Efficient Removal of Sr²⁺ by a Layered Metal Sulfide K_1.36Cd_1.12Bi_2.80S₆ via Ion Exchange

Abstract

As a fission product of ²³⁵U or ²³⁹Pu, ⁹⁰Sr is a β-emitting radionuclide with a relatively long half-life (t_1/2 = 28.9 years). Due to its high solubility, easy environmental mobility, and propensity for bioaccumulation within the food chain, the development of efficient materials for the selective capture of ⁹⁰Sr²⁺ is critical for the safe disposal of nuclear waste and environmental protection. In this study, a layered metal sulfide, K_1.36Cd_1.12Bi_2.80S₆ (denoted as KCBS), was synthesized via the high-temperature solid-phase method using K₂CO₃ as the potassium source. KCBS demonstrates high adsorption performance towards Sr²⁺, achieving a maximum adsorption capacity (q_m^Sr = 77 mg·g⁻¹). Moreover, it can maintain high adsorption efficiency (R^Sr > 84.15%) across a broad pH range of 2.98–12.01. In addition, KCBS exhibits the outstanding selectivity for Sr²⁺ removal in the presence of excessive Na⁺ ions and even in actual water samples. KCBS also possesses regenerability, maintaining its superior adsorption capacity for Sr²⁺ ions over three cycles. The mechanism study by energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) analyses indicates that the efficient Sr²⁺ capture is attributed to the ion exchange between Sr²⁺ and interlayer K⁺ ions in KCBS. This research further highlights the potential of layered metal sulfide ion exchange materials for radionuclide remediation.

1. Introduction

Nuclear energy plays a pivotal role in the global energy transition due to its high efficiency, stability, and low-carbon characteristics. However, the nuclear fuel cycle inevitably generates substantial volumes of radioactive liquid waste, for which its safe and long-term disposal is critical to the sustainable development of nuclear energy [1,2,3]. Strontium-90 (⁹⁰Sr), as a primary fission product of ²³⁵U and ²³⁹Pu, is one of the major radioactive sources in high-level radioactive liquid wastes [4,5]. ⁹⁰Sr is characterized by a long half-life (t_1/2 = 28.79 years), strong radioactivity (emitting high-energy β rays), and high heat release [6]. Notably, ⁹⁰Sr typically exists in the environment as ⁹⁰Sr²⁺ ions with high water solubility and mobility [7]. Due to its similar chemical properties with respect to Ca²⁺ ions, ⁹⁰Sr²⁺ can easily enter organisms through the food chain and accumulates in bones, constituting a persistent radiation risk [8]. In radioactive liquid waste, ⁹⁰Sr²⁺ frequently coexists with interfering ions such as Na⁺, Mg²⁺, and Ca²⁺ [9,10]. This poses a significant challenge for the selective removal of Sr²⁺ ions. In addition, ⁹⁰Sr serves as a β radiation source with applications in the field of industrial [11] and radiological medicine [12]. Consequently, the highly efficient removal and recovery of ⁹⁰Sr²⁺ from complex radioactive liquid wastes are of vital importance for the sustainable development of nuclear energy, ecological and environmental safety, and resource utilization.

Currently, various methods have been explored to remove Sr²⁺ from radioactive liquid wastes, primarily including solvent extraction, chemical precipitation, membrane filtration, and adsorption/ion exchange [13]. Among these, ion exchange is considered a more ideal treatment method due to its operational simplicity, high efficiency, and minimal secondary pollution generation [14,15]. Among the various ion exchange materials used for Sr²⁺ removal, organic ion exchange resins are widely applied. However, they face problems of high cost and long-term stability [16]. By contrast, inorganic ion exchange materials (such as titanates [17,18], clays [19], and zeolites [20]) offer significant advantages due to their outstanding chemical stability and easy post-treatment [21]. However, current inorganic ion exchangers still face some application limitations, including narrow pH ranges, limited adsorption capacity, and poor selectivity for Sr²⁺ [22,23].

