Enhanced Selective Adsorption of Rare Earth Ions with Ion-Imprinted Poly(hydroxamic acid) Interpenetrating Polymer Networks: Fabrication, Performance, and Mechanisms

Abstract

The separation of rare earth elements (REEs) is challenging due to their similar chemical properties. This study developed a series of novel polystyrene–ion-imprinted poly(hydroxamic acid) interpenetrating polymer networks (PS-IIPHAs) for the highly selective adsorption of La³⁺, Ce³⁺, and Y³⁺. The effects of the solution pH, contact time, initial concentrations, and temperature on the adsorption performance of the resins were systematically investigated. The results showed that adsorption equilibrium was reached within 4 h at a pH of 1.0, following the Langmuir isotherm, with maximum adsorption capacities of 2.425, 3.012, and 2.927 mmol/g for La³⁺, Ce³⁺, and Y³⁺, respectively. The resins exhibited excellent selectivity toward the template ions, with separation factors of 35.45 for Ce³⁺-La³⁺, 17.52 for Y³⁺-La³⁺, and 11.04 for Ce³⁺-Y³⁺. These results indicate PS-IIPHAs as promising adsorbents for the efficient, highly selective recovery of REEs.

1. Introduction

Rare earth elements (REEs) hold irreplaceable strategic significance in high-tech fields, such as new energy, electronic information, permanent magnet materials, catalysis, and environmental protection, due to their unique electronic structures and excellent physicochemical properties [1,2]. With the ongoing growth in demand for rare earth resources, their efficient separation and recovery have become critical topics in resource recycling and environmental governance. However, rare earth ions typically coexist and exhibit similar physicochemical characteristics, particularly their predominant trivalent oxidation states and comparable ionic radii, making their separation extremely challenging and costly [3,4]. Therefore, the development of novel functional materials with high selectivity is of great significance for the efficient separation of rare earth resources.

Current separation methods for REEs mainly include ion exchange and adsorption [5,6,7], solvent extraction [8,9], electrochemical separation [10], membrane separation [11,12], and chemical precipitation [12,13,14]. Among these, the ion exchange and adsorption method has demonstrated significant potential for REE separation, due to its operational simplicity, relatively low cost, and environmental friendliness [3,15]. It is currently the most effective approach, capable of separating all REEs [16]. The core efficacy of this method is highly dependent on the ion exchange and adsorbent materials employed. Ion exchange resins possess a three-dimensional cross-linked structure and specific functional groups, enabling the selective adsorption of rare earth ions through ion exchange or coordination interactions [17,18]. Alakhras [19] synthesized poly(amidoxime-hydroxamic acid) resins and investigated their kinetic performance for the removal of La³⁺, Nd³⁺, Sm³⁺, Gd³⁺, and Tb³⁺ ions from aqueous solutions. The study revealed that the adsorption process followed pseudo-second-order kinetics with correlation coefficients (R²) extremely close to unity, indicating that chemisorption is the rate-determining step. Furthermore, intraparticle diffusion analysis suggested that the sorption process is influenced by multiple steps, and the diffusion rate constants increased with the decreasing ionic radius of the lanthanide ions, following the order Tb³⁺ > Gd³⁺ > Sm³⁺ > Nd³⁺ > La³⁺. Iskander et al. [20] found that when hydroxamic acid monomers were directly polymerized, chain-transfer reactions of hydroxamic acid groups occurred during free-radical polymerization, resulting in only 29–32% of the hydroxamic acid groups retaining metal-complexation activity. Simultaneously, the polymers developed significant branching and cross-linking structures, severely constraining the adsorption capacity and mass transfer efficiency. Cao et al. [21,22] assembled polystyrene (PS) with poly(hydroxamic acid) (PHA) to prepare a novel polystyrene-poly(hydroxamic acid) (PS-PHA) copolymer and a novel PS-PHA interpenetrating polymer network (IPN). The PS-PHA copolymer achieved maximum adsorption capacities of 2.183, 2.592, and 2.884 mmol/g for La³⁺, Ce³⁺, and Y³⁺, respectively, while the PS-PHA IPN showed corresponding capacities of 2.013, 2.434, and 2.684 mmol/g for La³⁺, Ce³⁺, and Y³⁺, respectively. Using Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS), they confirmed that the nitrogen and oxygen atoms in hydroxamic acid groups (–C(=O)NHOH) can coordinate to rare earth ions, resulting in stable chemical adsorption. These results indicate that hydroxamic acid groups exhibit strong binding affinity for rare earth ions and represent a promising choice for constructing efficient rare earth adsorbents.

