Thorium(IV) adsorption onto multilayered Ti3C2Tx MXene: a batch, X-ray diffraction and EXAFS combined study
aLaboratory of Nuclear Energy Chemistry, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, People's Republic of China, bSchool of Chemistry, Biological and Materials Science, East China University of Technology, Nanchang 330013, People's Republic of China, cCollege of Science, Jiangxi Agricultural University, Nanchang 330045, People's Republic of China, and dSchool of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, People's Republic of China
*Correspondence e-mail: firstname.lastname@example.org, email@example.com
The interlayer regulation of layered environmental adsorption materials such as two-dimensional early transition metal carbides and carbonitrides (MXenes) plays an important role in their purification performance for specific pollutants. Here the enhanced uptake of ThIV by multilayered titanium carbides (Ti3C2Tx) through a hydrated intercalation strategy is reported. ThIV adsorption behaviors of three Ti3C2Tx samples with different c lattice parameters were studied as a function of contact time, pH, initial concentration, temperature and ion strength in batch experiments. The results indicated that the ThIV uptake was pH and ionic strength dependent, and the adsorption process followed the pseudo-second-order kinetics and the heterogeneous isotherm (Freundlich) model. Thermodynamic data suggested that the adsorption process of all MXene samples was a spontaneous endothermic reaction. The dimethyl sulfoxide intercalated hydrated Ti3C2Tx featured the largest interlayer space and exhibited the highest ThIV adsorption capacity (162 mg g−1 at pH 3.4 or 112 mg g−1 at pH 3.0), reflecting the significant increase in available adsorption sites from Ti3C2Tx interlayers. The adsorption mechanism has been clarified based on adsorption experiments and spectroscopic characterizations. An ion exchange process was proposed for the interaction between hydrated MXenes and ThIV, where H+ from surface [Ti−O]−H+ groups were the primary active sites on Ti3C2Tx. Extended X-ray absorption fine structure (EXAFS) fitting results, in combination with X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) analyses, clearly indicated that ThIV mainly formed the outer-sphere complexes on Ti3C2Tx surface through electrostatic interaction under strong acid conditions, while at pH > 3.0 the adsorption mechanism was determined by inner-sphere coordination and electrostatic interaction together.
As a long-lived natural radioactive element, thorium is a potential nuclear fuel for the next-generation reactors (e.g. liquid fluoride thorium reactor) with advantages of large abundance, low cost, low risk of nuclear proliferation and almost no waste (Wang, Brown et al., 2018; Wang, Liu et al., 2018). In addition to the nuclear industry, thorium and its compounds are widely used in the applications of efficient catalysis, refractory materials, high-quality lenses and aviation alloy manufacturing (Rao et al., 2006). In terms of environmental concerns, a significant amount of thorium-containing waste has been discharged by ore processing in the rare earth industry, coal burning in thermal power plants and the use of chemical fertilizers. The radioactive nature and heavy metal toxicity of thorium may lead to great hazards for public health and ecosystems (Zhou et al., 2021). In aqueous solutions, thorium usually exists in the form of stable ThIV, which can be deposited in bone, kidney and liver (Liu et al., 2019; Rao et al., 2006), thus causing permanent damage to the human body. In addition, ThIV is often used as a chemical analog of other tetravalent actinides to simulate their migration and transformation behavior in the environment (Chen & Wang, 2007). The development of novel materials featuring high uptake capacity, good stability and excellent selectivity for ThIV capture is urgently desired with regard to the effective utilization of thorium resources and radioactive pollution remediation.
