Ultrasonic degassing reinforced electrooxidation wastewater dechlorination device and method

Abstract

The invention discloses an ultrasonic degassing reinforced electrooxidation wastewater dechlorination device and method, and belongs to the technical field of electrochemical water treatment. The electrooxidation wastewater dechlorination device comprises a reaction tank and an ultrasonic generator for applying ultrasonic action to wastewater in the reaction tank, the reaction tank is provided with a chlorine evolution anode and a titanium-based cathode; the chlorine evolution anode comprises a pure titanium substrate, and the pure titanium substrate is sequentially coated with a noble metal oxide coating and a Bi5O7Cl coating from inside to outside. By preparing the high-activity chlorine evolution anode, the chlorine evolution efficiency is improved; chlorine generated by electrooxidation is discharged in time under the action of ultrasonic waves, so that the chlorine is prevented from being redissolved and converted into chloride ions again; meanwhile, ultrasonic waves can also act on water molecules and oxygen generated by oxygen evolution side reaction, so that various active species are generated, and chloride ions are converted into chlorine. The method can significantly improve the chloride ion removal rate, improve the current efficiency, reduce the electrode pollution risk, and prolong the service life of the electrode.

Description

Technical Field

[0001] This invention belongs to the field of electrochemical water treatment technology, specifically relating to an ultrasonic degassing-enhanced electro-oxidation wastewater dechlorination device and method. Background Technology

[0002] Chlorine-containing wastewater mainly originates from metallurgy, mining, chemical industry, machinery manufacturing, food processing, textiles, and the use of daily cleaning and disinfection products. The chloride ion concentration in industrial wastewater is generally higher than 2000 mg / L, and in metallurgical wastewater, it can even reach 10000 mg / L. High concentrations of chloride ions can cause corrosion and scaling in pipelines and facilities. If chlorine-containing wastewater is not properly treated, its discharge into the environment can lead to soil salinization and subsequent degradation of soil quality.

[0003] Currently, methods for removing chloride ions from wastewater mainly include chemical precipitation, ion exchange, membrane separation, and oxidation. Chemical precipitation primarily involves adding a precipitant to react with chloride ions, forming a precipitate to remove them. Ion exchange removes chloride ions by utilizing an ion exchanger to exchange ions with those in the water. Membrane separation separates water from other substances through sieving and charge repulsion. Oxidation primarily uses ozone or persulfate to generate active species that oxidize chloride ions, producing chlorine gas, thus removing chloride ions.

[0004] However, chemical precipitation and oxidation methods involve the addition of chemical reagents, and the resulting precipitates require further treatment. Ion exchange and membrane separation methods only transfer and concentrate chloride ions; the used ion exchange resins and membrane concentrates still require further treatment. In contrast, electro-oxidation utilizes the direct and indirect oxidation of the anode to convert chloride ions into chlorine gas, requiring no external chemical reagents. It is simple, easy to operate, and particularly advantageous for industrial wastewater with high salinity due to its good conductivity.

[0005] However, commonly used noble metal oxide anodes suffer from low chlorine evolution efficiency in electro-oxidation chlorine removal applications. Furthermore, electro-oxidation chlorine removal is plagued by oxygen evolution side reactions and chlorine re-dissolution, which reduce current efficiency and chloride ion removal rate. Therefore, it is necessary to prepare anode materials with high chlorine evolution activity and to improve and optimize the electro-oxidation chlorine removal process to address these issues. Summary of the Invention