To overcome the limitations, ion-imprinted technology and IPN technology have been introduced into adsorbent design. Ion-imprinted technology allows for the construction of three-dimensional cavities within the polymer matrix that are highly matched to template ions (such as La³⁺, Ce³⁺, Y³⁺) in terms of the spatial dimensions and functional group arrangement, thereby endowing the adsorbent with exceptionally high selective recognition capability for the target ions [23,24]. IPN technology, through the mutual interpenetration and entanglement of two or more polymer networks, can significantly enhance the mechanical strength, chemical stability, and pore structure of materials, providing an effective approach to address issues such as the high mass-transfer resistance and poor accessibility of active sites [25,26]. The synergistic application of these two technologies is expected to yield novel adsorbents that combine high selectivity, high adsorption capacity, and rapid adsorption kinetics.

In this study, a series of polystyrene–ion-imprinted poly(hydroxamic acid) interpenetrating polymer networks (PS-IIPHAs) were proposed and fabricated. Using macroporous polystyrene resin as the skeleton and employing ion-imprinted technology with rare earth ions (La³⁺, Ce³⁺, and Y³⁺) as templates, functional monomers containing hydroxamic acid groups were introduced and immobilized within the interpenetrating networks. This study systematically investigated the synthesis process of the resins and analyzed their physicochemical properties through various characterization techniques. The adsorption performance and mechanism with respect to rare earth ions were thoroughly explored, providing new material foundations and technical references for the green and efficient separation of rare earth resources.

2. Materials and Methods

2.1. Materials and Chemicals

All chemical reagents used in this research were of analytical grade and procured from Aladdin Industrial Corporation in Shanghai, China. Benzoyl peroxide (BPO) was purified via recrystallization before use. Stock solutions of rare earth metal ions were prepared individually by dissolving lanthanum nitrate hexahydrate (La(NO₃)₃·6H₂O), cerium nitrate hexahydrate (Ce(NO₃)₃·6H₂O), and yttrium nitrate hexahydrate (Y(NO₃)₃·6H₂O) in deionized water. Then, 1 mol/L nitric acid (HNO₃) was used to adjust the pH of the solutions.

2.2. Experimental Methods

2.2.1. Preparation of PS-IIPHAs

Polystyrene and allyl hydroxamic acid were synthesized according to the methods previously reported by our group [21,22]. As illustrated in Scheme 1, an excess of La(NO₃)₃·6H₂O (8.66 g), Ce(NO₃)₃·6H₂O (8.68 g), or Y(NO₃)₃·6H₂O (7.66 g) was added separately to allyl hydroxamic acid (5 g) in a small amount of distilled water. The mixture was stirred at room temperature under mildly acidic conditions (pH < 5.0) for 4 h to yield ion-imprinted allyl hydroxamic acid. Subsequently, polystyrene was swollen in the ion-imprinted allyl hydroxamic acid solution for 12 h (mass ratio of polystyrene to ion-imprinted allyl hydroxamic acid = 1:1). After filtering, the polystyrene resin containing ion-imprinted allyl hydroxamic acid was transferred into a 50 mL aqueous solution comprising 0.8 g divinylbenzene (DVB), 0.5 g BPO, and 0.1 g polyvinyl alcohol (PVA). The mixture was heated to 80 °C and stirred uniformly for 2 h, then heated to 85 °C for 4 h, and finally cured at 90 °C for 2 h to obtain pale-yellow uniform spheres. The resulting spheres were washed with water until neutral and then shaken twice in 1.6 mol/L HNO₃ at 30 °C for 6 h to remove the template ions. After filtration, the product was vacuum-dried to yield an ion-imprinted resin. The resins imprinted with La³⁺, Ce³⁺, and Y³⁺ as template ions are designated as PS-La-IIPHA, PS-Ce-IIPHA, and PS-Y-IIPHA, respectively. For the subsequent characterization and adsorption experiments, the resins were mechanically ground to a particle size of 150–250 μm.

2.2.2. Adsorption and Desorption Experiments

To evaluate the performance of the as-prepared PS-IIPHAs, batch adsorption experiments were conducted for La³⁺, Ce³⁺, and Y³⁺. The pre-swollen resin (containing 0.2 g dry resin) and the metal ion solution (50.0 mL) were mixed in a 100 mL conical flask and shaken at a temperature (T) for an appropriate contact time (t). The mixture was filtered, and the residual metal ion concentration in the filtrate was determined using a UV–Vis spectrophotometer (WFZ UV-2100, Unico (Shanghai) Instrument Co., Ltd., Shanghai, China) via a colorimetric method with specific chromogenic reagents for each rare earth ion, since the rare earth ion solutions are colorless and lack characteristic absorption bands. Calibration curves were pre-plotted using a series of standard rare earth ion solutions to ensure quantitative accuracy. Specifically, La³⁺ reacts with arsenazo III to form a blue complex under weakly acidic conditions (pH = 6.0–7.0), which was then colorimetrically measured at 656 nm; Ce³⁺ forms a blue complex with tribromoarsenazo under acidic conditions (pH = 1.0), and the determination was performed at 635 nm; Y³⁺ reacts with arsenazo III to generate a blue complex under acidic conditions (pH = 1.0–2.0), with the colorimetric measurement carried out at 651 nm. Each experiment was repeated at least three times to ensure measurement accuracy. The adsorption capacity of the resin was calculated according to Equation (1) [27,28].

