Preparation method of wide-pH-adaptive porous membrane and application thereof in uranium adsorption / enrichment

Abstract

The application discloses a preparation method of a wide-pH-adaptive porous membrane and application of the wide-pH-adaptive porous membrane in uranium adsorption and enrichment. First, mechanical force assisted ball milling modification treatment is performed on mixed-hydrated palygorskite clay, mixed-hydrated powder obtained through the treatment is used as a foam stabilizer, the foam stabilizer is mixed with a sodium alginate solution, soapberry powder and phytic acid at a certain proportion, and through an optimized foam template method, a casting membrane forming process and a post-treatment process, a porous membrane with a sponge-like porous structure and rich functional groups is successfully constructed. With the help of rich functional groups and high-activity rough interfaces on the surface of the porous membrane, the porous membrane exhibits excellent uranium separation efficiency and comprehensive use performance in a wide pH range. The technology provides an innovative path for safe and environmentally-friendly treatment of uranium-containing wastewater, uranium pollution prevention and control and resource enrichment, and can be further applied to related fields such as environmental protection, new materials and nuclear energy.

Description

Technical Field

[0001] This invention relates to a method for preparing a porous membrane and its application in uranium separation / enrichment, specifically providing a method for preparing a porous membrane with wide pH adaptability and its high-value application in uranium contamination control and uranium resource enrichment. Background Technology

[0002] With the continuous expansion of the uranium industry chain, including uranium mining, hydrometallurgical refining, and nuclear fuel processing, the resulting environmental and ecological risks have become increasingly prominent, posing a significant challenge to the sustainable development of nuclear energy. Among these, the environmental behavior and risk control of uranium-containing wastewater are gradually becoming key environmental issues affecting the green development of nuclear energy. The environmental risks of uranium stem not only from its radioactivity but also from its chemical toxicity as a heavy metal and its potential bioaccumulation effects. Furthermore, the speciation, migration, and transformation of uranium in the aquatic environment further exacerbate its ecological risks. Therefore, the persistence of uranium in the environment and its reactivation potential determine the long-term and complex nature of its pollution control.

[0003] Membrane separation technology has become a research hotspot in the field of uranium-containing wastewater treatment due to its advantages such as high separation efficiency, simple operation, no phase change, and avoidance of secondary pollution. Among them, porous membranes have shown significant advantages in uranium separation due to their unique structural characteristics: ordered interconnected channels enable efficient convective mass transfer, greatly improving the mass transfer rate; the membrane morphology facilitates continuous operation and regeneration; and it is easy to combine with various functional materials to achieve synergistic separation effects. An ideal porous uranium separation membrane should have high porosity and interconnected channels, a suitable pore size distribution, and abundant uranium recognition sites. Its separation performance is highly dependent on the choice of matrix material, film formation process, pore structure characteristics, and surface chemical properties.

[0004] Chinese invention patents CN120532317A and CN120586675A disclose a method for constructing a porous separation membrane using acid-etched powder of mixed-fiber attapulgite clay as a stabilizer, including a combination of processes such as foam template method, casting film formation, ion crosslinking, and carboxylation. However, the membrane exhibits a strong pH dependence on uranium separation performance, with its optimal separation efficiency limited to a narrow pH window of 4–5, and limited efficiency at low pH conditions. Therefore, in actual uranium separation processes, frequent adjustments to influent conditions (such as pH) are necessary to maintain this optimal operating range, significantly reducing the operational efficiency and continuous operation capability of the uranium separation process. Furthermore, during the heated oxalic acid etching treatment of mixed-fiber attapulgite clay, the total mass loss caused by mineral component dissolution and oxalate loss is no less than 30%, significantly limiting the efficient utilization of mixed-fiber mineral resources. Therefore, further strategies such as membrane composition optimization, surface chemical property control, and enhancement of the abundance of highly active functional groups are needed to improve its applicability over a wide pH range and achieve substantial breakthroughs in separation performance.

[0005] Based on the aforementioned technical shortcomings of porous membranes in uranium extraction from water, this invention proposes a novel porous membrane constructed using oxalic acid-modified mixed-fiber attapulgite clay powder as a foam stabilizer, through a combination of green water-based foam template method, casting film formation, ionic crosslinking, and acid activation. By combining increased sodium alginate concentration, controlled film-forming solution viscosity, and the introduction of phytic acid, this membrane possesses both a sponge-like porous structure and high-abundance uranium adsorption sites, effectively improving uranium adsorption efficiency and overall performance over a wide pH range. This technology or process provides a potential pathway for the controllable preparation of high-performance porous membranes and their application in the safe and environmentally friendly treatment of uranium-containing wastewater. Summary of the Invention

[0006] To address the shortcomings of existing mixed-dimensional mineral synergistic stabilization Pickering foam template methods in constructing high-performance porous membranes, such as significant mineral loss, strong pH dependence of uranium separation behavior, and poor separation efficiency under low pH conditions, this invention uses mechanically assisted oxalic acid-modified mixed-dimensional mineral powder as a foam stabilizer. By rationally controlling the composition, viscosity, foam stability, and functional group types of the Pickering foam film-forming solution, a wide pH-adaptive porous membrane can be constructed in a green and low-cost manner, achieving efficient separation and concentration of uranium in water.

