Composite adsorbent, preparation method and application thereof, and method for removing fluorine ions in photovoltaic wastewater

Abstract

The present application relates to the technical field of water treatment, in particular to a kind of composite adsorbent and its preparation method and application, method for removing fluorine ion in photovoltaic wastewater.The preparation method of composite adsorbent includes the following steps: citric acid and chitosan are mixed uniformly according to weight ratio of 5:4 to obtain mixed solution A.Aluminum chloride solution and mixed solution A are mixed uniformly according to volume ratio of 4:5 to obtain mixed solution B.Add alkali solution to mixed solution B until pH reaches 7.0-8.0 to obtain suspension C.After suspension C is aged, it is sequentially washed with water, washed with alcohol and centrifuged and freeze-dried to obtain composite adsorbent.The present application successfully synthesizes composite adsorbent with large specific surface area and reasonable pore size structure by loading aluminum oxide on chitosan and introducing citric acid as chelating agent.The composite adsorbent can effectively resist the interference of inorganic anions such as Cl ‑ 、SO4 2‑ And NO3 ‑ , maintain good defluorination effect, and is suitable for treating high-salt fluorine-containing wastewater.

Description

Technical Field

[0001] The present invention relates to the technical field of water treatment, and in particular to a composite adsorbent, a preparation method and application thereof, and a method for removing fluoride ions from photovoltaic wastewater. Background Art

[0002] It is extremely important to effectively treat fluorine-containing wastewater from the photovoltaic industry, reduce water pollution and ecological environment damage caused by the discharge of fluorine-containing wastewater, and reduce the adverse effects on animals, plants and humans.

[0003] Adsorption has become one of the most advantageous methods for deep fluoride removal due to its simplicity, wide applicability, high efficiency, low cost, and stable operation. Alumina is currently the most widely used and effective adsorption material. Alumina has a large surface area and surface hydroxyl sites, enabling effective adsorption of fluoride ions through electrostatic adsorption. However, existing activated alumina still has significant drawbacks, such as low adsorption capacity, short service life, and a narrow pH range.

[0004] After searching, the invention with application number CN202310514430.4 discloses a porous Al2O3-La2O3 composite material supported by a carbon fiber structure and its use, which is prepared and synthesized by combining gelation technology and calcination. However, the synthesis steps of the adsorption material are complicated, and the introduction of rare earth metals leads to high preparation costs. The invention with application number CN202010231050.6 discloses a preparation method and application of Zr / La co-modified cross-linked chitosan. This technology introduces chitosan into a composite adsorbent, and the rarity of Zr and La metals greatly limits the promotion and application of this technology. Through the above search, it can be seen that the adsorbents in the prior art not only have complex preparation processes and high preparation costs, but the adsorption effect of the prepared adsorbent on fluoride ions is also interfered with by other anions in photovoltaic wastewater, resulting in poor adsorption effect on fluoride ions. Summary of the Invention

[0005] In order to solve the technical problem that the adsorbent prepared by the prior art is easily affected by other anions in photovoltaic wastewater when adsorbing fluoride ions, resulting in poor treatment effect on fluoride ions, the present invention provides a composite adsorbent and its preparation method and application, and a method for removing fluoride ions from photovoltaic wastewater.

[0006] The present invention is implemented using the following technical solution: A method for preparing a composite adsorbent comprises the following steps: uniformly mixing 1% to 5% citric acid and 2g / L to 10g / L chitosan in a weight ratio of 5:4 to obtain a mixed solution A. Aluminum chloride solution with a molar concentration of 1mol / L to 5mol / L is added to mixed solution A and uniformly mixed to obtain a mixed solution B; the volume ratio of the aluminum chloride solution to mixed solution A is 4:5. Alkali solution is then dropwise added to mixed solution B until the pH of mixed solution B reaches 7.0-8.0, to obtain a suspension C. After aging, the suspension C is sequentially washed with water and alcohol multiple times, and finally centrifuged and freeze-dried to obtain the composite adsorbent.

[0007] As a further improvement of the present invention, the fluorine removal rate of the composite adsorbent in photovoltaic wastewater within the pH range of 3-7 is higher than 90%, and the fluorine ion concentration in the treated photovoltaic wastewater is lower than 1.5 mg / L.

[0008] As a further improvement of the present invention, the alumina fixed in the chitosan is amorphous alumina, and the loading amount of the amorphous alumina is 15%-35% in terms of aluminum element.

[0009] As a further improvement of the present invention, the alkali solution 1 includes one of NaOH, KOH, and NH3•H2O.

[0010] As a further improvement of the present invention, the molar concentration of the alkali solution 1 is 0.5 mol / L to 10 mol / L.

[0011] As a further improvement of the present invention, the deacetylation degree of chitosan is 60%-80%, and the molecular weight is 5×10 4 Da~5×10 5 Da.

[0012] As a further improvement of the present invention, the standing aging time is 10 hours to 20 hours, and the freeze-drying time is 12 hours to 24 hours.

[0013] The present invention also includes a composite adsorbent, which is prepared by the above-mentioned preparation method of the composite adsorbent. After adsorbing fluoride ions in photovoltaic wastewater, the composite adsorbent can be desorbed and regenerated by alkaline solution 2.

[0014] As a further improvement of the present invention, the second alkali solution includes one of NaOH and KOH; the molar concentration of the second alkali solution is 0.5 mol / L-10 mol / L.

[0015] As a further improvement of the present invention, the composite adsorbent has a porous structure with a pore volume of 0.2 cm 3 / g-0.8 cm 3 / g, specific surface area is 100cm 2 / g-800 cm 2 / g.

[0016] The present invention also includes an application of the composite adsorbent as described above, wherein the -NH2 group, -OH group, -CH2 group and -CO group in the composite adsorbent can resist Cl - 、SO4 2- and NO3 - The fluoride ions in photovoltaic wastewater can be removed by chemical adsorption.