In recent years, metal sulfide ion exchangers (MSIEs) have shown promising prospects in the field of radioactive contamination remediation [15,24,25,26]. MSIEs can remove Sr²⁺ ions through the ion exchange of Sr²⁺ with charge-balanced cations within interlayer spaces or channels of the anionic framework of MSIEs. S^2- in the anionic framework of MSIEs serves as Lewis soft base sites that are easily polarized, inducing metal–sulfur bond distortion and conferring structural flexibility, which results in the breathing effect and adaptive modulation of interlayer or channel spacing to facilitate Sr²⁺ entry and the ion exchange process [16]. To date, MSIEs for Sr²⁺ removal mainly are Sn-containing metal sulfides, such as the KMS series (e.g., K_2xMg_xSn_3-xS₆, q_m^Sr = 86.89 mg·g⁻¹ [27]), KTS series (e.g., K_2xSn_4-xS_8-x, q_m^Sr = 102 mg·g⁻¹ [28]) and its derivatives (e.g., Na₂Sn₃S₇, q_m^Sr = 80 mg·g⁻¹ [29] and Na_1.94Sn_2.87S₇, q_m^Sr = 88.9 mg·g⁻¹ [30]), FJSM-SnS series (e.g., (Me₂NH₂)_1.33(Me₃NH)_0.66Sn₃S₇, q_m^Sr = 65.19 mg·g⁻¹ [31] and [CH₃NH₃][Bmmim]Sn₃S₇, q_m^Sr = 59.41 mg·g⁻¹ [32]), and FJSM-KSnS (K₂Sn₂S₅, q_m^Sr = 59.88 mg·g⁻¹ [33]). However, research on Sn-free MSIEs for Sr²⁺ removal remains quite limited. Only a few examples, such as InS-1 ([CH₃CH₂NH₃]₆In₆S₁₂, q_m^Sr = 105.35 mg·g⁻¹ [34]), K-TaS₂ (K_0.33TaS₂, q_m^Sr = 37.95 mg·g⁻¹), and K-TbS₂ (K_0.36NbS₂, q_m^Sr = 79.25 mg·g⁻¹), have been reported [35]. In addition, the adsorption capacity and selectivity for Sr²⁺ capture by MSIEs are still unsatisfactory. Therefore, new MSIEs still require development for Sr²⁺ removal.

Herein, a layered metal sulfide K_1.36Cd_1.12Bi_2.80S₆ (denoted as KCBS) was prepared via the high-temperature solid-phase method using K₂CO₃ as the potassium source. KCBS exhibits a Bi₂Se₃-type structure, consisting of anionic layers of [Cd_1.12Bi_2.80S₆]_n^1.36n- and exchangeable K⁺ ions in the interlayer spaces. KCBS shows high adsorption capacity (q_m = 77 mg·g⁻¹) and rapid kinetic response (t = 30 min, R^Sr = 95.08%) for Sr²⁺ removal. It maintains high Sr²⁺ removal efficiency (R^Sr > 84.15%) in the pH range from 2.98 to 12.01, while preserving framework stability and achieving low Cd and Bi leaching rates (R_L < 0.02%). Notably, KCBS can effectively remove Sr²⁺ in the presence of excessive Na⁺ ions and various competing ions, as well as in actual water samples. Furthermore, adsorption and desorption experiments demonstrate that KCBS can be readily regenerated using 2 mol·L⁻¹ KCl solution as the eluent and maintain framework stability and Sr²⁺ removal efficiency after three regeneration cycles. Characterizations, including energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and powder X-ray diffraction (PXRD), reveal that the efficient Sr²⁺ removal is attributed to the ion exchange between Sr²⁺ and interlayer K⁺. This work provides a new metal sulfide scavenger for Sr²⁺ with high removal performance and highlights the development of metal sulfide ion exchange materials for radioactive contamination remediation.

2. Materials and Methods

2.1. Materials

S (99.50%), NaCl (AR), MgCl₂·6H₂O (AR), and HNO₃ (69%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). KCl (AR) was purchased from Tianjin Fengchuan Chemical Reagent Co., Ltd. (Tianjin, China). Bi₂S₃ (99.999%) was obtained from Adamas Reagent Co., Ltd. (Shanghai, China). Cd (AR) was purchased from Tianjin Jinke Fine Chemical Industry Research Institute Co., Ltd. (Tianjin, China). CaCl₂·2H₂O (74%) was purchased from Shanghai Sili Chemical Plant (Shanghai, China). CsCl (99.99%) was purchased from Shanghai Longjin Metallic Material Co., Ltd. (Shanghai, China). SrCl₂·6H₂O (AR) and NaOH (AR) were obtained from Guangfu Fine Chemical Research Institute (Tianjin, China). All reagents were used directly after purchase without undergoing any processing.

