Efficient capture of Sr²⁺ ions by a layered crystalline zirconium phosphate fluoride

Abstract

The effective remediation of radioactive strontium-90 (⁹⁰Sr) from complex aqueous environments remains challenging due to the inherent high solubility and migration propensity of Sr²⁺ ions. Herein, we synthesized hydrothermally a new two-dimensional (2D) crystalline zirconium phosphate fluoride [(CH₃)₂NH₂][Zr(PO₄)F₂] featuring a layered anionic architecture of [Zr(PO₄)F₂]_nⁿ⁻ with intercalated [(CH₃)₂NH₂]⁺ cations, which shows exceptional Sr²⁺ remediation capability. It possesses a high maximum Sr²⁺ adsorption capacity (q^Sr_m) of 161.48 mg g⁻¹ (higher than that of many inorganic crystalline adsorbents) and fast kinetics for Sr²⁺ capture (Sr²⁺ removal rate (R^Sr) of 94.89% within 1 min). Specifically, it maintains Sr²⁺ removal efficiency in the presence of competing Cs⁺, K⁺, Na⁺, Ca²⁺, Mg²⁺ ions and in actual aqueous systems including seawater (R^Sr = 79.06%). X-ray photoelectron spectroscopy (XPS) and thermodynamics confirm that spontaneous Sr²⁺ capture occurs through ion exchange processes, where the interlayered [(CH₃)₂NH₂]⁺ cations in [(CH₃)₂NH₂][Zr(PO₄)F₂] are exchanged with Sr²⁺. The compound [(CH₃)₂NH₂][Zr(PO₄)F₂] represents the first crystalline inorganic zirconium phosphate fluoride ion exchange material for radionuclide capture. This work provides a high-performance ion exchanger as a candidate for radiostrontium capture.

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Introduction

Strontium-90 (⁹⁰Sr), recognized as one of the most hazardous artificial radionuclides, predominantly exists as ⁹⁰Sr²⁺ ions in radioactive waste liquids. As a β-emitting radioisotope derived from the nuclear fission of ²³⁵U/²³⁹Pu, ⁹⁰Sr exhibits a long half-life (t_1/2 = 28.6 years) and high β-particle energy (0.546 MeV),^1,2 posing severe environmental and health risks due to its high mobility, radiotoxicity, and biotoxicity.^3,4 Its chemical similarity to Ca²⁺ facilitates bioaccumulation in human bone tissues, leading to bone cancer and leukemia.^5,6 On the other hand, ⁹⁰Sr has found significant implementation in therapeutic oncology as a radiation source for targeted bone cancer treatment.⁷ Concurrently, its sustained decay heat generation enables essential roles in radioisotope thermoelectric generators (RTGs), particularly for powering deep-space probes and remote terrestrial monitoring systems under extreme environmental conditions.^7,8 Thus, the rapid separation and recovery of ⁹⁰Sr are critical for radioactive waste management, human health protection, environmental sustainability, and resource recycling.⁷ However, ⁹⁰Sr waste liquids are extremely complex, typically containing abundant non-radioactive ions (e.g., Na⁺, K⁺, and Mg²⁺) and other fission products.⁹ Therefore, highly selective capture of Sr²⁺ from such complex radioactive matrices remains a significant challenge.

Current strategies for Sr²⁺ removal include solvent extraction,^10,11 chemical precipitation,¹² and adsorption/ion exchange.¹³ Among these, adsorption/ion exchange stands out for its cost-effectiveness, operational simplicity, and high efficiency.^14,15 Various ion exchange materials have been widely studied, such as zeolites,¹⁶ layered clay minerals,¹⁷ crystalline silicotitanates,¹⁸ porous coordination polymers,¹⁹ and metal chalcogenides.^20,21 Particularly, inorganic ion exchangers demonstrate potential for radionuclide remediation due to their structural robustness under extreme conditions.²² Metal phosphate compounds have employed as ion exchange materials for Sr²⁺ ion capture, attributed to their good thermostability, radiation resistance,²³ and inherent selective coordination capacity of phosphate groups toward multivalent cations.^23–25 Specifically, tetravalent metal orthophosphates exhibit superior ion exchange characteristics, particularly in nuclear waste treatment scenarios due to their thermal stability, radiation tolerance, and structural stability under acidic conditions.^23,25–29