[0006] To address the aforementioned problems, this invention provides an ultrasonically enhanced electro-oxidation wastewater dechlorination device and method. By preparing a highly active chlorine-evolving anode, the chlorine evolution efficiency is improved. The chlorine gas generated by electro-oxidation is promptly discharged through ultrasonic action, preventing chlorine gas from redissolving and re-converting into chloride ions. Simultaneously, ultrasonic waves can also act on water molecules and oxygen generated by the oxygen evolution side reaction, thereby producing various active species and converting chloride ions into chlorine gas.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows: The first objective of this invention is to provide an ultrasonically enhanced electro-oxidation wastewater dechlorination device, comprising a reaction tank and an ultrasonic generator for applying ultrasonic action to the wastewater in the reaction tank. The ultrasonic working head of the ultrasonic generator is inserted into the reaction tank or fixed to the outer wall of the reaction tank. The reaction tank is provided with a gas outlet. The reaction tank is connected to a gas absorption tank through the gas outlet. The gas absorption tank contains an absorbent liquid for absorbing chlorine gas, which is saturated lime water or sodium hydroxide solution. The reaction tank is connected to a peristaltic pump. Specifically, a flexible hose can be connected to the reaction tank, and the peristaltic pump is installed on the flexible hose. The peristaltic pump agitates the water in the reaction tank, making the various chemical reactions more uniform, thereby improving the water treatment effect. The reaction tank is equipped with a chlorine-evolving anode and a titanium-based cathode; the chlorine-evolving anode and the titanium-based cathode are connected to a power source; the chlorine-evolving anode consists of a pure titanium substrate, which is coated with a noble metal oxide coating and a Bi5O7Cl coating from the inside out; the titanium-based cathode can be a pure titanium cathode.

[0008] As a preferred technical solution, the method for preparing the chlorine-evolving anode includes the following steps: (1) Provide a pure titanium substrate with a noble metal oxide coating on the surface; more preferably, the noble metal oxide coating is an oxide coating containing ruthenium and iridium (such as RuO2, IrO2, etc.).

[0009] (2) Add bismuth salt to a solvent containing ethylene glycol, oleylamine, and ethanol to obtain a bismuth salt solution; preferably, the bismuth salt is bismuth nitrate, bismuth sulfate, or bismuth chloride; ethylene glycol in the bismuth salt solution has good solubility and stability, which can promote uniform dispersion of bismuth salt; oleylamine can prevent particle agglomeration and regulate the morphology of the material; at the same time, ethylene glycol and oleylamine can provide an alkaline environment for the preparation of Bi5O7Cl; ethanol, as a co-solvent, can improve the solubility of polar reagents and accelerate mass transfer; the specific operation method is as follows: first, mix ethylene glycol and oleylamine in a volume ratio of 1:1, then add bismuth salt to dissolve, then add ethanol and stir evenly to obtain a bismuth salt solution; add potassium chloride solution dropwise to the bismuth salt solution, and heat at a temperature of 60~80 °C to obtain the precipitate Bi5O7Cl; the reaction equation for the preparation of Bi5O7Cl is: .

[0010] Bi5O7Cl was dispersed in a mixed solution of water, isopropanol, and Nafion. The resulting mixture was coated onto the surface of a pure titanium substrate with a noble metal oxide coating. After drying under an infrared lamp, a chlorine-evolving anode was obtained. In the above method, Nafion, as a solvent, can promote the dispersion of Bi5O7Cl and improve the electrochemical performance of the coating. Isopropanol can promote the dissolution of Nafion, thereby obtaining a more uniformly dispersed coating solution. The present invention uses infrared lamp drying because infrared irradiation can accelerate solvent evaporation, and infrared rays have strong penetrability, which can reduce the internal temperature gradient of the material, resulting in better uniformity and stability of the Bi5O7Cl coating on the surface of the chlorine-evolving anode.

[0011] The second objective of this invention is to provide a method for dechlorinating wastewater by ultrasonic degassing enhancement, which utilizes the electro-oxidation wastewater dechlorination device described in the first objective; and includes the following steps: The wastewater to be treated is injected into the reaction tank; Power is supplied to the chlorine-evolving anode and titanium-based cathode. The chlorine-evolving anode converts chloride ions in the wastewater into chlorine gas. Simultaneously, an ultrasonic generator is used to ultrasonically treat the wastewater, thereby improving the chloride ion removal rate. Preferably, the current density of the power supply is 10-200 mA / cm². 2 The input power of the ultrasonic generator is 50-150W, and the ultrasonic frequency is 10-100kHz. The generated chlorine gas is collected and treated; specifically, the chlorine gas is discharged through the outlet into a gas absorption tank containing a gas absorption liquid, and then treated with saturated lime water or sodium hydroxide solution.