[0017] The present invention also includes a method for removing fluoride ions from photovoltaic wastewater, comprising the following steps: adding a composite adsorbent prepared by the composite adsorbent preparation method described above to the photovoltaic wastewater to be treated and stirring the mixture; while stirring, using a fluoride ion detection instrument to monitor the fluoride ion concentration in the photovoltaic wastewater in real time; if the fluoride ion detection instrument detects a fluoride ion concentration in the photovoltaic wastewater exceeding 1.5 mg / L, continuing to add the composite adsorbent to the photovoltaic wastewater to be treated until the fluoride ion detection instrument detects a fluoride ion concentration in the photovoltaic wastewater to be treated below 1.5 mg / L.

[0018] The technical solution provided by the present invention has the following beneficial effects:

[0019] (1) The preparation process of the composite adsorbent provided by the present invention is the first to introduce chitosan as a modifier into activated alumina, and a composite adsorbent with excellent adsorption performance is prepared through a specific synthesis process. This innovation not only solves the problems of low adsorption capacity and narrow pH application range of traditional activated alumina, but also significantly improves the stability and service life of the prepared composite adsorbent. At the same time, by introducing citric acid as a chelating agent, a composite adsorbent with a large specific surface area and a reasonable pore structure is successfully synthesized, so that the prepared composite adsorbent can achieve deep fluoride removal and meet increasingly stringent wastewater discharge standards. Moreover, the composite adsorbent can effectively resist Cl − 、SO4 2− and NO3 − It can eliminate the interference of inorganic anions such as fluorine and maintain good defluorination effect, and is suitable for treating high-salt fluoride-containing wastewater.

[0020] (2) The composite adsorbent of the present invention can effectively increase the exchangeable groups on the surface of the prepared composite adsorbent by loading amorphous alumina onto chitosan, thereby changing the specific surface area and improving the adsorption capacity. At the same time, the citric acid can not only acidify the chitosan, making it easier for the amorphous alumina to be loaded onto the chitosan, but also promote the formation of macroporous and mesoporous structures in the amorphous alumina, thereby improving the adsorption effect of the prepared composite adsorbent.

[0021] (3) The composite adsorbent of the present invention is prepared by acidification with citric acid followed by in-situ precipitation. It not only has high fluorine removal performance, but also has good environmental adaptability and economic efficiency. It provides an effective fluorine removal adsorbent for deep fluorine removal. After adsorption by the composite adsorbent of the present invention, the concentration of fluoride ions in photovoltaic wastewater is less than 1.5 mg / L, meeting the wastewater discharge standard. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is the appearance of the composite adsorbent prepared in Example 4 of the present invention.

[0023] Figure 2 These are electron microscope images of the composite adsorbent prepared in Example 4 of the present invention at different magnifications under a scanning electron microscope.

[0024] Figure 3 This is a picture obtained after analyzing the composite adsorbent prepared in Example 4 of the present invention in an EDS spectrometer.

[0025] Figure 4 This is a picture of the composite adsorbent prepared in Example 4 of the present invention taken under a transmission electron microscope.

[0026] Figure 5 This is the X-ray diffraction pattern of the composite adsorbent prepared in Example 4 of the present invention.

[0027] Figure 6 This is a graph showing the FT-IR spectra of the composite adsorbent prepared in Example 4 of the present invention before and after adsorbing fluoride ions in photovoltaic wastewater.

[0028] Figure 7 Schematic diagram of the defluorination efficiency of fluoride ions in photovoltaic wastewater by the composite adsorbents prepared in Example 4, Comparative Example 1 and Comparative Example 2 of the present invention.

[0029] Figure 8 Schematic diagram of the defluorination efficiency of fluoride ions in photovoltaic wastewater under different pH conditions of the composite adsorbent prepared in Example 4 of the present invention and Comparative Example 2.

[0030] Figure 9 This is a schematic diagram of the defluorination efficiency of the composite adsorbent prepared in Example 4 of the present invention for fluoride ions in photovoltaic wastewater under the interference of different anions.

[0031] Figure 10 Schematic diagram of the defluorination efficiency of fluoride ions in photovoltaic wastewater by the composite adsorbent prepared in Examples 4, 5 and 6 of the present invention. DETAILED DESCRIPTION

[0032] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0033] Example 1

[0034] With the development of the photovoltaic industry, photovoltaic wastewater has also increased. Photovoltaic wastewater is a type of wastewater with high salt content and contains a lot of fluoride ions. Excessive fluoride ions in water bodies not only endanger ecological safety, but also pose a threat to human health. In the existing technology, fluoride ions in photovoltaic wastewater are generally removed by physical adsorption. This physical adsorption method generally makes the adsorbent positively charged, and uses the positive charge to adsorb fluoride ions in photovoltaic wastewater. However, since the adsorption sites of the adsorbent are limited, and photovoltaic wastewater also contains a large amount of Cl - 、SO4 2- and NO3 - Therefore, when the existing adsorbents adsorb fluoride ions in photovoltaic wastewater by physical adsorption, Cl - 、SO4 2- and NO3 - These inorganic anions will also be adsorbed by the adsorbent, thus occupying many adsorbent sites. As a result, existing adsorbents are unable to effectively remove fluoride ions from photovoltaic wastewater.

[0035] To address this issue, this embodiment provides a method for preparing a composite adsorbent, comprising the following steps: uniformly mixing citric acid having a volume fraction of 1% to 5% and chitosan having a mass concentration of 2g / L to 10g / L in a weight ratio of 5:4 to obtain a mixed solution A. Aluminum chloride solution having a molar concentration of 1mol / L to 5mol / L is added to mixed solution A in a volume ratio of 4:5, and the mixture is uniformly mixed to obtain a mixed solution B. Alkali solution is then added dropwise to mixed solution B until the pH of mixed solution B reaches 7.0-8.0, to obtain a suspension C. After aging, suspension C is washed with water and alcohol multiple times, and finally centrifuged and freeze-dried to obtain a composite adsorbent.

[0036] In the above preparation process, we introduced chitosan as a modifier into activated alumina for the first time, and prepared a composite adsorbent with excellent adsorption performance through a specific synthesis process. This innovation not only solves the problems of low adsorption capacity and narrow pH application range of traditional activated alumina, but also significantly improves the stability and service life of the prepared composite adsorbent. At the same time, by introducing citric acid as a chelating agent, a composite adsorbent with a large specific surface area and a reasonable pore structure was successfully synthesized, so that the prepared composite adsorbent can achieve deep fluoride removal and meet increasingly stringent wastewater discharge standards. And the composite adsorbent can effectively resist Cl − 、SO4 2− and NO3 − It can eliminate the interference of inorganic anions such as fluorine and maintain good defluorination effect, and is suitable for treating high-salt fluoride-containing wastewater.

