Preparation method and application of modified hydrotalcite-like loaded nano zero-valent iron composite material

Abstract

The invention discloses a preparation method and application of a modified hydrotalcite-like compound loaded nano zero-valent iron composite material, and relates to the technical field of wastewater treatment. According to the specific technical scheme, the preparation method comprises the following steps: preparing nano silicon dioxide modified hydrotalcite-like compound (SiLDH) by adopting a coprecipitation method; the preparation method comprises the following steps: uniformly mixing SiLDH and a FeSO4. 7H2O solution in an ultrasonic manner, loading nano zero-valent iron (nZVI) on the SiLDH by adopting a NaBH4 reduction method under an anoxic condition, magnetically collecting and washing a black solid after the reaction is completed, and performing surface functionalization modification on the black solid by using glycine to obtain the glycine functionalization modified hydrotalcite loaded nano zero-valent iron composite material (Gly-SiLDH (at) nZVI). SiLDH with a large specific surface area is used as a carrier of nZVI, and glycine functionalization is used as a material surface to introduce an additional active site for adsorbing nitrate, so that the problem that nZVI is easy to agglomerate is solved, secondary pollution caused by metal modification is avoided, and the nitrate removal efficiency is greatly improved.

Description

Technical Field

[0001] The present invention relates to the technical field of wastewater treatment, and in particular to a preparation method and application of a modified hydrotalcite-like material loaded with nano-zero-valent iron composite material. Background Art

[0002] Groundwater provides drinking water for one-third of the global population, and the quality of groundwater is related to the stability of the ecosystem on the earth and the safety of organisms. With the development of industrialization and urbanization in China, the problem of groundwater pollution is becoming more and more serious. The China Ecological Environment Bulletin 2018 pointed out that nitrate (NO3 - -N) is one of the main exceeded indicators in groundwater in various regions of China. NO3 - -N in the environment mainly comes from the excessive use of agricultural nitrogen fertilizers and the intensification of industrial activities (electroplating, dyeing, tanning). The increase in its exposure will cause eutrophication of water bodies and algal blooms. Long-term human intake of NO3 - -N will increase the risk of methemoglobinemia and even cancer.

[0003] At present, the remediation technologies for groundwater NO3 - -N can be roughly divided into three categories: bioremediation technology, physicochemical remediation technology, and chemical remediation technology. Physicochemical methods such as electrodialysis, reverse osmosis, and ion exchange do not remove nitrate radicals essentially and have high costs, so they are less used in actual applications. Biological denitrification uses heterotrophic or autotrophic microorganisms to assimilate nitrate into biomass or dissimilate it into nitrogen to achieve the purpose of denitrification. Its removal effect is good, but the growth cycle of microorganisms is long, the conditions are harsh, and heterotrophic microbial denitrification requires an external carbon source, which may cause secondary pollution. Chemical methods introduce reducing agents to reduce nitrates in water to achieve the purpose of removal, and have the advantages of high removal efficiency, low cost, short repair cycle, etc., and are often used for in-situ remediation of polluted groundwater.

[0004] Nano-zero-valent iron (nZVI) has a large specific surface area, high reaction activity, strong permeability, low cost, and mild reaction conditions. It can quickly reduce nitrate directly or indirectly under acidic conditions, and about 90% of the products are ammonia nitrogen. nZVI has been used as an ideal filling material for injection wells and permeable reactive barriers and is often used for in-situ remediation of groundwater pollution sites.

[0005] However, there is a major difficulty in the nZVI technology: due to the extremely high surface energy of nZVI particles, they are prone to agglomeration and easy to oxidize to form a passivation layer in the water environment, thus limiting the application of this nano-metal material in the field of water pollution remediation.

[0006] To solve the above problems, the prior art modifies nZVI, for example, by surface coating with stabilizers, doping with a second metal, loading on the surface of porous materials, etc. Introducing stabilizers such as carboxymethyl cellulose and sodium alginate into the synthesis of nZVI particles can improve the dispersibility and mobility of nZVI in aqueous solutions, while an excess of some stabilizers may act as a shielding effect on nZVI, reducing the nitrate removal rate. Depositing a second metal catalyst such as silver, aluminum, magnesium, platinum, etc. on the surface of nZVI particles can enhance the reaction activity of nZVI, but the bimetallic materials synthesized by this method are still prone to aggregation and have a high cost. In addition, metal modification will cause secondary pollution. Uniformly distributing nZVI on the surface of some carriers can effectively reduce the aggregation of nZVI particles and increase the contact area with pollutants.

[0007] Layered double hydroxides (LDHs), also known as hydrotalcite-like compounds, have a large specific surface area and a layered structure, and the anions between their layers are exchangeable. Loading nZVI on the surface and interlayer pores of LDH materials can play a dispersing role and also have a certain adsorption capacity for anions such as nitrate, thereby efficiently removing nitrate in water. However, the large surface energy of LDH makes its surface structure very unstable and prone to aggregation, which limits the adsorption performance of LDH and its ability to provide active sites for nZVI as a carrier. Summary of the Invention

[0008] In view of the above problems, the present invention provides a preparation method and application of a modified hydrotalcite-like compound loaded with nano-zero valent iron composite material. This method uses nano-silica modified hydrotalcite as a carrier, and through nZVI loading and glycine functionalization modification, effectively reduces the aggregation phenomenon of nZVI, provides more adsorption sites for the removal of pollutants in water, avoids secondary pollution of water bodies, and realizes the efficient removal of nitrate.

