Method for recovering nitrogen and phosphorus in wastewater

Abstract

The invention belongs to the technical field of wastewater treatment, and provides a method for recovering nitrogen and phosphorus in wastewater. The recovery method comprises the following steps: adding magnesium ore into an anode reaction chamber, and injecting anolyte into the anode reaction chamber; injecting the wastewater into a cathode reaction chamber; an anode is arranged in the anode reaction chamber, a cathode is arranged in the cathode reaction chamber, the anode and the cathode are connected with a power supply for electrolysis, and an anode solution and a cathode solution are obtained; mixing the anode solution and the cathode solution to obtain struvite precipitate; the type of cathode is defined. The cathode is a composite cathode of the cell substrate and the palladium-lanthanum-rhodium alloy, and the palladium-lanthanum-rhodium alloy can catalyze nitrite to be converted into ammonium salt under the action of current, so that the yield of struvite is increased, and the recovery rate of nitrogen is increased; the palladium-lanthanum-rhodium alloy is a foil material, so that the material cost of the cathode is reduced, and the corrosion of a battery cell substrate is slowed down; and a suspension type titanium net bag is arranged in the anode reaction chamber, so that leakage of magnesium ore is avoided, and the magnesium ore is promoted to fully react with H < + > generated by an anode.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of wastewater treatment, in particular to a method for recovering nitrogen and phosphorus in wastewater. BACKGROUND

[0002] Phosphorus is a non-renewable resource. With the rapid development of economy and society, the uncontrolled mining and use of phosphate rock has led to a sharp decrease in phosphorus resources, and the problem of water eutrophication caused by phosphorus discharge has become increasingly prominent. Therefore, efficient removal and recovery of phosphorus in wastewater not only improves the water environment, but also alleviates the crisis of decreasing phosphorus resources.

[0003] Currently, the commonly used method for recovering phosphorus in wastewater is struvite (MgNH4PO4·6H2O, MAP for short) crystallization. The optimal pH value for struvite crystallization is 8-11, and this method requires the introduction of alkaline substances to adjust the pH value of the wastewater. However, due to the presence of buffering substances in the wastewater, a large amount of alkali needs to be added to form struvite crystalline precipitate. Electrochemical struvite precipitation technology is a new technology for phosphorus removal and recovery. Through cathode-mediated oxygen reduction and water reduction reactions, OH - is generated, creating a high-pH environment in the local area of the electrode, promoting the heterogeneous precipitation of struvite crystals on the cathode surface and nearby, and achieving the purpose of removing phosphorus. Electrochemical struvite precipitation technology has the advantages of high product purity and no need to add alkali, which can save a large amount of reagent cost. However, due to mass transfer problems, the efficiency of reactant diffusion to the local area of the electrode is limited, affecting the efficiency of phosphorus precipitation in the heterogeneous area. Moreover, the presence of H + in the anode solution can pass through the cation exchange membrane, preventing the creation of a high-pH environment in the cathode solution. In addition, for the recovery of nitrogen in wastewater, electrochemical struvite precipitation technology generally only recovers NH4 + in wastewater, and cannot recover nitrite nitrogen.

[0004] Therefore, how to improve the efficiency of electrochemical struvite precipitation technology and simultaneously recover nitrite nitrogen is of great significance for wastewater treatment. SUMMARY

[0005] The purpose of the present application is to provide a method for recovering nitrogen and phosphorus in wastewater to overcome the shortcomings of the prior art.

[0006] In order to achieve the above-mentioned purpose of the application, the present application provides the following technical solutions: The present application provides a method for recovering nitrogen and phosphorus in wastewater, comprising the following steps: 1) adding magnesium ore to the anode reaction chamber and injecting anode liquid into the anode reaction chamber; and injecting wastewater into the cathode reaction chamber; 2) an anode is arranged in the anode reaction chamber, and a cathode is arranged in the cathode reaction chamber, the anode and the cathode are connected to a power supply to perform electrolysis, and an anode solution and a cathode solution are obtained; 3) the anode solution and the cathode solution are mixed to obtain struvite precipitate; The cathode is a composite cathode of an electric core substrate and a palladium-lanthanum-rhodium alloy, the inside of the composite cathode is the electric core substrate, and the surface of the composite cathode is the palladium-lanthanum-rhodium alloy.