$$Q={\displaystyle \frac{\left(C_0-C\right)V}{m}}$$

(1)

where Q (mmol/g) represents the adsorption capacity; C₀ (mol/L) and C (mol/L) are the concentrations of rare earth metal ions at the initial time and at time t, respectively; V (L) is the volume of the experimental solution; and m (g) is the mass of dry resin.

To evaluate the preferential recognition of the imprinted cavities, binary competitive adsorption experiments were carried out. Here, 0.2 g of resin was equilibrated with 50.0 mL of a binary mixture of La³⁺-Ce³⁺, Ce³⁺-Y³⁺, or La³⁺-Y³⁺ (each ion 0.01 mol/L, pH = 1.0) at 40 °C for 4 h. The concentrations of both ions in the residual solution were determined using Atomic Absorption Spectrometry (AAS, WFX-200, Beifen-Ruili Analytical Instrument Co., Ltd., Beijing, China), and the selectivity coefficient (K) of the imprinted ion (ion 1) versus the competing ion (ion 2) was calculated according to Equation (2) [29].

$$K={\displaystyle \frac{Q_{\mathrm{e}1}/C_{\mathrm{e}1}}{Q_{\mathrm{e}2}/C_{\mathrm{e}2}}}$$

(2)

where Q_e1 and Q_e2 (mmol/g) represent the equilibrium adsorption capacities of the imprinted ion and the competing ion, respectively; C_e1 and C_e2 (mol/L) are the equilibrium concentrations of the imprinted ion and the competing ion, respectively.

Batch desorption experiments were conducted using 1.6 mol/L HNO₃. The ion-imprinted resins loaded with La³⁺, Ce³⁺, or Y³⁺ were immersed in HNO₃ at 30 °C for 4 h. Subsequently, the mixture was filtered, and the concentration of rare earth ions in the aqueous phase was measured. The desorption efficiency of rare earth ions from PS-IIPHAs was calculated according to Equation (3) [28].

$$R_{\mathrm{w}}={\displaystyle \frac{C_{\mathrm{w}}{V}_{\mathrm{w}}}{Qm}}\times 100\%$$

(3)

where R_w (%) is the desorption efficiency, C_w (mol/L) is the concentration of rare earth metal ions in eluent solutions, V_w (L) is the volume of eluent, Q (mmol/g) is the adsorption capacity, and m (g) is the mass of dry resin.

2.2.3. Characterization Methods

The C, H, and N elemental compositions of the ion-imprinted resins were analyzed using an elemental analyzer (Vario EL III, Elementar, Langenselbold, Germany). The specific surface area, pore volume, and average pore size were tested using the JW-BK132F analyzer (JWGB Instruments, Beijing, China), adopting the Brunauer–Emmett–Teller (BET) and Barret–Joyner–Halenda (BJH) methods. The morphology of the resins was determined via scanning electron microscopy (SEM) (MIRA 3, TESCAN, Brno, Czech Republic). Thermogravimetric analysis (TGA) data were collected using an SDT-Q600 analyzer (TA Instruments, New Castle, DE, USA). FTIR spectra were recorded using a Nicolet 6700 spectroscopy (Thermo Fisher Scientific, Waltham, MA, USA) to investigate the chemical functional groups. The binding energy of the resins was determined via XPS (ESCALAB 250Xi, Thermo Fisher Scientific, Waltham, MA, USA).

3. Results and Discussion

3.1. Characterization

3.1.1. Elemental Analysis

The elemental contents of polystyrene and the ion-imprinted resins obtained after separately imprinting with La³⁺, Ce³⁺, and Y³⁺ were determined using an elemental analyzer, and the results are presented in

Table 1. Results of the elemental analysis for PS-IIPHAs.

Resin	Contents (wt%)
	C	H	N
Polystyrene	91.89	7.785	ND
PS-La-IIPHA	85.95	7.408	1.05
PS-Ce-IIPHA	84.36	7.326	0.96
PS-Y-IIPHA	83.91	7.289	1.08

ND, not detected.

. The C and H contents of the resins are almost identical, indicating that their carbon skeletons are essentially the same, whereas the nitrogen content changes markedly; in particular, nitrogen is undetectable in polystyrene, but, during the formation of the interpenetrating networks, PHA diffuses into the polystyrene pores, raising the nitrogen content to 1.05% for the PS-La-IIPHA, 0.96% for the PS-Ce-IIPHA, and 1.08% for the PS-Y-IIPHA. These results confirm that nitrogen-containing functional groups have been successfully introduced onto the resins, thereby promoting the adsorption of rare earth ions.