Because of the large pre-concentration coefficient and facile operation, adsorption is one of the most attractive approaches for thorium extraction from aqueous solution. In the past few decades, a large number of solid-phase adsorbents such as metal oxide particles (Reiller et al., 2005; Rojo et al., 2009; Tan et al., 2007; Yusan et al., 2016), inorganic porous materials (Alahabadi et al., 2020; Sheng et al., 2008; Misaelides et al., 1995), functionalized mesoporous silica (Lebed et al., 2011; Yuan et al., 2014; Zhang, Ma et al., 2020), polymers (Yuan et al., 2019), organic–inorganic hybrids (Abbasizadeh et al., 2013; Kaygun & Akyil, 2007) and metal–organic framework (MOF) materials (Moghaddam et al., 2018; Zhang et al., 2017) have been widely examined to evaluate ThIV separation from artificial and/or environmental wastewater. Apart from the above materials, the layered adsorbents provide numerous surface atoms and have unique structures with intralayer covalent bonding and interlayer van der Waals bonding (Wang et al., 2020), which allow the facile regulation of surface and interlayer properties to prepare multifunctional adsorbents for ThIV elimination. Compared with the natural layered minerals (e.g. phlogopite, bentonite, illite and sepiolite) (Erden & Donat, 2017; Fralova et al., 2021; Wu et al., 2018; Zhang et al., 2016; Zhao, 2008), the emerging two-dimensional materials such as graphene oxide (Bai et al., 2014; Li, Yang et al., 2018) and layered sulfides (Li, Li et al., 2018; Xu et al., 2021) exhibit much higher thorium removal capacity and better selectivity under acidic conditions, which reflects the great potential of novel lamellar materials in radionuclide remediation.
Two-dimensional transition metal carbides and carbonitrides (MXenes) are a family of layered materials with hexagonal structure and the general chemical formula of Mn+1XnTx discovered in the past decade, where M represents an early transition metal, X represents C and/or N, Tx denotes surface termination groups, and n = 1–3 (Naguib et al., 2011, 2012). So far, MXene has been widely used in the application fields of catalysis, energy storage, sensing and environment (VahidMohammadi et al., 2021; Li et al., 2019), and the specific performance of this material largely depends on its chemical bonding and surface termination properties (Magnuson & Näslund, 2020; Naslund et al., 2021). Benefiting from the variable element composition, abundant oxygen-containing surface groups and good hydrophilicity, MXenes have proved to be promising environmental remediation materials in the field of water purification (Rasool et al., 2019; Ihsanullah, 2020; Chen et al., 2020). In terms of radionuclide elimination, various MXenes (e.g. Ti3C2Tx, V2CTx, T2CTx) and their composites and derivatives have been synthesized to investigate the corresponding adsorption behavior for CsI (Jun, Jang et al., 2020; Khan et al., 2019; Shahzad et al., 2020), SrII (Jun, Park et al., 2020), BaII (Mu et al., 2018), PdII (Mu et al., 2019), EuIII (Zhang, Wang et al., 2020), UVI (Wang, Song et al., 2018; Wang et al., 2016, 2021), ReVII/TcVII (Wang et al., 2019) and I− (Huang et al., 2020). For ThIV, only one preliminary study has reported its adsorption on Ti2CTx MXene (Li et al., 2019), and results indicated that the adsorption had a high distribution coefficient and was independent of ionic strength. However, the easily degradable nature of Ti2CTx in environmental atmosphere and water greatly limits its environmental application. Moreover, the rational utilization of the spontaneous intercalation behavior of MXenes is of great importance, because the uptake performance may be significantly improved by weakening the interlayer interaction of multilayer structures. Herein, we report the adsorption of ThIV on more stable Ti3C2Tx MXene as extended research. The effect of interlayer regulation of Ti3C2Tx on the adsorption behavior has been examined in detail by batch experiments. Furthermore, the underlying adsorption mechanism is also deciphered by EXAFS (extended X-ray absorption fine structure), XRD (X-ray diffraction) and FTIR (Fourier transform infrared) spectroscopy analyses.
All the chemicals used as-received in this study were analytical grade and obtained from Sinopharm Chemical Reagent Co. Ltd. A 10 g l−1 thorium stock solution was prepared by dissolving an appropriate amount of thorium nitrate pentahydrate [Th(NO3)4·5H2O] in deionized water acidified with nitric acid.
