Fluorine-doped porous carbon electrode and method for treating refractory wastewater

Abstract

The application relates to the technical field of industrial wastewater treatment, and discloses a fluorine-doped porous carbon electrode and a refractory wastewater treatment method, wherein the fluorine-doped porous carbon electrode for treating the refractory wastewater is a carbon matrix co-doped with fluorine and nitrogen; the carbon matrix has a gradient porous structure distributed along the thickness direction of the carbon matrix; the gradient porous structure has a macroporous side facing the inlet water; the gradient porous structure has a microporous side facing the outlet water; and the pore diameter of the gradient porous structure gradually decreases from the macroporous side to the microporous side. The fluorine-doped porous carbon electrode and the refractory wastewater treatment method have the advantages that the difference in electronegativity between fluorine and nitrogen is utilized to form an electron donor-acceptor pair in the carbon skeleton, target degradation of target fluorine-containing organic pollutants by persulfate is guided, and the traditional diffusion mass transfer is converted into convective mass transfer through the gradient porous structure, so that the path and time of pollutants from the water body to the active sites in the electrode are shortened.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of industrial wastewater treatment, and particularly relates to a fluorine-doped porous carbon electrode and a method for treating refractory wastewater. BACKGROUND

[0002] The pan-semiconductor industry (including integrated circuits, photovoltaics, display panels, etc.) produces complex refractory wastewater during the production process, which usually contains high concentrations of fluoride (including refractory fluorine-containing organic matter, such as perfluorooctanoic acid PFOA and perfluorooctane sulfonic acid PFOS), high salinity (especially chloride ions), nitrate / nitrite, and various heavy metals. Such wastewater is highly toxic and has poor biodegradability, making it extremely difficult to treat, and is a serious challenge in the field of industrial wastewater treatment.

[0003] Currently, the main methods for treating such wastewater include chemical precipitation, adsorption, advanced oxidation, membrane separation, and biological treatment. However, these methods have many limitations in practical application: the chemical precipitation method has limited removal effect on refractory organic fluorine and easily produces a large amount of fluorine-containing sludge, causing secondary pollution; the adsorption method has limited adsorption capacity, is difficult to regenerate, and has poor selectivity for complex components; advanced oxidation techniques such as Fenton and ozone oxidation can degrade some organic matter, but have low degradation efficiency for fluorine-containing organic matter under high-salinity and high-chloride conditions, and easily produce toxic byproducts (such as perchlorate and chlorite); membrane separation technology can separate salt and pollutants, but the concentrated water is difficult to treat, the membrane is severely contaminated, and the operation cost is high; and the biological treatment method is difficult to apply directly due to the high biological toxicity of the wastewater and the salt inhibition of microbial activity.

[0004] Electrochemical technology has shown good prospects in the treatment of refractory wastewater due to its high efficiency, cleanliness, and strong controllability. However, traditional electrochemical methods still face problems such as insufficient catalytic activity of electrode materials, limited selectivity, and poor durability when treating high-fluorine and high-salt wastewater in the pan-semiconductor industry. Common electrodes (such as platinum, boron-doped diamond, and metal oxide electrodes) are easily corroded and passivated in complex water quality, have low bond-breaking and defluorination efficiency for fluorine-containing organic matter, and have serious competition of side reactions (such as oxygen evolution and chlorine evolution), resulting in low current efficiency and high energy consumption. In addition, the existing electrochemical reactor has poor mass transfer performance, and the utilization efficiency of active substances (such as •OH) is low, which limits the rapid degradation and deep purification of pollutants. SUMMARY

[0005] To this end, the purpose of the present application is to solve the technical problem of efficient and targeted removal of target fluorine-containing organic pollutants in industrial wastewater containing high concentrations of fluorides, high salinity, high concentrations of nitrate / organic nitrogen and refractory organic pollutants generated during the production process in the general semiconductor industry. The present application provides a fluorine-doped porous carbon electrode and a method for treating refractory wastewater. The difference in electronegativity between fluorine and nitrogen elements forms an electron donor-acceptor pair in the carbon skeleton, enabling the active interface to guide the heterolysis of persulfate to generate singlet oxygen molecules. Singlet oxygen molecules preferentially attack C-F bonds, achieving targeted degradation of target fluorine-containing organic pollutants. The gradient porous structure converts traditional diffusion mass transfer into convective mass transfer to shorten the path and time of pollutants from the water body to the active sites inside the electrode, thereby improving the degradation reaction rate.

[0006] To solve the above technical problems, the present application provides a fluorine-doped porous carbon electrode for treating refractory wastewater, wherein the fluorine-doped porous carbon electrode is a carbon matrix co-doped with fluorine and nitrogen elements.

[0007] The carbon matrix has a gradient porous structure distributed along its thickness direction, wherein the gradient porous structure has a macroporous side facing the inlet water, a microporous side facing the outlet water, and the pore size of the gradient porous structure gradually decreases from the macroporous side to the microporous side.

[0008] Preferably, the atomic ratio of the fluorine element to the nitrogen element is 0.5:1 to 2:1.

[0009] Preferably, the atomic ratio of the fluorine element to the nitrogen element is 0.8:1 to 1.5:1.

[0010] Preferably, the gradient porous structure comprises:

[0011] The first layer is located at the macroporous side, and the pore size of the first layer is greater than 50 nm.

