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Article

Functionalized Polyethyleneimine Adsorbent for Efficient and Selective Uranium Extraction from Aqueous Solution

by
Huijun Yan
1,*,
Long Huo
2,
Hong Gao
1,
Xuanyi Li
1 and
Jianwei Bai
2,*
1
Department of Chemistry, Harbin University, Harbin 150086, China
2
Key Laboratory of Superlight Material and Surface Technology, College of Materials Science and Chemical Engineering, Harbin Engineering University, Ministry of Education, Harbin 150001, China
*
Authors to whom correspondence should be addressed.
Sustainability 2025, 17(13), 5953; https://doi.org/10.3390/su17135953
Submission received: 11 May 2025 / Revised: 24 June 2025 / Accepted: 26 June 2025 / Published: 28 June 2025
(This article belongs to the Section Sustainable Water Management)
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Abstract

The sustainable treatment of uranium-containing wastewater is of significant importance for environmental protection. This study reports a novel Polyethyleneimine-4-cyanobenzaldehyde/p-Phthalaldehyde-Amidoxime (PEI-PAC-AO) adsorbent for the effective extraction of uranium from aqueous solutions. The structural and performance characteristics of the adsorbents were analyzed through FT-IR, TGA, SEM, CA, and ICP-MS. Adsorption mechanisms were investigated using X-ray photoelectron spectroscopy (XPS), revealing that uranium adsorption is due to coordination with N and O atoms in the amidoxime groups. Batch adsorption experiments showed that PEI-PAC-AO exhibited excellent removal efficiency at pH 6. The static adsorption performance better fits the Langmuir model and pseudo-second-order kinetics. Adsorption results indicated that the removal extent of uranium ions remained at 80% after nine consecutive adsorption cycles using 0.5 M nitric acid as the eluent. These findings suggest that PEI-PAC-AO is a sustainable and promising material for the efficient removal of uranium from wastewater, offering a sustainable and environmentally friendly approach that contributes to environmentally responsible wastewater treatment strategies.

1. Introduction

With the increasing demand for industrial energy, the excessive use of traditional fossil fuels has caused serious environmental impacts [1]. Nuclear energy, being both clean and efficient and devoid of greenhouse gas emissions, is widely regarded as an indispensable pathway towards achieving carbon neutrality in the twenty-first century [2,3]. The development of nuclear energy is inseparable from uranium resources. However, the proven reserves of terrestrial uranium ore are relatively limited, scarcely adequate to satisfy the increasing demand [4]. In contrast, the ocean is rich in uranium resources; it is currently estimated to contain 4.5 billion tons [5]. The development of the nuclear industry generates a large amount of radioactive uranium-containing wastewater [6]. Additionally, uranium has an extremely long half-life and greater mobility compared to natural heavy metals, leading to more persistent pollution and hazards [7]. Therefore, developing methods to extract uranium from solutions holds significant importance [8]. However, the effective extraction of uranium from seawater remains a major challenge because of the extremely low concentration of uranium (≈3.3 ppb) and the coexistence of numerous metal cations in seawater.
There are various common methods for uranium enrichment and treatment, including membrane separation [9,10], chemical precipitation [11,12], ion exchange [13,14], biological treatment [15,16,17], and adsorption [18,19,20]. Among the various uranium enrichment and treatment techniques, the adsorption method stands out as the most promising approach for uranium separation, attributed to its remarkable efficiency and cost-effectiveness [21]. Currently, numerous adsorbent materials have been developed, primarily encompassing porous carbon-based materials [22], metal–organic framework materials [23,24], porous aromatic framework materials [25], and composites [26]. Significant progress has been made in the development of uranium adsorbent materials, yet numerous challenges persist. Issues such as the easy detachment of adsorbing groups, as well as the inherent limitations of the materials themselves, result in a sharp decline in performance and poor recyclability after multiple uses. To address the shortcomings of current materials, further exploration and development are still required.
Polyethylenimine (PEI) has been utilized in numerous fields such as dye adsorption [27], cell detection [28], phosphate removal [29], carbon dioxide capture [30], enrichment of rare earth elements [31], and identification of aromatic derivatives due to its strong chelating properties, hydrophilicity, biocompatibility, and the abundant amine groups on its macromolecular chain [32]. Its amine functional groups exhibit chelating effects on heavy metal cations and heavy metal cluster anions, resulting in excellent heavy metal adsorption performance. Currently, various PEI-based materials have received widespread attention in the removal of toxic heavy metals due to their unique properties, including high affinity, metal binding capacity, thermal stability, mechanical strength, and potential for reuse. Up to now, there have been numerous reports on the removal of heavy metals using PEI-based composite materials [33,34,35]. However, there are relatively few studies on their synthesis strategies and applications in uranium adsorption.
The amidoxime group, renowned for its rapid adsorption extent and high selectivity towards uranium, can significantly enhance uranium adsorption efficiency when integrated into a porous matrix, thereby augmenting its specific surface area [36]. Functionalized polymer materials, known for their outstanding characteristics, including a three-dimensional network structure, hydrophilic nature, water retention capabilities, and vast specific surface area, have emerged as prominent uranium adsorbents. Incorporating the amidoxime group into polyethyleneimine polymers effectively enhances their uranium adsorption performance [37]. This modification ensures uniform distribution of the amidoxime groups within a three-dimensional cross-linked hydrophilic network, facilitating the migration of UO22+ into the material’s interior and promoting synergistic interactions [38]. This approach addresses challenges related to material shaping, cost, and recyclability, offering promising prospects. Capitalizing on the modifiable characteristics of polyethylenimine, we conducted rigorous research experiments aimed at incorporating the amidoxime group, ultimately enhancing both the adsorption strength and selectivity capabilities of the material.
Here, for the first time, we used the low-cost 4-cyanobenzaldehyde (PA) as a PEI modification material, employing p-Phthalaldehyde (PC) as a crosslinker to create Polyethyleneimine-4-cyanobenzaldehyde/p-Phthalaldehyde (PEI-PAC). This was then followed by an oxime alkylation process, resulting in a Polyethyleneimine-4-cyanobenzaldehyde/p-Phthalaldehyde-Amidoxime (PEI-PAC-AO) adsorbent. The modified PEI chains, rich in amidoxime groups, confer excellent uranium adsorption performance on the material. Additionally, the amidoxime group binds strongly with the PEI chain, significantly enhancing the material’s recyclability stability, maintaining over 80% uranium removal efficiency after nine cycles. The structure and chemical groups of the adsorbents were characterized by FT-IR, TGA, SEM, and CA. The performance and adsorption mechanism were probed by ICP-MS and XPS, respectively. The adsorption kinetics, sorption isotherms, and reproducibility were also studied. Finally, the mechanism of sorption was explored by XPS analysis.

