Surface-Anchored Zirconium Phosphate via Polydopamine Coating on Ion-Exchange Resin for Rapid, High-Capacity Cs⁺ Capture

Abstract

In this study, a novel hybrid adsorbent polydopamine-based nano-zirconium phosphate coated resin (DPZrP) was successfully synthesized, where zirconium phosphate (ZrP) was surface-anchored onto a polystyrene ion-exchange resin (D001) via polydopamine (PDA) mediation. Characterization results indicated that PDA, acting as an interfacial bridge, not only achieved the stable loading of ZrP but also exerted a spatial confinement effect on ZrP through its polymeric cross-linked structure, thereby effectively suppressing the agglomeration of nanoparticles. Compared with pristine D001 and pure ZrP, the hybrid material DPZrP exhibited superior adsorption performance for Cs⁺. The adsorption capacity of DPZrP for Cs⁺ reached a theoretical maximum of 921.99 mg/g at 333 K. Adsorption kinetic studies indicated that adsorption equilibrium was reached within 120 min, and the reaction rate constant was 1.55 times that of DZrP. The pH effect experiment showed that DPZrP maintained Cs⁺ removal rates of 73.4% and 58.1% under strongly acidic (pH = 2) and strongly alkaline (pH = 12) conditions, respectively. When the molar ratio of Ca²⁺ to Cs⁺ was as high as 64, the Cs⁺ removal rate of DPZrP was 19.3% and 30.4% higher than those of DZrP and D001, respectively. Dynamic column experiments revealed that after treating 2000 bed volumes of simulated wastewater ([Cs⁺]₀ = 2.5 mg/L), the Cs⁺ concentration in the effluent remained below 0.5 μg/L, with breakthrough occurring at 3000 BV. After five consecutive adsorption–desorption cycles, the Cs⁺ removal rate of DPZrP remained at 88.4%. These studies confirmed the dispersion effect of PDA on ZrP, and the synthesized DPZrP possessed both rapid capture ability and high adsorption capacity for Cs⁺. Thus, it provides an efficient adsorbent for the safe purification of nuclear waste liquids.

1. Introduction

Under the dual pressures of the global energy crisis and the pursuit of carbon neutrality, nuclear energy is recognized as a low-carbon alternative to fossil fuels for meeting the world’s growing energy demand. However, with the rapid expansion of nuclear energy, various environmental risks have emerged, such as the discharge of large volumes of radioactive wastewater and severe nuclear leakage accidents [1,2]. Advanced purification of radioactive wastewater is a prerequisite for ensuring environmental safety and the sustainable development of nuclear energy. Among radioactive nuclides in such wastewater, cesium-137 (¹³⁷Cs) has attracted widespread attention due to its high γ-ray and β-particle activity, as well as its long half-life of 30.1 years [3,4]. Epidemiological studies have confirmed that ¹³⁷Cs can enter the food chain, accumulate in the human body, and thereby increase the risk of thyroid cancer and hematological malignancies [5]. Therefore, it is urgent to safely and effectively remove ¹³⁷Cs from radioactive wastewater.

Various separation techniques have been used to remove cesium from nuclear wastewater, including chemical precipitation [6], solvent extraction [7], ion exchange [8], membrane separation [9,10], and adsorption [11,12,13,14]. Among these methods, adsorption is generally recognized as one of the most effective and reliable for ¹³⁷Cs removal, and the selection of adsorbents is a key factor influencing adsorption performance. A range of adsorbent materials, such as ammonium phosphomolybdate [15,16], layered metal sulfides [17,18], graphene oxide [19], and Prussian blue analogs (PBAs) [20,21], have been widely applied in the treatment of ¹³⁷Cs-contaminated wastewater. However, their adsorption equilibrium time, adsorption capacity, and stability still need to be improved.

In recent decades, nano-zirconium phosphate (ZrP) has garnered extensive attention owing to its outstanding properties, including high adsorption capacity, rapid adsorption kinetics, excellent thermal stability, and radiation resistance [22,23,24]. However, ZrP exists as fine or ultrafine particles, which suffer from excessive pressure drop and poor mechanical strength—limitations that render it unsuitable for direct application in fixed-bed or other flow-through systems. To address the challenges of nanoparticle application in water treatment, other researchers have directly loaded functional nanoparticles onto ion exchange resins. For example, Wang et al. [23] immobilized ZrP on a polystyrene resin; at a Ca²⁺-to-Cs⁺ molar ratio of 32, the composite’s Cs⁺ removal rate increased from 16.2% (the rate of the resin carrier alone) to 40.2%, while simultaneously solving the separation issue of ZrP in aqueous solutions. Nevertheless, during direct loading, functional nanoparticles on the resin surface and within its pores tend to agglomerate. This agglomeration buries active sites and increases the adsorption mass transfer distance—both of which significantly weaken the nano-adsorbent’s ability to rapidly purify pollutants.

This issue directly compromises the unique nano-scale size and high reactivity of ZrP, thereby limiting the potential for engineering applications of functional nanocomposites.

