A Hydrofluoric Acid-Free Green Synthesis of Magnetic M.Ti2CTx Nanostructures for the Sequestration of Cesium and Strontium Radionuclide

Highlights An environmentally friendly MAX phase etching methodology was established Sodium hydroxide produced magnetic layered M.Ti2CTx nanostructure M.Ti2C-AIII exhibited exceptional Sr2+ and Cs+ removal capacities of 376 and 142.88 mg/g Highly efficient magnetic nanostructures permitted selective radionuclide removal Abstract MAX phases are the parent materials used for the formation of MXenes, and are generally obtained by etching using the highly corrosive acid HF. To develop a more environmentally friendly approach for the synthesis of MXenes, in this work, titanium aluminum carbide MAX phase (Ti2AlC) was fabricated and etched using NaOH. Further, magnetic properties were induced during the etching process in a single-step etching process that led to the formation of a magnetic composite. By carefully controlling etching conditions such as etching agent concentration and time, different structures could be produced (denoted as M.Ti2CTx). Magnetic nanostructures with unique physico-chemical characteristics, including a large number of binding sites, were utilized to adsorb radionuclide Sr2+ and Cs+ cations from different matrices, including deionized, tap, and seawater. The produced adsorbents were analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray energy dispersive spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS). The synthesized materials were found to be very stable in the aqueous phase, compared with corrosive acid-etched MXenes, acquiring a distinctive structure with oxygen-containing functional moieties. Sr2+ and Cs+ removal efficiencies of M.Ti2CTx were assessed via conventional batch adsorption experiments. M.Ti2CTx-AIII showed the highest adsorption performance among other M.Ti2CTx phases, with maximum adsorption capacities of 376.05 and 142.88 mg/g for Sr2+ and Cs+, respectively, which are among the highest adsorption capacities reported for comparable adsorbents such as graphene oxide and MXenes. Moreover, in seawater, the removal efficiencies for Sr2+ and Cs+ were greater than 93% and 31%, respectively. Analysis of the removal mechanism validates the electrostatic interactions between M.Ti2C-AIII and radionuclides.


Introduction
Nuclear energy is a prevalent, sustainable, cost-effective, and green source of energy [1]. Nevertheless, the environmental effect of producing nuclear power through nuclear fission reactions is an unsettled matter. Spent nuclear fuel that consists of a large amount of waste full of toxic radioisotopes poses a huge environmental challenge [2]. treatment, and the magnetic property was induced during the exfoliation process due to the formation of a Fe 3 O 4 -based composite. The produced magnetic materials were characterized and utilized for radionuclide Sr 2+ and Cs + removal, and the adsorption efficiencies in different matrices including seawater were evaluated.

Synthesis of M.Ti 2 CT x
To synthesize the magnetic M.Ti 2 CT x nanostructures, 0.25 g FeSO 4 ·7H 2 O was dissolved in 40 mL deionized water in a 100 mL beaker. Afterward, a certain amount of NaOH was inserted into the solution, and the solution was stirred to produce a homogenous suspension. A 0.2 g measure of Ti 2 AlC MAX phase was then added to the solution and reacted for 1 h at room temperature. The synthesis method of the Ti 2 AlC MAX phase is reported in our previous work [35]. The prepared suspension was filled into a Teflon-lined stainless autoclave and treated at 200 • C in an oven. After 12 h of hydrothermal treatment, the prepared material was washed repeatedly with DI water and ethanol. The obtained blackish/grayish residues were collected and dried at 60 • C overnight in a vacuum oven. The synthesis protocol was varied to optimize the process and achieve materials with high adsorption capacities for radionuclides.
As a reference material, Fe 3 O 4 magnetic particles and Alk-Ti 2 C sheet were also synthesized using the same procedure used for M.Ti 2 CT x -A III . However, for Fe 3 O 4 synthesis the Ti 2 AlC MAX phase was not added during synthesis. For Alk-Ti 2 C sheet fabrication, the Ti 2 AlC MAX phase was exfoliated in 5 M NaOH at 200 • C for 12 h in the absence of FeSO 4 .7H 2 O.