2. Materials and Methods

2.1. Materials

Diaminomaleonitrile (DM, AR), p-Phthalaldehyde (PC, AR), 4-cyanobenzaldehyde (PA, AR), Acetic anhydride (AA), and Sodium borohydrideand (AR) were purchased from Shanghai Haohong Scientific Co., Ltd. (Shanghai, China). Polyethylenimine (PEI, AR), Dimethyl sulfoxide (DMSO, AR), Hydroxylamine hydrochloride (NH2OH·HCl, AR), and Tetrahydrofuran (THF, AR) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). All reagents were used without further purification.

2.2. Preparation of PEI-PAC

The synthesis route is shown in Figure 1. The PEI-PAC was prepared as follows. Firstly, measure 10 mL THF and 0.5 g PEI and slowly add them to a 25 mL round-bottom flask. Add a magnetic stirring bar to the flask and maintain a water bath at 298 K. Stir thoroughly until the mixture in the flask becomes uniformly transparent. Then, while stirring, add 0.3 g of PA. After about 1 h, transfer the flask to an ice-water bath and stir until the temperature stabilizes. Quickly add 0.2 g PC, stir for 8 min, then pour into a mold to set. The solidified product was frozen for 24 h to maintain its shape. After setting, place the product in a conical flask and replace THF with water five times over two days. The final product is a white, porous, foam-like solid. Based on the mass and ratio of PA and PC added, name the synthesized materials PEI-PAC-1 to PEI-PAC-6, as shown in Table S1.

2.3. Preparation of PEI-PAC-AO

Dissolve PEI-PAC-1 to PEI-PAC-6 separately with 0.5 g of sodium borohydride in 60 mL of water. The flask was then shaken and reduced for 36 h on a shaker. Afterwards, the product was washed with distilled water to clean the residual sodium borohydride to obtain PEI-PAC-RA. Subsequently, carry out the oximation in a conical flask using 0.75 g of sodium hydroxide, 10 mL of methanol, 1.25 g of hydroxylamine hydrochloride, and 20 mL of distilled water, continuing to shake on an oscillator for 24 h to ensure complete reaction. After the amidoximation process, the substrate was washed with distilled water and placed in a refrigerator for freezing to maintain its shape. Finally, the frozen material was placed in a freeze dryer for drying to obtain the final samples, which were named PEI-PAC-AO-1 to PEI-PAC-AO-6. For more detailed experimental methods, please refer to the Supplementary Materials.

