A method for total quantification removal of pfas in an environmental water body

Abstract

The application discloses a full-quantitative treatment method for PFAs in an environmental water body. Wastewater containing PFAs is first filtered through a filter, and then is adsorbed through an adsorption tank A or an adsorption tank B containing active adsorbents, so that treated water meeting the standard is directly discharged. When the adsorption tank A or the adsorption tank B is saturated, NaOH solution is introduced for desorption, the obtained desorption liquid is introduced into a BDD reaction tank for electrochemical oxidation degradation, and then is returned to the adsorption tank A or the adsorption tank B for desorption. The obtained desorption liquid is introduced into the BDD reaction tank for electrochemical oxidation degradation again. Through the cyclic treatment of desorption-degradation, the desorption is completed. The adsorption tank A or the adsorption tank B after the desorption is completed is introduced into acid solution for regeneration of the active adsorbents. The desorption liquid obtained after the desorption is completed is first discharged into a storage tank, and then is returned to the adsorption tank A or the adsorption tank B for desorption after the adsorption tank A or the adsorption tank B is saturated, so as to enter the next cycle of desorption.

Description

Technical Field

[0001] This invention relates to a method for the complete removal of PFAs from environmental water bodies, belonging to the field of wastewater treatment technology. Background Technology

[0002] Perfluorinated compounds (PFAs) are a class of organic compounds in which fluorine atoms replace all hydrogen atoms in hydrocarbons. The high bond energy of the CF bond (484 kJ / mol) gives PFAs excellent hydrophobic and oleophobic properties as well as extremely high chemical stability, making them resistant to degradation under high temperatures, strong light, and biodegradation. Therefore, they are widely used in textiles, leather, coatings, chemicals, and food packaging. There are as many as 2060 commercially available PFAs globally. Perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA) and their salts are the final products of many PFA transformations and are the most common in the environment, thus attracting widespread attention from researchers. The stable structure and poor biodegradability of PFAs allow them to persist in the environment globally, causing widespread environmental pollution. PFAs enter organisms through drinking water, surface water, and other media, and accumulate to high concentrations in organisms through the food chain, posing a serious threat to human reproductive and immune systems.

[0003] Current methods for treating perfluorinated compounds include physical adsorption, ion exchange, nanofiltration, reverse osmosis, ozone oxidation, photocatalytic oxidation, and electrocatalytic oxidation. However, physical treatment methods do not achieve complete final disposal, and chemical oxidation methods, such as ozone oxidation, have low efficiency and long reaction times. Summary of the Invention

[0004] To address the current technical challenges in removing perfluorinated compounds (PFAs) from environmental water bodies, this invention aims to provide a comprehensive method for removing PFAs from environmental water bodies. This method employs modified activated carbon adsorption and enrichment, combined with BDD electrochemical oxidation to treat the adsorption eluent. This method can significantly improve degradation efficiency.

[0005] This invention is achieved by the following technical solution:

[0006] This invention provides a comprehensive treatment method for PFAs in environmental water. Wastewater containing PFAs is first passed through a filter, then through adsorption tank A or adsorption tank B containing an activated adsorbent. The treated water is then directly discharged. Once adsorption tank A or B is saturated, a NaOH solution is introduced for desorption. The resulting desorbate is then fed into a BDD reaction tank for electrochemical oxidation degradation. The desorbate is then returned to adsorption tank A or B for further desorption, and the resulting desorbate is again fed into the BDD reaction tank for electrochemical oxidation degradation. This desorption-degradation cycle is repeated until desorption is complete. After desorption, adsorption tank A or B is regenerated by introducing an acid solution. The regenerated NaOH solution is then used for the next wastewater adsorption cycle. The resulting desorbate is first discharged to a storage tank, and then returned to adsorption tank A or B after adsorption tank A or B is saturated for the next desorption cycle.

[0007] The comprehensive treatment method provided in this invention first uses an activated adsorbent to completely adsorb and remove PFAs from the wastewater, concentrating and enriching the PFAs in the activated adsorbent. Then, the PFAs enriched in the activated adsorbent are desorbed using NaOH solution, followed by electrochemical oxidation degradation using a BDD reactor. The desorption-degradation cycle is used to improve the desorption efficiency and enhance the regeneration adsorption activity of the activated adsorbent. After desorption is completed, the activated adsorbent is regenerated using an acid solution and can then be used for the adsorption of wastewater in the next cycle.

[0008] The inventors discovered that BDD has a high oxygen evolution potential, reaching 2.5-2.8V, and its degradation and removal rate of recalcitrant PFAc is higher than other advanced oxidants, reaching 100%. Moreover, during the BDD degradation process, due to factors such as mass transfer efficiency, the degradation efficiency of low-concentration organic matter is lower than that of low-concentration organic matter. For example, when COD is 100-500 mg / L, the degradation efficiency of BDD is several times or even tens of times that of 0-100 mg / L. Therefore, this invention uses BDD to treat concentrated waste liquid desorbed from activated carbon, which greatly improves efficiency and is more economical.

[0009] Furthermore, in this invention, one tank is used and the other is on standby. When adsorption tank A becomes saturated, it is switched to adsorption tank B. Adsorption tank A and adsorption tank B are used alternately for adsorption, desorption, and degradation processes in different cycles. That is, the two adsorption tanks are used for different stages of the adsorption, desorption, and degradation cycle, so that wastewater containing PFAs can be continuously treated.