2.2. Methods

2.2.1. Synthesis of K_1.36Cd_1.12Bi_2.80S₆ (KCBS)

The crystalline samples of KCBS were synthesized via solid-state reaction. Previously, K₂S₂ was employed as the potassium source for the preparation of K_1.83Cd_1.83Bi_2.17S₆ [36], a compound with a similar layered structure to that of KCBS. However, alkali metal polysulfides like K₂S₂ are susceptible to degradation upon air exposure, making them unsuitable for long-term storage unless kept inside a glove box. Furthermore, alkali metal polysulfides are commonly synthesized using liquid ammonia, posing operational risks [37,38]. To avoid these issues, K₂CO₃ was used instead of K₂S₂ as the potassium source in this study, and a pure phase of KCBS was successfully obtained under simple synthesis conditions. The synthesis procedure is as follows. A mixture of K₂CO₃ (0.45 mmol, 0.0622 g), Bi₂S₃ (0.5 mmol, 0.2571 g), Cd (0.4 mmol, 0.045 g), and S (1 mmol, 0.032 g) was thoroughly ground. The resultant powder mixture was loaded into a quartz tube (outer diameter of 12 mm, inner diameter of 10 mm, and length of 30 cm). The quartz tube was evacuated to a pressure below 100 Pa and then sealed, which underwent thermal treatment in a muffle furnace according to the following program: heating from room temperature to 350 °C within 12 h and holding at 350 °C for 12 h and then heating to 780 °C within 12 h and holding at 780 °C for 16 h and finally slowly cooling to 300 °C within 96 h, followed by natural cooling to room temperature. Subsequently, the solid product was repeatedly washed with deionised water and ethanol. During the washing process, a loss of the supernatant black fine microcrystals occurred. Then, the black plate-like crystals were collected (yield: 0.252 g, 74% based on Cd).

2.2.2. Characterization Techniques

Powder X-ray diffraction (PXRD) measurements were performed on a Rigaku Miniflex II diffractometer (Japan) using CuKα radiation (λ = 1.54178 Å) at 30 kV and 15 mA, with data collected across a 2θ range spanning 5° to 65°. Single-crystal X-ray diffraction data were acquired on a Rigaku diffractometer (Japan) utilizing graphite-monochromated MoKα radiation (λ = 0.71073 Å). X-ray photoelectron spectroscopy (XPS) measurements were conducted on an ESCALAB 250Xi spectrometer (Thermo Scientific, Waltham, MA, USA) using AlKα radiation. Thermogravimetric analysis (TGA) was carried out on a NETZSCH STA 449 F3 instrument (Netzsch, Germany) under a nitrogen atmosphere at a heating rate of 10 °C·min⁻¹. Morphological and elemental analyses were performed using a field-emission scanning electron microscope (FE-SEM, JSM-6700F, JEOL, Akishima, Tokyo, Japan) equipped with energy-dispersive X-ray spectroscopy (EDS) for elemental mapping. Elemental concentrations were determined by inductively coupled plasma mass spectrometry, including tandem mass spectrometry (ICP-MS/MS, Agilent 8900, Santa Clara, CA, USA) and conventional ICP-MS (Thermo XSeries II, Waltham, MA, USA), following stringent quality assurance protocols such as the use of calibration standards and repeated measurements.

2.2.3. Batch Adsorption Experiments

Ion Exchange Experiment: In a typical procedure, a measured amount of KCBS crystals was mixed with a Sr²⁺-containing solution at a solid-to-liquid ratio of 1000 mL·g⁻¹. Unless otherwise specified, all exchange experiments were conducted at room temperature in a shaking incubator operating at 160 rpm. After reaction, the supernatant was collected by natural settling followed by filtration through a 0.22 μm membrane, diluted with 2% HNO₃, and analyzed for residual Sr²⁺ concentration using ICP-MS/MS and ICP-MS. Solid samples were washed sequentially with deionized water and ethanol, air-dried at room temperature, and stored for subsequent characterization (PXRD, EDS, TG, etc.).