[0009] The present invention is realized through the following technical solutions:

[0010] A preparation method of a modified hydrotalcite-like compound loaded with nano-zero valent iron composite material of the present invention includes the following steps: preparing nano-silica modified hydrotalcite (SiLDH) by the co-precipitation method; ultrasonically mixing SiLDH with FeSO4·7H2O solution, and under anoxic conditions, using the NaBH4 reduction method to load nano-zero valent iron (nZVI) on SiLDH. After the reaction is completed, the black solid is collected by magnetic force, washed, and surface functionalized with glycine to obtain a glycine-functionalized modified hydrotalcite-like compound loaded with nano-zero valent iron composite material (Gly-SiLDH@nZVI).

[0011] A preparation method of a modified hydrotalcite-like compound loaded with nano-zero valent iron composite material includes the following steps:

[0012] (1) Preparation of SiLDH: 1.1. Add nano-silica into deionized water, stir evenly, and then ultrasonically treat for 20 - 30 min at room temperature to form a suspension, where the average particle size of the nano-silica is 45 - 55 nm; 1.2. Add CaCl₂ and AlCl₃·6H₂O into the nano-silica suspension, stir until completely dissolved, and then, under anoxic conditions, slowly drip 3 mol / L NaOH into the reaction vessel. During the dripping process, stir at a speed of 600 - 800 rpm and continuously introduce an inert gas, controlling the pH range in the reaction vessel to be 9 - 10; 1.3. After the reaction is completed, seal the reaction vessel and continue to stir for 8 h under the condition of a 65°C water bath. Subsequently, perform centrifugal separation, washing, drying, and grinding and sieving to obtain SiLDH.

[0013] (2) Preparation of Gly-SiLDH@nZVI: 2.1. After introducing an inert gas into deionized water, add FeSO₄·7H₂O and stir until dissolved, then add the SiLDH prepared in step (1). The mass ratio of FeSO₄·7H₂O to SiLDH is 2:1. Continuously introduce an inert gas, ultrasonically treat for 20 - 30 min at room temperature, and then continue to stir at a speed of 600 - 800 rpm to make the reaction system fully mixed and uniform; 2.2. Under the continuous introduction of an inert gas and vigorous stirring, drip a 1.5 mol / L NaBH₄ solution at a speed of 1 - 2 drops / s. After the dripping is completed, continue to stir for 20 - 30 min to ensure complete reaction, and then perform magnetic collection and washing; 2.3. Add the glycine solution prepared with deoxygenated deionized water to the above-mentioned black solid, ultrasonically treat for 20 - 30 min under anoxic and room temperature conditions, perform magnetic collection, washing, drying, and grinding and sieving to obtain the Gly-SiLDH@nZVI composite material.

[0014] Preferably, in step (1), the mass ratio of the nano-silica to the amount of deionized water used is 1 - 3 g:100 mL, the mass ratio of the nano-silica to the molar amount of Al is 33.3 - 100 g:1 mol, and the molar ratio of Ca to Al is 2 - 4:1.

[0015] More preferably, in step (1), the mass ratio of the nano-silica to the amount of deionized water used is 3 g:100 mL, the mass ratio of the nano-silica to the molar amount of Al is 100 g:1 mol, and the molar ratio of Ca to Al is 3:1.

[0016] Preferably, in step (2), the concentration of the glycine solution is 3 - 10 g / L.

[0017] More preferably, in step (2), the concentration of the glycine solution is 7 g / L.

[0018] Preferably, the specific operations of magnetic collection, washing, drying, and grinding and sieving are as follows: placing the reacted solution on a magnet for precipitation; washing it 2 - 3 times each with ethanol and deionized water successively; pre-freezing at -20 °C for 12 h and then drying under vacuum conditions at -110 to -95 °C for 10 - 12 h; grinding the obtained powder and sieving it through a 200-mesh sieve, and putting it into a vacuum bag for standby.

[0019] The application of a modified hydrotalcite-like material loaded with nano-zero-valent iron composite in treating nitrate pollution in groundwater includes the following steps: mixing the glycine-functionalized modified hydrotalcite-like material loaded with nano-zero-valent iron composite with the nitrate-containing polluted water body under the oscillation condition of 180 rpm for reaction to remove nitrate.

[0020] Preferably, the addition amount of the glycine-functionalized modified hydrotalcite-like material loaded with nano-zero-valent iron composite is 0.09 - 0.18 g of the glycine-functionalized modified hydrotalcite-like material loaded with nano-zero-valent iron composite added to every 30 mL of the nitrate-containing polluted water body; the initial concentration range of nitrate in the polluted water body is 50 - 200 mg / L.

[0021] Preferably, during the process of the glycine-functionalized modified hydrotalcite-like material loaded with nano-zero-valent iron composite removing nitrate, the reaction temperature is 15 - 35 °C; the oxygen condition of the nitrate-containing system is aerobic or anoxic; the initial pH range of the nitrate-containing system is 3 - 11.

[0022] Preferably, the anoxic condition means introducing an inert gas into the reaction vessel, discharging the air in the reaction vessel, and sealing it to achieve the anoxic condition of the reaction system; the inert gas is nitrogen.

[0023] The beneficial effects of the present invention include:

[0024] 1. The glycine-functionalized modified hydrotalcite-like material loaded with nano-zero-valent iron prepared by using nano-silica modified hydrotalcite-like material loading and glycine functionalization in the present invention solves the problems of easy aggregation and passivation of nZVI. The structure of this composite material makes the nZVI particles more dispersed, thereby increasing the contact area with pollutants and improving the removal efficiency.

[0025] 2. The glycine-functionalized modified hydrotalcite-like material loaded with nano-zero-valent iron prepared in the present invention breaks through the environmental limiting factors such as water body pH and oxygen in the application. Compared with nZVI, it has stronger stability and reaction activity. Whether under aerobic or anoxic conditions, and within the pH range of 3 - 11, it can rapidly remove nitrate, and the removal rate can reach 97.0% within 2 h. After the reaction, about 85% of the nitrate is converted into ammonia nitrogen.