[0012] The core of the ultrasonic degassing enhanced electro-oxidation wastewater dechlorination device and method provided by the present invention is: (1) the chlorine-evolving anode has excellent chlorine evolution efficiency under the combined action of Bi5O7Cl coating and noble metal oxide coating, which can improve the electro-oxidation dechlorination efficiency; (2) the chlorine-evolving anode converts chloride ions into chlorine gas through direct oxidation. At the same time, the anode electrolyzes water to generate hydroxyl radicals, which can also oxidize chloride ions to generate chlorine gas. (3) When ultrasound is applied to wastewater, cavitation bubbles are formed. The generated chlorine gas will enter the cavitation bubbles. Small bubbles quickly turn into large bubbles and leave the water surface. That is, ultrasound accelerates the discharge of chlorine gas and prevents it from redissolving and turning into chloride ions again. (4) Ultrasound acts on water and oxygen generated by the oxygen evolution side reaction, producing a variety of active substances and oxidizing chloride ions to produce chlorine gas, which improves the current efficiency of chloride ion to chlorine gas conversion. (5) At the same time, ultrasound can increase the shear force of water, shorten the residence time of bubbles generated on the electrode surface, reduce the detachment diameter of bubbles, and thus improve the utilization rate of active sites on the electrode surface and improve the treatment efficiency. (6) In addition, the high-frequency vibration of ultrasound can inhibit the adhesion of pollutants on the electrode surface and peel off the deposited pollutants. That is, ultrasound can reduce the risk of electrode contamination, improve treatment efficiency, and extend the service life of the electrode.

[0013] The relevant chemical reactions involved in the electro-oxidation process of this invention are as follows: Direct anodic oxidation for chloride ion removal:

[0014]

[0015] Hydroxyl radicals are generated at the anode to remove chloride ions:

[0016]

[0017]

[0018] Chlorine gas is redissolved and converted back into chloride ions:

[0019]

[0020] Ultrasonic waves act on water and react with oxygen produced by the oxygen evolution reaction at the anolyte to generate active species, thus removing chloride ions.

[0021]

[0022]

[0023]

[0024]

[0025]

[0026]

[0027] In summary, compared with the prior art, the advantages of the present invention are as follows: 1. Cl in the Bi5O7Cl coating of the chlorine-evolving anode - Under the excitation of the anodic electric field, it is oxidized to And further produce chlorine gas, while lacking Cl... - Bi5O7Cl and Cl in solution - It has strong binding force and can regenerate Bi5O7Cl, therefore Cl - Compensation can occur quickly, which guarantees... It can be produced continuously without destroying the structure of Bi5O7Cl; at the same time, Cl in the solution... - With lack of Cl - The bonding ability between Bi5O7Cl is much higher than that between Cl. - Adsorption on the surface of a common anode can therefore promote The generation of oxygen improves the chlorine evolution efficiency; moreover, the Bi5O7Cl coating can block the oxygen evolution sites on the surface of the noble metal oxide coating, suppress the oxygen evolution side reaction, and further improve the current efficiency of chlorine evolution at the anode.

[0028] 2. Since chlorine is readily soluble in water and can react with water to ultimately convert into chloride ions, the chlorine produced during the electro-oxidation dechlorination process will redissolve, reducing the chloride ion removal rate and current efficiency. This invention utilizes the degassing effect of ultrasound to accelerate the discharge of chlorine produced by electro-oxidation from the wastewater, thereby effectively alleviating the problem of chlorine redissolving and improving current efficiency and chloride ion removal rate.

[0029] 3. During the electro-oxidation process, while chloride ions lose electrons and are oxidized at the anode, hydroxide ions also lose electrons at the anode to generate oxygen. The competitive effect of the oxygen evolution side reaction reduces the current efficiency of electro-oxidation for chloride removal. The ultrasonic waves introduced in this invention can act on the oxygen generated by the oxygen evolution side reaction and convert it into active substances, which then oxidize chloride ions to generate chlorine gas, thereby realizing the "turning waste into treasure" of oxygen and improving current efficiency and chloride ion removal rate.

[0030] 4. The ultrasound introduced in this invention can promote the gas to leave the electrode surface and reduce the detachment diameter of the bubbles, thereby improving the utilization rate of the chlorine-evolving active sites on the electrode surface and increasing the processing efficiency.

[0031] 5. The ultrasound introduced in this invention can inhibit the adhesion of contaminants to the electrode surface and remove deposited contaminants, thereby reducing the risk of electrode contamination, improving processing efficiency, and extending the service life of the electrode.

