Preparation method of BC (at) PDA / NH2-MIL-53 (Fe) composite membrane, BC (at) PDA / NH2-MIL-53 (Fe) composite membrane and application of BC (at) PDA / NH2-MIL-53 (Fe) composite membrane

Abstract

The invention belongs to the technical field of photocatalytic materials. In particular to a preparation method of a BC (at) PDA / NH2-MIL-53 (Fe) composite membrane and the BC (at) PDA / NH2-MIL-53 (Fe) composite membrane and application thereof.The BC (at) PDA / NH2-MIL-53 (Fe) composite membrane comprises the steps that firstly, a bacterial cellulose membrane is purified through a sodium hydroxide solution, then, the bacterial cellulose membrane is placed in a Tris-HCl buffer solution with the pH value being 8.5, in-situ polymerization reaction is conducted for 24 hours through dopamine, the BC (at) PDA composite membrane is prepared, finally, the composite membrane is soaked in a ferric nitrate solution to adsorb iron ions, and then the BC (at) PDA / NH2-MIL-53 (Fe) composite membrane is obtained. According to the method, the BC (at) PDA substrate is loaded with NH2-MIL-53 (Fe), then the NH2-MIL-53 (Fe) is mixed with a DMF solution of 2-aminoterephthalic acid, a reaction is carried out for 8 hours at the temperature of 150 DEG C, washing and drying are carried out, and finally the target composite membrane is prepared.

Description

Technical Field

[0001] This invention belongs to the field of photocatalytic materials technology, specifically relating to the preparation method of BC@PDA / NH2-MIL-53(Fe) composite film, the BC@PDA / NH2-MIL-53(Fe) composite film and its application. Background Technology

[0002] With the acceleration of industrialization, nitrogen-containing heterocyclic compound pollution has become a significant issue in water environment management. Indole and quinoline, as typical nitrogen heterocyclic compounds, are widely found in industrial wastewater from coking, pharmaceutical, and dye industries. They are characterized by high toxicity, poor degradation, and easy accumulation, posing a serious threat to aquatic ecosystems and human health. Traditional physical adsorption and biodegradation methods have low removal efficiency for these pollutants and are insufficient to meet increasingly stringent emission standards.

[0003] Photocatalysis technology has attracted widespread attention due to its ability to directly degrade organic pollutants using solar energy. However, most photocatalysts currently studied are powdered materials, which suffer from problems such as difficulty in separation and recovery, easy agglomeration and deactivation, and short lifespan in practical applications. In addition, single-component photocatalysts often have high recombination rates of photogenerated electron-hole pairs, resulting in poor photocatalytic efficiency.

[0004] Metal-organic frameworks (MOFs) have shown great potential in photocatalysis due to their high specific surface area, tunable pore structure, and abundant active sites. NH₂-MIL-53(Fe), as an iron-based MOF, exhibits visible light response and good chemical stability. However, NH₂-MIL-53(Fe) has a wide band gap, a limited light absorption range, and its powder form makes it difficult to recycle. Loading NH₂-MIL-53(Fe) onto a support surface to prepare membrane materials is an effective way to solve the recycling problem, but enhancing its visible light absorption capacity and improving the separation efficiency of photogenerated carriers remain key issues that need to be addressed.

[0005] Polydopamine is widely used in the surface modification of functional materials due to its excellent adhesion properties, good biocompatibility, and unique photoelectric properties. Polydopamine can absorb visible light and generate photogenerated carriers through a π-π conjugated system, playing a role in photosensitization and electron transport in photocatalytic systems. Bacterial cellulose, as a natural polymer material, possesses a three-dimensional network structure, good mechanical strength, and environmental friendliness, making it an ideal carrier for preparing photocatalytic membranes.

[0006] Therefore, developing a composite membrane photocatalyst that combines the advantages of bacterial cellulose, polydopamine, and NH2-MIL-53(Fe), constructing an efficient heterojunction interface to promote the separation of photogenerated carriers, broadening the visible light response range, and enabling convenient recycling and reuse of the photocatalyst is of great scientific significance and application value. Summary of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a method for preparing BC@PDA / NH2-MIL-53(Fe) composite membranes, the BC@PDA / NH2-MIL-53(Fe) composite membranes, and their applications. This composite membrane constructs a highly efficient heterojunction photocatalytic system by loading NH2-MIL-53(Fe) metal-organic framework material onto the surface of polydopamine-modified bacterial cellulose, significantly improving the photocatalytic degradation efficiency of nitrogen heterocyclic compounds, while simultaneously solving the problem of difficult recovery of powdered photocatalysts.

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

[0009] The preparation method of BC@PDA / NH2-MIL-53(Fe) composite membrane includes the following steps:

[0010] First, the bacterial cellulose membrane was pretreated. The membrane was thoroughly rinsed with deionized water to remove surface culture medium and impurities, then immersed in a 1 mol / L sodium hydroxide solution in a 90°C water bath for 1 hour to remove surface nutrients and viable bacteria. After treatment, the membrane was removed and repeatedly rinsed with deionized water until the pH reached 7, yielding a purified bacterial cellulose membrane. This step effectively removed impurities from the bacterial cellulose surface, providing a clean substrate for the subsequent uniform loading of polydopamine.

[0011] Next, the BC@PDA composite membrane was prepared. 1 g of pretreated bacterial cellulose membrane was weighed and added to 10 mL of a tris(hydroxymethyl)aminomethane hydrochloric acid buffer solution with a pH of 8.5. An appropriate amount of dopamine hydrochloride was added under magnetic stirring, and after complete dissolution, the pH was adjusted to 8.5 with sodium hydroxide solution. The reaction was carried out at room temperature with continuous stirring at 200 r / min for 24 h. During the reaction, dopamine under alkaline conditions underwent self-polymerization to form polydopamine, which firmly adhered to the bacterial cellulose surface through multiple interactions such as hydrogen bonding, van der Waals forces, and π-π stacking. After the reaction, the membrane material was removed and washed three times alternately with deionized water and methanol to remove unreacted monomers and oligomers, yielding a black BC@PDA composite membrane. The introduction of polydopamine not only enhanced the light absorption capacity of the material but also provided abundant functional groups for the loading of NH2-MIL-53(Fe).

[0012] Then, a BP / NH2-MIL-53(Fe) composite membrane was prepared. 2 mmol of ferric nitrate nonahydrate was weighed and ultrasonically dissolved in 10 mL of dimethylformamide to obtain a clear iron salt solution. A 1 g wet BC@PDA composite membrane was immersed in the iron salt solution and magnetically stirred for 30 min at room temperature to allow the nitrogen- and oxygen-containing functional groups on the BC@PDA surface to fully complex and adsorb iron ions, forming solution A. This pre-adsorption process ensured a uniform distribution of iron ions on the membrane surface. Separately, an appropriate amount of 2-aminoterephthalic acid was dissolved in 10 mL of dimethylformamide to form solution B. Solution B was slowly added dropwise to solution A, and the mixture was slowly stirred for 30 min at room temperature to allow the ligands to fully coordinate with the iron ions. The mixture was transferred to a polytetrafluoroethylene-lined reactor, sealed, and reacted in an oven at 150 °C for 8 h. Under hydrothermal conditions, iron ions and 2-aminoterephthalic acid ligands form NH2-MIL-53(Fe) crystals through coordination bonds, which are then grown in situ on the BC@PDA surface. After the reactor cools naturally to room temperature, the composite membrane is removed and repeatedly rinsed with deionized water to remove residual reactants and uncoordinated substances. It is then washed three times alternately with methanol and deionized water, and finally dried at 40°C to obtain a dark brown BP / NH2-MIL-53(Fe) composite membrane.

