Method for electrochemically and synchronously synthesizing urea and struvite by utilizing nitrogen and phosphorus pollutants and waste gas

Abstract

The invention discloses a method for electrochemically and synchronously synthesizing urea and struvite by utilizing nitrogen and phosphorus pollutants and waste gas, and belongs to the technical field of sewage and waste gas treatment. The electro-catalysis process designed by the invention is as follows: the first part is an electro-catalysis oxidation stage: nitrogen and phosphorus in the sewage are oxidized into inorganic states as far as possible by using a strong oxidizing anode, meanwhile, the release of magnesium ions is promoted, and the specific steps are as follows: preparing the strong oxidizing anode, building an electrolytic tank device and carrying out the electro-catalysis oxidation process; and the second part is an electrocatalytic reduction stage: nitrate nitrogen, nitrite nitrogen and industrial waste gas (mainly containing CO2 gas) in the oxidized sewage are coupled with urea by using a strong reducing cathode, and struvite precipitate is generated, and the specific steps are as follows: preparing the strong reducing cathode, and carrying out an electrocatalytic reduction process. By means of the electrochemical system design, the purposes of resource conversion of nitrogen and phosphorus in sewage and carbon dioxide in waste gas and water purification are well achieved.

Description

Technical Field

[0001] The invention belongs to the technical field of sewage and waste gas treatment, and specifically relates to a method for electrochemically and synchronously synthesizing urea and struvite by utilizing nitrogen and phosphorus pollutants and waste gas. Background Art

[0002] Nitrogen and phosphorus pollution can cause eutrophication of water bodies, deteriorate water quality and damage aquatic ecosystems. Nitrogen is converted into toxic nitrites in the human body, which can not only cause hypoxia and suffocation, but may even cause cancer in the nerves, bones, brain, skin and intestines. Phosphorus can cause health problems such as liver damage and respiratory diseases. At the same time, despite the rapid development of renewable energy such as wind and solar energy, nearly 80% of energy still needs to be provided by burning fossil fuels, resulting in carbon dioxide emissions far exceeding the safe CO2 concentration limit of 350 ppm, causing global warming to cause the melting of polar glaciers and flooding cities, and the increase in sea acidity affecting the growth and reproduction of marine life. Huge threats.

[0003] For the three pollutants of nitrogen, phosphorus and carbon dioxide, the current nitrogen and phosphorus treatment technologies mainly include: biological method, ion exchange method, dialysis method, chemical precipitation method, etc. The biological method has high treatment efficiency and is environmentally friendly, but the reaction conditions need to be strictly controlled and the reaction cycle is long; the ion exchange method and dialysis method are efficient, but they only separate and concentrate the pollutants, and the subsequent treatment costs are high; and the chemical precipitation method requires the addition of a large amount of chemical reagents, which is easy to cause secondary pollution. Carbon dioxide treatment technology is mainly divided into two categories: capture and storage, and conversion. Capture and storage face a huge threat of leakage and high treatment costs; the microbial method in the conversion technology has a long treatment cycle and is only suitable for small-scale treatment. The thermal catalytic treatment consumes a lot of energy, which increases the environmental burden. If the carbon dioxide in the exhaust gas and the nitrogen and phosphorus in the sewage can be converted into high-quality fertilizers-urea and struvite through electrocatalysis, these problems can be effectively solved. Traditional industrial synthetic urea usually uses carbon dioxide and ammonia to react under high temperature and high pressure, which has the defects of low production efficiency, high energy consumption, and aggravated greenhouse effect. Existing struvite recovery technology also faces problems such as high operating costs and high energy consumption. Electrocatalytic technology can fill the gaps in the existing recovery of nitrogen, phosphorus and carbon dioxide. It can not only remove pollutants, but also provide a new green and efficient way to synthesize high-quality fertilizers, thus achieving the goal of turning waste into treasure. Summary of the invention

[0004] In view of the content of the background technology, the present invention provides a method for the electrochemical synchronous synthesis of urea and struvite using nitrogen and phosphorus pollutants and waste gas, aiming to use efficient and green electrochemical technology to utilize pollutants such as nitrogen and phosphorus in sewage and carbon dioxide in waste gas to synthesize high-quality fertilizers - urea and struvite, realizing the true meaning of "turning waste into treasure", and providing a new green and efficient way for the synthesis of urea and struvite, which is a treatment technology that kills two birds with one stone.

