A method for electrochemically synthesizing urea and struvite using nitrogen and phosphorus pollutants and waste gas

Abstract

The present invention discloses a method for electrochemically and synchronously synthesizing urea and struvite using nitrogen and phosphorus pollutants and waste gas, and belongs to the technical field of sewage and waste gas treatment. 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 into an inorganic state 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 performing 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 to urea, as well as the formation of struvite precipitation. The specific steps include: preparing a strong reducing cathode and performing 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.

Description

Technical Field

[0001] The invention belongs to the technical field of sewage and waste gas treatment, and particularly 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, deteriorate water quality, and damage aquatic ecosystems. Nitrogen is converted into toxic nitrites in the human body, leading to not only hypoxia and asphyxiation but also cancers of the nerves, bones, brain, skin, and intestines. Phosphorus can cause health problems such as liver damage and respiratory illnesses. Meanwhile, despite the rapid development of renewable energy sources such as wind and solar power, nearly 80% of energy still relies on the burning of fossil fuels, causing carbon dioxide emissions to far exceed the safe CO2 concentration limit of 350 ppm. This poses significant threats, including global warming, melting polar glaciers and flooding cities, and increasing ocean acidity, which affects the growth and reproduction of marine life.

[0003] Current technologies for treating nitrogen, phosphorus, and carbon dioxide (CO2) primarily include biological, ion exchange, dialysis, and chemical precipitation. Biological methods offer high efficiency and are environmentally friendly, but require strict control of reaction conditions and have long reaction cycles. Ion exchange and dialysis are highly efficient, but they only separate and concentrate the pollutants, leading to high subsequent treatment costs. Chemical precipitation, on the other hand, requires the addition of large amounts of chemical reagents, which can easily cause secondary pollution. Carbon dioxide treatment technologies are primarily categorized into capture and storage (CTS) and conversion. CTS faces significant leakage risks and high treatment costs. Among conversion technologies, microbial methods have long treatment cycles and are only suitable for small-scale treatment. Thermal catalytic treatment consumes a lot of energy, increasing the environmental burden. Electrocatalytic conversion of CO2 from waste gas, along with nitrogen and phosphorus from wastewater, into high-quality fertilizers—urea and struvite—would effectively address these issues. Traditional industrial urea synthesis typically involves reacting CO2 with ammonia at high temperature and pressure, resulting in low production efficiency, high energy consumption, and amplification of the greenhouse effect. Existing struvite recovery technologies also face high operating costs and energy consumption. Electrocatalytic technology can fill the gaps in existing nitrogen, phosphorus and carbon dioxide recovery. It not only removes pollutants, but also provides a new green and efficient way to synthesize high-quality fertilizers, achieving the goal of turning waste into treasure. Summary of the Invention

[0004] In response to 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. It aims 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. It is a treatment technology that kills two birds with one stone.

[0005] The present invention provides a method for electrochemically and synchronously synthesizing urea and struvite using nitrogen and phosphorus pollutants and waste gas, comprising the following steps:

[0006] Step 1: Prepare anode materials using RuO2, IrO2, and Ti4O7, and construct a double-chamber H-type electrolytic cell with graphite as the cathode; introduce nitrogen and phosphorus-containing wastewater and magnesite into the anode area, and connect the power supply to carry out the electrocatalytic oxidation reaction;

[0007] The main reaction formula in step 1 is:

[0008] Organic nitrogen → NO3 - + NO2 -

[0009] Organic phosphorus → PO4 3-

[0010] Magnesite → Mg 2+ ;

[0011] 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 the new anode. Exhaust gas containing carbon dioxide is introduced into the cathode area. The positive and negative electrodes are exchanged and a power supply is connected to perform an electrocatalytic reduction reaction to obtain urea and struvite.

