Method for removing polyfluoroalkyl pollutants by using out-of-phase monatomic photocatalyst

Abstract

The invention discloses a method for removing polyfluoroalkyl pollutants by using a heterogeneous monatomic photocatalyst, and relates to the technical field of pollutant treatment.The method comprises the steps that a heterogeneous monatomic photocatalyst precursor is prepared, specifically, silicon dioxide solid particles are prepared by adopting a Stber synthesis method, and the heterogeneous monatomic photocatalyst precursor is prepared by taking the silicon dioxide solid particles as a template agent; coating titanium dioxide, loading transition metal, adsorbing polyvinylpyrrolidone and secondarily coating silicon dioxide to obtain a precursor; according to the prepared heterogeneous monatomic photocatalyst, through hollow sphere structure design and monatomic transition metal loading, the efficiency of photocatalytic degradation of PFAS is remarkably improved, the specific surface area of the catalyst is increased through a hollow structure, the adsorption and enrichment capacity of the catalyst to PFAS molecules is enhanced, meanwhile, monatomic dispersed transition metal serves as an active center, and the adsorption and enrichment capacity of the catalyst to PFAS molecules is improved. According to the method, the PFAS can be fully subjected to nucleophilic substitution reaction with a C-F bond in the PFAS, and the degradation rate of the PFAS can be remarkably improved under the conditions that the pH is 5-7, the dosage of the catalyst is 0.5-3.0 g / L and the hydraulic retention time is 30-240 minutes.

Description

Technical Field

[0001] This invention relates to the field of pollutant treatment technology, specifically to a method for removing polyfluoroalkyl pollutants using a heterogeneous single-atom photocatalyst. Background Technology

[0002] With the rapid development of modern industry and technology, polyfluoroalkyl substances (PFAS), as a class of compounds with excellent thermal and chemical stability, are widely used in industrial production, daily life, and high-tech fields. From waterproof and breathable materials, non-stick cookware coatings, and fire-fighting foam to semiconductor manufacturing processes, PFAS are highly favored due to their unique physicochemical properties. However, it is precisely this widespread and continuous application that has led to the environmental release of PFAS worldwide, especially through industrial wastewater discharge, landfill leachate, and rainwater runoff into water bodies, causing serious water pollution problems. The persistence and bioaccumulation of PFAS enable them to exist in the environment for a long time and accumulate step by step through the food chain, ultimately posing a potential threat to human health. Therefore, PFAS pollution has become an important issue that urgently needs to be addressed in the global environmental science field.

[0003] However, traditional technologies for the treatment of PFAS pollution mainly include physical adsorption, chemical oxidation, biodegradation, and membrane separation. But these methods all have significant limitations in practical applications. Although physical adsorption is simple to operate, the adsorbent is easily saturated and difficult to regenerate, resulting in high treatment costs and the inability to completely remove PFAS. Although chemical oxidation can destroy some of the PFAS molecular structure, it usually requires harsh reaction conditions and a large amount of chemical reagents, and it will produce toxic byproducts. Biodegradation is limited by the types of microorganisms and environmental conditions, and its degradation efficiency for PFAS is generally low. Although membrane separation technology can effectively retain PFAS, it only transfers the pollutants to the concentrated aqueous phase, and further treatment is still required. These traditional methods generally fail to solve the problem of secondary pollution in the PFAS degradation process and are difficult to meet increasingly stringent environmental protection requirements.

[0004] Therefore, a method for the complete degradation and removal of polyfluoroalkyl contaminants using heterogeneous single-atom photocatalysts was developed. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a method for removing polyfluoroalkyl pollutants using a heterogeneous single-atom photocatalyst. This method significantly improves the efficiency of photocatalytic degradation of PFAS through the preparation of a heterogeneous single-atom photocatalyst with a hollow sphere structure and single-atom transition metal loading. The hollow structure increases the specific surface area of ​​the catalyst, enhancing its adsorption and enrichment capacity for PFAS molecules. Simultaneously, the single-atom dispersed transition metal acts as an active center, enabling it to fully undergo nucleophilic substitution reactions with the CF bonds in PFAS. Under conditions of pH 5-7, catalyst dosage 0.5-3.0 g / L, and hydraulic retention time 30-240 minutes, the PFAS degradation rate can be significantly improved, far exceeding that of traditional adsorption, chemical oxidation, and biodegradation methods. Furthermore, the photocatalytic reduction reaction directly breaks the CF bonds, avoiding secondary pollution. This provides a key solution for the efficient treatment of highly toxic and recalcitrant PFAS pollution.

[0006] To solve the above-mentioned technical problems, the present invention provides the following technical solution: a method for removing polyfluoroalkyl contaminants using a heterogeneous single-atom photocatalyst, the method comprising:

[0007] Preparation of heterogeneous single-atom photocatalyst precursor: Silica solid particles were prepared by Stöber synthesis, and the precursor was obtained by coating titanium dioxide, loading transition metal, adsorbing polyvinylpyrrolidone, and secondary coating silica using silica solid particles as template agent.

