Sr2+ sorption property of seaweed-like sodium titanate mats: effects of crystallographic properties

Abstract

Layered sodium titanate is a typical ion-exchanger for water purification aimed at removing cationic heavy metals and radionuclides. The material design of an ion-exchanger is effective for cation removal. For that purpose, understanding the basic impacts of crystallographic properties such as crystal size, morphology, and phase is critical for developing highly functional nanoscale ion-exchangers. In this study, we investigate the principal relationship between the crystallographic properties of seaweed-like sodium titanate mats (SSTs), which consist of a dititanate (H_xNa_2−xTi₂O₅) phase of nanofibers synthesised by the alkaline hydrothermal method and their Sr²⁺ sorption mechanism. A trititanate (H_xNa_2−xTi₃O₇) phase, which has a micro-sized fibre morphology, was also synthesised using the same method by adjusting the NaOH concentration. The SST demonstrates a high ion-exchange selectivity of Sr²⁺ against H⁺ and a high maximum sorption capacity (2 mmol g⁻¹), which was four times higher than that of the trititanate phase (0.49 mmol g⁻¹). In contrast, the trititanate phase, which is the comparison target, had a low Sr²⁺ ion-exchange selectivity and precipitated SrCO₃. We conclude that these differences in Sr²⁺ sorption mechanisms were derived from not only the unique morphology but also the crystal structure of sodium titanates. Although almost all of the Na⁺ in dititanate with lamellar structure was consumed by the ion-exchange reaction, some Na⁺ remained in the trititanate because there are two sites in the zigzag layered structure. These findings on the crystallographic properties of SST for Sr²⁺ sorption may contribute to the functionalisation of a nanoscale ion-exchanger.