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

다국어 초록 (Multilingual Abstract)

<P><B>Abstract</B></P> <P>Strontium ion (Sr<SUP>2+</SUP>) separation from seawater has attracted attention for radioactive pollutants removal and for Sr<SUP>2+</SUP> recovery. Herein, we synthesized titanate nanotubes (TiNTs) via a simple hydrothermal reaction, characterized their physicochemical properties, and systematically evaluated Sr<SUP>2+</SUP> 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 (260m<SUP>2</SUP>/g). Sr<SUP>2+</SUP> 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 97mg/g. Based on the Sr<SUP>2+</SUP> uptake and Na<SUP>+</SUP> release with a stoichiometric balance, the Sr<SUP>2+</SUP> sorption mechanism on TiNTs was ion exchange between Na<SUP>+</SUP> in the TiNT lattice and Sr<SUP>2+</SUP> in the solution phase, as confirmed by XRD and Raman analysis. Among the competitive ions, Ca<SUP>2+</SUP> significantly hindered Sr<SUP>2+</SUP> sorption on TiNTs, whereas Na<SUP>+</SUP> only slightly affected Sr<SUP>2+</SUP> sorption, despite the Na<SUP>+</SUP> exchange sorption mechanism. The effect of Ca<SUP>2+</SUP> on Sr<SUP>2+</SUP> sorption was evaluated by introducing a distribution coefficient (<I>K<SUB>d</SUB> </I>) as a critical factor in determining the selectivity, which revealed a slightly higher selectivity for Sr<SUP>2+</SUP>. The Sr<SUP>2+</SUP> adsorption-desorption test in a real seawater medium enabled the determination of <I>K<SUB>d</SUB> </I> and the concentration factor (CF) for co-existing matrix ions in seawater; these values were evaluated for Sr<SUP>2+</SUP> removal and recovery from seawater. TiNTs were regenerated by acid treatment and reused through consecutive adsorption-desorption experiments. While most studies addressing Sr<SUP>2+</SUP> sorption using TiNTs aimed for extraction from wastewater and radioactive wastewater, this study elucidated Sr<SUP>2+</SUP> sorption behavior under seawater conditions and provided insights into developing the removal and recovery processes from seawater.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Titanate nanotubes (TiNTs) were synthesized by a simple hydrothermal reaction. </LI> <LI> The sorption of strontium (Sr) on TiNTs rapidly occurred, achieving Sr uptake 97mg/g. </LI> <LI> Na had little effect on Sr sorption despite the sorption mechanism of the Na exchange. </LI> <LI> Ca significantly hindered Sr sorption on TiNTs among co-existing cations in seawater. </LI> <LI> TiNTs could be easily regenerated by acid treatment and reused for repeated cycles. </LI> </UL> </P>