[0010] In a preferred embodiment, the porosity of the active adsorbent is 40-75%, and the specific surface area is 1000-2000 m² / g.

[0011] In a preferred embodiment, the active adsorbent is selected from at least one of biochar, activated carbon, and activated carbon fiber.

[0012] In a preferred embodiment, the active adsorbent is first modified. The modification process involves adding the active adsorbent to a sulfuric acid solution, performing a first reaction, separating the solid and liquid phases, washing the resulting solid phase until the pH is >1, adding it to hydrogen peroxide, and performing a second reaction to obtain the final product. In actual operation, after the second reaction is completed, the solid and liquid phases are separated, the solid phase is washed until neutral, and then dried for later use.

[0013] In a further preferred embodiment, the concentration of the sulfuric acid solution is 1-2 mol / L.

[0014] In a further preferred embodiment, the volume ratio of the active adsorbent to the sulfuric acid solution is 1:1 to 2.

[0015] In a further preferred embodiment, the first reaction is carried out at 80-100°C for 1-2 hours.

[0016] A further preferred embodiment is that the hydrogen peroxide has a mass concentration of 5-10%.

[0017] Further preferred, the volume ratio of the washed solid phase to hydrogen peroxide is 1:1 to 2.

[0018] Further optimization involves a second reaction time of 12-24 hours.

[0019] The inventors discovered that in the aquatic environment, there are a large number of competing compounds and colloids that compete with anionic PFAs for adsorption sites during the adsorption process, thereby reducing the adsorption capacity of the adsorption material. For perfluorinated compounds, mesoporous adsorption has a higher adsorption rate than microporous adsorption. The acid activation modification of this invention can expand the pore size of activated carbon and increase the number of mesopores, thereby improving the adsorption efficiency of activated carbon and improving the adsorption selectivity. This ensures that PFA-containing wastewater can be completely adsorbed after being adsorbed by adsorption tank A or adsorption tank B containing activated adsorbent, resulting in treated water that meets the standards.

[0020] The active adsorbent modified by the above method of the present invention has a mesoporous content of 50-70% or more and a microporous content of <15%.

[0021] In a preferred embodiment, the NaOH solution contains 1-2% NaOH by mass.

[0022] In a preferred embodiment, during the process of introducing NaOH solution into adsorption tank A or adsorption tank B for analysis, ozone is introduced into the activated carbon tank of adsorption tank A or adsorption tank B at a concentration of 1-3 mg / L.

[0023] The inventors discovered that ozone can synergistically degrade PFAc with BDD, thereby improving the system's degradation efficiency. In particular, the intermediate products formed after BDD chain breakage can be significantly improved and the degradation load of BDD reduced by ozone synergistic treatment.

[0024] In a preferred embodiment, during the electrochemical oxidation degradation process in the BDD reaction cell, the BDD electrode is used as the anode, the Ti electrode is used as the cathode, the current density is 50-100 mA / cm2, the reaction temperature is 30-60℃, and the time for a single degradation is 0.5-2.5 h.

[0025] In a further preferred embodiment, the BDD electrode comprises a silicon carbide substrate, a gradient boron-doped SiC semiconductor transition layer disposed on the surface of the silicon carbide substrate, and a gradient boron-doped diamond semiconductor layer disposed on the surface of the gradient boron-doped SiC semiconductor transition layer. In the gradient boron-doped SiC semiconductor transition layer, the boron content decreases from top to bottom; in the gradient boron-doped diamond semiconductor layer, the boron content decreases from bottom to top.

[0026] In this invention, since organic fluorine in PFA-containing wastewater degrades into inorganic fluorine, which is highly corrosive, the use of the aforementioned silicon carbide substrate electrode can effectively extend the electrode's lifespan. Furthermore, the boron doping treatment of the SiC semiconductor in this invention yields a p-type semiconductor, which helps improve the substrate's conductivity, thereby increasing the current efficiency during the electrochemical oxidation process. Simultaneously, it further enhances the chemical bonding between the SiC substrate and the BDD coating, improving the service stability of the BDD composite coating material. The lower boron content in the bottom layer of the transition layer helps maintain the good chemical stability and thermal conductivity of the silicon carbide substrate; the higher boron content in the top layer enhances the chemical bonding between the transition layer and the BDD coating, improving the substrate's conductivity; the gradual decrease in boron content in the middle layer helps alleviate the hardness gradient between the substrate and the transition layer, ensuring good bonding between the transition layer and the substrate.

[0027] In a further preferred embodiment, the surface of the gradient boron-doped SiC semiconductor transition layer contains a plurality of grooves, the depth of which is 0.5–3 μm. These grooves are obtained by plasma etching of the gradient boron-doped SiC semiconductor transition layer.

[0028] The inventors discovered that etching the substrate surface of a graded boron-doped SiC semiconductor transition layer helps increase the specific surface area, further improving the current density at the film-substrate interface. Simultaneously, increasing the substrate surface roughness helps improve the diamond nucleation rate, further enhancing film-substrate adhesion and improving material performance. However, the inventors found that plasma etching is required, and the depth of the grooves must be controlled so that the grooves only achieve optimal performance on the surface of the graded boron-doped SiC semiconductor transition layer.

[0029] More preferably, the total amount of B atoms in the top of the gradient boron-doped SiC semiconductor transition layer is 10. 16 ~10 19 cm -3 The proportion of boron atoms directly doped between SiC lattices is 20%–30%, while the proportion of boron atoms in B4C is 70%–80%.