[0012] The third layer is located at the microporous side, and the pore size of the third layer is less than 2 nm.

[0013] The intermediate layer is located between the first layer and the second layer, and the pore size of the intermediate layer is 2 nm to 50 nm.

[0014] Preferably, the pore size of the third layer is greater than 0.5 nm and less than 2 nm.

[0015] Preferably, the pore size of the first layer is greater than 50 nm and less than 200 nm.

[0016] In another aspect, the present application provides a treatment method for recalcitrant wastewater, which uses a fluorine-doped porous carbon electrode as described above as the electrode of an electrochemical reactor, and comprises the following steps:

[0017] The recalcitrant wastewater to be treated is introduced into the electrochemical reactor and vertically penetrates the fluorine-doped porous carbon electrode, with the water flowing from the macroporous side to the microporous side;

[0018] A persulfate salt is added to the cathode region;

[0019] A forward current is applied between the cathode and the anode of the electrochemical reactor to perform electrocatalytic oxidation;

[0020] The concentration of fluoride ions in the effluent of the electrochemical reactor is detected in real time, and in response to the concentration of fluoride ions reaching a first preset threshold, the forward current is switched to a reverse current to regenerate the electrode;

[0021] The first preset threshold is set based on the target effluent fluoride ion standard.

[0022] Preferably, the treatment method for recalcitrant wastewater further comprises:

[0023] When the forward current is switched to the reverse current, a calcium salt is added to the reaction system to form calcium fluoride precipitate from the desorbed fluoride ions from the electrode;

[0024] The molar ratio of calcium ions in the added calcium salt to fluoride ions in the effluent is 1:1 to 1.5:1.

[0025] Preferably, the treatment method for recalcitrant wastewater further comprises:

[0026] In response to the concentration of fluoride ions reaching a second preset threshold, the reverse current is switched to a forward current;

[0027] The second preset threshold is less than the first preset threshold.

[0028] Preferably, the molar ratio of the added amount of persulfate salt to the target fluorine-containing organic pollutants is 10:1 to 100:1.

[0029] Preferably, the density of the forward current is 5 mA / cm² to 50 mA / cm²;

[0030] Preferably, the density of the reverse current is 2 mA / cm² to 20 mA / cm².

[0031] The above technical solutions of the present application have the following beneficial effects compared to the prior art:

[0032] The fluorine-doped porous carbon electrode of this invention utilizes the electronegativity difference between fluorine and nitrogen to form electron donor-acceptor pairs within the carbon framework, creating an active interface. This active interface guides heterolytic cleavage of persulfate, generating singlet oxygen molecules. These singlet oxygen molecules preferentially attack CF bonds, achieving targeted degradation of fluorine-containing organic pollutants. Furthermore, the gradient porous structure transforms traditional diffusion mass transfer into convective mass transfer, significantly shortening the path and time for pollutants to migrate from the water to the active sites within the electrode, thereby increasing the degradation reaction rate.

[0033] The recalcitrant wastewater treatment method of this invention applies a positive current to the active interface via electron donor and acceptor to directionally activate persulfate to generate singlet oxygen, thus achieving preferential attack on the C–F bond. Furthermore, based on real-time monitoring of fluoride ion concentration, a first preset threshold is set as the regeneration trigger signal, enabling regeneration upon saturation and effectively preventing continuous degradation of electrode performance. Specifically, through electrostatic repulsion and active interface reduction, fluoride ions and contaminants (such as PFOA carboxylate ions) enriched on the electrode are effectively desorbed to restore the active sites of the electrode. Attached Figure Description

[0034] To make the content of this invention easier to understand, the invention will be further described in detail below with reference to specific embodiments and accompanying drawings, wherein:

[0035] Figure 1 This is a schematic diagram of the structure of an electrochemical reactor in an embodiment of the present invention.

[0036] Figure 2 This is a simulation diagram of water diffusion as wastewater passes through a fluorine-doped porous carbon electrode in an embodiment of the present invention.

[0037] Figure 3 This is a schematic flowchart of a method for treating recalcitrant wastewater in an embodiment of the present invention.

[0038] Figure 4 This is a schematic diagram of a process for treating recalcitrant wastewater using a fluorine-doped porous carbon electrode when a positive current is applied, as described in an embodiment of the present invention.

[0039] Figure 5 This is a schematic diagram of a process for treating recalcitrant wastewater using a fluorine-doped porous carbon electrode when a reverse current is applied, as described in an embodiment of the present invention.

[0040] Explanation of reference numerals in the accompanying drawings: 1. Reaction tank; 111. Outlet; 112. Inlet; 121. Fluorine-doped porous carbon electrode; 122. Boron-doped diamond electrode; 131. First additive component; 132. Second additive component; 14. Detection component; 15. Power supply component; 16. Wastewater storage device; 17. Treatment device. DETAILED DESCRIPTION

[0041] The application will be further described below in conjunction with the drawings and specific embodiments so that those skilled in the art can better understand and implement the application, but the embodiments are not intended to limit the application. The refractory wastewater is an industrial wastewater generated in the production process of the semiconductor industry, which contains high-concentration fluorides (including inorganic fluoride ions and organic fluorides), high salinity (represented by chloride ions), high-concentration nitrate / organic nitrogen, and refractory organic pollutants. Among them, the refractory organic pollutants include target fluorine-containing organic pollutants. The target fluorine-containing organic pollutants are organic compounds in which the hydrogen atoms on the carbon-hydrogen chain are partially or completely replaced by fluorine atoms, and at least one of perfluorocarboxylic acids (such as perfluorooctanoic acid PFOA), perfluorosulfonic acids (such as perfluorooctanesulfonic acid PFOS, perfluorohexanesulfonic acid PFHxS), and fluorine-containing oligomers (such as perfluorobutyl alcohol).