Characterization
The surface morphology and structure of the Ti 2 AlC MAX phases and as-synthesized M.Ti 2 C-A III powders were analyzed using a field emission scanning electron microscope (SEM, S-4800, HITACHI, Tokyo, Japan). The samples were gold-coated with a Balzers' sputtering device prior to analysis with SEM. The X-ray powder diffraction spectra of the synthesized materials were recorded using Rigaku D/MAX 2500PC powder XRD (Rigaku, Tokyo, Japan) in a scan range of 2-80 • . The accelerating voltage and current were set at 40 kV and 200 mA, respectively, with a monochromatic Cu Kα radiation of wavelength (λ = 1.5405 Å). A superconducting quantum interference device magnetometer (Quantum Design, San Diego, CA, USA) was used for the magnetic characterization of Fe 3 O 4 and the final nanostructure. Inductively coupled plasma optical emission/mass spectrometry (ICP-MS, Perkin Elmer, Waltham, MA, USA) was used to analyze the strontium and cesium ion concentrations in the solutions. The surface area and pore size analyses were conducted using a Brunauer-Emmett-Teller (BET) analyzer and Barrett−Joyner−Halen (BJH) method, respectively. The BJH method was applied using a Micromeritics ASAP-2020 analyzer with a nitrogen gas adsorption-desorption isotherm at 77 K to determine the pore size distribution. XPS spectra of as-prepared magnetic composites were measured using a scanning X-ray micrograph (SXM: ULVAC-PHI II, Quantera, Kanagawa, Japan). For XPS spectra after Cs and Sr adsorption, the sample was prepared by inserting 10 mg of asprepared M.Ti 2 C-A III into a binary solution containing 5 ppm of both Sr 2+ and Cs + . After a 12 h reaction, the adsorbent was separated, washed, and dried in an oven at 50 • C under vacuum conditions.

Adsorption Experiments
To evaluate the radionuclides removal capabilities of the synthesized structures, adsorption testing was performed in batch experiments with specific pH and adsorbent amounts. The adsorbent was then removed and filtered, and the remaining concentrations of Sr 2+ and Cs + were estimated using inductively coupled plasma optical emission spectroscopy (ICP-OES, Perkin Elmer, Waltham, MA, USA) and ICP-MS, as appropriate. The absolute adsorption capacity and adsorption efficiency of the cations were calculated using the following Equations (1) and (2): where C o is the initial concentration in ppm, and C e is the final concentration of Sr 2+ and Cs + , respectively; V is the volume of solution in liter, m is the mass of the adsorbent (g); and Q e is the adsorption capacity of the cations. Experiments were performed to assess the influence of pH on both Cs + and Sr 2+ adsorption in a pH range 2.0-9.0. Batch experiments were carried out using different concentrations (10-1000 mg/L) at room temperature and pH of 6 for 6 h of contact time. Pseudo-first-order and pseudo-second-order kinetics were applied. Furthermore, the Langmuir and Freundlich isotherm models were used to analyze the data. A comparison of adsorption capacities was made for different types of adsorbents. In comparison with the newly synthesized M.Ti 2 CT x , a certain amount of identically adsorbent including graphene oxide (GO), Ti 3 C 2 T x MXene, or Fe 3 O 4 was introduced into 15 mL of 10 ppm pollutant solution. The experiments were performed under optimized experimental conditions. Samples were taken at different time intervals, diluted 10-fold, and analyzed using ICP-MS.
To validate the usefulness of the synthesized materials for radioactive ions adsorption, 10 mg M.Ti 2 CT x A III adsorbent was introduced into different matrices, e.g., distilled water, tap water, and seawater. The seawater was simulated using 10,400 ppm Na + , 390 ppm K + , 1270 ppm Mg 2+ , and 405 ppm Ca 2+ ions. The aliquot was drawn after 24 h reaction and analyzed using ICP-MS to determine the residual amount.