Isothermal Adsorption Experiment: In total, 10 mg of KCBS crystal samples was mixed with 10 mL of Sr²⁺ solutions with varying initial concentrations (C₀^Sr: 3.31 to 537 mg·L⁻¹). The mixture was shaken for 12 h. The adsorption data were fitted to the Langmuir, Freundlich, and Langmuir–Freundlich models for the calculation of the maximum adsorption capacity.

Adsorption Kinetics Experiment: In total, 50 mg of KCBS was added to 50 mL of Sr²⁺ solution (C₀^Sr = 6.45 mg·L⁻¹) and stirred magnetically. Aliquots (~1 mL) were taken at specific time intervals. After separation, the residual Sr²⁺ concentrations were measured. The kinetic data were analyzed employing pseudo-first-order and pseudo-second-order kinetic models.

Adsorption Thermodynamic Experiment: Experiments were carried out at 293, 313, and 333 K, with 10 mg of KCBS added to 10 mL of a Sr²⁺ solution (C₀^Sr = 10.58 mg·L⁻¹), which was magnetically stirred throughout the process.

Acid–Base Stability Experiment: Sr²⁺-containing solutions with different pH values ranging from 1 to 12 (C₀^Sr: 3.49–5.30 mg·L⁻¹) were prepared using nitric acid solution or sodium hydroxide solution. Then, 10 mg of KCBS was mixed with 10 mL of the above solutions and shaken for 12 h. The solid residues were washed with deionised water and ethanol and characterized by PXRD.

Competitive Experiment: Solutions with various molar ratios (M/Sr²⁺) were prepared (M = Cs⁺, Na⁺, Mg²⁺, or Ca²⁺). Mixed solutions containing multiple ions were also prepared. In addition, Sr²⁺ was spiked into various water samples, including tap water (Fuzhou, China), lake water (Qishan Lake, Fuzhou, China), river water (Wulong River, Fuzhou, China), and seawater (Dongshan Island, Zhangzhou, China), to simulate polluted water environments.

Adsorption Experiment after Irradiation: In total, 10 mg samples of KCBS irradiated with β and γ rays at doses of 100 kGy and 200 kGy were placed in 10 mL of Sr²⁺ solution (C₀^Sr = 8.07 mg·L⁻¹) for an adsorption study.

Adsorption and Desorption Experiment: In total, 250 mg of KCBS was added to 250 mL of 1000 mg·L⁻¹ Sr²⁺ solution and shaken for 12 h. The products after adsorption were washed with deionized water and ethanol, dried, and then characterized by EDS, Element Mapping, and PXRD. Then, the product was immersed in a 2 mol·L⁻¹ KCl solution at a solid-to-liquid ratio of 1000 mL·g⁻¹ and shaken for 12 h to desorb Sr²⁺. The desorbed sample was similarly washed with deionized water and ethanol, dried, and characterized. Finally, the adsorption and regenerative capacity were evaluated using the Sr²⁺ solution (C₀^Sr = 7.9 mg·L⁻¹) and 2 mol·L⁻¹ KCl solution. The adsorption–desorption–re-adsorption cycle was repeated three times to assess cycling stability.

3. Results

3.1. Crystal Structure and Characterization

Single-crystal X-ray analysis reveals that KCBS crystallizes in the space group P6₃/mmc (Tables S1–S3). Its structure features a two-dimensional anionic layer of [Cd_1.12Bi_2.80S₆]_n^1.36n- with K⁺ ions located in the interlayer spaces. [(Bi/Cd)S₆] octahedra form the Bi₂Se₃-type anionic layers of [Cd_1.12Bi_2.80S₆]_n^1.36n- (Figure 1a) by edge-sharing. Different from the previously reported K_1.83Cd_1.83Bi_2.17S₆ with only two crystallographically independent potassium sites (K1, K2) in the asymmetric unit, there are three crystallographically independent potassium sites (K1, K2, K3) in K_1.36Cd_1.12Bi_2.80S₆ (KCBS) (Table S4) [36]. In addition, compared with the anionic layers of [Cd_1.83Bi_2.17S₆]_n^1.83n– in K_1.83Cd_1.83Bi_2.17S₆, the anionic [Cd_1.12Bi_2.80S₆]_n^1.36n– layers in KCBS contain more Cd vacancies, resulting in a slightly higher K/Cd ratio. These structural differences are likely attributable to the distinct synthesis conditions. Previous research has demonstrated that the ion exchange performance of two-dimensional MSIEs exhibits an inverted U-curve rule with the interlayer d_S-S [35]. Specifically, when the d_S-S of MSIEs falls within the range of 4.676 to 6.403 Å, they may exhibit good ion exchange performance. The interlayer d_S-S in KCBS is 5.201 Å, which suggests that the current compound possesses significant ion exchange potential (Figure 1b).

