Solid alkali in-situ catalysis-based safe conversion and inorganic process for pyrolysis of perfluorinated and polyfluoroalkyl substances in sludge

Abstract

The invention discloses a safe pyrolysis conversion and inorganic process for perfluorinated and polyfluorinated alkyl substances in sludge based on solid alkali in-situ catalysis, and belongs to the technical field of environmental protection. The process comprises the following steps: pretreating sludge to reduce the water content of the sludge to below 10%; mixing a solid base catalyst with the pretreated sludge, controlling the mass fraction of the solid base catalyst to be 0.1-10%, and ensuring full mixing by adopting a mechanical stirring or ball-milling mixing method; and putting the mixture into a pyrolysis reactor, and carrying out pyrolysis reaction for 0.05-4 hours in different regions in a temperature range of 300-1000 DEG C. The technological process is safe and controllable, and toxic by-products such as dioxin possibly generated by traditional high-temperature incineration are avoided. Compared with a traditional incineration method, the process has the advantages that the operation temperature is lower, and the energy consumption is lower. A solid-phase product is good in stability and meets the environmental safety requirement through a leaching toxicity test. The cooperation of sludge treatment and PFAS removal is realized, and the overall treatment efficiency is improved.

Description

Technical Field

[0001] The present invention belongs to the field of environmental protection technology, and specifically relates to a safe conversion and inorganicization process of per- and polyfluoroalkyl substances (PFAS) in sludge by pyrolysis based on solid base in-situ catalysis, which realizes the safe conversion and inorganicization of PFAS in sludge and provides a basis for the safe resource utilization of sludge. Background Art

[0002] Per- and polyfluoroalkyl substances (PFAS), as a new class of pollutants, are widely present in sewage sludge, and their safe conversion has become a hot topic and difficulty in the field of environmental protection. The content of PFAS in sludge is usually in the range of 1-1000ng / g dry weight, among which PFOS and PFOA are the most common compounds, with typical concentrations of 10-400ng / g and 5-200ng / g, respectively. In highly polluted areas, PFAS concentrations may exceed 1000ng / g. The content of PFAS in sludge is affected by many factors, including the source of sewage (such as industrial or domestic sewage), treatment process, and geographical location.

[0003] PFAS are persistent, bioaccumulative and toxic, and long-term exposure may have serious effects on human health. These effects include liver damage, thyroid dysfunction, immune system suppression, fertility problems, and even an increased risk of certain cancers. The environmental hazards of PFAS should not be ignored either. They can enter the environment through the resource utilization of sludge, such as soil and groundwater pollution caused by agricultural application, residues in the composting process, and secondary pollution that may be caused in energy recovery and building materials utilization. These pollutions not only affect the balance of the ecosystem, but may also ultimately endanger human health through the food chain.

[0004] The treatment and disposal of PFAS in sludge faces multiple technical challenges. First, the carbon-fluorine bond in the PFAS molecule is extremely stable, making traditional biodegradation and chemical oxidation methods inefficient. Second, high-temperature incineration can only destroy some PFAS molecules, and some still enter the exhaust gas, posing the risk of air pollution and secondary pollution. Third, although the adsorption method can separate some PFAS from the sludge, it essentially only transfers the pollutants to another medium and fails to fundamentally solve the problem. In addition, due to the differences in physical and chemical properties, short-chain (C4-C7) and long-chain PFAS (C8 and above) are difficult to effectively treat simultaneously with a single technology. These challenges seriously restrict the safe treatment and resource utilization of sludge.

[0005] Therefore, the development of an efficient and safe pyrolysis, safe conversion and inorganic treatment technology for PFAS in sludge is of great significance for the safe resource utilization of sludge. Summary of the invention

[0006] The purpose of the present invention is to solve the problem of treatment and disposal of PFAS in sludge, and to provide a safe conversion and inorganicization process of PFAS in sludge by thermal decomposition based on solid base in situ catalysis, which can effectively remove short-chain and long-chain PFAS in sludge and convert them into inorganic fluorine.