Zirconium phosphate (ZrP) and its structural analogues have been extensively studied as classical materials for Sr²⁺ capture.^25,30,31 However, most of them exhibit poor Sr²⁺ selectivity in excessive competitive ions (such as Na⁺, K⁺, and Ca²⁺), severely compromising strontium capture.^32–35 For instance, crystalline α-Zr(HPO₄)₂·H₂O (α-ZrP) exhibits adsorption capacities of 64.10 mg g⁻¹ for Cs⁺ and 43.03 mg g⁻¹ for Sr²⁺.^32,33 Meanwhile, γ-Zr(PO₄)H₂PO₄·2H₂O (γ-ZrP) demonstrates enhanced uptake for both Rb⁺ (129.90 mg g⁻¹) and Sr²⁺ (114.78 mg g⁻¹), though excessive Na⁺/Ca²⁺ ions severely compromise strontium capture.³⁴ Kinetic analyses reveal that γ-ZrP's ion exchange rates follow the hierarchy of Cs⁺ > K⁺ > Rb⁺ > Sr²⁺. Recently, potassium-incorporated zirconium phosphate (K₂Zr(PO₄)₂) shows the Sr²⁺ capture property but with poor selectivity for Sr²⁺ due to the effect of competitive ions (Ca²⁺, Na⁺, and H⁺).³⁵ Additionally, introducing fluorine into phosphate structures facilitates the formation of highly stable Zr–F bonds, enhancing crystallinity and thermal stability and improving structural diversity and robustness.³⁶ Nevertheless, crystalline inorganic zirconium phosphate fluoride has never been studied for the removal of radioactive ions.

In this work, a new crystalline zirconium phosphate fluoride [(CH₃)₂NH₂][Zr(PO₄)F₂] is reported, which is characterized by a layered anionic framework of [Zr(PO₄)F₂]_nⁿ⁻ hosting intercalated [(CH₃)₂NH₂]⁺ cations. It exhibits exceptional Sr²⁺ capture across wide pH regimes (1.95–8.46). It has a high Sr²⁺ adsorption capacity (q^Sr_m = 161.48 mg g⁻¹) and rapid capture kinetics (removal rate R^Sr = 94.89% within 1 min). Further, it maintains Sr²⁺ removal efficiency in the presence of competing Cs⁺, K⁺, Na⁺, Ca²⁺, and Mg²⁺ ions. It is also demonstrated that the material can efficiently capture Sr²⁺ from actual aqueous systems (especially seawater), surpassing most reported solid sorbents. This study highlights the substantial potential of layered crystalline zirconium phosphate fluorides for radionuclide capture, while establishing a rational design paradigm for developing Sr²⁺-selective ion-exchange materials of crystalline zirconium phosphate fluorides.

Materials and methods

Materials

ZrOCl₂·8H₂O (98.00%, Shanghai Adamas Reagent Co., Ltd.), N,N-dimethylacetamide (abbreviated as DMA, AR, Sinopharm Chemical Reagent Co., Ltd.), 2-hydroxyphosphonoacetic acid (abbreviated as HPAA, 50%, Shanghai Adamas Reagent Co., Ltd.), hydrofluoric acid (abbreviated as HF, AR, ≥40 nt, Shanghai Adamas Reagent Co., Ltd.), HNO₃ (65–68%, China Pharmaceutical Chemical Reagents Co., Ltd.), NaOH (98.00%, Greagent Reagent Co., Ltd.), SrCl₂·6H₂O (AR, Guangfu Co., Ltd.), CsCl (99.99%, Shanghai Longjin Metal Materials Co., Ltd.), KCl (AR, Shanghai Titan Co., Ltd.), NaCl (AR, Sinopharm Chemical Reagent Co., Ltd.), CaCl₂·2H₂O (74–78%, Shanghai Songjiang Silian Chemical Co., Ltd.), MgCl₂·6H₂O (AR, Kelong Co., Ltd.). Ultrapure water was supplied by a certified laboratory water purification system (Model WP-UP-LH-10, Sichuan Water Treatment Equipment Co., Ltd.). All reagents were purchased directly for use without further purification.

Synthesis of [(CH₃)₂NH₂][Zr(PO₄)F₂]

[(CH₃)₂NH₂][Zr(PO₄)F₂] was synthesized through a solvothermal process using stoichiometric quantities of ZrOCl₂·8H₂O (2.4 mmol, 0.7735 g) and HPAA (2.4 mmol, 0.3745 g) in a mixed solvent system containing DMA (24 mL) and deionized water (5 mL). To modulate crystallization kinetics, HF (0.08 mL) was introduced as a mineralizer.³⁷ The reaction mixture was then acidified to pH 4 through precise addition of HNO₃ (1 mL), a critical parameter influencing layer assembly.

