Layered double hydroxides (LDHs) are promising compounds in a wide range of fields. However, exchange of CO 3 2− anions with other anions is necessary, because the CO 3 2− anions are strongly affixed in the LDH interlayer space. To elucidate the reason for the extremely high stability of CO 3 2− anions intercalated in LDHs, we investigated in detail the chemical states of CO 3 2− anions and hydrated water molecules in the LDH interlayer space by synchrotron radiation X-ray diffraction, solid-state NMR spectroscopy, and Raman spectroscopy. We found the rigidity of the network structure formed between the CO 3 2− anions, hydrated water molecules, and the hydroxyl groups on the metal hydroxide layer surface to be a crucial factor underlying the stability of CO 3 2− anions in the LDH interlayer space.

To understand the origin of the nitrate anion selectivity in layered double hydroxide (LDH; [Ni1−xAlx(OH)2]Clx) materials (i.e., Cl−-NiAl(x)LDH; x = 1/3 and 1/5) comprising Ni2+, Al3+, and exchangeable chloride anions, the equilibrium and kinetic properties of the chloride-to-nitrate anion exchange reaction were investigated in aqueous media. The anion exchange isotherms of the studied systems showed that the nitrate anion selectivity was significantly higher at x = 1/5 than at x = 1/3. The fine crystal structures of the LDH materials and the hydration states of the nitrate anions in their interlayer spaces were identified by synchrotron-radiation X-ray diffraction (SXRD) and Raman spectroscopy, respectively. The nitrate anion selectivities of Cl−-NiAl(x)LDH (x = 1/3 and 1/5) were determined by the relationship between the magnitudes of the thermal vibration parameters of the chloride and nitrate anions; the anion exchange reaction primarily gave NiAl(x)LDH containing anions with smaller thermal vibration parameters. Time-resolved SXRD measurements of the anion exchange reaction revealed that the exchange pathway differed depending on the nitrate anion selectivity. The nitrate anion removal ability and selectivity of Cl−-NiAl(x)LDH (x = 1/3 and 1/5) in artificial seawater were maintained, suggesting that Cl−-NiAl(1/5)LDH could be used for nitrate removal from contaminated seawater.