# Sorption of Eu (III) onto Nano-Sized H-Titanates of Different Structures
## 2 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 \({}^{\circ}\)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 \({}^{\circ}\)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 \({}^{\circ}\)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 \({}^{\circ}\)C for 48 h. Long titanate nanowires were synthesized at 170 \({}^{\circ}\)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 \({}^{\circ}\)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). \({}^{152}\)Eu (T\({}_{1/2}\) = 13.54 years) was added as tracer to measure europium content by \(\gamma\)-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.