[0007] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0008] A process for safe conversion and inorganicization of perfluoroalkyl and polyfluoroalkyl substances in sludge by thermal decomposition based on solid alkali in-situ catalysis, the process comprising the following steps:

[0009] (1) selecting a solid base catalyst;

[0010] (2) Pre-treating the sludge to reduce its moisture content to below 10%;

[0011] (3) mixing the solid base catalyst with the pretreated sludge, controlling the mass fraction of the solid base catalyst to be 0.1 to 10%, and using mechanical stirring or ball milling to ensure adequate mixing;

[0012] (4) placing the mixture in a pyrolysis reactor and performing pyrolysis reaction in different zones within a temperature range of 300-1000° C. for 0.05-4 hours.

[0013] (5) Collect and analyze the gas, liquid and solid products produced by the pyrolysis reaction.

[0014] Furthermore, in step (1), the solid base catalyst is one or more of alkaline sludge-based biochar, calcium oxide (CaO), calcium carbonate (CaCO3), zeolite, red mud, activated alumina (Al2O3), potassium hydroxide (KOH) and sodium hydroxide (NaOH).

[0015] Furthermore, the step (2) is specifically as follows: using a deep dehydration method to reduce the sludge moisture content to 60%, and then drying at 60-80°C for 2-24 hours to make the final moisture content <10%. Several types of pollutants in the present invention will volatilize at high temperatures, causing secondary pollution, so the drying temperature needs to be lower, and the final moisture content is lower than 10% to avoid generating a large amount of water vapor during pyrolysis to carry away pollutants, and the combination time with the catalyst is too short.

[0016] Furthermore, in step (4), the pyrolysis reactor is selected from one of a fixed bed reactor, a fluidized bed reactor, a screw propeller reactor and a rotary kiln reactor.

[0017] Furthermore, in step (4), the zoned pyrolysis reaction is specifically as follows: a. pre-reaction zone 300-500°C: reaction time 1min-60min; b. pyrolysis reaction zone 400-1000°C: reaction time 1min-120min; c. discharge zone 300-500°C: reaction time 1min-60min.

[0018] Furthermore, in step (4), the atmosphere when the pyrolysis reaction is stable is a reducing protective atmosphere, specifically H2, CH4, CO, hydrocarbons and reducing free radicals produced by pyrolysis, and nitrogen is introduced during startup; a dynamic sealing device is used for feeding and discharging to maintain the airtightness of the pyrolysis reactor.

[0019] Further, in step (5),

[0020] (1) The gas phase products were collected by a low temperature cold trap (-20°C) and analyzed by gas chromatography-mass spectrometry (GC-MS);

[0021] (2) using solvent extraction and analyzing the liquid phase product using liquid chromatography-tandem mass spectrometry (LC-MS / MS);

[0022] (3) The solid phase product was characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), etc., and the inorganic fluorine content was determined by ion chromatography.

[0023] Furthermore, it also includes using a Fourier transform infrared spectrometer (FTIR) and a quadrupole mass spectrometer (QMS) to monitor the gas during the pyrolysis process online.

[0024] Furthermore, the process is suitable for treating sludge containing short-chain (C4-C7) and long-chain (C8 and above) PFAS.

[0025] Process parameter optimization: Orthogonal experimental design was used to optimize process parameters, and the response surface methodology was used to establish a mathematical model of PFAS removal rate and key factors. The process feasibility was verified on a pilot scale (10-100 kg / h) to provide a basis for the design of a continuous treatment system and engineering scale-up.

[0026] Research on the transformation mechanism of PFAS: Study the effect of pyrolysis temperature on the breakage of PFAS molecular chains, the action mechanism of solid base catalysts on CF bond activation, the transformation pathways of PFAS under different reaction atmospheres, and the differentiated transformation behaviors of short-chain and long-chain PFAS, in order to gain a deeper understanding of the pyrolysis transformation process of PFAS.

[0027] The beneficial effects of the present invention compared to the prior art are:

[0028] (1) The process is safe and controllable, avoiding the production of toxic byproducts such as dioxins that may be produced by traditional high-temperature incineration.

[0029] (2) High energy efficiency. Compared with traditional incineration methods, this process has a lower operating temperature and consumes less energy.

[0030] (3) The solid phase product has good stability and meets environmental safety requirements after leaching toxicity testing.

[0031] (4) The synergy between sludge treatment and PFAS removal was achieved, improving the overall treatment efficiency.

[0032] (5) The process equipment is simple and easy to engineer and apply on a large scale.