Nanocoating technology provides an effective approach for the immobilization of ZrP nanoparticles and the engineering application of their composites. Through the charged chelating groups, such as amino and hydroxyl functional groups of biological proteins, the surface charging environment of the carrier is changed to promote the uniform dispersion of ZrP [25,26]. However, due to the loss of pore protection and poor interfacial compatibility between the organic carrier framework and nanoparticles, the resin carrier exhibits a weak binding force to the nanocoating, leading to easy loss of nanoparticles during application [27]. Therefore, enhancing the interfacial interaction between nanoparticles and the carrier and improving the chemical stability of nanoparticles are the keys to breaking through the bottleneck.

Dopamine contains both catechol and amino groups, providing a basis for strong binding forces. Moreover, its charged chelating structure can undergo a self-crosslinking reaction at room temperature to form polydopamine (PDA) with a polymer crosslinked network. This network exerts a network confinement effect on ZrP, enhancing the chemical stability and environmental safety of the nanocomposite [28,29]. Khalid et al. [30] prepared a polyvinyl alcohol nanocomposite membrane by coating TiO₂ with PDA. This approach improved the dispersibility of TiO₂ nanoparticles and increased the microbial removal rate of the composite membrane by 30%. Zhang et al. [31] successfully fabricated mussel-inspired polydopamine microspheres (PDA-Ms) with controllable sizes via a simple polymerization method. These microspheres achieved efficient chelation of lead ions under high-salt conditions, and their selectivity was 30 times higher than that of the commercial ion exchanger 001 × 7. Thus, we aim to use the adhesiveness of PDA to bridge the ion exchange resin and ZrP, achieving uniform dispersion of ZrP in the pores of the resin and thereby enabling efficient removal of Cs⁺.

Therefore, in this study, a polydopamine-based nano-zirconium phosphate-coated resin, DPZrP, was prepared via in situ synthesis. The microstructure and chemical composition of DPZrP were characterized using scanning electron microscope (SEM), energy dispersive spectroscopy (SEM-EDS), and a high-resolution transmission electron microscope (TEM). The capture and retention properties of DPZrP for Cs⁺ in aqueous solutions were investigated via batch adsorption experiments, with comparisons made to D001 and DZrP. These results confirm that DPZrP is a promising Cs⁺ adsorbent. The mechanism of Cs⁺ capture and retention by DPZrP was elucidated through the analysis of adsorption experimental data and characterization results. This study’s findings highlight the dispersion-enhancing effect of PDA on ZrP and provide an efficient Cs⁺ adsorbent for the purification and safe treatment of radioactive wastewater.

2. Materials and Methods

2.1. Reagent

D001 resin was purchased from Jiangsu Hecheng New Materials Co., Ltd. (Nanjing, China). Absolute ethanol (C₂H₅OH, 99.5%, 64-17-5), zirconium oxychloride octahydrate (ZrOCl₂·8H₂O, 13520-92-8), and phosphoric acid (H₃PO₄, 7664-38-2) were supplied by Sinopharm Chemical Reagent Co., Ltd. (Beijing, China). Dopamine (purity grade: BR) and Tris-base (purity grade: AR, 51-61-6) were obtained from Merck KGaA (Darmstadt, Germany). In this study, the non-radioactive isotope ¹³³Cs was used as a substitute for the radioactive isotope ¹³⁷Cs to avoid generating radioactive waste. Cesium nitrate (CsNO₃, 99.9%, 7789-18-6)—the source of ¹³³Cs—was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China).

2.2. Sample Preparation

Preparation of DP: First, 2.0 g of pretreated D001 resin, 0.265 g of copper (II) sulfate pentahydrate (CuSO₄·5H₂O), and 0.4 g of dopamine were sequentially added to a beaker containing 200 mL of Tris-HCl buffer solution (pH = 8.5, 50 mM). Next, 0.54 mL of 30% hydrogen peroxide (H₂O₂) was added, and the mixture was immediately protected from light and magnetically stirred for 50 min. Finally, the product was washed with deionized water until the filtrate reached neutrality, then dried to obtain DP.

Preparation of DPZrP: First, 8 g of zirconium oxychloride octahydrate (ZrOCl₂·8H₂O) was weighed and added to a beaker containing 200 mL of 10% ethanol-water solution, followed by magnetic stirring until completely dissolved. Next, 2.0 g of DP was added, and the mixture was transferred to a water bath; magnetic stirring was maintained at 333 K until the solution evaporated to dryness. The remaining solid was rinsed 3–5 times with deionized water and filtered to obtain an intermediate product. Subsequently, the intermediate product was added to a beaker containing 100 mL of 20% phosphoric acid (H₃PO₄) solution, and magnetic stirring was continued for 12 h. After the reaction, the solid was filtered out, washed with deionized water until neutral, and dried to prepare DPZrP.

Preparation of DZrP: The preparation procedure for DZrP is identical to that of DPZrP, except that DP is replaced with pristine D001 resin.

