Sorption of Eu ( III ) onto Nano-Sized H-Titanates of Different Structures

Hydrogen titanates (H-titanates) of different nanostructures (nanotubes, nanowires, nanosheets) have been synthesized by hydrothermal methods. The europium (III) sorption from aqueous solutions onto nano-sized H-titanates was studied as a function of contact time, pH values, and initial Eu (III) concentration in batch experiments. Reversibility of adsorption of europium has been investigated as well. Nano-sized H-titanates can be used for tri-valent f-elements removal in polluted water treatment due to fast and efficient sorption of Eu (III).


Introduction
The development of nuclear energy results in increasing discharge of radionuclides in natural environments due to accidents in nuclear power or reprocessing plants [1].Among different products, actinides and lanthanides are formed in spent nuclear fuel (SNF) and radioactive waste (RW).Thus, storage and reprocessing of SNF and RW can release these elements, as well as other components, into the natural water and soil environment.Transplutonium actinides (mainly, Am and Cm) and lanthanides formed in nuclear fuel during its operation are important hazardous elements due to their high toxicity and radioactivity.Therefore, it is very important to develop materials and methods for their effective removal from wastewater and groundwater.

Materials and Methods
The procedure to prepare various titanate nanostructures is described as follows.In all synthesis experiments, anatase TiO 2 particles (Sigma-Aldrich, USA) were used as the precursor.The short titanate nanotubes were synthesized following the conventional hydrothermal synthesis method [20,27].In a typical experiment, 0.5 g TiO 2 particles were dispersed in 80 mL 10 M NaOH solution and transferred into a 125 mL Teflon-lined stainless-steel autoclave.The hydrothermal synthesis was carried out at 130 • C for 48 h.The short titanate nanowires were synthesized under the same synthesis conditions and procedure, except that the temperature was set at 170 • C for 48 h.For the synthesis of titanate nanosheet structures, the NaOH concentration was lowered to 5 M, and the synthesis was carried out at 150 • C for 48 h.
The long titanate nanotube and nanowire samples were synthesized using a stirring hydrothermal method detailed elsewhere [20,28].In a typical experiment, 0.1 g TiO 2 particles were dispersed in 15 mL 10 M NaOH solution via continuous stirring.The solution with dispersed particles was then transferred into a 23 mL Teflon-lined stainless-steel autoclave with a magnetic stirrer placed inside.The autoclave was placed in a silicone oil bath heated to a designated synthesis temperature.During the synthesis, magnetic stirring rate was maintained at 500 rpm.Long titanate nanotubes were obtained after the stirring hydrothermal synthesis at 130 • C for 48 h.Long titanate nanowires were synthesized at 170 • C for 24 h.
After the above syntheses were completed, different forms of sodium titanate precipitates were produced and collected using a centrifuge.The as-synthesized precipitates were washed with DI water several times until the pH value of the water solution decreased to around 7. The collected precipitates were then soaked in a 0.6 M HCl solution for 72 h, under constant stirring, and collected by centrifugation.The sample was then washed by DI water as described above.The entire process of proton exchange was repeated 2-3 times until the pH of the solution reached around 6, which indicated that the sodium ions were completely exchanged by hydrogen ions to form hydrogen titanate, while the nanostructures of the sodium titanates were retained.
The morphology of synthesized H-titanates was studied by scanning electron microscopy.Specific surface area of the synthesized H-titanates samples was determined by N 2 sorption-desorption and further fitting with the BET (Brunauer-Emmett-Teller) model [29].
The 152 Eu radionuclide was used in this study to determine the content of europium in water solutions.All other commercial reagents were of analytical grade and used as received.All solutions were prepared with deionized MilliQ water.
All experiments were carried out at ambient atmospheric conditions and at room temperature (22-23 • C).
Radionuclide removal was studied in aqueous solutions at different pH values.Solutions were prepared with deionized water and pH values were adjusted with 0.1 M NaOH and 0.1 M HCl solutions after the addition of H-titanates.The solid to liquid ratio was 1 g L −1 in all experiments.Europium solutions were prepared by dilution of Eu (NO 3 ) 3 stock solution (natural content of stable isotopes). 152Eu (T 1/2 = 13.54 years) was added as tracer to measure europium content by γ-ray spectrometry (high purity germanium detector, GC 3020, Canberra, Australia).Total initial concentration of europium was ca. 10 −9 M. H-titanates were added first in water solution, then aliquots of Eu stock solution were added, and further pH values were adjusted.Then solutions were intensively shaken for few seconds and left on the orbital shaker.The solutions at pH 4 were sampled periodically to determine the optimal equilibration time-i.e., time when the steady-state is reached.All other samples were sampled after two days of experimentation, after re-measurement of the pH values.All samples were centrifuged at 40,000× g within 10 min to remove particles of H-titanates.Supernatants were then analyzed for the content of europium.Sorption isotherms were obtained in the same manner, but the initial concentration of Eu (III) was varied from 10 −9 M to 10 −4 M.