After homogenization via magnetic stirring at ambient temperature, the precursor solution was transferred to a 100 mL Teflon-lined autoclave and subjected to thermal treatment at 200 °C for 72 h under autogenous pressure. This prolonged aging period facilitated the formation of well-defined crystalline phases.

The resultant crystals were isolated by vacuum filtration, followed by sequential washing cycles with deionized water and ethanol (3 × 20 mL each) to remove residual organics. Subsequently, drying under ambient conditions yielded the target compound [(CH₃)₂NH₂][Zr(PO₄)F₂] (relative molecular mass (M) = 270.28 g mol⁻¹) as colorless plate-like crystals, where [(CH₃)₂NH₂]⁺ was generated by the thermal decomposition of DMA as the starting material and (PO₄)³⁻ was generated by the thermal decomposition of HPAA.³⁸ Yield: 20.13% (calculated based on Zr content). Elemental analysis (EA) validated phase purity: theoretical: C 9.02%, H 3.03%, N 5.26%. Experimental: C 8.97%, H 3.33%, N 4.83%.

Batch adsorption experiment

The adsorption performance evaluation of [(CH₃)₂NH₂][Zr(PO₄)F₂] was conducted in 25 mL polyethylene bottles with caps at room temperature. The experimental setup included: phase allocation, precise dosing of [(CH₃)₂NH₂][Zr(PO₄)F₂] (solid–liquid ratio m/V = 1 g L⁻¹) and Sr²⁺ standard solution into bottles, followed by continuous agitation on a magnetic stirrer for 12 h at ambient temperature. Post-adsorption, aliquots of the supernatant were extracted via syringe, filtered through 0.22 μm membranes, and diluted with 2% HNO₃ prior to Sr²⁺ concentration analysis by ICP-OES or ICP-MS. The post-adsorption solid phase was isolated from the aqueous medium via centrifugal separation at 8000 rpm for 10 min, followed by triple rinsing with deionized water to eliminate residual ions prior to subsequent analysis.

The Sr²⁺-exchanged derivative of [(CH₃)₂NH₂][Zr(PO₄)F₂] (designated as [(CH₃)₂NH₂][Zr(PO₄)F₂]-Sr) was synthesized via 12 h room-temperature agitation of 200 mg adsorbent in 200 mL of Sr²⁺ solution. The resultant [(CH₃)₂NH₂][Zr(PO₄)F₂]-Sr underwent comprehensive characterization (PXRD, EDS, SEM, and EA). Experimental particulars are described in the supplementary information (SI).

Characterization techniques

Elemental composition (C, H, N) was quantified via high-precision combustion analysis using a Vario EL III EA. Single-crystal X-ray diffraction analysis of [(CH₃)₂NH₂][Zr(PO₄)F₂] was performed on a Rigaku XtaLAB Synergy-R diffractometer equipped with a graphene-monochromated microfocus Mo K_α radiation source (λ = 0.71073 Å) at 100 K. Powder X-ray diffraction (PXRD) patterns were acquired at room temperature on a Rigaku Miniflex II system equipped with Cu K_α radiation (λ = 1.54178 Å), operating at 30 kV and 15 mA, with a 2θ scanning range of 5–65°. Elemental composition and spatial distribution were analyzed by energy-dispersive X-ray spectroscopy (EDS) coupled with scanning electron microscopy (SEM, JEOL JSM-6700F). Surface chemical states were investigated via X-ray photoelectron spectroscopy (XPS) on a ThermoFisher ESCALAB 240 Xi spectrometer using monochromatic Al K_α excitation. Solution pH measurements were conducted with a Leimage E-201F digital pH meter (Shanghai, China). The concentration of Sr²⁺ and other metal ions were quantificationally measured by inductively coupled plasma optical emission spectroscopy (ICP-OES, Thermo iCAP 7400) and inductively coupled plasma mass spectrometry (ICP-MS, Thermo XSeries II), respectively. Liquid–solid separation was achieved via centrifugation using a G16-WS benchtop centrifuge.

Results and discussion

Crystal structures

Single-crystal X-ray diffraction (SC-XRD) analysis determines that [(CH₃)₂NH₂][Zr(PO₄)F₂] crystallizes in the monoclinic space group P2₁/m. The asymmetric unit comprises one Zr atom, one PO₄ group, two F atoms, and one [(CH₃)₂NH₂]⁺ cation (Fig. S1). The Zr center adopts a distorted octahedral geometry, coordinated by four oxygen donors from distinct PO₄ groups (Zr–O: 2.054–2.079 Å) and two F atoms (Zr–F: 1.972–2.007 Å). The bond angles span 86.6(3)–93.5(3)° for O–Zr–F and 88.89(10)–91.11(10)° for O–Zr–O, reflecting minor octahedral distortion (Tables S1–S3).

