Introduction
Limited resource availability, uncontrolled population growth, and global environmental degradation have aroused interest in accessing sustainable clean energy resources. The nuclear fuel based on uranium is a new clean energy source, whose fission releases much more energy than that released by burning coal, effectively avoiding the emission of the greenhouse gas carbon dioxide [1, 2]. However, uranium-containing wastes from the nuclear fuel cycle or nuclear accidents are both radioactive and chemically toxic [3]. Uranium has a half-life of millions of years, and it can be easily transferred to the water environment when there is a slight change in the environmental condition, causing chronic pollution [4]. At the same time, large uranium reserves are necessary for energy security and greenhouse gas mitigation. Therefore, developing efficient technologies for the removing and recovering uranium from nuclear industry wastewater or other solutions for ensuring environmental safety and building up nuclear energy reserves is essential.
Current research efforts are heavily focused on extracting U(VI) (the predominant aqueous species of uranium) from diverse uranium-laden solutions, including nuclear industry wastewater and seawater, mainly concentrating on adsorption [5, 6], ion exchange [7], membrane separation [8, 9], and solvent extraction [10]. Among them, adsorption is the most common and effective approach for uranium extraction and removal of other organic and inorganic contaminants, owing to its cost-effectiveness and operational simplicity [11-15]. At present, various types of adsorbents have been successfully developed, including metal/covalent organic frameworks [6, 16], synthetic organic polymers [17], and carbon-based adsorbents [18]. However, most of these reported adsorbents are synthetic materials based on petrochemicals, which still increases global carbon emission during the production of nuclear industry.
In response to the current increasingly serious environmental problems, people are turning their attention from petrochemical products to biomass materials due to the latter’s many unique advantages such as eco-friendliness, abundant sources, and a large number of reactive groups that can interact with the target adsorbate [19, 20]. Hence, biomass-based adsorbents are emerging [21-23]. For example, Wang et al. synthesized amidoximated cellulose fibers by grafting polyacrylonitrile (PAN) onto cellulose fibers and amidoxime modification to extraction uranium from seawater [24]. Ye and his workers designed a new uranium extraction material by grafting phosphoric acid and amidoxime groups onto skin collagen fibers for uranium extraction from seawater [25]. On the other hand, aerogel adsorption materials have attracted extensive attention of many experts and scholars in recent years because of their high porosity, high specific surface area, and exceptional adsorption performance in the application of uranium extraction from aqueous solutions [26]. For instance, Chen et al. developed unique carbon-encapsulated zero-valent iron nanoparticles using konjac glucomannan-derived carbon aerogel for reduction-assisted uranium extraction [27]. In addition, other biomass aerogels, such as feather keratin [28], chitosan [29], and microalgae aerogels [30], have been widely reported and used for recovering uranium. Although the biomass materials are used in these reported biomass-based adsorbents, their preparation process includes complex multi-step chemical reactions and the usage of large amounts of petrochemical feedstocks. To pursue as green and low-carbon as possible for the uranium extraction process, developing biomass aerogels through a facile method and using as many biomass feedstocks as available is very significant.
Tannin, a naturally occurring polyphenol extracted from plants, ranks among the most abundant biomass feedstocks in nature. And the plant tannin includes abundant phenolic hydroxyls, which can form five-member rings to chelate multiple metal ions [31, 32]. Hence, the polyphenol-based adsorbent is a promising material for extracting uranium from various aqueous solutions. However, tannins cannot be directly applied for uranium extraction due to their high water solubility. Thus, they must be embedded within a solid matrix prior to use. The chitosan is also a biomass material with abundant active groups such as -OH and -NH2 groups, and has been used to prepare adsorbents for adsorbing metal ions from aqueous solutions [33-35]. Meanwhile, since the chitosan is water-soluble in acidic conditions, it usually also need to be solidified similarly. Based on the above statement, we believe that the preparation of biomass aerogels with a 3D network structure by facile cross-linking tannins with chitosan is promising in uranium extraction from seawater, which is expected to meet the green demands of uranium recovery process as much as possible.
