Method for treating ammonia-nitrogen wastewater based on three-dimensional electrode

Abstract

The invention belongs to the field of water pollution treatment, and particularly discloses a method for treating ammonia-nitrogen wastewater based on a three-dimensional electrode, which comprises the following steps: firstly, by taking cheap metal bismuth, sodium chloride and the like as raw materials, directly synthesizing a BiOCl shell layer on the surface of a pure bismuth particle in situ by adopting a wet chemical method to form a stable Bi-coated BiOCl core-shell structure, and preparing BiOCl particles; then the prepared BiOCl particles are used as a third electrode, a Ti net or a stainless steel plate is used as a cathode, a ruthenium-iridium-titanium mixed metal net is used as an anode, a three-dimensional electrode electrolysis device is constructed, the device can maintain high chlorine free radical generation efficiency based on catalytic adsorption synergy of the BiOCl particles under the severe conditions of low Cl, acidity and containing organic matter (humic acid and the like), the current efficiency is remarkable, and the device is suitable for large-scale industrial production. The method has the advantages that the pH is not required to be frequently adjusted or exogenous chlorine is not required to be added, the stability is excellent, the organic matter interference resistance is high, the ammonia nitrogen removal rate of 93% or above can be achieved, the performance is superior to that of an existing three-dimensional system such as activated carbon and metal oxide, and an innovative solution is provided for degradation-resistant ammonia nitrogen wastewater.

Description

Technical Field

[0001] This invention belongs to the field of water pollution control, specifically relating to a method for treating ammonia nitrogen wastewater based on a three-dimensional electrode. Background Technology

[0002] In the field of water pollution control, ammonia nitrogen, as a key pollutant in water bodies, easily leads to eutrophication and dissolved oxygen depletion, threatening the stability of aquatic ecosystems. Furthermore, high concentrations of ammonia nitrogen, when exposed to drinking water, may pose health risks to humans. Therefore, efficient treatment technologies for ammonia nitrogen have been a global research hotspot. Traditional biological methods for treating ammonia nitrogen wastewater suffer from low efficiency and easy microbial inactivation when dealing with recalcitrant wastewater characterized by low C / N ratios, high salinity, low pH, and high temperatures. Electrochlorination is used to remove ammonium ions (NH4+). + Mixed metal oxide (MMO) anodes are an important method because they can electrolyze chloride-containing compounds (Cl-). - In situ, free chlorine is generated in the wastewater, which then oxidizes (NH4+). + It is widely used because its principle is similar to the traditional breakpoint chlorination process, where free chlorine is in NH4+. + It plays a key role in the conversion to nitrogen gas and is particularly suitable for treating wastewater that is difficult to degrade biologically, such as wastewater with low carbon-to-nitrogen ratio, high salinity, low pH, and high temperature. It is considered an effective alternative to biological treatment methods.

[0003] However, MMO anodes have significant limitations in practical applications: ① Insufficient efficiency at low chlorine concentrations, especially when Cl in the wastewater... - At low concentrations, the amount of free chlorine generated at the MMO anode is insufficient, leading to a decrease in electrochlorination efficiency; this is because low Cl... - At certain concentrations, the oxygen evolution reaction (OER) competes with the chlorine evolution reaction (ClER), significantly reducing the Faraday efficiency of free chlorine formation; ② Under acidic conditions, the reaction kinetics are slow, and NH4+... + Oxidation releases protons, causing the system's pH to decrease. Under acidic conditions, free chlorine (such as HClO) and NH4+ precipitate. + The reaction kinetics of NH4 are significantly slowed down, further affecting the reaction kinetics of NH4. + The removal effect; ③ The electrode area is limited. The reaction interface of the two-dimensional electrode is limited, which leads to the ammonia nitrogen oxidation efficiency being restricted by the mass transfer process, especially in the treatment of low-concentration ammonia nitrogen wastewater. Summary of the Invention