[0007] Preferably, in the palladium-lanthanum-rhodium alloy, the mass fraction of palladium is 65-86%, the mass fraction of lanthanum is 3-5%, and the mass fraction of rhodium is 11-30%.

[0008] Preferably, the preparation method of the composite cathode comprises the following steps: The palladium-lanthanum-rhodium alloy is vacuum sputtered onto the surface of the electric core substrate under an argon atmosphere to obtain the composite cathode; The vacuum degree of the vacuum sputtering is 0.1-1 Pa, the power of the vacuum sputtering is 50-120 W, the time of the vacuum sputtering is 2-5 h, and the temperature of the electric core substrate during the vacuum sputtering is 200-300℃.

[0009] Preferably, the electric core substrate comprises iron-carbon alloy, titanium alloy, copper alloy, nickel alloy or aluminum alloy.

[0010] Preferably, the magnesium ore comprises periclase, magnesite or serpentine, and the particle size of the magnesium ore is 3-7 mm. The mass content of magnesium carbonate in the magnesium ore is ≥25%. The mass-volume ratio of the magnesium ore and the anode solution is 25-35 g:1 L.

[0011] Preferably, the anode solution is a sodium sulfate solution, and the concentration of the sodium sulfate solution is 45-60 mmol / L.

[0012] Preferably, the anode reaction chamber further comprises a titanium mesh pocket, the titanium mesh pocket is a cylindrical shape with an open top, and the titanium mesh pocket is suspendedly installed in the anode reaction chamber. The anode is arranged at the center of the titanium mesh pocket, the magnesium ore is arranged at the bottom of the titanium mesh pocket, and the anode does not contact the magnesium ore.

[0013] Preferably, the pore size of the titanium mesh pocket is 30-80 mesh.

[0014] Preferably, in step 2), the current density of the electrolysis is 5-14 A / m 2 , the electrolysis time is 180-300 min, and the electrolysis temperature is 25-30℃.

[0015] As preferred, the wastewater is nitrogen and phosphorus containing wastewater, the concentration of phosphorus is ≥70mmol / L, the concentration of nitrogen is ≥70mmol / L, and the molar ratio of nitrogen and phosphorus is 1:0.8-1.2. The nitrogen in the wastewater comprises ammonia nitrogen and nitrite nitrogen.

[0016] The beneficial effects of the present application include the following points: 1) The cathode of the present application is a composite cathode of an electric core substrate and a palladium lanthanum rhodium alloy. After the power is turned on, the palladium lanthanum rhodium alloy can catalyze the conversion of nitrite into ammonium under the action of current, thereby improving the yield of struvite. The present application can be applied to high-concentration ammonia nitrogen-containing wastewater and can also treat nitrite in the wastewater at the same time, thereby improving the nitrogen recovery rate. The palladium lanthanum rhodium alloy is a foil, which on the one hand reduces the material cost of the cathode, and on the other hand slows down the corrosion process of the electric core substrate.

[0017] 2) The present application is provided with a titanium mesh pocket in the anode reaction chamber. The aperture of the titanium mesh pocket is much smaller than the particle size of the magnesium ore, thereby avoiding the leakage of the magnesium ore and promoting the reaction between the magnesium ore and H + generated by the anode. + The titanium mesh pocket provides sufficient space for the electromagnetic stirring device, and the electromagnetic stirring can accelerate the reaction process between H + and the magnesium ore.

[0018] 3) The magnesium ore filled in the anode reacts with H - generated by the anode, thereby effectively avoiding the loss of OH - and forming a local high-pH environment on the surface of the cathode. The present application solves the problem of low recovery rate of nitrogen and phosphorus elements. DETAILED DESCRIPTION

[0019] The present application provides a method for recovering nitrogen and phosphorus in wastewater, comprising the following steps: 1) adding magnesium ore into an anode reaction chamber and injecting an anode liquid into the anode reaction chamber; and injecting wastewater into a cathode reaction chamber; 2) providing an anode in the anode reaction chamber and a cathode in the cathode reaction chamber, connecting the anode and the cathode to a power supply, and performing electrolysis to obtain an anode solution and a cathode solution; 3) mixing the anode solution and the cathode solution to obtain struvite precipitate; The cathode is a composite cathode of an electric core substrate and a palladium lanthanum rhodium alloy. The inside of the composite cathode is the electric core substrate, and the surface of the composite cathode is the palladium lanthanum rhodium alloy.

