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

Yoshifumi Kondo,Tohru Sekino,Tomoyo Goto

doi:10.1039/d1ra03088d

Yoshifumi Kondo, Tohru Sekino + Show 1 more

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https://doi.org/10.1039/d1ra03088d

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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 (HxNa2−xTi2O5) phase of nanofibers synthesised by the alkaline hydrothermal method and their Sr2+ sorption mechanism. A trititanate (HxNa2−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.

Highlights

94.7 90.6 167.8 12.2 a Determined by the multi-BET method by N2 adsorption; data ranging from p/p0 1⁄4 0.05 to 0.30
To investigate the effective optimal-synthesis conditions for seaweed-like sodium titanate mats (SSTs) mats, layered sodium titanates were synthesised by an alkaline hydrothermal process using 5–15 M NaOH solutions
Layered sodium titanates were tested as water puri cation materials for Sr2+ removal

Summary

Introduction

Numerous efforts have been undertaken to develop puri cation methods that can remove strontium from water effectively. We reported the synthesis of a unique seaweed-like sodium titanate mat (SST), which is composed of randomly distributed layered-sodium titanate nano bers, using a template-free alkaline hydrothermal process.[36] The SST showed a high Co2+ sorption capacity by ion-exchange reactions without self-precipitation of cobalt hydroxide in comparison with commercially available Na2Ti3O7 This previous study underscores the importance of understanding the reaction mechanism of cation removal on the functionalisation of sorbents, the synthesis conditions, such as the effect of the alkaline concentration on crystallographic properties of the SST, have not been sufficiently discussed.[36] Alkaline concentration (Na+) and solution pH are important factors for the formation of sodium titanate,[26] and optimisation of the material structure synthesis process is indispensable for improving its properties.

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Results & discussion

Conclusions