3. Results and Discussion

3.1. Characterization of Adsorbents

FT-IR characterization of the target polymer PEI-PAC and the raw materials is used to determine the synthesis status of PEI-PAC. As shown in Figure 2a, the characteristic peaks of PEI, PA, and PC can be clearly observed. By comparing the FT-IR spectra of PEI-PAC with those of the three reactants, a common amine absorption peak at 3433 cm−1 was found between PEI-PAC and PEI, indicating that some amine groups remain after the reaction [39]. Due to the different structures of the aldehyde group on the benzene ring side, PA and PC exhibit different absorption peaks at 1704 cm−1 and 1696 cm−1, respectively. At 2234 cm−1 in the figure, PC shows a strong absorption peak, which is attributed to the stretching vibration of the cyanide group and corresponds to PEI-PAC. The infrared spectrum confirms the successful synthesis of polymer PEI-PAC.
As shown in Figure 2b, PEI-PAC-3 was selected for infrared analysis of its sodium borohydride reduced PEI-PAC-RA and the final product PEI-PAC-AO. PEI-PAC was reduced to PEI-PAC-RA, and the C=N absorption peak at 1381 cm−1 disappeared. At the same time, the 2234 cm−1 cyanide absorption peak was retained, indicating that the reduction of C=N was relatively complete and had no effect on the cyanide group, which is consistent with expectations [40]. After the next step of the amidoximation reaction to PEI-PAC-AO, the cyanide absorption peaks present in PEI-PAC and PEI-PAC-RA disappear, followed by the appearance of three absorption peaks at C=N-OH, C-N, and N-O, corresponding to 1639 cm−1, 1381 cm−1, and 936 cm−1, respectively, indicating complete reduction and complete conversion of cyanide groups to amidoxime groups in the two-step transformation of PEI-PAC-AO.
The pyrolysis temperature of amidoxime groups is greatly affected by the matrix. To understand the thermodynamic stability of functionalized polyethyleneimine samples with different ratios and ensure that the adsorption performance of the samples is not affected, six samples of PEI-PAC-AO were dried in a vacuum drying oven at 333 K for 24 h, and their thermogravimetric curves were measured under a nitrogen atmosphere. As shown in Figure 3a, with the increase in temperature, the mass of the sample decreases, and there is a significant mass loss at around 349 K, indicating that the moisture in the sample begins to evaporate [41]. When the temperature reaches 573 K, there is a significant decrease in the quality of the sample, which is due to the decomposition of the amidoxime groups and amino groups containing nitrogen, oxygen, and hydrogen elements in the sample, followed by high-temperature carbonization of the benzene ring and carbon chain. The quality tends to stabilize at around 773 K. In Figure 3b, the DTG curve shows three obvious weight loss peaks can be observed at 273–373 K, 573–673 K, and 673–773 K, corresponding to the loss of attached water, decomposition of amidoxime groups and amino groups, and decomposition of benzene rings and carbon chains. It can be seen from the comparison of six different ratios of samples that, as the percentage of PEI increases, the quality loss becomes faster. At the same time, PEI-PAC-AO-1 decreased the most at 373 K, which may be due to the high proportion of polyethyleneimine in the sample, which adsorbed and retained more moisture. After 773 K, the remaining mass percentage of PEI-PAC-AO-4 is the lowest, which may be due to the low proportion of PC during synthesis, which failed to crosslink the polyethyleneimine chains well, resulting in a lower final carbon content of the sample. Through the analysis of TGA and DTG curves, it is shown that the material has good thermal stability and can be used in harsh environments.
Water contact angle tests were carried out to analyze the hydrophilicity of the adsorbent. As shown in Figure 4, the contact angle curves of functionalized polymers with different ratios over time are presented. It can be clearly seen that the contact angle from 180° to 0° decreases within 300 ms, indicating that the adsorbent has good hydrophilicity. At the same time, as the proportion of PC increases in each group, the time will decrease, while the proportion of PA has a more significant effect on the increase in time. The reason may be that the hydrophobic effect of the benzene ring reduces the overall hydrophilicity of the material, and the increase in the amidoxime groups will slow down this effect. The overall hydrophilicity is determined by the polyethylene imine, which contains a large number of amino groups that quickly form hydrogen bonds with water molecules. At the same time, the -NH2 and -OH groups produced after amidoxime reaction also have a certain enhancing effect on hydrophilicity. In summary, the PEI-PAC-AO material has strong hydrophilicity, and the addition of amidoxime groups also significantly enhances hydrophilicity.
The physical and microscopy images of the functionalized adsorbent PEI-PAC-AO are shown in Figure S1. It can be observed that the adsorbent has a light yellow columnar shape, which may be due to the color generated by the conjugated structure formed by the benzene ring and C=N. The electron microscopy image clearly shows that the material has a thin-film-like network structure and also has a relatively uniform micro-scale pore structure that extends longitudinally to the interior [42]. During adsorption, it can promote the solution’s entry into the interior of the material, indicating that the adsorbent has a good structure and provides sufficient conditions for its adsorption. On the other hand, X-ray diffraction patterns are used to indicate the crystalline structure of functionalized adsorbents. As shown in Figure S2, the spectrum of PEI-PAC-AO shows that the material has no obvious characteristic diffraction peaks and exhibits a continuous broad peak, reaching a peak at around 2θ = 22°, indicating that the material belongs to an amorphous structure. The result is also consistent with the scanning electron microscopy results.