Alternating arrangements of ZrO₄F₂ octahedra and PO₄ tetrahedra by corner-sharing mode generate a 2D anionic layer of [Zr(PO₄)F₂]_nⁿ⁻, in which each ZrO₄F₂ octahedron connects four PO₄ tetrahedra and each PO₄ tetrahedron links four ZrO₄F₂ octahedra (Fig. 1a). It is found that an eight-membered ring (8-MR) window with dimensions 4.63 × 5.04 Å is formed by two Zr atoms, two P atoms, and four O atoms in the anionic layer (Fig. 1b). Charge-balancing [(CH₃)₂NH₂]⁺ cations are intercalated between adjacent layers (Fig. 1c and d), with an interlayer spacing of 7.60 Å (defined as the shortest perpendicular distance between Zr atom of adjacent anionic layers, as defined in Fig. S1).³⁹ The relatively big interlamellar spacing and the presence of exchangeable [(CH₃)₂NH₂]⁺ cations provide the prerequisite for the Sr²⁺ capture via the ion exchange way.⁴⁰

Fig. 1 Crystal structural diagrams for the compound [(CH₃)₂NH₂][Zr(PO₄)F₂]. (a) A 2D anionic layer of [Zr(PO₄)F₂]_nⁿ⁻ viewed along the c-axis. (b) Ball-and-stick representation of the eight-membered ring in the anionic layer. (c) View of the packing diagram of [(CH₃)₂NH₂][Zr(PO₄)F₂] along the a-axis. (d) The anionic [Zr(PO₄)F₂]_nⁿ⁻ layers arrangement illustrating interlayer spacing (7.60 Å). Color scheme: Zr (cyan), P (purple), O (red), F (yellow), N (blue), C (grey).

Adsorption isotherms studies

Adsorption isotherm analysis serves as a fundamental methodology for characterizing the surface characteristics and binding affinity of adsorbents, while also determining their maximum adsorption capacity (q^Sr_m). The actual and theoretical (assuming that all available [(CH₃)₂NH₂]⁺ is fully exchanged by Sr²⁺) ion-exchange capacity is calculated by eqn (S1) and eqn (S2). The Sr²⁺ ions adsorption capacity of [(CH₃)₂NH₂][Zr(PO₄)F₂] was systematically evaluated through equilibrium isotherm studies. Experimental data were modeled using three classical adsorption isotherms: Langmuir (monolayer adsorption), Freundlich (heterogeneous surface), and Langmuir–Freundlich hybrid (multi-mechanism synergy).⁴¹ Nonlinear regression analysis reveals superior correlation for the Langmuir–Freundlich model (R² = 0.9858) compared to the Langmuir (R² = 0.9699) and Freundlich (R² = 0.8467) models (Fig. 2a, Tables S4 and S5, and eqn (S3)–(S5)). This statistical dominance indicates a complex adsorption mechanism involving both monolayer coverage and heterogeneous surface interactions.⁴¹ The derived q^Sr_m from the Langmuir–Freundlich model reaches 161.48 mg g⁻¹, which is higher than that of some crystalline zirconium-based adsorbents such as zirconium fluorophosphonate [(CH₃)₂NH₂]₂[ZrC₆H₄(CH₂PO₃)₂F₂] (SZ-7, 129.1 mg g⁻¹), [(CH₃)₂NH₂][ZrCH₂(PO₃)₂F] (SZ-4, 121 mg g⁻¹),⁴² and zirconium phosphates such as γ-ZrP (114.78 mg g⁻¹), K₂Zr(PO₄)₂ (52.83 mg g⁻¹), and α-ZrP (43.03 mg g⁻¹), as visually compared in Table S6 and Fig. 2b.^{20,21,42–46} Furthermore, the title compound [(CH₃)₂NH₂][Zr(PO₄)F₂] also outperforms some other inorganic adsorbents such as K_2xSn_4−xS_8−x (x = 0.65–1) (KTS-3, 102.00 mg g⁻¹)⁴⁷ and commercial AMP–PAN (16 mg g⁻¹).⁴⁸

Fig. 2 Adsorption performance diagrams of the compound [(CH₃)₂NH₂][Zr(PO₄)F₂]. (a) Sr²⁺ adsorption isotherms of [(CH₃)₂NH₂][Zr(PO₄)F₂] fitted by the Langmuir (black line), Langmuir–Freundlich (azure line), and Freundlich (blue line) models. (b) Comparison of the adsorption capacities of [(CH₃)₂NH₂][Zr(PO₄)F₂] with other reported inorganic adsorbents for Sr²⁺. (c) The kinetic curves of Sr²⁺ removal rate (R) by [(CH₃)₂NH₂][Zr(PO₄)F₂] plotted as the concentration (C_e) and R^Sr of Sr²⁺ ions vs. time (min). (d) Distribution coefficient (K^Sr_d, columns) and removal rate (R^Sr, line) at various solutions with different initial pH (0.92–12.92).

