A Two-Dimensional Thiotitanate Ion Exchanger with High Cs⁺ Removal Performance

Abstract

¹³⁷Cs is a persistent β/γ-emitter (t_1/2 = 30.1 years) generated from ²³⁵U and ²³⁹Pu fission. It is a critical challenge to efficiently capture ¹³⁷Cs⁺ for nuclear waste management due to its high solubility, environmental mobility, and propensity for biological accumulation. Herein, we prepare a two-dimensional (2D) thiotitanate Rb_0.32TiS₂·0.75H₂O (denoted Rb-TiS₂) using a special molten salt synthesis method, “Mg + RbCl”. Rb-TiS₂ can selectively capture Cs⁺ from aqueous solutions. Its structure features a flexible anionic thiotitanate layer with Rb⁺ as counter ions located at the interlayer spaces. As an ion exchanger, it possesses high adsorption capacity (q_m^Cs = 232.70 mg·g⁻¹), rapid kinetics (the removal rate R > 72% within 10 min), and a wide pH tolerance range (pH = 4–12) for Cs⁺ adsorption. Through a single-crystal X-ray structural analysis, we elucidated the mechanism of Cs⁺ capture, revealing the ion exchange pathways between Cs⁺ and Rb⁺ in Rb-TiS₂. This work not only provides an important reference for the synthesis of transition metal sulfides with alkali metal cations but also proves the application prospect of transition metal sulfides in radionuclide remediation.

1. Introduction

Nuclear energy, with its high energy density and low carbon emissions, is a key solution for addressing global energy demands and climate change. However, the radioactive waste generated simultaneously presents significant environmental and public health challenges [1,2,3,4,5]. Among the fission products of spent fuel, ¹³⁷Cs emerges as one of the particularly problematic radionuclides due to its long half-life (30.1 years), high decay heat, and potent β/γ radiation emissions [6]. It predominantly exists as the form of Cs⁺ cation. Its high-water solubility facilitates rapid migration through environmental media and entry into ecosystems via the hydrological cycle, posing a serious threat to environmental security and human health [7]. In addition, as alkali metal ions, Cs⁺ ions have similar chemical properties to K⁺ ions, which makes it easy to accumulate in living organisms, increasing the risk of radiation exposure, especially the potential damage to bone [8], thyroid [9], and germ cells [10]. Historical nuclear accidents, such as the Chernobyl accident, Goiânia radiation accident, and Fukushima nuclear accident, have amply demonstrated that large-scale releases of radiocesium can result in long-term environmental degradation and serious public health effects [11,12]. Further, ¹³⁷Cs can be applied in specific fields; for example, industrial irradiation [13], medical radiation therapy [14], radioisotope thermoelectric generators [15], etc. Therefore, the rapid and efficient removal of cesium from complex nuclear wastewater is of vital importance.

At present, the common methods for Cs⁺ ion removal include chemical precipitation, solvent extraction, membrane separation, and ion exchange [16]. Among these, chemical precipitation suffers from poor selectivity despite its operational simplicity [17]. Solvent extraction offers high efficiency but has prohibitive costs [18]. Membrane separation enables continuous operation yet faces challenges such as fouling and clogging [19]. By contrast, ion exchange techniques have garnered significant attention due to their high selectivity, operational simplicity, and no secondary pollution [20]. Ion exchange materials with exchangeable cations and specific recognition sites can efficiently adsorb Cs⁺ [20,21]. Particularly, metal sulfide ion exchange materials (MSIEs) have attracted research interest due to their numerous soft basic S²⁻ sites and flexible framework for Cs⁺ capture [22,23], such as KMS series (KMS-1: K_2xMn_xSn_3−xS₆ [24], KMS-2: K_2xMg_xSn_3−xS₆ (x = 0.5–1), etc. [25]), KTS series (KTSS: K_1.93Ti_0.22Sn₃S_6.43 [26], KTS-3: K_2xsn_4−xS_8−x (x = 0.65–1) [27], KTS-2: K₂Sn₂S₅ [28], etc.), and FJSM series (FJSM-SnS: (Me₂NH₂)_4/3(Me₃NH)_2/3Sn₃S₇·1.25H₂O [29], FJSM-GAS-1: [Me₂NH₂]₂[Ga₂Sb₂S₇]·H₂O [30], etc.).