Herein, we fabricated a biomass aerogel by the facile cross-linking of chitosan and bayberry tannins with glutaraldehyde. This aerogel maximizes the use of animal- and plant-derived biomass feedstocks. The as-prepared CS-BT aerogel has a 3D porous network structure and numerous reactive groups that can interact with uranium, allowing it to be used for uranium adsorption from aqueous solutions. Adsorption experiments were conducted to evaluate its adsorption performances, while kinetic and thermodynamic studies were carried out to propose the appropriate adsorption models. Finally, the adsorption mechanism was summarized via multiple characterization methods. This work contributes to further reducing the use of chemical feedstocks in the uranium recovery process, thereby reducing carbon emissions and promoting environmentally sustainable development.
Experiment
Materials
Uranyl nitrate (99%, AR) was offered by Hubei Chushengwei Chemical Co., Ltd. Glutaraldehyde (50 wt% aqueous solution, AR) and acetic acid (99%, AR) were purchased from Chengdu Kelong Chemical Co., Ltd. Chitosan (CS, BR, 85%, deacetylation degree ≥90%, Mw:~ 100000) was purchased from Shanghai Macklin Biochemical Technology Co., Ltd. Bayberry tannin (BT, industrial grade, 66%) was purchased from Guangxi Baise Forest Chemical Factory. Arsenazo III was obtained from Shanghai Linen Technology Development Co., Ltd. Uranium standard solution was offered by Beijing Institute of Chemical Metallurgy for Nuclear Industry.
Preparation of CS-BT aerogel
Firstly, the deionized water (600 mL) was added to a beaker (1 L) with BT (3 g) and stirred (300 r/min at 25 °C) for 2 h to completely dissolve BT. Then, CS (3 g) and acetic acid (6 mL) were added to the breaker and continuously stirred for 2 h. After completely dissolving CS, the glutaraldehyde (1.5 mL) was added to the beaker and put into the thermostatic shaking box (150 r/min at 45 °C) for 24 h. After that, the prepared sol was transferred into a plastic petri dish. Deionized water was added and decanted three times to thoroughly wash away any unreacted reagents.. Finally, after freezing in a refrigerator at -80 °C for 12 h, the CS-BT composite aerogel was obtained by freeze-drying for 72 h.
Characterizations of CS-BT aerogel
X-ray photoelectron spectroscopy (XPS) analysis was performed on an X-ray photoelectron spectrometer (Thermo Fisher Kalpha, USA) with a monochromatic Al X-ray radiation source. Fourier transform infrared (FT-IR) spectra were tested from 4000 to 500 cm-1 on an infrared spectrometer (Thermo Fisher Nicolet iS50, USA) with a potassium bromide tableting method. The microscopic morphology of samples was observed by field emission scanning electron microscopy (FE-SEM, CarlZeiss Ultra55, Germany) at 200 kV. Nitrogen adsorption and desorption isotherms were tested at the liquid nitrogen temperature with an automatic surface area and porosity analyzer (Micromeritics ASAP 2460, USA).
Adsorption of uranium by CS-BT aerogel
Test method of uranium ion and BT
Uranium ion standard solution (1000 mg·L-1), dilute hydrochloric acid solution (0.1 mol·L-1), and arsenazo III solution (1 g·L-1) were used to accurately prepare the uranium test solution with different concentrations of 0, 1, 2, 3, 4, and 5 mg·L-1 under different pH. After that, these solutions were measured on an enzyme marker (Molecular SpectraMax 190, USA) to establish a standard curve of uranium content based on absorbance.
On the other hand, BT solutions with different concentrations (0.02, 0.04, 0.06, 0.08, 0.1, 0.12, and 0.14 g·L-1) were also prepared. After that, these solutions were measured with an ultraviolet spectrophotometer (Hitachi UV-3900, Japan) within the wavelength range of 200 to 400 nm to establish a standard curve of BT content between absorbance.
