阅读说明：本技术 氯离子插层LDHs及其制备方法与应用 (Chloride ion intercalation LDHs and preparation method and application thereof ) 是由 赵春霞 张春辉 张冠腾 周郡 李天一 于 2021-08-16 设计创作，主要内容包括：本发明提供了一种氯离子插层LDHs及其制备方法与应用,该氯离子插层LDHs的制备方法包括如下步骤：(1)将氯化镍和氯化铁的混合盐溶液与氢氧化钠溶液同时滴加到反应器皿中,滴加过程控制反应pH值,滴加完毕后继续搅拌；(2)将步骤(1)所得混合溶液转移至微波合成仪,进行陈化反应；(3)反应结束后,将产物抽滤、洗涤、真空干燥。本发明采用微波辅助共沉淀双滴法成功制备了氯离子插层的镍铁LDHs(CNF-LDHs),明显简化了LDHs制备过程,缩短了制备时间,提高了硝酸盐选择吸附性能。(The invention provides chloride ion intercalated LDHs and a preparation method and application thereof, wherein the preparation method of the chloride ion intercalated LDHs comprises the following steps: (1) dripping the mixed salt solution of nickel chloride and ferric chloride and sodium hydroxide solution into a reaction vessel simultaneously, and controlling the reaction in the dripping process p H value, continuously stirring after the dropwise adding is finished; (2) transferring the mixed solution obtained in the step (1) to a microwave synthesizer for aging reaction; (3) after the reaction is finished, the product is filtered, washed and dried in vacuum. The invention successfully prepares the nickel-iron LDHs (CNF-LDHs), obviously simplifies the preparation process of LDHs, shortens the preparation time and improves the selective adsorption performance of nitrate.)

1. A preparation method of chloride ion intercalation LDHs is characterized by comprising the following steps:

(1) dripping the mixed salt solution of nickel chloride and ferric chloride and sodium hydroxide solution into a reaction vessel simultaneously, and controlling the reaction in the dripping processpH value, continuously stirring after the dripping is finished to obtain a mixed solution;

(2) transferring the mixed solution obtained in the step (1) to a microwave synthesizer for aging reaction;

(3) after the reaction is finished, the product is filtered, washed and dried in vacuum.

2. The method for preparing chloride ion intercalated LDHs as claimed in claim 1, wherein in step (1), the molar ratio of nickel chloride to ferric chloride is 3: 1-5: 1, and the reaction process is carried outpH value is 9-11;

in the step (2), the microwave reaction temperature is 100-140 ℃, and the microwave reaction time is 15-45 min.

3. The method for preparing chloride ion intercalated LDHs as claimed in claim 1, wherein in step (1), the molar ratio of nickel chloride to ferric chloride is 4:1, and the reaction process is carried outpH value is 10;

in the step (2), the microwave reaction temperature is 100 ℃, and the microwave reaction time is 15 min.

4. The method for preparing LDHs with intercalated chloride ions as claimed in claim 1, wherein in step (3), the LDHs are washed by distilled water and absolute ethyl alcohol through suction filtration, and dried in vacuum at 60 ℃ for 24 hours.

5. A preparation method of magnetic chloride ion intercalated LDHs is characterized by comprising the following steps:

(1) firstly, adding nano Fe into a reaction vessel containing deionized water₃O₄Magnetic core, ultrasonic dispersing, dripping the mixed salt solution of nickel chloride and iron chloride and sodium hydroxide solution into a reaction vessel, and controlling the reaction in the dripping processpH, continuously stirring after the dripping is finished to obtain a mixed solution;

(2) transferring the mixed solution obtained in the step (1) to a microwave synthesizer for aging reaction;

(3) after the reaction is finished, the product is filtered, washed and dried in vacuum.

6. The chloride ion intercalated LDHs prepared by the preparation method of any one of claims 1 to 5.

7. The use of the intercalated LDHs with chloride ions as described in claim 6 in the technology of adsorbing nitrate in water.