Nevertheless, the current research on MSIEs is predominantly focused on those based on main group metal ions such as In³⁺, Sn⁴⁺, and Sb³⁺ [20,22,31]. However, transition metal sulfides based on IVB group ions that share similar valence states and coordination modes with Sn have been scarcely explored as MSIEs. Thus far, the only example is a Ti-doped ion-exchange material that contains a trace amount of titanium [26]. Furthermore, it is notable that the alkali metal ions as the exchangeable cations in reported MSIEs are predominantly Na⁺ and K⁺, while the alkali ion Rb⁺ hardly serves as an exchangeable cation in MSIEs. According to the reported K₃Rb₃Zn₄Sn₃Se₁₃ [32] and KCsRb_1.2(NH₄)_2.71Sn₅Zn₄S₁₇ [33], however, Rb⁺ with a larger ionic radius also has mobile properties. Additionally, the crystal structures of some ion exchange materials after trapping radioactive ions remain unresolved [34,35], hindering the in-depth exploration of the structure–property relationships and impeding the design and synthesis of advanced materials. Therefore, expanding the diversity of exchangeable cations and designing novel transition metal sulfides are critical for developing advanced crystalline MSIEs with efficient Cs⁺ capture properties.

Herein, we report a two-dimensional (2D) thiotitanate Rb_0.32TiS₂·0.75H₂O (Rb-TiS₂) synthesized via a unique “Mg + RbCl” molten salt method [36], which overcomes the operational complexity and safety limitations of traditional solid-state synthesis for thiotitanates while enabling precise single-crystal structure determination. Rb-TiS₂ exhibits good adsorption performance, including a high adsorption capacity of 232.70 mg·g⁻¹, rapid kinetics (81.61% removal rate within 5 min), excellent selectivity, and good adsorption–desorption behavior. Notably, we also obtained the crystal structure of Cs_0.28TiS₂·0.6H₂O (denoted as Cs-TiS₂) after Cs⁺ capture by Rb-TiS₂, providing atomic-level visualization of the radioactive ion capture process. Then, according to elemental chemical analysis and other characterizations, the ion exchange mechanism of Rb-TiS₂ for Cs⁺ capture is elucidated. The flexible and tunable layered framework of Rb-TiS₂, along with its abundant and accessible S²⁻ sites, endows its excellent Cs⁺ ion adsorption properties. This work not only provides a new crystalline thiotitanate ion exchange material through a unique synthetic approach but also demonstrates the great potential of Group IVB transition metal sulfides for capturing radioactive cations.

2. Materials and Methods

2.1. Materials

Ti (99.00%), S (99.50%), NaCl (Analytical Research grade (AR)), MgCl₂·6H₂O (AR), and HNO₃ (69%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). RbCl (99.50%) was purchased from Adamas Reagent Co., Ltd. (Shanghai, China). Mg (99.5%) was purchased from Tianjin Fuchen Chemical Reagent Factory (Tianjin, China). KCl (AR) was purchased from Titan Co., Ltd. (Shanghai, 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). NaOH (AR) was purchased from Guangfu Co., Ltd. (Tianjin, China). All the chemicals were used without further purification.