3.1.2. N₂ Adsorption–Desorption Isotherms

The N₂ adsorption–desorption isotherms were employed to evaluate the specific surface area, pore volume, and pore-size distribution of PS-IIPHAs (Figure 1). The La³⁺-, Ce³⁺-, and Y³⁺-imprinted resins exhibited specific surface areas of 103.6, 82.5, and 97.3 m²/g, average pore diameters of 4.35, 3.87, and 3.46 nm, and pore volumes of 0.3164, 0.2845, and 0.3056 cm³/g, respectively. All three resins displayed typical Type IV isotherms with pronounced Type H3 loops, indicating a mesopore-dominated, structurally stable, and non-uniform pore architecture [30]. The emergence of the Type H3 loops is intimately linked to the ion-imprinting technique and the interpenetrating network structure. The imprinting process creates recognition cavities complementary to La³⁺, Ce³⁺, and Y³⁺ within the polymer matrix, while the interpenetrating networks enhance the pore connectivity and stability, jointly generating an adsorbent with a high surface area and well-suited pore-size distribution.

3.1.3. SEM Analysis

The surface morphology of the as-synthesized PS-IIPHAs was characterized via SEM. As shown in Figure 2, the polymer microspheres possess a well-developed porous structure, with a large number of cavities uniformly distributed across the surface. These cavities are characteristic of the ion-imprinted sites created during the synthesis process and serve as specific recognition elements for template ions. Furthermore, the interconnected pore channels observed in the image confirm the successful construction of the interpenetrating polymer network. These features confirm that the synthesized resin possesses an excellent mesoporous structure and a stable pore system, providing abundant active sites and fast diffusion channels for the efficient adsorption of rare earth ions.

3.1.4. TGA Results

TGA was used to evaluate the thermal behavior of the prepared PS-IIPHAs. As shown in Figure 3, the slight mass loss from room temperature to 150 °C is probably due to the desorption of the surface physically adsorbed water and residual solvent. A sharp weight loss occurred between 150 °C and 500 °C, resulting from the decomposition of the hydroxamic acid functional groups and the thermal degradation of the interpenetrating networks [22,31]. The results demonstrate that PS-IIPHAs possess outstanding stability over a wide temperature range.

3.1.5. FTIR Analysis

The structural changes in the La³⁺, Ce³⁺, and Y³⁺ ion-imprinted resins before and after adsorption of their respective template ions were characterized via FTIR, and the results are shown in Figure 4. The metal-free resin exhibits a broad intense band at 3438–3440 cm⁻¹, assigned to the overlapping N–H and O–H stretching vibrations of the hydroxamic acid moiety, and a strong peak at 1720 cm⁻¹ corresponding to C=O stretching, confirming that the hydroxamic acid functionality has been successfully incorporated into the polystyrene backbone [32,33]. Peaks at 3025 cm⁻¹, 2923 cm⁻¹, and 1601 cm⁻¹ are attributed to saturated C–H, unsaturated C–H, and aromatic C=C stretching vibrations, respectively, corroborating the basic resin structure [34]. After the adsorption of rare earth ions, the N–H and O–H double absorption peaks shift, due to the N and O atoms of the hydroxamic acid group coordinating with the metal ions. A slight change in the carbonyl C=O stretching peak is also observed, further suggesting that the carbonyl oxygen participates in coordination with the rare earth ions, forming a stable chelate structure [31].

3.1.6. XPS Analysis

To gain deeper insight into the interaction sites between PS-IIPHAs and La³⁺, Ce³⁺, and Y³⁺ during adsorption, Figure 5 and Figure S1 present the XPS spectra of PS-IIPHAs before and after adsorption. Compared with the spectrum before adsorption, new signals corresponding to La, Ce, and Y appear. This trend clearly indicates that La³⁺, Ce³⁺, and Y³⁺ are effectively adsorbed by PS-IIPHAs. The electron binding energies of the main elements before and after adsorption are listed in

Table 2. Electron binding energies of the main elements in PS-IIPHAs before and after adsorption of La³⁺, Ce³⁺, and Y³⁺.

Resin	Binding Energies (eV)
	C 1s	O 1s	N 1s	La 3d	Ce 3d	Y 3d
PS-La-IIPHA	284.58	531.99	400.39	ND	ND	ND
PS-La-IIPHA-La3+	284.59	532.78	400.13	837.36	ND	ND
PS-Ce-IIPHA	284.65	532.27	400.37	ND	ND	ND
PS-Ce-IIPHA-Ce3+	284.65	532.64	400.08	ND	885.96	ND
PS-Y-IIPHA	284.59	532.15	400.08	ND	ND	ND
PS-Y-IIPHA-Y3+	284.65	532.54	399.94	ND	ND	159.05

ND, not detected.