[0004] The purpose of this invention is to overcome the above-mentioned defects in the prior art by using a simple wet chemical method to synthesize BiOCl particles as the third electrode to construct a three-dimensional electrode electrolysis device. By utilizing the catalytic performance of BiOCl and the high reactivity of chlorine free radicals, the ammonia nitrogen oxidation efficiency is improved, providing a new approach for the efficient treatment of ammonia nitrogen wastewater.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is: a method for treating ammonia nitrogen wastewater based on a three-dimensional electrode, comprising the following steps: Step 1: Pretreatment: Based on the quality of the industrial wastewater, pretreatment is carried out using one or more of the following methods: ① Equalization-stage sedimentation: The wastewater is introduced into an equalization tank and retained for 4-8 hours to buffer fluctuations in ammonia nitrogen and COD concentrations, while allowing natural sedimentation; ② Screening for suspended solids: Large suspended solids are removed using a fine screen (5-10mm spacing); ③ Oil removal: If the wastewater contains oil, floating oil is first removed using a gravity oil removal tank, followed by air flotation with the addition of a demulsifier to remove emulsified oil, ensuring an oil content <10mg / L; ④ Heavy metal removal: Sodium sulfide or calcium hydroxide is added to cause heavy metals to precipitate as sulfides or hydroxides, ensuring the residual concentration meets requirements; ⑤ Acid-base neutralization: Acidic wastewater is neutralized with sodium hydroxide solution; alkaline wastewater is neutralized with sulfuric acid solution, ultimately adjusting the pH to 6-8. Step 2: Construction of the three-dimensional electrode electrolysis device: The three-dimensional electrode device uses a Ti mesh or stainless steel plate as the cathode, a ruthenium-iridium-titanium mixed metal mesh as the anode, and BiOCl particles filled between the cathode and anode as the third electrode. The filling amount of BiOCl particles is (80~260g / L). Step 3: Electrolysis Operation: Send the pretreated wastewater into the three-dimensional electrode electrolysis device, set the current density to 5~20 mA / cm², and the electrolysis time to 1.5~3.5h; check the ammonia nitrogen concentration every 20~60min, and after the ammonia nitrogen concentration meets the emission standard, discharge the wastewater in the device to the qualified discharge outlet.

[0006] The present invention also provides a method for preparing the above-mentioned BiOCl particles, comprising the following steps: (a) Pretreatment of pure Bi particles: screen pure Bi particles with a purity ≥99.9% and a particle size of 1~5mm, polish the surface of pure Bi particles with 600~5000 grit sandpaper, and then ultrasonically clean them with 0.5~1mol / LH2SO4 solution for 20~30min to remove impurities from the surface of pure Bi particles. Then wash them repeatedly with deionized water until the washing pH=6~8, and dry them at 60~80℃ for 8~12h for later use. (b) Synthesis of BiOCl particles: Prepare a 1-2 mol / L NaCl solution, adjust the pH to 1.6-2.8 with dilute H2SO4, add the pretreated pure Bi particles to the solution at a solid-liquid ratio of 1:20-1:50, place in a constant temperature water bath at 40℃-60℃, and magnetically stir at 200-500 rpm; monitor the pH every 0.5-1 h during the process, when the pH>3.0, add a small amount of hydrochloric acid to maintain the pH=1.6-2.8 environment, add 0.2-0.5 mL of 30% H2O2 solution every 2.5-3 h, and continue the reaction for 9-12 h to obtain particles with a uniform BiOCl shell on the surface; (c) Purification of BiOCl particles: Collect the solid product and wash it with deionized water and ethanol alternately 3 to 5 times to remove residual impurities; (d) Stabilization of BiOCl particles: After drying the cleaned particles at 60~80℃ for 8~12h, they are placed in a muffle furnace and calcined at 380~420℃ for 2~3h to enhance the bonding force between Bi and BiOCl shell.

[0007] The beneficial effects achieved by this invention are as follows: This invention uses inexpensive metallic bismuth and sodium chloride as raw materials. By controlling conditions such as pH, temperature, and the addition of oxidant (H2O2), combined with calcination, a BiOCl shell is directly synthesized in situ on the surface of pure bismuth particles using a wet chemical method, forming a stable Bi@BiOCl core-shell structure. The prepared BiOCl particles have a robust structure, excellent stability and recyclability, and are not easily pulverized or lost. They combine the conductivity of bismuth with the catalytic activity of BiOCl. This preparation method is simple, uses inexpensive and readily available raw materials, has low overall cost, and is suitable for industrial scale-up. This invention uses prepared BiOCl particles as the third electrode to construct a three-dimensional electrode electrolysis device. Based on the synergistic catalytic adsorption of BiOCl particles, this device can maintain high chlorine radical generation efficiency and significant current efficiency under harsh conditions of low Cl⁻, acidity, and organic matter (humic acid, etc.). It does not require frequent pH adjustment or addition of exogenous chlorine, and has excellent stability and strong resistance to organic interference. It can achieve an ammonia nitrogen removal rate of over 93%, which is superior to existing three-dimensional systems such as activated carbon and metal oxides. It provides an innovative solution for recalcitrant ammonia nitrogen wastewater. Attached Figure Description

