Polyethyleneimine modified chitosan / nano alpha-MnO2 composite foam as well as preparation method and application thereof

Abstract

The invention provides a preparation method of polyethyleneimine modified chitosan / nano alpha-MnO2 composite foam, which comprises the following steps: firstly, preparing alpha-MnO2 nanorods by a hydrothermal method, and then preparing a chitosan solution and a polyethyleneimine solution; the preparation method comprises the following steps: uniformly mixing a chitosan solution and a polyethyleneimine solution, then adding the alpha-MnO2 nanorod, stirring, then adding epichlorohydrin, and carrying out a pre-crosslinking reaction to obtain pre-crosslinked composite gel; and carrying out advanced freezing on the pre-crosslinked composite gel, and then carrying out freeze drying to obtain the polyethyleneimine modified chitosan / nano alpha-MnO2 composite foam. The invention also provides an application of the polyethyleneimine modified chitosan / nano alpha-MnO2 composite foam prepared by the preparation method in adsorption of uranium in an aqueous solution. The polyethyleneimine modified chitosan / nano alpha-MnO2 composite foam provided by the invention has low density and high elasticity, and has the advantage of efficiently removing uranium U (VI).

Description

Technical Field

[0001] The invention belongs to the technical field of preparation methods of nano materials, and specifically relates to a polyethyleneimine modified chitosan / nano α-MnO2 composite foam and a preparation method and application thereof. Background Art

[0002] Uranium is an important component of nuclear fuel and the "food" for the operation of nuclear reactors. However, uranium is radioactive and has a long half-life, which poses serious harm to human health and the ecological environment. The main sources of uranium radioactive pollution include uranium mining and processing, weapons manufacturing, nuclear safety accidents and nuclear waste treatment. On the other hand, nuclear industrial wastewater, uranium tailings leachate, seawater or salt lake brine are also important uranium resources. Conventional methods such as precipitation / co-precipitation, solvent extraction and membrane separation are costly or produce secondary pollution; while the adsorption method has the advantages of high efficiency, simple operation and low cost. The use of resin adsorption is an effective method to remove radioactive contamination.

[0003] Chitosan (CS) has good biocompatibility and biodegradability. There are a large number of amine and hydroxyl groups in its molecular chain, which can effectively bind different metal ions. CS has good solubility and can be easily made into adsorbent materials in different forms, such as powder, beads, fibers, foams or gels. In particular, CS foam has a highly developed pore structure and high specific surface area, which helps the adsorbate ions to diffuse quickly into the pores and fully contact with the active sites, thereby significantly improving the adsorption performance. However, the mechanical strength of pure chitosan foam is poor, and the porous structure is easy to collapse during the adsorption / regeneration process, which is not conducive to its practical application. The mechanical strength of chitosan can be significantly improved by combining it with inorganic materials (i.e. biochar / carbon, clay and metal oxides). MnO2 has good ion exchange, catalytic and electrochemical properties and is widely used in molecular sieves and battery electrodes. MnO2 is composed of tightly packed octahedral [MnO6] units, and different stacking methods form different forms of MnO2 crystals.

[0004] The functional groups of adsorbents are crucial to their adsorption performance. Polyethyleneimine (PEI) contains a large number of amine / imine groups, and its introduction into chitosan / manganese dioxide composite foam can significantly improve its adsorption performance for heavy metal ions. In addition, incorporating PEI into foam helps to construct a 3D porous network structure.

[0005] However, the prior art does not disclose a composite foam with good adsorption effect on uranium. Based on this, a polyethyleneimine modified chitosan / nano α-MnO2 composite foam and a preparation method and application thereof are proposed. Summary of the invention

[0006] The technical problem to be solved by the present invention is to provide a polyethyleneimine modified chitosan / nano α-MnO2 composite foam and its preparation method and application in view of the deficiencies of the above-mentioned prior art. The polyethyleneimine modified chitosan / nano α-MnO2 composite foam prepared by the present invention has low density and high elasticity, and is easy to be separated from water bodies, and has good application prospects in the fields of radioactive wastewater treatment.

[0007] In order to solve the above technical problems, the technical solution adopted by the present invention is: a method for preparing polyethyleneimine modified chitosan / nano α-MnO2 composite foam, the preparation method comprising the following steps:

[0008] S1, Synthesis of α-MnO2 nanorods;

[0009] S2, dissolving chitosan in acetic acid and stirring evenly to obtain a chitosan solution; dissolving polyethyleneimine in deionized water and stirring evenly to obtain a polyethyleneimine solution; mixing the chitosan solution and the polyethyleneimine solution evenly, then adding the α-MnO2 nanorods obtained in S1 and stirring, then adding epichlorohydrin, performing a pre-crosslinking reaction at a certain temperature, cooling to room temperature after the reaction, and obtaining a pre-crosslinked composite gel;

[0010] S3. Freeze the pre-crosslinked composite gel obtained in S2, and then freeze-dry it to obtain a primary product; wash the primary product with deionized water a and ethanol in turn, and then vacuum dry it to obtain a polyethyleneimine-modified chitosan / nano-α-MnO2 composite foam, and the polyethyleneimine-modified chitosan / nano-α-MnO2 composite foam is named PCM foam.

[0011] Preferably, the synthesis method of the α-MnO2 nanorods described in S1 is: dissolving KMnO4 in deionized water b and mixing evenly, then adding MnSO4·H2O and stirring to obtain a mixed solution; subjecting the mixed solution to a hydrothermal reaction, cooling to room temperature after the reaction to obtain a mixed product, first centrifuging the mixed product, then washing with deionized water c and ethanol in turn, and finally vacuum drying to obtain α-MnO2 nanorods.