[0026] 3. In the synthesis process of the hydrotalcite-like compound, the addition of nano-silica in the present invention can inhibit the growth of LDH particles, effectively improve the layered structure of LDH, and further better load nZVI. The glycine functionalization modification introduces -NH3 + , which can serve as additional active sites for nitrate nitrogen adsorption, thereby enhancing the nitrate adsorption capacity of the material.

[0027] In the present invention, the prepared Gly-SiLDH@nZVI composite material is used as the PRB filling filler for the dynamic remediation of nitrate in water bodies. The results show that the composite material has stronger remediation ability and longer service life than nZVI, reducing the material production and maintenance costs in practical applications. In addition, the raw material cost required for the preparation of the composite material in the present invention is low, and it is more suitable as the PRB reaction medium for removing nitrate pollution in groundwater. Description of the Drawings

[0028] Figure 1 is the image of the Gly-SiLDH@nZVI preparation device;

[0029] Figure 2 are the scanning electron microscope (SEM) images of LDH, SiLDH, nZVI, and Gly-SiLDH@nZVI;

[0030] Figure 3 are the X-ray diffraction (XRD) images of LDH, SiLDH, nZVI, and Gly-SiLDH@nZVI;

[0031] Figure 4 are the N2 adsorption-desorption isotherm (a) and pore size distribution (b) of nZVI and Gly-SiLDH@nZVI;

[0032] Figure 5 are the Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) images of Gly-SiLDH@nZVI;

[0033] Figure 6 is the comparison chart of the nitrate removal performance of LDH, nZVI, LDH@nZVI, and Gly-SiLDH@nZVI

[0034] Figure 7 is the influence of different calcium-aluminum ratios on nitrate removal;

[0035] Figure 8 is the influence of different glycine concentrations and nano-silica incorporation amounts on nitrate removal;

[0036] Figure 9 is the influence of the material dosage on nitrate removal;

[0037] Figure 10 Effect of initial pH on nitrate removal;

[0038] Figure 11 Effect of temperature on nitrate removal;

[0039] Figure 12 Effect of dissolved oxygen on nitrate removal;

[0040] Figure 13 Recycling diagram of Gly-SiLDH@nZVI;

[0041] Figure 14 Diagram of one-dimensional sand column simulation column device (a), comparison of effluent nitrate (b) and effluent total iron concentration of nZVI column and Gly-SiLDH@nZVI column (c), and effect of different flow rates on nitrate removal by Gly-SiLDH@nZVI column (d). Specific implementation mode

[0042] To make the objectives, technical solutions and advantages of the present invention clearer, the following further details the specific implementation modes of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the following embodiments are only the preferred embodiments of the present invention, not all of them. Based on the embodiments in the implementation modes, other embodiments obtained by those skilled in the art without creative efforts belong to the protection scope of the present invention. It is worth noting that the raw materials used in the present invention are all ordinary commercially available products, and no specific limitation is imposed on their sources. The technical and scientific terms used in the embodiments have the meanings commonly understood by those of ordinary skill in the technical field to which the present invention belongs.

[0043] A preparation method of a modified layered double hydroxide supported nano zero-valent iron composite material of the present invention prepares SiLDH by the co-precipitation method, ultrasonically mixes SiLDH with FeSO4·7H2O solution, and under anoxic conditions, loads nZVI on SiLDH by the NaBH4 reduction method. After the reaction is completed, the black solid is collected by magnetic force, washed, and surface functionalized and modified with glycine to obtain the Gly-SiLDH@nZVI composite material. The method includes the following steps:

[0044] (1) Preparation of SiLDH: 1.1. Add nano-silica into deionized water, stir evenly, and then ultrasonicate for 20 - 30 min at room temperature to form a suspension. Among them, the average particle size of the nano-silica is 45 - 55 nm; 1.2. Add CaCl2 and AlCl3·6H2O into the nano-silica suspension, stir until completely dissolved, and then, under anoxic conditions, slowly drip 3 mol / L NaOH into the reaction vessel. During the dripping process, stir at a rotation speed of 600 - 800 rpm and continuously introduce an inert gas to control the pH range in the reaction vessel to be 9 - 10; 1.3. After the reaction is completed, seal the reaction vessel and continue to stir for 8 h under the condition of a 65°C water bath. Subsequently, perform centrifugal separation, washing, drying, and grinding and sieving to obtain SiLDH. Among them, the mass ratio of the nano-silica to the amount of deionized water used is 1 - 3 g:100 mL, the molar ratio of the mass of the nano-silica to the number of moles of Al is 33.3 - 100 g:1 mol, and the molar ratio of Ca to Al is 2 - 4:1.

[0045] (2) Preparation of Gly-SiLDH@nZVI: 2.1. After introducing an inert gas into deionized water, add FeSO4·7H2O and stir until dissolved, and then add the SiLDH prepared in step (1). The mass ratio of FeSO4·7H2O to SiLDH is 2:1. Continuously introduce an inert gas, ultrasonicate for 20 - 30 min at room temperature, and then continue to stir at a rotation speed of 600 - 800 rpm to make the reaction system fully mixed and uniform; 2.2. Under continuous introduction of an inert gas and vigorous stirring, drip 1.5 mol / L NaBH4 solution at a speed of 1 - 2 drops / s. After the dripping is completed, continue to stir for 20 - 30 min to ensure complete reaction, and then perform magnetic collection and washing; 2.3. Add the glycine solution prepared with deoxygenated deionized water into the above black solid, ultrasonicate for 20 - 30 min under anoxic and room temperature conditions, perform magnetic collection, washing, drying, and grinding and sieving to obtain the Gly-SiLDH@nZVI composite material. Among them, the concentration of the glycine solution is 3 - 10 g / L.