[0037] The composite adsorbent prepared by the above method includes -NH2 group, -OH group, -CH2 group, and -CO group, which can resist Cl in photovoltaic wastewater. - 、SO4 2- and NO3 - The invention can eliminate the interference of inorganic anions such as fluorine and can specifically bind to fluoride ions in photovoltaic wastewater. The composite adsorbent in this embodiment is combined with fluoride ions by forming a chemical bond. This chemical adsorption method has a fast reaction speed, and the compound formed by the adsorbent on the adsorption surface has high stability and selectivity. The physical adsorption speed is relatively slow, and the thermodynamic parameters of the adsorption process are also small, which makes the reverse reaction of physical adsorption easy to occur. Therefore, the adsorbent of this embodiment can specifically bind to fluoride ions in photovoltaic wastewater by chemical adsorption, thereby improving the adsorption effect of fluoride ions in photovoltaic wastewater.

[0038] The composite adsorbent prepared by the above method uses natural high molecular polymer chitosan as a carrier, and the pores of chitosan are embedded with alumina. The pore volume of the composite adsorbent is 0.2 cm 3 / g-0.8 cm 3 / g, specific surface area is 100cm 2 / g-800 cm 2 / g. The pore volume is the volume of the micropores in the adsorbent, and is usually expressed as the volume of the adsorbent micropores per unit weight of the adsorbent. The pore volume is the effective volume of the adsorbent, which is the value calculated using the saturated adsorption capacity, that is, the volume of the adsorbate that the adsorbent can accommodate. Therefore, for the adsorbent, the larger the pore volume, the better the adsorption effect. As for the specific surface area, the larger the specific surface area, the better the adsorption capacity. Therefore, by measuring the pore volume and specific surface area of ​​the composite adsorbent prepared by the preparation method of this embodiment, it can be proved that the composite catalyst prepared in this embodiment has good adsorption capacity.

[0039] In this example, by loading amorphous alumina onto chitosan, the number of exchangeable groups on the surface of the prepared composite adsorbent is effectively increased, thereby changing the specific surface area and improving the adsorption capacity. Simultaneously, the addition of citric acid not only acidifies the chitosan, making it easier for the amorphous alumina to be loaded onto the chitosan, but also promotes the formation of macroporous and mesoporous structures in the amorphous alumina, thereby enhancing the adsorption efficiency of the prepared composite adsorbent.

[0040] The alumina fixed in the chitosan pores can be amorphous alumina, with a loading of 15% to 35% of the elemental aluminum. Amorphous alumina has a high specific surface area and a controlled pore structure, which can enhance the catalytic and adsorption capabilities of the composite adsorbent.

[0041] Among them, the deacetylated fluorine content of chitosan is 60%-80%, and the molecular weight is 5×10 4 Da~5×10 5 The static aging time can be 10-20 hours, and the freeze-drying time can be 12-24 hours. Freeze-drying helps the prepared composite adsorbent form a porous structure, thereby improving the adsorption effect of the prepared composite adsorbent.

[0042] The alkali solution (I) can be one of NaOH, KOH, or NH₃•H₂O. The molar concentration of the alkali solution (I) can be between 0.5 mol / L and 10 mol / L. By adding the alkali solution (I), the desired amorphous alumina can be obtained, and the amorphous alumina can be loaded onto the chitosan, thereby producing the desired composite adsorbent.

[0043] The composite adsorbent prepared by the above preparation method is a spherical material that can be desorbed and regenerated using a second alkali solution. This allows the composite adsorbent prepared in this embodiment to be recycled, improving its practicality. The second alkali solution can be either NaOH or KOH. The molar concentration of the second alkali solution can be between 0.5 mol / L and 10 mol / L.

[0044] Example 2

[0045] An application of a composite adsorbent, the composite adsorbent is obtained by the preparation method of Example 1. The -NH2 group, -OH group, -CH2 group, and -CO group in the prepared composite adsorbent can resist Cl - 、SO4 2- and NO3 - The fluoride ions in photovoltaic wastewater can be removed by chemical adsorption.

[0046] Example 3

[0047] A method for removing fluoride ions from photovoltaic wastewater comprises the following steps: (1) pouring the composite adsorbent prepared by the preparation method of Example 1 into the photovoltaic wastewater to be treated and stirring. (2) detecting the concentration of fluoride ions in the photovoltaic wastewater in real time using a fluoride ion detection instrument while stirring. If the fluoride ion detection instrument detects that the fluoride ion concentration in the photovoltaic wastewater is higher than 1.5 mg / L, continue adding the composite adsorbent to the photovoltaic wastewater to be treated until the fluoride ion detection instrument detects that the fluoride ion concentration in the photovoltaic wastewater to be treated is lower than 1.5 mg / L.

[0048] In this embodiment, the fluoride ion detection instrument may be a fluoride ion analyzer.

[0049] Several specific preparation processes of the composite adsorbent are listed below. Please refer to Example 4, Example 5, Example 6, Comparative Example 1 and Comparative Example 2 for details.

[0050] Example 4

[0051] A method for preparing a composite adsorbent comprises the following steps:

[0052] (1) Chitosan powder with a mass concentration of 5 g / L and a weight of 5 g was added to a citric acid solution with a volume fraction of 2% and a volume of 125 mL, and stirred at 25 °C at a stirring rate of 500 r / min to obtain a mixed solution A.

[0053] (2) A 100 mL aluminum chloride solution with a molar concentration of 1 mol / L was added to the mixed solution A and stirred evenly to obtain mixed solution B.

[0054] (3) Add 5 mol / L NaOH solution to the mixture B until the pH of the mixture B is between 7.0 and 8.0 to obtain suspension C.

[0055] (4) The suspension C was allowed to stand for 20 h, washed with water three times, and then alcoholized three times. Finally, it was centrifuged and freeze-dried for 24 h to obtain composite adsorbent 1.

