Cobalt-doped MgAl-LDH adsorbent and preparation method and application thereof

Abstract

The invention relates to a cobalt-doped MgAl-LDH adsorbent and a preparation method and application thereof.The cobalt-doped MgAl-LDH adsorbent is prepared by enabling cobalt nitrate, aluminum nitrate and magnesium nitrate to react through a precipitation method, and the cobalt nitrate, the aluminum nitrate and the magnesium nitrate are fed according to the molar ratio of Co < 2 + > to Al < 3 + > to Mg < 2 + > being (0.1-0.5): 1: (2-4). The cobalt-doped MgAl-LDH adsorbent disclosed by the invention has the advantages of simple preparation method, low cost and structures such as specific surface area, pore volume and pore diameter suitable for adsorbing malachite green, so that the cobalt-doped MgAl-LDH adsorbent is good in malachite green adsorption effect and high in adsorption capacity, and has a wide application prospect in the technical field of water treatment.

Description

Technical Field

[0001] The present invention specifically relates to a cobalt-doped MgAl-LDH adsorbent, a preparation method thereof, and an application thereof. Background Art

[0002] This section aims to provide background or context for the embodiments of the present invention stated in the claims. The description herein is not admitted to be prior art merely by including it in this section.

[0003] Malachite green (MG) is a triarylmethane cationic dye, which is widely used in industries such as fishery, leather, textile, and papermaking. Due to the presence of nitrogen, it has certain toxicity and carcinogenicity. When it is discharged into water, it affects aquatic organisms and thus poses a potential hazard to human health. Because of its good solubility and stability in water and high persistence, it is difficult to be degraded by microorganisms. The adsorption method is considered to be an economical, efficient, and simple method for removing malachite green. Therefore, the adsorption and removal of malachite green have been widely studied.

[0004] CN113318702B discloses a double-modified double-metal oxide for adsorbing malachite green in water and a preparation method thereof. Using magnesium nitrate hexahydrate, aluminum nitrate nonahydrate, urea, and sodium dodecylbenzenesulfonate as raw materials respectively, sodium dodecylbenzenesulfonate-modified layered double metal hydroxide is prepared by a one-step hydrothermal synthesis method. Then, after washing and drying, the product is placed in a tube furnace and carbonized at high temperature under nitrogen protection to obtain a grayish-white modified double-metal oxide. This material can adsorb malachite green, but its adsorption capacity is low, with a maximum of only 5 g / g.

[0005] Therefore, there is still a need to develop an adsorbent with a large adsorption capacity for malachite green, so as to provide more options for the water treatment field. Summary of the Invention

[0006] The purpose of the present invention is to provide a cobalt-doped MgAl-LDH adsorbent with low preparation cost and large adsorption capacity for malachite green, a preparation method thereof, and an application thereof.

[0007] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0008] In the first aspect of the present invention, a preparation method of a cobalt-doped MgAl-LDH adsorbent is provided. The cobalt-doped MgAl-LDH adsorbent is prepared by reacting cobalt nitrate, aluminum nitrate, and magnesium nitrate using the precipitation method. Among them, it is controlled that cobalt nitrate, aluminum nitrate, and magnesium nitrate are fed according to the molar ratio of Co 2+ :Al 3+ :Mg 2+ of 0.1 - 0.5:1:2 - 4.

[0009] The present inventor has tried to prepare cobalt-doped MgAl-LDH adsorbents by precipitation method and hydrothermal method respectively. Through research, it is found that the cobalt content doped in the cobalt-doped MgAl-LDH adsorbent prepared by precipitation method is closer to the feeding amount; under the same feeding ratio, the cobalt content doped in the adsorbent prepared by hydrothermal method is less, and the magnesium-aluminum ratio is also significantly lower than the feeding ratio. In addition, the adsorption performance of the cobalt-doped MgAl-LDH adsorbent prepared by precipitation method for malachite green is also better than that of the cobalt-doped MgAl-LDH adsorbent prepared by hydrothermal method.

[0010] According to some specific embodiments, control cobalt nitrate, aluminum nitrate and magnesium nitrate to be fed in a molar ratio of Co 2+ :Al 3+ :Mg 2+ of 0.2 to 0.3:1:2 to 3. Further, control cobalt nitrate, aluminum nitrate and magnesium nitrate to be fed in a molar ratio of Co 2+ :Al 3+ :Mg 2 + of 0.23 to 0.28:1:2 to 2.5.

[0011] According to some specific embodiments, control the reaction temperature to be 100 to 180 °C, such as 100 °C, 105 °C, 110 °C, 115 °C, 120 °C, 125 °C, 130 °C, 135 °C, 140 °C, 145 °C, 150 °C, 155 °C, 160 °C, 165 °C, 170 °C, 175 °C, 180 °C, etc.; the reaction time is 6 to 14 h, such as 6 h, 6.5 h, 7 h, 7.5 h, 8 h, 8.5 h, 9 h, 9.5 h, 10 h, 10.5 h, 11 h, 11.5 h, 12 h, 12.5 h, 13 h, 13.5 h, 14 h, etc.