Adsorption kinetics studies

The rapid capture capability of an adsorption material for radioactive Sr²⁺ ions is critical in nuclear emergency management. [(CH₃)₂NH₂][Zr(PO₄)F₂] demonstrated exceptional adsorption efficiency, achieving a Sr²⁺ removal rate of 94.89% within 1 min in neutral aqueous solutions (Sr²⁺ concentration: from 34.04 mg L⁻¹ to 1.74 mg L⁻¹, Fig. 2c, Table S7, eqn (S6)). The current compound approached adsorption equilibrium within a 5 min contact period, achieving remarkable Sr²⁺ removal efficiency (R^Sr = 98.88%). We systematically investigated the Sr²⁺ adsorption mechanism of it using pseudo-first-order and pseudo-second-order kinetic models (Table S8, Fig. S2, eqn (S7) and (S8)).⁴⁹ Quantitative analysis demonstrated superior correlation with the pseudo-second-order kinetic model.

pH effect on adsorption

The pH of aqueous solutions critically governs adsorption processes by mediating proton competition and material stability. To systematically evaluate these effects, the removal performance of [(CH₃)₂NH₂][Zr(PO₄)F₂] was investigated across a pH range of 0.92–12.92 (Fig. 2d, Table S9, eqn (S9)). The title compound exhibited exceptional adsorption capacities across the pH range of 1.95–8.46, with distribution coefficients (K^Sr_d) consistently exceeding 5 × 10³ mL g⁻¹ and R^Sr above 83.99%, accompanied by a moderate leaching of Zr (Fig. S3a). The leaching rates of Zr of [(CH₃)₂NH₂][Zr(PO₄)F₂] after being immersed in different acidic (pH of 1.95) or alkaline (pH of 11.87) solutions for 12 h were less than 6.60% (Fig. S3a). At pH 1.95–12.92, the layered anionic architecture of [Zr(PO₄)F₂]_nⁿ⁻ is stable, as no phase transition observed by PXRD (Fig. S3b). This indicates that [(CH₃)₂NH₂][Zr(PO₄)F₂] has relatively wide acid–base stability. Under strongly acidic conditions (pH = 0.92), K^Sr_d exhibits a substantial reduction to 4.4 × 10² mL g⁻¹, possibly attributed to competitive proton exchange, where H⁺ effectively displaces Sr²⁺ at [(CH₃)₂NH₂]⁺ sites. Under extreme acidity, concomitantly partially structural degradation of the anionic layers further diminished the ion-exchange capacity.³⁰ Subsequently, we employed the pH drift method for analysing the pH dependence of adsorption. The ΔpH (= pH₀ − pH_f) vs. pH₀ curve shows the pH_pzc value of 5.0 (Fig. S3c). It indicates that the surface of [(CH₃)₂NH₂][Zr(PO₄)F₂] is positively charged at pH < 5.0 and negatively charged at pH > 5.0. For low pH, the surface of [(CH₃)₂NH₂][Zr(PO₄)F₂] favours electrostatic attraction of anions, while for high pH, it favours the attraction of cations.

Competitive adsorption

The presence of high-concentration competing ions (Na⁺, K⁺, Mg²⁺, and Ca²⁺) poses significant challenges for selective Sr²⁺ removal in nuclear wastewater treatment. Thus, we systematically evaluated the Sr²⁺ selectivity of [(CH₃)₂NH₂][Zr(PO₄)F₂] under competing ions (Na⁺, K⁺, Cs⁺, Mg²⁺, and Ca²⁺) using separation factor (SF) and distribution coefficient (K_d) analyses. Notably, effective SF typically requires values exceeding 100.⁵⁰