[0007] The method for preparing a wide pH-adaptive porous membrane of the present invention is characterized by comprising the following steps: (1) Preparation of modified mixed-fiber powder and plant powder: Mixed-fiber attapulgite clay with a quartz sand mass fraction ≤1.0% and oxalic acid powder are thoroughly mixed at a mass ratio of 10:1.5-3.0. Then, it is dispersed in an ethanol-water mixed solution at a solid-liquid ratio of 1g:2mL and ball-milled at room temperature for 1-4 h. After separation and drying, the obtained mixed-fiber mineral-oxalate composite is passed through a 200-mesh sieve to obtain modified mixed-fiber powder (OMDPal). Among them, the mixed-fiber attapulgite clay (MDPal) has an attapulgite mass fraction of 20-50%, and is associated with minerals such as illite, hematite, anorthite, dolomite, calcite, gypsum and quartz, and the CaO mass fraction in the chemical composition is 8-10%. Mixed-fiber attapulgite clay with a quartz sand mass fraction ≤1.0% is obtained by dry deagglomeration and 200-mesh sieve treatment. In the ethanol-water mixed solution, the volume ratio of ethanol to water is 1:1 to 2:1; the ball-to-material ratio in the ball milling process is 15 to 20:1, the rotation speed is 400 to 500 rpm, and the mass ratio of grinding balls of different diameters is Φ80 large ball: Φ40 medium ball: Φ20 small ball = 3:5:2.

[0008] The presence of quartz sand in MDPal significantly degrades the colloidal properties and thixotropic properties of the film-forming solution, and may even cause localized stress concentration and rupture in the film layer. Therefore, a pretreatment process involving dry deagglomeration, cyclone separation, and 200-mesh sieving is employed to obtain MDPal with a quartz sand mass fraction ≤1.0%. However, even after this sand removal treatment, the number of active functional groups on the MDPal surface for uranium adsorption remains insufficient, requiring further surface modification or grafting to enhance the functional group abundance. Given the strong complexing ability of oxalate ions for uranium, this invention dissolves oxalic acid powder in an ethanol-water mixture (ethanol:water volume ratio of 1:1 to 2:1), and then adds it to MDPal for room-temperature mechanically assisted ball milling surface modification and nano-sizing treatment.

[0009] In a weakly acidic, alcohol-water system, dissolved oxalic acid exists as HC₂O₄. - and C2O4 2- In the presence of a mixed morphology, the positively charged metal sites at the edges of the mixed-dimensional minerals can, through ligand exchange and electrostatic adsorption, allow oxalate ions to replace the original hydroxyl groups and water molecules on the surface, forming stable inner-layer complexes and anchoring carboxyl groups to the solid-phase surface. Simultaneously, the alcohol-water system effectively inhibits the hydrolysis and dissolution of metal ions in the attapulgite structure, ensuring the stability of the mineral structure. By controlling the amount of oxalic acid added (15-30% of the MDPal mass) and using high-solid-phase mechanical grinding, oxalic acid can be firmly bound to the active sites on the surface of the mixed-dimensional minerals, effectively avoiding problems such as severe structural corrosion, reduced solid-phase yield, and significant loss of carboxyl groups into the liquid phase caused by heating and high-concentration oxalic acid liquid-phase corrosion reactions. Furthermore, the CaO mass fraction in the MDPal chemical composition is 8-10%, and oxalate ions in the solution react with metal cations to form calcium oxalate, magnesium oxalate, or aluminum oxalate precipitates, further retaining carboxyl groups in the solid phase, ultimately forming a modified mixed-dimensional mineral-oxalate complex.

[0010] In the room-temperature mechanically assisted oxalic acid modification of MDPal of this invention, the total mineral mass loss is ≤12.5%, far lower than the mass loss of over 30% caused by heated acid etching. For example... Figure 1 As shown in a and b, OMDPal retained its original rod / plate mixed-fiber morphology, and the mineral crystals exhibited higher dispersibility and degree of dissociation. This indicates that oxalic acid and ball milling effectively removed impurities and cementing agents between the rod / plate mixed-fiber structures, promoting the rearrangement and dispersibility of nanorod crystals and plate particles. The packing height of OMDPal (5g) was significantly higher than that of MDPal. Figure 1 e), which also confirms its lower bulk density and better dispersion of nano-sized mixed minerals. XRD pattern ( Figure 1c) Confirmed that the diffraction peak intensities of attapulgite and illite did not significantly decrease after modification, while the characteristic diffraction peaks of calcite, dolomite, and gypsum disappeared. Simultaneously, diffraction peaks of calcium oxalate appeared at 14.95°, 15.32°, 23.54°, 24.40°, 30.11°, and 38.18°; FTIR spectrum ( Figure 1 The newly added values ​​in d) are 1657, 1318, 781, and 666 cm. -1 The absorption peaks further confirmed the formation of oxalates such as calcium oxalate and magnesium oxalate, indicating that oxalate ions had been successfully introduced into the mixed-dimensional mineral system. Figure 1 As shown in e, unlike mixed-fiber attapulgite which turns white after being etched with heated acid, OMDPal remains brick red, and its chromaticity parameters... L*、a* and b* The values ​​did not change significantly (Table 1), indicating that the oxalic acid modification treatment did not cause significant dissolution of the mineral framework elements. Furthermore, as shown in Table 1, the specific surface area and pore volume of OMDPal increased significantly after modification treatment. Figure 1 f) Enhanced adsorption energy and contact area, and made particles more difficult to desorb through capillary action, thus improving foam stability; compared with MDPal, the relative contents of Al2O3, MgO, and Fe2O3 in OMDPal were slightly reduced, while the relative contents of SiO2 and CaO were increased, reflecting the high structural stability of the tetrahedral lamellar clay minerals, while the dissolved Ca 2+ The oxalic acid-encapsulated components are retained in the solid phase, providing a material basis for further functionalization. The characterization results confirm that mechanically assisted oxalic acid modification of MDPal provides stable structural support for subsequent foam stabilization, enhanced gas-liquid interface, and increased functional group abundance.

