Chitosan-citric acid crosslinked bamboo charcoal-based composite adsorbent, its preparation method and application

Abstract

This invention relates to a chitosan-citric acid crosslinked bamboo charcoal-based composite adsorbent, its preparation method, and its application, relating to the field of uranium-containing wastewater treatment technology. The preparation method includes: dissolving citric acid in deionized water and sonicating until completely dissolved; subsequently adding bamboo charcoal, chitosan, dipotassium hydrogen phosphate, and acetic acid solution sequentially, and stirring the mixture at room temperature; after solid-liquid separation, drying the resulting solid to obtain the composite adsorbent. This adsorbent overcomes the limitations of existing bamboo charcoal applications in uranium adsorption, such as limited adsorption capacity due to the single type of surface functional groups, significant performance degradation under extreme pH conditions, and insufficient structural stability and selectivity. This adsorbent can achieve highly efficient and selective adsorption of uranium in practical wastewater over a wide pH range.

Description

Technical Field

[0001] This invention relates to the field of uranium-containing wastewater treatment technology, and in particular to a chitosan-citric acid crosslinked bamboo charcoal-based composite adsorbent, its preparation method, and its application. Background Technology

[0002] Nuclear energy, with its high energy density and extremely low greenhouse gas emissions, is considered one of the most promising energy sources of the future. However, terrestrial uranium reserves are dwindling, making it difficult to meet the growing demand resulting from the expansion of global nuclear power capacity. At the same time, radioactive pollution generated during uranium mining, processing, and utilization poses a serious threat to the ecological environment. To address the dual challenges of uranium resource shortages and nuclear environmental remediation, developing efficient methods for treating low-concentration uranium-containing wastewater has become an urgent priority.

[0003] In the field of uranium-containing wastewater treatment, mature technologies such as ion exchange, membrane separation, and adsorption have been developed for many years. However, the practical application of these methods is largely limited by high operating costs and low removal efficiency. Studies have shown that adsorption methods perform well under acidic conditions (e.g., PA / PAO / SA gel beads achieve nearly 100% uranium removal in 20 mg / L uranium-containing solutions at pH 2.5-5.5), neutral conditions (PAA hydrogel effectively adsorbs uranium in uranium-containing water at pH 7), and alkaline conditions (SAIEF achieves highly efficient uranium removal in U(VI)-containing wastewater at pH 10.5), fully demonstrating their strong adaptability to complex water quality environments. However, due to the influence of pH on the uranyl ion speciation, maintaining the effective performance of uranium adsorbents in both acidic and alkaline environments remains a significant challenge.

[0004] Activated carbon is widely considered an ideal matrix for uranium adsorption materials due to its tunable surface properties, well-developed pore structure, and excellent mechanical stability. Bamboo-based biochar (BC), as an readily available material obtained through the pyrolysis of bamboo, is an environmentally friendly and economically viable option. As a cost-effective member of the activated carbon family, it has significant advantages in terms of economic efficiency and ecological compatibility. However, unmodified and unloaded BC suffers from low selectivity and limited adsorption capacity.

[0005] To improve its adsorption performance, existing technologies often modify the surface of BC by functionalization (such as introducing nitrogen- or oxygen-containing functional groups) to enhance its affinity for uranyl ions. However, some modification methods have limitations such as complex processes, high costs, or applicability only to specific conditions (such as seawater or a narrow pH range).

[0006] Therefore, there is an urgent need to develop a bamboo charcoal-based adsorbent that is easy to prepare, low in cost, and can efficiently and selectively remove uranium from wastewater over a wide pH range. Summary of the Invention

[0007] One of the objectives of this invention is to provide a method for preparing a chitosan-citric acid crosslinked bamboo charcoal-based composite adsorbent, in order to overcome the problems of poor selectivity, low adsorption capacity, and limited applicability in uranium-containing wastewater with a wide pH range in the prior art of bamboo-based biochar (i.e., bamboo charcoal).

[0008] To achieve the above objectives, the present invention employs the following technical solution: a method for preparing a chitosan-citric acid crosslinked bamboo charcoal-based composite adsorbent, comprising the following steps:

[0009] Citric acid (CA) is dissolved in deionized water and sonicated (e.g., sonicated for 15 minutes) until completely dissolved; then bamboo charcoal (BC), chitosan (CS), dipotassium hydrogen phosphate (DKP), and acetic acid solution are added sequentially, and the mixture is stirred at room temperature (e.g., stirred for 2 hours); after solid-liquid separation, the resulting solid is dried (e.g., dried at 80 °C for 12 hours) to obtain the composite adsorbent (ASBP).

[0010] Furthermore, the mass ratio of bamboo charcoal, chitosan, and citric acid is 1:1:1.

[0011] Furthermore, the total mass ratio of bamboo charcoal, chitosan, and citric acid to dipotassium hydrogen phosphate is 3:1. For example, when bamboo charcoal, chitosan, and citric acid are added in equal masses, the mass ratio of bamboo charcoal, chitosan, citric acid, and dipotassium hydrogen phosphate is 1:1:1:1.

[0012] Furthermore, each 0.3 g of bamboo charcoal corresponds to 0.3 g of chitosan, 0.3 g of citric acid, 0.3 g of dipotassium hydrogen phosphate, 0.3 mL of acetic acid solution, and 30 mL of deionized water. For example, the amounts are 0.3 g bamboo charcoal, 0.3 g chitosan, 0.3 g citric acid, 0.3 g dipotassium hydrogen phosphate, 0.3 mL acetic acid solution, and 30 mL deionized water.

[0013] The bamboo charcoal is made by crushing bamboo biomass (such as moso bamboo), carbonizing it at high temperature under an inert atmosphere, and then washing and drying it.

[0014] Specifically, the bamboo is dried (e.g., dried at 105°C), pulverized (e.g., cut into 3 mm square pieces and ground into powder), and then carbonized at 600°C for 3 hours in a nitrogen atmosphere (e.g., calcined at 600°C for 3 hours in a nitrogen atmosphere). The resulting product is washed with deionized water (e.g., washed three times) and dried at 80°C (e.g., dried at 80°C to constant weight) to obtain the bamboo charcoal.

[0015] Another object of the present invention is to provide a chitosan-citric acid crosslinked bamboo charcoal-based composite adsorbent, which is prepared by the above-described preparation method.

[0016] Furthermore, the aforementioned chitosan-citric acid crosslinked bamboo charcoal-based composite adsorbent can be applied to the treatment of uranium-containing wastewater. Specifically, the composite adsorbent is added to the wastewater to adsorb uranium from the wastewater.