[0008] Preparation of hollow sphere heterogeneous single-atom photocatalyst: The precursor was calcined to form a single-atom distribution of transition metal. The calcined product was dissolved in NaOH solution to dissolve the silica template. After centrifugation, washing and drying, a hollow sphere particle structure heterogeneous single-atom photocatalyst was obtained.

[0009] Adjusting the reaction conditions of PFAS-containing water: Introduce raw water containing PFAS pollutants into the raw water conditioning tank, and add sulfuric acid and sodium hydroxide through a pH adjustment dosing device to adjust the pH value of the raw water containing PFAS pollutants to 5-7.

[0010] Photocatalytic advanced reduction reaction to degrade PFAS: Adjusted raw water is sent into a photocatalytic reaction tower, heterogeneous single-atom photocatalyst is added to form a fluidized system, and the reaction is carried out under the irradiation of a low-pressure mercury lamp to degrade pollutants;

[0011] Solid-liquid separation and catalyst recycling: The mixture after the reaction is separated by a solid-liquid separation tower. The catalyst slurry is returned to the photocatalytic reaction tower by a reflux diaphragm pump to continue participating in the reaction. After separation, the supernatant, which does not contain the catalyst, is discharged from the drain outlet of the clean water area.

[0012] Furthermore, in the preparation of the heterogeneous single-atom photocatalyst precursor, silica solid particles are prepared using the Stöber synthesis method. The steps are as follows: an alcohol solvent is selected and mixed with deionized water to form a base solvent, wherein the alcohol solvent is one of ethanol, ethylene glycol or propanol, preferably ethanol, and the volume ratio of alcohol to deionized water is 4-10:1. A quaternary ammonium salt cationic surfactant is added to the base solvent, followed by ammonia water with a mass concentration of 28-30 wt% as a catalyst. Tetraethyl orthosilicate is slowly added dropwise while stirring, the volume of tetraethyl orthosilicate accounting for 4-5% of the total volume of the solvent, and the volume ratio of tetraethyl orthosilicate to ammonia water is 1.5-2:1. The total volume after mixing is placed in a constant temperature water bath at 30°C and stirred continuously for 10 hours. After the reaction is completed, solid particles are obtained by solid-liquid separation, and residual impurities are removed by washing with ethanol and deionized water. After drying, monodisperse silica solid particles are obtained.

[0013] Furthermore, in the preparation of the heterogeneous single-atom photocatalyst precursor, the amount of quaternary ammonium salt cationic surfactant is 0.5-3 wt% of the added tetraethyl orthosilicate.

[0014] Furthermore, in the preparation of the heterogeneous single-atom photocatalyst precursor, the step of coating titanium dioxide is as follows: adding silica solid particles to a mixed solvent of alcohol and acetonitrile to prepare a suspension with a silica solid particle mass concentration of 5 wt%, and the volume ratio of alcohol to acetonitrile in the mixed solvent is 2-4:1; adding an organic titanium source diluted with the alcohol and acetonitrile mixed solution to dilute the mixed solvent, wherein the volume ratio of the diluted mixed solvent to the titanium source is 10:1; and adding ammonia water with a mass concentration of 28-30 wt% as a catalyst, wherein the volume ratio of the titanium source to the ammonia water is 2-3:1, and the titanium source accounts for about 2% of the total volume of the solvent after adding the titanium source dilution solution; after stirring and reacting, centrifuging, washing, and drying are performed to obtain silica particles coated with titanium dioxide.

[0015] Furthermore, in the preparation of the heterogeneous single-atom photocatalyst precursor, the step of loading the transition metal is as follows: adding silica particles to deionized water to prepare an aqueous dispersion with a mass concentration of 2-5 wt%, adding a transition metal hydrochloride solution, wherein the transition metal is one of copper, iron, manganese, nickel or cobalt, preferably copper, so that the effective metal element accounts for 0.75-1.5 wt% of the silica particles by mass fraction, stirring continuously for 3-12 h, and then separating the solid and liquid to obtain SiO2 particles coated with titanium dioxide sol loaded with transition metal co-catalytic components.

[0016] Furthermore, in the preparation of the heterogeneous single-atom photocatalyst precursor, the steps of adsorbing polyvinylpyrrolidone and secondary coating with silica are as follows: particles loaded with transition metal are added back into deionized water, polyvinylpyrrolidone is added, the mass ratio of polyvinylpyrrolidone to the catalyst semi-finished product is 0.8-1:1, and the mixture is stirred for more than 12 hours to allow complete adsorption of polyvinylpyrrolidone. After centrifugation, particles adsorbed with polyvinylpyrrolidone are obtained, and silica is secondary coated on the surface of the particles adsorbed with polyvinylpyrrolidone by the Stöber synthesis method. After drying and grinding, the precursor is obtained.