Effects of solution pH and adsorbent mass on adsorption
Firstly, 20 mg· L-1 uranyl nitrate solutions with different pH (3.0, 4.0, 5.0, 6.0, 7.0, and 8.0) were prepared. Then, the as-prepared U(VI) solution (50 mL) and aerogel with different masses (0.005, 0.01, 0.02, 0.03, 0.04, 0.06, and 0.08 g) were placed in a conical flask and shaken in a thermostatic shaking box (150 r/min at 45 °C) for 24 h. After that, the solution was filtered, and the U(VI) concentration was tested and calculated. For each adsorption experiment, at least three repetitions were performed to minimize operator error. The adsorption capacity (Ac, mg·g-1) and removal rate (Rr, %) of U(VI) were calculated by the following equations:
Cyclic adsorption
Firstly, 20 mg of CS-BT aerogel, which had adsorbed uranium, was placed in a 150 mL conical flask. Then 50 mL of 0.1 mol·L-1 HCl solution was added to the conical flask for desorption at 25 °C for 24 h. After desorption was completed, the CS-BT aerogel was washed trice with 100 mL of distilled water and dried at 45 °C. This cycle was repeated five times and the adsorption capacity was calculated.
Adsorption kinetics
The kinetics experimental values were obtained from 20 mg·L-1 uranyl nitrate solution and linearly fitted by pseudo-first-order model and pseudo-second-order kinetics models, and they can be expressed by the following two equations, respectively:
The isothermal experimental values were linearly fitted by Langmuir and Freundlich isotherm models, which can be expressed by the following two equations, respectively:
Adsorption thermodynamics
The isothermal experimental values were obtained from uranyl nitrate solutions with different concentrations (20, 80 140, 240, and 300 mg·L-1) over a wide temperature range and linearly fitted by the following three equations, respectively:
Effects of coexisting ions on adsorption
20 mg·L-1 uranyl nitrate solution containing coexisting ions (2 mg·L-1 Cd2+, Ni2+,
Results and Discussion
Preparation and characterizations of CS-BT aerogel
The schematic diagram of the preparation of CS-BT aerogel is shown in Fig. 1a. First CS and BT completely dissolved to form a complex solution under stirring. Then, the glutaraldehyde, acting as a chemical cross-linking agent, was added to the complex solution, facilitating the grafting or or crosslinking between BT and CS. In this process, the glutaraldehyde could undergo a Schiff base reaction with -NH2 on CS and a Mannich reaction with C6 and C8 reactive hydrogens on BT to form covalent bonds, which ensures successful functionalization of CS by BT [32].
FTIR spectra of CS-BT aerogel and CS/BT mixture are shown in Fig. 1b. The band at 3418 cm-1 could be assigned to the stretching vibration of -NH2 and -OH, and the band at 2917 cm-1 was related to the stretching vibration of C-H. Moreover, the benzene skeleton vibration BT could also be observed at 1632 cm-1 and 1567 cm-1. However, compared with the CS/BT mixture, there was a significant difference in the peak intensity of CS-BT at 1632 cm-1, and it became stronger and shifted to 1619 cm-1. This can be attributed to the formation of C=N by the Schiff base reaction between -NH2 and aldehyde groups [36-38]. On the other hand, C=N bonds are usually not stable under a strong acidic environment. Hence, the BT leaching experiment of BT was carried out at different pH levels. As shown in Fig. 1c, the leaching rate of BT was less than 1% at pH > 3.0. This indicates that BT has been successfully grafted to CS.
The microscopic morphology of CS-BT aerogel was characterized by SEM. As shown in Fig. 2a-b, it could be clearly seen that the inner part of CS-BT aerogel is loose and porous, with a three-dimensional network structure, and the pore size is between ten and twenty microns. This indicates that CS-BT aerogel has the advantages of being light weight, having higher porosity, and having a large specific surface area, which increases the contact area with metal ions and the adsorption sites during adsorption. To further quantify the porous feature, the nitrogen adsorption and desorption isotherms for CS-BT aerogel were investigated. As presented in Fig. 2c, the nitrogen adsorption and desorption isotherms belonged to typical type IV. It was observed that the nitrogen adsorption volume increased gradually in the low-pressure region but sharply in the high pressure region, suggesting the presence of both micro- and mesopores in the synthesized CS-BT aerogel [17]. This resultant is consistent with SEM images. Moreover, the specific surface area of CS-BT aerogel calculated by the Brunauer-Emmett-Teller (BET) method was 26.06 cm2·g-1, and its average pore size was 10.33 nm. The presence of many a large number of pores and a high specific surface area in the CS-BT aerogel provides favorable circumstances for the diffusion of metal ions into its interior.
