# Sorption of Eu (III) onto Nano-Sized H-Titanates of Different Structures
## 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, 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., \(\equiv\)T-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 \({}^{\circ}\)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 \(\upmu\)mol m\({}^{-2}\) (91.8 \(\upmu\)mol g\({}^{-1}\)  
Figure 5: Distribution coefficients of Eu (III) onto H-titanates of different morphology. 14.0 mg g\({}^{-1}\)), nanowires short 0.45 umol m\({}^{-2}\) (46.8 umol g\({}^{-1}\), 7.1 mg g\({}^{-1}\)), nanowires long 0.39 umol m\({}^{-2}\) (12.9 umol g\({}^{-1}\), 2.0 mg g\({}^{-1}\)), nanotubes short 0.53 umol m\({}^{-2}\) (150.0 umol g\({}^{-1}\), 22.8 mg g\({}^{-1}\)), nanotubes long 0.41 umol m\({}^{-2}\) (64.8 umol 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.