Figure 1. (a) Schematic of [(Cd/Bi)S₆] octahedron and a Bi₂Se₃-type anionic layer of [Cd_1.12Bi_2.80S₆]_n^1.36n-. (b) Packing diagram of KCBS viewed along the a-axis, showing the layers stacking and interlayer disordered K⁺ ions, with the interlayer d_s–s indicated. (c) Experimental and simulated PXRD patterns of KCBS. (d) Photographs of KCBS and KCBS-Sr crystal samples, along with corresponding SEM images and elemental distribution maps.

The PXRD pattern of KCBS exhibits agreement with the simulated pattern from the single-crystal X-ray structure, confirming a pure phase (Figure 1c). The discrepancy in the intensity of some diffraction peaks is likely due to the preferred orientation. In total, 100 mg of KCBS crystal samples was immersed in 100 mL of a 1000 mg·L⁻¹ Sr²⁺ solution and shaken at room temperature for 12 h. The obtained adsorption product is denoted as KCBS-Sr. Elemental distribution maps (Figure 1d) and EDS (Figure S1a,b) results confirm the successful replacement of K⁺ by Sr²⁺. The surface morphology of the material, as observed by scanning electron microscopy (SEM), shows no significant change upon substitution of K⁺ with Sr²⁺ (Figure S2). The TGA curve reveals that KCBS exhibits approximately 2% mass loss between 25 °C and 125 °C (Figure S1c), which is attributed to the loss of adsorbed surface water. No significant mass change occurs between 125 °C and 700 °C, indicating the high thermal stability of KCBS. Meanwhile, KCBS-Sr exhibits a mass loss of approximately 7.3% from 20 to 500 °C, primarily attributable to the desorption of water molecules from the hydrated Sr²⁺ ions in it.

3.2. Adsorption Isotherm and Adsorption Kinetics Studies

The maximum adsorption capacity of KCBS for Sr²⁺ was investigated through systematic adsorption isotherm experiments. The relationship between Sr²⁺ adsorption amount (q_e^Sr, Equation (S1)) and equilibrium concentration (C_e^Sr) is illustrated (Figure 2a and Table S5). The isotherm data are fitted using the Langmuir (Equation (S2)), Freundlich (Equation (S3)), and Langmuir–Freundlich models (Equation (S4)) [39]. Among them, the Langmuir–Freundlich model exhibits the highest goodness-of-fit (R² = 0.98396), outperforming both the Langmuir model (R² = 0.97557) and the Freundlich model (R² = 0.96663) (Table S6). Calculations based on the Langmuir–Freundlich model indicate that the maximum adsorption capacity of KCBS for Sr²⁺ (q_m^Sr) is 77 mg·g⁻¹, which exceeds that of some reported Sr²⁺ adsorbents, such as Na₅Zn_3.5Sn_3.5S₁₃ (32.3 mg·g⁻¹) [40], Na_1.60Mg_0.33Sn_3.15S₇ (52.6 mg·g⁻¹) [41], and (Me₂NH₂)_1.33(Me₃NH)_0.66Sn₃S₇ (65.19 mg·g⁻¹) [31], as well as SnO₂-Sb₂O₅ (25.7 mg·g⁻¹) [42] and AMP-PAN (16 mg·g⁻¹) [43] (Table S7).

Figure 2. (a) Adsorption isotherms of KCBS for Sr²⁺ fitted using the Langmuir, Freundlich, and Langmuir–Freundlich models. (b) Kinetic curves of KCBS plotted with C_t^Sr (red line) and R^Sr (blue line) vs. time t (min). (c) Effect of temperature on Sr²⁺ adsorption by KCBS. (d) K_d^Sr (mL·g⁻¹) (column) and R^Sr (%) (dotted line plot) values of KCBS at different pH values.

