A method for generating hydroxyl radicals by electrochemical advanced oxidation method and its application

Abstract

The present invention discloses a method and application of generating hydroxyl radicals by electrochemical advanced oxidation method, the method comprises the following process: inserting a working electrode, a counter electrode and a reference electrode into an electrolyte solution and connecting an electrochemical workstation, wherein the working electrode is a carbon-based titanium dioxide electrode, the counter electrode is a graphite electrode, and the reference electrode is an Ag / AgCl electrode; during operation, the electrochemical workstation periodically applies a positive and negative alternating potential to the working electrode, when a positive potential is applied, the working electrode is in an anode mode, and adsorbed H2O2 is generated on the surface of the working electrode; when a negative potential is applied, the working electrode is in a cathode mode, and the adsorbed H2O2 generated in the anode mode is generated into hydroxyl radicals by an electron-accepting reaction. The method disclosed by the present invention does not require aeration or reagent addition, the reaction process is clean, energy consumption is saved, a large amount of ·OH can be generated quickly, and the wastewater treatment capacity is improved.

Description

Technical Field

[0001] The invention relates to the field of electrochemical technology, and in particular to a method and application of generating hydroxyl radicals by electrochemical advanced oxidation. Background Art

[0002] Cathodic electro-Fenton (EF) and anodic oxidation are both commonly used electrochemical advanced oxidation technologies (EAOPs), which treat difficult-to-degrade organic wastewater based on the production of active free radicals such as ·OH. ·OH has strong oxidizing properties (oxidation potential of 2.8V) and can undergo rapid chain reactions with most organic pollutants, non-selectively oxidizing harmful substances into CO2, H2O or mineral salts without causing secondary pollution. The degradation of organic matter in water is achieved by reacting the electrode on the cathode with O2 and water to generate H2O2 and then ·OH. O2 is usually pumped in by air. Due to the low solubility of O2, a large amount of air needs to be pumped in continuously, generating a large amount of energy consumption, even accounting for more than half of the energy consumption in the water treatment process. The air also contains gases such as CO2, which will also undergo reduction reactions with free radicals to generate some organic by-products, causing interference.

[0003] Anodic oxidation is based on an inactive anode, with low ·OH production and high energy consumption. The anode can directly decompose water to produce ·OH, which requires a high current density and is prone to electrode corrosion. The corrosion-resistant BDD anode has high manufacturing and use costs, which is not conducive to industrial applications. Our previous research found that at low potentials (1-1.2V), the anode can also first produce H2O2 and then decompose into ·OH. However, the rate at which the anode decomposes H2O2 is slow, and H2O2 will accumulate, while the rate at which the cathode decomposes H2O2 is four times that of the anode. There are also studies that increase the anode ·OH production by preparing Blue-TNT. During the reaction, the anode activity is maintained by performing cathode reduction in a reducing medium. This method has a small effect on the ·OH yield, less than doubled (BDD: 1.7*10 -14 M, Blue-TNT: 9.8*10 -15 M), and this material will still be deactivated after a period of operation and cannot meet the demand for high ·OH production. Summary of the invention

[0004] In order to solve the above technical problems, the present invention provides a method and application of generating hydroxyl radicals by electrochemical advanced oxidation method, so as to achieve the purpose of no need for aeration or addition of reagents, clean reaction process, saving energy consumption, and being able to quickly generate a large amount of ·OH, thereby improving the wastewater treatment capacity.

[0005] To achieve the above object, the technical solution of the present invention is as follows:

[0006] A method for generating hydroxyl radicals by electrochemical advanced oxidation method comprises the following steps:

[0007] The working electrode, the counter electrode and the reference electrode are inserted into the electrolyte solution and connected to the electrochemical workstation, wherein the working electrode is a carbon-based titanium dioxide electrode, the counter electrode is a graphite electrode, and the reference electrode is an Ag / AgCl electrode. During operation, the electrochemical workstation periodically applies a positive and negative alternating potential to the working electrode. When a positive potential is applied, the working electrode is in an anode mode, and adsorbed H2O2 is generated on the surface of the working electrode. When a negative potential is applied, the working electrode is in a cathode mode, and the adsorbed H2O2 generated in the anode mode is converted into hydroxyl radicals through an electron-accepting reaction.

[0008] In the above scheme, the applied positive potential is 1.0 to 1.2V, and the applied negative potential is -0.3 to -0.8V.