[0012] Preferably, the mass volume ratio of the KMnO4 and the deionized water b is 0.59 g:30 mL, the molar ratio of the KMnO4 and the MnSO4·H2O is 1:1.5, and the stirring time is 1 h; the temperature of the hydrothermal reaction is 140°C and the time is 12 h, the temperature of the vacuum drying is 70°C and the time is 12 h; the speed of the centrifugation is 5000 r / s and the time is 5 min; the washing times of the deionized water c and the ethanol are both 3 times; the temperature of the vacuum drying is 70°C and the time is 12 h.

[0013] Preferably, the mass volume ratio of chitosan and acetic acid in S2 is 1g:50mL, the mass fraction of the polyethyleneimine solution is 4%, and the volume ratio of the chitosan solution to the polyethyleneimine solution is 3:1; the mass ratio of the α-MnO2 nanorods and the chitosan is (0.5-2):1; the time for adding the α-MnO2 nanorods and stirring is 4h; the amount of epichlorohydrin used is 2mL; the temperature of the pre-cross-linking reaction is 60°C and the time is 1h.

[0014] Preferably, the freezing temperature in S3 is -20°C and the time is 3 hours; the freeze-drying pressure is 1Pa and the time is 48 hours; the washing times of the deionized water a and the ethanol are both 3 times; the vacuum drying temperature is 120°C and the time is 4 hours.

[0015] The present invention also provides a polyethyleneimine modified chitosan / nano α-MnO2 composite foam, which is prepared by the above-mentioned polyethyleneimine modified chitosan / nano α-MnO2 composite foam preparation method.

[0016] The invention also provides an application of polyethyleneimine modified chitosan / nano α-MnO2 composite foam in adsorbing uranium in aqueous solution.

[0017] Preferably, when the polyethyleneimine modified chitosan / nano α-MnO2 composite foam adsorbs uranium in an aqueous solution, the pH of the aqueous solution is first adjusted to 4.5.

[0018] Compared with the prior art, the present invention has the following advantages:

[0019] 1. The present invention utilizes PEI and α-MnO2 nanorods and chitosan to prepare ultra-light, highly elastic honeycomb PCM foam by freeze drying, which has the advantages of simple preparation, low cost and high efficiency in removing uranium (U(VI)).

[0020] 2. The cross-linked network and α-MnO2 nanorods formed by the PCM-40 foam prepared by the present invention improve the mechanical strength and reusability. After 5 adsorption-desorption cycles, the U(VI) adsorption capacity loss is less than 13%. The treatment results of the actual acid leaching solution of uranium ore show that PCM-40 has good adsorption performance for U(VI) and can selectively adsorb U(VI) from high-concentration coexisting metal ions.

[0021] 3. The polyethyleneimine modified chitosan / nano α-MnO2 (PCM foam) composite foam prepared in the present invention has its main components PEI and CS cross-linked by epichlorohydrin. PCM foam has a developed honeycomb pore structure and a large number of amine groups, which can significantly improve its adsorption rate and adsorption capacity for U(VI), while enhancing the mechanical strength and elasticity of the composite foam.

[0022] The present invention is further described in detail below with reference to the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 The invention discloses a preparation process of the polyethyleneimine modified chitosan / nano alpha-MnO2 composite foam.

[0024] Figure 2 FTIR spectra of (a) CS-PEI foam; (b) PCM-20; (c) PCM-40; (d) PCM-60; and (e) α-MnO2 nanorods.

[0025] Figure 3 is a SEM image of (a) CS foam; (b) CS-PEI foam; (c) PCM-20; (d) PCM-40; (e) PCM-60; and (f) α-MnO2 nanorods.

[0026] Figure 4 are the compressive stress-strain curves of CS-PEI foam, PCM-20, PCM-40, and PCM-60.

[0027] Figure 5 are optical images of (a) CS-PEI foam, PCM-20, PCM-40 and PCM-60; (b) magnified SEM image of PCM-40; (c) optical image of PCM-40; (d) bendable PCM-40; and (eg) photographs of PCM-40 under compression.

[0028] Figure 6 are X-ray diffraction patterns of (a) CS-PEI foam; (b) α-MnO2 nanorods; and (c) PCM-40.

[0029] Figure 7 It is a comparison chart of U(VI) adsorption performance of CS foam, CS-PEI foam, PCM-20, PCM-40, PCM-60 and α-MnO2 nanorods.

[0030] Figure 8 The effect of pH on the adsorption of U(VI) on CS-PEI foam and PCM-40.

[0031] Figure 9 is (a) U(VI) species at different pH (Visual MINTEQ 3.1 software); (b) Zeta potential curves of CS-PEI foam and PCM-40; (c) pH variation of U(VI) adsorption on CS-PEI foam and PCM-40.

[0032] Fig.10(a) is the SEM image of the surface of PCM-40 after adsorption of U(VI); (b)-(f) are the elemental mapping images of the surface of PCM-40 after adsorption of U(VI).

[0033] Fig.11 is the EDS spectrum of U(VI) loaded PCM-40.

[0034] Fig.12 FTIR spectra of PCM-40 and U(VI)-loaded PCM-40.

[0035] Fig.13 XPS spectra of (a) N1s and (b) O1s of PCM-40 before and after U(VI) adsorption; and (c) XPS spectra of U4f5 / 2 and U 4f7 / 2.

[0036] Fig.14 is the adsorption capacity of PCM-40 for 5 consecutive adsorption / desorption cycles.