[0030] The inventors discovered that by controlling the total amount of boron atoms within the range of this invention, the final semiconductor composite coating material exhibits optimal performance. If the boron doping level is high, a higher thermal diffusion temperature is required, which can lead to the oxidation of SiC to form SiO2 residues, and the poor conductivity of SiO2 ultimately affects the electrical properties of the substrate. If the boron doping level is too low, it cannot effectively improve the conductivity of the substrate and the film-substrate bonding performance. Furthermore, by controlling the proportion of boron atoms directly doped into SiC within the range of this invention, the overall performance of the resulting silicon carbide / boron-doped diamond semiconductor composite coating material is also better.

[0031] In a further preferred embodiment, the gradient boron-doped diamond layer comprises, from bottom to top, a boron-doped diamond bottom layer, a boron-doped diamond intermediate layer, and a boron-doped diamond top layer. The boron-doped diamond bottom layer has a uniform boron content, with a B / C ratio of 46,666-60,000 ppm (atomic ratio). The boron-doped diamond top layer has a uniform boron content, with a B / C ratio of 26,666-40,000 ppm (atomic ratio). The boron content in the boron-doped diamond intermediate layer decreases linearly from bottom to top, with the boron content at the bottom layer being the maximum and decreasing linearly to the boron content at the top layer.

[0032] In this invention, the boron-doped diamond bottom layer adopts a uniform boron content to maximize the conductivity of the coating, enhance the chemical bonding force between the BDD coating and the gradient boron-doped SiC transition layer, and further improve the film-substrate bonding performance. The boron-doped diamond top layer also adopts a uniform boron content to maximize the corrosion resistance of the top layer, effectively reduce the coating peeling rate, and improve the service life of the composite coating material. The boron-doped diamond intermediate layer adopts a linearly decreasing boron gradient, which allows for a natural transition between coatings, making it less prone to separation and breakage, and improving the bonding force.

[0033] In a further preferred embodiment, the BDD electrode is prepared by: immersing a silicon carbide substrate in a solution containing boron oxide powder, drying it to obtain a silicon carbide substrate coated with boron oxide powder, then subjecting the silicon carbide substrate to thermal diffusion treatment to obtain a gradient boron-doped SiC semiconductor transition layer, and then growing a gradient boron-doped diamond semiconductor layer on the silicon carbide substrate containing the gradient boron-doped SiC semiconductor transition layer by chemical vapor deposition.

[0034] More preferably, the heat diffusion treatment process is as follows: first, the temperature is raised to 200-650°C and held for 20-50 minutes; then, the temperature is raised to 1250-1400°C and held for 45-90 minutes; finally, the temperature is raised to 1450-1650°C and held for 30-60 minutes; the heat diffusion treatment atmosphere is air, oxygen, or nitrogen.

[0035] In a further preferred embodiment, after thermal diffusion treatment, the thermally diffused silicon carbide substrate is sequentially subjected to chemical cleaning and mechanical polishing treatment.

[0036] In a further preferred embodiment, the chemical cleaning process involves cleaning the surface of the thermally diffused silicon carbide substrate using a BOE solution or an HF solution.

[0037] In a further preferred embodiment, the mechanical polishing process involves heating the chemically cleaned silicon carbide substrate to 50–80°C, then coating it with paraffin wax. The paraffin-coated silicon carbide substrate is then symmetrically placed onto the surface of the sample block to ensure uniformity of the polishing thickness in different areas of the surface, followed by mechanical polishing. The substrate is then removed, cooled, and the paraffin wax is removed for cleaning. The polishing fluid has the following composition: water:alumina = (8–10):1; the polishing fluid rotation speed is 80–100 r / min; and the mechanical polishing time is 60–120 min.

[0038] Secondary cleaning processes, including chemical cleaning and mechanical polishing, can remove residual vitrified boron oxide from the transition layer surface, facilitating subsequent deposition of the BDD semiconductor coating.

[0039] In a further preferred embodiment, the silicon carbide substrate containing a gradient boron-doped SiC semiconductor transition layer is first subjected to plasma etching, and then a gradient boron-doped diamond semiconductor layer is grown by chemical vapor deposition. The plasma etching atmosphere includes an etching gas, an auxiliary gas, and a diluent gas. The etching gas is selected from SF6, CF4, CHF3, NF3, and Cl2, preferably SF6. The auxiliary gas is selected from HBr, O2, and Ar, preferably O2. The diluent gas is selected from He, Ne, and N2. The auxiliary gas accounts for 1-20% of the total gas flow rate, preferably 1-8.3%, and the diluent gas accounts for 15%-30% of the total gas flow rate, preferably 20%-30%. During the etching process, the cavity pressure is 4-10 Pa, the RF power adjustment range is 200-400 W, and the process time is 50-600 s.

[0040] A further preferred embodiment of the process for growing a graded boron-doped diamond semiconductor layer on a silicon carbide substrate containing a graded boron-doped SiC semiconductor transition layer via chemical vapor deposition is as follows: First, the silicon carbide substrate containing the graded boron-doped SiC semiconductor transition layer is placed in a suspension containing a mixture of nanocrystalline and / or microcrystalline diamond particles; ultrasonic treatment is performed, followed by drying; a silicon carbide substrate with surface-adsorbed nanocrystalline and / or microcrystalline diamond is obtained. Then, the silicon carbide substrate with surface-adsorbed nanocrystalline and / or microcrystalline diamond is placed in a chemical vapor deposition furnace, and hydrogen, boron-containing gas, and carbon-containing gas are introduced to perform chemical vapor deposition to grow a graded boron-doped diamond semiconductor layer. The temperature of the chemical vapor deposition is 600-1000℃, and the gas pressure is 10. 3 -10 4 Pa, time is 3-20h.