[0032] 6. Compared with conventional electro-oxidation dechlorination technology, this invention can significantly improve the chloride ion removal rate, increase current efficiency, reduce the risk of electrode contamination, and extend the service life of the electrode. Attached Figure Description

[0033] Figure 1 This is a schematic diagram of the ultrasonic degassing enhanced electro-oxidation wastewater dechlorination device of the present invention; Figure 2 This is another structural schematic diagram of the ultrasonic degassing enhanced electro-oxidation wastewater dechlorination device of the present invention; Figure 3 The diagram shows the bubble detachment diameter and chlorine gas discharge effect under (a) no ultrasound and (b) ultrasound conditions. Figure 4 This is a schematic diagram comparing the chlorine evolution process of a conventional anode (a) and the chlorine evolution anode (b) in this invention.

[0034] Figure labels: 1-Reaction tank; 2-Peristaltic pump; 3-Power supply; 4-Ultrasonic generator; 5-Gas absorption tank; 6-Chlorine anode; 7-Pure titanium cathode; 8-Ultrasonic working head; 9-Gas outlet. Detailed Implementation

[0035] The present invention will be further described below with reference to embodiments, so that those skilled in the art can better understand and implement the present invention, but the embodiments are not intended to limit the present invention.

[0036] In addition, unless otherwise specified, the preparation processes in the following embodiments are all conventional methods in the prior art, and therefore will not be described in detail; the raw materials used in this invention are all commercially available products.

[0037] Example 1 A schematic diagram of an ultrasonically degassing-enhanced electro-oxidation wastewater dechlorination device is shown below. Figure 1 and Figure 2 The system includes a reaction tank 1, a peristaltic pump 2, a power supply 3, an ultrasonic generator 4, and a gas absorption tank 5. The reaction tank 1 is equipped with a chlorine-evolving anode 6 and a pure titanium cathode 7 connected to the power supply. The ultrasonic generator 4 is connected to an ultrasonic working head 8, which is inserted into the reaction tank or fixed to the outer wall of the reaction tank. The reaction tank 1 is provided with a gas outlet 9, which is connected to the gas absorption tank 5. Figure 1 The ultrasonic working head is inserted into the reaction tank. Figure 2 The ultrasonic working head is fixed to the outer wall of the reaction tank.

[0038] Example 2 A method for dechlorinating wastewater by ultrasonic degassing-enhanced electro-oxidation includes the following steps: Inject the wastewater to be treated into the reaction tank; start the peristaltic pump; Power is supplied to the chlorine-electrode anode and the titanium-based cathode. The chlorine-electrode anode converts chloride ions in the wastewater into chlorine gas. At the same time, an ultrasonic generator is activated to apply ultrasonic action to the wastewater to improve the chloride ion removal rate. The generated chlorine gas is collected and treated.

[0039] The specific operating parameters and processing results are as follows: Simulated chlorine concentration in wastewater: 500 mg / L; Ultrasonic input power: 50W; Ultrasonic frequency: 20kHz; Current density: 50 mA / cm 2 ; Processing time: 10 minutes; Treatment effect: After treatment, the chloride ion concentration was 48 mg / L, and the removal rate was 90.4%.

[0040] The method for preparing the chlorine-evolving anode used in Example 2 is as follows: (1) A pure titanium substrate with a noble metal oxide coating is provided; in a specific embodiment, a commercially available ruthenium-iridium titanium electrode is selected. This electrode consists of an industrially pure titanium substrate and a noble metal oxide coating. The coating material contains ruthenium and iridium oxides, which have high catalytic activity, can reduce the overpotential of chlorine and oxygen evolution, and improve current efficiency. In wastewater treatment, chloride ions are converted into chlorine gas through direct oxidation. For those skilled in the art, a similar purpose can be achieved by coating the surface of the pure titanium electrode with other noble metal oxide coatings.