[0017] Furthermore, in the preparation of the hollow sphere heterogeneous single-atom photocatalyst, the steps for preparing the hollow sphere particle structure heterogeneous single-atom photocatalyst are as follows: the precursor is placed in air and calcined at 800℃-950℃ for 4-6 hours to complete the anatase phase transformation of titanium dioxide and the secondary dispersion of the supported transition metal co-catalyst components to form a single-atom catalyst. Then, a 0.5M NaOH solution is added, and the reaction is carried out at 90℃ with stirring for 6-9 hours to completely dissolve the silica template. The solid particles are obtained by centrifugation and washed with ethanol and deionized water to remove residual NaOH and dissolved silica products. After washing, the product is dried to obtain the heterogeneous single-atom photocatalyst with a hollow sphere particle structure.

[0018] Furthermore, in the photocatalytic advanced reduction reaction degradation of PFAS, during pollutant degradation, the raw water with a pH adjusted to 5-7 is pumped into the photocatalytic reaction tower via a booster pump. The photocatalytic reaction tower has a height-to-diameter ratio of 4:1 and adopts a bottom-in, top-out water inlet method. A cone of approximately 30° is installed at the bottom, and the bottom of the cone is connected to a reflux diaphragm pump that returns the heterogeneous single-atom photocatalyst from the solid-liquid separation tower. Before starting the reaction, the heterogeneous single-atom photocatalyst is prepared into a slurry of the required mass concentration and directly fed into the cyclone separator from the solid-liquid separation tower. Simultaneously, the booster pump and the reflux diaphragm pump are turned on to control the heterogeneous single-atom photocatalyst. The dosage of heterogeneous single-atom photocatalyst is 0.5-3.0 g / L. The reflux ratio of heterogeneous single-atom photocatalyst is adjusted to 100-400% of the influent flow rate by a reflux diaphragm pump to form a fully fluidized system. A low-pressure mercury lamp is installed inside the photocatalytic reaction tower to irradiate the catalyst and raw water in the photocatalytic reaction tower. At the same time, the hydraulic residence time of the raw water in the photocatalytic reaction tower is controlled to be 30-240 minutes. Under the irradiation of the low-pressure mercury lamp, the heterogeneous single-atom photocatalyst is excited to generate a strongly reducing species with hydrated electrons, which undergoes a nucleophilic substitution reaction with the CF bond in PFAS to degrade pollutants.

[0019] Furthermore, in the solid-liquid separation and catalyst recycling process, the specific process of solid-liquid separation is as follows: the mixed liquid after the reaction contains heterogeneous single-atom photocatalyst, PFAS degradation products and a small amount of unreacted pollutants, and flows by gravity from the upper outlet of the photocatalytic reaction tower into the solid-liquid separation tower. The height-to-diameter ratio of the solid-liquid separation tower is 4:1. The inlet of the mixed liquid is located at the lower part of the settling zone and is connected to the cyclone separator at the bottom of the solid-liquid separation tower. The mixed liquid is initially separated by the cyclone separator. The catalyst particles are thrown against the wall of the separator and settle to form a concentrated catalyst slurry. The supernatant enters the settling zone upward. The baffle plate three-phase separator at the upper part of the solid-liquid separation tower connects the settling zone and the clean water zone. The effective water volume of the clean water zone and the settling zone is ≥1:1. After the supernatant settles in the settling zone to remove trace amounts of catalyst, it enters the clean water zone through the baffle plate three-phase separator.

[0020] Furthermore, in the solid-liquid separation and catalyst recycling process, the specific process of catalyst recycling and product water discharge is as follows: the concentrated catalyst slurry separated by the cyclone separator is returned to the photocatalytic reaction tower through a reflux diaphragm pump connecting the bottom of the cyclone separator and the bottom of the cone hopper. The reflux ratio is controlled at 100-400% of the influent flow rate. After the clear liquid entering the purification zone is allowed to stand, the purified product water without catalyst is discharged from the drain outlet at the top of the purification zone.

[0021] Compared with existing technologies, this method for removing polyfluoroalkyl contaminants using heterogeneous single-atom photocatalysts has the following advantages:

[0022] I. This invention utilizes a heterogeneous single-atom photocatalyst, which significantly improves the photocatalytic degradation efficiency of PFAS through hollow sphere structure design and single-atom transition metal loading. The hollow structure increases the specific surface area of ​​the catalyst, enhancing its adsorption and enrichment capacity for PFAS molecules. Simultaneously, the single-atom dispersed transition metal acts as an active center, enabling it to fully undergo nucleophilic substitution reactions with the CF bonds in PFAS. Furthermore, under conditions of pH 5-7, catalyst dosage 0.5-3.0 g / L, and hydraulic retention time 30-240 minutes, the PFAS degradation rate is significantly improved, far exceeding that of traditional adsorption, chemical oxidation, and biodegradation methods. Moreover, the photocatalytic reduction reaction generates highly reducing hydrated electron radicals (e). aq - It undergoes a nucleophilic reaction with the CF bond to remove the F element, thus avoiding secondary pollution and providing a key solution for the efficient treatment of highly toxic and recalcitrant PFAS pollution.