2.3. Surface Features

The microstructure of the materials and the crystal structure of ZrP were analyzed using a scanning electron microscope (SEM, Regulus 8100, Hitachi, Tokyo, Japan), energy dispersive spectroscopy (SEM-EDS, Regulus 8100, Hitachi, Tokyo, Japan), and a high-resolution transmission electron microscope (TEM, HT7700, Hitachi, Tokyo, Japan). A zeta potential analyzer (Zetasizer Nano, Malvern Instruments Ltd., Malvern, UK) and X-ray photoelectron spectroscopy (XPS, Escalab 250Xi, Thermo Scientific, Waltham, MA, USA) were employed to investigate the role of each component in DPZrP during Cs⁺ adsorption, as well as the mechanisms by which pH and coexisting ions influence adsorption behavior. The crystal structure of the materials was characterized by an X-ray diffractometer (XRD, XTRA, Hitachi, Tokyo, Japan) over the 2θ range of 5–90°, with a scan rate of 5° 2θ min⁻¹ and an incident wavelength of 0.1542 nm. The crystal structure and functional group structure of the materials were analyzed by a Fourier transform infrared spectrometer (FT-IR, Nexus 870, Thermo Scientific, Waltham, MA, USA).

2.4. Batch Experiments

The removal rate of Cs⁺ by DPZrP was investigated via batch adsorption experiments. In a typical procedure, 0.25 g of DPZrP was added to an Erlenmeyer flask containing 50 mL of a 10 mg/L Cs⁺ solution. The flask was placed in a constant-temperature shaker and agitated at 200 rpm for 24 h at 298 K. The supernatant was collected, filtered through a 0.45 μm membrane filter, and the Cs⁺ concentration was determined using an atomic absorption/emission spectrometer. To evaluate its reusability, the Cs⁺-loaded DPZrP was desorbed with 50 mL of a mixed solution containing 1% HCl and 5% Ca(NO₃)₂.

To comprehensively investigate the Cs⁺ removal efficiency of DPZrP, experiments were designed to evaluate the effects of solution pH, coexisting ions, adsorption isotherms, and adsorption kinetics. For the experiment investigating the effect of initial concentration, Cs⁺ solutions with initial concentrations ranging from 5 to 100 mg/L were prepared, and 0.025 g of DPZrP was added to each solution for batch adsorption experiments. To assess the pH effect, solution pH was adjusted to 2.0–12.0 using 1% and 0.1% NaOH or HCl solutions, followed by batch adsorption experiments with the same dosage of DPZrP. For coexisting ion studies, the Cs⁺ solution was spiked with coexisting cations (Ca²⁺, Mg²⁺, K⁺, and Na⁺), where the molar ratios of coexisting ions to Cs⁺ were set to 0, 2, 4, 8, 16, 32, and 64, respectively. Adsorption isotherm experiments were conducted at initial Cs⁺ concentrations of 50–1400 mg/L and temperatures of 298 K, 318 K, and 338 K. In kinetic experiments, DPZrP, DZrP, or D001 resin was immersed in Cs⁺ solution, and the Cs⁺ adsorption capacity was calculated at time intervals ranging from 0 to 480 min. For dynamic adsorption experiments, wastewater containing 2.5 mg/L Cs⁺ and 25 mg/L Ca²⁺ was prepared. An adsorption column (150 mm × 10 mm) was packed with DPZrP to a wet apparent volume of 4 mL. The wastewater was introduced into the column via a peristaltic pump at a flow rate of 12 BV/h. Effluent samples were collected every 20 min using an automatic sampler to determine the Cs⁺ concentration.

3. Results

3.1. Characterization Analysis

The surface characteristics of DPZrP were observed via SEM to analyze the effects of ZrP and polydopamine loading on the material’s surface properties. As shown in Figure 1, both DPZrP and DZrP exhibit a regular spherical shape, with a diameter of 600–900 μm (determined by the sieving method), facilitating easy separation from aqueous solutions. Figure 1b shows the high-magnification SEM image of DPZrP, revealing a relatively smooth and flat surface with no nanoparticle agglomeration. This indicates that the polydopamine coating uniformly covers the surface of D001, and the functional groups of its unique chain structure effectively mitigate the agglomeration of zirconium phosphate, enabling its uniform loading on DPZrP [32]. Compared with D001, the surface of DZrP is rougher; high-magnification images reveal that ZrP combines with the layered structure of D001 to form a flake-like morphology. A large amount of Zr and P elements can be observed on the outer surfaces of both DPZrP and DZrP, confirming the successful loading of nano-ZrP [33]. The distribution of nano-ZrP in DPZrP and DZrP was further explored. By comparison, it was found that Zr and P elements in DPZrP are distributed within a spherical shell layer of a certain thickness, extending inward from the resin surface. It is indicated that the adhesion and confinement of the outer surface of DP hinder the diffusion of nano-ZrP into the resin, which is beneficial for shortening the mass transfer distance of target pollutants and enhancing adsorption kinetics. Additionally, Zr and P elements exhibit the same distribution location, indicating that all Zr⁴⁺ deposited into the material is converted into nano-ZrP. In contrast, Zr and P elements in DZrP are distributed throughout the entire material.

Figure 1. SEM images of (a–c) DPZrP and (d–f) DZrP, along with Zr and P elemental mapping of their outer surfaces and cross-sections.