The pristine and intercalated Ti3C2Tx samples were synthesized according to the literature (Wang et al., 2017). Briefly, a pristine Ti3C2Tx water suspension (Ti3C2Tx-H) was prepared by etching Ti3AlC2 with 15 wt% HF and washing with deionized water. Dry Ti3C2Tx sample (labeled as Ti3C2Tx-D) was obtained by filtering and vacuum drying Ti3C2Tx-H MXene at room temperature. The filtered Ti3C2Tx-H was dispersed in dimethyl sulfoxide (DMSO) for 2 days, and then washed with deionized water to prepare a water suspension of hydrated DMSO-intercalated Ti3C2Tx (labeled as Ti3C2Tx-DMSO-H). To avoid the slow oxidation, Ti3C2Tx-H and Ti3C2Tx-DMSO-H were stored in polypropylene bottles filled with Ar at 277 K (4°C) for later use. The mass concentrations of hydrated samples were measured by calculating the weight of MXene after filtering and drying the suspension with a fixed volume.
A series of adsorption experiments for ThIV uptake in aqueous solution by various Ti3C2Tx samples were performed as a function of contact time, pH, initial ThIV concentration (C0), temperature, ionic strength and other competing metal cations. The ThIV stock solution was diluted to initial concentrations ranging from 5 to 150 mg l−1 to carry out adsorption experiments by a batch method. Typically, 4 mg MXene was mixed with an appropriate amount of deionized water to keep the total volume of suspension as 5 ml in a beaker. Then 5 ml of solution containing ThIV or multi-metal ions was added such that the concentration of sorbent was 0.4 g l−1. The pH value of the solution was adjusted with small amounts of 0.1 M NaOH and 0.1 M HNO3, and then the solution was stirred at room temperature for a specified time. The solid adsorbent was removed after ThIV adsorption to obtain the supernatant sample by using a polyethersulfone syringe filter (0.22 µm, ANPEL Scientific Instrument Co. Ltd, Shanghai). The supernatant was diluted with 5 wt% HNO3 before the final concentration determination. For the ionic strength experiment, NaClO4 (0.1–500 mmol l−1) was selected as representative electrolytes. In the selective adsorption test, all initial concentrations of ThIV and other competing metal ions, including Co2+, Ni2+, Zn2+, Sr2+, La3+, Nd3+, Sm3+, Gd3+ and Yb3+, were 0.5 mmol l−1. The residual concentrations of thorium and other metal elements were determined by an inductively coupled plasma optical emission spectrograph (ICP-OES, Horiba JY2000-2, France).
For the above batch adsorption experiments, the uptake capacity Qe (mg g−1) and distribution coefficient Kd (ml g−1) were calculated using the following equations:
where C0 and Ce are the initial and final equilibrium concentration of cations, respectively, and V and m are the volume of solution and the mass amount of solid sorbent in the batch adsorption tests, respectively. All the adsorption experiments have been carried out at least twice, and the uncertainty of measurement is less than 5%.
The morphologies and chemical compositions of the MXene samples were analyzed by a Hitachi S-4800 field-emission scanning electron microscope equipped with Horiba 7593-H energy-dispersive X-ray spectroscopy (EDS). A Bruker D8 Advance diffractometer (Cu Kα radiation, λ = 1.5406 Å) was used to collect powder XRD patterns. The step size for XRD pattern scanning was 0.02°. A Bruker Tensor 27 spectrometer was used to measure the FTIR spectra of samples by a potassium bromide pellet method.