[0046] The present invention will be further described below in conjunction with specific embodiments.

[0047] Example 1

[0048] (1) Preparation of SiLDH: 1.1. Add 3 g of nano-silica into 100 mL of deionized water. After stirring evenly, ultrasonicate for 20 - 30 min at room temperature to form a suspension, wherein the average particle size of the nano-silica is 45 - 55 nm; 1.2. Add CaCl2 and AlCl3·6H2O into the nano-silica suspension. After stirring until completely dissolved, under anoxic conditions, slowly drip 3 mol / L NaOH into the reaction vessel. During the dripping process, stir at a rotation speed of 600 - 800 rpm and continuously introduce an inert gas, and control the pH range in the reaction vessel to be 9 - 10; 1.3. After the reaction is completed, seal the reaction vessel and continue to stir for 8 h under the condition of a 65°C water bath. Subsequently, perform centrifugal separation, washing, drying and grinding and sieving to obtain SiLDH. The mass ratio of the nano-silica to the amount of deionized water used is 3 g:100 mL, the mass ratio of the nano-silica to the molar amount of Al is 100 g:1 mol, and the molar ratios of Ca and Al are 2:1, 3:1, and 4:1 respectively.

[0049] (2) Preparation of Gly-SiLDH@nZVI: 2.1. After introducing an inert gas into 100 mL of deionized water, add 2.78 g of FeSO4·7H2O and stir until dissolved. Then add 1.117 g of SiLDH prepared in step (1). The mass ratio of Fe in FeSO4·7H2O to SiLDH is 2:1. Continuously introduce an inert gas. After ultrasonication for 20 - 30 min at room temperature, continue to stir at a rotation speed of 600 - 800 rpm to make the reaction system fully mixed and uniform; 2.2. Under the continuous introduction of an inert gas and vigorous stirring, drip 20 mL of 1.5 mol / L NaBH4 solution at a speed of 1 - 2 drops / s. After the dripping is completed, continue to stir for 20 - 30 min to ensure complete reaction. Subsequently, perform magnetic collection and washing; 2.3. Add the glycine solution prepared with deoxygenated deionized water to the above-mentioned black solid. Ultrasonicate for 20 - 30 min under anoxic and room temperature conditions, perform magnetic collection, washing, drying and grinding and sieving to obtain the Gly-SiLDH@nZVI composite material. Among them, the concentration of the glycine solution is 7 g / L.

[0050] The application of the Gly-SiLDH@nZVI composite material in this example is carried out according to the following steps: Under the oscillation condition of 180 rpm, mix the Gly-SiLDH@nZVI composite material with the nitrate-containing polluted water body for reaction to remove nitrate.

[0051] For the removal of nitrate, a water pollutant, the addition amount of the above-mentioned Gly-SiLDH@nZVI composite material is 0.09 g of the Gly-SiLDH@nZVI composite material added to every 30 mL of nitrate-polluted water, and the initial concentration of nitrate in the polluted water is 50 mg / L. Adjust the pH of the nitrate-polluted water to 7. After adding the material, place the reactor in a constant-temperature shaking incubator, adjust the temperature to 25 °C, and the reaction time is 4 h. Filter, and use ultraviolet spectrophotometry to measure the nitrate concentration in the solution.

[0052] According to the method of Example 1, the Gly-SiLDH@nZVI composite materials with different molar ratios of Ca and Al were tested for nitrate removal, and the results are as Figure 6 shown: when the molar ratio of Ca and Al is 3:1, the Gly-SiLDH@nZVI composite material has the highest nitrate removal, so the optimal molar ratio of Ca and Al is determined to be 3:1.

[0053] Example 2

[0054] Compared with Example 1, the only difference is: in step (1), 2 g of nano-silica is added to 100 mL of deionized water, that is, the mass ratio of the nano-silica to the amount of deionized water used is 2 g: 100 mL, the mass ratio of the nano-silica to the molar number of Al is 66.6 g: 1 mol, and the molar ratio of Ca and Al is 3:1.

[0055] In step (2), the concentration of the glycine solution is 7 g / L.

[0056] The remaining operations are the same as those in Example 1.

[0057] Comparative Example 1

[0058] Compared with Example 1, the only difference is: in step (1), 1 g of nano-silica is added to 100 mL of deionized water, that is, the mass ratio of the nano-silica to the amount of deionized water used is 2 g: 100 mL, the mass ratio of the nano-silica to the molar number of Al is 66.6 g: 1 mol, and the molar ratio of Ca and Al is 3:1.

[0059] In step (2), the concentration of the glycine solution is 7 g / L.

[0060] The remaining operations are the same as those in Example 1.

[0061] Comparative Example 2

[0062] Compared with Example 1, the only difference is: in step (1), no nano-silica is added, and glycine-functionalized layered double hydroxide supported nano-zero valent iron is prepared, labeled as Gly-LDH@nZVI, and the molar ratio of Ca and Al is 3:1.

[0063] The concentration of the glycine solution described in step (2) is 7 g / L.

[0064] The remaining operations are the same as those in Example 1.

[0065] Comparative Example 3

[0066] Compared with Example 1, the only difference is that the concentration of the glycine solution described in step (2) is 3 g / L.

[0067] In step (1), the mass ratio of the nano-silica to the amount of deionized water used is 3 g: 100 mL, the molar ratio of the nano-silica to Al is 100 g: 1 mol, and the molar ratio of Ca to Al is 3:1.

[0068] The remaining operations are the same as those in Example 1.