[0042] The conventional electrochemical advanced oxidation process (such as electro-Fenton) mainly relies on non-selective active species such as hydroxyl radicals (•OH), which has inherent defects in the treatment of fluorine-containing organic matter (such as target fluorine-containing organic pollutants): the attack efficiency of •OH on the C–F bond with high bond energy (about 486 kJ / mol) is low, the reaction kinetics is slow, it is difficult to achieve directional bond breaking, and toxic short-chain byproducts (such as perfluorobutyric acid, perfluoropentanoic acid, and other short-chain perfluorocarboxylic acids with carbon number less than 8) are easily produced. At the same time, in the presence of a large amount of chloride ions, •OH will preferentially react with Cl - to generate active chlorine radicals, resulting in ineffective quenching of the oxidant •OH, and further conversion of toxic byproducts such as chloramine and nitrosamine.

[0043] In addition, although the conventional three-dimensional electrode has a high specific surface area, the disordered pore structure leads to large mass transfer resistance, and the pollutants mainly rely on diffusion to reach the internal active sites, which is slow, resulting in low utilization rate of a large number of active sites inside the electrode. At the same time, the short-lived active species (such as •OH) generated on the surface of the electrode have self-quenching before diffusing to the target fluorine-containing organic pollutants, which seriously limits the reaction rate and current efficiency.

[0044] The application discloses a fluorine-doped porous carbon electrode 121 and a refractory wastewater treatment method.

[0045] Embodiment one: the embodiment of the application discloses an electrochemical reactor for treating refractory wastewater.

[0046] In application, reference is made to Figure 1The electrochemical reactor comprises a reaction tank 1, and a water inlet 112 and a water outlet 111 are arranged on the reaction tank 1. The water inlet 112 is connected with an external wastewater storage device 16, and is used for continuously pumping the wastewater to be treated into the reaction tank 1; and the water outlet 111 is used for discharging the treated water meeting the standard from the reaction tank 1 and further introducing the treated water into an external treatment device 17.

[0047] In actual application, the reaction tank 1 is provided with an anode and a cathode arranged oppositely, one electrode is a boron-doped diamond electrode 122 (for example, BDD produced by Beijing Wald Diamond Tool Co., Ltd.), and the other electrode is a fluorine-doped porous carbon electrode 121. Further, a power supply assembly 15 (for example, a bidirectional pulse power supply) is arranged between the anode and the cathode, and the power supply assembly 15 is electrically connected with the anode and the cathode, and is used for outputting a forward current (for performing electrocatalytic oxidation) or a reverse current (for electrode regeneration).

[0048] In actual implementation, the electrochemical reactor can further comprise a first adding assembly 131, a second adding assembly 132 and a detection assembly 14. Specifically, the first adding assembly 131 is used for adding persulfate (PMS) to the cathode region; the second adding assembly 132 is used for adding calcium salt into the reaction tank 1; and the detection assembly 14 is used for monitoring the concentration of fluorine ions in the effluent of the reaction tank 1 in real time.

[0049] The first adding assembly 131 comprises a first storage tank storing persulfate (for example, KHSO5), and a first metering pump is arranged at the output end of the first storage tank and used for adding persulfate to the cathode region. The persulfate is electrically activated at the cathode interface to generate singlet oxygen (O2), and the singlet oxygen selectively attacks the C-F bond in the target fluorine-containing organic pollutants, so that the organic fluorine is mineralized and the inorganic fluorine is released. 1 O2), and the singlet oxygen selectively attacks the C-F bond in the target fluorine-containing organic pollutants, so that the organic fluorine is mineralized and the inorganic fluorine is released.

[0050] The second adding assembly 132 comprises a second storage tank storing calcium salt (for example, calcium chloride solution or calcium hydroxide solution), and a second metering pump is arranged at the output end of the second storage tank and used for adding calcium salt into the reaction tank 1 in the electrode regeneration stage, so as to precipitate the desorbed fluorine ions in the reaction tank 1.

[0051] In addition, the detection assembly 14 comprises a sensor (for example, a fluorine ion selective electrode) installed at the water outlet 111, and is used for monitoring the concentration of fluorine ions in the effluent in real time and continuously.

[0052] Embodiment two: The embodiment of the present application discloses a fluorine-doped porous carbon electrode 121, which can be used as an electrode of the electrochemical reactor of embodiment one.

[0053] In application, in order to realize the targeted degradation of fluorine-containing organic matter in water, the fluorine-doped porous carbon electrode 121 of the embodiment is a carbon matrix co-doped with fluorine and nitrogen elements, and an electron donor-acceptor pair is formed in the carbon skeleton by virtue of the difference in electronegativity between fluorine and nitrogen elements.