[0009] Preferably, the positive potential applied is 1.0 V for 300 s, and the negative potential applied is -0.6 V for 100 s.

[0010] In the above scheme, the electrolyte solution is Na2SO4 solution.

[0011] In the above scheme, the preparation method of the carbon-based titanium dioxide electrode is as follows:

[0012] (1) ultrasonically clean and dry expanded graphite powder for later use;

[0013] (2) placing the pretreated expanded graphite powder into an aniline-ethanol solution with a volume ratio of 1:4, ultrasonically treating the solution, and then rinsing the solution with ultrapure water;

[0014] (3) adding the cleaned expanded graphite powder to a mixed acid solution of hydrochloric acid and sulfuric acid with a molar concentration ratio of 1:2, adding ammonium persulfate while stirring, stirring at low temperature, filtering and drying to obtain EG-PAN powder;

[0015] (4) Weighing a certain amount of EG-PAN powder, mixing it with tetrabutyl titanate and anhydrous ethanol, and stirring to obtain a mixed solution A;

[0016] (5) Anhydrous ethanol, deionized water, and concentrated nitric acid are mixed in a certain volume ratio and stirred to obtain a mixed solution B;

[0017] (6) adding the mixed solution B dropwise into the mixed solution A at a volume ratio of 1:1, mixing the two thoroughly and evenly, and then leaving them to stand at room temperature for a period of time until a non-flowing gel system is formed;

[0018] (7) drying the formed gel in an oven to obtain black-gray particles;

[0019] (8) grinding it into powder, placing it in a muffle furnace for calcination, and finally obtaining a self-supporting carbon-based TiO2 electrocatalytic material EG-PAN-TiO2 powder;

[0020] (9) Weigh a certain mass of EG-PAN-TiO2 powder, put it into a tablet pressing mold, and use a tablet press to press it into a sheet electrode; apply a certain volume of industrial conductive silver glue to the end of the graphite plate, connect it to the sheet electrode, and let it stand for a period of time to obtain a carbon-based titanium dioxide electrode.

[0021] Preferably, the carbon-based titanium dioxide electrode has a thickness of 2 mm and a length and width of 2 cm×2 cm.

[0022] In the above scheme, the sheet electrode end of the carbon-based titanium dioxide electrode is inserted into the electrolyte solution, the graphite plate end is connected to the working electrode end of the electrochemical workstation through the working electrode signal line, the counter electrode signal line is connected to the counter electrode end of the electrochemical workstation, and the reference electrode signal line is connected to the reference electrode end of the electrochemical workstation.

[0023] An application of an electrochemical advanced oxidation method to generate hydroxyl radicals, using the above-mentioned method of generating hydroxyl radicals by an electrochemical advanced oxidation method, utilizing the strong oxidizing property of the generated hydroxyl radicals to oxidize and degrade harmful substances in an electrolyte solution.

[0024] In a further technical solution, the harmful substance is cephalexin, methylene blue, methyl orange, antibiotics or tetracycline.

[0025] Through the above technical scheme, the method and application of generating hydroxyl radicals by electrochemical advanced oxidation method provided by the present invention have the following beneficial effects:

[0026] The present invention periodically applies a positive and negative alternating potential to the carbon-based titanium dioxide working electrode through an electrochemical workstation. When a positive potential is applied, the working electrode is in an anode mode, and adsorbed H2O2 is generated on the surface of the working electrode; when a negative potential is applied, the working electrode is in a cathode mode, and the adsorbed H2O2 generated in the anode mode is converted into hydroxyl radicals through an electron-accepting reaction. The product on the anode is used as a reactant on the cathode, which can increase the production of ·OH, does not require aeration, can effectively reduce energy consumption, and greatly improve the ability to treat wastewater. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art are briefly introduced below.

[0028] Figure 1 A schematic diagram of a device for generating hydroxyl radicals by an electrochemical advanced oxidation method disclosed in an embodiment of the present invention;

[0029] Figure 2 Schematic diagram of applied potentials for different electrode modes: (a) bipolar mode, (b) anodic mode, and (c) cathodic mode.

[0030] Figure 3 is the ·OH production under different modes;

[0031] Figure 4 The current and voltage conditions in the bipolar mode of the present invention;

[0032] Figure 5 This is the degradation rate diagram of cephalexin under different modes;

[0033] Figure 6 This is a graph showing the degradation rate of cephalexin from three experiments under the bipolar mode of the present invention.