Results and Discussions
3.1. Characterization of M.Ti 2 CT x -A III Ti 2 AlC MAX phase was successfully fabricated by high temperature (1350 • C) sintering of Al and TiC in a 1:2 ratio [36]. Further, the hydrothermal alkalization of Ti 2 AlC powder resulted in different types of magnetic structures. In a one-step hydrothermal treatment method, magnetic properties were induced during the alkalization process. To achieve the best magnetic properties and a well-exfoliated structure, synthesis conditions such as NaOH concentration (5-15 M), and treatment time (12-48 h) were varied; however, the temperature was kept unchanged at 200 • C). The obtained magnetic structures using the different synthesis conditions are shown in Table 1. The synthesized magnetic structures were used for radioactive cations Sr 2+ and Cs + removal from water and the results revealed that M.Ti 2 CT x -A III showed the highest removal efficiencies for both Sr 2+ and Cs + ions. M.Ti 2 CT x -A III was synthesized by the hydrothermal treatment of Ti 2 AlC in the presence of 5 molar sodium hydroxide and FeSO 4 at 200 • C for 48 h. Scanning electron microscopy showed different morphologies for the different exfoliated phases. Figure 1a shows the SEM images of the layered Ti 2 AlC MAX phase. After hydrothermal treatment, Ti 2 AlC changed into 2D sheet-like structures as shown in Figure 1b,c. The growth of the nanostructure depends on the synthesis conditions; changes in the alkalinity and synthesis time led to different structures of the final product. For the synthesis of M.Ti 2 CT x -A III , 5 M NaOH and 250 mg of FeSO 4 ·7H 2 O was sufficient to remove the layers of Al from Ti 2 AlC and induce maximum magnetization as corroborated by energy-dispersive X-ray spectroscopy (EDS) analysis and magnetic field-dependent magnetic measurements. Micron-sized Fe 3 O 4 particles with octahedron geometry were also observed in SEM images confirming the formation of a composite material (Figure 1d). A higher concentration of NaOH (10-15 M) could not create any definitive structural shape and further caused a decrease in the observed magnetization. Further, EDS measurements established the complete elimination of the Al layers from Ti 2 AlC as shown in Table S1; consequently, the newly developed structures were named M.Ti 2 CT x , where T x characterizes the surface functional groups such as -Na, -OH, and -O, and the presence of all elements in the EDS graph ( Figure 1e,f) [35]. Additionally, the elemental mapping in SEM-EDS analysis (Figure 1e) of the samples after Sr 2+ and Cs + adsorption (Sr 2+ @M.Ti 2 CT x -A III or Cs + @M.Ti 2 CT x -A III ) presented a uniform distribution of all characteristic elements, including the presence of significant amounts of Sr 2+ and Cs + .

