Perchlorate uptake and molecular mechanisms by magnesium/aluminum carbonate layered double hydroxides and the calcined layered double hydroxides

Abstract

A series of Mg/Al carbonate layered double hydroxides (LDHs) with different Mg/Al ratios and their calcined LDHs (CLDHs) were prepared as a promising adsorbent of perchlorate anionic pollutant. The LDHs, CLDHs and reconstructed LDHs were characterized by X-ray diffraction, SEM, Raman and FTIR spectra. The adsorption of perchlorate to the parent LDHs was very weak, and the isotherms fit well to the Freundlich model. The perchlorate uptake was independent of the positive charge density of the hydroxide layers because the interlayer carbonate was not significantly replaced by perchlorate. In contrast, the CLDHs exhibited a high adsorption capacity with a nonlinear isotherm, which was mainly driven by the structural memory effect of the LDHs with perchlorate as an interlayer anion. The perchlorate uptake by the CLDHs was enhanced as the positive layer charge density decreased, which cannot be explained by anion exchange mechanism alone. According to the Cl–O and O–H stretching vibrations of the reconstructed ClO4--LDHs, we proposed that perchlorate transfer a negative charge to positively hydroxide layers via electrostatic interactions, then the oxygen atoms of perchlorate are polarized by the surrounding hydroxyls and form strong hydrogen bonds with the hydroxide layers.