Furthermore, the Sr²⁺ adsorption kinetics of KCBS were investigated. The Sr²⁺ concentration rapidly decreases from 6.45 mg·L⁻¹ to 1.77 mg·L⁻¹ within 15 min, achieving a high removal efficiency (R^Sr, Equation (S5)) of approximately 95.08% at 30 min (Figure 2b and Table S8), which indicates a rapid and highly efficient adsorption process. The kinetic data are successfully fitted to both the pseudo-first-order and pseudo-second-order models (Equations (S6) and (S7), Figure S3 and Table S9).

3.3. Adsorption Thermodynamics Study

The thermodynamic behavior of KCBS for Sr²⁺ adsorption was investigated within the temperature range of 293 K to 333 K. The results indicate that the distribution coefficient (K_d^Sr, Equation (S8)) increases progressively with rising temperatures, suggesting that higher temperatures are more favorable for the adsorption process (Figure 2c and Table S10). Concurrently, relevant thermodynamic parameters are calculated based on thermodynamic models (Equations (S9)–(S11)). Both the enthalpy change (ΔH⁰) and entropy change (ΔS⁰) are positive values (ΔH⁰ = 56.52 kJ·mol⁻¹, ΔS⁰ = 0.27 kJ·mol⁻¹·K⁻¹). Therefore, all measured Gibbs free energy changes (ΔG⁰) are negative, confirming the spontaneity of this adsorption process. As the temperature increases from 293 K to 333 K, ΔG⁰ decreases gradually from −22.49 kJ·mol⁻¹ to −33.28 kJ·mol⁻¹, further indicating that higher temperatures are of benefit to Sr²⁺ adsorption.

3.4. Effect of pH on Sr²⁺ Adsorption

Considering the variable acidity and alkalinity of nuclear wastewater, solution pH critically affects the removal efficiency of radioactive nuclides. The adsorption performance of KCBS towards Sr²⁺ was systematically evaluated across a pH range of 2.01 to 12.01. The results show that KCBS maintains high R^Sr above 84.15% within a broad pH range of 2.98 to 12.01 (Figure 2d). Even under strongly acidic conditions (pH = 2.01), the R^Sr is 32.26%. Under strongly alkaline conditions (pH = 12.01), KCBS still exhibits good adsorption performance with an R^Sr of 98.42%, and the K_d^Sr reaches 6.27 × 10⁴ mL·g⁻¹. Furthermore, Cd and Bi leaching rates (R_L, Equation (S12)) in solutions after adsorption are below 0.02% throughout the pH range of 2.01 to 12.01, indicating excellent material stability (Figure S4b and Table S11). PXRD analyses confirm that the layered framework of KCBS remains intact after adsorption under various pH values (Figure S4a). KCBS shows outstanding and stable Sr²⁺ removal capability over a wide pH range from acidic to strongly alkaline, making it a promising candidate for treating nuclear wastewater with complex pH conditions.

3.5. Effect of Competitive Metal Ions

Nuclear wastewater contains substantial amounts of competitive ions such as Na⁺, Cs⁺, Ca²⁺, and Mg²⁺, for which their presence may significantly affect the removal of Sr²⁺. Therefore, the adsorption capacity of KCBS towards Sr²⁺ in the presence of competitive metal ions was systematically investigated. At different Na/Sr molar ratios from 7.03 to 131.35, K_d^Sr consistently remained above 10⁴ mL·g⁻¹, and R^Sr exceeded 92.2% (Figure 3a and Table S12). Notably, at a Na/Sr molar ratio of 7.03, the separation factor (SF_Sr/Na, Equation (S13)) of KCBS reaches 3789.77. These results demonstrate that KCBS can efficiently and selectively separate Sr²⁺ under high concentrations of Na⁺. Furthermore, the selective removal of Sr²⁺ was investigated under coexisting Cs⁺ and Na⁺. When the Sr/Cs/Na molar ratio is 1:2.32:72.30, R^Sr still reaches 89.95%. Even when the Sr/Cs/Na molar ratio increases to 1:4.69:137.94, R^Sr remains at 63.7% (Figure S5 and Table S13). This indicates that KCBS retains Sr²⁺ removal capacity in the presence of mixed monovalent alkali metal ions.