The L3 edge (16300 eV) EXAFS spectra of Th were collected at beamline 1W1B of the Beijing Synchrotron Radiation Facility (BSRF) using a silicon (111) double-crystal monochromator. Th(NO3)4·5H2O (reference) was measured in transmission mode while other samples were measured in fluorescence mode using a Lytle-type ion chamber detector. EXAFS oscillation data were extracted, analyzed and fitted using computer programs Athena and Artemis, which were parts of the IFEFFIT program package (Ravel & Newville, 2005). A k range of ∼1.8−11.5 Å−1 and a background frequency cutoff parameter (Rbkg) of 1.2 were used for acquisition of the Fourier transform of k3-weighted EXAFS data. To fit parameters such as coordination number (CN), atomic distance (R) and Debye–Waller factor (σ2), the theoretical phase and amplitude functions for the scattering pathways of Th–O and Th–N were calculated based on the crystal structures of Th(NO3)4·5H2O. The fitting procedures were carried out in R space from 1.2 to 3.5 Å for Th(NO3)4·5H2O and 1.2 to 3.0 Å for other samples with a fixed amplitude reduction factor (S02) of 0.97.
MXene samples of Ti3C2Tx-D, Ti3C2Tx-H and Ti3C2Tx-DMSO-H were prepared by drying, wetting and intercalating multilayered Ti3C2Tx. Their powder XRD patterns are shown in Fig. 1. The (002) peak of Ti3C2Tx-D is centered at 2θ of 8.8°, corresponding to a c lattice parameter (c-LP) of 20.0 Å. Because of the hydration effect and co-intercalation of H2O/DMSO molecules inside the MXene interlayer, the (002) peak position shifts to 6.8° and 4.6° for Ti3C2Tx-H and Ti3C2Tx-DMSO-H, respectively, which results in much larger c-LPs (25.9 and 38.1 Å). Furthermore, the good reproducibility of intercalated MXene preparation has been proved using Ti3C2Tx-DMSO-H as a demonstration (Fig. S1 in the supporting information). The SEM (scanning electron microscopy) images in Fig. 2 indicate that Ti3C2Tx-D, Ti3C2Tx-H and Ti3C2Tx-DMSO-H have similar morphology, i.e. they feature micrometre-sized bulk particles composed of nano-lamellar structures. The above results are in good agreement with our previous study (Wang et al., 2017).
The adsorption kinetics experiments of radioactive ThIV onto the three MXene samples were carried out at a concentration of 100 mg l−1 and pH 3.4, with the results shown in Fig. 3(a). The adsorption of ThIV on Ti3C2Tx-D reached a plateau in 30 min with an uptake capacity of 37 mg g−1. The hydrated and intercalated MXenes showed much higher ThIV removal performance but slower kinetics, and the equilibration times for Ti3C2Tx-H and Ti3C2Tx-DMSO-H were 90 and 300 min, respectively. Previous literature (Wang et al., 2017) and XRD measurements in this study (see below, Section 3.7) have demonstrated that the adsorption of cations by dry MXene sample mainly occurs on the external surface because its interlayer space is too narrow, while the hydrated and intercalated MXene could provide a large amount of interlayer adsorption sites besides the exterior surface adsorption sites; therefore Ti3C2Tx-H and Ti3C2Tx-DMSO-H exhibited much larger ThIV uptake capacities than Ti3C2Tx-D in this work. Our results suggest that the adsorption of ThIV inside MXene interlayers corresponds to a slower internal diffusion process; this is because the `deep' adsorption sites in multilayer MXene particles may require sufficient diffusion of ThIV in the confinement space.
In order to clarify the adsorption process, the pseudo-second-order kinetic model was used to analyze the experimentally observed kinetic data and the linear fitting results are shown in Fig. 3(b). The linearized form of the model is given as follows:
where Qe (mg g−1) and Qt (mg g−1) are the quantities of the sorbed ThIV at equilibrium time and time t, respectively, and k (g mg−1 min−1) is the pseudo-second-order adsorption rate constant. The model parameters and correlation coefficients obtained by the model are listed in Table 1.