[0041] (2) Preparation of Bi5O7Cl coating, including the following steps: A 7 mmol / L bismuth nitrate solution A was prepared using ethylene glycol and oleylamine as solvents (volume ratio 1:1), stirred continuously for 30 min, and heated to 60 °C. Take 150 mL of bismuth nitrate solution A, add 50 mL of anhydrous ethanol, and stir continuously for 20 min to obtain bismuth nitrate solution B. A 20 mmol / L potassium chloride solution was prepared using deionized water as the solvent. 50 mL of potassium chloride solution was added dropwise to bismuth nitrate solution B, and the mixture was stirred continuously and heated at 60 °C for 24 h. The precipitate was collected by centrifugation at 6000 rpm and washed repeatedly with chloroform, anhydrous ethanol, and deionized water. Finally, the precipitate was dried under vacuum at 60 °C for 24 h to obtain material Bi5O7Cl. 10 mg of Bi5O7Cl was dispersed in a mixed solution of 1400 µL of deionized water, 450 µL of isopropanol and 100 µL of Nafion (5 wt%). The solution was ultrasonically treated to obtain a spraying solution, which was then sprayed onto the surface of a pure titanium substrate with a noble metal oxide coating and dried under an infrared lamp to obtain a chlorine-evolving anode.

[0042] Example 3: In this embodiment, a chlorine-evolving anode prepared using the same method as in Example 2 is used for the ultrasonic degassing-enhanced electro-oxidation process for dechlorination of wastewater. The specific operating parameters and treatment effects are as follows: Simulated chlorine concentration in wastewater: 2000 mg / L; Ultrasonic input power: 60W; Ultrasonic frequency: 40kHz; Current density: 100 mA / cm 2 ; Processing time: 20 minutes; Treatment effect: After treatment, the chloride ion concentration was 92 mg / L, and the removal rate was 95.4%.

[0043] Comparative Example 1: Similar to Example 3, the difference is that no electric field is applied. Specific operating parameters and processing effects are as follows: Simulated chlorine concentration in wastewater: 2000 mg / L; Ultrasonic input power: 60W; Ultrasonic frequency: 40kHz; Current density: 0 mA / cm 2 ; Processing time: 20 minutes; Treatment effect: After treatment, the chloride ion concentration was 1880 mg / L, and the removal rate was 6%.

[0044] Comparative Example 2: Similar to Example 3, the difference is that ultrasound is not applied. Specific operating parameters and treatment effects are as follows: Simulated chlorine concentration in wastewater: 2000 mg / L; Ultrasonic input power: 0W; Ultrasonic frequency: 0kHz; Current density: 100 mA / cm 2 ; Processing time: 20 minutes; Treatment effect: After treatment, the chloride ion concentration was 1450 mg / L, and the removal rate was 27.5%.

[0045] The results of Comparative Example 2 show that, without the application of ultrasound, even with all other process parameters identical to those in Example 3, the chloride ion removal rate was extremely low, far lower than that in Example 3. To visually demonstrate the effect of ultrasound in chloride ion removal, combined with... Figure 3 To explain, Figure 3 The diagram illustrates the bubble detachment diameter and chlorine gas discharge effect under (a) no ultrasound and (b) ultrasound conditions. Figure 3 It can be seen that ultrasound can promote the gas to leave the electrode surface and reduce the detachment diameter of the bubbles, thereby improving the utilization rate of the chlorine-evolving active sites on the electrode surface and increasing the treatment efficiency. In addition, ultrasound applied to wastewater forms cavitation bubbles, and the generated chlorine gas enters the cavitation bubbles. The small bubbles quickly turn into large bubbles and leave the water surface. In other words, ultrasound accelerates the discharge of chlorine gas and prevents it from redissolving and being converted back into chloride ions.

[0046] Comparative Example 3: Similar to Example 3, except that the anode uses a commercially available ruthenium-iridium-titanium electrode.

[0047] The specific operating parameters and processing results are as follows: Simulated chlorine concentration in wastewater: 2000 mg / L; Ultrasonic input power: 60W; Ultrasonic frequency: 40kHz; Current density: 100 mA / cm 2 ; Processing time: 20 minutes; Treatment effect: After treatment, the chloride ion concentration was 312 mg / L, and the removal rate was 84.4%.

[0048] The results of Comparative Example 3 show that if the chlorine anode does not contain a Bi5O7Cl coating, the chloride ion removal rate will be lower than that of Example 3. Figure 4 This is a schematic diagram comparing the chlorine evolution process of a conventional anode (a) and the chlorine evolution anode (b) of this invention. Figure 4 It can be seen that the chlorine-evolving anode containing a Bi5O7Cl coating used in this invention is more readily reacted with Cl in the solution. - The combination can promote the production of Cl⁻ and improve the efficiency of chlorine precipitation.

