A method for removing nitrate from water bodies by using manganese-promoted layered metal oxide electrode membrane capacitive deionization

Abstract

This invention discloses a method for removing nitrates from water using manganese-induced layered metal oxide electrode membrane capacitive deionization, relating to the field of water treatment technology. The key technical point is: based on CuAl-LMO, Mn... x CuAl-LMO materials were used in membrane capacitor deionization (MCDI) desalination. Studies showed that when the Mn / Cu / Al ratio was 0.2:2.0:1.0, Mn... x CuAl-LMO has the highest specific capacitance (1695.15 F·g). ‑1 When the voltage is 1.2V and the initial KNO3 concentration is 2000 mg·L⁻¹ ‑1 At that time, Mn 0.2 The maximum salt adsorption capacity of CuAl-LMO can reach 115.74 mg·g. ‑1 The combined electrochemical and physical characterization results show that appropriate Mn doping of CuAl-LMO can effectively improve the structural properties of CuAl-LMO, thereby enhancing the desalination effect of MCDI. This provides a theoretical basis for layered metal oxides to be used as MCDI electrodes in practical applications.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of water treatment, more particularly, it relates to a method for removing nitrate in water by using manganese to promote layered metal oxide electrode film capacitive deionization. BACKGROUND

[0002] Excessive concentration of nitrate in water has a huge threat to the ecological environment and human health. For the ecological environment, it can cause eutrophication, hypoxia and reduction of river biodiversity, and seriously damage the balance of aquatic ecosystems. If too much nitrate is ingested by the human body, it can cause blue baby syndrome and increase the risk of cancer. In addition, excessive intake of nitrate can cause other serious chronic diseases, such as cardiovascular disease, diabetes and thyroid disease, etc.

[0003] Membrane capacitive deionization (MCDI) technology is an effective means for removing NO3 - based on electroadsorption and electrocatalysis. The process is to apply a voltage (generally not more than 2.0V) between the electrodes, and under the action of electric field force and concentration gradient, anions and cations in the solution migrate to the two poles, form a double layer on the electrode surface to remove ions, and then short circuit or reverse, so that the electrode is regenerated for the next cycle. MCDI is a membrane composite electrode formed by adding anion (cation) exchange membrane on the surface of the anode (cathode) electrode of the traditional capacitive deionization (CDI) device, which can effectively avoid the occurrence of the same ion exclusion effect of the CDI device, thereby significantly improving the current efficiency of MCDI to about 4 times that of CDI. Therefore, compared with the traditional CDI technology, it has many advantages such as high efficiency, energy saving and more times of electrode utilization.

[0004] The key to the effect of MCDI technology is the electrode material. The ideal MCDI electrode material should have a large specific surface area, uniform pore size, excellent electrical conductivity, hydrophilicity and cyclic stability, etc. Layered mixed metal oxides (LMOs) are obtained by calcining layered double hydroxides (LDHs). LMOs retain the high active surface area of LDHs and can reconstruct the layered structure by adsorbing anions in water. Due to its excellent performance such as positive charge supplement effect, pseudo-capacitance effect and large specific surface area, it has become a new hotspot in the research and application of MCDI electrode materials in recent years.

[0005] CuAl-LMO has the performance of high-efficiency electroadsorption of NO3 - In recent years, LDH-based mixed metal oxides as electrode materials have shown excellent electroadsorption performance in the application of MCDI. At the same time, studies have shown that LMO loaded with Pd / Cu catalyst can efficiently adsorb NO3 -Among various Faraday electrode materials, MnO2 is proved to be an extremely attractive electrode material for water desalination due to its customizable crystal structure, low cost, low water toxicity, high water environmental stability, high electrochemical activity and high theoretical specific capacity. Mn-doped CuAl-LMO can effectively improve the structural properties of CuAl-LMO (expand the interlayer spacing and increase the active sites), make the surface pores tend to microporous / mesoporous, increase the adsorption contact interface of NO3 - , thereby increasing the adsorption amount of NO3 - , expanding the interlayer spacing of CuAl-LMO and enhancing the adsorption amount of MCDI. SUMMARY

[0006] The purpose of the present application is to provide a method for removing nitrate in water body by using manganese-promoted layered metal oxide electrode film capacitive deionization, which realizes the purpose of removing nitrate in water body by preparing manganese-promoted layered metal oxide electrode and using it for MCDI.

