Sorption of radioactive anions $(TcO_4^-,I^-)$ on Mg/Al layered double hydroxide

Abstract

Hydrotalcite-like compounds have an anion-exchange capacity of around $3 meq g^{-1}$ and can be readily synthesized under laboratory conditions. These synthetic anionic clays are referred as layered double hydroxide (LDH). The sorption behavior of radioactive anions, $TcO_4^-$ and $I^-$, on calcined Mg/Al LDH was observed. Some studies to investigate the reaction equilibrium and mechanism for the sorption followed. The Mg/Al LDH with carbonate anion and x ($x = Al^{3+}/Al^{3+}+Mg^{2+}$) of 0.25 was synthesized in the laboratory. The calcined LDH was obtained by heating the synthesized LDH at $560℃ for 3 hours. $TcO_4^-$ and $I^-$ anions including radioisotope of Tc-99 and I-125 in aqueous solutions were used for sorption experiments. The $ReO_4^-$ was used to synthesize the reconstructed LDH. The Mg/Al LDH with carbonate anion, calcined LDH and reconstructed LDH with other anions were characterized by powder X-ray diffraction (XRD), FT-IR and $^{27}MAS$ NMR spectroscopies. Powder XRD pattern of synthesized Mg/Al LDH with carbonate anion is in good agreement with that of LDH cited in the literature. Heating at the temperature of 560℃ destroys the layered structure of LDH, losing 003 and 006 reflections in the XRD pattern of the calcined LDH. Contacting with aqueous $ReO_4^-$ or $I^-$ solutions reconstructs the layered structure, showing again 003 and 006 reflections in the XRD patterns of LDH$(ReO_4)_2$ and LDH$(I)_2$. FT-IR and $^{27}Al$ MAS NMR spectroscopies also confirm this destruction and reconstruction property of LDH. This "memory effect" may prove the calcined LDH to be useful for sorbing anionic species from industrial and nuclear wastes. The sorption experiments were carried out in an inert atmosphere and at a constant temperature of 25℃. The $TcO_4^-$, $ReO_4^-$ and $I^-$ solutions with various concentrations were prepared. The calcined LDH was added to the solutions and the solid-liquid suspension was stirred. The time dependence of sorption reac...