[0037] Fig.15 (a) Removal efficiency diagram and (b) distribution ratio diagram of various metal ions. DETAILED DESCRIPTION

[0038] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0039] The reagents used in the following examples and experiments are shown in Table 1.

[0040] Chitosan (CS, molecular weight 1.095×10 6 , deacetylation degree 75-85%), polyethyleneimine (PEI, molecular weight 1200-20000), potassium permanganate (KMnO4), manganese sulfate·H2O (MnSO4·H2O) and other analytical grade reagents were purchased from China National Pharmaceutical Group Shanghai Chemical Reagent Co., Ltd. U(VI) solution (1000 mg / L) was prepared by dissolving UO2(NO3)2·6H2O (AR) in deionized water.

[0041] Table 1 Experimental reagents

[0042]

[0043]

[0044] Example 1

[0045] This embodiment provides a method for preparing a polyethyleneimine modified chitosan / nano α-MnO2 composite foam, the preparation method comprising the following steps:

[0046] S1. Synthesis of α-MnO2 nanorods: 0.59 g KMnO4 was dissolved in 30 mL deionized water b and mixed evenly, and then MnSO4·H2O was added and stirred for 1 h to obtain a mixed solution; the mixed solution was transferred to a polytetrafluoroethylene-lined autoclave, and a hydrothermal reaction was carried out at a temperature of 140° C. for 12 h, and then cooled to room temperature (25° C.) after the reaction to obtain a mixed product, and the mixed product was first centrifuged at a speed of 5000 r / s for 5 min, and then washed with deionized water c and ethanol for 3 times respectively to remove impurities, and finally vacuum dried at a temperature of 70° C. for 12 h to obtain α-MnO2 nanorods;

[0047] The molar ratio of the KMnO4 to the MnSO4·H2O is 1:1.5;

[0048] S2, dissolving 1g of chitosan (CS) in 50mL of acetic acid and stirring evenly to obtain a chitosan solution; dissolving polyethyleneimine (PEI) in deionized water and stirring evenly to obtain a polyethyleneimine solution with a mass fraction of 4%; mixing the chitosan solution and the polyethyleneimine solution at a volume ratio of 3:1, then adding the α-MnO2 nanorods obtained in S1 and stirring for 4h, then adding 2mL of epichlorohydrin (ECH), performing a pre-crosslinking reaction at a temperature of 60°C for 1h, cooling to room temperature (25°C) after the reaction, and obtaining a pre-crosslinked composite gel;

[0049] The mass ratio of the α-MnO2 nanorods to the chitosan is 0.5:1; epichlorohydrin is used as a catalyst;

[0050] S3, the pre-crosslinked composite gel obtained in S2 is first frozen at a temperature of -20°C for 3 hours, and then freeze-dried at a pressure of 1Pa for 48 hours using an LGJ-10 freeze dryer, and further cross-linked during the drying process to obtain a primary product; the primary product is washed with deionized water a and ethanol for 3 times respectively, and then vacuum dried at a temperature of 120°C for 4 hours to obtain a polyethyleneimine modified chitosan / nano α-MnO2 (PCM) composite foam;

[0051] The polyethyleneimine modified chitosan / nano α-MnO2 composite foam obtained in this example is recorded as PCM-20.

[0052] Example 2

[0053] This embodiment provides a method for preparing a polyethyleneimine modified chitosan / nano α-MnO2 composite foam, the preparation method comprising the following steps:

[0054] S1. Synthesis of α-MnO2 nanorods: 0.59 g KMnO4 was dissolved in 30 mL deionized water b and mixed evenly, and then MnSO4·H2O was added and stirred for 1 h to obtain a mixed solution; the mixed solution was transferred to a polytetrafluoroethylene-lined autoclave, and a hydrothermal reaction was carried out at a temperature of 140°C for 12 h, and then cooled to room temperature (25°C) after the reaction to obtain a mixed product, and the mixed product was first centrifuged at a speed of 5000 r / s for 5 min, and then washed with deionized water c and ethanol for 3 times respectively to remove impurities, and finally vacuum dried at a temperature of 70°C for 12 h to obtain α-MnO2 nanorods; the molar ratio of the KMnO4 to the MnSO4·H2O was 1:1.5;

[0055] S2, dissolving 1g chitosan (CS) in 50mL acetic acid and stirring evenly to obtain a chitosan solution; dissolving polyethyleneimine (PEI) in deionized water and stirring evenly to obtain a polyethyleneimine solution with a mass fraction of 4%; mixing the chitosan solution and the polyethyleneimine solution at a volume ratio of 3:1, then adding the α-MnO2 nanorods obtained in S1 and stirring for 4h, then adding 2mL epichlorohydrin (ECH), performing a pre-crosslinking reaction at a temperature of 60°C for 1h, cooling to room temperature (25°C) after the reaction, and obtaining a pre-crosslinked composite gel; the mass ratio of the α-MnO2 nanorods to the chitosan is 1:1; epichlorohydrin is used as a catalyst;

[0056] S3, the pre-crosslinked composite gel obtained in S2 is first frozen at a temperature of -20°C for 3 hours, and then freeze-dried at a pressure of 1Pa for 48 hours using an LGJ-10 freeze dryer, and further cross-linked during the drying process to obtain a primary product; the primary product is washed with deionized water a and ethanol for 3 times respectively, and then vacuum dried at a temperature of 120°C for 4 hours to obtain a polyethyleneimine modified chitosan / nano α-MnO2 (PCM) composite foam;

[0057] The polyethyleneimine modified chitosan / nano α-MnO2 composite foam obtained in this example is recorded as PCM-40.