[0069] Comparative Example 4

[0070] Compared with Example 1, the only difference is that the concentration of the glycine solution described in step (2) is 5 g / L.

[0071] In step (1), the mass ratio of the nano-silica to the amount of deionized water used is 3 g: 100 mL, the molar ratio of the nano-silica to Al is 100 g: 1 mol, and the molar ratio of Ca to Al is 3:1.

[0072] The remaining operations are the same as those in Example 1.

[0073] Comparative Example 5

[0074] Compared with Example 1, the only difference is that the concentration of the glycine solution described in step (2) is 10 g / L.

[0075] In step (1), the mass ratio of the nano-silica to the amount of deionized water used is 3 g: 100 mL, the molar ratio of the nano-silica to Al is 100 g: 1 mol, and the molar ratio of Ca to Al is 3:1.

[0076] The remaining operations are the same as those in Example 1.

[0077] Comparative Example 6

[0078] Compared with Example 1, the only difference is that glycine is not used for functionalization modification in step (2), and nano-silica modified hydrotalcite-like compound supported nano-zero valent iron is prepared, denoted as SiLDH@nZVI.

[0079] In step (1), the mass ratio of the nano-silica to the amount of deionized water used is 3 g: 100 mL, the molar ratio of the mass of the nano-silica to Al is 100 g: 1 mol, and the molar ratio of Ca to Al is 3:1.

[0080] The remaining operations are the same as those in Example 1.

[0081] Comparative Example 7

[0082] Compared with Example 1, the only difference is that no nano-silica was added in step (1), and glycine was not used for functional modification in step (2), and a hydrotalcite-like nanoscale zero-valent iron was prepared, denoted as LDH@nZVI.

[0083] The molar ratio of Ca to Al is 3:1.

[0084] The remaining operations are the same as those in Example 1

[0085] Table 1. Parameter comparison of Examples 1-2 and Comparative Examples 1-7

[0086]

[0087] Example 3

[0088] (1) Screening of the incorporation amount of nano-silica and glycine concentration

[0089] For the products prepared in Example 2 and Comparative Examples 1-7, according to the application steps of the Gly-SiLDH@nZVI composite material in Example 1 above, the removal of nitrate in polluted water was investigated. As Figure 7 shown, under the condition that the molar ratio of Ca to Al is 3:1, with the increase of the incorporation amount of nano-silica and glycine concentration, the removal rate of nitrate by the Gly-SiLDH@nZVI composite material gradually increases. On the one hand, adding nano-silica to the LDH sample can prevent the aggregation of LDH particles, improve the dispersibility of the material, and thus provide a larger attachment area for nZVI; on the other hand, there is an electrostatic interaction between the ≡Si-O - groups on the surface of nano-silica and the positive charge end of the glycine zwitterion (+H3N-CH2-COO-), increasing the -NH3 + , -NH3 + on the material surface, and -NH3 --N has a stronger adsorption capacity. When the doping amount of nano-silica is 3 g and the glycine concentration is 7 g / L, the nitrate removal rate reaches 74.35%. Continuing to increase the doping amount of nano-silica and the glycine concentration is of little significance for improving the nitrate removal rate. Therefore, it is most appropriate to select the mass ratio of nano-silica to the molar number of Al as 100 g:1 mol and the glycine concentration as 7 g / L.

[0090] (2) Perform characterization and analysis on Gly-SiLDH@nZVI by SEM, XRD, BET, FTIR, XPS, etc. The results are as Figures 2 to 5 shown:

[0091] Figure 2 SEM images of LDH, SiLDH, nZVI and Gly-SiLDH@nZVI. LDH shows an uneven layered structure, with the lamellae stacking and intersecting with each other, and there is a certain degree of agglomeration on the surface; SiLDH obtained by modification with nano-silica has an obvious layered structure, showing a porous and relatively loose feature, indicating that nano-silica can effectively reduce the agglomeration of LDH and make it have a thinner layered structure; nZVI prepared by the NaBH4 reduction method presents a typical chain-like structure with serious agglomeration, and the nZVI particles are in a smooth spherical structure with a particle size between 50 - 100 nm; Gly-SiLDH@nZVI forms a loading structure, in which SiLDH shows an obvious flaky structure with a petal-like morphology, and spherical nZVI is successfully loaded on the surface and interlayer of LDH, improving the dispersion of nZVI.

[0092] Figure 3 XRD patterns of LDH, SiLDH, nZVI and Gly-SiLDH@nZVI. The strong peaks of LDH at 2θ of 18.34° and 20.32° indicate that LDH has high purity and good crystallinity. SiLDH has an XRD pattern similar to that of LDH, while the characteristic diffraction peak intensity of SiLDH is weaker, indicating that the addition of SiO2 results in a thinner layered structure and weaker crystallinity of LDH. From the XRD patterns of nZVI and Gly-SiLDH@nZVI, the characteristic peak of Fe is found at a diffraction angle of 44.67°, indicating that the nZVI particles are successfully loaded on LDH. 0

[0093] Figure 4 ​N2 adsorption - desorption isotherms (a) and pore size distribution diagrams (b) of nZVI and Gly - SiLDH@nZVI. nZVI shows a type II adsorption isotherm without a hysteresis loop, while Gly - SiLDH@nZVI presents a type IV adsorption isotherm with an H3 - type hysteresis loop, indicating that nZVI has a uniform microporous structure and Gly - SiLDH@nZVI has a mesoporous structure. It can be seen from the pore size distribution diagram that the pore size of nZVI is mainly concentrated in a relatively small range and the distribution is relatively narrow, while Gly - SiLDH@nZVI has an obvious pore volume distribution in a larger pore size range (such as 5 - 35 nm). It can be seen from Table 2 that Gly - SiLDH@nZVI has a larger specific surface area and a more abundant pore structure, which may be attributed to the fact that SiLDH itself has a large specific surface area, thus increasing the specific surface area of the composite material. In summary, Gly - SiLDH@nZVI has better surface properties than nZVI, which is beneficial to the adsorption of pollutants.