[0008] Figure 1 Images of the raw materials and BiOCl particles are provided. Figure 2 The XRD pattern of the BiOCl particle electrode is shown. Figure 3 The graph shows the stability analysis of the BiOCl particle electrode. Figure 4A comparison chart of the blank group and the experimental group for the degradation of ammonia nitrogen content in wastewater. Detailed Implementation

[0009] The embodiments of the present invention will be further described below with reference to specific examples. Example 1

[0010] For pure Bi particles ( Figure 1 -a) Pretreatment was performed to screen out particles with a purity ≥99.9% and a particle size of 3mm. The surface was first polished with 5000-grit sandpaper for 5 minutes, then ultrasonically cleaned with 1mol / L H2SO4 solution for 30 minutes to remove surface oxides and impurities. Subsequently, the particles were repeatedly centrifuged and washed with deionized water until the pH of the filtrate was 7. Finally, the particles were vacuum dried at 60℃ for 12 hours for later use. Figure 1 -b); Next, BiOCl particles were synthesized. A 1 mol / L NaCl solution was prepared, and the pH was adjusted to 2.0 with dilute H2SO4 as the reaction medium. Pretreated pure Bi particles were added to this solution at a solid-liquid ratio of 1:50 (g:mL). The mixture was placed in a 60℃ constant temperature water bath and magnetically stirred at 500 rpm for 12 hours. During the reaction, the pH was monitored every 1 hour. If the pH > 3.0, a small amount of hydrochloric acid was added to maintain the acidic environment. Every 3 hours, 0.5 mL of 30% H2O2 solution was added to promote the reaction of pure Bi with Cl. - Dissolved oxygen reacts to produce BiOCl, which is then stored for later use. After the reaction was completed, purification was performed, the solid product was collected, and washed three times alternately with deionized water and anhydrous ethanol to remove residual Cl⁻ and other impurities. Figure 1 -c), and then stabilization treatment was performed. After drying at 60℃ for 12 hours, the product was placed in a muffle furnace and calcined at 400℃ for 2 hours to obtain particles with a uniform BiOCl shell on the surface. Figure 1 -d), which is similar to pure Bi particles ( Figure 1 Compared to -a), the surface color changed, and a white coating was formed, which is consistent with the characteristics of BiOCl; The prepared particulate electrode was subjected to XRD testing, and the results are shown in the figure. Figure 2 It can be seen that the crystal diffraction peaks of the product are consistent with the diffraction peaks of the tetragonal BiOCl standard card (JCPCS No. 85-0861), and no other impurity peaks appear. Therefore, the surface of the prepared material is a single BiOCl crystal, indicating that the BiOCl film formed on the particle surface is relatively uniform. Example 2

[0011] Ammonia nitrogen removal performance test in water: Test conditions: Initial pH=7, NaCl=50mM, NH4+ + -N=200mgL-1 Humic acid = 100 mg / L -1 Reaction time = 180 min; Current density = 10 mA / cm² 2 6g of granules; Three-dimensional electrode electrolysis apparatus: Electrolysis experiments were conducted in a non-separated cylindrical glass beaker with a sample volume of 50 mL. The anode used was MMO, measuring 2.5 × 3.5 cm. 2 The anode and cathode were placed perpendicularly to each other with a 20mm gap, and 6g of BiOCl particles were filled in. A magnetic paddle was placed in the solution (to ensure uniform mixing and reduce sampling error). The power was then turned on and adjusted to a constant current of 10mA / cm. 2 Under these conditions, energy consumption is 0.01459 kWh / h. Energy consumption is calculated using the following formula:

[0012] In the formula: EC is the power consumption, kWh / g; Ecell is the applied voltage of the electrolysis device, V; I is the electrolysis current, A; t is the reaction time, h; CO is the pollutant concentration before electrolysis, mg / L; Ct is the pollutant concentration at time th, mg / L; V is the wastewater volume, L.