[0011] Furthermore, traditional organic surfactants used as foaming agents to prepare porous materials often face problems such as organic pollution, poor foam stability, and high cost. However, the pericarp of the soapberry fruit is rich in saponins and plant fibers, with saponins accounting for approximately 30%. It can serve as a green, surfactant-like agent to synergistically stabilize foam systems, and the plant fibers can be used as plant-based toughening and reinforcing materials for porous membranes. Therefore, in this invention, the pericarp of the soapberry fruit is washed, dried to constant weight, pulverized, and passed through a 200-mesh sieve to obtain plant powder. During the ultrasonic treatment in step (2), the abundant saponin components in the plant powder exhibit excellent foaming and foam stability performance, and are insensitive to changes in temperature, water hardness, and pH. Simultaneously, the plant fiber bundles it contains can play a skeletal support role in the gaps between the foam matrix, significantly enhancing the toughness and impact resistance of the foam, thereby synergistically improving the overall stability of the foam system. This plant powder possesses the integrated structure-function characteristics of both a "green foaming agent" and a "natural fiber filler," achieving multiple functions such as foaming, foam stabilization, and enhancing the gas-liquid interface.

[0012] (2) Preparation of Pickering foam film-forming solution: The modified mixed fiber powder and plant powder obtained in step (1) are mixed at a mass ratio of 15-20:1, and uniformly dispersed in deionized water at a solid-liquid ratio of 1g:10mL. The mixture is ultrasonically treated for 0.5h to obtain a suspension. The sodium alginate (SA) is a powder with a solution viscosity ≥0.02Pa·s at 20℃ and a solution viscosity of 10g / L. The ultrasonic treatment power is 200-300W and the frequency is 40-50KHz. The above suspension, sodium alginate / glycerol mixed aqueous solution and phytic acid are mixed at a volume ratio of 20:80:0.5-1.0, wherein the mass fractions of sodium alginate and glycerol in the sodium alginate / glycerol mixed aqueous solution are 2.5% and 0.6% respectively. After mixing, the mixture is stirred at 8000 rpm for 20min to obtain the Pickering foam film-forming solution.

[0013] Pickering foam template method is an ideal technical route for preparing high-performance porous membranes. The composition of the foam film-forming liquid, the morphology and dispersibility of solid particles directly affect the gas / liquid interface membrane strength, the overall stability of the foam, and the distribution of functional groups, which together determine the final separation performance and application value of the porous membrane. This invention utilizes OMDPal to stabilize the foam, fully leveraging the synergistic effect between rod-shaped and sheet-like particles, easily forming a dense interfacial network structure, thus creating a highly stable foam. This effectively avoids the limitation of single-dimensional minerals easily inducing material embrittlement when enhancing stiffness, achieving a balance between rigidity and toughness in the porous membrane while maintaining stiffness. Furthermore, the OMDPal suspension exhibits lower viscosity (rotational viscosity <50 mPa·s at 7% solid content) and weak thixotropy. Increasing the concentration of SA matrix in the final film-forming solution to 2.0 wt.% and increasing the OMDPal addition to 1.5–2.0 wt.% resulted in a Pickering foam film-forming solution that maintained good film-forming and rheological properties. Moreover, the increased density of carboxyl, hydroxyl, and oxalate groups in the system helps to improve uranium separation efficiency.

[0014] Rheological properties of different film-forming liquids ( Figure 2a) It can be seen that the initial viscosity of the 2% SA film-forming solution reaches 1700 mPa·s, while the viscosity of the film-forming solution increases to 1800 mPa·s after adding 2% OMDPal. No foam was observed under an optical microscope, confirming that single SA and OMDPal / SA film-forming solutions cannot encapsulate air bubbles. Further addition of 0.05% Sap powder for foaming increases the viscosity of the film-forming solution system (OMDPal / Sap / SA-1) to 3100 mPa·s. Although the system contains a large number of air bubbles with an average diameter of approximately 150 μm, separation occurs after casting. When 0.10% Sap powder and 0.5% phytic acid are introduced into the above system, the initial viscosity of the film-forming solution system (OMDPal / Sap / SA-2) stabilizes at 3500 mPa·s, containing a large number of uniformly dispersed air bubbles of approximately the same size with an average diameter of 72.74 μm. Figure 2 c,d) exhibit excellent cohesiveness, film-forming properties, and foam stabilization. Phytic acid is introduced to regulate the viscosity of the film-forming solution, and phosphate functional groups are also introduced. This film-forming solution exhibits higher shear stress in the high-shear region. Figure 2 (b) indicates that the foam's resistance to shear deformation is enhanced, which is beneficial for subsequent natural drying and pore formation. However, when the Sap powder content is further increased to 0.125%, the film-forming liquid system (OMDPal / Sap / SA-3) is rich in foam, but the initial viscosity reaches 4100 mPa·s, making casting difficult. This invention uses mixed clay minerals and plant-based active ingredients to synergistically stabilize water-based foam, resulting in a foam film-forming liquid with high viscoelasticity and excellent film-forming properties. This increases foam stability and liquid film strength, effectively preventing bubble coarsening and aggregation, ultimately preserving the hierarchical porous structure intact. Furthermore, the introduction of oxalate and phytic acid-rich adsorption functional groups into the system is beneficial for improving uranium separation efficiency.

[0015] (3) Preparation of porous membrane: The film-forming solution obtained in step (2) is cast into a membrane, and the thickness of the liquid film is controlled to be 0.2-0.4 cm. After drying at room temperature, the membrane is peeled off. The obtained membrane is completely immersed in a 2.0% (w / w) water-soluble calcium salt solution for 0.5-2 h, then removed and transferred to a 0.4 mol / L nitric acid solution. The membrane is ultrasonically treated for 5-10 min, and then rinsed with deionized water until the washing solution is neutral to obtain a porous membrane. The water-soluble calcium salt is at least one of calcium chloride, calcium iodide, calcium dihydrogen phosphate, and calcium nitrate.

