High-Efficiency Uranium Adsorption from Real Salt-Lake Brine Using Amine-Functionalized Lignin Microspheres

Abstract

The exploit of an efficient method for uranium (U) extraction is crucial for the development of nuclear energy. In this study, an aminated lignin-based microsphere (AL-PEI/GMS) was synthesized and used as an adsorbent for the recovery of hexavalent uranium (U(VI)) from salt-lake brine. The effects of adsorbent dosage, initial solution pH value, interfering ions, adsorption time, and temperature on the U(VI) adsorption performance of AL-PEI/GMSs were systematically investigated. The results show that when the adsorbent dosage was 2 g/L, the temperature was 45 °C, and the pH was 8, the adsorption capacity of AL-PEI/GMS for U(VI) could reach 256.4 mg/g. In addition, after five cycles, a high U(VI) adsorption efficiency of over 90% could still be achieved. Furthermore, through a fixed-bed system, AL-PEI/GMS could rapidly adsorb U(VI) from actual salt-lake brine. Therefore, the prepared AL-PEI/GMS is a competitive alternative material compared with other adsorbents in terms of efficiently recovering U(VI) from actual salt-lake brine.

1. Introduction

Uranium (U) is the main resource of nuclear energy [1,2,3]. However, terrestrial U(VI) reserves are limited, and their extraction not only poses environmental risks but is also susceptible to geopolitical disruptions [2,3,4,5]. Many countries are experiencing a shortage of U(VI) reserves or difficulties in U(VI) mining. Therefore, novel U(VI) extraction methods have been widely concerned, such as U(VI) extraction from seawater, which utilizes adsorbents to recovering U(VI) from seawater [6,7]. However, the extremely low concentration of U(VI), approximately 3.3 ug/L, in seawater leads to high extraction costs and low efficiency, and the adsorption capacity of the adsorbents needs to be improved [8,9]. In contrast, the concentration of U(VI) in salt-lake brines is relatively high, reaching several tens to hundreds of micrograms per liter [10]. Therefore, extracting U(VI) from salt-lake brine is a promising method used to ease the shortage of U(VI) resources, thereby promoting the development of a circular economy [11].

U(VI) in salt-lake brines mainly exists as uranyl carbonate complexes and coexists with various metal ions, resulting in complex speciation. This complexity presents significant challenges for extraction and underscores the urgent need to develop novel and efficient extraction methods [12]. The primary methods for U(VI) extraction from salt lakes include chemical leaching [13], electrochemical techniques [14], photocatalysis [15], and adsorption [16]. Among these, adsorption arises, increasing researchers’ attentions due to its advantages of low cost, operational simplicity, and environmental friendliness. Currently, biochar, metal nanoparticles, and covalent organic framework polymers have been widely researched for U(VI) adsorption. However, these adsorbents exhibit significant limitations in practical applications, particularly in fixed-bed systems. For instance, they are susceptible to interference from other ions in complex brine systems and prone to fragmentation and loss during prolonged adsorption, and they require large amounts of chemical reagents for desorption, which often leads to incomplete recovery. These issues severely restrict the efficiency of U(VI) resource recovery [17,18,19]. Recently, modifying the structure of U(VI) adsorbents has been widely researched. Researchers have synthesized new covalent organic framework (COF) materials, such as TzDva-COOH and TzDva-NH₂. These materials are modified with grafting carboxyl (-COOH) and amino (-NH₂) groups into a TzDva framework, thereby achieving effective adsorption of U(VI). These two materials exhibit adsorption capacities for U(VI) with 139.5 mg/g and 88.3 mg/g, respectively. In addition, high selectivity and good tolerance in salt environment have also been demonstrated. Therefore, this study provides theoretical guidance for the development of U(VI) recovery technologies based on covalent organic frameworks (COFs) [20].