[0041] More preferably, in the suspension containing nanocrystalline and / or microcrystalline diamond mixed particles, the mass fraction of diamond mixed particles is 0.01%-0.05%; the particle size of the diamond mixed particles is 5-30 nm, and the purity is ≥97%; the ultrasonic treatment time is 5-30 min.

[0042] More preferably, during the chemical vapor deposition, the percentage of carbon-containing gas in the total gas mass flow rate in the furnace is 0.5-10.0%, preferably 2-5%.

[0043] In a further preferred embodiment, during the chemical vapor deposition, the percentage of boron-containing gas in the total mass flow rate of the furnace is first controlled to be 0.069%-0.0884% to obtain a boron-doped diamond underlayer. Then, the boron doping concentration is reduced linearly until the percentage of boron-containing gas in the total mass flow rate of the furnace is 0.03968%-0.0593% to obtain a boron-doped diamond transition layer. Then, the percentage of boron-containing gas in the total mass flow rate of the furnace is controlled to be 0.03968%-0.0593% again to obtain a boron-doped diamond outer layer; thus, a gradient boron-doped diamond semiconductor layer is obtained.

[0044] In a preferred embodiment, the acid solution is a sulfuric acid solution, wherein the mass fraction of sulfuric acid in the sulfuric acid solution is 1-5%.

[0045] The process system used in the full-scale treatment method of PFAs in environmental water bodies of the present invention includes an adsorption tank, a BDD reaction tank, a circulating pump, an intermediate water tank, a filter, an ozone microporous aeration facility, an ozone generator, and an automatic control device consisting of pressure, pH, fluoride online detection, and electric valves for the circulating treatment process.

[0046] Principles and advantages

[0047] This invention provides a method for the complete removal of PFAs from environmental water bodies. First, activated carbon adsorption is used to completely adsorb and remove PFAs from wastewater, concentrating and enriching the PFAs in the activated carbon. Then, NaOH solution is used to precipitate the PFAs enriched in the activated carbon. Next, electrochemical oxidation degradation is carried out using a BDD reactor. The precipitation-degradation cycle is used to improve the precipitation efficiency and enhance the regeneration adsorption activity of the activated carbon. After the precipitation is completed, the activated adsorbent is regenerated by an acid solution and can be used for the adsorption of wastewater in the next cycle.

[0048] Because BDD has a high oxygen evolution potential of 2.5-2.8V, its degradation and removal rate of recalcitrant PFAc is higher than other advanced oxidants, reaching 100%. Moreover, during the BDD degradation process, due to factors such as mass transfer efficiency, the degradation efficiency of low-concentration organic matter is lower than that of low-concentration organic matter. For example, when COD is 100-500 mg / L, the degradation efficiency of BDD is several times or even tens of times that of 0-100 mg / L. Therefore, this invention uses BDD to treat concentrated waste liquid desorbed from activated carbon, which greatly improves efficiency and is more economical.

[0049] By combining the processes of this invention, the full-scale treatment of trace perfluorinated compounds in wastewater can be achieved. Compared with BDD degradation alone, the degradation efficiency can be increased by 8-12 times, and the investment and operating costs can be reduced. Attached Figure Description

[0050] Figure 1 The process flow diagram of this invention. Detailed Implementation

[0051] Example 1

[0052] In this embodiment, the BDD electrode set in the BDD reaction cell is a gradient boron-doped silicon-based electrode plate from Xinfeng Technology.

[0053] Wastewater containing PFAs (PFOS concentration of 15.3 μg / L, PFOA concentration of 19.6 μg / L, flow rate of 10 m / s) was treated. 3 The material first passes through a filter, then through a container filled with activated carbon (specific surface area 1862 m²). 2Adsorption tank A, with a porosity of 57.6% (including 21.9% mesopores), directly discharges treated water that meets standards. When the active agent in adsorption tank A becomes saturated, the pipeline is switched so that the wastewater filtered by the filter passes through adsorption tank B. Meanwhile, adsorption tank A is purged with a 2% NaOH solution for desorption. The resulting eluent is then fed into a BDD reaction tank for electrochemical oxidation degradation. After further desorption in adsorption tank A, the eluent is returned to the BDD reaction tank for electrochemical oxidation degradation, creating a desorption-degradation cycle. During the desorption process, ozone is introduced into adsorption tank A. The concentration of ozone introduced into the eluent was controlled at 2 mg / L. During the degradation process, the BDD electrode was used as the anode and the Ti electrode as the cathode. The current density was 60 mA / cm2, the reaction temperature was 40-50℃, and the time for a single degradation was 1.0 h. After repeating the eluent-degradation process about 8 times, the degradation was completed by online monitoring. The resulting eluent was a regenerated NaOH solution, which was first discharged to a storage tank. After the adsorption tank B was saturated, it was returned to the adsorption tank B to enter the next cycle of degradation. Meanwhile, a 4% sulfuric acid solution was introduced into the adsorption tank A to regenerate the active adsorbent in the adsorption tank A for later use.