[0056] Example 5

[0057] A method for preparing a composite adsorbent comprises the following steps:

[0058] (1) Chitosan powder with a mass concentration of 5 g / L and a weight of 5 g was added to a citric acid solution with a volume fraction of 2% and a volume of 125 mL, and stirred at 25 °C at a stirring rate of 500 r / min to obtain a mixed solution A.

[0059] (2) A 100 mL aluminum chloride solution with a molar concentration of 3 mol / L was added to the mixed solution A and stirred evenly to obtain a mixed solution B.

[0060] (3) Add 5 mol / L NaOH solution to the mixture B until the pH of the mixture B is between 7.0 and 8.0 to obtain suspension C.

[0061] (4) The suspension C was allowed to stand for 20 h, washed with water three times, and then alcoholized three times. Finally, it was centrifuged and freeze-dried for 24 h to obtain the composite adsorbent 2.

[0062] Example 6

[0063] A method for preparing a composite adsorbent comprises the following steps:

[0064] (1) Chitosan powder with a mass concentration of 5 g / L and a weight of 5 g was added to a citric acid solution with a volume fraction of 2% and a volume of 125 mL, and stirred at 25 °C at a stirring rate of 500 r / min to obtain a mixed solution A.

[0065] (2) A 100 mL aluminum chloride solution with a molar concentration of 5 mol / L was added to the mixed solution A and stirred evenly to obtain a mixed solution B.

[0066] (3) Add 5 mol / L NaOH solution to the mixture B until the pH of the mixture B is between 7.0 and 8.0 to obtain suspension C.

[0067] (4) The suspension C was allowed to stand for 20 h, washed with water three times, and then alcoholized three times. Finally, it was centrifuged and freeze-dried for 24 h to obtain composite adsorbent three.

[0068] Comparative Example 1

[0069] A method for preparing a composite adsorbent comprises the following steps:

[0070] (1) Chitosan powder with a mass concentration of 5 g / L and a weight of 5 g was added to a citric acid solution with a volume fraction of 2% and a volume of 125 mL, and stirred at 25 °C at a stirring rate of 500 r / min to obtain a mixed solution A.

[0071] (2) Add 5 mol / L NaOH solution to the mixture A until the pH of the mixture A is between 7.0 and 8.0 to obtain suspension C.

[0072] (4) The suspension C was allowed to stand for 20 h, washed with water three times, and then alcoholized three times. Finally, it was centrifuged and freeze-dried for 24 h to obtain composite adsorbent 4.

[0073] Comparative Example 2

[0074] A method for preparing a composite adsorbent comprises the following steps:

[0075] (1) A 100 mL aluminum chloride solution with a molar concentration of 5 mol / L was added to a 125 mL citric acid solution with a volume fraction of 2%. The mixture was stirred at 25°C at a stirring rate of 500 r / min to obtain a mixed solution A.

[0076] (2) Add 5 mol / L NaOH solution to the mixture B until the pH of the mixture B is between 7.0 and 8.0 to obtain suspension C.

[0077] (3) The suspension C was allowed to stand for 20 h, washed with water three times, and then alcoholized three times. Finally, it was centrifuged and freeze-dried for 24 h to obtain composite adsorbent V.

[0078] Performance Testing

[0079] In order to verify the properties and performance of the composite adsorbent provided in this embodiment, technicians verified the schemes of Example 4, Example 5, Example 6, Comparative Example 1 and Comparative Example 2 in subsequent tests and experiments, and tested the adsorption effect of the prepared composite adsorbent on fluoride ions.

[0080] (1) Pore volume and specific surface area of ​​composite adsorbent

[0081] The composite adsorbent prepared in Example 4 was placed in a pore volume and pore size distribution measuring instrument for measurement, and the pore volume of the composite adsorbent prepared in Example 1 was obtained to be 0.38 cm 3 / g, with a specific surface area of ​​230 cm 2 / g. And the picture of the composite adsorbent prepared in Example 1 is as follows Figure 1 As shown, by observing Figure 1 Combined with the above data, it can be proved that the composite adsorbent prepared in Example 1 has a porous structure and a high specific surface area, and therefore has a stronger adsorption capacity.

[0082] (2) Composition analysis of composite adsorbent

[0083] 2.1 Scanning electron microscopy experiments

[0084] The composite adsorbent prepared in Example 4 was placed under a scanning electron microscope for observation, as shown below: Figure 2 The data shown. Figure 2 (a) is an electron microscope image of the partial structure of the composite adsorbent under a scanning electron microscope at 100 μm. Figure 2 (b) is an electron microscope image of the partial structure of the composite adsorbent under a scanning electron microscope at 5 μm. Figure 2 (c) is an electron microscope image of the partial structure of the composite adsorbent in the scanning electrode at 10 μm.

[0085] 2.2 EDS energy spectrum analysis experiment

[0086] The composite adsorbent prepared in Example 4 was put into EDS spectrometer for analysis, and the obtained Figure 3 .in, Figure 3 The picture on the left is the analysis diagram of the group elements in the composite adsorbent. Figure 3 The picture on the right side shows the distribution of aluminum ions in the composite adsorbent.

[0087] By combining the scanning electron microscopy results and EDS energy spectrum analysis results of the composite adsorbent prepared in Example 4, it can be seen that: Figure 2 From (a), (b), and (c), we can see that the surface of the composite adsorbent prepared in Example 4 has many wrinkles and is distributed with slit-shaped or wedge-shaped pores. These structures effectively increase its specific surface area and provide structural support for the high adsorption capacity of the composite adsorbent. Figure 2 (a) and Figure 3 Analysis shows that the surface of the composite adsorbent prepared in Example 4 is evenly distributed with nano-alumina particles with a diameter between 1nm and 10nm, and the inner surface is uneven. The reason for the uneven inner surface may be the result of the complexation of alumina and chitosan.

