation and ultrasound. After extraction, the solution is filtered through a 0.22μm aqueous filter membrane to achieve solid-liquid separation.

7. The method according to claim 1, characterized in that, In step S4, fluorine resource recovery can be carried out using any of the following methods: When alkali metal additives are used, the fluorine-containing leachate is directly concentrated by evaporation and crystallization to obtain an enriched product containing alkali metal fluorides. When alkaline earth metal additives are used, a soluble alkaline earth metal salt corresponding to the fluoride is added to the fluoride-containing leachate to convert fluoride ions into insoluble alkaline earth metal fluorides and enrich them in the solid phase. After solid-liquid separation, washing and drying, the fluoride-enriched product is obtained.

8. The method according to claim 1, characterized in that, In step S4, the defluorinated granular activated carbon is washed multiple times with deionized water until the pH, conductivity and fluoride ion concentration of the washing solution meet the standards. After washing, the granular activated carbon is dried under vacuum, inert atmosphere or oven to obtain regenerated granular activated carbon. The regenerated granular activated carbon retains a single-point adsorption performance of PFOA of not less than 99%.

9. The method according to claim 1, characterized in that, The method is implemented using a continuous process: waste granular activated carbon loaded with alkali is laid on a conductive conveying carrier, and rapid heating, second-level constant temperature and cooling processes are completed sequentially in a closed reaction chamber. The residence time is controlled by the conveying speed and the length of the heating zone. The treated granular activated carbon continuously enters the water washing, solid-liquid separation and regeneration processes.

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