[0034] In the figure, 1. electrochemical workstation; 2. working electrode; 3. counter electrode; 4. reference electrode; 5. electrolyte solution. DETAILED DESCRIPTION

[0035] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the accompanying drawings in the embodiments of the present invention.

[0036] 1. Preparation of carbon-based titanium dioxide electrode

[0037] (1) The expanded graphite powder was ultrasonically cleaned with ultrapure water and anhydrous ethanol for 30 min, and then placed in a vacuum drying oven and dried at 80° C. for 8 h;

[0038] (2) 1 g of pretreated expanded graphite powder was placed in an aniline-ethanol solution with a volume ratio of 1:4 and subjected to ultrasonic treatment at 60 Hz for 30 min;

[0039] (3) adding the expanded graphite powder washed with ultrapure water to 75 mL of a 0.5 M / 1 M HCl / H2SO4 mixed acid solution, adding 7.6325 g of ammonium persulfate while stirring, continuing to stir at 6°C for 10 h, filtering, and drying at 80°C for 10 h to obtain EG-PAN powder;

[0040] (4) 1 g of EG-PAN powder, 7.5 ml of tetrabutyl titanate, and 22.5 ml of anhydrous ethanol were mixed and stirred with a magnetic stirrer for 20 min to form a mixed solution A;

[0041] (5) 22.5 ml of anhydrous ethanol, 3 ml of deionized water, and 0.6 ml of concentrated nitric acid were mixed and stirred with a magnetic stirrer for 20 min to form a mixed solution B;

[0042] (6) adding the mixed solution B dropwise into the mixed solution A at a volume ratio of 1:1 to fully mix the two, and then placing the mixture at 20° C. for a period of time until a non-flowing gel system is formed;

[0043] (7) drying the mixture in an oven at 105°C for 8 h to obtain black-gray particles;

[0044] (8) Grinding it into powder with a glass mortar, placing it in a muffle furnace and calcining it at 500 °C for 2 h to obtain a self-supporting carbon-based TiO2 electrocatalytic material EG-PAN-TiO2 powder;

[0045] (9) Weigh 0.5 g of the prepared EG-PAN-TiO2 powder and put it into a circular tablet pressing mold with a diameter of 30 mm. Use a tablet press to press the powder into a circular sheet with a thickness of 2 mm at a pressure of 20 MPa, and then cut it into a size of 2 cm × 2 cm. Use a pipette to absorb 10 μl of conductive silver glue and apply it to the end of a 0.5 cm × 6 cm × 0.2 cm graphite plate, connect it to the sheet electrode and let it stand for 30 minutes to finally obtain a carbon-based titanium dioxide electrode.

[0046] 2. Device Construction

[0047] like Figure 1 As shown, the prepared carbon-based titanium dioxide electrode is used as the working electrode 2, the graphite electrode is used as the counter electrode 3, and the Ag / AgCl electrode is used as the reference electrode 4. The sheet electrode end of the carbon-based titanium dioxide electrode is inserted into the electrolyte solution 5, and the graphite plate end is connected to the working electrode end of the electrochemical workstation 1 through the working electrode signal line. The counter electrode and the reference electrode are both inserted into the electrolyte solution 5, and the signal line of the counter electrode is connected to the counter electrode end of the electrochemical workstation 1, and the signal line of the reference electrode is connected to the reference electrode end of the electrochemical workstation 1.

[0048] 3. Generation of hydroxyl radicals

[0049] During the working process, the electrochemical workstation 1 periodically applies alternating positive and negative potentials to the working electrode. When a positive potential is applied, the working electrode is in the anode mode, and adsorbed H2O2 is generated on the surface of the working electrode; when a negative potential is applied, the working electrode is in the cathode mode, and the adsorbed H2O2 generated in the anode mode is converted into hydroxyl radicals through an electron-accepting reaction.

[0050] The reaction process is as follows:

[0051] anode:

[0052] cathode:

[0053] Among them, in the existing anode mode, this adsorbed H2O2 can also be decomposed, but when decomposing in the anode mode, it depends on the natural decomposition of H2O2, and its reaction equation is 2Ti-OH→2·OH, there is no electron gain or loss, and the reaction rate is slow; while the decomposition in the cathode mode accelerates the generation of ·OH by gaining electrons, and the reaction rate is greatly accelerated. The reaction method in the bipolar mode of the present invention can accelerate the conversion of H2O2 to ·OH. Most importantly, it can also provide reactants (H2O2) for the generation of ·OH in the cathode process, thereby avoiding aeration and the addition of H2O2. In addition, the method of the present invention has a low current density, can avoid electrode loss, and has strong reaction stability.