5
M.  Figure 1a shows the SEM images of the layered Ti2AlC MAX phase. After hydr mal treatment, Ti2AlC changed into 2D sheet-like structures as shown in Figure 1b, growth of the nanostructure depends on the synthesis conditions; changes in the alka and synthesis time led to different structures of the final product. For the synthe M.Ti2CTx-AIII, 5 M NaOH and 250 mg of FeSO4·7H2O was sufficient to remove the of Al from Ti2AlC and induce maximum magnetization as corroborated by energy-d sive X-ray spectroscopy (EDS) analysis and magnetic field-dependent magnetic me ments. Micron-sized Fe3O4 particles with octahedron geometry were also observed in images confirming the formation of a composite material (Figure 1d). A higher conc tion of NaOH (10-15 M) could not create any definitive structural shape and f caused a decrease in the observed magnetization. Further, EDS measurements estab the complete elimination of the Al layers from Ti2AlC as shown in Table S1; consequ the newly developed structures were named M.Ti2CTx, where Tx characterizes the s functional groups such as -Na, -OH, and -O, and the presence of all elements in th graph (Figure 1e,f) [35]. Additionally, the elemental mapping in SEM-EDS analysi ure 1e) of the samples after Sr 2+ and Cs + adsorption (Sr 2+ @M.Ti2CTx-AIII or Cs + @M.T AIII) presented a uniform distribution of all characteristic elements, including the pre of significant amounts of Sr 2+ and Cs + . Based on the initial assessments of radioactive cation removal by materials and phology examined by SEM data analysis, we selected only M.Ti2CTx-AIII for further acteristic analyses. In comparison with other exfoliated structures, M.Ti2CTx-AIII exh Based on the initial assessments of radioactive cation removal by materials and morphology examined by SEM data analysis, we selected only M.Ti 2 CT x -A III for further characteristic analyses. In comparison with other exfoliated structures, M.Ti 2 CT x -A III exhibited higher adsorption capacity for both Sr 2+ and Cs + ; thus, M.Ti 2 CT x -A III was selected for further physio-chemical characteristics studies. The XRD pattern of M.Ti 2 CT x -A III indicated the perseverance of crystalline structures after exfoliation at 200 • C and treatment with 5 M sodium hydroxide. After exfoliation, the intensity of the characteristic peak in Ti 2 AlC (2θ = 39.26 • ) decreased (red color spectrum Figure 2a). Furthermore, the peak at 2θ = 12.90 • moved to 2θ = 9.98 • in M.Ti 2 CT x -A III (Figure 2a). Fe 3 O 4 was synthesized for reference, and the XRD pattern of Fe 3 O 4 is shown in blue color in Figure 2a. Fe 3 O 4 was synthesized thorough hydrothermal treatment using 5 M NaOH at 200 • C for 48 h. The representative peaks in the XRD pattern, 2θ = 35 • , 30 • , 39 • , 57 • , and 18.07 • match with available literature and PDF reference code 01-089-0688. Further, the peak at 2θ =~18 • strongly suggests the octahedron morphology of Fe 3 O 4 nanoparticles [37]. Therefore, the peaks at 2θ =~18 • and~35 • in M.Ti 2 CT x -A III (green color spectrum in Figure 2a) represent the emergence of magnetic Fe 3 O 4 nanoparticles during the exfoliation process. The produced M.Ti 2 CT x -A III exhibited ferrimagnetic behavior, as evidenced by the magnetic field-dependent magnetization measurements. A characteristic magnetic hysteresis loop is shown in Figure 2b. At room temperature, M.Ti 2 CT x -A III showed a saturation magnetization of 10.69 emu/g, which is expectedly lower than that of pure Fe 3 O 4 (52.02 emu/g). The reduction in magnetization is understandable as non-magnetic Ti 2 CT x is present in large amounts in the magnetic composite M.Ti 2 CT x -A III .
higher adsorption capacity for both Sr 2+ and Cs + ; thus, M.Ti2CTx-AIII was selected for further physio-chemical characteristics studies. The XRD pattern of M.Ti2CTx-AIII indicated the perseverance of crystalline structures after exfoliation at 200 °C and treatment with 5 M sodium hydroxide. After exfoliation, the intensity of the characteristic peak in Ti2AlC (2θ = 39.26°) decreased (red color spectrum Figure 2a). Furthermore, the peak at 2θ = 12.90° moved to 2θ = 9.98° in M.Ti2CTx-AIII (Figure 2a). Fe3O4 was synthesized for reference, and the XRD pattern of Fe3O4 is shown in blue color in Figure 2a. Fe3O4 was synthesized thorough hydrothermal treatment using 5 M NaOH at 200 °C for 48 h. The representative peaks in the XRD pattern, 2θ = 35°, 30°, 39°, 57°, and 18.07° match with available literature and PDF reference code 01-089-0688. Further, the peak at 2θ = ~18° strongly suggests the octahedron morphology of Fe3O4 nanoparticles [37]. Therefore, the peaks at 2θ = ~18° and ~35° in M.Ti2CTx-AIII (green color spectrum in Figure 2a) represent the emergence of magnetic Fe3O4 nanoparticles during the exfoliation process. The produced M.Ti2CTx-AIII exhibited ferrimagnetic behavior, as evidenced by the magnetic field-dependent magnetization measurements. A characteristic magnetic hysteresis loop is shown in Figure 2b. At room temperature, M.Ti2CTx-AIII showed a saturation magnetization of 10.69 emu/g, which is expectedly lower than that of pure Fe3O4 (52.02 emu/g). The reduction in magnetization is understandable as non-magnetic Ti2CTx is present in large amounts in the magnetic composite M.Ti2CTx-AIII. The Brunauer-Emmett-Teller (BET) surface area of Ti2AlC MAX phase and M.Ti2CTx-AIII were measured, and the results showed a sudden increase in the surface area from 0.618 to 29.33 m 2 /g for the parent MAX phase and M.Ti2CTx-AIII composite, respectively ( Figure S1a). An unexpected and noteworthy rise in surface area in the after-exfoliation samples was potentially due to the elimination of Al layers and the formation of sheetlike structure in M.Ti2CTx-AIII. Furthermore, in a Barrett-Joyner-Halenda (BJH) plot the mean pore diameter of M.Ti2CTx-AIII was 32.04 nm ( Figure S1b). X-ray photoelectron spectroscopy (XPS) additionally revealed the formation of M.Ti2CTx-AIII and changes in all the elements states (Table S3). The complete spectra of M.Ti2CTx-AIII showed the presence of representative elements including Ti 2p, O 1s, C 1s, Fe 2p, and Na 1s (bottom spectrum in Figure 3). Furthermore, after adsorption of the radionuclides, peaks corresponding to Sr 3d and Cs 3d emerged in the M.Ti2CTx-AIII samples (blue color spectrum in Figure 3). The chemical changes that occurred in the different phases of adsorbent authenticate the successful syntheses of desired materials and loading of radionuclides onto materials. Furthermore, in regional peak fitting analysis of Sr 3d, two de-convoluted peaks were found at binding energies of 133.71 and 135.5 eV, which can be designated as Sr 3d5/2 and Sr 3d3/2 ( Figure S2). Moreover, we have observed a radical The Brunauer-Emmett-Teller (BET) surface area of Ti 2 AlC MAX phase and M.Ti 2 CT x -A III were measured, and the results showed a sudden increase in the surface area from 0.618 to 29.33 m 2 /g for the parent MAX phase and M.Ti 2 CT x -A III composite, respectively ( Figure S1a). An unexpected and noteworthy rise in surface area in the after-exfoliation samples was potentially due to the elimination of Al layers and the formation of sheet-like structure in M.Ti 2 CT x -A III . Furthermore, in a Barrett-Joyner-Halenda (BJH) plot the mean pore diameter of M.Ti 2 CT x -A III was 32.04 nm ( Figure S1b).
X-ray photoelectron spectroscopy (XPS) additionally revealed the formation of M.Ti 2 CT x -A III and changes in all the elements states (Table S3). The complete spectra of M.Ti 2 CT x -A III showed the presence of representative elements including Ti 2p, O 1s, C 1s, Fe 2p, and Na 1s (bottom spectrum in Figure 3). Furthermore, after adsorption of the radionuclides, peaks corresponding to Sr 3d and Cs 3d emerged in the M.Ti 2 CT x -A III samples (blue color spectrum in Figure 3). The chemical changes that occurred in the different phases of adsorbent authenticate the successful syntheses of desired materials and loading of radionuclides onto materials. Furthermore, in regional peak fitting analysis of Sr 3d, two de-convoluted peaks were found at binding energies of 133.71 and 135.5 eV, which can be designated as Sr 3d 5/2 and Sr 3d 3/2 ( Figure S2). Moreover, we have observed a radical decrease in Na 1s peak intensity after Sr 2+ adsorption onto M.Ti 2 CT x -A III , which could be due to the ion exchange of Sr 2+ with Na ions ( Figure S3). Further, there was a decrease in peak intensity and peak shifting in C 1s after adsorption of Sr 2+ and Cs + onto M.Ti 2 CT x -A III ( Figure S4). Further, in regional XPS data recording, we could not find the Cs 1s peak and this could be due to the presence of a comparatively small amount of Cs + in the M.Ti 2 CT x -A III . However, a signal of Cs 3d was found at around 690 eV. Furthermore, SEM-  Table S2. decrease in Na 1s peak intensity after Sr 2+ adsorption onto M.Ti2CTx-AIII, which could be due to the ion exchange of Sr 2+ with Na ions ( Figure S3). Further, there was a decrease in peak intensity and peak shifting in C 1s after adsorption of Sr 2+ and Cs + onto M.Ti2CTx-AIII ( Figure S4). Further, in regional XPS data recording, we could not find the Cs 1s peak and this could be due to the presence of a comparatively small amount of Cs + in the M.Ti2CTx-AIII. However, a signal of Cs 3d was found at around 690 eV. Furthermore, SEM-EDS analysis of the Cs-laden M.Ti2CTx-AIII sample exhibited the presence of a significant amount of Cs in it (1.52 Wt.%). Further details are given in Table S2.

