Chemical associations of selenium oxyanions in metal oxides derived from layered double hydroxides: Implication for the immobilization of radionuclides.

Layered double hydroxides (LDHs) can effectively stabilize Se oxyanions, yet the thermal stability of Se oxyanions incorporated into LDHs remains unclear. In this study, calcination products of three types of LDHs loaded with SeO2- 3 or SeO2-4 were analyzed using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), X-ray absorption fine structure spectroscopy (XAFS) and leaching tests. It has been found that SeO2-4 can be reduced to SeO2- 3 in the Fe-containing LDHs after calcination at temperatures above 450 °C. However, Se oxyanions in MgAl-LDHs retained the same valence following calcination. This was primarily due to the presence of Fe-OH laminates acting as electron donors in the calcination process. Only approximately 13% SeO2- 3 was associated with iron oxides. Most of SeO2- 3 was associated with Mg oxides through monodentate mononuclear and binuclear bidentate complexations in the case of MgFe-CLDH initially loaded with SeO2-4. In contrast, SeO2- 3 was associated with Mg oxides in the formation of both bidentate mononuclear and binuclear bidentate complexes for calcined MgFe-LDH initially loaded with SeO2- 3. For MgAl-CLDH, SeO2- 3 can associate with Mg oxides as binuclear bidentate complexes. As for the leaching process, the SeO2- 3 association with iron oxide to some extent contributes to the Se immobilization. However, the ion-exchanging behavior of Se oxyanions with other anions during rehydration of CLDHs posed a stronger influence on its leaching out than the influence of valence changing and complexes associations. These findings offer valuable insights into the stabilization of Se radionuclides using LDHs.