Competitive impacts of individual competitive Na⁺, K⁺, and Mg²⁺ ions concentrations on Sr²⁺ adsorption by [(CH₃)₂NH₂][Zr(PO₄)F₂] were quantified. At the Na⁺/Sr²⁺ molar ratio of 494.4 : 1, it maintains Sr²⁺ removal efficiency of 99.37% (K^Sr_d = 1.59 × 10⁵ mL g⁻¹, SF_Sr/Na = 2219.95), confirming excellent Na⁺/Sr²⁺ separation (Fig. 3a, Table S10, eqn (S10)). At the K⁺/Sr²⁺ molar ratio of 45.6 : 1, K^Sr_d reaches 3.96 × 10⁴ mL g⁻¹ with R^Sr of 97.54% (Fig. 3b, Table S11). Even under extremely excessive K⁺ (C^Sr₀ = 5.57 mg L⁻¹, C^K₀ = 294.91 mg L⁻¹, K⁺/Sr²⁺ molar ratio of 118.7 : 1), K^Sr_d reaches 2.69 × 10⁴ mL g⁻¹ (R^Sr = 96.41%). Notably, SF_Sr/K surpasses 100, confirming exceptional selectivity for Sr²⁺ over competing K⁺ ions. At the Mg²⁺/Sr²⁺ molar ratio of 20.1 : 1, K^Sr_d remains 1.17 × 10⁵ mL g⁻¹ (R^Sr = 99.15%, SF_Sr/Mg = 299.98), underscoring [(CH₃)₂NH₂][Zr(PO₄)F₂]'s resilience against the interference of alkaline earth metal ions (Fig. 3c, Table S12). However, when the Mg²⁺/Sr²⁺ molar ratios are more than 211.4 : 1, the Sr²⁺ removal of the current compound is interfered.⁵¹

Fig. 3 Selectivity performance diagrams of the compound [(CH₃)₂NH₂][Zr(PO₄)F₂]. K_d (columns) for Sr²⁺, K⁺, Na⁺, Mg²⁺ and R^Sr (line plot) of [(CH₃)₂NH₂][Zr(PO₄)F₂] at different molar ratios of Na/Sr (a), K/Sr (b), and Mg/Sr (c). (d) K_d for Sr²⁺, K⁺, Na⁺, Cs⁺ and R^Sr of [(CH₃)₂NH₂][Zr(PO₄)F₂] in mixed Na⁺, K⁺, Cs⁺, and Sr²⁺ solution at different molar ratios. (e) K_d for various metal ions and R^Sr of [(CH₃)₂NH₂][Zr(PO₄)F₂] in mixed Cs⁺, K⁺, Na⁺, Ca²⁺, Mg²⁺, and Sr²⁺ solution. (f) K^Sr_d and R^Sr of [(CH₃)₂NH₂][Zr(PO₄)F₂] in tap water, river water, lake river, and sea water.

Simultaneously, Sr²⁺ selectivity of [(CH₃)₂NH₂][Zr(PO₄)F₂] was systematically evaluated in mixed Na⁺, K⁺, Cs⁺, and Sr²⁺ solution and mixed Cs⁺, K⁺, Na⁺, Ca²⁺, Mg²⁺, and Sr²⁺ solution (Fig. 3d and e, Tables S13 and S14), respectively. In the mixed Na⁺, K⁺, Cs⁺, and Sr²⁺ solution, the material exhibits exceptionally high separation factors (SF_Sr/M > 100, M = Na⁺ and K⁺) for Sr²⁺ under the presence of monovalent competing ions. SF_Sr/M (M = Na⁺, K⁺) exceed 668.67 and 173.09 despite the presence of high concentrations for competing ions, whereas SF_Sr/Cs is merely 45.66. Nevertheless, in the mixed Cs⁺, K⁺, Na⁺, Ca²⁺, Mg²⁺, and Sr²⁺ solution, significantly diminished selectivity (SF_Sr/M < 100, M = K⁺, Cs⁺, Mg²⁺, and Ca²⁺) was observed in monovalent and divalent cation matrices. Remarkably, SF_Sr/Na values still remain elevated (464.99) with concurrent high distribution coefficient (K^Sr_d = 6.44 × 10³ mL g⁻¹) and removal efficiency (R^Sr = 86.57%).

Significantly, further validation in actual aqueous environments including tap water, lake water, river water, and seawater demonstrates consistently high R^Sr values of 99.09%, 98.34%, 98.97%, and 79.06%, respectively, while K^Sr_d values exceed 3.78 × 10³ mL g⁻¹ without exception (Fig. 3f, Table S15). The removal rate in actual seawater is higher than that of many reported adsorbents, such as K₂SbPO₆ (R^Sr = 1.17%)⁵² and K-HTNs (R^Sr = 35%)⁵³ at m/V = 1 g L⁻¹. These results unequivocally establish the maintenance of the removal capacity of [(CH₃)₂NH₂][Zr(PO₄)F₂] in complex contaminated matrices.