[0016] During the casting process, controlling the liquid film thickness to 0.2–0.4 cm balanced the water evaporation rate and the concentration gradient within the film. A film that is too thin (<0.2 cm) is prone to surface crusting and internal defects due to localized rapid drying; a film that is too thick (>0.4 cm) prolongs the drying time and increases the risk of stress cracking. A liquid film thickness of 0.2–0.4 cm ensures uniform water evaporation, resulting in a porous membrane with a smooth surface and uniform thickness, providing a foundation for subsequent uniform crosslinking. During the slow dehydration and curing process at room temperature, sufficient time is available for structural reorganization and local bonding between the components at the gas / liquid interface, forming a dense, continuous interfacial phase and improving the membrane's mechanical properties. Testing showed that the tensile strength, elongation at break, and flexural strength of the wet porous membrane reached 3.30 MPa, 62.21 MPa, and 101.72 MPa, respectively, demonstrating excellent mechanical properties and plastic deformation capacity. Crosslinking with a 2.0% calcium salt solution can prevent excessively high calcium concentrations from causing an overly dense calcium crosslinking layer to form instantaneously on the membrane surface, hindering ion exchange within the porous membrane, while also facilitating Ca2+ crosslinking. 2+ A gradual infiltration process is used to obtain a uniformly cross-linked structure along the thickness direction, which ultimately solidifies into a highly stable porous structure. For example... Figure 3 As shown in (a, b), the OMDPal / Sap / SA-Ca membrane exhibits a uniformly distributed sponge-like porous structure in its cross-section. The pore size is mostly concentrated around 100 µm, with nearly circular and irregular shapes predominating. The pore density is high, and the membrane has good interconnectivity. Based on the porous structure of this membrane, the active uranium-adsorbing functional groups in OMDPal, SA, PA, and Sap saponins are also uniformly anchored on the pore walls, providing a structural basis for the rapid and efficient capture of uranyl ions.

[0017] Furthermore, this invention systematically investigated the effects of different concentrations of nitric acid on the OMDPal / Sap / SA-Ca membrane. C HNO3 The effect of acid activation (0.1–0.5 mol / L) on uranium separation efficiency was investigated to clarify the promoting effect of acid activation on uranium separation efficiency and its applicable conditions. Figure 3 As shown in d, when C HNO3 The efficiency of uranium separation in a uranium solution with pH=2.5 when the concentration is increased from 0.1 mol / L to 0.4 mol / L is ( R The concentration increased significantly from 50.94% to 87.54%; after further increasing to 0.5 mol / L, R The trend towards stabilization indicates that the activation effect has reached a plateau within the range of acid treatment. Figure 3 e demonstrates the change in uranium separation efficiency of the OMDPal / Sap / SA-Ca membrane activated with the same concentration of nitric acid in a uranium solution at pH 3.2. It exhibits a more sensitive response to acid activation and can achieve separation efficiency even at lower acid concentrations. C HNO3A concentration >0.2 mol / L) achieves rapid performance improvement. Given that the subsequent high-rate uranium enrichment process requires elution with a high-concentration acid solution, 0.4 mol / L HNO3 is preferred for activation treatment to ensure the high efficiency and stability of the porous membrane in practical applications. Figure 3 As shown in c, after acid activation, the resulting OMDPal / Sap / SA-H membrane still maintains a sponge-like porous structure, and the roughness of the pore walls increases significantly. This leads to an increase in the specific surface area of ​​the membrane material and full exposure of the carboxyl and hydroxyl groups in SA and the highly reducing functional groups in SAP, thus providing more abundant active sites and enhancing the complexation ability with uranyl ions. Figure 3 As shown in f, the apparent density of the OMDPal / Sap / SA-H membrane is significantly lower than that of the aqueous medium, allowing it to float stably on the liquid surface. When immersed in aqueous solutions with pH values ​​of 1, 5, 9, and 13, and subjected to ultrasonic treatment (250 W, 50 kHz) for 2 hours, followed by static and intermittent shaking, the membrane maintained its structural integrity during a 30-day observation period. Ultrasonic cavitation did not cause significant degradation or damage, demonstrating excellent structural and chemical stability.

[0018] The porous membrane prepared by the above method is applied to uranium separation and enrichment, including the following steps: (1) The above porous membrane was added to uranium-containing water with a pH of 2.5–8.0 at a dosage of 0.75–1.0 g / L. The mixture was shaken or stirred at room temperature for 0.5–2 h. The uranium concentration in the solution after the reaction was monitored in real time. The uranium separation efficiency was calculated as follows: separation efficiency = (initial uranium concentration − uranium concentration after reaction) / initial uranium concentration × 100%. If the separation efficiency is ≥98%, the porous membrane is removed from the water body to obtain a uranium-rich membrane; If the separation efficiency is less than 98%, the porous membrane is removed and a new porous membrane is added back into the uranium-containing water body for repeated processing until the separation efficiency reaches more than 98%, thus obtaining a uranium-rich membrane.

[0019] (2) Immerse the uranium-rich membrane obtained in step (1) in 0.4 M nitric acid solution at a solid-liquid ratio of 2~5 g / L. After 5~30 min, take out the membrane and the resulting eluent is the uranium-rich solution.

[0020] The porous membrane can be repeatedly used for uranium separation and enrichment, and can be recycled at least 10 times.