8. Use according to claim 7, wherein the adsorption conditions are: initial NO in Water₃ ^-The concentration is 10 mg/L, the dosage of the chloride ion intercalation LDHs is 1.0 g/L, the adsorption time is 30min, and the magnetic stirring speed is 600 r/min.

9. The regeneration method of chloride ion intercalated LDHs as claimed in claim 6, wherein a microwave-assisted alkali liquor regeneration method is adopted, the desorption solution is prepared from 0.01mol/L sodium chloride and 0.025-0.1 mol/L sodium hydroxide, the temperature of desorption microwave is 60-100 ℃, and the time of desorption microwave is 10-30 min.

Technical Field

The invention relates to layered double hydroxides and a preparation method and application thereof, in particular to chloride ion intercalated LDHs and a preparation method and application thereof.

Background

In recent decades, nitrogen pollution in water has become a primary concern worldwide. Many international and governmental organizations have established frameworks to reduce the level of nitrate contamination in the environment and food products, and most countries have enacted relevant control standards. At present, nitrogen in surface water in China mainly comes from point source discharge of urban sewage treatment plants and surface source discharge of a large amount of nitrogen fertilizer used by agricultural departments. The total nitrogen is an important index for measuring the water quality standard in China, is abbreviated as TN and refers to a nitrogen-containing compound existing in water, wherein the content of organic nitrogen, nitrite nitrogen and ammonia nitrogen has little influence on the total nitrogen value; the nitrate nitrogen content is decisive for the total nitrogen value. Compared with the surface water environment IV-type standard, the tail water index of the sewage treatment plant in China meets the standard, and the total nitrogen index exceeds the standard more. The research shows that: in 738 parts of underground water nitrate data collected in China, the maximum value reaches 179 mg/L, the average value is 10.5 mg/L, about 27.4% of the data exceeds the national underground water quality standard. Therefore, reducing the total nitrogen concentration of the wastewater treatment plant tail water is a major concern in reducing nitrate nitrogen concentrations.

Conventional treatment techniques for nitrate in water bodies can be broadly divided into biotechnology and physicochemical technology. The biological technology mainly comprises a biological denitrification method and an activated sludge method, wherein the biological denitrification method needs sufficient organic carbon sources, is usually difficult to reach urban wastewater and needs a complex control process and continuous monitoring; the sludge particles in the activated sludge process are susceptible to factors such as carbon-nitrogen ratio, temperature, hydraulic load, and the like. The physical and chemical techniques can be divided into ion exchange, reverse osmosis, electrodialysis, adsorption, metal reduction, catalytic reduction, electrochemical reduction and the like, wherein the adsorption method is generally considered to be a better treatment method due to the advantages of easy operation, simple design and the like.

The main types of nitrate adsorbents in the field of water treatment are: activated carbon, clay minerals, zeolite, chitosan, agricultural wastes, industrial waste fly ash, slag and the like. However, all of the above adsorbents have problems of small capacity, low selectivity, and the like, and therefore nitrate adsorbents having a large adsorption capacity and a certain selective adsorption property are urgently required.

Layered Double Hydroxides (LDHs), namely hydrotalcite compounds, have the excellent characteristics of large specific surface area, positive laminate charge, interlayer anion exchange and the like. Meanwhile, the chemical composition of the LDHs is flexible and changeable, the metal elements and interlayer guest anions can be adjusted, and the LDHs materials with different physicochemical properties can be obtained through adjustment. Compared with other adsorbents, the LDHs have rich interlayer ions and functional groups (such as-OH), can remove metal cations through electrostatic adsorption, isomorphic substitution, hydroxide surface precipitation and surface complexation, can adsorb and remove anions in water through interlayer anion exchange, and also has better adsorption performance on charged organic pollutants.

The traditional LDHs preparation method comprises two main types of coprecipitation method and hydrothermal synthesis method, generally the preparation process is complex, the time consumption is long, and the selectivity to nitrate in water is poor.

Disclosure of Invention

The invention aims to provide chloride ion intercalated LDHs and a preparation method and application thereof, and solves the problems of complex preparation process, long time consumption and poor selectivity on nitrate in water of the existing LDHs.