[0017] Further, each 15-25 mg of composite adsorbent corresponds to 90 mL of wastewater. Preferably, each 15 mg of composite adsorbent corresponds to 90 mL of wastewater. For example, the composite adsorbent is 15 mg, and the wastewater is 90 mL.

[0018] The initial uranium concentration in the wastewater does not exceed 10 mg / L. For example, the initial uranium concentration in the wastewater is 10 mg / L, 5 mg / L, 1.286 mg / L, 1.0 ± 0.2 mg / L, or 1.0 ± 0.3 mg / L.

[0019] Further, the wastewater has a pH of 2-9 (e.g., pH = 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0 or 9.0, preferably pH = 6) and a temperature of 288-318 K (e.g., 288 K, 298 K, 308 K or 318 K, preferably 308 K).

[0020] Specifically, when the pH of the wastewater is 2-9, the removal rate of uranium by the composite adsorbent remains above 91%.

[0021] Furthermore, when the pH of the wastewater is 6 and the temperature is 308 K, the adsorption capacity (theoretical maximum adsorption capacity) of the composite adsorbent for uranium reaches more than 700 mg / g (e.g., 705.07 mg / g).

[0022] The composite adsorbent exhibits an adsorption capacity of uranium of 36 mg / g or higher (e.g., 36.79 mg / g) within 10 minutes of contact with wastewater.

[0023] Wherein, the uranium-containing wastewater is multi-ion radioactive wastewater (i.e., the uranium-containing wastewater also contains other metal cations, such as Al). 3+ Fe 3+ Ca 2+ Mn 2+ Pb 2+ Zn 2+ When one or more of the following are present, the partition coefficient (Kd) of the composite adsorbent for uranium reaches 3.0 × 10⁻⁶. 5 mL / g.

[0024] Furthermore, the application is carried out in a fixed-bed system. Specifically, the composite adsorbent is filled into a fixed-bed column, through which uranium-containing wastewater flows to achieve uranium adsorption and removal.

[0025] Among them, the fixed-bed process combined with the above-mentioned composite adsorbent can achieve highly efficient and selective adsorption of uranium in actual wastewater within a wide pH range (such as pH 2-9).

[0026] This invention synthesizes a bamboo charcoal-based composite adsorbent (ASBP) for the efficient removal of uranium from uranium-contaminated wastewater. This approach overcomes the limitations of bamboo charcoal (BC) in uranium adsorption applications, such as limited adsorption capacity due to the limited variety of surface functional groups, significant performance degradation under extreme pH conditions, and insufficient structural stability and selectivity. The adsorbent is prepared by crosslinking and immobilizing bamboo charcoal (BC) with chitosan (CS) and citric acid (CA), while simultaneously loading dipotassium hydrogen phosphate (DKP). Thermodynamic analysis shows that ASBP conforms to the Freundlich isotherm model, exhibiting multilayer adsorption behavior. At pH=6 and 308K, its maximum theoretical adsorption capacity reaches 705.07 mg / g, indicating that the adsorption process is spontaneous and endothermic. Kinetic analysis shows that ASBP conforms to a pseudo-second-order kinetic model, indicating that its adsorption mechanism is chemisorption. Tests showed that the adsorption efficiency of ASBP exceeded 91% over a wide pH range of 2-9. In selective adsorption experiments, the partition coefficient (Kd) of ASBP for uranium reached as high as 3.0 × 10⁻⁶. 5 The uranium concentration of ASBP is 17.8 times higher than that of interfering aluminum ions (mL / g). After six adsorption-desorption cycles, its adsorption and desorption efficiencies remained above 94.01% and 83.27%, respectively. Fixed-bed column experiments using actual wastewater (uranium mine leachate) showed that the effluent uranium concentration remained zero until 1159 bed volumes (11.013 L), with the breakthrough point occurring at 1201 bed volumes (11.418 L), and adsorption saturation equilibrium reached at 1486 bed volumes (14.126 L). Its saturated bed volume exceeds that of many previously reported wastewater uranium removal materials. Adsorption mechanism studies revealed that phosphate groups (including PO and P=O) in ASBP form complexes with uranyl ions, while hydroxyl, carboxyl, and amino groups synergistically participate in uranium adsorption, and CN bonds also participate in the process. A multifunctional synergistic system constructed by crosslinking bamboo charcoal with chitosan and citric acid and loading DKP significantly improved the uranium adsorption performance of ASBP. In summary, ASBP exhibits excellent affinity for uranium and has significant application potential in the efficient removal of uranium from real wastewater. Attached Figure Description

[0027] Figure 1 A schematic diagram of a scanning electron microscope image of bamboo charcoal (BC);

[0028] Figure 2 A schematic diagram of a scanning electron microscope image of the chitosan-citric acid crosslinked bamboo charcoal-based composite adsorbent (ASBP);

[0029] Figure 3 In the middle: (c) is a schematic diagram of the Fourier transform infrared spectra of BC and ASBP; (d) is a schematic diagram of the Raman spectrum of ASBP.

[0030] Figure 4 This is a schematic diagram of the specific surface area analysis of BC;

[0031] Figure 5 This is a schematic diagram of the specific surface area analysis of ASBP.

[0032] Figure 6 A schematic diagram of the thermogravimetric analysis curve of ASBP;

[0033] Figure 7 This is a schematic diagram of the particle size distribution of ASBP.

[0034] Figure 8 This is a schematic diagram illustrating the effect of adsorbent mass on the uranium adsorption performance of ASBP.

[0035] Figure 9 This is a schematic diagram showing the effect of initial pH on the uranium adsorption performance of ASBP and the Zeta potential analysis of ASBP.

[0036] Figure 10 A schematic diagram of the kinetic analysis of BC and ASBP;

[0037] Figure 11 A schematic diagram showing the thermodynamic properties of ASBP at 288K.

[0038] Figure 12 A schematic diagram showing the thermodynamic properties of ASBP at 298K.

[0039] Figure 13 A schematic diagram showing the thermodynamic properties of ASBP at 308K.

[0040] Figure 14 This is a schematic diagram illustrating the effect of competing metal cations on selective adsorption.

[0041] Figure 15 This is a schematic diagram of the adsorption-desorption cycle adsorption results;

[0042] Figure 16 A schematic diagram showing the adsorption results of ASBP in 90 ml of actual leachate;

[0043] Figure 17 This is a schematic diagram of a fixed bed system.