[0023] II. This invention achieves continuous recycling of heterogeneous single-atom photocatalysts through a solid-liquid separation and catalyst reflux process. After the reaction mixture is processed by a cyclone separator and a three-phase baffle plate separator, the concentrated catalyst slurry is refluxed back to the photocatalytic reaction tower via a reflux diaphragm pump. This ensures that the catalyst is fully fluidized and continuously participates in the reaction. Compared with traditional adsorbents that require frequent replacement or chemical oxidation methods that require continuous reagent addition, this catalyst can be reused after simple cleaning, significantly reducing material consumption and processing costs. At the same time, the hollow sphere structure of the catalyst can effectively retain PFAS degradation products, preventing them from being released back into the water and avoiding the risk of secondary pollution. This design achieves complete mineralization of PFAS pollutants through the synergistic effect of physical separation and photocatalytic reduction, combining economic efficiency and environmental friendliness.

[0024] Other advantages, objectives and features of the invention will be set forth in part in the description which follows, and in part will be apparent to those skilled in the art from the following examination or study, or may be learned from the practice of the invention. Attached Figure Description

[0025] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are merely some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without any creative effort.

[0026] Figure 1 A flowchart of a method for removing polyfluoroalkyl contaminants using a heterogeneous single-atom photocatalyst;

[0027] Figure 2 This is a flowchart illustrating the preparation of a heterogeneous single-atom photocatalyst for a method of removing polyfluoroalkyl contaminants using a heterogeneous single-atom photocatalyst.

[0028] Figure 3 This is a schematic diagram of a system for removing PFAS from water using a heterogeneous single-atom photocatalyst.

[0029] In the diagram: 1. Photocatalytic reaction tower; 2. Raw water conditioning tank; 3. Booster pump; 4. Reflux diaphragm pump; 5. Conical hopper; 6. Solid-liquid separation tower; 7. Settling zone; 8. Cyclone separator; 9. Baffle plate three-phase separator; 10. Low-pressure mercury lamp; 11. Water purification zone; 12. Drain outlet. Detailed Implementation

[0030] To further illustrate the technical means and effects of the present invention in achieving its intended purpose, the following detailed description of the specific implementation methods, structures, features, and effects of the present invention, in conjunction with the accompanying drawings and preferred embodiments, is provided below.

[0031] Example 1

[0032] Preparation of heterogeneous single-atom photocatalyst precursor: In the chemical preparation workshop of an electronics factory, silica solid particles were prepared using the Stöber synthesis method: 200 mL of ethanol and 40 mL of deionized water were mixed (volume ratio 5:1), and 0.3 g of a quaternary ammonium salt cationic surfactant (hexadecyltrimethylammonium bromide, accounting for 1.5 wt% of tetraethyl orthosilicate) was added. After stirring to dissolve, 12 mL of 28 wt% ammonia solution was added, followed by slow dropwise addition of 20 mL of tetraethyl orthosilicate (accounting for 4.5% of the total solvent volume, with a volume ratio of 1.67:1 to ammonia solution). The mixture was stirred in a 30°C constant temperature water bath for 10 minutes. After filtration, washing, and drying, monodisperse silica particles were obtained. Using these particles as templates, a 5wt% suspension was prepared by dispersing them in a mixture of 200 mL ethanol and 60 mL acetonitrile (volume ratio 3.3:1). 8 mL of ammonia and 18 mL of tetrabutyl titanate diluted with 180 mL of ethanol-acetonitrile mixture (volume ratio 3:1) were added. The mixture was stirred and reacted for 4 hours. After centrifugation, washing, and drying, TiO2 sol-coated SiO2 particles were obtained. The coated particles were dispersed in 100 mL of deionized water (3wt% mass concentration), and 0.075 g Cu was added. 2+ A CuCl2 solution (copper accounts for 1 wt% of the particle mass) was stirred at room temperature for 8 h. After filtration and washing, Cu-loaded particles were obtained. 4 g of PVP (mass ratio of PVP to particles 0.8:1) was added to the deionized water dispersion of the particles. After stirring for 12 h, the mixture was centrifuged and then dispersed in a mixture of 250 mL of ethanol and 40 mL of deionized water (volume ratio 6.25:1). 12 mL of ammonia and 18 mL of tetraethyl orthosilicate (volume ratio 1.5:1) were added. The mixture was stirred at 30 °C for 8 h. After drying and grinding, the precursor was obtained.

[0033] Preparation of hollow spherical heterogeneous single-atom photocatalyst: The precursor was placed in a muffle furnace and calcined at 900℃ for 4 hours in air atmosphere, and then ball-milled to a fineness of 800 mesh or higher. 5 g of the calcined product was added to 100 mL of 0.5 M NaOH solution, and stirred at 90℃ for 6 hours to dissolve the silica template. After centrifugation and washing until pH neutral, the product was dried at 60℃ to obtain hollow spherical Cu@TiO2 single-atom photocatalyst, which was stored in the dark for later use.

[0034] Adjusting reaction conditions for PFAS-containing water: Wastewater from an electronics factory containing 15 mg / L perfluorooctanoic acid (PFOA, a type of PFAS) enters the raw water equalization tank 2. Dilute sulfuric acid is added via an automatic pH dosing device to adjust the pH of the wastewater to 6.0 (neutral to slightly acidic). After adjustment, the wastewater is pumped into the photocatalytic reaction tower 1 by a booster pump 3. Figure 1 As shown.