It can be clearly seen that t/Qt has a good linear relationship with t for the adsorption kinetics of all three MXene samples, and the correlation coefficients (R2) are close to 1. Additionally, the calculated removal capacities of Ti3C2Tx-D, Ti3C2Tx-H and Ti3C2Tx-DMSO-H based on the model are similar to the experimental equilibrium capacities (Table 1). The pseudo-second-order kinetic model describes the removal of ThIV on Ti3C2Tx MXenes quite well, revealing that the kinetics are determined by two components which may be related to the diffusion of ThIV and the occupation of adsorption sites on MXene surface.
Fig. 4 represents the removal of ThIV by Ti3C2Tx-D, Ti3C2Tx-H and Ti3C2Tx-DMSO-H as a function of pH. The uptake of ThIV for the three MXene samples shows an overall increasing trend with the increase in solution pH, which is ascribed to the changes in the surface charge of MXene and the species of ThIV in the solutions. Since pristine Ti3C2Tx and its intercalated products had similar zero charge points around pH 2.0–2.5 (Ying et al., 2015), their surfaces were positively charged under very acidic conditions. In this case, the adsorption of cationic ThIV on Ti3C2Tx MXenes was greatly inhibited due to the electrostatic repulsion and the competition of the high concentration of H+. Nonetheless, Ti3C2Tx-DMSO-H still exhibited a considerable adsorption capacity (∼45 mg g−1) at pH 1.5, implying that its largest interlayer space promotes the ion exchange of protons on the MXene surface (in the form of [Ti–O]−H+) with ThIV. As the pH increased above 2.5, the surface of Ti3C2Tx became negatively charged, and the electrostatic interaction between adsorbent and ThIV resulted in an increase in adsorption capacity. The species distribution of ThIV as a function of pH was calculated using the Medusa/Hydra software. It can be seen from Fig. S2 that although there is no Th(OH)4 precipitation under the experimental conditions, the partial hydrolysis species of ThIV including Th(OH)3+ and Th(OH)22+ formed with the increase in pH. The fractions of Th(OH)3+ and Th(OH)22+ increase sharply at pH > 2.0 and pH > 3.0, respectively, which also contributes to the increase of ThIV adsorption capacity at high pH conditions.
The adsorption isotherm experiments were conducted to assess the effect of ThIV equilibrium concentration on the adsorption behavior of the MXenes. As shown in Fig. 5, over the initial concentration range of 5–150 mg l−1, the obtained maximum experimental adsorption capacities (Qmax) of ThIV for Ti3C2Tx-D, Ti3C2Tx-H and Ti3C2Tx-DMSO-H at pH 3.4 were 46, 126 and 162 mg g−1, while the corresponding Qmax at pH 3.0 were 17, 86 and 112 mg g−1, respectively. Ti3C2Tx-DMSO-H had the best ThIV enrichment performance among the three MXene samples, which is attributed to the most available active adsorption sites stemming from its largest interlayer space.
To further evaluate the adsorption mechanism, Langmuir and Freundlich isotherm models were used to fit the experimental data. The former mainly describes monolayer adsorption and the latter is suitable for the interpretation of multilayer adsorption. The equations of the two models are expressed as follows in order:
For the Langmuir model [equation (4)], Qm is the maximum adsorption capacity (mg g−1) corresponding to a complete monolayer coverage and kL is a constant indirectly related to the sorption capacity and energy of adsorption (l mg−1), which characterizes the affinity of the adsorbate to the adsorbent. As for the Freundlich model [equation (5)], Qe (mg g−1) is the equilibrium adsorption amount, kF and n are the Freundlich constants related to the adsorption capacity and the adsorption intensity, respectively. Based on Fig. 5 and the fitting parameters in Table 2, it is apparent that the Freundlich model could better describe the adsorption process of ThIV on the three MXene samples. Because the fitting of pH 3.0 and pH 3.4 led to similar results, we concluded that the heterogeneous adsorption of ThIV by Ti3C2Tx was a pH-independent intrinsic characteristic. The heterogeneous adsorption mechanism could be explained by the various termination groups (–OH, –O and –F) and unique multilayer structures of Ti3C2Tx. It was also noted that Ti3C2Tx-H and Ti3C2Tx-DMSO-H had larger values of parameter n than Ti3C2Tx-D, implying a more favorable adsorption of ThIV by the hydrated and intercalated MXene.