3.2. Characterization of Uranium Adsorption Properties

In order to investigate the influence of different ratios of PC and PA on the final adsorption effect, the samples with different ratios, as shown in Table S1, and their adsorption capacities, were calculated. As shown in Figure S3, it can be seen that the adsorption capacity of functionalized polyethyleneimine material increases with the increase in PC, while the adsorption capacity of the sample also increases [43]. However, the extent of improvement of uranium adsorption is not significant. This may be due to the introduction of cyano groups into the benzene ring, which increases the proportion of the benzene ring. At the same time, the benzene ring will reduce the water absorption performance of the material and increase the difficulty of adsorbing uranium ions in solution [44]. When the amount of PC added remains constant and the amount of PA added is increased from 0.2 g to 0.3 g, the ratio of PC to PA decreases with the increase in PA mass. However, this leads to an increase in the proportion of hydrophobic benzene rings in the functionalized material, resulting in a decrease in its adsorption capacity. Meanwhile, as shown in the figure, the adsorption capacity of the functionalized material is highest when the ratio of PC to PA is 6:4, which confirms the hypothesis in the SEM analysis that the adsorption effect is better when the micropores are abundant and regularly arranged in the functionalized adsorbent material. At the same time, the infrared characterization of the amidoxime group is more pronounced at this ratio, which is consistent with the results. Therefore, the sample with a PC to PA ratio of 6:4 was chosen as the optimal ratio, and subsequent experiments used this ratio of adsorbent as the control sample.
The solid–liquid ratio is an important reference for measuring the adsorption capacity of samples, and its impact on the adsorption performance of functionalized polyethyleneimine samples is shown in Figure 5. From the graph, it can be seen that, as the solid-liquid ratio increases from 0.05 g/L to 0.7 g/L, the uranium ion removal extent increases rapidly. When the solid–liquid ratio continues to increase to 0.8 g/L, the increase in adsorption extent begins to slow down, and, even though the solid–liquid ratio continues to increase, the adsorption capacity increases abnormally slowly until there is no significant change [45]. By calculation, it is speculated that the reason may be that, at the beginning, due to the small proportion of adsorbent, as the proportion increases, the adsorption sites on the surface of the adsorbent increase, allowing the functionalized sample to adsorb more uranium ions, and the adsorption amount to be almost linearly correlated. As the proportion of functionalized adsorbents continues to increase, the active center sites begin to shift from the amidoxime group, and the amino groups in the adsorbent begin to compete, resulting in a slowing down of the trend of increase. As the solid–liquid ratio continues to increase, competition among adsorption sites intensifies, and, at the same time, the concentration of uranium ions in the solution reaches an equilibrium state with the ability of the sites themselves, making it difficult for uranium ions to be better accepted by the sites, resulting in no further increase in removal efficiency. Therefore, considering the adsorption curve and removal extent of functional materials, as well as practical applicability, 0.8 g/L was chosen as the optimal solid–liquid ratio, and this ratio was also used in subsequent experiments.
Numerous studies have shown that the initial pH of the environment has a significant impact on the adsorption efficiency during uranium adsorption. To investigate the effect of pH on the adsorption of functionalized polyethyleneimine, the adsorption performance was tested at different pH levels. According to Figure 6a, as the pH increases from 2 to 6, the removal extent of uranium ions significantly increases. However, as the solution pH continues to increase, the removal extent begins to decrease. This indicates that the surface adsorption state of polyethyleneimine adsorbent changes with different pH values, thus exhibiting different physicochemical properties. To further clarify the effect of pH on functionalized polyethyleneimine materials, the existence of different forms of uranium ions in different environmental pH was derived. As shown in Figure 6b, when the pH value is less than 3, the vast majority of uranium exists in the form of UO22+, which is relatively stable and in line with the preservation rules of experimental uranium solutions. At the same time, due to the positive charge carried by this form and the repulsion between hydrogen ions released by chelation reactions in the solution and the adsorption group amidoxime group, the adsorption capacity is relatively low [46]. Under acidic conditions, the amidoxime group will protonate, thereby reducing the adsorption capacity [47]. However, as the pH value gradually increases, the neutralization reaction causes the concentration of