[0021] Figure 4 a represents the conditions at different pH values, with a solid-liquid ratio of 0.75 g / L and an initial uranium concentration of 8 × 10⁻⁶ g / L. ⁻4The separation efficiency of the porous membrane for U(VI) in solution at mol / L. The membrane exhibits excellent separation efficiency and stability over a wide pH range of 3.0-7.3, with a separation efficiency ( R The adsorption capacity has consistently remained above 94%, maintaining a high level. Qe The concentration reached 271.05 mg / g. Even at pH 2.5, high concentrations of H⁺ fiercely competed with uranyl ions for active sites. R The electrostatic repulsion between uranium and the negatively charged film surface increased to 86% at pH 7.75; however, at pH 7.75, the electrostatic repulsion between uranium and the negatively charged film surface was enhanced. R The uranium-rich membrane still achieves a separation efficiency of 89%, significantly outperforming other materials. This excellent broad-range adsorption performance is mainly attributed to the membrane's porous structure and rough pore walls, which fully expose the active sites, allowing for ideal electrostatic attraction, coordination, and redox synergy with uranium, thus achieving highly efficient capture. All uranium-rich membranes remain perfectly spherical, exhibiting good chemical stability and homogeneity. Figure 4 b represents different solid-liquid ratios ( m / V pH=2.5 and initial uranium concentration of 8×10 ⁻ 4 The separation efficiency of U(VI) at mol / L. With m / V The increase, Qe It shows a downward trend, while R Then it increases significantly. When m / V When ≥0.4g / L, R ≥83%, when m / V When =0.75g / L, Qe and R The concentrations reached 230 mg / g and over 91%, respectively, demonstrating excellent uranium capture capabilities. Further continuous adsorption separation-elution enrichment was conducted to investigate its recyclability, with results as follows: Figure 4 As shown in (c, d), during 10 consecutive separation-enrichment processes, the separation efficiency and elution efficiency remained above 98% and 97%, respectively, and the enrichment ratio reached 3.0, demonstrating excellent recycling performance.

[0022] XPS fine spectral analysis ( Figure 5 The results showed that after the OMDPal / Sap / SA-Ca membrane interacted with uranium, the area of ​​the characteristic peaks of C–C / C–H / C=C decreased, indicating that the carbon-containing functional groups were consumed or covered as electron donors during the separation process. The presence of coexistence of U(VI) and U(IV) on the membrane surface, in a ratio of approximately 6.63:1, further confirmed that the electron-rich carbocyclic structure transferred electrons to U(VI), reducing some U(VI) to insoluble U(IV). The calcium binding energy increased by 0.3 eV, suggesting the presence of Ca in the membrane framework. 2+A substitution reaction occurs with UO2²⁺; the binding energy of silicon increases slightly, indicating that the Si–O– metal bonds in the mixed-fiber minerals interact with uranyl groups, causing a fine-tuning of the silicon coordination environment. After activation with 0.4 mol / L nitric acid, the characteristic peak areas of C–C / C–H / C=C in the OMDPal / Sap / SA-H film increase significantly, indicating that acid etching exposes the bulk framework of sodium alginate and Sapindus saponins; the binding energies of C–O and C=O bonds decrease by 0.2 eV and 0.4 eV, respectively, and the O1s binding energy decreases by 0.2 eV, confirming that oxygen-containing functional groups undergo protonation or hydrogen bonding association, thereby improving the hydrophilicity and reactivity of the film surface and optimizing the interfacial properties. After interaction with uranium, the characteristic peak areas of C–C / C–H / C=C decreased significantly, indicating that the full exposure of the electron-rich carbide ring structure promoted electron transfer to U(VI), reducing more U(VI) to U(IV), with a ratio of approximately 3.65:1. The binding energies of C–O and C=O increased by 0.2 eV and 0.3 eV, respectively, and the O1s binding energy increased by 0.2 eV, indicating that the oxygen-containing functional groups coordinated with uranyl or uranium ions. Therefore, in summary, the separation of uranium by the OMDPal / Sap / SA-H membrane is dominated by enhanced surface coordination, with redox reactions and ion exchange providing synergistic assistance, resulting in superior separation performance and efficiency.

[0023] In summary, the present invention has the following advantages compared with the prior art: 1. This invention uses modified mixed-fiber powder obtained through mechanically assisted ball milling and nano-processing as a foam stabilizer, with a 2% sodium alginate solution as the continuous phase, and phytic acid added as a supplementary modifier and functional group enhancer. By optimizing the foam template method, casting film formation, and post-processing, a high-performance porous membrane is constructed. With its abundant functional groups and highly active rough interface, it significantly improves uranium separation efficiency and overall performance over a wide pH range.

[0024] 2. Based on reducing the loss of mineral content in mixed-fiber materials and enhancing the surface functionality and activity of the membrane, highly active groups / complexing ligands (such as carboxyl groups, oxalate groups, etc.) are effectively retained in the modified mixed-fiber powder. Furthermore, the pore structure and functional group abundance of the porous membrane are optimized by rationally controlling the composition and viscosity of the film-forming solution. This leads to the development of a high-performance porous membrane and its high-value application in uranium separation / enrichment, providing an innovative solution for uranium contamination control and resource enrichment.

[0025] 3. This invention uses environmentally friendly and abundant mixed-fiber attapulgite clay and plant-derived resources as main raw materials. It fully utilizes the crystal structure characteristics of mixed-fiber minerals and the integrated functional characteristics of Sapindus mukorossi powder, which combines "foaming" and "natural fiber filler," to synergistically improve the overall stability of the foam system. Through a low-carbon, economical, and compatible combined process, porous membranes can be constructed, enabling applications in environmental protection, new materials, and nuclear energy. Attached Figure Description

[0026] Figure 1 The images show SEM images (a, b), XRD patterns (c), FTIR patterns (d), packing height comparison (e), and N2 adsorption-desorption curves (f) of the mixed-fiber attapulgite clay and modified mixed-fiber powder in this invention.