[0094] Table 2. BET parameters of nZVI and Gly - SiLDH@nZVI

[0095]

[0096] Figure 5 FTIR and XPS diagrams of Gly - SiLDH@nZVI before and after use. From Figure 5 (a), the absorption peak near 3468 cm -1 is related to the O - H stretching vibration between the metal hydroxide layers and the N - H stretching vibration of glycine. The absorption peaks at 2950 cm -1 and 1630 cm -1 represent the stretching vibrations of alkyl C - H and the stretching vibration of interlayer water molecules OH, respectively. The absorption peak at 1105 cm -1 is attributed to the stretching vibrations of Si - O - Si and Si - O - H. The absorption peaks at 796 cm -1 and 478 cm -1 are caused by the stretching vibration of Fe - O and the lattice vibration of M - O (M = Ca or Al), respectively, indicating that nZVI has been successfully loaded onto SiLDH. From Figure 5 (b), the complete XPS spectra of Gly - SiLDH@nZVI before and after use show that the elements on the surface of Gly - SiLDH@nZVI are Fe, O, C, and N elements; Figure 5(c) is the high-resolution XPS spectrum of Fe 2p. The peaks at 711.2 eV and 724.4 eV correspond to the Fe 2p3 / 2 and Fe 2p1 / 2 orbitals of Fe(II), respectively. The peaks at 713.4 eV and 726.2 eV correspond to the Fe 2p3 / 2 and Fe 2p1 / 2 orbitals of Fe(III), respectively. The peaks at 716.8 eV, 728.5 eV and 720.5 eV, 732.7 eV are the satellite peaks of Fe(II) and Fe(III), respectively. The appearance of divalent iron and trivalent iron on the material surface is attributed to different degrees of oxidation during the preparation and storage of the material. Before the material was used, a weak characteristic peak of Fe 0 appeared at 707.3 eV, while the characteristic peak of Fe 0 disappeared in the XPS spectrum after use, indicating that Fe 0 was oxidized. At the same time, it can be seen from Figure 5 (d) that the content of Fe(III) increased from 48.86% to 71.06%, and the content of Fe(II) decreased from 48.25% to 28.94%, indicating that both Fe 0 and Fe(II) participated in the reaction during the use of the material. Figure (e) is the high-resolution XPS spectrum of C 1s. The C 1s spectrum shows three main peaks at binding energies of 288.9 eV, 286.6 eV and 284.8 eV, corresponding to C=O, C-O / C-H, and C-C / C-H, respectively. Figure (g) is the high-resolution XPS spectrum of O 1s. Before use, there were two main peaks at binding energies of 533.0 eV and 531.4 eV, corresponding to SiO2 / NH2CH2COOH and hydroxyl groups (Ca / Al-OH), respectively. After the material reacted with nitrate, a new characteristic peak belonging to Fe-O appeared at 529.9 eV.

[0097] Subsequent experiments were carried out on this basis.

[0098] (3) Influence of the dosage of the composite material on the removal effect

[0099] Add 30 mL of 50 mg / L nitrate solution with pH = 7 into 5 50-mL conical flasks respectively. Add the Gly-SiLDH@nZVI composite material at dosages of 3, 4, 5, 6, and 6.5 g / L respectively. After plugging the rubber stoppers, place the conical flasks in a constant temperature shaker and shake at 25 °C and 180 rpm. Take samples at specific times, filter, and measure the nitrate concentration in the solution. As Figure 9As shown, further increasing the dosage, the nitrate removal effect gradually increases. When the dosages are 5 g / L and 6 g / L, the removal rates of the composite material for nitrate reach 91.67% and 96.67%, respectively, and the reaction reaches equilibrium in about 2 h. Continuing to increase the dosage has little effect on improving the nitrate nitrogen removal rate. Considering the cost, 6 g / L is determined as the optimal dosage.

[0100] Subsequent experiments are carried out on this basis.

[0101] (4) Influence of initial pH on the removal effect

[0102] Add 30 mL of 50 mg / L nitrate solution to 5 50-mL conical flasks respectively. Use 0.1 mol / L hydrochloric acid and 0.1 mol / L NaOH solution to adjust the pH values of the water bodies to 3, 5, 7, 9, and 11 respectively. Add the Gly-SiLDH@nZVI composite material at a dosage of 6 g / L. After plugging with a rubber stopper, place the conical flasks in a constant temperature shaking incubator and shake at 25 °C and 180 rpm. Take samples at specific times, filter, and measure the nitrate concentration in the solution. As Figure 10 shown, under the conditions of the same dosage of Gly-SiLDH@nZVI composite material and initial nitrate concentration, the lower the pH value, the faster the nitrate removal rate in the first 60 min. In acidic solutions, the increase in nitrate removal rate can be attributed to more H + promoting the corrosion of nZVI and the generation of electrons, so that more electrons transfer from the Fe 0 core to NO3 - -N. Under the condition of pH = 11, within the first 30 min, its reaction rate is slightly lower than that under other pH conditions, but the removal rate of the composite material for nitrate reaches more than 97% at 3 h. This shows that the composite material performs well in removing NO3 - -N in a wide pH range and has the potential to become an effective material for repairing nitrate pollution in groundwater.