[0054] The nitrogen atom has 5 valence electrons, and the carbon has 4. When the nitrogen atom replaces the carbon atom into the crystal lattice in the form of graphite nitrogen, one extra electron is generated. The extra electron is delocalized into the pi conjugated system of the carbon skeleton, resulting in a significant increase in the electron cloud density of the carbon atoms around the nitrogen atom, forming a local electron-rich region. The electron-rich region exhibits Lewis base characteristics and tends to donate electrons.

[0055] Fluorine is the element with the highest electronegativity in the periodic table, much higher than carbon. When the fluorine atom is combined with carbon in the form of a semi-ionic bond C-F, the electronegativity of fluorine will suck electrons from the carbon skeleton, resulting in a sharp decrease in the electron cloud density of the carbon atoms around the fluorine atom, forming a local electron-deficient region. The electron-deficient region exhibits Lewis acid characteristics and tends to accept electrons. It is worth noting that adjacent electron-deficient and electron-rich regions are usually separated by several carbon atoms, which can ensure that the electric field generated by both regions can act on the same persulfate.

[0056] Fluorine / nitrogen co-doping breaks the original symmetry of the electron distribution of the carbon skeleton, forming a large number of asymmetric electron donor-acceptor pair active interfaces on the surface of the carbon skeleton. Unlike traditional catalysis (trying to Fenton reaction) which breaks the peroxide bond of persulfate by single electron transfer to generate indiscriminate attacking free radicals (such as •OH or SO4 - ), the electron donor-acceptor pair active interface can guide the heterolysis of persulfate. Specifically:

[0057] After the persulfate (HSO5 - ) is chemisorbed onto the electron donor-acceptor pair active interface, the peroxide bond (-O-O-) of the persulfate spontaneously bridges between the electron-rich region and the electron-deficient region, and the terminal oxygen atom is close to the electron-rich region, and the oxygen atom close to -SO3 - is close to the electron-deficient region. The electron-rich region provides electrons to the terminal oxygen atom of the peroxide bond, and the electron-deficient region attracts electrons from the oxygen atom close to -SO3 - , forming a cooperative electron push-pull effect from both ends in opposite directions, causing the peroxide bond (-O-O-) to break and the two electrons of the peroxide bond to completely deviate to the -SO3 - end, resulting in heterolysis and generating singlet oxygen molecule 1 O2 and sulfate ion SO4 2- . The chemical path can be simplified as: .

[0058] Singlet oxygen molecule is an excited state molecule with strong electrophilicity, which reacts through electrophilic addition or energy transfer pathway. It has higher reaction priority to organic molecules rich in strong electron-withdrawing groups (such as fluorine atoms) in the molecule. For example: the strong electron-withdrawing effect of fluorine atom makes the carbon atom in C-F bond show positive charge (i.e. electron-deficient state), therefore, the electrophilic singlet oxygen molecule will preferentially attack the carbon atom in the electron-deficient C-F bond, thereby achieving targeted degradation.

[0059] It is worth noting that the carbon matrix co-doped with boron and nitrogen elements has lower internal electric field strength and lower interface polarization degree due to the much lower electron-withdrawing ability of boron element than that of fluorine element. Therefore, the catalytic efficiency of the carbon matrix co-doped with boron and nitrogen elements and the activity and selectivity of singlet oxygen molecule are all inferior to those of the carbon matrix co-doped with fluorine and nitrogen elements in the embodiment. The carbon matrix co-doped with phosphorus and nitrogen elements has lower electron-donor-acceptor pair control precision and catalytic efficiency due to the large radius of phosphorus atom, which mainly causes lattice distortion and topological defects, and the synergistic mechanism is derived from structural reconstruction rather than electron push-pull effect. Meanwhile, the electron-donating ability of phosphorus is weaker than that of nitrogen, which leads to the electron-donor-acceptor pair of the carbon matrix co-doped with phosphorus and nitrogen elements being inferior to that of the carbon matrix co-doped with fluorine and nitrogen elements in the embodiment. The carbon matrix co-doped with sulfur and nitrogen elements has unstable electron-donating ability of sulfur atom and easy formation of uneven defects due to the tendency of sulfur atom to be located at the edge of carbon grid or to form ring structure. Therefore, the quality of the electron-donor-acceptor pair and the density of active interface of the carbon matrix co-doped with sulfur and nitrogen elements are all inferior to those of the carbon matrix co-doped with fluorine and nitrogen elements.

[0060] Further, in order to balance the catalytic performance and structural stability of the electrode, the atomic ratio of fluorine to nitrogen in the embodiment is 0.5:1 to 2:1.

[0061] When the F / N ratio is greater than or equal to 0.5, there is sufficient fluorine atom to pair with nitrogen atom to form effective electron-donor-acceptor pair and sufficient density of active interface to ensure the generation efficiency of singlet oxygen molecule and the selectivity of reaction pathway.

[0062] If the F / N ratio is less than 0.5, the proportion of fluorine is too low, and most of the nitrogen atoms have no fluorine atoms to form strong polarized active interface. Although the isolated nitrogen atom can create an electron-rich region, it cannot cooperate with fluorine to produce a push-pull effect, and the density of active interface is insufficient, which leads to the catalytic pathway possibly shifting from generating singlet oxygen molecule to generating •OH or SO4 - shift, which reduces the selectivity of the reaction, and further leads to the decrease of catalytic selectivity.