[0007] The above technical purpose of the present application is realized by the following technical scheme: a method for removing nitrate in water body by using manganese-promoted layered metal oxide electrode film capacitive deionization, comprising the following steps:

[0008] S1. Preparing manganese-promoted layered metal oxide electrode material;

[0009] S2. Preparing the electrode material into an electrode;

[0010] S3. Building a MCDI system;

[0011] S4. Removing nitrate in water body by the MCDI system.

[0012] The present application is further provided that: the specific operation of preparing manganese-promoted layered metal oxide electrode material is as follows:

[0013] (1) Mn x Preparation of CuAl-LDH: 9.00 mmol of Cu(NO3)2·3H2O, 4.50 mmol of Al(NO3)3·9H2O, 0.23-1.35 mmol of MnCl4·4H2O, 12.00 mmol of urea, 15 mL of dimethyl sulfoxide and 45 mL of deionized water are dissolved in a 100 mL beaker, magnetically stirred uniformly, the solution is transparent and uniform, transferred into a 100 mL hydrothermal reactor, reacted in a 110℃ oven for 12 h, after the reaction is completed, the precipitate is filtered, washed with deionized water until neutral, vacuum dried at 90℃ for 6 h, and Mn x CuAl-LDH is obtained.

[0014] (2) Preparation of Mn x CuAl-LMO: under vacuum conditions, Mnx CuAl-LDH was placed in a tube furnace, heated to 200℃ at 5℃ / min, then heated to 300℃ at 2℃ / min, and kept for 1h, to obtain Mn x CuAl-LMO, namely manganese-promoted layered metal oxide electrode material.

[0015] The application is further provided that: the specific operation of preparing the electrode material into an electrode is as follows:

[0016] The prepared electrode material, acetylene black and 10g / L PVDF aqueous solution are mixed in a ratio of 8:1:1 to prepare a viscous electrode slurry; after uniform stirring, the slurry is evenly coated on a titanium mesh by using a doctor blade, dried in a 45℃ vacuum oven for 1h, dried at 110℃ for 2h, and then sealed for 12h to obtain Mn x CuAl-LMO electrode.

[0017] The application is further provided that: the specific operation of preparing the electrode material into an electrode is as follows: x The active material mass of the CuAl-LMO electrode before use is 0.28±0.04g, and the specific operation of preparing the electrode material into an electrode is as follows: x The optimal value of x in the CuAl-LMO electrode is 0.2.

[0018] The application is further provided that: the MCDI system is composed of a beaker, a peristaltic pump, a MCDI module unit, a constant-current constant-voltage power supply and an online conductivity meter.

[0019] The MCDI module unit is composed of two PVC plastic end plates, an anode electrode, a rubber gasket, a cation exchange membrane, an anion exchange membrane, a rubber gasket and a cathode electrode.

[0020] The application is further provided that: the anode electrode is Mn x CuAl-LMO electrode, and the cathode electrode is an activated carbon (AC) electrode.

[0021] The application is further provided that: the specific operation of removing nitrate in a water body by the MCDI system is as follows:

[0022] The solution containing NO3 -1 in the beaker is pumped to the bottom of the MCDI module unit by a peristaltic pump with a flow rate of 20mL·min - , and then discharged from the top of the MCDI module unit, and finally pumped into the beaker, and a constant voltage is applied to the MCDI module unit by using a constant-current constant-voltage power supply; the conductivity of the solution containing NO3 - is monitored in real time by using a conductivity meter, and the salt adsorption amount, i.e. the removed nitrate content, is calculated.

[0023] The application is further provided with a calculation formula of the salt adsorption amount:

[0024]

[0025] SAC is the salt adsorption amount of the electrode (mg·g -1 ), C0 and C t are the initial and saturated adsorption concentrations of the KNO3 solution (mg·L -1 ), V is the solution volume (L), and m is the total mass of the effective electrode (g).

[0026] The application is further provided with an optimal value of the constant voltage, which is 1.2 V.