[0058] Example 3

[0059] This embodiment provides a method for preparing a polyethyleneimine modified chitosan / nano α-MnO2 composite foam, the preparation method comprising the following steps:

[0060] S1. Synthesis of α-MnO2 nanorods: 0.59 g KMnO4 was dissolved in 30 mL deionized water b and mixed evenly, and then MnSO4·H2O was added and stirred for 1 h to obtain a mixed solution; the mixed solution was transferred to a polytetrafluoroethylene-lined autoclave, and a hydrothermal reaction was carried out at a temperature of 140°C for 12 h, and then cooled to room temperature (25°C) after the reaction to obtain a mixed product, and the mixed product was first centrifuged at a speed of 5000 r / s for 5 min, and then washed with deionized water c and ethanol for 3 times respectively to remove impurities, and finally vacuum dried at a temperature of 70°C for 12 h to obtain α-MnO2 nanorods; the molar ratio of the KMnO4 to the MnSO4·H2O was 1:1.5;

[0061] S2, dissolving 1g chitosan (CS) in 50mL acetic acid and stirring evenly to obtain a chitosan solution; dissolving polyethyleneimine (PEI) in deionized water and stirring evenly to obtain a polyethyleneimine solution with a mass fraction of 4%; mixing the chitosan solution and the polyethyleneimine solution at a volume ratio of 3:1, then adding the α-MnO2 nanorods obtained in S1 and stirring for 4h, then adding 2mL epichlorohydrin (ECH), performing a pre-crosslinking reaction at a temperature of 60°C for 1h, cooling to room temperature (25°C) after the reaction, and obtaining a pre-crosslinked composite gel; the mass ratio of the α-MnO2 nanorods to the chitosan is 2:1; epichlorohydrin is used as a catalyst;

[0062] S3, the pre-crosslinked composite gel obtained in S2 is first frozen at a temperature of -20°C for 3 hours, and then freeze-dried at a pressure of 1Pa for 48 hours using an LGJ-10 freeze dryer, and further cross-linked during the drying process to obtain a primary product; the primary product is washed with deionized water a and ethanol for 3 times respectively, and then vacuum dried at a temperature of 120°C for 4 hours to obtain a polyethyleneimine modified chitosan / nano α-MnO2 (PCM) composite foam;

[0063] The polyethyleneimine modified chitosan / nano α-MnO2 composite foam obtained in this example is recorded as PCM-60.

[0064] Comparative Example 1

[0065] This comparative example provides a method for preparing CS-PEI foam, which comprises the following steps:

[0066] S1. Dissolve 1 g of chitosan (CS) in 50 mL of acetic acid and stir evenly to obtain a chitosan solution; dissolve polyethyleneimine (PEI) in deionized water and stir evenly to obtain a polyethyleneimine solution with a mass fraction of 4%; mix the chitosan solution and the polyethyleneimine solution at a volume ratio of 3:1, then add 2 mL of epichlorohydrin (ECH), perform a pre-crosslinking reaction at a temperature of 60°C for 1 hour, and cool to room temperature (25°C) after the reaction to obtain a pre-crosslinked composite gel;

[0067] S2. The pre-crosslinked composite gel obtained in S2 is first frozen at a temperature of -20°C for 3 hours, and then freeze-dried using an LGJ-10 freeze dryer at a pressure of 1 Pa for 48 hours, and further cross-linking is completed during the drying process to obtain a primary product; the primary product is washed with deionized water a and ethanol for 3 times respectively, and then vacuum dried at a temperature of 120°C for 4 hours to obtain CS-PEI foam.

[0068] Comparative Example 2

[0069] This comparative example provides a method for preparing CS foam. Compared with the method for preparing CS-PEI foam in comparative example 1, the difference between the method is that no polyethyleneimine solution is added in comparative example S1, and the remaining steps are the same as those in the preparation method in comparative example 1.

[0070] Experiment 1: Characterization of PCM foam

[0071] Figure 2 FTIR images of PCM-20, PCM-40 and PCM-60 prepared in Examples 1-3, CS-PEI foam and α-MnO2 nanorods prepared in Comparative Example 1, FTIR of CS-PEI foam ( Figure 2 (a) 3310cm -1 bands, indicating the joint contribution of -NH, -NH2 and -OH groups. Other characteristic bands include: 2930cm -1 and 2877cm -1 (CH asymmetric and symmetric stretching vibrations); 1640cm -1 (amide I band stretching vibration) and 1545 cm -1 (NH bending vibration). 892cm -1 (CN asymmetric stretching vibration) indicates the cross-linking reaction between ECH and the amino groups of PEI and CS. In addition, 1313 cm -1 and 1248cm -1 Related to -CO and -CN groups, respectively; 1408cm -1 is the CH bending vibration, and 1147cm -1 、1023cm-1 and 654cm -1 The peak at is related to the polysaccharide structure. FTIR of α-MnO2 nanorods ( Figure 2 (e) 708cm -1 and 594cm -1 The FTIR of PCM-20, PCM-40 and PCM-60 prepared in Examples 1-3 ( Figure 2 (b)-(d) all contain the characteristic peaks of CS-PEI foam and α-MnO2 nanorods. Compared with CS-PEI foam, the peak intensities of -NH2 and -OH groups have changed, and the peak at 3310 cm -1 The slight shift in the center indicated that there was interaction between α-MnO2 nanorods and the polymer matrix (CS and PEI).