In order to achieve the purpose, the technical scheme of the invention is as follows:

a preparation method of chloride ion intercalated LDHs comprises the following steps:

(1) dripping the mixed salt solution of nickel chloride and ferric chloride and sodium hydroxide solution into a reaction vessel simultaneously, and controlling the reaction in the dripping processpH value, continuously stirring after the dripping is finished to obtain a mixed solution;

(2) transferring the mixed solution obtained in the step (1) to a microwave synthesizer for aging reaction;

(3) after the reaction is finished, the product is filtered, washed and dried in vacuum.

The reaction equation is as follows:

4NiCl₂ + FeCl₃ + 10NaOH + 2H₂O → Ni₄Fe(OH)₁₀Cl·2H₂O + 10NaCl。

in the step (1)The molar ratio of the nickel chloride to the ferric chloride is 3: 1-5: 1, preferably 4:1, and the reaction processpThe H value is 9-11, preferably 10.

In the step (2), the microwave reaction temperature is 100-140 ℃, preferably 100 ℃, and the microwave reaction time is 15-45 min, preferably 15 min.

And (3) respectively filtering and washing the mixture by distilled water and absolute ethyl alcohol, and drying the mixture for 24 hours in vacuum at the temperature of 60 ℃.

The invention also provides a preparation method of the magnetic chloride ion intercalated LDHs, which comprises the following steps:

(1) firstly, adding nano Fe into a reaction vessel containing deionized water₃O₄Magnetic core, ultrasonic dispersing, dripping the mixed salt solution of nickel chloride and iron chloride and sodium hydroxide solution into a reaction vessel, and controlling the reaction in the dripping processpH value, continuously stirring after the dripping is finished to obtain a mixed solution;

(2) transferring the mixed solution obtained in the step (1) to a microwave synthesizer for aging reaction;

(3) after the reaction is finished, the product is filtered, washed and dried in vacuum.

The nano Fe₃O₄The magnetic core is prepared by a microwave-solvothermal method, and the preparation process comprises the following steps: adding polyethylene glycol into ethylene glycol, heating to dissolve, adding FeCl₃·6H₂O, dissolved with stirring, and then CH is added₃COONa·3H₂O, transferring the solution into a microwave reaction tube after the solution is uniformly mixed, putting the solution into a microwave synthesizer, and setting parameters for reaction; and after the temperature is naturally cooled to below 50 ℃, taking out the microwave tube, carrying out magnetic separation under an external magnetic field, washing for multiple times by using deionized water and ethanol, removing redundant solvent of the reaction, and drying in a vacuum drying oven at 60 ℃.

The application of the chloride ion intercalation LDHs in the technology of adsorbing nitrate in water has the following adsorption conditions: initial NO in Water₃ ^-The concentration is 10 mg/L, the dosage of the chloride ion intercalation LDHs is 1.0 g/L, the adsorption time is 30min, and the magnetic stirring speed is 600 r/min. The removal rate is 60.5 percent, the adsorption capacity is 6.53 mg/g, and the method is obviously superior to a urea methodAnd (5) preparing the LDHs.

The invention also provides a regeneration method of the chloride ion intercalated LDHs, which adopts a microwave-assisted alkali liquor regeneration method, wherein the desorption solution is prepared from sodium chloride and sodium hydroxide, the desorption microwave temperature is 60-100 ℃, and the desorption microwave time is 10-30 min.

The concentration of the sodium chloride solution in the desorption solution is 0.01mol/L, the concentration of the sodium hydroxide solution is 0.025-0.1 mol/L, preferably 0.1 mol/L, and the volume ratio of the sodium chloride solution to the sodium hydroxide solution is 1: 1. The preferred desorption microwave temperature is 100 ℃ and the desorption time is 30 min.

The (magnetic) chloride ion intercalated ferronickel LDHs (CNF-LDHs) adsorbent is prepared by adopting a microwave-assisted coprecipitation double-drop method, is used for removing low-concentration nitrate in water, has excellent nitrate adsorption performance and stable performance, and can realize better regeneration.