[0027] In summary, the application has the following beneficial effects:

[0028] (1) Mn is doped in CuAl-LMO to improve the structural characteristics, so as to improve the salt adsorption capacity of the MCDI system. Compared with CuAl-LMO, the introduction of Mn expands the interlayer spacing of CuAl-LMO, increases the active sites, improves the electrochemical performance, and greatly improves the desalination effect.

[0029] (2) Mn x The improvement mechanism of CuAl-LMO for the desalination performance of CuAl-LMO lies in the improvement of the structural characteristics and the improvement of the electrochemical performance. The incorporation of Mn into LMO can make the surface pores of LMO tend to be microporous / mesoporous, which is suitable for MCDI ion adsorption, can increase the adsorption contact interface of salt ions, and can expand the interlayer spacing of LMO and increase the accessible interface of salt ions. In addition, the rich multivalent state of Mn can effectively improve the electrochemical performance of the material by affecting the electronic properties, thereby enhancing the salt adsorption capacity.

[0030] (3) After doping Mn in CuAl-LMO, the specific capacitance is increased and the ion diffusion resistance is reduced. When the ratio of Mn / Cu / Al is 0.2:2.0:1.0, the specific capacitance of Mn 0.2 CuAl-LMO has a higher specific capacitance (1695.15 F·g -1 ), excellent pseudo-capacitance performance and good capacitance reversibility, and exhibits excellent MCDI desalination performance by inserting anions to reconstruct the layered LDHs structure.

[0031] (4) When the applied voltage is 1.2 V and the initial KNO3 solution concentration is 2000 mg·L -1 , the salt adsorption capacity of Mn 0.2 CuAl-LMO can reach 115.74 mg·g -1Its excellent MCDI desalination performance makes it a promising MCDI desalination electrode material, providing a new approach for wastewater desalination. Attached Figure Description

[0032] Figure 1 This is a diagram of the MCDI desalination system and its components according to the present invention;

[0033] Figure 2 (a) At a voltage of 1.2V and an initial KNO3 concentration of 500 mg·L⁻¹ -1 At that time, CuAl-LMO and Mn x MCDI adsorption curves of CuAl-LMO and (b) CuAl-LMO and Mn x The MCDI Ragone plot of CuAl-LMO (c) shows the voltage at 1.2 V and the initial KNO3 concentration at 1000 mg·L⁻¹. -1 At that time, CuAl-LMO and Mn x MCDI adsorption curves of CuAl-LMO and (d)CuAl-LMO and Mn x MCDI Ragone plot of CuAl-LMO;

[0034] Figure 3 In (a) the voltage range is 0.8-2.0V, and the initial KNO3 concentration is 2000 mg·L⁻¹. -1 At that time, CuAl-LMO and Mn x MCDI adsorption curves of CuAl-LMO and (b) CuAl-LMO and Mn x Salt adsorption capacity diagram of CuAl-LMO;

[0035] Figure 4 (a) At a voltage of 1.2V, the initial KNO3 concentration ranged from 250 to 2000 mg·L⁻¹. -1 At that time, Mn 0.2 SAC curves of CuAl-LMO and (b)Mn 0.2 MCDI Ragone plot of CuAl-LMO. Detailed Implementation

[0036] The following is in conjunction with the appendix Figures 1-4 The present invention will be described in further detail below.

[0037] Example 1: Constructing an MCDI system with a CuAl-LMO electrode as the anode

[0038] Preparation of CuAl-LDH: 9.00 mmol of Cu(NO3)2·3H2O, 4.50 mmol of Al(NO3)3·9H2O, 12.00 mmol of urea, 15 mL of dimethyl sulfoxide and 45 mL of deionized water were dissolved in a 100 mL beaker, magnetically stirred until uniform, the solution was transparent and uniform, transferred into a 100 mL hydrothermal reactor, reacted in a 110°C oven for 12 h, after the reaction was completed, the precipitate was filtered, washed with deionized water until neutral, and then vacuum dried at 90°C for 6 h.

[0039] Preparation of CuAl-LMO: under vacuum conditions, CuAl-LDH was heated to 200°C at a rate of 5°C / min in a tube furnace, then heated to 300°C at a rate of 2°C / min, and kept for 1 h to obtain CuAl-LMO.