hydrogen ions released by the chelation reaction between the hydrogen ions that create an acidic environment in the solution and the amidoxime group to gradually decrease. This results in the surface of the functionalized polyethyleneimine sample in the solution gradually becoming negatively charged, thereby exhibiting a gradually increasing adsorption capacity. When the pH value is 6, the adsorption performance and efficiency of the material reach their maximum due to the synergistic electrostatic and chelating effects between uranyl ions and functionalized polyethyleneimine materials. When the pH value exceeds 6, the increase in hydroxide ions in the environment leads to the hydrolysis of uranyl ions, which mainly exist in the forms of UO2(CO3)22− and UO2(CO3)34− [48]. This causes the negatively charged PEI-PAC-AO on the surface to repel these particles, increasing the difficulty of adsorption and reducing the removal extent of uranium ions. Eight data points with pH values ranging from 2 to 9 were selected for the experiment. At pH 6, the adsorption extent of PEI-PAC-AO reached 90%, and the removal extent of uranium ions was relatively high. Therefore, an environment with a pH value of 6 was chosen as the optimal value for PEI-PAC-AO to adsorb uranium ions, and subsequent experiments were conducted at this pH.
To understand the adsorption extent of PEI-PAC-AO, the adsorption behavior of the sample was studied at 25 °C for 20 h. The adsorption curve was plotted as shown in Figure 7a. It can be seen that the time required for PEI-PAC-AO adsorption to reach equilibrium is relatively long, which may be due to its physical properties. Although it contains abundant pore structures, only the surface is fully exposed, and the internal connectivity is poor. The adsorption extent of PEI-PAC-AO continues to rise before 500 min and reaches equilibrium at around 800 min, reaching 90% adsorption capacity. At this point, it is close to the maximum adsorption extent, and the adsorption capacity no longer increases as the adsorption time continues to increase.
To further determine the adsorption model of PEI-PAC-AO and understand its adsorption mechanism, the pseudo-first-order kinetic model, pseudo-second-order kinetic model, and Morris–Weber models were used for fitting. The data curves are shown in Figure 7b–d. The fitted data were analyzed to obtain Table S2. It can be seen that the first-order kinetic model R2 is 0.9042, while the second-order kinetic model R2 is 0.9826. The second-order kinetic model has an R2 closer to 1, indicating that the adsorption of uranium ions by PEI-PAC-AO is mainly a chemical adsorption process. Therefore, the second-order kinetic model fits better and is more in line with actual adsorption. The final calculated constant equilibrium adsorption capacity is 140.85 mg/g, and the value of K2 is 0.000026 g/(mg·min). The Morris–Weber model simultaneously indicates that there are two adsorption processes in the adsorption of uranium ions by PEI-PAC-AO, namely, external diffusion and permeation equilibrium processes. The K1 of PEI-PAC-AO is 4.48, indicating that it has a high diffusion extent, which may be due to the presence of more sites that promote uranium ion diffusion in the material, as well as a wide pore size distribution that accelerates the diffusion extent. However, a K2 value of 0.8459 indicates weak permeability, which also confirms the poor connectivity of internal pores analyzed earlier.
To evaluate the adsorption capacity of PEI-PAC-AO for uranyl ions and investigate its adsorption process, the adsorption capacity of PEI-PAC-AO was tested at different temperatures and uranium ion concentrations. Adsorption was carried out at initial concentrations of 25, 50, 75, 100, 125, 150, and 200 ppm at temperatures of 298 K, 303 K, and 308 K, respectively, and the adsorption amounts were measured. The simulation curves are shown in Figure S4. It can be clearly seen from the curve in the figure that, as the temperature increases from 298 K to 308 K, the curve rises and the adsorption capacity of the material increases. This is because the increase in temperature will increase the activity of the adsorption functional groups, promote the diffusion of uranyl ions, and reduce the time required to establish equilibrium. This also indicates that the process of PEI-PAC-AO adsorbing uranium ions is endothermic. At the same temperature, as the initial concentration of the uranium solution increases, the adsorption capacity also increases. To conduct an in-depth analysis of the adsorption process, the Langmuir, Freundlich, Dubinin Radushkevich, and Temkin models were used to fit the data and obtain the following image.
By analyzing and fitting the Figure 8a–d curves, the calculation results are shown in Table S3. Among the Langmuir, Freundlich, Dubinin Radushkevich, and Temkin models, the Langmuir model is closer to 1 in terms of R2 compared to the three models, indicating that the Langmuir hypothesis is more consistent with the PEI-PAC-AO adsorption process, that is, the monolayer dynamic adsorption model. The fitting results of the Temkin and Freundlich models also have a large R2, which is due to the uneven distribution of PEI-PAC-AO adsorption sites.