[0103] (5) Influence of temperature on the removal effect

[0104] Add 30 mL of 50 mg / L nitrate solution with pH = 7 to 4 50-mL conical flasks respectively. Add the Gly-SiLDH@nZVI composite material at a dosage of 6 g / L. After plugging with a rubber stopper, place the conical flasks in a constant temperature shaking incubator and adjust the reaction temperatures to 5, 15, 25, and 35 °C respectively. Shake at 180 rpm. Take samples at specific times, filter, and measure the nitrate concentration in the solution. As Figure 11As shown, when the temperature reached 35°C, the reaction reached equilibrium at 60 minutes, and the removal rate was 97.21%. When the temperature decreased to 15°C and 5°C, the reaction rate was significantly inhibited, and the NO3 - -N removal rates decreased to 89.54% and 81.65% respectively at 4 hours. Therefore, temperature is one of the important factors affecting the reaction rate.

[0105] (6) Influence of dissolved oxygen on removal effect

[0106] Add 30 mL of 50 mg / L nitrate solution with pH = 7 to two 50 mL conical flasks respectively. Pass nitrogen into one of the conical flasks for 30 minutes, add the Gly-SiLDH@nZVI composite material at a dosage of 6 g / L, immediately plug the rubber stopper, place the conical flask in a constant temperature shaking incubator, shake at 25°C and 180 rpm, take samples at specific times, filter, and measure the nitrate concentration in the solution. As Figure 12 shown, the removal efficiency of nitrate under anoxic conditions was slightly lower than that under aerobic conditions within the first 2 hours. The removal rates at 3 hours could all reach over 97%, indicating that this composite material can also exert its advantages in the groundwater environment under anoxic conditions.

[0107] Example 4

[0108] (1) Recycling

[0109] In this experiment, the reacted Gly-SiLDH@nZVI composite material was magnetically collected, rinsed several times with deionized water, and then re-added to 30 mL of 50 mg / L nitrate nitrogen solution for the second cycle test. According to the above method, this was cycled three times. The cyclic performance of Gly-SiLDH@nZVI and nZVI is as Figure 13 shown. The removal rate of Gly-SiLDH@nZVI for NO3 - -N decreased from 95.9% to 74.1% in the first two cycles, and rapidly decreased to 31.1% in the third cycle. At this time, most of the Fe in the material 0 was oxidized and consumed, resulting in a reduction in the number of active sites on the material surface. For nZVI, the removal rate of NO3 - -N was 34.9% in the second cycle, a 42.8% decrease compared to the NO3 - -N removal rate during the first use. During the recycling of nZVI, due to agglomeration and oxidation, electron transfer was blocked, the reaction activity decreased significantly, and the ability to remove nitrate also decreased accordingly. In contrast, there may be a synergistic effect among the components in Gly-SiLDH@nZVI, which is beneficial to electron transfer, thus enabling it to maintain a relatively high reaction activity.

[0110] (2) One-dimensional sand column simulation test

[0111] To explore the application potential of Gly-SiLDH@nZVI, a one-dimensional sand column simulation experiment was conducted. As Figure 14 (a) shows, coarse sand (0.40 - 0.85 mm) and fine sand (0.15 - 0.20 mm) were added to the reactor to simulate soils with different particle sizes. A 1.0 cm thick layer of Gly-SiLDH@nZVI was filled in the middle of the reactor. The influent nitrate-nitrogen solution concentration was 50 mg / L. A peristaltic pump was used to pump the pollutant solution into the reactor from the bottom at a constant flow rate. 2 mL samples were taken at different time intervals, filtered through a 0.22 μm membrane, and then analyzed. According to the Class III groundwater standard limit (GB / T 14848 - 2017), when the effluent NO3 - -N concentration reached 20 mg / L, it was defaulted that the composite material lost its remediation ability, and the running time of the reactor at this time was called the breakthrough time.

[0112] Through the one-dimensional sand column simulation experiment, the dynamic remediation performance of nZVI and Gly-SiLDH@nZVI composite materials for NO3 - -N with an influent concentration of 50 mg / L was studied. A peristaltic pump was used to control the influent flow rate, and the water entered from the lower end of the reactor. Samples were taken at regular intervals and the concentration of NO3 - -N in the effluent was measured. With the hydraulic retention time as the abscissa and the concentration of NO3 - -N in the effluent as the ordinate, a breakthrough curve was plotted. As Figure 14 (b) shows, when the filling material was nZVI, the nitrate effluent concentration was at a relatively low level (<5.0 mg / L) in the first 8 h, and the stable running time of the nZVI column was 8 h; subsequently, from 8 to 28 h, the nitrate concentration in the effluent of the nZVI column increased slowly, and after 28 h, the nitrate concentration in the effluent was in a dynamic equilibrium state, approaching the influent concentration, and the nZVI column reached saturated remediation. In contrast, the Gly-SiLDH@nZVI column ran stably for 12 h at the same influent flow rate, and then the nitrate concentration in the effluent gradually increased from 12 to 36 h, indicating that the composite material could more fully remediate nitrate-contaminated groundwater. When the running time was 36 h, the Gly-SiLDH@nZVI column reached the saturated remediation state. The nitrate concentration at the effluent end of the nZVI column increased to 20 mg / L at about 14 h, and this concentration exceeded the Class III groundwater quality standard specified in GB / T 14848 - 2017. At this time, the running time was called the breakthrough time, and it was considered that the nZVI material in the nZVI column lost its remediation ability. The breakthrough time of the Gly-SiLDH@nZVI column was extended to about 17.5 h, indicating that the Gly-SiLDH@nZVI composite material had a longer lifespan when applied in the simulation column.