[0040] The prepared CuAl-LMO electrode material, acetylene black and 10 g / L PVDF aqueous solution were mixed in a ratio of 8:1:1 to prepare a viscous electrode slurry; after uniform stirring, the slurry was evenly coated on a titanium mesh by using a doctor blade, dried in a 45°C vacuum oven for 1 h, dried at 110°C for 2 h, and then sealed to obtain a CuAl-LMO electrode.

[0041] The prepared electrode was used to build a MCDI system, which consisted of a beaker, a peristaltic pump, a MCDI module unit, a constant current and constant voltage power supply and an online conductivity meter; the MCDI module unit consisted of two PVC plastic end plates, an anode electrode, a rubber gasket, a cation exchange membrane, an anion exchange membrane, a rubber gasket and a cathode electrode. The anode electrode in the system was a CuAl-LMO electrode, and the cathode electrode was an activated carbon (AC) electrode.

[0042] Example 2: a MCDI system was built with Mn x MCDI system with CuAl-LMO electrode as anode

[0043] Mn x Preparation of CuAl-LDH: 9.00 mmol of Cu(NO3)2·3H2O, 4.50 mmol of Al(NO3)3·9H2O, 12.00 mmol of urea, 15 mL of dimethyl sulfoxide and 45 mL of deionized water were dissolved in a 100 mL beaker, magnetically stirred until uniform, the solution was transparent and uniform, transferred into a 100 mL hydrothermal reactor, reacted in a 110°C oven for 12 h, after the reaction was completed, the precipitate was filtered, washed with deionized water until neutral, and then vacuum dried at 90°C for 6 h.

[0044] Mn x Preparation of CuAl-LMO: under vacuum conditions, Mn xCuAl-LDH was heated to 200℃ at a rate of 5℃ / min, then heated to 300℃ at a rate of 2℃ / min, and kept for 1h to obtain Mn x CuAl-LMO.

[0045] The prepared Mn x CuAl-LMO electrode material, acetylene black and 10g / L PVDF aqueous solution were mixed in a ratio of 8:1:1 to prepare a viscous electrode slurry; after uniform stirring, the slurry was evenly coated on the titanium mesh by using a doctor blade, dried in a vacuum oven at 45℃ for 1h, dried at 110℃ for 2h, and then sealed for 12h to obtain Mn x CuAl-LMO electrode. The active material mass of the electrode before use was 0.28±0.04g.

[0046] The prepared electrode was used to build a MCDI system, which consisted of a beaker, a peristaltic pump, a MCDI module unit, a constant current and voltage power supply and an online conductivity meter; the MCDI module unit consisted of two PVC plastic end plates, an anode electrode, a rubber gasket, a cation exchange membrane, an anion exchange membrane, a rubber gasket and a cathode electrode. The anode electrode in the system was Mn x CuAl-LMO electrode, and the cathode electrode was an activated carbon (AC) electrode.

[0047] Example 3: MCDI desalination experiment

[0048] The MCDI system with CuAl-LMO electrode as anode built in Example 1 and Example 2 and the MCDI system with Mn x CuAl-LMO electrode as anode were used to remove nitrate respectively. Mn x CuAl-LMO electrode with x being 0.05, 0.10, 0.20 and 0.30 respectively to build a MCDI system.

[0049] The KNO3 solution with a concentration of 500mg·L -1 was pumped into the bottom of the MCDI module unit by a peristaltic pump with a flow rate of 20mL·min -1 , then discharged from the top of the MCDI module unit, and finally pumped into the beaker. A constant voltage of 1.2V was applied to the MCDI module unit by using a constant current and voltage power supply; it was found that all the prepared electrode devices rapidly decreased within 10min, which indicated that the electrode had excellent adsorption performance and could quickly adsorb salt ions onto the relatively charged electrode. As shown in Figure 2 (a) and (b), with the extension of adsorption time, the conductivity value of the KNO3 solution gradually decreased, and the curve flattened at 60min, at which time the electrode was in an electrosorption equilibrium state. CuAl-LMO, Mn 0.05CuAl-LMO, Mn 0.1 CuAl-LMO, Mn 0.2 CuAl-LMO and Mn 0.3 The salt adsorption capacity (SAC) of the CuAl-LMO electrode was 29.74 mg·g. -1 32.39 mg·g -1 35.77 mg·g -1 36.8 mg·g -1 31.26 mg·g -1 Therefore, it can be seen that Mn 0.2 CuAl-LMO has the largest salt adsorption capacity, although Mn 0.05 CuAl-LMO, Mn 0.1 CuAl-LMO and Mn 0.3 CuAl-LMO SAC has no Mn 0.2 The values ​​of CuAl-LMO are high, but all are higher than CuAl-LMO, indicating that doping CuAl-LMO with Mn has a positive effect on improving the desalination performance of MCDI.