[0020] In the present application, an electrochemical device is adopted, and struvite crystals are generated by using electrochemical method; the electrochemical device is preferably a double-chamber electrolytic cell, an anode is arranged in an anode reaction chamber, a cathode is arranged in a cathode reaction chamber, a through channel is arranged between the anode reaction chamber and the cathode reaction chamber, and a cation exchange membrane is arranged in the middle of the through channel to separate the anode reaction chamber and the cathode reaction chamber; and the anode reaction chamber is provided with a magnetic stirring device at the bottom.

[0021] In the present application, the mass fraction of palladium in the palladium-lanthanum-rhodium alloy is preferably 65-86%, further preferably 68-83%, and more preferably 72-76%; the mass fraction of lanthanum is preferably 3-5%, further preferably 3.5-4.5%, and more preferably 4%; and the mass fraction of rhodium is preferably 11-30%, further preferably 15-25%, and more preferably 20%.

[0022] In the present application, the preparation method of the composite cathode preferably comprises the following steps: The palladium-lanthanum-rhodium alloy is vacuum sputtered onto the surface of the cell substrate under an argon atmosphere, and the composite cathode is obtained; The vacuum degree of the vacuum sputtering is preferably 0.1-1 Pa, further preferably 0.3-0.8 Pa, and more preferably 0.5 Pa; the power of the vacuum sputtering is preferably 50-120 W, further preferably 70-100 W, and more preferably 80-90 W; the time of the vacuum sputtering is preferably 2-5 h, further preferably 3-4 h; and the temperature of the cell substrate during the vacuum sputtering is preferably 200-300 DEG C, further preferably 220-280 DEG C, and more preferably 250 DEG C.

[0023] In the present application, before the vacuum sputtering, the cell substrate is preferably sequentially polished, derusted and cleaned; the polishing is performed until the surface roughness Ra of the cell substrate is less than 0.1 μm; and the reagent used for derusting is preferably hydrochloric acid, and the volume fraction of the hydrochloric acid is preferably 5-15%, and further preferably 10%.

[0024] In the present application, the palladium-lanthanum-rhodium alloy is preferably a foil, which can reduce the material cost of the palladium-lanthanum-rhodium alloy and slow down the corrosion process of the cell substrate.

[0025] In the present application, the cell substrate preferably comprises an iron-carbon alloy, a titanium alloy, a copper alloy, a nickel alloy or an aluminum alloy.

[0026] In the present application, the magnesium ore preferably comprises periclase, magnesite or serpentine, and the particle size of the magnesium ore is preferably 3-7 mm, further preferably 4-6 mm, and more preferably 5 mm. The mass content of magnesium carbonate in the magnesium ore is preferably ≥ 25%, further preferably ≥ 26%, and more preferably ≥ 27%; The mass-volume ratio of the magnesium ore and the anolyte is preferably 25-35 g: 1 L, further preferably 28-32 g: 1 L, and more preferably 30 g: 1 L.

[0027] In the present application, the anolyte is preferably a sodium sulfate solution, and the concentration of the sodium sulfate solution is preferably 45-60 mmol / L, further preferably 50-55 mmol / L.

[0028] In the present application, the anode reaction chamber preferably further comprises a titanium mesh pocket, which is preferably a cylinder with an open upper portion, and the titanium mesh pocket is preferably suspended and installed in the anode reaction chamber. The anode is preferably placed at the center of the titanium mesh pocket, and the magnesium ore is preferably placed at the bottom of the titanium mesh pocket, and the anode does not contact the magnesium ore.