[0049] Example 4: In this embodiment, a chlorine-evolving anode prepared using the same method as in Example 2 is used for the ultrasonic degassing-enhanced electro-oxidation process for dechlorination of wastewater. The specific operating parameters and treatment effects are as follows: Simulated chlorine concentration in wastewater: 8000 mg / L; Ultrasonic input power: 100W; Ultrasonic frequency: 60kHz; Current density: 150 mA / cm 2 ; Processing time: 30 minutes; Treatment effect: After treatment, the chloride ion concentration was 132 mg / L, and the removal rate was 98.4%. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

Claims

1. A device for dechlorinating wastewater by ultrasonic degassing enhancement, characterized in that: The device includes a reaction tank and an ultrasonic generator for applying ultrasonic action to the wastewater in the reaction tank; the reaction tank is provided with an air outlet; the reaction tank is equipped with a chlorine-evolving anode and a titanium-based cathode; the chlorine-evolving anode and the titanium-based cathode are connected to a power source; the chlorine-evolving anode includes a pure titanium substrate, which is coated with a noble metal oxide coating and a Bi5O7Cl coating from the inside to the outside.

2. The ultrasonic degassing-enhanced electro-oxidation wastewater dechlorination device according to claim 1, characterized in that: The method for preparing the chlorine-evolving anode includes the following steps: (1) Provide a pure titanium substrate with a noble metal oxide coating on its surface; (2) Add bismuth salt to a solvent containing ethylene glycol, oleylamine and ethanol to obtain a bismuth salt solution; add potassium chloride solution dropwise to the bismuth salt solution and heat it at a temperature of 60~80 °C to obtain precipitate Bi5O7Cl; disperse Bi5O7Cl in a mixed solution of water, isopropanol and Nafion, coat the resulting mixture on the surface of a pure titanium substrate with a noble metal oxide coating, and dry it under an infrared lamp to obtain a chloride anode.

3. The ultrasonic degassing-enhanced electro-oxidation wastewater dechlorination device according to claim 2, characterized in that: In step (1), the noble metal oxide coating is an oxide coating containing ruthenium and iridium.

4. The ultrasonic degassing-enhanced electro-oxidation wastewater dechlorination device according to claim 2, characterized in that: In step (2), the bismuth salt is bismuth nitrate, bismuth sulfate or bismuth chloride.

5. The ultrasonic degassing enhanced electro-oxidation wastewater dechlorination device according to any one of claims 1 to 4, characterized in that: The reaction tank is connected to a gas absorption tank; the gas absorption tank is connected to the gas outlet of the reaction tank.

6. The ultrasonic degassing-enhanced electro-oxidation wastewater dechlorination device according to claim 5, characterized in that: The gas absorption tank contains an absorbent liquid for absorbing chlorine gas; the absorbent liquid is saturated lime water or sodium hydroxide solution.

7. The ultrasonic degassing-enhanced electro-oxidation wastewater dechlorination device according to any one of claims 1 to 4, characterized in that: The ultrasonic generator is connected to an ultrasonic working head, which is inserted into the reaction tank or fixed to the outer wall of the reaction tank.

8. The ultrasonic degassing-enhanced electro-oxidation wastewater dechlorination device according to any one of claims 1 to 4, characterized in that: The reaction tank is connected to a peristaltic pump.

9. A method for dechlorinating wastewater by ultrasonic degassing enhancement, characterized in that: It is carried out using the electro-oxidation wastewater dechlorination device as described in any one of claims 1 to 8; Includes the following steps: The wastewater to be treated is injected into the reaction tank; Power is supplied to the chlorine-electrode anode and the titanium-based cathode. The chlorine-electrode anode converts chloride ions in the wastewater into chlorine gas. At the same time, the ultrasonic generator is used to improve the chloride ion removal rate by ultrasonically treating the wastewater. The generated chlorine gas is collected and treated.

10. The method for dechlorinating wastewater by electro-oxidation according to claim 9, characterized in that: The current density of the power supply is 10-200 mA / cm². 2 The ultrasonic generator has an input power of 50-150W and an ultrasonic frequency of 10-100kHz.

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