[0013] The beneficial effects of this invention are as follows:

[0014] First, this invention successfully constructed a film-type photocatalytic material by loading NH2-MIL-53(Fe) onto the surface of BC@PDA, completely solving the problem of the difficulty in recycling traditional powder photocatalysts. The composite membrane has good mechanical strength and structural stability, and can be removed from the reaction system through simple physical separation. After washing and drying, it can be reused, significantly reducing operating costs and avoiding secondary pollution of the catalyst.

[0015] Secondly, the introduction of polydopamine plays multiple key roles in the composite membrane system. On the one hand, polydopamine acts as a molecular bridge connecting bacterial cellulose and NH2-MIL-53(Fe). Its abundant catechol and amino functional groups can interact with bacterial cellulose through hydrogen bonding and coordinate with iron ions in NH2-MIL-53(Fe), forming a stable chemical bonding interface. On the other hand, polydopamine possesses a unique π-π conjugated structure and broad-spectrum absorption characteristics, which can significantly broaden the visible light response range of the composite membrane. More importantly, a matched band structure is formed between polydopamine and NH2-MIL-53(Fe), constructing a Z-type heterojunction. Under visible light excitation, photogenerated electrons transfer from the conduction band of NH2-MIL-53(Fe) to the HOMO level of polydopamine, while holes remain in the valence band of NH2-MIL-53(Fe). This space charge separation mechanism effectively suppresses the recombination of electron-hole pairs, giving the composite membrane higher photocatalytic activity.

[0016] Third, the composite membrane prepared in this invention exhibits excellent photocatalytic degradation performance for nitrogen heterocyclic compounds. Under optimized conditions, the composite membrane achieves complete degradation of a 50 mg / L indole solution within 30 min of visible light irradiation, with a rate constant of 0.066 min⁻¹, significantly higher than that of single-component catalysts. For the more structurally stable quinoline, the degradation rate can also reach over 98% after 100 min of visible light irradiation. This performance is at an advanced level in similar studies. Furthermore, the composite membrane maintains high degradation efficiency in actual water bodies such as lake water, coal washing water, and coking wastewater, demonstrating good environmental adaptability.

[0017] Fourth, the composite membrane exhibits excellent stability and reusability. After six cycles, the degradation rates of indole and quinoline remained above 86% and 88%, respectively, indicating that the material maintained good structural integrity and catalytic activity during repeated use. XRD and SEM characterization confirmed that the crystal structure and morphology of the composite membrane did not change significantly after multiple cycles, further verifying its excellent stability.

[0018] Fifth, mechanistic studies show that, under the synergistic effect of visible light and potassium persulfate, the composite membrane mainly degrades nitrogen heterocyclic compounds by generating superoxide radicals, sulfate radicals, and holes. Among these, superoxide radicals and sulfate radicals play a dominant role, and this mechanism of synergistic oxidation by multiple active species ensures the efficient removal of recalcitrant organic matter. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the preparation process of BC@PDA / NH2-MIL-53(Fe) composite membrane.

[0020] Figure 2 The images show the XRD patterns of different samples. In the image, a is a comparison of the full XRD patterns of BC, BC@PDA, NH2-MIL-53(Fe), BC / NH2-MIL-53(Fe), and BP / NH2-MIL-53(Fe), and b is a magnified XRD pattern of NH2-MIL-53(Fe) and BP / NH2-MIL-53(Fe) in the range of 10° to 50°.

[0021] Figure 3 The images are scanning electron microscope (SEM) images of different samples, where a is the SEM image of BC, b is the SEM image of BC@PDA, and c and d are the SEM images of BP / NH2-MIL-53(Fe) at different magnifications.

[0022] Figure 4 The SEM-EDS elemental distribution map of BP / NH2-MIL-53(Fe).

[0023] Figure 5 The images are photographs of different samples, where a is BC, b is BC@PDA, c is BC / NH2-MIL-53(Fe), and d is BP / NH2-MIL-53(Fe).

[0024] Figure 6 The images show the XPS spectra of the samples, where a is the full spectrum scan of BP / NH2-MIL-53(Fe), b is the high-resolution spectrum of N 1s, c is the high-resolution spectrum of Fe 2p, d is the high-resolution spectrum of O 1s, and e is the high-resolution spectrum of C 1s.

[0025] Figure 7 The UV-Vis diffuse reflectance spectra of the samples are shown, where a is the absorption spectrum of BC@PDA, NH2-MIL-53(Fe) and BP / NH2-MIL-53(Fe), and b is the Tauc plot of the three samples used to calculate the band gap energy.

[0026] Figure 8 The electrochemical performance of the samples is characterized, where a is the AC impedance spectrum of BP / NH2-MIL-53(Fe), BC / NH2-MIL-53(Fe) and BC@PDA, b is the transient photocurrent response of the three samples, and c is the fluorescence spectrum of the three samples.

[0027] Figure 9 The graph shows the degradation curves of indole by different photocatalysts.

[0028] Figure 10 The graph shows the pseudo-first-order kinetics of indole degradation, where a is the kinetic curve of indole degradation by different catalysts, and b is a bar chart comparing the rate constants of different catalysts.

[0029] Figure 11 The graph shows the effect of different reaction conditions on indole degradation.

[0030] Figure 12 The figure shows the influencing factors of photocatalytic degradation of indole, where a represents the effect of PDA concentration, b represents the effect of the ratio of NH2-MIL-53(Fe) to BC@PDA, c represents the effect of BP / NH2-MIL-53(Fe) dosage, and d represents the effect of potassium persulfate concentration.

[0031] Figure 13 The graph shows the effect of environmental factors on indole degradation, where a represents the effect of humic acid concentration and b represents the effect of pH value.

[0032] Figure 14 The graph shows the degradation curves of quinoline by different photocatalysts.

[0033] Figure 15The graph shows the pseudo-first-order kinetics of quinoline degradation, where a is the kinetic curve of quinoline degradation by different catalysts, and b is a bar chart comparing the rate constants of different catalysts.

[0034] Figure 16 The graph shows the effect of different reaction conditions on the degradation of quinoline.

[0035] Figure 17 The graph shows the reusability of BP / NH2-MIL-53(Fe), where a represents the results of 6 cycles of indole degradation and b represents the results of 6 cycles of quinoline degradation.

[0036] Figure 18 The graph shows the degradation performance of BP / NH2-MIL-53(Fe) in different aqueous media, where a represents the effect of indole degradation in different aqueous media and b represents the effect of quinoline degradation in different aqueous media.

[0037] Figure 19 The graph shows the TOC removal rate, where a represents the change in TOC removal rate during the degradation of indole, and b represents the change in TOC removal rate during the degradation of quinoline.

[0038] Figure 20 Figure 1 shows the results of the free radical capture experiment. Figure 2 shows the effect of different capture agents on indole degradation, Figure 3 shows the comparison bar chart of indole degradation efficiency under different capture agents, and Figure 4 shows the XPS valence band spectrum of BC@PDA.

[0039] Figure 21 This is a schematic diagram of the mechanism of photocatalytic degradation of nitrogen heterocyclic compounds by BP / NH2-MIL-53(Fe). Detailed Implementation

[0040] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, in the following description, descriptions of well-known structures and technologies are omitted to avoid unnecessarily obscuring the concept of the invention.

[0041] All reagents and materials used in the examples were commercially available products or prepared according to conventional methods. Bacterial cellulose was purchased from Hainan Yide Food Co., Ltd., with a purity of 100%. Dopamine hydrochloride, tris(hydroxymethyl)aminomethane hydrochloride buffer, ferric nitrate nonahydrate, 2-aminoterephthalic acid, dimethylformamide, indole, quinoline, potassium persulfate, humic acid, p-benzoquinone, isopropanol, disodium ethylenediaminetetraacetate, and methanol were all analytical grade and purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. All solutions were prepared with deionized water.

[0042] Example 1: Preparation of BC@PDA / NH2-MIL-53(Fe) composite membrane

[0043] like Figure 1 As shown, the BC@PDA / NH2-MIL-53(Fe) composite membrane prepared in this embodiment is prepared by a three-step method, including bacterial cellulose pretreatment, polydopamine modification and in-situ loading of NH2-MIL-53(Fe).