. According to the table, the N 1s binding energy decreases, while the O 1s binding energy increases, after adsorption, suggesting that the resin forms stable chelates with the rare earth ions through the synergistic coordination of O and N atoms in the hydroxamic acid groups [28].

3.2. Adsorption and Desorption Performance

3.2.1. Effect of the Solution pH on the Adsorption

The solution pH is a critical factor influencing the adsorption of rare earth ions. It determines not only the dissociation state of hydroxamic acid functional groups, but also the speciation of rare earth ions and the surface charge of the resin. Figure 6 presents the adsorption capacities of PS-IIPHAs for their respective template ions as a function of pH. Since lanthanide elements undergo hydrolysis at a pH above 6.0 [35,36], the adsorption experiments were conducted within the pH range of 1.0 to 6.0. As shown in the figure, with an increasing pH, the adsorption capacities for La³⁺, Ce³⁺, and Y³⁺ exhibit an initial decrease followed by stabilization. At pH 1.0, the adsorption capacities for La³⁺, Ce³⁺, and Y³⁺ reached 1.43, 1.79, and 1.82 mmol/g, respectively. This phenomenon may be attributed to the pre-assembly structure formed between template ions and functional monomers under acidic conditions during the ion-imprinting process, enabling the imprinted cavities to maintain a high recognition capability for target ions even at low pH [37]. Additionally, the lower pH environment may optimize the swelling state of the resin, facilitating ion diffusion and adsorption within the imprinted cavities. As the pH increases, the adsorption of rare earth ions by PS-IIPHAs gradually stabilizes, indicating that the high electronegativity of PS-IIPHAs is favorable for REE adsorption. Therefore, pH 1.0 was selected as the optimal adsorption condition for the subsequent experiments.

3.2.2. Effect of the Contact Time on the Adsorption

Figure 7 illustrates the effect of the contact time on the adsorption capacity of PS-IIPHAs at 40 °C. As shown in Figure 7, all three ion-imprinted resins exhibited similar adsorption trends. In the initial stage (0–2 h), the adsorption capacity increased rapidly due to abundant unoccupied active sites on the resin surface and a strong mass-transfer driving force. From 2 to 4 h, the growth rate slowed down, as the active sites became gradually occupied, and the mass transfer resistance increased. When the adsorption time reached 4 h, the adsorption capacity stabilized, indicating that dynamic equilibrium had been achieved, and the active sites were saturated. This relatively short equilibrium time reflects the rapid recognition and adsorption ability of the ion-imprinted resins, which can be attributed to the spatial configuration match between the specific adsorption sites and the target ions [38,39], thereby reducing the diffusion paths and resistance. Therefore, 4 h was selected as the optimal adsorption time for the subsequent experiments to ensure sufficient equilibrium.

3.2.3. Effect of the Initial Metal Ion Concentrations on the Adsorption

Figure 8 shows the effect of the initial concentrations of La³⁺, Ce³⁺, and Y³⁺ in the range of 0.005–0.020 mol/L on the adsorption capacities of the corresponding ion-imprinted resins. With increasing initial concentrations, the adsorption capacities of all three rare earth ions first increased rapidly and then gradually leveled off. When the ion concentrations rose from 0.005 mol/L to 0.01 mol/L, the adsorption capacities increased significantly. As the concentrations increased to 0.020 mol/L, the rate of growth in the adsorption capacities gradually slowed, indicating that the resin approached saturation. This phenomenon can be attributed to the increase in the initial concentrations, enhancing the mass transfer driving force at the solution–resin interface and increasing the probability of contact between ions and active sites on the resin surface, thus promoting the adsorption process. As the adsorption sites gradually became occupied, the adsorption efficiency stabilized, indicating that the resin’s adsorption capacity for rare earth ions is finite.

3.2.4. Effect of the Adsorption Temperature on the Adsorption

Figure 9 systematically shows the variation in the adsorption capacity of PS-IIPHAs for the respective imprinted ions within the temperature range of 25 °C to 40 °C. The study reveals that, as the adsorption temperature increases from 25 °C to 40 °C, the adsorption capacity of PS-IIPHAs for the corresponding imprinted ions shows a steady rising trend. This indicates that the adsorption process is endothermic, and elevating the temperature favors adsorption; however, the rate of increase is very gradual. This result suggests that temperature has a minor influence on the adsorption capacity for metal ions.

3.2.5. Selective Adsorption Performance of PS-IIPHAs

To evaluate the selective recognition ability of the resins for rare earth ions, batch adsorption experiments were conducted in a mixed solution containing La³⁺, Ce³⁺, and Y³⁺, and the results are shown in

Table 3. Separation performance of PS-IIPHAs for La³⁺, Ce³⁺, and Y³⁺ in binary mixed solutions.