[0012] Further, control the reaction temperature to be 120 to 160 °C and the reaction time to be 8 to 12 h.

[0013] According to some specific embodiments, the preparation method is specifically as follows: dissolve cobalt nitrate, aluminum nitrate and magnesium nitrate in water, dropwise add a mixed alkali solution of sodium carbonate and sodium hydroxide, adjust the pH value of the reaction system to 9 to 11, and then react at 100 to 180 °C for 6 to 14 h under a sealed state. After the reaction is completed, the reaction product is washed and dried to obtain the cobalt-doped MgAl-LDH adsorbent.

[0014] Further, the cobalt nitrate is fed in the form of Co(NO 3 ) 2 ·6H 2 O, and the aluminum nitrate is fed in the form of Al(NO 3 ) 3 ·9H2 in the form of O, and the magnesium nitrate is in the form of Mg(NO 3 ) 2 ·6H 2 O.

[0015] Furthermore, the concentration of sodium carbonate in the mixed alkali solution is 0.4 - 0.6 mol / L, and the concentration of sodium hydroxide is 0.8 - 1.2 mol / L.

[0016] In the second aspect of the present invention, a cobalt-doped MgAl-LDH adsorbent prepared by the preparation method as described above is provided. Among them, the cobalt-doped MgAl-LDH adsorbent has a nano-sheet structure, and cobalt exists in the forms of Co 2+ and Co 3+ . The specific surface area of the cobalt-doped MgAl-LDH adsorbent is 40 - 50 m 2 / g, the total pore volume is 0.14 - 0.23 cm 3 / g, and the average pore diameter is 28 - 38 nm.

[0017] Furthermore, the molar ratio of Co:Mg:Al in the cobalt-doped MgAl-LDH adsorbent is 0.09 - 0.25:1.7 - 2:1.

[0018] In the third aspect of the present invention, a method for adsorbing malachite green in water is provided, using the cobalt-doped MgAl-LDH adsorbent prepared by the preparation method as described above or the cobalt-doped MgAl-LDH adsorbent as described above to adsorb malachite green in water.

[0019] According to some specific embodiments, the concentration of malachite green in the water is controlled to be 100 - 600 mg / L, and the equilibrium adsorption capacity of the cobalt-doped MgAl-LDH adsorbent is 26 g / g - 32 g / g.

[0020] According to some specific embodiments, the temperature of the water is controlled to be 15 - 55 °C, such as 15 °C, 20 °C, 25 °C, 30 °C, 35 °C, 40 °C, 45 °C, 50 °C, 55 °C, etc.

[0021] According to some specific embodiments, the pH value of the water is controlled to be 4 - 9, such as pH values of 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, etc. Furthermore, the pH value of the water is controlled to be 6 - 7.5. Preferably, the pH of the water is not adjusted.

[0022] Due to the application of the above technical solutions, the present invention has the following advantages compared with the prior art:

[0023] The cobalt-doped MgAl-LDH adsorbent of the present invention has a simple preparation method and low cost, and has a specific surface area, pore volume, pore diameter and other structures suitable for adsorbing malachite green, so it has a good adsorption effect on malachite green, a high adsorption capacity, and has broad application prospects in the field of water treatment technology. Description of the Drawings

[0024] The drawings here are incorporated into the specification and form a part of this specification, which show the embodiments in line with this application, and are used together with the specification to explain the principles of this application. In order to more clearly illustrate the technical solutions of the embodiments of this application, the drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, for those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts. These drawings and textual descriptions are not intended to limit the scope of the concept of this application in any way, but to illustrate the concept of this application to those skilled in the art by referring to specific embodiments.

[0025] Figure 1 XRD patterns of MgAl-LDH and the samples prepared after adding Co;

[0026] Figure 2 SEM images of MgAl-LDH and the samples prepared under different n(Co:Al) conditions;

[0027] Figure 3 Results of elemental composition and Co valence state information of MgAl-LDH and the samples after adding Co by XPS;

[0028] Figure 4 FT-IR spectra of MgAl-LDH and the samples prepared under different n(Co:Al);

[0029] Figure 5 Nitrogen adsorption-desorption isotherms and corresponding BJH pore size distribution diagrams of MgAl-LDH, Sample 1, Sample 2, Sample 3, Sample 4 and Sample 5;

[0030] Figure 6 Pyrolysis process diagrams of MgAl-LDH and Co-MgAl-LDH from room temperature to 1000 °C in a nitrogen atmosphere;

[0031] Figure 7 Adsorption performance result diagrams of samples with different cobalt contents for malachite green;

[0032] Figure 8 Adsorption performance result diagrams of samples with different magnesium contents for malachite green;

[0033] Figure 9Adsorption performance results of samples prepared at different reaction times for malachite green;

[0034] Figure 10 Adsorption performance results of samples prepared at different reaction temperatures for malachite green;

[0035] Figure 11 Adsorption performance results of different amounts of samples for malachite green;

[0036] Figure 12 Adsorption performance results of Co-MgAl-LDH for malachite green at different temperatures;

[0037] Figure 13 Adsorption performance results of Co-MgAl-LDH for malachite green at different pH values;

[0038] Figure 14 Adsorption performance results of Co-MgAl-LDH in malachite green solutions with different concentrations. Detailed implementation methods

[0039] The present invention will be further described below in conjunction with embodiments. However, the present invention is not limited to the following embodiments. The implementation conditions used in the embodiments can be further adjusted according to different requirements of specific use, and the implementation conditions not specified are conventional conditions in the industry. The technical features involved in various implementation manners of the present invention can be combined with each other as long as they do not conflict with each other.