[0063] When the F / N ratio is less than or equal to 2, excessive fluorination can be avoided to cause destruction of the pi conjugated system of the carbon skeleton, so as to maintain high conductivity of the electrode; at the same time, the C-F bond can be avoided to be too dense to cause material embrittlement and electrochemical corrosion, so as to ensure that the electrode can be stably operated for a long time in an electrochemical environment. Fluorine is a strong electron-withdrawing group, if the F / N ratio is greater than 2, excessive fluorine doping will excessively pull away the electrons of the carbon skeleton, seriously interfere with the pi conjugated system of carbon, cause the electronic conductivity of the electrode to drop sharply, and cannot efficiently perform electrocatalysis. Excessive fluorine may cover or surround the nitrogen sites, so that the two cannot effectively cooperate, causing the electrons to be unable to effectively transmit. In addition, a high fluorine content may cause a large number of unstable C-F bonds to be formed on the carbon skeleton, and the unstable C-F bonds are prone to break in an electrochemical environment, causing material corrosion and structure collapse, and shortening the service life of the electrode.

[0064] It is worth noting that, from the stoichiometry, when the F / N ratio is close to 1, it is most beneficial to form a pair of F-N active sites (i.e., effective catalytic centers) that are spatially adjacent and electronically synergistic, thereby maximizing the density of effective catalytic centers per unit area. In some embodiments, the atomic ratio of fluorine elements to nitrogen elements is 0.8:1 to 1.5:1, at which time the intrinsic catalytic activity of the electrode is higher, and the activation efficiency of the persulfate is higher. 1 O2 production rate is more significant, and has faster pollutant degradation kinetics. In some preferred embodiments, in order to avoid the binder covering the active sites or falling off, causing the electrode performance to decay, the fluorine-doped porous carbon electrode 121 of the present embodiment is a self-supporting film structure, and the material is a carbon matrix co-doped with fluorine elements and nitrogen elements at an atomic ratio of 1. The overall structure of the carbon matrix can realize a performance decay of less than 3% after 500 hours of continuous operation in a high-salt and strong-oxidizing environment; the removal rate of perfluorooctanoic acid (PFOA) is greater than 80%, and almost no short-chain byproducts are produced; even under the condition that the Cl - concentration is 5000 mg / L, the degradation efficiency of the target fluorine-containing organic pollutants decreases by less than 5%, significantly reducing the generation of chlorinated toxic byproducts.

[0065] In summary of the above embodiments, the fluorine-doped porous carbon electrode 121 and the refractory wastewater treatment method of the present application catalyze the persulfate through the active interface co-doped with fluorine elements and nitrogen elements to generate singlet oxygen, realize directional attack on the C-F bond,

[0066] In actual application, in order to solve the problems of large internal mass transfer resistance and low active site utilization rate of the traditional three-dimensional electrode, the carbon matrix of the present embodiment has a gradient porous structure distributed along the thickness direction thereof. The gradient porous structure has a macroporous side facing the inlet water and a microporous side facing the outlet water, and the pore size of the gradient porous structure gradually decreases from the macroporous side to the microporous side, which can force the refractory wastewater to be treated to vertically penetrate the fluorine-doped porous carbon electrode 121, and the refractory wastewater to be treated can be uniformly distributed in the gradient porous structure of the carbon matrix, thereby improving the utilization rate of the active sites of the electrode and the degradation efficiency of the pollutants.Figure 2 Water flows from the macroporous side and out of the microporous side, converting the traditional diffusion mass transfer into convective mass transfer, significantly shortening the path and time of pollutants from the water body to the active sites inside the electrode, and further improving the catalytic reaction rate (i.e. degradation rate).

[0067] Further, the gradient porous structure comprises: a first layer, an intermediate layer, and a third layer arranged in sequence.

[0068] The first layer is located on the macroporous side (i.e. the water inlet side), and the pore size of the first layer is greater than 50 nm, preferably, the pore size of the first layer is greater than 50 nm and less than 200 nm. The first layer can intercept suspended particles and macromolecular organic matter in the wastewater to be treated, prevent internal pore blockage, and ensure long-term stable operation of the reactor. In addition, the first layer can uniformly distribute the influent to the mesoporous region of the intermediate layer and the microporous region of the third layer, thereby ensuring the flux of the electrode.

[0069] The intermediate layer is located between the first layer and the third layer, and the pore size of the intermediate layer is 2 nm to 50 nm. The intermediate layer has a high specific surface area and also has good mass transfer performance, can adsorb and degrade most of the pollutants (such as perfluorooctanoic acid, the molecular length is about 2 nm), and realizes high capacity and high efficiency treatment. Perfluorooctanoic acid (PFOA) is efficiently adsorbed and enriched in the intermediate layer, and fully contacts with the active sites under the action of vertical convection, realizing the synergistic enhancement of catalytic reaction and mass transfer process.

[0070] The third layer is located on the microporous side (i.e. the water outlet side), and the pore size of the third layer is less than 2 nm. The third layer can further capture and degrade small molecule intermediates, and in a very small space, can greatly improve the collision frequency of pollutant molecules and active sites, to significantly improve the reaction rate, thereby ensuring that the effluent water quality meets the standard. Preferably, in order to ensure that the pore size is sufficient for water molecules and pollutants to enter, while further strengthening the confinement effect, and to avoid complete blockage due to too small pore size, the pore size of the third layer is greater than 0.5 nm and less than 2 nm.