[0044] Figure 18 A schematic diagram of the penetration curve of uranium in a fixed bed column filled with ASBP;

[0045] Figure 19 A schematic diagram showing the changes in the concentrations of four competing ions in a uranium solution passing through a fixed bed column;

[0046] Figure 20 This is a schematic diagram showing the changes in the concentrations of common metal elements (Ca, Mg, Mn, and Zn) in wastewater after passing through a fixed-bed system.

[0047] Figure 21 A schematic diagram of a scanning electron microscope-energy dispersive spectroscopy (EDS) image of BC;

[0048] Figure 22 This is a schematic diagram of an ASBP scanning electron microscope-energy dispersive spectroscopy (EDS) image.

[0049] Figure 23 This is a schematic diagram of a scanning electron microscope-energy dispersive spectroscopy (EDS) image of the ASBP-U.

[0050] Figure 24 In the middle: (d)-(f) are schematic diagrams of the selected region energy spectrum analysis results corresponding to BC, ASBP and ASBP-U respectively;

[0051] Figure 25 In the middle: (g) is a schematic diagram of the scanning electron microscope image of ASBP-U, and (h) is a schematic diagram of the scanning electron microscope image of D-ASBP;

[0052] Figure 26 In the middle: (i) is a schematic diagram of the element surface distribution analysis region of ASBP-U, and (j)-(p) are schematic diagrams of the characteristic distribution spectrum of each element of ASBP-U;

[0053] Figure 27 (a) is a schematic diagram of the Fourier transform infrared spectra of ASBP, ASBP-U, and D-ASBP; (b) is a schematic diagram of the X-ray photoelectron spectra of ASBP and ASBP-U. Figure 1 ;

[0054] Figure 28 Schematic diagram of X-ray photoelectron spectra of ASBP and ASBP-U Figure 2 ;

[0055] Figure 29 Schematic diagram of X-ray photoelectron spectra of ASBP and ASBP-U Figure 3 ;

[0056] Figure 30 Schematic diagram of X-ray photoelectron spectra of ASBP and ASBP-U Figure 4 . Detailed Implementation

[0057] To facilitate understanding by those skilled in the art, the present invention will be further described below with reference to embodiments. The content mentioned in the embodiments is not intended to limit the present invention. It should be noted in advance that the following embodiments were completed in a laboratory. Those skilled in the art should understand that the amounts of each component given in the embodiments only represent the ratio between the components, and are not specific limitations.

[0058] Chitosan (CS) is biodegradable, non-toxic, and rich in amino and hydroxyl functional groups, which can promote the adsorption of heavy metals through ion exchange or chelation mechanisms. Regarding uranium adsorption, uranyl ions (UO2)... 2+ The coordination between uranium and amino groups is particularly important because uranium exists primarily as uranyl cations in wastewater. However, its limited mechanical strength restricts its practical application. Numerous studies have shown that introducing crosslinking agents can improve the mechanical properties of CS adsorbents. Traditional crosslinking strategies, such as the use of glutaraldehyde, often lead to a significant decrease in uranium adsorption capacity due to the masking or consumption of active amino groups, while loosely bound glutaraldehyde molecules may also pose potential pollution risks. Citric acid (CA), as an economical, efficient, non-toxic, and environmentally friendly tricarboxylic acid crosslinking agent, has achieved a dual breakthrough in "enhanced stability" and "improved adsorption efficiency." CA interacts with the polymer network (especially the amino groups in chitosan) through its three carboxyl groups (-COOH), forming hydrogen bonds and ionic bonds, thereby significantly enhancing the structural stability of the material and making it a key focus in the development of high-efficiency crosslinking agents. The free -COOH groups remaining after crosslinking can serve as novel coordination sites for uranyl ions, synergistically enhancing adsorption capacity with the -NH2 groups of chitosan. This multidentate coordination promotes the formation of a dense three-dimensional ionic crosslinking network, contributing to improved mechanical strength and adsorption performance of the composite material. Furthermore, phosphate modification enhances the surface properties and selective coordination ability of the material, demonstrating significant potential for efficient U(VI) capture. In-situ phosphate modification using dipotassium hydrogen phosphate (DKP) effectively overcomes the limitations of traditional phosphorylation processes, which are complex and energy-intensive. By directly loading DKP into the CA-CS network, pretreatment and strong acid activation steps are eliminated, significantly simplifying the overall process. This strategy offers important insights for practical applications in industrial wastewater treatment.

[0059] Based on the above concept, to overcome the limitations of bamboo charcoal (BC) in terms of limited selectivity, low adsorption capacity, and restricted applicability in practical wastewater treatment scenarios, this invention develops a novel, simple, and environmentally friendly composite adsorbent (ASBP) based on chitosan-citric acid crosslinked dipotassium hydrogen phosphate / bamboo charcoal for efficient removal of uranium (U) from uranium-containing wastewater. The enhanced uranium adsorption performance and adsorption mechanism of ASBP were systematically studied using morphological (SEM), spectroscopic (FT-IR, Raman, XPS), structural (BET), and thermal analysis (TGA) methods.

[0060] The following specific examples will provide further details.

[0061] I. Preparation of composite adsorbent (ASBP) according to the following method

[0062] 1. Prepare materials and reagents

[0063] The bamboo was sourced from a bamboo product processing plant in Hunan Province. Chitosan (PubChem CID: 71853, degree of deacetylation ≥95%, viscosity 100-200 mPa·s), citric acid monohydrate (C6H8O7·H2O; CAS No.: 5949-29-1), and glacial acetic acid (CH3COOH; CAS No.: 64-19-7) were all purchased from Hunan Huihong Reagent Co., Ltd. Dipotassium hydrogen phosphate was provided by Tianjin Kemei Chemical Reagent Co., Ltd. All reagents used in this example were of analytical grade.

[0064] 2. Bamboo Charcoal Preparation Process

[0065] First, the bamboo was dried at 105 °C and then cut into 3 mm square pieces. These pieces were further ground to obtain a fine powder. The resulting bamboo powder was placed in a tube furnace and calcined at 600 °C for 3 hours under a nitrogen atmosphere to obtain bamboo charcoal. Subsequently, the bamboo charcoal was washed three times with deionized water and finally dried at 80 °C to constant weight.

[0066] 3. Preparation process of composite adsorbent (ASBP)

[0067] 0.3 g of citric acid was placed in a beaker, and 30 mL of deionized water was added. The mixture was sonicated for 15 minutes until the citric acid was completely dissolved. Then, 0.3 g of bamboo charcoal, 0.3 g of chitosan, 0.3 g of dipotassium hydrogen phosphate, and 0.3 mL of acetic acid solution were added to the beaker sequentially. The mixture was stirred at room temperature for 2 hours. After the reaction was complete, the solid and liquid phases were separated by centrifugation. The solid residue was then dried at 80°C for 12 hours to obtain the ASBP composite material.