Adsorptive Behavior of M.Ti2CTx
The presence of radioactive nuclides such as Sr 2+ and Cs + in wastewater is a major risk and serious threat to humans and other living organisms. Therefore, before disposal, the removal of Cs + and Sr 2+ from nuclear waste is very crucial. This work aims to examine the adsorptive performance of the synthesized magnetic adsorbent for Cs + and Sr 2+ . The synthesized magnetic M.Ti2CTx exhibited higher porosity, magnetic behavior, and the presence of surface functional groups such as ⎼Na, ⎼OH, ⎼O, and FeO. Structures with these properties could be used in the purification of water contaminated with heavy metal ions, especially cationic radionuclide removal from water. Therefore, the synthesized materials were tested against Sr 2+ and Cs + in batch adsorption tests, and the results are displayed in Table 2. The radionuclide adsorption from solution at certain concentrations (12.811 and 10.911 ppm for Cs + and Sr 2+ , respectively) was determined for eight different materials in batch adsorption tests. The adsorbent named M.Ti2CTx-AIII exhibited the highest efficiency for Cs + and Sr 2+ . The maximum removal efficiency for Cs + was 79.43%, which was higher than all other magnetite materials. Furthermore, in the case of Sr 2+ , all synthesized nanostructures exhibited excellent removal efficiency and M.Ti2CTx-AIII exhibited more than 99% removal. The nanostructure showed a higher affinity for divalent cations Sr 2+ with the highest removal efficiency between 96 and 99% as compared to monovalent radionuclides Cs + , which was between ~2 and 80%.