Adsorption and desorption

Reusability serves as a critical performance for ion exchange materials. Remarkably, the Sr-loaded phase [(CH₃)₂NH₂][Zr(PO₄)F₂]-Sr can be efficiently reused through elution with a 0.5 mol L⁻¹ KCl solution. The sample treated with KCl solution is named [(CH₃)₂NH₂][Zr(PO₄)F₂]-Sr-K. EDS analysis of the eluted material confirms near-complete substitution of Sr²⁺ by K⁺ ions (Fig. S4a and b). The eluted material still maintains exceptional Sr²⁺ adsorption capacity (R^Sr = 93.80%) over one adsorption–desorption cycle, with high desorption efficiencies (E^Sr = 91.47%) (Table S16, eqn (S11)). Structural integrity preservation is verified through PXRD patterns in cycled samples (Fig. S4c). [(CH₃)₂NH₂][Zr(PO₄)F₂] demonstrates reusability capabilities for the sequential capture of Sr²⁺ ions in radioactive waste management.

Adsorption mechanism

PXRD analysis validates no detectable phase transitions observed upon Sr²⁺ displacement (Fig. S5). Notably, the (002) diffraction peak of [(CH₃)₂NH₂][Zr(PO₄)F₂]-Sr shifts toward higher Bragg angles (Δ2θ = 0.46°) according to the Bragg equation. Corresponding SEM characterization reveals well-defined [(CH₃)₂NH₂][Zr(PO₄)F₂]-Sr crystals exhibiting pristine morphological integrity, evidenced by defect-free planar facets (Fig. S6).

Based on the above evidences, we hypothesized that this follows the ion exchange mechanism, wherein [(CH₃)₂NH₂]⁺ cations are displaced by Sr²⁺ (Fig. 4a). To validate this hypothesis, high-resolution XPS analysis of the pristine and Sr-loaded phases was conducted. All XPS spectra were corrected to the C 1s peak of adventitious carbon at 284.8 eV. XPS spectra reveal Sr 3d and P 2p signals overlap at ∼133.0 eV in [(CH₃)₂NH₂][Zr(PO₄)F₂]-Sr (Fig. 4b),⁵⁴ while emergent peaks at 360.0 eV (Sr 3s) and 270.0 eV (Sr 3p) provide unambiguous evidence of strontium integration, accompanied by definitive Sr 3d spectral signatures at binding energies of 134.6 eV (Sr 3d_5/2), 136.6 eV (Sr 3d_3/2) (Fig. 4c). At the same time, N 1s intensity attenuated at 402.1 eV after Sr²⁺ capture (Fig. 4d). Collectively, these results confirm that Sr²⁺ capture occurs via ion exchange between interlamellar [(CH₃)₂NH₂]⁺ cations and Sr²⁺.

Fig. 4 Adsorption mechanism diagrams of the compound [(CH₃)₂NH₂][Zr(PO₄)F₂]. (a) Mechanistic illustration of Sr²⁺ exchange within the anionic layers of [(CH₃)₂NH₂][Zr(PO₄)F₂]. (b) Comparative XPS survey spectra of pristine [(CH₃)₂NH₂][Zr(PO₄)F₂] (black) and [(CH₃)₂NH₂][Zr(PO₄)F₂]-Sr (blue). (c) Narrow scan XPS spectrum of Sr 3d of [(CH₃)₂NH₂][Zr(PO₄)F₂]-Sr. (d) Narrow scan XPS spectra of N 1s in pristine [(CH₃)₂NH₂][Zr(PO₄)F₂] (black) versus [(CH₃)₂NH₂][Zr(PO₄)F₂]-Sr (blue). (e) SEM micrographs at magnification of 2.0k, complemented by corresponding elemental distribution profiles of pristine [(CH₃)₂NH₂][Zr(PO₄)F₂] and [(CH₃)₂NH₂][Zr(PO₄)F₂]-Sr.

EDS analysis (Fig. S6f) confirmed successful capture of Sr²⁺ ions. Elemental mapping reveals a homogeneous distribution of Sr²⁺ in [(CH₃)₂NH₂][Zr(PO₄)F₂]-Sr (Fig. 4e). EA further reveals a concomitant reduction in nitrogen content from 4.83% to 0.43% after Sr²⁺ ion exchange (experimental: C 1.50%, H 2.11%, N 0.43%), quantitatively confirming that [(CH₃)₂NH₂]⁺ ions have been exchanged during ion exchange. Furthermore, the theoretical ion exchange capacity of [(CH₃)₂NH₂][Zr(PO₄)F₂] for Sr²⁺ is calculated as 162.10 mg g⁻¹ (eqn (S2)), whereas by experimental measurements the adsorption capacity of [(CH₃)₂NH₂][Zr(PO₄)F₂] for Sr²⁺ is 161.48 mg g⁻¹. This indicates that the interlamellar [(CH₃)₂NH₂]⁺ cations are almost completely exchanged by Sr²⁺ ions.