The preparation method disclosed by the invention is simple in synthetic process, less in time consumption, capable of obviously simplifying the preparation process of LDHs, shortening the preparation time, free of high-temperature calcination and addition of organic reagents, green and efficient.

According to the prepared chloride ion intercalated nickel-iron LDHs, interlayer ions are chloride ions, bimetal is nickel-iron, the LDHs belongs to chloride ion intercalation, a good nitrate adsorption effect can be realized through interlayer chloride ion exchange and surface adsorption, the selective adsorption performance of LDHs on nitrate in water is remarkably improved, and even if the LDHs is used for removing nitrate in actual surface water, the removal rate exceeds 60%. The removal effect on nitrate is obviously better than that of the commercial chlorine type 717 anion exchange resin.

The invention also prepares magnetic chloride ion intercalation ferronickel LDHs which is combined with the reactor under the control of an external magnetic field, so that the adsorption process is simpler, the effluent does not need to be separated from the nano adsorbent, the adsorbent is not lost, and the adsorbent is convenient to replace.

Meanwhile, the invention also provides a microwave-assisted alkali liquor regeneration method of the chloride ion intercalated ferronickel LDHs, which has short desorption time and high speed, reduces the desorption time from 4 hours to 30 minutes, reduces the sodium chloride concentration from 1.0 mol/L to 0.01mol/L, and increases the regeneration rate from 61.8% to 67.4%. The improvement of the regeneration method realizes the continuous efficient green removal of the nitrate in the water by the CNF-LDHs.

Drawings

FIG. 1 is an XRD spectrum of a chlorine ion intercalation CNF-LDHs sample.

FIG. 2 is an infrared spectrum of a chlorine ion intercalation CNF-LDHs sample.

FIG. 3 is a High Resolution Transmission Electron Micrograph (HRTEM) of a sample of chloride intercalated CNF-LDHs.

FIG. 4 is a graph showing the effect of adsorption time on nitrate removal in water.

FIG. 5 is a graph of the effect of initial concentration on nitrate removal in water.

FIG. 6 is a solutionpThe influence of H value on the removal effect of nitrate in water.

FIG. 7 is a graph showing the effect of coexisting ions on nitrate removal in water, in which: experiments No. 1, No. 2, No. 3 and No. 4 respectively correspond to Cl ions coexisting in the solution^-、SO₄ ^2-、CO₃ ^2-、PO₄ ^3-No. 5 is a control experiment containing only 10 mg/L nitrate.

FIG. 8 is a graph showing the effect of the amount of the added nitrate on the removal of nitrate from water.

Fig. 9 shows water sample adsorption effects of anzhou bridge (a), south liuzhuang (b), east wind park (c) and east lake (d).

Fig. 10 is a magnetic representation of magnetic CNF-LDHs, in which: the left side is prepared magnetic CNF-LDHs, and the right side is the right side wall of the bottle body which is attracted by the magnetic adsorbent in the bottle by the external magnetic field under the action of the magnet.

FIG. 11 is an infrared spectrum of magnetic CNF-LDHs.

FIG. 12 is a HRTEM image of a sample of magnetic CNF-LDHs.

Detailed Description

In the following examples, the various procedures and methods not described in detail are conventional methods well known in the art, and the reagents used are commercially available analytically or chromatographically pure, without indication of source and specification.