[0088] 2.3 Transmission electron microscopy experiments

[0089] The composite adsorbent prepared in Example 4 was analyzed under a transmission electron microscope to obtain Figure 4 . Figure 4 The middle picture is a picture of the composite adsorbent taken under a transmission electron microscope. Figure 4Analysis revealed that the nanoparticles loaded on the chitosan substrate exhibited clear lattice fringes, with a measured interplanar spacing of 0.3412 nm (3.412 Å). This nanoparticle closely matches the theoretical value of the (012) plane of α-Al2O3 (corundum, trigonal system) (JCPDS 46-1212: ~3.479 Å). The slight deviation may be due to lattice distortion or local stress in the nanoparticles. These results directly demonstrate that the alumina in the composite adsorbent exists in the form of a thermally stable α phase. The continuity and regular arrangement of the lattice fringes indicate that the alumina has zero crystallinity, and no obvious lattice defects or distortions were observed. This indicates that the loading of chitosan did not significantly destroy the crystal structure of Al2O3, and also demonstrates the successful preparation and structural stability of the composite adsorbent, providing a reliable basis for crystalline active sites for its applications such as adsorption or catalysis.

[0090] 2.4 XRD (X-ray diffraction) analysis

[0091] The crystal structure of the composite adsorbent prepared in Example 4 was analyzed by X-ray diffraction technology to obtain Figure 5 Through Figure 5 Analysis revealed broad diffraction peaks at 2θ = 11.2° and 19.8°, with diffuse peaks and low intensities, typical of the amorphous titanium structure of chitosan. This result indicates that the chitosan matrix did not undergo significant crystallization during the composite process, and the disordered arrangement of its molecular chains was retained. Sharp diffraction peaks were detected at 2θ = 31.8°, 45.6°, 56.4°, and 75.4°, which are highly consistent with the characteristic peaks of crystalline Al2O3 in the standard diffraction data of alumina (JCPDS series cards). For example, the 45.6° peak matches the (400) crystal plane of alumina (JCPDS 10-0425). The 75.4° peak corresponds to the high-angle diffraction peak of alumina (such as the (300) crystal plane in JCPDS 46-1212). By comparing with the standard cards, it was found that the relative intensity and peak position of the above-mentioned sharp diffraction peaks in all composite adsorbents and the relative intensity and peak position of the characteristic peaks of alumina in the standard cards had errors of less than ±0.3°, which indicates that the crystalline structure of alumina in the composite adsorbent is complete and there is no significant interference from impurities. This also directly proves that the alumina in the composite adsorbent in this Example 4 was successfully loaded onto chitosan, and the loading process did not cause damage to the structure of the chitosan matrix or alumina particles.

[0092] 2.5 Study on the adsorption mechanism of fluoride ions by composite adsorbent

[0093] In order to further explore the adsorption mechanism of fluoride ions by the composite adsorbent prepared in this example, the present application also conducted a comparative analysis of the Fourier transform infrared spectra (FT-IR) of the composite adsorbent prepared in Example 4 before and after adsorbing fluoride ions in photovoltaic wastewater, and obtained Figure 6 . Figure 6 Curve a in the middle is the FT-IR spectrum curve of the composite adsorbent before adsorbing fluoride ions in photovoltaic wastewater. Figure 6 Curve b in the middle is the FT-IR spectrum curve of the composite adsorbent after adsorbing fluoride ions in photovoltaic wastewater. Figure 6 Analysis shows that at 3221cm -1 The broad band at 2901cm indicates the presence of -OH and -NH stretching vibrations and is the main absorption peak. -1 The band at 1582 cm represents the -CH stretching vibration of -CH and -CH2. -1 The band at 1152cm corresponds to the -NH bending vibration of -NH2. -1 The band at 1061 cm corresponds to the stretching vibration of -OH. -1 The band at 1000 cm corresponds to the stretching vibration of -CO in -COH. In addition, the band at 1000 cm -1 and 500cm -1 The bands observed between the two bands may be attributed to the characteristic vibration of aluminum oxide. -1 Al-O stretching mode was observed.

[0094] Comparing the FT-IR spectra before and after the composite adsorbent, it was found that at 3221 cm -1 and 1582cm -1 The obvious shift was found at both ends, and the peak intensity was weakened. This is because the N atoms in NH and F - A coordination reaction occurs between them to form an N→F coordination bond. -1 The band at the position also shifts significantly and the peak intensity weakens. This is because -OH participates in the - The adsorption process. At 1061cm -1 The band at the position also shifted significantly after adsorption, and the peak intensity increased, which proves that -CO also participated in the adsorption of F. - The increase in peak intensity may be attributed to the fewer functional groups in the loaded adsorbent. -1 The AL-O stretching band corresponding to the adsorption shifts to 622 cm -1 This indicates that during the adsorption process, the Al-O bond and F - Surface complexation or ion exchange may occur between them.

[0095] In addition, it can be seen from observation that Figure 6 The FT-IR spectra before adsorption (curve a) and after adsorption (curve b) show significant differences in characteristic peak positions and intensities, which reveals the interaction between the functional groups on the surface of the composite adsorbent and the fluoride ions. -1 ), the broad absorption band at this location undergoes a significant red shift and weakens in intensity after adsorption, which is the main absorption peak. This change is due to the coordination of the lone-pair-rich N atoms in the -NH2 groups on the surface of the material with F-, forming a stable N→F coordination bond, which causes the vibration mode of the NH bond to change. It is worth noting that the NH2 group at 1582 cm -1 The -NH bending vibration peak at 1152 cm also shows a shift phenomenon, which further proves the key role of amino group in the adsorption process. -1 ) indicates that the -OH groups on the surface of the composite adsorbent form OH...F dynamic bonds with F- through hydrogen bonding, and this weak interaction may play an important role in the initial stage of adsorption.

[0096] Of particular note is the -1 The characteristic peak of the -CO group at the site undergoes a significant blue shift and abnormally enhanced intensity after adsorption. This phenomenon may be attributed to two mechanisms: on the one hand, F - The introduction of induces the deprotonation process of the carboxylic acid group, resulting in the enhancement of the C=O double bond characteristics; on the other hand, the destruction of the internal hydrogen bond network of the composite adsorbent after adsorption increases the vibration freedom of the -CO group. At the same time, the consumption of some functional groups may trigger the intramolecular charge rearrangement. The above reasons together lead to the abnormal increase of the absorption peak intensity.