[0072] The SEM images of the CS foam prepared in Comparative Example 2, the CS-PEI foam prepared in Comparative Example 1, the PCM-20, PCM-40 and PCM-60 prepared in Examples 1-3, and the α-MnO2 nanorods are shown in FIG. Figure 3(a)-Figure 3(f) . The CS foam prepared in Comparative Example 2 (shown in Figure 3 (a)) has a layered structure with an interlayer spacing of 15 to 70 μm and no 3D porous structure. The CS-PEI foam (shown in Figure 3 (b)) has a honeycomb macroporous structure, which can indicate that when PEI is cross-linked on the surface of α-MnO2 nanorods, it helps to form a 3D honeycomb pore structure. Figures 3 (c) to (e) show that PCM-20, PCM-40 and PCM-60 prepared in Examples 1-3, respectively, all have a developed honeycomb structure, which is conducive to the rapid penetration of U(VI) into the pores and the combination with the functional groups of the PCM foam. As the content of α-MnO2 nanorods increases, the foam surface is significantly roughened and the pore structure is more developed. An appropriate amount of α-MnO2 nanorods helps to improve the porous structure of the PCM foam. When the mass ratio of α-MnO2 nanorods to chitosan is 1:1, PCM-40 has a developed and uniform honeycomb structure, which can significantly enhance the adsorption of U(VI). However, PCM-60 with a higher content of α-MnO2 nanorods exhibits an irregular porous structure and agglomerations appear on the surface (as shown in Figure 3(e)).

[0073] The physical properties of the CS foam prepared in Comparative Example 2, the CS-PEI foam prepared in Comparative Example 1, and the PCM-20, PCM-40, and PCM-60 prepared in Examples 1-3 (Table 2) show that the CS-PEI foam and the PCM-20, PCM-40, and PCM-60 prepared in Examples 1-3 have high porosity (86.08-90.92%), which is consistent with the SEM observation results. As the content of α-MnO2 nanorods in the PCM foam increases, the specific surface area (SBET ), porosity (ρ) and pore volume (V ρ ) also increased (except PCM-60), but the polymer content decreased, so the swelling degree decreased.

[0074] Table 2 Structural characteristic parameters and elastic modulus (E) of synthetic foam

[0075]

[0076]

[0077] *Porosity,ε=V P ×ρ; where V P is the pore volume and ρ is the apparent density.

[0078] High mechanical strength and good flexibility are crucial for the practical application of foams. Figure 4 The compressive stress-strain curves of different foams are shown. Since the porous structure in the foam gradually collapses during the compression process, the curve shows a slow growth trend. The results confirm that the addition of α-MnO2 nanorods can enhance the compressive strength of the foam. When the strain is 60%, the porous skeleton of PCM-40 and PCM-60 foams is completely damaged, and the stress increases sharply. When the strain is 70%, the order of compressive stress is: PCM-60 (0.185MPa)>PCM-40 (0.095MPa)>PCM-20 (0.023MPa)>CS-PEI foam (0.009MPa), indicating that the compressive stress increases with the increase of α-MnO2 nanorod content, and the elastic modulus values ​​in Table 2 also follow the above rule.

[0079] Optical photographs of CS-PEI foam and PCM-20, PCM-40 and PCM-60 prepared in Examples 1-3, respectively ( Figure 5 (a) shows that PCM foams have different colors and a rough surface. High magnification SEM of PCM-40 ( Figure 5 (b)) shows that α-MnO2 nanorods are uniformly distributed in the polymer matrix along the 3D honeycomb pattern, and the size of the nanorods is 2 μm. Figure 5 (c)-(g) show that PCM-40 is ultralight, bendable and remarkably elastic, and can recover its original shape after the compressive stress is removed. In addition, PCM foam has a strong ability to absorb water during swelling. For example, the amount of water adsorbed reaches 29.34 gH2O / g PCM-40, and the amount of water adsorbed by PCM-40 remains stable after 5 repeated squeezing and expansion, indicating that repeated swelling does not destroy the microstructure.

[0080] XRD patterns of CS-PEI foam, α-MnO2 nanorods and PCM-40 Figure 6As shown. CS-PEI foam presents a semi-crystalline structure with characteristic peaks at 2θ=13.2°, 30.5° and 42.1°, while α-MnO2 nanorods have a typical crystalline structure with characteristic peaks at 2θ=12.5°, 17.7°, 28.5°, 34.4°, 34.4°, 41.8°, 49.8°, 56.2°, 60.1° and 69.2°, corresponding to the crystal planes of (110), (200), (310), (211), (301), (411), (600), (521), (002) and (741), respectively. The characteristic peaks of PCM-40 include those of CS-PEI foam and α-MnO2 nanorods, but the peak intensity of α-MnO2 nanorods is significantly reduced due to the presence of more than 50wt% of semi-crystalline polymer in PCM-40 foam.

[0081] Example 4

[0082] Application of polyethyleneimine modified chitosan / nano α-MnO2 composite foam in adsorbing uranium in aqueous solution.

[0083] Experiment 2: Uranium (U(VI)) adsorption experiment

[0084] 10.0 mg of each of the CS foam prepared in Comparative Example 2, the CS-PEI foam prepared in Comparative Example 1, the PCM-20, PCM-40 and PCM-60 prepared in Examples 1-3, and α-MnO2 nanorods were mixed with 50 mL of U(VI) solution and adsorbed by oscillation for 2 h at a set initial concentration, temperature and pH (C0 = 100 mg / L; T = 298 K; pH = 4.5). The initial pH of the solution was adjusted by adding a small amount of 0.01-1.0 mol NaOH or HCl. After sampling and filtering through a membrane filter, the U(VI) concentration was determined using azoarsenazo III spectrophotometry (UV-5100 ultraviolet-visible spectrophotometer, 650 nm).