Resin	Mixed Ions	Ce1 (mmol/L)	Ce2 (mmol/L)	Qe1 (mmol/g)	Qe2 (mmol/g)	K
PS-La-IIPHA	La3+-Ce3+	3.353	4.615	1.662	1.346	1.699
	La3+-Y3+	3.616	5.067	1.596	1.233	1.813
PS-Ce-IIPHA	Ce3+-La3+	1.058	8.075	2.236	0.481	35.45
	Ce3+-Y3+	1.510	6.625	2.123	0.844	11.04
PS-Y-IIPHA	Y3+-La3+	2.125	8.254	1.969	0.437	17.52
	Y3+-Ce3+	1.864	5.647	2.034	1.088	5.662

. For all ion-imprinted resins, the adsorption capacity for the template ion was higher than that for competing ions, and the selectivity coefficients K were all higher than 1, indicating good selective recognition ability. The highest separation factors of binary rare earth ions were 35.45 for Ce³⁺-La³⁺, 17.52 for Y³⁺-La³⁺, and 11.04 for Ce³⁺-Y³⁺, achieved by PS-Ce-IIPHA, PS-Y-IIPHA, and PS-Ce-IIPHA, respectively. These results demonstrate that the ion-imprinting technology successfully created recognition sites in the resin that matched the template ions, significantly enhancing the adsorption selectivity toward the target ions.

3.2.6. Desorption Performance

Batch desorption experiments were conducted on ion-imprinted resins saturated with adsorbed La³⁺, Ce³⁺, and Y³⁺ using 1.6 mol/L HNO₃ as the desorbent. The resins were shaken for desorption at 30 °C for 4 h, and the desorption was repeated three times. The results are shown in

Table 4. Results of the desorption experiments.

Metal Ion	E1 (%)	E2 (%)	E3 (%)
La3+	76.8	92.5	97.6
Ce3+	72.6	95.6	98.7
Y3+	80.9	96.9	98.1

. After three cycles, the desorption efficiencies of the La³⁺, Ce³⁺, and Y³⁺ ion-imprinted resins reached 97.6%, 98.7%, and 98.1%, respectively.

3.3. Adsorption Kinetics, Isotherms, and Thermodynamics

3.3.1. Kinetics

Adsorption kinetics studies are crucial for elucidating the rate laws of adsorption processes, revealing adsorption mechanisms, and identifying the rate-limiting step [40,41]. As a porous medium material, the adsorption process of resin generally consists of five stages, as follows: first, metal ions diffuse from the bulk solution to the boundary layer of the resin particle; then, they diffuse through the liquid film; subsequently, they diffuse from the resin surface to the internal active sites; forth, they undergo ion exchange or complexation with the functional groups within the resin; and, finally, if ion exchange occurs in the preceding step, the displaced ions diffuse outward [16,42]. The overall adsorption rate is influenced by the liquid film diffusion rate, the particle diffusion rate, and the chemical reaction rate, with the slowest step controlling the entire adsorption process. These models can be described by Equations (4), (5), and (6), respectively [21,22].

$$-\ln (1-F)=k_{1}t$$

(4)

$$1-3{(1-F)}^{2/3}+2(1-F)=k_{2}t$$

(5)

$$1-{(1-F)}^{1/3}=k_{3}t$$

(6)

where F = Q_t/Q_e, with Q_e and Q_t (mmol/g) representing the adsorption capacities at equilibrium and at time t (min), respectively; k₁, k₂, and k₃ (min⁻¹) denote the rate constants for the liquid film diffusion, particle diffusion, and chemical reaction, respectively.

Figure 10 illustrates the adsorption kinetics of PS-IIPHAs toward their corresponding imprinted ions (La³⁺, Ce³⁺, and Y³⁺). The kinetic parameters obtained from the linear regression are summarized in

Table 5. Parameters of different kinetic models.

Metal Ion	Kinetic Model	k	Intercept	R2
La3+	Liquid film diffusion	1.2791	−0.6320	0.9869
	Particle diffusion	0.2928	−0.1687	0.9864
	Chemical reaction	0.2487	−0.0623	0.9784
Ce3+	Liquid film diffusion	1.1195	−0.6721	0.9160
	Particle diffusion	0.2868	−0.2263	0.9623
	Chemical reaction	0.2475	−0.1123	0.9497
Y3+	Liquid film diffusion	1.0042	−0.3731	0.9925
	Particle diffusion	0.2723	−0.1737	0.9798
	Chemical reaction	0.2345	−0.0675	0.9408