Based on the thermodynamic analysis above, we can understand the energy changes in the PEI-PAC-AO adsorption process. The results obtained from the fitting calculation of Figure S5 are shown in Table S4. It is known from the table that the ΔG0 values (−0.93 kJ/mol, −1.22 kJ/mol, −1.50 kJ/mol) at the three temperatures are all less than 0, while the entropy change ΔS0 value (56.83 kJ/mol) is greater than 0, indicating that the uranium adsorption process is spontaneous. The ΔH0 value of PEI-PAC-AO is 16 kJ/mol, which theoretically proves that PEI-PAC-AO is endothermic in the uranium adsorption process, indicating that chemical adsorption plays the main adsorption role.
The key performance indicator for the practical application of adsorbent materials is selectivity. In real scenarios, there will be a large number of competing ions (such as K+, Ca2+, Na+, Mg2+ ions). Therefore, in order to study the selectivity of the functional polyethyleneimine material PEI-PAC-AO towards U (VI), multiple coexisting ions of the same concentration, Ca2+, K+, Mg2+, Na+, Ba2+, Sr2+, Bi2+, Co2+, and the same anion NO3−, were tested for their adsorption performance. As shown in Figure 9, the experimental results indicate that although there is interference from other ions, PEI-PAC-AO still has strong selective adsorption performance for uranium ions. Among them, K+ and Na+ are monovalent ions with weak competition ability with uranium ions, while Ca2+ and Mg2+ can form relatively stable complexes with the adsorbent, which has a certain impact on uranium selectivity. Ba2+ and Co2+ compete mainly with uranium in solution, and their Kd values differ by an order of magnitude compared to uranium, which can be attributed to the high selective adsorption of the amidoxime group. Table S5 shows the selectivity coefficient of uranium ions relative to other ions. It can be seen that the selectivity coefficient for single charge Na+ is the highest at 238.6, indicating the feasibility of the adsorbent for uranium enrichment in seawater. The metal ions that can form complexes with Ba and Co also exhibit cross-order selectivity coefficients, demonstrating that the material has high selectivity for uranium ions and indicating that PEI-PAC-AO can selectively adsorb uranium ions in complex environments.
To evaluate whether PEI-PAC-AO can be reused, which is an important factor considered in practice, experiments were designed for this purpose. As mentioned earlier, adsorption performance is closely related to pH, and desorption is carried out using desorption agents with different pH values. The experiment used 0.5 mol/L HCl, HNO3, NaOH, NaHCO3 solution, and H2O for desorption treatment. As shown in Figure 10a, the removal extent of acidic solution is significantly higher than that of alkaline solution, which may be due to their different working principles. In acidic solution, hydrogen ions are more likely to form hydrogen bonds with the lone pair electrons at the binding site of uranium ions and promote the disintegration of coordination. The alkaline eluent promotes desorption by increasing the pH of the solution and combining with uranium ions to form alkaline ions, which compete with the adsorbent. In the standard concentration acidic eluent, the dissociation degree of nitrate ions is higher. In fact, 0.5 M nitric acid is also the most efficient desorbent, with a desorption efficiency slightly higher than 0.5 M hydrochloric acid at 89%. Therefore, nitric acid is used as the subsequent desorbent.
PEI-PAC-AO was subjected to nine cycles of adsorption at 298 K. In order to maximize the release of adsorbed uranium ions while maintaining its original state, it was washed with nitric acid and filtered with distilled water each time. We calculated the uranium adsorption removal extent for each cycle. The results are shown in Figure 10b. It can be seen that PEI-PAC-AO showed a slightly larger decrease in the first two cycles. Based on the analysis of the morphology, it is speculated that the weak internal pore connectivity caused a shorter entry time of the eluent. However, as the number of cycles increased, the adsorption capacity decreased compared to the previous cycle, but slowly tended to stabilize, and the final adsorption effect was still around 80%. This proves that PEI-PAC-AO has stable adsorption performance and high reusability.
To analyze the adsorption process, the adsorption kinetics of PEI-PAC-AO were tested, and Figure S6 was obtained. It can be seen that PEI-PAC-AO has a larger initial slope compared to the adsorption curve, and about 80% of uranyl ions can be released within 200 min. After 400 min, desorption equilibrium is reached, and the elution extent reaches about 90%. Compared with other studies on the adsorption of amidoxime, PEI-PAC-AO has a higher level, and it is speculated that the reason may be due to the promoting effect of amino groups in polyethyleneimine. In summary, polymers can quickly resolve adsorbed uranium ions, which is beneficial for saving time and cost.