The TEM images of DPZrP and DZrP are shown in Figure 2. A large number of loaded nano-ZrP particles can be observed on the surfaces of both DPZrP and DZrP (Figure 2a,c). As seen in Figure 2c, the lattice structure of ZrP is present on the surface of DPZrP, and the measured lattice spacing is 0.3454 nm, which corresponds to the (112) crystal plane of α-ZrP. In contrast, no ZrP lattice was observed in the high-resolution transmission electron microscope (HRTEM) image of DZrP, indicating that the nano-ZrP loaded on DZrP is amorphous. This difference demonstrates that DP successfully promotes the formation of α-ZrP. Compared with amorphous zirconium phosphate, α-ZrP contains bound water in its interlayer structure, resulting in a larger interlayer spacing, which facilitates the entry of larger-radius Cs⁺ ions [34].

Figure 2. TEM and HRTEM images of (a,b) DPZrP and (c,d) DZrP.

The XRD patterns in Figure S1 revealed that D001, DP, and ZrP were amorphous in structure. However, the ZrP sample exhibited a broad diffraction peak at 33.7° 2θ, which suggested the presence of a low-crystallinity phase of ZrP [35]. Notably, DZrP displayed XRD curve characteristics similar to those of ZrP powder, suggesting that the in situ synthesis of ZrP on D001 resin did not alter its intrinsic crystal form. In contrast to other materials, DPZrP exhibited well-defined peaks at 19.7°, 24.9° and 33.7° 2θ, indexed to the (110), (112) and (020) planes of α-ZrP (PDF#22-1022). This indicated that α-ZrP with higher crystallinity was formed on the DP surface via the in situ synthesis method, which was associated with the addition of polydopamine. The surface functional groups of the composite materials were investigated via FT-IR spectra (Figure S1b) to analyze the changes in functional group structure during the preparation of DPZrP. The vibration peak of the Zr–O group at 505 cm⁻¹ and the characteristic peak corresponding to the PO₄³⁻ group at 1000 cm⁻¹ confirmed the successful loading of nano-ZrP onto DPZrP and DZrP. The absorption peaks at~1165 cm⁻¹ for D001, DP, DZrP, and DPZrP were derived from the sulfonic acid groups (–SO₃H) inside D001.

3.2. Performance Analysis

As shown in Figure 3a, when the initial Cs concentration in the solution increased from 5 mg/L to 10 mg/L, the removal rate increased significantly with the increase in concentration, which was mainly due to the strong mass transfer driving force generated by the concentration difference. As the initial phosphorus concentration increased to 100 mg/L, the active sites on the surface of the adsorbent tended to be saturated, so the removal rate generally showed a downward trend during the period of 10–100 mg/L. The Cs⁺ removal rate of D001 within 5–100 mg/L was comparable to that of DPZrP. The sulfonic acid groups in D001 inherently carry a strong negative charge, causing Cs⁺ to accumulate near the material via the Donnan effect; the ions are then immobilized within the material’s abundant pore structure, thereby enabling D001 to exhibit high Cs⁺ removal efficiency. The loading of nano-ZrP on the DP surface mainly addresses the agglomeration issue of nanomaterials and enhances the adsorption capacity and selective adsorption performance of the composite. Therefore, within the concentration range of 5–100 mg/L, there was no significant difference in Cs⁺ removal rate between DPZrP and D001.

Figure 3. (a) Effect of Cs⁺ concentration on adsorption performance; (b) initial pH on Cs⁺ removal rate; (c) zeta potential of DPZrP and its reference materials. (adsorbent dosage = 0.025 g, volume of Cs⁺-containing wastewater: 50 mL, Cs⁺ concentration: 10 mg/L, 298 K, 180 rpm).

Figure 3b,c show the Cs⁺ removal rates and zeta potentials of DPZrP and its reference materials under different pH conditions. As shown in Figure 3b, within the relatively wide pH range of 4–10, the adsorption behavior of DPZrP for Cs⁺ was basically unaffected by pH. In strongly acidic or alkaline environments with pH < 2 or > 12, the adsorption rates of ZrP powder and DZrP for Cs⁺ dropped to below 45%, while the adsorption rates of DPZrP for Cs⁺ maintained 73.4% (pH = 2) and 58.1% (pH = 12), respectively. Among all experimental materials, DP exhibited the lowest Cs⁺ removal rate, with a maximum of only 72% at pH = 6. This indicates that polydopamine hindered the mass transfer of Cs⁺ to the sulfonic acid groups on D001’s surface and within its pores, and that polydopamine itself has extremely limited direct adsorption capacity for Cs⁺.

Figure 3c shows the zeta potentials of DPZrP and its constituent materials. ZrP carries a positive charge at pH < 3, which impairs its Cs⁺ adsorption efficiency under this condition. D001 remains negatively charged across the pH range of 2–12 due to its intrinsic sulfonic acid groups [36], rendering its adsorption performance less susceptible to pH variations. Although DZrP is also negatively charged, its surface ZrP exhibits poor adsorption at pH < 3, and the resin’s internal pores are blocked by agglomerated ZrP. Consequently, DZrP displays a removal efficiency comparable to that of DP at pH = 2. DP itself has a limited capacity for Cs⁺ adsorption, and ZrP’s adsorption efficiency is low at pH < 3. However, DPZrP maintains a relatively high removal rate at pH = 2, indicating that polydopamine enhances the Cs⁺ adsorption capacity of ZrP under this condition.