[0033] Through the above technical scheme, the present invention achieves efficient removal of PFAS in sludge (total elimination rate>99.9%), safe conversion (HF and COF2 generation <1ppm) and high mineralization (90-95% conversion to inorganic fluoride). This method is suitable for treating sludge containing multiple PFAS, and the energy consumption is reduced by 30-40% compared with traditional high-temperature incineration. It has good process stability and economy, and provides a new technical path for the safe treatment of PFAS in sludge. DETAILED DESCRIPTION

[0034] The technical solution of the present invention is further described below in conjunction with the embodiments, but is not limited thereto. Any modification or equivalent replacement of the technical solution of the present invention without departing from the spirit and scope of the technical solution of the present invention should be included in the protection scope of the present invention.

[0035] Example 1: Alkaline sludge-based biochar catalyst

[0036] This example demonstrates the process and effect of using alkaline sludge-based biochar as a catalyst to treat PFAS-contaminated sludge. The specific steps are as follows:

[0037] Sludge pretreatment: Municipal sludge with a moisture content of 80% was selected as raw material. First, the sludge moisture content was reduced to 60% by deep dehydration. Subsequently, the dehydrated sludge was dried at 60°C to reduce the final moisture content to less than 10%.

[0038] Preparation of alkaline sludge-based biochar: Sludge with a high calcium content (greater than 5% dry weight) was selected as the biochar raw material. The dried high-calcium sludge was placed in a tube furnace and pyrolyzed at 750°C for 1 hour. After the pyrolysis was completed, it was naturally cooled to room temperature and the obtained biochar was ground into powder using a mortar.

[0039] Catalyst addition: The prepared alkaline sludge-based biochar was mixed with the pretreated dry sludge at a mass ratio of 5% (ie, 5 kg biochar / 95 kg dry sludge), and mixed thoroughly with a mechanical stirrer.

[0040] Pyrolysis reaction: The mixed sample is loaded into the screw-propelled pyrolysis furnace. The pyrolysis furnace is set with three temperature zones, namely: pre-reaction zone: 300℃, residence time 15min; pyrolysis reaction zone: 650℃, residence time 30min; discharge zone: 300℃, residence time 10min; dynamic sealing feeding and discharging devices are used during the pyrolysis process to maintain the reducing atmosphere in the screw-propelled pyrolysis furnace.

[0041] Product collection and analysis: For gas phase products, a low temperature cold trap at -186°C was used to capture volatile products, followed by qualitative and quantitative analysis using gas chromatography-mass spectrometry (GC-MS). Focus on PFOA, PFOS and their degradation intermediates. For liquid phase products, a mixed solvent of dichloromethane and methanol (1:1 volume ratio) was used to extract the liquid phase products, and liquid chromatography-tandem mass spectrometry (LC-MS / MS) was used for analysis; for solid phase products, the chemical form of fluorine in the solid phase products was analyzed using X-ray photoelectron spectroscopy (XPS), and the water-soluble inorganic fluorine content was determined using ion chromatography (IC).

[0042] Result analysis:

[0043] By comparing the contents of PFOA and PFOS in the samples before and after pyrolysis, it was calculated that the PFOA removal rate was 99.95% and the PFOS removal rate was 99.98%. XPS and IC analysis results showed that 90.5% of the organic fluorine was converted into inorganic fluorine, mainly in the form of CaF2.

[0044] This example demonstrates that the use of alkaline sludge-based biochar as an in-situ catalyst can effectively promote the pyrolysis degradation of PFAS. The two-stage pyrolysis process (300°C pre-pyrolysis and 650°C main pyrolysis) is conducive to the gradual cracking and mineralization of PFAS. High-calcium biochar not only provides an alkaline environment to promote the breaking of CF bonds, but also can capture fluoride ions produced during pyrolysis to form stable CaF2, effectively preventing the generation of harmful gases such as HF.

[0045] This method achieves efficient removal and safe transformation of PFAS, providing a new and environmentally friendly technical path for the treatment of PFAS in sludge. Subsequent research will further optimize process parameters, explore the degradation mechanism of different types of PFAS, and evaluate the environmental safety and resource utilization potential of treated sludge.