[0044] Step 1: Pretreatment of the bacterial cellulose membrane. Take several commercially available bacterial cellulose membranes and rinse them thoroughly with deionized water 3 to 5 times to remove residual culture medium and soluble impurities from the surface. Immerse the rinsed bacterial cellulose membranes completely in a 1 mol / L sodium hydroxide solution and heat in a 90°C water bath for 1 hour. The purpose of this alkaline treatment is to remove proteins, polysaccharides, and other biomolecules, as well as residual microbial cells, from the surface of the bacterial cellulose, while also moderately opening the hydrogen bond network between the cellulose molecular chains to increase the specific surface area of ​​the material. After treatment, remove the bacterial cellulose membranes and rinse them repeatedly with plenty of deionized water until the pH of the washing solution drops to around 7, becoming neutral. Cut the washed bacterial cellulose membranes into circular pieces with a diameter of 27 mm and store them in sealed bags for later use.

[0045] Step 2: Preparation of the BC@PDA composite membrane. Accurately weigh 1g of pretreated wet bacterial cellulose membrane and add it to a beaker containing 10mL of tris(hydroxymethyl)aminomethane hydrochloride buffer solution. Measure the initial pH of the solution using a pH meter and adjust it to 8.5 with dilute hydrochloric acid or sodium hydroxide solution. Under magnetic stirring, accurately weigh 70mg of dopamine hydrochloride and add it to the above solution, corresponding to a concentration of 7mg / mL. After the dopamine hydrochloride is completely dissolved, accurately adjust the pH of the system to 8.5 again with 1mol / L sodium hydroxide solution. Place the beaker on a magnetic stirrer, set the speed to 200r / min, and continue stirring the reaction at room temperature for 24h. During this process, the dopamine monomer is oxidized by oxygen in the air under a weakly alkaline environment, undergoing a self-polymerization reaction to generate polydopamine. The polydopamine molecule, rich in various functional groups such as catechol, catechin, and amino groups, is firmly adsorbed onto the surface of bacterial cellulose fibers through multiple non-covalent interactions, including hydrogen bonds, π-π stacking, and van der Waals forces, forming a uniform coating. After the reaction, the membrane material is removed, and its color changes from white to black, which is the characteristic melanin color of polydopamine. The membrane material is washed three times each with deionized water and methanol alternately to remove unreacted monomers, oligomers, and buffer salts adsorbed on the surface, yielding the BC@PDA composite membrane.

[0046] Step 3: Preparation of the BP / NH2-MIL-53(Fe) composite membrane. Accurately weigh 2 mmol of ferric nitrate nonahydrate (0.808 g) and place it in a 50 mL beaker. Add 10 mL of dimethylformamide and sonicate in an ultrasonic cleaner for 15 min until completely dissolved, yielding a clear, transparent, pale yellow iron salt solution. Immerse 1 g of the wet BC@PDA composite membrane prepared in Step 2 completely into the above iron salt solution and magnetically stir at 150 rpm for 30 min at room temperature. During this process, the oxygen- and nitrogen-containing functional groups such as the catechol hydroxyl and amino groups on the polydopamine surface coordinate with iron ions, resulting in the adsorption and enrichment of a large number of iron ions on the BC@PDA surface, forming an iron-containing precursor composite membrane, denoted as solution A. In another beaker, accurately weigh 2 mmol of 2-aminoterephthalic acid (0.362 g) and add 10 mL of dimethylformamide, stirring until completely dissolved to form solution B. Solution B was slowly added dropwise to solution A using a dropping funnel or pipette, at a rate of approximately 20 drops per minute. Magnetic stirring was maintained during the addition to ensure sufficient contact and coordination between the ligands and the iron ions adsorbed on the membrane surface. After the addition was complete, stirring was continued slowly at room temperature for 30 minutes. The entire mixture containing the membrane material was transferred to a 100 mL polytetrafluoroethylene-lined reactor. The reactor lid was tightened to ensure a good seal, and the reactor was then placed in a preheated 150°C thermostatic drying oven for hydrothermal reaction. The reaction was maintained at 150°C for 8 hours. Under hydrothermal conditions, iron ions and 2-aminoterephthalic acid self-assembled through metal coordination bonds and bridging with organic ligands to form NH2-MIL-53(Fe) crystals with a three-dimensional porous structure. These crystals grew in situ on the surface and within the pores of the BC@PDA membrane. After the reaction was complete, the power was turned off, and the reactor was allowed to cool naturally to room temperature before being carefully opened and the membrane material removed. At this point, the membrane color changed from black to dark brown. The membrane surface was repeatedly rinsed with plenty of deionized water to remove residual dimethylformamide, unreacted iron salts, and ligands. Then, it was washed three times each with methanol and deionized water, alternating between the two. The cleaned membrane material was placed in a vacuum drying oven and dried at 40°C for 12 hours until constant weight was achieved, yielding the BP / NH2-MIL-53(Fe) composite membrane product.

[0047] like Figure 2 As shown, the prepared material was characterized by XRD. From Figure 2It can be seen that pure bacterial cellulose exhibits sharp characteristic diffraction peaks at 2θ of 14.5° and 23.7°, corresponding to the (101) and (002) crystal planes of cellulose type I crystal, respectively. The XRD pattern of BC@PDA is basically consistent with that of pure BC, with characteristic peaks appearing at the same positions, indicating that the loading of polydopamine did not change the crystal structure of bacterial cellulose. This is because polydopamine is an amorphous polymer and does not produce obvious diffraction peaks. The XRD pattern of NH2-MIL-53(Fe) shows a series of characteristic diffraction peaks at positions such as 8.6°, 9.9°, 17.6°, and 19.9°, which are completely consistent with the standard pattern of NH2-MIL-53(Fe) reported in the literature, proving that the target crystal phase was successfully synthesized. The XRD patterns of BC / NH2-MIL-53(Fe) and BP / NH2-MIL-53(Fe) contain characteristic peaks of both bacterial cellulose and NH2-MIL-53(Fe), and the diffraction peak of NH2-MIL-53(Fe) increases with its content, indicating that NH2-MIL-53(Fe) was successfully loaded onto the support surface. Figure 2 The magnified view shows more clearly that the diffraction peaks of BP / NH2-MIL-53(Fe) are slightly shifted relative to pure NH2-MIL-53(Fe). This is due to lattice distortion caused by the strong interaction between polydopamine and NH2-MIL-53(Fe).

[0048] like Figure 3 The images shown are scanning electron microscope (SEM) images of different samples. Figure 3 As can be seen, the dried pure bacterial cellulose maintained a good three-dimensional network porous structure with uniform fiber diameter ranging from 50 nm to 100 nm. The fiber surface was smooth and flat, and the fibers intertwined to form a large number of pores. This unique network structure provides a large number of reaction sites and mass transfer channels for the loading of subsequent functional components. Figure 3 b shows the microstructure of BC@PDA. It can be clearly observed that after polydopamine coating, the originally independent bacterial cellulose fibers are bonded together by polydopamine to form fiber bundles, with multiple fibers agglomerated into clusters. Polydopamine forms a relatively dense coating on the fiber surface, making the fiber surface rough, which confirms that polydopamine successfully adheres to the surface of bacterial cellulose. Figure 3 c and Figure 3 d represents SEM images of the BP / NH2-MIL-53(Fe) composite film at different magnifications. (From low magnification...) Figure 3 As can be seen from c, NH2-MIL-53(Fe) crystals are uniformly and densely distributed on the surface of the BC@PDA substrate, with high coverage. (High magnification) Figure 3The image clearly shows the crystal morphology of NH2-MIL-53(Fe), which presents a regular octahedral spindle shape with a uniform crystal size distribution of about 100 nm. This nanoscale crystal has a large specific surface area, which is beneficial to the photocatalytic reaction.