. As shown in the table, the linear R² values for all three kinetic models are relatively high (close to 0.98), indicating that these models can adequately describe the experimental data, and the adsorption process may be simultaneously controlled by liquid film diffusion, particle diffusion, and chemical reaction. Moreover, the rate constants of the liquid film diffusion model are generally higher than those of the particle diffusion and chemical reaction models, suggesting that the imprinted resin surface possesses abundant recognition sites, which facilitate the rapid transport of rare earth ions through the liquid film. The adsorption process exhibits a typical “fast initial, slow later” characteristic [43]. In the initial stage, due to the large concentration gradient and the “memory effect” of surface imprinted cavities, rare earth ions are quickly captured by the resin surface. Subsequently, the ions gradually diffuse into the internal network of the resin via particle diffusion and interact with the internal functional groups. This mechanism indicates that ion imprinting not only enhances the adsorption selectivity through conformational matching, but also accelerates the diffusion of target ions within the resin due to the oriented channels formed by the imprinted cavities and the reduced spatial hindrance.

3.3.2. Isotherms

Adsorption isotherms describe the mathematical relationship between the equilibrium adsorption capacity and the concentrations of the metal ions in solution at constant temperature [44]. This relationship is significant for revealing the adsorption mechanism, evaluating the adsorbent performance, and estimating the maximum adsorption capacity [45,46]. To investigate the adsorption behavior of rare earth ions (La³⁺, Ce³⁺, and Y³⁺) onto PS-IIPHAs, this study employed the Langmuir and the Freundlich isotherms to fit and analyze the experimental data. Their linear forms are expressed as Equations (7) and (8) [47].

$${\displaystyle \frac{C_{\mathrm{e}}}{Q_{\mathrm{e}}}}={\displaystyle \frac{C_{\mathrm{e}}}{Q_{\mathrm{m}}}}+{\displaystyle \frac{1}{Q_{\mathrm{m}}{K}_{\mathrm{L}}}}$$

(7)

$$\ln Q_{\mathrm{e}}={\displaystyle \frac{1}{n}}\ln C_{\mathrm{e}}+\ln K_{\mathrm{F}}$$

(8)

where Q_e (mmol/g) is the equilibrium adsorption capacity, Q_m (mmol/g) is the maximum adsorption capacity, C_e (mol/L) is the equilibrium concentration, K_L (L/mol) is the Langmuir isotherm constant, and n and K_F are the Freundlich isotherm coefficients.

As shown in Figure 11 and

Table 6. Parameters of the Langmuir and the Freundlich isotherms.

Metal Ion	Langmuir Isotherm			Freundlich Isotherm
	KL (L/mol)	Qm (mmol/g)	R2	KF	n	R2
La3+	347.6	2.425	0.9975	0.0119	2.507	0.9807
Ce3+	563.2	3.012	0.9981	0.0170	2.535	0.9048
Y3+	523.4	2.927	0.9960	0.0172	2.472	0.8813

, the correlation coefficients of the Langmuir isotherm for La³⁺, Ce³⁺, and Y³⁺ are significantly higher than those of the Freundlich isotherm, indicating that the adsorption of rare earth ions by PS-IIPHAs likely follows a monolayer adsorption mechanism. This is consistent with the adsorption behavior of most hydroxamic acid-based resins toward rare earth ions [21,22,28,31].

3.3.3. Thermodynamics

The study of adsorption thermodynamics is significant for understanding the spontaneity, energy changes, and temperature effects of the adsorption process [48]. To investigate the thermodynamic adsorption behavior of PS-IIPHAs toward La³⁺, Ce³⁺, and Y³⁺, we calculated the thermodynamic parameters of the adsorption process using the temperature coefficient method, including Gibbs free energy change (ΔG), enthalpy change (ΔH), and entropy change (ΔS). The thermodynamic parameters can be calculated using Equations (9)–(11) [49,50].

$$K_{\mathrm{D}}={\displaystyle \frac{Q_{\mathrm{e}}}{C_{\mathrm{e}}}}$$

(9)

$$\mathsf{\Delta }G=-RT\ln K_{\mathrm{D}}$$

(10)

$$\ln K_{\mathrm{D}}={\displaystyle \frac{\mathsf{\Delta }S}{R}}-{\displaystyle \frac{\mathsf{\Delta }H}{RT}}$$

(11)

where K_D represents the standard thermodynamic equilibrium constant, C_e (mol/L) represents the equilibrium concentration, R (8.314 J/(mol·K)) is the gas constant, and T (K) is the temperature.

By plotting lnK_D versus T⁻¹ (Figure 12), ΔH and ΔS can be calculated. The relevant parameters are listed in

Table 7. Thermodynamic parameters for La³⁺, Ce³⁺, and Y³⁺ adsorption by PS-IIPHAs.

