Preparation and Characterisation of Mg–Al Layered Double Hydroxides

AMg–Al layered double hydroxide (Mg–Al LDH) has been modified with aromatic anions by a coprecipitation technique. This method is based on the anion-exchange characteristics of the hydrotalcite-type compound which intercalates various anions, including anions of organic acids, in the interlayer. The Mg–Al LDHs are intercalated with 2-naphthalene sulphonate (2-NS) and 2,6-naphthalene disulphonate (2,6-NDS2) ions, which contain a naphthalene ring with one and two sulphonate (–SO3) groups in their structures, respectively. They are formed by the dropwise addition of a mixed aqueous solution of Mg(NO3)2 and Al(NO3)3 (Al/Mg mole ratio = 1/3) to a solution containing the individual organic anion at a constant pH of 10.0. The Mg–Al LDHs modified with aromatic anion are expected to recognize and selectively uptake hazardous aromatic compounds, such as bisphenol A, from aqueous solutions. The composite materials prepared were characterized by X-ray diffraction, FT-IR spectroscopy and chemical analyses. 2-NS was observed to be intercalated into the interlayer almost fully up to the accommodation expected based on the neutralisation of the positive charge of the host lattice. The intercalation of 2,6-NDS2 was approximately 80 % of the expected value, however. IR analysis results show that the organic anions are intercalated into the Mg–Al LDH without any significant change in their intrinsic structure. The X-ray diffraction data suggest that the intercalation of the organic anions under consideration was accompanied by an expansion in the basal spacing of the LDHs. The X-ray diffraction patterns of the 2-NS-modified LDH suggested the formation of two types of LDHs showing different basal spacing at higher 2-NS content. On the other hand, the 2,6-NDS-modified LDH showed diffraction peaks, which corresponde