[0085] The adsorption performance of CS foam, CS-PEI foam, PCM-20, PCM-40 and PCM-60 prepared in Examples 1-3 and α-MnO2 nanorods on U(VI) is shown in Figure 2. Figure 7As shown in the figure, the order of adsorption capacity is: PCM-40>PCM-20>PCM-60>CS-PEI foam>CS foam>α-MnO2 nanorods, indicating that the mixing of an appropriate amount of α-MnO2 nanorods helps to form an ultra-thin polymer layer on the surface of CS-PEI, so that the active sites are fully exposed to bind uranyl ions. PCM-40 has the best adsorption performance for U(VI) due to its developed macroporous structure and rich functional groups. However, the excessive α-MnO2 nanorods in PCM-60 may hinder the effective contact between U(VI) and the active sites, resulting in reduced adsorption performance. In addition, the PEI content in PCM foam was optimized. The results showed that when the mass ratio of CS:α-MnO2 nanorods was 1:1 and the PEI content increased from 2wt% to 10wt%, the U(VI) adsorption capacity increased from 215.6mg / g to 285.2mg / g, but further increase in PEI content may lead to phase separation (difficult to form uniform foam). Among all foams, PCM-40 has the best adsorption effect on U(VI), so subsequent experiments focused on the adsorption performance of PCM-40.

[0086] Experiment 3: Effect of pH on U(VI) adsorption

[0087] pH affects the morphology of metal ions, functional group protonation / deprotonation and affinity of the adsorbent for metal ions. 10.0 mg of each of the CS-PEI foam prepared in Comparative Example 2 and the PCM-40 prepared in Example 2 were mixed with 50 mL of U(VI) solution and adsorbed under oscillation for 2 h at an initial concentration of C0 = 100 mg / L, pH 1-6, and temperature T = 298 K.

[0088] Effect of pH on U(VI) adsorption Figure 8 As shown in Figure 2. When pH < 4.0, the adsorption of U(VI) by different CS-PEI foams and PCM-40 increases with increasing pH and is stable at higher pH. When pH > 6.0, U(VI) tends to precipitate. PZC 5.2 and 5.5, respectively (Figure 9(a)), indicating that when the pH is lower than 5.2-5.5, the adsorbent surface carries a cationic charge (protonation of the amine group). At low pH, the charge repulsion of the protonated functional groups on U(VI) and the large amount of H +The strong competitive adsorption of ions leads to the weakening of the adsorption performance of U(VI). As the pH increases, the functional groups are gradually deprotonated, and the charge repulsion of cations is weakened, thereby promoting the coordination and complexation of N- and O-donor functional groups with uranyl ions. The Zeta potential curves of CS-PEI foam and PCM-40 at different pH values ​​are shown in Figure 9(b). When pH < 7.0, U(VI) mainly exists in the form of cations, and uranyl ions are aggregated and combined with amine / imine groups. As the pH value increases, the functional groups are deprotonated and aggregated structures are formed, which gradually increases the adsorption amount of U(VI).

[0089] Figure 9(c) shows the pH change curves of PCM-40 and CS-PEI foams during the adsorption of U(VI). When the initial pH was 1.5-3.0, the pH increased by 0.3-0.5 after adsorption. It was stable at pH 4.0-5.0 (the adsorbent had a buffering effect), and increased slightly at pH 5.5. The increase in pH was due to the presence of H + Bind (amino protonation) or release OH- ions.

[0090] Experiment 4: Adsorption selectivity experiment

[0091] The adsorption solution is a mixed solution containing 10 mg / L of metal ions (including U(VI), Th(IV), Eu(III), Fe(III), Al(III), Co(II), Pb(II), Ni(II) and Cu(II)). The actual uranium-containing wastewater may contain the above selected metal ions. The metal ion concentration was analyzed by inductively coupled plasma atomic emission spectrometry (ICP-OES, Nippon Jarrell Ash).

[0092] Actual radioactive wastewater contains metal ions that coexist with U(VI). The adsorption selectivity results (Table 3) show that PCM-40 has a higher distribution ratio (D) value for U(VI) and S U / M The value is between 2.9 and 6.7, indicating that PCM-40 has good adsorption selectivity and strong affinity for U(VI). According to the hard-soft acid-base (HSAB) theory: hard acids tend to react or bind with hard bases. Amine / imine groups are hard bases, while U(VI) is a hard acid (softness is -0.27), so amine / imine groups preferentially bind to U(VI). U / M The low value (or high affinity) is attributed to the Jahn–Teller effect of the Cu(II) complex. In addition, the high binding energy of the actinide 5f / 6d orbitals also contributes to the coordination of U(VI) with the PCM-40 functional groups.

[0093] Table 3 Selective adsorption of U(V) by PCM-40 and CS-PEI adsorbents from multicomponent solutions

[0094]

[0095] The performance comparison results of different adsorbents are shown in Table 4. The maximum adsorption capacity of PCM-40 was determined by the adsorption isotherm. PCM-40 has a higher maximum adsorption capacity (q m ) and fast adsorption kinetics. Among different adsorbents, PCM-40 is very competitive due to its fast adsorption, easy preparation and environmental friendliness, and is expected to be used in radioactive wastewater treatment. The inventors also conducted adsorption kinetics, adsorption isotherms and adsorption thermodynamics experiments, but due to space constraints, they are not described in detail.