[0068] II. Relevant experiments and characterization of the composite adsorbent (ASBP)

[0069] 1. Adsorption experiment

[0070] Uranium was adsorbed from 90 mL of uranyl carbonate solution (pH range: 2-9, initial uranium concentration range: 5-400 mg / L) using 15 mg ASBP within a temperature range of 288-318 K. The experiments were conducted with continuous stirring at 175 rpm for 0-12 hours. The pH of the solution was adjusted with HNO3 and sodium carbonate, and the initial pH was monitored in real time using a pH meter. The uranium concentration was quantified by spectrophotometry and inductively coupled plasma (ICP) analysis. The adsorption efficiency R (%) and adsorption capacity Qe (mg / g) of ASBP for uranium were calculated based on formulas (1) and (2), respectively. All adsorption experiments were performed in triplicate and independently repeated three times to ensure the reliability of the obtained data.

[0071] Formulas (1) and (2) are as follows:

[0072] (1)

[0073] (2)

[0074] In this equation: (mg / L) and (mg / L) represents the initial and equilibrium concentrations of hexavalent uranium in the solution before and after the adsorption reaction, respectively; V (L) represents the solution volume; m (g) is the dry mass of the adsorbent ASBP; (mg / g) characterizes the adsorption capacity of ASBP for hexavalent uranium at adsorption equilibrium.

[0075] This adsorption experiment serves as a benchmark method for subsequent experiments.

[0076] 2. Adsorption kinetics experiment

[0077] The adsorption kinetics of U(VI) at different time intervals were investigated under controlled conditions. Adsorption experiments were conducted at 298 K, with 90 mL of uranyl carbonate solution (pH 6.0, initial uranium concentration: 10 mg / L) treated with 15 mg ASBP and continuously oscillated at 175 rpm for 0 to 12 hours.

[0078] 3. Adsorption thermodynamics experiment

[0079] This experiment aimed to investigate the effect of temperature on the adsorption process. The experiment was conducted at three temperatures: 288 K, 298 K, and 308 K. Uranium was adsorbed from a 90 mL solution at pH 6.0 using 15 mg of ASBP adsorbent, with initial uranyl carbonate concentrations ranging from 5 to 400 mg / L.

[0080] 4. Selective adsorption experiment

[0081] This experiment prepared simulated uranium-containing wastewater containing multiple metal cations with the following initial concentrations: U(VI) 1.286 mg / L; Al(III) 2.265 mg / L; Ca(II) 320.25 mg / L; Fe(III) 2.872 mg / L; Mn(II) 11.74 mg / L; Pb(II) 0.147 mg / L; Zn(II) 5.713 mg / L. Under pH 6.0, 15 mg of ASBP was added to 90 mL of the simulated uranium-containing wastewater for adsorption experiments. The concentrations of metal ions before and after adsorption were quantitatively analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES), and the partition coefficient (K0) was determined. d The result is obtained from formula (3).

[0082] Formula (3) is as follows:

[0083] (3)

[0084] In the selective adsorption experiment, the partition coefficient Kd (unit: mL / g) is calculated using formula (3).

[0085] 5. Actual wastewater experiment

[0086] This experiment used uranium-containing wastewater collected from a uranium tailings pond as the research object. The actual wastewater had a pH range of 6.5-7, and the uranium concentration was adjusted to 1.0 ± 0.2 mg / L. Intermittent adsorption experiments and fixed-bed column experiments were conducted using ASBP. In addition, the concentrations of four divalent metal cations (Ca, Mg, Mn, and Zn) in the effluent before and after fixed-bed column treatment were quantitatively analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES).

[0087] 6. Characterization methods

[0088] To investigate the structural and performance changes of ASBP before and after uranium adsorption and to elucidate the potential mechanism of uranium adsorption by ASBP, this embodiment employed a variety of characterization techniques, including scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FT-IR) (instrument models are detailed in Table 1).

[0089]

[0090] The synthesis and characterization of ASBP are as follows:

[0091] The surface structural changes of bamboo charcoal (BC) after cross-linking with chitosan (CS) and citric acid (CA) and functionalization with dipotassium hydrogen phosphate (DKP) were analyzed using scanning electron microscopy (SEM). Figure 1As shown, ASBP retains the porous morphology and irregular pore structure characteristics of the original bamboo charcoal, which is consistent with the common characteristics of bamboo charcoal-based adsorbent materials. Furthermore, Figure 2 The increased surface roughness of ASBP indicates that the crosslinking of chitosan (CS) with citric acid (CA) forms a denser crosslinked network, further stabilizing the bamboo charcoal framework. Additionally, small granular protrusions were observed on the ASBP surface, which may correspond to dipotassium hydrogen phosphate (DKP) crystals formed during the modification process, potentially contributing to an increase in available active sites.

[0092] The structural characteristics of BC and ASBP were analyzed using Fourier transform infrared spectroscopy (FT-IR). The results are as follows: Figure 3 As shown in (c), the FT-IR spectrum of ASBP is at 1092 cm⁻¹. -1 (POC stretching vibration), 1154 cm -1 A new peak appeared at (P=O stretching vibration), and at 899 cm⁻¹. -1 The stretching vibration of PO at this location confirms the introduction of the phosphate group. Furthermore, at 3410 cm⁻¹... -1 The nearby absorption peaks broaden significantly, indicating that the vibrations of the -OH and -NH2 groups overlap due to the introduction of chitosan (CS). The asymmetric stretching vibration peak of the carboxyl group is at 1590 cm⁻¹. -1 The strength at this point increases significantly, which is attributed to the cross-linking reaction between the amino and carboxyl groups. At 1722 cm⁻¹ -1 The new shoulder peak observed corresponds to the stretching vibration of the C=O bond in carboxylic acids. 1258 cm⁻¹ -1 The peak at 1386 cm⁻¹ represents the stretching vibration of the CN bond, while the OH deformation vibration of the -CH₂-OH group is at 1386 cm⁻¹. -1 The presence of these groups was detected. These results indicate that CS and CA were successfully crosslinked, and DKP was successfully loaded onto BC, thus successfully synthesizing ASBP. Furthermore, ASBP retained the original functional groups in BC, such as carboxyl and hydroxyl groups, while simultaneously forming phosphate groups. This demonstrates that the functionalization of BC was achieved efficiently without altering its basic structure.