Adsorptive Behavior of M.Ti 2 CT x
The presence of radioactive nuclides such as Sr 2+ and Cs + in wastewater is a major risk and serious threat to humans and other living organisms. Therefore, before disposal, the removal of Cs + and Sr 2+ from nuclear waste is very crucial. This work aims to examine the adsorptive performance of the synthesized magnetic adsorbent for Cs + and Sr 2+ . The synthesized magnetic M.Ti 2 CT x exhibited higher porosity, magnetic behavior, and the presence of surface functional groups such as -Na, -OH, -O, and FeO. Structures with these properties could be used in the purification of water contaminated with heavy metal ions, especially cationic radionuclide removal from water. Therefore, the synthesized materials were tested against Sr 2+ and Cs + in batch adsorption tests, and the results are displayed in Table 2. The radionuclide adsorption from solution at certain concentrations (12.811 and 10.911 ppm for Cs + and Sr 2+ , respectively) was determined for eight different materials in batch adsorption tests. The adsorbent named M.Ti 2 CT x -A III exhibited the highest efficiency for Cs + and Sr 2+ . The maximum removal efficiency for Cs + was 79.43%, which was higher than all other magnetite materials. Furthermore, in the case of Sr 2+ , all synthesized nanostructures exhibited excellent removal efficiency and M.Ti 2 CT x -A III exhibited more than 99% removal. The nanostructure showed a higher affinity for divalent cations Sr 2+ with the highest removal efficiency between 96 and 99% as compared to monovalent radionuclides Cs + , which was between~2 and 80%.

Comparison with Other Materials
The M.Ti 2 CT x -A III nanostructure synthesized in this work was compared with other similar benchmark adsorbents, including 2D GO, 2D Ti 3 C 2 T x MXene, Fe 3 O 4 , and Alk-Ti 2 C sheet . The synthesis method for Ti 3 C 2 T x MXene is presented in our previous study [38]. The 2D GO nanosheets used in this work were produced by a modified Hummer's method [39] and further details are also provided in our previous work [37]. The synthesis method for the reference Fe 3 O 4 is also provided before in the material synthesis section. Alk-Ti 2 C sheet nanosheets were synthesized following the same procedure used for M.Ti 2 CT x -A III synthesis under different synthesis conditions. Alk-Ti 2 C sheet can be synthesized by treating 200 mg of Ti 2 AlC MAX phase in 5 M NaOH at 200 • C for 12 h [35]. In a comparison adsorption test, the M.Ti 2 CT x -A III showed the highest removal efficiency for Sr 2+ (99.60%) as compared to Fe 3 O 4 , GO, and Ti 3 C 2 T x MXene, which was 18.44, 94.70, and 16.60%, respectively (Figure 4a). For Cs + removal, M.Ti 2 CT x -A III also performed well among all other completive materials except Alk-Ti 2 C sheet . The Cs + removal efficiency was 73.40, 10.47, 33.68, and 37.49% for M.Ti 2 CT x -A III , Fe 3 O 4 , GO, and Ti 3 C 2 T x MXene, respectively (Figure 4b). Alk-Ti 2 C sheet showed excellent adsorption efficiency for both Cs + and Sr 2+ (93.77%, and 99.68%, respectively) as compared to M.Ti 2 CT x -A III . The main reason for higher removal efficacy is the well-exfoliated and well-defined structure of Alk-Ti 2 C sheet . Alk-Ti 2 C sheet was synthesized in only NaOH-solution in the absence of iron sulphate and thus the exfoliation was more efficient, and a comparatively shorter time was required for the Al layer to be etched out from the Ti 2 AlC phase. The well-defined structure and functional groups played important roles in a greater number of nuclides cations becoming entrapped and being captured. However, without magnetic properties, it was very difficult to remove Alk-Ti 2 C sheet from the water after contact with radionuclides. On the other hand, M.Ti 2 CT x -A III offers easy separation by using an external magnet after the adsorption experiment. Therefore, M.Ti 2 CT x -A III could be a better alternative option for the easy separation of radionuclide-loaded nanomaterials, and thus the discharge of nanoparticles into the environment can be circumvented. The stated results showed that M.Ti 2 CT x -A III has comparatively higher binding abilities against Sr 2+ and Cs + as compared to other benchmark materials. A comparison between the fabricated M.Ti 2 CT x -A III and previously reported materials is illustrated in Table 3.