[0005] The present invention provides a method for electrochemically synthesizing urea and struvite synchronously by using nitrogen and phosphorus pollutants and waste gas, comprising the following steps: Step 1: Prepare anode materials with RuO2, IrO2, and Ti4O7, use graphite as the cathode, and construct a double-chamber H-type electrolytic cell; introduce nitrogen and phosphorus-containing wastewater and magnesite into the anode area, and connect the power supply to carry out electrocatalytic oxidation reaction; The main reaction formula in step 1 is: Organic nitrogen → NO3 - + NO2 - Organic phosphorus → PO4 3- Magnesite → Mg 2+ ; Step 2: Prepare metal-loaded In(OH)3 nanomaterials. Based on the electrolytic cell in step 1, use In(OH)3 nanomaterials to replace the anode prepared by RuO2, IrO2, and Ti4O7 as a new cathode. Graphite is used as a new anode. The waste gas containing carbon dioxide is introduced into the cathode area. The positive and negative electrodes are exchanged and the power is turned on to perform an electrocatalytic reduction reaction to obtain urea and struvite. The main reaction formula in step 2 is: 2NO3 - + CO2+ 18H + +16e - → NH2CONH2 (urea) + 7H2O CO2+ 2NO2 - + 14H + +12e - → NH2CONH2 (urea) + 5H2O NH4 + + Mg 2+ + PO4 3- + 6H2O → Mg(NH4)PO4·6H2O (struvite).

[0006] Furthermore, the specific method for preparing the anode material with RuO2, IrO2 and Ti4O7 in step one is: using nickel foam as the matrix material of the electrode, taking equal masses of RuO2, IrO2 and Ti4O7 and adding them into a distilled water / ethanol dispersant, ultrasonically dispersing them into a uniform ink state, and then adding a conductive glue to coat it on the treated titanium plate to obtain RuO2, IrO2 and Ti4O7 nickel foam anode materials.

[0007] Furthermore, in step 1, the double-chamber H-type electrolytic cell uses 0.1 mmol / L Na2SO4 as the electrolyte, and the cathode area and the anode area are separated by a proton exchange membrane; the ratio of organic nitrogen to phosphorus in the nitrogen- and phosphorus-containing wastewater is 5:1.

[0008] The present invention adopts a traditional H-type electrolytic cell, the purpose of which is to concentrate the sewage in the anode area for treatment to improve the treatment efficiency. The anode adopts three strong oxidizing materials, RuO2, IrO2 and Ti4O7, which have strong corrosion resistance, good conductivity and high electrocatalytic activity. Among them, the RuO2 electrode has the advantages of low internal resistance, the IrO2 electrode has strong stability, high reversibility and strong wear resistance, and the Ti4O7 electrode is acid- and alkali-resistant and has the widest electrochemical window. After the DC power supply is turned on, the strong oxidizing anode is used to oxidize nitrogen and phosphorus in the sewage into inorganic states such as NO3 as much as possible through the way of electron transfer. - 、NO2 - NH4 + PO4 3- It provides raw materials for the subsequent electrocatalytic reduction process to couple high-quality fertilizers, while the anode area continuously produces H + Create an acidic environment to promote the precipitation of Mg from magnesite into the solution 2+ , creating a microenvironment for the formation of struvite.

[0009] Furthermore, the preparation method of the metal-loaded In(OH)3 nanomaterial in step 2 is: ultrasonically disperse the In(OH)3 powder in deionized water, add the metal M compound and stir, then add the sodium carbonate solution to obtain a suspension, filter, wash, dry and then calcine to obtain the metal M-loaded In(OH)3 nanomaterial.

[0010] Furthermore, the calcination is carried out in a H2 / Ar mixture at a calcination temperature of 250°C; and the metal M is Fe, Mn, or Co.

[0011] Furthermore, in step 2, the electrocatalytic reduction reaction is performed by pulse potential and constant potential to suppress side reactions and improve the yield of urea, with multiple potentials of -0.3 ~ -0.9 V vs. RHE, and the pH of the anode solution is adjusted to 9 by pulse potential.

[0012] The electrocatalytic reduction device of the present invention only needs to be simply modified on the basis of the original electrocatalytic oxidation device: In(OH)3 nanomaterials loaded with metal M are used to replace RuO2, IrO2, and Ti4O7 electrodes as new cathodes, and the chamber is transformed from the original anode area to the cathode area. The graphite plate is used as a new anode, and the chamber is transformed from the original cathode area to the anode area. The position of the positive and negative voltages is exchanged to complete the modification of the device, which effectively saves equipment costs. The waste gas containing carbon dioxide is introduced into the cathode area, and after the direct current power supply is turned on, the nitrate, nitrite and carbon dioxide gas in the sewage after electrocatalytic oxidation are coupled into urea as raw materials under the action of current; the pH of the anode area solution is adjusted to 8-10 by pulse potential, so that the magnesium ions precipitated in the magnesite and the ammonia nitrogen and phosphate radicals are promoted to form struvite precipitation.

