Condensation of chromate into dichromate within magnesium/aluminum layered double hydroxide: An experimental and theoretical study

This study investigated the speciation of hexavalent chromium (Cr(VI)) anion when inserted in layered double hydroxides (LDH). Nitrated magnesium/aluminum (Mg/Al-NO 3 ) LDH nanoparticles (NP) were synthesized via fast coprecipitation followed by hydrothermal treatment with a Mg/Al molar ratio of 2. The saturation with hexavalent chromium (Cr(VI)) was easily performed by mixing the colloidal dispersion of LDH NP and a solution of chromate ion. Comprehensive characterization using powder X-ray diffraction (PXRD), Raman, Fourier-transform infrared (FTIR), and X-ray photoelectron spectroscopies, scanning electron microscopy (SEM) and transmission electron microscopy (TEM), thermogravimetric analysis (TGA) were performed on LDH nanoparticles in dry state. Ultraviolet-visible spectroscopy (UV–vis) captured the evolution of the Cr(VI) speciation during the hydration of the solid into colloidal aqueous suspension. The structural characterization data of both hydrated CrO 4 2− -LDH in colloidal state and dried Cr 2 O 7 2− -LDH nanoparticles in the solid state was interpreted with the help of density functional theory (DFT) calculation. Cr(VI) was demonstrated to keep its oxidation state and remain in the interlamellar space and not inserted in the cationic layer. The existence of Cr 2 O 7 2− along with CrO 4 2− in the dry state was unexpected, particularly when it was surrounded by hydroxide molecules in the interlamellar space of LDH, as this species is known to exist only in acidic solution. CrO 4 2− became fully predominant upon hydration, indicating that study only on the dry solid, usually performed, could not be used to explain phenomenon in wet conditions such as the ionic exchange in LDH. • The comprehensive physico-chemical characterization of CrO 4 2− -LDH and Cr 2 O 7 2− -LDH is completed DFT calculations. • Two Cr(VI) species CrO 4 2− and Cr 2 O 7 2− in reversible equilibrium coexist in Mg/Al LDH interlayers