[0054] The specific process is:

[0055] 1. Through preliminary experiments, we found the potential (1.0V) at which the carbon-based titanium dioxide electrode produces the highest H2O2 in the anode mode and the optimal potential (-0.6V) at which the carbon-based titanium dioxide electrode decomposes H2O2 to produce ·OH in the cathode mode.

[0056] 2. Determine the time distribution of the cathode and anode potentials by observing the current changes during the reaction. This time distribution is related to the specific electrode and is affected by the electrode area. In this embodiment, the 2 cm × 2 cm working electrode is selected, and the appropriate distribution time is 1.0 V, 300 s, -0.6 V, 100 s. In this way, a periodic alternating positive and negative potential is applied to the working electrode, such as Figure 2 As shown in (a).

[0057] The following is a comparison of the production of ·OH in the bipolar mode of the present invention with the existing anode mode and cathode mode, wherein the anode mode is shown in FIG. Figure 2 As shown in (b), a potential of 1.0 V is continuously applied to the working electrode. Figure 2 As shown in (c), a potential of -0.6 V is continuously applied to the working electrode.

[0058] 4. Detection of OH

[0059] The ·OH concentration was determined by high performance liquid chromatography (HPLC-20AT) equipped with a C-18 reverse phase column and a UV detector at a wavelength of 300 nm. The mobile phase was a mixture of methanol and water (volume ratio of 4:6, pH adjusted to 2.01 with phosphoric acid). The flow rate of the mobile phase was 0.6 ml min -1, the single injection volume is 10 L. Salicylic acid was used as a probe to determine the generation of ·OH (the reaction formula of salicylic acid and ·OH: OHBA+·OH→2,3-DHBA+2,5-DHBA+ring cleavageproducts+……), and HPLC was used to quantitatively analyze the concentrations of the reaction products 2,3-dihydroxybenzoic acid (2,3-DHBA) and 2,5-dihydroxybenzoic acid (2,5-DHBA). After the sample passed through HPLC, the two peaks at around 16 and 22 minutes corresponded to the concentrations of the above two products, and the sum of the two concentrations was the ·OH concentration to be measured.

[0060] Reaction conditions: 100 mL of a solution containing 0.05 M Na2SO4 and 0.276 g / L salicylic acid, adjusted to pH 3 with H2SO4, reacted for 1 h, and stirred continuously during the reaction (300 r / min).

[0061] Figure 3 The ·OH production in different modes, wherein the cathode mode (constant -0.6V) is kept exposed to oxygen, so the ·OH production is relatively high, but this mode consumes a lot of energy, and air is introduced, which requires considerable energy consumption to provide enough oxygen. Compared with the anode only (0.8mg / L), the bipolar mode of the present invention (2.75mg / L) increases the ·OH production by more than 3 times.

[0062] Figure 4 The current and voltage in the bipolar mode of the present invention are shown in the figure. It can be seen from the figure that during the alternating process of the cathode and anode voltages, the current is relatively stable. The current size is about 3-10mA under the cathode voltage and about 2-10mA under the anode voltage. (The electrode area is 4cm 2 ), the current density of the reaction process does not exceed 2.5mA / cm 2 This low current density can effectively reduce the loss of anode graphite and prevent the dissolution of the anode during the reaction. Moreover, the current of each cathode and cathode cycle is relatively stable, which has the potential for continuous application.

[0063] 5. Application of Degradation of Organic Matter

[0064] A 100 mL solution containing 0.05 M Na2SO4 and 10.7 mg / L cephalexin was prepared, the pH was adjusted to 3 with H2SO4, and the solution was reacted in the above-mentioned bipolar mode for 90 min with continuous stirring (stirring rate of 300 r / min) during the reaction.