[0049] like Figure 4 The EDS elemental distribution diagram shown further confirms the composition of the BP / NH2-MIL-53(Fe) composite membrane. As can be seen from the diagram, carbon, nitrogen, oxygen, and iron are uniformly distributed on the membrane surface, without obvious areas of elemental enrichment. Carbon mainly originates from bacterial cellulose, polydopamine, and 2-aminoterephthalic acid ligands; nitrogen originates from the amino groups on polydopamine and its ligands; oxygen originates from the carboxyl groups on cellulose, polydopamine, and its ligands; and iron originates from the metal center of NH2-MIL-53(Fe). This uniform elemental distribution indicates that the components are well dispersed in the composite membrane, forming a homogeneous composite structure.

[0050] like Figure 5 The images shown are photographs of different samples. Pure bacterial cellulose membranes are translucent white and possess good toughness. After polydopamine modification, the BC@PDA membrane appears black, a characteristic color of polydopamine. The BC / NH2-MIL-53(Fe) membrane is light brown, while the BP / NH2-MIL-53(Fe) membrane is dark brown; the color changes visually reflect the composition of the different components. All membrane materials maintain good integrity and a certain degree of mechanical strength, facilitating removal from solution and reuse.

[0051] XPS characterization was performed to further investigate the interactions and electron transfer behavior among the components in the composite membrane. Figure 6 As shown in Figure a, the full spectrum of BP / NH2-MIL-53(Fe) clearly shows the characteristic peaks of C 1s, N 1s, O 1s and Fe 2p, which further confirms the successful preparation of the composite membrane. Figure 6 b is the high-resolution N 1s binding energy spectrum. NH₂-MIL-53(Fe) shows two peaks at 396.5 eV and 399.2 eV, which are attributed to the CN and NH bonds in the 2-aminoterephthalic acid ligand, respectively. Notably, compared to pure NH₂-MIL-53(Fe), the N 1s binding energy in BP / NH₂-MIL-53(Fe) shifts to higher energies by approximately 0.8 eV, indicating a decrease in the electron cloud density around the nitrogen atom. This phenomenon is attributed to the coordination interaction between the iron ion in NH₂-MIL-53(Fe) and the nitrogen atom in the adjacent polydopamine, resulting in electron transfer from the nitrogen atom to the iron ion. Figure 6c is the high-resolution spectrum of Fe 2p. The peaks at 724.6 eV and 711.6 eV are attributed to Fe 2p1 / 2 and Fe 2p3 / 2 of Fe³⁺. Comparison reveals that the binding energy of Fe 2p in BP / NH₂-MIL-53(Fe) shifts towards lower energies compared to pure NH₂-MIL-53(Fe), indicating an increase in the electron cloud density at the iron center. This is because the electron-rich nitrogen and oxygen atoms in polydopamine donate electrons to the iron center. Figure 6 d is the high-resolution spectrum of O 1s, with peaks at 529.9 eV and 531.1 eV corresponding to the C=O double bond in the ligand and the Fe-O bond in the metal center, respectively. The O 1s peak of BP / NH2-MIL-53(Fe) also shifted towards higher energies. Figure 6 The high-resolution spectrum of C 1s shows peaks at 285.6 eV, 284.1 eV, and 282.1 eV, which are attributed to CN(O), C=N, and C=C bonds on aromatic carbons, respectively. Similarly, the C 1s peak of BP / NH2-MIL-53(Fe) shifts towards higher energies compared to pure NH2-MIL-53(Fe). These XPS data strongly demonstrate the existence of chemical interactions and electron transfer between polydopamine and NH2-MIL-53(Fe). The interface between the two forms a heterojunction structure, and this interfacial effect facilitates the effective separation of photogenerated electrons and holes, which is a key factor in improving photocatalytic performance.

[0052] Example 2: Characterization of the optical and photoelectric properties of BC@PDA / NH2-MIL-53(Fe) composite film

[0053] To evaluate the light absorption performance of the composite film, ultraviolet-visible diffuse reflectance spectroscopy was performed. Figure 7 As shown in Figure a, NH2-MIL-53(Fe) exhibits significant light absorption in the visible light region, with an absorption edge around 600 nm, indicating that it is a visible-light-responsive photocatalyst. BC@PDA shows strong light absorption in both the ultraviolet and visible light regions, attributed to the extended π-conjugated system in the polydopamine molecule. When BC@PDA is combined with NH2-MIL-53(Fe), the light absorption capacity of BP / NH2-MIL-53(Fe) is significantly enhanced, with a noticeable red shift in the absorption edge, extending the light response range to the longer wavelength visible light region, thus greatly improving the utilization efficiency of sunlight. The band gap energy is calculated using the Tauc equation, as shown in Figure a. Figure 7As shown in Figure b, the relationship between the absorption coefficient α and the photon energy hν is converted into a graph of (αhν)² versus hν. The intersection of the linear extrapolation to the horizontal axis represents the band gap energy of the material. Calculations show that the band gap energy of BC@PDA is 2.2 eV, that of NH₂-MIL-53(Fe) is 2.05 eV, while the band gap energy of BP / NH₂-MIL-53(Fe) decreases to 1.68 eV. The reduction in band gap means that the material is more easily excited by visible light to generate photogenerated carriers, which corresponds to the enhanced light absorption capacity. This explains the improved photocatalytic activity of the composite film from the perspective of band structure.

[0054] To further investigate the photoelectrochemical properties of the composite film, electrochemical impedance spectroscopy, transient photocurrent response, and fluorescence spectroscopy were performed. Electrochemical measurements were conducted in a three-electrode system, using conductive glass modified with the sample as the working electrode, a platinum sheet as the counter electrode, and a saturated calomel electrode as the reference electrode. The electrolyte was a 0.5 mol / L sodium sulfate solution. Figure 8 In the AC impedance spectra shown in Figure a, all three samples exhibit a semi-circular arc shape on the Nyquist plot. The radius of the arc reflects the charge transfer resistance of the electrode surface. The smaller the arc radius, the smaller the charge transfer resistance and the faster the interfacial charge transport. It is evident from the figure that BP / NH2-MIL-53(Fe) has the smallest arc radius, significantly smaller than BC@PDA and BC / NH2-MIL-53(Fe), indicating that BP / NH2-MIL-53(Fe) has the lowest charge transfer resistance and the highest charge separation efficiency. This is attributed to the excellent conductivity of polydopamine and the heterojunction interface formed between it and NH2-MIL-53(Fe), which promotes the rapid transfer of photogenerated carriers.

[0055] like Figure 8 Figure b shows the transient photocurrent response curve, which measures the photocurrent generated by the sample under periodic switching illumination with visible light. When the light source is turned on, the sample absorbs photons and generates electron-hole pairs, which move directionally under an applied bias voltage to form a photocurrent. The intensity of the photocurrent directly reflects the generation and separation efficiency of photogenerated carriers. When the light source is turned off, the photogenerated carriers recombine rapidly, and the photocurrent drops to the baseline level. As can be seen from the figure, BP / NH2-MIL-53(Fe) exhibits the highest photocurrent intensity, demonstrating excellent stability and reproducibility in multiple switching cycles. The photocurrent intensity of BC@PDA is moderate, while that of BC / NH2-MIL-53(Fe) is the weakest. This result further confirms that the recombination of polydopamine with NH2-MIL-53(Fe) significantly promotes the separation and transport of photogenerated carriers.

[0056] like Figure 8Figure c shows the fluorescence spectroscopy results. Fluorescence emission originates from the radiative recombination of photogenerated electrons and holes. Stronger fluorescence intensity indicates a higher recombination rate of electron-hole pairs, and consequently, lower photocatalytic activity. The figure shows that BC / NH2-MIL-53(Fe) exhibits the highest fluorescence intensity, indicating severe recombination of photogenerated carriers. BC@PDA shows slightly lower fluorescence intensity, while BP / NH2-MIL-53(Fe) shows the lowest, significantly lower than the former two. This indicates that the recombination of photogenerated electrons and holes in BP / NH2-MIL-53(Fe) is effectively suppressed, allowing a large number of carriers to participate in the photocatalytic reaction. The combined results of electrochemical impedance spectroscopy, transient photocurrent, and fluorescence spectroscopy consistently demonstrate from different perspectives that the BP / NH2-MIL-53(Fe) composite film possesses excellent photogenerated carrier separation efficiency, which is the fundamental reason for its high photocatalytic activity.