[0035] Photocatalytic advanced reduction reaction to degrade PFAS: Adjusted wastewater enters photocatalytic reaction tower 1 (bottom inlet, top outlet, 30° conical hopper 5 at the bottom) with a height-to-diameter ratio of 4:1. Cu@TiO2 catalyst is prepared as a slurry and fed into cyclone separator 8 from solid-liquid separation tower 6. The catalyst concentration in the reaction system is controlled at 1.5 g / L. The bottom reflux diaphragm pump 4 (reflux ratio 300%) is turned on to fully fluidize the catalyst. A 375W low-pressure mercury lamp 10 (radiation intensity 100 mW / cm²) is installed inside photocatalytic reaction tower 1. 2 Under irradiation (within a radiation range of 400 mm), the catalyst is excited to produce hydrated electrons (e electrons). aq - The CF bond of PFOA undergoes a nucleophilic substitution reaction, gradually degrading the pollutant, and the hydraulic retention time is controlled at 240 min.

[0036] Solid-liquid separation and catalyst recycling: After the reaction, the mixed liquid flows from the top of the photocatalytic reaction tower 1 into the solid-liquid separation tower 6 with a height-to-diameter ratio of 4:1, and enters the bottom cyclone separator 8 from the bottom of the settling zone 7. The catalyst particles settle to form a concentrated slurry, which is returned to the photocatalytic reaction tower 1 for recycling via the reflux diaphragm pump 4. The supernatant enters the clean water zone 11 through the settling zone 7 and the baffle plate three-phase separator 9 (the volume ratio of the clean water zone 11 to the settling zone 7 is 1:1). The purified water is discharged from the drain outlet 12 at the top of the clean water zone 11.

[0037] In summary, using wastewater containing 15 mg / L perfluorooctanoic acid (PFOA) from an electronics factory as the target for treatment, a Cu@TiO2 hollow sphere single-atom photocatalyst was prepared. After adjusting the wastewater pH to 6.0, the PFOA was fluidized in photocatalytic reaction tower 1 with a dosage of 1.5 g / L and a 300% reflux ratio, and irradiated with a 375W low-pressure mercury lamp 10 for 240 min. This achieved efficient degradation of PFOA. The catalyst was recycled through a solid-liquid separation tower 6. The enhanced photocatalytic activity of copper single atoms generated strong reducing hydrated electrons to break CF bonds. The overall process is simple to operate, has mild conditions, and can efficiently remove PFOA pollutants.

[0038] Example 2

[0039] Preparation of heterogeneous single-atom photocatalyst precursor: In the chemical preparation workshop supporting the chemical industrial park, silica solid particles were prepared by the Stöber synthesis method: 200 mL of ethanol and 40 mL of deionized water were mixed (volume ratio 5:1), 0.3 g of cetyltrimethylammonium bromide (CTAB, 1.5 wt% of the subsequent tetraethyl orthosilicate mass) was added, and after stirring and dissolving, 12 mL of ammonia water with a mass concentration of 28 wt% was added. Then, 20 mL of tetraethyl orthosilicate (4.5% of the total volume of the solvent, volume ratio with ammonia water 1.67:1) was slowly added dropwise. Stirring was continued for 10 h in a constant temperature water bath at 30 °C. After the reaction ended, the solid was separated by suction filtration, washed alternately with ethanol and deionized water 3 times, and dried at 60 °C for 8 h to obtain monodisperse silica particles. Using these silica particles as a template, they were dispersed in a mixed solvent of 200 mL of ethanol and 60 mL of acetonitrile (volume ratio 3.3:1), and ultrasonic emulsification was carried out for 30 min to prepare a 5 wt% suspension. 8 mL of ammonia water with a mass concentration of 28 wt% was added. Separately, 18 mL of tetrabutyl titanate was diluted with a mixed solution of 180 mL of ethanol and acetonitrile (volume ratio 3:1) (dilution solvent to titanium source volume ratio 10:1), and slowly dropped into the above suspension (titanium source accounting for 2% of the total volume, volume ratio with ammonia water 2.25:1). Stirring reaction was carried out for 4 h, and after centrifugal separation, it was washed with ethanol 2 times and dried at 60 °C for 6 h to obtain SiO2 particles coated with TiO2 sol. The coated particles were dispersed in 100 mL of deionized water (mass concentration 3 wt%), and a CoCl2·3H2O solution containing 0.0 2+ 2+ 75 g of Co (cobalt element accounting for 1 wt% of the total mass of the particles) was added. Stirring was carried out at room temperature for 8 h. After suction filtration separation, it was washed with deionized water until there was no Cl - in the filtrate (detected by silver nitrate), and dried at 60 °C for 4 h to obtain Co-loaded TiO2 / SiO2 particles. 4 g of polyvinylpyrrolidone (PVP, mass ratio with the particles 0 Figure 2 Figure 2 .8:1) was added to the deionized water dispersion of the loaded particles, and stirring was carried out for 12 h to ensure complete adsorption of PVP. After centrifugal separation, the particles were dispersed in a mixed solution of 250 mL of ethanol and 40 mL of deionized water (volume ratio 6.25:1), 12 mL of ammonia water and 18 mL of tetraethyl orthosilicate (volume ratio 1.5:1) were added, and stirring was carried out in a constant temperature water bath at 30 °C for 8 h. After drying, it was ground to obtain the precursor, as shown