TABLE 1 list of instruments used

Example 1: preparation of CNF-LDHs

Taking 0.008 mol/L nickel chloride, namely 1.9015g, according to the nickel-iron ratio of 4: 1; 0.002 mol/L ferric chloride (0.5406 g) is dissolved in 10 mL distilled water at 60 deg.C under heating and stirring, and then ultrasonically dispersed and mixed for 20 min. Preparing 1mol/L sodium hydroxide solution, namely dissolving 4 g of sodium hydroxide in 100 mL of distilled water and uniformly stirring. The mixed salt solution of ferronickel and sodium hydroxide solution was slowly added dropwise to a beaker containing 40 mL of distilled water with vigorous stirring at room temperature. The mixed salt solution of ferronickel and sodium hydroxide solution are controlled by a peristaltic pump at a dropping speed and are used in the dropping processpH meter control reactionpThe H value is stabilized at 10 until the dropwise adding of the ferronickel mixed salt solution is finished, and the peristaltic pump is closed. Continuing stirring at room temperature for 2h, transferring the mixed solution into a microwave synthesis tube, placing into a microwave synthesizer, setting the microwave temperature at 100 deg.C for 15min, and reacting; and after the temperature is reduced to below 50 ℃, taking out the microwave tube, respectively carrying out suction filtration and washing on the microwave tube by using distilled water and absolute ethyl alcohol for a plurality of times, spreading a thin layer on the precipitate in a glass plane dish, and carrying out vacuum drying for 24 hours at the temperature of 60 ℃ to obtain powdery CNF-LDHs, wherein if the CNF-LDHs are agglomerated, the CNF-LDHs are required to be placed in an agate mortar and ground into powder for a plurality of times, which indicates that the moisture in the vacuum drying process is not extracted in time.

The structural characteristics of CNF-LDHs are characterized and analyzed by XRD, FTIR and HRTEM, and the results are shown in figures 1-3.

XRD characterization was performed using an X-ray diffractometer model D8 ADVANCE, manufactured by Bruker Corporation, Germany, under the following test conditions: the radiation source is a Cu target, the voltage is 40 KV, the current is 40 mA, the incident ray wavelength lambda =0.15418 nm, the scanning range (2 theta) is 10-90 degrees, the testing speed is 0.1 sec/step, and the scanning step is 0.02 degrees. As can be seen from fig. 1, the CNF-LDHs diffraction peak has a sharp peak shape, and 3 diffraction peaks with relatively large diffraction intensity appear in the low diffraction region, which correspond to the characteristic diffraction peaks of the (003), (006) and (012) crystal planes of the layered structure, respectively, and a characteristic peak of the hydrotalcite-like structure appears at about 60 °.

FTIR characterization Using Thermo FiThermo Nicolet iS5 from sher corporation was measured to measure the wavenumber range: 4000 cm^-1~400 cm^-1The sample is prepared by a KBr tabletting method, the mixing ratio is 1:200, and the sample is ground and tabletted. As can be seen from FIG. 2, the distance is 3439.92 cm^-1The near infrared band can be attributed to OH stretching vibration of brucite-like layer and interlayer water, at 1634.70 cm^-1The strong absorption at (a) is due to the hydroxyl deformation mode of the interlayer water molecules, and these results indicate that: the sample contained OH groups, water molecules were present in the intermediate layer, and the sample was at 657.27 cm^-1Is represented by Cl^-The special strong absorption peak of the type ferronickel LDHs is 1366.54 cm^-1Nearby CO appears₃ ^2-But not of a significant intensity, due to the CO contained in the solution during the preparation process₃ ^2-Impurities are inevitably inserted between the CNF-LDHs layers.

HRTEM characterization was tested using a field emission transmission electron microscope model Tecnai G2F 20, manufactured by FEI corporation, usa, under the following test conditions: acceleration voltage 200 KV, dot resolution 0.24 nm, information resolution: 0.14 nm, electron gun energy resolution: less than or equal to 0.7 eV. FIG. 3 is an HRTEM image of CNF-LDHs in which the crystals of LDHs are in the form of flakes having a transverse dimension of less than 50 nm. Because the LDHs crystals prepared by the coprecipitation method are aggregated, the size distribution of the sample is relatively wide. The median diameter was about 25 nm, which is essentially consistent with HRTEM observations. The prepared CNF-LDHs has obvious layered structure.

Example 2: preparation of CNF-LDHs

The ferronickel ratio is 3: 1; is used in the dropping processpH meter control reactionpThe H value is stabilized at 9; setting the microwave temperature at 120 ℃ for 30min to carry out reaction.

The rest of the procedure was the same as in example 1.

Example 3: preparation of CNF-LDHs

The ferronickel ratio is 5: 1; is used in the dropping processpH meter control reactionpThe H value is stable at 11; setting the microwave temperature at 140 ℃ for 45min to carry out the reaction.