2.2. Methods

2.2.1. Synthesis of Rb-TiS₂

The synthesis was performed by sealing a stoichiometric mixture of Ti (2 mmol), S (6 mmol), RbCl (4 mmol), and Mg (1 mmol) in an evacuated quartz tube (outer diameter 12 mm; inner diameter 10 mm) using an oxygen–hydrogen flame. The sealed ampoule was subjected to a programmed thermal treatment in a muffle furnace with the following temperature program: (i) gradual heating from room temperature to 900 °C over 24 h; (ii) maintaining at 900 °C for 48 h; (iii) controlled cooling to 550 °C at a rate of 7.3 °C·h⁻¹; (iv) isothermal annealing at 550 °C for 10 h; and then (v) natural cooling to ambient temperature. The resulting product was thoroughly washed with deionized water and absolute ethanol, followed by air-drying, yielding lustrous black-gold plate-like crystals.

2.2.2. Characterization Techniques

Morphological and elemental analyses were conducted using a field-emission scanning electron microscope (FE-SEM, JSM-6700F, JEOL, Akishima, Japan) equipped with energy-dispersive X-ray spectroscopy (EDS) for elemental mapping. X-ray powder diffraction (PXRD) was performed using a Rigaku Miniflex II X-ray diffractometer (Rigaku, Tokyo, Japan) with Cu K_α (λ = 1.54178 Å) operating at 30 kV and 15 mA, collecting data over a 2θ range of 5–65°. Single-crystal diffraction data were collected with a Rigaku diffractometer (XtaLAB Synergy Custom, Rigaku, Tokyo, Japan) with graphite monochromatic CuK_α (λ = 1.54178 Å). Thermogravimetric analysis (TGA) was carried out on a NETZSCH STA 449 F3 (Netzsch, Selb, Germany) instrument under N₂ atmosphere with a heating rate of 10 °C·min⁻¹. X-ray photoelectron spectroscopy (XPS) was conducted on an ESCALAB 250Xi (Thermo Scientific, Waltham, MA, USA) spectrometer (AlKα). The pH value of the solution was measured with a PHS-2F pH meter (PHS-2F, Shanghai, China). Elemental concentrations were quantified using inductively coupled plasma optical emission spectroscopy (ICP-OES, Thermo 7400, Thermo Scientific, Waltham, MA, USA) and inductively coupled plasma mass spectrometry (ICP-MS, Thermo XSeries II, Thermo Scientific, Waltham, MA, USA) with appropriate quality control measures, including calibration standards and replicate analyses.

2.2.3. Batch Adsorption Experiments

In the ion exchange experiments, non-radioactive Cs⁺ was used as a substitute for radioactive Cs⁺ ions, and all tests were carried out at room temperature. A typical ion exchange experiment was performed in a 25 mL polyethylene bottle, where the ion exchange material was mixed with a Cs⁺-containing aqueous solution at a solid-to-liquid ratio (V/m) of 1000 mL·g⁻¹. Then, the bottle was put on the shaking table for a certain time and then taken out and stood for about 3 min. The supernatant was filtered through a 0.22 μm syringe filter and diluted with 2% HNO₃ to an appropriate concentration for analysis by ICP-MS or ICP-OES. The solid sample was washed several times with deionized water and ethanol followed by natural drying, and then subjected to PXRD, EDS, and TG analyses.

Isothermal adsorption experiments: Cs⁺ solutions with varying concentrations were prepared using deionized water. An amount of 10 mg of Rb-TiS₂ sample was added to 10 mL of the solution and oscillated for approximately 6 h. The solution was then analyzed using ICP-MS to measure the change in Cs⁺ concentration. The adsorption data were fitted using the Langmuir, Freundlich, and Langmuir–Freundlich models to calculate the maximum adsorption capacity.

Kinetic experiments: An amount of 50 mg of Rb-TiS₂ sample was added to 50 mL of a 7 ppm Cs⁺ solution and stirred using a magnetic stirrer to ensure full solid–liquid contact. Approximately 0.3 mL of the mixture was sampled at regular intervals (2, 5, 10, 15, 20, 30, 40, 60, 120, 240, and 480 min), and the Cs⁺ concentration was measured. The kinetic data were fitted using the pseudo-first-order and pseudo-second-order models.