[0029] In the present application, the pore size of the titanium mesh pocket is preferably 30-80 mesh, further preferably 40-70 mesh, and more preferably 50-60 mesh. The particle size of the magnesium ore is much larger than the pore size of the titanium mesh pocket, which avoids the magnesium ore from leaking out of the titanium mesh pocket and affecting the reaction process of the magnesium ore and H + .

[0030] In the present application, the current density of the electrolysis in step 2) is preferably 5-14 A / m 2 , further preferably 6-12 A / m 2 , and more preferably 7-10 A / m 2 ; the electrolysis time is preferably 180-300 min, further preferably 190-280 min, and more preferably 200-260 min; and the electrolysis temperature is preferably 25-30℃, further preferably 26-29℃, and more preferably 27℃.

[0031] In the present application, the wastewater is preferably nitrogen and phosphorus-containing wastewater, the concentration of phosphorus is preferably ≥ 70 mmol / L, further preferably ≥ 75 mmol / L, and more preferably ≥ 80 mmol / L; the concentration of nitrogen is preferably ≥ 70 mmol / L, further preferably ≥ 75 mmol / L, and more preferably ≥ 80 mmol / L; and the molar ratio of nitrogen to phosphorus is preferably 1:0.8-1.2, further preferably 1:0.9-1.1, and more preferably 1:1. The nitrogen in the wastewater preferably comprises ammonia nitrogen and nitrite nitrogen.

[0032] In the present application, the working principle of the electrolysis in step 2) is that the anode decomposes H2O into O2 and H + and releases electrons, the electrons reach the cathode through an external circuit, and H + reacts with the magnesium ore in the anode reaction chamber to generate Mg 2+A titanium mesh bag is placed inside the anode reaction chamber to hold the magnesium ore, allowing the magnesium ore to fully react with the H2 produced at the anode. + The reaction thus avoids H + The electrons pass through the cation exchange membrane to the cathode; the cathode uses the electrons obtained to decompose H2O into H2 and OH-. - The obtained electrons and the palladium-lanthanum-rhodium alloy in the composite cathode are used to remove NO2 from the wastewater. - Converted to NH4 + At the same time, OH is generated - The pH value in the cathode reaction chamber is affected by OH. - It is generated and improved.

[0033] In this invention, during the electrolysis process described in step 2), the reaction process at the anode is preferably as follows: ①2H2O-4e - →4H + +O2↑ ②2MgCO3+4H + →2Mg 2+ +2H2O+2CO2↑, reaction ② is the dissolution reaction of magnesium ore; The preferred reaction process at the cathode is: ③4H2O+4e - →4OH - +2H2↑ ④NO2 - +6e - +7H + →H2O+NH4 + +OH - .

[0034] In this invention, the preferred reaction process for step 3) of struvite precipitation is as follows: Mg 2+ +NH4 + +H n PO4 (3-n)- +nOH - +(6-n)H₂O→MgNH₄PO₄·6H₂O↓ (n=0, 1, 2). Because Mg 2+ The cation exchange membrane passes through slowly, which slows down the struvite precipitation process. In step 3), the anolyte and catholyte solutions are poured out and mixed to accelerate the struvite precipitation process.

[0035] The technical solutions provided by the present invention will be described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0036] The electrochemical device used in the examples and comparative examples of the present application is a double-chamber electrolytic cell, which comprises an anode reaction chamber and a cathode reaction chamber, the volume of each of the anode reaction chamber and the cathode reaction chamber is 1500 mL, a through channel is arranged between the anode reaction chamber and the cathode reaction chamber, a cation exchange membrane is arranged in the middle of the through channel to separate the anode reaction chamber and the cathode reaction chamber; a titanium mesh pocket is arranged in the anode reaction chamber, the titanium mesh pocket is a cylindrical mesh pocket with an open upper portion and is suspendedly installed in the anode reaction chamber, the aperture of the titanium mesh pocket is 50 mesh; the anode is a ruthenium-yttrium-titanium electrode, the anode is arranged at the center position of the titanium mesh pocket, a magnesium ore is placed at the bottom of the titanium mesh pocket, and the magnesium ore is not in contact with the anode; a magnetic stirring device is arranged at the bottom of the anode reaction chamber, and the rotating speed of the magnetic stirring device is controlled to be 140 rpm.