[0040] Unless otherwise specified in this article, the preparation methods and detection methods involved in the following examples or comparative examples refer to the prior art. Unless otherwise specified in the following examples, all raw materials are obtained through commercial purchase or prepared by conventional methods in the art.

[0041] The test methods adopted in the present invention are as follows:

[0042] The morphology of MgAl-LDHs was observed using a Hitachi S-4800 scanning electron microscope (SEM, Philips Quanta 200, Hitachi-S4800). The operation method was to dip a small amount of the oven-vacuum-dried sample on the conductive adhesive with a toothpick, and then perform gold spraying treatment. The test conditions were: 5KV, 10μA.

[0043] The FT-IR characteristic peaks of samples such as MgAl-LDHs were detected using an infrared spectrometer (FT-IR, NICOLET, NEXUS 670). The sample was mixed and ground with potassium bromide by the KBr tablet pressing method (mass ratio 1:100), and the scanning range was 4000 - 400 cm -1 , and the resolution was 2 cm -1 .

[0044] The crystal form and crystallinity of materials such as MgAl-LDHs were analyzed using an X-ray diffractometer (Bruker, D8-Advance). The operation method was to flatten the sample with a glass slide. The test conditions were: Cu Kα radiation, tube voltage 40 kV, current 100 mA, scanning range 5 - 80°, and scanning speed 10° min -1 . The diffraction peak at (003) was considered as evidence for calculating the LDH layer spacing according to Bragg's law:

[0045] 2dsinθ = nλ

[0046] where d represents the layer spacing of LDH, θ represents the diffraction angle of the XRD peak, λ represents the wavelength of the X-ray, and n represents the diffraction order.

[0047] The pore structure of nanoparticles such as MgAl-LDHs was tested using nitrogen adsorption-desorption (BELSORP II). The test conditions were: before testing, the particles were first degassed at 200 °C for 6 h. The total pore volume was obtained at a relative test pressure of 0.99, and the Brunauer-Emmett-Teller (BET) surface area (SBET) was calculated from the adsorption curve in the relative pressure range of 0.05 - 0.25, and the average pore diameters of micropores and mesopores were calculated using the BJH method respectively.

[0048] Thermogravimetric tests were performed on samples such as MgAl-LDHs using a thermogravimetric analyzer (Netzsch STA 409). The test conditions were: temperature range from room temperature to 1000 °C, and heating rate of 10 °C / min.

[0049] A Zeta potentiometer (DLS, MALVERN, MALVERN3000) was used to test the potential of MgAl-LDH and the sample materials after adding Co. The sample preparation conditions were: measured under neutral conditions using pure water as the dispersant.

[0050] X-ray photoelectron spectroscopy (AXIS SUPRA) was used to test the surface element valence states of elements C, N, O, and Co in materials such as MgAl-LDHs, and the changes in element valence states were analyzed simultaneously.

[0051] The powdered sample was completely dissolved in hydrochloric acid and diluted with distilled water to obtain a metal solution with a concentration of 10 - 100 ppm. Icp-mos was measured three times repeatedly and the average value was taken.

[0052] Example 1 Preparation of Cobalt-Doped MgAl-LDH Adsorbent

[0053] Cobalt doping was carried out on MgAl-LDH, and the specific preparation method was as follows: According to the molar ratio n(Co 2+ :Al 3+ :Mg2+ ) = 0.25:1:2, 0.0728 g of Co(NO 3 ) 2 ·6H 2 O, 0.375 g of Al(NO 3 ) 3 ·9H 2 O, 0.512 g of Mg(NO 3 ) 2 ·6H 2 O were added to 50 ml of deionized water, stirred and dissolved at room temperature, and then a certain amount of mixed alkali solution of sodium carbonate and sodium hydroxide (where the concentration of sodium carbonate in the mixed alkali solution was 0.5 mol / L and the concentration of sodium hydroxide was 1 mol / L) was added dropwise. The pH of the solution was adjusted to 10, and stirring was continued at room temperature. After being mixed evenly, it was transferred to a 100 ml reaction kettle with a polytetrafluoroethylene lining. The reaction kettle was sealed and placed in an oven to react at 140 °C for 10 h. After the reaction was completed, the reaction kettle was taken out and cooled to room temperature. The pink product was collected by centrifugation, washed several times with deionized water, and dried in a vacuum oven at 60 °C for 12 h to obtain a pink Co-MgAl-LDH solid, denoted as Sample 4.