Adsorption performances
Effects of solution pH on adsorption
pH is closely related to the chemical activity of metal ions and the ionization of adsorbent functional groups [39]. Hence, the effect of solution pH on the uranium adsorption was investigated. As shown in Fig. 3a, the adsorption rate of 0.06 g CS-BT aerogel on U(VI) increased gradually when the solution pH increased from 3.0 to 5.0. The adsorption efficiency was smaller when the pH was lower, which was because due to the fact that the surface amino groups of CS-BT aerogel were positively charged after protonation, increasing the electrostatic repulsion between U(VI) and amino groups, thus reducing the adsorption effect of CS-BT aerogel. When pH value increased from 5.0 to 8.0, the adsorption rate almost reached equilibrium, with maximum efficiency of approximately 99%, which may be attributed to the fact that the uranyl ion in the solution is positively charged [(UO2)x(OH)y]z+ [40], and the degree of protonation for amino groups decreases, thus weakening the electrostatic repulsion to promote the adsorption capacity of uranium. And the CS-BT aerogel was gradually deprotonated, and more carboxyl groups were dissociated into the carboxylate ions with negative charge, which provided more active sites for the adsorption of U(VI). The adsorption rate is higher at pH 5.0, so the subsequent experiments were carried out at pH 5.0.
Effect of CS-BT aerogel mass on adsorption
The amount of adsorbent determines the number of adsorption active sites during operation. As shown in Fig. 3b, the adsorption rate increased significantly with increasing mass of CS-BT aerogel, with 98% at 0.02 g CS-BT aerogel. The adsorption rate of CS-BT aerogel was higher than that of previously reported aerogels, such as feather keratin (92%) [28], chitosan/amidoxime polyacrylonitrile (91%) [29], and microalgae (85%) aerogels [30]. The adsorption rate no longer increased when the adsorbent mass continuously increased. However, the adsorption capacity kept decreasing with the increase in the adsorbent amount. This is because at the same concentration of U(VI), the continuous increase in the mass of adsorbent caused a decrease in the adsorption capacity per unit mass of aerogel. Based on adsorption efficiency and cost considerations, CS-BT aerogel was quantified as 0.02 g in the subsequent experiments. Moreover, the cyclic adsorption was also performed. As shown in Fig. 3c, the adsorption capacity only slightly reduced, from 44.6 to 37.4 mg·g-1, showing a potential reusability.
Adsorption kinetics
Adsorption kinetics is an effective method for assessing the efficiency of an adsorbent. The adsorption kinetics curves of CS-BT aerogel are shown in Fig. 4a. With the increase of adsorption time, the adsorption rate and capacity also significantly improved. After 180 min, the adsorption rate was exceeded 80%. Subsequently, the adsorption almost reached equilibrium at about 600 min, with the adsorption rate of 95.09% and adsorption capacity of 45.4 mg·g-1, respectively. Initially, U(VI) rapidly bound bind to the active adsorption sites on the surface of CS-BT aerogel, showing a higher adsorption rate. Then, U(VI) spread and migrated from the outside to the inside of CS-BT aerogel, and the adsorption rate continued to grow due to the high ion concentration difference between the inside and outside of CS-BT aerogel. As more active sites on CS-BT aerogel were gradually bound by U(VI), the adsorption rate no longer changed significantly, reaching adsorption equilibrium.