3.3. Possible Mechanisms

Using XPS to analyze the elements of functionalized polyethyleneimine samples before and after uranium adsorption, as shown in Figure 11a, it can be seen that there are three distinct peaks before and after adsorption, namely, C, N, and O elements, with values of 284 eV, 398 eV, and 525 eV, showing no significant changes. However, two characteristic peaks of U 4f appeared at 379.8 eV and 391.6 eV, indicating that PEI-PAC-AO had adsorbed uranium ions. Afterwards, the O 1s, N 1s, and U 4f electron spectra in PEI-PAC-AO were fitted and peak-separated, resulting in the fitting results shown in Figure 11b. In the U4f peak, two characteristic peaks at 379.8 eV and 391.6 eV were clearly observed, which were caused by the electron splitting of uranium at U 4f5/2 and U 4f7/2. In addition, the peak values of U (IV) are concentrated at 381.77 and 392.60 eV, while U (VI) is concentrated at 383.4 and 394.3 eV, indicating that PEI-PAC-AO reduces U (VI) to U (IV) for adsorption. To further investigate the adsorption mechanism of PEI-PAC-AO, peak separation was performed on O 1s and N 1s before and after adsorption, as shown in Figure 12. Through comparison, it was found that there were three significant changes in the peak binding energy before and after adsorption. In the O 1s peak, the binding energy shifted by 0.09 eV, from 531.44 eV to 531.53 eV. This is due to the lone pair electrons provided by the combination of uranium ions and hydroxyl oxygen in the amidoxime group, which changed the binding energy, while the peak at 531.1 eV did not change. In the N 1s peak, there are two peak shifts in the binding energy, from 399.3 eV to 399.41 eV and from 400.75 eV to 400.64 eV, respectively, due to the adsorption of uranium ions by ammonia in the amidoxime group and ammonia in the amino group, which affects the binding energy. At the same time, the peaks at 398.45 eV and 405.9 eV remain unchanged, indicating that U (VI) has interacted with the amidoxime group. From the XPS high-resolution spectra of N 1s and O 1s mentioned above, it can be seen that the binding energy between N and O in the amidoxime group shifts to higher positions, indicating a decrease in the electron cloud density around these atoms after adsorption of uranium ions. Therefore, the amidoxime group, as an electron donor, plays an important role in deepening the color of the adsorbent material after adsorption.

4. Conclusions

In summary, a novel adsorbent PEI-PAC-AO was successfully prepared via polyethyleneimine functionalization and cross-linking, with the aim of extracting and removing uranium from aqueous solutions. It shows a fine removal extent for uranium ions. The adsorption process fits the Langmuir model thermodynamically and the pseudo-second-order kinetic model kinetically, indicating monolayer and chemical adsorption. In ion-competition experiments, PEI-PAC-AO has a strong preference for U (VI) ions. Its removal extent is much higher than other ions, and the distribution coefficient of U (VI) ions is also higher, showing its excellent selective adsorption ability. After nine adsorption–desorption cycles, the removal efficiency is still above 80%, reflecting its good stability and reusability. From the XPS analysis of the adsorbent, before and after capturing uranyl ions, the adsorption is mainly due to the coordination between uranium and the N and O atoms of the amidoxime group. These results underscore the potential of PEI-PAC-AO as a sustainable, efficient, fast-acting, and cost-effective material for uranium removal, contributing to environmentally responsible wastewater treatment solutions and holding promise for the industrial application of uranium extraction from seawater.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/su17135953/s1. Figure S1: Images before and after adsorption and microscopy images of PEI-PAC-AO. Figure S2: XRD curve of PEI-PAC-AO. Figure S3: The effect of different raw material ratios on material adsorption efficiency. Figure S4: Effect of initial uranium concentration on the adsorption of U (VI) onto PEI-PAC-AO at different temperatures. Figure S5: Van’t Hoff plot between lnKd and 1/T. Figure S6: Desorption curve of PEI-PAC-AO in HNO3. Table S1: The original material quality added for the synthesis of PEI-PAC. Table S2: Dynamics fitting data. Table S3: Model constants and determinable coefficient values. Table S4: Thermodynamic parameter ΔG0, ΔH0, ΔS0. Table S5: The selectivity coefficient of the materia.

Author Contributions

Conceptualization, X.L.; Software, H.Y., L.H. and H.G.; Validation, X.L.; Investigation, L.H. and H.G.; Data curation, L.H., H.G. and X.L.; Writing—original draft, H.Y.; Writing—review & editing, J.B.; Supervision, J.B.; Funding acquisition, J.B. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Key Research and Development Plan Guided Project of Heilongjiang Province (GZ20230004), Harbin Science and Technology Plan Self-funded Project (2022ZCZJCG031), Key Project of Natural Science Foundation of Heilongjiang Province (ZD2021E005). And The APC was funded by [ZD2021E005].

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data supporting the findings of this study are available from the corresponding author upon reasonable request.