[0054] After activated carbon adsorption, the effluent PFOS concentration was 33.6 ng / L, PFOA concentration was 29.1 ng / L, and the activated carbon adsorption capacity was 48.4 mg / g. The initial PFOS concentration in the activated carbon washing and regenerated alkaline solution was 66.5 mg / L, and PFOA concentration was 61.9 mg / L. After BDD degradation, the PFOS concentration in the regenerated alkaline solution was 1.17 μg / L, and PFOA concentration was 1.02 μg / L. The average energy consumption for BDD degradation of fluorides was 12.1 kWh / g fluoride.

[0055] Example 2

[0056] The BDD used in this Example 2 is the same as that used in Example 1.

[0057] In Example 2, granular activated carbon was first mixed with activated adsorbent in a 2 mol / L sulfuric acid solution at a solid-liquid volume ratio of 1:2. The mixture was reacted at 45°C for 2 hours. The sulfuric acid was then poured off, and the adsorbent was washed until the pH of the washing solution was >1. Then, 10% hydrogen peroxide was added at a solid-liquid ratio of 1:2, and the mixture was reacted for 24 hours. The resulting product was then washed, dried, and ready for use. After the above modification, the mesopore content of the activated adsorbent increased from 21.9% to 66.5%, and the total porosity was 73.4%.

[0058] Wastewater containing PFAs (PFOS concentration of 15.3 μg / L, PFOA concentration of 19.6 μg / L, flow rate of 10 m / s) was treated. 3 The activated carbon ( / h) first passes through a filter, then through a container filled with the modified granular activated carbon (specific surface area of ​​2689 m²). 2Adsorption tank A (with a porosity of 73.4%, of which 66.5% is mesoporous) directly discharges treated water that meets standards. When the active agent in adsorption tank A is detected to be saturated, the pipeline is switched so that the wastewater filtered by the filter passes through adsorption tank B, while adsorption tank A is purged with a 2% NaOH solution for desorption. The resulting eluent is then fed into a BDD reaction tank for electrochemical oxidation degradation, and then returned to adsorption tank A for further desorption. The resulting eluent then re-enters the BDD reaction tank for electrochemical oxidation degradation, thus performing a desorption-degradation cycle. During the desorption process, ozone is introduced into adsorption tank A. The concentration of ozone introduced into the eluent was controlled at 2 mg / L. During the degradation process, the BDD electrode was used as the anode and the Ti electrode as the cathode. The current density was 60 mA / cm2, the reaction temperature was 40-50℃, and the time for a single degradation was 2.5 h. After repeating the eluent-degradation process 20 times, the solution was monitored online. The eluent obtained after the eluent was desorbed was a regenerated NaOH solution, which was first discharged to a storage tank. After the adsorption tank B was saturated, it was returned to the adsorption tank B to enter the next cycle of eluent. Meanwhile, a 4% sulfuric acid solution was introduced into the adsorption tank A to regenerate the active adsorbent in the adsorption tank A for later use.

[0059] After activated carbon adsorption, the effluent PFOS concentration was 16.9 ng / L, PFOA concentration was 15.2 ng / L, and the activated carbon adsorption capacity was 95.8 mg / g. The initial PFOS concentration in the activated carbon washing and regenerated alkaline solution was 139.8 mg / L, and PFOA concentration was 140.7 mg / L. After BDD degradation, the PFOS concentration in the regenerated alkaline solution was 0.54 μg / L, and PFOA concentration was 0.51 μg / L. The average energy consumption for BDD degradation of fluorides was 7.62 kWh / g fluoride.

[0060] Example 3

[0061] The BDD electrode used in Example 3 is prepared as follows:

[0062] 99.99% pure boron oxide powder was dissolved in anhydrous ethanol, placed in a silicon carbide matrix, and heated and dried until the solvent was dry. The sample was then placed in a tube annealing furnace for step heat treatment. The heat treatment atmosphere was nitrogen. The temperature was first raised to 200°C and held for 30 min, then raised to 1250°C and held for 60 min, and finally raised to 1450°C and held for 30 min.

[0063] After thermal diffusion, the transition layer surface undergoes a secondary cleaning process, which includes chemical cleaning and mechanical polishing. The chemical cleaning process involves using BOE solution to clean the sample surface. The mechanical polishing process involves placing the sample block on a heated platform and heating it to 60°C, uniformly coating it with paraffin wax, and then symmetrically placing the sample block on the surface to ensure uniformity of the polishing thickness in different areas. Mechanical polishing is then performed. The sample block is then removed, cooled, and the paraffin wax is removed for cleaning. The polishing solution has a water:alumina ratio of 10:1, a polishing speed of 100 r / min, and a mechanical polishing time of 60 min.

[0064] The thickness of the gradient boron-doped SiC semiconductor transition layer is 2.98 μm, and the total amount of B atoms at the top of the transition layer is 2 × 10⁻⁶. 18 cm -3 The proportion of B atoms directly doped in SiC is 23.1%, while the proportion of B atoms in B4C is 76.9%.

[0065] Then, a silicon carbide substrate with a gradient boron-doped SiC semiconductor transition layer on its surface is placed in a suspension containing a mixture of nanocrystalline and / or microcrystalline diamond particles; ultrasonic treatment is performed, followed by drying; a substrate material with nanocrystalline and / or microcrystalline diamond adsorbed on its surface is obtained; the mass fraction of the diamond mixture particles in the suspension containing nanocrystalline and / or microcrystalline diamond particles is 0.02%; the particle size of the diamond mixture particles is 5-10 nm, and the purity is ≥97%; the ultrasonic treatment time is 30 min.