In the purification of radioactive wastewater, cations such as Ca²⁺, Mg²⁺, Na⁺, and K⁺ are common. These positively charged cations share similar chemical properties with Cs⁺, which exerts adverse effects on Cs⁺ purification. Therefore, it is necessary to evaluate the impact of these coexisting ions on Cs⁺ adsorption.

Figure 4 presents the Cs⁺ removal efficiency of DPZrP in the presence of coexisting ions. The order of influence of the four coexisting ions on Cs⁺ adsorption (from strongest to weakest) was Ca²⁺ > Mg²⁺ > K⁺ > Na⁺, which may be attributed to the higher charge number of divalent metal ions. In the presence of competing ions, the Cs⁺ selectivity of D001 decreases sharply: when the Ca²⁺/Cs⁺ molar ratio (mol/mol) increases from 8 to 64, its removal rate drops from 81.1% to 3.9%; when the Mg²⁺/Cs⁺ molar ratio (mol/mol) increases from 8 to 64, the removal rate decreases from 73.0% to 12.3%. This indicates that traditional resins almost lose their Cs⁺ capture ability under the shielding effect of high-valence cations. When the Ca/Cs molar ratio (mol/mol) was 64, the removal rates of DZrP and DPZrP were 19.3% and 35.3% higher than that of D001, respectively; under the same concentration of Mg²⁺, their removal rates were increased by 15.2% and 30.4%, respectively. This phenomenon occurs because D001 adsorbs Cs⁺ primarily through ion exchange between protons on its sulfonic acid groups –SO₃H and Cs⁺. This non-specific mechanism operates with all positively charged cations. In contrast, nano-ZrP exhibits selective adsorption for Cs⁺ [23,37], thereby enhancing the Cs⁺ adsorption performance of DZrP and DPZrP in the presence of coexisting ions. Additionally, compared with DZrP, DPZrP shows further enhanced selective adsorption for Cs⁺. This is because the ZrP loaded on DPZrP is of the α-type, whereas that on DZrP is amorphous zirconium phosphate. Compared with amorphous zirconium phosphate, α-ZrP has better selective adsorption performance for Cs⁺ [34].

Figure 4. Effect of coexisting ions (a) Ca²⁺, (b) Na⁺, (c) Mg²⁺ and (d) K⁺ on Cs⁺ adsorption by D001, DZrP and DPZrP. (adsorbent dosage = 0.025 g, volume of Cs⁺-containing wastewater: 50 mL, Cs⁺ concentration: 10 mg/L, 298 K, 180 rpm).

The kinetic data were fitted using the pseudo-first-order (PFO) kinetic model, pseudo-second-order (PSO) kinetic model, and intra-particle diffusion model, with the results presented in Figure 5. As observed from Figure 5a, D001 reached adsorption equilibrium for Cs⁺ at 120 min, with an adsorption capacity of 97.6 mg/g. Sulfonic acid groups impart D001 with a permanent negative charge, enabling it to enrich metal cations. However, while the resin’s complex pore structure provides additional adsorption sites, it also reduces the mass transfer rate of Cs⁺ into the resin—prolonging the time required to reach reaction equilibrium. Loading nano-ZrP onto D001 provided more adsorption sites for Cs⁺: DZrP’s adsorption capacity increased to 131.4 mg/g, and the reaction equilibrium time was extended to 180 min. This is likely because the loaded nano-ZrP penetrates the resin interior, occupying pore space and impairing the mass transfer rate of Cs⁺ to the composite material. This prolongs the equilibrium time and further validates the SEM-EDS analysis results. DPZrP exhibited an adsorption capacity of 130.7 mg/g for Cs⁺, with the reaction reaching equilibrium at 120 min. Although polydopamine formation affects the pore structure on D001’s surface, the pores are not completely blocked. Moreover, polydopamine mitigates nano-ZrP agglomeration, exposes more adsorption sites, and confines nano-ZrP to the composite resin surface and a spherical shell layer of specific thickness—shortening the mass transfer distance. Consequently, DPZrP achieves an adsorption capacity comparable to that of DZrP, while its adsorption equilibrium time is reduced by 30 min.

Figure 5. The adsorption kinetic models for Cs adsorption by (a,d) D001, (b,e) DZrP and (c,f) DPZrP fitted by pseudo-first-order model and pseudo-second-order model and intraparticle diffusion model. (adsorbent dosage = 0.25 g, volume of Cs⁺-containing wastewater: 500 mL, Cs⁺ concentration: 80 mg/L, 298 K, 180 rpm).

Based on the above analysis, it was found that three components—nano-ZrP, D001, and polydopamine—jointly participate in the Cs⁺ adsorption process of DPZrP. Specifically, polydopamine uniformly disperses ZrP and confines it to the spherical shell layer on the resin surface; the deprotonated sulfonate group –SO₃⁻ of D001 enriches Cs⁺ near the material; and both nano-ZrP and the sulfonic acid groups of D001 directly adsorb Cs⁺.