[0093] Figure 3(d) shows the Raman spectrum of ASBP. At 1352 cm⁻¹ -1 The peak observed at that point corresponds to the D band (A) of the carbon material. 1g This pattern is closely related to structural defects or disorder within the ASBP. This phenomenon may be attributed to incomplete carbonization of the BC matrix or local structural perturbations caused by CSCA / DKP modification. At 1589 cm⁻¹ -1 The peak observed at that point corresponds to the G band (E) of the carbon material. 2gThe pattern reflects the crystallinity or structural order of ASBP. Furthermore, at 2500-3000 cm⁻¹... -1 The peaks within the range are related to CH stretching vibrations. Taken together, these observations suggest that ASBP possesses a carbon-based framework characterized by numerous defects and partially ordered structural features.

[0094] The BET analysis results for ASBP are shown below. Figure 4 , 5 The specific surface areas of BC and ASBP were 3.96 m² / g and 3.32 m² / g, respectively, with average pore sizes of 6.95 nm and 7.17 nm, respectively. Both materials exhibited mesoporous structures. It was observed that after CSCA crosslinking and DKP loading, the specific surface area decreased slightly, while the average pore size increased.

[0095] Figure 6 Thermogravimetric analysis (TGA) curves of ASBP are presented. As shown in the figure, the mass loss of ASBP mainly occurs between 30 °C and 175 °C, and only slightly decreases. This is mainly due to the evaporation of moisture inside the material, reflecting the hydrophilicity of ASBP. Significant mass loss is observed between 175 °C and 428 °C with increasing temperature, which is closely related to the thermal decomposition of oxygen- and phosphorus-containing functional groups, including the decomposition of carboxyl, hydroxyl, and phosphate groups. Between 428 °C and 800 °C, the mass loss of ASBP decreases by approximately 9.48%, showing a trend towards stabilization. This behavior is attributed to the stable cross-linked network formed by CSCA and the structural enhancement effect brought about by DKP modification. TGA analysis shows that ASBP retains approximately 50.3% of its initial mass at 800 °C. This residual mass can be partially explained by the inherent thermal stability of the BC skeleton and the further enhanced thermal stability achieved through the cross-linking and fixation of CA and CS.

[0096] Granularity analysis was performed on ASBP to assess its granularity distribution characteristics. For example... Figure 7 As shown, the particle size distribution follows a normal exponential pattern. Gaussian curves are typically used to fit the average particle size. Based on the Gaussian curve fitting, the average particle size of ASBP was determined to be 288.25 nm, which corresponds to the most concentrated particle size range. A dominant peak was observed between 200 and 400 nm, indicating that ASBP exhibits typical characteristics of nanoparticle-based adsorbent materials, generally suitable for uranium removal from wastewater.

[0097] III. Analysis of Experimental Results

[0098] 1. Analysis of the influence of initial pH value and adsorbent dosage

[0099] Figure 8 The adsorption efficiency of ASBP for uranium under different dosages was demonstrated. The uranium adsorption rate gradually increased with increasing ASBP dosage. The adsorption rate reached its maximum of 98.29% when the ASBP dosage reached 15 mg. Within the dosage range of 15-25 mg, the adsorption rate tended to stabilize and then decreased slightly. The observed decrease in adsorption efficiency can be attributed to the agglomeration of adsorbent particles at high dosages. This phenomenon hinders the mass transfer process of the adsorbate at the solid-liquid interface, mainly due to the mutual shielding of active sites between particles and the reduction of the effective surface area.

[0100] Figure 9 The effect of the initial pH of the uranium solution on the adsorption efficiency of BC and ASBP was demonstrated. Within a wide pH range of 2–9, the uranium removal rate of ASBP consistently remained above 91%, while the maximum adsorption rate of the original BC under the same conditions was only 45.2%, significantly lower than that of ASBP. Between pH 2 and 4, the uranium adsorption rate of ASBP increased from 91.81% to 97.41%. Within the pH range of 4–9, the adsorption efficiency remained stable between 97.22% and 97.75%. Notably, the uranium adsorption rate of ASBP gradually increased with increasing pH in the acidic range (pH 2–4), rising from 91.81% to 97.41%, and remained stable above 97.22% within the pH range of 4–9. Furthermore, the data indicate that with increasing solution pH, functional groups such as -OH and -COOH are mainly protonated under acidic conditions, leading to a significant negative shift in the zeta potential of ASBP. Within the pH range of 2–4, UO2… 2+ It is the main uranium species in solution, and its concentration decreases with increasing pH. During this process, the positive charge on the ASBP surface gradually decreases, thereby reducing UO2. 2+ The electrostatic repulsion between (UO2)2(OH)2 and ASBP enhances uranium adsorption efficiency. Within the pH range of 4-5, (UO2)2(OH)2 2+ and UO2OH + These become the dominant species in solution, leading to increased electrostatic repulsion between these cationic forms and positively charged ASBP. Simultaneously, H... + Ions and UO2 2+ Competition for adsorption sites leads to a slight decrease in uranium adsorption efficiency. Within the pH range of 5-8, new uranium species are formed, such as (UO2)2CO3(OH)3. - UO2(CO3)2 2- and UO2(CO3)3 4-These anionic species enhance the electrostatic attraction of the ASBP active sites, leading to improved adsorption efficiency. Furthermore, when the pH exceeds 8, the ASBP surface becomes negatively charged, resulting in a slight decrease in adsorption efficiency due to electrostatic repulsion, but it still remains above 96.98%. In summary, the adsorption rate of ASBP remains stable over a wide pH range of 2–9. This stability can be attributed to the cross-linking and immobilization effects of CS and CA, which significantly enhance the structural stability of the material, providing a solid foundation for its application in complex wastewater system treatment.

[0101] 2. Analysis of Adsorption Kinetics and Adsorption Thermodynamics Experimental Results

[0102] Adsorption kinetics studies not only provide in-depth insights into the uranium adsorption mechanism at the solid-liquid interface but also quantitatively elucidate the effect of the contact time between the adsorbent and the uranium-containing solution on the adsorption capacity. The adsorption efficiencies of synthesized BC and ASBP for uranium (VI) were evaluated through comparative analysis. Figure 10 As shown, there are significant differences in the adsorption performance of the two materials, with the modified material exhibiting a significantly enhanced adsorption capacity. BC reached near equilibrium within 20 to 720 minutes, with an adsorption capacity of 35.59 mg / g. In contrast, ASBP reached an adsorption capacity of 36.79 mg / g in just 10 minutes, approximately 1.77 times the equilibrium value of BC. Kinetic model analysis results (as shown in Table 2) indicate that the adsorption behavior of BC for U(VI) better conforms to the pseudo-first-order kinetic model (R1² = 0.996), suggesting that the process is mainly controlled by physical mechanisms such as pore diffusion. The pseudo-second-order kinetic model (R2² = 0.996) further supports this model. 2 The R² = 0.992 also indicates that chemical effects play a secondary role in BC, with physical and chemical processes occurring simultaneously. However, the CSCA / DKP modified composite material mainly follows a pseudo-second-order kinetic model (R² = 0.983), indicating that chemisorption is the dominant rate-limiting mechanism. This shift in kinetic behavior is attributed to the introduction of functional groups such as phosphates and hydroxyl groups through surface modification, which significantly enhances the chemical interaction between the material and U(VI), thereby increasing its affinity for uranium.