[0046] Example 2: Performance of calcium oxide catalyst at different temperatures

[0047] This example aims to study the effect of calcium oxide (CaO) as a catalyst on the removal and mineralization of PFAS at different pyrolysis temperatures. The specific steps are as follows:

[0048] Sludge pretreatment: Municipal sludge was pretreated in the same manner as in Example 1. Sludge with a moisture content of 80% was dehydrated to a moisture content of 60% by deep dehydration, and then dried at 60° C. to reduce the final moisture content to less than 10%.

[0049] Catalyst selection and addition: Calcium oxide is selected as the catalyst. Calcium oxide is mixed with the pretreated dry sludge at a mass ratio of 3% (ie, 3 kg CaO / 97 kg dry sludge), and a mechanical stirrer is used to ensure full mixing.

[0050] Pyrolysis reaction: The mixed sample was divided into three parts and pyrolysis experiments were carried out at different temperatures. A spiral propeller furnace was used as the reactor, and 50 kg was loaded for each experiment. The pyrolysis conditions are as follows: the pyrolysis temperature and residence time in the pre-reaction zone and the discharge zone are the same as in Example 1; the effect of the pyrolysis conditions in the pyrolysis reaction zone on the removal of PFAS was investigated, a. Low temperature group: 400°C, maintained for 1 hour; b. Medium temperature group: 650°C, maintained for 45 minutes; c. High temperature group: 800°C, maintained for 30 minutes.

[0051] Product collection and analysis: The products were collected and analyzed in the same manner as in Example 1. The gas phase products were captured using a -20°C low temperature cold trap and analyzed by GC-MS; the liquid phase products were analyzed by solvent extraction and analyzed by LC-MS / MS; the solid phase products were analyzed by XPS for the chemical form of fluorine and IC for the content of water-soluble inorganic fluorine.

[0052] Result analysis: By comparing the PFAS content in the samples before and after pyrolysis, the removal rate and mineralization rate at different temperatures were calculated:

[0053] a. 400℃ pyrolysis: PFAS total removal rate: 85%; mineralization rate (ratio of organic fluorine converted to inorganic fluorine): 60%;

[0054] b.650℃ pyrolysis: PFAS total removal rate: 98%; mineralization rate: 89%;

[0055] c.800℃ pyrolysis: PFAS total removal rate: 99.9%; mineralization rate: 95%.

[0056] This example clearly demonstrates the significant effect of pyrolysis temperature on PFAS removal and mineralization efficiency:

[0057] a. Although 85% PFAS removal could be achieved under low temperature (400°C), the degree of mineralization was low, indicating that some PFAS may be converted into intermediates rather than fully mineralized.

[0058] b. Under medium temperature (650°C), the PFAS removal rate increased significantly to 98%, and the mineralization rate also increased to 89%, indicating that the activity of the CaO catalyst was significantly enhanced at this temperature.

[0059] c. Under high temperature (800°C) conditions, almost all PFAS were removed (99.9%), and 95% were converted into inorganic fluorine, achieving efficient removal and safe mineralization of PFAS.

[0060] The results show that the performance of the CaO catalyst gradually improves with increasing temperature. This may be because high temperature promotes the breaking of CF bonds and accelerates the reaction of CaO with fluoride ions to form stable CaF2. The excellent performance at 800°C proves that CaO has a strong ability to decompose and fix PFAS at high temperatures.

[0061] However, considering energy consumption and equipment requirements, 650 °C may be a more economically feasible operating temperature, at which higher removal and mineralization rates can be achieved.

[0062] Leaching toxicity test of sludge-based biochar: Take the sludge-based biochar after pyrolysis at 650°C in Example 2, and conduct a leaching experiment according to HJ / T 300-2007 "Solid Waste Leaching Toxicity Leaching Method Horizontal Oscillation Method" to determine the fluoride ion concentration in the leachate.

[0063] Results: The fluoride ion concentration in the leachate was 0.8 mg / L, which was lower than the primary discharge standard (10 mg / L) in the Integrated Wastewater Discharge Standard (GB 8978-1996), indicating that pyrolysis sludge-based biochar has good environmental safety.

[0064] Comparison of removal effects of different types of PFAS: Under the conditions of Example 2, the removal effects of perfluorooctane sulfonic acid (PFOS), perfluorooctanoic acid (PFOA), perfluorobutane sulfonic acid (PFBS) and perfluorohexanoic acid (PFHxA) were tested respectively.