[0071] Embodiment three: the embodiment discloses a preparation method of a fluorine-doped porous carbon electrode 121, which can be used as the fluorine-doped porous carbon electrode 121 of embodiment two.

[0072] The preparation method of the fluorine-doped porous carbon electrode 121 of the embodiment comprises steps: step SS11 to step SS13.

[0073] Step SS11: mix the fluorine source, the nitrogen source and the carbon source in the solvent to obtain a homogeneous precursor solution.

[0074] In application, polyvinylidene fluoride (PVDF) is used as the fluorine source and the carbon source; a nitrogen-containing high polymer (such as polyaniline PANI) is used as the nitrogen source; and N,N-dimethylformamide (DMF) is used as the solvent.

[0075] In actual application, 1.6 g of polyvinylidene fluoride and 0.8 g of polyaniline are weighed and dissolved in 20 ml of N,N-dimethylformamide to obtain a first mixture. Then, the first mixture is subjected to a water bath at 60°C and magnetically stirred for 12 h to obtain a homogeneous precursor solution.

[0076] In actual implementation, the homogeneous precursor solution is a viscous black spinning solution.

[0077] Step SS12: injecting the homogeneous precursor solution into a mold to form an embryo with a gradient porous structure.

[0078] In application, the homogeneous precursor solution is injected into a stainless steel mold, the bottom of which is made of red copper and the side is made of thermal insulation material. Then, the mold is placed on a cold table of a constant temperature bath at -40°C, and the bottom is unidirectionally cooled for 4 h to form a unidirectional cooling and axial temperature gradient system, which guides the growth of ice crystals along the temperature gradient direction to obtain an embryo with a gradient porous structure.

[0079] In actual application, during the cooling process in the environment at -40°C, the homogeneous precursor solution undergoes directional solidification and phase separation, the DMF solvent forms ice crystals, and the PVDF / PANI is enriched along the temperature gradient direction to form an embryo with a gradient porous structure along the thickness direction. The gradient porous structure has a network of interconnected, three-dimensionally connected pore channels, and the pore size gradually decreases from the bottom to the top of the gradient porous structure. In some embodiments, the network of pore channels exhibits a sponge-like, reticular morphology dominated by macropores and gradually branching into mesopores and micropores.

[0080] In actual implementation, the bottom has the lowest temperature and the largest supercooling degree, and ice crystals are formed first. Since the side is thermally insulated, heat can only be vertically upward. The ice crystals formed at the bottom grow along the heat flow direction, i.e., from the bottom (the coldest) to the top (warmer).

[0081] Step SS13: drying and carbonizing the embryo to obtain a fluorine-doped porous carbon electrode 121.

[0082] In application, the embryo is transferred to a freeze dryer (which needs to be precooled to -50°C) at -50°C. Under the condition of a vacuum degree <10 Pa, the ice crystals in the embryo are sublimated by continuous drying for 48 h to obtain a precursor skeleton with a gradient porous structure.

[0083] In actual application, the dried precursor skeleton is placed in a tube furnace, nitrogen (N2, 99.999%) is continuously passed at a flow rate of 100 mL / min, and the temperature of the reaction system is raised from room temperature to 300°C at a rate of 2°C / min under the protection of nitrogen, and then the temperature of the reaction system is raised from 300°C to 900°C at a rate of 5°C / min, and carbonization is carried out at 900°C for 2 h, and then the temperature is naturally cooled to room temperature (generally 23°C), thereby obtaining a fluorine-doped porous carbon electrode 121 with a gradient porous structure along the thickness direction and co-doped with fluorine and nitrogen elements.

[0084] In actual implementation, the gradient porous structure of the fluorine-doped porous carbon electrode 121 has a pore size gradually decreasing from 150 nm to 50 nm and then to 1 nm along the thickness direction. Further, the fluorine-doped porous carbon electrode 121 contains C, N, and F elements, the atomic content of F element is about 8.5 at%, the atomic content of N element is about 6.2 at%, and the F / N atomic ratio is about 1.37. Among them, the F element mainly exists in the form of semi-ionic bond C-F, and the N element mainly exists in the form of graphite nitrogen. Further, the specific surface area of the fluorine-doped porous carbon electrode 121 is 1250 m² / g.

[0085] Example Four: The present embodiment discloses a treatment method for refractory wastewater, which can be realized by using the electrochemical reactor of Example One or by using the fluorine-doped porous carbon electrode 121 of Example Two as the electrode of the reactor.

[0086] Reference Figure 3 The treatment method for refractory wastewater of the present embodiment includes steps SS21 to SS24. Further, the treatment method of the present embodiment can further include steps SS25 to SS26.

[0087] Step SS21: The refractory wastewater to be treated is passed into the electrochemical reactor, and the refractory wastewater to be treated penetrates the fluorine-doped porous carbon electrode 121 vertically, with the water flow direction flowing in from the macropore side and flowing out from the micropore side.

[0088] In application, the refractory wastewater to be treated is uniformly passed into the electrochemical reactor. After the refractory wastewater to be treated enters the reaction tank 1 through the water inlet 112, it penetrates the negative electrode and finally flows out of the reaction tank 1 from the water outlet 111.