[0040] Preparation of hollow spherical heterogeneous single-atom photocatalyst: The precursor was placed in a muffle furnace and calcined at 900℃ for 4 hours in air atmosphere. After natural cooling, it was ball-milled to a mesh size of 800 or finer. 5 g of the calcined product was added to 100 mL of 0.5 M NaOH solution and reacted at 90℃ with stirring for 6 hours to completely dissolve the silica template. The solid particles were separated by centrifugation and repeatedly washed with deionized water until the pH was neutral. After drying at 60℃ for 6 hours, Co@TiO2 single-atom photocatalyst with hollow spherical structure was obtained. It was then bagged and stored in a light-proof and dry special container.

[0041] Adjusting the reaction conditions of PFAS-containing water: Wastewater containing 15 mg / L perfluorooctanoic acid (a type of PFAS) discharged from the chemical industrial park first enters the raw water conditioning tank 2. An online pH monitor and an automatic dosing pump are installed in the tank. Dilute sulfuric acid is added to the tank through the dosing pump to adjust the pH value of the wastewater to 6.0 (a neutral to slightly acidic environment, which is conducive to the stable existence of reducing species in the photocatalytic reaction). The adjusted wastewater is then transported to the photocatalytic reaction tower 1 by the booster pump 3.

[0042] Photocatalytic advanced reduction reaction for PFAS degradation: Adjusted wastewater enters photocatalytic reaction tower 1 (with a bottom-in, top-out inlet, and a 30° conical hopper 5 at the bottom) with a height-to-diameter ratio of 4:1. Before starting the reaction, the Co@TiO2 catalyst is prepared into a slurry of the required concentration and directly fed into the cyclone separator 8 from the solid-liquid separation tower 6. Simultaneously, the booster pump 3 and the reflux diaphragm pump 4 are turned on, controlling the catalyst dosage in the reaction system to 1.5 g / L. The reflux ratio is adjusted to 300% by the reflux diaphragm pump 4 at the bottom of the conical hopper 5, allowing the catalyst to be fully fluidized under the water flow, forming a uniform suspension system. A 375W low-pressure mercury lamp 10 (radiation intensity 100 mW / cm², radiation range 400 mm) is installed inside the photocatalytic reaction tower 1. After the light source is turned on, the Co@TiO2 catalyst is excited by ultraviolet light, generating hydrated electrons (e-electrons) with strong reducing properties. aq - The raw water undergoes a nucleophilic substitution reaction with the CF bonds of PFOA in the wastewater, gradually degrading the pollutants. The hydraulic residence time of the raw water in the photocatalytic reaction tower 1 is controlled at 240 minutes to ensure a complete reaction.

[0043] Solid-liquid separation and catalyst recycling: The reaction mixture (containing Co@TiO2 catalyst, PFOA degradation products, and a small amount of unreacted pollutants) flows by gravity from the outlet at the top of the photocatalytic reaction tower 1 into the solid-liquid separation tower 6 with a height-to-diameter ratio of 4:1. The mixture enters the cyclone separator 8 at the bottom of the tower from the bottom of the settling zone 7. Under centrifugal force, the catalyst particles are thrown against the wall and settle to form a concentrated slurry. This slurry is then returned to the photocatalytic reaction tower 1 by the reflux diaphragm pump 4 connecting the bottom of the cyclone separator 8 and the bottom of the conical hopper 5, continuing to participate in the reaction. The supernatant flows upward into the settling zone 7, and after further separation by the baffle plate three-phase separator 9 (the effective water volume ratio between the clean water zone 11 and the settling zone 7 is 1:1), it enters the clean water zone 11. The clear liquid entering the clean water zone 11 is allowed to stand briefly and then discharged from the drain outlet 12 at the top of the clean water zone 11. Figure 3 As shown.

[0044] In summary, for wastewater containing 15 mg / L perfluorooctanoic acid (PFOA) in a chemical industrial park, a Co@TiO2 hollow sphere single-atom photocatalyst was prepared. The pH of the wastewater was adjusted to 6.0 and fed into photocatalytic reaction tower 1. The catalyst dosage was 1.5 g / L, and a fluidized state was formed with a 300% reflux ratio. Under irradiation by a 375W low-pressure mercury lamp 10 for 240 min, the strongly reducing species generated by cobalt single-atom excitation reacted with PFOA, gradually degrading the pollutants. The catalyst was then recycled through a solid-liquid separation tower 6, which improved the degradation effect on PFOA pollutants.