[0103]

[0104] Equilibrium adsorption isotherms are commonly used to determine the maximum adsorption capacity of an adsorbent, and the Langmuir and Freundlich models are frequently used to fit experimental data. Figure 11-13 As shown in Table 3, under pH 6 conditions, the Freundlich model exhibited a higher regression fit (R²) during the ASBP fitting process. F 2 ), compared to the correlation coefficient (R²) of the Langmuir model L 2The results indicate that the adsorption behavior is more consistent with the Freundlich model. This suggests that the adsorption of U(VI) on the heterogeneous surface of ASBP involves multilayer adsorption, with adsorption energy and affinity unevenly distributed across the surface. Under pH 6 conditions, the theoretical maximum adsorption capacity of ASBP for uranium increased from 443.65 mg / g to 576.92 mg / g as the temperature varied from 288 K to 298 K. Furthermore, at 308 K, the theoretical maximum adsorption capacity of ASBP for uranium reached 705.07 mg / g, exceeding most previously reported adsorbents for uranium-containing wastewater treatment.

[0105]

[0106] 3. Analysis of the influence of competitive metal cations on selective adsorption

[0107] In multi-ion radioactive wastewater, the effectiveness of an adsorbent depends on its adsorption selectivity for uranium. The adsorption selectivity of ASBP was investigated in a mixed solution containing multiple metal cations. Figure 14 As shown, the ASBP allocation coefficient for uranium (K) d Up to 3.0 × 10 5 mL / g, significantly exceeding the observed values ​​for other metal cations. Among the tested metal ions, Al... 3+ K d The value is closest to that of uranium, but the partition coefficient of uranium is still higher than that of Al. 3+ It is 17.8 times higher. In addition, the mixed solution also contains trivalent metal ions such as Fe. 3+ and divalent ions such as Ca 2+ Mn 2+ Pb 2+ and Zn 2+ Al 3+ The selective adsorption of Al can be explained by the charge-radius ratio (Z / r). Although Al 3+ It exhibits a higher charge density (Z / r), but its electron affinity for phosphate groups is lower than that of UO2. 2+ Weak. Furthermore, the selectivity of ASBP for high Z / r ions is limited by the coordination space configuration, resulting in its limited effectiveness against Al. 3+ The adsorption efficiency of Fe is lower than that of uranium. In environments with a pH value higher than 2.5, Fe... 3+ It mainly exists as Fe(OH)3 precipitate, with only a small amount existing as Fe(OH)3. 2+ It exists in hydrate form. The formation of this precipitate enhances the Fe... 3+ Physical adsorption occurs on the adsorbent surface. However, the binding affinity between the precipitate and ASBP is significantly lower than that of uranium chemocoordination, which further reduces the affinity for Fe. 3+ K d Value. Meanwhile, Pb2+ The adsorption of uranium likely relies primarily on weak electrostatic interactions, i.e., physisorption, which makes it less competitive compared to the chemisorption of uranium. This observation further demonstrates the significant advantages of ASBP in treating complex wastewater systems, effectively reducing interference from coexisting ions.

[0108] 4. Cyclic performance evaluation and analysis of ASBP

[0109] The cycling performance of ASBP was systematically evaluated through repeated adsorption and desorption experiments. Specifically, uranium was adsorbed onto 90 mL of uranium solution (pH = 6, CO = 5 mg / L) using 15 mg ASBP at 298 K. Subsequently, desorption was performed at 318 K using 30 mL of a desorption buffer composed of 1 mol / L NaCl and 2.5 g / L NaHCO3. The experimental results are as follows: Figure 15 As shown in the figure, after six adsorption-desorption cycles, the adsorption efficiency of ASBP for uranium decreased slightly from 99.77% to 94.01%, a decrease of only 5.76%. The desorption efficiency gradually decreased with increasing cycle number, but remained above 83.27% throughout the entire test cycle, with the highest desorption efficiency at 92.74% in the first cycle. Notably, the smaller desorption liquid volume (30 mL) significantly enhanced the uranium enrichment effect, thus simplifying the subsequent recovery process. Furthermore, the desorption process is expected to play a crucial role in practical wastewater treatment applications. The results indicate that ASBP has significant application potential in the field of practical wastewater treatment.

[0110] IV. Conducting batch scale-up experiments and real wastewater fixed-bed experiments