The kinetic equations of the PFO model, PSO model, and intraparticle diffusion model correspond to Equations (1)–(3), respectively, with the relevant fitting parameters summarized in Table 1 and Table S1. The fitting results indicate that the adsorption process of Cs⁺ onto D001, DZrP, and DPZrP is better described by the pseudo-first-order (PFO) kinetic model, as it exhibits the lowest Root Mean Square Error (RMSE) and chi-square (χ²) values along with the highest R-Square (R²) value, demonstrating the best goodness-of-fit [38]. This finding further suggests that the adsorption of Cs⁺ onto these materials is predominantly governed by physisorption. The adsorption capacity of DZrP was 1.35 times that of D001, and its reaction rate constant was 37% lower. For DPZrP, its adsorption capacity increased to 1.33 times that of D001, and its reaction rate constant was 1.55 times that of DZrP.

Table 1. Fitting parameters of adsorption kinetics.

Models	DPZrP	DZrP	D001
qe(exp) (mg/g)	130.65	131.589	97.6
Pseudo-first-order model
qe(cal) (mg/g)	128.82	131.44	97.17
k1 (min−1)	0.034	0.022	0.035
R2	0.981	0.997	0.994
RMSE (mg/g)	6.707	1.745	3.102
Chi square (χ2)	44.983	3.046	9.620
Pseudo-second-order model
qe(cal) (mg/g)	137.92	155.05	107.80
k2 (g·mg−1·min−1)	0.0004	0.0002	0.0004
R2	0.959	0.996	0.988
RMSE (mg/g)	4.484	1.851	4.248
Chi square (χ2)	20.108	3.425	18.048

Pseudo-first-order kinetic equation:

$$q_t=q_e (1-{\mathrm{e}}^{-kt})$$

(1)

Pseudo-second-order kinetic equation:

$${\displaystyle \frac{t}{q_t}}={\displaystyle \frac{1}{k_2{q}_e^2}}+{\displaystyle \frac{t}{q_e}}$$

(2)

Intraparticle diffusion model:

$$q_t=k_1{t}^{1/2}$$

(3)

where: q_t: Adsorption capacity at time t (mg/g); q_e: Equilibrium adsorption capacity (mg/g); k: Pseudo-first-order kinetic rate constant (min⁻¹); k₂: Pseudo-second-order kinetic rate constant (g·mg⁻¹·min⁻¹) k₁: Diffusion coefficient rate constants (mg/(g⋅min^1/2)).

To investigate the effect of temperature on Cs⁺ adsorption by DPZrP and evaluate its Cs⁺ adsorption capacity, isothermal adsorption experiments were performed on DPZrP and D001 at 298 K, 313 K, and 333 K, respectively. The experimental data were fitted using the Langmuir and the Freundlich isothermal models, with the fitting results presented in Figure 6.

Figure 6. (a–c) Langmuir, Freundlich and Sips isotherms for DPZrP; (d–f) corresponding fits for D001. (adsorbent dosage = 0.025 g, volume of Cs⁺-containing wastewater: 50 mL, 298 K, 180 rpm).

The equilibrium adsorption capacity of DPZrP for Cs⁺ initially increased with rising Cs⁺ concentration and then tended to stabilize. Its maximum adsorption capacity for Cs⁺ increased with increasing temperature, indicating that the adsorption process is endothermic. At 333 K, the maximum theoretical adsorption capacity of DPZrP reaches 921.99 mg/g, which is significantly higher than that of other Cs⁺ adsorbents (Table 2) and D001 (496.54 mg/g). This temperature independence arises from D001’s Cs⁺ adsorption mechanism: proton exchange between –SO₃H and Cs⁺. Unlike mechanisms involving the formation or cleavage of chemical bonds, ion exchange primarily relies on electrostatic interactions and thus exhibits weak temperature dependence. It can be concluded that DPZrP exhibits a significantly higher Cs⁺ adsorption capacity than D001, attributable to the excellent Cs⁺ adsorption performance of zirconium phosphate [39]. Furthermore, the confinement effect of the polydopamine coating alleviates nano-ZrP agglomeration and facilitates the formation of α-ZrP (which demonstrates superior Cs⁺ adsorption performance), thereby enhancing the Cs⁺ adsorption efficiency of DPZrP.

Table 2. Comparison of Cs adsorption capacities and kinetics by adsorbents.