Effect of Solution pH
The pH of a solution usually plays a crucial role in the adsorption of metal ion contaminants in liquid phase, as the removal is a pH-dependent process. Therefore, the influence of the solution's pH on Sr 2+ and Cs + was assessed in this work. The adsorption efficiency of M.Ti 2 CT x -A III for Sr 2+ and Cs + was performed at various pH values ranging from pH = 2 to 9. The findings revealed that at pH = 2, both Sr 2+ and Cs + adsorption were as low as 60.58 and 16.40%, respectively, but increased significantly at pH values ranging from 3 to 9 to 99.97 and 60.0%, respectively (Figure 4c). The increase in removal affinity evidently demonstrated the influence of Sr 2+ and Cs + ionic form and also the surface characteristics of the M.Ti 2 CT x -A III material used. M.Ti 2 CT x -A III demonstrated low removal efficiency at lower pH of the solution, where the surface charges on M.Ti 2 CT x -A III were reduced, perhaps due to the competition of positive charges and M + ions [25]. Hydroxyl group protonation on M.Ti 2 CT x -A III is possibly a cause for this, as it produces repulsive forces in highly acidic media with very low pH. These results strongly indicate that the adsorption of Sr 2+ and Cs + ions onto M.Ti 2 CT x -A III is a pH-dependent phenomenon.

Effect of Contact Time
The synthesized M.Ti 2 CT x -A III nanostructures demonstrated exceptional adsorption behavior with fast sorption kinetics for Sr 2+ and Cs + ions. For the adsorption kinetics test, a binary solution containing Sr 2+ and Cs + ions was prepared, and a certain amount of M.Ti 2 CT x -A III was introduced into the solution and agitated for 12 h. The results showed that >97% of total Sr 2+ (8.55 ppm) was adsorbed in just 15 min and adsorption equilibrium was achieved in 1 h. In the case of Cs + , the adsorption process was also fast, as, in the first 15 min,~68% Cs + (7.91 ppm) was adsorbed and achieved equilibrium state in 1 h. Further, sorption kinetics models such as the Lagergren-pseudo-first order and secondorder adsorption kinetics models were applied on obtained sets of data to obtain insight about fast kinetics and possible interaction mechanism between solid-liquid phases of the adsorbent and adsorbate (Figure 5a,b). Among the applied kinetic models of adsorption, the pseudo-second-order kinetic model fitted very well with the data sets of Sr 2+ @M.Ti 2 CT x -A III and Cs + @M.Ti 2 CT x -A III as compared to the pseudo-first-order model. This rapid adsorption of radionuclide ions may be due to the large surface area and porosity and highly occupied empty binding sites on the M.Ti 2 CT x -A III [50]. Moreover, the calculated equation parameters were close to the experimental results and validated the process of adsorption. The adsorption capacity (Q t ) calculated by second-order kinetics for Sr 2+ and Cs + was 12.78 and 9.15 mg/g, respectively, which was close to the experimental adsorption density of 12.77 and 8.99 mg/g, respectively. Additionally, the regression coefficient value (R 2 ) was 1.0 and 0.999 for Sr 2+ and Cs + , respectively. Thus, the above results obtained from the kinetic test indicated that the adsorption of both Sr 2+ and Cs + onto M.Ti 2 CT x -A III was a chemical interaction with a rate-limiting step.  In nuclear power plants, management of nuclear waste is imperative. Therefore, the capability of the synthesized magnetic nanostructures for nuclear waste treatment was evaluated to establish their efficacy. Accordingly, the adsorption of Cs + and Sr 2+ cations in DI, tap, and seawater, filled with coexisting ions, was analyzed. The competing ions, such as Ca 2+ , Mg 2+ , K + , and Na + , were inserted, with concentrations similar to the matrices. The removal efficiencies in tap water and simulated seawater were very good compared to the control experiments. The results obtained from the experiments are illustrated in Table 5. The adsorption of Sr 2+ and Cs + was performed in both single and binary (Sr 2+ + Cs + ) solution, Sr 2+ exhibiting excellent removal efficiency in all matrices. The Cs + removal efficiency was lower than that of Sr 2+ ; furthermore, in DI water, the adsorption efficiency was ~93% but reduced to ~70 and ~31% in tap and seawater, respectively. The removal efficiencies were lower in seawater, which is due to the presence of competitive cations in simulated