Acknowledgments

The authors thank the valuable comments of anonymous reviewers and editor.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Figure 1. Synthesis process of PEI-PAC-AO.
Figure 1. Synthesis process of PEI-PAC-AO.
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Figure 2. (a) FT-IR spectra of PEI, PEI-PAC, PC, and PA; (b) FT-IR spectra of PEI-PAC, PEI-PAC-RA, and PEI-PAC-AO.
Figure 2. (a) FT-IR spectra of PEI, PEI-PAC, PC, and PA; (b) FT-IR spectra of PEI-PAC, PEI-PAC-RA, and PEI-PAC-AO.
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Figure 3. Thermogravimetric curves of PEI-PAC-AO ((a) TGA curve; (b) DTG curve).
Figure 3. Thermogravimetric curves of PEI-PAC-AO ((a) TGA curve; (b) DTG curve).
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Figure 4. Contact angle curve of PEI-PAC-AO.
Figure 4. Contact angle curve of PEI-PAC-AO.
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Figure 5. Different solid–liquid ratios of uranium adsorption amount and removal extent of PEI-PAC-AO.
Figure 5. Different solid–liquid ratios of uranium adsorption amount and removal extent of PEI-PAC-AO.
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Figure 6. (a) Adsorption of PEI-PAC-AO at different pH; (b) the existence state of uranyl ion in different pH environments.
Figure 6. (a) Adsorption of PEI-PAC-AO at different pH; (b) the existence state of uranyl ion in different pH environments.
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Figure 7. (a) Influence of contact time on uranium adsorption properties of PEI-PAC-AO; (b) first-order kinetic model fitting curve; (c) second-order kinetic model fitting curve; (d) Morris–Weber simulation fitting curve.
Figure 7. (a) Influence of contact time on uranium adsorption properties of PEI-PAC-AO; (b) first-order kinetic model fitting curve; (c) second-order kinetic model fitting curve; (d) Morris–Weber simulation fitting curve.
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Figure 8. Thermodynamic fitting curve ((a) Langmuir model curve; (b) Freundlich model curve; (c) Temkin model curve; (d) D-R model curve).
Figure 8. Thermodynamic fitting curve ((a) Langmuir model curve; (b) Freundlich model curve; (c) Temkin model curve; (d) D-R model curve).
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Figure 9. (a) Coexistence ion competition experiment of PEI-PAC-AO adsorbent; (b) distribution coefficient.
Figure 9. (a) Coexistence ion competition experiment of PEI-PAC-AO adsorbent; (b) distribution coefficient.
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Figure 10. (a) Desorption efficiency under different desorption agents; (b) adsorption–desorption cycle experiment.
Figure 10. (a) Desorption efficiency under different desorption agents; (b) adsorption–desorption cycle experiment.
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Figure 11. (a) XPS full-scan spectra of before and after uranium adsorption; (b) high-resolution U4f of PEI-PAC-AO(U).
Figure 11. (a) XPS full-scan spectra of before and after uranium adsorption; (b) high-resolution U4f of PEI-PAC-AO(U).
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Figure 12. XPS absorption spectra of PEI-PAC-AO before and after adsorption ((a) O1s before uranium absorption; (b) O1s after uranium absorption; (c) N1s before uranium absorption; (d) N1s after uranium absorption).
Figure 12. XPS absorption spectra of PEI-PAC-AO before and after adsorption ((a) O1s before uranium absorption; (b) O1s after uranium absorption; (c) N1s before uranium absorption; (d) N1s after uranium absorption).
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Yan, H.; Huo, L.; Gao, H.; Li, X.; Bai, J. Functionalized Polyethyleneimine Adsorbent for Efficient and Selective Uranium Extraction from Aqueous Solution. Sustainability 2025, 17, 5953. https://doi.org/10.3390/su17135953

AMA Style

Yan H, Huo L, Gao H, Li X, Bai J. Functionalized Polyethyleneimine Adsorbent for Efficient and Selective Uranium Extraction from Aqueous Solution. Sustainability. 2025; 17(13):5953. https://doi.org/10.3390/su17135953

Chicago/Turabian Style

Yan, Huijun, Long Huo, Hong Gao, Xuanyi Li, and Jianwei Bai. 2025. "Functionalized Polyethyleneimine Adsorbent for Efficient and Selective Uranium Extraction from Aqueous Solution" Sustainability 17, no. 13: 5953. https://doi.org/10.3390/su17135953

APA Style

Yan, H., Huo, L., Gao, H., Li, X., & Bai, J. (2025). Functionalized Polyethyleneimine Adsorbent for Efficient and Selective Uranium Extraction from Aqueous Solution. Sustainability, 17(13), 5953. https://doi.org/10.3390/su17135953

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