Results
The morphology of synthesized H-titanates revealed by scanning electron microscopy is presented in Figure 1.

Results
The morphology of synthesized H-titanates revealed by scanning electron microscopy is presented in Figure 1.Values of the specific surface area of the synthesized H-titanates samples are presented in Table 1.Short names of the studied samples given in Table 1 are used throughout the text.Values of the specific surface area of the synthesized H-titanates samples are presented in Table 1.Short names of the studied samples given in Table 1 are used throughout the text.The dependence of the Eu (III) removal from aqueous solution at different pH 4 versus time is shown in Figure 2. The steady state has been reached within 10 min of experiments in all cases.The equilibrium fraction of the sorbed europium (III) is more than 98%.
The dependence of the Eu (III) removal from aqueous solution at different pH 4 versus time is shown in Figure 2. The steady state has been reached within 10 min of experiments in all cases.The equilibrium fraction of the sorbed europium (III) is more than 98%.Strong sorption of cations can lead to their irreversible fixation.We have performed desorption studies after sorption experiments by shifting pH values to more acidic.During desorption studies the supernatant solutions were not changed, but only small portions of HCl solutions were added to shift the pH values to lower values.The results for all the investigated samples are presented in Figure 4.The example scheme of pH shifting (arrows) is presented in Figure 4a for short nanotubes.The dependence of the Eu (III) removal from aqueous solution at different pH 4 versus time is shown in Figure 2. The steady state has been reached within 10 min of experiments in all cases.The equilibrium fraction of the sorbed europium (III) is more than 98%.Strong sorption of cations can lead to their irreversible fixation.We have performed desorption studies after sorption experiments by shifting pH values to more acidic.During desorption studies the supernatant solutions were not changed, but only small portions of HCl solutions were added to shift the pH values to lower values.The results for all the investigated samples are presented in Figure 4.The example scheme of pH shifting (arrows) is presented in Figure 4a for short nanotubes.Strong sorption of cations can lead to their irreversible fixation.We have performed desorption studies after sorption experiments by shifting pH values to more acidic.During desorption studies the supernatant solutions were not changed, but only small portions of HCl solutions were added to shift the pH values to lower values.The results for all the investigated samples are presented in Figure 4.The example scheme of pH shifting (arrows) is presented in Figure 4a for short nanotubes.Investigated H-titanates have different morphology and specific surface areas (see Table 1).Thus, it is useful to compare specific sorption properties of these materials, e.g., distribution coefficient (Kd).We calculated the Kd values with Equation (1): where Kd-is the distribution coefficient, mL m −2 ; %-fraction of a sorbed cation; V-volume of the sample, mL; S-specific surface area, m 2 g −1 ; m-mass of the sorbent, g.The resulting values of Kd are presented in Figure 5.The values of sorbed Eu (III) fraction expressed in percentage units demonstrate almost no difference between different H-titanates at pH values above 4 (see Figure 2).However, Kd values differ significantly-at least order of magnitude between long nanowire (NWL) and long nanotube (NTL) samples.The reason is the sensitivity of Kd values to very high (close to 100%) or very low (close to 0%) sorption percentage.In this range, the Investigated H-titanates have different morphology and specific surface areas (see Table 1).Thus, it is useful to compare specific sorption properties of these materials, e.g., distribution coefficient (Kd).We calculated the Kd values with Equation (1): where Kd-is the distribution coefficient, mL m −2 ; %-fraction of a sorbed cation; V-volume of the sample, mL; S-specific surface area, m 2 g −1 ; m-mass of the sorbent, g.The resulting values of Kd are presented in Figure 5.The values of sorbed Eu (III) fraction expressed in percentage units demonstrate almost no difference between different H-titanates at pH values above 4 (see Figure 2).However, Kd values differ significantly-at least order of magnitude between long nanowire (NWL) and long nanotube (NTL) samples.The reason is the sensitivity of Kd values to very high (close to 100%) or very low (close to 0%) sorption percentage.In this range, the uncertainty of radioactivity measurements is even bigger than the "(100−%)" term in Equation ( 1).Thus, Kd values cannot be correctly used for the investigated system at pH > 4.
Appl.Sci.2019, 9, x FOR PEER REVIEW 6 of 11 uncertainty of radioactivity measurements is even bigger than the "(100−%)" term in Equation ( 1).Thus, Kd values cannot be correctly used for the investigated system at pH > 4.