[0089] Step SS22: Adding persulfate to the cathode region.

[0090] In application, the persulfate is not directly used for oxidizing pollutants, but is electrocatalytically activated on the active interface co-doped with fluorine and nitrogen elements to generate singlet oxygen which actually plays an oxidizing role.

[0091] In practical applications, in order to provide sufficient peroxymonosulfate to completely degrade the target fluorine-containing organic pollutants, avoid the waste of oxidants (i.e. peroxymonosulfate), and prevent side reactions such as self-quenching of active species caused by excessive concentration of peroxymonosulfate, the molar ratio of the added amount of peroxymonosulfate to the target fluorine-containing organic pollutants in the embodiment is 10:1 to 100:1.

[0092] Step SS23: A forward current is applied between the cathode and the anode of the electrochemical reactor to perform electrocatalytic oxidation reaction.

[0093] In applications, a forward current is applied, and the fluorine-doped porous carbon electrode 121 serves as the cathode of the electrochemical reactor. Referring to Figure 4 , after the peroxymonosulfate (HSO5 - ) is chemisorbed on the active interface of the electron donor-acceptor pair, it undergoes heterolysis to generate singlet oxygen molecules 1 O2 and sulfate ions SO4 2- . The electrophilic singlet oxygen molecules preferentially attack the C-F bonds in the nearby adsorbed target fluorine-containing organic pollutants, mineralizing them into CO2, H2O and F - .

[0094] In practical applications, in order to ensure that the electrocatalytic oxidation reaction (i.e. peroxymonosulfate activation and organic matter degradation) has sufficient driving force (i.e. electron transfer rate) to efficiently activate peroxymonosulfate and maintain a high reaction rate; at the same time, avoid excessive current from causing water to be electrolyzed to generate O2, which competes with the peroxymonosulfate activation reaction for electrons, reducing current efficiency and increasing ineffective energy consumption; in addition, avoid the destruction of the carbon skeleton and C-F bonds by a strong electric field or activation reaction, leading to the deactivation of the electrode material, the density of the forward current in the embodiment is 5 mA / cm² to 50 mA / cm².

[0095] Step SS24: Real-time detection of the concentration of fluoride ions in the effluent of the electrochemical reactor, and in response to the concentration of fluoride ions reaching a first preset threshold, switching the forward current to a reverse current to regenerate the electrode.

[0096] In applications, the system is started, and a forward current is applied to perform electrocatalytic oxidation reaction to degrade fluorine-containing organic matter. The concentration of fluoride ions in the effluent from the effluent outlet 111 of the initial reaction tank 1 is very low (e.g. 0.2 mg / L). As the system operates, the adsorption of fluorine by the cathode tends to saturate, and the catalytic efficiency begins to decline, and the concentration of fluoride ions in the effluent gradually increases. When it is detected that the concentration of fluoride ions in the effluent reaches or exceeds the first preset threshold, it indicates that the adsorption of the cathode is close to saturation.

[0097] Further, the first preset threshold is based on a target effluent fluoride ion standard. To ensure absolute stability and compliance with a safety margin, the first preset threshold is less than the target effluent fluoride ion concentration (for example, the first preset threshold is 0.8 mg / L, and the target effluent fluoride ion concentration is 1.0 mg / L) to ensure that the final effluent meets the standard requirements at any time. Specifically, the first preset threshold includes the following formula:

[0098] ;

[0099] ; in which, is the first preset threshold; is the target effluent fluoride ion concentration; is the flow rate of the difficult-to-degrade wastewater to be treated into the electrochemical reactor; is the volume of the reaction tank 1; is the delay time of the detection assembly 14; is the concentration of the target fluorine-containing organic pollutant in the difficult-to-degrade wastewater to be treated.

[0100] It is worth noting that when the water inlet load of the water inlet 112 is high, or the concentration of the target fluorine-containing organic pollutant in the difficult-to-degrade wastewater to be treated is high, the first preset threshold is increased to ensure the pass rate of the effluent. When the water inlet load of the water inlet 112 is reduced, or the concentration of the target fluorine-containing organic pollutant in the difficult-to-degrade wastewater to be treated is reduced, the first preset threshold is reduced to reduce the number of regenerations, thereby avoiding unnecessary damage to the electrode material caused by the electrochemical regeneration process.

[0101] In actual application, when the concentration of fluoride ions in the effluent reaches the first preset threshold, the direction of the current is switched in time, so that the fluorine-doped porous carbon electrode 121 changes from a cathode to an anode. At this time, the fluorine-doped porous carbon electrode 121 has a positive charge, which produces a static repulsive force on the negatively charged pollutant intermediates (such as PFOA carboxylate) and fluoride ions, causing them to desorb and restore the active sites of the electrode, thereby achieving in-situ regeneration of the electrode.

[0102] In actual implementation, in order to provide sufficient energy to effectively desorb the pollutant intermediates and fluoride ions enriched on the fluorine-doped porous carbon electrode 121 and achieve electrode regeneration, while avoiding mechanical damage or excessive reduction of the carbon skeleton of the electrode caused by excessive reduction (such as the generation of a large amount of H2 gas bubbles): , the density of the reverse current in the present embodiment is 2 mA / cm² to 20 mA / cm².