[0111] Batch adsorption experiments were conducted using 90 mL of real leachate obtained from uranium ore, with 15 mg of ASBP as the adsorbent. The initial uranium concentration in the leachate was adjusted to 1.0 ± 0.3 mg / L by controlling the amount of uranium added. Experimental results are as follows: Figure 16 The results showed that the material achieved an adsorption efficiency of 98.05% for uranium, significantly reducing the uranium concentration from 1.286 mg / L to 25 μg / L after treatment. To evaluate the practical applicability of ASBP, 1 L and 2 L of uranium ore leachate were treated with 0.3 g and 0.6 g of ASBP, respectively. After the first adsorption cycle, the uranium concentrations decreased to 197.94 μg / L and 179 μg / L, respectively. After the second adsorption stage, uranium was completely removed from both samples, with concentrations below the national emission standard of 50 μg / L. Accurately predicting the breakthrough curve of the target metal ion in the fixed-bed system is a crucial step in the system design and operation planning. Figure 17 and Figure 18A schematic diagram of a continuous fixed-bed system filled with 3 g ASBP (bed volume: 9.5 mL) and the corresponding concentration distribution of uranium percolate in the effluent are shown. During the initial operation at a bed volume of 1159 (BV, equivalent to 11.013 L), the uranium concentration in the effluent remained below the detection limit. The breakthrough point occurred at bed volume 1201 (BV, equivalent to 11.418 L), at which point the uranium concentration in the effluent reached 89.09 μg / L, corresponding to a breakthrough rate of 8.9%. Adsorption saturation equilibrium was reached at bed volume 1486 (BV, equivalent to 14.126 L), where the uranium concentration in the effluent rose to 1.05 mg / L. Subsequently, Figure 19 The adsorbent material was intermittently eluted using two different eluents: 1% HNO3 and a solution consisting of 1 mol / L NaCl + 2.5 g / L NaHCO3. Elution with 1% HNO3 was completed after 52 bed volumes (BV, equivalent to 52 mL), with a peak uranium concentration of 66.937 mg / L observed at 13 mL. Complete elution was achieved after 78 bed volumes (equivalent to 78 mL) of treatment with 1 mol / L NaCl + 2.5 g / L NaHCO3, at which point the maximum uranium concentration reached 75.91 mg / L at 52 mL. Furthermore, Figure 20 The changes in the concentrations of common metal elements (Ca, Mg, Mn, and Zn) in wastewater after passing through a fixed-bed system are shown. The concentrations of Ca, Mg, Mn, and Zn decreased from 684.5 mg / L, 83.15 mg / L, 10.505 mg / L, and 10.07 mg / L to 492.15 mg / L, 53.3 mg / L, 5.907 mg / L, and 5.671 mg / L, respectively, with corresponding removal efficiencies of 28.11%, 35.9%, 43.77%, and 43.70%. These removal rates are significantly lower than those for uranium. This indicates that even in real wastewater, ASBP maintains high selectivity for uranium adsorption. In conclusion, ASBP materials not only demonstrate highly efficient uranium purification capabilities (processing capacity exceeding 1200 bed volumes) in actual uranium-containing wastewater, but their reusability further highlights their engineering application potential in the field of radioactive wastewater treatment.

[0112] V. Adsorption Mechanism of ASBP

[0113] To investigate the changes in the surface structure of ASBP after adsorption of U(VI), this embodiment uses scanning electron microscopy (SEM) for characterization. Figure 21 , 22SEM-EDS images of BC and ASBP are shown. After DKP loading and CSCA crosslinking treatment, the phosphorus (P) content of BC increased by 0.9%, and the potassium (K) content increased by 1.51%. Furthermore, a significant increase in oxygen (O) content was observed, indicating the introduction of a large number of oxygen-containing functional groups. Figure 23 As shown, SEM-EDS analysis of ASBP-U (representing the adsorbent after uranium adsorption) showed that its uranium (U) mass percentage was 3.28%, confirming the successful adsorption of U on ASBP. Figure 24 Selected area scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) results for BC, ASBP, and ASBP-U are presented respectively. Furthermore, Figure 25 Image (g) shows the morphological characteristics of ASBP-U, indicating that its particle surface exhibits a rough structure, some pores are filled, and localized agglomeration occurs, which may be due to U adsorption. Meanwhile, Figure 25 Image (h) shows the SEM morphology of D-ASBP (representing the adsorbent after regeneration cycle), revealing no obvious cracks and an intact pore structure, indicating that ASBP has good regeneration potential. The SEM-Mapping results after adsorption are shown below. Figure 26 As shown in (i)-(p), U is uniformly distributed on the surface of the material, except for C, N, O, K and P elements in the raw materials, further confirming the uniform synthesis of ASBP and the uniform distribution of its functional groups.

[0114] This embodiment also explores the coordinating groups that may be involved in the ASBP adsorption of uranium. For example... Figure 27 As shown in the FT-IR spectrum of (a), compared to ASBP, ASBP-U exhibits higher spectral density at approximately 890 cm⁻¹. -1 A new strong peak appears at [location], corresponding to the O=U=O vibration band, confirming the successful adsorption of uranium by ASBP. The carboxylic acid C=O shoulder peak (1722 cm⁻¹) -1 The disappearance of ) and CN (1258 cm) -1 ) and P=O (1154cm) -1 The weakening of vibration, and PO (899 cm) -1 ) and CH / CP mode (715 cm) -1 The disappearance of the ) indicates bond breaking. These observations strongly suggest that the carboxyl and phosphate groups in ASBP participate in the complexation with uranium, and that the CN bond also participates in the uranium adsorption process. Furthermore, the infrared spectrum of D-ASBP shows that after six regeneration cycles, ASBP still retains the characteristic peaks of OH, CN, and P=O, explaining its high adsorption efficiency. However, a weak O=U=O characteristic peak was detected in D-ASBP, which may be due to incomplete desorption and a small amount of residual U(VI) in the ASBP.

[0115] Figure 27 Figure (b) shows the XPS spectra of ASBP before and after U(VI) adsorption, highlighting the characteristic signals of C 1s, O 1s, N 1s, P 2p, and U 4f. Figure 28 As shown in (c), the appearance of the U 4f peak in the ASBP-U spectrum provides clear evidence for uranium binding. Specifically, the U 4f peak... 7 / 2 The signal appeared at 382.70 eV and 383.91 eV, while U 4f 5 / 2 The signals at 393.92 eV and 394.18 eV further support the efficient adsorption of uranium by ASBP. Figure 28 In the C 1s spectrum of ASBP shown in (d), peaks corresponding to CC / C=C, COP, and C=O were observed at 284.8 eV, 286.67 eV, and 288.67 eV, respectively. The presence of the COP peak provides evidence for the phosphorylation of ASBP. In the ASBP-U spectrum, the intensity of the CC / C=C peak decreased, while the COP and C=O peaks shifted to 286.07 eV and 288.06 eV, respectively, indicating that the phosphate and carboxyl groups are significantly involved in the adsorption process. Figure 29 The N 1s spectrum in (e) reveals three low-intensity peaks located at 399.67 eV (CN bond), 401.24 eV (-NH2), and 402.16 eV (-NH3). + These peaks all disappeared after uranium adsorption, indicating that a limited number of amino groups participated in the uranium adsorption process. In the ASBP O 1s spectrum (e.g....), Figure 29 In (f), hydroxyl (-OH) and phosphoryl oxide (P=O) contribute peaks at 533.08 eV and 531.53 eV, respectively. In the ASBP-U spectrum, the intensities of the -OH and P=O peaks decrease, and their binding energies shift to 533.36 eV and 531.68 eV, respectively. Furthermore, a new low-intensity peak appears at 532.24 eV, which can be attributed to the splitting shift of the P=O peak at 531.68 eV caused by complexation with uranyl ions. Figure 30 The p 2p spectra of ASBP and ASBP-U shown are deconvolved into two distinct peaks, corresponding to p 2p. 1 / 2 and P 2p 3 / 2Before adsorption, the characteristic peaks at 133.46 eV and 134.36 eV were attributed to PO and P=O bonds, respectively. After adsorption, these peaks shifted to 133.71 eV and 134.61 eV, respectively, further confirming the complexation and coordination interactions between phosphate groups and uranyl ions. XPS analysis was consistent with FT-IR results. During uranium adsorption, the phosphate groups (including PO and P=O bonds) enriched on the ASBP material surface interacted with UO2 through lone pairs. 2+ Through specific coordination, stable uranium-phosphate complexes are formed. The binding energy of these complexes is significantly higher than that of single-ion interactions. This complexation dominates the adsorption process, allowing pre-bound H+ ions to... + Ions are exchanged with UO2 2+ The substitution enhances the material's adaptability to acidic, neutral, and alkaline environments and improves the ASBP's adsorption selectivity for uranium. The carboxyl groups retained from BC (-COOH) and newly introduced carboxyl groups from ASBP bind to UO2 in monodentate (η¹) and bidentate (η²) modes. 2+ Coordination occurs, and amino and hydroxyl groups also participate in the uranium adsorption process. Crosslinking and solidification of CA and CS further enhance adsorption stability. The multifunctional synergistic system formed by the crosslinking of CS and CA and the introduction of DKP significantly improves the uranium adsorption performance of ASBP.