[0066] A silicon carbide substrate with surface-adsorbed nanocrystalline and / or microcrystalline diamond is placed in a chemical vapor deposition furnace. Hydrogen, boron-containing gas, and carbon-containing gas are introduced. First, the percentage of boron-containing gas in the furnace mass flow rate is controlled to be 0.0884% to obtain a boron-doped diamond bottom layer. Then, the boron doping concentration is reduced linearly until the percentage of boron-containing gas in the furnace mass flow rate is 0.0593% to obtain a boron-doped diamond intermediate layer. Then, the percentage of boron-containing gas in the furnace mass flow rate is controlled to be 0.03968% to deposit a boron-doped diamond top layer. This yields a gradient boron-doped diamond semiconductor layer.

[0067] The carbon-containing gas accounts for 3.0% of the total gas mass flow rate in the furnace, the boron-doped diamond deposition temperature is 800℃, and the gas pressure is 10. 3 The deposition time was 10 h. The thickness of the gradient boron-doped diamond semiconductor layer was 10.24 μm.

[0068] In Example 3, granular activated carbon was first mixed with activated adsorbent in a 2 mol / L sulfuric acid solution at a solid-liquid volume ratio of 1:2. The mixture was reacted at 45°C for 2 hours. The sulfuric acid was then poured off, and the adsorbent was washed until the pH of the washing solution was >1. Then, 10% hydrogen peroxide was added at a solid-liquid ratio of 1:2, and the mixture was reacted for 24 hours. After washing, the adsorbent was dried and ready for use. The mesoporous content of the modified activated adsorbent increased from 21.9% to 66.5%.

[0069] Wastewater containing PFAs (PFOS concentration of 15.3 μg / L, PFOA concentration of 19.6 μg / L, flow rate of 10 m / s) was treated. 3 The wastewater first passes through a filter, then through adsorption tank A containing the modified granular activated carbon (specific surface area of ​​2689 m² / g, porosity of 73.4%, of which 66.5% is mesoporous). The treated water is then directly discharged. When the activated carbon in adsorption tank A becomes saturated, the pipeline is switched so that the filtered wastewater passes through adsorption tank B, while adsorption tank A is purged with a 2% NaOH solution for desorption. The resulting purging solution is then fed into a BDD reactor for electrochemical oxidation degradation. The purging solution is then returned to adsorption tank A for further desorption, and then back into the BDD reactor for electrochemical oxidation degradation, thus completing the desorption-degradation cycle. The process involves several steps. During the desorption process, ozone is introduced into adsorption tank A, with the concentration of ozone in the desorption solution controlled at 2 mg / L. During the degradation process, a BDD electrode is used as the anode and a Ti electrode as the cathode, with a current density of 60 mA / cm², a reaction temperature of 40-50℃, and a single degradation time of 2 hours. After repeating the desorption-degradation process 16 times, the solution is monitored online. The desorption solution obtained after the desorption process is a regenerated NaOH solution, which is first discharged to a storage tank. After adsorption tank B becomes saturated, the solution is returned to adsorption tank B to enter the next cycle of desorption. Meanwhile, a 4% sulfuric acid solution is introduced into adsorption tank A to regenerate the active adsorbent in adsorption tank A for later use.

[0070] After activated carbon adsorption, the effluent PFOS concentration was 25.3 ng / L, PFOA concentration was 18.4 ng / L, and the adsorption capacity was 96.1 mg / g. The initial PFOS concentration in the activated carbon washing and regenerated alkaline solution was 140.1 mg / L, and PFOA concentration was 142.8 mg / L. After BDD degradation, the PFOS concentration in the regenerated alkaline solution was 0.42 μg / L, and PFOA concentration was 0.37 μg / L. The average energy consumption for BDD degradation of fluorides was 6.3 kWh / g fluoride.

[0071] Example 4

[0072] In this Example 4, all other conditions are the same as in Example 3, except that the gradient boron-doped SiC semiconductor transition layer is plasma etched and then subjected to chemical vapor deposition during the BDD electrode preparation process. The etching gas is SF6, the auxiliary gas is O2, and the diluent gas is N2. The auxiliary gas accounts for 5% of the total gas flow rate, and the diluent gas accounts for 15% of the total gas flow rate. During the etching process, the cavity pressure is 5 Pa, the radio frequency power is 300 W, and the process time is 300 s. A groove structure with a depth gradually decreasing from the bottom center to the edge is obtained, and the etching depth is 2 μm.