[0027] Figure 2 Rheological curves (a, b) of different film-forming liquids prepared for this invention, optical microscope images (c) and their foam diameter distribution curves (d).

[0028] Figure 3 SEM images (ac) of the OMDPal / Sap / SA-Ca membrane and OMDPal / Sap / SA-H membrane prepared for this invention, the effect of different concentrations of nitric acid treatment on their U(VI) separation performance (d, e), and digital photographs (f) of the chemical stability test of the OMDPal / Sap / SA-H membrane.

[0029] Figure 4 The separation performance of the OMDPal / Sap / SA-H membrane prepared in this invention for U(VI) (a- pH boundary curve, b- effect of solid-liquid ratio) and its recycling performance (c, d) are shown in the examples.

[0030] Figure 5 XPS spectra of the porous membrane prepared in this invention before and after uranium separation, wherein (a) C1s spectrum, (b) O1s spectrum, (c) U4f spectrum, (d) Si2p spectrum, and (e) Ca2p spectrum. Detailed Implementation

[0031] The following typical examples further illustrate the preparation method of the wide pH adaptable porous membrane of the present invention and its application in uranium separation and enrichment.

[0032] The mixed attapulgite clay was subjected to dry deagglomeration and dispersion, cyclone separation, and 200-mesh sieve treatment to remove sand. The obtained sand-removed mixed attapulgite clay (quartz sand mass fraction ≤1.0%) was mixed with oxalic acid powder at a mass ratio of 10:1.5, and then dispersed in an ethanol-water mixed solution at a solid-liquid ratio of 1g:1mL. The mixture was ball-milled in a closed system at room temperature for 1 h. The resulting product was separated, dried at 50℃, and sieved through a 200-mesh sieve to obtain modified mixed attapulgite powder, labeled as OMDPal-1, for later use.

[0033] The obtained sand-free mixed attapulgite clay (quartz sand mass fraction ≤1.0%) was mixed with oxalic acid powder at a mass ratio of 10:3.0 until homogeneous. Then, it was dispersed in an ethanol-water mixed solution at a solid-liquid ratio of 1g:2mL and ball-milled in a closed container at room temperature for 4 h. The resulting product was separated, dried at 60℃, and passed through a 200-mesh sieve to obtain modified mixed fiber powder, labeled as OMDPal-2, for later use.

[0034] Under stirring conditions of 500 rpm, 100 g of sodium alginate and 24 g of glycerol were uniformly dispersed in 3.88 L of deionized water and stirred continuously for at least 24 h to obtain a homogeneous sodium alginate / glycerol mixed solution for later use.

[0035] The uranium separation performance of the prepared porous membrane was evaluated by adsorption experiments. Specifically, the porous membrane was added to a uranium-containing solution and then placed in a constant-temperature shaker. After shaking at 160 rpm and 25°C for 24 h, the supernatant was collected, and the uranium concentration in the liquid phase was analyzed by UV-Vis spectroscopy (wavelength 652 nm). The uranium separation efficiency was calculated according to equations (1) and (2). R (%) and adsorption capacity (%) Q e (mg / g).

[0036] In the formula, C 0 (mg / L) and C e (mg / L) represent the initial and final uranium concentrations in the uranium-containing solution, respectively. V (mL) represents the volume of the uranium-containing aqueous solution. m (mg) represents the mass of the porous membrane added.

[0037] Comparative Example 1 2.0 g of OMDPal-1 powder was uniformly dispersed in 20 mL of deionized water and sonicated for 0.5 h to obtain a suspension. 20 mL of the above suspension was mixed with 80 mL of a sodium alginate / glycerol mixed solution and stirred at 8000 rpm for 20 min to obtain the OMDPal / SA film-forming solution. Its rheological properties are shown in [reference needed]. Figure 2 (a, b), with a rheological viscosity of 1800 mPa·s, and no foam was observed under an optical microscope.

[0038] Comparative Example 2 Sapindus mukorossi pericarp was washed, dried to constant weight, pulverized, and passed through a 200-mesh sieve to obtain Sapindus mukorossi powder (Sap). 1.5 g of OMDPal-2 and 0.1 g of Sap were mixed and uniformly dispersed in 20 mL of deionized water, and sonicated for 0.5 h to obtain a suspension. 20 mL of the above suspension was mixed with 80 mL of a sodium alginate / glycerol mixed solution, and stirred at 8000 rpm for 20 min to obtain the OMDPal / Sap / SA-1 film-forming solution. Its rheological properties are shown in [reference needed]. Figure 2 (a, b) The viscosity of the film-forming solution is 3100 mPa·s, containing a large number of bubbles with a diameter of about 150 μm. However, after casting, the solution precipitates within 0.5 h.

[0039] Comparative Example 3 The pericarp of Sapindus mukorossi was washed, dried to constant weight, pulverized, and passed through a 200-mesh sieve to obtain Sapindus mukorossi powder (Sap). 1.5 g of OMDPal-2 and 0.10 g of Sap were mixed and uniformly dispersed in 20 mL of deionized water, and sonicated for 0.5 h to obtain a suspension. 20 mL of the above suspension, 80 mL of sodium alginate / glycerol mixed aqueous solution, and 0.5 mL of phytic acid were mixed and stirred at 8000 rpm for 20 min to obtain an OMDPal / Sap / SA-2 film-forming solution. Its rheological properties are shown in [reference needed]. Figure 2 (a, b), with a rheological viscosity of 3500 mPa·s, containing a large number of approximately uniformly dispersed bubbles with an average diameter of 72.74 μm. Figure 2 c, d).

