Adsorption mechanism of ReO$_4^−$ on Ni–Zn layered hydroxide salt and its application to removal of ReO$_4^−$ as a surrogate of TcO$_4^−$
Résumé
The long-lived anionic radionuclide of 99Tc (pertechnetate ion TcO4−) is highly mobile under oxic conditions, and therefore long-term removal and immobilization of TcO4− from radioactively contaminated water have been major challenges. Mg–Al layered double-hydroxides (LDH) have been often investigated to remove anions from waste water. In this study, we propose Ni–Zn layered hydroxide salt (LHS) as an alternative adsorbent for anionic radionuclides. We prepared and characterized Ni–Zn LHS with the nominal composition of Ni0.77Zn0.46(OH)2(OCOCH3)0.46·0.96H2O, using X-ray diffraction (XRD) pattern, scanning electron microbe (SEM) and extended X-ray absorption fine structure (EXAFS) spectroscopy. The formation of a layered structure was confirmed by XRD, and plate-like crystals of the Ni–Zn LHS were observed in SEM images. Analysis of Ni K-edge and Zn K-edge EXAFS spectra supported the structure proposed by previous works, where Ni ions form brucite-type layers with vacancies, and Zn ions were located at tetrahedral sites above and below the vacancy sites of the Ni hydroxide layers. The prepared Ni–Zn LHS was used for adsorption experiments of ReO4−, as a surrogate of TcO4−, in aqueous solutions with various initial Re and sodium salt concentrations. The maximum adsorption amount of Re was estimated at 127.7 mg/g (6.86 × 10−4 eq/g) by fitting adsorption isotherm of ReO4− to Langmuir plot. The adsorption of ReO4− at neutral pH was a reversible process by anion exchange, and decreased with increasing Cl−, NO3− and SO42− in solution. EXAFS analysis indicated that ReO4− was adsorbed as an outer-sphere complex on Ni–Zn LHS. The Ni–Zn LHS is a more robust adsorbent for ReO4− than the Mg–Al LDH in terms of solution pH and tolerance to competing anions, and may be an effective alternative to the traditional and more limited method of removing aqueous TcO4− by reductive precipitation.