Acid–Base Stability Tests: Solutions containing Cs⁺ ions with different acidity and alkalinity were prepared using CsCl, NaOH, and HNO₃ and the pH value of the solution was measured using a pH meter. An amount of 10 mg of Rb-TiS₂ sample was added to 10 mL of the solution, respectively, and oscillated for approximately 6 h. The ion concentration in the solution was measured using ICP-MS. The solid sample was washed, dried, and analyzed with PXRD.

Competitive Experiments: The Cs–Na (molar ratio = 1:7.53, 1:105, 1:300 and 1:516), Cs–Ca (1:10.64) and Cs–Mg (1:4.80) solutions containing a single competitor ion were prepared by the corresponding halide salts, in which the concentration of Cs⁺ ions was about 6 mg·L⁻¹. In addition, by adding a small amount of Cs⁺ ions in tap water, river water (Wulong River), and lake water (Qishan Lake), the polluted environment water body was simulated. An amount of 10 mg of Rb-TiS₂ sample was added to each solution and stirred for 6 h. Afterward, the solutions were analyzed using ICP-MS or ICP-OES.

Adsorption–Desorption Experiments: (1) Desorption Experiments: An amount of 50 mg of the Rb-TiS₂ sample was treated with 50 mL of an approximately 1000 ppm Cs⁺ solution to obtain the adsorbed product (denoted as Cs-TiS₂). An amount of 20 mg of the Cs-TiS₂ sample was immersed in 20 mL of a 0.5 M KCl and 0.5 M RbCl solution for about 6 h, respectively, followed by PXRD analysis and further ion exchange experiments. (2) Adsorption–Desorption Experiments: An amount of 500 mL Cs⁺ ion solution (about 20 ppm) and 500 mL of a 1 M KCl solution were prepared for adsorption and desorption experiments, respectively. After each adsorption analysis test, the liquid sample was measured using ICP-MS; the solid sample was washed with deionized water and ethanol, dried, and weighed and part of the sample was extracted for the PXRD test.

3. Results

3.1. Synthetic Optimization

Currently, alkali metal-containing titanium sulfides (e.g., K₄Ti₃S₁₄ [37] and K₂TiS₃ [38]) are typically synthesized via traditional high-temperature solid-state methods. These approaches require the pre-preparation of polysulfide precursors using liquid ammonia and must be conducted under strict glovebox conditions, resulting in complex procedures and significant safety risks. Additionally, the intercalation method (e.g., K_xTiS₂ [39]) involves the initial preparation of TiS₂ single crystals through iodine vapor transport, followed by secondary reactions to introduce alkali metal ions. This process involves high energy consumption and is technically demanding.

In this study, we use a synthesis strategy based on the “Mg + RbCl” molten salt method [36], which enables the direct preparation of millimeter-scale alkali-metal-ion-containing plate-like crystals. The unique advantage of this method lies in the direct generation of abundant S²⁻ and Mg²⁺ ions through the redox reaction between sulfur (S) and magnesium (Mg). Excess S²⁻ ions provide the thermodynamic driving force for the formation of alkali metal-containing titanium sulfide. Using this strategy, we successfully synthesized Rb-TiS₂ crystals (Figure 1a). Compared to traditional methods, this approach eliminates the need for liquid ammonia, cumbersome glovebox operations, and multi-step synthesis, significantly enhancing safety and efficiency.

To further validate the critical role of Mg, we conducted control experiments under identical reaction conditions without Mg. Interestingly, even in the presence of excess RbCl without Mg, only golden TiS₂ crystals (Figure 1b) were obtained, rather than the target product. The corresponding PXRD (Figure 1c) and EDS data (Figure S1) confirm the indispensable role of Mg in the synthesis of Rb-TiS₂.