[0040] The above-mentioned OMDPal / Sap / SA-2 film-forming solution was cast into a film with a thickness controlled at 0.3 cm. After drying at room temperature for 48 hours, the film was peeled off. The resulting film was then immersed in a 2.0% calcium chloride solution for 2 hours, removed, and rinsed with deionized water until the washing solution was neutral, yielding a porous membrane. Its cross-sectional microstructure is shown in the figure. Figure 3 As shown in (a, b), the solid-liquid ratio is 0.75 g / L and the uranium concentration is 8 × 10⁻⁶ g / L. -4 Under the condition of mol / L, at pH=2.5 and 3.2, the separation efficiency of this porous membrane for uranium is 45.6% and 50.5%.

[0041] Comparative Example 4 The pericarp of Sapindus mukorossi was washed, dried to constant weight, pulverized, and passed through a 200-mesh sieve to obtain Sapindus mukorossi powder (Sap). 1.5 g of OMDPal-2 and 0.125 g of Sap were mixed and uniformly dispersed in 20 mL of deionized water, and sonicated for 0.5 h to obtain a suspension. 20 mL of the above suspension, 80 mL of sodium alginate / glycerol mixed aqueous solution, and 0.5 mL of phytic acid were mixed and stirred at 8000 rpm for 20 min to obtain the OMDPal / Sap / SA-3 film-forming solution. Its rheological properties are shown in [reference needed]. Figure 2 (a, b) The film-forming liquid contains a large amount of foam, but the viscosity is 4100 mPa·s, which makes it difficult to cast the film.

[0042] Example 1 (1) Repeat the preparation process of the porous membrane in Comparative Example 3, and crosslink it with 2% calcium nitrate solution to obtain OMDPal / Sap / SA-Ca membrane.

[0043] (2) The above OMDPal / Sap / SA-Ca membranes were immersed in 0.1, 0.2, 0.3, 0.4 and 0.5 mol / L nitric acid solutions, respectively, and ultrasonically treated for 5 min at a power of 300 W and a frequency of 40 kHz. Then, the membranes were rinsed with deionized water until the washing solution was neutral to obtain a series of porous membranes. Figure 3 As shown in d, with a solid-liquid ratio of 0.75 g / L and a uranium concentration of 8 × 10⁻⁶ g / L, -4 At mol / L and pH 2.5, the uranium removal efficiencies of porous membranes activated with 0.1, 0.2, 0.3, 0.4 and 0.5 mol / L nitric acid were 51.1%, 68.3%, 77.7%, 87.5% and 88.1%, respectively.

[0044] Example 2 (1) Repeat the preparation process of the porous membrane in Comparative Example 3 to obtain the OMDPal / Sap / SA-Ca membrane.

[0045] (2) The above OMDPal / Sap / SA-Ca membranes were immersed in 0.1, 0.2, 0.3, 0.4 and 0.5 mol / L nitric acid solutions, respectively, and ultrasonically treated for 10 min at a power of 200 W and a frequency of 50 kHz. Then, the membranes were rinsed with deionized water until the washing solution was neutral to obtain a series of porous membranes. Figure 3 As shown in e, with a solid-liquid ratio of 0.75 g / L and a uranium concentration of 8 × 10⁻⁶ g / L, -4 At mol / L and pH 3.2, the uranium removal efficiencies of porous membranes activated with 0.1, 0.2, 0.3, 0.4 and 0.5 mol / L nitric acid were 66.9%, 90.1%, 95.8%, 97.6% and 97.6%, respectively.

[0046] Example 3 (1) Repeat the preparation process of the porous membrane in Comparative Example 3, except that: 20 mL of the above suspension, 80 mL of sodium alginate / glycerol mixed aqueous solution and 1.0 mL of phytic acid are mixed to prepare a membrane solution, which is then cast and crosslinked to obtain an OMDPal / Sap / SA-Ca membrane.

[0047] (2) The above OMDPal / Sap / SA-Ca membrane was immersed in 0.4 mol / L nitric acid solution and sonicated for 5 min at a power of 200 W and a frequency of 50 kHz. The membrane was then rinsed with deionized water until the washing solution was neutral, yielding the OMDPal / Sap / SA-H membrane. Its chemical stability was evaluated as follows: Figure 3 As shown in f. Figure 4 As shown in Figure a, with a solid-liquid ratio of 0.75 g / L and a uranium concentration of 8 × 10⁻⁶ g / L, -4 Under conditions of mol / L, and within a wide pH range of 3.5 to 6.6, this membrane exhibits ultra-high adsorption capacity and good stability. R It has consistently maintained a high level of over 99%. Qe It reached 271.05 mg / g; at pH=3.0 and 7.75, R They still reached 94.5% and 89.0%.

[0048] Example 4 (1) Repeat the preparation process of the porous membrane in Comparative Example 3 to obtain the OMDPal / Sap / SA-Ca membrane.

[0049] (2) The above OMDPal / Sap / SA-Ca membrane was immersed in 0.4 mol / L nitric acid solution and ultrasonically treated for 10 min at a power of 300 W and a frequency of 40 kHz. Then, the membrane was rinsed with deionized water until the washing solution was neutral to obtain the OMDPal / Sap / SA-H membrane. Its cross-sectional microstructure is as follows. Figure 3 As shown in (c), the effect of porous membranes on U(VI) separation performance under different solid-liquid ratios is as follows. Figure 4 As shown in b, when m / V When =0.75g / L, Qe and R Reaching concentrations of 230 mg / g and over 91% respectively, they demonstrate excellent uranium capture capabilities.

[0050] Example 5 (1) Repeat the preparation process of the porous membrane in Example 3 to obtain the OMDPal / Sap / SA-Ca membrane.