Lignin, as the second most abundant regeneration biomass material in the plant kingdom [21], consists of a three-dimensional network structure formed by phenol hydroxyl, amino, and other functional groups. However, due to structural and other limitations, it requires chemical modification to improve its adsorption efficiency for U(VI) [22,23]. Therefore, this study employed aminated lignin-based microspheres (AL-PEI/GMS) to investigate their application in U(VI) recovery from salt-lake brine. To evaluate the U(VI) adsorption performance of AL-PEI/GMS, abundant experiments were carried out under various conditions, including solution pH values, the presence of interfering ions (such as Na⁺, Ca²⁺, Mg²⁺, and F⁻), and adsorption time. Furthermore, the regeneration performance of the material was investigated, and its U(VI) recovery efficiency was evaluated using actual salt-lake brine and a fixed-bed system, which is crucial for practical applications. This study investigates the changes in the uranium adsorption performance of AL-PEI/GMS under a weakly alkaline, real, and complex environment of salt-lake brine. Furthermore, dynamic adsorption experiments using a fixed-bed system were conducted to provide an additional option for the industrial application of uranium extraction from salt lakes. This study aligns with the principles of green chemistry under the “dual carbon” goals, facilitating not only the high-value utilization of lignin but also the sustainable development of the nuclear energy industry.

2. Experiments

2.1. Experimental Instruments and Reagents

The details are described in S1 of Supplementary Information [24].

2.2. Preparation and Characterization of AL-PEI/GMS

A total of 0.75 g of alkali lignin (Shanghai, China), 0.225 g of sodium alginate (Chengdu, China), 0.15 g of polyethylene glycol (Mw = 4000) (Chengdu, China), 3 g of polyethyleneimine (Mw = 600) (Shanghai, China), and 0.3 g of sodium dodecylbenzene sulfonate (Chengdu, China) were dissolved in 15 mL of deionized water and stirred at a constant temperature of 60 °C for 60 min. Then, 120 mL of liquid paraffin (Chengdu, China) was added to the mixture to initiate an emulsification reaction. After 30 min of emulsification, 3.5 mL of epichlorohydrin (Chengdu, China) was added for crosslinking, and the reaction was continued at 60 °C for another 120 min. The resulting microspheres were soaked in acetone (Chengdu, China) and repeatedly washed with anhydrous ethanol (Chengdu, China) until the washing solution became clear and transparent. Finally, the product was dried at 45 °C to obtain the aminated lignin-based microsphere adsorbent, AL-PEI/GMS [24]. Figure 1 is a schematic diagram of the preparation process of AL-PEI/GMS.

The surface element distribution was estimated with the EDS elemental mapping method. The morphology was observed using a scanning electron microscope (SEM) (TESCAN Group, a. s., Brno, Czech Republic). Fourier transform infrared (FT-IR) spectra were collected with a Fourier transform infrared spectrometer (Thermo Fisher Scientific Inc., Madison, WI, USA). Structural properties were determined by nitrogen adsorption using a specific surface area analyzer (Micromeritics Instrument Corporation, Norcross, GA, USA). X-ray photoelectron spectroscopy (XPS) spectra were acquired with an Axis Ultra DLD spectrometer (Kratos Analytical Ltd., Manchester, UK) [24].

2.3. Adsorption Experiments

U(VI) solution was placed in a 100 mL conical flask, and then AL-PEI/GMS and AL were added, respectively, and mixed thoroughly. The mixture was then shaken at 180 r/min for 2 h. At 20 min intervals, 8 to 10 mL of the solution was sampled. All samples were filtered through a 0.45 μm membrane, and the concentration of U(VI) in the solution was determined by inductively coupled plasma–optical emission spectrometry (ICP-OES). The adsorption efficiency and adsorption capacity were calculated using Equations (1) and (2).

$${\mathrm{Q}}_{\mathrm{e}}={\displaystyle \frac{({\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{e}})\mathrm{V}}{\mathrm{m}}}$$

(1)

$$\mathrm{R}={\displaystyle \frac{{\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{C}}_0}}\times 100\mathrm{\%}$$

(2)

where Q_e—adsorption capacity at equilibrium (mg/g); C_e—solution U(VI) concentration at equilibrium (mg/L); C₀—solution U(VI) concentration at the initial time (mg/L); m—mass of adsorbent (mg); and V—volume of U(VI) solution at the time of adsorption (mL).