3.2. Structural Description and Removal Mechanism

Rb_0.32TiS₂·0.75H₂O (Rb-TiS₂, Tables S1 and S2) exhibits significant differences in structure compared with the previously reported RbTiS₂ [40]. Rb-TiS₂ crystallizes in the space group P6₃/mmc (a = b = 3.4210(1) Å, c = 17.4272(10) Å), while the anhydrous RbTiS₂ (ICSD#: 77990) crystallizes in the space group R3m (a = b = 3.43(1) Å, c = 24.2(1) Å). A detailed comparison of their powder XRD patterns is provided in the Supporting Information (Figure S2). In Rb-TiS₂, [TiS₆] octahedra share edges to form a two-dimensional anionic layer of [TiS₂]_n^0.32n− (Figure 2a). The adjacent layers exhibit different orientations, which are stacked in an ABAB sequence, forming a staggered arrangement (Figure 2b). The interlayer space is occupied by disordered Rb⁺ ions and lattice water molecules. Rb⁺ ions are located at two distinct sites of Rb(1) and O(1)/Rb(2) (Figure S3). The Cs⁺ adsorbed product, Cs_0.28TiS₂·0.60H₂O (Cs-TiS₂, Tables S1 and S2), also crystallizes in the P6₃/mmc space group (a = b = 3.4177(2) Å, c = 17.8784(17) Å), retaining the anionic layered structure of pristine Rb-TiS₂, while Rb⁺ ions are replaced by Cs⁺ ions. Notably, the interlayer distance increases from 5.864 Å in Rb-TiS₂ to 6.087 Å in Cs-TiS₂ after Cs⁺ capture (Figure 2c), likely due to the larger ionic radius of Cs⁺ (ionic radius: 1.69 Å) compared to that of Rb⁺ (ionic radius: 1.48 Å) [41]. This confirms the 2D flexible layered structure of Rb-TiS₂.

3.3. Characterization of Rb-TiS₂ and Cs-TiS₂

TG shows that the actual mass loss of Rb-TiS₂ and Cs-TiS₂ at 200 °C is 7.85% and 6.97%, respectively (Figure S4), which is close to the theoretical values of loss of their corresponding lattice water molecules in the structure (8.83% and 6.76%). Experimental PXRD patterns of Rb-TiS₂ and Cs-TiS₂ agree with the simulated ones from the corresponding single-crystal X-ray structures (Figure 1c and Figure S5), respectively. Furthermore, the (002) basal Bragg peak positions of Cs-TiS₂ shift to lower angles compared to that of Rb-TiS₂ (Figure 3a), consistent with the observed expansion of interlayer distances. EDS and elemental mapping analyses reveal that both Rb-TiS₂ and Cs-TiS₂ maintain well-defined crystal morphologies and uniform elemental distributions (Figure 3b). In the XPS spectrum of Rb-TiS₂, characteristic peaks of Rb 3d_5/2 and Rb 3d_3/2 are observed at 109.4 eV and 110.9 eV, respectively (Figure 3c,d). By contrast, these Rb peaks disappear in the XPS spectrum of Cs-TiS₂, while new peaks corresponding to Cs 3d_5/2 and Cs 3d_3/2 appear at 724.35 eV and 738.35 eV, respectively (Figure 3e). In summary, single-crystal analysis, PXRD, EDS, and XPS results collectively demonstrate that the Cs⁺ capture mechanism in Rb-TiS₂ is ion exchange between Rb⁺ and Cs⁺ ions. The 2D flexible layered structure of Rb-TiS₂ contains abundant soft basic S²⁻ sites, which exhibit a stronger affinity for Cs⁺ ion than Rb⁺ ion according to the Lewis acid–base theory [42], enabling the effective capture of Cs⁺.