[0057] Example 3: Preparation and performance evaluation of BP / NH2-MIL-53(Fe) composite membranes with different PDA concentrations

[0058] This embodiment investigates the effect of polydopamine concentration on the photocatalytic performance of the composite membrane. The composite membrane was prepared according to the method in Example 1. In step two, the amount of dopamine hydrochloride added was varied to prepare three different concentrations: 5 mg / mL, 7 mg / mL, and 9 mg / mL, while other conditions remained constant. The photocatalytic degradation performance of indole by the three composite membranes was tested using the same method.

[0059] The specific procedures for the photocatalytic degradation experiment are as follows: 50 mL of a 50 mg / L indole solution was added to a 200 mL jacketed glass reactor, followed by the addition of 1 g / L of a composite membrane photocatalyst. The pH of the solution was adjusted to 5. The reactor was connected to a circulating cooling water system to maintain a constant reaction temperature. Under light-protected conditions, the reactor was magnetically stirred for 60 min to allow the indole to reach adsorption-desorption equilibrium on the catalyst surface. Then, 10 mM potassium persulfate was added, and a 300 W xenon lamp equipped with a 420 nm filter was immediately turned on to obtain visible light. Under illumination, 1 mL samples were taken every 5 min, filtered through a 0.45 μm polyethersulfone membrane, and the indole concentration was detected using high-performance liquid chromatography (HPLC). The chromatographic conditions were: C18 reversed-phase column, mobile phase of methanol and water mixture, flow rate of 1 mL / min, detection wavelength of 254 nm, and column temperature of 30 °C.

[0060] like Figure 12As shown in Figure a, when the PDA concentration was 5 mg / mL, the composite membrane exhibited a 92.3% degradation rate of indole within 30 min. When the PDA concentration was increased to 7 mg / mL, the degradation rate reached 100%, demonstrating optimal performance. Further increasing the PDA concentration to 9 mg / mL resulted in a decrease in the degradation rate to 85.7%. This is because an appropriate amount of polydopamine can enhance visible light absorption and promote electron transport, but excessive polydopamine forms an overly thick coating on the membrane surface, hindering light transmission and reducing the effective exposure area of ​​NH2-MIL-53(Fe) crystals. Furthermore, excessive polydopamine may also lead to its own aggregation, which is detrimental to electron transfer. Therefore, 7 mg / mL is the optimal PDA concentration for this system.

[0061] Example 4: Preparation and performance evaluation of composite membranes with different NH2-MIL-53(Fe) loading ratios

[0062] This embodiment investigates the effect of the mass ratio of NH2-MIL-53(Fe) to BC@PDA on the performance of the composite membrane. Following the method of Example 1, the amount of NH2-MIL-53(Fe) generated was controlled in step three by varying the amount of 2-aminoterephthalic acid ligand, resulting in the preparation of three composite membranes with mass ratios of BC@PDA to NH2-MIL-53(Fe) of 0.5:1, 1:1, and 1:2.5. Photocatalytic degradation of indole was performed under the same conditions as in Example 3.

[0063] like Figure 12 As shown in b, when the mass ratio is 0.5:1, i.e., when the NH2-MIL-53(Fe) content is high, the degradation rate of indole by the composite membrane is 88.5%. The degradation rate reaches its maximum of 100% when the mass ratio is 1:1. When the mass ratio increases to 1:2.5, i.e., when the NH2-MIL-53(Fe) content is relatively low, the degradation rate decreases to 79.2%. This is because NH2-MIL-53(Fe) is the main photocatalytic active component; too low a content leads to insufficient active sites, limiting the photocatalytic reaction. While too high a content of NH2-MIL-53(Fe) increases the number of active sites, it may cause localized crystal aggregation, reducing crystal dispersion and effective utilization. Furthermore, excessive NH2-MIL-53(Fe) also increases the membrane's mass and brittleness, which is detrimental to practical applications. Therefore, a 1:1 mass ratio achieves the optimal synergistic effect between polydopamine and NH2-MIL-53(Fe).

[0064] Example 5: Optimization of catalyst dosage

[0065] This example studies the effect of the dosage of BP / NH2-MIL-53(Fe) composite membrane on photocatalytic performance. The composite membrane was prepared according to Example 1 and the optimal conditions. In the photocatalytic experiment, the dosage of the catalyst was varied to 0.2 g / L, 0.4 g / L, 0.6 g / L, 1.0 g / L, and 1.4 g / L, while other conditions remained constant.

[0066] like Figure 12 As shown in Figure c, the degradation rate of indole gradually increased with the catalyst dosage from 0.2 g / L to 1.0 g / L, increasing from 65.3% to 100% at 30 min. This is because increasing the catalyst dosage provides more active sites and light absorption centers, accelerating the photocatalytic reaction. However, when the dosage continued to increase to 1.4 g / L, the degradation rate did not increase further, but instead decreased slightly to 97.8%. This may be due to the excessive catalyst films blocking each other in the reactor, reducing the light penetration depth and effective irradiation area. Additionally, excessive suspended solids may also reduce the light transmittance of the solution. Considering both economics and practicality, 1.0 g / L is the optimal catalyst dosage.

[0067] Example 6: Optimization of potassium persulfate concentration

[0068] This embodiment investigates the effect of potassium persulfate concentration on photocatalytic performance. Composite membranes were prepared according to Example 1 and the optimal conditions, and photocatalytic experiments were conducted by varying the concentration of potassium persulfate to 3 mM, 5 mM, 10 mM, and 15 mM.

[0069] like Figure 12 As shown in Figure d, without potassium persulfate, the degradation rate of indole by BP / NH2-MIL-53(Fe) at 30 min was only 25.2%, indicating that relying solely on photocatalysis is inefficient. Adding 3 mM potassium persulfate significantly increased the degradation rate to 72.5%. Further increasing the potassium persulfate concentration to 10 mM resulted in a 100% degradation rate. However, when the potassium persulfate concentration was further increased to 15 mM, the degradation rate slightly decreased to 96.3%. This is because potassium persulfate plays a dual role in the photocatalytic system: on the one hand, it acts as an electron acceptor, capturing photogenerated electrons and inhibiting electron-hole recombination; on the other hand, it decomposes under photoexcitation and iron ion activation to generate sulfate radicals, a highly reactive species that can directly oxidize and degrade organic pollutants. However, excessive potassium persulfate consumes the generated sulfate radicals, similar to the quenching effect of excess hydrogen peroxide on hydroxyl radicals in the Fenton system. Therefore, 10 mM is the optimal potassium persulfate concentration.

[0070] Example 7: Effect of hydrothermal reaction time

[0071] This embodiment investigates the effect of hydrothermal reaction time on the growth of NH2-MIL-53(Fe) crystals and the performance of the composite film. Following the method of Example 1, the hydrothermal reaction time was varied in step three to 6 h, 8 h, and 10 h, while other conditions remained constant.

[0072] XRD and SEM characterization revealed that after 6 hours of hydrothermal reaction, characteristic diffraction peaks of NH2-MIL-53(Fe) appeared but were weak, with small and unevenly dispersed crystals. After 8 hours, the diffraction peak intensity significantly increased, and the crystal morphology became regular and the crystal size uniform. After 10 hours, the diffraction peak intensity increased slightly, but some crystals showed growth and aggregation. Photocatalytic performance testing showed that the composite film prepared after 8 hours of reaction exhibited the best degradation effect on indole, reaching 100% degradation rate within 30 minutes, while the degradation rates of samples prepared after 6 and 10 hours were 83.6% and 92.4%, respectively. This indicates that appropriate hydrothermal time is beneficial for the full growth and uniform distribution of NH2-MIL-53(Fe) crystals; too short a time results in incomplete crystals, while too long a time may lead to crystal aggregation.