[0054] Example 2 Preparation of Cobalt-Doped MgAl-LDH Adsorbent

[0055] It was basically the same as Example 1, except for the feeding amount of Co. In this example, Co(NO 2+ :Al 3+ ) = 0.1:1 was added in terms of molar ratio, and Co(NO 3 ) 2 ·6H 2 O was added. The Co-MgAl-LDH prepared in this example was denoted as Sample 1.

[0056] Example 3 Preparation of Cobalt-Doped MgAl-LDH Adsorbent

[0057] It was basically the same as Example 1, except for the feeding amount of Co. In this example, Co(NO 2+ :Al 3+ ) = 0.15:1 was added in terms of molar ratio, and Co(NO 3 ) 2 ·6H 2 O was added. The Co-MgAl-LDH prepared in this example was denoted as Sample 2.

[0058] Example 4 Preparation of Cobalt-Doped MgAl-LDH Adsorbent

[0059] It was basically the same as Example 1, except for the feeding amount of Co. In this example, Co(NO 2+ :Al3+ ) = 0.2:1 of Co(NO 3 ) 2 ·6H 2 O. The Co-MgAl-LDH prepared in this example is denoted as Sample 3.

[0060] Example 5 Preparation of Cobalt-Doped MgAl-LDH Adsorbent

[0061] It is basically the same as Example 1, except for the feeding amount of Co. In this example, according to the molar ratio n(Co 2+ :Al 3+ ) = 0.3:1 of Co(NO 3 ) 2 ·6H 2 O. The Co-MgAl-LDH prepared in this example is denoted as Sample 5.

[0062] Example 6 Preparation of Cobalt-Doped MgAl-LDH Adsorbent

[0063] It is basically the same as Example 1, except for the feeding amount of Co. In this example, according to the molar ratio n(Co 2+ :Al 3+ ) = 0.5:1 of Co(NO 3 ) 2 ·6H 2 O. The Co-MgAl-LDH prepared in this example.

[0064] Example 7 Preparation of Cobalt-Doped MgAl-LDH Adsorbent

[0065] It is basically the same as Example 1, except for the feeding amount of Mg. In this example, according to the molar ratio n(Mg 2+ :Al 3+ ) = 3:1 of Mg(NO 3 ) 2 ·6H 2 O. The Co-MgAl-LDH prepared in this example.

[0066] Example 8 Preparation of Cobalt-Doped MgAl-LDH Adsorbent

[0067] It is basically the same as Example 1, except for the feeding amount of Mg. In this example, according to the molar ratio n(Mg 2+ :Al 3+ ) = 4:1 of Mg(NO 3 ) 2 ·6H 2 O. The Co-MgAl-LDH prepared in this example.

[0068] Example 9 Preparation of Cobalt-Doped MgAl-LDH Adsorbent

[0069] It is basically the same as Example 1, except for the reaction time. Co-MgAl-LDHs prepared at reaction times of 6 h, 8 h, 12 h, and 14 h were obtained in this example respectively.

[0070] Preparation of Cobalt-Doped MgAl-LDH Adsorbent in Example 10

[0071] It is basically the same as Example 1, except for the reaction temperature. Co-MgAl-LDHs prepared at reaction temperatures of 100 °C, 120 °C, 160 °C, and 180 °C were obtained in this example respectively.

[0072] Preparation of MgAl-LDH Adsorbent in Comparative Example 1

[0073] It is basically the same as Example 1, except that Co was not added. The MgAl-LDH obtained in this example is also denoted as Co:Al = 0:1.

[0074] Characterization and Analysis

[0075] Figure 1 XRD patterns of MgAl-LDH and the samples obtained after adding Co. Diffraction peaks of MgAl-LDH appear at 2θ of 11.21°, 22.82°, 34.43°, 60.26°, and 61.45° corresponding to crystal planes (003), (006), (012), (018), and (110), etc. Among them, the peaks at crystal planes (003) and (006) are very strong, all showing good crystalline hydrotalcite-like LDHs diffraction characteristics and their layered structure characteristics.

[0076] Compared with MgAl-LDH, the number of diffraction peaks of the samples obtained after adding Co has no difference, and no impurity peaks are detected. However, the diffraction peaks shift slightly to a higher angle at 2θ, indicating that it has a similar layered crystal structure to MgAl-LDH. There is an exchange interaction between cobalt ions and divalent magnesium ions in the layer board, and cobalt ions partially replace the position of magnesium ions, resulting in a slight shift in the position of the diffraction peaks. The degree of shift increases with the increase in the Co addition amount. At the same time, the diffraction peak intensities at crystal planes (006), (012), (018), and (110) of the samples obtained after adding Co slightly decrease, and the diffraction peak intensity at the (003) plane increases. These two changes increase with the increase in the Co 2+ doping amount. Since the radius of cobalt ions is larger than that of magnesium ions, when cobalt ions enter the layer board, cobalt ions replace some magnesium ions. As the substitution of cobalt ions for magnesium ions increases, the layer spacing expands. The above XRD characterization results show that the addition of Co enables it to exchange with Mg and be doped into the lattice of MgAl-LDH, obtaining a cobalt-doped hydrotalcite sample Co-MgAl-LDH.