[0013] Methods for measuring ammonia nitrogen concentration: The national environmental protection standard document "Determination of Ammonia Nitrogen in Water - Nessler's Reagent Spectrophotometric Method" (HJ 535—2009) was adopted. The specific operation is as follows: 1. Calibration curve: Add 0.00, 0.50, 1.00, 2.00, 4.00, 6.00, 8.00 and 10.00 mL of the solution to eight 50 mL colorimetric tubes, respectively. Ammonia nitrogen standard working solutions with corresponding ammonia nitrogen contents of 0.0, 5.0, 10.0, 20.0, 40.0, 60.0, 80.0 and 100 μg were prepared, and water was added to the mark. 1.0 mL of potassium sodium tartrate solution was added, and the solution was shaken well. Then, 1.5 mL of Nessler's reagent was added, and the solution was shaken well. After standing for 10 min, the absorbance was measured at a wavelength of 420 nm using a 20 mm cuvette and water as a reference using a UV-Vis spectrophotometer. A calibration curve was plotted with the absorbance after blank correction as the ordinate and the corresponding ammonia nitrogen content (μg) as the abscissa. 2. Sample determination: Before starting the electrolysis reaction, 0.5 mL of water sample was taken as the first test sample. After the electrolysis reaction started, 0.5 mL of water sample was taken every 30 min as the test sample. The absorbance of the pretreated water sample was measured according to the same steps as the calibration curve. Example 3

[0014] Stability evaluation of BiOCl particle electrode: Based on Example 2, the performance test for removing ammonia nitrogen from water was performed 60 times in a cyclic manner. The ammonia nitrogen degradation rate was as follows: Figure 3 As shown, the stability of BiOCl particles is maintained at around 91%~94%, indicating excellent stability.

[0015] Comparative Example 1 Under the test conditions of Example 2, a blank group (two-dimensional electrode) was set up and its performance was compared with that of the BiOCl experimental group; The results are as follows Figure 4 As shown, the ammonia nitrogen removal rate in the blank group was 39%, while the ammonia nitrogen removal rate in the BiOCl experimental group was 93%. The ammonia nitrogen removal rate of the BiOCl experimental group was more than twice that of the blank group, which proves the performance of BiOCl particle electrode in promoting ammonia nitrogen oxidation in water.

[0016] Comparative Example 2 Based on Example 2, the Cl- concentration was adjusted to 100, 150, and 200 mM, respectively, while other conditions remained unchanged; the ammonia nitrogen degradation rate is shown in Table 1 below. Table 1

[0017] As shown in Table 1, the ammonia nitrogen oxidation efficiency gradually increases with the increase of chlorine concentration, indicating that the present invention can treat both low-chlorine and high-chlorine wastewater with significant effect.

[0018] Comparative Example 3 Based on Example 2, the BiOCl particles were replaced with the following common existing electrode materials for comparative experiments; A: activated carbon; B; Fe3+-TiO2@CGS; C: Particles made from iron-based catalysts, with an iron-to-carbon ratio of 3; D: It is made by mixing cellulose acetate coated activated carbon and granular activated carbon in a mass ratio of 1:2.

[0019] The data comparison is as follows: The pH changes are shown in Table 2: Table 2

[0020] The ammonia nitrogen degradation rate (%) is shown in Table 3: Table 3

[0021] As shown in Tables 2 and 3, during the ammonium oxidation process, the pH drops rapidly to the acidic range (e.g., pH < 4.0). At this point, the reaction rate of free chlorine (e.g., Cl2, HClO) with NH4+ is extremely slow, resulting in low ammonia nitrogen removal efficiency. Compared to other electrodes, this invention can efficiently generate chlorine free radicals (Cl) in a low-chlorine environment, which have a high reaction rate with NH4+, thus achieving efficient oxidative removal of ammonium under acidic conditions.