3.4. Adsorption Isotherm Study

The adsorption capacity (q, Equation (S1)), a critical parameter for evaluating the performance of an adsorbent, was determined through isothermal adsorption experiments to assess the maximum Cs⁺ uptake by Rb-TiS₂. The experimental data are summarized in Table S3. The adsorption isotherm data are fitted using the Langmuir (Equation (S2)), Freundlich (Equation (S3)), and Langmuir–Freundlich models (Equation (S4)) [43], and the corresponding adsorption constants and parameters are listed in Table S4. The Langmuir–Freundlich model exhibits the highest correlation coefficient (R² = 0.993), outperforming the Langmuir (R² = 0.991) and Freundlich (R² = 0.903) models, indicating that the adsorption process of Cs⁺ on Rb-TiS₂ is best described by the Langmuir–Freundlich model (Figure 4a). Based on this model, the maximum Cs⁺ adsorption capacity of Rb-TiS₂ is calculated to be 232.70 mg·g⁻¹, which surpasses that of many commonly reported Cs⁺ adsorbents, such as sulfide KMS-1 (226 mg·g⁻¹) [24], oxide NaFeTiO (52.8 mg·g⁻¹) [44], MOF Nd-BTC (86 mg·g⁻¹) [45], zeolite (Turkish samples: 89.18 mg·g⁻¹) [46], and commercial AMP-PAN (ammonium molybdophosphate-polyacrylonitrile) (81 mg·g⁻¹) [47] (the more compounds are compared in Table S5).

3.5. Adsorption Kinetics Studies

Adsorption kinetics is a crucial parameter for evaluating the adsorption rate of an adsorbent, whereas rapid kinetics is essential to minimize the adverse effects of radiation exposure. The kinetic data for Rb-TiS₂ at room temperature are summarized in Table S6. Remarkably, Rb-TiS₂ achieves a removal efficiency (R, Equation (S5)) of 72.16% within 10 min and reaches adsorption equilibrium in approximately 40 min (Figure 4b). The kinetic data are fitted using the pseudo-first-order (Equation (S6)) and pseudo-second-order models (Equation (S7)); the latter exhibits a higher correlation coefficient (R² = 0.997, Figure 4c and Table S7), indicating that the adsorption process is predominantly governed by chemisorption. The adsorption kinetics of Rb-TiS₂ significantly outperforms that of many reported adsorbents, such as KMS-1/r-GO (34% in 1 h) [48], CdSnSe-1 (71.06% in 4 h) [49], and K-SGU-45 (28% in 15 h) [50].

3.6. Effect of pH on the Cs⁺ Adsorption

Given the diverse origins of radioactive waste liquids and the significant pH variations among different waste streams, we systematically evaluated the pH-dependent performance of Rb-TiS₂. The experimental results are summarized in Table S8 and the PXRD results are shown in Figure S6a. Within the pH range of 3–12, Rb-TiS₂ maintains good Cs⁺ adsorption performance, with distribution coefficients (K_d^Cs, Equation (S8)) consistently exceeding 10³ mL·g⁻¹ (Figure 4d). However, at pH values below 3, the crystal color of Rb-TiS₂ undergoes a noticeable change (Figure S6b), accompanied by an increased leaching of Ti (Figure S6c).