[0097] At 646cm -1 The Al-O stretching vibration corresponding to the composite adsorbent adsorbs F - Back offset to 622cm -1 This indicates that during the adsorption process, the Al-O bond in the composite adsorbent may have reacted with the F - Surface complexation or ion exchange occurs between them.

[0098] The above analysis reveals that the present invention's multidimensional adsorption mechanism, revealed through FT-IR spectroscopy, is based on the composite adsorbent. The multidimensional adsorption mechanism is as follows: amino groups dominate chemical adsorption through coordination bonds, hydroxyl groups participate in physical adsorption through hydrogen bonds, and carboxylic acid groups assist in ion capture through electrostatic interactions, achieving deep fluoride removal. This multi-faceted synergistic mechanism not only explains the material's excellent adsorption performance but also provides a theoretical basis for optimizing material selectivity through subsequent functional group modification.

[0099] Combined reaction mechanism: amino groups dominate chemical adsorption through coordination bonds, F- forms hydroxyl groups through hydrogen bonds with Al-O bonds, and carboxylic acid groups assist ion capture through electrostatic effects to achieve deep fluoride removal. It can be summarized that the composite adsorbent prepared by the present invention uses chemical adsorption to form coordination bonds as the main adsorption function, and physical adsorption assists in the adsorption and removal of fluoride ions in photovoltaic wastewater. The reasons can be analyzed from the following content:

[0100] (1) Selective chemical adsorption dominated by coordination bonds

[0101] The lone pair of electrons (-NH2) in the amino group preferentially forms a strong coordination bond (N→F) with F⁻. This specific interaction stems from F⁻'s high charge density (small ionic radius, 0.133nm) and strong electronegativity, enabling it to form a stable covalent coordination with the electron cloud of the N atom. The larger ionic radii of SO⁄²⁻ (0.230nm) and NO⁃⁻ (0.264nm) lead to steric hindrance, making it difficult for them to approach the amino group's active site. Cl⁻ (0.181nm), while similar in size, has a weaker polarizability (polarizability 3.0×10⁻²). 4 cm³ vs F⁻ of 1.04×10⁻² 4 cm³), so Cl - In summary, the composite adsorbent prepared by the present invention has excellent resistance to interference from the loading matrix and is suitable for effectively adsorbing and removing fluoride ions in high-salinity photovoltaic wastewater.

[0102] (2) Size selectivity of hydrogen bonding

[0103] When hydroxyl groups (-OH) participate in adsorption through OH•••F hydrogen bonds, the spherical structure of F⁻ (hydrated radius 3.52Å) can form a multidentate hydrogen bond network. However, the tetrahedral structure of SO⁻ (hydrated radius 3.8Å) results in poor hydrogen bonding compatibility. While Cl⁻ (hydrated radius 3.32Å) is similar in size, its lower charge density (-1 compared to F⁻'s -1) weakens the hydrogen bond strength. Therefore, the composite adsorbent prepared by this invention can specifically bind to fluoride ions even in the presence of numerous inorganic anions, achieving the goal of adsorbing and removing fluoride ions.

[0104] (3) Charge density matching of electrostatic interaction

[0105] Deprotonation of the carboxylic acid group (-COOH) forms -COO⁻: F⁻'s high charge density (-1 / 0.133nm³ = 5.64 e / nm³) creates a strong electrostatic attraction with -COO⁻. However, the charge densities of SO₄²⁻ (2.67 e / nm³), Cl⁻ (3.05 e / nm³), and NO⁻ (2.37 e / nm³) are significantly lower than F⁻, weakening the Coulombic interaction. Therefore, the composite adsorbent prepared by this invention can specifically bind to fluoride ions even in the presence of numerous inorganic anions, thereby achieving the purpose of adsorption and removal of fluoride ions.

[0106] (4) Ion exchange specificity of the aluminum-oxygen bond The ion exchange process of the Al-O bond preferentially replaces F⁻: F⁻ and OH⁻ have similar ionic radii (F⁻ 1.33Å vs OH⁻ 1.40Å), which can achieve lattice matching. The large sizes of SO4²⁻ (2.30Å) and NO3⁻ (2.64Å) lead to an increase in the lattice distortion energy barrier. The size difference of Cl⁻ (1.81Å) leads to an increase in the exchange activation energy (about 25 kJ / mol). Therefore, the composite adsorbent prepared by the present invention can achieve specific binding to fluoride ions even in the presence of many inorganic anions, thereby achieving the purpose of adsorbing and removing fluoride ions.

[0107] (3) Defluorination efficiency of fluoride ions in photovoltaic wastewater

[0108] In order to verify the removal effect of the composite adsorbent prepared by the present invention on fluoride ions in photovoltaic wastewater, the following verification experiment is performed.

[0109] 3.1 Verification of the removal effect of fluoride ions from photovoltaic wastewater by composite adsorbents prepared in different examples and comparative examples

[0110] First, simulate the environment of photovoltaic wastewater in the laboratory: prepare high-grade pure sodium fluoride to obtain a 10 mg / L standard sodium fluoride solution, take 3 portions of 1 mg / L standard sodium fluoride solution, and each portion has a volume of 1 L. And adjust the pH of the 3 portions of standard sodium fluoride solution to 5.0. Then use the prepared composite adsorbent to adsorb and remove fluoride ions in photovoltaic wastewater: put 1g of composite adsorbent 1 prepared in Example 4 into the first portion of standard sodium fluoride solution; put 1g of composite adsorbent 4 prepared in Comparative Example 1 into the second portion of standard sodium fluoride solution; put 1g of composite adsorbent 5 prepared in Comparative Example 2 into the third portion of standard sodium fluoride solution and stir for 30 minutes. Measure the residual fluoride ion concentration in the solution: take samples separately and use instruments to determine the concentration of residual fluoride ions in three portions of sodium fluoride solutions to which different composite adsorbents are added. Then calculate the defluorination efficiency of the three groups of composite adsorbents on the three portions of standard sodium fluoride solutions, and get the following: Figure 7 The data shown.

