Synergistic removal system and method for nitrite nitrogen and nitrate nitrogen in carbon steel passivation wastewater

Abstract

The invention discloses a system and a method for synergistically removing nitrite nitrogen and nitrate nitrogen in carbon steel passivation wastewater. The system comprises a catalytic filter tower and an electrochemical reduction reactor, a catalytic filter tower water inlet and a catalytic filter tower water outlet are formed in a catalytic filter tower body, a coarse filter screen, a fine filter screen, a multi-element alloy catalytic layer and a fine filter screen are sequentially arranged in the flowing direction of wastewater, and the multi-element alloy catalytic layer is filled with multi-element alloy catalyst filler; the electrochemical reduction reactor body is provided with an electrochemical reduction reactor water inlet and an electrochemical reduction reactor water outlet, and the electrochemical reduction reactor water inlet is communicated with the catalytic filter tower water outlet; four vertical porous copper-based composite electrodes as cathodes and four vertical stainless steel plate / titanium-based coating electrodes as anodes are arranged in the electrochemical reduction reactor body. The problem that two nitrogen pollutants with different characteristics are difficult to efficiently remove at the same time in the same system is solved through the process combination of catalytic reduction nitrate nitrogen removal and electrochemical reduction nitrate nitrogen removal.

Description

Technical Field

[0001] This invention relates to the field of industrial wastewater treatment technology, and in particular to a system and method for the synergistic removal of nitrite and nitrate nitrogen from carbon steel passivation wastewater. Background Technology

[0002] Passivation wastewater from carbon steel pipelines is a typical difficult-to-treat wastewater generated during the construction of marine engineering projects such as Floating Production Storage and Offloading (FPSO) systems. Its notable characteristic is the presence of high concentrations of nitrite nitrogen (NO2). -N) and nitrate nitrogen (NO3) These two nitrogen pollutants have very different properties. Nitrite nitrogen is unstable under acidic conditions and is easily decomposed or reacted, while nitrate nitrogen can be converted into nitrogen gas or the intermediate product nitrite nitrogen under reducing conditions.

[0003] Existing technologies struggle to achieve synergistic and efficient removal of both nitrite and nitrate. Biochemical methods, with their high concentrations, strongly inhibit denitrifying microorganisms; chemical reduction methods, while effective for nitrate, face competition for reducing agents due to the presence of high nitrite concentrations, leading to increased treatment costs and complex reaction pathways. Electrochemical methods, while promising, suffer from low selectivity and efficiency in nitrate reduction using conventional electrodes, and the coexistence of nitrite interferes with the reduction process. Therefore, developing a synergistic technology capable of selectively removing nitrite followed by deep reduction of nitrate is a pressing technical challenge in this field. Summary of the Invention

[0004] To overcome the shortcomings of the prior art, the main objective of this invention is to provide a system and method for the synergistic removal of nitrite and nitrate nitrogen in carbon steel passivation wastewater, which solves the problem of the difficulty in achieving the synergistic and efficient removal of nitrite and nitrate nitrogen in carbon steel passivation wastewater.

[0005] To achieve the aforementioned objectives, the technical solution adopted by this invention includes: A system for the synergistic removal of nitrite and nitrate nitrogen from carbon steel passivation wastewater is disclosed. The system comprises a catalytic filter tower and an electrochemical reduction reactor. The catalytic filter tower body is provided with a catalytic filter tower inlet 1 and a catalytic filter tower outlet 6. A coarse filter screen 2, a first fine filter screen 3, a multi-element alloy catalyst layer 4, and a second fine filter screen 5 are arranged sequentially along the wastewater flow direction. The multi-element alloy catalyst layer 4 is filled with multi-element alloy catalyst packing material. The electrochemical reduction reactor body is provided with an electrochemical reduction reactor inlet 7 and an electrochemical reduction reactor outlet 10. The electrochemical reduction reactor inlet 7 is connected to the catalytic filter tower outlet 6. The electrochemical reduction reactor body is provided with four vertical porous copper-based composite electrodes 9 as cathodes and four vertical stainless steel / titanium-based coated electrodes 8 as anodes.

[0006] The multi-element alloy catalyst filler is a multi-element alloy of four metallic elements: copper, cobalt, nickel, and iron.

[0007] The molar ratio of the four metallic elements copper, cobalt, nickel and iron is 1:3:2:1 to 2:2:2:1.

[0008] The coarse filter 2 is a 100-mesh stainless steel mesh, and the first fine filter 3 and the second fine filter 5 are 500-mesh stainless steel meshes.