[0012] The main reaction formula in step 2 is:

[0013] 2NO3 - + CO2+ 18H + + 16e - → NH2CONH2 (urea) + 7H2O

[0014] CO2+ 2NO2 - + 14H + + 12e - → NH2CONH2 (urea) + 5H2O

[0015] NH4 + + Mg 2+ + PO4 3- + 6H2O → Mg(NH4)PO4·6H2O (struvite).

[0016] Furthermore, the specific method for preparing the anode material with RuO2, IrO2 and Ti4O7 in step one is: using nickel foam as the base 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-like state, and then adding a conductive glue to coat the treated titanium plate to obtain RuO2, IrO2 and Ti4O7 nickel foam anode materials.

[0017] Furthermore, the double-chamber H-type electrolytic cell in step 1 uses 0.1 mmol / L Na2SO4 as the electrolyte, and the cathode region and the anode region are separated by a proton exchange membrane; the ratio of organic nitrogen to phosphorus in the nitrogen- and phosphorus-containing wastewater is 5:1.

[0018] 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 connecting to the DC power supply, the strong oxidizing anode is used to oxidize nitrogen and phosphorus in the sewage into inorganic forms such as NO3 as much as possible through 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 encourage magnesite to continuously precipitate Mg into the solution 2+ , creating a microenvironment for the formation of struvite.

[0019] 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 sodium carbonate solution to obtain a suspension, filter, wash, dry and then calcine to obtain the metal M-loaded In(OH)3 nanomaterial.

[0020] 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.

[0021] Furthermore, in step 2, the electrocatalytic reduction reaction is enhanced by suppressing side reactions through pulse potential and constant potential, with multiple potentials ranging from -0.3 to -0.9 V vs. RHE, and the pH of the anode solution is adjusted to 9 through pulse potential.

[0022] The electrocatalytic reduction device of the present invention requires only a simple modification of the existing electrocatalytic oxidation device: the RuO2, IrO2, and Ti4O7 electrodes are replaced with metal M-loaded In(OH)3 nanomaterials to form a new cathode, and the chamber is transformed from the original anode region to the cathode region. The graphite plate serves as the new anode, and the chamber is transformed from the original cathode region to the anode region. The positive and negative voltage positions are swapped to complete the device modification, effectively saving equipment costs. Exhaust gas containing carbon dioxide is introduced into the cathode region, and after connecting to a DC power supply, the nitrate, nitrite, and carbon dioxide gas in the wastewater after electrocatalytic oxidation are coupled as raw materials to form urea under the action of the current. The pH of the solution in the anode region is adjusted to 8-10 through pulsed potential, which promotes the magnesium ions precipitated in the magnesite to react with ammonia nitrogen and phosphate to form struvite precipitates.

[0023] The nitrogen source used in the electrocatalytic synthesis of urea in the process of the present invention is nitric nitrogen obtained through 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. The electrocatalytic reaction process mainly relies on the transfer of electrons to produce chemical reactions, without the need to add additional chemical agents, thus 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. Unlike 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.

[0024] To address the two major challenges of nitrogen and phosphorus pollution in water and the greenhouse effect, the present invention designs an electrocatalytic process as follows: The first part is the electrocatalytic oxidation stage: a strongly oxidizing anode is used to oxidize nitrogen and phosphorus in wastewater to an inorganic state as much as possible, while promoting the release of magnesium ions. The specific steps include preparing a strongly oxidizing anode, constructing an electrolytic cell, and conducting the electrocatalytic oxidation process. The second part is the electrocatalytic reduction stage: a strongly reducing cathode is used to couple nitrate nitrogen, nitrite nitrogen, and industrial waste gas (primarily containing CO2) in the oxidized wastewater with urea, thereby generating struvite precipitates. The specific steps include preparing a strongly reducing cathode and conducting the electrocatalytic reduction process. This electrochemical system design effectively achieves the resource conversion of nitrogen and phosphorus in wastewater and carbon dioxide in waste gas, as well as water purification.

[0025] This technology has the following advantages:

[0026] (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.