The adsorption of ThIV onto the MXene samples was carried out at 280, 290, 300 and 310 K to investigate the effect of temperature. The uptake capacities of Ti3C2Tx-D, Ti3C2Tx-H and Ti3C2Tx-DMSO-H increased with increasing temperature [Fig. 6(a)], suggesting that a higher temperature is favorable for the adsorption of ThIV by Ti3C2Tx. Thermodynamic evaluation of an adsorption process is necessary to conclude whether the process is spontaneous or not. The Gibb's free energy change (ΔG0, kJ mol−1), enthalpy change (ΔH0, kJ mol−1) and entropy change (ΔS0, J mol−1 K−1) can be calculated by equations (6) and (7):
where R is the universal gas constant (8.314 J mol−1 K−1), T is absolute temperature (K) and Kd represents the distribution coefficient (ml g−1). Combining equations (6) and (7), ΔH0 and ΔS0 can be determined from the linear relationship of lnKd versus 1/T, described as follows:
Fig. 6(b) shows the above linear plots and Table 3 lists the thermodynamic parameters calculated from the fitting lines. ΔG0 is negative at all the conditions applied, confirming that the adsorption of ThIV by Ti3C2Tx is spontaneous. Ti3C2Tx-DMSO-H has the most negative ΔG0 among the three MXene samples, which reflects the thermodynamically favorable nature of the hydrated intercalated MXene for ThIV adsorption. ΔH0 > 0 indicates that the adsorption is an endothermic process. Ti3C2Tx-D has a larger ΔH0 than the other two samples, which might be related to the hydration/dehydration states of MXene. The positive value of ΔS0 suggests the increased randomness at the adsorbent/adsorbate interface during the adsorption. Additionally, compared with the thermodynamic data of previously studied hydrated Ti2CTx (Li et al., 2019), Ti3C2Tx-H renders a lower ΔH0 and ΔS0, which indicates that the affinity of Ti3C2Tx to ThIV is not as strong as that of Ti2CTx, although the two MXenes have similar surface termination and layer structure.
3.6. Effect of ionic strength and selectivity test
Particular attention was also paid in this study to ThIV adsorption under various ionic strengths and competing cations to evaluate the potential of Ti3C2Tx in practical applications of radionuclide separation and thorium purification.
As can be seen in Fig. 7, the removal efficiency of ThIV by the hydrated MXenes decreased with the increase in ionic strength, which reflected that the adsorption process might be controlled by the ion exchange mechanism. Nevertheless, Ti3C2Tx-DMSO-H still exhibited good ThIV uptake performance at the medium ionic strength conditions. The Kd extracted from Fig. 7 was found to be larger than 5000 ml g−1 when the ionic strength was ≤0.1 mol l−1, confirming Ti3C2Tx-DMSO-H is an excellent adsorbent for ThIV removal from common wastewater. It is interesting that the removal efficiency of Ti3C2Tx-D increased with the increase in ionic strength, which is contrary to the behavior of the hydrated Ti3C2Tx MXenes. A reasonable explanation is that the presence of a high concentration of Na+ in aqueous solution may activate the dried Ti3C2Tx, leading to the intercalation of hydrated ions at the edge of nano-lamellar MXene, thereby increasing the adsorption sites for ThIV.