The rest of the procedure was the same as in example 1.

Example 4: adsorption experiments

Main influencing factors in the process of adsorbing nitrate by CNF-LDHs are researched and optimized, and the main influencing factors are compared by using a national reagent chlorine type 717 anion exchange resin, which is shown in figures 4-8.

(1) The adsorption time has a great influence on the adsorption of low-concentration nitrate in water by CNF-LDHs, the first 10 min is a rapid adsorption stage, and the slow adsorption stage is within 10 min to 30 min. Treatment of initial NO₃ ^-100 mL of water sample with the concentration of 10 mg/L, the adding amount of the water sample is 1.0 g/L, the adsorption time is 30min, the magnetic stirring rotating speed is set to be 600 r/min, the removal rate is 60.5%, and the adsorption capacity is 6.53 mg/g, which is shown in figure 4.

(2) The initial concentration of the nitrate has influence on CNF-LDHs, and when the initial concentration is 2 mg/L, the adding amount is 1.0 g/L, the adsorption time is 30min, and the magnetic stirring rotating speed is 600 r/min, the removal rate of the CNF-LDHs is up to 87.3 percent, which is shown in figure 5.

(3) The initial pH value of the solution has a great influence on the CNF-LDHs adsorption effect, and when the initial pH value of the solution is 8, the CNF-LDHs removal rate is 52%. The adsorption effect of the chlorine type 717 anion exchange resin is not greatly influenced by the initial pH value of the solution, and the removal rate is 56.9% when the pH value is 7. See fig. 6.

(4) Interfering ion Cl^-、SO₄ ^2-、CO₃ ^2-、PO₄ ^3-The effect on the adsorption effect of the chlorine type 717 anion exchange resin is large, and the effect on the CNF-LDHs for adsorbing the nitrate is shown as follows: high-valence anions have a large influence on adsorption, and low-valence anions have a small influence on adsorption. See fig. 7, where: experiments No. 1, No. 2, No. 3 and No. 4 respectively correspond to Cl ions coexisting in the solution^-、SO₄ ^2-、CO₃ ^2-、PO₄ ^3-No. 5 is a control experiment containing only 10 mg/L nitrate.

(5) When the concentration of the nitrate in the water is 10 mg/L, the removal rate of the nitrate is gradually increased along with the increase of the adding amount of the CNF-LDHs. When the adding amount is 4.0 g/L, the nitrate removal rate reaches 89.6 percent, and the removal rate is not obviously improved when the adding amount is continuously increased. The chlorine type 717 anion exchange resin shows the same trend as CNF-LDHs, and the adsorption effect on nitrate in the simulated wastewater is slightly better than CNF-LDHs. See fig. 8.

(6) Actual surface water sample treatment effect: 4 actual surface water samples are selected for processing, and are respectively selected from Anzhou Zhen Anzhou Daqiao in Anxin county, New Xiandan, Baiyang lake south Liuzhang, Baoding City lotus pool area Dongfeng park and Donghu park.

The adsorption effect of the CNF-LDHs and the chlorine type 717 anion exchange resin on the actual water samples of the anzhou bridge, the south Liuzhuang, the east wind park and the east lake park is shown in the attached drawing 9, along with the increase of the adding amount, the removal rate of the CNF-LDHs and the chlorine type 717 anion exchange resin on the nitrate in the actual water samples of the anzhou bridge and the south Liuzhuang is obviously increased, when the adding amount is 2.0 g/L, the removal rate of the nitrate in the water samples of the anzhou bridge is respectively 61.9% and 55.5%, the removal rate of the nitrate in the water samples of the south Liuzhuang is respectively 60.8% and 50.8%, the removal rate of the nitrate in the east wind park is respectively 66.7% and 56.76%, and the removal rate of the nitrate in the east lake park is respectively 64.7% and 53.3%. As various coexisting anions in an actual water sample interfere the adsorption process, the CNF-LDHs has better adsorption effect and stability compared with chlorine type 717 anion exchange resin, mainly because the CNF-LDHs has better nitrate selectivity and more stable physical property and chemical property.