[0027] (2) Strong controllability. By choosing different electrode materials, selective control of reaction products can be achieved, which makes product generation more flexible and helps to meet different needs.

[0028] (3) Easy to operate. The equipment is simple in design and easy to operate and maintain, which reduces production and maintenance costs and improves ease of use.

[0029] (4) Efficient resource utilization. It can not only effectively remove pollutants from sewage and exhaust 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.

[0030] This invention combines high efficiency, environmental friendliness, and ease of operation, aligning with environmental protection principles. It holds great promise for development, particularly in the treatment of wastewater and waste gas. By converting pollutants into valuable products, it furthers the environmental protection principles of resource utilization, harmlessness, and reduction, contributing to the achievement of sustainable development goals. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 Figure 2 is a diagram of the electrocatalytic oxidation device.

[0032] Figure 2 Figure 2 is a diagram of an electrocatalytic reduction device.

[0033] Figure 3 This is a diagram showing the effect of RuO2 titanium-based electrode oxidizing organic nitrogen and phosphorus during the electro-oxidation process.

[0034] Figure 4 This is a diagram showing the effect of IrO2 titanium-based electrode oxidizing organic nitrogen and phosphorus during the electro-oxidation process.

[0035] Figure 5 This is a diagram showing the effect of Ti4O7 titanium-based electrode oxidizing organic nitrogen and phosphorus during the electro-oxidation process.

[0036] Figure 6 This is a graph showing the concentration of magnesium ions deposited at the anode.

[0037] Figure 7 Performance diagram of electrochemical synthesis of urea. DETAILED DESCRIPTION

[0038] To make the objects, technical solutions, and advantages of the present invention more clear, the technical solutions of the present invention will be described clearly and completely below in conjunction with the examples. Where specific conditions are not specified in the examples, the experiments were carried out under conventional conditions or the conditions recommended by the manufacturer. Where the manufacturer of the reagents or instruments is not specified, all are commercially available conventional products.

[0039] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art in the art. 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" as used herein includes any and all combinations of one or more of the associated listed items.

[0040] Example

[0041] A method for electrochemically and synchronously synthesizing urea and struvite using nitrogen and phosphorus pollutants and waste gas, comprising:

[0042] 1. Preparation of three types of RuO2, IrO2, and Ti4O7 nickel foam anode materials by coating method

[0043] Porous nickel foam was cut into the desired shape (approximately 5cm x 5cm) to serve as the electrode substrate. 10mg each of RuO2, IrO2, and Ti4O7 powders were added to a dispersant containing 20µL of distilled water and 900µL of ethanol. Ultrasonic dispersion was then performed until the mixture reached a uniform ink-like consistency. 80µL of conductive Nafion adhesive was then added and coated onto a pre-treated titanium plate. Each layer was then baked under vacuum at 200°C for 4 hours. This was repeated three times to obtain the three types of RuO2, IrO2, and Ti4O7 nickel foam anode materials.

[0044] 2. Electrocatalytic oxidation

[0045] RuO2, IrO2, and 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, and the electrolyte was 0.1 mmol / L Na2SO4. Nitrogen- and phosphorus-containing wastewater to be treated (groundwater from 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 to construct an H-type electrocatalytic oxidation electrolytic cell device ( 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 + Create an acidic environment, which prompts magnesite to continuously precipitate magnesium ions.

[0046] 3. Preparation of cathode

[0047] 3 g of In(OH)3 powder was dispersed in 10 mL of deionized water and sonicated for 20 minutes. 0.2 mol of ferric chloride hexahydrate was added and stirred, followed by the addition of 2 mL of a 0.5 mol / L aqueous solution of Na2CO3. The resulting suspension was stirred for a long time, filtered, and surface impurities were washed with deionized water. The suspension was then dried in an oven and calcined at 250°C in a H2 / Ar mixture for 1 hour to obtain Fe-supported In(OH)3 nanomaterials.