[0037] Example 1

[0038] The iron-carbon alloy (the mass percentage of carbon in the iron-carbon alloy is 2%) is polished with a 500-mesh sandpaper until the surface roughness Ra of the iron-carbon alloy is less than 0.1 μm. The iron-carbon alloy is soaked in hydrochloric acid with a volume fraction of 10% for 3 min, and then the iron-carbon alloy is washed with water until the washing liquid is neutral. Then, the iron-carbon alloy is cleaned with an ultrasonic cleaning instrument at a frequency of 30 kHz for 5 min, and is dried to obtain a cell base. In an argon atmosphere, the alloy components with a mass content of 25% rhodium, 3% lanthanum and 72% palladium are used to perform vacuum sputtering on the surface of the cell base at 0.5 Pa and 100 W, and the temperature of the cell base during the vacuum sputtering process is 250°C. The vacuum sputtering is performed for 3 h, and then the temperature is lowered to 40°C in an argon atmosphere to obtain a composite cathode with an iron-carbon alloy inside and a palladium-lanthanum-rhodium alloy on the surface.

[0039] The composite cathode is arranged in the cathode reaction chamber, 1 L of nitrogen and phosphorus-containing wastewater is injected into the cathode reaction chamber, the nitrogen includes ammonia nitrogen and nitrite nitrogen, the total concentration of nitrogen in the nitrogen and phosphorus-containing wastewater is 70 mmol / L, the concentration of ammonia nitrogen is 60 mmol / L in terms of nitrogen, and the concentration of nitrite nitrogen is 10 mmol / L in terms of nitrogen; the concentration of phosphorus in the nitrogen and phosphorus-containing wastewater is 70 mmol / L. The magnesium ore is magnesite (purchased from Haicheng Xinhemagnesium Product Co., Ltd.), and the mass content of magnesium carbonate in the magnesite is 27%, and the addition amount of the magnesite is 30 g. 1 L of an anode solution is injected into the anode reaction chamber, and the anode solution is a 50 mmol / L sodium sulfate solution. The anode and the composite cathode are connected to a power supply, the current density is 7 A / m 2 The electrolysis is performed for 200 min. After the electrolysis is completed, the anode solution and the cathode solution are poured into the same container, and are stirred at a rotating speed of 140 rpm for 5 min, and are left to obtain struvite precipitate.

[0040] Example 2

[0041] Different from example 1 is that the alloy composition of the palladium lanthanum rhodium alloy is: 30% rhodium, 5% lanthanum and 65% palladium in mass content.

[0042] Comparative example 1

[0043] Different from example 1 is that the palladium lanthanum rhodium alloy is omitted, and only the iron-carbon alloy is used as the cathode and placed in the cathode reaction chamber.

[0044] Comparative example 2

[0045] Different from example 1 is that the alloy composition for vacuum sputtering is: 25% rhodium and 75% palladium in mass content.

[0046] Comparative example 3

[0047] Different from example 1 is that the alloy composition for vacuum sputtering is: 3% lanthanum and 97% palladium in mass content.

[0048] Comparative example 4

[0049] Different from example 1 is that the alloy composition for vacuum sputtering is: 3% lanthanum and 97% rhodium in mass content.

[0050] Comparative example 5

[0051] Different from example 1 is that the rhodium electrode is used as the cathode and placed in the cathode reaction chamber.

[0052] Comparative example 6

[0053] Different from example 1 is that the nitrite nitrogen in the nitrogen-containing phosphorus wastewater does not contain nitrite nitrogen, and the concentration of ammonia nitrogen is 70 mmol / L in terms of nitrogen.

[0054] The struvite precipitates obtained in examples 1-2 and comparative examples 1-6 are naturally air-dried and weighed, and the weight of the struvite is shown in table 1.