To evaluate the ion selectivity of the MXene adsorbents, a competing adsorption experiment was also performed in a mixed solution containing ThIV and nine other metal cations. The initial concentration of each metal ion was 0.5 mmol l−1. As shown in Fig. 8(a), Ti3C2Tx-D had almost no selectivity because the adsorption performance of ThIV (mmol g−1) was comparable with other metal ions. The ThIV uptake capacities at pH 3.0 for Ti3C2Tx-H and Ti3C2Tx-DMSO-H were 0.33 and 0.40 mmol g−1, and at pH 3.4 they were 0.39 and 0.45 mmol g−1, respectively, whereas the uptake capacities for other competing ions were as low as less than 0.07 mmol g−1. The selectivity coefficients, which we defined as the ratio of Kd of ThIV to that of the competing ions, were calculated to be larger than 8.3 and 9.4 for Ti3C2Tx-H and Ti3C2Tx-DMSO-H at pH 3.0, respectively, suggesting a desirable selectivity of the hydrated and intercalated Ti3C2Tx for ThIV.
To clarify the underlying adsorption mechanism, the MXene samples loaded with ThIV were vacuum dried and then subjected to SEM, EDS and XRD characterization. SEM images in Figs. 2(d)–2(f) indicated that the multilayer structures of Ti3C2Tx MXene were well retained after adsorption. EDS results in Figs. 2(g)–2(i) clearly confirmed that the content of Th in the MXene samples increased rapidly with the expansion of Ti3C2Tx interlayer space, which is consistent with the results of batch adsorption experiments. The comparison of XRD patterns can reflect the interaction behavior between layered materials and guest ions. According to Fig. 9(a) and Fig. S3, the (002) peak position of Ti3C2Tx-D remained unchanged after the adsorption, suggesting that no hydrated ThIV ions entered the interlayers, and the uptake of ThIV only occurred on the exterior surface of MXene. For hydrated MXenes including Ti3C2Tx-DMSO-H and Ti3C2Tx-H, the (002) peak shifted to 6.6° after the adsorption, revealing the successful intercalation of ThIV in these samples. Since the solution pH had a slight decrease after the adsorption, we attribute the intercalation to the replacement of hydrated H+ by ThIV in the interlayers of MXene, namely, there may be an ion exchange reaction. An estimation of the size of intercalated ions was carried out by calculating the enlarged interlayer space (1/2 Δc-LP) between Th-loaded Ti3C2Tx-DMSO-H/Ti3C2Tx-H and Ti3C2Tx-D samples. The dimension of intercalated ThIV was calculated to be about 3.4 Å, which is smaller than the previously reported size of intercalated uranyl ions (3.9 Å) (Wang et al., 2017). This difference in size is reasonable because the hydrated thorium ion has a spherical configuration while the hydrated uranyl ion prefers equatorial plane coordination. Fig. 9(b) shows the effect of pH on the c-LP of Th-loaded Ti3C2Tx-DMSO-H. As the reaction pH decreased, the c-LP of MXene decreased from 26.7 to 25.1 Å, and the intensity of the (002) peak was also significantly weakened. This result shows that the interlayer adsorption amount and the chemical species of ThIV under different pH together determine its intercalation behavior inside MXene interlayers.
The EXAFS technique can probe the local environment around Th atoms (Shi et al., 2014), and therefore provide important information for understanding the microscopic mechanism of ThIV adsorption on the Ti3C2Tx MXene. The Th L3 edge k3-weighted EXAFS spectra of the Th-loaded hydrated MXenes and the references are shown on the left side of Fig. 10. The EXAFS data for the Ti3C2Tx-D sample are not available due to the poor quality of the spectrum caused by low ThIV adsorption capacity. The oscillation periods of Ti3C2Tx-DMSO-H at pH 2.0 are consistent with that of ThIV aqueous solution, but are significantly different from Th(NO3)4·5H2O, implying the presence of highly hydrated ThIV ions in the adsorbent and there were no coordinated nitrates around Th atoms. The amplitudes at k > 8.5 Å become lower under higher pH (e.g. pH 3.0 and 3.4), which reflects a lack of long-range order of ThIV in the MXene interlayers. Fig. 10 (right side) represents the associated Fourier transform (FT) spectra in R space, and the fitting results are shown in Table 4. The FT spectra