[0050] The voltage was set to 1.2V to increase the initial KNO3 solution concentration to 1000 mg·L⁻¹. -1 The material was subjected to MCDI testing. For example... Figure 2 As shown in (c) and (d), the conductivity of all prepared electrode devices decreased rapidly within 10 min. With increasing adsorption time, the conductivity gradually decreased, and the curve flattened at 60 min, indicating that the electrode was in electro-adsorption equilibrium. CuAl-LMO, Mn 0.05 CuAl-LMO, Mn 0.1 CuAl-LMO, Mn 0.2 CuAl-LMO and Mn 0.3 The salt adsorption capacity (SAC) of the CuAl-LMO electrode was 36.51 mg·g⁻¹. -1 46.54 mg·g -1 48.56 mg·g -1 51.07 mg·g -1 and 39.57 mg·g -1 Experimental results show that increasing the solution concentration increases the salt adsorption capacity of the material, and Mn x The SAC of CuAl-LMO is higher than that of CuAl-LMO and Mn 0.2CuAl-LMO still has the largest salt adsorption capacity. It is proved again that doping Mn in CuAl-LMO can expand the LMO interlayer spacing, increase the accessible interface of anions, increase active sites, and enhance the electrochemical performance, thereby enhancing the salt adsorption capacity, and when the ratio of Mn / Cu / Al is 0.2:2.0:1.0, the desalination performance is the best.

[0051] In order to further study the effect of different voltages and solution concentrations on the Mn 0.2 CuAl-LMO electrode MCDI desalination performance. The voltage range is set to 0.8-2.0V, and the KNO3 solution concentration range is 250-2000mg·L -1 MCDI test is carried out. As shown in Figure 3 (a), the voltage range is set to 0.8-2.0V, and the initial KNO3 solution concentration is 2000mg·L -1 , the curve in the figure sharply decreases within 10min, and reaches a stable state at 150min. As shown in Figure 3 (b), with the increase of applied voltage, the Mn 0.2 The salt adsorption capacity (SAC) of CuAl-LMO increases from 88.9mg·g -1 to 115.74mg·g -1 , but when the voltage exceeds 1.6V, the salt adsorption capacity (SAC) decreases obviously. This is because the electrochemical reduction of the material mainly follows the mechanism of H* interatomic indirect reduction, when the voltage exceeds 1.6V, strong side reactions occur, i.e. water electrolysis; at the same time, sacrificial anode reaction occurs, and the electrode material begins to dissolve, which hinders the progress of MCDI reaction. Therefore, 1.2V is the optimal voltage of Mn 0.2 CuAl-LMO in MCDI desalination.

[0052] As shown in Figure 4 (a), the voltage is set to 1.2V, and the KNO3 solution concentration is 250mg·L -1 , 500mg·L -1 , 1000mg·L -1 and 2000mg·L -1 , the SAC of Mn 0.2 CuAl-LMO is 33.73mg·g -1 , 36.80mg·g -1 , 51.07mg·g -1 and 115.74mg·g -1 . From Figure 4As can be seen from (b), the curve increases to the right, which means that the higher the solution concentration, the higher the adsorption capacity and removal rate of KNO3, and the easier the formation of EDLs. The higher the solution concentration, the faster the migration speed of salt ions to the inter-electrode spacing channel. However, when the increase of ions in the solution is much larger than the adsorption capacity, the ion adsorption in the pores is saturated, resulting in a decrease in removal efficiency.

[0053] The specific embodiments are only an explanation of the present application, which is not a limitation of the present application. Those skilled in the art can make modifications to the embodiments without creative contribution after reading the specification, as long as the modifications are within the scope of the claims of the present application.