[0073] Wastewater containing PFAs (PFOS concentration of 15.3 μg / L, PFOA concentration of 19.6 μg / L, flow rate of 10 m / s) was treated. 3 The activated carbon ( / h) first passes through a filter, then through a container filled with the modified granular activated carbon (specific surface area of ​​2689 m²). 2 Adsorption tank A (with a porosity of 73.4%, of which 66.5% is mesoporous) directly discharges treated water that meets standards. When the active agent in adsorption tank A is detected to be saturated, the pipeline is switched so that the wastewater filtered by the filter passes through adsorption tank B, while adsorption tank A is purged with a 2% NaOH solution for desorption. The resulting eluent is then fed into a BDD reaction tank for electrochemical oxidation degradation, and then returned to adsorption tank A for further desorption. The resulting eluent then re-enters the BDD reaction tank for electrochemical oxidation degradation, creating a desorption-degradation cycle. During the desorption process, ozone is introduced into adsorption tank A. The concentration of ozone introduced into the eluent was controlled at 2 mg / L. During the degradation process, the BDD electrode was used as the anode and the Ti electrode as the cathode. The current density was 60 mA / cm2, the reaction temperature was 40-60℃, and the time for a single degradation was 1.75 h. After repeating the eluent-degradation process 14 times, the solution was monitored online. The eluent obtained after the eluent was desorbed was a regenerated NaOH solution, which was first discharged to a storage tank. After the adsorption tank B was saturated, it was returned to the adsorption tank B to enter the next cycle of eluent. Meanwhile, a 4% sulfuric acid solution was introduced into the adsorption tank A to regenerate the active adsorbent in the adsorption tank A for later use.

[0074] After activated carbon adsorption, the effluent PFOS concentration was 16.3 ng / L, PFOA concentration was 14.9 ng / L, and the adsorption capacity was 98.2 mg / g. The initial PFOS concentration in the activated carbon washing and regenerated alkaline solution was 139.2 mg / L, and PFOA concentration was 142.3 mg / L. After BDD degradation, the PFOS concentration in the regenerated alkaline solution was 0.91 μg / L, and PFOA concentration was 0.84 μg / L. The average energy consumption for BDD degradation of fluorides was 5.56 kWh / g fluoride.

[0075] Comparative Example 1

[0076] Other conditions were the same as in Example 1, except that the wastewater containing PFAs was passed through a filter and then directly degraded in a BDD reactor until qualified water was obtained. The test showed that the PFOS concentration in the effluent from the BDD reactor was 0.28 ng / L and the PFOA concentration was 0.33 ng / L. The average energy consumption for BDD degradation of fluoride was 71 kWh / g fluoride.

Claims

1. A method for the complete treatment of PFAs in environmental water bodies, characterized in that: Wastewater containing PFAs is first passed through a filter, and then adsorbed by adsorption tank A or adsorption tank B containing active adsorbent. The treated water is then directly discharged. When adsorption tank A or adsorption tank B is saturated, NaOH solution is introduced for desorption. The resulting desorbate is then fed into a BDD reaction tank for electrochemical oxidation degradation. The desorbate is then returned to adsorption tank A or adsorption tank B for further desorption. After this cycle of desorption and degradation, the desorption is completed. After the desorption is completed, adsorption tank A or adsorption tank B is then purged with an acid solution to regenerate the active adsorbent. The wastewater is used for the next cycle of adsorption. The solution obtained after the analysis is a regenerated NaOH solution, which is first discharged to the storage tank. After the adsorption tank A or adsorption tank B is saturated, it is returned to adsorption tank A or adsorption tank B to enter the next cycle of analysis. During the electrochemical oxidation degradation process in the BDD reaction tank, the BDD electrode is used as the anode and the Ti electrode is used as the cathode. The current density is 50-100 mA / cm2, the reaction temperature is 30-60℃, and the time for a single degradation is 0.5-2.5 h. The BDD electrode consists of a silicon carbide substrate, a gradient boron-doped SiC semiconductor transition layer disposed on the surface of the silicon carbide substrate, and a gradient boron-doped diamond semiconductor layer disposed on the surface of the gradient boron-doped SiC semiconductor transition layer. In the gradient boron-doped SiC semiconductor transition layer, the boron content decreases from top to bottom; in the gradient boron-doped diamond semiconductor layer, the boron content decreases from bottom to top.

2. The method for the complete treatment of PFAs in environmental water bodies according to claim 1, characterized in that: The porosity of the active adsorbent is 40-75%, and the specific surface area is 1000-2000 m² / g; The active adsorbent is selected from at least one of biochar, activated carbon, and activated carbon fiber.

3. The method for the complete treatment of PFAs in environmental water bodies according to claim 2, characterized in that: The active adsorbent is first modified by adding the active adsorbent to a sulfuric acid solution for the first reaction, followed by solid-liquid separation. The resulting solid phase is washed until the pH is greater than 1, and then added to hydrogen peroxide for the second reaction. The concentration of the sulfuric acid solution is 1-2 mol / L; The volume ratio of the active adsorbent to the sulfuric acid solution is 1:1~2; The first reaction is carried out at 80-100℃ for 1-2 hours. The mass concentration of the hydrogen peroxide is 5-10%; The volume ratio of the washed solid phase to hydrogen peroxide is 1:1~2; The second reaction takes 12-24 hours.

4. The method for the complete treatment of PFAs in environmental water bodies according to claim 1, characterized in that: The NaOH solution contains 1-2% NaOH by mass.

5. A method for the complete treatment of PFAs in environmental water bodies according to claim 1 or 4, characterized in that: During the process of introducing NaOH solution into adsorption tank A or adsorption tank B for analysis, ozone is introduced into adsorption tank A or adsorption tank B at a concentration of 1-3 mg / L.