[0045] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for removing polyfluoroalkyl contaminants using a heterogeneous single-atom photocatalyst, characterized in that, The method includes: Preparation of heterogeneous single-atom photocatalyst precursor: Silica solid particles were prepared by Stöber synthesis, and the precursor was obtained by coating titanium dioxide, loading transition metal, adsorbing polyvinylpyrrolidone, and secondary coating silica using silica solid particles as template agent. Preparation of hollow sphere heterogeneous single-atom photocatalyst: The precursor was calcined to form a single-atom distribution of transition metal. The calcined product was dissolved in NaOH solution to dissolve the silica template. After centrifugation, washing and drying, a hollow sphere particle structure heterogeneous single-atom photocatalyst was obtained. Adjusting the reaction conditions of PFAS-containing water: Introduce raw water containing PFAS pollutants into the raw water conditioning tank (2), and add sulfuric acid and sodium hydroxide through the pH adjustment dosing device to adjust the pH value of the raw water containing PFAS pollutants to 5-7. Photocatalytic advanced reduction reaction to degrade PFAS: The adjusted raw water is sent into the photocatalytic reaction tower (1), a heterogeneous single-atom photocatalyst is added to form a fluidized system, and the reaction is carried out under the irradiation of a low-pressure mercury lamp (10) to degrade the pollutants; Solid-liquid separation and catalyst recycling: The mixture after the reaction is separated by a solid-liquid separation tower (6), and the catalyst slurry is returned to the photocatalytic reaction tower (1) by a reflux diaphragm pump (4) to continue to participate in the reaction. After separation, the supernatant is discharged from the drain outlet (12) of the clean water area (11) without the catalyst.

2. The method for removing polyfluoroalkyl contaminants using a heterogeneous single-atom photocatalyst according to claim 1, characterized in that, In the preparation of the heterogeneous single-atom photocatalyst precursor, silica solid particles are prepared by the Stöber synthesis method. The steps are as follows: an alcohol solvent is selected and mixed with deionized water to form a base solvent, wherein the alcohol solvent is one of ethanol, ethylene glycol or propanol, preferably ethanol, and the volume ratio of alcohol to deionized water is 4-10:

1. A quaternary ammonium salt cationic surfactant is added to the base solvent, followed by ammonia water with a mass concentration of 28-30 wt% as a catalyst. Tetraethyl orthosilicate is slowly added dropwise while stirring, the volume of tetraethyl orthosilicate accounting for 4-5% of the total volume of the solvent, and the volume ratio of tetraethyl orthosilicate to ammonia water is 1.5-2:

1. The total volume after mixing is placed in a constant temperature water bath at 30°C and stirred continuously for 10 hours. After the reaction is completed, solid particles are obtained by solid-liquid separation, and residual impurities are removed by washing with ethanol and deionized water. After drying, monodisperse silica solid particles are obtained.

3. The method for removing polyfluoroalkyl contaminants using a heterogeneous single-atom photocatalyst according to claim 2, characterized in that, In the preparation of the heterogeneous single-atom photocatalyst precursor, the amount of quaternary ammonium salt cationic surfactant used is 0.5-3 wt% of the added tetraethyl orthosilicate.

4. The method for removing polyfluoroalkyl contaminants using a heterogeneous single-atom photocatalyst according to claim 2, characterized in that, In the preparation of the heterogeneous single-atom photocatalyst precursor, the step of coating titanium dioxide is as follows: adding silica solid particles to a mixed solvent of alcohol and acetonitrile to prepare a suspension with a silica solid particle mass concentration of 5 wt%, and the volume ratio of alcohol to acetonitrile in the mixed solvent is 2-4:1; adding an organic titanium source diluted with the mixed solution of alcohol and acetonitrile to dilute the mixed solvent, wherein the volume ratio of the diluted mixed solvent to the titanium source is 10:1; and adding ammonia water with a mass concentration of 28-30 wt% as a catalyst, wherein the volume ratio of the titanium source to the ammonia water is 2-3:1, and the titanium source accounts for about 2% of the total volume of the solvent after adding the titanium source dilution solution; after stirring and reacting, centrifuging, washing, and drying are performed to obtain silica particles coated with titanium dioxide.

5. The method for removing polyfluoroalkyl contaminants using a heterogeneous single-atom photocatalyst according to claim 4, characterized in that, In the preparation of the heterogeneous single-atom photocatalyst precursor, the step of loading the transition metal is as follows: adding silica particles to deionized water to prepare an aqueous dispersion with a mass concentration of 2-5 wt%, adding a transition metal hydrochloride solution, wherein the transition metal is one of copper, iron, manganese, nickel or cobalt, preferably copper, so that the effective metal element accounts for 0.75-1.5 wt% of the silica particles by mass fraction, stirring continuously for 3-12 h, and then separating the solid and liquid to obtain SiO2 particles coated with titanium dioxide sol loaded with transition metal co-catalytic components.