Example 5: desorption experiment

(1) Alkali liquor regeneration method:

taking 100 mL of solution with the nitrate content of 10 mg/L and the CNF-LDHs dosage of 1.0 g/L, adsorbing for 30min under the experimental condition of normal-temperature magnetic stirring rotation speed of 600 r/min, taking supernatant, filtering, measuring the mass concentration of the nitrate in the solution, calculating the removal rate for one time, and performing nine groups of parallel experiments. And filtering and drying the CNF-LDHs adsorbed and saturated in the nine groups of parallel experiments, and respectively storing for later use.

When preparing the desorption solution, the volume ratio of the NaCl solution to the NaOH solution is always 1:1 (the total volume is 40, 70 and 100 mL respectively), and different desorption experiment conditions are set under the conditions: NaCl concentrations of 0.01, 0.1 and 1.0 mol/L, NaOH concentrations of 0.1, 0.5 and 1.0 mol/L, and desorption times of 2, 4 and 6 h.

And desorbing the CNF-LDHs subjected to adsorption saturation in nine groups of parallel experiments, continuously adding the CNF-LDHs into 100 mL of solution with the nitrate content of 10 mg/L for secondary adsorption after the desorption is finished, measuring the mass concentration of the nitrate in each solution after the adsorption is finished, and calculating the secondary removal rate and the regeneration rate.

When the concentration of sodium chloride is 1mol/L, the concentration of sodium hydroxide is 0.1 mol/L, the total volume of desorption solution (NaCl: NaOH =1: 1) is 100 mL, and the desorption time is 4 h, the regeneration rate of CNF-LDHs is 65.3%.

(2) The microwave-assisted alkali liquor regeneration method comprises the following steps:

taking 100 mL of solution with the nitrate content of 10 mg/L and the CNF-LDHs dosage of 1.0 g/L, adsorbing for 30min under the experimental condition of normal-temperature magnetic stirring rotation speed of 600 r/min, taking supernatant, filtering, measuring the mass concentration of the nitrate in the solution, calculating the removal rate for one time, and performing nine groups of parallel experiments. And filtering and drying the CNF-LDHs adsorbed and saturated in the nine groups of parallel experiments, and respectively storing for later use.

Different microwave desorption experimental conditions are set: the concentration of sodium chloride in the desorption solution is set to be 0.01mol/L, the desorption solution is prepared with NaOH solutions (0.025, 0.05 and 0.1 mol/L) with different concentrations according to the volume ratio of 1:1, nine groups of CNF-LDHs subjected to parallel experiment adsorption saturation are desorbed, the desorption microwave temperatures are 60, 80 and 100 ℃, and the desorption microwave time is 10, 20 and 30 min.

And continuously adding the solution into 100 mL of solution with the nitrate content of 10 mg/L for secondary adsorption after desorption is finished, measuring the mass concentration of the nitrate in each solution after adsorption is finished, and calculating the secondary removal rate and the regeneration rate.

When the concentration of NaOH is 0.1 mol/L, the microwave temperature is 100 ℃, and after desorption is carried out for 30min, the secondary removal rate of nitrate after CNF-LDHs desorption is 37.6 percent, and the regeneration rate is 67.4 percent. The microwave-assisted alkali liquor method has short desorption time and high desorption speed.

Example 6: preparation of magnetic CNF-LDHs (Fe)₃O₄@CNF-LDHs）

Preparation of nano Fe by microwave-solvothermal method₃O₄The magnetic core is prepared by the following steps: 0.7536 g of polyethylene glycol is added into 15 mL of ethylene glycol, heated and dissolved at 65 ℃, and 0.6751 g of FeCl is added under the heating of water bath₃•6H₂Dissolving with magnetic stirring, and adding 2.0260 g CH₃COONa•3H₂O, transferring the solution into a microwave reaction tube after the solution is uniformly mixed, putting the solution into a microwave synthesizer, and setting parameters for reaction; and after the temperature is naturally cooled to below 50 ℃, taking out the microwave tube, carrying out magnetic separation under an external magnetic field, washing for many times by using deionized water and ethanol, removing redundant solvent of the reaction, and drying in a vacuum drying oven at 60 ℃ for later use.