[0009] The porous copper-based composite electrode 9 is prepared by in-situ electrochemical etching-electroplating technology, including the following steps: using a pretreated copper substrate as the cathode and a graphite rod as the anode, placing it in an electrolyte containing copper salt and auxiliary electrolyte, and constructing a porous copper-based composite electrode on the cathode surface by controlling the current density.

[0010] The electrolyte is a mixed solution of copper sulfate and sulfuric acid, with a copper ion concentration of 0.3 mol / L and a sulfuric acid concentration of 0.3 mol / L. The electrochemical etching-electroplating conditions are a current density of 30 mA / cm². 2 Temperature 35℃, time 30 minutes.

[0011] A method for the synergistic removal of nitrite and nitrate nitrogen from carbon steel passivation wastewater includes the following steps: Step 1) The carbon steel passivation wastewater is passed through a catalytic filter tower filled with multi-element alloy catalyst packing. In the presence of a reducing agent, the nitrite nitrogen in the wastewater is catalytically reduced to nitrogen gas. Step 2) The effluent treated in Step 1) is introduced into an electrochemical reduction reactor. The electrochemical reduction reactor uses a porous copper-based composite electrode as the cathode and reduces nitrate nitrogen in the wastewater to nitrogen gas through electrochemical reduction.

[0012] In step 1), the multi-element alloy catalyst filler is selected from a multi-element alloy of four metal elements: copper, cobalt, nickel, and iron, with a molar ratio of 1:3:2:1 to 2:2:2:1.

[0013] In step 1), the reducing agent is sodium formate, and the amount of sodium formate added is 2:1 molar ratio with nitrite nitrogen.

[0014] In step 1), the pH of the wastewater is adjusted to 3.0-5.0, and the hydraulic retention time is 30 minutes. In step 2, the cathode current density is 50 mA / cm². 2 .

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. By combining the processes of "catalytic reduction for nitrite removal + electrochemical reduction for nitrate removal", the problem of simultaneously and efficiently removing two nitrogen pollutants with very different characteristics in the same system is solved. The process path is clear and does not interfere with each other. 2. Thorough removal of nitrite at the front end allows the reducing substances generated by the electrochemical cathode at the back end to be used almost entirely to attack nitrite molecules, greatly improving the current efficiency and reaction rate of nitrite reduction. 3. The multi-element alloy catalyst used is low in cost and has good stability; the porous copper-based composite cathode has a large specific surface area and high catalytic activity. The two work together to ensure the efficient and stable operation of the system. 4. The main pollutants are all converted into harmless N2, avoiding secondary pollution, and the system does not require the addition of large amounts of chemical agents. Attached Figure Description

[0016] Figure 1 : Process flow diagram of the system of this invention; Figure 2 A is a schematic diagram of a catalytic filter tower device; Figure 2 B is a schematic diagram of an electrochemical reactor device; Figure 2 The markings are as follows: 1-Catalytic filter tower inlet, 2-Coarse filter screen, 3-First fine filter screen, 4-Multi-element alloy catalyst packing, 5-Second fine filter screen, 6-Catalytic filter tower outlet, 7-Electrochemical reactor inlet, 8-Stainless steel plate / titanium-based coated anode, 9-Porous copper-based composite cathode, 10-Electrochemical reactor outlet; Figure 3 The degradation curve of nitrite by a multi-element alloy catalyst; Figure 4 A represents the curves of catalytic filtration and synergistic electrochemical degradation of nitrite and nitrate nitrogen at pH 3.0. Figure 4 B represents the synergistic electrochemical degradation curves of nitrite and nitrate nitrogen under pH 5.0 catalytic filtration; Figure 5 : Degradation curves of nitrate nitrogen by different electrochemical cathode electrodes. Detailed Implementation

[0017] The embodiments described below are exemplary descriptions of key experimental evidence and are not intended to limit the core content and application scope of this invention due to the amount of evidence. It should be noted that all the accompanying drawings and corresponding descriptions merely illustrate the concept, principles, and representative experimental evidence of the disclosed embodiments of this invention. Where the chain of evidence is complete, it is unnecessary to show all the specific details and extended details of the various embodiments listed in this invention.

[0018] Unless otherwise defined, the technical terms used in the following embodiments have the same meaning as commonly understood by those skilled in the art to which this invention pertains.