[0065] Cephalexin was detected by HPLC: injection volume 30 μL, wavelength 262 nm, flow rate 1.0 ml / min. The mobile phase was 25% acetonitrile 75% water 1L plus 1.35 ml of phosphoric acid. Figure 5As shown, the reaction rate constant has been marked in the figure. It can be seen from the figure that under the bipolar mode (1.0V / -0.6V), the degradation rate of cephalexin reaches 81.8%, which is lower than the cathode oxygen mode (degradation rate 96.1%), but more than doubled than the anode mode (degradation rate 37.60%). The bipolar mode was repeated three times. Figure 6 As shown, the reduction in the degradation rate of cephalexin did not exceed 1%, indicating that this model had excellent stability.

[0066] The above description of the disclosed embodiments enables one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for generating hydroxyl radicals by electrochemical advanced oxidation, characterized in that: The process includes the following: Inserting a working electrode, a counter electrode and a reference electrode into an electrolyte solution and connecting them to an electrochemical workstation, wherein the working electrode is a carbon-based titanium dioxide electrode, the counter electrode is a graphite electrode, and the reference electrode is an Ag / AgCl electrode; During the working process, an alternating positive and negative potential is periodically applied to the working electrode through the electrochemical workstation. When a positive potential is applied, the working electrode is in anode mode, and adsorbed H2O2 is generated on the surface of the working electrode; when a negative potential is applied, the working electrode is in cathode mode, and the adsorbed H2O2 generated in the anode mode is converted into hydroxyl radicals through an electron-accepting reaction; the applied positive potential is 1.0V for 300s, and the applied negative potential is -0.6V for 100s.

2. The method for generating hydroxyl radicals by electrochemical advanced oxidation method according to claim 1, characterized in that: The electrolyte solution is Na2SO4 solution.

3. The method for generating hydroxyl radicals by electrochemical advanced oxidation method according to claim 1, characterized in that: The preparation method of the carbon-based titanium dioxide electrode is as follows: (1) Ultrasonic cleaning and drying of expanded graphite powder for later use; (2) placing the pretreated expanded graphite powder into an aniline-ethanol solution with a volume ratio of 1:4, ultrasonically treating it, and then rinsing it with ultrapure water; (3) adding the washed expanded graphite powder to a mixed acid solution of hydrochloric acid and sulfuric acid with a molar concentration ratio of 1:2, adding ammonium persulfate while stirring, stirring at low temperature, filtering and drying to obtain EG-PAN powder; (4) Weigh a certain amount of EG-PAN powder, mix it with tetrabutyl titanate and anhydrous ethanol, and stir to obtain a mixed solution A; (5) Mix anhydrous ethanol, deionized water, and concentrated nitric acid in a certain volume ratio and stir to obtain a mixed solution B; (6) Adding the mixed solution B dropwise into the mixed solution A at a volume ratio of 1:1, the two are fully mixed, and then allowed to stand at room temperature for a period of time until a non-flowing gel system is formed; (7) drying the formed gel in an oven to obtain black-gray particles; (8) Grinding it into powder, placing it in a muffle furnace for calcination, and finally obtaining a self-supporting carbon-based TiO2 electrocatalytic material EG-PAN-TiO2 powder; (9) Weigh a certain mass of EG-PAN-TiO2 powder, put it into a tablet pressing mold, and use a tablet press to press it into a sheet electrode; apply a certain volume of industrial conductive silver paste to the end of the graphite plate, connect it to the sheet electrode, and let it stand for a period of time to obtain a carbon-based titanium dioxide electrode.

4. The method for generating hydroxyl radicals by electrochemical advanced oxidation method according to claim 3, characterized in that: The carbon-based titanium dioxide electrode has a thickness of 2 mm and a length and width of 2 cm×2 cm.

5. The method for generating hydroxyl radicals by electrochemical advanced oxidation method according to claim 3, characterized in that: The sheet-shaped electrode end of the carbon-based titanium dioxide electrode is inserted into the electrolyte solution, the graphite plate end is connected to the working electrode end of the electrochemical workstation through the working electrode signal line, the counter electrode signal line is connected to the counter electrode end of the electrochemical workstation, and the reference electrode signal line is connected to the reference electrode end of the electrochemical workstation.

6. An application of an electrochemical advanced oxidation method to generate hydroxyl radicals, using the method of generating hydroxyl radicals by an electrochemical advanced oxidation method as claimed in claim 1, characterized in that: The strong oxidizing property of the generated hydroxyl free radicals is utilized to oxidize and degrade harmful substances in the electrolyte solution; the harmful substances are cephalexin, methylene blue, methyl orange or tetracycline.

Images