Desorption reagent was ultimately selected for the U(VI) desorption experiment as reported in our previous study [24]. U(VI) solution was mixed with adsorbent AL-PEI/GMS, and after a 2 h adsorption reaction, the adsorbent was filtered and dried. The saturated adsorbent was then mixed with 1 M desorption reagent solution, and the desorption reaction was carried out in a shaker at 180 r/min for 24 h. The desorption efficiency was calculated according to Equation (3).

$$\mathrm{R}={\displaystyle \frac{{\mathrm{C}}_2}{{\mathrm{C}}_0-{\mathrm{C}}_1}}\times \mathrm{100\%}$$

(3)

where C₀—concentration of solution U(VI) at initial time (mg/L); C₁—concentration of solution U(VI) after adsorption (mg/L); and C₂—concentration of solution U(VI) after desorption (mg/L).

2.4. U(VI) Adsorption via Fix-Bed System

Fixed-bed column experiments were performed using a chromatography column with a bed height of 30 cm and an inner diameter of 1.0 cm, and the fixed bed was filled with 4 g of AL-PEI/GMS. Actual salt-lake brine, collected from Qinghai Yanhu Industry Co., Ltd., was pumped through a peristaltic pump into the column at a flow rate of 0.5 mL/min, and then the effluent from the column was collected in aliquots of 8 mL, and the concentration of U(VI) was determined to calculate the adsorption efficiency and adsorption capacity.

3. Results and Discussion

3.1. Exploration of U(VI) Adsorption Performance of AL-PEI/GMS

AL-PEI/GMS exhibits high U(VI) adsorption efficiency with 92.13%, which is much higher than that of AL (42.66%), indicating the promoting effect of the amination process for lignin on U(VI) adsorption. The effect of adsorbent dosage is shown in Figure 2A, and it illustrates that U(VI) adsorption by AL-PEI/GMS is accelerated by larger adsorbent dosage. This is attributed to the increase in active sites enhancing adsorption efficiency. Moreover, the improved adsorption efficiency can boost treatment efficiency, reduce equipment costs, drive its transition from laboratory research to large-scale application, and is particularly suitable for meeting various complex needs in industrial scenarios [25,26]. Noticeably, a high U(VI) adsorption capacity (2.26 mg/g) and efficiency (88.41%) are obtained at 2 g/L of AL-PEI/GMS, and the adsorption capacity decreased sharply as the adsorbent dosage increased from 2 to 10 g/L. Therefore, 2 g/L of AL-PEL/GMS was used in the following experiments.

Table 1. The concentrations of major ions in salt-lake brine samples.

Major Ions	Concentration (μg/L)
Mg2+	6.41 × 107
Ca2+	1.69 × 106
K+	1.67 × 106
UO22+	165

presents the concentrations of major interfering ions in the salt-lake brine sample. As shown in the table, the concentrations of Mg²⁺, Ca²⁺, and K⁺ are much higher than that of UO₂²⁺. Additionally, the presence of F⁻ can also interfere with the adsorption process. Therefore, it is necessary to explore the interference effect of these ions. Figure 2B describes the effect of interferential ions. When the concentrations of Mg²⁺, Ca²⁺, K⁺, and F⁻ increased from 2.5 g/L to 50 g/L, the adsorption efficiency of AL-PEI/GMS for U(VI) was slight decreased. This is because these ions may compete with UO₂²⁺ for active sites, and F⁻ can combine with uranyl ions to form stable complexes, both of which lead to a decrease in the adsorption efficiency of uranyl ions [27]. Although F⁻ exhibited the most intense impact, the adsorption efficiency can still approach 70%. Compared with the adsorption efficiency of 92.13% when no interfering ions are added, the decrease in adsorption efficiency is slight. This indicates that AL-PEI/GMS can tolerate the presence of high concentrations of interfering ions in salt-lake brine, demonstrating strong adaptability. As shown in Figure 2C,D, the adsorption capacity and separation factor (SF_U(VI)) of AL-PEI/GMS for three types of metal ions and UO₂²⁺ are presented. It can be concluded that the order of its anti-interference ability against the three metal ions is Mg²⁺ < K⁺ < Ca²⁺. Furthermore, the SF_U(VI) values between UO₂²⁺ and each of the three ions all exceed 150,000, which demonstrates that AL-PEI/GMS exhibits high anti-interference ability against these three metal ions and can be applied in the high-salinity environments of salt lakes.