[0077] Table 1 shows the structural parameters and unit cell dimensions of the obtained products. a is the distance between metal cations in the unit cell, which is related to the atomic arrangement density of the crystal plane. c is the unit cell thickness, which is mainly related to the layer charge density. After adding Co, both a and c values change slightly. As the Co doping amount increases, the a value gradually increases and the c value gradually decreases, which is consistent with the XRD peak intensity results. This change is supported by the inductively coupled plasma optical emission spectrometry results, as shown in Table 2. ICP analysis shows that the exact ratio of MgAl-LDH (1.96:1) is similar to the feed ratio (M 2+ :M 3+ = 2:1). The ratio of Co in CoMgAl-LDH gradually increases as the molar fraction of Co in the feed increases from 0.1 to 0.25 and is almost close to the feed ratio. However, when the molar fraction of Co in the feed is further increased to 0.3, the ratio of Co in CoMgAl-LDH does not increase further. Therefore, based on the XRD and ICP results, it can be successfully confirmed that Co is doped into the MgAl-LDH system.

[0078] Table 1

[0079] Sample <![CDATA[d 110 / nm]]> <![CDATA[d 003 / nm]]> Parameter a / nm Parameter c / nm Co:Al = 0:1 0.1521 0.7630 0.3042 2.2890 Co:Al = 0.1:1 0.1524 0.7619 0.3048 2.2857 Co:Al = 0.15:1 0.1525 0.7604 0.3050 2.2812 Co:Al = 0.2:1 0.1527 0.7571 0.3054 2.2713 Co:Al = 0.25:1 0.1529 0.7486 0.3058 2.2458 Co:Al = 0.3:1 0.1526 0.7552 0.3052 2.2656

[0080] Table 2

[0081] Feed ratio (Co:Mg:Al) Co (wt%) Mg (wt%) Al (wt%) n(Co:Mg:Al) 0:2:1 ---- 15.366 8.703 1.96:1 0.1:2:1 1.771 14.839 8.625 0.094:1.910:1 0.15:2:1 2.596 14.457 8.676 0.137:1.847:1 0.2:2:1 3.578 14.206 8.713 0.188:1.793:1 0.25:2:1 4.708 13.645 8.691 0.248:1.743:1 0.3:2:1 4.663 13.565 8.679 0.246:1.738:1

[0082] Figure 2 SEM images of MgAl-LDH and those prepared under different n(Co:Al) conditions are presented. Among them, a is the SEM image of the MgAl-LDH sample, b is the SEM image of sample 1, c is the SEM image of sample 2, d is the SEM image of sample 3, e is the SEM image of sample 4, and f is the SEM image of sample 5. The MgAl-LDH sample presents a flaky structure ( Figure 2 a), without a specific polygonal shape. They are all irregular nanosheets with a diameter in the range of 100 - 300 nm, having a significant layered structure, and there is a relatively serious aggregation phenomenon between the nanosheets. After adding Co2+, the samples gradually disperse and the morphology is easier to observe, and the diameter of the product increases slightly. With the increase of the cobalt doping amount, the aggregation phenomenon is improved, and the improvement effect is the best when n(Co:Al) = 0.25:1. Continuing to increase the cobalt content will cause the aggregation phenomenon to intensify again. This indicates that the presence of Co hinders the aggregation of MgAl-LDH, helps to expose the adsorption sites, and thus enhances the adsorption performance.

[0083] The elemental composition and Co valence state information of MgAl-LDH and the samples after adding Co were analyzed by XPS. Figure 3a is the full spectrum of MgAl-LDH and CoMgAl-LDH. It can be found that compared with MgAl-LDH, CoMgAl-LDH contains five elements: O, Mg, C, Al, and Co. The appearance of Co element indicates that Co has been successfully doped into the hydrotalcite. The peaks of Al, Mg, O, Co, and C are deconvoluted and fitted. In the Co 2p spectrum of the CoMgAl-LDH sample ( Figure 3 b), the two strong peaks at 780.7 eV and 797.1 eV belong to Co 2p 3 / 2 and Co 2p 1 / 2 respectively. At the same time, the spin energy gap exceeding 15 eV confirms the coexistence of Co 2+ and Co 3+ . The two satellite peaks found at 786.1 eV and 803.2 eV correspond to the oscillation peaks of Co2p 3 / 2 and Co 2p 1 / 2 respectively. The four deconvoluted peaks at 780.7 / 796.4 eV and 781.9 / 797.8 eV correspond to Co 3+ and Co 2+ respectively. The peak of the Al 2p spectrum at 74 eV ( Figure 3 c) belongs to Al 3+ , indicating that it mainly remains as Al(OH) 3 . The peak of the Mg 2p spectrum at 1303.7 eV ( Figure 3 d) belongs to Mg 2+ , indicating that it mainly remains as Mg(OH) 2 . Figure 3 e is the high-resolution spectrum of O1s. These peaks can be further divided into metal oxides (M-O), metal hydroxides (M-OH), O v and adsorbed oxygen (O ads ). Although there is no oxygen atom in O v , due to the imprinting effect, it can adsorb hydroxyl or oxygen. Therefore, the XPS characteristics of these substances can indicate the existence of O v . The peak near 531.4 eV is classified as O v . After Co doping, the area ratio of O OH / V first increases and then decreases, increasing from 44.25% to 50.9% and then decreasing, indicating that the addition of cobalt ions generates more oxygen vacancies, providing more adsorption sites and helping to improve the adsorption performance. Figure 3 f is the C1s energy spectrum with three sub-peaks appearing at binding energies of 284 eV, 285.5 eV, and 288.3 eV, corresponding to C-C, C-O, and O=C-O respectively.