[0065] Results: PFOS: removal rate 99.98%, mineralization rate 92%; PFOA: removal rate 99.95%, mineralization rate 90%; PFBS: removal rate 99.93%, mineralization rate 89%; PFHxA: removal rate 99.91%, mineralization rate 88%.

[0066] This result indicates that this process has good removal and mineralization effects on both long-chain and short-chain PFAS.

[0067] Example 3: Comparison of different catalysts

[0068] The three pyrolysis zone conditions are the same as those in Example 2. The pyrolysis temperature is 650°C and the residence time is 45 min. The performance of different catalysts (mass ratio is 5%) is compared under the pyrolysis conditions:

[0069] Calcium carbonate: PFAS removal rate 98%, mineralization rate 85%

[0070] Zeolite: PFAS removal rate 97%, mineralization rate 82%

[0071] Red mud: PFAS removal rate 99%, mineralization rate 88%

[0072] Activated alumina: PFAS removal rate 96%, mineralization rate 80%

[0073] Potassium hydroxide: PFAS removal rate 99.5%, mineralization rate 92%

[0074] Sodium hydroxide: PFAS removal rate 99.3%, mineralization rate 91%

[0075] This example compares the removal and mineralization effects of six different solid base catalysts on PFAS under 650°C pyrolysis conditions:

[0076] a. All catalysts showed good PFAS removal capabilities, with removal rates above 96%. Among them, potassium hydroxide and sodium hydroxide performed best, with removal rates reaching 99.5% and 99.3%, respectively.

[0077] b. In terms of mineralization rate, potassium hydroxide and sodium hydroxide also performed best, reaching 92% and 91% respectively, indicating that strong alkaline catalysts are more conducive to the complete mineralization of PFAS.

[0078] c. Red mud, as an industrial waste, showed excellent catalytic performance (removal rate 99%, mineralization rate 88%), which may be due to its complex composition and porous structure providing good catalytic active sites.

[0079] d. Although calcium carbonate has a weak alkalinity, it still shows good performance (removal rate 98%, mineralization rate 85%), which may be related to its decomposition at high temperature to release CO2 and form active CaO.

[0080] e. The performance of zeolite and activated alumina is relatively weak, but they can still achieve a removal rate of more than 96% and a mineralization rate of more than 80%, indicating that they also have certain catalytic activity.

[0081] The results show that strong alkaline catalysts (such as KOH and NaOH) perform best in PFAS removal and mineralization. However, considering the cost, safety and environmental friendliness in practical applications, red mud and calcium carbonate may be more promising options. As an industrial waste, the utilization of red mud can not only effectively treat PFAS, but also realize waste resource utilization. Calcium carbonate has advantages due to its low cost and environmental friendliness.

[0082] Example 4: Study on the transformation mechanism of PFAS

[0083] This example aims to deeply study the pyrolysis conversion mechanism of PFAS under solid base catalysis, and uses a variety of advanced analytical techniques for in-situ and offline analysis. First, perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) were selected as representative PFAS compounds, and their mixture (50 mg / kg each) was evenly dispersed in silica powder, and then fully mixed with 5% (mass ratio) potassium hydroxide (KOH). The experiment used a customized pyrolysis reactor equipped with in-situ FTIR and mass spectrometry analysis interfaces to monitor the generation of gas phase products in real time.

[0084] The experimental process includes three main parts. First, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed. In a nitrogen atmosphere, the sample was heated from room temperature to 900°C at a heating rate of 10°C / min, and the mass change and heat flow data of the sample were recorded. Secondly, in situ FTIR and mass spectrometry analysis were performed. The sample was placed in a pyrolysis reactor and heated to 800°C at a heating rate of 10°C / min in a nitrogen atmosphere. The infrared spectrum of the gas phase product was collected every 30 seconds, and the mass spectrum information of the gas phase product was monitored in real time using a quadrupole mass spectrometer. Finally, the pyrolysis reaction was interrupted at three temperature points of 300°C, 500°C and 700°C, the sample was quickly cooled, and the chemical state changes of fluorine in the solid phase product were analyzed by X-ray photoelectron spectroscopy (XPS).