When CNF-LDHs are prepared, nano Fe is added₃O₄Adding magnetic core into the mixed solution, and generating nano Fe in the CNF-LDHs₃O₄As magnetic core to make CNF-LDHs grow on nano Fe₃O₄And the CNF-LDHs has good magnetic response. The preparation process comprises the following steps: adding 0.1 g of nano Fe₃O₄And (3) carrying out ultrasonic dispersion on the magnetic core in a beaker filled with 40 mL of deionized water for 2 min, and slowly dropwise adding a nickel-iron mixed salt solution and a sodium hydroxide solution into the beaker by adopting a peristaltic pump. The rest of the procedure was the same as in example 1.

The magnetic CNF-LDHs infrared spectrum is shown in FIG. 11 at 3424.26 cm^-1And 1631.26 cm^-1Corresponds to the vibration absorption peak of OH groups at 1362.36 cm^-1Nearby CO appears₃ ^2-Characteristic absorption peak of (1), sample at 627.35 cm^-1Is represented by Cl^-The specific strong absorption peak of the Ni-Fe LDHs has a wave number of 496.66 cm^-1Corresponding to Fe₃O₄The vibration absorption peak of the medium Fe-O bond.

A HRTEM image of the magnetic CNF-LDHs sample is shown in FIG. 12, Fe₃O₄The nanoparticles are regularly spherical and successfully coated by CNF-LDHs.

Taking prepared Fe₃O₄The @ CNF-LDHs composite material is added into a solution with the initial concentration of nitrate of 5 mg/L, the adding amount is 1.0 g/L, the mechanical stirring speed is 200 r/min, and after adsorption is carried out for 30min at normal temperature, the removal rate is 40.2%.

Comparative example 1: microwave-assisted preparation method of traditional urea method

The zinc-aluminum LDO is prepared by the following specific processes:

respectively weighing 0.006 mol of ZnCl₂And 0.002 mol of AlCl₃•6H₂O (molar ratio Zn)²⁺:Al³⁺And =3: 1) is dissolved in 20 mL of solution formed by mixing ethylene glycol and deionized water according to a certain ratio (3:7, 6:4, 8.5:1.5), the solution is stirred and dissolved, and urea is weighed according to the ratio of the concentration of urea to the concentration of total metal ions (4: 1,5:1,6: 1) and then dissolved in the mixed solution to be uniformly mixed. Transferring 20 mL of the mixed solution into a microwave synthesis tube, starting a microwave synthesizer, setting parameters such as temperature (120 ℃, 140 ℃, 160 ℃), time, stirring, power and the like, and starting operation; after the temperature is naturally cooled to below 50 ℃, taking out the microwave tube, and repeatedly pumping, filtering and washing with deionized water and absolute ethyl alcohol for a plurality of times respectively until the filtrate ispH value is neutral, the precipitate is spread in a glass plane dish to form a thin layer, and the thin layer is dried in a vacuum drying oven at 60 ℃. And (3) calcining the dried sample in a muffle furnace at 550 ℃ for 3h, and storing the zinc-aluminum LDO sample obtained after the calcination for later use.

The preparation process of the nickel-iron LDO is the same as the above.

Interlayer anions of LDHs prepared by a traditional urea method are carbonate, LDO is formed after high-temperature calcination, and nitrate in water is adsorbed by using the structure memory effect of LDHs, so that the aim of removing the carbonate is fulfilled. During the calcination process, carbonate between LDHs layers is not completely removed, and the calcination temperature is higher, so that LDHs precursors are not completely converted into LDO, but a large amount of spinel with a compact structure is generated, and CO in the air after calcination is finished₂And the nitrate enters the LDO layers to occupy adsorption sites, so that the nitrate adsorption effect is poor. When the adding amount is 1.0 g/L and the rotating speed is 180 r/min, nitrate with the initial concentration of 10 mg/L is adsorbed for 4 hours, and the average adsorption capacity is only 0.56 mg/g.