[0073] Comparative Example 1: Preparation and Performance Testing of Pure NH2-MIL-53(Fe)

[0074] To verify the roles of bacterial cellulose and polydopamine in composite membranes, pure NH2-MIL-53(Fe) powder was prepared in this comparative example. Following the method in step three of Example 1, but without adding BC@PDA, ferric nitrate nonahydrate solution was directly mixed with 2-aminoterephthalic acid solution and subjected to a hydrothermal reaction to obtain a yellow-brown powdery NH2-MIL-53(Fe).

[0075] like Figure 9 As shown, under the same photocatalytic conditions, pure NH2-MIL-53(Fe) only achieved a 58.2% degradation rate of indole after 30 minutes. Figure 10 The kinetic fitting results show that the rate constant for the degradation of indole by pure NH2-MIL-53(Fe) is 0.025 min⁻¹. This performance is significantly lower than that of the BP / NH2-MIL-53(Fe) composite membrane (0.066 min⁻¹), representing only 37.9% of the latter. Furthermore, pure NH2-MIL-53(Fe) powder requires centrifugation or filtration to be separated from the solution after the reaction, a cumbersome process prone to loss. This comparative example clearly demonstrates that the introduction of the BC@PDA support not only improves photocatalytic activity but also solves the problem of difficult recovery of powdered catalysts.

[0076] Comparative Example 2: Preparation and Performance Testing of BC / NH2-MIL-53(Fe)

[0077] To investigate the role of polydopamine in the composite membrane, a polydopamine-free BC / NH2-MIL-53(Fe) was prepared in this comparative example. Following the method of Example 1, the polydopamine modification process was omitted in step two, and pretreated bacterial cellulose was directly used instead of BC@PDA for NH2-MIL-53(Fe) loading in step three.

[0078] like Figure 9 As shown, BC / NH2-MIL-53(Fe) achieved a degradation rate of 78.3% for indole after 30 min, with a rate constant of 0.045 min⁻¹. This performance is superior to pure NH2-MIL-53(Fe) but significantly lower than BP / NH2-MIL-53(Fe). From Figure 8 It can be seen that the photocurrent intensity and charge transfer efficiency of BC / NH2-MIL-53(Fe) are lower than those of BP / NH2-MIL-53(Fe), while the fluorescence intensity is higher, indicating that photogenerated carrier recombination is more severe. This suggests that polydopamine plays a key role in the composite film, not only enhancing visible light absorption but, more importantly, promoting the effective separation of electron-hole pairs by forming a heterojunction interface.

[0079] Comparative Example 3: Preparation and Performance Testing of BC@PDA

[0080] To evaluate the photocatalytic ability of BC@PDA alone, this comparative example prepared BC@PDA according to steps one and two of Example 1, without performing the NH2-MIL-53(Fe) loading in step three, and used it directly for photocatalytic experiments.

[0081] like Figure 9 As shown, BC@PDA exhibited a degradation rate of only 18.2% for indole after 30 min, with a rate constant of 0.0049 min⁻¹. Although polydopamine possesses certain photocatalytic activity, its performance is significantly lower than that of the composite membrane containing NH₂-MIL-53(Fe). This indicates that NH₂-MIL-53(Fe) is the main photocatalytically active component in the composite membrane system, providing a large number of catalytically active sites. The role of polydopamine is primarily to assist in light absorption and electron transport; only through the synergistic effect of both can highly efficient photocatalytic performance be achieved.

[0082] Example 8: Performance study of BP / NH2-MIL-53(Fe) in degrading quinoline

[0083] Quinoline is a more stable nitrogen heterocyclic compound than indole, possessing a closed conjugated system, making it more difficult to degrade. This example studies the photocatalytic degradation performance of quinoline by a BP / NH2-MIL-53(Fe) composite membrane. The composite membrane was prepared according to Example 1 and the optimal conditions. The photocatalytic experimental conditions were the same as those for indole degradation, but the reaction time was extended to 100 min.

[0084] like Figure 14 As shown, BP / NH2-MIL-53(Fe) exhibited excellent degradation effects on quinoline at a concentration of 50 mg / L, achieving a degradation rate of 98.8% after 100 min. In contrast, the degradation rates of BC / NH2-MIL-53(Fe), NH2-MIL-53(Fe), and BC@PDA were only 55.2%, 34.6%, and 13.4%, respectively. Figure 15 The kinetic analysis shows that the rate constant for quinoline degradation by BP / NH2-MIL-53(Fe) is 0.034 min⁻¹, which is 5.7 times, 11.3 times, and 34 times that of BC / NH2-MIL-53(Fe), NH2-MIL-53(Fe), and BC@PDA, respectively. These results fully demonstrate that the BP / NH2-MIL-53(Fe) composite membrane is not only effective for indole but also exhibits excellent catalytic performance for the more recalcitrant quinoline, showing broad application prospects.

[0085] like Figure 16 As shown, similar to the degradation of indole, the quinoline concentration remains essentially unchanged under conditions of no light, no catalyst, or no potassium persulfate, indicating that quinoline is extremely stable at room temperature. Only with the combined action of BP / NH2-MIL-53(Fe), visible light, and potassium persulfate can efficient degradation of quinoline be achieved. This further demonstrates the synergistic effect of the system of this invention.

[0086] Example 9: The Influence of Environmental Factors on Photocatalytic Performance

[0087] The presence of multiple coexisting substances in actual wastewater can affect photocatalytic performance. This example investigated the effects of humic acid concentration and pH on the degradation of indole by BP / NH2-MIL-53(Fe). Humic acid is a widely distributed organic compound in natural water bodies and may compete with target pollutants for reactive species during photocatalysis.

[0088] like Figure 13 As shown in Figure a, indole was completely degraded within 30 minutes in the absence of humic acid. The degradation rate slightly decreased to 96.8% after the addition of 1 mg / L humic acid. With increasing humic acid concentrations to 3 mg / L, 5 mg / L, and 7 mg / L, the degradation rate gradually decreased to 89.5%, 81.7%, and 72.3%, respectively. This is because humic acid molecules contain numerous functional groups such as carboxyl, hydroxyl, and phenolic hydroxyl groups, which can react with sulfate radicals and superoxide radicals, consuming some of the active species. Humic acid may also compete with indole for adsorption sites on the catalyst surface. However, it is noteworthy that even in the presence of 7 mg / L humic acid, the degradation rate remained above 72%, indicating that BP / NH2-MIL-53(Fe) has strong anti-interference capabilities.

[0089] like Figure 13 As shown in b, the pH value of the solution has a significant impact on the photocatalytic performance. At pH 5, indole is completely degraded within 30 min, exhibiting the best performance. The degradation rate is 88.1% at pH 3, 67.3% at pH 7, and further decreases to 45.6% and 28.9% at pH 9 and 11, respectively. This is because under acidic conditions, indole molecules exist in a neutral form and are easily oxidized and degraded. Simultaneously, potassium persulfate is more easily activated under acidic conditions to generate sulfate radicals. Under alkaline conditions, indole undergoes deprotonation and acquires a negative charge, causing electrostatic repulsion with the negative charge on the catalyst surface, reducing adsorption and degradation efficiency. Furthermore, alkaline conditions are also unfavorable for the activation of potassium persulfate. Therefore, weakly acidic conditions are the optimal pH range for this system.