As shown in Figure 3A, the effect of the reaction time was explored. In the first 120 min, the adsorption capacity of AL-PEI/GMS increased rapidly, and negligible increases were observed as the reaction time further increased, indicating that a saturated adsorption capacity can be obtained within 120 min. As shown in Figure 3B,C, the adsorption processes of the two adsorbents were analyzed using the pseudo-first-order and pseudo-second-order kinetic model (Equations (4) and (5)). The fitted parameters are presented in

Table 2. The kinetic fitting parameters.

Adsorbent	Pseudo-First Order			Pseudo-Second Order
	q1/(mg·g−1)	k1/min−1	R2	q2/(mg·g−1)	k2/g·mg−1·min−1	R2
AL-PEI/GMS	1.59	0.0539	0.8781	2.71	0.0295	0.9914
AL	0.67	0.0552	0.8876	1.21	0.0717	0.9894

. The correlation coefficients of the pseudo-second-order kinetics for the two adsorbents are higher than those of the pseudo-first-order kinetics, indicating that the adsorption process is more consistent with the pseudo-second-order kinetic model [28].

The Webber–Morris kinetic model (Equation (6)) was used to further compare the adsorption performance of AL-PEI/GMS and AL, respectively, and both of the adsorbents exhibit a three-stage adsorption process. As presented in

Table 3. The Webber–Morris fitting parameters.

Adsorbent	k1/(mg·g−1·min−2)	k2/(mg·g−1·min−2)	k3/(mg·g−1·min−2)
AL-PEI/GMS	0.2958	0.0867	0.0013
AL	0.1458	0.0543	0.0033

, the slope values of the three stages follow the order k₁ > k₂ > k₃, which exactly confirms the adsorption kinetic character, which gradually decreased during the adsorption process. This result is due to the more and more active sites occupied by uranium when the rection time is prolonged. Furthermore, both k₁ and k₂ values of AL-PEI/GMS are higher than those of AL, indicating that AL-PEI/GMS outperforms AL in terms of adsorption rate and capacity during the surface diffusion and mesopore diffusion stages [29].

$$\ln \left({\mathrm{Q}}_{\mathrm{e}}-{\mathrm{Q}}_{\mathrm{t}}\right)=\mathrm{l}\mathrm{n}{\mathrm{Q}}_{\mathrm{e}}-{\mathrm{k}}_1\mathrm{t}$$

(4)

$${\displaystyle \frac{\mathrm{t}}{{\mathrm{Q}}_{\mathrm{t}}}}={\displaystyle \frac{1}{{\mathrm{k}}_2{\mathrm{Q}}_{\mathrm{e}}^2}}+{\displaystyle \frac{\mathrm{t}}{{\mathrm{Q}}_{\mathrm{e}}}}$$

(5)

$${\mathrm{Q}}_{\mathrm{t}}={\mathrm{k}}_{\mathrm{p}}{\mathrm{t}}^{{\displaystyle \frac{1}{2}}}+\mathrm{C}$$

(6)

where Q_e—adsorption capacity at equilibrium (mg/g); Q_t—adsorption capacity at moment t (mg/g); t—adsorption reaction time (min); k₁—quasi-secondary kinetic rate constant (min⁻¹); k₂—quasi-secondary kinetic rate constant (g/(mg·min)); k_p—the Webber–Morris kinetic rate constant (mg/(g·min^−1/2)); and C—constant.