[0051] (2) The above OMDPal / Sap / SA-Ca membrane was immersed in 0.4 mol / L nitric acid solution and sonicated for 5 min at a power of 200 W and a frequency of 50 kHz. The membrane was then rinsed with deionized water until the washing solution was neutral to obtain the OMDPal / Sap / SA-H membrane. The solid-liquid ratio was 0.75 g / L, and the uranium concentration was 8 × 10⁻⁶ g / L. -4 At mol / L and pH=3.5, the recycling performance of this membrane is as follows: Figure 4 As shown in (c, d), during 10 consecutive separation-enrichment processes, the separation efficiency and elution efficiency remained above 98% and 97%, respectively, and the enrichment ratio reached 3.0.

Claims

1. A method for preparing a porous membrane with wide pH adaptability, characterized in that, Includes the following steps: (1) Preparation of modified mixed fiber powder and plant powder: Mixed fiber attapulgite clay with quartz sand mass fraction ≤1.0% and oxalic acid powder at a mass ratio of 10:1.5~3.0, and then dispersed in an ethanol-water mixed solution at a solid-liquid ratio of 1g:1~2mL. The mixture is ball-milled in a closed environment at room temperature for 1~4 h. After separation and drying, the obtained mixed fiber mineral-oxalate complex is passed through a 200-mesh sieve to obtain modified mixed fiber powder. Take Sapindus mukorossi pericarp, wash, dry to constant weight, pulverize and pass through a 200-mesh sieve to obtain plant powder. (2) Preparation of foam film-forming solution: The modified mixed fiber powder and plant powder obtained in step (1) are mixed at a mass ratio of 15-20:1, and uniformly dispersed in deionized water at a solid-liquid ratio of 1g:10mL. The mixture is ultrasonically treated for 0.5h to obtain a suspension. The above suspension, sodium alginate / glycerol mixed aqueous solution and phytic acid are mixed at a volume ratio of 20:80:0.5-1.0, wherein the mass fractions of sodium alginate and glycerol in the sodium alginate / glycerol mixed aqueous solution are 2.5% and 0.6%, respectively. After mixing, the mixture is stirred at 8000rpm for 20min to obtain foam film-forming solution. (3) Preparation of porous membrane: The film-forming liquid obtained in step (2) is cast into a membrane, and the thickness of the liquid membrane is controlled to be 0.2-0.4 cm. After drying at room temperature, the membrane is peeled off. The obtained membrane is completely immersed in a 2% water-soluble calcium salt solution and soaked for 0.5-2 hours. Then it is taken out and transferred to a 0.4 mol / L nitric acid solution. It is ultrasonically treated for 5-10 minutes. Then the membrane is rinsed with deionized water until the washing solution is neutral to obtain a porous membrane.

2. The method for preparing a wide pH-adaptive porous membrane as described in claim 1, characterized in that: In step (1), the mass fraction of attapulgite in the mixed attapulgite clay is 20-50%, and it is associated with illite, hematite, anorthite, dolomite, calcite, gypsum and quartz minerals, and the mass fraction of CaO in the chemical composition is 8-10%.

3. The method for preparing a wide pH-adaptive porous membrane as described in claim 1, characterized in that: In step (1), the mixed attapulgite clay with a quartz sand mass fraction ≤1.0% is obtained by dry deagglomeration and dispersion, cyclone separation and 200-mesh sieve treatment; in the ball milling process, the ball-to-material ratio is 15~20:1, the rotation speed is 400~500rpm, and the mass ratio of grinding balls of different diameters is Φ80 ball: Φ40 ball: Φ20 ball = 3:5:

2.

4. The method for preparing a wide pH-adaptive porous membrane as described in claim 1, characterized in that: In step (1), the volume ratio of alcohol to water in the ethanol-water mixed solution is 1:1 to 2:1; the drying temperature is 50 to 60°C.

5. The method for preparing a wide pH-adaptive porous membrane as described in claim 1, characterized in that: In step (2), the sodium alginate is a powder with a solution viscosity ≥0.02 Pa·s at 20℃ and a 10 g / L solution viscosity; the ultrasonic treatment power is 200~300 W and the frequency is 40~50 KHz.

6. The method for preparing a wide pH-adaptive porous membrane as described in claim 1, characterized in that: In step (3), the water-soluble calcium salt is at least one of calcium chloride, calcium iodide, calcium dihydrogen phosphate, and calcium nitrate.

7. A porous membrane prepared by the method according to any one of claims 1 to 6.

8. The application of the porous membrane according to claim 7 in uranium separation and enrichment includes the following steps: (1) The porous membrane was added to uranium-containing water with a pH of 2.5–8.0 at a dosage of 0.75–1.0 g / L, and the reaction was carried out at room temperature with shaking or stirring for 0.5–2 h. The uranium concentration in the solution after the reaction was monitored in real time, and the uranium separation efficiency was calculated as follows: separation efficiency = (initial uranium concentration − uranium concentration after reaction) / initial uranium concentration × 100%. If the separation efficiency is ≥98%, the porous membrane is removed from the water body to obtain a uranium-rich membrane; If the separation efficiency is less than 98%, the porous membrane is removed and a new porous membrane is added back into the uranium-containing water body for repeated processing until the separation efficiency reaches more than 98% to obtain a uranium-rich membrane. (2) Immerse the uranium-rich membrane obtained in step (1) in 0.4 M nitric acid solution at a solid-liquid ratio of 2~5 g / L. After 5~30 min, take out the membrane and the resulting eluent is the uranium-rich solution.

9. The application of the porous membrane according to claim 8 in uranium separation and enrichment, characterized in that: The porous membrane can be repeatedly used for uranium separation and enrichment, and can be recycled at least 10 times.

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