To deepen the insight into the adsorption mechanism of CS-BT aerogel, the kinetics data were further fitted by the pseudo-first-order and pseudo-second-order kinetics models. The fitting results are shown in Fig. 4b-c and Table 1. The correlation coefficients (R2) of pseudo-first-order and pseudo-second-order kinetics models were 0.957 and 0.999, respectively. The theoretical equilibrium adsorption capacity (38.034 mg·g-1) fitted by the pseudo-first-order kinetics model was much smaller than the experimental adsorption capacity (48.311 mg·g-1). In contrast, the pseudo-second-order kinetics model had relatively good fitting results to the kinetics data, and the theoretical equilibrium adsorption capacity (48.309 mg·g-1) was very close to the actual adsorption capacity. It suggests that the adsorption process of CS-BT aerogel is more consistent with the pseudo-second-order kinetics model, thus a chemical adsorption reaction [41].
| Kinetics models | Pseudo-first-order | Pseudo-second-order | ||||
|---|---|---|---|---|---|---|
| qe (mg g-1) | K1 (g mg-1 min-1) | R2 | qe (mg g-1) | K2 (g mg-1 min-1) | R2 | |
| Parameters | 38.034 | 0.011 | 0.957 | 48.309 | 5.621×10-4 | 0.999 |
Adsorption isotherms
The adsorption isotherms of U(VI) on CS-BT aerogel are shown in Fig. 5a. With the increasing concentration of U(VI), the adsorption capacity of CS-BT aerogel rapidly improved, and then gradually slowed down. At the low concentration of U(VI), the adsorbent had relatively abundant active adsorption sites, which allowed rapid and efficient adsorption. As the U(VI) concentration increased, the adsorption sites on CS-BT aerogel became progressively saturated by U(VI), thus reducing the U(VI) concentration difference between adsorbent and solution. The increment of adsorption capacity of CS-BT aerogel gradually slowed down until adsorption equilibrium was achieved. With increasing temperature, the adsorption capacity increased, with 390.11 mg·g-1 at 328.15 K. It indicates that high temperature is conducive to the adsorption of CS-BT aerogel toward uranyl ions. The adsorption isotherms data were further nonlinearly fitted by using Langmuir and Freundlich isothermal models. They are based on monolayer adsorption on a homogeneous surface, and multilayer adsorption on a heterogeneous surface, respectively [42, 43]. The fitting results are shown in Fig. 5b-c and Table 2. R2 of Freundlich model was higher than that of Langmuir model, with 0.991 and 0.988 at 328.15 K, respectively. It suggests that the isotherms are well fitted by the Freundlich model, and the adsorption process of U(VI) occurs a multilayer adsorption dominated mechanism on the heterogeneous surface of CS-BT aerogel [44, 45].
| Isothermal model | Temperature (K) | Langmuir | Freundlich | ||||
|---|---|---|---|---|---|---|---|
| qm (mg g-1) | KL (L mg-1) | R2 | KF (L mg-1)1/n | n | R2 | ||
| Parameters | 298.15 | 586.27 | 0.004 | 0.974 | 10024.0 | 1.609 | 0.974 |
| 308.15 | 627.67 | 0.004 | 0.980 | 9441.8 | 1.571 | 0.981 | |
| 318.15 | 780.00 | 0.003 | 0.983 | 7711.8 | 1.458 | 0.986 | |
| 328.15 | 814.42 | 0.003 | 0.988 | 7727.8 | 1.449 | 0.991 | |
Adsorption thermodynamics
The equilibrium adsorption capacity of CS-BT aerogel toward U(VI) at different temperatures is shown in Fig. 6a. As the temperature increased, the equilibrium adsorption capacity exhibited a significant upward trend, indicating that the elevated temperature facilitated the adsorption of U(VI) by the CS-BT aerogel. Figure 6b showed the liner fitting curve of lnK0 versus 1/T. According to this curve, the thermodynamic data can be obtained, as shown in Table 3. The Gibbs free energies is negative at the four experimental temperatures, indicating U(VI)adsorption on the CS-BT aerogel spontaneously occur and is thermodynamically feasible. The chelation between -OH, -NH2, and C=O groups on CS-BT aerogel and U(VI) served as one of the driving forces to promote the spontaneous occurrence of the adsorption process. The negative enthalpy change during the adsorption process indicated that the adsorption of U(VI) by the CS-BT aerogel was an exothermic reaction. Consequently, increasing the temperature did not significantly enhance the adsorption efficiency. The entropy change is positive, demonstrating that the adsorption of CS-BT aerogel toward U(VI) is a disordered increasing process on the solid-liquid surface [46].