Adsorbents	Capacity (mg/g)	Kinetics (min)	Ref.
DPZrP	921.99	120	This study
Crown ether-functionalized polyimide nanofiber membranes	85.23	180	[10]
ZnFe Prussian blue analogs	621.11	120	[1]
Prussian blue analogue-polymeric carbon nitride nanorods hybrid	376.41	180	[40]
Dual crown ether-modified chitosan	172.11	540	[11]
Dual-template ion-imprinted membrane with polyacrylonitrile framework	20	200	[41]
MIL-125-NH2(Ti)/etched silicon carbide/carboxymethyl cellulose composite aerogel	366.43	60	[5]
Prussian blue analogs	381.71	120	[4]
Hollow microspheres CMPs	4.75	10	[42]
Polyoxometalate-ionic liquid composites	400.0	180	[3]

The Langmuir, Freundlich, and Sips isothermal adsorption models—corresponding to Equations (4)–(6), respectively—were employed to fit the experimental data, with all fitting parameters compiled in Table 3. The fitting results demonstrate that the Langmuir model exhibits a significantly higher value than the Freundlich and Sips models; consequently, this model proves to be more appropriate for characterizing the adsorption thermodynamic behavior of DPZrP and D001 towards Cs⁺. This finding indicates that the adsorption of Cs⁺ onto DPZrP and D001 primarily follows a monolayer adsorption mechanism.

Table 3. Fitting parameters of the adsorption isothermal model for Cs⁺ by DPZrP and D001.

		Langmuir			Freundlich			Sips
	T (K)	Qe (mg/g)	KL (L/mg)	R2	Kf (mg/g)	n	R2	Qe (mg/g)	Ks (Lns/mgns)	R2
DPZrP	298	716.74	0.010	0.984	84.87	3.298	0.956	716.77	−10.56	0.980
	313	820.96	0.008	0.986	78.98	3.031	0.948	820.97	−39.23	0.982
	333	921.99	0.009	0.994	90.46	3.019	0.950	922.00	−63.13	0.993
D001	298	491.90	0.011	0.957	64.60	3.425	0.962	491.92	−4.851	0.945
	313	488.07	0.013	0.954	71.32	3.606	0.937	488.11	−10.11	0.941
	333	496.54	0.011	0.964	65.04	3.427	0.956	496.57	−5.261	0.954

Langmuir model equation:

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

(4)

Freundlich model equation:

$$q_{\mathrm{e}}=lg{K}_{\mathrm{F}}+{\displaystyle \frac{1}{n}}lg{C}_{\mathrm{e}}$$

(5)

Sips model equation:

$$q_e={\displaystyle \frac{Q_e{K}_s{C}_e^{ns}}{1+K_s{C}_e^{ns}}}$$

(6)

where: q_e: Adsorption capacity of Cs⁺ at solution equilibrium (mg/g); Q_e: Maximum adsorption capacity of the material (mg/g); K_L: Langmuir adsorption equilibrium constant; C_e: Concentration of Cs⁺ at solution equilibrium (mg/L); K_F: Freundlich adsorption equilibrium constant; K_s: Sips adsorption equilibrium constant.

To evaluate the practical application potential of DPZrP, dynamic adsorption experiments were conducted using simulated wastewater containing 2.5 mg/L Cs⁺ and 25 mg/L Ca²⁺. The experimental results are presented in Figure 7a. With the effluent flow rate controlled at 12 BV/h, for DPZrP, the Cs⁺ concentration in the effluent increased when the effluent volume reached 2000 BV, and breakthrough occurred when the effluent volume increased to 3000 BV. In contrast, under the same flow rate, the D001 resin exhibited breakthrough at an effluent volume of 1000 BV. These results indicate that DPZrP has a superior continuous wastewater treatment capacity compared to D001. The enhanced performance of DPZrP is attributed to the role of polydopamine: it promotes the formation of α-ZrP and confines nano-ZrP to the spherical shell layer on the resin surface. This not only improves DPZrP’s adsorption capacity but also addresses the reduction in adsorption rate caused by the direct loading of ZrP onto D001. Specifically, the increase in Cs⁺ concentration in the 2000 BV effluent (without breakthrough) occurs because the adsorption sites of nano-ZrP in the composite resin are partially depleted, reducing DPZrP’s ability to deeply purify Cs⁺. However, the –SO₃⁻ inside the resin can still provide additional adsorption sites until the effluent volume reaches 3000 BV, at which point all adsorption sites are exhausted, leading to breakthrough.

Figure 7. (a) Dynamic column adsorption experiment and (b) adsorption–desorption cycle experiment of DPZrP and D001.

The reusability of DPZrP was evaluated using a mixed desorption solution of 1% HCl + 5% Ca(NO₃)₂, with the results presented in Figure 7b. After five adsorption–desorption cycles, the Cs⁺ adsorption rate of DPZrP remained at 88.4%. In contrast, the adsorption rate of D001 resin dropped to 31.8% after the same number of cycles. These findings indicate that DPZrP exhibits excellent reusability, which effectively enhances the material’s economic viability for practical applications.

3.3. Mechanism Analysis

The adsorption mechanism of Cs⁺ on DPZrP was investigated via XPS, with the results presented in Figure 8. In the full XPS spectrum of DPZrP (Figure 8a), the C 1s and O 1s peaks originate from the styrene-diene-benzene copolymer of the D001 matrix; the S 2p peak is derived from the sulfonic acid groups within the D001 resin. The presence of an N 1s peak confirms the successful loading of polydopamine onto the DPZrP surface. Additionally, the loading of nano-ZrP is evidenced by the appearance of Zr 3d and P 2p peaks in the DPZrP full spectrum. The detection of a Cs 3d peak in the DPZrP spectrum after Cs⁺ adsorption further verifies that DPZrP has adsorbed Cs⁺. To elaborate further, the high-resolution Cs 3d spectra of DPZrP components post-Cs⁺ adsorption were compared, as shown in Figure 8b. As observed, polydopamine exhibits negligible interaction with Cs⁺. In contrast, after Cs⁺ adsorption by the –SO₃H of D001 and nano-ZrP, the Cs 3d peak shifts toward higher binding energy by 0.21 eV and 0.28 eV, respectively. This binding energy shift confirms that Cs⁺ is adsorbed by both the –SO₃H and nano-ZrP.