[0113] In view of the potential environmental risks associated with the application of nano-iron particles, the concentration of total iron in the effluent of the one-dimensional simulated column experiment was monitored. As Figure 14 (c) shows, within the running time of 2 - 18 h, the total iron concentration in the effluent of the nZVI column was between 4 - 7.5 mg / L, generally higher than that of the Gly-SiLDH@nZVI column (<3.5 mg / L), indicating that fewer iron ions migrated out from the Gly-SiLDH@nZVI column. As the running time extended, the materials in the column gradually failed, and the total iron concentration in the effluent maintained within the range of 2 - 4 mg / L.

[0114] By changing the rotational speed of the peristaltic pump, the effect of different influent flow rates on nitrate nitrogen removal was studied. As can be seen from Figure 14 (d), the breakthrough times corresponding to the flow rates of 0.76, 0.98, and 1.36 mL / min were 20 h, 17.5 h, and 14 h respectively. At low flow rates, the Gly-SiLDH@nZVI composite material could fully contact and react with nitrate, so the stable running time was longer. However, due to the continuous erosion of the water flow, the materials were gradually depleted, so the final treated water volume was less at low flow rates. As the flow rate increased, the amount of nitrate solution flowing through the packed material per unit time in the device increased, and the composite material in the reaction zone failed to remove the nitrate in the solution in time, resulting in the outflow of the nitrate solution that had not been treated in time, thus the effluent concentration of nitrate nitrogen was higher.

[0115] Table 3 One-dimensional simulated column operation parameters

[0116]

Claims

1. A preparation method of a modified hydrotalcite-supported nano zero-valent iron composite material, characterized in that, It includes the following steps: (1) Preparation of nano-silica modified hydrotalcite-like material SiLDH: Add CaCl2 and AlCl3·6H2O to the nano-silica suspension and stir until completely dissolved. Under anoxic and room temperature conditions, slowly dropwise add NaOH solution to control the pH of the reaction system to 9 - 10, and synthesize the nano-silica modified hydrotalcite-like material by the coprecipitation method; (2) Preparation of glycine-functionalized modified hydrotalcite-like material loaded with nano-zero-valent iron composite Gly-SiLDH@nZVI: After ultrasonically mixing SiLDH with FeSO4·7H2O solution, under anoxic conditions, load nZVI on SiLDH by the NaBH4 reduction method. After the reaction is completed, magnetically collect and wash the black solid, and perform surface functionalization modification on it with glycine to obtain the Gly-SiLDH@nZVI composite.

2. The preparation method according to claim 1, characterized in that: In step (1), the preparation of the SiLDH specifically includes the following steps: 1.1 Add nano-silica to deionized water, stir evenly, and ultrasonically treat for 20 - 30 min at room temperature to form a suspension, wherein the average particle size of the nano-silica is 45 - 55 nm; 1.2 Slowly dropwise add 2.8 - 3.2 mol / L NaOH into the reaction vessel, stir at a rotation speed of 600 - 800 rpm during the dropping process, and continuously introduce an inert gas to control the pH range in the reaction vessel to 9 - 10; 1.3 After the reaction is completed, seal the reaction vessel and continue to stir in a water bath at 60 - 70 °C for 8 - 12 h, and then perform centrifugal separation, washing, drying and grinding and sieving to obtain SiLDH.

3. According to the preparation method described in claim 1, it is characterized in that: The preparation of the Gly-SiLDH@nZVI composite in step (2) specifically includes the following steps: 2.1 After introducing an inert gas into deionized water, add FeSO4·7H2O and stir until dissolved, and then add the SiLDH prepared in step (1), wherein the mass ratio of FeSO4·7H2O to SiLDH is (1.8 - 2.2):

1. Continuously introduce an inert gas, ultrasonically treat for 20 - 30 min at room temperature, and then continue to stir at a rotation speed of 600 - 800 rpm to make the reaction system fully mixed and uniform; 2.2 Under continuous introduction of an inert gas and vigorous stirring, dropwise add 1.2 - 1.6 mol / L NaBH4 solution at a rate of 1 - 2 drops / s. After the dropping is completed, continue to stir for 20 - 30 min to ensure complete reaction, and then perform magnetic collection and washing; 2.3 Add the glycine solution prepared with deoxygenated deionized water to the above-mentioned black solid, ultrasonically treat for 20 - 30 min under anoxic and room temperature conditions, and magnetically collect, wash, dry and grind and sieve to obtain the Gly-SiLDH@nZVI composite.

4. The preparation method according to claim 1, characterized in that, In step (1), the mass ratio of the nano-silica to the amount of deionized water is 1-3 g: 95-105 mL, the molar ratio of the mass of the nano-silica to Al is 33.3-100 g: 0.95-1.05 mol, and the molar ratio of Ca to Al is 2-4:

1.

5. The preparation method according to claim 1, characterized in that: The concentration of the glycine solution is 3-7 g / L.

6. The preparation method according to claim 1 or 3, characterized in that, The specific operations of magnetic collection, washing, drying and grinding and sieving are as follows: placing the reacted solution on a magnet for precipitation; washing with ethanol and deionized water 2-3 times each in sequence; pre-freezing at -20 to -40 °C for 8-12 h, and then drying under vacuum conditions at -110 to -95 °C for 10-12 h; grinding the obtained powder and sieving it through a 180-220 mesh sieve, and putting it into a vacuum bag for standby.

7. The preparation method according to claim 1 or 3, characterized in that: The anoxic condition refers to introducing an inert gas into the reaction vessel, exhausting the air in the reaction vessel, and sealing it to achieve the anoxic condition of the reaction system; the inert gas is nitrogen. The Gly-SiLDH@nZVI composite material prepared by the preparation method according to any one of claims 1-6.

9. Use of the Gly-SiLDH@nZVI composite material prepared by the preparation method according to any one of claims 1 to 6 in treating nitrate pollution in groundwater, characterized in that: The application of the described Gly-SiLDH@nZVI composite material in the removal of nitrate from groundwater.

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