[0111] Through Figure 7 Analysis shows that the defluorination efficiency of the composite adsorbent prepared in Example 4 for fluoride ions in a standard sodium fluoride solution is 92.3%, the defluorination efficiency of the composite adsorbent prepared in Comparative Example 1 for fluoride ions in a standard sodium fluoride solution is 35.2%, and the defluorination efficiency of the composite adsorbent prepared in Comparative Example 2 for fluoride ions in a standard sodium fluoride solution is 43.2%. Among them, the composite adsorbent prepared in Comparative Example 1 did not add aluminum chloride solution during the preparation process compared to the composite adsorbent prepared in Example 4. The composite adsorbent prepared in Comparative Example 2 did not add chitosan during the preparation process compared to the composite adsorbent prepared in Example 4. Judging from the defluorination efficiency of the composite adsorbents prepared in Example 4, Comparative Example 1 and Comparative Example 2 for fluoride ions in photovoltaic wastewater, the defluorination efficiency of the composite adsorbent prepared in Example 4 for fluoride ions in photovoltaic wastewater is much better than the defluorination efficiency of the composite adsorbents prepared in Comparative Examples 1 and Comparative Examples 2. This proves that the composite adsorbent prepared in Example 4 by loading alumina on chitosan has a good defluorination effect on fluoride ions in photovoltaic wastewater.

[0112] 3.2 Verification of the defluorination effect of the composite adsorbent prepared in this example on fluoride ions in photovoltaic wastewater at different pH values

[0113] First, simulate the photovoltaic wastewater environment: prepare high-grade pure sodium fluoride to obtain a 10 mg / L standard sodium fluoride solution. Take six portions of this 1 mg / L standard sodium fluoride solution, each with a volume of 1 L. Adjust the pH of two portions of this standard sodium fluoride solution to 3.0, the pH of another two portions to 5.0, and the pH of the remaining two portions to 7.0. Fluoride ions in the photovoltaic wastewater are then adsorbed and removed using the prepared composite adsorbent: 1 gram of composite adsorbent 1 prepared in Example 4 is added to the first portion of a standard sodium fluoride solution (pH 3.0); 1 gram of composite adsorbent 5 prepared in Comparative Example 2 is added to the second portion of a standard sodium fluoride solution (pH 3.0); 1 gram of composite adsorbent 1 prepared in Example 4 is added to the third portion of a standard sodium fluoride solution (pH 5.0); and 1 gram of composite adsorbent 5 prepared in Comparative Example 2 is added to the fourth portion of a standard sodium fluoride solution (pH 5.0). 1g of the composite adsorbent 1 prepared in Example 4 was added to the fifth portion of a standard sodium fluoride solution (pH 7.0), and 1g of the composite adsorbent 5 prepared in Comparative Example 2 was added to the sixth portion of a standard sodium fluoride solution (pH 7.0). The six mixed solutions were stirred for 30 minutes. Samples were taken and the concentrations of residual fluoride ions in the six sodium fluoride solutions with different pH values ​​were measured using an instrument. The defluorination efficiency of fluoride ions in the six sodium fluoride solutions with different pH values ​​was calculated, and the results were as follows: Figure 8 The data shown.

[0114] Through Figure 8 Data analysis shows that the composite adsorbent 1 prepared in Example 4 has a defluorination efficiency of over 90% for fluoride ions in photovoltaic wastewater within the pH range of 3 to 7, while the composite adsorbent 5 prepared in Comparative Example 2 gradually decreases its adsorption capacity for fluoride ions in photovoltaic wastewater with increasing pH. This demonstrates that the composite adsorbent prepared by the preparation method of this example broadens the pH range applicable to existing composite adsorbents for adsorbing fluoride ions in photovoltaic wastewater, thereby improving the practicality of the composite adsorbent prepared by the preparation method of this example.

[0115] 3.3 Verification of the adsorption effect of the composite adsorbent prepared in this example on fluoride ions under the interference of different anions

[0116] First, prepare a 20mg / L sodium fluoride solution of high-grade pure sodium fluoride. Then select 4 portions of 20mg / L sodium fluoride solutions with a volume of 50mL for standby use, and adjust the pH of the 4 portions of sodium fluoride solutions to 5.0. Add 50mL of ultrapure water to the first sodium fluoride solution to obtain liquid one to be tested. Add 50mL of 1mol / L sodium chloride solution to the second sodium fluoride solution to obtain liquid two to be tested. Add 50mL of 1mol / L sodium sulfate solution to the third sodium fluoride solution to obtain liquid three to be tested. Add 50mL of 1mol / L sodium nitrate solution to the fourth sodium fluoride solution to obtain liquid four to be tested.

[0117] 1 g of the composite adsorbent 1 prepared in Example 4 was added to each of the four test liquids. After stirring for 30 minutes, samples were taken from each of the four test liquids, and the concentrations of residual fluoride ions in the four test liquids were measured by an instrument. The defluorination efficiency of the composite adsorbent 1 in the four test liquids was calculated, and the results were as follows: Figure 9 shown.

[0118] Through Figure 9 Analysis shows that the defluorination efficiency of the composite adsorbent prepared in Example 4 for fluorine ions in the first test liquid is about 92.3%; the defluorination efficiency of the composite adsorbent prepared in Example 4 for fluorine ions in the second test liquid is about 91.5%; the defluorination efficiency of the composite adsorbent prepared in Example 4 for fluorine ions in the third test liquid is about 90.3%; and the defluorination efficiency of the composite adsorbent prepared in Example 4 for fluorine ions in the fourth test liquid is about 92.8%. From the above data, it can be seen that in the presence of Cl - 、SO4 2- and NO3 -In the presence of these inorganic anions, the composite adsorbent prepared in Example 4 still exhibited a high defluorination efficiency for fluoride ions in photovoltaic wastewater. This demonstrates that the composite adsorbent prepared in this example has excellent resistance to interference from complex matrices and is suitable for adsorbing and removing fluoride ions in high-salinity photovoltaic wastewater.

