Adsorption mechanism of ReO4- on Ni-Zn layered hydroxide salt and its application to removal of ReO4- as a surrogate of TcO4-

The long-lived anionic radionuclide of Tc-99 (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)center dot 0.96H(2)O, 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 x 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.