In simulated nuclear wastewater containing competing ions (Na⁺, K⁺, Cs⁺, Ca²⁺, and Mg²⁺), (CH₃)₂NH₂[Zr(PO₄)F₂] maintained Sr²⁺ removal efficiency. Ion exchange constitutes a stoichiometrically constrained process wherein charge balance is preserved through equivalent substitution of exchanged cations.⁵⁵ The Sr²⁺ selectivity of (CH₃)₂NH₂[Zr(PO₄)F₂] may arise from the electrostatic valence-dependency rule: adsorbent affinity scales with cation charge density under constant selectivity coefficients and concentrations of competing cations.⁵⁶ Consequently, the [Zr(PO₄)F₂]_nⁿ⁻ anionic layers may exhibit preferential binding toward divalent Sr²⁺ over monovalent Na⁺/K⁺. This material achieves SF_Sr/Mg exceeding 100, possibly because the hydrated Sr²⁺ diameter (8.24 Å) is smaller than competing cations (Mg²⁺, 8.56 Å).⁵¹

Thermodynamics of Sr²⁺ adsorption on [(CH₃)₂NH₂][Zr(PO₄)F₂] was further investigated between 293–353 K (Table S17, Fig. S7). The K_d values increased with temperature, indicating enhanced adsorption affinity at higher temperatures. The thermodynamic parameters were also calculated using the thermodynamic model (eqn (S13)–(S15)).^57,58 Thermodynamic parameters reveal positive ΔH⁰ (29.62 kJ mol⁻¹) and ΔS⁰ (0.18 kJ mol⁻¹ K⁻¹), confirming an entropy-driven process. Negative ΔG⁰ values (−22.10 to −32.55 kJ mol⁻¹) demonstrate spontaneous adsorption across the temperature range. The decreasing ΔG⁰ with rising temperature further support favourable adsorption at elevated temperatures. Notably, operational expenditure increases at higher temperatures, necessitating balanced temperature selection.

Conclusions

Here, a new layered zirconium phosphate fluoride ([(CH₃)₂NH₂][Zr(PO₄)F₂]) was synthesized hydrothermally. This material achieves a maximum Sr²⁺ adsorption capacity of 161.48 mg g⁻¹ (higher than that of many inorganic crystalline adsorbents) and fast kinetics (R^Sr = 94.89% within 1 min). It exhibits exceptional pH tolerance (operational range: 1.95–8.46). This material maintains Sr²⁺ removal efficiency in the presence of competing Cs⁺, K⁺, Na⁺, Ca²⁺, and Mg²⁺ ions. Particularly, it can maintain R^Sr of 79.06% even in seawater. XPS and thermodynamics confirm that spontaneous Sr²⁺ adsorption occurs through ion exchange with interlayered [(CH₃)₂NH₂]⁺ cations in the current zirconium phosphate fluoride. This work provides a promising adsorbent for capture of radiostrontium in radioactive waste management.

Author contributions

Z. Y. Chen: data curation, writing – reviewing and editing, validation, formal analysis, investigation, and software. S. Z. Liu: formal analysis and data curation. S. J. Li: formal analysis. Z. H. Chen: formal analysis and visualization. L. Yang: data curation. S. M. Zhang: data curation. H. Y. Sun: funding acquisition, reviewing and editing, supervision, and project administration. M. L. Feng: conceptualization, funding acquisition, review and editing, visualization, supervision, and project administration. X. Y. Huang: review and editing, and project administration. All authors have read and agreed to the published version of the manuscript.

Conflicts of interest

The authors declare no conflict of interest.

Data availability

Supplementary information: experimental section (batch adsorption experiments, equations, crystal structures section, and characterization section), and additional tables (Tables S1–S17) and figures (Fig. S1–S7). See DOI: https://doi.org/10.1039/D5CE00784D.

CCDC 2466544 contains the supplementary crystallographic data for this paper.⁵⁹

Acknowledgements

This research was funded by the National Natural Science Foundation of China (No. 22325605, U21A20296, and 22406185), the Natural Science Foundation of Fujian Province (No. 2024J08105), and the Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDB1170000).

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