[0019] The nitrite nitrogen concentration detection method of this invention adopts GB 7493-87 "Determination of Nitrite Nitrogen in Water - Spectrophotometric Method", and the nitrate nitrogen concentration detection method adopts HJ / T 346-2007 "Determination of Nitrate Nitrogen in Water - Ultraviolet Spectrophotometric Method (Trial)". The commercial Ti / RuO2-IrO2 mesh electrode and the unetched electrode are both commercially available.

[0020] The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0021] A synergistic removal system for nitrite and nitrate nitrogen in carbon steel passivation wastewater, the process flow of which is as follows: Figure 1 As shown, the schematic diagram of the system device of the present invention is as follows. Figure 2 As shown, it includes: 1) Catalytic Filter Tower: The catalytic filter tower body is equipped with a catalytic filter tower inlet 1 and a catalytic filter tower outlet 6. The catalytic filter tower inlet is used to receive carbon steel passivation wastewater. Along the wastewater flow direction, a coarse filter screen 2, a first fine filter screen 3, a multi-element alloy catalyst layer 4, and a second fine filter screen 5 are sequentially arranged. The multi-element alloy catalyst layer 4 is filled with multi-element alloy catalyst packing. The multi-element alloy catalyst packing is selected from a multi-element alloy of four metal elements: copper, cobalt, nickel, and iron (molar ratio 1:3:2:1). The coarse filter screen 2 and the fine filter screens 3 and 5 are 100-mesh and 500-mesh stainless steel meshes, respectively. The function of the filter screens is to filter impurities and pollutants of different particle sizes.

[0022] 2) An electrochemical reduction reactor, comprising an inlet 7 and an outlet 10. The inlet 7 is connected to the outlet 6 of the catalytic filter tower via a pipe. The reactor body contains four vertical porous copper-based composite electrodes 9 as cathodes for reducing nitrate nitrogen and four vertical stainless steel / titanium-coated electrodes 8 as anodes for oxidizing the water (electrode spacing 1 cm). The effective volume of the electrochemical reduction reactor is 1 L.

[0023] Example 1 A porous copper-based composite electrode for efficient electrochemical reduction of nitrate nitrogen was prepared by in-situ electrochemical etching-electroplating technology. The electrode should have a micron / nano hierarchical pore or nanowire structure to provide high specific surface area and excellent electrocatalytic activity.

[0024] Copper substrate: foamed copper, dimensions 2 cm × 5 cm × 0.1 cm.

[0025] Electrolyte: A mixed aqueous solution consisting of copper sulfate (CuSO4·5H2O) and sulfuric acid (H2SO4) (copper ion concentration of 0.3 mol / L; sulfuric acid concentration of 0.3 mol / L).

[0026] Power supply: Electrochemical workstation (electrochemical etching-electroplating conditions: current density 30 mA / cm²) 2 (Temperature 35℃, time 30 minutes).

[0027] Auxiliary electrode: A graphite rod serves as the counter electrode (anode).

[0028] Operating steps: Step 1: Use 400-grit, 800-grit, and 1200-grit sandpaper in sequence to polish the surface of the copper substrate to remove the oxide layer and contaminants.

[0029] Step 2: After polishing, ultrasonically clean the copper substrate in acetone, anhydrous ethanol and deionized water for 10 minutes each to thoroughly remove surface grease and impurities, and then dry it with nitrogen.

[0030] Step 3: Prepare a mixed aqueous solution of 0.3 mol / L copper sulfate and 0.3 mol / L sulfuric acid as the working electrolyte for electrochemical etching-electroplating.

[0031] Step 4: Immerse the pretreated copper substrate as the cathode and the graphite rod as the anode in the electrolyte solution described above. Connect to an electrochemical workstation and conduct the reaction in constant current mode. Control parameters: Current density: 30 mA / cm² 2 Processing time: 30 minutes.

[0032] Step 5: After the reaction is complete, remove the electrode and rinse it with plenty of deionized water to remove the electrolyte adhering to the surface. Dry it in a vacuum drying oven at 60 °C for 2 hours to obtain a porous copper-based composite electrode.

[0033] Example 2 Waste liquid from carbon steel pipeline, 1L. Water quality: pH = 4.0, NO2 -N = 207 mg / L, NO3 -N = 194 mg / L, Chemical Oxygen Demand (COD) = 11767 mg / L, Cl = 1500 mg / L.