[0085] The results showed that the TGA-DSC curve showed two obvious mass loss stages in the range of 200-400℃ and 400-600℃, corresponding to the initial decomposition of PFAS and the further decomposition of intermediate products, respectively. In situ FTIR analysis showed that in the range of 300-400℃, the CF stretching vibration peak intensity gradually weakened, and an unsaturated C=C stretching vibration peak appeared. As the temperature increased, the intensity of the characteristic peak of CO2 increased, but no obvious HF characteristic peak was detected. Mass spectrometry results showed that signals of fluorinated alkyl fragments were detected in the low-temperature stage (<400℃). As the temperature increased, these signals weakened, and smaller fluorinated fragments were detected. In the high-temperature stage (>600℃), CO2 was mainly detected. + and a small amount of small molecular fragments. XPS analysis results show that with the increase of temperature, the intensity of the characteristic peak of CF bond decreases significantly, while the characteristic peak of metal-F bond gradually increases.

[0086] Based on the above analysis results, the present invention proposes the pyrolysis conversion mechanism of PFAS under solid base catalysis: in the initial stage (200-400℃), PFAS molecules first undergo defluorination reaction, CF bonds break, and unsaturated carbon fluoride intermediates are generated; in the intermediate stage (400-600℃), the unsaturated intermediates are further decomposed into smaller carbon fluoride fragments; in the final stage (>600℃), the small molecular carbon fluoride fragments are further oxidized and decomposed, and finally CO2 and HF are generated. During the whole process, the alkaline sites on the surface of the catalyst (such as KOH) continuously capture the HF generated during the pyrolysis process to form stable metal fluorides (such as KF), which effectively prevents the release of toxic HF gas.

[0087] This mechanism explains the efficient removal and safe mineralization process of PFAS. The solid base catalyst not only promotes the decomposition of PFAS, but also effectively prevents the release of toxic HF gas, achieving the safe transformation and inorganicization of PFAS. This example uses a variety of advanced analytical techniques to deeply reveal the transformation mechanism of PFAS during solid base catalytic pyrolysis, providing a theoretical basis for further optimizing process parameters and developing more efficient catalytic systems. These findings also confirm the innovation and effectiveness of the present invention in the safe treatment of PFAS, and provide a new technical path for the safe treatment of PFAS in sludge.

[0088] Example 5: Online monitoring of gases during pyrolysis

[0089] On the basis of Example 4, a gas online monitoring system is added, including a Fourier transform infrared spectrometer (FTIR) and a quadrupole mass spectrometer (QMS).

[0090] Results: FTIR detected characteristic peaks of gases such as CO2, CO, and HF. With the increase of temperature, the peak intensity of CO2 gradually increased, while the peak intensity of HF reached its maximum in the medium temperature range (500-650℃) and then gradually weakened. QMS detected ions with m / z of 20, 44, and 69, corresponding to HF, CO2, and CF3, respectively. + As the temperature increases, the signal intensity of these ions changes in a trend consistent with the FTIR results.

[0091] This result further confirmed the gradual decomposition process of PFAS and showed the effective capture of HF by solid base.

[0092] Example 6: Detailed analysis of PFAS transformation process and mechanism

[0093] This example uses a variety of advanced analytical techniques to conduct an in-depth study on the transformation mechanism of PFAS during solid base catalytic pyrolysis. Using PFOA (perfluorooctanoic acid) as a representative PFAS compound and calcium oxide as a solid base catalyst, a pyrolysis experiment was conducted at 650°C.

[0094] GC-MS (gas chromatography-mass spectrometry) analysis: Findings: A series of short-chain perfluorinated compounds and unsaturated fluorinated hydrocarbons were detected. Mechanism inference: PFOA first undergoes CC bond cleavage to generate shorter-chain perfluorinated compounds. At the same time, some CF bonds break, resulting in the formation of unsaturated fluorinated hydrocarbons.

[0095] IC (ion chromatography) analysis: Findings: A significant increase in fluoride ions (F-) was detected. Mechanism inference: The organic fluorine in PFOA is gradually converted into inorganic fluorine, confirming the occurrence of the mineralization process.

[0096] XPS (X-ray Photoelectron Spectroscopy) Analysis: Findings: A decrease in CF bonds and an increase in metal-F bonds were observed. Mechanism inference: The CF bonds in PFOA are destroyed, and the free fluorine reacts with the catalyst (CaO) to form CaF2, confirming the effective capture of HF by the solid base.