As shown in Figure 4A, the effect of pH variation in the U(VI) solution on the AL-PEI/GMS adsorption efficiency can be divided into three stages. As the pH increased from 2 to 8, the adsorption efficiency was rapidly enhanced from 27.32% to 93.55%. However, the adsorption efficiency abruptly decreased to 87.55% as the solution pH further increased to 10. Figure 4B shows that the isoelectric point of the AL-PEI/GMS is 5.4. When pH < 5.4, the surface of AL-PEI/GMS is positively charged. Figure 4C (calculated by Visual MINTEQ) indicates the variation in U(VI) species with pH variation. When pH < 8, the dominant species are positive (UO₂²⁺, (UO₂)₃(OH)₅⁺, and (UO₂)₄(OH)₇⁺). As the solution pH increases to 11, the electricity of dominant species becomes negative (UO₂)₃(OH)₇⁻ and UO₂ (OH)₃⁻). Therefore, when the solution pH is lower than 5.4, the low adsorption efficiencies of AL-PEI/GMS result from the static repulsion between the positive surface of the AL-PEI/GMS and U(VI) species. In the range from 5.4 to 8, AL-PEI/GMS with a negative surface strongly attracts U(VI) species (UO₂²⁺, (UO₂)₃(OH)₅⁺, and (UO₂)₄(OH)₇⁺) [30], leading to a high adsorption efficiency. In addition, the decrease in the adsorption efficiencies, as the solution pH becomes higher than 8, is due to the static repulsion between the negative surface of the AL-PEI/GMS and U(VI) species. However, AL-PEI/GMS still exhibits excellent adsorption performance within the pH range of 4 to 10, which is well-suited to the actual conditions of U(VI) extraction from salt lakes, indicating that AL-PEI/GMS holds great potential for recovering U(VI) from salt-lake brine.

As shown in Figure 4D, at a given temperature, the adsorption capacity of AL-PEI/GMS for U(VI) first increases with the rise in initial concentration and then gradually approaches equilibrium. Specifically, when the reaction temperature is 45 °C, the adsorption capacity increases from 4.48 mg/g to 165.57 mg/g as the initial concentration of U(VI) increased from 10 to 500 mg/L. No obvious increase in the adsorption capacity was observed when the initial concentration of U(VI) increases further. In addition, the significant effect of the adsorption temperature on uranium adsorption indicates that the adsorption of U(VI) by AL-PEI/GMS may be related to chemical adsorption.

According to the above results, the Langmuir and Freundlich isotherm model was fitted based on Equations (7) and (8), as shown in Figure 4E,F. As can be seen, the Langmuir isotherm model exhibits higher R² for the adsorption process of U(VI), indicating that the adsorption mainly proceeds through a homogeneous monolayer process [31]. Further, the theoretical maximum adsorption capacities (Q_m) were 48.18, 84.29, and 256.4 mg/g, and the corresponding equilibrium constants (K_L) were 4.15 × 10⁻⁴, 5.66 × 10⁻⁴, and 5.81 × 10⁻⁴ L/mg, at the adsorption temperatures of 25 °C, 35 °C, and 45 °C, respectively.

$${\mathrm{Q}}_{\mathrm{e}}={\displaystyle \frac{{\mathrm{Q}}_{\mathrm{m}}{\mathrm{K}}_{\mathrm{L}}{\mathrm{C}}_{\mathrm{e}}}{1+{\mathrm{K}}_{\mathrm{L}}{\mathrm{C}}_{\mathrm{e}}}}$$

(7)

$${\mathrm{Q}}_{\mathrm{e}}={\mathrm{K}}_{\mathrm{F}}{\mathrm{C}}_{\mathrm{e}}^{\mathrm{n}}$$

(8)

where Q_e—adsorption capacity at equilibrium (mg/g); K_L—Langmuir model equilibrium constant (L/mg); Q_m—maximum adsorption capacity for saturated adsorption of adsorbent monolayer (mg/g); C_e—solution U(VI) concentration at equilibrium (mg/L); K_F—Freundlich model equilibrium constant ((mg/g)·(mg/L)^−1/n); and n—Freundlich adsorption index.