[0034] Operating steps: Step 1: Adjust the pH of the water sample precisely to 3.0 using sulfuric acid. Pump it into the catalytic filter tower at a flow rate of 2 BV / h. (Catalytic filter tower...) Figure 2 A): The tower is packed with 100 mL of Cu-Co-Ni-Fe multi-element alloy catalyst packing (Cu:Co:Ni:Fe = 1:3:2:1), and sodium formate is added as a reducing agent (molar ratio to nitrite nitrogen is 2:1). The hydraulic retention time is 30 minutes, and samples are taken at the tower outlet every 30 minutes.

[0035] Step 2: Pump the effluent from Step 1 into the electrochemical reduction reactor. Electrochemical reactor ( Figure 2 B): Effective volume 1L, cathode is a porous copper-based composite electrode prepared according to Example 1 (effective area 20 cm²). 2 The anode is a stainless steel plate / titanium-based coated electrode. The cathode current density is controlled at 50 mA / cm². 2 The electrode spacing is 1 cm, and magnetic stirring is used.

[0036] Analysis and monitoring: Sampling was conducted at regular intervals, and NO2 was determined using national standard methods. -N, NO3 -N.

[0037] like Figure 3 As shown, after catalytic reduction filtration for 0.5 h, nitrite nitrogen decreased from 207 mg / L to 11 mg / L. After entering the electrochemical reduction reactor for 1.5 h, nitrate nitrogen decreased from 194 mg / L to 3 mg / L, meeting the discharge standard "Integrated Wastewater Discharge Standard (DB12 / 356-2018)" (total nitrogen < 70 mg / L). This indicates that the catalytic reduction and electrochemical oxidation synergistic system of the present invention has excellent catalytic reduction and oxidation performance.

[0038] Example 3 The procedure was essentially the same as in Example 2, except that the ratio of the multi-element alloy filler to the pH was changed: Cu:Co:Ni:Fe = 1:3:2:1 and Cu:Co:Ni:Fe = 2:2:2:1. The pH was also changed: pH = 3.0 and pH = 5.0. The conditions in Table 1 were followed.

[0039] Table 1

[0040] Operating procedures: Follow only step 1 of Example 2 to investigate the removal of nitrite nitrogen under different conditions.

[0041] Analysis and monitoring: Sampling was conducted at regular intervals, and NO2 was determined using national standard methods. -N.

[0042] like Figure 4 As shown in Figure A, at pH 3.0, after catalytic reduction and filtration of Cu:Co:Ni:Fe = 1:3:2:1 and Cu:Co:Ni:Fe = 2:2:2:1 for 0.5 h, the nitrite nitrogen levels decreased to 11 mg / L and 56 mg / L, respectively, indicating that an increased Co content is beneficial to the catalytic reduction of nitrite nitrogen. Figure 4As shown in Figure B, at pH 5.0, after catalytic reduction filtration of Cu:Co:Ni:Fe = 1:3:2:1 and Cu:Co:Ni:Fe = 2:2:2:1 for 0.5 h, the nitrite nitrogen levels decreased to 46 mg / L and 87 mg / L, respectively. This indicates that pH has a significant impact on the catalytic reduction of nitrite nitrogen. Furthermore, at pH 3.0, the nitrite nitrogen after catalytic reduction of Cu:Co:Ni:Fe = 1:3:2:1 meets the discharge standard "Integrated Wastewater Discharge Standard (DB12 / 356-2018)" (total nitrogen < 70 mg / L).

[0043] Example 4 The effluent from the tower treated in step 1 of Example 2 (i.e., low nitrite and high nitrate nitrogen water sample) was processed under the same reactor and operating conditions (current density 50 mA / cm²). 2 (Processing time 3 hours) Comparison of different cathode electrodes: the porous copper-based composite electrode of this invention, the commercial Ti / RuO2-IrO2 mesh electrode, and the unetched electrode. Evaluation index: Nitrogen removal rate.

[0044] Analysis and monitoring: Sampling was conducted at regular intervals, and NO3 was determined using national standard methods. -N.

[0045] Results analysis: like Figure 5 As shown, the removal effects of the porous copper-based composite electrode of the present invention, the commercial Ti / RuO2-IrO2 mesh electrode, and the unetched electrode on nitrate nitrogen were compared. After 1.5 h, the porous copper-based composite electrode, the commercial Ti / RuO2-IrO2 mesh electrode, and the foamed copper electrode removed nitrate nitrogen respectively, and the residual nitrate nitrogen concentrations were 2.91 mg / L, 66.93 mg / L, and 101.07 mg / L, respectively. This indicates that the porous copper-based composite electrode of the present invention has excellent nitrate nitrogen removal performance and meets the discharge standard "Integrated Wastewater Discharge Standard (DB12 / 356-2018)" (total nitrogen < 70 mg / L).