Figure 3. (a) K_d (mL·g⁻¹) (column) and R (%) (dotted line plot) of KCBS for Sr²⁺ and Na⁺ removal in solutions with different Na/Sr molar ratios. (b) K_d (mL·g⁻¹) (column) and R (%) (dotted line plot) of KCBS for Sr²⁺ and Mg²⁺ removal in solutions with different Mg/Sr molar ratios. (c) K_d^Sr (mL·g⁻¹) (column) and R(%) (dotted line plot) of KCBS for Sr²⁺ removal in tap water (TW), river water (RW), lake water (LW), and sea water (SW). (d) K_d^Sr (mL·g⁻¹) and R^Sr (%) (dotted line plot) of KCBS samples before and after 100/200 kGy β/γ irradiation.

Competitive adsorption experiments were also carried out to assess the interference of divalent ions Ca²⁺ and Mg²⁺ on Sr²⁺ removal by KCBS. When the Mg/Sr molar ratios range from 11.06 to 19.17, K_d^Sr remains above 10³ mL·g⁻¹, with R^Sr exceeding 87.3%. Increasing the Mg/Sr ratio to 43.77 leads to a significant decline in K_d^Sr to 993 mL·g⁻¹ and R^Sr to 49% (Figure 3b, Table S14). When the Ca/Sr ratio is 3.54, K_d^Sr surpasses 10⁴ mL·g^−1, and R^Sr reaches 98.82%. Once the ratio exceeds 5.64, however, K_d^Sr drops below 10³ mL·g⁻¹, and R^Sr is lower than 56.6% (Figure S6 and Table S15). These results confirm that KCBS effectively removes Sr²⁺ in the presence of low concentrations of Mg²⁺ or Ca²⁺. Even within the complex system featuring molar ratios of Sr/Na/Cs/Mg/Ca at 1:75.82:2.27:6.97:3.16, the R^Sr of KCBS is 67.0% (Table S16). However, the above results indicate that the presence of high-concentration Mg²⁺ and Ca²⁺ significantly affects the removal efficiency of Sr²⁺.

To simulate the actual Sr²⁺ removal efficiency in actual environmental water samples, low-concentration Sr²⁺ (C₀^Sr: 3.028–3.43 mg·L⁻¹) was artificially added to tap water, river water, lake water, and sea water to evaluate the adsorption performance of KCBS for Sr²⁺. The results demonstrate that KCBS exhibits outstanding Sr²⁺ adsorption performance in tap water and river water, with K_d^Sr being more than 10⁴ mL·g⁻¹ and R^Sr being higher than 96.68% (Figure 3c and Table S17). In lake water and seawater, K_d^Sr values are 916 mL·g⁻¹ and 1.22 × 10³ mL·g⁻¹, respectively, with corresponding R^Sr values of 47.8% and 55.0%.

3.6. Study on the Irradiation Stability of KCBS

In the process of treating radioactive wastewater, adsorbents are often subjected to high levels of radiation, which can damage their structural integrity and reduce their capacity to remove target radionuclides. Consequently, the Sr²⁺ adsorption performance of KCBS was investigated using samples before and after irradiation with 100/200 kGy β-rays and 100/200 kGy γ-rays. By comparing the PXRD patterns of the original and irradiated samples, it is confirmed that KCBS can maintain structural stability after strong irradiation, exhibiting no obvious collapse or transformation (Figure S7). Furthermore, the K_d^Sr of KCBS after β irradiation exceeds 10⁵ mL/g, with R^Sr exceeding 99.43%. By contrast, KCBS samples subjected to 100 and 200 kGy γ irradiation exhibit K_d^Sr at the level of 10³ mL·g⁻¹, with a corresponding R^Sr of 64.3% and 63.1%, respectively (Figure 3d and Table S18). This indicates that β irradiation does not affect the Sr²⁺ removal capacity of KCBS, whereas γ irradiation causes a decline in the Sr²⁺ adsorption performance of KCBS.