[0013] The nitrogen source used in the electrocatalytic synthesis of urea in the process of the present invention is nitric nitrogen obtained by electrocatalytic oxidation of organic nitrogen wastewater. The raw material for industrial synthesis of urea is N2, and the bond energy of N≡N is nearly five times that of N=O. The reaction process of the present invention can be carried out at room temperature and pressure, which solves the key problem of high energy consumption in the prior art. In the electrocatalytic reaction process, chemical reactions mainly occur by the transfer of electrons, and no additional chemical agents need to be added, thereby avoiding secondary pollution and having little negative impact on the environment. In addition, most nitrogen and phosphorus wastewater is in an organic state. The present invention removes organic matter by preparing a strong oxidizing anode material, quickly degrades organic pollutants into an inorganic state, and then reduces CO2 by preparing a highly selective cathode material to achieve the purpose of purifying the water body. Different from traditional treatment processes, this method not only removes toxic and harmful substances, but also converts them into recyclable resources, effectively reducing environmental pollution while realizing waste resource utilization and promoting sustainable development.

[0014] In response to the two major problems of nitrogen and phosphorus pollution in water bodies and the greenhouse effect, the electrocatalytic process designed by the present invention is as follows: the first part is the electrocatalytic oxidation stage: using a strong oxidizing anode to oxidize nitrogen and phosphorus in sewage to inorganic states as much as possible, while promoting the release of magnesium ions. The specific steps include: preparing a strong oxidizing anode, building an electrolytic cell device, and conducting an electrocatalytic oxidation process. The second part is the electrocatalytic reduction stage: using a strong reducing cathode to couple nitrate nitrogen, nitrite nitrogen and industrial waste gas (mainly containing CO2 gas) in the oxidized sewage with urea, as well as the formation of struvite precipitation. The specific steps include: preparing a strong reducing cathode and conducting an electrocatalytic reduction process. Through such an electrochemical system design, the purpose of resource conversion of nitrogen and phosphorus in sewage and carbon dioxide in waste gas and water purification can be well achieved.

[0015] This technology has the following advantages: (1) Strong environmental protection. Renewable energy (such as solar energy, wind energy and tidal energy) is used to provide electricity for electrochemical technology. No additional chemical agents are required, which significantly reduces the risk of secondary pollution and reduces the negative impact on the environment.

[0016] (2) Strong controllability. Selective control of reaction products can be achieved by choosing different electrode materials, which makes product generation more flexible and helps to meet different needs.

[0017] (3) Easy to operate. The equipment is simple in design and easy to operate and maintain, which reduces production and maintenance costs and improves convenience of use.

[0018] (4) Efficient resource utilization. It can not only effectively remove pollutants from sewage and waste gas, but also convert these pollutants into valuable chemical products such as urea and struvite. This is an effective way to "turn waste into treasure" and promote the efficient utilization of resources.

[0019] The invention combines the characteristics of high efficiency, green environmental protection and simple operation, conforms to the concept of environmental protection, and has good development prospects in the field of sewage and waste gas treatment. By converting pollutants into valuable products, it further deepens the environmental protection ideas of resource utilization, harmlessness and reduction, and contributes to the realization of sustainable development goals. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 Figure 2 is a diagram of an electrocatalytic oxidation device.

[0021] Figure 2 This is a diagram of the electrocatalytic reduction device.

[0022] Figure 3 This is a diagram showing the effect of RuO2 titanium-based electrode oxidizing organic nitrogen and phosphorus during the electro-oxidation process.

[0023] Figure 4 This is a diagram showing the effect of IrO2 titanium-based electrode oxidizing organic nitrogen and phosphorus during the electro-oxidation process.

[0024] Figure 5 This is a diagram showing the effect of Ti4O7 titanium-based electrode oxidizing organic nitrogen and phosphorus during the electro-oxidation process.

[0025] Figure 6 This is the concentration diagram of magnesium ions precipitated at the anode.

[0026] Figure 7 Performance diagram of electrochemical synthesis of urea. DETAILED DESCRIPTION

[0027] In order to make the purpose, technical scheme and advantages of the present invention clearer, the technical scheme of the present invention will be clearly and completely described below in conjunction with the embodiments. If the specific conditions are not specified in the embodiments, they are carried out according to the conventional conditions or the conditions recommended by the manufacturer. If the manufacturer of the reagents or instruments is not specified, they are all conventional products that can be purchased commercially.