[0046] The above embodiments only describe a portion of the specific implementation methods of the present invention in detail, and are not limited to the embodiments disclosed herein. Furthermore, the substantive content protected by the present invention is not limited thereto. Any other modifications, equivalent substitutions, improvements, etc., made based on the principles and techniques of the present invention without departing from its design scope are all within the protection scope of the present invention.

Claims

1. A system for simultaneous removal of nitrite and nitrate in carbon steel passivation wastewater, characterized in that, The system comprises a catalytic filter tower and an electrochemical reduction reactor, the catalytic filter tower body is provided with a catalytic filter tower water inlet (1) and a catalytic filter tower water outlet (6), a coarse filter screen (2), a first fine filter screen (3), a multi-element alloy catalytic layer (4), and a second fine filter screen (5) are sequentially arranged in the direction of wastewater flow, the multi-element alloy catalytic layer (4) is filled with multi-element alloy catalyst fillers; the electrochemical reduction reactor body is provided with an electrochemical reduction reactor water inlet (7) and an electrochemical reduction reactor water outlet (10), the electrochemical reduction reactor water inlet (7) is communicated with the catalytic filter tower water outlet (6), and four vertical porous copper-based composite electrodes (9) are arranged in the electrochemical reduction reactor body as cathodes, and four vertical stainless steel plate / titanium-based coating electrodes (8) are arranged as anodes.

2. The system for simultaneous removal of nitrite and nitrate from carbon steel passivation wastewater according to claim 1, characterized in that, The multi-element alloy catalyst fillers are multi-element alloys of copper, cobalt, nickel and iron.

3. The system for simultaneous removal of nitrite and nitrate from carbon steel passivation wastewater according to claim 2, characterized in that, The molar ratio of the four metal elements of copper, cobalt, nickel and iron is 1:3:2:1~2:2:2:

1.

4. The system for simultaneous removal of nitrite and nitrate from carbon steel passivation wastewater according to claim 1, characterized in that, The coarse filter screen (2) is a 100-mesh stainless steel screen, and the first fine filter screen (3) and the second fine filter screen (5) are 500-mesh stainless steel screens.

5. The system for simultaneous removal of nitrite and nitrate from carbon steel passivation wastewater according to claim 1, characterized in that, The porous copper-based composite electrode (9) is prepared by in-situ electrochemical etching-plating technology, which comprises the following steps: taking a pretreated copper substrate as a cathode, a graphite rod as an anode, and placing them in an electrolyte containing copper salt and auxiliary electrolyte, and constructing a porous copper-based composite electrode on the surface of the cathode by controlling the current density.

6. The system for simultaneous removal of nitrite and nitrate from carbon steel passivation wastewater according to claim 5, characterized in that, The electrolyte is a mixed solution of copper sulfate and sulfuric acid, the copper ion concentration is 0.3 mol / L; the sulfuric acid concentration is 0.3 mol / L; the electrochemical etching-electroplating conditions are current density 30 mA / cm 2 , temperature 35℃, time 30 minutes.

7. A method for the simultaneous removal of nitrite and nitrate nitrogen from carbon steel passivation wastewater, characterized in that, The method comprises the following steps: Step 1) passing the carbon steel passivation wastewater through the catalytic filter tower filled with multi-element alloy catalyst fillers, and catalytically reducing nitrite nitrogen in the wastewater into nitrogen in the presence of a reducing agent; Step 2) introducing the effluent treated in step 1) into the electrochemical reduction reactor, and reducing nitrate nitrogen in the wastewater into nitrogen by electrochemical reduction with the porous copper-based composite electrode as the cathode.

8. The method for simultaneous removal of nitrite and nitrate from carbon steel passivation wastewater according to claim 7, characterized in that, The multi-element alloy catalyst fillers in step 1) are selected from multi-element alloys of copper, cobalt, nickel and iron, and the molar ratio is 1:3:2:1~2:2:2:

1.

9. The method for simultaneous removal of nitrite and nitrate from carbon steel passivation wastewater according to claim 7, characterized in that, The reducing agent in step 1) is sodium formate, and the molar ratio of sodium formate to nitrite nitrogen is 2:

1.

10. The method for simultaneous removal of nitrite and nitrate from carbon steel passivation wastewater according to claim 7, characterized in that, The pH value of the wastewater in the step 1) is adjusted to 3.0-5.0, the hydraulic retention time is 30 minutes, the cathode current density in the step 2 is 50 mA / cm 2 .

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