| T (K) | ΔH0 (K ·J ·mol-1) | ΔS0 (J mol-1 K-1) | ΔG0 (K· J· mol-1) |
|---|---|---|---|
| 298.15 | -1.109 | 0.004 | -2.352 |
| 308.15 | -1.109 | 0.004 | -2.412 |
| 318.15 | -1.109 | 0.004 | -2.398 |
| 328.15 | -1.109 | 0.004 | -2.507 |
Effect of coexisting ions
In practical applications, a variety of coexisting ions are inevitably present in uranium-containing aqueous solutions, and they can also compete with the U(VI) for adsorption, thus affecting the removal efficiency of U(VI) [47]. To assess the influence of coexisting ions on the adsorption effect of U(VI), the removal efficiency of U(VI) was investigated within 120 min in the coexistence solution of different ions. As shown in Fig. 6c, Pb2+, Cd2+, Ni2+, Fe3+, and
Adsorption mechanism
The microscopic morphology of CS-BT aerogel adsorbed U(VI) was also characterized by SEM. As shown in Fig. 7a-b, the blocky solid particles could be observed in the pores and walls of CS-BT aerogel, which should be the uranium aggregate. Meanwhile, the peak of uranium element is also shown in the energy spectrum of Fig. 7c. The average elemental percentages of the energy spectra obtained from the three different sites are shown in Table 4, in which the percentage of uranium is as high as
| Element | C | O | S | U |
|---|---|---|---|---|
| Proportion (%) | 66.50 ± 2.69 | 14.89 ± 3.87 | 3.25 ± 0.94 | 15.36 ± 5.20 |
To further characterize the structure change of CS-BT aerogel after adsorbing U(VI), FTIR and XPS were carried out. As shown in Fig. 7d, a new peak could be observed at 898 cm-1 after the adsorption of CS-BT aerogel toward U(VI), which can be assigned to the characteristic peak of U(VI). The peak at 3419 cm-1 corresponded to -OH and -NH2 groups, which became wider, and the phenolic -OH group obviously shifted from 1407 cm-1 to 1389 cm-1. It suggests that there is a strong interaction between -OH groups and U(VI). Moreover, a new U 4f peak also could be seen from XPS total spectra in Fig. 7e. And the O 1s XPS high-resolution spectra in Fig. 7f showed that the peaks of C=O and C-O shifted to higher binding energy (from 531.37 to 532.2 eV, and 532.76 to 533.05 eV, respectively) [48], which once again confirms the interaction between CS-BT aerogel and U(VI). These results indicate that the adsorption of CS-BT aerogel toward U(VI) is mainly achieved through the chelation between -OH groups and U(VI) [49]. Based on the above analyses, a possible adsorption mechanism is proposed during the adsorption of CS-BT aerogel toward U(VI) and shown in Fig. 8.
Conclusion
In this work, we have fabricated a biomass CS-BT aerogel with porous structure by crosslinking chitosan and bayberry tannin using glutaraldehyde. The cross-linked aerogel confers excellent aqueous stability in water, solving the disadvantage that chitosan and bayberry tannin cannot be directly used in uranium extraction from aqueous solutions due to their inherent water solubility. Because there are several -OH groups on CS-BT aerogel that can chelate with U(VI), CS-BT aerogel shows an excellent adsorption performance toward U(VI), with the maximum adsorption capacity of 140 mg·g-1 (at 298.15 K, pH=5.0). The kinetic study reveals that its adsorption is a chemical adsorption process dominated by multilayer adsorption. And the thermodynamic study shows that the adsorption process is spontaneous and exothermic. These results demonstrate that the CS-BT aerogel is a promising material for extracting uranium from natural seawater and nuclear industry wastewater. Unlike traditional adsorbent materials, this CS-BT aerogel is made from almost exclusively biomass, reducing carbon emissions during the preparation, and contributing to the sustainability of the nuclear industry.
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