[0055] Table 1 weight of struvite obtained in examples and comparative examples

[0056] As can be seen from the results in table 1, the composite cathode with the inner electrode core substrate and the outer palladium lanthanum rhodium alloy and the reasonable control of the element ratio of the palladium lanthanum rhodium alloy can convert the nitrite nitrogen in the wastewater into struvite to recover nitrogen and improve the nitrogen conversion rate. After electrolysis, the cathode surface of comparative example 1 is corroded, and the cathode surface of examples 1-2 is not corroded.

[0057] The above merely describes the preferred embodiments of the present application, and it should be pointed out that, for those skilled in the art, several improvements and refinements can be made without departing from the principles of the present application, and these improvements and refinements should also be considered as falling within the protection scope of the present application.

Claims

1. A method for recovering nitrogen and phosphorus from wastewater, characterized by, The method comprises the following steps: 1) adding magnesium ore into an anode reaction chamber, and injecting an anolyte into the anode reaction chamber; and injecting wastewater into a cathode reaction chamber; 2) an anode is arranged in the anode reaction chamber, and a cathode is arranged in the cathode reaction chamber; the anode and the cathode are connected to a power supply to perform electrolysis, so as to obtain an anode solution and a cathode solution; 3) mixing the anode solution and the cathode solution to obtain struvite precipitate; The cathode is a composite cathode of an electric core base and a palladium-lanthanum-rhodium alloy, the electric core base is arranged in the interior of the composite cathode, and the palladium-lanthanum-rhodium alloy is arranged on the surface of the composite cathode.

2. The method for recovering nitrogen and phosphorus in wastewater according to claim 1, characterized by, In the palladium-lanthanum-rhodium alloy, the mass fraction of palladium is 65-86%, the mass fraction of lanthanum is 3-5%, and the mass fraction of rhodium is 11-30%.

3. The method for recovering nitrogen and phosphorus in wastewater according to claim 2, characterized by, The preparation method of the composite cathode comprises the following steps: The palladium-lanthanum-rhodium alloy is vacuum sputtered on the surface of the electric core base under an argon atmosphere, so as to obtain the composite cathode; The vacuum degree of the vacuum sputtering is 0.1-1 Pa, the power of the vacuum sputtering is 50-120 W, the time of the vacuum sputtering is 2-5 h, and the temperature of the electric core base during the vacuum sputtering process is 200-300 DEG C.

4. The method for recovering nitrogen and phosphorus in wastewater according to claim 2 or 3, characterized by, The electric core base comprises an iron-carbon alloy, a titanium alloy, a copper alloy, a nickel alloy or an aluminum alloy.

5. The method of claim 4, wherein the method is characterized by, The magnesium ore comprises periclase, magnesite or serpentine, and the particle size of the magnesium ore is 3-7 mm; The mass content of magnesium carbonate in the magnesium ore is greater than or equal to 25%; The mass-volume ratio of the magnesium ore and the anolyte is 25-35 g: 1 L.

6. The method of claim 5, wherein the method is characterized by, The anolyte is a sodium sulfate solution, and the concentration of the sodium sulfate solution is 45-60 mmol / L.

7. The method for recovering nitrogen and phosphorus in wastewater according to claim 5 or 6, characterized by, The anode reaction chamber further comprises a titanium mesh pocket, the titanium mesh pocket is a cylindrical shape with an open upper portion, and the titanium mesh pocket is suspendedly arranged in the anode reaction chamber; The anode is arranged at the center of the titanium mesh pocket, the magnesium ore is arranged at the bottom of the titanium mesh pocket, and the anode does not contact the magnesium ore.

8. The method of claim 7, wherein the method is characterized by, The pore size of the titanium mesh pocket is 30-80 mesh.

9. The method of claim 8, wherein the method is characterized by, Step 2) the current density of the electrolysis is 5-14 A / m 2 the electrolysis time is 180-300 min, and the electrolysis temperature is 25-30°C.

10. The method of claim 1, wherein the method is characterized by, The wastewater is nitrogen and phosphorus-containing wastewater, the concentration of phosphorus is greater than or equal to 70 mmol / L, the concentration of nitrogen is greater than or equal to 70 mmol / L, and the molar ratio of nitrogen to phosphorus is 1:0.8-1.2; The nitrogen in the wastewater comprises ammonia nitrogen and nitrite nitrogen.

Images