As, Se, and Re sorption by Mg-Al layered double hydroxides

Layered double hydroxides (LDHs) have high anion exchange capacities and are easily synthesized in the laboratory, giving them considerable potential as adsorbents for anionic contaminants in the environment. In this study, sorption of arsenate, selenate, and perrhenate by uncalcined Mg-Al LDH (Mg:Al ratio 2.4: 1) was evaluated. To investigate sorption mechanisms, the substrate was added to the solution both as a pre-wetted slurry (allowing 24 hours for rehydration), and as a dry powder. Sorption results varied for different anionic complexes. For selenate and perrhenate, an incubation time was required for the dry material to achieve the same K-d value as the pre-wetted material. For arsenate, however, both wetted and dry materials initially had high K(d)s, which decreased with time. These behaviors are attributed to the structural reconstruction of the calcined LDH material in solution. Se and Re are not highly sorbed by the phases present in the calcined material, but are incorporated as interlayer anions in the LDH structure upon hydration and reconstruction, while As may be sorbing onto the dehydration products, and being partially released during rehydration.