Layered double hydroxides(LDHs), also called anionic clay minerals or hydrotalcite-like compounds(HTIcs), are a family of hydroxylated hydrate compounds of general formula;
$[M(II)_{1-x}M(III)_x(OH)_2][X^{m-}_{x/m}\cdot{nH_2O}]$
In these compounds, the metal cations are located in $M(OH)_2$, brucite-like sheets that are positively charged when M(II) sites are occupied by M(Ⅲ) instead, then anions can be located in between the hydroxylated sheets in order to compensate for positive charge, and hydrated by water molecules.
Several LDHs(Mg-Al-$CO_3$ systems) of varying $Mg^{2+}$/$Al^{3+}$ ratios were synthesized by a coprecipitation method, and were characterized by X-ray powder diffraction, FT-IR, $^{13}C$-CP MAS NMR, $^{27}AI$-MAS NMR, TGA/DSC, ICP-AES and other methods. X-ray powder patterns represent the products were layered structure materials. FT-IR and $^{13]C$-CP MAS NMR indicate that the carbonate anion was lied in a symmetrical site between two adjacent layers in parallel direction. $^{27}AI$-MAS NMR spectra show only single resonance due to octahedrally coordinated aluminum. According to TGA/DSC thermograms, LDHs were generally characterized by two endothermic transitions. The first transition was obserbed around 100-280℃ due to dehydration of interlayer water and the second around 280-500℃ due to dehydroxylation and decarbonation. Then the homogeneous mixed metal oxides formed.
The rehydration reactions of calcined LDHs in aqueous chromate solution were investigated with various reaction conditions such as chromate concentration, calcination temperature and time, x value, competition of other anions, atmospheric conditions and pH at 298˚K. From the results, the mixed metal oxides, calcined at 340 - 760℃ were readily rehydratedin aqueous chromate solution to reconstruct original layered structure and optimum calcination temperature range were 560 - 600℃. The rehydration reaction of calcined LDHs were Langmuir type adsorption with single reaction site and not affected by calcination time. The maximum amount of adsorption was 2.5 mmol·$g^{-1}$ when x were 0.25 and 0.27. The adsorption of chromate anion was strongly affected by the anions which had high charge density and the inhibition of carbon dioxide in air was highly pH dependent.
The rehydration reactions of calcined LDHs in aqueous salt solution are expected to apply for removal and fixation of toxic anions in industrial wastewater.