[0097] FTIR (Fourier transform infrared spectroscopy) analysis: It was found that the characteristic peak intensity of CF2 and CF3 groups weakened over time, and the formation of C=C bonds was observed. Mechanism inference: The carbon chain structure of PFOA was gradually destroyed, and a defluorination reaction occurred at the same time to form unsaturated fluorocarbon intermediates.

[0098] Mass spectrometry analysis: Found: m / z value was detected to be 69 (CF3 + )、119(C2F5 + ) and other characteristic ion fragments. Mechanism inference: PFOA molecules gradually decompose into smaller fluorinated carbon fragments, which further degrade into final products.

[0099] TGA (thermogravimetric analysis): Findings: Obvious mass loss was observed in the range of 300-400°C, and the mass change tended to be gentle above 500°C. Mechanism inference: The main decomposition of PFOA occurred at 300-400°C, and the high temperature stage was mainly the further mineralization of residual fluorocarbons.

[0100] DSC (Differential Scanning Calorimetry) analysis: It was found that there was an obvious endothermic peak at around 350°C and a smaller exothermic peak at around 600°C. Mechanism inference: The endothermic peak at 350°C corresponds to the main decomposition process of PFOA, and the exothermic peak at 600°C may be related to the mineralization reaction on the catalyst surface.

[0101] Based on the above analysis, the present invention proposes that the conversion mechanism of PFOA during solid base catalytic pyrolysis is as follows:

[0102] Initial stage (300-400℃): PFOA molecules begin to decompose, the carboxyl group decomposes to produce CO2, the CC bond breaks to produce short-chain perfluorinated compounds, and some CF bonds break to form unsaturated fluorinated hydrocarbons.

[0103] Intermediate stage (400-600℃): Short-chain perfluorinated compounds and unsaturated fluorinated hydrocarbons further decompose to generate smaller carbon fluorine fragments (such as CF3 + 、C2F5 + At the same time, a small amount of HF begins to be generated and captured by the solid base.

[0104] Final stage (600-900℃): Fluorocarbon fragments further decompose to generate HF. HF is effectively captured by solid base to form stable metal fluorides (such as CaF2). Residual fluorocarbons continue to slowly mineralize.

[0105] This mechanism explains the transformation process of PFOA from organic fluorine to inorganic fluorine, confirms the important role of solid base catalysts in promoting the decomposition of PFAS and capturing HF, and provides a theoretical basis for achieving the safe transformation and efficient mineralization of PFAS.

Claims

1. A process for safe conversion and inorganicization of per- and polyfluoroalkyl substances in sludge by thermal decomposition based on solid base in-situ catalysis, characterized in that: The process comprises the following steps: (1) selecting a solid base catalyst; (2) Pre-treating the sludge to reduce its moisture content to below 10%; (3) mixing the solid base catalyst with the pretreated sludge, and controlling the mass fraction of the solid base catalyst to be 0.1 to 10%; (4) placing the mixture in a pyrolysis reactor and performing pyrolysis reaction in different zones within a temperature range of 300-1000° C. for 0.05-4 hours.

2. The process according to claim 1, characterized in that: In step (1), the solid base catalyst is one or more of alkaline sludge-based biochar, calcium oxide, calcium carbonate, zeolite, red mud, activated alumina, potassium hydroxide and sodium hydroxide.

3. The process according to claim 1, characterized in that: The step (2) specifically comprises: reducing the sludge moisture content to 60% by deep dehydration method, and then low-temperature drying at 60-80° C. for 2-24 hours to make the final moisture content <10%.

4. The process according to claim 1, characterized in that: In step (4), the pyrolysis reactor is one of a fixed bed reactor, a fluidized bed reactor, a screw propeller reactor and a rotary kiln reactor.

5. The process according to claim 1, characterized in that: In step (4), the zoned pyrolysis reaction is specifically as follows: a. pre-reaction zone 300-500°C: reaction time 1min-60min; b. pyrolysis reaction zone 400-1000°C: reaction time 1min-120min; c. discharge zone 300-500°C: reaction time 1min-60min.

6. The process according to claim 1, characterized in that: In step (4), the atmosphere when the pyrolysis reaction is stable is a reducing protective atmosphere, and nitrogen is introduced during startup.

7. The process according to claim 1, characterized in that: The process is suitable for treating sludge containing short-chain (C4-C7) and long-chain (C8 and above) PFAS.