Figure 8. (a) Full XPS spectrum; (b) Cs 3d peak without coexisting ions; (c) Cs 3d peak in the presence of coexisting ions.

Figure 8c presents the high-resolution Cs 3d spectrum of DPZrP after Cs⁺ adsorption under coexisting ion conditions. The Cs 3d₅/₂ peak in the spectrum can be deconvoluted into two sub-peaks (designated as Peak A and Peak B), where Peak A corresponds to Cs⁺ adsorbed by ZrP and Peak B corresponds to Cs⁺ adsorbed by –SO₃H groups. As the Ca²⁺/Cs⁺ molar ratio (mol/mol) increased from 0 to 4 and 8, the area of Peak A (ZrP-adsorbed Cs⁺) increased from 37.1% to 49.2% and 94.6%, respectively. In contrast, the area of Peak B (–SO₃H-adsorbed Cs⁺) decreased from 62.9% to 50.8% and 5.5%, respectively. This result indicates that Ca²⁺ significantly interferes with Cs⁺ adsorption by the –SO₃H groups in D001, whereas ZrP maintains selective adsorption for Cs⁺. Additionally, compared with Ca²⁺, the change in peak area ratio induced by Na⁺ is significantly smaller, indicating that Na⁺ exerts a negligible impact on the Cs⁺ adsorption performance of –SO₃H groups.

Figure S2 presents the FT-IR spectra of DPZrP and D001 before and after Cs⁺ adsorption. No obvious changes occurred in the FT-IR spectra of D001 before and after adsorption. In contrast, the intensity of the characteristic peak of the PO₄³⁻ group at ~1000 cm⁻¹ in DPZrP was significantly weakened after Cs⁺ adsorption, which further confirmed the specific binding between Cs⁺ and zirconium phosphate.

Combined with the adsorption performance experimental data and characterization analysis results, the adsorption mechanism of DPZrP for Cs⁺ is summarized as follows: D001 enriches Cs⁺ at the material interface and adsorbs Cs⁺ via ion exchange between its –SO₃H groups and Cs⁺; ZrP selectively adsorbs Cs⁺ through its PO₄³⁻ groups; The primary role of polydopamine is to promote the formation of α-ZrP, enabling uniform distribution of nano-ZrP on the spherical shell layer of the DPZrP surface. This addresses nanoparticle agglomeration and enhances the loading stability of nano-ZrP.

4. Conclusions

In this study, DPZrP was fabricated via an in situ synthesis method. The morphology, structure, and elemental distribution of the material were systematically characterized using multiple analytical techniques, including SEM, TEM, HRTEM, EDS and XRD. Additionally, XPS and FT-IR were employed to further investigate the underlying adsorption mechanism. The results demonstrated that DPZrP exhibits a regular spherical morphology with a diameter ranging from 600 to 900 μm, which facilitates efficient solid–liquid separation. HRTEM and XRD characterizations confirmed that ZrP exists in the α-crystalline phase and is uniformly dispersed within a spherical shell layer of specific thickness on the resin surface, with no nanoparticle agglomeration observed. Within the pH range of 4–10, DPZrP’s Cs⁺ adsorption was not affected by pH; under strongly acidic (pH = 2) and strongly alkaline (pH = 12) conditions, the Cs⁺ removal rates were 73.4% and 58.1%, respectively. When the Ca²⁺/Cs⁺ molar ratio was 64, DPZrP’s Cs⁺ adsorption rate was 19.3% and 30.4% higher than that of DZrP and D001, respectively. Adsorption kinetics showed that equilibrium was reached within 120 min, with a reaction rate constant 1.55 times that of DZrP. DPZrP’s Cs⁺ adsorption capacity increased with rising temperature, reaching a theoretical maximum of 921.99 mg/g at 333 K. Dynamic adsorption tests revealed that DPZrP could continuously and deeply purify 2000 bed volumes (BVs) of simulated Cs⁺ wastewater; beyond this, the effluent Cs⁺ concentration increased, with breakthrough occurring at 3000 BV. After five adsorption–desorption cycles, the Cs⁺ adsorption rate remained at 88.4%, demonstrating good regenerability and dynamic adsorption performance. The adhesion and confinement effects of PDA enable uniform distribution of ZrP on the resin surface. This not only increases the number of active adsorption sites but also prevents nano-ZrP from diffusing into the resin interior. Consequently, DPZrP’s reactivity and Cs⁺ adsorption capacity are enhanced, the mass transfer distance of the target pollutant is shortened, and the adsorption rate is improved.