[0096] Table 4 Comparison of adsorption performance of various chitosan-based adsorbents

[0097]

[0098]

[0099] Adsorption mechanism:

[0100] Fig.10 (a) is the SEM image of PCM-40 after adsorption of U(VI), (b)-(f) are the elemental mapping images of the surface of PCM-40 after adsorption of U(VI), Fig.11 This is the EDS spectrum of U(VI)-loaded PCM-40, indicating that U(VI) is uniformly distributed on the surface of PCM-40 after adsorption. There are other elements on the surface of PCM-40, such as Mn, C, N and O. Among them, Mn and O elements are uniformly distributed and the images are similar, indicating that α-MnO2 nanorods are uniformly embedded in PCM-40. U and N elements are arranged along the porous skeleton composed of polymer (PEI and CS) and α-MnO2 nanorods, indicating that both the polymer matrix and α-MnO2 nanorods contribute to the adsorption of U(VI). EDS shows that U(VI) can be effectively adsorbed, and U(VI) peaks appear at different positions, and the U(VI) content is about 22.5wt%.

[0101] FTIR spectra of PCM-40 before and after U(VI) adsorption Fig.12 After adsorption of U(VI), 3310cm -1 and 1313cm -1 The peak intensity at 1640cm -1 and 1545cm -1 The peaks at 1614 cm -1 and 1513cm -1 . At 880cm -1 A new peak appears at , which is related to the symmetric stretching vibration of uranyl ions. The above results indicate that both -OH and -NH groups participate in the adsorption of U(VI).

[0102] PCM-40 N1s( Fig.13 (a)) at 397.69eV (CN), 398.51eV (-NH / -NH2), 399.96eV (NH3 + ) presents three main peaks. After U(VI) binding, the peaks at 397.69eV, 398.51eV and 399.96eV move to 397.91eV, 398.68eV and 399.76eV respectively, and the corresponding peak intensities (or peak areas) also change. In addition, a strong peak of UN bond appears at 400.42eV. O1s spectrum ( Fig.13 (b)) Two peaks appear at 532.28eV (C-OH) and 529.87eV (Mn-O). After U(VI) binding, the peak intensity of C-OH decreases and the peak moves slightly, indicating that C-OH participates in the complexation of U(VI). And a new peak appears at 532.38eV, accounting for 12% of the total peak area, which may be related to the formation of -OU bond. XPS of U( Fig.13 (c)) shows two peaks at 392.73 eV and 381.86 eV, which are respectively attributed to U 4f 5 / 2 and U 4f 7 / 2 , indicating that the adsorbed U(VI) is oxidized uranium.

[0103] In addition to N and O complexation, U(VI) adsorption can also be carried out through an ion exchange mechanism. At low pH values, PCM functional groups are protonated, thus binding U(VI) by ion exchange. At pH 5.0, the functional groups are highly deprotonated, thus adsorbing uranium by coordination or chemical adsorption. Based on the FTIR and XPS results, strong complexation between U(VI) and functional groups with different coordination structures was observed. Uranyl ions were coordinated with two amine nitrogen donors and two hydroxyl oxygen donors to form a tetradentate complex, and the coordination of U(VI) with the functional groups of PCM-40 was a monolayer chemical adsorption. The E value calculated by the DR model was greater than 8 kJ / mol, indicating that the adsorption of U(VI) by PCM was chemical adsorption. In addition, the honeycomb structure of PCM-40 provides a large specific surface area, and the α-MnO2 nanorods fully expose the surface polymer matrix, which is conducive to the binding of uranyl ions. At the same time, the honeycomb structure of PCM-40 allows U(VI) to quickly penetrate into the cross-linked network to enhance the adsorption of U(VI). In fact, the adsorption mechanism is very complex and more research work is needed.

[0104] Experiment 5: Reusability of PCM-40

[0105] PCM-40 after adsorption of U(VI) was regenerated by 0.5M NaHCO3 or 0.3M HNO3, stirred at 150rpm for 2h, filtered and separated, washed with deionized water, dried under vacuum at 65℃, and reused. PCM-40 after adsorption of U(VI) can be effectively desorbed by 0.50M NaHCO3 solution: 92% can be desorbed in 60min contact time, indicating that UO2 2+ With CO3 - (i.e. UO2(CO3)2 2- and UO2(CO3)3 4- Forming a soluble anion complex. In fact, PCM-40 loaded with U(VI) can also be desorbed by 0.35M HNO3 (desorption rate>90%). However, since HNO3 is highly corrosive, NaHCO3 was used for desorption. Fig.14 It was shown that the adsorption capacity gradually decreased during 5 adsorption-desorption cycles; however, the final adsorption capacity loss was less than 12%, indicating the reusability of PCM-40 foam.

[0106] Experiment 6: Treatment of actual acid leaching solution of uranium ore

[0107] Uranium ore acid leaching solution (ALS) was obtained from a uranium industry company in Fuzhou. The pH of ALS was adjusted to 4.5 (optimal pH for U(VI)) by NaOH, and excess metal ions (such as Al(III) and Fe l(III)) were precipitated. Other experimental conditions were the same as those for static adsorption (pH = 4.5; PCM-40 addition (SD) = 0.20 g / L; t = 120 min; T = 298 K), and the concentration of each metal ion in the solution was measured by ICP.