Adsorption Isotherm
Adsorption isotherm models were further assessed to obtain an understanding of the adsorption of radionuclides. Separate sets of adsorption experiments were performed for each nuclide's cations at different initial concentrations. The maximum adsorption densities of M.Ti 2 CT x -A III for Sr 2+ and Cs + at saturation point were 357.60 and 140.42 mg/g, respectively. Adsorption isotherm models, such as Langmuir and Freundlich isotherms, were applied to the experimentally obtained data ( Table 4). The Langmuir isotherm model with higher regression coefficient (R 2 ) values fitted well as compared to the Freundlich isotherm model (Figure 5c,d). The Langmuir isotherm model, with calculated maximum adsorption capacities of 376.05 and 142.88 mg/g for Sr 2+ and Cs + , respectively, indicate that the radionuclides were adsorbed as a monolayer on M.Ti 2 CT x -A III . The adsorption capacities calculated from the Langmuir isotherm were close to the obtained experimental values (Table 4). In nuclear power plants, management of nuclear waste is imperative. Therefore, the capability of the synthesized magnetic nanostructures for nuclear waste treatment was evaluated to establish their efficacy. Accordingly, the adsorption of Cs + and Sr 2+ cations in DI, tap, and seawater, filled with coexisting ions, was analyzed. The competing ions, such as Ca 2+ , Mg 2+ , K + , and Na + , were inserted, with concentrations similar to the matrices. The removal efficiencies in tap water and simulated seawater were very good compared to the control experiments. The results obtained from the experiments are illustrated in Table 5. The adsorption of Sr 2+ and Cs + was performed in both single and binary (Sr 2+ + Cs + ) solution, Sr 2+ exhibiting excellent removal efficiency in all matrices. The Cs + removal efficiency was lower than that of Sr 2+ ; furthermore, in DI water, the adsorption efficiency was~93% but reduced to~70 and~31% in tap and seawater, respectively. The removal efficiencies were lower in seawater, which is due to the presence of competitive cations in simulated seawater. The higher concentration of Mg 2+ and Ca 2+ could be responsible for the decrease in Sr 2+ , and Na + and K + might influence the Cs + adsorption. The results showed a higher affinity for divalent cations over single-valent cations, as we experienced in previous experimental tests. Overall, the results established remarkable adsorption efficiency of M.Ti 2 CT x -A III .

Conclusions
In this work, we have performed etching of the Ti 2 AlC phase using a green hydrothermal alkalization process to etch out the Al layer. The magnetic properties were successfully incorporated during A-layer etching. The resulting M.Ti 2 CT x -A III exhibited sheet-like morphology with abundant surface-terminal groups. The said characteristics and unique morphology marked it as an outstanding material for Sr 2+ and Cs + adsorption. M.Ti 2 CT x -A III was able to adsorb Sr 2+ and Cs + swiftly and efficiently in numerous matrices with very high removal efficiencies, including deionized, tap, and seawater. In the selective removal test, the fast and excellent adsorption capacity of Cs + and Sr 2+ in seawater (325.59 and 1014.02 µg/g, respectively) validates the potential of the synthesized material for practical applications. The radionuclide Sr 2+ and Cs + removal procedure was dependent on the pH of the solution, monolayer adsorption process, and rate-limiting parameters, and the maximum adsorption capacities of M.Ti 2 CT x -A III was 376.05 and 142.88 mg/g for Sr 2+ and Cs + , respectively. These findings suggest that the etching of Ti 2 AlC by adopting fluoride-free procedure might be an unconventional but feasible approach to preparing nanomaterials for environmental applications. This research also advances the use of innovative 2D nanomaterials to address radioactive waste remediation challenges.

Supplementary Materials:
The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/nano12183253/s1, Figure S1. (a) The BET adsorption/desorption isotherm and (b) BHJ pore size distribution graph of M.Ti 2 CT x .AIII; Figure S2. The XPS peak fitting analysis of Sr 3d in M.Ti 2 CT x .AIII after radionuclides adsorption; Figure S3. The XPS peak fitting analysis of Na 1s (a) before and (b) after radionuclides adsorption in M.Ti 2 CT x .AIII; Figure S4. The XPS peak fitting analysis of O 1s and C 1s before (a,c) and after (b,d) radionuclides adsorption, respectively, in M.Ti 2 CT x .AIII; Table S1. Elemental composition of M.Ti 2 CT x .AIII measured in SEM-EDS analysis; Table S2. Elemental composition of M.Ti 2 CT x .AIII after Sr 2+ and Cs + adsorption, measured in SEM-EDS analysis; Table S3. Elemental composition (Atomic%) of as-prepared M.Ti 2 CT x .AIII and after Sr 2+ and Cs + adsorption, measured in XPS analysis.