To further clarify the effect of reaction temperature on the adsorption process, the equilibrium constant K_L obtained from the Langmuir model was converted into the dimensionless constant K⁰, and the thermodynamic parameters for the adsorption of U(VI) onto AL-PEI/GMS were calculated using Equations (9) and (10) [32]. According to the calculations, at environmental temperatures of 25 °C, 35 °C, and 45 °C, the ΔG⁰ values for the adsorption of U(VI) by AL-PEI/GMS are −11.39, −12.56, and −13.04 kJ/mol, respectively. The ΔH⁰ and ΔS⁰ values for the adsorption process are 13.38 kJ/mol and 83.44 J/(mol·K), respectively. The above results indicate that the adsorption of U(VI) by AL-PEI/GMS is a spontaneous, endothermic, and entropy-increasing process [32].

$${∆}_{\mathrm{r}}{\mathrm{G}}_{\mathrm{m}}^0=-\mathrm{R}\mathrm{T}\mathrm{l}\mathrm{n}{\mathrm{K}}^0$$

(9)

$$\mathrm{l}\mathrm{n}{\mathrm{K}}^0=-{\displaystyle \frac{{∆}_{\mathrm{r}}{\mathrm{H}}_{\mathrm{m}}^0}{\mathrm{R}\mathrm{T}}}+{\displaystyle \frac{{∆}_{\mathrm{r}}{\mathrm{S}}_{\mathrm{m}}^0}{\mathrm{R}}}$$

(10)

where Δ_rG_m⁰—molar Gibbs free energy change in the reaction in standard state (kJ/mol); Δ_rH_m⁰—molar enthalpy change in the reaction in standard state (kJ/mol); Δ_rS_m⁰—molar entropy change in the reaction in standard state (J/(mol·K)); K⁰—standard dimensionless equilibrium constant or thermodynamic equilibrium constant; R—gas constant (8.314 J/(mol·K)); and T—reaction temperature (K).

The comparison of the adsorption capacity values (from Langmuir data) between AL-PEI/GMS and other adsorbent materials is presented in

Table 4. The comparison of material properties.

Material	pH	Temperature/K	Time/min	Qe/mg·g−1	Ref.
Carbon nanotubes	5	298	160	150	[16]
Biochar composites	4	318	1440	52.63	[17]
Co0.5Mn0.5Fe2O4	6	298	60	104	[18]
COF-TpTHA/CNF	6	298	300	177.9	[19]
TzDva-COOH	6	318	240	139.5	[20]
TzDva-NH2	8	318	240	88.3	[20]
AL-PEI/GMS	8	318	120	256.4	This work

. It can be found that AL-PEI/GMS exhibits superior adsorption capacity and a relatively faster adsorption rate, thus demonstrating certain application potential.

3.2. Industrial Application of AL-PEI/GMS

The desorption results are displayed in Figure 5A, and it can be seen that the desorption efficiency of AL-PEI/GMS reached 96.46% with Na₂CO₃ solution, which was significantly higher than that of H₂SO₄, HNO₃, and NaOH solutions: 41.32%, 49.79%, and 62.57%. This is because the complexation stability constant of CO₃²⁻ is significantly higher than that of OH⁻ and SO₄²⁻. Meanwhile, the weakly alkaline environment of the sodium carbonate solution balances the deprotonation state of amino groups and the dissolution stability of UO₂²⁺. Furthermore, the sodium carbonate solution causes less damage to the lignin structure, which ensures the reusability of the adsorbent [33]. Therefore, Na₂CO₃ solution was used as the eluent for the subsequent cycling experiments. The results of regeneration are shown in Figure 5B. As illustrated, the adsorption efficiency of AL-PEI/GMS decreased from 93.00% to 90.26% after five cycles, and the desorption efficiency also decreased from 95.62% to 88.93%. The above results indicate that AL-PEI/GMS is highly reusable.