[0108] PCM foam was used to treat actual uranium-containing acid leaching solution (ALS) for adsorption. The initial pH of ALS was 2.1, the U(VI) concentration was 0.96 g / L, and it contained other high-concentration ions, such as Si(IV) 81.30 g / L, Al(III) 36.41 g / L, Fe(III) 3.75 g / L, and Ti(IV) 5.65 g / L, as well as some alkaline earth metal ions (Table 5). To improve the adsorption effect of U(VI), the pH of ALS was first adjusted to 4.5. The concentrations of metal ions (such as Si(IV), Al(III), Fe(III), Mg(II), and Mn(II)) were reduced by 25-80% due to precipitation, but the residual concentration was still high. During the precipitation process, the loss of U(VI) was less than 10%. Fig.15 Displays the removal efficiency and distribution ratio of various metal ions. Fig.15(a) shows that compared with other metal ions, the removal efficiency of U(VI) is higher, reaching 98.2%, indicating that PCM-40 has good adsorption performance for U(VI). Although some metal ions have strong competitive adsorption with U(VI) and occupy the active sites of the adsorbent, the total adsorption capacity of metal ions is as high as 5.89mmol / g, which is much larger than the adsorption capacity of U(VI) (1.23mmol / g) in a single-component solution. The removal efficiency is affected by the initial metal concentration in the ALS, so the distribution coefficient (K d ) further highlights the affinity of the adsorbent for U(VI). Fig.15 (b) Displays the K of U(VI) d The value (278 L / g) was much higher than that of Fe(III) (36 L / g) and other metal ions (less than 11 L / g), indicating that PCM-40 can selectively adsorb U(VI) from high concentrations of coexisting metal ions.

[0109] Table 5 Composition of metal ions in acidic leachate at pH 2.1 and pH 4.5

[0110]

[0111] The above is only a preferred embodiment of the present invention and does not limit the present invention in any way. Any simple modification, change and equivalent change made to the above embodiment according to the technical essence of the invention still falls within the protection scope of the technical solution of the present invention.

Claims

1. A method for preparing polyethyleneimine modified chitosan / nano α-MnO2 composite foam, characterized in that: The preparation method comprises the following steps: S1, Synthesis of α-MnO2 nanorods; S2, dissolving chitosan in acetic acid and stirring evenly to obtain a chitosan solution; dissolving polyethyleneimine in deionized water and stirring evenly to obtain a polyethyleneimine solution; mixing the chitosan solution and the polyethyleneimine solution evenly, then adding the α-MnO2 nanorods obtained in S1 and stirring, then adding epichlorohydrin, performing a pre-crosslinking reaction at a certain temperature, cooling to room temperature after the reaction, and obtaining a pre-crosslinked composite gel; S3, freezing the pre-crosslinked composite gel obtained in S2, and then freeze-drying it to obtain a primary product; washing the primary product with deionized water a and ethanol in turn, and then vacuum drying it to obtain a polyethyleneimine-modified chitosan / nano α-MnO2 composite foam.

2. The method for preparing a polyethyleneimine modified chitosan / nano α-MnO2 composite foam according to claim 1, characterized in that: The synthesis method of α-MnO2 nanorods described in S1 is: KMnO4 is dissolved in deionized water b and mixed evenly, and then MnSO 4. H2O is stirred to obtain a mixed solution; the mixed solution is subjected to a hydrothermal reaction, and after the reaction, it is cooled to room temperature to obtain a mixed product, the mixed product is first centrifuged, then washed with deionized water c and ethanol in sequence, and finally vacuum dried to obtain α-MnO2 nanorods.

3. The method for preparing a polyethyleneimine modified chitosan / nano α-MnO2 composite foam according to claim 2, characterized in that: The mass volume ratio of the KMnO4 and the deionized water b is 0.59 g:30 mL.

4. The molar ratio of H2O is 1:1.5, and the stirring time is 1 hour; the temperature of the hydrothermal reaction is 140°C and the time is 12 hours, and the temperature of the vacuum drying is 70°C and the time is 12 hours; the speed of the centrifugation is 5000r / s and the time is 5min; the washing times of the deionized water c and the ethanol are both 3 times; the temperature of the vacuum drying is 70°C and the time is 12 hours.

4. The method for preparing a polyethyleneimine modified chitosan / nano α-MnO2 composite foam according to claim 1, characterized in that: The mass volume ratio of chitosan and acetic acid in S2 is 1g:50mL, the mass fraction of the polyethyleneimine solution is 4%, and the volume ratio of the chitosan solution to the polyethyleneimine solution is 3:1; the mass ratio of the α-MnO2 nanorods and the chitosan is (0.5-2):1; the time for adding the α-MnO2 nanorods and stirring is 4h; the amount of epichlorohydrin used is 2mL; the temperature of the pre-cross-linking reaction is 60°C and the time is 1h.

5. The method for preparing a polyethyleneimine modified chitosan / nano α-MnO2 composite foam according to claim 1, characterized in that: The freezing temperature in S3 is -20°C and the time is 3h; the freeze-drying pressure is 1Pa and the time is 48h; the washing times of the deionized water a and the ethanol are both 3 times; the vacuum drying temperature is 120°C and the time is 4h.

6. A polyethyleneimine modified chitosan / nano α-MnO2 composite foam, characterized in that: The foam is prepared by the method for preparing a polyethyleneimine-modified chitosan / nano α-MnO2 composite foam as described in any one of claims 1 to 5.

7. Use of the polyethyleneimine modified chitosan / nano α-MnO2 composite foam as claimed in claim 6 in adsorbing uranium in aqueous solution.

8. The use of a polyethyleneimine modified chitosan / nano α-MnO2 composite foam according to claim 7, characterized in that: When the polyethyleneimine modified chitosan / nano α-MnO2 composite foam adsorbs uranium in an aqueous solution, the pH of the aqueous solution is first adjusted to 4.5.

Images