[0028] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art of the present invention. The terms used in the specification of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention. The term "and / or" used herein includes any and all combinations of one or more related listed items.

[0029] Example A method for electrochemically synthesizing urea and struvite synchronously using nitrogen and phosphorus pollutants and waste gas, comprising: 1. Preparation of three types of RuO2, IrO2 and Ti4O7 nickel foam anode materials by coating method The porous nickel foam was cut into the required shape (about 5cm×5cm in size) as the base material of the electrode. 10 mg of RuO2, IrO2, and Ti4O7 powders were taken respectively, added to a dispersant containing 20 µL of distilled water and 900 µL of ethanol, and dispersed into a uniform ink state by ultrasonication. Then 80 µL of conductive glue Nafion was added and coated on the treated titanium plate. After each layer was coated, it was baked at 200°C in vacuum for 4 h. The coating was repeated three times to obtain three types of RuO2, IrO2, and Ti4O7 nickel foam anode materials.

[0030] 2. Electrocatalytic oxidation RuO2, IrO2, Ti4O7 nickel foam anode materials were used as anodes, graphite plates (5 cm × 5 cm) were used as cathodes, the cathode and anode regions were separated by a proton exchange membrane, the electrolyte was 0.1 mmol / L Na2SO4, nitrogen- and phosphorus-containing wastewater to be treated (groundwater in Zhanggong District, Ganzhou City, with organic nitrogen and phosphorus contents of approximately 80 mg / L and 17 mg / L, accounting for approximately 75% and 60% of the total nitrogen and total phosphorus, respectively) and magnesite were introduced into the anode region, and an H-type electrocatalytic oxidation electrolytic cell device was constructed ( Figure 1 ); connect direct current to provide ultra-low voltage (-0.6 V vs. RHE). Under the action of current, organic nitrogen and phosphorus in the anode area are oxidized to inorganic state, providing raw materials for subsequent coupling of high-quality fertilizers. During the electrolysis process, H + An acidic environment is created, prompting magnesite to continuously precipitate magnesium ions.

[0031] 3. Preparation of cathode 3 g of In(OH)3 powder was dispersed in 10 mL of deionized water and ultrasonicated for 20 min. 0.2 mol of ferric chloride hexahydrate was added and stirred. Then 0.5 mol / L Na2CO3 aqueous solution (2 mL) was added. The suspension was obtained, and after long-term stirring, it was filtered and the surface impurities were washed with deionized water. It was dried in an oven and finally calcined at 250°C in a H2 / Ar mixture for 1 h to obtain Fe-supported In(OH)3 nanomaterials.

[0032] 4. Electrocatalytic reduction The H-type electrocatalytic oxidation electrolytic cell device was slightly modified on the basis of the original structure. The prepared Fe-loaded In(OH)3 nanomaterials replaced the RuO2, IrO2, and Ti4O7 electrodes as the new cathode, and the chamber was transformed from the original anode area to the cathode area; the graphite plate was used as the new anode, and the chamber was transformed from the original cathode area to the anode area. The position of the positive and negative voltages was exchanged to complete the modification of the device ( Figure 2 ). The industrial waste gas (mainly containing CO2 gas) is introduced into the cathode area, and a DC power supply is connected to provide an ultra-low voltage (-0.6 V vs. RHE). Under the action of the current, the nitrate, nitrite and carbon dioxide gas in the wastewater after the second step of oxidation are coupled as raw materials to form urea; the pH of the anode solution is adjusted to about 9 by pulse potential, so that the magnesium ions precipitated in the magnesite react with ammonia nitrogen and phosphate to form struvite precipitation.

[0033] During the electro-oxidation process, the acidic environment generated at the anode causes magnesite to continuously precipitate magnesium ions. During the electro-reduction, pulse voltage, that is, positive and negative voltage cycles, are used to adjust the pH of the cathode solution to about 9, which causes magnesium ions, ammonia nitrogen and phosphate to produce struvite precipitation.

[0034] Figure 3 , 4 5 are the effects of electrocatalytic oxidation of organic phosphorus and organic nitrogen by three different anodes, RuO2, IrO2 and Ti4O7. It can be seen that RuO2 and Ti4O7 have significant oxidation efficiency for organic nitrogen and organic phosphorus. After 80 minutes of electrocatalytic oxidation, the oxidation rate reached about 90%. The oxidation efficiency of IrO2 was slightly lower, but still maintained above 70%, indicating that the three anodes, RuO2, IrO2 and Ti4O7, have certain considerable oxidation performance in electrocatalytic oxidation of organic nitrogen and organic phosphorus. Figure 7 The graph shows the change of nitrogen selectivity and Faraday efficiency with voltage in the electrochemical synthesis of urea. The change of nitrogen selectivity with voltage is not obvious. The Faraday efficiency of urea shows a "sawtooth" change trend with the increase of voltage, reaching a peak of 62% at the optimal potential of -0.6 V vs. RHE. It can be seen that the metal Fe-loaded In(OH)3 nanomaterial has excellent reduction performance.