Metal Ion	ΔG (KJ/mol)	ΔH (KJ/mol)	ΔS (J/(mol·K))
La3+	−6.524	20.705	86.99
Ce3+	−6.597	8.4778	48.16
Y3+	−6.433	20.779	86.94

. As shown in the table, the ΔG values for the adsorption processes of La³⁺, Ce³⁺, and Y³⁺ are all negative, indicating that the adsorption processes are spontaneous. The positive ΔH indicates that the process is endothermic and that an elevated temperature is favorable for adsorption, consistent with the aforementioned conclusions. The positive ΔS indicates a high affinity between the rare earth ions and the resin [51].

3.4. Comparison of PS-IIPHAs with Other Adsorbents

In this work, the maximum adsorption capacities of PS-IIPHAs for La³⁺, Ce³⁺, and Y³⁺ reached 2.425, 3.012, and 2.927 mmol/g, respectively. Compared with other adsorbents reported in the literature (

Table 8. Comparison of various adsorbents for La³⁺, Ce^3+/4+, and Y³⁺ adsorption.

Adsorbent	Adsorption Capacities (mmol/g)			Reference
	La3+	Ce3+/4+	Y3+
Asahi SQS-6	/	0.034 a	0.082	[52]
D113 resin	0.68	0.78 b	0.44	[31]
Dowex 50WX8	0.212	0.169 b	0.135	[53]
Ion-imprinted polymer using acrylamide as a monomer (IIP-AM)	0.862	/	/	[54]
Ion-imprinted polymer using methacrylic acid as a monomer (IIP-MAA)	0.582	/	/	[54]
La(III) imprinted polymer (La-IIP)	0.452	/	/	[55]
Lewatit MDS 200 H	0.210	0.173 b	0.121	[53]
PAAHA	2.10	2.38 b	3.98	[31]
Kenaf cellulose-based PHA ligand	1.86	1.74 b	/	[56]
Phosphorylated porous phenolic resin (PO-PPR-1)	/	/	0.724	[57]
PS-PHA copolymer	2.183	2.592 b	2.884	[21]
PS-PHA IPN	2.013	2.434 b	2.684	[22]
Purolite C160	0.250	0.177 b	0.110	[53]
Solvent-impregnated resin (SIR)	0.349	/	/	[58]
SIR coated with PVA crosslinked by glutaraldehyde (SIR-GA)	0.252	/	/	[58]
PS-IIPHAs	2.425	3.012 b	2.927	This work

/, not reported; ^a Ce⁴⁺ and ^b Ce³⁺_.

), our materials exhibited significantly higher adsorption capacities for rare earth ions. This advantage is attributed to the synergistic effect of the IPN and ion-imprinted technology, which provides abundant accessible sites and specific recognition cavities. These results demonstrate that PS-IIPHAs are promising adsorbents for the efficient uptake of rare earth ions from aqueous solutions.

4. Conclusions

This study successfully developed novel adsorbents, PS-IIPHAs, to address the challenge of efficiently and selectively separating rare earth ions which share similar chemical properties. The material exhibited excellent comprehensive performance, and the conclusions are summarized as follows:

(1)

A resin with specific recognition cavities and a stable mesoporous structure was successfully prepared using polystyrene as the skeleton via ion-imprinted and IPN technology. Characterization confirmed that the interpenetrating networks provided a high specific surface area and abundant pore channels, while ion-imprinting formed sites within the polymer that matched the template ions (La³⁺, Ce³⁺, and Y³⁺) in terms of the spatial configuration and functional groups.

(2)

Under optimal conditions (pH = 1.0, t = 4 h), the maximum adsorption capacities of PS-IIPHAs for La³⁺, Ce³⁺, and Y³⁺ reached 2.425, 3.012, and 2.927 mmol/g, respectively, which are significantly higher than most adsorbents reported in the literature. The highest separation factors were 35.45 for Ce³⁺-La³⁺ by PS-Ce-IIPHA, 17.52 for Y³⁺-La³⁺ by PS-Y-IIPHA, and 11.04 for Ce³⁺-Y³⁺ by PS-Ce-IIPHA, demonstrating enhanced adsorption selectivity toward the target ions. Using 1.6 mol/L HNO₃ as the eluent, the desorption efficiencies for the rare earth ions reached above 97% after three desorption cycles.

(3)

The adsorption kinetics conformed to a model jointly controlled by liquid film diffusion, particle diffusion, and chemical reaction, exhibiting a “fast-then-slow” characteristic. The adsorption was better described by the Langmuir isotherm, indicating a predominant monolayer process. The thermodynamic parameters (ΔG < 0, ΔH > 0, ΔS > 0) demonstrate that adsorption occurs as a spontaneous and endothermic process.

In summary, PS-IIPHAs are novel adsorbent materials that combine high adsorption capacity with high selectivity, demonstrating potential for application in REE separation and recovery.