6. The method for removing polyfluoroalkyl contaminants using a heterogeneous single-atom photocatalyst according to claim 5, characterized in that, In the preparation of the heterogeneous single-atom photocatalyst precursor, the steps of adsorbing polyvinylpyrrolidone and secondary coating with silica are as follows: particles loaded with transition metal are added back into deionized water, polyvinylpyrrolidone is added, the mass ratio of polyvinylpyrrolidone to catalyst semi-finished product is 0.8-1:1, and the mixture is stirred for more than 12 hours to allow complete adsorption of polyvinylpyrrolidone. After centrifugation, particles adsorbed with polyvinylpyrrolidone are obtained, and silica is secondary coated on the surface of the particles adsorbed with polyvinylpyrrolidone by Stöber synthesis. After drying and grinding, the precursor is obtained.

7. The method for removing polyfluoroalkyl contaminants using a heterogeneous single-atom photocatalyst according to claim 6, characterized in that, The steps for preparing the hollow spherical heterogeneous single-atom photocatalyst are as follows: The precursor is placed in air and calcined at 800℃-950℃ for 4-6 hours to complete the anatase phase transformation of titanium dioxide and the secondary dispersion of the supported transition metal co-catalyst components to form a single-atom catalyst. Then, a 0.5M NaOH solution is added, and the mixture is heated at 90℃ with stirring for 6-9 hours to completely dissolve the silica template. Solid particles are obtained by centrifugation and washed with ethanol and deionized water to remove residual NaOH and dissolved silica products. After washing, the mixture is dried to obtain the heterogeneous single-atom photocatalyst with a hollow spherical particle structure.

8. The method for removing polyfluoroalkyl contaminants using a heterogeneous single-atom photocatalyst according to claim 7, characterized in that, In the process of photocatalytic advanced reduction reaction degradation of PFAS, when degrading pollutants, the raw water with a pH value adjusted to 5-7 is transported to the photocatalytic reaction tower (1) through a booster pump (3). The height-to-diameter ratio of the photocatalytic reaction tower (1) is 4:1, and a bottom-inlet, top-outlet water inlet is adopted. A cone (5) with an angle of about 30° is installed at the bottom. The bottom of the cone (5) is connected to a reflux diaphragm pump (4) for refluxing heterogeneous single-atom photocatalyst from the solid-liquid separation tower (6). Before starting the reaction, the heterogeneous single-atom photocatalyst is prepared into a slurry of the required mass concentration and directly fed into the cyclone separator (8) from the solid-liquid separation tower (6). At the same time, the booster pump (3) and the reflux diaphragm pump (4) are turned on. The dosage of heterogeneous single-atom photocatalyst is controlled at 0.5-3.0 g / L. The reflux ratio of heterogeneous single-atom photocatalyst is adjusted to 100-400% of the influent flow rate by the reflux diaphragm pump (4) to form a fully fluidized system. A low-pressure mercury lamp (10) is installed inside the photocatalytic reaction tower (1) to irradiate the catalyst and raw water in the photocatalytic reaction tower (1). At the same time, the hydraulic residence time of the raw water in the photocatalytic reaction tower (1) is controlled to be 30-240 minutes. Through the irradiation of the low-pressure mercury lamp (10), the heterogeneous single-atom photocatalyst is excited to generate a strong reducing species with hydrated electrons, which undergoes a nucleophilic substitution reaction with the CF bond in PFAS to degrade pollutants.

9. A method for removing polyfluoroalkyl contaminants using a heterogeneous single-atom photocatalyst according to claim 8, characterized in that, In the solid-liquid separation and catalyst recycling process, the specific process of solid-liquid separation is as follows: the mixed liquid after the reaction contains heterogeneous single-atom photocatalyst, PFAS degradation products and a small amount of unreacted pollutants. It flows into the solid-liquid separation tower (6) by gravity from the upper outlet of the photocatalytic reaction tower (1). The height-to-diameter ratio of the solid-liquid separation tower (6) is 4:

1. The inlet of the mixed liquid is set at the lower part of the settling zone (7) and connected to the cyclone separator (8) at the bottom of the solid-liquid separation tower (6). The mixed liquid is initially separated by the cyclone separator (8). The catalyst particles are thrown to the wall and settle to form a concentrated catalyst slurry. The supernatant enters the settling zone (7) upward. The baffle three-phase separator (9) at the upper part of the solid-liquid separation tower (6) connects the settling zone (7) and the clean water zone (11). The effective water volume of the clean water zone (11) and the settling zone (7) is ≥1:

1. After the supernatant settles in the settling zone (7) to remove trace amounts of catalyst, it enters the clean water zone (11) through the baffle three-phase separator (9).

10. A method for removing polyfluoroalkyl contaminants using a heterogeneous single-atom photocatalyst according to claim 9, characterized in that, In the solid-liquid separation and catalyst recycling, the specific process of catalyst recycling and product water discharge is as follows: the concentrated catalyst slurry separated by the cyclone separator (8) is returned to the photocatalytic reaction tower (1) by the reflux diaphragm pump (4) connecting the bottom of the cyclone separator (8) and the bottom of the cone bucket (5). The reflux ratio is controlled to be 100-400% of the influent flow rate. After the clear liquid entering the water purification zone (11) is allowed to stand, the purified product water without catalyst is discharged from the drain outlet (12) at the top of the water purification zone (11).

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