6. The method for the complete treatment of PFAs in environmental water bodies according to claim 1, characterized in that: The surface of the gradient boron-doped SiC semiconductor transition layer contains several grooves, the depth of which is 0.5~3μm; At the top of the gradient boron-doped SiC semiconductor transition layer, the total amount of B atoms is 10. 16 ~10 19 cm -3 The proportion of boron atoms directly doped between the SiC lattice is 20-30%, while the proportion of boron atoms in B4C is 70-80%. The gradient boron-doped diamond layer, from bottom to top, includes a boron-doped diamond bottom layer, a boron-doped diamond intermediate layer, and a boron-doped diamond top layer. The boron-doped diamond bottom layer has a uniform boron content, with a B / C ratio of 46,666-60,000 ppm (atomic ratio). The boron-doped diamond top layer also has a uniform boron content, with a B / C ratio of 26,666-40,000 ppm (atomic ratio). The boron content in the boron-doped diamond intermediate layer decreases linearly from bottom to top, with the boron content at the bottom layer being the maximum and decreasing linearly to the top layer.

7. A method for the complete treatment of PFAs in environmental water bodies according to claim 1 or 6, characterized in that: The method for preparing the BDD electrode is as follows: a silicon carbide substrate is immersed in a solution containing boron oxide powder and dried to obtain a silicon carbide substrate coated with boron oxide powder. Then, the silicon carbide substrate coated with boron oxide powder is subjected to thermal diffusion treatment to obtain a gradient boron-doped SiC semiconductor transition layer. Then, a gradient boron-doped diamond semiconductor layer is grown on the silicon carbide substrate containing the gradient boron-doped SiC semiconductor transition layer by chemical vapor deposition. The heat diffusion treatment process is as follows: first, the temperature is raised to 200~650℃ and held for 20~50 min; then, the temperature is raised to 1250~1400℃ and held for 45~90 min; finally, the temperature is raised to 1450~1650℃ and held for 30~60 min; the heat diffusion treatment atmosphere is at least one of air, oxygen, and nitrogen. After thermal diffusion treatment, the thermally diffused silicon carbide substrate is subjected to chemical cleaning and mechanical polishing treatment in sequence. The chemical cleaning process involves cleaning the surface of the thermally diffused silicon carbide substrate with BOE solution or HF solution. The mechanical polishing process involves heating a chemically cleaned silicon carbide substrate to 50-80°C, then coating it with paraffin wax. The paraffin-coated silicon carbide substrate is then symmetrically placed onto the surface of a sample block for mechanical polishing. After removal and cooling, the paraffin wax is removed, and the sample is cleaned. The polishing solution has a water:alumina ratio of 8-10:1; the polishing speed is 80-100 r / min; and the mechanical polishing time is 60-120 min. A silicon carbide substrate containing a gradient boron-doped SiC semiconductor transition layer is first plasma etched, and then a gradient boron-doped diamond semiconductor layer is grown by chemical vapor deposition. The plasma etching atmosphere includes an etching gas, an auxiliary gas, and a diluent gas. The etching gas is selected from SF6, CF4, CHF3, NF3, and Cl2; the auxiliary gas is selected from HBr, O2, and Ar; and the diluent gas is selected from He, Ne, and N2. The auxiliary gas accounts for 1-20% of the total gas flow rate, and the diluent gas accounts for 15%-30% of the total gas flow rate. During the etching process, the cavity pressure is 4-10 Pa, the RF power is adjustable from 200-400 W, and the process time is 50-600 s. The process of growing a graded boron-doped diamond semiconductor layer on a silicon carbide substrate containing a graded boron-doped SiC semiconductor transition layer via chemical vapor deposition is as follows: First, the silicon carbide substrate containing the graded boron-doped SiC semiconductor transition layer is placed in a suspension containing a mixture of nanocrystalline and / or microcrystalline diamond particles; ultrasonic treatment is performed, followed by drying; a silicon carbide substrate with surface-adsorbed nanocrystalline and / or microcrystalline diamond is obtained. Then, the silicon carbide substrate with surface-adsorbed nanocrystalline and / or microcrystalline diamond is placed in a chemical vapor deposition furnace, and hydrogen, boron-containing gas, and carbon-containing gas are introduced to perform chemical vapor deposition to grow a graded boron-doped diamond semiconductor layer. The temperature of the chemical vapor deposition is 600-1000℃, and the gas pressure is 10. 3 -10 4 Pa, time is 3-20h; In the suspension containing nanocrystalline and / or microcrystalline diamond mixed particles, the mass fraction of diamond mixed particles is 0.01%-0.05%; the particle size of the diamond mixed particles is 5-30 nm, and the purity is ≥97%; the ultrasonic treatment time is 5-30 min. During the chemical vapor deposition process, the percentage of carbon-containing gas in the total gas mass flow rate within the furnace is 0.5-10.0%. During the chemical vapor deposition process, the percentage of boron-containing gas in the total mass flow rate of the furnace is first controlled to be 0.069%-0.0884% to obtain a boron-doped diamond underlayer. Then, the boron doping concentration is reduced linearly until the percentage of boron-containing gas in the total mass flow rate of the furnace is 0.03968%-0.0593% to obtain a boron-doped diamond transition layer. Then, the percentage of boron-containing gas in the total mass flow rate of the furnace is controlled to be 0.03968%-0.0593% again to obtain a boron-doped diamond outer layer; thus, a gradient boron-doped diamond semiconductor layer is obtained.

8. The method for the complete treatment of PFAs in environmental water bodies according to claim 1, characterized in that: The acid solution is a sulfuric acid solution, and the mass fraction of sulfuric acid in the sulfuric acid solution is 1-5%.

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