Claims

1. A method for removing nitrates from water using manganese-induced layered metal oxide electrode membrane capacitance deionization, characterized in that: Includes the following steps: S1. Preparation of manganese-promoted layered metal oxide electrode materials: (1) Mn x Preparation of CuAl-LDH: Weigh 9.00 mmol Cu(NO3)2·3H2O, 4.50 mmol Al(NO3)3·9H2O, 0.23-1.35 mmol MnCl4·4H2O, 12.00 mmol urea, 15 mL dimethyl sulfoxide, and 45 mL deionized water and dissolve them in a 100 mL beaker. Stir magnetically until homogeneous and transparent. Transfer the solution to a 100 mL hydrothermal reactor and react in an oven at 110 °C for 12 h. After the reaction, filter the precipitate, wash with deionized water until neutral, and then vacuum dry at 90 °C for 6 h to obtain MnCl4·4H2O. x CuAl-LDH; (2) Mn x Preparation of CuAl-LMO: Under vacuum conditions, Mn x CuAl-LDH was placed in a tube furnace and heated to 200°C at a rate of 5°C / min, then heated to 300°C at a rate of 2°C / min and held for 1 h to obtain Mn. x CuAl-LMO, or manganese-promoted layered metal oxide electrode material; S2. Fabricating electrodes from electrode materials: The prepared electrode material, acetylene black, and 10 g / L PVDF aqueous solution were mixed in a ratio of 8:1:1 to prepare a viscous electrode slurry. After uniform stirring, the slurry was evenly coated onto a titanium mesh using a scraper, dried in a vacuum oven at 45℃ for 1 h, then dried at 110℃ for 2 h, and finally sealed and stored to obtain Mn. x CuAl-LMO electrode; S3. Set up the MCDI system; S4. Remove nitrates from water using the MCDI system.

2. The method for removing nitrates from water using manganese-induced layered metal oxide electrode membrane capacitance deionization according to claim 1, characterized in that: The Mn x The active material mass of the CuAl-LMO electrode before use is 0.28 ± 0.04 g, and the Mn... x The optimal value of x in the CuAl-LMO electrode is 0.

2.

3. The method for removing nitrates from water using manganese-induced layered metal oxide electrode membrane capacitance deionization according to claim 2, characterized in that: The MCDI system consists of a beaker, a peristaltic pump, an MCDI module unit, a constant current and constant voltage power supply, and an online conductivity meter. The MCDI module unit consists of two PVC plastic end plates, an anode electrode, a rubber gasket, a cation exchange membrane, an anion exchange membrane, a rubber gasket, and a cathode electrode.

4. The method for removing nitrates from water using manganese-induced layered metal oxide electrode membrane capacitance deionization according to claim 3, characterized in that: The anode electrode is Mn x The CuAl-LMO electrode has an activated carbon (AC) cathode electrode.

5. The method for removing nitrates from water using manganese-induced layered metal oxide electrode membrane capacitance deionization according to claim 4, characterized in that: The specific operation for removing nitrates from water using the MCDI system is as follows: The NO3-containing beaker was emptied using a peristaltic pump. - The solution was pumped to the bottom of the MCDI module unit, then discharged from the top of the MCDI module unit, and finally pumped into a beaker. A constant voltage was applied to the MCDI module unit using a constant current and constant voltage power supply. The NO3 content was monitored in real time using a conductivity meter. - The amount of salt adsorbed can be calculated by measuring the conductivity of the solution, which is the amount of nitrate removed.

6. The method for removing nitrates from water using manganese-induced layered metal oxide electrode membrane capacitance deionization according to claim 5, characterized in that: The formula for calculating the salt adsorption capacity is as follows: ； SAC represents the electrode salt adsorption capacity (mg·g). -1 ), C0 and C t Each contains NO3 - The initial and adsorption saturation concentrations of the solution (mg·L) -1 V is the solution volume (L), and m is the total effective electrode mass (g).

7. The method for removing nitrates from water using manganese-induced layered metal oxide electrode membrane capacitance deionization according to claim 6, characterized in that: The optimal value of the constant voltage is 1.2V.

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