[0022] Comparative Example 4 Based on Comparative Example 3, humic acid was removed while other conditions remained unchanged. The data comparison is as follows: The pH changes are shown in Table 4: Table 4

[0023] The ammonia nitrogen degradation rate (%) is shown in Table 5: Table 5

[0024] Based on the data in Tables 2, 3, 4, and 5, it can be seen that organic matter such as humic acid can easily clog the pores of activated carbon or poison the catalytic sites. However, the present invention still maintains a high ammonia nitrogen removal rate in wastewater containing humic acid, high salt, and low C / N ratio, and has a strong resistance to organic interference.

[0025] Comparative Example 5 Stability comparison and evaluation of BiOCl particle electrodes with existing electrode materials: Based on Comparative Example 3, the experiment was repeated five times. The data are shown in Table 6, indicating that BiOCl particles have higher stability as the third electrode material compared with existing electrode materials. Table 6

[0026] The above specific embodiments are used to explain and illustrate the present invention, but not to limit the present invention. Any modifications and changes made to the present invention within the spirit and scope of the claims shall fall within the protection scope of the present invention.

Claims

1. A method for treating ammonia nitrogen wastewater based on a three-dimensional electrode, characterized in that: Includes the following steps: Step 1: Pretreatment: Based on the quality of the industrial wastewater, pretreatment is carried out using one or more of the following methods: ① Equalization-stage sedimentation: The wastewater is introduced into an equalization tank and retained for 4-8 hours to buffer fluctuations in ammonia nitrogen and COD concentrations, while allowing natural sedimentation; ② Screening for suspended solids: Large suspended solids are removed using a fine screen; ③ Oil removal: If the wastewater contains oil, floating oil is first removed using a gravity oil removal tank, followed by air flotation with the addition of a demulsifier to remove emulsified oil, ensuring an oil content <10mg / L; ④ Heavy metal removal: Sodium sulfide or calcium hydroxide is added to cause heavy metals to precipitate as sulfides or hydroxides, ensuring the residual concentration meets requirements; ⑤ Acid-base neutralization: Acidic wastewater is neutralized with sodium hydroxide solution; alkaline wastewater is neutralized with sulfuric acid solution, ultimately adjusting the pH to 6-8. Step 2: Construction of the three-dimensional electrode electrolysis device: The three-dimensional electrode device uses a Ti mesh or stainless steel plate as the cathode, a ruthenium-iridium-titanium mixed metal mesh as the anode, and BiOCl particles filled between the cathode and anode as the third electrode, with a filling amount of BiOCl particles of 80-260 g / L. Step 3: Electrolysis Operation: Send the pretreated wastewater into the three-dimensional electrode electrolysis device, set the current density to a constant current mode of 5-20mA / cm², and the electrolysis time to 1.5-3.5h; check the ammonia nitrogen concentration every 20-60min, and after the ammonia nitrogen concentration meets the emission standard, discharge the wastewater in the device to the qualified discharge port.

2. The method according to claim 1, characterized in that: The method for preparing the BiOCl particles includes the following steps: (a) Pretreatment of pure Bi particles: screen pure Bi particles with a purity ≥99.9% and a particle size of 1~5mm, polish the surface of pure Bi particles with 600~5000 grit sandpaper, and then ultrasonically clean them with 0.5~1mol / LH2SO4 solution for 20~30min to remove impurities from the surface of pure Bi particles. Then wash them repeatedly with deionized water until the washing pH=6~8, and dry them at 60~80℃ for 8~12h for later use. (b) Synthesis of BiOCl particles: Prepare a 1-2 mol / L NaCl solution, adjust the pH to 1.6-2.8 with dilute H2SO4, add the pretreated pure Bi particles to the solution at a solid-liquid ratio of 1:0-1:50, place in a constant temperature water bath at 40℃-60℃, and magnetically stir at 200-500 rpm; monitor the pH every 0.5-1 h during the process, when the pH>3.0, add a small amount of hydrochloric acid to maintain the pH=1.6-2.8 environment, add 0.2-0.5 mL of 30% H2O2 solution every 2.5-3 h, and continue the reaction for 9-12 h to obtain particles with a uniform BiOCl shell on the surface; (c) Purification of BiOCl particles: Collect the solid product and wash it with deionized water and ethanol alternately 3 to 5 times to remove residual impurities; (d) Stabilization of BiOCl particles: After drying the cleaned particles at 60~80℃ for 8~12h, they are placed in a muffle furnace and calcined at 380~420℃ for 2~3h to enhance the bonding force between Bi and BiOCl shell.

Images