[0090] Example 10: Application of BP / NH2-MIL-53(Fe) in different aqueous media

[0091] To evaluate the application potential of composite membranes in practical wastewater treatment, this example investigated the degradation performance of BP / NH2-MIL-53(Fe) on indole and quinoline in different aqueous media. Four water samples were selected: deionized water, lake water, coal washing water, and coking wastewater. The lake water was taken from the surface water of a local lake and contained natural organic matter and minerals. The coal washing water and coking wastewater were taken from coal chemical enterprises and were used after filtration and dilution pretreatment. Indole and quinoline solutions with a concentration of 50 mg / L were prepared in each of the four water samples, and 1 g / L of catalyst and 10 mM potassium persulfate were added. The pH was adjusted to 5, and photocatalytic experiments were conducted.

[0092] like Figure 18 As shown in Figure a, when deionized water is used as the medium, BP / NH2-MIL-53(Fe) achieves a 100% degradation rate of indole within 30 minutes. The degradation rate is 92.2% in lake water, 85.6% in coal washing water, and 76.3% in coking wastewater. Figure 18 As shown in b, the degradation of quinoline follows a similar pattern, with degradation rates of 98.8%, 90.3%, 83.2%, and 75.2% in deionized water, lake water, coal washing water, and coking wastewater at 100 min, respectively. Although the degradation efficiency decreased in actual water samples, it remained at a high level overall. This is because inorganic ions such as chloride ions and carbonate ions, as well as organic matter in actual water bodies, react with reactive species, reducing their effective concentration. Simultaneously, suspended particles in the water body scatter and absorb light, reducing light utilization. Nevertheless, BP / NH2-MIL-53(Fe) still exhibits good adaptability in complex water conditions, demonstrating its potential in practical applications.

[0093] Example 11: Performance Evaluation of Reusability

[0094] The stability and reusability of photocatalysts are important indicators for evaluating their practical value. This example studies the recycling performance of a BP / NH2-MIL-53(Fe) composite membrane. The composite membrane was prepared according to Example 1 and optimal conditions, and experiments were conducted on the photocatalytic degradation of indole and quinoline. After the reaction, the composite membrane was removed from the solution, washed three times alternately with deionized water and methanol, and then vacuum-dried at 40°C for use in the next cycle experiment. The reaction time for indole was 30 min, and the reaction time for quinoline was 100 min.

[0095] like Figure 17 As shown, the BP / NH2-MIL-53(Fe) composite membrane maintained high catalytic activity after six cycles. The degradation rate of indole decreased slightly from 100% in the first cycle to 86.3% in the sixth cycle, and the degradation rate of quinoline decreased from 98.8% to 88.8%. The slight decrease in degradation rate may be attributed to the following reasons: first, a small amount of NH2-MIL-53(Fe) crystals detached from the support surface during repeated washing and drying, resulting in catalyst loss; second, some active sites were occupied by degradation products or intermediates, leading to a reduction in available active sites; and third, a small amount of iron ions may have dissolved after multiple light and redox cycles. Nevertheless, the composite membrane still maintained a degradation rate of over 85% after six cycles, demonstrating its good structural stability and reusability, and its economic advantages in practical applications.

[0096] Example 12: TOC Removal Rate Test

[0097] Total organic carbon (TOC) is an important indicator for evaluating the degree of mineralization of organic pollutants. This example tested the TOC removal rate during the degradation of indole and quinoline by BP / NH2-MIL-53(Fe). The TOC values ​​of the reaction solution at different times were measured using a total organic carbon analyzer, and the TOC removal rate was calculated.

[0098] like Figure 19 As shown in Figure a, for a 50 mg / L indole solution, although the indole molecules were completely degraded after 30 min, the TOC removal rate was only 15.3%, far lower than the pollutant removal rate. This indicates that indole was not completely mineralized into carbon dioxide and water, but rather transformed into small molecule intermediates. With the reaction time extended to 60 min, 90 min, 120 min, and 180 min, the TOC removal rate gradually increased to 32.5%, 48.7%, 61.3%, and 76.2%, respectively. This suggests that the intermediates were gradually mineralized under continuous photocatalytic oxidation, but complete mineralization requires a relatively long time.

[0099] like Figure 19As shown in b, the mineralization process of quinoline is slower. At 100 min, the degradation rate of quinoline reached 98.8%, but the TOC removal rate was only 40.6%. Extending the reaction time to 200 min increased the TOC removal rate to 68.5%. This is attributed to the stable bicyclic conjugated structure of the quinoline molecule, with the nitrogen atom in a closed conjugated system; ring opening and complete mineralization require stronger oxidation conditions and a longer reaction time. Nevertheless, the TOC removal rate of the system in this invention is relatively high compared to similar studies, demonstrating its strong mineralization ability for organic pollutants.

[0100] Example 13: Experimental and Mechanistic Study of Free Radical Scavenging

[0101] To clarify the reaction mechanism of BP / NH2-MIL-53(Fe) photocatalytic degradation of nitrogen heterocyclic compounds, this example identifies the main active species through a radical scavenging experiment. p-benzoquinone was selected as the superoxide radical scavenger, isopropanol as the hydroxyl radical scavenger, disodium ethylenediaminetetraacetate as the hole scavenger, and methanol as the sulfate radical scavenger. Under standard photocatalytic degradation conditions of indole, the effects of each scavenger on the degradation efficiency were investigated by adding 1 mM p-benzoquinone, 1 mM isopropanol, 10 mM disodium ethylenediaminetetraacetate, or 1 mM methanol, respectively.

[0102] like Figure 20 As shown in a and 20b, indole was completely degraded within 30 min without the addition of a scavenging agent. The degradation rate was 96.2% after the addition of isopropanol, showing only a slight decrease, indicating that hydroxyl radicals played a minimal role in the reaction. The degradation rate decreased to 82.3% after the addition of disodium ethylenediaminetetraacetate, indicating that vacancies played a role. However, the degradation rate decreased significantly to 38.2% and 50.4% after the addition of p-benzoquinone or methanol, respectively, indicating that superoxide radicals and sulfate radicals were the main reactive species in the reaction. This result is consistent with the characteristics of the potassium persulfate advanced oxidation system; sulfate radicals have a high redox potential of 2.5 to 3.1 V, exhibiting extremely strong oxidizing power and selectivity, making them an effective reactive species for degrading recalcitrant organic matter.

[0103] Based on experimental results and theoretical analysis, the mechanism of photocatalytic degradation of nitrogen heterocyclic compounds by BP / NH2-MIL-53(Fe) is proposed, such as... Figure 21As shown, under visible light irradiation, NH2-MIL-53(Fe) and polydopamine are simultaneously excited to generate photogenerated electron-hole pairs. The conduction band of NH2-MIL-53(Fe) is at -1.03 eV, and the valence band is at 1.02 eV. The LUMO level of polydopamine is -1.4 eV, and the HOMO level is 0.8 eV. According to the Z-type heterojunction mechanism, photogenerated electrons from the conduction band of NH2-MIL-53(Fe) transfer to the HOMO level of polydopamine and recombine with holes in the HOMO. Thus, photogenerated electrons are mainly concentrated in the LUMO level of polydopamine, while photogenerated holes are mainly concentrated in the valence band of NH2-MIL-53(Fe). This space charge separation effectively suppresses simple recombination of electron-hole pairs and prolongs carrier lifetime. The electrons in the LUMO level of polydopamine have strong reducing power and can reduce dissolved oxygen to superoxide radicals. Holes in the valence band of NH2-MIL-53(Fe) possess strong oxidizing capabilities. They can directly oxidize organic matter adsorbed on the catalyst surface, or react with potassium persulfate to generate sulfate radicals. The generated superoxide radicals, sulfate radicals, and holes work together to oxidize and degrade nitrogen heterocyclic compounds through multiple pathways, ultimately mineralizing them into carbon dioxide, water, and inorganic nitrogen compounds.

[0104] Test Results Summary and Analysis

[0105] Table 1 lists the performance data of all examples and comparative examples for indole degradation under standard conditions for 30 min.