[0035] The embodiments described above only express several preferred embodiments of the present invention, and the descriptions thereof are relatively specific and detailed, but are not intended to limit the present invention. It should be noted that for those skilled in the art, the present invention may also have various changes and modifications, and any modifications, equivalent substitutions, improvements, etc. made within the concept and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for electrochemically synthesizing urea and struvite using nitrogen and phosphorus pollutants and waste gas, characterized in that: The steps include: Step 1: Prepare anode materials with RuO2, IrO2, and Ti4O7, use graphite as the cathode, and construct a double-chamber H-type electrolytic cell; introduce nitrogen and phosphorus-containing wastewater and magnesite into the anode area, and connect the power supply to carry out electrocatalytic oxidation reaction; The main reaction formula in step 1 is: Organic nitrogen → NO3 - + NO2 - Organic phosphorus → PO4 3- Magnesite → Mg 2+ ; Step 2: Prepare metal-loaded In(OH)3 nanomaterials. Based on the electrolytic cell in step 1, use In(OH)3 nanomaterials to replace the anode prepared by RuO2, IrO2, and Ti4O7 as a new cathode. Graphite is used as a new anode. The waste gas containing carbon dioxide is introduced into the cathode area. The positive and negative electrodes are exchanged and the power is turned on to perform an electrocatalytic reduction reaction to obtain urea and struvite. The main reaction formula in step 2 is: <h2 style=";text-align:left;direction:ltr">2NO3<h2 style=";text-align:left;direction:ltr"> - <h2 style=";text-align:left;direction:ltr"> + CO2 + 18H<h2 style=";text-align:left;direction:ltr"> + <h2 style=";text-align:left;direction:ltr"> +16e<h2 style=";text-align:left;direction:ltr"> - <h2 style=";text-align:left;direction:ltr"> → NH2CONH2 + 7H2O CO2 + 2NO2 - + 14H + + 12e - → NH2CONH2 + 5H2O NH4 + + Mg 2+ + PO4 3- + 6H2O → Mg(NH4)PO4·6H2O。 2. The method for electrochemically synthesizing urea and struvite using nitrogen and phosphorus pollutants and waste gas according to claim 1, characterized in that: The specific method for preparing anode materials with RuO2, IrO2 and Ti4O7 in step one is: using nickel foam as the matrix material of the electrode, taking equal masses of RuO2, IrO2 and Ti4O7 and adding them into distilled water / ethanol dispersant, ultrasonically dispersing them into a uniform ink state, and then adding conductive glue to coat them on the treated titanium plate to obtain RuO2, IrO2 and Ti4O7 nickel foam anode materials.

3. The method for electrochemically synthesizing urea and struvite by using nitrogen and phosphorus pollutants and waste gas according to claim 1, characterized in that: Step 1: The double-chamber H-type electrolytic cell uses 0.1 mmol / L Na2SO4 as the electrolyte, and the cathode area and the anode area are separated by a proton exchange membrane; the ratio of organic nitrogen to phosphorus in the nitrogen- and phosphorus-containing wastewater is 5:

1.

4. The method for electrochemically synthesizing urea and struvite using nitrogen and phosphorus pollutants and waste gas according to claim 1, characterized in that: The preparation method of the metal-loaded In(OH)3 nanomaterial in step 2 is as follows: ultrasonically disperse the In(OH)3 powder in deionized water, add the metal M compound and stir, then add the sodium carbonate solution to obtain a suspension, filter, wash, dry and calcine to obtain the metal M-loaded In(OH)3 nanomaterial.

5. The method for electrochemically synthesizing urea and struvite using nitrogen and phosphorus pollutants and waste gas according to claim 4, characterized in that: The calcination is carried out in a H2 / Ar mixture at a calcination temperature of 250°C; the metal M is Fe, Mn, or Co.

6. The method for electrochemically synthesizing urea and struvite using nitrogen and phosphorus pollutants and waste gas according to claim 1, characterized in that: In step 2, the electrocatalytic reduction reaction is performed by pulse potential and constant potential to suppress side reactions and improve the yield of urea, with multiple potentials of -0.3 ~ -0.9 V vs. RHE, and the pH of the anode solution is adjusted to 9 by pulse potential.

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