# Sorption of Eu (III) onto Nano-Sized H-Titanates of Different Structures
## 3 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.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Sample** & **Specific Surface Area, m\({}^{2}\) g\({}^{-1}\)** \\ \hline Nanowires short, NWS & 104 \\ Nanowires long, NWL & 33 \\ Nanotubes short, NTS & 283 \\ Nanotubes long, NTL & 158 \\ Nanosheets, NS & 148 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Specific surface area of the investigated H-titanates.  
Figure 1: SEM images of H-titanates of different structures: (**a**) nanotubes short; (**b**) nanotubes long; (**c**) nanowires short; (**d**) nanowires long; (**e**) nanosheets.  
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 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.  
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.  
Figure 3: Removal (%) of europium (III) from aqueous solutions at different pH values.  
Figure 2: Time dependence of Eu (III) removal from aqueous solutions at pH 4 by H-titanates of different morphology.  
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):  
\[Kd=\frac{\%/(S\cdot m)}{(100-\%)/V} \tag{1}\]  
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,  
Figure 4: Results of Eu (III) sorption–desorption experiments for: (a) nanotubes short; (b) nanotubes long; (c) nanowires short; (d) nanowires long; (e) nanosheets.  
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.