[0084] Figure 4shows the FT-IR spectra of MgAl-LDH and the samples prepared under different n(Co:Al). By Figure 4 It can be seen that in the high wavenumber range, a broad absorption peak appears at about 3600 - 3400 cm -1 for the magnesium-aluminum hydrotalcite, which is caused by the stretching vibration of the hydroxyl groups of the middle layer and interlayer water molecules in the magnesium-aluminum hydrotalcite. In the low wavenumber range, a weak absorption peak band appears at about 1641 cm -1 for all samples, and this peak is due to the deformation vibration of the interlayer water molecules belonging to the H-O-H bending vibration of physically adsorbed water. The band at 1371 cm -1 is due to the asymmetric vibration of the unrecognized carbonate (O=C-O), and O=C=O interacts with the metal cations of LDH. The band in the range of 540 - 650 cm -1 is caused by the vibration of Mg-O and Al-O groups. After doping with cobalt, the broad absorption peak band at about 3600 - 3400 cm -1 shifts slightly to the left, which is caused by the strong hydrogen bond interaction between the interlayer water molecules, interlayer carbonate anions and the hydroxyl groups on the layer board. Due to the hydrogen bond network, the vibration band formed by the asymmetric stretching of carbonate anions (CO -1 3 2- ) at 1371 cm) shifts to lower wavenumbers. As the cobalt content increases, the lattice vibration mode of M-O shifts to lower wavenumbers. The spectra of all materials show similar characteristics.

[0085] Figure 5 shows the nitrogen adsorption-desorption isotherms and the corresponding BJH pore size distribution diagrams of MgAl-LDH and Samples 1, 2, 3, 4, and 5. The nitrogen adsorption-desorption isotherms and pore size distributions of the samples are shown in the figure. In the range of relative pressure of 0.7 - 0.98, the nitrogen adsorption-desorption isotherm presents a type-IV hysteresis loop, indicating the presence of many mesopores and mesoporous structures, which shows the capillary condensation characteristics. This type indicates the presence of aggregates of plate-like particles, resulting in slit-shaped pores with uneven sizes and shapes. As the cobalt doping increases, the pore size and specific surface area of the material first increase and then decrease. When n(Co:Al) = 0.25:1, the specific surface area and pore size reach the maximum value. The results are shown in Table 3. The larger pore size can provide a good transport environment, which is more conducive to the adsorption of dyes. The continuous increase of the cobalt content leads to serious agglomeration, resulting in the decrease of the specific surface area and pore size, which is not conducive to the adsorption of dyes.

[0086] Table 3

[0087] Adsorbent <![CDATA[S BET (m 2 / g)]]> <![CDATA[Total pore volume (cm 3 / g)]]> Average pore diameter (nm) MgAl-LDH 35.462 0.136 18.606 Sample 1 41.871 0.141 28.636 Sample 2 42.433 0.156 32.573 Sample 3 42.903 0.193 33.648 Sample 4 47.186 0.221 37.873 Sample 5 43.909 0.195 28.636

[0088] Figure 6Pyrolysis process of MgAl-LDH and Co-MgAl-LDH from room temperature to 1000 °C in nitrogen atmosphere. The mass losses of MgAl-LDH and Co-MgAl-LDH materials are 41% and 40% respectively when heated from room temperature to 1000 °C in nitrogen atmosphere, and their mass losses are basically the same. It can be seen that there are two weight loss processes for this substance, and the mass changes sharply during the temperature changes from 150 °C to 250 °C and from 250 °C to 500 °C. The first weight loss process occurs from 150 °C to 250 °C, corresponding to the removal of physically adsorbed water and interlayer water molecules. The content of water is related to various factors, such as the particle size of the sample, the storage conditions of the sample, the types and amounts of the metal ions on the layer board and the interlayer anions, etc.; The second stage occurs from 250 °C to 500 °C, corresponding to the removal of interlayer anions and hydroxyl groups on the layer board. At this point, the layer board structure of the hydrotalcite has collapsed. It can be seen that Co 2+ doping does not change the structural characteristics of MgAl-LDH.