[0116] In summary, this embodiment developed a bamboo charcoal-based composite adsorbent (ASBP) for the efficient removal of uranium from wastewater by cross-linking and immobilizing chitosan (CS) and citric acid (CA) and loading dipotassium hydrogen phosphate (DKP) onto a bamboo charcoal (BC) matrix. Experimental results showed that ASBP achieved a uranium adsorption efficiency exceeding 91% over a wide pH range of 2-9, exhibiting significantly enhanced adsorption capacity and pH adaptability compared to pure biochar. The uranium adsorption behavior of this material was well described by the Freundlich isotherm model, confirming a multilayer adsorption mechanism. The theoretical maximum adsorption capacity reached 705.07 mg / g at pH 6 and 308 K, and the adsorption process was determined to be spontaneous and endothermic. Selective adsorption experiments showed that the partition coefficient (K0) of ASBP for uranium was [not specified in the original text]. d ) is 3.0×10 5The uranium concentration (mL / g) was significantly higher than that of other coexisting metal ions, further demonstrating its excellent selectivity for uranium. After six adsorption-desorption cycles, the adsorption efficiency decreased from 99.77% to 94.01%, a reduction of only 5.76%. Meanwhile, the desorption efficiency remained above 83.27%, reaching a maximum of 92.74%, indicating excellent repeatability and good cycling performance. Furthermore, fixed-bed column experiments using uranium ore percolate showed that within a bed volume of 1159 (BV, equivalent to 11.013 L), the uranium concentration in the effluent was consistently below the detection limit. The breakthrough point occurred at 11.418 L (1201 BV), and the adsorption saturation point reached 14.126 L (1486 BV). The adsorption mechanism of ASBP for uranium was elucidated using post-adsorption characterization techniques (including FT-IR and XPS). The main mechanism involves the complexation and coordination between P=O and PO functional groups and uranyl ions in phosphate-enriched ASBP. Furthermore, hydroxyl, carboxyl, and amino functional groups synergistically participate in uranium adsorption, and CN bonds also play a role in this process. The environmentally friendly crosslinking agent CA significantly improves the structural stability and adsorption capacity of ASBP through hydrogen bonding and ionic interactions between carboxyl and amino groups in chitosan. Simultaneously, DKP loading enhances the surface properties and selective coordination ability of the material. In summary, this bamboo charcoal-based composite adsorbent, characterized by its simple, environmentally friendly, and green synthesis method, demonstrates significant practical application potential in treating various nuclear wastewaters.

[0117] The above embodiments are preferred implementations of the present invention. In addition, the present invention can be implemented in other ways. Any obvious substitutions without departing from the concept of the present technical solution are within the protection scope of the present invention.

Claims

1. A method for preparing a chitosan-citric acid crosslinked bamboo charcoal-based composite adsorbent, characterized in that, Includes the following steps: Citric acid was dissolved in deionized water and sonicated until completely dissolved. Then, bamboo charcoal, chitosan, dipotassium hydrogen phosphate, and acetic acid solution were added in sequence and stirred at room temperature. After solid-liquid separation, the resulting solid was dried to obtain the composite adsorbent. The mass ratio of bamboo charcoal, chitosan and citric acid is 1:1:1; The ratio of the total mass of bamboo charcoal, chitosan, and citric acid to the mass of dipotassium hydrogen phosphate is 3:

1. The bamboo charcoal is made by crushing bamboo biomass, carbonizing it at high temperature under an inert atmosphere, and then washing and drying it.

2. The preparation method of the chitosan-citric acid crosslinked bamboo charcoal-based composite adsorbent according to claim 1, characterized in that: Each 0.3 g of bamboo charcoal corresponds to 0.3 g of chitosan, 0.3 g of citric acid, 0.3 g of dipotassium hydrogen phosphate, 0.3 mL of acetic acid solution, and 30 mL of deionized water.

3. A chitosan-citric acid crosslinked bamboo charcoal-based composite adsorbent, characterized in that: It is prepared by the preparation method described in claim 1 or 2.

4. The application of the chitosan-citric acid crosslinked bamboo charcoal-based composite adsorbent according to claim 3 in the treatment of uranium-containing wastewater, characterized in that: The composite adsorbent is added to the wastewater to adsorb uranium in the wastewater.

5. The application of the chitosan-citric acid crosslinked bamboo charcoal-based composite adsorbent according to claim 4 in the treatment of uranium-containing wastewater, characterized in that: Each 15-25 mg of composite adsorbent corresponds to 90 mL of wastewater.

6. The application of the chitosan-citric acid crosslinked bamboo charcoal-based composite adsorbent according to claim 4 in the treatment of uranium-containing wastewater, characterized in that: The wastewater has a pH of 2-9 and a temperature of 288-318 K.

7. The application of the chitosan-citric acid crosslinked bamboo charcoal-based composite adsorbent according to claim 4 in the treatment of uranium-containing wastewater, characterized in that: When the pH of the wastewater is 6 and the temperature is 308 K, the adsorption capacity of the composite adsorbent for uranium reaches more than 700 mg / g.

8. The application of the chitosan-citric acid crosslinked bamboo charcoal-based composite adsorbent according to claim 4 in the treatment of uranium-containing wastewater, characterized in that: The composite adsorbent is filled into a fixed bed column, and uranium-containing wastewater is allowed to flow through the fixed bed column to achieve the adsorption and removal of uranium.

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