3.7. Adsorption and Desorption

To investigate the regeneration performance of KCBS-Sr, adsorption–desorption cycling experiments were conducted. Initially, the desorption efficiency of Sr²⁺ from KCBS-Sr was explored using a 2 mol·L⁻¹ KCl solution as the eluent. The 2 mol·L⁻¹ KCl solution can effectively desorb Sr²⁺ from KCBS-Sr, yielding the product noted as KCBS-Sr-K (Figure 4a and Figure S8b). The PXRD patterns show that the layered framework of the current metal sulfide remains stable throughout this process (Figure S8a). Accordingly, removal and desorption efficiencies are systematically studied across three adsorption cycles. R^Sr remains consistently above 99.94% throughout the three cycles, with desorption efficiency (E^Sr, Equation (S14)) approaching 100% (Figure 4c and Table S19). Furthermore, PXRD patterns reveal that samples maintain their layered structure after each adsorption and regeneration cycle, showing no signs of collapse (Figure 4b). Consequently, KCBS enables simple and efficient regeneration using the 2 mol·L⁻¹ KCl solution while preserving its Sr²⁺ adsorption efficiency, demonstrating its suitability as a regenerable, recyclable adsorption material.

Figure 4. (a) SEM image and elemental distribution maps of KCBS-Sr-K. (b) PXRD patterns of KCBS during three adsorption–desorption cycles. (c) R^Sr (%) (red column) and E^Sr (%) (grey column) during three adsorption–desorption cycles.

3.8. Adsorption Mechanism

To elucidate the mechanism of Sr²⁺ capture by KCBS, XPS spectra of KCBS and KCBS-Sr were acquired (Figure 5a). In KCBS, the characteristic peaks of K 2p_3/2 and K 2p_1/2 were clearly observed at approximately 292.5 eV and 295.2 eV, respectively (Figure 5b). After Sr²⁺ adsorption, these K 2p characteristic peaks disappeared in the KCBS-Sr sample, while characteristic Sr 3d_5/2 and Sr 3d_3/2 peaks appeared at around 133.8 eV and 135.5 eV, respectively (Figure 5c). Furthermore, the PXRD pattern reveals that after capturing Sr²⁺, the (002) diffraction peak at a 2θ angle of 7.561° exhibits a pronounced shift towards a lower 2θ angle of 6.418° (Figure 5d), indicating an expansion of the interlayer spacing from 11.682 to 13.761 Å. This expansion is primarily attributed to the Sr²⁺ ion radius (4.12 Å) being considerably larger than that of K⁺ (3.31 Å) [44], leading to interlayer spacing expansion after Sr²⁺ enters the interlayer. This behavior is similar to the Sr²⁺ ion exchange of KMS-2 [16]. In summary, combining EDS, XPS, and PXRD results confirms that KCBS achieves effective Sr²⁺ capture through ion exchange between interlayer K⁺ and Sr²⁺ (Figure 5e).

Figure 5. (a) Survey spectra of KCBS and KCBS-Sr. (b) X-ray photoelectron spectra of potassium for KCBS and KCBS-Sr. (c) X-ray photoelectron spectrum of strontium for KCBS-Sr. (d) PXRD patterns of KCBS-Sr, pristine KCBS, and simulated KCBS. (e) Illustration of the KCBS ion exchange of Sr²⁺.

4. Conclusions

In this study, a metal sulfide ion exchanger, K_1.36Cd_1.12Bi_2.80S₆ (KCBS), was successfully synthesized for the efficient removal of Sr²⁺ from water environments. The structure of KCBS features the anionic layers of [Cd_1.12Bi_2.80S₆]_n^1.36n- with K⁺ ions located in the interlayer spaces. KCBS exhibits a high Sr²⁺ adsorption capacity of 77 mg·g⁻¹ and rapid adsorption kinetics, achieving 95.08% Sr²⁺ removal within 30 min. It maintains outstanding stability and adsorption performance across a broad pH range (2.98–12.01), with extremely low leaching of cadmium and bismuth. Crucially, KCBS with good renewability can be reused for three adsorption–desorption cycles without structural degradation, maintaining over 99.94% removal efficiency. Mechanism studies confirm that Sr²⁺ removal primarily relies on ion exchange with interlayer K⁺ ions, concurrently accompanied by significant interlayer spaces expansion due to Sr²⁺ intercalation. This study comprehensively demonstrates that KCBS represents a promising inorganic ion exchange material for Sr²⁺ capture. This finding provides significant reference for developing advanced metal sulfide ion exchange materials for radionuclide remediation. Although MSIEs presently exhibit excellent adsorption properties for Sr²⁺ at the laboratory stage, several issues remain to be addressed. For instance, synthetic methods for metal sulfides require further exploration to produce structurally novel and high-performance MSIEs. In addition, composite MSIEs with good mechanical stability and ease of handling should be further developed to meet the demands of practical applications.