Discussion
The obtained materials demonstrate different morphology, specific surface area, and sorption capacities towards Eu (III) ions.It must be noted that X-ray diffraction analysis demonstrates that all the materials are mixtures of different H-titanates and admixtures of titanium oxides.
The effect of pH on Eu (III) sorption percentage is shown in Figures 3 and 4. A pronounced rise of Eu (III) sorption from 0 to 100% is observed within pH values 2-4.The typical pH range of the trivalent metal cations sorption edge on different oxide surfaces is within 2-6 [3,[30][31][32].Thus, Htitanates demonstrate very strong binding with Eu (III).
The high values of the trivalent f-elements (lanthanides and actinides) removal (Figures 2 and 3) can be usually explained by their high tendency to sorb onto different mineral surfaces and by formation of strong inner-sphere complexes with different surface hydroxyl groups (e.g., ≡Ti-OH) [3,33].The spectroscopic studies and quantum-chemical calculations are ongoing to determine the sorbed complexes of Eu (III) onto H-titanates.
The investigated samples of H-titanates demonstrate total reversibility of Eu sorption (Figure 4).This is again typical for the sorption by formation of inner-sphere complexes with surface hydroxyl groups.This is similar to the commonly known hydrolysis reactions; hydrolysis species (like complexes with surface groups) form with pH increasing, but can be decomposed by increasing the acidity, i.e., pH decreasing.Europium is considered as a chemical analogue of trivalent actinides, which was confirmed in several sorption studies with different materials [34][35][36].Thus, H-titanates can be applied for reversible extraction of trivalent lanthanides and actinides.For example, trivalent f-elements can be sorbed at pH 7-8 (typical for many natural waters) and then be removed in acidic conditions (pH < 3) to recycle the sorbent.Additionally, sorbents with the adsorbed trivalent felements can be removed from the solution by centrifugation or filtration.After that, they can be annealed at temperatures above 300 °C to form very stable TiO2 phases in which lanthanides or actinides can be encapsulated.Thus, the investigated H-titanates can serve as a precursor of the stable matrix for the immobilization of trivalent actinides.