To evaluate the adsorption performance of AL-PEI/GMS under practical application conditions, brine from an actual salt lake was used as the U(VI) solution. As shown in Figure 5C, the adsorption efficiency of AL-PEI/GMS for U(VI) reached 85.38%, which is significantly higher than that of AL (30.99%), further demonstrating the strong practical applicability of the adsorbent. The introduction of amino groups highly enhances the coordination capacity and anti-interference ability of active sites and effectively optimizes the surface charge of the adsorbent material to endow it with a wide pH adaptability. In addition, micro-spherization improves the mass transfer efficiency of the adsorbent [34]. The cyclic experiments were conducted again on salt-lake brine, with the results shown in Figure 5D. The adsorption efficiency of AL-PEI/GMS for U(VI) decreased from 86.55% to 79.00%, while its desorption efficiency decreased from 90.22% to 85.89%. This indicates that AL-PEI/GMS still possesses high regeneration capacity in salt-lake brine, which can help reduce costs in industrial applications. The results of the fixed-bed adsorption experiment are shown in Figure 5E. As shown, abundant feedstock streams can be effectively treated after a 5036 min reaction, indicating promising potential of AL-PEI/GMS for uranium recovery in fixed-bed system [35]. To further investigate the dynamic adsorption process, the Thomas model was applied to fit the breakthrough curves of the dynamic experiments [35]. As shown in Figure 5F, the correlation coefficient R² was greater than 0.94. According to Equation (11), the theoretical adsorption capacity is relatively high (187.2 mg/g), demonstrating the high efficiency of the AL-PEI/GMS for U(VI) extraction from actual salt-lake brine.

$$\mathrm{l}\mathrm{n}({\displaystyle \frac{{\mathrm{C}}_0}{{\mathrm{C}}_{\mathrm{t}}}}-1)=-{\mathrm{k}}_1{\mathrm{C}}_0\mathrm{t}+{\displaystyle \frac{{\mathrm{k}}_1{\mathrm{q}}_{\mathrm{c}\mathrm{a}\mathrm{l}}\mathrm{m}}{\mathrm{Q}}}$$

(11)

where C₀—initial mass concentration of U(VI) at the time of water intake (mg/L); C_t—mass concentration of U(VI) at the time of water discharge (kJ/mol); k₁—rate constant of Thomas model (mL/(min·mg)); q_cal—adsorption capacity at equilibrium (mg/g); m—mass of adsorbent (g); Q—inlet water flow rate (mL/min); and t—inlet time (min).

The interaction mechanism between the main functional groups of AL-PEI/GMS and U(VI) is shown in Figure 6. The main active groups of AL-PEI/GMS for capturing U(VI) are amino groups introduced on its surface. Essentially, the N atoms of the amino groups combine with the central U atom of UO₂²⁺ to form coordinate bonds. If there are sufficient amino groups, multiple amino groups can synergistically coordinate with a single UO₂²⁺ to form a chelate, which further enhances the binding stability [36].

4. Conclusions

In this study, an aminated lignin-based microsphere adsorbent (AL-PEI/GMS) was used. The results show that AL-PEI/GMS can effectively recover U(VI) from salt-lake brine. The research results indicate that the amino groups in AL-PEI/GMS are responsible for the efficient uranium adsorption performance through an electron attraction process. Batch adsorption experiments revealed that AL-PEI/GMS has a fast adsorption rate for U(VI), which conforms to the pseudo-second-order kinetic model and is suitable for efficient and rapid adsorption processes. This material is applicable under neutral and alkaline conditions (pH = 8.0), and high concentrations of common ions in salt-lake brine have a minimal impact on it, making it suitable for the complex environment of uranium extraction from salt-lake water. Furthermore, the Langmuir isotherm model and thermodynamic data indicated that the adsorption of U(VI) onto AL-PEI/GMS was a spontaneous, endothermic process with increased entropy and monolayer adsorption characteristics. Finally, dynamic adsorption experiments were conducted under fixed-bed conditions, and the results confirm that this adsorbent has good U(VI) removal efficiency. The above experimental results demonstrate that the synthesized AL-PEI/GMS is an adsorbent for U(VI) with great application prospects in actual salt-lake environments. In addition, it is easy to regenerate and reuse, showing great potential in practical applications. However, the limitation of this study is that the preparation conditions of AL-PEI/GMS are relatively harsh, and it is unclear whether it can be produced and used on a large scale. In subsequent studies, the material preparation conditions need to be optimized.