Chromate uptake characteristics of the pristine layered double hydroxides of Mg with Al

The layered double hydroxides (LDHs) of Mg with Al readily scavenge dissolved chromate ions under a wide range of conditions. While the chromate uptake is quantitative in the LDH containing 33 mol% Al, the uptake is only 58% of the stoichiometric value in the LDH containing 25 mol% Al. This indicates that the lower symmetry of the NO3− ions in the LDH with 33 mol% Al facilitates the intercalation of chromate ions even under conditions of equilibration with excess dissolved nitrate ions. The chromate uptake obeys the Langmuir adsorption isotherm suggesting that the entire interlayer region of the LDH behaves like a surface. This surface is structural rather than morphological as the chromate uptake correlates negatively with the BET surface area of the LDHs.

ATR measurements show the lifting of the degeneracy of the ν3 (E′) mode (1340 cm−1 and 1420 cm−1) of NO3− in the [Mg–Al–NO3]0.33 LDH indicating the C2v coordination symmetry of the NO3− ion as opposed to D3h in the x = 0.25 composition. The orientation of the NO3− ion profoundly affects the chromate uptake characteristics of the two LDHs.Figure optionsDownload as PowerPoint slide