[0119] 3.4 Verification of the defluorination effect of composite adsorbents prepared with different aluminum ion concentrations on fluoride ions in photovoltaic wastewater

[0120] First, simulate the environment of photovoltaic wastewater in the laboratory: prepare high-grade pure sodium fluoride to obtain a 10 mg / L standard sodium fluoride solution, take 3 portions of 1 mg / L standard sodium fluoride solution, and each portion has a volume of 1 L. And adjust the pH of the 3 portions of standard sodium fluoride solution to 5.0. Then use the prepared composite adsorbent to adsorb and remove fluoride ions in photovoltaic wastewater: put 1g of composite adsorbent 1 prepared in Example 4 into the first portion of standard sodium fluoride solution; put 1g of composite adsorbent 2 prepared in Example 5 into the second portion of standard sodium fluoride solution; put 1g of composite adsorbent 3 prepared in Example 6 into the third portion of standard sodium fluoride solution and stir for 30 minutes. Measure the residual fluoride ion concentration in the solution: take samples separately and use instruments to determine the concentration of residual fluoride ions in three portions of sodium fluoride solutions with different composite adsorbents added. Then calculate the defluorination efficiency of the three groups of composite adsorbents on the three portions of standard sodium fluoride solutions, and get the following: Figure 10 The data shown.

[0121] Through Figure 10 The data analysis shows that the defluorination efficiency of fluoride ions in photovoltaic wastewater prepared by the composite adsorbent pair prepared in Example 4 is about 92.3%, the defluorination efficiency of fluoride ions in photovoltaic wastewater prepared by the composite adsorbent pair prepared in Example 5 is about 94.5%, and the defluorination efficiency of fluoride ions in photovoltaic wastewater prepared by the composite adsorbent pair prepared in Example 6 is about 96.8%. According to the above data, during the preparation of the composite adsorbent, when the concentration of aluminum ions increases from 1.0 mol / L to 5.0 mol / L, the defluorination efficiency of fluoride ions in photovoltaic wastewater by the composite adsorbent also increases with the increase of the aluminum ion concentration. And during the preparation of the composite adsorbent, when the concentration of aluminum ions is within the range of 1.0 mol / L to 5.0 mol / L, the defluorination efficiency of the composite adsorbent for fluoride ions in photovoltaic wastewater is greater than 90%, and the concentration of fluoride ions in the treated photovoltaic wastewater is measured to obtain the concentration of fluoride ions in the photovoltaic wastewater after defluorination by the composite adsorbent. It is lower than 1.5 mg / L. This shows that the composite adsorbent prepared in this example has a good defluorination effect on fluoride ions in photovoltaic wastewater.

[0122] The above describes the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the foregoing embodiments. The foregoing embodiments and description merely illustrate the principles of the present invention. Various changes and modifications may be made to the present invention without departing from the spirit and scope of the present invention. Such changes and modifications are intended to fall within the scope of the present invention. The scope of protection claimed by the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a composite adsorbent, characterized in that: The -NH2 group, -OH group, -CH2 group, and -CO group in the composite adsorbent can all specifically bind to fluoride ions in photovoltaic wastewater; and the preparation method of the composite adsorbent comprises the following steps: Mix 1% to 5% citric acid and 2g / L to 10g / L chitosan in a weight ratio of 5:4 to obtain a mixed solution A; A 1 mol / L to 5 mol / L aluminum chloride solution was added to the mixed solution A, and the mixture was mixed to obtain a mixed solution B; the volume ratio of the aluminum chloride solution to the mixed solution A was 4:5; Then, an alkali solution is added dropwise to the mixed solution B until the pH of the mixed solution B reaches 7.0-8.0 to obtain a suspension C; The suspension C is allowed to stand for aging, and then washed with water and alcohol for multiple times, and finally centrifugally freeze-dried to obtain the composite adsorbent.

2. The method for preparing the composite adsorbent according to claim 1, wherein: The composite adsorbent has a fluorine removal rate of higher than 90% in photovoltaic wastewater within a pH range of 3-7, and the fluorine ion concentration in the treated photovoltaic wastewater is lower than 1.5 mg / L.

3. The method for preparing the composite adsorbent according to claim 1, wherein: The alumina fixed in the chitosan is amorphous alumina, and the loading amount of the amorphous alumina is 15%-35% in terms of aluminum element.

4. The method for preparing the composite adsorbent according to claim 1, wherein: The alkali solution 1 includes one of NaOH, KOH, and NH3·H2O; the molar concentration of the alkali solution 1 is 0.5 mol / L~10 mol / L.

5. The method for preparing the composite adsorbent according to claim 1, wherein: The chitosan has a deacetylation degree of 60%-80% and a molecular weight of 5×10 4 Da~5×10 5 Da; And / or, the static aging time is 10h-20h, and the freeze-drying time is 12h-24h.

6. A composite adsorbent, characterized in that The composite adsorbent is prepared by the preparation method of the composite adsorbent according to any one of claims 1 to 5. After adsorbing fluoride ions in photovoltaic wastewater, the composite adsorbent can be desorbed and regenerated by alkaline solution 2.

7. The composite adsorbent according to claim 6, wherein The second alkali solution includes one of NaOH and KOH; the molar concentration of the second alkali solution is 0.5 mol / L-10 mol / L.

8. The composite adsorbent according to claim 6, wherein The composite adsorbent has a porous structure and a pore volume of 0.2 cm 3 / g-0.8 cm 3 / g, specific surface area is 100cm 2 / g-800 cm 2 / g.

9. Use of the composite adsorbent according to any one of claims 6 to 8, characterized in that: The -NH2 group, -OH group, -CH2 group and -CO group in the composite adsorbent can resist Cl in photovoltaic wastewater. - 、SO4 2- and NO3 - The fluoride ions in photovoltaic wastewater can be removed by chemical adsorption.

10. A method for removing fluoride ions from photovoltaic wastewater, characterized in that: It includes the following steps: Pour the composite adsorbent prepared by the preparation method of the composite adsorbent according to any one of claims 1 to 5 into the photovoltaic wastewater to be treated and stir; While stirring, the concentration of fluoride ions in the photovoltaic wastewater is detected in real time by a fluoride ion detection instrument: if the fluoride ion detection instrument detects that the fluoride ion concentration in the photovoltaic wastewater is higher than 1.5 mg / L, continue to add the composite adsorbent to the photovoltaic wastewater to be treated until the fluoride ion detection instrument detects that the fluoride ion concentration in the photovoltaic wastewater to be treated is lower than 1.5 mg / L.

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