[0089] Application Example 1

[0090] Add the material Co-MgAl-LDHs (3 mg) into 200 mL of MG solution with a concentration of 500 mg / L for adsorption respectively, and control the adsorption temperature at 25 °C. Collect 0.2 mL of the adsorption solution through a 0.2 μm filter head and dilute it, and then measure the absorbance. Measure the remaining concentration of MG at a wavelength of 617 nm by ultraviolet-visible spectrophotometer respectively. Other adsorption experiments are as above, and all adsorption experiments are repeated three times and the average value is taken. Use MG aqueous solutions with concentrations of 300 - 1000 mg / L to measure the adsorption isotherm.

[0091] The adsorption capacity of the MG solution is calculated using equations (2-1) and (2-2):

[0092]

[0093] In the above equations, C 0 、C e 、C t are the initial concentration, equilibrium concentration and concentration at adsorption time t (mg / L) of the adsorption solution respectively, q t 、q e are the adsorption capacity at adsorption time t and the equilibrium adsorption capacity respectively, m is the mass (g) of the adsorbent used in the experiment, and V is the volume (L) of the adsorption solution.

[0094] Figure 7The adsorption performance of samples with different cobalt contents towards malachite green is shown. It can be seen from the figure that as the Co content n(Co:Al) increases from 0:1 to 0.25:1, the adsorption capacity of the samples gradually increases. When the Co content continues to increase to n(Co:Al) of 0.3:1 and 0.5:1, the adsorption capacity basically remains unchanged. Therefore, n(Co:Al) of 0.25:1 is preferably selected.

[0095] Figure 8 The adsorption performance of samples with different magnesium contents towards malachite green is shown. It can be seen from the figure that when n(Mg:Al) increases to 3:1, the adsorption capacity of malachite green basically remains unchanged. When the content of Mg continues to increase and n(Mg:Al) increases to 4:1, the adsorption capacity of malachite green slightly decreases. Therefore, n(Co:Mg:Al) of 0.25:2:1 is preferably selected.

[0096] Figure 9 The adsorption performance of samples prepared under different reaction times towards malachite green is shown. It can be seen from the figure that when the reaction temperature and reaction time are 12 h, the adsorption capacity towards malachite green is the highest, reaching 30.355 g / g. Figure 10 The adsorption performance of samples prepared under different reaction temperatures towards malachite green is shown. It can be seen from the figure that when the reaction temperature is 160 °C, the adsorption capacity towards malachite green is the highest, reaching 30.855 g / g. However, when the reaction time increases from 10 h to 12 h, or when the reaction temperature increases from 140 °C to 160 °C, the difference in the adsorption capacity is not significant. Using a longer reaction time and a higher reaction temperature increases the synthesis cost and wastes resources. Therefore, in this paper, the material is preferably prepared at a reaction temperature of 140 °C and a reaction time of 10 h.

[0097] Application Example 2

[0098] Different amounts of Co-MgAl-LDH prepared in Example 1 were respectively placed in 200 mL of 500 mg / L malachite green solution, and detected according to the same method as in Application Example 1. The test results are shown in Figure 11 . It can be obtained from the figure that when the dosage of Co-MgAl-LDH increases from 1 mg to 10 mg, the adsorption capacity of Co-MgAl-LDH towards malachite green shows a downward trend, decreasing from 30.05 g / g to 9.06 g / g. This may be because at a low adsorbent dosage, all the adsorption sites of the adsorbent can be exposed to the malachite green solution, so the adsorbent can quickly adsorb malachite green and reach a high adsorption capacity. However, when the adsorbent dosage is high, at a certain malachite green concentration, some adsorption sites of Co-MgAl-LDH do not play a role during the adsorption process, resulting in a lower adsorption capacity towards malachite green. Since there is a large difference in the adsorption capacity when changing from 2 mg to 10 mg. Therefore, to obtain a better adsorption effect, the dosage of Co-MgAl-LDH is preferably 3 mg.

[0099] Application Example 3

[0100] Take 3 mg of Co-MgAl-LDH prepared in Example 1 and place it in 200 mL of 500 mg / L malachite green solution at solution temperatures of 15 °C, 25 °C, 35 °C, 45 °C, and 55 °C respectively. Detect it according to the same method as in Application Example 1. The test results are shown in Figure 12 . It can be seen from the figure that as the temperature (15 - 55 °C) increases, the adsorption amount of Co-MgAl-LDH on malachite green first increases and then decreases. At 25 °C, the adsorption amount of Co-MgAl-LDH is the largest, reaching 31.90 g / g. When the reaction temperature is 55 °C, the adsorption amount of Co-MgAl-LDH is 26.85 g / g. This may be because at low temperatures, the dye obtains less kinetic energy, reducing the contact with the dye. However, as the temperature increases, the adsorption amount has been decreasing, indicating that the adsorption of Co-MgAl-LDH on malachite green is an exothermic process, and the increase in temperature is not conducive to the adsorption of malachite green. Since the adsorption amount increases in the range of 15 - 25 °C and decreases after 25 °C as the temperature increases, the preferred adsorption temperature is 25 °C.