3.7. Competitive Adsorption Studies

Considering that Cs⁺ ions in actual waste liquids often coexist with various interfering ions, we conducted competitive adsorption experiments with individual competing Na⁺, Ca²⁺, and Mg²⁺ ions. The data are summarized in Table S9. When individual competitive Na⁺, Ca²⁺, and Mg²⁺ ions exist in a small amount, Rb-TiS₂ can still efficiently remove Cs⁺ ions, and its K_d^Cs and R^Cs values are greater than 10³ mL·g⁻¹ and 80%, respectively (Figure 5a). Further, the effect of a high concentration of Na⁺ ions on Cs⁺ removal is investigated (Table S10). As illustrated in Figure 5b, even when the Cs⁺/Na⁺ molar ratio reaches 1:105, the separation factor (SF_Cs/Na, Equation (S9)) remains at a high level of 138. And at Cs⁺/Na⁺ molar ratios of 1:300, K_d^Cs values of Rb-TiS₂ could still reach the level of 10³ mL·g⁻¹, whereas in the Cs⁺/Na⁺ molar ratio range of 1:7.53–1:516, the R^Na are always at very low levels (R^Na < 10%, Table S10). This shows that Rb-TiS₂ has a high selectivity for Cs⁺ ion and a good Cs/Na separation effect. This selective ion exchange behavior can be attributed to two factors: (1) size-matching effects and (2) differential affinity toward S²⁻ soft basic sites. According to the Lewis acid–base theory, as the radius of alkali metal ions increases (Na⁺: 0.95 Å < Rb⁺: 1.48 Å < Cs⁺: 1.69 Å [41]), the soft acid–soft base interaction between them and S²⁻ becomes stronger. Consequently, Cs⁺, which is closer to the radius of Rb⁺ ion and has a stronger affinity for S²⁻, is more likely to replace the interlayer Rb⁺ ion, while Na⁺ has a weaker exchange capacity due to its smaller size and weaker affinity. In addition, in the simulated polluted tap water, river water (Wulong River), and lake water (Qishan Lake), R^Cs are all greater than 84% (Figure S9 and Table S11). This indicates that Rb-TiS₂ can effectively remove Cs⁺ ions when the natural water body is polluted with a low concentration of Cs⁺.

3.8. Adsorption and Desorption

The elution cycle ability of an adsorbent is the key index in practical application. To evaluate this, desorption experiments were conducted on Cs-TiS₂ using 0.5 M RbCl and 0.5 M KCl solutions as the eluent, respectively. The sample treated with RbCl solutions is named Rb-Cs-TiS₂ and the sample treated with KCl is named K-Cs-TiS₂. EDS analysis reveals uniform elemental distribution in the Rb-Cs-TiS₂ and K-Cs-TiS₂ samples. Among them, there is a small amount of Cs⁺ ion residue in K-Cs-TiS₂, while there is almost no Cs⁺ ion residue in Rb-Cs-TiS₂ (Figure 5c and Figure S7). Moreover, the powder diffraction peak of Rb-Cs-TiS₂ is in good agreement with that of Rb-TiS₂ (Figure S8), indicating that the framework structure remains unchanged. Considering the cost of the eluent, a 1 M KCl solution was selected as the eluent to carry out the elution cycle experiment (Figure S10, Tables S12 and S13) in the two elution cycles, and the adsorption rate and desorption rate of Cs⁺ (Equation (S10)) were maintained at a high level (R^Cs/E^Cs > 92%). These results further demonstrate the current framework flexibility and practical application potential of this material.

4. Conclusions

In this work, we developed a 2D thiotitanate (Rb-TiS₂) for efficient Cs⁺ removal, which is synthesized using a unique “Mg + RbCl” molten salt method. This approach addresses the limitations of traditional solid-state synthesis, such as operational complexity and high safety risks, providing a valuable reference for the synthesis and design of alkali-metal-ion-containing transition metal sulfides. Notably, Rb-TiS₂ represents the first example of alkali-metal-ion-containing ternary Group IVB MSIEs. Rb-TiS₂ exhibits high adsorption capacity, broad pH activity range, rapid kinetics, excellent selectivity, and good adsorption–desorption behavior for Cs⁺ ion removal. Importantly, single-crystal analysis combined with XPS and EDS characterization techniques unambiguously reveals that the Cs⁺ capture arises from ion exchange between Cs⁺ and Rb⁺ ions located in the interlayer spaces of the Rb-TiS₂. The efficient capture of Cs⁺ ions is enabled by the abundance of soft basic S²⁻ sites, size-matched and exchangeable Rb⁺ ions, appropriate interlayer spacing, and the flexible tunable layered structure of Rb-TiS₂. This work offers a valuable reference for synthesizing and designing alkali-metal-ion-containing transition metal sulfides. Moreover, it deepens the understanding of ion exchange mechanisms in these materials and paves the way for designing new materials for nuclear waste management and environmental remediation.