[0106] Table 1. Photocatalytic degradation performance of indole by different samples

[0107] Sample number Sample Name Degradation rate (%) Rate constant (min⁻¹) Example 1 <![CDATA[BP / NH2-MIL-53(Fe)]]> 100 0.066 Comparative Example 1 <![CDATA[NH2-MIL-53(Fe)]]> 58.2 0.025 Comparative Example 2 <![CDATA[BC / NH2-MIL-53(Fe)]]> 78.3 0.045 Comparative Example 3 BC@PDA 18.2 0.0049

[0108] Table 1 clearly shows that the degradation performance of the BP / NH2-MIL-53(Fe) composite membrane is significantly better than that of all comparative samples. Compared with pure NH2-MIL-53(Fe), the rate constant of the composite membrane is increased by 164%, demonstrating the synergistic effect of the BC@PDA carrier. Compared with BC / NH2-MIL-53(Fe), the performance of the composite membrane is improved by 46.7%, demonstrating the key role of polydopamine. These data quantitatively verify the superiority of the technical solution of this invention.

[0109] Based on the research results of all the above embodiments, the following conclusions can be drawn:

[0110] First, this invention successfully prepared a BC@PDA / NH2-MIL-53(Fe) composite membrane photocatalyst, which has a well-defined composition, structure, and morphology. Bacterial cellulose provides a three-dimensional network support framework, polydopamine serves as a functional interface layer connecting the support and the photocatalytically active component, and NH2-MIL-53(Fe) crystals are uniformly distributed on the membrane surface.

[0111] Second, the optimal preparation conditions were determined through system parameter optimization: a polydopamine concentration of 7 mg / mL, a BC@PDA to NH2-MIL-53(Fe) mass ratio of 1:1, a hydrothermal reaction temperature of 150℃, and a reaction time of 8 h. The composite membrane prepared under these optimal conditions exhibited the best photocatalytic performance.

[0112] Third, the BP / NH2-MIL-53(Fe) composite membrane exhibits excellent photocatalytic degradation performance for nitrogen heterocyclic compounds. Under conditions of 1 g / L catalyst dosage, 10 mM potassium persulfate concentration, and pH 5, it completely degrades a 50 mg / L indole solution within 30 min of visible light irradiation and degrades 98.8% of a 50 mg / L quinoline solution within 100 min. This performance is significantly superior to that of single-component catalysts and the unmodified polydopamine composite membrane.

[0113] Fourth, the composite membrane exhibits excellent stability, reusability, and environmental adaptability. Even after six cycles, it maintains a degradation efficiency of over 85% and retains high activity in actual water samples such as lake water, coal washing water, and coking wastewater, demonstrating promising prospects for practical application.

[0114] Fifth, mechanistic studies show that the high photocatalytic activity of the composite membrane stems from multiple synergistic effects. Polydopamine broadens the visible light response range and enhances light absorption. The Z-shaped heterojunction formed by polydopamine and NH2-MIL-53(Fe) promotes the spatial separation of photogenerated electrons and holes, inhibiting carrier recombination. The superoxide radicals, sulfate radicals, and holes generated during the reaction synergistically oxidize and degrade organic pollutants; the combined effect of multiple active species ensures highly efficient degradation performance.

[0115] The BC@PDA / NH2-MIL-53(Fe) composite membrane photocatalyst and its preparation method provided by this invention successfully solve the problems of traditional powder photocatalysts, such as difficulty in recycling, severe recombination of photogenerated carriers, and low visible light utilization. It exhibits excellent performance and application potential in the photocatalytic degradation of nitrogen heterocyclic compounds in water, and provides a new technical approach for the treatment of recalcitrant organic pollutants in industrial wastewater.

Claims

1. A method for preparing a BC@PDA / NH2-MIL-53(Fe) composite membrane, characterized in that... Includes the following steps: Step 1, Pretreatment of bacterial cellulose membrane: Rinse the bacterial cellulose membrane with deionized water, then soak it in a 1 mol / L sodium hydroxide solution and treat it in a 90℃ constant temperature water bath for 1 hour. After taking it out, rinse it with deionized water until the pH value is 7 to obtain the purified bacterial cellulose membrane. Step 2, Preparation of BC@PDA composite membrane: Weigh 1g of the bacterial cellulose membrane treated in Step 1 and add it to 10mL of tris(hydroxymethyl)aminomethane hydrochloride buffer solution with a pH of 8.

5. Add dopamine hydrochloride under magnetic stirring, adjust the pH to 8.5 with sodium hydroxide solution, and stir at 200r / min for 24h at room temperature. After the reaction is completed, take it out and wash it three times alternately with deionized water and methanol to obtain the BC@PDA composite membrane. Step 3, Preparation of BP / NH2-MIL-53(Fe) composite membrane: Weigh 2 mmol of ferric nitrate nonahydrate and sonicate it into 10 mL of dimethylformamide. Add 1 g of wet BC@PDA composite membrane to the ferric nitrate nonahydrate precursor solution and stir magnetically for 30 min at room temperature to allow BC@PDA to fully adsorb iron ions to form solution A. 2-Aminoterephthalic acid was dissolved in 10 mL of dimethylformamide to form solution B. Solution B was slowly added dropwise to solution A. The mixture was stirred slowly at room temperature for 30 min. The mixture was then transferred to a reaction vessel and reacted at 150 °C for 8 h. After naturally cooling to room temperature, the composite membrane was removed, rinsed repeatedly with deionized water, and then washed three times alternately with methanol and deionized water. The membrane was dried at 40 °C to obtain the BP / NH2-MIL-53(Fe) composite membrane.

2. The preparation method according to claim 1, characterized in that, In step two, the concentration of dopamine hydrochloride is 5 mg / mL to 9 mg / mL.

3. The preparation method according to claim 2, characterized in that, In step two, the concentration of dopamine hydrochloride is 7 mg / mL.

4. The preparation method according to claim 1, characterized in that, In step three, the mass ratio of BC@PDA composite membrane to NH2-MIL-53(Fe) is 0.5:1 to 1:2.

5.

5. The preparation method according to claim 4, characterized in that, In step three, the mass ratio of BC@PDA composite membrane to NH2-MIL-53(Fe) is 1:

1.

6. The preparation method according to claim 1, characterized in that, In step three, the amount of 2-aminoterephthalic acid used is 2 mmol.

7. The preparation method according to claim 1, characterized in that, The hydrothermal reaction in step three takes 6 to 10 hours.

8. A BC@PDA / NH2-MIL-53(Fe) composite membrane, characterized in that, The composite membrane is prepared by the preparation method according to any one of claims 1 to 7. The composite membrane is composed of three components: bacterial cellulose, polydopamine and NH2-MIL-53(Fe), wherein NH2-MIL-53(Fe) is uniformly loaded on the surface of BC@PDA. The band gap energy of the composite membrane is 1.6 eV to 1.7 eV, and NH2-MIL-53(Fe) exhibits a uniformly distributed octahedral spindle morphology with a size of 80 nm to 120 nm.

9. The application of the BC@PDA / NH2-MIL-53(Fe) composite film according to claim 8 in the photocatalytic degradation of nitrogen heterocyclic compounds, characterized in that, The nitrogen heterocyclic compound is indole or quinoline, and the photocatalytic degradation conditions are as follows: the dosage of the composite membrane is 0.2 g / L to 1.4 g / L, the concentration of potassium persulfate is 3 mM to 15 mM, the pH of the solution is 3 to 7, the light source is a xenon lamp with a wavelength greater than 420 nm.

10. The application according to claim 9, characterized in that, The preferred conditions for photocatalytic degradation are as follows: the dosage of the composite membrane is 1 g / L, the concentration of potassium persulfate is 10 mM, the pH of the solution is 5, and under these conditions, the degradation rate of indole solution with a concentration of 50 mg / L reaches 100% after irradiation with visible light for 30 min, and the degradation rate of quinoline solution with a concentration of 50 mg / L reaches more than 98% after irradiation with visible light for 100 min.

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