exhibit a strong peak at around 2 Å (without phase-shift correction), which can be ascribed to the first oxygen coordination shell surrounding the ThIV atom. The average Th—O distance was fitted as 2.46 Å with a CN of 10.7 for ThIV aqueous solution, which is in good agreement with the literature (Dähn et al., 2002; Rothe et al., 2002; Zhang et al., 2017). The sample of Ti3C2Tx-DMSO-H at pH 2.0 had 10.1 oxygen atoms at 2.46 Å. Although the CN is slightly reduced due to the confinement of the MXene interlayer, this result is similar to that of ThIV aqueous solution, which indicates that electrostatic interaction is dominant during the adsorption process under low pH conditions. When the solution pH increased to 3.0 and 3.4, the Th—O distance shifted to 2.43 Å, while concomitantly the CN decreased to 9.5 and 8.9, respectively. It has been reported that the compact Th—O bond at 2.40–2.42 Å and the CN of 8.0–9.0 usually correspond to the formation of stable inner-sphere complexes for ThIV (Seco et al., 2009; Zhang et al., 2017). Therefore, our fitting results indicate that the contribution of surface complexation to ThIV adsorption on Ti3C2Tx cannot be ignored at such pH conditions. Additionally, the fitting parameters of Ti3C2Tx-H are very close to that of Ti3C2Tx-DMSO-H at pH 3.4, reflecting that the different intercalated guest in this study (water or a mixture of water and DMSO molecules) will not significantly affect the underlying adsorption mechanism. The FTIR spectra in Fig. 11 provide further information to support the above conclusion. After ThIV uptake, the intensity of the absorption peak at 649 cm−1 decreased and the absorption peak at 561 cm−1 clearly shifted to 568 cm−1 for both hydrated MXenes, whereas the absorption peaks for Ti3C2Tx-D were almost unchanged. The absorption bands in the range of 560–650 cm−1 can be ascribed to the Ti–O interaction of Ti3C2Tx MXene (Peng et al., 2014). The changes of FTIR spectra apparently demonstrate the presence of strong affinities between Ti–O and ThIV ions in Ti3C2Tx-H and Ti3C2Tx-DMSO-H. Combined with the fact that the ionic strength could influence the uptake capacity of MXene, we conclude that under higher pH conditions (pH > 3.0), the adsorption of ThIV on hydrated Ti3C2Tx is jointly determined by inner-sphere complexation and electrostatic interaction.
In summary, this work systemically studied the adsorption behavior of ThIV by dry, hydrated and intercalated Ti3C2Tx MXene samples. Because of the full utilization of the active sites inside the interlayers, the intercalation adsorption of ThIV was dominant for the hydrated MXenes, while only external surface adsorption was available for the dry MXene, so Ti3C2Tx-DMSO-H and Ti3C2Tx-H exhibited higher uptake capacities and slower kinetics. Our work demonstrated that the elimination of ThIV by Ti3C2Tx was a spontaneous endothermic reaction which followed a heterogeneous adsorption model. Ti3C2Tx showed a lower affinity for binding ThIV compared with its titanium-based analog Ti2CTx, and the adsorption process could be influenced by the ionic strength. Nevertheless, Ti3C2Tx-DMSO-H still exhibited considerable removal capacity and desirable selectivity for ThIV at moderate concentration of ionic strength and with competing cations, reflecting its application potential in the purification of thorium-containing radioactive wastewater. Additionally, by analyzing the coordination environment and adsorption sites, we confirmed that the adsorption of ThIV on the hydrated Ti3C2Tx MXenes was determined by a combination of electrostatic interaction and surface complexation. These findings can greatly enrich our understanding of the microscopic interactions between MXenes and actinide ions.
‡These authors contributed equally to this work.
We are grateful to the staff of Beijing Synchrotron Radiation Facility for the EXAFS measurement.
This work was supported by the National Science Fund for Distinguished Young Scholars (grant No. 21925603), the National Natural Science Foundation of China (grant Nos. 22176190, U20B2019, 21906020 and 11875004) and Youth Innovation Promotion Association CAS (2021010).
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