[0048] 4. Electrocatalytic Reduction

[0049] The original H-type electrocatalytic oxidation electrolytic cell device was slightly modified. 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 Industrial waste gas (primarily containing CO2) is introduced into the cathode region, and a DC power supply is connected to provide an ultra-low voltage (-0.6 V vs. RHE). Under the action of the current, nitrates, nitrites, and carbon dioxide gas in the wastewater after the second oxidation step are coupled as raw materials to form urea. The pH of the anode solution is adjusted to around 9 through pulsed potential, which promotes the magnesium ions precipitated in the magnesite to react with ammonia nitrogen and phosphate to form struvite precipitation.

[0050] During the electro-oxidation process, the acidic environment generated at the anode prompts the magnesite to continuously precipitate magnesium ions. During the electro-reduction process, the pH value of the cathode solution is adjusted to about 9 through pulse voltage, that is, positive and negative voltage cycles, which prompts magnesium ions, ammonia nitrogen and phosphate to produce struvite precipitation.

[0051] Figure 3 、 4 Figures 5 and 6 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%. This shows that RuO2, IrO2, and Ti4O7 have considerable oxidation performance in electrocatalytic oxidation of organic nitrogen and organic phosphorus. Figure 7This is a graph showing the changes in nitrogen selectivity and Faraday efficiency with voltage in the electrochemical synthesis of urea. The nitrogen selectivity does not change significantly with voltage, and the Faraday efficiency of urea shows a "sawtooth" trend with increasing 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.

[0052] The embodiments described above merely represent several preferred embodiments of the present invention. While the descriptions are relatively specific and detailed, they are not intended to limit the present invention. It should be noted that those skilled in the art will readily appreciate that the present invention is susceptible to various variations and modifications. Any modifications, equivalent substitutions, or improvements within the scope of the present invention are intended to be included within the scope of protection 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 using RuO2, IrO2, and Ti4O7, and construct a double-chamber H-type electrolytic cell with graphite as the cathode; introduce nitrogen and phosphorus-containing wastewater and magnesite into the anode area, and connect the power supply to carry out the electrocatalytic oxidation reaction; The specific method for preparing the anode material with RuO2, IrO2, and Ti4O7 in step 1 is as follows: using nickel foam as the base material of the electrode, taking equal masses of RuO2, IrO2, and Ti4O7 and adding them to a distilled water / ethanol dispersant, ultrasonically dispersing them until they are uniformly ink-like, then adding a conductive adhesive and coating them on a treated titanium plate to obtain RuO2, IrO2, and Ti4O7 nickel foam anode materials; 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 the new anode. Exhaust gas containing carbon dioxide is introduced into the cathode area. The positive and negative electrodes are exchanged and a power supply is connected to perform an electrocatalytic reduction reaction to obtain urea and struvite. The preparation method of the metal-loaded In(OH)3 nanomaterial in step 2 is as follows: ultrasonically dispersing In(OH)3 powder in deionized water, adding a metal M compound and stirring, then adding a sodium carbonate solution to obtain a suspension, filtering, washing, drying, and then calcining to obtain the metal M-loaded In(OH)3 nanomaterial; the metal M is Fe, Mn, or Co; 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: In step 1, the double-chamber H-type electrolytic cell uses 0.1 mmol / L Na2SO4 as the electrolyte, and the cathode and anode regions are separated by a proton exchange membrane; the ratio of organic nitrogen to phosphorus in the nitrogen- and phosphorus-containing wastewater is 5:

1.

3. The method for electrochemically synthesizing urea and struvite using nitrogen and phosphorus pollutants and waste gas according to claim 1, characterized in that: The calcination was carried out in a H2 / Ar mixture at a calcination temperature of 250°C.

4. 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 controlled by pulsed potential and constant potential to suppress side reactions and improve the yield of urea. The potential is multi-staged from -0.3 to -0.9 V vs. RHE, and the pH of the anode solution is adjusted to 9 by pulsed potential.

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