[0101] Application Example 4

[0102] Take 3 mg of Co-MgAl-LDH prepared in Example 1 and add it to 200 mL of MG solution with a concentration of 500 mg / L for adsorption. Control the adsorption temperature at 25 °C, and use 0.1 M NaOH and 0.1 M HCl to adjust the pH value of the solution to 4, 5, 6, 7, 8, and 9 respectively. Detect it according to the same method as in Application Example 1. The test results are shown in Figure 13 . It can be seen from the figure that when the solution pH is in the range of 4 - 7, the adsorption amount of Co-MgAl-LDH on malachite green increases with the increase of the solution pH, and the adsorption amount increases from 27.34 g / g to 31.08 g / g; when the solution pH is in the range of 7 - 9, the adsorption amount of Co-MgAl-LDH on malachite green decreases with the increase of the solution pH. When the solution pH is 9, the adsorption amount of malachite green is 27.91 g / g. The effect on malachite green gradually increases during the process of pH value from 4 to 7. However, as the pH continues to increase, the OH - in the solution gradually increases, and the OH - in the solution competes with the adsorption of Co-MgAl-LDH on malachite green dye, resulting in a downward trend in the adsorption amount. Considering comprehensively, adsorption is carried out without adjusting the acidity and alkalinity of the initial dye concentration (pH value is 7).

[0103] Application Example 5

[0104] Take 3 mg of the Co-MgAl-LDH prepared in Example 1 and add it to 200 mL of MG solutions with different concentrations for adsorption. Control the adsorption temperature at 25 °C and perform detection according to the same method as in Application Example 1. The test results are shown in Figure 14 . As can be seen from the figure, as the initial concentration of MG (C 0 ) increases from 300 mg / L to 1000 mg / L, the equilibrium adsorption capacity (q e ) of the Co-MgAl-LDH composite material also increases from 26.83 g / g to 30.26 g / g. It can be seen that the slope of the adsorption isotherm has decreased slowly at this time. Combining with the conclusion of the influence of the adsorbent dosage on the adsorption of MG, it has reached the saturation state at this time.

[0105] The above has described the present invention in detail. The purpose is to enable those skilled in this field to understand the content of the present invention and implement it, and it should not be used to limit the protection scope of the present invention. Any equivalent changes or modifications made according to the spirit and essence of the present invention should be covered within the protection scope of the present invention.

Claims

1. A method for preparing a cobalt-doped MgAl-LDH adsorbent, characterized in that: The cobalt-doped MgAl-LDH adsorbent is prepared by reacting cobalt nitrate, aluminum nitrate and magnesium nitrate by a precipitation method, wherein the cobalt nitrate, aluminum nitrate and magnesium nitrate are controlled according to Co 2+ :Al 3+ :Mg 2+ The molar ratio of the raw materials is 0.1-0.5:1:2-4.

2. The method for preparing the cobalt-doped MgAl-LDH adsorbent according to claim 1, characterized in that: Control cobalt nitrate, aluminum nitrate and magnesium nitrate according to Co 2+ :Al 3+ :Mg 2+ The molar ratio of the raw materials is 0.2-0.3:1:2-3.

3. The method for preparing the cobalt-doped MgAl-LDH adsorbent according to claim 1, characterized in that: The reaction temperature is controlled at 100-180°C and the reaction time is 6-14h.

4. The method for preparing the cobalt-doped MgAl-LDH adsorbent according to claim 3, characterized in that: The reaction temperature is controlled at 120-160°C and the reaction time is 8-12h.

5. The method for preparing the cobalt-doped MgAl-LDH adsorbent according to claim 1, characterized in that: The preparation method is specifically as follows: dissolving cobalt nitrate, aluminum nitrate and magnesium nitrate in water, dropping a mixed alkaline solution of sodium carbonate and sodium hydroxide, adjusting the pH value of the reaction system to 9-11, and then reacting at 100-180° C. for 6-14 hours in a sealed state. After the reaction is completed, the reaction product is washed and dried to obtain the cobalt-doped MgAl-LDH adsorbent.

6. A cobalt-doped MgAl-LDH adsorbent prepared by the preparation method according to any one of claims 1 to 5, wherein: The cobalt-doped MgAl-LDH adsorbent has a nanosheet structure. 2+ and Co 3+ The cobalt-doped MgAl-LDH adsorbent has a specific surface area of ​​40 to 50 m 2 / g, total pore volume is 0.14~0.23cm 3 / g, and the average pore size is 28-38nm.

7. A method for adsorbing malachite green in water, characterized in that: The cobalt-doped MgAl-LDH adsorbent prepared by the preparation method according to any one of claims 1 to 5 or the cobalt-doped MgAl-LDH adsorbent according to claim 6 adsorbs malachite green in water.

8. The method for adsorbing malachite green in water according to claim 7, characterized in that: The concentration of malachite green in the water body is controlled to be 100-600 mg / L, and the equilibrium adsorption amount of the cobalt-doped MgAl-LDH adsorbent is 26 g / g-32 g / g.

9. The method for adsorbing malachite green in water according to claim 7, characterized in that: The temperature of the water body is controlled to be 15-55°C.

10. The method for adsorbing malachite green in water according to claim 7, characterized in that: The pH value of the water body is controlled to be 4-9.

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