[0103] Step SS25: When the forward current is switched to the reverse current, calcium salt is added to the reaction system to form calcium fluoride precipitate from the desorbed fluoride ions from the electrode, as described in reference Figure 5 .

[0104] In application, the dissolved fluorine ions in water are converted into calcium fluoride precipitates, such as CaF2. The solid CaF2 has large density and good stability, which not only can precipitate and remove the desorbed fluorine ions in time, prevent the fluorine ions from circulating and accumulating in the reactor, avoid the re-adsorption of the fluorine ions on the electrode surface or the formation of CaF2 fouling layer, and further protect the electrode performance, but also can provide a basis for subsequent enrichment and recovery of fluorine resources.

[0105] In actual application, the molar ratio of calcium ions in the added calcium salt to fluorine ions in the effluent is 1:1 to 1.5:1.

[0106] Step SS26: Switching the reverse current to the forward current in response to the concentration of fluorine ions reaching the second preset threshold.

[0107] In application, the current is switched back to the forward direction according to the second preset threshold, realizing closed-loop control of treatment and regeneration. After 20 complete treatment and regeneration cycles, the comprehensive performance decay rate of the fluorine-doped porous carbon electrode 121 is less than 2%.

[0108] In actual application, the second preset threshold is set based on the first preset threshold and the expected fluorine ion concentration when the electrode regeneration is completed. Specifically, the second preset threshold is less than the first preset threshold, and the second preset threshold is less than or equal to the expected fluorine ion concentration when the electrode regeneration is completed.

[0109] Obviously, the above embodiments are only examples for clarity and do not limit the embodiments. For those skilled in the art, other different forms of changes or variations can be made on the basis of the above description. Here, all the embodiments need not and cannot be exhausted. The obvious changes or variations derived therefrom are still within the protection scope of the present application.

Claims

1. A fluorine-doped porous carbon electrode, characterized by, The fluorine-doped porous carbon electrode is a carbon matrix co-doped with fluorine and nitrogen elements, and is used as an electrode of an electrochemical reactor for electrocatalytic oxidative degradation of fluorine-containing organic wastewater. The atomic ratio of the fluorine element to the nitrogen element is 0.5:1 to 2:

1. The carbon matrix has a gradient porous structure distributed along the thickness direction thereof, a macroporous side facing the inlet side of water, a microporous side facing the outlet side of water, and a pore size gradually decreasing from the macroporous side to the microporous side.

2. The fluorine-doped porous carbon electrode according to claim 1, wherein, The atomic ratio of the fluorine element to the nitrogen element is 0.8:1 to 1.5:

1.

3. The fluorine-doped porous carbon electrode according to claim 1, wherein, The gradient porous structure comprises: a first layer located at the macroporous side, the pore size of the first layer being greater than 50 nm; a third layer located at the microporous side, the pore size of the third layer being less than 2 nm; an intermediate layer located between the first layer and the third layer, the pore size of the intermediate layer being 2 nm to 50 nm.

4. The fluorine-doped porous carbon electrode according to claim 3, wherein the pore size of the third layer is greater than 0.5 nm and less than 2 nm; and / or the pore size of the first layer is greater than 50 nm and less than 200 nm. The treatment method comprises the following steps: introducing the difficult-to-degrade wastewater to be treated into the electrochemical reactor, and making the difficult-to-degrade wastewater to be treated vertically penetrate the fluorine-doped porous carbon electrode, the water flow direction being from the macroporous side to the microporous side; 5. A method for treating recalcitrant wastewater, characterized by, adding peroxymonosulfate to the cathode region; applying a forward current between the cathode and the anode of the electrochemical reactor to perform electrocatalytic oxidation reaction; real-time detecting the concentration of fluoride ions in the effluent of the electrochemical reactor, and switching the forward current to a reverse current to regenerate the electrode in response to the concentration of fluoride ions reaching a first preset threshold value; the first preset threshold value is set based on a target effluent fluoride ion standard. The treatment method further comprises: when the forward current is switched to the reverse current, adding calcium salt to the reaction system, so that the fluoride ions desorbed from the electrode form calcium fluoride precipitate; 6. The method of treating recalcitrant wastewater of claim 5, wherein, the molar ratio of calcium ions in the added calcium salt to fluoride ions in the effluent is 1:1 to 1.5:

1. The treatment method further comprises: switching the reverse current to the forward current in response to the concentration of fluoride ions reaching a second preset threshold value; 7. The method of treating recalcitrant wastewater of claim 5, wherein, the second preset threshold value is less than the first preset threshold value. The molar ratio of the added amount of peroxymonosulfate to the target fluorine-containing organic pollutant is 10:1 to 100:

1. The density of the forward current is 5 mA / cm² to 50 mA / cm²; 8. The method of claim 5, wherein the refractory wastewater is a wastewater containing a compound selected from the group consisting of a phenol compound, a nitro compound, a chlorinated hydrocarbon, a heavy metal, and a cyanide compound. and / or, the density of the reverse current is 2 mA / cm² to 20 mA / cm².

9. The method of claim 5, wherein the refractory wastewater is a wastewater containing a compound selected from the group consisting of a phenol compound, a nitro compound, a chlorinated hydrocarbon, a heavy metal, and a cyanide compound. ​ ​

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