Strontium ion (Sr2+) separation from seawater by hydrothermally structured titanate nanotubes: Removal vs. recovery

Strontium ion (Sr2+) separation from seawater has attracted attention for radioactive pollutants removal and for Sr2+ recovery. Herein, we synthesized titanate nanotubes (TiNTs) via a simple hydrothermal reaction, characterized their physicochemical properties, and systematically evaluated Sr2+ sorption behavior under various reaction conditions corresponding to seawater environments. The synthesized TiNTs exhibited a fibril-type nanotube structure with a high specific surface area (260 m2/g). Sr2+ adsorption on TiNTs rapidly occurred following a pseudo-second-order kinetic model and was in good agreement with the Langmuir isotherm model, indicating a maximum adsorption capacity of 97 mg/g. Based on the Sr2+ uptake and Na+ release with a stoichiometric balance, the Sr2+ sorption mechanism on TiNTs was ion exchange between Na+ in the TiNT lattice and Sr2+ in the solution phase, as confirmed by XRD and Raman analysis. Among the competitive ions, Ca2+ significantly hindered Sr2+ sorption on TiNTs, whereas Na+ only slightly affected Sr2+ sorption, despite the Na+ exchange sorption mechanism. The effect of Ca2+ on Sr2+ sorption was evaluated by introducing a distribution coefficient (Kd) as a critical factor in determining the selectivity, which revealed a slightly higher selectivity for Sr2+. The Sr2+ adsorption-desorption test in a real seawater medium enabled the determination of Kd and the concentration factor (CF) for co-existing matrix ions in seawater; these values were evaluated for Sr2+ removal and recovery from seawater. TiNTs were regenerated by acid treatment and reused through consecutive adsorption-desorption experiments. While most studies addressing Sr2+ sorption using TiNTs aimed for extraction from wastewater and radioactive wastewater, this study elucidated Sr2+ sorption behavior under seawater conditions and provided insights into developing the removal and recovery processes from seawater.