Discussion
The obtained materials demonstrate different morphology, specific surface area, and sorption capacities towards Eu (III) ions.It must be noted that X-ray diffraction analysis demonstrates that all the materials are mixtures of different H-titanates and admixtures of titanium oxides.
The effect of pH on Eu (III) sorption percentage is shown in Figures 3 and 4. A pronounced rise of Eu (III) sorption from 0 to 100% is observed within pH values 2-4.The typical pH range of the trivalent metal cations sorption edge on different oxide surfaces is within 2-6 [3,[30][31][32].Thus, H-titanates demonstrate very strong binding with Eu (III).
The high values of the trivalent f-elements (lanthanides and actinides) removal (Figures 2 and 3) can be usually explained by their high tendency to sorb onto different mineral surfaces and by formation of strong inner-sphere complexes with different surface hydroxyl groups (e.g., ≡Ti-OH) [3,33].The spectroscopic studies and quantum-chemical calculations are ongoing to determine the sorbed complexes of Eu (III) onto H-titanates.
The investigated samples of H-titanates demonstrate total reversibility of Eu sorption (Figure 4).This is again typical for the sorption by formation of inner-sphere complexes with surface hydroxyl groups.This is similar to the commonly known hydrolysis reactions; hydrolysis species (like complexes with surface groups) form with pH increasing, but can be decomposed by increasing the acidity, i.e., pH decreasing.Europium is considered as a chemical analogue of trivalent actinides, which was confirmed in several sorption studies with different materials [34][35][36].Thus, H-titanates can be applied for reversible extraction of trivalent lanthanides and actinides.For example, trivalent f-elements can be sorbed at pH 7-8 (typical for many natural waters) and then be removed in acidic conditions (pH < 3) to recycle the sorbent.Additionally, sorbents with the adsorbed trivalent f-elements can be removed from the solution by centrifugation or filtration.After that, they can be annealed at temperatures above 300 • C to form very stable TiO 2 phases in which lanthanides or actinides can be encapsulated.Thus, the investigated H-titanates can serve as a precursor of the stable matrix for the immobilization of trivalent actinides.
Fitting of the sorption isotherms with the Langmuir equation gives the following values for the maximum sorption capacity of the investigated samples: nanosheets 0.62 µmol m −2 (91.8 µmol g −1 , 14.0 mg g −1 ), nanowires short 0.45 µmol m −2 (46.8 µmol g −1 , 7.1 mg g −1 ), nanowires long 0.39 µmol m −2 (12.9 µmol g -1 , 2.0 mg g -1 ), nanotubes short 0.53 µmol m -2 (150.0 µmol g -1 , 22.8 mg g -1 ), nanotubes long 0.41 µmol m −2 (64.8 µmol g −1 , 9.8 mg g −1 ).The sorption capacity values of H-titanates of different morphology relative to the surface area are very close.That means affinity to Eu (III) and number of sorption sites per surface area are similar for the investigated samples.However, more precise studies are required to understand the difference.Due to the significant difference in specific surface area, the mass sorption capacity values (per g) differ from sample to sample.
Table 2 represents the comparison of Eu (III) sorption capacity of the investigated samples with some other materials.Different authors reported sorption capacity either as mg g −1 or as mol g −1 .
In Table 2 we have recalculated respective values by using the atomic mass of stable europium, equal to 151.964 according to the latest version of IUPAC Technical Report [37], and rounded the values to the first decimal place (the original numbers are presented as published).As it can be seen from Table 2, H-titanates are not superior materials for Eu sorption.However, fast and reversible sorption of different cations [21][22][23][24][25][26] and possibility for further transformation to a very stable TiO 2 make this materials perspective for applications in purification of water solutions.

Figure 2 .
Figure 2. Time dependence of Eu (III) removal from aqueous solutions at pH 4 by H-titanates of different morphology.

Figure 2 .
Figure 2. Time dependence of Eu (III) removal from aqueous solutions at pH 4 by H-titanates of different morphology.The removal of europium (III) from aqueous solutions at different pH values by H-titanates of different structures at steady state is shown in Figure 3.The complete recovery of Eu (III) occurs at pH 4 for all investigated H-titanates.

Figure 2 .
Figure 2. Time dependence of Eu (III) removal from aqueous solutions at pH 4 by H-titanates of different morphology.

Figure 5 .
Figure 5. Distribution coefficients of Eu (III) onto H-titanates of different morphology.

Figure 5 .
Figure 5. Distribution coefficients of Eu (III) onto H-titanates of different morphology.

Table 1 .
Specific surface area of the investigated H-titanates.

Table 1 .
Specific surface area of the investigated H-titanates.